The SGR for these faults ranges from high, or sand-prone (left) to low, or shale-prone (right). Empirical studies in producing fields have demonstrated a relationship between SGR and seal behavior.
The left-most fault displaces Tertiary sandstone and the gouge consists solely of sand. The center fault displaces Jurassic shale and limestone and the gouge consists of both shale and limestone. The right-most fault displaces a Jurassic limestone-shale sequence and the gouge consists of a 1 m thick layer of shale. Only small amounts of the more brittle limestones are incorporated in the gouge.
Note that both the brittle and ductile strata are incorporated into the fault zone. There is no simple "smearing" of ductile shales into the fault. The process is much more complex. Both brittle and ductile, both sand and shale, control the sealing behavior of the fault gouge.
This type of analysis is not applicable to all structural styles, all basins, and all prospects. Some structural styles such as strike-slip or foreland fold and thrust belts are dominated by brittle failure. Some plays such as the Rotliegendes in the southern North Sea are dominated by diagenesis and cataclasis. See the SEALS COURSE outline.