Subsurface aquifer heterogeneities of Lower Triassic clastic sediments in central Germany

Abstract

Abstract A combined study of facies and diagenesis variations was carried out with the aim to understand small-scale heterogeneities in porosity and permeability of sandstones within a 219 m completely cored Middle Buntsandstein succession from central Germany. The well Erfurt 1/12 (EF-FB 1/12) allows studying aquifer quality variations by taking 49 plug samples of fresh rock material for thin section analyses and petrophysical measurements. This potential Buntsandstein aquifer is composed of varying portions of sandstones, mudstones and sandstone-mudstone interlayers, which were deposited on a large terminal fan system and in a large playa-lake in the basin centre. A fluvial channel and a sandflat depositional environment can be distinguished. Both are composed of varying amounts of channel-, sandsheet- and floodplain sediments of mainly massive, cross-bedded, horizontally laminated and ripple cross-bedded sandstones. Best aquifer potential occurs in the horizontally laminated and cross-bedded sandstones within channels of the fluvial environment, and in the massive sandstones of channels in sandflat deposits. Aquifer quality does not decrease with increasing depth or stratigraphic position. Instead, it is controlled by grain size and diagenetic evolution. Four diagenesis types were distinguished: (1) cementation type (CT), (2) leaching type (LT), (3) illite type (IT) and (4) mixed type (MT). Best aquifer quality was found in the leaching type (LT) consisting of larger grain sizes (medium sand), which led to the formation of a large primary pore network. Our data suggest a strong relation of hydraulic parameters not only with different facies types, architectural elements and depositional environments, but particularly with compaction and cementation during burial, and dissolution by meteoric water during subsequent uplift history. Thereby, present horizons with good hydraulic properties relate to this complex interaction of sedimentary facies and diagenetic evolution, but are restricted to only very local areas with no relevance for basin-wide fluid flow in the subsurface.