Gas geochemistry of the Cerro Pabellón geothermal plant: protocol constraining thermal and redox conditions

Abstract Geothermal heat extraction at a binary plant, and associated plant lifetime, can be optimized through fluid monitoring of geothermal gases and natural system gas discharges. A novel gas monitoring tool for geothermal exploration and production-stage decision-making is presented, based on research conducted at Cerro Pabellón, Chile, following its internationally celebrated opening in 2017. The gas phase was studied over a period of 5 years using direct gas condensate sampling and an innovative application of principal component analysis combined with the uniform manifold approximation and projection (UMAP) method. Monitoring results show that gas composition varies during cooling of the vapor phase, with oxidation state (RH) increasing as vapor fraction (%) decreases. A subsequent increase in HCO₃ 2 ⁻ concentration was detected in condensate liquids measured by ion chromatography, reflecting progressive gas–liquid interaction. According to UMAP clustering and gas composition analysis, the N₂/Ar ratio in gas samples represents an effective pathfinder to monitor geothermal plant physico-chemical parameters. Gas compositional clusters of CO₂-rich gases (> 96.0%), mainly represented by the N₂/Ar ratio (N 2 /O 2 :CH 4 /CO 2 ), show two clear trends in gas mixture fluctuations: cooling associated with a relatively hot (315 ± 45 °C) Apacheta-like gas mixture derived from the hydrothermal reservoir, relatively cooler fluids (Md = 252 ± 78 °C) varying in redox conditions (Md (LogH 2 /H 2 O) = − 3.33 ± 0.64 RH). These estimated thermo-redox conditions in tandem with vapor N₂/Ar ratio fluctuate distinctly along each vapor separation line segments (C, F, J, M), correlating strongly on vapor fraction (19–100 vol.%), with progressive cooling until reaching temperatures of 140–160 °C at the plant steam–brine separators. Gas compounds in equilibrium with CO₂-rich fluids represent critical indicators of reservoir and plant-scale processes. Monitoring and control of these gas compounds at geothermal plants can improve long-term maintenance strategies and sustainability of binary power plants through integration of condensate liquid surveillance and improved geostatistical analysis.