Palaeogeographic mapping to understand the hydrocarbon and CO2 storage potential of the post-rift Warnbro Group, offshore Vlaming Sub-basin, southern Perth Basin, Australia

Abstract

The Lower Cretaceous Gage Sandstone and South Perth Shale are a prospective reservoir-seal pair in the Warnbro Group, offshore Vlaming Sub-basin, Western Australia. Plays include post-breakup pinch-outs and 4-way dip closures. A sequence stratigraphic analysis incorporating seismic interpretation, well log analysis and new biostratigraphic data characterised the reservoir-seal pair. Palaeogeographic mapping reveals multiple regressive–transgressive cycles which infilled the central palaeodepression on the Valanginian Unconformity. Within the deltaic South Perth Supersequence, the Gage Lowstand Fan (lithostratigraphically referred to as the Gage Sandstone) is a sand-rich submarine fan system and ranges from canyon-confined inner fan to basin–plain middle fan deposits. Major sediment contributions were from north–south-trending canyons adjacent to the Mandurah Terrace. Detailed seismic facies mapping and well log analysis of the Gage Lowstand Fan show that distal middle fan sand sheets and stacked channelised sands in the inner fan may provide an extensive reservoir of good to excellent quality. Seal quality varies greatly and may explain the lack of exploration success at some structural closures. A re-evaluation of the regional seal determined the extent of deepwater shale facies that provides an effective seal for the underlying submarine fans. 3D geological modelling confirms that the reservoir is suitable for hydrocarbon entrapment and CO2 storage. Migration path analysis identified the presence of multiple structural and stratigraphic closures at the top of the reservoir. Previous petroleum systems modelling concluded that some source rock maturation probably post-dates deposition of the effective seal thereby allowing structural closures to be charged with hydrocarbons. Uncertainties potentially comprising hydrocarbon preservation and CO2 storage include reactivation of large syn-rift faults that may breach top seal, the potential absence of base seal, effective seal thickness, and reservoir thickness and quality.