The Eromanga Basin is a Early Jurassic - Late Cretaceous basin.
The Eromanga Basin encloses the multi-aquifer system of the Great Artesian Basin and overlies late Palaeozoic and older basins. It consists of a broad downwarp with two main depocentres — the Poolowanna Trough and the Cooper region.
|Age||Early Jurassic - Late Cretaceous|
|Area in South Australia||360 000 km2 (139 000 sq miles)|
|Exploration Well Density||1 well per 436 km2 (1 well per 168 sq. miles)|
|Depth to target zone||1200-3000m|
|Thickness||Up to 3000m|
|Hydrocarbon shows||Commercial discoveries of oil from almost every unit from the Poolowanna to the base Cadna-owie Formation in the Cooper region; elsewhere shows in the Poolowanna Formation.|
|First commercial discovery||1976 gas (Namur 1) 1978 oil (Strzelecki 3)|
|Identified reserves||Cooper region only, elsewhere - nil.|
|Undiscovered resources (50%)||2.4 x 106 kL (15.1 mmbbl); Western Eromanga Basin|
8.4 x 106 kL (52.8 mmbbl) ERD estimate June 2006
|Production||Cooper region only - refer to Cooper Basin chapter; elsewhere nil.|
|Depositional setting||Productive non-marine sequence overlain by non-productive marine, marginal marine, and non-marine sediments.|
|Reservoirs||Braided and meandering fluvial, shoreface and lacustrine turbidite sandstones.|
|Regional structure||Broad, four-way dip closed anticlinical trends in regional sag basin.|
|Seals||Lacustrine - floodplain shales and basin-wide volcanogenic sandstones.|
|Source rocks||Underlying Cooper Basin and siltstone; Birkhead and Murta formations' siltstone and coal.|
|Number of wells||2351 (381 development/appraisal exclusively Eromanga targets) mostly in Cooper region|
Seismic line km
|105408 2D; 17421 3D km2 (115023 km)|
Structurally, the Eromanga Basin is divided into two by the NE-trending Birdsville Track Ridge, a complex of related domes and ridges. The Poolowanna Trough in the NW contains a thick sand-dominated sequence in comparison to the Cooper region where intercalated shale and siltstone units occur.
Since 1959 over 2000 wells have penetrated the Eromanga Basin sequence and over 100 000 km of seismic has been acquired. Exploration has concentrated in the Cooper region. A new phase of exploration for oil in the Eromanga Basin commenced in 2002 in the 27 new licences resulting from the expiry of PELs 5 and 6 in 1999. Most new entrant explorers are currently targeting Eromanga Basin oil plays.
The upper non-marine sequence (Winton Formation) was rapidly deposited — up to 1100 m over ~8 million years. A period of erosion in the Late Cretaceous, caused by a switch in drainage from the Cooper region to the Ceduna Depocentre on the rifted southern margin, was followed by deposition of the non-marine Cainozoic Lake Eyre Basin.
In the Poolowanna Trough, principal reservoirs occur in the Poolowanna Formation (variable reservoir quality) and Algebuckina Sandstone.
In the Cooper region, where the Nappamerri Group regional seal is thin or absent, oil and gas pools are found in coaxial Permian–Mesozoic structures (in some cases from the Patchawarra to Murta formations).
Trapping mechanisms within the Eromanga Basin are dominantly structural (anticlines with four-way dip closure or drapes over pre-existing highs) with a stratigraphic component (e.g. Hutton–Birkhead transition, Poolowanna, McKinlay Member and Murta Formation). Eromanga structures in South Australia are rarely filled to spill with oil — net oil columns are relatively thin compared to the height under closure (due to poor sealing characteristics).
Cooper Basin region of South Australia
The USGS (schenk et al 2016) has recently estimated the undiscovered conventional oil and gas potential of the Eromanga Basin in this area of South Australia. Table 1 summarises the results of this assessment. Further potential has also been assessed for the underlying Cooper Basin.
Table 1 Undiscovered conventional oil and gas resources of the Cooper Basin region, Eromanga Basin, South Australia (USGS, May, 2016)
Western Eromanga Basin
Areas to the west have had minimal exploration effort, with only one sub-economic discovery. Oil found here would probably be sourced from the Poolowanna and Birkhead formations or the underlying Pedirka and Simpson basins. Estimates of undiscovered resources in the western Eromanga are best carried out by a method that uses available geological data and Monte Carlo type statistical techniques to calculate, as a probability distribution, the undiscovered resources for each play.
Maps for each play, taking into account distribution of source, seal and reservoir were constructed. Figure 7 summarise these for the Poolowanna Formation, Hutton Sandstone and Namur–Algebuckina sandstones plays respectively.
Table 1 summarises the results of the assessment of the undiscovered resources of the western Eromanga Basin at various probability levels.
