Several gas geothermometers exist that have applications in the geothermal industry. Most are based on the CO2-CH4-H2-H2O system but none involve H2S, a gas that is always present in geothermal systems.

The equilibrium reactions for water-magnetite-pyrite-pyrrhotite-H2S-CO2-CH4 were first described by Norman et al. (1998) to explain approaching equilibrium from the hot and cold sides. Equilibrium is based on the reaction:

2Fe3O4 + 12H2S + CO2 = 6FeS2 + 10H2O + CH4

Fluid-rock equilibria modeling using PHREEQC v.2.10 confirms that the H2S content increases approximately five fold for every 50?C increase in temperature. The H2S concentration increases from 0.0003 mole % at 200?C to 0.008 mole % at 300?C. A formula to calculate temperature based on H2S concentration is presented for the 200-300?C range and is valid for non-boiling low-salinity fluids.

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