|Age||Late Carboniferous – Middle Triassic|
|Area in South Australia||35 000 km2 (13 510 sq. miles), total area 130 000 km2|
|Depth to target zones||1250–3670 m|
|Hydrocarbon shows||Widespread over 8 formations|
|First commercial discovery||1963 gas (Gidgealpa 2)|
|Production (12 months to June 2004)||3.283 Gm3 sales gas, 227.8 kt LPG, 332.06 ML condensate, 384.21 ML crude oil, 20.42 kt ethane (Cooper and Eromanga basins)|
|Reservoirs||Fluvial, deltaic, shoreface sandstones|
|Regional structure||Faulted anticlines|
|Seals||Lacustrine shale, coal|
|Source rocks||Carbonaceous shale, thick (up to 30 m) coal|
|Depth to oil/gas window||1250 m|
|Number of wells||8 deep geothermal wells, petroleum wells ~1800|
|Seismic line km||75 633 2D; 8774 km2 3D|
The intracratonic Cooper Basin represents a Late Carboniferous to Triassic depositional episode terminated at the end of the Middle Triassic with widespread compressional folding, regional uplift and erosion. It lies unconformably over early Palaeozoic sediments of the Warburton Basin and Mid Carboniferous Big Lake Suite granites and is overlain disconformably by the central Eromanga Basin.
Three major troughs (Patchawarra, Nappamerri and Tenappera) are separated by structural ridges (Gidgealpa–Merrimelia–Innamincka (GMI) and Murteree) associated with the reactivation of NW-directed thrust faults in the underlying Warburton Basin. (You can see the structural elements on top Warburton Basin, Cooper Basin, South Australia and a seismic cross-section of the Cooper Basin area showing C, P and Z seismic horizons.) The major tectonic episode separating the Cooper and Warburton basins is interpreted to be the Devonian–Carboniferous Alice Springs Orogeny. The three troughs contain up to 2500 m of Permo-Carboniferous to Triassic sedimentary fill overlain by as much as 1300 m of Jurassic to Tertiary cover.
The basin contains a number of non-marine depositional sequences within the Late Carboniferous to Late Permian Gidgealpa Group and Late Permian to Middle Triassic Nappamerri Group.
The Cooper Basin unconformably overlies flat lying to compressively deformed Cambro-Ordovician Warburton Basin strata and Carboniferous granitic intrusives (see the geological summary of the Cooper Basin). The unconformity is mapped as the Z seismic horizon in the seismic cross-section. The oldest units are the Late Carboniferous to Early Permian Merrimelia Formation and Tirrawarra Sandstone which comprise terminoglacial and glaciofluvial systems deposited unconformably on a glacially scoured landscape. The Tirrawarra Sandstone represents braided fluvial to fan-delta deposits overlain by peat swamp and floodplain facies of the Patchawarra Formation. Locally (eg Pondrinie Field), Merrimelia aeolianite forms a major gas reservoir.
Two lacustrine shale units (Murteree and Roseneath Shales) with intervening fluviodeltaic sediments (Epsilon and Daralingie formations) were deposited during a phase of continued subsidence. Early Permian uplift led to erosion of the Daralingie Formation and underlying units from basement highs.
The Late Permian Toolachee Formation was deposited on the Daralingie unconformity surface and is overlain conformably by Late Permian to Middle Triassic Arrabury Formation (comprising the Callamurra, Paning and Wimma Sandstone members) and the Middle to early Late Triassic Tinchoo Formation.
Permian coal measures and shales are the principal hydrocarbon source rocks in the region. Locally, the Toolachee Formation is the richest source unit. The Patchawarra Formation is considered the other major source unit, especially the lower shales and coals. The lacustrine Murteree and Roseneath Shales have little hydrocarbon source potential. Thin, laterally discontinuous coals represent the best source rocks of the upper Nappamerri Group, whilst shales tend to be organically lean. The lower Nappamerri Group is coal-poor, contains kerogen that tends to be oxidised, and any source rocks are humic-rich and gas-prone.
Multi-zone high-sinuosity fluvial sandstones form poor to good quality reservoirs. The main gas reservoirs occur primarily within the Patchawarra Formation (porosities up to 23.8%, average 10.5%; permeability up to 2500 mD) and Toolachee Formation (porosities up to 25.3%, average 12.4%; permeability up to 1995 mD). Shoreface and delta distributary sands of the Epsilon and Daralingie formations are also important reservoirs. Oil is produced principally from low-sinuosity fluvial sands within the Tirrawarra Sandstone (porosities up to 18.8%, average 11.1%; permeability up to 329 mD). Towards the margin of the Cooper Basin, oil is also produced from the Patchawarra Formation and from fluvial channel sands in the Merrimelia Formation in Malgoona Field.
The Callamurra Member of the Arrabury Formation is conventionally regarded as a regional seal, but nevertheless contains economic oil and gas reservoirs in some areas and is a leaky seal in others. Low-sinuosity fluvial sandstones of the Paning and Wimma Sandstone members form economic oil and gas reservoirs, and high-sinuosity fluvial sandstone of the Tinchoo Formation reservoirs oil.
Intraformational shale and coal form local seals in the major reservoir units. Beneath the Daralingie unconformity are two important early Permian regional seals — the Roseneath and Murteree Shales. The Roseneath Shale is the top seal of the Epsilon Formation, and the Murteree Shale seals the Patchawarra Formation. A younger regional seal is provided by the Triassic Arrabury Formation.
Gravestock, DI, Hibburt, JE and Drexel, JF (Eds) 1998. The petroleum geology of South Australia. Volume 4: Cooper Basin. South Australia. Department of Primary Industries and Resources. Petroleum Geology of South Australia Series.