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 detected over many active geothermal systems. The production of these gases, and of N2O, can be a€ected by both geothermal and biological processes. In this investigation, soil gas and soil-gas ¯uxes were measured at the Cove Fort-Sulphurdale geothermal ®eld in Utah, which produces steam from both liquid- and vapor-dominated portions of the resource. The objectives were to determine the sources of these gases, the factors controlling their production, and the potential application to surface exploration and reservoir evaluation. Flux measurements were made in both summer and winter to evaluate and to quantify variations in seasonal noise. Carbon dioxide in soil gas, and in ¯uxes from the soil to the atmosphere during the summer sampling were dominated by soil respiration processes. During the winter, a geothermal compo- nent was visible. Methane ¯uxes were small negative values during the summer months, re¯ecting methanotrophic oxidation of atmospheric CH4 and, possibly, geothermal CH4 in the soils. Nitrous oxide in soil gas and in soil-gas ¯uxes to the atmosphere also varied seasonally. Surpris- ingly high concentrations were observed at locations directly above the steam cap. We suggest thatNH3 produced in the geothermal reservoir by the Haber reaction was seeping upward where it was biologically oxidized toNO3 ÿ. This oxidation, and possible localized biological reduction of NO3 ÿ to N2, produced moderate amounts of N2O, averaging three times typical background ¯ux rates and ten times background over the central portion of the geothermal area. There were higher ¯uxes of CO2, CH4 and N2O over the steam cap and the surrounding area, relative to background values. The high ¯ux may re¯ect seepage of gas along faults that intersect the more extensive liquid-dominated portion of the reservoir. Nitrous oxide mea- surements in soil gas and soil-gas ¯uxes to the atmosphere o€er promise as an exploration and Geothermics 29 (2000) 637±670 www.elsevier.com/locate/geothermics 0375-6505/00/$20.00 # 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S0375-6505(00)00036-5 * Corresponding author. Fax: +1-303-273-3629. E-mail address: rklusman@mines.edu (R.W. Klusman). reservoir characterization tool. # 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.