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Frontiers in Earth Science ·

Volume 14 - 2026 | https://doi.org/10.3389/feart.2026.1861447

Effects of pressure on the stable carbon isotope composition of gas generated from continental type-I and type-III kerogen under semi-closed conditions

  • Yanju Li1,2*corresponding author
  • Hao Yang3
  • Zhonghong Ning4
  • Shengling Jiang1
  • Biyang Feng1
  • Xiulan Zhu1
  • Wanchang Zhang3
  • Xianghe Ma1
  1. 1School of New Energy, Longdong University, Qingyang, China
  2. 2College of Geographical Science, Qinghai Normal University, Xining, China
  3. 3School of Engineering and Technology, China University of Geosciences Beijing, Beijing, China
  4. 4Second Oil Production Plant, Changqing Oilfield Company, PetroChina, Qingyang, China
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Abstract

Hydrous semi-closed pyrolysis was performed to clarify how water pressure (WP, 50–1,200 bar) and lithostatic pressure (LP, 125–2000 bar) control molecular composition and stable carbon isotopes of thermogenic gas from lacustrine Type I and Type III kerogen, and to verify whether pressure triggers carbon isotope reversal. Immature calcareous shale (Type-I, Ro = 0.40%) and low-mature carbonaceous mudstone (Type-III, Ro = 0.68%) from the Liaohe Shahejie Formation were heated at 450 °C and 480 °C, respectively, with a constant pyrolysis duration of 10 h. All produced hydrocarbon gases display normal isotope sequences δ 13 C 1 <δ 13 C 2 <δ 13 C 3; no isotope reversal was observed under all experimental pressure conditions. For oil-prone Type-I kerogen: moderate water pressure (50–190 bar) increases δ 13 C 1 by accelerating oil cracking, while WP > 190 bar restrains secondary oil cracking and generates 12C-enriched methane, ethane, and propane. LP exerts dual controls: 125–625 bar LP gradually shifts the reactor toward a closed system and boosts oil cracking to form 13 Crich C 1 –C 2, whereas 625–2,000 bar LP suppresses oil cracking and lightens all gaseous δ 13 C values. For gas-prone Type-III kerogen, elevated WP facilitates primary kerogen cracking; progressive mixing of early-formed 13 C-enriched initial gas and late-stage 12 C-depleted cracking gas causes continuous depletion in 13 C 1. LP of 125–625 bar weakens hydrocarbon expulsion and hinders kerogen cracking, yielding heavier methane isotopes, while LP above 625 bar imposes negligible impacts on kerogen pyrolysis. Distinct pressure responses between the two kerogen types originate from their divergent hydrocarbon generation pathways (secondary oil cracking vs. primary kerogen breakdown). Our work demonstrates that pressure must be incorporated into natural gas maturity assessments using carbon isotope proxies, and confirms the favorable deep hydrocarbon resource potential of continental lacustrine source rocks in eastern China.

Keywords

1 Introduction

Stable carbon isotope geochemistry of thermogenic gaseous hydrocarbons has been extensively studied and widely applied to the evaluation of natural-gas maturity (Chung et al., 1988; James, 1990; Rooney et al., 1995; Tang et al., 2000; Dai et al., 2016; Ahmed et al., 2025; Ahmed et al., 2026). It has also become an important tool in shale-gas exploration because isotope reversals are commonly associated with mature and highly productive shale gas systems (Burruss and Laughrey, 2010; Chatellier et al., 2011; Tilley and Muehlenbachs, 2013). However, many studies have shown that maturity is not the only factor controlling the carbon-isotope composition of natural gas; kerogen type (Berner et al., 1995; Farouk et al., 2025a; Farouk et al., 2025b), post-accumulation biodegradation (Kinnaman et al., 2007; Meng et al., 2017), gas mixing (Burruss and Laughrey, 2010; Dai et al., 2012), and isotope fractionation during expulsion and migration (Takahashi and Suzuki, 2017; Sherif et al., 2025; Ping et al., 2017; Shuai et al., 2018; Zhang et al., 2017) can also affect the carbon-isotope composition of natural gas. In addition, isotopic fractionation of gaseous hydrocarbons may also be influenced by pressure.

Previous studies have focused mainly on the effect of pressure on hydrocarbon generation from organic matter (Hill et al., 1996; Carr et al., 2009; Wu et al., 2016; Wu et al., 2018a), whereas isotopic fractionation of gas under high-temperature/high-pressure conditions has received much less attention. Some researchers suggested that pressure can influence methane carbon-isotope fractionation by affecting the rates of methane-forming reactions, whereas its effect on ethane and propane is minor (Hill et al., 1996). Jiang et al. (1998) reported partially reversed carbon-isotope sequences in gases generated from peat at 1–2 GPa and 400 °C. Du et al. (2003) showed that pressure significantly affected gas isotopic composition under high-temperature (500 °C–700 °C) and high-pressure (1–3 GPa) conditions. In contrast, kinetic modeling by Shuai et al. (2006) suggested that pressure has a negligible effect on the carbon-isotope composition of methane. Xie et al. (2016) showed that methane became enriched in 13C as pressure increased from 200 to 900 bar and concluded that pressure affects methane carbon-isotope fractionation by changing the rates of methane-forming reactions, consistent with Hill et al. (1996). Overall, previous conclusions regarding pressure effects on isotopic fractionation of gaseous hydrocarbons remain inconsistent, possibly because the pressure applied in pyrolysis experiments differs from that experienced by source rocks in natural settings.

In geological basins, the pressure borne by source rocks mainly comprises lithostatic pressure and hydrostatic pressure (Hunt, 1996). Lithostatic pressure is generated by the load of the overlying strata and contributes to diagenesis and porosity reduction (Sun et al., 2015). Hydrostatic pressure is generated by pore water and can retard hydrocarbon generation and the aromatization of organic matter, pore water occupies intergranular pore space, reduces molecular collision frequency of organic macromolecules, and stabilizes unsaturated aromatic intermediates to slow down aromatization and heavy hydrocarbon cracking (Carr et al., 2009; Uguna et al., 2015; 2016). The respective effects of water pressure and lithostatic pressure on the carbon-isotope composition of gaseous hydrocarbons therefore require separate evaluation.

