Geothermal energy, a clean, continuously available and sustainable energy source, will help boost the current renewable energy supply and, simultaneously, reduce the dominant contribution of fossil fuels to the total energy supply in Italy [1]. Efficient exploitation of this resource, however, requires a characterization of the subsurface region, through integration of multi-parameter datasets, to mitigate the risks of drilling unsuccessful geothermal wells.

The InGEO project (Innovation in GEOthermal resources and reserves potential assessment for the decarbonization of power/thermal sectors, www.ingeo.cnr.it) seeks to develop an innovative exploration workflow integrating geological, geophysical and petrophysical datasets. It focuses on the northern sector of the Northern Apennine buried-structures belonging to the Romagna and Ferrara Folds (RFF), where a thermal anomaly attributable to deep fluid circulation within the deep-seated Mesozoic carbonate sequences, was identified [2].

With an ongoing study focusing on the reconstruction of a 3D geological model of the RFF region [3], this study develops the research by jointly interpreting previous geophysical datasets [4-5] geographically constrained within the RFF region. The novelty consists on the application of a machine learning algorithm for jointly re-interpreting geophysical datasets. The similarities among the geophysical datasets within the RFF are classified by applying the Fuzzy c-means method, which uses the Euclidean distance measure. The findings include the 3D spatial distribution of derived classes and are validated with the 3D geological model of the RFF [3] and laboratory data obtained on rock samples analyses [6].

The resulting 3D geophysical model contributes to the delineation and constraint of shallow and deep structural features within the RFF. This information will be used as input parameters for the development of a thermal model and the implementation of an open-source and web-based GIS tool that will assess the deep geothermal resource potential for both hydrothermal resources and closed-loop deep heat exchangers solutions in Italy, but with potential to extend the approach in different geological contexts. The workflow of InGEO project will be used as a decision support system for developing geothermal projects in Italy.

InGEO is a PRIN 2022 PNRR Project and has received funding from the European Union, Next Generation EU.

References

[1] International Energy Agency (IEA) Italy 2023 Energy Policy Review. https://www.iea.org/reports/italy-2023

[2] Pasquale et al., 2013. Evidence for thermal convection in the deep carbonate aquifer of the eastern sector of the Po Plain, Italy. Tectonophysics 594, 1-12.

[3] Cortassa et al., 2024. Integrated geological modelling for assessing geothermal potential in the Romagna and Ferrara Folds (Italy). 43° National Conference GNGTS, Bologna, 11-14 February 2025.

[4] Nouibat et al., 2023. Ambient-noise wave-equation tomography of the Alps and Ligurian-Provence basin. J. Geophys. Res., 128, e2023JB026776.

[5] Zahorec, et al., 2021. The first pan-Alpine surface-gravity database, a modern compilation that crosses frontiers, Earth Syst. Sci. Data, 13, 2165–2209.

[6] Slupski et al., 2025. The importance of measuring thermal and acoustic properties on rock analogues in geothermal potential assessment studies: the example of Northern Apennines Triassic carbonate platform and underlying basement rock. 43° National Conference GNGTS, Bologna, 11-14 February 2025.

How to cite: Basant, R., Tesauro, M., Cortassa, V., Gola, G., Nanni, T., Galgaro, A., and Manzella, A.: Machine Learning-Based Joint Interpretation of Geophysical data for the geothermal potential assessment in the Romagna and Ferrara Folds (Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12018, https://doi.org/10.5194/egusphere-egu25-12018, 2025.

Geothermal energy is emerging as a promising renewable power source in the global energetical transition toward a clean energy future. The role of shallow and deep tectonic structures on fluid circulation and heat transfer is of primary importance to better target high-temperature geothermal reservoirs. In extensional context, the complexity of geothermal processes results from the coexistence of multiple structures whose activity evolves through time: normal to oblique faults, detachments and transfer zones. This complex structural setting leads to major questions: What are the main pathways driving fluid flow and heat transfer in high-temperature geothermal systems? What is the plumbing of the system at depth? What is the origin of geothermal fluids and recharge potential of the system? To address these issues, we propose a coupled study of an active system, i.e. the Larderello geothermal system (Tuscany, Italy), and its fossil, exhumed equivalent, i.e. the Elba Island and the Boccheggiano area (Italy). This region is located in a back-arc extensional context, following the eastward retreat of the west-verging Adriatic slab that started about 35 Ma ago. Deformation, that migrates eastward with the slab retreat, has been accommodated by low-angle detachments and associated normal faults. Such structures allowed the exhumation of metamorphic core complexes (MCCs) and the emplacement of plutonic bodies whose ages decrease eastward. Therefore, Elba Island with its exhumed detachments and MCC can be considered as a fossil equivalent of the active Larderello geothermal system. Through this project, we propose a Thermo-Hydro-Mechanical (THM) model of the Larderello geothermal system constrained by the available geophysical and geochemical data. Additional constraints at depth will be then provided by fieldwork observations (structural, mapping, sampling) and laboratory analysis (isotopes, elemental composition, fluid inclusion) on mineralized fault zones mainly from Elba Island. The disclosed part at EGU 2025 will focus on the preliminary modelling activities of Larderello. Firstly, a geological model is built with the PETREL software based on the available borehole data and interpreted seismic lines or cross-sections, which enables to display the 3D lithostratigraphic sequence and the structural geometries at depth. It highlights the complexity of this system related to boudinage favored by low-angle normal faults, which caused important thickness variations (up to disappearance) of some geological units through space, as well as important normal fault offsets, and flat horizons associated to shear zones at depth. Subsequently, the THM numerical model is solved with the COMSOL Multiphysics software by employing the above-mentioned geological model as the main geometrical framework. Presented numerical results focus on the plumbing of the system (different types of faults and their crosscutting relationships) and its role on heat transfer and fluid flow processes. Furthermore, different water recharge scenarios are also investigated. The chosen physical parameters involved in fluid flow, heat transfer and poroelasticity phenomena together with their implications on the numerical solutions are discussed. This on-going and multidisciplinary work participates to assess the geothermal potential, identify new exploitable areas and estimate the lifetime of high-temperature geothermal systems, fitting the global context of carbon-free energy development.

How to cite: Lagardère, C., Do Couto, D., Verlaguet, A., Jolivet, L., Leroy, S., and Gola, G.: Thermo-Hydro-Mechanical modelling of the Larderello geothermal system, Tuscany, Italy: Emphasis on the role of tectonic structures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17687, https://doi.org/10.5194/egusphere-egu25-17687, 2025.

How to cite: Lanari, R., Bonini, M., Sembroni, A., Papeschi, S., Del Ventisette, C., Smith, A., Lupi, M., and Montanari, D.: Topographic signature of magmatic emplacement: the case of the Larderello-Travale Geothermal area (Northern Apennines, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7956, https://doi.org/10.5194/egusphere-egu25-7956, 2025.

Seismic activity in Tuscany, Italy, is driven by the interplay between complex tectonics and fluid flow processes. Fluid-driven seismic sequences are typically associated with high-enthalpy geothermal regions, such as the Larderello-Travale Geothermal Field (LTGF). To better understand the regional tectonic setting, we derive a detailed catalogue of earthquake hypocenters and magnitudes from a seismic network consisting of 30 permanent seismic stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and 30 temporary stations deployed in Tuscany in the framework of a specific acquisition survey  (TEMPEST), during a period of one year (from September 2020 to September 2021).

We applied an automated processing routine including a machine‐learning (ML) phase picker, PhaseNet, and the Gaussian Mixture Model Association (GAMMA) algorithm, a sequential earthquake association and location workflow. We initially obtained nearly 1 million P-phases and 2 million S-phases, yielding around 5k detected events. We then located the events with NonLinLoc and applied quality metrics to filter out potential false detections (22%) and recognize the high-quality solutions, which represented 30% of the initial 5k locations. The high-quality catalogue has been relocated with the Double-Difference software scrtdDD to better constrain earthquake clusters and to improve the robustness of  the subsequent analysis of the seismic sources in the region.

We identified a five-day-long sequence of 203 earthquakes with magnitudes Mw ranging from 0.4 to 2.0, oriented NW-SE, north of Monte Amiata (Southern Tuscany). We observe a temporal upward migration of the events between 10 to 5 km depth towards SE. We have obtained seven high-quality focal mechanisms from first-motion polarities by applying a Bayesian approach (Mtfit). Our results suggest the occurrence of a seismic swarm with predominant normal faulting mechanisms and average dipping angles of 50° NE. Our solutions could be associated with a normal fault system that is compatible with the local geology.

How to cite: Porras, J., Michailos, K., Savard, G., Montanari, D., Saccorotti, G., Bonini, M., Del Ventisette, C., and Lupi, M.: Seismicity in central Tuscany, Italy: Insights from a regional seismic deployment, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15476, https://doi.org/10.5194/egusphere-egu25-15476, 2025.

Unlocking the geothermal potential of Britain’s Mississippian carbonate platforms (MCP) could significantly advance the UK transition to net-zero carbon emissions. Geothermal energy offers a reliable and clean heat source, yet comprehensive assessments of geothermal resources remain limited, despite the urgency of decarbonization efforts. The MCP, equivalent to Early Carboniferous limestones (ECL), are linked to thermal springs in Bath, Bristol, the Taff Valley (South Wales), and the Peak District (UK). Their bulk permeability, however, is highly variable, with fluid flow largely governed by faults and fractures. While Belgium and the Netherlands have successfully harnessed similar formations for geothermal energy, the MCP in Britain remain underexplored. Limited seismic data and sparse boreholes reaching the top of the MCP highlight the critical need for further investigations.

Our main focus in this presentation addresses the connectivity of the fault and fractures  in the MCP,through multi-scale data, to provide valuable insights into geothermal energy potential. This study integrates regional, local, and outcrop-scale data from the MCP. Fault and fracture maps are developed using: (a) seismic reflection data and published geological maps, and (b) fieldwork imagery. In situ stress and pore pressure data are drawn from legacy onshore hydrocarbon wells, wireline logs, British Geological Survey reports, and recent publications. Fault stability analyses, incorporating Normalised slip (Ts) and dilation (Td) tendencies, provide insights into likely fault and fracture behaviour. By addressing uncertainties in input parameters, the study evaluates their implications for geothermal resource exploration.

Key findings in this study include: (a) a detailed characterization of fracture network connectivity and patterns in the MCP, (b) refined understanding of regional-to-local fault-fracture interpretations, and (c) permeability estimates under prevailing stress conditions. The integration of outcrop and subsurface data enhances the reliability of the interpretations, bridging the gap between field observations and geological modelling. Additionally, insights into the role of subsurface stress regimes and their impact on fault stability will provide valuable guidance for optimizing drilling strategies and mitigating operational risks.

How to cite: Aditama, M., Hollis, C., Huuse, M., and Healy, D.: Multi-scale fault and fracture networks of Mississippian carbonate platforms (MCP): implications for extracting geothermal energy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4250, https://doi.org/10.5194/egusphere-egu25-4250, 2025.

The Buda Thermal Karst is characterised by three geographically and geologically separated discharge areas along the Danube in the Buda Hills (Erőss et al, 2012 etc.). These contain lukewarm, lukewarm - warm and warm springs, respectively. This research aims to find an answer to the question of the separation and differences of these discharge areas and their significance in geothermal exploration under the confined Pest side.

Along with the datasets used in previous numerical simulations (Szijártó et al, 2021 etc.), the Supervisory Authority for Regulatory Affairs Hungary (SARA) has made available a new three-dimensional geological model of the study area. This updated model enables more accurate numerical simulations.

In the numerical simulations of the recent study, we use the previous datasets and we are creating five two-dimensional cross-sections by Comsol Multiphysics. The evaluation includes a flow system and temperature analysis that relies on a comparison between model results and on-site measurement data. The primary objectives of the modelling include:

(1) evaluating the connection between lukewarm and warm springs of the Central system and the thermal waters in the deep confined karst areas of Pest

(2) assessing the structural separation of this system from adjacent Northern and Southern areas and its implications for geothermal exploration risks.

This research is directly linked to geothermal risk assessment by testing how numerical model outputs can be integrated into the development of general methodologies for geological risk analysis.

Preliminary findings suggest that the thermal springs of the Buda Thermal Karst Central system are connected to the regional discharge area at the Danube River. This indicates that the most favourable conditions for geothermal utilization can be found in areas near the Danube on the Pest side. However, these areas are also critical for protecting the water of the thermal baths, emphasizing the need to incorporate risk assessment into geothermal planning. The modelling results offer multiple applications concerning data integration. In this research, the theoretical temperature distribution estimated through statistical analysis of the model outputs can be used to predict the performance of geothermal projects and evaluate the geological feasibility of potential developments. This integrated approach highlights the importance of balancing geothermal energy utilization with conserving natural thermal resources.

This research was carried out within the framework of the project RRF-2.3.1-21-2022-00014 of the Climate Change Multidisciplinary National Laboratory.

Supported by the EKÖP-24 University Excellence Scholarship Program of the Ministry for Culture and Innovation from the Source of the National Research, Development and Innovation Fund. Contract number: ELTE/15380/1(2024)

References

Erőss, A., Mádl-Szőnyi, J., Surbeck, H., Horváth, Á., Goldscheider, N., & Csoma, A. É. (2012). Radionuclides as natural tracers for the characterization of fluids in regional discharge areas, Buda Thermal Karst, Hungary. Journal of Hydrology, 426, 124-137.

Szijártó, M., Galsa, A., Tóth, Á., & Mádl-Szőnyi, J. (2021). Numerical analysis of the potential for mixed thermal convection in the Buda Thermal Karst, Hungary. Journal of Hydrology: Regional Studies, 34, 100783, pp.2.

How to cite: Tóthi, T., Mádl-Szőnyi, J., Markó, Á., Csicsek, L. Á., Szilágyi, I., and Szijártó, M.: Connections between Thermal Springs and Deep Geothermal Potential in the Buda Thermal Karst System, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4392, https://doi.org/10.5194/egusphere-egu25-4392, 2025.

The Asal-Ghoubbet Rift is a less-than-one-million-year-old area characterized by both magmatic and tectonic activity. It is underlain by very thin crust and exhibits exceptionally high temperatures at reachable depths, making it highly favorable for geothermal energy development. However, despite the high temperatures, geothermal exploitation in this region faces significant challenges, mostly related to sustainable access to fluids, including generally low permeability, low flow rates, and highly saline fluids. Some of the identified sites have proven unproductive, which can be attributed to a limited understanding of the subsurface geothermal system.

This resource is challenging to evaluate, and a reliable 3D geological model is therefore essential. This paper presents the first static model of the geothermal site in the Asal-Ghoubbet region, which serves as a foundation for numerical simulations. To build the geological model, a combination of well data, surface geological observations, geophysical studies, and structural knowledge was employed. As the data are not equally spread across the considered area, four geological cross-sections, representative of the subsurface structures, were created and used as input data for the modeling. The geometry and properties of the subsurface structures were interpolated using the Discrete Smooth Interpolation (DSI) method, implemented through the SKUA/GOCAD software. The resulting 3D model consists of four main stratigraphic units and 17 normal fault planes aligned parallel to the rift.

Numerical modeling of temperature and fluid circulation was performed using the TemisFlow basin modeling software, which integrated the geometry of the geological model built in SKUA, the lithological fill, and a representation of the lithosphere in the study area. Several scenarios were tested to address uncertainties regarding the depth and extent of the magmatic chamber, which influence heat transfer by conduction. Additionally, uncertainties in the hydraulic properties of faults and lithofacies, which control fluid infiltration and consequently heat transfer by convection and advection, were considered. The results were calibrated against measured temperatures in wells.

These findings highlight the main controlling factors of the geothermal system in the Asal Rift, provide a 3D visualization of heat transfer and fluid circulation, and will enable future quantification of the geothermal potential of the area.

How to cite: Abdi Ali, A., Bonté, D., Souque, C., Traby, A., and Nader, F.: 3D Numerical Modeling of the High-Temperature Geothermal System in Asal-Ghoubbet Rift, Djibouti, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3115, https://doi.org/10.5194/egusphere-egu25-3115, 2025.

The island of Pantelleria, located in the Sicily Channel, is a volcanic system characterized by peralkaline (pantelleritic) magmatism and recurrent explosive eruptions, which have produced prominent caldera structures. Active geothermal manifestations, including gas emissions, hot springs, and elevated temperatures detected in deep boreholes, underscore its potential as a geothermal resource.

To elucidate the island's subsurface and enhance the understanding of its volcanic structures, geothermal reservoirs, and the pathways of hydrothermal fluid circulation, a magnetotelluric (MT) survey was performed, including 78 independent measurements. The resulting three-dimensional resistivity model delineates the geological and geothermal framework down to a depth of 2.5 km below sea level.

The resistivity anomalies imaged by the 3D electrical model can be associated with specific geological processes and physical conditions within the geothermal system, accurately identifying the main volcano-tectonic structures, particularly the rims of the La Vecchia and Cinque Denti calderas which drive the hydrothermal system's fluid circulation and storage.

This study highlights the fundamental role of caldera structures in influencing hydrothermal processes and improves our understanding of Pantelleria's geothermal potential. The results provide valuable insights for targeted exploration and sustainable exploitation of the island's geothermal resources.

How to cite: Isaia, R., Sposato, M., Di Giuseppe, M. G., Troiano, A., De Paola, C., and Di Maio, R.: Three-Dimensional Resistivity Imaging of Pantelleria Island: Insights into its Geothermal Potential, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20152, https://doi.org/10.5194/egusphere-egu25-20152, 2025.

Meteoric water infiltrates deeply into high-relief mountain ranges, heats up along its circulation path due to the background geothermal gradient and eventually discharges at lower elevation as thermal springs. Enabling such topographically-driven circulation depends on parameters such as permeability, hydraulic head, and thermal buoyancy of the rising water. Brittle deformation zones, especially active fault zones, often provide high-permeability fluid pathways due to repeated slip and refracturing. A systematic 4D analysis of such fault systems can therefore aid in identifying prospective sites in orogenic belts for more detailed geothermal exploration by 3D seismics and drilling.

Our ongoing GeoTex project investigates the geothermal potential of the Rhône Valley, SW Switzerland – a geothermally active Alpine zone with thermal springs, regional faults, and enhanced seismicity. Using structural field observations, seismological data, and remote sensing, we characterise fault geometries, kinematics and fault rocks in the vicinity of known thermal springs. Observable paleo-fluid pathways marked by hydrothermal veins and rock alteration are treated as analogues for recent thermal water circulation and are linked to major Alpine structures in the underlying basement units, such as large-scale strike-slip faults or the axial planes of uplifting basement domes. We identify three geodynamic domains with distinct fault characteristics: (1) A domain on the NW flank of the valley floor characterized by a NW–SE oriented maximum principal stress, high seismicity, and a pervasive network of strike-slip dominated faults; (2) a zone encompassing the valley floor with dilatant zones along strike-slip fault segments; and (3) a zone on the southern flank of the valley floor subjected to recent NE–SW extension expressed by dominantly normal to transtensional faulting focal mechanisms.

