User:Ashton-bridges/Cooper Basin

The Cooper Basin is a northeast-trending structural rift basin that is located in northeastern South Australia and southwestern Queensland. It is about 130,000 km2 in size and is one of the largest Gondwana intracratonic rift basins in Australia. This basin consists of Permo-Triassic age and is located near other basins in central and eastern Australia which include Galilee, Arckaringa, and Pedirka basins. There are three major troughs of this basin that are the major structural features. These troughs include the Patchawarra, Nappamerri, and Tenappera troughs. The northwest-trending Karmona-Naccowlah boundary divides the Cooper Basin into northern and southern sections. The northern Cooper Basin contains thin Permian section whereas the southern Cooper Basin contains thick Permian section.

Tectonic Setting
The first discovery of the Cooper Basin took place in 1959 when the Innamincka 1 well drilled and found evidence for the basin. The Cooper Basin contains three major troughs which include the Patchawarra, Nappamerri, and Tenappera troughs. All of these troughs have Permo-Carboniferous to Triassic section that has a thickness of 2,500 m and overlain by up to 1,300 m of Jurassic to Tertiary cover. The structural ridges of this basin include the Gidgealpa, Merrimelia, Innaminka, and Murteree which separate the troughs from each other.

The basin is located in the northeastern part of South Australia that extends into southwestern Queensland. Beyond the basin lies the Eromanga Basin, which is part of the Great Artesian Basin. The unconformity which distinguishes the basin from older rocks is the Alice Springs Orogeny.

The basin is relatively compressional, and its development is mostly influenced by older basement structural characteristics. Its deposition has been disrupted by various features in thickness, facies distribution, and sedimentation breaks. As a consequence of several tectonic events, complex structures formed with younger structures forming over the older features. Subsequently, major phases of uplift are a key factor for paleocurrent trends and basin fill during the basin's development.

Depositional History
The Cooper Basin's depositional history began during the late Carboniferous. At this time, the Antarctic ice sheet reached a maximum volume before it started to back away during the early Permian. This brought a massive amount of ice that consisted of marine and non-marine sediments into the Gondwana basin. The mixed lake-fluvial deposits of the Roseneath, Epsilon, and Murteree, or REM, were strongly dominated by the basin structure and water masses. The Murteree Shale is in between two formations with the Epsilon Formation on top and the Patchawarra Formation on bottom. This unit contains black, grey, and brown argillaceous siltstone and fine-grained sandstone that gets sandier in the southern part of the basin. Also, fine-grained pyrite, muscovite, and carbonaceous siltstone are all significant to the Murteree Shale. Then, the Roseneath Shale is a combination of shales and siltstones that lie on top and in between the Epsilon Formation. This unit consists of light to dark brown-grey siltstones and mudstones with fine-grained pyrite and light brown sandstone interbeds.

The paleo-environment evolution of the basin that occurred during the deposition of the Roseneath and Murteree shales happened in four major parts. For the first part, the Gondwana ice sheet formed over most of the basin during the late Carboniferous. The Merrimelia Formation intersected with the Gondwana glacier maximum which occurred when the Cooper Basin was located in the high southern polar latitudes of Australia. The second part occurred during the Permo-Carboniferous and consisted of the Merrimelia ice sheet getting smaller and retreating. Then, the coarse grained fluvial deposits went into the basin continuously which led to deposition of the Tirrawara sandstones. During the early Permian, the third part involved a transformation into the Patchawarra Formation which contained lacustrine and cold climate fluvial deposition with peat swamps. Lastly, for the fourth part, which took place during the Late Permian, had the development of a fresh water lake in which the Roseneath and Murteree shales deposited into.

Stratigraphy
The Cooper Basin contains glacial, fluvial, and lacustrine strata that is of Permian-Triassic age. The basin's stratigraphy is divided into two groups which are the Gidgealpa Group and the Nappamerri Group. The basin fill consists mainly of non-marine depositional units of the late Permian Gidgealpa group and the late Permian to middle Triassic Nappamerri group. Underneath the basin sequence, there are younger granitoids.

Various features characterize the stratigraphic history of the Cooper Basin. A very high geothermal gradient, and a tectonic evolution that was dominated by older basement structures reactivating. Downwarped areas of the basin contained crustal unconformities over active features that was driven by crustal buckling with small uplifts and thrusting. Thick deltaic sediments formed in lower areas. The rates of sedimentation were strongly affected by the rate of uplift of the source areas surrounding the basin which were largely due to buckling. Subsequently, these features showed that the basin is closely affiliated with a compressional tectonic regime.

Natural Resources
The Cooper Basin produces a large volume of Australia's natural gas and is the largest onshore petroleum province in Australia. The first commercial Permian gas discovery for this basin occurred in 1963 by the Gidgealpa 2 well.

In 2015, the basin consisted of 82 oil fields and 98 gas fields in South Australia. Also, there were 84 oil fields and 158 gas fields in Queensland. It provides gas for the East Coast Gas Market. This gas is being produced from the Nappamerri trough, where the sediments have experienced a large amount of heating.

The source rocks for the hydrocarbons are thought to be the terrestrial coal measure sediments of the Permian Gidgealpa Group. The formations of the Gidgealpa Group in the Patchawarra trough contain beds with an average of 1-5% volume of dispersed organic matter. If the total volume of dispersed organic matter is linked to the generation of hydrocarbons, then most of the Gidgealpa Group formations are great sources for liquid hydrocarbons.

From the Patchawarra and Toolachee formations, the liquid hydrocarbons are lighter than the basin's hydrocarbons in other units. This proposes that there are two separate organic sources. Therefore, there must be two different petroleum systems.