Petroleum Related Rock Mechanics

Erling Fjær , ... Rasmus Risnes , in Developments in Petroleum Science, 2021

3.4.4 Rock salt

Rock salt is precipitated from sea water and may occur in the Earth as extensive salt beds or interstratified with, for example, sedimentary rocks. The mineralogical composition of natural rock salts varies from very homogeneous (99 % halite; NaCl) to heterogeneous mineral associations. In many areas, salt domes are found, such as beneath the Ekofisk field in the North Sea, where the underlying salt has a strong impact on the reservoir stresses (see Section 3.1). Salt may also be found above reservoirs, such as in the Gulf of Mexico area and offshore Brazil. Sometimes, salt is found to impose drilling problems. Salt has very low permeability and is therefore of interest for long-term storage of hazardous waste.

Salt grains (or crystals) can be between 1 and 50   mm in size. Virgin rock salt is usually characterised by very low porosity (< 0.5–1.0 %), which in some cases may be less than 0.1 %. A significant portion of the pore volume may occur as closed voids containing gas, brine or both. Pore sizes are in the nanometre to micrometre range. Permeability of virgin rock salt in the Earth is probably in the nanoDarcy range or lower (Cosenza and Ghoreychi, 1993). Ultra low permeability of natural intact rock salt enables us to hold this rock impermeable in many practical situations. The negligible permeability of rock salt is also attributed to healing processes and creep taking place under in situ conditions (Horseman, 1988).

A practical problem of measuring porosity and permeability is the solubility of rock salt in the liquids usually used in laboratory routine work. Therefore organic fluids or inert gas is often used for permeability tests. Laboratory measured permeabilities and porosities may be much larger than those representative for field conditions.

The value of Young's modulus in rock salt as obtained in a conventional static test is rate-sensitive. To reduce the effect of rate sensitivity, Young's modulus is usually measured during unloading-reloading paths, yielding E-values of 10–30   GPa for various types of rock salt. Poisson's ratio ranges between 0.15 and 0.4 being 0.2–0.3 on the average (Hansen et al., 1984).

Some rock salt types have tight cementation and are quite competent, whereas others are loosely cemented and can be crushed by hand pressure. Uniaxial compressive strength C 0 typically ranges from about 15   MPa to 35   MPa. Tensile strength T 0 varies from less than 1   MPa to 2–3   MPa. Low resistance against tensile stresses is one of the characteristic features of rock salt. The ratio C 0 / T 0 can be above 20 (Silberschmidt and Silberschmidt, 2000). The angle of internal friction ranges from 40° to 65°. Confining pressure remarkably increases the ductility of rock salt. Axial strain measured at failure in the confined regime can reach 10–25 % (Lux and Rokahr, 1984).

The plastic behaviour of rock salt is linked to very significant creep behaviour. This phenomenon can be explained microscopically by a dislocation glide mechanism (Munson and Wawersik, 1991; Fokker and Kenter, 1994) and can be modelled macroscopically in analogy with time-dependent metal plasticity. The amount of creep strain increases with increasing deviatoric stress and increases strongly with increasing temperature.

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Salt- and shale-detached gravity-driven failure of continental margins

Mark G. Rowan , in Regional Geology and Tectonics (Second Edition), 2020

Diapir dissolution

Halite and some of the other salt minerals are highly soluble, so dissolve in the presence of undersaturated water. This occurs primarily in the phreatic zone due to the circulation of meteoric groundwater (e.g. Warren, 2016) and thus is characteristic of nonmarine environments. When the halite dissolves, nonhalite lithologies in the layered evaporite sequence remain as a layer of caprock at the top of the diapir. On continental margins, caprock is common onshore, may develop during major sea-level lowstands on the shelf, and is largely absent in deepwater settings. If the salt is exposed at the sea floor, dissolution occurs, but salt diapirs in deepwater are almost always covered by a thin veneer of hemipelagic mud that protects the halite from dissolution.

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Core Analysis

Colin McPhee , ... Izaskun Zubizarreta , in Developments in Petroleum Science, 2015

4.7.3 Halite

Halite forms an important cement in certain formations—especially those with a high formation water salt concentration. The challenge in cleaning samples containing halite is to remove salt from the pore water and yet not dissolve the natural halite cement.

