Seismic Expression of Structural and Stratigraphic Traps Controlling Bitumen Accumulation in the Dahomey Basin, Nigeria

Abstract

The Dahomey Basin in southwestern Nigeria hosts one of Africa’s largest onshore bitumen deposits, largely controlled by structural and stratigraphic traps formed during the basin’s rift–drift evolution. This study applies integrated seismic refraction and Multichannel Analysis of Surface Waves (MASW) techniques to delineate and characterize bitumen-bearing horizons. Fifty-one seismic profiles were acquired along E–W traverses using a 24-geophone spread and processed with SeisImager 2D software for P- and S-wave velocity modeling. This study focuses specifically on the Makun Field, located in the southern sector of the eastern Dahomey Basin, where extensive bitumen seepages and Afowo sandstone exposures occur. The velocity models reveal three distinct lithological units, with P-wave velocities ranging from 300–2800 m/s and S-wave velocities from 86–1890 m/s, enabling discrimination of topsoil, saturated sands, and bituminous sands. Two major bitumen-bearing horizons were mapped, a shallow horizon (2–15 m depth, 6–12 m thick) and a deeper horizon (40–130 m depth, up to 45 m thick). Fault-assisted migration pathways and stratigraphic continuity of Afowo sand bodies sealed by Araromi shales strongly influenced the distribution of these horizons. These results confirm the existence of a functioning petroleum system and highlight exploration zones suitable for both surface mining and thermal in-situ recovery methods. This work improves understanding of the spatial distribution and geometry of bitumen reservoirs, offering a framework for optimized exploration and sustainable resource development.

 Keywords: Bitumen accumulation, seismic refraction, hydrocarbon exploration, velocity modeling.