oa Fault-zone Localisation in High-porosity Sandstone Reservoirs and Impact on Flow Efficiency

We present results from a study of the distribution of deformation bands and larger ultracataclastic faults in a high-porosity sandstone reservoir outcrop analogue. We show that tectonic loading path and the nature of the stress changes causing deformation may strongly influence strain distribution. Localisation of deformation onto a smaller number of larger ultracataclastic fault zones is more likely in an extensional context than a compressional one because of the work-hardening nature of deformation band formation: In the extensional context, the differential stress required to continue deforming deformation bands is much less than in the compressional case, and may be achieved by subtle differences in stress variation around faults or small fluid pressure changes. Using new permeability measurements of host rock and deformation structures, we test models for the impact of these structures on flow rates in a producing field. Calculations of flow efficiency show that a small number of larger ultracataclastic faults may severely impede flow during production. Low-displacement deformation bands, with a much less reduced permeability however, will have a much lower impact on flow rates despite their higher densities. The impact of small deformation bands is likely to be greatest when produced by tectonic shortening.