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 and 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 the Fika Shale Formation is only partly within it. The “oil window”, deduced from the mean vitrinite reflectance profile, occurs between 1,270m ‐ 2,600m in the Kanadi well, and between 1,985m ‐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.
ABSTRACT. In Niger, uranium occurs in upper Palaeozoic and lower Mesozoic continental sedimentary basins west of the Air mountains, but the source of the uranium had not been identified. Geochemical studies and fissiontrack observations on alkaline ignimbrites preserved in two Palaeozoic anorogenic centres in Ai'r show that uranium is concentrated in the matrix and on secondary iron-oxide coatings surrounding lithic and crystal fragments. Based on variable Th/U ratios and degree of oxidation, it is concluded that the original ignimbrite field was enriched in uranium, but that a considerable proportion was leached during the weathering of the volcanic pile. Tectonic uplift, anorogenic magmatism, followed by weathering and erosion of the volcanic cover, with sedimentation in nearby continental basins, have all contributed to the development of uranium mineralization in Niger. The petrological and geochemical similarities between the Palaeozoic ring complexes in Niger and the Nigerian Mesozoic ring structures suggest that sedimentary uranium deposits may also be found in Nigeria if the tectonic and sedimentological controls were favourable.Enriched concentrations of uranium have been discovered in the exposed granitic roof zones of the Nigerian subvolcanic centres, with Th/U ratios approaching unity. Thus local vein deposits of uraninite, as well as dispersed uranium in recent sedimentary horizons, could be discovered particularly in the drainage systems entering the Chad Basin.
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