Mudstone microstructure is poorly understood even though it has a profound impact upon permeability. Mudstone permeability is critical in defining the rate at which oil and gas leak through oil and gas field caprocks, and the effectiveness of aquicludes utilised in protecting clean confined groundwater supplies and isolating wastes. During burial, mudstone permeability is controlled initially by the depositional fabric and mineralogy and then by mechanical compaction. A secondary stage of (chemical) compaction and permeability evolution will occur during progressive burial when diagenetic reactions, such as smectite-to-illite and clay-carbonate reactions, occur as a result of the faster reaction rates at higher temperatures and the overstepping of thermodynamic reaction boundaries. This research is intended to address the links between microstructure and mineralogy of mudstones, permeability, pore size distribution and their bulk sealing capacity. These data will be placed in the context of the geological history in terms of depth, temperature, pressure and effective stress histories.