Abstract - Shale smears occur on the surfaces of tectonic faults contained within lithified sequences of sandstones and shales. The smears occur on sandstone/sandstone fault surfaces and may extend from footwall to hangingwall cutoffs of the shale source layer. The nature and systematics of shale smears have been examined in working quarries exposing Westphalian sandstone/shale sequences, which are offset by faults of up to 15m throw. The continuity of shale smears has been examined for a range of values of Shale Smear Factor (SSF), which is the ratio of fault throw to vertical thickness of the offset shale layer. When this ratio is 7 or less, shale smears are continuous on sandstone/sandstone surfaces between base shale footwall cutoffs and top shale hangingwall cutoffs. Continuous smears, in 2-D sections, may occur at SSF values of up to 50. Shale smears are formed by one or a combination of three mechanisms: abrasion, shear and injection. Previous descriptions have been restricted to occurrences on growth faults in soft sediments where the shear mechanism is dominant.
In contrast to fault populations, which have well defined scaling
properties, the mechanisms of shale smearing are not sufficiently understood
to establish their scaling laws and to extrapolate outcrop observations
to larger scale faults. The shale smears on fault surfaces examined in
outcrop are assumed to be equivalent to seals in the subsurface context
and the observations are of most relevance to sealing of sub-seismic populations
of intra-reservoir fault surfaces rather than to trap closures. The degree
of sealing of fault surfaces due to shale smearing depends not only on
the gross sandstone/shale ratio but also on the distribution of shale units
within the sequence and on their individual thicknesses.
In: The Geological Modelling of Hydrocarbon Reservoirs and
Outcrop Analogues (edited by Flint, S. and Bryant, I. D.) Special Publication
of the International Association of Sedimentologists 15, 113-123,
1993.