To understand the geological properties of a supercritical geothermal reservoir, we investigated a granite-porphyry system as a natural analog. Quartz veins, hydrothermal breccia veins, and glassy veins are present in Neogene granitoids in NE Japan. The glassy veins formed at 500-550 degrees C under lithostatic pressures, and then pressures dropped drastically. The solubility of silica also dropped, resulting in formation of quartz veins under a hydrostatic pressure regime. Connections between the lithostatic and hydrostatic pressure regimes were key to the formation of the hydrothermal breccia veins, and the granite-porphyry system provides useful information for creation of fracture clouds in supercritical geothermal reservoirs. (C) 2016 Published by Elsevier Ltd.