The Rausu field is the most active geothermal field in the Shiretoko Peninsula, Eastern Hokkaido, Although the discharge area of geothermal fluid does not widely spread in the field, the underground temperature is regarded as the high class in the geothermal fields of Hokkaido. The Rausu field facing the Nemuro Channel at the central part of the Shiretoko Peninsula, is composed of hornblende porphyry and Miocene members. The former intruded into the latter showing monoclinic structure as a whole, The Miocene members dip toward the Nemuro Channel. The geothermal system of the Rausu field is considered as follows. 1) Meteoric water penetrates into the Miocene members, and is removed from the ridge of the Shiretoko Peninsula toward the Nemuro Channel. 2) The geothermal fluid, which is heated meteoric water, is reserved and removed to the geomorphic surface through fractures of the hornblende porphyry. The discharge area is bounded by the flexural zone of the Miocene members in the east. 3) In the discharge area, the maximum underground temperature is estimated at more than 200°C.
The Sengan area including Onuma, Matsukawa and Kakkonda power plants (total capacity of 82MWe) is one of the largest geothermal areas in Japan. The andesitic magma of Quarternary in age is assumed to be ultimate geothermal heat source in this area, because high temperature thermal manifestations with acidic alteration zones are distributed around the andesite volcanoes such as Yakeyama and Hachimantai. Exploitation data in the Matsukawa, Kakkonda and Onuma indicate the geothermal reservoirs are composed of various kinds of fractures developed in the NeogeneTertiary formations (sedimentary and pyroclastic rocks). High temperature fluids may rise from deep horizon (probably basement rocks) along the faults and are stored or circulated in the fractured-type reservoir. Recent results of geothermal exploration show there is no positive evidence indicating the presence of deep and widely distributed high temperature hydrothermal system which have been considered to be origin for shallower hydrothermal systems found in the exploited areas. Geothermal resources of above 150 °C stored in the Neogen Tertiary formations can be roughly calculated to be about 4, 000MWe, 30 years by the volumetric method.
The Matsukawa and Kakkonda geothermal areas are located in the central part of the Hachimantai volcanic region, which occupies northern part of the Tohoku backbone range. The Matsukawa geothermal area is characterized by the vapor dominated system and made up of assemblies of cap rock and reservoir rock. Geologic formations in this area are classified into three units, from oldest to youngest, marine sedimentary rocks of Miocene age, Pliocene-Pleistocene Tamagawa Welded Tuffs, and Quaternary Matsukawa andesite. The geothermal fluids are reserved in fractures of the Miocene rocks and lower Tamagawa Welded Tuffs. In contrast to them, the Matsukawa andesite acts as a cap rock because of its low permeability and causes geothermal manifestations to be poor on the surface. The Kakkonda geothermal area, about 7 km southwest of Matsukawa, is known as a hot water dominated system. Geothermal manifestations such as fumaroles and hot springs are widely distributed in this area because of absense of cap rocks and occurrence of steeply dipping faults. The area is underlain chiefly by the Miocene rocks, which are divided into the Obonai, Kunimitoge, Takinoue-onsen and Yamatsuda formations in ascending order. Geothermal fluids are encountered in high permeable fractures in the trough of a syncline, sheared zone in the wing of a asymmetrical anticline, and major faults.
Geothermal system in the eastern Hachimantai geothermal area is investigated based upon the results of geological and geochemical survey including bore hole drilling. The isothermal map at the sea level suggests that the center of geothermal activity is situated near the Toshichi hot springs, just south of the Hachimantai volcano. The temperature decreases gradually to the north and southeast. This fact indicates that the geothermal heat source in this area is different from that of the Matsukawa geothermal area, situated about 5km south of the studied area. Drilling and geochemical data suggest the existence of following three geothermal reservoirs of fracture type. The first one is situated in the limited area at about 300m depth, which is filled acidic hot water saturated with H2S and CO22 gases. The second one is developed in the Miocene formation of 500 to 800m depth with Na-SO4 type geothermal fluid with medium temperature. The last one is probably filled with Na-Cl type geothermal fluid with high temperature and it is assumed to be developed in deeper part of this area. The wide distribution of wairakite in 400 to 1000m depth indicate that the reservoir temperature with Na-Cl type geothermal fluid have been falled about 100°C with the passage of time.
