Research On The Development Laws Of Shale Diagenetic Fracture On The Basis Of Desiccation Crack Of Muddy Sediments

Fractures are divided into structural fractures and non structural cracks,there are a lot of researches on the structural fractures and the research on the non structural cracks is very weak.The shale diagenetic fracture whose development and distribution is much wider than the structural fractures at the microscopic scale is a very important non-structural cracks and of great significance to the occurrence and development of shale gas.In this paper,the problem that the development laws of diagenetic fracture is not clear was focused,the similarity of the development laws between the diagenetic fracture and the desiccation crack was considered,the method of field observation and research of the desiccation crack of modern mud sediment and the method of physical simulation experiment were used,the main controlling factors and development laws of shale diagenetic fracture were studied,a geological model for the development of shale diagenetic fracture was established,mainly achieved the following understandings:The development of the desiccation crack in the modern mud sediment is random and isotropic,the parameters of the lengths,the widths,the angles follow a normal distribution roughly,the geometry of desiccation crack includes triangle,quadrangle,pentagon and hexagon,the inflection points are mainly three line points and four line points.The desiccation crack appeared at the point firstly in which the stress is unbalanced,the development of the desiccation crack can be divided into four stages:the development of the main crack,the development of the secondary crack,the steady state and the fixed state.In the process of growth of the desiccation crack,the growth angles of the main crack is 0?60 degrees and the growth angles of the secondary crack is 60?120 degrees.In addition,the sediment thickness,the drying temperature,the clay mineral content and the salinity are the main controlling factors of the development of the desiccation crack.The reason for the formation of the shale diagenetic fracture and the desiccation crack is the change of the volume.When the volume of shale or mud sediment change,the volume change of various minerals is not the same,then cracks produce.The comparative study shows that the formation of shale and the development of the shale diagenetic fracture and the desiccation crack which are both controlled by holes and large particles are random,their morphology is similar and their difference is the diagenetic fracture is also under the control of bedding and original cracks.Therefore,we can use the theory of “present is the key to the past” for studying diagenetic fracture with the help of desiccation crack.The development of shale diagenetic fracture is random,Compared with the structural fractures,the diagenetic fracture have more crack branches,the fracture surface is rough and the direction of the law is poor.Holes,large particles,bedding and original crack control the development of diagenetic fracture obviously.The diagenetic fracture can be divided into main cracks and secondary cracks,the growth angles of the main crack are mainly 60 degrees and the growth angles of the secondary crack are mainly 120 degrees.The development of shale diagenetic fracture controlled mainly by the shale thickness,formation temperature,the content of clay minerals and water salinity,a quantitative mathematical prediction model can be established between the parameters of the diagenetic fracture and the main control factors.The physical model of shale diagenetic fracture is mainly divided into three types: controlling model of holes or large particles,controlling model of the beddings or the original cracks,stochastic growth model.