When evaluating faults for their sealing and barrier potential, it is important to know that different processes can occur along their lengths. These processes can variably impact the petrophysical properties of the fault(s). An obvious example is cementation which often occurs in local dilational jogs. The cements in Figure 1 are patchily developed, sometimes being only found on one side of each deformation band.

An example of how processes can change with different lithologies long a strike-slip fault are shown on Figure 2. At point 2, cataclastic (grain size reduction) deformation bands develop in relatively clean deltaic sandstones. A few metres along the fault, the sand becomes more clay rich - evidenced by the occurrence of desiccation cracks (top of the top right inset in Figure 2). Deformation is taken up by folding as opposed to faulting - these drag folds are a good kinematic indicator - of a dextral (movement to the right) strike slip fault in this case.

Figure 1. Fluorite cement along deformation bands in high porosity (aeolian) sandstones (Permian Hopeman Sandstone, UK). These are patchy and sometimes only spread out on one side of the fault.

Figure 2. Different processes along the same strike-slip fault, floor of Kirkmaky valley, Azerbaijan. Rock was excavated in the foreground to look for slickenlines but none were found.

The deformation band in the cored sandstone in Figure 3 is partly cataclastic (pencil point) but most of it (dark grey part) contains clays and finer minerals as a result of the band passing though finer grained (more clay rich), red colored sands. There is likely to be greater porosity and permeability reaction across the dark grey part than across the light coloured cataclastic part due to the presence of clays.

Figure 3. Deformation band in cored sandstone. Pencil points to cataclastic part of the fault, whereas the darker grey part is is clay rich deformation band.