Abstract
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.
Highlights
Salt deposits contain liquids and gases of varying composition and concentration
It appears plausible that the CO2 and CH4 gases in the salt rocks are related to the Neogene volcanic activity, where the CO2 originated directly from the magmatic source and the CH4 was formed from organic material in the vicinity of the basaltic dikes
We conclude that the permeability of the salt rocks in the testing area is significantly increased through disturbance by the excavation process and that micro-cracks are produced which dominate the gas flux in the rocks
Summary
Salt deposits contain liquids and gases of varying composition and concentration. The gases present in salts are mainly mixtures of different components with regionally differing compositions and quantities. In the North-German Basin, gases consist mainly of hydrocarbons and N2 with lower concentrations of CO2, whereas in the Werra-Fulda deposits CO2 is the dominant gas (Müller, 1958). In the WerraFulda deposits, over 90 % of these gases are bound at the grain boundaries along inter-crystalline fissures and fractures as free gas (Hofrichter, 1976). The encountered gases bound in the evaporites must be monitored carefully. The aim of the study is to better understand the fixation and the migration of gases in evaporites and to make statements on the amount, the origin and the evolution of gas in salt beds of a potash mine
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