Rock Physics Modelling for Seismic Reservoir Characterisation in the Agbada Formation

This study develops and applies rock physics models for linking seismic attributes to reservoir properties in the Agbada Formation of the Niger Delta, South South Nigeria. Rock physics modelling provides the quantitative framework needed to translate seismic velocities and impedances into porosity, clay content, fluid saturation, and permeability estimates, enabling the conversion of seismic data into actionable reservoir characterisation products for field development. This study integrates core analysis, petrographic data, and well log measurements from five development wells in a producing onshore Niger Delta field to build and calibrate rock physics models. The Hertz-Mindlin and Biot-Gassmann frameworks are applied to model the velocity-porosity-fluid relationship for clean and shaly sands. Fluid substitution using Gassmann equations predicts seismic response for different fluid scenarios to guide AVO modelling. Empirical rock physics templates are constructed from crossplots of Vp-Vs ratio versus acoustic impedance. Seismic AVO inversion is performed and compared with modelled responses. Findings reveal that Agbada Formation sands show a well-defined rock physics trend with Vp-Vs ratio of 1.75 to 2.05 for gas sands and 2.15 to 2.45 for brine sands, providing discrimination potential in seismic attribute space. Gassmann fluid substitution predicts Class II AVO for the gas-bearing sands, consistent with observed seismic amplitude behaviour. Permeability estimation from resistivity-derived water saturation and porosity log-derived inputs achieves acceptable accuracy against core permeability measurements (R-squared of 0.71). The study provides a calibrated rock physics framework for the Agbada Formation applicable to quantitative seismic interpretation across the Niger Delta.

Keywords: rock physics, seismic reservoir characterisation, Agbada Formation, Gassmann, AVO modelling.