Assessment of Hydrocarbon Microseepage Using Soil Gas Survey Over an Onshore Block in the Anambra Basin, South East Nigeria

Surface geochemical prospecting through soil gas analysis provides a cost-effective preliminary screening tool for identifying subsurface hydrocarbon accumulations, and the Anambra Basin in South East Nigeria hosts several unexplored blocks where soil gas surveys have not been applied. This study conducts a soil gas survey over a 50-square-kilometre portion of an onshore block in Imo and Anambra states to detect hydrocarbon microseepage and identify areas warranting deeper geophysical investigation. Soil gas samples were collected from auger holes drilled to 0.8 metres depth at 200 by 200 metre grid nodes, with a total of 125 sample points. Samples were analysed for methane, ethane, propane, and heavier hydrocarbon gas concentrations using portable gas chromatography in a field laboratory. Background methane levels are established from a reference area where surface geology precludes any petroleum accumulation. Anomalous methane concentrations exceeding twice background levels are identified at 22 sample locations clustered in two distinct zones in the northern and central areas of the survey block. Wet gas anomalies characterised by elevated ethane and propane ratios at eight of these locations are considered indicative of thermal maturity in the source rock and suggest a petroleum system is active. The anomaly zones are spatially correlated with structural closures interpreted from available seismic reflection data in the block. Keywords: soil gas survey, hydrocarbon microseepage, Anambra Basin, geochemical prospecting, petroleum system