Assessment of reservoir temperatures of thermal springs of the northern areas of Pakistan by chemical and isotope geothermometry

Chemical and isotope geothermometers, i.e. the Na–K, K–Mg, quartz and δ¹⁸O(SO₄–H₂O), have been applied to estimate the reservoir temperature of the thermal springs in the northern areas of Pakistan. The chemical types of the thermal waters and the effects of mixing of shallow cold water with the thermal end-members are discussed. These waters are neutral to slightly alkaline and have low dissolved contents. Sodium is the dominant cation in almost all the cases. In terms of anions, the hot waters of Budelas are of the SO₄ type, those of Tatta Pani are of mixed character (SO₄ and HCO₃), and the waters from the remaining areas show HCO₃ domination. An absence of tritium in Tatta Pani and Tato thermal springs indicates that they do not have any contribution of shallow young water. In the case of the Murtazabad springs, the wide range of tritium concentrations, negative correlations with surface temperature and Cl, and positive correlation between Na and Cl show that the shallow cold groundwater is mixing with thermal water in different proportions. For the mixed water of Murtazabad thermal springs, ‘isochemical modelling’ using the Na–K, K–Mg and quartz geothermometers indicates an equilibrium temperature in the range 185–200 °C. The δ¹⁸O(SO₄–H₂O) geothermometer gives relatively low temperatures for three springs, whereas two samples are close to the 185–200 °C temperature interval. The reservoir temperatures of Tatta Pani springs (100–120 °C), determined by Na–K and quartz geothermometers, are in good agreement. The δ¹⁸O(SO₄–H₂O) geothermometer gives a relatively higher range (140–150 °C) for most of the Tatta Pani springs. For Tato spring, the isotope and chemical geothermometers (except for the K–Mg) agree on an equilibrium temperature of about 170 °C. Reservoir temperatures of the remaining minor fields are not conclusive due to the lack of sufficient data.     

Measurement and interpretation of terrestrial heat flow in Israel

Sixty-eight new determinations of terrestrial heat flow in Israel have a range of 0.17-11.07 μcal/cm²s. The average value of deep conductive heat flow in the undisturbed complex of the Arabo-Nubian Massif is 0·94 μcal/cm²s; it is least affected by circulation of groundwater. This value is only slightly higher than the heat flow of 0·88 μcal/cm²s in the Levantine Basin of the Mediterranean Sea. Several values that exceed 2·0 μcal/cm²s are due either to (probable) deep hydrothermal activity or to small domal structures of the basement.Within the sedimentary sequence which blankets the crystalline massif, terrestrial heat flow is often redistributed by circulating groundwater. Recharge regions, particularly Judean-Samarian Galilee, where cool meteoric waters percolate into the subsurface have anomalously low heat flow, ranging from 0·17 to about 1·0 μcal/cm²s. Part of the original deep thermal flux in those regions is intercepted at moderate depths by the recharge flow, and is carried into deeper aquifers of the Foothills, Coastal Plain, or the Jordan-Dead Sea Rift. Movement of groundwater occurs mainly along faults.Deep faults associated with the Jordan-Dead Sea Rift system act as conduits for hot waters ascending from deep confined aquifers. The most tangible surface expression of the convective hydrothermal system are the numerous warm to hot springs, emerging along the margins of the Rift. However, the waters emerging on the surface as the warm and hot springs are a minor fraction of the convective system. Most of the ascending thermal waters are absorbed by shallow aquifers with lower hydraulic potential. Such regions are characterized by anomalously high heat flow; several values exceed 2 and one value is 11 μcal/cm²s.     

Hydrogeochemical exploration of geothermal prospects in the Tecuamburro Volcano region, Guatemala

