Fault Propagation, Mechanical Layering and Fault Zone Evolution

Robert Knipe¹, Michael Welch¹, Stephen R. Freeman¹, Russell K. Davies², Christine Souque¹, Simon D. Harris¹, and Christian Tueckmantel^1
1Rock Deformation Research, Leeds, United Kingdom
²Rock Deformation Research USA, McKinney, TX

Faults in multilayered sequences are rarely planar features, but more commonly complex sub-seismic architectures that are critical in assessing the flow behaviour of faults in reservoirs. We introduce some new simple models based on detailed outcrop mapping aimed at improving the prediction of complex fault zones. The outcrop work of (contraction and extension) fault systems, in a range of sedimentary environments, highlights the importance of lens development and the mechanical stratigraphy in fault zone evolution. The unique properties of the layers control the orientation of individual fault segments that link via short-cuts to form more planar slip surfaces while preserving the complex lens architecture. This process may occur without rotation of the beds, or where two faults propagate towards each other folding of the layers, creating a lens with strongly rotated beds, before fault propagation can occur. The geometry and distribution of these fault bound lenses and the bed rotation in fault zones is particularly important to the development of flow pathways or barriers. Two models are presented to describe these processes:

The Quadshear model extends the established Trishear model through upward and downward propagation of faults in layered sequences. The stacking sequences of different mechanical properties impacts the deformation processes (folding vs failure/faulting) and controls the distribution and amount of shale smearing, impacting on flow behaviour. The FaultZ model considers fault short-cutting in a sequence with different mechanical properties. The model is used to assess the location and intensity of fault zone complexity in zones developed under different deformation conditions. The models provide simple predictive methods to define the fault zone architecture. The complexity of these fault zones impacts strongly on the flow pathways.

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