Issuu

Geological Society of Australia

ABSTRACTS Numbers

Jointly sponsored by the

Petroleum Exploration Society of Australia and the

Geological Society of Australia

ADELAIDE iVIovember 9 - 11th, 1982

EROMANGA BASIN SYMPOSIUM

JOINTLY SPONSORED BY THE GEOLOGICAL SOCIETY OF AUSTRALIA AND THE PETROLEUM EXPLORATION SOCIETY OF AUSTRALIA (S.A. BRANCHES)

ABSTRACT VOLUME

EDITED BY P.S. MOORE & T.J. MOUNT

GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS NO. 5 1982

SYMPOSIUM COMMITTEE

Mr Terry Barr (Social Manager), SANTOS Limited Mr Michael Cobb (Treasurer), S.A. Dept. Mines & Energy Dr David Gravestock (Secretary), S.A. Dept. Mines & Energy Dr Peter Moore (Convenor), Delhi Petroleum Pty. Ltd. Dr Trevor Mount (Programme Manager), Delhi Petroleum Pty. Ltd. Mr Graham Pitt (Public Relations), Western Mining Corporation Mr Tony Williams (Senior Whip), SANTOS Limited

ACKNOWLEDGEMENTS

Ms Sue Buckley (Publications), Delhi Petroleum Pty. Ltd. Mr Philip Burnett (Satchel), Delhi Petroleum Pty. Ltd. Mr Mark Griffiths (Core displays), S.A. Dept. Mines & Energy Mr Ross Skerman (Company displays), Delhi Petroleum Pty. Ltd. Ms Carolyn Wakefield (Typing), SANTOS Limited

SPONSORS

A list of sponsors is included at the back of this volume.

PUBLICATIONS SALES

Copies of this volume can be purchased from: Geological Society of Australia Inc., Challis House, 10 Martin Place, SYDNEY, N.S.W. 2000 or from the South Australian Divisions of either PESA or GSA.

lii

PREFACE

The Mesozoic Eromanga Basin covers a large area of Australia, spanning four states. Principal areas of outcrop are widely separated, east and west, while knowledge of the basin in the central areas has formerly relied on subsurface information from shallow artesian bores. In the last decade, however, a large amount of subsurface data has accumulated from the search for hydrocarbons. Significant discoveries include oil flows at Poolowanna, Dullingari, Strzelecki, Merrimelia and Jackson, and gas at Namur. All of these discoveries have been made since 1977.

% ^ ^

Exploration of the Eromanga basin by oil companies has been accompanied by substantial Federal and State government research into the basin. Private consulting groups have also been active. As a result, an urgent need has arisen to integrate this knowledge, and rationalise the study of the Eromanga Basin to provide a solid foundation for further work. The Eromanga Basin Symposium, held in Adelaide on 9~llth November, 1982, is the first step in this direction.

Abstracts of the papers to be delivered to the Eromanga Basin Symposium are set out in the following pages, in probable order of presentation. Minor changes have been made to abstracts to maintain uniformity and for clarity. All copy was typed by Carolyn Wakefield of SANTOS Limited.

iv CONTENTS

KEYNOTE ADDRESSES

PAGE

SPRIGG, R.C. The long lead-up to commerlcal oil discovery in the Mesozoic Eromanga Basin

ARMSTRONG, J.D. & BARR, T.M. The Eromanga Basin

r STRATIGRAPHY AND STRUCTURE

MOSS, F.J. The Central Eromanga Basin Project - a contribution to the regional study of the Eromanga and underlying basins

MOORE, P.S. Mesozoic geology of the Simpson Desert region, northern South Australia

WILTSHIRE, M.J. Late Triassic and Early Jurassic sedimentation in the Great Artesian Basin

WILTSHIRE, M.J. Revision of Eromanga Basin limits

YOUNGS, B.C. & BOOTHBY, P.G. The Middle Permian - Middle Triassic rocks of the southwestern Cooper Basin

GRAVESTOCK, D.I. Jurassic to Lower Cretaceous stratigraphy of the Eromanga Basin - problems and progress in subsurface correlation

PAGE

AMBROSE, G., SUTTILL, R. & LAYERING, I. A review of the Early Cretaceous Murta Member in the southern Eromanga Basin

FORBES, B.G. Margin of the Eromanga Basin South Australia: a review

MOORE, P.S. & PITT, G.M. Cretaceous of the southwestern Eromanga Basin: stratigraphy, facies variations and petroleum potential

KRIEG, G.W. Stratigraphy and tectonics of the Dalhousie Anticline, southwest Eromanga Basin

WAKE-DYSTER, K.D. The relationship between the Thomson Syncline and the underlying Barcoo Trough

PINCHIN, J. & ANFILOFF, V. The Canaway Fault and its effects on the Eromanga Basin

PALYNOLOGY

McKELLAR, J-L. Late Triassic ('Rhaetian') and Jurassic palynostratigraphy of the Surat Basin

BURGER, D. Palynology of the Eromanga Basin and its applications

PAGE DETTMAN, M.E., FILATOFF, J. & PRICE, P.L. The Eromanga and Surat Basins: a palynostratigraphical perspective

GEOPHYSICS

ANFILOFF, V. Gravity features in the Eromanga Basin

SPENCE, A.G. & FINLAYSON, D.M. Resistivity structure of the central Eromanga Basin and underlying sequences from magnetotelluric soundings

LOCK, J. Basement structure under the central Eromanga Basin from seismic refraction studies

RUMPH, B. Seismic data from the Eromanga Basin

OIL SHALES AND THE TOOLEBUC FORMATION

SCHEIBNEROVA, V. Foraminifera of the marine Cretaceous of the Great Artesian Basin with special reference to the Toolebuc Formation

OZIMIC, S. Depositional environment of the Toolebuc Formation and its equivalents, Eromanga Basin, Australia

GLIKSON, M. Cynobacterial mats and other bacteria: major contributors to the formation of Lower Cretaceous Toolebuc oil shales

vli

PAGE RILEY, K.W. & SAXBY, J.D. Organic matter and vanadium in the Toolebuc Formation of the Eromanga Basin

EKSTROM, A., LOEH, A. & DALE, L. The petroporphyrins in the oil shale from the Julia Creek deposit

SAXBY, J.R. Geochemistry of oil shale in the eastern Eromanga Basin

OZIMIC, S. The significance of gamma-ray anomalies in the Cretaceous Toolebuc Formation facies and in their lateral equivalents, Eromanga and Carpentaria Basins

SOURCE ROCKS

COOK, A.C. Organic facies in the Eromanga Basin

McKIRDY, D,M. Aspects of the source rock and petroleum geochemistry of the Eromanga Basin

MATURATION AND GENERATION

PITT, G.M. Geothermal gradients in the Eromanga-Cooper Basin region

KANTSLER, A.J., COOK, A.C. & ZWIGULIS, M. Maturation patterns in the Eromanga Basin

PASSMORE, V.L. & BOREHAM, C.J. Subsidence and associated source rock maturation for the central Eromanga Basin

viii

PAGE MIGRATION AND ENTRAPMENT

HOLMES, JSome aspects of the theory of hydrodynamic entrapment of oil and gas

HABERMEHL, M.A. Aspects of regional groundwater movement and hydrocarbon migration in the Eromanga Basin

MORIARTY, K.C. & WILLIAMS, A.F. Hydrocarbon flushing in the Eromanga Basin - fact or fallacy

SENIOR, B.R. Landsat interpreted structure and groundwater flow within the Eromanga Basin

HYDROCARBON FIELDS

BOWERING, O.J.W, & HARRISON, D.M. The Merrimelia oil and gas field

PATON, I.M. The Birkhead Formation: petroleum reservoir

a Jurassic

MOUNT, T.J. Geology of the Dullingari Murta oilfield

POSTER DISPLAYS

AMBROSE, G.J. & FLINT, R.B. Mesozoic stratigraphy - southwestern margin of the Eromanga Basin

PAGE BUCKLEY, R.C. Environmental aspects of exploration in the Eroraanga Basin arid zone

HUNTING GEOLOGY AND GEOPHYSICS (AUST.) PTY. LTD. Landsat interpreted structure and groundwater flow within the Eromanga and Surat Basins

100

LIST OF SPONSORS

101

AUTHOR INDEX

102

STRATIGRAPHIC INDEX

103

KEYNOTE ADDRESSES

THE LONG LEAD-UP TO COMMERCIAL OIL DISCOVERY IN THE MESOZOIC EROMANGA BASIN

Reg. C. Sprigg

Consultant Geologist, Arkaroola Village, via PORT AUGUSTA, S.A. 5700

The Great Artesian Basin is devoid of obvious hydrocarbon seepages. Most of its deeper sediments are non-marine. None of the widespread thousands of Artesian water bores produced a single flow of oil or gas. However, small volumes of methane were known to escape with artesian water from the Coonananna and Yandama bores lying northeast of Lake Frome and elsewhere. Early geologists regarded the Artesian Basin as a structurally stable, Mesozoic development; a simple saucer-like depression interrupted only by buried basement ridges and local warpings. Dr R.L. Jack in 1924 first mapped definite fold structure at Haddon Downs in the northeast corner of the State in the course of underground water investigations. His factual geological cross-sections disclosed 200 metres of structural turn-over in Tertiary outcrop, overlying with strong erosional unconformity a still steeper-limbed Late Cretaceous anticline. The significance of this Mesozoic-Tertiary growth structure went almost unrecognised for another 30 years. Dr Woolnough, visiting the area in 1927, missed this and, at Innamincka, reported only flat-lying Tertiary mesas surrounding a central core of "gneissic granite". This was presumably the giant Innamincka dome development in thick Mesozoic and Palaeozoic sediments. The first Eromanga crude oil "discovery" came fortuitously in 1925 in drilling the Longreach Town water bore. Lander Oil Company moved in and, two years later, confirmed the discovery by producing several barrels of waxy crude oil from lower Mesozoic sands overlying granite basement. 1927 saw growing eastern States' interest in the conversion of coal to oil. This led to authoritative pronouncements that Australia's extensive coal basins may well source significant petroleum, a fact which most geologists seemed unable to accept for another 30 years or more. This was also despite Cosmo Newberry's discovery in 1890 of traces of free oil in the Leigh Creek, Triassic coal measures, and the discovery of petroliferous gas at Roma in 1900. Oriomo Oil Ltd. subsequently took over regional exploration about Longreach, and then, late in the 1930s, Shell Development (Queensland) extended geological exploration in this area and carried gravity-magnetic surveys far across southwestern Queensland. In the mid 1940s, Zinc Corporation reported favourably on the natural gas potential of the Frome Embayment and centred their initial activities on the Joulnie anticline. Frome Broken Hill Company then expanded gravity-magnetic surveys across Innamincka Dome (then unrecognised) to Haddon Downs and to the Flinders Ranges. Interpretation of up to 3000 metres of supra-basement sediments was noted south of Innamincka. Seven wells were drilled by the early

1950s without encouragement. In the late 1940s, Dr Ivan Tschebotarev, of the South Australian Mines Department, reviewed hydro-chemical aspects of artesian basin waters and concluded, on the basis of his Russian oil-field experience, that the best place to seek "Jurassic" oil in South Australia was in the northeast. In 1952, W.D. Mott summarised oil and gas shows in artesian wells and noted them to be widespread in Queensland, thereby providing new encouragement. Still it was the 1954 discovery of oil in the Mesozoic at Rough Range, Western Australia, that was to provide real exploration impetus. John Bonython then formed SANTOS Limited, for whom Reg Sprigg's Geosurveys of Australia Ltd. operated and progressively took up most of the central-western basin as major companies lost interest. An anticline was mapped and drilled near Oodnadatta to disclose free oil traces in the Jurassic and Lower Cretaceous. Interest was then transferred to Haddon Downs and Innamincka where groups of anticlines were shown to extend far back into Queensland (Sprigg, 1958). The major drainage occupied the broader synclines. BMR seismic then confirmed the Oodnadatta and Haddon anticlines in depth, after which SANTOS farmed-out a half interest in all its acreage to Delhi Taylor Corp. of Dallas. The drilling of Innamincka and Beetoota anticlines thereafter revealed oil showings at a number of levels within the Jurassic as well as gas showings in Innamincka Permian. Low oil prices and poor test results accorded the Jurassic shows little interest. Five wells later, gas discovery in the underlying Permian deflected interest almost completely away from the JuraCretaceous for the next decade or so. Commonwealth Government exploration subsidies in the late 1950s greatly encouraged more sophisticated oil exploration technology. This culminated in widespread hydrocarbon discoveries across Australia, as well as grossly improving basin understanding generally. By 1970, Brooks of C.S.I.R.O. and others were clearly regarding Australia's sedimentary coaly detritus as potential sourcing material for petroleum generation. Coal hydrocarbons were seen to change abruptly in the sub-bituminous coal range to produce petroleum-type hydrocarbons from components of waxy leaf cuticles, pollen and spore casings. They noted further that oil occurs in the Cooper infra-basin where the coals are of lower rank (80-85% carbon). The central deeper Eromanga Basin also proved to fit this category. Thomas (1982) concluded further that land plants contribution to oil formation related particularly to accumulation in the paralic-deltaic or marginal marine environment to which a number of siltstone-shale sequences of the Eromanga Jurassic related. It was concluded that generation and migration of petroleum fluids was primarily a function of time, temperature and depth of burial. Furthermore, Thomas (1982) noted that pre-Jurassic coal measures are deficient in exinite materials and are therefore mainly gas-prone. In contrast, Jurassic to Tertiary coal-rich sediments often contain abundant exinite and may have substantial potential to generate oil in commercial quantities. This is the result of the dominance of conifers in the swamp floras of those periods, together with the evolution of flowering plants in the Late Cretaceous. Poolowanna Eromanga oil was discovered in the Simpson Desert by the SANTOS-Delhi-WMC partners in 1977. The first commercial oil flow was 2400 bpd from the Hutton Sandstone in Strzelecki 3 in 1978. In 1979, oil flowed at 450 bpd from the Murta Member in Dullingari North 1. A series

of discoveries have followed, Including Big Lake, Cuttapirrie, Jackson, Marabooka, McKinlay, Merrimelia, Moorari, Naraur and Wackett, in reservoirs all the way from the 'basal' Jurassic up into the Early Cretaceous Coorikiana Sandstone. A broad Jurassic-Cretaceous "oil window" appears now well established in relation to temperature/depth parameters. Timing of structuring in relation to migration is clearly important. Undoubtedly, the thicker, more rapid deposition of sediments climaxed in Late Cretaceous to earliest Tertiary time during which burial would have been sufficiently deep to generate hydrocarbons and activate their expulsion. Too, the full significance of hydrodynamic flushing in stratigraphic entrapment or oil escape has still to be assessed but the evidence of hydrocarbons in a mound spring in the vicinity of Lake Eyre indicates that it could be a significant factor. Import parity pricing of crude oil introduced by the Commonwealth Government in the mid 1970s has acted to greatly stimulate ongoing oil search. Continued sound pricing will undoubtedly result in far more widespread discovery.

THE EROMANGA BASIN

J.D. Armstrong and T.M, Barr

SANTOS Limited, 39 Grenfell Street, ADELAIDE, S.A. 5000

The Eromanga Basin, of immense area and volume, has been explored for hydrocarbons since 1924. Covering 400,000 square miles it has recently become prominent in the Australian petroleum scene with 11 significant discoveries being made since 1975. These discoveries have led to the Eromanga Basin Symposium, which indicates the emerging awareness amongst petroleum explorers of the basin's new significance. A collection of 40 papers being presented by a wide range of earth scientists directly concerned with the basin compares somewhat favourably with the 24 papers presented at the national APEA conference earlier in the year. The hydrocarbon potential of the Eromanga Basin was largely ignored in the past by the majority of earth scientists. In reviewing the literature only a few authors, for example R.C. Sprigg and G.D. Williams, expressed a positive view of the prospect of finding hydrocarbons in the Eromanga Basin. The Namur 1 gas discovery in 1976 confirmed the optimism of Sprigg and Williams to some extent, but it wasn't until substantial oil flows were obtained from the Strzelecki 3 well in 1978, and from the Dullingari North 1 well in 1979 that attention began to be more seriously focused on the basin. Discoveries have followed since 1978 with finds such as Wackett, Dullingari, Cuttapirrie, Merrimelia, Jackson and Jackson South. With the completion in 1982 of a liquids pipeline from Moomba to Stony Point it is expected that in 1983, 3.5 million barrels of oil will be produced from fields in the South Australian Eromanga Basin. If it is assumed that there could be an undrilled prospect every 200 square miles over the central 50% of the basin, then the Eromanga Basin could contain 1000 prospects yet to be investigated. The 1000 prospects represents some 7.5 million feet of drilling and using the cost of $225 per foot, which i s a cost that makes allowance for both seismic and drilling, then this 7.5 million feet equates with exploratory work worth $1.7 billion. This exploration is significant from a commercial viewpoint in that Australia's economy has remained relatively buoyant in the last 10 years due to the substantial production from the Gippsland Basin. However, this production will decline during the last 2 decades of this century and there will be an ever-widening gap between the demand for petroleum-based fuels and indigenous production.

Successful exploration of the Eromanga Basin could contribute to diminishing this gap. For example, applying a 1 In 20 success to the hypothetical 1000 undrllled prospects, and assuming that half of the discoveries were oil with Moonle Oil Field size recoverable reserves, then there could be some 500 million barrels of recoverable oil to be discovered In the basin. The Eromanga Basin Is large and petroleum has accumulated In almost all of the Jurassic reservoirs In the section at one location or another. If exploration Is to be encouraged, exploration companies need security of tenure of exploration permits and continued world market pricing for the petroleum produced. To achieve success, earth scientists should look forward with a breadth of vision commensurate with the potential of the basin.

