**4. Results**

#### *4.1. Depositional Environments Characterization*

The first step of the interpretation was the identification of reflection discordances and lapouts such as onlaps, downlaps, offlaps, toplaps, and truncations [51]. The direction of the presented profile is SW–NE, which is parallel to the main deposition direction (see Figure 1).

A detailed interpretation of the chronostratigraphic horizons and the Wheeler diagram are presented in Figure 2. The heterogeneous sequence exhibits high diversity and complexity in its chronostratigraphic horizons. The whole analyzed interval, which spans approximately 170 ms (about 300 m), consists of 17 depositional sequences and 18 sequence boundaries (SB).

**Figure 2.** Detailed interpretation of (**a**) chronostratigraphic horizons in structural domain showed against the migrated seismic section (**b**) and Wheeler diagram transformed from seismic data. (**c**) Model presenting a chronostratigraphic nature of the stratigraphic units (Reproduced from Qayyum et al. [52], John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists, 2015, Page: 340).

Within the interpreted seismic interval (Figure 3, red frame), it was possible to indicate two kinds of depositional sequences connected to different types of sequence boundaries: the type I depositional sequence with sequence boundary no 1 (SB1) and the type II depositional sequence with sequence boundary no 2 (SB2).

Depositional sequences (type I) linked to SB1 are characterized by subaerial unconformities. These sequences change their character basinward into correlative conformities. The configuration of chronostratigraphic horizons of type I enables the identification of architectural elements that are typical of falling stage system tract (FSST) deposits that manifest hydrocarbon potential (incised valleys, slope fans and basin floor fans) [53]. These elements are also characterized by anomalous values of seismic attributes (see next subsection). The complete type I sequence consists of four depositional systems: falling stage system tract (FSST: Figure 3, brown—slope fan, yellow—basin floor fan), lowstand system tracks (LST: Figure 3, pink—lowstand wedge), transgressive system tracks (TST: Figure 3, green) and highstand system tract (HST: Figure 3, orange). In the bases of these sequences, it was possible to identify the borders of SB1, which are characterized by terrestrial and marine erosion [47].

**Figure 3.** Detailed interpretation of (**a**) chronostratigraphic horizons in structural domain and (**b**) Wheeler diagram transformed from seismic data.

Sequence boundaries (SB2) that are associated with type II deposition manifest the aggradational deposition of the falling stage system tract (FSST). At the shelf margin, aggradational deposition can be found, but the shelf area is not intensively eroded. Additionally, at the slope and basin floor, there is a lack of fans. The influence of tectonic activity is visible. At the base of the slope, and also basinwards (e.g., slope fans—sf; basin floor fans—bff), there exist areas where there is a lack of older deposits that cannot be linked to a gap in sedimentation, but can be linked to the erosional cut (underwater erosion). Three levels/areas with high erosional relief can be defined and are generated by a cohesive debris floor (mudflow). Such processes might be linked to early forced regression [51]. The type II depositional sequence consists of an early falling stage system tract (FSST, Figure 3, purple), which corresponds to forced regression in type I sequences (or, e.g., forced regression of the shelf margin wedge), TST (Figure 3, green) and HST (Figure 3, orange). The lower boundary of these sequences (SB2) is characterized by an erosional surface in a proximal shelf zone. Basinwards, this boundary becomes correlative conformity.

Higher up in the profile, after each type I depositional sequence, a change in the depositional axis can be observed. The character of the sequence elements is also modified (type II depositional sequence). The direction of the siliciclastic material transport is SW, but the sea level drop is not rapid, and the shelf area is not substantially eroded. These circumstances do not favor fan creation within the slope and basin floor. At the shelf margin, aggradational deposition of falling stage system tracts (FSSTs) takes place.

The facies that dominate the shelf (mainly in the SW part of the cross-section) are transgressive deposits (TST, Figure 3, green), their architecture indicates shelf edge progradation. A substantial part of these deposits are highstand deltas (Figure 3, orange) that are built of clinoforms and deposits corresponding to the shallow sea level.

The heterogeneous complex has retrogradational–progradational characteristics with significant thickness changes in specific depositional elements and a diverse architecture. This is a result of few components that changed over time: sediment flux directions, subsidence and eustatic sea level changes. The primary influence on the deposition must have been related to relative sea level changes linked to subsidence rate fluctuations. The high distribution rate in the sedimentary basin seems to limit fluvial erosion and move the shelf margin forward into the area of higher subsidence (depocenter).

Some of the depositional sequences are characterized by the lack of a link between the shelf and deep basin sediments (see Figure 4a,b, around the second "TST" from the left). This is the result of an erosional cut in the older deposits caused by younger deposits. This suggests that the rebuilding of the sedimentary basin architecture took place, which is most likely linked to tectonic processes. We observe a shift in the basin axis, which is crucial information from a prospecting point of view.

#### *4.2. Seismic Signatures for Unconventional Targets Prospecting*

The interpretation of possible prospecting targets took into account the results of the sweetness attribute and spectral decomposition. These two attributes give similar results, but their images are slightly di fferent. The sweetness attribute (Figure 4c) reveals more details that are associated with the definition of the attribute and instantaneous character. The value of the attribute is assigned to each sample at every time point. The amplitude image of the frequency slice (Figure 4d) is more blurred than for the sweetness attribute. This is a result of the averaging e ffect of the FFT computed within the specific time window. Nonetheless, for the presented low frequency volume (20 Hz), it is possible to indicate the anomalous zones. These zones are a ffiliated with the attenuation of high frequencies, which results in a shift in the dominant frequencies in the lower part of a spectrum. The presented image indicates such regions (Figure 4d).

The zones of the highest hydrocarbon potential are linked to the FSST, especially the slope fans and basin floor fans (Figure 4a, yellow; Figure 4c,d, bright colors—high amplitudes). Elements of the transgressive sequence are also prominent (Figure 4a, brown). The transgressive sequence is built of the fine clastic material and can play the role of a caprock (see Figure 4, "TST"—green). There also exist anomalies suggesting hydrocarbon saturation in a transgressive sequence at the shelf, but they have limited volume (Figure 4, "TST"—green, Figure 4c,d, bright colors—high amplitudes). The high-resolution seismic image enabled us to indicate the potential hydrocarbon saturation in the incised valley (see Figure 4, "IVF").

From a hydrocarbon prospecting point of view, transgressive deposits might be prominent exploration targets (TST, Figure 4a, green, Figure 4b). The main prism of these sediments is deposited in the SW part of the section, near the shelf. The interpretation of the seismic reflection pattern in the zone of maximal thickness indicates development towards the shelf. However, TST deposits manifest a retrogradational character and hence cannot be the most prominent from a hydrocarbon prospecting point of view. The most promising areas for hydrocarbon prospecting are as follows: (1) transgressive sequences that fill incised valleys; (2) onshore barriers in the central part of a profile; (3) transgressive deposits covering slope fans.
