Silica Geothermometers Research Articles

Determining recharge area in ULUBELU geothermal field, LAMPUNG, Indonesia using stable isotope data

The Ulubelu geothermal field is located in Lampung Province, Indonesia, about 125 km west of Bandar Lampung City. This field is utilized as a power plant with an installed capacity of 220 MW operated since 2012. The Ulubelu geothermal system is a high enthalpy water-dominated reservoir and has temperature around 265 °C. This study aims to determine the elevation of the recharge area in the Ulubelu geothermal system based on the values of δ2H and δ18O isotopes to support the geothermal use sustainability. Major ions, δ2H, δ18O, and δ13C(DIC) isotopes analysis were carried out on hot springs, lake water, cold springs, and rainwater. The content of the major anions and cations indicates that the hot springs from the Way Panas Group are chloride-type water originating from the reservoir meanwhile the Danau Hijau and Pagar Alam groups are condensates acid-sulphate fluids. Moreover, the calculation using a Na/K and silica geothermometer shows that the reservoir has a temperature around 229 ± 4 °C. Analysis of the δ2H and δ18O isotopes content informs that the origin meteoric water seeped into the geothermal reservoir was δ18O −6.65‰ and δ2H −39.5‰ ranging from 1108 to 1570 masl elevation. Overlaying with the surface terrain map depicts that the recharge area originates from the peak of Mt. Rendingan in the north of Ulubelu and Mt. Sula in the west with 42.9 km2 total area. In conclusion, this study elucidates the recharge area elevation in the Ulubelu geothermal system, offering crucial insights for sustainable geothermal resource management and reservoir origin understanding, thus highlighting its potential for sustainable energy production.

Assessment of possibility of finding geothermal energy resources around Mount Meru in northern Tanzania using classical solute geothermometry

This study uses classical solute geothermometry analysis to estimate the temperature and circulation depth of the geothermal reservoir around Mount Meru. Five springs affected by magmatic gases from a deep magmatic heat source are investigated; these include two hydrothermal springs. Results from silica geothermometers show that estimated reservoir temperatures range from 70 to 110 °C; this corresponds to a low-temperature hydrothermal system. Since Mount Meru is an active volcano, one would expect a high-temperature hydrothermal system; the low temperatures are ascribed to the mixing of hydrothermal and cold recharge waters. The geothermal reservoir is estimated to be about 1.3–1.4 km deep. Thus, the study shows that a low-temperature geothermal energy resource can be found on the eastern flank of Mount Meru.

Genesis Mechanisms of Geothermal Resources in Mangkang Geothermal Field, Tibet, China: Evidence from Hydrochemical Characteristics of Geothermal Water

The Mangkang geothermal field, distributed in the Mediterranean–Himalayas geothermal belt, hosts abundant hot springs whose geneses remain unclear. To determine the hydrochemical characteristics, reservoir temperature, circulation and recharge depths, and water–rock interactions of the geothermal water in the geothermal field, this study analyzed hydrochemical compositions and isotopes (2H, 3H, and 18O), conducted a PHREEQC simulation, and established a conceptual model to illustrate the genesis of geothermal resources in the Mangkang field. Based on the study of hot springs in Meipu, Qvzika, and Zulongpu villages and Rumei town, the following results are reported: The orifice temperatures of these hot springs vary between 18 °C and 67.5 °C. The hydrochemical composition analysis results indicate that the geothermal water in the hot springs is of hydrochemical type HCO3-Ca·Mg. Moreover, the geothermal water has high HBO2 and Na+ concentrations, suggesting protracted water runoff and strong water–rock interactions during its evolution. According to the mineral–water solubility equilibrium and silica geothermometers, it is estimated that the reservoir temperature of the Zulongpu hot spring is 47 °C and other hot springs have much higher reservoir temperatures of 116–130 °C. As indicated by geothermal gradients, annual temperatures, and reservoir temperatures, the geothermal water in Meipu and Qvzika villages has the greatest circulation depth, up to 3600–4300 m, followed by that in Rumei town (3700–4000 m) and Zulongpu village (~1500 m). The 2H-18O isotopic analysis of the geothermal surface water revealed that the geothermal water originates from meteoric water. The recharge elevation was inferred to be ~4700–4900 m. Moreover, the low 3H values (<1 Tu) suggest that the geothermal water is older than 40 years. The PHREEQC inverse simulation results indicated that the variation in the hydrochemical composition of the geothermal water results from the precipitation of chalcedony and dolomite, the absorption of NaX, and the loss of CaX2 during migration and storage.