STRATIGRAPHY AND STRUCTURE

THE CENTRAL EROMANGA BASIN PROJECT

A CONTRIBUTION TO

THE REGIONAL STUDY OF THE EROMANGA AND UNDERLYING BASINS

F.J. Moss

Bureau of Mineral Resources, P.O. Box 378, CANBERRA CITY, A.C.T. 2601

The Bureau of Mineral Resources in cooperation with the Geological Survey of Queensland is providing new regional information on the structure and depositional history of the Eromanga and underlying Cooper, Galilee and Adavale Basins in southwestern Queensland. The information being obtained is particularly relevant to a better understanding of the petroleum prospectivity of the area covered by the central part of the Eromanga Basin where recent discoveries of oil and gas in the Eromanga and Cooper Basin sequences have stimulated a renewal of petroleum exploration activity in the area. The Central Eromanga Basin Project (Harrison et al., 1980) was initiated after preliminary studies of the structure, hydrodynamics and hydrocarbon potential of the area by Senior & Habermehl (1980). By the end of 1982 approximately 1500 km of regional 6-fold C.D.P. seismic reflection traverses recorded to 20s, up to 300 km long, will have been recorded over major structures in the Eromanga Basin, the eastern margin of the Cooper Basin, the southwestern Galilee Basin and the underlying Adavale Basin with its associated troughs. The seismic traverses are being tied to existing petroleum exploration wells for which synthetic seismograms have been produced to assist in identifying reflectors and integrated with older seismic data to provide good quality structural and stratigraphic information. Some of the older analogue seismic data which has been transcribed to digital form has also been reprocessed. The seismic cross-sections are released through the Government Printing Office, Canberra. Seismic refraction, gravity, magnetic and magnetotelluric surveys are providing additional information on both sedimentary features and basement. LANDSAT imagery studies have provided new perspective on many regional features particularly when used in conjunction with seismic and gravity information. Geochemical and source-rock maturation studies are also providing information on the generation and migration of hydrocarbons. The Warrabin Trough, containing sediments ranging in age from the lateral equivalents of the Devonian Buckabie Formation and of the Middle Devonian Cooladdi Dolomite to possible Early Devonian, is up to 3000 m thick. The sequence is folded and faulted by high-angle reverse faults which were active during the Late Carboniferous. The Canaway Ridge was uplifted at that time and separated the trough from the main part of the Adavale Basin (Pinchin & Senior, in press). The Barcoo Trough, another Devonian trough, lies north of the Warrabin Trough to which it is connected by a flat-lying to gently folded sequence of Devonian sediments. The trough

145* 30'

I44*00'

I42°30'

— 27°00

Petroleum exploration (tied to traverses)

Concealed margin of Ad a vale Basin and associated troughs

- 4 0 0 j j m / s 2gravity (267 t/m 3 density )

anomaly

Areas with gravity (2 67 t/m* density)

<-400jum/s2

Seismic contour

Traverses

•

BMR

I960

BMR

1981

well

BMR 1962 (proposed )

100 km —J

-i2e°oo' 26/Q/59

Central Eromanga Basin P r o j e c t - m a i n structural units, gravity low areas ft BMR seismic traverses

10 which underlies the Thomson Syncllne contains up to 1450 m of Devonian sediments previously considered to be part of the Cooper Basin sequence. The Quilpie Trough which lies to the south of the main Adavale Basin contains a similar package of sediments to the Warrabin Trough. Direct correlation between the Adavale Basin and its associated troughs is not possible because of faulting at the margins of the troughs, basement uplifts and the lack of deep wells in the troughs. Thus their petroleum potential is generally unknown. The eastern extents of the Permian and Triassic Cooper Basin sediments have been defined. Cooper Basin sediments are not present over the southern part of the Warrabin Trough as previously interpreted. Petroleum source—rock geochemistry indicates that potential gas~prone kerogens are widespread within the northeast coal—measure facies of the Cooper Basin sequence. The Jurassic-Cretaceous Eromanga Basin sequence shows a number of reflections which provide information on the stratigraphy and structure. Shoaling and channelling of the Toolebuc Formation, and coal and carbonaceous shale within the basal Winton Formation are clearly evident from reflection character. Post-Cretaceous movement due to differential compaction and minor tectonic rejuvenation may have given surface expression to some of the deep-seated faults. LANDSAT imagery indicates low relief in the Eromanga Basin sediments which are deeply weathered and mainly covered by surficial sediments. The LANDSAT data enables faults to be traced over long distances between seismic traverses. The concluding phase of the Central Eromanga Basin Project will be undertaken in 1983 with interpretation of the seismic and related data and publication of results.

References HARRISON, P.L., MATHUR, S.P., MOSS, F.J., PINCHIN, J. & SENIOR, B.R., 1980: Central Eromanga Basin Project, program proposals 1980-1982. Aust., Bur. Miner. Resour., Geol. Geophys., Rec. 1980/32 (unpubl.)PINCHIN, J. & SENIOR, B.R., 1982: The Warrabin Trough, western Adavale Basin. Geol. Soc. Aust., J. (in press). SENIOR, B.R. & HABERMEHL, M.A., 1980: Structure, hydrodynamics and hydrocarbon potential, central Eromanga Basin, Queensland, Australia. BMR J. Aust. Geol. Geophys., 47-55.

11 MESOZOIC GEOLOGY OF THE SIMPSON DESERT REGION, NORTHERN SOUTH AUSTRALIA

P.S. Moore

Delhi Petroleum Pty. Ltd., 33 King William Street, ADELAIDE, S.A. 5000

The uppermost Mesozoic unit in the Simpson Desert region is the coalbearing Winton Formation, of Early to Late Cretaceous age. This rests conformably on a sequence of interbedded calcareous sandstones and siltstones of the Mackunda Formation. The Mackunda Formation in turn rests conformably on a fine-grained, marginal-marine sequence comprising the Allaru Mudstone, Toolebuc Formation and Wallumbilla Formation. The top of the Cadna-owie Formation corresponds to the major ' C seismic reflector, which separates predominantly fine-grained sediments above from the sandier sequences below. The Cadna-owie Formation is a coarsening-upwards unit grading from interbedded mudstone and siltstone at the base, into fine-grained calcareous sandstone in the upper portion. It rests with apparent conformity on the very thick, braidedfluvial Algebuckina Sandstone. The Algebuckina Sandstone, as presently defined, has an age range of Early Jurassic to Early Cretaceous. In the central portion of the Poolowanna Trough, the lower part of the formation appears to pass laterally into a sequence of interbedded sandstone, siltstone, shale and coal known as the Poolowanna Beds. The Poolowanna Beds contain two distinctive microfloral assemblages, one of Early Jurassic age (J1 and lowermost J2-3 zones) and the other of Middle Jurassic age (Lower J5-6 zone). Thus, it appears that all of the J4 palynological zone is missing in this area, and that a major hiatus separates the 'Upper' and 'Lower' Poolowanna Beds. The Poolowanna Beds rest unconformably on the Peera Peera Formation. The Peera Peera Formation consists of interbedded shale, siltstone and sandstone. The formation is of Middle to Late Triassic age, and contains Moolayember, Ipswich and Basal Bundamba microfloral assemblages. It is best developed in the central, deepest part of the Poolowanna Trough, and appears to extend eastwards at least as far as the Birdsville Track Ridge. In the central portion of the Poolowanna Trough, the Peera Peera Formation rests with apparent conformity on the Walkandi Formation (new name). The Walkandi Formation, of Triassic age, consists of interbedded shale, siltstone and minor sandstone. Fine-grained lithologies are variable in colour with pale-grey, grey-green, maroon, brown and brick-red varieties all recorded. While the sequence is lithologically similar to parts of the Nappamerri Formation in the Cooper Basin, the age relationship between the two units is unclear. In addition, the Walkandi and Nappamerri Formations are separated by a 150-200 km wide zone of non-deposition or erosion along the Birdsville Track Ridge.

12 Various nomenclature has been used to describe Mesozoic and Permian sediments within the Simpson Desert region. The Pedirka Basin clearly refers to an Early Permian depositional cycle, comprising the Crown Point and Purni Formations. Triassic sediments of the Peera-Peera and Walkandi Formations are assigned to the Simpson Desert Basing on the basis that these formations are separated from underlying and overlying strata by significant unconformities. A traditional definition for the Eromanga Basin is therefore applied, with the base of the Eromanga Basin taken at the base of the 'Lower' Poolowanna Beds, of earliest Jurassic (Jl) age. There are many unsolved stratigraphic problems associated with the Mesozoic sequence in the Simpson Desert region. However, most should be resolved following comprehensive analysis of the results of Walkandi 1, Erabena 1 and Kuncherinna 1 (northern South Australia), Thomas 1 (southwestern Northern Territory) and Adria Downs 1 (southwestern Queensland).

13 LATE TRIASSIC AND EARLY JURASSIC SEDIMENTATION IN THE GREAT ARTESIAN BASIN

M-J. Wiltshire

Consulting Petroleum Geologist, Wiltshire Geological Services, c/- Australian Mineral Foundation Inc., P.O. Box 97, GLENSIDE, S.A. 5063

Exploration drilling for petroleum In the Great Artesian Basin In the past 20-odd years has resulted In significant discoveries of oil and gas In the Surat Basin and the Eromanga Basin. Analysis of sediments Intersected during drilling In the Surat Basin has led to a revised Interpretation of the Early Jurassic sequence In that basin. In particular, the Evergreen Formation oolitic Ironstones are Interpreted as the products of a large, shallow, warm freshwater lake. A modern analogue Is the oolitic Iron-rich sedimentation now occurring In Lake Chad. Sandbodles within the Evergreen Formation are products of slight increases in the average gralnslze of detrltal feed to the lake. Some distributary channel sandstones remain, but most sands were reworked and winnowed in dune, beach and lake bar environments. Often sands are beach/bar products at the top, and pass almost imperceptibly into distributary sandbodles at the base. The top of Evergreen Formation transition into Hutton Sandstone deposition was produced by a similar Increase in average gralnslze of detrltal input to the Evergreen lake. The supply of coarser material persisted, however, where previously it had only occurred as pulses. The basal 200-odd feet of Hutton Sandstone lithologies in the Surat Basin were deposited in lake shoal bar, beach and dune environments, not in a fluvial environment. Analyses of Jurassic sediments reported f rom the western Eromanga Basin (Bowerlng, 1982; Poll, 1981; Barr & Youngs, 1981) indicates that the 'basal Hutton' and 'basal Jurassic' of the western Eromanga Basin is possibly the depositional equivalent of the Surat Basin Early Jurassic sequence up to the top of the lower part of the Hutton Sandstone. The higher energy sediments of the 'Upper Hutton' sequence of the Eromanga Basin are then direct correlates of the upper part of the Hutton Sandstone in the Surat Basin. Ages of the basal Jurassic sequence in the Simpson Desert area (Bowerlng, 1982) imply the absence of the 'Upper Hutton' sequence west of the Birdsville Track Ridge. If this is proven, large stratigraphic/ structural oil plays are possible in the 'Upper Hutton' of the western Eromanga Basin, where the Blrkhead Formation should overlap and seal the zero edge of the upper part of the Hutton Sandstone, in the vicinity of the Birdsville Track Ridge.

Figure 1;

Simpson Desert area

Stratigraphlc relationships, Great Artesian Basin

Cooper

Basin

Adavale

Basin

Surat

Basin

References BARR, T.M. & YOUNGS, B.C., 1981: Cuttapirrie 1, an oil discovery in the Early Jurassic of the Eromanga Basin. APEA J., 21(1), 60-70. BOWERING, O.W.J., 1982: Hydrodynamics and hydrocarbon migration - a model for the Eromanga Basin. APEAJ., 22(1), 227-36. POLL, J.J.K., 1981: The significance of the southwest Eromanga Basin oil and gas discoveries (Central Australia). APEA J., 21(2), 33-8.

REVISION OF EROMANGA BASIN LIMITS

M.J. Wiltshire

Consulting Petroleum Geologist, Wiltshire Geological Services, c/- Australian Mineral Foundation Inc., P.O. Box 97, GLENSIDE, S.A. 5063

Within the stratigraphic code there is no provision for formal definition of sedimentary basins, nor has any convention been adopted to allow for such definition. Consequently basins are defined informally, by usage, and such definitions should be dynamic, developing with the growth of geologic knowledge. For the past thirty years that area of the Australian Great Artesian Basin lying west of the Nebine Ridge and Eulo Shelf in central western Queensland and south of the Euroka Arch in northwestern Queensland has been referred to as the Eromanga Basin. The terms 'sub-basin' and •infra-basin' have also been (incorrectly) applied to the region. The Informally defined lower and upper limits to the stratigraphic sequence of the Eromanga Basin have become established as the base of Jurassic sediments and the top of preserved Cretaceous sediments. Figure 1;

Regions and infra-basins of the Great Artesian Basin

16 There are several (underlying) infra-basins in the region of the Eromanga Basin, These infra-basins include the Pedirka, Cooper, Adavale and Galilee Basins. All contain some sediments of the Permo-Carboniferous to mid-Triassic depocycle, overlying older Palaeozoic sediments and basement rocks. These infra-basins have been variously and informally defined to contain sediments ranging in age up to early Middle Triassic. A period of mid-Triassic folding, uplift and erosional peneplanation caused some redistribution of the older sediments into thin Middle Triassic to Late Triassic sedimentary sequences. The depocentres of these later Triassic sediments were produced by sedimentary compaction and post-tectonic adjustments that post-date the mid-Triassic tectonism. The later Triassic sediments are now known to be widely distributed as a relatively thin veneer which extends far beyond the areal extent of any one of the infra-basins. The sequence apparently developed as a precursor to the more general sedimentation of the Eromanga Basin which began in Early Jurassic time. The thickest developments of the Late Triassic sediments occur in the areas of thicker Jurassic sediments. An outlier of these sediments forms part of the coal-bearing sequence at Leigh Creek. Due to dogged pursuit of Permian exploration plays which depended heavily if not totally on a component of *C' horizon (near base of Cretaceous) to 'P' horizon (near top of Permian) thinning, most exploration wells drilled in the Eromanga Basin in the past twenty years have avoided the areas of thicker Jurassic development, so the late Middle Triassic and Late Triassic section has long been overlooked by the exploration companies active in the area. Since the drilling of Poolowanna 1 in 1977, with the penetration in that well of a thick Early Jurassic and Late Triassic section containing significant oil shows, the wide distribution of the Late Triassic section has been recognised. The section is recognised as far west as Macumba 1, and as far east as Cuttapirrie 1, though it may extend considerably further east. Hydrocarbons have often been observed in association with these sediments in the subsurface, and the potential economic significance of the section is now becoming more generally appreciated. Exploration and evaluation of the entire section will be aided by general recognition of the potential of all components of the section, and this appreciation will be enhanced by incorporation of this Late Triassic sequence into the existing basin framework in the area. On the basis of area of distribution, age range and lithofacies, the later Triassic sediments are here grouped with the Eromanga Basin sediments of Jurassic and Cretaceous age. The Eromanga Basin is re-defined to incorporate all Mesozoic sediments which post-date the mid-Triassic tectonic event. The areal extent of the Eromanga Basin is not significantly changed by this re-definition of stratigraphic range.

THE MIDDLE PERMIAN - MIDDLE TRIASSIC ROCKS OF THE SOUTHWESTERN COOPER BASIN

Bridget C. Youngs and Peter G. Boothby South Australian Oil & Gas Corporation, P.O. Box A70, NORTH ADELAIDE, S.A. 5006

The Triassic strata of the Cooper Basin, traditionally viewed as caprocks to the Permian reservoirs, have received little attention until recently when flow rates of up to 2850 BOPD at Merrimelia renewed interest in these sediments. Papalia (1969) proposed the term Nappamerri Formation for what he believed to be Early-Middle Triassic rocks conformably overlying the Late Permian Toolachee Formation. It is now recognised that the Nappamerri Formation is Middle Permian-Middle Triassic in age (Foster, 1982). Recent drilling in the northern Cooper Basin has revealed younger Triassic sediments overlying the original Nappamerri Formation (Barr & Youngs, 1981). These younger units are similar in lithology to the Late Triassic "Peera-Peera Formation" in the Simpson Desert Basin (Moore, 1982). This present study covers the entire Triassic section and that part of the Middle-Late Permian which overlies the top coal of the Toolachee Formation (Fig. 2). These rocks are over 500 m thick in the northeast and thin, mainly by erosion, to a zero edge which approximately parallels the Permian edge in the west but lies to the north of it over the southern Cooper Basin (Fig. 1). Four informal lithological units, different from those of Papalia (1969), have been recognised on logs from South Australia and southwestern Queensland. They are best developed in the north (see Beanbush 1, Fig. 2). Unit 1 is the lowermost, covering the whole subcrop area and is a fine-grained, ?alluvial facies with only 10-30% sandstone. Owing to erosion. Units 2, 3 and 4 occupy progressively smaller areas. Unit 2 (Late Permian - Early Triassic) is similar to Unit 1 but Unit 3 (undated) represents a different regime consisting of thick, ?braided-fluvial deposits with over 90% sandstone in some areas. The Middle Triassic Unit 4 reverts to lower energy levels and is represented by interbedded sandstones and shales. Palynological dates are scarce but, on lithological evidence, the sequence appears conformable. Hydrocarbon discoveries in Units 1 and 2 (Burke 3, Gidgealpa 13, Merrimelia Field) together with the potential for good reservoirs in Units 3 and 4 point to the need for more study.

i4r do ^ ^ if

2r 00

LATE PERM.-TRIA8SIC STRATA ABSENT CONTOUR INTERVAL 50 METRES

SCALE 10

in KILOMETRES 20

• '

Figl

BEANBUSH 1 MMA RAY

SONIC

Fig. 1

Isopachs in metres

LOCATION MAP

DEPTH BELOW K.I. •800

Fig. 2 Lithological units in Beanbush 1 and Dilchee 1

^TOOLACHEE^ P FORMATION

•^^'FORMATION jisil

References

BARR, T.M- & YOUNGS, B.C., 1981: Cuttapirrie 1 - an oil discovery In the Early Jurassic of the Eromanga Basin. APEA J., 21(1), 60-70. EVANS, P.R., 1966: Mesozoic stratigraphic palynology in Australia. Australas. Oil Gas J., 12(6), 58-63. FOSTER, C.B., 1982: Review of the time frame for the Permian of Queensland. Geol. Soc. Aust., Abstracts, 21. MOORE, P.S., 1982: Mesozoic geology of the Simpson Desert region, northern South Australia. This volume. PAPALIA, N., 1969:

Tlie Nappamerri Formation.

APEA J., 9(1), 108-10.

20 JURASSIC TO LOWER CRETACEOUS STRATIGRAPHY OF THE EROMANGA BASIN, SOUTH AUSTRALIA

PROBLEMS

AND PROGRESS IN SUBSURFACE CORRELATION

D.I. Gravestock

South Australian Department of Mines and Energy, P.O. Box 151, EASTWOOD, S.A. 5063

Study of older Eromanga Basin strata over the South Australian Cooper Basin region is hampered by poor seismic definition between the Upper Permian and Lower Cretaceous. Wells for the most part are drilled on structural highs so that the degree of completeness of the Mesozoic section remains largely unknown. Nevertheless, an integrated study of palynological, petrophysical and lithological data has clarified aspects of early Eromanga Basin history. Jurassic to Neocomian strata are thickest north of the approximate preserved limit of Permian rocks. Early to Middle Jurassic differential subsidence which shaped the southwest Cooper Basin margin shed reworked Permo-Triassic and older detritus into the developing basin. Elsewhere, more uniform deposition was modified by persisting ancient structural trends. In contrast with the Early to Middle Jurassic isopach (ranging from a total of 30 m to 300 m), thickness variations of Late Jurassic to Neocomian strata (300 m to 450 m) are slight, suggesting a period of relative tectonic quiescence prior to the Aptian marine transgression. Stratigraphic and structural relationships near the southwest Cooper Basin margin are complex. Locally, a Middle to Late Jurassic unit of unknown areal extent and thickness has been included in the Hutton Sandstone; it is a time-equivalent of part of the Injune Creek Group in the Surat Basin. Overlying units are everywhere younger than current correlation implies; there is no stratigraphic basis for diachroneity at the base of the overlying unit but strong evidence exists for local disconformities on structural highs near the southwest margin. Elsewhere, petrophysical logs indicate a more complete, possibly conformable sequence. Although sands throughout the studies section consist almost entirely of mineralogically mature, slightly feldspathic quartzarenite, traces of volcanogenic detritus appeared in the Late Jurassic and persisted into the Neocomian. Reworking of ancient volcanic rocks, or supply from a distant, contemporaneous volcanically active provenance is indicated. Systematic logging and petrological study of cores and cuttings is being carried out as exploration drilling proceeds. This, when integrated with palynological and petrophysical data will help define and ultimately map the Jurassic sand units within a stratigraphic framework.

Acknowledgement s Permission from Delhi Petroleum Pty. Ltd., SANTOS Limited and Cooper Basin Associates to incorporate confidential data is gratefully acknowledged. The author publishes with the permission of the DirectorGeneral of the South Australian Department of Mines and Energy.

