Geothermal Play Research Articles

Geothermal resources of Hungary: a play-based review

In this paper we review the geothermal resources of Hungary using the geothermal play concept and its scale-dependent hierarchical categorization. The Pannonian Basin belongs to an extensional domain play type, with two main plays: the porous Neogene basin fill sediments and the fractured-karstified Palaeozoic and Mesozoic basement rocks. Both plays can be divided into hydrostatic and overpressured play segments. The most widely utilized play level in the hydrostatic segment of the porous Neogene sediment play comprises laterally extended highly permeable sandstone layers storing low salinity thermal water with average flow rate around 900 l/min and temperatures between 50 °C and 100 °C in the depth range of 1–2 km. The other prosperous play level is represented by the fractured-karstified Mesozoic carbonates in the hydrostatic segment of the basement play having elevated temperatures (90–120 °C) and several thousand l/min flow rates. The play level related to fractured Palaeozoic rocks is less important due to its limited extent. The overpressured segment of the basement play may have high flow rates, but still an untapped resource, where exploitation is limited by high overpressure, high temperature (170–200 °C), and highly saline hot waters.

Expert-driven geothermal play fairway analysis for low to medium temperature hidden systems in northwestern Luzon Island, Philippines

The Philippines is the third largest geothermal power-producing country in the world. However, additional opportunities exist to expand the geothermal portfolio of the country with the exploitation of low- and moderate- temperature resources, as well as assessment and development of hidden geothermal systems (i.e., those with no active surface manifestations). As part of the Geothermal Development Initiative (GEODE) funded through the U.S. Department of State award to the University of Nevada, Reno, a collaboration between the Philippines Renewable Energy Management Bureau (REMB) of the Department of Energy and the University of Nevada in Reno, Great Basin Center for Geothermal Energy occurred from September 2022 to March 2024. As part of this collaboration, an expert-driven geothermal Play Fairway Analysis (PFA) was conducted in the northwestern portion of Luzon Island of the Philippines utilizing existing data sets available to the REMB. The goal of this PFA is to identify areas with greater relative geothermal favorability, determine where the data gaps are, and assist in the discovery of hidden geothermal systems. A geothermal PFA requires the compilation and transformation of datasets into appropriate formats (e.g., tables, shapefiles, and raster files) and evaluation of each dataset to determine how it can be used as a proxy for the key attributes of a geothermal system (e.g., heat and permeability). Once the data are organized, they are weighted for integration into the GIS software to develop a favorability model for heat and permeability. The attribute models are then combined to produce two PFA models: the conventional hydrothermal low- to moderate- temperature geothermal favorability and the hidden systems favorability. The data difference between the conventional geothermal favorability and hidden systems models is that hot springs are not used for the hidden systems model. The conventional favorability model of northwestern Luzon Island indicates areas of higher geothermal favorability in the central and southern sections of the study area. Many of the favorable areas identified in the conventional model already have active leases for geothermal exploration, lending more confidence to the methodology and model, and suggesting that the areas with no current leases and high geothermal favorability could represent future exploration targets. The hidden systems model shows favorable zones in the north, central and southern sectors, and can serve as a tool to decide the areas for exploration of potential hidden geothermal systems, particularly, in the Ilocos Norte, Apayao, and Nueva Viscaya regions.

An introduction to the Bowland Shale Formation, UK: processes and resources

Abstract This volume showcases recent geological, geophysical and geochemical research on the Carboniferous Bowland Shale Formation. The Bowland Shale is a relatively thick and extensive Palaeozoic black shale unit with a long history of debate and controversy in the UK. The Bowland Shale is proven in many of the key Carboniferous basins in the Midlands, northern England and North Wales, and represents a significant, near-continuous temporal record spanning 16 Myr including the important mid-Carboniferous boundary (Mississippian–Pennsylvanian). Since the first geological surveys in the late nineteenth century, the Bowland Shale has been of interest for a variety of reasons, including the search for Irish-type Pb–Zn base metal mineral deposits, and as a source rock in conventional hydrocarbon systems. In the mid-2000s, attention turned to the Bowland Shale as a target for unconventional hydrocarbon extraction, shale gas, following success in the USA. This placed the Bowland Shale at the centre of a series of interconnected controversies and debates from the local to national scale. The geological credibility of the purported shale gas resource in the UK was – and continues to be – highly contentious. This volume contributes to a more updated view of the Bowland Shale, covering topics such as sedimentary, geochemical and physical properties and processes, basin-forming events, hydrocarbon prospectivity, mineralization and heat and fluid flow in the subsurface. The volume also includes a field guide to some of the key localities in the UK. With the benefit of hindsight offered by the latest generation of research, the early regional shale gas assessments failed to attach sufficient weight to the compositional heterogeneity, structural complexity, compartmentalization and highly variable exhumation, erosion and palaeo-heat flow history of the Bowland Shale. The following topics are identified as promising avenues for future research beyond shale gas: (1) the role of the Bowland Shale in the context of mineral systems; (2) the role of the Bowland Shale as a cap rock in the Lower Carboniferous limestone geothermal play and potentially as an analogue of CO 2 or radioactive waste storage in the UK; and (3) pathways and mechanisms for weathering, alteration and trace element release from the Bowland Shale into the surface and/or subsurface environment.

