The present state of geothermal development in Japan

Geothermics (;~7 o) - SPECIALISSU~ 2 U. N. Symposiumon the Developmentand Utilization of Geothermal Resources. Pisa xq7o. Vol. 2, Part I

The Present State of Geothermal Development in Japan K. SATO w

ABSTRACT Before World War II, small geothermal test plants were r~ade in Beppu in 1925 and in Otake in 1926, both located in Kyushu, for converting geothermal heat energy in the volcanic areas to electric power, based on the idea that volcanoes have enormous heat energy as seen in volcanic explosions. However, these trials were not successful. In 1950, the Geological Survey of Japan began research and investigation of geothermal activities and resources. Among 0;e areas where preliminary investigation was made, eighteen areas had been selected for detailed investigation since 1950 by the Geological Survey, prefectural governments and private companies. In this paper, the data on general geology, thermal indications, the methods of prospecting, subsurface structure, the size and depth of wells, physical and chemical properties of geothermal products, utilization and development programrues in twelve areas will be explained. Among them, Matsukawa area, in northeast I'apan has a geothermal power plant of 20.000 kW constructed by Nihon lukako Co., Ltd. in 1966 and Otake area, Kyushu, has a power plant of 13,000 kW constructed by Kyushu Electric Power Co., Ltd. in 1967. To obtain more amount of geothermal steam, they are doing detailed survey in outside areas. In addition to these areas, four areas are under investigation for development. To promote geothermal development in lapan, the |apan Geothermal Energy Association was established in 1960. Since that time it works to collect data ongeothermal resources, pu blishing the journal named • Chinetsu (Geothermal Energy). and making educational campaigns for geothermal development. The Committee of Geothermal Resources organized in this Association is doing research on the problem of the estimation of stored heat in the Japanese Islands. Japan has an area of approximately 360 thousand square kilometers comprising the four islands of Hokkaido, Honsu, Shikoku and Kyushu, and 67 per cent of the area is highland. The climate is w a r m and humid with an average annual precipitation of 1600 mm which amounts to 600 billion tons, of which 80 per cent is believed to flow off as surface water. Under these circumstances it is only natural that hydroelectricity, should have played the most important role in the earlier phase of the development of power resources. At present, the hydroelectric potential of lapan is estimated to be 36.5 million k W and approximately half of this is already developed and the remaining 46 per cent is still unexploited. These remaining areas, however, are unfavorable areas; the develop* Geothermal Resources Committee, lapan Geothermal Energy Association, I-3 Yuraku-eho, Chiyoda-ku, Tokyo. (a) K. SA'ro, H. NAKAMX.rgX(Geological Survey of lapan); T. HrrosuOl, M. SnmA'rA (Electric Power Development Co.), K. WA'rANABE (Mitsubishi Metal Mining Co.); T. K~'~o (Toehigi Prefectural Governmen0 and T. HA'f^smD^ (Kyushu Electric Power Co.I.

ment cost will be considerably high and the pace of hydroelectric development is now very slow. With steam power generation, on the other hand, the cost of oil burning generators have decreased significantly because of the development of techniques, and more steam power plants have been built than hydroelectric plants since 1957. Recently, hydroelectric power has remained more or less constant while steam power is increasing at a rapid rate and ator~ic power plants are about to be developed. Japan lacks fuel resources; both oil and uranium must be imported from overseas. The percentage of imported energy resources is estimated to be 76.4, 82.4, and 89.4 per cent in the years 1970, 1975, and 1980 respectively. Thus it is necessary for Japan to utilize all her domestic energy resources to the full: geothermal energy has been investigated during the last ten years and with the success of Matsukawa and Otake power stations, it is of great interest to various phases of industry. This report was prepared by the members (1) of the Geothermal Resources Committee, the Japan Geothermal Energy Association. Ten areas have been selected from the surveyed parts of the country (Figure. 1) and the outline of the results of investigation is reported together with the problems related to 'the geothermal energy development of Japan. O u t l i n e of t h e r e s u l t s of i n v e s t i g a t i o n

KUTCHARO GEOTHERMAL AREA This area is located in the eastern part of Hokkaido Island, north Japan, and is a part of the Akan National Park. This area is located in the Chishima volcanic zone and there are many volcanic cones of various sizes, and caldera lakes (Figure 2). Kutcharo area is in the midst of a larg e caldera which is considered to have been formed on Diluvium. In this ealdera, several volcanic cones have erupted, one of which is still active with many steaming vents, and a large number of hot-springs, fumaroles and altered zones are distributed. The western part of the caldera is covered with thick collapsed deposits and lake deposits, and in the 155

eastern part, there are many volcanic cones which consist of pyroxene andesite or dacite lava and effusive materials such as agglomeratic tuff, and it is inferred that these lava and effusive materials act as cap-rock. This area is situated on a fault zone which is believed to be the path of heat flow; one of the faults was formed by the 1938 earthquake. At first, three places were expected to be developed for geothermal projects, but after several geological surveys, the eastern part of the caldera was selected and investigated. The principal geological investigations which were conducted at the Kutcharo area are as follows. ~7

Electric prospecting (conducted by Compagnie Generale de Geophysique, investigated depth lO00 m) --Measurement of temperature distribution in sub-surface ground. Gravitational prospecting, magnetic prospecting. Chemical prospecting (for hot-springs, fumaroles).

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--Boring (coring; measurement of temperature in the hole; chemical analysis of underground water in the hole; zoning based on chemical and physical properties of core). (Figure 3).

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As a result of the above investigations on selected places, it is presumed that the thickness of the effusive materials reaches 1000 m to 1500 m. Promising steam reservoirs exist in the parts deeper than 1000 m.

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Four borings for geological purposes were drilled at the site (depths 250 m, 300 m, 500 m, 1000 m, diameters at the bottom of holes 49 mm to 60 ram).

GK-2 EL.I85.0t

A small amount of steam (temperature 107°C, amount 1.0 t/h, pressure 2.5 kg/cm2 in closed valve, small content of H=S) and a large amount of hot water (amount 13.0 t/h, alkaline, deposits scale easily which consist of soft calcite and aragonite) were obtained. At present, we have no plans to develop this area for geothermal power, but the possible utilization of the large amount of hot water is being considered. NORTH

HACHIMANTAI

AREA

Location

The north Hachimantai geothermal area (the Toroko-Onuma area) is in Akita Prefecture, near the boundary of Akita and Iwate Prefectures, northeast Japan (longitude E-141°, latitude N-40°), As shown in Figure 4, the area lies about 25 kilometers south of Hanawamachi, the biggest town in the vicinity and a railway station on the National Railway.

4O 40

1~3 157 156 150

Geography and topography

I-

The area is located on the western side of the Oubackbone range and the elevation is 800 to 950 meters. In the surrounding area, there are many mountains such as Hachimantai (1631 m), Yakeyama (1366 m), Shibakuradake (1180 m), etc. The area is well known as one of volcanic regions in Japan, but at present, Ya-

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LAKE (

FIG. 4. - area.

Index

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AMI'I~t,t~I

m a p o] the north H a c h i m a n t a i geothermal

157'

keyama is the only volcano which shows some activity. In this area, there a number of swamps, large and small, and hot springs. The Hachimantai area is designated as a National Park and is well known as a scenic spot for its natural beauty.

