Takigami geothermal system, northeastern Kyushu, Japan

Geothermics 29 (2000) 191±211 Takigami geothermal system, northeastern Kyushu, Japan Shigetsugu Furuya a, *, Michio Aoki a, Hiroki Gotoh a, Teruo Takenaka b a Idemitsu Oita Geothermal Co., Ltd, 2862-12, Nogami, Kokonoe-machi, Oita 879-4802, Japan b New Energy and Industrial Technology Development Organization (NEDO), 3-1-1 Higashi-Ikebukuro, Toshima-ku, Tokyo 170-6030, Japan Received 27 November 1997; accepted 30 August 1999 Abstract The Takigami geothermal reservoir is bounded by a system of faults and fractures oriented along two main directions, north to south and east to west. The Noine fault has a large vertical displacement and trends north to south, dividing the subsurface characteristics of resistivity, permeability, temperature and reservoir depth. The Takigami geothermal uid has a near neutral ph and is of the Na±Cl type, with a chloride content ranging from 400 to 600 ppm. The southwestern part of the area has the highest subsurface temperature, up to 2508C. The deep uid originates from the southwest, and ow is mainly to the north and partly to the east along faults and fractures, decreasing in temperature with increasing lateral ow. # 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. Keywords: Reservoir; Geology; Geochemistry; Takigami; Japan 1. Introduction Central Kyushu is cut by a volcano-tectonic depression that has developed within a tensile stress eld since the Neogene, resulting in Plio-Pleistocene to * Corresponding author. E-mail address: furuya@idemitsu-geo.co.jp (S. Furuya). 0375-6505/00/$20.00 # 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S0375-6505(99)00059-0

192 S. Furuya et al. / Geothermics 29 (2000) 191±211 Recent volcanism (Hase et al., 1985). The northeastern part of Central Kyushu, known as the Hohi region, is one of the most active geothermal areas of Japan (Fig. 1). This geothermal region is associated with a negative Bouguer anomaly referred to as the Shishimuta low-gravity area. The Takigami geothermal area is located on the eastern margin of this gravitational low (Fig. 2). Although the Takigami system lies within the very active Hohi geothermal region, there are no surface manifestations in the immediate area. The nearest hot springs are located 1±2 km north and east of the area (Fig. 3). Geothermal exploration in the Takigami area started in 1979 with various surveys and drilling, as discussed below. The 25 MW Takigami power plant opened in November 1996 (Fig. 3). Idemitsu Oita Geothermal Co., Ltd produces the geothermal steam and the Kyushu Electric Power Co., Inc. generates electrical power from this steam. The purpose of this paper is to review and discuss the geothermal structure of the Takigami area based on information obtained from surface surveys and well drilling. Fig. 1. Map of the Hohi geothermal region, northeast Kyushu, showing the location of the Takigami area, the major Quaternary volcanoes and the Otake±Hatchobaru geothermal area.

S. Furuya et al. / Geothermics 29 (2000) 191±211 193 Fig. 2. Compiled gravity map (by NEDO) in and around the Takigami geothermal area (Bouguer anomalies: density=2.3 g/cm 3, contour interval=5 mgals). 2. Geophysical exploration In and around the Takigami area, gravity and electromagnetic prospecting has been conducted since 1979, while resistivity logging began in 1981 (Aoki, 1988). The Takigami area is located on the eastern margin of a steep gravitational low. The easternmost part of the geothermal area extends to a local gravitational high, while the westernmost portion of the eld has a value close to 35 mgal (Fig. 2).

