The continually-discovered large volumes of marine hydrocarbons indicate a huge exploration potential within the carbonate rocks of Ordovician in the Tarim Basin, northwestern China. The Middle–Upper Ordovician (O) carbonate source rocks are at a highly mature stage. These rocks presently contain a low amount of total organic carbon (TOC ≤0.5%) (TOC means the present-day TOC). A mass balance approach is used to identify the source rocks that have expelled hydrocarbons. The characteristics and origin of hydrocarbon expulsion from low-TOC source rocks are studied, which are significant for oil exploration in the basin. The results showed that a low-TOC O source rock having expelled hydrocarbons was type I and had a very narrow oil window. With a high rock maturity and a high original TOC (TOC) value exceeding 1%, nearly 80% of the generated hydrocarbons were expelled. The content of gammacerane and C steranes in the oil and source rock extracts were relatively lean, with a V-shaped distribution of regular steranes, suggesting a possible genetic relation between the low-TOC source rocks and the marine oil in the Tabei area. Hydrogen-rich planktonic algae or acritarchs were the main hydrocarbon parent materials, distributed in the subsiding platform-slope facies. Organic matter was preserved under reducing conditions, and source rocks were formed with a favorable kerogen type and a moderate hydrocarbon generation potential. This study illustrates the hydrocarbon expulsion of low TOC source rocks in northern Tarim Basin, which is greatly significant for analyzing the genesis of marine crude oil in Tarim Basin, and evaluating the distribution of marine source rocks. This research method is theoretically significant for oil and gas exploration in the same type of highly-mature carbonate paleobasins.

中文翻译：

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塔里木盆地中上奥陶统低有机质碳酸盐岩烃源岩特征及成因

塔里木盆地奥陶系碳酸盐岩不断发现大量海相油气，勘探潜力巨大。中上奥陶统碳酸盐岩烃源岩处于高度成熟阶段。这些岩石目前含有少量的总有机碳（TOC≤0.5%）（TOC是指现在的TOC）。质量平衡方法用于识别排出碳氢化合物的烃源岩。研究低TOC烃源岩排烃特征及成因，对盆地石油勘探具有重要意义。结果表明，排烃的低TOC O烃源岩为I型烃源岩，生油窗口很窄。岩石成熟度高，原始有机碳（TOC）值超过1%，生成的烃类近80%被排出。石油和烃源岩提取物中γ蜡烷和C甾烷含量相对较少，甾烷呈V形分布，表明塔北地区低TOC烃源岩与海相石油可能存在成因关系。富氢浮游藻类或原质是主要的烃母质，分布于沉降台坡相。还原条件下有机质得到保存，烃源岩具有良好的干酪根类型和中等的生烃潜力。该研究揭示了塔里木盆地北部低TOC烃源岩的排烃情况，对于分析塔里木盆地海相原油成因、评价海相烃源岩分布具有重要意义。该研究方法对于同类型高成熟碳酸盐岩古盆地油气勘探具有重要的理论意义。