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Timestamp: 2019-04-21 10:44:26+00:00

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is the focus of article 3. Along the way, we shall also discuss whether some of the encapsulated gases in salt can be considered samples of the ambient atmosphere that have been held in brine inclusions since the salt bed was first precipitated? And, as a corollary, we will come to a discussion of how did some of the occluded gases first enter or remobilize through the salt mass during the long history of burial and salt flow (halokinesis) experienced by all ancient evaporite units?
Various gases such as, carbon dioxide, nitrogen, methane, hydrogen and hydrogen sulfide, can occur in significant volumes in and around domal salt masses or bedded evaporite deposits, as seen in numerous documented examples in mines and drilling blowouts in Louisiana, New Mexico, Germany, Poland and China (Figures 1, 2; Table 1). Gases are held in pressurized pockets in the salt that, if intersected, can create stability and safety problems during an expansion of operations in an active salt mine or during petroleum drilling, especially if the pockets contain significant levels of toxic or flammable gases, sufficient to drive rockbursts or gassy outbursts into the adjacent opening. A gas outburst (or rockburst) is defined as an unexpected, nearly instantaneous expulsion of gas and rock salt from a mine production face, normally resulting in an expanded open cavity in the salt. Outburst cavity shapes are generally metre- to tens of metre-scale combinations of conical, cylindrical, hemispherical, or elongated shapes with an elliptical cross section decreasing in diameter away from the opening (Figure 1). Many mapped examples in salt mines of the US Gulf coast have the shape of a cornucopia (Molinda, 1988).
In the case of blowouts during oil-well drilling, there are two dominant styles of overpressured-salt encounters. The first, and the main focus in blowout discussions this article) is when gassy fluid outbursts occur internally in the salt unit as it is being drilled. Generally, this happens on the way to a test a deeper subsalt target, or less often on the way to test as series of intrasalt beds. Once intersected, pressures in such intrasalt pockets tend to bleed off and so decrease in hours to days as pressure profiles return to normal (Finnie, 2001; Warren 2016; Chapter 8). Providing the drilling system was designed to deal with short-term high-pressure outbursts, drilling can continue toward the target. The other type of gas outburst encountered when drilling salt is located in or near the periphery of a salt mass or bed, especially where the drill bit breaks out on the other side of a salt mass into a highly overpressured and fractured fluid reservoir. Such intersections allow the drill stem to connect with a large highly-overpressured volume of fluids, with the open fractures facilitating extremely high rates of fluid flow into the well bore. This type of breach draws on a significant fluid volume and a resulting blowout can continue unabated for weeks or months.
Perhaps one the most impressive examples of this type of blowout, and the ability of evaporite unit to seal and maintain an overpressured subsalt pressurized cell, comes from the Alborz 5 discovery in Central Iran (Figure 2; Morley et al., 2013; Gretener, 1982; Mostofi and Gansser, 1957). Earlier wells testing the Alborz Anticline had failed to reach target due to drilling difficulties coming from “an extremely troublesome evaporite section[i] that continually menaced drilling and caused numerous sidetrack operations.” So difficult was drilling through this stressed Upper Red Formation salt unit that it had taken eight months for a previous well to drill through some 170 metres of evaporitic sediments to reach the Qom target. Later wells testing a Qom Fm. target, like Aran-1 to the south of the Alborz anticline, did not intersect thick stressed halite above the Qom Fm., only an anhydrite layer that perhaps was the dissolution residues of former halokinetic salt mass (pers obs.). The discovery well in the Alborz anticline (Alborz 5) had drilled through some 2296 m of middle to late Tertiary clastics and some 381 metres of Oligo-Miocene salines in the lower part of the Upper Red Formation and made up of siliciclastics, banded salt, anhydrite (Figure 3). On its way to the blowout point, the lower part of the well trajectory had penetrated normally to slightly overpressured dirty salt (halokinetic) and then penetrated some 5 cm into the fractured subsalt Qom Limestone (Oligo-Miocene). On August 26, 1956, the entire drill string and mud column were blown back out the hole and many metres into the air. At that time, the mud pressure was 55 MPa (8,000 psi) at a reservoir depth of 2700 m (8,800 ft), a pressure depth ratio of 20.5 kPa/m or 0.91 psi/ft (a lithostatic value!). Over 82 days, the well released 5 million barrels of oil and a large, but unknown quantity of gas before it self-bridged and the flow died on November 18, 1956. The temperature of the oil at the surface was measured at 115°C and at the time of the blowout the mud column density was 2.07 x 103 kg/m3 (129 lb/ft3)(see Figure 3). This type of subsalt overpressured gas occurrence illustrates salt’s ability to act as a highly effective seal holding back huge volumes of highly overpressured fluid. Associated occluding processes are discussed in an earlier series of Salty Matters articles dealing with salt as a seal, especially the article published March 13, 2016.
Much of the occluded gas in a salt body, prior to release into a mine opening or well bore, is held within inclusions within salt crystals or in intercrystalline positions between the salt crystals. Gas-entraining rock salt, was known from salt mines of Poland and in East Germany since the 1830s and described as knistersalz (literally translates as “crackling salt”). In many mines, walking on knistersalz releases gas as little popping sounds from underfoot. The pressure of the shoe adds a little more stress to an already gas-stressed fragment of salt (Roedder, 1972, 1984). Dumas (1830) first described such “popping salt from Wieliczka, Po­land, and concluded that gas was evolved, presumably from compressed gas inclusions, upon dissolving the salt. Further details on the occurrence were given by Rose (1839). As we shall see, this type of salt can cause serious mine accidents when large volumes of salt explo­sively and spontaneously decrepitate into the mine openings as rockbursts. Dumas (1830) and Rose (1839) found the released gas from "popping " salt in Germany to be inflammable. Bun­sen (1851, p. 251) found 84.6 % CH4 in the gas released during the dissolution of Wieliczka salt, while in many early mines in Germany the occluded gas phase is dominated by nitrogen or carbon dioxide (see Article 2).
Knistersalz will "pop" sporadically once placed in water, releasing pressurized gas bubbles as the salt matrix dissolves. This simple demonstration of gas presence is also the foundation for one method of determining the gas content of a rock salt sample (Hyman, 1982). The sometimes rather energetic "pops" that can occur as gases are released from a gas-enriched rock salt sample attest to the high pressures under which the gases are occluded. Pressures postulated in knistersalz can be near-lithostatic and even higher depending on local stresses, related to the low creep limits of rock salt, particularly around mine openings. According to Hoy et al. 1962, CO2-bearing gas mixtures in the knistersalz of the Winnfield salt dome (Louisiana, USA) is under a pressure of 490 - 980 bar (49 - 98 MPa) at 0°C. Similar values (500 - 1000 bar or 50 - 100 MPa) are given by Hyman (1982) for gas bubbles held in rock salt in various Louisiana salt domes. For example, during exploratory drilling in one such Louisiana salt dome, methane gas was released from the salt under a pressure of 62 bar (6.2 MPa) at a flow rate of 1.2 m3/hr (Iannachione et al., 1984).
Mining causes a pressure drop in the rock salt as it is extracted from a working face and such pressure drops can change the phase of a fluid occluded in salt, or change the solubility of a gas dissolved in such a fluid. Carbon dioxide, in particular, is susceptible to a phase change because its critical point is close to some ambient mining conditions. As long as CO2 is present above 1070 psi (7.4 MPa) and below 31°C (88°F; critical point), it will be in a liquid phase. Such conditions are not typical in salt mines in the US. However, CO2 generally exists as a liquid in rock salt in many German potash mines (Gimm, Thoma and Eckart, 1966). When mining drops the pressure (from lithostatic to near atmospheric) the CO2 phase will change to a gas, causing abrupt expansion. The sudden change also results in a 5 to 6°C cooling, as measured in regions near large outbursts (Wolf, 1966). The solubility of gases dissolved in brine also changes when mining. For example, the solubility of methane in brine is extremely low at atmospheric pressure and so is released as gas bubbles from a brine issuing from rock salt fissures upon mining, as observed in a number of US Gulf Coast salt mines (Iannacchione and Schatzel, 1985).
Pressures released during an outburst result in velocities at the outburst throat which can be very large and locally approach sonic velocities (Ehgartner et al., 1998). Velocities of more than 152 m/sec (500 ft/s) have been recorded in vertical airways some distance from rockbursts in Germany. Velocities at the rockburst site would be even higher. Narrow throat characteristic of some rockbursts can result in throttling. However, associated pressure waves are not strong enough to cause the observed levels of equipment destruction, since they are of a magnitude similar to those found in blasting. Rather, observed damage associated with rockbursts is due to flying debris in the pressure wave as the quantities of rock thrown out by the burst have high kinetic energy (Wolf, 1966).
Given the relatively impermeable nature of bedded and halokinetic salt, occluded gases generally are not released from their containment unless mining or drilling activities intercept (1) a gas-filled fissure zone, an area where the voids between the salt crystals are interconnected, (2) a mechanically unstable zone of gas-enriched salt that disaggregates, releasing its entrained gases (a blowout), or (3) as the mine or the drill bit enters some other relatively permeable geologic anomaly (Kupfer, 1990).
