Geochemical characteristics of crude oils in the Zhijing area, Ordos Basin:Implication for oil accumulation in the Yan’an Formation

Geochemical research on the crude oils in the strata shallower than the Chang-6 member of Yanchang Formation in the Zhidan-Jingbian area was carried out using the full hydrocarbon gas chromatography and saturated hydrocarbon chromatography-mass spectrometry. Our results show that the source-related indexes of these oils are similar, which suggest aparent material imported by mixed organic matter and a sedimentary environment of source rocks with weak oxidation and reduction and poor water column stratification. Their maturity indexes are also similar and indicate that all of them are mature oils. However, the distribution characteristics of n-alkanes and phase fractionation indexes are obviously different between the oils in the Yan’an and Yanchang formations. The former were characterized by the disorderly distribution of n-alkanes below C₁₈, "dislocation" occurring at C₁₁, and the values of toluene/n-heptane generally less than 0.2. The latter were characterized by the relatively regular distribution of normal alkanes and the values of toluene/n-heptane generally greater than 0.2. In Yan’an Formation, the maturities of crude oils near the ancient river channel are generally lower than those far from the ancient river channel, and the oil maturity distribution on the plane is very disorderly. According to the analysis, the studied oils in the area are expected to originate from the Chang 7 source rocks in the central basin. The abnormal n-alkanes distribution of the oils in Yan’an Formation is related to the separation and mixing of light-heavy components of oils during the vertical migration along faults/fractures. The oils from the Chang 7 source rocks first migrated laterally through sand body towards the northeast within Yanchang Formation, and then migrated vertically along faults/fractures and charged into Yan’an Formation. The distribution of oil reservoirs within Yan’an Formation is controlled by the distribution of oil-migration-related faults/fractures, which is the cause that oil reservoirs are still developed in Yan’an Formation far away from the ancient rivers.