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

Zhang Xinshun; Xiao Kunye; Wang Jingchun; Wang Li; Du Yebo; Hu Jie; Hu Ying; Wang Yifan

摘要

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 (R-o), 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).

机译：中非裂缝系统的Doseo抑郁症的特点是反转疲软，脱局和脱裂，这是非洲石油和天然气勘探的重要陆上边境地区。基于近年来钻孔的新井数据，通过分析TOC，热解，Vitrinite反射率（RO）全面研究了Doseo抑郁症中的下白垩纪源岩。通过分析，对有机质类型，丰度，成熟度和形成环境进行了全面研究了），碳同位素和气相色谱 - 质谱（GC-MS）源岩样品，以及发现原油样品的GC-MS实验。此外，通过埋葬历史模拟推断出主要的碳氢化合物产生和排出期。研究结果在三个方面揭示了调查结果。首先，在Doseo凹陷的下白垩统计中有三套源岩，即Doba形成，Kedeni形成和Mangara形成，其中后两个是该地区的主要源岩。 Kedeni形成源岩，平均TOC为3.96％和I型I-II1型的有机物，处于成熟阶段。 Mangara形成源岩，平均TOC为3.52％，II型的有机物质均为成熟至高成熟阶段。其次，所有三组源岩都是低减少和低氧化环境开发的湖泊面。它们的特征通常是甾烷/磁链比和低萜状物/磁链比，并且含有来自混合起源的有机物，包括水生低生物和陆地高等植物。从马加拉形成到kedeni形成，源岩的碳同位素值变得更轻;源自高等植物的有机物表现出降低比例，而衍生自下生物体（例如藻类）的有机物更丰富。发现的原油具有类似的有机物质的特点，它们主要来自Kedeni形成和Mangara形成源岩，在Doseo抑郁症的中心处具有相对高的成熟度。第三，Kedeni形成源岩在抑郁症内的油生成成熟，而Mangara形成源岩在抑郁症的凹陷中心和成熟的油生成中的气体产生高成熟。 Doseo凹陷内的源岩石在110 mA以来的碳氢化合物产生成熟，并且在早期白垩纪（105-95mA）和早期古烯（66-40mA）中展示了两个烃产生和排出峰。

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

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