Fluid flow processes in the BEPPU geothermal system, Japan

The Beppu geothermal system is centred beneath the late Quaternary volcanoes of Tsurumi and Garandake at the northern end of the Ryukyu volcanic arc. The deep fluid has a temperature of at least 250–300°C, and an inferred chloride concentration of 1400–1600 mg/kg. Apart from fumarolic areas near the summits of the two volcanoes, most thermal activity occurs at low elevation along the two main outflow paths towards the coast. The hot spring waters of downtown Beppu have originated from outflow along the Asamigawa Fault, with their chemistry indicating predominantly dilution of the deep fluid by groundwater. The second outflow zone towards the hot spring area of downtown Kamegawa coincides with a ridge of lavas. Here boiling, steam loss, and subsequent mixing with steam-heated groundwaters have significantly modified both the deep fluid and host rocks. The area of the geothermal system above 200°C is at least 15 km² at sea level, and the total natural heat output is inferred to be at least 250 MW. Most of this heat output occurs as subsurface hot water outflows towards the coast due to the 1300 m of topographic relief across the system.     

Long-term changes associated with exploitation of the Beppu hydrothermal system, Japan

Exploitation of the Beppu hydrothermal system started mainly in the lowland areas as early as the 1880s, and by the 1920s the number of wells had increased to about 1000. This caused the piezometric head of the thermal groundwater to draw down, and seawater to intrude into the thermal groundwater aquifer near the coast. A second flurry of exploitation occurred during the 1960s–70s, by which time there were over 2300 wells, and the mass and heat flows had increased due mainly to the discharge of high-temperature chloride waters in the highland areas. This caused a decline in piezometric head of the deep chloride water, a decline in the subsurface flow of chloride water towards the lowlands, and intrusion of steam-heated shallow water into the chloride water layer.     

An oxygen isotope study of silicates in the larderello geothermal field, Italy

Stable-isotope analyses were carried out on hydrothermal minerals sampled from the deep metamorphic units at Larderello, Italy. The ∂¹⁸O values obtained for the most retentive minerals, quartz and tourmaline, are from + 12.0‰ to + 14.7‰ and 9.9‰, respectively, and indicate deposition from an ¹⁸O-rich fluid. Calculated ∂¹⁸O values for these fluids range from + 5.3‰ to + 13.4‰. These values, combined with available fluid inclusion and petrographic data, are consistent with the proposed existence of an early thermal fluid of probable magmatic origin and a late meteoric water. Mixing between these two fluids occurred locally.     

Isotopic and chemical composition of parbati valley geothermal discharges, North-West Himalaya, India

The isotopic compositions of the waters discharged from Parbati Valley geothermal areas indicate a higher altitude meteoric origin, with discharge temperatures reflecting variations in the depth of penetration of the waters to levels heated by the existence of a ‘normal’ geothermal gradient. On the basis of mixing models involving silica, tritium, discharge temperatures and chloride contents, deep equilibration temperatures of 120–140°C were obtained for Manikaran, possibly reaching 160°C at even greater depth. Geothermometers based on sulfate-water ¹⁸O exchange and gas reactions point to similar temperatures. Exceptionally high helium contents of the discharges correspond to apparent crustal residence times of the waters in the order of 10–100 Ma; relative nitrogen-argon contents support a largely meteoric origin of the waters with a possible fossil brine, but no detectable magmatic component.     

Fluid-inclusion evidence for previous higher temperatures in the miravalles geothermal field, Costa Rica

Heating and freezing data were obtained for liquid-rich secondary fluid inclusions in magmatic quartz, hydrothermal calcite and hydrothermal quartz crystals from 19 sampled depths in eight production drill holes (PGM-1, 2, 3, 5, 10, 11, 12 and 15) of the Miravalles geothermal field in northwestern Costa Rica. Homogenization temperatures for 386 fluid inclusions range from near the present measured temperatures to as much as 70°C higher than the maximum measured well temperature of about 240°C. Melting-point temperature measurements for 76 fluid inclusions suggest a calculated salinity range of about 0.2–1.9 wt% NaCl equivalent. Calculated salinities as high as 3.1–4.0 wt% NaCl equivalent for 20 fluid inclusions from the lower part of drill hole PGM-15 (the deepest drill hole) indicate that higher salinity water probably was present in the deeper part of the Miravalles geothermal field at the time these fluid inclusions were formed.     

