Report 01/2013 Geothermal Exploration Best Practices Geology Exploration Drilling Geochemistry Geophysics IFC - International Finance Corporation Developers Workshop Izmir, Turkey, 18.-19.11.2013 IGA ACADEMY global.geothermal.knowledge IGA ACADEMY Report Vol. 1 / Dec. 2013 Geothermal Exploration Best Practices – Geology, Exploration Drilling, Geochemistry, Geophysics Editors R. BRACKE, C. HARVEY, H. RUETER Publisher IGA Service GmbH, Bochum, Germany © 2013 by IGA Service GmbH. All rights reserved. IGA ACADEMY Report comprises textbook materials of advanced trainings for geothermal professionals conducted by lecturers who are approved by the International Geothermal Association - IGA. These materials and the individual contributions contained in are protected under copyright by IGA Service GmbH and by the contributors. Photocopying and Derivative Works Single photocopies of single articles may be made for personal use as allowed by national copyright laws. 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About IGA ACADEMY is a Trademark of the IGA Service GmbH, Bochum, Germany; IGA Service GmbH is the business and service company of the International Geothermal Association. Cover Images GZB – International Geothermal Centre, Bochum, Germany. Further information www.geothermal-energy.org Contents Report-Nr. Author Title 0101-2013 I. MOECK Classification of geothermal plays according to geological habitats 0102-2013 I. KARAMANDERESI Characteristics of Geothermal Reservoirs in Turkey 0103-2013 T. BACKERS Borehole Geomechanics and Well Design 0104-2013 R. BRACKE, Geothermal Drilling Best Practices: The Geothermal translation of D. VOLLMAR, conventional drilling recommendations - main potential challenges V. WITTIG 0105-2013 H. RÜTER, Geothermal Exploration Best Practices, geophysical methods, A. DONAT, seismic, general aspects S. BAUER 0106-2013 H. RÜTER, Geothermal Exploration Best Practices, geophysical methods, A. DONAT, seismic, equipment S. BAUER 0107-2013 H. RÜTER, Geothermal Exploration Best Practices, geophysical methods, A. DONAT, seismic, data acquisition S. BAUER 0108-2013 H. RÜTER, Geothermal Exploration Best Practices, geophysical methods, A. DONAT, seismic, data processing S. BAUER 0109-2013 H. RÜTER, Geothermal Exploration Best Practices, geophysical methods, A. DONAT, seismic, geological interpretation S. BAUER 0110-2013 G.P. HERSIR, Resistivity surveying and electromagnetic methods Ó.G. FLÓVENZ 0111-2013 C.C. HARVEY Water-Rock Interaction, Alteration Minerals and Mineral Geothermometry 0112-2013 N. GÜLEC Isotope and gas geochemistry of geothermal systems Moeck: Classification of geothermal plays according to IGA Academy Report 0101-2013 geological habitats Supported by: Classification of geothermal plays according to geological habitats I. Moeck Faculty of Science, University of Alberta, Edmonton, AB T6G 2E3, Canada Abstract: Geothermal resources can be found in a large variety of geological environments ranging from volcanic arcs to sedimentary basins to crystalline rock provinces. Two large groups can be distinguished between all geothermal resources types: Either the heat transport is dominated by convection leading to accumulated heat, or the heat transport is conduction dominated leading to distributed heat. Considering the source of heat, heat transport, and thermal energy storage capacity of rock to store heat or to host hot fluids, it becomes obvious that geologic controls influence significantly the placement and formation of geothermal resources. The new geothermal play concept follows the basic approach of general resources assessment to group exploration targets according geologically controlled tiers, connected by similar geodynamic, sedimentary, magmatic and/or tectonic evolution. This lecture introduces into the geothermal play concept, play analysis and play assessment comprised by meaningful natural groups placing this concept directly into a geologic context. Worldwide case studies are demonstrated and exemplify the application of this new geothermal play concept. and it does not help in decision making for the 1 The play type concept appropriate exploration strategy. Two major In his article about exploration plays, well known problems rise from such an unspecific classification petroleum explorationist Harry Doust (2010) refers to scheme: the first problem is that these criteria are not the almost mythical status of “plays” in the known prior to exploration, therefore we cannot hydrocarbon industry. Playmakers are considered as classify a geothermal resource before exploration and heroes of the industry, and the successful play feeds therefore we cannot estimate the success of an everlasting legends about incredible discoveries, exploitation venture. The second problem is that low, exploration efforts and glory men. Basically, the play shallow, medium, deep are adjectives that invite us to type concept is a fundamental part of resources guess a whole suite of subjective interpretations and assessment in hydrocarbon industry. The analysis and ultimately numbers. understanding of play types is recognized as essential to strategically focused and