We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagen- esis conditions similar to those ofdeeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of syn- kinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195°C to 268°C. Permeability measurements on plugs ori- ented parallel with the principal axes of the finite strain ellip- soid showthat the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sand- stone (10–2 md for Y against 10–3 md for X, Z, and the pro- tolith, measured at a confining pressure of 20 bars). Micro- structural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest perme- ability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines repre- senting the local slip direction on the fault surface.Wesuggestthat a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithol- ogies in deep diagenesis conditions.
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