Geothermal power development at the Oguni field, central Kyushu, is planned to begin in the year 2001 with a double flash system generating 20 MWe. The Oguni reservoir has been delineated by systematic geothermal surveys, well tests and reservoir engineering studies. The fractured reservoir is horizontally layered and divided into northern and southern portions. Both of them have NaCl dominant fluids ranging from 200 to 240 degrees C. The northern reservoir covers a large area including the Takenoyu Fault Zone and has a relatively high permeability (kh approximate to 80-230 darcy-m). By contrast, the southern reservoir covers a relatively small area and has limited transmissivity. The southern reservoir has a higher pressure (1 MPa) than the northern reservoir, indicating little connectivity between them. Based on numerical calculations, as well as the surface topography and environmental aspects, the production and reinjection zones have been separated, and a large part of the necessary fluid will be produced from the northern reservoir along the Takenoyu Fault and a small part will be taken from the southern reservoir. The separated water will be reinjected into the northernmost part of the northern reservoir, in order to prevent a cold sweep problem for production. (C) 2000 CNR.