Isotopic and chemical evidence for water sources and mixing in the Cerro Pando geothermal area, Republic of Panama

The Cerro Pando geothermal area in Chiriqui Province is situated just to the south of the continental divide in western Panama. Three groups of thermal springs are associated with lineations in a complex of late Tertiary and Quaternary extrusives. Spring temperatures reach maximum values of 66°C at Los Pozos, 67°C at Cotito and 41°C at Catalina; flow-rates are low, ranging up to 1.5 l/s. However, total heat output is estimated at around 7 MW from calculations incorporating measured spring discharges with river-bed discharges inferred from stream conductivity anomalies. In all cases the spring hydrochemistries become of a more dominantly Na-Cl character as mineralization increases; the highest salinities are found in samples from test boreholes in which 4500 mg/l Cl⁻ has been measured. ¹⁸O/¹⁶O and ²H/¹H data for all thermal springs are roughly colinear, plotting on the δ-diagram with a slope around 3.6 and intersecting the meteoric water line within the compositional range of local surface water. Moreover, δ¹⁸O data are found to correlate with Cl⁻ concentrations, although separate linear trends represent the Los Pozos/Cotito and the Catalina groups of springs. These data are interpreted as indicating that deep thermal ground water feeds the thermal spring systems, with differing isotopic compositions and/or salinities in the Los Pozos/Cotito and Catalina groups. These end-member compositions have evolved by sub-surface steam loss, possibly without any δ ¹⁴O shift due to water - rock exchange. The observed spring compositions are all mixtures between the deep thermal and shallow cool end-member ground waters. The importance of resolving mixing relationships before applying geothermometric calculations is illustrated.     

Oxygen and hydrogen isotopes in deep thermal waters from the south meager creek geothermal area, british columbia, canada

Deuterium and oxygen-18 (¹⁸O) have been measured in deep thermal, shallow thermal and non-thermal water samples collected at various times between 1982 and 1989 from the Meager Creek area, with the aim of assessing the origin of the thermal waters. The isotopic composition of the reservoir waters (δ¹⁸O = −13‰ and δD = −114.8‰) was calculated from data on post-flash deep thermal waters, using a two-stage steam loss model. The reservoir composition shows an oxygen shift of 2.4‰ relative to the local meteoric water line. The composition of the recharge, obtained by removing the oxygen shift, is isotopically heavier than the average local meteoric waters, suggesting that the recharge may be from an area to the west of Mt Meager where isotopically heavier ground-waters are likely to be found. The small δ¹⁸O shift of the deep high-temperature waters is indicative of dominance of fracture-related permeability in the reservoir. Analysis of the chemistry and the temperature of the waters from hot springs and shallow thermal wells suggests that these waters have evolved from the deep geothermal waters through dilution by meteoric waters and about 40°C adiabatic cooling (steam loss).     

Hydrothermal alteration in the Aluto-Langano geothermal field, Ethiopia

The hydrothermal mineral assemblages found in eight wells (with a depth range of 1320–2500 m) of the active geothermal field of Aluto-Langano (Ethiopia) indicate a complex evolution of water-rock interaction processes. The zone of upflow is characterized by high temperatures (up to 335°C) and the presence of a propylitic alteration (epidote, calcite, quartz and chlorite, as major phases) coexisting with calcite and clay minerals. The zone of lateral outflow is characterized by mixing of deep and shallow waters and the occurrence of a calcite-clay alteration that overprints a previous propylitic assemblage. Clay minerals have a mushroom-shaped zonal distribution consistent with the present thermal structure of the field. Microprobe analyses have been carried out on chlorite and illite in order to apply several geothermometers. Most of the chlorite is iron-rich chlorite. It is found that the temperatures calculated from the chlorite geothermometer (159–292°C) after Cathelineau and Nieva [Contrib. Mineral. Petrol. 91, 235–244 (1985)] are in good agreement with in-hole measured temperatures (155–300°C). In the upflow zone, temperatures calculated from this geothermometer (217–292°C), together with fluid inclusion data of Valori et al. [Eur. J. Mineral. 4, 907–919 (1992)], and computed saturation indices of alteration minerals, indicate thermal stability or slight heating. On the other hand, evidence of a significant cooling process (up to 171°C) in the outflow zone is provided by the comparison between fluid inclusion homogenization temperature (240–326°C) and in-hole temperature (155–250°C). The apparent salinities (0.8–2.3 wt% NaCl eq.) of the fluid inclusions are generally higher than the salinity of the present reservoir fluid (0.29–0.36 wt% NaCl eq.). Clay minerals (illite, smectite, Ill/S mixed layers, vermiculite and chloritic intergrades) generally occur at temperatures consistent with their stability fields.     

