Geothermal Prospect Research Articles

Shallow and deep geothermal water sources identification in Unai geothermal field, Gujarat, India with applications of Magnetotelluric (MT)

The aim of this study is to conduct and examine the results of a Magnetotelluric (MT) survey in Unai, Gujarat, India for geothermal prospect identification. At Unai, the data was acquired, processed, and interpreted in the form of 2D and 3D MT surveys. Data were acquired at 56 stations in uniformly spaced profiles along the ENE-WSE direction (geological strike direction) with a station spacing of 2 km. Qualitative and quantitative data understanding has been done for geothermal prospect identification. Deep and shallow 1D and 2D models of electrical resistivity were made, which suggests the presence of a geothermal aquifer. This is corroborated by polar and skew tipper diagrams. The one-dimensional Occam models and two-dimensional conjugate gradient models confirm the presence of a low resistivity anomaly near the surface ranging from the depth of 100–700 m. This suggests the presence of a shallow geothermal zone. Apart from the shallow geothermal prospect, this study also delineates a deep conductive body which may be attributed to a matured geothermal reservoir. For a better understanding of the prospect geoelectrical cross sections are analysed from a depth of 10 m to 15 km. The MT cross-sections show the presence of geological features such as a series of faults. These faults form horst and graben in nature and may act as a good trap for the shallow reservoirs.

Delineation of geological structures of Arta geothermal prospect in Djibouti based on the gravity data analysis and interpretation

The Arta geothermal prospect is located in the southern part of the Gulf of Tadjoura, 45 km west of Djibouti city. This study uses 398 gravity data stations collected by the Djiboutian Office for Geothermal Energy Development (ODDEG) from February to May 2018. The geothermal activity of the Arta area is revealed by surface thermal manifestations, such as near-boiling fumaroles (98 °C). This study aims to delineate the subsurface geological structures responsible for surface geothermal manifestations and define the depth and geometry of the geothermal reservoir in the study area. A Complete Bouguer anomaly was obtained using a Bouguer density of 2.55 g/cm3, derived as an average density from the F-H, G-H and CVUR methods. A regional and residual anomaly trend separation method was applied to focus on the structures of interest in the study area, and a residual Bouguer anomaly map was obtained. Edge detection filters, including Horizontal Derivatives (HD) and an Improved Normalized Horizontal tilt angle (INH), were used to examine the gravity data for edge identification. The density was then modelled in three-dimensional (3-D), utilizing the VOXI Earth Modeling Tool performed in Geosoft Oasis Montaj. The model was constrained using estimated lower (-0.35 g/cm3) and upper (0.65 g/cm3) bound density values from the expected density values of the local geology. The Bouguer and residual anomaly maps show a high-density body in the central part, surrounded by low-anomaly structures. The high-density body is interpreted as an intrusive body covering the basement of the study area. The anomalies imaged from HD and INH indicate a significant number of high gradient anomalies trending N-S, NE-SW, and NW-SE, corresponding to the trend of the geological structures of the Arta geothermal prospect. These high-gradient anomalies can be interpreted as faults, contact zones, or geological boundaries. The 3-D model imaged a high-density anomaly trending N-S in the central part at 2 and 3 km below sea level with a volume of about 126 km3 and an average block density of 2.66 g/cm3, interpreted as magma intrusion or intrusive volcanic rocks. The 3-D vertical cross-sections revealed that the intrusion is visible at depth and that some intrusions could be seen at the surface and near surface. The findings of this gravity study will contribute to the overall and future geothermal exploration and development of the Arta geothermal prospect.

Integrated geophysical methods used to explore geothermal potential areas in Siogung-Ogung, North Sumatra, Indonesia

The manifestations of some hot springs in Siogung-Ogung, North Sumatra, Indonesia have increased speculation of likely geothermal sources. Therefore, integrated geochemical and geophysical methods were employed to identify the geothermal prospect of the area. Two-dimensional electrical resistivity and geomagnetic methods were used for the geophysical survey. The geochemical survey used three concentration measurements: geothermometer silica (SiO2), geothermometer Na-K, and geothermometer Na-K-Ca. A Wenner Schlumberger array with a 5-meter electrode spacing was used to acquire the 2-D resistivity data, which was processed using Res2Dinv software. The geomagnetic method was performed with a proton precession magnetometer, and the data were processed using Surfer to produce the magnetic residual map. The 2-D electrical resistivity results show that the area has low resistivity values (1-700 Ωm). The resistivity values from 1 to 100 Ωm could be due to the presence of hot waters in alluvium, and the resistivity values > 400 indicate andesite rock, which can function as a hot water conductor from the source. The magnetic residual map shows geomagnetic values from 150 nT to 360 nT, which infer the potentiality of geothermal within the study area. The geochemical results show that the reservoir temperature is 572 °C. Based on the integrated results, the study area has promising geothermal potential.