The most frequent and largest rockbursts and gas outflows from subsurface salt occurred in the Werra mining district in former East Germany. Gimm and Pforr (1964) report that rockbursts occurred every day in the Werra region. If one also includes potash mines in the Southern Harz region, more than 10,000 outbursts were recorded up till the 1960s in the German salt mines (Dorfelt, 1966). The 1953 Menzengraben(Potash Mine No. 3) rockburst blew out some 100,000 metric tons of fractured rock salt (approximately 1.6 million cubic feet). This may well be the world’s largest rockburst in terms of cavity size (Gimm, 1968). In an earlier incident in the same region in 1886, the shaft Aschersleben II was flooded with water and gas as it reached a depth of 300 m. A pilot hole drilled from the temporary bottom of the shaft into the underlying Stassfurt rock salt, hit a gas pocket, releasing a combination of H2S—CH4—N2 gases, which then escaped under high pressure for some two hours carrying with it an NaCl brine to the height of a “house” above the shaft floor before the outflow abated. The shaft was abandoned (Baar, 1977).
In 1887 the shaft Leopoldshall III, at Stassfurt, had been sunk through the caprock, and into the Zechstein salt to a total depth of 412 m subsurface, when it hit a gas pocket containing H2S, and four miners were killed by gas escape. Subsequently, in 1889, seven more were killed during shaft construction in the same mine. In 1895, a large volume of CO2 was released from rock salt at a depth of 206 m during the sinking of the Salzungen shaft (Gimm 1968, p. 547). Numerous other outbursts of gas occurred in the same Werra-Fulda district with most mines operating at depths greater than 300 meters, with outbursts responsible for a number of deaths both below and above ground. According to Gimm (1968, p. 547), since 1856, toxic gases were also encountered during the sinking of a number of other shafts in the Stassfurt area. Gropp (1918) documents 106 gas occurrences in German potash mines for the period 1907 to 1917, at depths of ≈300 meters and greater. Many of these gassy encounters caused casualties, particularly in salt dome mines of the Hannover area where several of the potash mines were abandoned due to dangerous gas intersections (Barr, 1977).
Less severe examples of gas outbursts and rockbursts transpired in other salt mines around the world (Figure 2). More than 200 gas outbursts with ejected rock salt volumes up to 4500 tons have occurred in the Upper Kama potash deposits of Russia (Laptev and Potekhin, 1989). Baltaretu and Gaube (1966) reported sudden gassy outbursts in potassium salt deposits in Rumania. Outbursts in Polish salt mines were noted by Bakowski (1966). Potash mines in England and Canada also exhibited outbursts (Table 1; Schatzel and Dunsbier, 1988) with the most recent case being a gassy outburst that caused a fatality in the Boulby mine in July 2016.
Major rockbursts, tied to methane releases, occurred in Louisiana in four of the 5-Island salt mines exploiting the crestal portions of subcropping salt domes (Belle Isle, Cote Blanche, Weeks Island, and Jefferson Island) with the exception of Avery Island. Gassy outbursts, of mostly CO2, also occurred at the Winnfield salt mine, Louisiana (Table 1). Rockburst diameters range from a few inches up to over 50 ft. Cavity heights range from several inches to several hundred feet. Smaller rockburst and cavities in the Five-Island mines were ordinarily not reported (Kupfer,1990). Only the more gas-inclusion-rich salt decrepitates in these mines, and the concave curvatures of the walls are such that the resulting slight additional confining force from the concavity keeps the remaining salt from decrepitating further (Figures 1, 4; Roedder, 1984).
The larger outburst shapes tended to be cornucopian in shape, whereas the shorter ones were conchoidally shaped with symmetrical dimensions (Figure 4). Outbursts approaching several hundred feet high were documented in the Jefferson Island and Belle Isle mines. The disaster at Belle Isle mine in 1979, in which five miners died, proved that high-pressure methane in large quantities could be released near instantaneously during a rockburst. It was estimated that more than 17,000 m3 (600,000 ft3) of methane was emitted by the 1979 outburst (Plimpton, et al.,1980). At the former Morton mine at Weeks Island, an even larger gas emission apparently occurred in connection with a rockburst. It was estimated that as much as 1,020 m3 (36,100 ft3) of salt was released as 1.4 million m3 (50 million ft3) of gas filled the former Morton Mine (MSHA,1983). If the limited number of sample points represent a well-mixed mine atmosphere, the gas alone would occupy approximately 17,000 m3 (600,000 ft3) in the salt at lithostatic pressure (Plimpton, et al.,1980).
Outbursts occurred during mining in all three of the mines at Weeks Island - the “old” Morton mine (the site of the now abandoned U.S. Strategic Petroleum Reserve), the Markel mine, and the “new” Morton mine. Perhaps the largest outburst at the “new” Morton mine occurred on October 6, 1982, in the southwest corner of the 1200-ft level, close to the edge of the dome. A balloon with an attached measuring string is typically used to estimate the height of the major vertical outbursts. A balloon went up more than 30 m (100 ft) into an outburst some 10 m (35 ft) wide (MSHA, 1983). Outbursts in the old Morton mine occurred only in the larger lower level (-800 ft) of the two level mine outside the vertically projected boundary of the upper (-600 ft) level. A similar trend was noted at Jefferson Island where no gas outbursts occurred in the upper level of the mine. The outbursts observed at the Jefferson Island mine were in the same relative position at both the 1300-ft and 1500-ft levels. This is attributed to the near vertical orientation of a very gassy zone of salt (Iannacchione, et al., 1984). Structural continuity (banding) is nearly vertical in many Gulf coast salt dome diapirs, except where the top of the dome has mushroomed. As a result, horizontal runs of outbursts have reportedly been small, and unlikely to connect caverns separated by 100 ft or more (Thoms and Martinez, 1978.).
The geometry of the gas pockets is not well known. Thoms & Martinez (1978) argued that prior to the rockburst the gas is concentrated in vertical, cylindrical zones or pockets, which were created and elongated by the upward movement of the salt. Mapping in the Five-Island mines shows that the rockbursts are often aligned along structural trends . At Winnfield (Hoy et al., 1962), and possibly at Belle Isle (Kupfer,1978), the outbursts occur close to the edge of the dome. In other cases (e.g., Cote Blanche and Belle Isle) the outbursts follow structural trends such as shear zones within the dome (Kupfer, 1978). In all cases, there is an association between methane gas occurrence and other anomalous features such as dirty salt, sediment inclusions and oil or brine seeps (see article 2).
Rockbursts are not limited to gassy intersections in domal salt. High-pressure pockets of inert gas, typically nitrogen, are documented in bedded potash mines (Carlsbad, NM), and combustible gases (methane)and fluids (brine and oil) in potash mines in Utah (Djahanguiri, 1984). The Cane Creek potash mine (Utah). exploiting halokinetic salts sandwiched by the bedded formations of the Paradox Basin, had a history of fatalities and extensive equipment damage as a result of rockbursts (Westfield, et al., 1963). In contrast, no gassy outbursts were reported during the construction and operation of the Waste Isolation Pilot Plant in the bedded salts of southeastern New Mexico. During WIPP construction, routine drilling ahead of the road-header checked for gas, but found very little (Munson, 1997).
In my opinion, some gas pockets in domal salt can be related to the diagenetic process creating a caprock, where metahne and H2S are typical byproducts. In others, the gases are related to the burial history and recrystallisation (partially preserving primary nitrogen), while in yet others, the gas release is related to heating and alteration especially of the hydrated salts (hydrogen) and associated fracturing related to igneous intrusion (CO2). In some cases, gases were encountered in fracture systems of cap anhydrite close to the top or edge of the salt dome; such fracture systems apparently had connections to the groundwater as the gassy outbursts were followed by water of varying salinity. In other cases, fracture systems headed by a gas cap connected the expanding mine to overlying aquifers and ongoing salt dissolution was facilitated. But, in most cases of rockburst located within the interior of a salt mass, the majority of the intersected gas pockets are isolated, as once the burst occurred most cavities tended to receive little if any subsequent recharge, so gas and brine outflow rates tended to decrease to zero across hours to days (Loffler, 1962). The relationship between the type of gas, its position in the salt, and possible lithological associations are documented and discussed in detail in articles 2 and 3.
The physics that drives rock and gas outbursts in an expanding mine-face or shaft is relatively straightforward. In the petroleum industry, it constitutes a process set that is already well documented as the cause of many salt-associated gassy blowouts such as Alborz 5 (Figure 3; Warren, 2016 – Chapter 8 for detail on pressure distribution in and about a salt mass). Oilfield blowouts associated with salt occur when pore pressures in fluids in the drilled rock approach or even exceed lithostatic and the weight of mud in the approaching borehole is not sufficient to hold back this overpressured fluids entering and escaping up the borehole (Figure 3). Spindletop and other famous caprock blowouts in the early days of salt dome drilling in Texas and Louisiana are famous examples of this process (Figure 5). Ehgartner et al. (1998) argue that the same pressure release occurs as an expanding mine face approaches a gassy zone in the mined salt. Once the pressure is reduced by the approach of the mine face, the release of gas formerly held in place by lithostatic pressure within a homogenously stressed salt mass will release, the enclosing rock salt will lose cohesion and so a rockburst (gas outburst) occurs (Figure 6).
How is the gas held and distributed within salt at the micro and mesoscale (microns to metres)?
