Abstract
The Zechstein-2-Carbonates represent one of the most prolific hydrocarbon systems of Central Europe. Carbonate reservoir quality is primarily controlled by mineralogy, with dolomite representing moderate-to-good porosities and calcite commonly representing low porosities. Current models suggest that this calcite is the result of a basin-wide phase of dedolomitization. The calcium (Ca) source for the dedolomites is thought to be derived from the fluids liberated during gypsum-to-anhydrite conversion. We present an easy-to-use and generally applicable template to estimate the dedolomitization potential of these fluids. Depending on reaction stoichiometry, salinity, and temperature, we estimate that between2.8⁎10−3 m3and6.2⁎10−3 m3of calcite may replace dolomite for each m3of anhydrite created. Within the constraints dictated by the environment of the late Permian Zechstein basin, we estimate that about5⁎10−3 m3of dedolomite is created for each m3of anhydrite. Mass balance constraints indicate that fluids derived from gypsum-to-anhydrite conversion account for less than 1% of the observed dedolomite in most of the studied industry wells from northern Germany.
Highlights
The Zechstein-2-Carbonate (Ca2) of the Southern Permian Basin represents one of the major gas plays in northern Germany
Reservoir quality is mainly controlled by mineralogy
Where the mineralogy is dominated by dolomite, reservoir quality is predicted to be moderate to good and poor where the mineralogy is mainly calcitic [1,2,3]
Summary
The Zechstein-2-Carbonate (Ca2) of the Southern Permian Basin represents one of the major gas plays in northern Germany. Dissolution of dolomite took place and led to local creation of secondary porosity, especially in the lower slope facies [9, 27] In this model the origin as well as the migration path of the CO2 explains the spatial distribution of dedolomite within the Zechstein-2Carbonates. Following the initial proposal by Clark [9], calcium sources for the Zechstein dedolomites have been largely attributed to the fluids released during gypsum-to-anhydrite conversion of the over- and underlying anhydrite sequences (A2 and A1) [9, 21, 23, 27] During this process the gypsum sequence loses 49% of its volume in the form of intercrystalline water [31, 32]. The results of this study may have direct implications for dedolomitization processes in other carbonate evaporite successions worldwide
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