Mineralogical and Hydraulic Characteristics of the Wasabizawa Geothermal Field, Akita Prefecture, Japan.

摘要

The Wasabizawa geothermal power station (42 MWe) is under construction since May 2015, which will be a newly constructed first large-scale geothermal power plant after 23 years of constructing stagnation in Japan since commencement of commercial operation of the Takigami geothermal power station (27.5 MWe) in 1996. This area is known to be rich in old caldera geography and commercial hot springs. Extensive geothermal explorations, including the promotional surveys carried out by New Energy and Industrial Technology Development Organization (NEDO), has been performed for the area. However, since the stratum older than the Neogene in which the geothermal reservoir is formed is covered with thick Quaternary volcanic sediments, the pre-Tertiary basement rock could not be confirmed as the outcrops in the area, and therefore the detailed geological structure previous Neogene has not been fully delineated. Furthermore, the fact that the geology consisted primarily of homogeneous lithofacies of granite was making it difficult to elucidate the detailed geological structure. During the construction, 11 new production and reinjection wells were drilled, and new rock samples were obtained. Alteration minerals of the cuttings were identified by X-Ray diffraction analysis. Furthermore, the pressure transient tests were carried out using the new wells. We studied a geological and geothermal structure based on the newly available mineralogical and hydrological data. The result of the study suggests that the geology of the site could be divided into several alteration zones. A small amount of prehnite can only be identified at the zone with comparatively high permeability, and particularly concentrates at the lost circulation point and its surroundings during well drilling. The characteristics of those hydraulic and mineral distributions may be closely related to the geotectonic history of the caldera that occurred around the site during the late Neogene to the Quaternary. The geological and geothermal structure of this area was revised for the future make-up well drilling.