The Nigorikawa geothermal system is located in the Nigorikawa caldera, southwest Hokkaido. The basement rocks are dominantly sediments, including limestone, while Tertiary rocks are mainly andesitic volcanics. Geothermal waters in this area can be divided into four groups on the basis of their relative Cl and SO₄ contents (i.e., hot springs outside the Basin, hot springs inside the Basin, hot water derived from faults in the pre-Tertiary Kamiiso Group, and hot water derived from the fracture zone related to the caldera wall). Although the SO₄ concentration of the thermal waters is controlled by anhydrite solubility in the deep formations, there are no trends indicating its dissolution. In the shallow hot spring waters, SO₄ concentration decreases by mixing with groundwater. Isotopic data suggest that the geothermal water is formed by simple mixing of meteoric water with deep hot water having a large magmatic component and/or altered sea water. Relative He, Ar and N₂ contents of Nigorikawa geothermal fluids indicate that they are mixtures of magmatic-derived gas and atmospheric air dissolved in groundwater. Based on trends in the Cl-enthalpy relationship, two endmembers have been identified in the Nigorikawa system, i.e., original deep fluid and zero chloride, shallow steam-heated water; a third endmember is now present due to the reinjection of high chloride waters from the geothermal power development. The reservoir liquid was saturated with calcite, resulting in CaCO₃ scale deposition during production. However, after a prevention system using a CaCO₃ scale inhibitor was completed in 1985, the CaCO₃ scaling problem was solved. Stevensite scale has recently precipitated in the two-phase pipeline and waste water pipelines related to well ND-1, due to an incursion of Mg-rich low enthalpy water.

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Chemical and isotopic data are summarized for fluids from newly drilled wells in the Sumikawa geothermal system, northern Honshu, Japan, and the geochemical characteristics of the reservoir are discussed. The fluids are mainly of the Na-Cl type with a near neutral pH, and measured temperatures in wells often exceed 300°C. The total dissolved salt concentration is low (<3000 mg/kg), less than other high temperature geothermal systems in Japan. The chemical and isotopic compositions of the fluids vary from well to well. One well (S-2) produces both acid sulfate-chloride and weakly acid pH fluids from two different feed zones, with these fluids possibly related to a local magmatic component. The Sumikawa geothermal waters are diluted by a relatively shallow heated (∼200°C) water that contains sulfate plus bicarbonate but nil chloride. The hydrogen and oxygen isotopic compositions of the total fluid indicate that the fluids are of meteoric origin, with an oxygen isotope shift of less than 2‰. The SO₄ in the meteoric-dominated neutral pH waters comes from sulfur in rocks, which is essentially a mixture of supergene and fossil marine SO₄. On the basis of the N₂, Ar and He values in the vapor fraction, the chemical and isotopic variation of the fluids appears to be due to different degrees of minor magmatic contribution of volatiles to the hydrothermal fluid. Despite the meteoric-dominant source of most water, the magmatic contribution of volatiles results in fluids that are relatively immature and out of chemical equilibrium with the host rocks and alteration minerals.

