Hydrothermal gas equilibria: the H 2O-H 2-CO 2-CO-CH 4 system

Abstract

The difficulty in measuring reservoir gas concentrations in geothermal systems often forces the use of gas ratios in a separated vapor phase to investigate reservoir conditions. Measured CO/CO 2 and H 2/H 2O ratios of fumarolic fluids and vapors from geothermal wells representative of twenty-two different hydrothermal systems are consistent with theoretical values obtained from either of two commonly used redox buffers, indicating that CO and H 2 attain chemical equilibrium in the hydrothermal reservoir. Use of different f O 2 -buffers has little effect on these functions. Many measured CH 4/CO 2 ratios are, instead, inconsistent with theoretical values obtained with any redox buffer. Since CH 4/CO 2 ratios are strongly affected by redox conditions in the gas equilibration zone, this disagreement between measured and theoretical values likely indicates that either no unique f O 2 -buffer is active in all the hydrothermal environments or that CH 4 is not in equilibrium with the other gases. The weight of CH 4 on the 3log(X CO/X CO 2 ) + log(X CO/X CH 4 ) function is relatively small. Therefore this function and the log(X CO/X CO 2 ) − log(X H 2 /X H 2O ) function, both of which are independent upon redox conditions, were used. These functions gave reasonable estimates of the equilibrium temperature and either the fraction of separated steam or the fraction of condensed steam in each sample. From these data, the CO/CO 2, H 2/H 2O, and H 2/CO ratios in the hypothetical single saturated vapor phase were calculated and used to investigate f O 2 and f CO 2 distributions in the considered twenty-two hydrothermal systems. Recalculated f CO 2 values are generally consistent, within one-half log-unit, with the full equilibrium function of Giggenbach (1984), Giggenbach (1988) although production of thermometamorphic CO 2 might locally take place. It is evident that no unique f O 2 -buffer is active in all the hydrothermal environments. This fact imply that CH 4 could have attained chemical equilibrium with other gas species in the H 2O-H 2-CO 2-CO-CH 4 system.