Seismic expression of paleokarst morphologies and associated Siderolithic infill in the Geneva Basin, Switzerland: Implications for geothermal exploration

The Geneva Basin is witnessing intense geothermal exploration campaigns arising from the Cantonal and Swiss federal energy transition strategies towards an ultimate net-zero future by 2050. However, a major challenge to successful geothermal exploration in the basin are uncertainties regarding the tectonostratigraphic framework of the basin, especially in relation to subsurface morphologies of significance to geothermal prospectivity. This was demonstrated when the newly drilled GEo-02 geothermal exploration borehole surprisingly penetrated a c. 158 m Siderolithic reservoir unit known to infill paleokarst structures in the Molasse Basin. Therefore, we aim to understand the seismic character diagnostic of these paleokarst features and their infill in subsurface geophysical dataset. This was achieved by interpreting industrial 2D seismic reflection and borehole dataset integrated with satellite imagery and outcrop analogues. The seismic dataset reveals these paleokarsts as depressions along the Cretaceous-Paleogene transition (K-Pg) and is characterized by three distinct end-member morphologies: (1) fault-controlled type, (2) doline type associated with dissolution/collapse and (3) complex type, atop the Lower Cretaceous carbonates. However, the karst infill is characterized by different seismic facies exhibiting either a higher or lower seismic reflectivity depending on the karst-end member type. This is related to differences in the acoustic properties of the karst infill, overlying lower molasse and the Lower Cretaceous carbonates hosting the karst. The GEo-02 borehole reveals that the Siderolithic infill consists of a variety of lithologies including quartz-rich sandstone, polygenic breccias and conglomerates, terra rossa and lateritic deposits. The evolution of the paleokarst features in the study area is influenced by the structural re-organization of the Molasse Basin in relation to the Alpine orogeny. This resulted in its emergence and change in base-level promoting the dissolution of the carbonate substratum by meteoritic water subsequently generating the paleokarst morphologies. Considering the regional geothermal gradient, the foredeep of the Geneva Basin that exhibit subsurface manifestations of this Siderolithic reservoirs may present new opportunities for an underground thermal energy storage. However, the presence of karstified intervals may pose geohazard risks to geothermal exploration during drilling operations targeting deeper structures in the basin.

Assessment of Bedugul Geothermal Prospect Using A Numerical Reservoir Modeling

A numerical modeling was carried out to generate a numerical model of the Bedugul geothermal field located in Bali Island, Indonesia. This study presents a natural state model and updates the conceptual model of Bedugul based on published geological, geophysical, geochemical, and well data. The numerical simulation of the Bedugul geothermal field has been developed using the previous conceptual model and well data. The Bedugul reservoir model is properly aligned with actual Pressure and Temperature well data. Thus, the natural state model is used to update the Bedugul conceptual model. The main updated points of the conceptual model were the location of the heat source to be beneath Mount Tapak, adding the flow fluid pattern as outflow and upflow location, caprock, reservoir, recharge, discharge, and adding iso-temperature distribution. Based on the numerical model result, the Bedugul geothermal field is a waterdominated geothermal system. The results of this model have been applied to the heat stored method with the Monte Carlo simulation to generate a probabilistic distribution of the reserve potential estimation. The probabilistic for P50 of probable and proven reserve estimation for thirty years are 136.3 MW and 33.4 MW, respectively. Keywords: Bedugul, numerical model, natural state, resource assessment, geothermal

San Felipe Geothermal Prospect: A Previously Unrecognized Hydrothermal System on the Northeastern Coast of the Baja California Peninsula, México

