The Fluid Geochemistry and Reservoir Model for the Kakkonda Geothermal System, Obtained by Nedo's Deep-Seated Geothermal Reservoir Survey, Japan

Abstract

In the discharge test of WD-1b in 1998, liquid samples showed pH of 2.0-4.5 and chloride concentration of ca. 10,000ppm at the weir box. In the Kakkonda geothermal field, deep wells (with excess enthalpy) generally discharge liquid of pH 3-5 at the weir box, whereas shallow wells discharge liquid of pH 7-8. ‰D-‰18O and He-Ar-N2 compositions show that the geothermal fluid is mainly meteoric water, and that a magmatic fluid is a rare contribution. The acidity of WD-1b and other deep wells is presumed to originate from the H2S-HSO4-SO4 system depending on fluid temperature and/or from a negligible HCl generated by hydrolysis of chloride salts in the reservoir. We estimate by comparing ‰D (H2O) in fluid inclusions to the reservoir waters that the hypersaline metal-rich brine sampled from WD-1a was trapped in Kakkonda Granite during crystallization, and that a small amount of meteoric water permeated into a heat conduction zone in the Kakkonda Granite. A schematic model for the Kakkonda geothermal system is constructed from these results of NEDOís project.