That free gas and gas in inclusions occur simultaneously in salt masses is undeniable, numerous examples come from salt mines and salt cores (Table 1). Gases are held in evaporite salts in three ways (Hermann and Knipping, 1993); 1) Crack- and fissure-bound gases, 2) Mineral-bound gases, a) intracrystal, b) intercrystal, and 3) Absorption-bound gases. Type 1 occurrences, as the name suggests, are defined by gas accumulations in open fractures and fissures, typically in association with brine. Some occurrences are tied to pressurized aquifers, others are isolated local accumulations within the salt. Intracrystal gas occurs as bubbles, some elongate, some rounded in brine inclusions that are fully enclosed within a crystal (typically halite). At the micro (thin section-SEM scale), intracrystalline gases typically form as a few to aggregates of small bubbles, arranged along crystallographic axes or planes, with bubble diameters in the range 1 to 100 µm. Intercrystalline gases occupy the boundary planes of crystals in contact with one another, that is intercrystalline gases occupy polyhedral porosity. According to Hermann and Knipping (1993), up to 90% of the mineral-bound CO2gas mixtures in the salt rocks of the Werra-Fulda mining district is likely intercrystalline, and the remaining 10% is intracrystalline. Absorption bonding is likely an independent form of gas fixation in salt. Adsorptive bonding describes the ability of solids, especially clays, and crystalline compounds to store gas on their surfaces in the form of layered molecules, most would term this a subset of microporous gas storage in a shale.
[i]The stresses in and around and in salt structures can be high and troublesome to stabilize, even today and is an indication of the ongoing dynamic nature of salt flow and recrystallisation in the subsurface.Therefore, if borehole fluid pressure is lower than salt strength during drilling, stress relaxation may significantly reduce open-hole diameters. In some cases, relaxation causes borehole restrictions even before drilling and completion operations are finished and casing has been set.
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Finnie, A. B., 2001, A Case Study of High Pressure Brine Flows within the Zechstein Supergroup of the Southern North Sea: SPE Paper 67781 Presented at the SPE/IADC Drilling Conference held in Amsterdam, the Netherlands, 27 February-1 March 2001.
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Gimm, W., and H. Pforr, 1964, Breaking Behavior of Salt Rock Under Rockbursts and Gas Outbursts: In: 4th International Conference on Strata Control and Rock Mechanics, Columbia University, NY, May 4-8, 1964.
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Within a framework of fluids breaching a subsurface salt body, the breached salt can be a bed of varying thickness, or it can have flowed into a variety of autochthonous and allochthonous salt masses. Autochthonous salt structures are still firmly rooted in the stratigraphic level of the primary salt bed. Allochthonous salt structurally overlies parts of its (stratigraphically younger) overburden and is often no longer connected to the primary salt bed (mother-salt level).
The principal documented mechanism enabling leakage across bedded salt in the diagenetic realm is dissolution, leading to breaks or terminations in salt bed continuity. Less often, leakage across a salt unit can occur where bedded salt has responded in a brittle fashion and fractured or faulted (Davison, 2009). In hydrocarbon-producing basins with widespread evaporite seals, significant fluid leakage tends to occur near the edges of the salt bed. For example, in the Middle East, the laterally continuous Hith Anhydrite (Jurassic) acts as a regional seal to underlying Arab Cycle reservoirs and carbonate-mudstone source rock. The high efficiency of the Hith seal creates many of the regions giant and supergiant fields, including Ghawar in Saudi Arabia, which is the largest single oil-filled structure in the world. Inherent maintenance of the evaporite's seal capacity also prevents vertical migration from mature sub-Hith source rocks into potential reservoirs in the overlying Mesozoic section across much of Saudi Arabia and the western Emirates. However, toward the Hith seal edge are a number of large fields supra-Hith fields, hosted in Cretaceous carbonates, and a significant portion of the hydrocarbons are sourced in Jurassic carbonate muds that lie stratigraphically below the anhydrite level (Figure 1).
The modern Hith Anhydrite edge is not the depositional margin of the laterally extensive evaporite bed. Rather, it is a dissolution edge, where rising basinal brines moving up and out of the basin have thinned and altered the past continuity of this effective seal.
The process of ongoing dissolution allowing vertical leakage near the edge of a subsurface evaporite interval, typifies not just the edge of bedded salts but also the basinward edges of salt units that are also halokinetic. The dissolution edge effect of the Ara Salt and its basinward retreat over time are clearly seen along the eastern edge of the South Oman Salt Basin where the time of filling of the Permian-hosted reservoir structures youngs toward the west (Figure 2).
Once formed, salt diapirs tend to focused upward escape of basinal fluid flows: as evidenced by: (1) localized development of mud mounds and chemosynthetic seeps at depopod edges above diapirs in the Gulf of Mexico (Figure 3a); (2) shallow gas anomalies clustered around and above salt diapirs in the North Sea and (Figure 3b); (3) localized salinity anomalies around salt diapirs, offshore Louisiana and with large pockmarks above diapir margins in West Africa (Cartwright et al., 2007). Likewise, in the eastern Mediterranean region, gas chimneys in the Tertiary overburden are common above regions of thinned Messinian Salt, as in the vicinity of the Latakia Ridge (Figure 4).
Whenever a salt weld or touchdown occurs, fluids can migrate vertically across the level of a now flow-thinned or no-longer-present salt level. Such touchdowns or salt welds can be in basin positions located well away from the diapir edge and are a significant feature in the formation of many larger base-metal and copper traps, as well as many depopod-hosted siliciclastic oil and gas reservoirs (Figure 5: Warren 2016).
Any caprock indicates leakage and fractional dissolution have occurred along the evaporite boundary (Figure 5). Passage of an undersaturated fluid at or near the edge of a salt mass creates a zone of evaporite dissolution residues, which in the case of diapiric occurrences is called usually called a “caprock,” although such diagenetic units do not only form a “cap” or top to a salt structure.
Historically, in the 1920s and 30s, shallow vuggy and fractured caprocks to salt diapirs were early onshore exploration targets about topographic highs in the Gulf of Mexico (e.g. Spindletop). Even today, the density of drilling and geological data derived from these onshore diapiric features means many models of caprock formation are mostly based on examples in Texas and Louisiana. Onshore in the Gulf of Mexico, caprocks form best in dissolution zones at the outer, upper, edges of salt structures, where active cross-flows of meteoric waters are fractionally dissolving the salt. However, rocks composed of fractional dissolution residues, with many of the same textural and mineralogical association as classic Gulf of Mexico caprocks, are now known to mantle the deep sides of subvertical-diapirs in the North Sea (e. g., lateral caprock in the Epsilon Diapir) and define the basal anhydrite (basal caprock) that defines the underbelly of the Cretaceous Maha Sarakham halite across the Khorat Plateau in NE Thailand (Figure 5; Warren, 2016).
All “caprocks” are fractionally-dissolved accumulations of diapir dissolution products and form in zones of fluid-salt interaction and leakage, wherever a salt mass is in contact with undersaturated pore fluids (Figure 6). First to dissolve is halite, leaving behind anhydrite residues, that cross-flushing pore waters can then convert to gypsum and, in the presence of sulphate-reducing bacteria, to calcite. If the diapir experiences another growth pulse the caprock can be broken and penetrated by the rising salt. This helps explain fragments of caprock caught up in shale sheaths or anomalous dark-salt zones, as exemplified by less-pure salt-edge intersection units described as dark and anomalous salt zones in the Gulf of Mexico diapirs (as documented in Article 1).
"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". Extrapolating from their static halite pressure experiments they inferred that halite, occurring at depths of more than ≈3 km and temperatures above 200 °C, has a uniform intrasalt pore system filled with brine, and therefore relatively high permeabilities.
In the real world of the subsurface, salt seals can hold back significant hydrocarbon columns down to depths of more than 6-7 km (see case studies in Chapter 10 in Warren, 2016 and additional documentation the SaltWork database). Based on a compilation of salt-sealed hydrocarbon reservoirs, trans-salt leakage across 75-100 metres or more of pure salt does not occur at depths less than 7-8 km, or temperatures of less than 150°C. In their work on the Haselbirge Formation in the Alps, Leitner et al. (2001) use a temperature range >100 °C and pressures >70 MPa as defining the onset of the dihedral transition.
It seems that across much of the mesogenetic realm, a flowing and compacting salt mass or bed can maintain seal integrity to much greater depths than postulated by static halite percolation experiments. In the subsurface, there may be local pressured-induced changes in the halite dihedral angle within the salt mass, as seen in the Ara Salt in Oman, but even there, there is no evidence of the total km-scale salt mass transitioning into a leaky aquifer via changes in the halite dihedral angle (Kukla et al., 2011). But certainly, as we move from the diagenetic into the metamorphic realm, even thick pure salt bodies become permeable across the whole salt mass. Deeply buried and pressured salt ultimately dissolves as it transitions into various meta-evaporite indicator minerals and zones (Chapter 14, Warren, 2016).
When increasing pressure and temperature changes the halite dihedral angle in the diagenetic realm, then supersaturated hydrocarbon-bearing brines can enter salt formations to create naturally-hydrofractured "dark-salt". As we discussed in Article 2, pressure-induced changes in dihedral angle in the Ara Salt of Oman create black salt haloes that penetrate, from the overpressured salt-encased carbonate sliver source, up to 50 or more meters into the adjacent halite (Schoenherr et al. 2007). Likewise, Kettanah, 2013 argues Argo Salt of eastern Canada also has leaked, based on the presence of petroleum-fluid inclusions (PFI) and mixed aqueous and fluid inclusions (MFI) in the recrystallised halite (Figure 7 - see also Ara “black salt” core photos in Article 2 of this series).
