Effects of paleosedimentary environment on organic matter enrichment in a saline lacustrine rift basin - A case study of Paleogene source rock in the Dongpu Depression, Bohai Bay Basin

Abstract Organic-rich shale, the third member of the PaleogeneShahejie Formation (Es3), is considered as an excellent lacustrine source rock in the Bohai Bay Basin. However, studies on how paleosedimentary environments affect organic matter enrichment are still lacking. In this study, cores of the Es3 shale in the Dongpu Depression (DD) nearly 200 m in length were collected at well Pu 115 to investigate organic matter enrichment mechanism. Results suggest that the Es3 shale was deposited under reductive conditions based on the redox index (V/(V + Ni)) and the large amount of pyrite. The Mg/Sr and Fe/Mn ratios indicate a generally arid climate with a frequently changing paleowater depth. The paleosalinity analyses indicate the Es3 shale developed under fresh-brackish conditions. The Ba content ranges from 75 to 435 ppm (avg. 284 ppm), showing a moderate paleoproductivity. Correlation analyses among the total organic carbon (TOC) and major and trace element ratios show that the TOC rises with the increasing water reduction degree. With increasing water paleosalinity, the TOC increases initially and then decreases, reaching a maximum TOC at a paleosalinity of 11‰. In addition, the TOC increases initially and then decreases with increasing detrital input, indicating that a moderate detrital input would be beneficial for organic matter enrichment. The redox conditions, paleosalinity and quantity of detrital input are the key factors controlling the organic matter enrichment. Further analyses show that the redox conditions were controlled by the paleoclimate, and the paleosalinity and terrestrial detrital input quantity were affected by the paleowater depth. The root factors controlling the organic matter enrichment are paleoclimate and paleowater depth in the saline lacustrine rift basin.