In China, source rocks deposited in continental environments have generated most of the country’s petroleum resources (Fan et al., 1980; Fu et al., 1988; Grice et al., 1998). Different terrestrial depositional settings have produced a wide range of source rocks, including lacustrine evaporites in the Jianghan Basin (Xiong et al., 2005), lacustrine oil shales of the Ordos Basin (Zhang et al., 2015), and coal-measure strata in the Turpan Basin (Huang et al., 1991). Continental organic-rich shales are an important component of these source rocks and play a significant role in established petroleum systems. Consequently, investigating the carbon-isotope composition of gaseous hydrocarbons generated from continental organic-rich shales is important for shale-gas exploration and for assessing deep petroleum potential in China.

Prior pyrolysis approaches include open, semi-open, semi-closed and closed systems, with distinct advantages and limitations, open-system pyrolysis continuously expels all products and ignores secondary cracking; closed systems trap all generated fluids and overestimate late-stage gas generation; semi-open setups permit continuous fluid loss but fail to mimic natural episodic hydrocarbon expulsion. The semi-closed hydrous pyrolysis adopted here sets a fixed expulsion threshold (20% of lithostatic pressure), allowing partial retention of generated oil/gas to replicate subsurface source-rock fluid retention conditions (Wu et al., 2016; Wu et al., 2018b; Takahashi and Suzuki, 2017). Water also exerts a marked influence on the carbon-isotope ratios of thermogenic gases (Gao et al., 2014; Wu et al., 2022). These observations suggest that semi-closed hydrous pyrolysis is more suitable for simulating the isotopic evolution and possible isotope reversal of natural gas.

To avoid biodegradation of the generated gas, pyrolysis temperatures of 450 °C and 480 °C were selected. Type-I kerogen is oil-prone and reaches intense secondary oil cracking at 450 °C; Type-III kerogen has higher thermal activation energy for primary gas generation, so 480 °C was applied to achieve equivalent hydrocarbon transformation extent and guarantee comparable pyrolysis progress between two kerogen types. Based on the issues outlined above, continental type-I and type-III kerogen were pyrolyzed under semi-closed conditions at water pressures of 50–1,200 bar and lithostatic pressures of 125–2000 bar, to decouple water and lithostatic pressure effects on molecular and carbon isotopic composition of gaseous hydrocarbons. The results have important implications for assessing shale-gas and deep-petroleum potential in China.

2 Materials and methods

2.1 Samples

Calcareous shale samples were collected from the Shu 13 drill core in the Shahejie Formation of the Liaohe Basin. The Liaohe Depression is a typical intracontinental rift lacustrine basin developed on the North China Craton, dominated by extensional tectonics during Paleogene deposition. The Shahejie Formation, the primary hydrocarbon source sequence of the basin, was deposited under alternating deep to semi-deep lacustrine reducing environments with frequent carbonate interlayers and terrigenous clastic input. Their geochemical parameters are listed in Table 1; analytical methods were described previously by Wu et al. (2018a). The geochemical data indicate that the calcareous shale contains immature type-I kerogen (VR = 0.40%Ro; Tmax = 434 °C). The high hydrogen index (HI = 792) further confirms that it is an oil-prone source rock (Table 1; Figure 1). Carbonaceous mudstone was collected from the Tao 10 drill core in the Shahejie Formation of the Liaohe Basin. This sample is mature (VR = 0.68%Ro; Tmax = 439 °C) and has a hydrogen index of 179, indicating the presence of type-III kerogen (Figure 1).

TABLE 1. Geochemical parameters of the simulation samples.

WellStratumDepth (m)LithologyTOC (%)HI (mg/g)OI (mg/g)S1 (mg/g)S2 (mg/g)Tmax (oC)VR (% Ro)Type of kerogenδ13C (vs ‰Pdb)
Shu13Es31,387–1,390Calcareous shale8.73792230.4269.24340.40I−27.9
Tao10Es33,404–3,412Carbonaceous mudstone32.2179136.2857.844390.68III−24.5
Hydrogen index (HI)versus oxygen index (OI) (**(A)**, after Peters et al., 1996), and HI versus Rock-Eval T<sub>max</sub> (**(B)**, after Espitalié et al., 1985; Meinhold et al., 2013) for the calcareous shale and carbonaceous mudstone.
Figure 1. Hydrogen index (HI)versus oxygen index (OI) ((A), after Peters et al., 1996), and HI versus Rock-Eval Tmax ((B), after Espitalié et al., 1985; Meinhold et al., 2013) for the calcareous shale and carbonaceous mudstone.

2.2 Experimental methods

2.2.1 Pyrolysis

Semi-closed pyrolysis experiments were conducted using a WYMN-3 high-temperature/high-pressure (HTHP) system (Figure 2), following the procedures of Wu et al. (2016), Wu et al. (2018a), Wu et al. (2018b) and Zhang et al. (2020). The original samples were crushed to approximately 10 mm and air-dried. About 30 g of sample fragments were used for each experiment. The reactor was evacuated, a specified lithostatic pressure was applied to the sample, and water was then introduced to establish the desired water pressure. The reactor was heated from room temperature to 200 °C at 1.5 °C/min, then to 300 °C at 5 °C/min, and finally to the target temperature, which was maintained for 10 h. Gas expelled from the reactor during pyrolysis was collected in a cold trap. After the experiment, the gas was released into a pre-evacuated gas holder, and its volume was measured at room temperature using a saturated brine displacement system. Experimental conditions, including water pressure, lithostatic pressure, and pyrolysis temperature, were set and recorded automatically. Details of the experimental conditions and hydrocarbon expulsion procedure are given by Wu et al. (2016), Wu et al. (2018b).