De-risking superhot EGS development through 3D play fairway analysis: Methodology development and application at Newberry Volcano, Oregon, USA

The DErisking Exploration for multiple geothermal Plays in magmatic ENvironments (DEEPEN) project aims to de-risk exploration of geothermal plays in magmatic systems, with a focus on superhot and supercritical geothermal systems. This article presents advancements in geoscientific assessment of superhot enhanced geothermal system (EGS) resources as part of the DEEPEN project, particularly in exploration and identification of these systems. While there exists a general consensus on key components of conventional hydrothermal systems, there has not been a prior consensus on such components for EGS, let alone superhot EGS. The DEEPEN project identified these key components for superhot EGS plays, and used them to modify the traditional geothermal play fairway analysis (PFA) approach to identify favorable areas in superhot EGS plays in 3D. This was done through modeling both favorability and uncertainty separately for each key component of a superhot EGS play, incorporating diverse 3D geoscientific datasets, including geologic features, models produced from direct observations, single inversions, and joint inversions. The PFA is applied to Newberry Volcano as a form of validation. The results appear to validate the methodology, aligning with expectations derived from conceptual modeling, and highlighting the area targeted for EGS stimulation well NWG 55-29 as favorable, and suggesting potential additional areas worthy of exploration at Newberry.

Hydrothermally altered shear zones: A new reservoir play for the expansion of deep geothermal exploration in crystalline settings

Our study investigates the genesis of granitic reservoirs formed within a brittle shear zone in central Finland, evaluating their feasibility as deep geothermal targets. We employ a suite of laboratory-based experiments and cutting-edge analytical mineral techniques to determine the petrophysical and thermal properties of targeted granitic rocks, elucidating their formation processes. The most favorable reservoir properties were observed in granites affected by cataclasis and mineral dissolution, leading to a notable secondary porosity of ∼20 %. Reservoir quality is largely controlled by the pore network morphology. Alongside fractures, interconnected moldic, sieve, and interparticle pores contribute to substantial permeability of ∼5 × 10−14 m2 (50mD), even under high confining pressures of 50MPa (∼2km). A new geothermal play is here presented: brittle shear zones that have undergone high-temperature (∼200–300 °C) hydrothermal alteration. These findings will significantly enhance our capacity to identify and exploit prolific permeable zones in deep crystalline settings globally, thereby making a substantial contribution to our ongoing transition to cleaner energy resources.

Geothermal play fairway analysis, part 1: Example from the Snake River Plain, Idaho

