Potential for coal-derived gaseous hydrocarbons in the middle Benue Trough of Nigeria

About 6, 000m of alternating marine, paralic, and continental sediments of Cretaceous to Tertiary ages are present in the Benue Trough. Conspicuous amongst these are coal-bearing strata, of which those in the Middle Benue Trough have attained a high- to medium-volatile bituminous rank. Coal beds are now known to be major sources of associated and non-associated gases, and increasingly are becoming exploration targets in many parts of the World. In the Middle Benue Trough, the Turonian Awgu Formation about 400-m thick, is the main coal-bearing unit. Maceral analysis of samples from coal seams and from organic matter-rich interseam sediments obtained from four boreholes and two outcrop sections allowed three petrographic facies to be identified, namely: (1) A vitrinite-fusinite coal facies, which is vitrinite-dominated with minor amounts of liptinite and variable amounts of inertinite; the content of mineral matter is low. (2) A trimaceritic coal facies, which is vitrinite-dominated with moderate amounts of liptinite and inertinite; the content of mineral matter is low. (3) A shaley coal facies,which is dominated by mineral matter with variable amounts of vitrinite, liptinite and inertinite. Vitrinite reflectance values measured on these samples generally increase with depth from 0. 74 - 1.25Rm%. At these values, large amounts of volatile matter ave yielded in the form of methane, carbon dioxide and water during progressive devolatization of humic organic matter. Organic matter in the underlying dark-black, carbonaceous marine shales of the Cenomanian Ezeaku and Albian Arufu/Uomba/Gboko Formations may also bt at the rank of medium-volatile bituminous coal, and probably reaches the low-volatile bituminous stage. These would correspond with the interval of maximum gas generation and the generation of methane, accompanied by strong aromatisation and ring condensation.