鹿児島県霧島地熱地域大霧地熱開発地区の地質学的手法による地熱構造解析

抄録

The Kirishima geothermal area is located in the Kagoshima graben which is a volcanic-tectonic depression. The Ogiri geothermal field is one of the active geothermal zones in the Kirishima geothermal area. A thick pile of volcanic rocks of Pleistocene age, reaching 2, 500m in thickness, deposited in the area. These volcanic rocks overlie the Cretaceous basement rock of the Shimanto group which is depressed to a depth of more than 1, 300m in the area. Features of the geothermal resources in the Ogiri field are of the typical fracture type. The Ginyu fault accompanied by hot water reservoirs is the target for developing geothermal resources in the field.
Geological surveys such as predicting underground temperatures, analysing fracture systems, clarifying permeable zone, calculating drilling depth intersecting the Ginyu fault and evaluating characteristics of the Ginyu fault reservoir have been made during the process of exploration and development in the area. The underground temperatures were predicted by the studies on the ditribution of alteration minerals and fluid inclusions. The fracture systems were analyzed by measuring thermo-remnant magnetism of the cores and mineral assembleges of the alteration minerals and quantity of the hydrothermal vein minerals in the cuttings. Based on the results of the fracture analysis, multiple regression analysis were efficiently used to estimate the subsurface location of the Ginyu fault.
As a result of the geological surveys, all of the production wells drilled in the Ogiri field intersected the Ginyu fault reservoir successfully, and steam production equivalent to 30 MW of electricity could be secured. On the basis of the informations obtained, excellent reservoir models in the Ogiri field were developed.