Physical, petrographical and 3D imaging of normal faults damage zone (Soultz granite)
Abstract
Structure of a fault zone is charaterised by a fault core (gouge, cataclasite, mylonite), a damage zone (small faults, fractures, veins fold) and a protolith (Caine et al., 1996). We can clearly describe these structures in the Soultz-sous-Forêts granite (HDR Project, France). There is a correspondance between these structural and petrogrphycal descriptions (Genter, 1989, Jaquemont, 2002), but also with the petrophysical properties. The petrographycal study revealed three types of material : a fresch granite, a pervasive alteration granite and a fractured-altered granite. The first type of alteration is characterised by transformation of primary minerals like biotite, amphiboles and plagioclase. Biotite is mainly altered in chlorite and sometimes in celadonite, epidote, hydrogarnet, and carbonates. Plagioclase is mainly altered in corrensite, calcite and/or illite. The second type of alteration is influenced by fluid circulations in fractures and is located in the matrix of the damaged zone around. The main minerals present in the fractures were phyllosilicates (illite, chlorite, tosudite..), carbonates and quartz. Petrophysical study showed a large variation of physical properties (porosity, P-wave velocities, thermal and electrical conductivity) of the granite. We focused on porosity and noticed a large variation of the mercury injection porosity measurements between 0.2 to 10.3 %. There was a clear relationship between alteration type and porosity values. Pervasive alteration samples were very low porosity samples (around 0.5 %) whereas the porosity of fracture alteration type samples ranged between 1 % and 10 %. Permeability measurements on very porous samples indicated a very low permeability. This could be explained by the amount of illite that controls the fluid pathways around 0.1µm in the rock matrix. Computered X-ray microtomography were performed on the three types of granite. We particularly looked at the fractures and the damaged zone. We looked at capillary flow in a fracture and 3D evolution of the fluid from the fracture to the damage zone to deterrmine the pathways to one zone to an other. This study confirme that each zone of the fault acts as a specific fluid pathway where the structure, the petrography and the physical properties are clearly defined.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....12914S