Strontium Consentration and Isotopic Ratio in the Sumikawa and Ohnuma Geothermal Field, Japan

Abstract

Concentrations and isotopic ratios of Sr in the Sumikawa and Ohnuma geothermal fields were determined for various types of waters (production fluids from geothermal wells, river water and groundwater) and rocks. The study was conducted to assist in modeling fluid movement and characterizing waterrock interactions in these geothermal systems. The following results were obtained: 1) At Sumikawa, Sr isotopic ratios are significantly lower (less than 0.7041) in the Quaternary volcanic rocks than in the Tertiary marine sediments (up to 0.7075). Hence, they become higher northward reflecting the distribution of Tertiary black shales. 2) Sr isotopic ratios of the geothermal fluids are strongly affected by those of the rocks. In particular, ratios of the discharged waters from the Sumikawa wells are similar to those of rocks around the feed zones. It can be concluded that water-rock interaction occurs in the vicinity of the wells. 3) The logarithm of the Sr concentration in the reservoir fluids and the reciprocal of their reservoir temperature show a linear relationship. Also, Sr versus Ca concentrations in the fluids show a linear relationship. Chemical equilibrium calculations show that the reservoir fluid is saturated with respect to anhydrite. This means that the Sr and Ca concentrations in the fluids are controlled by the solubility of this mineral and by Sr partitioning.