Mathematical analysis of structural fracture scale in shale based on core fracture observation of the second member of Kongdian Formation in Cangdong Sag
Taking the structural fractures in the Kong 2 Member shale of the Cangdong Depression as the research object, we focused on using core data and using core observation, mathematical analysis and software fitting to carry out fracture type identification, structural fracture scale calculation and analysis of influencing factors. and other research work. Based on the measured data of structural fractures on the shale core of the Kong 2 Member, the ellipse fitting method based on the least squares method was used to establish the elliptical standard equation of the longitudinal and horizontal sections of the representative structural fractures, and obtain the central axis and minor axis of the fracture ellipse. , and then calculate the long axis of the fracture ellipse based on the triangle similarity principle, thereby establishing the ellipsoid standard equation of representative structural fractures. The scale of other structural fractures on the core is calculated by analogy with the scale of representative structural fractures. The results show that: Kong 2 member shale The types of medium fractures include structural fractures and non-structural fractures, among which structural fractures include tensional cracks and shear cracks; the established ellipsoid standard equation reflects that their scale is at the meter level, and their horizontal extension length ranges from 0.13 to 4.55 m, with an average value of 1.15 m , the longitudinal extension length is 0.20 m, and the opening is 0.58 mm; non-structural fractures include bedding fractures, bedding fractures, dolomitization fractures and drainage fractures. Factors affecting the scale of structural fractures in the Kong 2 Member shale include faults, rock properties, layer thickness and burial depth, among which faults control the occurrence of structural fractures, and as the distance from faults increases, the average horizontal extension length of structural fractures decreases; pages rich in carbonate minerals or felsic minerals The lithology of the rock is highly brittle, and structural cracks are more likely to occur and expand. Therefore, the average horizontal extension length of structural fractures is the largest in dolomitic shale, followed by felsic shale; when the shale lithology is constant, the average horizontal extension length of structural fractures The horizontal extension length increases with the increase of layer thickness, and the two have a good positive correlation; when the shale burial depth increases, the surrounding rock pressure increases, making it difficult for cracks to generate and expand. The average horizontal extension length of structural fractures There is a decreasing trend.