Spectral decomposition of seismic data: Unlocking lithological insights into clastic sediments interpretation within fold-and-thrust settings of the northern Outer Carpathians, Poland

The research problem was to differentiate clastic sediments in the flysch sequences of the fold-and-thrust setting from the Outer Carpathians in 2D seismic data. The complex tectonics of the thrust belts hinder the preservation of relative amplitudes in seismic imaging, complicating interpretation and, in turn, leading to relatively unsuccessful hydrocarbon prospecting in the nappe-building sedimentary formations. To compensate for tectonic complications that affect accurate seismic data interpretation, velocity anisotropy was applied in the processing sequence. With this method, more reliable structural images for the two exploration targets were produced. Geological information from nearby wells validated the results of the anisotropic processing. To further differentiate specific members of the clastic sequence, spectral decomposition methods were applied, as amplitude variations alone provide insufficient constraints to distinguish separate clastic beds. Spectral analysis helped further differentiate the thick flysh sequence (sandstones intercalated with shales) from the thin-layered shale-dominated intervals. Additionally, the concept of frequency analysis for hydrocarbon detection was tested. The effect of the frequency drop beneath the reservoir yielded no valuable results, but the applicability of spectral decomposition for thickness analysis and, consequently, for lithological differentiation was demonstrated. • Anisotropic seismic data processing enables more accurate structural imaging in fold-and-thrust belt settings • Spectral decomposition method can help in thickness differentiation, which supports the lithological interpretation of seismic data • Spectrograms generated along the seismic horizons can be interpreted in terms of varying thicknesses of sandstone beds in a flysch sequence • Frequency shadows as direct hydrocarbon indicators play, if any, a subordinate role in the thrust bed setting