Salty Matters

The Blog is written by me, John Warren. Once every three or four weeks or so I will post an article or two on an evaporite topic that has piqued my interest. On the Saltwork Publications webpage (under "the Works") there is a growing library of pdfs and epubs based on these blogs. These articles on the website have much higher resolution extractable graphics in than in the blog. There is also a link to this set of pdfs and epubs on the home page (www.saltworkconsultants.com).

Gases in Evaporites Part 3 of 3; Where do gases generate and reside at the scale of a salt mass or salt bed

John Warren - Saturday, December 31, 2016

So far we have looked at gas distribution and origins in evaporites at micro and mesoscales and have now developed sufficient understanding to extrapolate to the broader scale of architecture for a large body of salt in an evaporite. We shall do this in a classification framework of extrasalt versus diagenetic periphery versus intrasalt gas in a halokinetic salt mass (Figure 1).


Extrasalt gas and brine intersections

This type of gas intersection is perhaps the most damaging to a salt mine operation and tends to occur when a gas release is encountered in an expanding mining operation, or a drill hole, that lies near the salt body edge and intersects nonsalt sediments. Extrasalt fluids can be either normally pressured or overpressured depending on the connectivity of the plumbing in the extrasalt reservoir. Salt because of its excellent seal potential tends not to leak or leak only slowly, so facilitating significant pressure buildup (Warren, in press)

The gas inflow from this type of extrasalt breach in a salt mine is typically accompanied, or followed by, a brine release that sometimes cannot be plugged, even by a combination of grouting and brine pumping. Brine inflow rates in this scenario tend to increase with time as ongoing salt dissolution is via ongoing undersaturated water crossflows and the mine or the shaft is ultimately lost to uncontrollable flooding of gas blowouts in an oil well with poor pressure control infrastructure and planning. This type of edge intersection is why a number of early attempts to construct shafts for potash mines in western Canada failed in the middle of last century. It is why freeze curtains are considered the best way to contract a shaft for a potash mine. Examples of this type of gas/brine intersection are usually tied to telogenetic fluid entry from substantial aquifer reservoirs outside the main salt mass and are discussed in detail in Warren, (2016, Chapter 13) and as a type of salt anomaly association discussed in Warren (in press).

The extrasalt source and potential inflow volume of this form of gas (mostly methane and co-associated brine) is largely tied to maturity of hydrocarbon source rocks located external to the salt mass in both suprasalt and subsalt positions (Figure 1). In the past, unexpected extrasalt intersections of pressurised gas reservoirs during oil well drilling lead to spectacular blowouts or “gushers”, especially in situations where the salt held back a significant volume of fluid held in open fractures beneath or adjacent to a salt seal (Table 1). The fluid-focusing effects of suprasalt dome drape and associated extensional falling and gas leakage also mean “gas clouds” are common above salt domes (Warren, 2016, in press). Low σhmin leads to upward gas migration through fracturing (Dusseault et al., 2004). So, in the supradome extrasalt position, simultaneous blowout and lost circulation conditions can be encountered, as well as the problem of severely gas-cut drilling fluids. The volumes of gassy liquids held in pressurised extrasalt reservoirs can be substantial so blowouts or “gushers” can be difficult to control, as was the case with the world-famous subsalt Qom (1956) and suprasalt Macondo (2010) blowouts (Table 1). Methane and gassy liquids generated by organic maturation tend to be the dominant gases found in this situation.

 

Caprock and other salt periphery-held gases

This style of gas occurrence is in part related to gases sourced in maturing extra-salt sediments but also taps gases that are the result of the diagenetic processes that create caprocks. Caprocks are alteration and dissolution haloes to both bedded and halokinetic salt masses and so are distinct gas reservoirs compared to extrasalt sediments (Warren, 2016; Chapter 7). They are compilations of fractionated insolubles left behind at the salt dissolution interface as the edge of halite mass liquefies. Accordingly, caprocks are zoned mineralogically according rates of undersaturated fluid crossflow and in part responding to variable rates of salt rise and resupply. Anhydrite (once suspended in the mother salt) accretes at the dissolution front. Ongoing undersaturated crossflow at the outer contact of the anhydrite residue carapace alters anhydrite to calcite via bacterially- or thermochemically-driven sulphate reduction, with hydrogen sulphide as a by-product. Additional sulphate reduction can occur in the extrasalt sediment both at or near the caprock site, but also deeper or more distal positions in the extrasalt, so sulphate reduction can be a major source of the H2S gas found in the salt periphery. H2S can also migrate in a c from sulphate reduction in maturing sediments located some depth below the salt.

Dissolution that facilitates caprock also drives the creation of vugs and fractures in the caprock, and is one of the primary controls on reservoir poroperm levels in various caprock oil and gas reservoirs discovered in the 1920s in the US Gulf Coast. Methanogenic biodegradation of the same hydrocarbons, which facilitate sulphate reduction, can generate CO2 in the caprock and extrasalt sediments (Clayton et al., 1997)

Many salt mine problems in Germany in the early days of shaft sinking for salt mining were related to unexpected shallow gas outflows confronted within caprock-hosted gas-filled vugs and fractures encountered by the mine shaft on the way to a potash ore target (Gropp, 1919; Löffler, 1962; Baar, 1977). Likewise, the highly unpredictable distribution of gases in the shallow caprocks and salt peripheries of the US Gulf Coast were the cause of some spectacular blowouts such as Spindletop (1901) (Table 1). Because the volume of held liquids is more limited in the vugs and fractures in a caprock compared to fractured subsalt reservoirs, the rate of fluid escape in a “caprock-fed” gusher tends to lessen and even self-bridge more rapidly than when salt is sealing a fractured overpressured subsalt reservoir (days or weeks versus months). As such these intersections, if isolated from extrasalt reservoirs as not such a problem in the drilling of oil wells. In simpler, less environmentally conscious, early days of oilwell drilling in East Texas in the 1920s, “gushers” were often celebrated, tourist spots and considered a sign of the potential wealth coming to the country being drilled.

Intrasalt gas

This type of accumulation/intersection is often described as an intrasalt gas pocket and is typified by a high rate of gas release, that in a mine is accompanied by a rockburst, followed by a waning flow that soon reaches negligible levels as the pocket drains (see article 1 in this series). Intrasalt gas pockets can create dangerous conditions underground and lives can be lost, but in many cases after the initial blowout and subsequent stabilisation, the mine operations or oil-well drilling can continue. Gas constituents and relative proportions are more variable in intrasalt gas pockets compared to gases held in the extrasalt and the periphery. Extra-salt gases are typically dominated by methane with lesser H2S and CO2, periphery gases by H2S and methane, while intrasalt gases can be dominated by varying proportions of nitrogen, hydrogen or CO2. Methane can be a significant component in some intrasalt gas pockets, but these occurrences are usually located in salt anomalies or fractures that are in current or former connection with the salt periphery.

Gas types and sources at the local and basin scale

The type of gas held within and about a salt mass in a sedimentary basin is broadly related to position in the mass and proximity to a mature source rock. Herein is the problem, most of the gases that occur in various salt-mass related positions (intrasalt, extrasalt and periphery) can have multiple origins and hence multiple sources.

Accumulations of gas with more than 95 vol.% N2 are found in most ancient salt basins and the great majority of these accumulations are hosted in intersalt and subsalt beds, with the gas occurring in both dispersed and free gas forms in the salt, as in many Zechstein potash mines of Germany and the Krasnoslobodsky Mine in the Soligorsk mining region of Russia (Tikhomirov, 2014). Nitrogen gas today constitutes around 80% of earth atmosphere where it can result from the decay of N-bearing organic matter (proteins). Ultimately, nitrogen speciates from aqueous mantle fluids in oxidised mantle wedge conditions in zones of subduction and in terms of dominance in planetary atmospheres it indicates active plate tectonics (Mikhail and Sverjensky, 2014). Nitrogen in the subsurface is large unreactive compared to oxygen and so tens to stay in its gaseous form while oxygen tens to combine into a variety of minerals. When held in a salt bed, nitrogen can be captured from the atmosphere during primary halite precipitation and stored in solution in a brine inclusion so creating a dispersed form of pressurised nitrogen. When buried salt recrystallizes during halokinesis, with flow driven by via pressure solution, inclusion contents can migrate to intercrystalline positions and from there into fractures to become free gas in the salt.

Methane gas captured in and around a salt mass as both dispersed and be gas typically mostly comes from organic maturation. The maturing organic matter can be dispersed in the salt during primary halite precipitation, it can be held in intersalt source beds (as in the Ara Salt of Oman), or it can migrate laterally to the salt edge, along with gases and fluids rising from more deeply buried sources. Thus, the presence of oil, solid bitumen and brine inclusions, with high contents of methane in halite, does not unequivocally point to the presence of oil or gas in the underlying strata, it can be locally sourced from intersalt beds as in the Ara Salt. However, a geochemical aureole can be said to occur if hydrocarbons in the halite-hosted inclusions can genetically be linked with reservoired oil or gas. The presence of methane in salt anomalies in Louann Salt mines in the US Gulf Coast and some mines in Germany is likely related to organic maturation of deeply buried extrasalt source rocks with subsequent entrapment during halokinesis and enclosure of allochthon-suture sediments.

