Some proponents of geothermal energy have described this energy source as renewable, but many geothermal fields show declines in output as exploitation proceeds. On the other hand, those who would call geothermal a depletable energy source have to explain how some mature fields are able to produce with negligible declines and no apparent limit to the amount of recoverable energy. The confusion arises out of the attempt to describe geothermal resources using inappropriate conceptual models of how resources behave. This paper presents a conceptual model that is better suited to describe the expected performance of a geothermal field over its entire life cycle. The paper also describes a set of terms that can be used to quantify geothermal resources and to guide plans for development.

The Rye Patch geothermal field is a classic Basin-and-Range resource with a "blind" anomaly, moderate temperature and "deep circulation" as the source of heat. This paper presents a conceptual model of the field based on lithologic, geophysical and temperature logs as well as production, injection and pressure interference data from 8 deep wells drilled to date. For the hottest well in the field (405°F), we have conducted well bore heat transfer modeling to resolve a major observed discrepancy between the flowing and static temperature profiles, the latter being shown to be affected by downflow of cooler water. The conceptual model has been developed to fit all available resource information, and had been used to estimate reserves: a minimum of 36 M Wand a most-likely level of 64 MW for a 20-year project life. Based on well test data, the maximum gross power production capacity from the existing wells is estimated at 21 MW. The existing wells are capable of supplying the 12.5 MW plant installed at the site if one or two additional wells are drilled for injection.

Existing geologic models of geothermal systems work reasonably well for the exploration of systems clearly associated with active, or recently active, volcanic centers. The poor results of exploration drilling undertaken during the last few years in the non-volcanic systems of the western United States, however, indicate that the model being used for exploring these systems requires significant modification to be useful. Based on considerations of local and regional geology, as well as on drilling results, it is proposed that the presence of large tilted fault blocks, characteristic of extended terrains, are essential to the development of this type of non-volcanic geothermal system.