Natural gas origin and accumulation of the Changxing–Feixianguan Formation in the Puguang area, China

Songfeng Li; Xiaoxia Liu; Peixin Zhang; Yinan Zhao

doi:10.1515/geo-2022-0658

Article Open Access

Natural gas origin and accumulation of the Changxing–Feixianguan Formation in the Puguang area, China

Songfeng Li , Xiaoxia Liu , Peixin Zhang and Yinan Zhao

Published/Copyright: January 21, 2025

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From the journal Open Geosciences Volume 17 Issue 1

Abstract

Investigating the origin and accumulation of natural gas is crucial for investigating fields of gas and oil. This study can provide effective guidance for future strategic decisions. In this study, the Changxing–Feixianguan Formation gas reservoir in the Puguang area was employed as a case study. The gas source strata were evaluated through analysis of the geochemical characteristics of natural gas. The period of accumulation and reservoir type were identified by examining the inclusion and burial history characteristics. The results revealed that (1) the Changxing–Feixianguan Formation was a high-sulfur gas reservoir, primarily characterized by oil-type gas. Influenced by thermal sulfate reduction, it exhibited partial characteristics of a mixed gas; (2) the natural gas of the Changxing–Feixianguan Formation demonstrated a favorable biogenic relationship with the carbon isotope characteristics of the Longtan Formation and Maokou Formation kerogen, and its primary gas source was Permian source rock; and (3) the Changxing–Feixianguan Formation gas reservoir experienced three oil and gas charging events, namely, the Middle Triassic, Late Jurassic, and Late Cretaceous. Natural gas was primarily extracted from ancient reservoirs and has evolved into light oil, condensate gas, wet gas, and dry gas. These findings serve as a foundation for investigating the accumulation of natural gas within the Changxing–Feixianguan Formation in the Puguang area. In future natural gas exploration efforts, it is recommended to concentrate on identifying paleostructures that may have formed ancient oil reservoirs to enhance the precision of natural gas exploration.

Keywords: carbon isotope; gas source comparison; inclusion; accumulation period; Puguang area

1 Introduction

During the global energy transition from high carbon to low carbon, the natural gas industry is crucial for the harmonized progress of renewable energy sources. Therefore, it is necessary to further promote basic and consistent technical research on complicated gas deposits through technological innovation and integration to achieve critical discoveries in new fields and strata.

China is among the globally leading ten importers of natural gas. Most of its natural gas reserves are located in the western regions, particularly in Sichuan, Xinjiang, and Shaanxi. According to data released by the China Bureau of Statistics in 2022, the national natural gas production was 2,211 × 10⁸ m³. Notably, Sichuan Province contributed significantly to national production, accounting for approximately one-fourth of the country’s total natural gas output, with a production of 561.2 × 10⁸ m³.

The gas source field in Puguang is notable for being the largest marine-integrated high-sulfur gas field in China. Its exploration and development hold a unique position and play a crucial role in China’s energy development strategy [1,2]. The early exploration and development of the Puguang gas field have largely focused on marine strata [3,4,5,6]. In recent years, significant progress has been made in the exploration of continental strata [7,8]. Nevertheless, the gas field in that region remains the primary gas source for the national key project “Sichuan gas to the east,” with the marine strata of the Changxing–Feixianguan Formation serving as the main gas-producing strata for natural gas. However, as natural gas is increasingly explored and developed, there has been a decline in the success rate of drilling, and the understanding of the gas source and accumulation period remains unclear. The primary factors contributing to this disparity are insufficient geochemical data and absence of rigorous research methodologies, concerning neither the variability of an individual geochemical index nor the intricacies of geological conditions within the study region [9,10,11]. Therefore, it is crucial to conduct a comparative investigation of the accumulation period and sources of gas in the Changxing–Feixianguan Formation, as it holds significant guidance for future research in the Puguang area.

In this study, we conducted a thorough examination of 10 natural gas component samples, 12 natural gas carbon isotope samples, and 21 kerogen carbon isotope samples to explore the nature, origin, and relationship between the natural gas sources. Via investigating the consistency in temperature and salinity profiles of 12 inclusion samples, combined with the burial history analyses, we successfully determined the accumulation period of natural gas.

