ABSTRACT

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.