What Is Geothermal Energy?
Key facts
- A geothermal resource is more than underground heat alone.
- Temperature, depth, permeability, fluids and reservoir behaviour all influence development potential.
- Geothermal resources occur in volcanic, sedimentary, shallow and other geological settings.
- The resource and the technology used to develop it are separate concepts.
What is a geothermal resource?
Geothermal energy is heat associated with the Earth and its subsurface. A geothermal resource is a particular occurrence of that heat that may be technically accessible and useful.
A geothermal reservoir is more specific. It is the subsurface volume from which heat and, in conventional geothermal systems, geothermal fluids can be produced.
A geothermal system describes the wider geological setting. Depending on the resource, this can include the heat source, reservoir, fluid pathways, fractures, recharge zones and geological seals or caprock where these are present.
Technical resource assessments sometimes make a further distinction between the resource base, the resource and the reserve. The resource base represents the broader thermal energy present in the subsurface. A resource is the portion considered potentially recoverable, while reserves are the more narrowly defined portion demonstrated to be recoverable under specified technical, economic and regulatory conditions.
For a general understanding of geothermal energy, the important point is simpler: heat alone does not make a viable geothermal resource. It must occur under conditions that allow useful energy to be accessed.
Read more:
- USGS, A Review of Methods Applied by the U.S. Geological Survey in the Assessment of Geothermal Resources (pdf)
- USGS, Assessment of Geothermal Resources of the United States - 1978 (pdf)
What determines the quality of a geothermal resource?
Geothermal resources are shaped by several interacting characteristics. A high temperature is useful, but it does not by itself determine whether a resource can be developed.
Temperature and depth
Temperature influences what a geothermal resource can be used for. Higher-temperature resources can support electricity generation and high-temperature industrial applications, while lower-temperature resources can provide heating and cooling. Depth matters because reaching deeper resources generally increases drilling requirements and technical complexity. A moderate-temperature resource at accessible depth, with good fluid flow and permeability, may therefore be more attractive than a much hotter resource that is substantially deeper or harder to access.Heat flow and geothermal gradient
The geothermal gradient describes how temperature changes with depth. In many continental settings, the geothermal gradient is roughly 25-30°C per kilometre (about 72-87°F per mile, or 14-17°F per 1,000 feet) although local values can be considerably lower or higher depending on geology, tectonic setting, heat flow, rock properties and groundwater movement. Heat flow is different. It describes the rate at which heat passes through a unit area of the Earth's crust. The relationship between heat flow and geothermal gradient also depends on the thermal conductivity of the rock. Low-conductivity formations can produce a relatively steep temperature gradient without unusually high heat flow, while highly conductive rocks transfer heat more readily. This is particularly important when comparing volcanic settings with sedimentary basins. A favourable geothermal gradient does not automatically mean the underlying geological conditions are identical. Read more:- Guillou-Frottier et al., Heat flow, thermal anomalies, tectonic regimes and high-temperature geothermal resources
- DOE/NREL Geothermal Resource-Base Assessment (pdf)
Fluids
Fluids transport heat through many geothermal reservoirs. In conventional hydrothermal systems, water or steam circulates naturally through permeable rock and fractures before being produced through geothermal wells. The chemistry of these fluids is also important. Salinity, dissolved minerals and gases can affect well design and plant operation. They can contribute to scaling, corrosion and the need for specific materials or fluid-management strategies. Resources with strong heat but insufficient natural fluid circulation may require a different development approach.Enhanced Geothermal Systems
Permeability and porosity
Porosity describes the amount of pore space within a rock. Permeability describes how easily fluids can move through interconnected pore spaces or fractures. The distinction is important. A rock may contain substantial pore space but still transmit fluids poorly. Sedimentary reservoirs often rely strongly on matrix porosity and permeability, while volcanic and crystalline geothermal systems can depend heavily on fracture networks and faults. Understanding the pathways available to geothermal fluids is therefore fundamental to resource assessment.Reservoir size, recharge and long-term behaviour
Resource assessment also considers the size of the reservoir and how it responds to production over time. Natural recharge can replenish geothermal fluids, while reinjection is widely used to return produced fluids to the subsurface and support reservoir management. Long-term resource performance depends on the interaction between heat extraction, pressure, temperature, fluid movement and recharge.What Is Geothermal Energy?
Where are geothermal resources found?
Geothermal resources occur in a broad range of geological environments.
Their distribution reflects the interaction of heat flow, geological structure, groundwater and the thermal properties of the crust.
