9. Review of Geothermal Resources

  1. William H. K. Lee
  1. James R. Mcnitt

Published Online: 18 MAR 2013

DOI: 10.1029/GM008p0240

Terrestrial Heat Flow

Terrestrial Heat Flow

How to Cite

Mcnitt, J. R. (1965) Review of Geothermal Resources, in Terrestrial Heat Flow (ed W. H. K. Lee), American Geophysical Union, Washington, D.C.. doi: 10.1029/GM008p0240

Author Information

  1. California Division of Mines and Geology, San Francisco

Publication History

  1. Published Online: 18 MAR 2013
  2. Published Print: 1 JAN 1965

ISBN Information

Print ISBN: 9780875900087

Online ISBN: 9781118668825



  • Geothermal resources;
  • Magmatic processes;
  • Natural thermal fluid;
  • Physical characteristics of thermal systems;
  • Thermal fluid preservation


The geothermal power plants operating in Italy, New Zealand, and the United States have proved the practical value of using natural steam and hot water as a source of energy within the economic framework of these countries. The success of these projects has stimulated additional exploration within the three countries, and has initiated or intensified other investigations, principally in Iceland, Japan, Russia, and Mexico.

All the thermal areas under development occur in regions of late Cenozoic volcanic activity, and their heat is probably derived from shallow intrusive bodies. These intrusives are found in two contrasting tectonic environments: (1) regions of Quaternary uplift of several hundreds of meters and (2) regions of late Tertiary and Quaternary subsidence of several thousands of meters. These tectonic environments determine the local structural and stratigraphic characteristics of the thermal fluid reservoirs. Structure and permeability of the reservoir greatly influence the thermal gradients measured in an area and the heat content of the thermal fluid. In permeable reservoirs, thermal gradients approximate the boiling point curve of water. Gradients as high as 800°C/km have been measured at depths of 30 meters in impermeable rock above the reservoir, but deeper drilling has shown that such high gradients do not persist. A gradient of 180°C/km, however, has been measured to a depth of 1000 meters. Thermal fluid enthalpies as high as 690 kcal/kg have been reported.

Chemical investigations have determined some of the complex reactions that occur between the thermal fluid, which is predominantly of the sodium chloride type, and its enclosing reservoir rock. These data aid in determining the paths of migration of the thermal fluid through the reservoir. In addition, isotopic studies indicate that probably 95% of the thermal fluid discharged from a given area is meteoric in origin. The exploration of thermal areas has been supplemented by numerous geophysical techniques including gravity, magnetic, and heat flow surveys. These investigations, although providing specific geologic information on hydrothermal systems, still leave unanswered fundamental questions about heat transfer within the crust.