Organic geochemical characteristics of Eocene crude oils from Zhanhua Depression, Bohai Bay Basin, China

Geochemical studies of crude oil and source rock play an important role in future exploration in Zhanhua Depression. In this study, thirty-one oil samples collected from Shahejie Formation in Zhanhua Depression, Bohai Bay Basin, NE China have been geochemically analyzed and their organic geochemical characteristics have been applied to differentiate groups of oils. These oil samples can be classified into two families based on multiple biomarker proxies and stable carbon isotopic values. Family I is characterized by a low ratio of pristane over phytane (Pr/Ph < 0.7), a relatively high ratio of phytane over n-C18 (Ph/n-C18), varying ratios of gammacerane over C30 hopane (Ga/C30H) and C22/C21 tricyclic terpane, and a low ratio of C19/C23 tricyclic terpane. Family II is marked by a relatively high Pr/Ph ratio (0.7–1.6), relative low ratios of Ph/n-C18 and C22/C21 tricyclic terpane, and avarying ratio of C19/C23 tricyclic terpane. Both families I and II within these crude oils can be subdivided into two families based on different values of stable carbon isotopic composition of individual n-alkanes. Moreover, the potential source rocks of oil samples in Family I and Family II were likely derived from the upper Es4 member and Es3 member, respectively, based on the correlation of organic geochemical characteristics of the oils and source rocks. The results of oil–source rock correlation provide insight into the process from oil generation to migration and to final accumulation, providing a better understanding of factors controlling oil–gas distribution for prediction of sweet spots.