Deformation-band networks at Buckskin Gulch, Utah, and the Big Hole fault, Utah, both formed in the Navajo Sandstone with similar initial porosity and permeability, at similar burial depths, and result in similar reductions in effective permeability. However, the band networks at Buckskin Gulch, which formed in a contractional tectonic setting, appear to be much more areally extensive and are not associated with any discrete faults having displacements greater than at most a few meters and more likely only a few tens of centimeters. In contrast, the bands at Big Hole fault are generally limited to the damage zone of a about 25-m (82-ft) displacement normal fault formed in a locally extensional environment. These results suggest that deformation bands in well core from extensional settings may be indicative of discrete damage zones associated with normal faults, whereas deformation bands in well core from contractional settings may be indicative of much more are ally extensive deformation-band networks. The band networks in both cases will affect similar reductions in reservoir effective permeability, but only in the latter case will the affected area be sufficiently large to affect well performance.