Name

Geology The geological survey has shown that this area consists mainly, of Pliocene-Ouaternary lava and pyroclastic rocks which cover the Middle Tertiary basement rock groups (Miocene). These Tertiary basement rocks are the so-called <~Green Tuff ), and are mainly exposed on the northern side of the Shibukuro and Kumazawa rivers. The other area is covered by Quaternary lava flow (two py. roxene andesite). The Miocene basement rocks mainly consist of mudstone, sandstone, conglomerate, tuff, tuff breccia, lapilli tuff, propylite, propylitic agglomerate, rhyolite etc. The mudstone of the formation yields marine fauna, such as mollusca, echinoid and foraminifera. These basement rocks seem to be Middle to Lower Miocene, namely, Funakawa or Onnagawa stage. Lower Miocene - earlier Pliocene rocks are composed of acid volcanic rocks, <~Tamagawa dacite ~ and a two pyroxene andesite lava flow which consists of 3-4 horizons. They are most widely distributed in the south-west part of this area and seem to extend into northeastern and eastern parts. In the northeastern area, however, there are no exposures of these formations on the surface due to erosion, or covering by Quaternary lava flow. The .upper and uppermost formations consist of lake sediments and Quaternary lava flows (andesite lava flow and mud flow which has 4-5 horizons). These Quaternary lava flows are mainly divided into two groups, the Hachimantai lava group and the Yakeyama lava group. The geology is shown in Figure 5.

Geothermal indications on the surface a) Numerous thermal springs are known all over the area, and also there are many fumaroles and solfatara. The chemical compositions of these were surveyed by MAEnA and ABE (Geological Survey of lapan) in luly, 1969. For reference, the temperature and pH of main hot springs are shown in the following list. The nature of the thermal waters will be investigated in detail toward the end of this year. b) Altered zones have been discovered widely throughout the area, especially in Goshogake, Fukenoyu, Niiyamazawa, Tamagawa, Sakebizawa, (these are hot springs) and Yakeyama (volcano). c) Regarding the above-mentioned geothermal indications on the surface, it is considered that systematic distribution is seen, for instance, in trends such as N-S, E-W, NW-SE, etc. 158

pH

Temp.

Shibari

No. 1 No. 2

8.15 8.15

47.5 °C 43.5

Zenikawa

No. I No. 2 No. 3

8.18 8.30 7.22

98.0 96.5 86.5

2.85

57.0

Akagawa Sumikawa

No. 1 No. 2 No. 3

2.10 3.20 3.44

75.0 80.0 62.0

Fukenoyu

No. 1 No. 2 No. 3

2.75 3.00 7.40

97 0 80.0 83.0

Obuka

5.40

51.0

Goshogake N o . 1 No. 2

2.00 4.70

89.0 64.0

Tohichi

3.10

Tamagawa

No. 1 No. 2

2.60 1.15

Sakebisawa

No. 1 No. 2

2.10 2.15

87.0 :

82.0 97.0 93.0 96.0

Past exploration work 1965 - - Geological reconnaissance survey in the aorthern area of Hachimantai (scale 1/5000) Electrical reconnaissance survey in the northern area of Hachimantai (Schlumberger type D.C. resistivity 1966 ~

method) Geological reconnaissance survey (scale 1/5000), sou.

them area Electrical reconnaissance survey (D. C. method), sou them area 1967 - - Detailed electrical survey (D. C. method) in small part of northern area Drilling for structure survey No. 2 hole (depth 870 m, spindle type boring machine) - Detailed geological survey 1968 -- Production well (Two wells, rotary machine) No. 2 A (depth 850 m) No. 3 (depth I042 m) Drilling for structure survey flour holes, wire line method) No. 4 (depth 700 m) No. 9 (depth 700 m) No. 10 (depth 700 m) No. 11 (depth 446 m) 1969 (planned) -- Photographic geological survey l~-oduction well (two wells, rotary machine): No. 3A (depth 1500 m); No. 3B (depth 1500 m) -- Geological and mineralogical examination of altered zone and its secondary minerals

- - Geochemical examination of thermal water

Electrical survey For the Schlumberger type D.C. resistivity method survey, 1 1 lines were cut with a total length of 69.5 kilometers, and measurements were done at 223 points in all. Electrode spacing was 250 meters with survey depth to about 600 meters. Main lines are shown in Figure 5 (dotted lines show the area of a detailed survey conducted in 1967). Several low anomalous areas under

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10 f2-m resistivity were found (Figure 5). Geologically, such a n o m a l o u s areas seem to be altered zones and also alternating aquifers a n d confining beds. The results of the electrical survey on the surface and also in the drill hole agree w i t h the geological data of the cores, particularily w i t h regard to alteration. The review of the data of e x p l o r a t i o n seems to indicate that there is a close relationship between h y d r o t h e r m a l alteration and low resistivity.

Drilling work By the a b o v e - m e n t i o n e d exploration (geological and electrical survey), it was r e c o m m e n d e d that the TorokoO n u m a area is w o r t h y of investigation to discover thermal reservoirs. In addition, this area is strategically located from the s t a n d p o i n t of transportation, w a t e r supply, construction of p o w e r station, etc.

Preliminary drilling was carried out for the examination of the geological structure before the production well was sunk. So that drilling w o r k was started in August, 1967. - - A summary of the drilling work is given in Table I. Core drilling was done by a rotary machine (in this case, partial coring) and wire line method (spindle type machine, all coring). Locations of the holes are shown in Figure 5 and drill logs of Nos. 2, 2A, 3, 4, 9 and 10 holes are illustrated in Figures 6, 7, 8, 9, 10 and 11. - - Nos. 2, 4, 9, 10 and 11 holes were drilled by the wire line method quickly, and continuous coring was quite useful for geological examination. The adoption of an oversized bit, reformation of corebar tel, study of chrome mudwater and improvement of the water way of the bit were made for the application of the wire line drilling method in geothermal exploration.

Geological structure A l t h o u g h available data are few and not yet studied fully, at present from the a b o v e survey it seems that

TAaLS 1. - - Summary o/ drilling works in the north Hachimantai geothermal area. Hole

Elevation Depth (m) (a.s.1.) meters Proposed Actual

Method Machine Contractor

Period Commenced

No. 2

975

I000

870.04 Ordinary Aug. 17 Spindle type Tone 1967 TFL TEL Drilling Co. Ltd.

No. 2A

955

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850.00 Rotary machine Aug. 16 Draw works 5 7/8" 1968 UTDW Teiseki Sakuse Co.

Ltd. No. 3

934

1000

1042.00 Rotary machine Aug, 4 Draws works 1968 OH700 Teiseki Sakusei Co. Ltd.

No. o

684

700

700.80 Wire iine method May 2 Tone TEL 1968 Ote Kaihatsu Co. Ltd.

No. 10

672

700

No. 4

811

700

No. 11

826

700

No. 3a

965

1500

No. 3b

970

1500

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6½"

Teiseki Sakusei Co. Ltd. i 60

Description

Andesite (Hachimantai lava), Silt stone, Dacite or Andesitic tuff Tuff breccia, Andesitic lapilli tuff - T u f f - Tuff breccia. Altered andesite, Dacitic welded tuff Oct. 23 Andesite (Hachimantai lava), Al- demh 829 m loss of tered rock (Andesitic lapilli tuff?), 1968 Altered andesite, Andesitic lapilli mudwatcr tuff, Andesitic tuff breccia (with partly Mudstone breccia), Dacitic tuff breccia, Andesite, Welded tuff Nov. 12 Andesite (Hachimantai lava) ,Altered rock (Andesitic lapilli tufiq 1968 Altered andesite, Andesitic lapilli tuff, Andesitic tuff breccia (with mudstone breccia partly). Dacite, Basaltic andesite, Dacitic lapilli tuff. Welded tuff. June 14 Andesite (Hachimantai lava). Andesitc lapilli tuff-tuff breccia. 1968 Mudstone. Dacitic tuff - lapi!!! tuff Feb. 15 1968

