Comparison of the Natural Fracture Systems in Low-Permeability Sandstones of the
San Juan, Piceance, Raton, and Green River Basins
John C. Lorenz and Scott P. Cooper
Sandia National Laboratories, Albuquerque, NM
Subsurface sandstones in four studied Rocky Mountain basins are universally fractured. Matrix permeabilities are in the microdarcy range, but the permeability of matrix-fracture systems, measured by well tests, is typically in the millidarcy range and highly anisotropic. Significant natural fractures have been documented by cores taken from as deep as 18,300 ft, but not all fracturing creates economic reservoirs. The most common fractures are vertical, extension fractures, filled or partially filled by quartz, calcite, and locally with kaolinite. Reservoirs in the middle of basins and away from faults and folds typically contain widely spaced (a few tens of feet) regional fractures. These fractures have been locally reactivated and/or enhanced where they overlie deep-seated basement faults, and such reactivated systems have been the most productive targets. Average fracture spacing may be less than a foot in these systems, and shear reactivation of irregular fractures props them open. Unfortunately, such fault-related fractures also commonly contain mobile water. Reservoirs near overthrust basin margins locally display near-horizontal shear fractures, and reservoirs in complex structural positions consist of highly fractured rock. Widespread conjugate fractures are present in some strata and offer potentially rewarding, interconnected fracture systems, but are difficult to predict and document. Exploiting fractured reservoirs is difficult: most attempts to produce gas from deviated holes have fared poorly even though great numbers of fractures were intersected, and hydraulic-fracture stimulations are as likely to damage natural-fracture permeability as to enhance it.
AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming