Characteristics of Cretaceous Outcrop Shales and Coals from the Nigeria Frontier Basins as Hydrocarbon Source Rocks: Insights from Open System Pyrolysis

Abstract

ABSTRACT The long-term hydrocarbon explorations in the West and Central African Rift Systems (WCARS) basins have given rise to the recent interest shown by local and foreign investors in exploring for hydrocarbons in the Nigerian Frontier Basins. The basins, which include Anambra, Bida, and Dahomey, share similar tectonic histories and are filled with other WCARS basins. In the present study, hydrocarbon generation characteristics of Cretaceous outcrop coals, shales and coaly-shales from Anambra, Bida and Dahomey Basins are investigated. Herein, open system pyrolysis is employed and is complemented with bulk geochemistry and organic petrography. From the results, it was shown that organic matter types in the rocks vary, and they range from type II/III (oil and gas prone) to type III (gas prone). The studied coals and coaly-shale have higher HI (255361 mg HC/g TOC) than the shales (108–273 mg HC/g TOC), and are all thermally immature. The source rocks are dominated by vitrinite/huminite, but with significant liptinite contents (23–40%). Despite the low thermal maturity level of the source rocks, liquid hydrocarbons in the form of exsudatinite have been generated in the Dahomey Basin. Pyrolysis results indicate that as the heating rate increases, the corresponding temperature to the maximum hydrocarbons generated also increases. Also, the faster the pyrolysis heating rate is, the higher the hydrocarbon generation rate. Further, the highest cumulative hydrocarbon yield was obtained at 10 °C/min, and the least was obtained at 50 °C/min. The studied shales have a narrow distribution of activation energies compared to coals. The differences in the activation energy distributions in the source rocks are primarily controlled by the nature of the contributing source materials (i.e., macerals). In addition, the shift in activation energies in the Dahomey source rock towards low values indicates earlier hydrocarbon generation compared to others. The lower hydrocarbon generation temperatures and faster kerogen transformation rate in the Dahomey shale are proven by the presence of exsudatinite. Based on the HI, liptinite contents, and coal ranks, the coals have the potential to generate liquid hydrocarbons. The study will open up new exploration directions in the Nigerian Frontier Basins and contribute to the existing knowledge in the WCARS basins.