Aperture-size scaling variations in a low-strain opening-mode fracture set, Cozzette Sandstone, Colorado

The aperture-size distribution of a set of opening-mode fractures in the Cozzette Sandstone is complex and cannot be described uniquely by a single equation. This study of horizontal core includes aperture-size data of macrofractures measured using a hand lens and microfractures measured using scanning electron microscope-based cathodoluminescence (SEM-CL). Macrofractures are poorly cemented, and total fracture strain is an order of magnitude lower than the smallest strain values of most previously described vein sets. Macrofractures in this set therefore resemble typical joints, as opposed to veins, in most respects. Although intragranular microfractures compose about 98% of the microfracture population, orientation, fracture fill, and fracture trace characteristics suggest that only transgranular microfractures can be shown to be genetically related to macrofractures. Aperture-size data below ∼0.012 mm and above ∼1 mm follow a log-normal distribution. Intermediate-size data, though relatively sparse, suggest a power-law aperture-size distribution. These results support the notion that this fracture set is an intermediate case between typical veins and joints. The results also suggest that processes leading to size distributions commonly associated with veins (i.e., power laws) may be present in joint sets, but over a more limited range, bound at small size-scales by grain-scale effects and above by mechanical layering effects.