Geochemistry and oil-gas generation modeling of the Late Cretaceous shales from the Chad (Bornu) Basin, Northeast Nigeria

Abstract

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 %Ro 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.