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10.1021/acsomega.4c05874
Article
Generation Potential and Kinetics of Gaseous Hydrocarbons from Upper Paleozoic Carboniferous Benxi Formation Mudstone in Ordos Basin, China
Liu Min †‡§
Kong Qingfen ∥
Wang Huaichang ∥
Zhang Hui ∥
https://orcid.org/0009-0004-7165-7437
Guo Huijuan *†‡
https://orcid.org/0000-0003-4164-9677
Wang Yunpeng †‡
Shi Shuyong †‡
Wang Qiang †‡
Liu Jinzhong †‡
Peng Ping’an †‡
† State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
‡ CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
§ University of Chinese Academy of Sciences, Beijing 100049, China
∥ Research Institute of Exploration and Development, PetroChina Changqing Oilfield, Xi’an 710018, China
* Email: guohuijuan@gig.ac.cn.
05 09 2024
17 09 2024
9 37 3908839099
24 06 2024
27 08 2024
18 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Industrial quantities of tight gas from the Carboniferous Benxi Formation have been found in the southern Ordos Basin. The source and contributions from mudstone and coal in the Benxi Formation to the tight gas are still unclear, and the hydrocarbon generation potential and kinetics of the Benxi Formation mudstone have rarely been reported, which has halted resource evaluation of tight gas. Confined pyrolysis experiments were performed to determine the yields and kinetic parameters for gaseous hydrocarbon formation for a representative sample of a Benxi Formation mudstone from the Ordos Basin, with a hydrogen index (HI) of 137 mg/g TOC and Tmax of 434 °C. The maximum yield of C1–C5 hydrocarbons is 143 mg/g TOC. For samples with similar HI, Upper Paleozoic mudstone samples might have a higher total gas generation potential than Upper Paleozoic coal samples because of the more reducing environment during mudstone deposition, which is beneficial for the preservation of sedimentary lipids. Even though the HI of Upper Paleozoic coals is apparently higher than that of Upper Paleozoic mudstones, they have a similar late gas generation potential. Basin modeling shows that the amount of natural gas generated from Benxi Formation mudstone increases southward. Coaly source rocks from the Ordos Basin with a lower oil generation potential have more negative δ13C1 values when the vitrinite reflectance is lower than 1.4–1.7%. This phenomenon might be related to the more negative δ13C of wet gas or the small hydrocarbon molecules incorporated into the kerogen compared with the components generating primary cracking gases. This study deepens the understanding of gaseous hydrocarbons generated from mudstone and coal in transitional depositional environments and provides the key parameters for tight gas resources in this area.

National Natural Science Foundation of China 10.13039/501100001809 42473032, 41803043 Theory of Hydrocarbon Enrichment under Multi-Spheric Interactions of the Earth NA THEMSIE04010104 Basic and Applied Basic Research Foundation of Guangdong Province 10.13039/501100021171 2021A1515012562 State Key Laboratory of Organic Geochemistry 10.13039/501100011215 SKLOG2020-1 Chinese Academy of Sciences 10.13039/501100002367 XDA14010103 document-id-old-9ao4c05874
document-id-new-14ao4c05874
ccc-price
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pmcIntroduction

The Ordos Basin is one of the largest petroliferous basins in China with abundant natural gas resources.1 Eight giant tight gas fields have been discovered so far, mainly in the central to northern parts of the basin, including the Sulige, Yulin, Wushenqi, Daniudi, Shenmu, Zizhou, and Mizhi gas fields in the Upper Paleozoic strata and the Jingbian gas fields in the Lower Paleozoic strata.2 Gases produced from the Upper Paleozoic are mainly sourced from coal-measured source rocks (coal, mudstone, and limestone) that are widely distributed in the Carboniferous Benxi Formation and the Permian Taiyuan and Shanxi formations.3 Industrial quantities of natural gas (tight sandstone gas) flow from the Benxi Formation in the southern part of the basin.4 It is still unclear about the source and contributions of mudstone and coal in the Benxi Formation to the tight gas. The coal seams in the southern part are generally too thin (0–2 m) to expel sufficient amounts of natural gas,5 so one of the main sources of tight gas might be the Benxi Formation mudstone.6