2 Geological settings

The gas resource field in Puguang is situated within Xuanhan and Daxian counties in Dazhou City, Sichuan Province. Geographically, it is located in the northeastern part of the Sichuan Basin, north of the front fold belt within the Dabashan thrust nappe structural belt, and southern region of the gentle fold belt of the central Sichuan area. The field has primarily experienced the Indosinian, Yanshan, and Himalayan movements (Figure 1) [12,13]. During the Paleozoic and Mesozoic eras, numerous sets of source rocks, such as carbonate rocks, argillaceous rocks, and coal, developed in the area. These source strata overlap vertically and cross each other in a plane, resulting in a complex oil and gas system in the region. Consequently, the target energy types may be interlaced and mixed within the formation [14,15,16]. Additionally, due to the presence of multiple periods of hydrocarbon generation, reservoir formation, adjustment, and transformation, it is extremely challenging to trace the origin of the gas (Figure 2) [12,13].

Figure 1

Tectonic zoning of the Sichuan Basin [12,13].

Figure 2

Stratigraphic column section of the Eastern Sichuan Basin [12,13], where PL represents the Penglai Formation; SN represents the Suining Formation; SXM represents the Shaximiao Formation; ZLJ represents the Ziliujing Formation; XJH represents the Xujiahe Formation; LKP refers to the Leikoupo Formation; JLJ refers to the Jialingjiang Formation; FXG refers to the Feixianguan Formation; CX denotes the Changxing Formation; LT denotes the Longtan Formation; MK denotes the aokou Formation; QX denotes the Qixia Formation; HL denotes the Huanglong Formation; HJD signifies the Hanjiadian Formation; XHB signifies the Xiaoheba Formation; LMX signifies the Longmaxi Formation; WF represents the Wufeng Formation; LX represents the Linxiang Formation; BT represents the Baota Formation; SZP represents the Shizipu Formation; MT denotes the Meitan Formation; HHY denotes the Honghuayuan Formation; TZ denotes the Tongzi Formation; XXC denotes the Xixiangchi Formation; LWM signifies the Longwangmiao Formation; CLP refers to the Canglangpu Formation; QZS refers to the Qiongzhusi Formation; DY denotes the Dengying Formation; and DST represents the Doushantuo Formation.

3 Methods

The chemical attributes of natural gas resources are critical for studying their origin and source and present a comprehensive scientific underpinning for the creation and storage of hydrocarbons [17,18,19,20,21]. Notably, the carbon isotope of natural gas is the most significant index of its genetic class. In general, the carbon isotope of methane (δ ¹³ C ₁) in natural gas increases with increasing thermal evolution, which makes it a better index to measure the maturity of natural gas. The ethane carbon isotope (δ ¹³ C ₂) of natural gas is typically close to the carbon isotope composition of the parent material, making it a valuable index for determining the type of gas-source rock. Therefore, δ ¹³ C ₁ and δ ¹³ C ₂ can be utilized to effectively identify the genetic type of natural gas [22,23].

The study of 10 natural gas component samples, 12 natural gas carbon isotope samples, and 21 kerogen carbon isotope samples facilitated the discussion of the types, genesis, and gas source relationship of natural gas. The constituents of the natural gas were analyzed using a gas chromatograph of SCION 456-GC manufactured by Tianmei Yituo from the USA, where high-purity helium was utilized as the carrier gas. The examination was performed using a double thermal conductivity detector. Finnigan MAT-252 MS was utilized for testing the isotopic compositions of carbon and hydrogen within natural gas, which was originally manufactured by Finnigan Mat (Germany). We standardized the carbon and hydrogen concentrations of each constituent to international benchmarks (Vienna Pee Dee Belemnite for carbon and Vienna standard mean ocean water for hydrogen) with a standard deviation of 3‰.

Fluid inclusions are rich in the information of oil and gas accumulation and migration. One of the most significant aspects of fluid inclusion research is the homogenization temperature of the inclusions [24]. By measuring the homogenization temperature of primary or secondary inclusions in sandstone strata, it is possible to trace the paleogeothermal temperature and hydrocarbon-filling period of the host strata [25]. To analyze the inclusions in the multi-period accumulation process in superimposed basins, it is necessary to comprehensively understand the statistical distribution of the homogenization temperature of inclusions and the regularity of the peak temperature distribution integrated with the constraints of the tectonic thermal evolution history. This is necessary for objectively understanding the significance of the peak temperature of inclusions as well as the corresponding oil and gas-filling period and paleo-geothermal environment.

