ABSTRACT

Fluid inclusion thermometry has been applied to hydrothermal quartz and anhydrite collected from cores and cuttings from the Hatchobaru geothermal system, Kyushu, Japan. The principal area of development (projected to the surface) is bounded by the NW-trending Komatsuike and Hatchobaru faults. Subsurface temperatures are closely estimated by the minimum homogenization temperature of fluid inclusions. Since there are few directly measured thermal profiles under stable (non-flowing) conditions, fluid inclusion data at Hatchobaru provide the best (and sometimes only) indication of present day pre-drilling temperature patterns. The estimated high temperature zone (above 270°C) at the production level (about 1000 m below surface and 100 m in altitude) is elongated about 900 m, and is 100 m wide, corresponding to the location of the NW-trending Komatsuike fault; chemical components such as pH and Cl content of the reservoir fluid also show a similar elongated pattern. Temperature profiles in the main developed area are of two types, convective and conductive. The convective profiles are constrained by the boiling point curve (taken from the water table in the well). Lower temperature, conductive profiles (14°C/100 m thermal gradient at about 200°C, at sea level) exist on the margin of the system as well as close to the central upflow zone. At depths less than 400 m, the convective temperature profile rapidly changes to the lower temperature conductive profile. This is probably due to boiling of upflowing fluid, resulting in a pressure drop and subsequent shallow mixing with cool, marginal water. Limited permeabilities away from the fault also contribute to the rapid transition from convective to conductive gradients. This indicates that fracture development is one of the most important factors in controlling the subsurface temperature distribution in such a geothermal system hosted by andesite volcanics.