This paper presents an application of a micromechanical model to the analysis of cavity excavation in heterogeneous rocks by taking into account influences of mineralogical compositions. The Callovo-Oxfordian argillite is considered here in the context of feasibility studies for geological storage of nuclear waste storage. Based on the microstructural analysis, a micromechanical model is first proposed using nonlinear homogenization techniques for heterogeneous materials. The argillite is seen as a three phase composite with elastoplastic clay matrix, elastic damaged calcite grains and elastic quartz grains. Using the proposed model, the variations of elastic and plastic properties of material with mineralogical compositions at different depths are systematically taken into account. Comparisons between experimental data and model's predictions are presented. The proposed model, implemented into a finite element computation code, is finally applied to the analysis of excavation of a vertical shaft.