In order to clarify the role of fracture permeability in a reservoir, the properties and formation mechanism of fractures were investigated on core samples from the Uenotai geothermal field, Japan. The fractures were located and identified from the records of lost circulation while drilling. Productive fractures are classified into a dominant high-angle (average: 77 degrees) group, and a less common low-angle (average: 28 degrees) group. The high-angle fractures were formed as extensional hydrothermal fractures in response to fault-valve behavior triggered by fluid pressure activation under lithostatic conditions. The low-angle fractures formed as extensional and/or extensional shear fractures under hydrostatic-lithostatic transitional conditions, with subhorizontal sigma(1) stress. Both kinds of fractures were accompanied by hydrothermal mineral deposition.