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Exploration of geothermal resources in the Tatun volcanic region, Taiwan, Republic of China
Geothermics (r~,7o) - SWCIAL ISSUE ", U. N. Symposium on the Development and Utilization of Geothermal Resources, Pisa r97o. Vol..~, Part t
Exploration of Geothermal Resources in the Tatun Volcanic Region, Taiwan, Republic of China T. TSUNG FENG *
~
K. KONG H U A N G *
ABSTRACT
The Tatun volcanic region is located on the northern tip of Taiwan. It covers an area of about 320 square kilometers. It is composed of andesitic lava flows and pyroclastics. The basement rocks are Miocene sediments. In view of the strong surface thermal manifestations in the region, it was chosen as the first area in Taiwan for geothermal exploration. Exploration work was commenced in November, 1965 and has been carried on until now by the Mining Research and Service Organization. About 150 square kilometers on a scale of 1 to 10,000 have been mapped geologically. The total surveyed area will be 200 square kilometers. Chemical analyses have been-made for hot springs, fumaroles, and drillhole discharges. A onemeter-depth temperature survey was completed in early 1967. Although this survey did not give very much information on the temperature anomaly at depth, it can help delineate the boundaries of individual hot areas and may belp trace out certain structural trends. Both electric resistivity survey and magnetic survey have I/een applied in this volcanic region in search of hot hydrothermaLly altered zones. But the results are difficult to interpret, for the region is characterized by high relief and heterogeneous petrographic composition. There were 58 small (1 7/8" - 3" in diameter) and shallow (47 - 155 meters in depth) gradient wells drilled in the region. The total footage was 6625 meters. These wells proved to be useful for siting future exploration wells. Up to date, 7 exploration wells ranging from 4,11 to 1005 meters in depth have been drilled. A femperature as high as 240 oC has been measured at the bottom of the 1005 meters deep well. A 1500 meters deep exploration well is under drilling and is to be completed in August, 1969.
Introduction The utilization of geothermal steam as a possible energy source in T a i w a n was first suggested in 1962 by P. P. SMXTH, engineering consultant of an A m e r i c a n foreign aid agency in T a i w a n . In view of the high ¢x>tentials of heat energy in the young volcanic regions. the Chinese G o v e r n m e n t has decided to investigate the possibility of developing geothermal resources in Taiwan. The T a t u m volcanic region in northern T a i w a n has been selected as the first area for geothermal exploration due to the high intensity of thermal manifestations and activities in the volcanic rocks. The exploration project, w h i c h began in N o v e m b e r 1965, is still
* Mining Research and Serv. Organization, Ministry of Economic Affairs, 261, Roosevelt Road, Sect. III, Taipei, Taiwan.
150
being carried on by the Mining Research and Service O r g a n i z a t i o n of the Ministry of Economic Affairs. in cooperation with the T a i w a n P o w e r C o m p a n y and the Chinese Petroleum Corporation. Field exploration has benefited from helpful advice and c o m m e n t s from foreign experts w h o have visited this geothermal region at different times. T h e present p a p e r gives a general review of the exploration w o r k that has so far been acc o m p l i s h e d in this geothermal region. T h e i s l a n d of T a l w a n
Taiwan is one of the important islands bordering the western Pacific Ocean. [t is n o w one province of the Republic of China and lies 150 kilometers off the Fukian coast of the Chinese mainland, separated from the latter by the Taiwan Strait which is only 50 to I00 meters deep. The island of Taiwan is a spindle shape, with the longitudinal axis extending roughly north-south for a length of 385 kilometers and a maxi m u m width of 143 kilometers. It occupies a total area of 35,960 kilometers. The main island of Taiwan is the site of Tertiary. geosynclinal sedimentation on. a pre-Tertiary metamorphic basement. The sedimentary pile in the geosyncline attains a m a x i m u m thickness of more than 10.000 meters. This geosynclinal trough has a general north-south trend, and the main structure of the rocks is that of an elongated arc convex toward the west. A dominant volcanic cycle began at the end of the early Pleistocene orogeny which is the most important d;astrophism ,~n Taiwan. Great quantities of andesite or dacite lavas and pyroclastics extruded from a number of centers distributed in the northern part of the Taiwan island. Tatun volcano group Q u a t e r n a r y volcanoes arc d i s t r i b u t e d mostly in northern T a i w a n and on the volcanic islets off the northeastern coast of T a i w a n . The most p r o m i n e n t volcanic region is the Tatun Volcano G r o u p , ranging from 10 to 15 kilometers north of Taipei city (Figure 1).
