SPATIAL DISTRIBUTION OF DEFORMATION BANDS AND PROCESSES OF FORMATION WITHIN POROUS SANDSTONE: SOUTHEASTERN UTAH

Individual deformation bands within porous sandstones reduce permeability and form barriers to fluid flow. A network of deformation bands can destroy reservoir connectivity and impose compartmentalization; however the extent of compartmentalization depends on spatial arrangement. Consequently, the spatial arrangement of deformation bands affects fluid flow within sandstone reservoirs, but it is unknown whether deformation bands occur at random locations or whether they follow systematic patterns.

This study, undertaken at Arches National Park, examined the spatial distribution of deformation bands in the Moab Member of the Jurassic Entrada Sandstone. Field observations accurately quantified spatial relationships between deformation bands across six orders of magnitude (<1 cm to ~10 km). A scanline 9.4 km long was constructed within the Moab Member, to measure spacing and orientation for individual deformation bands (N = 1467). Correlation count methodology was used to quantify the spatial arrangement of deformation bands.

Correlation count analyses show that deformation bands within the Moab Member are clustered at two different ranges of length scale. Spatial arrangement is a natural fractal at short length scales (cm to m). However, the spatial arrangement of deformation bands is non-fractal but non-random at long length scales (m to km). Down-plunge projection shows that at long length scales, regions of abundant deformation bands correspond with hinge zones of subtle tectonic folds.

The fractal nature of deformation bands, evident at short length scales, may be attributed to strain hardening mechanisms of deformation band development. Deformation bands interact with one another due to strain hardening, resulting in spontaneous self organization. At long length scales, deformation band occurrence is controlled by lateral variation in strain magnitude due to other deformation processes. Locations of deformation bands are non-random at all length scales considered, but spatial organization is externally imposed at long length scales and self organized at short length scales.