Application of geothermometric and hydrochemical methods to the investigation of thermal water of sources in the Northeastern of Algeria case of Setif city

Thermal water in northeastern Algeria (Setif) is a promising, sustainable resource of energy. To date, thermal water has not been used as a source of renewable energy in Algeria as no clear strategy has yet been developed for its use. In this study, eight samples of thermal water springs distributed within the area of Setif city were assessed, and their physical and chemical parameters (such as temperature, potential of hydrogen, electrical conductivity, and major ionic composition, including the K+, Na+, Ca2+, Mg2+, Cl−, SO42− and HCO3−) were measured. The results show that the temperature, potential of hydrogen and the electrical conductivity range between 30 and 52 °C (as measured in field), 6.92 and 7.35, and 1170 and 3160 µS/cm, respectively. Furthermore, the results indicate the existence of four hydrogeochemical facies dominating the hydrogeological system, which are SO4–Ca, Na–SO4, HCO3–Na and Cl–Na. The interaction with Jurassic limestone and the existence of evaporates along the upwelling system, respectively, control this. Due to the high variation of temperature, different geothermometers were used to estimate the geothermal reservoir temperature, the application of silica geothermometer (Quartz), because it is the most suitable in our case, gives temperatures estimated at the origin of the order of 80° C. It is higher than that measured at emergence, it reflects an average dissipation of 48° C. According to an analysis of these findings and the geothermal gradient in the area, two aquifer systems of the thermal complex are supplied by a reservoir that is stored in fissured Jurassic limestone that is more than 2600 m deep.

Chemical geothermometers and mixing models to understand the thermal aptitudes in the management of Bouhanifia and Saida geothermal resources, northwest of Algeria

The paper aims to determine the geothermal characteristics and the mixing possibility of Bouhanifia and Saida geothermal resource because of its importance in tourist activities and energy investigations by using Geothermometry and geochemical approaches. The methodology of our work is based on physico-chemical analysis and thus is validated by the execution of plots and graphs that identify the mixing with cold water and define the geothermal behavior. Geothermal water is characterized by natural pH ranging between 5.86 and 7.03. Furthermore, by cation domination water type Cl−, Na+, Ca+ for Bouhanifia and Cl−, SO42−, Mg2+ for Saida, the Silica geothermometers give lower temperatures variation between 12 and 106 °C. On the other hand, the estimated temperature from cationic geothermometers ranges between 51 and 236 °C. The geothermal is mixed with cold water from saline and carbonated formations. Results showed that the studied geothermal water chemistry and Geothermometry are more affected by local geology and tectonic than regional perspectives. Therefore, the water is influenced by polluted shallow interactions and is classified as a low enthalpy category.

ANALISIS KARAKTERISTIK FLUIDA DAN ESTIMASI TEMPERATUR RESERVOIR MENGGUNAKAN GEOINDIKATOR Na-K-Mg DI KECAMATAN TOMPASO BARU

Geothermal is a source of thermal energy contained in hot water, rock, water vapor, and other gases and minerals that are genetically inseparable from a geothermal system. This energy comes from heating water and rocks and other elements contained in geothermal energy stored in the earth's crust. Energy needs are increasing, various efforts are made to take advantage of the potential that exists in Indonesia, one of which is geothermal potential. Geothermal energy is a renewable natural resource with high potential and is one of the preferred energy sources in energy diversity. The existence of geothermal sources, marked by the presence of geothermal manifestations emerging to the surface, one of which is in Tompaso Baru District. The fluid samples were then tested in the laboratory, then data processing was carried out using the fluid geochemical method in the form of calculating the percentage of the Na-K-Mg geoindicator to determine the type of fluid and a geothermometer to determine the estimated reservoir temperature. By using the calculation of the percentage of chemical content from the laboratory test results Na/1000+K/100+√Mg and plotting a ternary diagram of a partial equilibrium waters type of geothermal fluid, namely a condition where the fluid has partially changed so that it is not fully in equilibrium, and calculations using a silica geothermometer the estimated reservoir temperature is 191.1078ºC, which is the type of reservoir with medium temperature.

