Geophysical Prospecting, 2005, 53, 667–676 Seismic anisotropy of shales C.M. Sayers ∗ Schlumberger DCS, 1325 South Dairy Ashford Road, Houston, TX 77077, USA Received January 2004, revision accepted October 2004 ABSTRACT 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 δ parameter, is sufficient to explain the difference between the small-offset normal-moveout velocity and vertical velocity, and to inter- pret 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. δ 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 δ, and a change in the sign of δ, 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 δ. 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 δ, while for fixed W 400 , δ increases with increas- ing 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 max 200 = √ 10/8π 2 and W max 400 = 3 √ 2/8π 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 max 400 /W max 200 . INTRODUCTION Shales are a major component of sedimentary basins (Jones and Wang 1981) and play an important role in fluid flow and seismic-wave propagation because of their low permeability and anisotropic microstructure. Banik (1984) studied 21 data sets from the North Sea and found an excellent correlation between the occurrence of depth errors obtained from sur- face seismic data and the presence of shales in the subsurface. Failure to account for anisotropy in seismic processing may also lead to errors in normal-moveout (NMO) correction, dip- ∗ E-mail: csayers@houston.oilfield.slb.com moveout (DMO) correction, migration and amplitude-versus- offset (AVO) analysis. Many shales encountered in the subsurface can be described, to a good approximation, as being transversely isotropic with a vertical axis of rotational symmetry. A transversely isotropic medium has five independent elastic stiffnesses. Taking the x 3 -axis to lie along the axis of rotational symmetry, the non- vanishing elastic stiffness coefficients are c 11 = c 22 , c 33 , c 12 = c 21 , c 13 = c 31 = c 23 = c 32 , c 44 = c 55 and c 66 = (c 11 − c 12 )/2 in the conventional two-index notation (Nye 1985). Since an isotropic medium can be described by two elastic constants, a transversely isotropic medium has three anisotropy param- eters. In the following, the three anisotropy parameters ε, γ C  2005 European Association of Geoscientists & Engineers 667