Structural interpretation of the south-western flank of the Anambra Basin (Nigeria) using satellite-derived WGM 2012 gravity data

Abstract

The Anambra Basin in the west of the lower Benue Trough is one of Nigeria's most petroliferous provinces. The subsurface structure has been extensively studied using various geophysical methods except for the mapping of structural lineaments. For the present study, the satellite-derived World Gravity Map 2012 (WGM 2012) global gravity model was utilized to study structural features located at different depths. Upward continuation at the height of 30 km was used to extract the regional anomaly from the Bouguer anomaly. The residual anomaly was determined after the deduction of the regional gravity anomaly from the Bouguer anomaly. The total horizontal derivative (THD) of the complete Bouguer, regional, and residual anomalies was used to map the lineaments. The Bouguer gravity anomaly ranges from −58 to more than +28 mGal and presents two generic trends in the E-W and NNE-SSW directions. The regional Bouguer anomaly reveals a Moho depth of 38.0 km from the CRUST1.0 model. The amplitude pattern of the residual gravity anomaly was used to distinguish seven high residual anomalies (HR1-HR7) and four low residual anomalies (HL1-HL4). The residual anomalies of HR1-HR7 are inferred to be hypabyssal igneous rocks. The low residual anomalies of HL1-HL4 are interpreted to be Paleogene and Cretaceous deposits of the Niger Delta complex, and the Bende-Ameki, Imo, Nsukka, Ajali, Mamu, and Nkporo formations. Based on THD analysis, the major structural trends are found to be in NE-SW and NW-SE directions. In addition, 2.5-dimensional forward modelling using the residual Bouguer anomaly along profile CD was performed, providing a variation of the basement depth between 3.5 km and 6.5 km. Therefore, the sedimentary thickness is adequate for the recommended minimum depth for source rock formation and petroleum accumulation with respect to the other petroleum system elements.