Iune 22 1968

Sep. 2 1968

Sep. 3 1968

Oct. 8 1968

Sep. 25 1968

Oct. 23 1968

July 23 1969

Aug. 5 1969

Rem~rk.~

Completed

Andesite (Hachimantai lava), Dacite, Mudstone, Dacitic lapilli tuff, Muddy tuff, Lapilli tuff, Mudstone and Tuff alternation Andesite (Hachimantai lava). Andesitic tuff. lapilli - tuff breccia Mudstone. Dacitic lapiIli tuff, An desite, Tuff breccia. Andesite (Hachimantai lava), Andesitic lapilli tuff, Tuff breccia. Mudstone, Andesitic lapilli tuff Welded tuff

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Fro. 8. - - Data on exploration well No. 3. there is a fault trending approximately E-W in the Toroko-Onuma area. According to the core investigation of test and production holes, distinct differences in geology are seen between the southern part (including Nos. 2, 2A, 3 and 11 holes) and the northern part (Nos. 4, 9 and 10 holes) in the Toroko-Onuma area (Figures 12, 13). In this area, however, the fault has been buried under the Hachimantai lava flow (Quaternary) and does not crop out on the surface. From the data of drill core, the formations occupying the two sides of this fault are not the same (Figure 14). The writer considers that there are many N-S and E-W trending faults in this area; that is, block movement. The said fault, therefore, may be considered one of them, and the southern part has apparently sunk more than the northern part. In this connection, however, there is a possibility of the said depression having been caused by cald.era, Secondary minerals As regards alteration, although few data are available at this time, especially on the surface, from rough 162

X-ray examinations of the cores from Nos. 2, 2A and 3 wells, laumontite, wairakite, epidote, calcite and prehnite have been found. The alteration has a striking resemblance to that of Wairakei (New Zealand), but appears to differ from that of Matsukawa (|apan). At present, a detailed examination is being made at Mitsubishi's central research laboratory. Temperature distribution Provisional isotherms were drawn from the seven holes drilled (1967-68) in the Toroko-Onuma area. The isotherms show close relationships with the volcanic center, surface manifestations, altered areas and electrical anomalies. These have confirmed the distribution of rising curves trending from north to south as shown in Figure 15. Plans ]or the future In November, 1968, No. 2A well succeeded in obtaining geothermal steam of about 25 tons per hour and 65 tons of hot water per hour. The steam has continued to come out ever since. The results of chemical ant)

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physical examinations are not described in this report, but will be given at another opportunity. By the end of this year, if the data of No. 3A and No. 3B wells warrant proceeding with geothermal steam production, the construction of a 10,000 kW power station will be undertaken as soon as possible for completion possibly by November, 1971. MATSUKAWA GEOTHERMAL AREA

Matsukawa geothermal area is situated on the southern part of Mt. Iwate, an active volcano included in the Hachimantai volcanic region. It is 50 km from the city of Morioka and about one hour by car from the city. Geology

This area is covered by debris and lava flow of Marumori volcano and Matsukawa andesite of the Quaternary age. The basement of Quaternary volcanic rocks is not found around the area, except in two small areas located on the north and southwest sides of this area, where Tamagawa welded tuff formation and Yamatsuda formation consisting of Tertiary. marine sediments are exposed along river banks. This area is characterized by topography like caldera, and by wide distribution of altered rocks within 3 X 0.7 kin. In spite of such a remarkable development of rock alteration, thermal manifestations are so few that only several hot 164

! 35"c

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-- ~o~-

well No. 10.

springs, the temperature of which is 40-70°C, issue from altered rocks. Geothermal investigations

They were made by the Japan Metals and Chemicals C., Ltd. (older name, Azuma Kako Co.) and the Geological Survey of Japan. Thanks to success in obtaining geothermal steam, [apan's first geothermal plant of 20,000 kW was constructed in this area in 196 I. Now it is generating at full capacity, though it was put into operation with 9000 kW at the beginning. As the results of geothermal exploration including geological survey, geophysical prospecting and drilling are explained in other papers in detail, only the process of geothermal development in this area is given here as summarized in Table 2. a) Drilling done by the Matsuo village office for thermal water (from 1952 to 1955). The Matsuo village office drilled seven holes for thermal water for balneological purposes with the help of the Matsuo sulphur mine from 1952 to 1955. From this survey, it became clear that this area has high underground temperatures such as 200°C or more at a depth of 160 m, though thermal manifestations are poor on the surface. "This was the beginning of geothermal exploration and after that this area was noted as one of the most promising geothermal fields in lapan.

TABLE 2.

-

-

Geothermal investigation in the Matsukawa area

Content Exploration well No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 Geological surv.~y ,

Electric prospectin? Seismic prospecting Exploration wcq GS-I AR-I BR-I Exploration wel! BR-2 Production well MR-I MR-2 Exploration well No. 21 Production well MR-3 MR-4 Seismic prospecting Exploration well No. 22 Production well MR-5 Seismic prospecting Production well MR-6

Depth 163.5m 159.3 m 327.0 m 88.7m 57.0 m 205.6m 164.0 m

Year

Remarks

1952 ~, 1952 1953 ~ Matsuo village 1953 i office 1954 1954 1955 , 1957 Geological Survey 1958 • 1959 •

214.5 m 1960 325.0 m ~ 450.0 m •

•

N.M.C.

570.0 m 1961 945.0 m 1963 1080.0 m 1964

M.R.C. •

600.0 m 1964

•

thermal power station of 20,000 kW was erected at the Matsukawa area in 1966. d) Recent investigation. Two productive wells 1200 m deep have been newly drilled since 1967, after the power station was put into operation, successfully obtaining steam at above 50 t / h respectively. Besides drilling, they also made seismic surveys, in 1967 and 1968 to obtain information on the geological structure extending over a wide area. They are planning to make the next productive well in the near future. TAKINOKAMI

GEOTHERMAL

AREA

The Takinokami geothermal area, 8 km southeast of the Matsukawa area, is situated along the upper stream of the Kakkonda river flowing around the western side of the Iwate volcano and discharging into the Shizukuishi basin. Geology

1207.O m 1964 1500.0 m 1964 1967

•

600.0 m 1967

•

1200.0 m 1967 1968

•

1200.0 m 1968

•

This area is included in the Hachimantai volcanic region as is Matsukawa, except that Tertiary welded tuff and marine sediments constituting the basement

A~ V

b) Geothermal investigation made by the Japan Metals and Chemicals Co. Ltd., in cooperation with the Geological Survey (from 1957 to 1960). After a preliminary survey was made by the Japan Metals and Chemicals Co. in 1956, the Matsukawa area was put under co-operative investigation of the company and Geological Survey from 1957 to 1960. During this time, the Geological Survey conducted a geological survey, electric survey, seismic survey, drilled a research well of 214 m deep, and carried out electrical and temperature logging, while the lapan Metals and Chemical Co. drilled three holes (AR-1 325 m deep, BR-1 450 m deep and BD-2 570 m deep). From the data obtained by co-operative investigation, it was concluded that geothermal fluid was stored in Tertiary marine sediments called Yamatsuda formation situated at a depth of about 1000 m and that a possibility of geothermal development would be expected in this area. c) Drilling of productive wells by the Japan Metals and Chemicals Co. for the Research Development Corporation (from 1963 to 1964). After examining the results obtained from the investigation, the Research Development Corporation subsidized the company to make productive wells. From 1963, drilling work was begun and owing to the success in getting a large amount of the steam from three wells (MR-I 945 m deep, MR-2 1080 m deep and MR-3 1027 deep), except one unsuccessful well (MR-r 1501 m deep), a geo-