In conventional hot Soxhlet cleaning with toluene and methanol, the water content will evaporate rapidly in contact with toluene, precipitating its salt content into the pore volume. Removal of precipitated salt from the pore space using methanol will simultaneously remove native halite cements and alter the porosity and permeability properties of the core.

The selected cleaning method must remove the formation water and its salts without significantly affecting the native halite cement content. Methanol will dissolve both, so it is recommended to use iso-propyl alcohol (IPA) as the solvent. It is impossible to avoid some dissolution of the native halite, but the relatively large surface area of the precipitated salts will cause them to be dissolved more rapidly than cementing halite, which has a relatively small exposure surface. As direct application of water-miscible solvents (e.g. methanol and IPA) with saline pore brine can often cause precipitation, the initial stage of cleaning should therefore attempt to remove a significant volume of water prior to application of IPA.

Before attempting to prepare samples containing halite, a cleaning study should be performed to assess any damage to the core and the optimum cleaning method (Fig. 4.28). This would normally involve flush cleaning plugs with toluene at elevated temperatures (70–80   °C), then IPA, using cyclic periods of injection and stasis (to allow diffusion). A thin end trim is sliced off of one end of the plug and examined under the SEM. If this shows salt precipitate, the IPA cleaning is continued until no precipitated salts remain. When the salt precipitates have been cleared, the plugs are flushed with methanol until the effluent shows no precipitation to silver nitrate. Measuring porosities after each cleaning treatment, in conjunction with SEM analysis, can help define the optimum cleaning method.

Figure 4.28. Example flow chart for cleaning pre-study for halite-cemented samples.

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Large Coal-Derived Gas Fields and Their Gas Sources in the Tarim Basin

Jinxing Dai , et al., in Giant Coal-Derived Gas Fields and their Gas Sources in China, 2017

1.4 Caprock

Gypseous salt rock acts as the major caprock in the Kuqa depression (Lü et al., 2000); i.e., Paleogene Kumugeliemu gypseous salt rock to the west of the Kuqa River (Kelasu tectonic zone) and Neogene Jidike gypseous salt rock east of the Kuqa River (Zhao et al., 2012) (Figure 4.7).

Figure 4.7. Isopach map of Neogene gypseous salt rock superimposed by a map showing hydrocarbon reservoir distribution in the Kuqa depression

 (after Research Institute of Tarim Oilfield, 2013)

Gypseous salt rock suffering no large tectonic movement and fractures shows good sealing performance. As per experimental measurements, both gypsum rock and salt rock demonstrate great breakthrough pressure and median pressure, and the gas column height (which could be theoretically sealed) may reach 577–2039 m. The existence of huge Paleogene gypseous salt rock and uncompacted mudstone overburdened with abnormal pressure is the prerequisite to overpressure gas reservoirs in Keshen. The structural traps would probably not be destroyed by some minor faults on account of the large thickness of this set of caprock. Thus, the premium caprock, composed of alternate layers of gypseous salt rock, mudstone, and uncompacted mudstone, is crucial to the preservation of Keshen overpressure gas fields. The reservoir-seal assemblage composed of this caprock and huge Lower Cretaceous sandstone is essential to the formation of large gas fields in the whole zone.

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Karst Geomorphology

A. Frumkin , in Treatise on Geomorphology, 2013

6.32.1 Introduction

Rock salt (referred to as 'salt'), composed primarily of halite – NaCl, is the most common highly soluble evaporite rock. Although general karst features in carbonate rocks and salt may be quite similar, there are several important differences:

•

halite is the most soluble common mineral, 360   g   l−1 at 25   °C (Langer and Offermann, 1982), which is two orders of magnitude higher than gypsum, and three orders of magnitude larger than limestone;

•

in contrast to carbonates, salt solubility does not depend on dissolved gases or acidity;

•

salt dissolution rate is rapid (Simon, 1981); the rapid evolution of salt karst renders it suitable as possible real-world models for evolution of carbonate-karst systems;

•

because of the high equilibrium solubility, rates are controlled by mass-transport rather than by surface reaction;

•

rock salt is weak and erodible, allowing rapid abrasion;

•

salt has a lower Young's modulus than carbonate rocks; therefore, it is very ductile, that is, it tends to flow under pressure, promoting halokinetic processes, such as diapir rising, deformation, and annealing of voids;

•

intrusion of water, especially where the incoming discharge exceeds the outflow capacity, reduces the shear strength of potential failure surfaces within the salt; under such conditions, flow rates of rock salt may increase dramatically; and

•

salt rock density, approximately 2.1   g   cm−3, is lower than most other sedimentary rocks. This causes buoyant upward flow of deeply buried salt, forming salt diapirs (also known as plugs or domes).