Based on the examination of the homogenization temperature and salinity characteristics of 12 inclusion samples extracted from well PG2, combined with an analysis of the burial history, a period of natural gas accumulation was established. The inclusion samples were evaluated for homogenization temperature and salinity using a hot and cold platform that encompassed a minimum temperature of −100°C and a maximum temperature of 350°C, as well as a polarizing microscope equipped with a long-focus objective. For accurate determination of the uniform temperature, the heating rate should be carefully controlled, with a suggested range of 1–3°C/min when the inclusions approach homogenization. In the process of salinity determination, it was crucial to consider initial dissolution and freezing point temperatures.

4 Results

4.1 Geochemical characteristics of natural gas

4.1.1 Characteristics of natural gas compositions

The analysis of ten natural gas compositions from the Changxing–Feixianguan Formation in the Puguang area indicated that the methane content was quite high, with an average of approximately 76%. Furthermore, approximately 60% of the samples exhibited H₂S contents greater than 5%, indicating the presence of high-sulfur gas reservoir characteristics (Table 1).

Table 1

Composition of the natural gas in the Changxing–Feixianguan Formation

Number	Well	Horizon	Compositions (%)
Number	Well	Horizon	CH₄	C₂H₆	CO₂	H₂S
1	PG2	T₁f	75.90	0.22	7.83	15.45
2	PG2	P₂ch	75.03	0.33	8.73	15.41
3	MB6	T₁f	72.31	0.02	10.14	16.16
4	MB3	P₂ch	81.47	0.09	4.18	12.67
5	MB2	T₁f	52.68	0.21	30.32	0.00
6	DW1	P₂ch-T₁f	70.05	0.03	11.78	12.84
7	LJ1	T₁f	62.39	0.02	27.92	1.40
8	LJ1	P₂ch	76.18	0.05	4.16	19.01
9	XQX1	T₁f	99.53	0.30	0.10	0.00
10	M1	P₂ch	93.97	0.40	5.30	0.00

4.1.2 Characteristics of natural gas carbon isotopes

The δ ¹³ C ₁ values of natural gas from the Changxing–Feixianguan Formation in the Puguang area were predominantly between −33.18 and −30.02‰, with an average of −31.11‰. Moreover, the δ ¹³ C ₂ values were mainly concentrated between −33.83 and −27.67‰, with an average of −30.40‰ (Table 2). The values of −27.5 and −29‰ were considered the boundaries for distinguishing between coal-derived gas, mixed gas, and oil-type gas, and the maturity stage was classified based on different δ ¹³ C ₁ values. The natural gas from the Changxing–Feixianguan Formation primarily consisted of oil-type gas and partly mixed gas, both of which are in the over-mature stage (Figure 3).

Table 2

δ ¹³ C ₁ and δ ¹³ C ₂ values of the natural gas in the Changxing–Feixianguan Formation

Number	Well	Horizon	δ ¹³ C ₁ (‰)	δ ¹³ C ₂ (‰)
1	CY83	T₁f	−30.93	−33.83
2	CY84	T₁f	−32.26	−31.24
3	CY84	T₁f	−31.65	−31.19
4	CY84	P₂ch	−30.02	−31.49
5	CY84	P₂ch	−30.67	−28.9
6	CY84	P₂ch	−30.43	−30.26
7	MB2	T₁f	−33.18	−32.25
8	PG2	T₁f	−30.93	−28.51
9	PG2	T₁f	−30.49	−29.07
10	PG2	T₁f	−30.96	−28.81
11	PG2	P₂ch	−30.05	−27.67
12	PG9	T₁f	−31.8	−31.67

Figure 3

Genetic types of the natural gas in the Changxing–Feixianguan Formation.

4.1.3 Characteristics of kerogen carbon isotopes

The kerogen carbon isotope of the source rocks served as a crucial indicator of the gas-source correlation. The δ ¹³ C values of the Longtan Formation in the Puguang area, which were derived from kerogen, were predominantly between −28.65 and −26.33‰, with an average of −27.57‰. Similarly, the δ ¹³ C values of the Maokou Formation were primarily between −30.35 and −26.57‰, with a mean of −27.92‰ (Table 3).