Schematic diagram of the WYMN−3 HTHP instrument (after Zhang et al., 2020).
Figure 2. Schematic diagram of the WYMN−3 HTHP instrument (after Zhang et al., 2020).

2.2.2 Carbon isotopic composition analysis

The molecular composition of expelled gas was analyzed following Wu et al. (2018a). The analytical uncertainty of gas yields was ±2% for each component, and the accuracy of vitrinite reflectance measurements was ±0.03%Ro.

Stable carbon-isotope compositions were analyzed using a Finnigan MAT Delta Plus mass spectrometer interfaced with an HP 5890 II gas chromatograph. Helium was used as carrier gas, and gaseous compounds were separated on an HP-PLOT-Q fused silica column (30 m × 0.32 mm × 20 μm). After chromatographic separation, the gases were converted to CO2 in a combustion interface and introduced into the mass spectrometer. The GC oven temperature was programmed from 35 °C to 80 °C at 8 °C/min and then to 260 °C (held for 10 min) at 5 °C/min. One milliliter of gas from each sample bottle was analyzed for carbon-isotope composition. Reported δ13C values represent the average of three measurements; additional duplicate or triplicate analyses were carried out when C2–C5 contents were very low. Analytical precision was ±0.5‰.

3 Results and discussion

3.1 Gas molecular composition

Gas yields for the calcareous shale and carbonaceous mudstone pyrolyzed under water pressures of 50–1,200 bar and lithostatic pressures of 125–2000 bar at 450 °C and 480 °C, respectively, under semi-closed conditions are listed in Table 2 and shown in Figures 3, 4. The major hydrocarbon gases generated from both shales were CH4 to n-C5H12 (C1–C5), together with C2H4, C3H6, i-C4H10, and i-C5H12. The main non-hydrocarbon gases were N2, H2, CO, and CO2, whereas H2S was negligible and is therefore not listed in Table 2.

TABLE 2. Molecular indices and stable isotope compositions of gases generated from type-I and type-III kerogen.

Experiment no.Temp a /Time (°C/h)LP b (bar)WP c (bar)Molecular composition (mmol/g TOCorginal)Carbon isotopic composition (δ13C vs. V-PDB, ‰)Molecular indices (mole basised)VR d
N2H2COCO2CH4C2H6C3H8iC4nC4iC5nC5C6+CH4C2H6C3H8CO2C1/C2C1/C3C2/C3
Calcareous shale (shu 13)δ13Ckerogen (‰ PDB) = −32.9111450/481,500500.05110.10730.02566.39461.44660.62870.49070.07670.17370.02560.05620.0143−42.0−32.2−29.50.872.302.951.281.96
1121200.04600.11760.02566.68601.46190.64920.48050.07150.15840.02560.05110.0133−40.6−31.5−29.60.912.253.041.351.94
1131900.05620.12780.02566.62461.49770.67470.53160.08690.16860.02560.05620.0138−39.3−31.0−29.61.392.222.821.271.96
1143250.04090.09710.02047.02851.33410.66450.48560.07150.14820.02040.05110.0128−39.5−31.6−29.81.042.012.751.371.97
1154750.04090.08180.02046.93651.27280.61850.47540.06640.14820.02040.05110.0128−41.3−32.1−29.71.362.062.681.301.94
1167000.04090.06130.02047.03871.09390.54180.40890.05110.12260.01530.04090.0102−42.1−32.2−29.70.862.022.681.332.01
1177000.03580.06650.02047.04891.10920.53670.42430.06130.13290.02040.04600.0112−42.4−32.7−30.10.892.072.611.26/
1181,0000.03580.05620.01536.93130.93030.46000.35780.05110.10730.01530.03580.0092−42.7−32.5−29.80.982.022.601.292.07
1191,2000.02560.04600.01537.05400.85360.39360.30160.04600.09200.01530.03070.0077−43.2−32.6−29.91.232.172.831.312.21
Carbonaceous mudstone (tao 10)δ13Ckerogen (‰ PDB) = −26.7611480/481,500500.17050.02500.03170.79910.16520.10490.07500.01830.02810.01030.02100.0429−31.7−26.4−24.4−11.21.572.201.402.08
6121200.17190.02900.03300.82050.17190.10890.07810.01920.02950.01070.02500.0438−32.7−26.5−24.1−11.11.582.201.392.07
6131900.17590.02720.02810.82990.15040.09290.06700.01610.02500.00940.02720.0455−32.9−26.1−23.9−11.11.622.241.392.05
6143250.17320.02500.02460.89060.12990.08040.05760.01380.02140.00800.02410.0388−32.5−25.8−24.2−11.01.622.261.402.07
6154750.16700.01650.02320.94550.11830.07590.05450.01340.02050.00760.01920.0335−33.3−26.2−24.4−10.81.562.171.392.12
6167000.17460.01380.02231.00540.11790.07280.05220.01250.01960.00710.01740.0317−34.1−26.8−24.4−11.01.622.261.39/
6177000.17190.01520.02101.05180.11700.07370.05180.01210.01880.00710.01610.0304−35.3−27.2−24.1−10.71.592.261.422.24
6181,0000.16920.01250.01881.07590.09870.06120.04380.01070.01650.00630.01520.0290−35.5−27.0−23.8−11.01.612.251.402.33
6191,2000.16790.00980.03171.15850.08040.06030.03040.01830.02810.01030.01290.0254−36.3−27.3−24.5−11.01.332.641.982.47

Temp, Temperature.

LP, Lithostatic Pressure.

WP, Water Pressure.

VR, Vitrinite Reflectance.

