This thesis presents work performed to advance our understanding of the mechanical conditions in shale leading to crack initiation and propagation, and the influence of anisotropy, microstructure and composition in the development of fracture networks. In situ synchrotron X-ray imaging was used to acquire images of the behaviour of an organic rich Kimmeridge Clay while it was heated at a constant rate, until kerogen transformed into gas causing fracture networks to develop. Three dimensional computed tomography and two dimensional radiographic acquisitions were alternatively used throughout the experiment. Electron microscopy was also used to characterize the microstructure of the sample before and after the experiment.The tomographic data was analyzed to calculate the strain fields using digital volume corre-lation. The coefficients of thermal expansion and the nonlinear responses in different direc-tions were calculated. The strain fields show that the preferential microstructural sites for crack nucleation are the laminated organic matter. These regions host kerogen inclusion which transforms into gas. The growth and the propagation of cracks are governed by the development of the maximum principal strains which primarily occurs in the clay rich phase.Finally, the measurements allow for the first time the quantification of the opening displacements at the crack level and the estimation of the preferential mode of fracture which acts in the direction perpendicular to the bedding plane.