GEOLOGICAL SURVEY CIRCULAR 892 

A es~ment. of .. Low.-Tempera j .ture 

Geothermal Resources of the 

Un1iited States-1982 

Prepared in cooperation with the U.S. Department of Energy 

Assessment of Low-Temperature 

Geothermal Resources of the 

United States-1982 

Marshall J. Reed, Editor 

GEOLOGICAL SURVEY CIRCULAR 892 

Prepared in cooperation with the U.S. Department of Energy 

The first quantitative estimation of the thermal energy 

recoverable from low-temperature (less than 90° C) 

geothermal systems within the United States 

1983 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Assessment of low-temperature geothermal resources of the United States--1982 

(Geological Survey Circular 892) 

"Prepared in cooperation with the U.S. Department of Energy." 

Bibliography: p. 6-8, 15-16, 29-30, 39-40, 64-65, 73 

Supt. of Docs. no.: I 19.4/2:892 

1. Geothermal resources--united States. I. Reed, Marshall J. II. United 

States. Geological Survey. III. United States. Department of Energy. 

IV. Series: United States. Geological Survey. Circular 892. 

QE75.C5 no. 892 557 .3s 83-600196 

[GB1199.6J [553. 7'0973J 

Free on application to Branch of Distribution, U.S. Geological Survey 

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Page 

Introduction, by Marshall J. Reed --------------------------------------- 1 

Regional heat flow and temperature gradients, by Manuel Nathenson, 

Marianne Guffanti, John H. Sass, and Robert J. Munroe -------------- Methods for assessing low-temperature geothermal resources, by Michael L. 

Sorey, Manuel Nathenson, and Christian Smith ----------------------- Low-temperature geothermal resources in the Western United States, by 

Robert H. Mariner, Charles A. Brook, Marshall J. Reed, James D. 

Bliss, AmyL. Rapport, and Randolph J. Lieb ------------------------ Low-temperature geothermal resources in the Central and Eastern United 

States, by Michael L. Sorey, Marshall J. Reed, Duncan Foley, and Joel 

L. Renner ----------------------------------------------------------- Summary, by Marshall J. Reed --------------------------------------------

ILLUSTRATIONS 

Figure 1. Plot of temperature-versus-depth relation used to define 

9 

17 

31 

51 

67 

Page 

low-temperature geothermal resources ---------------------- 2 

2. Maps showing major geologic provinces of the United States --- 4 

3. Temperatures required for uses of geothermal water ----------- 6 

4. Heat-flow map of the conterminous United States -------------- 11 

5. Temperature-gradient map of the conterminous United States --- 14 

6. Sketches illustrating conceptual models for types of 

hydrothermal-convection systems in which low-temperature 

geothermal-resource areas in category 1 (isolated thermal 

springs and wells) occur ----------------------------------- 19 

7. Sketches illustrating conceptual models for types of 

hydrothermal-convection systems in which low-temperature 

geothermal-resource areas in category 2 (delineated 

thermal reservoirs) occur ---------------------------------- 20 

8. Sketches illustrating conceptual models for types of 

conduction-dominated systems in which low-temperature 

geothermal-resource areas in category 3 (sedimentary 

basins) and category 4 (coastal plains) occur -------------- 21 

9-12. Plots of: 

9. Idealized temperature profiles in hydrothermalconvection systems with lateral leakage and within 

sedimentary basins -------------------------------- 22 

10. Reservoir area versus the optimum area per well for 

reservoirs with leaky confining beds -------------- 25 

11. Reservoir area versus the optimum area per well for 

reservoirs with nonleaky confining beds ----------- 26 

12. Usable temperature drop versus reservoir or input 

temperature for various direct-use applications --- 28 

13. Map showing low-temperature geothermal systems in the 

Western United States ------------------------------------- 32 

14. Map showing identified low-temperature geothermal-resource 

areas and areas containing undiscovered geothermal 

resources in the Central United States -------------------- 52 

15. Map showing identified low-temperature geothermal-resource 

areas and areas containing undiscovered geothermal 

resources in the Eastern United States -------------------- 53 

iii 
Figure 16. Plot of cumulative frequency versus reservoir temperature 

for 1,346 hydrothermal-convection systems in the United 

Table 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

States ---------------------------------------------------- 71 

17. Plot of Monte Carlo distribution of resource for lowtemperature geothermal systems in the United States ------- 72 

TABLES 

Page 

Typical values of heat flow and temperature gradient in parts 

of the conterminous United States --------------------------- 10 

Thermal conductivities of common rock types ------------------- 12 

Categories of low-temperature geothermal-resource areas ------- 18 

Estimated reservoir values and thermal energies of identified 

low-temperature geothermal systems in the Western United 

States ------------------------------------------------------ 41 

Summary of the identified and undiscovered accessible resource 

base in geologic provinces of the Western United States ----- 39 

Summary of energies for low-temperature geothermal systems in 

the Western United States ----------------------------------- 39 

Reservoir parameters and thermal energies of identified lowtemperature geothermal resources in the Central United 

States ------------------------------------------------------ 58 

Reservoir parameters and thermal energies of identified lowtemperature geothermal resources in the Eastern United States 61 

Summary of energies for identified low-temperature geothermal 

systems in the Central United States ------------------------ 62 

Summary of energies for identified low-temperature geothermal 

systems in the Eastern United States ------------------------ 62 

Estimates of undiscovered low-temperature geothermal resources 

in the Central and Eastern United States -------------------- 63 

Summary of energies by State for identified low-temperature 

geothermal systems in ,the United States --------------------- 70 

Summary of energies for identified and undiscovered lowtemperature geothermal systems in the United States --------- 70 

Comparison of the total beneficial heat from low-temperature 

geothermal systems with that from other energy sources ------ 72 

iv 
Length: 

Area: 

Volume: 

Mass: 

Flow rate: 

Temperature: 

Temperature gradient: 

Energy: 

Power or work: 

Heat flow: 

Thermal conductivity: 

CONVERSION FACTORS 

1 centimeter (cm)=0.3937 inch (in.} 

1 meter (m}=3.281 feet (ft} 

1 kilometer (km)=0.6214 mile (mi} 

1 m2=10.76 ft 2 

1 km2=0.3861 mi 2 

1 li~er (L}= 0.~642 gallon (gal} 1 km =0.2399 mi 

1 kilogram (kg}=2.205 pounds (lb) 

1 L/s = 15.85 gal/min 

degrees celsius (°C)=5/9(degrees Fahrenheit (°F]-32) 

Kelvins (K)=°C+273.15 

1°C/km=0.05486°F/100 ft 

1 joule (J)=O 2390 calorie (cal) 

1 J=9.485x1o-! British thermal unit (Btu) 

1 f=2.777x10- watt-hoy§ (W"hr) 

10 8 J=0.9485 quad (10 ~tu) 1 MWt for 30 yr=9.461x10 J 

1 watt (W)=1 J /s 

1 meg.awatt (MW)=3.154x1ol3 J/yr 

1 mW/m2

2

=2.390x10-8 cal/cm·s 

1 mW/m =2.390x1o- 2 heat-flow unit (HFU) 

1 W/m·K=2.390 mcal/cm·s· 0 c 

v 

Introduction 

By Marshall J. Reed 

CONTENTS 

Abstract -------------------------

Background ----------------------- Terminology ---------------------- Methodology ---------------------- Resource utilization -------------

Acknowledgments ------------------

References cited ----------------- Additional references ------------

ABSTRACT 

Page 

1 

1 

2 

2 

3 

6 

6 

7 

The geothermal-resource assessment presented 

here is the first quantitative estimation of the thermal 

energy recoverable from low-temperature (less than 

90°C) geothermal systems within the United States. 

This assessment, based on data available through April 

1982, includes estimates of accessible resource base 

(geothermal energy in the ground), resource (energy 

that might be recoverable at the surface), and 

beneficial heat (energy that might be usable in a 

specific application). The minimum temperature for 

low-temperature geothermal resources was defined as 

10°C above the mean annual air temperature at the 

surface and increasing by 25°C/km with depth. 

Systematic variations in heat flow and temperature 

gradient permitted the division of the United States 

into western, central, and eastern regions; within each 

of these regions, the low-temperature geothermal 

resources were divided into hydrothermal-convection 

and conduction-dominated systems. 

Quantitative estimates were made for the 

geothermal energy available in undiscovered as well as identified systems, and the results are tabulated by 

region, State, and geologic province. Identified lowt~mperature geothermal systems in the Unite~rtates 

contain an acces~~le resource base of 27x10 J, a resource of 87x10 J, and a beneficial heat of 41 GW t 

for 30 years. Undiscovered low-temperature 

geothermal systems are estimated to con~n an additional accessipge resource base of 7 .2x10 J, a 

resource of 66x10 J, and a beneficial heat of 30 GWt 

for 30 years. 

1 

BACKGROUND 

Resource assessment is the estimation of the 

amount of a given raw material that might be 

produced from the Earth and used economically at a 

future time. The present assessment of geothermal 

resources in the United States estimates the thermal 

energy that might be recoverable from lowtemperature (less than 90°C) geothermal reservoirs. 

Using a newly developed uniform methodology applied 

to the most accurate data available through April 

1982, this assessment provides a scientific basis for 

decisions about national energy policy and offers some guidance for resource-development strategy. The 

overall goal of this assessment is to provide a 

comprehensive framework for future geothermalresource development. 

This is the first quantitative assessment of lowtemperature geothermal resources to be conducted by 

the U.S. Geological Survey (USGS). An earlier (1978) 

geothermal assessment included a qualitative 

discussion of low-temperature geothermal waters in 

the United States (Sammel, 1979); however, the data 

available at that time were not adequate for a 

quantitative assessment. The present geothermalresource assessment is an extension and expansion of 

the inventory by Sammel and of the discussions of 

conduction-dominated thermal regimes by Diment and 

others (1975) and Sass and Lachenbruch (1979). 

In 1978, the Division of Geothermal Energy of 

the U.S. Department of Energy began to fund studies, 

covering all the States, to investigate low- and 

intermediate-temperature geothermal systems; 

information gathered in these studies was provided to 

the USGS. The list of additional references at the end 

of this chapter includes a series of State geothermal- resource maps and the major reports of the State 

studies. Other studies, primarily carried out within 

the Geothermal Research Program and the Regional 

Aquifer Systems Analysis Program of the USGS, have 

provided additional information on low-temperature 

geothermal systems. Information on water chemistry, 

temperature, flow rate, and other parameters 

measured at many low-temperature geothermal sites 

was stored in the computer-based GEOTHERM 

information system (Teshin and others, 1979) 

maintained by the USGS. The GEOTHERM system 

enabled the assessment team to manipulate the data in 

various ways for the more than 2,500 geothermal 

systems that were considered. 
TERMINOLOGY 

The terminology used in this report is based on the review by Muffler and Cataldi (1978). "Resource 

base" is defined as the total geothermal energy in the 

Earth's crust. "Accessible resource base" is defined as all the geothermal energy between the Earth's surface 

and a specified depth in the crust. "Resource," or recoverable energy, is defined as that part of the 

accessible resource base that is producible at the 

wellhead under reasonable assumptions of future 

economics and technology (Muffler and Guffanti, 

1979). The energy calculations for accessible resource 

base and resource were made for a reference 

temperature of 15°C (standard reference temperature 

of White and Williams, 1975, and Muffler, 1979), which 

is the average of the mean annual air temperatures in 

the United States. "Beneficial heat" is defined as that 

part of the resource that is usable in a specific 

application; beneficial heat is a function of the 

temperature drop within the application system, and 

an empirical relation between temperature drop and 

reservoir temperature is used in this report to 

calculate beneficial heat. 

Use of the term "accessible resource base" is 

limited in this report to porous and permeable 

geothermal reservoirs that can produce water to carry 

thermal energy to the surface. This same limitation 

was applied by Brook and others (1979) in their 

assessment of hydrothermal-convection systems at 

temperatures equal to or greater than 90°C. Adoption 

of this limitation reflects a judgment that only lowtemperature geothermal systems with high 

permeability will be economically competitive in the 

foreseeable future. In this assessment, depth to the 

resource is limited bJ: the minimum-temperature 

function, defined as 10 C above the mean annual air 

temperature at the surface and increasing by 25°C/km 

with depth. Thus, this assessment considers the 

geothermal energy to a maximum depth of 3.2 km. 

For example, in an area with a mean annual air 

temperature of 12°C, spring-water temperature must 

exceed 22°C, and water temperature at a depth of 1 

km must exceed 47°C (22°+25°). Figure 1 illustrates 

these relations. 

Adoption of the lower temperature limit 

excludes from consideration an enormous amount of 

shallow ground water in the United States; average 

ground-water temperatures from 5 to 15 m deep are 5° 

to 7°C above the mean annual air temperature (Gass 

and others, 1979, fig. 1). It is recognized that such 

shallow waters may be useful as a source of thermal 

energy in specific applications, but these cases are 

judged to be exceptional. Similarly, the definition of 

the lower temperature limit at depth virtually 

restricts this assessment to areas having anomalous 

concentrations of heat associated either with 

hydrothermal-convection or with conductiondominated systems within deep sedimentary basins or 

beneath coastal plains. 

METHODOLOGY 

Nathenson and others (this volume) discuss 

regionally significant temperature-gradient 

2 

measurements to depths of 2 km and present a map 

showing the regional variation of these gradients (see 

fig. 4). Delineation of the regional variations in 

temperature gradients and in heat flow provides a 

background against which to recognize anomalous 

concentrations of thermal energy that may include 

low-temperature geothermal systems. These 

temperature-gradient and heat-flow data exhibit a systematic variation across the United States and 

provide a basis for division of the country into 

western, central, and eastern regions for a discussion 

of geothermal resources. 

'\. '\. Low-temperature 

" " geothermal reservoirs 

'\. '\. 

'\. ' ' ' en 

'~ '\. a: 

w '\.~(9 '\. 

t-

"~ w '\. ~ ~~ ' ...J 

0 yt '\ 

52 ~ '\ 

~ 2 '\ '\ 

:I: '\ '\ 

t- Normal ground water ' a.. Y' w '\ 0 ' '\ A' 

3 ' ' X' 

0 15 30 45 60 75 90 

TEMPERATURE, IN DEGREES CELSIUS 

Figure !.-Temperature-versus-depth relation used to 

define low-temperature geothermal resources. 

Upper temperature limit is 90°C, following 

usage of Muffler (1979); lower temperature 

limit is defined as 10°C above the mean annual 

air temperature at the surface, increasing by 

25°C/km with depth. Dashed lines X-X' and YY' show minimum geothermal-resource temperatures re~ired for mean annual air temperatures of 0 and 23°C, which are the limits for 

air temperatures considered in this assessment. For example, for a mean annual air 

temperature of 12°C, the minimum surfacespring temperature is 22°C (point A), and the 

line A-A' gives the minimum temperature at 

any depth. Mean annual air temperatures are from Kincer (1941, p. 703), supplemented by information for Alaska from Johnson and Hart- man (1969, pl. 35) and for Hawaii from Blumenstock and Price (1978). 
Low-temperature geothermal systems can be 

divided into two types, hydrothermal convection and 

conduction dominated, on the basis of the major 

mechanism of heat transfer (Sorey, Nathenson, and 

Smith, this volume). Both types of geothermal systems 

occur in each region. Relatively small volume 

hydrothermal-convection systems predominate in the 

western region (Mariner and others, this volume); the 

Western States also contain all the intermediate- and 

high-temperature hydrothermal-convection systems 

identified in previous assessments (Nathenson and 

Muffler, 1975; Renner and others, 1975; Brook and 

others, 1979). In the central region, a few conductiondominated low-temperature geothermal systems of 

relatively large volume account for the bulk of· the 

Nation's low-temperature identified accessible 

resource base (Sorey, Reed, and others, this volume). 

A few small-volume low-temperature geothermal 

systems of both types are identified in the eastern 

region (Sorey, Reed, and others, this volume). Figure 2 

shows the regions and geologic provinces used for this 

assessment. 

The detailed assessment methodology is 

presented by Sorey, Nathenson, and Smith (this 

volume). The calculation of identified accessible 

resource base uses a volumetric method. -For lowtemperature geothermal systems with only limited 

information av~ilable, a standard minimum reservoir 

volume of 1 km was assumed; this assumption reflects 

a judgment about the average size of a system that 

supplies only a few isolated springs or wells. 

Calculation of the resource determines the 

energy recoverable from a low-temperature reservoir over a period of 30 years without fluid injection into 

the reservoir. The resource value depends on the 

number of evenly spaced wells that can maintain 

production at a constant flow rate for a 30-year period 

with a maximum drawdown of 152 m. A similar 

approach was used for an assessment of geopressured 

geothermal systems (Papadopulos and others, 1975; 

Wallace and others, 1979). In the analysis here, the 

proportion of the accessible resource base that is 

recoverable as a resource increases as the resource 

calculation, the proportion of the accessible resource 

base recoverable from a reservoir in 30 years ranges 

from a minimum of 0.1 percent for regional aquifers in 

large sedimentary basins (Sorey, Reed, and others, this 

volume) to a maximum of 25 percent for small-area 

reservoirs. The upper limit of 25-percent recovery 

from the accessible resource base is derived from the 

heat-sweep analysis by Nathenson and Muffler (1975). 

Calculations of the beneficial heat are based on 

an analysis of recently published information that 

provides measured energy-utilization factors and heatrejection temperatures. A reservoir temperature of 

25°C (l0°C above the average mean annual air 

temperature of 15°C) is the lower limit considered in 

these calculations; values are in watts thermal (W t>· 

The United States has a broad range in the climatic 

conditions that control some of the uses of geothermal 

energy. Mean annual air temperature ranges from 

-12°C in northern Alaska (0°C is the lowest air 

temperature considered in this report) to 23°C in 

southern Texas and Hawaii; in addition, the central 

region has extremely large seasonal variations in air 

temperature. It is possible that water below 25°C can 

3 

be used economically in certain localities and at 

certain times of the year, even though water of lower 

temperatures is omitted here from the calculations of 

beneficial heat. 

Uncertainties in the energy estimates of this 

assessment are expressed as standard deviations. The 

uncertainty in the identified accessible resource base 

results from uncertainties in estimates of the 

temperature, area, and thickness for each reservoir. 

Minimum, maximum, and most likely values were assumed for each of these parameters to create a 

triangular probability density from which the mean and 

standard deviation were calculated analytically (Brook 

and others, 1979, fig. 4); this calculation assumes that 

temper~ture, area, and thickness are statistically 

independent variables (Nathenson, 1978, app. 1). In the 

calculations of resource and beneficial heat, however, 

additional nonlinear parameters are used, and the 

standard deviation cannot be calculated analytically; 

thus, only the mean values are listed (see tables 4, 7, 

and 8). To determine the mean and standard deviation 

for the totals of identified accessible resource base, 

resource, and beneficial heat in the summary tables (5, 

9, 10, and 12), a Monte Carlo computer simulation was used that created 400 random values of each 

parameter within the triangular probability density. 

The simpler analytical result was well suited to the 

calculation of energies for individual systems, but for 

the summary of energies by temperature category or 

region the more complex Monte Carlo calculations 

were necessary to obtain standard deviations. The 

values for identified accessible resource base from the 

Monte Carlo calculations differ slightly from those 

calculated analytically, but the differences are not 

significant (well within the standard deviation). 

Estimates of the minimum, maximum, and most likely 

values of the distributed parameters for each system 

have been made by Reed and others (1983). 

RESOURCE UTILIZATION 

Anderson and Lund {1979) discussed many 

specific legal and economic factors related to the 

development of low-temperature geothermal energy in 

the United States. A similarly detailed discussion is 

beyond the scope of this report; our assessment 

presents only an estimate of the resource that will be 

available in the foreseeable future within an undefined 

framework of legal and economic factors. 

Direct use of low-temperature geothermal 

water can supply the energy needs of many processes 

that now depend on fossil fuels, as shown in figure 3. 

Some of these_ uses involve direct consumption of 

thermal water rather than an exchange of heat, and so 

the method of calculation of beneficial heat does not 

apply. Three low-temperature geothermal reservoirs 

in China currently provide energy for electrical 

generating plants (Reed and Bliss, 1983), but this use 

of low-temperature geothermal water is not 

considered at present to be economical in the United 

States. 

In the past, the use of geothermal water in the 

United States was primarily for hot-water baths and 

pools (balneology). After 1920, however, the 

abundance of inexpensive natural gas for heating baths 

and pools caused a rapid decline in the use of natural 
HAWAII 

t· ---fd ------- ---~· 

~ C::::::7 '\)<?c> 

·"' 

v ·~· 1~5" 

A 0 250 500 KILOMETERS 

Figure 2.-Major geologic provinces of the United States (modified from the physiographic provinces of 

Fenneman, 1946), showing division of the country into three regions for discussion in this assess- ment. ~ Alaska and Hawaii (western region). .!!_, Conterminous United States (western, central, 

and eastern regions). 

4 
-----t---~··· 

\ 

\1 

c.n 

WESTERN REGION 

B 

0 250 500 KILOMETERS 

Figure 2.-Continued 
hot water. Some use of geothermal water for space 

heating dates from before 1890 in such areas as Boise, 

Idaho, but interest in this application has been rather 

slight until the 1970's. 

This assessment estimates that identified lowtemperature geothermal systems in the United ¥tates 

contain an acces\igle resource base of 27xto2 J, a resource of 87x10 J, and a beneficial heat of 41 GWt 

for 30 years; undiscovered low-temperature 

geothermal systems are estimated to con1fJn an additional accessipJe resource base of 7 .2x10 J, a resource of 66x10 J, and a beneficial heat of 30 GWt 

for 30 years. The current estimated use of lowtemperature geothermal energy requires only a small 

part of the identified beneficial heat. Installed uses in 

the United States at the end of 1980 consisted of 790 

MW t for enhanced oil recovery in Montana, North 

Dakota, and Wyoming; 1 MWt for balneology; and 110 

MWt for agricultural, residential, and industrial needs 

(estimated from Oliver, 1981, and M. J. Reed, unpub. 

data, 1981). From a 1980 survey, the use of 

geothermal energy with reservoir temperatures less 

than 90°C in countries other than the United States is 

estimated at 2.2 GWt for balneology and 1.7 GWt for 

all other needs (from Gudmundsson and Palmason, 

1981). 

Temperature 

(OC) 

90 

Uses 

Drying of stock fish 

Intense deicing operations 

80 Space heating 

70 

60 

50 

40 

30 

20 

Greenhouse heating and milk pasteurization 

Refrigeration (lower limit) 

Vacuum distillation of ethanol 

Animal husbandry 

Combined space and bed heating of greenhouses 

Mushroom growing 

Enhanced oil recovery (lower limit) 

Soil warming 

Water for winter mining in cold climates 

Balneology and deicing (lower limit) 

Fish hatching and fish farming 

Figure 3.-Temperatures required for uses of geothermal water (from L(ndal, 1973). 

6 

ACKNOWLEDGMENTS 

The preparation of this report was made 

possible through the valuable assistance of many 

contributors from Government agencies, academic 

institutions, and private industry, and was aided by 

generous people in almost every State of the country. 

Colleagues in the USGS and in the Minerals 

Management Service of the U.S. Department of 

Interior have contributed to a better understanding of 

this resource. Much of the information necessary for 

this assessment was developed by the Geothermal 

Research Program and the Regional Aquifer Systems 

Analysis Program of the USGS. 

This work benefited greatly from the advice and 

counsel of an outstanding advisory group. Deep 

appreciation is expressed to Robert L. Christiansen, 

Wendell A. Duffield, Robert 0. Fournier, Arthur H. 

Lachenbruch, Donald R. Mabey, Leland L. Mink, L. J. 

Patrick Muffler, Franklin W. Olmsted, Edward A. 

Sammel, Alfred H. Truesdell, and Donald E. White, 

who participated in this group. In conclusion, I thank 

Robert A. Gray and John W. Salisbury, formerly of the 

U.S. Department of Energy, Division of Geothermal 

Energy, for their continuing encouragement. 

REFERENCES CITED 

Anderson, D. N ., and Lund, J. W ., eds., 1979, Direct 

utilization of geothermal energy: A technical 

handbook: Geothermal Resources Council 

Special Report 7. 

Blumenstock, D. I., and Price, Saul, 1978, Hawaii, in 

Climates of the states: Detroit, Gale Research 

Co., v. 1, p. 266-294. 

Brook, C. A., Mariner, R. H., Mabey, D. R., Swanson, 

J. R., Guffanti, Marianne, and Muffler, L. J.P., 

1979, Hydrothermal convection systems with 

reservoir temperatures ~ 90°C, in Muffler, L. J. 

P ., ed., Assessment of geothermal resources of 

the United States-1978: U.S. Geological 

Survey Circular 790, p. 18-85. 

Diment, W. H., Urban, T. C., Sass, J. H., Marshall, R. 

J., Munroe, R. J., and Lachenbruch, A. H., 1975, 

Temperatures and heat contents based on 

conductive transport of heat, in White, D. E., 

and Williams, D. L., eds., Assessment of 

geothermal resources of the United States1975: U.S. Geological Survey Circular 726, p. 

84-103. 

Fenneman, N. M., 1946, Physical divisions of the 

United States: Washington, U.S. Geological 

Survey, scale 1:7,000,000. 

Gass, T. E., Purdin, W. H., and Armitage, D. M., 1979, 

Economic impact of using nonmetalic materials 

in low to intermediate temperature geothermal 

well construction, volume II: National Water 

Well Association Contract Report 485874-S to 

Brookhaven National Laboratory (BNL 51224), v. 2, UC-66d, 115 p. 

Gudmundsson, J. S., and P~lmason, G., 1981, World 

survey of low-temperature geothermal energy 

utilization: Reykjavik, Iceland, Orkustofnun 

Report OS81005/JHD02, 148 p. 

Johnson, P. R., and Hartman, C. W., 1969, 

Environmental atlas of Alaska: Fairbanks, 
University of Alaska, Institute of Arctic 

Environmental Engineering Report 9/69, 83 p. 

Kincer, J. B., 1941, Climate and weather data for the 

United States, in Climate and man: 1941 

Yearbook of Agriculture: Washington, U.S. 

Department of Agriculture, p. 685-747. 

L(ndal, Baldur, 1973, Industrial and other applications 

of geothermal energy, in Armstead, H. C. H., 

ed., Geothermal energy, review of research and 

development: Paris, United Nations 

Educational, Scientific and Cultural 

Organization, p. 135-148. 

Muffler, L. J. P., ed., 1979, Assessment of geothermal 

resources of the United States-1978: U.S. 

Geological Survey Circular 790, 163 p. 

Muffler, L. J. P., and Cataldi, R., 1978, Methods for 

regional assessment of geothermal resources: Geothermics, v. 7, no. 2-4, p. 53-89. 

Muffler, L. J. P., and Guffanti, Marianne, 1979, 

Introduction, in Muffler, L. J. P., ed., 

Assessment of-geothermal resources of the 

United States-1978: U.S. Geological Survey 

Circular 790, p. 1-7. 

Nathenson, Manuel, 1978, Methodology of determining 

the uncertainty in the accessible resource base 

of identified hydrothermal convection 

systems: U.S. Geological Survey Open-File 

Report 78-1003, 50 p. 

Nathenson, Manuel, and Muffler, L. J. P., 1975, 

Geothermal resources in hydrothermal 

convection systems and conduction-dominated 

areas, in White, D. E., and Williams, D. L., eds., 

Assessment of geothermal resources of the 

United States-1975: U.S. Geological Survey 

Circular 726, p. 104-121. 

Oliver, R. E., ed., 1981, Progress report, June 1981: 

U.S. Department of Energy Geothermal 

Progress Monitor 5, 93 p. 

Papadopulos, S. S., Wallace, R. H., Jr., Wesselman, J. 

B., and Taylor, R. E., 1975, Assessment of 

onshore geopressured-geothermal resources in 

the northern Gulf of Mexico basin, in White, D. 

E., and Williams, D. L., eds., Assessment of 

geothermal resources of the United States1975: U.S. Geological Survey Circular 726, p. 

125-146. 

Reed, M. J., and Bliss, J. D., 1983, Geothermal 

resources and their small-scale development, in 

Meyer, R. F., and Olson, J. C., eds., The 

resource potential of small energy sources: 

New York, McGraw-Hill in press • 

Reed, M. J., Mariner, R. H., Brook, C. A., and Sorey, 

M. L., 1983, Selected data for low-temperature 

(less than 90°C) geothermal systems in the 

United States; . reference data for U.S. 

Geological Survey Circular 892: U.S. 

Geological Survey Open-File Report 83-250, 129 

p. 

Renner, J. L., White, D. E., and Williams, D. L., 1975, 

Hydrothermal convection systems, in White, D. 

E., and Williams, D. L., eds., Assessment of 

geothermal resources of the United States1975: U.S. Geological Survey Circular 726, p. 

5-57. 

Sammel, E. A., 1979, Occurrence of low-temperature 

7 

geothermal waters in the United States, in 

Muffler, L. J.P., ed., Assessment of geothermal 

resources of the United States-1978: U.S. 

Geological Survey Circular 790, p. 86-131. 

Sass, J. H., and Lachenbruch, A. H., 1979, Heat flow 

and conduction-dominated thermal regimes, in 

Muffler, L. J.P., ed., Assessment of geothermal 

resources of the United States-1978: U.S. 

Geological Survey Circular 790, p. 8-11. 

Schurr, S. H., and Netschert, B. C., 1960, Energy in the 

American economy, 1850-1975: An economic 

study of its history and prospects: Baltimore, 

Johns Hopkins Press, 774 p. 

Teshin, V. N., Swanson, J. R., and Orris, G. J., 1979, 

GEOTHERM--geothermal resources file: 

Geothermal Resources Council Transactions, v. 

3, p. 721-724. 

Wallace, R. H., Jr., Kraemer, T. F., Taylor, R. E., and 

Wesselman, J. B., 1979, Assessment of 

geopressured-geothermal resources in the 

northern Gulf of Mexico basin, in Muffler, L. J. 

P ., ed., Assessment of geothermal resources of 

the United States-1978: U.S. Geological 

Survey Circular 790, p. 132-155. 

White, D. E., and Williams, D. L., eds., 1975, 

Assessment of geothermal resources of the 

United States-1975: U.S. Geological Survey 

Circular 726, 155 p. 

ADDITIONAL REFERENCES 

The following were used as general references in the 

preparation of one or more chapters of this report. A 

series of geothermal-resource maps was produced by 

several States with coordination by the Division of 

Geothermal Energy of the U.S. Department of Energy 

and by the U.S. National Oceanic and Atmospheric 

Administration. 

MAPS 

Gosnold, W. D., Jr., and Everson, D. A., 1982, 

Geothermal resources of Nebraska: Lincoln, 

University of Nebraska, Conservation and 

Survey Division, scale 1:500,000. 

Harris, K. L., 1981, Geothermal resources of North 

Dakota: Grand Forks, North Dakota Geological 

Survey, scale 1:500,000. 

Heasler, H. P., Buelow, K. L., Decker, E. R., Hinckley, 

B. S., and Spencer, S. A., 1982, Geothermal 

resources of Wyoming: Laramie, Wyoming 

Geological Survey, scale 1:500,000. 

Higgins, C. T., 1980, Geothermal resources of 

California: California Division of Mines and 

Geology California Geologic Data Map Series 

Map 4, scale 1:500,000. 

Korosec, M. A., Kaler, K. L., Schuster, J. E., 

Bloomquist, R. G., Simpson, S., and Blackwell, 

D. D., 1981, Geothermal resources of 

Washington: Washington Division of Geology 

and Earth Resources Geologic Map 25, scale 

1:500,000. 

Mitchell, J. C., Johnson, L. L., and Anderson, J. E., 

Geothermal resources of Idaho, pl. 1 of 

Geothermal investigations in Idaho, part 9, 
Potential for direct heat application of 

geothermal resources: Idaho Department of 

Water Resources Water Information Bulletin 30, 

scale 1:500,000. 

Murphy, P. J., 1980, Geothermal resources of Utah: 

Salt Lake City, Utah Geological and Mineral 

Survey, scale 1:500,000. 

Pearl, R. H., 1980, Geothermal resources of 

Colorado: Colorado Geological Survey Map 

Series 14, scale 1:500,000. 

Peterson, N, V., Priest, G. R., Black, G. L., Brown, D. 

E., and Woller, N. M., 1982, Geothermal 

resources of Oregon: Portland, Oregon 

Department of Geology and Mineral Industries, 

scale 1:500,000. 

Stavnes, S. A., and Steeples, D. W., 1982, Geothermal 

resources of Kansas: Lawrence, Kansas 

Geological Survey, scale 1:500,000. 

Sonderegger, J. L., and Bergantino, R. N., 1981, 

Geothermal resources map of Montana: 

Montana Bureau of Mines and Geology 

Hydrogeologic Map 4, scale 1:1,000,000. 

Swanberg, C. A., 1980, Geothermal resources of New 

Mexico: Las Cruces, New Mexico State 

University, New Mexico Energy Institute, scale 

1:500,000. 

Trexler, D. T., Koenig, B. A., and Flynn, Thomas, 1979, 

Geothermal resources of Nevada and their 

potential for direct utilization: Reno, Nevada 

Bureau of Mines and Geology, scale 1:500,000. 

Witcher, J. C., Stone, Claudia, and Hahman, W. R., 

1982, Geothermal resources of Arizona: 

Tucson, Arizona Bureau of Geology and Mineral 

Technology, scale 1:500,000. 

REPORTS 

Berry, G. W., Grim, P. J., and Ikelman, J. A., 1980, 

Thermal springs list for the United States: U.S. 

National Oceanic and Atmospheric 

Administration Key to Geophysical Records 

Documentation 12, 59 p. 

Biggane, J. H., 1981, The low temperature geothermal 

resource of the Yakima region-a preliminary 

report: Washington Division of Geology and 

Earth Resources Open-File Report 81-7, 70 p. 

Costain, J. K., 1979, Geothermal exploration methods 

and results-Atlantic Coastal Plain, in A 

symposium of geothermal energy and its direct 

uses in the eastern United States: Geothermal 

Resources Council Special Report 5, p. 13-22. 

Glover, Lynn, III, 1979, General geology of the east 

coast with emphasis on potential geothermal 

energy regions: A detailed summary, in A 

symposium of geothermal energy and its direct 

uses in the eastern United States: Geothermal 

Resources Council Special Report 5, p. 9-11. 

Gries, J. P ., 1977, Geothermal applications on the 

Madison (Pahasapa) aquifer system in South 

Dakota: Rapid City, South Dakota School of 

Mines and Technology Report 100/1625-2, 85 p. 

Harris, K. L., Howell, F. L., Winczewski, L. M., 

Wartman, B. L., Umphrey, H. R., and Anderson, 

S. D., 1981, An evaluation of hydrothermal 

resources of North Dakota; phase 2 final 

technical report: Grand Forks, University of 

North Dakota, Engineering Experiment Station 

8 

Bulletin 81-05-EE8-02, 180 p. 

Head, W. J., Kilty, K. T., and Knottek, R. K., 1979, 

Maps showing formation temperatures and 

configurations of the tops of the Minnelusa 

Formation and the Madison Limestone, Powder 

River Basin, Wyoming, Montana, and adjacent 

areas: U.s. Geological Survey Miscellaneous 

Investigations Series Map 1-1159. 

Hull, D. A., ed., 1980, Progress report on activities of 

the low-temperature resource assessment 

program 1979-1980: Oregon Department of 

Geology and Mineral Industries Open-File 

Report 0-80-4, 79 p. 

Jorgensen, D. G., and Signor, D. C., 1981, Plan of 

study for the central Midwest regional aquifer 

system analysis in parts of Arkansas, Colorado, 

Kansas, Missouri, Nebraska, New Mexico, 

Oklahoma, South Dakota, and Texas: U.S. 

Geological Survey Water-Resources 

Investigations 81-206, 28 p. 

Korosec, M. A., and Schuster, J. E., eds., 1981, The 

1979-1980 geothermal resource assessment 

program in Washington: Washington Division of 

Geology and Earth Resources Open-File Report 

81-3, 270 p. 

