Two types of fluids are encountered in the Onikobe geothermal reservoir: one is neutral (pH = 6.7-7.8) and the other is acidic (pH = 2.6-3.3) in the ambient condition of the separated waters. Acidic and neutral fluids are locally isolated from one another. On the basis of a spatial relationship between the acidic fluid zone and the acidic alteration zone (pyrophyllite and/or diaspore), it is suggested that acidic fluid might be upwelling along the faults forming the horst structure. Based on the conceptual model of the acidic fluid, reaction-path simulation (chemical equilibrium approach) and coupled thermal, hydrological and chemical (THC) simulation were performed to understand the formation mechanism and the chemical behavior of acidic fluid at the Onikobe geothermal field. Volcanic gases from rhyolitic magma mix with hydrothermal fluid originating from meteoric water at deep depths near Well GO-11, which may be located in the center of the upwelling zone. The fluid reacts with granitic rocks along the faults to form NaCl-type fluid (pH 5.2 at 500°C and 70 MPa) with SO₂(aq). During the upwelling process, the fluid decreases in temperature and pressure, and HCl(aq) dissociates into H⁺ and Cl^-, and SO₂(aq) disproportionates into SO₄^2- and HS^-. The fluid pH is lowest and a large amount of quartz precipitates near the critical point. When the fluid reaches the reservoir, it reacts with the reservoir rocks and the pH rises, stabilizing at about pH 3.5 (pH buffered by the pyrophyllite-quartz-chlorite assemblage). This fluid may be the acidic fluid in the Onikobe reservoir. Further downstream in the fluid flow (200-250°C), the pH rises above 4.5. Some HSO₄^- ions become SO₄^2- which reacts with Ca²⁺. The fluid becomes supersaturated with respect to anhydrite (anhydrite precipitates). As a result, the SO₄^2- concentration decreases to about 1/2 to 1/3. The conceptual model in the Onikobe acidic geothermal system was well reproduced by the reaction- path and the THC simulations. Both results were basically in agreement. The formation mechanism and the chemical behavior of the acidic fluid in the fracture were understood. However, there was a difference in the pH distribution in the fluid upwelling process, and the total HS^- concentration of the reaction-path simulation was lower than the measured value. This may be due to the initial gas compositions from the magma and a limitation of the reaction-path simulation as equilibrium calculation.

Okubo et al. (2023) presented Curie point depths (CPD) of the Japanese Islands. We applied the CPD data of Okubo et al. (2023) to the Hatchobaru geothermal field and compared it with other data to evaluate the results of our analysis. We here define Dcmag as a CPD calculated by Curie point analysis of magnetic data and Dchole as a CPD estimated by drillhole temperature – depth curve, respectively. The Dcmag of the Hatchobaru geothermal field was approximately 6.5 to 9.0 km below sea level. This is generally consistent with the Dchole. Although it is necessary to compare the results with other geoscience data for local temperature anomalies, the Curie point analysis is expected to contribute to the geothermal system modeling.