Conceptual Hydrogeological Model of the Wasabizawa Geothermal Field, Akita Prefecture, Japan

Abstract

A conceptual hydrogeologic model of the Wasabizawa geothermal field was developed prior to the application of a numerical simulation. This conceptual model was based on an updated analysis of a significant amount of data gathered during recent extensive exploration and well testing by New Energy and Industrial Technology Development Organization (NEDO). Except for the eastern quarter of the Wasabizawa area, which has a very thick deposit of Tertiary pyroclastics (Ohtoriyazawa Formation), the area is considered to be part of the southeastern extension of the Okumaemori uplift with NEand NW-trending faults. Well test results show that fluid production is from fractures in the lower Ohtoriyazawa Formation (wells WZ-1 and WZ-5), and metamorphic and granitic basement rocks in the uplifted zone (wells WZ-2, WZ-6, WZ-7, WZ-9 and YO-3). Based on the chemistry of geothermal fluids produced during flow tests, two water types were defined in the Wasabizawa area: low Cl waters (about 400mg/l at reservoir conditions) were produced from wells WZ-1, WZ-6, and possibly WZ-5; high Cl waters (1,000 ~ 2,000mg/l) were produced from wells WZ-2, WZ-7, WZ-9, and YO-3. Both types are thought to migrate through different paths that are controlled by high-permeability zones along NW-trending faults, as indicated by subsurface temperature contours above ñ200 meters (msl). Temperature distribution at shallow depths also shows a high-temperature trend toward the Akinomiya area, indicating a possible subsurface discharge toward the southwest. All temperature maps show a horizontal component of recharge from the SE direction. Static temperature profiles in the Wasabizawa wells indicate that there is upflow as well as the horizontal flow. It is, therefore, suggested that the geothermal fluids migrate into the area from a deep source located in the direction of Mt. Yamabushi and Mt. Takamatsu.