Evolution of reservoir properties in the Lower Triassic aquifer sandstones of the Thuringian Syncline in Central Germany
The Buntsandstein (Lower Triassic) aquifer sandstones of the Thuringian Syncline are important for water supply, geothermal energy and storage wells. The aim of this study was to analyse the control factors of the diagenetic and petrophysical properties of the Buntsandstein, and particularly the influence of varying depositional parameters, thermal history, subsidence and exhumation. The Thuringian Buntsandstein records fluvial, sandflat, lacustrine and eolian environments. The dominant reason for porosity loss in the sandstones is mechanical compaction. The permeabilities depend on their predominant depositional facies: fine grained lacustrine sandstones in the Lower Buntsandstein have low permeabilities (about 4 mD); conversely, Middle Buntsandstein sandflat, fluvial and eolian sandstones are moderately to highly permeable (27 – 108 mD). Thermal maturity data of Carboniferous, Permian and Triassic sediments provide information about the maximum burial depth of the Buntsandstein rocks. Between 1800 m and 3800 m of overburden must have been eroded from the Thuringian Syncline. The thickness of the Zechstein to Lower Jurassic succession is insufficient to explain the level of thermal maturity. A substantial cover of strata younger than Lower Jurassic must have been present over most of the Thuringian Syncline. The erosion of this cover must have occurred near the end of the Cretaceous period during a widespread inversion event. The pore space and the mineral surfaces exposed to the pore space of the four main facies types were studied using digital image analysis on thin sections and BET measurements. The abundance of large pores influences the permeability of a rock, as well as pore density and porosity. As a result of different types of mineral surfaces exposed to the pore space, the sandstones of the four facies types will exhibit distinctly different reactions upon contact with fluids.