Diverse Anhydrous Pyrolysis Analyses for Assessment of the Hydrocarbon Generation Potential of the Dukla, Silesian, and Skole Units in the Polish Outer Carpathians

Abstract

The study presents the results of investigations into various types of anhydrous pyrolysis aimed at determining the kinetic parameters of hydrocarbon generation processes from source rocks. Surface outcrop samples from the Silesian, Dukla, and Skole units, characterized by a low level of thermal maturity, were used as experimental material. The samples predominantly represented the Menilite Beds from the aforementioned three units, but also included Istebna, Lgota, Verovice, and Spas beds, which exhibit significantly lower parameters that describe generation properties. The anhydrous pyrolysis experiments provided information on the rate of organic matter decomposition (TG/DSC), the degree of conversion (Rock-Eval), the quality of the obtained products (Py/GC), and the isotopic composition of the gaseous products (Py/GC/IRMS). Chromatographic analyses confirmed the oil-prone nature of kerogen contained in the Menilites from the Dukla Unit (Tylawa area), the Silesian Unit (Iwonicz fold), and the Skole Unit, revealing an equal share of all hydrocarbon fractions: C₁–C₉, C₁₀–C₁₅, and C₁₅₊. Through the integration of pyrolytic studies conducted on potential source rocks in the polish Outer Carpathians, a new type of information was obtained regarding the rate of organic matter decomposition, as well as the fractional and isotopic composition of the pyrolysis products. The set of obtained results was used to estimate the activation energy and characterize the potential source levels. The innovative aspect of this approach involved the isotopic characterization of gaseous products generated during thermal degradation of the source rocks. These data were subsequently used to establish genetic correlations with natural gases accumulated in hydrocarbon reservoirs of the Carpathian region. It has been demonstrated that pyrolysis using PY-GC-IRMS can yield results comparable to those obtained through generation in natural geological conditions.

1. Introduction

Anhydrous pyrolysis studies, including Rock-Eval analysis, TG/DSC and Py-GC/FID, employ complementary techniques to monitor and characterize the products generated during pyrolysis. Each method provides distinct yet synergistic insights into the thermal behavior and hydrocarbon potential of source rocks. These methods make it possible to provide more detailed information on the quality of source rocks within petroleum systems [1,2,3,4,5]. Such compilation studies were already initiated by the authors of this paper in the analysis of the decomposition of organic matter contained in the source rocks of the Menilite beds, which are regarded as the main source of hydrocarbon generation in the Outer Carpathians [6,7,8,9,10]. However, the aforementioned studies were limited in scope. This article presents a comprehensive range of research, notably expanding upon previous studies by incorporating analyses of the isotopic composition of pyrolysis products for the first time. Understanding the mechanism of organic matter decomposition is very helpful in simulating generative processes in the Carpathian basin [11].

Rock-Eval analysis is a rapid screening of organic matter type and maturity. It quantifies parameters, which are essential for evaluating the generative potential of source rocks and allows the selection of samples for detailed testing [3]. Thermogravimetric analysis (TG/DTG) enables precise monitoring of mass loss and thermal events during heating, revealing the decomposition stages of organic and mineral components [12]. Py-GC/FID (Pyrolysis–Gas Chromatography with Flame Ionization Detection) method identifies specific hydrocarbon compounds, enabling detailed characterization of the kerogen structure and the types of hydrocarbons that can be generated [10]. Py-GC/IRMS method enables online measurement of the δ¹³C isotopic composition in the gaseous products obtained from pyrolysis of the rock sample. The results of this work not only allow for more precise information on the generation character of the source rocks but also provide information necessary for correlating the source rocks with the discovered gas accumulations [13,14,15].

The conducted investigations, carried out within the INNKARP project, included selected samples from all three tectonic units where potential source rocks occur: the Menilite beds, as well as the Istebna, Verovice, Lgota, and Spas beds. The Menilite beds were most abundantly represented in the study as they are characterized by the highest generative potential, although differing depending on facies type, as has been repeatedly emphasized in [11,16,17,18,19].

2. Research Material

The study encompassed rock samples collected from outcrops within three tectonic units: the Dukla, Silesian, and Skole units. For the purposes of interpretation and evaluation of generative conditions, based on the results of the thermal decomposition studies of organic matter, a total of 19 samples were used in this paper (from the Dukla Unit: four Menilite Beds and one Transition Beds samples; from the Silesian Unit: five Menilite Beds samples and one sample each from Istebna, Lgota, Verovice beds and one sample of tectonic melange; from the Skole Unit: four Menilite Beds and one Spas Beds samples). The locations of the outcrops are shown on the map (Figure 1). The choosing of localisation and lithostratigraphic formation for sampling was connected with the criterium of hydrocarbon residual potential presence and low thermal maturity. Surface samples of the Menilite Beds from the Dukla Unit are characterized by higher levels of thermal alteration. Nevertheless, they are still at such a thermal transformation stage that they show considerable generative potential [20,21,22,23,24].

The burial depth of the Menilite Beds varies significantly across the Carpathian units, ranging from surface exposures to over 4000 m in the Silesian and Skole units. The present study was conducted exclusively on surface samples collected from the three tectonic units where the Menilite shales are exposed. T_max values obtained from Rock-Eval pyrolysis indicate low thermal maturity levels in these samples. Although T_max-derived temperatures correspond to burial depths of approximately 3–9 km, the current burial depth of the Menilite Formation in these areas is estimated at 2–3 km. This discrepancy reflects the complex tectonic evolution and differential uplift across the Carpathians. Importantly, the consistently low maturity observed in surface samples across various units supports the validity of using outcrop material for geochemical characterization. Despite regional differences in burial history, the Menilite shales remain largely unaltered, making them suitable for comparative studies of source rock properties and early diagenetic processes [22,23].