194 S. Furuya et al. / Geothermics 29 (2000) 191±211 Fig. 3. Map showing the location of geothermal wells and hot springs around the Takigami area. The cross-section line (X±X') for Fig. 4 is also shown. A north-to-south striking fault system was therefore thought to exist in the area, related to the large-scale volcano-tectonic depression (Kamata, 1985). Resistivity surveys and logging in wells show that the electrical structure is composed mainly of three layers. The surface ( rst) layer has a resistivity of 30± 500 ohm-m, the intermediate (second) layer has an extremely low resistivity of 1± 10 ohm-m, while the bottom (third) layer is relatively more resistive than the second layer, and ranges from 30 to 500 ohm-m. The three-layer structure extends laterally in the area. The second, conductive layer is shallow and thin in the east and deepens and becomes thicker to the west. This second resistivity structure

S. Furuya et al. / Geothermics 29 (2000) 191±211 195 Fig. 4. Generalized geologic cross-section through the Takigami area showing (a) stratigraphy, (b) distribution of the hydrothermal alteration, and (c) measured down-hole temperatures, deduced isotherms and location of the hot-water entries (modi ed from Yamamoto, 1988). Lithology is described in Table 1.

196 S. Furuya et al. / Geothermics 29 (2000) 191±211 Table 1 Stratigraphy and lithology of rocks in the Takigami geothermal area Time Formation Lithology Quaternary Pleistocene Noine-dake volcanic rocks Pyroxene-hornblende andesite/dacite lava and pyroclastics Kusu formation Pumice/sandy tu Ajibaru formation Hornblende andesite Lava/brecciated lava Takigami formation Plagioclase megacrystic dacite Lava/pyroclastics Andesite lava/pyroclastics Tu aceous sandstone/siltstone Neogene Pliocene Usa group Andesite lava/pyroclastics Dacite lava/pyroclastics Tu aceous sandstone/siltstone

S. Furuya et al. / Geothermics 29 (2000) 191±211 197 Fig. 5. Conceptual geologic structure of the Takigami area. The traces of wells and faults, and the locations of hot water entries to wells are shown.

198 S. Furuya et al. / Geothermics 29 (2000) 191±211 correlates with a layer of pyroclastic rocks and a zone of strong hydrothermal alteration. 3. Geologic setting The subsurface geology of the area was studied from drill cores and cuttings (Furuya, 1988; Yamamoto, 1988). A generalized geological cross-section of the Takigami area is shown in Fig. 4(a). A thick layer of Quaternary volcanic and associated rocks overlies the Tertiary Usa Group. The Quaternary volcanic rocks are classi ed into four formations from top to bottom, comprising the Noine-dake volcanic rocks, and the Kusu, Ajibaru and Takigami Formations. These Quaternary units consist of layers of andesitic or dacitic volcanic rocks. The Tertiary Usa Group is composed mainly of altered andesite lava ows and pyroclastic rocks that have been propylitically altered. Even the deepest well in the area, TT-1 (3000 m), does not reach the Pre-Tertiary basement rocks (Table 1). There are few outcrops of the deep formations at the surface because young volcanic products cover the area extensively. The conceptual geologic structure of the area is shown in Fig. 5. Two sets of fault/fracture systems were identi ed mainly from studies of lineaments and correlation of subsurface stratigraphy. One system strikes almost north to south, and the other east to west. The north-to-south trending Noine fault zone is important because it divides the area into eastern and western parts. The vertical displacement of the faults in this zone is estimated to be more than 1000 m, based on a comparison of the depths to the Usa Group on the western and eastern sides of the fault zone. The Takigami Formation that overlies the Usa Group is approx. 1000 m thick on the downfaulted western side, but it is thin (approx. 200 m) on the eastern side of the Noine fault zone [Fig. 4(a)]. The east-to-west striking faults were con rmed by a study of surface lineaments. One of these, the Teradoko fault, can be seen in the outcrop. These east-to-west striking faults are estimated to have smaller vertical displacements than those of the north-to-south striking faults. High permeability zones and uid discharge points in wells of the Takigami geothermal system appear to be located along both of these fault sets. 4. Hydrothermal alteration There are few areas of surface hydrothermal alteration. Samples of drill cores and cuttings from the wells were studied mainly by X-ray di raction analysis. Based on the classi cation of Utada (1980), the subsurface alteration zones consist mainly of intermediate alteration minerals [Fig. 4(b)]. This intermediate alteration type consists of four zones, i.e. the zeolite, montmorillonite, mixed-layer clay minerals, and illite plus chlorite zones. The locations of the upper limit of the zeolite minerals laumontite and wairakite mimic

S. Furuya et al. / Geothermics 29 (2000) 191±211 199 Fig. 6. Temperature pro le of NE-5 well. Two measured down-hole temperature pro les under static conditions (1,2), uid inclusion minimum temperatures (3), and boiling point-with-depth curve for water (BPCW) are shown. Note that the down-hole temperatures do not reach the boiling temperature at any depth. This is con rmed in all wells in the area. Stratigraphy and alteration zones are shown in the left columns, and the thermal structure of the three layers is shown to the right. ST=standing time.

