Estimating the elastic properties of mica and clay minerals

The orientation distribution of the platelets of the phyllosilicate minerals, including mica clays, significantly affects the elastic velocities and anisotropy of shales. Despite a substantial number of publications on shale elasticity in general and, more specifically, on the elastic properties of clay minerals (phyllosilicates) and clay mineral aggregates with varying degree of the platelet preferred orientation, our understanding of these properties is still rather limited. Elastic properties of mudrocks, influenced by the presence of variously oriented phyllosilicates, such as muscovite, illite, and illite/smectite platelets, can be well-constrained if not accurately estimated by a combination of microstructural observations and semiempirical rock-physics models for conventional shale. The orientation distribution data from published literature are used to relate the maximum platelet pole orientation density to the double median orientation angle 2 tau with respect to the bedding plane, both of which relate to the platelet alignment strength. The 2 tau angle is an equally intuitive orientation distribution parameter, which we rely upon in a heuristic approach to model bedding-normal anisotropic elastic moduli and P- and S-wave velocities of zero-porosity clay (matrix) aggregates as a function of the median orientation angle of clay platelets composing these aggregates. It is demonstrated that zero-porosity illite-dominated aggregates with perfect platelet alignment (2 tau = 0 degrees) should have the platelet-normal elastic moduli of C-33 = 30.0 +/- 1.0 GPa, C-44 = 7.3 +/- 0.2 GPa, and a V-P/V-S ratio of approximately 2.0 +/- 0.07. The platelet-normal moduli and mineral density of muscovite are significantly greater than those of illite, whereas the respective moduli of the smectite-dominated clay composite are smaller (and the velocity ratio is greater) than those estimated for illite-dominated clays. Finally, it is also shown that the platelet-normal velocity ratio gradually decreases in the smectite, illite, and muscovite series.