Volcanic and tectonically active settings
Many high-temperature geothermal fields occur in areas affected by volcanism and tectonic activity. Magma and elevated crustal heat can create favourable temperatures relatively close to the surface. Faults and fractures can provide pathways for water to circulate through hot rock. These geological settings commonly host hydrothermal geothermal resources, where heat, fluid and permeability occur naturally together. Such systems have supported major geothermal development in Iceland, Indonesia, New Zealand, Japan, the Philippines, East Africa, the western United States, Mexico and elsewhere. Volcanic activity, however, is not required for geothermal development.Sedimentary basins and stable continental settings
Sedimentary basins can contain large, laterally extensive formations with favourable porosity and permeability. Deep aquifers within these basins can reach temperatures suitable for district heating, industrial heat and, under favourable conditions, power generation. In many regions, decades of oil and gas exploration have also created extensive subsurface datasets that can support geothermal resource assessment. Stable continental settings may lack obvious surface geothermal manifestations, but useful temperatures can still occur at depth. Local heat flow, sediment thickness, rock conductivity, groundwater circulation and geological structures influence the resource conditions. This is why geothermal potential should not be understood simply as a map of active volcanoes.Read more:
- NREL, Estimate of the Geothermal Energy Resource in the Major Sedimentary Basins in the United States (pdf)
What types of geothermal resource settings are there?
Geothermal resources are described using several overlapping geological and technical classifications.
ThinkGeoEnergy uses resource setting as the broad editorial term where useful, while retaining familiar terms such as hydrothermal resources and sedimentary geothermal resources where they are clearer for readers.
There is no requirement that every geothermal occurrence fit neatly into one rigid classification system.
Hydrothermal geothermal resources
Hydrothermal resources contain naturally occurring heat, geothermal fluid and sufficient permeability for fluids to circulate through the subsurface. These resources form the basis of most conventional geothermal power generation and many direct-use systems operating today. They can contain hot water, steam or a mixture of both. Hydrothermal Geothermal SystemsRead more:
- U.S. Department of Energy, Geothermal Glossary
Sedimentary and aquifer geothermal resources
Sedimentary geothermal resources occur within sedimentary basins, often in water-bearing formations with useful porosity and permeability. These resources can be extensive and can be particularly attractive for heating applications where suitable aquifers occur beneath or near areas of heat demand. Their development depends on temperature, depth, reservoir thickness, fluid chemistry, permeability and the ability to sustain production and reinjection. Sedimentary geothermal is not simply a lower-temperature form of volcanic geothermal. It is a different geological setting with its own reservoir characteristics and development considerations.Geopressured geothermal resources
Geopressured geothermal resources occur in deep sedimentary formations containing hot fluids under unusually high pressure. The fluids can contain three potential forms of recoverable energy: thermal energy, hydraulic pressure and, in some settings, dissolved natural gas such as methane. These resources have been studied for decades, particularly in the US Gulf Coast region. Geopressured geothermal has seen limited commercial deployment and remains a specialised resource setting rather than a widely commercialised geothermal category.Read more:
- NREL, Geopressured Geothermal Resource and Recoverable Energy Estimate for the Wilcox and Frio Formations, Texas (pdf)
Hot rock resources
Large quantities of geothermal heat are stored in rock that lacks the natural permeability or fluid circulation needed for conventional hydrothermal production. Historically, these resources were often described as Hot Dry Rock (HDR). The term still appears in geothermal literature, but hot rock is often a clearer description of the resource condition. Technologies such as Enhanced Geothermal Systems (EGS) can be used to create or improve fluid pathways in these settings. This distinction is important: hot rock describes the resource condition; EGS describes an approach to developing it.Enhanced Geothermal Systems
Shallow geothermal resources
The shallow subsurface provides relatively stable thermal conditions that can be used for heating and cooling. These temperatures are influenced strongly by the atmosphere, solar energy and seasonal heat storage rather than being simply the shallow end of deep geothermal heat flow. Geothermal or ground-source heat pumps use electricity to move heat between buildings and the ground. The shallow thermal environment is the resource. The heat pump and associated ground or groundwater system are the technology used to access it.Geothermal Heat Pumps
Very-high-temperature, superhot and supercritical resources
Very-high-temperature geothermal represents one of the most technically demanding areas of geothermal development. Superhot geothermal broadly refers to geothermal resource conditions at exceptionally high temperatures. Supercritical, by contrast, describes a thermodynamic state of a fluid in which temperature and pressure exceed its critical point. The terms therefore describe related but different concepts and should not be treated as exact synonyms. Very-high-temperature resources could provide significantly greater energy from individual wells, but they also create major challenges for drilling, well construction, reservoir measurement, materials and surface equipment. Because terminology and temperature boundaries are not yet applied uniformly across the geothermal sector, ThinkGeoEnergy does not use a single universal temperature threshold on this introductory resource page.Read more:
- International Energy Agency, Superhot Geothermal
- International Energy Agency, The Future of Geothermal Energy (pdf)
How are geothermal resources classified?
Temperature is one of the most common ways to describe geothermal resources.
Terms such as low-temperature, medium-temperature and high-temperature geothermal are widely used, but the boundaries between them vary between classification systems.
This means that a temperature range described as “medium temperature” in one technical source may be classified differently elsewhere.
Enthalpy is another important geothermal concept. It describes the energy content of a fluid and can be useful when evaluating geothermal production and power-generation options.
Because enthalpy is a fluid property, however, it may be difficult to define accurately before wells provide direct information about subsurface fluids.