22 A REVIEW OF THE EARLY CRETACEOUS MURTA MEMBER IN THE SOUTHERN EROMANGA BASIN

G. Ambrose^, R. Suttill2 and I. Lavering3

^SANTOS Limited, 39 Grenfell Street, ADELAIDE, S.A. 5000 ^South Australian Oil and Gas Corporation, 226 Melbourne Street, NORTH ADELAIDE, S.A. 5006 ^Esso Australia Ltd., 127 Kent Street, SYDNEY, N.S.W. 2000

In the study area the Murta Member (Mooga Formation) was deposited in an asymmetric basin formed by compaction of the underlying Cooper Basin sequence. It is a mainly fine-grained lacustrine sequence intervening between braided fluvial sediments of the Namur Sandstone Member (Mooga Formation) and the overlying, marginal-marine 'Transition Beds'. The lower contact is transitional and marks a period of degradation of the 'Namur' braided fluvial regime prior to the onset of lacustrine conditions. Core data indicate a gradational upward passage from the Murta Member into the basal Transition Beds and there is no conclusive evidence of marine influence until about midway through the latter. Regional log correlations show that the sequence intertongues with the Namur Sandstone Member on a regional scale and it is thus proposed that the unit retain its status as a member of the Mooga Formation. A reference section is assigned to Dullingari 9 where the sequence is subdivided into four units. The basal unit (Unit 1) is an upwardcoarsening-lacustrine delta sequence capped by delta-front sands and occasional distributary channel deposits. Lower distal deposits contain minor pelagics but consist mainly of thin, graded-silt fine-sand beds deposited by density currents which traversed a gently shelving lake floor. Unit 2 contains similar graded fine sand and silt beds deposited in relatively shallow water. This unit is succeeded by a sub—lacustrine fan sequence comprising graded beds which show an upward increase in thickness and sand content (Unit 3). The basinward progradation of the fan sequence, which extends over an area of about 5000 sq. km, was basically a shallowing trend which culminated locally in the development of a thin, elongate, shoreline sand. Graded beds characterise Unit 4 which marks a return to off-shore sedimentation. The sequence fines upwards into the basal Transition Beds probably reflecting deeper water conditions prior to flooding of the lake by the sea.

BRAIDED AND

FLUVIAL NAMUR

THIN MURTA

SANDSTONE

SILTSTONES

INTERMEDIATE LACUSTRINE

•

FACIES

PROXIMAL DISTAL

SOUTHERN EROMANGA BASIN - MURTA MEMBER

REGIONAL LITHOFACIES

LACUSTRINE

FACIES

On a regional scale the Murta Member decreases in thickness and sand content from the north-northeast to the southwest. This trend reflects a regional depositional pattern whereby the 'Namur' braided fluvial regime contributed large volumes of sediment to the 'Murta' lake on the northern and eastern basin margins* Sandy proximal delta and fan sequences, together with minor marsh-bay facies have been cored in the Jackson Field. The sequence prograded from this area towards the south-southwest and fine-grained sediment and occasional coarse-grained sands were transported large distances into the basin by density flows (i.e. turbidity currents). To date the Murta Member has hosted two oil discoveries. The largest is the Dullingari-Murta Field which is a combined structural-stratigraphic play. Lenticular winnowed shoreline sands provide the main reservoir in the field and subtle structural movements have apparently controlled porous sand development. Proximal turbidite sand flows and delta distributary channels have only produced small oil flows, largely due to the presence of quartz-calcite cements and limited reservoir continuity. The Jackson Field discovery occurs in a relatively sandy sequence deposited close to source areas. The main reservoir is a proximal deltafront deposit, possibly modified by wave action. The oil at Dullingari and Jackson is interpreted to have been sourced from silty shales within the Murta sequence which are locally enriched in exinite (mainly alginite, sporinite and cutinite), particularly in the lower part of the sequence. With reference to the regional depositional model (see figure) hydrocarbon potential of the Murta Member is summarised as follows:

the

Distal lacustrine facies: reservoir sands are sparse. poor;

contains adequate source rocks but Prospectivity is regarded as moderate to

Intermediate lacustrine facies: source rocks are present and potential reservoirs include shoreline, delta-front, distributary channel and turbidite facies. Prospectivity is regarded as high;

Proximal lacustrine facies: reservoir quality sandstones are relatively abundant in this zone and prospectivity is regarded as high.

Overall the style of sedimentation in the Murta sequence lends itself to stratigraphic entrapment of hydrocarbons but it is considered that, at this stage of exploration, combined structural-stratigraphic plays of the Dullingari type are the most viable targets.

25 MARGIN OF THE EROMANGA BASIN SOUTH AUSTRALIA;

A REVIEW

B.G. Forbes

South Australian Geological Survey, Department of Mines and Energy, 191 Greenhill Road, PARKSIDE, S.A. 5063

The Eromanga Basin occupies most of northeastern South Australia. Early geological work in the Eromanga Basin in South Australia was related to the search for groundwater. In publications prior to about 1978 the term Great Artesian Basin was commonly used to refer to the Eromanga Basin in South Australia. Some important early contributions were by Rawlinson (1878) who predicted a large interior groundwater basin, Hudleston and Howchin (1884-1895: early palaeontology), Lockhart Jack (1930: stratigraphy and structure). Ward (1945: water supplies) and Sprigg (1958: petroleum potential). Geological maps of the Basin at 1:63,360 scale first appeared in 1961; more recent geological maps have been produced at 1:250,000 scale. Current stratigraphic nomenclature is based on geological mapping by Freytag, Forbes, Wopfner, and others (1965-1970) and the biostratigraphy of Ludbrook (1966, 1978). Stratigraphic units in common use on Geological Survey maps are (from oldest to youngest): Algebuckina Sandstone, a Jurassic clean fluvial sandstone and conglomerate and main artesian aquifer; Cadna-owie Formation, Early Cretaceous (Neocomian to Aptian) fluvial to shallow marine brown sandstone and siltstone, with Mount Anna Sandstone Member; Bulldog Shale, marine Aptian to Albian shale, claystone and concretionary limestone; Oodnadatta Formation, marine Albian claystone, sandstone and limestone, with Coorikiana Member (revised to Coorikiana Sandstone Thomson, 1980), Wooldridge Limestone Member and Mount Alexander Sandstone Member; Winton Formation. Although the Bulldog Shale and Oodnadatta Formation have been grouped together as Marree Subgroup (Thomson, 1980), it is probable that the original Marree Formation (now revised to Marree Subgroup) did not include the upper sandstone (Mount Alexander sandstone Member) in the Oodnadatta Formation. It is recommended that Marree Subgroup refer to the sequence in the original Marree Formation, with the exception of the basal Trinity Well Sandstone Member, which is probably equivalent to the Mount Anna Sandstone Member of the Cadna-owie Formation. In future geological mappipg, an endeavour should be made to map the Mackunda Formation (probably equivalent to the Mount Alexander Sandstone Member of the Oodnadatta Formation) which lies above the Marree Subgroup and below the Winton Formation.

26 CRETACEOUS OF THE SOUTHWESTERN EROMANGA BASIN: STRATIGRAPHY, FACIES VARIATIONS AND PETROLEUM POTENTIAL

P.S. Moore^ and G.M. Pitt2 ^Delhi Petroleum Pty. Ltd,, 33 King William Street, ADELAIDE, S.A, 5000 ^Western Mining Corporation Ltd., 168 Greenhlll Road, PARKSIDE, S.A. 5063

The Cretaceous stratigraphy of the Eromanga Basin was recently reviewed by Senior et al. (1978). Unfortunately their report was unable to include data from South Australia, thus leaving a large and critical gap in our understanding of the sequence. This paper presents the first major attempt to bridge the gap, and reconcile South Australian outcrop stratigraphy with subsurface data derived from PELs 5 & 6 in South Australia and ATP 259P in southwestern Queensland. The subsurface stratigraphy of PELs 5 & 6 and ATP 259P is shown in Figure 1. The Wlnton and Mackunda Formations are recognised over the entire area, and overlie a finer-grained sequence dominated by shallow-marine mudstones. These in turn overlie the sandy Cadna-owle Formation (otherwise known informally as the 'Transition Beds'). The marine mudstone sequence contains the dark, organic-rich Toolebuc Formation over most of the study area, and is thus subdivided into the Allaru Mudstone (above) and the Wallumbilla Formation (below). Around the margins of the Eromanga Basin, the Toolebuc Formation is absent due to a f a d e s change. Instead, the marine mudstone sequence is interrupted by the glauconltic Coorlklana Sandstone. In this case the mudstones are subdivided into the Oodnadatta Formation (above) and the Bulldog Shale (below). In a few wells, the Coorlklana Sandstone and Toolebuc Formation are both developed, leading to a natural overlap in the stratigraphic nomenclature (Fig. 1). In these wells, the top of the Coorlklana Sandstone occurs 70-100 m below the base of the Toolebuc Formation. F a d e s changes recorded in the sequence are primarily a response to proximity to the margins of the Eromanga Basin. Thickness variations reflect the influence of four major depocentres - the Poolowanna, Patchawarra, Nappamerrl and Wlndorah Troughs. In addition, the thickness of the preserved Wlnton Formation in southwestern Queensland is strongly Influenced by Late Cretaceous or Tertiary uplift and erosion. The two most prospective units in the Cretaceous above the geophysical 'C' horizon (top Cadna-owle Formation) are probably the Toolebuc Formation and the Coorlklana Sandstone. Despite the bulk of the sequence being Immature in traditional terms, gas shows have been recorded in both formations, and fluorescence is commonly reported in the Coorlklana Sandstone.

The excellent source potential of the Toolebuc Formation has long been recognised. The unit Is the target of oil shale exploration In several states, and it is known to pass laterally into the Julia Creek oil shales in Queensland. A lack of porosity and permeability in the black, organic-rich shales is a recognised problem, however where the unit is calcareous, or in certain tectonic settings, the potential for production from fractured shales exists. The Coorikiana Sandstone first produced gas in 1981, during an open-hole drill-stem test in Strzelecki 4. Gas flowed to surface at a rate too small to measure, confirming that hydrocarbons had been generated in the sequence. In May, 1982, Strzelecki 8 tested the same sand unit nearby, and produced gas to surface at the rate of 9,900 cu. m. (350,000 cu. ft.) per day.

References SENIOR, B.R., MOND, A. & HARRISON, P.L., 1978: Geology of the Eromanga Basin. Aust., Bur. Miner. Resour., Geol. Geophys., Bull., 167.

Figure 1:

Subsurface stratigraphy of the southwestern Eromanga Basin.

Basin Margin Sequence

Age B = = = Late Cenomanian Cretaceous

Winton

Formation

Mackunda

Formation

Oodnadatta Early

Albian

u W) XI in

0) cu

Cretaceous

Formation

Neocomian

Allaru

Mudstone

Toolebuc Formation

Coorikiana Sandstone Wallumbilla

u u

Aptian

Basinal Sequence

Bulldog Shale

Cadna-owie

Formation

28 STRATIGRAPHY AND TECTONICS OF THE DALHOUSIE ANTICLINE, SOUTHWEST EROMANGA BASIN

G.W. Krieg

South Australian Department of Mines and Energy, 191 Greenhill Road, PARKSIDE, S.A. 5063

The Dalhousie Anticline (Fig. 1) is a roughly oval-shaped, regional structure which displays a considerable interval of folded and faulted Eromanga Basin sequence and also an assemblage of late Cainozoic deposits that illustrates the tectonic development of the feature. The exposed anticlinal succession passes up conformably through a Neocomian-Cenomanian sequence of transgressive marginal-marine sandstone/ silstone (Cadna-owie Formation), shallow-marine shale with calcareous sandstone interbeds (Marree Subgroup) and regressive altered sandstone/ claystone (Winton Formation), to a disconformably overlying PaleoceneEocene fluviatile sandstone (Eyre Formation) which has been strongly silcreted (Cordillo Silcrete). A terrestrial sandy carbonate/dolomitic limestone (Mount Willoughby Limestone) occurs in shallow incisions in the silcrete. Within the breached anticline a concentrically disposed toposequence of Cainozoic units has been deposited in response to uplift, crestal break-up of the anticline and major artesian spring development. This topo-sequence consists of gypcreted gravel (Alinerta Gravel), lacustrine clay/dolomite (Dalhousie Beds) and mound-spring clay, silt, and locally, coarse sand (unnamed deposits). The structure of Dalhousie Anticline appears, from reconnaissance seismic data, to have a simple, steadily curving form, but the smaller scale folds and faults within and marginal to the anticline suggest an essentially polygonal form controlled by fairly straight faults and monoclinal flexures. Anticlinal growth along these intersecting lineaments may have begun as early as latest Eocene and was well established during the Oligocene and early Miocene, but the major uplift did not occur until the late Tertiary and Quaternary. To explain the concentric distribution, vertical separation, and still-water depositional component of the Cainozoic topo-sequence, intermittent pulses of uplift alternating with periods of quiescence, and initial crestal break-up occurring over the whole anticline simultaneously is suggested. Anticlinal growth is still active today. The relevance of the Dalhousie Anticline to the tectonic history of the deeper parts of the Eromanga Basin is open to question as the seismically-derived structure contour patterns, and gravity and magnetic patterns, posssibly indicate structurally different basement regimes.

Fig. 1

EROMANGA BASIN IN SOUTH A U S T R A L I A REGIONAL SETTING, DALHOUSIE ANTICLINE SCALE TOO

200

Q u a t e r n a r y : mainly

-300 J

400

sand

U n d i f f e r e n t i a t e d Cainozoic : sand, a l l u v i u m ,

Early

T e r t i a r y : mainly

silcreted

Mesozoic : mudstone, siltstone,

Pre Mesozoic

Faults

500 K I L O M E T R E S

deposits

sand

sandstone

colluvium

THE RELATIONSHIP BETWEEN THE THOMSON SYNCLINE AND UNDERLYING BARCOO TROUGH

K.D. Wake-Dyster

Bureau of Mineral Resources, P.O. Box 378, CANBERRA CITY, A.C.T. 2601

Seismic surveys have generally been made over the margins of the Thomson Syncline and over basement highs such as the Galway, Chandos and Thunda structures. These anticlinal trends were the main targets for petroleum exploration in the Eromanga and Cooper Basin sequences. Deep seismic reflections recorded in the central region were interpreted to indicate a very thick Permian sequence. However some doubts were cast on the identification of the age of the sediments in the lower part of the section since reconnaissance gravity data indicate that a major low over the Thomson Syncline is similar to that over the Warrabin Trough which is known to contain Devonian sediments. Seismic traverses recorded over the syncline by the Bureau of Mineral Resources in 1980 (Wake-Dyster & Pinchin, 1981) and in 1981 (Sexton & Taylor, in prep.) clearly show the major unconformity which has been associated with the top of the Devonian sequence elsewhere in the area. The presence of Devonian sediments was confirmed from the drilling of XL Barcoo Junction 1 well. This trough of Devonian sediments was named the Barcoo Trough by Pinchin & Senior (1981). The Barcoo Trough is a structural remnant of the formerly widespread Adavale Basin sequence. It was formed by the structural uplift of the Canaway Ridge and other associated basement highs, which cause major deformation resulting in folding and faulting of the Devonian sequence. During the mid-Carboniferous Kanimblan Orogeny anticlines which had been formed in the Devonian sequence were eroded to produce a widespread erosional platform. A thin veneer of Devonian sediments connects the Barcoo Trough to the Warrabin Trough in the south. Cooper Basin sediments are unevenly distributed over the Barcoo Trough as a result of further basement movements during the Permian and Triassic. The absence of coal measures in some areas can be clearly inferred from the presence of good quality deep reflections which would otherwise have been masked. Eromanga Basin sediments of Jurassic to Late Cretaceous age were deposited conformably over a large widespread area. Movements of basement blocks during the Tertiary have structurally deformed the Eromanga Basin sequence to produce low-amplitude folds within the Thomson Syncline, the deepest part of the Eromanga Basin in the area.

31 There is up to 4200 m of sediments in the area, 1450 in of which lie in the Barcoo Trough. This trough is north-trending, and it has a smaller sub-basin on its western flank abutting the Windorah Anticline. In the Thomson Syncline depth contours to the reflector associated with the Wyandra Sandstone Member in the Lower Cretaceous Cadna-owie Formation indicate general coincidence of the depositional axis with that of the underlying Barcoo Trough sequence. The axis of the Thomson Syncline, determined from surface mapping as coincident with the Thomson River, lies west of the axi s of the Barcoo Trough. The recent seismic data and those from previous seismic surveys are being further analysed to provide information on the evolution of the basins and their petroleum prospectivity as part of the 'Central Eromanga Basin Project'.

References PINCHIN, J. & SENIOR, B.R., 1982: The Warrabin Trough, Western Adavale Basin. Geol. Soc. Aust. J. (in press). SEXTON, M.J. & TAYLOR, F.J.: Central Eromanga Basin seismic survey Queensland, 1981: Operational report. Aust., Bur. Miner. Resour., Geol. Geophys., Rec. (in prep.). WAKE-DYSTER, K.D. & PINCHIN, J., 1981: Central Eromanga Basin seismic survey, Queensland, 1980: Operational report. Aust., Bur. Miner. Resour., Geol. Geophys., Rec. 1981/22 (unpubl.).

THE CANAWAY FAULT AND ITS EFFECT ON THE EROMANGA BASIN

J. Plnchln^ and V. Anflloff2

^Flower Doery Buchan Pty. Ltd., 30 Greenough Circuit, KALEEN, A.C.T. 2607 ^Bureau of Mineral Resources, P.O. Box 378, CANBERRA CITY, A.C.T. 2601

The Canaway Fault is a 250 km long north-trending normal fault within the Eromanga Basin, Queensland. It forms the eastern margin of the Canaway Ridge which separates the Cooper Basin from the Galilee Basin. Two combined seismic and gravity traverses of the Bureau of Mineral Resources' Eromanga Basin program were recorded across the Canaway Fault in 1980 (Wake-Dyster & Pinchin, 1980). Detailed analysis of these two traverses has enabled a new interpretation to be made of the structure and timing of the fault. The Canaway Fault is near-vertical and is in places associated with granitic intrusions. During the Early Devonian a palaeo-ridge here cut across the Adavale Basin, but later Devonian deposition was continuous across this ridge. The Canaway Fault began with large movements in the Carboniferous; following which, a long period of erosion, then deposition within the Cooper and Galilee Basins reduced the elevation difference across the fault. During deposition of the Eromanga Basin sequence there was almost continuous movement of the Canaway Fault, and this movement continued after deposition ceased, ending in the late Middle Tertiary. The Canaway Fault is unlikely to have acted as a total barrier to eastwards migration of hydrocarbons from the depo-centres of the Eromanga Basin, because the fault-displacement of Jurassic reservoir rocks was only 47 m in Late Cretaceous times when migration is considered to' have begun (Bowering, 1982). Furthermore the fault has a rounded, monoclinal, shallow structure, which may mean that sedimentary sequences are not abruptly terminated here. Thus, if hydrocarbons have migrated into this region, the Jurassic rocks to the east of the fault may be almost as prospective as those to the west.

