Impact of Paleosalinity, Dilution, Redox, and Paleoproductivity on Organic Matter Enrichment in a Saline Lacustrine Rift Basin: A Case Study of Paleogene Organic-Rich Shale in Dongpu Depression, Bohai Bay Basin, Eastern China

Abstract

Saline, lacustrine rift basins (SLRBs) can contain very productive shale oil plays, in which "sweet spots" are closely related to organic-rich shale intervals. However, organic matter enrichment in the shale intervals within SLRBs and the controlling factors have not been well-explored in past research. In this study, well Pu6-65 within the Dongpu Depression of the Bohai Bay Basin was systematically cored and analyzed for basic geochemistry, trace elements, clay mineral content, and original total organic carbon (TOC) to understand the relationships between TOC and paleosalinity, dilution, redox, and paleoproductivity. Results indicate that the Es 3 U and Es 3 L (upper and lower third member of the Paleogene Shahejie Formation) in the Dongpu Depression mainly developed in a brackish and weak anoxic to anoxic depositional environment, in which surface water featured relatively low paleoproductivity. In comparison, the dilution degree is stronger in the Es 3 L shale. Paleosalinity, dilution, redox, and paleoproductivity together controlled the organic matter enrichment in the Es 3 U and Es 3 L shales, while the paleosalinity, dilution, and redox condition were much more significant controlling factors than the paleoproductivity. The degree of organic matter enrichment first increases and then tends to decrease with increasing paleosalinity; the enrichment degree gradually decreases with the increasing of the dilution degree and increases with the increasing of the anoxia degree. Therefore, future exploration of organic-rich shales in the Dongpu Depression should target shale intervals developed in the environment of moderate salinity, weak dilution, and strong reducibility. This study not only improves the theory of the organic matter enrichment but also provides guidance for predicting the organic-rich shale distribution in the Dongpu Depression and in other SLRBs around the world.

Depositional model for mixed carbonate-clastic sediments in the Middle Cambrian Lower Zhangxia Formation, Xiaweidian, North China

“…However, chemical characteristics have been proven to be a useful tool in differentiating these two depositional environments of limestones. Water circulation on restricted platform is limited by geographic conditions, and thus results in the increasing salinity of water and formation of a reducing environment (Hu et al, 2018). On the contrary, open platform is actually similar to normal carbonate shallow marine, and here, circulation of seawater is unrestricted.…”

Key factors controlling shale oil enrichment in saline lacustrine rift basin: implications from two shale oil wells in Dongpu Depression, Bohai Bay Basin

“…For example, the salinity of the depositional water in Well PS18-8 was much more varying and higher than that of Well PS18-1, resulting in significant variations in the kerogen types in Well PS18-8. Generally, in water with high salinity, the terrestrial higher plant input should be relatively small, while the lower halophilic aquatic organism input should be relatively great (Carbonel 1988;Hu et al 2018b). Therefore, regarding the kerogen types, more shale samples of Well PS18-8 are type I, while few samples from Well PS18-1 are (Fig.…”

Organic-Inorganic Geochemical Characteristics of the Upper Permian Pusige Formation in a High-Saline Lake Basin, Tarim Basin: Implications for Provenance, Paleoenvironments, and Organic Matter Enrichment

“…Another quantitative index for the redox state is the Cu/Zn ratio [77][78][79]. With a decrease in oxygen fugacity, vertical zonation from Cu enrichment towards Zn enrichment can develop in water columns during depositions [79].…”