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

Abstract

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 cm(STP)3 gas was liberated. Due to the relatively large permeability of the disturbed salt rocks (4×10-17 to 4×10-18 m2), 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 δ13CCO2-isotopic composition (−7.8 ‰ to 6.7 ‰) indicates a magmatic source, whereas 13C∕12C of CH4 (−22.2 ‰ to −21.3 ‰) is typical for a thermogenic gas. We assume that CO2 and CH4 are related to volcanic activity, where they isotopically equilibrated at temperatures of 513 to 519 ∘C about 15–16 Ma ago.