1998 Volume 20 Issue 3 Pages 209-224
We performed fluid inclusion study of an investigation well (WD-la; 3, 729 m in total depth) in the Kakkonda geothermal field, Japan. WD-la was drilled in the Quaternary Kakkonda granite from 2, 860 m through 3, 729 m in depth which is the heat source of the Kakkonda geothermal system. The boundary between hydrothermal convection and heat conduction zones was found around 3, 100 m by temperature loggings. We discussed thermal structure in the Kakkonda granite and permeation of meteoric water into the Kakkonda granite. Temperature of the Kakkonda granite shallower than 3, 100 m can be evaluated roughly from homogenization temperature of the liquid-rich inclusion which has a minimum salinity in every depth. Since the Kakkonda granite deeper than 3, 100 m is less permeable at greater depths, meteoric water permeates less into the Kakkonda granite. The evidence is as follows: (a) Minimum salinities of the liquid-rich inclusions shallower than 3, 250 m are close to 0 wt. %, although those deeper than 3, 300 m increase. (b) Homogenization temperature of the liquid-rich inclusion, which has minumum salinity, increases just slightly between 2, 750 m and 3, 250 m, then they increase steeply deeper than 3, 300 m. (c) According to the result of gas analysis for fluid inclusions by laser Raman microprobe spectroscopy, CO2 and H2S were not detected shallower than 3, 150 m, although they were detected deeper than 3, 350 m. We also built a geothermal model of the Kakkonda field based on the fluid inclusion study, showing thermal structure and fluid flow in the Kakkonda geothermal system.
