Organic geochemical characteristics of the Lower Cretaceous Abu Gabra Formation in the Great Moga oilfield, Muglad Basin, Sudan: Implications for depositional environment and oil-generation potential

摘要

Organic-rich sediments within the Abu Gabra Formation from three wells in the Great Moga oilfield were analyzed using organic geochemistry and organic petrology. The analyzed samples generally contain more than 2.0 wt.% TOC and have a very good to excellent hydrocarbon generative potential. This is supported by high bitumen extract and hydrocarbon (HCs) yields with values exceeding 4000 and 2000 ppm, respectively. The Abu Gabra also have moderate to high hydrogen index (HI) values of 287-865 mg HC/g TOC and large amounts of amorphous organic matter and alginite, consistent with oil-prone Types Ⅰ and Type Ⅱ kerogen. Vitrinite reflectance (0.59-0.72) %Ro and pyrolysis T_(max) (430-438 ℃) indicate an early oil window stage. This is supported by bitumen/TOC ratios (0.04-0.09) and biomarker thermal maturity parameters with equilibrium C_(32) homohopane 22S/(22S + 22R) ratios (0.50-58), moretane/hopane (0.11-018) and C_(29)ββ/(ββ + αα) (0.53-0.73) and 20S/(20S + 20R) ratios (0.26-0.48). The biomarkers are characterized by a dominance of low to medium molecular weight n-alkane compounds with significant waxy alkanes (n-C_(25)-n-C_(34)), moderately high Pr/Ph ratios (1.17-2.51), high abundance of C_(27) regular steranes, high C_(27)/C_(29) regular sterane ratios, the presence of tricyclic terpanes and relatively low sterane/hopane ratios. These data indicate that the organic-rich sediments of the Abu Gabra Formation contain a mixture of aquatic (algal and bacterial) and terrigenous organic matter, deposited in a lacustrine environment and preserved under suboxic conditions.