A geomechanical analysis of the formation and evolution of Polygonal Fault Systems

The development of Polygonal Fault Systems (PFS) remains poorly understood despite extensive study for over two decades. These systems of exclusively normal faults are developed over wide areas of many basins worldwide and are believed to influence caprock integrity and hydrocarbon reservoir quality, whilst also potentially presenting shallow drilling hazards. A seemingly obvious conclusion from their layer-bound nature and significant lateral extent is that their origin must be governed by the constitutive behaviour of the host sediments. Establishing specific causative mechanisms has, however, proven difficult. The aim of this research project is to first assess some existing arguments for PFS genesis and this is complimented by a review of modification of structure in soft rocks and discussion of how this facilitates changes in both shear and compaction. The approach in this work is to utilise geomechanical forward modelling to demonstrate how such changes might lead to PFS formation. The concepts of this approach are discussed with focus on the adopted computational framework and selected constitutive model. An argument for polygonal fault genesis is presented that is founded on diagenetically induced shear failure, and the requirements for incorporating this into the constitutive model are described. Recovery of realistic PFS geometries is demonstrated as validation of the geomechanical argument and the competency of the computational approach. Conceptual sensitivity studies at the field scale are undertaken to better understand what processes, reactions and conditions might control fault genesis and propagation. When observed in mapview polygonal faults commonly intersect bedding planes at a wide variety of azimuths which reflects an inferred horizontally isotropic state of stress. Occasionally, this so-called 'planform geometry' is modified by the presence of tectonic faults, slopes or salt structures. Therefore, there is the possibility that polygonal faults may be useful paleostress indicators. This is explored using the characterised materials in simple models featuring anisotropic horizontal stress conditions