Wilkins, Scott J.1, Stephen J. Naruk1
(1) Shell International Exploration and Production Inc, Houston, TX
ABSTRACT: Sealing, but Active Faults: The Role of Frictional Stability and Transient Dilation?
Faults characterized as critically stressed or active, where the ratio of resolved shear to normal stress is close to or exceeds the maximum frictional strength, are believed to experience dilation. Presumably, this dilation enhances the hydraulic conductivity of the fault zone relative to aseismic faults, or "dead" faults that are not critically stressed. While critically stressed faults are expected to leak -and thus reduce the amount of trapped hydrocarbons- we document numerous active faults, in a variety of tectonic and depositional settings, that effectively trap large oil and gas columns. We suggest that a possible solution to this conundrum may be related to the frictional stability of the fault zone (aseismic or seismic slip) and the transient dilatancy that occurs during seismic slip. Numerous experiments suggest that transient dilation is associated with seismic slip, whereby faults experience a brief episode of dynamic dilation just behind the tip of a propagation rupture pulse. This mechanism of dilation is not effective in draining oil and gas reservoirs, and thus differs from more permanent dilation mechanisms (hydraulic fracturing within the fault core or creation of a permeable damage zone). The lack of transient dilation and stress drop during asesmic slip presumably prevents mechanisms that enhance fluid-flow. While many have used a combination of stress data and fault geometry to evaluate the possibility of dilation and resultant leakage, the mechanisms of frictional sliding and dilation have not been critically evaluated in these contexts.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.