The volcanic rocks occupy an area of nearly 320 square kilometers and the highest mountain peak reaches 1120 meters above sea level. There are ten and more eruptive centers in this volcano group. They are generally aligned in a northeast trend in two or three rows, suggesting a northeast fracture pattern in the underlying rocks that allow escape of magma. This volcanic region is divided by the northeast-trending Chinshan fault into a northwest zone of more basic andesite and a sou-
Agglomeratic and tuffaceous breccias, tufts, lahars, and other detrital rocks constitute the other major portion of the volcanic rocks. Lower Miocene clasfic sediments form the basement of the volcanic rocks. They are represented mostly by whitish grey to light grey orthoquartzites and arkosic sandstones. The sandstone is medium to coarse-grained, massive to thick-bedded, and porous. Dark grey shale and siltstone constitute a minor portion of the Lower Miocene formations.
U Fumeroles Hot Springs I Hsinpeitou ~. Tahuangfsui $ Taehuan9
N
_l
9 ! z S ~ s , kM
'J
Yangmin~shan S Chutzehu s HsiaoyuKong ? Mat$OO
/
a TayuKeng g Sanchungchiao ,o Szehumn~tzepin9 " K.nfzep~ng
,z Chinshan To~ou
.
0
PEITOU
Youn9 Volcanoes: z 5hamaoshan z Chihsing.~n Z Tungkuashan ~r Huangtsuisnan
Fro. I. -- Tatun volcanic region theast zone of less basic andesite in which are four young volcanoes and all the hot springs and fumaroles of the region. The four youngest volcanoes generally Iie in a ENE belt about 10 kilometers long. Beginning from the west, they are named as Shamaoshan (643.2 m), Chihsingshan (1119.6 m), Tungkuashan (883.8 m), and Huangtsuishan (911.5 m). In spite of the young age, no volcano is known to be active in the historical record. The volcanic rocks in the Tatun volcanic region are composed mainly of andesitic lavas, varying from dens~ andesite to glassy and autobrecciated andesite.
There are nearly 100 hot springs scattered throughout the entire island of Taiwan. The hottest and mos~ impressive groups of hot springs and fumaroles are located in the Tatun volcanic region. Numerous hot springs, fumaroles, soIfataras, and mud pools are observed in the region. The thermal areas are confined to a northeast belt which crosses the southeast to the town of Chinshan on the northeast coast. This thermal belt is about 18 kilometers long and 3 kilometers wide. The Chinshan fault, which is a thrust fault dipping southeast, forms the northern limit of the thermal belt. The Kanchiao fault, which is of similar trend and na151
ture as the Chinshan fault, defines the southern boundary of this belt. The rocks in the thermal belt are hydrothermally altered by Quaternary volcanism. Hydrothermal rock alteration is more prominent in the volcanic rocks but much less marked in the sandstones. Argillization and opalization are the most characteristic features in the altered volcanic rocks. Silicification is more pronounced in the sandstones. The common hydrothermal minerals are kaolinite, opal, cristobalite, pyrite, sulphur, alunite, calcite, quartz, and tridymite. Geothermal exploration project Geothermal investigation in Taiwan is practically based on the same general principles and methods known in" most other geothermal fields of the world. A geologic mapping program in the Tatun volcanic region was started in February, 1967. Field mapping is carried out on a 1:10,000 topographic base map which was prepared from aerial surveys. The total surveyed area covers 200 square kilometers. Up to October 1969, about I50 square kilometers had been mapped in detail and the structural and petrologic features carefully studied and evaluated. A detailed geologic description of the region will be given in a separate paper by C. H. CrIEN. A one-meter-depth temperature survey of the thermal belt was completed in early 1967. This survey covered an area of 80 square kilometers on a 1:25,000 base map in the southwestern part and 38 square kilometers on a 1:10,000 base map in the northeastern part. In the former area, ]54 holes were drilled, averaging 4.45 temperature measurements per square kilometer. In the latter area, 478 holes were drilled, averaging 12.6 temperature measurements per square kilometer. This survey did not yield much information on deep temperature anomalies, yet it may help delineate the boundaries of individual hot areas as well as trace certain structual trends throughout the thermal belt. Chemical analyses have been made for hot springs. fumaroles, and drillhole discharges. Resistivity survey is used now to locate the hot zones at depth. Schlumberger electrode configuration is employed to measure the resistivity, In electric profiling, current electrode spacing of AB= 1000 meters is made; while in vertical electric sounding, AB(max.)= 4000 meters. In normal resistivity survey a greater probing depth is generally accomplished by larger electrode spacing. The high topographic relief and the rugged terrain in the Tatun volcanic region make the use of large electrode separation very difficult if not impossible. The dipole-dipole method which offers a greater probing depth with less electrode spacing has been under consideration to supplement the normal type of resistivity work. Magnetic survey is also used to explore the geothermal belt in this volcanic region. This survey is gen152