Because of its high solubility and weakness, rock-salt karst is ephemeral. On geologic timescales, the salt is usually recycled to the sea as the surrounding rocks pass through diagenetic into metamorphic stages (Talbot and Pohjola, 2009). Ancient former salt sequences that were completely dissolved can be recognized from their metamorphosed remnants or their megabreccias (Jackson et al., 2003). In humid climates, salt destruction is very rapid; hence, salt rarely crops out, and most dissolution occurs below surface. The dissolution may operate even without guiding disruptions and forced through-flow across the salt unit. Intensive dissolution can occur along the salt front border by an aggressive aquifer, and isolated chamber voids may form wherever concentrated aggressive water is in contact with the salt. The low mechanical strength of salt may induce frequent rock failure; hence, chambers collapse once they reach a critical roof span. Salt karst dynamics induce serious geomorphic hazards, particularly under anthropogenic environmental changes.

The unique geomorphology of salt karst (as compared to better-studied carbonate) reflects these factors, as well as their interaction. This chapter discusses the particular dynamics and peculiarities of salt karst.

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Salt Minerals in Saline Soils and Salt Crusts

Florias Mees , Tatiana V. Tursina , in Interpretation of Micromorphological Features of Soils and Regoliths (Second Edition), 2018

2 Halite

Halite in soils occurs mainly as coatings or cements composed of anhedral crystals, which is observed both in thin sections ( Tursina et al., 1980; Tursina & Yamnova, 1987; Shahid & Jenkins, 1994; Mees, 2003; Smith et al., 2004; Mees & Singer, 2006) and in SEM images (Driessen, 1970; Eswaran & Carrera, 1980; Eswaran et al., 1980, 1983; Vergouwen, 1981a; Eswaran, 1984; Gerasimova et al., 1996) (Fig. 1A). The general absence of well-developed crystal faces has been related to the strongly hygroscopic nature of halite, resulting in partial dissolution following the absorption of water (Eswaran & Carrera, 1980; Eswaran et al., 1980); this aspect of halite behaviour can be investigated using ESEM and low-temperature SEM (cf. Wierzchos et al., 2012). A lack of euhedral forms can be expected for occurrences of highly soluble salts unless they formed very recently (e.g., Buck et al., 2006), but xenotopic fabrics are clearly exceptionally common for halite in soils.

Figure 1. Halite. (A) Halite coating (arrow) composed of anhedral crystals (Otjomongwa pan, Namibia; F. Mees, unpublished data) (PPL). (B) Euhedral cubic halite crystals (Ha), enclosed by xenotopic blödite cement (Bl) (Timimoun, Algeria; F. Mees, unpublished data) (XPL). (C) Halite crystals with high concentration of fluid inclusions (Ha), with variations between bands parallel to the crystal faces, in a surface crust with associated kainite (non-cubic crystals; Ka) (southern Aral Sea basin, Uzbekistan; F. Mees, unpublished data) (PPL). (D) Detail of halite crystal with fluid inclusions, illustrating their cubic shape (southern Aral Sea basin, Uzbekistan; F. Mees, unpublished data) (PPL).

Euhedral to subhedral cubic crystals are largely confined to surface crusts or efflorescences (Eswaran et al., 1980; Vergouwen, 1981a; Herrero Isern et al., 1989; Mees & Stoops, 1991; Vizcayno et al., 1995; Joeckel & Ang Clement, 1999; Gore et al., 2000; Chernousenko et al., 2003; Mees & Singer, 2006; Buck et al., 2011) (Fig. 1B and C). These halite crystals generally formed from brines that covered the surface. However, cubic halite crystals have also been described as forming a meniscus cement in surface crusts (Pueyo Mur, 1978/1979, 1980). In salt lake studies, thick accumulations of cubic crystals, transformed to beds of interlocking equant crystals during diagenesis, have been interpreted as subarial precipitates, formed by capillary evaporation of groundwater (Bobst et al., 2001; Lowenstein et al., 2003).