The Snake River Plain (SRP) volcanic province overlies the track of the Yellowstone hotspot, a thermal anomaly that extends deep into the mantle. Most of the area is underlain by a basaltic volcanic province that overlies a mid-crustal intrusive complex, which in turn provides the long-term heat flux needed to sustain geothermal systems. Previous studies have identified several known geothermal resource areas within the SRP. For the geothermal study presented herein, our goals were to: (1) adapt the methodology of Play Fairway Analysis (PFA) for geothermal exploration to create a formal basis for its application to geothermal systems, (2) assemble relevant data for the SRP from publicly available and private sources, and (3) build a geothermal PFA model for the SRP and identify the most promising plays, using GIS-based software tools that are standard in the petroleum industry.The study focused on identifying three critical resource parameters for exploitable hydrothermal systems in the SRP: heat source, reservoir and recharge permeability, and cap or seal. Data included in the compilation for heat source were heat flow, distribution and ages of volcanic vents, groundwater temperatures, thermal springs and wells, helium isotope anomalies, and reservoir temperatures estimated using geothermometry. Reservoir and recharge permeability was inferred from the analysis of stress orientations and magnitudes, post-Miocene faults, and subsurface structural lineaments based on magnetics and gravity data. Data for cap or seal included the distribution of impermeable lake sediments and clay-seal associated with hydrothermal alteration below the regional aquifer. These data were used to compile Common Risk Segment maps for heat, permeability, and seal, which were combined to create a Composite Common Risk Segment map for all southern Idaho that reflects the risk associated with geothermal resource exploration and identifies favorable resource tracks.Our regional assessment indicated that undiscovered geothermal resources may be located in several areas of the SRP. Two of these areas, the western SRP and Camas Prairie, were selected for more detailed assessment, during which heat, permeability, and seal were evaluated using newly collected field data and smaller grid parameters to refine the location of potential resources. These higher resolution assessments illustrate the flexibility of our approach over a range of scales.

Gas Emissions and Subsurface Architecture of Fault‐Controlled Geothermal Systems: A Case Study of the North Abaya Geothermal Area

AbstractEast Africa hosts significant reserves of untapped geothermal energy. Exploration has focused on geologically young (<1 Ma) silicic calderas, yet there are many sites of geothermal potential where there is no clear link to an active volcano. The origin and architecture of these systems are poorly understood. Here, we combine remote sensing and field observations to investigate a fault‐controlled geothermal play located north of Lake Abaya in the Main Ethiopian Rift. Soil gas CO2 and temperature surveys were used to examine permeable pathways and showed elevated values along a ∼110 m high fault, which marks the western edge of the Abaya graben. Ground temperatures are particularly elevated where multiple intersecting faults form a wedged horst structure. This illustrates that both deep penetrating graben bounding faults and near‐surface fault intersections control the ascent of hydrothermal fluids and gases. Total CO2 emissions along the graben fault are ∼300 t d−1; a value comparable to the total CO2 emission from silicic caldera volcanoes. Fumarole gases show δ13C of −6.4‰ to −3.8‰ and air‐corrected 3He/4He values of 3.84–4.11 RA, indicating a magmatic source originating from an admixture of upper mantle and crustal helium. Although our model of the North Abaya geothermal system requires a deep intrusive heat source, we find no ground deformation evidence for volcanic unrest or recent volcanism along the graben fault. This represents a key advantage over the active silicic calderas that typically host these resources and suggests that fault‐controlled geothermal systems offer viable prospects for geothermal exploration.

Exploratory analysis of machine learning techniques in the Nevada geothermal play fairway analysis

Play fairway analysis (PFA) is commonly used to generate geothermal potential maps and guide exploration studies, with a particular focus on locating and characterizing blind geothermal systems. This study evaluates the application of machine learning techniques to PFA in the Great Basin region of Nevada. Following the evaluation of various techniques, we identified two approaches to PFA that produced promising results, 1) supervised Bayesian probabilistic neural networks to generate geothermal potential maps with confidence intervals, and 2) unsupervised principal component analysis paired with k-means clustering to generate both cluster maps to help identify spatial patterns, as well as new combined feature inputs. We applied these techniques to perform a comparative analysis between two principal sets of geological and geophysical features related to permeability and heat and a set of positive (known geothermal resources) and negative training sites (known drill sites with unsuitable geothermal conditions). We found that these methods constrain previously unrecognized feature controls on geothermal favorability, many of which are spatially organized within the extent of cluster groups and the major structural-hydrologic domains of the study area. Furthermore, we utilized exploratory unsupervised modeling to highlight spatial relationships between input data and predictive output results of our supervised modeling. Finally, we demonstrate how our models compare to the previous Nevada PFA and how the rapid insights these machine learning techniques offer may support future assessments of both known and undiscovered blind geothermal systems in the Great Basin region of Nevada and beyond.