200 S. Furuya et al. / Geothermics 29 (2000) 191±211 the current temperature contours, about 150 and 2008C, respectively. These zones are also controlled by the geologic structure [Fig. 4(a) and 4(b)]. The intermediate alteration minerals are likely to be formed by the present near-neutral ph geothermal uid that also precipitated vein minerals such as anhydrite, calcite and the zeolite group minerals. In particular, there is a large amount of laumontite and wairakite in and around productive fractures (Hayashi et al., 1988). Acid alteration minerals such as pyrophyllite and kaolinite are found in the area of the basin-like structure (Fig. 5). These acid minerals are inferred to have formed prior to the intermediate type of alteration, since the distribution of acid minerals is not concordant with the intermediate type of alteration minerals nor Fig. 7. Subsurface temperature distribution in the Takigami geothermal area.

S. Furuya et al. / Geothermics 29 (2000) 191±211 201 Table 2 Chemical compositions of liquid discharged from Takigami geothermal wells and hot springs around the area Sample (symbol) a Sampling temp. b (8C) ph (208C) Li Na K Mg Ca (ppm) Cl SO 4 HCO 3 CO 3 B SiO 2 TT-1 120 9.4 1.6 443 41.0 0.1 20.0 525 261 ± 50 4.8 437 TT-2 124 9.0 1.6 466 47.1 0.1 18.8 547 238 ± 53 5.6 365 TT-7 110 9.1 2.1 475 63.3 0.2 20.6 640 154 11 53 7.0 441 TT-8 100 9.2 1.7 488 57.4 0.1 12.3 585 251 13 47 5.2 473 TT-10 98 8.9 1.4 455 23.0 0.1 17.9 545 229 38 38 6.2 306 TT-13 121 8.9 2.8 514 93.3 0.1 14.2 758 108 36 36 9.1 675 TT-14 123 9.1 2.6 502 89.0 0.1 8.4 785 95 ± 53 9.8 593 TT-16 104 8.8 2.6 510 73.0 0.1 9.2 732 191 61 12 8.9 647 Noya (1) 38 9.1 ± 48 0.3 0.1 1.0 9 10 56 26 ± 45 Lakeside 3 (2) 68 8.0 ± 38 1.3 0.1 1.2 14 9 52 24 c 116 Lakeside 5 (3) 52 8.0 ± 34 1.0 0.6 2.4 9 5 96 ± ± 137 Okue (4) 48 7.6 ± 8 5.6 1.7 3.0 2 2 51 ± ± 109 a Chemical concentration of each sample analyzed by ICP spectrometer. b The liquid phase from geothermal wells was separated at the sampling temperature and collected after cooling. No steam lost from liquid after separation. c Less than 0.1 ppm.

202 S. Furuya et al. / Geothermics 29 (2000) 191±211 Table 3 Gaseous composition of the Takigami geothermal well discharges Gas composition in steam CO 2 H 2 S N 2 H 2 CH 4 Well name Date Separation temperature (8C) Gas in steam (vol. %) Gas in total discharge a (mmol/mol) TT-1 01/1984 120 0.05 85.1 5.5 7.9 0.04 0.8 0.10 TT-2 06/1987 124 0.07 75.7 7.0 15.9 0.2 0.9 0.11 TT-7 06/1987 110 0.08 84.7 9.0 5.3 0.06 0.77 0.17 TT-8 04/1988 100 0.12 71.8 5.9 21.0 0.1 0.51 0.27 TT-14 06/1987 123 0.10 83.1 11.0 4.5 0.09 1.2 0.22 TT-13 05/1987 104 0.11 67.6 4.1 27.5 0.09 0.7 0.29 a Total gas concentration in total discharge is calculated from gas concentration in steam and steam fraction at separation. Analytical method: gas in steam was absorbed by KOH solution. H 2 S and CO 2 analyzed by titration; other residual gases analyzed by gas chromatography.