Subsurface occurrences of euhedral cubic crystals are only rarely reported (Gibson et al., 1983; Amit & Yaalon, 1996; Joeckel & Ang Clement, 1999; Van Hoesen et al., 2001; Schiefelbein et al., 2002; Buck et al., 2006; Moghiseh & Heidari, 2012). They include occurrences of skeletal cubic crystals, interpreted to indicate a high degree of supersaturation (Buck et al., 2006; see also Eswaran & Drees, 2004). Another example are subhedral cubic crystals that formed within the groundmass by displacive and partly incorporative growth (Hussain &Warren, 1989; Amit & Yaalon, 1996). Occurrences of halite crystals in the groundmass are rare, because of the high solubility of the mineral.

A subordinate but common crystal habit of halite in soils is a fibrous form (see Kooistra, 1983). The crystals are perfectly straight or curved, elongated parallel to one of the crystallographic axes. Halite of this type mainly forms along or near the soil surface, where it develops as parallel fibres, perpendicular to the surface (Hanna & Stoops, 1976; Eswaran et al., 1980; Bullock et al., 1985). Random orientations (Joeckel & Ang Clement, 1999), subsurface occurrences (Hanna & Stoops, 1976; Yamnova, 2016) and fibrous cements in sedimentary gypsum crusts (Pueyo Mur, 1978/1979) have also been described. The development of fibrous forms of halite in soil environments has been related to fast drying (Eswaran et al., 1980), capillary evaporation (Joeckel & Ang Clement, 1999) and the continual localised upward movement of soil solutions (von Hodenberg & Miotke, 1983). Fibrous halite perpendicular to a surface is also common in efflorescences on building materials (e.g., Arnold & Kueng, 1985). In this context, it is interpreted to form in conditions with low water content of the covered material, low rates of water supply from the substrate and low evaporation rates (Arnold & Zehnder, 1985).

Shorter prismatic forms (Eswaran et al., 1980) seem to represent an intermediate stage in the development of halite fibres. Another related form is elongated crystals occurring as radial aggregates (Gore et al., 2000).

Features related to the dissolution of halite, other than xenotopic fabrics, include the presence of rounded cavities (Tursina et al., 1980; Chernousenko et al., 2003). Joeckel and Ang Clement (1999) describe more or less cubic hollows in the centre of crystal faces as dissolution features (see also Taher & Abdel-Motelib, 2015), related to the hygroscopic nature of the mineral or to contact with dilute solutions derived from the surface. Other possible examples are pitted surfaces (Gore et al., 2000) and grooves parallel to the longitudinal axis of fibrous crystals (Joeckel & Ang Clement, 1999). Halite casts include angular indentations along the base of silty intercalations in mudstone, recording a period of near-surface displacive halite growth followed by associated halite dissolution and deposition of coarse-grained sediments during flooding (Eriksson et al., 2005).

Other features displayed by halite crystals are the presence of fluid inclusions, which is indicative of fast growth (Fig. 1C and D), and a 'stair-step' pattern along the underside of a surface crust, which has been attributed to growth rather than dissolution (Joeckel & Ang Clement, 1999). A central tubular cavity was observed for elongated halite crystals by von Hodenberg and Miotke (1983), who suggest that it acted as a capillary during crystal growth.

Partial or complete replacement of lenticular gypsum crystals by halite has been described by Hussain and Warren (1989), without commenting on the processes that would explain this relationship between unrelated phases. An association of halite with remains of roots has also been reported (Tursina et al., 1980; Tursina & Yamnova, 1987; Young, 1987; Tovey & Dent, 2002), but the significance of this feature is not apparent. Canfora et al. (2016) imply that halite distribution in surface crusts can be determined by prior biogenic gypsum precipitation around cyanobacterial filaments.

The presence of xenotopic halite coatings has a sealing effect on the underlying soil material (Driessen, 1970; Vergouwen, 1981a; Vizcayno et al., 1995). This type of halite can also act as a cement that binds soil aggregates (Hanna & Stoops, 1976; Eswaran et al., 1980; Schroeder et al., 1984; Zhang & Wang, 1987; Shahid & Jenkins, 1994; Joeckel & Ang Clement, 1999), including vadoze meniscus cements between sand grains (McLaren, 2001). The type of aggregation may vary with salt mineral composition, whereby spheroidal shapes are recognised for halite-dominated materials and a vermiform morphology for thénardite-cemented aggregates (Tursina et al., 1980).

