Contrast of elastic properties between rock layers as a mechanism for the initiation and orientation of tensile failure under uniform remote compression

Natural fracture systems are often influenced by the presence of different layers with contrasting rock mechanical properties. Understanding the distribution of stress between different mechanical layers is essential to the interpretation of fracture networks within these systems. A simple model of elastic stress states within mechanically layered media is presented as a description of fracture formation within layered rock. These stress states are based on analytic solutions in three dimensions for an idealized system of bonded, planar, isotropic, homogeneous, linear-elastic layers in equilibrium with a uniform remote stress. Properties of these solutions indicate that tensile stress and failure will result in some layers in response to remote compression alone given sufficient contrast in elastic properties between the different layers. In this case, tensile stress arises without the requirement for an addition body force such as internal fluid pressure and will concentrate within either the stiffest or softest layers according to the relative amount or layer-parallel versus layer-perpendicular compression. Lithification and bonding of layers under remote stress leads to residual stresses within layers that enhance tensile failure of stiffer than average layers under remote compression. Any change in orientation of the remote stress may lead to significant differences in both stress magnitude and orientation between different layers. In turn, the mode and orientation of fractures within different layers may significantly differ despite forming under the same remote stress.