The elastic velocity and anisotropy of shales are significantly affected by the orientation distribution of clay minerals including mica. A lot of research has been done on elastic properties of clay and clay aggregates with varying degree of the platelet preferred orientation. However, our understanding regarding the properties are still limited. In this study, we use both experimental and empirical rock physics models (RPM) for conventional shale to constrain elastic properties of muscovite and illite. The orientation distribution data from the published literature is used to relate the maximum platelet pole density to the median orientation angle . We then propose a heuristic approach to model anisotropic elastic moduli and P- and S-wave velocities of zero-porosity clay (matrix) aggregates as a function of the median orientation angle.
Introduction
Shales are phyllosilicate mineral-rich heterogeneous rocks that can be recognized as a source rock, cap rock and a reservoir. The anisotropy of shales can be a combined effect of clay platelet preferred orientation (Kaarsberg, 1959 ; Hornby et al., 1994 ; Sayers, 1994), kerogen, and crack alignment at various scales (Vernik, 1997). The results of multiple laboratory experiments show that the anisotropy of shales can be approximated as transverse isotropy (TI) with a rotational symmetry axis normal to bedding plane.
Preferred orientation of clay platelets normally forms during mechanical compaction with some recrystallization at the advanced stages of diagenesis (Wenk, 2007). The process of recrystallization can either enhance or reduce the alignment depending on the stress history. The degree of clay mineral preferred orientation can be expressed as the maximum pole density (MPD) in multiples of a random distribution – m.r.d. (Wenk, 1985), in which higher values reflect higher degrees of preferred orientation and the values can go until infinity for the perfect bedding-plane alignment. The orientation distribution (OD) of clay minerals along with mica significantly affects the anisotropic elastic moduli of TI shales. The information on elastic properties of clay minerals is very crucial in doing rock physics modeling and seismic interpretation, and pore pressure prediction.
