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Abstract: Fractured Reservoirs in Ciastic Rocks: Differences Between a Basement- Cored Structure and a Detached Fold Belt

Terry Engelder, Michael R. Gross, Amgad Younes

The Elk Basin anticline, Wyoming-Montana, has an order of magnitude more structural relief than structures of the Appalachian Plateau, New York.

Despite its structural relief the Elk Basin anticline shows very little macroscopic evidence for layer-parallel shortening vs. more than 10% for the subtle Appalachian Plateau folds. Elk Basin anticline is a passive drape fold extending over a tongue of basement punching up into the sedimentary cover. On the other extreme, the detached fold belt of the Appalachian Plateau remained in compression during most, if not all, of the Alleghanian layer-parallel shortening event. The joint pattern in Elk Basin is dominated by fold-parallel sets. The joint pattern in the Appalachian Plateau is dominated by fold-perpendicular sets. These two joint patterns are consistent with states of stress that suppress layer-parallel shortening in the former case and favor it in the latter case.

Curvy cross joints are unambiguous records of the change in stress field orientation. Such structures in the clastic rocks of Elk Basin indicate a 10° to 15° clockwise reorientation of the stress field during later stages of fold development. The early to synfolding propagation of fold-parallel joints is indicated by their attitude normal to bedding on both limbs of the Elk Basin anticline. Fold-parallel joints are also rotated during strike-slip motion on later, vertical faults cutting subperpendicular to the anticlinal axis. Finally, the fracture spacing index for fold-parallel joints in various formations at Elk Basin is less than for cross fold joints of the Appalachian Plateau.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela