Pressure interference tests have been used to determine the permeability structure of the Sumikawa reservoir. Interference tests between wells S-4 and KY-1 have indicated the presence of a very high permeability (140 md) north-south channel in the altered andesite layer. Pressure buildup data from well SN-7D have provided indications of a high transmissivity (kh - 18 daxy-meters) reservoir located in the granodiorite layer, lack of pressure response in nearby shutin Sumikawa wells implies that the reservoir penetrated by SN-7D is isolated from the shallower reservoir in the altered andesites. The ìaltered andesiteî and the ìgranodioriteî formations constitute the principal geothermal aquifers at Sumikawa. Pressure interference tests (wells KY-1 and SB-2, and wells KY-2 and SB-3) have also confiied the presence of moderately high transmissivity (- 2 darcy-meters) dacitic layers in the ìmarine-volcanic complexî formation. Because of its low vertical permeability, the ìmarine volcanic complexî formation constitutes an attractive target for the reinjection of waste geothermal fluids.

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