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The physical and chemical characteristics of the Oku-aizu geothermal system, northeast Japan, are reviewed, with these data used to construct a geochemical model of the system. The exploration of this area started in 1974, with a total of 39 wells being drilled through fiscal year 1990. A total of 509 t/h of dry steam (165°C) was confirmed during the simultaneous production test, equivalent to about 55 MW of electric power. The bedrock of the production zones are Miocene formations composed mainly of rhyolitic lavas and pyroclastics. Pre-Tertiary basement unconformably underlie these formations. The uppermost portion of the system is comprised of a Quaternary lacustrine deposit which was penetrated by shallow rhyolite intrusions at 0.2 to 0.5 Ma. This lacustrine deposit is thought to be related to a subsidence structure. Two different types of hydrothermal alteration are recognized. One is related to submarine volcanic activity in the Miocene, and is composed of chlorite and sericite. The other is a product of the present geothermal system, and has advanced argillic alteration zones at the surface and an argillic zone at depth. Alteration minerals observed in the advanced argillic zone are smectite and mordenite with a minor amount of sericite, alunite and kaolinite. Those in the argillic zone are smectite, interlayered illite-smectite, kaolinite, zeolite and K-feldspar. Anhydrite is common in the deeper portion of the system at higher temperature. The highest measured temperature is 341°C, and temperatures higher than 300°C are quite common in production zones. Isotherms at −1200 m ASL open to the SE, which may indicate the direction of the heat source. From the deepest drilled levels to about −200 m ASL, the thermal gradient is small, indicating the existence of non-boiling convection over this depth range. The uppermost part of the convection cell is limited by a sealed zone caused by argillic alteration and a Miocene mudstone formation. The system is considered to have had little boiling in its natural state before exploration. However, it is presently boiling around geothermal wells due to the pressure drop caused by reservoir fluid discharge. The composition of the reservoir fluid is characterized by high salinity (about 2 wt%) and a large amount of non-condensible gas (about 1 wt% CO₂ and 250 mg/kg H₂S). This, as well as its very high underground temperature, is consistent with isotopic indications of a magmatic component to the discharge of the system. This idea is also concordant with the B/Cl and Br/Cl ratios. Small amounts of base metal sulfide mineralization (pyrite, sphalerite, galena, chalcopyrite etc.) are present at the level of the production zone. This is considered to be a product of the present geothermal system, judging from its mode of occurrence. Significant concentrations of precious metals (Au ≤ 116 mg/kg, Ag ≤ 3.49 wt%) were found in sulfide scales collected from surface two-phase lines.

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Idemitsu Geothermal Co., Ltd. has been exploring for geothermal energy in Takigami, Oita, Japan, since 1979. This paper presents a model of the Takigami geothermal system, which has no surface manifestations in the immediate area. The chemical and isotopic compositions of well discharges, hot springs and ground water in and around the Takigami geothermal system are analyzed to define the geothermal system. Judging from temperature and salinity, there are two portions to the reservoir in the system: the western reservoir has a high temperature of about 250°C and salinity of about 600 ppm Cl; the eastern reservoir has a temperature of about 200°C and a lower salinity of about 450 ppm Cl. Fluid in the eastern reservoir is a mixture of that of the western reservoir and water stored in the fomation. Chemical and thermal equilibrium has been reached in the reservoirs in spite of their relatively low temperature. The recharge area of the Takigami geothermal system is located to the south. The geothermal fluid flows from south to north through the system. Almost no reservoir fluid reaches the hot springs from beneath the Takigami area, because a montmorillonite zone functions as a “cap rock” on the reservoir.

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Kyushu Electric Power Co. Inc., has conducted geothermal development in the Kujyu volcanic area of central Kyushu since the 1950s, and three power stations are now in operation, including Hatchobaru I and II (55 MW each) and Otake (12.5 MW). The geothermal fluid in the Otake-Hatchobaru system is recharged by meteoric water from the southeast, is heated at depth, and is subsequently discharged to the northwest along fractures in the Otake-Hatchobaru system. In the Hatchobaru area, reinjection of waste hot water began in 1973 for environmental reasons. However, soon after, this reinjection began to affect the reservoir, with the temperature gradually decreasing due to interference between the reinjection zone and steam production zone. As a result, the distance between production wells and new reinjection wells is now kept as large as possible. This has stopped the trend of falling reservoir temperature. However, it is also apparent that reinjection of hot water is necessary to help recharge the reservoir, and maintain pressures in the production zone.

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Fluid inclusion thermometry has been applied to hydrothermal quartz and anhydrite collected from cores and cuttings from the Hatchobaru geothermal system, Kyushu, Japan. The principal area of development (projected to the surface) is bounded by the NW-trending Komatsuike and Hatchobaru faults. Subsurface temperatures are closely estimated by the minimum homogenization temperature of fluid inclusions. Since there are few directly measured thermal profiles under stable (non-flowing) conditions, fluid inclusion data at Hatchobaru provide the best (and sometimes only) indication of present day pre-drilling temperature patterns. The estimated high temperature zone (above 270°C) at the production level (about 1000 m below surface and 100 m in altitude) is elongated about 900 m, and is 100 m wide, corresponding to the location of the NW-trending Komatsuike fault; chemical components such as pH and Cl content of the reservoir fluid also show a similar elongated pattern. Temperature profiles in the main developed area are of two types, convective and conductive. The convective profiles are constrained by the boiling point curve (taken from the water table in the well). Lower temperature, conductive profiles (14°C/100 m thermal gradient at about 200°C, at sea level) exist on the margin of the system as well as close to the central upflow zone. At depths less than 400 m, the convective temperature profile rapidly changes to the lower temperature conductive profile. This is probably due to boiling of upflowing fluid, resulting in a pressure drop and subsequent shallow mixing with cool, marginal water. Limited permeabilities away from the fault also contribute to the rapid transition from convective to conductive gradients. This indicates that fracture development is one of the most important factors in controlling the subsurface temperature distribution in such a geothermal system hosted by andesite volcanics.