Hydrogen sulphide gas (H2S) is a commonplace free gas component in regions of bacterial and thermogenic sulphate reduction. Like methane, much of its genesis is tied to organic maturation products (and sulphate reduction processes), and like methane, it can be held in salt seal traps, or in peripheral salt regions, or in intrasalt and intersalt positions and like metyhane if it escapes and ponds in an air space its release can be deadly (Table 1; Luojiazhai gas field, China). Because both bacterial and thermochemical sulphate reduction requires organic material or methane, there is a common co-occurrence of the two gases. Caprock calcite phases are largely a by-product of bacterial sulphate reduction, so there is an additional association of H2S with caprock-held occurrences. This form of H2S, along with CO2, created many problems in the early days of shaft sinking in German salt mines. More deeply sourced H2S tend to be a production of thermochemical sulphate reduction in regions where pore fluid temperatures are more than 110°C.

Detailed study of CO2 and its associated geochemical/mineralogic haloes shows much of the CO2 held in Zechstein strata of Germany has two main sources; 1) Organic maturation and 2) carbonate rock breakdown especially in magmatic hydrothermal settings (Fischer et al., 2006). The organic-derived CO2 endmember source (with δ13C near -20‰) is present in relatively low concentrations, whereas large CO2 concentrations are derived from an endmember source with an isotope value near 0‰. Although the latter source is not unequivocally defined by its isotopic signature, such “heavy” CO2 sources are most likely attributed to heating-related carbonate decomposition processes. This, for example, explains the CO2-enriched nature of salt mines in parts if the former East Germany where Eocene intrusives are commonplace (Shofield et al., 2014).

Hydrogen (H2) gas distribution as a major component varies across salt basins and is especially obvious in basins with significant levels of carnallite and other hydrated potassic salts. This association leads to elevated radiogenic contents tied to potassic salt units, with hydrogen gas likely derived from the radiogenic decomposition of water (see article 2 in this series). The water molecules can reside in hydrated salts or in brine inclusions in salt crystals.

Summary

Various proportions of gases (N2, CH4, CO2, H2S, H2) held in salt as dispersed and free gas occur in all salt basins. But at the broad scale, certain gases are more common in particular basin and tectonic positions. Methane is typically enriched in parts of a basin with mature source rocks, but can also have a biogenic source. Likewise, H2S is tied to zones of organic breakdown, especially in zones of either bacterial or thermochemical sulphate reduction. CO2 can occur in salt in regions of organic degradation, but is most typical those of parts of a salt basin where igneous processes have driven to thermal and metamorphic decomposition of underlying carbonates (including marbles). Nitrogen because of its inert nature is a commonplace intrasalt gas and comes typically from zones of organic decomposition with dispersed nitrogen becoming free gas with subsequent halokinetic recrystallisation. Ongoing salt flow can drive the distribution of all dispersed salt stored gases into free gas (gas pocket) positions.

References

Baar, C. A., 1977, Applied salt-rock mechanics; 1, The in-situ behavior of salt rocks: Developments in geotechnical engineering. 16a.

Clayton, C. J., S. J. Hay, S. A. Baylis, and B. Dipper, 1997, Alteration of natural gas during leakage from a North Sea salt diapir field: Marine Geology, v. 137, p. 69-80.

Dusseault, M. B., V. Maury, F. Sanfilippo, and F. J. Santarelli, 2004, Drilling around salt: Stresses, Risks, Uncertainties: Gulf Rocks 2004, In 6th North America Rock Mechanics Syposium (NARMS), Houston Texas, 5-9 June 2004, ARMA/NARMS 04-647.

Fischer, M., R. Botz, M. Schmidt, K. Rockenbauch, D. Garbe-Schönberg, J. Glodny, P. Gerling, and R. Littke, 2006, Origins of CO2 in Permian carbonate reservoir rocks (Zechstein, Ca2) of the NW-German Basin (Lower Saxony): Chemical Geology, v. 227, p. 184-213.

Gropp, 1919, Gas deposits in potash mines in the years 1907-1917 (in German): Kali and Steinsalz, v. 13, p. 33-42, 70-76.

Löffler, J., 1962, Die Kali- und Steinsalzlagerstätten des Zechsteins in der Dueutschen Deomokratischen Republik, Sachsen: Anhalt. Freiberg. Forschungsh C, v. 97, p. 347p.

Mikhail, S., and D. A. Sverjensky, 2014, Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere: Nature Geoscience, v. 7, p. 816-819.

Schofield, N., I. Alsop, J. Warren, J. R. Underhill, R. Lehné, W. Beer, and V. Lukas, 2014, Mobilizing salt: Magma-salt interactions: Geology, v. 42, p. 599-602.

Tikhomirov, V. V., 2014, Molecular nitrogen in salts and subsalt fluids in the Volga-Ural Basin: Geochemistry International, v. 52, p. 628-642.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released Feb. 2016: Berlin, Springer, 1854 p.

Warren, J. K., in press, Salt usually seals, but sometimes leaks: Implications for mine and cavern stability in the short and long term: Earth-Science Reviews.

 

 

Red Sea metals: what is the role of salt in metal enrichment?

John Warren - Friday, April 29, 2016

 

Introduction

Work over the past four decades has shown many sediment-hosted stratiform copper deposits are closely allied with evaporite occurrences or indicators of former evaporites, as are some SedEx (Sedimentary Exhalative) and MVT (Mississippi Valley Type) deposits (Warren, 2016). Some ore deposits, especially those that have evolved beyond greenschist facies, can retain the actual salts responsible for the association, primarily anhydrite relics, in proximity to the ore. Such deposits include the Zambian and Redstone copper belts, Creta, Boleo, Corocoro, Dzhezkazgan, Kupferschiefer (Lubin and Mansfeld regions), Largentière and the Mt Isa copper association. All these accumulations of base metals are associated with the formation of a burial-diagenetic hypersaline redox/mixing front, where either copper or Pb-Zn sulphides tended to accumulate. Mechanisms that concentrate and precipitate base metal ores in this evaporite, typically halokinetic, milieu are the topic of upcoming blogs. Then there are deposits that are the result from hot brine fluids, tied to dissolving evaporites and igneous activity, mixing and cooling with seawater, so precipitating a variety of hydrothermal salts, sometimes in including economic levels of copper, lead and zinc (Warren, 2016)

In this article, I focus on one such hypersaline-brine deposit, the cupriferous hydrothermal laminites of the Atlantis II Deep in the Red Sea and look at the role of evaporites in the enrichment of metals in this deposit. It is a modern example of a metalliferous laminite forming in a brine lake sump on the deep seafloor where the brine lake and the stabilisation of the precipitation interface is a result of the dissolution of adjacent halokinetic salt masses. Most economic geologists classify the metalliferous Red Sea deeps as SedEx deposits, but the low levels of lead and high levels of copper, along with its stratigraphic position atop seafloor basalts, place it outside the usual Pb-Zn dominant system that typifies ancient SedEx deposits. Some economic geologists use the Red Sea deeps as analogues for volcanic massive sulphides, and some argue it even illustrates aspects of some stratiform Cu accumulations. Many such economic geology studies have the propensity to ignore the elephant in the room; that is the Red Sea deeps are the result of brine focusing by a large Tertiary-age halokinetically-plumbed seafloor brine association. This helps explain the large volume of metals compared to Cyprus-style and mid-ocean ridge volcanic massive sulphides (Warren 2016, Chapters 15 and16).

In my mind what is most important about the brine lakes on the deep seafloor of the Red Sea is the fact that they exist with such large lateral extents only because of dissolution of the hosting halokinetic slope and rise salt mass. Seismic surveys conducted in the past decade in the Red Sea show extensive salt flows (submarine salt glaciers) along the whole of the Red Sea Rift (at least from 19–23°N; Augustin et al., 2014; Feldens and Mitchell, 2015)). In places, these salt sheets flow into and completely blanket the axial region of the rift. Where not covered by namakiers, the seafloor comprises volcanic terrain characteristic of a mid-ocean spreading axis. In the salt-covered areas, evidence from bathymetry, volume-balance of the salt flows, and geophysical data all seems to support the conclusion that the sub-salt basement is mostly basaltic in nature and represents oceanic crust (Augustin et al., 2014).

 

The Rift

The Red Sea, located between Egypt and Saudi Arabia, represents a young active rift system that from north to south transitions from continental to oceanic rift (Rasul and Stewart, 2015). It is one of the youngest marine zones on Earth, propelled by an area of relatively slow seafloor spreading (≈1.6 cm/year). Together with the Gulf of Aqaba-Dead Sea transform fault, it forms the western boundary of the Arabian plate, which is moving in a north-easterly direction (Figure 1; Stern and Johnson, 2010). The plate is bounded by the Bitlis Suture and the Zagros fold belt and subduction zone to the north and north-east, and the Gulf of Aden spreading center and Owen Fracture Zone to the south and southeast. The Red Sea first formed about 25 Ma ago in response to crustal extension related to the interface movements of the African Plate, the Sinai Plate, and the Arabian Plate (Schardt, 2016). The present site of Red Sea rifting is controlled, or largely overprinting, on pre-existing structures in the crust, such as the Central African Fault Zone. In the area between 15° and 20° along the rift axis, active seafloor spreading is prominent and is characterized by the formation of oceanic crust with Mid-Ocean Ridge Basalt (MORB) composition for the last 3 Ma (Rasul and Stewart, 2015). In contrast, the northern portion of the Red Sea sits in a magmatic continental rift in which a mid-ocean ridge spreading centre is just beginning to form. That is, the split in the crust that is the Red Sea is unzipping from south to north (Figure 1).

