JOINTS IN DEFORMATION BAND SHEAR ZONES

Deformation band shear zones (DBSZs) are strain-hardened fault zones marked by reduced porosity and permeability and grain-scale cataclasis. DBSZs are a preferred deformation mechanism in porous sandstones, and they often form interlocking networks in three dimensions. Recent work has focused on the role of DBSZs in compartmentalizing porous sandstone reservoirs. However, DBSZs in Jurassic Navajo Sandstone on the Kaibab uplift of southern Utah contain systematic joints that may restore permeability and connectivity throughout the Navajo Sandstone. Steeply dipping joints within DBSZs strike N45W, approximately perpendicular to the regional Laramide shortening direction, and are interpreted as unloading or release joints. The spacing of joints localized within DBSZs is a function of DBSZ thickness. The fracture spacing index (FSI, the slope of the regression line on a plot of layer thickness versus joint spacing) is 0.96 with a correlation coefficient of 0.86. The frequency distribution of DBSZ-confined joints is log-normal. We conclude that DBSZ-confined joints show the same general spacing and distribution characteristics as joints controlled by primary lithological mechanical stratigraphy, despite the fact that DBSZs are secondary structural features oblique to primary sedimentary layers.

Spacing and distribution of joints that traverse DBSZs clearly are important factors in assessing the permeability and connectivity of sandstone reservoirs compartmentalized by DBSZs. Determination of exact timing of the formation of such jointing is essential to evaluate whether deformation bands served as seals or conduits at given times in given reservoirs.