Spatial arrangement of deformation bands and processes of formation within porous sandstone : Arches National Park, Utah

Abstract

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. 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 arrangement of deformation bands in the Moab Member of the Jurassic Entrada Sandstone. Field observations of this unit accurately quantified spatial relationships between deformation bands across 6 orders of magnitude (1 cm to 10 km). A 9.4-km-long scanline was constructed within the Moab Member, to measure spacing and orientation for individual deformation bands (N = 1467). Correlation count methodology was used to determine the spatial arrangement of deformation bands. Correlation count analyses show that deformation bands within the Moab Member are natural fractals (self organized) at short length scales (cm to m). Although the spatial arrangement of deformation bands also is non-random at long length scales (m to km), spatial arrangement is not fractal (externally organized). 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 overall strain hardening during deformation band development. Deformation bands interact with one another due to strain hardening, and this results in spontaneous self organization. At long-length scales, deformation band occurrence is controlled by lateral variation in strain magnitude due to other deformation processes. Deformation bands are non-random in space, but the spatial organization is externally imposed