ABSTRACT

The Woodside Energy operated North West Shelf asset covers over 5200 km², with 15 gas fields producing across multiple Triassic to Jurassic aged reservoirs. To support further exploration and infill opportunity maturation, several seismic wave-equation-based amplitude variation with offset (WEB-AVO) studies have been completed, utilising newly reprocessed 3D seismic data. These studies are targeting specific risks, such as reservoir or gas presence, and uncertainties associated with gaswater-contact (GWC) depth and initially in place (GIIP) volume estimates. The WEB-AVO inversion solves directly for the reciprocals of bulk and shear moduli, compressibility (κ) and shear compliance (M), together with density (ρ). Compressibility is very sensitive to porosity, saturation, shale volume and lithology changes when compared to traditional elastic attributes and can be highly effective for addressing GWC uncertainty. A phased approach was employed in each study, initially building a regional rock physics understanding before focussing on specific targets. With WEB-AVO inversion a key initial step is 1D inversion at well control to ensure the robust recovery of the compressibility and shear compliance information required from the seismic data. A 2D arbitrary line WEB-AVO pilot intersecting offset well control was the next stage to characterise the reservoir(s) away from the 1D inversion tests and then optimise inversion parameters spatially. In the final stage a complete 3D inversion for the whole area was completed and this used the parameterisation optimised from the earlier 1D and 2D inversion testing. These products have significantly improved understanding of exploration and infill opportunity risks and volume uncertainties.