Deep geothermal structure and targets of future development in the Hatchobaru geothermal field; Hatchobaru chinetsu chitai no shinbu chinetsu kozo to shinbu kaihatsu no target

Survey was made on the farther deep geothermal structure to cope with the drop in output of a 110MW geothermal power station which is under commercial operation in the Hatchobaru Field, Japan. Well drilling and gravity exploration were made with precision to grasp the deep subsurface structure of this field, while resistivity structural analysis, isotopic survey and other fluid geochemical surveys were conducted to grasp the existence of geothermal fluid in the depths. From the result of gravity exploration and resistivity profile, it was presumed that the geothermal fluid of steam-producing area in this field was not what had ascended from the depths of the present production area, and that it originated in the liquid having ascended from the depths where the gravitational basement steeply deepened in the eastern part of this field. In both its central and northern parts, the subsurface temperature is so influenced by the reduced hot water as to be lower than 200{degree}C at smaller depths than 1300m. It is, however, 300{degree}C in and above the basement rock at larger depths than 2000m. Heated in the depths, the deep hot water is reserved in the deeper volcanic rock than 2000m, which is judged to lie in a crush zone above the basement rock. At the center of this field, there exists a depressed structure formed from the NE-SW running fault. Around it, there possibly exists a crush zone as a reservoir. 10 refs., 18 figs.