Salt and Methane Generation Initiated by Membrane Polarisation

The existence of deep marine hypersaline anoxic basins (DHAB) has been well-documented starting with the MedRiff Project in the Eastern Mediterranean. We suppose that there is analogy between the recent and ancient DHABs. This premise allows us to hypothesize that some methane accumulations in geological reservoirs may have been generated by historical euryhaline bacteria. The extreme life conditions of the bacteria and the facieses, as found in currently existing supersaturated salt brines DHABs, may have also existed in the geological past. Since salt basins overlap some of the most productive gas provinces, this article aims to introduce a new approach to salt and methane generation. It highlights the need to reconsider the classical approach to salt and methane generation due to new observations. Hereby we describe a new mechanism for DHAB generation due to membrane polarization. These phenomena generate a surface on which seawater of normal salinity meets the underneath brine of high salinity, and there is no diffusion between them. Hence we presume that non-crystalized, over-pressured, salty brine is the appropriate material to trap and host methane. Following overburden by deposited basin sediments, this viscous, gas-saturated brine can be an engine for diapir formation, which is prior to the crystalline phase. This new idea redefines our search for salt and methane deposits yet it requires further research and consideration, along with the new approach of salt diapir formation in specific salt basins.


Inspiration and Research History
The first recorded firedamp explosion occurred on 9 September 1664 in Hallstatt (Austria), in a salt mine and not in a coal mine (Harris, 1908). Prior to the installation of a proper ventilation system in Wieliczka Salt Mine (Poland), skilled workers were responsible for burning off the methane that would accumulate in the ceilings of the mine chambers. In Solotvina Salt Mine (Aknaszlatina, Ukraine), the methane escaping from salt was used for illumination (Wanek, 2008) in the early 19 th century. Nowadays salt mines are equipped with extensive ventilation systems to prevent similar firedamp explosions.
As for the classical view on salt formation, the name "evaporite" itself expresses that salt is generally seen a product of evaporation by dry -out of sedimentary basins (Ochsenius&Van't Hoff theory in Arrhenius&Lachman, 2003). Further evaporitic minerals, gypsum, potash, etc. result from this phenomenon (Clarke, 1920). It is also wellknown that numerous hydrocarbon accumulations are connected to salt basins. For successful exploration, it is crucial to understand the deposition and generation of evaporites, also how the subsequent deformation history can influence salt basin petroleum systems (Thomas, 2008).
The structural dynamics of salt systems is tackled in both theoretical and applied science literature (Jackson et al., 1994), yet describing when and how salt bodies may have been generated, has not been unambiguously completed until now. Arrhenius & Lachman (Arrhenius & Lachman, 2003) point out that no large-scale salt deposits are being formed under current geological conditions.

Unusual Physical Properties
The means to spot these brines was the sonic response while surveying the Mediterranean. In several deep zones (3,000-4,000 m), the bottom presented a "flat" reflection when the sonic signal had encountered a DHAB layer. An important feature of the DHAB is its high viscosity inasmuch as the "flat" reflection surface can and does support deposited fine grain sediments (Corselli et al., 1995).
DHABs seem to have high viscosity, bearing properties of a non-Newtonian fluid, as if solid. Anomalous superficial tension was found between normal salinity seawater and salt brine layers supporting the deposited sediments.
Researchers found abruptly increasing conductivity at the brine surface (Corselli et al., 1995). For example, conductivity in the Discovery Basin varies from 45 to 125 mS/cm ( Figure 1). In the same basin, salinity abruptly increases at the same depth from 40 ppt to 120 ppt ( Figure 1).

Biological and Chemical Characteristics
Along with physical properties, biological characteristics were also investigated by MEDRIFF.
Methanogenesis in brines has been described in several articles. Borin et al. (2008) mention that methanogenesis greatly exceeds sulphate reduction in most of the saline layers in Urania DHAB where extremely high bacterial abundance varies from layer to layer. The researchers labelled these as "hot-spots" of microbial activity, which can occur in geological time scale, too.
For the DHABs studied in the Mediterranean Sea, Karisiddaiah (2000) published data on methane concentration between 128-2692x10 3 nM (2.048-43.072 mg/L). In Lake Madee DHAB (Eastern Mediterranean), an intensive and stratified bacterial ecosystem was identified (Yakimov et al., 2013). Besides the bacterial phylogeny, a detailed chemical composition of the brine was documented layer by layer.

Discussion
There is sufficient evidence that DHABs exist. The followup questions are: 1. Why does diffusion not work across the "flat" reflection? 2. How is that surface formed and maintained? 3. What sediment load can it support? 4. Can this phenomenon occur over geological timescale?  Wherever critical micella concentration (CMC) of normal salinity sea water (as a neglected colloid system) is reached, polar particles will aggregate and generate coagulation and flocculation. This process creates a potential field in which positive ions (Ca 2+ and Mg 2+ ) will be forced to migrate. This causes the increased conductivity detected (Figure 1.), which is a phenomenon called membrane polarization (Ward 1990). The flocculation of argillaceous particles in the presence of the Ca 2+ and Mg 2+ ions is a process described in the article "Relative factors influencing membrane filtration effects in geologic environments" by Frederick A. F. Berry (1969).

