This project consists of integrated experimental and theoretical work to investigate conditions for formation of deformation bands: thin tabular zones of shear and/or volumetric deformation, found in sandstones. While all deformation band types have been identified in sandstone (compaction, compactant shear, isochoric shear, dilatant shear and dilation), reports of compaction and dilation bands are relatively uncommon. A theoretical framework, based on bifurcation theory, predicts conditions required to form deformation bands; results suggest a relationship between the band type and the stress conditions that produced it. Thus, it is hypothesized that the observed deformation band type in the field, and its orientation relative the maximum principal stress direction, can be used to infer the approximate value of the intermediate principal stress, relative to the other principal stresses. Theoretical results also suggest that specialized stress states are required to form compaction or dilation bands, possibly explaining why these bands are rarely reported. This project conducts a systematic evaluation of the theoretical framework, by conducting a comprehensive suite of true triaxial laboratory experiments, over the spectrum of material responses (brittle to ductile behavior), stress conditions (low to high mean stress), and deviatoric stress states (different relationships between the intermediate, maximum and minimum principal stresses.) True triaxial tests enable independent control of all three principal stresses, which is required to determine the influence of the intermediate principal stress on deformation band formation, to assess theoretical predictions.
Localized deformation occurs in the Earth's crust on all scales; therefore, understanding why these bands of deformed material form is of fundamental importance in the earth sciences. The high porosity sandstones studied here are useful in understanding deformation histories: the sedimentary bedding structure allows identification of shear deformation, while the high porosity allows for compaction or dilation. By determining the band type (compaction or dilation, with or without shear) and the band orientation, the mathematical framework of this project can be used to infer the field stress conditions that produced the band (in particular, determination of the approximate value of the intermediate principal stress, which is difficult to determine using traditional methods). Due to reduced porosity and reduced permeability, compaction bands can act as barriers to fluid flow, while dilation bands (consisting of increased porosity) could act as fluid conduits. Thus, these bands are likely to affect natural and forced fluid flows in the host rock. Good fluid flow is required in fluid extraction applications, such as oil production; improving domestic oil production will reduce dependence on foreign oil. Alternatively, storage applications, such as carbon dioxide sequestration require fluid containment.
