--> Phoenix South: A New Petroleum System in the Bedout Sub-Basin

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Phoenix South: A New Petroleum System in the Bedout Sub-Basin

Abstract

Drilling of Phoenix South-1 (PS-1) in 2014 in the Bedout Sub-basin, NW Shelf Australia, supports an active oil dominated petroleum system in an area previously thought to be gas prone. Four independent oil columns were encountered in the Lower-Mid Triassic Keraudren Fm over a 315m gross interval, with a combined 151m oil column. PS-1 tested a 4-way dip closure, in an overall fault bounded structure controlled by N-S oriented faults covering an area of ~45km2. Closure to the north and south is dip-controlled, spilling to the north towards the Phoenix-1 well and the undrilled Roc closure to the south. The depositional environment for the Lower Keraudren Fm reservoir sands has been interpreted historically as fluvial channel sands to nearshore marine or even pro-deltaic sequences. Analysis of palynology, XRD, SWCs and FMI logs from the dominantly fine-medium grained sandstone packages in PS-1 indicates a likely non-marine to marginal marine sequence. The hydrocarbon charged interval in PS-1 appears to be sealed by a highly cemented, fine grained clastic zone related to a marine incursion. The individual reservoir units have overlying 20–40m claystones acting as intra-formational seals to the oil columns encountered. Several faults and fractures were recognised on the well FMI image, and together with an uncharacteristically low fracture gradient that may be bounded at the low end by frictional equilibrium conditions, may have implications for development. Analyses of oil samples collected by MDT from three of the reservoir zones in the Keraudren indicate a mixed terrestrial-marginal marine source facies. Isotope characterization of the oils and related gases point to a complex maturity and migration story for the different reservoirs. Comparison of the various geochemical characteristics of the oils, including biomarkers and isotopes, to published data suggests that the source rock is not related to known petroleum systems on the North West Shelf. Although the proven Keraudren will be the focus of future regional exploration and drilling, several other plays are recognised within the sub-basin that have the potential to be charged by the same source, including the Permian and Lower to Middle Jurassic. The recent discovery at Phoenix South-1 has highlighted the potential for non-traditional oil plays in deeper Triassic reservoirs to be effective and shows the potential for other sparsely explored areas of the North West Shelf to rise from the ashes of obscurity.