Maurath, Gary, and Eckstein, Yoram, 1981, Heat flow 

and heat production in north-western 

Pennsylvania: Geothermal Resources Council 

Transactions, v. 5, p. 103-106. 

Motyka, R. J., and Moorman, M. A., 1981, 

Reconnaissance of thermal spring sites in the 

Aleutian Arc, Atka Island to Becherof Lake: 

Geothermal Resources Council Transactions, v. 

5, p. 111-114. 

Motyka, R. J., Moorman, M. A., and Reeder, J. W., 

1980, Assessment of thermal springs in southern 

Southeastern Alaska-preliminary results and 

evaluation: Alaska Division of Geological and 

Geophysical Surveys Open-File Report AOF127,66p. 

Pearl, R. H., 1979, Colorado's hydrothermal resource base-an assessment: Colorado Geological 

Survey Resources Series 6, 144 p. 

Smith, D. L., and Dees, W. T., 1981, Low-grade 

geothermal resources in northern Louisiana: 

Geothermal Resources Council Transactions, v. 5, p. 127-128. 

Sonderegger, J. L., and Schmidt, F. A., 1981, 

Geothermal resources in Montana: Montana 

Academy of Science Proceedings, v. 40, p. 50-

62. 

Steeples, D. W., and Bickford, M. E., 1981, Piggyback 

drilling in Kansas: An example for the 

Continental Scientific Drilling Program: Eos 

(American Geophysical Union Transactions), v. 

62, no. 18, p. 473-476. 

Steeples, D. W ., and Stavnes, S. A., eds., 1982, 

Assessment of the geothermal resources of 

Kansas: Lawrence, Kansas Geological Survey, 

final report to U.S. Department of Energy, 

Geothermal Division, 261 p. 

Thomas, D. M., Cox, M. E., Lienert, B. R., Kauahikaua, 

J. P., and Mattice, M. D., 1980, Preliminary 

geothermal assessment surveys for the State of 

Hawaii: Geothermal Resources Council 

Transactions, v. 4, p. 185-188. 
Regional Heat Flow and Temperature Gradients 

By Manuel Nathanson, Marianne Guffanti, 

John H. Sass, and Robert J. Munroe 

CONTENTS 

Abstract -------------------------- Introduction ----------------------

Background ------------------------ Distributions of heat flow and 

temperature gradients in the 

United States ------------------- Low-temperature geothermalresource assessment -------------

References cited ------------------

ABSTRACT 

Page 

9 

9 

9 

10 

13 

15 

To assess the potential for low-temperature 

geothermal resources in regional conductive thermal 

environments, a knowledge of temperature gradients 

to depths of about 2 km is required. Regional 

variations in temperature gradient, which reflect 

corresponding regional variations in heat flow, thermal 

conductivity, or both, result in some uncertainties in 

the derivation of deep thermal-gradient data from 

near-.c;urface (100-250-m depth) heat flows. A contour 

map of regional heat flow in the conterminous United 

States shows that heat flow in the West is generally 

higher than in the East. A temperature-gradient map, 

based on data from 240 drill holes generally deeper 

than 600 m, indicates the same sort of first-order 

variation in geothermal-resource potential as does the 

heat-flow map, although there also are some important 

differences between these two maps. Large areas are 

without data on both maps, but either map can be used 

to identify promising geothermal-resource areas or 

areas where more reconnaissance work is needed. 

INTRODUCTION 

For the assessment of low-temperature 

geothermal resources in the United States, regional 

heat flow and temperature gradients assume a much 

greater importance than for intermediate- and hightemperature resources. For low-temperature 

geothermal energy, a favorable combination of high 

regional heat flow, low thermal conductivity, and a 

good aquifer can result in an exploitable resource at 

depths of 2 km or less. However, the depths of 

9 

occurrence for high-temperature geothermal energy 

derived from conductive thermal gradients without 

hydrothermal convection are so great that economical 

extraction becomes unlikely. 

This chapter briefly reviews heat flow and 

temperature gradients to provide a background for 

presentation of maps of heat flow and deep 

temperature gradients in the United States and of a 

table of thermal conductivities. These maps help to 

delineate areas favorable for the occurrence of lowtemperature geothermal resources and have been used 

to assign average temperature gradients for the 

estimation of reservoir temperatures for some geothermal systems (Sorey, Reed, and others, this 

volume). 

BACKGROUND 

The vertical conductive heat flow .9.. given by 

q=k (dT \' -- dz) 

( 1) 

where k is the conductivity and dT/dz is the vertical 

temperature gradient. The temperature gradient is 

determined by measuring the temperature at various 

depths in a drill hole and calculating a gradient (for 

example, Sass and others, 1971). Thermal 

conductivities, which are commonly measured in the 

laboratory on core or cuttings, generally range from 

1. 7 to 3.5 W /m K for consolidated rocks, although 

values as low as 0.8 W /m K and as high as 8 W /m K 

also occur (Roy and others, 1981). Table 1 lists typical 

values for regional heat flow and temperature 

gradients in the United States. 

Birch and others (1968) showed that for granitic 

plutonic rocks in the Northeastern United States, a 

plot of the measured surface heat flow .9.. versus the 

measured radioactive heat production A defines a straight line: -

( 2) 

where D is the slope of the line, in units of depth. The 

reduced heat flow .9r is the heat flow obtained by 

extrapolating the Plot of 9. versus A to zero radioactive heat production. Typical values for 
Table I.-Typical values of heat flow and temperature 

gradient in parts of the conterminous United 

States 

[All values assume a thermal conductivity of 2.5 

W fm·~ and a radioactive heat production of 2.1 

J.lw /m (after Lachenbruch and Sass, 1977)] 

Reduced Heat-production Heat Temperature 

Region heat flow thickness flow gradient 

(mW/m 2

) (km) (mWtm 2) ("C/km) 

Sierra Nevada---------- 17 10 38 15 

Eastern United States-- 34 7.5 49 20 

Basin and Range------- 67 10 88 35 

Battle Mountain high 

(part of the 84 10 105 42 

Basin and Range) 

radioactive heat production in felsic grystallinebasement rocks range frogn 1 to 3 W /m , although 

values as high as 8 W /m are also known. The g-A 

relation was interpreted by Birch and others (1968) to 

indicate that the heat flow measured at the surface is 

made up of one component of heat flow ~ from the 

mantle and lower crust and another component of heat 

flow DA due to the radioactivity of the upper crust. 

The parameter D can be related to the thickness of a 

layer of rock with constant heat production A below 

which heat flow is constant and equals the reduced 

heat flow ~· Other distributions of radioactivity with 

depth also satisfy equation 2; a model in which A 

decreases exponentially with depth was proposed to 

maintain the validity of equation 2 under the effects 

of differential erosion (Lachenbruch, 1968, 1970). 

Different regions have been found to have 

characteristic values of ~ and D (for example, Roy 

and others, 1968a, b; Lachenbruch, 1968), and on this 

basis the conterminous United States can be divided 

into regions of characteristic heat flow. Table 1 lists 

the values of .9.r and D for these regions (Lachenbruch 

and Sass, 1977): Within most such regions,~ remains 

constant, whereas the measured surface heat flow may 

vary from place to place owing to variations in 

radioactive heat production of the crust. The value 

used in table 1 for radioactively generated he~t flow in 

the Eastern United States is 16 mW/m , which 

represents a substantial fraction of the measure surface heat flow. For the tectonically young parts of 

the Western United States, the data for g are quite 

high on the average, and the g-A data show 

considerable scatter (Lachenbruch and Sass, 1977), so that a linear g-A relation cannot be defined. Some of 

the heat flow in all the regions is attributable to 

crustal radioactivity, but other large~cale processes 

also are involved. The high mean value is most likely 

related to deep~eated tectonic processes, such as 

crustal extension and associated magmatism, whereas 

the large scatter is probably due to hydrothermal 

convection in the uppermost few kilometers of the 

crust, and to associated hot~pring activity. 

10 

DISTRIBUTIONS OF HEAT FLOW 

AND TEMPERATURE GRADIENTS 

IN THE UNITED STATES 

The temperature-versus-depth relation in the 

upper 2 km of the crust can be estimated from either 

heat-flow or temperature-gradient data. In many 

areas, heat flows have been determined from data 

collected in drill holes less than 150 m deep, and 

although the. measured gradients may appear to be 

conductive, some heat flows are probably affected by 

hydrothermal convection and ground-water flow below 

the drill hole. If, however, the conductive heat flow is 

representative of the region and if a model can be 

developed for the variation in thermal conductivity 

with depth, then temperatures to depths of 2 km can be predicted from shallow heat-flow measurements 

alone. In most of the conterminous United States, 

however, it is difficult to fulfill both these 

requirements, owing to an insufficient number of 

internally consistent heat-flow determinations or to 

incomplete knowledge of the thermal conductivity to 

the required depths. 

A more direct method of estimating deep 

subsurface temperatures is by extrapolating measured 

gradients. However, if the depths of interest lie 

significantly below the depth for which temperature 

measurements are available, this extrapolation 

becomes uncertain, and variation in conductivity must 

be accounted for. When the thermal conductivity has 

not been measured or cannot be estimated with 

confidence, the temperature data should be from drill 

holes sufficiently deep that any changes in thermal 

conductivity between the bottom of the hole and the 

target depth will not be significant. 

The heat-flow map (fig. 4) of Sass and others 

(1981, fig. 13.4) shows contours. of surface heat flow 

based on more than 1,000 determinations. The specific 

data are not shown, but a map of them together with a 

fairly complete reference list may be found in Sass and 

others (1981). The United States east of the 100th 

meridian is gene~ally characterized by a heat flow of 

40 to 60 m W /m , with some local regions of higher 

heat flow in New England and on the Atlantic Coastal 

Plain. Heat flow west of the 100th meridian appears 

to vary more and to be higher overall than in the Eas~; 

the mean heat flow in the West is about 80 mW/m • 

Within the West, areas of relatively low heat flow 

occur in the western Sierra Nevada, southern Nevada, 

and parts of the Colorad~ Plateaus, whereas heat flow 

greater than 100 mW/m characterizes the Southern 

Cascade Mountains, the Battle Mountain high, and the 

Rio Grande Rift. On a regional scale it is unlike¥ that 

conductive heat flow can exceed 150 mW/m , and 

higher values indicate some form of hydrothermal 

convection. 

An empirical approach to predicting heat flow 

in areas of little or no conventional heat-flow data was 

developed by Swanberg and Morgan (1978, 1980; see 

Sass and others, 1981), who discovered a statistical 

correlation between the silica geotemperature of 

ground waters and heat flow within 1-degree blocks of 

latitude and longitude for which silica geotemperature 

and heat flow are both well documented and have 

small scatter. They extended this empirical relation 
.-

.-

2.5 

Heat-flow contours 

• )2.5 

- 1.5-2.5 

WEST EAST D o.75-1.5 1.0-1.5 

mmmllill < 0·

75 < 1.0 

• (0 

0 250 500 KILOMETERS 

~~/ ' ~ 

IV ('·-".1""··'\ \ , ' ~ "'-

\ \/,.ft.- ., ,;::./";;{. 

~v 

____..... 

Figure 4.-Heat-fiow map of the conterminous United States (from Sass and others, 1981, fig. 13.4). Contours are in heat-flow units. 
to areas with few heat-flow measurements and 

predicted heat-flow anomalies for several such areas. Some of their predictions-namely, on the Atlantic 

Coastal Plain, in southeastern Utah, and in parts of 

Nebraska-have been confirmed by subsequent heatflow measurements, whereas others (for example, in 

the Central Valley of California) appear to represent 

something other than high heat flow (see J. K. Costain, 

in Sass and others, 1981, p. 533-539; C. A. Swanberg 

and Paul Morgan, in Sass and others, 1981, p. 540-544). 

The silica-geotemperature/heat-flow relation has thus 

had some success in predicting heat-flow anomalies on 

a regional basis, and the anomalies predicted by this 

method are worth investigating with conventional 

techniques. However, because the method relies on a 

statistical approach involving data averaged over 1-

degree blocks of latitude and longitude or larger areas, 

and because the physical basis of the relation has yet 

to be established, the silica-geothermometer/heatflow method probably has only a limited applicability 

to reconnaissance exploration for low-temperature 

geothermal resources. If thermal conductivities were more or less 

uniform or well known on a regional scale, the heatflow map in figure 4 could be used to characterize 

Table 2.-Thermal conductivities of common rock 

~ 

[All values in watts per meter-Kelvin] 

Rock type Range Mean 

Andesite--------------- 1.35-4.86 3.7 

Basalt----------------- 1.12-2.38 1.8 

Dolomite--------------- 4.0-5.9 4.5 

Gabbro---------------- 1.80-3.60 2.6 

Gneiss----------------- 1.69-5.75 3.7 

Granitic rocks-------- 2.1 -5.0 3.6 

Limestone-------------- 1.30-5.80 3.6 

Marble----------------- 2.02-6.52 4.3 

Quartzite-------------- 2.33-7.45 4.9 

Rhyolite--------------- 1. 58-4.33 3.0 

Rock salt-------------- 5.3-7.2 5.4 

Sandstone------------- 1. 5-4.3 2.9 

Shale------------------ 1.2-2.9 2.0 

Tuff------------------- .91-3.20 2.1 

12 

temperature gradients. Table 2 lists representative 

values of the thermal conductivities of watersaturated rocks in various parts of the United States. 

The ranges and means are only approximate and have 

been generalized from various sources, including Clark 

(1966), Roy and others (1981), and J. H. Sass and R. J. 

Munroe (unpub. data, 1982). 

Several observations should be made in relation 

to the data listed in table 2: 

1. For most rock types, the thermal conductivity 

varies enormously. For some rock types in a 

given locality or region, however, most values 

may fall within a relatively narrow range of 

about 20 to 30 percent of the mean. Mean values 

commonly vary from region to region, and so the 

literature values used for estimates of heat flow 

and for derivation of temperature gradients must 

be chosen with care. 

2. For quartz-rich rocks, the bulk thermal 

conductivity varies widely with the content of 

such low-conductivity minerals as feldspars and 

with the porosity, and so it is difficult to 

generalize regional means. 

3. Literature values for shale are unreliable. 

Argillaceous sedimentary rocks represent 

possibly the most difficult media for the 

measurement of thermal conductivity. They are 

fissile and, in many places, poorly consolidated, 

and it is almost impossible to maintain them in 

their natural physical state after removal from 

the ground. They also are anisotropic, and so 

measurements of thermal conductivity on 

crushed samples or drill cuttings (the most 

common current method) will generally be in 

error because such measurements represent a 

geometrically weighted average conductivity 

rather than the actual vertical conductivity. 

Blackwell and others (1981) discussed some of the 

implications of this type of error to measured 

heat-flow values from the Great Plains. In the 

context of low-temperature geothermal 

resources, suspect literature values for the 

thermal conductivity of shale are irrelevant if 

the temperatures of interest are entirely within a shale section; however, if gradients are 

extrapolated from sand to shale or vice versa, 

the predicted temperatures can be greatly in 

error. 4. Generalized literature values of thermal 

conductivity can be used to estimate the 

variation in conductivity with depth and thus, as 

mentioned previously, to facilitate extrapolation 

of temperature gradients for most crystalline 

terranes and a restricted class of sedimentary 

terranes. For carbonate rocks, the ratio of 

limestone to dolom~te in a, given section must be 

known. In sand~hale sections, an accurate 

estimate of the sand/shale ratio is required, and 

in sedimentary basins where the sand/shale ratio 

varies laterally, gradients in these sections may 

vary by a factor of 2 for the same regional heat 

flow. 

Several maps of temperature gradients in the 

United States have been constructed. The American 

Association of Petroleum Geologists and U.S. 
Geological Survey (1976) prepared a map of gradients 

calculated primarily from temperature measurements 

at a single depth in oil, gas, and water wells and from 

assumed values of the mean annual air temperature 

(see Guffanti and Nathenson, 1980, fig. 2). Vaught 

(1980) used the data for Michigan to point out various 

problems with the accuracy of this data set in that 

area and thus showed that the map must be interpreted 

with care. Kron and Heiken (1980a, b) used data from 

the heat-flow literature for drill holes deeper than 50 

m to construct a map of temperature gradients. 

Although they omitted data for any drill hole with 

temperatures that were obviously disturbed, some 

shallow drill holes with either high or low temperature 

gradients are most probably influenced by underlying 

hydrothermal convection. Although meaningful 

estimates of thermal budgets and deep temperatures 

can be obtained from groups of such shallow heat-flow 

data (for example, Sass and others, 1971; Brott and 

others, 1976), simple linear extrapolation of thermal 

gradients from such data generally is misleading. 

Guffanti and Nathenson (1980, fig. 1) 

constructed a temperature-gradient map based on data 

from drill holes generally deeper than 600 m, using 

data that appeared to represent conductive heat 

transfer, to obtain a representation of regional, 

background thermal gradients. Data from drill holes 

at sites in or adjacent to known hydrothermalconvection systems were omitted. In drill holes where 

the gradient varied with depth, an overall gradient was 

chosen as the average of straight-line segments, 

approximately weighted by depth interval. Although, 

this value may not exactly reflect the temperatures at 

all depths, it can be a good approximation of these 

temperatures, provided the temperature-gradient 

contrasts over large depth intervals are not too 

great. As part of their study, Guffanti and Nathenson 

(1981) made a systematic search of the compilation by 

Spicer (1964) to extract the deepest, least disturbed, 

and most areally representative temperature logs. 

Figure 5 shows the map of Guffanti and 

Nathenson (1980) but with added data from Blackwell 

and Steele (1981), Dashevsky and McClung (1980), M. 

C. Gardner (written commun., 1981), Hodge and others 

(1981), Jessop and Judge (1971), Judge and Beck (1973), 

W. S. Keys and D. E. Eggers (written commun., 1980), 

Leonard and Wood (1980), McClung (1980), Perry and 

others (1980), Roy and others (1980), Sass and others 

(1981), J. H. Scott and J. J. Daniels (written commun., 

1980), Shearer (1979), and Urban and others (1978). An 

important characteristic of these deep temperature 

gradients is that few of the high gradients shown on the map by Kron and Heiken (1980b) are confirmed by 

the deeper data. In part, this difference reflects the 

smaller number of deep drill holes used by Guffanti 

and Nathenson (1980), but it also reflects the 

improbability of very high gradients persisting to 

depths of 600 m except in geothermal areas, as well as 

the local-areal extent of most high-temperature 

thermal anomalies. It should be emphasized that the 

map (fig. 5) is highly generalized and that in areas 

between temperature-gradient contours, both higher 

and lower values may be measured on a local scale, 

especially at shallow (less than 300 m) depths. 

The temperature-gradient map (fig. 5) reflects 

the combined effects of heat flow and thermal 

conductivity. Comparison with the heat-flow map (fig. 

4) shows a general coincidence of temperature 

gradients with heat flow. Gradients less ~han 25°C/km 

and heat flow less than 63 mW/m (1.5 HFU) 

predominate east of the 100th meridian, whereas 

gradients greater than 25°C/km and a heat flow 

greater than 63 mW/m2 are common in the West. 

Within the East, part of the southern Appalachians 

region stands out as a thermal low in terms of both 

heat flow and temperature gradients, whereas in parts 

of the Atlantic Coastal Plain, higher than average heat 

flow is expressed by higher temperature gradients. 

High temperature gradients in the Northwestern 

United States and in parts of Colorado and Wyoming 

approximately correspond to areas of high heat flow. 

Virtually no heat-flow determinations exist on which a 

comparison can be based in western Texas, where 

temperature gradients are low, or in the Gulf Coastal 

Plain, where inland gradients are high. 

This general correspondence between heat flow 

and temperature gradients suggests that thermal 

conductivities cluster around some average value on a 

regional scale, despite smaller scale variations in 

lithology. Some variations in conductivity, however, are related to regional geologic features, and some 

temperature-gradient anomalies mirror geologic 

environments but not heat flow. For example, 

relatively high temperature gradients occur in western 

Pennsylvania and West Virginia, primarily owing to the 

low thermal conductivity of the thick sequence of 

Devonian shale in those States; however, this is not a 

region of high heat flow except for a small area in 

south-central New York. Some anomalous 

temperature gradients are related to local thermalconductivity extremes that are not significant on a regional scale; for example, a 13°C/km gradient in 

eastern Utah relects the local presence of highconductivity salt. 

LOW-TEMPERATURE GEOTHERMAL-RESOURCE 

ASSESSMENT 

Low-temperature geothermal resources are defined partly in relation to regional background 

values of heat flow and temperature gradient. The 

low-temperature geothermal resources assessed in this 

volume occur in permeable aquifers that have 

temperatures greater than those defined by a 

minumum of 10°C above the local mean annual air 

temperature at the surface, increasing by 25°C/km 

with depth to a maximum of 90°C (see Reed, this 

volume, fig. 1). The value of 25°C/km corresponds to 

the temperature radient based on an average heat 

flow of 63 m W /m and a thermal conductivity of 2.5 

W /m · K for felsic crystalline rocks. This thermal 

regime is appropriate for stable continental 

environments and is an upper limit for large areas of 

the Eastern United States, as depicted on the 

temperature-gradient map (fig. 5).Gradients higher 

than 25°C/km occur in regions of high heat flow and in 

areas of normal heat flow containing a thick sequence 

of such low-conductivity rocks as shale and basalt. 

The low-temperature limit used in this assessment screens from consideration geologic environments with 

13 
.... 

~ 

0 

80' 15' ------~-

\\ \.~' J__------\ \1 

I \ ' I \ 

\ II I ' \ 

··T···--T·--

~ 1 \ 

r----'--+--1-----z 

h~....._.~ 

EXPLANATION 

o (15°C/km 

~ 15-24 

A 25-34 

• 35-39 0 250 500 KILOMETERS 

• 40-44 

D ) 45 

Figure 5.-Temperature-gradient map of the conterminous United States (based on Guffanti and Nathenson, 1980, fig. 1). 

----
normal heat flow and average conductivity, and thus 

excludes areas containing vast amounts of relatively 

cool shallow ground water; it also constrains to 

reasonable values the drilling depths required to reach 

adequate temperatures for nonelectrical uses. 

The temperature~radient map (fig. 5) broadly 

highlights areas with gradients greater than 25°C/km 

where useful temperatures can be found at drillable 

depths. East of the 100th meridian, an area in western 

Pennsylvania, parts of the Atlantic Coastal Plain, and 

areas inland of the Gulf of Mexico coast all have 

higher than average temperature gradients. Much of 

the West has high gradients, although depths to 

basement are shallow in many places; obvious 

exceptions are the San Joaquin Valley and the Los 

Angeles basin in California, the Williston basin in 

North Dakota, and smaller basins in Wyoming, 

Colorado, and New Mexico. 

To be considered a resource, not only must the 

temperatures be adequate, but also there must be 

indication of sufficient permeability to supply longterm production (Sorey, Nathenson, and Smith, this 

volume). Mariner and others (this volume) and Sorey, 

Reed, and others (this volume) survey the available 

hydrologic data to estimate reservoir thicknesses, 

transmissivities, and confining-bed properties for 

aquifers that exceed the minimum-temperature 

criterion. For most aquifers, actual temperature data 

were used; however, for some areas the data shown on 

the temperature~radient map (fig. 5) were used to 

assign average gradients for an estimation of reservoir 

temperatures. 

Superimposed on the regional gradients are 

anomalies caused by hydrothermal convection. The 

low-temperature resources identified by Mariner and 

others (this volume) include some that have hot springs 

at the surface and are clearly associated with 

hydrothermal-convection systems. Other resources 

are defined by high temperatures in wells; for these 

resources, the heat-flow and temperature~radient 

maps (figs. 4, 5) are useful for deciding whether the 

system reflects regional conductive heat flow and 

temperature gradients, or is likely to require 

convection to give the temperatures measured in wells 

at the depth shown. 

REFERENCES CITED 

American Association of Petroleum Geologists and 

U.S. Geological Survey, 1976, Geothermal 

gradient map of North America: Washington, 

U.S. Government Printing Office, scale 

1:5,000 ,ooo. 

Birch, Francis, Roy, R. F., and Decker, E. R., 1968, 

Heat flow and thermal history in New England 

and New York, in Zen, E-an, White, W. S., 

Hadley, J. B., and Thompson, J. B., Jr., eds., 

Studies of Appalachian geology: northern and 

maritime: New York, Interscience, p. 437-451. 

Blackwell, D. D., and Steele, J. L., 1981, Heat flow 

and geothermal potential of Kansas: Kansas 

State Agency Contract 949 final report, 69 p. 

Blackwell, D. D., Steele, J. L., and Steeples, D. W ., 

1981, Heat flow determinations in Kansas and 

their implications for Midcontinent heat flow 

15 

patterns abs. : Eos (American Geophysical 

Union Transactions), v. 62, no. 17, p. 392. 

Brott, C. A., Blackwell, D. D., and Mitchell, J. C., 

1976, Heat flow study of the Snake River Plain 

region, Idaho, pt. 8 of Geothermal 

investigations in Idaho: ldahoDepartment of 

Water Resources Water Information Bulletin 30, 

195 p. 

Clark, S. P., Jr., 1966, Thermal conductivity, in Clark, 

S. P ., Jr., ed., Handbook of physical constants: 

Geological Society of America Memoir 97, p. 

459-482. 

Dashevsky, S. S., and McClung, W. S., 1980, Summary 

of temperature logging of Crisfield, Maryland 

geothermal test hole, in Costain, J. K., Glover, 

Lynn, m, and Sinha, A:-K., eds., Evaluation and 

targeting of geothermal energy resources in the 

southeastern United States: July 1-8eptember 

30, 1979: Blacksburg, Virginia Polytechnic 

Institute and State University Report VPI-8U78ET-27001-7, p. A22-A32. 

Guffanti, Marianne, and Nathenson, Manuel, 1980, 

Preliminary map of temperature gradients in 

the conterminous United States: Geothermal 

Resources Council Transactions, v. 4, p. 53-56. 

--1981, Temperature-depth data for selected deep 

drill holes in the United States obtained usimr 

maximum thermometers: U.S. Geological 

Survey Open-File Report 81-555, 100 p. 

Hodge, D. S., De Rito, R., Hifiker, K., Morgan, P., and 

Swanberg, C. A., 1981, Investigations of lowtemperature geothermal potential in New York 

state: Los Alamos National Laboratory Report 

LA-8960-MS, 7 4 p. 

Jessop, A. M., and Judge, A. S., 1971, Five 

measurements of heat flow in southern 

Canada: Canadian Journal of Earth Sciences, v. 

8, no. 6, p. 711-716. 

Judge, A. S., and Beck, A. E., 1973, Analysis of heatflow data-several boreholes in a sedimentary 

basin: Canadian Journal of Earth Sciences, v. 

10, no. 10, p. 1494-1507. 

Kron, Andrea, and Heiken, Grant, 1980a, Geothermal 

gradient map of the United States: Geothermal 

Resources Council Transactions, v. 4, p. 69-71. 

--1980b, Geothermal gradient map of the United 

States: Los Alamos Scientific Laboratory Map 

LA-8476-MAP, scale 1:5,000,000. 

Lachenbruch, A. H., 1968, Preliminary geothermal 

model of the Sierra Nevada: Journal of 

Geophysical Research, v. 73, no. 22, p. 6977-

6989. 

--1970, Crustal temperatures and heat production: 

Implications of the linear heat-flow relation: 

Journal of Geophysical Research, v. 75, no. 17, 

p. 3291-3300. 

Lachenbruch, A. H., and Sass, J. H., 1977, Heat flow in 

the United States and the thermal regime of the 

crust, in Heacock, J. G., ed., The Earth's 

crust: - American Geophysical Union 

Geophysical Monograph 20, p. 626-675. 

Leonard, R. B., and Wood, W. A., 1980, Geothermal 

gradients in the Missoula and Bitteroot valleys, 

west-central Montana: U.S. Geological Survey 

Water-Resources Investigations 80-89, 15 p. 
McClung, W. s., 1980, Temperature logs of observation 

wells in the Coastal Plain of Georgia, in 

Costain, J. K., and Glover, Lynn, Ill, Evaluation 

and targeting of geothermal energy resources in 

the southeastern United States, October 1, 

1979-March 31, 1980: Blacksburg, Virginia 

Polytechnic Institute and State University 

Report VPI-8U-78ET-27001-8, p. B22-B30. 

Perry, L. D., Higgins, G. P., and McKinney, M. M., 

1980, Heat flow and heat generation in the 

Atlantic Coastal Plain, in Costain, J. K., and 

Glover, Lynn, ill, eds., Evaluation and targeting 

of geothermal energy resources in the 

southeastern United States: October 1, 1979-

March 31, 1980: Blacksburg, Virginia 

Polytechnic Institute and State University 

Report VPI-8U-78ET-27001-8, p. B114-B153. 

Roy, R. F., Beck, A. E., and Touloukian, Y. S., 1981, 

Thermophysical properties of rocks, chap. 12 of 

Toulokian, Y. S., Judd, W. R., and Roy, R. F., 

eds., Physical properties of rocks and 

minerals: New York, McGraw-Hill, p. 409-502. 

Roy, R. F., Blackwell, D. D., and Birch, Francis, 

1968a, Heat generation of plutonic rocks and 

continental heat-flow provinces: Earth and 

Planetary Science Letters, v. 5, no. 1, p. 1-12. 

Roy, R. F., Decker, E. R., Blackwell, D. D., and Birch, 

Francis, 1968b, Heat flow in the United 

States: Journal of Geophysical Research, v. 73, no. 16, p. 5207-5221. 

Roy, R. F., Taylor, B., Pyron, A. J ., and Maxwell, J. 

C., 1980, Heat-flow measurements in the state 

of Arkansas: Final report: Los Alamos 

Scientific Laboratory Report LA-8569-MS, 15 p. 

Sass, J. H., Blackwell, D. D., Chapman, D. S., Costain, 

J. K., Decker, E. R., Lawver, L. A., and 

Swanberg, C. A., 1981, Heat flow from the 

crust of the United States, chap. 13 of 

Touloukian, Y. S., Judd, W. R., and Roy, R. F., 

16 

eds., Physical properties of rocks and 

minerals: New York, McGraw-Hill, p. 503-548. 

Sass, J. H., Lachenbruch, A. H., Munroe, R. J., Greene, 

G. W., and Moses, T. H., Jr., 1971, Heat flow in 

the western United States: Journal of 

Geophysical Research, v. 76, no. 26, p. 6376-

6413. 

Sass, J. H., Munroe, R. J., and Stone, Claudia, 1981, 

Heat flow from five uranium test wells in westcentral Arizona: U.S. Geological Survey OpenFile Report 81-1089, 42 p. 

Shearer, C. R., 1979, A regional terrestrial heat-flow 

study in Arizona: Socorro, New Mexico 

Institute of Mining and Technology, Ph. D. 

thesis, 184 p. 

Spicer, H. C., 1964, A compilation of deep earth 

temperature data, U.S.A., 1910-1945: U.S. 

Geological Survey Open-File Report [64-147]. 

Swanberg, C. A., and Morgan, Paul, 1978/79, The linear 

relation between temperatures based on the 

silica content of groundwater and regional heat 

flow: A new heat flow map of the United 

States: Pure and Applied Geophysics, v. ll7, p. 

227-241. 

--1980, The silica heat flow interpretation technique: 

Assumptions and applications: Journal of 

Geophysical Research, v. 85, no. B12, p. 7206-

7214. 

Urban, T. C., Diment, W. H., and Nathenson, Manuel, 

1978, East Mesa geothermal anomaly, Imperial 

County, California: Significance of 

temperatures in a deep drill hole near thermal 

equilibrium: Geothermal Resources Council 

Transactions, v. 2, pt. 2, p. 667-670. 

Vaught, T. L., 1980, Temperature gradients in a 

portion of Michigan: A review of the usefulness 

of data from the AAPG geothermal survey of 

North America: U.S. Department of Energy 

Report DOE/NV /10072-1, 39 p. 
Methods for Assessing Low-Temperature 

Geothermal Resources 

By Michael L. Sorey, Manuel Nathenson, and Christian Smith1 

CONTENTS 

Abstract -------------------------- Introduction ---------------------- Categories of low-temperature 

geothermal-resource areas -------

Determination of accessible resource 

base ---------------------------- Determination of resource --------- Determination of beneficial heat --

Undiscovered geothermal resources -

Acknowledgments -------------------

References cited ------------------

ABSTRACT 

Page 

17 

17 

18 

20 

22 

27 

28 

29 

29 

Low-temperature geothermal resources exist in 

systems dominated by hydrothermal convection and by 

heat conduction. Most identified low-temperature 

geothermal-resource areas occur in hydrothermalconvection systems that were delineated solely on the 

basis of a single thermal spring or well, and for 

resource-assessment purposes a standard reservoir 

volume was assigned to these areas. Other types of 

low-temperature geothermal-resource areas for which 

actual reservoir volumes could be determined occur in 

hydrothermal-convection systems and in conductiondominated systems within sedimentary basins and 

beneath coastal plains. In this assessment, mean 

values for the thermal energy stored in each identified 

low-temperature reservoir were obtained from 

estimates of triangular probability densities for the 

reservoir area, thickness, and temperature. Mean 

values of the thermal energy recoverable at the 

surface depend on estimates of the number of 

production wells each reservoir can support over a 

period of 30 years. An assumed development plan, 

with evenly spaced wells producing at 31.5 L/s at a maximum drawdown of 152 m, was used to generate 

curves that relate reservoir area and hydrologic 

properties to the optimum well spacing. The optimum 

well spacing is shown to increase with reservoir area 

but to be relatively insensitive to the length of the 

1university of Utah Research Institute, Earth 

Science Laboratory, Salt Lake City, Utah; currently 

with Chevron Resources Co., San Francisco, Calif. 

17 

development period and the fraction of time during a 

given period that fluid production actually occurs. 

Finally, estimates of the amount of recoverable energy 

that can be used in applications at the surface were 

obtained as a function of reservoir temperature. 

INTRODUCTION 

Assessment of geothermal resources involves 

determination of the location, size, and geologic 

characteristics of each resource area to calculate the 

accessible resource base (thermal energy stored in the 

reservoir) and the resource (thermal energy 

recoverable at the wellhead). Identified lowtemperature geothermal-resource areas must meet the 

criteria that a reservoir with sufficient permeability 

to supply long-term production exists and that 

reservoir temperatures exceed a defined temperaturedepth relation (see fig. 1). In this chapter, the types of 

hydrothermal-convection and conduction-domina ted 

systems within which low-temperature geothermalresources occur are discussed, and the methods used to 

estimate accessible resource base, resource, and 

beneficial heat (recovered thermal energy usable in 

applications at the surface) are described. A rationale 

is also presented for estimating undiscovered 

geothermal resources in various geologic 

environments. 

The statistical basis for resource estimates in 

this assessment is similar to that used by Brook and 

others (1979), with minor exceptions as noted. In 

contrast to the work of Brook and others (1979), 

however, in which recoverable thermal energy was 

determined by using a fixed recovery factor of 25 

percent of the stored thermal energy, the methodology 

used in this assessment involves estimation of the 

number of production wells a reservoir can support for 

a period of 30 years with a maximum drawdown of 152 m. Recovery factors based on this methodology are 

less than 25 percent except for small-volume 

reservoirs. 

Identified low-temperature geothermal resources occur mostly in areas where subsurface 

temperatures in permeable rock layers are above the 

normal or background temperatures at corresponding 

depths. At any given locality, one or more of the 

following factors may give rise to such a geothermal resource: (1) high regional heat flow, (2) young 
magmatic intrusions, (3) a thick sequence of lowthermal-conductivity rocks overlying an aquifer, (4) 

upward circulation of thermal fluid along faults, or (5) 

updip flow within areally extensive aquifers. In areas 

where these factors are unimportant, the temperature 

gradient is generally so low that drilling to resource 

temperatures is either uneconomical or impractical. 

A useful distinction can be made between a geothermal reservoir and a geothermal system. A 

"geothermal reservoir" is considered to be a 

geometrically defined volume of permeable rock from 

which thermal energy in water can be extracted. 

