Page 46 - Geothermal Energy Systems Exploration, Development, and Utilization

22 1 Reservoir Definition
this approach is highly dependent on economical indicators that are not linked
to geology (price of energy, incentives politics for access to renewable energies,
etc.), reference to present-day parameters will be provided for the different types of
reservoirs.

1.3.1
The Geology of Potential Heat Sources

To get heat is the first condition for defining a geothermal reservoir. How can we
explore potential heat sources? It has been shown that thermal boundary conditions
(the mean annual surface temperature, temperatures at depth estimated from the
P and S velocity anomalies) and thermal properties of the main lithologies and
structure at depth enable the first calculation of extrapolated temperature at depth
and thus the delineation of potential zones of high-thermal gradient.
Such zones can also be determined through a geological empiric approach. Heat
is transferred within the crust through two mechanisms:
• The main active and permanent phenomenon at the scale of the continental
crust is the conduction of heat. In conduction, heat moves through the material
from a hotter to a cooler zone. The feasibility and intensity of such transfer is
directly linked to the thermal properties of the mineral constituting the rock that
is evaluated as the thermal conductivity. As continental crust is heterogeneous
and a result of the superposition of layers with different conductivity properties
(stacked allochthonous units over autochthonous cover sequence or basement in
orogenic zones, sedimentary basins over basement within intracratonic zones,
etc.), conduction will not be homogenous at the scale of the whole continental
crust. Highly conductive zones such as fractured granites will be explored with
interest while refractory units such as mafic units will be considered as potential
thermal insulator.
• Inconvection,heat is transported bythe movement of hot material.The ascent and
emplacement of a granitic body or of a volcanic dyke network is a typical example
of convection where heat is transferred from deep source and then dissipated by
conduction in the host rocks at shallow level. Contact metamorphism is a direct
expression of the elevation of temperature with respect to extreme geothermal
gradients reaching 500 C for granites emplaced at around 5 km depth. Globally,
◦
convection leads to anisotropic diffusion of heat; the movement of hot material
being, most of the time, controlled by the permeability system of the continental
crust, mainly fracture network.
The past or present geodynamic context gives a first-order constrain on the loca-
tion of favorable and unfavorable geodynamic sites for high geothermal gradients.
Conduction is directly controlled by the thickness, heterogeneity, and composition
of the continental crust, whereas convection processes are mainly located within
active zones of magmatism and metamorphism.
Rift in accretionary systems are characterized by thinned crust and lithosphere, in
relation with asthenospheric doming and upwelling. This definition covers both