Fine-grained carbonate formation and organic matter enrichment in an Eocene saline rift lake (Qianjiang Depression): Constraints from depositional environment and material source

Carbonate-rich fine-grained rocks in saline lakes have become potential targets for shale oil and gas exploration. However, few studies have investigated the processes of carbonate formation and organic matter enrichment in hypersaline environments. The Qianjiang Depression is a hypersaline rift lake basin that contains carbonate-rich fine-grained rocks in the inter-salt strata of the Qianjiang Formation. In this study, sedimentological observations and multiple geochemical analyses were conducted on the inter-salt strata in the sixth and eighteenth rhythmites of the Qianjiang 4 Member. The results showed that the middle inter-salt stratum in the sixth rhythmite is rich in dolomite, calcite, and organic matter, whereas the inter-salt stratum in the eighteenth rhythmite is rich in dolomite and sulfate minerals and low in organic matter. Comparisons among multiple redox parameters revealed that the middle inter-salt stratum in the sixth rhythmite was formed in a deep saline lake, while the inter-salt stratum in the eighteenth rhythmite was formed in a shallow saline lake. Further, sedimentary observations and analyses of δ34S and trace and rare-earth elements revealed that the inter-salt stratum in the sixth rhythmite contained a significant amount of volcanic ash, whereas that in the eighteenth rhythmite was influenced by hot brine derived from older strata. Therefore, the differing environments and material inputs notably affected the carbonate crystallization and organic matter enrichment in these strata. This study suggests that the input of volcanic materials is conducive to improving biological productivity in saline lakes, and anoxic conditions with moderate salinity are conducive to the preservation of organic matter. These findings provide theoretical guidance for future self-sourced inter-salt shale oil reservoir explorations.