The Salt

The rift basement is covered a thick sequence of middle Miocene evaporites that precipitated in the earlier hydrographically isolated stage of rifting (Badenian – Middle Miocene). The maximum thickness of rift-fill sediments, including halokinetic salt, is around 8,000 m in the Morgan basin in the southern Red Sea (Farhoud, 2009; Ehrhardt et al., 2005). Girdler and Southren (1987) conclude that Miocene evaporites first accumulated on Red Sea transitional crust but must have later flowed downdip to now cover parts of the axial zone (basaltic) of the Plio-Pleistocene oceanic crust. At latitudes of 20° to 23° N, transform fracture zones provide focused passage-ways for salt flow. They also enable the involvement of dissolving salt in axial hydrothermal circulation, so producing pools of dense hot brines and the topographic isolation of spreading segments into evaporite-enclosed deeps (Feldens and Mitchell, 2015). So today, flow-like features cored by Miocene evaporites are situated along the axis of the Red Sea atop younger magnetic seafloor spreading anomalies. However, not all brine seeps occur in or near the deep axis of the Red Sea on the downdip edge of flowing Miocene salt, some occur in much shallower suprasalt positions nearer the coastal margins of the Red Sea, in waters just down dip of actively-growing well-lit coral reefs (Batang et al., 2012).


Six salt flows, most showing rounded fronts in plan-view, with heights of several hundred meters and widths between 3 and 10 km, are seen in high-resolution bathymetry and DSDP core material around Thetis Deep and Atlantis II Deep, and between Atlantis II Deep and Port Sudan Deep (Figure 2; Feldens and Mitchell, 2015; Mitchell et al., 2010). Relief on the underlying volcanic basement surface likely controls the positions of individual salt flow lobes. On the flow surfaces, along-slope and downslope ridge and trough morphologies have developed parallel to the local seafloor gradient, presumably due to the extension of the hemiplegic sediment cover or strike-slip movement within the evaporites.

Some sites with irregular seafloor topography are observed close to the flow fronts, interpreted to be the result of dissolution of Miocene evaporites, which contributes to the formation of brine lakes in several of the endorheic deeps (Feldens and Mitchell, 2015). Based on the vertical relief of the flow lobes, deformation is still taking place in the upper part of the evaporite sequence. Considering the salt flow that creates the Atlantis II Deep in more detail, strain rates due to dislocation creep and pressure solution creep are estimated to be 10−14 sec-1 and 10−10 sec-1, respectively, using given assumptions of grain size and deforming layer thickness (Feldens and Mitchell, 2015). The latter strain rate is comparable to strain rates observed for onshore salt flows in Iran and signifies flow speeds of several mm/year for some offshore salt flows. Thus, salt flow movements can potentially keep up with Arabia–Nubia tectonic half-spreading rates across large parts of the Red Sea (Figure 1)


The Deeps

Beneath waters more than a kilometre deep, along the deep rift axis, there are 26 brine pools and deeps, some of which are underlain by metalliferous sediments (Figure 3; Blanc and Anschutz 1995, Blum and Puchelt, 1991). Because of varying size, age, and formation history between the various deeps, Ehrhardt and Hübscher (2015) discriminate between central and northern Red Sea deeps. The larger central Red Sea deeps are located in the axial trough and are separated by inter-trough zones. They are floored by young basaltic crust and exhibit magnetic anomalies not older than 1.7 Ma. The northern Red Sea deeps are smaller and form only isolated deeps within the axial depression. Some of them are accompanied by volcanic activity. Many of the central Red Sea deeps contain bottom-water brines and metalliferous sediments, pointing to hydrothermal circulation of seawater (Schmidt et al., 2015). The largest and most prominent deep is the Atlantis II Deep, located in the central part of the Red Sea in the vicinity of other large deeps such as the Chain Deep and Discovery Deep. Other prominent deeps are the Tethys and Nereus Deeps further north, but still in the central part of the Red Sea.


Historically, the various deeps along the Red Sea rift axis are deemed to be initial seafloor spreading cells that will accrete sometime in the future into a continuous spreading axis. Northern Red Sea deeps are isolated structures often associated with single volcanic edifices in comparison to the further-developed and larger central Red Sea deeps where small spreading ridges are locally active (Ehrhardt and Hübscher, 2015). But not all deeps are related to initial seafloor spreading cells, and there are two types of ocean deeps: (a) volcanic and tectonically impacted deeps that opened by a lateral tear of the Miocene evaporites (salt) and Plio-Quaternary overburden; (b) non-volcanic deeps built by subsidence of Plio-Quaternary sediments due to evaporite subrosion (dissolution) processes. Type b) deeps develop as evaporite collapse structures (Figure 4: Ehrhardt and Hübscher, 2015). In contrast, the type (a) volcanic deeps can be correlated with their positions in NW–SE-oriented segments of the Red Sea, which are daylighted volcanic segments. The N–S segments, between these volcanically active NW–SE segments, is called a “non-volcanic segment” as no volcanic activity is known, in agreement with the magnetic data that shows no major anomalies. Accordingly, the deeps in the "nonvolcanic segments" are evaporite collapse-related structures creating discontinuities and brine breakout zones in and atop the salt sheets without the need for a seafloor spreading cell.

Such evaporite collapse-type ocean deeps are not limited to the non-volcanic segments, as subrosion processes driven by upwells in hydrothermal circulation are possible at any part of the axial depression, especially along fault damage zones. The combined interpretation of bathymetry and seismic reflection profiles gives further insight into the nature of lateral salt gliding in the Red Sea. Salt rises are typically present where the salt flows above basement faults. The internal reflection characteristic of the salt changes laterally from reflection-free to stratified, which suggests significant salt deformation during the salt deposition. Acoustically-transparent halite accumulated locally and evolving rim synclines were filled by stratified evaporite-related facies. (Figure 5)


Both types of deeps, as defined by Ehrhardt and Hübscher (2015), are surrounded by thick halokinetic masses of Miocene salt with brine chemistry in the bottom brine layer that signposts ongoing halite subrosion and dissolution. Red Sea deeps were discovered in the 1960s at a time when lateral translation of salt (gliding and spreading) and the formation of density stratification that define deepsea hypersaline anoxic lakes (DHALS) were not known (Warren, 2016). Today, with our knowledge of seeps and hypersaline seafloor depressions in halokinetic terranes on the slope and rise in the Gulf of Mexico and accretionary ridges in the parts of the Mediterranean Sea, we now know that the brine-filled deeps on the floor of the Red Sea are just another example of DHALs. What is most interesting in the chemical make-up Red Sea DHALS are the elevated levels of iron, copper and lead that occur in some deeps, especially the deepest and one of the most hypersaline set of linked depressions known as the Atlantis II deep (Figure 6).


Brine Chemistry in Red Sea DHALS

Most Red Sea deeps contain waters with somewhat elevated salinities, compared to normal seawater. Bulk chemistry of major ions in bottom brines from the various Red Sea DHALS are covariant and are derived by dissolution of the adjacent and underlying Miocene halite (Figure 7; replotted from Schmidt et al., 2015).


Mineralization in Red Sea DHALS

Economically, the most important brine pool is the Atlantis II Deep; other smaller deeps, with variable development of metalliferous muds and brine sumps, include; Commission Plain, Hatiba, Thetis, Nereus, Vema, Gypsum, Kebrit and Shaban Deeps (Figure 3; Chapter 15, Warren 2016). Laminites of the Atlantis II Deep are highly metalliferous, while the Kebrit and Shaban deeps are of metalliferous interest in that fragments of massive sulphide from hydrothermal chimney sulphides were recovered in bottom grab samples (Blum and Puchelt, 1991). All Red Sea DHALS are located in sumps along the spreading axis, in the region of the median valley. Most of these axial troughs and deeps are also located where transverse faults, inferred from bathymetric data, seismic, or from continuation of continental fracture lines, cross the median rift valley in regions that are also characterised by halokinetic Miocene salt. Not all Red Sea deeps are DHALS and not all Red Sea DHALS overlie metalliferous laminites.

The variably metalliferous seafloor deeps or deepsea hypersaline anoxic lakes (DHALs) in the deep water axial rift of the Red Sea define the metalliferous end of a spectrum of worldwide DHALs formed in response to sub-seafloor dissolution of shallowly-buried halokinetic salt masses. What makes the Red sea deeps unique is that they can host substantial amounts of metal sulphides, and, as Pierre et al. (2010) show, a Red Sea deep without the seafloor brine lake, is not significantly mineralised.

In my opinion, it is the intersection of the DHAL setting with an active to incipient midocean ridge (ultimate metal source), and a lack of sedimentation in the DHAL, other than hydrothermal precipitates (including widespread hydrothermal anhydrite), that explains the size and extent of the Atlantis II deposit. Its salt-dissolution-related brine hydrology, with a lack of detrital input, changes the typical mid-ocean massive-sulphide ridge deposit (with volumes usually around 300,000 and up to 3 million tonnes; Hannington et al., 2011) into a more stable brine-stratified bottom hydrology, which can fix metals over longer time and stability frames, so that the known sulphide accumulation in the Atlantis II Deep today has a metal reserve that exceeds 90 million tonnes.


