Geochemical Characteristics and Genetic Origin of Tight Sandstone Gas in the Daning–Jixian Block, Ordos Basin

Abstract

Tight sandstone gas constitutes a strategically significant resource in the exploration of unconventional hydrocarbon systems. Current understanding of the geochemical composition and genesis of tight sandstone gas in the Daning–Jixian Block, southeastern Ordos Basin, is insufficient, which hampers a comprehensive assessment of its resource potential. This study is the first to systematically investigate the geochemical characteristics and genetic origin of high-maturity tight sandstone gas in the southeastern Ordos Basin’s Daning–Jixian Block. Gas specimens were systematically acquired from multiple stratigraphic units within the reservoir interval and subjected to compositional and carbon–hydrogen isotope analysis. Compared with other gas fields in the Ordos Basin, the Daning–Jixian Block has higher average methane concentration, and notably lower ethane and propane concentrations; its average δ¹³C₁ and δ²H-CH₄ is heavier, while δ¹³C₂ and δ¹³C₃ are lighter. Based on the δ¹³C₁-δ²H-CH₄ diagram, all gas samples from the block and other basin gas fields fall into the geothermal, hydrothermal and crystalline gas domain, indicating gas genesis associated with over-mature organic matter interacting with external hydrogen. Milkov genetic diagram analysis reveals that the natural gas consists of primarily early-stage kerogen-cracking gas, with a minor contribution from crude oil-derived gas originating from Carboniferous–Permian source rocks. Notably, samples from Daning–Jixian exhibit a unique δ¹³C₁ > δ¹³C₂ reversal, attributed to mixing effects between gas from highly mature kerogen and gas from secondary cracking of crude oil. Consequently, ethane carbon isotopes alone are insufficient for definitive genetic classification. These findings provide a new geochemical interpretation framework for analogous high-maturity tight gas reservoirs.

1. Introduction

The growing global demand for clean energy has positioned unconventional energy resources as a focal point in energy research [1]. Tight sandstone gas, a subtype of tight gas and a significant component of unconventional natural gas resources, predominantly occurs in sandstone reservoirs characterized by permeability ranging from low to ultra-low levels [2]. Due to the inherently tight geological characteristics of such reservoirs, natural gas production from individual wells is generally low without artificial stimulation. Consequently, large-scale hydraulic fracturing and reservoir engineering techniques are essential to achieve economically viable gas extraction [3,4,5]. Tight sandstone gas is widely distributed across numerous petroliferous basins worldwide with abundant resources [6], serving as a significant replacement for conventional oil and gas energy. The Ordos Basin, recognized as a pivotal hydrocarbon basin in China, hosts extensive tight sandstone gas reserves with significant exploitation potential [7], particularly in the Daning–Jixian area, which has attracted widespread attention and in-depth investigation [8,9].

In 2014, gas tests conducted in the sandstone reservoirs within the Shanxi Formation of the Daning–Jixian Block yielded remarkable production rates, marking a breakthrough in the prospecting and exploitation of coal-derived tight gas reservoirs within this region [10]. The Shan 2³ gas layer recorded an initial average daily gas production of 23,469.23 m³/d in the first year [11]. Previous studies in the Daning–Jixian area have primarily focused on petrological characteristics, pore structure, reservoir physical properties, and their controlling factors [12,13,14,15], providing crucial insights for reservoir characterization and sweet-spot prediction.

The formation of tight-sandstone gas accumulations is controlled by a suite of interrelated factors, encompassing the organic-rich source rocks’ hydrocarbon generation potential, the diagenetic compaction and cementation processes that reduce reservoir permeability, and mechanisms of gas migration and accumulation [16,17,18]. Diverse source rocks—including marine and terrestrial organic-rich mudstones and shales, as well as coal seams—provide abundant gas sources for tight sandstone gas systems. Although substantial work has focused on the geochemistry and origins of tight gas in the Ordos Basin (e.g., in the Zizhou, Mizhi, Yulin, Sulige, Wushenqi, and Shixi gas fields) [19,20], most studies have concentrated on the central-northern basin sectors, resulting in few published studies on the geochemical mechanisms in the southeastern Daning–Jixian Block. Comprehensive investigation of geochemical signatures of tight sandstone gas can elucidate its formation mechanisms and provide guidance for future exploration and development in the Daning–Jixian area. The novelty lies in providing the first comprehensive genetic interpretation of high-maturity tight sandstone gas in this block, offering a scalable reference for similar highly mature to over-mature coal-measure tight gas systems.

