Quantification of the systemic permeability of geothermal reservoirs is essential to the assessment of their economic feasibility. Here we investigate experimentally the role of fractures on the permeability of a 400 m-thick unit of Permo-Triassic sandstone from the EPS-1 exploration borehole in the Upper Rhine Graben near Soultz-sous-Forets (France). The permeability of initially intact sandstone samples was measured before and after the introduction of a through-going tensile fracture. While the permeability of the fracture-free samples varied over five orders of magnitude between 1 x 10(-19) and 1 x 10(-14) m(2), the presence of through-going fractures increased sample permeability to between 8 x 10(-14) and 4 x 10(-12) m(2). Using the fracture aperture of open fractures provided by borehole televiewer data, we model the equivalent permeability down the borehole to be between 7 x 10(-18) and 3 x 10(-12) m(2), which is in agreement with values of hydraulic conductivity determined using borehole tests. Overall, these equivalent permeability values are not sufficiently high to sustain hydrothermal convection at Soultz-sous-Forets, highlighting the need for continued anthropogenic stimulation.