On the Elastic Anisotropy of Shales

The elastic properties of shales, and their anisotropic character, have been tested more extensively with the introduction of the large group of lithologies which are collectively named gas shales. With respect to a traditional view of “shales = claystones”, gas shales have certainly introduced a more complex compositional mix, with a strong presence of carbonate material and, more limitedly organic matter. The direct impact is a distinct new trend in the relationship between elastic anisotropy and clay content, with probable implications on the constitutive equation of shales, as well as on the interplay between stiffness tensor and particle alignment. It is certainly noteworthy the fact that introducing isotropically (i.e. randomly) carbonate material in the architecture of an anisotropic composite has increased anisotropy significantly. In addition, we currently do not observe a direct relationship between the amount of organic material and elastic anisotropy. The implication is that the key to a general anisotropic “hardening” in gas shales relies in an additional parameter which is currently not understood.