15 / 2021-10-24 12:49:17

Origin of Chang 8 Crude Oil from Yanchang Formation in Pingliang Area, Southwestern Margin of Ordos Basin, China

Ordos Basin, origin of crude oil, geochemical characteristics, Chang 8 crude, Chang 7

Pingliang area is located at the southwestern edge of the basin. In recent years, a certain scale of oil reservoirs has been discovered in Chang 8 of Yanchang Formation in the area. In addition, Chang 7 has discovered a black mudstone distribution of 10-30 m. Research on the organic geochemical characteristics of Chang 7 source rocks in Pingliang area revealed that the average TOC of Chang 7 source rocks in the area was 3.23%, the average content of chloroform bitumen "A" was 0.44%, and the abundance of organic matter was high. The identification of the microscopic components of kerogen showed that the types of organic matter were types I and II₁, which had great potential for hydrocarbon generation. However, the vitrinite reflectance measurement shows that Ro is 0.52% on average, and the source rock maturity is low. The characteristics of biomarker compounds of Chang 7 source rock and Chang 8 crude oil were analyzed by gas chromatography-mass spectrometry. The results showed that: Chang 7 source rock has obvious advantages in n-alkanes parity and low maturity, while Chang 8 crude oil has high maturity. The characteristics of steranes and terpanes are the same as those of n-alkanes, and both show that the maturity of Chang 8 crude oil is higher than that of Chang 7 source rock in this area. Therefore, the possibility of Chang 8 crude oil supplying the local Chang 7 source rock is ruled out. By analyzing the geochemical characteristics of Chang 7 source rock in the middle of the basin, it is found that the geochemical characteristics of Chang 8 crude oil in the area are related to it, indicating that the crude oil comes from Chang 7 source rock in the middle of the basin.