For each domain, we developed conceptual structural models helping to identify present-day fluid pathways. Integrating hydrochemical data (indicative of deep-geothermal fluid circulation) into our models, allows us to refine our understanding of such fluid pathways and to predict potential locations of blind active geothermal systems throughout the Rhône Valley and other Alpine settings.

How to cite: Herwegh, M., Schmid, T., Berger, A., Diehl, T., Madritsch, H., Diamon, L., Wanner, C., and van den Heuvel, D.:  Detecting blind geothermal systems in orogenic belts: Structural controls on paleo-hydrothermal fluid pathways in the Rhône Valley, SW Switzerland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10192, https://doi.org/10.5194/egusphere-egu25-10192, 2025.

The Ruilin-Hongye region in Hualien County, Taiwan, represents a promising site for the development of non-volcanic geothermal energy resources. This study adopts an integrated approach, combining geological, geophysical, and geochemical analyses to assess geothermal potential and delineate reservoir characteristics. The region is located within the Yuli Belt, a complex metamorphic zone dominated by quartz-mica schists with interspersed mafic rock bodies. Geophysical surveys, particularly magnetotelluric (MT) surveys, have identified four shallow geothermal reservoirs at depths ranging from several hundred meters to approximately 1.5 kilometers. These reservoirs are distinguished by fractured and permeable rock formations, facilitating efficient hydrothermal fluid storage and circulation, which are crucial for geothermal energy development.

Geochemical analyses indicate a favorable thermal gradient across the region, supported by recharge from meteoric waters and groundwater inflow along local fault and fracture networks. Borehole temperature profiles combined with simulation data suggest that the reservoirs could support a generation capacity of approximately 14 MWe, positioning the area as a viable candidate for geothermal power production. The study also developed a conceptual 3D geological model, offering a detailed subsurface map that highlights high-potential zones and enables more targeted exploration and drilling strategies.

Our findings contribute to Taiwan’s renewable energy goals by demonstrating the feasibility of non-volcanic geothermal systems as a clean and sustainable energy source. This research underscores the importance of integrating geophysical and geochemical data to accurately characterize subsurface conditions, reduce exploration uncertainties, and optimize resource extraction. Future work should prioritize additional drilling and comprehensive monitoring to confirm production capacity and refine the development model, potentially advancing the role of non-volcanic geothermal systems in Taiwan's energy portfolio. By using such integrated methodologies, this study aims to mitigate development risks and enhance the efficiency of geothermal energy projects.

Keywords: Hualien, non-volcanic geothermal, Reservoir characteristics, Resource evaluation

How to cite: Wang, Y.-J., Yang, C.-H., Tan, C.-H., Yu, C.-W., Su, Z.-Z., Chen, C.-H., and Lin, H.-H.: Integrating Geophysical and Geochemical Insights for Geothermal Resource Evaluation in Taiwan's Ruilin-Hongye Region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3908, https://doi.org/10.5194/egusphere-egu25-3908, 2025.

Time-lapse absolute and relative gravity surveys, are directly sensitive to the mass redistribution in the subsurface. These well-established methods allow for the monitoring of geothermal fields and for assessing the sustainability of the anthropogenic activities (water injection and extraction).

 We apply the hybrid gravimetry method, in order to characterize the fluid redistribution within the subsurface. The hybrid gravimetry method combines absolute and relative microgravity time-lapse measurements and continuously recorded gravity time series. At Theistareykir geothermal field (Northern Iceland), we collected yearly time-lapse data at 26 fixed locations, and we recorded continuous data with 2 superconducting gravimeters (SGs), deployed within the hydrothermal field. Data acquisition started just before the beginning of operation of the powerplant in 2017, allowing us to monitor the hydrothermal system behaviour before and during anthropogenic perturbation.

After 7 years of data collection, we present here the gravity datasets (discrete and continuous) from 2017 to 2024. In particular, we show the continuous SGs signals and we detail the modelled gravity contributions (e.g., local tidal model, atmospheric loading, instrumental drift, ground deformation contribution). These modelled contributions are subtracted from the raw signal, to obtain gravity residuals. These residuals are then compared to the results from the microgravity and absolute surveys. Our datasets evidence a gravity decrease towards the extraction area of the geothermal field, and gravity increase towards the injection area. Furthermore, maps of 2017-2024 microgravity residuals, namely after correction of vertical ground deformation, display localized areas of gravity decrease (-60 µGal), that can be associated to low permeability zones in the production area, as well as small gravity increase (about +15 µGal) towards north (around the centre of injection). The gravity increase trend appears to be controlled by a well-known fissure swarm that crosses the area of Theistareykir.

In order to identify and quantify the hydrothermal processes from the observed gravity variations, we develop numerical functionalities for multi-phase fluid flow models using CSMP++ (Weis et al., 2014) to predict the gravity responses of simplified hydrothermal systems.

