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

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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 Roebuck Basin and the adjoining Beagle and Barcoo sub-basins are underexplored areas on Australia’s North West Shelf that are undergoing renewed exploration interest since the discovery of oil at Phoenix South 1 in 2014 and subsequent hydrocarbon discoveries in the Bedout Sub-basin. A well folio of 24 offshore wells across the Beagle, Bedout, Rowley and Barcoo sub-basins has been compiled as part of Geoscience Australia’s hydrocarbon prospectivity assessment across the region. It consists of composite well log plots and well correlations that summarise lithology, lithostratigraphy, Geoscience Australia’s newly acquired biostratigraphic and geochemical data as well as results of petrophysical analysis. A revised sequence-stratigraphic interpretation, key petroleum system elements and drilling results are also documented. The wells dominantly target Triassic shoreward facies (Keraudren Formation) as the primary reservoir objective and Jurassic fluvial-deltaic (Depuch Formation) and/or Lower Cretaceous sandy deltaic facies as the secondary objective. The Keraudren Formation sandstones are sealed intra-formationally either by discontinuous units and/or by the regional Cossigny Member. The Jurassic Depuch Formation sandstones are sealed by regional Lower Cretaceous mudstones. Both charge and structure have been identified as critical issues in the Roebuck Basin. In the Beagle Sub-basin, seal integrity and migration pathways are also considered high risk. Well correlations have identified differences in the basin history and provide insights into the distribution of facies and other characteristics of the Jurassic and Triassic successions.