The influence of seismic velocity distribution on the depth imaging of the sub-Zechstein horizons in areas affected by salt tectonics: a case study of NW Poland

Abstract

Lateral changes in the thickness of strata and petrophysical parameters within the Zechstein succession (salt pillows and domes) can cause many problems in seismic exploration of the aeolian Rotliegend formations, which are prospective for hydrocarbons. An assessment of the influence of halokinesis on the seismic imaging of the sub-Zechstein strata in NW Poland (Obrzycko–Szamotuły area, to the SW of the Mid-Polish Swell) utilised time-to-depth conversion with different, seismic-geological models. Various, seismic velocities were used in models for the Zechstein and the Mesozoic successions, namely velocities, dependent on the thickness of particular rock successions, on their depths, and velocities, determined from seismic inversion. The results show opposite reflection patterns for the seismic section imaged in the time and depth domains. The synclinal arrangement of the strata boundaries in the depth model is represented by convex-upwards reflection events on the seismic section. The pull-up of reflection events, associated with the sub-Zechstein strata, observed on the seismic sections, is mainly a result of both the greater thickness of the Zechstein salt within the salt structures (pillows, diapirs) and the increase in velocity contrast between the salt body and the Mesozoic strata.