Reservoirs containing low-temperature (and hightemperature) geothermal resources commonly are 

surrounded by cooler rocks that are also permeable and 

hydraulically connected to the reservoir; thus, water 

may flow between the reservoir and surrounding rocks 

in the natural state. Such reservoirs exist as parts of 

larger "geothermal systems" involving circulation of 

meteoric water downward from recharge areas and 

upward toward discharge areas, commonly with lateral 

leakage of thermal water into permeable formations 

adjacent to the upflow conduits. In the broadest sense, a geothermal system could also be construed to include 

a heat source of either magmatic or nonmagmatic 

origin. Although the reservoir is the producible part of 

the geothermal system, the response of the reservoir 

to development may be significantly affected by the 

nature of its connection with the rest of the 

geothermal system. 

CATEGORIES OF LOW-TEMPERATURE 

GEOTHERMAL-RESOURCE AREAS 

Low-temperature geothermal resources occur in 

two types of geothermal systems-hydrothermal 

convection and conduction dominated. In 

hydrothermal-convection systems, upward circulation 

of water transports thermal energy to reservoirs at 

shallow depths or to the surface. These systems 

commonly occur in regions of active tectonism and 

above-normal heat flow, such as much of the Western 

United States. In conduction-dominated systems, 

upward circulation of fluid is less important than the 

existence of high vertical temperature gradients in 

rocks that include aquifers of significant lateral 

extent. These conditions occur beneath many deep 

sedimentary basins throughout the United States. 

For each type of geothermal system, two 

categories of low-temperature geothermal-resource areas are recognized (table 3). Each low-temperature 

Table 3.-Categories of low-temperature geothermalresource areas 

Category Setting Example 

Hydrotherma 1-convec t ion sys terns 

Isolated thermal springs and wells------- Pagosa Springs, Colorado 

Delineated thermal reservoirs------------ Klamath Falls, Oregon 

Conduction-dominated systems 

Sedimentary basins----------------------- Powder River Basin, Wyoming 

Coastal plains--------------------------- Delmarva Peninsula, Virginia 

18 

geothermal-resource area identified in this assessment 

is assigned to one of these four categories to convey 

additional information about resource characteristics. Figures 6 through 8 illustrate 

conceptual models of geothermal systems related to 

all these categories. Additional discussions of the 

various types of geothermal systems, including those in 

which low-temperature geothermal resources occur, 

were presented by Muffler and others (1979). 

Most of the identified low-temperature 

geothermal-resource areas associated with 

hydrothermal-convection systems fall into category 1 

(isolated thermal springs and wells). In such areas, the 

only evidence that a geothermal reservoir exists at 

depth is a single thermal spring or group of closely 

spaced springs, or a well that produces thermal 

water. In the Western United States, thermal springs 

commonly occur along normal faults, whereas in the 

Eastern United States, thermal springs occur in regions 

of folded and thrust-faulted rocks. Figure 6 shows 

three possible models of fluid circulation in such areas; 

other models were presented by Breckenridge and 

Hinckley (1978) and Hobba and others (1979). Although 

reservoir volumes and associated thermal energies may 

vary greatly from area to area, for localities where 

data on subsurface conditions are too few or absent, a 

standard reservoir volume of 1 km3 was assigned. 

Low-temperature geothermal-resource areas in 

category 2 (delineated thermal reservoirs in 

hydrothermal-convection systems) are generally 

characterized by the upflow of thermal water along 

faults and its subsequent lateral movement into 

aquifers at relatively shallow depths (fig. 7). There 

may or may not be an associated discharge of thermal 

springs at the surface, and the shallow thermal aquifer 

may be underlain by a hotter reservoir at greater 

depths. Temperature profiles in wells drilled in such 

areas generally show high gradients above the thermal 

aquifer and temperature reversals below; figure 9A 

illustrates such a temperature profile along with the 

25°C/km minimum-gradient criterion used in this 

assessment to identify low-temperature geothermalresource areas. For resource areas in category 2, 

reservoir volumes were estimated from available data 

on reservoir areas and thicknesses; such data were 

provided by test drilling, geophysical surveys, or 

simply by the distribution of thermal springs within the same geologic province. 

The lateral-leakage model (fig. 7 A) is applicable 

to many low-temperature geothermal-resource areas 

in the Basin and Range province and the Snake River 

Plain, for example, near Klamath Falls, Oregon, and 

Boise, Idaho. Test drilling near Marysville, Montana, 

has delineated an intermediate-temperature 

hydrothermal-convection system related to a bedrock 

high (see fig. 7B) within a stock in the Boulder 

batholith (Blackwell and Baag, 1973). Although 

detection of systems of this type is hampered by 

absence of surface manifestations, many such 

occurr~nces are likely within the Boulder and Idaho 

batholiths and in parts of central Alaska where 

thermal springs are associated with granitic plutons 

(Miller and others, 1975). This bedrock-high model is 

also applicable to areas within the Basin and Range 

province, such as Grass Valley, Nevada, where heat-
flow data and exploratory drilling indicate that lowtemperature geothermal reservoirs exist in fracturedbedrock highs just below the contact with the 

overlying less permeable valley fill (Welch and others, 

1981). 

Additional models of hydrothermal-convection 

systems in which low-temperature geothermal 

resources occur may be developed as data from future 

exploration become available. For example, the basinconstriction model (fig. 7 A) has been suggested for 

geothermal areas in the Rio Grande Rift in New 

Mexico (Morgan and others, 1981), although none of 

these areas has been adequately drilled and tested as yet. 

Low-temperature geothermal resources in 

conduction-dominated systems occur within 

sedimentary basins (category 3) and beneath coastal 

plains (category 4). Identified geothermal-resource 

areas in category 3 exist in the Central United States 

within the Great Plains and Wyoming Basin geologic 

provinces, where thick layers of low-thermalA 

conductivity shale and relatively high temperature 

gradients occur above regionally continuous carbonate 

and sandstone aquifers (fig. SA). An idealized 

temperature profile within a sedimentary basin (fig. 

98) illustrates that aquifers must occur at depths 

sUfficient for temperatures to exceed the minimumtemperature criterion. Thus, many basins east of the 

Great Plains are not identified as containing lowtemperature geothermal resources because either the 

thickness of the sediment is insufficient or its thermal 

conductivity is too high to produce aquifer 

temperatures above our minimum-temperature 

criterion. In contrast, within some parts of the Great 

Plains, such as the Denver Basin in western Nebraska, 

ground-water flowing updip in a regional aquifer 

results in high conductive temperature gradients and 

heat flow in the overlying sediment, so that aquifer 

temperatures exceed the minimum-temperature 

criterion at relatively shallow depths (Gosnold and 

Everson, 1981). 

Low-temperature geothermal-resource areas in 

c 

Figure 6.-Conceptual models for types of hydrothermal-convection systems in which low-temperature geothermal-resource areas in category 1 (isolated thermal springs and wells) occur. !!J Fault plane. _!!, Deep 

reservoir. .£, Margin of anticline. Arrows indicate direction of fluid circulation; shading shows location 

of reservoir containing low-temperature geothermal resources. 

19 
category 4 have been identified along the Atlantic and 

Gulf Coastal Plains. The conceptual model shown for 

this category (fig. 8_!!) involves a thick sedimentary 

layer underlain by an intrusive body that generates an 

elevated heat flow by radioactive decay. Although 

widespread occurrence of such intrusive bodies along 

the Atlantic coast has been proposed (Costain and 

others, 1980), delineation of such areas is limited by an 

absence of deep drill holes. Within the Gulf Coastal 

Plain, identified low-temperature geothermal-resource 

areas along the Balcones/Ouachita structural trend in 

central Texas are not associated with buried intrusive 

bodies but may involve a component of thermal water 

derived from updip migration from deeper zones. 

DETERMINATION OF ACCESSIBLE 

RESOURCE BASE 

The accessible resource base for each 

geothermal system inventoried in this report is given 

by 

A 

/ 

/ 

/ 

/ 

/ 

I 

/ 

/ 

/ 

c 

(1) 

where g8 is the accessible resource base, P£ is the 

volu?!etr1c specific heat of rock plus water (2.6 

J/cm ·°C), a is the reservoir area, d is the reservoir 

thickness, !.-is the reservoir temperature, and !ref is 

the reference temperatur~ (15°C). The volumetric 

specific heat of 2.6 J/cm •

0 c is a weighted average 

value calculated for the rock types and porosities 

found in low-temperature geothermal-resource areas. 

The reference temperature of 15°C is used for the 

entire United States. 

The statistical methods outlined by Brook and 

others (1979) were used to quantify the uncertainties 

in calculations of accessible resource base, resource, 

and beneficial heat. The use of triangular probability 

densities, involving estimates of the minimum, 

maximum, and most likely values for reservoir 

temperature, area, and thickness, enables calculation 

of the mean and standard deviation of the accessible 

B 

Figure 7 .-Conceptual models for types of hydrothermal-convection systems in which low-temperature geothermal-resource areas in category 2 (delineated thermal reservoirs) occur. A, Lateral leakage. _!!, Bedrock 

high. C, Basin constriction. Arrows indicate direction of fluid circulation; shading shows location of 

reservoir containing low-temperature geothermal resources. 

20 
A 

B 

Figure B.-Conceptual models for types of conduction-dominated systems in which low-temperature geothermal-resource areas in category 3 (sedimentary basins, A) and category 4 (coastal plains, B) occur. 

Arrows indicate direction of fluid circulation; shading shows location of reservoir containing lowtemperature geothermal resources. 

21 
resource base for individual areas and for all resource areas to be calculated. These estimates were also 

used to calculate probability distributions for the total 

accessible resource base, resource, and beneficial 

heat, using a Monte Carlo computer program similar to 

that described by Nathenson (1978). Such probability 

distributions establish confidence limits for each 

energy total. 

The mean identified accessible resource base 

for each low-temperature geothermal area is 

calculated by substituting the mean values into 

equation 1: 

( 2) 

where ·v=ad. The mean value of each variable, which is 

calculatedas the arithmetic average of the minimum, 

maximum, and most likely values, is not necessarily 

equal to the most likely value. Equations for 

determining the standard deviation of each variable 

and for the accessible resource base weregiven by 

Nathenson (1978). The identified accessible resource 

base for all areas equals the sum of the values of ~ 

for each area. The overall standard deviation equals 

the square root of the sum of the squares of the 

individual standard deviations. 

Methods of estimating the reservoir area, 

thickness, and temperature for the various categories 

en 

a: 

w \ 1-

w \ 

::;;! \ 0 

..J } S2 

z ~\ 0~ (\ 

:I: ~\ 1-

a.. 2 '?\ w 

c \ 

\ 

\ 

\ 

3 

0 15 30 45 60 75 90 

of low-temperature geothermal-resource areas are 

discussed by Mariner and others (this volume) and 

Sorey, Re1d, and others (this volume). The mean value 

of 1.0 km for the standard reservoir volume applied 

to resource areas in category 1 was calculated from 

minimum, maximum, ~nd most likely estimates of 

0.01, 2.0, and 1.0 km , respectively, which reflect 

limiting values for reservoir volumes in the models 

discussed previously for these categories. Although 

actual reservoir volumes in most low-temperature 

geothermal-resource areas where this standard volume 

is applied will probably differ from the mean value 

used here, it was assumed that the total identified 

accessible resource base for all such areas can be 

estimated by using the standard volume for each area. 

DETERMINATION OF RESOURCE 

The "resource" is that part of the accessible 

resource base that can be produced at the wellhead 

under reasonable assumptions of future economics and 

technology (Muffler and Cataldi, 1978). Thus, the 

methodology used to make resource estimates should 

be based on assumptions regarding development 

schemes that could reasonably be followed now or in 

the foreseeable future. No attempt is made in this 

assessment to estimate "reserves," which represent 

that part of the identified geothermal resource that 

can be extracted legally and economically at present 

(Muffler and Cataldi, 1978), because the required 

en 

a: 

w 

1-

w 

::;;! 

0 

..J 

S2 

z 

:I: 

5:: 2 w 

c 

15 30 45 60 75 90 

TEMPERATURE, IN DEGREES CELSIUS 

A B 

Figure 9.--ldealized temperature profiles in hydrothermal-convection systems with lateral leakage (A) and 

within sedimentary basins (B). Identified low-temperature geothermal resources exist where temperatures in aquifers exceed the minimum-temperature criterion (10°C above mean annual temperature plus 

25°C/km) used in this assessment, as shown by straight lines. 

22 
specifications of reservoir, production, and economic 

data are beyond the scope of this report. 

The simplest procedure for estimating the 

resource in each identified low-temperature 

geothermal-resource area is to multiply the accessible 

resource base by a fixed recovery factor !e· This 

approach was followed in previous assessments of 

intermediate- and high-temperature hydrothermalconvection systems, using !e=0.25, a value based on an 

energy-recovery process involving injection of cold 

water into the reservoir to replace the hot water 

withdrawn during production. Nathenson (1975) 

estimated that as much as 50 percent of the thermal 

energy in a uniformly permeable reservoir is 

recoverable in such a heat-sweep process but 

suggested using £,e=0.25 to account for permeability 

variations, including the parts of a reservoir that may 

be unproductive. Resource determinations based on 

this method do not depend on the time scale over which development occurs. 

The method used here to calculate recoverable 

energy involves estimation of the number of wells each 

reservoir can support over a development period of 30 

years, assuming that cold water will not be injected 

into the reservoir. Although injection of produced 

fluids after surface utilization may be legally required 

to protect the environment in certain areas, lower 

reservoir temperatures and larger reservoir areas 

make injection schemes for energy recovery less likely 

in low- than in intermediate- and high-temperature 

geothermal-resource areas. The method used in this 

resource assessment allows for induced recharge of 

water from permeable regions surrounding each 

thermal reservoir as reservoir pressure declines. Thus, 

the recovery factor approaches 0.25 over 30 years for 

small-area reservoirs. 

The resource is given by 

(3) 

where .9.wH is the resource, (p.£)f is 3

the volumetric 

specific heat of the fluid (4.1 J?cm ·°C), N is the 

number of production wells, _g_ is the average 

volumetric discharge of each production well, and P is 

the development period. Fluid temperatures at the 

wellhead are assumed to equal the corresponding 

reservoir temperatures; the reference temperature is 

15°c. 

To determine optimum values of N and .9_, 

several reservoir parameters must be known, and 

economic and engineering aspects of the process for 

which the resource is to be used must be considered. 

A detailed analysis of well-field design for each 

reservoir is beyond the scope of this assessment. 

Instead, a simplified production plan was considered 

for which the optimum value of the number of 

production wells can be determined for each reservoir 

by specifying a limited number of reservoir 

parameters. 

The production plan assumed here consists of 

regularly spaced wells on a square grid, discharging at 

31.5 L/s for 30 years, with a cumulative drawdown at 

the center of the production field of 152 m; these 

23 

conditions are representative of the well performances 

required for commercial development. The specified 

drawdown of 152 m applies to a decline in water level 

within a well or a decrease in wellhead pressure 

corresponding to a decline of 152 m in the piezometric 

surface for a flowing well. On the basis of this 

production plan, the number of wells that would 

produce a drawdown of 152 m at the center of the 

reservoir after 30 years is given by the ratio of the 

reservoir area!. to the area per well !.w· The area per 

well is the square of the distance between adjacent 

wells. 

For a given reservoir area and well spacing, the 

cumulative drawdown at the center of the area is the sum of the drawdowns due to each interfering well. 

For values of !.w less than the optimum, cumulative 

drawdown at the center of the reservoir exceeds 152 

m; for values of !.w greater than the optimum, 

cumulative drawdown at the center of the reservoir is 

less than 152 m. Determination of the optimum well 

spacing depends on the specified ratio of discharge to 

drawdown; discharge-drawdown combinations with the 

same ratio yield the same optimum well spacing. 

Drawdown calculations are based on the 

exponential integral solutions developed by Theis 

(1935) for artesian aquifers with nonleaky confining 

beds and by Hantush (1960) for artesian aquifers with 

leaky confining beds. Similar calculations were 

discussed by Papadopulous and others (1975) and 

Wallace and others (1978) for assessments of 

geopressured geothermal resources in the northern 

Gulf of Mexico Basin. In contrast with Papadopulous 

and others (1975), however, it was assumed in this 

assessment that the lateral boundaries of lowtemperature geothermal reservoirs are connected 

hydraulically to adjacent regions of permeable rock. 

Strictly speaking, the resource calculations in this 

assessment apply to reservoirs whose areas are square; 

application of the methodology to reservoirs of 

markedly different shape requires some adjustments, 

as noted below. 

Reservoir parameters that affect the 

calculation of optimum well spacing include the area, 

transmissivity, and compressibility. Reservoir 

transmissivity T is the product of the hydraulic 

conductivity K and the thickness; hydraulic 

conductivity, inturn, is a function of the permeability 

of the rock and the density and viscosity of the 

thermal fluid. The effects of reservoir compressibility 

and fluid compressibility can be included in the 

dimensionlgss storag~ coefficient~ which ranges from 

about 10- to 10- for most confined (artesian) 

aquifers (Lohman, 1972). To reduce the required 

number of ca~ulations for this analysis, a constant 

value for .§.=10 was used throughout because changes 

in this parameter were found to have only a secondorder effect on determinations of the optimum well 

spacing. 

Production from a reservoir can induce leakage 

of fluid into the reservoir from adjacent confining 

beds. The rate of induced leakage is related to the 

product of the hydraulic conductivity and specific 

storage ~) for each confining bed; the specific 

storage equals the storage coefficient divided by the 

thickness of the confining bed. Although values of K 
and §s range over several orders of magnitude for 

different rock types, the product KSs is more tightly 

constrained. In this assessment, confining beds 

adjacent to geothermal reservoirs consist primarily of 

shale, clay, or pyroclastic rocks. Data on ~ values 

for confining beds in most identified low-temperature 

geothermal-resource areas are absent except for those 

within sedimentary basins in the northern Great Plains, 

for which modeling studies of regional aquifer systems 

yield values for the predominantly shale confining beds 

(Konikow, 1976; Woodward-Clyde Consultants, 1980; 

Downey, 1982). Values of~ from these studies and 

values for nonindurated fine-grained deposits typical 

of confining beds in some identified low-temperature 

geothermal-resourcr5areas (Jopgson\ 1968) range from 

approximately 10- to 10- s- ; less indurated 

sedimentary rocks generally have higher ~ values. 

Two sets of curves that relate the optimum 

area per well to reservoir area and transmissivity are 

presented in figures 10 and 11. As discussed above, for a given reservoir area and transmissivity, the 

corresponding value of .!!.w indicates the spacing of 

wells producing at 31.5 Lts for which the cumulative 

drawdown at the center of the reservoir after 30 years 

would be 152 m. The curves in figure 10 are for the 

case of induced leakage from confining beds above and 

below the reservoir; the curves in figure 11 are for the 

case of impermeable confining beds. Comparison of 

these two sets of curves indicates that optimum well 

spacing is significantly smaller for reservoirs with 

leaky confining beds than for those with nonleaky 

confining beds. However, additional calculations 

carried out for other values of confining-bed 

properties indic~te that for reservoir areas of less than 

about 1,000 km , optimum well spacing is insensitive 

to variations in KSs within the range noted in the 

previous paragraph. Identified low-temperature 

geothermal-resou2ce areas with reservoirs larger than 

about 1,000 km occur only in sedimentary-basin 

environments for which the parameters indicated in 

figure 10 are applicable. Accordingly, the curves in 

figure 10 were used to estimate optimum well spacings 

for all reservoirs with leaky confining beds. 

Transmissivities for which well-spacing curves were determined range from 0.0005 to 0.02 m2 /s for 

reservoi~ with leaky confining beds and from 0.001 to 

0.01 m /s for reservoirs with nonleaky confining 

beds. Measured and estimated T values for reservoirs 

in resource areas identified in this assessment ~ll 

within this range. For T less than about 0.0005 m /s 

for reservoirs with leaky confining beds and 0.001 m2/s 

for reservoirs with nonleaky confining beds, the 

drawdown due to a single well approaches the 152-m 

limit after 30 years of production. Transmissivity 

values for each reservoir area were selected on the 

basis of available hydrologic and geologic data, as 

discussed by Mariner and others (this volume) and 

Sorey, Nathenson, and Smith (this volume). 

Resource estimates for each identified lowtemperature geothermal reservoir are based on use of 

the curves in figures 10 and 11 to determine the 

optimum area per well <.!.w> from specifications of 

reservoir area (a), transmissivity (T), and the presence 

or absence of leaky confining beds:- The corresponding 

estimate of the number of production wells (N) is given 

24 

by !f!..w· Methods used to quantify the uncertainty in 

resource determinations follow those used for 

determination of the accessible resource base in that 

triangular probability densities were calculated from 

minimum, maximum, and most likely estimates for !_, 

!_w, and !· An additional source of uncertainty in these resource estimates relates to the validity of the 

assumption that permeable connection exists 

throughout the reservoir. Although the areas over which aquifer temperatures meet the minimumtemperature criterion can be reasonably well 

delineated, not enough is known about the associated 

hydrologic conditions in most places to be certain that 

the entire low-temperature geothermal-reservoir area 

is sufficiently permeable to yield fluid at rates close 

to that assumed in the development plan. Therefore, a 

procedure was followed similar to that used with the 

recovery-factor approach of Brook and others (1979) of 

introducing a constant k to adjust for nonuniform 

transmissibility, including-unproducible regions within 

each reservoir. The corresponding probability density 

for k was based on minimum, maximum, and most 

likely values of 0, 1.0, and 0.5, respectively. The 

effects of this factor are to decrease estimates of the 

number of wells each reservoir can support and to 

increase the confidence limits on estimates of the 

resource and beneficial heat. 

The mean number of wells each reservoir can 

support is given by kal_!w, and the mean resource from 

equation 3 becomes 

Equation 4 was used in resource calculations for the 

identified low-temperature geothermal-resource areas 

in categories 2 through 4 for which actual reservoir 

areas could be estimated. A different method was 

used to estimate the resource for areas in category 1. 

For3 these areas, the standard reservoir volume of 1.0 

km was assumed, and the resource was calculated as 25 percent of the corresponding accessible resource 

base. 

For the production plan assumed here, the 

number of wells each reservoir can support does not 

increase in proportion to the reservoir area because 

the optimum area per well increases as the reservoir area increases owing to drawdown interference 

between wells. This increase results in considerably 

lower recovery factors for large- than for small-area 

reservoirs. As reservoir area decreases, however, 

induced recharge of water from surrounding regions 

becomes more important, and breakthrough of cold 

water in production wells rather than drawdown 

interference may limit recovery factors. To allow for 

this effect, the upper limit of the recovery factor 

.9.wHf.g_R is assumed to be 0.25. Thus, recovery factors are at or near 0.25 for the smallest area reservoirs in 

this assessment, which occur in hydrothermalconvection systems, and are near 0.001 for the largestarea reservoirs, which occur within sedimentary 

basins. 
C/) 

a: 

w 

..... 

w 

~ 

0 

_J 

i2 

w 

a: 

<( 

::J 

0 

C/) 

:z 

..1 

_J 

w 

~ 

a: 

w 

a.. 

<( 

w 

a: 

<( 

~ 

::J 

~ 

i= 

a.. 

0 

103 

102 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

:: 

10-1L_~ __ L6_LLQ~_L_L~_L~~U---~_L~LLLU~--~-L~LLLU~--~-L~~~ 

1 0° 1 01 1 02 1 03 

RESERVOIR AREA, IN SQUARE KILOMETERS 

Figure 10.-Reservoir area.!. versus optimum area per well !.:w for reservoirs with leaky confining beds, based on 

a production plan involving evenly spaced wells proaucing for 30 years at 31.5 L/s with a cumulative 

drawdown o~ 152 m. L reservoir transmissivity (in square meters per second); dashed portion of curve for 

T=0.0005 m /s involves fewer than five wells to produce ~e allowable drawdown. Drawdown comput.,.-

Tions were based on a reservoir storage coe[~iciynt .§.of 10 and a value for the product of hydraulic co"l.-

ductivity and specific storage KSs of 6x10- s- for each of two confining beds. 

25 
en 

a: 

w 

1-

w 

~ 

0 

...J 

:;;2 

w 

a: 

103 

~ 102 

0 

en 

z 

...J .. 

...J 

w 

~ 

a: 

w 

a.. 101 

< w 

a: 

< 

~ 

:::> 

~ 

i= 

a.. 

0 

10° / 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

101 

/ 

/ 

/ 

/ 

/ 

102 103 

RESERVOIR AREA, IN SQUARE KILOMETERS 

105 

Figure 11.--Reservoir area .! versus the optimum area per well.!w for reservoirs with nonleaky confining beds, 

based on a production plan involving evenly spaced wells producing for 30 years at 31.5 L/s with a cumulative drawdown of 152 m. L reservoir transmissivity (in square meters per second); dashed portion of curve for !,=0.001 m /s involves fewer than five wells to produc4 allowable drawdown. Drawdown computations were based on a reservoir storage coefficient S of 10- and a value for the product of hydraulic 

conductivity and specific storage ~ of 0 for each of two confining beds. 

26 
Several additional factors can be noted in 

regard to the resource determinations in this 

assessment. The first factor is that, for small-area 

reservoirs, the effects of lateral-boundary conditions 

may be important. These boundaries were assumed to 

connect the reservoir to additional regions of 

permeable rock. It may be that in some areas the 

reservoir boundaries are impermeable or behave as 

constant-pressure sources, as in the case of a fault 

conduit that connects a shallow with a deep reservoir 

(Benson and others, 1981). Although these conditions 

could be allowed for in specific areas by adjusting the 

value of a upward for impermeable boundaries and 

downward-for constant-pressure boundaries, we have 

not done so here because reservoir boundaries have not 

yet been adequately tested in any low-temperature 

geothermal-resource area. 

For reservoirs whose areal configuration is 

elongate rather than square, well-spacing 

determinations based on an assumption of evenly 

spaced wells in a square grid encompassing the same 

total area can lead to overly conservative estimates of 

the optimum well spacing. Allowance must be made in 

some areas for greater distances between wells and 

the center of the reservoir and, thus, for less 

interference. Such an allowance was made for some 

reservoirs within sedimentary basins by adjusting the 

values of !_w, estimated from the curves in figure 10 

downward by a factor of 2. 

The resource estimates obtained by the method 

used in this assessment depend on the assumed 

development period of 30 years. For a given reservoir, 

the number of wells that yield a specified maximum 

drawdown would not differ greatly for develpment 

times somewhat longer or shorter than 30 years 

because the rate of drawdown caused by each well 

decreases rapidly over time. Therefore, the method 

used here defines an optimum rate of energy recovery 

that is drawdown dependent but that could be 

sustained for periods longer or shorter than 30 years. 

Fluid production from low-temperature 

reservoirs for many direct-heat applications is carried 

out in a cyclic pattern corresponding to variations in 

the energy demand at the surface. This procedure 

introduces a load factor that represents the fraction of 

time during a given period when energy production and 

use occur; load factors are ordinarily integrated over significant periods of time (commonly 1 year). For the 

same installed energy-production capacity, the total 

energy produced at the wellhead over a period of 30 

years is less for small than for large load factors. The 

method used in this assessment for resource estimates 

assumes a load factor of 1.0. A limited number of 

·drawdown computations were carried out for load 

factors less than 1.0. Results of these computations 

and other theoretical considerations indicate that resource estimates equal to those in this assessment 

would be obtained for load factors less than 1 if the 

drawdown specification of 152 m were assumed to 

represent the average drawdown at the center of the 

reservoir between discharge and recovery cycles, 

because the drawdown at each well is proportional to 

the discharge rate. Thus, production schemes with 

different load factors that yield the same total fluid 

production over a given period will cause the same average reservoir drawdown. 

27 

DETERMINATION OF BENEFICIAL HEAT 

For geothermal resources, it is important to 

distinguish between thermal energy above some 

reference state and thermal energy comparable to that 

from another fuel. For resources above 150°C, the 

amount of wellhead thermal energy convertible to 

electricity can be calculated as a function of the resource temperature (for example, Nathenson, 1975; 

Brook and others, 1979), and the values can then be 

compared with the amount of electricity produced 

from fossil fuels. For low- and intermediatetemperature geothermal resources, the concept of 

beneficial heat was introduced by Nathenson and 

Muffler (1975); ''beneficial heat" is the energy applied 

by a user to a specific process. Brook and others 

(1979) calculated the beneficial heat as a fixed 

fraction of the wellhead thermal energy. Because of 

the importance of this quantity for assessments of 

low-temperature geothermal resources, the basis for 

this calculation is refined here. 

The mean beneficial heat .9ben is given by 

~en=(P£.)f(l<a/~)QPfl.!_, (5) 

where .9h n is the thermal energy (in MW t for 30 

years), k~f is the volumetric specific heat of water, Q 

is the mass produced, P is the duration of the 

development period, and fit is the usable temperature 

drop that occurs as energy is extracted in some 

process, such as home heating. For example, in the 

geothermal heating system at Lavey, Switzerland, the 

water comes out of the production well at 62°C, 

enters a heat exchanger at 58°C, and leaves it at 35°C 

(Rybach, 1979). Because the heat transferred from the 

geothermal fliud to the exchanger is the same as the 

heat transferred to the heating system on the other 

side of the exchanger, the usable temperature drop for 

calculating the beneficial heat is 58°-35°=23°C. 

To establish the dependence of the usable fit on 

the resource temperature, the data for five direct:Use 

applications are plotted in figure 12 as a function of 

reservoir temperature. The bar marked "8" is for the 

downhole heat exchangers used at Klamath Falls, 

Oregon, in closed-loop residential heating systems; the 

usable temperature drop is low relative to the other 

applications because flow rates are high enough at 

low fl t's to supply all the energy needed. The line 

marked "7" is for a relation proposed by Engen (1978) 

for the temperature change obtainable from a heat 

exchanger used for home heating under reasonable 

economic assumptions. The available data indicate 

that Engen's line underestimates beneficial-heat 

temperature drops; a better fit is given by a line with 

the equation 

(6) 

The upper end of this line is constrained by the data, 

whereas the intercept at flt=0°C at a resource temperature of 25°C is determined by the nationwide 
average mean annual temperature of 15°C plus the 

10°C required for a spring at the surface to be 

considered a resource. If equation 6 were used for a· 

specific location, the parameters would have to be 

adjusted for the local mean annual temperature; this 

degree of detail is beyond the scope of this 

assessment. 

Few data are available to characterize the !:it-t 

relation over the range 25°-60°C. Uses other than 

home heating are mentioned by Reed (this volume); 

however, no data are readily available to plot in figure 

12. Point 3, for a greenhouse project, does conform to 

CJ) 

=> 60 

U5 

...J 

w 

u 

CJ) 50 w 

w 

a: 

(.!) 

w 

0 40 

~ 

a: 

0 

a: 30 

0 

w 

a: 

=> 1- 20 

<( 

a: 

w 

~ 

~ 10 w 

1-

w 

...J 

~ QL-----~~--L-L-__ L_ ____ L_ ____ L_ __ __ 

~ 0 20 40 60 80 100 120 

RESERVOIR TEMPERATURE, IN DEGREES CELSIUS 

Figure 12.-Usable temperature drop t versus reser- voir or input temperature t for various directuse applications, showing -empirically derived 

line used in this assessment for effective temperature drop as a function of reservoir temperature. 1, Reykjavik, Iceland, municipal 

heating system (Palmason and Zoega, 1970); 2, 

proposed U.S. district heating using waste heat 

from central generating station (Karkheck and 

others, 1977); 3, Susanville, California, greenhouse (Boren, 1979); 4, Oregon Institute of 

Technology, Klamath Falls, Oregon, heating 

system (Purvine, 1974); 5, Mont de Marson, 

France, heating system (supplemental energy is 

added when outside temperature falls below 

6°C; Huxtable and others, 1980); 6, Lavey, 

Switzerland, heating system (Rybach, 1979); 7, 

estimated temperature change for economic 

heat exchanger to be used for home heating 

(Engen, 1978); 8, Klamath Falls, Oregon, downhole heat exchanger (Culver and Reistad, 1978). 

28 

the data available for home heating. At the lower 

temperatures, geothermal energy can be used in 

combination with a heat pump for home heating. By 

using the hotter source water, the electricity needed 

to drive the heat pump can be decreased (Reistad and 

Means, 1980a, b). Another method involving a heat 

pump is the use of geothermal energy for heating down 

to a certain outside temperature (and heating load) and 

use of the heat pump in combination with the 

geothermal energy below this temperature (Jaud, 

1980). Both of these schemes enable the use of lower 

temperature water; however, it is difficult to assign a 

usable temperature drop to the geothermal water to 

provide data for the lower temperatures in figure 12. 

The units for reporting beneficial heat are 

megawatts thermal (MWt) for 30 years, and the values 

obtained represent energy that might actually be used 

in applications at the surface. For comparison with 

other forms of energy, the overall efficiency of those 

other forms in direct-use applications should be 

considered. The overall efficiency for a fossil fuel is 

the energy inputted to the process divided by the 

heating value of the fuel. For natural gas, about 50 

percent of the energy in the gas is actually available 

for space heating (Beller, 1975); for electric-resistance 

heating, the efficiency is nearly 100 percent in the 

heater, but the overall efficiency is lower because the 

central-station efficiency is about 33 percent for a 

modern fossil-fueled plant (Beller, 1975). Thus, 100 

MW1 of beneficial heat from a geothermal system is 

equivalent to 100 megawatts electric (MW e> if 

electricity were used for heating. 

In assessing the benefits available from lowtemperature geothermal resources, the potential 

benefits from cascading high-temperature waters were 

not included. Karkhek and others (1977) proposed 

adjusting the condensation temperatures of central 

generating stations to 100°C, so that energy could be 

made available for district heating; similar schemes 

could be developed for multiple use of a geothermal resource. Quantifying the benefits of such schemes is 

possible only when some have actually been built, and 

no attempt is made to calculate the benefits here. 

UNDISCOVERED GEOTHERMAL RESOURCES 

The "undiscovered accessible resource base" 

represents the accessible thermal energy stored in 

reservoirs that are inferred to exist but as yet 

undiscovered. It includes: (1) Thermal energy in 

aquifers within sedimentary basins and beneath coastal 

plains, where the existing data are insufficient to 

allow any quantitative assessment; (2) additional 

thermal energy due to upward revisions of reservoir 

volume and temperature estimates for identified lowtemperature geothermal-resource areas; and (3) 

thermal energy in systems whose locations are as yet 

unknown. The undiscovered accessible resource base 

for various geologic and physiographic provinces is 

estimated below, along with the undiscovered resource 

and beneficial heat. 

For many of the sedimentary basins within 

which low-temperature geothermal resources were identified in a particular regional aquifer, 

corresponding undiscovered resources were assumed to 
exist in another aquifer or group of aquifers within the 

same basin. For example, in the Denver Basin in 

northeastern Colorado, low-temperature geothermal 

resources were identified in sandstone of the 

Cretaceous Dakota Group because sufficient data on 

temperature gradient, stratigraphy, and transmissivity 

exist to make a quantitative assessment. 

Undiscovered resources in this basin were inferred to 

exist in deeper Paleozoic aquifers for which fewer 

temperature and hydrologic data are available. In such 

areas, estimates of the undiscovered accessible 

resource base, resource, and beneficial heat were 

made by multiplying the corresponding estimates for 

the associated identified low-temperature geothermal 

resources by an assumed ratio of undiscovered to 

identified reservoir areas. 

Along the Gulf and Atlantic Coastal Plains, 

undiscovered resources are inferred to exist on the 

basis of limited evidence of favorable conditions, such 

as high measured temperature gradients, thick 

sequences of low-conductivity sediment, or 

geophysical evidence for buried intrusive bodies that 

may have radiogenic heating. Particularly in the Gulf 

Coastal Plain in parts of Texas, Louisiana, and 

Mississippi, available temperature-gradient 

information suggests that large areas containing lowtemperature geothermal resources in sandstone 

aquifers may exist, but additional data are required to 

confirm and delineate individual reservoirs. 

