This work presents results from a series of triaxial compression tests on two quartz sands (differing principally in grain shape), at confining pressures high enough to cause grain breakage during shearing. Tests are performed inside an x-ray scanner, which allows specimens to be imaged non-destructively as they deform. Observation of the acquired images clearly shows different mechanisms of deformation, including shearing, dilation, compaction and grain breakage. These mechanisms are investigated quantitatively through 3D measurements of local porosity, as well as strain (obtained by 3D Digital Image Correlation), which is analysed in terms of volumetric and shear components. These tools allow the transition between macroscopically dilative (typically of a dense sand at low mean stress) and compactive behaviour to be investigated. The analysis reveals that at the high end of the confining pressure range studied (100 to 7000 kPa) the more rounded sand deforms with highly localised shear and volumetric strain-the porosity fields show a dilative band within which a compactive region (due to grain crushing) grows. The more angular material shows shear strain localisation, however its faster transition to compactive behaviour (due to a higher propensity for individual grains to crush) translates to much more distributed compactive volumetric strain.

The damage zone of three small faults in the Navajo and Page formations, located on the NE side of San Rafael Swell, Utah, USA, are studied. Scanlines and microstructural analyses are used to document three distinct deformation events: (1) an early phase creating cataclastic deformation bands, during which most of the displacement on the fault took place; (2) a fracturing event with opening and shearing along fractures; and (3) fluid flow and local calcite precipitation along the NW-trending faults. Microstructural characterization of deformation structures shows complex interaction between deformation bands, fractures and the calcite precipitation. Calcite cement is observed as veins in the host rock and along cataclastic bands, with varying amount of cataclastic material floating within the veins. In addition to patchy calcite cement in the host rock, extensive poikilotopic cementation is observed to extend into cataclastic bands with a low degree of cataclasis. However, some cataclastic bands with a high degree of cataclasis show almost no cementation. Development of deformation bands and their link to fracturing affected the flow field, from a fault baffle to a conduit. Calcite cementation reveals the flow paths, before cementation recreated the fault baffle.

a b s t r a c tPetrophysical and mechanical properties of sandstone reservoirs are likely to change as a result of faulting. In this paper, we investigate the distribution of deformation features (structures) such as fractures and deformation bands in the Navajo and the Entrada sandstones in the fault core and damage zones of two faults in two localities in southeast (Cache Valley) and central (San Rafael Swell) Utah. These two localities had different degree of calcite cementation and hence are of interest to study the mechanical and petrophysical properties of these localities, in order to find out the impact of cementation on these properties and their possible relations. We have performed in-situ measurements by Tiny-Perm II and Schmidt hammer to examine the distribution of permeability and strength/elasticity of rock within the damage zone of these faults. We have studied the statistical relation between (i) TinyPerm II measurements and Schmidt hammer values, (ii) permeability and uniaxial compressive strength, and (iii) permeability and Young's modulus of deformed rocks. The statistical results demonstrate that there are correlations between the studied parameters, but the dependencies vary with the degree of calcite cementation in mineralogically similar sandstones (quartz sandstone). Statistical results demonstrate to first approximation that an exponential law is more suitable for description of the relations (i), (ii) and (iii) of non-cemented Navajo sandstone whereas for cemented Navajo sandstone these relations are better approximated by power law.