In contrast to cementation by halite, the development of elongated halite crystals may cause separation of soil aggregates (Smith et al., 2004). This includes the uplift of surface material, if deposition of eolian material following salt formation can be excluded (Hanna & Stoops, 1976). Crystal size has been mentioned as a factor for the degree of disruption of soil material, with large halite crystals having a stronger effect than smaller thénardite crystals (Tursina et al., 1980), although various other factors obviously also need to be considered (e.g., Rodriguez-Navarro & Doehne, 1999). A disruptive effect is implied for halite occurrences that are described as being displacive, formed within cracks in gravel-sized grains (Amit et al., 1993; Amit & Yaalon, 1996) or along cleavage planes and other structures in shales (Aref et al., 2002). In other studies, no disruptive effect of halite growth is observed, whereas a close association between halite crystals and altered quartz grain surfaces is interpreted to suggest that the main effect of the presence of salts is an increase of quartz dissolution rates (Young, 1987).

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Salt Minerals in Saline Soils and Salt Crusts

Florias Mees , Tatiana V. Tursina , in Interpretation of Micromorphological Features of Soils and Regoliths, 2010

2 Halite

Halite in soils occurs mainly as coatings or cements composed of anhedral crystals, which is observed both in thin sections ( Tursina et al., 1980; Tursina & Yamnova, 1987; Shahid & Jenkins, 1994; Mees, 2003; Smith et al., 2004; Mees & Singer, 2006) and in SEM images (Driessen, 1970; Eswaran & Carrera, 1980; Eswaran et al., 1980, 1983; Eswaran, 1984; Vergouwen, 1981a; Gerasimova et al., 1996) (Fig. 1A). The general absence of well-developed crystal faces has been related to the strongly hygroscopic nature of halite, resulting in partial dissolution following the absorption of water (Eswaran & Carrera, 1980; Eswaran et al., 1980). A lack of euhedral forms can be expected for occurrences of highly soluble salts unless they formed very recently (e.g. Buck et al., 2006), but xenotopic fabrics are clearly exceptionally common for halite in soils.

Fig. 1. Halite. (A) Halite coating (arrow) composed of anhedral crystals (Otjomongwa pan, Namibia; F. Mees, unpublished data) (PPL). (B) Euhedral cubic halite crystals (Ha), enclosed by a xenotopic bloedite cement (Bl) (Timimoun, Algeria; F. Mees, unpublished data) (XPL). (C) Halite crystals with a high concentration of fluid inclusions (Ha), with variations between bands parallel to the crystal faces, in a surface crust with associated kainite (non-cubic crystals; Ka) (southern Aral Sea basin, Uzbekistan; F. Mees, unpublished data) (PPL).

Euhedral to subhedral cubic crystals are largely confined to surface crusts or efflorescences (Eswaran et al., 1980; Vergouwen, 1981a; Herrero Isern et al., 1989; Mees & Stoops, 1991; Vizcayno et al., 1995; Joeckel & Ang Clement, 1999; Gore et al., 2000; Chernousenko et al., 2003; Mees & Singer, 2006) (Fig. 1B, 1C). These halite crystals generally formed from brines that covered the surface. However, cubic halite crystals have also been described as forming a meniscus cement in surface crusts (Pueyo Mur, 1978/1979, 1980). In salt lake studies, thick accumulations of cubic crystals, transformed to beds of interlocking equant crystals during diagenesis, have been interpreted as subaerial precipitates, formed by capillary evaporation of groundwater (Bobst et al., 2001; Lowenstein et al., 2003).

Subsurface occurrences of euhedral cubic crystals are only rarely reported (Gibson et al., 1983; Amit & Yaalon, 1996; Joeckel & Ang Clement, 1999; Van Hoesen et al., 2001; Schiefelbein et al., 2002; Buck et al., 2006). They include occurrences of skeletal cubic crystals, interpreted to indicate a high degree of supersaturation (Buck et al., 2006; see also Eswaran & Drees, 2004). Another example is subhedral cubic crystals that formed within the groundmass by displacive and partly incorporative growth (Hussain & Warren, 1989; Amit & Yaalon, 1996). Occurrences of halite crystals in the groundmass are rare, because of the high solubility of the mineral.

