Geochemical Modeling of Acidic Fluid Formation at the Shiramizugoe Geothermal System, Japan

Abstract

Acidic waters separated from the Shiramizugoe wells are Cl-SO4 type with pH less than 3 in the ambient condition. The acidity in the Shiramizugoe area is likely to be caused by volcanic gases injecting (mixing) into the geothermal system from magmatic source or hydrolysis of native sulfur. In this study, mechanism of acidic fluid formation in the Shiramizugoe area was studied by geochemical simulation approach. In modeling of magmatic origin, the process has been simulated by the injection of volcanic gases into meteoric water, followed by upwelling to the reservoir while cooling and interaction with reservoir rock. Chemical compositions and alteration minerals in the Shiramizugoe acidic reservoir was successfully reproduced when simulation was done without precipitation of acidic alteration minerals such as kaolinite and pyrophyllite, which are not observed in drill cuttings from the acidic wells. In modeling of hydrolysis of native sulfur, acid fluid formation is simulated through native sulfur hydrolysis in neutral fluid followed by reaction with reservoir rock. HS- and SO42- are formed by hydrolysis of native sulfur and acidic fluid are reproduced. However, very high HS- concentration was calculated as compared with the Shiramizugoe acidic fluid. Hydrolysis of native sulfur could not reproduce the acidic data in the Shiramizugoe area. Accordingly, the Shiramizugoe acidic fluid might be formed by magmatic gas injection.