لخّصلي

Catalog of geothermal play types based on geologic controls
Inga S. Moeck n
Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3
article info
Article history:
Received 11 July 2013
Received in revised form
21 April 2014
Accepted 10 May 2014
Available online 12 June 2014
Keywords:
Geothermal systems
Classification scheme
Convection dominated geothermal plays
Conduction dominated geothermal plays
Geological controls
abstract
The key element in the characterization, assessment and development of geothermal energy systems is
the resource type. Throughout the past 30 years many resource type schemes and definitions were
published, based on temperature and thermodynamic properties. An alternative possibility to cataloging
geothermal energy systems is by their geologic characteristics, referred to as geothermal plays. Applied
to worldwide case studies, a new catalog is developed based on the effects of geological controls and
structural plate tectonic positions on thermal regime and heat flow, hydrogeologic regime, fluid
dynamics, fluid chemistry, faults and fractures, stress regime, and lithological sequence. Understanding
geologic controls, especially of geothermal plays without surface expression, allows the comparison with
hydrocarbon reservoirs through their ratio of porosity and permeability. This analog has implications on
site-specific, first class exploration strategies and reservoir improvement through technologies specifi-
cally suitable for unconventional sustainable energy reservoirs. This article aims to introduce geothermal
plays to a wide geoscientific community and to initiate a geologically based cataloging of geothermal
resources. With this new catalog of geothermal plays, it will be ultimately possible to transfer lessons
learned not only within one specific catalog type, but also technology from geothermal plays to
unconventional hydrocarbon plays and vice versa.
& 2014 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 867


2. Geologic perspective on geothermal play systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 869


3. Geothermal plays in relation to plate tectonic setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 869


4. Geologic controls on geothermal plays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 871
   4.1. Convection-dominated geothermal plays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 871
   4.1.1. Magmatic geothermal plays–volcanic field and plutonic type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 871
   4.1.2. Non-magmatic geothermal plays – extensional domains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873
   4.2. Conduction dominated geothermal plays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874
   4.2.1. Igneous geothermal plays– basement type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875
   4.2.2. Non-magmatic geothermal plays – intracratonic basins and orogenic belts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875


5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 878


6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 879
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880


7. Introduction
   Geothermal energy provides commercial base-load electricity
   from conventional hydrothermal resources for more than 100 years,
   with a global installed electricity generation of 10,751 MWel [1]
   and direct use of 50,583 MWth [2]. Whereas these prime geothermal
   systems are limited to tectonically active areas or regions with active
   Contents lists available at ScienceDirect
   journal homepage: www.elsevier.com/locate/rser
   Renewable and Sustainable Energy Reviews
   http://dx.doi.org/10.1016/j.rser.2014.05.032
   1364-0321/& 2014 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
   Abbreviations: MWel, Megawatt electric; MWth, Megawatt thermal; EGS,
   Enhanced Geothermal Systems; MPa, Megapascal; HDR, Hot Dry Rock; GHG,
   Greenhouse gas; CV, Convection dominated heat transfer regime; CD, Conduction
   dominated heat transfer regime n Tel.: þ1 7804923265; fax: þ1 7804922030.
   E-mail address: [email protected]
   Renewable and Sustainable Energy Reviews 37 (2014) 867–882volcanism, the concept of Enhanced or Engineered Geothermal
   Systems (EGS) has significantly increased the world-wide geothermal
   potential by technology reservoirs where the stored thermal energy
   can be extracted from subsurface even in areas of low or moderate
   heat flow. Tester et al. [3] claim that EGS resources could technically
   provide 100,000 MWel cost-competitive electric energy in the USA by