Investigation of the main reservoir of the Tulehu geothermal system (Indonesia) using 3-D inversion of MT data

Tulehu geothermal prospect area is located in Maluku Province, Indonesia. The existence of the geothermal system in this area is recognized by the appearance of hot springs on the western side of Mt Eriwakang and the northern side of Mt. Huwe. Most of these hot springs occur along inferred geological structures. The geological conditions of this area are dominated by Quaternary volcanic rocks, limestone, alluvium, and metamorphic rocks. Several authors have conducted several studies to propose various geothermal conceptual models, from structurally controlled to volcanic-hosted geothermal systems. However, the geoscientific data supporting the hypotheses are not comprehensive yet. There are still gaps in the data coverage as well as in the interpretation of geological, geochemical, and geophysical data, so a comprehensive study should be done before deciding on further stages. Therefore, this paper proposes a new conceptual model of the Tulehu geothermal area by reducing the existing data gap. Interpretation of remote sensing data, geological field observations, age dating analysis, reanalysis of geochemical data, and additional MT data covering Mt. Eriwakang are conducted to reconstruct the conceptual model. Graben-like structures are identified based on the geological data where the product of Mt. Eriwakang is in the middle. Based on the age dating data, the product of Mt. Eriwakang erupted in the Quaternary time (413±6 kA), so it is still quite potential as a heat source. This indication is also supported by the trend of Cl, HCO3, and SiO2, which describe the possible flow from Mt. Eriwakang to the surrounding hot springs. Based on the Na-K-Mg and Silica geothermometers, the reservoir temperature is estimated at 210-240 °C. A three-dimensional (3-D) MT inversion was applied to all the existing and new MT data to obtain a 3-D subsurface resistivity model. The oceanic bathymetry data was also incorporated to mitigate the possible coastal effect from the surrounding seawater to the MT data. The new 3-D MT inversion results show a good agreement with the geological and geochemical indications. The updome-shaped pattern of the subsurface resistivity distribution beneath Mt. Eriwakang supports the geological and geochemical data analysis results concerning the indications of the prospects area around Mt. Eriwakang. These interpretations can strengthen the recommendation to focus on the area around Mt. Eriwakang for further development.

Geothermometry in High-Temperature Reservoirs of the Geothermal Springs in Southern Thailand: Insights from Cations and Silica Geothermometers

Geothermal springs have provided a unique opportunity to study the geothermal system of geological processes. A reservoir temperature estimation based on the chemical geothermometers is vitally essential for assessing the exploration and development of geothermal resources. The paper represents the various techniques of geothermometers with comparisons between the silica (quartz and chalcedony) and the cation geothermometers (Na–K–Ca and K–Mg) for the high exit temperature (temp. ≥55°C) of geothermal springs in southern Thailand. The Na–K–Ca geothermometer presented more elevated reservoir temperatures than the K–Mg, silica and chalcedony geothermometers, about 20–30◦C. The preliminary assumed difference between the geothermometers may indicate that the shallow subsurface conditions are mixed with groundwater

Genesis, evolution, speciation and fluid-mineral equilibrium study of an unexplored geothermal area in Northeast Himalaya, India

Arunachal Pradesh geothermal area, located in the Northeast Himalayan geothermal province is an unexplored geothermal area in India due to its rugged terrain and inaccessibility. In this study, a detailed geochemical characterization of the thermal waters is carried out to elucidate the source of the dissolved constituents and also to determine the subsurface reservoir temperature. The thermal waters are found to be nearly neutral to slightly alkaline (pH = 7.15 to 8.40) having highly variable EC values (EC ranges from 204 to 3600 µS/cm). Most of the thermal waters are mixed cation (Na, Ca)-HCO3-SO4 type whereas a few of them are of Na-HCO3 type. Silicate weathering and ion-exchange processes are found to be the two important mechanisms controlling the concentrations of the dissolved solutes in the thermal waters. The temperature dependant (20 to 198 °C) speciation analysis of the major ions (Na, K, Ca and Mg) shows that the composition of the reservoir fluid is dominated by free Na+ ions followed by K+, Ca2+ and Mg2+. The sulphate complex is found to be majorly dominated by both CaSO4 and MgSO4 whereas carbonate is mainly controlled by CaCO3 and CaHCO3+ species. Environmental stable isotope (δ18O, δ2H) data confirms the meteoric origin of these thermal waters. The isotope composition of the thermal waters in this region is found to be depleted compared to the non-thermal water samples indicating a high altitude precipitational recharge for these thermal waters. The altitude of the recharge estimated from the δ18O-altitude plot reveals that the precipitation occurring at the Gorichen Peak (altitude of 6858 m), the highest peak of the Arunachal Pradesh, is the main source of recharge for these thermal waters. Of different silica geothermometers, the quartz geothermometer gives the most conservative estimate (71 to 133 °C) of subsurface reservoir temperature. The quartz geothermometer gives a lower estimate of reservoir temperature due to mixing with non-thermal water. However multicomponent geothermometrical modelling results give a tightly constrained subsurface temperature i.e. 160±2 °C which may be taken as the probable reservoir temperature in this unexplored geothermal region. The study highlights the potential of integrated isotope-geochemical studies in expanding the geothermal energy utilization in India.

