Abstract Clay content is a first-order control on the mechanical and fluid-flow properties of fault rocks. The effects of deformation and also diagenesis are modified by the presence of clay in impure sandstones, although our understanding of the results of such changes is not well constrained. Because a lack of data for fault rocks in impure sandstones limits our ability to assess fault seal risk, a study was undertaken to investigate the effects of physical and diagenetic processes on these parameters in the Otway Basin on the southern margin of Australia. Fault rocks formed in impure reservoir sandstones from the eastern and western Otway Basin exhibit distinct geomechanical and capillary properties caused by differing clay content and distribution, overprinted by regional differences in diagenesis and geohistory. In the eastern Otway Basin, grain mixing and shear-induced clay compaction have increased fault capillary threshold pressures relative to host reservoir strata. These processes have led to a greater proportion of rigid framework grain contact, generating increased fault friction coefficients relative to the host reservoir rocks. Fault strands tend to form dense clusters as a result of strain hardening and preferential localization of new faults in weaker reservoir sandstone. Mechanical and diagenetic processes in fault rocks in impure sandstones from the western Otway Basin have significantly altered physical and geomechanical properties as a result of increased quartz dissolution and precipitation aided by lower clay contents. Here, faults exhibit increased friction coefficient and capillary threshold pressures because of more efficient grain packing, suturing of quartz grains, and fracture healing likely resulting from local diffusive mass-transfer processes. Phyllosilicate framework fault rocks from both regions appear significantly stronger than their host reservoirs as a direct result of syn- and post deformational physical and diagenetic processes. These findings have direct implications for understanding the micromechanics of deformation in impure sandstones, for physical property evolution during and postfaulting, and for geomechanical prediction of fault reactivation. In a regional context, the regeneration of fault strength influences stress distribution in regional top seals through localized rotation of stress trajectories and increased differential stress, which has resulted in fracturing and loss of hydrocarbons.

Abstract Postcharge fault reactivation may cause fault seal breach. We present a new methodology for assessment of the risk of reactivation-related seal breach: fault analysis seal technology (FAST). The methodology is based on the brittle failure theory and, unlike other geomechanical methods, recognizes that faults may show significant cohesive strength. The likelihood of fault reactivation, which is expressed by the increase in pore pressure (Δ P ) necessary for fault to reactivate, can be determined given the knowledge of the in-situ stress field, fault rock failure envelope, pore pressure, and fault geometry. The FAST methodology was applied to the fault-bound Zema structure in the Otway Basin, South Australia. Analysis of juxtaposition and fault deformation processes indicated that the fault was likely to be sealing, but the structure was found to contain a residual hydrocarbon column. The FAST analysis indicates that segments of the fault are optimally oriented for reactivation in the in-situ stress field. Microstructural evidence of open fractures in a fault zone in the subsurface in an offset well and an SP (self-potential) anomaly associated with a subseismic fault cutting the regional seal in the Zema-1 well support the interpretation that seal breach is related to fracturing.