A subordinate but common crystal habit of halite in soils is a needle-shaped form (see Kooistra, 1983). The crystals are perfectly straight or curved, elongated parallel to one of the crystallographic axes. Halite of this type mainly forms along or near the soil surface, where it develops as parallel fibres, perpendicular to the surface (Hanna & Stoops, 1976; Eswaran et al., 1980; Bullock et al., 1985). Random orientations (Joeckel & Ang Clement, 1999), subsurface occurrences (Hanna & Stoops, 1976) and fibrous cements in sedimentary gypsum crusts (Pueyo Mur, 1978/1979) have also been described. The development of fibrous forms of halite in soil environments has been related to fast drying (Eswaran et al., 1980), capillary evaporation (Joeckel & Ang Clement, 1999) and the continual localised upward movement of soil solutions (von Hodenberg & Miotke, 1983). Fibrous halite perpendicular to a surface is also common in efflorescences on building materials (e.g. Arnold & Kueng, 1985). In this context, it is interpreted to form in conditions with low water content of the covered material, low rates of water supply from the substrate and low evaporation rates (Arnold & Zehnder, 1985).

Shorter prismatic forms (Eswaran et al., 1980) seem to represent an intermediate stage in the development of halite fibres. Another related form is elongated crystals occurring as radial aggregates (Gore et al., 2000).

Features related to the dissolution of halite, other than xenotopic fabrics, include the presence of rounded cavities (Tursina et al., 1980; Chernousenko et al., 2003). Joeckel and Ang Clement (1999) describe more or less cubic hollows in the centre of crystal faces as dissolution features, related to the hygroscopic nature of the mineral or to contact with dilute solutions derived from the surface. Other possible examples are pitted surfaces (Gore et al., 2000), and grooves parallel to the longitudinal axis of fibrous crystals (Joeckel & Ang Clement, 1999). Halite casts include angular indentations along the base of silty intercalations in mudstone, recording a period of near-surface displacive halite growth followed by associated halite dissolution and deposition of coarse-grained sediments during flooding (Eriksson et al., 2005).

Other features displayed by halite crystals are the presence of fluid inclusions, which is indicative of fast growth (Fig. 1C), and a 'stair-step' pattern along the underside of a surface crust, which has been attributed to growth rather than dissolution (Joeckel & Ang Clement, 1999). A central tubular cavity was observed in elongated halite crystals by von Hodenberg and Miotke (1983), who suggest that it acted as a capillary during crystal growth.

Partial or complete replacement of lenticular gypsum crystals by halite has been described by Hussain and Warren (1989), without commenting on the processes that would explain this relationship between unrelated phases. An association of halite with remains of roots has also been reported (Tursina et al., 1980; Tursina & Yamnova, 1987; Young, 1987; Tovey & Dent, 2002), but the significance of this feature is not apparent.

The presence of xenotopic halite coatings has a sealing effect on the underlying soil material (Driessen, 1970; Vergouwen, 1981a; Vizcayno et al., 1995). This type of halite can also act as a cement that binds soil aggregates (Hanna & Stoops, 1976; Eswaran et al., 1980; Schroeder et al., 1984; Zhang & Wang, 1987; Shahid & Jenkins, 1994; Joeckel & Ang Clement, 1999). The type of aggregation may vary with salt mineral composition, whereby spheroidal shapes are recognised for halite-dominated materials and a vermiform morphology for thenardite-cemented aggregates (Tursina et al., 1980).

In contrast to cementation by halite, the development of elongated halite crystals may cause a separation of soil aggregates (Smith et al., 2004). This includes the uplift of surface material, if deposition of aeolian material following salt formation can be excluded (Hanna & Stoops, 1976). Crystal size has been mentioned as a factor for the degree of disruption of soil material, with large halite crystals having a stronger effect than smaller thenardite crystals (Tursina et al., 1980), although various other factors obviously also need to be considered (e.g. Rodriguez-Navarro & Doehne, 1999). A disruptive effect is implied for halite occurrences that are described as being displacive, formed within cracks in gravel-sized grains (Amit et al., 1993; Amit & Yaalon, 1996) or along cleavage planes and other structures in shales (Aref et al., 2002). In other studies, no disruptive effect of halite growth is observed, whereas a close association between halite crystals and altered quartz grain surfaces is interpreted to suggest that the main effect of the presence of salts is an increase of quartz dissolution rates (Young, 1987).

