Rotliegend Auk Formation sandstones form the second most productive reservoir in the 30/24 fields and the major source of aquifer pressure support. In the Argyll Field, where the Zechstein oversteps beyond the Rotliegend on to Devonian continental sandstones, there were initial difficulties distinguishingbetween the Rotliegend and the Devonian.
Four facies associations are present in the Rotliegend Auk Formation: aeolian slipface sands, aeolian wind-ripple sands, water-lain Weissliegend sands and other water-lain conglomerates, breccias and sands. Five reservoir zones consist of different proportions of these facies which infill topography and onlap the Argyll high. Their distribution and character reflect periods of sediment supply, subsidence and fluctuating climatic conditions towards the margin of an interior continental basin.
The Auk Formation as a whole forms a high quality reservoir at depths of 3000–4000 m. The best intervals, with Darcy permeabilities, consist of coarse-grained Weissliegend sands. Two of the four initial wells in the Ardmore (Argyll) Field redevelopment are planned to target the Rotliegend reservoir.