Table 3

δ ¹³ C values of the kerogen in the Longtan and Maokou Formations

Number	Well	Horizon	δ ¹³ C (‰)
1	MB3	P₂l	−28.56
2	MB3	P₂l	−28.18
3	MB3	P₂l	−27.84
4	MB3	P₂l	−28.18
5	MB3	P₂l	−26.33
6	MB3	P₂l	−26.5
7	PG5	P₂l	−28.65
8	PG5	P₂l	−28.16
9	PG5	P₂l	−27.48
10	PG5	P₂l	−27.49
11	PG5	P₂l	−26.67
12	PG5	P₂l	−26.82
13	QX3	P₁m	−26.73
14	QX3	P₁m	−26.57
15	QX3	P₁m	−29.91
16	QX3	P₁m	−28.15
17	QX3	P₁m	−27.21
18	QX3	P₁m	−30.35
19	QX3	P₁m	−27.7
20	QX3	P₁m	−27.15
21	QX3	P₁m	−27.53

4.2 Characteristics of inclusion

The homogenization temperature of inclusions typically represents the minimum temperature at which inclusions are trapped, i.e., the temperature at which hydrocarbons migrate into the reservoir, and the peak temperature denotes the peak period of the secondary migration of oil and gas [26,27]. As an illustration, examining the Feixianguan Formation through the analysis of 12 inclusion samples from PG2 well revealed a wide range of homogenization temperatures, spanning from 95 to 215°C (Figure 4), with two evident primary peaks. The first principal peak temperature was between 105 and 115°C, with an average of 110°C, whereas the second primary peak temperature laid between 145 and 155°C, with an average of 150°C. Additionally, there was a third notable peak temperature that spanned from 185 to 195°C, with an average of 190°C.

Figure 4

Homogenization temperature of the inclusion in the Feixianguan Formation for the PG2 well.

Based on the phase characteristics of the inclusions observed under a microscope, it was preliminarily determined that there were three periods of hydrocarbon fluid injection in the inclusions. The first-period inclusions were primarily identified in early fractured calcite or granular sparry calcite (Figure 5a), which were either liquid or solid (asphalt inclusions) with irregular shapes. The homogenization temperature of these inclusions ranged from 95 to 130°C, while their frequency value was high, and the main peak was noticeable at approximately 110°C. The second-period inclusions mainly occurred in late-period continuous-crystal calcite, calcite veins, and cave quartz crystals (Figure 5b). These inclusions were primarily gas–liquid two-phase inclusions, and they exhibited various shapes, such as square, rectangular, round, oval, and long oval. The homogenization temperature was within the range of 130–170°C, during which time the frequency attained its highest value, and the primary peak was evident at approximately 150°C. The third-period inclusions were primarily observed in calcite veins and cave quartz crystals (Figure 5c). These inclusions were predominantly of the gas or gas–liquid two-phase variety and were distributed in elongated ellipses, circles, rectangles, and irregular shapes. The homogenization temperature for this phase was between 170 and 215°C, with the primary peak evident at approximately 190°C.

Figure 5

Characteristics of the inclusions in the Feixianguan Formation from the PG2 well. (a) First period, (b) second period, and (c) third period.

The main basis for hydrocarbon accumulation analysis is the salinity of inclusions, such as the homogenization temperature. The salinity and homogenization temperature characteristics of the inclusions can be mutually confirmed [28,29]. The salinity characteristics of 12 fluid inclusion samples from PG2 well were consistent with the homogenization temperature characteristics. Specifically, the NaCl mass fraction of the first-phase inclusions was between 12.9 and 13.3%, with a relatively concentrated distribution. The NaCl mass fraction of the second-period inclusions was relatively wide, mainly between 5.32 and 9.84%. The NaCl mass fraction of the third-period brine inclusions was concentrated between 11.1 and 11.7%.

5 Discussion

5.1 Natural gas types and origin

The natural gas present in the Changxing–Feixianguan Formation of the Puguang area was primarily a high-sulfur gas reservoir, which was characterized by oil-type gas and partly mixed gas. The occurrence of mixed gases may be attributed to the thermochemical sulfate reaction (TSR). Underground oil and gas can crack owing to increased geothermal temperature and undergo redox reactions with sulfate to produce H₂S, calcite, and new organic sulfur compounds, resulting in a high sulfur content in the gas reservoir of the Changxing–Feixianguan Formation in the Puguang area [30].

