Petroleum Generative Potentials of the Upper Cretaceous Sediments within Afikpo Syncline, Southeastern Nigeria

Abstract

The present study assessed the petroleum potential of the upper Cretaceous stratigraphic interval within the Afikpo Basin. This basin has remained under-investigated relative to the neighbouring Anambra Basin and the prolific Niger Delta. Sixteen shale samples obtained from the stratigraphic interval were subjected to Total Organic Carbon determination (TOC) and Rock-Eval pyrolysis. The TOC values for all the samples range from 0.97 to 13.80 wt% (an average of 4.14 wt%). Almost all the samples have TOC values above 2.0 wt%, indicating a very good to excellent level of organic enrichment for hydrocarbon generation. The genetic potential (S1 + S2) of the shales ranges from 0.47 to 8.26 mg HC/g rock. This suggests that the samples have poor to excellent generative potential for hydrocarbons. Meanwhile, the bivariate plot of S2 against TOC shows that most of the samples analysed have a poor to fair level of petroleum generation, except for samples OK-4 and OK-7 that exhibit good potential for hydrocarbon generation. Furthermore, HI for the shales ranges from 16 to 101 mgHC/g TOC, with an average of 36.25 mgHC/g TOC. Thirteen samples have HI below 50 mgHC/gTOC, suggesting type IV kerogen, while three samples have HI above 50 mgHC/gTOC, suggesting type III gas-prone kerogen. The samples are in Type III gas-prone and Type IV kerogen, as shown by the plots of the hydrogen index against thermal maturity. Type IV kerogen doesn't hold hydrocarbons and is mostly inert organic matter in the form of polycyclic aromatic hydrocarbons. The thermal maturity (Tmax) values of the shales range from 401 to 429°C (mean = 410°C), indicating that the shales are thermally immature. The plots of Tmax against HI further affirm thermal immaturity. In conclusion, shales within the study area have good organic matter, poor generative potential, and are thermally immature.