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Salty MattersJohn Warren -Thursday March 24, 2016 as a Fluid Seal: Article 4 When and where salt leaks, implications for waste storage 400 350 200 0 40 km 300 250 Iran flowed into a variety of autochthonous and allochthonous salt 450 Arabian Gulf C.I. = 50 ft masses. Autochthonous salt structures are still firmly rooted in Manifa Hith Edge 500 the stratigraphic level of the primary salt bed. Allochthonous North eld Abqaiq Qatif salt structurally overlies parts of its (stratigraphically younger) Fazran Bahrain Rostum overburden and is often no longer connected to the primary salt Awali Maydan Abu Al Bukhoosh Fateh Mahzam bed (mother-salt level). Nasr Dukhan Id El Shargi SW Fateh Bul Umm Shaif Hanine DOHA Ghawar DUBAI Breaches in bedded (non-halokinetic) salt Qatar El Bunduq Zakum The principal documented mechanism enabling leakage across Mubarraz ABU DHABI Hair Ghasha Haradh Dalma bedded salt in the diagenetic realm is dissolution, leading to United breaks or terminations in salt bed continuity. Less often, leak- Saudi Arabia Ruwais Bab Arab age across a salt unit can occur where bedded salt has responded Emirates Oil eld with Jurassic source, Bu Hasa in a brittle fashion and fractured or faulted (Davison, 2009). In reservoir and cap hydrocarbon-producing basins with widespread evaporite seals, 200 Asab 250 500 450 300 400 350 significant fluid leakage tends to occur near the edges of the salt Oil eld with non-Jurassic source Shah or reservoir or cap bed. For example, in the Middle East, the laterally continuous Figure 1. Hith Anhydrite isopach showing thickening from the UAE west into Saudi Arabia and some of the main elds producing from Jurassic reservoirs in this region (in Hith Anhydrite ( Jurassic) acts as a regional seal to underlying part after Alsharhan and Kendall, 1994). Note the lack of Jurassic reservoirs to the east Arab Cycle reservoirs and carbonate-mudstone source rock. The of the Hith erosional edge showing the eectiveness of the evaporite in holding high efficiency of the Hith seal creates many of the regions giant back substantial oil and gas columns sourced in Hanifa mudstones (after Warren, 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 ba- 100 km sins worldwide (Article 3). When salt does leak or transmit fluid, it does so in one of two ways: 1) by the entry of undersaturat- ed waters (Article 1 in this series) and; 2) by temperature and pressure-induced changes in its dihedral angle, which in the di- agenetic realm is often tied to the development of significant South Oman Salt Basin 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 (“in- side the salt”). However, due to dihedral angle changes at greater Today depths and pressures, a significant portion of leaking fluid pass- Mid Tertiary ing through more deeply-buried altering salt is external. By the Base Tertiary onset of greenschist facies metamorphism, this is certainly the Not Base Mid-Cret. case (Chapter 14 in Warren, 2016) mapped Base Jurassic

Figure 2. Distribution and timing of charge in the oil and gas elds along the Diagenetic fluids driving salt leakage are highly productive eastern margin of the South Oman Salt Basin is controlled by the retreating dissolution edge of the Neoproterozoic Ara Salt and the underly- external to the salt mass ing Huqf Fm. source rock, which became mature in the Cretaceous. Oil elds Within a framework of fluids breaching a subsurface salt body, shaded red, stars indicate gas elds (after Terken et al., 2001). the breached salt can be a bed of varying thickness, or it can have Page 1 www.saltworkconsultants.com

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 diapir- ic structures Once formed, salt diapirs tend to focused upward escape of ba- sinal 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 anom- alies clustered around and above salt diapirs in the North Sea and (Figure 3b); (3) localized salinity anomalies around salt di- apirs, 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 Tertia- ry 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-lon- ger-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.

Northwest Southeast and supergiant fields, including Ghawar in Saudi Arabia, which 2.0 Gas chimney Salt breached is the largest single oil-filled structure in the world. Inherent and leaking maintenance of the evaporite’s seal capacity also prevents vertical 2.5 migration from mature sub-Hith source rocks into potential res- ervoirs in the overlying Mesozoic section across much of Saudi Arabia and the western Emirates. However, toward the Hith seal 3.0 Messinian salt edge are a number of large fields supra-Hith fields, hosted in

ime (second) 3.5 Cretaceous carbonates, and a significant portion of the hydrocar- T bons are sourced in Jurassic carbonate muds that lie stratigraph- wo-way ically below the anhydrite level (Figure 1). T 4.0 Latakia Ridge The modern Hith Anhydrite edge is not the depositional margin of the laterally extensive evaporite bed. Rather, it is a dissolution 4.5 edge, where rising basinal brines moving up and out of the basin have thinned and altered the past continuity of this effective seal. 5.0 Leading thrust ahead 3 km of the Latakia Ridge The process of ongoing dissolution allowing vertical leakage near Figure 4. Gas chimney present above the leading thrust of the Latakia Ridge (After the edge of a subsurface evaporite interval, typifies not just the Bowman, 2011). edge of bedded but also the basinward edges of salt units Page 2 www.saltworkconsultants.com

Zone of overpressure higher beneath alous salt zones in the Gulf of Mexico salt canopy diapirs (as documented in Article 1). Diagenetic fluids driving salt leakage are internal to the salt mass Fluid entry in relation to chang- es in the dihedral angle of is well documented (Article 2). It was first recorded by Lewis and Holness Salt Weld (1996) who postulated, based on their Fault zone or gumbo, commonly overpressured static-salt laboratory experiments; Halokinetic Salt Overpressured zone (shale dominant) Caprock and brine halo zones from salt dissolution Zone of uid transfer across a salt weld or “In sedimentary basins with normal (pore waters tends to be reducing & high salinity) a zone of thinned (residual salt < 10 m thick) geothermal gradients, halite bodies at Figure 5. Zones of leakage and insoluble residue (caprock, lateral caprock and basal caprock) are created at depths exceeding 3 km will contain a the salt edge in zones of undersaturated circum-salt uid focus (after Warren, 2016).

