Salt and Methane Generation Initiated by Membrane Polarisation

Abstract

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.

Review of the Ochsenius Theory for Salt Generation in the Transylvanian Basin, Romania

“…It is obvious that such a huge, basinwide salt amount cannot have been generated simply by evaporation [19] . Some other known processes are responsible for the salt deposit formation, as an alternative those described by Unger & LeClair [5] . Gas occurrences in the salt deposits show parallel salt and methane generation, mentioned in Unger&LeClair publication [9] .…”

“…The complicated geological build-up and evolution of the Transylvanian Basin is well documented in several upto-date publications [5][6][7][8]14,15] (Figure 4). The Transylvanian Miocene sedimentary basin was part of the Central Paratethys and it is considered as a piggyback, cold basin, with a thick lithosphere [14] in contrast with the Pannonian Basin, with thin lithosphere of elevated thermic gradient.…”

“…The Transylvanian Basin hosts important gas reserves in the post-salt deposits with characteristic salt tectonics that have been studied by several researchers starting from 1911 until nowadays [6][7][8][14][15][16][17][18] . The origin of the exclusively methane reserves is a matter of standing debate since a secondary gas source is expected to be found in addition to the classic biogenic source, to amount to the actual methane reserves and production [5] .…”

“…If we have salt from 4a. [8] ) precipitation by supercritical water [4] or from buried deep hypersaline brines [5] , then the HST is continuous; there is no LST in the sedimentary sequence build-up.…”

“…A new salt generation model was presented by Unger&LeClair [5] describing the deep hypersaline brine generation model as initiated by membrane polarisation, and generating a semipermeable membrane through a critical coagulation process. Reverse osmosis takes place releasing the water through this membrane; as a result, hypersaline brine is formed under the membrane, and salt will form in geological time scale.…”