References BOWERING, O.W.J., 1982: Hydrodynamics and hydrocarbon migration - a model for the Eromanga Basin. APEA J., 22(1), 227-236. WAKE-DYSTER, K. & PINCHIN, J., 1981: Central Eromanga Basin seismic survey, Queensland, 1980: Operational Report. Aust., Bur. Miner< Resour., Geol. Geophys., Rec. 1981/22 (unpubl.). " "

PALYNOLOGY

34 LATE TRIASSIC ('RHAETIAN^) AND JURASSIC PALYNOSTRATIGRAPHY OF THE SURAT BASIN

J^L. McKellar

Geological Survey of Queensland, G.P.O. Box 194, BRISBANE, QLD. 4001

In the Jurassic of eastern Australia, the principal palynological zonation in use is that devised by Evans (1963, 1966). This scheme, which was based primarily on data derived from the Surat and Eromanga Basins, has been variously modified (e.g. Burger, 1968; Burger & Senior, 1979) as new distribution data have become available. In the Surat Basin, the development of a more formal zonation has been limited to the Early Jurassic with the work of Reiser & Williams (1969). However, the zonations of de Jersey (1975, 1976), which were developed mainly for the sequence in the lower part of the Moreton Basin, also have application in the basal succession (Precipice Sandstone and Evergreen Formation) of the Surat Basin. They enable comparisons to be made between the two basins which demonstrate that deposition of the basal sediments of the Precipice Sandstone, at least in the Eddystone 1:250,000 Sheet area (northwestern Surat Basin), began in 'Rhaetian' (Late Triassic) times when the basal units of the Moreton Basin (Aberdare Conglomerate, Raceview Formation, and basal Helidon Sandstone) were laid down (McKellar, 1978). Deposition was then abruptly terminated and the early Liassic in the Surat Basin was characterised by a period of non-deposition and/or erosion. Subsequently, in the middle Liassic, accumulation of the Precipice Sandstone was reinitiated. This is comparable to the situation described by de Jersey (1976) in the Moreton Basin where a time-break occurs in the basal Helidon Sandstone. Within that unit, the magnitude of the hiatus increases westward and the 'Rhaetian' strata at its base are overlain by early to middle and middle Liassic strata, in respectively the Lowood-Walloon and (further west) in the Toowoomba areas. In terms of the palynostratigraphic subdivision of the Jurassic of the Great Artesian Basin, progress has been hindered by the absence of detailed taxonomic studies of the spore-pollen floras, particularly those of Middle to Late Jurassic age. Moreover, the key to the development of a more refined palynological zonation rests principally with the succession in the Surat Basin, rather than with the generally condensed and often incomplete sequences of the adjacent Eromanga Basin. It is to this end that research at the Geological Survey of Queensland is currently being directed. Of particular significance, the preliminary data derived from studies show that two biostratigraphically important genera, Contignisporites and Cicatricosisporites appear at considerably levels than previously recorded.

these lower

35 Contignisporites cooksonae^ the index used in definition of palynological unit J5 (Evans, 1966), first occurs in the Eurombah Formation, well below its previously documented level of appearance in the upper part of the overlying Walloon Coal Measures. On the other hand, specimens comparable with Cicatricosisporites australiensis first appear at approximately the same level in the basal Gubberamunda Sandstone as Aequitriradites verrucosusy the principal species used to define palynological unit J6 (Burger & Senior, 1979). Previously, Cicatricosisporites has not been recorded below the upper part of the overlying Orallo Formation and its appearance (primarily that of C. australiensis) has been used together with the appearances of several other species in the upper Orallo Formation - basal Mooga Sandstone to define the base of the Murospora florida Zone and the C. australiensis Subzone within it (Burger, 1973; Burger & Senior, 1979).

References BURGER, D., 1968: Stratigraphy and palynology of upper Mesozoic sections in some deep wells in the Surat Basin, Queensland. Aust., Bur. Miner. Resour., Geol. Geophys., Rec., 1968/24 (unpubl.) BURGER, D., & SENIOR, B.R., 1979: A revision of the sedimentary and palynological history of the northeastern Eromanga Basin, Queensland. Geol. Soc. Aust., J., 26, 121-132. EVANS, P.R., 1963: The application of palynology to stratigraphy in Australia. Proc. 2nd Symp. Dev. Pet. Resour. Asia & Far East Min. Resour. Dev. Ser., 18, 285-290. EVANS, P.R., 1966: Mesozoic stratigraphic palynology in Australia. Australas. Oil Gas J., 12, 58-63. DE JERSEY, N.J., 1975: Miospore zones in the lower Mesozoic of southeastern Queensland; Campbell, K.S.W. (ed.) Gondwana Geology (papers presented at the 3rd Gondwana Symposium, Canberra, 1973). ANU Press, Canberra, 159-172. DE JERSEY, N.J., 1976: Palynology and time relationships in the lower Bundamba Group (Moreton Basin). Qld. Gov. Min. J., 77, 460-465. McKELLAR, J.L., 1978: Palynostratigraphy of samples from GSQ Eddystone 1. Qld. Gov. Min. J., 7^, 424-434. REISER, R.F. & WILLIAMS, A.J., 1969: Palynology of the Lower Jurassic sediments of the northern Surat Basin, Queensland. Qld., Geol, Surv., Publ., 339, Palaeontol. Pap., 15.

PALYNOLOGY OF THE EROMANGA BASIN AND ITS APPLICATIONS

D. Burger

Bureau of Mineral Resources, P.O. Box 378, CANBERRA CITY, A.C.T. 2601

Palynologlcal studies In the Eromanga Basin contribute towards (1) explaining cyclic patterns of sedimentation In the basin, (2) establishing accurate ages for Jurassic and basal Cretaceous sedimentary formations, and (3) reconstructing Jurassic and Early Cretaceous paleoenvlronments and palaeogeography• (1) The standard sequence of the Eromanga Basin consists of alternate coarse-grained and fine-grained rock units. This cyclic development Is believed to originate from global eustatlc Jurassic and Early Cretaceous sea level changes (see Cooper, 1977; Vail et al., 1977), because palynologlcal correlations have shown them to synchronise with sedimentary cycles In the Surat Basin, which Exon & Burger (1981) attributed to the same cause. Seven full cycles are outlined In the Eromanga Basin, each Including basal coarse-grained sediments, and an upper fine-grained sequence, corresponding with low and high global sea levels respectively. (2) The sedimentary sequences In the two basins are correlated by means of 6 Jurassic and 8 Early Cretaceous spore and pollen zonal Intervals (Evans, 1966; Dettmann & Playford, 1969; Burger, 1980), and their fit against the global sea level curve according to Exon & Burger (1981) provides new evidence for the ages of rock units (see diagram). Cycle 1 Includes the basal Hutton Sandstone and a (discontinuous) mudstone, possibly the western continuation of the Evergreen Formation, and corresponds with units J1 - lower J2-3 (Hettanglan-Toarclan). Cycle 2 Includes the upper Hutton-Blrkhead sequence and corresponds with upper units J2-3 to lower unit J5 (Aalenlan-Bathonlan). Cycle 3 Includes the Adorl-Westbourne Interval and corresponds with upper unit J5 (Callovlan). Cycle 4 Includes the Hooray-lower Cadna-owle Interval (Oxfordlan-Hauterlvlan). It cannot be subdivided, but In the Surat Basin, In closer proximity to the open ocean, the correlative Interval Is subdivided Into a lower cycle 4, corresponding with unit J6 (Oxfordlan-Klmmerldglan), and an upper cycle 5a, corresponding with the

Cicatricosisporites

australiensis

and Foraminisporis

wonthaggiensis

Subzones (Berrlaslan-Hauterlvlan)• Cycle 5 Includes the Wyandra-Doncaster sequence In Queensland, and the Wyandra-lower Bulldog sequence In South Australia, and corresponds with the

Foraminisporis

asymmetricus

Snhzone

and

Osmundacidites

duhius

Zone

(Barrenuab-Aptlan). Cycle 6 Includes the lower Coreena Member In Queensland, and the upper Bulldog Shale In South Australia, and corresponds with the Crybelosporites striatus Zone (early Alblan)• Cycle 7 Includes the upper Coreena-Mackunda sequence In Queensland, and the Oodnadatta Formation In South Australia, and corresponds with the

Coptospora

paradoxa

and

Phimopollenites

pannosus

Zones

(middle

and

late Alblan). Cycle 8 Is not fully preserved, and Includes the Wlnton Formation In Queensland and South Australia, and may correspond with the Appendicisporites distocarinatus Zone (early Cenomanlan).

Stratigraphy o f Eromanga and Surat Basins, palynological zones* and global sea l e v e l changes INTERNATIONAL STAGE

GLOBAL EUSTATIC CHANGES

EROMANGA South Australia

BASIN Queensland

PALYNOLOGICAL ZONATION

Winton Mackunda Allaru Toole buc Goreena

A. d i s t o c a r i n .

Doncaster

Osrn, dubius

Wyandra

FJL asymm.

Cadna-owie

Gadna-owie

Winton Oodnadatta Wooldridge Goorikiana Bulldog

SURAT BASIN

P, pannosus Zj^ p a r , / C .

str.

F. wonthagg.

6 5b

Cm austral.

Westbourne Adori 3irkhead

Hut ton

unit J5

unit JA unit J2-3 unit J1

Kingull Mooga

Hooray unit J6

Goreena-Griman Gk. Doncaster Minmi Nullawurt

Orallo

Gubberamunda Westbourne Springbok Walloon Eurombah Hutton Evergreen Precipice

38 (3) Globally, the sea level gradually rose during the Jurassic and Early Cretaceous. The Jurassic sea did not inundate the Eromanga Basin, but high sea levels during the Toarcian and Callovian are thought to correspond with marginal facies, brackish spinose acritarchs in Hutton and Westbourne assemblages. Erosion and/or nondeposition at the eastern and northern basin margins in unit J6 are attributed to low sea levels during the Oxfordian. Isopachs indicate that the Hutton Sandstone was formed by a river valley system draining towards the Surat Basin, indicating an eastern corridor to the open ocean. A marine transgression during the Hauterivian is believed to have caused brackish conditions in the Longreach and Windorah areas, also establishing a northern corridor to the open ocean east of the Euroka Arch, creating brackish-marine conditions in the Gilberton-Hughenden area (cycle 4). During the Barremian a sea arm extended across the northern and eastern basin margin, gradually expanding until it reached its acme in the late Aptian, flooding the entire basin (cycle 5). During the Albian the eastern corridor was blocked, and the Toolebuc/Allaru inland sea was in contact with the open ocean only via the Carpentaria Basin to the north.

References BURGER, D., 1980: Palynological studies in the Lower Cretaceous of the Surat Basin, Australia. Aust., Bur. Miner. Resour., Geol. Geophys., Bull., 189. COOPER, M.R., 1977: and importance.

Eustacy during the Cretaceous: its implications Palaeogeogr., Palaeoclimatol., Palaeoecol., 22, 1-22.

DETTMANN, M.E., & PLAYFORD, G., 1969: Palynology of the Australia Cretaceous: a review: Campbell, K.S.W. (ed.) Stratigraphy and Palaeontology: Essays in Honour of Dorothy Hill. ANU Press, Canberra, 174-210. EVANS, P.R., 1966: Mesoizoic stratigraphic palynology in Australia. Australas. Oil Gas J., 12, 58-63. EXON, N.F. & BURGER, D., 1981: Seidmentary cycles in the Surat Basin and global changes of sea level. BMR J. Aust. Geol. Geophys., 153-9. VAIL, P.R., MITCHUM, R.M. & D THOMSON, S. , 1977: Seismic stratigraphy and global changes of sea level. Part 4; Global cycles of relative changes of sea level; Payton, C.E. (ed.) Seismic stratigraphy applications to hydrocarbon exploration. Am. Assoc. Pet. Geol., Mem., 26, 83-97.

39 THE SURAT AND EROMANGA BASINS: A PALYNOSTRATIGRAPHICAL PERSPECTIVE

M.E. Dettmann, J. Fllatoff and P.L. Price,

C.S.R. Oil and Gas Division, A.M.P. Place, 10 Eagle Street, BRISBANE, QLD. 4000

With the detailed exploration of the Eromanga Basin since 1978 it is perhaps timely to review the palynostratigraphy of the non-marine and near-shore marine Mesozoic section of eastern Australia. The last comprehensive review was that of Evans in 1966 although several studies of more localised geographical and stratigraphical extent have been published on parts of this section since then. There has been little interest in developing a biostratigraphy for the early to mid Triassic "red-bed" and sandstone sequence beneath the Surat Basin (Rewan and Clematis Formations) or for the Nappamerri Formation underlying the Eromanga Basin. This is a reflection of the previously perceived poor petroleum prospectivity of these sediments. Although the hydrocarbon potential of the mid Triassic of the Roma area is established (gas is produced from the Showgrounds Sandstone, and the Moolayember Formation has source potential) this section is generally thin and has not been subdivided palynologically. Sediments of Late Triassic age are limited in their extent in eastern Australia. They are almost entirely absent from the Surat Basin region, and the eastern and central Eromanga Basin area. In the western Eromanga Basin region Late Triassic microfloras have been described in Leigh Creek and, more recently, have been identified overlying the Pedirka Basin sequence. The best known microfloras of Late Triassic age in eastern Australia are those of the Ipswich Coal Measures and basal Bundamba Group of the Clarence-Moreton region. Neither of these sections completely spans the late Middle Triassic to the basal Jurassic and thus the microfloral succession of the Late Triassic in eastern Australia is incomplete. The non-marine Jurassic sediments mark the beginning of bona fide Surat-Eromanga Basin sedimentation and to date have proved to be the most oil productive of the Australian onshore successions. The palynostratigraphy applied to this section is a modification of Evans' 1966 units. With the more extensive sample coverage now available, they have proved difficult to apply and further subdivide because of the vagaries of the interrelationship of facies and plant ecology. The Early Cretaceous witnessed the onset of near-shore marine conditions in both the Surat and Eromanga Basins. The spore-pollen palynostratigraphy has proved to be more stable and consistent, suffering

40 less from variation in plant geography. In the more marine sections (especially in the Eromanga Basin) the miospore zonation is complemented by a subdivision defined on phytoplankton (dinoflagellates and acritarchs). The mid Cretaceous section (that above the Toolebuc Formation and its equivalents) has little representation in the Surat Basin, and has been subject to only limited palynological investigation in the Eromanga Basin. Palynology still has a significant contribution to make in prospecting for hydrocarbons in the largely non-marine Mesozoic sequence of the Surat and Eromanga Basin regions by the detailed refinement and application of palynostratigraphy and palynofacies studies.

GEOPHYSICS

GRAVITY FEATURES IN THE EROMANGA BASIN

V. Anflloff Bureau of Mineral Resources, P.O. Box 278, CANBERRA CITY, A.C.T. 2601

The gravity field over the Eromanga Basin provides Information on the development of troughs, faults and on the basin's relationship to adjoining areas. A major change In free"~air level across the Cork Fault may have Important Implications for the evolution of the basin as a whole (Anflloff, 1982). A rectilinear pattern of elongate gravity lows suggests Palaeozoic rifting along a pre-existing system of crustal sutures. Some patterns extend beyond the basin. Implying that Proterozolc basement occurs throughout the area. Subsidence Into the rifts was very gradual, and may have been accompanied by upwelllng of the lower crust, and the production of granites under the larger troughs. Various narrow basement highs such as the Neblne Ridge, may represent "bridges" along which compression is transmitted laterally. During the latter part of the Phanerozoic, a compression network may have caused a pattern of differential isostatic adjustments, producing growth faults and a multiplicity of simple structures which dislocated the original rift network.

References ANFILOFF, v., 1982: Elevation and gravity profiles across Australia Some implications for tectonism. BMR J. Aust. Geol. Geophys.. 47-54. —

7. -

RESISTIVITY STRUCTURE OF THE CENTRAL EROMANGA BASIN AND UNDERLYING SEQUENCES FROM MAGNETOTELLURIC SOUNDINGS

A.G. Spence and D.M. Flnlayson

Bureau of Mineral Resources, Geology & Geophysics, P.O. Box 378, CANBERRA CITY, A.C.T. 2601 Magnetotellurlc data from the central Eromanga Basin Indicate that one-dimensional resistivity models are appropriate for the region. The uppermost Jurassic-Cretaceous Eromanga Basin sequence contains flowing aquifers and has an average section resistivity in the range 1.3 to 6.8 ohm m with a mean of 3.60 ohm m . This overlies the older sedimentary sequences of the Cooper (Permo-Triassic) and Adavale (Devonian) Basins with resistivities in the range 9 to 398 ohm m , which present good resistivity contrasts for determining sub-surface morphology at the base of the Eromanga sequence. The depth extent of the older sequences is not well resolved because of the overlying highly conductive layers but it appears to be greater than that determined from seismic-reflection data in the Coonavalla Syncline and Warrabin Trough. Basement rocks of the Thomson Fold Belt have resistivities in the range 760 to 3781 ohm m with an average of 1323 ohm m . This resistivity is evident to depths of 40-60 km and consequently represents the conductivity of the Earth's crust in the region. No highly conductive layers were detected within this crustal sequence. In the depth range 60 to 150 km resistivities decrease to less than 5 ohm m . Such resistivities at 150 km depth are in broad agreement with data obtained from magnetometer array studies in southeastern Australia, which are explained by other authors in terms of a 5% partial basalt melt.

References MOORE, R.F., KERR, D.W., VOZOFF, K . & JUPP, D.L.B., 1977: Southern Cooper Basin magnetotellurlc survey. South Australia, 1974. Aust., Bur. Miner. Resour., Rec. 1977/41 (unpubl.). SPENCE, A . G . & FINLAYSON, D.M., 1982: The resistivity structure of the crust and upper mantle in the central Eromanga Basin, Queensland, using magnetotellurlc techniques. Geol. Soc. Aust. J., in press. WHITELY, R.J. & POLLARD, P.C., 1971: A combined deep resistivity and magnetotellurlc sounding in the Eromanga Basin, Queensland. Search, 103-5. WOODS, D.V. & LILLEY, F.E.M., 1980: Anomalous geomagnetic variations in the concentration of the telluric currents in southwest Queensland. Geophys. J . (R. Astronom. Soc.), 62, 675-89.

Seismic

travne

Magneto

- Telluric

Oil exploration Limits

site

well

of seismic

Southern

boundary

of Cooper

Basin

traverse

Site locations

w Sift I

10 II

Resistivity

amptitvH

45 BASEMENT STRUCTURE UNDER THE CENTRAL EROMANGA BASIN FROM SEISMIC REFRACTION STUDIES

J. Lock

Bureau of Mineral Resources, Geology and Geophysics, P-0. Box 378, CANBERRA CITY, A.C.T. 2601

In 1980 and 1981 the Bureau of Mineral Resources, Geology and Geophysics (BMR) conducted seismic surveys in the central Eromanga Basin, which included continuous seismic reflection and coincident refraction recording along a line extending f rom Mt. Howitt 1 well in the west to Cheepie in the east (Fig. 1). Refraction stations were spaced at 1.875 km intervals out to distances of 75 km to study the velocity structure of the Eromanga sequences, underlying basins and basement. All seismic refraction lines were reversed. Data at the western end of the line (Mt. Howitt 1 to Tallyabra) are presented in this interpretation. The representative velocity model from first arrival data presented here, is simple but allows several conclusions to be drawn about basin and basement structure with depth. No strong primary reflection branches are observed at short distances (less than 20 km) therefore the velocity of the basin sequence is interpreted as increasing continuously from 2.3 km/sec at the surface to 4.8 km/sec at 2.4 km depth (Fig. 2). Arrivals between 8 and 20 km have a lower apparent velocity than those recorded at greater distances. This indicates that the low-grade Ordovician metasediments encountered in Mt. Howitt 1 may extend to the east under the Eromanga Basin. Arrivals beyond 20 km are from basement, where velocity increases relatively rapidly to 5.9 km/sec at 7 km depth. Clear impulsive first arrivals persist with appreciable amplitudes out to 100 km. At greater distances amplitudes are greatly reduced, arrivals become emergent and their nature changes. This implies structure where basement velocity reduces from 5.9 km/sec at 7 km depth to 5.7 km/sec at 7.5 km. A feature of seismic refraction data in this region is a sequence of clear later arrivals which, in general, consist of an energy packet of the same shape as the initial onset but often larger amplitudes. There are at least three such arrivals identifiable on any record section and as many as six may be identified on some. These later arrivals are observed beyond 10 km to at least 40 km and possibly to 60 km, though unambiguous identification of these arrivals is difficult at larger distances. Their amplitude and arrival time characteristics indicate they are not multiple reflections from the basin/basement interface. They are probably refracted arrivals multiply reflected at the surface. Reflection coefficients for a range of angles of incidence at a low velocity surface layer and ray tracing (carried out to determine whether a velocity model of this type can explain these arrivals in this manner) will be presented. These arrivals further constrain the velocity variation with depth in the uppermost crust.

n at Ptrmo- Trimatit Coo/tar ana QaMaa Ba$mt Margin of Adavaia Basin Fault cutting Bromanga Batin and oMar rockt . FauH cutting pra Bromanga Batin rocks TTT

futt

%SI>otpoint

cutting Adavala Basin rocks

Zaro ttructura contour on bat, of BoHing Downs Group (datum MSL)

ShotlBBO

—SHottBBI

Stat»,nt tpacad at - t B?Skm Mnm

Fig.1 Location VELOCITY 1

diagram.

km/isec 3

Fig. 2 Velocity/depth profile from Mt. Howitt No.1 Well to Eromanga.