For a general resource guide, it is therefore more useful to describe temperature, geology and reservoir conditions explicitly rather than imposing one universal temperature or enthalpy classification.
The intended application also matters. Resources that are not hot enough for conventional electricity generation can still have substantial value for district heating, industrial processes, agriculture and other thermal uses.
Direct Use of Geothermal Energy
How are geothermal resources assessed?
Geothermal exploration and resource assessment combine different sources of subsurface information.
These can include:
- geological mapping and structural analysis;
- geochemical analysis of waters and gases;
- geophysical surveys;
- temperature and heat-flow data;
- existing wells and subsurface datasets;
- exploration drilling and well testing; and
- conceptual and numerical reservoir modelling.
From geothermal resource to usable energy
The characteristics of a geothermal resource influence which technologies can be used to develop it.
A naturally permeable hydrothermal resource can be developed with conventional production and reinjection wells.
A hot formation with insufficient natural permeability may be suitable for an Enhanced Geothermal System (EGS).
Closed-Loop Geothermal Systems (CLGS) can circulate a working fluid through sealed wells to exchange heat with hot rock or sedimentary formations without producing geothermal reservoir fluids.
Sedimentary aquifers can be developed with well doublets to provide district heating or other thermal applications.
Shallow thermal resources are commonly accessed with geothermal heat pumps.
The same geothermal resource may also support different energy applications depending on temperature, location and demand.
Overviews:
- Hydrothermal Geothermal Systems
- Enhanced Geothermal Systems
- Closed-Loop Geothermal Systems
- Geothermal Heat Pumps
Geothermal resources around the world
Geothermal resources occur worldwide, but their characteristics and accessibility vary substantially.
Historically, geothermal electricity generation has concentrated in countries with favourable high-temperature resources and established geothermal industries.
Geothermal heating already uses a broader range of resource settings, including sedimentary aquifers and lower-temperature resources.
New drilling, reservoir and heat-extraction technologies are widening the range of resources that may become technically accessible.
ThinkGeoEnergy's database and mapping tools are intended to connect this evergreen resource information with current geothermal projects, power plants, countries and technologies.
Dynamic data module: Geothermal development around the world. Populate from TGE database; include a visible data-updated timestamp. Do not hard-code changing figures in evergreen prose.
Global Geothermal Power Plant Map
Frequently asked questions about geothermal resources
What is a geothermal resource?
A geothermal resource is an occurrence of heat within the Earth that has geological and physical characteristics that may allow the energy to be recovered and used. What is the difference between a geothermal resource and a geothermal reservoir? A geothermal resource is the broader occurrence of potentially usable geothermal heat. A geothermal reservoir is the specific subsurface volume from which heat and, in conventional systems, geothermal fluids can be produced.What makes a good geothermal resource?
Temperature is important, but so are depth, permeability, fluids, reservoir size, chemistry, recharge and geological conditions.Are geothermal resources found everywhere?
Heat exists beneath the Earth's surface everywhere, but useful geothermal resources are not equally accessible. Geological conditions determine the depth, temperature and characteristics of the resource and the technologies required to develop it.What is a hydrothermal geothermal resource?
A hydrothermal resource contains naturally occurring geothermal heat, fluid and sufficient permeability to allow fluids to circulate through the reservoir.What is a geopressured geothermal resource?
A geopressured geothermal resource is a deep sedimentary formation containing hot fluids under unusually high pressure, often with dissolved natural gas.What is a sedimentary geothermal resource?
Sedimentary geothermal resources occur in sedimentary basins, often within porous and permeable aquifers where temperature and depth are suitable for heat extraction.Does geothermal always require underground water?
No. Conventional hydrothermal systems rely on naturally occurring geothermal fluids, but hot rock resources can also be developed using engineered technologies where natural fluids or permeability are insufficient.How hot does a geothermal resource need to be?
There is no universal minimum temperature. The useful temperature depends strongly on the application. Electricity generation generally requires higher temperatures than direct heating, while shallow geothermal systems can support heating and cooling at much lower temperatures.How deep are geothermal resources?
Geothermal resources range from the shallow subsurface to several kilometres depth. The depth required depends on local heat flow, geology, temperature and the intended application. Sources and further reading- U.S. Geological Survey - A Review of Methods Applied by the U.S. Geological Survey in the Assessment of Geothermal Resources (pdf)
- U.S. Geological Survey - Assessment of Geothermal Resources of the United States - 1978 (pdf)
- U.S. Department of Energy - Geothermal Glossary
- National Renewable Energy Laboratory - Estimate of the Geothermal Energy Resource in the Major Sedimentary Basins in the United States (pdf)
- National Renewable Energy Laboratory - Geopressured Geothermal Resource and Recoverable Energy Estimate for the Wilcox and Frio Formations, Texas (pdf)
- International Energy Agency - The Future of Geothermal Energy (pdf)
- Guillou-Frottier et al. Heat flow, thermal anomalies, tectonic regimes and high-temperature geothermal resources