| There is wide black shale developing in the Wufeng-Longmaxi Formation,which is in the northern margin of the Sichuan basin.In this paper,the representative sections and well of different areas in the northern margin of the Sichuan Basin are selected.Through the detailed field geological survey,the thin section identification,the scanning electron microscope observation,the TOC content analysis,the rock geochemistry analysis,the Zircon U-Pb dating and the Sr isotope analysis,this paper attempts to explore the sedimentary structure pattern and seawater environment in sedimentary period of Wufeng-Longmaxi Formation.Furthermore,the main control factors of organic matter accumulation of black shale in the research area are discussed.The following conclusions and understandings have been reached:(1)In the Late Ordovician-Early Silurian,the middle,south,east and north of Sichuan basin and the north area of the Micangshan were squeezed from the surrounding edge,resulting in the lack of Wufeng-Longmaxi Formation in varying degrees.Under the background of the extrusion tectonic,several main structures appeared in the Sichuan basin and its surrounding margin,which are the "Xixiang Rise" in the Micangshan area in northern Sichuan,and the "Chuanzhong Uplift" in the central Sichuan area,“Yichang Rise” in Western Hubei to Eastern Chongqing region,and “Qianzhong Uplift” in southern Sichuan to northern Guizhou area.In the Dabashan area of northeast Sichuan Basin,it shows the tectonic background of the extensional fault depression of the passive continental margin.(2)Controlled by the tectonic background of extrusion around the surroundings and tension in northeast Sichuan,there is a "Changning Sedimentary Center" in southern Sichuan,a "Shizhu Sedimentary Center" in western Hubei and eastern Chongqing,and a sedimentary area in the northern slope of Sichuan basin which is adjacent to the "Dabashan-South Qinling fault depression" during the sedimentary period of Wufeng Formation.Among them,Wufeng formation in Changning area is mainly siliceous shale,carbonaceous mudstone lithofacies assemblage;In Fuling Jiaoshiba area,it is mainly carbonaceous shale lithofacies;In Shizhu area,it is mainly siliceous shale phase;In northern slope area of Sichuan basin,it is mainly the combination of siliceous shale,carbonaceous shale and siliceous lithofacies assemblage;Overall,siliceous content increases from south to north.In the sedimentary period of Longmaxi Formation,compared with Wufeng formation,there is still "Changning Sedimentary Center" in southern Sichuan,and expanding in the east-west upward range.In western Hubei and eastern Chongqing area,there is still "Shizhu Sedimentary Center".In addition,"Wuxi Sedimentary Center" was found in the slope sedimentary area of the northern Sichuan Basin.The "black shale section" in the lower part of Longmaxi Formation is mainly the combination mudstone and carbonaceous shale lithofacies in Changning area;In Shizhu area,it is mainly the assemblage of carbonaceous shale and clay shale;In the northern slope area of Sichuan Basin,it is mainly siliceous shale,carbonaceous shale and silty shale lithofacies assemblage.The overall performance is that the trend of silty sand increases from south to north.(3)The distribution of TOC content in black shale of Wufeng-Longmaxi Formation in Sichuan Basin is obviously controlled by sedimentary-tectonic pattern.Under the influence of "Xixiang Rise",the thickness of black shale high TOC section(TOC > 2.0%)of Wufeng-Longmaxi Formation in Micangshan area is obviously smaller than that in the northern slope area of Sichuan Basin,while the thickness of black shale high TOC section in "Changning Sedimentary Center","Shizhu Sedimentary Center" and "Wuxi Sedimentary Center" is obviously thicker than the outside of sedimentary center.The most developed area in the high TOC section is concentrated in the Wuxi 2 well area,which may be caused by the fact that the Wuxi2 well is located not only in the slope area of the northern Sichuan Basin,but also in the "Wuxi Sedimentary Center" during the sedimentary period of Longmaxi Formation.(4)There is no significant difference in the existing forms of organic matter about black shale between Wufeng and Longmaxi Formation,mainly the following types:(1)The bacteria and algae,which belongs to the structural bacteria and algae in sapropel,is composed of the remains of low organisms with certain structure,single cell or multicellular and so on.(2)Dispersed massive organic matter,which belongs to the bituminous plastids in the components of sapropel,is in the shape of massive and fissure development.(3)Interstitial organic matter,the pore frame of mold hole after dissolution and shedding of strawberry-like pyrite,belongs to asphaltene in sapropel,a kind of gap organic matter,which is used as cement of pyrite framboids.(4)Dispersed flake organic matter,belongs to unstructured bacteria and algae in sapropel.(5)Thin membranous organic matter,outer shell of pyrite framboids.In addition to the above forms,there are nano-scale organic matter dispersed in the matrix,and microlayer or strip-like organic matter formed by alternating deposition with minerals.(5)The main controlling factors of organic matter accumulation in different periods in the sedimentary process of each typical section in the research area are clarified,and it is found that the influence of extensional tectonic background on organic matter accumulation in northeast Sichuan is mainly reflected in the following points:(1)On the one hand,the seepage CH4 of cold seep seepage event under extensional tectonic background can consume oxygen in water column to enhance the dysoxic and anoxic state of water column,which is beneficial to the preservation of organic matter.On the other hand,seepage Ba can promote the primary productivity of marine life and is beneficial to the production of organic matter.(2)Extensional fault depression controls the distribution of pyroclastic materials in Dabashan area,which ensures that only a small amount of pyroclastic material is accepted in the northern slope area of Sichuan Basin during the sedimentary period of Wufeng Formation,which is beneficial to the production of organic matter.(3)the northern slope area of the Sichuan Basin is maintained in a deep water environment in the middle to late stage of Longmaxi Formation,which is beneficial to the preservation of the organic matter.In the Longmaxi Formation,a large amount of terrigenous detrital is diverted to the fault depression area of the South Qinling through the deep-water turbidite or gravity flow,which is beneficial to the accumulation and preservation of the organic matter.(6)On the margin of the old lands or uplifts around Sichuan Basin,seasonal upwelling are widely developed in the slope area of the northern part of the basin,while the area far from the uplifts and slope area shows a restricted water environment.(7)Due to the influence of extensional fault depression tectonic in the northern slope area of Sichuan Basin,Longmaxi Formation of Yueliangping section "the model of high primary productivity and anoxic bottom water promoting organic matter accumulation under the action of cold seep seepage in upwelling area" is the most beneficial to the accumulation of organic matter in black shale.The second is "the model of high primary productivity and 'euxinic wedge' coordinated maintenance of organic matter accumulation under the action of cold seep seepage in upwelling area" developed in Wufeng Formation of Yueliangping section.The third one is "high primary productivity and rapid subsidence protection organic matter accumulation model" developed in the middle and upper part of "black shale section" of Longmaxi Formation in Qiaoting section.In addition,there is also the "model of the medium primary productivity and clay minerals rapid subsidence protection organic matter accumulation in the restricted areas of the basin" of the Wufeng Formation of the XX1 well. |
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