Critical state finite element models of contractional fault-related folding: Part 2. Mechanical analysis

Finite element modeling of contractional fault-related folding with critical state mechanics concepts reveals fundamentally different behaviors at two levels, depending on (1) mechanical stratigraphy and (2) initial fault geometry. Deviatoric plastic strain rate and strain patterns show that ‘strong’ layers, like normally consolidated sandstone or overconsolidated shale, develop strain localization in form of narrow plastic shear bands. ‘Soft’ layers like normally consolidated shale fail to develop localizations and respond by diffuse, distributed plastic strain. Inter-layered overburdens display both deformation modes. Models with listric thrust fault seeds develop basement-cored anticlines in the hangingwall that are characterized by locally tensile stress regimes within overall compressive regional stress fields. The anticline crests of these models show evidence for strain softening. By contrast, anticlines emerging in detached fault-related fold models are subject to locally compressive stress regimes, consistent with the applied far-field shortening. These anticline crests strain-harden during deformation. Stresses resulting from the finite element models deviate from stress estimates based on integrated densities. The discrepancy is small for basement-involved models but significant for detached style models, suggesting that stress predictions based on integrated densities may be inaccurate for contractional systems. First-order predictions of plastic deformation suggest that footwall synclines in basement-involved fold styles may host reservoirs with relatively poor quality owing to horizontal and vertical compaction. Anticlines found in detached deformation styles are bounded by compactive domains whereas the crestal regions are less affected by volumetric plastic deformation thus producing relatively better reservoir quality.

► Fault-related folding finite element models with critical state mechanics
► Basement-cored anticlines reveal local tensile stress regime.
► Detached folds form in overall compressive stress regime.
► Folding style controls hardening versus softening in fold crests.
► Relationships among hardening, softening, reservoir quality, and structural position.