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Deformation band analysis, Hillside Canyon, Utah
Deformation band shear zones play an important role in the migration of hydrocarbons and hydrothermal fluids through permeable sandstone reservoirs, acting as either conduits or barriers. Formed as planar zones of shear accommodation in porous materials by porosity collapse and grain-scale cataclasis, deformation bands significantly reduce the porosity and permeability across the width of the band. Deformation bands have been studied extensively in porous sandstones, however there are little data available in regards to how their characteristics vary between sandstones and conglomerates. These structures were studied in Hillsdale Canyon, Utah, located on the western margin of the Colorado Plateau, where they are found in quartz-rich litharenites of the Late Cretaceous Wahweap Formation. The orientation of deformation is consistent with the south-directed compressional force of the Rubys Inn thrust fault formed by collapse of the Marysvale volcanic complex to the north. This field area provides an exceptional opportunity to observe changes in deformation band strain characteristics through varying lithologies. Seventeen outcrop exposures of sandstone and conglomerate were quantified in terms of orientation, magnitude of offset, thickness, spacing, and areal density. Deformation bands most often occur in sandstones as single bands ranging from 1 to 10 millimeters in width and in the conglomerates as layers of millimeter wide bands forming zones of subparallel bands up to 8 centimeters thick. Thin section examination performed on fifteen collected samples revealed the porosity is significantly reduced from 20-30% in the host rock to 3-11% in the deformation bands. Scanning electron microscope analysis further revealed zones of very fine-grained cataclasis. Average grain size of the host rock measured in thin section, 0.36 millimeters for the sandstones and 0.28 millimeters for conglomerates, is reduced to an average of 0.15 millimeters in the deformation bands. Some samples contain a zone of increased porosity through intense fracturing and grain dissolution along a transitional zone between the undeformed host rock and deformation band. After heavy rain in the field area, the host rock adjacent to the deformation bands appeared darker indicating preferential wetting along the structures. A comparison of data from sandstones and conglomerates revealed differences in patterns of deformation, including spacing and thickness, which can be observed in outcrop across juxtaposed lithologies. These new data enhances the understanding of deformation band formation in sandstones and conglomerates allowing geologists to construct a more accurate subsurface model to aid in the prediction of fluid migration through sandstone and conglomerate reservoirs.
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