S. Furuya et al. / Geothermics 29 (2000) 191±211 203 with the current temperature. These acid minerals occur only within the Takigami formation and the Usa Group. 5. Subsurface temperature The subsurface temperature distribution in the Takigami area was estimated from the results of temperature recovery tests, shut-in temperature and uid inclusion measurements. Homogenization temperatures of uid inclusions are particularly useful to estimate the pre-drilling subsurface temperatures at the points of hot-water entry (Browne et al., 1976). Taguchi (1982) showed that inclusions tended to have a wide range of homogenization temperatures at a given depth in less active geothermal systems, with the minimum temperature being quite close to the present temperature. In the Takigami area, the minimum homogenization temperature of uid inclusions at each depth agrees well with measured temperatures (Fig. 6). The thermal structure of the Takigami system is basically composed of three layers. The rst layer is isothermal (508C) due to cold meteoric water circulation. This layer corresponds to fresh and fractured Quaternary volcanic rocks. The second layer is impermeable and has a steep and constant thermal gradient of about 208C/100 m. The temperature pro le in this layer indicates that there is conductive heat ow (Fig. 6). This layer corresponds to the montmorillonite and mixed-layer clay zones. The third layer is characterized by high temperatures that range from 1608C in the northeast to 2508C in the southwest. Temperature logs of wells show convective pro les in the third layer. All measured and estimated temperatures lie below the boiling point curve of water, indicating there is liquid only in the natural system at the depths encountered by drilling. The third layer corresponds to the Tertiary andesite lava ows (the Usa Group), and correlates approximately with the illite-chlorite zones. The subsurface temperature tends to increase towards the southwest, with the depth of the third layer becoming deeper. The temperature distribution indicates that the maximum temperatures encountered by drilling in the area increase from the northeast (reservoir temperature of 160±1708C) to the southwest (240±2508C), with the hottest zone in the vicinity of wells TT-13 and TT-14 (Fig. 7). 6. Fluid chemistry A geochemical model of the Takigami area was discussed by Takenaka and Furuya (1991). Here we summarize the essential uid chemistry of well discharges. The composition of well discharges, discharge enthalpies and sampling temperatures are shown in Table 2. The compositions of discharged steam from the production wells are listed in Table 3. The Takigami geothermal uid has a near-neutral ph and is of the Na±Cl type

204 S. Furuya et al. / Geothermics 29 (2000) 191±211 Fig. 8. (a) Relative Na, K, Mg and (b) Cl, SO4, HCO3 concentration of waters listed in Table 2. The full equilibrium line and the region of immature water are from Giggenbach (1988). The geothermal wells are shown as solid circles, hot springs as open circles.

S. Furuya et al. / Geothermics 29 (2000) 191±211 205 (Fig. 8; Table 2). The chloride concentration of the reservoir uid is dilute, ranging from approx. 400 to 600 ppm (Fig. 9). This relatively low salinity and the low non-condensable gas concentration in steam characterize the Takigami geothermal uids (Tables 2 and 3). All hot springs around the Takigami area are rich in bicarbonate. The low chloride concentrations of these springs indicate that they are heated but immature water [Fig. 8(a); Table 2], with only a small component of deep uid in these surface discharges [Fig. 8(b)]. The relative anion compositions of the well discharges show a linear relation, with the relative concentration of sulfate increasing with decreasing chloride concentration [Fig. 8(b)], indicating that the diluent is relatively sulfate-rich. The well discharges of high chloride and low sulfate concentrations have high enthalpies at the southwestern part of the system. On the other hand, discharges of low chloride and high sulfate correspond to lower discharge enthalpies associated with wells in the central to northeastern part of the system. An increase in the solubility of anhydrite with decreasing temperature may be partially responsible for the increase in sulfate associated with the decrease in temperature, since there is no evidence for a sulfate-rich uid in the system at present. The anhydrite chemical geothermometer indicates temperatures similar to other chemical geothermometers and to the measured and enthalpy temperatures (Table 4), indicating the geothermal uids are just saturated with anhydrite. The Takigami reservoir varies systematically in uid composition and in Fig. 9. Chloride±enthalpy diagram of initial discharge compositions; C values are corrected for steam loss from measured enthalpy temperatures. The e ect of dilution by water at about 1008C can be seen.

