**1. Introduction**

The terms "heterolithic bedding" or "heterogeneous sequence" are used commonly in the description of sedimentary series that are built of interlayered packets (laminae or lenses) of sandstone, mudstone and claystone. The heterogeneous sequence is not only a reservoir rock, but also a source rock that exhibits a high concentration of total organic matter [1–4]. The thicknesses of the separate intervals can be various, from millimeters to decimeters [5,6]. Considering the ratio of sandstone to mudstone, the thickness of the layers and the frequency of their appearance (i.e., the frequency of sandstone layers appearing in mudstone intervals), various types of heterogeneous sequences can be defined. The increasing interest in the heterogeneous sequence in the Carpathian Foredeep is due to the recent discoveries of gas fields [7–9]. Gas flows are present for intervals of low porosity and permeability, which, in well log analysis, are associated with low gas saturation. These intervals are challenging for well log interpretation. The low resolution of well log data and the high clay content associated with these sequences strongly influence the results. The data often sugges<sup>t</sup> increased water content in the heterogeneous sequence, while high dry gas flow is measured in gas tests. For this reason, an interpretation of the heterogeneous sequence based on other geophysical data and comprehensive depositional architecture analysis is needed.

The Carpathian Foredeep is part of a huge Pannonian Basin System (PBS) that is the subject of extensive geological analysis in the aspects of conventional and unconventional reservoirs. The Carpathian Foredeep area is not as detail di fferentiated in terms of stratigraphy and lithology as other parts of the PBS [10–13].

In our work, we present a seismostratigraphic-based approach for indicating the possible gas-bearing intervals within the heterogeneous complex. These sequences make standard well log interpretation di fficult due to the substantial horizontal and vertical variations in terms of lithology and position in a sedimentary basin. Moreover, low-resolution seismic data cannot be used for the reliable interpretation of these sequences within the Miocene sediments [14]. The seismic data presented in this study, however, have a higher than average resolution, therefore enabling a detailed depositional interpretation. In comparison to the previously published works [7,8,15] the presented data were designed to increase the resolution of a seismic image significantly. Only with such detailed seismic data is it possible to proceed with depositional analysis, which, in terms of heterogeneous sequences, yields accurate results.

Chronostratigraphic interpretations of the high-resolution seismic data and Wheeler diagrams (chronostratigraphic horizons identification and flattening) enable the definition of geometrical relations within the analyzed interval. With such an approach, it was possible to define the elements of the depositional architecture, such as slope fans, basin floor fans and barriers, then identify the depositional sequences [16,17]. Such an analysis is crucial for understanding the depositional architecture of the wider area, as well as facial change definitions that might enable us to understand the paleogeography and paleoenvironment of the Miocene succession, which could lead to hydrocarbon prospecting. The number of conventional hydrocarbon reservoirs within the Miocene formations recently started to decease, and fewer conventional prospects have been discovered in the last few years, hence the need to change the approach and begin an interpretation focused on unconventional, heterolithic targets [18]. Such a situation enforces the need for the development of an alternative methodology suited to heterogeneous sequences.

In order to indicate the possible gas-saturated intervals, we incorporated the spectral decomposition technique, which uses the concept of attenuative properties in relation to frequency modulations of gas-bearing sequences [19]. Similarly, we incorporated a sweetness attribute (based on instantaneous trace analysis), which was proven to give reliable results in clastic deposits [20].

The final set of hydrocarbon prospects was verified by the identification of the depositional architecture elements and their position within the sedimentary basin. Simultaneously, attribute analysis was performed in order to cross-check the prospected target areas. The results imply that, for the heterogeneous sequence definition, the chronostratigraphic approach plays a key role and should be routinely applied to the Miocene sediments of the Carpathian Foredeep.

#### **2. Geological Setting and Data Description**

#### *2.1. Geological Setting, the Survey Area*

The study area is located in southern Poland within the central part of the Carpathian Foredeep area (Figure 1).

