Page 50 - Geothermal Energy Systems Exploration, Development, and Utilization

26 1 Reservoir Definition
well documented in the European Variscan belt where high paleogradients de-
termined from mineral assemblages show that a regional geothermal system,
responsible for many ore deposits (Au, U, etc.), has been generated during the
late orogenic evolution of this collision belt (Bouchot et al., 2005). The melting
of large mid-crustal zones has been enhanced by the fertility of the crust rich
in radioelements and hydrated minerals generating large volume of migmatites
and granites over a long period, from 360 to 300 Ma (Ledru et al., 2001). This
situation reflects probably what is occurring within the Tibet Plateau – crustal
thickening resulting from the collision between Asia and India being responsible
for the development of migmatitic layers at depth. Taking this time delay related
to the progressive re-equilibration of the isotherms in the thickened crust, such
collision plate boundaries can be considered as favorable zones for high geother-
mal gradients. Moreover, like in the case of active margins, the concentration of
radioelement-rich geological units (differentiated granites, uranium-bearing sed-
imentary basins, volcanic ash flows, overthrust Precambrian radiogenic granites,
etc.) in the upper crust contributes to the thermal budget of the continents over
several hundreds of million years.
The location of high geothermal gradients in the vicinity of transform margins
and of thermal anomalies along continental-scale strike-slip faults can be related
to thickening processes inherited from an early stage of collision, or linked to
zones of pull-apart extension (that can be assimilated to the general case of rift
systems), or a combination of both processes. In the case of the San Andreas Fault
and its satellites in Nevada, it seems that the dominant feature for exploration
at the regional scale is the presence of structural discontinuities bordering such
pull-apart basins (Figure 1.14, Faulds, Henry, and Hinz, 2005; Faulds et al.,
2006).
Within plates, out of these plate boundaries, the lithosphere is considered as
stabilized and the main mechanism of heat transfer is conduction. Depending on
its composition (i.e., conductivity of its main lithologies) and thickness, geothermal
−1
◦
gradients vary between 15 and 25 Ckm . The main source of thermal anomalies
is the presence of highly radiogenic lithologies such as alkaline and aluminous
granites, uranium-bearing sedimentary basins, or highly conductive materials
(massive sulfide). The radioactive decay is the cause of heat anomalies in the
vicinity and at the apex of these radiogenic bodies, generally of small to medium
amplitude and wavelength (Figure 1.5). This is the model on which exploration
of deep geothermal resources is done presently in the Southern Australian craton
(McLaren et al., 2002; Hillis et al., 2004). Highly radiogenic Precambrian granites
−3
(∼16 mW m ), outcropping in large ranges and found laterally at the base of
a Paleozoic sedimentary basins resting unconformably over this basement, are
considered as the source of local thermal anomalies that are superposed to a regional
anomaly know as the south Australian heat flow anomaly (SAHFA) (McLaren et al.,
2003; Chopra and Holgate, 2005). Paralana hot springs are observed along the main
faulted contact between the basement and cover sequences and uranium-bearing
sediments deposited during the erosion of the radiogenic Precambrian granites are
presently exploited by in situ recovery (Berveley mine). The company Petratherm