Reservoir interpretation of Rotliegend seismic reflections in the central part of the Mid-Polish Trough.

The search for gas reservoirs in the deep central parts of the Mid-Polish Trough remains an open question because of fragmentary knowledge of the area caused by the small amount of available borehole data and poor seismic imaging of sub-Zechstein formations on vintage seismic data. 2D seismic data acquired in 2011 reveals seismic events related to strongly indurated sandstone intercalation found within fine-grained playa-lake deposits in the axial part of the Mid-Polish Trough. Seismic full-wave modelling has been carried out along recorded seismic 2D line in order to analyse seismic signatures from potential reservoir layers of variable porosity and gas saturation. This has allowed to evaluate the possibility of use of seismic methods to detect potential reservoir sandstones located in the deep parts of the basin below a thick Zechstein-Mesozoic cover, in which the actual spatial distribution of Rotliegend formations is only hypothetical. The analyses were focused on impact of porosity changes and variations in content of minerals with high bulk modulus on the P-wave velocity and seismic response. 2D seismic modelling have revealed that under specific geological conditions, playa formation may contain a sandstone layer – with a porosity of approx. 5% and acoustic impedance values not distinctly different from those observed for the surrounding rocks. As the result sandstone reflections will be undetectable through seismic methods. Promising results have been obtained by comparison of synthetic sections against a seismic profile recorded in the other deep part of the Mid-Polish Trough. The similarity of seismic signatures on synthetic section calculated for layer with porosity of 9% and observed on actual section implies that these events are related to a porous sandstone layer. This interpretation is confirmed by borehole data showing a thin layer of aeolian sandstone with 11% porosity. [ABSTRACT FROM AUTHOR]