5.2 Analysis of gas source

Based on the characteristics of the δ ¹³ C ₁ and δ ¹³ C ₂ averages for the specific energy in Puguang, Jiannan, Tieshan, and other regions in northeastern Sichuan, the δ ¹³ C ₂ of the Xujiahe Formation was notably heavier, indicating that it was coal-formed gas. Conversely, the natural gas present in other strata was predominantly of oil-formed origin. However, the natural gas found in other strata appeared to be oil-formed gas. However, the δ ¹³ C ₁ and δ ¹³ C ₂ characteristics of the Changxing–Feixianguan, Longtan, and Maokou formations were relatively similar, suggesting homologous natural gas. The similar characteristics of δ ¹³ C ₁ and δ ¹³ C ₂ in the Longmaxi Formation and Carboniferous strata may also indicate homologous natural gas (Figure 6). The natural gas in the Changxing–Feixianguan Formation and Xujiahe Formation was believed to have distinct origins, as their compositions differed. Despite sharing similarities with the natural gas of the Longmaxi Formation and Carboniferous strata, the gas of the Changxing–Feixianguan Formation exhibited characteristics of block aggregation. Consequently, it was unlikely that the specific gas in the Changxing–Feixianguan Formation originated from the strata below the Permian. If this were the case, its carbon isotope characteristics would remain consistent.

Figure 6

Origin of the natural gas in the northeast of Sichuan Basin.

A comparison of the δ ¹³ C ₂ characteristics of natural gas with the kerogen carbon isotope (δ ¹³ C) characteristics of the source rocks can serve as an effective means of identifying the source of natural gas. An extensive body of analytical data indicated that the δ ¹³ C ₂ of natural gas was typically 1–3‰ lighter than that of the source rock kerogen and δ ¹³ C, and natural gas and source rock kerogen within this range were homologous [31]. When conducting gas source correlation, the δ ¹³ C ₂ of natural gas can be compared with kerogen δ ¹³ C after adding an average of 1.5‰. The δ ¹³ C ₂ of natural gas in the Changxing–Feixianguan Formation in the Puguang area ranged from −33.83 to −27.67‰, with an average of −30.3‰ (based on 12 samples). The δ ¹³ C of kerogen in the Longtan Formation ranged from −28.65 to −26.33‰, with an average of −27.57‰ (based on 12 samples). The δ ¹³ C of kerogen in the Maokou Formation ranged from −30.35 to −26.57‰, averaging at −27.92‰ (based on nine samples). The comparative analysis indicated that the natural gas in the Changxing–Feixianguan Formation demonstrated an excellent biogenic relationship with kerogen in the Longtan and Maokou Formations (Figure 7).

Figure 7

Gas source comparison in the Puguang area.

5.3 Accumulation analysis

Currently, researchers typically identify the formation period of inclusions and the corresponding oil and gas-filling period by analyzing the characteristics of brine inclusions, such as occurrence, fluorescence color, and homogenization temperature, as well as by considering the diagenesis period. However, the temperature recorded by inclusions may have a normal change interval. Therefore, to conduct an inclusive analysis of the multiperiod accumulation process in superimposed basins, it is imperative to have a comprehensive understanding of the statistical distribution of the homogeneous temperature of the inclusions and the regularity of the peak temperature distribution combined with the constraints of the tectonic thermal evolution history. This can objectively comprehend the significance of the peak temperature of inclusions as well as the corresponding oil and gas-filling periods and paleo-geothermal environments.

The homogenization temperature and salinity characteristics of the 12 inclusion samples from well PG2 indicate that in Feixianguan Formation, the reservoir of gas has experienced three cycles of oil and gas filling, making it a multistage oil and gas reservoir. Through a burial history analysis of well PG2, it was inferred that these three cycles of gas and oil filling occurred during the Middle Triassic, Late Jurassic, and Late Cretaceous periods (Figure 8). Both the Feixianguan and Changxing Formations were reef-bank facies gas reservoirs with similar accumulation characteristics, and they have experienced three cycles of oil and gas filling. The gas reservoir within the Changxing–Feixianguan Formation exhibited low maturity in the mid-Triassic, corresponding to an early maturity period, and was primarily composed of light oil reservoirs. In the Late Jurassic, maturity increased and advanced to the late maturity period, characterized by both light oil and condensate gas reservoirs. By the Late Cretaceous, the formation had reached a near-maximum burial depth in the area and entered the over-mature period, primarily comprising wet gas and dry gas reservoirs (Table 4). The nature of the gas reservoir indicated that the natural gas of the Changxing–Feixianguan Formation was primarily derived from the cracking of ancient reservoirs and has undergone the evolutionary processes of light oil, condensate gas, wet gas, and dry gas.