Both these cases of dark-salt leakage (Ara and Argo salts) occur well within the salt mass, indicating the halokinetic salt has leaked or transmitted fluids within zones well away from the salt edge. In the case of the Argo salt, the study is based on drill cuttings collected across 1500 meters of intersected salt at depths of 3-4 km. Yet, at the three km+ depths in the Argo Salt where salt contains oil and bitumen, the total salt mass still acts a seal, implying it must have regained or retained seal integrity, after it leaked. Not knowing the internal fold geometries in any deeply buried salt mass, but knowing that all flowing salt masses are internally complex (as seen in salt mines and namakiers), means we cannot assume how far the hydrocarbon inclusions have moved within the salt mass, post-leakage. Nor can we know if, or when, any salt contact occurred with a possible externally derived hydrocarbon-bearing fluid source, or whether subsequent salt flow lifted the hydrocarbon-inclusion-rich salt off the contact surface as salt flowed back into the interior of the salt mass.
Thus, with any hydrocarbon-rich occurrence in a halokinetic salt mass, we must ask the question; did the salt mass once hydrofracture (leak) in its entirety, or did the hydrocarbons enter locally and then as the salt continued to flow, that same hydrocarbon-inclusion-rich interval moved into internal drag and drape folds? In the case of the Ara Salt, the thickness of the black salt penetration away from its overpressured source is known as it is a core-based set of observations. In the Ara Salt at current depths of 3500-4000 m, the fluid migration zones extend 50 -70 meters out from the sliver source in salt masses that are hundreds of metres thick (Kukla et al., 2011; Schoenherr et al., 2007).
So how do we characterize leakage extent in a buried salt mass without core?
Dark salt, especially if it contains hydrocarbons, clearly indicates fluid entry into a salt body in the diagenetic realm. Key to considerations of hydrocarbon trapping and long-term waste storage is how pervasive is the fluid entry, where did the fluid come from, and what are the likely transmission zones in the salt body (bedded versus halokinetic)?
Their conclusions are based on lab experiments on static salt and extrapolation to a combination of mud log and wireline data collected from a number of wells that intersected salt allochthons in Louann Salt in the Gulf of Mexico. Their lab data on changing dihedral angles inducing leakage or percolation in static salt confirms the experiments of Holness and Lewis (1996 – See Article 2). But they took the implications of dihedral angle change further, using CT imagery to document creation of interconnected polyhedral porosity in static salt at higher temperatures and pressures (Figure 8). They utilise Archies Law and resistivity measures to calculate inferred porosity, although it would be interesting what values they utilise for cementation exponent (depends on pore tortuosity) Sw and saturation exponent. Assuming the standard default values of m = 2 and n =2 when applying Archies Law to back calculate porosity spreads in halite of assumed Sw are likely incorrect.
They then relate their experimental observations to wireline measurements and infer the occurrence of interconnected pores in Gulf of Mexico salt based on this wireline data. Key to their interpretation is the deepwater well GC8 (Figure 9), where they use a combination of a resistivity, gas chromatograms, and mud log observations to infer that hydrocarbons have entered the lower one km of a 4 km thick salt section, via dihedral-induced percolation.
I have a problem in accepting this leap of faith from laboratory experiments on pure salt observed at the static decimeter-scale of the lab to the dynamic km-scale of wireline-inferred observations in a salt allochthon in the real world of the offshore in deepwater salt Gulf of Mexico. According to Ghanbarzadeh et al., 2015, the three-part gray background in Figure 9 corresponds to an upper no-percolation zone (dark grey), a transition zone (moderate grey) and a lower percolation zone (light grey). This they then infer to be related to changes in dihedral angle in the halite sampled in the well (right side column). Across the data columns, what the data in the GC8 well show is: A) Gamma log; allochthon salt has somewhat higher API values at depths shallower than 5000 m; B) Resistivity log, a change in resistivity to higher values (i.e., lower conductivity) with a change in the same cross-salt depth range as seen in the gamma log, beginning around 5100 m; C) Gas (from sniffer), shows a trend of decreasing gas content from the base of salt (around 6200 m) up to a depth around 4700 m, then relatively low values to top salt, with an interval that is possibly shalier interval (perhaps a suture - see below) that also has a somewhat higher gas content ; D) Gas chromatography, the methane (CH4) content mirrors the total gas trends, as do the other gas phases, where measured; E) Mud Log (fluorescence response), dead oil is variably present from base of salt up to 5000 m, oil staining, oil cut and fluorescence (UV) are variably present from base salt up to a depth of 4400 m.
On the basis of the presented log data, one can infer the lower kilometer of the 4 km salt section contains more methane, more liquid hydrocarbons, and more organic material/kerogen compared to the upper 3 km of salt. Thus, the lower section of the salt intersected in the GC8 well is likely to be locally rich in zones of dark or anomalous salt, compared with the overlying 3 km of salt. What is not given in figure 9 is any information on likely levels of non-organic impurities in the salt, yet this information would have been noted in the same mud log report that listed hydrocarbon levels in the well. In my opinion, there is a lack of lithological information on the Gulf of Mexico salt in the Ghanbarzadeh et al. paper, so one must ask; "does the lower kilometer of salt sampled in the GC8 well, as well as containing hydrocarbons also contain other impurities like shale, pyrite, anhydrite, etc. If so, potentially leaky intervals could be present that were emplaced by sedimentological processes unrelated to changes in the dihedral angle of the halite (see next section).
Giving information that is standard in any mud-log cuttings description (such as the amount of anhydrite, shale, etc that occur in drill chips across the salt mass), would have added a greater level of scientific validity to to Ghanbarzadeh et al.'s inference that observed changes in hydrocarbon content up section, was solely facilitated by changes in dihedral angle of halite facilitating ongoing leakage from below the base of salt and not due to the dynamic nature of salt low as the allochthon or fused allochthons formed. Lithological information on salt purity is widespread in the Gulf of Mexico public domain data. For example, Figure 10 shows a seismic section through the Mahogany field and the intersection of the salt by the Phillips No. 1 discovery well (drilled in 1991). This interpreted section, tied to wireline and cuttings information, was first published back in 1995 and re-published in 2010. It shows intrasalt complexity, which we now know typifies many sutured salt allochthon and canopy terrains across the Gulf of Mexico salt province. Internally, Gulf of Mexico salt allochthons, like others worldwide, are not composed of pure halite, just as is the case in the onshore structures discussed in the context of dark salt zones in article 1. Likely, a similar lack of purity and significant structural and lithological variation typifies most if not all of the salt masses sampled by the Gulf of Mexico wells listed in the Ghanbarzadeh et al. paper, including the key GC8 well (Figure 9). This variation in salt purity and varying degrees of local leakage is inherent to the emplacement stage of all salt allochthons world-wide. It is set up as the salt flow (both gravity spreading and gravity gliding) occurs at, or just below the seafloor, fed by varying combinations of extrusion or thrusting, which moves salt out and over the seabed (Figure 11).
Salt, when it is flowing laterally and creating a salt allochthon, is in a period of rapid breakout (Figure 11; Hudec and Jackson, 2006, 2007; Warren 2016). This describes the situation when a rising salt sheet rolls out over its base, much in the same way a military tank moves out over its track belt. As the salt spreads, the basal and lateral salt in the expanding allochthon mass, is subject to dissolution, episodic retreat, collapse and mixing with seafloor sediment, along with the entry of compactional fluids derived from the sediments beneath. Increased impurity levels are particularly obvious in disturbed basal shear zones that transition downward into a gumbo zone (Figure 12a), but also mantle the sides of subvertical salt structures, and can evolve by further salt dissolution into lateral caprocks and shale sheaths (Figure 6).
In expanding allochthon provinces, zones of non-halite sediment typically define sutures within (autosutures; Figure 12b) or between salt canopies (allosutures; Figure 12c). These sutures are encased in halite as locally leaky, dark salt intervals, and they tend to be able to contribute greater volumes of fluid and ongoing intrasalt dissolution intensity and alteration where the suture sediment is in contact with outside-the-salt fluids. Allochthon rollout, with simultaneous diagenesis and leakage, occurs across intrasalt shear zones, or along deforming basal zones. In the basal part of an expanding allochthon sheet the combination of shearing, sealing, and periodic leakage creates what is known as “gumbo,” a term that describes a complex, variably-pressured, shale-rich transition along the basal margin of most salt allochthons in the Gulf of Mexico (Figure 12a). Away from suture zones, as more allochthon salt rolls out over the top of earlier foot-zones to the spreading salt mass, the inner parts of the expanding and spreading allochthon body tend toward greater internal salt purity (less non-salt and dissolution residue sediment, as well as less salt-entrained hydrocarbons and fluid inclusion).
At the salt's upper contact, the spreading salt mass may carry its overburden with it, or it may be bare topped (aka open-toed; Figure 11). In either case, once salt movement slows and stops, a caprock carapace starts to form that is best developed wherever the salt edge is flushed by undersaturated pore waters (Figure 6). Soon after its emplacement, the basal zone of a salt allochthon acts a focus for rising compactional fluids coming from sediments beneath. So, even as it is still spreading, the lower side of the salt sheet is subject to dissolution, and hydrocarbon entry, often with remnants of the same hydrocarbon-entraining brines leaking to seafloor about the salt sheet edge. As the laterally-focused subsalt brines escape to the seafloor across zones of thinned and leaky salt or at the allochthon edge, they can pond to form chemosynthetic DHAL (Deepsea Hypersaline Anoxic Lake) brine pools (Figure 3a). Such seep-fed brine lakes typify the deep sea floor in the salt allochthon region of continental slope and rise in the Gulf of Mexico and the compressional salt ridge terrain in the central and eastern Mediterranean. If an allochthon sheet continues to expand, organic-rich DHAL sediments and fluids become part of the basal shear to the salt sheet (Figure 12a).
