ABSTRACT: Natural Fracture Patterns in Sandstones of the Frontier Formation, Southwestern Wyoming

LAUBACH, STEPHEN E., University of Texas at Austin, Austin, TX, and JOHN C. LORENZ, Sandia National Laboratories, Albuquerque NM

Sets of fractures having north, northeast, and east to east-northeast strikes are present in outcrops of Cretaceous Frontier Formation sandstone near Kemmerer, Wyoming. Fractures are perpendicular to bedding and predate local folds and faults; fractures may be present in Frontier reservoirs in the Green River basin to the east. The oldest fractures strike north and likely result from lateral stresses associated with subsidence of the north-trending Cretaceous foredeep. Fractures with an easterly strike are younger; they cut across or abut north-striking fractures where the two sets occur together. Their orientation may reflect east-west shortening during Cretaceous/early Tertiary orogenesis. Although this arrangement of fractures appears simple, large, interwell-scale outcrops reveal ttributes that could have a profound affect on the way such a network transmits fluid. The two fracture sets occur together only locally; more commonly, adjacent beds contain only one or the other set.

Fractures generally terminate within beds, so networks are vertically partitioned. Beds of similar composition and thickness have a range of fracture spacings, so spacing cannot be predicted from bed thickness. In addition to contrasts in fracture density between adjacent beds, variations in spacing occur within beds. Fractures are also arranged in swarms that range from simple elongated, lenticular zones to irregularly shaped patches and fronts. Fracture spacing may change abruptly from tens of meters to less than 10 cm. For beds of similar composition and thickness, fracture connectivity can range from 0 to 100%. These examples show that even apparently simple regional fractures, having what superficially appears to be a "sugar cube"-type pattern, can have drastic variation in orien ation, density, and connectivity. These attributes could govern success or failure of drilling operations designed to exploit production of hydrocarbons from fractures. In the Frontier Formation, such patterns need to be taken into account in exploration strategies and engineering evaluations.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)