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The chemical and physical features of the deeper part of the Fushime geothermal system (about 1000–2000 m depth), where temperatures exceed 300°C, have been revealed in the course of its exploration. The fluids discharged from wells are saline, and the maximum Cl concentrations of the reservoir waters are similar to that of seawater. The waters are depleted in Mg and SO₄ but are enriched in K, Ca, Fe, Mn, Zn, Pb, SiO₂, etc., over those of seawater, suggesting that the geothermal fluid originates from high temperature seawater-rock interaction. Relatively long term discharge testing shows that excess enthalpy conditions (i.e. two-phase feed) commonly develop, and that isothermal boiling has also occurred in the high temperature reservoir. The fluid chemical compositions that are the least disturbed by these physical processes caused by testing were selected from the data base for study. They indicate that boiling and dilution predominate in the undisturbed reservoir. Variations in the K and Ca concentrations of the waters suggest that the precipitation of K-bearing minerals and the dissolution of Ca-bearing minerals occur in the reservoir. Fluid-mineral equilibria for the Fushime reservoir waters were calculated without allowing for redox reactions for dissolved gases (CH₄-CO₂, H₂-H₂O and N₂-NH₃), because allowing for these reactions results in an extreme discrepancy between analytical CH₄, H₂ and SO₄ concentrations and those calculated. Calculations show that the fluids are close to anhydrite saturation and are close to equilibrium with both Na/K-feldspars. However, apparent undersaturation with respect to calcite is indicated. Higher pH values are calculated for the reservoir waters in relatively lower temperature wells (the measured pH values are also higher than those of the higher temperature wells). They are approximately in equilibrium with K-feldspar and K-mica at reservoir conditions. In contrast, the lower pH waters in wells with higher temperature are not calculated to be in equilibrium with this pair. The latter is inconsistent with the observation that these two minerals are common as alteration products. The numerical back titration into the fluids of sphalerite and galena, which are observed in scale deposited in the wells and surface equipment, results in a pH increase in the high temperature reservoir water. This reconciles the disagreement between the observed presence of these minerals and fluid composition of high-temperature wells. Thus, the precipitation of sphalerite and galena is the most likely source of the acidity.

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The geothermal fluids in seven Japanese geothermal systems are tested for attainment of aqueous and gaseous equilibrium. The pH of fluids in the geothermal reservoir is approximately buffered by the assemblage K-feldspar–K-mica–quartz. (Na⁺)/(K⁺) and (Na⁺)/√(Ca²⁺) activity ratios are thermodynamically approximated by reactions between albite and K-feldspar, and between albite and anorthite (or Ca-zeolites), respectively. The (Mg²⁺)/(K⁺)² activity ratio of high temperature geothermal fluids of Japan can be, represented by the reaction involving Mg-chlorite and K-bearing silicate minerals, though at lower temperatures other reactions may be responsible. The geothermal fluids are also commonly saturated with respect to anhydrite and calcite. A small amount of steam loss in the reservoir does not significantly affect the aqueous composition of the fluids. The partial pressure of CO₂ is controlled by the reaction involving calcite, K-bearing silicate minerals, and albite or Ca-zeolite in geothermal systems which are not affected by steam loss and dilution. Equilibrium between CH₄, CO₂ and H₂ is attained at high temperatures but not maintained to lower temperatures in most Japanese geothermal systems. The H₂/H₂S ratio is probably equilibrated with Fe-bearing minerals. Gaseous compositions are very good indicators to identify processes in the geothermal reservoir, such as boiling and dilution. Lastly, the major aqueous composition and pH of Japanese neutral Na-Cl type geothermal fluid are predictable if two variables (e.g., temperature and one of the cation activities) are provided.

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