The permeable crust: Geohydraulic properties down to 9101 m depth

Both boreholes of the German Continental Deep Drilling Program encountered a fluid-conducting crust over a 9101 m vertical profile. Low matrix permeability was observed on the laboratory scale (geometric mean value 7×10−20 m2 with something more than one decade standard deviation) and in situ higher values of the order of 5×10−18 to 3×10−16 m2. With respect to the scatter of measured matrix permeability there was no lithology dependence observed, whereas permeability parallel to the foliation (geometric mean value 3×10−19 m2) is significant higher than those perpendicular to the foliation (2×10−20 m2). With increasing depth the in situ permeability varies within a few orders of magnitude, showing, however, no clear depth dependence. The in situ permeability increases over 3 orders of magnitude while the effective pressure decreases by 50 MPa, whereas in contrast, the permeability of laboratory core specimens changes by 1 order of magnitude. The pilot drill hole “Vorboh-rung” and the main drill hole “Hauptbohrung” communicate through a network of conductive fractures at the bottom hole level of the Vorbohrung as well as between both bottom hole sections. The formation pressure increases with a mean gradient of 11.5 MPa km−1 from surface to 103 ± 3 MPa to 9101 m depth. Overhydrostatic increasing pore pressure and so a strong stress reduction can be excluded as a reason for the low permeability decrease observed at the bottom of the hole. Formation pressure remains hydrostatic with respect to probable increasing salinity.