In EGS projects, fault zones are considered as the structures controlling deep flow at the reservoir scale. Using a large set of petrophysical properties (porosity, density, permeability, thermal conductivity [TC]) measured on cores collected along the EPS-1 borehole, a model of fault zone is proposed to describe them. A fault zone is a complex structure, showing different parts with different kinds of deformations and/or materials that could explain chemical and physical processes observed during fluid-rock interactions. The different parts composing the fault zone are: (1) the fault core or gauge zone; (2) the damage zone; (3) and the protolith. They are usually heterogeneous and show different physical properties. The damage zone is a potential high permeability channel and could become the main pathway for fluids if secondary minerals seal the fault core. Porosity is the lowest within the protolith, between 0.5 and 1%, but can go up to 15% in the fault zone. Permeability ranges from 10 À20 m 2 in the fresh granite to, at least, 10 À15 m 2 in the fault core, and TC ranges from 2.5 W K À1 m À1 to 3.7 W K À1 m À1 . Finally, variations in specific surface are set over two orders of magnitude. If the lowest values usually characterize the fresh granite far from fault zones, physical properties could show variations spread over their whole respective ranges within these fault zones.

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Thanks to thermal conductivity maps, obtained from Optical Scanning method, and porosity maps, inferred from thermal conductivity maps, we have studied petrophysical heterogeneities commonly present in a granitic and sandstone geothermal reservoir (fault zone and permeable layers, respectively). The maps allowed determination of thermal conductivity and porosity variation to millimeter resolution, at a core scale. They permitted precise quantification and determination of the size of petrophysical heterogeneities (thermal conductivity and porosity) induced by rock variability..

This study, based on petrophysics, is linked to the geothermal project developed at Soultz-sous-Forêts, in northeastern France. In such a context, understanding and modelling the thermo-hydro-mechanical and chemical processes implies a good knowledge of the porosity network geometry and its evolution during alteration. Various physical properties (permeability, specific surface and porosity) were measured on samples from the protolith and damage zones of fault zones in the geothermal reservoir exploited at Soultz-sous-Forêts. Measured porosity values vary from 0.2 to 10%, and using structural parameters (threshold values, specific surface and porosity distribution), three groups are identified. Each of them shows a specific pore shape distribution and the porosity increase within a given group is assumed to be associated with an increase of pore number rather than an enlargement or shape modification.