Organic petrography and petroleum source rock evaluation of the Cretaceous Mamfe Formation, Mamfe basin, southwest Cameroon

The Mamfe basin is thought to be a rift splay segment of the southern Benue Trough and is genetically link to the Abakaliki and Anambra sub-basins in Nigeria. In the Mamfe basin very little studies have been done on maceral characterization and hydrocarbon generative potential of the Cretaceous black shale. Organic petrography and Rock–Eval pyrolysis studies were done on the black shale outcropping in the basin. The objectives were to determine the types of macerals present, and their relative abundance as well as their hydrocarbon generation potential; to assess the black shale's source rocks richness and quality; and to evaluate the source rock thermal maturity and the relationship between maturity and hydrocarbon generation. The results of this investigation indicated that the maceral content include relatively low percentages of vitrinite (4.9%), inertinite (3.4%) and scare liptinite (0.5%). The total organic carbon of the samples vary from 0.17 to 4.53 wt%, (mean = 1.11 wt%) indicating poor to good source rocks generative potential while the S1 values ranges from 0.04–1.25 mg HC/g rock, (mean = 0.23 mg HC/g rock), indicating poor–fair source rocks generation potential. Most of the samples are mature-peak mature for gaseous hydrocarbon generation. Based on the organic petrography and geochemical analyses performed on the black shale samples of the Mamfe Formation, they are described as mainly humic type III-reworked/oxidized type IV kerogen. The genetic potential of these source rocks is low and free hydrocarbon are absent. Given the prevalence of type III/IV kerogen, thermal maturity levels and hydrocarbon yield, it can be concluded that the source rocks of the Mamfe Formation have some moderate to fair potential for gaseous hydrocarbon which have not yet been generated at the present outcrop levels.