PyGeoT: A tool to automate mineral selection for multicomponent geothermometry

Multicomponent solute geothermometry is a useful tool for estimating the temperature of a deep geothermal reservoir prior to exploration drilling. The method uses full water analyses to determine the temperature at which the saturation indices of an assemblage of reservoir minerals reflect equilibrium with the water. One of the challenges with this method is selecting the mineral assemblage to be used for the computations. A new pre- and post-processing tool, PyGeoT, has been developed to answer this question. PyGeoT (i) automates the selection of the mineral phases to be used with the multicomponent geothermometry software GeoT-iGeoT, (ii) performs sensitivity analyses, and (iii) graphs the results for visualization of the data. The performance of PyGeoT and several classical solute geothermometers is tested using simulated and real water compositions from low-medium temperature geothermal systems. A reactive transport model reveals that PyGeoT provides reasonable reservoir temperature estimates even when only partial chemical equilibrium is reached, which is useful for exploring for low-medium temperature systems. In addition to estimating reservoir temperature, PyGeoT has the potential to identify the silica polymorph controlling SiO2 solubility and alteration minerals without site-specific mineralogical analysis, which is a clear advantage compared to silica geothermometers. Limitations of the method and classical geothermometers are discussed using both synthetic and real data. Finally, recommendations for the application of multicomponent geothermometry are presented.

Analisis Karakteristik Fluida dan Temperatur Reservoir Dengan Menggunakan Diagram Ternary Cl-Li-B Pada Manifestasi Panas Bumi Di Desa Kaleosan, Minahasa Utara

Research on determining the fluid and temperature of the geothermal reservoir in Kaleosan village, Kalawat district, North Minahasa regency, the hot springs are 100 ml. Determination of the characteristics of geothermal fluids is carried out using the Cl-Li-B triangle diagram. The Cl-Li-B triangular diagram is used to determine the origin, boiling, and dilution of geothermal reservoir fluids. Reservoir temperature estimation is done by using silica geothermometer equation. Measurements of Li, B and Ca elements were carried out using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and measuring SiO2 compounds using Visible Spectroscopy. Measurement of Cl elements using acid-base titration method at the PT. Indonesian WLN. Based on laboratory results and data processing and field sampling, the dominant type of chloride (Cl) concentration was obtained. The results showed that the geothermal springs in Kaleosan Village are in immature water, which indicates that the geothermal reservoir fluid has undergone dilution with other elements. The geothermal system in Kaleosan Village is dominated by water and hot springs come out in the outflow zone.

Hydrogeochemical Characteristics and Geothermometry of Hot Springs in the Tensile Tectonic Region Leizhou Peninsula and Hainan Island in South China

