Geochemical characteristics of Yan 9 crude oil and oil-source correlation in western Jingbian Oil Field, Ordos Basin

The geochemical characteristics of the Yan 9 reservoir were analyzed by means of petrophysical properties, hydrocarbon group composition, saturated hydrocarbon chromatography and gas chromatography-mass spectrometry in order to clarify the source of oil in the Yan 9 reservoir in the west of Jingbian Oil Field. The geochemical characteristics of Chang 7 source rocks in the study area and the Zhidan area were compared, and the source of Yan 9 oil was studied in combination with regional geological background. The results showed that the saturated hydrocarbon content of Yan 9 crude oil is above 60%, and the n-alkanes are distributed with a single mode with C₂₁ as the main carbon peak, C₃₀ hopane as the dominant biomarker, followed by C₂₉ hopane. Regular steroids are mainly C₂₉, while C₂₇ and C₂₈ are similar, showing an asymmetric "V" distribution. The Yan 9 crude oil has similar geochemical characteristics and unified material source and evolution degree. It is a mature crude oil derived from a source deposited in a weak oxidation-reduction environment under freshwater conditions. Oil-source correlation showed that the source rocks of the Chang 7 reservoir in the Jingbian Oil Field are related to the Yan 9 crude oil, and the geochemical characteristics of the Chang 7 source rocks in the Zhidan area, which is located in the center of the lacustrine basin, are obviously different from the Yan 9 crude oil. The formation of Yan 9 oil is mainly due to the maturation of Chang 7 source rocks in this area. The oils were transported by superimposed sand bodies and fracture systems, migrated vertically to the Yan 9 reservoir, and then migrated laterally to traps for accumulation.