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rocks of Quaternary volcanoes, crop out as a core of ~nticline along the Kakkonda river trending from northwest to southeast (Figure 16). Both wings are covered by younger volcanic rocks; by Matsukawa andesite on the eastern side and by lava flow of Nyuto volcano on the western side. The Yamatsuda formation of the Miocene age is subdivided into five beds (Y:Y,), overlying a small scale of hard shale named Sakamoto formation. The geological structure of the Yamatsuda formation is characterized by two kinds of structural features; one is folding, mentioned above, another, is a fracture zone trending northeast to southwest developed in the southern part of this area. Thermal manifestations are many in this area and there are about ninety hot springs besides two fumarolic areas.

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Geothermal investigation by the Geological Survey o:

Japan Parallel to the geothermal investigation in the Matsukawa area, the Geological Survey collected data on geological sequences and the structure of Tamagawa welded tuff and Yamatsuda formation in this area, because they are expected to be situated under the Matsukawa area. After that, the Geological Survey carried out geothermal investigation including detailed geological survey, electric survey, temperature survey at a depth of 30 m and drilling of a research well 400 m deep. for the purpose of clarifying geothermal conditions in the Yamatsuda formation, and other older formations. According to the result of the electric survey made along the line cutting through the anticlinal axis on the

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A temperature survey measured at a depth of 30 m and made at about fifty points (Figure 17) was carried out to find out the relation of thermal manifestations and geological structure, ludging from the distribution map of isothermal lines, there are two high temperature zones. One is that along the folding axis, the

north side of the river, a vertical distribution of resistivity curves, representing high values in dacite of the upper part of the Yamatsuda formation, low values in marine sediments and again high values in green tuff, was obtained for understanding the subsurface structure in this area.

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other is that along the fracture' zone through the fumarolic area in the southern part of this area. However, there remains the question of why low temperature exists between these two high temperature zones. To clarify the above-mentioned problem and occurrences of geothermal fluids in the Yamatsuda formation and other older formations, a research well 400 m deep was drilled in 1967 outside the high temperature zone occupying the southern part of this area. The geological column of this well is shown in Figure 18. In this column, dacite which was found below 310 m is what appeared unexpectedly and this is considered to be an intrusive rock corresponding to 168

Y~, exposed near the entrance of the Takinokami hot spring. During that drilling, mud water was lost at depths of 50, 119 and 145 m and steam with hot water spouted from the bore hole at depths of 169, 213 and 228 m. It is interesting that lekage of mud water occurred mostly in sandstone and tuffaceous beds of Yamatsuda formations, but scarcely in dacite. Though the maximum temperature was 200 °C at the bottom of the hole, steam and hot water did not come out from the well because cracks in the rocks were all closed by iron casing pipes which had penetrated to a depth of 230 m. And yet, the amount of discharge of products changes periodically in summertime, though a

stable flow of steam is obtained in winter. To obtain data on such geyser activities as seen in this well, an analysis of CI and measurement of flow discharge of hot water were made this summer, ludging from the result as shown in Figures 19a and 19b, it can be said that the unstable flow of geothermal fluid is caused by the inflow of underground water into the the bore hole. In conclusion, it is summarized that the Yamatsuda formation has comparatively many cracks in the part of sediments consisting of sandstone and tuff, but few in dacite; therefore, the low temperature zone has been formed by the effect of an intrusive body of dacite with poor cracks• On the basis of data obtained from the investigation by the Geological Survey, the Japan Metals and Chemicals Co. Ltd. is making a plan of geothermal exploration including seismic survey and test drilling in this area. ONIK~JBE

FnG.

TEMPERATURE o~ 0

50

100

AREA

The Onik~Sbe area is situated in the northwestern part of Miagi Prefecture, northeast Japan. This area is a wide basin, the so called Onik6be basin, extending roughly 9 km from north to south and 7 km from west to east. Near the center of this basin, there is the Ka-

17. - - Distribution map of under&round temperature in the Takinokami geothermal area.

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such as tuff, agglomerate and lava. Quaternary volcanic rocks are found partly in and around the basin, and one of them, dacite, intrudes in the southern part of the basin and forms mount Takahinata. There are many faults in and around the basin. Most of them seem to correspond to the fracture zone by the depression of the basement. Hot springs and fumaroles issue in the lake deposits in and around the basin. Their distribution is controlled by the geological structure of the basement trending NE and NW. Besides, fumaroles are distributed in the Katayama area and its surroundings, and hot springs are located enclosing the fumarolic area. Generally, hot springs of the southern group are higher in temperature than the northern one and in the southwest part of the basin, there are two famous geysers, called Fukiaga and Miyazawa. At the Katayama geothermal area, there is a widely eroded basin with an elevation of about 530 meters, covered completely by a hydrothermal altered zone extending roughly 3 km from east to west and 2 km from north to south. The base rock is granite, on which ~een tuff and late Tertiary deposits are distributed. The Tertiary deposits consist of volcanic complex, such as andesitic tuff, dacitic tuff, rhyolitic tuff, andesitic agglomerate, andesite lava, dacite lava and rhyolite lava.

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tayama geothermal area, in which many hot springs and fumaroles are distributed (Figure 20). For the purpose of developing geothermal power generation, the Katayama geothermal area has been surveyed by E.P.D.C. since 1962. To study the geological structure and the characteristics of the underground steam, various types of prospectings were carried out from the end of 1962 to 1968. As a result of these investigations and studies, it was seen that Onik6be area is a promising geothermal field. So, at present, the drilling of productive wells and the study of the characteristics of the steam are under way to realize the plan of development.

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The OnikObe basin consists of granite and Tertiary green tuff overlain by Tertiary volcanics, sedimentary rocks, and Quaternary volcanic lake deposits. The granite and green tufts form the basement of this area and are mostly distributed surrounding the OnikSbe basin. The volcanics and sedimentary rocks are as follows: mudstone, conglomerate, and volcanic complex, 170

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Besides, in the eastern part of this area, dacite lava which flowed out from mount Takahinata is distributed. According to boring data (Figure 21), the thicknesses of layers are as follows: Geology Dacite (lava) Volcanic complex Green tuff Granodiorite

Thickness 0-

20 m

300. 900 m 600 m ±

Distribution limited to the eastern part of this area all over this area

Process of survey 1962- 1965:

Preliminary survey Topographic survey, surface geological survey, surface temperature survey, electrical prospecting, 6 exploratory borings. 1966 - 196g : Feasibility survey Seismic prospecting, 3 exploratory steam wells.

The first productive well (No. 10) was drilled from December 1968 to April 1969 and a second well (No. 1 I) is now under way.

Geophysical prospectings At the center of this area, it is inferred that two main faults run parallel in a NW-SE direction and a horst appears bordered by the faults. The altered zone stretches in the same direction. ]udging from above-mentioned data, in the Katayama area, the dacite of mount Takahinata is related to the source of heat~ And it is presumed that two main faults are the path of hot water or steam and the upper altered layer of volcanic complex has a function of cap rock, and the cracked or fractured zone in the volcanic complex and green tuff forms a reservoir for the steam.

