Hydrocarbon potentials of sediments of the Ikom-Mamfe embayment, Southeastern Nigeria and Western Cameroon

Abstract Outcrop mapping, lithofacies and geochemical analyses of sediments of the Ikom-Mamfe Embayment across Nigerian-Cameroon border were carried out to determine the depositional environments and hydrocarbon generating potentials. The standard geochemical analyses carried out on the shale samples are Total Organic Carbon (TOC in wt%), Soluble Organic Matter (SOM) and column chromatographic analysis used to selectively elute the saturates, aromatics and polar components of the SOM. Whole oil analysis (using GC-FID 7890A model) was used to characterize the organic matter. Ten lithofacies identified from exposed sections in the Mamfe Formation were further grouped into four facies association based on their stratigraphic distribution. The four facies associations identified include FA-1, the alluvial-braided multi-storey channel belt complex deposits (making up the major constituent of the Etuko-Okoyong Member – EOM/IKM); FA-2, fluvio-lacustrine (heterolithic) deposits (constituting the Nfaitok Member); FA-3, sandy meandering-braided channels deposits and FA-4, meandering (point bar) fluvio-deposits (both making up the Manyu Member). Results from the geochemical analyses show that TOC range from 0.24 to 4.98% with an average of 1.64% indicating poor to excellent source rocks. The environment of deposition of the source rocks were characterized by anoxic to oxic based on the Pristane/Phytane (Pr/Ph) ratio (0.72–4.38). The high pristane-low phytane content, variable TOC and SOM values alongside the predominance of low to high molecular weight compounds (n-C17 to n-C23), terrigenous aquatic ratio (0.49–0.80) and Carbon Preference Index values (0.68–1.35), all indicate mixed (Type II and III) organic matter source for the shales. The prospect for hydrocarbon generation for the investigated shale units is placed hydrocarbon generation for the shale units investigated exist in the following order of significance: Nfaitok>Etuko-Okoyong>Manyu, with the lacustrine black shales of Nfaitok member being most prolific. Assessment of the maturity parameters (Odd to Even Predominance, Carbon Preference Index and ratio of saturate and aromatics components) indicate immature to marginal mature source rocks with a likelihood of having expelled some hydrocarbons.