Petroleum geochemistry of Cretaceous outcrops from the Calabar Flank, southeastern Nigeria

Abstract

A combined geochemical and molecular characterisation of a wide section of Cretaceous outcrop sedimentary rocks (with no significant effects of weathering) from the Calabar Flank, southeastern Nigeria has been undertaken for petroleum potential evaluation. Rock-Eval pyrolysis and lipid biomarkers show organic matter (OM) to contain varying proportions of marine and continental materials. OM content in the samples is variable. Low values of total organic carbon (TOC) ranging from 0.01to 9.49% with varying extractability (44–4215 ppm), low hydrogen indices (HI = 10–190 mg hydrocarbons (HC)/g TOC, Tmax in the range 414 °C–460 °C, and vitrinite reflectance values from 0.41% to 0.47% Ro were obtained indicating immature to marginally mature terrestrially derived OM of type III kerogen. Awi Formation of Aptian-middle Albian age with an average genetic potential of 4.3 kg HC/ton rock, has the highest potential for oil/gas. 13C/12C ratios of the kerogen, biomarker distribution pattern, and some specific compound ratios (Ts/Tm, oleanane/C30-hopane, C31-22S/22R + 22S homohopane, and moretane/C30-hopane are useful in determining the source and thermal maturity of the OM. The Coniacian–Campanian–Maastrichian black shales of New Netim and Nkporo Formations differ significantly from other formations because of the presence of oleanane which may serve to delineate the late Upper Cretaceous boundary.