Sedimentological and geochemical characterization of the Upper Permian transitional facies of the Longtan Formation, northern Guizhou Province, southwest China: Insights into paleo-environmental conditions and organic matter accumulation mechanisms

Characterized by high TOC content and large cumulative thickness, the Upper Permian marine-continental transitional facies of shales are widely distributed in southwest China, and are currently becoming a prospective area for shale gas exploration and production. Compared to marine shales, the organic matter (OM) accumulation model and paleo-environmental conditions remain unclear in transitional settings. In this study, core descriptions, thin section and scanning electron microscopy (SEM) observations, X-ray diffraction (XRD) mineralogical analysis, and a major, trace, and rare earth element analysis were integrated to document the varying lithofacies, paleo-environmental conditions, and OM accumulation mechanisms for the Upper Permian transitional facies of the Longtan Formation in northern Guizhou Province, southwest China. Six major lithofacies, including carbonaceous mudstones, siliceous-clay mixed mudstones, argillaceous mudstones, silty shales, muddy siltstones, and calcareous-clay mixed shales, were identified. The XRD analysis and SEM images suggest that clay (average = 51.2 wt%) and siderite (average = 17.3 wt%) are the dominant minerals in the Longtan Formation, which is different from the mineralogical composition of typical marine shales, which are generally dominated by quartz (e.g. Horn River Shale in the Western Canada Sedimentary Basin, and the Barnett Shale in the Fort Worth Basin). Furthermore, the sedimentological and geochemical analyses suggest that the paleo-environmental conditions and OM accumulation mechanisms of the transitional facies of the Longtan Formation are different from those of marine shales, and can be summarized as follows: (1) terrestrial plant fragments mainly contribute to the TOC contents of the Longtan Formation; (2) paleoproductivity is generally the major influencing factor of OM accumulation in marine shales, but is relatively low during the deposition of the Longtan Formation; (3) terrestrial OM input, redox conditions and sedimentation rate are the major factors controlling OM accumulation; and (4) most of the silica content has detrital origin instead of biogenic origin in the Longtan Formation.