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Abstract: Reservoir Potential of Buried Valley Wall Stress-Release Fractures in Carbonate and Clastic Rocks

SASOWSKY, IRA D., University of Akron

Valley wall stress-release fracturing is a process which increases permeability in modern erosional elastic and carbonate settings. The secondary (and sometimes tertiary) permeability which develops is several orders of magnitude higher than the primary rock permeability, and is both heterogeneous and anisotropic. In clastic settings this enhanced permeability has been recognized as open fractures observable in adits, increased water-well productivity with proximity to the valley wall, and valley-aligned local anticlines. In carbonate settings preferential karstification occurs parallel to the valley walls, showing extreme concordance with even minor topographic variations.

As a special type of unconformity-related reservoir, such zones, if preserved, have the potential for high productivity. As with most unconformity-related reservoirs, enhanced permeability decreases with depth from the erosional surface. However, the modern systems provide useful analogs for focusing exploration more clearly than just with depth.

For example, in the carbonate systems, permeability enhancement is greatest on the down-dip sides of valleys. In both types of settings permeability development is greatest in the valley parallel direction, suggesting that directional drilling might be useful for exploitation of reserves.

The East Fork Obey River, in Fentress County Tennessee, is an extensively studied modem system that has developed in a mixed carbonate /clastic terrane. Cavernous porosity was evaluated on the Eastern and Western valley walls of this deeply incised stream, with a basin area of 184 square kilometers. Two classes of cavernous porosity were recognized, with the bulk of cavernous porosity developing on the Eastern side. Exploration in a similar, but buried, setting will reveal whether such porosity can be usefully preserved.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah