DIAGENESIS AND FLUID EVOLUTION OF DEEPLY BURIED PERMIAN (ROTLIEGENDE)GAS-RESERVOIRS, NORTHWEST GERMANY

Depositional environment and tectonic setting were important in the di agenesis and evolution of reservoir properties in the Rotliegende sequ ence of the North German Basin. Facies belts paralleling the edge of a central saline lake controlled the distribution of early and shallow burial cements. Lake shoreline sands with radial chlorite cement show the best reservoir properties in the study area. Juxtaposition of Rotl iegende deposits against either Carboniferous Coal Measures or Late Pe rmian (Zechstein) evaporites by faulting resulted in cross-formational fluid exchange. The introduction of fluids from Carboniferous Coal Me asures into Rotliegende reservoirs produced intense clay cementation, significantly reducing rock permeabilities. Influx of Zechstein fluids favored precipitation of late carbonate and anhydrite cements. Cross- formational and fault-related fluid flow was enhanced during periods o f fault activity.