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. 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. 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. 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. 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. 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. 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. 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.
Most part of the western region of Sengan geothermal area is occupied by the Tamagawa Welded Tuffs which are associated with caldera collapses during Plio-Pleistocene time and Quaternary andesitic volcanoes (Yakeyama and Hachimantai). Many thermal manifestations are situated on northern part of this area. The Tamagawa Welded Tuffs have been greatly altered by alkaline hydrothermal solutions and borehole data show the Neogene Tertiary formations overlain by the Tuffs have been metamorphosed into hornfels facies. These facts reveal the presence of intrusive bodies in the Neogene Tertiary formations derived from caldera magma. Low bottom temperature in boreholes (max. 2km in depth) drilled through the condensed tuffs in the “Tamagawa Caldera”, however, indicates the caldera magma body have cooled at the present time. The Quaternary andesitic volcanism which occurred after eruption of the Tamagawa Welded Tuffs have been closely related to the high temperature geothermal activities in this area, because borehole temperature increases gradually toward the Yakeyama and Hachimantai volcanoes. Geochemical and geophysical data indicate the high possibility of the deep-seated hydrothermal systems in the pre-Tertiary basement rocks developed in the Hanawa depression zone extending N-S direction which is underlain between two volcanoes, Hachimantai and Yakeyama.
Four almost independent geothermal areas are distributed in the Kurikoma region Each of them is around Late Quaternary volcanoes; namely, the Onikobe Caldera, the Narugo volcano, the Takamatsudake volcano, and the Kurikomayama volcano. Hence, the magma reservoirs, which probably exist or existed beneath these volcanoes, are considered to be heat sources of the four geothermal areas. In the center of each geothermal area, a high temperature two phase zone (water & steam) is developed at a shallow depth. The temperature of the shallow two phase zone is controlled by the boiling point for depth of the geothermal fluid. The ground temperature decreases away from the center. The geothermal fluid beneath these high temperature areas is lighter than the surrounding lower temperature fluid and consequently at deeper levels the fluid pressure, compared at the same depth below the sea level, is lower beneath these high temperature areas than beneath their surroundings. As a result geothermal fluid flows toward the high temperature areas at depth. The flow pattern of geothermal fluid beneath each geothermal area is further controlled by the topography and the geological structure of the area. In the Kurikoma region, Neogene formations and pre-Tertiary basement rocks and distributed at a comparatively shallow depth; hence, fractures in these rocks are important as the reservoir of the geothermal fluid.
1) Initial Geothermal Model Geothermal Survey shows that Onikobe Katayama Geothermal Area is situated just above Anticline Axis; NW SE. Besides, during deposition of Omkobe Group, uplift along the Axis continued till deposition of Iwanazawa Formation in Pleistocene. Electric Resistivity Survey shows that the Geothermal Area is situated above the top of the Horst structure of Electric Basement. Depth of the Horst top is about 200-250m. Resistivity of the Electrical Basement is above 100fm; Whereas the Top part shows 3-4Ωm. Fault structure is presumed to be on both sides of the Horst, named F3, F4 fault. Low resistivity Area developed along the Faults and the top of down step faults. Broadily speaking, F3 and F4 Fault line correspond to Okunoin geothermal alteration zone and Katayama geothermal alteration zone. Based on these datum, at the first stage of exploration, Initial Geothermal Model was drawn up. It showsthat geothermal fluid rise up through the faults of both sides of the Horst is reserved in the top part of the Horst horizontally. Actually, several exploratory well got steam at 150-300m depth. With the progress of the exploration, it was recognized that geothermal reservoir did not always existhorizontally, but linearly along a fault. 