Relating natural heterogeneities and rheological properties of rocksalt: New insights from microstructural observations and petrophyisical parameters on Messinian halites from the Italian Peninsula

Abstract

The importance and economic interest of rocksalt as well as its influence on tectonics and applicative purposes such as mining, hydrocarbons extraction, and nuclear waste storage are well known. Careful characterization of physical and chemical properties of rocksalt is fundamental as the rocksalt behavior may influence its potential use for applicative purposes. Mechanical and rheological properties of rocksalt have been extensively studied in the past. However, the role of natural heterogeneities within rocksalt and their effect on salt rheology have not been investigated quantitatively. Here we present a comprehensive salt facies study on Messinian rocksalt from several Italian sites (Volterra Basin, Tuscany, Caltanissetta Basin, Sicily and Crotone Basin, Calabria). Four salt facies end members have been identified and analyzed by optical analyses. The main facies-defining characteristics resulted to be the primary salt crystal abundance, crystal size, roundness and orientation, as well as the clay inclusion contents. Three out of four facies were placed on an evolutionary path from an "immature," with respect to the deformation history, to a "mature," rocksalt. So we observed, with increasing rocksalt maturity, a progressive disappearing of primary crystal remnants, increasing crystals elongation and iso-orientation and decreasing in crystal size. This trend has been confirmed by differential stress calculation from subgrain size. Through seismic waves velocity measurements and uniaxial compressive runs, specific salt facies were tested. Results of the investigations demonstrate that the facies parameters have a distinct influence on the rocksalt petrophysical parameters like P-and S-waves velocity, dynamic and static Young Modulus, elastic limit, and strain at peak. Finally, this study allowed to suggest the subdivision of Volterra's salt sequence in three different units that have been subjected to variable deformation degree in response to the different salt characteristics.

Mineralogy, microstructures and geomechanics of rock salt for underground gas storage

“…A study by Speranza et al (2016) linked natural heterogeneities and the “maturity” of rock salt to rheological properties and concluded that rock salt type, that is, textural characteristics, mineralogical properties and fluid inclusion type and abundance, needs to be taken into account in studies and designs for rock salt applications. The rheological behavior of rock salt is impacted by impurities (solid inclusions) in halite through high rates of creep due to accelerated diffusion rates at halite‐solid phase interfaces (Závada et al, 2015).…”

Multi-Temporal Satellite Interferometry for Fast-Motion Detection: An Application to Salt Solution Mining

“…On a geological point of view, the area of interest is located in the Volterra basin, a NW-SE-oriented Miocene-Pliocene extensional basin developed in response to the eastward migration of the Apennine thrust-belt [47]. The basin is filled by a sedimentary succession ~2500 m thick [48] that includes an evaporitic Messinian succession composed of gypsum and halite layers alternated with conglomerates, sandstone, and claystone [49]. The upper part of this Messinian sequence, known as "Saline di Volterra formation," hosts laminitic gypsum, turbiditic gypsarenite, microcrystalline gypsum, and halite with intercalations of clay, silt, and sandstones [48].…”

“…The upper part of this Messinian sequence, known as "Saline di Volterra formation," hosts laminitic gypsum, turbiditic gypsarenite, microcrystalline gypsum, and halite with intercalations of clay, silt, and sandstones [48]. The Saline di Volterra formation has a variable thickness ranging from a few meters to 200 m, depending on the position within the Volterra basin [49]. The whole sequence is unconformably overlaid by Pliocene marine clays ("Blue Clays"), which represent the closing sedimentation of the basin and widely outcrop in the investigated area (Figure 2).…”

“…Little information is available to reconstruct the subsurface setting of the Messinian sequence in the area of interest. Nicolich et al [50] investigated the area of the Buriano mining concession through high resolution 3D seismic surveys, discovering the presence of cavities in the saline layer at a depth of 150 m to 300 m. Burgalassi et al [51] stated that the productive salt levels are 10 to 40 m thick and are found at a depth ranging from 60 to 400 m. Speranza et al [49,52] analyzed the rheological properties of the salt levels in the mining area of Saline di Volterra. The salt samplings were collected from deep cores which intersected the salt deposit between 122 and 165 m below surface, for a total thickness of 43 m. These authors recognized four salt facies, with variable percentage of halite and gypsum layers and variable grain size of the crystals.…”

“…The salt samplings were collected from deep cores which intersected the salt deposit between 122 and 165 m below surface, for a total thickness of 43 m. These authors recognized four salt facies, with variable percentage of halite and gypsum layers and variable grain size of the crystals. Speranza et al [49] estimated that halite represents the 40% of the total volume of the Messinian series.…”