Page 66 - Geothermal Energy Systems Exploration, Development, and Utilization

42 2 Exploration Methods
include subsurface borehole imaging and analysis, gravity surveys, resistivity
studies, and seismic reflection data (as discussed more thoroughly in geophysical
section). The geological and geophysical data sets can then be synthesized to
generate sophisticated 3D geological and stress/strain models of the geothermal
system, which can provide critical information for targeting favorable drilling sites
and ultimately developing a geothermal system. It is important to note, however,
that in some settings (highly vegetated areas or areas of low relief in basin interiors,
passive continental margins, etc.), field-based geologic studies yield limited results.
In such areas, analysis of the stratigraphic and structural framework and delineation
of geothermal reservoirs depends largely on geophysical investigations and their
geologic interpretation.
Recent studies of fields in the western United States and western Turkey have
revealed several favorable structural settings for geothermal activity in these young
largely amagmatic extensional domains. Such settings, investigated by analysis
of the 3D geometry and kinematic evolution of fault systems, include discrete
steps in fault zones or belts of intersecting, overlapping, and/or terminating
faults (Figure 2.1; Faulds et al., 2006). In addition, most fields are associated with
steeply dipping faults and, in many cases, with Quaternary faults. The structural
settings favoring geothermal activity generally involve subvertical conduits of highly
fractured rock along fault zones oriented approximately perpendicular to the least
principal stress. Features indicative of these settings that may be helpful in guiding
exploration include (i) major steps in range-fronts, (ii) interbasinal highs, (iii)
mountain ranges consisting of relatively low, discontinuous ridges, and (iv) lateral
terminations of mountain ranges (Faulds et al., 2006). Even magmatic systems
such as those in Iceland are also controlled by tensional fractures (Gudmundsson,
1999, 2000). However, dilational fault segments are not the only type of conduit for
hydrothermal systems. It is known, for example, from other fields such as the EGS
site in the Coso geothermal system, that critically stressed faults can control fluid
flow (Sheridan and Hickman, 2004).
However, the above examples represent amagmatic or magmatic geothermal
systems in which tectonic features are relatively well exposed on the surface. In
these cases, surface features that constrain the location, geometry, and controls of
the geothermal system can be synthesized with geophysical data and extrapolated
into the subsurface to define the extent of the geothermal reservoir. This is not
possible in many parts of the world with significant geothermal potential, including
some tectonically active regions in relatively moist, highly vegetated areas (e.g.,
Larderello, Italy; Oregon, USA), and deep sedimentary basins on passive continental
margins or in continental interiors (e.g., Paris and North German basins).
In areas with limited surface exposures, several methods must be combined to
generate an integrated geological model. These include a comprehensive assess-
ment of the geodynamic history of the region, various geophysical investigations,
and quantification of both the geothermal gradient and stress/strain along inter-
preted faults or within sediment-hosted geothermal reservoirs. Critical geophysical
methods include gravity, MT, resistivity, and seismic reflection surveys. The gravity
and seismic reflection data would collectively indicate the location of major faults