Historically, in the 1920s and 30s, shallow Saline seep Saline A. Fault conduit vuggy and fractured caprocks to salt diapirs seeps were early onshore exploration targets about topographic highs in the Gulf of Mexico (e.g. Lateral Thick basin-wide undersaturated bedded salts Spindletop). Even today, the density of drilling in ow (100s m - km thick) and geological data derived from these onshore diapiric features means many models of cap- Escaping basinal rock formation are mostly based on examples waters Limit of saline waters in Texas and Louisiana. Onshore in the Gulf Anhydrite nodules accrete as from immediate dissolution Halite and bittern nodular mosaic (insolubles) salts dissolve on upper side of basal of bedded halite mixed with of Mexico, caprocks form best in dissolution along underside anhydrite basinal waters (greenbed halo) Base of of bed zones at the outer, upper, edges of salt struc- salt lower Direction of interface retreat Na, Cl, K, (Mg?) created by salt dissolution tures, where active cross-flows of meteoric wa- ions permeate out of Halite undersaturated unit via inter-nodule water seeps through the Basal anhydrite ters are fractionally dissolving the salt. How- Basal permeability basal anhydrite bed anhydrite ever, rocks composed of fractional dissolution Anhydrite dissolves on ≈1m underside of bed; organics residues, with many of the same textural and aquifer and other residues, including sulphides,

mineralogical association as classic Gulf of op of Khok accumulate T Mexico caprocks, are now known to mantle the Kruat Group deep sides of subvertical-diapirs in the North Basinwide Mode. Basal Anhydrite Maha Sarakham, Basal Anhydrite , as an example of basal caprock formed via mesogenetic uid ushing as the rise of basinal waters of the Khorat group is focused along the salt’s underside Sea (e. g., lateral caprock in the Epsilon Diapir) and define the basal anhydrite (basal caprock) Crestal caprock Upper B. (active phreatic) that defines the underbelly of the Cretaceous Permian diapiric Maha Sarakham halite across the Khorat Pla- Surface teau in NE Thailand (Figure 5; Warren, 2016). Teritary salt Lower Cret. Upp. Cret. All “caprocks” are fractionally-dissolved accu- 9/2-8TS4 mulations of diapir dissolution products and form in zones of fluid-salt interaction and 9/2-8TS4 leakage, wherever a salt mass is in contact with T3

Lower Cret. Lower Lateral undersaturated pore fluids (Figure 6). First to caprock T4 (meso- dissolve is halite, leaving behind anhydrite res- Jur. genetic) idues, that corss-flushing pore waters can then Jurass. convert to gypsum and, in the presence of sul- Diapir

Triassic Anhydritic phate reducing bacteria, to calcite. If the diapir Basement lateral caprock Basement experiences another growth pulses the caprock Epsilon diapir Epsilon diapir detail Fluid ow model can be broken and penetrated by the rising Diapir Mode. Epsilon (squeezed) diapir, North Sea, as an example of lateral caprock formed via mesogenetic salt. This helps explain fragments of caprock uid shing and crestal caprock via active phreatic ushing caught up in shale sheaths or anomalous dark- Figure 6. Caprock also forms in non-crestal positions. A) Basal anhydrite beneath Maha Sara- salt zones, as exemplified by less-pure salt-edge kham salt, Thailand (after Warren, 2016) B) Lateral caprock- Epsilon diapir, North Sea (after intersection units described as dark and anom- Jackson and Lewis, 2012).

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Waymouth A-45 Waymouth A-45 ultimately dissolves as it transitions into various meta-evaporite (3645 m) (3645 m) indicator minerals and zones (Chapter 14, Warren, 2016). When increasing pressure and temperature changes the halite dihedral angle in the diagenetic realm, then supersaturated hy- drocarbon-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 over- Waymouth A-45 (3410 m) pressured salt-encased carbonate sliver source, up to 50 or more meters into the adjacent halite (Schoenherr et al. 2007). Like- wise, 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 re- crystallised 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) oc- cur 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,