erally carried out on a regional scale to outline the hydrothermally altered areas related to volcanic activities. Considerable difficulties have been encountered in the final interpretation of both electric resistivity and magnetic results, as the Tatun volcanic region is characterized by high relief and heterogeneous petrographic composition over most areas. The geophysical exploration will be described in another paper by W. T. Cr~ENG. One of the main objectives in the geothermal exploration of the Tatun area is to locate the upper surface of the basal Miocene sandstones below the mantle of the volcanic rocks. It is believed that large quantities of hot fluid could be stored in the porous zone at the contact of the volcanic and sedimentary rocks. Seismic refraction survey has been suggested to map the boundary between the andesitic volcanic rocks and the Miocene sandstones. Arrangements are being made for this type of seismic survey. After preliminary geological, geochemical, and temp.erature studies, four out of the 13 thermal areas in the Tatun volcanic region were given priority in the geothermal exploration project. They are, in order of field investigations: Tahuangtsui (including Shuangchungchi), Matsao, Szehuangtzeping (including Kengtzeping), and Chinshan. Shallow gradient wells were drilled in these four areas, the maximum depth being 160 meters. Deep exploratory wells were drilled in Tahuangtsue and Matsao to test the reservoirs, to study the underground structures and temperatures, and to measure the initial discharges. The general depth of the deep wells range from 500 to 1500 meters. Exploration in Tahuangtsui area Geothermal investigations in the Tahuan~tsui area commenced in March 1966. This area is at the southwest end of the thermal belt at an elevation of about 200 meters. This area was chosen because the hot springs there cover the greatest areal extent and the one-meter-depth survey revealed the highest surface temperature anomalies. In addition, this area is only 5 kilometers from Hsinpeitou and is readily accessible from Taipei city. The hot springs and fumaroles rise mostly from the Lower Miocene sandstones which are only partly covered by thin hydrothermally altered volcanic debris. The Lower Miocene white sandstones are interbedded with thin shale beds. The sediments are buried by volcanic tuff breccias and interbedded lava from the northeast. In Tahuangtsui 29 two-inch (1 7/8" to 3") gradient wells were drilled to depths ranging from 47 to 155 meters. The geological and temperature logs have been shown by CHEN (1967). He also separately compiled isothermal contour maps for 60, 80, 100 and 120°C in which two high centers were indicated, marking local ascent of steam or hot water at the northeast end
of the hot spring and fumarolie area. The highest temperature recorded in the gradient wells was 175 °C at 155 meters. Eight of the wells tapped steam at depths ranging from 39 to 155 meters, with temperatures from 120 to 175 °C. Five wells had discharges ranging from 0.4 to 2.6 tons per hour with enthalpies of 379-543 kcal/kg. The main producing horizons are in the coarse-grained sandstones. Both the pores and the fissures in the sandstones provide good conduits for the passage of steam. The discharges are mostly wet steam with high water content and pH value around 3. After the completion of the shallow gradient holes, three deep exploratory wells were drilled: E-101 (519.5 m), E-102 (630 m) and E-103 (1000 m), in which the maximum temperatures were respectively 198, 175 and 172 °C at depths of 250, 353, and 1000 meters. However these temperatures were measured only 9 hours after drilling ceased and, in E-103, about 153 hours later, so it can be assumed that the ground temperatures are higher than those measured at Tahuangtsui. Of the three deeper wells, E-101 discharges 5.4 tons per hour with 40% steam at a well-head pressure of 1.6 kg/cmZG, and E-102, 7.4 tons per hour with 10.1% steam at 1.2 kg/cm-~G. E-103 yields 35.6 tons per hour with about 12.2% steam, but it discharged for only a few days due to corrosion of the casing pipe by highly acid water. Corrosion tests are being made to find suitable corrosion resistant material for the pipes in this field. An electrical resistivity survey was made at Tahuangtsui during March and April of 1967, and another between August, 1967 and April, 1968 with increased electrode separations and regular traverse patterns. It was found that the sandstone formations have low resistivity and the zone of low resistivity is also consistent with the thermal area and the geological structures. A magnetic survey was conducted at the same time as the second resistivity survey was carried on. This survey showed that magnetic anomalies are associated with the andesites and pyroclastie rocks, and low values are in the hydrothermally altered area and the area of Tertiary sediments. E x p l o r a t i o n in M a t s a o area
The Matsao thermal area lies on the east of the Chihsingshan volcano and on the west of the Mafengshan volcano. Both volcanoes are composed of dark grey two-pyroxene hornblende andesite, but the former is much younger. The andesites alternate with pyroclastic rocks, and a number of eruptive stages can be differentiated. Between the two volcanoes is a flat, 300-400 meter wide valley floored by stratified volcanic detritus which has been hydrothermally altered. Steam emerges from the slopes and hot springs come out along the gullies going towards east and south. The western scarp of the Mafengshan is a possible fault scarp along which the western part of that volcano collapsed, pos-
