Development Characteristics Of Natural Fractures And Their Effects On Shale Gas Preservation In The Changning Area,Sichuan Basin

The Lower Paleozoic Wufeng-Longmaxi Formation in Changning area is the main area of shale gas development in Sichuan Basin.Natural shale fractures are developed,which is an important reservoir space and seepage channel of shale gas,affecting the productivity and exploitation effect of shale gas.In this paper,the development characteristics,main controlling factors,longitudinal and planar distribution of shale fractures in the Wufeng-Longmaxi Formation and their effects on shale gas preservation in Changning area are studied by using field,core,thin section,well logging,seismic and experimental data analysis,using the methods of reservoir geomechanics and geophysics.The natural fractures of Marine shales in Wufeng-Longmaxi Formation in Changning area are mainly tectonic fractures and diagenetic fractures,and abnormally high pressure fractures are underdeveloped.Among them,the structure including Bedding shear fractures,Interlayer shear fractures and Intralayer open fractures;Diagenetic fractures are mainly foliated fractures,and shrinkage fractures are less developed.The fracture strike is mainly NWW-SEE direction and NEE-SWW direction,and the average fracture density is 1.86/m,indicating good fracture effectiveness.Fracture development is affected by tectonic action,rock mechanical layer thickness,brittle mineral content and TOC content,among which tectonic action is the key factor affecting the degree of fracture development.In the longitudinal direction,the natural fracture intensity of Longmaxi Formation is in descending order of Long1-1~1,Long1-1~2,Long1-1~3and Long1-1~4.In plane,Shuanglong-Luochang syncline has the highest degree of fracture development,followed by Jianwu syncline,and Tiangongtang anticline has a relatively low degree of fracture development.Structural deformation is an important factor affecting the preservation of shale gas in Changning area.The preservation conditions of shale gas in different structural units have certain differences.The Jianwu syncline is dominated by ClassⅠtype deformation belt,which has the best preservation conditions.The Tiangongtang anticline is dominated by Class Ⅱ deformation belt,which has good shale gas preservation conditions.The Shuanglong-Luochang syncline is mainly a Type Ⅲ deformation zone with the worst shale gas preservation conditions.