Figure 7. Halite crystals of the Weymouth-A45 well showing halite crystal aggregates with implying it must have regained or retained seal integrity, after it sutured polyhedal contacts (A, B). C) shows perpendicular cleavage planes and assemblages leaked. Not knowing the internal fold geometries in any deeply of primary aqueous uid inclusions (AFI) between cleavages planes that are parallel to growth zones, as well as a linearly oriented assemblages of secondary petroleum uid buried salt mass, but knowing that all flowing salt masses are in- inclusions (PFI) trapped along healed fractured zones (after Kettanah, 2013). The sampled ternally complex (as seen in salt mines and namakiers), means we halite is once again impermeabe although it contains hydrocarbon inclusions. cannot assume how far the hydrocarbon inclusions have moved stable interconnected brine-filled porosity, resulting in permea- within the salt mass, post-leakage. Nor can we know if, or when, bilities comparable to those of sandstones”. Extrapolating from any salt contact occurred with a possible externally derived hy- their static halite pressure experiments they inferred that halite, drocarbon-bearing fluid source, or whether subsequent salt flow occurring at depths of more than ≈3 km and temperatures above lifted the hydrocarbon-inclusion-rich salt off the contact surface 200 °C, has a uniform intrasalt pore system filled with brine, and as salt flowed back into the interior of the salt mass. therefore relatively high permeabilities. Thus, with any hydrocarbon-rich occurrence in a halokinetic salt In the real world of the subsurface, salt seals can hold back signif- mass, we must ask the question; did the salt mass once hydrof- icant hydrocarbon columns down to depths of more than 6-7 km racture (leak) in its entirety, or did the hydrocarbons enter lo- (see case studies in Chapter 10 in Warren, 2016 and additional cally and then as the salt continued to flow, that same hydrocar- documentation the SaltWork database). Based on a compilation bon-inclusion-rich interval moved into internal drag and drape of salt-sealed hydrocarbon reservoirs, trans-salt leakage across folds? In the case of the Ara Salt, the thickness of the black salt 75-100 metres or more of pure salt does not occur at depths less penetration away from its overpressured source is known as it is a than 7-8 km, or temperatures of less than 150°C. In their work core-based set of observations. In the Ara Salt at current depths on the Haselbirge Formation in the Alps, Leitner et al. (2001) of 3500-4000 m, the fluid migration zones extend 50 -70 meters use a temperature range >100 °C and pressures >70 MPa as de- out from the sliver source in salt masses that are hundreds of fining the onset of the dihedral transition. metres thick (Kukla et al., 2011; Schoenherr et al., 2007). It seems that across much of the mesogenetic realm, a flowing So how do we characterize leakage extent in and compacting salt mass or bed can maintain seal integrity to much greater depths than postulated by static halite percolation a buried salt mass without core? experiments. In the subsurface, there may be local pressured-in- Dark salt, especially if it contains hydrocarbons, clearly indicates duced changes in the halite dihedral angle within the salt mass, fluid entry into a salt body in the diagenetic realm. Key to con- as seen in the Ara Salt in Oman, but even there, there is no ev- siderations of hydrocarbon trapping and long-term waste stor- idence of the total km-scale salt mass transitioning into a leaky age is how pervasive is the fluid entry, where did the fluid come aquifer via changes in the halite dihedral angle (Kukla et al., from, and what are the likely transmission zones in the salt body 2011). But certainly, as we move from the diagenetic into the (bedded versus halokinetic)? metamorphic realm, even thick pure salt bodies become perme- In an interesting recent paper documenting and discussing salt able across the whole salt mass. Deeply buried and pressured salt leakage, Ghanbarzadeh et al., 2015 conclude:

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isolated 100 pores No percolation (no leakage) 90

80 Ex. I Experiment I 70 60 (Experiment I) 660 µm No

Isolated tetrahehral Ø tetrahehral Isolated 50 T = 100°C P 20MPa leakage Experiment II connected 40 Ex. II pores angle ( θ ) Dihedral 30

20

Percolation 10 (leakage) 0 (Experiment II) θ > 60° Leakage Porosity (%) θ ≤ 60° T = 275°C P 100MPa (percolation) Ø polyhedral Connected A. 0 1 2 3 4 5 B. Figure 8. Halite dihedral angle changes and the creation of connected intercrystalline polyhedal pores at higher temperatures and pressures (after Ghanbarzadeh et al., 2015). A) Hydrostatic experiments on synthetic performed at P = 20 MPa and T = 100°C (Exp-I) and P =100 MPa and T = 275°C (Exp-II). Shows their 3D reconstruction of the pore network at assumed static textural equilibrium; all edges of the 3D volumes correspond to 660 mm. The skeletonized pore network was extracted from the reconstructed 3D volume; colored according to local pore-space-inscribed radii, with warmer colors indicating larger radii. B) Distribution of apparent dihedral angles in the two experiments plotting the Exp-I and Exp-II ponints in a porosity to dihedral angle plot space ,with the percolation threshold (brown versus white shading) calculated from static pore-scale theory . Left side inset and y axis shows the details of automated dihedral angle extraction from 2D images.They show also the median value of dihedral angles and the estimated errors based on the 95% condence interval. Bottom inset and x axis shows their calculated porosity in natural rock salt inferred from resistivity logs using Archies Law.

