Geologic Model of the Ginyu Reservoir in the Kirushima Geothermal Fleld, Southern Kyushu, Japan

Abstract

The Ginyu area is one of the active geothermal zones in the Kirishima geothermal area. The Ginyu fault running through the. center of the Ginyu area, which is the target of the geothermal development, provides a high angle passage for the geothermal fluids moving from the deep reservoir, which means that the reservoir of the Ginyu area is the typical fracture type. The features of the Ginyu fault reservoir are summarized as follows. l)Direction of the fault: ENE-WSW strike, dipping steeply NW. 2)Width of the fault: all of the penetrated point are projected to a width of about 150m. 3)Character of the cuttings collected from the fault zone by drilling; formed by highly altered rocks mainly composed of quartz, wairakite, and adularia. It is assumed that most of the geothermal fluids are derived from meteoric water around the Kirishima area. Descending water has percolated downward along cracks and circulated to the base rock of the Shimanto group, becoming heated and flowing up through the fault-fissure system and then restored in the volcanic rocks and lacustrine sediments of the Quarternary period.