Data pertinent to pore-water composition in Opalinus Clay in the Mont Terri and Mont Russelin anticlines have been collected over the last 20 years from long-term in situ pore-water sampling in dedicated boreholes, from laboratory analyses on drillcores and from the geochemical characteristics of vein infills. Together with independent knowledge on regional geology, an attempt is made here to constrain the geochemical evolution of the pore-waters. Following basin inversion and the establishement of continental conditions in the late Cretaceous, the Malm limestones acted as a fresh-water upper boundary leading to progressive out-diffusion of salinity from the originally marine pore-waters of the Jurassic low-permeability sequence. Model calculations suggest that at the end of the Palaeogene, pore-water salinity in Opalinus Clay was about half the original value. In the Chattian/Aquitanian, partial evaporation of sea-water occurred. It is postulated that brines diffused into the underlying sequence over a period of several Myr, resulting in an increase of salinity in Opalinus Clay to levels observed today. This hypothesis is further supported by the isotopic signatures of SO₄ ²⁻ and ⁸⁷Sr/⁸⁶Sr in current pore-waters. These are not simple binary mixtures of sea and meteoric water, but their Cl⁻ and stable water-isotope signatures can be potentially explained by a component of partially evaporated sea-water. After the re-establishment of fresh-water conditions on the surface and the formation of the Jura Fold and Thrust Belt, erosion caused the activation of aquifers embedding the low-permeability sequence, leading to the curved profiles of various pore-water tracers that are observed today. Fluid flow triggered by deformation events during thrusting and folding of the anticlines occurred and is documented by infrequent vein infills in major fault structures. However, this flow was spatially focussed and of limited duration and so did not markedly affect the bulk pore-water.

Magnetic pore fabrics (MPFs) are a promising approach to explore the 3D pore space of rocks. There exist empirical relationships between the orientation, degree, and shape of MPF with pore space geometry and permeability anisotropy. Nevertheless, the precise nature of these relationships remains elusive. A common assumption in MPF studies is that ferrofluid uniformly fills the pore space, establishing a constant nanoparticle density and magnetic properties in space and time. However, this assumption is challenged by observations of particle sedimentation and time‐dependent magnetic properties, which render the quantitative interpretation of MPFs challenging. This study explores the physical and chemical stability of ferrofluid, mobility of particles after impregnation, and the magnetic properties of impregnated rocks over time. Water‐based ferrofluids are physically more stable than oil‐based ferrofluids, though with higher magnetic variability. In impregnated rock, magnetic nanoparticles display a certain degree of mobility, resulting in changes in MPF degree and shape. When ferrofluid is mixed with epoxy, particles are less mobile, though some changes were observed both during polymerization and aging. The susceptibility of ferrofluid‐epoxy mixtures is lower than for ferrofluid and carrier liquid at the same concentration. Interestingly, the susceptibility of impregnated rock increases over time, regardless of the ferrofluid used for the impregnation. Understanding and controlling these processes will enhance the reliability of MPF interpretations and increase the applicability of the method.