Hydrochemistry and isotopes are studied for characterizations of geothermal waters, groundwaters, and surface waters in Leizhou Peninsula (LZ) and the northern part of Hainan Island (HN) across the tensile-tectonic Qiongzhou Strait, southern China. Water chemistry results showed that most geothermal waters are of alkaline and HCO3-Na types, while shallow groundwater samples were significantly different with HCO3-Ca, HCO3-Mg, and HNO3-Ca types. Stable isotope data indicated that the source of LZ geothermal waters is of atmospheric precipitation, while HN geothermal water has a slight “oxygen drift”. Cation geothermometers and silica geothermometers could appropriately predict reservoir temperatures. Based on multicomponent mineral equilibria and quart geothermometer without steam loss, reservoir temperatures were estimated to be in the range of 65.0°C to 165.0°C at an average circulation depth of 2023 m, substantiated with data from a thousand-meter-depth borehole. The saturation indexes (SI) simulated by PHREEQC and Na-K-Mg triangle diagram revealed that the chemical components in the geothermal waters are possibly derived from water-rock interactions at high temperatures. Four simulation paths were established to calculate the mass flux of minerals by PHREEQC program. The hydrothermal circulation process of the volcanic rocks on both sides of the Qiongzhou Strait was similar, but the water-mineral reactions of Leizhou Peninsula were more intense. During the high-temperature hydrothermal deep circulation in the central part of Hainan Island, the molar transfer was low and the degree of mineral reaction is weak. Finally, the geothermal genetic mechanism for the study area of the extensional geostructure was proposed. The findings mark the characteristics for the tensile-tectonic region in slight “oxygen shift”, low values of 87Sr/86Sr for possible deep source from the mantle, and higher reservoir temperatures at shallower circulation depths.

Hydrogeochemistry and reservoir characterization of the Konya geothermal fields, Central Anatolia/Turkey

A conceptual model with water samples from ten geothermal fields (İsmil, Ilgın (Çavuşcugöl), Tuzlukçu-Akşehir, Seydişehir and Kavakköy, Hüyük, Ereğli-Akhüyük, Kadınhanı, Cihanbeyli, Karapınar and Beyşehir) in the province of Konya defined the geothermal system. Carbonates, quartzite and marbles of Paleozoic metamorphics are the reservoir rocks and the heating sources are igneous rock intrusions and geothermal gradient. The variable thermal water (CaMgHCO3, CaSO4, NaSO4, CaHCO3, CaNaHCO3, NaCl and CaNaClHCO3) had EC and temperature between 177.8 and 56,100 μS/cm and between 18.3 and 44 °C, respectively. Ca2+ in geothermal fluids are associated with marble and carbonate rocks and the high chloride shows direct connection with deep geothermal system, and prolonged contact with evaporite rocks. Sulphate originates from dissolution of and oxidation of sulphate and sulphur-bearing minerals. The high As, B, F and Mn concentration in some thermal water samples were determined as 85 μg/l, 148.56 mg/l, 3.01 mg/l and 208.13 mg/l, respectively. Reservoir temperatures computed by Na/K geothermometers were between 85.37–158.89 °C for Akşehir thermal waters and 58.78–90.45 °C for Ereğli thermal waters. The maximum reservoir temperature of other geothermal waters was 75 °C by the silica geothermometers.

Fluid geochemistry and geothermal anomaly along the Yushu-Ganzi-Xianshuihe fault system, eastern Tibetan Plateau: Implications for regional seismic activity

Hydrothermal activity and seismicity are abundant along the Yushu-Ganzi-Xianshuihe fault system (YGXFS) in the eastern Tibetan Plateau. However, the processes and factors that control hydrothermal fluid circulation at a regional scale remain less constrained, and the relationship between geothermal anomaly and seismic activity is poorly known. In this study, we report an integrated dataset of thermal water chemistry (δ18OH2O, δDH2O, and δ13C of dissolved inorganic carbon (DIC)) to identify the origin, circulation path, and reservoir properties of hydrothermal fluids and then reveal the relationship between fluid circulation and seismic activity. The thermal waters are mainly classified as Na-HCO3 type with minor Ca-Na-HCO3 and Na-HCO3-Cl types. The δ18OH2O (−18.7‰ to −14.5‰) and δDH2O (−142‰ to −110‰) values indicate recharge of meteoric waters, which infiltrated along the YGXFS to 2–6 km depths and underwent mixing with deep fluids prior to their discharge at the surface. A mass balance model based on δ13CDIC values (−4.3‰ to 5.9‰) and DIC contents of thermal waters reveals the involvement of CO2-bearing fluids from mantle and thermo-metamorphic decarbonation of crustal marine limestones in tectonic CO2 outgassing from the active fault. The estimated reservoir temperatures (85–336 °C) based on silica geothermometer of the thermal waters decrease from southeast to northwest along the YGXFS. Notably, the epicenters of strong earthquakes (Ms ≥ 6) are mostly located in the transition zone of geothermal anomalies according to spatial distribution of the reservoir temperatures. Combined with geological and geophysical information, we suggest that the heterogeneity of crustal thermal structure and the upwelling of deep fluids may play an essential role in the distribution of thermal springs and the high frequency of earthquakes along the YGXFS.

