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Granulation caused by cataclasis reduces porosity and permeability in indurated quartz sandstones of the Simpson Group. Pore aperture size in granulated rock also is drastically decreased because breakage of grains and spalling of quartz overgrowths leads to a bimodal distribution of grain sizes: subangular to well-rounded, fine- to medium-grained quartz in an angular silt- and clay-sized quartz matrix. This mechanical reduction in pore aperture size creates the potential for fault-sealing traps. Granulation occurs along individual or anastomosing seams up to a few millimeters thick and is more pervasive near major faults. Undisturbed blocks, up to 30 cm across, of porous and permeable sandstone commonly are bounded in three dimensions by planar seams of tight granulated ock. This cellular nature influences the entrapment of hydrocarbons, as shown in asphalt quarries, and could affect the degree and type of diagenesis.
Because of the abundance of granulated material associated with major faults, it appears possible that faults may serve as seals even when sandstone is faulted against sandstone.
In friable sandstones studied, the clay coatings binding the grains are disrupted, fragmented, and then rearranged by grain rotation along insignifcant faults with about 15-cm throw, leading to a pronounced decrease in porosity and permeability as well as significant reduction in pore aperture size. The pore aperture size change associated with these friable sandstones in fault zones is not sufficient to constitute a seal.
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