Potential for surface gas flux measurements in exploration and surface evaluation of geothermal resources

Anomalous concentrations of CO2 and, to a lesser extent, CH4 have been dete cted over many active geothermal systems. The production of these gases, an d of N2O, can be affected by both geothermal and biological processes. In t his investigation, soil gas and soil-gas fluxes were measured at the Cove F ort-Sulphurdale geothermal field in Utah, which produces steam from both li quid- and vapor-dominated portions of the resource. The objectives were to determine the sources of these gases, the factors controlling their product ion, and the potential application to surface exploration and reservoir eva luation. Flux measurements were made in both summer and winter to evaluate and to quantify variations in seasonal noise. Carbon dioxide in soil gas, a nd in fluxes from the soil to the atmosphere during the summer sampling wer e dominated by soil respiration processes. During the winter, a geothermal component was visible. Methane fluxes were small negative values during the summer months, reflecting methanotrophic oxidation of atmospheric CH4 and, possibly, geothermal CH4 in the soils. Nitrous oxide in soil gas and in so il-gas fluxes to the atmosphere also varied seasonally. Surprisingly high c oncentrations were observed at locations directly above the steam cap. We s uggest that NH3 produced in the geothermal reservoir by the Haber reaction was seeping upward where it was biologically oxidized to NO3-. This oxidati on, and possible localized biological reduction of NO3- to N-2, produced mo derate amounts of N2O, averaging three times typical background flux rates and ten times background over the central portion of the geothermal area. T here were higher fluxes of CO2, CH4 and N2O over the steam cap and the surr ounding area, relative to background values. The high flux may reflect seep age of gas along faults that intersect the more extensive liquid-dominated portion of the reservoir. Nitrous oxide measurements in soil gas and soil-g as fluxes to the atmosphere offer promise as an exploration and reservoir c haracterization tool. (C) 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.