sibly due to hydrothermal alteration. The location of the Matsao thermal activities may be in part related to this. A total of 14 gradient holes have been drilled in Matsao for temperature measurements, generally less than 160 meters deep. Three of the holes blow steam. One of these, G207, has the highest measured temperature of 174°C at 147.5 m depth, and produced 1.9 tons per hour with a steam ratio of 85% and a shutin well-head pressure of 6.6 kg/cm2G. Following the shallow drilling program, five exploratory wells have been drilled at the eastern base of the Chihsingshan and one at the western base of the Mafengshan. All these wells passed through mainly hornblende andesite of the Mafengshan type with pyroclastic material on top. The lava flow of the area has not been penetrated through and the depth to the basement is unknown. The six deep wells drilled in Matsao are E201 (572 m), E202 (441 m), E203 (1005 m), E204 (577 m), E205 (1500 m), and E206 (301.5 m). The maximum temperatures measured were respectively 236, 169,240, 208, 240, and 72 °C at depths of 544, 303, 1000, 577, 1500 and 300 meters. E201 discharges 3.3 tons per hour with a steam ratio of 77.2% and a shut-in wellhead pressure of 9.3 kg/cm-"G. E202 discharges 2.7 tons per hour with a steam ratio of 81% and a shut-in wellhead pressure of 6.0 Kg/cm'~G. E203 had a shut-in wellhead pressure of 12.0 k~/cm~-G, but, due to incomplete cementing of the casing, it produced only 1.8 tons per hour. E205 was completed early in October, 1969. The preliminary test shows that it discharges 13.15 tons per hour with a steam ratio of 83.4 at a well-head pressure of 0.9 kg/cm2G. E207 is still under drilling and is 190 meters deep at present. There is less sulfur in evidence at Matsao and the pH values of waters from the discharging deep holes range from 4.6 to 7. The high temperature and high steam ratio and the less acid waters seem to prove that Matsao is a favorable area for initial development of geothermal power. Based on the present knowledge of this geothermal region, Matsao has been considered as the most suitable area for producing steam to run a pilot plant of 10 MW or so. Resistivity and magnetic surveys have been conducted in Matsao. The lowest resistivity zone and the lowest magnetic anomalies appear to be close to the existing hydrothermal activity near the surface and gradually become deeper toward the center of the Chihsingshan volcano. Exploration areas
in
Szehuangtzeping
and
Chinshan
Investigations have not been so extensively carded out in Szehuangtzeping (includine Kengtzeping) and Chinshan. A total of 13 shallow gradient holes have been drilled in Szehuangtzeping and Kengtzeping. The depths vary between 42 and 151 meters, the highest temperature of 140°C was recorded at 97 meters in well G 302 153
located south of the visible hot area. Andesite of the Huangtsuishan type was encountered in most of these holes. The fumarolic areas in Szehuangtzeping and in Kengtzeping may be connected at depth as one single system. The drilling of a 1000-meter hole between these two areas is under way at present. The data from this hole may clarify the relationship between these two areas and may also indicate the temperature and chemistry of the deeper water feeding these areas. Only two shallow gradient holes have b ~ n drilled in Chinshan with depths ranging from 100 to 110 meters. The rocks encountered are mainly Lower Miocene sandstones. One hole recorded a maximum temperature of 106 °C at 95 m. A large m o u n t of hot water with pH value of 2 flushed o u t . f r o m the hole and caused much trouble to the drilling operation. The shallow drilling program has thus been suspended.
canic region by regional resistivity survey and deep drilling so that the development of geothermal power can be made on an extensive scale to insure large production. Development drilling has at present been concent'rated in the Matso area in the hope that sufficient quantities of steam can be produced for the establishment of a pilot plant of 10 to 20 MW to study the technical problems in the utilization of geothermal steam. The applications of seismic survey to determine the depth to the Miocene basement sandstone and of the dipole-dipole method to obtain great probing depth are being studied. Other important thermal spring areas in southern Taiwan will be systematically investigated to study their potentiality for geothermal exploration.
Conclusion
McNITT J. R. 1967 - - Report of the United Nations technical assistance mission on geothermal resources in China (Tai-
REFERENCES
Geothermal investigations in the Tatun volcanic region are generally carded out on the basis of individual thermal areas. There is a close relationship between past and present hydrothermal areas and the Ouaternary volcanism on a regional scale. This indicates that the geothermal resources are of reasonable magnitude and continuity, which warrant exploration on a large scale. Efforts will be made to study the relationships between the individual thermal areas in the Tatum vol-
154
wan), HEAL¥ |. 1968 ~ Geothermal investigations in Taiwan. MRSO Report, 85.
ToNGIOanl E., M~z^ F., F~t,R^ G. C., Vrso A. 1968 - Some considerations and suggestions on the possibility of exploration of geothermal energy in Taiwan. Ho C. S., LEE C. N. 1963 - - Economic minerals of Taiwan. Geol. Surv. Taiwan.
NAKAMURAH. 1967 - - Report on the survey of hot springs in Taiwan. MRSO. Wane D. E., TRUESDELLA. H. 1969 - - Geothermal resources of Taiwan. An evaluation of existing data. U.N. Rep. PALMASON G. 1969 ~ Geophysical exploration in the Tatun geothermal areas, Northern Taiwan. U.N. Rep.
