Capturing the structural and compositional variability of Opalinus Clay: constraints from multidisciplinary investigations of Mont Terri drill cores (Switzerland)

Abstract

Opalinus Clay is considered a potential host rock for radioactive waste disposal. The Jurassic claystone formation is composed of several facies and subfacies types, which are characterized by varying amounts of quartz, carbonates (cements and fossils) and clay minerals. To provide samples for ongoing experimental and numerical studies, a complete core section was drilled in the Mont Terri rock laboratory. The lithological and structural variability (including tectonic fault structures) from borehole BAD-2 was investigated using a multidisciplinary approach including high-resolution geoelectric in situ borehole measurements, mineralogical/geochemical and (micro)structural analyses.The lithological–compositional variability was captured by successfully applying a modified subfacies approach, which is independently confirmed by the geochemical data and ERT (Electrical Resistivity Tomography) measurements. The variability on the cm to dm scale perpendicular to bedding can be determined based on the mean resistivity and variation of amplitude. In particular, the facies transitions could be precisely located. The new results suggest that both shaly facies types form the homogenous part of the investigated section, whereas the sandy facies and especially the carbonate-rich sandy facies represent the more heterogeneous lithofacies types of the Opalinus Clay. The varying resistivity can be attributed to differences in clay mineral and carbonate content. Regarding the structural variability, brittle faults were observed with varying frequency throughout the investigated section. Most fault planes occur in the shaly facies types, some of them concentrate along heterogeneities on the subfacies scale. The striking reproducibility of the measurements and observations was confirmed by a comparison with boreholes drilled in parallel, indicating a rather low compositional–structural variability parallel to bedding. The applied multidisciplinary approach is well suited to depict the vertical and lateral variability of a claystone formation, allowing an assessment of the degree of homogeneity/heterogeneity based on the subfacies concept.

Strain Partitioning and Frictional Behavior of Opalinus Clay During Fault Reactivation

“…The shaly facies (Fig. 1b, d) is composed of rather homogeneous, dark-grey, laminated, argillaceous claystones mainly consisting of clay minerals (illite, illite-smectite kaolinite and chlorite) with silt to fine-sand sized bioclasts (e.g., fossil shells) and detrital quartz and feldspar grains dispersed throughout the clay-rich matrix (Kneuker and Furche 2021;Lauper et al 2021). The OPA sandy facies (Fig.…”

“…Brittle deformation of clay-rich rocks is favored with decreasing clay mineral content and is further influenced by the present stress field and burial history (e.g., Gutierrez et al 1996;Ingram and Urai 1999;Nygård et al 2006;Bourg 2015). At the Mont Terri URL, several sub-units have been distinguished within the formation, which vary in grainsize, color as well as the proportions of the main mineral components: phyllosilicates, quartz and carbonates (Hostettler et al 2017;Lauper et al 2018Lauper et al , 2021Kneuker and Furche 2021). Accordingly, OPA is generally subdivided into a carbonate-rich sandy, a shaly and a sandy lithofacies type at the URL (Thury and Bossart 1999;Pearson et al 2003).…”

Effects of pH and exposure time to alkaline solutions on the mineralogy of the Opalinus Clay from the lower sandy facies of Mont Terri site

“…These observations underline the significant heterogeneity of the sandy facies of the Opalinus Clay in terms of mineralogy, in agreement with the observations by Lerouge (2014) and Ould Bouya (2014). Indeed, the lower sandy facies of the Mont Terri site is highly heterogeneous and can be divided into three subfacies according to Lauper et al (2018) and Kneuker and Furche (2021).…”

“…Indeed, during the operational phase of the repository, alkaline plumes resulting from the degradation of concrete structures, necessary for stability, can contaminate the underground water (Fernández et al, 2009;Fernández et al, 2010;Ye et al, 2014), further affecting the chemo-hydro-mechanical behavior of claystone through modifying its mineralogy. The dissolution of clay miner-als as well as other main minerals, i. e., quartz and calcite (Kneuker and Furche, 2021;Lauper et al, 2018), will occur. Hence, it is of paramount importance to investigate the impact of alkaline solutions on the mineralogy of claystones, especially in terms of calcite and quartz dissolutions.…”

Failure mode transition in Opalinus Clay: a hydro-mechanical and microstructural perspective

“…This appears to be associated with the contrasting mineral stiffnesses of the constituents such as hard calcite and quartz grains, and softer phyllosilicates. In contrast to the shaly facies of OPA from the MT-URL, the sandy facies is characterised by a heterogeneous mineralogy distribution at the macroscale (Lauper et al, 2021;Kneuker and Furche, 2021) and the microscale (Houben et al, 2014). This heterogeneity is also manifested in the distribution and the style of deformation within this facies (Schuster et al, 2021).…”