Eromanga Basin

Summary

Age Early Jurassic – Late Cretaceous
Area in South Australia 360 000 km2 (139 000 sq. miles)
Depth to target zones 1200–3000 m
Thickness Up to 3000 m
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; elsewhere — nil
Undiscovered resources (50% prob.) 2.4 x 106 kL (15.1 mmbbl); western Eromanga Basin 8.4 x 106 kL (52.8 mmbbl) (PIRSA estimate June 1996)
Production Cooper region — refer to Cooper Basin; elsewhere nil
Basin type Intracratonic
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 anticlinal trends in regional sag basin

Structural setting

The Eromanga Basin covers 1 000 000 km2 of central–eastern Australia, 360 000 km2 of which lie in South Australia. The Eromanga Basin encloses the multi-aquifer system of the Great Artesian Basin.

In South Australia, the Eromanga Basin overlies late Palaeozoic and older basins. It consists of a broad downwarp with two main depocentres — the Poolowanna Trough and the Cooper region — containing up to 3000 m of sediment. The central Eromanga Basin is overlain by the Tertiary to Recent Lake Eyre Basin. Eromanga Basin units crop out extensively on the western and southern margins.

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.

Stratigraphy

Eromanga Basin stratigraphy can be divided into three sequences — lower non-marine, marine and upper non-marine. The Eromanga Basin unconformably overlies Permo-Triassic infrabasins, the Cambro-Ordovician Warburton, Amadeus and Officer basins and Proterozoic basement in South Australia.

The base Eromanga Basin unconformity has been mapped over the entire basin in South Australia and a digital data set is now available.

In the Cooper region, the lower non-marine sequence consists of intertonguing braided fluvial sandstones (Hutton and Namur sandstones), lacustrine shoreface sandstone (McKinlay Member) and meandering fluvial, overbank and lacustrine sandstone, siltstone, shale and minor coal (Poolowanna, Birkhead and Murta formations).

In the Poolowanna Trough, the Poolowanna Formation (up to 130 m thick) is overlain by a thick sand-dominated unit (Algebuckina Sandstone). West of the northern Birdsville Track Ridge, Birkhead and Murta Formation shales pinch out into Algebuckina Sandstone. Algebuckina Sandstone crops out on the western and southern basin margin.

The non-marine sequence is succeeded conformably by a sequence reflecting transition from non-marine to marginal marine to open marine shale and sandstone. The basal unit, Cadna-owie Formation, is of significance for exploration as the top of the unit approximates a distinctive seismic reflector — the C horizon mappable over the entire basin. PIRSA has compiled a basin-wide C horizon data set from company seismic maps. The C horizon mapping now extends into Queensland, Northern Territory and New South Wales as a result of the National Geoscience Mapping Accord Cooper and Eromanga Basin project.

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.

Facies

The Poolowanna and Birkhead formations contain organic-rich shales that are oil-prone and in places at peak maturity for oil generation. Lateral migration from these source areas has also been postulated. Elsewhere in the basin, the presence of thick Poolowanna, Birkhead and Murta formations is critical to evaluation of oil source potential. The marine sequence and upper non-marine sequence are immature for hydrocarbon generation over much of the basin. The underlying Simpson and Pedirka basins also contain mature source rocks and are well placed to charge Eromanga Basin reservoirs.

Principal reservoirs in the Cooper region are the braided fluvial Hutton and Namur sandstones (porosities up to 25%, permeability up to 2500 mD). Meandering fluvial reservoirs are also important (Poolowanna and Birkhead formations), lacustrine shoreface (McKinlay Member) and lacustrine turbidite (Murta Formation) sandstones. Detailed petrophysical data is available from PIRSA for all of these Eromanga Basin reservoirs in the Cooper region.  In the Poolowanna Trough, principal reservoirs occur in the Poolowanna Formation (variable reservoir quality) and Algebuckina Sandstone (good–excellent reservoir quality). Elsewhere in the Eromanga Basin, the Algebuckina Sandstone has excellent reservoir properties and is a major artesian aquifer.

Seals consist of intraformational siltstones and shales of the Poolowanna, Birkhead and Murta formations in the Cooper region. In the Poolowanna Trough, they consist of intraformational siltstone and shale in the Poolowanna Formation and siltstone of the Cadna-owie Formation. Elsewhere in the basin, potential seals include the Cadna-owie Formation and Bulldog Shale – Wallumbilla Formation).

More information on the Eromanga Basin

Key Reference
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.