Chemical and isotopic analyses of thermal and nonthermal waters and of gases from springs and fumaroles are used to evaluate the geothermal potential of the Tecuamburro Volcano region, Guatemala. Chemically distinct geothermal surface manifestations generally occur in separate hydrogeologic areas within this 400 km² region: low-pressure fumaroles with temperatures near local boiling occur at 1470 m elevation in a sulfur mine near the summit of Tecuamburro Volcano; non-boiling acid-sulfate hot springs and mud pots are restricted to the Laguna Ixpaco area, about 5 km NNW of the sulfur mine and 350–400 m lower in elevation; steam-heated and thermal-meteoric waters are found on the flanks of Tecuamburro Volcano and several kilometers to the north in the andesitic highland, where the Infernitos fumarole (97°C at 1180 m) is the primary feature; neutral-chloride hot springs discharge along Rio Los Esclavos, principally near Colmenares at 490 m elevation, about 8–10 km SE of Infernitos. Maximum geothermometer temperatures calculated from Colmenares neutral-chloride spring compositions are 180°C, whereas maximum subsurface temperatures based on Laguna Ixpaco gas compositions are 310°C. An exploration core hole drilled to a depth of 808 m about 0.3 km south of Laguna Ixpaco had a bottom-hole temperature of 238°C but did not produce sufficient fluids to confirm or chemically characterize a geothermal reservoir. Hydrogeochemical data combined with regional geologic interpretations indicate that there are probably two hydrothermal-convection systems, which are separated by a major NW-trending structural boundary, the Ixpaco fault. One system with reservoir temperatures near 300°C lies beneath Tecuamburro Volcano and consists of a large vapor zone that feeds steam to the Laguna Ixpaco area, with underlying hot water that flows laterally to feed a small group of warm, chloriderich springs SE of Tecuamburro Volcano. The other system is located beneath the Infernitos area in the andesitic highland and consists of a lower-temperature (150–190°C) reservoir with a large natural discharge that feeds the Colmenares hot springs.     

Geochemistry of thermal waters on the island of Ischia (Campania, Italy)

The stratigraphic and structural situation on the island of Ischia (southern Italy), the recent volcanic activity and the presence of hot springs and fumaroles, suggest the existence of a geothermal field. The chemical and isotopic compositions of the waters from several springs and wells were examined to obtain information on deep temperatures and to formulate a geothermal model of the island. δD values range from −33.60 to −12.50‰ and δ¹⁸O from −7.10 to −1.71‰, relative to SMOW. These variations have mainly been attributed to the presence of seawater, as confirmed by the general shift to more positive values with the increase of Cl content. Water-rock reactions, evaporation and subsurface boiling also contribute to the δ¹⁸O−δD trend. The chemical analyses reveal the presence of alkaline sulphate chloride water (seawater), bicarbonate waters and waters interpreted as the result of mixing. The chemical and isotopic composition of the latter are dependent on water-rock interactions, water circulation rates and eventual evaporation and condensation phenomena. The silica geothermometer, which seems to be the most suitable for determining the deep temperatures of these waters, gave values of about 200°C, even for mixing models. Our data suggest the following geothermal model: the heat flow heats up a deep reservoir, causing steam to rise through faults and fractures and transfer heat to a shallower aquifer. The temperatures of 200°C obtained by the geothermometers are not the maximum reservoir temperatures, but are probably water-rock equilibrium temperatures for the shallower aquifers. The high boron contents and the isotopic data confirm the presence of steam in the system.     

Preliminary studies of some geothermal areas in India

In order to assess the geothermal resources of the hot springs located in different tectonic regions of India, preliminary geophysical, geochemical and tritium studies were undertaken in Puga valley (Ladakh), Ratnagiri and Kolaba Districts (West Coast) and Bhimbandh (Bihar) areas. The studies indicate that out of the three areas investigated, the Puga valley is the most promising because of its higher geothermal gradient, association of spring waters with magmatic components, its higher estimated reservoir temperature (≥ 200°C) and probable larger available supply of groundwater.     

Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215±10 °C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean T_h of 213 °C; 0.2–0.4 wt.% NaCl_eq.). Relict quartz–adularia–illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for “pulses” of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T_h values of 210±40 °C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with T_h values of 201±6 °C. Low-pH fluids in the near-surface setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous “silica residue”.     

Sodium/lithium ratio in water applied to geothermometry of geothermal reservoirs

We propose here a new geothermometer for natural waters. Analyses from many explored geothermal fields allow us to define two empirical thermometric relationships.One is for waters of low to moderate salinity (Cl⁻< 0·3 M) log Na/Li = 1000/T −0·38 and one for marine waters and brines (Cl⁻ > 0·3 M) log Na/Li = 1195/T + 0·38 These relationships, which at present are not well understood, result mainly from the increase of Li concentrations in waters with temperature.Equation (a) proved to be adequate for spring waters from mostly known geologic origin; this is an important feature in geochemical surveys for geothermal prospecting.Furthermore, when comparison between springs and drillhole chemistry of a given geothermal field is possible, the Na/Li geothermometer gives more reliable temperature estimates from the spring compositions than do classical geothermometers.     