successful exploration Let us have a look to one example: “moderate ventures. temperature” is described as between 90-150°C (Muffler, 1979), 125-225°C (Hochstein, 1988), 100- Amazingly, the geothermal community is lacking 200°C (Benderitter and Cormy, 1990) and 180-230°C such a concept although the utilization of geothermal (Sanyal, 2005). Global explorationists may struggle heat and energy looks back to a much longer history with a classification scheme that leads to such a wide than hydrocarbon production. Instead we range of temperature between 90-230°C. Obviously exhaustingly try to classify geothermal resources this concept is globally not applicable: in a standard according to temperature (low-, medium-, high- sedimentary basin 230°C is not in economically temperature), to depth (shallow, medium-shallow, drillable depth while at volcanic systems this medium-deep and deep) or to hydrothermal versus temperature might be accessed in a few couples of petrothermal systems. Obviously such a classification hundreds of meters. Important factors as porosity, focuses on the end-user or the probable utilization permeability, type of heat source, distribution of heat concept that may be demanded at a certain site. It (vertical or horizontal) are completely neglected in clearly prevents, however, from analog comparisons such an approach. The ultimate question is: what are because a certain temperature can be found in all we actually looking for? Is it temperature, or flow kinds of geological environments at various depths rate, or fluids in place? What answers need to be Moeck: Classification of geothermal plays according to IGA Academy Report 0101-2013 geological habitats provided by geothermal exploration, and how can we and permeability enhancement. There are clearly learn from geothermal field to field worldwide? physical limits to extract thermal fluids from any Eventually how to avoid over-estimates and under- rock types and main focus of play assessments is estimates of resources? on the description of the reservoir unit therefore. An alternative way to categorize geothermal  The heat charge system, comprising the type of resources may be based on a globally applicable play heat source and the heat transport, expelling heat type concept (Moeck, submitted). A play type in from deeper to shallower parts in a geothermal petroleum geology represents a particular system. stratigraphic or structural geological setting, defined  The regional topseal or caprock, a low by source rock, reservoir rock and trap. Translated to permeability unit which traps the thermal fluids at geothermal systems, a play type might be defined by a (litho)stratigraphic level and concentrates steam, the heat source and the geological controls on heat liquids or thermal fluids in specific locations, transport and thermal energy storage capacity. The allowing commercial exploitation either without or geological setting not only controls the play type but with EGS treatments. also the most appropriate exploration and heat recovery technologies.  The timely relationship of the above four ingredients, for example that a pluton intrudes into Certainly the geothermal community would benefit a porous rock formation, which is then covered by from a consistent globally applicable framework. low-permeability rock as fine grained mélange at a Moreover a logical and consistent framework for a forearc region of a subduction zones. geothermal play type catalog needs to be simple enough to communicate important aspects of  The play fairway, the geographic area over which geothermal energy potential to both non-experts and the play is believed to extend as for example the the general public. Additionally, it must be size of an intrusion in diameter and depth, or a comprehensive enough to provide a geological fault zone hosting vast volumes of circulating framework to cover the whole range of possible fluids. The mapping-out of the play fairway geothermal systems applicable for experts in industry belongs to the essential task in the early and academia. Plays can best help if they comprise exploration phase and conceptual model building. meaningful natural groups that can be used both for reliable analog comparison and for exploration 1.2 Geologic tiers in geothermal plays decision making in a specific geologic context. In this Key defining elements of this play type catalog are lecture we will learn about a new catalog of whether heat transfer is dominated by conduction or geothermal play types based on geologic controls. convection, and the characteristics of the heat source, reservoir and host rock, porosity-permeability 1.1 Play definition structure and fluid types. This new catalog is Before we talk about geothermal play types in explicitly not based on temperature, depth or particular it might be indispensable to define a play. hydrothermal versus petrothermal systems but on A play may initially be defined as a model in the geological tiers (Moeck, submitted; Moeck, in press). mind of a geologist of how a number of geological Geothermal plays can be separated at the system factors might generate a recoverable geothermal