8. However, more effort in research and development is needed
   to realize this goal. Successful reservoir production from geothermal
   systems depends mainly on the appropriate selection of exploration
   methods. The decision for these appropriate exploration methods
   might depend on the type of geothermal energy system foreseen for
   heat and power production and necessitates a classification system
   for geothermal system types. A geothermal system is generally
   classified by its geological, hydrogeological and heat transfer char-
   acteristics, while a geothermal resource is formed by an economically
   sufficient amount of heat concentration in drillable depth of Earth's
   crust [4]. The term sufficient may dependent on technology devel-
   opment resulting in modern viable geothermal reserves that were
   not economic in the past. When it comes to geothermal prospects,
   resources and reserves, it is obvious that clear terms and definitions
   are required to provide reliable and comparable reserve estimation
   analogous to the classification schemes developed for petroleum
   resources. According to the Petroleum Resources Management Sys-
   tem [5], reserves are classified as commercially recoverable resources
   and contingent resources are less certain because of some commer-
   cial or technical hurdle resulting in a lower confidence level for
   eventual production. The lowest level in this classification scheme is
   represented by prospective resources, which are estimated but
   undiscovered accumulation of potentially recoverable heat (i.e. prior
   to drilling). Unrecoverable resources are classified as not being
   commercially producible at the present point in time. While one
   portion the unrecoverables may become recoverable in the future
   with changing commercial and evolving technological circumstances,
   another portion may never be recovered due to physical or chemical
   constraints in the reservoir [5]. From this classification perspective,
   the lowest unit in a bottom up approach is the geologically based so-
   called “play type”, which leads to prospects and ultimately to
   reserves. A play type in petroleum geology represents a particular
   stratigraphic or structural geological setting, defined by source rock,
   reservoir rock and trap [6]. Translated to geothermal systems, a play
   type might be defined by the heat source, the geological controls on
   the heat migration pathway, heat/fluid storage capacity and the
   potential for economic recovery of the heat. Ultimately the geological
   habitat does not only control the play type but also the decision for
   applied heat recovery technology.
   The new interest in geothermal energy resources is tied to the
   question of economic risks and the production potential of
   individual geothermal resource types. Quantifying the chance of
   development and field production involves feasibility studies and
   utilization concepts for the economic development of specific
   geothermal systems. From this perspective, it is important to note
   that a geothermal resource is part of a geologic system where
   geologic factors such as lithology, faults, fractures, stress field,
   diagenesis, rock mechanics, fluid chemistry and geochemistry
   control key parameters, such as high porosity and high perme-
   ability domains, fluid flow, lateral and vertical temperature dis-
   tribution and overall reservoir behavior during injection and
   production. A site specific appropriate field development should
   therefore be based on a profound understanding of the geologic
   controls of a geothermal play involving a suite of modern site
   specific exploration techniques. A clear and widely understandable
   new catalog of geothermal plays is required to fulfill the aims of
   exploration in reducing the risk of non-productive wells and
   guiding best choice reservoir technology to ultimately produce
   thermal energy on an economically sustainable level. The need for
   a new catalog may also emerge for two major reasons: (I) The recent
