Page 68 - Geothermal Energy Systems Exploration, Development, and Utilization

44 2 Exploration Methods
and areas of structural complexity, such as overlapping faults and fault inter-
sections, where fracture density would likely be greatest. The MT and resistivity
data would permit assessment of which faults or stratigraphic horizons accommo-
dated significant fluid flow. Evaluation of the geodynamic history, GPS geodetic
data, any available borehole data (e.g., breakouts and FMI (Formation Micro Im-
age) imagery), and known regional fracture patterns would then help to quantify
stress/strain along interpreted faults and fractures and thus determine which
structures would likely accommodate dilation or shearing and related maximum
fluid flow. The geothermal gradient would dictate at what levels these fluids would
provide a viable geothermal resource. In deep sedimentary basins in tectonically
quiescent areas, highly permeable sedimentary horizons, at levels deep enough
to record sufficient temperatures, typically provide the most favorable geothermal
reservoirs (e.g., mid-Jurassic Dogger Limestone and lower Triassic sandstone in
Paris basin (Dezayes et al., 2008) and lower Permian sandstone in North German
basin (Moeck et al., 2009)). In tectonically active regions, fault zones are commonly
the most important targets as they can channel geothermal fluids from deep levels
in the crust to relatively shallow reservoirs, thus providing a more accessible and
more economical resource.

2.3
Relevance of the Stress Field for EGS

It is important to note, however, that exploration not only involves the delineation
of systems and zones favorable to geothermal development but also requires the
characterization of the subsurface to optimize the development of EGS sites. Some
of the crucial aspects are determination of the stress field and the transference of
lithostratigraphy to mechanical stratigraphy in terms of reservoir geomechanics.
If the stress field is known and the general mechanical behavior of rock types is
considered, the access and further treatments of the reservoir rock can be optimally
planned.
The stress fields analyzed for a region of interest are often found to be predictable
by lithospheric scale models assuming an overall control the forces exerted by the
boundaries of tectonic plates and their relative movements. However, there are
also examples of significant deviations from that overall trend at the regional scale
(Hillis and Reynolds, 2000). In some cases, these large-scale stress orientations
hold for basement level rocks; the stress regime in the sedimentary cover seems
to be detached from the basement, for example, in the Central North Sea, where
basement rocks show similar stress orientations to the rest of Europe, while for
the sedimentary cover in the basin stress orientations varied widely (Cowgill et al.,
1995). Similarly, detached stress orientations have been described for the Canadian
shelf (Yassir and Bell, 1994) and for the Gulf of Mexico (Yassir and Zerwer, 1997).
Generally, the regional stress field can be derived on the basis of several different
types of data, as suggested by Zoback (1992) for the World Stress Map (Reinecker,
Heidbach, and Mueller, 2003) and consequently used for regional studies (e.g., the