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Source Rock Characteristics, Burial History Reconstruction and Hydrocarbon Generation Modeling of Late Cretaceous Sediments in the Chad (Bornu) Basin, Northeastern Nigeria.

Abstract

The Late Cretaceous Gongila and Fika Formation sediments are believed to be the major source rocks in the Chad (Bornu) Basin. Pyrolysis and open system pyrolysis-gas chromatography (Py-GC) as well as organic petrography and one-dimensional basin modeling were performed on samples from wells located in the basin. The results provide information on organic matter quantity and quality as well as burial and thermal histories of the source rocks in the framework of the basin evolution. This is to improve our understanding of the hydrocarbon generation potential of the source rocks. A total of 127 cutting samples from Kanadi-1, Kemar-1, Kinasar-1, Tuma-1 and Kuchalli-1 wells were pyrolyzed by Weatherford Source Rock Analyzer-TPH/TOC (SRA) instrument. The TOC values of the sequences mainly range from 0.42 to 4.9 wt.%. The average values of Tmax and vitrinite reflectance indicate that samples from the wells have reached maturities corresponding to early to peak oil generation. 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 modeling was performed to analyse the hydrocarbon generation and expulsion history of the sediments in the basin based on the reconstruction of the burial/thermal maturity histories. 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 modeled 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.