Exploration of Geothermal Resources in the Tatun Volcanic Region, Taiwan, Republic of China T. TSUNG FENG *
~
K. KONG H U A N G *
ABSTRACT
The Tatun volcanic region is located on the northern tip of Taiwan. It covers an area of about 320 square kilometers. It is composed of andesitic lava flows and pyroclastics. The basement rocks are Miocene sediments. In view of the strong surface thermal manifestations in the region, it was chosen as the first area in Taiwan for geothermal exploration. Exploration work was commenced in November, 1965 and has been carried on until now by the Mining Research and Service Organization. About 150 square kilometers on a scale of 1 to 10,000 have been mapped geologically. The total surveyed area will be 200 square kilometers. Chemical analyses have been-made for hot springs, fumaroles, and drillhole discharges. A onemeter-depth temperature survey was completed in early 1967. Although this survey did not give very much information on the temperature anomaly at depth, it can help delineate the boundaries of individual hot areas and may belp trace out certain structural trends. Both electric resistivity survey and magnetic survey have I/een applied in this volcanic region in search of hot hydrothermaLly altered zones. But the results are difficult to interpret, for the region is characterized by high relief and heterogeneous petrographic composition. There were 58 small (1 7/8" - 3" in diameter) and shallow (47 - 155 meters in depth) gradient wells drilled in the region. The total footage was 6625 meters. These wells proved to be useful for siting future exploration wells. Up to date, 7 exploration wells ranging from 4,11 to 1005 meters in depth have been drilled. A femperature as high as 240 oC has been measured at the bottom of the 1005 meters deep well. A 1500 meters deep exploration well is under drilling and is to be completed in August, 1969.
Introduction The utilization of geothermal steam as a possible energy source in T a i w a n was first suggested in 1962 by P. P. SMXTH, engineering consultant of an A m e r i c a n foreign aid agency in T a i w a n . In view of the high ¢x>tentials of heat energy in the young volcanic regions. the Chinese G o v e r n m e n t has decided to investigate the possibility of developing geothermal resources in Taiwan. The T a t u m volcanic region in northern T a i w a n has been selected as the first area for geothermal exploration due to the high intensity of thermal manifestations and activities in the volcanic rocks. The exploration project, w h i c h began in N o v e m b e r 1965, is still
* Mining Research and Serv. Organization, Ministry of Economic Affairs, 261, Roosevelt Road, Sect. III, Taipei, Taiwan.
150
being carried on by the Mining Research and Service O r g a n i z a t i o n of the Ministry of Economic Affairs. in cooperation with the T a i w a n P o w e r C o m p a n y and the Chinese Petroleum Corporation. Field exploration has benefited from helpful advice and c o m m e n t s from foreign experts w h o have visited this geothermal region at different times. T h e present p a p e r gives a general review of the exploration w o r k that has so far been acc o m p l i s h e d in this geothermal region. T h e i s l a n d of T a l w a n
Taiwan is one of the important islands bordering the western Pacific Ocean. [t is n o w one province of the Republic of China and lies 150 kilometers off the Fukian coast of the Chinese mainland, separated from the latter by the Taiwan Strait which is only 50 to I00 meters deep. The island of Taiwan is a spindle shape, with the longitudinal axis extending roughly north-south for a length of 385 kilometers and a maxi m u m width of 143 kilometers. It occupies a total area of 35,960 kilometers. The main island of Taiwan is the site of Tertiary. geosynclinal sedimentation on. a pre-Tertiary metamorphic basement. The sedimentary pile in the geosyncline attains a m a x i m u m thickness of more than 10.000 meters. This geosynclinal trough has a general north-south trend, and the main structure of the rocks is that of an elongated arc convex toward the west. A dominant volcanic cycle began at the end of the early Pleistocene orogeny which is the most important d;astrophism ,~n Taiwan. Great quantities of andesite or dacite lavas and pyroclastics extruded from a number of centers distributed in the northern part of the Taiwan island. Tatun volcano group Q u a t e r n a r y volcanoes arc d i s t r i b u t e d mostly in northern T a i w a n and on the volcanic islets off the northeastern coast of T a i w a n . The most p r o m i n e n t volcanic region is the Tatun Volcano G r o u p , ranging from 10 to 15 kilometers north of Taipei city (Figure 1).
The volcanic rocks occupy an area of nearly 320 square kilometers and the highest mountain peak reaches 1120 meters above sea level. There are ten and more eruptive centers in this volcano group. They are generally aligned in a northeast trend in two or three rows, suggesting a northeast fracture pattern in the underlying rocks that allow escape of magma. This volcanic region is divided by the northeast-trending Chinshan fault into a northwest zone of more basic andesite and a sou-
Agglomeratic and tuffaceous breccias, tufts, lahars, and other detrital rocks constitute the other major portion of the volcanic rocks. Lower Miocene clasfic sediments form the basement of the volcanic rocks. They are represented mostly by whitish grey to light grey orthoquartzites and arkosic sandstones. The sandstone is medium to coarse-grained, massive to thick-bedded, and porous. Dark grey shale and siltstone constitute a minor portion of the Lower Miocene formations.