References

Weis, P., Driesner, T., Coumou, D., Geiger, S. (2014): Hydrothermal, multiphase convection of H2O-NaCl fluids from ambient to magmatic temperatures: a new numerical scheme and benchmarks for code comparison. - Geofluids, 14, 3, 347-371.
https://doi.org/10.1111/gfl.12080​

How to cite: Giuliante, B., Jousset, P., Riccardi, U., Pivetta, T., Hinderer, J., Weis, P., Krawczyk, C., and Mortensen, A.: Subsurface masses monitoring at Theistareykir geothermal field, Iceland, using hybrid gravimetry., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15606, https://doi.org/10.5194/egusphere-egu25-15606, 2025.

Supercritical geothermal systems offer significant potential for enhanced energy production to support the energy transition due to their high enthalpies and the unique properties of supercritical fluids. However, development and utilisation of such promising resources are still in their infancy and face scientific uncertainties, financial and environmental risks, and engineering challenges. Supercritical hydrothermal reservoirs are inherently associated with proximity to magmatic plumbing systems below them. However, it is not yet well understood the extent to which degassed magmatic volatiles may contribute to supercritical hydrothermal reservoir fluid compositions.

CO₂ (and its species) constitutes the largest proportion of gas content from degassing magmas and hydrothermal fluids. Basaltic host rocks are closely associated with many high-temperature mafic magmatic systems around the world and are prime candidates to host supercritical geothermal reservoirs due to their highly reactive mineral compositions and vesicularity. Previous carbon capture and storage (CCS) focused fluid-rock interaction experiments have shown that basalt facilitates carbonation reactions, forming stable carbonate minerals and enabling the sequestration of atmospheric carbon. Here, we extend the scope of experimental investigations of CO₂ brine interaction with basalt to simulate and quantify the effects of magmatic CO₂ infiltration into a supercritical hydrothermal reservoir.

Two supercritical flow-through experiments were performed at 400°C and 500 bar using a high P-T titanium alloy autoclave system at the experimental hydrothermal geochemistry laboratory in GNS Science, New Zealand. In both experiments the same crushed basalt with a fraction size of 355-500 µm was used. In Experiment 1, distilled water and CO₂ brine was injected for 42 days, while in Experiment 2, distilled water, CO₂ and NaCl brine was injected for 49 days. Daily reacted effluent samples were analysed for major cations, anions, and trace elements by ICP-OES, IC and ICP-MS. Results show the development of alteration fronts across the host rock sample along the reactor’s depth in both Experiments 1 and 2. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) on the solid products from Experiment 1 reveal extensive dissolution of primary bytownite and secondary precipitation of chlorite, blocky calcite, and sphene covering the grain surfaces at the fluid entry point. Calcite precipitation was restricted to the fluid entry location whereas chlorite precipitation was observed along the entire reacted sample, decreasing in amount with distance from the fluid entry location. The penetrative dissolution of glass and primary bytownite deep into the basaltic grain alongside precipitation of secondary mineralogy suggests intact basaltic reservoir may experience fluid pathway evolution as CO₂ brines move through them under supercritical conditions.

How to cite: Rybak, D. T., Sajkowski, L., Kilgour, G., Mountain, B., Kamiya, A., McNamara, D. D., and Kavanagh, J. L.: Experimental investigation of Basalt-CO2 brine interactions at supercritical conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12162, https://doi.org/10.5194/egusphere-egu25-12162, 2025.

The Juisui Geothermal Field, located in eastern Taiwan, is renowned for its hot springs, which are spread along the foothills and riverbanks. Covering only about 12 km², the area features a diverse range of springs, each with distinct characteristics, reflecting its location along a subduction mélange.

Around 15 to 10 Ma, turbidites at the edge of the Eurasian continent were affected by the rifting of the South China Sea. As a result, these rocks were subducted eastward, descending to depths of 40 to 60 km beneath the Philippine Sea Plate. The mantle wedge and oceanic plate’s basic/ultrabasic rocks mixed with sediments and experienced temperatures of up to 550 ℃ and pressures of 17 kBar (Tsai et al., 2013; Baziotis et al., 2017). These rocks were then rapidly uplifted to the surface due to orogeny. As a result, the host rocks in this area are not only quartz-mica schist, but also include serpentinite, meta-gabbro, epidote amphibolite, and Glaucophane schist. Shallow drilling can reach extremely high temperatures— for instance, the JS-5G well reaches 180℃ at just 190 m deep, and the highest temperature recorded was 204 ℃ at 1,428m in the CPC-JS-2 well.