The origin of hypersaline liquid in the quaternary kakkonda granite, sampled from well WD-1a, kakkonda geothermal system, Japan

Hypersaline metal-rich liquid (ca. 40 wt% total chloride species) was obtained from a depth of 3708 m in the Kakkonda geothermal system. Sampling of well WD-1a was conducted by reverse circulation after a standing time of about 196 hours (with temperature recovering to >500°C). Tritium content and the relationship between δD and δ¹⁸O showed that the river water that was circulated in the well had mixed with an isotopically heavy fluid during the standing time. Phase separation occurred during temperature recovery, concentrating the hypersaline liquid in the bottom of the well. This original hypersaline liquid has a salinity of about 55 wt% NaCl eq., consisting of Na–Fe–K–Mn–Ca chloride, rich in Zn and Pb but poor in Cu, Au and Ag. The fluid originates from the Kakkonda granite and mixed with circulating water from the well in a zone of fine fractures induced by thermal stress during drilling.     

Hydrology of the Greater Tongonan geothermal system, Philippines, as deduced from geochemical and isotopic data

Fluids in the Greater Tongonan geothermal system exhibit a large positive ¹⁸ O shift from the Leyte meteoric water line. However, there is also a significant shift in ²H. The δ²H−δ¹⁸O plot shows that the geothermal fluids may be derived by the mixing of meteoric water (δ¹⁸O = −7‰ and δ²H = −41‰) with local magmatic water (δ¹⁸O = 10 ± 2‰ and δ²H = −20 ± 10‰). The most enriched water in the Greater Tongonan system, in terms of δ¹⁸O, δ²H and Cl, is comprised of approximately 40% magmatic water. Baseline isotope results support a hydrogeochemical model in which there is increasing meteoric water dilution to the southeast, from Mahiao to Sambaloran and towards Malitbog. The Cl−δ¹⁸O plot confirms that the geothermal fluid in Mahanagdong, further southeast, is distinct from that of the Mahiao-Sambaloran-Malitbog system.     

Geothermal resources of Hachijojima

Hachijojima is a gourd-shaped volcanic island in the Pacific Ocean. Nishiyama and Higashiyama volcanoes consist of basalt lava and associated pyroclastic rocks. A promising geothermal resource was found in south Higashiyama, associated with an uplift of Tertiary rocks consisting of mainly andesite lava and related pyroclastic rocks, overlain by Quaternary volcanic rocks. Steep high-temperature (over 250°C) and high-pressure gradients occur in the deeper portion of the system near the Tertiary–Quaternary contact, indicating the presence of a cap rock. The cap rock formed by deposition of hydrothermal minerals. Geothermal fluid ascends from the deeper portions to shallow depths along vertical fractures through the cap rock. These vertical fractures form the geothermal reservoir in the Tertiary formation. Three wells were drilled into these vertical fractures, and approximately 30 t/h of superheated steam was obtained from each well during flow tests. The geothermal fluid is mainly a mixture of seawater and meteoric water in an approximate ratio of 1 to 2, based on chemical analyses, with a portion of volcanic gas included. At present a 3.3 MW_e, geothermal power plant is being constructed here.     

Chemical and isotopic composition of geothermal discharges from the Puyehue-Cordón Caulle area (40.5°S), Southern Chile

