Inside Auk—the Anatomy of an Eolian Oil Reservoir

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Abstract
Auk field is located in the central North Sea and produces oil from Permian Zechstein carbonates and the underlying Rotliegend sandstones. The Rotliegend is 150 to 500 m thick and can be divided into five discrete episodes of desert sedimentation. Deposits of the upper three episodes are oil bearing, with eolian slipface sands forming the main producing intervals. The earliest deposits (unit 5) are localized waterlain conglomerates, which possibly infill topography on the unconformity with the Devonian. They are overlain by a large wedge-shaped mass of eolian slipface sands (unit 4) that onlaps the Devonian. Unit 3 represents a change in eolian deposition with a marked increase in wind-ripple laminated strata. Their abundance probably indicates more variable winds, and thin conglomerates of interdunal dolomite clasts suggest periodically wet conditions. This depositional unit varies in thickness by about 100%, with thick areas corresponding to the stacked deposits of slipfaceless draa and thin areas to stacked interdunes. Dune slipface sands within the draa accumulations are orientated toward the east and cannot be correlated over hundreds of meters between wells in a cross-wind direction. Unit 2 contains a still greater proportion of wind-ripple laminated sands and waterlain deposits. Its geometry is dominated by a large depositional mound -7 km across wind, 87 m thick, and thinning to 10 m in its adjacent interdunes. The mound is located above an interdunal thin of unit 3. Several intervals of dune and probably draa slipface sands occur where the mound is thickest, and a distinctive body of fine-grained eolian sands mantles its northern flank. The uppermost unit (1; Weissliegend) comprises waterlain mass-flow sands that partially infill interdunal lows and appear to represent reworking of a largely abandoned erg by rain water. Intercalated organic-rich shales and dolomites contain indications of evaporitic conditions, desiccation, and deflation and yield no evidence of marine faunas. The succeeding marine Zechstein gently inundated remaining topography.

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