Undiscovered resources in regions characterized 

by the occurrence of hydrothermal-convection systems are estimated as multiples of the corresponding 

identified resources. Where identified lowtemperature geothermal-resource areas in category 1 

(standard reservoir volume assumed) occur, 

undiscovered resources could exist in similar systems 

whose locations are unknown and in known systems 

whose temperature or volume is larger than assumed. 

Upward revision of reservoir temperature is possible 

where the measured spring temperature was used 

instead of geothermometric calculations. Upward 

revision of reservoir volume is possible if both a deep 

circulation system and a zone of shallow lateral 

leakage or circulation within bedrock highs exist. In 

regions containing identified low-temperature 

geothermal-resource areas in category 2, similar 

undiscovered resources are inferred to occur in areas 

with similar geologic conditions. 

No estimates are included here of lowtemperature geothermal resources available in the 

form of waste water from powerplants utilizing water 

from higher temperature geothermal systems. This 

omission avoids overlap or double counting with 

respect to the resource estimates in previous 

assessments. Although the magnitude of lowtemperature geothermal energy potentially available 

from such sources is not likely to be quantitatively 

significant, the costs of utilizing these resources are 

likely to be relatively low where powerplants already 

exist. 

ACKNOWLEDGMENTS 

In developing the methodology used in this 

assessment, we have benefited greatly from 

29 

discussions with the authors of the other chapters in 

this volume and with members of the advisory group 

acknowledged by Reed (''Introduction," this volume). 

We are also indebted to Jeff Hullen of the Earth 

Science Laboratory, University of Utah Research 

Institute, for the development of figures 6 through 8. 

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Report BNL 50483, 193 p. 

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Muffler, L. J. P., and Cataldi, R., 1978, Methods for 

regional assessment of geothermal resources: Geothermics, v. 7, no. 2-4, p. 53-89. 

Muffler, L. J. P., Costain, J. K., Foley, Duncan, 

Sammel, E. A., and Youngquist, Walter, 1979, 

Nature and distribution of geothermal energy, 

in Anderson, D. N. and Lund, J. W., eds., Direct 

utilization of geothermal energy: A technical 

handbook: Geothermal Resources Council 

Special Report 7, p. 1-1 to 1-15. 

Nathenson, Manuel, 1975, Physical factors determining 

the fraction of stored energy recovered in 

hydrothermal convection systems and 

conduction-dominated areas: U.S. Geological 

Survey Open-File Report 75-142, 47 p. 

--1978, Methodology of determining the uncertainty 

in the accessible resource base of identified 

hydrothermal convection systems: U.S. 

Geological Survey Open-File Report 78-1003, 50 

p. 

30 

Nathenson, Manuel, and Muffler, L. J. P., 1975, 

Geothermal resources in hydrothermal 

convection systems and conduction-dominated 

areas, in White, D. E., and Williams, D. L., eds., 

Assessment of geothermal resources of the 

United States-1975: U.S. Geological Survey 

Circular 726, p. 104-121. 

Palmason, G., and Zoega, J., 1970, Geothermal energy 

deveopments in Iceland 1960-1969, in United 

Nations Symposium on the Development and 

Utilization of Geothermal Resources, Pisa, 

1970: Geothermics, Special Issue 2, v. 2, pt. 1, 

p. 73-76. 

Papadopulos, S. S., Wallace, R. H., Wesselman, J. R., 

and Taylor, R. E., 1975, Assessment of onshore 

geopressured-geothermal resources in the 

northern Gulf of Mexico basin, in White, D. E., 

and Williams, D. L., eds., Assessment of 

geothermal resources of the United States1975: U.S. Geological Survey Circular 726, p. 

125-146. 

Purvine, W. D., 1974, Utilization of thermal energy at 

Oregon Institute of Technology Klamath Falls, 

Oregon, in Lienau, P. J ., and Lund, J. W ., eds., 

Multipurpose use of geothermal energy: 

Klamath Falls, Oregon Institute of Technology, 

Geo-Heat Utilization Center, p. 179-191. 

Reistad, G. M., and Means, Paul, 1980a, Heat pumps 

for geothermal applications: Availability and 

performance: U.S. Department of Energy 

Report DOE/ID/12020-T1, 73 p. 

---1980b, Unitary water source heat pumps for 

geothermal applications: Availability, 

performance and design: Geothermal Resources 

Council Transactions, v. 4, p. 601-604. 

Rybach, Ladislaus, 1979, Geothermal resources: An 

introduction with emphasis on low-temperature 

reservoirs, in A symposium on geothermal 

energy and its direct uses in the eastern United 

States: Geothermal Resources Council Special 

Report 5, p. 1-7. 

Theis, C. V., 1935, The relation between the lowering 

of the piezometric surface and the rate and 

duration of discharge of a well using groundwater storage: American Geophysical Union 

Transactions, pt. 2, p. 519-524. 

Wallace, R. E., Jr., Kraemer, T. F., Taylor, R. E., and 

Wesselman, J. R., 1979, Assessment of 

geopressured-geothermal resources in the 

northern Gulf of Mexico basin, in Muffler, L. J. 

P ., ed., Assessment of geothermal resources of 

the United States-1978: U.S. Geological 

Survey Circular 790, p. 132-155. 

Welch, A. H., Sorey, M. L., and Olmsted, F. H., 1981, 

The hydrothermal system in southern Grass 

Valley, Pershing County, Nevada: U.S. 

Geological Survey Open-File Report 81-915, 193 

p. 

Woodward-Clyde Consultants, 1980, Well-field 

hydrology: San Francisco, 414 p. Technical 

report for the environmental impact statement on the Energy Transportation System, Inc., 

proposed coal slurry pipeline. [Available from 

U.S. Bureau of Land Management, Special 

Projects Staff, 555 Zang St., Denver, CO 

80228.] 
Low-Temperature Geothermal Resources 

in the Western United States 

By Robert H. Mariner, Charles A. Brook, Marshall J. Reed, 

James D. Bliss, Amy L. Rapport, and Randolph J. Lieb 

CONTENTS 

Abstract --------------------------

Introduction ---------------------- Distribution and geologic setting 

of low-temperature geothermal 

systems ------------------------- Central Alaska ---------------- Southeastern Alaska ----------- Aleutian Islands and Peninsula 

Hawaii ------------------------ Olympic Mountains -------------

Cascade Range -----------------

Coast Ranges ------------------

Central Valley ----------------

Sierra Nevada ----------------- Transverse Ranges -------------

Peninsular Ranges -------------

Salton Trough -----------------

Basin and Range ---------------

Oregon Plateaus ---------------

Columbia Plateaus ------------- Western Snake River Plain -----

Eastern Snake River Plain -----

Northern Rocky Mountains ------

Middle Rocky Mountains --------

Southern Rocky Mountains ------

Colorado Plateaus -------------

Rio Grande Rift --------------- Determination of accessible resource 

base, resource, and beneficial 

heat ---------------------------- Estimation of temperatures --------

Estimation of area and thickness --

Use of tables --~-----------------­

Undiscovered accessible resource 

base ---------------------------- Summary --------------------------- Acknowledgments -------------------

References cited ------------------

Page 

31 

31 

34 

34 

34 

34 

34 

34 

35 

35 

35 

35 

35 

35 

35 

35 

36 

36 

36 

36 

36 

36 

36 

36 

36 

37 

37 

38 

38 

ABSTRACT 

Most of the 1,084 low-temperature (less than 

90°C) geothermal systems identified in the Western 

United States are characterized by hydrothermal 

convection; conduction-dominated systems are 

identified only in the Columbia Plateaus (8 systems) 

and the Salton Trough (1 system). The identified 

accessible resource base for all low-temperature 

geothermal sr~tems in the Western United States is 

about 310x10 J. The resource associated with tGese 

identified thermal reservoirs is about 31x101 J, 

corresponding to a beneficial heat of 13.7 GWt for 30 

years. Hydrothermal-convection systems account for 

96 percent of this resource; conduction-dominated 

systems contain approximately a third of the identified 

accessible resource base, and about 1 percent of this 

energy can be extracted as a resource under the 

proposed development plan. The undiscol~red 

accessible resource base is estimated at 480x10 J; 

thus, the total accessible resource base available from 

identified and undiscovered low-temperature 

geotherwal systems in the Western United States is 

790x101 J. 

INTRODUCTION 

The assessment of low-temperature geothermal 

systems in the Western United States (fig. 13) is 

presented in this chapter in terms of the accessible 

resource base, resource, and beneficial heat. To be 

included in this assessment, springs or free-flowing 

wells must discharge water at least 10°C warmer than 

the mean annual air temperature for a given locality 

(see Reed, "Introduction," this volume), and nonflowing 

wells must have a water temperature at depth that 

exceeds the sum of 10°C above mean annual air 

temperature plus the product of the depth and the 

·"adient 25°C/km. The GEOTHERM computer file 

feshin and others, 1979) maintained by the U.S. 

eological Survey in Menlo Park, California, formed 
A 

---l-J- 1 \ 

\ .. t., •• 

\, 

0 250 

100" 

··-··-··-··-··-··-··-··-· _ .. ., .. -··-··-··--··-~-·· 

500 KILOMETERS 

I 

i 

i 

I 

, 

i 

i 

I 

i 

i 

L---------- i 

i 

i 

i 

I 

I 

I 

I 

I 

I 

~------

1 

I 

I 

I 

I 

I 

I __ L-·---·-1 

I 

i 

i 

I 

I 

I 

i 

I 

I 

I 

I 

i 

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I 

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Figure 13.-Low-temperature geothermal syste"ls in the Western United States. Major geologic provinces are 

identified in figure 2. Dots, isolated systems; triangles, systems with delineated areas. ~ Conterminous United States • ..!!_, Alaska and Hawaii. 

32 
"c2. 0 

,·~ 'J 

,.--. ~..0'&£ .... · 

HAWAII 

.~-

:B 

L 

Figure 13.-continued 

33 
the data base for this assessment. A total of 2,000 

records were identified for low-temperature 

geothermal occurrences in the Western United States; 

about 46 percent (927) of these records were 

ultimately considered t~ represent isolated systems 

(reservoir volume, 1 km ), whereas the remaining 54 

percent are distributed among 157 systems3of large 

area (reservoir volume, more than 1 km ). The 

distribution of low-temperature geothermal systems 

was determined by plotting the locations of thermal 

springs and wells on maps at a scale of 1:250,000. 

Point sources or clusteW of springs or wells distributed 

over an area of 4 km or less are considered to be 

associated with an isolated system. All isolat~d 

systems are assigned reservoir volumes of 1 km ; 

groups of wells ~r springs distributed over areas of more than 4 km are assumed to represe~t systems 

having reservoir volumes of more than 1 km . A total 

of 1,084 low-temperature reservoirs were thus 

identified. Hydrothermal-convection systems 

predominate in the Western United States; fewer 

conduction-dominated systems have been identified 

here than in the Central and Eastern United States 

(Sorey, Reed, and others, this volume). 

Extrapolation of a curve of cumulative 

frequency versus reservoir temperature for 

hydrothermal-convection systems with reservoir 

temperatures above 90°C (Brook and others, 1979, fig. 

11) indicates that 902 hydrothermal-convection 

systems should be present in the temperature range 

20°-90°C. The number of observed hydrothermalconvection systems (1,075) differs from the number of 

predicted systems (902) for several reasons. Data on 

approximately 20 new systems with reservoir 

temperatures slightly above 90°C, identified during 

this assessment, were not included in this curve. The 

addition of these systems to the lower temperature 

end of the curve would increase its slope and thus 

increase the number of systems expected in the lowtemperature range. At least 25 of the intermediateand high-temperature reservoirs assessed by Brook and 

others (1979) have low-temperature aureoles that are 

evaluated in this assessment. If these systems are 

subtracted from the observed low-temperature system 

total, the number of identified low-temperature 

hydrothermal-convection systems is reduced to 1,041-

still 139 more than predicted from an extrapolation of 

the curve. This difference is due in part to the 

shortage of identified thermal reservoirs in the 

temperature range 90°-100°C (Brook and others, 1979) 

and may also indicate that some of the reservoirs 

listed as isolated in this assessment are parts of larger 

reservoirs. 

DISTRIBUTION AND GEOLOGIC SETTING OF 

LOW-TEMPERATURE GEOTHERMAL SYSTEMS 

Geothermal systems are widely distributed 

throughout the Western United States and occur in 

diverse geologic settings. Much of the region is 

characterized by active tectonism and volcanism and 

generally has higher than normal heat flow; these 

conditions are favorable for the occurrence of 

geothermal systems. For simplicity of discussion, the 

western region is divided into geologic provinces, as shown in figures 2 and 12. 

34 

Central Alaska 

Most of the thermal springs in central Alaska 

are situated in an east-west-trending zone between 

latitudes 64° and 68° N. They are thought to result 

from deep circulation along faults in or associated 

with Mesozoic and Tertiary granitic plutons (Miller and 

others, 1975). A total of 15 intermediate- and hightemperature systems were identified in the province 

by Brook and others (1979); 25 isolated lowtemperature hydrothermal-convection systems are 

identified in this assessment. 

Southeastern Alaska 

Thermal springs in southeastern Alaska are 

associated with faults and thus are believed to result 

from deep circulation. One high- and six 

intermediate-temperature geothermal systems were 

identified in the province by Brook and others (1979); 

five isolated low-temperature geothermal systems are 

identified in this assessment. 

Aleutian Islands and Peninsula 

Although numerous hydrothermal-convection 

systems would be expected in association with the 

active Alaskan volcanoes, only six high-temperature 

systems were identified by Brook and others (1979). 

More recently, Motyka and others (1981) sampled 

springs associated with 18 additional hydrothermalconvection systems and reported that 15 of these 

systems have reservoir temperatures of more than 

90°C and that at least seven additional thermal springs 

may exist in the province. We have identified only 

three isolated low-temperature geothermal reservoirs 

in the province, but many systems may be masked by 

near-surface cold water. 

Hawaii 

Geothermal resources in the Hawaiian province 

have been identified only at the crater and along the 

East Rift Zone of Kilauea Volcano on the Island of 

Hawaii (Brook and others, 1979). The one lowtemperature geothermal resource identified in the 

Kapoho area of the East Rift Zone is apparently 

associated with the underlying high-temperature 

hydrothermal-convection system. Undiscovered lowtemperature geothermal resources in the province may 

occur in other rift zones associated with the shield 

volcanoes on Hawaii and Maui. The repeated 

emplacement of basaltic dikes in the rift zones may 

provide local near-surface heat sources. Several 

potential low-temperature geothermal sites have been 

studied (Thomas and others, 1982). 

Olympic Mountains 

The Olympic Mountains of northwestern 

Washington consist of late Mesozoic to Tertiary 

sedimentary and volcanic rocks that have been 

complexly deformed and weakly metamorphosed 

(Tabor and Cady, 1978). A complex assemblage of 

mostly gneissic amphibolite and quartz diorite forms 

the basement. Heat flow is low, and only two isolated 
hydrothermal-convection systems are identified in the 

province. Geothermal systems in the province are probably confined to faults and fractures. 

Cascade Range 

The Cascade Range is an active volcanic chain 

that overlies a subduction zone along the Pacific 

Northwest. The mountains consist of a thick pile of 

dominantly andesitic flows, overlain locally by large 

stratovolcanoes (Hammond, 1979). In the northern 

Cascades, Mesozoic and older crystalline basement is 

exposed at the surface over extensive areas and is 

overlain only locally by large Quaternary 

stratovolcanoes. In the central Cascades, the volcanic 

rocks are under lain locally by Eocene marine 

sedimentary rocks and Mesozoic sedimentary and 

volcanic rocks that in some places include 

serpentinite. In the southern Cascades, the preCenozoic crystalline basement is overlain by Miocene 

to Quaternary basalt flows and andesitic to dacitic 

stratovolcanoes. Small granodiorite plutons have 

intruded and created metamorphic aureoles in early 

Tertiary volcanic rocks in the northern and central 

Cascades. 

A total of 36 isolated low-temperature 

hydrothermal-convection systems are identified in the 

province; 13 intermediate- and high-temperature 

systems were identified by Brook and others (1979). 

Coast Ranges 

The Coast Ranges of central and northern 

California consist of Mesozoic and Cenozoic 

sedimentary rocks that have been folded and faulted 

into northwest-trending mountains (Page, 1966). The 

Clear Lake volcanic field, adjacent to the producing 

steam field at The Geysers, has been the center of 

silicic volcanic activity during much of the Quaternary 

(Donnelly-Nolan and others, 1981). The province 

contains 46 identified isolated geothermal systems and 

tw~ systems with reservoir volumes of more than 1 

km. 

Central Valley 

The Central Valley of California is a deep 

asymmetric basin filled with Cenozoic sediment. Only 

two isolated low-temperature geothermal systems 

have been identified in the province. The province 

appears to have little geothermal potential, although 

geopressured-geothermal systems may occur in the 

deeper parts of the basin. 

Sierra Nevada 

The Sierra Nevada is a westward-tilted fault 

block consisting mostly of Mesozoic granitic rocks. Of 

the 20 identified geothermal systems in the province, 

19 are isolated and appear to be limited to fractures. 

Transverse Ranges 

The Transverse Ranges consist of various rock 

35 

types that have been thrust faulted and folded into 

east-west-trending ranges of mountains. Thermal 

springs and wells in the eastern part of the province 

occur in a granitic and metamorphic basement 

complex and are apparently restricted to faults and 

fractures. In contrast, thermal waters in the western 

part of the province occur predominately in clastic 

sedimentary rocks and, for the most part, do not occur 

along faults. There are 23 isolated and three larger 

area low-temperature geothermal systems identified in 

the province. 

Peninsular Ranges 

The Peninsular Ranges of southern California are dominated by granitic and metamorphic terranes. 

A total of 29 isolated low-temperature geothermal 

reservoirs are probably confined to faults and 

fractures in the crystalline rock, and \of the 4 larger 

systems are small in volume (2.5 km or less). The 

large-volume (38 km3) reservoir in the San Jacinto 

Valley is poorly defined. 

Salton Trough 

The Salton Trough of southern California 

includes the Imperial and Coachella Valleys. This 

province, which marks the transition from the 

divergent plate boundary of the East Pacific Rise to 

the transform plate boundary of the San Andreas fault 

system (Puis and others, 1982), is characterized by 

active tectonism, recent volcanism, and high heat 

flow. A total of 10 intermediate- and hightemperature hydrothermal-convection systems have 

been identified in the province (Brook and others, 

1979). A conduction-dominated low-temperature 

geothermal system, with an estimated reservoir area 

of 800 km2, occupies the east third of the Imperial 

Valley. In addition, a total of 18 hydrothermalconvection systems are identified, 3

of which 3 have 

reservoir volumes of more than 1 km • 

Basin and Range 

Much of the Western United States lies within 

the Basin and Range geologic province, which has a 

heat flow generally higher than normal (Sass and 

others, 1981) and is characterized by extensional 

tectonism (Atwater, 1970; Scholz and others, 1971). 

The combination of range-front faults and sedimentfilled basins is favorable for the occurrence of 

geothermal systems (Sorey, Nathenson, and Smith, this 

volume). Young silicic volcanic centers along the east 

and west margins of the province provide localized 

heat sources for hydrothermal-convection systems. 

Most thermal waters in the province result from 

deep circulation. Normal faults provide near-surface 

conduits for the circulating waters and thus control 

the positions of most of the identified hydrothermalconvection systems (figs. 5, 6). Basin-fill sediment 

may act as a thermal blanket that traps heat in 

relatively shallow aquifers beneath large areas of some of the basins. Leakage away from fault conduits is 

probably the source of the thermal waters in these 

aquifers. 
Numerous geothermal systems, widely 

distributed throughout the northern part of the Basin 

and Range province, reflect the active seismicity of 

the region. Hydrothermal-convection systems are generally localized along faults, and lateral leakage 

appears to be limited owing to the relatively narrow 

basins and thin basin fill. In contrast, lateral leakage 

is more common in the southern part of the province, 

where the basins are much larger and generally 

deeper. A total of 471 low-temperature hydrothermalconvection systems are identified in the Basin and 

Range, of which 376 are isolated systems. 

Oregon Plateaus 

The Oregon Pia tea us province is structurally 

transitional between the Basin and Range on the south 

and the Columbia Plateaus on the north. Rocks 

exposed in the province range from late Paleozoic and 

Mesozoic marine strata and Mesozoic intrusive rocks 

to late Tertiary volcanic and volcaniclastic rocks 

(Baldwin, 1981). The 40 low-temperature geothermal 

systems identified in the province appear to be 

controlled by normal faults and probably reflect dee~ 

circulation in a region of near-normal heat flow. Only 

five systems in this province were i1entified as having 

reservoir volumes greater than 1 km • 

Columbia Plateaus 

The Columbia Plateaus province of Washington 

and Oregon is constructed of flood basalt of Miocene 

to Pliocene age. The basalt flows have a maximum 

thickness of about 2 km and are mantled by 

Pleistocene sediment, as much as 350 m thick 

(Swanson and others, 1979). Although the regional 

heat flow is normal for the Western United States and 

shallow ground waters are cold, deep wells in eastern 

Washington have penetrated warm water in aquifers at 

two depth intervals. The shallower aquifer, which occurs at depths of 300 to 700 m, appears to be 

restricted in area; the subsurface extent of the deeper 

aquifer, which occurs between depths of 800 and 1,500 

m, is unknown. Thermal waters in the Washington part 

of the province appear to be conductively heated. 

Besides the 8 conduction-dominated systems identified 

in Washington, 15 hydrothermal-convection systems are identified along the south edge of the province in 

Oregon. Preliminary, unpublished data collected for 

the U.S. Department of Energy indicate that the 

Hanford area identified in this assessment may be 

significantly larger than was estimated here. 

Western Snake River Plain 

The Snake River Plain of southern Idaho is 

divided into a western and an eastern province on the 

basis of distinct geologic settings. The western 

province is a northwest-trending grabenlike structure, 

partly filled with late Tertiary silicic volcanic rocks 

and clastic sedimentary rocks (Malde and Powers, 

1962). The province has higher than normal heat flow, 

and 32 low-temperature geothermal systems are 

identified. Most of the thermal waters apparently rise 

along normal faults and spread laterally into the basin 

36 

fill; 13 geotherral systems with reservoir volumes of 

more than 1 km are recognized. 

Eastern Snake River Plain 

The Eastern Snake River Plain province is a broad northeast-trending downwarp, partly filled with 

young basalt flows. Heat-flow values measured in 

shallow (less than 200 m) wells in the ~stern Snake 

River Plain are low (less than 20 m W /m ) because of 

cold-water movement in the extensive Snake Plain 

aquifer (Brott and others, 1976). The abundance of 

young basalt, however, suggests that thermal 

anomalies may exist at least locally. A total of 20 

low-temperature geothermal systems are identified in 

the province; 16 of these are isolated systems. 

Northern Rocky Mountains 

The Idaho batholith in central Idaho and the 

Boulder batholith in southwestern Montana together 

make up most of the Northern Rocky Mountains 

province. The province contains 135 isolated lowtemperature geothermal systems that are probably 

controlled by fault and joint patterns in the crystalline 

rocks and six larger area systems. 

Middle Rocky Mountains 

The Middle Rocky Mountains are characterized 

by diverse and complex geology; thrusting, normal 

faulting, and folding are all recognized. A total of 25 

widely scattered low-temperature geothermal systems are identified in the province. These systems are 

apparently structurally controlled, and all but one 

system are probably of limited extent. Heat flow is 

normal, and water temperatures are determined by 

circulation depths. 

Southern Rocky Mountains 

The Southern Rocky Mountains of Colorado and 

New Mexico consist of a wide variety of rocks, ranging 

from Precambrian crystalline basement to Cenozoic 

volcanic rocks. The province has a normal heat flow 

and is not seismically active. Of the 34 identified 

geothermal systems, 33 are isolated, and the province 

does not appear to have much geothermal potential. 

Colorado Plateaus 

The Colorado Plateaus province is an area of 

relatively undeformed Paleozoic and Mesozoic 

sedimentary rocks. The province is seismically 

inactive and has low heat flow. Low-temperature 

geothermal systems are identified only in that part of 

the province in Utah; a total of 30 such systems, of 

which 29 are isolated, are recognized. Although young 

volcanic features in northern Arizona and New Mexico 

are possible geothermal targets, the province has little 

identified geothermal potential. 

Rio Grande Rift 

The Rio Grande Rift, as used here, extends 
from western Texas to the upper Arkansas Valley of 

Colorado. The province, particularly in New Mexico, 

has high heat now and contains several thermal 

anomalies. It is divided into several interconnected 

partly filled structural basins. Harder and others 

(1980) and Morgan and others (1981) proposed that 

these thermal anomalies are the result of forced 

convection driven by ground-water now through the 

interconnected basins. Thermal springs and surface 

heat-now anomalies are thought to occur where the 

ground-water now is constricted at the discharge 

areas of the basins. Temperatures within individual 

geothermal systems are determined by the depths of 

water circulation and the geothermal gradient. A 

total of 44 isolated and four larger area lowtemperature geothermal reservoirs are identified in 

the province. 

DETERMINATION OF ACCESSIBLE RESOURCE 

BASE, RESOURCE, AND BENEFICIAL HEAT 

The methodology used here in determining the 

identified accessible resource base for lowtemperature geothermal systems in the Western 

United States is essentially the same as that used by 

Brook and others (1979, equations 1, 1a) in evaluating 

intermediate- and high-temperature hydrothermalconvection systems. This methodology requires that 

minimum, maximum, and most likely values be 

determined for reservoir temperature, area, and 

thickness. These three estimated values for each 

reservoir variable are assumed to fit a trisngular 

probability density and are used to calculate a mean 

and standard deviation. These mean values are used, 

in turn, to calculate the identified accessible resource base, resource, and beneficial heat; uncertainties 

associated with the magnitudes of these quantities are expressed in terms of standard deviations. 

The resource (wellhead thermal energy) for 

each identified thermal reservoir is calculated either 

(1) by assuming a 25-percent recovery fact~r for those 

reservoirs with a standard volume of 1 km , or (2) by 

estimating the number of production wells that a reservoir with a larger than standard volume can 

support for 30 years with a cumulative drawdown of 

152 m. The number of production wells is determined 

by the reservoir area and the optimum well spacing, 

which, in turn, is a function of reservoir transmissivity 

and the properties of the confining beds. For most 

reservoirs, the existence of leaky confining beds can 

reasonably be inferred, although measured transmissi~ity values are unavailable. Transmissivity 

values are estimated principally on the basis of the 

discharge rates of springs and wells. Systems with 

springs or wells that individually discharge less tran 

3.3 L/s are assigned a transmissivity of 0.0025 m /s, 

those with discharges of a few hundred to a few 

thfusand liters per minute a transmissivity of 0.005 m /s, and those wi1h greater discharges a transmissivity of 0.01 m /s; a few systems that have 

wells with very large 1ischarges are assigned 

transmissivities of 0.02 m /s. If qow rates are unavailable, a transmissivity of 0.005 m Is is assumed. 

Beneficial heat is calculated by applying 

equation 6 of Sorey, Nathenson, and Smith (this 

volume), who also give a detailed discussion of the 

methodology and all the assumptions and equations 

used in calculating the identified accessible resource 

base and resource. 

ESTIMATION OF TEMPERATURES 

Minimum, maximum, and most likely reservoir 

temperatures are estimated from the temperatures 

measured in springs or wells and from calculated 

subsurface temperatures based on geothermometry. 

The mean temperature is considered to be 

characteristic of the entire reservoir, although both 

cooler and warmer temperatures may occur locally 

within the reservoir. For isolated geothermal systems 

and for a few systems with reservoir volumes only 

slightly larger than 1 km3, the minimum temperatures 

are the measured temperatures, whereas the maximum 

and most likely values are from geothermometric 

calculations. For reservoirs of garger than the 

standard volume (greater than 1 km ), the minimum, 

maximum, and most likely temperatures are generally 

the temperatures measured in wells. 

Where the basic assumptions of water-rock 

equilibrium on which geothermometry is based 

(Fournier and others, 197 4) are not satisfied, the 

maximum and most likely temperatures are inferred 

from geologic information. For reservoirs with 

insufficient data and for those with geothermometric 

temperatures below the measured temperatures, the 

maximum and most likely temperatures are set equal 

to the spring or well temperature. 

The geothermometers used in this assessment 

are the Na-K-Ca, quartz conductive, and chalcedony. 

Although these geothermometers may not be reliable 

at low temperatures, owing to slow rates of waterrock reaction, these data were used as the best 

available estimates in the absence of measured 

reservoir temperatures. Selection of the appropriate 

Na-K-Ca temperature followed the rules given by 

Fournier and Truesdell (1973). A magnesium 

correction (Fournier and Potter, 1978) was applied to 

the Na-K-ca ~eothermometer where a correction of more than 1 C was indicated. Precise rules for 

selection of the quartz or chalcedony geothermometer 

in all terranes are not available. Arn6rsson (1975) 

demonstrated that, in basaltic terranes in Iceland, 

chalcedony controls dissolved-silica concentrations in 

reservoirs with temperatures below 130°C, whereas 

quartz limits dissolved-silica concentrations in 

reservoirs with temperatures above 180°C. Between 

130° and 180°C, either quartz or chalcedony could be 

the mineral that controls the dissolved-silica 

concentration. In granitic terranes the solubility of 

quartz apparently limits the dissolved-silica 

concentration down to 80°C, whereas the solubility of 

chalcedony limits dissolved-silica concentrations at 

lower temperatures (R.O. Fournier, oral commun., 

1981). Thermal-spring waters discharged from granitic 

terranes generally have Na/Ca weight ratios of more than 20, whereas ratios of less than 20 are 

characteristic of most other terranes. For this 

assessment, the quartz geothermometer is used only 

where the Na/Ca ratio in thermal water equals or is 

greater than 20 and where the Na-K-Ca 

37 
geothermometer indicates a temperature above 80°C. 

Interpretation of the temperatures estimated 

from silica geothermometers is further complicated by 

the increased solubility of silicate minerals at high pH 

(greater than 8.5). This is a common problem for 

thermal waters discharging in granitic terranes. The 

solubility of quartz or chalcedony is a function of both 

temperature and pH. At a pH between 4 and 8, the 

solubility of quartz or chalcedony is almost solely a function of temperature, and the effects of pH are 

negligible. At a pH greater than 8.5, however, the 

solubility of quartz or chalcedony increases sharply 

with rising pH at any given temperature. The 

equations that relate dissolved-silica concentrations to 

the temperature of quartz-water or chalcedony-water 

equilibrium (Fournier, 1977) are valid only for waters 

of near-neutral pH. In high-temperature geothermal 

reservoirs, thermal waters are buffered at near- neutral pH by silicate-mineral/water reactions. At 

low temperatures, however, the water is not buffered, 

and the pH may exceed 9 or even 10. Accordingly, the 

dissolved-silica concentration of alkaline waters 

cannot be used directly in silica geothermometers; 

instead, a computer program is used to calculate the 

pH at successively higher temperatures until 

equilibrium with quartz or chalcedony is theoretically 

achieved; these equilibrium temperatures are then 

used in place of the temperatures calculated from 

quartz or chalcedony geothermometers. This method 

generally gives temperatures near those estimated 

from the Na-K-Ca geothermometer. 

ESTIMATION OF AREA AND THICKNESS 

The mean volume of a geothermal reservoir is 

the product of the mean estimated area and the 

thickness. Reservoir areas for large geothermal 

systems are based predominately on the distribution of 

thermal springs and wells as plotted on 1:250,000-scale 

maps. Mean reservoir thickness is assumed to be 0.25 

km unless the data indicate otherwise; this mean 

thickness is based on estimated minimum, maximum, 

and most likely permeable thicknesses of 0.1, 0.4, and 

0.25 km, respectively. 

Isolated geothermal reserv~irs are assigned a mean standard volume of 1 km on the basis of 

theoretical calculations by M. L. Sorey (oral commun., 

1981). This standard volume is obtained from 

minimum, m~ximum, and most likely volumes of 0.01, 

2, and 1 km , respectively. This approach probably 

overestimates the volume of reservoirs in granitic or other low-permeability rock in which the thermal 

fluids are restricted to narrow faults or fractures, but 

underestimates the volume of isolated geothermal 

reservoirs in sedimentary and basin-fill terranes. 

USE OF TABLES 

Table 4 lists mean values for the reservoir 

parameters temperature, area, and thickness, and 

calculated values for the accessible resource base, 

resource, and beneficial heat, for identified lowtemperature geothermal systems in the Western 

United States. The table is arranged by State, and 

large-volume systems are listed individually. Isolated 

38 

geothermal systems are not listed individually; instead, 

the accessible resource base, resource, and beneficial 

heat for these systems are given as totals for the 

respective geologic provinces in each State. Complete 

chemical data and estimates of the reservoir 

temperature, area, thickness, transmissivity, well 

spacing, and type of system have been reported by 

Reed and others (1983). The range of total dissolved 

solids is included in the "Comments" to table 4 as a 

guide to environmental planning associated with water 

disposal after the usable heat has been extracted. 

Slightly to highly saline waters are discharged by 

approximately 30 percent of the large-volume lowtemperature hydrothermal-convection systems in the 

Western United States. 

All identified low-temperature geothermal- resources areas are classified according to the 

categories of Sorey, Nathenson, and Smith (this 

volume, figs. 5-7; table 3). Systems associated with 

fault zones (fig. 5) have been identified in all geologic 

settings and are by far the most common type of lowtemperature geothermal system in the Western United 

States. Lateral-leakage systems (fig. 6) are 

particularly common in the Basin and Range province, 

the Snake River Plain, the Rio Grande Rift, and the 

Salton Trough. Conduction-dominated systems (fig. 7) 

have been identified only in the Columbia Plateaus and 

the Salton Trough. 

UNDISCOVERED ACCESSIBLE RESOURCE BASE 

Sammel (1979) listed areas favorable for the 

discovery and development of low-temperature 

geothermal resources. Many of these same areas are 

evaluated in this assessment; however, several areas remain unevaluated because the data are insufficient. 

Unevaluated areas make up part of the undiscovered 

accessible resource base, the rest of which is assumed 

to be either in identified thermal reservoirs that may 

be larger or warmer than determined here, or in 

reservoirs with no surface discharge. The 

undiscovered accessible resource base was estimated 

for each geologic province by multiplying the 

identified accessible resource base of that province by 

factors of 1, 2, 3, 5, or, rarely, 10; our estimates are listed in table 5. The factor for each province was 

based on geologic and hydrologic characteristics, the 

number of systems identified in the province, and our own judgment; we recognize that large uncertainties 

may be associated with these estimates. The resource and beneficial heat were estimated by assuming the 

same ratio of resource (or beneficial heat) to 

accessible resource base as that determined for 

identified geothermal reservoirs in the region. 

SUMMARY 

The identified accessible resource base for both 

hydrothermal-convection and conduction-dominated 

low-temperature geothefgtal systems in the Western 

Unite18States is 310x10 J (table 6). Of this total, 

75x10 J is contained in hydrothermal-convration 

systems with standard reservoir volumes, 128x10 J is 

in hydrothermal-convection systems with l~rger than 

standard reservoir volumes, and 102x10 J is in 
Table 5.-Summary of the identified and undiscovered 

accessible resource base in geologic provinces of 

the Western United States 

[All values calculated analytically. All values are rounded to two significant figures or, if the first 

digit is 1, to three significant figures; this rounding 

represents a range of 0.5 to 5 percent in the accura- cy of the total value] 

Geologic province 

Access i b 1 e resource base 

(1018 J) 

I dent i fi ed 

Aleutian Islands and Peninsula-------- 0.35 

Central Alaska----------------------- 2.60 

Southeastern A 1 ask a------------------ • 58 

Basin and Range----------------------- 107 

Cascade Range-------------------------- 3. 5 

Coast Ranges-------------------------- 3. 7 

Co 1 or ado P 1 ateaus---------------------

Co 1 umbi a Plateaus---------------------

Ce nt ra 1 Va 11 ey------------------------

Hawa i i--------------------------------

1.56 

78 

.094 

1.70 

Olympic Mountains--------------------- .29 

Oregon Plateaus----------------------- 6.1 

Pen i nsu 1 ar Ranges--------------------- 5. 7 

Rio Grande Rift----------------------- 5.4 

Southern Rocky Mountains-------------- 3.1 

Middle Rocky Mountains---------------- 1.80 

Northern Rocky Mountains-------------- 15.4 

Sa 1 ton Trough------------------------- 29 

Sierra Nevada------------------------ 3.6 

Eastern Snake River Plain------------- 5.7 

Western Snake River Plain------------- 28 

Transverse Ranges--------------------- 2. 7 

To ta 1---------------------------- 310 

Undiscovered 

3.5 

13.0 

2.9 

210 

10.5 

11.1 

1.56 

78 

.094 

5.1 

.29 

12.2 

5. 7 

27 

3.1 

1.80 

15.4 

29 

3.6 

11.4 

28 

2. 7 

480 

conduction-dominated systems. Table 6 lists the 

distribution by temperature range of the identified 

resource and beneficial heat. Hydrothermalconvection systems account for only 65 percent of the 

identified accessible resource base but for 97 percent 

of the identified resource and be&ficial heat. The 

total identified resource is 31x10 J, and the total 

beneficial heat is 13.7 GWt. 