Clarifying the hydrocarbon generation potential and characteristics of coal-measured source rocks will help to accurately evaluate gas resources.7 Kinetic parameters obtained from short-term pyrolysis experiments on kerogen can be effectively used to simulate hydrocarbon generation over geological time.7−9 The kinetics of hydrocarbon generation of coal and mudstone samples from the Permian Shanxi and Taiyuan formations have been reported by previous studies.3,5,8,10 As the thermal maturity of the Benxi Formation source rocks is the highest among the Upper Paleozoic source rocks, it is difficult to obtain immature Benxi Formation samples appropriate for a pyrolysis study. Therefore, pyrolysis results from mudstone samples from the Benxi Formation have rarely been reported.

In the southern Ordos Basin, the sedimentary environments of the Benxi, Taiyuan, and Shanxi formations are different. During the period in the Carboniferous when the Benxi Formation was deposited, east–west tensile stress and basement subsidence resulted in a large-scale marine transgression. The sedimentary environment in the eastern Ordos Basin was barrier island coastal deposition with the deposition of marine-influenced mudstones and coal seams. The scale of seawater intrusion continued to increase during the deposition of the Permian Taiyuan Formation, resulting in a coastal sedimentary environment dominated by limestone. A continental lake-delta sedimentary system was developed in the basin during the period of the Permian when the Shanxi Formation was deposited.11 Wang et al. reported the kinetic study of hydrocarbon generation of a Taiyuan Formation mudstone, but the sample used was from an outcrop, so the organic matter composition might have been influenced by weathering.5

The Southern Ordos Basin is characterized by high thermal maturity, with a vitrinite reflectance (Ro) exceeding 2.0%. While traditionally the hydrogen index (HI) has been used to assess the gas generation potential, previous studies suggest a decoupling between HI and late gas generation in coals with such a high Ro (>2.0%).12 The focus may need to be shifted toward the influence of liquid hydrocarbon content and composition within the coal itself. Compared to coals, the Ordos Basin coal-measured mudstones generally exhibit lower HI values,8 reflecting distinct depositional environments and organic matter compositions. However, a critical knowledge gap remains regarding the relative late-stage gas generation potential of these mudstones compared to coals. Additionally, the key factors controlling this potential are not fully understood.

The dynamic depositional environments within the Southern Ordos Basin, characterized by relatively shallow water depths, have resulted in significant geochemical heterogeneity within both coals and mudstones. This heterogeneity manifests as substantial variations in total organic carbon (TOC) and HI.5,8 While previous research has often focused on representative basin-average TOC and HI values, a significant portion of the basin likely contains outlier samples with geochemical properties deviating from the mean. Understanding the gas generation potential of these outliers is essential for a comprehensive evaluation of the basin’s gas resources. In this work, a low-maturity Benxi Formation mudstone core sample was selected for a high-pressure gold-tube pyrolysis experiment so as to obtain the kinetic parameters of petroleum gas generation. The kinetic parameters were applied to the geological situation through basin modeling so as to simulate the gas generation processes of the Benxi Formation mudstone in the Ordos Basin. The hydrocarbon gas generation potentials of coal-measured source rocks of the Upper Paleozoic strata from the Ordos Basin were collected from the literature. The objectives of this study are to (1) evaluate the hydrocarbon potential of the Benxi Formation mudstone; (2) compare the hydrocarbon generation potential and carbon isotopes of gaseous hydrocarbons generated from the pyrolysis of Upper Paleozoic coal-measured source rocks from the Ordos Basin; and (3) compare the hydrocarbon generation process of the Benxi Formation mudstones in the northern, middle, and southern Yishan slopes in the Ordos Basin.

