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