scale into two large groups referring to heat transport: resource at a specific structural position in a certain either the heat transport is dominated by convection geologic setting. Comparable with the general play leading to accumulated heat, or the heat transport is concepts described by Allan and Allan (2005) these conduction dominated leading to distributed heat. geological factors must be capable of providing the Whether convection or conduction dominates essential ingredients of a geothermal play, namely: depends primarily on the characteristics of the heat  The reservoir unit, porous or fractured enough to source and the distribution of permeability within the store thermal fluids and yielding them to the well host rocks at the system scale (Bogie et al., 2005; bore at commercial rates. Man-made reservoirs Lawless et al., 1995). It is important to recognize that units must be capable for stimulation treatments as convection and conduction are end-members of a fracturing or acidizing (Enhanced Geothermal heat transfer continuum. Conductive intervals always Systems-EGS). The more brittle the reservoir unit exist in localized parts of a convective regime, while the better the requisite for hydraulic stimulation convective intervals can sometimes exist within 2 Moeck: Classification of geothermal plays according to IGA Academy Report 0101-2013 geological habitats conductive systems. For example, gravity-driven geothermal systems (Gianelli and Grassi, 2001). They convection might occur within a discrete aquifer include all known ‘high temperature’ (>200°C) within a conduction-dominated system in steep geothermal reservoirs shallower than 3,000 m. These mountainous terrain where recharge zones are at a invariably lie adjacent to plate tectonic margins or in higher elevation than discharge sites. Alternatively, regions of active tectonism (Nukman and Moeck, buoyancy variations due to different concentrations 2013), active volcanism (Bogie et al., 2005), young of fluid salinity can result in local convection. plutonism (< 3 Ma), or regions with elevated heat flow due to crustal thinning during extensional Geothermal play types in convection-dominated tectonics (Faulds et al., 2009; Faulds et al.,2010). In systems are grouped into “Magmatic”, “Plutonic” and convection-dominated geothermal plays, heat is “Fault-controlled in Extensional Domains” referring transported efficiently from depth to shallower to the nature of the dominant heat source and tectonic reservoirs or the surface by the upward movement of setting (Moeck, submitted). Geothermal play types in fluid along permeable pathways. Laterally extensive, conduction-dominated systems are grouped into porous high-permeability formations act as the “Intracratonic Basins”, “Orogenic Belts with primary reservoirs. Convection-dominated Adjacent Foreland Basins” and “Basement types” geothermal plays are grouped primarily according to divided according to the dominant permeability the nature of the heat source. control; lithofacies, fractures, or a combination of both. In the next chapters the geologic controls on Favorable tectonic settings for convection-dominated each play type are described and constrained by real- Geothermal Play Types include (I) magmatic arcs above subduction zones in convergent plate margins world case studies. This new catalog of geothermal (e.g. the Indonesian Sunda Arc or the Philippine- play types provides a range of generic conceptual Japan Arc); (II) divergent margins located within models that serve as basis for refinement through oceanic (e.g. the Mid-Atlantic Ridge) or appropriate exploration methods. intracontinental settings (e.g. East African Rift); (III) transform plate margins with strike-slip faults (e.g. 2 Convection dominated plays the San Andreas Fault in California or Alpine faults in New Zealand); and (IV) intraplate ocean islands Convection-dominated Geothermal Play Types formed by hot spot magmatism (e.g. Hawaii) include those often referred to as ‘viable’ or ‘active’ Figure 1: Plate tectonic setting of major geothermal play types upon which this catalog is based. CV1 = convection-dominated magmatic type; CV2 = convection-dominated plutonic type; CV3 = convection-dominated extensional domain type; CD1 = conduction-dominated intracratonic type; CD2 = conduction-dominated orogenic belt type; CD3 = conduction-dominated basement type. The map shows the locations of examples of production projects developed from the different Geothermal Play Types in relation to plate tectonic setting (from Moeck, submitted). (Geothermal fields from http://geothermal-powerplant.blogspot.com; Plate tectonic map based on Frisch and Löschke, 2003 3 Moeck: Classification of geothermal plays according to IGA Academy Report 0101-2013 geological habitats It is possible for different types of convection- Figure 2: Generic model of an extrusive magmatic dominated plays to lie geographically close to each play associated with active volcanism (from Moeck, other where the structural setting varies over short in press) distance scales. 