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.     

The geothermal zone of lake Assal (F.T.A.I.), geochemical and experimental studies

A geothermal model of Lake Assal region (F.T.A.I.) was derived from in situ observations and chemical and isotopic studies of hot and cold springs.According to this model, the lake water supply is provided mainly by sea water infiltrating from Ghoubbet el Kharab and, for a small part, by highly mineralized hot spring water of meteoric origin.Temperatures at depth were determined by means of various methods (among them a new one). These temperatures were found to range from 150 to 170°C.A geochemical experimental study of this model is under way.     

The thermal springs of bockfjord, svalbard: occurrence and major ion hydrochemistry

The Troll and Jotun thermal springs of northern Svalbard, with temperatures of up to 25.6°C, are derived from a major fault forming the junction between Devonian sandstones and Proterozoic marbles, mica schists and gneisses. The Troll waters are dominated by Na–HCO₃ compositions and the Jotun waters by Na–Cl compositions. The pristine thermal water source has a sub-neutral pH and is highly reducing. Taken at face value, common geothermometers suggest temperatures at depth of 130–180°C for the Troll springs (corresponding to a depth of 1.6–2.3 km), with 10–30% thermal water diluted by 70–90% cold water. Such geothermometers may, however, be inappropriate to the cool, high CO₂ waters of Bockfjord, and real temperatures at depth and dilution factors are probably considerably lower. The salinity of the thermal water appears to be only partially derived from water–rock interaction; Br\Cl ratios suggest that seawater or possibly evaporites may be a source of chloride salinity.     

Development and role of geothermal resources in India

The geothermal resources discovered in India consist of warm/hot water systems. Medium-temperature waters and reversal of temperature at depth were observed in Puga, Manikaran and the West Coast geothermal areas after exploratory drilling. Such resources can be utilized only for non-electrical applications after detailed technical—economic feasibility studies. The presence of medium-temperature (90–140°C) springs in the cold, remote and steep Himalayan terrains and of lower temperature springs (100°C) in the hot and variable climate of the Peninsular and Coastal regions further restrict full utilization of these resources, with the exception of Cambay, West Coast and Tatapani—Jhor areas. After careful study a list of direct utilizations is proposed for future consideration and the development of the main geothermal resources in India.     

A comparison of the silica and Na-K-Ca geothermometers for thermal springs in Utah

The silica and Na-K-Ca chemical geothermometers do not always agree (within 25°C) when applied to hot springs in Utah. Some of the differences can be explained by mixing of hot and cold ground waters. However, the geology of the areas in which the springs arise must also be taken into account.     

The source and heating mechanism for the Ahram, Mirahmad and Garu thermal springs, Zagros Mountains, Iran

The Ahram, Mirahmad and Garu low-temperature geothermal springs in the Zagros Mountains, Boshehr Province, Iran, emerge along the Ghatar-Kazeroon fault. The average temperature of the springs is about 40 °C and the waters have appreciable amounts of dissolved solids and hydrogen sulfide. Based on chemical analyses, including stable isotopes of the thermal waters and data interpretations, and on a comparison with fresh water springs and wells in the study area, we conclude that the hot waters are of meteoric origin. Because of the prevailing geothermal gradient, the waters are heated as they circulate deep in the system through joints, fractures and the Ghatar-Kazeroon fault. During their deep circulation, the waters come into contact with Hormoz Series evaporites and the oilfield brines, resulting in an increase in dissolved ion concentrations.     

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.     