2. Geological Setting

The Daning–Jixian Block is situated on the eastern margin of the Ordos Basin. The Upper Paleozoic stratigraphic sequence in this area consists, in ascending stratigraphic order, of the Carboniferous Benxi Formation, followed by the Permian Taiyuan, Shanxi, Shihezi, and Shiqianfeng Formations, with a cumulative sedimentary thickness of approximately 800 m [21].

In the study area, the Upper Paleozoic source rocks are classified as coal-bearing successions deposited in marine–terrestrial transitional settings, dominated by Carboniferous–Permian coal seams, carbonaceous mudstones, and dark mudstones [22]. The principal gas-bearing intervals for tight sandstone gas include the Benxi and Taiyuan Formations, along with the Shan2, Shan1, and He8 Members. The reservoirs are buried at depths of 1745–2680 m, with measured permeability and porosity averaging 0.46 mD and 6.9% [23].

3. Sampling and Analytical Methods

Distinct well sites within the study area exhibit variability in their production layers. Natural gas samples were harvested at the wellhead and subsequently submitted to Geological Experiment Testing Center of Xi’an Shiyou University for determinations of gas composition and C-H isotope ratios. The natural gas samples collected at the wellhead were pure gas that had undergone prior gas–water separation at the production facility, ensuring the absence of formation water, free water, or condensate. Since the samples were already free of moisture and liquid hydrocarbons, no additional drying process was required. Gas composition was ascertained via an Agilent 7890B gas chromatograph in accordance with the Chinese National Standard “Analysis of Natural Gas Composition—Gas Chromatography” (GB/T 13610-2020) [24]. The analytical precision for CH₄ was determined to be ±0.5%, ensuring the accuracy and reliability of the compositional analysis results. Carbon and hydrogen isotopes were analyzed using an isotope ratio mass spectrometer (Isoprime VISION), adhering to the petroleum and natural gas industry standards “Analysis Method for Carbon and Oxygen Isotopes in Organic Matter and Carbonate” (SY/T 5238-2019) [25,26] and “Analysis of Hydrogen Isotope in Petroleum, Rock Extracts, and Kerogen”(SY/T 7313-2016) [27]. For carbon isotope determination, a continuous flow analysis approach was employed. Carbon and hydrogen isotope values are expressed in per mil (‰), referenced against the VPDB and SMOW international standards, respectively. Analytical results are systematically compiled in

Table 1. Gas composition testing results (unit: %).

Formation	Type	CO2	N2	CH4	C2H6	C3H8	i-C4H10	n-C4H10	i-C5H12	n-C5H12	C2+	C1–5	Dryness Index
Benxi Formation	min	0.15	0.00	93.26	0.09	0.01	0.00	0.00	0.00	0.00	0.12	93.38	99.32
	max	6.62	0.61	98.86	0.62	0.03	0.02	0.00	0.00	0.00	0.65	99.23	99.87
	aver	3.10	0.20	96.32	0.37	0.02	0.00	0.00	0.00	0.00	0.39	96.71	99.60
Taiyuan Formation	min	1.52	0.00	97.51	0.20	0.01	0.00	0.00	0.00	0.00	0.21	97.76	99.63
	max	2.24	0.28	98.06	0.34	0.02	0.00	0.00	0.00	0.00	0.36	98.39	99.79
	aver	1.85	0.08	97.78	0.28	0.02	0.00	0.00	0.00	0.00	0.30	98.07	99.70
Shan2 Member	min	1.28	0.00	97.33	0.22	0.01	0.00	0.00	0.00	0.00	0.23	97.61	99.24
	max	2.15	0.44	98.05	0.71	0.04	0.00	0.00	0.00	0.00	0.75	98.28	99.77
	aver	1.83	0.14	97.54	0.47	0.02	0.00	0.00	0.00	0.00	0.49	98.03	99.50
Shan1 Member	min	0.48	0.00	97.64	0.30	0.01	0.00	0.00	0.00	0.00	0.31	98.08	99.11
	max	1.64	0.70	98.45	0.79	0.06	0.01	0.01	0.01	0.00	0.88	99.00	99.68
	aver	1.03	0.39	98.01	0.53	0.04	0.00	0.00	0.00	0.00	0.58	98.58	99.42
He8 Member	min	0.61	0.29	97.04	0.31	0.02	0.00	0.00	0.00	0.00	0.33	97.37	99.08
	max	2.32	1.48	97.90	0.84	0.06	0.01	0.01	0.00	0.00	0.91	98.68	99.66
	aver	1.17	0.69	97.44	0.65	0.05	0.00	0.01	0.00	0.00	0.70	98.14	99.29
No. 8 Coal	min	1.22	0.06	93.91	0.05	0.00	0.00	0.00	0.00	0.00	0.05	93.98	100
	max	5.92	0.20	97.94	0.43	0.02	0.00	0.00	0.00	0.00	0.45	98.39	100
	aver	4.74	0.10	95.00	0.11	0.00	0.00	0.00	0.00	0.00	0.11	95.11	100