   development in Enhanced Geothermal System (EGS) technologies
   produces tangible pilot projects for heat and power generation from
   low-enthalpy resources, thereby extending the worldwide geother-
   mal potential, and (II) the growing political-social request for renew-
   able energy to reduce climate gas emission.
   Throughout the past 30 years many catalog schemes and
   definitions for geothermal resources have been published, mainly
   based on temperature and thermodynamic properties. Tempera-
   ture has been the essential measure of the quality of the resource,
   and geothermal play systems have been divided into three
   different temperature (or enthalpy) play types: low-temperature,
   moderate-temperature and high-temperature [7–13]. There are,
   however, no uniform temperature ranges for these types (Table 1).
   Lee [14] pointed out that temperature and enthalpy alone are
   inconsistent and insufficient to catalog geothermal plays and
   suggests a catalog scheme by the specific exergy of a geothermal
   fluid as a measure of its ability to do a work. The term exergy is
   used in thermodynamics to define the amount of energy that is
   available to be used during a process that brings the system into
   equilibrium [15]. Lee [14] developed a specific exergy index as the
   ratio of the specific exergy of a given geothermal play to the
   specific exergy in the saturated steam at a pressure of 9 MPa. Lee's
   geothermal play catalog has some advantages, as it directly relates
   to relevant properties of the produced thermal fluid at the well-
   head. However, it does not consider geological–hydrogeological
   aspects such as geological setting, controls on fluid flow, fluid
   chemistry and possible mineral precipitation in reservoir rock or in
   technical installations below and above the ground surface. All of
   these factors can impair the energy production and overall economic
   utilization of a geothermal resource. Moreover, Lee's [14] concept
   requires access to both temperature and pressure estimates for actual
   conditions at the wellhead; thus his catalog scheme can only be
   applied after drilling the first well. A geothermal play catalog and
   assessment scheme should, however, also be applicable before
   drilling for assessment and site specific field development.
   Williams et al. [16] point out that it is still a substantial
   requirement that a resource assessment provides a logical and
   consistent framework that is simplified enough to communicate
   important aspects of geothermal energy potential to both non-
   experts and the general public. One possible solution may be to
   Table 1
   Catalog scheme of geothermal resources by temperature according to different authors (compilation modified from Lee [14]).
   Muffler [8] (1C) Hochstein [9] (1C) Benderitter and Cormy [12] (1C) Haenel et al. [10] (1C)
   Low enthalpy o90 o125 o100 o150
   Moderate enthalpy 90–150 125–225 100–200 –
   High enthalpy 4150 4225 4200 4150
   Sanyal [13] Non-electrical (1C) Very low (1C) Low (1C) Moderate (1C) High (1C) Ultra high (1C)
   o50–100 100–150 150–180 180–230 230–300 4300
   868 I.S. Moeck / Renewable and Sustainable Energy Reviews 37 (2014) 867–882avoid cataloging geothermal plays by temperature and simply
   state the range of temperatures at the individual site.
   Due to technological development, in particular in EGS tech-
   nology, currently there are more geothermal systems that are
   potentially economical than there were 30 years ago. Therefore, a
   new cataloging scheme for geothermal play systems should
   characterize geologic controls on geothermal resources, recogniz-
   ing that future technological developments may alter quantitative
   boundaries and definitions based on temperature. A catalog of
   geothermal system plays should not be mistaken with a geother-
   mal system classification, which is preferably used for financial
   reporting schemes aiming to distinguish between different
   degrees of certainty and project maturity (G. Beardsmore, 2013,
   personal communication).