Geophysical surveys discovered low resistivity values that, in combination with geochemical and structural data, indicate the presence of a previously unrecognized geothermal system with a possible maximum size potential similar to that of the presently installed capacity of the Cerro Prieto geothermal field. The only evidence of a hydrothermal system in the San Felipe Valley are warm groundwater wells and four intertidal hot springs. Exploration at San Felipe was part of a research project that included geological, geochemical and geophysical studies, which indicated the presence of low resistivity anomalies and high temperature at depth; the geological survey provided evidence of active fault systems that may act as channels for convective heat transport. Estimation of the energy potential of the San Felipe prospect using the heat in place method yielded a 50% probability of more than 300 MW that would add to the 570 MW of the Cerro Prieto geothermal field and can supply almost 40% of the state of Baja California electricity demand. The Baja California Peninsula is not connected with the national grid and has one of the highest electricity prices in Mexico, which should be an incentive to develop this clean energy source. The results of this work support the recommendation to pursue further advanced exploration of this prospect.

Integration of Geophysical and Geospatial Techniques to Evaluate Geothermal Energy at Siwa Oasis, Western Desert, Egypt

Environmental degradation is reducing crop productivity in many regions of Egypt. Moreover, unsustainable surface water drainage contributes to salinized soil conditions, which negatively impact crops. Egypt is seeking solutions to mitigate the problem of surface water drawdown and its consequences by exploring renewable and sustainable sources of energy. Geothermal energy and the desalination of saline water represent the only solutions to overcoming the fresh water shortage in agricultural industry and to providing sustainable fresh water and electricity to villages and the Bedouin livelihood. In Egypt, the Siwa Oasis contains a cluster of thermal springs, making the area an ideal location for geothermal exploration. Some of these thermal springs are characterized by high surface temperatures reaching 20 °C to 40 °C, and the bottom-hole temperatures (BHT) range from 21 °C to 121.7 °C. Pre-Cambrian basement rocks are usually more than 440 m deep, ranging from 440 m to 4724.4 m deep. It is this feature that makes the Siwa Oasis locality sufficient for geothermal power production and industrial processes. This study utilized both the Horner and the Gulf of Mexico correction methods to determine the formation temperatures from BHT data acquired from 27 deep oil wells. The present study revealed a geothermal gradient ranging from 18 to 42 °C/km, a heat flux of 24.7–111.3 mW/m2, and a thermal conductivity of 1.3–2.65 W/m/k. The derived geothermal, geophysical, and geological layers were combined together with space data and the topographic layer to map relevant physiographic variables including land surface elevation, depth to basement, lineament density, land surface temperature, and geologic rock units. The ten produced variables were integrated in GIS to model the geothermal potential map (GTP) for the Siwa Oasis region. According to the model, both the eastern side and north and northeastern portions of the study region contain high and very high geothermal potential energy. Combining bottom-hole temperature measurements with satellite remote sensing and geospatial analysis can considerably enhance geothermal prospecting in Egypt and other East African areas that have geologically and tectonically similar settings. In addition to identifying sustainable resources needed for food production, this research has implications for renewable energy resources as well.

Maximizing geothermal prospects: Unveiling Paraná Basin’s potential through advanced GIS and Multicriteria Decision Analysis (MCDA)

Geothermal energy is now clearly an important option for the Brazilian energy grid due to the demand for renewable energy sources. The northern and central regions of the Paraná Basin (Brazil) were identified as prospects for additional research into the region’s geothermal energy potential in this context by prior geothermal studies. A thorough investigation based on geological, geophysical, and thermal data is necessary to identify regions with a high potential for geothermal energy. When used by various authors to execute these tasks, the Multicriteria Decision Analysis (MCDA) approach built on Geospatial Information Systems (GIS) has shown considerable promise. The weights of the following parameters-rock type, fault proximity, heat flow, magnetic anomaly, and gravimetric anomaly-were determined in this study using the Analytic Hierarchy Process (AHP) approach, which was incorporated into the MCDA. Five favorability maps were created using the data pertaining to these criteria after they had been processed, standardized, and classed. From ”very low” to ”very high” levels, there were five classes for each selected parameter. From them, we created a map of the geothermal potential based on the list of criteria and the weights assigned to each. In order to verify the findings, we compared the locations of hot springs and aquifers considered indicators of geothermal activity with those of the places with higher potential. Using only one data source, the results showed that four regions had a high index of geothermal favorability, one of which had been previously identified as promising. As a result, the first map produced in the research area using the MCDA-GIS technique is shown in this paper. The strategy proposes a novel way to direct future research to deepen and thoroughly explore these four promising areas.

Prospective geothermal well and heat catchment locations in Nigeria: Remote sensing interpretations and field validation

Geothermal energy is a renewable energy that is environmentally friendly and will help reduce greenhouse emission resulting from the burning of fossil fuels. Nigeria has numerous geothermal surface manifestations like hot and warm springs in most parts of the country that has not been exploited or explored due to the initial high cost of exploration. Most research conducted on geothermal energy prospects in Nigeria considered specific geographical sections. Due to the geological features of Nigeria, the direct and indirect tectonic activities, there is the need to map the thermal anomalies over Nigeria to determine likely geothermal wells and ground heat catchment in Nigeria. Forty years remote sensing dataset (1980–2019), was obtained from the MERRA-2 for three hundred and two (302) locations across Nigeria. The acquired thermal parameters were processed using known models. The data was also analyzed statistically and spatially using the Statistical Package for Social Sciences (SPSS) and Quantum Geographic Information System (QGIS). The results of show that the Gummel-Kumaganum areas of chad basin, Owode-igbo ora areas of the Dahomey basin, Belli area of the basement complex in Taraba state, potiskum areas of the Upper benue Basin, Ekpoma-kwale, itu areas of the Niger delta basin are middle geothermal wells with the terrestial radiation of < −702 W/m2 while the terrestial radiation >200 W/m2 in the Gummi area of the sokoto basin, Hunkuyi area of the basement complex in the northeast, Gashua area of the chad basin, Ozubulu-idah area of Anambra basin, Atijere area of the Dahomey basin, Agbasa, Omoko, Akamkpa area of Niger delta basin, Shaki area of the southwest basement complex are heat catchment regions. Other heat catchment areas had been identified for standalone energy generation. Also, the thermal anomalies in those areas were significant. The validation of the result was achieved via benchmarking similar geothermal well around the globe and ground truthing at Ijebu-Ode Nigeria. Based on the thermal reversal depth (TRD) concept, Ijebu-Ode may have a deep geothermal well with temperatures pattern similar to geothermal wells in other parts of the globe. It is recommended that ground measurement should be carried out in the basement complex to cater for geothermal systems whose mechanism is based on conduction.

Identification of Mount Sirung Geothermal Potential based on Land Surface Temperature and 3D Gravity Model

According to the Ministry of Energy and Mineral Resources 2021 data, first ranks in the list of 10 provinces with the lowest electrification ratio in Indonesia. One of the geothermal prospect areas in East Nusa Tenggara is Mount Sirung. This research was conducted in August 2022 which aims to identify geothermal systems. Gravity data was obtained from the GGMPlus 2013 with a total of 3819 data. Land Surface Temperature (LST) is used as supporting data with a surface temperature approximately 26.1 – 29.5°C because there are manifestations of hot springs at Mount Sirung. Based on the derivative analysis, there are four trajectories in the northwest-southeast direction with reverse faults and normal faults as the geothermal control system of Mount Sirung. The results of 3D gravitational inversion modeling are estimated that there is clay interspersed with breccia with a density of 2.34 – 2.39 g/cm3 as clay cap at 0 – 600 m, and lava interspersed with sandy tuff as a reservoir with a density of 1.98 – 2.03 g/cm3 at 700 – 1400 m. Based on these results and discussions, Mount Sirung is proven to have geothermal potential which can be utilized as a source of electrification in East Nusa Tenggara.

Probe Screening Techniques for Rapid, High-Resolution Core Analysis and Their Potential Usefulness for Energy Transition Applications

