Quantitative Gravity Interpretation for Sediment Thickness Estimation in the Dahomey Basin

This study applies quantitative gravity interpretation methods to estimate sediment thickness and basement geometry in the Dahomey Basin, southwestern Nigeria, providing constraints for petroleum system modelling and resource assessment. The Dahomey Basin is a West African passive margin basin that extends along the Nigerian coast in Lagos and Ogun States. Although relatively small in Nigeria, it contains Cretaceous to Tertiary sedimentary sequences with reported hydrocarbon shows, but its subsurface architecture is incompletely characterised due to sparse well penetration. This study processes and analyses a regional gravity dataset for the Nigerian onshore and offshore Dahomey Basin comprising 245 ground stations and EGM2008 satellite gravity. Standard gravity reductions produce the complete Bouguer anomaly. Isostatic residual anomaly computation removes long-wavelength effects of crustal compensation. Two-dimensional forward modelling constrained by REFLEX seismic profiles and available well tops produces basin geometry models along five profiles. Three-dimensional inversion using Parker-Oldenburg iterative method estimates basin depth across a 15,000 square kilometre area. A density contrast between sedimentary fill and basement of 0.4 grams per cubic centimetre is calibrated using well data. Findings reveal maximum sediment thickness of 8.5 kilometres in the offshore depocenter, thinning rapidly to 1.5 to 3.0 kilometres at the basin margins onshore. A northeast-trending basement ridge separates the main depocentre from a secondary onshore sub-basin. The gravity-derived isopach map provides the structural framework for a petroleum system burial history model indicating moderate maturation of Campanian source rocks in the deepest basin areas. The study recommends acquisition of new 2D seismic lines over the identified deep sub-basin.

Keywords: gravity interpretation, sediment thickness, Dahomey Basin, basement geometry, petroleum system.