Gaseous hydrocarbons produced from calcareous shale and carbonaceous mudstone during pyrolysis: CH<sub>4</sub> **(A)**, C<sub>2</sub>H<sub>6</sub> **(B)**, C<sub>3</sub>H<sub>8</sub> **(C)**, C<sub>4</sub>H<sub>10</sub> **(D)**, C<sub>5</sub>H<sub>12</sub> **(E)**, and total C<sub>1‐5</sub> gaseous hydrocarbons **(F)**.
Figure 3. Gaseous hydrocarbons produced from calcareous shale and carbonaceous mudstone during pyrolysis: CH4 (A), C2H6 (B), C3H8 (C), C4H10 (D), C5H12 (E), and total C1‐5 gaseous hydrocarbons (F).
Ternary plots of **(A)** methane:ethane:propane, **(B)** ethane:propane:butanes and **(C)** n-pentane:n-butane:i-butane (molar basis, after Kotarba and Lewan, 2013).
Figure 4. Ternary plots of (A) methane:ethane:propane, (B) ethane:propane:butanes and (C) n-pentane:n-butane:i-butane (molar basis, after Kotarba and Lewan, 2013).

Marked differences in the yields of individual gaseous hydrocarbons were observed between type-I and type-III kerogen in both water-pressure and lithostatic-pressure experiments (Figure 3), confirming that gas generation under hydrous pyrolysis varies with kerogen type (Lewan and Ruble, 2002). In addition, type-I kerogen pyrolyzed at 450 °C generated more gas than type-III kerogen pyrolyzed at 480 °C, suggesting that the gas from type-I kerogen included an oil-cracking component. Lewan and Ruble (2002) proposed that 355 °C for 72 h represents the late stage of oil generation; thus, pyrolysis of type-I kerogen at 450 °C likely produced abundant oil-cracking gas, whereas gas-prone type-III kerogen would generate much less oil-cracking gas. This interpretation is consistent with the observations of (Wu et al., 2016; Wu et al., 2018b).

For type-I kerogen, less methane (Figure 3A), similar amounts of ethane (Figure 3B), and more propane (Figure 3C), n-butane (Figure 3D), and n-pentane (Figure 3E) were generated in the water-pressure experiments than in the lithostatic-pressure experiments. These results indicate that water pressure may retard the cracking of long-chain n-alkanes at 450 °C, consistent with the conclusion of Wu et al. (2016) that water pressure retards oil cracking. The total yield of C1–C5 gases (Figure 3F) follows the methane trend because methane is the dominant component. For type-III kerogen, however, the same trends were not observed for methane, ethane, propane, n-butane, or n-pentane between water-pressure and lithostatic-pressure experiments, supporting the interpretation that the gas generated from type-III kerogen is predominantly primary kerogen-cracking gas rather than oil-cracking gas.

Figure 3 shows the relative proportions of methane, ethane, and propane in gases generated from calcareous shale and carbonaceous mudstone under water pressures of 50–1,200 bar and lithostatic pressures of 125–2000 bar at 450 °C and 480 °C, respectively, under semi-closed conditions. In the methane:ethane:propane ternary diagram, the gases can be divided into four groups. The proportion of methane in gas from type-I kerogen is higher than that in gas from type-III kerogen in both water-pressure and lithostatic-pressure experiments (Figure 4A). In addition, for both kerogen types, gas from the water-pressure experiments contains less methane than gas from the lithostatic-pressure experiments. For oil-prone kerogens such as type-I, methane is generated mainly by oil cracking (Tissot and Welte, 1984). The relatively low methane yield and low dryness of hydrocarbon gas in the water-pressure experiments again suggest that water pressure retards oil cracking. This observation may also imply that lithostatic pressure enhanced oil cracking under the present experimental setup. However, this inference differs from the expectation based on Le Chatelier’s principle, according to which hydrocarbon-generation reactions are endothermic volume-expansion reactions that should be retarded by high pressure in sedimentary basins (Atkins and Paula, 2002; Carr et al., 2009; Uguna et al., 2016). In this study, the aim of the lithostatic-pressure experiments was to evaluate isotope effects under a constant pyrolysis temperature and water pressure, with the hydrocarbon-expulsion threshold set at 20% of the imposed lithostatic pressure. As lithostatic pressure increased from 125 to 625 bar, the experimental conditions therefore evolved gradually from semi-closed toward closed. Sandvik et al. (1992) suggested that little or no hydrocarbon expulsion may change reaction mechanisms by accelerating secondary oil cracking because of increased molecular collision frequency (Mallinson et al., 1992). Thus, the apparent enhancement of oil cracking over 125–625 bar likely reflects the change in experimental conditions rather than the direct effect of increasing lithostatic pressure. In the 625–2000 bar range, where the system behaved as closed, increasing lithostatic pressure retarded oil cracking and reduced gas yields (Wu et al., 2018a). Overall, the methane:ethane:propane proportions clearly distinguish both kerogen type and experimental condition.

In the ethane:propane:butanes ternary plot (Figure 4B), gases enriched in methane (Figure 4A) also show relatively high ethane proportions, corresponding to C2/C3 ratios of 2.3 or greater. Gas from type-III kerogen is slightly enriched in butanes. However, unlike the methane:ethane:propane plot, the ethane:propane:butanes ternary diagram does not clearly distinguish kerogen type or experimental condition.

In the n-pentane:n-butane:i-butane ternary plot (Figure 4C), gases from water-pressure experiments on type-III kerogen show relatively high n-pentane contents, consistent with the slight butane enrichment indicated by the ethane:propane:butanes plot (Figure 4B). In addition, gas from type-I kerogen shows a relatively high n-butane proportion, whereas gas from type-III kerogen shows a relatively high i-butane proportion. Similar to Figure 4B, however, the n-pentane:n-butane:i-butane ternary plot does not provide a clear distinction between kerogen types or experimental conditions.