“The observed hydrocarbon distributions in rock salt require that interpretation is the deepwater well GC8 (Figure 9), where they percolation occurred at porosities considerably below the static use a combination of a resistivity, gas chromatograms, and mud threshold due to deformation-assisted percolation. Therefore, the log observations to infer that hydrocarbons have entered the design of nuclear waste repositories in salt should guard against lower one km of a 4 km thick salt section, via dihedral-induced deformation-driven fluid percolation. In general, static perco- percolation. lation thresholds may not always limit fluid flow in deforming environments.” I have a problem in accepting this leap of faith from laboratory experiments on pure salt observed at the static decimeter-scale Their conclusions are based on lab experiments on static salt of the lab to the dynamic km-scale of wireline-inferred obser- and extrapolation to a combination of mud log and wireline vations in a salt allochthon in the real world of the offshore in data collected from a number of wells that intersected salt al- deepwater salt Gulf of Mexico. According to Ghanbarzadeh et lochthons in Louann Salt in the Gulf of Mexico. Their lab data al., 2015, the three-part gray background in Figure 9 corresponds on changing dihedral angles inducing leakage or percolation in to an upper no-percolation zone (dark grey), a transition zone static salt confirms the experiments of Holness and Lewis (1996 (moderate grey) and a lower percolation zone (light grey). This – See Article 2). But they took the implications of dihedral an- they then infer to be related to changes in dihedral angle in the gle change further, using CT imagery to document creation of halite sampled in the well (right side column). Across the data interconnected polyhedral porosity in static salt at higher tem- columns, what the data in the GC8 well show is: A) Gamma peratures and pressures (Figure 8). They utilise Archies Law log; allochthon salt has somewhat higher API values at depths and resistivity measures to calculate inferred porosity, although shallower than 5000 m; B) Resistivity log, a change in resistivity it would be interesting what values they utilise for cementation to higher values (i.e., lower conductivity) with a change in the exponent (depends on pore tortuosity) Sw and saturation expo- same cross-salt depth range as seen in the gamma log, beginning nent. Assuming the standard default values of m = 2 and n =2 around 5100 m; C) Gas (from sniffer), shows a trend of decreas- when applying Archies Law to back calculate porosity spreads in ing gas content from the base of salt (around 6200 m) up to a halite of assumed Sw are likely incorrect. depth around 4700 m, then relatively low values to top salt, with an interval that is possibly shalier interval (perhaps a suture - see They then relate their experimental observations to wireline below) that also has a somewhat higher gas content ; D) Gas measurements and infer the occurrence of interconnected pores chromatography, the methane (CH4) content mirrors the total in Gulf of Mexico salt based on this wireline data. Key to their gas trends, as do the other gas phases, where measured; E) Mud Page 5 www.saltworkconsultants.com

emplaced by sedimentological processes unrelated to changes in the dihedral an- gle 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 leak- age 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 Mexi- co public domain data. For example, Fig- ure 10 shows a seismic section through the Mahogany field and the intersection of the salt by the Phillips No. 1 discov- ery well (drilled in 1991). This interpret- ed 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 Mexi- Figure 9. Petrophysical observations using wireline well logs and mud logs data to co salt province. Internally, Gulf of Mex- constrain the uid distribution and connectivity in 4 km of saltintersected in the ico salt allochthons, like others world- well GC8 from the deep water Gulf of Mexico (after Ghanbarzadeh et al., 2015). (A) wide, are not composed of pure halite, Gamma-ray log, (B) electrical resistivity, (C) total hydrocarbons gas, (D) gas just as is the case in the onshore struc- chromatography, (E) hydrocarbon indications in the mud log (FL, uorescence; tures discussed in the context of dark salt OS, oil stain; DO, dead oil; and OC, oil cut) in mud logs, and (F) the dihedral angle zones in article 1. Likely, a similar lack of inferred from experimental data. Shading around each curve shows the measure- purity and significant structural and lith- ment error and average uctuations in data. ological variation typifies most if not all of the salt masses sampled by the Gulf of Log (fluorescence response), dead oil is variably present from Mexico wells listed in the Ghanbarzadeh base of salt up to 5000 m, oil staining, oil cut and fluorescence et al. paper, including the key GC8 well (Figure 9). This variation (UV) are variably present from base salt up to a depth of 4400 m. in salt purity and varying degrees of local leakage is inherent to the emplacement stage of all salt allochthons world-wide. It is On the basis of the presented log data, one can infer the lower set up as the salt flow (both gravity spreading and gravity glid- kilometer of the 4 km salt section contains more methane, more ing) occurs at, or just below the seafloor, fed by varying combi- liquid hydrocarbons, and more organic material/kerogen com- nations of extrusion or thrusting, which moves salt out and over pared to the upper 3 km of salt. Thus, the lower section of the salt the seabed (Figure 11). 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, when it is flowing laterally and creating a salt allochthon, salt. What is not given in figure 9 is any information on likely is in a period of rapid breakout (Figure 11; Hudec and Jackson, levels of non-organic impurities in the salt, yet this information 2006, 2007; Warren 2016). This describes the situation when a would have been noted in the same mud log report that listed rising salt sheet rolls out over its base, much in the same way a hydrocarbon levels in the well. In my opinion, there is a lack of military tank moves out over its track belt. As the salt spreads, lithological information on the Gulf of Mexico salt in the Ghan- the basal and lateral salt in the expanding allochthon mass, is barzadeh et al. paper, so one must ask; “does the lower kilometer subject to dissolution, episodic retreat, collapse and mixing with of salt sampled in the GC8 well, as well as containing hydrocar- seafloor sediment, along with the entry of compactional fluids bons also contain other impurities like shale, pyrite, anhydrite, derived from the sediments beneath. Increased impurity lev- etc. If so, potentially leaky intervals could be present that were els are particularly obvious in disturbed basal shear zones that

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transition downward into a gumbo zone (Figure 12a), but also mantle the sides of subvertical salt structures, and can evolve by further salt disso- lution 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 contrib- ute greater volumes of fluid and ongoing intra- salt dissolution intensity and alteration where the suture sediment is in contact with outside- the-salt fluids. Allochthon rollout, with simul- taneous diagenesis and leakage, occurs across intrasalt shear zones, or along deforming basal zones. In the basal part of an expanding alloch- thon sheet the combination of shearing, sealing, and periodic leakage creates what is known as Figure 10. Interpreted seismic across the Mahogany Discovery, Gulf of Mexico (after Harrison et al., 2010). This was the petroleum industry's rst commercial subsalt oil development in the Gulf of Mexico and even in this 1990s vintage “gumbo,” a term that describes a complex, vari- seismic, the intrasalt complexity is evident both in the seismic section and along the well trajectory. Internally, Gulf of Mexico halite is not made up of halite with only hydrocarbons as impurities. ably-pressured, shale-rich transition along the