2.1.2 Intrusive magmatic plays 2.1 Magmatic play type An active magma chamber does not always produce A Magmatic Geothermal Play (CV1) is distinguished volcanism. Influenced by active faulting, deep rooted by a shallow, intense heat source in the form of a magmas can intrude beneath flat terrain with no young magma chamber. A relatively shallow magma volcanism, but with upflow of liquid and the chamber is the dominant feature in all Magmatic formation of hot springs, fumaroles, boiling mud Geothermal Plays. The chamber’s parental melts, pools and other geothermal surface manifestations. recharge of basalt, and crystallized melts control fluid chemistry, fluid flow and the overall geothermal system. Ultimately, the placement of the magma chamber relative to the surrounding terrain controls the geometry of the geothermal system and affects the hydraulic head of steam and brine. Faults can act as seals or conduits, playing a role in the formation of reservoir compartments or hydrothermal convection, while accommodation zones of faults can sustain Figure 3: Generic model of an intrusive magmatic enhanced vertical permeability and channel play associated with active faulting (from Moeck, in hydrothermal plumes (Rowland and Sibson, 2004). press) 2.1.1 Extrusive magmatic plays Hence, the Taupo Volcanic Zone (New Zealand) is also an example of this sort of Geothermal Play Such plays can be identified in regions with active (Bogie et al., 2005). basaltic volcanism at divergent plate margins (e.g. Iceland), basaltic to andesitic volcanism along island 2.2 Plutonic play type – CV2 arcs (e.g. Java), or recent andesitic to dacitic volcanism (e.g. South American Andes or Taiwan). A Plutonic Geothermal Play (CV2) incorporates a This play type may include an upflow zone and an heat source in the form of a crystalline rock enriched outflow zone, provided the topography of the volcano in heat generating elements or a young, crystallized supports this zonation (Williams et al., 2011; but still cooling, intrusive igneous body. Such Play Giggenbach, 1992; Hochstein, 1988). The outflow is Types are located where surrounding mountain generally modified from the original fluid, and has a ranges provide high recharge rates of circulating lower temperature and higher pH than the upflow due meteoric water, driving a hydrothermal system with to lateral migration (with associated heat loss) and possible vapor partition above the hot rock. This play loss of gases (during boiling) towards the flank of the type can co-exist with Magmatic play types and is volcano (Hochstein, 1988). Vertically extensive, low- typically located along continent-continent permeability, clay-rich layers in steep terrain, such as convergent margins with recent plutonism, such as andesitic strato-volcanoes, can cap high temperature the southern periphery of the Alpine Orogeny. reservoirs. 2.2.1 Plutonic play without recent volcanism The placement of felsic plutons is characteristic for mature subduction zones and decaying volcanism in continental crust. 4 Moeck: Classification of geothermal plays according to IGA Academy Report 0101-2013 geological habitats The K-horizon sits above a granite intrusion emplaced during a Pliocene extensional event (1.3- 3.8 Ma). Melts emplaced during a subsequent Pleistocene (0.3-0.2 Ma) magmatic event provide the primary heat source, while low-angle normal faults from the Pliocene event control the recharge of meteoric water into the system. 2.3 Extensional domain play type – CV3 Figure 4: Generic model of a plutonic play without In an Extensional Domain Geothermal Play (CV3) recent volcanism (from Moeck, in press) the mantle is elevated due to crustal extension and thinning. The elevated mantle provides the principal This play type can be found therefore in fields with source of heat for geothermal systems associated with declining volcanism and fore- or back-arc regions of this Play Type. The resulting high thermal gradients fold-thrust belts along subduction zones. An example facilitate the heating of meteoric water circulating is the Geyers geothermal field in California where a through deep faults or permeable formations. long history of subduction during Mesozoic and early Cenozoic was followed by strike-slip offset along the San Andreas fault during the late Cenozoic (Argus, 2001). This Andean-type continental margin produced forearc deposits, volcanism and accretion of terranes and mélange. A silicic pluton emplaced in early Quaternary representing the heat source. The 7 Figure 6: Generic model of a fault controlled km deep and 14 km in diameter large pluton is extensional domain play with elevated mantle due to overlain by porous greywacke sandstone representing active crustal extension (from Moeck, in press) the reservoir rock which is overlain by a low permeable caprock formation preventing the steam to Examples of geological settings hosting Extensional percolate from the reservoir formation upwards. Domain Geothermal Plays include the Great Basin Since the recharge volume from meteoric water is not (Western USA), Western Turkey, pull-apart basins sufficient for steam production, sewage from adjacent along the Sumatra Fault Zone, and the African Rift. communities is injected