Depositional environment and organic matter enrichment of the lower Cambrian Niutitang shale in western Hubei Province, South China

Abstract

Abstract 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 western Hubei Province, middle Yangtze Block, 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 sedimentological analyses, including systematic observations of the color of fresh rocks, grain size, sedimentary textures and structures, biogenic and diagenetic (e.g., veins and pyrite) characteristics in cores and thin sections with mineralogical analyses by X-ray diffraction (XRD), and changes in geochemical proxies, including TOC (wt.%), Mo EF , U EF , Ni/Al and Zn/Al, suggest that 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 Niutitang shale changed from the siliceous shale-dominated lithofacies association (LA1), to the argillaceous shale-dominated lithofacies association (LA2), and then back to the siliceous shale-dominated lithofacies association (LA1), 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. The low Mo/TOC values of the siliceous shale and siliceous-clay mixed shale lithofacies are similar to the Mo/TOC values of marine sediments in Framvaren Fjord, suggesting that the study area was located in a moderately restricted basin during deposition of the Niutitang shale. The Fe/Ti and Al/(Al + Fe) ratios and the Al–Fe–Mn diagram combined with our detailed petrographic studies and the good correlation between TOC and quartz contents are indicative of a mainly biogenic origin with a minor detrital source for the silica in the Niutitang shale. TOC is well correlated to Mo EF , U EF , Ni/Al and Zn/Al values in LA1, but it is only moderately correlated to Mo EF and U EF values and weakly correlated to Ni/Al and Zn/Al values in 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. The correlations between TOC content and clay minerals within the different lithofacies associations suggest that adsorption of carbon components by clay minerals promoted the accumulation of organic matter to various extents in LA2 but probably did not impact organic matter enrichment in LA1.