The Red Sea DHAL evaporite-metal-volcanic association underlines why vanished evaporites are significant in the formation of giant and supergiant base metal deposits. Most thick subsurface evaporites in any tectonically-active metalliferous basin tend to flow and ultimately dissolve. Through their ongoing flow, dissolution and alteration, chloride- and sulphate-rich evaporites can create stable brine-interface conditions suitable for metal enrichment and entrapment. This takes place in subsurface settings ranging from the burial diagenetic through to the metamorphic and into igneous realms. An overview of a selection of the large-scale ore deposits associated with hypersaline brines tied to dissolving/altered and "vanished" salt masses, plotted on a topographic and salt basin base, shows that the majority of evaporite-associated ore deposits lie outside areas occupied by actual evaporite salts (Figure 8; see Warren Chapters 15 and 16 for detail). Rather, they tend to be located at or near the edges of a salt basin or in areas where most or all of the actual salts have long gone (typically via subsurface dissolution or metamorphic transformation). This widespread metal-evaporite association, and the enhancement in deposit size it creates, is not necessarily recognised as significant by geologists not familiar with the importance of "the salt that was." So evaporites, which across the Phanerozoic constitute less than 2% of the world's sediments, are intimately tied to (Warren, 2016):

 

  • All supergiant sediment-hosted copper deposits (halokinetic brine focus)
  • More than 50% of world’s giant SedEx deposits (halokinetic brine focus)
  • More than 80% of the giant MVT deposits (sulphate-fixer & brine)
  • The world's largest Phanerozoic Ni deposit
  • Many of the larger IOCG deposits (meta-evaporite, brine and hydrothermal)
References

 

Augustin, N., C. W. Devey, F. M. van der Zwan, P. Feldens, M. Tominaga, R. A. Bantan, and T. Kwasnitschka, 2014, The rifting to spreading transition in the Red Sea: Earth and Planetary Science Letters, v. 395, p. 217-230.

Batang, Z. B., E. Papathanassiou, A. Al-Suwailem, C. Smith, M. Salomidi, G. Petihakis, N. M. Alikunhi, L. Smith, F. Mallon, T. Yapici, and N. Fayad, 2012, First discovery of a cold seep on the continental margin of the central Red Sea: Journal of Marine Systems, v. 94, p. 247-253.

Blanc, G., and P. Anschutz, 1995, New stratification in the hydrothermal brine system of the Atlantis II Deep, Red Sea: Geology, v. 23, p. 543-546.

Blum, N., and H. Puchelt, 1991, Sedimentary-hosted polymetallic massive sulphide deposits of the Kebrit and Shaban Deeps, Red Sea.: Mineralium Deposita, v. 26, p. 217-227.

Ehrhardt, A., and C. Hübscher, 2015, The Northern Red Sea in Transition from Rifting to Drifting-Lessons Learned from Ocean Deeps, in N. M. A. Rasul, and I. C. F. Stewart, eds., The Red Sea: Berlin Heidelberg, Springer p. 99-121.

Ehrhardt, A., C. Hübscher, and D. Gajewski, 2005, Conrad Deep, Northern Red Sea: Development of an early stage ocean deep within the axial depression: Tectonophysics, v. 411, p. 19-40.

Farhoud, K., 2009, Accommodation zones and tectono-stratigraphy of the Gulf of Suez, Egypt: a contribution from aeromagnetic analysis: GeoArabia, v. 14, p. 139-162.

Feldens, P., and N. C. Mitchell, 2015, Salt Flows in the Central Red Sea, in N. M. A. Rasul, and I. C. F. Stewart, eds., The Red Sea: Springer Earth System Sciences: Berlin Heidelberg, Springer p. 205-218.

Girdler, R. W., and T. C. Southren, 1987, Structure and evolution of the northern Red Sea: Nature, v. 330, p. 716-721.

Hannington, M., J. Jamieson, T. Monecke, S. Petersen, and S. Beaulieu, 2011, The abundance of seafloor massive sulfide deposits: Geology, v. 39, p. 1155-1158.

Pierret, M. C., N. Clauer, D. Bosch, and G. Blanc, 2010, Formation of Thetis Deep metal-rich sediments in the absence of brines, Red Sea: Journal of Geochemical Exploration, v. 104, p. 12-26.

Rasul, N. M. A., and I. C. F. Stewart, 2015, The Red Sea: Springer Earth System Sciences, Springer, 638 p.

Rowan, M. G., 2014, Passive-margin salt basins: hyperextension, evaporite deposition, and salt tectonics: Basin Research, v. 26, p. 154-182.

Schardt, C., 2016, Hydrothermal fluid migration and brine pool formation in the Red Sea: the Atlantis II Deep: Mineralium Deposita, v. 51, p. 89-111.

Schmidt, M., R. Al-Farawati, and R. Botz, 2015, Geochemical Classification of Brine-Filled Red Sea Deeps, in N. M. A. Rasul, and I. C. F. Stewart, eds., The Red Sea: Berlin Heidelberg, Springer-Verlag, p. 219-233.

Stern, R. J., and P. R. Johnson, 2010, Continental lithosphere of the Arabian Plate: a geologic, petrologic, and geophysical synthesis: Earth Science Reviews, v. 101, p. 29-67.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released Feb. 2016: Berlin, Springer, 1854 p.


 

 

 

 

 

 

Salt as a Fluid Seal: Article 4 of 4: When and where it leaks - Implications for waste storage

John Warren - Thursday, March 24, 2016

 

In the three preceding articles on salt leakage, we have seen that most subsurface salt in the diagenetic realm is a highly efficient seal that holds back large volumes of hydrocarbons in salt basins worldwide (Article 3). When salt does leak or transmit fluid, it does so in one of two ways: 1) by the entry of undersaturated waters (Article 1 in this series) and; 2) by temperature and pressure-induced changes in its dihedral angle, which in the diagenetic realm is often tied to the development of significant overpressure and hydrocarbon migration (Article 2). The other implication linked to the two dominant modes of salt leakage is the source of the fluid entering the leaking salt. In the first case, the fluid source is external to the salt ("outside the salt"). In the second case, it can be internal to the main salt mass ("inside the salt"). However, due to dihedral angle changes at greater depths and pressures, a significant portion of leaking fluid passing through more deeply-buried altering salt is external. By the onset of greenschist facies metamorphism, this is certainly the case (Chapter 14 in Warren, 2016) 

Diagenetic fluids driving salt leakage are external to the salt mass

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).

Breaches in bedded (non-halokinetic) salt

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).


 

Leakages associated with the margins of discrete diapiric structures

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).



Leakage of sub-salt fluids associated with salt welds and halokinetic touchdowns

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).


Caprocks are leaky

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).

2. Fluids that are internal to the salt mass

Fluid entry in relation to changes in the dihedral angle of halite is well documented (Article 2). It was first recorded by Lewis and Holness (1996) who postulated, based on their static-salt laboratory experiments;

"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)?

In an interesting recent paper documenting and discussing salt leakage, Ghanbarzadeh et al., 2015 conclude:

“The observed hydrocarbon distributions in rock salt require that percolation occurred at porosities considerably below the static threshold due to deformation-assisted percolation. Therefore, the design of nuclear waste repositories in salt should guard against deformation-driven fluid percolation. In general, static percolation thresholds may not always limit fluid flow in deforming environments.”

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.

Worldwide, the biggest problem with converting existing salt mines to low to medium level nuclear waste storage facilities is that all salt mines are relatively shallow, with operating mine depth controlled by temperatures where humans can work (typically 300-700 m and always less than 1.1 km). This relatively shallow depth range, especially at depths above 500 m, is also where slowly-circulating subsurface or phreatic waters are dissolving halite to varying degrees, This is where fluids can enter the salt from outside and so create problematic dark-salt and collapse breccia zones within the salt. In the long-term (hundreds to thousands of years) these same fluid access regions have the potential to allow stored waste fluids to escape the salt mass,

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.


Now what?

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).

References

Alsharhan, A. S., and C. G. S. C. Kendall, 1994, Depositional setting of the Upper Jurassic Hith Anhydrite of the Arabian Gulf; an analog to Holocene evaporites of the United Arab Emirates and Lake MacLeod of Western Australia: Bulletin-American Association of Petroleum Geologists, v. 78, p. 1075-1096.

Andresen, K. J., M. Huuse, N. H. Schodt, L. F. Clausen, and L. Seidler, 2011, Hydrocarbon plumbing systems of salt minibasins offshore Angola revealed by three-dimensional seismic analysis: AAPG Bulletin, v. 95, p. 1039-1065.

Bowman, S. A., 2011, Regional seismic interpretation of the hydrocarbon prospectivity of offshore Syria: GeoArabia, v. 16, p. 95-124.

Cartwright, J., M. Huuse, and A. Aplin, 2007, Seal bypass systems: American Association Petroleum Geologists - Bulletin, v. 91, p. 1141-1166.

Davison, I., 2009, Faulting and fluid flow through salt: Journal of the Geological Society, v. 166, p. 205-216.

Dooley, T. P., M. R. Hudec, and M. P. A. Jackson, 2012, The structure and evolution of sutures in allochthonous salt: Bulletin American Association Petroleum Geologists, v. 96, p. 1045-1070.

Ghanbarzadeh, S., M. A. Hesse, M. Prodanović, and J. E. Gardner, 2015, Deformation-assisted fluid percolation in rock salt: Science, v. 350, p. 1069-1072.

Gillhaus, A., 2010, Natural gas storage in salt caverns - Summary of worldwide projects and consequences of varying storage objectives and salt formations, in Z. H. Zou, H. Xie, and E. Yoon, eds., Underground Storage of CO2 and Energy, CRC Press, Boca Raton, Fl., p. 191-198.

Harrison, H., and B. Patton, 1995, Translation of salt sheets by basal shear: Proceedings of GCS-SEPM Foundation 16th Annual Research Conference, Salt Sediment and Hydrocarbons, Dec 3-6, 1995, p. 99-107.

Holly Harrison, Dwight ‘Clint’ Moore, and P. Hodgkins, 2010, A Geologic Review of the Mahogany Subsalt Discovery: A Well That Proved a Play (The Mahogany Subsalt Discovery: A Unique Hydrocarbon Play, Offshore Louisiana): Search and Discovery Article #60049 Posted April 28, 2010, Adapted from presentation at AAPG Annual Convention, 1995, and from an extended abstract prepared for presentation at GCS-SEPM Foundation 16th Annual Research Conference, “Salt, Sediment and Hydrocarbons,” December 3-6, 1995.