Large amounts of non-hydrocarbon gases, mainly CO2, N2, CO, and H2, were generated during pyrolysis (Table 2). Carbon dioxide is the most abundant non-hydrocarbon gas in both water-pressure and lithostatic-pressure experiments for both kerogen types. Nevertheless, type-III kerogen, despite having a higher O/C ratio, generated less CO2 than type-I kerogen. Previous studies have shown that CO2 in natural gas may derive from thermogenic evolution of organic matter, decomposition of carbonate minerals, and, in some cases, microbial processes (Jenden et al., 1993; Kotarba, 1987; Kotarba, 2012; Sheng et al., 1997; Paull et al., 2000; Kotarba and Rice, 2001; Kotarba and Lewan, 2013). Microbial contributions are not relevant in the present experiments. The δ13C values of CO2 in water-pressure and lithostatic-pressure experiments with type-I kerogen range from +0.86‰ to +1.39‰, indicating that the CO2 was derived mainly from carbonate minerals in the calcareous shale, because thermogenic CO2 from organic matter is typically isotopically lighter (−12‰ to −8‰) than CO2 derived from carbonate decomposition (−5‰ to +4‰) (Hunt, 1996; Sheng et al., 1997; Paull et al., 2000; Kotarba and Lewan, 2004). In contrast, the δ13C values indicate that CO2 generated from type-III kerogen was derived mainly from organic matter.

Both humic and sapropelic organic matter can generate substantial amounts of nitrogen during thermogenic transformation (Krooss et al., 1995; Littke et al., 1995; Jurisch et al., 2012; Heim et al., 2012). NH4-rich illite can also release nitrogen during intense fluid-rock interaction (Lüders et al., 2005; Mingram et al., 2005). Some studies have further shown that nitrogen yield correlates positively with maturity when VR is below 4.0%Ro (Lewan and Kotarba, 2014). In the present study, type-III kerogen generated much more nitrogen than type-I kerogen. This may reflect the higher pyrolysis temperature used for type-III kerogen and/or a higher content of NH4-rich illite in the carbonaceous mudstone than in the calcareous shale (Wu et al., 2018a).

Hydrogen concentrations were markedly higher in gases generated from type-I kerogen than from type-III kerogen. In pyrolysis experiments conducted below 500 °C, molecular hydrogen is thought to derive mainly from cleavage of heteroatomic bonds and demethylation (Li et al., 2015; 2017). At temperatures above 500 °C, aromatization and condensation are inferred to be the main mechanisms of hydrogen generation (Li et al., 2015; 2017). In closed or semi-closed systems, H2 may also be consumed during secondary reactions, such as thermal cracking and alkene hydrogenation (Lewan, 1997). The greater amount of hydrogen generated from type-I kerogen is therefore probably related, at least in part, to its higher atomic H/C ratio (Table 1).

3.2 Carbon isotopic composition

The carbon-isotope compositions of gases generated from calcareous shale and carbonaceous mudstone under water pressures of 50–1,200 bar and lithostatic pressures of 125–2000 bar at 450 °C and 480 °C, respectively, under semi-closed conditions are listed in Table 2 and shown in Figures 5, 6. The δ13C values of the initial kerogen are also shown in Figure 5. As illustrated in Figure 6, the δ13C values of gaseous hydrocarbons approach those of the original kerogen with increasing carbon number. Propane generated from type-I and type-III kerogen is approximately 0‰–1‰ and 0‰–2‰ heavier than the corresponding initial kerogen, respectively. The gases can be divided into two groups in the carbon-isotope distribution plot (Figure 5): methane, ethane, and propane generated from type-III kerogen are isotopically heavier than those generated from type-I kerogen, supporting the conclusion that the initial kerogen exerts a primary control on the isotopic composition of the gases it generates.

Carbon-isotope composition of methane, ethane, and propane plotted against the reciprocal carbon number of gases generated from calcareous shale and carbonaceous mudstone.
Figure 5. Carbon-isotope composition of methane, ethane, and propane plotted against the reciprocal carbon number of gases generated from calcareous shale and carbonaceous mudstone.
Ternary plots of the stable carbon-isotope composition of gases generated in water-pressure and lithostatic-pressure experiments for type-I kerogen **(A)** and type-III kerogen **(B)**.
Figure 6. Ternary plots of the stable carbon-isotope composition of gases generated in water-pressure and lithostatic-pressure experiments for type-I kerogen (A) and type-III kerogen (B).

Methane, ethane, and propane generated from both type-I and type-III kerogen display a normal carbon-isotope sequence (δ13C1 < δ13C2 < δ13C3) in both water-pressure and lithostatic-pressure experiments. No isotope reversal was observed in the present study (Figure 5), indicating that neither water pressure nor lithostatic pressure causes partial isotope reversal in gases generated from type-I or type-III kerogen under semi-closed conditions at water pressures of 50–1,200 bar and lithostatic pressures of 125–2000 bar at 450 °C and 480 °C, respectively. These normal isotope distributions are consistent with the kinetic isotope effects expected during thermogenic gas generation from kerogen. An exception is that propane in three lithostatic-pressure experiments on type-I kerogen, and in several water-pressure and lithostatic-pressure experiments on type-III kerogen, is isotopically heavier than the original kerogen (Figure 5). These exceptions also occurred at WP and LP experiments of type-III kerogen (Figure 5). Such anomalies may reflect thermodynamic effects, as suggested by Galimov (1985), Galimov (2006). In addition, Lorant et al. (1998) showed that the δ13C values of individual gaseous hydrocarbons increase and diverge at high maturity under closed conditions, indicating that C2–C5 compounds can become more enriched in 13C than the original kerogen. Given the high pyrolysis temperatures used in this study (450 °C and 480 °C), the observation that propane is isotopically heavier than the initial kerogen is therefore reasonable.

Comparison of methane, ethane, and propane δ13C values plotted against the reciprocal of carbon number for the water-pressure and lithostatic-pressure experiments (Table 2; Figure 6) shows that ethane and propane generated under 410–2000 bar lithostatic pressure are isotopically heavier than those generated in the water-pressure experiments for type-I kerogen (Figure 6A). Hill et al. (1996) and Xie et al. (2016) suggested that pressure has little effect on the carbon-isotope ratios of ethane and propane under confined (closed) conditions. In the present study, however, water pressure and lithostatic pressure had clearly different effects on the stable carbon-isotope composition of gases generated from type-I kerogen under semi-closed conditions.

