The lower part of the first member of the Shahejie formation (Es1x) as a source rock for oil found in Lixian Slope, Raoyang Sag, Bohai Bay Basin, Northern China

Abstract

The Es1x sub-member (lower part of the first member of Shahejie Formation), which consists mainly of dark oil shale and mudstone, is well developed in Lixian Slope, Raoyang Sag, Bohai Bay Basin, northern China. The samples from Es1x sub-member were subjected to kerogen petrographic microscope, Rock-Eval, extractable organic material (EOM), and vitrinite reflectance (R o) analyses, respectively. In addition, some samples respectively from the Es3x and Es1x source rocks as well as six crude oil samples from Shahejie Formation of Lixian Slope were subjected to gas chromatography–mass spectrometry (GC–MS) analyses. Based on these geochemical analyses and petrographic analyses, this study provides an overview on the type, richness, thermal maturity, and hydrocarbon generation potential of organic matter in Es1x sub-member, as well as a successful oil–source correlation in Lixian Slope. The total organic carbon (TOC) content of Es1x sub-member samples varies from 0.07 to 6.20%, suggesting that this interval is heterogeneous but contains excellent source rocks. This is further corroborated by hydrocarbon yields (S 2) and extractable organic matter (EOM). Both maceral composition and HI values indicate that organic matter is mostly derived from aquatic organisms. This is supported by n-alkane-isoprenoid ratios (pristane/n-C17 alkane (Pr/n-C17) versus phytane/n-C18 alkane (Ph/n-C18)). The microscope of kerogen in Es1x sub-member samples shows that the mixed-type I/III kerogen is dominant. This is supported by the hydrogen index (HI) values of Es1x sub-member samples. Vitrinite reflectance (R o) values of Es1x source rock samples are between 0.32 and 0.97%, with most of them being lower than 0.7% indicating immature to low maturity levels. T max and production index (PI) values are in line with R o value-based maturity assignments. The n-alkanes and biomarker distributions in crude oils and Es1x source rock samples indicate presence of low-maturity oils derived from organic matter most likely deposited under anoxic, strongly reducing conditions in a lacustrine, brackish to saline/hypersaline water environment. This similarity suggests that crude oils of the Lixian Slope were predominantly sourced from the Es1x sub-member. Es3x (lower part of the third member of Shahejie Formation) source rock samples exhibit different properties that are more in line with freshwater depositional environments.