THE EVOLUTION OF DEFORMATION BAND FAULT CORE, AND ITS INFLUENCE ON DEFORMATION BAND FAULT ZONE SEALING CHARACTERISTICS

Deformation band faults in porous sandstone have a fault core consisting of glassy looking pods associated with polished through-going slip surfaces and minor anastomosing slip surfaces. Deformation band growth is well understood but little is known about slip surface and fault core nucleation and propagation. A series of normal faults with <30m maximum offset have been investigated in a variety of porous aeolian and fluvial sandstones. From detailed outcrop mapping the structure, geometry, and composition of fault core is highly variable along strike. Distinct fault components can be recognised within all the sandstones: zones of deformation bands, condensed deformation bands, breccia pods and polished slip surfaces. Through-going slip surfaces occur on one or both sides of the fault core and are striated, highly polished and occasionally fractured. Minor slip surfaces, which form anastomosing networks in condensed deformation bands, are first seen at low offsets (< 0.5 m) and are abandoned at higher offsets. Local fault growth processes (cataclasis, pore-space collapse, pressure solution, microfracturing) result in fault core with variable grain size (sub µm – 0.5 cm), grain sorting, and porosity (1-15 %). Increased grain crushing is observed adjacent to slip surfaces. The permeability of the fault core (0.07- 400md) is reduced with respect to host rock permeability (400-2000 md).

Fault-core thickness is a large uncertainty in calculations of fluid flow in fault zones. Our data show that the thickness is highly variable along strike and does not correlate with either the amount of slip or the number of slip surfaces. The thickness of the fault core is likely to be dependent on local growth processes, specifically the linkage of fault segments. This suggests that correlations of fault permeability with throw do not accurately represent along strike and down dip variability of deformation band faults. Simple calculations of two-phase flow properties based on measured porosity and permeability values suggest that deformation band faults containing fault core are likely barriers to two-phase flow. More data on the variability of fault core thickness and its petrophysical properties needs to be collected in order to characterise population statistics for models of deformation band fault fluid flow properties.