Cretaceous Source Rocks and Thermal Maturation Patterns in the Nigerian Inland Basins: Implications for Deepwater Exploration

Samuel O. Akande
University of lIorin, lIorin, Nigeria

Development of inland rift basins in Nigeria was consequent to the continental separation of Africa and South America during the Early Cretaceous times. These basins with up to 5000m of Cretaceous sediments are contiguous with those of the East Niger, Chad and the Sudan Republic. In these basins, at least two major petroleum systems: the Lower Cretaceous fluvio-lacustrine and the Upper Cretaceous to Lower Tertiary marine dominated petroleum systems are proven.

A regional study of the source beds by incident light microscopy and geochemical analysis suggest the possibilities of Neocomian to Aptian synrift source beds as components of the petroleum systems in the Dahomey Basin and the Upper Benue Trough. Synrift lacustrine source beds are dominated by oil prone Type II kerogens in the Upper Benue and oil prone Type I assemblages at some intervals in the Dahomey Basin. In contrast, there is a prevalence of gas prone Type III kerogens over the oil prone Types I and II in the Upper Cretaceous source facies suggesting more land derived terrigenous organic matter. Maturity levels of the successions vary, with potential areas for oil and gas generation confined to deeper >2000m levels in coreholes and synclines, and more mature and overmature areas predominantly on the structural highs in the Benue Basins. Hydrocarbons generated from the Cretaceous sources have greater chances of being discovered in the deep offshore Cretaceous and Tertiary reservoirs if sufficient migration pathways were available prior to and during the growth of the Niger delta petroleum system.