Geothermal Development in Hachijojima

Abstract

Hachijojima is a gourd shaped volcanic island in the Pacific Ocean. Nishiyama volcano and Higashiyama volcano consist of basalt lava and associated pyroclastic rocks. A promising geothermal resource was found in south Higashiyama, associated with an uplift of Tertiary rocks consisting of mainly andesite lava and related pyroclastic rocks, overlain by Quaternary volcanic rocks. Steep high-temperature (over 250 Åé ) and high-pressure gradients occur 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 of hydrothermal minerals. Geothermal fluid ascends from the deeper portions to shallow depths along vertical fractures through the cap rock. These vertical fractures form the geothermal reservoir in the Tertiary formation. Three wells were drilled into these vertical fractures, and approximately 30t/h of superheated steam from each well was obtained during flow tests. The geothermal fluid is mainly a mixture of seawater and meteoric water in an approximate ratio of 1 to 2, based on chemical analysis, with a portion of volcanic gas included. 3.3 MW geothermal power station has been operating since March 1999.