Table 2: Undiscovered recoverable oil resources of the western Eromanga Basin
UNDISCOVERED POTENTIAL 106 kL (mmbbl)
Probability that the ultimate potential will exceed the stated value:
DEM is not currently involved in any Eromanga Basin research projects.
Several National Parks and Wildlife reserves overlie the Eromanga Basin Reserve Land. (South Australia Reserved Land figure).
Exploration is permitted in the Simpson Desert Regional Reserve, the Innamincka Regional Reserve, the Witjira National Park, Tallaringa Conservation Park and the Lake Eyre National Park. Exploration is not permitted in the Coongie Lakes National Park, the Simpson Desert Conservation Park or the No-Go Special Management Zone of the Innamincka Regional Reserve.
Compulsory relinquishment of roughly 36% (19 150 km2) of the areas in current Cooper region PELs will precede competitive bidding from 2009. In anticipation of the calling for work program bids in 2009, negotiations opened in 2006 to develop a conjunctive Indigenous land access agreement (ILUA) for the regions already covered with land access agreements resulting from earlier RTN proceedings. These negotiations involve the Aboriginal Legal Rights Movement, the three native title parties already familiar with the RTN process, the South Australian Government, and petroleum exploration and production company representatives (through the South Australian Chamber of Mines and Energy).
The draft framework agreement established in 2006 will be the subject of necessary formal negotiations as required by the Native Title Act 1993, with a hope to formalise ILUAs for the whole of the Cooper Basin region in 2007.
Sales et al (2015) has concluded that best prospects for new significant oil discoveries are on the western flank of the Cooper Basin. Two plays are evident and may be subtle. The dominant play is in the structurally subtle Namur Sandstone, while the secondary is the mid-Birkhead Formation which is stratigraphically subtle and may be difficult to delineate, depositionally complex and a challenge to produce.
The recent Kangaroo-1 exploration well drilled in September 2016 intersected a 20 metre gross oil column in the Birkhead Formation with a calculated oil flowrate of 320 bopd. The oil discovery in Kangaroo-1 have expanded the prospectivity of the north western flank further west than previous exploration, highlighting significant under explored areas.
Altmann MJ and Gordon HM, 2004. Oil on the Patchawarra Flank - some implications from the Sellicks and Christies oil discoveries. In: Boult, P.J., Johns, D.R. and Lang, S.C. (Eds), PESA’s Eastern Australasian Basin Symposium II, Adelaide 2004. Petroleum Exploration Society of Australia. Special Publication, pp. 29-34.
Boult PJ, Lanzilli E, Michaelsen BH, McKirdy DM and Ryan MJ, 1998. New model for the Hutton/Birkhead reservoir seal couplet and the associated Birkhead-Hutton(!) petroleum system. APPEA Journal, 38(1):724-744.
Cotton TB, Scardigno MF and Hibburt JE eds, 2006. The petroleum geology of South Australia. Vol. 2: Eromanga Basin. 2nd edn. South Australia. Department of Primary Industries and Resources. Petroleum Geology of South Australia Series.
Gravestock DI, Moore PS and Pitt GM eds, 1986. Contributions to the geology and hydrocarbon potential of the Eromanga Basin. Geological Society of Australia. Special Publication, 12.
Kramer L, McKirdy DM, Arouri KR, Schwark L and Leythaeuser D, 2004. Constraints on the hydrocarbon charge history of sandstone reservoirs in the Strzelecki Field, Eromanga Basin, South Australia. In: Boult PJ, Johns DR and Lang SC eds, PESA’s Eastern Australasian Basin Symposium II, Adelaide 2004. Petroleum Exploration Society of Australia. Special Publication, pp. 589-602.
McKirdy DM, Arouri KR and Kramer L, 2005. Conditions and effects of hydrocarbon fluid flow in the subsurface of the Cooper and Eromanga Basins. University of Adelaide report on ARC SPIRT Project C39943025 for PIRSA and Santos Ltd. South Australia. Department of Primary Industries and Resources. Report Book, 2005/00002.
Nakanishi T and Lang SC, 2002. Constructing a portfolio of stratigraphic traps in fluvial–lacustrine successions, Cooper–Eromanga Basin. APPEA Journal, 42(1):65-82.
O’Neil BJ Ed, 1989. The Cooper and Eromanga Basins, Australia. Proceedings of the Cooper and Eromanga Basins Conference, Adelaide, 1989. Petroleum Exploration Society of Australia, Society of Petroleum Engineers, Australian Society of Exploration Geophysicists (SA Branches).
Sales, M., Altmann, M., Buick, G., Dowling, C. Bourne, J., Bennett, A. 2015 Subtle oil fields along the Western Flank of the Cooper/Eromanga petroleum system Extended Abstract APPEA 2015.
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