U Fumeroles Hot Springs I Hsinpeitou ~. Tahuangfsui $ Taehuan9
N
_l
9 ! z S ~ s , kM
'J
Yangmin~shan S Chutzehu s HsiaoyuKong ? Mat$OO
/
a TayuKeng g Sanchungchiao ,o Szehumn~tzepin9 " K.nfzep~ng
,z Chinshan To~ou
.
0
PEITOU
Youn9 Volcanoes: z 5hamaoshan z Chihsing.~n Z Tungkuashan ~r Huangtsuisnan
Fro. I. -- Tatun volcanic region theast zone of less basic andesite in which are four young volcanoes and all the hot springs and fumaroles of the region. The four youngest volcanoes generally Iie in a ENE belt about 10 kilometers long. Beginning from the west, they are named as Shamaoshan (643.2 m), Chihsingshan (1119.6 m), Tungkuashan (883.8 m), and Huangtsuishan (911.5 m). In spite of the young age, no volcano is known to be active in the historical record. The volcanic rocks in the Tatun volcanic region are composed mainly of andesitic lavas, varying from dens~ andesite to glassy and autobrecciated andesite.
There are nearly 100 hot springs scattered throughout the entire island of Taiwan. The hottest and mos~ impressive groups of hot springs and fumaroles are located in the Tatun volcanic region. Numerous hot springs, fumaroles, soIfataras, and mud pools are observed in the region. The thermal areas are confined to a northeast belt which crosses the southeast to the town of Chinshan on the northeast coast. This thermal belt is about 18 kilometers long and 3 kilometers wide. The Chinshan fault, which is a thrust fault dipping southeast, forms the northern limit of the thermal belt. The Kanchiao fault, which is of similar trend and na151
ture as the Chinshan fault, defines the southern boundary of this belt. The rocks in the thermal belt are hydrothermally altered by Quaternary volcanism. Hydrothermal rock alteration is more prominent in the volcanic rocks but much less marked in the sandstones. Argillization and opalization are the most characteristic features in the altered volcanic rocks. Silicification is more pronounced in the sandstones. The common hydrothermal minerals are kaolinite, opal, cristobalite, pyrite, sulphur, alunite, calcite, quartz, and tridymite. Geothermal exploration project Geothermal investigation in Taiwan is practically based on the same general principles and methods known in" most other geothermal fields of the world. A geologic mapping program in the Tatun volcanic region was started in February, 1967. Field mapping is carried out on a 1:10,000 topographic base map which was prepared from aerial surveys. The total surveyed area covers 200 square kilometers. Up to October 1969, about I50 square kilometers had been mapped in detail and the structural and petrologic features carefully studied and evaluated. A detailed geologic description of the region will be given in a separate paper by C. H. CrIEN. A one-meter-depth temperature survey of the thermal belt was completed in early 1967. This survey covered an area of 80 square kilometers on a 1:25,000 base map in the southwestern part and 38 square kilometers on a 1:10,000 base map in the northeastern part. In the former area, ]54 holes were drilled, averaging 4.45 temperature measurements per square kilometer. In the latter area, 478 holes were drilled, averaging 12.6 temperature measurements per square kilometer. This survey did not yield much information on deep temperature anomalies, yet it may help delineate the boundaries of individual hot areas as well as trace certain structual trends throughout the thermal belt. Chemical analyses have been made for hot springs. fumaroles, and drillhole discharges. Resistivity survey is used now to locate the hot zones at depth. Schlumberger electrode configuration is employed to measure the resistivity, In electric profiling, current electrode spacing of AB= 1000 meters is made; while in vertical electric sounding, AB(max.)= 4000 meters. In normal resistivity survey a greater probing depth is generally accomplished by larger electrode spacing. The high topographic relief and the rugged terrain in the Tatun volcanic region make the use of large electrode separation very difficult if not impossible. The dipole-dipole method which offers a greater probing depth with less electrode spacing has been under consideration to supplement the normal type of resistivity work. Magnetic survey is also used to explore the geothermal belt in this volcanic region. This survey is gen152
erally carried out on a regional scale to outline the hydrothermally altered areas related to volcanic activities. Considerable difficulties have been encountered in the final interpretation of both electric resistivity and magnetic results, as the Tatun volcanic region is characterized by high relief and heterogeneous petrographic composition over most areas. The geophysical exploration will be described in another paper by W. T. Cr~ENG. One of the main objectives in the geothermal exploration of the Tatun area is to locate the upper surface of the basal Miocene sandstones below the mantle of the volcanic rocks. It is believed that large quantities of hot fluid could be stored in the porous zone at the contact of the volcanic and sedimentary rocks. Seismic refraction survey has been suggested to map the boundary between the andesitic volcanic rocks and