1) Electric prospecting (conducted by the Compagnie Generale de Geophysique) as one of the preliminary studies of the geothermal development: an electric survey was carried out in the southern part of the OnikObe basin, during the period from May to luly, 1 9 6 3 The major purpose of this study was to clarify the subsurface geological structure. The results obtained are as follows: o the stratum can be classified in terms of resistivity into two layers, namely, low resistivity (5-20 ohm-m) and high resistivity (100 ohm-m); 171

- - green tuff has high resistivity and its depth is 250-800 meters in the Katayama area; - - at the center of this area, a central horst appears bordered by two parallel faults; - - the resistivity is lowest (3-8 ohm-m) in the vicinity of the horst. This fact can easily be explained by the strong hydrothermal manifestations of this area and the two faults should be the source of the hydrothermal activity.

the best point for drilling productive wells. The results obtained are as follows: - - the stratum can be classified in terms of velocity into three layers, namely, 3.1 or less km/sec layer, 3.1-3.5 km/sec layer and 3.5 or more km/sec layer; - - their thicknesses are 200-400 m, 400-500 m and 600 m ± in descending order; - - the locations of faults and fractured zones in the geothermally altered zone are clarified to a certain extent.

2) Seismic prospecting (reflective method): a seismic survey was carried out in the central part of the Katayama area, during the period from April to Iune, 1968. The major purpose of this study w a s to find

OO-I

Boring and wells Six exploratory borings, three exploratory steam wells and a productive well were drilled during the pc-

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FIG. 21. - -

172

riod from May, 1963 to April 1969. The results are shown in the columnal sections. NASU GEOTHERMALAREA Nasu volcano, 1917 m, situated along the boundary of Tochigi and Fukusima prefectures in the northern corner of the Kwanto plain is a strati-volcano extending over 10 km to the east-west and 25 km to the north-south. On the southeastern slope of Nasu volcano, there are many springs which are used for balneological purposes. It takes about three hours by train and car from Tokyo. Nasu volcano is composed of nine volcanic bodies connected in a north-south direction and among them. Chausu-dake is an active volcano with fumarolic activity at the top. Basement rocks of the volcano consists of Mesozoic granite and Tertiary andesite and tuff breccia which crop out on the western side of this area. This is due to the large fault with a north-south trend on the eastern side of this area; the western side of the fault where Nasu volcano was born was depressed and thus the eastern part was covered by thick volcanic lavas and ejectas while basement rocks have remained on the surface in the western part. The fumaroles of Chausu-clake have a temperature of 100-150°C, and contain volcanic gases such as HCI, SO2 and H~S. On the contrary, the fumaroles and tlqerreal water distributed on the southeastern slope of the

volcano having thirty issue points are characterized by a low content of C1- and a relatively high content of SO, 2- with intermediate to acid properties. Geothermal investigation was carried out by the Tochigi prefectural government. In 1963, the Tochigi prefectural government started geothermal investigation including geological survey, temperature survey with eight shallow bore holes of 100 to 190 m deep, electric survey and one test well 900 m deep with the help of the Geological Survey of Japan, together with mesurements of precipitation, discharge flow of the river and thermal waters, and change of groundwater level. Fumaroles and hot springs distributed on the southeastern slope of Nasu volcano issue from tuff breccia constituting the basal part of younger volcanic rocks. The investigated area is located near the Yumoto hot spring, having small scale fumaroles, a natural flow of thermal water and self-spouting wells of hot water (Figure 1). After the geological survey was made, shallow bore holes were drilled to obtain data on the distribution of underground temperatures in this area (Figure 22). Then an electric survey was tried:to estimate the depth to the basement rocks. From the result obtained from the temperature survey, it was expected that the geothermal fluids of this area were stored in fractures developed along the Nigatogawa fiver. To examine this idea, a test well 900 m deep was drilled on the left bank of the river. However, this exploration was unsuccessful because the underground temperature was not

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FiG. 22. -- Distribution map of underground temperature in the Nasu geothermal area. 173

as high as expected, though basement rocks consisting of Tertiary formation and granite were found at a depth of below 525 m and 841 m respectively. To detect fractures developed .along the river, the government is making plans to drill on the right bank at present.

GM-2 El.,..14 I 2 m

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OSHIRAKAWA GEOTHERMAL AREA

The Oshirakawa area is located in the north eastern part of Hakusan volcano situated in the middle of lapan. The geological conditions of this area are as follows: the rocks distributed around the proposed area are composed of granite, quartz porphyry and sandstone slate alteration. Granite and andesite are distributed as intrusive bodies into the alteration of sandstone and slate. Some parts of the boundary between granite and the sedimentary rock are altered and weathered by thermal metamorphism, and the sublimation of sulfur can be seen. Sedimentary rocks of sandstone and slate are siliceous and very hard. The geological structure of this formation is generally simple with the bedding planes trending N60 ° 90°E and dipping 3°-60°SE. A major fault was not found by the surface geologic investigation. Small hot springs or ~/apors and altered zones are found here and there. The investigations conducted at this site are as follows: - - S u r f a c e geological survey (conducted by EPDC) - - Measurement of tmnpcrature distribution in subsurface ground (conducted by EPDC) Core borings (temperature measurement in the holes. chemical analysis of hot spring water and other core analyseo-~ 6 borings, total 1420 m (Figure 23).

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As the result of borings, the steam was ejected from each hole. The temperature in these bore holes is 18°C (minimum) to 205 °C (maximum), the pressure of steam flow is 0.25 kg/cm2G to 1.5 kg/em2G and the quantity i3 6 t/h at the biggest hole. ludging from the result of the investigations conducted, the mechanism of the geothermal activity is presumed to be as follows.

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a) The source of the geothermal energy may be reiated to the past volcanic activity of Mt. Hakusan (the distance between this site and the volcano. Hakusan. is estimated to be about 3 km). b) The steam reservoir may not exist as a horizontal layer and it is presumed that the steam comes up through the main cracks in the bed rocks such as granite or sedimentary rocks. c) Therefore, the geothermal phenomena are especially strong at the boundary between two different rocks.

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t[)TAKE GEOTHERMAL AREA In 1949, the Kyushu Electric C o m p a n y started a p r e l i m i n a r y survey at several areas in Kyushu, for the purpose of generating electricity by geothermal energy. 174

FIG. 23. - - Geological column o~ exploration well GM-2. Pressure (shut-in) 2.0 kg, cm'-'; pressure (open) 0.25 kg/cmZ: amount o/ discharge 0.01 t/h; temperature at the ori/ice 95 "C

In 1953-1956 they began investigation at Otake geothermal field and drilled four test wells, but could not develop superheated steam and the project was discontinued. In 1961, they began the study of the use of geothermal energy to generate electricity by the flashed steam from a steam-water mixture, and exploration of the Otake geothermal field was put into operation. Also during 1963-1966 five productive wells (Otake No. 6-No.10) were drilled and had very powerful spouts• Construction of the power plant was started in |anuary 1966 and was completed in about 19 months. The plant went into commercial operation in Aug. 1967. It now generates 11,000 kW of electricity (Table 3).

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Location and topography

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The Otake geothermal field is located.6 km northwest of Mt. Kuju, the highest peak in the Kuiu volcano group, and is formed of the three areas of Otake, Komatsu and Hatchobaru (Figures 24, 25). This field and its surroundings have a basin topography surrounded by many dome-shaped volcanoes and the river Kusu flows north of the central part of the field.

