Geoelectrical resistivity imaging of shallow oil sands in the Eastern Dahomey Basin (SW Nigeria): Implication for heavy oil exploration and production

The Eastern Dahomey Basin hosts an extensive reserve of oil sands, a significant portion of which is unexplored and located within the southwestern part of Nigeria. Here, we use detailed outcrop mapping and Electrical Resistivity Tomography (ERT) to investigate the physical and geological characteristics of the oil sands and surrounding lithological units within the Eastern Dahomey Basin in SW Nigeria. The oil sands in the studied outcrop belong to the Turonian to Maastrichtian sandstones of the Afowo Formation which is characterized by active bitumen seepage along the length of the outcrop. The ERT results reveal lateral and vertical variability of the overburden and oil sands units in the study area. The oil sands are characterised by two unique geoelectric anomalies. Each of these anomalies has higher resistivity values (>3713 Ωm) compared to the low resistivity values of the topsoil/laterite overburden (160–2790 Ωm) and the underlying clay dominated interval (160–1576 Ωm). Importantly, the oil sands have variable thicknesses of up to 20 m and are located at shallow depths (~1–5 m below ground level). Hence, the oil sands can be exploited using surface mining techniques. This study provides a detailed assessment into the application of a non-invasive geophysical techniques for characterising oil sands near an active seep system, with implications for drawing up exploitation strategies.