Stephen J. Naruk1
(1) Shell EP Technology & Research, Houston, TX
Abstract: Fault Seal Evaluation and Prediction
Fault-seal prediction is a long-standing problem of both exploration and producing assets. In most plays, cross-fault juxtaposition of reservoir beds with shales is assumed to be the main cause of fault seals. The pressure-sealing capacity of those reservoir-on-reservoir fault contacts which do seal, is usually attributed to the seal capacity of the gouge, which can be described by two fundamentaly different types of equations; clay smear potential, or shale gouge ratio. Clay smear potential describes the gouge's seal capacity in terms of a physically continuous, wedge-shaped smear of viscous clay or shale. The shale gouge ratio describes the gouge seal capacity in terms of a homogeneous mixture of the of the wall-rock sands and shales. In this type of equation, the mechanics of faulting are not considered. The gouge's seal capacity is thought to be inversely proportional to the sand/shale ration of the faults' wall rocks.
In mature fields, well data, 3D seismic data, and advanced interpretation tools make it possible to accurately characterize the geometry and kinematics of faults, their bounding lithologies, and the in situ pressure differences across them. Empirical studies incorporating all of these data show that most demonstrably sealing faults in fact include significant areas of reservoir sand juxtaposed with reservoir sand. These sealing sand-on-sand contacts occur in both hydropressured and geopressured reservoirs. They are less common in geopressured reservoirs, but this may simply be due to the inherently lower net/gross in geopressures. Conversely, many sand-on-shale fault traps are demonstrably underfilled with respect to the structurally highest possible sand-on-sand cross-fault leak point. Thus the gouge itself appears to be controlling the trapped column heights in both sand-on-sand and sand-on-shale fault traps.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana