Abstract

Deformation-band shear zones can severely compartmentalize oil and gas reservoirs by drastically reducing permeability in thin, extensive sheets. This study presents data showing that subsurface deformation bands can develop closely spaced systematic joints that breach the bands and that terminate at the boundaries between the cataclastic deformation-band material and the surrounding sandstone. Observations at six field sites on the Colorado Plateau reveal the influence of deformation-band orientation on joint orientation, the relationship between deformation-band thickness and joint spacing, and the origin and timing of development of joints in deformation-band shear zones associated with major structures. Joints in deformation-band shear zones are perpendicular to the deformation bands, joint spacing is a linear function of deformation-band thickness, and the slope of the best-fit linear regression reflects the degree of joint saturation. A correspondence between tectonically induced principal strain axes and joint orientations implies that joints developed simultaneously with, or soon after, the deformation-band shear zones that they occupy. Deformation bands in subsurface reservoirs are therefore likely to contain joints that breach the nearly impermeable deformation-band shear zones, connecting compartments of undeformed sandstone and restoring reservoir permeability.

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