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Sedimentary rocks

S.K. Haldar , in Introduction to Mineralogy and Petrology (Second Edition), 2020

6.7.3.1 Mineral composition, origin, and classification of evaporite rocks

The initial stages of evaporation and concentration of sea water allow the secretion of Ca carbonate in the form of aragonite, high-magnesium calcite or calcite, and the process ends with dolomite. The dolomite subsequently suppresses the Ca carbonates, as explained in Section 6.7.2.2. The salt concentration increases to about 3.5 times with the succeeding evaporation of water, and the salinity of sea water rises to approximately 120%. The mineral gypsum begins to crystallize at this stage at a temperature of 30°C and continues until the concentration of salt in water does not grow to 4.8 times higher than in normal seawater salinity (Table 6.6). The secretion of anhydrite commences above this concentration at temperature of 30°C. Necessary increase in concentration in relation to the normal concentration of sea water and the sequence of secretion of singular evaporite minerals at a temperature of 30°C are listed in Table 6.6.

Table 6.6. Limit values necessary to increase the concentration of sea water at 30°C for the extraction of minerals evaporites (Füchtbauer and Müller, 1970).

Mineral excreted Increase in the concentration of seawater
Calcite, aragonite, dolomite To 3.5 times
Gypsum 3.5–4.8 times
Anhydrite 4.8–9.5 times
Halite 9.5–11 times
K-MG salt &gt;60 times

The secretion of Ca sulfates (gypsum and anhydrite) can take place from solutions of small concentration at temperatures much higher than 30°C. Gypsum, for example, is excreted at a temperature of 58°C from the water with normal salinity, and anhydrite secretes much above that temperature. On the other hand, gypsum and anhydrite can secrete at lower temperatures, if the mother solutions contain a high salinity. The anhydrite begins to exude at a temperature of 60°C from sea water with normal concentration. The same secretion begins at 20°C from the water with 7 times higher concentrations at arid saline or sabkha. The secretion of anhydrite, gypsum, halite, and K-Mg salt is directly dependent on temperature and salinity of water. Normal salinity requires high temperature, and with increasing salinity the secretion of evaporites is possible at lower temperatures, especially in saline or sabkha, and salt lakes.

Gypsum, anhydrite, and halite evaporite sedimentary rocks can be found much more likely than evaporite rocks containing K-Mg salt.

Gypsum is excreted in the closed shallow-sea water, and salt lakes in the initial stages of drying sabkha. The initial salinity might not reach the concentration suitable for the secretion of anhydrite (Table 6.6). The extract of gypsum or anhydrite depends primarily on the concentration (salinity) of water and environments in shallow-sea water, or protected shallow or salt lake, and evaporites sabkha conditions.

Anhydrite (sabkha anhydrite) is excreted in large amounts in association with early diagenetic dolomite in coastal saline or sabkha at temperatures of about 25°C–35°C in conditions of dry climate and strong evaporation of water, which significantly increased the concentration of Ca sulfate and salinity at approximately 4–7 times higher than normal salinity of sea water.

Halite (rock salt, Fig. 1.20) excreted mostly in close marine shallow waters, saline (sabkha) and occasional salt lakes which during dry periods left without water in the form of layered cyclic sequence. Such sequences often destroyed completely by diapirism, which are very prone to salt deposits. In diapirism, by plastic injection in roof sediments a significant part of the salts can dissolve. In subaquatic conditions, that is, the closed shallow sea and salt lakes, salt crystals in evaporation grow at the surface of water, particularly intense at the contact water-sediment and within the sediment due to the relatively slow growth of crystals and slightly elevated salinity, resulting in large crystals of halite.

Diapirism is an anticlinal fold on sedimentary layers in which a mobile core, such as salt or gypsum, has pierced through the more brittle overlying rocks.

The primary porosity in the salt sediments is directly dependent on the dimensions of the crystal and place of their origin. The mechanical compaction can be very different compared to the thickness of salt deposits as it depends on the pore waters and the waters in the surrounding sediments outside evaporite deposits. Migration of highly concentrated fluid from the salt deposits in the water layer or from the water in the residue can cause dolomitization of limestone deposits, and cementing early diagenetic salt and other deposits in the form of extraction of gypsum, anhydrite, and calcite. In such cementation, the primary textural–structural features of evaporites deposits may be preserved very well in the form of laminations, stratifications, and nodules.

The evaporite complexes, specially halite and gypsum deposits, are excellent insulator rocks beneath, which often can be found significant amount of oil and gas accumulation from the standpoint of geology and petroleum resources.

