Investigating the Accuracy of Formation Evaluation in Low-Contrast Low-Resistivity Pay Zones in the Western Coastal Swamp Depobelt, Delta State, South South Nigeria

Low-contrast low-resistivity pay zones represent a class of hydrocarbon-bearing intervals that are systematically underestimated or misclassified as wet sands by conventional log interpretation workflows, leading to significant missed pay in Niger Delta exploration and appraisal programmes. This study investigates the sources of low-contrast low-resistivity pay character and develops improved formation evaluation approaches for this pay type in the Western Coastal Swamp Depobelt, Delta State. Eighteen low-resistivity pay zones confirmed as productive by well tests or production history were examined using a suite of wireline log data including triple-combo, Nuclear Magnetic Resonance, dipole sonic, and formation microimager logs from six wells. The low resistivity origin was classified as attributable to four mechanisms: conductive clay minerals within sand pore spaces, pyrite microinclusions detected by density-neutron crossplot responses, finely laminated sand-shale sequences below log resolution, and elevated formation water salinity. Bimodal pore system analysis using NMR T2 spectra confirmed moveable hydrocarbons in all confirmed pay zones despite resistivity values as low as 1.8 ohm-metres. An improved evaluation workflow incorporating Dual Water model parameters calibrated to NMR clay-bound water volumes and a lamination correction algorithm using Thomas-Stieber analysis is validated against the confirmed pay zone dataset. Application of the new workflow reduces missed pay volume by 38 percent compared to conventional Archie-based evaluation. Keywords: low-resistivity pay, formation evaluation, Coastal Swamp Depobelt, Delta State, NMR log.