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FIG. 24. - - Index map oJ the Otake geothermal area.

The basement rocks clarified so far in the area are the lower Pleistocene Hohi volcanic complex, which

TABLE 3. - - Physical and chemical properties of steam and hot water i~ the Otake area. Omke

Well N o .

Item

Bore Diameter Bore D e p t h S h u t in P r e s s u r e Max

Pressure

Steam Flow e-

H o t W a t e r Flow Total Total Non-condensable Gases in the S t e a m o..~ Z'~

e-

<

inch m kg/cm:

kg, c m -~ t/h t'h t/h kcal/h

Hatchobaru

6

7

8

9

10

1

2

8 500 4.3

8 350 11.3

8 346 4.6

8 550 8.0

8 600 6.6

8 785 20.0

8 739

over

over

over

7.0 17 (i) 0 (1) 17 1.1 x I0 r

15.0 29 (a) 61 (a) 90 2.7 × 10:

7.0 30 ( 0 135 (1) 165 3.8 x 107

over 10.0 19 (a) 187 (1) 206 3.8 x 10 r

17.8 53 (1) 193 (1) 246 6.1 x 10 r

wt %

0.46

0.25

0.30

0.44

0.05

CO~ H~S 02, N~, O t h e r s Total

J • • •

95.03 0.57 4.40 100

96.65 0.65 2.70 100

93.48 2.02 4.50 100

96.02 1.3g 2.60 100

93.00 1.00 6.00 100

pH To~lSolid C1-SiO2

ppm J ~

5;0 3.7 <1.0 0.4

5.1 20D <1.0 0.7

5,7 13.0 <1.0 0,7

5.0 3.4 <1.0 0.6

5.3 15.0 9.0 0.0

8.4 2750 2540 1010 414 15.0 4.8 670 70 200

8.0 3510 3530 1760 525 17.0 6.0 920 I00 96

6.7 3500 3810 1630 668 20.7 1 I0 940 10.0 145

8.0 5100 4030 1720 612 31.2 7.8 1060 140 95

pH Conductivity T o t a l Solid CI-SiO2 Ca + + Mg ÷ + Na + K* SO,----

!tmhos ppm ~ ~ , , • ~ *

-38 (-~) 60 (2) 98 3.3 × 10 r

w

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m

5400

m

4720

1900

m

680 I m

140

(i) At the separator, p r e s s u r e 2.1 k g / c m 2 G . t~) At the separator, pressure 2.5 k g / c m ~ G .

175

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Map o# production wells in the Otake area.

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consists of alteration of two pyroxene andesite or hornblend-bearing two pyroxene andesite lavas and their tuff breccias crop out locally in the valley which is eroded by the river Kusu. It has been confirmed by drillholes, however, that the complex develops more widely and deeply below sea level with the thickness of about 1000 meters. Furthermore, the complex overlies the Miocene sediments of the Kusu group, which is the basement rock distinguished so far underneath the field and is intercalated with pyroclastics frequently. They have been extruded later on by the MiddleUpper Pleistocene Kujyu volcanic complex which is mostly viscous lava of hornblende andesites making so many lava domes surrounding the Otake geothermal field. The complex is partly intercalated with the Handa pumice flow of the same type of rock. Of these complexes, the tuff breccias in the Hohi complex and slightly indurated sediments of the Kusu group, both with relatively high porosity, may fairly be presumed to be the reservoir elsewhere in the field. The geological structure of this area, covered widely with Holocene volcanic ash, has not been thoroughly clarified yet. The prominent NW fault which may possibly be the main breeding fault passes through the west side of Sujiyu hot springs, besides which several faults are presumed to cut the field in NW and EW directions. The faults cut the Misokobushi-yama lava, the base of the Kujyu volcanic complex, and are covered with the lava of the younger Kujyu volcanic complex. The marked altered rocks are observed along the faults and fissures or points probably due to these faultings, and obviously these weak lines are believed to be the passages for geothermal fluids. From the viewpoint of the stability range of secondary minerals due to hydrothermal alteration, the altered rocks can be classified into seven zones; alunite zone, kaolin zone, pyrophylUte-kaolin zone, pyrophyllite zone, montrnorillonite zone, zeolite zone and chlorite zone.

Geothermal mani]estations In addition to the marked altered rocks as mentioned above, there are surface geothermal manifestations such as hot springs or steam-fumaroles (called e ligoku >>) named Otake-ligoku, Komatsu-|igoku, Hizenyu hot spring and Sujiyu hot spring on the right bank of the Kusu river. They are also arranged frequently along or near the faults, and offer valuable underground information. We also measured the underground temperature over the field at a depth of 1.5 meters. The temperature distribution was the same as the results which are usually obtained in ordinary geothermal areas.

Geophysical exploration 1) Magnetic exploration: for measuring instrument, a vertical magnetometer of the saturated iron core

type was used. 550 measuring stations were set up at intervals of 150 meters. The distribution of high magnetic zones and low magnetic zones is irregular. And there are two low magnetic zones (less than 0.34 c.g.s.e.m.u.) on the east side of the Kusu river. It can be considered that the low magnetic zone indicated by magnetic observation is directly connected with the altered zone, because the geology in Otake is composed of andesite down to the depth of about 1 kin. 2) Electrical exploration: traverse lines were set in a N-S direction at intervals of 300-450 meters and vertical electric sounding using the Schlumberger method was applied. The resistivity survey was carried out with the improved Gish-Rooney apparatus. The low resistivity layer is divided into three blocks and the thickness of the low resistivity layer is in the range of 200-800 meters. A low resistivity value may indicate the existence of an altered zone, and the value is in the range of 3-100 ohm-m. The horizontal extent of the altered zone is presumed to be larger than the area covered by traverse lines and the boundary has not been clarified yet. 3) Gravity exploration: 1000 measuring points were instituted at intervals of 150-200 meters in the Otake geothermal field and about 150 points in the surrounding areas. The North American gravimeter and Worden gravimeter were used for the gravitational measurement. A high gravity anomaly was recognized at the Otake geothermal field, and it was presumed that this anomaly was caused by the uplifted parts of the basement. Also the surface geothermal indications are scattered around the boundary of gravity anomaly and they are both closely connected. From the gravity anomaly, it is supposed that fault-like structures run northwest to southeast in the Otake field. 4) Seismic exploration: at the Hatchobaru area, the effectiveness of seismic exploration was tested. The traverse line was set in a NE-SW direction and its length was 1600 meters; the interval between receiving points was 20 meters and the interval of the shot points was 60 meters. As a result, it was confirmed that two faults run from NW to SE in the Hatchobaru area. The part between the two faults is presumed to be the crushed zone and forms a good hot water path.

Hydrothermal ]eatures This field is composed of andesite. Though the original rock is compact and impermeable, permeability increases as it undergoes alteration. The coefficient of permeability of strongly altered rock is about 10" cm/sec, but in the case of medium or weak altered rock, it is 10"~-10"7 cm/sec. 177

Since the degree of alteration of underground rock is variable, the mean value was assumed as 10"6 em/sec to calculate the seepage of the water from the surrounding rocks to the drillhole. The amount of seepage is very small compared with the discharge amount from the wellhead. During drilling, we frequently met cracks, where drilling water escaped. It is believed that more hot water is supplied through these cracks than by permeation. The mineral deposition of No. 6 drillhole is calcium carbonate, while No. 7 drillhole next to it has silica deposition. We cannot explain this phenomenon by assuming that hot water is supplied from the same s~ratum, if we do not take two independent cracks into consideration. The temperature of steam from a magma reservoir will be lowered as it ascends through fissures, because ascending steam supplies heat to the surrounding rocks, and it encounters low temperature meteoric water. Thus it remains, in cracks as hot water. If the hydrostatic pressure of the water is less than the saturation pressure of hot water, it will be discharged from the drillhole as wet steam. The hot water in the cracks is considered to be flowing, but the flow pattern is not well known, since the distribution of the cracks is not yet clear.

