PETROPHYSICALLY SELECTIVE COMPACTION BANDS IN NAVAJO SANDSTONE

Tectonic compaction of highly porous sandstone can localize into tabular discontinuities known as compaction bands. Although rare, such bands locally occur in the highly porous Navajo Sandstone in southern Utah. We have investigated bands that are E and W dipping, locally parallel to dune cross-strata, and coexistent with cataclastic shear deformation bands in stacked dune sequences. The compaction bands involve crushing of well-rounded quartz grains in addition to pressure solution at grain contacts. The occurrence of compaction bands is closely related to sedimentary facies and associated petrophysical properties of these eolian deposits. The eolian sequences show vertical variations in poro-perm properties, and the compaction bands are extremely selective and prefer the parts of the dunes with the highest porosity and permeability, i.e. the lower (but not basal) parts of the sequences. We have used a mini-permeameter (Tiny-perm) to quantify the sensitivity of compaction bands to poro-perm properties, which show that Tiny-perm values corresponding to 8-10 darcy are required for compaction bands to form in this area. Although the conversion from Tiny-perm values to permeability values is uncertain and the numbers quoted here possibly too high, the values indicate that very high permeability (and porosity) values are required. Our field observations show that additional shearing reduces this sensitivity. Hence, while compaction bands are localized to the most porous and permeable lower part of the dune sequences, compactional shear bands are more persistent, crosscutting several dune sequences in some cases. The findings support the idea that compaction bands are rare because they require very high porosity and permeability. Where found, compaction bands involve significant permeability reduction with respect to their host rock and have been claimed to be able to compartmentalize reservoirs and trap fluid. However, the selectivity of compaction bands with respect to permeability described here suggest that they equally well may have a positive effect, improving sweep as they slow down flow in the most permeable and channelizing parts of the reservoir.