Sedimentology and geochemistry of the Upper Permian Longtan and Dalong formations in the Lianyuan Sag, Xiangzhong Depression, South China: Implications for paleoclimate, provenance, tectonic setting, and organic matter accumulation

Abstract

The organic-rich shales in the Upper Permian Longtan Formation (LF) and Dalong Formation (DF) are important hydrocarbon source rocks and have become key targets for shale gas exploration in the Yangtze area, South China. To investigate the provenance, tectonic setting, paleoenvironmental conditions, and mechanisms of organic matter (OM) accumulation, this study presents an integrated petrological and geochemical study of a continuously cored well in the Xiangzhong Depression, South China. The LF was deposited in marine–continental transitional facies under a hot and humid climate with intense chemical weathering. OM is predominantly terrigenous-derived, consisting mainly of vitrinite and inertinite. Multiple geochemical proxies, together with pyrite framboid analyses, suggest that oxic, brackish water conditions, low paleoproductivity, and a high influx of terrestrial material prevailed during the deposition of the LF. In contrast, the DF comprises fully marine sediments, reflecting a depositional setting influenced by a rapid sea-level rise and progressive climatic aridification. These changes resulted in a saline, anoxic water column with enhanced primary productivity and reduced weathering intensity. The relationships between total organic carbon (TOC) content and geochemical indicators suggest that paleoclimate and terrigenous influx are the dominant factors controlling the OM accumulation in the LF, whereas upwelling-induced high productivity was the dominant factor driving OM enrichment in the DF. This study provides a comprehensive dataset on the LF and DF, which exhibit markedly different depositional environments, and aims to address the gap in the evaluation of Permian shale gas potential in the Middle Yangtze region.