Geothermal gas geochemistry in a source-pathway-reservoir framework: review, applications, and limitations

Geothermal gases serve as sensitive recorders of heat sources, fluid pathways, and reservoir conditions in geothermal systems. However, their original signals are often obscured by multi-endmember mixing, gas-water partitioning, phase separation, and sampling biases, limiting the reliability of traditional diagram-based discrimination methods. This review applies a Source-Pathway-Reservoir (SPR) evidence-chain framework to systematically evaluate sampling targets (fumaroles, hot springs, geothermal wells, and soil gases) and the interpretive boundaries of major gas components and multiple isotope systems. Noble gases, owing to their chemical inertness, offer unique advantages in source discrimination and reservoir sealing assessment, whereas reactive gases (CO 2 , H 2 S, H 2 , CH 4 , N 2 ) are more susceptible to shallow alteration and should be interpreted with caution. The core of multi-isotope integration lies in distinguishing deep-source signals from process-induced modifications. Practical applications include reservoir temperature estimation, heat-source identification, fault-controlled fluid pathway analysis, and dynamic monitoring. Future efforts should move incrementally toward a more quantitative framework that incorporates air correction, endmember constraints, mixing modeling, and multi-indicator cross-validation, rather than relying solely on qualitative diagram-based interpretation. This review outlines the essential components of such a framework and identifies current gaps.