The Puyehue-Cordón Caulle area (40.5°S) hosts one of the largest active geothermal systems of Southern Chile, comprising two main thermal foci, Cordón Caulle and Puyehue. Cordón Caulle is a NW-trending volcanic depression dominated by fumaroles at the top (1500 m) and boiling springs at the northwest end (1000 m). In the latter, the alkaline-bicarbonate composition of the springs with low Mg (<0.06 mg/l) relative to the local meteoric waters (5 mg/l), low chloride (<60 mg/l), high silica (up to 400 mg/l) and δ¹⁸O–δD values close to the Global Meteoric Water Line (GMWL), in combination with the large outflow (100 l/s), suggest the existence of a secondary steam-heated aquifer overlying a main vapor-dominated system at Cordón Caulle. Subsurface temperatures of the secondary aquifer are estimated to be about 170–180 °C (corrected silica geothermometers). The Puyehue thermal area, on the other hand, includes Mg-rich hot springs discharging along stream valleys, with maximum temperatures of 65 °C and a δ¹⁸O–δD signature resembling the local meteoric composition, which suggests that the surface manifestations contain a reservoir component that is strongly diluted by meteoric waters. Topographic/hydrologic and chemical characteristics suggest that Cordón Caulle and Puyehue represent two separate upflows.     

Isotopic evidence for magmatic and meteoric water recharge and the processes affecting reservoir fluids in the palinpinon geothermal system, Philippines

Stable isotopic compositions of meteoric and geothermal waters indicate that the Palinpinon geothermal system of Southern Negros is fed by a parent water that originated from a mixture of local meteoric (80%) and magmatic (20%) waters. The meteoric water has an isotopic concentration of −8.5‰ and −54‰ in ¹⁸O and ²H, respectively, which corresponds to an average infiltration altitude of about 1000 m above sea level. With exploitation of the system and injection of wastewaters to the reservoir, the stable isotopic composition became heavier due to significant mixing of geothermal fluids with injection waters. Incursion of cooler meteoric waters, which is confirmed by the presence of tritium, also leads to the formation of acid-sulfate waters. Stable isotopes are effective as “natural tracers” to determine the origin and mixing of different fluids in the reservoir.     

Chemical and isotopic changes in shallow groundwater caused by exploitation of the Wairakei geothermal field, New Zealand

Chemical data obtained in 1983/84 and 1994 from groundwater monitor wells at Wairakei are reviewed and interpreted in conjunction with physical changes in the deeper system. The composition of the waters is controlled by the proportion of deep chloride, shallow meteoric and steam heated components. The deep component dominates the central production area, a bicarbonate meteoric water is predominant in the peripheral areas, and steam-heated water is more evident in between, particularly near surface steam discharge and hot ground. The major change observed is a decrease of the deep component. Some wells show effects of evaporation.     

Geochemical study of fluids on Lesbos island, Greece

Sixty-five water samples and seven associated gas samples have been collected on Lesbos island. The lithology and structural setting have resulted in two main types of hydrological circulation: a shallow circulation hosting low-salinity cold waters and a deeper one, hosting high-salinity hot waters that often emerge in thermal springs near the coast. The cold waters are characterized by Ca(Mg)-HCO₃(SO₄) composition, while the thermal waters generally have an Na-Cl composition. The chemical features of the former can be explained by their circulation in the ophiolite-bearing phyllitic basement and volcanic rocks. Waters circulating in the ultramafic layers of the basement are richer in Mg than the waters whose circulation is mainly within marble levels or volcanic rocks. The Na-Cl thermal waters are characterized by salinities ranging from 1910 to 35,700 mg/kg. As indicated by previous hydrogeochemical and isotopic studies, the Na-Cl composition of the thermal waters on Lesbos is the result of mixing between shallow meteoric waters and marine waters. While interacting with the minerals of the geothermal reservoir, the saline waters retain the Na/Cl sea water ratio but become enriched in Ca²⁺ and depleted in Mg²⁺ with respect to sea water.Processes of hydrothermal alteration at depth are activated by a gas phase enriched in CO₂, which reaches the geothermal reservoir by rising along the deep fractures of the basement. Thermodynamic calculations based on hydrothermal alteration processes occurring at the estimated temperatures of the geothermal reservoir (about 120 °C) indicate that the thermal waters of Lesbos are in equilibrium with talc and dolomite.     

Rock-water interactions in the Fenton Hill, new Mexico, hot dry rock geothermal systems I. fluid mixing and chemical geothermometry

The chemistry of fluids circulated through an artificially-stimulated, hot dry rock (HDR) fractured geothermal reservoir system in granitic rock is described in terms of mixing phenomena, geothermometry, and approach to saturation with reservoir rock minerals. Based on the similar dynamic behavior of Na⁺, K⁺, Li⁺, CI⁻, and B species and other isotopic evidence, the presence of a concentrated in-situ pore fluid was identified. Mixing and displacement of this in-situ fluid with meteoric make-up water is responsible for the observed behavior of the major dissolved species in the circulated fluid of this HDR system.     