replenishing the geothermal In general, segmented faults are more favorable for system. geothermal systems than large faults with large 2.2.2 Plutonic play with recent volcanism offsets. The local stress regime and its orientation relative to fault geometry has a controlling impact on An example is the Lardarello geothermal system, permeability pathways, with faults oriented which is controlled by the interaction between perpendicular to the minimum compressive stress igneous rocks and faults. direction more likely to be permeable (Barton et al., 1997). Belts of intermeshing, overlapping, or intersecting faults, such as step-over regions, fault terminations and accommodation zones, often provide high permeability pathways through closely spaced, breccia dominated fracture networks (Faulds et al., 2010). In the Western USA, for example, most known geothermal fields are located at step-over regions or relay ramps (Faulds et al., 2012), while Figure 5: Generic model of a plutonic play geothermal systems are relatively rare along associated with recent volcanism (from Moeck, in displacement maxima or on the mid-segments of press) faults. Lardarello is known for its recent volcanism with 3 Conduction dominated plays occasional phreatic eruptions. This play includes a vapor-dominated layer (H-horizon) above a fluid- Conduction-dominated Geothermal Play Types (Fig. dominated layer (K-horizon) (Bertini et al., 2006). 1) include all of what could be called ‘passive’ 5 Moeck: Classification of geothermal plays according to IGA Academy Report 0101-2013 geological habitats geothermal systems due to an absence of fast Faults do not naturally channel heat in their natural convective flow of fluids or short-term variations in condition in conduction-dominated Play Types. fluid dynamics. These Play Types dominate within However, faults can play an important role as a fluid passive tectonic plate settings where there has been conduit or barrier during production from these no significant recent tectonism or volcanism. In these geothermal reservoirs, and may cause settings, temperature increases steadily (although not compartmentalization of the reservoir into separate necessarily linearly) with depth. Conductively heated fault blocks. Lithofacies (a rock unit formed in a geothermal reservoirs with potentially economic certain depositional environment affecting grain size, temperatures are located at greater depth than pore geometry and mineralogy), diagenesis and convectively heated geothermal reservoirs. Economic karstification greatly influence reservoir quality. viability, therefore, is closely linked to the Evaluation of fault and lithofacies characteristics geothermal gradient. Gradients higher than the global should therefore be primary goals of exploration average can be found in regions of high heat flow (for within these Geothermal Play Types. example, due to elevated concentrations of heat generating elements in the crust), or where overlying 3.1 Intracratonic basin type – CD1 strata are thermally insulating (Beardsmore and Cull, An Intracratonic Basin Geothermal Play (CD1) 2001). incorporates a reservoir within a sedimentary sequence laid down in an extensional graben or Conduction-dominated Geothermal Play Types can thermal sag basin. be sub-divided according to the natural porosity– permeability ratio within the potential reservoir rock, and the absence or presence of producible natural reservoir fluids. Conduction-dominated Play Types in this Guide are divided into Intracratonic Basin Type, Figure 7: Generic model of an intracratonic basin Orogenic Belt Type, and Basement Type. Favorable play and sub-plays therein due to the typical tectonic settings for conduction-dominated formation of several troughs or sub-basins. Similar Geothermal Play Types include (I) extensional, structure is characteristic for inactive rift basins and divergent margins and grabens, or lithospheric graben systems (from Moeck, in press) subsidence basins such as the North German Basin or Intracratonic basins that originate from lithospheric the Otway Basin in Australia; (II) foreland basins thinning and subsidence are commonly divided into within orogenic belts, such as the Molasse Basin several troughs or sub-basins (Salley, 2000). The north of the Alps, or the Western Canadian long geological history of intracratonic basins usually Sedimentary Basin east of the Rocky Mountains; (III) produces a several kilometer thick sediment fill that crystalline basement underlying thermally insulating spans a wide range of depositional environments that sediments, such as the Big Lake Suite Granodiorite may include fluvial siliciclastics, marine carbonates, beneath the Cooper Basin in Australia. muds and evaporites. Lithology, faulting, and Conduction-dominated Geothermal Play Types with diagenesis control the pattern of high and low low permeability potential reservoirs such as tight porosity domains (Wolfgramm et al., 2009; Hartmann sandstones, carbonates or crystalline rock can only be and Beaumont, 2000), and are themselves strongly developed using engineered geothermal systems influenced by basin evolution and