Fractured Continental Crust : Permeability, Fluid-Flow, And Connectivity Of Fractures In The Crystalline Basement

Abstract

The permeability 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 of a representative volume of that system. Permeability decreases with depth. Permeability can be derived from hydraulic well test data in deep boreholes. Only a handful of such deep wells exist on a worldwide basis. Consequently, few data from hydraulically tested wells in crystalline basement are available to the depth of 4–5 km. The permeability of upper crust 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 values ranging over nine log-units from 10-21 to 10-12 m². This large variance also decreases with depth, and at 4 km depth, a characteristic value for the permeability is 10-15 m². At depths below the deepest wells down to the brittle ductile transition zone, evidence of permeability 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. The existence of deep circulating systems (E.G. Thermal springs), huge water table fluctuations due to earth tides, or long-lasting hydraulic tests show that the fracture pore space is interconnected on a large scale and that the crust reacts hydraulically coherent.