Geothermal Resource­Base Assessment

Geothermal Resource­Base AssessmentPrevious analyses have suggested that the amount of thermal energy available for Enhanced Geothermal System (EGS) development is enormous. However, these earlier works did not use detailed geologic information – and, as a result, the methodologies employed and resulting resource estimates 2­3 were, by necessity, somewhat simplified.

This study utilizes published geologic and geophysical data for the United States to calculate the stored thermal energy (or “heat in place”) on both a national and state level, at depths from 3 to 10 km. The methodology, resource types considered, and the resource­ base calculations are included in this chapter. Recoverability, or useful energy, is discussed in Chapter 3 of this report. A depth of 3 km was selected as a cutoff for upper depth because, outside of the periphery of active magmatic and hydrothermal systems, temperatures in excess of 150°C at less than that depth are rare.

Several classes of geothermal resources are discussed in this chapter (Table 2.1). In earlier analyses – USGS Circular 726, USGS Circular 790, and USGS Circular 1249  – the geothermal resource was divided into four major categories: hydrothermal, geopressured, magma, and conduction ­dominated (Enhanced Geothermal Systems or Hot Dry Rock). The resource classes that are discussed in this report include

1) sedimentary Enhanced Geothermal Systems (EGS)

2) basement EGS,

3) geo-pressured ­geothermal systems

4) coproduced fluids (hot aqueous fluids that are produced during oil and gas production). Brief mention is also made of supercritical/volcano (i.e., igneous) geothermal systems.

There is overlap of some of these categories, which will be explained in the discussion that follows.