Thermal degradation products indicate that the Lower Cretaceous source rocks—specifically the Spas and Verovice shales—exhibit low thermal maturity in surface outcrops. Owing to their limited transformation, detailed characterization of hydrocarbon generation, particularly molecular and isotopic composition, provides a robust basis for genetic correlation with gases accumulated in the Carpathian reservoirs. This approach enables the identification of potential links between organic matter type, diagenetic conditions, and the presence of thermogenic gases in deeper tectonic structures.

Table 1. Results of Rock-Eval, Py-GC/FID, TG, and PY-GC/IRMS analyses.

Formation					Sample ID	Rock-Eval
						Tmax	S1	S2	S3	PI	PC	RC	TOC	HI	OI	MINC
Dukla Unit
Menilite Beds (Upper Eocene–Oligocene)					1D	430	0.30	45.07	1.73	0.01	3.91	3.25	7.16	629	24	0.27
Menilite Beds (Upper Eocene–Oligocene)					2D	437	0.52	37.87	0.58	0.01	3.26	3.74	7.00	541	8	0.09
Menilite Beds (Upper Eocene–Oligocene)					3D	427	3.49	82.75	0.46	0.04	7.22	6.32	13.54	611	3	0.36
Menilite Beds (Upper Eocene–Oligocene)					4D	433	0.27	45.76	0.59	0.01	3.86	2.76	6.62	691	9	0.11
Transition Beds (Oligocene)					5D	434	0.39	24.63	0.26	0.02	2.12	2.65	4.77	516	5	0.14
Silesian Unit
Menilite Beds (Oligocene)					1Sl	429	0.39	8.04	0.13	0.05	0.73	2.18	2.91	276	4	0.12
Tectonic Melange (Oligocene)					2Sl	431	0.15	9.62	0.08	0.02	0.83	1.83	2.66	362	3	0.06
Menilite Beds (Oligocene)					3Sl	425	1.37	38.32	0.21	0.03	3.34	3.26	6.6	581	3	0.26
Menilite Beds (Oligocene)					4Sl	410	2.70	52.69	1.93	0.05	4.71	4.1	8.81	598	22	0.07
Menilite Beds (Oligocene)					5Sl	421	0.14	10.37	1.6	0.01	0.97	2.52	3.49	297	46	0.09
Menilite Beds (Oligocene)					6Sl	422	5.14	144.04	1.14	0.03	12.5	5.59	18.09	796	6	0.11
Istebna Beds (Senonian–Paleocene)					7Sl	427	0.02	0.29	0.23	0.06	0.05	0.49	0.54	54	43	0.07
Lgota Beds (Albian)					8Sl	430	0.03	0.77	0.25	0.04	0.08	0.81	0.89	87	28	0.60
Verovice Beds (Barrenian–Albian)					9Sl	422	0.10	11.06	0.3	0.01	0.96	2.38	3.34	331	9	0.04
Skole Unit
Menilite Beds (Upper Eocene–Oligocene)					1Sk	417	0.92	37.75	3.78	0.02	3.44	4.07	7.51	503	50	0.24
Menilite Beds (Upper Eocene–Oligocene)					2Sk	410	0.60	33.54	2.81	0.02	3.07	4.73	7.80	430	36	0.33
Menilite Beds (Upper Eocene–Oligocene)					3Sk	418	0.63	50.78	3.86	0.01	4.48	4.5	8.98	565	43	0.20
Menilite Beds (Upper Eocene–Oligocene)					4Sk	400	4.20	106.58	2.03	0.04	9.34	4.63	13.97	763	15	0.08
Spas Beds (Barrenian–Albian)					5Sk	425	0.05	6.03	0.23	0.01	0.58	1.2	1.78	339	13	1.30
Sample ID	Py-GC/FID				Py-GC/IRMS			TG/DTG
								40–300 °C			300–650 °C			650–1050 °C
	Yield indicator	C1–C9	C10–C15	C15+	δ13C-C1	δ13C-C2	δ13C-C3	Weight loss	Onset–Tmax–Endset [°C]		Weight loss	Onset–Tmax–Endset [°C]		Weight loss	Onset–Tmax–Endset [°C]
Dukla Unit
1D	31.16	68.60	19.99	11.41	−39.57	−34.62	−34.15	2.00	40-104-220		9.88	338-453-610		5.00	650-709-745
2D	24.57	69.43	17.90	12.67	−37.93	−32.55	−31.57	0.69	40-104-210		8.73	346-462-609		5.23	650-685-729
3D	20.12	31.67	21.04	47.30	−39.47	−31.35	−30.42	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
4D	22.99	46.00	23.79	30.20	−41.54	−34.25	−34.45	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
5D	8.38	72.85	18.48	8.66	−38.50	−32.05	−31.95	0.71	40-93-200		7.52	350-462-604		4.72	650-677-730
Silesian Unit
1Sl	3.27	36.72	33.67	29.60	−37.36	−32.07	−31.44	2.95	40-84-180		7.72	300-491-558		3.82	650-734-790
2Sl	8.92	41.70	46.76	11.53	−37.82	−32.55	−33.63	1.27	60-104-210		7.17	366-469-594		4.01	650-751-815
3Sl	13.31	32.44	27.91	39.65	−38.76	−33.71	−32.92	1.91	40-99-180		10.01	360-448; 515-650		6.70	650-713-750
4Sl	9.17	50.07	28.39	21.54	−42.84	−33.62	−33.00	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
5Sl	8.63	61.32	27.78	13.90	−37.20	−32.17	−32.08	2.29	40-116-190		7.15	350-437; 502-600		3.89	650-674-720
6Sl	45.58	21.92	20.78	57.31	−43.18	−35.64	−35.71	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
7Sl	0.33	93.20	6.36	0.43	n.d.	n.d.	n.d.	2.09	60-81-260		6.37	360-509-608		2.31	n.o.
8Sl	0.48	76.26	22.38	1.36	n.d.	n.d.	n.d.	2.56	60-89-190		5.26	380-499-603		3.37	670-728-768
9Sl	2.09	64.31	25.21	10.47	−40.42	−33.10	−33.76	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
Skole Unit
1Sk	35.71	26.49	54.76	18.78	−35.47	−28.85	−28.78	3.65	40-112-215		10.03	360-442; 502-560		7.64	670-729-775
2Sk	14.58	34.37	34.49	31.13	−38.19	−32.25	−31.04	4.07	40-111-200		10.01	330-437; 502-600		8.09	650-725-770
3Sk	43.92	52.88	26.09	21.03	−40.00	−34.48	−34.37	5.06	60-123-220		9.87	325-443-590		5.68	650-697-780
4Sk	55.09	15.88	18.76	65.36	−42.40	−34.70	−35.52	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
5Sk	1.49	60.98	26.27	12.76	−43.75	−34.15	−31.98	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.