Unfortunately, Ghanbarzadeh et al., 2015 did not consider the likely geological implications of salt allochthon emplacement mechanisms and how this likely explains much of the geological character seen in wireline signatures across wells intersecting salt in the Gulf of Mexico. Rather, they assume the salt system and the geological character they infer as existing in the lower portions of Gulf of Mexico salt masses, are tied to post-emplacement changes in salt's dihedral angle in what they consider as relatively homogenous and pure salt masses. They modeled the various salt masses in the Gulf of Mexico as static, with upward changes in the salt purity indicative of concurrent hydrocarbon leakage into salt and facilitated by altered dihedral angles in the halite. A basic tenet of science is "similarity does not mean equivalence." Without a core from this zone, one cannot assume hydrocarbon occurrence in the lower portions of Gulf of Mexico salt sheets is due to changes in dihedral angle. Equally, if not more likely, is that the wireline signatures they present in their paper indicate the manner in which the lower part of a salt allochthon has spread. To me, it seems that the Ghanbarzadeh et al. paper argues for caution in the use of salt cavities for nuclear waste storage for the wrong reasons.
Is nuclear waste storage in salt a safe, viable long-term option?
Worldwide, subsurface salt is an excellent seal, but we also know that salt does fail, that salt does leak, and that salt does dissolve, especially in intrasalt zones in contact with "outside" fluids. Within the zone of anthropogenic access for salt-encased waste storage (depths of 1-2km subsurface) the weakest points for potential leakage in a salt mass, both natural and anthropogenic, are related to intersection with, or unplanned creation of, unexpected fluid transmission zones and associated entry of undersaturated fluids that are sourced outside the salt (see case histories in Chapter 7 and 13 in Warren, 2016). This intersection with zones of undersaturated fluid creates zones of weakened seal capacity and increases the possibility of exchange and mixing of fluids derived both within and outside the salt mass. In the 1-2 km depth range, the key factor to be discussed in relation to dihedral angle change inducing percolation in the salt, will only be expressed as local heating and fluid haloes in the salt about the storage cavity. Such angle changes are tied to a thermal regime induced by long-term storage of medium to high-level radioactive waste.
I use an ideal depth range of 1-2 km for storage cavities in salt as cavities located much deeper than 2 km are subject to compressional closure or salt creep during the active life of the cavity (active = time of waste emplacement into the cavity). Cavities shallower than 1 km are subject to the effects of deep phreatic circulation. Salt-creep-induced partial cavity closure, in a salt diapir host, plagued the initial stages of use of the purpose-built gas storage cavity known as Eminence in Mississippi. In the early 1970s, this cavity was subject to a creep-induced reduction in cavity volume until gas storage pressures were increased and the cavern shape re-stabilised. Cavities in salt shallower than 1 km are likely to be located in salt intervals that at times have been altered by cross flows of deeply-circulating meteoric or marine-derived phreatic waters. Problematic percolation or leakage zones (aka anomalous salt zones), which can occur in some places in salt masses in the 1-2 km depth range, are usually tied to varying combinations of salt thinning, salt dissolution or intersection with unexpected regions of impure salt (relative aquifers). In addition to such natural process sets, cross-salt leakage can be related to local zones of mechanical damage, tied to processes involved in excavating a mine shaft, or in the drilling and casing of wells used to create a purpose-built salt-solution cavity. Many potential areas of leakage in existing mines or brine wells are the result of poorly completed or maintained access wells, or intersections with zones of “dark salt,” or with proximity to a thinned salt cavity wall in a diapir, as documented in articles 1 and 2 (and detailed in various case studies in Chapters 7 and 13 in Warren 2016).
In my opinion, the history of extraction, and intersections with leakage zones, during the life of most of the world’s existing salt mines means conventional mines in salt are probably not appropriate sites for long-term radioactive waste storage. Existing salt mines were not designed for waste storage, but to extract salt or potash with mining operations often continuing in a particular direction along an ore seam until the edge of the salt was approached or even intersected. When high fluid transmission zones are unexpectedly intersected during the lifetime of a salt mine, two things happen; 1) the mine floods and operations cease, or the flooded mine is converted to a brine extraction facility (Patience Lake) or, 2) the zone of leakage is successfully grouted and in the short term (tens of years) mining continues (Warren, 2016).
For example, in the period 1906 to 1988, when Asse II was an operational salt mine, there were 29 documented water breaches that were grouted or retreated from. Over the long term, these same water-entry driven dissolution zones indicate a set of natural seep processes that continued behind the grout job. This is true in any salt mine that has come “out of the salt” and outside fluid has leaked into the mine. “Out-of-salt” intersections are typically related to fluids entering the salt mass via dark-salt or brecciated zones or shale sheath intersections (these all forms of anomalous salt discussed in article 1 and documented in the case studies discussed in Chapter 13 in Warren 2016).
I distinguish such “out-of-salt” fluid intersections from “in-salt” fluid-filled cavities. When the latter is cut, entrained fluids drain into the mine and then flow stops. Such intersections can be dangerous during the operation of a mine as there is often nitrogen, methane or CO2 in an "in-the-salt” cavity, so there is potential for explosion and fatalities. But, in terms of long-term and ongoing fluid leakage “in-salt” cavities are not a problem.
Ultimately, because “out-of-salt” fluid intersections are part of the working life of any salt mine, seal integrity in any mine converted to a storage facility will fail. Such failures are evidenced by current water entry problems in Asse II Mine, Germany (low-medium level radioactive waste storage) and the removal of the oil formerly stored in the Weeks Island strategic hydrocarbon facility, Texas. Weeks Island was a salt mine converted to oil storage. After the mine was filled with oil, expanding karst cavities were noticed forming at the surface above the storage area. Recovery required a very expensive renovation program that ultimately removed more than 95% of the stored hydrocarbons. And yet, during the active life of the Weeks Island Salt Mine, the mine geologists had mapped “black salt” occurrences and tied them to unwanted fluid entries that were then grouted. Operations to block or control the entry of fluids were successful, and salt extraction continued apace. This information on fluid entry was available well before the salt mine was purchased and converted to a federal oil storage facility. However, in the 1970s when the mine was converted, our knowledge of salt properties and salt's stability over the longer term was less refined than today.
Another potential problem with long-term waste storage in many salt mines, and in some salt cavity hydrocarbon storage facilities excavated in bedded (non-diapiric) salt, is the limited thickness of a halite beds across the depth range of such conventional salt mines and storage facilities. Worldwide, bedded ancient salt tends to be either lacustrine or intracratonic, and individual halite units are no more than 10-50 m thick in stacks of various saline lithologies. That is, intracratonic halite is usually interlayered with laterally extensive carbonate, anhydrite or shale beds, that together pile into bedded saline successions up to a few hundred metres thick (Warren 2010). The non-halite interlayers may act as potential long-term intrasalt aquifers, especially if connected to non-salt sediments outside the halite (Figure 13). This is particularly true if the non-salt beds remain intact and hydraulically connected to up-dip or down-dip zones where the encasing halite is dissolutionally thinned or lost. Connection to such a dolomite bed above the main salt bed, in combination with damaged casing in an access well, explains the Hutchison gas explosion (Warren, 2016). Also, if there is significant local heating associated with longer term nuclear waste storage in such relatively thin (<10-50 m) salt beds, then percolation, related to heat-induced dihedral angle changes, may also become relevant over the long-term (tens of thousands of years), even in bedded storage facilities in 1-2 km depth range.
Creating a purpose-built mine for the storage of low-level waste in a salt diapir within the appropriate depth range of 1-2 km is the preferred approach and a much safer option, compared to the conversion of existing mines in diapiric salt, but is likely to be prohibitively expensive. To minimise the potential of unwanted fluid ingress, the entry shaft should be vertical, not inclined. The freeze-stabilised “best practice” vertical shaft currently being constructed by BHP in Canada for its new Jansen potash mine (bedded salt) is expected to cost more than $1.3 billion. If a purpose-built mine storage facility were to be constructed for low to medium level waste storage in a salt diapir, then the facility should operate at a depth of 800-1000m. Ideally, such a purpose-built mine should also be located hundreds of metres away from the edges of salt mass in a region that is not part of an area of older historical salt extraction operations. At current costings, such a conventionally-mined purpose-built storage facility for low to medium level radioactive waste is not economically feasible.
This leaves purpose-built salt-solution cavities excavated within thick salt domes at depths of 1-2 km; such purpose-built cavities should be located well away from the salt edge and in zones with no nearby pre-existing brine-extraction cavities or oil-field exploration wells. This precludes most of the onshore salt diapir provinces of Europe and North America as repositories for high-level nuclear waste, all possible sites are located in high population areas and can have century-long histories of poorly documented salt and brine extraction and petroleum wells. Staying "in-the-salt" over the long-term would an ongoing problem in these regions (see case histories in chapters 7 and 13 in Warren, 2016 for a summary of some problems areas).
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The geological term “black salt” covers a variety of salt textures and associated mechanisms of formation. The term “black” salt also has a non-technical culinary association (kala namak) but, other than in the footnote, I will not discuss it further in this series of articles. The geological descriptor “black” or “dark” salt is widely used in the US salt mining industry as a pointer to possible zones of current or past natural fluid entry into the salt mass. Colouring fluids can be brine, oil or gas, often with solid impurities dominated by shale, anhydrite or calcite-dolomite. These intrasalt “black” or “dark” salt zones in a mine were also referred to as “shear” zones and considered pointers to what are often unstable regions, liable to fluid entry, gassy outbursts and roof or wall collapse. “Shear”, “black” and “dark” salt zones are better described under the broader term “anomalous salt” zones, many of which were or are in fluid contact with the enclosing non-salt sediment mass (Kupfer, 1990).
