Origin of Silica for the Permian Gufeng Formation and Its Relation to Organic Matter Enrichment in the Lower Yangtze Region: A Case Study of Well GD1

The Lower Permian Gufeng Formation is a significant target for shale gas exploration in the Lower Yangtze region, as well as a favorable source rock for various types of natural gas accumulations. However, the silica origins in its organic-rich shale and chert, and their relation with organic matter enrichment have not been well understood. Based on analyses of X-ray diffraction (XRD), major, trace, and rare earth elements, silicon-oxygen isotopes, and total organic carbon (TOC) from the Gufeng shale and chert samples in the Lower Yangtze region, this study investigates the paleo-marine environment, silica origin, and organic matter enrichment of the Gufeng Formation, with a focus on data from the Well GD1. The results indicate that the Gufeng Formation was deposited in a deep-water shelf environment, forming a stratigraphic sequence that transitions upward from chert to shale, with generally high organic matter content. Intermittent upwelling occurred in the chert interval, which weakened during the shale development stage. The paleoproductivity of the chert interval was higher than that of the shale interval, whereas the shale interval exhibited higher detrital input compared to the chert. The silica in both the chert and shale of the Gufeng Formation has multiple sources, including terrestrial input, biogenic activity, and the transformation of clay minerals. Early transgression and upwelling supplied abundant nutrients, promoting the siliceous organisms proliferation and organic matter enrichment, leading to the formation of the lower chert unit. Subsequent regression and increased terrestrial input caused a lithological transition from chert to shale. The weakening of upwelling and changes in the depositional water environment altered the sources of quartz, resulting in silica in the upper shale interval of the Gufeng Formation being derived from terrestrial input, siliceous organisms and clay mineral transformation. Coupled patterns of silica origin and organic matter accumulation were established for chert and shale lithotypes, providing important guidance for shale gas exploration in the Lower Yangtze region.