Figure 8

Gas reservoir accumulation period in the Feixianguan Formation for the PG2 well.

Table 4

Analysis of the gas reservoir in the Feixianguan Formation for the PG2 well

Period of homogenization temperature (℃)	Average of homogenization temperature (℃)	Inclusion type	Color and shape	Salinity (%)	Reservoir forming time	Reservoir type
100–120	110	Liquid, solid	Irregular, light blue fluorescence	12.86–13.30	Mid-Triassic	Light oil reservoir
140–160	150	Mainly gas‒liquid two phase, followed by liquid phase	Square, rectangle, circle, ellipse, long ellipse	5.32–9.84	Late Jurassic	Reservoir of light oil and condensate gas
170–210	190	Mainly gas‒liquid two phase, followed by gas phase	Long oval, round, rectangular	11.10–11.70	Late Cretaceous	Reservoir of wet and dry gases

The Changxing–Feixianguan Formation gas reservoir exhibited favorable conditions for a source–reservoir–cap assemblage. The Longtan Formation source rocks were spatially stable and could yield substantial quantities of oil and gas, with a maximum oil generation intensity of 42 × 10⁸ m³/km² [32]. The Changxing–Feixianguan Formation comprised high-quality reef- and shoal-facies dolomite reservoirs [33] and formed multiple structural–lithologic composite traps over the paleo-uplift [34]. The Jialingjiang–Leikoupo Formation thick gypsum–salt layer functioned as a regional cap rock [34]. The regional faults connected the source rocks of the Longtan Formation to the reservoirs of the Changxing–Feixianguan Formation, but did not extend upward to cut the cap rocks of the Jialingjiang–Leikoupo Formation, thereby preserving the structural–lithologic composite trap integrity [35]. Hence, the exploration of natural gas in the Changxing–Feixianguan Formation of the Puguang area should prioritize the location of ancient structures that may have formed paleo-reservoirs and conduct fine characterization and interpretation.

6 Conclusions

The Changxing–Feixianguan Formation was a reservoir of high-sulfur natural gas. The values of δ ¹³ C ₁ for this gas were primarily between −33.18 and −30.02‰, while the values of δ ¹³ C ₂ were primarily between −33.83 and −27.67‰. This gas was primarily of the oil-formed variety, which was affected by the TSR and partially by the mixed-gas variety.
The carbon isotope ratios of δ ¹³ C ₁ and δ ¹³ C ₂ in natural gas from the Changxing–Feixianguan, Longtan, and Maokou Formations were remarkably similar. In particular, the δ ¹³ C ₂ value of natural gas from the Changxing–Feixianguan Formation exhibited a strong correlation with the kerogen δ ¹³ C characteristics of the Longtan and Maokou Formations. This finding supported the notion that the primary gas source was derived from Permian source rocks.
The Changxing–Feixianguan Formation gas reservoir was subjected to three instances of gas and oil charging: the Middle Triassic, Late Jurassic, and Late Cretaceous. The natural gas present within the reservoir was believed to have originated from the cracking of ancient reservoirs and has undergone a process of evolution involving light oil, condensate gas, wet gas, and dry gas.

These findings of the investigation conducted in the Puguang area can provide a foundation for the investigation of natural gas accumulation within the Changxing–Feixianguan Formation. Future natural gas exploration efforts should focus on identifying the paleostructure that may have served as an ancient oil reservoir to enhance the precision of natural gas exploration.

Funding information: This work was funded by the doctoral research project of the Henan University of Urban Construction, China (No. KQ2022012, KQ2023019, KH2024012, KH2024013, and KH2024024) and the Key Scientific Research Project of Henan Higher Education Institutions (24B410001).
Author contributions: Songfeng Li contributed significantly to the analysis, review, and editing of this manuscript. Xiaoxia Liu, Peixin Zhang, and Yinan Zhao helped perform data curation. Each author has thoroughly reviewed and concurred with the finalized version of the manuscript prior to publication.
Conflict of interest: The authors assert the absence of any conflicts of interest.

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Received: 2023-09-08

Revised: 2023-12-13

Accepted: 2024-06-10

Published Online: 2025-01-21

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/geo-2022-0658

Keywords for this article

carbon isotope; gas source comparison; inclusion; accumulation period; Puguang area

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