Geologic Framework of Continental Shelf off Northwest Australia

Abstract

Oil exploration on the continental shelf off northwest Australia has been significantly active since oil was discovered at Barrow Island in 1964. The search has accelerated so that sufficient data have been accumulated to allow a reasonable gross analysis of the shelf's geologic framework. Reconnaissance aeromagnetic, gravity, and seismic surveys have provided evidence of substantial thicknesses of sedimentary rocks and revealed the presence of several major structural-depositional features. The geologic significance of these features has been partly rationalized by the integration of well and seismic data, and by interpolation of established onshore geology. Two offshore wells, isolated from geologic control, have provided vital stratigraphic information. Ashmore Reef No. 1 revealed a substantially complete Tertiary-Upper Cretaceous carbonate-clay sequence, a thin Lower Cretaceous-Upper Jurassic section containing detritus from underlying Upper Jurassic basic lavas, and part of a thick Triassic sedimentary sequence. Legendre No. 1 penetrated Tertiary, Cretaceous, and J rassic sedimentary sequences with gross stratigraphic similarities to sequences in parts of the Carnarvon basin; oil was discovered in Lower Cretaceous sandstone. The shelf has a block-faulted basement substructure of Precambrian rocks related in part to the extension of major structure in Precambrian rocks of the northwest Australian mainland. The geologic superstructure of the shelf consists of warped and faulted Phanerozoic sequences which have two major strike components, one subnormal and the other subparallel with the present coastline. The subnormal trend is considered to be related to rejuvenated Precambrian structure and the subparallel trend was superimposed subsequently by later Phanerozoic tectonic events of predominantly noncompressional origin.