DESCRIPTION AND QUANTIFICATION OF VEIN ALTERATIONS - PALAEOPERMEABILITIES IN THE SOULTZ-SOUS-FORETS GRANITE (BAS-RHIN, FRANCE)

Souitz-sous-Forets is one of the designated deep geothermal 'Hot Dry R ocks' test sites. Three boreholes have been drilled: GPK1, GPK2 which are the geothermal fluid transport system and a reference hole EPS1 wh ich has been fully cored. A large database for EPS1 has been collected using cores as well as BHTV borehole wall imagery studies. These deta iled 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 o f this study is to describe and understand the mechanisms of fluid cir culation that were responsible for the propagation of alteration throu gh fractures (vein alterations). Observed correlations between the par ameters: fracture orientation, thickness, density and nature of minera l phases occurring within fractures, demonstrate that: (1) the mineral s which seal fractures define three general types of alteration (quart z-illite, calcite-chlorite, hematite). (2) fracturing, although genera lly occurring in two main systems (N005 degrees E, 70 degrees W et N17 0 degrees E, 70 degrees E) is not homogeneously distributed with depth . Ten other families of fractures have been described throughout the b orehole. Clear relationships between the mineral assemblage of fractur es, their depth occurrence and their orientation have been detected. S ixteen levels (of constant characteristics) have been thus identified by dividing the granite according to these criteria, into zones of dif ferent depth. It allows the calculation of the equivalent hydraulic pr operties 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 averag e permeability of the whole core is 1,5.10(-6) m/s. The study of vein alteration in the Soultz EPS1 core shows that different fracture netwo rks are present throughout the core, implying that the estimation of t he average palaeopermeability is not representative of that on a local scale and not constant with time.