Hydrogeochemistry and groundwater circulation in the Xi’an geothermal field, China

Geothermal waters from the Tertiary aquifers located at 1000–3000 m beneath Xi’an city, Shaanxi Province, China, show unique isotopic composition as compared to local groundwaters from shallower Quaternary aquifers. Positive oxygen shifts of as much as 8‰ VSMOW are observed, while the corresponding δ²H values remain essentially constant at about −80‰ VSMOW, which is significantly different from those of waters in the Quaternary aquifers with a mean δ²H value of −60‰ VSMOW. The strong ¹⁸O shift is a result of isotope exchange between geothermal water and carbonate minerals such as calcite over a residence time of several thousand years up to 30,000 years, based on ¹⁴C dating. A comparison of the isotopic composition of geothermal waters with neighbouring groundwater units on both sides of the Guanzhong Basin indicates that the geothermal reservoirs are recharged by rain that falls on the northern slope of the Qinling Mountains, south of the Xi’an geothermal field, but not from the North Mountains to the north of the field. Based on chemical geothermometers the highest temperature estimated for the Tertiary aquifers of the Xi’an area is around 130 °C.     

Geothermal prospecting by geochemical methods on natural gas and water discharges in the vulsini mts volcanic district (central Italy)

The Latera and Torre Alfina geothermal fields were discovered in the Vulsini Mts district (central Italy) in the 70s. The fluid produced by the two geothermal systems is a high pCO₂ (around 7 MPa) sodium chloride solution (T.D.S. is 9200 ppm at Latera and 7800 at Torre Alfina), with high SiO₂ and H₃BO₃ contents. The fluid temperature taken at well bottom is about 155°C at Torre Alfina, whereas at Latera it ranges from 200 to over 350°C. In spite of these temperatures, recorded in producing wells, previous geochemical prospectings using geothermometers in natural thermal manifestations had predicted temperatures no higher than 140°C in all the Vulsini district. This contrasting feature between real temperatures and those evaluated during prospecting is caused by the fast circulation of large amounts of meteoric waters in the aquifer located in the shallow parts of the carbonate reservoir formations, and by the short interaction between the latter and the deep geothermal fluids.In the present study a new geochemical survey on thermal and cold springs, stream samples, as well as natural gas emissions has been carried out. A critical review of the main geothermometers, some considerations about the hydraulic behavior of the reservoir formations, and the cross comparison between NH₄⁺/B ratio, pCO₂ and SiO₂ content in both cold and thermal waters, have led to the conclusion that in the Vulsini Mts there are no shallow anomalous areas apart from those already discovered at Latera and Torre Alfina.The present method could be successfully applied in other geothermal systems, where the potential reservoir is represented by carbonate formations.     

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).     

Hot spring activity in the Dakongbeng geothermal area, China

The Dakongbeng geothermal area, whose hot springs reach a temperature of up to 96°C, has been considered one of the potential high-temperature hydrothermal systems in south-west China. The concentration of dominant cations and anions indicates an NaHCO₃ type of thermal water, whose major constituents in decreasing order are: Na>K>Ca>Mg, HCO₃>SiO₂>Cl>SO₄. On the basis of the silica geothermometer, cation geothermometers, gas geothermometer and activity diagram, the reservoir temperature is estimated at about 200°C. All the thermal waters have originated from meteoric water of a higher altitude that circulated as ground water at considerable depth along faults. The stability of their contents of Cl, SiO₂, δD, δ¹⁸O and of the Cl/B, Na/Li ratios suggests that the main heat loss process is through steam loss. The geochemistry of the initial liquid has been estimated by single and continuous steam loss. On the basis of its geologic and geographic setting, the Dakongbeng geothermal area appears to belong to the Himalayan geothermal belt and is thus regarded as an area of interest for further study.     

Geothermal reservoir temperatures estimated from the oxygen isotope compositions of dissolved sulfate and water from hot springs and shallow drillholes

The oxygen isotope compositions of dissolved sulfate and water from hot springs and shallow drillholes have been tested as a geothermometer in three areas of the western United States. Limited analyses of spring and borehole fluids and existing experimental rate studies suggest that dissolved sulfate and water are probably in isotopic equilibrium in all reservoirs of significant size with temperatures above ca. 140°C and that little re-equilibration occurs during ascent to the surface. The geothermometer is, however, affected by changes in δ¹⁸O of water due to subsurface boiling and dilution and by addition of sulfate of nearsurface origin. Methods are described to calculate the effects of boiling and dilution. The geothermometer, is applied to thermal systems of Yellowstone Park, Wyoming, Long Valley, California, and Raft River, Idaho to estimate deep reservoir temperatures of 360, 240, and 142°C, respectively.