Sealing Potential of Normal Faults: Comparison between Simple Models and Outcrop Analogs
E. J. M. Willemse, D. D. Pollard, and A. Aydin
Fault sealing can control the size of some hydrocarbon accumulations, influence the number of wells required to drain reserves and affect implementation of enhanced recovery schemes. The fault sealing potential is composed of three components: (a) the slip distribution on a fault, which governs how reservoir and non-reservoir rocks are juxtaposed across the fault; (b) the spatial continuity of the fault plane(s); and (c) the hydraulic properties of the fault gouge. We develop a simple model to study the first two and compare the results with outcropping faults.
Commonly fault slip is assumed to follow an approximately linear variation, from a maximum at the fault center to zero at the fault tip line. Analyses based on this model show that juxtaposition may be well developed in the central fault area. The sealing potential due to juxtaposition usually is poor close to the fault tip-line where layers remain largely in contact if the fault throw is less than the layer thickness.
Detailed outcrop studies show that most faults are discontinuous and consist of numerous side-stopping segments. Even if the fault surfaces are fully sealing, fluids can locally flow across such fault zones by exploiting the relay zones. Slip distributions are symmetric along central segments, but asymmetric along outer segments. Regions immediately adjacent to relays are also prone to cross-flow because of the low offset. The emerging picture is a series of small, asymmetric footwall closures rather than a single large trap. Differentiation between continuous and segmented faults using seismic data remains challenging. Meanwhile irregular slip variations and structure contour patterns suggesting multiple closures can be used to signal a possible reduction in sealing potential.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California