Core analysis techniques have traditionally been used mainly for hydrocarbon reservoir applications. However, the same techniques are equally applicable to reservoir issues associated with energy transition, such as geothermal prospects, carbon geosequestration, and hydrogen storage. Traditionally, much core analysis has been performed successfully using core plugs. However, this approach has certain drawbacks: (1) the selected plugs may not necessarily be representative of the full range of lithologies, (2) key features (e.g., thin naturally cemented or fractured zones) may be missed, (3) high-resolution detail at the lamina scale may be missed, (4) depth shifting to well logs may not be sufficiently accurate, and (5) this strategy may be more sensitive to missing core. In this paper, we highlight the usefulness of probe core analysis techniques on slabbed core and powdered samples. For many reservoirs relevant to energy transition, it is crucial to have a high-resolution continuous record of petrophysical properties so that key features are not missed. Probe measurements are less destructive, without the need to cut core plugs, and provide: (1) high-resolution data at the lamina scale so that key features and small-scale heterogeneities can be identified, (2) improved depth matching to well-log data, and (3) rapid, cost-effective data. We describe examples highlighting some different probe techniques. While some techniques are well known, such as probe permeability, others, such as probe acoustics, probe luminance (from linear X-ray measurements), and probe magnetics, are less familiar to core analysts but are well suited for analyzing cores from reservoirs associated with energy transition as well as hydrocarbons. For example, potential geothermal prospects involve studying igneous and metamorphic samples (where the main radiogenic heat sources reside) as well as sedimentary samples, and differences in the magnetic susceptibility signals using a small, portable magnetic probe can quickly differentiate the different rock types. Probe acoustics can be used to (1) rapidly identify anisotropy by orienting the acoustic transmitter-receiver bracket in different directions, (2) identify open microfractures via longer transit times, and (3) produce high-resolution porosity profiles after correlation of transit times with some representative plug or well-log porosity data. Probe luminance and associated linear X-ray images, which are related to density, can indicate small-scale heterogeneities that may impact permeability variation and anisotropy and may not be seen from mere visual observations of the slabbed core surface.

Gravity inversion by second order approximation applied to the Styrian Basin (Austria)

AbstractAn approximative second order gravity inversion scheme by a truncated power series expansion is applied to derive the thickness of the Neogene, mostly clastic, sedimentary section of the Styrian Basins in South–East Austria, which are sub‐basins of the Pannonian Basin System. These sub‐basins with a derived thickness of up to 4 km are of interest for geothermal exploitation because of the increased geothermal gradient and heat flow observed in the Pannonian Basin in general and a geothermal gradient of 4°–5°/100 m measured in some wells in the Styrian Basin. The Styrian Basin also has been an area for hydrocarbon exploration in the past 50 years, with oil and gas show encountered in several exploration wells and one sub‐commercial gas discovery. The Miocene and Plio–Pleistocene volcanism in the Styrian Basin caused by Miocene crustal thinning is discussed in terms of the influence to the gravity inversion taking the aeromagnetic field into account. The volcanism is of relevance for the geothermal prospectivity but poses problems for the single layer‐based gravity inversion scheme. Results are discussed from a computational side comparing observed and calculated gravity fields but also the match with well data is discussed. In terms of gravity inversion methodology, the presented can be viewed as an approximative fast‐track approach.

2D magnetotelluric inversion based on ResNet

In this study, a deep learning algorithm was applied to two-dimensional magnetotelluric (MT) data inversion. Compared with the traditional linear iterative inversion methods, the MT inversion method based on convolutional neural networks (CNN) does not rely on the selection of the initial model parameters and does not fall into the local optima. Although the CNN inversion models can provide a clear electrical interface division, their inversion results may remain prone to abrupt electrical interfaces as opposed to the actual underground electrical situation. To solve this issue, a neural network with a residual network architecture (ResNet-50) was constructed in this study. With the apparent resistivity and phase pseudo-section data as the inputs and with the resistivity parameters of the geoelectric model as the training labels, the modified ResNet-50 model was trained end-to-end for producing samples according to the corresponding production strategy of the study area. Through experiments, the training of the ResNet-50 with the dice loss function effectively solved the issue of over-segmentation of the electrical interface by the cross-entropy function, avoided its abrupt inversion, and overcame the computational inefficiency of the traditional iterative methods. The proposed algorithm was validated against MT data measured from a geothermal field prospect in Huanggang, Hubei Province, which showed that the deep learning method has opened up broad prospects in the field of MT data inversion.

