Lithofacies control on deformation bands

Deformation bands are strain localisation features common in highly porous clastic sedimentary bodies. They form as predominately planar features with lateral extents commonly of a few metres and thicknesses frequently less than a centimetre. Deformation bands affect reservoir quality as they can have significantly reduced permeability compared to that of the host rock (typically 3-5 orders of magnitude lower). As such, the frequency and arrangement of multiple deformation bands within a reservoir sandbody can have implications for recovery rates. Despite the potential negative implications, our current ability to predict the presence of deformation bands is largely limited to their proximity to larger-scale (seismically resolvable) fault structures. However, using outcrop examples from the Triassic, aeolian-fluvial Sherwood Sandstone Group in the Cheshire Basin, this study is able to demonstrate a link between host lithofacies types and the frequency of deformation bands. Specifically, this study has shown that deformation bands are most likely to occur within the highly permeable aeolian facies types, notably grainflow and grainfall facies (constituent aeolian sand dune facies types). Deformation bands observed in aeolian facies types also have significantly larger lateral extents and are more likely to occur in complex morphologies compared to those identified in non-aeolian facies types. A new working classification scheme based on deformation band geometry is proposed that aims to provide information on the anisotropy resulting from multiple variable arrangements of deformation bands. This new scheme is complimentary to the kinematic and dominate mechanism of deformation schemes currently adopted.