UNITED STATES MINERAL RESOURCES 



GEOTHERMAL RESOURCES 



By L. J. P. Muffler 



CONTENTS 



Page 



Abstract of conclusions 251 



Introduction 251 



Exploitation 252 



Environmental considerations 253 



Geologic Environments 254 



Resources and problems 254 



Prospecting techniques 258 



References cited 259 



FIGURES 



27. 



28. 



Page 



Graph showing growth of geothermal generating 



capacity by countries, 1900-72 252 



Graph showing weight percent of steam produced 

 by flashing water at various temperatures — 255 



TABLE 



Principal utilization (other than for electricity) 

 of geothermal resources 



253 



ABSTRACT OF CONCLUSIONS 



The geothermal resource base is defined as all the heat 

 above 15°C in the earth's crust, but only a small part of this 

 resource base can properly be considered as a resource. The 

 magnitude of the geothermal resource depends on the evalua- 

 tion of many physical, technological, economic, environmental, 

 and governmental factors. The physical factors that control 

 the distribution of heat at depth can be evaluated, at least 

 rudely. More tenuous are the assumptions of technology, eco- 

 nomics, and governmental policy. These assumptions are cri- 

 tical to geothermal resource estimation, and differences 

 among them are in great part responsible for the vast range 

 in magnitude among different geothermal resource estimates. 



Utilization of a greater proportion of the geothermal re- 

 source base depends on achieving one or more of the fol- 

 lowing items : 



1. Technological advances that would allow electrical gen- 



eration from low-temperature reservoirs. 



2. Breakthroughs in drilling technology that would permit 



low-cost drilling of holes to depths greater than 3 km. 



3. Development of techniques of artificial stimulation that 



would increase the productivity of geothermal reser- 

 voirs. 



4. Expansion of the use of low-grade geothermal resources 



for such purposes as space heating, product processing, 

 agriculture, and desalination. 



INTRODUCTION 



Geothermal energy, in the broadest sense, is the 

 natural heat of the earth. Temperatures in the 

 earth rise with increasing depth. At the base of 

 the continental crust (25-50 km), temperatures 

 range from 200°C to 1,000°C (Lachenbruch, 1970) ; 

 at the center of the earth (6,371 km), they range 

 perhaps from 3,500 °C to 4,500 °C. Most of the earth's 

 heat is far too deeply buried ever to be tapped by 

 man. Although drilling has reached 71/2 km and may 

 some day reach 15-20 km, the depths from which 

 heat might be extracted profitably are unlikely to 

 be greater than 10 km. Even in this outer 10 km, 

 most of the geothermal heat is far too diffuse ever 

 to be recovered economically (White, 1965). Conse- 

 quently, most of the heat within the earth, even at 

 depths of less than 10 km, cannot be considered an 

 energy resource. 



Geothermal energy, however, does have potential 

 economic significance where heat is concentrated 

 into restricted volumes in a manner analogous to 

 the concentration of valuable metals into ore de- 

 posits or of oil into commercial petroleum reser- 

 voirs. At present, economically significant concen- 

 trations of geothermal energy occur where elevated 

 temperatures are found in permeable rocks at depths 

 less than 3 km. The thermal energy is stored both 

 in the solid rock and in water and steam that fill 

 pores and fractures. This water and steam serve to 

 transfer the heat from the rock to a well and thence 



U.S. GEOL. SURVEY PROF. PAPER 820 



261 