**Figure 1.** Geological map of the studied area: (**a**) generalized map of the Carpathian system range, after Picha [21] (modified); (**b**) location of the research area against the range of the Carpathian Foredeep in Poland; ranges of geological units according to Por˛ebski and Warchoł [22].

The oldest structural stage in the research area is represented by a series of Neoproterozoic anchimetamorphic rocks. The Ediacaran age of that complex is confirmed by the results of micropaleontological analyses carried out on samples from the boreholes [23,24]. The middle stage is composed of Meso-Paleozoic rocks of a considerable summary thickness up to 2000 m. In the southern part of the analyzed region, directly on the Ediacaran interval, the Lower Paleozoic deposits (Ordovician and Silurian) are situated, but their ranges are not well documented by deep boreholes and insignificant thicknesses (based on well log data and seismic interpretation between 20–100 m) [25,26]. Higher up in the profile lie the carbonate series of Devonian and Carboniferous sediments [27,28]. The Mesozoic interval is represented by carbonate and clastic Triassic sediments, and above them lie the carbonate complexes of the Jurassic and Cretaceous periods [29–31].

The younges<sup>t</sup> structural stage is formed by the Miocene formations (Badenian–Sarmatian), which were initially deposited in the Carpathian Foredeep basin. The sedimentary basin of the Carpathian Foredeep was a fragment of a large foreland graben basin stretching along the whole Carpathian arc. The complex of autochthonous Miocene strata in the research area can be divided into three main units: the Lower Badenian clastic sub-evaporite series, the Upper Badenian evaporite series and the Upper Badenian–Sarmatian clastic series [32,33].

The sub-evaporite sediments, distinguished as the Skawina Formation [34], are mainly represented by a set of claystone and mudstone, with thicknesses up to a few dozen meters. The genesis of the Badenian evaporite series is connected with the Badenian Salinity Crisis (BSC), which probably commenced due to a rather sudden drop in sea level due to global cooling [35–38]. The sulphate rocks (gypsum and anhydrite) are distinguished as the Krzy ˙zanowice Formation [34]. The siliciclastic sediments, classified as the Machów Formation [32–34], are an essential part of the autochthonous Miocene profile in the analyzed region of the Carpathian Foredeep. This formation manifests significant lithofacial diversity. The Machów Formation is assigned to NN6/NN7 calcareous nano plankton zones, although some authors qualify the uppermost part of this formation up to NN9 [39]. A detailed seismostratigraphic analysis of the middle part of the Machów Formation is the main subject of this article.

#### *2.2. Definition of Heterogeneous Sequence*

Lis and Wysocka [40], on the basis of well data analysis, show that the Carpathian Foredeep region is defined by three main types of heterogeneous sequences:


The specific types of bedding are characteristic of the heterogeneous sequence, predominantly flaser, wavy and lenticular laminations that are created by the interchanging deposition form traction and suspension. During the traction-based sedimentation, sand and silt fractions dominate with small-scale cross-bedding or planar bedding. The suspension-based sedimentation favors the deposition of mudstone and claystone without lamination, as well as flaser, wavy or lenticular laminations.

The heterogeneous sequence is commonly found in the Miocene sediment profile in the central part of the Carpathian Foredeep. The sequences might be present in many depositional environments and at various depths, even though they are mostly associated with shallow and tidal regions [41–45]. In the study area, the heterogeneous sequences are typical for the studied part of the Miocene complex.

In the analyzed interval, we can define the existence of the heterolithic facies with all depositional zones, from shallow to deep water environments. The interpreted interval reveals diverse and dynamic facial changes observed at the chronostratigraphic horizons and defined in the Wheeler diagram. The geometrical analysis of these chronostratigraphic horizons enables uniformly defined diagnostic elements of the depositional architecture, such as shelf, shelf margin, slope and basin floor, associated to the sedimentation environments [46,47].
