## 1 Introduction  
The initiation and evolution of micro-cracking morphology in a shale gas reservoir depends not only on the hydraulic treatments but also on the mechanical properties of the rock matrix and beddings. Being different from isotropic rock, finely bedded internal structures in shale formation may result in much more complex anisotropic characteristics and thus may induce different fracturing morphologies. Previous investigations found that laminated structures could affect rock anisotropy. For instance, [PERSON] et al. [1] tested the layered rock specimens from the area of Boyeong, Asan, Yeoncheon, and observed a significant variation of anisotropy in deformation and strength. [PERSON] and [PERSON] [2] observeda significant reduction of compressive strength if the shear failure of beddings occurred earlier than matrix failure. [PERSON] et al. [3] found that the joint friction coefficient plays a key role in the residual strength of transversely isotropic rocks rather than the uniaxial compressive strength (UCS) in uniaxial compression. [PERSON] et al. [4] proposed that the anisotropy of the elastic modulus induced by layered structures could affect the estimation of the in-situ stress field. In addition to experimental investigations, [PERSON] et al. [5,6] investigated the inherent anisotropy of Tournemire shale by introducing a higher-order microstructure tensor into a mathematical model. Their investigations indicated that the deformation and the failure criterion of sedimentary rocks could be described by angle-dependent rock parameters in a theoretical method. The rock strength anisotropy induced by laminated structures is very complex [7,8]. Therefore, the bedding strength parameters should be carefully included in the investigation on the mechanical properties and failure of laminated shale formations. Macro-fracture or fault morphology has been observed at a geological scale. At this scale, the degree of rock anisotropy induced by layered structures was investigated through long historical environmental impacts and tectonic stress [9,10]. [PERSON] et al. [11] discussed the relationship between the degree of shale anisotropy and tectonic stress and concluded that the prominent triaxial anisotropy of shale appears at a low confining pressure. [PERSON] et al. [12] found that the tectonic stress of rock formation plays a critical role in the pore compressibility of low permeable porous medium, which is directly related with oil saturation. In the laboratory, the mineral composition of Longmaxi shale was found to affect the stress-strain responses of shale [13], and the beddings in shale formation could also affect the pore pressure and crack propagation path during hydraulic fracturing [14]. In engineering, the stability of wellbore is strongly affected by the orientation between the laminated structures and the axis of wellbore [15]. Therefore, bedding strength parameters may vary with geological conditions but strongly affect the mechanical behaviors of laminated shale. Micro-cracking morphology has been observed in micro-scale experiments. Digital image processing [16], acoustic emission [17] and other advanced techniques have been widely used to observe the initiation and evolution of micro-cracks and their impacts on the mechanical characteristics and fracturing failure of rocks. With the help of a micro computerized tomography (micro-CT) scanner and scanning electron microscopy (SEM) imaging techniques, [PERSON] et al. [18] observed that a lot of micro-tensile cracks occur in uniaxial compression and found that beddings play a key role in the micro-crack propagation in shale formation. Real-time ultrasonic detecting and post-test CT scanning were used to explore the relationship among the total energy and the elastic energy, as well as the dissipated energy in shale formations [19]. These techniques and observations make a great contribution to the understanding of micro-cracking morphology and the fracturing mechanisms.