9. Geologic perspective on geothermal play systems
   In contrast to the straightforward definition of hydrocarbon
   play systems, which are clearly defined by their source rock,
   reservoir and trap, geothermal play systems are lacking such a
   clear set of geological features. Instead, geothermal play systems
   appear in diverse geologic environments and theoretically all over
   the world. For geothermal resource utilization, important factors
   are how much heat is stored at a drillable depth and if this heat is
   producible at an economic rate for a specific project. Pioneering
   work in describing and cataloging geothermal systems was done
   by Manfred Hochstein in the late 1980s. After 30 years of devel-
   opment in geothermal technology, however, it is time, to extend
   Hochstein's catalog to incorporate EGS.
   The American Geosciences Institute defines a geothermal
   system generally as [17]:
   “Any regionally localized geological settings where naturally
   occurring portions of the earth's internal heat flow are transported
   close enough to the earth's surface by circulating steam or hot
   water to be readily harnessed for use”.
   Since this definition refers only to convective geothermal
   resources, Williams et al. [16] broadened this definition to include
   also conductive geothermal resources:
   “A geothermal system is any localized geologic setting where
   portions of the Earth's thermal energy may be extracted from a
   circulating fluid and transported to a point of use”.
   This definition still excludes the concept of EGS, where the
   geothermal play system conditions are enhanced from previous
   non-economic to economic conditions. The key point for EGS is
   that the ratio of the temperature to the flow rate (or production
   and injection rate) must be given for an economic use. Although
   the quantitative meaning of economic might change through time,
   the terms flow rate, temperature and economics must be linked
   for a modern geothermal system definition. The definition of
   Williams et al. [16] should therefore be extended as follows:
   “A geothermal system is any localized geologic setting where
   portions of the Earth's thermal energy may be extracted from
   natural or artificially induced circulating fluids transported to a
   point of use. Enhanced Geothermal Systems are portions of the
   Earth crust where the ratio of flow rate and fluid temperature is
   naturally too low for economic use, and therefore the flow rate must
   be increased to a sufficient flow rate/temperature ratio by enhancing
   the natural permeability through technological solutions”.
   In EGS the circulating fluid can be the natural fluid if a
   hydrothermal system is hosted by a low permeability formation,
   or it can be an artificial (i.e. injected) fluid if the formation of the
   geothermal system does not contain enough fluid volume for heat
   extraction (referred to as Hot Dry Rock or petrothermal system).
   Referring to the revised definition of a geothermal system
   above, an alternative possibility is classifying geothermal play
   systems by their geologic setting. Recent attempts in categorizing
   geothermal plays are the play fairway analyses of hydrothermal
   systems in the United States, where the geographic extent of
   favorable settings is defined [18], or the play concept of rift zones
   where repeating sets of prospects with common characteristics
   define a play group [19]. From a structural geology perspective, a
   catalog theme can be guided by the plate tectonic setting, for
   example, whether the play system is related to convection or
   conduction dominated heat transfer and if the geothermal play
   system is magmatic or non-magmatic. Understanding and char-
   acterizing the geologic controls on geothermal plays has been an
   ongoing focus on different scales, from plate tectonics (e.g. [20,21])
   to local tectonics/structural geology [22]. In fact, the geologic
   setting has a fundamental influence on the potential temperature,
   on the fluid composition, the reservoir characteristics and whether
   the geothermal play is a convective or conductive system.
   In particular, a structural geological understanding helps to
   better interpret geophysical data and to identify favorable settings
   for drilling [23]. Essential parameters are the stress field and
   reservoir geomechanics, since the orientation of the current stress
   field has an impact on fluid flow along faults and ultimately on the
   permeability anisotropy in fractured reservoirs [24]. The stress field is
   also crucial for EGS development because technology reservoirs,
   particularly the technology of reservoir stimulation, aim to increase
   the permeability by generating additional fractures [25]. Orientation
   and growth of these artificial fractures are strongly controlled by the
   stress field and geomechanical rock properties, which need to be
   understood prior to stimulation and defining injection rates. A more
   important factor than generating fractures through stimulation might
   be keeping the induced fractures open during production and
   subsequent formation pressure drop. The analysis of the fault
   reactivation potential by the slip and dilation tendency technique
   helps in risk assessment during injection in general, which also
   includes re-injection [26]. A quantitative structural geology evalua-
   tion involving 3D structural geological modeling, stress field analysis
   and fault stress modeling is therefore a fundamental part in
   geothermal field development from exploration to drilling to reser-
   voir engineering [27].
   This work will review these aspects along with a newly
   developed catalog scheme based on the author's work experience
   in different geologic-geothermal settings. This catalog involves
   both convective and conductive dominated geothermal plays.
   Special emphasis is given to geothermal exploration that provides
   site-specific guidelines for geothermal systems, especially EGS.