Geological Background

The Ordos Basin covers an area of 32 × 104 km2 and is located in northern-central China (Figure 1). It is an intracratonic basin with vast oil and gas reservoirs.1 The Ordos Basin is divided into six major tectonic depositional units: the Weibei uplift zone in the south, the Yimeng uplift zone in the north, the Western edge thrust belt and the Tianhuan depression in the west, the Yishan slope in the center, and the Jinxi folding belt in the east.13 During the Upper Paleozoic, the sedimentary environment gradually changed from a marine facies to a marine-continental transitional facies and then to a continental facies.14 The Upper Paleozoic strata in the study area consist of, from the bottom to the top, the Upper Carboniferous Benxi Formation and the Cisuralian Taiyuan and Shanxi formations (Figure 2).

Figure 1 Map showing the locations of the sampled wells and the main tectonic units in the Ordos Basin, China. The locations of wells Shen-27 (Sh-27), M-115, and Y-88 used for experiments on the hydrocarbon generation process and calculation of mass generated are also shown. Reproduced with permission from ref (8) Copyright 2018 ELSEVIER.

Figure 2 Generalized stratigraphic column of the Upper Paleozoic strata in the Ordos Basin. Reproduced with permission from ref (8) Copyright 2018 ELSEVIER.

During the Late Carboniferous, when the Benxi Formation was deposited (and also the time-equivalent Yanghugou Formation in the western Ordos Basin), the sedimentary system of both the western and eastern Ordos Basins was dominated by a marine-continental transitional environment.15,16 The contact relationship between the underlying Ordovician limestone and the Benxi Formation is a parallel unconformity.16 Coal seams, mudstones, sandstones, and limestones were deposited in the Benxi Formation (Figure 2). The thickness of the Benxi Formation mudstone ranges from 5–30 m in the eastern Ordos Basin to 15–30 m in the southern part.16,17 The maximum thickness of the Benxi Formation mudstone is 40 m to the southeast of Yinchuan, which is located to the west of the central uplift in the Ordos Basin (Figure 1).16 The Benxi Formation mudstone sample used for the pyrolysis experiments was collected from well R-14 near Yinchuan (Figure 1).

In the Late Paleozoic, the Ordos Basin was affected by the Caledonian orogeny and was uplifted by a small amount, with erosion of about 50 m of strata. From the Late Carboniferous to the Late Triassic (320–205 Ma), the basin experienced large-scale subsidence of 2800–3300 m. At the end of the Late Triassic, the basin was affected by the Indosinian orogeny (205–195 Ma), and the strata were denuded by 50–100 m. During the Yanshan orogeny in the Late Jurassic (157–145 Ma), the strata were uplifted and denuded by about 280–400 m. In the Early Cretaceous (145–95 Ma), the basin underwent further subsidence. Finally, in the late Early Cretaceous, large-scale uplift and denudation occurred due to the influence of the Himalayan orogeny, with erosion of about 1800–2100 m sediments.18,19

Samples and Methods

Samples

Rock-eval data of 80 Benxi Formation mudstone samples were collected from the published literature (15 samples from the northeastern Ordos Basin reported by Wang et al.,20 31 samples from the central-eastern Ordos Basin reported by Guo et al.21, and 34 samples from the Changqing oilfield company, mostly from the western Ordos Basin). These data, and new Rock-eval data on the sample from well R-14 analyzed in this work, show that the hydrocarbon generation potentials of the mudstones, of low-middle thermal maturity, from the Yanghugou Formation in the western Ordos Basin and the Benxi Formation in the eastern Ordos Basin are similar, based on the HI and temperature of maximum hydrocarbon generation during pyrolysis (Tmax; Figure 3). Yanghugou Formation mudstones in the western Ordos Basin and Benxi Formation mudstones in the eastern Ordos Basin were both deposited in a marine-continental transitional environment,15,16 so the mudstone sample provided by the Changqing oilfield company is representative of the Benxi Formation mudstone samples of the Ordos Basin (Figure 3) and is appropriate for thermal simulation of hydrocarbon generation.

