Geothermal Resources of Japan

Characteristics of geothermal resources of Japan are summarized with special attention given to their geological distributions. Japan has four potential types of geothermal resources: natural dry steam, high-enthalpy thermal water in volcanic areas; low-enthalpy thermal water in sedimentary basins in non-volcanic regions; hot dry rock; and volcanic heat. Among these, the high-enthalpy thermal water in volcanic areas is of the greatest interest at present because most of Japanese geothermal resources are distributed in a big volcano-tectonic depressed region which was formed in the Miocene period. The depressed region is filled mainly with Neogene tertiary volcanic rocks in thicknesses of one or two kilometers and overlain by the clusters of volcanic cones of Quaternary age. Such a structure is apparently conducive to the formation of high-enthalpy thermal water which requires a reservoir structure and a sufficient supply of heat and water. National reserves of the natural dry steam and high-enthalpy thermal water, which may be utilized for commercial power generation, are calculated to be more than ten million kW for at least a thousand years taking into account only production wells not deeper than 1.5 km. It is demonstrated that prospecting hereafter should be focused on the deeply seated geothermal resourcesmore » which may occur in pre-Miocene fractured sedimentary rocks hidden beneath the Miocene volcanic rocks of the Neogene volcano-tectonic depressed area in Japan. 39 references.« less