High heat flow at the SW passive margin of the Gulf of California

AbstractNew high heat flow data confirm the presence of a heat source in the passive margin of the southern Baja California Peninsula that is not linked with volcanic activity. The high heat discharge is spatially associated with low seismic velocity, crustal thinning, intense seismic activity, documented local extension, shallow Curie Point Depth and silica geothermometer temperatures above 100°C. The highly oblique rifting regime in the centre of the Gulf of California generates a transtensional setting conducive to local extension in its southwest margin. Similar high heat flow in a rifted margin has been reported only in the Gulf of Aden that is also an obliquely spreading ridge. The intense heat discharge, with heat flow values as high as 613 mW/m2, indicates the presence of a heat source consistent with mantle upwelling. Heat transfer calculations indicate that the heat source emplacement would be at most 2 Ma old.

Analysis of fluid characteristics and estimation of geothermal reservoir temperature in Kaleosan Area, North Minahasa Regency

The appearance of manifestation characterizes the existence of geothermal potential. This research was conducted in the Kaleosan area of North Minahasa Regency, in North Sulawesi Province, Indonesian, which has a manifestation of hot water with a temperature of 90°C with pH of 6.89, and there are silica deposits around it, all of which are characteristics of a water-dominated geothermal reservoir. This research was carried out by analyzing the characteristics of the surface fluid samples of the Kaleosan hot water manifestations. Analysis of fluid characteristics using the Q-SO4-HCO3and Na-K-Mg. From the Q-SO4-HCO3diagram, it is found that the geothermal fluid in Kaleosan is a type of chloride. The high chloride content is thought to be due to the increase in geothermal fluids containing CO2and condensing in shallow aquifers. From the Na-K-Mg diagram, it is obtained that the Kaleosan hot springs are immature water where this manifestation fluid has undergone dilution and cooling by meteoric water during its trip to the surface. From the quartz silica geothermometer, the reservoir temperature estimate is 236.6°C, which indicates that the geothermal reservoir in the Kaleosan area is a high enthalpy geothermal system.

An approach for heat flow determination in the absence of geothermal gradient measurements: west Anatolia example

In western and southwestern Anatolia, the geothermal gradient cannot be accurately measured in many wells due to the complex horizontal water cycle. The aim of this study is to calculate the geothermal gradients by using a new approach for such wells. In this case, the first step is to calculate the geothermal gradients in order to estimate the heat flow values around the wells. Naturally, a direct heat flow estimate cannot be made for wells without a geothermal gradient. However, the heat flow values can be indirectly obtained by different methods. Curie point depth, silica geothermometer, and magnetotelluric conductive layer depth are some of these indirect methods. In this study, we used the heat flow data based on the Curie depth. In this approach, Curie temperature gradients are calculated using the Curie heat flow values and thermal conductivities for problematic wells. An empirical relationship between the Curie temperature gradient and the correctly measured temperature gradients in the region was established. With this relationship, it is possible to make an approach from the current Curie temperature gradient to the normal temperature gradient. In this way, heat flow can also be calculated for areas where no geothermal gradient can be obtained, and heat flow distribution over the whole area can be given. This proposed approximation, to obtain thermal gradient and heat flow, can also be extended to any other region.

The La Laja spring system in the Argentine Precordillera: A conceptual model based on geochemical and isotopic data

The La Laja Spring System (LLSS) (31.34°S; 68.48°W) in the Argentine Precordillera is well-known for its low enthalpy thermal baths since pre-Columbian times and its surroundings full of travertine fields. This work focuses on geochemical and isotopic analysis of the thermal waters and their hydrological implications. We found a system with partially equilibrated waters with dominant Cl−–Na+ and subordinated HCO3−Ca2+(Mg2+) and SO42−Ca2+compositions, which seem affected to the basin fill composed of thick Cenozoic synorogenic deposits overlying early Paleozoic carbonate successions. Several cationic geothermometers suggest a deep reservoir with average temperatures reaching ~170 °C, while the silica geothermometers suggest a shallow reservoir at 91 °C. The average temperature of the springs is 28 °C and the low-geothermal gradients (~27 °C/km) in the region indicate a relatively deeper (~5500 m) reservoir and another shallower (~2500 m). The source of heat is related to the orogenic belt because this region is on top of the flat-slab subduction of the South-Central Andes, with an absence of active volcanism. Hence, this spring system is mainly affected by the relatively low-geothermal gradient within the Eastern Precordillera. The geochemical and isotopic characteristics O18/O16 as well as H2/H1 ratios of waters indicate meteoric source associated with evaporation in relation to strong aridity and secondary processes modifying the chemistry of waters is likely to be erased by any thermal end-member signature. Our conceptual model for the LLSS indicates the N–S trending Villicum-Zonda regional thrust and the minor accommodation faults within the Eastern Precordillera domain seem to behave as the preferential zone for recharging and latter thermal water rising.