Figure 3 Relationship between the temperature of maximum hydrocarbon generation during pyrolysis (Tmax) and HI for Benxi Formation mudstone samples in the Ordos Basin. Reproduced with permissions from ref (21) Copyright 2023 American Chemical Society and ref (20) Copyright 2023 ELSEVIER.

Kerogen Isolation from the Mudstone Sample

The small size of the gold tubes meant that only small amounts of sample could be placed in the tube. Additionally, the low HI and total organic content (TOC) of the mudstone sample (Table 1) made it difficult to obtain sufficient gas from the pyrolysis experiment for the measurement of the molecular and carbon isotope composition of the hydrocarbon gases. To increase the amount of gas products, kerogen was extracted from the mudstone sample and used for thermal simulation. The mudstone sample was crushed to 200 mesh and treated with HCl and HF to remove carbonate and silicate minerals. Then the sample was washed with distilled water to a neutral pH and dried in an oven at 60 °C to obtain concentrated kerogen.22

Table 1 Rock-eval Parameters of the Benxi Formation Mudstone from Well R-14 (C2b = Upper Carboniferous Benxi Formation) Used for High-Pressure Gold-Tube Pyrolysisa

depth (m)	lithology	calculated reflectance (Rc) %	TOC %	S1 mg/g	S2 mg/g	Tmax °C	HI mg/g TOC	
2006.75	mudstone	0.64*	1.7	0.1	2.6	434	137	
2006.75	kerogen	0.59*	60.9	5.4	96.0	430	158	
a Notes: The symbol “*” indicates that the reflectance values are calculated reflectance values. The calculated reflectance was calculated using the formula: %Rc = (0.013 × Tmax) – 5.26

Rock-eval Analysis

Rock-eval pyrolysis of about 5–10 mg of the kerogen powder was conducted using a Vinci Rock-eval 6 instrument. The parameters S1, S2, S3, and Tmax were measured, and TOC, oxygen index, and HI were calculated based on the measured parameters.

Confined Gold-Tube Pyrolysis Experiments

The gold tubes had an outer diameter of 4 mm, a wall thickness of 0.25 mm, and a length of 50 mm. Before loading the kerogen samples, gold tubes were welded on one end. Then, the gold tubes were heated to 800 °C to remove any residual organic materials. Under a flow of argon gas, 20–100 mg of powdered kerogen was loaded into each gold tube, and then they were sealed and weighed. The pyrolysis experiment was conducted following the procedure outlined in ref (23).

Qualitative Analysis of Pyrolysis Products

After thermal simulation, the gold tubes were wiped clean and placed into a customized glass vacuum tube, and the gold tube was then punctured to collect the gas components. The glass tube system was connected to an Agilent 7890N gas chromatograph (GC) for the analysis of the gas components. In order to measure both hydrocarbon and nonhydrocarbon gases, the GC was equipped with eight columns and three detectors. The gas composition was analyzed using the method detailed in ref (23).

After analysis of the C1–C5 hydrocarbon gases, the volatile C6–14 compounds were cooled and collected in a 4 mL vial in a liquid nitrogen environment. Then, 3 mL of n-pentane was injected into the vial. The gold tube was cut into small pieces, immersed in n-pentane, and ultrasonically extracted for 2 min. Before C6–14 analysis, an internal standard of deuterated n-C24 was added to the solution for quantification. C6–14 hydrocarbon measurements were made using another Agilent 6890 GC instrument equipped with a 50 m × 0.32 mm × 0.4 μm CP-Sil5CB column and a flame ionization detector. The GC oven was programmed from 40 °C (held for 5 min) to the final 290 °C (held for 50 min) at a heating rate of 4 °C/h.