The undiscovered accessible resource bflfe for 

low-temperature geothermal systems is 480x10 J, on 

the basis of our judgment guided by geologic and 

hydrologic factors and by the distribution of identified 

geothermal systems. Assuming the same ratios of 

resource to accessible resource base and of beneficial 

heat to accessible resource base as observed in the 

identi{Wd systems, the undiscovered resource is 

48x10 J, and the beneficial heat in the undiscovered 

component is 21 GWt. Thus, the total lowtemperature accessible resource base, resource, and 

benefic[ftl heat ip the Western United States are 

790x10 J, 79x10 8 J, and 35 GWt, respectively. 

39 

Table 6.-Summary of energies for low-temperature 

geothermal systems in the Western United States 

[Systems in national parks are omitted. All means and 

standard deviations calculated by the Monte Carlo 

method. All values are rounded to two significant 

figures or, if the first digit is 1, to three significant 

figures; this rounding represents a range of 0.5 to 5 

percent in the accuracy of the total value J 

Type of system 

Identified 

Hydrotherma 1 convection 

Oe 1 i neated areas 

1 ess than so· C 

so· to 70"c 

70" to 90"c 

Number of 

systems 

Subtota 1----------- 148 

I so 1 a ted Systems 

1 ess than so· c so· to 7o"c 

70" to 90"c 

Subtota 1----------- 921 

Conduction dominated 

1 ess than so· C 

so· to 70"c 

70" to 90"c 

Subtota 1--------------

Tot a 1-------------------- 1, 078 

Undiscovered---------------- ---

Accessible 

resource base 

(1018 J) 

128±S.1 

Resource 

(1018 J) 

4.6:!:0.42 

5.3:!:0.SS 

1.18:!:0.192 

11.1±0.72 

6.3±0.189 

7.9±0.31 

4.8±0.27 

Beneficial 

heat 

(MWt for 30 yr) 

1,600:!:181 

2,600!280 

620!10S 

4,800±3SO 

1,9S0±81 

3,900±1S7 

2,S00±149 

7S±l.26 19.0±0.4S 8,400±230 

.62±0.139 210±S7 

.2S±0.170 120±93 

• 32±0 .149 166±78 

102±16.7 1.19±0.27 S00±134 

310±17 .s 31±0.89 13,700±440 

480 48 21,000 

ACKNOWLEDGMENTS 

Plots of thermal-spring and well locations at a 

scale of 1:250,000 were provided by Alan M. Mikuni 

and Brian S. Bennett of the U.S. Geological Survey, 

Menlo Park, California; this assessment could not have 

been accomplished without their willing assistance. 

REFERENCES CITED 

Arn6rsson, Stef,n, 1975, Application of the silica 

geothermometer in low temperature 

hydrothermal areas in Iceland: American 

Journal of Science, v. 275, no. 7, p. 763-784. 

Atwater, Tanya, 1970, Implications of plate tectonics 

for the Cenozoic tectonic evolution of western 

North America: 'Geological Society of America 

Bulletin, v. 81, no. 12, p. 3513-3535. 

Baldwin, E. M., 1981, Geology of Oregon (3d ed.}: 

Dubuque, Iowa, Kendall/Hunt, 170 p. 

Brook, C. A., Mariner, R. H., Mabey, D. R., Swanson, 

J. R., Guffanti, Marianne, and, Muffler, L. J. 

P., 1979, Hydrothermal convection systems with 

reservoir temperatures ~90°C, in Muffler, L. J. 

P ., ed., Assessment of geothermal resources of 

the United States-1978: U.S. Geological 

Survey Circular 790, p. 18-85. 

Brott, C. A., Blackwell, D. D., and Mitchell, J. C., 
1976, Heat flow study of the Snake River Plain 

fegiOO, Idaho, pt. 8 of Geothermal 

investigations in Idaho: IdahoDepartment of 

Water Resources Water Information Bulletin 30, 

195 p. 

Donnelly-Nolan, J. M., Hearn, B. C., Jr., Curtis, G. H., 

and Drake, R. E., 1981, Geochronology and 

evolution of the Clear Lake Volcanics, in 

Donnelly-Nolan, J. M., and McLaughlin, R. J., 

eds., Research in the Geysers-Clear Lake 

geothermal area, northern California: U.S. 

Geological Survey Professional Paper 1141, p. 

47-60. 

Fournier, R. 0., 1977, Chemical geothermometers and 

mixing models for geothermal systems, in 

Barbier, Enrico, ed., The application of nuclear 

techniques to geothermal studies: Geothermics, 

v. 5, no. 1-4 (special issue), p. 41-50. 

Fournier, R. 0., and Potter, R. W ., II, 1978, A 

magnesium correction for the Na-K-Ca 

geothermometer: Geochimica et Cosmochimica 

Acta, v. 43, no. 10, p. 1543-1550. 

Fournier, R. 0., and Truesdell, A. H., 1973, An 

empirical Na-K-Ca geothermometer for natural 

waters: Geochimica et Cosmochimica Acta, v. 

37, no. 5, p. 1255-1275. 

Fournier, R. 0., White, D. E., and Truesdell, A. H., 

1974, Geochemical indicators of subsurface 

temperature-part 1, Basic assumptions: U.S. 

Geological Survey Journal of Research, v. 2, no. 

3, p. 259-262. 

Fuis, G. S., Mooney, W. D., Healy, J. H., McMechan, G. 

A., and Lutter, W. J., 1982, Crustal structure of 

the Imperial Valley region, California, in The 

Imperial Valley earthquake of October 15, 

1979: U.S. Geological Survey Professional 

Paper 1254, p. 25-50. 

Hammond, P. E., 1979, A tectonic model for evolution 

of the Cascade Range, in Armentrout, J. M., 

Cole, M. R., and TerSest, Harry, Jr., eds., 

Cenozoic Paleogeography of the western United 

States: Pacific Coast Paleogeography 

Symposium 3: Los Angeles, Society of 

Economic Paleontologists and Mineralogists, 

Pacific Section, p. 219-237. 

Harder, Vicki, Morgan, Paul, and Swanberg, C. A., 

1980, Geothermal resources in the Rio Grande 

Rift: Origins and potential: Geothermal 

Resources Council Transactions, v. 4, p. 61-64. 

Malde, H. E., and Powers, H. A., 1962, Upper Cenozoic 

stratigraphy of western Snake River Plain, 

Idaho: Geological Society of America Bulletin, 

v. 73, no. 10, p. 1197-1220. 

Miller, T. P., Barnes, Ivan, and Patton, W. W., Jr., 

1975, Geologic setting and chemical 

characteristics of hot springs in west-central 

Alaska: U.S. Geological Survey Journal of 

Research, v. 3, no. 2, p. 149-162. 

40 

Morgan, Paul, Harder, Vicki, Swanberg, C. A., and 

Daggett, P. H., 1981, A groundwater convection 

model for the Rio Grande Rift geothermal 

resources: Geothermal Resources Council 

Transactions, v. 5, p. 193-196. 

Motyka, R. J., Moorman, M. A., and Liss, S. A., 1981, 

Assessment of thermal spring sites Aleutian 

Arc, Atka Island to Becherof Lake-preliminary 

results and evaluation: Alaska Division of 

Geological and Geophysical Surveys Open-File 

Report 144, 173 p. 

Page, B. M., 1966, Geology of the Coast Ranges of 

California, in Coast Ranges province, chap. 6 of 

Bailey, E.-H., ed., Geology of northern 

California: California Division of Mines and 

Geology Bulletin 190, p. 255-276. 

Reed, M. J., Mariner, R. H., Brook, C. A., and Sorey, 

M. L., 1983, Selected data for low-temperature 

(less than 90°C) geothermal systems in the 

United States; reference data for U.S. 

Geological Survey Circular 892: U.S. 

Geological Survey Open-File Report 83-250, 129 

p. 

Sammel, E. A., 1979, Occurrence of low-temperature 

geothermal waters in the United States, in 

Muffler, L. J.P., ed., Assessment of geothermal 

resources of the United States-1978: U.S. 

Geological Survey Circular 790, p. 86-131. 

Sass, J. H., Blackwell, D. D., Chapman, D. S., Costain, 

J. K., Decker, E. R., Lawver, L. A., and 

Swanberg, C. A., 1981, Heat flow from the 

crust of the United States, in Touloukian, Y. S., 

Judd, W. R., and Roy, R:-F., eds., Physical 

properties of rocks and minerals: New York, 

McGraw-Hill, p. 503-548. 

Scholz, C. H., Barazang, Mauwia, and Sbar, M. L., 

1971, Late Cenozoic evolution of the Great 

Basin, western United States, as an ensialic 

interarc basin: Geological Society of America 

Bulletin, v. 82, no. 11, p. 2979-2990. 

Swanson, D. A., Wright, T. L., Hooper, P. R., and 

Bentley, R. D., 1979, Revisions in stratigraphic 

nomenclature of the Columbia River Basalt 

Group: U.S. Geological Survey Bulletin 1457-G, 

p. G1-G59. 

Tabor, R. W., and Cady, W. M., 1978, Geologic map of 

the Olympic Peninsula: U.S. Geological Survey 

Miscellaneous Investigations Series Map 1-994, 

scale 1:125,000, 2 sheets. 

Teshin, V. N., Swanson, J. R., and Orris, G. J., 1979, 

GEOTHERM-geothermal resources file: 

Geothermal Resources Council Transactions, v. 

3, p. 721-724. 

Thomas, D. M., Lienert, B. R., Cox, M. E., Kauahikaua, 

J. P., and Mattice, M., 1982, Hawaii geothermal 

resource assessment: 1982: Geothermal 

Resources Council Transactions, v. 6, p. 63-66. 
TABLE 4 
Table 4.-Estimated reservoir values and thermal energies of identified low-temperature geothermal resources 

in the Western United States 

[Systems in national parks have been excluded from calculations of resource and beneficial heat. All means and 

standard deviations calculated analytically. All values are rounded to two significant figures or, if the first 

digit is 1, to three significant figures; this rounding represents a range of 0.5 to 5 percent in the accuracy of 

the total value. See table 3 for the categories of low-temperature geothermal systems. TDS, total dissolved 

solids: (1) Less than 1,000 mg/L; (2) 1,000 to 10,000 mg/L; (3) more than 10,000 mg/L. I., assumed reservoir. 

transmissivity; !!.w' area per well for equally spaced production wells] 

42 
Geologic province 

Geothennal area 

Category 

Aleutian Islands and Peninsula province 

Isolated systems----------

Centra 1 Alaska pro vi nee 

I so 1 a ted systems----------

Southeastern Alaska pro vi nee 

Isolated systems----------

Basin and Range province 

Isolated systems----------

Agua Caliente-------------

Bowie-----"---------------

Buckeye Valley------------

Buckhorn------------------

Cactus Flat---------------

Chandler------------------

Gila Bend-----------------

Harquaha 1 a Plain----------

Hyder Valley--------------

Palomas PT ai n-------------

Rainbow Valley------------

Sulphur Springs Valley----

Basin and Range province (northern) 

I sol a ted systems---------- 1 

North of Likely-----------

Susanvi 11 e----------------

Basin and Range province (southern) 

I sol a ted systems---------- 1 

Desert Center-------------

Eastern Long Valley-------

McCoy Wash----------------

Saline Valley-------------

Twentynine Palms----------

Cascade Range pro vi nee 

!sol a ted systems----------

Mean 

reservoir 

temperature 

(·cl 

48 

42 

40 

55 

53 

58 

55 

50 

53 

53 

48 

55 

65 

68 

35 

40 

32 

56 

55 

Mean 

reservoir 

area 

(km2l 

22 

ll3 

150 

53 

33 

no 

27 

100 

200 

100 

100 

100 

5.0 

6. 7 

123 

33 

19.3 

7.3 

63 

Mean Mean 

reservoir accessible 

thickness resource base 

(km) (lo18 J) 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.50 

.25 

.25 

.25 

ALASKA 

0.35±0.098 

2.6±0.26 

.58±0 .ll6 

ARIZONA 

4.8±0.35 

.47±0.162 

1. 96±0.86 

2 .4±0.80 

1.39±0.66 

.83±0.35 

6.1±1.97 

.69±0.33 

2.3±0.95 

5.0±1.96 

2.5±0.98 

2.2±1.04 

2.6±1.04 

CALIFORNIA 

1.03±().15 

.161±0.069 

.23±0.086 

1.95±0.21 

1.60±0.45 

1.08±0.66 

.21±0.098 

.194±0.059 

1.65±0.83 

• 71:t0.24 

43 

Resource 

nol8Jl 

0.088 

.65 

.145 

1.21 

.106 

.172 

.149 

.23 

.180 

.30 

.157 

.20 

.25 

.22 

.193 

.23 

0.26 

.038 

.054 

.49 

.063 

.042 

.046 

.046 

.089 

.179 

Beneficial 

Heat 

(MWt for 30 yr l 

43 

320 

71 

550 

47 

68 

58 

llO 

84 

147 

74 

90 

115 

102 

84 

107 

101 

19 

1'8 

200 

20 

16 

12 

22 

42 

88 

Conments 

3 systems. 

25 systems. 

5 systems. 

51 systems. 

5 wells, no depth data; TOS (1,2), 

.!_=0.01 m2;s, !w=l km2. 

Poorly defined system: 4 wells, no depth 

data or flow rates; TDS (1), 

.!_=0.01 m2;s, !w=l.8 km2. 

3 wells: 1 well 84•c and 214m deep; other 

2 wells: saline (1,000 mg/Ll, cooler, 

and lack depth data; T=O.Ol m2/s, 

!w=l .6 km2. -

2 wells; TDS (1), .!_=0.01 m2;s, !w=l km2. 

12 wells: 1 well, 489 m deep; TDS (2), 

.!_=0.01 m2/s, !w=l km2. 

8 wells (30°-62.C), to 914 m deep; 

TOS (1,2), .!_=0.01 m2/s, !w=2.3 km2. 

2 wells, no depth data; TOS (2), 

.!_=0.01 m2;s, !w=l km2. 

3 wells, no depth data; TOS (1 l, 

.!_=0.01 m2/s, !w=l.8 km2. 

ll wells, 31 to 162 m deep; TDS ( 1), 

.!_=0.01 m2;s, !w=2.3 km2. 

9 wells: 2 wells are 73 and 145m deep; 

TOS (1 ), .!_=0.01 m2/s, !w=l.8 km2. 

3 wells, no depth data; TOS (1 ), 

.!_=0.01 m2;s, !w=l.8 km2. 

Poorly defined system: 2 wells, 412 and 

670 m deep; TOS (1 ), T=O.Ol m2/s, 

!w=l.8 km2. -

16 systems. 

2 wells and 1 spring (maximum 44 •c l: 

wells, 34 and 61 m deep; TDS (1 l, 

.!_=0.0025 m2/s, !w=l km2. 

Numerous shallow wells (36°-63°C), 90 to 

180m deep; TDS (1 ), T=0.0025 m2/s, 

!w=l km2. -

26 systems. 

5 wells (3o·-35.Cl, 69 to 183m deep; 

TOS (2), .!_=0.005 m2;s, !w=2.33 km2. 

Springs east and southeast of Hot Creek, 

1 eakage from high-temperature system; 

TDS (2), .!_=0.005 m2;s, !w=l.2 km2. 

Wells (3l 0-33.C), 84 to 178m deep; no 

chemical data, .!_=0.005 m2/s, !w=l km2. 

4 springs on northeast-side of valley; 

TDS (1 ), .!_=0.005 m2/s, !w=l km2. 

5 wells (40°-67.C), no depth data; 

TDS (1), .!_=0.005 m2;s, !w=l.8 km2. 

6 systems • 
:Jeol ogle pro vi nee 

Geothennal area 

:entral Valley pro vi nee 

!sol a ted systems----------

:oast Ranges pro vi nee 

!sol a ted systems----------

Agua Caliente-------------

Paso Robles---------------

~eni nsul ar Ranges provi nee 

Isolated systems----------

Diamond Va 11 ey------------

Domeni goni Valley---------

Palm City-----------------

San Jacinto Valley--------

Sa 1 ton Trough pro vi nee 

!sol a ted systems----------

Desert Hot Springs--------

Eastern Imperial Va 11 ey---

Lower Borrego Va 11 ey------

Northwestern Sal ton Sea---

Sierra Nevada pro vi nee 

!sol a ted systems----------

Sierra Valley-------------

Transverse Ranges provi nee 

I sol a ted systems----------

Urbita Hot Springs--------

Harlem Hot Springs--------

Ojai Springs--------------

Rio Grande Rift pro vi nee 

Category 

!sol a ted systems----------

Southern Rocky Mountains pro vi nee 

Isolated systems----------

Glenwood Springs----------

Hawaii an pro vi nee 

Kapoho--------------------

Basin and Range province 

Isolated systems----------

Mean 

reservoir 

temperature 

("C) 

43 

72 

42 

53 

52 

45 

67 

57 

42 

43 

50 

45 

50 

68 

51 

48 

Mean Mean 

reservoir reservoir 

area thickness 

(km2l (km) 

3. 7 .25 

4.3 .25 

5. 7 .25 

5.7 .25 

10.0 .25 

150 .25 

32 .25 

800 .30 

27 .25 

30 .25 

78 .25 

10.7 .25 

9.0 .25 

15.3 .25 

5.0 .25 

78 .25 

Mean 

accessible 

resource base 

(1018 J) 

0 .094±{).028 

3.5±0.28 

.068±0.024 

.160±{).058 

2.4±0.21 

.099±(l.054 

.141 ±{).060 

.24±0.103 

2.9±1.19 

1.21 ±0.154 

1. 06±{). 38 

26±10.4 

.46±0.138 

.55±0.187 

1. 78±{).20 

1. 78±0.88 

1. 78±0.179 

.21±0.063 

.20±0.074 

.53±0.196 

COLORADO 

0.39±0.101 

3.0±0.26 

.ll7t0.039 

HAWAII 

1. 70±0. 57 

IDAHO 

1.28±0.189 

44 

Resource 

(1018J) 

0.024 

1.56 

.016 

.038 

.59 

.023 

.033 

.052 

.094 

.30 

.120 

.23 

.062 

.062 

.41 

.050 

.44 

.045 

.048 

.094 

0.098 

. 75 

.028 

0.054 

0.32 

Beneficial 

Heat 

(MWt for 30 yr) 

6.6 

370 

6.5 

20 

250 

9.2 

15.6 

24 

40 

129 

61 

109 

25 

26 

188 

23 

187 

19.1 

22 

48 

44 

340 

12.6 

24 

153 

Conments 

2 systems. 

46 systems. 

3 springs (38"-46"Cl; no chemical data, 

.!.=0.005 m2/s, ~=1 km2. 

3 springs (39"-43"C); TDS (2), 

.!.=0.005 m2/s, ~=1 km2. 

29 systems. 

1 well, 27m deep; TDS (2), .!.=0.005 m2/s, 

~=1 km2. 

3 wells (37"-49"C), 6 to 34m deep; 

TDS (1,2), .!.=0.005 m2/s, ~=1 km2. 

2 wells (36"Cl; TDS (2), T=0.005 m2/s, 

~=1 km2. -

Springs (38" -43"C l and wells (26" -49"C), 

depths to 186 m; TDS (1), 

.!.=0.005 m2/s, ~=2.08 km2. 

15 systems. 

14 wells, 5 to 69 m deep; TDS (1 ), 

. .!.=0.005 m2/s, ~=1.17 km2. 

Numerous wells (30" -55"C l, some artesian, 

100 to 300m deep; TDS (2), 

.!.=0.005 m2/s, ~=9.03 km2. 

9 wells (31"-38"C), 45 to 102m deep; 

TDS (1), .!.=0.005 m2/s, ~=1.07 km2. 

Numerous wells (32"-43"C), only one well 

depth ava i1 ab 1 e (118 m); TDS ( 1 l, 

.!.=0.005 m2/s, ~=1.08 km2. 

19 systems. 

Wells (25"-35"C), 7 to 335m deep; TDS (1), 

(1 well 94"C, has 1,370 mg/Ll, 

.!.=0.0025 m2/s, ~=3.33 km2. 

23 systems. 

7 wells (41"-51"C), 53 to 297m deep; 

TDS (1 ), .!.=0.005 m2/s, ~=1 km2. 

3 wells (43"-54"C), 42 to 152m deep; 

no chemical rlata, T=0.005 m2/s, 

~=1 km2. -

5 springs ( 39" -51 •c l; TDS ( 1 , 2 l, 

.!.=0.005 m2/s, ~=1 km2. 

5 systems. 

33 systems. 

Several springs (32"-51 "C); TDS (3), 

.!.=0.01 m2/s, ~=1 km2. 

2 springs near sea 1 evel ( 33" and 36"C l and 

3 wells (37"-52"C), 44 to 96 m deep; 

TDS (1,2l,.i=o.0025 m2/s, ~=2.93 km2. 

12 systems. 
Geologic province 

Geothennal area 

Goose Creek-Oakley--------

Lower Raft River----------

Malad City----------------

Marsh Creek---------------

Raft River----------------

Soda Springs--------------

Category 

Middle Rocky Mountains province 

I sol a ted systems----------

Northern Rocky Mountains pro vi nee 

Isolated systems---------- 1 

Boi 1 i ng Springs-----------

Cascade-------------------

Holdover Hot Springs------

Mi dva 1 e-------------------

Punkin Corners------------

Sunflower-Riverside-------

Eastern Snake River Plain provi nee 

I sol a ted systems----------

Newda 1 e-------------------

Rock Creek-Artesian City--

South ffnerican Falls------

Tyhee ---------------------

Western Snake River Plain pro vi nee 

Isolated systems----------

Boise Front---------------

Bruneau-Grandview---------

Buhl----------------------

Clover Creek--------------

Dry Creek-----------------

Emmett Valley-------------

Glenns Ferry-King Hill----

Mean 

reservoir 

temperature 

("C) 

63 

40 

33 

60 

47 

59 

80 

67 

62 

52 

48 

75 

57 

53 

30 

35 

67 

40 

45 

68 

43 

24 

35 

Mean 

reservoir 

67 

62 

41.0 

8.3 

220 

5. 7 

6.0 

ll. 7 

7.0 

14.3 

11.7 

5.0 

57 

108 

7. 7 

9.3 

57 

750 

58 

21 

6. 7 

12.3 

37 

Mean 

reservoir 

thickness 

(km) 

0.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

Mean 

accessible 

resource base 

(1018 J) 

2.1±0.71 

1.01±0.61 

.49±0.31 

.24±0.100 

4.5±1.72 

.163±0.087 

.33±0 .139 

10.1±0.49 

. 25±0.087 

.39±0.168 

.22±0.071 

.34±0.170 

.25tO.ll5 

.19~±0.068 

1.22±0.172 

1.54±0.60 

2.7±0.98 

.075±0.028 

.121±0.049 

1.28±0.165 

1.90±0.84 

12.2±3.8 

1.14±0.49 

.7?.±0.26 

.123±0.048 

.072±0.034 

.48±0.176 

45 

Resource 

(1018J) 

0.138 

.20 

.075 

.057 

.183 

.038 

.082 

2.5 

.060 

.076 

.051 

.067 

.055 

.046 

• 30 

.ll7 

.127 

.018 

.029 

.32 

.186 

.23 

.099 

.135 

.029 

.015 

.028 

Beneficial 

Heat 

(MWt for 30 yr) 

69 

77 

21 

28 

79 

18.9 

37 

1,220 

32 

39 

25 

31 

24 

24 

130 

57 

60 

3. 7 

9.1 

134 

95 

88 

42 

70 

11.9 

0 

8.9 

Comments 

spring (47"C) and 5 wells (3Z"-47"C), 

some artesian, 259 to 326m deep; 

TOS (1 ), !=0.005 m2/s, !w=l.55 km2. 

5 wells (32"-40"C), depths to 352m; 

TDS (1), !=0.01 m2/s, !w=l km2. 

3 springs (25"-27"C), poorly defined 

reservoir; TDS (2), T=0.005 m2/s, 

~=1 km2. -

Springs and 1 well (60"C and 136m deep); 

TDS (1 ), !=0.01 m2/s, ~=1 km2. 

Numerous wells (generally 20"-40"C), 50 to 

200m deep: 1 well (77"r.) 1 km deep; 

TDS (1 ,2), !=0.005 m2/s, ~=2.3 km?.. 

2 C02-charged springs, system temperatures 

speculative; TDS (2), T=0.005 m2/s, 

~=1 km2. -

4 systems. 

94 systems. 

6 springs (32"-86"C); TDS (1 ), 

!=0.005 m2/s, !w=1 km2. 

2 springs (39"-71"Cl and 1 well (43"C) 15"' 

deep; TDS (1), !=0.005 m2/s, !w=l km2. 

2 springs (46"C); TDS (1 ), T=0.005 m2/s, 

~=1 km2. -

spring (50"Cl and 2 wells (28"C), depths 

to 316m; TDS (1), T=0.005m2/s, 

~=1 km2. -

2 wells (26"-35"C), 58 m deep; TDS (1), 

.!_=0.005 m2ts, ~=1 km2. 

springs (43"-65"C), along both sides of 

Middle Fork, Salmon River; TDS (1 ), 

!=0.005 m2/s, ~=1 ~m2. 

16 systems . 

Wells (21"-37"C), 56 to 200m deep, in 

area extending across Teton River; 

TDS (1), !=0.005 m2/s, !w=l.39 km2. 

Many wells (23"-4l"C), 24 to 335m deep; 

TDS (1 ), !=0.005 m2/s, !w=l.80 km2. 

Wells (27"-33"C), depths to 164m; 

TDS (1 ), !=0.01 m2/s, !w=l km2. 

6 wells (20"-4l"C), 60 to 152m deep; 

TDS (1 ), !=0.005 m2/s, !w=l km2. 

17 systems. 

More than 25 wells, most are 100 to 450 m 

deep; 1 well 90"C and 1,174 m deep; 

TDS (1), !=0.005 m2/s, !w=l.13 km2. 

More than 100 high-discharge wells (?0"-

84"Cl; TDS (1 ), T=O.Ol m2/s, 

~=5.94 km2. -

Springs (to 70"Cl and wells (28"-63"C), 

34 to 332m deep; TDS (1), 

!=0.005 m2/s, !w=l.19 km2. 

spring (27"Cl and 3 wells (43"-47"C), 

50 to 256 m deep; TDS ( 1 l , 

!=0.005 m2/s, ~=1 km2. 

5 wells (25" -30"C), 90 to 120 m deep; 

TDS (1 ), !=0.005 m2/s, !w=l km2. 

2 wells (24"Cl, 9 to 54 m deep; no 

chemical data, !=0.005 m2/s, !w=l k112. 

Wells (23"-38"Cl, 73 to 396m deep; 

TDS (1 ), !=0.00?5 m2/s, !w=1.64 ~m?. 
Geo 1 ogi c pro vi rice 

Geothermal area 

Category 

Mt. Bennett Hills---------

Murphy-Givens-------------

Na t-Soo-Pah ---------------

Salmon Falls Creek--------

West Snake River Plain----

Middle Rocky Mountains pro vi nee 

I sol a ted systems----------

Northern Rocky Mountains pro vi nee 

Isolated systems----------

Basin and Range province 

I sol a ted systems----------

Abel-Chimney Springs------

Alkali Springs------------

Arnol dson Spring----------

Artesia Lake--------------

Ash Meadows---------------

Ash Springs---------------

Bartine Hot Springs-------

Beatty-Hicks--------------

Bennett's Spring----------

Black Canyon--------------

Blue Eagle Spring---------

Caliente------------------

Carl in--------------------

Comstock------------------

Dixie Valley--------------

Duckwater-----------------

El ko ----------------------

El y-Lackawa na-------------

Fallon South--------------

2 

Mean 

reservoir 

temperature 

("C) 

72 

35 

43 

33 

32 

47 

58 

23 

83 

30 

36 

57 

72 

25 

50 

35 

80 

83 

77 

25 

44 

75 

40 

73 

Mean 

reservoir 

area 

(km2l 

30 

150 

26 

17.7 

570 

54 

6. 7 

6.3 

20 

100 

5.0 

6.7 

16.7 

13.0 

5. 7 

10.0 

5. 7 

5.0 

5.3 

53 

7. 7 

7.3 

9. 7 

10.7 

Mean 

reservoir 

thickness 

(km) 

0.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.25 

.2<; 

.25 

Mean 

accessible 

resource base 

(1018 J) 

1.10±0.35 

1.95±0.69 

.47±0.175 

. 21 ±0.085 

6.1±2.0 

MONTANA 

0.29±0.072 

3.6±0.28 

NEVADA 

11.9±{).31 

l.ll±O.Sl 

.188±0.070 

.033tO.Oll 

.88±0.40 

.98±0.34 

.068±0.024 

.182±0.075 

.61±0.24 

.084±0.027 

.129±0.058 

.132±0.056 

.24±0.096 

.22±0.073 

.22±0.066 

.35±0.161 

.146±0.056 

.29±0.091 

.159±0.068 

.40±0.25 

46 

Resource 

(1018J) 

0.145 

.063 

.103 

.050 

.078 

0.074 

.91 

3.0 

.081 

.044 

.008 

.146 

.083 

.016 

.043 

.112 

.016 

.030 

.031 

.056 

.052 

.051 

.034 

.034 

.067 

.035 

.088 

Beneficial 

Heat 

(MWt for 30 yr) 

76 

19.8 

42 

14.3 

19.8 

30 

4ll 

1,310 

35 

22 

o.o 

79 

17.6 

5.3 

21 

58 

0.0 

13.7 

10.0 

30 

28 

27 

o.o 

14.4 

36 

13.3 

46 

Comments 

Springs (57"C) and wells (36"-68"Cl, 

to 483 m deep; TDS (1), T=0.005 m2/s, 

~=1.08 km2. -

Springs and wells (25"-56"C): wells 

39 to 457 m deep; TDS (1), 

I_=0.005 m2/s, ~=2.27 km2. 

Springs and wells near 36"C, depths to 

236m; TDS (1), I_=O.Ol m2/s, ~=1 km2. 

Wells (25"-30"C), 150 to 250m deep; 

no chemical data, T=O.Ol m2/s, 

~=1 km2. -

Numerous irrigation wells, near 25"C, 

100m deep; TDS (1 ), T=0.005 m2/s, 

~=7 .42 km2. -

4 systems. 

41 systems. 

143 systems. 

4 springs (37"-66"Cl and 1 artesian well 

(60"Cl <;22m deep; TOS (1 ), 

I_=0.005 m2/s, ~=1.2 km?. 

Several springs ( 37" -60"C); TDS (2 l, 

I_=0.005 m2/s, ~=1 km2. 

3 springs (22" -23"C); no chemical data, 

I_=O.Ol m2/s, ~=1 km2. 

3 shallow wells (21"-28"C), 18 to 165m 

deep; TDS (1), !=0.005 m2/s, ~=1 km2. 

More than 30 springs and shallow wells 

(26"-34"C): wells less than 200m; 

TDS (1), I_=O.Ol m2/s, ~=1.27 km2. 

2 high-discharge springs (32" and 36"C); 

TDS (1), I_=O.Ol m2/s, ~=1 km2. 

spring (44"C) and 1 well (47"Cl 148m 

deep; TDS (1), T=O.OOS m2/s, 

~=1 km2. -

4 springs (26" -43"C l; TDS ( 1 l, 

I_=0.005 m2/s, ~=1 km2. 

2 springs (2<;"Cl and 4 shallow wells 

(23"-28"C), depths to 100m; no 

chemical data, I_=O.OOS m2/s, ~=1 km2. 

Springs (25"-33"C); TDS (2), T=0.005 m2/s, 

~=1 km2. -

3 springs (22" -28"C l and 1 well (22"C l 56 m 

deep; TDS (1), I_=O.Ol m2/s, ! wl km2. 

spring (63"C l and 4 wells (42" -67"C ), 15 

to 41 m rteep; TDS (1), T=O.Ol m2/s, 

~=1 km2. -

spring (79"Cl; TDS (1), T=0.005 m2/s, 

~=1 km2. -

well (77"C), 914 m deep; no chemical 

data, !=0.005 m2/s, ~=1 km2. 

2 wells (22" and 23"C), fil m rteep; 

TDS (1), I_=O.OOS m2/s, ~=1.27 km2. 

3 springs (32"Cl; TDS (1), T=0.005 m2/s, 

~=1 km2. -

2 springs; TDS (1), I_=0.005 m2/s, ~=1 km2. 

2 springs (30" and 35"Cl; TDS (1 ), 

I_=0.005 m2/s, ~=1 km2. 

artesian well (70"C l, no chemical or 

depth data; I_=O.OOS m2!s, ~=1 km2. 
Geo 1 ogi c pro vi nee 

Geothennal area 

Fish Lake Valley----------

Fly Ranch-----------------

Gabbs---------------------

Gamble Ranch Spring-------

Golconda Hot Springs------

Goose Creek---------------

Grass Valley--------------

Hawthorne-----------------

Hill's Wann Spring--------

Hobo Hot Spring-----------

Hot Creek Canyon----------

Hot Springs Point---------

Huffaker Narrows----------

Humboldt Wells------------

Indian Springs------------

Las Vegas-----------------

Moan a---------------------

Moapa Springs-------------

Moniter-------------------

North Dixie Valley--------

Panaca--------------------

Pinyon Hills--------------

Pleasant Valley-----------

Preston Springs-----------

Reese River Valley--------

Rose Creek----------------

Category 

Mean 

reservoir 

temperature 

(·cl 

27 

58 

53 

38 

84 

40 

67 

47 

56 

60 

53 

72 

52 

48 

26 

26 

55 

47 

22 

43 

32 

50 

30 

23 

59 

48 

Mean Mean 

reservoir reservoir 

area thickness 

(km2) (km) 

42 0.25 

21.7 .25 

7. 7 .25 

17.3 .25 

5.0 .25 

21.0 .25 

20.0 .20 

53 .25 

8.3 .25 

5.3 .25 

13.3 .25 

10.7 .25 

40.0 .25 

6.3 .25 

9. 7 .25 

21 .25 

25.7 .25 

10.3 .25 

5. 7 .25 

40.0 .25 

12.0 .25 

15.0 .25 

17.3 .25 

5.3 .25 

10.3 .25 

7.0 .25 

Mean 

accessible 

resource base 

(1018 J) 

0.33±0.182 

.61:!:0-26 

.19]±0.063 

. 26±0.115 

• 22±0.076 

.34±0.124 

.54±0.171 

1 .1 0±0.46 

.22t0.ll3 

.157±0.053 

.33±0.167 

.39±0.125 

.95±0.40 

.137±0.051 

.069±0.01'4 

.150±0.085 

.67±0. 32 

.22±0.082 

.026±0.010 

• 74±0.29 

.133±0.047 

.34±0.105 

.169±0.055 

.028tO.Oll 

.30±0.1 01 

.152±0.063 

47 

Resource 

(lolBJl 

0.043 

.093 

.045 

.050 

.053 

.057 

.060 

.073 

.052 

.037 

.078 

.085 

.102 

.032 

.015 

.035 

.090 

.047 

.006 

.068 

.031 

.062 

.031 

.007 

.064 

.036 

Beneficial 

Heat Corrments 

(MWt for 30 yr) 

3.9 

45 

21 

18 

29 

22 

31 

32 

25 

18.3 

37 

45 

47 

14.3 

0.9 

2.0 

43 

21 

0.0 

28 

8.1 

28 

6.5 

o.o 

31 

15.8 

Springs (25°-30°C); TDS (2), T=0.005 m2/s, 

~=1.08 km2. -

At least 5 shallow wells (2o•-25•cl, 

depths less than 100 m; T=0.005 m2/s, 

~=1 km2. -

3 wells (52.-68.C), depths to 139m; no 

chemical data, T=O.OO~ m2/s, 

~=1 km2. -

springs 121• and 44•c1 and 1 well (24.Cl 

64 m deep; TDS (1), .!_=0.005 m2/s, 

~=1 km2. 