Hudec, M. R., and M. P. A. Jackson, 2006, Advance of allochthonous salt sheets in passive margins and orogens: American Association Petroleum Geologists - Bulletin, v. 90, p. 1535-1564.

Hudec, M. R., and M. P. A. Jackson, 2007, Terra infirma: Understanding salt tectonics: Earth-Science Reviews, v. 82, p. 1-28.

Jackson, C. A. L., and M. M. Lewis, 2012, Origin of an anhydrite sheath encircling a salt diapir and implications for the seismic imaging of steep-sided salt structures, Egersund Basin, Northern North Sea: Journal of the Geological Society, v. 169, p. 593-599.

Kettanah, Y. A., 2013, Hydrocarbon fluid inclusions in the Argo salt, offshore Canadian Atlantic margin: Canadian Journal of Earth Sciences, v. 50, p. 607-635.

Kukla, P., J. Urai, J. K. Warren, L. Reuning, S. Becker, J. Schoenherr, M. Mohr, H. van Gent, S. Abe, S. Li, Desbois, G. Zsolt Schléder, and M. de Keijzer, 2011, An Integrated, Multi-scale Approach to Salt Dynamics and Internal Dynamics of Salt Structures: AAPG Search and Discovery Article #40703.

Leitner, C., F. Neubauer, J. L. Urai, and J. Schoenherr, 2011, Structure and evolution of a rocksalt-mudrock-tectonite: The haselgebirge in the Northern Calcareous Alps: Journal of Structural Geology, v. 33, p. 970-984.

Lewis, S., and M. Holness, 1996, Equilibrium halite-H2O dihedral angles: High rock salt permeability in the shallow crust: Geology, v. 24, p. 431-434.

Schoenherr, J., J. L. Urai, P. A. Kukla, R. Littke, Z. Schleder, J.-M. Larroque, M. J. Newall, N. Al-Abry, H. A. Al-Siyabi, and Z. Rawahi, 2007, Limits to the sealing capacity of rock salt: A case study of the infra-Cambrian Ara Salt from the South Oman salt basin: Bulletin American Association Petroleum Geologists, v. 91, p. 1541-1557.

Terken, J. M. J., N. L. Frewin, and S. L. Indrelid, 2001, Petroleum systems of Oman: Charge timing and risks: Bulletin-American Association of Petroleum Geologists, v. 85, p. 1817-1845.

Thrasher, J., A. J. Fleet, S. J. Hay, M. Hovland, and S. Düppenbecker, 1996, Understanding geology as the key to using seepage in exploration: the spectrum of seepage styles, in S. D., and M. A. Abrams, eds., Hydrocarbon migration and its near-surface expression, AAPG Memoir 66, p. 223-241.

Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

Warren, J. K., 2016, Evaporites: A Compendium (ISBN 978-3-319-13511-3) Released Feb. 22 2016: Berlin, Springer, 1854 p.


 

Salt as a Fluid Seal: Article 2 of 4: Internal fluid source

John Warren - Wednesday, January 20, 2016

 

Black Salt: as an indicator of overpressure

The previous article in this series on salt leakage focused on black and dark salt created by ingress or interaction of undersaturated waters with relatively shallow halokinetic salt masses, with entry zones often tied to intervals of salt shear. The resulting black or dark salt textures are one style of “anomalous” salt. This article looks at fluid entry into salt in subsurface intervals of high pore pressure, exemplified by the “black salt” in the Ara salt seals of Oman. Such intervals are often tied to burial-pressure and temperature-related changes to the dihedral angle of salt (halite).


Dihedral angle changes and the permeability of salt

Permeability in intercrystalline pore networks in re-equilibrating and crystallising subsurface salt is tied to the dihedral angle  at solid-solid-liquid triple junctions (Figure 1; Lewis and Holness, 1996, Holness and Lewis, 1997). When the halite dihedral angle is higher than 60° under static laboratory conditions, this contact angle equates to the maintenance of closure of polyhedral grain boundaries by halite precipitation, and so at these lower temperatures both bedded and halokinetic recrystallized salt is impermeable (Schenk and Urai, 2004; Holness and Lewis, 1996). In this temperature range, the small amount of brine present in the salt is distributed in micrometer-sized isolated fluid inclusions at termini of salt crystal polygon apices. In contrast, when the solid-solid-liquid interfaces of increasingly heated and pressurised polyhedral halite attain dihedral angles that are less than 60° then the fluid-inclusion filled intercrystal cavities link up and the salt mass becomes permeable.


At burial temperatures >100°-150°C and pressures of 70 MPa or more, the dihedral angle has decreased to values <60°, driving a redistribution of the fluid into a thermodynamically stable network of connected, fluid-filled channels or fused fluid strings at grain boundary triple junctions. This transition may be related to the observation by Peach and Spiers (1996) that, during natural deformation of rocksalt at great depths, salt undergoes natural hydraulic fracturing or dilatancy. The dihedral angle is, therefore, a thermodynamic property that changes with pressure P and temperature T. Holness and Lewis’s experiments demonstrated that buried salt masses, subject to high pressures and elevated temperatures, can acquire intercrystalline or polyhedral permeability comparable to associated with intergranular permeability in sand.

This typically occurs at higher temperatures and pressures where intercrystal water positions link within flowing or static, but texturally re-equilibrated, salt and so creates continuous fluid strings along evolving intercrystalline junctions in the burial-recrystallised salt. The newly attained intercrystal configuration allows penetration and throughflow of hot, dense brines or hydrocarbons into and through the altered mass of salt polyhedrons. In Oman has created characteristic haloes of black salt about pressurised salt-encased carbonate slivers (next section).

At the same time as a recrytallising salt mass passes into the earlier stages of the greenschist facies, the salt is dissolving and altering to sodic scapolite (Warren, 2016; Chapter 13). Thus, through the later stages of diagenesis and into early to medium grades of metamorphism, the salt and its daughter products may act as sources and conduits for flow of chloride-rich metalliferous brines and salt slurries. This occurs as bedded and halokinetic salt evolves from a dense impermeable salt mass into permeable salt with higher dihedral angles and so explains salt’s significant role in the creation of many of massive base metal and IOCG deposits (Warren, 2016; Chapters 15, 16).


Black salt and overpressure in Oman

The transition in dihedral angle with increasing pressure and temperature explains the occurrence of black (bitumen-charged) haloes in salt encasing some carbonate-sliver reservoirs in the Ara Salt of Oman (Figure 2; Kukla et al., 2011a, b). Once this recrystallization occurs, the previous lower P&T mosaic halite loses its ability to act as an aquitard or aquiclude (seal) and can instead serve as a permeable conduit for escaping highly-pressurised and hydrocarbon-rich formation waters. According to Lewis and Holness, the depth at which the recrystallization occurs may begin as shallow as a few kilometres (Figure 1). But, their pressure bomb laboratory-based static-salt experiments did not completely encompass the ability of natural salt to pressure creep and self-seal by longer-term diffusion-controlled pressure solution (Warren 2016, Chapter 6). Even if the changing dihedral angles alter and open up permeability at such shallow depths, there is no guarantee that subsequent flowage associated with pressure solution will not re-anneal these new pores. The ability of salt to continue to act as a highly efficient hydrocarbon seal to depths of 6-10 km means, in my opinion, that bedded salt does may become a relative aquifer until attaining depths of 6-10 km or more. This occurs certainly at temperatures and pressures where the sequence is entering the greenschist realm. In extremely overpressured situations the transition of dihedral angles is much shallower, as in the 40-50m thick black salt rims that typify the salt-encased hydrocarbon-charged carbonate stringers in the Ara Salt of Oman (Kukla et al., 2011b). Once it does transform into polyhedral halite, a former aquiclude becomes an aquifer flushed by chloride-rich brines, likely carrying other volatiles.

A release of entrained inclusion (±intercrystalline) water at temperatures > 300-400°C (early greenschist) influences the textures of deeply buried halite. Most of the inclusions in chevron halite and other inclusion-rich cloudy primary salts are due to entrained brine inclusions and not mineral matter. Figure 3 plots the weight loss of various types of halite during heating. It clearly shows cloudy (inclusion-rich) halite releases up to 5 times more brine (0.2-0.5 wt%) than clear coarsely crystalline halite. An analysis of all fluids released during heating shows carbon dioxide and hydrogen contents are much lower than the water volumes: CO2/H2O < 0.01 and H2/H2O < 0.005. Organic compounds, with CH4, are always present (<0.05% H2O), and are twice as abundant in cloudy halite. There are also traces of nitrogen and, in some samples, hydrogen sulphide and sulphur dioxide (Zimmermann and Moretto, 1996).


The influence of overpressure driving changes in the dihedral angle of pressurised salt is most clearly seen in black-salt encased Late Neoproterozoic to early Cambrian intra-salt Ara (stringer) reservoirs of the South Oman Salt Basin (Figures 2, 4, 5; Kukla et al., 2011b). These carbonate bodies are isolated in salt and frequently contain low-permeability dolomites and are characterised by high initial hydrocarbon production rates due to overpressure. But not all stringers are overpressured, and a temporal relationship is observed defined by increasingly overpressured reservoirs within stratigraphically younger units. There are two separate pressure trends in the stringers; one is hydrostatic to slightly-above hydrostatic, and the other is overpressured from 17 to 22 kPa.m−1, almost at lithostatic pressures (Figure 4).


