The Eromanga Basin covers 1 000 000 km² of central–eastern Australia, 360 000 km² of which lies 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.

Petroleum exploration commenced in the 1950s when licences covering the Cooper and Eromanga basins were first acquired by Santos, who went against conventional wisdom that commercial accumulations of oil would not be found in Mesozoic formations within the Great Artesian Basin.

Initial exploration involved surface mapping, stratigraphic drilling, aerial surveys, gravity and aeromagnetic surveys and seismic. The first petroleum well was drilled in 1959 and Cooper Basin gas was discovered in 1963.

The first commercial hydrocarbon to flow from the Eromanga Basin was gas produced from Namur 1 in 1976 (Cooper region). Oil was discovered in 1977 with an uneconomic flow from Poolowanna 1 in the Poolowanna Trough. The first economic oil flow was recorded from Strzelecki 3 (Cooper region) in the following year and this stimulated a major oil exploration program.

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

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. Play analysis has been conducted by ERD for the Poolowanna Formation and Algebuckina Sandstone in the Poolowanna Trough region.

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 petroleum exploration as the top of the unit approximates a distinctive seismic reflector — the C horizon mappable over the entire basin. In 1998 DEM compiled a basin-wide C horizon data set from company seismic maps. Extending into Queensland, Northern Territory and New South Wales, as part of the National Geoscience Mapping Accord Cooper and Eromanga Basin project.

Vertical migration of oil from Permian (Cooper Basin) source rocks has been widely accepted as the principal source of most Eromanga-reservoired oil (in the Cooper region). Both Cooper and Eromanga mature source rocks have contributed to oil accumulations in the region, however each oil accumulation needs to be considered on its merits with respect to the extent of mixing from Permian and Mesozoic sources. The Poolowanna and Birkhead formations contain organic-rich shales that are oil-prone and in places at peak maturity for oil generation, but volumetrically any oil generated from them is almost negligible in comparison with that from Permian source rocks. Lateral migration from these source areas has been proven on the Western Flank of the Cooper area in particular.

Phase separation is likely to have a major influence on the occurrence of liquid versus gas in the Cooper and Eromanga Basins (Hall et al, 2016). The fluids are a dew point system with primarily gas at depth. As hydrocarbons migrate into shallower reservoirs, the pressures reduce to below the saturation pressure and liquids separate out. Saturation pressure depends both on the gas to liquids ratio and the mutual miscibility of the two phases, which is in turn dependent on their composition.

Most oils in the Cooper area of the Eromanga Basin are now relatively dead with little or no gas. This is due to water washing in strong aquifer drive reservoirs, removing the light ends.

In the western Eromanga 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). Oil is also reservoired in meandering fluvial (Poolowanna and Birkhead formations), lacustrine shoreface (McKinlay Member), lacustrine turbidite (Murta Formation) and shallow marine (Coorikiana Sandstone) sandstones. Detailed petrophysical data is available from DEM for all of these Eromanga Basin reservoirs. A schematic section showing typical petroleum traps of the Eromanga Basin is shown in Eromanga Basin figure 6

In the Cooper region the structural framework of the Eromanga Basin is largely inherited from mild but widespread compression, regional tilt and erosion in the Late Triassic which produced the Nappamerri unconformity surface (N seismic horizon). Regional downwarping commenced in the Early Jurassic. Across the basin, Tertiary W–E compression reactivated Palaeozoic structures.

Seals consist of intraformational diagenetic sandstones, 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).

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). The structure at the Coorikiana level is characterised by a desiccated fault pattern with no obvious preferred orientation and small irregular shaped fault blocks, with faulting predominately extensional in nature, providing for normal displacement.

Eromanga structures in South Australia are rarely filled to spill with oil — net oil columns are relatively thin compared to the height under closure, often due to poor sealing characteristics, but also due to late structural movements tilting closures.

Cooper Basin region of South Australia

Petroleum exploration in the Eromanga Basin in South Australia has traditionally concentrated in the portion underlain by the highly productive Cooper Basin, a relatively mature area. In addition, there is the likelihood that a significant amount of the oil found in this area has been sourced from the underlying Cooper Basin.

The USGS (Schenk et al 2016) estimated the undiscovered conventional oil and gas potential of the Eromanga Basin in South Australia and Queensland, as shown in their Figure 1 below. Table 1 summarises the results of this assessment. Further potential has also been assessed for the underlying Cooper Basin.

Figure 1. Map showing locations of five conventional assessment units in the Eromanga Basin Province, Australia (Schenk et al 2016)

Table 1  Undiscovered conventional oil and gas resources of the Cooper Basin region, Eromanga Basin (USGS, May 2016)

The initial 2001 right to negotiate (RTN) agreements in the Cooper Basin for the CO-98 PEL application areas were groundbreaking in Australia as the first conjunctive agreements (covering exploration and production) that provide certainty in enabling any explorer, the Aboriginal parties, and the state being able to benefit from any commercial discoveries made.

The state has also commenced the RTN process for additional PELs in the Eromanga and Arrowie basins and it is likely that existing RTN agreements will continue to form a practical precedent. Any agreements will provide for protection of the heritage and cultural interests of the Aboriginal parties.

The first conjunctive ILUA in Australia in a proven petroleum province was concluded in February 2007 with the Yandruwandha–Yawarrawarrka native title claimants over a major portion of the South Australian Cooper Basin. This will provide greater certainty and expedite the grant of PELs in a way that remains fair to native title claimants and sustainable in relation to exploration and production investment. The application of conjunctive ILUAs will enable land access more quickly and with lower transaction costs than serial RTN proceedings. The successful implementation of conjunctive ILUAs for Cooper Basin petroleum exploration and production will serve as a model for analogous agreements elsewhere in the state.

Negotiations in respect to formalising conjunctive petroleum ILUAs with the remaining two native title claimant parties in the Cooper Basin are progressing.

In summary, conjunctive ILUAs are proposed as an evolutionary, additional, alternative to the RTN process already working comparatively well in South Australia. Indeed, conjunctive ILUAs will be an attractive incentive to achieve competitive bids, with explorers knowing the terms of land access prior to lodging bids.

A number of applications are being held over the western part of the Eromanga Basin pending resolution of native title issues.

Figure 8 shows the licence status at the time of publication. Use this link for further information on holders of petroleum tenements in South Australia.

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