Wayang Windu geothermal field, which is situated in quaternary volcanic rock, has been delivering 227 MW of electricity and is planned to be developed in near future. Isotope and geochemistry study has been done to support understanding of the system. Samples were taken from thermal manifestations and production wells (two-phase and dry steam wells) to analyse its isotope and chemical compositions. Most of the hot springs are bicarbonate type and have neutral pH values, except for steam heated waters found near fumarolic fields, which are acid sulfate type waters. Deep brine fluids are of mature chloride type, while other deep dilute fluids are bicarbonate type indicates the existance of condensate layer. Solute and gas geothermometer calculation showed good agreement with measured temperature, i.e., 280–300 °C. However, gas-gas equilibria calculation showed increasing T and decreasing y (steam fraction) compare to previous study which indicates contribution form hotter source with high liquid saturation. Aside from different recharge area of thermal manifestation and production wells, the isotopes composition also shows physical processes in reservoir, i.e., adiabatic boiling, steam separation at different temperature (140–260 °C) and steam fraction (0.30–0.65), and also mixing process with meteoric water.
- gas equilibria