SEISMIC DATA FROM THE EROMANGA BASIN

B. Rumph

Delhi Petroleum Pty. Ltd., 33 King William Street, ADELAIDE, S.A. 5000

Delhi Petroleum Pty. Ltd. and SANTOS Limited have held exploration licences In northeastern South Australia and southwestern Queensland since 1958. Multi-fold seismic recording began In the early 1970s and over 35,000 km have been recorded. The Vlbrosels source was first used In 1973. Over the last five years approximately 28,500 km of multi-fold data have been recorded In the licence areas. Including approximately 6,800 km and a 3D survey sole-risked by the South Australia Oil and Gas Corporation Pty. Ltd. The poster display and paper will present a regional cross-section of the western Eromanga Basin and cross-sections of recent oil discoveries to Illustrate Improvements in data quality with modern seismic techniques, and different structural and tectonic styles in the Eromanga Basin. Recent oil discoveries in the Eromanga Basin have indicated the potential of this section, and the need for detailed high-quality seismic data and adequate velocity control to define the fields. Although these discoveries are largely structurally controlled, a significant stratigraphlc component has been recognised. Interpretation techniques are being developed to enable more detailed definition of the structure and tectonic history of the basin, and their relationship to hydrocarbon accumulations. The challenge of developing stratigraphlc exploration techniques to expand the exploration horizons will require imaginative thinking from all explorationists.

OIL SHALES AND THE TOOLEBUC FORMATION

FQRAMINIFERA OF THE MARINE CRETACEOUS OF THE GREAT AUSTRALIAN BASIN WITH SPECIAL REFERENCE TO THE TOOLEBUC FORMATION

Viera Scheibnerova

Geological and Mining Museum, 36 George Street, SYDNEY, N.S.W. 2000

Marine Cretaceous deposits of the Great Australian Basin contain over 100 species of foraminifera represented by some 60 genera (37 agglutinated and 65 calcareous forms). The upper part of the section proved to contain oil shale deposits in the Queensland portion of the Eromanga Basin. Oil shales occur immediately below the Toolebuc Formation represented by several lithological types including calcareous and bituminous siltstones and mudstones with subordinate limestone. Scheibnerova & Byrnes (1977) concluded that the Toolebuc Formation in New South Wales represents a phase of slow deposition during one of the several wide-spread transgressions in Early Cretaceous time, with accumulation of planktic foraminifera and/or fish remnants and ammonites. However, in New South Wales the Toolebuc Formation in the Department of Mines boreholes (stratigraphic cores) lacks some of the characteristic features such as planktic foraminifera, limestone lithology, gamma-ray anomaly and Inoceramus prisms. Scheibnerova & Byrnes (1977) considered other features to aid diagnosis of the Toolebuc Formation in New South Wales. More recent studies of newly recovered material in northwest New South Wales contributed some very relevant material to this problem. These are presented and discussed in more detail (BMR Urisino 1, WRC Urella Downs 1 and WRC Birrigulpar 1 in New South Wales, and BMR Augathella 5, 6 and 7 in Queensland).

References SCHEIBNEROVA, V. & BYRNES, J.G., 1977: Correlation of Cretaceous lithostratigraphic, lithological, palaeontological and foraminiferal units in the Great Artesian Basin. N.S.W., Geol. Surv., Q. Notes, 27, 1-11.

DEPOSITIONAL ENVIRONMENT OF THE TOOLEBUC FORMATION AND ITS EQUIVALENTS, EROMANGA BASIN, AUSTRALIA

S. Ozlinic

Division of Continental Geology, Bureau of Mineral Resources, Geology and Geophysics, P.O. Box 378, CANBERRA CITY, A.C.T. 2601

The widespread Early Cretaceous oil shale-bearing Toolebuc Formation facies, and their lateral equivalents (Fig. 1) which (Wooldridge Limestone Member and 'Urisino beds') lack oil shale, are interpreted to have been deposited: in and towards the southern boundary of a late Albian transgressive Toolebuc sea; on a northerly dipping palaeoslope;

and

in a quiet anaerobic, relatively deep-water marine environment in the north, grading to a higher energy aerobic, shallow-water marine to brackish environment in the south and southwest. The conditions envisaged are those of a 'positive water balance basin' (Demaison & Moore, 1980) in which saline water entered over the narrow Euroka Arch in the north and fresh water from the basin hinterland flowed northwards out of the area (Fig. 1). The sea was therefore most likely stratified with a permanent halocline below a layer of fresh water. Such conditions would have: enhanced prolific productivity in the euphotic zone; led to permanent or intermittent oxygen depletion in the lower part of the water column of the deeper parts of the basin; limited the establishment of normal benthonic marine fauna; favoured preservation of organic matter in the northern and central parts of the basin; and effectively prevented the growth and preservation of organic matter in the southern and southwestern parts of the Toolebuc sea where, along the shelf and shelf margins shallow higher energy and mostly oxidizing conditions prevailed. The areal distribution of the Toolebuc Formation and its lateral equivalents reflects the interplay of basin geometry, water circulation and sedimentary processes. The conditions which favoured oil shale deposition were apparently terminated with the return of normal marine environment, probably the result of increase in saltwater inflow arising from rising sea level and possible variations in conditions limiting the productivity in the euphotic zone.

Toohbuc F^ormotion - korogenous fades A

_ ^

_ r

•^

Major data points (Oiimic, t992)

. , « Tooiebuc Format,on-kerogenous focias B

Extent of Tooiebuc Formation ^^ ^^^ ^qui>^tant$

Wooldridgt Limatton* Odambar

Urisino b9d$

South

North A . BASIN

EROMANGA BASIN

1 it CrtO <

EUROKA

SOOkm 1

ARCH

. —

— »

Fig.I

Outflowing frastter (itss dtnsej water k^flowing tahne (denser) water

Oitfribution of Toolebuc Formotion facitt and lateral tquivalentsi crota-sactiofl •howing hypo»h«ticol conditions in th« batin during kerogenout «hol« dtpotition.

Acknowledgement s This abstract Is published with permission of the Director, Bureau of Mineral Resources, Geology & Geophysics, Canberra. Support of this research was provided under the National Energy Research Development and Demonstration Programme administered by the Commonwealth Department of National Development and Energy. References DEMAISON, G.J. & MOORE, G.T., 1980: Anoxic environments and oil source bed genesis. Am. Assoc. Pet. Geol., Bull., 64, 179-209. OZIMIC, S., 1982; Depositional environment of the oil shale-bearing Cretaceous Toolebuc Formation and its equivalents, Eromanga Basin, Australia: Proc. 15th Oil Shale Symp., Golden, Colorado, U.S.A. Colorado School of Mines (in press).

CYANOBACTERIAL MATS AND OTHER BACTERIA; MAJOR CONTRIBUTORS TO THE FORMATION OF THE LOWER CRETACEOUS TOOLEBUC OIL SHALES

M. Gllkson Centre for Resource and Environmental Studies, Australian National University, CANBERRA, A.C.T. 2600

The oil shales of the Toolebuc Formation consist of Irregularly spaced calcareous laminations alternating with black organic-rich laminae suggestive of "algal-mats". Cyanobacterla of Nostoc and Anabaena types were encountered In SEM observations of whole-rock fragments. TEM observations of the organic matter (OM) concentrates revealed a wealth of microbial remains. These bacteria were most likely responsible for the degradation of the primary OM leading to the production of the amorphous OM that Is visible with optical microscopy. The so-called "amorphous OM" In TEM observations consists of mesh-structured "palaeoprotelns", humlc acids and more or less homogeneous OM of llpldlc nature. The 13C values of the OM from the laminated oil shales gives values of -27.8% to -29.0%, Indicating a bacterial lipid source as the major contributor at the final stage of deposition. High concentrations of the trace metals vanadium, molybdenum and nickel are associated with the oil shales. The higher accumulations of molybdenum were found to be present in samples with a high organic nitrogen content. A dependancy of N—fixing cyanobacterla on the presence of molybdenum has been previously demonstrated. The presence of pyrlte within the calclte indicates that CaC03 was deposited in the anoxic zone. The irregularity in extension and dimensions of the calcareous laminae is not compatible with seasonal precipitation. It is more likely to be the result of mlcroenvlronmental changes brought about by the cyanobacterla upon reaching high densities which led to depletion of the nitrogen source and to their subsequent death. The wealth of planktonlc foramlnlferal and coccolith remains suggests a highly productive aerated upper water layer and an anoxic water-sediment Interface as evident from the abundance of pyrlte within the remains of organisms as well as in the surrounding matrix.

ORGANIC MATTER AND VANADIUM IN THE TOOLEBUC FORMATION OF THE EROMANGA BASIN

Riley and J.D. Saxby CSIRO Division of Fossil Fuels, P.O. Box 136, NORTH RYDE, N.S.W, 2113

Since being discovered in 1966, the Julia Creek oil shale deposit has been noteworthy for its high vanadium content. The oil shale is contained within the Cretaceous Toolebuc Formation which consists of limestone, coquinite, labile sandstone, siltstone, mudstone and calcareous shale (Exon & Senior, 1976). Possibly up to one million square kilometres of the Eromanga and Carpentaria Basins contain oil shale, although the thickness of the Toolebuc Formation is usually less than 75 metres. This paper aims to give an overview of vanadium distribution and geochemistry, not just at Julia Creek, but throughout the Eromanga Basin. Thirteen samples of organic-rich sediments and weathered surface material from widely separated sites in the Toolebuc Formation have been investigated. Two similar samples from the southern Carpentaria Basin are also included. Sample details as well as basic carbon, vanadium and porphyrin analytical data are given in Table 1. Specific objectives of the research were to examine the distribution of vanadium, the association of vanadium with carbonaceous material and the composition of the organic matter itself. The major conclusions are as follows: i)

The concentration of vanadium in 14 widely separated oil shale samples from the Toolebuc Formation varies from 250 to 3200 yg g""^. In a sample of extensively weathered Julia Creek shale 2600 yg g"^ are present. All these contents are much higher than tynical average levels for shales (130 yg sandstones (20 yg g"^), limestones (20 yg g"^) and igneous rocks (135 yg g"^) (Bowen, 1966). Vanadium concentrations tend to be higher in northern parts of the Eromanga Basin and lower in the southeast.

ii) Vanadium occurs in a number of chemical forms in the Toolebuc Formation (Riley & Saxby, 1982). Most is present as a relatively labile absorbed or precipitated species, possibly hydrated oxides or metal vanadates. Vanadium also occurs in the silicate structure of micas or clays and, in some areas, significant amounts of organically-bound vanadium are present. Particularly high concentrations of organically-bound vanadium have been found at Julia Creek and the sample from 906 m at Mayneside has about half its vanadium chemically-bound within the kerogen. These samples contain the lowest proportion of labile vanadium (approx. 35%).

Table 1

Sample Details and Carbon, Vanadium and Porphyrin Analyses

SAMPLE LOCATION

ORGANIC C (%)

CARBONATE C (%)

TOTAL V (yg

METALLATED* PORPHYRIN ) (Pg g-1)

Augathella No. 7: 1460l6'12"E, 25042'16"S, 75.15-75.3 m

5.9

0.3

250

Bedourie No. 1: 139°20'30"E, 24°15'40"S, 107.36-108.36 m

5.9

2.0

750

Boulla No. 3A: 140O43'30"E, 22°47'00"S, 32.05-32.29 m

13.1

6.2

1900

Boulla No. lOB: 140°46'05"E, 22°42'25"S, 42.97-44.00 m

9.6

7.3

1650

Burketown No. 1: 139°32'E, 18O04'S, Core 5, 394.7 m

6.9

8.0

1000

Charleville No. 5: 146°02'04"E, 26°23'17"S, 60.95-61.05m

5.5

<0.1

250

Dobbyn No. 2: 140°13'E, 19°11'S, Core 6, 114.3 m

16.3

6.4

3200

Jericho No. 11: 145°31'18"E, 23°51'30"S, 61.93-62.15 m

11.7

3.0

1600

Julia Creek, exploration drilling (1980) 30-32 m

13.7

4.5

1900

350

Weathered coquinite from outcrop, St. Elmo Structure, Julia Creek

0.1

9.3

2600

<0.005

Longreach No. 6: 145°19'16"E, 23°37'35"S, 29.50-29.65 m

6.6

<0.1

300

Mayneside No. 1: 142°31'E, 23^35'S, Core 1, 905.6 m

11.4

7.5

1800

730

Springvale No. 9: 140^37•50"E, 23°08'10"S, 134.00-135.11 m

13.7

6.8

2500

Tambo No. 38: 145^39'07"E, 24°15'00"S, 58.90-59.00 m

10.6

3.3

1850

Tambo No. 44: 146°09'25"E, 24°58'22"S, 17.25-17.50 m

7.7

1.2

300

calculated as vanadyl desoxophylloerythroetioporphyrln

(M.W. « 542)

ill) Vanadium Is probably mobilised from the organic matter by weathering and concentrated In adjacent inorganic host rocks. This mobilisation occurs at varying depths due to oxidation and biological activity associated with water movement through aquifers of the Great Artesian Basin for which the Toolebuc Formation acts as one of the confining beds. Iv)

Mobilisation of vanadium appears to occur before there Is significant alteration to the average chemical composition of the organic material. Elemental analyses of Toolebuc kerogen are quite similar over wide areas of the formation.

Acknowledgements The collaboration of CSR Limited and the Bureau of Mineral Resources in the provision of samples and in discussions is gratefully acknowledged. Some of the samples used in this research were obtained during a joint CSIRO/BMR project supported under the National Energy Research, Development and Demonstration programme administered by the Commonwealth Department of National Development.

References BOWEN, H.J.M., 1966: London.

Trace Elements in Biochemistry.

Academic Press,

EXON, N.F. & SENIOR, B.R., 1976: The Cretaceous of the Eromanga and Surat Basins. BMR J. Aust. Geol. Geophys., 33-50. RILEY, K.W. & SAXBY, J.D., 1982: Association of organic matter and vanadium in oil shale from the Toolebuc Formation of the Eromanga Basin, Australia. Chem. Geol., in press.

THE PETROPORPHYRINS IN THE OIL SHALE FROM THE JULIA CREEK DEPOSIT

A. Ekstrom, H. Loeh, and L. Dale,

CSIRO Division of Energy Chemistry, Lucas Heights Research Laboratories, Private Mail Bag 7, SUTHERLAND, N.S.W- 2232

The compounds solvent-extracted from Julia Creek oil shale with chloroform have been examined by a variety of techniques including chromatography and mass spectrometry. These studies have provided the following results: 1.

The oil shale contains significant concentrations of elements such as iron, copper, chromium, molybdenum and zinc which are present as metal-organic complexes. However, the major metal organic species present are vanadium and nickel porphyrins.

The concentration of the solvent extractable forms of the nickel and vanadium porphyrins show marked variations with depth in the deposit, although these forms are generally associated with the oil shale.

At present it appears that the nickel and vanadium are complexed to different types of porphyrin compounds. This observation may indicate that these two species have different origins in the deposit.

The vanadium porphyrins are an extraordinarily complex mixture which includes compounds tentatively identified as phylloerythrin and chlorin derivatives. In addition, porphyrins of a molecular weight in the range 1000-1200 are present in significant amounts, as are compounds containing vinyl substituent groups.

The presence of these thermally and chemically quite unstable compounds in the deposit suggests that despite its age, the deposit has undergone very little maturation.

GEOCHEMISTRY OF OIL SHALE IN THE EASTERN EROMANGA BASIN

J.D. Saxby

CSIRO Division of Fossil Fuels, P.O. Box 136, NORTH RYDE, N.S.W. 2113

The largest and most widespread oil shale deposit in Australia occurs in the Cretaceous Toolebuc Formation of the Eroraanga Basin. An understanding of the geochemistry of this deposit will assist not only in future exploration but also in solving problems arising during possible utilisation. Results from a study of the eastern margin of the Eromanga Basin are the subject of this paper. Samples have been taken from fifteen shallow holes drilled between Longreach and Charleville, a distance of over 300 km. The holes were located far enough into the basin, away from the outcrop, for the Toolebuc Formation to be encountered beyond the extensively weathered surface zone. The Toolebuc Formation was continuously cored and 154 samples were chosen so as to adequately reflect all significant vertical variations. All cores and cuttings v^re megascopically examined and their lithologies correlated with wire-line logs (Ozimic, 1981a, 1981b; Stephenson, 1982). The following analyses have been carried out: moisture, total carbon, carbonate carbon, hydrogen, nitrogen, total sulphur, pyritic sulphur, sulphate sulphur and oil, water, char and gas -f loss from a small scale pyrolysis procedure. Tlie drilling, logging and analytical work form part of a CSIRO-Bureau of Mineral Resources joint research project on exploration methodology applicable to a widespread oil shale deposit such as that within the Eromanga Basin. Major conclusions are as follows: i)

The thickness of Toolebuc oil shale varies considerably along the eastern margin of the Eromanga Basin from Longreach to Charleville. Only low-grade oil shale has been intersected. The most significant zone was 2 m near Tambo yielding 43 litres/tonne. In general organic contents and oil yields decrease towards the south.

ii)

Organic matter from the sampled holes does not vary greatly in composition. Atomic H/C values fall in the range 0.9 to 1.3, significantly lower than most oil shales including Rundle and Green River (1.5-1.6). If it is assumed that completely aromatic and aliphatic kerogens have H/C ratios of 0.6 and 1.8 respectively, then Toolebuc organic matter appears to be approximately 60% aromatic. Nitrogen content of the carbonaceous material is approximately 2.5%, while organic sulphur varies up to 10% with the most organic-rich samples containing approximately 3.2%.

iii) On average, 27% of the Toolebuc organic matter is converted to oil on pyrolysis. 20% gas is also produced and 53% remains as char in the spent shale. Evidently this low conversion to oil is a

reflection of the high content of insoluble aromatics in the unheated shale. The proportion of kerogen converted to oil rather than gas or char increases with organic carbon content. iv)

A precise determination of total nitrogen can be used as an indicator of organic carbon (and thence oil yield) in a specific area.

The sulphur content of Toolebuc shale oils results mainly from the higher than average organic sulphur content of the kerogen. This sulphur is probably present as sulphide and disulphide bonds, as well as in heterocyclic aromatic rings.

vi)

Metal carbonate contents in the Toolebuc "limestone" vary from zero to 67% and are not directly related to oil yields.

vii) Moisture contents in ground, air-dried samples vary from 1.4 to 9.9%. As expected, carbonate-rich, clay-poor samples are lowest in water lost up to 105^C. Water obtained on retorting varies up to 10.5% and can be considered as the sum of moisture, water released from clays and a small amount produced from decomposition of oxygen-functional groups in kerogen. Water released above 400®C during retorting may assist in hydrogenating shale oil. viii)

Although most of the wells in this study were positioned so that Toolebuc samples were not extensively weathered, some changes in mineralogy, re-distribution of metals (such as vanadium) and bacterial alteration of kerogen cannot be excluded.