T_max—temperature at which the maximum amount of hydrocarbons is generated during cracking of kerogen [°C]; S₁—content of free hydrocarbons [mg HC/g rock]; S₂—amount of hydrocarbons released during cracking of kerogen [mg HC/g rock]; S₃—amount CO₂ released during cracking of kerogen [mg CO₂/g rock]; PC—pyrolytic carbon content [wt.%.]; RC—residual carbon content [wt.%]; TOC—total organic carbon content [wt.%]; HI—hydrogen index [mg HC/g TOC]; OI—oxygen index [mg CO₂/g TOC]; MINC—mineral carbon content [wt. %]. Yield indicator—calculated as peak area divided by weight of sample, then multiplied by 10⁻⁶. C₁–C₉, C₁₀–C₁₅, C₁₅₊—calculated as proportions of hydrocarbon fractions as percentage of a whole [%]. δ¹³C-C₁—carbon isotopic composition of methane [‰ vs. PDB]. δ¹³C-C₂—carbon isotopic composition of ethane [‰ vs. PDB]. δ¹³C-C₃—carbon isotopic composition of propane [‰ vs. PDB]. Weight loss—loss of sample weight during TG analysis [%]. Onset–Tmax–Endset—onset temperature indicates the point at which mass loss begins; Tmax represents the temperature at which the maximum rate of mass loss occurs; endset temperature marks the completion of the reaction [°C]. n.d.—not determined.

Table 1. Results of Rock-Eval, Py-GC/FID, TG, and PY-GC/IRMS analyses.

