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

Abstract

Anomalous concentrations of CO 2 and, to a lesser extent, CH 4 have been detected over many active geothermal systems. The production of these gases, and of N 2O, can be affected by both geothermal and biological processes. In this investigation, soil gas and soil-gas fluxes were measured at the Cove Fort-Sulphurdale geothermal field 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 fluxes from the soil to the atmosphere during the summer sampling were 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 CH 4 and, possibly, geothermal CH 4 in the soils. Nitrous oxide in soil gas and in soil-gas fluxes to the atmosphere also varied seasonally. Surprisingly high concentrations were observed at locations directly above the steam cap. We suggest that NH 3 produced in the geothermal reservoir by the Haber reaction was seeping upward where it was biologically oxidized to NO 3 −. This oxidation, and possible localized biological reduction of NO 3 − to N 2, produced moderate amounts of N 2O, averaging three times typical background flux rates and ten times background over the central portion of the geothermal area.There were higher fluxes of CO 2, CH 4 and N 2O over the steam cap and the surrounding area, relative to background values. The high flux may reflect seepage of gas along faults that intersect the more extensive liquid-dominated portion of the reservoir. Nitrous oxide measurements in soil gas and soil-gas fluxes to the atmosphere offer promise as an exploration and reservoir characterization tool.