Geochemical characteristics and oil-source correlation of crude oils in 6th and 7th members of Yanchang Formation, Fuxian area, Ordos Basin

The geochemical characteristics and oil-source correlation of crude oils in the sixth (Chang 6) and seventh (Chang 7) members of Yanchang Formation in the Fuxian area of the Ordos Basin were studied based on physical properties, group composition, biomarker compounds and oil-source correlation. The Chang 6 and Chang 7 crude oils in the study area are similar in physical properties, both of which are light with a good fluidity. The average mass fraction of saturated hydrocarbons is 71.46%, the average mass fraction of aromatics is 13.27%, and the contents of non-hydrocarbons and asphaltenes are relatively low. The saturated to aromatics ratios of the crude oils are high, averaging 5.43. The n-alkane distribution is dominated by a single peak at C15 and low carbon numbers. The C30 hopane is prominent in biomarker compounds. The regular sterane distribution is anti-"L" type, that is, dominated by C29. Both of the Chang 6 and Chang 7 crude oils are mature. The sedimentary parent material type is lower hydrobiont and contains some terrestrial higher plants, which are deposited in a weakly reduced freshwater-brine water terrestrial sedimentary environment. The oil-source correlation shows that the Chang 6 and Chang 7 crude oils are closely related to the Chang 73 source rocks in this area; however, the relationship with the dark mudstones in the upper part of Chang 7 and the Chang 73 source rocks in the Zhidan area in the basin center is poor. Therefore, it is believed that the crude oil is mainly from the Chang 73 source rocks in this area.