Formation					Sample ID	Rock-Eval
						Tmax	S1	S2	S3	PI	PC	RC	TOC	HI	OI	MINC
Dukla Unit
Menilite Beds (Upper Eocene–Oligocene)					1D	430	0.30	45.07	1.73	0.01	3.91	3.25	7.16	629	24	0.27
Menilite Beds (Upper Eocene–Oligocene)					2D	437	0.52	37.87	0.58	0.01	3.26	3.74	7.00	541	8	0.09
Menilite Beds (Upper Eocene–Oligocene)					3D	427	3.49	82.75	0.46	0.04	7.22	6.32	13.54	611	3	0.36
Menilite Beds (Upper Eocene–Oligocene)					4D	433	0.27	45.76	0.59	0.01	3.86	2.76	6.62	691	9	0.11
Transition Beds (Oligocene)					5D	434	0.39	24.63	0.26	0.02	2.12	2.65	4.77	516	5	0.14
Silesian Unit
Menilite Beds (Oligocene)					1Sl	429	0.39	8.04	0.13	0.05	0.73	2.18	2.91	276	4	0.12
Tectonic Melange (Oligocene)					2Sl	431	0.15	9.62	0.08	0.02	0.83	1.83	2.66	362	3	0.06
Menilite Beds (Oligocene)					3Sl	425	1.37	38.32	0.21	0.03	3.34	3.26	6.6	581	3	0.26
Menilite Beds (Oligocene)					4Sl	410	2.70	52.69	1.93	0.05	4.71	4.1	8.81	598	22	0.07
Menilite Beds (Oligocene)					5Sl	421	0.14	10.37	1.6	0.01	0.97	2.52	3.49	297	46	0.09
Menilite Beds (Oligocene)					6Sl	422	5.14	144.04	1.14	0.03	12.5	5.59	18.09	796	6	0.11
Istebna Beds (Senonian–Paleocene)					7Sl	427	0.02	0.29	0.23	0.06	0.05	0.49	0.54	54	43	0.07
Lgota Beds (Albian)					8Sl	430	0.03	0.77	0.25	0.04	0.08	0.81	0.89	87	28	0.60
Verovice Beds (Barrenian–Albian)					9Sl	422	0.10	11.06	0.3	0.01	0.96	2.38	3.34	331	9	0.04
Skole Unit
Menilite Beds (Upper Eocene–Oligocene)					1Sk	417	0.92	37.75	3.78	0.02	3.44	4.07	7.51	503	50	0.24
Menilite Beds (Upper Eocene–Oligocene)					2Sk	410	0.60	33.54	2.81	0.02	3.07	4.73	7.80	430	36	0.33
Menilite Beds (Upper Eocene–Oligocene)					3Sk	418	0.63	50.78	3.86	0.01	4.48	4.5	8.98	565	43	0.20
Menilite Beds (Upper Eocene–Oligocene)					4Sk	400	4.20	106.58	2.03	0.04	9.34	4.63	13.97	763	15	0.08
Spas Beds (Barrenian–Albian)					5Sk	425	0.05	6.03	0.23	0.01	0.58	1.2	1.78	339	13	1.30
Sample ID	Py-GC/FID				Py-GC/IRMS			TG/DTG
								40–300 °C			300–650 °C			650–1050 °C
	Yield indicator	C1–C9	C10–C15	C15+	δ13C-C1	δ13C-C2	δ13C-C3	Weight loss	Onset–Tmax–Endset [°C]		Weight loss	Onset–Tmax–Endset [°C]		Weight loss	Onset–Tmax–Endset [°C]
Dukla Unit
1D	31.16	68.60	19.99	11.41	−39.57	−34.62	−34.15	2.00	40-104-220		9.88	338-453-610		5.00	650-709-745
2D	24.57	69.43	17.90	12.67	−37.93	−32.55	−31.57	0.69	40-104-210		8.73	346-462-609		5.23	650-685-729
3D	20.12	31.67	21.04	47.30	−39.47	−31.35	−30.42	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
4D	22.99	46.00	23.79	30.20	−41.54	−34.25	−34.45	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
5D	8.38	72.85	18.48	8.66	−38.50	−32.05	−31.95	0.71	40-93-200		7.52	350-462-604		4.72	650-677-730
Silesian Unit
1Sl	3.27	36.72	33.67	29.60	−37.36	−32.07	−31.44	2.95	40-84-180		7.72	300-491-558		3.82	650-734-790
2Sl	8.92	41.70	46.76	11.53	−37.82	−32.55	−33.63	1.27	60-104-210		7.17	366-469-594		4.01	650-751-815
3Sl	13.31	32.44	27.91	39.65	−38.76	−33.71	−32.92	1.91	40-99-180		10.01	360-448; 515-650		6.70	650-713-750
4Sl	9.17	50.07	28.39	21.54	−42.84	−33.62	−33.00	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
5Sl	8.63	61.32	27.78	13.90	−37.20	−32.17	−32.08	2.29	40-116-190		7.15	350-437; 502-600		3.89	650-674-720
6Sl	45.58	21.92	20.78	57.31	−43.18	−35.64	−35.71	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
7Sl	0.33	93.20	6.36	0.43	n.d.	n.d.	n.d.	2.09	60-81-260		6.37	360-509-608		2.31	n.o.
8Sl	0.48	76.26	22.38	1.36	n.d.	n.d.	n.d.	2.56	60-89-190		5.26	380-499-603		3.37	670-728-768
9Sl	2.09	64.31	25.21	10.47	−40.42	−33.10	−33.76	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
Skole Unit
1Sk	35.71	26.49	54.76	18.78	−35.47	−28.85	−28.78	3.65	40-112-215		10.03	360-442; 502-560		7.64	670-729-775
2Sk	14.58	34.37	34.49	31.13	−38.19	−32.25	−31.04	4.07	40-111-200		10.01	330-437; 502-600		8.09	650-725-770
3Sk	43.92	52.88	26.09	21.03	−40.00	−34.48	−34.37	5.06	60-123-220		9.87	325-443-590		5.68	650-697-780
4Sk	55.09	15.88	18.76	65.36	−42.40	−34.70	−35.52	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.
5Sk	1.49	60.98	26.27	12.76	−43.75	−34.15	−31.98	n.d.	n.d.		n.d.	n.d.		n.d.	n.d.

T_max—temperature at which the maximum amount of hydrocarbons is generated during cracking of kerogen [°C]; S₁—content of free hydrocarbons [mg HC/g rock]; S₂—amount of hydrocarbons released during cracking of kerogen [mg HC/g rock]; S₃—amount CO₂ released during cracking of kerogen [mg CO₂/g rock]; PC—pyrolytic carbon content [wt.%.]; RC—residual carbon content [wt.%]; TOC—total organic carbon content [wt.%]; HI—hydrogen index [mg HC/g TOC]; OI—oxygen index [mg CO₂/g TOC]; MINC—mineral carbon content [wt. %]. Yield indicator—calculated as peak area divided by weight of sample, then multiplied by 10⁻⁶. C₁–C₉, C₁₀–C₁₅, C₁₅₊—calculated as proportions of hydrocarbon fractions as percentage of a whole [%]. δ¹³C-C₁—carbon isotopic composition of methane [‰ vs. PDB]. δ¹³C-C₂—carbon isotopic composition of ethane [‰ vs. PDB]. δ¹³C-C₃—carbon isotopic composition of propane [‰ vs. PDB]. Weight loss—loss of sample weight during TG analysis [%]. Onset–Tmax–Endset—onset temperature indicates the point at which mass loss begins; Tmax represents the temperature at which the maximum rate of mass loss occurs; endset temperature marks the completion of the reaction [°C]. n.d.—not determined.

Geological Setting

The eastern sector of the Polish Outer Carpathians represents an accretionary wedge composed of multiple nappes. Each nappe records sedimentary successions of separate basins that were later deformed during the Carpathian orogeny [25,26]. In the research area, the tectonic boundary between the Magura and Silesian nappes is clearly developed: the Siary Zone of the Magura Nappe is thrust over both the Dukla and Silesian nappes [27,28], while the southern part of the Silesian Nappe forms the Gorlice Fold [4]. The stratigraphic record ranges from the Cretaceous to the Miocene and is dominated by deep-marine flysch deposits [29].

