Argillaceous rocks have a complex and heterogeneous structure at different scales. At the scale of the mineral inclusions embedded in a clay matrix, the deformation generally induces microcracking and material damage. Modelling the latter requires to take into account microscale characteristics and their effect on the micromechanical response. This response can be used in double scale approach to predict material behaviour at larger scale. Thus, heterogeneous microstructures of a claystone are generated with a distribution of morphological properties satisfying experimental observations. The overall microscale material behaviour under solicitation is obtained by numerical homogenisation. Then, the variability of the material response is studied with regard to small-scale characteristics. In terms of deformation and failure, a dominant shear deformation mode and decohesion between grains are observed. The decohesion induces microcracking in the microstructure and strain softening of its overall response.