The black staining of the halite is caused by intragranular microcracks and grain boundaries filled with solid bitumen formed by the alteration of oil (Figures 2, 5). The same samples show evidence for crystal plastic deformation and dynamic recrystallization. Subgrain-size piezometry indicates a maximum differential paleostress of less than 2 MPa. Under such low shear stress, laboratory-calibrated dilatancy criteria suggest that oil can only enter the rock salt at near-zero effective stresses, where fluid pressures are very close to lithostatic. In Schoenherr et al.’s (2007b) model, the oil pressure in the carbonate stringer reservoirs reservoir increases until it is equal to the fluid pressure in the low, but interconnected, porosity of the Ara Salt, plus the capillary entry pressure (Figure 5). When this condition is met, oil is expelled into the rock salt, which dilates and increases its permeability by many orders of magnitude. Sealing capacity is lost, and fluid flow will continue until the fluid pressure drops below te minimal principal stress, at which point rock salt will reseal to maintain the fluid pressure at lithostatic values. Inclusion studies in the halite indicate ambient temperatures at the time of entry were more than 90°C, implying hydrocarbons could move into interconnected polyhedral tubes in the halite. These conduits were created in response to changes in the polyhedral angle in the halite in response to elevated temperatures (Lewis and Holness, 1996).


Hydrocarbon-stained “black salt” can extend up to 100 metres from the pressurised supplying stringer into the Ara salt of Oman (Figure 2, 5). It indicates a burial-mesogenetic pressure regime and is not the same process set as seen in the telogenetic “black salt” regions of the onshore Gulf of Mexico. The latter is created by dissolution, meteoric water entry, and clastic contamination, as in the crests of nearsurface diapirs such as Weeks Island (Warren 2015). An Ara stringer enclosed by oil-stained salt but now below the lithostatic gradient likely indicates a later deflation event that caused either complete (C) or partial (E) loss of overpressures. Alternatively, stringers showing overpressure, but below the lithostatic gradient (E), might be explained by regional cooling or some other hitherto unexplained mechanism (Figure 4a; Kukla et al., 2011a, b).

Structural, petrophysical and seismic data analysis suggests that overpressure generation in the Ara is driven initially by rapid burial of the stringers in salt, with a subsequent significant contribution to the overpressure from thermal fluid effects and kerogen conversion of organic-rich laminites with the stringer bodies. If the overpressured stringers come in contact with a siliciclastic minibasin, they will deflate and return to hydrostatic pressures (A) in Figure 4. When the connection between the minibasin and the stringers is lost, they can regain overpressures because of further oil generation and burial (A’). If hydrocarbon production in undeflated stringers stops relatively early, the fluid pressures do not reach lithostatic pressures (B). If hydrocarbon generation continues, the fluid pressures exceed the lithostatic pressure (red star), leading to dilation and oil expulsion into the rock salt to what is locally known as “black salt” (D and E).

As well as these examples of overpressure associated with older evaporites, overpressure readily develops in salt-sculpted Tertiary basins. For example, overpressure occurs in salt shear (gumbo) transitions beneath some, but not all, shallow salt allochthons in Green and Mahogany Canyon regions in the Gulf of Mexico (Beckman, 1999: Shaker 2008). Where salt allochthons are climbing the stratigraphy, subsalt sealing and associated overpressure can occur beneath the salt mass at shallower levels than is observed in overpressured shale basins.

In terms of extension and compression regimes within a single allochthon tongue, Shaker (2008) noted that in extensional regions in halokinetic basins the magnitude and direction of the principal stresses are controlled by sediment load, salt thickness, and salt emplacement-displacement history. Therefore, the maximum principal stress is not necessarily represented by the sheer weight of the overburden, as is usually assumed in quiescent terranes. Salt buoyancy often acts upward and has the tendency to accelerate and decelerate the principal stress above and below the salt, respectively. A distinctive shift of the pore pressure envelopes and normal compaction trends takes place across the salt body in several wells drilled trough salt below minibasins in the Mississippi Canyon, Green Canyon, and Garden Banks areas of the Gulf of Mexico. A lower pore pressure gradient has been observed below the salt and a higher gradient above the salt barrier. On the salt-rooted minibasin scale, a high-gradient was also observed in areas where the salt was emplaced and a lower gradient where the salt withdrew (Shaker and Smith, 2002). On the other hand, in the compressional portion of a salt allochthon system, lateral stress generated by the salt movement piling up salt at the foot of the slope acts as the maximum principal stress, whereas the load of sediment represents the minimum stress.


Extreme overpressuring is commonplace in subsalt settings in the Gulf of Mexico at depths of 3000-4000 m and its variability creates drilling problems, as evidenced by the BP Horizon spill and explosion on April 20, 2010. Gas generated at greater depths in these regions can be trapped under the salt seal at pressures approaching lithostatic. It means drilling under the allochthonous salt on the Gulf Coast slope can intersect undercompacted sediments that are moderately to extremely overpressured and friable (Hunt et al., 1998). The influence of highly effective Jurassic salt seals on pressure gradients in the Neogene stratigraphy of the Gulf of Mexico is seen in the increased mud weights typically required for safe drilling, once an evaporite allochthon is breached by the drill (Table 1). Many wells intersecting salt allochthons in the deepwater realm of the Gulf of Mexico and the circum-Atlantic Salt basins are overpressured at some depth below the base of salt with mud weights controlling pressures ranging from 14 to 17.5 ppg.

Implications

This and the previous article (Warren, 2015) demonstrate that black salt is a form of anomalous salt that indicates salt has leaked, however, the locations and conditions where leakage has occurred are distinct. The black salt encountered in the salt mines of the US Gulf Coast are indicative of meteoric water entry in relatively shallow conditions in regions where the salt is in contact with the surrounding shales of muds that enclose the diapir salt core. In other words, fluid entry is from the outside of the salt mass and fluids move into the salt from its edges and likely enhance  the porosity in the intercrystalline salt. In contrast, the black salt occurrences in the Ara Salt of Oman are indicative of overpressure haloes, generated internally via hydrocarbon and fluid expulsion in carbonate slivers, which are are fully encased in salt. This creates naturally hydrofractured envelopes in the salt mass in zones where pressure and temperature induced changes in the dihedral angle has generated intercrystalline fluid strings within the recrystallised polyhedral halite. The two settings of black salt formation are distinct.

There is not a single mechanism that creates black salt in a halokinetic salt mass. We shall discuss the implications of this in the next article which will include a look at leakage models in halokinetic salt systems both in terms of their seal integrity and the implications for short and  long term storage of hydrocarbons and nuclear waste. 

References

Beckman, J., 1999. Study reveals overpressure sources in deep-lying formations. Oil and Gas Journal, September: 137.

Holness, M.B. and Lewis, S., 1997. The structure of the halite-brine interface inferred from pressure and temperature variations of equilibrium dihedral angles in the halite-H2O-CO2 system. Geochimica et Cosmochimica Acta, 61(4): 795-804.

Hunt, J.M., Whelan, J.K., Eglinton, L.B. and Cathles III, L.M., 1998. Relation of shale porosities, gas generation, and compaction to deep overpressures in the US Gulf Coast. In: B.E. Law, G.F. Ulmishek and V.I. Slavin (Editors), Abnormal pressures in hydrocarbon environments. American Association Petroleum Geologists Memoir 70, Tulsa, OK, pp. 87-104.

Kukla, P., Urai, J., Warren, J.K., Reuning, L., Becker, S., Schoenherr, J., Mohr, M., van Gent, H., Abe, S., Li, S., Desbois, Zsolt Schléder, G. and de Keijzer, M., 2011a. An Integrated, Multi-scale Approach to Salt Dynamics and Internal Dynamics of Salt Structures. AAPG Search and Discovery Article #40703 (2011).

Kukla, P.A., Reuning, L., Becker, S., Urai, J.L. and Schoenherr, J., 2011b. Distribution and mechanisms of overpressure generation and deflation in the late Neoproterozoic to early Cambrian South Oman Salt Basin. Geofluids, 11(4): 349-361.

Lewis, S. and Holness, M., 1996. Equilibrium halite-H2O dihedral angles: High rock salt permeability in the shallow crust. Geology, 24(5): 431-434.

O'Brien, J. and Lerche, I., 1994. Understanding subsalt overpressure may reduce drilling risks. Oil and Gas Journal, 92(4): 28-29,32-34.

Peach, C. and Spiers, C.J., 1996. Influence of crystal plastic deformation on dilatancy and permeability development in synthetic salt rock. Tectonophysics, 256: 101-128.

Schenk, O. and Urai, J.L., 2004. Microstructural evolution and grain boundary structure during static recrystallization in synthetic polycrystals of sodium chloride containing saturated brine. Contributions to Mineralogy and Petrology, 146: 671-682.

Schoenherr, J., Littke, R., Urai, J.L., Kukla, P.A. and Rawahi, Z., 2007a. Polyphase thermal evolution in the Infra-Cambrian Ara Group (South Oman Salt Basin) as deduced by maturity of solid reservoir bitumen. Organic Geochemistry, 38(8): 1293-1318.

Schoenherr, J., Urai, J.L., Kukla, P.A., Littke, R., Schleder, Z., Larroque, J.-M., Newall, M.J., Al-Abry, N., Al-Siyabi, H.A. and Rawahi, Z., 2007b. Limits to the sealing capacity of rock salt: A case study of the infra-Cambrian Ara Salt from the South Oman salt basin. Bulletin American Association Petroleum Geologists, 91(11): 1541-1557.

Shaker, S., 2008. The double edged sword: The impact of the interaction between salt and sediment on subsalt exploration risk in deep water. Gulf Coast Association of Geological Societies Transactions, 58: 759-769.

Warren, J.K., 2015. Salt as a fluid seal: Article 1,  Salty Matters blog; First published on Dec 19, 2015; www.saltworkconsultants.com.

Warren, J.K., 2016. Evaporites: A compendium (ISBN 978-3-319-13511-3) Released Feb. 2016. Springer, Berlin, 1854 pp.

