[en] Detailed examination by mineralogical and petrological techniques has enabled a detailed characterisation of the lithologies of the Oxford Clay, Corallian Beds and the Kimmeridge Clay beneath the Harwell Research Site. Information obtained has revealed the nature of the bulk mineralogy, pore-types, pore-surface mineralogy and post-depositional alteration of the rocks. Diagenesis has played an important part in determining the mineralogy, porosity and fabric of the rocks and has had the greatest variation of effects in the Corallian Beds, determining the phases now in contact with groundwater. It is these authigenic phases that are of key interest in assessing the behaviour of radionuclides which may be released into the local groundwater systems. The importance of the different pore-types characterised during this investigation and of the mineral phases lining these potential pathways for groundwater movement are discussed in detail at the end of this report. Diagenesis has reduced primary porosity in many of the Corallian rocks by calcite precipitation. In such rocks where a cohesive cement is present, groundwater flow must occur along large-scale fractures and more slowly along intercrystalline grain-boundary cracks. (author)