Structure of Fault Zones in Sandstone and Its Effect on Permeability

SEEBURGER, DONALD A., Chevron Oil Field Research Co., La Habra, CA, A. AYDIN, Purdue University, W. Lafayette, IN, and J. L. WARNER and R. E. WHITE, Chevron Oil Field Research Co., La Habra, CA

Faults in clean porous sandstone occur as thin, 3-D deformed zones containing crushed grains. Field observations and laboratory measurements show that fault zones have a profound effect on fluid flow. Field observations indicate that most fault zones across which there have been more than a few cm of displacement develop a slip surface that is bounded on one side by the deformed zone in which porosity, grain sizes, and the degree of sorting of the sand grains are reduced, and on the other side by nearly undeformed rock. Permeability along this slip surface could be large, as for a fracture, allowing fluid migration in all directions parallel to the slip surface. Laboratory measurements confirm that permeability in the deformed, low porosity zone is low. Fault zone permeability could t us be extremely anisotropic with low permeability on one side due to the deformed zone, and high permeability parallel to the fault on the other side related to the through-going slip surface. The slip surface may occur more often on the hanging wall side of the deformed zone providing asymmetry in permeability distribution and, thus, asymmetry in trapping and migration potential. This asymmetry could explain many oil-water-fault relations such as those reported from the Niger Delta.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)