Overpressure and shale properties: Stress unloading or smectite‐illite transformation?

Abstract

Standard methods for estimating pressure from seismic velocity need to be modified on the Gulf of Mexico (GoM) shelf because overpressure is often not due simply to undercompaction, but is at least partly due to smectite‐illite (S‐I) transformation. There are two end‐member methods for estimating pressure from velocity for these shales. One assumes that the velocity‐stress dependence of the shales during compaction is unaffected by the transformation, and that the effect of the transformation is to increase the pressure and reduce the net stress. The reduction in net stress implies that the sediment is inside its mechanical yield envelope on an elastic unloading stress path. Therefore this method must cope with the problem of determining the paleo‐maximum stress experienced by the sediment. The second method assumes that the smectite‐illite transformation changes velocity‐stress behavior of the shale, but that the transformation does not cause a reduction in net stress. The second model is apparently inconsistent with unloading, which is probably seen in some wells on the GoM shelf. I suggest here that the second model can accommodate reductions of net stress if the S‐I transformation involves mechanical failure. The transformation thus might reduce net stress while still allowing the sediment to be on a yield envelope rather than on an elastic path within the yield envelope. GoM shelf wells also have compositional variations in sonic‐density trends that seem to favor the second method over the first elastic unloading model.