**3. Methods**

## *3.1. Depositional Analysis*

The method is based on seismic sequence stratigraphy and depositional sequence characterization. A depositional sequence is understood as a lithostratigraphic unit that was deposited during a specific depositional episode in a sedimentary basin. Each seismic horizon is associated with a certain position within a depositional sequence. The chronostratigraphic horizons can be linked with a specific element of the paleoenvironment within the sedimentary basin.

The seismic volume was subject to depositional sequence interpretation. It should be noted that the sequence stratigraphy analysis based on seismic volume is less detailed than the interpretation based on well and outcrop data.

The first step of the analysis was to define a set of seismic sections to find those in directions parallel to the transportation of sediments to the basin. For the analysis, several sections were chosen. The area of analysis was limited to an exact time range by two seismic horizons. Afterwards, the apparent dip in seismic reflections was computed, and the resulting seismic volume enabled us to proceed to the next step, which was a chronostratigraphic horizon extraction. According to the information about the apparent dip and its direction, it was possible to apply a semi-automatic algorithm that tracks the seismic horizon in a given time range. The analysis was performed several times to adjust the parameters and take into account the resolution of the seismic data.

The defined chronostratigraphic horizons were transformed into the Wheeler domain by flattening [16,17] in order to analyze their relative temporal position. Wheeler diagrams were constructed for several seismic sections for the identification and visualization of the directions of deposition, hiatuses (a lack of deposition or erosion periods) and lateral continuities of depositional sequences.

With such an approach, it was possible to identify local changes in depositional directions that affected the exact shape and geometry of the chronostratigraphic horizons. The results are presented for the chosen cross-section.

## *3.2. Seismic Analysis*

Seismic attributes were incorporated for possible gas-bearing zone interpretation. The subject of the analysis is a clastic sequence; hence, attributes that are suited to such environments give the best results. The sweetness attribute is calculated by dividing the instantaneous amplitude by the square root of the instantaneous frequency [48] and is sensitive to changes in amplitude and changes in frequency, both of which are considered direct hydrocarbon indicators (DHI) in clastic sequences [49]. The sweetness attribute might highlight thick, clean reservoirs and is sensitive to hydrocarbon content due to the incorporation of an instantaneous frequency [50]. This is a robust attribute, and its calculation is fast and efficient.

The second tool incorporated was spectral decomposition based on Fast Fourier Transform (FFT). The transform, based on the set of sine and cosine functions, decomposes the seismic trace to a time–frequency domain, in which specific frequency components hold the fraction of the amplitude of the original seismic trace. In this way, the filtering of a seismic signal is performed. The decomposition is performed in the time window; its length can be adjusted to the seismic resolution, but should not be too small in order for the decomposition to be stable. The interpretational value of the frequency volumes is that the reasoning can be limited to lower frequencies that tend to hold information about the hydrocarbon saturation.
