Subsurface Thermal Structure Revealed by Fluid Inclusion Thermometer at the Otake Geothermal Field, Kyushu

Abstract

Homogenization temperatures of fluid inclusions found in secondary hydrothermal minerals such as quartz, anhydrite and subordinate calcite from the Otake geothermal field, Kyushu, have been measured. The homogenization temperatures in the reservoir have a relatively narrow range of 15-30°C. Except in the case where inclusions may have trapped a gas phase, most of the highest homogenization temperature are on or slightly below the surface boiling point curve. This suggests the state of the maximum subsurface temperature with an activity index of 100 prior to the explorations. However, the subsurface temperature just before the exploration seems to have been nearly equal to the lowest value of the homogenization temperature at each depth. The temperature of the reservoir is estimated to have been 200-220°C at depths of 300-500m, having an activity index of about 90. For the reasons, it appears that the Otake geothermal system had cooled down about 15-30°C from the highest activity level even prior to the exploration and has since undergone further cooling. The contour map of the subsurface temperatures forms a slightly deformed mushroom in shape. This is due to the inclination of the main conduit in a south-east direction. The fluid conduit is also supported by the distribution of alteration minerals.