D. M. D. James writes: the combination of 'tip-line' bifurcation with 'asperity' bifurcation as set out by Childs, Watterson and Walsh is a significant step forward in fault modelling. The authors correctly point out the implications of their model for analysis of lateral connectivity of hydrocarbon reservoirs using simple juxtaposition concepts appropriate to a single zero-thickness fracture, but do not mention the equally important implications of their model for vertical connectivity between source rock and reservoir. Contrary to the authors' statement, field experience of interlayered sands and shales typical of many Tertiary deltas shows that fault throws far greater than individual thicknesses of potential reservoirs do not necessarily result in fault seal. Moreover, 'sand-on-sand' contacts commonly seal, 'sand-on-shale' contacts commonly leak, and the fault zone must commonly act as both a vertical migration route and lateral barrier, both to hydrocarbons and pore-water, and do so simultaneously.
Figure 1 illustrates the 'shingled gouge' model that I developed to account for these relationships and which I believe complements the model of Childs et al, although not requiring tip-line bifurcation. It postulates that gouges develop as a result of episodic fault movement during fault growth through material of variable elastic properties and angles of internal friction—the former governing the ratio of horizontal to vertical stress and the latter governing the hade of the developing fault tip. Shaly gouge lenses originate largely by injection into transtensile portions of the fault plane in poorly consolidated sediments. Both shaly and sandy gouge more commonly result