Ternary plots of the stable carbon-isotope composition of gases generated in the water-pressure and lithostatic-pressure experiments for type-I kerogen clearly show that δ13C1, δ13C2, and δ13C3 values are lower in the water-pressure experiments than in the lithostatic-pressure experiments (Figure 6A). This trend indicates that water pressure inhibits oil cracking and reduces the cracking extent, as the activation energy for carbon-carbon bond cleavage follows the sequence:12C–12C < 13C–12C < 13C–13C. Elevated water pressure stabilizes heavy hydrocarbon molecules and restricts secondary cracking of crude oil. Under such conditions, only low-energy 12C–12C bonds break preferentially, which lowers the δ13C1, δ13C2, and δ13C3 values. For type-III kerogen, δ13C1 and δ13C2 are lower in the water-pressure experiments than in the lithostatic-pressure experiments, whereas δ13C3 is higher (Figure 6B). These results suggest that water pressure promoted type-III kerogen cracking, producing 13C-enriched propane and 12C-enriched methane and ethane, whereas lithostatic pressure retarded type-III kerogen cracking, producing 13C-enriched methane and ethane but 12C-enriched propane. The contrasting behavior of δ13C1, δ13C2, and δ13C3 in type-I and type-III kerogen indicates that the effects of water pressure and lithostatic pressure on hydrocarbon generation depend on generation stage and reaction pathway; for example, water pressure retards oil cracking but promotes cracking of type-III kerogen.

Figure 6B shows only slight differences in the δ13C values of ethane and propane between gases from the water-pressure and lithostatic-pressure experiments for type-III kerogen. Overall, ethane from the water-pressure experiments is slightly isotopically lighter than that from the lithostatic-pressure experiments, whereas propane is slightly heavier. These results indicate that water pressure and lithostatic pressure exert subtly different effects on the carbon-isotope composition of gases generated from type-III kerogen. Takahashi and Suzuki (2017) showed that, during pyrolysis of coals, gas δ13C values shift when the simulation condition changes from semi-open to semi-closed. In this study, all water-pressure experiments were conducted under semi-closed conditions, whereas the lithostatic-pressure experiments evolved progressively toward closed conditions. The slight isotopic differences between water-pressure and lithostatic-pressure experiments may therefore reflect differences in the degree of openness of the reaction system.

3.3 The effect of water and lithostatic pressure on δ13C values of gas from type-I kerogen

Stable carbon-isotope compositions of gases generated in the water-pressure experiments for type-I kerogen are plotted against water pressure in Figure 7A. Methane δ13C values range from −43.2‰ to −39.3‰, with a maximum carbon isotope fractionation of 3.9‰. δ13C1 became progressively heavier as water pressure increased from 50 to 190 bar and then steadily lighter as pressure increased further. A similar trend was reported by Xie et al. (2016), who observed initial 13C enrichment followed by depletion in methane generated from oil under 200–900 bar at 425 °C. The heaviest methane in the present study (−39.3‰) occurred at 190 bar Hill et al. (1996) observed an approximately 2‰ fraction in δ13C1 during oil pyrolysis at 90–2000 bar and 380 °C, whereas Xie et al. (2016) reported a shift of about 3.5‰ at 390 °C–425 °C and 200–900 bar. The larger methane isotopic fractionation observed here, occurring at lower pressure, may reflect the fact that gas generated from type-I kerogen in the water-pressure experiments was a mixture of primary gas and oil-cracking gas. Primary gas may have been generated during the temperature-ramping stage, and the higher pyrolysis temperature used in this study may also have contributed to the stronger isotopic effect. As discussed in Section 3.1, the gas from type-I kerogen pyrolyzed at 450 °C was dominated by oil-cracking gas. The increase in methane yield therefore reflects an increase in the degree of crude-oil cracking, which explains why δ13C1 follows the methane-yield trend over 50–190 bar. Previous studies have suggested that high water pressure (>200 bar) retards oil cracking (Xie et al., 2016; Wu et al., 2016), implying a lower degree of oil cracking. This interpretation is consistent with the decrease in δ13C1 at 325–1,200 bar (Figure 7A). Ethane δ13C values range from −32.6‰ to −31.0‰, with a maximum carbon isotope fractionation of 1.6‰. δ13C2 became heavier as water pressure increased from 50 to 190 bar and then remained nearly constant at higher pressure. Propane δ13C values changed only slightly with increasing water pressure. These results are consistent with Hill et al. (1996), who concluded that pressure exerts little influence on the carbon-isotope ratios of ethane and propane. This suggests that water pressure has only a minor effect on ethane and propane isotopes under semi-closed conditions.

Stable carbon-isotope compositions and (i-C4 + i-C5)/(n-C4 + n-C5) ratios of gases generated in water-pressure and lithostatic-pressure experiments for type-I kerogen **(A,B)** and type-III kerogen **(C,D)**.
Figure 7. Stable carbon-isotope compositions and (i-C4 + i-C5)/(n-C4 + n-C5) ratios of gases generated in water-pressure and lithostatic-pressure experiments for type-I kerogen (A,B) and type-III kerogen (C,D).

