AAPG Annual Convention and Exhibition

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Fault Zone Complexity: Impact on Seal Analysis and Prediction

Abstract

The deformation complexity along a fault length in cross-section may vary as a function of the mechanical properties of the stratigraphic section cut by the fault. The array of minor structures that are often present within a damage zone around the principal fault slip surface are generally at a sub-seismic scale that may impact flow resistance across the fault. This presentation reports progress made on predicting the distribution and styles of fault zone complexity from a combination of detailed, high resolution outcrop mapping and numerical modelling that includes consideration of the mechanical properties in the faulted stacking sequence. This integrated approach recognizes the different mechanical responses of lithological units and packages that define a vertical mechanical heterogeneity (VMH). The degree of heterogeneity influences the fault zone complexity. Outcrop examples of fault zone evolution are from a range of outcrops with differences in the facies and VMH. The observations support the model that much of the finite fault zone architecture, seen in the hanging wall or footwall of seismic scale faults, is developed early during the throw history across premonitory shear zones that define the early fault structure. By incorporating the expected VMH of the stratigraphic sequence at the time of deformation, we have generated and calibrated modelled fault architectures and evolution against the detailed outcrop geometries. These models, using the Fault Modeller software, have successfully reproduced critical aspects of the observed fault architectures, especially with respect to the location of more complex deformation domains along the fault. For example, domains where minor faults concentrate, where local folding promotes more ductile deformation and areas more prone to fault lens formation, can all be related to the mechanical stacking and heterogeneity present. The results provide a new platform for constraining and predicting the distribution of complexity in fault zones and generating important input for assessing the impact of fault zone architecture on cross fault fluid flow behaviour. The input information used for the modelling is a depth log of lithologies and properties and defines a mechanical stratigraphy for the time of deformation. These fault zone architectures can be used to improve models of flow across the faults.