and

Table 2. Gas isotope testing results.

Formation	Type	δ 13C1 (‰)	δ 13C2 (‰)	δ 13C3 (‰)	δ 13CiC4 (‰)	δ 13CnC4 (‰)	δ 13CCO2 (‰)	δDC1 (‰)
Benxi Formation	min	−29.30	−36.70	−39.20	−34.50	−38.40	−2.70	−161.80
	max	−27.70	−29.30	−31.70	−31.50	−28.70	5.20	−169.70
	aver	−28.40	−34.23	−36.17	−33.50	−34.95	1.85	−165.13
Taiyuan Formation	min	−29.30	−35.50	−32.90	−39.50	−27.30	−4.50	−165.80
	max	−28.30	−32.80	−30.60	−22.50	−23.00	1.30	−169.50
	aver	−28.78	−34.36	−32.00	−29.90	−25.10	−0.58	−167.54
Shan2 Member	min	−29.20	−36.70	−37.70	−42.90	−35.60	−4.70	−165.80
	max	−27.20	−31.80	−31.80	−22.50	−22.40	−1.20	−170.90
	aver	−28.49	−35.44	−35.97	−33.16	−29.60	−2.71	−168.16
Shan1 Member	min	−28.70	−30.90	−31.40	−30.00	−29.50	−7.80	−166.40
	max	−26.20	−28.60	−27.90	−15.70	−24.60	−2.70	−169.60
	aver	−27.70	−30.13	−29.88	−24.75	−26.93	−5.00	−168.40
He8 Member	min	−27.90	−32.80	−33.80	−29.30	−34.40	−13.30	−166.10
	max	−26.50	−31.30	−30.60	−22.90	−20.50	−0.50	−168.20
	aver	−27.36	−31.96	−32.18	−27.25	−30.04	−5.44	−167.23

[28].

4. Discussion

4.1. Natural Gas Composition Characteristics

In the tight sandstone gas reservoirs from the Benxi Formation to the He8 Member in the Daning–Jixian Block, the methane (CH₄) content consistently exceeds 93%, with an average value higher than the 95% methane content reported for coalbed methane from the No. 8 coal seam of the Taiyuan Formation in this area. The ethane (C₂H₆) concentration ranges between 0.09% and 0.84%, while propane (C₃H₈) levels vary from 0.01% to 0.06%. The ethane-to-propane ratio falls within 9.00–30.00, and the dryness coefficient (C₁/∑C₁–C₃) is universally above 99%, classifying the gas as dry gas. Comprehensive analysis of methane concentration, dryness coefficient, and the ethane/propane ratio indicates that the natural gas from the Daning–Jixian Block shares characteristics with the Daniudi gas field (Ordos Basin) and is classified as coal-derived cracked gas (Table 1) [28,29].

Comparative assessment with other gas fields across the Ordos Basin indicates that the Daning–Jixian Block exhibits a comparatively higher average methane concentration, whereas the Linxing and Sulige gas fields demonstrate relatively lower methane levels. The average ethane and propane concentrations in the Daning–Jixian Block are notably lower than those in other gas fields; specifically, the Mizhi and Sulige gas fields register higher ethane concentrations, and the Linxing block shows elevated propane levels. Furthermore, the dryness coefficient of the gas in the Daning–Jixian Block is significantly greater than that observed in other gas fields. The Shixi Block, owing to its geographical proximity to the Daning–Jixian Block, displays analogous profiles in hydrocarbon gas concentrations [19,20,28,29,30,31] (Figure 1).

