A comparative study of representative 2D microstructures in Shaly and Sandy facies of Opalinus Clay (Mont Terri, Switzerland) inferred form BIB-SEM and MIP methods

Abstract

A combination of Broad-Ion-Beam (BIB) polishing and Scanning Electron Microscopy (SEM) has been used to study qualitatively and quantitatively the microstructure of Opalinus Clay in 2D. High quality 2D cross-sections (ca. 1 mm2), belonging to the Shaly and Sandy facies of Opalinus Clay, were investigated down to the nanometre scale. In addition Mercury Intrusion Porosimetry (MIP) and X-Ray powder Diffraction experiments have been used to extend characterization of the microstructure to the mm–cm scale on bulk volume sample material. Interestingly, both end-member samples of the Opalinus Clay show qualitatively similar mineralogy and pore characteristics as well as a comparable pore size distribution and pore morphology within the different mineral phases and mineral aggregates. Differences between the facies are mainly due to variations in mineral size and mineral amount present in the alternating layers of the different facies. Six different porous mineral phases have been identified and the pores have been subdivided into ten different pore types. Pores visible in the SEM images are most abundant in the clay matrix and these seem to follow a power law distribution with a power law exponent of ca. 2.25 independent of the sample location. Furthermore, all common mineral grains show characteristic porosity, pore shape and pore size distribution in 2D and are proposed to be considered as elementary building blocks for Opalinus Clay. Combined these homogeneous elementary building blocks make up the heterogeneous fabric of the different facies of Opalinus Clay. Based on extrapolation of the power law size distribution in the clay matrix below SEM resolution results in a porosity of 10–25% for clay rich layers (60–90% of clay matrix), whereas sand and carbonate layers show an extrapolated porosity of 6–14%. These extrapolated porosities are in agreement with water-loss and physical porosity measurements performed on bulk material of comparable samples.