Stratigraphic and structural architecture across the central North West Shelf – implications for Triassic petroleum systems

Abstract

The Triassic succession in the Roebuck Basin and parts of the adjacent sub-basins is the subject of a regional mapping program at Geoscience Australia that addresses stratigraphic and structural aspects of exploration risk. Seismic horizons of regional significance are integrated into a new tectonostratigraphic framework that also includes lithostratigrapy, gross depositional facies and tectonic extension phases. The main Triassic depocentre extends over the Beagle Sub-basin and north-east Exmouth Plateau, with a re-entrant into the Bedout Sub-basin. A smaller north-east oriented depocentre is located over the outer Rowley Sub-basin and extends into the Barcoo Sub-basin. The Triassic succession thins markedly between these depocentres over a regional palaeogeographic high. Above a basal marine mudstone, the Triassic succession is mainly fluvio-deltaic, but outboard the uppermost Triassic succession it is carbonate-rich. The Rowley depocentre is unusual in that it comprises Lower–Middle Triassic volcanics and Middle–Upper Triassic carbonate-rich facies. Syn-depositional half graben in the Barcoo Sub-basin extend part-way into the adjacent Roebuck Basin. The Roebuck Basin structural architecture is simpler than the neighbouring sub-basins. The contiguous development of fluvio-deltaic facies across the main Triassic depocentre indicates that the Middle Triassic petroleum system may extend beyond the Bedout Sub-basin. Syn-depositional extension may have formed isolated sub-basins favourable for source rock deposition in the Barcoo Sub-basin. The Triassic carbonate play may also be of interest, as carbonate build-ups are locally well imaged in seismic profiles.