HR: 09:00h
AN: U41A-03 INVITED     [Abstracts]
TI: Overpressure and Shale Properties on the Gulf of Mexico Shelf: Stress Unloading or Smectite-Illite Transformation?
AU: * Katahara, K W
EM: keith@spinexp.com
AF: Spinnaker Exploration Company, 1200 Smith Street, Suite 800, Houston, TX 77002
AB: Standard methods for estimating pressure from seismic velocity need to be modified on the GoM shelf because overpressure is often not due to undercompaction, but is at least partly due to smectite-illite (S-I) transformation. There are two end-member methods for estimating pressure from velocity for such shales. One assumes that the velocity-stress dependence of the shales during compaction is unaffected by the transformation, and that the transformation only increases the pressure and reduces the net stress. The reduction in net stress implies that the sediment is inside the mechanical yield envelope on an elastic unloading stress path. Therefore this method must also determine the maximum paleo-stress experienced by the sediment. The other method assumes that S-I transformation changes the velocity-stress behavior of the shale, but does not cause a reduction in net stress. Crossplots of shale density and sonic transit time show regularities that are inconsistent with the first model. The very high overpressures observed in some wells are inconsistent with the second model. However, the second model can accommodate reductions of net stress if the S-I transformation involves mechanical failure. The transformation thus might reduce net stress while still allowing the sediment to be on a yield envelope rather than on an elastic path within the yield surface.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5102 Acoustic properties
SC: Union [U]
MN: 2005 Joint Assembly