Lars Stemmerik 1,2*, Kasper H. Blinkenberg 1, Ingrid P. Gianotten 3, Malcolm S.W. Hodgskiss 4, Aivo Lepland 4, Päärn Paiste 5, Israel Polonio 3, Nicholas M.W. Roberts 6 and Niels Rameil 3

1 Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.

2 Department of Arctic Geology, University Centre in Svalbard, P.O. Box 156, N-9171, Longyearbyen, Norway.

3 Aker BP ASA, Strandveien 4, N-1366 Lysaker, Norway.

4 Geological Survey of Norway, Leiv Eirikssons vei 39, N-7040 Trondheim, Norway.

5 Department of Geology, Ravila 14a, University of Tartu, Tartu, 50411, Estonia.

6 Geochronology and Tracers Facility, British Geological Survey, Nottingham, NG12 5GG.

* Corresponding author:

The preserved Zechstein succession on the Utsira High in the NE part of the Norwegian North Sea is 25-100 m thick and is dominated by shelf carbonates. Internal subdivision of the succession is based on the recognition of key surfaces in petrophysical logs and cores, and suggests that the carbonates mainly consist of ZS2 and ZS3 deposits and that younger ZS4 and ZS5 deposits are only locally preserved. The carbonates have undergone early, syn-depositional dolomitization followed by later dolomite recrystallization and calcitization. Calcitization, interpreted as dedolomitization, is restricted to the upper part of the ZS3 carbonate unit and based on U/Pb dating took place during the Triassic, with a later phase of recrystallization linked to mid-Jurassic uplift. Both dedolomitization and dolomite recrystallization relate to fresh-water infiltration with the resetting of dO18 values prior to the Late Jurassic drowning of the Utsira High. The reservoir quality of the carbonates is directly linked to post-depositional meteoric diagenesis, and the best reservoir properties are recorded in intervals dominated by recrystallized dolomites in ZS2 and lower ZS3 carbonates. Dedolomitization significantly reduced porosity in the upper ZS3 carbonates.

Key words: Upper Permian, Zechstein Supergroup, reservoir potential, carbonate-evaporite systems, dedolomitization, U/Pb geochronology, Utsira High, Norwegian North Sea.

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