Deformation band development as a function of intrinsic host-rock properties in Triassic Sherwood Sandstone

Deformation bands significantly alter the local petrophysical properties of sandstone reservoirs, although it is not known how the intrinsically variable characteristics of sandstones (e.g. grain size, sorting and mineralogy) influence the nature and distribution of deformation bands. To address this, cataclastic deformation bands within fine- and coarse-grained Triassic Sherwood Sandstone at Thurstaston, UK were analysed, for the first time, using a suite of petrographical techniques, outcrop studies, helium porosimetry and image analysis. Deformation bands are more abundant in the coarse-grained sandstone than in the underlying fine-grained sandstone. North- and south-dipping conjugate sets of cataclastic bands in the coarse-grained sandstone broadly increase in density (defined by number/m2) when approaching faults. Microstructural analysis revealed that primary grain size controls deformation band density. Deformation bands in both coarse and fine sandstones led to significantly reduced porosity, and so can represent barriers or baffles to lateral fluid flow. Microstructural data show preferential cataclasis of K-feldspar grains within the host rock and deformation band. The study is of direct relevance to the prediction of reservoir quality in several petroleum-bearing Lower Triassic reservoirs in the near offshore, as deformation band development occurred prior to Carboniferous source-rock maturation and petroleum migration.