Geothermal System in the Eastern Hachimantai Geothermal Area, Northeast Japan

Abstract

Geothermal system in the eastern Hachimantai geothermal area is investigated based upon the results of geological and geochemical survey including bore hole drilling. The isothermal map at the sea level suggests that the center of geothermal activity is situated near the Toshichi hot springs, just south of the Hachimantai volcano. The temperature decreases gradually to the north and southeast. This fact indicates that the geothermal heat source in this area is different from that of the Matsukawa geothermal area, situated about 5km south of the studied area. Drilling and geochemical data suggest the existence of following three geothermal reservoirs of fracture type. The first one is situated in the limited area at about 300m depth, which is filled acidic hot water saturated with H₂S and CO₂2 gases. The second one is developed in the Miocene formation of 500 to 800m depth with Na-SO₄ type geothermal fluid with medium temperature. The last one is probably filled with Na-Cl type geothermal fluid with high temperature and it is assumed to be developed in deeper part of this area. The wide distribution of wairakite in 400 to 1000m depth indicate that the reservoir temperature with Na-Cl type geothermal fluid have been falled about 100°C with the passage of time.