Geochemical characteristics of marine shale oil and oil-source correlations in Ordos Basin

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.