Unravelling organic rich shale formation using geochemical proxies in the Doushantuo Formation, South China

Abstract

The black shale of the Doushantuo Formation provides important insights into the Ediacaran geological environment and organic carbon enrichment with shale gas potential and exploration. As a typical marine shale with high organic carbon content and considerable thickness, it also holds significant resource potential. In this study, shale samples from multiple water depths and depositional environments across the middle Yangtze block, were examined by well log analysis, field surveys, and lithological divisions. We identified four distinct systems tracts within the second member of Doushantuo Formation (DST-2). High-resolution total organic carbon (TOC) profiles recorded several discrete enrichment pulses. These pulses vary systematically across systems tracts and water depths. This pattern suggests changes in organic-matter supply and preservation and these changes are driven by shifts in water-column redox structure. They are also linked to episodic upwelling. Together, these factors control organic carbon accumulation. Enrichment occurred primarily during transgressive phases, driven by varying water column redox conditions and persistent upwelling. Notably, deeper-water shales showed stronger organic matter enrichment than shallow-water shales did, highlighting the importance of deep-water environments for shale gas exploration. By integrating a sequence-stratigraphic framework with high-resolution TOC data across a nearshore–offshore transect, we constrain the timing and controls of episodic organic carbon enrichment in DST-2. The results highlight transgressive-phase enrichment driven by redox stratification and persistent upwelling; and demonstrate that deep-water settings consistently hosted higher TOC and thicker organic-rich intervals, thereby refining most prospective facies and intervals for shale-gas exploration.