Gas geothermometers are based on equilibrium chemical reactions between gaseous
species. For each reaction considered a thermodynamic equilibrium constant may be
written, where the concentration of each species is represented by his partial
pressure in vapor phase.
The gas-gas equilibrium in geothermal fields with two phase-components should not
reflect the real gas composition present in the reservoir. It depends from many
factors like gas/steam ratio. It is assumed that there is no re-equilibration of
the chemical species from the source or sources to wellhead. The fluids analyzed
are those collected at the well head.
In geothermal fields the concentrations (or ratios) of gases like CO2, H2S, H2, N2,
NH3, and CH4 are controlled by temperature. Because of that, data from gas have
been used to study a correlation between the relative gas concentrations and the
temperature of the reservoir using the D’Amore and Panichi (1980) geothermometer
based on partial pressures of CO2, H2S, CH4, H2, where CO2 is externally fixed.
Hydrocarbon compounds in fumarolic gases result less abundant (up to one order of
magnitude) with respect to those measured in gases sampled from the productive
wells. This compositional difference is likely to be caused by the partial
dissolution into the superficial aquifer of hydrocarbons which fed the fumaroles,
since these compounds are characterized by a higher solubility with respect to that
of the other inert gases (mainly due to their higher molecular weight). On the
contrary, productive-well fluids, directly derived from the geothermal reservoir,
are not affected by this "scrubbing" process. Nevertheless, light hydrocarbon
compounds, such as methane, ethane, propane, propene, i-butane and i-butene, show
very similar solubility, thus the equilibrium reactions among them, depending on
their reciprocal ratios and not on their absolute abundances, result almost
independent from both phase transfer processes and the influence of superficial
aquifer. Therefore, it is reasonable to consider that the application of
geothermometric techniques based on thermodynamic equilibrium of organic gases is a
reliable tool to evaluate the temperature of deep systems even by adopting the
hydrocarbon composition of natural discharges.