Structural Response Of Fracture Structure And Shale Pore Of Longmaxi Formation In Wuxi Area,Chongqing

Wuxi area in Chongqing is located in the front of nandaba mountain.The structure of this area is extremely complex,which makes the geological study of Longmaxi formation shale gas more difficult.Based on the detailed field investigation and experimental test,this paper uses the theory of structural geology and reservoir analysis technology to conduct in-depth study on the structural characteristics and the shale reservoir structure of Longmaxi formation,and discusses the influence of structural action on shale gas accumulation.The results show that:The study area can be divided into four sub structural units,namely,xiaoyangba-Chengkou thrust belt,Miaopo-mingtongjing fold belt,Tiexi-Wanyuan fold belt and Hekou-Tianba fold belt,which have experienced the formation stage of Proterozoic basement,the extension and rifting stage of Sinian Early Triassic continental margin,the collision and orogeny stage of the middle late Triassic early Jurassic and the intracontinental orogeny since the middle Late Jurassic There are four tectonic evolution stages such as stage.Longmaxi formation shale has a large development thickness(40?80m),strong structural deformation,and structural styles such as fold,fracture and friction surface;the organic matter content in shale is between 2.5% and 5%,and the maturity is between 1.5%?2.5%,mostly type I;mainly composed of quartz(average 84.46%),clay minerals(average 9.9%)and feldspar(average 3.29%),occasionally pyrite,siderite,anatase and other minerals.The pores in shale can be divided into organic pores,mold pores,intragranular pores and intergranular pores,some of which are deformed.The fractures can be divided into three types: one group of fractures,two groups of fractures and three groups of fractures.In addition,there are also organic matter shrinkage fractures and structural fractures;most of the fractures are filled by calcite vein.The pore size distribution curve of shale has obvious “three peaks” characteristics,the peaks are 0.4 nm,1 nm and 1.6 nm respectively;the proportion of mesoporous pore volume is obviously increased,while the specific surface area of micropore is absolutely dominant;the average pore size is significantly increased;the fractal dimension of pore is relatively large,with an average of 2.6026.From both sides of the structural belt,the total pore volume,total specific surface area,fractal dimension of pores and methane adsorption amount show a trend of decreasing first and then increasing;while the average pore size shows a trend of increasing first and then decreasing;the total gas content gradually decreases from bottom to top,which may be related to the organic matter content.The pore structure characteristics of Longmaxi formation shale in different structural positions have great changes.From the structural belt to both sides,the total pore volume,total specific surface area,pore fractal dimension and methane adsorption capacity show a trend of decreasing first and then increasing;while the average pore diameter,on the contrary,has a trend of increasing first and then decreasing,which may be related to the development of faults and folds in the structural belt;methane adsorption The total gas content decreased from the bottom to the top,which may be related to TOC content.The tectonic subsidence process of Longmaxi formation shale in the study area can be divided into four stages: the slow subsidence stage from the Early Silurian to the early Devonian,the slow uplift stage from the early Devonian to the Middle Permian,the fast subsidence stage from the Middle Permian to the Middle Cretaceous and the fast uplift stage from the Middle Cretaceous to the present.During this period,it experienced the primary hydrocarbon generation from the Early Silurian to the early Devonian and the secondary hydrocarbon generation from the Middle Permian to the Middle Cretaceous,generating a lot of gas.Complex structural features may lead to the escape of shale gas and the destruction of early formed shale gas reservoirs,but it is still possible to form better shale gas reservoirs in the wide and gentle fold wings.