Fault sealing is one of the key factors controlling hydrocarbon accumulations and can be a significant influence on reservoir behavior during production. Fault seal is, therefore, a major exploration and production uncertainty that requires a rigid, systematic framework within which to quantify the geological risk of trapping hydrocarbons. One of the key uncertainties in this risking procedure is the breaching of structurally bound traps due to the formation of structural permeability networks. Considering a population of faults and fractures, those that are critically stressed are more prone to act as conduits for fluid transmission. Evaluation and mapping of fault seal breach through such networks involves integration of in-situ stress conditions, pore pressure, fault architecture and fault geomechanics.

Geomechanical characterization of well-lithified fault rocks from the Otway Basin and the Northwest Shelf demonstrates that faults can exhibit significant cohesive strength and that fault reactivation and trap breach is influenced by the development of shear, tensile and mixed-mode fractures. The mechanics of the reactivation process are influenced by grain strength and fault morphology. Mercury injection capillary pressures of cataclastic faults indicate a seal capacity of 2400 psi. Following reactivation, seal capacity is reduced ∼95% due to the development of a highly connected fracture network. The tensile strength of such healed faults allows failure to occur by shear, tensile and mixed mode fracturing. These data suggest simple application of Byerlee's Law may not always be applicable when predicting reactivation induced fault seal failure. Consequently, geomechanical tools used to predict trap breach via reactivation that assume cohesionless frictional failure are likely to significantly underestimate seal reactivation risk.

The impact of structurally risking traps using Byerlee and laboratory-derived fault data is demonstrated using the Fault Seal Risk Web approach. Application of geomechanical fault data results in a significant reappraisal of prospect structural risk due to consideration of fault healing.

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