Gas Chemistry and Subsurface Temperature Estimation in the Hveragerdi High-Temperature Geothermal Field, SW-Iceland

Abstract

Five gas geothermometers based on the concentrations of CO2, H2S, H2, CH4, N2 and Ar in fumaroles and wet-steam well steam are applied to estimate subsurface temperature in the Hveragerdi high-temperature geothermal field, SWIceland. The results for fumaroles indicate that the calculated subsurface temperatures decrease from the northern part to the southern part part of the field. The CO2 - geothermometer gives the highest temperature values, an average of 256?C for the northern part, and 247?C for the southern part. The H2S - geothermometer reveals aquifer temperature of 211?C for the northern part, and 203?C for the southern part. The H2 - geothermometer gives an average subsurface temperature of 229?C for the northern part, and 184?C for the southern part, which agrees excellently with the measured temperatures in wet-steam wells. The measured borehole temperatures in the field range from 215?C to 230?C for the northern part, and from 167?C to 198?C for the southern part. The CO2 /H2 - geothermometer gives the lowest subsurface temperature values, an average of 203?C for the northern part, and 143?C for the southern part. The CO2 /N2 - geothermometer gives 249?C for the northern part and 235?C for the southern part. For data from wells, the CO2 - geothermometer, the H2S - geothermometer, and the H2 - geothermometer, give an average subsurface temperature of 247?C for the northern part and 246?C for the southern part, 213?C for the northern part and 220?C for the southern part, and 217?C for the northern part and 216?C for the southern part, respectively. The CO2 /H2 - geothermometer indicates an average subsurface temperature of around 200?C for both the northern part and the southern part. The CO2 /N2 - geothermometer gives an average subsurface temperature of 180?C for the northern part and 259?C for the southern part. The discrepancy between the estimated subsurface temperatures obtained by various gas geothermometers has been explained in this paper. By integration of solute geothermometry results, mixing model studies and the gas geothermometry results, the maximum subsurface temperatures of the Hveragerdi high-temperature geothermal system may be considered to be about 240-260?C.