PERMEABILITY AND FLUID FLOW IN THE UPPER CONTINENTAL CRUST

The permeability (k [m²]) of fractured crystalline basement of the upper continental crust is an intrinsic property of a complex system of rocks and fractures that characterizes the flow properties. Permeability can be derived from hydraulic well test data in deep boreholes. Few data from hydraulically tested wells in crystalline basement are available to the depth of 4 – 5 km. The permeability of upper crust decreases with depth and varies over a very large range depending on the predominant rock type at the studied site and the geological history of the drilled crystalline basement.

Hydraulic tests in deep boreholes in the continental crystalline basement revealed permeability (k) values ranging over nine log-units from 10⁻²¹ −10⁻¹² m². This large variance also decreases with depth and at 4 km depth a characteristic value for the permeability k is 10^-15 m².

Permeability varies with time due to deformation related changes of fracture aperture and fracture geometry and as a result of chemical reaction of flowing fluids with the solids exposed along the fractures. The time dependence of k is difficult to measure directly and it has not been observed in hydraulic well tests. Evidence of permeability variation with time can be found in surface exposures of rocks fractured at depth. Exposed hydrothermal reaction veins are very common in continental crustal rocks and witness fossil permeability and its variation with time.