Detecting and characterizing a hidden geothermal system in the Mt. Endut area, Indonesia, through integrated soil-gas component analyses

A hidden geothermal system (HGS) is a specific, complex geothermal system that does not have surface thermal manifestations but has significant potential for energy resources. Despite the difficulty of exploration of such systems because of their concealed features, detection and development of these systems can significantly contribute to increasing geothermal power generation. This study aimed to detect and characterize an HGS in the Mt. Endut area, Banten, west Indonesia, and to clarify its relationship with the adjacent, conventional medium-temperature Cikawah Geothermal System (CGS). Mt. Endut area is classified as a hidden system because a thick low-permeability cap rock and sealed fault segments suppress surface discharge, with no evidence of a depressed water table or an overlying cold aquifer. Under such sealing conditions, moderately low-solubility CO₂ and short-lived radon isotopes (²²²Rn and ²²⁰Rn) can migrate upward through minor fractures while liquid flow remains impeded, making them effective tracers of concealed fluid pathways. To evaluate this mechanism, an integrated soil-gas geochemical approach was employed, combining CO₂ flux, δ¹³C-CO₂, and radon isotopes measurements with interpretations from existing geophysical datasets, a complete Bouguer anomaly (CBA) map, and magnetotelluric (MT) data. The soil-gas survey revealed large variabilities in the CO₂ flux (15 to 1399 g m ⁻² day⁻¹) and δ¹³C-CO₂ (−18.71 to −9.75‰) caused by a complex mixture of biogenic and hydrothermal gas sources. The locations of high-flux zones of hydrothermal-dominated mixture (HDM) CO₂ were strongly correlated with the Handeleum dextral fault having a permeable structure. The ²²²Rn/²²⁰Rn ratio was effective for differentiating gas source depths and attributing deep-seated ²²²Rn anomalies to major fault zones and advective transport. A key finding was the role of the NNE–SSW Endut normal fault as a distinct structural boundary separating the Endut HGS from the CGS, supported by a coincident gravity low and a steep MT resistivity gradient. Moreover, the results suggest that the two systems are driven by distinct heat sources, a deep magmatic body for the Endut HGS and a shallower andesitic intrusion for the CGS, and delineate three new potential areas warranting future exploration. Consequently, this study identifies robust geochemical signatures for both the concealed and conventional geothermal systems by high CO₂ flux of HDM origin coexisting with shallow-source ²²²Rn, and underscores the effectiveness of this integrated approach in reducing exploration uncertainty for HGS detection and characterization. These result can contribute to the development of geothermal resources in Indonesia and other volcanic areas.