A REVIEW OF THE HYDROCARBON HABITAT OF THE EASTERN AND CENTRAL BARROW — DAMPIER SUB-BASIN, WESTERN AUSTRALIA

Abstract

The most completely known section of the Barrow- Dampier Sub-basin in the northern Carnarvon Basin of the Northwest Shelf comprises three depositional super- cycles spanning the Triassic to the Tertiary. Each cycle is made up of an initial transgressive section of mainly fine-grained clastics overlain by a thick, extensive, off- lapping sequence of coarse-grained deposits. The transgressive sedimentary package typically contains a coarse basal unit overlain by a thick, argillaceous unit, whereas the progradational package changes character in each cycle, representing increasingly open marine conditions as the depocentre and its palaeogeography evolved. Continental siliciciastics at the end of the Triassic Supercycle contrast with the marine-marginal marine siliciciastics at the end of the Jurassic-Neocomian Supercycle and the prograding Tertiary carbonate wedge of the youngest cycle. Each of these gross sequences has a distinctive seismic signature upon which are superimposed stratigraphic features reflecting basin evolution from a broad intra-continental depocentre to a mature, passive continental margin basin.

In the area east of Barrow Island, potential hydrocarbon source rock quality and richness varies between each cycle but potential source beds frequently occur at similar levels within each supercycle. The Dingo Claystone within the Jurassic-Neocomian depositional package contains by far the thickest and most extensive potential sources in the area and is likely to be the source for most of the hydrocarbon liquids discovered to date in the northern Carnarvon Basin (with the probable exclusion of the majority of the Rankin Platform condensates).

The occurrence of oils of mixed composition and considerable variability beneath the Muderong Shale regional seal in areas of low thermal maturity suggests that many of the hydrocarbon liquids have undergone considerable vertical migration and have also a complex genesis. Furthermore, saturate-rich liquid hydrocarbons overprinting an older biodegraded oil are recognised in a number of wells along the basin margin hingeline. The likely migration and entrapment model for the majority of hydrocarbons discovered to date in the area under review involves dynamic charging of reservoirs, mainly during the Tertiary. Two main pulses of generation and migration are recognised in the eastern portion of the sub -basin, and a third phase is probably occuring at present-day, west of Barrow Island.