On the geothermal system of the northern Hachimantai area

Abstract

The Mitsubishi Onuma geothermal power plant, having 10 MW generator, has been running since 1973 in northern Hachimantai geothermal field, and make an offer the low cost power to Mitsubishi Akita refinery. The geology of this field consists of Quaternary volcanic rocks and Neogene sedimentary and volcanic rocks. Many volcanoes are distributed on the up-heaved block built up by fault systems and folding structure. The distribution of geothermal alteration minerals, for example zeolites, sericite and others, shows that two volcanic or geothermal centers were existed along the anticlinal axial belt of folding structure in the graven and on the upheaved block, but according to the isogeotemperature map compiled from many deep wells, the present geotemperature in the latter center is about 100°C lower than the former center. Thus, it is considered that the productive geothermal reservoir was formed during the geo-history, through which the meteoric water has been infiltrated in proportion to the up heaved movement.