Relationship of hydraulic parameters with diagenetic evolution and depositional preconditions (Buntsandstein, Central Germany)

The Lower Triassic Buntsandstein is an economically important clastic underground reservoir and aquifer unit in the Central European Basin (e.g. for hydrocarbon reservoirs, gas storage, geothermal energy use, drinking water supply). Its quality mainly controls its prospectivity, storage capacity and exploitability. Thus, predictions for the realization of economic intentions depend most notably on a substantial understanding of the parameters that control reservoir quality, such as facies and diagenetic alterations resulting from fluid-rock interaction. Therefore, research of even small-scale relationships between hydraulic heterogeneities and rock properties is necessary.

The study area is the Thuringian Syncline, which is a small sub-basin of the North German Basin located at its southern margin. One of its major aquifers is built from siliciclastic sediments of the Buntsandstein, which are characterized by rapid changes of depositional environments from channel to sandflat to lacustrine depositions resulting in large heterogeneities at a relatively small scale (few to some hundred meters). Furthermore, burial history and subsequent basin inversion and uplift led to only minor depths of 700 to 1000 m in the center of the syncline and an exposure at the surface at syncline margins, which allows for the exploration of the recent impact of meteoric water infiltration vs. former burial evolution on aquifer quality. We combined a petrographic study focusing on mineral composition and diagenesis with a study of depositional facies and linked the results with petrophysical data like permeability and porosity. The corresponding dataset consists of measurements on more than 300 plug samples from 12 wells and additionally more than 400 thin sections.

All in all, the Buntsandstein exhibits a very complex relationship of hydraulic parameters with diagenetic evolution in relation to depositional preconditions. For example, high amounts of channel deposits in the sandstones result in better aquifer qualities. The same holds true for increasing grain sizes. Finally, there is a major influence of telodiagenetic processes and meteoric water infiltrations. Thus, major pathways for fluid flow are not solely controlled by sedimentary facies, but also by present-day cement dissolution and mineral alteration, especially in the vadose zone.