Anatomy of a Normal Fault
with Shale Smear
Atilla Aydin and Yehuda Eyal
Some faults are fluid pathways but others are barriers. The latter type well
known in the oil and gas industry and attributed to granulation and shale smear.
Fault
zone granulation has been the focus of many recent studies, but shale
smearing remains relatively obscure. We describe the geometry and structure of a
normal
fault
with shale smear in a 1500 m thick sedimentary sequence of Gambrian
to Neogene age in a graben l0 km west of Elat southern Israel. The
fault
has a
trace length of about 2 km and is marks entirely by what remains of a formation
made up of a 60 m lower shale unit, 25 m of middle carbonates, and 35 m of upper
shale. Both shale units had have been stretched over a planar discontinuity
defined by the footwall cut-off planes of the underlying sandstone and limestone
units for 250 m, the magnitude of the normal slip. Thus, the
fault
geometry and
the position of the shale units reveal a smearing process by which the shale
units reduce thier hickness or nearly vanish by thinning perpendicular to the
fault
and stretching parallel to the
fault
. ln a few exposures, the lower shale
unit is reduced from 60 m to a thickness less than 0.5 m. The middle carbonates
display boudinage and form discontinuous lenses along the
fault
. The impact of
the intense continuous deformation, the discontinuous deformation by the faults,
joints and veins of the shale and surrounding competent rocks, and mixing of the
shale with adjacent permeable units, on the hydraulics of the
fault
zone and its
sealing potential need to be, carefully evaluated.
This study improves the present knowledge about how fault
zones may
incorporate shales therein act as lateral seals for hydrocarbons, and when and
how this sealing potential may be breached.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California