Stable carbon-isotope compositions of gases generated in the lithostatic-pressure experiments for type-I kerogen are plotted against lithostatic pressure in Figure 7B. Methane δ13C values range from −42.3‰ to −38.3‰, with a maximum carbon isotope fractionation of 4.0‰. δ13C1 became progressively heavier as lithostatic pressure increased from 125 to 625 bar and then became progressively lighter above 975 bar. Ethane and propane show the same overall trend (Figure 7B). Lorant et al. (1998) suggested that the δ13C values of individual gaseous hydrocarbons diverge under closed conditions but converge under open conditions. Takahashi and Suzuki (2017) reported that, at the same maturity, methane, ethane, and propane generated under semi-closed conditions are isotopically lighter than those generated under semi-open conditions during pyrolysis of Eocene Yubari coal. These studies indicate that the openness of the reaction system significantly affects gas isotope compositions. In the present study, the pyrolysis conditions evolved gradually from semi-closed to closed as lithostatic pressure increased from 125 to 625 bar. Little or no hydrocarbon expulsion may change reaction mechanisms by accelerating secondary oil cracking (Sandvik et al., 1992). Accordingly, the isotopic enrichment observed over 125–625 bar can be attributed to enhanced oil cracking as the system became more closed. In the 625–2000 bar range, lithostatic pressure retarded oil cracking and lowered the δ13C values of gaseous hydrocarbons as pressure increased.

3.4 The effect of water and lithosatic pressure on δ13C values of gas from type-III kerogen

Previous studies have shown that type-III kerogen is gas-prone organic matter (Tissot and Welte, 1984; Clayton, 1991; Whiticar, 1996; Hunt, 1996). The mechanism of gas generation in carbonaceous mudstone may therefore differ from that in calcareous shale. Stable carbon-isotope compositions of gases generated in the water-pressure experiments for type-III kerogen are plotted against water pressure in Figure 7C.

Stable carbon isotope composition of gases generated from WP experiments of type-III kerogen were plotted in Figure 7C against the water pressure ranged from 50 to 1,200 bar. Methane δ13C values range from −36.3‰ to −31.7‰, with a maximum carbon isotope fractionation of 4.6‰. δ13C1 decreased as water pressure increased from 50 to 190 bar, remained nearly constant over 190–325 bar, and then decreased further above 325 bar (Figure 7C). In contrast, ethane and propane showed little systematic variation with water pressure. Based on the molecular-composition data, we infer that the gases generated from type-III kerogen at 480 °C are mainly primary kerogen-cracking gases. A decreasing trend in δ13C1 has also been reported by Lorant et al. (1998), Tang et al. (2000), Tocque et al. (2005), and Wang et al. (2013). By analyzing δ13C2–δ13C3 versus ln (C2/C3), Wang et al. (2013) showed that decreasing δ13C1 values are characteristic of the primary kerogen-cracking stage. Galimov, (2006) suggested that a natural sample selected for pyrolysis retains a prior transformation history, so the residual organic matter becomes relatively enriched in 13C and initially generates 13C-rich gas that subsequently mixes with newly generated 13C-depleted cracking gas. Lorant et al. (1998) further proposed that the initial decrease in δ13C1 may be related to the formation of 13C-rich methane from heteroatomic bonds (e.g., C–O and C–S bonds) at the beginning of kerogen cracking. The carbonaceous mudstone used here, with VR = 0.68%Ro (Table 2), indicates that the kerogen is in the oil-generation stage, as also supported by the maceral data (Figure 1). The interpretation proposed by Galimov (2006) is therefore reasonable for explaining the decrease in δ13C1 in the water-pressure experiments on type-III kerogen. Combined with the discussion in Section 3.1, the results suggest that water pressure promoted kerogen cracking, and that the observed decrease in δ13C1 reflects mixing between initially generated 13C-rich gas and newly generated 13C-depleted cracking gas as the contribution of the latter increased with water pressure.

Stable carbon-isotope compositions of gases generated in the lithostatic-pressure experiments for type-III kerogen are plotted against lithostatic pressure in Figure 7D. Methane δ13C values range from −35.2‰ to −31.3‰, with a maximum carbon isotope fractionation of 3.9‰. δ13C1 became progressively heavier as lithostatic pressure increased from 125 to 625 bar and then remained nearly constant at higher pressure. Ethane and propane also remained nearly constant. Previous studies have shown that removal of generated products from heated samples (Solomon et al., 1990) and increasing the amount of molecular hydrogen in the reactor (Hershkowitz et al., 1983) can promote kerogen cracking. In the 125–625 bar lithostatic-pressure range, the effect of hydrocarbon expulsion gradually weakened, which retarded primary kerogen cracking and reduced the amount of newly generated 13C-depleted gas. Consequently, the mixture contained a greater proportion of initially generated 13C-rich gas, leading to progressively heavier δ13C1 values with increasing lithostatic pressure over this interval. In the 625–2000 bar range, pressure had little effect on primary gas generation, and the δ13C values of the mixed gas remained nearly constant.

The marked differences in (i-C4 + i-C5)/(n-C4 + n-C5) ratios between gases generated from type-I and type-III kerogen (Figure 7) are consistent with the view that iso-alkane/normal-alkane ratios are largely independent of source material (Hunt et al., 1980; Mango, 1997). In addition, because n-alkanes and iso-alkanes differ in thermal stability, the (i-C4 + i-C5)/(n-C4 + n-C5) ratio can also be used as the index of gas maturity. In the present study, methane carbon-isotope trends are broadly similar to the trends of iso-alkane/n-alkane ratios, supporting our interpretation of the effects of water pressure and lithostatic pressure on the stable isotope composition of gases generated from type-I and type-III kerogen.

3.5 Geological application

Pyrolysis experiments are generally used to simulate the thermal transformation of source rocks through the combined effects of time and temperature (Lopatin, 1971; Waples, 1980). Although some studies have attempted to relate laboratory simulation temperatures to natural burial temperatures, such relationships may not be universally applicable because of differences in experimental conditions. Pressure, however, can be used to approximate burial depth. Previous studies suggested that oil cracking begins at depths of 5–6.7 km in basins with normal geothermal gradients (Hayes, 1991; Horsfield et al., 1992), corresponding to water pressures of 525–700 bar and lithostatic pressures of 1,115–1,500 bar, respectively (Powers, 1967; Bruce, 1984). In the present study, water pressures of 50–1,200 bar and lithostatic pressures of 125–2000 bar were used to simulate hydrocarbon generation during thermal transformation of source rocks and to investigate the effects of pressure on the stable carbon-isotope composition of gases generated from continental type-I and type-III kerogen.

