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

摘要

The third (Es-3) member of the Eocene Shahejie Formation is the main hydrocarbon source rock interval in the Raoyang Sag of the Bohai Bay Basin. The lower Es-3 (Es-3(L)) and upper Es-3 (Es-3(U)) 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 Es-3(L) submember and saline and hydrologically closed lakes with stable water column stratification likely during deposition of the Es-3(U) submember. Carbon isotope values of organic matter, trace elements and biomarker parameters suggest that the Es-3(L) submember had moderate productivity, with a significant contribution from terrigenous organic matter whereas the Es-3(U) 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 Es-3(U) submember were best for preserving organic matter. Comparison of the models of source rock deposition of the Es-3(L) and Es-3(U) submembers indicates that the redox conditions play an important role in the formation of organic-rich source rocks in the Raoyang Sag.