Abstract: Brine Evolution Deduced from Fluid Inclusions, Permian Zechstein, Germany

Erik W. Zwart, Jacques L.R. Touret

The Zechstein 2 formation is an important gas reservoir in the northwestern part of Germany. It contains a carbonate sequence (Ca2) that is embedded within two thick evaporitic anhydrite sequences (A1 and A2). Diagenetic evolution of the carbonates has been influenced by descending and upwelling evaporitic brines of which the remnants were trapped as inclusions in late diagenetic minerals. Aqueous and gaseous inclusions could be observed in calcite, fluorite and anhydrite. The aqueous inclusions contain liquid, vapor and, depending on the occurrence, a solid phase while the gaseous inclusions are single phase at room temperature. The salt system and the salinity of the inclusions seem to be related to the evolution of the basin. Most inclusions in relatively early calcite contain the ystem H₂O-CaCl₂-NaCl. Later diagenetic calcites (bright cathodoluminescence) and late fluorite show increasing NaCl concentrations. In late fluorite, that fills open vugs and fractures, occur aqueous inclusions with high salt concentrations and secondary trails of carbon dioxide-methane inclusions. We tentatively interpret the methane inclusions as remnants of migrating hydrocarbons during a later stage of diagenesis. Anhydrite, one of the last diagenetic minerals to grow, contains inclusions (two solids, one liquid) belonging to a salt system that is not yet fully understood but that possibly points to an influx of KCl in the formation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994