Deep geothermal resources survey program: Igneous, metamorphic and hydrothermal processes in a well encountering 500 degrees C at 3729 m depth, Kakkonda, Japan

The exploration well WD-1a was drilled to a depth of 3729 m in the Kakkonda geothermal field, northeast Japan, using efficient borehole cooling techni ques. The well penetrated an entire shallow hydrothermal convection zone, a n entire contact metamorphic aureole and part of a neo-granitic pluton. The recovered temperature of the well indicates a boiling point-controlled pro file up to 380 degrees C to a depth of 3100 m, and a conduction-controlled profile with a very high gradient from 3100 m to the bottom of the hale, wh ere the temperature is 500 degrees C. WD-1a may be the first geothermal wel l that encountered 500 degrees C, which exceeds the conventional hydrostati c boiling-point curve. An inflection point of the temperature-depth profile at 3100 m and about 380 degrees C reflects the brittle-plastic boundary. T he brittle-plastic boundary constrains the maximum depth of fracture format ion, and the fracture distribution constrains the maximum depth of hydrothe rmal convection. (C) 1998 CNR. Published by Elsevier Science Ltd.