10. Geothermal plays in relation to plate tectonic setting
    Plate tectonic settings have a fundamental influence on the
    characteristics of a geothermal play. The thermal regime and heat
    flow, hydrogeologic regime, fluid dynamics, fluid chemistry, faults
    and fractures, stress regime and lithological sequence are all
    controlled by the plate tectonic framework and are critical for
    understanding the geothermal play system. The thermal state of
    the crust at active plate boundaries is distinct from that in other
    large-scale geological provinces, such as tectonically quiescent
    settings (e.g. cratons), major fault zones (active or inactive), or
    deep, sedimentary basins (intracontinental or in front of orogenic
    zones).
    In general, geothermal plays are dominated either by a con-
    vection or conduction heat transfer regime. Convection-dominated
    geothermal plays – often referred to as viable or active geothermal
    play systems due to their fluid dynamics [28] – host high enthalpy
    resources and occur at plate tectonic margins, or settings of active
    tectonism or volcanism (Fig. 1). Convection of thermal fluids
    induced by a heat source or elevated heat flow transports heat
    I.S. Moeck / Renewable and Sustainable Energy Reviews 37 (2014) 867–882 869from deeper levels to the surface. Structural controls have a major
    effect on fluid flow pathways in convection-dominated systems.
    In high temperature play systems, fluid flow velocities are faster
    than in low temperature resources [29]. Several factors and
    processes influence convection within a geothermal play. Besides
    a high temperature gradient, high permeability (410-
    14 m2
    ;
    10 mD) is necessary to allow significant convection, whereas in
    low permeability layers (o10-
    15 m2
    ; 1 mD) only minor or no
    convection occurs [9]. Generally, a high geothermal gradient,
    natural fluid flow and fluid dynamics characterize convection-
    dominated geothermal plays.
    In contrast, conduction-dominated geothermal plays host low
    to medium enthalpy resources, which can also be called passive
    geothermal play systems due to the absence of fast convective
    flow of fluids and less short-term fluid dynamics. These systems
    are located predominately at passive tectonic plate settings where
    no significant recent tectonism or volcanism occurs. Here, the
    geothermal gradient is average, thus this type of geothermal play
    is located at greater depth than convection-dominated geothermal
    systems. Conduction-dominated geothermal plays in low perme-
    ability domains such as tight sandstones, carbonates or crystalline
    rock require EGS technology to be utilized on an economic level.
    Faults can still play an important role in these systems as a fluid
    conduit or barrier during production and may induce compart-
    mentalization of the system into separate fault blocks. Lithofacies,
    diagenesis, dissolution processes including karstification and frac-
    tures play a major role for reservoir quality evaluation comparable
    to oil and gas plays.
    An important factor in understanding the occurrence of con-
    vection and conduction-dominated play systems is distinguishing
    between igneous and non-magmatic geothermal plays. These
    terms refer to the heat source and tectonic activity. Igneous play
    systems can induce both conduction and convection-dominated
    geothermal plays. The difference is that conduction-dominated
    systems in or close to igneous rocks are related to high radiogenic
    heat production (typically high heat producing element rich
    granites), but no active volcanism and minor or no active tecton-
    ism occurs. Alternatively, convection-dominated magmatic plays
    require a magma chamber as the heat source in volcanic and
    tectonically active areas. In conduction-dominated igneous plays,
    large volumes of natural fluids are absent. These “dry” systems
    require EGS technology for hydraulic fracturing and injection
    induced circulation of fluids to transfer heat from depth to surface.
    Fluids play an important role in geothermal system utilization,
    since they are necessary for transporting heat from the reservoir to
    the surface. The volume of produced fluids determines whether a
    geothermal play system is economic. The appropriate balance
    between production and injection of thermal fluids influences
    the economic life-time of a geothermal reservoir. Moreover, the
    fluid chemistry has major effects on the efficiency and life-time of
    a reservoir and the material selection of technical installations to
    minimize phenamona such as corrosion and mineral precipitation
    (i.e. scaling). It is, therefore, important to understand the reservoir
    fluids' origin, chemistry, recharge characteristics, and meteoric
    water content. Hochstein [9] points to the influence of steep
    topography in geothermal play systems, which cause large
    volumes of meteoric water recharge into convective geothermal
    plays via high infiltration rates. The influence of steep terrain on
    the hydraulic head is not only significant in volcanic field settings
    as in Hochstein's concept but also in sedimentary basin settings
    Fig. 1. Geothermal fields installed worldwide in a plate tectonic setting. Geothermal play types with example fields: CV – Convection dominated heat transfer,
    CD – conduction dominated heat transfer. (List of geothermal fields from http://geothermal-powerplant.blogspot.com; www.thinkgeoenergy.com; Zheng and Dong, 2008 [30]; plate
    tectonic map based on Frisch and Loschke, [31]).
    870 I.S. Moeck / Renewable and Sustainable Energy Reviews 37 (2014) 867–882