No abstract

Triassic petroleum systems on the central North West Shelf Rollet et al.

No 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.

The inboard areas of the Otway Basin, particularly the Shipwreck Trough, are well explored and a petroleum-producing province. However, outboard in water depths greater than 500m, the basin is underexplored with distant well control and sparse 2D reflection seismic data coverage. The presence of a successful petroleum province onshore and in shallow waters raises the question as to whether these plays may extend further outboard into the deep-water areas. In the deep-water area, structural complexity and poor imaging of events in the legacy seismic data have resulted in interpretation uncertainty and consequentially a high-risk profile for explorers. The 2020 Otway Basin seismic program acquired over 7000-line km of 2D reflection seismic data across the deep-water Otway Basin. In addition, over 10000km of legacy 2D seismic data were reprocessed to improve the tie between the inboard wells and the new seismic grid. This new dataset provides the first clear insight into the structural and stratigraphic framework of this frontier area, including better imaging of the sedimentary section and the lower crust, increased structural resolution and improved calibration of the outboard seismic reflectors via ties to the inboard wells. Interpretation of the new data has led to an improved assessment of the structural elements and the extension of regional supersequences into the deep-water areas. These refinements have been used as input into petroleum systems modelling work and will provide a foundation for future work to understand petroleum prospectivity, including the distribution of source, reservoir and seal facies.