Mechanisms for initiation and termination of extraordinarily high total organic carbon in the Longmaxi Formation, Yangtze Platform, South China

Abstract

The Early Silurian Longmaxi Formation in the Yangtze Platform of South China hosts exceptional shale gas reservoirs, particularly in intervals where total organic carbon (TOC) exceeds 3 wt%. However, the initial TOC values and termination timing of these “extraordinarily high TOC” intervals differ markedly among localities, especially between inner- and outer-shelf settings. In this study, we integrate graptolite biostratigraphy, carbon isotope chemostratigraphy, and multiple geochemical proxies (e.g., major–trace elements and sulfur isotopes) from drill cores in Yichang, Hubei Province, South China (outer shelf). We establish a refined chronostratigraphic framework, identifying an extraordinarily high TOC interval (>3 wt%) and a subsequent high TOC interval (2–3 wt%), and then compare these intervals with representative inner-shelf sections. Our results show that at the base of the Longmaxi Formation in Yichang, TOC values are notably lower than those in the inner-shelf Changning area, Sichuan Province, South China, whereas the extraordinarily high TOC interval extends to a higher stratigraphic level in Yichang. Paleoenvironmental proxies indicate that extraordinarily high TOC intervals formed in a euxinic setting with higher primary productivity, and moderate restriction, whereas the high TOC interval reflects an anoxic setting, lower productivity, and stronger restriction. By determining sedimentation rates, sulfur isotope values, paleogeographic setting, and the influence of the Kwangsian Orogeny, we propose two sedimentary models to explain TOC variability in the Longmaxi Formation. The Sedimentation Rate–H₂S Concentration Model highlights how rapid sedimentation and abundant availability of ³⁴S-enriched H₂S promote extraordinarily TOC enrichment in inner-shelf settings, while slower burial and limited H₂S availability inhibit TOC accumulation in outer-shelf locations. The Kwangsian Orogeny–Hydrographic Restriction Model associates tectonic uplift with enhanced hydrographic restriction, reducing sulfate supply and driving the diachronous termination of high TOC intervals across the platform. These insights refine our understanding of early Silurian black-shale deposition and offer practical guidance for future shale-gas exploration in South China.