Lacustrine conditions control on the distribution of organic-rich source rocks: An instance analysis of the lower 1st member of the Shahejie Formation in the Raoyang Sag, Bohai Bay Basin

Abstract

The lower 1st Member (Es1L) of the Oligocene Shahejie Formation in the Raoyang Sag, the particularly noteworthy source rock interval in the Jizhong Subbasin, were analyzed with sedimentological and geochemical techniques to describe the distribution of organic-rich source rocks and reveal the lacustrine environment and lake types during source rocks deposition. Results of the Raoyang Sag demonstrate that the organic-rich source rocks are mainly distributed in the north-central region with a maximum thickness of up to 120 m in the Maxi Subsag and gradually decreases to the southern tectonic regions such as Liuxi Subsag. These horizontal and vertical heterogeneities are attributed to depositional environments and lake types during the sedimentary period of the Es1L. Lakes in the Raoyang Sag are generally characterized by saline water during the Es1L deposition. The samples representing the south part of the Raoyang Sag display high pristane/phytane (Pr/Ph), C19/C23 TT, and C24 Tet/C26 TT ratios as well as low gammacerane index, 4-methyl sterane index, and sterane/hopane ratio. These findings suggest that hydrological open lake type has unstable water-column stratification, less reducing bottom water, and significant terrigenous supply. However, the samples representing the north-central part of the Raoyang Sag display high gammacerane index, 4-methyl sterane index, and sterane/hopane ratio as well as low Pr/Ph, C19/C23TT, and C24 Tet/C26 TT ratios. A strong correlation exists between δ13C and δ18O, indicating hydrological closed lake-type with excellent water stratification, anoxic bottom water, and enhanced algae productivity. This lake-type is of great significance for continuing research on organic matter enrichment in the Bohai Bay Basin.