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.

 

 

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.

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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.


 


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