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The Northwest shelf of Australia extends over 1,000 mi (1,600 km) in a northeast-southwest direction and averages more than 200 mi (320 km) in width; it can be divided, from southwest to northeast, into the Carnarvon basin, the offshore Canning basin, the Browse basin, and the Bonaparte basin. Each of these is further divided into subbasins based on stratigraphic and structural boundaries. Sedimentary thicknesses are probably in excess of 30,000 ft (9,000 m).
Structurally, the entire Northwest shelf is dominated by Early to mid-Jurassic rifting. Although the tensional tectonic style predominates, compressional features are present, probably resulting in part from rebound of the tensional stress and possibly from strike-slip movement in the basement. In the Bonaparte basin, salt movement has created both piercement and deep-seated salt structures.
Proven petroleum reservoirs of the Northwest shelf are Permian, Triassic, Jurassic, and Cretaceous sandstones. The major petroleum source is Upper Jurassic shale, which has generated both oil and gas in the Carnarvon and Browse basins. Gas in Permian sandstones in the Bonaparte basin probably has a source within the Permian.
By the end of 1983, approximately 200 exploratory wells had been drilled on the Northwest shelf, for a drilling density of less than one well per 1,000 mi2 (2,600 km2). Over 100 of those wells are in the Carnarvon basin, which covers less than 20% of the total area of the shelf. Significant discoveries have been made in the Carnarvon, Browse, and Bonaparte basins, but only the Carnarvon is currently producing; its proved reserves are estimated at 462 million bbl of oil and condensate, 155 million bbl of LPG and 11 tcf of gas (1982 statistics).
Except for the Carnarvon basin, where well density is still low, the Northwest shelf is essentially unexplored. All of the basins are indicated to have most of the elements required for the generation and accumulation of petroleum. Recently announced discoveries in widely divergent areas of the shelf have generated renewed interest in this large unexplored offshore area and may stimulate the exploration activity necessary to make the Northwest shelf a major petroleum province of the future.
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