A) Plug-fed Thrust B) Plug-fed extrusion C) Source-fed thrust Salt plug Salt plug Thick overburden Autochthonous source layer Future thrust

Shortening Extrusive advance Thrust advance (tectonic) Diapir roof thrust Thrust advance (tectonic)

Partial burial Breakout Open-toed advance Open-toed advance Open-toe Open-toed advance Open-toe

Breakout Breakout Full burial Breakout Thrust advance (driven Burial Thrust advance (driven Burial Thrust advance by salt expulsion) Thin roof by salt expulsion) Thin roof

Thick Thick Thick overburden Sheet pinned overburden Sheet pinned overburden Sheet pinned

Figure 11. Emplacement of salt sheets (allochthons; after Hudec and Jackson 2006). Most salt sheets advance by a time-related sequence of extrusion, open-toed advance, and thrusting. These modes may combine in many ways over the life span of a salt sheet, but three evolutionary sequences (or lineages) are particularly common. (a) Plug-fed extrusions. (b) Plug-fed thrusts, and (c) Source-fed thrusts (see also Figure 12a for detail on basal shear).

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At the salt’s upper Anadarko #1 Amoco #1 Vermilion Unocal #1 contact, the spreading Vermilion 349 Ship Shoal 356 360 salt mass may carry its Sealevel Sealevel overburden with it, or it may be bare topped 1 1 (aka open-toed; Figure 5,000 ft 2 1 11). In either case, once 3 2 2000 m 4 2 salt movement slows Rotated, dissolved and 3 and stops, a caprock 10,000 ft sheared base of salt 3 atop ”gumbo” 4 carapace starts to form 5 6 4000 m that is best developed 7 4 wherever the salt edge 15,000 ft 8 9 is flushed by undersatu- 5 10 rated pore waters (Fig- 6 6000 m ure 6). Soon after its 20,000 ft 7 emplacement, the basal 8 15,000 ft Fault gouge Condensed rafted section correlates zone of a salt alloch- 9 (shale sheath) with layer 5 and older thon acts a focus for ris- 25,000 ft ing compactional fluids A) Rollout model of base of salt and creation of underlying gumbo (can become coming from sediments a suture if adjacent salt canopies overlap beneath. So, even as it is

Diapir pinched shut Minor suturing Apical allosuture line still spreading, the lower Passive diapir Frontal escarpment side of the salt sheet is Extension Aggrading peripheral plain acts as allochthon Terminal suture point subject to dissolution, buttress (R>A) Peripheral plain and hydrocarbon entry, Allosuture Shortening often with remnants Feeder of the same hydrocar- Salt sheet Salt Sheet A feeder bon-entraining brines Suture dips toward leaking to seafloor about overiding sheet Feeder the salt sheet edge. As Salt Sheet B the laterally-focused Autosutures localised by Time Major suturing subsalt brines escape to Fold belt parallel weak zones in roof with apical suture line Apical allosuture line the seafloor across zones Aggrading peripheral Terminal suture point plain acts as allochthon of thinned and leaky Peripheral plain buttress (R>A) salt or at the allochthon edge, they can pond to Autosuture Feeder form chemosynthetic Salt Sheet A Salt expelled Basal suture point DHAL (Deepsea Hy- from beneath overriding lobe Boudinaged and torn persaline Anoxic Lake) sediments in suture zone Feeder brine pools (Figure 3a). Listric allosuture Salt Sheet B B) Autosutures (zones of internal C) Allosutures (zones of internal Such seep-fed brine lakes typify the deep sea non-salt sediment and black salt) non-salt sediment and black salt) floor in the salt alloch- Figure 12. Geological settings that can create “leaky” base of salt or intrasalt sediment sutures. A) Cross section view thon region of conti- of salt allochton with a basal thrust fault de ning the rising compressional nose of the salt sheet. Relevant Gulf of nental slope and rise in Mexico wells are placed schematically on this section (after Harrison and Patton (1995). B) Autosutures: the roof of a salt sheet shortens where sheet advance is buttressed by peripheral plain or by another salt sheet. Overriding auto- the Gulf of Mexico and sutures may initiate at any zone of weakness in the roof, especially pre-existing reactive diapirs formed by previous the compressional salt stretching as the salt sheet spreads. As it is overridden from the rear, the front of the roof depressed into the salt. C) ridge terrain in the cen- Asymmetric allosutures. As one salt sheet overridesthe other, sediments in the suture are depressed, stretched, and tral and eastern Medi- dismembered to form boudins in the salt. These sediments eventually tear o from the basal suture line (B anc C terranean. If an alloch- after Dooley et al., 2012). thon sheet continues to expand, organic-rich basal margin of most salt allochthons in the Gulf of Mexico DHAL sediments and (Figure 12a). Away from suture zones, as more allochthon salt fluids become part of the basal shear to the salt sheet (Figure rolls out over the top of earlier foot-zones to the spreading salt 12a). mass, the inner parts of the expanding and spreading allochthon body tend toward greater internal salt purity (less non-salt and Unfortunately, Ghanbarzadeh et al., 2015 did not consider the dissolution residue sediment, as well as less salt-entrained hydro- likely geological implications of salt allochthon emplacement carbons and fluid inclusion). Page 8 www.saltworkconsultants.com

