Facies and aquifer characterization of the German triassic Buntsandstein in central Germany
The Lower Triassic Buntsandstein in the Hesse Trough and in the Thuringian Syncline is a major aquifer in Thuringia and adjacent areas. The depositional system is characterized by a playa-lake to lake setting with some aeolian phases in the Lower Buntsandstein and fluvial, sandflat, and lacustrine to aeolian deposits in the Middle Buntsandstein. The aim of the study is to understand the influence of small-scale facies heterogeneities on fluid-flow pathways. For this purpose Lithofacies Types and Associations in outcrops and cores were studied and statistically characterized, with special emphasis on the thickness, depth and width ratios of architectural elements as well as of inclination and orientation of bounding surfaces. To predict aquifer qualities, the spatial distribution and connectivity in relation to variations in petrophysical properties (porosity and permeability) were studied. With the exception of aeolian deposits, especially Facies Associations with poor petrophysical properties have a wide lateral distribution. This indicates that potential barriers, while thin, show a wider lateral continuity than potential aquifers. Based on this dataset small-scale generic 3D-models of different depositional environments were developed, and fluid flow pathways with regard to the sedimentary structures were modelled. Results show that sedimentary structures influence the fluid flow only in the case of high differences in permeability, e.g. in sand layers intercalated in muds. Minor permeability differences within sedimentary units only slow down fluid flow, but do not cause deviations from the flow direction.