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

摘要

Abstract. In order to better understand both the fixation andmigration of gases in evaporites, investigations were performed in fivehorizontal boreholes drilled in an underground potash seam. One of the fiveboreholes was pressurised with Ar and the pressure signal and chemical gascomposition were then monitored in the other holes. A further gas samplefrom a separate borehole was characterised for the chemical composition andfor noble gas and carbon isotopic compositions to conclude on the origin andevolution of the gas in the salt rocks. Additionally, in order to determinethe total gas amount in the salt rocks, a potash-bearing salt sample wasdissolved 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 m~(2) ), which is about 3–4 ordersof magnitude higher than in undisturbed salt rocks, we assume that themigration of injected Ar most likely takes place along micro-cracks produced during the mining process. The geogenic gas concentrations found in theobservation holes correlate directly to the Ar concentration, suggestingthat 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 δ 13 C CO 2 -isotopic composition ( ?7.8 ‰ to 6.7 ‰) indicates a magmatic source, whereas ~(13)C∕~(12)C of CH_(4) ( ?22.2 ‰ to ?21.3 ‰)is typical for a thermogenic gas. We assume that CO_(2) and CH_(4) are related to volcanic activity, where theyisotopically equilibrated at temperatures of 513to 519 ~(°) C about 15–16 Ma ago.