Geothermal Play Types along the East Africa Rift System: Examples from Ethiopia, Kenya and Tanzania

Based on geophysical, geological and geochemical investigations carried out in the last decade, we reviewed three major geothermal plays that well represent the different structural, volcanological and hydrogeological realms that can be encountered in the East African Rift System (EARS). Alalobeda (Ethiopia) and Menengai (Kenya) are examples of typical geothermal plays of the Eastern Branch of EARS. The former is a fault-leakage-controlled geothermal play located in a graben structure. The heat source is likely deep-seated, widespread magmatism, associated with the lithosphere thinning that regionally affects this area. The reservoir temperature of the water-dominated system ranges from 185 to 225 °C. Menengai can be classified as a convection-dominated magmatic play type. The heat source could be a magmatic intrusion located beneath a caldera. A shallow, liquid-dominated reservoir (with temperatures of 150–190 °C) and an intermediate-deep reservoir, hosting steam and liquid (with temperatures of 230–340 °C), were detected. The Kilambo-Ilwalilo play (Tanzania) is an example of geothermal play of the Western Branch of EARS. It is in a half-graben realm where a regional fault controls the ascending groundwater flow. Reservoir temperatures are about 110–140 °C, and the heat source is provided by lithosphere thinning. The results of this study provide helpful guidelines for future studies on the geothermal resources in the rift.

Tectonics and hydrogeochemical features of geothermal waters in Tangyin Rift SE Taihang Mountain

Geothermal exploration in the Tangyin rift is gaining very important attention due to the increased demand for electricity and space heating in the Hebi City area. Due to geological and tectonic settings of rifting extension, volcanism and a thin lithosphere, it possesses a considerable amount of geothermal potential. Efforts to replace sporadic coal use with clean energy sources like gas, geothermal and electricity have helped reduce air pollutants brought about by many household activities. The present work conducts in-depth structural analysis, geochemistry (major and trace elements), and stable isotope analyses of geothermal waters in an attempt to decipher their hydrogeochemical evolution and their influence on the geothermal potential of Tangyin rift. Correlation analyses are used to determine the geochemical mechanisms that control the geochemistry of geothermal waters. The results indicate that large faults' deformational model, permeability, fluid transportation, and fluid flow direction are influenced by major faults trending NNE-SSW. Furthermore, the waters are classified into five types of hydrogeochemical: Ca-HCO3, Na-Cl–SO4, Ca-Mg-HCO3, Na-HCO3 and Ca-Cl. The Na-Cl-SO4 waters are connected with the Hebi volcano and ascend directly. Subsurface activities including cold water mixing and water-rock interaction have a higher impact on the region's Ca-HCO3 shallow ground waters. The geothermal system is characterized by convection-dominated geothermal play coupled with volcanism, replenished by meteoric fluids flowing through fractures and faults from Taihang Mountain and affected by CO2-rich waters from deeper levels. The reservoir is defined as immature water by the Giggenbach Na-K-Mg triangular diagram and mineral saturation indices (SI) that estimate an average reservoir temperature of 220 °C. Finally, the conceptual hydrogeological model for the Tangyin rift geothermal system was developed which can be used for studying the target for geothermal energy development.

Pismanta geothermal play in the iglesia basin related to transverse fault systems in the andean orogen

The intermontane Iglesia Basin constitutes the northern Uspallata-Calingasta-Iglesia tectonic depression that holds the Pismanta geothermal field, genetically connected with the basin evolution and structure. Understanding the local structure associated with this geothermal field and linking this system to the regional tectonics is essential to characterize thermal water rising in this area. To address this issue, we carried out a field-based structural analysis and an examination of sedimentary outcrops at the Pismanta area. Also, we linked surface structural data to the subsurface basin structure by means of a previously interpreted seismic reflection line. Our integrated analysis reveals that the Pismanta geothermal field is located in an area of favorable lithology at the intersection of WNW- and NNE-trending thrust faults. These jointly developed a permeability anomaly zone at the Pismanta locality that allows hot underground water rising from deep sedimentary sequences. This observation explains why the thermal springs are only locally concentrated in the study area; a key aspect that was not taken into account in previous models for the Pismanta geothermal field. These results and the overall basin tectonic evolution allowed us to define a new and more complex structural geothermal play model at the Pismanta field.