the Miocene sandstones. Arrangements are being made for this type of seismic survey. After preliminary geological, geochemical, and temp.erature studies, four out of the 13 thermal areas in the Tatun volcanic region were given priority in the geothermal exploration project. They are, in order of field investigations: Tahuangtsui (including Shuangchungchi), Matsao, Szehuangtzeping (including Kengtzeping), and Chinshan. Shallow gradient wells were drilled in these four areas, the maximum depth being 160 meters. Deep exploratory wells were drilled in Tahuangtsue and Matsao to test the reservoirs, to study the underground structures and temperatures, and to measure the initial discharges. The general depth of the deep wells range from 500 to 1500 meters. Exploration in Tahuangtsui area Geothermal investigations in the Tahuan~tsui area commenced in March 1966. This area is at the southwest end of the thermal belt at an elevation of about 200 meters. This area was chosen because the hot springs there cover the greatest areal extent and the one-meter-depth survey revealed the highest surface temperature anomalies. In addition, this area is only 5 kilometers from Hsinpeitou and is readily accessible from Taipei city. The hot springs and fumaroles rise mostly from the Lower Miocene sandstones which are only partly covered by thin hydrothermally altered volcanic debris. The Lower Miocene white sandstones are interbedded with thin shale beds. The sediments are buried by volcanic tuff breccias and interbedded lava from the northeast. In Tahuangtsui 29 two-inch (1 7/8" to 3") gradient wells were drilled to depths ranging from 47 to 155 meters. The geological and temperature logs have been shown by CHEN (1967). He also separately compiled isothermal contour maps for 60, 80, 100 and 120°C in which two high centers were indicated, marking local ascent of steam or hot water at the northeast end
of the hot spring and fumarolie area. The highest temperature recorded in the gradient wells was 175 °C at 155 meters. Eight of the wells tapped steam at depths ranging from 39 to 155 meters, with temperatures from 120 to 175 °C. Five wells had discharges ranging from 0.4 to 2.6 tons per hour with enthalpies of 379-543 kcal/kg. The main producing horizons are in the coarse-grained sandstones. Both the pores and the fissures in the sandstones provide good conduits for the passage of steam. The discharges are mostly wet steam with high water content and pH value around 3. After the completion of the shallow gradient holes, three deep exploratory wells were drilled: E-101 (519.5 m), E-102 (630 m) and E-103 (1000 m), in which the maximum temperatures were respectively 198, 175 and 172 °C at depths of 250, 353, and 1000 meters. However these temperatures were measured only 9 hours after drilling ceased and, in E-103, about 153 hours later, so it can be assumed that the ground temperatures are higher than those measured at Tahuangtsui. Of the three deeper wells, E-101 discharges 5.4 tons per hour with 40% steam at a well-head pressure of 1.6 kg/cmZG, and E-102, 7.4 tons per hour with 10.1% steam at 1.2 kg/cm-~G. E-103 yields 35.6 tons per hour with about 12.2% steam, but it discharged for only a few days due to corrosion of the casing pipe by highly acid water. Corrosion tests are being made to find suitable corrosion resistant material for the pipes in this field. An electrical resistivity survey was made at Tahuangtsui during March and April of 1967, and another between August, 1967 and April, 1968 with increased electrode separations and regular traverse patterns. It was found that the sandstone formations have low resistivity and the zone of low resistivity is also consistent with the thermal area and the geological structures. A magnetic survey was conducted at the same time as the second resistivity survey was carried on. This survey showed that magnetic anomalies are associated with the andesites and pyroclastie rocks, and low values are in the hydrothermally altered area and the area of Tertiary sediments. E x p l o r a t i o n in M a t s a o area
The Matsao thermal area lies on the east of the Chihsingshan volcano and on the west of the Mafengshan volcano. Both volcanoes are composed of dark grey two-pyroxene hornblende andesite, but the former is much younger. The andesites alternate with pyroclastic rocks, and a number of eruptive stages can be differentiated. Between the two volcanoes is a flat, 300-400 meter wide valley floored by stratified volcanic detritus which has been hydrothermally altered. Steam emerges from the slopes and hot springs come out along the gullies going towards east and south. The western scarp of the Mafengshan is a possible fault scarp along which the western part of that volcano collapsed, pos-
