Geophysical Mapping of Hydrocarbon Microseepage Anomalies Over Cretaceous Reservoirs in the Anambra Basin, UNN Area

Surface geochemical and geophysical detection of hydrocarbon microseepage provides a low-cost, non-invasive approach for direct hydrocarbon indication in frontier and under-drilled basin settings. This study applies soil gas radon and magnetic susceptibility surveys to map hydrocarbon microseepage anomalies over Cretaceous reservoir sequences in the Anambra Basin around the University of Nigeria, Nsukka area. Radon gas concentrations were measured in soil at 5 m depth along twelve traverses using a RAD7 electronic radon detector at 10 m station spacing. Magnetic susceptibility measurements were simultaneously collected using a KT-10 kappameter to identify diagenetic alteration halos associated with hydrocarbon seepage. Radon anomaly maps reveal linear zones of elevated soil gas concentration with values exceeding background by 150 to 400 percent. These anomalous zones exhibit a spatial coincidence with northeast-trending structural features mapped from aeromagnetic data. Magnetic susceptibility lows (diagenetic chimneys) are identified at the apex of inferred anticlinal closures, consistent with recent models of hydrocarbon-induced magnetic mineral dissolution. The combined microseepage signature defines three priority anomaly clusters that are spatially associated with modeled subsurface closures in the Nkporo Sandstone. Spectral analysis of the anomaly patterns suggests a fault-controlled migration pathway for hydrocarbons from depth. The integrated approach provides a compelling case for targeted seismic acquisition and drilling in the identified anomaly zones. This study contributes a direct hydrocarbon indicator dataset to the Anambra Basin exploration database.

Keywords: hydrocarbon microseepage, radon survey, magnetic susceptibility, Anambra Basin, direct detection