Hydrogeology and hydrogeochemistry of the geothermal systems and its direct use application: Balçova-Narlıdere geothermal system, İzmir, Turkey

The Balçova-Narlıdere geothermal system, located in western Turkey, is an extensional domain type geothermal play. Geological, hydrogeological, and geothermal studies have been done in the Balçova-Narlıdere geothermal field since 1960. As a result of these studies, production and research wells were drilled in the area by both the public and private sectors. Of the 37 wells drilled in the Balçova-Narlıdere geothermal field, 26 were drilled by İzmir Geothermal Energy Company Inc. (16 productions, 4 re-injection, 4 gradientst, 2 unused) and 11 by the İzmir Governorship Investment Monitoring and Coordination Department and companies in the private sectors. There are two reservoirs, one shallow and one deep, in the geothermal field. Well depths are less than 200 m in the shallow reservoir. The deep production wells have depths ranging from 400 to 1,100 m and reservoir temperatures reaching 140 °C. The electrical conductivity (EC) values in the shallow production wells range from 1,200 to 1,500 µS/cm. EC values in the deep production wells vary from 1,871 to 2,025 µS/cm, and all geothermal fluids in the field are mineral-rich waters. In the İzmir geothermal district heating system, the newest technologies are used, and operational costs are very low. İzmir Geothermal Energy Company Inc. has been operating since 1996 and has reached 38,460 residences. The current capacity of the system is approximately 160 MWt. Cooling systems continue to spread in Turkey, focusing on electric air conditioning systems and geothermal heating systems, but both economic and applicability problems have slowed the expansion of cooling systems. However, İzmir Geothermal Energy Company Inc. began operating its first geothermal cooling application in Balçova in 2018, cooling 1,900 m2 of the indoor area by lithium bromide absorption and 90/85 °C geothermal temperature regime by supplying 6/9 °C clean cold water to the coolers in the buildings. These results show that the extensional domain type geothermal system in the Balçova-Narlıdere region is suitable for both heating and cooling applications.

Discovery and analysis of a blind geothermal system in southeastern Gabbs Valley, western Nevada, USA

This study assessed the potential for a high-temperature blind geothermal system in southeastern Gabbs Valley, an area with no previous geothermal exploration, by utilizing geothermal play fairway analysis. Gabbs Valley is a structurally complex, tectonically active basin occupying a transtensional displacement transfer zone. Multiple lines of evidence suggest the presence of a blind geothermal system, including collocated intersecting gravity gradients, magnetic-low, low-resistivity, 2-m temperature anomaly, and 130–140 °C geothermometers obtained from nearby agricultural wells. Six 150 m-deep temperature-gradient holes were drilled to target the geophysical and thermal anomalies. Two holes produced bottom-hole temperatures of 114.5 °C and 124.9 °C. Drilling results establish the discovery of a blind geothermal system. Power estimates based on resource conceptual models of the geothermal system suggest a median (P50) capacity of ∼16 MWe.

Distribution of geothermal arsenic in relation to geothermal play types: A global review and case study from the Anatolian plate (Turkey)

Arsenic has a natural cycle as it travels underground. It can mix with geothermal fluid in different ways under the control of magmatic and tectonic processes. Geogenic arsenic is present in many geothermal fields in the world at concentrations above the limits set for human health. The arsenic content of geothermal fluids is also related to the concept of geothermal play type, which forms geothermal systems, because the natural processes that form the geothermal system also control the arsenic cycle. In this study, an attempt is made to explain the relationship between the geothermal play type concept and geothermal arsenic circulation. For this purpose, geothermal field examples are given from around the world and Turkey. The result shows that arsenic concentrations can reach significant levels along with plate tectonic boundaries in the world. When arsenic concentrations were evaluated, the effect of major faults on the Anatolian Plate was clearly seen. Also, in the Anatolian plate where volcano-sedimentary units are common, geothermal fluids caused more effective alteration along with structural control and increased arsenic concentrations in geothermal systems. This interaction between structural elements, geothermal fluid, and the arsenic cycle shows that the concept of play type in geothermal systems should also be taken into consideration. It was determined that the places with high arsenic values are located within the convective-non-magmatic extensional geothermal play types such as Western Anatolian Extensional System and the North Anatolian Fault. The concept of play type in geothermal systems includes all systematic and external factors that make up these processes. For this reason, it is very important to evaluate the play type classification together with the arsenic cycle.

