Gas geothermometry and chemical equilibria of fluids from selected geothermal fields

Abstract

It is only relatively recently that gas chemistry has been widely used for geothermal systems. In this report, methods of collecting gas from fumaroles and wells and the chemical analysis of gas are described. Several empirical and thermodynamic gas geothermometers have been proposed in the past few years. Most of these are reviewed and evaluated mainly with data from Icelandic geothermal fields. The chemical reactions, in which gases take part in reservoirs, are discussed in detail. The WATCH aqueous speciation program is used to simulate the behaviour of minerals, which are considered to react with gas. The mineral buffer pyrite + magnetite + epidote + prehnite is suggested to control the concentration of hydrogen sulphide in a one (liquid) phase reservoir fluid. It is plausible that zoisite + prehnite + quartz + calcite control the carbon dioxide concentration in the reservoir fluid. Calculations show that carbon dioxide, hydrogen sulphide and hydrogen geothermometers are relatively efficient for Icelandic geothermal fields. Magmatic gases entering the reservoir will disturb the chemical equilibrium in the reservoir and this is reflected in the results obtained by the gas geothermometers. Condensation and separation of steam will increase the gas concentration and cause high results for gas geothermometers; the converse is true for the removal of hydrogen sulphide and hydrogen. In such cases, the co-application of different methods is more useful. The Fischer-Tropsch reaction does not seem ....