Textural Indices and Depositional Characteristics of Tar Sand Deposits in the Eastern Dahomey Basin, Nigeria

Background and Objective: The Eastern Dahomey Basin in Southwestern Nigeria holds significant but underexplored bitumen-rich sandstone deposits, particularly in Ondo State. Despite their evident hydrocarbon potential, these formations have received limited attention in terms of reservoir quality assessment. This study investigates the textural properties and depositional environments of bitumen-impregnated sandstones from eight locations, Isowa, Aba Sokoto, Egbe, Agbabu, Ilubirin, Mile 2, Loda, and Ipetu Irele, to evaluate their suitability as hydrocarbon reservoirs. Materials and Methods: Field mapping and sample collection were conducted across the eight sites. Granulometric analyses were carried out to determine particle size distributions, supported by statistical evaluation of grain size parameters including mean size, standard deviation, skewness, and kurtosis. These metrics were employed to infer sediment transport processes and depositional settings. No additional subheadings were used to maintain a concise methodological flow. Results: The analyzed samples are predominantly composed of medium sands (0.9-1.1 φ), moderately to well sorted (σ₁ = 0.7-0.9), with positive skewness (Sk₁ = +0.1 to +0.3) and mesokurtic to slightly leptokurtic kurtosis (KG = 1.0-1.2). These characteristics indicate deposition under moderately high-energy conditions, including deltaic, beach ridge, and shallow marine environments. Uniform sediment properties across the sampled locations suggest a consistent sediment source or stable depositional regime. Notably, surface bitumen seepages and staining, along with interbedded shales acting as seals, indicate hydrocarbon migration and entrapment potential. Conclusion: This study highlights the moderate textural maturity and favorable depositional environments of the eastern Dahomey Basin’s bitumen-rich sandstones, supporting their potential as viable hydrocarbon reservoirs. The presence of sealing shales enhances their reservoir integrity. Future studies should integrate petrophysical and geochemical analyses to further delineate reservoir quality and economic viability.