Estimates of Curie point depth, geothermal gradient and near-surface heat flow of Bida and environs, Nupe Basin, Nigeria, from high-resolution aeromagnetic data

The Nupe Basin is a northwest–southeast (NW–SE) trending intracratonic sedimentary basin in the central part of Nigeria with its coordinates defined by longitude 5°00′ to 8°00′ E and latitude 7° 00′ to 11° 00′ N. Spectral inversion of the high-resolution aeromagnetic data was carried out to estimate the Curie point depth (CPD), geothermal gradient and heat flow regime of parts of the Nupe Basin, Nigeria. The estimate of regional temperature distribution and the identification of thermally active regions within the study area were made from the spectral analysis of high-resolution aeromagnetic data which were divided into 16 overlapping spectral blocks to obtain the top and centroid depths of the magnetic sources which were later used to calculate the heat flow characteristics. The estimated Curie point depths range from 21.43–26.71 km with an average CPD value of 24.54 km, while the geothermal gradient ranges between 21.72 °C km−1 and 27.07 °C km−1 with an average value of 24.39 °C km−1. Similarly, the estimated heat flow regime of the study area ranges from 54.30 m Wm−2 to 67.67 m Wm−2 with an average value of 59.41 m Wm−2. The range of estimated geothermal parameters in the study area is an indication that the area is not a volcanic or thermally active region and therefore has little potentials for geothermal energy exploitation. The geothermal play of the area, therefore, was revealed to be of the conduction-dominated play type, typically a combination of the basement and intracratonic basin geothermal play types.

Structural assessment and geochemistry of thermal waters at the Cerro Machin Volcano (Colombia): An approach to understanding the geothermal system

Through a detailed structural assessment of faults, carried out by remote sensing and a slip-dilation tendency analysis along with water geochemistry of thermal waters, it was possible to characterize the geothermal system of Cerro Machin Volcano (CMV) located in the western flank of the Cordillera Central of Colombia. The study shows that the geothermal system presents a favorable structural setting for the transport of fluids (e.g. fault intersection), where the structural component is marked by faults striking NW-SE, NE-SW, and N-S, controlling its deformational model, permeability and therefore, the fluid flow path. In addition, hydro-geochemistry allows identifying two sets of water (i) Na-HCO3 and (ii) Ca-HCO3 types. Na-HCO3 springs are representatives of a more direct upflow in the system, where fluid uses critically stressed areas to ascend. In contrast, Ca-HCO3 springs are more strongly affected by subsurface process such as mixing and water-rock interaction. The geothermal system is characterized of a convection-dominated geothermal play associated with recent volcanism, which is recharged by meteoric waters that flow through fractures and faults and is strongly influenced by a CO2-rich fluid coming from deeper levels. The reservoir occupies a fracture zone within the Cajamarca complex with an estimated temperature of around 220 °C, according to solute and multicomponent geothermometry.

A three-dimensional temperature model of the Acoculco caldera complex, Puebla, Mexico, from the Curie isotherm as a boundary condition

Three-dimensional temperature simulations of the Acoculco caldera complex are undertaken to improve our understanding of its thermal regime. This volcanic caldera has been considered a Conduction-Dominated Crystalline-Rock geothermal play based on evidences of low permeability of the formations hosting the heat source. An estimated Curie isotherm at depth is obtained from the de-fractal method to establish the background heat flow in the area. Additionally, local thermal anomalies and radiogenic heat generation are considered. The description of the structures controlling the heat flow is based on limited borehole data along with field observations, it includes stratigraphic columns and fracture zones. The heat transfer equation is solved in steady state with a Finite Volume numerical method and a Conjugate Gradient algorithm. The resulting computational model constitutes a workable instrument to test assumptions on the thermal regime in the Acoculco caldera that can be gradually constrained, as more data become available. Numerical results show that shallow and localized thermal anomalies (750 °C) lead to geothermal gradients as high as those measured in the field. The thermal anomalies turn out between 3300 and 3800 m below ground surface as the mean depth, shallower than those assumed in previous 1D thermal models in this caldera complex.

