Geologic Model of the "Vapor-Dominated" Reservoir in Yunotani Geothermal Field, Kyushu, Japan

Abstract

The Yunotani geothermal field, located in the Aso caldera, has abundant surface manifestations characteristic of vapor dominated systems. Two slim wells drilled near active fumarolic areas have been producing dry steam from a 500 meter deep flashing reservoir dominated by a allow chloride, bicarbonates/sulphate hot water which was initially tapped along with the steam during early production. This reservoir is considered the result of the mixing of meteoric water and high enthalpy, partially condensed steam which is rising along a fault zone forma steam reservoir that overlies, a boiling, high chloride, deep water reservoir. This deep water is recognized at the surface in the hot water that emerges at low elevation hot springs four kilometers west of Yunotani, where it emerges after mixing with surface waters.