Shales are anisotropic. Most definitions of shale in-corporate this attribute, either by referring to fissility and existence of cleavage planes, or to anisotropic texture resulting in anisotropy of physical properties on many length scales. Definitions of shale scatter though; some focus on a high content of clay minerals as characteristic of a shale, while others consider a large amount of fine grains (< 2μm) as sufficient. In a rock mechanical context, it is natural to define shale as a rock in which clay minerals constitute the load-bearing framework. This means that “gas shales” in oil-field terminology are, strictly speaking, not shales according to a geological or a geomechanical perspective. Still, these materials have a lot in common with classically defined shales (e.g., low permeability) and anisotropy.

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