ABSTRACT

Idemitsu Geothermal Co., Ltd. has been exploring for geothermal energy in Takigami, Oita, Japan, since 1979. This paper presents a model of the Takigami geothermal system, which has no surface manifestations in the immediate area. The chemical and isotopic compositions of well discharges, hot springs and ground water in and around the Takigami geothermal system are analyzed to define the geothermal system. Judging from temperature and salinity, there are two portions to the reservoir in the system: the western reservoir has a high temperature of about 250°C and salinity of about 600 ppm Cl; the eastern reservoir has a temperature of about 200°C and a lower salinity of about 450 ppm Cl. Fluid in the eastern reservoir is a mixture of that of the western reservoir and water stored in the fomation. Chemical and thermal equilibrium has been reached in the reservoirs in spite of their relatively low temperature. The recharge area of the Takigami geothermal system is located to the south. The geothermal fluid flows from south to north through the system. Almost no reservoir fluid reaches the hot springs from beneath the Takigami area, because a montmorillonite zone functions as a “cap rock” on the reservoir.