Diageneseevolution von Ober-Rotliegend II Sandsteinen eines Tight-Gas Feldes in Ostfriesland (NW Deutschland)

In the Southern Permian Basin (SPB) in northern Germany, tight gas reservoirs in aeolian sediments represent the main target for the Rotliegend gas exploration. The diagenetic evolution of these rocks is the result of fluid-rock interactions, which lead to extensive mineral formation processes after deposition and during burial of the rocks. Thus, the reservoir properties of sandstones were strongly affected. Although the diagenesis of the Rotliegend in the relevant study area was the subject of previous studies, the mineral formation processes in connection with the structural evolution (salt-diapirism and basin inversion) and migration of hydrocarbons are not completely understood. Therefore, the aim of this work is the reconstruction of the temporal and spatial diagenetic evolution of Upper Rotliegend II rocks exposed in three wells of a tight gas field in East Frisia, as well as to assess the importance of hydrocarbon migration for diagenetic mineral reactions. In the target area Rotliegend sandstones were buried to about 4300-4650 m depth. Core and data material were used to carry out petrographic and geochemical investigations on Upper Rotliegend II sandstones for such reconstruction. Existing data sets of this area were compiled, re-evaluated and completed. Eodiagenetic processes in the Upper Rotliegend II sandstones were mainly controlled by the depositional environment and the occurrence of Na+-, Ca+-, Mg2+- and HCO3--dominated pore fluids. These processes include the formation of hematite and illite grain coatings, anhydrite (gypsum), dolomite, calcite and quartz overgrowths. The mesodiagenesis includes the most extensive mineral formation and can be related to tectonic events combined with fluid transport (descending fluids from overlaying rock units and ascending HC-rich fluids from the Carboniferous). The reservoir quality was strongly reduced within the mesodiagenesis by the formation of pore filling minerals. Mesodiagenetic minerals comprise the precipitation of dolomite, ankerite, calcite, quartz, anhydrite, barite, illite, chlorite and antase. A significant factor for diagenetic processes and therefore for the reservoir quality plays also facies and grain size distribution of the sandstones. Sandstones with strong alternation of aeolian and fluvial/lacustrine facies contain more carbonate cement than illite, whereas the amount of illite increases in sandstones with only aeolian facies. Both mineral phases contribute to a significant loss of permeability and porosity. Coarse-grained sandstones are characterized by larger pore spaces and thus have a better porosity and permeability. The results of this study may help to better understand the diagenetic evolution of continental Rotliegend II sandstones in more detail. A generalization of Rotliegend diagenesis in other tight gas fields is difficult because of the complexity of diagenesis, which reflects an interaction of tectonic, fluid composition and facies

Diageneseevolution von Ober-Rotliegend II Sandsteinen eines Tight-Gas Feldes in Ostfriesland (NW Deutschland)

In the Southern Permian Basin (SPB) in northern Germany, tight gas reservoirs in aeolian sediments represent the main target for the Rotliegend gas exploration. The diagenetic evolution of these rocks is the result of fluid-rock interactions, which lead to extensive mineral formation processes after deposition and during burial of the rocks. Thus, the reservoir properties of sandstones were strongly affected. Although the diagenesis of the Rotliegend in the relevant study area was the subject of previous studies, the mineral formation processes in connection with the structural evolution (salt-diapirism and basin inversion) and migration of hydrocarbons are not completely understood. Therefore, the aim of this work is the reconstruction of the temporal and spatial diagenetic evolution of Upper Rotliegend II rocks exposed in three wells of a tight gas field in East Frisia, as well as to assess the importance of hydrocarbon migration for diagenetic mineral reactions. In the target area Rotliegend sandstones were buried to about 4300-4650 m depth. Core and data material were used to carry out petrographic and geochemical investigations on Upper Rotliegend II sandstones for such reconstruction. Existing data sets of this area were compiled, re-evaluated and completed. Eodiagenetic processes in the Upper Rotliegend II sandstones were mainly controlled by the depositional environment and the occurrence of Na+-, Ca+-, Mg2+- and HCO3--dominated pore fluids. These processes include the formation of hematite and illite grain coatings, anhydrite (gypsum), dolomite, calcite and quartz overgrowths. The mesodiagenesis includes the most extensive mineral formation and can be related to tectonic events combined with fluid transport (descending fluids from overlaying rock units and ascending HC-rich fluids from the Carboniferous). The reservoir quality was strongly reduced within the mesodiagenesis by the formation of pore filling minerals. Mesodiagenetic minerals comprise the precipitation of dolomite, ankerite, calcite, quartz, anhydrite, barite, illite, chlorite and antase. A significant factor for diagenetic processes and therefore for the reservoir quality plays also facies and grain size distribution of the sandstones. Sandstones with strong alternation of aeolian and fluvial/lacustrine facies contain more carbonate cement than illite, whereas the amount of illite increases in sandstones with only aeolian facies. Both mineral phases contribute to a significant loss of permeability and porosity. Coarse-grained sandstones are characterized by larger pore spaces and thus have a better porosity and permeability. The results of this study may help to better understand the diagenetic evolution of continental Rotliegend II sandstones in more detail. A generalization of Rotliegend diagenesis in other tight gas fields is difficult because of the complexity of diagenesis, which reflects an interaction of tectonic, fluid composition and facies