Paleoenvironmental Characteristics of Paleogene Lacustrine Source Rocks in the Western Bozhong Sag, Bohai Bay Basin, China: Evidence from Biomarkers, Major and Trace Elements

Abstract

AbstractThe organic matter (OM) enrichment mechanisms and depositional environment characteristics of lacustrine source rocks in western Bozhong Sag, Bohai Bay Basin (BBB) remain controversial. To address these issues, based on Rock‐Eval pyrolysis, kerogen macerals, H/C and O/C ratios, GC‐MS, major and trace elements, the Dongying Mbr 3 (E3d3), the Shahejie Mbrs 1 and 2 (E2s1+2), and the Shahejie Mbr 3 (E2s3) source rocks in the western Bozhong Sag were studied. The above methods were used to reveal their geochemical properties, OM origins and depositional environments. The results indicate that the E2s1+2 and E2s3 are excellent source rocks, and the E3d3 of the second good quality. E3d3 source rocks were formed under a warm and humid climate, mainly belong to fluvial/delta facies, the E3d3 sediments have formed under weakly oxidizing and freshwater conditions. Comparatively, the depositional environments of E2s1+2 source rocks were arid and cold climate, representing saline or freshwater lacustrine facies, and the sediments of E2s1+2 belong to anoxic or suboxic settings with large evaporation and salinity. During the period of E2s3, the climate became warm and humid, indicating the freshwater lacustrine facies, and E2s3 was characterized by freshwater and abundant algae. Moreover, compared with other intervals, the OM origin of E3d3 source rocks has noticeable terrestrial input. The OM origin of the E2s1+2 and E2s3 are mainly plankton and bacteria. Tectonic subsidence and climate change have affected the changes of the depositional environment in the western Bozhong Sag, thus controlling the source rocks distribution, the geochemical characteristics in the three intervals of lacustrine source rocks have distinct differences. Overall, it is effective to evaluate the paleoenvironmental characteristics of source rocks by biomarkers, major and trace elements. The established models may have positive implications for research of lacustrine source rocks in offshore areas with few drillings.