Abstract: A breakthrough has been made in exploration of shale oil in marine strata in the western margin of Ordos Basin. Compared with the publicly reported shale oil layers, the organic carbon content of marine shale is lower in Wulalike Formation. Therefore, it is of great significance to determine whether marine shale oil in Wulalike Formation is generated in situ or originated from external sources. The results show that the marine shale oil of Wulalike Formation has high saturated hydrocarbon content, high ratio of saturated hydrocarbon to aromatic hydrocarbon, and high ∑nC_21-/∑nC₂₂₊, which are 88.35% -93.96%, 9.63-18.57 and 4.51-5.58, respectively. Moreover, the shale oil has a significant advantage from C₂₇ sterane, with a relatively high ratio of tricyclic terpene to hopane (0.89-1.09), a low ratio of hopane to sterane (0.91-1.10), and light carbon isotope of whole oil (-31.8 ‰—-31.6 ‰). In Wulalike Formation, the shale oil shows a positive distribution sequence of carbon isotopes. That is, the carbon isotope values of saturated hydrocarbon (δ¹³C_saturated), aromatic hydrocarbon (δ¹³C_aromatic) and non-hydrocarbon (δ¹³C_non) are shown as δ¹³C_saturated<δ¹³C_aromatic<δ¹³C_non. Additionally, the n-alkanes have light single molecule carbon isotopes, ranging from -35.7 ‰ to -30.4 ‰, which gradually becomes lighter with the increase of carbon number, revealing that the shale oil of Wulalike Formation has obvious marine crude oil characteristics. The biomarker parameters of C₂₉20S/(20S+20R) and C₂₉αββ/(ααα+αββ) for saturated hydrocarbons are 0.37-0.47 and 0.58-0.62, respectively. The equivalent vitrinite reflectance converted based on the methyl phenanthrene ratio F1 and F2 is 0.86%-1.09%, indicating that the shale oil of Wulalike Formation is in the mature stage. The comprehensive analysis of saturated and aromatic biomarkers, carbon isotopes of single molecule hydrocarbons, carbon isotopes of whole oil and maturity show that the Wulalike Formation shale oil is different from Yanghugou Formation marine terrestrial transitional source rocks and Yanchang Formation lacustrine source rocks, but most similar to Wulalike Formation marine shales in terms of geochemical characteristics. Therefore, Wulalike Formation is dominated by a typical self-generating and self-storing shale oil, which is of great significance for exploration of the Paleozoic marine shale oil in Ordos Basin and even in North China. In addition, the study results also demonstrate that shale oil reservoirs can be developed in the distribution area of marine source rocks with low organic matter contents.

Key words: Ordos Basin, Wulalike Formation, marine shale oil, geochemical characteristics, oil-source correlations

摘要： 鄂尔多斯盆地西缘在海相地层中获得了页岩油勘探突破。与公开报道的形成页岩油的烃源岩相比,乌拉力克组海相页岩的有机碳含量偏低,明确海相页岩油是原位生成还是外源运移具有重要意义。研究结果表明:乌拉力克组海相页岩油具有高饱和烃含量(88.35% ~93.96%)、高饱和烃与芳香烃比值(9.63~18.57)、高∑nC_21-/∑nC₂₂₊(4.51~5.58);具有明显的C₂₇甾烷优势,较高的三环萜烷与藿烷比值(0.89~1.09),低的藿烷与甾烷比值(0.91~1.10),较轻的全油碳同位素分布(-31.8 ‰ ~-31.6 ‰)。乌拉力克组页岩油具有正序列的碳同位素分布,即饱和烃碳同位素(δ¹³C_saturated)、芳香烃碳同位素(δ¹³C_aromatic)和非烃碳同位素(δ¹³C_non)表现为δ¹³C_saturated＜δ¹³C_aromatic＜δ¹³C_non,且正构烷烃中单体烃的碳同位素偏轻,为-35.7 ‰ ~-30.4 ‰, 且随碳数增加逐渐变轻, 这揭示出乌拉力克组页岩油具有明显的海相原油特征。页岩油饱和烃中表征成熟度的生物标志化合物参数C₂₉20S/(20S+ 20R)和C₂₉αββ/(ααα＋αββ)分别为0.37~0.47和0.58~0.62,基于芳香烃中甲基菲比值F1和F2换算的等效镜质体反射率为0.86% ~1.09%, 表明乌拉力克组页岩油处于成熟阶段。对生物标志化合物、单体烃和全油碳同位素、成熟度等进行综合分析认为,乌拉力克组页岩油与羊虎沟组海陆过渡相烃源岩、延长组湖相烃源岩的地球化学特征均存在较大差异,而与乌拉力克组海相页岩的地球化学特征最为相似。因此,乌拉力克组页岩油为典型的自生自储型页岩油,这一认识对鄂尔多斯盆地、乃至整个华北地区古生界海相页岩油勘探具有重要意义。此外,研究结果也表明,在低有机质含量的海相烃源岩分布区可以发育页岩油油藏。

关键词: 鄂尔多斯盆地, 乌拉力克组, 海相页岩油, 地球化学特征, 油源对比