Significance of Hydraulic Conductivity as Precondition to Fluid Flow in Fractured and Faulted Crystalline Basement and Its Impact on Fluid-Rock Interaction Processes

Abstract

The permeability of the fractured upper continental crust is an inherent property of a complex system of rocks, fractures and fault-systems that characterizes the flow properties of that system. The permeability of the crystalline basement rocks decreases with depth. Permeability can be derived from hydraulic well test data in deep boreholes. Only a handful of such deep wells exist even on a worldwide basis. Consequently, very few data exist to the depth of 4 – 5 km, which is most interesting for geothermal energy use. Generally, distance and permeability of faults could be derived from long lasting pumping-tests. Derived permeabilities of the upper crust vary over a very large range depending on the predominant rock type in the bore and the occurrence of fracture and fault systems in crystalline basement. The permeability of the crystalline basement varies with time due to deformation related changes of aperture and fracture or fault geometry and as a result of chemical reaction of flowing fluids with the solids exposed along the fractures. Particularly dissolution and precipitation of minerals contribute to the variation of the permeability with time. The time dependence of κ is difficult to measure directly. At depths below the deepest wells to the brittle ductile transition zone evidence of permeability variation with time can be found in surface exposures of rocks originally from this depth. Exposed hydrothermal reaction veins are very common in continental crustal rocks and witness fossil permeability and its variation with time. A quantitative description of permeability variation with time in the deeper parts of the brittle part of the continental crust is not possible at present.