Zimmermann, J.L. and Moretto, R., 1996. Release of water and gases from halite crystals. European Journal of Mineralogy, 8(2): 413-422.


 

 

Salt ablation indicators; how flowing salt dissolves at the surface

John Warren - Tuesday, March 10, 2015

When a salt sequence is in contact with undersaturated waters it dissolves to leave behind layers of salt dissolution residues and breccias. The process of salt dissolution is most obvious and most visually stunning is where solution collapse dolines or stoping karst chimneys daylight, as was discussed in an earlier blog dealing with salt collapse features forming today in the vicinity of Uralkali’s Solikamsk 2 mine. Although spectacularly catastrophic, such features are not typical of an area where a halokinetic salt mass is flowing and ebbing across the landsurface. The continual resupply of salt prevents such features forming. Instead, stratiform or stratabound dissolution residue layers and perhaps breccia horizons, along with salt welds, are the more typical indicators of the former presence of a flowing and dissolving salt mass. In this essay I illustrate this by focusing on the geological development of dissolution features within and about three current or former halokinetic salt masses (namakiers), currently outcropping at the surface in central Iran. The three salt features are Kuh-e-Namak (Qom), Kuh-e-Namak (Shurab) and Kuh-e-Gach (Saveh). Listed in this order they illustrate a sequence of dissolution from an initial salt fountain or namakier, through supra-salt overburden foundering, to a time when all at-surface halite has dissolved and only a carapace of gypsum and clay residues remain. Similar features are seen in the rock record, wherever salt ablation breccias occur.

Namakier is a term suggested by Martin Jackson utilising the comparison to an ice glacier, where namak is the Persian (Farsi) word for salt. Gach is Farsi for gypsum. while kuh means mountain. So, the term kuh-e-namak when translated means “mountain of salt.” There are many such mountains of salt (namakiers) composed of Oligo-Miocene salt in Central Iran and mountains of Neoproterozoic/Cambrian salt in the Zagros fold belt, located further to the south and southeast of Qom. This means it is unwise to describe any salt glacier as kuh-e-namak, without a geographic suffix to better define its location. Likewise there are a number of mountains of gypsum, which are typically the remnants of a namakier, at the stage where all the halite has dissolved to leave behind a carapace of gypsum residues, which outline the former extent of the salt tongue. Gypsum residues too will ultimately dissolve, as is occurring about the current edges of Kuh-e-Gach (Saveh) in central Iran (Figure 1).

 

Qom Kuh is one of a number of salt diapirs currently emergent along releasing bends in the Qom-Saveh basin, which are regions that have been pulled apart along major dextral transpressive faults, crossing and defining the margin of the Qom-Saveh Basin in the central plateau of Iran. The other two features discussed in this essay, Kuh-e-Namak (Shurab) and Kuh-e-Gach Saveh are located on the current thrust margin of the basin and so have been cut-off from the mother salt supply, rather than directly above the current salt area (Figure 1). Interestingly, Kuh-e-Namak (Qom) is only ten or so kilometres away from the site of the famous 1956 blowout site at the Alborz 5 well atop the Alborz anticline (Figure 1). With an estimated volume of 5 million barrels of oil escaping at Alborz 5 over 82 days (until the blowout self-bridged), the salt-sealed and salt-cored fractured Qom carbonate (Oligo-Miocene) reservoir remains the feeder to one of the world’s largest blowouts. For comparison, Macondo in the Gulf of Mexico (another salt-related blowout) had an estimated total volume of escaped fluids of around 4.9 million barrels. We shall discuss salt-related blowouts and salt seal integrity in a future blog. For now, we shall focus on what typifies the geology where a namakier flows over the landsurface and then starts to dissolve.

 

Kuh-Qom, is still located in a supplying salt mass and so is actively flowing or fountaining salt to the surface, while the other two salt features are in varying stages of collapse and dissolution (Figure 1). Qom-Kuh is located on a releasing bend of an offset in the Qom Fault that crosscuts the plunging nose of the Alborz Anticline (Figure 2). The Qom Fault intersects a number of normal faults in the vicinity of Kuh Qom which are exposed to the east of Qom Kuh, where it extends beneath alluvium to the northwest of the diapir (Figures 1, 3). Splays of oblique-reverse and oblique-normal faults offset the nearby asymmetric salt-cored thrust anticline, known as the Alborz Anticline (where the Alborz 5 blowout occurred). These faults are well exposed in the 60m-high ephemeral-stream eroded cliff that defines the eastern flank of Qom Kuh. The adjacent ephemeral-river bench has bevelled into the Upper Red Formation marls at the 920m above sea level (Figure 3b). Faults are clearly seen in the zone separating the diapir from the hill created by the northern limb outcrop of the anticline. The largest of these faults has an irregular trace that was superimposed on a ductile dextral strike-slip shear zone before it offset the river bench some 1.5 m down to the east (Figure 3a). Today the crest of the salt fountain of Qom Kuh rises to 1235 m asl and is some 315 m above the surrounding plateau (Figure 3d).

 

The Qom Kuh namakier can be divided into two main components: (1) a smooth hemispheric dome or summit with a diameter of 2.5 km and surrounded by, (2) a dissected and dissolving apron of salt allochthonous flowing atop Upper Red Formation and Recent gravels (Figure 3b, d). The margin of this rim is defined by large blocks of salt-buoyed exotics, mostly Eocene volcanics, that have slide down the salt mass to accumulate as a pile of blocks about the salt edge (Figure 3a-c). Steps in the salt topography indicate that the extruded salt overflows a collar of Upper Red Formation up-tilted around the inferred diapiric vent. This raised collar is highest beneath the NE shoulder of Qom Kuh, where it is exposed in windows up to 1100 m asl and lowest in its SW corner (Figure 3; Talbot and Atabi, 2004; Cosgrove et al., 2009). At the Qom-Kuh summit the salt mass is characterised by diffusion karst and variably covered by up to a metre of clay-dominant residuals, along with rare cm-sized clasts of anhydrite and limestone.

The river cliff truncates the eastern end of a sedimentary bedrock collar (Figure 3a, b). The bench and cliff exposes the apron of allochthonous salt, which is up to 200 m thick and about 0.75 km long to the west of the summit, and some 2 km to the south (Figure 3d). The underlying rhombic vent, through which Qum Kuh extrudes, is interpreted to be a pull-apart between normal faults that hard-link a releasing offset along a regional transpressive strike-slip fault, which trends west–east (Figure 2b). In profiles though the summit, Qum Kuh can be considered as a pile of recumbent fold nappes of gneiss-like and mylonite-like Oligocene and Miocene salts that have extruded from depth and the gravity spread over recent alluvial gravels (Figure 3c).

An ephemeral stream still flows anticlockwise around most of Qom Kuh, and is still eroding the cliff on the east side of Qom Kuh. Broken pottery lying on the river-bevelled bench to the east, and exposed by wind-deflation of the soils overridden by the southern namakier, suggest that the river eroded the salt of Qom Kuh in historical times (≈ 10,000 years ago according to Talbot and Atabi, 2004). Salts dissolved from the Qom Kuh namakier are now being re-precipitated as Holocene evaporite beds in the adjacent playa low, which is possibly situated atop an active salt withdrawal basin (Figures 2, 3b).

 

The rhombic transtensional nature of the salt neck to this pull-apart graben structure that allows the ongoing salt supply to the Kuh-Qom salt fountain is seen more clearly in a now inactive rhomboid neck , which now outcrops at Kuh-e-Gach, Saveh, a highly dissolved namakier remnant (Figures 1, 4). Only the nodular gypsum carapace and the underlying reducing brine halo remain to define the position of the former namakier (Figure 4a-d). The leaching of the namakier salt created dense plumes of reducing brines that coloured the underlying redbeds grey, as it converted iron in the redbeds from its ferrous to ferric state (Warren, 2008). Clearly, at the other end of the dissolution spectrum from an active salt fountain, the actual evaporite mass (both halite and gypsum residues) will disappear, as at Kuh-e-Gach (Saveh). Once the reducing brine source is gone, it is also likely the greybeds will transition back into redbeds in this semiarid climate. All in all, not much evaporite evidence of a former salt fountain will remain as mineral salts in the namakier-influenced stratigraphy.  


As a general rule, the upper portion of an actively flowing salt mound is covered by residual soils, along with a variety of gypsum textures growing in the insoluble components (suffusion karst). Small ephemeral features, including shallow diffusion caves, are typical of the salt glacier surface, but in an active namakier the salt is flowing too fast to preserve them for more than a few hundred to a few thousand years depending on the lateral extent and rate of extrusion of the diapir. The karstic carapace atop the moving and dissolving namakier is dominated by soils, typically composed of a combination of dissolution residues, water-carried deposits and wind transported dust (loess). As the moving salt dissolves, dolines develop in the surface of these soils, they enlarge with time as the soils thicken. These non-halite features remain in place and can even accrete as they are carried downslope by underlying flowing salt evan as it dissolves. Dolines below the surface of these soils enlarge and the soils thicken as the moving salt dissolves in its passage away from the vent. Ultimately, once the supply of mother salt is depleted the at-surface namakier salt dissolves to leave behind a gypsum-encrusted surficial layer, no more than a metre or two thick (Figure 5; Warren, 2008).