Hydrogeochemistry and isotopic assessment of a new geothermal prospect in rancho Nuevo (Guanajuato-Central Mexico)

Mexico has five high-temperature geothermal fields, three of which are located in an active volcanic arc known as the Trans-Mexican Volcanic Belt in central Mexico. In this geological province, there are numerous hot springs, many of which have not been studied yet. It is the case of the Rancho Nuevo area, which is composed of two hot springs and thermal groundwater wells. The present study focuses on the hydrogeochemical and isotopic characterization of the Rancho Nuevo geothermal system. According to the results cationic exchange, silicates and carbonates rock alteration processes are occurring. A cooling of thermal water is also carried out caused by a mixing process between deep Na-Cl type water and shallow Na-HCO3. In addition, the most relevant process identified in the study area is the water-rock interaction and corroborated by the stable isotope results showing enrichment of δ18O. All these processes are enhanced by the thermal activity in the region. The CO2 and N2 concentrations confirm the geothermal activity in the Rancho Nuevo area. The CH4 concentration is also typical of geothermal fluids and may represent a magma degassing.Finally, the reservoir temperature was estimated at 225 °C ± 15 °C based on silica, cation, and integrated multicomponent solute geothermometers. Therefore, the Rancho Nuevo geothermal area can be defined as a high-temperature system and as a very interesting prospect; however, for geothermal resource production, the calcium carbonate deposition in the wells shall be considered.

Geothermal energy prospect for decarbonization, EWF nexus and energy poverty mitigation in East Africa; the role of hydrogen production

The affordability and availability of water and energy have a huge impact on food production. Research has shown that there exists a direct and indirect link between power production and clean water generation. Hence the inclusion/importance given to the energy-water-food (EWF) nexus in the United Nations' sustainable development goals. Acknowledging the importance of decarbonization to the global future, there exists a gap in literature on the development of models that can enhance the EWF nexus, reduce energy poverty, and achieve 100% renewable energy in the electricity sector. Therefore, the technical and economic prospect of geothermal energy for bridging the aforementioned gaps in existing works of literature is presented in this study. The energy poverty/wealthy status of a country has been confirmed to have a significant impact on economic development, as economic development is largely reflected in the food-water availability. Ditto, this study is focused on the interconnectivity of the EWF nexus while incorporating global decarbonization targets. Geothermal energy is of the utmost significance in East Africa due to its abundant potential and distinctive geological features. Located in the Great Rift Valley, the region has an abundance of geothermal reservoirs, making it an ideal location for geothermal power generation. This study is novel as a comprehensive assessment framework for energy poverty is developed and innovative models utilizing primarily the geothermal resource in the East African region to mitigate this problem are proposed and analyzed. The role of hydrogen generation from critical excess electricity production is also analyzed. The East Africa region is considered the case study for implementing the models developed. A central renewable energy grid is proposed/modelled to meet the energy demand for seven East African countries namely; Ethiopia, Tanzania, Uganda, Djibouti, Comoros, Eritrea, and Rwanda. This study considers 2030, 2040, and 2050 as the timestamp for the implementation of the proposed models. The hybrid mix of the biomass power plant, solar photovoltaic (PV), pumped hydro storage system and onshore wind power is considered to furthermore show the potency of renewable energy resources in this region. Results showed that the use of geothermal energy to meet energy demands in the case study will mitigate energy poverty and enhance the region's EWF.

Subsurface structural control of geothermal resources in a magmatic rift: gravity and magnetic study of the Tulu Moye geothermal prospect, Main Ethiopian Rift

Since the Quaternary, extension and magmatism in the Main Ethiopian Rift (MER) have been mainly focused into narrow magmatic segments that have numerous volcanic centers and caldera collapses that offer favorable conditions for the occurrence of geothermal resources. However, the subsurface structure of the volcanic systems (0–10 km) and their link to the distribution of shallow geothermal resources remain unclear. To investigate the role of subsurface structures on the occurrence of these resources, we conducted gravity and magnetic studies combined with geological constraints within the Tulu Moye Geothermal Prospect (TMGP), one of the current geothermal prospects in the central MER associated with caldera collapses. Gravity data from the Global Gravity Model plus (GGMplus 2013) and ground magnetic data transformed into residual and derivative maps reveal that shallow magmatic intrusions occur under the volcanic centers (Tulu Moye, Bora, and Bericha). Our interpretation along with recent magnetotelluric model suggests that only the intrusion beneath Tulu Moye is currently magmatically active and includes partial melt, consistent with it being a primary heat source for the geothermal system. A new caldera formation model is proposed where the TMGP hosts an older large caldera (about 25 km diameter) within which there are several smaller nested caldera systems associated with the Bora, Bericha, and Tulu Moye volcanoes. Along with existing geologic, seismic, and magnetotelluric studies, our gravity and magnetic analysis indicate the interaction between NNE-SSW (rift-parallel) and NW-SE (cross-rift) trending faults, along with shallow magmatic intrusions and caldera systems, suggesting that such a large geothermal system is possible under these conditions.

