Other
Scientific paper
Jun 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49.1307a&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 49, Issue 6, pp.1307-1325
Other
11
Scientific paper
Calibration of five gas geothermometers is presented, three of which used CO 2 , H 2 S and H 2 concentrations in fumarole steam, respectively. The remaining two use CO 2 / H 2 and H 2 S / H 2 ratios. The calibration is based on the relation between gas content of drillhole discharges and measured aquifer temperatures. After establishing the gas content in the aquifer, gas concentrations were calculated in steam formed by adiabatic boiling of this water to atmospheric pressure to obtain the gas geothermometry functions. It is shown that the concentrations of CO 2 , H 2 S and H 2 in geothermal reservoir waters are fixed through equilibria with mineral buffers. At temperatures above 230°C epidote + prehnite + calcite + quartz are considered to buffer CO 2 . Two buffers are involved for H 2 S and H 2 and two functions are, therefore, presented for the geothermometers involving these gases. For waters containing less than about 500 ppm chloride and in the range 230-300°C pyrite + pyrrholite + epidote + prehnite seem to be involved, but pyrite + epidote + prehnite + magnetite or chlorite for waters above 300°C and waters in the range 230-300°C, if containing over about 500 ppm. The gas geothermometers are useful for predicting subsurface temperatures in high-temperature geothermal systems. They are applicable to systems in basaltic to acidic rocks and in sediments with similar composition, but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones. The geothermometry results may be used to obtain information on steam condensation in upflow zones, or phase separation at elevated pressures. Measured aquifer temperatures in drillholes and gas geothermometry temperatures, based on data from nearby fumaroles, compare well in the five fields in Iceland considered specifically for the present study as well as in several fields in other countries for which data were inspected. The results of the gas geothermometers also compare well with the results of solute geothermometers and mixing models in three undrilled Icelandic fields.
Arnórsson Stefán
Gunnlaugsson Einar
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