Using probabilistic shale smear modelling to relate SGR predictions of column height to fault-zone heterogeneity

Fault-seal analysis in hydrocarbon exploration often involves prediction of the sealing capacity of fault rock at reservoir–reservoir juxtapositions on subsurface faults. A proxy property, such as Shale Gouge Ratio (SGR), is mapped on to the fault surface, and then SGR is either (a) calibrated by observations of known sealing faults, to define its sealing capacity (empirical approach), or (b) assumed to be equal to the composition of the fault rock, for which a database of capillary threshold pressures is available from cores (deterministic approach). The deterministic approach implicitly assumes that capillary pressures measured on centimetre-scale samples are representative of seismically mappable faults, for example that faults of intermediate SGR are equivalent to phyllosilicate framework fault rocks.

This contribution builds on earlier outcrop and modelling work to suggest an alternative explanation for the observed progressive increase in sealing capacity on faults of increasing SGR. Stochastic models of disrupted shale smears display the same pattern of increasing sealing capacity as SGR increases. These models have a bimodal ‘fault rock’ composed only of sealing shale smears and non-sealing matrix and, yet, at intermediate SGR the predicted column heights are similar to those normally ascribed to intermediate composition fault rocks. The resulting ‘fault-seal envelope’ in the models is a statistical estimate of the maximum trappable column height, dependent on the random occurrence of a gap in the smeared fault surface.