Springs (74°C); TDS (1 ), .!_=0.005 m2/s, 

~=1 km2. 

springs and shall ow wells ( 21• -43•c l, 

to 75 m depth; TDS ( 1), T=0.005 m2/s 

~=1 km2. -

Low-temperature water 1 eak i ng from Leach 

Hot Springs high-temperature system; 

TDS (1 ), .!_=0.0025 m2/s, ~=2.33 km2. 

At least 6 shallow wells (23°-34.C), 

depths near 200 m: 1 well ( ~1·c l with 

unknown deptl!; TDS (1 ), T=0.005 m2/s, 

~=1.55 km2. -

2 springs (23• and 28.Cl; TDS (1 ), 

.!_=0.005 m2/s, ~=1 km2. 

2 spring areas; TDS (1 ), T=0.005 m2/s, 

~=1 km2. -

At least 5 springs (33°-82.C); TDS (1 ), 

.!_=0.01 m2/s, ~=1 km2. 

spring area (55•-6o•cl and 1 well (75•cJ 

125m deep, may overlie hightemperature system; TDS ( 1 l, 

.!_=0.005 m2/s, ~=1 km2. 

Springs and shallow wells, leakage from 

Steamboat Springs high-temperature 

system; TDS (2), T=O.D05 m2/s, 

~=1.27 km2. -

2 wells (36° and 57"C)' no depth data; 

TDS (1), .!_=0.005 m2/s, ~=1 km2. 

4 springs (25°-26.C); TOS (1 ), 

.!_=0.005 m2/s, ~=1 km2. 

Marginally thennal (26.C), high-discharge 

wells; TDS (1), .!_=0.01 m2/s, ~=1 km2. 

Many shallow wells in Reno (most are 

30•-go•c), depths to 214m: water 

used for space heating; TDS ( 1 l, 

.!_=0.005 m2/s, !w=l.07 km2. 

Springs (28°-32.C), subsurface 

temperatures are speculative; TDS (1 ), 

.!_=0.005 m2;s, ~=1 km2. 

2 wells 122•c l, no depth data; TOS ( 1), 

.!_=0.005 m2/s, ~=1 km2. 

5 springs (28°-78.Cl; TDS (1), 

.!_=0.005 m2/s, ~=1.27 km2. 

spring (32.C) and 3 wells (23°-24.C), 

32 to 41 m deep; TOS (1), limestone 

aquifer, .!_=0.01 m2;s, ~=1 km2. 

4 springs and wells, temperatures to so•c; 

TDS (1,2), .!_=0.005 m2/s, ~=1 km2. 

Shallow wells (22°-28.C), 5 to 51 m 

deep; TDS (1), .!_=0.005 m2/s, ~=1 km2. 

2 wells 122• and 24°C), 120 and 124m 

deep; TOS (1), !_=0.005 m2/s, ~=1 km2. 

2 springs {so· and 53.C); TDS {1 ), 

.!_=0.005 m2/s, ~=1 km2. 

Wells (28.C) 30m deep; TDS (2), 

.!_=0.005 m2/s, ~=1 km2. 
Mean Mean Mean Mean 

Geologic province reservoir reservoir reservoir accessible Resource Beneficial 

Category 

(lo18Jl Heat Co11111ents 

Geothermal area temperature area thickness resource base 

("C) (km2l (km) (1018 J) (MWt for 30 yr) 

Soldier Meadows----------- 64 11.7 0.25 0.30±0.107 0.075 38 Many springs (to 54"C ), at northwest end 

of Black Rock Desert; TDS (1), 

!.=0.005 m2/s, !w=l km2. 

South Las Vegas----------- 30 72 .25 .70±0.23 .041 8. 7 Marginally warm wells (25" -27"C ), 

100 to 150m tleep; TOS (1), 

!.=0.005 m2/s, ~=1.7 km2. 

Wheeler Ranch------------- 26 6.0 .25 .043±0.016 .010 0.6 2 wells (26"C), 61 m deep; no chemical 

data, !.=0.005 m2/s, ~=1 km2. 

NEW MEXICO 

Basin and Range pro vi nee 

I sol a ted systems---------- 1. 78±0.177 0.45 185 26 systems. 

Gil a Hot Springs---------- 47 63 .25 1.30±0.61 .092 40 Springs and shall ow wells (27" -68"C); 

TDS (1), 1 well (32"Cl 183m deep, 

discharges Na-Ca-S04 type water with 

2,400 mg/L TOS, T=0.005 m? /s, 

~=1.4 km2. -

Colorado Plateaus province 

Isolated systems---------- .55±0.084 .135 43 12 systems. 

Rio Grande Rift pro vi nee 

I sol a ted systems---------- 1.93±0.21 .48 210 27 systems. 

Laguna Puebl 0------------- 27 18.3 .25 .139±0.050 .025 2.3 Saline springs ( 20" -30"C l; no detai 1 ed 

chemical analyses, !.=0.005 m2/s, 

~=1 l:m2. 

San Ysidro---------------- 43 65 .25 1.20±0.52 .128 52 Springs (20"-30"Cl and wells (maximum 6l"C) 

585 m deep; TDS (1,2,3), !.=0.005 m2/s, 

~=1.19 l:m2. 

Socorro Canyon------------ 38 17.7 .25 .27±0.152 .058 21 Springs in galleries (32"-43"Cl; TOS (2), 

!.=0.005 m2/s, ~=1 km2. Warm wells 

north of Socorro are barely nonthermal 

by our temperature-depth criterion. 

Soda Dam------------------ 75 6.0 .25 .23±0.093 .055 29 Springs (48"-SO"C), water is probably 

leaking from the Valles Caldera hightemperature system; TOS (2), 

!.=0.005 m2/s, ~=1 km2. 

OREGON 

Basin and Range province 

I sol a ted systems---------- 1.34±0.163 0.33 132 22 systems. 

Klamath Basin------------- 30 117 .25 1.14±0.66 .062 13.2 Wells (21"-90"C), 25 to 344m deep: 2 wells 

(90"Cl 127m deep, ( 22"C) 344 m deep; 

TDS (1), !.=0.005 m2/s, ~=1.8 km2. 

Klamath Fall s-Ol ene Gap--- 60 32 .25 .93±0.36 .122 60 Springs (65"-74"Cl and wells (21"-89"C), 

to 550 m deep; TDS (1), T=0.005 m2/s, 

~=1.07 km2. -

Klamath Hi 11 s------------- 53 20 .25 .51±0.23 .092 43 Wells (22" -93"C l, temperatures do not 

change uniformly with depth, 2 wells 

(93"C) 86 m deep, (30"Cl 235m deep; 

TDS (1), !.=0.005 m2/s, !w=l km2. 

Warner Valley------------- 58 40 .25 1.13±0.63 .104 51 20 springs and wells (20"-93"C), depths to 

285m; TDS (1), !.=0.005 m2fs, 

~=1.14 km2. 

Cascade Range pro vi nee 

I sol a ted systems---------- 1.82±0.23 .45 230 15 systems. 

Columbia Plateaus province 

I sol a ted systems---------- .95±0.155 .24 107 ll systems. 

Cove---------------------- 43 7.3 .25 .135±0.052 .032 13.0 Springs and wells (20"-42"C); TDS (1 ), 

!.=0.005 m2/s, ~=1 km2. 

Hot Lake------------------ 67 7.3 .25 .25±0.097 .058 23 Springs (30"-BO"Cl and wells (25" -82"C): 

no 'depth data; TOS (1 l, !.=0.005 m2/s, 

~=1 km2. 

La Grande----------------- 71 13.7 .25 .50±0.21 .101 53 Wells (26"-28"Cl, no depth data; TDS (1), 

!.=0.005 m2/s, ~=1 km2. 

Rhinehart- Imb 1 er---------- 55 11.0 .25 .29±0.128 .062 30 Wells (22"-3l"C), no depth data; TDS (1), 

!.=0.005 m2/s, ~=1 km2. 

48 
Mean Mean Mean Mean 

Geologic province reservoir reservoir reservoir accessible Resource Beneficial 

Category Comments 

Geothermal area temperature area thickness resource base (lol8J 1 Heat 

("C) (km2l (km) (1018 J) (MWt for 30 yr) 

Oregon Plateaus province 

I sol a ted systems---------- 2.5±().22 0.63 260 35 systems. 

Baker Spring-------------- 45 7.7 .25 .151±0.059 .036 15.1 2 springs (21 •c) and 1 well (23"C); 

TDS (1), .!.=0.005 m2/s, !w=l km2. 

Burns--------------------- 52 14.0 .25 .34±0.145 .066 31 Wells (25"-30"C), some are artesian, 

depths about 30 m; T=O.OOS m2/s, 

!_w=l km2. -

Powell Buttes------------- 45 63 .25 1.24±0.55 .040 17.0 Marginal well temperatures (20"-32°C), no 

depth data; TDS (1 ), T=0.0025 m2/s, 

!_w=2.55 km2. -

Southeast Harney Lake----- 55 30 .25 . 78±0.32 .068 32 Springs (2o•-5l.C), possibly associated 

with high-temperature system; TDS (2), 

.!.=0.005 m2/s, !w=l.08 km2. 

Warm Springs Valley------- 55 27 .25 . 70±0.32 .104 50 Springs and artesian wells (2o·-26°C), 

on northeast side of Harney Lake; 

TDS (1), .!.=0.005 m2/s, !,w=l.03 km2. 

Western Snake River Plain province 

I sol a ted systems---------- 1 • 25±0.082 .062 31 2 systems • 

Vale Well----------------- 40 4.3 .25 .070±().024 .017 6.3 2 wells (36• and 40.C), 212 m deep: 

possibly associated with a hightemperature system; TDS (1), 

.!.=0.005 m2/s, !w=l km2. 

TEXAS 

Rio Grande Rift pro vi nee 

I sol a ted systems---------- 1 .27±0. 174 0.28 133 12 systems. 

UTAH 

Basin and Range provi nee 

I sol a ted systems---------- 4.4±0.29 1.11 400 80 systems. 

Central Bear Valley------- 38 7.3 .25 .111 ±0.046 .025 9.1 2 wells (25• and 28.C), 91 and 155 m deep, 

near Great Salt Lake; TDS (3), 

.!.=0.005 m2/s, !w=l km2. 

Crater-Saratoga Hot Springs 43 14.0 .25 .26±0.081 .030 12.4 2 springs (32• and 43•c1 and 7 wells 

( 21 • -35 •c), 15 to 27 m deep; TDS (2) • 

.!.=0.0025 m2/s, !w=l km2. 

Crystal Hot Spring-------- 55 5.3 .25 .139±().056 .033 15.5 spring (58.C) and 1 well (28"C) 

251 m deep; TDS (2), .!.=0.005 m2/s, 

!w=l km2. 

Delta--------------------- 25 633 .25 4.1±1.95 .055 0.0 Marginally thermal wells (22"-29"C), 

91 to 264m deep; TDS (1,2), 

.!.=0.005 m2/s, !w=7. 1 km2. 

Deseret Livestock--------- 23 5. 7 .25 .028±0.009 .007 o.o 3 springs (22"-24"C); TDS (1,2), 

.!.=0.005 m2/s, !w=l km2. 

Goshen Valley------------- 50 11.7 .25 .26±0.139 .035 15.9 2 wells (23" and 26"C), no depth data; 

TDS (1), .!.=0.0025 m2/s, !w=l km2. 

Granger-Mud Flats--------- 25 100 .25 .65±0. 22 .029 o.o Springs (29"C) and wells (26"-28"C), 114 to 

194m deep; TDS (1,2,3), T=O.OOS m2/s, 

!w=l.63 km2. -

Kaysvi 11 e-Farmi ngton------ 23 18.7 .25 .101 ±().058 .022 0.0 Marginally thermal wells (24"-29"C), to 

372m deep; TDS (1), T=0.005 m2/s, 

!_w=l km2. -

Kennecott-Asarco Wells---- 30 6.0 .25 .058±0.021 .014 2.9 6 wells (25" -31 •c), no depth data; 

TDS (3), .!.=0.005 m2/s, !w=l km2. 

Meadow-Hatton Hot Springs- 48 15.0 .25 .32±0.134 .059 26 5 springs (22"-4l"C) and 1 well (67"C) 

27m deep; TDS (2), .!.=0.005 m2/s, 

!_w=l km2. 

Mi 1 ford------------------- 33 48 .25 .58±0. 29 .044 12.8 Area of marginally warm wells (to 26"C), 

approximately 20 km southwest of 

Roosevelt Hot Springs high-temperature 

system; .!.=0.005 m2/s, !w=l.39 km2. 

Newcastle----------------- 52 20 .25 .48±0.26 .112 52 3 wells (23" -95"C), 76 to 152 m deep; 

.!.=0.01 m2/s, !. =1 km2. 

49 
Mean Mean Mean Mean 

Geo 1 ogi c pro vi nee reservoir reservoir reservoir accessible Resource Beneficial 

Category Co11111ents 

Geothennal area temperature area thickness resource base nol8Jl Heat 

("C) (km2l (kml (1018 J) (MWt for 30 yr) 

North Salt Lake City------ 37 53 0.25 0. 75±0.40 0.077 26 Springs (55"Cl and 7 wells (21"-42"C), 35 

to 305m ~eep; TDS (1,2,~), 

.!.=0.005 m /s, !w=l.l7 km • 

Ogden Flats--------------- 30 350 .25 3 .4±1.65 .031 6.6 Marginally thermal wells ( 20" -25"C l, to 

358m deep; TDS (1), T=0.0025 m2/s, 

!w=l0.3 km2. -

Southern Cache Valley----- 27 270 .25 2.0±1.22 .033 3.0 Marginally thennal wells from (2l"Cl 50 m 

deep to (49"C) 1,580 m deep; TDS (1 l, 

.!.=0.0025 m2/s, !w=5.8 km2. 

Tule Valley--------------- 30 28.3 .25 .28±0.138 .050 10.6 5 springs (25"-28"Cl and 1 well (3l"C) 

13m deep; TDS (2), T=0.005 m2/s, 

~=1 km2. -

Wendover (northeast)------ 28 15.0 .25 .130±0.044 .031 4.8 4 wells (24"-28"C}, 51 to 90 m deep; 

TDS (2), .!.=0.01 m2/s, ~=1 km2. 

Colorado Plateaus pro vi nee 

Isolated systems---------- .65±0.083 .163 35 17 systems. 

Ashley Valley------------- 45 18.3 .25 .36±0.125 .065 28 irrigation well (20"Cl and 3 wells 

(44"-49"C), 1.3 km deep; TDS (1,2), 

.!.=0.005 m2/s, ~=1 km2. 

Middle Rocky Mountains province 

Isolated systems---------- • 045±0. 0132 .0111 0.41 2 systems. 

Midway-------------------- 52 10.0 .25 .24±0.096 .052 24 2 springs (39"C}; TDS (2), .!.=0.005 m2/s, 

~=1 lcm2. 

WASHINGTON 

Cascade Range pro vi nee 

Isolated systems---------- 1.01±0.129 0.24 97 15 systems. 

Co 1 umbi a Plateaus pro vi nee 

Connell-Cunningham-------- 39 700 .15 6. 5±2 .4 .124 45 10 wells (25" -47"C l, 300 to 760 m deep; 

.!.=0.005 m2/s, ~=8.2 km2. 

Ephrata------------------- 30 280 .27 2.9±1.17 .046 9.7 5 wells (25"-30"C}, 140 to 360m deep; 

.!.=0.005 m2/s, ~=5.7 km2. 

Hanford------------------- 72 700 .27 28±9.6 .30 155 Several test wells (60"-75"Cl, 800 to 1,700 

m deep; .!.=0.005 m2/s, ~=8.2 km2. 

Moses Lake---------------- 33 1,000 .25 ll.9;t3.7 .093 27 5 wells (28"-35"C}, 150 to 300m deep, and 

1 oil test well (66"C l 1,343 m 

deep; .!.=0.005 m2/s, ~=12 km2. 

Odessa-------------------- 33 280 .25 3.4±1.20 .056 16.2 4 wells (28"-35"C}, 100 to 300m deep; 

.!.=0.005 m2/s, ~=5.7 km2. 

Othell 0------------------- 48 280 .27 6.5±2.8 .101 45 5 wells (25"-54"C}, 300 to 925 m deep; 

.!.=0.005 m2/s, ~=5.7 km2. 

Walla Walla River--------- 39 320 .25 4.9;tl.76 .073 27 spring (22"C l and 7 wells (25" -4l"C}, 

250 to 470 m deep; T=O.OOS m2/s, 

~=6.3 km2. -

Yakima-------------------- 34 930 .25 11.3±3.8 .089 26 More than 15 wells (25" -36"C ), 300 to 480 m 

deep; .!.=0.005 m2/s, ~=12 km2. 

Olympic Mountains pro vi nee 

I sol a ted systems---------- • 29±0.090 2 systems • 

WYOMING 

Middle Rocky Mountains province 

I so 1 a ted systems---------- 0.89±0.098 0.22 90 14 systems. 

50 
Low-Temperature 

Central and 

Geothermal Resources in the 

Eastern United States 

By Michael L. Sorey, Marshall J. Reed, Duncan Foley1, and Joel L. Renner2 

CONTENTS 

Abstract -------------------------- Introduction ---------------------- Low-temperature geothermal-resource 

areas in the Central United 

States -------------------------- Low-temperature geothermal-resource 

areas in the Eastern United 

States -------------------------- Atlantic Coastal Plain --------

Appalachian Mountains ---------

Sedimentary basins ------------ Calculations of accessible resource 

base, resource, and beneficial 

heat ---------------------------- Estimates of identified accessible 

resource base, resource, and 

beneficial heat ----------------- Undiscovered geothermal resources -

Acknowledgments -------------------

References cited ------------------

ABSTRACT 

Page 

51 

51 

54 

55 

55 

56 

56 

56 

57 

63 

63 

64 

Identified low-temperature (less than 90°C) 

geothermal resources in the Central and Eastern 

United States occur primarily in regional aquifers 

within sedimentary basins in the Great Plains and in 

aquifers beneath the Gulf and Atlantic Coastal 

Plains. Additional resources occur in hydrothermalconvection systems associated with thermal springs in 

the Wyoming Basins and Ouachita geologic provinces 

and in the Appalachian Mountains. The total 

1University of Utah Research Institute, Earth 

Science Laboratory, Salt Lake City, Utah. 

2Gruy Federal, Inc., Arlington, Virginia; currently 

with Ernest K. Lehmann and Associates, Inc., 

Minneapolis, Minnesota. 

51 

accessible resource base for identified lowtemperature geothermal reservoirs in the C~¥tral and 

Eastern United States is estimated at 27x10 J. The 

total identified resource is estimated at 55x1o18 J, 

from which 28 GW1 for 30 years of beneficial heat 

could be obtained. Estimates of the total undiscovered 

accessible 2Isource bai~ resource, and beneficial heat are 6.7x10 J, 18x10 J, and 9.2 GWt for 30 years, 

respectively. 

INTRODUCTION 

Within the Central and Eastern United States, 

low-temperature geothermal· resources occur in 

various geologic settings. The central region, as outlined in figure 14, is bordered on the east by the 

Mississippi River and on the west by the Rocky 

Mountains. The Wyoming Basins geologic province is 

included in the central region because of the 

occurrence of geothermal resources within several 

sedimentary basins. The eastern region (fig. 15) lies 

east of the Mississippi River. Figure 2 shows the 

geologic provinces within these two regions. 

This chapter provides a discussion of the general 

geothermal settings and the criteria used to identify 

low-temperature geothermal-resource areas in the 

Central and Eastern United States, and estimates the 

accessible resource base, resource, and beneficial heat 

for each geothermal reservoir. The methods used to 

obtain these estimates are discussed by Sorey, 

Nathenson, and Smith (this volume), and additional 

details are given here. Energy totals are presented for 

identified low-temperature geothermal reservoirs in 

hydrothermal-convectionand conduction-dominated 

systems over three ranges of reservoir temperature, as 

well as estimates of the undiscovered geothermal resource in various geologic provinces. Selected 

references to the maps and reports that provided the 

information utilized in this assssment are given by 

Reed ("Introduction," this volume). The stratigraphic 

nomenclature used in this chapter may not necessarily 

be that adopted by the U.S. Geological Survey. 
40° 

30° 

0 

110° 

Carbonate aquifer 

Area with temperature 

above or below minimum-temperature 

criterion 

Area with undiscovered low-temperature 

geothermal reSources 

o Resource area less than 100 km2 

• Resource area with thermal spring 

100° 

0 250 500 KILOMETERS 

Figure 14.-ldentified low-temperature geothermal-resource areas and areas containing undiscovered geothermal resources in the Central United States. Numbers refer to individual geothermal-resource 

areas listed in table 7; for sedimentary basins, numbers are positioned near deepest part of each basin. 

52 
soo 

• 

0 

0 250 500 KILOMETERS 

EXPLANATION 

Resource area less than 100 km2 

Resource area with thermal spring 

Area with undiscovered lowtemperature geothermal resources 

Deep sedimentary basin with low 

thermal gradient 

Figure 15.-Identified low-temperature geothermal-resource areas and areas containing undiscovered geothermal resources in the Eastern United States. Numbers refer to individual geothermal-resource areas listed in table 8. 

53 
LOW-TEMPERATURE GEOTHERMAL-RESOURCE 

AREAS IN THE CENTRAL UNITED STATES 

The categories of low-temperature geothermalresource areas identified in this assessment are discussed by Sorey, Nathenson, and Smith (this 

volume). Isolated thermal springs and wells (category 

1, table 3) must meet the criterion that the estimated 

reservoir temperature ·be less than 90°C and at least 

10°C above the local mean annual air temperature. 

For identified low-temperature geothermal-resource 

areas within sedimentary basins (category 3) and 

beneath coastal plains (category 4), a permeable 

reservoir with a temperature less than 90°C and 

greater than the minimum-temperature criterion given 

by Reed ("Introduction," this volume) must exist. Lowtemperature geothermal-resource areas in each of 

these three categories occur in the Central United 

States. 

The stratigraphic and hydrologic conditions 

within sedimentary basins of the Great Plains geologic 

province are complex. In most areas, water wells 

produce from numerous formations between the Upper 

Cretaceous shale and crystalline basement. For the 

purposes of this assessment, however, one or two 

regional aquifers are identified within each basin; 

these aquifers are considered to be a combination of 

one or more permeable formations that are 

hydraulically connected. Identified aquifers in 

Cretaceous rocks are denoted as sandstone of the 

Dakota Group, and in Paleozoic rocks as carbonates 

(limestone and dolomite) of the Madison Group or 

Arbuckle Group, except in the Bighorn and Wind River 

Basins in Wyoming, where we have identified 

Pensylvanian sandstone of the Tensleep Formation as 

the reservoir rock because of the relative abundance 

of data on its thermal, hydrologic, and chemical 

characteristics. Most of these data were collected 

during oil and gas exploration, and present-day 

production of thermal water from these sedimentary 

basins is used primarily in secondary oil recovery. 

Selection of particular basins and related areas 

of thick sedimentary accumulations for geothermalresource assessment was based primarily on evidence 

for temperature gradients greater than 25°C/km above 

the Lower Cretaceous and Paleozoic aquifers. 

Wherever possible, measured temperature gradients 

were used along with bottom-hole temperature data 

and lithologic information to assign average 

temperature gradients and to estimate aquifer 

temperatures in each area. In most areas, reservoir 

temperatures were assumed to increase linearly with 

depth, and a depth range over which reservoir 

temperatures meet the minimum-temperature 

criterion was established. For example, if the average 

vertical temperature gradient above a dipping aquifer 

is 35°C/km and the mean annual air temperature is 

10°C, low-temperature geothermal resources would be 

assessed in this aquifer at temperatures of 45° to 90°C 

between depths of 1 and 2.3 km. Areas in which 

aquifer temperatures exceed 90°C at greater depths 

exist in most of our identified sedimentary basins. 

Within much of the northern Great Plains 

provincA in Montana, Wyoming, North Dakota, and 

South Dakota, identified low-temperature geothermal 

54 

resources exist in the Dakota and Madison aquifers at 

depths between approximately 0.5 and 3 km. These 

regional aquifer systems are recharged primarily in 

outcrop areas in the Black Hills and the Bighorn 

Mountains, and flow is continuous bet ween the Powder 

River, Kennedy, and Williston basins. Hydraulic heads 

substantially above the land surface allow flowing 

wells to be completed in the Madison and Dakota 

aquifers; in parts of Montana and North Dakota, heads 

in the Madison aquifer are as much as 300 m above 

land surface. Although salinity generally increases 

with depth over most of the geothermal-resource area, 

the contents of total dissolved solids are less than 

10,000 mg/L. In addition to usage in secondary oil 

recovery and potential usage for coal-slurry 

transmission, Madison aquifer water is currently being 

produced for geothermal applications at several places 

in South Dakota (for example, Martinez, 1981). 

Abnormally high temperature gradients and heat 

flow exist in the Denver basin in western Nebraska and 

the Kennedy basin in South Dakota and north- central 

Nebraska. These conditions could reflect convective 

heat transfer associated with updip flow within ·a 

single aquifer and leakage of warm water from a 

deeper aquifer into a shallower one. Within the 

Denver basin in western Nebraska, temperature 

gradients as high as 60°C/km above the Dakota ~quifer 

and conductive heat flows as great as 85 m W /m have 

been measured (Gosnold and Everson, 1981). 

Theoretical analyses of heat and fluid flow suggest 

that ground water flowing updip in the Dakota aquifer 

at rates of about 1 m/yr could generate the measured 

thermal regime west of the Chadron-Cambridge arch. 

In north-central Nebraska and south-central South 

Dakota (east of the Chadron-Cambridge arch in the 

Kennedy basin), measured gradients above the Dakota 

aquifer exceed2

100°C/km, and conductive heat flows 

of 100 m W /m and higher have been determined. 

These data suggest that, in some parts of this area, 

warm water is leaking from the Madison into the 

Dakota aquifer through a subcrop connection (Schoon 

and McGregor, 1974; Gosnold and Everson, 1981). 

In western Wyoming, identified low-temperature 

geothermal-resource areas are associated with both 

hot springs and sedimentary basins. Hot springs occur 

adjacent to major uplifts, in many places along small 

anticlinal folds on the flanks of the bordering mountain 

blocks (Breckenridge and Hinckley, 1978). In the 

Bighorn Basin, vertical flow over such anticlinal 

structures results in measured temperature gradients 

of 30° to 50°C/km above the Tensleep aquifer. In 

southern Wyoming, geothermal-resource areas are 

associated with regional uplifts, such as the Rock 

Springs and Rawlins uplifts; over most of the adjacent 

basins, depths to aquifers in Cretaceous and older 

sedimentar~ rocks are so great that the temperature 

exceeds 90 C. 

In parts of Kansas and Oklahoma, geothermal 

resources were assessed within carbonate rocks of the 

Arbuckle Group of Cambrian and Ordovician age. 

Identified areas occur within the Anadarko and 

Hugoton basins and along the Nemaha ridge, a major 

crustal fracture zone along which relatively high 

temperature gradients have been measured (fig. 4; 

Blackwell and Steele, 1981). The Arbuckle Group 
contains significant oil and gas reserves and has been 

used as a reservoir to dispose of industrial liquid waste 

and saline water produced in conjunction with 

hydrocarbon production (U.S. Environmental 

Protection Agency, 197 4). The Arbuckle contains no 

freshwater within the low-temperature geothermalresource area outlined in figure 14. 

Along the Balcones-Ouachita structural trend, as outlined by Woodruff and Caran (1981) in central 

Texas, the buried Ouachita Mountains are covered by a 

southeastward-thickening wedge of Mesozoic and 

Cenozoic sedimentary rocks. Cretaceous rocks form 

major stratigraphically controlled thermal aquifers 

(Woodruff and McBride, 1979). Two thermal regimes 

are postulated along the Balcones-Ouachita trend: A 

cooler, fresher water regime resulting from water 

moving downdip from recharge zones west of the 

thermal areas, and a warmer, more saline regime 

controlled by updip migration of water from deeper 

parts of the Gulf of Mexico coastal embayment. Five 

counties were identified as having low-temperature 

geothermal resources in Cretaceous sandstone 

aquifers. Additional resources may exist along the 

Balcones-Ouachita trend, but the data do not permit 

their delineation as identified low-temperature 

geothermal-resource areas. 

In western Arkansas, identified low-temperature 

geothermal resources occur in areas of thermal springs 

and within coastal-plain sediment. Thermal springs in 

the Ouachita province, including those at Hot Springs 

National Park and Caddo Gap, are associated with 

tightly folded and thrust-faulted rocks. Studies by 

Bedinger and others (1979) and Steele and Wagner 

(1981) indicated that chemical compositions are similar in all the springs in this province and suggested 

that circulation systems feeding the springs occur 

largely in silica-rich sandstone and chert formations. 

Little is known, however, about the configuration of 

associated low-temperature geothermal reservoirs in 

these areas. In southwestern Arkansas, brines 

containing more than 4,000 mg/L b~omide ion are present over an area of about 3,000 km (Collins, 1974; 

Carpenter and Trout, 1978). Wells in this area produce 

hypersaline water from a highly permeable 20-m-thick 

oolitic limestone near the top of the Jurassic 

Smackover Formation. Low-temperature geothermal 

resources have been identified in seven fields 

containing brines with temperatures between 70° and 

90°C at depths of 1.7 to 2.1 km. The Smackover 

Formation dips south, and in northern Louisiana and 

eastern Texas, temperatures in the formation are 

above our upper limit of 90°C. 

In other parts of the Central United States, 

regional aquifers are known to exist within 

sedimentary basins or beneath coastal plains but were not identified as containing low-temperature 

geothermal resources. In such areas, either the depth 

of the aquifer is too shallow, or the corresponding 

temperature gradient is less than 25°C/km. Areas of 

relatively low thermal gradient (see fig. 5) occur in 

western Texas, Minnesota, Iowa, and Missouri. Lowtemperature geothermal-resource areas may exist 

within the Gulf of Mexico coastal embayment in 

southeastern Texas and Louisiana, where temperature 

gradients are generally greater than 25°C/km and 

55 

upward leakage of higher temperature brines from 

under lying geopressured zones may be occurring, as 

suggested by Sammel (1979). Because of the absence 

of data on specific reservoirs, this area is assumed to 

contain undiscovered low-temperature. geothermal resources. 

LOW-TEMPERATURE GEOTHERMAL-RESOURCE 

AREAS IN THE EASTERN UNITED STATES 

Most of the Eastern United States is 

characterized by lower than average heat flow and by 

rocks of moderate to high thermal conductivity 

(crystalline rocks and quartz-rich sedimentary rocks). 

This combination of features limits the areas where 

low-temperature geothermal resources are likely to be 

found. As shown in figure 15, thermal waters meeting 

our minimum-temperature criterion were identified in 

two geologic settings-the Appalachian Mountains (in 

the Valley and Ridge, Blue Ridge, Piedmont, and New 

England geologic provinces) and the Atlantic Coastal 

Plain. The existing data base is small owing to limited 

exploration; future exploration may result in the 

identification of additional low-temperature 

geothermal resources in the Eastern United States. 

Atlantic Coastal Plain 

A sequence of nearly flat lying sedimentary 

rocks, ranging in age from Late Jurassic to Holocene, 

overlies metamorphic and igneous basement rocks 

beneath the Atlantic Coastal Plain (Brown and others, 

1972, 1979). The sedimentary sequence generally 

thickens to the east, and its greatest thickness lies off 

shore. In the course of exploration for geothermal 

resources, more than 50 holes, drilled to depths of at 

least 300 m, have been used to determine heat flow 

and temperature gradient along the Atlantic Coastal 

Plain from New Jersey to Georgia (Lambiase and 

others, 1980). These widely spaced holes indicate that 

heat transfer through the sedimentary section is 

dominated by conduction and that convective heat 

transfer is restricted to near-surface zones of 

moderate vertical permeability. The available 

information was used to calculate the temperature at 

the top of bedrock (Lambiase and others, 1980), in 

order to determine areas that meet our minimumtemperature criterion. These areas have a thickness 

of sedimentary rock greater than 1 km ~d a moderate 

heat flow in the range 63-79 m W /m • At depths 

greater than 1 km, Cretaceous marine and continental 

sedimentary rocks with a significant shale content and 

a low thermal conductivity provide an increased 

temperature gradient. Water-saturated discontinuous 

sand lenses within these Lower Cretaceous 

sedimentary rocks are potential geothermal reservoirs. 

Along the Atlantic Coastal Plain, four areas in 

Virginia and two areas in North Carolina have been 

identified as containing low-temperature geothermal 

resources. In each area, temperature gradients exceed 

30°C/km, and the reservoir size was estimated from 

the inferred lithology of the basement rocks. Gravity 

and magnetic maps of the Atlantic Coastal Plain 

indicate that silicic plutonic rocks may underlie the 
geothermal-resource areas, and Costain and others 

(1980) suggested that radiogenic heat production in 

these rocks is the source of the anomalous 

temperature gradients and heat flow. On the basis of 

this model, jhe areas in Virginia were estimated to 

covfr 50 km each, and the areas in North Carolina 90 

km each. Schubert and Johnson (1980) suggested that 

geothermal-resource areas in this part of the Coastal 

Plain are larger and not restricted to zones over 

radiogenic plutons; however, evidence -of lower heat 

flow and temperature gradients in wells between the 

identified areas indicate that the geothermal-resource 

areas are not regional in scale. 

Appalachian Mountains 

Thermal springs in the Eastern United States are associated with fault and fracture zones in several 

provinces of the Appalachian Mountains. Early 

descriptions of these springs dealt with their 

therapeutic and recreational values (Moorman, 1867; 

Crook, 1899; Fitch, 1927). Despite some inaccuracies 

in reported spring locations and characteristics, 

comparisons of these descriptions with more recent 

data indicate some changes in flow rate and 

temperature over time. Recent chemical analyses, 

available for most major springs in the Appalachian 

Mountains (Hobba and others, 1976, 1979), were used in 

this assessment. 

The locations of these springs are controlled 

mostly by the structural setting and, to a lesser 

extent, by lithology. The springs occur in areas of 

steeply dipping folded rocks that are transected by 

nearly vertical east-west-trending fracture zones. 

Correlation of springs with topographic lows, or gaps, 

apparently results from the fact that easily eroded 

areas correspond to zones containing many fractures, 

which, in turn, provide the increased vertical 

permeability needed to establish a hydrothermalconvection system. The warm springs in the 

Appalachians that were considered in this assessment 

issue from sandstone or carbonate rocks exposed in the 

steeply dipping limbs of anticlinal folds (Hobba and 

others, 1979). Chemical analyses of the warm-spring 

waters issuing from carbonate rocks exhibit 

consistently low concentrations of dissolved silica and 

high concentrations of magnesium and calcium, which 

indicate that the flow of warm water is restricted to 

the carbonate rocks. Analyses of waters from springs 

issuing from fractured sandstone show higher 

concentrations of dissolved silica and lower 

concentrations of magnesium and calcium, which 

indicate that flow is restricted to the sandstone beds. 

