U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3082

2008

Assessment of Moderate- and High-Temperature 

Geothermal Resources of the United States

Introduction

The U.S. Geological Survey 

(USGS) has recently assessed the 

electric power generation potential of 

conventional geothermal resources in 

the United States. These resources are 

concentrated in the States of Alaska, 

Arizona, California, Colorado, Hawaii, 

Idaho, Montana, Nevada, New Mexico, 

Oregon, Utah, Washington, and Wyoming, which contain all 241 identified 

moderate-temperature (90 to 150°C; 194 

to 302°F) and high-temperature (greater 

than 150°C) geothermal systems located 

on private or accessible public lands. 

Geothermal power plants at The Geysers in northern California. Currently, the United States has an installed 

and utilized power production capacity of more than 2,500 Megawatts-electric (MWe) from geothermal plants 

located in Alaska, California, Hawaii, Idaho, Nevada, and Utah. (USGS photograph by Julie Donnelly-Nolan.) 

S cientists with the U.S. 

Geological Survey (USGS)

recently completed an 

assessment of our Nation’s 

geothermal resources. 

Geothermal power plants 

are currently operating in six 

states: Alaska, California, 

Hawaii, Idaho, Nevada, 

and Utah. The assessment 

indicates that the electric 

power generation potential 

from identified geothermal 

systems is 9,057 Megawattselectric (MWe), distributed 

over 13 states. The mean 

estimated power production 

potential from undiscovered 

geothermal resources is 

30,033 MWe. Additionally, 

another estimated 517,800 

MWe could be generated 

through implementation 

of technology for creating 

geothermal reservoirs in 

regions characterized by 

high temperature, but low 

permeability, rock formations. 

(Geothermal systems located on closed 

public lands, such as national parks, were 

not included in the assessment.) Electricpower potential was also determined 

for seven low-temperature (less than 

90°C) systems in Alaska for which local 

conditions make electric power generation feasible. In addition, the assessment 

also includes a provisional estimate of 

the power generation potential from the 

application of unconventional, Enhanced 

Geothermal Systems (EGS) technology 

in Arizona, California, Colorado, Idaho, 

Montana, Nevada, New Mexico, Oregon, 

Utah, Washington, and Wyoming. This 

assessment benefited from cooperation 

and coordination with the Department of 

Energy (DOE); Bureau of Land Management (BLM); the University of Nevada, 

Reno; the University of Utah; Idaho 

National Laboratory; Lawrence Berkeley 

National Laboratory; state and local 

agencies; and the geothermal industry.

Identified Geothermal Systems

Currently, the United States 

has an installed and utilized power 

production capacity of more than 

2,500 Megawatts-electric (MWe) from 

geothermal plants located in Alaska, 

California, Hawaii, Idaho, Nevada, and 

Utah. The nearly 15,000 Gigawatt-hours 

(GWh) of geothermal power generated 

in 2005 constituted 25% of domestic 

nonhydroelectric renewable electrical 

power generation. (Power generation 

of 1 MWe provides 8.77 GWh of 

electricity in 1 year.) The results of the 

new assessment for the power generation 

potential from identified geothermal 

systems yield a mean total of 9,057 MWe
with a 95% probability of 3,675 MWe

and a 5% probability of 16,457 MWe

(table 1). The distribution of the 

individual systems across the study area 

is shown in figure 1. State totals were 

derived from summations of volumetric 

models for the thermal energy and 

electric generation potential of each 

individual geothermal system (Muffler, 

1979; Williams and others, 2008). The 

results of the assessment indicate that 

full development of identified systems 

alone could expand geothermal power 

production by approximately 6,500 MWe 

and to seven additional states. The 

distribution of identified geothermal 

resources among the 13 states with

identified geothermal resources is shown 

graphically in figure 2A. California, with 

large producing geothermal fields at The 

Geysers, the Salton Sea, and Coso, has 

59.7% of the total resource, followed by 

Nevada with 15.4% and Alaska with 7.5%. 

Undiscovered Geothermal 

Resources

Undiscovered geothermal resources 

were assessed for the same states in 

which the identified moderate- and 

high-temperature geothermal systems are 

located, based on a series of Geographic 

Information Systems (GIS) statistical 

models for the spatial correlation of Figure 1. Map showing the location of identified moderate-temperature and high-temperature 

geothermal systems in the United States. Each system is represented by a black dot.

Figure 2. Pie charts illustrating the distribution of (A) identified, (B) undiscovered and (C) Enhanced Geothermal Systems (EGS) resources (mean estimates) 

among the western states. Alaska and Hawaii were not included in the assessment of EGS resources because of a lack of information in those states.

