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Late Cretaceous Fika Formation shale was collected and analyzed from six wells in northeastern Nigeria's Chad (Bornu) Basin. The characteristics and the oil and gas-generating potential of the source are discussed based on combined geochemistry investigations and basin models. The abundance of marine phytoplanktonic-bacteria and land-plant suggested by biological markers indicate that the kerogen in the Fika shale Formation is a mixture of Types II and III, which is not compatible with the current low Rock-Eval HI findings of less than 200 mg HC/g TOC. The chemical and optical maturity parameters suggest that the Fika shales throughout the studied wells are in the early maturity stage of the oil window near the gas window point. Therefore, thermal maturation is the critical factor that significantly reduces the original HI values and modifies Types II and III to produce oil and gas. Based on the kinetic database models for Types II and III kerogen mixtures, the initial conversion of kerogen to oil has been occurring since the Late Cretaceous in the Kanadi-1 and Tuma-1 wells and since the Eocene in the Kinasar-1, Kuchalli-1, and Faltu-1 wells with transformation ratios (TR) between 10–31% and 10–50%, respectively. Furthermore, the oil was expelled from the Fika shale source rock in the Kanadi-1 and Tuma-1 wells at the end of Late Cretaceous, with TRs of 65–70%. From that period up to the present, the retained oil was cracked into wet gas in the gas window of 1.10–1.30 Easy %R o with peak TR ratios greater than 65%. The distributions of thermal maturation, conversion ratio, and thickness suggest that the Fika source rock could generate substantial amounts of oil and gas in the southwest part of the basin. Therefore, the southwest part of the Chad (Bornu) Basin is considered a frontier exploration target. • Late Cretaceous Fika shales from six wells in the Chad (Bornu) Basin, were collected and analyzed. • Fika shales contain phytoplanktonic-bacteria and land-plant, consistent with a mixture of Types II and III. • The Fika shales are currently in the early maturity stage of the oil window to near the gas window point. • The southwest part of the Chad (Bornu) Basin is considered a frontier oil and gas exploration targets. [ABSTRACT FROM AUTHOR]

Trace element contents in isolated kerogen from Upper Cretaceous sediments within Gongila and Fika formations in the Chad (Bornu) Basin, northeastern Nigeria were determined using Inductively-coupled plasma mass spectrometer (ICP-MS), in order to infer the origin of the organic matter and the paleo-redox conditions during their sedimentation. The concentrations of the elements in the kerogen samples varied from 1.01 to 24,740 ppm. The distribution of elements shows that Fe is the most abundant element in Chad (Bornu) Basin kerogen, followed by Ce. Among the biophile elements, V is the most abundant, followed by Ni and Co in that order. Statistical evaluation of the elemental composition data shows that As, Ce, Pb, V, Cr, Fe, Co, Ni and U exhibit good positive correlations with each other. Molybdenum, on the other hand displays no obvious correlation with most of the trace elements determined including TOC, but has good positive correlation with TS and negative correlation with Tmax, Ce and Th, which suggests that the concentration of Mo decreases with increasing maturity and vice versa. Some trace element concentrations and their ratios suggest mixed marine and terrigenous source input for the organic matter (kerogen) in Chad (Bornu) Basin. More so, the concentrations of redox-sensitive elements, such as V, Ni, Cu, Cr Mo and Mn, in the kerogen samples suggest dysoxic bottom water conditions within the Gongila and Fika sediments. Cross-plots of V and Ni and V/(V + Ni) ratio also indicate that the organic matter of these samples was deposited in slightly reducing environments. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fika shales were deposited in suboxic marine conditions. [•] Aquatic algae and microorganisms represent major contribution in organic matter with minor terrigenous input. [•] Moderate salinity stratification water conditions are also evident in the Fika shales. [•] An active continental margin tectonic setting for the provenance of the Fika shales. [ABSTRACT FROM AUTHOR]

The shallow marine shales of the Cretaceous formations namely Yolde, Dukul, Jessu, Sekuliye and Numanha ranging in age from Cenomanian to Coniacian within the Yola Sub-basin in the Northern Benue Trough, northeastern Nigeria were analysed to provide an overview on their hydrocarbon generation potential. This study is based on pyrolysis analysis, total organic carbon content (TOC), extractable organic matter (EOM), biomarker distributions and measured vitrinite reflectance. The present-day TOC contents range between 0.24 and 0.71 wt. % and Hydrogen Index (HI) values between 8.7 and 113 mg HC/g TOC with Type III/IV kerogens. Based on the present-day kerogen typing, the shale sediments are expected to generate mainly gas. Biomarker compositions indicates deposition in a marine environment under suboxic conditions with prevalent contribution of aquatic organic matter and a significant amount of terrigenous organic matter input. Organic matter that is dominated by marine components contains kerogens of Type II and Type II-III. This study shows that the organic matter has been affected by volcanic intrusion and consequently, have reached post-mature stage of oil generation. These higher thermal maturities levels are consistent with the vitrinite reflectance ranging from 0.85 to 2.35 R o % and high T max ( 440–508 °C ) values as supported by biomarker maturity ratios. Based on this study, a high prospect for major gas and minor oil generation potential is anticipated from the shallow marine Cretaceous formations from Yola Sub-basin. [ABSTRACT FROM AUTHOR]