Neil T. Grant, 2020. "Stochastic modelling of fault gouge zones: implications for fault seal analysis", Integrated Fault Seal Analysis, S. R. Ogilvie, S. J. Dee, R. W. Wilson, W. R. Bailey
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Fault zones are complex, and show considerable variability in both structure and the distribution of associated fault rocks within the fault core: the zone that localizes most strain and displacement. It is the fault-core gouge zone and associated slip surfaces which provide the cross-fault seal when permeable layers are juxtaposed. Predicting the sealing properties of fault gouge zones is difficult but often required when evaluating faults in exploration prospects. A stochastic modelling approach is described to help better understand the compositional controls on fault gouge seal potential. The model is populated with a random assemblage of four fault rock components: shale smears, shaly gouge, cataclastic gouge and low-strain host-rock lenses. Harmonic averaging of permeability and arithmetic averaging of Vshale are then used to upscale the properties, and to propose a simple permeability–Vshale model for fault rocks. Practical application of the model is discussed by developing an empirical link between standard well-log data and associated fault rock effective permeability. This new approach has the potential to offer a simple well-log-based fault seal model. The utility of the model is demonstrated with a case study, comparing the results to those generated using other published techniques.
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Faults commonly trap fluids such as hydrocarbons and water and therefore are of economic significance. During hydrocarbon field development, smaller faults can provide baffles and/or conduits to flow. There are relatively simple, well established workflows to carry out a fault seal analysis for siliciclastic rocks based primarily on clay content. There are, however, outstanding challenges related to other rock types, to calibrating fault seal models (with static and dynamic data) and to handling uncertainty.
The variety of studies presented here demonstrate the types of data required and workflows followed in today's environment in order to understand the uncertainties, risks and upsides associated with fault-related fluid flow. These studies span all parts of the hydrocarbon value chain from exploration to production but are also of relevance for other industries such as radioactive waste and CO2 containment.