Soultz-sous-Forets is one of the designated deep geothermal "Hot Dry Rocks" test sites. Three boreholes have been drilled: GPK1, GPK2 which are the geothermal fluid transport system and a reference hole EPS1 which has been fully cored. A large database for EPS1 has been collected using cores as well as BHTV borehole wall imagery studies. These detailed data present an excellent opportunity to study the structural and mineralogical properties of the Soultz granite over a depth interval of 810 m. This granite has been strongly altered by fluid percolations which have completely sealed the majority of fractures. The purpose of this study is to describe and understand the mechanisms of fluid circulation that were responsible for the propagation of alteration through fractures (vein alterations). Observed correlations between the parameters: fracture orientation, thickness, density and nature of mineral phases occurring within fractures, demonstrate that: (1) the minerals which seal fractures define three general types of alteration (quartz-illite, calcite-chlorite, hematite). (2) fracturing, although generally occurring in two main systems (N005 degrees E, 70 degrees W et N170 degrees E, 70 degrees E) is not homogeneously distributed with depth. Ten other families of fractures have been described throughout the borehole. Clear relationships between the mineral assemblage of fractures, their depth occurrence and their orientation have been detected. Sixteen levels (of constant characteristics) have been thus identified by dividing the granite according to these criteria, into zones of different depth. It allows the calculation of the equivalent hydraulic properties of the rock. The palaeopermeabilities and characteristics of the three types of alteration are variable both within a single depth zone, and throughout the entire length of the EPS1 section. The average permeability of the whole core is 1.5.10 (super -6) m/s. The study of vein alteration in the Soultz EPS1 core shows that different fracture networks are present throughout the core, implying that the estimation of the average palaeopermeability is not representative of that on a local scale and not constant with time.

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