Studies on the Fluid Chemistry in the Fushime Geothermal System, Japan

The Fushime field is characterized by a high temperature reservoir ( e x d n g 350 "C ) and high salinity. The some fluids collected from eleven prochction wells also show acidic pH (lower than pH4.5). The chloride concentration rdcu la ted by using a Na-K-Ca geothermometer ranges from 1 1600 to 22000 mg/kg in the reservoir. Furthermore, K, Ca and SiO, concentrations were higher and Mg, SO, concentrations were extremely lower when compared to the seawater value. Some fluids showing a low pH had a comparatively high concentraticn af metal ions such as Pb and Zn. As a result, sulfik minerals such as galena and sphalerite were precipikted in pipelines and on control valves duringa discharge test. Isotopic data indicate that the fluids were mixed between local meteoric water and altered seawater. The fluids contain noncondensible gases with less than 1 .O vol %, and the most dominant gas is CO, which ranges from 17.3 to 284 mmol/kg in steam. Judging from non-reactive gas composition such as N2. Ar, and He, it is probable that the fluids contain very little crustal or magmatic gas.