Synthetic AVO Gathers

Part 6, Part 6: Elastic and AVO

Learning objectives

Build a synthetic angle gather for a shale over a reservoir
Run Gassmann fluid substitution and rebuild the gather
Watch the top reflection walk through the AVO classes
Read the deviation from brine as a direct hydrocarbon indicator

The Payoff: A Gather That Answers to Fluid

Everything in this chapter converges here. We build a real angle gather, a fan of traces at increasing incidence angle, for a shale sitting on a porous reservoir sand. Then we change the pore fluid and let the physics rebuild the gather.

The fluid control is not a cosmetic dimmer. It runs Gassmann fluid substitution on the reservoir frame. Gassmann says that when you swap one fluid for another the rock frame's shear modulus $\mu$ does not change (a fluid has no shear strength), but the saturated bulk modulus $K_{sat}$ sat and the bulk density $\rho$ both do. Replace brine with gas and $V_p$ p drops while $V_p/V_s$ collapses. Those new elastic properties change the reflection coefficient at every angle, and the gather redraws.

Walking the Classes

Start with brine and the sand is stiff: the top reflection is a weak Class I peak that barely stands out from the shale. Substitute oil and the point drifts toward Class II, near zero at the stack but building a trough at far offsets. Substitute gas and the sand softens hard: the top reflection becomes a bright Class III trough that brightens as the offset grows. One rock, three fluids, three completely different gathers, and only the physics changed.

The faint ghost behind the gather, and the dashed curve on the amplitude plot, are always the brine response. That is deliberate. On real data you never see the fluid directly; you see the deviation from the water-wet baseline. The gap between the coloured gather and its grey brine shadow is the direct hydrocarbon indicator, the thing AVO exists to measure. This is the fit-for-purpose lesson at its sharpest: a convolutional stack would have shown you one amplitude, but only the elastic, angle-dependent model reveals whether that amplitude means gas.