Enrichment mechanisms of organic matter in the marine slope facies shales from the Niutitang Formation in the western Xuefeng Uplift

The lower Cambrian organic-rich shales are widely distributed in the middle-upper Yangtze platform, which are also the main target layers for shale gas exploration and extraction. Revealing the enrichment mechanisms of organic matter in these shales is of great significance for understanding the relationship between ocean bio-environmental systems and Earth resources. Through systematical geochemical analyses, this study investigated the lower Cambrian Niutitang Formation slope facies shales at the western Xuefeng Uplift, to elucidate the paleoenvironmental characteristics and their main controlling factors. The results show that the Niutitang Formation shales generally have high TOC contents, with higher TOC contents in the lower part than those in the upper part. Sedimentological geochemistry analyses indicate that the Niutitang Formation formed under high paleo-productivity and anoxic-anaerobic even sulfurized conditions. The formation generally has a lower degree of reduction in the late period of sedimentation than at earlier times, and was deposited in a moderately hydrologically restricted setting, with weak terrestrial input and hydrothermal activities. The enrichment of organic matter in the Niutitang Formation slope facies shales was mainly controlled by the redox conditions, however upwelling currents also contributed to the primary input of organic matter. During the early deposition stage of the Niutitang Formation, the nutrients brought by upwelling currents promoted primary paleo-productivity, resulting in the booming of algal organisms and subsequent bacterial sulfate reduction (BSR) and anoxic even sulfurized environments. During the late deposition stage of the Niutitang Formation, a drop in sea level led to a weaker degree of reduction and lower contents of TOC. Our results are of great significance to the enrichment mechanisms of marine organic-rich shales in petroliferous basins.