Permeability differences between surface‐derived and deep drillhole core samples

Laboratory tests reveal that the permeability of samples obtained from deep boreholes is often lower and more sensitive to pressure than the permeability of common surface‐derived crystalline rocks reported in the literature. We attribute the differences in permeability behavior to the fact that surface rocks have histories of unloading, weathering and retrograde metamorphism which are not comparable to that of the deeper rocks. Weathering products that line cracks and pores of surface rocks and make these openings more difficult to close as pressure increases may account for the relatively low pressure‐sensitivity of permeability. Stress‐relief cracking in the borehole samples can also reduce the pressure sensitivity. These results have important implications for models that incorporate assumptions about the transport properties of rock at depth, such as models of heat transport or fluid pressure buildup, because many models are based on the properties of common surface‐derived rocks. Other physical properties that are controlled by cracks and pores, such as seismic velocity and electrical resistivity, may be similarly affected by differences between surface‐derived and deep rocks.