Geology in the Nigorikawa geothermal field, Mori-Machi, Hokkaido, Japan

The Nigorikawa geothermal field is located in a krakatoan type caldera forming an approximate circular basin about 3 km in diameter. Geothermal manifestation concentrates in the northern half portion of the Nigorikawa Basin. The main rock units are composed of the pre-Tertiary Kamiiso Group, Miocene Ebiyagawa Formation and Pleistocene Nigorikawa caldera deposits. Geothermal fluids are stored in fractured zones of the caldera wall deposits and limestone and transverse faults of the Kamiiso Group. Based on porosity and permeability of the reservoir, the Nigorikawa geothermal system is divided into three zones of shallow warm water reservoir, high temperature water convective zone and solid (rock) conductive zone.