Amgad I. Younes¹

(1) Stanford University, Stanford, CA

ABSTRACT: A New Method For Calculating Fault Seal Potential by Shale Smearing

Evaluation of shale smear along sealing faults helps prediction of structural traps and assessment of reservoir performance under variable pressure. Current methods are based on the calculation of the fault throw and the thickness of the faulted shale layers. These methods assume that the smeared shale maintains a constant thickness and rheology over the faulted interval, therefore providing only a qualitative measure. Outcrop examples of shale-smeared faults show a different geometry, however. There, individual shale layers thin asymptotically until they are disconnected beyond a given offset that increases with the softness and thickness of the shale layers. This distance is determined to be 4-6 times the source layer thickness.

We suggest a new method based on analysis of faults with shale smear preserved in both the hanging and foot -walls. For each example, the curvature of the shale was traced as it smears along the fault, plotted on an x-y grid then fit to an equation in the form: S(z) =2.S t0 /cosq.e -bx/sinq. This equation predicts the smear thickness and the volume of shale in the fault zone, taking into account fault angle (q), source layer thickness (t0), shale stiffness (b), and distance in the dip normalized to fault offset (z). Higher b values represent stiffer shales. The method suggested can be integrated over the entire stratigraphic section of interest while honoring the variation in any of the parameters included. It assumes equal contribution from the hanging wall and footwall, and it provides a more accurate and quantitative measure of shale smearing.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado