Page 135 - Geothermal Energy Renewable Energy and The Environment

Resource Assessments 121

Discovered Undiscovered
Economic Reserve

Resource
Accessible Sub- economic

Residual Resource base

Inaccessible

FIGUre 7.1 Nomenclature and parameters used to delineate geothermal resources. (Modified from
Williams, C. F., Reed, M. J., and Mariner, R. H., A Review of Methods by the U.S. Geological Survey in the
Assessment of Identified Geothermal Resources, U.S. Geol. Survey Open File Report 2008–1296, 2008.)

it is too remote from a transmission system. If a transmission line were brought in, for whatever
reason, the site would then be economically viable. In other cases, the reservoir may be exploitable
using a particular technology that is difficult to access and therefore the resource is not likely to be
brought online in the near future. In some instances, some reservoirs may not be economically via-
ble under existing tax and incentive structures, but would be with a small change in either regulatory
framework. In all of these cases, there is nothing about their physical attributes that makes these
reservoirs unsuitable for use. Rather, the immediate set of economic circumstances makes them
economically marginal or unsuitable for immediate development. Since circumstances can change
relatively quickly, these types of occurrences are added to the overall formal resource.
A so-called residual exists that represents occurrences that are theoretically accessible, in the
sense that they could be drilled or otherwise accessed, but which pose significant challenges for
development and are not seen to be economically sustainable. Finally, there is that portion of the
total geothermal energy budget that simply is not accessible using any existing or foreseeable tech-
nology. The combined total of all of these sources of heat is called the resource base.
In summary, the resource base is made up of all heat, both accessible and inaccessible that we
wish to consider. The resource is that portion of the resource base that is technologically accessible,
whether or not it is currently economically suitable for development under current market condi-
tions. The reserve is that portion of the resource base that is accessible and can be economically
developed.
It is important to realize that all of these values are mutable. The resource base can change
significantly depending on the amount of data available to characterize the resource, the state of
knowledge of the geological environment and how a resource occurs. The estimated resource base
for oil, for example, increased from 600 billion barrels in the early 1940s to over 3900 billion
barrels in 2000 (Wood and Long 2000). This does not mean that the amount of oil on the planet