Cooper Basin

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Introduction

The Cooper Basin is a large sedimentary basin located in East-Central Australia, specifically the south west corner of Queensland and the north east corner of South Australia. In terms of size, the Basin covers an area of almost 50,000 square miles (130,000 square km) and is the largest onshore province in Australia, as show in figure 1a. The cooper basin contains source rock derived from the Middle Triassic period, and is mainly composed of coal and carbonaceous shale. The overall hydrocarbon potential is very prominent for both oil and gas as well.

(1a) Location of the Cooper Basin.

History of the Cooper Basin

The Cooper Basin is one of the most important on-shore petroleum and natural gas deposits in Australia and has been for an extended period of time. The oil and gas target location is located 1,250 m below the surface and was originally discovered in the 1960s, along with several other larger deposits that are located off-shore. The first commercial discovery of gas occurred in 1963 with the Gidgealpa Oil and Gas Field , and a second major natural gas field was found soon after at nearby Moomba. In 1969 a natural gas pipeline was completed from Moomba to Adelaide, and in 1976 natural gas began to be supplied to the Sydney area by a second pipeline. Petroleum exploration in the 1980s resulted in new discoveries of both oil and gas, and as of today It includes Australia's largest onshore oil field, the Jackson Oil Field. This field was discovered in 1981. Pipelines transport gas to the major markets of Brisbane, Adelaide and Sydney. Overall about 1,800 petroleum wells have been drilled.

Geologic Risks & Uncertainties

The basin’s Permian-age shales have a non-marine (lacustrine) depositional characteristics and the shale gas appears to have elevated CO2 content, both factors adding risk to these shale gas and shale oil plays. There is also some uncertainty involving key components including source rock UEP (ultimate expulsion potential), timing of peak generation and expulsion, and expected migration losses. However, none of these significantly affect the Cooper Basin as a whole and occur sporadically throughout the large area of land.

Petroleum Systems

Source Rock

(2a) Graphic showing the source rock that makes up the Cooper Basin.

The Toolachee and Patchawarra formations represent the principal source rocks in the Basin. These are comprised of coals and carbonaceous shales deposited in fluvial deltaic and peat swamp environments, great hydrocarbon indicators, and show good to very good oil and gas source potential. Additional source rocks include the gas prone lacustrine Roseneath and Murteree shales, as well as coals and carbonaceous shales of the Daralingie and Epsilon formations. As shown in the graphic on the right (2a), there are multiple formations that formed the cooper basin.

Trap

Exploration for hydrocarbons in the Cooper Basin has led to the discovery of oil and gas in both structural and structural/stratigraphic traps. However, the hydrocarbon potential of the Basin has largely been addressed by drilling the crest of the major anticlines. Discoveries containing a significant stratigraphic or fault component have usually been associated with major structural trends within the Basin.

Seal

There are four different types of sealing lithofacies that come together to form the seal of the Cooper Basin. There are sandstones, lacustrine silts and clay, and floodplain mudstones. Each of these seals support different maximum seal capacities and hydrocarbon seal column heights. The greatest seal capacity, able to support an oil column height of I ,353 m or a gas column of 727 m is in lacustrine clays and silts. These four distinct seals are also classified as Regional Seals.

Reservoir Types

The main commercial reservoirs for gas in the Cooper Basin are in the Patchawarra and Toolachee Formations and, to a lesser degree, the Epsilon Formation. Commercial reservoirs also exist in the Merrimelia Formation, the Tirrawarra Sandstone, and the Daralingie and Arrabury Formations. There are multiple time periods that contain hydrocarbons, as well as different types of reservoirs. When looking at the Cambrian, Permian, and the Jurrasic/Crusteaceous period, the target formations are located in convential reservoirs. On the other hand, there are also Permian basins containing coal and shale parent rocks that are considered unconventional.

Current & Future Examination of the Basin

(3a) Seismic cross-sections of the Cooper Basin.

Due to the Basin's massive size, the production of the now 1,846 wells is doing quite well. New seismic acquisition techniques (Poole et al 2013) has shown that the environmental footprint can be reduced, while at the same time allow clear imaging of channels in the Permian sequence. The seismic cross-sections can be viewed in figure 3a. Cooper Basin unconventional activity is mainly at the exploration appraisal stage and initially focused on three plays in three distinct regions—the Roseneath-Epsilon-Murteree (REM) shale play, the basin-centered gas (BCG) play and the deep coal play.

Petroleum and Facility Engineering

As of now, the majority of the Cooper Basin consists of shallow target formations. This allows for the easy extraction of oil at relatively high flow rates, specifically when using Horizontal Drilling. Through the use of horizontal drilling, Oil and Gas companies are able to reach a much larger area that contains hydrocarbons at a slightly increased cost, due to their shallow nature. Currently, many fields located in the Cooper Basin utilize this technique as it decreases the payback period from the well investment.

References

  • Mackie, S. (2015, September). History of Petroleum Exploration and Development in the Cooper and Eromanga Basins. https://library.seg.org/doi/10.1190/ice2015-2194999
  • Stanmore, P. (2019, November 15). Case studies of stratigraphic and fault traps in the Cooper Basin, Australia - PESA - Petroleum Exploration Society of Australia. Retrieved December 14, 2020, from https://pesa.com.au/the_cooper__eromanga_basins_australia_p361-369-pdf/
  • AAPG. Petroleum Exploration Society of Australia (PESA)
  • South Australia Department for Energy and Mining. https://www.energymining.sa.gov.au/petroleum/prospectivity/cooper_basin
  • Technically Recoverable Shale Oil and Shale Gas Resources: Australia. (2015). https://www.eia.gov/analysis/studies/worldshalegas/pdf/Australia_2013.pdf