Membrane Polarisation; Surface Tension; Reverse
The membrane generated by this polarisation will function as an osmotic, semipermeable membrane, allowing only certain ions to pass through. Considering the above mentioned depth (~3000m) and pressure (>300atm ~4,500psi), the phenomenon will be one of a reverse osmotic process. This facilitates the solvent, i.e. water to pass upward through this membrane into a region of normal seawater concentration.
Meanwhile, ions remain behind, thus increasing their concentration beneath the membrane. By this process, hypersaline brine DHAB is generated, reaching very high salinity and viscosity. Diffusion between normal seawater and brine is blocked by the surface tension created via membrane polarisation.
If the phenomenon described above is repeated pending on CMC, membrane polarisation cycles are formed. This yields a layered brine separated by thin argillaceous films.

DHABs in Geological Frame
According to the MedRiff Consortium (1995), this viscous brine is able to support deposited sediments. Over geological time, a large number of DHABs might be buried and isolated within a sedimentary basin. These hidden, over-pressured and layered brine pools of high viscosity bear non-Newtonian fluid properties. They behave as solid bodies between the sedimentary deposits. They have been able to preserve their internal properties, and support large, basin-fill sedimentary sequences.

Buried DHABs Hosting Methane
If the methane content of current DHABs continues to be confirmed (Karisiddaiah, 2000 andYakimov et al., 2013), then substantial volumes of biogenic methane are likely to be trapped in the buried brines (~40mg/L).
Thus, primary bacterial generation ~40 mg/L and secondary acetate decomposition ~30 mg/L will increase the aggregated methane concentration to ~70mg/L in buried brine. As a consequence, these brines can serve as hydrocarbon resources.
In case of the buried DHABs, the volume of these brines as non-Newtonian fluids will determine the trapped methane quantity in the ancient DHABs. In geological timescale the thickness of such buried DHABs may be higher than the actual brines, because it depends on the basin filling rate.

New Hypotheses on Diapir Formation
As methane-hosting buried brine is generated (DHAB) and covered by a significant volume of sediments, the substantial weight of the deposits will create overpressure in the brine layers throughout geological time. It is likely that the initial equilibrium of this almost solid, non-Newtonian fluid can reverse as a fluid caused by the impact of an earthquake. As a consequence, the energy transferred to this liquid will breach the covering deposits, penetrating into them. This uplift of the brine from the buried DHABs will incorporate rock fragments of the covering deposits. The over-pressured, methane saturated brine on the bubble point will release the gas and the remaining water into the reservoir packages of the covering sediments (Figure 3. Posepny, 1871).  The de-gasification and dehydration of the hypersaline ascending brine suffer volume collapse when the surrounding-and covering breccia is formed. This loss of material initiates salt crystallization, meanwhile preserving the folds generated during the penetration and plastic ascent. The incorporated foreign bodies and rock fragments from the covering deposits can be found in the crystalline salt diapir Figure 4. 2. Sea water of normal salinity is not merely a solution but also a colloid system. At a particular depth, micella reach critical concentration. Duly, physical properties: conductivity, salinity concentration and osmotic pressure will change abruptly. We presume that the phenomenon of membrane polarisation is responsible for the increase of conductivity due to the presence of argillaceous ions of negative charge and the positive Ca 2+ and Mg 2+ ions.
3. Membrane polarisation creates a semipermeable membrane and generates a reverse osmotic process due to hydrostatic pressure. This is the phenomenon responsible for the increase of the salinity beneath the membrane layer; this is how DHABs are generated. This semipermeable membrane is responsible for the surface tension between normal sea water and brine, serving as a physical barrier against diffusion.
4. The oversaturated, viscous brine is an ideal environment both for the halophile bacteria, i.e. methane generating bacteria and for acetate decomposition. Throughout these processes, methane generation reaches 70 mg/l (CH 4 ~ 40 mg/l bacterial + ~30 mg/l acetate decomposition).

5.
As found by MedRIFF, the viscous brine surface can support deposited sediments. This allows the brines to be trapped and buried over geological time, generating an over-pressured, non-crystallized body of high methane and acetate concentration (~70mg/l).
6. As non-Newtonian fluids, DHABs could grow in volume along geological time to dimensions vaster than those of the currently known ones; 7. Once the buried, over-pressured brine hosting methane is exposed to tectonic events, the over-pressured brine will breach the seal and expel the trapped methane into the sandy sedimentary layers above it. As a result, crystalline salt diapirs are formed.