Abstract
Polygonal faults are thought to potentially compromise the integrity of regional caprocks and are widely developed in mudrocks that are targeted for unconventionals exploration. Polygonal Fault Systems (PFS) contain networks of exclusively normal faults that intersect bedding planes at a wide variety of azimuths. This fact alone suggests a non-tectonic origin and recent arguments focus on a constitutive control on PFS formation. Geomechanical forward modelling has the potential to shed light on the evolution of material and stress state as geological structures form. The evolution of polygonal faults is studied here using the finite strain forward modelling code ELFEN FM. More specifically the recently suggested diagenetic trigger for PFS formation is investigated, and the process of incorporating this into a critical-state based model is outlined and discussed. The results demonstrate the formation of PFS in both 2D plane-strain and full 3D simulations as validation of both the genetic argument and the computational approach. PFS are additionally observed to be sensitive to subtle horizontal stress anisotropy e.g. around salt diapirs or larger tectonic faults. As such, there is the potential for these fault networks to act as ‘paleo-stress piezometers’. This study investigates this by incorporating subtle stress anisotropy into the model and observing the influence on fault alignment. Results indicate that even for subtle stress anisotropy, preferential fault alignment is observed and this is consistent with observation.
1. INTRODUCTION
The controls on the formation of Polygonal Fault Systems (PFS) remain poorly understood and are still widely debated, despite having been studied for over two decades. These unusual networks of normal faults, which are confined to specific stratigraphic intervals termed ‘tiers’, have been identified on seismic data from over 100 basins worldwide and frequently cover vast areas [1]. The presence of these faults on passive continental margins and particularly basin floors, where regional dips are close to zero, is curious as it implies that they must have formed in regions of low tectonic activity. Hence, the presence of shear failures in what are inferred to be essentially K0 conditions (zero lateral strain) is counter-intuitive. The confinement to tiers coupled with the suggestion that tectonic forces have minimal influence is strongly indicative of a constitutive control on the formation of PFS. Understanding the nature of PFS and the mechanism(s) by which they form is important as all known examples discovered to date are exclusively found in very fine-grained sediments which in some cases form the areal caprock. Therefore, there is the potential for PFS to act as a ‘seal bypass’ [2], and hence they have implications for subsurface fluid migration and therefore hydrocarbon exploration [3]. Additionally, the presence of PFS in the shallow subsurface means that they may present a drilling hazard and compromise efforts aimed at recovery of hydrocarbons. Finally, it has been noted that PFS are often sensitive to subtle horizontal stress anisotropy induced by tectonic faults, slopes or salt diapirs.
