Exploration during the past 10 years on the Northwest Shelf of Australia provides an excellent example of the use of modern technology to evaluate the hydrocarbon potential of a large offshore region.

The area under concession consists of about 140,000 sq mi and comprises the greater part of five separate sedimentary basins or subbasins. Water depths range from 20 mi to more than 1,500 m, and are greater than 175 m in over half the total concession area.

Exploration commenced in 1964 with seismic surveys which have continued every year to the present. The first drilling commenced in 1967, and to the end of 1973 a total of 36 exploration wells has been drilled. Of these, nine can be classed as hydrocarbon discoveries, with several being potentially commercial fields despite relatively deep water and long distances from shore. In addition, nine successful appraisal wells have been completed.

The utilization of both drilling and seismic data has enabled a meaningful geologic interpretation of this previously unexplored area.

The Paleozoic history of the offshore area is largely unknown. During the early Mesozoic, rifting on a continental scale resulted in the development of a typical pull-apart tectonic regime. Extensive Triassic to Middle Jurassic fluviodeltaic sedimentation was terminated in Late Jurassic time by major faulting followed by a widespread marine transgression. Upper Jurassic and Lower Cretaceous sedimentary rocks are mainly marine clastic, passing upward to predominantly carbonate sedimentation.

Tertiary sediments are thick and widespread, showing a transition from sands in the nearshore areas to carbonates farther out on the shelf.

The main hydrocarbon accumulations have been found in reservoirs ranging in age from Late Triassic to Late Cretaceous. These discoveries can be attributed largely to the successful application of geologic models produced as a result of close cooperation between geologists and geophysicists.

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