Geochemical characteristics of crude oil and oil-source correlation of Shahejie Formation in Linnan Sub-Sag, Huimin Sag, Bohai Bay Basin

The Huimin Sag is a significant petroleum unit in the Jiyang Depression. This work reports a systematic study of the geochemical characteristics of 20 crude oil samples and 4 source rock samples collected from the Linnan Sub-Sag of the Huimin Sag and interpretations of maturity, sedimentary environment and organic matter sources. Oil-source rock relationship was established by using the biomarker parameters of the source rock and crude oil samples. To classify the oils and to interpret the oil-source correlation, various effective parameters were selected and processed using cross-plots and cluster analysis. The results showed that the crude oils have entered the mature stage, with continental-lacustrine-basin hybrid parent materials and a weakly oxidizing to weakly reducing depositional environment. Most of the crude oils had a high content of saturated hydrocarbon, low contents of resins and asphaltene, "platform" bimodal chromatograms of n-alkanes, low content of gammacerane and high content of 4-methyl steranes. According to the differences of the geochemical characteristics of certain areas, crude oils were divided into the south zone oils and the north zone oils. Compared with the south zone oils, the north zone oils possessed lower ratios of 4-methyldecane/C₂₉R and Ts/Tm, and higher ratios of C₂₉ norhopane/C₂₉ Ts, 1,2,7-/1,2,6-trimethylnaphthalene and phenanthrene/∑methyl phenanthrenes. The organic matter type of the source rocks of the fourth member of Shahejie Formation in the Linnan Sub-Sag is type Ⅲ, and the abundance of organic matter is low. The organic matter of the source rocks of the third member of Shahejie Formation is dominated by type Ⅱ, and the abundance of organic matter is high. The lower and middle sections of the third member of Shahejie Formation are the main source rocks of the sub-sag. By systematically comparing the geochemical characteristics of the crude oils and the source rocks, it was found that the crude oils from the third and fourth members of Shahejie Formation of the Linnan Sub-Sag came from the source rocks of the lower and middle sections of the third member of Shahejie Formation.