Data from faults with core recovery offshore Norway and from outcrops in study areas onshore demonstrate the development and continuity of smear along large (seismic-scale) faults. Smear along these faults is typically associated with thick (tens of metres) shale source layers and fault segments that are slightly offset, where the overlap between the segments creates an extensional dip relay. It is demonstrated that rock types other than shale, such as coal, siltstones and carbonates, may smear and thereby contribute to the low- permeable fault gouge. A critical threshold for the shale smear factor (SSF), given by the fault throw divided by the thickness of the shale source layer, is established to separate continuous and discontinuous smears. An SSF ≤4 is likely to correspond to a continuous smear along large faults and thereby to a sealing membrane on the fault surface. For small (subseismic) faults, a continuous smear can be maintained for both shale and coal source layers for much higher smear factors compared with large faults. The continuity of smear associated with small faults also displays a greater variation (SSF in the range of 1–50), to become less predictable than smear along large faults.