Equilibrium halite-H2O dihedral angles: High rock-salt permeability in the shallow crust?

Abstract

Shallow deposits of halite are well known to be impermeable to brines and hydrocarbons, yet recent studies of deeply buried salt point toward considerable water-halite interaction. We have resolved this apparent paradox via experimental determinations of the dihedral angle for the (static) halite-H 2 O system at temperatures of 20 to 300 °C and pressures from atmospheric to 2 kbar. The halite-brine dihedral angle, which controls pore-fluid connectivity and hence permeability, is found to be a sensitive function of both temperature and pressure. In sedimentary basins with normal geothermal gradients, halite bodies at depths exceeding 3 km will contain a stable interconnected brine-filled porosity, resulting in permeabilities comparable to those of sandstones. Our results explain recent observations of major salt-fluid interactions at depth, and suggest that deep-rooted salt diapirs may act as conduits for basinal formation water. They also have important implications for the flow of brines around heat-producing canisters of high-level nuclear waste buried in natural rock salt.