Deciphering Hydrochemistry and Fluid-Mineral Equilibria from Characteristic Low-Enthalpy Geothermal Waters of Himalaya and Eastern India

Geothermal energy, with associated low-carbon emissions compared to conventional fossil fuels, is presently one of the potential renewable energy sources. The decryption of the chemistry of thermal manifestations of geothermal systems, in terms of relative abundances of various chemical facies, is mandatory before launching systematic geothermal exploration in a given area as it provides valuable information on the basic characteristics of the geothermal reservoir. For the present study, three clusters of hot springs have been selected -1) Shyok-Nubra valleys geothermal prospect in NW Himalaya, 2) Subansiri valley geothermal areas in NE Himalaya and 3) the hot spring system occurring along Munger-Saharsa Ridge Fault Zone (MSRF) in the Eastern part of India. The study of these geothermal systems with low to moderately high temperatures of 35−75°C, has been carried out with two main objectives. The first objective is to estimate reservoir temperature and other characteristics in the three above-mentioned hot spring groups using conventional chemical geothermometry techniques for the first-hand assessment of the quality of the geothermal resource and postulating its possible application. The second objective of the study is to understand the distribution of stable isotopes (δ2H and δ18O) in the studied geothermal systems to reveal reservoir-related hydrological aspects of the geothermal system. The three groups of hot springs show large variations in total dissolved solids (TDS) from <150 to about 1800 mg/L. Enigmatic and inexplicably low TDS values of 126 to 150 mg/Lare reported from the three Bhimband hot springs along MSRF Zone. These springs discharge Ca−Mg−HCO3 type water. The dearth of dissolved ionic species in these springs indicates lack of water-rock interaction in Bhimband area. Silica is the most abundant solute species and corresponds to a reservoir temperature of about 75°C. It may be inferred that relatively inert lithology in the reservoir and very high water-to-rock ratio might have rendered the observed chemical signatures to Bhimband hot springs. Shyok-Nubra springs at Changlung and Panamik are predominantly Na-HCO3 type with TDS values of about 1700 and 550 mg/L, respectively. These hot springs give an indication of water-rock interaction in different types of lithology at temperatures of 120 to >150°C resulting in the observed chemical differences between the two. Thermal water at Taksing in NE Himalaya is also Na−HCO3 type. Chetu hot spring with TDS of >1100 mg/Lis anomalous in two ways. First, it has a very high SO4 content of 358 mg/Land second, it has the lowest silica value. There is a possibility that the Chetu hot spring has its chemistry influenced by the dissolution of sulfates of Ca and Na. Preliminary stable isotope study indicates that the geothermal fluid is derived from meteoric sources. The lack of any indication for positive δ18O-shift suggests that reservoir temperatures of the springs are generally low. Keywords: Thermal Waters, Hydrogeochemistry, Reservoir Temperature, Stable Isotopes, Himalayas, Bihar

Delineating reservoir area using geologic remote sensing analysis, Jizan (Saudi Arabia): Implications for geothermal exploration

Jizan Province is considered one of the potential geothermal areas in Southwest Saudi Arabia. However, the prospective geothermal reservoir is still not delineated and the geothermal energy reserve is not estimated properly. Therefore, this study aims to delineate the geothermal prospect area using remote sensing-based analysis. Permeable areas, the high anomaly of Land Surface Temperature (LST), and the existence of granitic rock were the parameters used in the remote sensing analysis. Digital Elevation Models (DEM) SRTM and Landsat 8 images were used as the data source. Automatically- and manually-extracted structural lineaments supported by fractures/faults interpretation were integrated to generate a final map of the permeable areas defined in the study area (with three main permeable zones; Zones 1, 2 and 3). The average value of five LST maps and the Natural Different Building Index (NDBI) were used to investigate the high anomaly of LST. Final fuzzy logic integration of the defined permeable area, high LST anomaly, and the existence of granitic rock allowed a unique opportunity for the delineation of the prospective geothermal reservoir area. Remote sensing results were validated using the field-based studies performed in the same area. Three zones are suggested as prospective geothermal potential areas (Zones A, B, and C), where Zone A is the most favourable due to its estimated largest area and is interpreted to belong to the same geothermal system as Zone C. This study improves the understanding of the prospective geothermal reservoir area in Jizan based on the delineation of high permeable zones, and high anomaly LST. The spatial distribution of radiogenic elements in the rock units is also considered an indicator of the heat source of the geothermal system.