In a somewhat related fashion, the term “black salt” is used by the oil industry in Oman and Europe to indicate subsurface zones of overpressured salt, where natural hydrofracturing has occurred, and hydrocarbons have penetrated up to 100 m into the sealing salt mass. Hence its dark color (naturally hydrofractured salt and its textures are the focus of the second article in this series on salt leakage). Fluid entry in this type of “black’ salt is ascribed to temperature-related changes in the dihedral angle of the halite crystals in “black” salt zones. In a similar fashion, the term “black” salt was used in a recent paper in Science by Ghanbarzadeh et al. (2015) and the dihedral angle changes are ascribed to temperature increases in halokinetic salt intervals in the offshore Gulf of Mexico. There the authors argue temperature increases have changed the intercrystalline dihedral angle in a salt mass, and so facilitated the entry of fluids from adjacent strata into the salt body.
So, the term “black” salt is used in the geological community without reference to geological criteria that can separate what I consider are at least two distinct styles of “black” or “dark” salt formation and leakage. One type of salt leakage occurs when the salt is relatively shallow and subject to dissolution driven by the entry of meteoric and other near-surface undersaturated waters into folded and refolded shear (anomalous) salt zones in and about salt stems and décollements. This typically occurs when the flowing salt crest is relatively shallow and tends to occur in regions where the leaking “black” salt zone is in contact with the nonsalt boundary edges of the halokinetic salt mass. This process set ultimately leads to an accumulation of insoluble residues (clays, anhydrite, gypsum, calcite, etc.) that define a unit called a caprock. The term “cap” is somewhat of a misnomer as “caprock” units also form on the sides and undersides of a salt mass, wherever the salt unit is in contact with undersaturated cross-flowing formational waters (Warren, 2016). The other type of “black salt, exemplified by the Ara salt in Oman is related to deeper salt burial, salt flow and an association with intrasalt pressurized fluids (a focus of next article in this series on salt leakage). Accordingly, if we are not to confuse styles of “black” salt genesis (meteoric or undersaturated fluid entry versus intrasalt overpressures) then a better non genetic term should be used to describe zones of "black" or "dark" salt. Although less euphonious, the better term is “anomalous” salt. This describes all zones within halite-dominant intervals with features that are not typical of the bulk of the main diapiric salt mass (Kupfer, 1990).
In this first article we look at various types of anomalous salt in salt mines, largely related to the entry of, or interaction with, undersaturated relatively shallow formation waters. The next article focuses on salt leakage and "black" salt related to overpressure. Then, as we shall see in the third article on salt leakage, there are significant implications of occurrences of anomalous salt with respect to practicalities of safe intrasalt storage and fluid contamination with respect to separating the two types of black salt. This is especially so when working in the subsurface without the luxury of core or mine wall exposures. Ignoring the origins of “black” or “dark” salt, and the associated implications for wireline interpretation, means any conclusions in terms of waste storage outcomes and/or hydrocarbon seal potential, by generalizing lab-based experimental results on leaking salt to all “black” salt occurrences in halokinetic settings, will be somewhat confused (e. g. Ghanbarzadeh et al., 2015).
A shear zone in a diapiric structure forms where adjacent parts of a salt structure are moving (rising or falling) at different rates. Such zones tend to dominate the perimeter of a salt structure across which salt mass is rising or falling with respect to the adjacent sediment and so grade outward from the salt spine into a boundary “shale sheath”. Older shear zones and shale sheaths also are commonplace in re-folded intervals within a salt stem. Mapping of these zones by Balk (1953), Muehlberger and Claubaugh (1968) and Kupfer (1976) across many salt mines showed salt in a diapir must flow at different rates at different times. Otherwise, the complex and highly variable internal refolded drape and napkin folds seen in diapirs in all the world’s salt mines could not form. Figure 2 illustrates some internal complexities the diapir scale typifying various diapiric salt structures across the world and the dominantlyvertical flow fabrics in diapir stems and subhorizonatal flow textures in overhangs and salt tongues. Figure 3 shows that same vertical dominance (biaxial elongation) of salt crystals from cores collected in diapir stems cored various salt mines, while Figure 4 shows the typical vertical banding fold style that typifies diapir stems.
Walden and Jacoby (1963) were the first to call attention to a Gulf Coast anomalous salt zone. They documented a fault zone in the Avery Island salt mine that separated the region of salt being mined, across an anomalous zone, from the domal core. To call attention to the zonal ductile, not brittle, nature of intradiapir salt flow, Kupfer, 1974 changed the description of such anomalous zones from "fault” zones to "shear” zones and concluded most intradiapir shear zones were not faulted zones (defined by brittle fracture offset). In a later paper, he suggested abandoning of the genetic and misleading term "shear zone" and proposed replacement with the broader nongenetic term "anomalous salt zone" (Kupfer, 1990).
Compositions--Potash/magnesium, high anhydrite content, very black salt (made up of disseminated fluid and solid impurities.
The terms “anomalous salt” and “anomalous zones” as defined by Kupfer (1990), are based on observations across the various Five Island salt mines of South Louisiana (Figure 1). As later refined in Kupfer et al. (1998), anomalous salt is a rocksalt zone that deviates from what are considered typical domal salt. Typical Gulf Coast rocksalt according to Kupfer is reasonably pure halite (97%+/- 2%), with minor amounts of disseminated anhydrite (CaSO4) being the primary non-halite impurity. Grainsize is considered to be uniform with grain diameters of 3 – 10 mm (0.12 – 0.39 inches). With continued mapping of Five Island mines, Kupfer et al. (1998) and Looff et al. (2010) documented an even wider variety of anomalous salt zone characteristics and concluded that the creation of anomalous zones need not be related to faulting or shearing, but also can be related to fluid entry and salt dissolution (Figure 5). Anomalous salt can be defined by impurity content, structure, colour, or other features. Anomalous features may not have sharp contacts or uniform thickness, and most are not continuous over long distances. Individual anomalous features commonly disappear for tens of metres (hundreds of feet) only to appear over some horizontal distance. The internal salt fabric of a salt dome is always composed of both typical (volumetrically dominant) and anomalous salt. Kupfer (op. cit.) noted that other salt deposits, including horizontally bedded nonhalokinetic salt deposits in the Permian of West Texas and the Devonian of Western Canada, all have anomalous zones of various origins.
Further work in both the salt mines and salt cavern storage industry has increasingly invoked the concept of anomalous features, anomalous zones and boundary shear zones although there is still a significant confusion over the appropriate use of the terminology (Looff et al., 2010). Because of the flow experienced by diapiric salt, most anomalous salt features parallel the near vertical internal banding of the salt. Many anomalous salt features may create zones of differing creep, strength, or dissolution characteristics that can impact the solution mining and operation of a salt storage cavern and some are tied to zones of problematic fluid entry in a mine. An anomalous zone is any zone in a salt diapir that contains 3 or more dissimilar anomalous features (Kupfer, 1990). The term “anomalous” implies nothing regarding the genesis of the zone. While many anomalous zones may extend laterally over hundreds of metres in length, they are variable in nature, near vertical, and parallel to layering (Figure 5). Typical widths are poorly known but are commonly in the order of 30-50m; however individual structures or anomalous features within the anomalous zone may be as thin as millimetres.
Boundary Shear Zones (BSZ) and Edge Zones (EZ) are the two types of anomalous zones that have a genetic interpretation (Looff et al., 2010). Boundary shear zones are those zones that bound an active salt spine where the salt experiences increased shear stress due to differential salt movement. An edge zone is similar to a boundary shear zone except, instead of being internal within the dome, it is confined to the periphery of the salt stock. Anomalous salt is not restricted to shear zones, however within and about as diapir edge one can expect most anomalous salt to be associated with shear zones (Kupfer, 1990; Looff, 2000).
Anomalous zones within a salt spine are in many cases the remnants of relict BSZ’s from older spines incorporated into younger active salt spines and this especially obvious with those boundary zones associated with clastic impurities (Figure 6). Boundary shear zones and edge zones around the dome tend to be more problematic for storage caverns as they are likely to contain greater occurrences of anomalous salt, higher impurity content (including gas and brine) and structural features that may degrade salt quality and enable leakage. Thus salt caverns can be constructed within boundary salt zones, but if possible, they should be avoided as they can result in non-optimal operating conditions, long-term operational difficulties and in the most severe cases contribute to the loss of cavern integrity (Looff et al., 2010). In the case of edge zones, additional distance to the edge of the salt dome needs to be maintained not only to cover any uncertainty regarding the placement of the edge of salt with respect of mine workings but also to account for the potential for degraded salt quality and to provide a sufficient pillar of good quality salt between the mine or cavern wall and the edge of dome.
A top-of-salt boundary between aggradational and dissolutional components atop diapirs in the Five Islands salt landscape typically coincides with underlying anomalous zones of differential shear within the underlying diapir typically indicated by “black” or “dark” salt zones in the various diapirs (Kupfer, 1976; Lock, 2000). Where such interior anomalous “black” salt zones have intersected the edge of the salt mass, they tend to create intervals with a greater propensity for water entry or gas outbursts and unstable roof zone liable to slabbing and collapse (Figure 6). Such anomalous zones can leak water into a mine, and over the longer term create stability problems, as illustrated by problems in; the now abandoned Weeks Island oil storage facility, the Avery Island Salt Mine, and the likely association of a subvertical zone with anomalous salt, and the enhanced fluid entry that occurred during the Lake Peigneur collapse, which was tied to 1980 flooding of the former Jefferson Island Salt Mine. Today, only two of the former mines in the Five Island Salt Dome trend remain unflooded. For a more detailed discussion of these and other salt leakage scenarios tied to undersaturated fluid entry into salt mines and caverns, see case histories in Chapter 13, Warren 2016.