Few data are available for natural gas in the Liaohe Basin at depths of 5–6.7 km, although previous studies have shown that gases from depths greater than 3.0 km are dominated by hydrocarbons, typically with C1/C1–5 ratios greater than 0.85 and only minor H2S, N2, H2, and CO2 contents (Zhang et al., 1995). Compared with these natural gases, the pyrolysis gases generated in this study contain more C2–C5 hydrocarbons, CO2, and N2. One possible explanation is that post-generation migration enriches natural gas in methane (Prinzhofer and Huc, 1995). The high CO2 contents of the pyrolysis gases may be related to reactions between bitumen and water dissolved in bitumen (Lewan, 1997), although such reactions can also occur in natural systems. Kotarba and Lewan, (2013) additionally emphasized the high aqueous solubility of CO2 and its diagenetic reactivity with minerals. Because CO2 is generated earlier than hydrocarbons, it also has sufficient time to escape from natural gas accumulations. Differences in migration behavior between CO2 and hydrocarbons may therefore contribute to methane enrichment in natural gas. Considerable amounts of H2 were generated during pyrolysis, whereas natural gases from the Liaohe Depression contain only trace H2. Because H2 is highly reactive, it can be consumed in secondary reactions; moreover, its low molecular weight favors escape from natural gas. The high nitrogen yields in the experiments may also reflect the high pyrolysis temperatures, consistent with the observation of Lewan and Birdwell (2013) that nitrogen yield correlates positively with maturity when VR is below 4.0%Ro.

Reported δ13C values of methane, ethane, and propane in natural gas from the Liaohe Basin range mainly from −48.9‰ to −31.9‰, −38.7‰ to −23.4‰, and −29.8‰ to −19.6‰, respectively (Wang et al., 2015). The maximum carbon isotopic fractionations of methane, ethane, and propane in natural gases are much larger than those observed in the gases generated here. Biodegradation and oxidation may account for the large isotopic ranges in natural gas from the Liaohe Depression (Wang and Xu, 1992; Chen et al., 1998). Although all pyrolysis gases in this study fall within the isotopic range of natural gases, this does not necessarily validate the experimental results, nor does it justify direct comparison between laboratory gases and natural gases. In general, pyrolysis gases are relatively enriched in 13C compared with natural gas generated from the same source rock at the same maturity (Tang et al., 2000).

Our results show that water pressure and lithostatic pressure exert significant effects on the δ13C values of gases generated from continental type-I and type-III kerogen under semi-closed conditions, indicating that pressure must be considered when stable carbon-isotope compositions are used to assess natural-gas maturity. The results also suggest that continental type-I and type-III kerogen have good potential for deep petroleum generation, with important implications for deep petroleum exploration in China.

4 Conclusion

  • Under semi-closed hydrous pyrolysis (water pressure: 50–1,200 bar; lithostatic pressure: 125–200 bar), gases generated from continental Type-I and Type-III kerogen both display a normal carbon isotope sequence δ13C1<δ13C2<δ13C3. Neither water pressure nor lithostatic pressure triggers carbon isotope reversal under the experimental conditions of this study.
  • Water pressure uniformly inhibits secondary oil cracking of Type-I kerogen: δ13C1 rises moderately at 50–190 bar and gradually lightens above 190 bar. Lithostatic pressure presents two-stage regulation on oil cracking: 125–625 bar facilitates oil breakdown and enriches gas in 13C, while pressures exceeding 625 bar restrain cracking and reduce δ13C values of methane, ethane and propane. This isotopic variation is fundamentally controlled by the differing cleavage activation energies of 12C–12C, 13C–12C and 13C–13C bonds.
  • For gas-prone Type-III kerogen, rising water pressure accelerates primary kerogen cracking; continuous mixing of early 13C-rich initial gas and late 12C-depleted cracking gas leads to progressive depletion of δ13C1. Within 125–625 bar lithostatic pressure, hydrocarbon expulsion weakens, primary kerogen degradation is suppressed, and methane becomes isotopically heavier; lithostatic pressure above 625 bar barely alters the carbon isotopic composition of C2–C3 hydrocarbons.
  • Kerogen type dominates the pressure response of gas isotopes (Type-I dominated by oil secondary cracking, Type-III dominated by primary kerogen cracking), while the openness of pyrolysis system acts as a secondary regulating factor. The differential isotopic responses to two pressure types demonstrate that burial pressure must be incorporated into natural gas maturity evaluation models, and continental lacustrine sequences in eastern China possess promising deep hydrocarbon exploration potential.

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Footnotes

Edited by: Ramanathan Alagappan, Independent Researcher, Tiruchirapalli, India

Reviewed by: Sherif Farouk, Egyptian Petroleum Research Institute, Egypt

Yinhua Pan, Guangxi Normal University, China

Statements

Conflict of interest

Author ZN was employed by Changqing Oilfield Company, PetroChina.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was supported by the National Natural Science Foundation of China (Grant Number: 52274012), Industry Support Plan for University of Gansu (Grant Number: 2024CYZC-55), and the Innovation Foundation of Higher Education in Gansu Province (Grant Number: 2025A-271).

Data availability

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

YL: Methodology, Writing – original draft. HY: Conceptualization, Writing – review and editing. ZN: Supervision, Writing – original draft. SJ: Formal Analysis, Writing – review and editing. BF: Data curation, Writing – review and editing. XZ: Writing – review and editing. WZ: Validation, Writing – original draft. XM: Writing – original draft, Software.

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Citation

Li, Y., et al. (2026). Effects of pressure on the stable carbon isotope composition of gas generated from continental type-I and type-III kerogen under semi-closed conditions. Frontiers in Earth Science, Volume 14. https://doi.org/
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Copyright © 2026 Li, Yang, Ning, Jiang, Feng, Zhu, Zhang and Ma.

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*Correspondence: Yanju Li

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