Shales are phyllosilicate mineral rich mudrocks that act as a source rock, a reservoir and a cap rock. The significance of understanding its minerals is vital for any conventional or unconventional hydrocarbon reservoirs. Shales are highly anisotropic complex heterogeneous rocks. The anisotropy in the shales is mostly caused by a unified effect of clay platelet preferred orientation (Hornby et al. 1994) cracks, pores and kerogen alignment (Vernik and Liu, 1997). Results from different laboratory measurements indicate that shales are largely transversely isotropic (TI) for all stages of compaction.
The clay minerals are generally fine-grained and it is not possible to find a single large enough crystal
for acoustic measurements. For any type of rock physics modeling or anisotropy analysis, individual elastic moduli of clay minerals, their orientation distribution functions (ODF) and their volume fractions are imperative. Understanding the importance, several authors have proposed theoretical (Sayers, 2005; Bayuk et al. 2007) experimental (Kathahara, 1996), empirical (Castagna et al, 1985; Tosaya, 1982) and heuristic/semi-empirical (Vernik and Kachanov, 2010) approaches to estimate elastic properties of clay aggregates with different preferred orientations. However, better understanding for constraining the elastic properties is still needed.
The orientation of clay mineral is generally expressed in the terms of MPD (maximum pole density) in m.r.d. (multiples of random distribution) (Wenk, 1985). A higher value of m.r.d. signifies a higher degree of preferred orientation. In this study, we use ODF data from X-ray goniometry for different shales to devise a relationship between P- and S-wave velocities for zero interparticle porosity minerals to their preferred orientation. We then compute elastic tensors for the most common phyllosilicate shale minerals such as muscovite, illite and illite/smectite aggregates.