The Dukla Nappe crops out in the eastern Outer Carpathians, particularly between Dukla and Wetlina along the southern and eastern Polish border. Westward, it gradually passes beneath the Magura Nappe and becomes exposed only in tectonic windows. Noticeable lithological contrasts between rocks in these windows and those typical of the Dukla Nappe have led various researchers to distinguish different lithostratigraphic units (e.g., Sub-Grybów and Grybów beds, Grybów marls/shales, Sub-Cergowa marls, and Cergowa beds). These were sometimes interpreted as belonging to separate tectonic units and therefore were given different local names [30,31]. The deposits forming the Dukla Nappe date from the Early Cretaceous to the Oligocene [31,32].

The Silesian Unit is the most petroleum-rich nappe of the Polish Carpathians. Its central structure, the axial synclinorium, contains up to 3000 m of the Oligocene Menilite–Krosno deposits [18]. The Menilite shales act as the primary source rock, being composed mainly of type II kerogen and, in some places, the highly oil-prone type II-S kerogen [25,33]. Their thickness varies between 50 and 250 m, depending on the occurrence of sandstone complexes such as the Magdalena or Cergowa sandstones. Cretaceous units (Istebna, Lgota, Verovice, and Spas beds) may serve as secondary, gas-prone source rocks due to the prevalence of type III kerogen [24]. The Silesian Unit hosts the largest hydrocarbon accumulations in the region, including the Gorlice, Osobnica, Bóbrka–Rogi, Strachocina, and Grabownica fields [19].

The Skole Nappe occupies the outermost position of the Polish Outer Carpathians. It is composed of several elongated thrust sheets and folds. Its youngest succession corresponds to the Oligocene Menilite–Krosno series, which includes the Menilite Formation (dominated by black to dark brown non-calcareous shales) and the overlying Oligocene–Miocene Krosno Formation (consisting mainly of yellow-weathering calcareous sandstones and mudstones) [34]. The Menilite shales are rich in terrigenous type III kerogen, making them predominantly gas-prone and of low oil potential [35].

3. Analytical Methods

Sample preparation and analyses (such as Rock-Eval, Py-GC/FID, Py-GC/IRMS) were performed in the Oil and Gas Geochemistry Laboratory (Oil and Gas Institute—National Research Institute). Thermogravimetric (TG/DTG) analyses were performed in the Laboratory of Geochemical Engineering (Silesian University of Technology).

3.1. Rock-Eval

As a first step, Rock-Eval analysis was performed using a RE6 Turbo model apparatus (Vinci Technologies, Nanterre, France) equipped with an FID (for hydrocarbon analysis) and two IR detectors (for CO and CO₂ analysis), applying the Bulk Rock method and the basic cycle [3,10].

3.2. TG/DTG

The TG/DTG analysis was carried out using a STA 449 F3 Jupiter (NETZSCH, Selb, Germany) analyzer. The measurement was conducted while heating powdered samples in the temperature range of 40–1030 °C, with a temperature increment of 10 °C/min. A dynamic flow of inert gas (nitrogen) at a rate of 50 mL/min was applied in the temperature range of 40–650 °C, whereas above 650 °C, a flow of synthetic air was used. The heating program was designed to approximate the regime of the Rock-Eval pyrolysis analysis: heating from 40 °C to 300 °C (10 °C/min, working gas N₂); isothermal at 300 °C (20 min, working gas N₂); heating from 300 °C to 650 °C (10 °C/min, working gas N₂); isothermal at 650 °C (20 min, working gas N₂); and finally, heating to 1030 °C (10 °C/min, working gas O₂/N₂).

Ground and sieved rock samples of approximately 20 mg were placed in ceramic (Al₂O₃) crucibles [12].

3.3. Py-GC/FID

The studies using pyrolytic gas chromatography (Py-GC) were performed with an analytical system consisting of a multi-shot pyrolyzer EGA/PY-3030D (Frontier Laboratories, Koriyama, Japan) coupled with a GC-2010 Plus gas chromatograph (Shimadzu, Kyoto, Japan) equipped with a flame ionization detector (FID). The system included a cryogenic trap cooled by liquid nitrogen via a MicroJet Cryo-Trap MJT-1030Ex, installed before the chromatographic column. The sample, prepared to a granulation analogous to that used for Rock-Eval analysis (below 0.2 mm), was weighed directly into an analytical sample cup, then inserted into the autosampler and placed in the pyrolyzer. Approximately 10 mg of sample was introduced into the pyrolysis furnace, where it was pyrolyzed in a helium atmosphere at a programmed temperature and duration. The thermal decomposition products of the analyzed sample were collected in the cryogenic trap connected to the GC column, then directed to the detector after pyrolysis and identified using the FID [1,7,10,36]. A yield indicator (calculated from Py-GC/FID analysis based on peak area per 1 mg of sample weight × 10⁻⁶) was used to determine the product-generating capacity of samples.

3.4. Py-GC/IRMS

For the determination of the carbon isotopic composition of pyrolysis products, a Delta V Advantage isotope ratio mass spectrometer (Thermo Scientific, Bremen, Germany) was used, coupled with a Trace GC Ultra gas chromatograph and a Pyroprobe 6150 pyrolyzer (CDS Analytical, Oxford, PA, USA). Pyrolysis of samples, placed in quartz tubes, was conducted at a temperature of 500 °C and maintained for 10 s. The heating rate was 20 °C per millisecond. The gaseous pyrolysis products were separated on the Trace GC Ultra chromatograph using a HP-PLOT/Q capillary column that was 30 m long, with a 0.32 mm diameter. The temperature program started at 30 °C (held for 6 min), followed by an increase to 210 °C (held for 5 min). The injector temperature was set at 150 °C. The successive separated gas components exiting the column were combusted in the reactors of the GC IsoLink device (1000 °C) and subsequently introduced into the IRMS (Delta V Advantage) [13].

4. Results and Discussion

To evaluate the nature of thermal decomposition simulating generative processes of organic matter (specifically in terms of reaction rate, temperature range, and the quantitative and qualitative composition of the resulting products), this study utilized data from 19 samples of source rocks. Table 1 presents results obtained from three additional pyrolytic methods, each employing compatible thermal decomposition ranges. These complementary techniques were selected to ensure consistency in evaluating the breakdown behavior of organic matter under controlled heating conditions.

By incorporating multiple pyrolysis approaches, the study enhances the reliability of the thermal characterization. Each method contributes unique analytical strengths, whether in detecting specific compound classes, refining kinetic parameters, or improving resolution across temperature intervals. Comparative analysis across these methods allows for a more nuanced interpretation of organic matter transformation and hydrocarbon generation potential.

4.1. Characteristics of Organic Matter from the Dukla Unit

Samples from the Dukla Unit exhibited the highest total organic carbon (TOC) content, ranging from 4.77 to 13.54%. They also showed the highest hydrogen index (HI) values, between 516 and 691 mg HC/g TOC, which clearly indicates an oil-prone generative potential. The Menilite beds samples from the Dukla Unit simultaneously exhibited the lowest oxygen index (OI), which suggests low-oxygen conditions in the depositional environment of the source material (Table 1). Their level of thermal maturity was slightly higher than that observed in the Silesian Unit samples from surface outcrops. This means that such a high hydrogen index, responsible for the high hydrocarbon productivity, could have been even slightly higher in pre-generational conditions (initial conditions).

The Menilite samples from surface outcrops in the Dukla Unit, in analyses conducted by pyrolysis coupled with gas chromatography (Py-GC/FID), showed a high content of hydrocarbons in the C₁–C₉ range (Figure 2). This observation may indicate two key aspects: firstly, the occurrence of secondary cracking processes at temperatures up to 500 °C; and secondly, the oil-and-gas-prone nature of these sediments.

It is particularly noteworthy that the Menilite samples from the Dukla Unit exhibit remarkable homogeneity, both in terms of the quantity of hydrocarbons generated and their fractional composition. This consistency suggests a uniform source rock quality and supports the interpretation of these deposits as having significant generative potential for both liquid and gaseous hydrocarbons. Detailed results are provided in Table 1.

For a comprehensive summary of the transformations occurring during the most critical stage of kerogen decomposition in the TG experiments, the data corresponding to the temperature interval of 300–650 °C are presented in Table 1. This table includes the mass losses observed within this range, along with the characteristic temperatures of the respective reactions. The onset temperature indicates the point at which mass loss begins, while the endset temperature marks the completion of the reaction. T_max represents the temperature at which the maximum rate of mass loss occurs, as determined from the differential TG (DTG) curve. In certain cases, two T_max values are provided when multiple distinct peaks are apparent on the DTG curve (whereby they are separated with a semi-colon). From the perspective of organic matter breakdown, the temperature interval of 300–650 °C is of primary interest. Within this range, under an inert gas atmosphere, pyrolysis of the organic matter takes place. However, interpreting the organic content in the sample requires accounting for the presence of other thermally reactive constituents. Within this same temperature interval, advanced decomposition of certain clay minerals may also occur [37].

In the case of samples from the Dukla Unit, thermal analysis (TG/DTG) revealed the absence of significant amounts of mineral matter disrupting the decomposition of kerogen. TG/DTG analyses performed on selected samples representing the Dukla Unit within the temperature range of 300–650 °C (Table 1) confirmed the high homogeneity of the organic matter with respect to the rate of its thermal decomposition. The organic matter contained in the Menilite shales of the Dukla Unit undergoes decomposition between 338 and 610 °C, with maximum hydrocarbon generation occurring at temperatures ranging from 453 to 462 °C. Within this interval, the mass loss ranges from 7.52% to 9.88%. When these results are integrated with data obtained from other pyrolytic techniques (Rock-Eval and Py-GC/FID), it indicates that this mass loss corresponds to hydrocarbon generation from 1 g of rock in amounts ranging from 24.63 mg to 45.07 mg, with a fractional composition of 68% of C₁–C₉, 20% of C₁₀–C₁₅, and 11% of C₁₅₊, as exemplified by sample 3D. These samples are also characterized by a high yield indicator in Py-GC, reaching a value of 31.16 (Figure 3). This indicator is proportional to both the mass loss and the pyrolyzable organic carbon determined by the Rock-Eval analysis (45.07 mg/g rock). The geochemical homogeneity of these samples is further evidenced by the very similar temperatures at which the maximum rate of organic matter decomposition occurs (Figure 4) and by the low threshold temperatures initiating generative processes. These lower temperature weight losses were most significant in the Skole unit samples and were less for the Dukla samples of the Menilite shale and the transition beds. The main region of weight loss, occurring from about 300 °C to 650 °C, was because of the loss of hydrocarbon material and the evolution of gases and oil vapor. The oil shale samples from the Dukla Unit exhibit a one-step thermal decomposition in the main weight loss area suggesting a one-step evolution of hydrocarbon volatiles from the oil shale, whereas the Menilite Beds from the Silesian Unit (3SL and 5SL) samples exhibit a two stage decomposition in the range of 350–650 °C, representing a two stage evolution of hydrocarbon material. The determination of whether the decomposition is single-stage or two-stage depends on the type of oil shale.

