Monitoring Fumarole Gas Chemistry in the Theistareykir Geothermal Field, NE Iceland

Abstract

The chemical and isotopic composition of fumarole steam is reported for the Theistareykir geothermal field in NE Iceland. The samples were collected annually from selected fumaroles in the years 2012 to 2018 as part of a baseline monitoring of surface activity. The first 45 MWe unit of the Theistareykir geothermal power plant was commissioned in December 2017, and the second 45 MWe unit came online in April 2018. In addition to annual samples for analyses of major gas components and stable water isotope analyses, samples were collected for the determination of stable gas isotopes (He, Ne, Ar, C, N) in 2014. The gas composition suggests considerable variability within the field, both in terms of estimated reservoir temperatures as determined by gas geothermometry and fluid origin as determined from stable hydrogen and oxygen isotopes. Estimated temperatures range from 250 to 320°C. The Theistareykir fluids are much more depleted in deuterium than local groundwater. The gas isotopes suggest superficial origin of N2 and Ar, but δ13C values are typical for the Icelandic mantle plume. The 3He/4He ratio is higher in Theistareykir than in nearby Krafla and Námafjall – and somewhat higher than what is expected for MORB. During the monitoring period, little changes have been observed in gas composition and overall surface activity of the Theistrareykir field, although geothermal manifestations constantly form and disappear. The monitoring period covers a few years of drilling and well testing, but only half a year of production from the Theistareykir field and will provide a valuable base line for future monitoring of the field. Since production started from the field, the hydrogen and oxygen isotopes of steam from one fumarole have changed substantially, suggesting that the fumarole now draws steam from a different aquifer. This is most likely a result of pressure change in the reservoir due to production.