Sedimentology and Diagenesis of the Zechstein Ca2 Carbonate, Late Permian, Northwest Germany

Abstract

Carbonates of the second Zechstein cycle, the Stassfurt Carbonate (Ca2), constitute north Germany's most prolific carbonate gas play. The underlying Werra Anhydrite (A1) of the first Zechstein cycle and the overlying Basal Anhydrite (A2) of the second Zechstein cycle enclosed the reservoir. An erosional sequence boundary present at the top of the A1 sulfate platform separates the A1 from the overlying Ca2 carbonate platform. Ca2 thickness ranges from 30 to 80 m on the platform, 40 to 250 m on the slope, and 10 to 40 m in the basin. Following flooding of the underlying A1 sulfate platform (Ca2 maximum flooding), Ca2 shallow-water carbonates prograded basinward. Platform, upper slope, middle slope, and lower slope facies are recognized and are subdivided into 27 subfacies. Systematic trends in facies distribution are apparent on depositional dip-oriented well log cross sections. Trends in vertical succession of subfacies types, along with the good correlation between Ca2 facies and A1 thickness, enable mapping of the updip and downdip limits of the facies. Diagenesis is important in determining Ca2 reservoir quality. Reservoir rock is typically fair to good where it is dolomite. Early calcitization (dedolomitization) of the Ca2 yields poor to non-reservoir rocks. Although calcitization in generalmore » increases basinward, lateral prediction of mineralogy within individual Ca2 facies is difficult. A multivariate statistical investigation has been initiated to determine parameters, which are important in predicting trends in calcite distribution.« less