Analysis of Oil Source of Oil Sands in Yanghugou Formation in Yindongzi Area, Ordos Basin

【Objective】The origin of crude oil in the oil sands of the Carboniferous Yanghugou Formation in the Yin Dongzi Area, southern segment of the western margin of the Ordos Basin, remains controversial. This study aims to investigate the source of crude oil in the Yanghugou Formation oil sands and provide theoretical support for the further advancement of oil and gas exploration in the region.【Methods】Systematic analyses of carbon isotopes and biomarker characteristics were conducted on oil sand and source rock samples from the Taiyuan Formation, Yanghugou Formation, and Wulalike Formation in the study area.【Results】①The Yanghugou Formation oil sands exhibit a distinct UCM hump, and the carbon isotope profiles of the group components show inversion, indicating that the reservoir has undergone biodegradation. ②The UCM hump coexists with intact n-alkanes; the δ¹³C of the asphaltene fraction is from -29.83‰ to -27.52‰, suggesting a mixture of marine crude oil and coal-derived oil; The sterane isomerization parameters C₂₉ββ/(αα+ββ) and C₂₉20S/(20S+20R) range from 0.52 to 0.56 and 0.49 to 0.54, respectively, while R_ob (F₁) ranges from 0.96% to 1.27%, and R_ob (F₂) ranges from 0.86% to 1.08%. These parameters indicate differences in maturity and suggest that the Yanghugou Formation oil sands are a mixture of marine crude oil and coal-derived oil at different maturities. ③Using parameters such as terpane and sterane series compounds, which are less affected by biodegradation, oil sands from the Yanghugou Formation and source rocks were classified under the same maturity conditions. The results suggest that the Yanghugou Formation oil sands are primarily sourced from the Wulalike Formation source rocks, with minor contributions from the Type I coal-measure source rocks in the Yanghugou Formation, as defined in this study.【Conclusions】This study, through carbon isotope and biomarker analyses, concludes that the Yanghugou Formation oil sands are a mixture of marine crude oil from the Wulalike Formation and coal-derived oil from the Yanghugou Formation. The study eliminates the influence of biodegradation, enhancing the reliability of the results and providing new insights for oil and gas exploration in the southern segment of the western margin of the Ordos Basin.