Hydrochemical, D–O–Sr isotopic and electromagnetic characteristics of geothermal waters from the Erdaoqiao area, SW China: Insights into genetic mechanism and scaling potential

The Erdaoqiao area has been the hot area of geothermal resource prospecting in southwestern China. Significance difference of scaling potential has been found between left and right riverbanks of the Erdaoqiao area. However, there still has no interpretation for the scaling difference since the genetic mechanism of geothermal waters remains unclear. In this study, hydrochemical, D–O–Sr isotopic and electromagnetic (EM) approaches were integrated to clarify the genetic mechanism and quantitively evaluate scaling potential of geothermal waters. All geothermal waters belonged to Ca–Na–HCO3 type and were determined by the weathering and/or dissolution of carbonates (66.87%) and silicates (31.53%) with cation exchange based on hydrochemical and Sr isotopic analyses. Na–K–Mg triangle diagram indicated they were immature water mixed by shallow cold water. Quartz geothermometer indicated the shallow reservoir temperatures (after cold–water mixing) of left riverbank (88.46 °C–90.51 °C) were higher than those of right riverbank (61.20 °C–87.89 °C). Silica – Enthalpy methods revealed the initial geothermal reservoir temperatures and cold-water mixing ratio were 102.03 °C–138.93 °C and 65.24%–85.87% in the right riverbank, 144.11 °C–146.76 °C and 73.08%–74.79% in the left riverbank of the Erdaoqiao area. D-O isotopes proposed the geothermal waters were recharged by atmospheric precipitation in the elevation of 4957 m to 5118 m. Therefore, the genetic model of geothermal waters in the Erdaoqiao area can be constructed. Stronger calcite scaling in left riverbank was attributed to deeper circulation identified by higher reservoir temperature and electromagnetic exploration. Quantitively evaluation demonstrated that the weight of scaling during 48 h in left riverbank (2968 kg) was four times higher than that in right riverbank (748 kg). The achievements of this study contribute to geothermal exploration in the Erdaoqiao and other similar areas worldwide.

Upscaling of geological properties in a world-class carbonate geothermal system in France: From core scale to 3D regional reservoir dimensions

The greater Paris area has some 12 million inhabitants and 48 heating network production units that exploit the heat capacity of a 1.5 km deep aquifer, the Bathonian limestone. This is one of the most productive aquifers in the world for district heating, with an annual output of about 1.7 TWh of energy. The current challenge for Paris is to triple the number of heating networks using geothermal energy so as to reduce dependence on fossil fuels (40% in 2020 in France). As no detailed geothermal reservoir model is available, drilling involves geological risk. A recent well, drilled at Grigny (20 km south of Paris), turned out to be a failure in transmissibility because the bed of permeable limestones it tapped into was very thin (meter-scale). The main aim of this study is to create a digital database and a 3D geological model of this aquifer to minimize geological risks and optimize the location of future geothermal operations around Paris. By compiling data from 168 wells, a high-resolution 3D geological model of 360 km3 size is constructed (about 40 km x 50 km x 0.2 km), made up of 12.2 million cells and displaying sedimentary facies, sequence stratigraphy, porosity (Φ) and permeability (k). About 20% of the oolitic and bioclastic facies are of good reservoir quality (Φ > 13% and k > 350 mD), especially in two targeted, high-quality reservoir sequences. These facies of interest probably correspond to giant dunes and a shoal/barrier prograding from east to west. In these facies, permeable zones are generally 4 m thick and form patches of 1600 m x 1100 m, on average, elongated perpendicular to the depositional slope. 2D and 3D maps of temperature, salinity, porosity, transmissivity, and permeability allow us to understand the areas of interest for geothermal exploration, as demonstrated around Grigny. This model helps us to apprehend better the heterogeneous character of the reservoir for geothermal prospection and to reduce the risk of future doublets during well implantation. Detailed local models may be extracted to anticipate better the implantation of new doublets in areas with already densely spaced existing wells.