Molecular and isotopic signatures of indigenous and non-indigenous hydrocarbons in Campanian sediments from the Tano Basin, Ghana

Abstract

The Campanian source strata of the Tano Basin, Ghana, have garnered exploration interest based on the recent discoveries of hydrocarbon shows in some offshore wells. This study reports on the bulk geochemical composition (Total organic carbon content and Rock-Eval pyrolysis parameters), molecular markers, and stable carbon isotope (δ¹³C) composition of eleven Campanian sediments from TP–1 and Ankobra–1 well, a shallow and deep-water well in the Tano Basin respectively. Based on the analysis of total organic carbon content and Rock-Eval pyrolysis parameters (S1, S2, and Tmax), it was found that the TP-1 Campanian sediments contain indigenous hydrocarbons. Contrariwise, the Campanian samples from the Ankobra-1 well are mainly composed of non-indigenous hydrocarbons, possibly originating from offshore drilling fluid contaminants. Distribution of saturated and aromatic hydrocarbons, including normal alkanes, acyclic isoprenoids, steranes, naphthalenes, phenanthrenes, dibenzothiophene, dibenzofuran, fluorenes, and their alkyl derivatives, as well as stable carbon isotope (δ¹³C) values, were analyzed to gain insight into the biological source input, depositional environment, and thermal maturity of the organic matter (OM) in the sediments. The molecular and isotope results indicated that the OM in the indigenous hydrocarbons is from mixed organic sources, with a predominance of terrestrial plants, whereas lower marine organisms dominate the non-indigenous hydrocarbons. Moreover, the molecular ratios obtained from our analysis indicated that the OM present in the indigenous hydrocarbons was deposited under suboxic conditions in a marginal marine environment. Contrarily, the non-indigenous hydrocarbons exhibit a strong association with marine carbonate facies. Additionally, the maturity assessment based on biomarker ratios, indicated that the OM within the Campanian sediments from TP-1 well is in the early phase of oil generation whereas the OM in the non-indigenous hydrocarbons is in the peak-late mature phase. In conclusion, our geochemical findings have revealed the hydrocarbon potential and heterogeneity of hydrocarbons within the Tano Basin's Campanian source strata induced by non-indigenous hydrocarbons. The geochemical data obtained from our study provide evidence of the potential influence of non-indigenous hydrocarbons on the misrepresentation of source input, depositional environment, and thermal maturity of organic matter (OM) in source rock analysis.