Geochemical considerations suggest that 

reservoir temperatures are not substantially higher 

than the measured surface temperatures at most 

eastern thermal springs; observed temperatures range 

from 18° to 41 °C. The occurrence of these springs in 

areas of average heat flow and relatively low 

temperature gradients (Costain and others, 1976; Perry 

and others, 1979) indicates that the depths of 

hydrothermal circulation are generally between t and 

3 km. A standard reservoir volume of 1 km was assumed for each spring because of an absence of data 

on actual reservoir configurations. 

56 

Sedimentary Basins 

Three large sedimentary basins (dashed lines, fig. 

15) in parts of the Appalachian basin (in the 

Appalachian Plateaus geologic province) centered in 

western Pennsylvania may contain low-temperature 

geothermal resources because the basin is deep and 

contains a thick sequence of Devonian shale of low 

thermal conductivity. Average measured temperature 

gradients to a depth of 2 km range from 25° to 

32°C/km in the region (fig. 5), although data are 

unavailable with which to delineate productive 

aquifers beneath the Devonian shale. Consequently, 

only undiscovered geothermal resources are estimated 

here for the Appalachian basin. 

In the Michigan and Illinois basins, temperature 

gradients measured in deep wells are less than 

25°C/km. Gradients greater than 30°C/km reported 

for some wells less than 1 km deep appear to be in 

error (Vaught, 1980a, b). The relatively low gradients 

in these basins apparently reflect low heat flow and 

the absence of thick accumulations of sediment of low 

thermal conductivity. Even though future exploration 

or adoption of a minimum-temperature criterion 

different from that used in this assessment could 

result in identification of aquifers in these basins 

containing low-temperature waters acceptable for 

energy development, no identified or undiscovered 

low-temperature geothermal-resource areas in these 

basins are included in this assessment. 

CALCULATIONS OF ACCESSIBLE RESOURCE BASE, 

RESOURCE, AND BENEFICIAL HEAT 

The mean accessible resource base for each 

identified low-temperature geothe~mal reservoir in the 

Central and Eastern United States was calculated from 

equation 2 of Sorey, Nathenson, and Smith (this 

volume). Estimates of the minimum, maximum, and 

most likely values for reservoir area, thickness, and 

temperature define the probability densities and mean 

values for these quantities, and the mean and standard 

deviation for the accessible resource base. For lowtemperature geothermal-resource areas in category 1 

(table 3), which includes isolated thermal springs in the 

northern Great Plains, Wyoming Basins, and Ouachita 

provinces and the Appala::rian Mountains, a mean reservoir volume of 1.0 km was assumed. For lowtemperature geothermal-resource areas within large 

sedimentary basins (category 3), variations in 

temperature, salinity, and hydrologic properties 

necessitated division of the total reservoir areas into 

subareas to improve the accuracy in estimating stored 

and recoverable thermal energy. 

Methods used to· calculate the resource in each 

identified low-temperature geothermal reservoir are discussed by Sorey, Nathenson, and Smith (this 

volume). For low-temperature geothermal-resource 

areas in category 1, a recovery factor of 0.25 was 

applied to the accessible resource base to obtain the resource estimate. For identified low-temperature 

geothermal-resource areas in other categories, 

resource determinations involve estimates of the 

number of production wells each reservoir can support 
over a period of 30 years in the absence of fluid 

injection. A development plan is assumed, consisting 

of evenly spaced wells producing at 31.5 L/s for 30 

years, with a cumulative drawdown at the center of 

the reservoir of 152 m. The number of production 

wells is given by the reservoir area divided by the 

optimum area per well <.!!.w> for this development plan. 

Estimates of the mmimum, maximum, and most 

likely values of .!!.w were obtained from the wellspacing curves in figure 10, which relate optimum area 

per well to reservoir area and transmissivity. Most 

likely values of .!!.w correspond to estimates of the 

most likely values of transmissivity; minimum and 

maximum values of .!!.w correspond to estimates of the 

maximum and minimum values of transmissivity, 

respectively. Reservoirs in identified low-temperature 

geothermal-resource areas within sedimentary basins 

and beneath coastal plains are assumed to be bounded 

by leaky confining shale beds, for which the curves in 

figure 10 apply. 

Most of the data on the hydrologic properties of 

regional aquifers within the sedimentary basins 

assessed in this volume were obtained from reports 

prepared by the U.S. Geological Survey as part of a program of regional-aquifer-system analysis. 

However, the degree of detail and the accuracy of the 

hydrologic information available for different areas 

vary considerably. The best data set for sedimentary 

basins exists for the Madison and Dakota aquifers in 

the northern Great Plains, where a large fraction of our estimated low-temperature geothermal resource 

occurs (Woodward-Clyde Consultants, 1980; MacCary 

and others, 1981; Downey, 1982). In other sedimentary 

basins for which data on aquifer transmissivity and 

confining-bed properties are more limited, it was 

necessary to rely partly on comparisons with similar 

formations in the northern Great Plains to obtain 

estimates of the required hydrologic parameters. 

For aquifers in Cretaceous sand within the 

Balcones-Ouachita structural trend in Texas, adequate 

data exist from well tests to assign transmissivity 

values to each reservoir. For low-temperature 

geothermal-resource areas along the Atlantic Coastal 

Plain, results of well tests and laboratory 

measurements on cores from a 1-km-deep well drilled near Cambridge, Maryland (Harold Meisler, unpub. 

data, 1981), were used to assign a transmissivity value 

to each reservoir, assuming a composite reservoir 

thickness of 30 m in each area. 

For low-temperature geothermal-resource areas 

that were divided into subareas because of variations 

in reservoir properties, separate estimates of the 

accessible resource base, resource, and beneficial heat 

were obtained for each subarea. In addition to 

reservoir area and transmissivity, two factors 

influenced the choice of values of .!!.w for each resource area or subarea: (1) For subareas that are connected hydraulically to adjacent subareas, the total 

connected reservoir area was used with the curves in 

figure 10 to estimate .!!.w; (2) for subareas forming narrow bands around the margins of certain basins and 

bounded by permeable regions at temperatures above 

and below the minimum-temperature criterion, values 

of .!!.w were adjusted downward by a factor of 2 to 

reflect less interference between wells than would 

57 

occur in square reservoirs with the same area. Mean values of reservoir area, area per well, and 

temperature were used in equation 4 of Sorey, 

Nathenson, and Smith (this volume) to calculate the 

mean resource for each identified low-temperature 

geothermal-resource area. The correction factor (J9 in 

this equation accounts for the likelihood that these 

reservoirs are not uniformly permeable and may 

contain regions of low-permeability rock. Minimum, 

maximum, and most likely values of k are 0, 1.0, and 

0.5, respectively. An example that demonstrates the 

need to consider such a correction factor is provided 

by the results of drilling and testing wells completed in 

the Madison aquifer in the Powder River basin of 

Wyoming. Wells that penetrate cavities associated 

with the paleokarst system developed near the top of 

the limestone show relatively high transmissivity and 

production, whereas wells that do not intercept 

cavities show relatively low transmissivity and 

production (Kelly and others, 1981). 

The amount of a resource that can be utilized in 

applications on the surface is termed the "beneficial 

heat." Beneficial-heat estimates for each identified 

low-temperature geothermal reservoir were obtained 

from equations 5 and 6 of Sorey, Nathenson, and Smith 

(this volume). Equation 6 is based on an empirically 

derived relation between reservoir temperature and 

the temperature drop as energy is extracted from the 

geothermal water. The lower reservoir-temperature 

limit for beneficial-heat calculation is taken as 25°C 

for all resource areas. . Units of megawatts thermal for 30 years are used to report beneficial heat estimates, whereas both 

accessible r~source base and resource are reported in 

units of 10 J. /or comparison, 1 MWt for 30 years 

equals 9.461x101 J. The use of units of megawatts 

thermal for 30 years for beneficial heat facilitates 

comparisons with equivalent thermal-energy 

requirements from other fuels. 

ESTIMATES OF IDENTIFIED ACCESSIBLE 

RESOURCE BASE, RESOURCE, 

AND BENEFICIAL HEAT 

Tables 7 and 8 list the results of our quantitative 

assessment of low-temperature geothermal resources 

in the Central and Eastern United States. Estimates 

of reservoir characteristics and thermal energy. are 

listed by State; for each State, separate totals are 

given for areas within which a particular reservoir is 

continuous and for groupings of thermal springs within 

the same geologic province. For sedimentary basins 

that were subdivided into subareas, the area-weighted mean reservoir temperature is given along with the 

total mean reservoir area and the mean reservoir 

thickness; multiplication of these estimates yields the 

listed value for mean accessible resource base. For 

groupings of thermal springs, the range in mean 

reservoir temperature among the spring areas is listed, 

but no entry is made for reservoir area and thickne~ 

because a standard mean reservoir volume of 1.0 krn 

was assumed in each case. For all low-temperature 

geothermal-resource areas, the standard deviation for 

each estimate of the accessible resource base is 

specified. 
Table 7 .-Reservoir parameters and thermal energies of identified low-temperature geothermal resources in the 

Central United States 

[All means and standard deviations calculated analytically. Values are rounded to two significant figures or, if 

the first digit is 1, to three significant figures; this rounding represents a range of 0.5 to 5 percent in the 

accuracy of the total value. Map numbers refer to locations in figure 13. Categories of low-temperatur~geothermal-resource areas are listed in table 3. For areas in category 1, a mean reservoir volume of 1.0 km was 

assumed, and the resource was calculated as 25 percent of the accessible resource base. Resource and beneficial heat were calculated from equations 4 and 6 (Sorey, Nathenson, and Smith, this volume). Beneficial heat 

is assumed to be zero for reservoir temperatures less than or equal to 25, C. TDS, total dissolved solids: (1) 

Less than 1,000 mg/L; (2) 1,000 to 10,000 mg/L; (3) 10,000 to 50,000 mg/L; (4) more than 50,000 mg/L. _g_, 

flow rate; L reservoir transmissivity; G, average temperature gradient above reservoir; !.w' mean area per 

well] 

Geologic pro vi nee Map Category No. Geothennal area 

Ouachita provi nee 

18 I sol a ted systems--------

18 Hot Springs National Park 

Gulf Coastal Plain province 

19 Brine fields------------

Great Pl afns province 

14 Isolated systems--------

14 Washakie-Sand Wash basin 

Isolated systems--------

Canon City embayment----

Denver basin------------

Great Plains province 

Denver basin------------

8 Anadarko basin----------

9 Nemaha ridge 

Great Plains province 

North-central Montana--- Central Montana Wi 11 i ston basin Powder River basin 

1 North-central Montana---

2 Central Montana 3 Williston basin 

4 Powder River basin 

Mean 

reservoir 

temperature 

("C) 

89 

60 

40 

77 

40 

71 

67 

61 

Mean Mean Mean accessible 

reservoir' reservoir resource base area thickness (1018 J} 

(km2l (km} 

ARKANSAS 

0.151 t0.031 

.123±0.050 

60 .020 .23±0.01'5 

COLORADO 

0.32±0.090 

1,170 .152 21±4.3 

.183±0.052 

260 .091 1.54±0.38 

56,000 .091 660±81 

KANSAS 

2,900 0.091 17.4±3.0 

65,000 .173 1,640±260 

MONTANA 

177,000 0.37 8,900±530 

115,000 .091 1,260±163 

58 

Beneficial 

Resource Heat Comments (1018 J} (MWt for 30 yr) 

0.038 7.4 4 systems, springs ( 25" -35"C); 

TOS (1). 

l system, 47 spring orifices (58" -66"C ); !1,=40 l/s. 

.054 29 Penneable limestone reservoir in 

the Smackover Fonnatlon; 

TDS (4), ~=33"C/km. 

0.080 38 3 systems, springs (52"-58"C) in 

Washakie-Sand Wash basin; 

TDS (1,2), !1,=0.8-10 L/s. 

.124 61 Reservoir in sandstone of the 

Dakota aquifer; TDS (2,3), 

§.=30-35"C/km, !.=0.001 m2/s, 

aw=26 tm2. 

.046 16.5 3 systems, springs (25"-48"C) in 

Canon City embayment-Pueb 1 o 

area; TDS (2}, !l_=0.3-1.6 L/s. 

.024 9.2 Reservoir in sandstone of the 

Dakota aquifer; TDS (2}, 

G=45"C/km, T=0.001 m2ts, 

~=16.4 km2-:-

2.4 1,240 Reservoir in sandstone of the 

Dakota a qui fer; TDS ( 2, 3}, 

G=35"C/km, T=O.OOl m2/s, 

~=70 km2. -

0.064 24 Reservoir in sandstone of the 

Dakota aquifer; TDS {2,3}, 

G=45"C/km, T=O.OOl m2/s, 

~=70 lcm2. -

3.2 1,660 Reservoir in carbonate rocks of the 

Arbuckle aquifer; TDS (4}, 

§.=30-40"C/km, !,=0.001 m2ts. 

!l-1=70 km2. 

11.6 6,000 Reservoir in carbonate rocks of the 

Madison aquifer; TDS (2,3,4}, 

G=30-35"C/km, 

f=0.005-0.02 m2/s, !l-1=49 tm2. 

4.6 2,300 Reservoir in sandstone of the 

Dakota aquifer; TDS 12,3,4}, 

G =30-40"C/km, T=O.OOl m /s} 

~=70 km2, -
Map 

No. 

Geologic province 

Geothennal area 

Wyoming Basins pro vi nee 

Category 

10 Big Horn Basin----------

17 

Great Plains province 

Denver basin------------

Kennedy basin 

Great Plains province 

Williston basin---------

Williston basin---------

Great Plains province 

Anadarko basin----------

Nemaha ridge 

Great Plains province 

Williston basin---------

Kennedy basin 

Williston basin---------

Kennedy basin 

Isolated. systems--------

Gulf Coastal Plain province 

20 Hunt County-------------

21 Navarro County----------

22 Limestone County--------

23 Falls County------------

24 Caldwell County---------

Mean 

reservoir 

temperature 

("C) 

40 

50 

63 

62 

87 

51 

50 

52 

75 

51 

64 

63 

Mean 

reservoir 

area 

{km2 l 

620 

112,000 

128,000 

57,000 

62,000 

86,000 

55,000 

320 

1,170 

2,400 

1,230 

1,290 

Mean 

reservoir 

th.ickness 

(km) 

.122 

Mean accessible 

resource base 

(1018J) 

4.9±1.37 

NEBRASKA 

0.091 930±64 

NORTH DAKOTA 

0.366 5,800±470 

.091 628±75 

OKLAHOMA 

0.31 3,600±620 

SOUTH DAKOTA 

0.183 1 ,490±154 

.061 31 Ot57 

.083±0.021 

TEXAS 

0.046 1.39±0.30 

.091 16.7±2.6 

.076 17 .lt3.2 

.152 24±4.3 

.152 24t3. 7 

59 

Resource 

(1018 J) 

.037 

3.4 

7.5 

2.3 

3.9 

3.9 

1. 76 

.021 

0.033 

.098 

.093 

.176 

.182 

Beneficial 

Heat Comments 

(MWt for 30 yr) 

14 

1,550 

3,800 

1,150 

2,100 

1, 790 

800 

2.5 

15.3 

52 

42 

89 

92 

Reservoir in sandstone of the 

Tensl eep aquifer; TDS (2 l, 

G=35"C/km, T=O.OOl m2/s, 

!"w=26 km2. -

Reservoir in sandstone of the 

Dakota Group; TDS(2,3), 

G=45-80"C/km, T=O.OOl m2/s, 

!"w=70 km2. -

Reservoir in carbonate rocks of the 

Madison aquifer; TDS (3,4), 

G=30-35"C/km, 

!=0.005-0.02 m2/s, !w=49 km2. 

Reservoir in sands tone of the 

Dakota aquifer; TDS (2,3,4), 

G=30-40"C/km, T=O.OOl m2/s), 

~=70 km2. -

Reservoir in carbonate rocks of the 

Arbuckle aquifer; TDS (4), 

G=30-35"C/km, T=O.OOl m2/s, 

!"w=70 km2. -

Reservoir in carbonate rocks of the 

Madison a qui fer; TDS (1,2), 

G=30-40"C/km, 

!=0.005-0.01 m2/s, !w=49 km2. 

Reservoir in sandstone of the 

Dakota aquifer; TDS (2), 

G=35-80"C/km, T=O.OOl m2/s, 

!"w=54-70 km2. -

3 systems, springs (22"-3l"C) in 

the Black Hills; TDS (2), 

.Q.=57 -670 L/s. 

Reservoir in Woodbine Sands 

aquifer; TDS (1,2), 

G=37"C/km, T=O.OOl m2/s, 

~=22 km2. -

Reservof r in Woodbine Sands 

aquifer; TDS (1,2 l, 

G=48"C/km, T=O.OOl m2/s, 

!"w=44 km2. -

Reservoir in Woodbine Sands 

aquifer; TDS (1,2), 

G=43"C/km, T=O.OOl m2/s, 

~=57 km2. -

Reservoir in Houston-Trinity Sands 

aquifer; TDS (1,2 l, 

G=5l"C/km, T=0.003 m2/s, 

!"w=22 km2. -

Reservoir in Houston-Trinity Sands 

aquifer; TDS (1,2), 

G=43"C/km, T=0.003 m2/s, 

!"w=22 km2. -
Mean Mean Mean Mean accessible Beneficial Geologic provi nee Map reservoir reservoir reservoir Resource No. Category resource base nolB Jl Heat Co11111ents 

Geothennal area temperature area thickness (1018 J) (MWt for 30 yr) ("C) (km2l (kml 

WYOMING 

Great Plains province 

Isolated system---------

I sol a ted system---------

Powder River basin------ 69 27,000 .152 

Powder River basin------ 70 24,000 0.091 

Denver basin------------ 80 8,800 .091 

Wyoming Basins province 

10 I sol a ted systems--------

11 Isolated systems--------

13 I sol a ted systems--------

10 Big Horn Basin---------- 43 6,000 .122 

11 Wind River basin-------- 65 7,300 .122 

12 Rock Springs uplift----- 65 2,800 .152 

13 Rawlins uplift---------- 65 6,700 .152 14 Washakie-Sand Wash basin 15 Shirley Basin 

16 laramie basin 

In the "Comments" to tables 7 and 8, additional 

information used to make estimates of the accessible resource base and resource is listed. For lowtemperature geothermal-resource areas within 

sedimentary basins and beneath coastal plains, this 

information includes the formation name, the range in 

total dissolved solids, the range in assumed 

transmissivity, the range in average temperature 

gr.adient within the sedimentary section above each 

reservoir, and the range in mean values used for the area per well. For low-temperature geothermalresource areas in category 1, the number of springs 

and the range in flow rate are also listed. Most of the source data and the results of intermediate 

calculations have been given by Reed and others 

(1983). 

60 

0.071:t{).032 0.0178 7.2 1 system, spring (42"Cl in Powder River basin; TDS (1 ), 

.Q.=50 l/s. 

.035±0.0169 .0087 1.38 system, spring (28"Cl in Denver 

basin; TDS (1 l, .Q.=1.4 l/s •. 

580:t64 4.1 2,100 Reservoir in carbonate rocks of the 

Madison aquifer; TDS (1,2), 

G=30-35"C/km, 

!=0.005-0.02 m2/s, !w=27 km2. 

310±55 2.1 1,070 Reservoir in sandstone of the Dakota aquifer; TDS (2), 

§.=30-40"C/km, .!_=0.001 m2/s, 

!w=39 km2. 

136±23 .90 480 Reservoir in sandstone of the 

Dakota aquifer; TDS (2), 

G=35"C/km, T=O.OOl m2/s, 

~=39 km2. -

.33:t{).090 .083 33 5 systems, springs (24"-64"Cl in 

the Big Horn Basin, 

TDS (1,2), .Q.=25-270 l/s. 

.33:t{).071 .082 28 6 systems, spring (24"-53"C) in 

the Wind River basin; 

TDS (1,2), .Q.=O.J-35 l/s. 

.35:t{).l03 .086 42 3 systems, springs (31"-87"Cl 

near the Rawlins uplift and 

Shirley Basin; TDS (1,2), 

.Q.=6.3-140 L/s. 

53±6.5 .39 158 Reservoir in sandstone of the 

Tensleep aquifer; TDS (2), 

G=30-60"C/km, T=O.OOl m2/s, 

~=26 km2. -

115±29 .86 430 Reservoir in sandstone of the 

Tensleep aquifer; TDS (2), 

G=35"C/km, T=O.OOl m2/s), 

~=26 km2. -

56±12.4 .167 84 Reservoir in sandstone of the 

Dakota aquifer; TDS (2,3), 

G=35"C/km, T=O.OOl m2/s, 

~=52 km2. -

133±32 .53 270 Reservoir in sandstone of the 

Dakota aquifer; TDS ( 2, 3 l, 

G=35"C/km, T=O.OOl m2/s, 

~=40 km2.-

Mean values of the total resource and beneficial 

heat for each reservoir also represent summations over all subareas within each low-temperature geothermal- resource area. Corresponding standard deviations for 

the resource and beneficial heat are not listed because 

they were calculated for ·only a few individual 

reservoirs. Typical values of the standard deviations 

for the resource and beneficial heat in thermal-spring areas are 70 percent of the corresponding mean 

values. For low-temperature geothermal-resource 

areas within sedimentary basins and beneath coastal 

plains, the standard deviations for the resource and 

beneficial heat are about 50 percent of the 

corresponding means for undivided areas and 20 

percent of the corresponding means for areas that 

were subdivided. These calculations show that the 
Table B.-Reservoir parameters and thermal energies of identified low-temperature geothermal resources in the 

Eastern United States 

[All means and standard deviations calculated analytically. Values are rounded to two significant figures or, if 

the first digit is 1, to three significant figures; this rounding represents a range of 0.5 to 5 percent in the 

accuracy of the total value. Map numbers refer to locations in figure 13. Categories of low-temperatur~geothermal-resource areas are listed in table 3. For areas in category 1, a mean reservoir volume of 1.0 km was 

assumed, and the resource was calculated as 25 percent of the accessible resource base. Resource and beneficial heat were calculated from equations 4 and 6 (Sorey, Nathenson, and Smith, this volume). Beneficial heat 

is assumed to be zero for reservoir temperatures less than or equal to 25, C. TDS, total dissolved solids: (1) 

less than 1,000 mg/L; (2) 1,000 to 10,000 mg/L; (3) 10,000 to 50,000 mg/L; (4) more than 50,000 mg/L • .Q, fiow 

rate; L reservoir transmissivity; Q, average temperature gradient above reservoir; !.w' mean area per well] 

Map 

No. 

37 

31 

32 

36 

29 

30 

35 

25 

26 

27 

28 

Geologic province 

Category 

Geothennal area 

Piedmont pro vi nee 

Isolated system--------

New England Provi nee 

Isolated system--------

New England provi nee 

Isolated system--------

Blue Ridge provi nee 

Isolated system--------

Atlantic Coastal Plain province 

Stumpy Point-----------

Engl ehard--------------

Va 11 ey and Ridge pro vi nee 

I sol a ted systems-------

Atlantic Coastal Plain province 

Oak Hall---------------

Wallops Island---------

Tas 1 ey-----------------

Smith Point------------

Va 11 ey and Ridge pro vi nee 

Isolated systems-------

33 Northeast springs 

34 Southeast springs 

Mean 

reservoir 

temperature 

("C) 

81 

76 

66 

77 

61 

46 

Mean Mean Mean accessible 

reservoir reservoir resource base 

area thickness (1018 J) 

(km2 l (km) 

GEORGIA 

0.050±0.020 

MASSACHUSETTS 

0. 01 56±0. 0063 

NEW YORK 

0.0183±0.0074 

NORTH CAROLINA 

0.068±0.028 

90 .030 .47±0.112 

90 .030 .44±0.104 

VIRGINIA 

0.31±0.044 

50 .030 .20±0.049 

50 .030 .24±0.059 

50 .030 .181 ±0.044 

50 .030 .121 ±0.030 

WEST VIRGINIA 

0 .089±0.0162 

61 

Beneficial 

Resource 

(1018 J) Heat Co11111ents 

(MWt for 30 yr) 

0.0124 3. 7 1 system, spring (34"C); TDS (1), 

Jl=57 L/s. 

0.0039 0.0 system, spring (2l"C); 

TDS(l), 

0.0046 0.0 system, spring (22"C); TDS (1 ), 

Q.=6.7 L/s. 

0.0170 6.6 1 system, spring ( 44 "C); TDS (1 ) , 

Q=0.4 L/s 

.070 38 Reservoir in Lower Cretaceous 

sandstone aquifers; TDS (3), 

G=37"C/km, T=0.002 m2/s, 

~=5.2 km2.-

.065 35 Reservoir in Lower Cretaceous 

sandstone aquifers; TDS ( 3), 

G=37"C/km, T=0.002 m2/s, 

~=5.2 km2.-

0.077 13.2 10 systems, springs (22"-40"C), 

TOS (1), Jl=3.8-200 L/s. 

.047 24 Reservoir in Lower Cretaceous 

sandstone aquifers; TDS (3), 

Q_=3l"C/km, !_=0.002 m2/s, 

~=3.3 km2. 

.057 30 Reservoir in Lower Cretaceous 

sandstone aquifers; TDS (3), 

G=3l"C/km, T=0.002 m2/s, 

~=3.3 km2.-

.042 21 Reservoir in Lower Cretaceous 

sandstone aquifers; TDS (3), 

G=3l"C/km, T=0.002 m2/s, 

~=3.3 km2.-

.028 12.2 Reservoir in Lower Cretaceous 

sandstone aquifers; TDS (3}, 

G=3l"C/km, T=0.002 m2/s, 

~=3.3 km2.-

0.022 0.0 5 systems, springs (20" -23"C), 

TDS (1 ), Jl=6.3-100 L/s. 
standard deviations for resource and beneficial heat 

represent a larger fraction of the corresponding mean 

values than do those for the accessible resource base, 

a result that reflects the larger uncertainty in 

recoverable-energy estimates. This degree of 

uncertainty decreases, however, as the number of 

subareas or the number of individual thermal-spring areas grouped together increases. Similarly, the 

degree of uncertainty associated with the mean values 

of nationwide resource and beneficial-heat totals is 

significantly less than that associated with the mean 

values of these quantities for individual reservoirs. 

Resource estimates for low-temperature 

geothermal-resource areas with isolated thermfd 

springs or wells (category 1) average about 0.04x10 

J. For comparison, the spring-flow rate that would 

yield this amount of thermal energy over a 30-year 

period is 160 L/s (at 45°C). With few exceptions, total 

flow rates at individual spring areas in the Central and 

Eastern United States are much less than 160 L/s, and 

the resource estimates listed in tables 7 and 8 are 

larger than the energy that could be obtained by 

tapping only the natural spring flow. 

Resource estimates for sedimentary basins are 

significantly larger than corresponding estimates for 

other types of low-temperature geothermal-resource 

areas. For example, for the Madison a~~fer in eastern 

Montana, the total resource is 11.6x10 J. Although 

this value represents a recovery of only about 0.1 

percent of the stored thermal energy, approximately 

1,800 wells spaced at distances of about 7 km would be 

required to realize this resource estimate. Obviously, 

no single development scheme would be considered on 

such a scale. An alternative development, however, 

involving well fields with closer spacings near each 

population center could result in similar total energy 

recovery. To accomplish this goal, well fields of 30 

wells each would be required near approximately 60 

towns located in the geothermal-resource area. Thus, 

the methods used in this assessment appear to yield 

realistic estimates of the thermal energy that could 

potentially be recovered from regional aquifers within 

sedimentary basins. 

Tables 9 and 10 list the mean values and standard 

deviations of the total accessible resource base, 

resource, and beneficial heat for identified lowtemperature geothermal-resource areas in the Central 

and Eastern United States; subtotals are listed for 

hydrothermal-convection and conduction-dominated 

systems. For resource and beneficial-heat estimates, 

subtotals are listed for three temperature ranges. The 

total resource in conduction-dominated syste?l_~ of the 

Central and Eastern United States is 55x10 J; 93 

percent of this total is in thermal reservoirs at 

temperatures of 50° to 90°C. The mean value of the 

total refgurce in hydrothermal-convection systems is 

0.60x10 J; 34 percent of this total is in reservoirs at 

temperatures of 50° to 90°C. From the data listed in 

tables 7 and 8, it is clear that the dominant sources of 

low-temperature geothermal energy for areas east of 

the Rocky Mountains are regional aquifers within deep 

sedimentary basins in the Central United States. 

Although the thermal energy stored in such reservoirs 

is huge (27xto21 J), less than 1 percent of this energy 

is recoverable at the surface under the proposed 

Table 9.-Summary of energies for identifled lowtemperature geothermal systems in the Central 

United States 

62 

[Systems in national parks are omitted. All means and 

standard deviations calculated by the Monte Carlo 

method. All values are rounded to two significant 

figures or, if the first digit is 1, to three significant 

figures; this rounding represents a range of 0.5 to 5 

percent in the accuracy of the total value] 

Type of system 

Hydrotherma 1 convection 

I so 1 a ted Systems 

less than so·c 

so· to 7o"c 

70" to 9o"c 

Number of 

systems 

Subtota 1----------- 31 

Conduction dominated 

Sedimentary bas ins and 

coastal plains 

less than so·c 

so· to 7o"c 

70" to 90"c 

Accessible 

resource base 

(1018 J) 

1.84:!:0.191 

Subtotal------------- 27 27 ,000:!:1,010 

Tot a 1-------------------- 58 27, 000:!:1, 010 

Resource 

(1018 J) 

0.25!0.038 

.161!0.042 

.046!0.30 

.46!0.064 

4.3:!:0. 78 

32:!:3.3 

19.0:!:2.5 

55!4.27 

55:!:4.27 

Beneficial 

heat 

(MWt for 30 yr) 

73!16.3 

78!21 

25:!:17.0 

176:!:32 

1,630:!:330 

15,800:!:1,640 

10,300!2,100 

28,000:!:2,100 

28,000:!:2,100 

Table 10.-Summary of energies for identified lowtemperature geothermal systems in the Eastern 

United States 

[Systems in national parks have been omitted. All means and standard deviations calculated by the 

Monte Carlo method. Values are rounded to two 

significant figures or, if the first digit is 1, to three 

significant figures; this rounding represents a range 

of 0.5 to 5 percent in the accuracy of the total value] 

Accessible Beneficial 

Number of Resource 

Type of system resource base heat 

systems (1018 J) (1018 J) (MWt for 30 yr) 

Hydrotherma 1 convection 

Isolated Systems 

less than so·c 0.138:!:0.022 24:!:6.5 

so· to 7o"c 

70" to 90"c 

Subtota 1----------- 31 0.55:!:0.060 .138:!:0.022 24:!:6.5 

Conduct ion dominated 

Coastal plains 

1 ess than so"c .029:!:0.015 12.7:!:6.4 

so· to 7o"c .096:!:0.035 48:!:17 .4 

70" to 90"c .199:!:0.057 106:!:35 

Subtota 1-------------- 1.63!0.175 .32:!:0.070 167:!:35 

Tot a 1-------------------- 25 2 .2:!:0 .150 .46:!:0.062 191!32 
development plan. The total resource for the eastern 

region is significantly lower than for the central 

region, a difference ·that reflects an absence of 

exploration activity and the lower heat flow and higher 

thermal conductivity in most of the Eastern United 

States. 

UNDISCOVERED GEOTHERMAL RESOURCES 

Estimates of undiscovered low-temperature 

geothermal resources in the . Central and Eastern 

United States were made for aquifers within some sedimentary basins and beneath coastal plains and for 

reservoirs associated with thermal-spring areas in the 

Appalachian Mountains. Although low-temperature 

geothermal energy may exist in these areas, additional 

information is needed to identify individual reservoirs 

and their characteristics. Favorable indications 

include measured or inferred temperature gradients 

greater than 25°C/km, thick sedimentary 

accumulations, evidence of upward leakage of thermal 

water in nearby areas, and geophysical evidence of 

radiogenic granitic intrusive bodies beneath aquifers. 

Table 11 lists estimates of the undiscovered 

accessible resource base, resource, and beneficial heat 

for seven low-temperature geothermal-resource areas in the Central and Eastern United States. For each 

region, the basis for these estimates is a comparison of 

the reservoir areas and volumes considered favorable 

for the existence of low-temperature geothermal 

reservoirs with those of identified geothermal systems 

that are similar geologically. For sedimentary basins, 

undiscovered resources were estimated for many 

basins that also contain an identified low-temperature 

geothermal reservoir, and for the Appalachian basin in 

western Pennsylvania and the Raton basin in 

southeastern Colorado for which no corresponding 

estimates of the identified geothermal resource have 

been made. Estimates of the total undiscovered 

accessible resource base and resource in sedimentary 

basins are less than those for identified lowtemperature geothermal resources within sedimentary 

basins and thus reflect the degree to which resource 

areas of this type can be delineated on the basis of 

existing data. 

Undiscovered geothermal resources in the 

Balcones-Ouachita structural trend in Texas are estimated to be twice the identified resources. For 

the Gulf of Mexico coastal region in eastern Texas, 

Louisiana, and Mississippi, where temperature 

gradients (fig. 4) are gre:rter than 35°C/km, resource 

areas totaling 12,000 km were assumed in estimating 

undiscovered geothermal resources. 

Geologic settings similar to those in identified 

warm-spring areas in the Appalachian Mountains occur 

elsewhere in each subprovince of this region. An 

extension of the valley-and-ridge style of folding also 

underlies the Champlain River valley in Vermont and 

New York. The absence of warm springs in 

nonmountainous areas of tightly folded rocks suggests 

that vertical fracture zones and hydraulic gradients 

imposed by topographic variations may be necessary 

for the occurrence of warm springs in the 

Appalachians. Undiscovered geothermal resources 

equal in magnitude to identified resources were 

estimated for the States where warm springs occur. 

63 

Table 11.--Estimates of undiscovered low-temper!!ture 

geothermal resources in the Central and Eastern 

United States 

[Values are rounded to two significant figures or, if the 

first digit is 1, to three significant figures; this 

rounding represents a range of 0.5 to 5 percent in 

the accuracy of the total value] 

Accessible Beneficial 

Resource 

Resource area resource base heat 

( 1olB J) (1018 J l (MWt for 30 yr) 

Central United States 

Denver basin ------------------ 3,500 

Raton basiJ.i ----------------- 75 

Anadarko basin ---------------- 1, 900 

Rawlins and Rock Springs uplifts; 380 

Washakie, Shirley, and Laramie basins 

Ba leones-Ouachita structura 1 trend 170 

Gulf Coastal PlaiJ.i ---------- 170 

Eastern United States 

Atlantic Coastal Plain-------- 4.9 

Appalachian Mountains ---------

Appa 1 ach ian basi J.l -----------

Tota 1----------------------

.56 

490 

6,700 

6.7 

.34 

5.0 

.70 

1.16 

1.16 

.93 

.14 

1.90 

18.0 

3,300 

174 

2,700 

350 

580 

580 

480 

24 

1,000 

9,200 

!./ Estimates based on a total resource area of 2,100 ki containing aquifers 

in Cretaceous and Pa 1 eozoi c sedimentary rocks, in comparison with resource areas 

in the Denver basin in Colorado. 

21 Estimates based on a total resource area of 12,000 ki dispersed within 

the ~rea of 120,000 km2 outlined in figure 13, in comparison with resource areas 

in the Balcones-Ouachita structural trend. 

}_/ Estimates based on a tot a 1 resource area of 37,000 ki containing aquifers 

in Paleozoic sedimentary beneath Devonian shale, in comparison with resource areas 

in the Anadarko basin-Nemaha ridge in Kansas. 

Along the Atlantic Coastal Plain, thick sediment 

and high temperature gradients are known to be 

present only in the areas of Stumpy Point, North 

Carolina, and the Delmarva Peninsula, Virginia. 

Undiscovered geothermal reservoirs were assumed to 

have more than 3 times the identified reservoir volume 

in each of those areas. 

