Palaeoenvironment characteristics of the Ediacaran Doushantuo Shale, Yichang area, South China: Implications for organic matter accumulation mechanism

Abstract

The organic-rich Ediacaran Doushantuo Shale have been identified as a high-quality source rock and a main shale gas play in the Yichang area, South China, which was characterized by high TOC content, high thermal maturity, enormous cumulative thickness, and relatively high contents of brittle minerals. As the oldest shale gas stratum, the paleo-environmental characteristics and organic matter (OM) accumulation mechanism remain unclear in this special marine setting. In this study, core descriptions, TOC content, X-ray diffraction (XRD) mineralogical analysis, scanning electron microscopy (SEM) observation, and elemental analysis, including the major, trace, and rare earth elements, were combined to identify lithofacies, reconstruct paleo-environmental conditions, and document organic matter (OM) accumulation mechanisms for this potential Ediacaran Doushantuo Shale in the Yichang area, South China. Four major shale lithofacies were identified based on TOC content and mineral composition. The geochemical redox proxies, such as the enrichment factors of Uranium (U-EF) and ratio of organic carbon to total phosphorus (Corg/P), indicate that the Doushantuo Shale was probably deposited in a dysoxic-anoxic conditions, which is consist with the interpretation from size distribution of pyrite framboids. The terrigenous influxes suggest the Doushantuo Shale was deposited in arid climate in absence of chemical weathering. Meanwhile, the productivity proxies (biogenic SiO2, Ba, and P/Al) indicate that the organic-rich Doushantuo Shale was deposited under a relatively low primary productivity. In addition, hydrothermal input has limited influence on organic matter (OM) accumulation of the Doushantuo Shale, which was also reported in the Qinglinkou Section of Yichang area. It can be observed that the organic matter (OM) accumulation mechanism of the Doushantuo Shale, deposited after the Marinoan “Snow Earth” glaciation, in the Yichang area, South China, was mainly controlled by the redox conditions, which is much different from typical marine shales and transitional shales.