Acknowledgements Support for this research was provided under the National Energy Research, Development and Demonstration Programme administered by the Commonwealth Department of National Development. NERDDC Project 78/2616 on oil shale methodology is a joint CSIRO/BMR investigation. The analytical assistance of N. Watson and R. Judd is also gratefully acknowledged. References OZIMIC, S., 1981a: Stratigraphic drilling in the Cretaceous Toolebuc Formation in the southern and eastern Eromanga Basin, 1980. A contribution to BMR/CSIRO, NERDDC Project 78/2616. Aust., Bur. Miner. Resour., Geol. Geophys., Rec., 1981/12. OZIMIC, S., 1981b: Stratigraphic drilling in the Cretaceous Toolebuc Formation in the Charleville district, southern Queensland, June 1981. A contribution to BMR/CSIRO NERDDC Project, 78/2616. Aust., Bur. Miner. Resour., Geol. Geophys., Rec., 1981/38. STEPHENSON, A.E., 1982: Stratigraphic drilling in the Cretaceous Toolebuc Formation in the southern Eromanga Basin, 1981. A contribution to BMR/CSIRO NERDDC Project, 78/2616. Aust., Bur. Miner. Resour., Geol. Geophys., Rec., in press.

61 Typical correlation between geochemical and geophysical parameters Tambo No, Al. TTarbonate , . Organif Carton(%y Carbon .Pyfitf. (%) 1 2 • 2 6 .10 ! 2 \

THE SIGNIFICANCE OF GAMMA-RAY ANOMALIES IN THE CRETACEOUS TOOLEBUC FORMATION FACIES AND IN THEIR LATERAL EQUIVALENTS, EROMANGA AND CARPENTARIA BASINS

S. Ozimic

Division of Continental Geology, Bureau of Mineral Resources, Geology and Geophysics, P,0. Box 378, CANBERRA CITY, A.C.T. 2601

In boreholes, the presence of the Toolebuc Formation has in the past been suspected from a marked isolated peak or set of peaks on gamma-ray logs. The anomaly is easily recognised, and correlates with the Toolebuc Formation kerogenous facies throughout the central and northern Eromanga Basin, and onshore Carpentaria Basin. In the southern part of the Eromanga Basin in BMR Urisino 1 stratigraphic hole an anomaly occurs in the upper part of the Coreena Member equivalent, but not in the overlying Toolebuc Formation equivalent 'Urisino beds' (Ozimic, 1982). Palynological dating found the age of sediments giving the gamma-ray anomaly in BMR Urisino 1 to be slightly older than the Toolebuc Formation in central and northern parts of the Eromanga Basin (Burger, 1981). Examination of the gamma-ray logs of a number of petroleum exploration wells, stratigraphic holes and water bores in South Australia, New South Wales, Northern Territory and Queensland shows: that the gamma-ray anomaly possibly rises in the succession from the southern to the northern parts of the Eromanga Basin, from the upper part of the Coreena Member equivalent to the upper beds of the Toolebuc Formation kerogenous facies (Fig. 1); that the character of the anomaly changes from weak, broad and serrated, along the central and northern basin margins to intensive, narrow, single or double peaked in the centre of the basin; that where kerogenous shales (oil shales) have been proven they are associated with a gamma-ray anomaly; the reverse proposition is not true for the whole basin, as the anomaly below the Toolebuc Formation equivalent 'Urisino beds' in the southern part of the basin demonstrates (Fig. 1); and that the intensity of the gamma-ray opposite kerogenous shales (oil shales) does not bear a simple relationship to oil-shale grade. The great areal extent of the gamma-ray anomaly is thought to be evidence for widespread partial and intensive reducing conditions (during the deposition of upper Coreena Member equivalent in the south; the

Wooldrldge Limestone Member in the southwest and the Toolebuc Formation In the north) suitable for fixation of soluble radioactive matter Into insoluble minerals as well as for preservation of organic matter constituting the bulk of the oil shale component of the Toolebuc Formation•

Acknowledgement s This abstract is published with permission of the Director, Bureau of Mineral Resources, Geology & Geophysics, Canberra. Support of this research was provided under the National Energy Research Development and Demonstration Programme administered by the Commonwealth Department of National Development and Energy.

References BURGER, D., 1981: Palynology of three N.E.R.D.D.C. stratigraphic bores in the Eromanga Basin (Preliminary report). Aust., Bur. Miner. Resour., Geol. Geophys., Rec., 1981/32. OZIMIC, S., 1982: Depositional environment of the oil shale-bearing Cretaceous Toolebuc Formation and its equivalents, Eromanga Basin, Australia: Proc., 15th Oil Shale Symp., Golden, Colorado, U.S.A. Colorado School of Mines (in press).

Rock Units X-log ALLARU MUDSTONE

TOOLEBUC FORMATION

COREENA MEMBER

LOCALITY

MAP

Coquinite

Kerogenous $hal9 (oil shots)

Shale

Silly

sands

Giouconie - bearing

Fig.l

Eromanga

Basin, Gamma-ray anomaly correlation.

sands

SOURCE ROCKS

66 ORGANIC FACIES IN THE EROMANGA BASIN

A.C. Cook

Geology Department, University of Wollongong, P.O. Box 1144, WOLLONGONG, N.S.W. 2500

The organic matter In the sedimentary rocks of the Eromanga Basin Is dominated by coals and coal-related sedimentary rocks, but a significant part of the sequence contains a component of marlne-sourced organic matter. The coals and These are:

the

coal-related

organic

matter

show

four major

fades.

Basal Jurassic. Coals are Inertlnlte-rlch but contain abundant exlnlte. Exlnlte contents are typically In the range 5-10% with vltrlnlte dominate over sporenlte and reslnlte. Associated sediments have a high content of Inertlnlte but vltrlnlte Is also abundant.

Walloon-type coals. These are coals which contain little or no Inertlnlte and exceptionally large amounts of exlnlte. Suberlnlte Is the most abundant exlnlte maceral but cutlnlte and reslnlte are also unusally abundant. Related sedimentary rocks also contain abundant vltrlnlte and exlnlte. This f a d e s Is characteristic of the Blrkhead Formation but similar assemblages occur in some of the overlying Jurassic units.

Wlnton-type coals. These coals contain abundant band vltrlnlte but are characterised by the presence of small amounts of spore-rich duroclarite.

The basal Jurassic suite of coals shows some similarities to the coals in the Toolachee Formation but are distinguished by the abundance of reslnlte and the paucity of Botryococcus-related alginlte. The marine units in the lower part of the Cretaceous contain a significant content of marine phytoplankton, but the majority of the organic matter in most of these units is of higher plant origin. The organic matter in the Jurassic part of the sequence is the probable source of at least some of the oil in the Mesozoic reservoirs.

ASPECTS OF THE SOURCE ROCK AND PETROLEUM GEOCHEMISTRY OF THE EROMANGA BASIN

D.M. McKirdy

Australian Mineral Development Laboratories, Fletnington Street, FREWVILLE, S.A. 5063

This investigation of the source-rock potential and petroleum geochemistry of the Eromanga Basin is based on organic geocheraical analyses of rock, oil and gas samples from more than 30 wells in South Australia, Queensland and the Northern Territory. Interpretation of the analytical data is facilitated by dividing the basin into six provinces, broadly delineated by the structural framework of the underlying PermianTriassic basins. Each province has had a distinctive thermal history, as shown by the depth-reflectance studies of Kantsler and Cook (1980). Regional variation in the maturation state and hydrocarbon-generating potential of Jurassic and Cretaceous shales and siltstones is assessed using vitrinite reflectance, extract yield and composition, various alkane parameters, and the petrology of the dispersed organic matter. Units with the best oil-source potential are the Birkhead Formation and Murta Member (good to excellent). Fair to good oil sources include the shale/siltstone lithofacies of the Poolowanna Beds, basal Jurassic and Namur Sandstone Member. Woody-herbaceous material (including sporinite, resinite, cutinite, suberinite and resinous vitrinite) is the major hydrocarbon precursor throughout the Jurassic section. Kerogen in the Murta Member, Transition Beds, and Wallumbilla Formation has a significant algal component (phytoplankton and bituminite). The 'liptinite facies' of the source rock largely determines both the nature of its primary hydrocarbon product and the level of organic maturity necessary for hydrocarbon genesis to begin. Adequate maturation levels for the generation of waxy oil from terrigenous organic matter (vitrinite R^ max ^ 0.7%) have been attained by Early and Middle Jurassic sediments in the central Pedirka Basin area, and over much of the southern Cooper Basin area. Less mature Jurassic and Early Cretaceous sediments containing significant amounts of labile exinite (viz. resinite, suberinite) are potential sources of light oil and condensate. Gas generation from such organic matter commences at RQ max = 0.6%. Jurassic and Cretaceous oils in the Eromanga Basin are of three main types: 'heavy', waxy paraffinic (37-41^ API, 19-41^C pour point) e.g. Poolowanna 1 (Poolowanna Beds), Jackson 1 (Westbourne, Hutton); intermediate paraffinic-naphthenic (45-49^ API, 8-12^ pour point) e.g. Strzelecki 3 (Birkhead, Hutton); and light, low wax paraffinic (49-57^ API, 1 O^C pour point) e.g. Cuttapirrie 1 (basal Jurassic), Merrimelia 8, 10 (Namur, Hutton), Dullingari North 1 (Murta). The

bimodal distribution of pristane/p^tane values displayed by these oils is explicable in terms of early (RQ max = 0.5-0.7%, pr/ph = 3-4) and normal (R^ max > 0.7%, pr/ph = 5-6) expulsion from source beds containing organic matter of higher plant origin. Appreciable differences exist in the extent of their subsequent thermal alteration in the reservoir. With the exception of the gas at Namur (and possibly some of the gas at Dullingari, Wackett and Naccowlah), Mesozoic oil and gas in the Eromanga Basin are chemically and isotopically distinct from Permian hydrocarbons in the underlying Cooper Basin. However, the presence of bicyclic sesquiterpanes, probable biomarkers of resinite and/or essential oil precursors, is common to both Mesozoic and Permian oils.

MATURATION AND GENERATION

70 GEOTHERMAL GRADIENTS IN THE EROMANGA -

COOPER BASIN REGION

G.M. Pitt

Western Mining Corporation Limited, 168 Greenhill Road, PARKSIDE, S.A. 5063

Borehole temperature measurements are generally available from four sources: wireline log runs, drillstem tests, static-pressure surveys and temperature logs. Of these, wireline logs are the most commonly available on a non-confidential, regional basis. The present study considered thermal data from DSTs and wireline logs of some 250 petroleum wells in the Eromanga-Cooper region. Bottom-hole temperatures were extrapolated to a conservative, estimated formation temperature. From these data, "basement" (or base Permian)-to-surface geothermal gradients were calculated. The gradients are also considered conservative but accurate to about 5%. Figure 1 shows the distribution of "basement"-to-surface geothermal gradients. The contours show a NE-SW and SE-NW structural control and a good correlation with regional tectonic structure. The GidgealpaMerrimelia-Innamincka (GMI) trend is immediately apparent (A-A), as is the Jackson-Barrolka trend (B-B) which divides the thermally-different northern and southern Cooper Basins. Six provinces are recognised: northern Cooper Basin (gradient >4^C/100m), Daralingie-Moomba-Wackett Area (4.0-5.5), Patchawarra Trough (<4.0), northeastern Pedirka (>4.0), southwestern Pedirka (<4.0) and the Thunda-Yongala Area (<4.2). In the northeast and southeast of the study area Permian is thin to absent. Data from these areas are not compatible with that in Figure 1 as they are usually based upon readings from water wells producing from the Jurassic aquifers. Such gradients often greatly exceed the overall "basement"-to-surface values. Intermediate wireline logging runs preceed the usual setting of casing in the Cretaceous Transition Beds (Cadna-owie Formation). These, with the few appropriate shallow DSTs allow the calculation of Late Jurassic-tosurface and (combined with total-depth readings) "basement"-to-Jurassic gradients. A consistent zonal distribution of the difference between the latter two has been mapped. Despite a poor data distribution it shows a clear correlation with the regional structure of the Cooper Basin. Gradients in the study area (3.2-6^C/100 m) are high by world standards for intracratonic basins (the mean is some 3^C/100 m). Maturation modelling by the author supports the view, based on vitrinite reflectance studies, that the current high temperatures are a geologically recent phenomenon. Such modelling also suggests that generation in the Late Jurassic-Early Cretaceous sequences commenced no earlier than the mid-Tertiary and is proceeding most strongly at the present. This will have an important bearing on the role of hydrodynamic migration and trapping in the Eromanga Basin and the prospectivity of "young" structures.

F I G U R E 1 : GEOTHERMAL

GRADIENT, " B A S E M E N T " TO S U R F A C E

UNIT=°C/100M

C.I

0 25°C/100M

MATURATION PATTERNS IN THE EROMANGA BASIN

A.J. Kant8lerl>3^ ^^Q^ Cook^ and M. Zwigulls^

^Department of Geology, University of Wollongong, Northfields Road, WOLLONGONG, N.S.W. 2500 ^Delhi Petroleum Pty. Ltd., 33 King William Street, ADELAIDE, S.A. 5000 ^Present address: Shell Development (Australia) Pty. Ltd., 140 St. George Terrace, PERTH, W.A. 6000

Vitrinite reflectance data from 70 wells which penetrate the JurassicCretaceous Eromanga Basin sequence above the Permian-Triassic of the Cooper and Pedirka Basins vary from less than 0.3% in parts of the Cretaceous Winton Formation to more than 1,7% in some intersections of the Jurassic Hutton Sandstone. Isoreflectance surfaces show varying relationships to regional structure and present—day temperature gradient variation. Vitrinite reflectance modelling studies indicate a highly variable thermal history. Basin-wide high heat flow is related primarily to high levels of crustal heat production. The Jurassic-Cretaceous was characterised by a phase of lower heat flow, and subsidence was accompanied by the local development of temperature 'highs' on prominent structural trends in response to lateral dewatering of the sub-basins. Late Tertiary-Recent artesian flow has created temperature anomalies within the Eromanga section. Model results suggest that in many areas the high present-day temperature gradients (up to eO^C/km) are a relatively recent phenomenon. Coal beds and land-plant related organic detritus are present throughout the section which includes at least three potential source-rock horizons. The onset of significant hydrocarbon generation followed the rapid deposition of the Cenomanian Winton Formation. However, host rocks for most Eromanga Basin oil and gas discoveries are associated with relatively low levels of organic maturity, implying a source either from underlying, mature, Permian coal-bearing sequences or from a more mature Jurassic section in a more basinward setting. Maturation maps at the top of several potential Jurassic-Cretaceous source intervals indicate that most commercial Eromanga Basin oil discoveries will be found flanking mature Jurassic sequences such as those in the Nappamerri Trough.

73 SUBSIDENCE AND ASSOCIATED SOURCE ROCK MATURATION FOR THE CENTRAL EROMANGA BASIN V.L. Passmore and C.J. Boreham Bureau of Mineral Resources, P.O. Box 378, CANBERRA CITY, A.C.T. 2601 Geochemical analysis of 41 wells In southwestern Queensland confirms the hydrocarbon prospectivity of source rocks in Jurassic and Cretaceous sediments. Prospective source rocks of both ages were found to be present over most of the central Eromanga Basin region. In contrast to the underlying Cooper, Galilee and Adavale Basins, the source rocks in the Eromanga Basin appear to be largely oil prone, characterised by a predominence of type I and II kerogens in the prime source sequences. The Jurassic formations in particular appear to be most prospective with source potentials ranging from fair to very good. Figure 1;

Cross section, central Eromanga Basin

BASIN

EROMANGA

Sea

SURAT BASIN

leve/"^

{';\ L Cretoceous [y^] E CretaCiOui

n/o/9 Jurassic Permian/Triossic

Most of the basin's subsidence and sediment deposition occurred in the mid-Cretaceous, corresponding to a period of active fault movement; areas of high organic maturation generally coincide with the thickest intervals of Late Cretaceous sediments. This pattern is complicated, however, by a lateral change in geothermal gradients of up to 20®C across the region. Vitrinite reflectance and headspace gas analysis data indicate that within the basin a minimum depth of burial of around 1200 m is required for source rocks to reach the main threshold of oil generation. The relatively thin sedimentary pile and low geothermal gradients east of the Canaway Fault suggest source rocks in this region are unlikely to have reached the main oil generative zone or 'oil window'. Generation and migration of hydrocarbons in the area west of the Canaway Fault probably occurred no earlier than the Late Cretaceous or Tertiary.

References SENIOR, B.R. & HABERMEHL, M.A., 1980: Structure, hydrodynamics and hydrocarbon potential of the central Eromanga Basin, Queensland, Australia. BMR J. Aust. Geol. Geophys., ^7-56. TISSOT, B.P. & WELTE, D.H., 1978: Petroleum formation and occurrence - a new approach to oil and gas exploration. Springer-Verlag, Berlin.

MIGRATION AND ENTRAPMENT

SOME ASPECTS OF THE THEORY OF HYDRODYNAMIC ENTRAPMENT OF OIL AND GAS

J . W . Holmes

School of Earth Sciences, Flinders U n i v e r s i t y , Sturt R o a d , BEDFORD P A R K , S . A . 5042

1 , The dependence of the capillary pressures upon the pore size distTTbutions of the oil-bearing beds; Hubbert (1969, 1953) developed an e x p r e s s i o n , from first principles, to represent the pressure change (p^) from the oil to the water side of the curved oil/water interface. It is Pe

^ ^ ^^^^ ,

(1)

where R is a length characteristic of the grain sizes of the sedimentary bed, a is the interfacial tension (surface tension). Bis the characteristic angle made by the water/oil interface where it contacts the m i n e r a l grain surfaces and c is a c o n s t a n t . This constant has to convert the grain size to a representative and characteristic pore size. It is found that R/r = c = 8 , where r is a size of pores associated with grains of size R . In host rocks composed of sandstone with R = 10"^ m , the capillary pressure could not exceed 2kP^ if there were some pore spaces as large as that i n d i c a t e d . H e r e , following H u b b e r t , I have used a cos 6 = 0.25 Nm"^. By c o n t r a s t , in a contiguous fine-grained b e d , the grain size might be represented by R = 10"^ m . Then the capillary pressure would be 200 kP^, This capillary pressure can then be visualised as a pressure experienced by the oil-substance, if it were present in the fine-grained r o c k , tending to make it move towards the s a n d s t o n e . Conversely, it is the pressure required to be exerted on the oil in the sandstone to make it enter the m u d - s t o n e , which would have a porosity of 0 . 3 , entirely w a t e r - f i l l e d , yet be an excellent cap-rock. 2 . Conditions for an equipotential volume of the oil p o o l . Hubbert also drew attention to the force vectors that describe the actual flow of the oil and the ambient water in the reservoir r o c k . In g e n e r a l , the force on the ambient water is given by

i ^ dx

j ^ dy

(2)

where (})w is the total potential of the ambient w a t e r . For an isotropic a q u i f e r , the streamlines, parallel to F , are at directions normal to the

77 isopotential surfaces. Similarly for the oil, but the actual direction of its force vector, Fl, Is usually nearly at right angles to Fy. The condition where F l is exactly at right angles to F^, depicted in Figure 1, defines an oil pool at rest, i.e., it is in a trap.

