Sedimentary Model And Geological Significance Of Carbonaceous Shale In The Early Cambrian,northern Jiangsu And Its Surroundings

The early Cambrian was a critical period for the global biological,climatic and environmental changes.A set of organic-rich shale with high content of total organic carbon was deposited in the whole Yangtze area during the early Cambrian.Sedimentology research on the organic-rich shale of early Cambrian can not only provide scientific basis for the restoration of the evolution process of paleoclimate and paleoenvironment in the early Cambrian,but also provide certain theoretical basis for the exploration of shale gas in the Yangtze area.In this paper,the lower Cambrian black carbonaceous shale from XYZ-1 well,a newly drilled well in Xuyi,Jiangsu province,was selected as the main research object.Combined with peripheral region field profiles and the contrast of upper-middle Yangtze region,using the field outcrop,core observation,analysis,testing data,such as element geochemistry,total organic carbon(TOC),total sulfur(TS)?XRD analysis and scanning electron microscopy(FE-SEM),to carry out the study of sedimentary characteristics,depositional environment,geochemistry and reservoir characteristics for the black carbonaceous shale in the lower Cambrian.The main achievements and realizations of the paper are as following:The "stone coal" with thickness of 8.1 m at the bottom of XYZ-1well is used as the marker bed to carry out the stratigraphic correlation of the Lower Cambrian System in the Yangtze Region,and the bottom of the Mufushan formation corresponds to the Stage 3 of the Series 2 epoch in the Cambrian period.The main lithology of the Mufushan formation is black and dark gray mudstone,carbonaceous mudstone,argillaceous limestone,micritic limestone,and intercalated with carbonate breccia,terrigenous clastic limestone,dolomitic limestone,marlite and andesite.Develop primary sedimentary structures such as horizontal bedding,turbidite lamina,rhythmic layering,graded bedding,deformation bedding,bidirectional cross bedding and miniature wave-ripple cross bedding,in which turbidite sediment is concentrated in the upper lithological section.Carbonate cementation occurs in the slump turbidity channel conglomerate,with normal graded bed sequence and scouring contact at the bottom.Combined with the regional tectonic and sedimentary background,the sedimentary period of the Mufushan formation in the study area belongs to the sedimentary system of shallow sea-tidal flat-platform system.The tidal flat system develops the intertidal zone mudflat facies,mixed flat and platform facies,while the shallow sea system is dominated by the underwater fan,slump turbidity channel,shallow sea.Therefore,it is speculated that the southern Nanjing platform provides carbonate provenance and the northern Jiaoliao platform provides siliciclastic sources.Based on the comprehensive analysis of element geochemistry,organic carbon(TOC)and total sulfur(TS),the paleoclimate,paleosalinity and redox conditions of the early Cambrian in the study area were systematically analyzed.The results show that the average value of V/(Ni +V)is 0.72,the average value of Ni/Co is 6.98,the average value of U/Th is 1.30,and the average value of V/Sc is 61.66.The redox index shows an anoxic/vulcanized sedimentary environment.Based on the analysis of reduction sensitive element such as Mo,U,V and framboidal pyrites,two strong euxinic events were distinguished at the bottom and the top of the Mufushan formation.The average value of TOC in fine grain deposition of Mufushan formation is 5.02 %,in which 84.6 % of the samples are more than 2 %.The content of brittle minerals is between 32 % and 88 %,with an average of 59.75 %.Developed three main pore types: organic pore,Inorganic pore and fracture.The organic pore is mainly developed in the layer where the TOC value more than 2 %.The inorganic pores are mainly intergranular pores,intraparticle pores and dissolved pore,the pore size is more than 2 ?m,and the maximum is 50 ?m.Microfractures are widely developed.The fracture width of structural fractures is generally 20-500 nm,while the width of diagenetic fractures is 50 nm-200 nm,diagenetic fractures length more than 10 ?m.The Lower Yangtze area has good source rock conditions and the conditions for the development of high-quality reservoirs.The deposition conditions of anoxia / vulcanization provide a guarantee for the enrichment and preservation of organic matter.Therefore,there is a potential for shale gas exploration in the Lower Yangtze area.Through the above research,the main innovative achievements of the paper are mainly reflected in two aspects:1.The black carbonaceous shale of the early Cambrian Mufushan formation was formed in a restricted-anoxic bay environment.Compared with upper-middle Yangtze area,the anoxic environment lasts longer and reaches the second peak in the late period.It is considered that the area is mainly composed of shallow sea-tidal flat-platform system,and the Nanjing platform and Jiaoliao platform may have provided carbonate rock cataclastic and terrigenous cataclastic for Chuhe sag respectively.The special genetic environment has laid a good geological foundation for shale gas accumulation.2.The euxinic event is the main cause of TOC enrichment in the black carbonaceous shale of lower Yangtze,and the TOC in black carbonaceous shale in the euxinic event section reaches 21.27 % at the highest.At the same time,the pore types are diverse and the content of brittle minerals is high.Compared with upper-middle Yangtze area,this area has better hydrocarbon generation conditions,good storage capacity and potential for shale gas exploration.