ACKNOWLEDGMENTS 

Information on low-temperature geothermal 

resources in the Central and Eastern United States was generously provided by numerous individuals. In this 

regard, we gratefully acknowledge the assistance of 

William D. Gosnold (University of Nebraska, Lincoln}, 

Joe S. Downey and D. H. Lob meyer (U.S. Geological 

Survey, Denver, Colorado), Debra W. Struhsacker 

(Chevron Minerals Company, Denver), Richard H. 

Pearl (Colorado Geological Survey, Denver), Donald C. 

Signor (U.S. Geological Survey, Lawrence, Kansas), 

Bern S. Hinckley and Henry Heasler (University of 

Wyoming, Laramie), Charles M. Woodruff (University 

of Texas, Austin), Harold Meisler and Henry Trapp 

(U.S. Geological Survey, Trenton, New Jersey), Donald 

W. Steeples (Kansas Geological Survey, Lawrence), and 
Joseph J. Lambiase (Marathon Oil Company, Littleton, 

Colorado). 

We are also indebted to our colleague, Edward A. 

Sammel, for his pioneering work on the assessment of 

low-temperature geothermal resources and his advice 

and criticism during each phase of this effort. 

REFERENCES CITED 

Bedinger, M. S., Pearson, F. J ., Reed, J. E., Sniegocki, 

R. T., and Stone, C. G., 1979, The waters of Hot 

Springs National Park, Arkansas-their nature 

and origin: U.S. Geological Survey Professional 

Paper 1044-C, p. Cl-C33. 

Blackwell, D. D., and Steele, J. L., 1981, Heat flow 

and geothermal potential of Kansas: Report for 

Kansas State Agency under Contract No. 949. 

Breckenridge, R. M., and Hinckley, B. S., 1978, 

Thermal springs of Wyoming: Geological Survey 

of Wyoming Bulletin 60, 104 p. 

Brown, P. M., Brown, D. L., Reid, M. S., and Lloyd, 0. 

B., Jr., 1979, Evaluation of the geologic and 

hydrologic factors related to the waste-storage 

potential of Mesozoic aquifers in the southern 

part of the Atlantic Coastal Plain, South 

Carolina and Georgia: U.S. Geological Survey 

Professional Paper 1088, 37 p. 

Brown, P. M., Miller, J. A., and Swain, F. M., 1972, 

Structural and stratigraphic framework, and 

spatial distribution of permeability of the 

Atlantic Coastal Plain, North Carolina to New 

York: U.S. Geological Survey Professional Paper 

796, 79 p. 

Carpenter, A. B., and Trout, M. L., 1978, 

Geochemistry of bromine-rich brines of the Dead 

Sea and southern Arkansas, in Johnson, K. H., 

and Russell, J. A., eds., Thirteenth annual forum 

on the geology of industrial minerals: Oklahoma 

Geological Survey Circular 79, p. 78-88. 

Collins, A. G., 197 4, Geochemistry of liquids, gases 

and rocks from the Smackover Formation: U.S. 

Bureau of Mines Report of Investigations 7987, 

84 p. 

Costain, J. K., Glover, Lynn, ITI, and Sinha, A. K., 

1980, Low-temperature geothermal resources in 

the eastern United States: Eos (American 

Geophysical Union Transactions), v. 61, no. 1, p. 

1-3. 

Costain, J. K., Keller, G. V., and Crewdson, R. A., 

1976, Geological and geophysical study of the 

origin of the warm springs in Bath County, 

Virginia: Blacksburg, Virginia Polytechnic 

Institute and State University Report for U.S. 

Department of Energy under Contract E-(40-1)-

4920. 

Crook, J. K., 1899, The mineral waters of the United 

States and their therapeutic uses: New York, 

Lea Brothers &: Co., 588 p. 

Downey, J. S., 1982, Geohydrology of the Madison 

aquifers and associated rocks of the northern 

Great Plains in parts of Montana, South Dakota, 

North Dakota, and Wyoming: U.S. Geological 

Survey Open-File Report 82-914, 124 p. 

Fitch, W. E., 1927, Mineral waters of the United States 

64 

and American spas: New York, Lea & Febiger, 

799 p. 

Gosnold, w. D., Jr., and Everson, D. H., 1981, 

Usefulness of heat flow data in regional 

assessment of low temperature geothermal resources with special reference to Nebraska: 

Geothermal Resources Council Transactions, v. 

5, p. 79-82. 

Hobba, W. A., Jr., Chemerys, J. C., Fisher, D. W., and 

Pearson, F. J., Jr., 1976, Geochemical and 

hydrologic data for wells and springs in thermal 

spring areas of the Appalachians: U.S. 

Geological Survey Open-File Report 76-550, 34 

p. 

Hobba, W. A., Jr., Fisher, D. W., Pearson. F. J., Jr., 

and Chemerys, J. C., 1979, Hydrology and 

geochemistry of thermal springs of the 

Appalachians: U.S. Geological Survey 

Professional Paper 1044-E, p. E1-E36. 

Kelly, J. E., Papadopulos. S. S., and Burnham. W. L., 

1981, The evolution of a double-transmissivity 

concept for the Madison aquifer system, in 

Annual Rocky Mountain Ground Water 

Conference, lOth, 1981, Proceedings: Laramie, 

University of Wyoming, p. 74-75. 

Konikow, L. F., 1976, Preliminary digital model of 

groundwater flow in the Madison Group, Powder 

River Basin and adjacent areas, Wyoming, 

Montana, South Dakota, North Dakota, and 

Nebraska: U.S. Geological Survey WaterResources Investigations 63-75, 44 p. 

Lambiase, J. J., Dashevsky, S. S., Costain, J. K., 

Gleason, R. J., and McClung, W. S., 1980, 

Moderate-temperature geothermal resource 

potential of the northern Atlantic Coastal 

Plain: Geology, v. 8, no. 9, p. 447-449. 

MacCary, L. M., Cushing, E. M., .and Brown, D. L., 

1981, Potentially favorable areas for large-yield 

wells in the Red River Formation and Madison 

Limestone in parts of North Dakota, South 

Dakota, and Wyoming: U.S. Geological Survey 

Open-File Report 81-220, 29 p. 

Martinez, J. A., 1981, Geothermal development of the 

Madison Group aquifer-a case study: 

Geothermal Resources Council Transactions, v. 

5, p. 541-543. 

Moorman, J. J., 1867, The mineral waters of the 

United States and Canada: Baltimore, Kelly &: 

Piet, 507 p. 

Perry, W. C., Costain, J. K., and Geiser, P. A., 1979, 

Heat flow in western Virginia and a model for 

the origin of thermal springs in the folded 

Appalachians: Journal of Geophysical Research, v. 84, no. B12, p. 6875-6883. 

Reed, M. J., Mariner, R. H., Brook, C. A., and Sorey, 

M. L., 1983, Selected data for low-temperature 

(less than 90°C) geothermal systems in the 

United States; reference data for U.S. Geological 

Survey Circular 892: U.S. Geological Survey 

Open-File Report 83-250, 129 p. 

Sammel, E. A., 1979, Occurrence of low-temperature 

geothermal waters in the United States, in 

Muffler, L. J. P ., ed., Assessment of geothermal 

resources of the United States-1978: U.S. 

Geological Survey Circular 790, p. 86-131. 
Schoon, R. A., and McGregor, D. J., 1974, Geothermal 

potentials in South Dakota: South Dakota 

Geological Survey Report of Investigations 110, 

76 p. 

Schubert, C. E. and Johnson, W. J., 1980, Hot dry rock 

geothermal evaluation, Cris-Wall site, eastern 

shore of Maryland and Virginia: D'Appolonia 

Consulting Engineers, Inc., report for Los Alamos 

Scientific Laboratory under Contract 4-X29-

7745G-1, 4 v. 

Steele, K. F., and Wagner, G. H., 1981, Warm springs 

of the Western Ouachita Mountains, Arkansas: 

Geothermal Resources Council Transactions, v. 5, p. 137-140. 

U.S. Environmental Protection Agency, 1974, 

Compilation of industrial and municipal injection 

wells in the United States: Washington, Report 

EPA-520/9-74-020, 2 v. 

Vaught, T. L., 1980a, An assessment of the geothermal 

resources of Indiana based on existing geologic 

65 

data: U.S. Department of Energy Report 

DOE/NV /10072-3, 38 p. 

--1980b, Temperature gradients in a portion of 

Michigan: A review of the usefulness of data 

from the AAPG Geothermal Survey of North 

America: U.S. Department of Energy Report 

DOE/NV /10072-1, 39 p. 

Woodruff, C. lVI., and McBride, M. W., 1979, Regional 

assessment of geothermal potential along the 

Balcones and Luling-Mexia-Talco fault zones, 

central Texas: U.S. Department of Energy 

Report DOE/ET/28375-1, 145 p. 

Woodward-Clyde Consultants, 1980, Well-field 

hydrology: San Francisco, 414 p. Technical 

report for the environmental impact statement 

on the Energy Transportation System, Inc., 

proposed coal slurry pipeline. [Available from 

U.S. Bureau of Land Management, Special 

Projects Staff, 555 Zang St., Denver, CO 80228.] 

Summary 

By Marshall J. Reed 

CONTENTS 

Abstract -------------------------

Discussion -----------------------

Thermal regimes ------------------ Methodology of assessment --------

Western United States ------------

Central United States ------------

Eastern United States ------------

Conclusions ---------------------- References cited -----------------

ABSTRACT 

Page 

67 

67 

67 

68 

68 

68 

69 

69 

73 

This assessment of low-temperature geothermal 

resources in the United States presents a quantitative 

estimate of the energy available from identified and 

undiscovered reservoirs at temperatures less than 

90°C. Changes in the world price of petroleum and 

the increasing price of natural gas have led to a 

growing interest in the use of low-temperature 

geothermal energy. Quantitative estimates made for 

the geothermal energy available in identified and 

undiscov~led systems give an accr~sible resource base 

of 27x10 J, a resource of 87x10 J, and a beneficial 

heat of 42 GWt for 30 years. 

DISCUSSION 

The principal source of thermal energy in the 

temperature range 10°-90°C within the United States 

is the burning of natural gas and No. 2 diesel oil; 

electrical resistance heating is also a common source 

of thermal energy in this temperature range. Changes 

in the world price of petroleum and the increasing 

price of natural gas have led to a growing interest in 

the use of low-temperature geothermal energy and 

have clearly indicated the need for an evaluation of 

low-temperature geothermal resources. Knowledge of 

the quantity, distribution, and potential of these 

resources is critical to their development. To 

determine these factors, the U.S. Geological Survey, 

with the support of the u.s. Department of Enerax, 

has evaluated low-temperature (less than 90 C) 

geothermal resources in the United States on the basis 

of information available through April 1982. This 

67 

assessment complements the earlier estimates of 

intermediate- and high-temperature geothermal 

resources by White and Williams (1975) and Muffler 

(1979). 

The terminology and methodology used in this 

assessment make careful distinctions among the 

accessible resource base (energy stored in the ground), 

the resource (energy recoverable at the wellhead), and 

the beneficial heat (energy usable in a specific 

application). The resource, which is a factor of major 

significance in any assessment, is the quantity that can 

be most easily compared with other energy sources. 

The accessible resource base for this assessment 

of low-temperature geothermal resources is restricted 

to the upper 10 km of the crust, as suggested by 

Muffler and Cataldi (1978). It is also constrained by a 

minimum-temperature criterion and by the 

requirement that the reservoirs be permeable (see 

Reed, "Introduction," this volume). Using this 

definition of accessible resource base, a total resource 

of 34x1o21 J was calculated for identified and 

undiscovered low-temperature geothermal systems in 

the United States. From this energy in the Earth, a 

resource of 153x1o18 J could be recovered in 30 years, 

and the resulting beneficial heat is 72 GWt for 30 

years. 

THERMAL REGIMES 

The discussion of heat flow, thermal 

conductivity, and temperature gradients in the United 

States identifies the thermal regimes within which 

low-temperature geothermal systems exist (Nathenson 

and others, this volume). Conductive heat transfer 

through rock is the dominant mechanism for the 

movement of thermal energy in the crust of the 

Earth. However, in parts of the crust where faults, 

fractures, or other high-permeability structures 

provide steeply dipping channels, warm water may 

move toward the surface, carrying thermal energy in 

hydrothermal-convection systems. The types of lowtemperature geothermal systems have been divided 

into conduction dominated or hydrothermal 

convection, depending on the principal mode of heat 

transfer within them. 

Maps of heat flow and thermal gradient (figs. 4, 

5) show a systematic variation across the conterminous 
United States that reflects differences in the 

underlying rocks and the level of tectonic and 

magmatic activity in different regions of the 

country. This assessment of low-temperature 

geothermal resources was conducted by dividing the 

country into western, central, and eastern regions so 

as to group the geothermal systems by similar geologic 

environments that reflect regional differences in the 

thermal structure of the crust. The map of 

temperature gradients (fig. 5) was combined with 

available geologic data to provide a basis for estimates 

of the range in depth for some identified geothermal 

systems and of the probable extent of undiscovered 

geothermal systems. 

METHODOLOGY OF ASSESSMENT 

The accessible resource base in low-temperature 

geothermal reservoirs was estimated by the volumetric 

method used previously for hydrothermal-convection 

systems greater than 90°C (Brook and others, 1979). 

Geothermal reservoirs identified only by isolated hot 

springs or thermal wells, or from limited subsurface 

inf~rmation, were assigned an estimated volume of 1 

km • New methods have been developed to calculate 

the resource and beneficial heat available from lowtemperature geothermal reservoirs (Sorey, Nathenson, 

and Smith, this volume). The method for calculating 

the resource is based on a production model with 

equally spaced wells, producing at 31.5 L/s, for a 30-

year period with a cumulative drawdown of 152 m. 

This production model assumes a natural recharge of 

water in the system as the reservoir pressure declines, 

but does not include injection of produced water after 

thermal energy has been extracted. Well spacings that 

result in the allowable drawdown are a function of the 

reservoir area and hydrologic properties. With this 

method of resource calculation, the proportion of the 

accessible resource base that can be recovered from a 

reservoir in 30 years ranges from a minimum of 0.001 

for regional aquifers in large sedimentary basins 

(Sorey, Reed, and others, this volume) to a maximum 

of 0.25 for small-area reservoirs. The upper limit of 

the recovery factor, 0.25, from the accessible resource 

base is derived from the heat-sweep analysis of 

Nathenson and Muffler (1975). 

The estimate of beneficial heat is based on an empirical relation derived from the characteristics of 

several low- and intermediate-temperature geothermal 

installations. This relation indicates that the usable 

temperature difference is approximately 60 percent of 

the difference between the reservoir temperature and 

25°C (see Sorey, Nathenson, and Smith, this volume, 

equations 5 and 6). 

WESTERN UNITED STATES 

Cenozoic tectonic activity in the Western United 

States created many faults with a high vertical 

permeability that commonly have hydrothermalconvection systems associated with them. Little 

information is available on the reservoirs in 927 

isolated hydrothermal-convection systems; these 

sys~ems are here assigned a standard volume of 1 

km • Six hydrothermal-convection systems in national 

68 

parks were excluded from the calculations of resource 

and beneficial heat because they are not available for 

development; the resource for the 921 remaining 

reservoirs was calculated by using a recovery factor of 

0.25 for a period of 30 years. Enough information is 

available for 157 systems in the Western United States 

to estimate the area of each reservoir individually. 

The resource for these areas was calculated by using 

the equally-spaced production-well method of Sorey, 

Nathenson, and Smith (this volume). Of these larger 

systems, most are hydrothermal-convection systems, 

but one in the Salton Trough province of California and 

eight in the Columbia Plateaus province of Washington are conduction-dominated geothermal systems. The 

nine conduction-dominated systems have nearly 

horizontal zones of high permeability where water 

moving laterally absorbs thermal energy from the 

surrounding rock. 

For the 1,075 hydrothermal-convection systems 

identified by Mariner and others (this volume), an 

estimated resource of 30x1o18 J could be recoveredfR 

30 years from an accessible resource base of 200x10 

J. From the nine reservoirs in conduction11fminated 

systems, an estimated resource of 1.19xl01 J could 

be recovered in ~o years from an accessible resource base of 102x10 J. The beneficial heat from all 

identified systems in the Western United States is 13.7 

GWl fog 30 years from a total identified resource of 

31x o1 J and 1 total identified accessible resource base of 310x101 J (table 6). 

The estimate of the undiscovered accessible 

resource base has a much greater uncertainty than 

that of the identified accessible resource base because 

so little information is available. Some identified 

geothermal reservoirs may be larger in volume than 

estimated in this assessment. An undiscovered 

component of energy in identified systems and the 

energy contained in undiscovered systems were 

estimated by geologic province in an attempt to partly 

reduce this uncertainty. Undiscovered systems were considered to be similar in geologic and hydrologic 

characteristics to the identified systems in the same 

province (table 5). The total accessible resource base, 

including identified and undiscovered components, for 

low-temperature geothi~mal resources in the Western 

United States is 790x10 J. 

CENTRAL UNITED STATES 

The Central United States has had little tectonic 

activity during the Cenozoic, and few hydrothermalconvection systems are found there. The heat now map (fig. 4) shows that the central region is 

transit~onal between the below-average to average (61 

m WI m ) heat flow in the East and the high heat flow 

in the West. The existence of low-temperature 

geothermal reservoirs in the central region depends 

primarily on the occurrence of rocks with low thermal 

conductivity. The Central United States includes 

many large basins containing thick sequences of 

Paleozoic and Mesozoic sedimentary rocks. Oil and 

gas exploration wells have been drilled into most of 

these basins, and several aquifers have been found that 

contain low-temperature geothermal water. Two very 

extensive conduction-dominated systems have been 
identified in this assessment (Sorey, Reed, and others, 

this volume}: The Madison aquifer in Mississippian 

limestone and the Dakota aquifer in Cretaceous 

sandstone, which both have great areal extent within 

the basins of the northern and central Great Plains 

province. These large-area aquifers dominate the 

energy estimates for low-temperature geothermal resources in the United States. 

An estimated resource of 55xlo18 J could be 

recover~jn 30 years from an accessible resource base 

of 27x10 J in the reservoirs of the 23 conductiondominated geothermal systems identified in the 

Central ~nited States, and an estimated resource of 

0.46x10 J could be recovered in 30 I~ars from an accessible resource base of 1.84x10 J in the 

reservoirs of the 29 hydrothermal-convection systems 

in the same region. No estimate was made of the 

resource or beneficial heat available from the 

hydrothermal-convection system in Hot Springs 

National Park, Arkansas. The beneficial heat from all 

identified systems in the Central United States ~ 28 

GWt for 30 years from a total resource of ~~x10 8 J 

and a total accessible resource base of 27x10 J. 

The energy contained in undiscovered geothermal 

systems was estimated by comparing areas considered 

favorable for the existence of low-temperature 

geothermal reservoirs with identified geothermal 

systems that are similar in geologic and hydrologic 

characteristics (table 11}. The total accessible 

resource base, including identified and undiscovered 

components, for low-temperature ge~tpermal 

resources in the central United States is 34x10 J. 

In the course of this assessment, large amounts 

of thermal energy at reservoir· temperatures above 

90°C contained in deep aquifers of the Anadarko, 

Powder River, and Williston basins were excluded from 

consideration. This enerw is in water with 

temperatures of between 90 and 150°C containing 

more than 100,000 mg/L total dissolved solids at 

depths greater than about 2.5 km (MacCary, 1981}. 

The great depth and high concentration of dissolved 

solids make economical recovery of this energy 

unlikely. 

EASTERN UNITED STATES 

The Eastern United States had little tectonic 

activity during the Cenozoic, and the few 

hydrothermal-convection systems found in this region 

occur along areas of pre-Cenozoic folding and faulting 

in the Appalachian Mountains. The efstern region has 

below-average to average (61 mW/m } heat flow, and so the existence of conduction-dominated lowtemperature geothermal reservoirs depends on the 

presence of thick sequences of rocks of low thermal 

conductivity. The eastern region, however, includes 

large areas of high-thermal-conductivity crystalline 

rock exposed at the surface or at shallow depth, and so the available area for low-temperature geothermal 

systems is greatly reduced. Oil and gas exploration 

wells have been drilled into the Atlantic Coastal Plain 

and into most of the sedimentary basins, but all the 

explored basins except the Appalachian basin fall 

below the minimum-temperature criterion. In the 

Appalachian basin, drilling has penetrated only about 

2.2 km (Joel Renner, written commun., 1981}, although 

stratigraphic information from outside the basin 

suggests that an undiscovered low-temperature 

geothermal reservoir may exist at greater depth in 

pre-Devonian sedimentary rocks. 

An estimated resource of 0.32x1o18 J could be 

recovered i~JO years from an accessible resource base 

of 1.63x10 J in the six conduction-dominated 

geothermal systems identified in the Eastern U~\fed 

States, and an estimated resource of 0.138x10 J 

could be recovered in 30 years from an accessible resource base of 0.55x1018 J in the 19 hydrothermalconvection systems in the same region. The beneficial 

heat from all identified low-temperature geothermal 

systems in the Eastern United States is 191 MWt f?£8

30 

years from a total identified resource of 0.46x10 J 

and a

1

1otal identified accessible resource base of 

2.2x10 J. 

Undiscovered geothermal systems were 

considered to be similar in geologic and hydrologic 

characteristics to identified systems (table 11}. The 

undiscovered geothermal system anticipated in the 

Appalachian basin has no counterpart in the identified 

conduction-dominated systems of the eastern region, 

and so similarities are drawn to identified geothermal 

systems in the central region. The total accessible resource base, including identified and undiscovered 

components, for low-temperature geotherwal 

resources in the Eastern United States is 490x101 J 

(dominated by the undiscovered component in the 

Appalachian basin}. 

69 

CONCLUSIONS 

This assessment of low-temperature geothermal 

resources in the United States refines and extends the 

qualitative estimate by Sammel (1979}. Extensive data 

collected by the State-cooperative projects of the U.S. 

Department of Energy, Division of Geothermal Energy 

(see "Additional References" in Reed, "Introduction," 

this volume), led to the identification of many more 

low-temperature geothermal systems within the 

favorable areas listed by Sammel (1979}. Table 12 

summarizes the identified low-temperature 

geothermal energy in each State. 

Table 13 identifies the contributions from both 

hydrothermal-convection and conduction-dominated 

geothermal systems for the three regions of the 

country and summarizes the undiscovered component 

of low-temperature geothermal energy. Reservoirs in 

hydrothermal-convection systems make up the 

greatest proportion (97 percent) of identified lowtemperature geothermal-resource areas, but the 

smaller average volume of these systems accounts for 

their contribution of only 1 percent to the accessible 

resource base. However, the small size of most 

re~ervoirs in hydrotbermahconvection systems also 

leads to a relatively high average recovery factor 

(0.15), and they account for 36 percent of the 

identified resource. The beneficial heat calculated for 

hydrothermal-convection systems is 32 percent of the 

total identified beneficial heat, a proportion that 

reflects the lower average reservoir temperature of 

these systems. Although reservoirs in conductiondominated systems are much less numerous, some are 
extremely large, and taken together they constitute 64 

percent of the total low-temperature geothermal 

resource in the United States. Because use of the 

proposed development plan to calculate the resource results in lower recovery factors for larger area 

reservoirs, the average recovery factor for 

conduction-dominated systems is only 0.002. The 

greater depth to most of these reservoirs and the 

attendant higher average temperature explains their 

contribution of 69 percent of the total beneficial heat 

for identified geothermal systems. 

Evaluation of low-temperature geothermal 

energy in the U~~ed States gives an estimated resource of 153x10 J (tab~ll3). This is much lower 

than the estimate of 2.4x10 J for the total resource 

in hydrothermal-convection systems with reservoir 

temperatures above 90°C (Brook and others, 1979), 

Table 12.-Summary of energies by State for identified 

low-temperature geothermal systems in the 

United States 

[Systems in national parks have been omitted from 

calculations of the resource and beneficial heat. All 

means calculated analytically. Values are rounded 

to two significant figures or, if the first digit is 1, to 

three significant figures; this rounding represents a 

range of 0.5 to 5 percent in the accuracy of the 

total value] 

4\ccess ib le Benefica 1 

State Number of resource base Resource heat systems (1018 J) (1018 J) (MWt for 30 yr) 

A 1 ask a------------ 33 3.5 0.88 430 

Arizona----------- 63 33 3.6 1,640 

Arkansas--------- 6 .50 .092 36 

Ca 1 i forni a-------- 203 54 5.6 2,000 

Co 1 orado---------- 49 690 3.6 1,800 

Georgi a----------- 1 .050 .012 3. 7 

Hawaii------------ 1.70 .054 24 

ld aho------------ 171 55 6.1 2,800 

Kansas----------- l, 700 3.3 1,700 

Massachusetts----- .016 .004 .o 

Mont an a----------- 52 10,200 17.3 8,800 

Nebraska---------- 930 3.4 1,550 

Nevada----------- 191 28 5.5 2,400 

New Mexico-------- 70 7.4 1.32 580 

New York---------- 1 .018 .005 .0 

North Caro 1 ina---- .98 .152 80 

North Dakota------ 6,400 9.8 5,000 

Ok 1 ahoma--------- 3,600 3.9 2,100 

Oregon------------ 99 15.0 2. 7 1,200 

South Dakota------ 5 1,800 5.7 2,600 

Texas------------- 17 84 .86 420 

Utah-------------- 118 20 2.1 690 

Virgin i a---------- 14 1.05 .25 100 

Washington-------- 25 77 1.12 450 

West Virginia---- 5 .088 .023 .0 

Wyoming----------- 34 1,400 g_5 4,800 

Tota 1-------- 1,168 27,000 87 41,000 

70 

primarily because of the lower recovery factors 

calculated in this assessment. The ratio of the 

undiscovered to the identified. component for the 

accessible resource base in hydrothermal-convection 

systems is 5.0 for intermediate- and high-temperature 

systems (Brook and others, 1979) and 1.8 for lowtemperature systems (table 13); this difference 

reflects the greater number of oil, gas, and . water 

wells that have been drilled in areas with favorable 

geology for the discovery of low-temperature 

geothermal resources. However, areas with favorable 

geology for intermediate- and high-temperature 

geothermal resources have fewer wells and less 

information available, and so larger areas exist with 

the possibility for undiscovered systems. In the 

estimates of the accessible resource base for lowtemperature geothermal energy, the higher ratio of 

the undiscovered to the identified component in 

hydrothermal-convection systems (1.8) than in 

conduction-dominated systems (0.25) also reflects the 

much greater drilling activity in and greater 

subsurface information on areas where undiscovered 

conduction-dominated systems may exist. 

A total of 1,126 low-temperature hydrothermalconvection reservoirs are identified in this 

assessment. Water chemistry was evaluated for these 

systems, many of which have estimated geochemical 

Table 13.-Summary of energies for identified and undiscovered low-temperature geothermal systems 

in the United States 

[Omitted are seven hydrothermal-convection systems 

in national par~ that contain an accessible resource base of 0.76x10 J. All means and standard deviations ~lculated by the Monte Carlo method. Values 

are rounded to two significant figures or, if the first 

digit is 1, to three significant figures;· this rounding 

represents a range of 0.5 to 5 percent in the accuracy of the total value] 

Accessible Beneficial Number of Resource Type of system resource base heat systems (1018 J) (1018 J) (MWt for 30 yr) 

Identified Hydrotherma 1 convection 

Western region ----- 1,069 200<;5.3 30!0.85 13,200!:420 

Central region ----- 3l 1.84!0.191 .46!0.064 176!32 

Eastern region ---- 19 .55!0.060 .138!0.022 24!6.5 

Subtotal ----------- 1,119 200!:5. 3 31!0.85 13,400:!:420 

Conduction dominated 

Western region ----- 102:!:16.7 1.19:!:0.27 500!:134 

Central region ----- 27 27 ,000:!:1010 55!:4.2 28,00~2100 

Eastern region ----- 1.63:!:0.175 .32:!:0.069 167!35 

Subtotal ----------- 42 27,00~1010 56:!:4.2 29,000:!:2100 

Total ----------------- 1,161 27 ,000:!:1010 87:!:4.3 42,000!:2100 

Undiscovered ------------- 7,200 66 30,000 

Grand tota 1---------- 34,000 153 72,000 
temperatures above the measured reservoir 

temperatures. Difficulties exist in the interpretation 

of chemical geothermometers ·at low temperatures 

(see Mariner and others, this volume), and the 

estimates of maximum and most likely reservoir 

temperatures are subject to large errors. To evaluate 

the overall temperature distribution of hydrothermalconvection systems, the mean temperatures for the 

220 intermediate- and high-temperature systems 

identified by Brook and others (1979) were combined 

with the mean temperatures of the systems in this 

assessment on a semilogarithmic plot of cumulative 

frequency versus temperature (fig. 16). The 

distribution of the 1,346 identified hydrothermalconvection systems shown in figure 16 approximates 

the equation N = exp [(318.26-T)/41.49], where N is the 

total number-of geothermal systems above a certain 

reservoir temperature and T is the temperature (in 

degrees Celsius). This equation (as determined by a 

least-squares fit to the data) is nearly identical to that 

for intermediate- and high-temperature systems alone 

(Brook and others, 1979, fig. 11). The natural 

distribution of a resource of this type could be 

expected to fit some kind of exponential equation; if 

the distribution of temperatures for the total 

>-

u 

z 

w 

1000 

~ 100 w 

a: 

u.. 

w 

> 

~ :::> 

~ 

:::> 

u 10 

.. 

population of geothermal systems is a similar 

exponential function, then the identified 

hydrothermal-convection systems are representative 

of that total population. 

The 38 low-temperature conduction-dominated 

geothermal systems were evaluated similarly to the 

assessment of geopressured-geothermal systems in the 

northern Gulf of Mexico basin (Wallace and others, 

1979). The recovery factors, based on similar methods 

of calculating the resource, are also comparable for 

geopressured-geothermal and low-temperature 

conduction-dominated geothermal systems. 

The distribution of low-temperature geothermal 

energy as a function of reservoir temperature is 

important for the potential uses of this resource. The 

value of geothermal water depends strongly on its 

temperature because water near the higher limit of 

the temperature range (near 90°C) can be used for 

more applications and carries more energy per unit 

mass than water near the lower limit of temperature. 

Figure 17 summarizes the distribution of lowtemperature geothermal energy in three temperature 

ranges. To determine the resource energy within these 

temperature ranges, a Monte Carlo computer 

simulation was performed. Values for the reservoir 

·~ .. .. . . .. .. . 

1~---------L----------~--------~-----------L----------L---------~--~~--~ 

0 100 200 300 

RESERVOIR TEMPERATURE, IN DEGREES CELSIUS 

Figure 16.--Cumulative frequency versus reservoir temperature for 1,346 hydrothermal-convection systems in 

the United States, including 1,126 low-temperature geothermal systems in this assessment and 220 intermediate- and high-temperature systems from Brook and others (1979). Straight line is least-squares fit to 

data, described by equation given in text. 

71 
>- 1--

::::i 

m 

~ 0.5 m 

0 

a: 

CL. 

ENERGY, IN 1018 JOULES 

Figure 17 .-Monte Carlo distribution of resource for low-temperature geothermal systems in the United States. 

parameters of temperature, area, thickness, and 

production-well spacing were generated by using the 

minimum, maximum, and most likely values to 

simulate triangular probability distributions. The 

thermal energies listed in tables 6, 9, 10, and 13 were 

also calculated by the Monte Carlo method; these 

values are slightly higher than those calculated 

analytically, but both types of calculations agree 

within the standard deviations listed. 

The standard deviations given in this assessment 

indicate the uncertainties associated with the 

estimated ranges of parameters that were allowed to 

vary. Some parameters were assumed to be constant 

for the calculations, but assignment of a distribution 

to these parameters would add to the overall 

uncertainty of the results. The totals for large 

numbers of systems have smaller associated 

uncertainties because the aggregation reduces the 

overall standard deviation. As in the assessments by 

White and Williams (1975) and Muffler (1979), the 

total-energy estimates are considered more reliable 

than the estimates for any one reservoir. 

Important gains can be made in the use of lowtemperature geothermal energy for suitable 

applications, and a significant contribution to the 

Nation's energy needs could result from increased use 

of this resource. Table 14 compares the energy 

available from the total (identified and undiscovered) 

beneficial heat of low-temperature geothermal 

resources with the calculated amounts from other 

Table 14.-Comparison of the total beneficial heat from low-temperature geothermal systems with that 

from other energy sources 

Energy source 

Energy content 

per unit quantity 

Natural gas-------- 3.7xl07 J/standard m3 

No. 2 diesel oil--- 4.4xlo7 J/kg 

Bituminous coal---- 7 3.0xl0 J/kg 

Burner efficiency 

(Beller, 197 5) 

72 

0.48 

.44 

.44 

Resource quantity needed to 

provide a beneficial heat 

of 27 GWt for 30 years in 

a space-heating application 

3.8xlo12 standard m3 

3.5xlo12 kg 

5.2xlo12 kg 
energy sources needed to provide the same beneficial 

heat. Each process that involves transfer of this heat 

to air, water, or other substances will have an 

efficiency factor. To calculate the beneficial heat 

available from each energy source, the efficiencies for 

combustion in residential space heating were used 

(Beller, 1975); other applications may have much 

different efficiencies. 

Future low-temperature geothermal-resource 

assessments will have the advantage of a longer 

history of exploration and development. The data base 

will be enlarged by future exploration, and a better 

understanding of the operating efficiencies of 

geothermal installations will refine our knowledge of 

beneficial heat. The resolution of existing 

uncertainties concerning reservoir temperature will 

require further investigations into chemical 

geothermometers and the measurement of equilibrium 

temperatures in available wells. It is anticipated that, as more information is acquired, State or regional 

agencies will wish to refine the estimates of 

geothermal energy within their own boundaries. With 

that possibility in mind, the authors of this volume 

have attempted to provide a basic methodology and 

examples of its application, so that future geothermal- resource assessments can build on this base. 

REFERENCES CITED 

Beller, 1\1., ed., 1975, Sourcebook for energy 

assessment: Brookhaven National Laboratory 

Report BNL 50483, 193 p. 

Brook, C. A., Mariner, R. H., Mabey, D. R., Swanson, 

J. R., Guffanti, Marianne, and Muffler, L. J. P., 

1979, Hydrothermal convection systems with 

reservoir temperatures ~ 90°C, in Muffler, L. J. 

P., ed., Assessment of geothermal resources of 

73 

the United States-1978: U.S. Geological Survey 

Circular 790, p. 18-85. 

MacCary, L. M., Cushing, E. M., and Brown, D. L., 

1981, Potentially favorable areas for large-yield 

wells in the Red River Formation and Madison 

Limestone in parts of North Dakota, South 

Dakota, and Wyoming: U.S. Geological Survey 

Open-·File Report 81-220, 29 p. 

Muffler, L. J. P., ed., 1979, Assessment of geothermal 

resources of the United States-1978: U.S. 

Geological Survey Circular 790, 163 p. 

Muffler, L. J. P., and Cataldi, R., 1978, Methods for 

regional assessment of geothermal resources: 

Geothermics, v. 7, no. 2-4, p. 53-89. 

Nathenson, Manuel, and Muffler, L. J. P., 1975, 

Geothermal resources in hydrothermal 

convection systems and conduction-dominated 

areas, in White, D. E., and Williams, D. L., eds., 

Assessment of geothermal resources of the 

United States-1975: U.S. Geological Survey 

Circular 726, p. 104-121.. 

Sammel, E. A., 1979, Occurrence of low-temperature 

geothermal waters in the United States, in 

Muffler, L. J. P., ed., Assessment of geothermal 

resources of the United States-1978: U.S. 

Geological Survey Circular 790, p. 86-131. 

Wallace, R. H., Jr., Kraemer, T. F., Taylor, R. E., and 

Wesselman, J. B., 1979, Assessment of 

geopressured-geothermal resources in the 

northern Gulf of Mexico basin, in Muffler, L. J. 

P ., ed., Assessment of geothermal resources of 

the United States-1978: U.S. Geological Survey 

Circular 790, p. 132-155. 

White, D. E., and Williams, D. L., eds., 1975, 

Assessment of geothermal resources of the 

United States-1975: U.S. Geological Survey 

Circular 726, 155 p. 
GPO 687-040/101