Conceptual model of the north Hachimantai-Yakeyama geothermal area.
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概要
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Based on the geothermal exploration conducted at the northern part of Mt. Hachimantai, Akita Prefecture, new conceptual model of geothermal reservoir in the area has been established.<BR> 1) The area is composed of Quaternary volcanics and thick piled Tertiary formations. Pre-Tertiary basement rock is not identified even by the deepest drilling. Many volcanoes are located on the uplifted zone which forms a folded block cut by series of faults.<BR> 2) A large low gravity zone, elongated graben structure trending N-S direction, forms Hanawa to Tanzawa lake. An E-W trending volcanic chain, connecting Mt. Yakeyama and Mt. Hachimantai summit, disturbes the graben. Geothermal manifestations are distributed either inside of the graben or along the vocanic chain.<BR> 3) Judging from the subsurface isothermal map of the area, two units of large convection cell related to Mt. Hachimantai and Mt. Yakeyama are formed The heat sources in the area are probably located beneath of those volcanoes, respectively.<BR> 4) A characteristic feature of the reservoir is the presence of steam cap (vapor dominant part) at the top of water dominated reservoir. Quaternary formations of andesite lava and lacustrine sediment have an important role of cap rocks for making steam cap in the reservoir.<BR> 5) Geothermal fluid has been stored in the open fractures like druse as observed in ore veins. During drilling work, a lost circulation occurs frequently at high temperature zone suggesting the area is fairly fractured. However, fractures are selfsealed with vein materials at low temperature zone.<BR> 6) Geothermal waters are mostly neutral and mainly composed of sodium chloride, but acidic hot water gushed out from some wells in the area. The behavior of acidic solution in the reservoir is not known yet.<BR> 7) Considering from the distribution of altered minerals, high temperature part would be widely extended in the past. It is seemed that the subsurface temperature along river or fault system is gradually cooling down through geologic time.
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