Origin of Crude Oils in the Lower Yanchang Formation of Yan’an Area in Ordos Basin, China

The clastic rocks of the Yanchang Formation in the Ordos Basin display poor physical properties but are rich in petroleum resources, exhibiting significant exploration potential. However, due to the existence of multiple sets of oil-bearing formations, hydrocarbon-generating formations, and a large longitudinal span, elucidating the correspondence between crude oil and source rocks is vital for further exploration. This study concentrates on the Lower Yanchang Formation of Triassic in the Yan'an area of the Ordos Basin, aiming to perform refined oil-source correlation. The findings indicate that the relative contents of diasteranes [DiaC₂₇βα (20S + 20R), DiaC₂₈βα (20S + 20R), and DiaC₂₉βα (20S + 20R)] are instrumental in categorizing hydrocarbon sources. By examination of the biomarker characteristics of crude oils in the Lower Yanchang Formation, it is detected that there are variations in hydrocarbon source contributions across the Wuqi, Shunning, and Xihekou areas and among different formations. In conclusion, the oils within the lower assembly in the study area primarily originate from a composite source, which includes Chang-7 black mudstone in the center of the lake basin, Chang-9 black mudstone in the Wuqi region, and Chang-9 dark mudstone in other areas. The Chang-7 dark mudstone in other areas plays a minor role in the lower assembly crude oil genesis, while the Chang-10 dark mudstone has a negligible influence on the lower assembly oil source of the Yanchang Formation.