Predicting distribution of remobilized aeolian facies using sub-surface data: the Weissliegend of the UK Southern North Sea

Abstract

Aeolian sandstones of the Permian Rotliegend Group comprise the principal gas reservoirs of the Southern North Sea Basin. The upper portion of the reservoir interval comprises a unit informally called the ‘Weissliegend’. This unit comprises mass-flow and in-situ soft-sediment deformed deposits formed when the Zechstein Sea catastrophically flooded the Rotliegend sand sea. The Weissliegend typically exhibits poorer reservoir properties than the underlying Rotliegend reservoirs and its thickness and distribution are notoriously difficult to map. Hanging vertical sections on a regionally extensive, intra-Rotliegend super-surface has led to the recognition of a preserved topography at the top of the Rotliegend. The thickness differences within the unit between the super-surface and the top Rotliegend (termed here the Upper Aeolian Unit, UAU) represent the original dune topography modified by flood-related processes, and provide an insight into the nature and scale of the aeolian bedforms that existed within the basin prior flooding. The relationship between known Weissliegend distribution and this topography provides a predictive tool for understanding Weissliegend distribution. Mapping the preserved topography reveals preserved bedforms up to 85 m high, with a bedform spacing of 8 to 10 km. The dunes deposited ‘transverse type’ strata but the scale and spacing of the bedforms are too big compared with modern transverse dunse. Instead a more complex model is proposed. The Weissliegend has highly variable thickness (0–26 m) and is commonly thicker within the interdune hollows. The model for a preserved topography at the top of the Rotliegend is further supported by thickness trends in the overlying Zechstein units which increase into the proposed interdune lows.