206 S. Furuya et al. / Geothermics 29 (2000) 191±211 Table 4 Comparison of discharge enthalpy, enthalpy temperature, and temperatures measured and estimated from uid inclusion and chemical geothermometers Sample Enthalpy (kj/kg) T E a (8C) T meas T FI b T QA T CH c T NKC T CaSO4 c TT-1 871 212 212 207 216 209 194 198 TT-2 846 199 205 203 204 196 201 200 TT-7 946 221 217 209 216 203 213 217 TT-8 925 216 215 ± 222 212 215 215 TT-10 758 179 169 184 191 180 163 200 TT-13 1030 239 246 237 248 250 244 235 TT-14 1013 235 238 234 245 236 248 250 TT-16 1013 235 241 233 245 241 232 234 a Quartz (T QA ), chalcedony (T CH ) and alkali (T NKC ) geothermometers are from Truesdell (1983) and Giggenbach (1988), and anhydride (T CaSO4 ) from Marshall and Slusher (1968). T E means temperate estimated from total enthalpy of discharged uid. b These homogenization temperatures are minimum values of measurements. c Temperatures for geothermal wells are calculated from the total discharge concentration. temperature from southwest to northeast (Figs. 8 and 9). A chloride-enthalpy diagram helps to determine geothermal processes, such as mixing, boiling and conductive heat loss (Fournier, 1977). Chloride concentrations of the well discharges are corrected for steam loss, and the enthalpy of the deep uid, as plotted, is derived from the measured temperature of the feed point of the well, or is calculated from geothermometry of the well discharges (Table 4). The reservoir compositions of uid discharged from the wells de ne a dilution trend. The highest chloride water occurs at a temperature of about 2508C, whereas extrapolation of the dilution trend to zero chloride indicates a diluent at a temperature of about 1008C (Fig. 9). The southwestern part of the reservoir, with uid represented by the discharge of TT-14, has the highest salinity (about 600 ppm Cl) and the highest temperature (about 2508C) in the Takigami geothermal system. This reservoir is located in the western, downfaulted zone and is regarded as the parent reservoir uid in the geothermal system. The uid in the central part of the reservoir has a lower salinity and enthalpy compared to that in the southwest. The chloride concentration is about 450 ppm and the temperature is about 2008C. The lowest enthalpy uid comes from TT-10 in the northern area of the Noine fault, and has a low chloride content (Figs. 5 and 9). 7. Framework of the Takigami geothermal system The characteristics of the Takigami geothermal system are closely related to the geological, mineralogical and hydrological control on the three thermal layers, which in turn determine the subsurface thermal structure. The highly permeable fracture system in the third layer, which is regarded as a fractured type of

S. Furuya et al. / Geothermics 29 (2000) 191±211 207 Fig. 10. (a) Temperature distribution at 1000 m below sea level. (b) The distribution of chloride concentration obtained from well discharges. There are no chemical data in the northern part. The chloride content of well NE-5 is almost the same as NE-11. The iso-chloride contours partially cross the isotherms because the elevation of feed points is di erent from those for the isotherms plotted in the gure.

208 S. Furuya et al. / Geothermics 29 (2000) 191±211 reservoir, is the channel for geothermal uid ow and provides feed points to the wells. The present subsurface horizontal uid ow pattern in the Takigami area has been analyzed on the basis of geothermal indicators. The distribution of chloride concentrations and the temperatures in the deep reservoir are shown in Fig. 10. The iso-chloride contours indicate that the chloride concentrations in the reservoir uid decrease from southwest to northeast, as do the temperatures. The subsurface uid probably ows in the same direction, southwest to northeast, and maintains chemical and thermal equilibrium with alteration minerals (Takenaka and Furuya, 1991). From the view point of reservoir engineering, the Takigami geothermal system is best described as having two parts, eastern and western. There are signi cant di erences in terms of subsurface temperature distribution, depth of fractured reservoirs, and permeabilities. The boundary is probably the Noine fault zone. The eastern part of the reservoir system is shallow (700±1100 m depth) and has Fig. 11. Three-dimensional block model of the Takigami geothermal system (view from the southeast).

