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Abstract
The presented study explores the salt dissolution potential of fluids around a salt diapir in the Transylvanian town of Ocna Mures, Romania, which is facing land-collapse hazards related to salt mining activities. A structural three-dimensional (3D) model of the salt diapir, the adjacent basin sediments, and the mining galleries was developed based on existing maps, borehole data, own field observations, and geological publications of the Transylvanian Basin. The salt dissolution potential was estimated from 2D vertical thermohaline flow and transport model scenarios along the southeastern flank of the diapir. Results showed that the following factors increase the salt dissolution capacity along the upper 180 m of the diapir: (1) the presence of more permeable Quaternary alluvial sediments in connection with a fault zone of higher permeability along the diapir, and (2) the presence of more permeable sandstone units within the Miocene sediments in the east of the diapir, which provide freshwater access to the upper parts of the diapir. Thermohaline simulation with viscosity variation of the fluid, instead of a constant viscosity, influences the resulting salt fluxes by up to 50% within studied temperature ranges of 10–60 °C in the model domain. The range of theoretical dissolution rates along the upper 180 m of the diapir supports the hypothesis that cavern collapse is more likely to occur where cavern side walls have already been mined to almost no remaining side walls of rock salt, which is the case in the southeastern part of the diapir.
Highlights
Salt diapirs and the surrounding sediments are often involved in a variety of human activities such as salt mining, exploration and storage of hydrocarbons (e.g. Beckman and Williamson 1990; Hudec and Jackson 2007), and repositories for radioactive waste material (Ludwig et al 2001)
Circulation of fluids around a salt diapir plays a crucial role in the dissolution process, and the flow pattern itself is strongly influenced by the density variation of the fluids due to gradients of salt concentrations and temperature, respectively
Salinity increase in the fault zone along the diapir typically leads to a downward flux due to the higher densities in almost all the simulated scenarios (Fig. 6)
Summary
Salt diapirs and the surrounding sediments are often involved in a variety of human activities such as salt mining, exploration and storage of hydrocarbons (e.g. Beckman and Williamson 1990; Hudec and Jackson 2007), and repositories for radioactive waste material (Ludwig et al 2001). Circulation of fluids around a salt diapir plays a crucial role in the dissolution process, and the flow pattern itself is strongly influenced by the density variation of the fluids due to gradients of salt concentrations and temperature, respectively. Evans et al (1991) studied the effects of thermal and concentration gradients on two hypothetical salt domes. Due to large variations in salinities and temperature, Magri et al (2009) suggest to consider variable fluid viscosity in modeling studies around a salt dome in Schleswig-Holstein, Germany. They showed that using a constant fluid viscosity in the studied large-scale setting leads to overestimated salinity gradients
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