A SUMMARY OF THE PETROLEUM GEOLOGY OF THE CARNARVON BASIN

Abstract

The Carnarvon Basin, an elongate crustal depression along the central West Australian coast line, contains a sedimentary record from the Silurian to the Recent. The southern part of the basin consists mainly of an area of downtaulted Palaeozoic sub-basins. The northern Carnarvon Basin is a major Mesozoic downwarp, containing a thich section of Triassic, Jurassic and Neocomian sediments. None of these is present in the south. All of the northern part of the basin and much of the southern Palaeozoic basins is covered by a blanket of Early Cretaceous shales and Late Cretaceous and Tertiary carbonates.

The Palaeozoic was a time of tectonic stability when shallow marine shelf sedimentation predominated. The major structural trends now observed in the basin were developed during a Jurassic tensional regime. This was related to break-up of the continental margins when block faulting developed around the edges of the Precambrian Shield. Within the resulting Exmouth-Barrow-Dampier downvtarp there accumulated over 8000 m of Jurassic and Neocomian marine and fluvial sediment. During the Aptian, epeiric seas inundated the basin and by the Late Cretaceous, shelf carbonate sedimentation had become established.

Progressive subsidence of the continental shelf has since allowed the accumulation of 3000 m of carbonate sediments along the present-day shelf edge.

The present cycle of petroleum exploration in the basin began in 1952 when West Australian Petroleum Pty Limited (WAPET) was formed to operate on leases held by Ampol Petroleum Limited. A non-commercial oil discovery at Rough Range −1 followed in 1953, but it was not until 1964 that commercial quantities of oil were found at Barrow Island. In July 1971 the Burmah group of companies drilled North Rankin −1 the first of a series of giant gas-condensate discoveries on the Rankin Platform.