Failure envelopes for well-lithified cataclastic fault rocks from the Otway Basin, Australia, where fault reactivation is a significant risk to trap integrity, have been determined through triaxial testing. Geomechanical analyses indicate that cemented cataclasites 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 fracturing process are influenced by grain strength and cataclasite morphology. Cemented cataclasites are more prone to failure than are reservoir sandstones under low differential stress conditions, as a result of a relatively low cohesive strength and higher friction coefficient. As such, the geomechanical property differential between cataclastic faults and undeformed reservoir strata may impact significantly on seal integrity during reactivation. Intact cataclasite seal capacity exceeds 2400 psi (16.5 MPa). Following reactivation seal capacity is reduced about 95% as a result of the development of a highly connected fracture network. The tensile strength. of these cataclastic faults allows failure to occur by shear, tensile, and mixed-mode fracturing. This suggests that geomechanical tools used to predict trap breaching by reactivation that assume cohesionless frictional failure may significantly underestimate seal risk. Determination of fault seal risk can, therefore, be significantly enhanced by multidisciplinary research efforts combining field- and laboratory-scale geomechanical analysis with microstructural and petrophysical property description.