Palaeogeographic reconstructions: Revealing the lithological variability of the Gage Sandstone and South Perth Shale, Offshore Vlaming Sub-basin

Brief description

The Early Cretaceous Gage Sandstone and South Perth Shale are a prospective reservoir-seal pair in the Vlaming Sub-basin. Plays include post-breakup pinch-outs of the Gage Sandstone against the Valanginian Unconformity with the South Perth Shale forming a top seal. The Gage reservoir has core porosities of 9.5-23.9% and permeabilities from 52-1340 mD. It was deposited in palaeotopographic lows of the Valanginian breakup unconformity and is the lowstand component of the thick deltaic South Perth (SP) Supersequence. To characterise the reservoir-seal pair, a sequence stratigraphic analysis was conducted by integrating 2D seismic interpretation, well log analysis and new biostratigraphic data. Palaeogeographic reconstructions for the SP Supersequence were derived by mapping higher-order prograding packages and documenting changes in relative sea level and sediment supply. High resolution reconstructions of the Gage reservoir were based on seismic facies mapping. The Gage reservoir forms part of a sand-rich submarine fan system and ranges from canyon-confined inner fan deposits to middle fan deposits on a basin plain. Directions of sediment supply are complex, with major sediment contributions from a northern and southern canyon adjacent to the Mandurah Terrace. The characteristics of the SP Supersequence differ between the north and south due to variations in palaeotopography and sediment supply. Palaeogeographic reconstructions for the SP Supersequence portray a complex early post-rift depositional history in the central Vlaming Sub-basin. They reveal a series of regressions and transgressions leading to infilling of the palaeodepression.

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