Porosity and pore-size distribution are relevant determining factors for a number of rock properties and states. The presented data contribute to the interpretation of permeability, porewater chemistry, diffusion coefficients and various geomechanical parameters and moduli.
Porosity of the Opalinus Clay was quantified by laboratory methods and geophysical log analyses and the results show consistent values in the range 0.10 to 0.13, with only a low correlation with current burial depth. Local excursions to lower porosities occur mainly in the upper part of the formation.
In the Jurassic section, a good correlation exists between clay-mineral content and porosity. Exceptions include (Fe-)oolitic lithologies, reef limestones of the «Herrenwis Unit» and silty/sandy beds in the Wedelsandstein Formation.
Apart from the clay-mineral content, the nature of the other constituents (e.g. carbonate/quartz ratio) also affects porosity, and the degree of cementation affects compaction behaviour and therefore porosity. The Opalinus Clay has a lower porosity than other units of comparable clay-mineral content in the confining units, which is probably because of its high deposition rate and therefore much shorter time over which early diagenetic cementation could occur.
The pore-size distribution in clay-rich rocks such as the Opalinus Clay shows a peak at pore radii in the range of only a few nm. In contrast, the pore-size distribution of limestones shows a broad peak in the range of 40 – 100 nm. Marls, i.e. mixtures between clay and carbonate minerals, show bimodal distributions. Thus, the mean pore size is negatively correlated with the clay-mineral content.
The external surface area correlates linearly with the clay-mineral content and cation-exchange capacity.