sibly due to hydrothermal alteration. The location of the Matsao thermal activities may be in part related to this. A total of 14 gradient holes have been drilled in Matsao for temperature measurements, generally less than 160 meters deep. Three of the holes blow steam. One of these, G207, has the highest measured temperature of 174°C at 147.5 m depth, and produced 1.9 tons per hour with a steam ratio of 85% and a shutin well-head pressure of 6.6 kg/cm2G. Following the shallow drilling program, five exploratory wells have been drilled at the eastern base of the Chihsingshan and one at the western base of the Mafengshan. All these wells passed through mainly hornblende andesite of the Mafengshan type with pyroclastic material on top. The lava flow of the area has not been penetrated through and the depth to the basement is unknown. The six deep wells drilled in Matsao are E201 (572 m), E202 (441 m), E203 (1005 m), E204 (577 m), E205 (1500 m), and E206 (301.5 m). The maximum temperatures measured were respectively 236, 169,240, 208, 240, and 72 °C at depths of 544, 303, 1000, 577, 1500 and 300 meters. E201 discharges 3.3 tons per hour with a steam ratio of 77.2% and a shut-in wellhead pressure of 9.3 kg/cm-"G. E202 discharges 2.7 tons per hour with a steam ratio of 81% and a shut-in wellhead pressure of 6.0 Kg/cm'~G. E203 had a shut-in wellhead pressure of 12.0 k~/cm~-G, but, due to incomplete cementing of the casing, it produced only 1.8 tons per hour. E205 was completed early in October, 1969. The preliminary test shows that it discharges 13.15 tons per hour with a steam ratio of 83.4 at a well-head pressure of 0.9 kg/cm2G. E207 is still under drilling and is 190 meters deep at present. There is less sulfur in evidence at Matsao and the pH values of waters from the discharging deep holes range from 4.6 to 7. The high temperature and high steam ratio and the less acid waters seem to prove that Matsao is a favorable area for initial development of geothermal power. Based on the present knowledge of this geothermal region, Matsao has been considered as the most suitable area for producing steam to run a pilot plant of 10 MW or so. Resistivity and magnetic surveys have been conducted in Matsao. The lowest resistivity zone and the lowest magnetic anomalies appear to be close to the existing hydrothermal activity near the surface and gradually become deeper toward the center of the Chihsingshan volcano. Exploration areas
in
Szehuangtzeping
and
Chinshan
Investigations have not been so extensively carded out in Szehuangtzeping (includine Kengtzeping) and Chinshan. A total of 13 shallow gradient holes have been drilled in Szehuangtzeping and Kengtzeping. The depths vary between 42 and 151 meters, the highest temperature of 140°C was recorded at 97 meters in well G 302 153
located south of the visible hot area. Andesite of the Huangtsuishan type was encountered in most of these holes. The fumarolic areas in Szehuangtzeping and in Kengtzeping may be connected at depth as one single system. The drilling of a 1000-meter hole between these two areas is under way at present. The data from this hole may clarify the relationship between these two areas and may also indicate the temperature and chemistry of the deeper water feeding these areas. Only two shallow gradient holes have b ~ n drilled in Chinshan with depths ranging from 100 to 110 meters. The rocks encountered are mainly Lower Miocene sandstones. One hole recorded a maximum temperature of 106 °C at 95 m. A large m o u n t of hot water with pH value of 2 flushed o u t . f r o m the hole and caused much trouble to the drilling operation. The shallow drilling program has thus been suspended.
canic region by regional resistivity survey and deep drilling so that the development of geothermal power can be made on an extensive scale to insure large production. Development drilling has at present been concent'rated in the Matso area in the hope that sufficient quantities of steam can be produced for the establishment of a pilot plant of 10 to 20 MW to study the technical problems in the utilization of geothermal steam. The applications of seismic survey to determine the depth to the Miocene basement sandstone and of the dipole-dipole method to obtain great probing depth are being studied. Other important thermal spring areas in southern Taiwan will be systematically investigated to study their potentiality for geothermal exploration.
Conclusion
McNITT J. R. 1967 - - Report of the United Nations technical assistance mission on geothermal resources in China (Tai-
REFERENCES
Geothermal investigations in the Tatun volcanic region are generally carded out on the basis of individual thermal areas. There is a close relationship between past and present hydrothermal areas and the Ouaternary volcanism on a regional scale. This indicates that the geothermal resources are of reasonable magnitude and continuity, which warrant exploration on a large scale. Efforts will be made to study the relationships between the individual thermal areas in the Tatum vol-
154
wan), HEAL¥ |. 1968 ~ Geothermal investigations in Taiwan. MRSO Report, 85.
ToNGIOanl E., M~z^ F., F~t,R^ G. C., Vrso A. 1968 - Some considerations and suggestions on the possibility of exploration of geothermal energy in Taiwan. Ho C. S., LEE C. N. 1963 - - Economic minerals of Taiwan. Geol. Surv. Taiwan.
NAKAMURAH. 1967 - - Report on the survey of hot springs in Taiwan. MRSO. Wane D. E., TRUESDELLA. H. 1969 - - Geothermal resources of Taiwan. An evaluation of existing data. U.N. Rep. PALMASON G. 1969 ~ Geophysical exploration in the Tatun geothermal areas, Northern Taiwan. U.N. Rep.