In the Py-GC/IRMS studies, the carbon isotopic composition was determined for methane, ethane, and propane generated during simulated hydrocarbon generation. These values indicate the thermogenic origin of hydrocarbons. For samples from the Dukla Unit (five samples), the carbon isotopic composition in methane ranges from −41.5 to −37.9‰. The δ¹³C values for ethane fall within the range of −34.6 to −31.3‰, while for propane the isotopic composition ranges from −34.4 to −30.4‰. Natural gas formed from marine organic matter (type II kerogen) has lower δ¹³C values than gas formed from terrigenous (lacustrine) or mixed organic matter. Under similar conditions of thermal maturity, differences in carbon isotope composition of methane, ethane, and propane may suggest differences in organic matter composition (marine, terrigenous, lacustrine, and mixed) [38]. Carbon isotopic compositions of methane, ethane, and propane are summarized on plots (Figure 5 and Figure 6), which show also theoretical values of δ¹³C and the thermal maturity (vitrinite reflectance) of source organic matter [39].

4.2. Characteristics of Organic Matter from the Silesian Unit

The Menilite Beds within the Silesian Unit exhibit significantly greater variability in generative properties. Such diversity in the generative potential of the Menilite shales within the Silesian Unit has been highlighted by research findings concerning the genetic correlation of accumulated hydrocarbons in discovered deposits [40,41,42,43]. The findings reveal that the Menilite shales display considerable heterogeneity with respect to their source rock properties, degree of thermal evolution, and ability to generate hydrocarbons. This diversity likely stems from differences in depositional environments, organic input, and post-depositional geological processes, which have influenced the type and quantity of hydrocarbons generated and subsequently accumulated in the subsurface traps. Most of these samples are characterized by a lower TOC content, ranging from 2.66 to 8.81%, except for one sample from the Monastyrec area (6Sl), where the TOC content reaches as high as 18.09%, and the generative potential attains a value of 144.04 mg HC/g TOC. In this sample, the highest hydrocarbon yield was also recorded, exceeding 45, with a fractional composition of 21.92% of C₁–C₉, 20.78% C₁₀–C₁₅, and 57.31% of C₁₅₊. This sample contains typically oil-prone source organic matter, which additionally stands out in the isotopic composition studies by having the lowest δ¹³C values in ethane and propane, indicating the lowest generation stage in the range of 0.6–0.7% on the vitrinite reflectance scale. On the other hand, such very low isotopic composition values combined, especially for 4Sl and 6Sl samples, with a high hydrogen index suggest a significant contribution of type I-II organic matter. This provides a clue for the genetic assessment of gases.

TG/DTG analyses conducted on samples representing the Menilite Beds of the Silesian Unit in the temperature range of 300–650 °C (Table 1) revealed a greater diversity of organic matter in terms of the rate of its thermal decomposition. The organic matter of the Menilites from the Silesian Unit decomposes in the temperature range of 300 to 650 °C, with the maximum number of hydrocarbons generated at temperatures between 437 and 499 °C. In this range, the mass loss varies from 7.12 to 10.01%. This means that such mass loss is equivalent to hydrocarbon generation from 1 g of rock in the amount of 8.04 mg to 38.32 mg, with a fractional composition of 32.44% of C₁–C₉, 27.91% C₁₀–C₁₅, and 39.65% of C₁₅₊ for the sample from the Iwonicz fold (sample 3Sl) (Table 1).

These samples show a very wide range of yield indices, from 3.27 to 45.58. The highest indices were obtained in samples 6Sl and 3Sl (13.31) (Figure 3). This index is proportional to mass loss as well as to the pyrolytic carbon content in the Rock-Eval analysis. Such diversity is also reflected in all pyrolytic results, indicating a varied source material with a changing proportion of terrestrial and marine input. The fractional composition of the pyrolysis products points to a higher share of hydrocarbons in the C₁₀–C₁₅ range than in samples from the Dukla Unit. The maximum generation rate is shifted towards higher temperatures, which may indicate a higher activation energy (Figure 4).

Thermal decomposition in TG analysis frequently shows a two-stage character in case of samples from the Silesian Unit. The example of TG/DTG analysis result is shown in Figure 7 for sample 3Sl from the Iwonicz fold. In the range of 40–300 °C, a mass loss of 1.91% is observed due to the dehydration of clay minerals, while in the range of 300–650 °C the pyrolysis of organic matter overlaps with a peak originating from pyrite decomposition (T_max approx. 515 °C). The decomposition of organic matter begins at 360 °C and continues up to 650 °C, with a maximum at around 448 °C and a total mass loss of 10.01%. As seen, due to the presence of a mineral substance (in this case, pyrite), the total mass loss in the range of 300–650 °C cannot be considered as related solely to pyrolysis of organic matter. To properly interpret the thermograms of rocks containing dispersed organic matter, it is necessary to consider the transformations taking place during the third stage of the experiment, which proceeds in the temperature range of 650–1050 °C under oxidizing conditions. Within this interval, combustion of the residual organic matter remaining after pyrolysis occurs. The greater the mass loss and the stronger the exothermic effect, the higher the proportion of organic matter in the sample composition. Combustion begins at approximately 650 °C, whereas the completion of this process occurs at different temperatures depending on the amount of organic substance and other reactions taking place within this temperature range. In the case of sample 3Sl (Figure 7), combustion of the residual organic matter ends at 750 °C. The total mass loss in the entire 650–1050 °C range amounts to 6.70%, which is consistent with the residual carbon (RC) value from the Rock-Eval analysis of 3.26%.

