Accumulation model and geochemistry characteristics of oil occurring from Jurassic coal measures in the Huangling mining area of the Ordos Basin, China

The Ordos Basin is an important intracontinental sedimentary basin in China, containing a significant amount of coal, oil, and natural gas. This study analyzed the sedimentary environment, sedimentary facies, parent material type, maturity, and carbon isotopic composition of the coal-bearing organic matter using gas chromatography–mass spectrometry (GC–MS) and stable isotope ratio mass spectrometry. The source of oil occurring in the No. 2 coal seam of the Jurassic Yan’an Formation (An-1 oil) and its accumulation model were also investigated. The results show that the relative abundances of C 27 , C 28 , and C 29 steranes in the An-1 oil are 43.8%, 33.0%, and 23.2%, respectively. The tricyclic terpanes, C 29 20S/(20S + 20R), and C 29 ββ /( ββ+αα ) contents of the An-1 oil are 31.4%–34.8%, 0.85 and 0.81, respectively. Pr/n-C 17 , Ph/n-C 18 , and Pr/Ph values are 0.34, 0.42, and 0.87, respectively. Biomarker parameters indicate that the An-1 oil mainly comes from the plankton source rock deposited in the freshwater lake facies and a reducing environment, which has evolved to maturity. The correlation of oil-oil indicates that the An-1 oil is homologous to the Chang-7 oil (Chang-7 member of the Triassic Yanchang Formation). The correlation of oil-source rock presents that the An-1 oil is generated from the Yanchang Formation (Chang-6 and Chang-7 source rocks) and occurred in the coal seam during the stage of stratum uplift since the Early of Late Cretaceous. The distribution characteristics of δ 13 C group components in the An-1 oil and Chang-7 oil also reveal the fractionation phenomenon during the migration of crude oil.