 

Once the supply from the mother salt layer ceases, an at-surface namakier stops its outward expansion and begins to shrink to ultimately subside and disappear. This occurs over time scales of hundreds to thousands of years post-flow, to leave behind a salt-ablation carapace. As the salt dissolves, soils that previously had accumulated about to namakier edge, now touch down over the whole extent of the former salt glacier. Ultimately, karstification and salt core collapse becomes so pervasive all the way across the former extent of the namakier core so that only the edges of the relict plug still show any positive relief (Figure 6). Salt plug ruins and overburden blocks once buoyed by the salt mass are  broken and disturbed, while a halo of salt-buoyed exotic blocks defines the former extent of the salt tongue. Overall, in the local Iranian landscape underlain by salt, inactive and shrunken plugs (as at Shurab - Figure 7) tend to be negative landscape features (depressions). 

 

Although not leaving behind much in the way of salt mineral remnants, the salt-buoyed transport of exotic blocks, which can be hundreds of metres in diameter, and the remnants of lithologies that were plucked and buoyed by the salt as it made its way to the surface are the best evidence for the former presences of a namakier mass. Such a horizon or layer of exotic blocks can also be accompanied by, and typically is immediately underlain by, a disturbed layer of scattered and rotated overburden blocks. This style of mega-breccia creation is seen in the foundering and collapse blocks of Qom Limestone in the salt-cored breached anticline that defines the Shurab diapir (Figure 7).


Today, to the south of the Qom Basin, in the Persian (Arabian) Gulf there are numerous salt-cored islands in various stages of namakier dissolution, such as Hormuz, Das and Yas islands (Figure 8). These once fountaining extrusive salt masses are now largely inactive, with only minor at-surface evaporite residues present, mostly as gypcretes and caprock remnants. Highest parts of the diapir-cored islands are typically covered by dissolution breccias that cap the still dissolving salt core below. Halokinetic Precambrian (Hormuz) salt beneath the breccia carapace is pervasively karstified and where relatively shallow, as at Hormuz Island, is covered by suffusion karst and crosscut by tube caves. Active flow of the squeezed salt in the various island cores seems to have ceased sometime in the Miocene, so that terrains of exotic blocks of meta-igneous and dolomite litologies now outline much of the surface expression of former outcropping salt masses, Across the Gulf region this stratiform complex of residues and large salt-buoyed blocks is termed the "Hormuz complex." But what it actually is a series of salt ablation breccias. Today these regions of outcropping Hormuz Complex are heavily eroded and partially covered by Neogene shoal water carbonates and marine-cut platforms, as in Das and Hormuz islands (Figure 8a, b)


Texturally near-identical Neoproterozoic megabreccias, breccias and breccia trains crop out in the Flinders Ranges of South Australia (Figure 9; Dalgarno and Johnson, 1968; Lemon, 1985). The salt no longer remains in these features, but they are clearly remnants of what were once diapiric structures (Hearon IV et al, 2015). Many breccias in anticlinal cores of the this halokinetic terrane are still located at or near the level of the former mother salt bed (Callanna Beds), with current outcrop patterns largely indicative of the positions of deeper basement faults (Backé et al., 2010). Today, the breccias define the polytectonic remnants of former autochthonous salt pillows or glide planes and inversion structures, rather than true salt-cored diapirs. Some structures, such as the Oratunga and Wirrealpa diapirs, still preserve evidence of their earlier formative extension phase and show subcircular patterns of outcrop and breccia wings at allochthonous stratigraphic positions well above the level of the original mother salt bed. Almost all the transtratal (halokinetic) breccias line up along major regional shears and faults (welds) surrounded by jostling depopods or minibasins, indicating that the mother salt bed was flowing as the basin was under extension. The salt was then remobilised during the inversion phase and the transition from haselgebirge to rauhwacke textures. Lemon (1985, 1988, 2000), Dyson (2004) and Rowan and Vendeville (2006) clearly illustrate synkinematic controls on sedimentary facies and thicknesses adjacent to many of these breccia masses in the Flinders Ranges (Warren, 2015).

In summary, when a namakier shrinks it does so via at-surface dissolution of the salt mass, much in the same way a glacier retreats as it melts). Likewise, the outer edge of a salt glacier expands and contracts like the outer edge of an ice glacier. When the salt mass retreats it leaves behind a jumbled mass of material it once buoyed, which can include megabreccia blocks tens of metres across, as well as the insolubles of its carapace and insoluble intrasalt layers. These chaotic breccias are similar to a diapiric collapse breccia, but contain a much more polymict assemblage of clasts than an evaporite collapse breccia (after bedded salt) and often show evidence of mechanical reworking of portions of the breccia material by waves or currents. A separate term is probably needed to distinguish them from the more general term diapiric breccia; I call them salt ablation or salt-retreat breccias (Table 1; Warren, 2015).

 

As we saw in the successive stages of diapir dissolution and retreat exposed in the Qom-Saveh Basin of Central Iran, wherever an active but shrinking salt tongue subcrops or lies beneath a dissolution-derived gypcrete carapace, adjacent larger clasts accumulate by breakup of the soft overburden and by stacking of fragments and rafts formerly held within the now dissolved salt matrix. The resulting namakier breccia is composed of a coarse, unsorted, heterolithic (polymict) rubble supported by a fine-grained calcareous matrix dominated by solution flour. This material can be mixed with fluvial and alluvial material (note the fans fed by alluvial streams that cross the ablation zone in Figure 2a) or marine carbonate debris from times when the sea encroached on the salt tongue (as at Hormuz Island; Figure 8).

Ongoing dissolution of the namakier leaves behind a rubble moraine at the level of the former salt allochthon. Thus, salt ablation breccias associated with salt cores are jumbles of exotic material carried to the surface by upwelling salt, which is then combined with the disrupted remnants of the any brittle overburden that once covered an earlier now-dissolved salt tongues. The mixing of contemporary sediment (alluvial or marine) with salt-buoyed megabreccia blocks and insolubles in the salt, and the stratiform circum-stem or circum-weld nature of its occurrence, is what distinguishes a salt ablation breccia from a diapiric breccia formed by subsurface salt dissolution and from breccias that result from the dissolution of a salt bed (Table 1; Warren 2015). 

 

References

Backé, G., G. Baines, D. Giles, W. Preiss, and A. Alesci, 2010, Basin geometry and salt diapirs in the Flinders Ranges, South Australia: Insights gained from geologically-constrained modelling of potential field data: Marine and Petroleum Geology, v. 27, p. 650-665.

Cosgrove, J. W., C. J. Talbot, and P. Aftabi, 2009, A train of kink folds in the surficial salt of Qom Kuh, Central Iran: Journal of Structural Geology, v. 31, p. 1212-1222.

Dalgarno, C. R., and J. E. Johnson, 1968, Diapiric structures and late Precambrian-early Cambrian sedimentation in Flinders ranges, South Australia: American Association Petroleum Geologists, Memoir, v. 8, p. 301 -314.

Dyson, I. A., 2004, Christmas tree diapirs and the development of hydrocarbon reservoirs; A model from the Adelaide Geosyncline, South Australia: Salt-sediment interactions and hydrocarbon prospectivity: concepts, applications and case studies for the 21st Century. Papers presented at the 24th Annual Gulf Coast Section SEPM Foundation Bob F. Perkins Research Conference, Houston Tx, December 5-8, 2004 (CD publication), p. 133-165.

Hearon IV, T. E., M. G. Rowan, K. A. Giles, R. A. Kernen, C. E. Gannaway, T. F. Lawton, and J. C. Fiduk, 2015, Allochthonous salt initiation and advance in the northern Flinders and eastern Willouran ranges, South Australia: Using outcrops to test subsurface-based models from the northern Gulf of Mexico: Bulletin American Association Petroleum Geologists, v. 99, p. 293-331.

Hurford, A. J., H. R. Grunau, and J. Stöcklin, 1984, Fission track dating of an apatite crystal from Hormoz Island, Iran: Journal of Petroleum Geology, v. 7, p. 365-380.

Kent, P. E., 1987, Island salt plugs in the Middle East and their tectonic implications, in I. Lerche, and J. J. O'Brien, eds., Dynamical geology of salt and related structures, v. 3-37, Academic Press, New York.

Lemon, N. M., 1985, Physical Modelling of Sedimentation Adjacent to Diapirs and Comparison with Late Precambrian Oratunga Breccia Body in Central Flinders Ranges, South Australia: American Association Petroleum Geologists Bulletin, v. 69, p. 1327 - 1328.

Lemon, N. M., 1988, Diapir recognition and modelling with examples from the Late Proterozoic Adelaide Geosyncline, Central Flinders Ranges, South Australia: Doctoral thesis, Univesity of Adelaide.

Lemon, N. M., 2000, A Neoproterozoic fringing stromatolite reef complex, Flinders Ranges, South Australia: Precambrian Research, v. 100, p. 109-120.

Morley, C. K., B. Kongwung, A. A. Julapour, M. Abdolghafourian, M. Hajian, D. Waples, J. Warren, H. Otterdoom, K. Srisuriyon, and H. Kazemi, 2009, Structural development of a major late Cenozoic basin and transpressional belt in central Iran: The Central Basin in the Qom-Saveh area: Geosphere, v. 5, p. 325-362.

Rowan, M. G., and B. C. Vendeville, 2006, Foldbelts with early salt withdrawal and diapirism: Physical model and examples from the northern Gulf of Mexico and the Flinders Ranges, Australia: Marine and Petroleum Geology, v. 23, p. 871-891.

Talbot, C. J., and P. Aftabi, 2004, Geology and models of Qum Kuh central Iran: Journal of Geological Society of London, v. 161, p. 1-14.

Warren, J. K., 2008, Salt as sediment in the Central European Basin system as seen from a deep time perspective (Chapter 5.1), in R. Littke, ed., Dynamics of complex intracontinental basins: The Central European Basin System, Springer-Verlag, Berlin-Heidelberg, p. 249-276.

Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p.

 


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