The elastic anisotropy of shales

Abstract

Shales constitute about 75% of the clastic fill of sedimentary basins and have a decisive effect on fluid flow and seimic wave propagation because of their low permeability and anisotropic microstructure. The elastic stiffness of a shale with partially oriented clay particles is expressed in terms of the coefficients W1mn in an expansion of the clay-particle orientation distribution function in generalized Legendre functions. Application is made to the determination of the anellipticity of shales. For transverse isotropy the anellipticity quantifies the deviation of the P wave slowness curve from an ellipse and is shown to depend on a single coefficient W400 in the expansion of the clay-particle orientation distribution function. Strongly aligned clay particles result in a positive value of W400 and a positive anellipticity, in agreement with the majority of field measurements. Numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given. -from Author