Upper Permian carbonates at the northern edge of the Zechstein basin, Utsira High, Norwegian North Sea

The first detailed description of Zechstein carbonates and their diagenesis along the northern margin of the northern Zechstein basin is based on core material from four wells on Utsira High in the Norwegian sector of the North Sea. It provides a generic link to the better known southern Zechstein basin, and northwards to the Late Permian East Greenland basin. The Zechstein succession consists of shale-carbonate cycles, and anhydrite only occurs in the uppermost preserved cycle, believed to correlate to Zechstein 4. The eleven carbonate facies reflect deposition in shallow shelf and arid shoreline environments, and resemble Zechstein-2 and -3 carbonate facies from the southern Zechstein basin. The carbonates are pervasively dolomitized by an early texture-preserving dolomite characterized by δ18O values between −6‰ and +2.4‰ similar to dolomites in the southern Zechstein basin, and δ13C values between −0.7‰ and +5‰ which is significantly lower than time-equivalent dolomites from the southern Zechstein basin. Later, subaerial exposure and fresh water infiltration during the late Permian or early Triassic caused replacement of the upper part of the dolomite by crystalline, fabric destructive calcite (dedolomite). The dedolomitized zone is up to 23 m thick and calcite crystals are characterized by moderately negative δ18O values in the range −8‰ to −5.5‰, characteristic for meteoric water. The Utsira High carbonates thus share a depositional history with the classical Zechstein carbonates whereas their diagenesis seems to have more in common with the time equivalent succession in East Greenland. Early leaching and dolomitization enhanced porosity whereas subsequent late Permian - early Triassic subaerial exposure and widespread dedolomitization had profound negative impact on reservoir properties of the Zechstein carbonates.