Geochemistry of the Nigorikawa geothermal system, southwest Hokkaido, Japan

抄録

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.