The Fault System and Geological Structure of the Oguni Geothermal Field, Central Kyushu in Japan

Abstract

The Oguni geothermal field lies in a large basin referred to as the Beppu-Kuju Subsidence Zone, and is located on the northwestern boundary of the Kuju Uplift Belt, and the southwest rim of the Shishimuta Subsidence Belt. Many faults and fracture systems were inferred to be present along the steep slope of the basement on the margin of the Kuju Uplift Belt with a strike thought to be NW-SE. The Takenoyu Fault runs through the central part of the Oguni field, and is thought as the representative fault that strikes NW-SE trend. The Takenoyu fault has vertical offset of over 400 meters. Itís appeared that there is high-angle reverse fault in the north of the Takenoyu Fault by geologic correlation of the wells. A pair of these faults forms a local horst structure. These faults form a nearly vertical zone of high permeability with several hundreds meters wide as the geothermal reservoir. The in situ stress field estimated by the drilling induced tensile fracture data in the Oguni field approximately conforms to seismogenic-stress and general tectonic stress in the central Kyushu. The direction of the principal geological structure in the Oguni field is also in accordance with that of maximum compressive stress (Û1).