Fracture Swarms in Low-Permeability-Sandstone Gas Reservoir Rocks in New Mexico, Colorado, and Wyoming: Implications for Exploration and Development

LAUBACH, S. E., Bureau of Economic Geology, University of Texas at Austin, Austin, TX

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 are difficult to measure directly. The Upper Cretaceous Pictured Cliffs Sandstone (San Juan basin, Colorado and New Mexico) and Frontier Formation (Green River basin, Wyoming) are low-permeability gas reservoirs for which production and core data indicate locally important fracture permeability. 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 this study, fracture patterns were characterized by mapping five large exposures of Pictured Cliffs and Frontier sandstone along the margins of the San Juan and Green River basins. A set of fractures, interpreted as having formed in flat rocks prior to local Tertiary folding and uplift, is recognized in both formations. In contrast to the conventional model 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 have only rare fractures. These less fractured domains may be as muc as several hundred 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 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.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)