The Effect of Low Aspect Ratio Pores On the Seismic Anisotropy of Shales

Sayers, Colin M. (Schlumberger)



As a result of the increasing stress that develops during burial, plate-shaped clay minerals in shales tend to align with planes oriented approximately perpendicular to the maximum stress direction. This partial alignment results in shale anisotropy, and this needs to be quantified to reliably extract reservoir fluid, lithology and pore pressure from seismic data and to understand time-to-depth conversion errors and non-hyperbolic moveout. The low aspect ratio pores between clay particles play an important role in determining the character of the anisotropy of shales and can be represented by a normal compliance BN and shear compliance BT that describe the deformation of the interparticle regions under an applied stress. The relations among the various anisotropy parameters for shales depend on the ratio BN/BT of these low aspect ratio pores. For perfectly aligned clay particles, Thomsen''s anisotropy parameter ? is a function only of the shear compliance BT, but ? and ? increase with increasing BN/BT. The presence of a fluid with non-zero bulk modulus in the regions between clay particles acts to decrease BN and may lead to significant reductions in e and d for sufficiently high fluid bulk modulus.