Reconstruction of linear dunes from ancient aeolian successions using subsurface data: Permian Auk Formation, Central North Sea, UK

Abstract

A series of well logs and cores penetrating the predominantly aeolian Auk Formation, Permian Rotliegend Group, Central North Sea, UK, have been evaluated to determine the morphology and style of migratory behaviour of the original dune bedforms, the overall depositional environment, and to assess implications for reservoir heterogeneity. This has been achieved by detailed facies analysis of subsurface datasets and by comparison of the observed sedimentary styles of accumulation to analogous modern aeolian dune fields. Aeolian bedform type, morphology, detailed migratory behaviour, and the nature of the accumulation surface have been interpreted. Analysis of the facies architecture of preserved cross-bedded sets and cosets indicates accumulation on a dry substrate via the migration and climb of large linear bedforms that possessed low-angle inclined lower plinths, up to 15?m thick. Dune plinth elements are dominated by wind-ripple and reworked wind-ripple strata, and were preferentially preserved as successive bedforms migrated over one another at low angles. Packages of grainflow-dominated strata representative of accumulation on the higher part of the bedform lee slope are less common and tend to be preserved mostly in the upper parts of large cosets of strata (~30?m thick). Large linear bedforms were separated by dry interdune areas. Although the primary direction of sand transport was along the elongated crests of the bedforms, a secondary component of transverse motion enabled the lateral migration and preferential preservation of lee-slope deposits that arose from a minor oblique component of bedform migration. In places, the architecture records the preservation of small barchanoid dune deposits, either within interdune depressions or superimposed on the lower flanks of the large linear bedforms. The preserved aeolian facies types exert a primary control on reservoir quality. Few previous studies have documented linear dunes in ancient successions; these findings represent a valuable case example.