We have used a reliable, updated database of ultrasonic measurements on organic mudrocks that contains components {Cij} of the transversely isotropic elastic tensor, and hence the Thomsen’s anisotropy parameters ϵ and δ, to establish a more useful upper bound of feasible values for C13, and hence δ in unconventional mudrock reservoirs than that introduced earlier. The new upper bound δmax=ϵ is consistent with elliptical anisotropy and represents a much better constraint for these mudrocks. We derive the upper bound C13max (and hence δmax) from Postma’s inequality applicable to a thin-layered effective medium composed of kerogen and inorganic phases. The anisotropic moduli of such a composite as given by the Postma-Backus model are well-matched by the experimental data at least in the bedding-normal direction. As for the lower bound δmin, which was suggested by others, we confirm its reliability and usefulness for organic mudrocks.

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