Application of gravity method in determination of hydrocarbon potentials in parts of the Anambra Basin, Southeastern Nigeria

Abstract

The renewed quest to boost Nigeria's dwindling reserves through an aggressive search for oil and gas deposits has re-ignited the need to re-evaluate the hydrocarbon potentials of the Anambra Basin. Aerogravity data are a low-cost geophysical tool deployed in mapping regional basement structures and determination of basement depths and sedimentary thickness in frontier basin exploration. In this study, aerogravity data covering some areas of the Anambra Basin, Southeast Nigeria have been quantitatively interpreted using forward and inverse modelling approaches. The absolute deliverables of the study included the determination of the thickness of the sedimentary basin, density contrasts, and the geological models that will provide details on geological structures capable of retaining hydrocarbon. Before interpretation, the zipped data was extracted and enhanced, gridded, and the regional anomalies were isolated from the residual anomaly by a second-order polynomial fit. Edge detection filters precisely the vertical derivative was applied to the Bouguer gravity grid to locate the edges and contacts of geological structures in the basin. Four profiles denoted as Profiles 1, 2, 3, and 4 were selected on the Bouguer gravity contour map based on the contour shapes for modelling. Results from forward and inverse modelling revealed that faulted anticlines were observed in models 1, 3 and 4, while faulted syncline was observed in model 2. The average estimated sedimentary thickness for profiles 1, 2, 3 and 4 were 5.7 km, 8.4 km, 6.3 km, and 2.7 km, with density contrast of anomalous bodies of 0.18 g/cm3, 0.48 g/cm3, 0.84 g/cm3 and 0.17 g/cm3 respectively. In terms of hydrocarbon potentials, these depth ranges and density distribution suggest a promising basin. The results of this study can therefore be used as an affordable and useful decision-making tool to precisely map subsurface features and define sedimentary thicknesses for hydrocarbon potential that can be economically beneficial to increase foreign exchange revenues in the state and federal government