Other uses of hot water A few hotels were using some of the natural hot springs. (Kawara, Otake, Komatsu steam-fumarole), before the Otake geothermal field was exploited. According to the progress of exploitation, the hot water is being piped to the lizobaru reservoir, which is about 4.5 km from the Otake power plant by aqueduct, for water pollution prevention purposes. The aqueduct is made of reinforced concrete, (1.0 meter wide, 1.1 meters deep) and water flows gravitationally. About 170 t/h of hot water is being led off from the aqueduct to private houses, sanatoriums, hotels and a botanical garden. The amount is about 100 t/h to private houses (5 villages, 130 homes), 30 t/h to 4 sanatoriums, 10 t/h to two hotels and 30 t/h to the botanical garden. The hot water is mostly used for heating, bathing and cooking, but the botanical garden in lizobaru is using it for plant-culture. The rest of the hot water is fed into the lizobaru reservoir. In the future, we have plans to use it for the improvement of agriculture in cold areas. Future plans of development The maximum geothermal energy development will be controlled by the total area of the field, the interval between adjacent wells, the average output of a well, and the amount of underground water flow and heat flow from the deeper part. The smallest of the above factors

178

limits the maximum output. The average power of a well we assumed to be 2000 kW for the Otake area, 3000 kW for Komatsu and Hatchobaru areas. As a whole, we predict the optimum limit of power to be 180,000 kW. In 1968, two productive wells were drilled successfully in the Hatchobaru area. We have plans to construct a new power plant there. TAKENOYU GEOTHERMAL

AREA

Takenoyu geothermal area is situated in the northern part of Kumamoto Prefecture in Kyushu, 7 km from the Otake geothermal area beyond Mt. Waita, one of the Ouaternary volcanoes included in the Kuju volcanic group.

Geology This area consists of HOhi volcanic complex which is known as a reservoir of geothermal fluids in the Otake area, and Yamakawa tuff breccia, both of which are covered by younger Misokobushi lava and lava dome of Waita volcano (Figure 26). The Hrhi volcanic complex is found on the north side of the Takenoyu fault running through Takenoyu and Haganoyu hot springs in a east-west direction, while the Yamakawa tuff breccia is mainly distributed on the south side of the fault. Besides the Takenoyu fault which is considered to extend in a southeast direction through the center of Waita volcano and the Otake geothermal area, there may be several faults in this area. The geographic distribution of volcanic bodies in the Kuju volcanic region suggests that their arrangements have been controlled by subsurface structure formed in the basement rocks. Along the Takenoyu fault, there are many fumaroles in the Takenoyu and Haganoyu areas and thermal water named Shinyu issues from Yamakawa tuff breccia, located on the south side of and 400 m from the fault. Geothermal exploration by the Yahata Iron Company Ltd. In 1961 and 1962, the Yahata Iron Company Ltd. drilled two test. bore holes for the purpose of geothermal exploration. The bore holes were drilled in the Takenoyu area; on the north side of the fault and in the area where the HOhi volcanic complex is distributed. Soon after blow out began, well A-1 stopped emission of steam and hot water because of the precipitation of calcium carbonate deposited in the bore hole, while well B-1 discharging 0.92 t/h of steam and 1.72 t/h of hot water at the beginning is still emitting, though chemical and physical properties have changed. However, the company gave up the exploration in 1963 for financial reasons and after that, the Kumamote prefectural government did measurements of flow discharge of steam and hot water emitted from well B-I.

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Geological map o/ the Takenoyu geothermal area. 179

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Geothermal investigation by the Geological Survey of Japan and the Kumamoto pre/ectural government In order to obtain data on the occurrence of geothermal fluid in this area, the Geological Survey carried out geothermal investigation including geological mapping, electric and gravity surveys and test drilling from 1965 to 1968. I.n addition to the investigation by the Geological Survey, the prefectural government also made a temperature survey by drilling five shallow holes 100 m deep in 1968. According to the result of the gravity surveys (Figure 27) made in 1964 and 1965, it is reported that fumarolic and hot spring areas distributed on the westside of Waita volcano are included in anomalous areas 180

of low gravity. surveys (Figure resistivity layer a thickness of

Then, from the result of the electric 28), it has become clear that a low is distributed all over this area with 100-150 m. Five shallow wells made 1

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by the prefectural government were drilled for the study of not only the underground temperature of this area, but also the geological properties of the low resistivity layer. The result of drilling showed that Yamakawa tuff breccia and HShi volcanic complex have suffered alteration, as far as to the depth of 100 m from the surface, though the temperature becomes lower in the outside area going away from Takenoyu and Haganoyu. Thus, whether such rock alteration is related to the present geothermal activity or not remains a problem. In the autumn of 1968, a research well 310 m deep was drilled by the Geological Survey at a point between Takenoyu and Haganoyu and on the south side of the fault (Figure 29). According to the observation of core samples, the boundary of Yamanoyu tuff

breccia and HShi volcanic complex is found at the depth of 200 m. The former is altered remarkably and the latter also contains zeolite and calcite veins. The underground temperature is relatively low in the former, but as soon as the bore hole reached the latter, the temperature increased rapidly and mud water for drilling was soon lost. The maximum temperature was 200°C at the bottom of the hole. As it became difficult to continue drilling on account of the escape of mud water and the rapid increase of temperature in the bore hole, drilling work was abandoned at a depth of 310 m, changing the scheduled depth from 400 m. After a 2 inch throttled pipe was put in the hole, the emission of steam and hot water began without artificial lifting of the water column. Shut-in pressure at the orifice is 10.5 kg/cm2 181

and the amount of discharge of hot water is 26 t/h at 2.0 kg/cm:, though the amount of steam has not yet been measured. Based on the idea that the geothermal conditions concerning the reservoir are better on the south side of the faut than on the north side, two productive wells 500 m deep will be drilled by the prefectural government next year.

The rocks distributed around the proposed site are composed of various volcanic products such as tuff, tuff breccia, agglomerate and andesite. Beneath the volcanic products, granite which is presumed as bed rock may exist.

Gi--i

IBUSUKI GEOTHERMALAREA The Ibusuki area is located in. the southern part of the Satuma peninsula in KyOshQ island. This area is located in the northwestern part of the Ata caldera which is one of the two largest calderas in south KyflshO and is noted for volcanism in the past. This caldera was formed in the shape of a large ellipse, but most of it has fallen into Kagoshima bay; consequently the caldera wall which can be seen near the Ibusuki geothermal area is only a part of this caldera. As mentioned above, the Ibusuki area is situated in the inside part of the Ata caldera, where many small calderas, volcanos and craters, which show typical volcanic topography exist. The representative calderas and volcanoes are as follows:

EL. 152.00 m_ A

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No. t - Lake Ikeda. This lake is the largest fallen caldera in this area. The western side of this caldera has overlapped on to the Ata caldera. According to the result of echo sounding, the maximum depth is 180 m below sea level and there are three small craters at the bottom of this lake.

