Potential For Surface Gas Flux Measurements In Exploration And Surface Evaluation Of Geothermal Resources

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Journal Article: Potential For Surface Gas Flux Measurements In Exploration And Surface Evaluation Of Geothermal Resources

Abstract
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 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 CH4 and, possibly, geothermal CH4 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 NH3 produced in the geothermal reservoir by the Haber reaction was seeping upward where it was biologically oxidized to NO3-. This oxidation, and possible localized biological reduction of NO3- to N2, produced moderate amounts of N2O, averaging three times typical background flux rates and ten times background over the central portion of the geothermal area.There were higher fluxes of CO2, CH4 and N2O 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.

Authors 
Ronald W. Klusman, Joseph N. Moore and Michael P. LeRoy








Published Journal 
Geothermics, 2000





DOI 
10.1016/S0375-6505(00)00036-5


 

Citation

Ronald W. Klusman,Joseph N. Moore,Michael P. LeRoy. 2000. Potential For Surface Gas Flux Measurements In Exploration And Surface Evaluation Of Geothermal Resources. Geothermics. (!) .