Geothermal resources of Hachijojima

Hachijojima is a gourd-shaped volcanic island in the Pacific Ocean. Nishiya ma and Higashiyama volcanoes consist of basalt lava and associated pyroclas tic rocks. A promising geothermal resource was found in south Higashiyama, associated with an uplift of Tertiary rocks consisting of mainly andesite l ava and related pyroclastic rocks, overlain by Quaternary volcanic rocks. S teep high-temperature (over 250 degrees C) and high-pressure gradients occu r in the deeper portion of the system near the Tertiary-Quaternary contact, indicating the presence of a cap rock. The cap rock formed by deposition o f hydrothermal minerals. Geothermal fluid ascends from the deeper portions to shallow depths along vertical fractures through the cap rock. These vert ical fractures form the geothermal reservoir in the Tertiary formation. Thr ee wells were drilled into these vertical fractures, and approximately 30 t /h of superheated steam was obtained from each well during flow tests. The geothermal fluid is mainly a mixture of seawater and meteoric water in an a pproximate ratio of 1 to 2, based on chemical analyses, with a portion of v olcanic gas included. At present a 3.3 MWe, geothermal power plant is being constructed here, (C) 2000 CNR, Published by Elsevier Science Ltd. All rig hts reserved.