Evolution of Paleoenvironment and Mechanism of Organic Matter Enrichment of the Datangpo Formation in Northeast Guizhou

Key words: 
• Cryogenian /
• Datangpo Formation /
• Paleoclimate /
• Redox /
• Primary Productivity /
• Organic Matter Enrichment

Abstract: [Objective] The sedimentary environment evolution and organic matter enrichment mechanisms of the Datangpo Formation in northeastern Guizhou lack systematic investigation. This study aims to reconstruct the paleoenvironmental evolution sequence and clarify the main controlling factors of organic matter enrichment through high-resolution geochemical analysis. [Methods] A total of 43 core samples from the Datangpo Formation in Well KZ8 in northeastern Guizhou were systematically analyzed for total organic carbon (TOC) and major and trace element geochemistry. The samples cover Member I (manganese carbonate rocks and manganese-bearing shales), Member II (black shales), and Member III (grey shales). [Results] The study reveals that Member I of the Datangpo Formation was deposited under a background of limited post-glacial climatic warming, within a moderately to strongly restricted basin. This interval is characterized by high-frequency alternations between ferruginous and sulfidic reducing conditions, accompanied by significant hydrothermal activity and intermittent marine incursions. The relatively high TOC content in the manganese carbonate sub-member was primarily controlled by high productivity driven by hydrothermal and seawater-derived nutrient inputs. In contrast, productivity in the manganese-bearing shales sub-member declined due to limited nutrient supply, yet its TOC content did not decrease significantly owing to effective preservation under sulfidic conditions. Additionally, some organic matter in Member I was consumed during manganese mineralization. Member II was deposited in a strongly restricted environment under warm climatic conditions, featuring persistent sulfidic reduction and high terrigenous input. The efficient coupling between productivity sustained by enhanced terrigenous nutrients and superior preservation conditions within the strongly restricted sulfidic water column led to peak TOC content. Member III, however, exhibits very low TOC content due to collapsed productivity and completely oxic water conditions. [Conclusion] This study clarifies the pattern of organic matter enrichment mechanisms co-controlled by “productivity-preservation conditions” and their evolution alongside environmental changes in the study area, providing direct insights for regional shale gas exploration. These findings are based on data from a single well; thus, their regional representativeness requires further verification through additional drilling. Moreover, the quantitative contributions of various environmental factors to TOC have not been precisely determined. Future work should focus on establishing a more universal enrichment model.