Page 45 - Geothermal Energy Systems Exploration, Development, and Utilization

1.3 Conceptual Models of Geothermal Reservoirs 21
the source of a granitic complex that has been emplaced between 3.8 and 1.3
Ma. A long-lived hydrothermal activity is recorded in this area by both fossil
(Plio-Quaternary ore deposits) and active (Larderello geothermal field) systems
(Dini et al., 2004). In Central-Eastern Europe, the Pannonian basin is characterized
since Middle Miocene by an upwelling of the asthenosphere and thinning of the
lithosphere, responsible for coeval rifting in the basin and compression in the
flanking Carpathian and Dinaric belts (Huismans, Podladchikov, and Cloetingh,
2001). In Turkey, the collision between the Arabian and Eurasion plates has
induced the westward escape of the Anatolian block, which is accommodated by
the right-lateral movement of the Anatolian fault network. Much of the geothermal
activity appears to be focused along kinematically linked normal and strike-slip
fault systems most commonly within E-W-trending grabens (Seng¨ or, Gorur, and
Saroglu, 1985; Ercan, 2002).
Besides these active tectonic zones, other positive anomalies are mainly dis-
tributed along a series of intracontinental grabens that cut the western European
platform, corresponding to the west European rift system (D` ezes, Schmid, and
Ziegler, 2004). These rift structures, the upper Rhine graben, the Limagne system,
the Rhˆ one valley, a part of Provence, the Catalonia, and the Eger grabens, were
created in the Oligocene as a result of the thinning of the continental crust. Among
these structures, the Rhine graben has been intensively studied over the last 10
years for its potential. It is about 300 km long, with an average width of 40 km,
limited by large-scale normal faults. The post-Paleozoic sediments of the western
European platform have overlain the Hercynian basement, which is made of gran-
ite, granodiorite, or other related basement rocks (Edel and Fluck, 1989). This area
– characterized by a thin continental crust and a Moho at 25 km depth – shows
a Tertiary volcanism that occurred in the form of isolated volcanoes of alkaline
composition related to a mantle magmatic activity (Wenzel and Brun, 1991).
This preliminary analysis shows that the thermal aspect of the geothermal
systems is directlylinked and controlled bythe past and present geodynamiccontext.
This framework provides a first-order constrain on the location of favorable and
unfavorable geodynamic sites for the exploration of potential geothermal reservoirs.
In order to define conceptual models, these different contexts will be reviewed and
complemented by an evaluation of the main properties of the potential reservoir in
terms of porosity, permeability, fluid flow with respect to the stress field.

1.3
Conceptual Models of Geothermal Reservoirs

From a geological point of view, geothermal reservoirs are heated and pressurized
water and/or vapor accumulations from which heat can be extracted from the
underground to the surface. From a technical, environmental, and economic
approach, the geothermal reservoir can be defined by the cost-efficiency of this
extraction depending on the temperature, depth and size of the accumulation, the
fluid flow, and the industrial process under which it will be processed. Although