Study On Paleo-environmental Reconstruction And Organic Matter Accumulation Of The Lower Cambrian Niutitang Formation In Northern Guizhou

The Niutitang Formation shale is characterized by high total organic carbon contents,high maturity and thickness in northern Guizhou.However,the lack of understanding on organic matter accumulation mechanism restricts the shale gas exploration and development.Based on sedimentology,sequence stratigraphy,logging geology,isotope chemistry and inorganic(organic)geochemistry,this paper was carried out the paleo-environmental reconstruction and organic matter accumulation mechanisms via combination of geological,logging,drilling and geochemical test data and made the following results and understanding:(1)In consideration of research on sequence comparison and sedimentary evolution of the Niutitang Formation based on the outcrop and surface wells in northern Guizhou,optimizing new shale gas drillings and geologic profiles with completely sequence strata have been done.These drillings and profiles are used to make the sedimentrary and sequence stratigraphic correlation.The results show that during deposition of the Niutitang Formation,tectonic paleogeographic pattern is characterized by northwest high and southeast low;the Niutitang Formation can be classified as tertiary sequence SQ1 and incomplete tertiary sequence SQ2;during the early SQ1,siliceous rock deposits is dominated,and carbonaceous shale deposits when transgression,and silty mudstone deposits when regression.(2)X-ray diffraction(Clay),main(trace)elements test,iron speciation and stable carbon isotope for 35 samples have been completed.Combined with scanning electron microscope,thin-section and core description,five sedimentary microfacies have been classified based on the 13 lithofacies.Moreover,the geochemistry characteristics of different samples from various sedimentary microfacies were compared.Study indicates that the Niutitang Formation in the study area of deep water shelf is dominated by anoxia with episodes of euxinic,high production,high burial efficiency,weak-moderate water restriction and a lot of sedimentary events such as anoxia,upwelling and hydrothermal events,which is mainly characterized by hydrothermal-anoxic deposits.But for the areas of shallow water shelf,sedimentary model is mainly dominated by hydrothermal water plus normal sea water,and sedimentary environment is mainly oxic conditions,weak production and moderate water restriction.(3)With analysis of Mo-U covariation in the Niutitang Formation,comparison with Mo-U patterns from modern marine system(Peruvian shelf margin)and combination of fluctuation in stable carbon isotope values,this work suggests that during deposition of the upper Niutitang Formation,oxic conditions with anoxia redox interface oscillation is shown.(4)based on sedimentary evolution within sequence framework,study indicates that during deposition of the Niutitang Formation,most of the study areas is characterized by organic matter accumulation under anoxic conditions,but organic matter accumulation patterns from relative high terrain regions(such as the neighbouring area of submarine uplift)are shown as:Accumulation via anoxia?High production promotes organic matter accumulation?Sedimentary events maintain organic matter accumulation?Variation in relative sea level controls organic matter accumulation?Low production hard to drive organic matter accumulation.