Environmental reconstruction and formation mechanisms for lacustrine source rocks of the third member of the Shahejie Formation in the Raoyang Sag, Bohai Bay Basin, eastern China

Abstract

The third (Es3) member of the Eocene Shahejie Formation is the main hydrocarbon source rock interval in the Raoyang Sag of the Bohai Bay Basin. The lower Es3 (Es3L) and upper Es3 (Es3U) submembers display clear differences in source rock quality. The formation mechanisms of the source rocks are investigated via geochemical methods using 60 samples from three wells to reconstruct the paleo-environment during deposition. The major element parameters exhibit changes in paleoclimate from humid to arid. Indicators such as Sr/Ba, B/Ga, the gammacerane index and isotopic data suggest fresh–brackish and hydrological open lakes with unstable water column stratification likely occurred during deposition of the Es3L submember and saline and hydrologically closed lakes with stable water column stratification likely during deposition of the Es3U submember. Carbon isotope values of organic matter, trace elements and biomarker parameters suggest that the Es3L submember had moderate productivity, with a significant contribution from terrigenous organic matter whereas the Es3U submember had slightly enhanced productivity, with no or minor contributions from terrigenous organic matter. Furthermore, the pristane/phytane ratio and the enrichment of Mo and U indicate that the euxinic bottom-water conditions (sulfidic) of the lakes during deposition of the Es3U submember were best for preserving organic matter. Comparison of the models of source rock deposition of the Es3L and Es3U submembers indicates that the redox conditions play an important role in the formation of organic-rich source rocks in the Raoyang Sag.