Characterization of Deformation Bands Associated with Normal and Thrust Stress States in the Navajo Sandstone, Western US

Deformation bands can reduce the bulk permeability of the rocks that contain them. The magnitude of reduction can be predicted using harmonic averaging provided the spatial characteristics, thickness distributions and permeability of the bands and the host rock can be constrained. The types of bands at Buckskin Gulch (BSG) and at the Big Hole Fault (BHF) are different, and the bands at the BHF tend to be thinner and more non-random than those at BSG. These differences, which will cause differences in effective permeability, are due to some as yet unresolved combination of faulting environment and/or burial depth.

In Utah the Jurassic Navajo sandstone contains deformation bands that formed under different stress states. At BSG bands formed during Cretaceous thrusting at < 2 km, while at BHF bands formed during Late Cretaceous/Early Tertiary normal faulting at ~1.8-3.5 km. Bands at BSG occur as a conjugate set of shear bands forming at 40-50 degrees to the maximum principal stress, with a set of compaction bands normal to that stress. Shear bands along the BHF also occur as a conjugate set, with most bands defining the damage zone of that fault. Compaction bands are not observed.

Thicknesses of bands from BSG show a slightly skewed distribution, with a peak at 1-2 mm, while those from the BHF show a highly-skewed distribution, with a modal thickness <1 mm. The spatial distributions at both sites have a maximum spatial correlation at a length scale of 0.15 m. However, the correlation dimension of the bands (BHF=0.7 BSG = 0.9) indicates that the distribution is more random at BSG.

The occurrence of bands is markedly different at BSG and BHF, indicating that the bulk permeability at those sites will be different. This presentation attempts to constrain the parameters that are responsible for these differences.