Geology and Petroleum Potential of Exmouth Plateau Area Off Western Australia

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The Exmouth Plateau lies oceanward of Australia's Northwest Shelf petroleum province, in water depths of 800 to 2,000 m, and is flanked by the abyssal plains of the Wharton basin. The plateau is dominated by the northeast-trending Exmouth Plateau arch and Kangaroo syncline. The southwestern and northwestern margins are fault-bounded escarpments, but the northern margin consists of spurs and subplateaus separated by deep troughs. Magnetic basement is as deep as 10 km below sea level, but shallow crystalline basement and igneous bodies are present around the seaward margins. A crustal thickness of about 20 km, similar to that under the Northwest Shelf, is deduced from gravity data. The outer limit of continental crust is roughly coincident with the 4,000-m isobath. Interpretation of seismic profiles indicates that up to 5,000 m of Paleozoic strata and up to 5,000 m of younger strata overlie basement. The sediments have been gently folded, and northeast-trending block faults affect the pre-Late Jurassic sequence. The sediments beneath the Exmouth Plateau are considered to have been deposited in part of the Carnarvon basin. This part formed a north-facing embayment in Gondwanaland which received detrital sediments from the south until Early Cretaceous time. At least, 3,000 m of mainly paralic and shallow-marine detrital sediments were deposited from Permian to Middle Jurassic times. Rifting probably commenced in the Late Permian. It recurred in the Late Triassic, except in the northeast of the area and on the adjacent Rankin platform where it culminated in the early Late Jurassic. Prevailing hypotheses suggest that during the early Late Jurassic seafloor spreading around northeast-trending ridges formed the plateau's northwestern and northern margins, by rifting and transform faulting, respecti ely. About 1,000 m of shallow-marine and deltaic detrital sediment, derived from the south and east, covered the block-faulted surface in Late Jurassic and Early Cretaceous times. The plateau's southwestern margin formed by faulting, probably transform faulting, in the Early Cretaceous. About 200 m of marine terrigenous sediment was deposited in the middle Cretaceous, and 500 to 1,000 m of carbonate sediment in the Late Cretaceous and Cenozoic. In the Miocene, the rate of subsidence exceeded the rate of sedimentation and, thereafter, the Exmouth Plateau arch and Kangaroo syncline took their present form. Potential petroleum source rocks (especially Paleozoic to Cretaceous shale and siltstone) and reservoir rocks (especially Triassic and Neocomian sandstone) appear to be present. Numerous Triassic-Jurassic fault blocks, and a large Neocomian delta in the south, are obvious exploration targets.