AMGAD I. Younes1,
(1) Stanford University, Stanford, CA
(2) Stanford University
Abstract: Segmentation of normal faults in the dip direction and its significance in fault seal analyses
Fault segmentation along strike is commonly observed in outcrop but segmentation along dip direction is difficult to observe in the field and to resolve in seismic. We argue that sense of fault segmentation has considerable implications for fault rock formation and should be carefully considered in any fault seal analysis.
Along-dip segmented faults form echelon arrays consistently stepping perpendicular to fault strike, towards either the hanging wall or the footwall. Faults step at contacts of competent and incompetent layers. Continued slip on fault segments produces a coalesced single fault strand. During fault coalescence, the deformation within the steps produces two distinct fault rock types depending on the sense of steps. In the hanging wall steps, deformation is dominated by shale smearing which involves the rotation of the incompetent layers until they are smeared parallel to the fault (smearing). In contrast, deformation along footwall steps forces the shale out of the step (extrusion), and eventually becomes dominated by brecciation of the brittle hanging and footwall rocks. In addition, our data shows that the thicker incompetent layers cause larger steps, predominantly, for hanging wall steps. Therefore, the dip angle of coalesced fault segments may become shallower (for hanging wall steps) or steeper (for footwall steps) depending on the ratio of competent to incompetent layers. Subsurface examples from the Gulf of Mexico will be discussed.
The hydraulic properties and sealing capacity of a fault may then depend on the fault step type and associated fault rock material; smeared shale or breccia.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana