Abstract:

The Chang 9 Member of Ansai area in Ordos basin has produced high-yield crude oil, which has good exploration prospects. The geochemical characteristics of Chang 9 crude oil and Chang 7 and Chang 9 source rocks in Ansai area were studied by using organic geochemical and biomarker analysis methods, and then oil-source correlation analysis was carried out. The results show that the average Pr/Ph value of Chang 9 crude oil is 1.86, formed in a weak reduction-weak oxidation environment. The ααα-20R C27-C28-C29 sterane configuration is inverted “L” type, indicating a mixed source dominated by terrestrial higher plants. C29sterane  20S/(20S+20R) is 0.48-0.51 and C29 sterane ββ/(αα+ββ) is 0.52-0.53, indicating that the crude oil is mature. The organic matter types of Chang 7 source rocks are mainly Ⅱ-Ⅲ and Ⅲ, deposited in a weak reduction-weak oxidation environment, and generally entered the peak oil generation period. The mass fraction of regular steranes is dominated by C29>C28>C27, indicating that they are closely related to the input of terrestrial higher plants. The organic matter types in Chang 9 are mainly Ⅱ-Ⅲ and Ⅲ, which was deposited in a weak reduction-weak oxidation environment, and has generally entered the peak oil generation period. The content of C29 in regular steranes is the highest, indicating that it is closely related to the terrestrial higher plant composition. According to the oil-source correlation, the crude oil of Chang 9 Member in Ansai area is a mixed source of Chang 7 and Chang 9 source rocks.

Key words: oil-source correlation, geochemistry, biomarker, Yanchang Formation, Ordos basin