Origin and Evolution of Gas in Salt Beds of a Potash Mine

Zimmer, Martin; Strauch, Bettina; Zirkler, Axel; Niedermann, Samuel; Vieth-Hillebrand, Andrea

In order to better understand both the fixation and migration of gases in evaporites, investigations were performed in five horizontal boreholes drilled in an underground potash seam. One of the five boreholes was pressurised with Ar and the pressure signal and chemical gas composition were then monitored in the other holes. A further gas sample from a separate borehole was characterised for the chemical composition and for noble gas and carbon isotopic compositions to conclude on the origin and evolution of the gas in the salt rocks. Additionally, in order to determine the total gas amount in the salt rocks, a potash-bearing salt sample was dissolved in water and from the mass of 1 kg salt sample, 9 cminline-formula M1inlinescrollmathml ( normal STP ) normal 3 23pt18ptsvg-formulamathimgb258f901284aedd26a7d392710d23a7f adgeo-54-15-2020-ie00001.svg23pt18ptadgeo-54-15-2020-ie00001.png gas was liberated. Due to the relatively large permeability of the disturbed salt rocks (inline-formula M2inlinescrollmathml normal 4 × normal 10 - normal 17 46pt14ptsvg-formulamathimg731e3a60b8ff2a73d86de1da9faedd60 adgeo-54-15-2020-ie00002.svg46pt14ptadgeo-54-15-2020-ie00002.png to inline-formula M3inlinescrollmathml normal 4 × normal 10 - normal 18 46pt14ptsvg-formulamathimg67ea06c76f76340dee98f22f9f839c5e adgeo-54-15-2020-ie00003.svg46pt14ptadgeo-54-15-2020-ie00003.png inline-formulam2), which is about 3–4 orders of magnitude higher than in undisturbed salt rocks, we assume that the migration of injected Ar most likely takes place along micro-cracks produced during the mining process. The geogenic gas concentrations found in the observation holes correlate directly to the Ar concentration, suggesting that they were stripped from the rocks in between the holes. According to the He-isotopes (0.092 Ra), a small contribution of mantle gas can be found in the geogenic salt gas. The inline-formula M5inlinescrollmathml chem italic δ normal 13 normal C normal CO normal 2 42pt18ptsvg-formulamathimg23d46fa230938dcb5c2ce4daaffffb57 adgeo-54-15-2020-ie00004.svg42pt18ptadgeo-54-15-2020-ie00004.png -isotopic composition (inline-formula−7.8 ‰ to inline-formula6.7 ‰) indicates a magmatic source, whereas inline-formula13C∕12C of inline-formulaCH4 (inline-formula−22.2 ‰ to inline-formula−21.3 ‰) is typical for a thermogenic gas. We assume that inline-formulaCO2 and inline-formulaCH4 are related to volcanic activity, where they isotopically equilibrated at temperatures of 513 to 519 inline-formulaC about 15–16 Ma ago.

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Zimmer, Martin / Strauch, Bettina / Zirkler, Axel / et al: Origin and Evolution of Gas in Salt Beds of a Potash Mine. 2020. Copernicus Publications.

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