Hydrocarbon origin and distribution regularity in Fulin sub-Sag, Zhanhua Sag, Bohai Bay Basin

Based on the studies of biomarker distribution and composition of crude oils in the Fulin sub-Sag of the Zhanhua Sag of the Bohai Bay Basin, there are 3 types of crude oil in the study area. The crude oils of type Ⅰ mainly exist in the east of the sub-sag, characterized by low Pr/Ph ratio, high gammacerence content as well as low diasterane and 4-methyl-sterane contents. They are originated from source rocks in the upper subsection of the 4th member of the Shahejie Formation in the east of the sub-sag. The crude oils of type Ⅱ mainly locate in the center of the sub-sag, featured by high Pr/Ph ratio, low gammacerence content and high diasterane content. The origin for oil and gas is different vertically. Hydrocarbon in the middle and shallow reservoirs is mainly originated from source rocks in the lower subsection of the 3rd member of the Shahejie Formation in the Gunan sub-Sag while hydrocarbon in the deep reservoir is mainly originated from source rocks in the lower subsection of the 3rd member of the Shahejie Formation in the Fulin sub-Sag. The crude oils of type Ⅲ mainly distribute around fault belts in the central sub-sag, featured by low maturity, high Pr/Ph ratio, medium gammacerence content, high C₂₈ sterane content as well as low pregnane and diasterane contents, which show mixed-oil characteristics of source rocks from both the 1st member of the Shahejie Formation in the Gunan sub-Sag and the lower subsection of the 3rd member of the Shahejie Formation in the Fulin sub-Sag.