Introduction to the geothermal play and reservoir geology of the Netherlands

Abstract The Netherlands has ample geothermal resources. During the last decade, development of these resources has picked up fast. In 2007 one geothermal system had been realised; to date (1 January 2019), 24 have been. Total geothermal heat production in 2018 was 3.7 PJ from 18 geothermal systems. The geothermal sources are located in the same reservoirs/aquifers in which the oil and gas accumulations are hosted: Cenozoic, Upper Jurassic – Lower Cretaceous, Triassic and Rotliegend reservoirs. Additionally, the yet unproven hydrocarbon play in the Lower Carboniferous (Dinantian) Limestones delivered geothermal heat in two geothermal systems. This is in contrast to the Upper Cretaceous and Upper Carboniferous with no producing geothermal systems but producing hydrocarbon fields. Similar to hydrocarbon development, developing the geothermal source relies on fluid flow through the reservoir. For geothermal application a transmissivity of 10 Dm is presently thought to be a minimum value for a standard doublet system. Regional mapping of the geothermal plays, with subsequent resource mapping, by TNO discloses the areas with favourable transmissivity within play areas for geothermal development. The website www.ThermoGis.nl provides the tool to evaluate the geothermal plays on a sub-regional scale. The Dutch geothermal source and resource portfolio can be classified using geothermal play classification of, for example, Moeck (2014). An appropriate adjective for play classification for the Dutch situation would be the predominant permeability type: matrix, karst, fracture or fault permeability. The Dutch geothermal play is a matrix-permeability dominated ‘Hot Sedimentary Aquifer’, ‘Hydrothermal’ or ‘Intra-cratonic Conductive’ play. The Dutch ‘Hot Sedimentary Aquifer’ play is subdivided according to the lithostratigraphical annotation of the reservoir. The main geothermal plays are the Delft Sandstone and Slochteren Sandstone plays.

On the geothermal potential of crustal fault zones: a case study from the Pontgibaud area (French Massif Central, France)

The present study aims to understand the potential of a new and novel type of geothermal play system for high temperature and electricity production: crustal fault zones (CFZ). According to geological and geophysical data, the Pontgibaud fault zone (French Massif Central) is suspected to host an active hydrothermal system at a depth of a few kilometers. The deep geometry of the fault zone and the permeability distribution are the main unknown parameters that are required to assess the geothermal potential of the Pontgibaud site. Structural and thin-section observations, laboratory permeability and connected porosity measurements and X-ray micro-tomography observations suggest that the hydrothermal system behaves like a double matrix-fracture permeability reservoir. Numerical modeling in which we varied the fault dip and the ratio between the fault zone permeability and host rock, R, was performed. Results indicate that three main convective regimes can be identified (weak convection, single cellular-type convection and bicellular convection). For a sufficiently high fault zone permeability (> 1 × 10−15 m2), buoyancy-driven flow creates a positive thermal anomaly of several tens of °C at a depth of 2–5 km. For a vertical fault zone, the thermal anomaly is larger for higher R values. Numerical models, then applied to the geologically constrained Pontgibaud fault zone, show that a temperature of 150 °C at a depth of 2500 m can be obtained for a fault zone permeability of 1.6 × 10−14 m2. Based on a multi-disciplinary approach, this work establishes a potential predictive tool for future high-temperature geothermal operations within basement rocks hosting large-scale fault systems.

Las Pailas geothermal field - Central America case study: Deciphering a volcanic geothermal play type through the combination of optimized geophysical exploration methods and classic geological conceptual models of volcano-tectonic systems

Sustainable exploitation strategies of high-enthalpy geothermal reservoirs in a volcanic geothermal play type require an accurate understanding of key geological structures such as faults, cap rock and caldera boundaries. Of same importance is the recognition of possible magmatic body intrusions and their morphology, whether they are tabular like dikes, layered like sills or domes. The relative value of those magmatic bodies, their age, shape and location rely on the role they play as possible local heat sources, hydraulic barriers between reservoir compartments, and their far-reaching effect on the geochemistry and dynamics of fluids. Obtaining detailed knowledge and a more complete understanding at the early stages of exploration through integrated geological, geophysical and geochemical methods is essential to determine promising geothermal drilling targets for optimized production/re-injection schemes and for the development of adequate exploitation programs. Valuable, extensive geophysical data gathered at Las Pailas high-enthalpy geothermal field at northwestern Costa Rica combined with detailed understanding of the geological structures in the underground may represent a sound basis for an in-depth geoscientific discussion on this topic. Currently, the German cooperation for the identification of geothermal resources in Central America, implemented by the Federal Institute for Geosciences and Natural Resources (BGR), supports an international and interdisciplinary effort, driven by the Instituto Costarricense de Electricidad (ICE) with different international and national research institutions, including the Leibniz Institute for Applied Geophysics (LIAG). The discussions and joint studies refer to the optimized utilization of geophysical and geological methods for geothermal exploration in the Central American region, using the example of Las Pailas Geothermal Field. The results should contribute to a better understanding of the most appropriate geothermal exploration concepts for complex volcanic field settings in Central America.