MECHANISMS OF DIFFERENTIAL ENRICHMENT OF ORGANIC MATTER ENRICHMENT OF THE LOWER CAMBRIAN NIUTITANG SHALE IN THE UPPER YANGTZE PLATFORM, SOUTH CHINA

It is important to consider the influences of depositional environment on lithofacies characteristics, lithofacies distribution patterns, and organic matter enrichment when defining reservoir properties and predicting “favorable reservoirs” and “sweet spots”. In this study, we examined the Niutitang shale of the early Cambrian Niutitang Formation in the upper Yangtze platform, to understand how depositional environment controlled organic matter accumulation in the Niutitang shale. Through a combination of detailed sedimentologic investigations with geochemical analyses, we identified four primary lithofacies, siliceous shale, siliceous-clay mixed shale, calcareous shale, and argillaceous shale, from the Y1 drill core. The siliceous shale and siliceous-clay mixed shale were deposited in euxinic and anoxic deep water with high surficial paleoproductivity which primarily controlled by long term global sea level changes, and that the calcareous shale and argillaceous shale were deposited in shallow and oxygen-rich water conditions with low energy and low paleoproductivity in the surface water. The variation of the dominated lithofacies associations upward suggesting a variation of sedimentary environment from euxinic–anoxic deep-water environment (highstand), to relatively oxic and shallow water environment (lowstand), and then back to euxinic–anoxic deep-water environment (highstand). This may reflect changes in sea level and mechanisms of organic matter enrichment during sea level highstand and lowstand. TOC is well correlated to MoEF, UEF, Ni/Al and Zn/Al values in the siliceous shale-dominated lithofacies association (LA1), but it is only moderately correlated to MoEF and UEF values and weakly correlated to Ni/Al and Zn/Al values in the argillaceous shale-dominated lithofacies association (LA2). The correlation relationships suggest that good preservation in euxinic–anoxic water conditions and high primary paleoproductivity were the major controlling factors of organic matter enrichment, whereas organic matter accumulation during sea level lowstand was mainly controlled by water redox conditions because primary paleoproductivity remained persistently low in the more restricted basin with shallow water and limited nutrient upwelling.