The Sumikawa geothermal field is located in the Hachimantai volcanic area in northern Honshu, Japan. Various investigations have been carried out in the area in recent years to assess the suitability of the field for electrical power production. This paper summarizes the principal characteristics of the natural fluid circulation system at Sumikawa and relates them to local geological structure. These features are deduced primarily from static ( shut - in ) pressure determinations in wells in the field and from a limited amount of pressure-transient testing. It is concluded that extensive natural two-phase flow prevails at intermediate depths at Sumikawa, and that a very high permeability zone is present below - 1 km depth containing fluids with temperatures exceeding 300'C. This zone is a suitable target for further study and possible future development.

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