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Abstract: The Structure, Content, and Growth of Fault Zones Within
Sedimentary Sequences and Their Effects on Hydrocarbon Flow
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 attempt to explain established quantitative correlations between fault displacement and fault rock thickness. Despite the importance of normal 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, this talk presents a quantitative model for fault zone evolution 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 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 recognizes the inherent complexities of natural fault zones.
This model for fault zone structure 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 recognized 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. New approaches are briefly described which are capable of incorporating the flow 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 behave as barriers or baffles to flow.
Faults are represented as planes in conventional reservoir cellular models and yet they contain fault rocks with permeabilities that differ from those of the host rock. From Manzocchi et al. (1999). Petroleum Geoscience, 5, 53-63.
Faults in a reservoir model in which across-fault sequence juxtaposition is explicitly included in the model geometry, with fault rock properties represented by cross-fault transmissibility multipliers on the cell faces along the fault surface (using the method of Manzocchi et al. 1999).