The author distinguishes between geothermal resource base, accessible geothermal resource base, geothermal resource, and geothermal reserve. Conditions for periodically updating the assessment of geothermal energy resources include: increased data from expanded exploration and drilling; development of improved and new technologies for exploration, evaluation, extraction, and use; rapid evolution of geothermal knowledge; and the increased role of geothermal energy in response to changing economic, social, political, and environmental conditions, particularly an increasing awareness of the limits to petroleum and natural gas resources. Accordingly, the U.S. Geological Survey (USGS) plans by the end of 1978 to update its 1975 assessment of the United States' geothermal resource, with increased emphasis on several items. The USGS's joint evaluations of geothermal resource-assessment techniques in the last year with the National Electric Agency of Italy (ENEL) under U.S. Energy Research and Development Agency sponsorship identified a number of problems, one of which was how to formulate geothermal recovery factors for systems producing by intergranular vaporization and by intergranular flow. The first formulation is fairly rigorous; the author solicits the reservoir engineering community's help in improving the estimate of the second. 3 figs., 11 refs.

The geothermal resource assessment presented is a refinement and updating of USGS Circular 726. Nonproprietary information available in June 1978 is used to assess geothermal energy in the ground and, when possible, to evaluate the fraction that might be recovered at the surface. Five categories of geothermal energy are discussed: conduction-dominated regimes, igneous-related geothermal systems, high-temperature (> 150/sup 0/C) and intermediate-temperature (90 to 150/sup 0/C) hydrothermal convection systems, low-temperature (