Pore morphology and distribution in the Shaly facies of Opalinus Clay (Mont Terri, Switzerland): Insights from representative 2D BIB–SEM investigations on mm to nm scale

We used Broad-Ion-Beam polishing in combination with Scanning Electron Microscopy to study the microstructure and porosity in the Shaly facies of Opalinus Clay (northern Switzerland). With resolutions of a few nm, we studied areas of around 1 mm2, sufficiently large to be statistically representative for the microstructure. In three samples from a 1 m long drill core from the Mt Terri laboratory, a qualitative description of the microstructure was combined with quantitative description of porosity. Eight different mineral phases and six different pore types are identified, in the clay matrix, siderite, pyrite framboids and fossil shells respectively. Pore sizes in the clay matrix have a power law distribution with an exponent of 2.4. Porosity inferred from the BIB–SEM method is comparable to that measured using mercury porosimetry. Based on micrographs made perpendicular and parallel to the bedding we show that pores and minerals are homogeneously distributed in 3D, and present a microstructural model for the Shaly facies of Opalinus Clay. This model defines predictable homogeneous porous and non-porous regions with similar pore characteristics. Combined, these form the heterogeneous fabric of Opalinus Clay (Shaly facies).