The 1993 stimulation tests carried out in well GPK1 at Soultz, Alsace, France, showed that out of the thousands of fractures cross-cutting the granite, only a limited number of natural fractures contributed to fluid flow. A comparison between the petrophysical properties of the rock and the interpretation of the hydraulic test results is proposed as a means of determining the distribution of the natural permeable fractures. Gamma-ray spectral logs and other geophysical logs (caliper, bulk density, P-wave slowness, and photoelectric factor) revealed some petrographical variations within the Soultz massive granite, as well as altered/fractured zones that act as preferential pathways for fluid flow as indicated by flow logs. Fluid losses during circulation tests have been estimated on the basis of statistical analysis of gamma-ray readings, and of potassium and thorium contents.