EGS resource accessible by drilling is mainly dependent of the deep-seated geology to
minimum depths ranging between 1 and 5km. The main favorable geological conditions in
France and Europe correspond to the hottest geothermal areas, e.g. first order
anomalies, characterized by recent volcanic activity or magmatic intrusion, deep
sedimentary basins, deep fractured basement or granites having high heat production
derived from natural radioactive elements.
Based on the litho-tectonic characteristics observed on the EGS Soultz site (Rhine
graben, France), a first classification of the EGS types in France and in Western
Europe was carried out. The prerequisite conditions for an EGS system site are
related to sufficient in-situ temperatures, favorable seismo-tectonic conditions and
adequate hard rock lithologies. For temperature higher than 180°C, an estimation of
the EGS potential of Europe was calculated with a simple volumetric method based on
temperature extrapolated at 5km depth.
In France and in Western Europe, two main types of EGS system have been evidenced
according to their geodynamical context: (1) the western European rift system which
contains the Upper Rhine graben and then the Soultz and the Basel sites, and (2) some
complex back-arc basins located inside the Alpine belt and related to subduction
process (Pannonian basin, Tuscany). As a result, the hottest zones matching with the
Tertiary graben location represent a minimum area of about 30,000km² in France. Other
promising EGS types occurred in Europe and correspond mainly to deep sedimentary
basins and their underlying
Paleozoic basement. New areas corresponding to second order geothermal anomalies or
hidden potential geothermal structures were also investigated in terms of deep seated
geology based on geophysical data mainly. For example highly populated areas located
in the central part of the Paris basin in France were studied through gravity and
magnetic data combined with deep borehole information. A series of both non magnetic
and low density zones was delineated below the Mesozoic and the Cenozoic sediments of
the Paris basin.
These large-scale basement areas could mainly correspond to deep Paleozoic granites,
gneiss or migmatites potentially able to produce heat.