Sedimentary Environment and Organic Matter Enrichment Mechanism of the Silurian Gaojiabian Black Shales in the Lower Yangtze Region, South China

A set of organic-rich black shales has developed in the lower Gaojiabian Formation (Lower Silurian) in the Lower Yangtze region, South China. However, limited research on its paleoenvironment and the mechanisms of organic matter enrichment has hindered further analysis of shale gas exploration prospects in this area. Utilizing samples from the Akidograptus ascensus to Cystograptus vesiculosus graptolite biozones in the Gaojiabian Formation obtained from well SY-1, we analyzed geochemical elements and pyrite framboids to reconstruct paleoenvironmental characteristics and paleoproductivity, revealing factors influencing organic matter enrichment. The results indicate that the total organic carbon (TOC) content and paleoproductivity levels of the shale are both high, with a significant enrichment of redox-sensitive elements. Additionally, the pyrite framboids are well developed, characterized by small particle sizes and a narrow range of variation. This study reveals that during this period, the region generally developed a perennial oxygen minimum zone (P-OMZ) environment, accompanied by transient euxinic conditions during the Akidograptus ascensus and the early stages of Parakidograptus acuminatus and Cystograptus vesiculosus. This situation represents the extension of the Rhuddanian Oceanic Anoxic Event (R-OAE) into the Lower Yangtze region, where the water body exhibited moderate restrictions. Compared to paleoproductivity indicators, there is a stronger positive correlation between TOC and the redox-sensitive elements vanadium (V), molybdenum (Mo), and uranium (U) in the samples. This finding indicates that the P-OMZ and euxinic environments of the Lower Yangtze Sea were the key factors influencing organic matter enrichment in the Akidograptus ascensus to Cystograptus vesiculosus biozones. Consequently, the overall pattern of organic matter enrichment was predominantly determined by preservation conditions.