Petrophysical Characterisation of Low-Resistivity Pay Zones in the Agbada Formation, Offshore Nigeria

This study investigates the petrophysical characteristics and hydrocarbon evaluation of low-resistivity pay zones in the Agbada Formation of offshore Nigeria, addressing a well-recognised but inadequately resolved petroleum evaluation challenge. Low-resistivity pay occurs in many Agbada Formation reservoirs due to conductive clay distribution, fine-grained laminated sands, and fresh formation water, leading to systematic underestimation of hydrocarbon saturation by conventional Archie analysis. Identifying and correctly evaluating low-resistivity pay is essential for accurate resource calculation and field development decisions in the Niger Delta. This study analyses an integrated dataset from three offshore wells including multi-frequency dielectric, nuclear magnetic resonance, and image logs alongside conventional suites and core data. Thomas-Stieber and Lambda plot methods are applied to identify laminated shaly sand architecture. Modified Archie and Waxman-Smits equations incorporating cation exchange capacity from NMR data improve saturation estimation. Comparison between corrected petrophysical evaluation and core-derived hydrocarbon saturation validates the improved models. Findings reveal that low-resistivity pay intervals of 15 to 45 metres net thickness are present in two of three wells, characterised by deep resistivity below 2 ohm-metres but mobile hydrocarbon on NMR T2 distribution, free fluid indicator data, and oil staining in core. Thomas-Stieber analysis confirms lamination porosity of 24 to 31 percent in these intervals. The corrected Waxman-Smits model improves hydrocarbon pore volume estimates by 35 to 52 percent over Archie. The study contributes an enhanced petrophysical workflow for low-resistivity pay evaluation in the Agbada Formation and recommends mandatory NMR logging in suspected low-resistivity pay intervals.

Keywords: low-resistivity pay, petrophysics, Agbada Formation, NMR log, hydrocarbon saturation.