Organic petrography and trace element geochemistry of organic black shales in the Kribi Campo sub-Basin, West Africa: Implication for petroleum source rock evaluation and depositional environment

Abstract

The aim of this study was to characterise the maceral properties, depositional environment and petroleum source rock properties of black shales deposits of the lower Mundeck Formation of the Douala Kribi Campo sub-basin from organic petrography and trace element geochemical studies. The study showed that the Kribi Campo shales are typically dominated by the liptinite group of macerals, followed by vitrinite and the inertinite group respectively. The liptinite group is primarily represented by lamalginite, bituminite and solid bitumen, while vitrinite consists primarily of collotelinite, vitrodetrinite, and corpogelinite. The inertinite maceral group is the least abundant and is represented by fusinite, semifusinite, pyrolytic carbon, funginite and inertodetrinite. The constituents of the maceral in the shales point to a type II and III kerogen which are proficient of generating oil and gas. Based on vitrinite reflectance, the shales may be classified as being subbituminous A/B to subbituminous and A/high volatile bituminous C in rank. In terms of petroleum production, these shales are regarded as thermally immature with an indicated Tmax temperature (from programmed pyrolysis) of 421 °C.Trace element ratios of Sr/Ba for the studied shales shows values between 0.18 to 0.34 indicating that the shales were deposited in both fresh water and estuary environment. The estuary environment may have resulted from marine contamination to fresh water environment as this is seen only in one shale sample. The V/Ni and V/V+Ni ratio of the studied shales is >1 and ranges from 0.60–0.94 respectively, revealing their deposition in an anoxic environment.