Geothermal water-rock equilibria at Palinpinon, Philippines: Insights from major reactive gas constituents.

Eleven producing wells of the Palinpinon geothermal field have been monitored for major reactive gas components over the period 1998-2010. Based on H2O, CO2, H2S, H2, and CH4 relative concentrations, we estimated redox conditions (controlled by the Fe(II)-Fe(III) buffer), a possible equilibrium mineralogical assemblage (made up by epidote-prehnite-quartz-calcite-pyrite-pyrrhotite), deep equilibration temperatures (up to 350°C), and CO2 partial pressures (up to 12 bar) for the deepest, liquid-dominated portions of the reservoir.