Analysis Of Distribution Characteristics And Main Control Factors Of Joint Shear Crack Angle In Nanchuan Area Longmaxi Formation Shale

As the shale of Longmaxi formation in Lower Silurian,which located in the area of Nanchuan,Chongqing,being the research object of this thesis,using the method of field observation,indoor statistic analysis,mechanics experiment and microscopic observation,and according to the basic idea of combining theory with practice,the joint shearing angle crack development characteristics and main controlling factors of inquiry in study area of Longmaxi shale is discussed,the results showed that:Research shows that the structure in Nanchuan area can be divided into 3 structurallayers,i.e.,I(the Caledonian period),II(the Hercynian-Indosinian period),and III(the Yanshanian period).The joint formed in two tectonic periods:in the early and middle period of Yanshanian period,being affected by the stress field of SSE-NNW,the study area formed the conjugate joints with NW and SE directions.In Himalayan period,being affected by extension of NWW-SEE which formed conjugate joints with NWW?SEE?NW and SE directions.There are four extension directions of joint in the study area,they are NE?NEE?NWW and NNW,the most advantaged is NE,the joint inclination is larger,78% of them is 60~90°,most of them are high inclined joints.The distribution of the joint shear angle is mainly between 20° and 44.5°,the average is 34.17°,70% of them are larger than 30°,9% of them are smaller than 25°.The development direction of joint shear crack angle is NWW?NW and NE,NE and NW are more advantaged.The development of joint shear crack angle under different stress conditions was simulated by three axial stress test,the results showed that: The higher the confining pressure is,the smaller the micro cracks will occur in the shale samples,and the failure is mainly caused by shear failure,the lower the confining pressure is,the more obvious the fragmentation characteristics of shale are,and the main reason is the splitting failure.The mechanical properties of shale control the development of joint shear crack angle,the influence of the stress on the shear crack angle,the number of joint shear angle increases with the increase of stress,the mechanical properties of rock also restrict the size distribution of joint shear crack angle,which makes the difference of the shear crack angle of different test samples.According to the characteristics of joint shear crack angle,the influence of structural conditions,rock composition and mechanical properties of rock on the shear crack angle is analyzed: Structural conditions are the key to the formation of shale fractures,the shape,direction and density of the joint shear angle are controlled from the two aspects of regional tectonic evolution and structural characteristics.The feature of rock composition is an important aspect of the development of the fracture angle,the fracture density of rock increases with the increase of mineral content,such as quartz,there is a negative correlation between the size of the rock fracture and the rock brittle coefficient,with the increase of shale brittleness coefficient,the joint shear crack angle decreases.The mechanical properties of rock are the internal factors that affect the development of joint shear crack angle,which controlled the distribution characteristics of joint shear crack angle,and there is a positive correlation between the fracture angle of shale and shale Poisson's ratio,that is,the larger the Poisson's ratio of shale,the larger the crack angle of shale joint increases.