FRACTURE DISTRIBUTION IN AND AROUND INTRUSIVE ROCKS IN THE FUSHIME GEOTHERMAL FIELD, JAPAN : EVIDENCE FROM THE FMI LOGGING

摘要

Fracture systems in and around an igneous intrusive body and/or in a volcanic depression can develop in the volcanic regime, but are rarely examined with a case history. Geothermal exploration and exploitation could give a chance to examine such a fracture system, because it is considered as one of main geothermal reservoir targets. Main geothermal reservoirs in the Fushime geothermal system develop in fractured zones in and around a dacite intrusion. The Fushime geothermal field is located in the southern Kyushu, Japan, where lots of volcanic features can be seen, including active volcanoes, volcano-tectonic depression, hot springs, and so on. The dacite intrusion is found in the center of the depression delineated by some major faults. Fracture systems in the Fushime geothermal system are considered to be affected by both of the intrusion and depression-bounding major faults. As a matter of fact, drillholes in the field often encountered main geothermal reservoirs in the vicinity of the intrusion and/or major faults. It is, therefore, of great importance to know the characteristics of these fractures for the exploitation of the field. FMI loggings were carried out in the three production wells, encountering the dacite intrusion. Dominant fracture azimuth, fracture density, and other features in FMI images are examined in terms of the relation with the intrusion and major faults.