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 The Zema prospect, located in the Otway Basin of South Australia, hosts an interpreted 69-m (226-ft) paleohydrocarbon column. Two faults are significant to prospect integrity. The main prospect-bounding fault (Zema fault) shows a significant change in orientation along strike, with some parts of the fault trending northwest-southeast and other parts trending east-west, all at a consistent dip of about 70°. The fault shows a complex splay and associated relay zone at its western tip. An overlying fault shows a similar northwest-southeast trend. Shale volume ( V shale ) derived from the gamma-ray log was tied to seismic horizon data in order to model across-fault juxtaposition and shale gouge ratio on the Zema fault. Shale volumes of greater than 40% correspond with paleosol shale lithotypes identified in the core that are characterized by high mercury injection capillary entry pressures of 55 MPa (8000 psi), capable of supporting gas columns far beyond the structural spillpoint of the trap. V shale values of 20–40% correspond to silty shale lithotypes characterized by mercury capillary entry pressures equivalent to gas column heights of less than 30 m (100 ft). Sands correspond with V shale values of less than 20%. Juxtaposition modeling of the Pretty Hill reservoir interval that is displaced across the Zema fault against the Laira Formation seal demonstrates the existence of both sand-on-sand juxtaposition and sand-on-silty shale juxtaposition above the paleofree-water level. Hence, juxtaposition alone cannot explain the observed paleocolumn. It is therefore likely that fault damage processes on the fault plane were responsible for holding back the original 69-m (226-ft) column. Shale gouge ratio values show a gradual decrease from 32% at the top of the fault trap to less than 14% at the structural spillpoint. The fault damage zone is likely to consist of phyllosilicate framework rock types. Because the Zema trap was not filled to structural spillpoint, it is likely that the percentage of shale gouge in the fault zone not only provided the original sealing mechanism but also limited the original column height. This is supported by fault zone capillary entry pressures calculated from shale gouge ratio values, which indicate that the fault zone is only capable of supporting a maximum column height of 72 m (236 ft), just 3 m (10 ft) more than the interpreted column height of 69 m (226 ft). Geomechanical analysis shows that the northwest-southeast-trending parts of the faults are optimally orientated in the in-situ stress field for reactivation. A spontaneous potential (SP) anomaly in the Zema-1 well, which was recorded in a northwest-southeast-striking fault damage zone through the seal, confirms the existence of open, permeable fracture networks. These are likely to have been generated by recent reactivation that caused the breach and subsequent leakage of the entire original hydrocarbon column.

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.

Abstract The Penola Trough of the Otway Basin, South Australia, is host to five economic gas fields containing an estimated 120 bcf of original gas in place in fault-related traps. However, throughout this trough, many other fault-dependent traps contain paleocolumns or partial paleocolumns. In 2001, the Balnaves 1 well discovered a semibreached structure. This structure was originally thought to be low risk because its associated fault was optimally oriented to seal with respect to the interpreted present-day maximum horizontal stress direction. On subsequent analysis of the wellbore image data, an open conductive fracture network was observed in the seal around the main bounding fault. We propose that perturbations of the regional stress field around preexisting faults may open a fracture network in the seal. This hypothesis is tested for the Laira Formation (cap seal) using the finite-difference distinct-element method (DEM). To our knowledge, this technique has not previously been used to assess seal integrity. The DEM has been used before for estimating perturbations around faults. The current work first summarizes and expands previous investigations of the perturbations developed in the two-dimensional (2-D) (horizontal) local maximum (σ 1 ) and minimum (σ 3 ) stress magnitudes produced around a single fault, it then uses this understanding to create and assess a 2-D DEM Penola Trough model. For a single fault, the magnitude of perturbations were examined as a function of k = σ 1 /σ 3 , θ (the angle between σ 1 direction and the fault strike), friction angle Φ, and fault stiffness j kn and j ks . The magnitude of stress perturbations are highly sensitive to k , θ, and Φ, but less sensitive to fault stiffness. This insight is applied to horizontal 2-D models to identify areas of potential cap rock failure. In a 2-D study of the Penola Trough areas of high shear stress are modeled where breached hydrocarbon columns are known to occur. We interpret areas of high shear stress to be zones of fractured rock and possible cap rock failure. Predicting zones of cap rock failure using DEM models could prove to be a very useful exploration tool in locations where cap rocks are known to be brittle and have suffered recent tectonic strain.