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The Three Dimensional Structure of Deformation Bands Within High Porosity Sandstones From Pore to Outcrop Scale Observations

Abstract

Strain within high porosity sandstones is commonly localized within deformation bands (DBs): narrow tabular regions of disaggregated, rotated and/or crushed grains, formed under a spectrum of failure mechanisms. Given the influence DBs are inferred to exert over production, in addition to the strong coupling between fault rock petrophysical properties and the hydromechanical behaviour of the reservoir, ascertaining the flow characteristics of DBs is of considerable interest. However, despite being paid significant attention, both the structural architecture of faulted intervals dominated by DBs, as well as the nature of their respective flow fields remain difficult to resolve. These difficulties may in part be attributed to the limited resolution and dimensionality of conventional methods of structural and petrophysical characterization: The reliance upon bulk hydraulic conductivity tests to determine DB permeability can be a major criticism levied at previous studies, with such measurements comprising an amalgamation of host and fault rock permeability, thus being incapable of representing the spatial variability in petrophysical properties observed in thin section. And though 2D microstructural characterization may give some indication of lateral variability, the inability to account for anisotropy within the vector normal to the field of view, as well as sparse phenomenological validation of micro flow processes inferred from such analyses introduces further uncertainties. This poor knowledge of inter-pore communication is mirrored at outcrop scale, with the restrictive exposure of DB clusters and zones making the characterization of their structural architecture an inherently difficult task. Here we share results from on-going work in which a combination of 3D imaging systems and image based modeling is employed to address these deficiencies. At pore scale, novel microtomographic imaging of fluid diffusion coupled with pore-scale flow simulations are used to characterize the 3D petrophysical structure of deformation bands. To complement these microscale observations, we deploy digital outcrop methods to exposures with an intrinsically high degree of 3D control over the fault network. It is hoped that the greater scope of observations in terms of both dimension and scale provided by these techniques may lead to an improved understanding of the hierarchical nature of petrophysical compartmentalization within cataclastically deformed sandstone intervals.