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The Impact of Dedolomitization on Reservoir Quality of the Upper Permian Zechstein 2 Carbonate, NW Germany


The Upper Permian Zechstein 2 Carbonate (Ca2), one of the most prolific gas reservoirs in northwestern Germany, shows fair to excellent reservoir quality when dolomite and decreased reservoir quality when calcite. The majority of calcite is related to an intermediate burial dedolomitization phase. Dedolomitization, the replacement of dolomite by calcite, is accompanied by calcite cementation and represents the most intense porosity- and permeability-destroying stage during the diagenetic history of the Ca2. Most of the gas fields within the German Zechstein fairway are located in the mainly dolomitized platform environment. The remaining fields are in the mainly dedolomitized slope environment. Significant exceptions to this generalization occur along the slope, where porous dolomitic zones enhance reservoir quality in otherwise tighter dedolomitized carbonates. Dedolomitized zones also decrease reservoir quality in platform lithofacies types carbonates. The spatial distribution of the gas-bearing dolomite zones is challenging to predict between wells. Detailed petrography and areal mapping of the dolomite/calcite ratio of wells indicates that most of the dolomitic zones within the slope occur along the upper slope. The middle and lower slope show dolomite contents of ca. 10% and are thus lower reservoir quality than the upper slope. On the platform a regional pattern is observed with intertidal lithofacies types consisting mainly of dolomite while more open marine shallow subtidal lithofacies types showing a higher degree of dedolomitization. The occurrence of dedolomite present within platform lithofacies types is essentially limited to the lowstand wedge (LSW), which overlies upper slope carbonates basinward of the late transgressive and highstand Ca2 platform carbonates. Porosity-permeability cross-plots, coded for mineralogy and lithofacies, indicate a strong dependency of reservoir quality to the stratigraphic architecture of the Ca2. The dedolomitizing fluids may originate from the overlying (A2) and/or underlying (A1) anhydrite layer. Calcium-rich pore fluids, released during the transformation of gypsum to anhydrite and pressure solution of anhydrite might have entered the Ca2 via a conductive fracture network, preferentially dedolomitizing lithofacies types with initially high reservoir quality.