The remaining liquid hydrocarbons (C14+) were solvent-extracted with a mixture of dichloromethane:methanol (93:7) using a Soxhlet extractor for 72 h. The extracts were dried and weighed three times, which was recorded as the C14+ yield.

Analysis of Stable Carbon Isotopes of Gases

A small fraction of the pyrolytic gas was transferred by the gas injection needle to an Agilent 6890 N connected to an Isoprime 100 mass spectrometer for measuring the stable carbon isotopes of CH4, C2H6, and C3H8. Various volumes of gas were injected into the Agilent 6890N to ensure that the peak height/area of the measured gas was comparable to that of the standard gas. The GC was fitted with a Poraplot Q column (27.5 × 0.32 mm × 10 μm). The carrier gas was helium, and the GC oven was programmed from an initial 60 °C (held for 5 min) to a final 190 °C (held for 7 min) at a heating rate of 25 °C/min.

1D Basin Modeling

Petromod basin simulation software (IES, Germany) was used for the 1D basin modeling. Based on actual geological data, a geological model and a mathematical model were established to simulate the geological burial history, thermal history, and hydrocarbon generation history. In general, the geological parameters used for basin modeling included geological layers, thickness, heat flow, event type (deposition and erosion), TOC, HI, and kinetics parameters.24,25 Geochemical parameters obtained through experimentation, including HI, TOC, and the kinetic parameters of hydrocarbon generation obtained by pyrolysis experiments, were added to the geological model in order to calculate the hydrocarbon generation history. The quantity and process of hydrocarbon generation of the Benxi Formation mudstone in the Shen-27, M-115, and Y-88 wells in the north, middle, and south of the Yishan slope (Figure 1) were simulated.

Results

Source Rock Geochemical Characteristics

The TOC contents of the Benxi Formation mudstone sample and kerogen isolated from the mudstone sample were 1.7 and 60.9%, respectively (Table 1). The Tmax values of the mudstone sample (434 °C) and the kerogen (430 °C) were very similar and gave calculated reflectances (Rc) of 0.64% for the mudstone sample and 0.59% for the kerogen sample,25 indicating that the sample is of early oil window thermal maturity. The HI values of the kerogen (158 mg/g TOC) are higher than those of the mudstone sample (137 mg/g TOC). The relationship between Tmax and HI shows that the organic matter of the mudstone sample is type II2-III (Figure 3).

Gaseous Hydrocarbon Yield during Pyrolysis Experiment

In general, there are three main methods to obtain a reflectance value: directly measure the Ro using a microscope, or measure the reflectance value and then transform it into a vitrinite conversion index to calibrate the kinetic maturation parameters,8,24−29 and model the reflectance value via EASY%Ro modeling.27 A previous study suggested that the EASY%Ro model is suitable for calculating reflectance values in confined gold-tube pyrolysis experiments.30 In this study, the EASY%Ro method was used to calculate the EASY%Ro values using the KINETICS 2000 program.

For the Benxi Formation mudstone, the yield of CH4 increased with temperature (Figure 4a). The final yields of CH4 are 142.7 mg/g TOC under a heating rate of 2 °C/h and 114.5 mg/g TOC under a heating rate of 20 °C/h. The final yield of the total gaseous hydrocarbon C1–C5 is 143.0 mg/g TOC under the heating rate of 2 °C/h and 115.1 mg/g TOC under the heating rate of 20 °C/h (Figure 4e). The yields of hydrocarbon gases under the heating rate of 2 °C/h are higher than the yields under the 20 °C/h heating rate (Figure 4a,e). The yield of C2–C5 first increased and then decreased with an increasing pyrolysis temperature (Figure 4b–d). At 432 °C under a heating rate of 2 °C/h (EasyRo = 1.7%), the yield of C2 was the highest (12.8 mg/g TOC; Figure 4b). Under the heating rate of 2 °C/h, the highest yields of C3 and C4+5 (5.3 and 3.4 mg/g TOC) were attained at 432 °C (EasyRo = 1.7%) and 408 °C (EasyRo = 1.4%), respectively (Figure 4c,d). The highest yield of C2–5 is 21.3 mg/g TOC (Table 2). The yield of H2S first increases and then decreases at both heating rates (Figure 4f). The maximum H2S yields are 57.3 mg/g TOC under the heating rate of 20 °C/h and 41.6 mg/g TOC under the heating rate of 2 °C/h.

