Depositional Environment And Organic Matter Accumulation Of The Lower Cambrian Niutitang Formation Shale In Northern Guizhou

The lower Cambrian Niutitang Formation shale developed in Northern Guizhou is characterized by high total organic carbon contents,large shale thickness and high organic matter maturity.However,due to the unclear exploration of organic matter enrichment in Niutitang Formation,the shale gas development and utilization in Cengong block has been restricted.This paper combines geological data,well logging data and geochemical data to study the Niutitang Formation shale.With sedimentology,organic geochemistry,elemental geochemistry and so on as the theoretical guidance to reconstruct depositional environment and to study the enrichment patterns of organic matter.The major conclusions are summarized below.(1)The shale of Niutitang Formation was deposited in the continental shelf slope environment.Combined with thin-section and core description,seven lithofacies have been classified.Moreover,the geochemistry characteristics of different samples from various depositional microfacies were compared.Two types of shale lithofacies are formed under the action of strong hydrodynamic conditions,which are carbonaceous silty mudstone and silty sand-containing carbonaceous mudstone,which belong to the shallow water continental shelf microfacies deposition.Under moderate hydrodynamic conditions,carbonaceous silty shale and silty carbonaceous shale are formed.Siliceous rock and siliceous shale are formed under weak hydrodynamic conditions,which belong to deep cement continental shelf deposition.(2)The organic matter of the Niutitang Formation shale is concentrated in the middle and lower parts,which was formed by the seawater transgression period and the warm and humid climate.The initial transgression caused an increase in the volume of water that could be accommodated,forming siliceous shale and siliceous rock at the bottom.The Niutitang Formation in the study area of deep water shelf is dominated by anoxia with episodes of euxinic,high production,high burial efficiency and hydrothermal events,which is mainly characterized by hydrothermal-anoxic deposits.For the areas of shallow water shelf,depositional model is mainly dominated by hydrothermal water plus normal sea water,and depositional environment is mainly oxic conditions,weak production and moderate.(3)Study indicates that during deposition of the Niutitang Formation,organic matter accumulation patterns are shown as: Accumulation via anoxia ? High production promotes organic matter accumulation ? Variation in relative sea level controls organic matter accumulation ? Low production hard to drive organic matter accumulation.During this period,there were many changes of main enrichment factors,which controlled the formation and distribution of organic-rich mud shale with the changes of paleoclimate characteristics and depositional environment.The formation model of organic shale in Niutitang Formation mainly relies on the rapid rise of sea level in the early stage,hydrothermal activities and land source input to provide nutrients for the water,and the surface productivity increases sharply.The anoxic and sulfuric environment at the bottom formed by strong hydrothermal activities which results in the accumulation and preservation of organic matter.