FAILURE MODES, DEFORMATION MECHANISMS, DILATION TENDENCY, SLIP TENDENCY, AND FAULT-CONTROLLED FLUID CONDUITS (Invited Presentation)

Faults typically nucleate in ideal orientations with respect to local stress and rock strength conditions at the time of initiation, which can result in complicated fault geometries. Although normally thought of as shear fractures, faults can originate with failure in a range of modes including components of tensile, hybrid, shear, compactive shear, and compactive failure. Non-planarity of faults is often caused by variation in failure mode, which in turn is dictated by mechanical stratigraphy interacting with the ambient stress field. Although faults tend to become smoother with increasing displacement and asperity removal, complicated fault shapes may remain active if the stress state is consistent. Complex fault shapes that form as a result of mechanical stratigraphic variation occur over a range of scales (bed- to formation-scale), and larger irregularities will persist to larger displacements. A fault (fault zone) may experience volume gain, volume conservation, and volume loss simultaneously depending on position along the fault’s surface. A combination of dilation tendency and slip tendency analysis of faults at initial failure or reactivation can guide identification of locations of positive dilation or volume gain, negative dilation or volume loss, and non-dilational or volume neutral deformation. Examples from central and west Texas are used to illustrate the correspondence between dilation and slip tendency patterns and failure (or reactivation) behavior, and to explore interrelated concepts of mechanical stratigraphy, failure mode, resolved stress analysis, and fluid transmission.