Geothermal energy has becoming rapidly perceived as a viable alternative energy source available with the current technology as well as solar energy. For a long time, geothermal energy has been utilized in various forms. High temperature steam and hot water have been mostly used for an electric power generation. World-wide today, total power of geothermally fired electric generating plant amounts to approximately 1, 100 MW. On the other hand, intermediate and low temperature geothermal fluids also have been consumed for space-heating and industrial processes in certain locations of the world. However, the full potential of electric and non-electric application of geothermal resources has not yet been realized, and only a small part of geothermal energy stored within the earth's crust has been tapped. Remaining enormous amounts of natural heat will be made available with the further exploration and technological research and development. Under present technology, use of geothermal energy for electric power generation is limited to the hydrothermal convection system having the temperature above 180°C. Among this, energy of natural steam has best quality, but the existence of this type of energy may be expected within only a part of whole high temperature convection system. Other hydrothermal convection systems are much more abundant geothermal resources. Research and development of technology for the efficient utilization of hydrothermal system having temperature below 180°C as well as hot water system (above 180°C) are needed. Especially, development of technology of electric power generation from intermediate temperature (90°C-180°C) geothermal fluid and multiple use of this type of geothermal resource must be enforced. Geothermal deposites in sedimentary basin are also potentially valuable geothermal resources which contain enormous amounts heat. In a high heat-flow area, deep-seated layered strata produce hot-water having intermediate temperature above 90°C, but, in a normal heat-flow area, water-bearing strata at depths of more than 2 km show temperature above 60°C, which is hot enough for space-heating and other non-electric use. In former case, reservoirs in subsiding sedimentary basin sometimes show abnormally high fluid pressure, and said to be geopressured. Hot dry rock deposit without associated hydrothermal system is expected as other potential geothermal deposit and may become an attractive geothermal resource if technology extracting the stored heat is developped. Research and development of exploitation technology for this type of deposit are now world widely under way.
Characteristics of geothermal resources of Japan are summarized with special interests in their geological distributions. There are four kinds of hopeful geothermal resources in Japan i.e. super-heated steam high enthalpy thermal water in volcanic area, low enthalpy thermal water in sedimentary basin in non-volcanic region, hot dry rock and volcanic heat. Amongst them, the high enthalpy thermal water in volcanic area is most hopeful at present. It may be resulted from the fact that most of Japanese geothermal resources are distributed in a big volcano-tectonic depressed region which was formed at miocene. The depressed region is filled mainly with Neogene Tertiary volcanic rock in one or two kilometers thick and overlain by the clusters of volcanic cones of Qnaternary age. Such structure is probably preferable to the formation of high enthalpy thermal water which requires reservoir structure and the sufficient supplies of heat and water into it. National reserves of the superheated steam and high enthalpy thermal water which is able to be utilized for commeraly power generation are calculated to several ten thousands million kW. thousand year at least as far as the production wells are not deeper than 1.5 kilometers. Writer demonstrated that the attention of the prospecting hereafter should be focussed to the deeply seated geothermal resources which may be occured in pre-miocene fructured sedimentary rocks hidden beneath the miocene volcanic rocks of the neogene volcanotectonic depressed area in Japan.
The Geological Survey of Japan started to carry out the basic geothermal survery at the beginning of 1973 FY. In that survey, the selected thirty geothermal areas shown in the figure 1 are being surveyed by geological, geophysical, and geochemical methods to develop geothermal exploitation at each area. This is the first systematic project conducted by the Government for geothermal exploration in Japan. In each area, hydrothermally altered zones are mapped and the measurement of natural heat discharge including shallow temperature survey is done. Geophysical prospecting (electric and gravimetric methods) and geological mapping are applied at some of them. Collecting chemical data of hot spring is also done at the most of places. The results of these exploration work are utilized to decide the sites for the proceeding drilling. At each area, a few wells of about five hundred meters depth are drilled by the government to know the underground geothermal structure. This drilling project started in 1974 FY. At some geothermal areas in foreign countries, using geophysical techniques the successful results were reported. However, generally speaking, the technique of geothermal exploration is not well developed. Recently, the Geological Survey is concentrated also to develop exploration technique.
Robert S. DIETZ is a unique geologist. His new hypotheses always brought forth controversies, but were finally accepted as a whole. His most prominent paper on the spreading sea-floor theory appeared in 1961. He managed thereafter to establish a new geosynclinal concept in terms of plate tectonics. DIETZ's proposal may be summarized as follows : 1. Continental rise and shelf couplet constitutes eu-and mio-geosynclines, respectively. 2. Eugeosynclinal sediments mainly deposited on the oceanic crust, while miogeosynclinal ones on the continental crust. 3. An orogeny is the geological expression of the collapse of continental rise sediments resulting from either the new subduction of the oceanic crust or the contient-continent collision. 4. Even Flysch facies indicate commonly the collapse of the continental rise, while Molasse facies appear much later than the former. 5. Real geosynclinal sediments were derived exclusively from the continent, hence the geanticlinal continent has never existed. DIETZ's proposal is probably nothing peculiar to most of geologists at the present, but not at the time. His proposal was felt to be more than revolutionary by the ordinary geologists. All his papers have much been in the discussion column. However, the progress of the affair is very instructive. The present author can learn the following matters through the discussions. 1. It may be difficult that a new hypothesis tides over the limit of a certain group such as university, country and so on. 2. It may be also difficult to evaluate fairly a pioneer's achievements.