The Cretaceous series in the Chad Basin, NE Nigeria: Source rock potential and thermal maturity

The Chad Basin is the largest intracratonic basin in Africa, and one-tenth of its surface area lies in NE Nigeria. Thermal maturation (mean vitrinite reflectance) and organic-geochemical (Rock-Eval pyrolysis) studies have been carried out on Cretaceous shales from the Bima Formation, Gongila Formation Fika Shale Formation. Samples were derived from the Kanadi and Albarka exploration wells, which are located about 76 km apart in the Nigerian sector of the Chad Basin. The organic matter is predominantly gas prone (Type III kerogen). Its thermal maturity, assessed from vitrinite reflectance and Tmax, indicates that the Gongila Formation and the Bima Formation are within the ''oil window'', while Fika Shale Formation is only partly within it. The ''oil window'', deduced from the mean vitrinite reflectance profile, occurs between 1,270-2,600m in the Kanadi well, and between 1,985-3,690m in the Albarka well, with respective maturation gradients of 0.52 log%Rm/km (0.41%Rm/km) and 0.41 log%Rm/km (0.34%Rm/km). Eroded thicknesses amount to about 1 km. The upper part of the Fika Shale Formation has the best source-rock potential in terms of its organic carbon content, but has low thermal maturity. The Gongila Formation is poor in organic carbon and its hydrocarbon potential may already have been exhausted. The Bima Formation is well within the ''oil window'', but has only limited potential as a source rock, because of the presence of clastic and inert materials. The potential for gas accumulations, however, exists.