Microstructural study of the Praid Salt Diapir (Transylvanian basin, Romania) and its implication on deformation history and hydrogen storage potential

The Transylvanian basin is one of the major Tertiary sedimentary basins in the Carpathian-Pannonian region. Its thick sedimentary fill contains prominent Middle Miocene age salt that forms major diapir structures at the basin margins. The microstructural characteristics of the rock salt represent one of the main factors that determines the potential of a salt body for storage of hydrogen. The main aim of this study is to extend our understanding of the deformation mechanism of Praid rock salt located at the eastern margin of the Transylvanian basin. Based on petrography, we identified two types of rock salt: (1) layered salt with rather uniform grain size distribution showing alternation of greyish (clay mineral bearing) and white (clear halite) layers, and (2) massive grey salt with large, elongated halite crystals, accompanied by sub-micrometer size grains of halite. To shed light on the microstructure of the rock salt, we performed electron backscatter diffraction (EBSD) mapping, and studied gamma-irradiated samples both in the massive and layered salt samples. Dislocation creep and pressure solution creep were identified which acted concurrently in the Praid rock salt. The total strain rate falls between 1.2 and 1.3×10⁻¹⁰ s⁻¹. The results of this study reveal a complex deformation history of the salt body where coexisting and migrating fluids have played an important role. The outcome of this project contributes to the hydrogen storage potential assessment for the Transylvanian salt and to a better understanding of the structural evolution of the Transylvanian basin.