Hawaii

Alaska

Oreg

Idaho

Calif Nev

Ariz N Mex

Colo

Wyo

Utah

Mont Wash

0 200 400

0 300 600

Miles

Kilometers

A. Identified Geothermal Resources

Arizona

0.29%

Colorado

0.33%

Hawaii

2.00%

Idaho

3.68%

Montana

0.65%

Nevada

15.36%

New Mexico

1.88%

Oregon

5.96%

Utah

2.03%

Washington

0.25%

Wyoming

0.43%

Alaska

7.47%

California

59.67%

B. Undiscovered Resources

Arizona

3.47%

California

37.76%

Colorado

3.68%

Hawaii

8.11%

Idaho

6.23%

Montana

2.57%

Nevada

14.53%

New Mexico

4.94%

Oregon

6.30%

Utah

4.87%

Washington

1.00%

Wyoming

0.58%

Alaska

5.95%

C. Enhanced Geothermal Systems

Arizona

10.56%

California

9.30%

Colorado

10.15%

Idaho

13.11%

Montana

3.27%

Nevada

19.86%

New

Mexico

Oregon

12.05%

Utah

9.11%

Washington

1.25%

Wyoming

0.59%

10.75%
geological factors that facilitate the 

formation of geothermal systems. The 

mean estimated power production 

potential from undiscovered resources 

located on private and accessible 

public lands is 30,033 MWe, with a 

95% probability of 7,917 MWe and 

a 5% probability of 73,286 MWe. As 

illustrated in figure 2B, compared to the 

identified resources, a larger fraction of 

the undiscovered geothermal resources 

are located outside California. This 

reflects both the limited degree of 

exploration and development in States 

other than California and Nevada and 

the uniqueness of the vapor-dominated 

geothermal reservoir at The Geysers in 

northern California, which contributes 

Enhanced Geothermal Systems

Conventional geothermal resources 

depend on hydrothermal fluid circulation 

that arises only with the convergence 

of high temperatures—due either to 

magmatism or other tectonic processes 

that elevate temperature gradients in 

the Earth’s crust—and permeability, 

typically fracture permeability produced 

as a result of active faulting (Duffield 

and Sass, 2003). Enhanced Geothermal 

Systems (EGS) are geothermal resources 

that require some form of engineering to 

develop the permeability necessary for 

the circulation of hot water or steam and 

the recovery of heat for electrical power 

generation. Because exploitation of EGS 

resources incorporates the augmentation 

or creation of permeability in place, the 

presence of elevated temperatures at 

drillable depths is the dominant factor 

controlling the quality of the resource.

Under the assumption of continued 

successful implementation of EGS 

technology, models for the extension of 

geothermal energy recovery techniques 

into regions of hot but low permeability 

crust yield an estimated mean electric 

power resource on private and accessible 

public land of 517,800 MWe (table 

1), with a 95% probability of 345,100 

MWe and a 5% probability of 727,900 

MWe. This is approximately half of 

the current installed electric power 

generating capacity in the United States 

and an order of magnitude larger than 

the conventional geothermal resource. 

This estimate does not include Alaska 

and Hawaii, because there is not enough 

information to accurately estimate 

crustal temperatures in those States on 

a regional basis. With EGS technology 

at an early stage of development (DOE, 

2008), the assessment results should be 

considered provisional.

The high crustal heat flow 

favorable for EGS development is more 

uniformly distributed across the western 

United States, and this is reflected 

in the distribution of the resource 

among the states, as shown in figure 

2C. The EGS resource distribution, 

although large in total magnitude, is 

also relatively diffuse. In contrast to 

power production from conventional 

geothermal reservoirs, which is often 

concentrated at 10 to 20 MWe per km2

of field area, the EGS resource outside 

of the high-temperature margins of 

Figure 3. Example map from one of a series of 28 spatial models showing the relative favorability of occurrence 

for geothermal resources in the western contiguous United States. The other models differ in details but show 

generally similar favorability patterns. Warmer colors equate with higher favorability. Identified geothermal 

systems are represented by black dots.

approximately 1,000 MWe to the 

identified geothermal resource for the 

State but is unlikely to be matched by 

any equivalent occurrences on private 

or accessible public lands elsewhere 

in the United States. The undiscovered 

resources results indicate that additional 

exploration could add substantially 

to the total of identified geothermal 

resources and further expand geothermal 

power production. As indicated by the 

geothermal favorability map shown 

in figure 3, regions with significant 

geothermal potential but few identified 

geothermal systems include northeastern 

Nevada, western Utah, southern Idaho, 

eastern Oregon, and parts of New Mexico 

and Colorado.

