Organic geochemical characteristics of Lower Cretaceous source rocks and crude oils in Doseo Depression of Central African rift system

Abstract The Doseo depression in the Central African Rift System is characterized by weak inversion, strong strike-slip and rifting, which is an important onshore frontier area for oil and gas exploration in Africa. Based on the data of new wells drilled in recent years, the Lower Cretaceous source rocks in the Doseo depression were investigated comprehensively in respect of organic matter type, abundance, maturity and formation environment, through the analysis of TOC, pyrolysis, vitrinite reflectance (Ro), carbon isotope and gas chromatography-mass spectrometry (GC-MS) of source rock samples, together with the GC-MS experiment of discovered crude oil samples. Furthermore, the main hydrocarbon generation and expulsion periods were inferred through burial history simulation. The study results reveal the findings in three aspects. First, there are three sets of source rocks in the Lower Cretaceous of the Doseo depression, i.e. Doba Formation, Kedeni Formation and Mangara Formation, of which the latter two are the main source rocks in the area. The Kedeni Formation source rocks, with the average TOC of 3.96% and the organic matters of type I–II1, are at low-mature to mature stage. The Mangara Formation source rocks, with the average TOC of 3.52% and the organic matters of type II1, are at mature to high-mature stage. Second, all the three sets of source rocks are lacustrine facies developed in low-reducing and low-oxidizing environment. They are typically characterized by low sterane/hopane ratio and low terpane/hopane ratio, and contain organic matters derived from mixed origins, including aquatic lower organisms and terrestrial higher plants. From the Mangara Formation to the Kedeni Formation, the carbon isotope value of source rocks becomes lighter; the organic matters derived from higher plants exhibit a reducing proportion, while the organic matters derived from lower organisms (e.g. algae) are more abundant. The discovered crude oils are found with similar characteristics of organic matters, and they are mainly sourced from the Kedeni Formation and Mangara Formation source rocks with relatively high maturity in the center of the Doseo depression. Third, the Kedeni Formation source rocks are mature for oil generation within the depression, while the Mangara Formation source rocks are high mature for gas generation in the center of the depression and mature for oil generation at the margin of the depression. The source rocks within the Doseo depression became mature for hydrocarbon generation since 110 Ma, and have demonstrated two hydrocarbon generation and expulsion peaks in the early Late Cretaceous (105–95 Ma) and the Early Paleogene (66–40 Ma).