Chemistry and isotopic composition of Rotliegend and Upper Carboniferous formation waters from the North German Basin

Element concentrations and isotopic composition (δD, δ18O, δ15N of ammonium, 87Sr/86Sr) have been determined in highly-saline formation waters from the Rotliegend Altmark gasfield and the Upper Carboniferous Husum–Schneeren gasfield in the North German Basin. Both reservoirs have been covered by huge Zechstein evaporite caps since the Late Permian. Formation waters from the Altmark gasfield are characterized by high total dissolved solids and are enriched in δ18O relative to meteoric waters and either show positive or negative δD values which typically follow the proposed δD vs. δ18O paths for seawater evaporation. Formation waters stored in Upper Carboniferous sandstones show much more variable element concentrations and δ18O and δD values than the Rotliegend Formation waters. The Br vs. Cl concentrations of the brines suggest seawater evaporation being the principal source of salinity at both sites. The ammonium concentrations in the analyzed brines are highly variable and the δ15N isotopic composition of dissolved ammonium is controlled either by the isotopic composition of the gas in the reservoir (Rotliegend) or that of the host rocks (Upper Carboniferous). At both sites, early formation waters interacted with the sedimentary host rocks after deposition. 87Sr/86Sr ratios suggest that re-equilibration between fluids and their host rocks occurred in pre-Triassic times. There is no chemical or isotopic evidence that original formation waters which occur in sandstone below the overlying Zechstein evaporites were flushed out or mixed by later meteoric water as has been documented in deep-seated Mesozoic and other Paleozoic aquifers of the North German Basin.

Research Highlights
► Formation waters provide information on fluid evolution in sedimentary basins.
► Deep-seated aquifers sealed by huge salt covers seem to preserve original formation waters.
► Aquifers with no sealing covers may contain younger fluids than the original formation waters.