mechanisms and how this likely explains much of the geologi- varying combinations of salt thinning, salt dissolution or inter- cal character seen in wireline signatures across wells intersecting section with unexpected regions of impure salt (relative aquifers). salt in the Gulf of Mexico. Rather, they assume the salt system In addition to such natural process sets, cross-salt leakage can be and the geological character they infer as existing in the lower related to local zones of mechanical damage, tied to processes portions of Gulf of Mexico salt masses, are tied to post-emplace- involved in excavating a mine shaft, or in the drilling and casing ment changes in salt’s dihedral angle in what they consider as of wells used to create a purpose-built salt-solution cavity. Many relatively homogenous and pure salt masses. They modeled the potential areas of leakage in existing mines or brine wells are the various salt masses in the Gulf of Mexico as static, with upward result of poorly completed or maintained access wells, or inter- changes in the salt purity indicative of concurrent hydrocarbon sections with zones of “dark salt,” or with proximity to a thinned leakage into salt and facilitated by altered dihedral angles in salt cavity wall in a diapir, as documented in articles 1 and 2 (and the halite. A basic tenet of science is “similarity does not mean detailed in various case studies in Chapters 7 and 13 in Warren equivalence.” Without a core from this zone, one cannot assume 2016). hydrocarbon occurrence in the lower portions of Gulf of Mexico salt sheets is due to changes in dihedral angle. Equally, if not In my opinion, the history of extraction, and intersections with more likely, is that the wireline signatures they present in their leakage zones, during the life of most of the world’s existing salt paper indicate the manner in which the lower part of a salt al- mines means conventional mines in salt are probably not appro- lochthon has spread. To me, it seems that the Ghanbarzadeh et priate sites for long-term radioactive waste storage. Existing salt al. paper argues for caution in the use of salt cavities for nuclear mines were not designed for waste storage, but to extract salt waste storage for the wrong reasons. or potash with mining operations often continuing in a partic- ular direction along an ore seam until the edge of the salt was approached or even intersected. When high fluid transmission Is nuclear waste storage in salt a safe, viable zones are unexpectedly intersected during the lifetime of a salt long-term option? mine, two things happen; 1) the mine floods and operations Worldwide, subsurface salt is an excellent seal, but we also know cease, or the flooded mine is converted to a brine extraction fa- that salt does fail, that salt does leak, and that salt does dissolve, cility (Patience Lake) or, 2) the zone of leakage is successfully especially in intrasalt zones in contact with “outside” fluids. grouted and in the short term (tens of years) mining continues Within the zone of anthropogenic access for salt-encased waste (Warren, 2016). storage (depths of 1-2km subsurface) the weakest points for po- tential leakage in a salt mass, both natural and anthropogenic, are For example, in the period 1906 to 1988, when Asse II was an related to intersection with, or unplanned creation of, unexpect- operational salt mine, there were 29 documented water breaches ed fluid transmission zones and associated entry of undersatu- that were grouted or retreated from. Over the long term, these rated fluids that are sourced outside the salt (see case histories in same water-entry driven dissolution zones indicate a set of nat- Chapter 7 and 13 in Warren, 2016). This intersection with zones ural seep processes that continued behind the grout job. This is of undersaturated fluid creates zones of weakened seal capacity true in any salt mine that has come “out of the salt” and outside and increases the possibility of exchange and mixing of fluids de- fluid has leaked into the mine. “Out-of-salt” intersections are rived both within and outside the salt mass. In the 1-2 km depth typically related to fluids entering the salt mass via dark-salt or range, the key factor to be discussed in relation to dihedral angle brecciated zones or shale sheath intersections (these all forms of change inducing percolation in the salt, will only be expressed anomalous salt discussed in article 1 and documented in the case as local heating and fluid haloes in the salt about the storage studies discussed in Chapter 13 in Warren 2016). cavity. Such angle changes are tied to a thermal regime induced I distinguish such “out-of-salt” fluid intersections from “in-salt” by long-term storage of medium to high-level radioactive waste. fluid-filled cavities. When the latter is cut, entrained fluids drain I use an ideal depth range of 1-2 km for storage cavities in salt into the mine and then flow stops. Such intersections can be as cavities located much deeper than 2 km are subject to com- dangerous during the operation of a mine as there is often ni- pressional closure or salt creep during the active life of the cavity trogen, methane or CO2 in an “in-the-salt” cavity, so there is (active = time of waste emplacement into the cavity). Cavities potential for explosion and fatalities. But, in terms of long-term shallower than 1 km are subject to the effects of deep phreat- and ongoing fluid leakage “in-salt” cavities are not a problem. ic circulation. Salt-creep-induced partial cavity closure, in a salt Ultimately, because “out-of-salt” fluid intersections are part of diapir host, plagued the initial stages of use of the purpose-built the working life of any salt mine, seal integrity in any mine con- gas storage cavity known as Eminence in Mississippi. In the ear- verted to a storage facility will fail. Such failures are evidenced ly 1970s, this cavity was subject to a creep-induced reduction by current water entry problems in Asse II Mine, Germany in cavity volume until gas storage pressures were increased and (low-medium level radioactive waste storage) and the removal the cavern shape re-stabilised. Cavities in salt shallower than 1 of the oil formerly stored in the Weeks Island strategic hydro- km are likely to be located in salt intervals that at times have carbon facility, Texas. Weeks Island was a salt mine converted to been altered by cross flows of deeply-circulating meteoric or ma- oil storage. After the mine was filled with oil, expanding karst rine-derived phreatic waters. Problematic percolation or leakage cavities were noticed forming at the surface above the storage zones (aka anomalous salt zones), which can occur in some plac- area. Recovery required a very expensive renovation program es in salt masses in the 1-2 km depth range, are usually tied to that ultimately removed more than 95% of the stored hydrocar- Page 9 www.saltworkconsultants.com

