Petroleum geochemistry of the Cretaceous Numanha Shales (Formation), Yola Sub-basin, Northern Benue Trough, NE Nigeria

Abstract

Petroleum geochemical characterisation of the Cretaceous Numanha Shales (Formation) Yola Sub-basin, Northern Benue Trough, northeastern Nigeria has been undertaken to provide an overview on the origin, richness, quality, hydrocarbon generation potential and paleodepositional conditions. The study is based kerogen pyrolysis, biomarkers, palynofacies and vitrinite refelctance. The total organic carbon (TOC) contents is generally below 1% and thus, classified the samples as having fair to relatively good source generative potential. The kerogen is predominantly of Type III and Type IV indicative of mainly gas with limited liquid hydrocarbon generating potential. Vitrinite reflectance, biomarkers and pyrolysis Tmax indicate that the samples are thermally mature and entered early mature to peak oil window stage. Molecular biomarkers indicates that the samples were deposited under suboxic marine environment with a major contribution of aquatic algae and microorganisms and significant amount of terrigenous organic matter input. This suboxic condition is recognized as one of the controlling factor for the organic matter preservation during the sedimentation. High concentrations of regular sterane C27, indicating suboxic conditions, typical of marine environment of deposition. The high thermal maturities experienced by the samples have influenced the nature of the organic matter thereby cracking them into thermogenic gas due to Tertiary volcanic intrusion konwn to be present in the Yola Sub-basin.