Abstract - Existing models for the growth of fault zones associated with normal faulting of sedimentary sequences range from conceptual models for fault zone architecture, incorporating components such as fault core and damage zone, through to a variety of fault wear models that explain established quantitative correlations between fault displacement and fault rock thickness.
Despite the importance of faults in a variety of application areas, no unified model for fault zone evolution has been developed which incorporates the broad range of fault-related features and processes.
Exploring links between the scaling of different fault zone components and fault displacement, a quantitative model for fault zone evolution has recently been developed which attempts to reconcile fault zone structure with the repetitive operation of a small number of processes, including fault segmentation and refraction, and asperity removal.
This model relates fault zone components characterised by different amounts of shear strain (where strain is measured as the ratio of displacement to thickness) to stages in a kinematic model, in which low shear strain structures, such as fault relays, become fault-bound lenses and eventually fault rock with increasing strain.
This model helps to reconcile the main characteristics of fault zones developed within a broad range of host rock sequences and at different deformation conditions, but still recognises the inherent complexities of natural fault zones. The model is also consistent with recent studies of high quality outcrops which illustrate how the combined effect of host rock rheology and prevailing deformation processes is capable of generating the full range of fault rock types, including those which have a major impact on hydrocarbon flow, such as shale/clay smears within poorly consolidated sediments through to shaley fault gouges within lithified sediments. The incorporation of either shale smears or shaley gouge within fault zones contained in siliciclastic sequences, is now recognised as one of the principal means of forming some fault-bounded traps and can have a major impact on intra-reservoir flow. Existing empirical constraints demonstrate that fault rock permeabilities decrease with increasing clay fraction and provide a means of predicting fault rock permeabilities in the subsurface. Typically the clay fraction in fault rock is assumed to be equivalent to the clay fraction of the sequence which has moved past a point on a fault (a value which is sometimes referred to as the Shale Gouge Ratio), and permeability is calculated by assuming a particular transformation between clay fraction and permeability, which can vary with various factors, such as burial depth and clay type.
Despite the inherent complexities of fault zones, new approaches are briefly described which are capable of incorporating the effects of faults in both hydrocarbon exploration and production models. Recently published studies show that these methods provide an improved basis for modelling faults contained within reservoir production or hydrocarbon migration flow models of siliciclastic sequences, in which faults usually behave as barriers or baffles to flow. Some studies have highlighted interesting, though at first glance counter-intuitive, results. These studies show, for example, that for a suite of synthetic Brent-type faulted reservoirs, produced by water injection along their flanks and crestal production wells, the effect of faults which are parallel to flooding directions is to decrease sweep efficiency and, consequently, recovery factors, whilst faults which are perpendicular to predominant flow directions can increase sweep efficiency, with consequent increases in recovery factors.
Bulletin for Applied Geology, 13, 59-62, 2009.