THE PETROLEUM GEOLOGY OF THE OUTER DAMPIER SUB-BASIN

Abstract

The Dampier Sub-basin represents the northern part of a depositional downwarp along the Western Australian coast within the greater Carnarvon Basin. The sub-basin can be separated into an inner and outer section by the depositional Lewis Trough, which drilling and seismic results indicate to have been active since at least earliest Jurassic times.

The Dampier Sub-basin originated as an intracratonic depocentre at the end of the Carboniferous and has developed progressively into a marginal basin at the present day. The oldest sediments penetrated to date in the outer area are fluviatile Upper Triassic clastics. Well data have shown that sedimentation continued without a break from the Late Triassic until the late Middle Jurassic, with gradually increasing marine influences. This phase of deposition was terminated by uplift in the Early Callovian, resulting in the emergence of various parts of the basin. These areas were transgressed at different stages, but by the late Early Cretaceous a marine environment was firmly established over the whole region.

Eleven hydrocarbon accumulations have been discovered to date in the Outer Dampier Sub-basin where the primary hydrocarbon generating section is believed to consist of pre- Upper Cretaceous shales, particularly in the Lewis Trough. The feature of major relevance to the petroleum geology is the Rankin Platform where the main discoveries occur in Triassic to Lower Jurassic reservoirs. Trapping is provided primarily by the drape and differential compaction of Cretaceous shales over the pre-tectonic horsts, but the water level in individual fields appears to depend on a combination of both drape and fault trapping. In the Angel Field, on the Madeleine Trend, hydrocarbons occur in Tithonian sands within a fold structure sealed by conformable Cretaceous shales.