OIL

POOL

Figure 1: The disposition of the oil pool, the critical angle of tilt, ( ac) the water streamlines and its force vector (F^) and the oil equipotential lines and its force vector (Fl). The potential of the oil in its trap Is (f) * l , which is, of course, constant for all the volume of the oil pool. The water streamlines and the oil equipotential lines in the oil/water zone beneath the oilpool equipotential volume are parallel. The critical angle of tile, ac» is given by

tan a^

P,w V^L

d<i>

w dx

(3)

where p and pT are the densities of the ambient water and the oil, respectively and d(()^/dx is the horizontal component of the ambient water's hydraulic gradient at the location of the trap in the aquifer. This angle (a^) to be interpreted as the angle, with respect to horizontal, made by the plane interface between the oil and the ambient water. 3. The nature of the oil/water interface. It should be of some practical importance to explore the nature of the oil/water interface, as influenced by the pore-size distribution of the host rock. Figure 2 shows a plot of the water/air (gas) volume fractions that could be established by standard soil physics techniques for a characteristic sandstone. Also shown plotted is the line for water/oil volumetric fractions derived from the former curve. These lines are to be interpreted as the water volumetric fractions of the sandstone as a

function of height above the water saturation datum for « 0. For example, for the water/oil system there would be a vertical zone 4 m thick over which the water content would decline from 0.250 m^ m-3 (total porosity) at saturation to about 0.035 m^ m""^ and the oil content would Increase from zero to 0.215 m^

Water content

(m^m"^)

Figure 2: The water content of a sandstone reservoir for a water/oil and a water/air (gas) system In the vertical zone of contact above the water saturation datam, c})^ = 0. Reference HUBBERT, M.K., (1969): The theory of groundwater motion and related papers. Hafner Publishing Company.

79 ASPECTS OF REGIONAL GROUNDWATER MOVEMENT AND HYDROCARBON MIGRATION IN THE EROMANGA BASIN

M.A. Habermehl

Division of Continental Geology, Bureau of Mineral Resources, Geology and Geophysics, P.O. Box 378, CANBERRA CITY, A.C.T. 2601

The Eromanga Basin is a constituent sedimentary basin of the hydrogeological Great Artesian Basin (Habermehl, 1980). Confined aquifers occur in continental quartzose sandstones of Jurassic and Cretaceous age in the Eromanga Basin. Intervening confining beds consist of siltstone and mudstone; a thick argillaceous sequence of sediments of marine origin and Cretaceous age forms the main confining unit. The basin is, in places, 3000 m thick, and forms a large synclinal structure, uplifted and exposed along its eastern marginal zone, and minor recharge takes place in the western margin. Natural discharge occurs from springs in the southwestern, western and southern margins; most springs are associated with structural features. Large-scale groundwater movement, interpreted from potentiometric maps which were prepared from long term and periodic measurements on large numbers of flowing artesian waterwells in the basin, is generally directed to the west, southwest and south in the main part of the basin. In the western part regional groundwater movement is towards the southeast and south; in the most northern part a northerly flow direction exists. Epeirogenic uplift of the eastern recharge areas and lowering of the southwestern area during Late Cretaceous and Tertiary times established the present flow regime. The hydraulic gradient steepened slightly in the Late Quaternary as the spring levels in the southwestern margin were lowered. Groundwater development by flowing artesian waterwells since about 1880 caused significant changes of the potentiometric surface (Habermehl, 1980). The artesian water is of meteoric origin, as shown by environmental isotope analysis. Groundwater movement is slow, and residence times of the water are large, but groundwater salinities are low, generally between 500 and 1500 mg/1 total dissolved solids, and increase only slightly along the flowpaths. Hydrochemical differences characterise different aquifers and regional groundwater flow patterns, particularly in the southwestern part, where groundwater derived from the eastern recharge area is characterised by Na-HC03-Cl and water from the western recharge area by Na-Cl-S04. Waters in the central part of the basin are mainly of the Na-HC03-Cl and Na-HC03-Cl-S04 type. Hydrocarbons have been generated from relatively abundant source rocks in the marginally mature to mature Jurassic and Cretaceous sedimentary sequence since Late Cretaceous time, and their migration could be influenced by groundwater flow. A model of hydrodynamic migration of

hydrocarbons, and their possible accumulation and entrapment in suitable structural zones of near barriers of stratigraphic or diagenetic origin was proposed by Senior & Habermehl (1980). Gas samples were taken at the surface from about 50 long (several decades) established, flowing artesian waterwells (which were 'randomly' drilled in contrast to petroleum exploration wells) in the central part of the Eromanga Basin in 1980. Methane concentrations from the upper, main Lower Cretaceous-Jurassic aquifer (Cadna-owie Formation/Hooray Sandstone) range up to about 100,000 microlitres per litre, and the sum of the hyrocarbons ethane to heptane range up to about 2250 microlitres per litre. Several samples contain relative high values for the hydrocarbons ethane to heptane, which suggests that the groundwater acts as a migration agent for hydrocarbons derived from the adjoining source rocks or from existing reservoirs.

References HABERMEHL, M.A., 1980: The Great Artesian Basin, Australia. Geol. Geophys., 9-38.

BMR J. Aust.

SENIOR, B.R. & HABERMEHL, M.A., 1980: Structure, hydrodynamics and hydrocarbon potential, central Eromanga Basin, Queensland, Australia. BMR. J. Aust. Geol. Geophys., 5, 47-55.

HYDROCARBON FLUSHING IN THE EROMANGA BASIN -

FACT OR FALLACY

K.C. Moriarty and A.F. Williams

SANTOS Limited, 39 Grenfell Street, ADELAIDE, S.A. 5000

Application of Hubbert's equation to the hydrodynamic regime of the Eromanga Basin yields angles of tilt of the oil-water interface of about 0.1^. Since mapped closures on oil fields such as Jackson, Merrimelia, Dullingari and Strzelecki are at least or more it is evident that the existing hydrodynamic regime cannot flush oil from these or any other structures with similar closures. Review of the history of the Eromanga Basin indicates very early development of the flow regime as we see it today with its origin possibly as far back as the Early Cretaceous. However, palaeo-gradients would have been substantially lower than today's, especially during and prior to the accepted oil migration phase (Mid-Late Cretaceous to Early Tertiary; Poll, 1981) and structure with dips of no more than 0.03® would be effective traps. Uplift along the eastern margin in the Tertiary raised the intake areas and increased gradients to levels at or about those we see today. Tectonism in the central basin accompanied this uplift such that as gradients increased so did structural closure, maintaining ability to trap oil. It is therefore implied that, providing some structural closure was present before the oil migration phase, hydrodynamic flushing of such traps by both past and present groundwater flow regimes in the Eromanga Basin was impossible. This, together with the fact that large numbers of stratigraphic traps exist within the reservoirs indicates that long distance migration of petroleum within the Eromanga Basin is highly improbable. Although this phenomenon has been used to explain occurrences of residual hydrocarbons in mound-spring sediments at the outlet of the basins (Bowering, 1982), such occurrences can be explained by solution transport. The source potential of the basin has been estimated to be high and the growing number of discoveries support this. It is concluded that most of the hydrocarbons generated are still trapped within the basin and the problem is now to find traps which existed during the migration phase or earlier.

References BOWERING, O.J.W., 1982: Hydrodynamics and hydrocarbon migration - a model for the Eromanga Basin. APEA J., 21(1), 227-36. HUBBERT, M.K., 1953: Entrapment of petroleum under hydrodynamic conditions. Am. Assoc. Pet. Geol, Bull., 37, 1954-2026. POLL, J.J.K., 1981: The significance of the southwest Eromanga Basin oil and gas discoveries. APEA J., 21(2), 33-8.

LANDSAT INTERPRETED STRUCTURE AND GROUNDWATER FLOW WITHIN THE EROMANGA BASIN

B.R. Senior

Hunting Geology and Geophysics (Australia) Pty. Limited, P.O. Box 365, FYSHWICK, A.C.T. 2609

Introduction The effectiveness of Landsat Interpretation of structure In the Eromanga Basin sequence was recently demonstrated by Senior & Habermehl (1980). They observed a close association between some linear features and selsmlcally determined faults. In places llnears could be used to 'extend' the strike lengths of known faults. This technique was later used by Plnchln & Senior (1982) to connect faults between widely spaced seismic lines, and formed a basis for planning the location of BMR seismic traverses In the central portion of the Eromanga Basin. With the advent of good quality Landsat Imagery covering the entire Eromanga Basin the opportunity existed for a basln-wlde assessment of structure. The Ideas generated by Senior & Habermehl (1980) were applied by HGG(A) on a regional scale, and results Integrated with structure derived from available geological and seismic Investigations.

Interpretation Procedure Approximately 90 false-colour Landsat dlaposltlves were purchased from the Australian Landsat Station, Belconnen, A.C.T. Cloud-free winter scenes with low sun angles were chosen wherever possible, as these highlight structural features In the flat landscapes which typify the physiography of the Eromanga Basin region. These scenes were re-photographed to 35 mm format and enlarged to 1:250,000 scale using projection equipment developed by HGG(A). The basic data to the Eromanga Basin Landsat study are linear and curvilinear features. In addition there are broad linear zones which were too diffuse to be annotated with a single line or series of sub-parallel lines and are depicted with a stipple (Fig. 1). Frequently these zones are coincident with alluvial belt s, or with zones of differing soil or vegetation cover. The continuity of these zones indicate their probable structural origin, and in places they are coincident with or grade laterally into definite fault systems. Eroslonal landforms and associated drainage patterns in the Eromanga Basin are carved from very widespread duricrusts, and deposltlonal landforms are frequently coincident with syncllnes. According to Idnurm St Senior (1978), weathering of the Eromanga Basin sequence began in the Late Cretaceous and continued until the terminal Palaeocene.

Following a phase of gentle structural rejuventation and fluvial deposition along synclinal downwarps, widespread siliceous duricrusts developed during the middle Tertiary. Both weathered crusts have responded to subsequent gentle warping by brittle fracture, with preservation of dip slopes on gently tilted cuestiform landforms. Dips are usually small (less than 5°) but quite strong deformation occurs adjacent to fault-bounded limbs of some folds. For example, the west limb of the Kihee Anticline has 12° to 18° dips preserved in indurated weathered rocks (Fig. 1). Geomorphic criteria of Senior (1977) were used to identify landforms indicative of folds throughout the region and are depicted by a stipple in Figure 1. In some instances smaller areas of probable structural closure, within large upwarps, can be identified. In favourable areas the Landsat data enable the identification of quite small anticlines, the lower limit being structures with an axial length of about 5 km.

Conclusions This integrated Landsat study has led to the probable Identification of numerous previously unknown folds and faults in the Eromanga Basin, and has linked other known geological features with basin-wide structural domains. Many of the linear features identified are probably the subtle surface expression of much stronger faults and folds within underlying basins and 'basement'. Very numerous linear features lack supporting geological or geophysical data to be rated as faults. Many of these lie sub-parallel to fault or fold trends, and thus enhance the structural 'grain' of the region. Many of these linear features may in fact be minor faults or fractures, which have displacements which are too small to be resolved in most of the existing seismic data. The relationship between faults and groundwater flow are considered important in entrapment of hydrocarbons within these basins. The fault-barrier model proposed by Senior & Habermehl (1980) indicates that displacements as little as 15 m, if at right angles to groundwater flow, could lead to zones of groundwater stagnation and possible associated hydrocarbons. Folds with four-way closure or stratigraphic barriers may also obstruct groundwater flow, particulary if orientated normally to the prevailing flow direction. Overview maps at one million scale, a portion of which is reproduced in Figure 1, illustrate the range of structural conditions which may influence migration and entrapment of hydrocarbons. The oil discoveries in Jurassic and Lower Cretaceous rocks of the Jackson Anticline in southwest Queensland appear to support the underlying philosophy behind this Landsat study. This anticline has strong intersecting north and northwest-trending axes, favourable groundwater flow directions and high geothermal gradients. A marked mid-Cainozoic faulting and folding episode is indicated by displaced weathered profiles, and is an additional attractive feature enhancing the prospectivity of this structure. A glance at the Landsat study shows analogous areas throughout these basins which appear to have prerequisite structural, hydrodynamic and geothermal parameters conducive to hydrocarbon generation migration and entrapment.

I42®30'

-ZT-CX)'

28*00' Londsat

Inltrprtted llntar

^ ^

ftaturi

Groundwater flow

direction

Foult

Structurally

Anticline

Groundwater stagnation

zone

Petroleum

well

Water

Syncllne Strike

ond

dip

FIGURE GENERALISED EROMANGA

upwarped

exploration

bore

PORTION

BASIN LANDSAT STUDY

area

85 References IDNURM, M . & SENIOR, B.R., 1978: Palaeomagnetic ages of Late Cretaceous and Tertiary weathered profiles in the Eromanga Basin, Queensland'. Palaeogeogr> Palaeclimatol> Palaeoecol., 24, 263-77. PINCHIN, J . & SENIOR, B.R., 1982: The Warrabln Trough, western Adavale Basin. Geol. Soc. Aust. J., 29, in press. SENIOR, B.R., 1977: Landform development, weathered profiles and Cainozoic tectonics in southwest Queensland. Ph.D. Thesis, University of N.S.W., Sydney, (unpubl.). SENIOR, B.R. & HABERMEHL, M.A., 1980: Structure, hydrodynamics and hydrocarbon potential, central Eromanga Basin, Queensland, Australia. BMR J . Aust. Geol. Geophys., 5, 47-55.

HYDROCARBON FIELDS

THE MERRIMELIA OIL AND GAS FIELD

O.J.W. Bowering^ and D.M. Harrison^

^Delhi Petroleum Pty. Ltd., 33 King William Street, ADELAIDE, S.A. 5000 ^Alliance Oil Development Aust. N.L., 30 Collins Street, MELBOURNE, VIC. 3000

The Merrlmella 1 well was drilled as an exploratory well for Permian gas, following the discovery of a commercial gas field in the nearby Gidgealpa structure. It was designed as a stratigraphic test well on the flank of a large northeast-trending anticlinal structure which seismic data indicated to be bald of Permian sediments at the crest. The well penetrated "Gidgealpa Group" sediments and recorded two small uneconomic gas flows from sands within what is now known as the Patchawarra Formation. Four subsequent wells were drilled for Permian gas, and of these, only Merrimelia 5 was completed as a potential gas producer. Merrimelia 2 recorded the first positive indications of hydrocarbons from the Triassic in the Cooper Basin. Later discoveries of oil and gas in several fields in sediments of the Eromanga Basin sequence led to the upgrading of the prospectivity of Jurassic and Early Cretaceous sandstones. Merrimelia 6 was drilled at the crest of the Merrimelia structure as a structural test for hydrocarbons in the Eromanga sequence, and recorded commercial flows of oil from the Namur and Button sandstones and gas from the Nappamerri Formation. Merrimelia 7, located downdip from Merrimelia 6, recorded commercial flows of both oil and gas from the Nappamerri Formation. The Merrimelia field thus became the first within the Cooper and Eromanga Basins to be hydrocarbon productive from Jurassic, Triassic and Permian reservoirs. Merrimelia 8 which produced substantial flows of oil and gas from three separate Mesozolc reservoirs provides further evidence of the great potential of the Merrimelia field. Like other oil fields of the Eromanga Basin, Merrimelia is a structural accumulation with some apparent stratigraphic control on trapping.

THE BIRKHEAD FORMATION: A JURASSIC PETROLEUM RESERVOIR

I.M. Paton

SANTOS Limited, 39 Grenfell Street, ADELAIDE, S.A. 5000

The Birkhead Formation is a fluvial, lacustrine and coal-swamp deposit forming the lower part of the Middle Jurassic (JA to J6 age) Injune Creek Group which is flanked conformably above and below by higher energy braided-fluvial stream clastics. Overlying the Cooper Basin region the Birkhead Formation ranges up to 400 feet in thickness. The formation occurs over a significant portion of the Eromanga Basin although it is notably absent west of the Birdsville Track Ridge in South Australia. Previously the Birkhead Formation has been of principal interest to oil explorers because of its mappable acoustic impedance character, seal to underlying Hutton Sandstone oil reservoirs and fair to excellent source rock characteristics. Recent major oil recoveries and flows in the Moorari, Strzelecki, Big Lake and Merrimelia Field s have shown the Birkhead Formation to be ranked as one of the important oil reservoir targets in the Eromanga Basin sequence. Although all these discoveries to date have been defined on structural highs the local stratigraphic development and distribution of the reservoirs will play a dominant role in their field development. The Birkhead Formation overlying the South Australian Sector of the Cooper Basin can be divided into three major units: Unit A, the uppermost unit, is dominantly sequence of generally limited development;

fluvial

sand/shale

Unit B, the middle unit, is principally a lacustrine-coal swamp sequence of finely laminated siltstones, shales, fine grained sandstones and coals; and Unit C, the lowermost unit, is a fluvial sequence of shoestring to limited stacked-channel sand development. It is this unit that generally has the best reservoir development in the Birkhead Formation. Reservoir qualities of the fluvial channel sands in the Birkhead Formation vary from poor to fair with the varying amounts of argillaceous matrix and siliceous overgrowth present. The shoestring, low energy, channel sand nature and generally poor to fair reservoir qualities of the sands makes exploration for Birkhead structural/stratigraphic traps in their own right of high risk. However, in combination with the development of other Eromanga Basin reservoirs and/or Permian targets, the Birkhead Formation will remain an important secondary reservoir target.

BIRKHEAD

FORMATION PROFILE SONIC

te^a

BRAIDED STREAM ENVIRONMENT

BIRKHEAD ^ F O R M A T I O N UNIT A FLUVIAL ENVIRONMENT UNIT B COAL SWAMP TO LACUSTRINE ENVIRONMENT

UNIT C FLUVIAL ENVIRONMENT

HUTTON

SANDSTONE BRAIDED STREAM > ENVIRONMENT

MAJOR H Y D R O C A R B O N DISCOVERIES BIRKHEAD FORMATION WELL

YEAR

WACKETT 1

1978

NAPPACOONGEE STRZELECKI

1979 1979

MOORARI

1980

BIG LAKE

1982

MERRIMELIA

1982

DST ^

RESULTS

D.S.T. 3 ( 5 4 5 8 - 5 5 5 8 ) Q 2.2MMCFD. at 5 3 0 P S I . 7 / 1 6 " TC. Rcc. 5 0 0 ' GCM. • D.S.T. 5 ( 5 3 5 2 ' - 5373 ) GTS in 12 mins. at 6PSI Rcc. 2 0 0 ' Oil, 3 0 0 0 ' GCW. • D.S.T 2 ( 5 4 2 4 ' - 5 5 0 7 ) Rcc. 2 1 0 ' O . 3 1 0 ' GCOM. • D.S.T 2 ( 7 0 5 0 ' - 7112 ) G T S at RTSTM R c c . 62-3 bbls Oil. 17 bbls Mud. • D.S.T 1 ( 6 3 9 6 ' - 6 4 5 4 ) GTS at RTSTM. Q 150 BOPD. • D.S.T 5 ( 6 1 1 1 ' - 6 1 8 2 ) GTS at RTSTM R c c . 15 bbls Oil, 5 1 ' A P L 1050' GCO.

References

McINTYRE, S., 1980: The hydrocarbon potential of the Jurassic and Lover Cretaceous sediments from five wells in the Eromanga Basin, South Australia^ B.Sc. (Hons) thesis, Univ. Adelaide (unpubl*). SENIOR, B.R., MOND, A., & HARRISON, P.L., 1978: Geology of the Eromanga Basin. Aust., Bur. Miner. Resour., Geol. Geophys., Bull, 167. WILTSHIRE, M.J., 1979: Eromanga Basin, Australia petroleum prospects. Wiltshire Geological Services, (unpubl.). Aust. Miner. Foundation, Adelaide.

