Seismic anisotropy of shales

Shales are a major component of sedimentary basins, and they play a decisive role in fluid flow and seismic-wave propagation because of their low permeability and anisotropic microstructure. Shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid, lithology and pore pressure from seismic data, and to understand time-to-depth conversion errors and non-hyperbolic moveout. A single anisotropy parameter, Thomsen's delta parameter, is sufficient to explain the difference between the small-offset normal-moveout velocity and vertical velocity, and to interpret the small-offset AVO response. The sign of this parameter is poorly understood, with both positive and negative values having been reported in the literature. delta is sensitive to the compliance of the contact regions between clay particles and to the degree of disorder in the orientation of clay particles. If the ratio of the normal to shear compliance of the contact regions exceeds a critical value, the presence of these regions acts to increase delta, and a change in the sign of delta, from the negative values characteristic of clay minerals to the positive values commonly reported for shales, may occur. Misalignment of the clay particles can also lead to a positive value of delta. For transverse isotropy, the elastic anisotropy parameters can be written in terms of the coefficients W-200 and W-400 in an expansion of the clay-particle orientation distribution function in generalized Legendre functions. For a given value of W-200, decreasing W-400 leads to an increase in delta, while for fixed W-400, delta increases with increasing W-200. Perfect alignment of clay particles with normals along the symmetry axis corresponds to the maximum values of W-200 and W-400, given by W-200(max) = root 10/8 pi(2) and W-400(max) = 3 root 2/8 pi(2). A comparison of the predictions of the theory with laboratory measurements shows that most shales lie in a region of the (W-200, W-400)-plane defined by W-400/W-200 <= W-400(max)/W-200(max).