Page 63 - Geothermal Energy Systems Exploration, Development, and Utilization

2.2 Geological Characterization 39
understanding of geomechanics in the subsurface. The geological characterization
must therefore also include various methods that constrain the stress field of a
reservoir and elucidate the stress states along faults slated for stimulation.
In summary, geothermal exploration for EGS means, on the one hand, that a
reservoir should be understood as a part of a complex geosystem and, on the other
hand, it is part of a complex mechanical rock response in the subsurface react-
ing – either positive or negative – to all manipulations that need to be done from
exploration over reservoir access to exploitation. Consequently, geothermal explo-
ration for EGS should encompass a broad palette of approaches from geosystem
analysis to reservoir characterization to reservoir geomechanics.
This chapter describes geological criteria, as far as they have been defined, and the
most common geological, geochemical, and geophysical methods of geothermal
prospecting, as well as trends and requirements for future developments.

2.2
Geological Characterization

Choosing a favorable location for a potential EGS site requires careful consideration
of the geologic setting, including heat flow, stratigraphy, and structural framework.
Although high permeability is critical for a conventional geothermal system, it is
not required for an EGS site because the flow rate and productivity of a well can
be increased artificially. Nonetheless, the delineation of suitable geologic settings
is the most important aspect of EGS reservoir exploration. Exploration strategies
must target accessible and extractable sources of thermal water in large enough
quantities to promise sustainable power and/or heat extraction.
The geological settings for geothermal reservoirs can vary widely. High enthalpy
systems typically occur in magmatic, extensional, or transtensional settings. Mag-
matic settings include arcs (e.g., Central America and parts of the Mediterranean),
both continental and oceanic rifts (e.g., Basin and Range and Iceland, respectively),
hot spots (e.g., Hawaii), and transtensional pull-aparts in strike-slip fault systems
(e.g., Salton Trough, California). However, high enthalpy geothermal systems are
also relatively common in amagmatic extensional and transtensional settings, as
most evident in the Basin and Range province (USA) and western Turkey (Akkus¸
et al., 2005), where normal fault systems are the primary control on geothermal
activity. Lower enthalpy systems are also found in the above settings as well as
in relatively quiescent tectonic environments, deep sedimentary basins (e.g., Paris
and North German Basins), and convergent plate margins (e.g., Alpine orogenic
belt; Hurter and Haenel, 2002).
Although the general tectonic settings favorable for geothermal activity are well
known, the detailed lithologic and structural controls on individual systems are
generally not well characterized. It is, however, crucial in geothermal exploration
to identify geological units and structures that host hydrothermal fluids. These
reservoirs can be governed by either pore space (i.e., a high porosity) or fractures
(i.e., high fracture density). Fluid pathways are critical for the productivity of a