Based on reservoir characterization investigations conducted in the Yanchang exploration block of the southeastern Ordos Basin by Zhou et al. [32], dark mudstones and coal measures in the Benxi Formation and Shan2 Member serve as the primary source rocks. The Benxi Formation and Shan2 Member of the Daning–Jixian Block belong to source-in situ accumulation, whereas Shan1 Member and He8 Member, situated above the main source rocks, represent near-source accumulation with primarily vertical charging through secondary migration. Along the migration pathway in coal-measure systems where CO₂ is consumed by methanogenesis or mineral reactions, N₂ concentration tends to increase while CO₂ concentration decreases [30]. The generally lower CO₂ and higher N₂ content observed in He8 Member and Shan1 Member suggest vertical migration of natural gas into these intervals (Figure 2) [33].

4.2. Stable Isotopic Characteristics

In the tight sandstone gas reservoirs of the Daning–Jixian Block, the content of propane, and especially butane and heavier hydrocarbons, is very low. Consequently, the measured stable carbon isotope values for these components are for reference only (Table 2). The natural gas in the Upper Paleozoic tight sandstone gas reservoirs of the Daning–Jixian Block exhibits average δ¹³C₁, δ¹³C₂, and δ¹³C₃ values of −28.20‰, −33.64‰, and −33.77‰, respectively. The average δ¹³C₁ value is heavier compared to other gas fields, while the average δ¹³C₂ and δ¹³C₃ values are significantly lighter [19,20,28,29,30,31] (Figure 3a–c). In contrast, coalbed methane (CBM) in this region exhibits significantly lighter δ¹³C₁ values, spanning −72.35‰ to −27.43‰, with mean values of −40.53‰ (Coal Seam No. 5) and −41.88‰ (Coal Seam No. 8) [34]. During the over-mature stage (Ro > 2.0%), organic matter mainly generates methane, also known as dry gas [35]. Due to the very high maturity and the dry composition of the natural gas in the study area, hydrogen isotopes were only detected for methane, with values primarily ranging from −161.8‰ to −170.9‰ and an average of −167.34‰. The abundance of other components is too low for reliable hydrogen isotope analysis; any detected values are for reference only. The average δ²H-CH₄ value is heavier than that of other gas fields [19,20,28,29,30,31] (Figure 3d). Theoretically, methane carbon isotope values decrease along the gas migration pathway. However, the relatively heavier isotopic values observed in He8 Member and Shan1 Member compared to other layers suggest a complex scenario involving both dynamic fractionation and migration fractionation effects [28,30].

4.3. Analysis of Origin and Genetic

Based on the analysis of natural gas compositional data and hydrocarbon carbon and hydrogen isotopic compositions, combined with classical diagnostic diagrams, the genetic origin of natural gas can be discriminated and interpreted. Building upon previous research, this study further conducts a targeted analysis by employing diagnostic diagrams specifically applicable to the study area, aiming to provide more accurate insights into the genesis and origin of natural gas in the Daning–Jixian Block.

4.3.1. Genetic Analysis

The carbon isotope diagram (δ¹³C₁–δ¹³C₂–δ¹³C₃) established by Dai Jinxing et al. [36] was utilized for comparative analysis of the Daning–Jixian area, and other gas fields in the Ordos Basin revealed distinct characteristics in the Daning–Jixian Block. The natural gas samples from this block did not plot within any defined genetic identification zones, yielding inconclusive results [28]. Furthermore, on the diagrams based on δ¹³C₁ and C₁/(C₂ + C₃) proposed by Bernard et al. [37] and Whiticar [38], the natural gas from the Daning–Jixian Block is plotted within or adjacent to the high-maturity thermogenic gas domain characteristic of Type III kerogen [28]. Consequently, the application of these templates for genetic identification of the tight sandstone gas in the Daning–Jixian Block also exhibits limitations.

In the present study, the classical Whiticar diagram based on δ¹³C₁ and δ²H-CH₄ was employed [38]. Results demonstrate that all gas samples from the Daning–Jixian Block and other gas fields across the Ordos Basin fall within the domain of geothermal, hydrothermal, and crystalline gases. This suggests that the gas genesis is related to the interaction of over-mature organic matter with external hydrogen [20] (Figure 4).

Milkov et al. [39] further revised previous genetic identification diagrams using a global dataset of 20,621 natural gas samples, encompassing 76 countries and regions across six continental landmasses. On the Milkov diagram, the gas samples from the study area entirely fall within the field of thermogenic gas with relatively high maturity (Figure 5a). The gas samples from other areas in the Ordos Basin demonstrate distribution within the thermogenic gas domain on the Milkov diagram, exhibiting comparatively lower thermal maturity. Notably, samples from fields like Daniudi and Zizhou also exhibit characteristics associated with oil-associated gas. The genetic identification diagram based on methane carbon and hydrogen isotopes also shows that the tight sandstone gas samples from Ordos Basin belong to thermogenic gas, with the Daning–Jixian Block exhibiting higher maturity (Figure 5b).