This study analyzed 40 meteoric fluid samples to map the regional meteoric water line, and collected 77 spring and geothermal well  samples during different seasons. The salinity of the hot spring water in the Juisui area ranges from 0.44 psu to 9 psu, while total dissolved solids (TDS) range from less than 520 mg/L to 6,550 mg/L. Bicarbonate concentrations range from 400 mg/L to 6,500 mg/L. Hydrogen isotope analysis suggests that the recharge for the springs may come from mountains at elevations of 830 to 1,100 m. These meteoric waters undergo different types of circulation: shallow circulation is heated by the geothermal gradient, which leads to lower concentrations and places the water on the meteoric water line. Deep circulation hot spring, characterized by Na-K ratios, falls between the greywacke line and the 1:1 line, indicating the mixing of deep brine or metamorphic water, and is far removed from the seawater 27:1. A few hot spring samples have especially high bicarbonate concentrations, indicating past flow through marble. Even when sampling from the same well (GSMMA-RS-1) at different times, there’s significant variation in salinity (ranging from 0.58 psu to 6.89 psu), which highlights the strong heterogeneity of the fluid properties— a key feature of the subduction Mélange. A positive oxygen isotopic shift suggests that deep circulation fluids have undergone water-rock interactions with various surrounding rocks in high-temperature environments.

In terms of mineral saturation, the Saturation Index for both calcite and aragonite in the region is either saturated or near saturation. This suggests that scaling could be an issue in future development, and strategies to prevent it should be considered.

How to cite: Lu, Y.-C., Song, S.-R., Peng, T.-R., Song, T.-J., and Lee, J.-C.: Chemical Characteristics of Geothermal Fluids in the Subduction Mélange: Insights from the Juisui Geothermal Field, Taiwan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16574, https://doi.org/10.5194/egusphere-egu25-16574, 2025.

How to cite: Markó, Á. and Pedretti, D.: PHREI – A simplified Python based Graphical User Interface for PHREEQC to model to chemical clogging processes in reinjection wells , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3613, https://doi.org/10.5194/egusphere-egu25-3613, 2025.

Geochemical methods are extensively used in geothermal exploration and exploitation phases, played a major role in both the identification and utilization of resources. In regions where geothermal systems are concealed or located at significant depths, soil gas surveys become indispensable. These methods focus on detecting anomalous concentrations of hydrothermal gases within the soil atmosphere, providing key insights into subsurface geothermal activity. Previous studies in the western area of La Palma island (Canary Islands, Spain) identified the highest geothermal potential of the studied areas. Consequently, more detailed investigations were conducted in the zones with the most significant anomalies to better characterize their potential for economic exploitation. A detailed geochemical survey with an average measurement spacing of ~12 m was carried out in an area of 0.11 Km² at Puerto Naos. A total of 561 sites were sampled at 40 cm depth using a metallic probe. Gas samples were collected with 60 cc hypodermic syringes and stored in 10 cc glass vials for subsequent laboratory analyses. Spatial distribution maps of diffuse He, H₂, CH₄ and CO₂ emission and δ¹³C-CO₂ were constructed to study the presence of enhanced vertical permeability areas related to high temperature hydrothermal activity at depth. The main CO₂, H₂ and δ¹³C-CO₂ anomalies reveal two well-defined zones located in the southeast and west of the study area. In contrast, He highest values are observed in the northern and southern regions. These patterns may be attributed to secondary processes, including interactions with coastal water and the varying reactivity and mobility of the analyzed species. The spatial distribution of soil gases in Puerto Naos confirms a relative enrichment of and H₂, He, CH₄ and CO₂ in the soil gas atmosphere, suggesting a significant contribution from deep-seated sources. These studies aid in identifying permeable zones and potential upflow areas associated with geothermal system structures, thereby facilitating a more efficient subsequent phase of subsurface exploration.

How to cite: Melián, G. V., Hei Ho, C., Henry, L., Plaza, K., Pérez, N. M., Taño Ramos, D., Trujillo Vargas, L., Ramos Delgado, C., Cartaya, S., Arencibia, M., Gironés, A., Asensio-Ramos, M., Padrón, E., Hernández, P. A., and Padilla, G. D.: Soil gas CO2, He, H2 and isotopic ratios for surface geothermal exploration in Puerto Naos, La Palma, Canary Islands. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13599, https://doi.org/10.5194/egusphere-egu25-13599, 2025.

Geothermal systems become viable when conditions, like appropriate temperature ranges, geological structures, and petrophysical properties, exist less than 3-5 km near surfaces. Assessing these factors is crucial for identifying economically viable geothermal resources. A key aspect of this evaluation is understanding subsurface electrical resistivity, which plays a pivotal role in characterizing geothermal systems. This study focuses on investigating geothermal resources and sites for geothermal power plants in the Tatun Volcano Group (TVG) in Northern Taiwan. Magnetotelluric (MT) methods, in particular, have emerged as a fundamental and powerful tool in geothermal exploration and subsurface architecture beneath geothermal areas. By performing 3D resistivity inversion of MT data collected from 47 measurement stations, this research develops a comprehensive model of subsurface electrical resistivity. Cross-sections and maps at various elevations are generated from this model to identify resistivity patterns essential for geothermal sites and to construct geological cross-sections for advancing geothermal exploration, such as further drilling.

