VARIATIONS IN AQUIFER CHARACTERISTICS AND -EVOLUTION OF THE GERMAN BUNTSANDSTEIN FACIES OF THE THURINGIAN BASIN, EAST GERMANY

The Lower Triassic Buntsandstein is a major aquifer in Thuringia and adjacent areas in central Germany. In the Thuringian Basin the investigation of its reservoir characteristics is part of a BMBF sponsored project on integrated fluid dynamics in sediment basins (INFLUINS).

The aquifer sediments are composed of fluvial, eolian and lacustrine facies which exhibit differences in reservoir quality. These differences are linked with facial and mineralogical controls causing regional to small scale variations of reservoir properties. In order to understand reservoir property differences and their linkage to fluid flow pattern an investigation of facies and diagenesis of the sandstones was undertaken comparing infiltration areas in outcropping Buntsandstein at the north-western basin margin (Eichsfeld) and subsurface Buntsandstein in wells in the basin center (area of Erfurt).

In the basin center the Lower Buntsandstein is dominated by lacustrine deposits, the Middle Buntsandstein by fluvial sediments with some eolian sandstones in the lower part and massive fluvial channel and sandflat sandstones in the upper part. At basin margins a strong increase of eolian deposits occurs as well as thickness reductions and erosion in the Middle Buntsandstein, but increasing thickness in the lowermost Lower Buntsandstein and the highest Middle Buntsandstein. Preferential pathways of fluid migration occur in intervals dominated by well-sorted fluvial and eolian sediments. Thereby sandstones with minor amounts of cements show high permeabilities, whereas low permeabilities are commonly related to the almost total filling of the primary porespace by authigenic minerals. Investigations on pore space geometries (abundance and interconnectivity of small and large pores) and grain surfaces allow an advanced understanding of reservoir properties and reactive surfaces in these rocks. In the basin center permeability of Lower Buntsandstein is low, whereas permeabilities in the Middle Buntsandstein formation are strongly enhanced. At the margin of the Thuringian Basin the influence of meteoric water is strongly affecting dissolution and alteration of minerals.

Investigations of fluid pathways and diagenesis form the basis for later modelling of reservoir evolution and fluid flow within the Thuringian Basin.