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).

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

John Warren - Wednesday, January 20, 2016

 

Black Salt: as an indicator of overpressure

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


Dihedral angle changes and the permeability of salt

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


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

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

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


Black salt and overpressure in Oman

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

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


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


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


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

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

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

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


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

Implications

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

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

References

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


 

 

Trackback Link
http://www.saltworkconsultants.com/BlogRetrieve.aspx?BlogID=13454&PostID=647336&A=Trackback
Trackbacks
Post has no trackbacks.

Recent Posts


Tags

perchlorate Weeks Island salt mine Paleoproterozoic Oxygenation Event Crescent potash 13C enrichment magadiite Zaragoza Koeppen Climate solikamsk 2 endosymbiosis anomalous salt zones LIP Mars York (Whitehall) Mine Ure Terrace Hell Kettle Lake Peigneur Deep capillary zone Belle Isle salt mine lunette crocodile skin chert lithium brine evaporite karst dark salt sulfate Hadley cell: Calyptogena ponderosa climate control on salt seawater evolution gassy salt brine lake edge natural geohazard hydrological indicator DHAB K2O from Gamma Log Mulhouse Basin vestimentiferan siboglinids salt ablation breccia CaCl2 brine 18O enrichment hydrothermal karst Lop Nur mine stability gas in salt Atlantis II Deep salt trade Lomagundi Event gem bedded potash Kalush Potash Archean Muriate of potash snake-skin chert Kara bogaz gol High Magadi beds MgSO4 enriched lithium carbonate supercontinent chert lapis lazuli zeolite CO2 Ceres Schoenite Catalayud hectorite epsomite Patience Lake member Mixing zone anthropogenically enhanced salt dissolution collapse doline nacholite Quaternary climate gas outburst halophile halogenated hydrocarbon Lop Nor jadarite extrasalt lot's wife deep meteoric potash Messinian geohazard venice Mega-monsoon cryogenic spring salts sulphur Sulphate of potash lithium battery Thiotrphic symbionts doline halotolerant marine brine mummifiction Mesoproterozoic evaporite-hydrocarbon association vadose zone Corocoro copper evaporite circum-Atlantic Salt Basins Deep seafloor hypersaline anoxic lake potash ore organic matter Neutron Log brine evolution meta-evaporite North Pole Neoproterozoic Musley potash intrasalt sedimentary copper bischofite Platform evaporite source rock salt mine Sumo rockburst Badenian Stebnik Potash causes of major extinction events lazurite End-Permian cauliflower chert salt leakage, dihedral angle, halite, halokinesis, salt flow, water in modern-day Mars Warrawoona Group authigenic silica carnallitite halite Proterozoic Stebnyk potash H2S methane well log interpretation astrakanite Large Igneous Magmatic Province SOP CO2: albedo mass die-back alkaline lake carbon cycle Salar de Atacama anthropogenic potash Ingebright Lake subsidence basin hydrogen Hadley Cell knistersalz extraterrestrial salt MOP Five Island salt dome trend methanogenesis Pilbara salt suture well logs in evaporites tachyhydrite Ripon palygorskite Danakhil Depression, Afar sinjarite Prairie Evaporite trona mirabilite Dead Sea caves Lamellibrachia luymesi 18O Prograde salt Hyperarid allo-suture Dead Sea karst collapse nuclear waste storage MgSO4 depleted Boulby Mine namakier potash freefight lake Belle Plain Member sepiolite halokinetic stevensite Precambrian evaporites well blowout End-Cretaceous vanished evaporite causes of glaciation seal capacity dihedral angle 13C Red Sea gypsum dune hydrothermal potash black salt hydrohalite basinwide evaporite potash ore price African rift valley lakes NPHI log Neoproterozoic Oxygenation Event End-Triassic Phaneozoic climate methanotrophic symbionts salt tectonics wireline log interpretation Ethiopia halite-hosted cave Dead Sea saltworks retrograde salt sinkhole Clayton Valley playa: MVT deposit Noril'sk Nickel kainitite RHOB Zabuye Lake McArthur River Pb-Zn brine pan GR log salt seal Great Salt Lake nitrogen Enceladus waste storage in salt cavity intersalt water on Mars Seepiophila jonesi SedEx base metal sulphate dissolution collapse doline stable isotope eolian transport Ganymede Magdalen's Road cryogenic salt solar concentrator pans McMurdo Sound NaSO4 salts SO2 Koppen climate salt karst ozone depletion recurring slope lines (RSL) HYC Pb-Zn Europe Jefferson Island salt mine halocarbon Dallol saltpan Density log Lake Magadi Bathymodiolus childressi saline giant oil gusher flowing salt carbon oxygen isotope cross plots sodium silicate Pangaea deep seafloor hypersaline anoxic basin Turkmenistan DHAL salt periphery blowout Karabogazgol ancient climate silicified anhydrite nodules sulfur evaporite-metal association phreatic explosion evaporite dissolution saline clay Realmonte potash Stolz diapir Gamma log antarcticite silica solubility auto-suture phreatomagmatic explosion Evaporite-source rock association

Archive