Hydrochemistry and geothermometry of thermal waters from UAE and their energetic potential assessment

Two main low-enthalpy geothermal fields are present in the UAE: Mubazzarah-Ain Faidha (Al-Ain city) in the Abu Dhabi Emirate and Ain-Khatt in the Ras Al-Khaimah (RAK) Emirate. The chemistry of nineteen waters was analyzed to calculate geothermal reservoir temperatures using cation and silica geothermometers, and to describe the source of recharge of the hot waters from isotopic data. In total, 19 water samples (16 from wells, 2 from springs AF and KT1 and one sea water) were collected and analyzed for major anion and cation concentrations. The isotope and chemical data suggest that the hot waters from Al-Ain (Mubazzarah and Ain Faidha) are almost entirely recharged by rainwater, which originates at higher altitude, heats up through deep water circulation, and travels to the surface through fault structures within the study area. The waters are mainly Na-Cl type, and the discharge temperatures range from 32.5 °C to 49 °C. The Na-K geothermometers predict a geothermal reservoir temperature of 151 °C for the Mubazzarah-Ain Faidha hot waters and 112 °C for the Ain-Khatt hot waters with estimated depth of water penetration of 3.8 km and 2.6 km, respectively. The existing known geothermal resources in the Mubazzarah and Ain Khatt areas can be immediately and sustainably used for low-temperature, direct-use geothermal applications. A stochastic Monte Carlo simulation was performed to estimate the geothermal energy stored in the two main low-enthalpy geothermal fields for 25 years and 50 years. The geothermal electricity potential at Mubazzarah-Ain Faidha is ∼16.5 MWe and ∼8.27 MWe for 25 and 50 years, respectively. The geothermal electricity potential at Ain Khatt is ∼291 kWe and ∼146 kWe for 25 and 50 years, respectively. These preliminary potential assessments are subject to further delineation drilling and availability of field performance data and considered that the geothermal power plant has either a single or multiple ORC systems due to the very low temperature.

Medium to low enthalpy geothermal reservoirs estimated from geothermometry and mixing models of hot springs along the Malawi Rift Zone

We investigated the major ion and stable isotope geochemistry and used silica and cation concentrations to assess the geothermometry of hot springs along the Malawi Rift Zone (MRZ). The MRZ is a magma-poor rift, where potential geothermal energy is postulated from elevated heat flow and the occurrence of hot springs. Our objectives were to (1) estimate hot spring reservoir temperatures, (2) identify processes that could affect hot spring water during the ascent from their geothermal reservoirs, and (3) identify the most promising areas for geothermal energy generation. New geochemistry data from 27 hot springs were analyzed and classified as Na-HCO3, Na-SO4, or Na-Cl (SO4) water types controlled mainly by the mineralogy of the host rocks. The hot springs in the MRZ are associated with the local meteoric waters that infiltrate at depth and are heated by an anomalously high geothermal gradient. Our findings indicate that a parent geothermal fluid is mixed and diluted with a proportion of more than ∼45 % of cold water from shallow aquifers. Additionally, other processes such as water-rock interaction, and CO2 outgassing take place during the ascent of the geothermal fluid to the surface. The quartz conductive silica geothermometer indicates reservoir temperatures that range from 60 °C to 130 °C, with enthalpies ranging from 250 kJ/kg to 537 kJ/kg, respectively. However, the Na-K cation geothermometer indicates reservoir temperatures from 92 °C to 190 °C with enthalpies from 388 kJ/kg to 808 kJ/kg. Three areas of potential geothermal energy generation were identified: (1) The Chiweta area, (2) Nkhotakota area, and (3) the Shire graben area. We conclude that the hot springs along the MRZ have geothermal energy capabilities of medium to low enthalpy that can be exploited for electricity generation or other direct uses.