Geochemical characteristics of organic carbon and pyrite sulfur in Ordovician-Silurian transition shales in the Yangtze Platform, South China: Implications for the depositional environment

Abstract

Global climate change and tectonism are principal causes of spatial and temporal differences in Ordovician-Silurian (OS) transition sedimentary environments of China's Upper Yangtze Platform. However, the relative significance of each of these drivers is not always obvious. This paper describes results of a geochemical study of a drill core section (Well X3) recovered from Xiang'e Submarine High in the central part of the Upper Yangtze Platform. This work focused on shale deposits of the Wufeng Formation and Longmaxi Formation that accumulated during the OS transition. Geochemical characteristics, including total organic carbon (TOC), pyrite S (Spy), stable carbon and sulfur isotopes, and Mo and U enrichment values, were combined with pyrite framboid size distribution data to reconstruct the OS paleoenvironmental evolution of this region of the Upper Yangtze Platform. The studied succession appears to record the transition from lower Katian-Hirnantian shale deposited under sulfate-limited conditions to Rhuddanian organic-rich shale and finally to organic-depleted Aeronian sediment. The depositional history of these deposits was controlled by changes in water depth and watermass chemistry as a result of changing sea level. The sea-level history of the studied succession was largely controlled by global climate change related to Hirnantian glaciation and Rhuddanian warming and tectonism associated with the Kwangsian Orogeny that resulted in uplift of the Xiang'e Submarine High. Similar geochemical investigation of the Pengshui and Shuanghe sections, also in the Upper Yangtze Platform, demonstrate the dominant role of global climate change and tectonism in controlling organic carbon content and thickness of potential shale gas targets. Specifically, the low concentrations of organic carbon in thin shale deposits proximal to uplifted regions of the basin pose a major risk to shale gas exploration.