Preliminary source rock evaluation, paleo-depositional environment and hydrocarbon generation potential of the cretaceous organic-rich outcrops of Mayo-Figuil River, Babouri-Figuil Basin, Northern Benue Trough (Yola arm) Cameroon: Insights from bulk geochemistry

Abstract

Babouri-Figuil sedimentary basin is considered the most northerly and one of the most attractive intracontinental Cretaceous basins in North Cameroon, which contains organic-rich deposits presumed to be oil shale levels. The fieldwork carried out as part of this study enabled to delineate six lithological units mainly consisting of black shales, marls, orange clay, black to grey clays and limestones. Twenty-two samples from the studied section were subjected to bulk organic (Rock Eval pyrolysis) and inorganic (biophiles XRF-trace element) geochemistry to investigate their hydrocarbon generation potential. Field data and organic geochemical analysis of organic matter reveal that three of the six lithological units are potential hydrocarbon source rocks; these are units 1 (1.50 ≤ TOC ≤3.98%), 2 (0.47 ≤ TOC ≤1.17%) and 4 (TOC = 24.74%) consisting mainly of black to grey clays and black shales. These source rocks are discontinuously distributed throughout the studied section with a cumulative stratigraphic thickness of 275 cm. The organic matter contained in unit 1 is mainly Type II/III (182 ≤ HI ≤ 382 mg HC/g TOC) with a significant contribution in Type II kerogen. It is mature (443 °C ≤ Tmax ≤448 °C) at the base (MF-1, -2, -3 and -4) and immature (Tmax = 416 °C < 435 °C) at the top (MF-5). Unit 2 contains early-mature type III organic matter (98 ≤ HI ≤ 155 mg HC/g TOC; 438–439 °C). Organic matter in unit 4 is also early-mature and has generated mainly oil (Type I or IIS). The combination of proxies of biophile trace elements (cross plots Ni/Co vs. V/Ni and V vs. Ni) indicates that this organic matter is of mixed source, both marine and terrestrial origin, deposited under oxic-dysoxic conditions. Thus, subject to further geochemical analyses and given the pessimistic predictions from outcrop samples, these units are conventional effective source rocks in place of the early reported immature oil shale levels.