Why Shales Are Anisotropic

Two Ways to Make a Shale Directional

Backus averaging in the last section made a rock anisotropic by stacking contrasting layers. Shales are the most anisotropic rocks in most sedimentary basins, and layering is only part of the reason. There are two distinct sources, and telling them apart matters because they behave differently. The first is extrinsic: a shale is often finely interbedded with silt or sand laminae, and that alternation is exactly the Backus problem of Part 9.2. Compute it and you get a VTI medium with a Thomsen $\gamma$ of around 0.1 for a moderate sand-shale contrast. The second is intrinsic: the clay platelets that make up the shale matrix are themselves flat, and during deposition and compaction they rotate to lie flat, stacking like a deck of cards with their soft direction vertical. A wave polarised in the plane of the platelets crosses stiff, well-bonded sheets and runs fast; a wave crossing the platelets meets the compliant gaps between sheets and runs slow. This is anisotropy of the solid frame itself, present in a hand specimen with no visible bedding at all.

How Much of Each

The distinction is not academic, because the two add. The extrinsic part scales with how strongly the shale is interlaminated and can be computed from the layer moduli by Backus. The intrinsic part scales with how aligned the clay fabric is, which is set by clay content, compaction, and the platy habit of the specific clay minerals; it is largest in well-compacted, clay-rich shales and weaker in silty or poorly aligned ones. Measured shales routinely show Thomsen $\epsilon$ and $\gamma$ of 0.1 to 0.3 and sometimes more, larger than the 50/50 layered stack of Part 9.2 produced on its own. That gap is the fingerprint of the intrinsic fabric: the layering alone does not account for the observed anisotropy, so the aligned clay platelets are carrying the rest. A useful diagnostic is that intrinsic fabric anisotropy grows with burial as compaction improves the platelet alignment, whereas the extrinsic layering part is fixed once the beds are deposited.

The Anisotropic Background

Whichever source dominates, the practical fact is the same and it is not a niche correction. Shale is the seal and the overburden almost everywhere, which means most reservoir reflections are read through, or against, an anisotropic shale. Ignore that anisotropy and depths pulled from seismic velocities come out wrong, moveout looks non-hyperbolic for reasons that get blamed on the wrong thing, and an AVO gradient measured against a shale background carries an offset that has nothing to do with fluid. The shale is not the target, but it sets the stage the target is seen on. This is also the honest boundary of what a scalar rock-physics model can promise: the isotropic Gassmann and contact models of the earlier parts describe the reservoir sand well, but the medium above it has a direction, and reconciling that is an anisotropic-seismics problem, developed in the Seismic Modeling course, Part 7. Our next move is a different axis entirely. We have spent this part on how a rock varies with direction; the remaining sections turn to how the same rock varies with frequency, starting with why velocity itself disperses.

References

• Sayers, C. M. (2005). Seismic anisotropy of shales. Geophysical Prospecting, 53(5), 667-676.
• Thomsen, L. (1986). Weak elastic anisotropy. Geophysics, 51(10), 1954-1966.