--> Detection of Fault-Zone Seals in Gulf Coast Normal Faults, South Texas, by R. R. Berg; #90986 (1994).

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Abstract: Detection of Fault-Zone Seals in Gulf Coast Normal Faults, South Texas

Robert R. Berg

Fault seals are formed by ductile deformation of interbedded shales and sands along fault surfaces. In cores from the Wilcox, the deformed sediments are sheared zones (clay smears or gouge) that display high-angle shear fabrics in single or conjugate sets. Ductile flow homogenized the original sediments and resulted in low permeabilities of less than 0.1 md and a uniform distribution of small pores. Consequently, sheared zones are effective seals against adjacent reservoirs. Oil-water capillary pressures are on the order of 480 kPa (70 psi) at 50% saturation and indicate a trapping capacity of 164 m (540 ft) of oil column when the oil-water density difference is 0.3 g/cm3. In contrast, normal-fault surfaces result from extension and will leak during periodic movement.

Structures observed in cores are reflected in dip logs. Sheared zones are characterized either by dips that are variable in magnitude and direction (chaotic pattern) or by more uniform dips that are distinctly different from structural dips. Faults are identified by dip patterns that denote normal drag with regularly increasing dips toward the fault surface. Examples of dip interpretation in the Vicksburg section show that reservoirs against sheared zones are sealed whereas reservoirs adjacent to fault surfaces are poorly sealed or not sealed. Pressure studies support the conclusion that non-sealing faults are conduits for water flow. These interpretations suggest that sealing faults do not depend on juxtaposition of shales across the fault but rather on the supply of uncompacted shal that will flow to form sheared zones along faults.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994