ABSTRACT

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.