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.

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