Gongola Basin Crust-Mantle Structural Analysis For Hydrocarbon Investigation Using Isostatic Residual Gravity Anomalies

Abstract

The isostatic residual gravity anomaly data of Gongola basin was computed by removing from the Bouguer gravity anomaly field, a regional gravity field calculated for compensation of masses of surface topography. The regional gravity field was computed using the Airy-Heiskanen isostacy model with compensation at the crust-mantle discontinuity. The free air, residual isostatic anomaly, density distribution maps and 2.5D gravity models were utilized in the definition of the basin’s crust-mantle structures. Results showed that: (1) the free air anomaly depicted the edge effect anomaly which correlates with the slope and rise at the NE zone. (2) The isostatic residual anomaly showed three zones: the overcompensated (NE-SE), undercompensated (NW-SW) while the hydrostatic equilibrium zone was found to be the transition between the undercompensated and the overcompensated zones. The distribution of rock densities within the exposed crust is not unimodal. The main component is the granodioritic ones 2.65 - 2.72g / cm³ consisting of plutonic rocks. The granitic component 2.60 - 2.64g / cm³ is common within the subsurface. The high velocities (6.2km/ s) and the modeled high densities from 2.78 - 2.83g / cm³ suggest that the materials comprises high-grade metamorphic mafic intrusive and volcanic rocks. (3) The change in the crustal thickness along the profiles due to intra sediment and basement intrusive provide a measure of the amount of heating that occurred during rifting. Thus, the gravity across the rift system is found to be due to upwelling of low density hot materials which has also affected the hydrocarbon in place. (4) The crustal thickness together with the depositional architecture (e.g. porosity and permeability) should be used to determine the subsurface fluid circulation in terms of hydrocarbon exploration within the rifted basin. The intrusives are potential targets for solid minerals.

Key word: Bouguer, Airy Heiskanen model, isostatic residual, free air, isostatic regional gravity, 2.5D gravity model.