4.3.2. Origin Analysis

The primary cracking of kerogen marks the stage of abundant methane generation, leading to a rapid increase in the methane-to-ethane concentration ratio. In contrast, crude oil-cracked gas shows a significant increase in propane content. Therefore, the cross-plot of ln(C₁/C₂) versus ln(C₂/C₃) is often used as a key indicator for genetic characterization of natural gas [40,41,42]. The natural gas from the Daning–Jixian Block demonstrates markedly elevated ln(C₁/C₂) and ln(C₂/C₃) relative to other gas fields, a phenomenon attributed to its advanced thermal evolution stage. The variation range of ln(C₂/C₃) differs among formations, with the Benxi Formation and Shan1 Member showing a larger range, suggesting that the tight gas accumulation was predominantly sourced from the primary cracking of kerogen, with a potential minor contribution from crude oil-cracked gas (Figure 6).

Previous research results indicate the presence of carbonaceous asphalt inclusions in the study area, interpreted as thermally mature residues from crude oil cracking during the terminal Early Cretaceous. During this period, the paleo-temperature in the range of 160–200 °C caused the complete transformation of crude oil into cracked gas and carbonaceous asphalt [43]. Geochemical cross-plots of samples from the study area show an intermediate position, suggesting contribution from both Type II and Type III kerogen-derived gases [28]. This finding is consistent with research results from the Zhidan–Ganquan area [42]. In summary, the Daning–Jixian Block gas accumulations originate from early-stage kerogen cracking in Carboniferous–Permian source rocks, with a subordinate contribution from crude-oil cracking.

4.4. Stable Carbon Isotope Series

Previous studies have shown that a reversed sequence of primary stable carbon isotopes and δ¹³C₁ values greater than −30‰ are key indicators for identifying inorganic genesis gas [44,45,46]. The distribution patterns of among light hydrocarbons (C₁–C₃) can be influenced by various secondary processes such as mixing, high temperatures, bacterial oxidation, and diffusion [47]. Significant variations in carbon isotopic compositions are observed across different strata in the Daning–Jixian Block. Gases from the Benxi Formation and the Shan2 Member predominantly exhibit a fully reversed sequence (δ¹³C₁ > δ¹³C₂ > δ¹³C₃). In contrast, samples from the Taiyuan Formation, along with some from the Shan 1 Member and He 8 Member, display a partially reversed sequence (δ¹³C₁ > δ¹³C₂ < δ¹³C₃). Compared to other gas fields in the Ordos Basin, the Upper Paleozoic natural gases from the Dongsheng, Linxing, Daniudi, Yulin, Zizhou, and Mizhi gas fields, located in the northern and eastern parts of the basin, primarily show a normal carbon isotopic sequence (δ¹³C₁ < δ¹³C₂ < δ¹³C₃). A partially reversed sequence (δ¹³C₁ < δ¹³C₂ > δ¹³C₃) is observed in some samples from the central basin areas, including the Sulige and Wushenqi gas fields, as well as parts of the Shixi Block [19,20,28,29,30,31].

This comparative analysis reveals that the Daning–Jixian Block in the southeastern basin exhibits characteristics distinct from other gas fields. All samples from this block display a δ¹³C₁ > δ¹³C₂ reversal. Furthermore, the difference between δ¹³C₂ and δ¹³C₃ values is generally small, whereas a more substantial isotopic discrepancy exists between δ¹³C₁ and the heavier homologues (δ¹³C₂, δ¹³C₃) (Figure 7).

A systematic southward increase in source rock maturity is observed within the Ordos Basin. Consequently, the carbon isotope distribution pattern of methane and its homologues evolves gradually from a positive carbon isotope sequence in the north, to a partially reversed sequence, and finally to a fully reversed sequence (i.e., a negative carbon isotope sequence) [47,48]. This trend can be attributed to the commingling of natural gases with distinct genetic origin [49]. Geochemically, the southeastern Ordos Basin features source rocks enriched in Type III kerogen, which is currently within the high maturity to over-mature thermal evolution stage, resulting in significant dry gas generation [22]. Therefore, based on an integrated assessment, the reversal of stable carbon isotopes observed in the study area’s multi-stratigraphic natural gases is interpreted to be associated with the elevated thermal maturity of the kerogen and mixing with crude oil-cracking gas.

