Geothermal Exploration Best Practices IGA ACADEMY

156 Pages·2013·10.66 MB·English

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.
Permission of the Publisher and payment of a fee is required for all other photocopying, copying for advertising
or promotional purposes and all forms of document delivery.
Permission of the Publisher is required for all derivative works, like compilations and translations.
For information on how to seek permission call the IGA Secretariat: [phone] (Germany).
Electronic Storage or Usage
Electronic storage and usages of any material, including any article or part of an article, contained in this report
requires permission of the Publisher.
Notice of Publication
No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of
products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions
or ideas contained in the training material herein.
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