Sub-surface clay samples are difficult to characterize using conventional methods so non-invasive Nuclear Magnetic Resonance (NMR) techniques were used to evaluate in a preserved state the pore structure, porosity, water mobility, and affinity of various clay systems. Within the CLAYWAT project launched by the NEA Clay Club, some of the most advanced NMR techniques were applied to samples from 11 clay-rich sedimentary formations (Boom Clay, Yper Clay (both Belgium); Callovo-Oxfordian shale, Upper Toarcian (both France); Opalinus Clay from two sites (Switzerland); Queenston Fm., Georgian Bay Fm., Blue Mountain Fm. (all Canada); Boda Clay (Hungary); and Wakkanai Fm. and Koetoi Fm. (Japan)). The degree of induration within this suite of samples varies substantially, resulting in a wide porosity range of 0.02–0.6. The key finding is the determination of pore-size distribution by NMR cryoporometry in the range of 2 nm–1 μm with the native fluid present in the pore space for most samples. The water volume in pore sizes of <2 nm could also be measured, thus providing a full description of the porosity system. A specific preparation by sample milling was applied to the preserved original cores minimizing disturbances to the samples in terms of water loss. The water content measured by NMR relaxation was comparable to values obtained by drying at 105°C. In general, the narrow T2 distributions indicate that water was diffusing throughout the pore network during the magnetization lifetime, implying that T2 distributions cannot be considered as proxies for the pore-size distributions. For the set of samples considered, the T1/T2 varied between 1.7 and 4.6, implying variable surface affinity. Finally, for most samples, a pore-shape factor of ~2.4, intermediate between a sheet (1) and a cylinder (4), was deduced.