bons. And yet, during the active life of the Weeks Island Salt Mine, the surface mine geologists had mapped “black non-salt salt” occurrences and tied them to unwanted fluid entries that were salt then grouted. Operations to block or control the entry of fluids were suc- cessful, and salt extraction continued apace. This information on fluid -en try was available well before the salt mine was purchased and converted to a federal oil storage facility. However, Geotechnically favourable Geotechnically in the 1970s when the mine was con- salt) is enclosed by (cavern (diapiric) Thick bedded salt verted, our knowledge of salt proper- ties and salt’s stability over the longer term was less refined than today. fault Worldwide, the biggest problem with converting existing salt mines to low to medium level nuclear waste stor- age facilities is that all salt mines are relatively shallow, with operating mine depth controlled by tempera- tures where humans can work (typi- cally 300-700 m and always less than 1.1 km). This relatively shallow depth (cavern not fully enclosed) (cavern

range, especially at depths above 500 less favourable Geotechnically Thin bedded salt Salt breccia (tectonic) m, is also where slowly-circulating subsurface or phreatic waters are dis- non-salt area in uenced solving halite to varying degrees, This by cavern cavern is where fluids can enter the salt from rock-salt outside and so create problematic Figure 13. Schematic summarising geotechnically favourable and less favourable scenar- dark-salt and collapse breccia zones ios for waste storage in salt (after Gillhaus, 2010; Warren 2016). within the salt. In the long-term (hundreds to thousands of years) these same fluid access regions have lated to heat-induced dihedral angle changes, may also become the potential to allow stored waste fluids to escape the salt mass, relevant over the long-term (tens of thousands of years), even in bedded storage facilities in 1-2 km depth range. Another potential problem with long-term waste storage in many salt mines, and in some salt cavity hydrocarbon storage Now what? facilities excavated in bedded (non-diapiric) salt, is the limited Creating a purpose-built mine for the storage of low-level waste thickness of a halite beds across the depth range of such con- in a salt diapir within the appropriate depth range of 1-2 km is ventional salt mines and storage facilities. Worldwide, bedded the preferred approach and a much safer option, compared to ancient salt tends to be either lacustrine or intracratonic, and the conversion of existing mines in diapiric salt, but is likely to individual halite units are no more than 10-50 m thick in stacks be prohibitively expensive. To minimise the potenial of unwant- of various saline lithologies. That is, intracratonic halite is usually ed fluid ingress, the entry shaft should be vertical, not inclined. interlayered with laterally extensive carbonate, anhydrite or shale The freeze-stabilised “best practice” vertical shaft currently be- beds, that together pile into bedded saline successions up to a ing constructed by BHP in Canada for its new Jansen potash few hundred metres thick (Warren 2010). The non-halite inter- mine (bedded salt) is expected to cost more than $1.3 billion. If layers may act as potential long-term intrasalt aquifers, especially a purpose-built mine storage facility were to be constructed for if connected to non-salt sediments outside the halite (Figure 13). low to medium level waste storage in a salt diapir, then the fa- This is particularly true if the non-salt beds remain intact and cility should operate a depth of 800-1000m. Ideally, such a pur- hydraulically connected to up-dip or down-dip zones where the pose-built mine should also be located hundreds of metres away encasing halite is dissolutionally thinned or lost. Connection to from the edges of salt mass in a region that is not part of an area such a dolomite bed above the main salt bed, in combination of older historical salt extraction operations. At current costings, with damaged casing in an access well, explains the Hutchison such a conventionally-mined purpose-built storage facility for gas explosion (Warren, 2016). Also, if there is significant local low to medium level radioactive waste is not economically fea- heating associated with longer term nuclear waste storage in sible. such relatively thin (<10-50 m) salt beds, then percolation, re- Page 10 www.saltworkconsultants.com