Wyoming

Utah

Washington

New Mexico

Montana

California

Nevada

Colorado

Oregon

Arizona

Idaho
Printed on recycled paper

USGS Geothermal Resources Assessment 

Team—Colin F. Williams, Marshall J. Reed, 

Robert H. Mariner, Jacob DeAngelo, S. Peter 

Galanis, Jr. 

Edited by James W. Hendley II

Graphic design by Jeanne DiLeo

For more information contact:

U.S. Geological Survey

Colin F. Williams: colin@usgs.gov,

345 Middlefield Road, Mail Stop 977

Menlo Park, CA 94025 

This Fact Sheet and any updates to it are

available online 

at http://pubs.usgs.gov/fs/2008/3082/

identified geothermal systems averages 

approximately 0.5 MWe per km2. 

However, continued advances in EGS 

technology, particularly with respect 

to creation of reservoirs at great depth 

and improved thermal energy recovery, 

could add substantially to the resource 

estimates (DOE, 2008).

EGS are not the only type of unconventional geothermal resource. Previous 

assessments (see for example, Muffler, 

1979) indicated significant unconventional geothermal resource potential 

associated with fluids in deep sedimentary basins of the United States. These 

unconventional geothermal resources 

will be assessed in a future study.

 Geothermal resources have the 

potential to play a much more significant 

role in our Nation’s energy mix. This 

assessment of geothermal resources 

in the United States is only part of the 

USGS effort to help ensure our Nation’s 

energy future.

References

Department of Energy Geothermal Technologies 

Program, 2008, An evaluation of enhanced 

geothermal systems technology, 37 p. [http://

www1.eere.energy.gov/ geothermal/pdfs/

evaluation_ egs_tech_2008.pdf, last accessed 

Sept. 5, 2008].

Duffield, W.A., and Sass, J.H., 2003, Geothermal energy—clean power from the Earth’s 

Heat: U.S. Geological Survey Circular 1249, 

36 p. [http://pubs.usgs.gov/circ/2004/c1249/]

Muffler, L.P.J., 1979, Assessment of geothermal resources of the United States—1978, 

U.S. Geological Survey Circular 790, 163 p.

Williams, C.F., Reed, M.J., and Mariner, R.H., 

2008, A review of methods applied by the 

U.S. Geological Survey in the assessment of 

identified geothermal resources: U.S. Geological Survey Open-File Report 2008-1296 

[http://pubs.usgs.gov/of/2008/1296/]

State N F95 F50 Mean F5 F95 F50 Mean F5 F95 F50 Mean F5

Alaska 53 236 606 677 1,359 537 1,428 1,788 4,256 NA NA NA NA

Arizona 2 4 20 26 70 238 775 1,043 2,751 33,000 52,900 54,700 82,200

California 45 2,422 5,140 5,404 9,282 3,256 9,532 11,340 25,439 32,300 47,100 48,100 67,600

Colorado 4 8 11 30 67 252 821 1,105 2,913 34,100 51,300 52,600 75,300

Hawaii 1 84 169 181 320 822 2,027 2,435 5,438 NA NA NA NA

Idaho 36 81 283 333 760 427 1,391 1,872 4,937 47,500 66,700 67,900 92,300

Montana 7 15 51 59 130 176 573 771 2,033 9,000 16,100 16,900 27,500

Nevada 56 515 1,216 1,391 2,551 996 3,243 4,364 11,507 71,800 101,300 102,800 139,500

New Mexico 7 53 153 170 343 339 1,103 1,484 3,913 35,600 54,400 55,700 80,100

Oregon 29 163 485 540 1107 432 1,406 1,893 4,991 43,600 61,500 62,400 84,500

Utah 6 82 171 184 321 334 1,088 1,464 3,860 32,600 46,500 47,200 64,300

Washington 1 7 20 23 47 68 223 300 790 3,900 6,300 6,500 9,800

Wyoming 1 5 31 39 100 40 129 174 458 1,700 2,900 3,000 4,800

Total 248 3,675 8,356 9,057 16,457 7,917 23,739 30,033 73,286 345,100 507,000 517,800 727,900

Identified Resources (MWe) Undiscovered Resources (MWe) Enhanced Geothermal Systems (MWe)

[All electric power generation figures are calculated on a basis of 30 years of production. F95 represents a 95% chance of at least the amount tabulated; other 

fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. N is the number of identified geothermal systems 

included in the estimate].

Table 1. Electric power generation potential in Megawatts-electric (MWe) from identified and undiscovered geothermal resources and 

Enhanced Geothermal Systems in the western United States.