Depth to basement estimation from aerogravity data over the Southeastern part of Niger Delta region of Nigeria

Abstract

This study presents a geological interpretation of aerogravity anomalies over the southeastern part of the Niger Delta Basin, Nigeria, with a focus on identifying subsurface structures related to hydrocarbon and mineral prospecting. High-resolution aerogravity data, collected by Fugro Airborne Surveys Canada between 2005 and 2010, were processed using Oasis Montaj (v7.0.1), Surfer (v16.6), and ArcGIS (v10.4) software. The study employed various data filtering and enhancement techniques, including Fast Fourier Transform (FFT) for spectral analysis, Euler deconvolution, and 2D forward modeling. The Butterworth filter, with a central wave number of 1900 and degree 2, was used to separate regional and residual anomalies. The regional gravity anomaly map exhibited a Bouguer gravity range from -7.3 mGal to 39.4 mGal, with prominent trends in NE-SW, N-S, and NW-SE directions, indicating deep-seated subsurface structures. The residual gravity map, filtered with a central wave number of 3 and degree 8, revealed structural features including linear, ellipsoidal, and irregular-shaped anomalies extending primarily in the N-S, NE-SW, and NW-SE directions. Euler deconvolution analysis showed depth ranges from a minimum of less than 34.4 m to a maximum of over 8,228 m, identifying subsurface fault structures aligned with the prevailing lineament patterns of the Niger Delta. Radial power spectrum analysis, applied to eight subdivided blocks of the study area, identified three depth ranges: shallow, intermediate, and deep sources. The 2D forward modeling using the GM-SYS package delineated the basement topography and subsurface structures across five key profiles, revealing undulating basement features with depths varying from approximately 6,000 m to over 10,000 m. Significant geological features were identified, including a major dyke intrusion along Profile 1 and domal uplift features along Profile 2, suggesting mantle plume activity. The results indicate a complex subsurface architecture characterized by variations in rock density and structural configurations, suggesting the presence of geological conditions favorable for hydrocarbon accumulation and mineral deposits. Regions with high-frequency anomalies, characterized by the concentration of short wavelength anomalies, such as the Oban Massif, are associated with dense rock formations like granite, granodiorite, gneiss and migmatite and may contain valuable minerals and rare earth elements. The high-frequency anomalies around Ikot Ekpene, Aba, and Degema areas may be due to high density materials of lateral extent.  Overall, the study demonstrates the effectiveness of using Bouguer gravity data in subsurface geological mapping and resource exploration. Blocks D1-D8 are recommended for further investigation for hydrocarbon exploration because they exhibit depths greater than 4000 m.