THE ORIENTATION CONSIDERATION: A REVISED NUMERICAL MODEL OF JOINT DEVELOPMENT

Cataclastic deformation band shear zones have the potential to compartmentalize petroleum reservoirs in porous sandstone. However, deformation bands in many structural settings contain joints that terminate at boundaries with surrounding sandstone, and these joints may restore connectivity. Understanding factors that control development and spacing of joints within deformation bands is essential to predicting reservoir-scale permeability. Existing models of jointing in stiff mechanical layers assume layer-parallel extension, but this assumption is generally incorrect where the stiff mechanical layers are deformation bands. Our revised numerical model predicts that the angle between deformation bands and the extension direction has a significant effect on joint development, a prediction supported by field data from jointed deformation bands.

Our model relates joint spacing to the thickness and elastic properties of the jointed layer and the bounding layers, the degree of strain, and the angle (θ) between the jointed layer and the extension direction. Joint spacing increases as θ increases, and θ reaches an upper limit above which joints cannot develop. The value of this limit depends on Young's modulus and tensile strength of the stiff layer and the degree of strain. Reasonable values for elastic moduli of stiff deformation bands bounded by relatively weak sandstone result in an upper θ limit of approximately 40°. Deformation bands that are more than 40° oblique to a concurrent or later extension direction should not develop joints.

We gathered joint spacing, layer thickness, and orientation data from jointed deformation bands at six field sites in southern Utah. At each site, deformation bands contain joints that are perpendicular to the S₂ (intermediate principal stretch) direction associated with deformation band formation, implying that joints developed almost immediately after deformation bands. At all field sites, deformation bands that are more than 40° oblique to the S₂ direction do not contain joints perpendicular to S₂. The influence of θ, the angle between a deformation band and a concurrent or later extension direction, is a crucial consideration in predicting the presence and abundance of joints in deformation band shear zones in subsurface petroleum reservoirs.