Page 65 - Geothermal Energy Systems Exploration, Development, and Utilization

2.2 Geological Characterization 41
can also become fairly expensive depending on the geological environment and the
depth anomaly of interest.
If the temperature field of an area is well constrained, the characterization of
the potential reservoir rock is of utmost importance. Fluid-hosting rock types
with a high porosity include sedimentary facies, such as sandstone-rich alluvial
fans within sedimentary basins, or some volcanic rocks, such as vesicular basalt
or some ignimbrites. The largest geothermal fields currently under exploitation
reside in rocks that range from limestone to shale and volcanic rock to granite.
Volcanic rocks are the most common host for geothermal reservoirs. The lateral
extent and deposition of sedimentary and volcanic lithologies is, however, limited
and – especially in the case of sedimentary basins – structurally controlled. Highly
porous sandstones with low clay content are common for alluvial fans and channel
deposits in alluvial plains. Also, aeolian and barren sandstones are highly porous
successions. Because the placement of alluvial fans and fluvial deposits are typically
strongly controlled by syntectonic processes or paleorelief, the tectonic and geody-
namic history of a basin may reflect the possible depocenters of potential reservoir
rock types. As such, facies types and highly porous sandstone sequences therein
are of limited spatial extent and strongly tied to structural highs and graben flanks.
Generally, potential reservoir rocks are limited to the basin rim, whereas the basin
center deposits encompass low porous finest grained siliciclastics, evaporites, or
limestone, depending on climatic conditions and general evolution of the basin. In
the case of the South Permian Basin System in Central Europe, the southern flank
of the basin system more likely contains potential reservoir rocks due to erosion
of volcanic rocks of the initial basin phase and proximity of the high mountain
ranges of the Variscan orogeny. Primary reservoir rocks, however, can be altered or
cemented during diagenesis, basin inversion, and related hydrothermal phases, or
can simply be eroded. Thus, the comprehensive geodynamic history of a geological
system needs to be analyzed to delineate potential reservoir rocks hosting fluids
and/or potential for hot dry-rock treatment.
While the hosting of thermal fluids can be controlled by facies, as described
above, the channelways for fluids are instead controlled by structures (e.g., faults
and fractures) in many geothermal systems. Moreover, in tectonically active regions,
such as the largely amagmatic extensional Basin and Range province in western
North America, highly permeable and porous fault breccia may actually host
geothermal reservoirs in highly faulted areas (Curewitz and Karson, 1997; Faulds
et al., 2006). It is therefore critical to determine which types of structures and which
segments of faults are most favorable for providing fluid pathways in geothermal
systems. Such structures must be characterized in known geothermal systems
in order to guide exploration for new systems and expansion of existing systems
(whether conventional or EGS). To facilitate this important characterization of
structural controls, integrated field-based geological and geophysical studies are
necessary. The field-based studies involve detailed geologic mapping of structures
(e.g., faults, fractures, and folds), stratigraphic units, and surficial geothermal
features, as well as kinematic analysis of faults, such that the geometry and
kinematic evolution of controlling fault systems are defined. Geophysical methods