The Amadeus basin is a structural depression extending easterly from the Canning basin in Western Australia to the Great Artesian basin in Queensland. It covers about 80,000 square miles and contains up to 30,000 feet of late Proterozoic and early Paleozoic marine and continental sediments.

The marine cycle of deposition started in late Proterozoic time and terminated in late Ordovician. No unconformity is present at the base of the Paleozoic sequence and both Proterozoic and Paleozoic beds are unaltered except along the northern margin of the basin where low grade metamorphic facies were developed in

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the sediments during the Pertnjara orogeny in Devonian time.

The marine sequence consists of clastics, carbonates, and evaporites. They were deposited in shallow water, shelf environments as is shown by algal growths, biohermal and biostromal carbonates, abundant ripple marks and abundant cross laminations in sandstones, and by widespread coquinoid facies in Ordovician shales. Silled or barred basins with restricted circulation of marine waters existed from time to time during the marine cycle as is shown by salt deposits in the upper Proterozoic and Cambrian sections and by thick accumulations of dark shales with abundant pyrite in late Proterozoic and Ordovician sediments.

The marine cycle of deposition was terminated by the Pertnjara orogeny. This orogenic episode created a welt north of the Amadeus basin and a bordering foredeep whose depoaxis follows the present northern margin of the basin. Marine sediments were stripped from the rising welt, transported southward, and dumped into the subsiding foredeep where they now form a thick apron of poorly sorted, coarse clastic deposits.

Salt tectonics has played an important role in the growth of structures in the northern Amadeus basin. Thick salt deposits in the Bitter Springs formation of late Proterozoic age constituted a semi-plastic layer near the base of the Proterozoic sequence. Sedimentary loading on this layer produced flowage and initiated salt anticlines and salt domes. These structures grew during late Proterozoic and early Paleozoic deposition as is shown by crestal stratigraphic convergence and local unconformities confined to one structure. The evaporite layer also provided a "lubricated zone" along which slippage was localized during the Pertnjara orogeny, and it may be responsible in part for the large nappes and overthrusts along the northern margin of the basin.

The anticlines and salt domes initiated by salt flowage were formed early in the history of deposition and thus constituted potential traps for hydrocarbons long before the Pertnjara orogeny. However, folding during the Pertnjara orogeny greatly increased structural relief on the anticlines and thereby created traps having large volumetric capacities.

Two of these large structures have been tested with encouraging results. The Exoil-Magellan-United Canso groups have discovered a large wet-gas accumulation, possibly with an appreciable oil leg, in Ordovician reservoirs on the Mereenie anticline in the western part of the Amadeus basin. They also discovered a non-commercial gas accumulation in Proterozoic sediments on the Ooraminna anticline in the eastern part of the Amadeus basin. A third test of a small structure near the Ooraminna anticline encountered non-commercial oil shows in Cambrian sediments. In addition, the Australian Bureau of Mineral Resources discovered oil-saturated sands in Ordovician sediments penetrated by a well being drilled as a test for phosphates. With the exception of shallow water bores, no other wells have been drilled to date in this basin.

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