Organic Geochemistry, Burial History and HC Generation Modelling of Upper Cretaceous Sediments in the Chad (Bornu) Basin

Abstract

Summary The Upper Cretaceous Gongila and Fika Formation sediments, which are believed to be the major source rocks in the Chad (Bornu) Basin, were analysed using organic geochemistry and petrology. The total organic carbon (TOC) contents of the sediments range from 0.42 to 4.90%. The samples analysed have vitrinite reflectance in the range of 0.58 – 1.39 % Ro and pyrolysis Tmax in the range of 429 – 475 oC indicate that the Gongila and Fika sediments contain mature to late mature organic matter. Moderate oil-generating potential is anticipated from the sediments with fairly high hydrogen indices (150 – 250 mg HC/g TOC). This is supported by their Py-GC (S2) pyrograms with n-alkane/alkene doublets extending beyond n-C30. The sediments are dominated by Type II and Type III kerogen and are thus considered oil and gas prone (mainly gas). One-dimensional basin modelling was performed to analyse the hydrocarbon generation and expulsion history of the Upper Cretaceous sediments in the Chad (Bornu) Basin based on the reconstruction of the burial/thermal maturity histories. This is to improve our understanding of the of hydrocarbon generation potential of the source rocks. Calibration of the model with measured vitrinite reflectance (Ro) and borehole temperature data reveals that the present-day heat flow in the Chad (Bornu) Basin varies from 55.0 mW/m2 to 60.0 mW/m2 and paleo-heat flow value at approximately 68 mW/m2. The source rocks of the Gongila and Fika Formation are presently at a stage of oil, condensates and gas generation with thermal maturity ranging from 0.58% to 1.39% Ro. The modelled burial history also suggest that maximum burial occurred in the late Miocene and that erosion might have been the cause of the thinning of the Tertiary sediments in the present time.