On the origin and evolution of geothermal fluids in the Patuha Geothermal Field, Indonesia based on geochemical and stable isotope data

Abstract

Volcano-hosted, vapor-dominated geothermal systems have great potential for power generation, although to date, such systems discovered globally remain limited in number. Understanding of the physical and chemical properties of geothermal fluids (water and gas) in vapor-dominated systems is critical for the sustainable development of geothermal resources. This study aims to clarify the origins, water–rock interactions, and chemical evolution of geothermal fluids during migration from a reservoir to the surface by selecting the Patuha geothermal field (PGF) in West Java, Indonesia as a case study. The PGF is characterized by a vapor-dominated system that originated from the subduction of the Indian–Australian plate beneath the Eurasian plate. In total, 26 water and 12 gas samples from production wells with 1,424–2,004 m depth, and fumaroles were analyzed for major anions, cations, trace elements, stable isotopes, and gas components to interpret phenomena occurring in deep reservoirs. Ternary diagrams of Cl–SO4–HCO3 ionic compositions suggest that the H2S and CO2 gases are condensed near the surface and changed to sulfate and bicarbonate by mixing with groundwater. Products of water–wall rock interactions appeared in the area with acidic water, which has mainly leached aluminum, accelerated pyrite oxidation, and increased iron concentration in the water. High fluoride concentration at a fumarole site (95.9 mg/L) implies HF gas supply from the deep-seated magmatic plume that is a geothermal source of the PGF system. Oxygen and hydrogen isotopes reveal that meteoric water is the main source of this system, and Na–K–Mg diagrams indicate immaturity of the reservoir water. Through evaporation and mixing with the magmatic waters, the waters have enriched heavy isotopic values, ascend along major faults towards the surface, and partly discharge at hot springs and fumaroles. High temperatures of the reservoir and gas-source in the subducted Indian–Australian plate are estimated based on the high CO2 and H2S concentrations and the high N2/Ar ratios, respectively. By integrating the analysis results of the water and gas samples, the well temperature data, and surface geology, the volcanic activity under a crater was estimated as the heat source and to have essential functions with the faults in the formation and fluid system of the vapor-dominated PGF.