Petroleum potential of the Gage submarine fan, offshore Perth Basin, Western Australia from paleogeographic mapping and geological modelling
Authors / CoAuthors
The Vlaming Sub-basin is an elongated, north-south trending Mesozoic depocentre in the Perth Basin, Western Australia containing up to 14 km of sediments. A number of potential plays at different stratigraphic levels were identified by the previous exploration. Although potential plays were described for the Early Cretaceous Gage Sandstone overlain by the South Perth Shale seal, they were not successfully explored in any detail. The exploration primarily focused on structural traps in the syn-rift succession due to the assumption that all hydrocarbons were generated prior to or at the time of the breakup. Petroleum systems modelling undertaken in 2007 concluded that some source rocks in the sub-basin reached maturity after the deposition of the South Perth Shale seal, therefore making plays in the post-breakup Gage reservoir prospective. Deposited as a lowstand component of the deltaic South Perth Supersequence, the Gage Lowstand Fan (previously referred to as the Gage Sandstone) infilled paleotopographic lows of the Valanginian breakup unconformity. Characteristics of the reservoir-seal pair were derived from sequence-stratigraphic analysis and seismic facies mapping by integrating 2D seismic interpretation, well log analysis and new biostratigraphic data. Palaeogeographic reconstructions for the South Perth Supersequence reveal a series of regressions and transgressions that infill the paleodepressions. The Gage reservoir is a sand-rich submarine fan system and ranges from canyon-confined inner fan deposits to middle fan deposits on a basin plain. Sand sheets in the distal middle fan and stacked channelized sands in the inner fan may provide an extensive reservoir of good to excellent quality. Characteristics of the South Perth Shale seal vary greatly across the basin and may account for some dry wells. A re-evaluation of the regional seal determined the extent of the pro-delta shale facies within the South Perth Supersequence, which provides an effective seal for the underlying Gage reservoir. 3D geological modelling provided a better understanding of the reservoir heterogeneity. Flow path analysis identified multiple stratigraphic closures at the top of the Gage reservoir, with the most favourable located in the Rottnest Trough.