The influence of siliciclastic content on the strength and deformation behavior of rock salt – Constraints from thermomechanical experiments

Halite forms the main constituent of rock salt, which is regarded as a possible host rock for nuclear waste repositories and storage caverns. The deformation behavior of pure halite and rock salt has been revealed by microfabric analyses of naturally and experimentally deformed samples. Such studies, however, are rare for rock salt with significant amount of secondary phases (e.g., siliciclastics, clay, anhydrite), although these are also common in nature. In order to determine the influence of siliciclastic material on the deformation behaviour of rock salt, we performed deformation experiments using rock salt samples with variable siliciclastic content (1, 7, 38 and 53 vol. %). The experiments were conducted under bulk flattening strain, elevated temperature (345 °C), low differential stress (< 4.6 MPa), and a strain rate of 10-7 s-1. To gain inside in the 3D distribution of the siliciclastic material and in the deformation mechanisms of the constituent minerals, computer tomographic (CT), microstructural and electron backscatter diffraction (EBSD) analyses were applied to both initial and experimentally deformed samples. The EBSD and microstructural data suggest that, independent of the amount of siliciclastic content, the deformation of the halite matrix was largely accommodated by subgrain formation without subgrain rotation recrystallization. The deformation of the siliciclastic domains, on the other hand, was entirely brittle. CT images show open fractures, oriented sub-perpendicular to the least principal stress, σ3. An increase in the siliciclastic content leads to an increase in differential stress of the halite matrix. The new results suggest that the barrier properties of rock salt is significantly reduced by larger content of siliciclastic material, particularly in cases where the siliciclastic parts and their fractures are interconnected making pervasive ascendant fluid transport possible in these lithological units. Future thermomechanical experiments of impure rock salt should focus on the effect of confining pressure, which is expected to reduce the number and width of open fractures in the siliciclastic domains.