This leaves purpose-built salt-solution cavities excavated within presentation at AAPG Annual Convention, 1995, and from an thick salt domes at depths of 1-2 km; such purpose-built cavi- extended abstract prepared for presentation at GCS-SEPM ties should be located well away from the salt edge and in zones Foundation 16th Annual Research Conference, “Salt, Sediment with no nearby pre-existing brine-extraction cavities or oil-field and Hydrocarbons,” December 3-6, 1995. exploration wells. This precludes most of the onshore salt dia- pir provinces of Europe and North America as repositories for Hudec, M. R., and M. P. A. Jackson, 2006, Advance of alloch- high-level nuclear waste, all possible sites are located in high thonous salt sheets in passive margins and orogens: American population areas and can have century-long histories of poorly Association Petroleum Geologists - Bulletin, v. 90, p. 1535-1564. documented salt and brine extraction and petroleum wells. Stay- Hudec, M. R., and M. P. A. Jackson, 2007, Terra infirma: Un- ing “in-the-salt” over the long-term would an ongoing problem derstanding : Earth-Science Reviews, v. 82, p. 1-28. in these regions (see case histories in chapters 7 and 13 in War- ren, 2016 for a summary of some problems areas). Jackson, C. A. L., and M. M. Lewis, 2012, Origin of an anhydrite sheath encircling a salt diapir and implications for the seismic References imaging of steep-sided salt structures, Egersund Basin, Northern Alsharhan, A. S., and C. G. S. C. Kendall, 1994, Depositional set- North Sea: Journal of the Geological Society, v. 169, p. 593-599. ting of the Upper Jurassic Hith Anhydrite of the Arabian Gulf; Kettanah, Y. A., 2013, Hydrocarbon fluid inclusions in the Argo an analog to Holocene evaporites of the United Arab Emirates salt, offshore Canadian Atlantic margin: Canadian Journal of and Lake MacLeod of Western Australia: Bulletin-American Earth Sciences, v. 50, p. 607-635. Association of Petroleum Geologists, v. 78, p. 1075-1096. Kukla, P., J. Urai, J. K. Warren, L. Reuning, S. Becker, J. Schoen- Andresen, K. J., M. Huuse, N. H. Schodt, L. F. Clausen, and L. herr, M. Mohr, H. van Gent, S. Abe, S. Li, Desbois, G. Zsolt Seidler, 2011, Hydrocarbon plumbing systems of salt minibasins Schléder, and M. de Keijzer, 2011, An Integrated, Multi-scale offshore Angola revealed by three-dimensional seismic analysis: Approach to Salt Dynamics and Internal Dynamics of Salt AAPG Bulletin, v. 95, p. 1039-1065. Structures: AAPG Search and Discovery Article #40703. Bowman, S. A., 2011, Regional seismic interpretation of the hy- Leitner, C., F. Neubauer, J. L. Urai, and J. Schoenherr, 2011, drocarbon prospectivity of offshore Syria: GeoArabia, v. 16, p. Structure and evolution of a rocksalt-mudrock-tectonite: The 95-124. in the Northern Calcareous Alps: Journal of Struc- Cartwright, J., M. Huuse, and A. Aplin, 2007, Seal bypass sys- tural Geology, v. 33, p. 970-984. tems: American Association Petroleum Geologists - Bulletin, v. Lewis, S., and M. Holness, 1996, Equilibrium halite-H2O di- 91, p. 1141-1166. hedral angles: High rock salt permeability in the shallow crust: Davison, I., 2009, Faulting and fluid flow through salt: Journal of Geology, v. 24, p. 431-434. the Geological Society, v. 166, p. 205-216. Schoenherr, J., J. L. Urai, P. A. Kukla, R. Littke, Z. Schleder, J.- Dooley, T. P., M. R. Hudec, and M. P. A. Jackson, 2012, The M. Larroque, M. J. Newall, N. Al-Abry, H. A. Al-Siyabi, and Z. structure and evolution of sutures in allochthonous salt: Bulletin Rawahi, 2007, Limits to the sealing capacity of rock salt: A case American Association Petroleum Geologists, v. 96, p. 1045-1070. study of the infra-Cambrian Ara Salt from the South Oman salt basin: Bulletin American Association Petroleum Geologists, v. Ghanbarzadeh, S., M. A. Hesse, M. Prodanović, and J. E. Gard- 91, p. 1541-1557. ner, 2015, Deformation-assisted fluid percolation in rock salt: Science, v. 350, p. 1069-1072. Terken, J. M. J., N. L. Frewin, and S. L. Indrelid, 2001, Petroleum systems of Oman: Charge timing and risks: Bulletin-American Gillhaus, A., 2010, Natural gas storage in salt caverns - Sum- Association of Petroleum Geologists, v. 85, p. 1817-1845. mary of worldwide projects and consequences of varying stor- age objectives and salt formations, in Z. H. Zou, H. Xie, and Thrasher, J., A. J. Fleet, S. J. Hay, M. Hovland, and S. Düppen- E. Yoon, eds., Underground Storage of CO2 and Energy, CRC becker, 1996, Understanding geology as the key to using seepage Press, Boca Raton, Fl., p. 191-198. in exploration: the spectrum of seepage styles, in S. D., and M. A. Abrams, eds., Hydrocarbon migration and its near-surface ex- Harrison, H., and B. Patton, 1995, Translation of salt sheets by pression, AAPG Memoir 66, p. 223-241. basal shear: Proceedings of GCS-SEPM Foundation 16th An- nual Research Conference, Salt Sediment and Hydrocarbons, Warren, J. K., 2010, Evaporites through time: Tectonic, cli- Dec 3-6, 1995, p. 99-107. matic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268. Holly Harrison, Dwight ‘Clint’ Moore, and P. Hodgkins, 2010, A Geologic Review of the Mahogany Subsalt Discovery: A Warren, J. K., 2016, Evaporites: A Compendium (ISBN 978-3- Well That Proved a Play (The Mahogany Subsalt Discovery: 319-13511-3) Released Feb. 22 2016: Berlin, Springer, 1854 p. A Unique Hydrocarbon Play, Offshore Louisiana): Search and Discovery Article #60049 Posted April 28, 2010, Adapted from

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John Warren, Chief Technical Director SaltWork Consultants Pte Ltd (ACN 068 889 127) Kingston Park, Adelaide, South Australia 5049 www.saltworkconsultants.com

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