Theoretical and Empirical Analysis of Electrical Anisotropy in Niger Delta Sediments: Implications for Resistivity-Based Water Saturation Estimation

Water saturation estimation from borehole resistivity measurements in Niger Delta sandstone reservoirs is subject to systematic errors attributable to electrical anisotropy arising from fine lamination structures at sub-log-resolution scale, yet the theoretical framework for anisotropy correction has not been developed and validated against Niger Delta core data, leading to potential misclassification of thin-bedded pay intervals. This research develops an original theoretical extension of the Thomas-Stieber model for laminated sand-shale systems that accounts for the full anisotropy tensor of Niger Delta thinly bedded sequences, including the contribution of pyrite cement and conductive clay lamina orientations. A new anisotropy correction algorithm derived from the theory is implemented in MATLAB and applied to 12 wells with available resistivity image logs, full-tensor dielectric logs, and core-derived Archie parameters. Comparison of water saturation estimates from uncorrected vertical resistivity logs, the conventional Thomas-Stieber model, and the new anisotropy-corrected model is conducted against core-measured water saturation values from 8 of the 12 wells. The new model reduces the RMS error in water saturation by 34 percent compared to uncorrected log analysis and by 19 percent relative to the conventional Thomas-Stieber approach. The implications for hydrocarbon in-place estimation are substantial: application to two re-evaluated field sectors increases net pay by between 8 and 23 percent after anisotropy correction. The research provides the theoretical and empirical basis for a standardised anisotropy correction workflow for water saturation estimation in Niger Delta thin-bed reservoirs. Keywords: electrical anisotropy, water saturation, Niger Delta, thin beds, Thomas-Stieber model