Geochemical Evaluation of Thermal Maturity of the Cretaceous Sediments in the Calabar Flank based on the Distribution of the Polynuclear Aromatic Hydrocarbons

Abstract

The thermal maturity parameter used in the evaluation of the hydrocarbon generative potential of the sedimentary organic matter is based largely on the extent of molecular transformation of the biological markers due to geothermal stress which is a function of depth or burial history. In particular the transformations leading to the formation of Polynuclear Aromatics usually occur at the end of diagenesis up to the onset of catagenesis. . An in-depth study of the transformation leading to the formation of polynuclear aromatic steroidal hydrocarbons revealed a step by step thermally mediated aromatization of the non-aromatic biogenic species during diagenetic/catagenetic evolution of organic matter in an anoxic environment. Since the formation of PAHs is a heat mediated process which increases with depth, then its concentration or the amount per unit mass of the rock sample can be used to estimate the thermal maturity of organic matter in the sediment .In light of the above, this work has evaluated the distribution profile of the PAHs in the Cretaceous sediments of the Calabar Flank using Gas Chromatograph interfaced to a Mass Spectrometer (GC-MS),an analytical tool combination widely used in the study of petroleum geochemistry. The results of the study based on the most dominant polynucear aromatic species namely, anthracene and phenanthrene revealed that Nkporo Shale(NS)samples from various locations in the Basin had a average PAH of 0.05mg/Kg (anthracene) and 0.06mg/Kg (phenanthrene).While the PAH value of Ekenkpon Shale(ES) samples was on the average 0.01mg/Kg and 0.02mg/Kg for anthracene and phenanthrene respectively. New Netim Marl(NNM) however gave the PAH value of only 0.001mg/Kg for phenanthrene and nil for anthracene .On the other hand Awi Sandstone (AS), Mfamosing Limestone (ML) and Nkporo Mudstone(NM) all fell within the non-detectable range. Based on the foregoing, PAH values can be used to evaluate the maturation level of organic matter in the sediments.