106 300A II

No. 2 - Lake Unagiike. This caldera has been formed by explosion.

330

II

II No. 3 - Yamakawa bay. Same as No. 2 Lake. The above three calderas are located on a line running N60°W.

A

550

400-

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No. 4 - Ikezoko. This is the ancient central crater of Ikezoko volcano, whose complete figure cannot be seen today, but the somma at the northeastern part of Ikezoko remains. This somma is called ~ Gongen somma >). The Electric Power Development Co. (hereafter EPDC) executed geological investigations at this ~ Gongen s o m m a , as follows: orospecting (conducted by the Compagnie Genetale de Geophysique). Total 117 electrical soundings. -- Gravimetric prospecting (conducted by the Geological Survey of Japan). --Measurement of temperature distribution in sub-surface ground (conducted by EPDC). Core borings (temperature measurement in the holes, chemical analysis of hot water and other core analyses). 3 borings, total 1500 m (Figure 30). Surface geological investigations.

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FIG. 30. m Geological column o[ exploration well GI-I. 1) Andesire: 2) Tu[[ breccia: 3) Tu[[.

ludging from the result of electric prospecting and. borings, the geology of this site is divided into three layers; the first layer with 200-300 ohm-m is tuff deposited on the ground, the second layer with 4-30 ohm-m is composed of altered tuff breccia, agglomerate and andesite, and the third layer with 100 ohm-m is considered to be the bed rock. The first layer with a thickness of about 50-100 m is composed of altered clay and is dry. The second layer with a thickness of about 300 m is composed of altered volcanic products, and abundant hot water is reserved in this layer. Steam was ejected from one of three bore-holes which were drilled in the second layer. The pressure of the steam flow was about 0.5 kg/cm2G, but the quantity was not measured. The remaining two boreholes encountered the reservoir of hot water at the second layer, but could not obtain steam ejection. Th..e temperatures of water were 18°C (minimum) to 143°C (maximum) in the holes. P r o b l e m s related to t h e d e v e l o p m e n t of g e o t h e r m a l e n e r g y in J a p a n

There are more than 90 areas of hot springs with water temperature exceeding 90 °C. Thus there are many localities with potential geothermal 'energy, but there are only two commercially exploited areas and their production is as yet small compared with the potentiality. The development of geothermal energy is being actively pursued in |apan at present, but there are many problems related with these projects, the major problems are as follows:

The quantity and the nature of Japanese geothermal energy resources Volcanism is active in Tapan and there are many hot springs and fumaroles: also precipitation is 1600 mm in annual average which amounts to 600 billion tons, and thus it is inferred that the total geothermal energy reserve would be very. promising. The geothermal energy reserve of }'apan has been calculated many times, and the recent survey made by the Geothe..rmal Resources Committee, Ta~an Geothermal Energy Association, estimates that power generation up to ten million kW will be possible in the future when various related technology is developed sufficiently. A limitation concerning ~eothermal energy develop. ment is that the utilization is restricted to the area of the resource. Also the value of the resources varies according to whether it is used as steam or hot water. The cost of geothermal fluid is the least expensive among the steam available in the world used for power generation and it is approximately 100 yen (30 US cents) per ton. In spite of the cheap unit value, it will be valuable if there are large quantities and they can be obtained continuously for long periods

It is advantageous that naturally spouting geothermal fluids can be used, but at the same time the amount of fluid available from one well is small, several tens to one hundred tons per hour being the maximum output. Therefore many producing wells at appropriate intervals become necessary. Thus the area of geothermal development must be sufficiently large. Also since there are Iimits to the available steam from one area, it will not be possible to develop large scale generation as in the case of oil-burning power stations, and many small scale stations must be established in geothermal areas. Compared to other natural resources such as oil or metals, geothermal resources will last longer once they are developed. As the development of geothermal energy for power generation has a short history in Japan, there are no data concerning the longevity of the resources, but judging from the history of hot springs which have been utilized since oldest times, it is inferred that, provided the development is done with scientific control, the resources should last for very long periods. The decrease of production would be unavoidable due to corrosion, deposition of scale and other occurrences, but with careful maintenance and in some cases drilling of new wells, it should be possible to maintain production for a long time. In the case of oil development, the ratio of reserve (R) and annual production (P) R/P must be kept above a certain level, and prospecting must be carried on in order to ensure a certain level of production. In the ease of prospecting and development for geothermal resources in [apan, however, the objective is to increase production rather than maintain the present level.

Problems related to prospecting for geothermal resources Many of the geothermal areas with surface manifestations are either designated as national parks or already developed as hot spring resorts. Thus there are many restrictions in these areas for the establishment of power generating facilities. The prospecting therefore must be carried out in areas at distances from the center of national parks, and therefore in [apan the normal method of surveying from the surface manifestations cannot always be done. In general, these resources must be developed in areas far from conspicuous thermal manifestations on the surface. Many of the geothermal areas in lapan are located in areas of Quaternary to Recent volcanic activity and various youn~ volcanic rocks are distributed irregularly near the surface. The structure of the basement rocks underlying these volcanic rocks varies, and thus the types of reservoirs of geothermal fluids differ with different geological conditions. The commercially operatin, geothermal areas of Matsukawa and Otake have quite d~fferent geological conditions, In order to search for geothermal areas suited to power generation under the above conditions, geother183

real manifestations must be analysed, all available information on geothermy obtained, the geological structure of the geothermal areas clarified, the model of the reservoir type controlled by these structures det?ermined, and then the most efficient prospecting for these areas carried out. Various methods are employed for the prospecting and development of natural resources. Systems of exploration works are established for various types of oil, gas, metal, and non-metal deposits. As for geothermal resources, the effectiveness of various methods is being tested and efforts are being made to obtain information on these methods. In Japan, geophysical methods have been applied with varying success, but the application of geochemical methods must be thoroughly investigated in the near future. Nevertheless, methods of prospecting will be developed together with the increase of knowledge concerning the occurrence of heat in the reservoirs. Regional surveys for the purpose of securing large quantities of steam for power generation should be effective from a structural point of view, but in Japan, a large scale regional survey has not been carried out owing to various limitations. Problems related to development and utilization of geothermal energy Iapan has a long history of geothermal utilization for bathing. The hot springs are fully developed as resorts for the public, and those in scenic areas are regarded highly. Hot springs are being prospected and developed in many areas, but these are much smaller, in scale compared to those for power generation.

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Since a large portion of the energy resources of Japan is dependent on imports, the development of geothermal energy is being attempted in order to secure domestic sources of energy and also to obtain an inexpensive and stable source of electricity. Some of these projects have specific purposes such as supplying power for metal refining. Under these circumstances, the success in power generation at Matsukawa and Otake together with favorable results elsewhere indicate the possibility of the future development of geothermal power generation in Japan. However, for large scale exploration of geothermal energy such as power generation, many factors must be considered. As mentioned earlier, hot spring resorts and the central parts of national parks must be avoided, and also there are laws governing small scale exploitation for hot spring baths but no laws for large scale operation. These must be worked out for the smooth development of geothermal power. As geothermal resources are one of the few natural resources of Japan, they must be utilized to the full, not only for power generation, but also for resorts, heating, and other uses and thus contribute to the general development of the area concerned; these are being considered. On the other hand, the potentiality of geothermal power generation and the degree of the contribution to the development of the electric industry is being studied from various angles. For this, the amount and nature of the presently known geothermal resources must be analysed, the method of prospecting must be established, and obstacles to the development and utilization must be removed; efforts are being made in this direction