GEOLOGY OF THE DULLINGARI MURTA OILFIELD

T.J. Mount

Delhi Petroleum Pty. Ltd., 33 King William Street, ADELAIDE, S.A. 5000

Oil was first discovered in the Murta Member of the Mooga Formation in the Dullingari North 1 well, 70 km east of Moomba, in August, 1979. Early geological models for the field (Mount, 1981) were limited by a lack of data, but there are now over 40 wells in the area which make possible a detailed analysi s of the accumulation. The principal Murta reservoirs are lake-beach or bar deposits; thin but highly permeable sandstones that are explained by persistence of low-energy winnowing of sediment at stillstands of a regressive lake margin. They are best developed in a narrow belt adjacent to a prominent structural lineament that cuts NW-SE across the Dullingari Dome and which is interpreted as the trace of an Early Cretaceous growth—fault. Slumping of the fault block was to the northeast, into the deeper lake. In contrast to the traditional 'uplift-enhances-winnowing' model it is here suggested that gentle subsidence of the slump block was responsible for a reduced rate of regression of the lake shoreline across the dome, creating the stillstand. A feature of the Murta sediments is a series of upward-coarsening arenaceous cycles that were originally considered to be possible delta-distributary lobes. These are now explained as sub-lacustrine turbidite deposit s. Comparisons are with Gilbert—type deltas where sediment-laden river water, being more dense than the lake water, advances across the lake floor as turbid underflows or density currents, possibly accompanied by grain-flow. A palaeogeographic reconstruction is offered that shows a series overlapping clastic lobes that were built out into the Murta Lake.

The ubiquitous interlamination of the Murta f a d e s resembles coarse varves and reflects an interplay between autochthonous lacustrine sedimentation and episodic clastic deposition (turbidites), mediated by pulses in lake level and regional epeirogenic events, but overprinted by local tectonics; such as at Dullingari. Source-rock studies are quoted that also emphasise the fine lamination of the Murta sediments; light-coloured silty and arenaceous turbidites contain terrestrially derived DOM, such as abbraded intertinite of allochthonous origins, while the intervening dark carbonaceous siltstones are rich in exinite of lacustrine-algal, authochthonous, origins. Furthermore, the dark beds feature horizontal feeding burrows (presumably annelids or athropods) that are often infilled with light silt, while the light layers have vertical burrows, infilled with black muds, and interpreted as escape burrows, providing further support for episodic deposition.

93 Vitrlnite reflectance data rate the Murta sediments at Dullingarl as 'marginally-mature' for oil generation which probably occurred In the mid to late Tertiary, possibly Into the Holocene. The alglnltes are an obvious and local source for the oil with very short migration pathways from the silts to the Intercalated sandstone reservoirs. Petrography on the sandstones points to the Importance of dlagnetlc and secondary solution processed to the development and modification of reservoir properties, especially porosity. The Murta reservoirs not only have structural and stratlgraphlc (plnchout) limits, but they are the first to be described In the Cooper sector that may also have a dlagenetlc component (carbonate/slllca cements) to the trapping mechanism. It Is a principal conclusion of the studies of the Murta Member at Dullingarl that attention to detail Is now essential to define meaningful trends In oilfields of the Eromanga Basin. The old 'layer-cake' concepts of the Eromanga stratigraphy can no longer apply and should be replaced by multlfaceted models where success will be measured by emphasis on the most subtle details of f a d e s , carefully measured against accurate time frames and Integrated with the finest scales of tectonic analysis.

References MOUNT, T.J., 1981: Dullingarl North 1, an oil discovery In the Murta Member of the Eromanga Basin. APEA J., 21(1), 71-7.

GAWLER

BLOCK

PALi£OGEOGRAPHY and SECTION DULLINGARI AREA in the EARLY

CRETACEOUS

NEOCOMIAN

VIEWED

FROM

ACROSS

THE

AREA

DULLINGARI NOW

FIELD

OCXMJPIED B Y

LOOKING

LAKE

AND

WEST,

E Y R E , TO T H E GAWLER

SOUTHWEST

BLOCK

FIG-5 IF

POSTER DISPLAYS

MESOZOIC STRATIGRAPHY - SOUTHWESTERN MARGIN OF THE EROMANGA BASIN G.J. Ambrose^ and R.B. Fllnt^ ^SANTOS Limited, 39 Grenfell Street, ADELAIDE, S.A. 5000 -Geological Survey of South Australia, 191 Greenhill Road, PARKSIDE, S.A. 5061.

On the southwestern margin of the Eromanga Basin (in the Oodnadatta"Billa Kalina" region) the Jurassic-Cretaceous sequence consists of the basal Algebuckina Sandstone (Mooga Formation-Namur Sandstone Member equivalent), Cadna-owie Formation ('Transition Beds' equivalent). Bulldog Shale, Coorikiana Sandstone, Oodnadatta Formation including the Wooldridge Limestone Member, and Winton Formation. This region includes the type sections for most of these units. The late Jurassic-?Early Cretaceous Algebuckina Sandstone consists mainly of fluviatile, fine to medium-grained sandstones and conglomeratic sandstones capped locally by ?lake shoreline or bar deposits. Wopfner et al. (1970) proposed a regional disconformity between this sequence and the overlying Cadna-owie Formation. In the field the contact is often difficult to interpret although minor erosion is observed in a few localities. However, the following data, acquired during recent mapping programmes, lend support to the concept of a weathering event separating the two units on the southwestern basin margin. Firstly, the Algebuckina Sandstone is invariably kaolinised and the degree of weathering often decreases vertically. This in situ weathering profile (cf. Wopfner et al., 1970) is occasionally capped by a silcrete horizon which is well exposed along the margins of the Peake and Denison Ranges. Secondly, acid porphyry clasts are locally abundant in the Algebuckina Sandstone and are far more weathered than those observed in the Cadna-owie Formation. It is unlikely that these highly weathered clasts could have survived prolonged transport from the Gawler Craton source area and hence at least some degree of in situ weathering is implied. The Cadna-owie Formation consists of mainly marginal-marine fine sandstones and siltstones which are commonly ferruginous in outcrop. The Mt. Anna Sandstone Member is a fan-delta facies comprising coarse-grained conglomeratic sandstones characterised by abundant clasts of acid volcanics. This unit, which forms a wedge of sediment abutting the Gawler Craton, dominates the sequence in the Billa Kalina region (Ambrose & Flint, 1981). Deposition of marine shales (Bulldog Shale) dominated in the Aptian-Albian (Ludbrook, 1966). Basal bouldery shales, which crop out extensively in the study area, have quartzite clasts containing Devonian fossils not recorded elsewhere in South Australia. Flint et al. (1980) suggested the boulders were transported from the Cobar area by Permian

ice and later reworked Into the Bulldog Shale by debris flows. The regressive Coorikiana Sandstone consists of fine-grained, micaceous, silty sandstones containing abundant burrows and tracks. There follows a succession of shales and siltstones (Oodnadatta Formation) which are mainly marine. The Wooldridge Limestone Member (Toolebuc Formation equivalent) contains abundant ammonites in outcrops north of Oodnadatta. From the late Albian into the Cenomanian, non-marine influences became important in the upper part of the Oodnadatta Formation and this trend continued into the largely non-marine Winton Formation. In outcrop many of the Mesozoic lithologies have been altered by Tertiary weathering; modifying processes include bleaching, kaolinisation, ferruginisation and silicification.

References AMBROSE, G.J. & FLINT, R.B., 1981: Explanatory notes, Billa Kalina 1;250,000 geological map, SH53-7. S. Aust., Geol. Surv. FLINT, R.B., AMBROSE, G.J. & CAMPBELL, K.S.W., 1980: Fossiliferous Lower Devonian boulders in Cretaceous sediments of the Great Australian Basin. R. Soc. S. Aust., Trans., 104, 57-65. LUDBROOK, N.H., 1966: Cretaceous biostratigraphy of the Great Artesian Basin in South Australia. S. Aust., Geol Surv., Bull., 40. WOPFNER, H., FREYTAG, I.B. & HEATH, G.R., 1970: Basal JurassicCretaceous rocks of western Great Artesian Basin, South Australia: stratigraphy and environment. Am. Assoc. Pet. Geol., Bull., 54, 353-416. —

ENVIRONMENTAL ASPECTS OF EXPLORATION IN THE EROMANGA BASIN ARID ZONE

R.C. Buckley AMDEL, P.O. Box 114, EASTWOOD, S.A. 5063

A large proportion of the Eromanga Basin lies under arid dunefields (Buckley, 1981a). The environmental aspects of exploration in such areas are very different from those in wetter and more densely inhabited regions. The main impacts of exploration derive from the extensive track cutting required for geophysical survey and drilling programmes. The immediate effect s of cutting tracks, namely removal of vegetation and increased sand mobility, are localised and relatively insignificant. More important are secondary disturbances associated with the increased access to four-wheel drive vehicles: these include an increase in fire frequency, introduction and spread of weeds, and destruction of native fauna (Buckley, 1982). The impact of fire is not likely to be detrimental as long as the fires are limited in area and occur in winter. Shooting, etc., is confined to areas adjacent to tracks. Hence the main requirement for 'environmental hygiene' in uninhabitated arid dunefields is to monitor and eradicate weeds, as far as this is possible. Besides the main dunefield areas, exploration in the western Eromanga Basin may involve operations on stony or gibber plains, sandplains, ranges and outcrops, salt lakes, and watercourses and floodout areas. Tracks cut in sandplain and gibber areas remain visible for decades. 'Spinifex' hummock grass and mulga or gidyea woodlands over tussock grasses on sandplain carry fire well, and summer wildfires are a potential hazard in areas which have not been burnt recently. Watercourses and flood plains are the most fragile areas in the arid zone. Disruption of surface hydrology can have major impacts on the vegetation. Much of the arid zone fauna is concentrated in such areas, and such modification of the vegetation therefore has a greater impact on the fauna than in sandrldge areas. Watercourses are also particularly susceptible to introduction of weeds which can persist there through droughts which would eradicate them in the sandridges (Buckley, 1981b). Ranges and outcrops also have a relatively rich and easily disturbed fauna: waterholes in particular represent fragile ecosystems and should be avoided. Besides surface disturbances as outlined above, drilling can contaminate potable groundwater if aquitards between sweet and saline aquifers are perforated. Groundwater investigations are frequently a standard part of exploration geology, but particular attention should be paid to sealing exploration drill holes in artesian and subartesian areas.

99 References J. Arid Environ*,

BUCKLEY, R.C., 1981a: 4, 91-101.

Central Australian sandrldges.

BUCKLEY, R.C., 1981b: Soc, 62, 369-79.

Allen plants in central Australia.

Bot. J. Linn.

BUCKLEY, R.C., 1982: Use and conservation of central Australian dunefields. Biol. Conserv., 22, 197-205.

100

LANDSAT INTERPRETED STRUCTURE AND GROUNDWATER FLOW WITHIN THE EROMANGA AND SURAT BASINS

Hunting Geology and Geophysics (Aust.) Pty. Ltd., P.O. Box 365, FYSHWICK, A.C.T. 2609

The methodology and results obtained from the HGG(A) integrated Landsat study of the Eromanga and Surat Basins are presented in this poster display. Approximately 90 false-colour Landsat diapositives provide complete coverage to these basins. Cloud-free winter scenes with low sun angles were selected wherever possible, as these tend to enhance structural features in flat landscapes which are typical of the basins. The Landsat scenes were rephotographed and projected enlargements annotated onto 1:250,000 scale drainage bases. Linear, circular and bedding traces were identified. Additional data incorporated into these 111 'factual' map sheets includes computer-generated groundwater flow-directions within Jurassic and Cretaceous aquifers, and subsurface control points (petroleum exploration wells, wireline-logged water bores and shallow stratigraphic drill holes). Geothermal gradients and Bouguer anomalies were added to complete the 'factual' data, and provide a basis for structural interpretation. Duplicate series of the 'factual* map sheets were prepared, and were used as base maps for the structural interpretation. Linear features were classified as faults wherever supportive geological or geophysical evidence exists. Areas of anomalous drainage or areas having landform characteristics denoting possible structural upwarps were identified. Groundwater flow-direction were modified in detail to conform with the structural interpretation. This study has led to the identification of numerous previously unknown probable folds and faults in the Eromanga and Surat Basins, and has linked other known geological features with basin-wide structural domains. Many of the linear features identified are probably the subtle surface expression of much stronger faults and folds within underlying basins and 'basement*. Very numerous linear features lack supporting geological or geophysical data to be rated as faults. Many of these lie sub-parallel to fault or fold trends, and thus enhance the structural 'grain' of the region. Many of these linear features may in fact be minor faults or fractures, which have displacements which are too small to be resolved in most of the existing seismic data. The relationships between faults and groundwater flow is considered important in entrapment of hydrocarbons within these basins. The fault-barrier model indicates that displacements as little as 15 m, if at right angles to groundwater flow, could lead to zones of groundwater stagnation and possible associated hydrocarbons. Folds with four-way closure or stratigraphic barriers may also obstruct groundwater flow, particularly if oriented normally to the prevailing flow direction. The overview map illustrates the range of structural conditions which may influence migration and entrapment of hydrocarbons.

101 LIST OF SPONSORS

Sponsorship from the following organisations Is gratefully acknowledged.

AAR Limited Australian Aquitalne Petroleum Australian D.S.T. Company Pty. Ltd. Brunswick Resources N.L. Core Laboratories (Aust.) Qld. Ltd. Crusader Resources N.L. Cultus Pacific N.L. Delhi Petroleum Pty. Ltd. Esso Exploration and Production (Aust.) GES Pty. Ltd. Hartogen Exploration Pty. Ltd. Offshore Oil N.L. SANTOS Limited S.A. Dept. Mines and Energy S.A. Oil and Gas Corporation Pty. Ltd. Tenesco Oil and Minerals Total Exploration (Aust.) Pty. Ltd. Vamgas Limited Western Mining Corporation

102

AUTHOR INDEX

AMBROSE, G.J.: ANFILOFF, v.: ARMSTRONG, J.D.:

22, 96 32, 42 5

BARR, T.M.: BOOTHBY, P.O.: BOREHAM, C.J.: BOWERING, O.J.W.: BUCKLEY, R.C. BURGER, D.:

5 17 73 88 98 36

COOK, A.C.:

LOEH, H.:

McKELLAR, J.L.: McKIRDY, D.M. :

34 67

MOORE, P.S.: MORIARTY, K.C.! MOSS, F.J.: MOUNT, T.J.:

11, 27 81 8 92

OZIMIC, S.:

51, 62

PASSMORE, V.L.: PATON, I.M.: PINCHIN, J.: PITT, G.M.: PRICE, P.L.:

73 89 32 27, 70 39

66, 72

DALE, L.: DETTMANN, M.E.:

58 39

EKSTROM, A.:

FILATOFF, J.: FINLAYSON, D.M.! FLINT, R.B.: FORBES, B.G.:

39 43 96 25

GLIKSON, M.: GRAVESTOCK, D.I.:

54 20

79 HABERMEHL, M.A.: 88 HARRISON, D.M.: 76 HOLMES, J.: HUNTING GEOL. & GEOPHYS.: 100 KANTSLER, A.J.: KRIEG, G.W.:

72 28

LAYERING, I.; LiOC K. 1 • •

22 45

RILEY, K.W.: RUMPH, B.:

55 47

SAXBY, J.R.: SCHEIBNEROVA, V.: SENIOR, B.R.: SPENCE, A.G.: SPRIGG, R.C.: SUTTILL, R.:

55, 59 50 82 43 2 22

WAKE-DYSTER, K.D.: WILLIAMS, A.F.: WILTSHIRE, M.J.:

30 81 13, 15

YOUNGS, B.C.:

ZWIGULIS, M.:

103 STRATIGRAPHIC INDEX

ABADARE CONGLOMERATE: ADORI SANDSTONE: ALGEBUCKINA SANDSTONE: ALINERTA GRAVEL: ALLARU MUDSTONE:

34 14, 3 6 , 37 1 1 , 1 4 , 25, 96 28 1 1 , 2 6 , 2 7 , 3 6 , 3 7 , 61,

BIRKHEAD FORMATION: BUCKABIE FORMATION: BULLDOG SHALE: BUNDAMBA GROUP:

1 3 , 14, 36, 37, 6 6 , 6 7 , 8 9 , 8 25, 2 6 , 27, 36, 37, 96, 97 39

CADNA-OWIE FORMATION:

CLEMATIS FORMATION: COOLADDI DOLOMITE: 'COORIKIANA MEMBER': COORIKIANA SANDSTONE: CORDILLO SILCRETE: COREENA MEMBER: CROWN POINT FORMATION:

11, 14, 22, 25, 26 27, 28, 31, 3 6 , 37, 6 7 , 7 0 , 8 0 , 96 39 8 (see COORIKIANA SANDSTONE) 4 , 25, 26, 27, 37, 96, 97 28 36, 37, 6 1 , 6 2 , 63 12

DALHOUSIE BEDS: DONCASTER MEMBER:

36,

EUROMBAH FORMATION: EVERGREEN FORMATION: EYRE FORMATION:

35, 37 1 3 , 1 4 , 3 4 , 3 6 , 37 28

GRIMAN CREEK FORMATION: GUBBERAMUNDA SANDSTONE!

37 1 4 , 37

HELIDON SANDSTONE: HOORAY SANDSTONE: BUTTON SANDSTONE:

34 14, 36, 3 7 , 80 3, 13, 1 4 , 2 0 , 3 6 , 3 7 , 67, 7 2 , 8 8 , 90

INJUNE CREEK GROUP: IPSWICH COAL MEASURES:

20, 39

KINGULL MEMBER:

LEIGH CREEK COAL MEASURES:

MACKUNDA FORMATION: 1 1 , 2 5 , 2 6 , 2 7 , 3 6 , 37 'MARREE FORMATION': (see MARREE SUBGROUP) MARREE SUBGROUP: 2 5 , 2 6 , 2 7 , 28 MINMI MEMBER: 37 'MOOGA SANDSTONE': (see MOOGA F O R M A T I O N ) MOOGA FORMATION: 1 4 , 2 2 , 35, 37, 9 2 , 96 M O O L A Y E M B E R FORMATION: 39 MOUNT ALEXANDER SANDSTONE MEMBER: 25

38,

104 MOUNT ANNA SANDSTONE MEMBER: MOUNT WILLOUGHBY LIMESTONE: MURTA MEMBER:

25. 96 28 3. 22, 23, 24,

NAMUR SANDSTONE MEMBER: NAPPAMERRI FORMATION: NULLAWURT SANDSTONE MEMBER:

22, 24, 67, 88, 11. 17. 39, 88 37

OODNADATTA FORMATION: ORALLO FORMATION:

25, 26, 27, 36, 14, 35, 37

PATCHAWARRA FORMATION: PEERA PEERA FORMATION: POOLOWANNA BEDS: PRECIPE SANDSTONE: PURNI FORMATION:

88 11. 12, 17 11, 12, 67 14, 34, 37 12

RACEVIEW FORMATION: REWAN FORMATION:

34 39

SHOWGROUNDS SANDSTONE: SPRINGBOK SANDSTONE:

39 14, 37

TOOLACHEE FORMATION: TOOLEBUC FORMATION:

17, 18, 66 19, 12, 26, 27, 50, 51, 52, 54, 59, 60, 61, 62. (see CADNA-OWIE 25

'TRANSITION BEDS': TRINITY WELL SANDSTONE MEMBER: WALKANDI FORMATION: WALLOON COAL MEASURES: WALLUMBILLA FORMATION: WESTBOURNE FORMATION: WINTON FORMATION: WOOLDRIDGE LIMESTONE MEMBER: WYANDRA SANDSTONE MEMBER:

11. 12 14. 35, 37, 66 11. 26, 27, 67 14, 36, 37, 38, 67 11. 25, 26, 27, 28. 66, 72, 96, 97 25, 37, 51, 52, 63. 31, 36, 37

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