The ethane carbon isotope signature (δ¹³C₂) serves as a key proxy for discriminating between coal-derived gas and oil-derived gas, employing a widely recognized benchmark wherein values heavier than −28‰ are diagnostic of the former, and values lighter than −28‰ suggest the latter [47]. However, all δ¹³C₂ values in the Daning–Jixian Block are less than −28‰ (Figure 8). This is attributed to the exceptionally high thermal maturity, which has caused reversal of the ethane carbon isotope values, leading to the lighter ethane carbon isotope signatures observed [29]. This aligns with the findings of Peng et al. [50], who reported that both coal-derived gas and oil-derived gas exhibit alkane gas carbon isotope sequence reversal during the high-maturity-to-over-mature stage. Consequently, the ethane carbon isotope value alone cannot be reliably used for delineation of the natural gas provenance in the study area.

5. Conclusions

This study systematically investigates the geochemical characteristics and genetic mechanisms of natural gas in the Paleozoic sandstone reservoirs of the Daning–Jixian Block, southern Ordos Basin, based on molecular composition and stable isotope (δ¹³C_1–3, δD-CH₄) data. The key conclusions are as follows:

(1)

Comparative assessment with other gas fields across the Ordos Basin indicates that the Daning–Jixian Block exhibits a comparatively higher average methane concentration. The average ethane and propane concentrations in the Daning–Jixian Block are notably lower than those in other gas fields. Furthermore, the dryness coefficient of the gas in the Daning–Jixian Block is significantly greater than that observed in other gas fields. The generally lower CO₂ and higher N₂ content observed in He8 Member and Shan1 Member suggest vertical migration of natural gas into these intervals.

(2)

The natural gas in the Upper Paleozoic tight sandstone gas reservoirs of the Daning–Jixian Block exhibits average δ¹³C₁, δ¹³C₂, and δ¹³C₃ values of −28.20‰, −33.64‰, and −33.77‰, respectively. The average δ¹³C₁ value is heavier compared to other gas fields, while the average δ¹³C₂ and δ¹³C₃ values are significantly lighter. The average δ²H-CH₄ value is heavier than that of other gas fields. The relatively heavier isotopic values observed in He8 Member and Shan1 Member compared to other layers suggest a complex scenario involving both dynamic fractionation and migration fractionation effects.

(3)

The high maturity of kerogen in the Daning–Jixian area limits the application of some commonly used genetic identification diagrams. The classical Whiticar diagram based on δ¹³C₁ and δ²H-CH₄ was employed, revealing that all gas samples from the Daning–Jixian Block and other gas fields across the Ordos Basin fall into the domain of geothermal, hydrothermal, and crystalline gases, which suggests that the gas genesis is associated with the interaction of over-mature organic matter with external hydrogen. The Milkov diagram indicates that natural gases sampled from the other producing fields in the Ordos Basin consistently fall within the canonical coal-derived gas field with relatively lower maturity, with some samples from fields like Daniudi and Zizhou also exhibiting characteristics of oil-associated gas. In contrast, the study area contains thermogenic gas with higher maturity. The Daning–Jixian Block gas accumulations originate from early-stage kerogen cracking in Carboniferous–Permian source rocks, with a subordinate contribution from crude-oil cracking.

(4)

The Daning–Jixian Block exhibits characteristics distinct from other gas fields. All samples from this block display a δ¹³C₁ > δ¹³C₂ reversal. This is interpreted to be associated with the elevated thermal maturity of the kerogen and mixing with crude oil-cracking gas. This process causes the reversal and lighter values of ethane carbon isotopes. Consequently, the ethane carbon isotope value alone cannot be reliably used for delineation of the natural gas provenance in the study area.

These findings not only deepen the academic understanding of tight sandstone gas accumulation mechanisms in high-maturity coal-measure systems but also provide critical practical guidance for industrial exploration and development. First, they offer a reliable basis for resource potential evaluation and sweet-spot prediction—essential for guiding industrial exploration deployment in the Daning–Jixian Block and analogous regions. Second, the identification of geochemical tracers and the interpretation of δ¹³C₁ > δ¹³C₂ reversal mechanisms enable industrial sectors to accurately characterize gas accumulation processes, supporting targeted reservoir stimulation and optimization of development schemes to address challenges posed by ultra-low permeability.