Abstract A comprehensive geochemical and petrographic study was performed on 23 clay-rich shale samples collected from 18 wells drilled in the western part of the Ordos Basin to investigate the characteristics and thermal maturity of organic matter as well as the paleo-depositional environment of the Chang 7 Member within the Triassic Yanchang Formation. The samples are characterized by moderate to very high organic carbon (TOC) contents ranging from 0.24% to more than 24.83% (2.72% on average) and total sulfur (TS) contents of 0.06% to 6.51%. Rock-Eval S1 and S2 values range from 0.13 to 3.70 and 0.62 mg HC/g rock to 92.50 mg HC/g rock, respectively, implying good to very good hydrocarbon generation potential for most of the samples. Rock-Eval data and petrographical observations reveal that the kerogen is mainly composed of type II to type III, consisting of a mixture of aquatic and terrigenous organic matter. The aliphatic fractions show variable monomodal to bimodal distributions of normal alkanes. Different parameters such as TOC/TS and molecular biomarker data imply that the Chang 7 member is a typical clay-rich source rock deposited in a sulphate-poor lacustrine to fluvio-deltaic environment under oxic to sub-oxic conditions with substantial input of organic matter derived from higher land plants, mainly conifers. Low values of gammacerane further indicate a low salinity of the paleo-lake. Average Tmax values of 445 °C, vitrinite reflectance values between 0.68% and 0.88% biomarker parameters, such as sterane ratios of C29 ααα 20S/(20S + 20R) and C29 ββ/(ββ + αα) and maturity indices based on aromatic compounds, indicate a thermal maturity within the oil window for the studied samples, showing some regional variability within the western Ordos Basin.

中文翻译：

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鄂尔多斯盆地中西部延长组长七段古沉积环境、成因及有机质特征

摘要 对鄂尔多斯盆地西部 18 口井采集的 23 个富黏土页岩样品进行综合地球化学和岩相研究，研究了鄂尔多斯盆地西部有机质特征、热成熟度和古沉积环境。三叠系延长组7段。样品的特点是有机碳 (TOC) 含量从 0.24% 到超过 24.83%（平均为 2.72%）和总硫 (TS) 含量为 0.06% 到 6.51%。Rock-Eval S1 和 S2 值的范围分别为 0.13 至 3.70 和 0.62 mg HC/g 岩石至 92.50 mg HC/g 岩石，这意味着大多数样品具有良好到非常好的生烃潜力。Rock-Eval资料和岩相观测表明干酪根主要由II型至III型组成，由水生和陆源有机物质的混合物组成。脂肪族馏分显示正构烷烃的可变单峰至双峰分布。TOC/TS 和分子生物标志物数据等不同参数表明，长 7 段是典型的富粘土烃源岩，沉积在贫硫酸盐湖相至河流三角洲环境中，在含氧到亚氧条件下，有机质大量输入来源于高等陆生植物，主要​​是针叶树。γcerane 的低值进一步表明古湖的盐度低。445 °C 的平均 Tmax 值，镜质体反射率值介于 0.68% 和 0.88% 之间的生物标志物参数，例如 C29 ααα 20S/(20S + 20R) 和 C29 ββ/(ββ + αα) 的甾烷比率以及基于芳香族化合物的成熟度指数,