To clarify the distribution characteristics of laminas and cracks in continental shale in the Yanchang Formation of the Ordos Basin, a high-definition camera, super depth of field microscope, IPP software, fractal geometry, Monte Carlo simulation and other means were used to observe and characterize the laminas and cracks of continental shale samples in the Chang 7₂ oil group. Bright and dark laminas were classified, and the mineral components of bright and dark laminas were compared by X-ray diffraction. The thickness distribution of bright and dark laminas at macro, 20 times and 50 times magnification were respectively analyzed, and the distribution rules of average laminar thickness of different thickness classes (below 1 mm, 1~10 mm and above 10 mm) were analyzed. The distribution characteristics of crack width, spacing and dip angle were characterized, and a two-dimensional mesoscopic structure model of continental shale was established based on the distribution rules of different types of laminas. The results show that: (1) The minerals in the bright lamina are mainly brittle minerals (80%), such as quartz and feldspar, while the dark lamina is mainly composed of 40% brittle minerals and 40% clay minerals. (2) The fitting result between total laminas thickness and quantity of continental shale laminas follows an exponential function, the fitting between bright laminas thickness and quantity of bright laminas follows a power function, and the fitting between dark laminas thickness and quantity of dark laminas follows a logarithmic function. (3) With increasing observation scale, the frequency of thin laminas(below 1 mm) increases, and the frequency of medium laminas (1~10 mm) and thick laminas (above 10 mm) decrease. There is a linear relationship between the logarithm of different kinds of laminar thickness classes and corresponding average laminar thickness. The distribution of laminar thickness has a fractal feature. (4) The crack width is distributed in the mesoscopic range, with a mean and standard deviation of 0.271 mm and 0.114 mm, respectively, which is relatively concentrated. The mean and standard deviation of crack spacing are 8.115 mm and 8.448 mm, which is relatively uniform. The fitting result between the absolute value of the crack dip angle and the quantity of cracks follows a logarithmic function.

Clarify the mechanical properties of different laminations and the fracture mechanism of continental shale under in-situ stress can provide theoretical basis for more comprehensive evaluation of the fracability of continental shale oil reservoir. The Chang 7₂ continental shale was used to investigate the mechanical properties of laminations and the effect of natural structure on the crack propagation of the shale. The XRD and thin section tests show that the laminations contain two types: bright sandy lamination with void structure and dark muddy lamination with layer structure. The real-time CT uniaxial compression tests were conducted to investigate the differences of mechanical properties between the muddy lamination and sandy lamination. It found that the uniaxial compression strength and elastic modulus of the sandy lamination are higher, forming a simple crack with large opening, and the Poisson's ratio of the muddy lamination is large, forming obvious lateral deformation and more secondary cracks. On this basis, the cuboid-shaped continental shale specimens were tested under true triaxial compression conditions to study the effect of laminations and interface cracks on crack propagation combining AE and CT techniques. It found that nascent cracks connected laminations and interface cracks to form fracture network under appropriate loading condition, tensile cracks developed in sandy lamination and shear cracks occurred in muddy lamination because of deformation dissonance and brittleness index differences, and more secondary cracks formed in muddy lamination with smaller fracture toughness. Moreover, the combination relationships between nascent and natural cracks mainly conclude direct penetration and deflection, which is affected by the filling degree and morphology of interface cracks and the relationship of lamination types. These conclusions show that laminar continental shale is conducive to forming complex fracture network, which can provide a theoretical basis for the proposal of indicators and methods for fracability evaluation.