The Impact of Fault-Related Fluid Flow on the Permeability of Siliciclastic Reservoirs

Q. J. Fisher, R. J. Knipe, M. Casey, and S. D. Harris
Rock Deformation Research Group, School of Earth Sciences
University of Leeds, Leeds LS2 9JT

        Fault-related fluid flow has frequently been held responsible for the altering the porosity and permeability of siliciclastic petroleum reservoirs. Microstructural analysis of >2000 faults from >100 petroleum reservoirs in a wide variety of sedimentary basins has been used to provide an indication of the importance of this process. The presence of cements within dilatant faults has been used as an indication as to whether or not they acted as significant conduits for mineralizing fluids. Faults formed in poorly lithified sediments are rarely mineralized, because the wall rock is too weak to support extensive dilation. Instead, such faults either do not affect fluid flow or act as barriers. 
        Mineralisation is generally only found within faults that formed either while the reservoir was being uplifted or after considerable sediment lithification. In such cases, the volume of fluid within the rocks is generally too small to carry sufficient quantities of fluid to significantly reduce to permeability of the undeformed reservoir. Locally such cemented faults can themselves, however, reduce the permeability of the reservoir.
        A simple numerical model for quartz cementation has been linked with results from rock deformation experiments to provide an indication as to the conditions that are likely to lead to a fault acting as a conduit for fluid flow. Rocks deposited slowly under high geothermal gradients may fault to produce conduits for fluid flow at shallow depth, when porosities are high. Faulting of rocks deposited rapidly under low geothermal gradients is unlikely to result in the formation of conduits until very deep burial when the porosity of the sedimentary sequence, and hence its fluid content, is low. 
        In our numerous studies, we have yet to find an example of where the permeability of an undeformed siliciclastic reservoir rock has been significantly reduced as a result of along-fault fluid flow. If such examples do exist we predict that they are more likely to be found in basins with high geothermal gradients.

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