It should be noted here that a significant part of the samples from the Silesian Unit (sample 3Sl, 5Sl, 7Sl, and 8Sl) contain inorganic components detectable by the TG method. These include pyrite, siderite, and quartz, with the presence of both pyrite and siderite influencing the kerogen decomposition process in the samples. In the case of the Istebna beds sample (7Sl), the organic matter content is so low (the mass loss in the 300–650 °C range results from siderite decomposition) that it is not possible to provide a T_max value for pyrolysis.

The isotopic composition of the pyrolysis products for samples from the Silesian Unit shows values ranging from −43.2 to −37.2‰ for methane. The values for ethane are in the range of −35.6 to −32.1‰, while for propane the isotopic composition ranges from −35.7 to −31.4‰ (Figure 8). In particular, samples 6Sl and 4Sl stand out, as its carbon isotope composition in methane, ethane, and propane is the lowest of all the samples from this unit (mentioned above). The much lower variability of δ¹³C of propane compared to methane and ethane is also notable.

Despite similar thermal maturity, the differences between samples result from the composition of the original organic matter. In particular, sample 6SL indicates a significant contribution of algal-type material (Type I), characterized by a high hydrogen index. A high HI (reaching 796 mg HC/gTOC) in 6SL suggests excellent potential for generating liquid hydrocarbons (oil) upon maturation which is supported by the results of PY-GC.

4.3. Characteristics of Organic Matter from the Skole Unit

The last group of samples comprised those from the Skole Unit, which are characterized by the lowest thermal maturity and high generative potential, in the range of 33.54–106.578 mg HC/g rock (Table 1). Such high source parameters were confirmed by Py-GC/FID analyses, where the hydrocarbon yield index was very high, ranging from 14.51 to 55.09, particularly for sample 4Sk. Only one sample from Spas Beds, 5Sk, had a low yield index that was reaching 1,49. The fractional composition of the products shows a considerably higher proportion of heavier hydrocarbons such as C₁₀–C₁₅ and C₁₅₊, relative to lighter hydrocarbons such as C₁–C₉ (Figure 9).

The initial discrepancies can be attributed to the nature of the research material itself, which consisted of Menilite shales as well as the Spas beds. These layers exhibit distinctly different source rock characteristics, as confirmed by the fractional and isotopic composition of pyrolysis products [44,45].

In the thermogravimetric analysis, these samples also exhibit the highest mass loss, exceeding 10%, and the onset of generation occurs from 325 °C, which would suggest low activation energy of the kerogen, likely associated with its molecular structure [46]. The highest mass loss occurs at lower temperatures, and the termination of generative processes also occurs at lower temperatures. All of these factors allow us to infer a low activation energy for the kerogen of the Menilite beds in the Skole Unit [47,48,49,50].

The example of the results of a TG analysis for a sample from the Skole Unit is shown in Figure 10. A high content of organic matter is characteristic here, which is manifested by significant mass loss in the range of 300–650 °C. Kerogen decomposition reaches the fastest rate at a temperature of 437 °C. An important component influencing the course of the thermogram in the analyzed sample is siderite (FeCO₃). The thermal dissociation of siderite, initially breaking down into FeO and CO₂, occurs within the range of approximately 480–550 °C. For the sample studied, the T_maxTG value for siderite decomposition is found to be 502 °C. From the compiled data, it follows that the lowest maturity of organic matter is found in sample 2Sk from the Skole Unit (T_maxTG = 437 °C), while the highest is observed in sample 8Sl from the Lgota beds of the Silesian Unit (T_max = 499 °C).

Five samples from the Skole Unit analyzed by Py-GC/IRMS show δ¹³C values in methane ranging from −43.8 to −35.5‰. The δ¹³C values of ethane vary from −34.7 to −28.8‰, while for propane the isotopic composition ranges from −35.5 to −28.8‰.

Samples from the Skole Unit exhibit significant variability in the isotopic composition of pyrolysis products (Figure 5, Figure 6 and Figure 8). Assuming a uniform level of thermal maturity across the analyzed rock samples, this diversity suggests differences in the original organic material and the conditions under which it was deposited. Within this group, sample 1Sk stands out due to its exceptionally high δ¹³C values in methane, ethane, and propane. This may indicate either secondary cracking processes or a source material with an aromatic character [51,52,53]. Organic matter rich in aromatic compounds tends to yield isotopically heavier gases. Additionally, this sample contains the highest concentration of hydrocarbon fractions in the C₁₀–C₁₅ range.

5. Conclusions

Integration of pyrolytic studies performed on potential source rocks from the Western Carpathians provided new types of information regarding the rate of organic matter decomposition, as well as the fractional and isotopic composition of the pyrolysis products obtained. Such new data may be used with previous works for new insight into the Polish Carpathian petroleum system:

• In the Silesian Unit, the Menilite beds samples showed greater diversity, demonstrated both in the fractional composition of the generated products (showing an oil- and-gas-prone character) and in the decomposition rates of the organic matter, which is directly related to a higher activation energy of the kerogen compared to that in the Dukla and Skole Units.
• The Menilite beds in the Skole Unit are characterized by the lowest activation energy and the lowest temperature for the onset of generative processes.
• The activation energy for the Menilite beds of the Dukla Unit is found to be intermediate between the values for the Silesian and Skole Unit. The samples also exhibited the greatest homogeneity in terms of their geochemical character.
• Isotopic compositions of methane, ethane, and propane are similar to natural gases present in regional deposits (e.g., Węglówka and Potok fold deposits—thermogenic gases generated in oil window).