S. Furuya et al. / Geothermics 29 (2000) 191±211 209 well-developed fractures giving a high permeability (50±100 darcy-m). The temperature ranges from 160 to 2108C. On the other hand, the western part of the reservoir is deeper (1500±2000 m depth) and has a lower permeability (5±30 darcy-m) and higher temperature (230±2508C). This two-part reservoir accounts for the di erence in the chemical composition of the geothermal uid, and the variation in the hydrology and thermal structure of the system (Takenaka et al., 1995). The reservoir evaluation of the Takigami system was made ve times (phase 1 to phase 5) before starting commercial operation in 1996. The reservoir evaluation of phase 4 (in 1987) was composed of long-term production-injection tests and a 3-dimensional simulation to predict the change of pressure and temperature in the system following exploitation (Fig. 11). The geothermal reservoir corresponds to the same layer for production and reinjection, but communication between them is small due to the low permeable zone suggested from the interference tests of wells. The low permeable zone is estimated to be located on the northeastern side Fig. 12. Schematic cross-sectional model of the Takigami geothermal system (modi ed from Takenaka and Furuya, 1991).

210 S. Furuya et al. / Geothermics 29 (2000) 191±211 of the Noine fault, which results in the di erent trend of uid chemistry for TT-10 and other wells (Fig. 9). Based on these tests, it was concluded that a production of 1850 t/h of geothermal uid should be able to be maintained for more than 30 years with little drop in reservoir pressure and temperature in the production zone (Gotoh, 1990). Fig. 12 shows the schematic model of the Takigami geothermal system in a southwest to northeast cross-section. 8. Conclusions The characteristics of the Takigami geothermal system can be summarized as follows (Fig. 12): Two types of fault/fracture occur in the area. One has a northto-south strike with large displacement (Noine fault zone). The other has an eastto-west strike with small displacement associated with the movements of faults. The geothermal reservoir is divided into two parts, the deeper, higher temperature southwest portion and the cooler, more shallow and fractured northeast portion. The southwestern reservoir uid has a high temperature (about 2508C), high salinity (Cl, 600 ppm) and ascends laterally towards the northeast. The uid in the central-to-northeastern portion of the reservoir has a moderate temperature (range from 200 to 2108C) and salinity (Cl, about 450 ppm). Based on the uniformity of calculated and measured temperatures, chemical and thermal equilibrium is maintained in the reservoir despite the relatively low temperatures. The geothermal uid ows toward the north and northeast from the southwest. There is no evidence for any signi cant boiling. During the lateral ow the hot uid mixes with a low-chloride, sulfate-rich water that has a temperature of about 1008C. The montmorillonite zone acts as a cap rock and prevents shallow circulating groundwater from penetrating deeper into the geothermal reservoir. Acknowledgements The authors thank Dr M. Sasada and Dr J.W. Hedenquist of the Geological Survey of Japan for their valuable suggestions and encouragement during this study. They also thank Professor M. Hayashi (Kyushu Sangyo University), Professor S. Taguchi (Fukuoka University) and sta members of the Idemitsu Oita Geothermal Co., Ltd for their contributions. References Aoki, M., 1988. Geophysical exploration at Takigami geothermal eld in Oita Prefecture, Japan, Abst., Int. Geothermal Symp., Kumamoto and Beppu, Japan 567±568. Browne, P.R.L., Roedder, E., Wodzicki, A., 1976. Comparison of past and present geothermal waters from a study of uid inclusions, Broadlands eld, New Zealand. In: Proc. Int. Symp. Water±Rock Interaction, Prague 1974, pp. 140±149.

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