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Deep-Sea Sediments

Reinhard Hesse , Ulrike Schacht , in Developments in Sedimentology, 2011

8 Effects of Evaporite Dissolution on Pore-Water Chemistry

Halite dissolution in the vicinity of salt domes and evaporite layers is the main, although not the only source of high-salinity NaCl and (Ca, Na 2)Cl2 brines which represent the high-salinity end member of pore-water profiles in terms of salinity variations, the counterpart to the low-salinity profiles. Typically they occur at greater depths in sedimentary basins that are encountered in deeper wells. However, increases in chlorinity at relatively shallow depth within the realm of early diagenesis have been reported from a number of oceanic drill sites of the DSDP and ODP in regions known to be underlain by evaporites, for example, the Mediterranean Sea (McDuff et al., 1978; Sayles et al., 1972), the Red Sea (Manheim et al., 1974), and Atlantic continental margins at the Blake Ridge Diapir in ODP Site 996 (Egeberg, 2000), off the Guyanas (Waterman et al., 1972), Namibia (Sotelo and Gieskes, 1978), Morocco (Couture et al., 1978; Gieskes et al., 1980), and the Milano Dome in ODP Sites 970A,B in the Eastern Mediterranean (De Lange and Brumsack, 1998). In some of these, the increase in chloride concentration is not matched by the sodium increase, for example, at Site 374 in the Balearic Basin of the Western Mediterranean (McDuff et al., 1978), indicating dissolution of other complex chlorides (Fig. 9.48). At this site, the rare magnesium-rich mineral lueneburgite [Mg3(PO4)2B2O(OH)4⋅   6H2O] has been detected (Müller and Fabricius, 1978).

Figure 9.48. Interstitial water profiles for DSDP Site 374 in the Balearic Basin of the western Mediterranean Sea, indicating dissolution of evaporite minerals at 380 mbsf (redrawn and modified from Gieskes, 1983, after McDuff et al., 1978). Lithology: I, marls; II, nannofossil ooze; III, dolomitic marls; IV, gypsum, anhydrite; V, halite.

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Survival of subsurface microbial communities over geological times and the implications for astrobiology

Helga Stan-Lotter , in Model Ecosystems in Extreme Environments, 2019

5 Extraterrestrial halite

Extraterrestrial halite has been identified in Martian meteorites ( Treiman et al., 2000), in the Murchison and other carbonaceous meteorites (Barber, 1981) and in the Monahans meteorite, together with sylvite (KCl) and water inclusions (Zolensky et al., 1999). Postberg et al. (2009) found that about 6% of the ice grains from the plumes of Saturn's moon Enceladus are containing roughly 1.5% of a mixture of sodium chloride, sodium carbonate, and sodium bicarbonate. Recent images from the Mars Reconnaissance Orbiter showed evidence for seasonal emergence of liquid flows down steep rocky cliffs in summer, termed Recurring Slope lineae (RSL), which would be consistent with briny liquid water emerging from underground reservoirs on Mars (McEwen et al., 2011; Ojha et al., 2015). There is evidence for various brines on Jupiter's moon Europa that are composed primarily of water and salts (Muñoz-Iglesias et al., 2013). All of these discoveries make the consideration of potential habitats for halophilic life in space intriguing.

As one example, Enceladus is considered here because it is of special interest and has even been called "the best (current) astrobiology target in the Solar System" (McKay et al., 2014). Enceladus is Saturn's sixth largest moon, only 252   km in mean radius. Hydrothermal vents spew water vapor and ice particles from an underground ocean beneath the icy crust of Enceladus (https://saturn.jpl.nasa.gov/science/enceladus/, accessed February 2017; Fig. 4). These plumes include organic compounds, volatile gases, carbon dioxide, carbon monoxide, and as mentioned various salts. With its global ocean, unique chemistry and internal heat, Enceladus has become a promising lead in the search for worlds where life could exist. Tsou et al. (2012) suggested a mission called LIFE (Life Investigation For Enceladus). They pointed out that acquisition of samples would be comparatively easy, since in a low cost fly-by mission sufficient material from the plumes could be obtained for analyses on Earth—no landing would be necessary, and the total mission time would be 13.5   years.

Fig. 4

Fig. 4. Illustration of the interior of Saturn's moon Enceladus showing a global liquid water ocean (dark blue) between its rocky core and ice crust. The South polar region with active jets is indicated. Thickness of layers shown is not to scale.

 Image credit: NASA/JPL-Caltech.

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