Previous 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.