The Shale Gouge Ratio (SGR) algorithm uses the clay distribution through the wall-rocks, together with the fault displacement, to estimate an 'average' clay content at each part of the fault zone, completely ignoring the detailed fault-rock distribution. This average value is often correlated with particular fault-rocks observed in small-scale samples. However, probabilistic modelling of shale smear distributions shows that these can produce a variable sealing capacity, dependent on the arrangement of disrupted smears in the fault zone. The resulting SGR vs column height relationship is analogous to the conventional empirical calibration of SGR vs buoyancy pressure. However, it arises with only two components in the model fault zone: infinitely sealing clay smears and non-sealing remainder. Variable capillary threshold pressure of fault-rock is not required to explain the trend of trapped column height with SGR.


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