2) Fracture Type Geothermal Model In the case of Onikobe Katayama Geothermal Area, low electrical resistivity part corresponds to highlyaltered zone, characterized by clay. Temperature Gradient is high in this part, but steam production is not got always. If it get steam, theproduction is intermittently. Production part is deeper than low electric resistivity part. It is situated in the electric basement. It'shigh resistivity is above>100Ωm Lost circulation points in the drilling are important indicator that show production zone directly. Figures 11, 12, 13 show lost circulation point and temperature in drilling of the pilot borings and production wells. Horizontal distribution of production zone is shown along K, Z, Y, R, and another fault. Geothermal fluid rise up from deeper part over 1000m depth, and reserve the tension faults and fractures caused by uplift movement along NW-SE anticline axis, and the faults intersect with them. It is supposed that another end of the faults connects with recharge area. Geothermal fluid rises through fault system is reserved at 1200m depth (L3 formation; from reflection method), 800m depth (L2 formation), 250-300m depth (L1 formation) on the way. The past experience shows that the most excellent production zones are close to the intersect points of the fault and L1, L2, L3 formation. Main production zone of Onikobe Geothermal Power Plant was L1 formation. According to the deeper part, GO-10 (1, 350m depth) and GO-11 (1, 300m depth); which diameters are as same as production well were bored. Both exploratory well were measured high temperature and got steam about 30-40 ton/hr. Actually the temperature of 288°C was measured at 1, 290m depth in GO-11. But the hot water with steam had very high HC1 acidity, pH 3.3 and 2.6. After 4 months from the first production, pieces of casing material rushed out from GO-11, and the steam product volume decreased to 30% of the former. It was presumed that the bottom part of the borehole collapsed. From these information, it was judged that deep well in Katayama geothermal area did not suit for generation. Therefore main production zone was decided to L1 formation L2 formation has been used to reinjection zone. After that, carried various examination, we obtained the conclusion that acid problem can avoid with separated drilling from the Horst. We drilled several directional wells which length are 1, 150-1, 500m and displacement are 500-700m. It was succeeded to produce steam. The volume of steam from these directional wells amount to 70% of total nowadays. It's pH value is 8.0-8.2.
Geothermal system in the Yuzawa-Ogachi area is investigated on the basis of the results of geological and geochemical survey including bore hole drilling.The isothermal map at 500 meters below the sea level suggests that the center of geothermal activity is beneath Mt. Yamabushidake and the Uenotai area. The temperature decreases gradually to the northeast and southwest. This fact indicates that the geothermal heat source in this area will be the magma reservoir beneath Late Quaternary volcanoes, namely, Takamatsudake and Oyasudake volcanoes.The geothermal reservoir of this area was previously known at the Uenotai and the Akinomiya areas which were near the boundary of the uplifted and subsided zones of basement rocks.This time, N57-Y0-3 bore hole located in the uplifted zone caught the geothermal reservoir in the basement granitic rocks. Therefore, the geothermal reservoir of this area is probably distributed widely in and surroundings of the uplifted zone.
The geothermal system of the West Kirishima field is discussed on the basis of geological structure, hydrothermal alteration, underground temperature and a circulation system of geothermal field. This field is situated in a volcano-tectonic depression “Kagoshima graben” which trends north-south and relates to the block-faulting after deposition of the Shimanto Group. The volcanic activity began in the Pliocene and continued during the Quaternary, so andesitic lava flows and pyroclastics accumulated in enormous thickness, more than 2, 000m in the depression area. Thermal manifestations such as fumaroles, hot springs and altered zones are aligned along fault systems of NE-SW, NW-SE and partially N-S trends. Quaternary volcanic rocks were extensively subjected to hydrothermal alteration. The alteration zones are classified into various types such as montmorillonite zone-mixed layered clay mineral zone-chlorite zone-chlorite sericite zone in increasing temperature order from ground surface to deeper parts. The upper level of the crystallization of quarts and wairakite corresponds to the present isotherms of about 100°C and 200°C respectively, which suggests that the hydrothermal system of this area is a rather young system. Impermeable zones, which are mainly composed of montmorillonite filling cracks and joints of andesitic rocks, play an important role, especially to prevent the mixing of descending cold ground water on the circulation system of geothermal fluid. The fault systems in Makizono and Ebino-Iino lavas under the impermeable zone play the main geothermal reservoir in this field.
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