Hydrogeochemistry and geothermometry of Changal thermal springs,Zagros region,Iran

This study addresses the hydrogeochemistry of thermal springs that emerge from the Asmari limestone in a gorge at Changal Anticline in the vicinity of the Salman-Farsi dam. The Changal thermal springs vary in temperature between 28 and 40 °C. Chemical and isotopic compositions of the thermal waters suggest two distinct hydrogeological systems: a deep, moderate-temperature (∼40 °C) geothermal system recharged by deeply circulating meteoric waters, and a shallow cold aquifer system related to local groundwater. The source geothermal fluid temperature was calculated using different geothermometers and mineral saturation indexes. Based on chemical and isotopic data, it is hypothesized that: (1) mixing occurs between the ascending geothermal water and shallow cold water; (2) the resulting thermal waters reaching surface are a mixture of 80% local, shallow meteoric water and 20% geothermal water; and (3) the circulation depth of the meteoric water is about 1500 m. The thermal reservoir temperature is estimated to be between 70 and 80 °C according to calculations using different geothermometers and computation of saturation indices for different solid phases.     

Takigami geothermal system, northeastern Kyushu, Japan

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 fluid 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 250°C. The deep fluid originates from the southwest, and flow is mainly to the north and partly to the east along faults and fractures, decreasing in temperature with increasing lateral flow.     

Exploration of Lahendong geothermal field in North Sulawesi, Indonesia

This paper describes the progress made in developing the geothermal resources at Lahendong, North Sulawesi, Indonesia for utilization in power generation. Exploration of the whole region included a geophysical survey undertaken exclusively by the Volcanological Survey of Indonesia (VSI). A temperature survey at various depths was conducted through gradient boreholes. The results show that the area of anomalous temperature corresponds to the area of low resistivity revealed by the seismic survey. Two shallow exploratory boreholes (300–400 m) drilled by VSI confirmed the existence of the resources. The deep reservoir in Lahendong field extends over an area of 10 km²; the upper parts of the reservoir are presumed to be water dominated (temperatures in excess of 200°C) and to overlie a zone of hot chloride water at an undetermined depth. The potential of Lahendong field is estimated to about 90 MW.In Pelita IV (1984–1989), the fourth 5-year plan, the State Electricity Public Corporation plans to construct a 30 MW geothermal power-plant in the Lahendong field.     

Hydrology,hydrochemistry and geothermal potential of El Chichón volcano-hydrothermal system,Mexico

Fluid and heat discharge rates of thermal springs of El Chichón volcano were measured using the chloride inventory method. Four of the five known groups of hot springs discharge near-neutral Na–Ca–Cl–SO₄ waters with a similar composition (Cl ∼ 1500–2000 mg kg⁻¹ and Cl/SO₄ ∼ 3) and temperatures in the 50–74 °C range. The other group discharges acidic (pH 2.2–2.7) Na–Cl water of high salinity (>15 g/L). All five groups are located on the volcano slopes, 2–3 km in a straight line from the bottom of the volcano crater. They are in the upper parts of canyons where thermal waters mix with surface meteoric waters and form thermal streams. All these streams flow into the Río Magdalena, which is the only drainage of all thermal waters coming from the volcano. The total Cl and SO₄ discharges measured in the Río Magdalena downstream from its junction with all the thermal streams are very close to the sum of the transported Cl and SO₄ by each of these streams, indicating that the infiltration through the river bed is low. The net discharge rate of hydrothermal Cl measured for all thermal springs is about 468 g s⁻¹, which corresponds to 234 kg s⁻¹ of hot water with Cl = 2000 mg kg⁻¹. Together with earlier calculations of the hydrothermal steam output from the volcano crater, the total natural heat output from El Chichón is estimated to be about 160 MWt. Such a high and concentrated discharge of thermal waters from a hydrothermal system is not common and may indicate the high geothermal potential of the system. For the deep water temperatures in the 200–250 °C range (based on geothermometry), and a mass flow rate of 234 kg s⁻¹, the total heat being discharged by the upflowing hot waters may be 175–210 MWt.