Laboratory observations of the hydraulic behavior of a permeable fracture from 3800 m depth in the KTB pilot hole

A thorough search through core recovered from the German Continental Deep Drilling Program (KTB) pilot hole in the vicinity of the high-permeability zone near 3800 m depth revealed three intervals containing undisturbed fractures that had the appearance of high-permeability fluid conduits. We report here a detailed study in the laboratory of one of those fractures, an epidote-filled fracture in amphibolite, aimed at determining if the fracture was fluid conducting at in situ stresses and, if so, what characteristics of the fracture allowed it to remain open over geologic times. We found that the presence of the fracture made the core sample 4-7 orders of magnitude more permeable than the host rock at confining pressures near 100 MPa. Fine scale measurements of the shape of the fracture aperture, made with a specially designed profilometer, suggest that water flows through a distributed network of very small (order of 1 mm) high-aspect-ratio (order of 10) channels that cover 30-70% of the fracture surface. In contrast, fresh, well-mated fractures in granite at comparable conditions and for a comparable amount of fracture closure are slightly more compliant normal to the fracture plane and more prone to concentrate flow into channels than the mineralized fracture, suggesting fundamentally different hydraulic conduction mechanisms in fresh versus mineralized fluid-conducting fractures.