The study highlights three potential geothermal areas with a high-resistivity core beneath a low-resistivity clay cap. Moreover, this study establishes several cross-sections across the north tip of Taiwan. A comparison between magnetization-derived airborne magnetic surveys, gravity-derived density models, and current 3D resistivity models leads to completing these cross-sections. They manifest subsurface architecture and elucidate the structural development. These results offer important new information for developing geothermal research in the TVG area.

How to cite: Nguyen, T. L. C., Huang, W.-J., Chen, C.-C., Yen, C.-S., Chen, C.-H., and Tong, L.-T.: Application of Magnetotelluric (MT) Methods in Geothermal Exploration and Geostructural Investigation: A Case in Tatun Volcano Group, Northern Taiwan , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10278, https://doi.org/10.5194/egusphere-egu25-10278, 2025.

How to cite: Savard, G., Munoz-Burbano, F., Cusin, H. T., Lupi, M., Finger, C., Löer, K., Villiger, L., and Shakas, A.: Geothermal exploration with passive geophysical methods in the canton of Thurgau, Switzerland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21379, https://doi.org/10.5194/egusphere-egu25-21379, 2025.

Geothermal energy is a sustainable and environmentally friendly solution for power generation and district heating/cooling, offering continuous availability throughout the day and year. Despite its global potential, targeted strategies are essential for advancing geothermal resource exploitation.

The InGEO project ("Innovation in GEOthermal resources and reserves potential assessment for the decarbonisation of power/thermal sectors"; www.ingeo.cnr.it) seeks to develop an innovative exploration workflow integrating geological, geophysical, thermophysical, and other datasets to enhance the characterization of potential geothermal reservoirs. This approach can support strategic planning, with scientific information voted to exploit deep geothermal resources in Italy.

Deep-seated carbonate reservoirs, forming the basement of sedimentary basins, are key targets for geothermal development in Italy, they are the main focus of the analyzed case study. In the eastern Po Plain, the buried Romagna and Ferrara Folds (RFF)—stretching from the Emilia Folds to the Adriatic coast and from the northern Apennines to the undeformed Po foreland—show significant geothermal gradient variations, indicative, in some cases of low gradient, of possible convective heat flow in deep carbonate units. Pasquale et al. (2013) reported low geothermal gradients (14 °C/km) within the carbonate reservoir and higher gradients (53 °C/km) in overlying impermeable formations, confirming thermal convection within Mesozoic carbonate units.

To investigate this area, we digitised and analysed a large amount of data, considering over 200 seismic surveys (VIDEPI database, www.videpi.com), 700 deep boreholes (>1500 m deep; CNR database, www.geothopica.igg.cnr.it), and 160 borehole logs (sonic and lithological; Livani et al., 2023), covering ~22,500 km². This extensive dataset underpins the development of a detailed 3D geological model that delineates the thickness variations of major lithological units to a depth of ~10 km. Seismic reflection interpretations, constrained by available well stratigraphy, were used to identify key lithological unconformities.

The resulting 3D geological model represents a fundamental tool for assessing the basin's geothermal potential and refining exploration workflows applicable to analogous basins. The final obtained geothermal model will serve as a benchmark for evaluating geothermal resources and as input for testing the consistency of various geophysical datasets (Basant et al., 2025) and an open-source, web-based GIS tool for multiple applications.

The InGEO project is part of the PRIN 2022 PNRR initiative and is funded by the European Union’s Next Generation EU program.

References

Basant R., Cortassa V., Tesauro M., Gola G., Nanni T., Galgaro A., and Manzella A. Joint interpretation of geophysical data for evaluating the geothermal energy potential in the Romagna and Ferrara Folds (Italy). Gruppo Nazionale per la Geofisica della Terra Solida (43° National Conference GNGTS), Bologna, 11-14 February 2025 (Abstract).

Pasquale, V. Chiozzi, P., and Verdoya, M., 2013. Evidence for thermal convection in the deep carbonate aquifer of the eastern sector of the Po Plain, Italy. Tectonophysics 594, 1-12. 15.

Livani, M., Petracchini, L., Benetatos, C., Marzano, F., Billi, A., Carminati, E., et al.,2023. Subsurface geological and geophysical data from the Po Plain and the northern Adriatic Sea (north Italy). Earth System Science Data Discussions, 2023, 1-41.

How to cite: Cortassa, V., Tesauro, M., Basant, R., Gola, G., Nanni, T., Galgaro, A., and Manzella, A.: A 3D Geological Model of the Romagna and Ferrara Folds, (Eastern Po Plain) for advanced deep geothermal exploration, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12131, https://doi.org/10.5194/egusphere-egu25-12131, 2025.