In the walls of the now-flooded Weeks Island salt mine, Kupfer (1976) noted that wide black beds of the internal “shear” zone are unusual and not found over most of the rest of the mine where salt was extracted. In places, the anomalous zone beds contain black clay (Room J-21), orange sandstone (S-20), and other fragments of clastic material (Paine et al., 1965). These clastic remnants typically occur as balls or roundish blebs ranging in size up to tens of cm in diameter. Petroleum leaked out of seams in this black salt zone and seams in the surrounding salt; the escaping fluid ranged from yellow grease and heavy, blue oil to very light, straw-yellow distillate. Methane and carbon dioxide were also common. The width (surmised) and structural complexity of the anomalous zone suggest that internal salt movement continued after a clastic boundary sheath-zone was incorporated into the salt stock (Figure 7).
The cause of the drainage and abandonment of the Weeks Island oil storage facility was an active subsidence sinkhole some 10 metres across and 10 metres deep, first noted near the edge of the SPR facility in May 1992, and perhaps reaching the surface about a year earlier. The growing doline depression was located on the south-central portion of the island, directly over a subsurface trough, which was obvious in the top-of-salt contours based on former mine records before conversion to a hydrocarbon storage facility (Figure7; Neal and Myers, 1995). Earlier shallow exploratory drilling around the Department of Energy service and production shafts in 1986 had identified the presence of irregularities and brine-filled voids along the top of salt mass across this region. A second, much smaller sinkhole was noticed in early February 1995, but it did not constitute a serious threat as it lay outside the area of cavern storage.
The first sinkhole occurs in a position of sharp change in landform slope (transition from high island to gully fill) and lies atop the projected alignment of what is known as Shear Zone E (a dark salt zone) in the underlying salt (Autin and McCulloh, 1995). Neal (1994) pointed out that Kupfer’s 1976 map of that part of the Weeks Island salt mine, located beneath the first sinkhole, was defined by black salt (also shown as Figure 8 which is based on the more recent Kupfer et al. (1998) map). Miners always avoided such “black” salt or “dark” salt zones in the various subsurface workings and the lateral extent of workings in many of the Five Island mines extended only as far as intersections with significant “black” or “dark” salt regions (Figure 6 & 7).
The volume of the first Weeks Island sinkhole (estimated as 650 m3 when first noted), its occurrence over a trough in the top of salt, and its position directly above the oil-filled mine caverns, meant it was of urgent concern to the SPR authorities, especially in terms of the stability of the roof of the storage cavern. This feature did not form overnight; it lies atop a shear zone that formed during the diapiric rise of the salt and capped by a rockhead valley containing Pleistocene sediment fill. Salt extraction during mine operations probably created tension across the shear zone, thereby favouring fracture enlargement in the anomalous salt zone, as early perhaps as 1970 (Figure 6; Waltham et al., 2005). Eventually, an incursion of undersaturated groundwater traversed the fracture zone across some 107 m, from a level equivalent to the rockhead down to the mine where it emerged. Over time, ongoing dissolution enlarged a void at the top of the anomalous salt zone, creating the collapse environment for the sinkhole first noted at the land surface in 1991. Investigations were undertaken in 1994 and 1995 into the cause of active at-surface sinkholes verified that water from the aquifer above the Weeks Island salt dome was seeping into the underground oil storage chamber at the first sinkhole site (Figures 7& 8; Neal and Myers, 1995; Neal et al., 1995, 1997). Drainage and decommissioning of the Weeks Island facility followed.
Beginning in 1994, and continuing until the abandonment of the facility, saturated brine was injected directly into the throat of the first sinkhole, which lay some 75 metres beneath the surface. This essentially arrested further dissolution and bought time for DOE (Department of Energy) to prepare for the safe and orderly transfer of crude oil to another storage facility. To provide added insurance during the oil transfer stage, a “freeze curtain” was constructed in 1995. It consisted of a 54 well installation around the principal sinkhole, which froze the overburden and uppermost salt to a depth of 67 metres (Figure 9; Martinez et al., 1998). Until the mine was filled with brine and its hydrocarbons removed, this freeze wall prevented groundwater flow into the mine via the region of black salt around the sinkhole. Dealing with this sinkhole was costly. Mitigation and the removal and transfer of oil, including the dismantling of infrastructure (pipelines, pumps, etc.), cost a total of nearly US$100 million; the freeze curtain itself cost nearly $10 million.
Following oil fill in 1980-1982, the Weeks Island facility had stored some 72.5 million BBL of crude oil in abandoned mine chambers. Then in November 1995, the Department of Energy (DOE) initiated oil drawdown procedures, along with brine refill and oil skimming, plus numerous plugging and sealing activities. In 1999, at the end of this recovery operation, about 98% of the crude oil had been recovered and transferred to other SPR facilities in Louisiana and Texas; approximately 1.47 MMBL remains in the now plugged and abandoned mine workings. In hindsight, based on an earlier leak into the mine, while it was an operational mine, and the noted presence of black salt in a shear zone in the mined salt, one might fault the initial DOE decision to select this mine for oil storage. In 1978 groundwater had already leaked into a part of the mine adjacent to the sinkhole and this was forewarning of events to come (Martinez et al., 1998). Injection of cement grout into the flow path controlled the leak into the operation mine at that time, but it could just as easily have become uncontrollable and formed a sinkhole then.
The most recently risen part of the Jefferson Island stock crest is now 250 m (800 ft) higher than the adjacent flat-topped salt mass, which is also overlain by a cap rock (Figure 10). The boundary separating the spine from the less active portion of the crest is a finer-grained and a “shale-rich” anomalous zone, penetrated by the former Jefferson Island mine workings. It defined a limit to the extent of salt mining in the diapir, which was focused on extracting the purer salt within the Jefferson Island spine. The spine and its boundary “shear” are reflected in the topography of the Jefferson Island landscape, with a solution lake, called Lake Peigneur, defining the zone of shallower salt created by the active spine. There on November 20, 1980, one of the most spectacular sinkhole events associated with oilwell drilling occurred atop the Jefferson Island dome just west of New Iberia. Lake Peigneur disappeared as it drained into an underlying salt mine cavern and a collapse sinkhole, some 0.91 km2 in area, developed in the SE portion of the lake (Figure 11; Autin, 2002; Warren 2016). In the 12 hours following the first intersection the underlying mine had flooded and the lake was completely drained. The lake is about 2.4 km in diameter, has a bean-shaped configuration, with a topographic promontory along the southeast shore of the lake rising to more than 23 m above sea level and the surrounding delta plain (Figure 10).
Drainage and collapse of the lake began when a Texaco oilrig, drilling from a pontoon in the lake, breached an unused section of the salt mine some 1000 feet (350 metres) below the lake floor (Figure 11). Witnesses working below ground described how a wave of water instantly filled an old sump in the mine measuring some 200 ft across and 24 feet deep. This old sump was in contact with a zone of anomalous “black” (shear zone) salt. The volume of floodwater engulfing the mine corridors couldn’t be drained by the available pumps. At the time of flooding the mine had four working levels and one projected future level. The shallowest was at 800 feet, it was the first mined level and had been exploited since 1922. The deepest part of the mine at the time of flooding was the approach rampways for a planned 1800 foot level. Some 23-28 million m3 of salt had been extracted in the preceding 58 years of mine life. The rapid flush of lake water into the mine, probably augmented by the drainage of natural solution cavities in the anomalous salt zone and associated collapse grabens below the lake floor, meant landslides and mudflows developed along the perimeter of the Peigneur sinkhole, so that post flooding the lake was enlarged by 28 ha.
With water filling the mine workings, the surface entry hole in the floor of Lake Peigneur quickly grew into a half-mile-wide crater. Eyewitnesses all agreed that the lake drained like a giant unplugged bathtub—taking with it trees, two oil rigs (worth more than $5 million), eleven barges, a tugboat and a sizeable part of the Live Oak Botanical Garden. It almost took local fisherman Leonce Viator Jr. as well. He was out fishing with his nephew Timmy on his fourteen-foot aluminium boat when the disaster struck. The water drained from the lake so quickly that the boat got stuck in the mud, and they were able to walk away! The drained lake didn’t stay dry for long, within two days it was refilled to its normal level by Gulf of Mexico waters flowing backwards into the lake depression through a connecting bayou (Delcambre Canal, aka Carline Bayou) former what was a waterfall with the highest drop in the Stat of Louisiana. Since parts of the lake bottom had slumped into the sinkhole during the collapse, the final water level in some sections of the lake was higher than before relative to previous land features. This ground movement and subsidence left one former lakefront house aslant under 12 feet of water.
Implications for other salt mines with anomalous salt zone intersections.
The Peigneur disaster had wider resource implications as it detrimentally affected the profitability of other salt mines in the Five Islands region (Autin, 2002). Even as the legal and political battles at Lake Peigneur subsided, safe mining operations at the nearby Belle Isle salt mine came into contention with public perceptions questioning the structural integrity of the salt dome roof. During ongoing operations, horizontal stress on the mineshaft near the level where the Louann Salt contacts the overlying Pleistocene Prairie Complex across a zone of anomalous salt had caused some mine shaft deterioration. Broad ground subsidence over the mine area was well documented and monitored, as was near continuous ground water leakage into the mine workings. The Peigneur disaster meant an increased perception of continued difficulty with mine operations and an increased risk of catastrophic collapse was considered a distinct possibility. In 1985, a controlled flooding of the Belle Isle Salt m\Mine was completed as part of a safe closure plan.
