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Fracture or Band? - A Transitional Type of Deformation Feature With Surprising Flow Effects


Fractures in rocks can be nature’s fluid superhighways, playing a role that enhances both natural and anthropogenic fluid transport. In contrast, deformation bands, including shear- and compaction-bands, are generally understood to act as fluid-flow baffles. A common perception is that bands and fractures are distinctly-different responses. We describe experimentally-created features that exhibit the textural characteristics of bands, but many of the flow effects of fractures, and which appear to be a transitional type of deformation. The resulting complex spatial arrays within the cylindrical rock samples (selected for depositional heterogeneity to create array complexity) show a clear relationship with the lithological layering of the carbonate (laminite & coquina) materials’ contrasting-textures. These material controls are similar to observed relationships with lithological boundaries commonly observed with macro-scale natural fractures. On x-ray tomographic imaging, the features are localized planar zones with density lower than the surrounding rock, and so they are dilational, like fractures. The array of features is incompletely connected through the 3D space, and so fluid flow along the entire sample must pass through some matrix and any features that are appropriate. Fluid-flow experiments, observed with 4D neutron tomographic imaging, reveal complex patterns of fluid motion. During the initial saturation of an air-filled sample with water, the water enters the sample using only a few of the fractures that reach the end, allowing the matrix to imbibe fluid from the features, competing with the strong imbibition effects related to the features themselves. During a subsequent experimental step, we observe, in the now-saturated samples, that the pressurized fluid passes readily from sample end to end using only a few of the features, bypassing almost all of the matrix volume, revealing that the features represent enhanced flow pathways. This latter observation uses the contrast of neutron absorption between the two isotopic forms of water, D2O and H2O, during a fluid-replacement process. Post-experiment investigations, via thin sectioning and SEM images, reveal that the features are not the expected open cracks, but instead are filled with broken grains and complex arrangements of mixed particle sizes, like are typically seen in bands. Digital-rock methods, based on the SEM-observed textures, calculate flow properties that agree with the inferences from the experimental flow observations. This study has significant implications concerning the assumptions usually made relative to the multi-phase characteristics of flow-enhancing features that are assumed to be classical ‘open’ fractures. The work also indicates that the distinction - in terms of process, or in terms of flow effects - between fractures and bands, is not as clear-cut as has been assumed.