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Source Rock Characterisation and Numerical Modelling of Burial and Temperature History of the Upper Cretaceous Sediments, Chad (Bornu) Basin, NE Nigeria

Abstract

The Upper Cretaceous sediments in the Chad (Bornu) Basin were characterised using an integration of geochemical analyses and organic petrography. This is to provide a detailed overview of the characteristics of the sediments as potential source rocks. 1D basin modelling was also carried out to provide information on the uplift and erosion histories of the basin as well as timing of hydrocarbon generation and expulsion. The Gongila and Fika sediments in the Chad (Bornu) Basin have TOC contents up to 2.37 wt.% and Hydrogen Index (HI) values largely below 200 mg HC/g TOC, suggesting that the sediments generally have only fair generative potential and contain Type III and Type IV kerogens. This is evidenced by Previous HitpalynofaciesNext Hit observations whereby the Previous HitkerogenNext Hit is characterised by large amounts of non-fluorescent AOM and phytoclasts. However, pyrolysis GC data indicate that some of the investigated samples, especially from the Fika shale contain mixed Type III/II Previous HitkerogenTop, suggestive of mixed oil and gas potential, although gas being more dominant. The Gongila and Fika sediments have vitrinite reflectance and Tmax values in the range of 0.57–1.79 % and 433–498 °C, respectively, indicating early-mature oil window to late stage of gas window. Detailed geochemical analyses and organic petrography provide evidence for a mixed aquatic algae and terrigenous organic matter input deposited under environmental conditions that were mostly suboxic. Biostratigraphy and isotopic compositions revealed that the Fika sediments were essentially deposited in environments that ranged from coastal deltaic through fluvio-marine to shallow inner neritic. 1D basin modelling revealed that the Upper Cretaceous sediments underwent continuous burial as the early Tertiary sediments accumulate; reaching a maximum burial depth in the late Miocene. The models assume that the heat flow values in the study area vary widely from location to location, probably in response to varying geothermal gradient occasioned by burial depth and proximity to contact aureole. The high heat flow in the basin could be as a result of the West and Central African rifting in the Cretaceous and the Maastrichtian tectonic episode (tensional deformation), which preceded the igneous intrusion. The models also show that the rate of hydrocarbon generation in the Upper Cretaceous sediments fall sharply towards the end of Maastrichtian, probably in response to the Maastrichtian deformation episode.