Molecular Biomarker Constraints on Organic Matter Provenance and Thermal Maturity of the Late Cretaceous Fika and Gongila Shales, Chad Basin, Northeastern Nigeria

Abstract

Previous organic geochemical studies of the hydrocarbon source potential of the Upper Cretaceous Fika and Gongila shales have mainly used bulk geochemical techniques, whereas the contribution of molecular biomarkers to source-rock characterisation remains relatively limited. Therefore, this study integrates total organic carbon (TOC), Rock-Eval pyrolysis, and aliphatic and aromatic biomarker data to determine the source of organic matter, reconstruct depositional conditions, and assess the thermal maturity of shale intervals from the Fika and Gongila Formations penetrated by the Wadi-1 exploratory well. The analysed shales contain relatively low organic carbon contents (TOC <2 wt.%), indicating organic-lean source rocks with poor to fair hydrocarbon-generating potential and a predominance of Type III kerogen. Biomarker distributions reveal distinct differences between the two rock units. The Fika Formation is characterised by mixed organic input, with a relatively greater aquatic contribution, and was deposited under predominantly open-marine conditions with fluctuating oxic to anoxic bottom-water conditions. In contrast, the Gongila Formation contains a stronger terrestrial organic-matter signature, reflecting sediment accumulation in transitional marginal-marine environments under largely anoxic conditions. Thermal-maturity indicators derived from Rock-Eval Tmax values and aromatic-hydrocarbon isomer ratios demonstrate progressive maturation with increasing burial depth. Three maturity zones are recognised within the studied succession: an immature interval in the upper Fika Formation (935-1860 m), a mature interval encompassing the lower Fika and upper Gongila Formations (1860-2240 m), corresponding to the main oil window, and an overmature interval within the deeper Gongila Formation (2240-3190 m), which has reached the dry-gas-generating stage. These findings provide new biomarker constraints on the origin, depositional evolution, and thermal history of the Upper Cretaceous source rocks of the Chad Basin and contribute to a more robust evaluation of their petroleum potential.