AAPG ACE 2018

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Lithofacies Control on Deformation Band Frequency and Geometry: Example From the Sherwood Sandstone Group, UK

Abstract

Deformation bands are strain localisation features common in highly porous clastic sedimentary bodies. Deformation bands often have permeabilities significantly lower than the host rock (up to five orders of magnitude lower) and many examples are recognised within reservoir fields around the world (e.g. Gulf of Mexico and North Sea). As such, they represent potential negative features for reservoir quality and ultimately well production, and as they occur on a sub-seismic scale the distribution can be difficult to predict and faithfully represent in reservoir models. 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, eolian-fluvial Sherwood Sandstone Group, this study is able to demonstrate a link between host lithofacies types and the frequency of deformation bands.

Individual deformation bands observed have at outcrop have highly variable geometries, with thicknesses ranging from grain-scale (sub-millimetre) up to a metre, and lateral extents ranging from 7 cm to 45 m. As well as variations between individual bands, four distinct architectural assemblages are recognised and a new classification scheme is proposed; i) isolated, ii) conjugate, iii) ladders, iv) chaotic.

Of all the lithofacies types present, the eolian grainfall and grainflow lithofacies (the constituent lithofacies in eolian sand dune elements) were proportionally the most likely to host deformation bands (>97% of the total observed). The deformation bands present in these two eolian facies types were more likely to be larger and more laterally persistent than deformation bands hosted by other lithofacies types. The preferential occurrences of deformation bands within eolian dune elements results in significant reservoir compartmentalisation in higher permeability hosts, and may indicate that more favourable reservoir conditions may be found in other facies of relatively lower permeabilities that may, at first assessment, represent a less attractive exploration target.