Opalinus Clay is the shale currently under investigation as the host formation for geological radioactive waste disposal in Switzerland. Its hydromechanical response has been widely studied, and the experimental results show a range of values whose dispersion needs to be clarified. This work aims to explain the dispersion in the literature results by correlating the hydro-mechanical response to the mineralogical variability of the tested specimens. Based on published microstructural studies, the Opalinus Clay shale is herein schematised as a sequence of two kinds of layers: the shaly (high in clay-mineral content) and the sandy (low in clay-mineral content) layers. The mineralogical composition, porosity and hydromechanical parameters are assigned to each layer type. The one-dimensional compressibility and the elastic response of combinations of layers are derived adopting an analytical solution for the stratified, transversely isotropic medium. The drained elastic properties are used to calibrate the approach, and the undrained elastic properties are derived and compared to literature data. Empirical correlations between the layer composition and the geomaterial strength are also drawn. It is shown that by adopting a layered structure with an alternation of two kinds of layers, most of the variability in the studied geomechanical properties of Opalinus Clay can be captured.