A Theoretical Framework for Integrated Geophysical-Petrophysical Characterisation of the Deep Sedimentary Basins of Nigeria: Implications for Undiscovered Resource Estimation

Nigeria's undiscovered hydrocarbon and mineral resources are distributed across multiple deep sedimentary basins whose subsurface characterisation remains fundamentally limited by the theoretical disconnect between geophysical observables and petrophysical formation properties, particularly at depths exceeding 4 kilometres where conventional empirical relationships derived from shallower systems lose predictive validity. This research develops and tests an integrated theoretical framework that bridges geophysical inversion outputs with rock physics models specifically calibrated for deep Nigerian basin conditions. The framework synthesises rock physics theories of cementation, diagenesis, and fluid substitution with empirical data from deep exploration wells in the Anambra, Chad, and Niger Delta basins to develop basin-specific transforms between seismic velocities, density, electrical resistivity, and reservoir quality parameters. A Bayesian inversion scheme incorporating the new theoretical transforms is applied to reinterpret deep seismic and gravity data across the three basins. The results fundamentally revise the understanding of deep reservoir quality distribution, demonstrating that diagenetic overprinting modelled through the new framework reduces predicted deep porosity by 18 to 35 percent relative to simple extrapolation of shallow empirical models. Probabilistic resource estimates derived from the framework-constrained subsurface models are compared with estimates from the Nigerian Upstream Petroleum Regulatory Commission to assess the magnitude of upward or downward resource revisions. The research contributes an original, validated theoretical foundation for geophysical characterisation of deep Nigerian basins that addresses a critical gap in the national petroleum system knowledge base. Keywords: rock physics, deep basins, resource estimation, Bayesian inversion, petrophysics