Subsidence over the nearby Avery Island salt mine (operated by Cargill Salt) has been monitored since 1986 when small bead-shaped sinkholes were initially noticed in the above mine region. Subsidence monitoring post-1986 defined a broad area of bowl-shaped subsidence, within associated areas of gully erosion (Autin, 2002). Avery mine is today the oldest operating salt mine in the United States and has been in continual operation since the American Civil War. The mine underwent a major reconstruction and a improved safety workover after the Lake Peigneur disaster. Subsidence is still occurring today along the active mine edge, which coincides with a topographic saddle above an anomalous salt zone, which is located inside the mined salt area. At times, ground water has seeped into the mine, and there are a number of known soil gas anomalies and solution dolines on the island. These are natural features that predate mining. Much of the subsidence on Avery Island is a natural process as differential subsidence occurs atop any shallow salt structure with the associated creation of zones of anomalous salt (Warren, 2016). Dating of middens and human artifacts around salt-solution induced, water-filled depressions atop the dome, shows dissolution-induced subsidence is a natural process, as are short episodes of lake floor collapse, slumping and the creation of water-filled suprasalt dolines (circular lakes). Such landscape events and their sedimentary signatures have histories that extend back well beyond the 3,000 years of human occupation documented on Avery Island (Autin, 2002).
Compared to the other salt domes of the Five Islands region of Louisiana, the Cote Blanche Island salt mine has benefited from a safe, stable salt mine operation throughout the mine life (Autin, 2002). Reasons for this success to date are possibly; (i) mining operations have not been conducted as long at Cote Blanche Island as other nearby domes, (ii) the Cote Blanche salt dome may have better natural structural integrity than other islands, thus allowing for greater mine stability (although it too has anomalous salt zones, a salt overhang, and other structural complexities), and (iii) the Cote Blanche Salt Mine is surrounded by more clayey (impervious) sediments than the other Five Islands diapirs, all with sandier surrounds, perhaps allowing for lower rates of undersaturated fluid crossflow and greater hydrologic stability.
And so, today, we know that anomalous salt zones near diapirs crests are often tied to subvertical fault or shear zones, and that many are also associated with the presence of past crossflows of undersaturated waters. Across the various US Gulf Coast mines (present and past) the anomalous (“shear”) salt zones within diapirs are known to be potential problematic leakage zones and so are avoided, if possible, during mining operations. This style of black salt distribution and the potential for intrasalt leakage must be taken into account when near-crestal and shallower portions of domes are to be utilised for any fluid or waste storage. Without an understanding of the significance of such “black” salt or anomalous salt layers, there are potential undefined leakage problems within some salt structures (Looff et al., 2010; Warren 2016).
Autin, W. J., 2002, Landscape evolution of the Five Islands of south Louisiana: scientific policy and salt dome utilization and management: Geomorphology, v. 47, p. 227-244.
Autin, W. J., and R. P. McCulloh, 1995, Quaternary geology of the Weeks and Cote Blanche islands salt domes: Gulf Coast Association of Geological Societies Transactions, v. 45, p. 39-46.
Balk, R., 1953, Salt Structure of Jefferson Island Salt Dome, Iberia and Vermilion Parishes, Louisiana: Bulletin American Association Petroleum Geologists, v. 37, p. 2455-2474.
Kupfer, D., 1976, Shear zones inside Gulf Coast salt stocks help to delineate spines of movement: Bulletin American Association of Petroleum Geologists, v. 60, p. 1434-1447.
Kupfer, D. H., 1974, Boundary shear zones in salt stocks: in Fourth Symposium on Salt. Northern Ohio Geological survey, v. 1, p. 215-225.
Kupfer, D. H., B. E. Lock, and P. R. Schank, 1998, Anomalous Zones Within the Salt at Weeks Island, Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 48, p. 181-191.
Lock, B. E., 2000, Geologic Mapping of Salt Mines in Salt Diapirs: Approaches and Examples from South Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 50, p. 567-582.
Looff, K. M., 2000, Geologic and Microstructural Evidence of Differential Salt Movement at Weeks Island Salt Dome, Iberia Parish, Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 50, p. 543-555.
Looff, K. M., K. M. Looff, and C. Rautman, 2010, Salt spines, boundary shear zones and anomalous salts: Their characteristics, detection and influence on salt dome storage caverns: Paper presented at Solution Mining Research Institute Spring 2010 Technical Conference, Grand Junction, Colorado, USA, 26-27 April 2010, 23 p.
Martinez, J. D., K. S. Johnson, and J. T. Neal, 1998, Sinkholes in Evaporite Rocks: American Scientist, v. 86, p. 38.
Muehlberger, W. R., and P. S. Clabaugh, 1968, Internal Structure and Petrofabrics of Gulf Coast Salt Domes: AAPG Memoir, v. 8, p. 90-98.
Neal, J. T., 1994, Surface features indicative of subsurface evaporite dissolution: Implications for storage and mining: Solution Mining Research Institute, Meeting paper, 1994 Spring meeting, Houston Texas.
Neal, J. T., S. Ballard, S. J. Bauer, B. L. Ehgartner, T. E. Hinkebein, E. L. Hoffman, J. K. Linn, M. A. Molecke, and A. R. Sattler, 1997, Mine-Induced Sinkholes Over the U.S. Strategic Petroleum Reserve (SPR) Storage Facility at Weeks Island, Louisiana: Geologic Mitigation Prior to and During Decommissioning, SAND96-2387A.: Presented at 6th Multidisciplinary Conference on Sinkholes and the Engineering & Environmental Impacts of Karst, Springfield, Missouri, April 6-9, 1997. Sandia National Laboratories, Albuquerque, NM.
Neal, J. T., S. J. Bauer, and B. L. Ehgartne, 1995, Sinkhole Progression at the Weeks Island, Louisiana, Strategic Petroleum Reserve (SPR) Site: Solution Mining Research Institute, Fall Meeting, San Antonio, Texas, October 1995. Sandia National Laboratories, Albuquerque, NM.
Neal, J. T., and R. E. Myers, 1995, Origin, Diagnostics, and Mitigation of a Salt Dissolution Sink-hole at the U,S. Strategic Petroleum Reserve Storage Site, Weeks Island Louisiana,: Sandia National Laboratories, Albuquerque, NM. Report Sandia SAND95-0222C Paper presented at the Fifth International Symposium on Land Subsidence, The Hague, October 1995. Proceedings of the Fifth International Symposium on Land Subsidence, IAHS Publ. No. 234.
Paine, W. R., M. W. Mitchell, R. R. Copeland Jr., and L. d. A. Gimbrede, 1965, Frio and Anahuac Sediment Inclusions, Belle Isle Salt Dome, St. Mary Parish, Louisiana: American Association Petroleum Geologists - Bulletin, v. 49, p. 616-620.
Walden, W., and C. H. Jacoby, 1963, Exploration by horizon­tal drilling at Avery Island, Louisiana, in A. C. Bersticker, ed., Symposium on Salt (First): Cleveland, OH, Northern Ohio Geo­logical Society, p. 367-376.
Waltham, T., F. Bell, and M. Culshaw, 2005, Sinkholes and Subsidence: Karst and Cavernous Rocks in Engineering and Construction: Berlin Heidelberg, Springer Praxis Books, 382 p.
Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released Jan-Feb. 2015: Berlin, Springer, 1854 p.
 Pungent-smelling condiment Kala Namak (black salt) is widely used in South Asia, it consists primarily of sodium chloride with trace impurities of sodium sulphate, sodium bisulphate, sodium bisulphite, sodium sulphide. Kala Namak is also known as Himalayan Black Salt, Sulemani Namak, Bit Lobon , Kala Noon or as Bire Noon in Nepal. Its characteristic smell and taste is mainly due to its elevated sulfur content, which to a western nose is reminiscent of rotten eggs, largely due to the presence of greigite. The various iron impurities impart a brownish pink to dark violet colour to the coarse translucent crystals and, when ground into a powder, transform into a light purple to pink color.
Traditionally, mined salt was transformed from the raw natural form of salt into commercially-sold kala namak through a reductive chemical process. This heating transforms some of the naturally occurring iron oxidew and sodium sulfates in the raw salt into pungent hydrogen sulfide and sodium sulfide daughter products (along with greigite.[ The various sulphate salt impurities in the halite typify the partially recrystallised meteoric overprints that typify textures and structures in nearsurface salt residues in the Himalayan thrust belt (see Richards et al., 2015 for documentation of the geological and structural characteristics of this salt – this article can be downloaded from the publications page on this website).
Historically, the transformation of Himalayan thrust belt salt into kala namak involved firing the raw salt in a furnace for 24 hours, while sealed in a ceramic jar containing charcoal along with small quantities of harad seeds, amla, bahera, babul bark, or natron. The fired salt was then cooled, stored, and aged prior to sale. Kala namak is still prepared in this manner in northern India with production concentrated in Hisar district, Haryana. Although the kala namak can still be produced from natural salts with the required compounds, it is now common to now manufacture it synthetically using halite from non-Himalayan sources. This is done through combining sodium chloride with smaller quantities of sodium sulfate, sodium bisulfate and ferric sulfate, which are then chemically reduced with charcoal in a furnace. Reportedly, it is also possible to create similar products through reductive heat treatment of sodium chloride, 5–10% of sodium carbonate, sodium sulfate, and some sugar.

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