Rock physics-driven quantitative seismic reservoir characterization of a tight gas reservoir: a case study from the Lower Indus Basin in Pakistan

The Lower Goru (LG) sandstone formation of Cretaceous age is a widely distributed proven hydrocarbon producing reservoir in the Lower Indus Basin, Pakistan. The tight early Cretaceous LG massive-sand (MAS) sandstone unit has low porosity and low permeability, which makes lithologies and litho-facies discrimination difficult. Based on a rock physics-driven quantitative seismic reservoir characterization approach, we accurately predict lithology and fluid saturations defined through litho-facies image mapping for the MAS sandstone reservoir unit. The workflow starts by preparing data for consistent petrophysical interpretation and rock physics analysis. Rock physics modelling generates a consistent set of elastic logs and establishes the link between reservoir properties and seismic amplitudes via elastic properties. Fluid substitution analysis indicates that gas sand separation from seismic data is difficult to achieve by conventional inversion methods due to very low porosity and stiffness of the rock frame which exhibits an AVO class I response. Therefore, we employed a novel joint impedance and facies inversion (Ji-Fi®) technology and jointly inverted for acoustic impedance and seismic litho-facies images. Ji-Fi produced a significantly better facies (gas sand) image and identified a potential sweet spot for a future appraisal and development well location.