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Natural fracture distributions in sinuous, channel-fill sandstones of the Cedar Mountain Formation, Utah

¹ Sandia National Laboratories, Albuquerque, New Mexico 87185; jcloren{at}sandia.gov
² Sandia National Laboratories, Albuquerque, New Mexico 87185
³ Sandia National Laboratories, Albuquerque, New Mexico 87185

John Lorenz bounced in and out of school and worked for the U.S. Geological Survey and the Peace Corps (Morocco) before landing at Sandia in 1981 to work on the Multiwell Experiment Project in the Piceance basin and other reservoir characterization projects. He received a B.A. degree from Oberlin College (1972), an M.Sc. degree from the University of South Carolina (1975), and a Ph.D. from Princeton University (1981).

Scott Cooper is a senior member of the technical staff at Sandia National Laboratories. He received his B.S. degree from the South Dakota School of Mines and Technology (1997) and his M.S. degree in geology from the New Mexico Institute of Mining and Technology (2000). His current research focuses on natural fracture systems and reservoir characterization.

William Olsson received his Ph.D. in geology from the University of Illinois in 1973. After working 5 years with the U.S. Bureau of Mines in Minneapolis, he migrated to Sandia National Laboratories. He has worked on the rock mechanics aspects of nuclear waste disposal and the geomechanics of reservoirs.

A set of regional natural fractures, present in the sandy to conglomeratic, fluvial, channel-fill deposits of the Cedar Mountain Formation (east-central Utah) has a consistent west-northwest strike regardless of the local axial orientations of the sinuous channels. The fracture-producing stresses were not significantly refracted by the mechanical-property contrast between the channel-fill sandstones and the encasing overbank mudstones. In addition, fracture spacing along the sinuous channel axes is relatively constant between one-half and one-third of the bed thickness for both large fractures that cut the full thickness of the channel deposits and for smaller fractures in the thinner, component beds. Fracture spacing was apparently not affected by the variations in stress amplification that commonly result from differently oriented stiff inclusions in a ductile matrix. Therefore, in the absence of other structures, fracture intensity and the orientation of fracture-related maximum horizontal permeability in sinuous elongated reservoirs will be relatively constant regardless of the orientation of the long axis of the reservoir. Whether maximum permeability trends along, oblique, or across such reservoirs, and the relative drainage efficiency of horizontal versus vertical wellbores drilled into them, will vary only with the local trend of the channel axis.