Porosity and pore-size distribution are relevant determining factors for a number of rock properties and states, such as permeability (Section 5.6), porewater chemistry (Section 5.4), diffusion coefficients (Section 5.8), and various geomechanical parameters (Section 5.5). The three-dimensional architecture and connectivity of different pore types also determine the anisotropy of many rock properties. Clay-rich rocks in general, and the Opalinus Clay in particular, are characterised by a relatively large porosity but, at the same time, extremely small pore apertures (the average value for the Opalinus Clay is around 2 – 3 nm).