Characteristics of deformation bands and relationship to primary deposition: an outcrop study from the Wirral, north-west England

The gross environment of deposition has been recognised as a major influence on the development of deformation bands. Deformation bands represent local zones of grainsize reduction/crushing and fusing that develop in response to the accommodation of stress, and typically develop in sandstones. As they can result in crosscutting planes of low permeability compared to undeformed rock, deformation bands can degrade reservoir quality, and have an adverse impact on the performance of economically important hydrocarbon reservoirs, aquifers or potential repositories for carbon capture. Outcrop of the Middle Triassic (Anisian) Sherwood Sandstone Group from Thurstaston Hill and West Kirby (Wirral, north-west England) comprises a series of stacked dune and interdune facies, with rare heterolithic fluvial associations (channel and channel lag). Aeolian dunes are characterised by relatively large-scale, low- to high-angle cross-sets with common bimodal ‘pinstripe’- type lamination, with interdunes typified by planar and ripple-laminated sandstone and silty sandstone. A striking feature is the development of deformation bands which are locally pervasive. Initial data collection shows that deformation bands are more common in grainfall and grainflow facies, becoming rare/not observed in trough, planar cross-bedded and pebbly sandstone facies. Their morphology has been categorised into 4 distinct classes based on the spatial density and relationship between individual deformation bands (see image). The permeability of these features, assessed in the field by mini-permeameter, does not appear to be related to the class of the deformation band. These features have a high permeability contrast with the surrounding sandstone, and permeability values up to two orders of magnitude lower than undeformed sandstone is indicated. Where present in reservoir rocks such as the Leman Sandstone of the North Sea, or regional aquifers such as the Wilmslow Sandstone onshore, present day bulk permeabilities may be higher in fluvial facies with no deformation bands, rather than silt and clay-poor aeolian facies that host these features. Deformation bands may have a stronger influence on fluid flow toward the end of the production history of a well or field, when reservoir pressures are depleted. These observations, could mean that reappraisal of reservoirs is required to optimise production in declining fields/aquifers.