AMGAD I. Younes1,
Atilla Aydin2
(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