subsidence rates. (EGS) technology. Although EGS techniques might Lithology, faults and the stress field control be applied to improve the productivity of any permeability and its anisotropy. geothermal reservoir, development of many Geothermal plays are located in different basin conduction-dominated geothermal systems depends portions depending on the internal present-day strongly on them. Through the application of EGS structure of the basin. Formations above salt diapirs techniques, non-commercial reservoir conditions (for might provide suitable geothermal reservoirs for example, rocks with naturally low transmissivity or district heating because high thermal conductivity of storativity) might be improved. The in situ stress field salt rock causes local positive thermal anomalies in is a critical parameter for EGS technology because the overburden (Norden and Förster, 2006). the successful planning and management of large- Formations in deeper parts of the basin might provide scale injection and hydraulic stimulation requires suitable reservoirs for power and heat production, knowledge of stress direction and magnitudes (e.g. provided they can produce at a flow rate of about 70 Moeck, 2012; Moeck and Backers, 2011). 6 Moeck: Classification of geothermal plays according to IGA Academy Report 0101-2013 geological habitats kg/s or more (Tester et al., 2007). In all potential Typical fluids are high-Cl brines (referred as basinal sedimentary reservoirs, primary porosity (affected by fluids) or HCO3-rich fluids (referred as infiltration deposition through lithofacies or biofacies) and water). secondary porosity (affected by diagenesis) have a major influence on the fluid storage capacity. 3.2 Orogenic belt type – CD2 Potential reservoir units are terrestrial sedimentary An Orogenic Belt Geothermal Play (CD2) rocks, such as aeolian and fluvial siliciclastic incorporates a sedimentary reservoir within a sequences, and shallow to deep marine sediments foreland basin or orogenic mountain belt. from carbonate sequences to shale and pelagic clays. Figure 8: Generic model of an orogenic belt play with adjacent foreland basin with typical conductive thermal structure (red isotherms), groundwater flow paths and discharge temperatures (blue arrows). The deeper parts of the foreland basin may provide targets for sedimentary geothermal reservoirs (from Moeck, in press) Sedimentary sequences in foreland basins are Hutcheon, 2001). Figure 8 illustrates a typical influenced by significant crustal subsidence (up to conduction-dominated, locally convectively several kilometers) towards the orogen due to the disturbed, thermal structure in an orogenic zone. weight of the thickened crust of the orogenic belt and loading of erosional products from the mountain belt 3.3 Basement type – CD3 on the non-thickened crust. The result of this process The key feature of a Basement Geothermal Play is downward bending of the non-thickened (CD3) is a faulted or fractured crystalline (usually lithosphere, forming areas of local extension and granitic) rock with very low natural porosity and normal faulting in an overall compressional plate permeability but storing vast amounts of thermal tectonic setting (Cacace et al., 2013). The wedge energy. shape of foreland basins results in a progressive deepening of potential aquifer rocks towards the orogen, with an associated increase in temperature. Faults and reef complexes provide prime reservoir targets in carbonate rocks of the Bavarian Molasse Basin, Germany, (Cacace et al., 2013) while highly permeable and porous sandstone in the Williston Figure 9: Generic model of a basement play in Basin in Saskatchewan, Canada, and North Dakota, crystalline rock with geologic controls on U.S.A., also provide potential geothermal reservoir temperature (from Moeck, in press) targets (Bachu and Burwash, 1991). Such low porosity-low permeability rocks underlie Within the orogenic mountain belt itself, the large areas of continents but require reservoir conductive thermal regime can be locally disturbed development by EGS techniques to allow circulation where groundwater infiltration cools the rock mass. between injector and producer wells using the hot Groundwater flow and thermal gradient are both rock mass as a heat exchanger (Cuenot et al., 2008). strongly influenced by extreme relief and resulting Fractured crystalline rocks attain potentially hydraulic head (Bachu and Burwash, 1991; Toth, economic temperatures through elevated heat flow or 2009). The great depth and small width of mountain thermal insulation in the overburden. Heat flow is belt valleys result in relatively shallow penetration of likely to be elevated if underlying rocks have recharge water, discharging in valley floors or on elevated radiogenic heat production from heat shallow valley slopes (Toth, 2009). Conductive producing elements such as thorium or uranium. thermal gradients can vary from about 15-20°C/km beneath high mountains at to about 30-50°C/km Since crystalline rocks are generally not natural beneath deep valleys (Craw et al., 2005; Grasby and aquifers, fluids need to be injected both to improve 7