Fracture permeability is of primary importance to producibility of gas from many low-permeability-sandstone gas reservoirs in the western United States, but the location, orientation, spacing, and connectedness of fractures in the subsurface is difficult to measure directly. The Upper Cretaceous Pictured Cliffs Sandstone (San Juan Basin, Colorado and New Mexico) and Frontier Formation (Green River Basin, Wyoming) form low-permeability gas reservoirs for which production data indicate locally important fracture permeability. In the wells studied, fractures are present in core or visible on borehole-imaging logs. Insight into the types of fracture patterns that may occur in these rocks could help guide exploration, completion, and stimulation strategies.

Maps of fractures provide the most accurate representation of fracture spatial distribution, trace length, connectivity, and size and shape of fracture-bounded blocks. In mis study, fracture patterns were characterized by mapping large exposures of Pictured Cliffs and Frontier sandstone along the margins of the San Juan and Green River Basins. A set of fractures is recognized in both formations that is interpreted as having formed in flat-lying rocks prior to local Tertiary folding and uplift. In contrast to the conventional map pattern of regional fractures, in which fractures are arranged in regularly spaced, orthogonal arrays, these fractures occur in discrete swarms separated laterally by domains that either lack fractures or that have only rare fractures. These less fractured domains may be as much as several hundreds of meters wide. Within swarms, fractures are well interconnected along the length of the swarm, but poorly interconnected across the width of the swarm. Fractures between swarms are commonly isolated. For the Pictured Cliffs and Frontier outcrops that were studied, the fracture pattern is self-similar over scales ranging from outcrop to interwell scale with fractal dimension D = 1.2. In the low-permeability sandstones of the Rocky Mountain region, irregularly spaced fracture swarms are potential targets for gas exploration and should be incorporated into fractured-reservoir models.

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