Figure 4 Yields of gases with increasing pyrolysis temperature under the heating rates of 2 and 20 °C/h for Benxi Formation mudstone in the Ordos Basin: CH4 (a), C2 (b), C3 (c), C4+5 (d), C1–C5 (e), and H2S (f).

Table 2 Yields of Hydrocarbons and H2S from the Pyrolysis Experiments for the Benxi Formation Mudstone

sample	temperature °C	EASY%Ro	C1 mg/g TOC	C2	C3	C4	C5	C1–5	C2–5	H2S	C6–14	C6+	C14+	
2 °C/h	 	 	 	 	 	 	 	 	 	 	 	 	 	
R14–1	336	0.7	1.14	0.79	0.32	0.19	0.029	2.5	1.33	0.061	37.0	141.0	104.0	
R14–2	360	0.9	4.1	3.3	2.1	0.85	0.16	10.5	6.4	0.32	63.0	195.0	132.0	
R14–3	384	1.1	10.1	6.6	3.9	1.70	0.63	22.9	12.8	0.78	66.2	186.0	119.9	
R14–4	408	1.4	18.8	11.0	4.8	2.4	1.00	38.0	19.2	1.97	30.7	98.9	68.2	
R14–5	432	1.7	28.1	12.8	5.3	2.3	0.84	49.4	21.3	6.2	18.9	39.1	20.2	
R14–6	456	2.1	43.3	11.9	4.3	1.49	0.21	61.2	17.9	14.2	14.6	29.2	14.6	
R14–7	480	2.5	63.5	5.6	0.40	0.64	0.0015	70.1	6.6	34.7	5.0	20.5	15.5	
R14–8	504	3.0	79.2	1.63	0.078	0.0033	0.0005	80.9	1.71	41.6	5.1	16.5	11.4	
R14–9	528	3.5	99.1	1.07	0.053	0.011	0.0005	100.2	1.13	24.7	3.0	18.9	15.9	
R14–10	552	3.9	120.01	0.59	0.011	0.0007	0.0006	120.5	0.61	23.0	1.60	20.4	18.8	
R14–11	576	4.2	138.4	0.32	0.0034	0.0003	0.0004	138.7	0.32	19.2	2.7	11.3	8.6	
R14–12	600	4.5	142.7	0.25	0.0027	 	0.0003	143.0	0.25	21.1	4.4	10.7	6.3	
20 °C/h	 	 	 	 	 	 	 	 	 	 	 	 	 	
R14–13	336	0.6	0.09	0.08	0.071	0.02	0.0023	0.26	0.17	0.52	8.9	54.1	45.2	
R14–14	360	0.7	1.13	0.7	0.32	0.19	0.029	2.3	1.19	3.2	18.0	101.3	83.3	
R14–15	384	0.8	2.7	2.1	1.20	0.56	0.13	6.8	4.0	5.5	31.2	143.6	112.4	
R14–16	408	1.0	6.1	5.0	2.7	1.31	0.46	15.6	9.4	6.5	64.0	204.6	140.6	
R14–17	432	1.2	13.5	9.1	4.2	2.1	0.85	29.7	16.2	10.5	27.0	127.1	100.2	
R14–18	456	1.5	19.0	11.6	5.1	2.6	1.06	39.3	20.3	20.0	10.2	73.5	63.2	
R14–19	480	1.8	35.2	12.8	5.4	2.3	0.63	56.3	21.2	32.9	7.2	38.1	30.9	
R14–20	504	2.2	48.9	11.4	3.6	1.28	0.14	65.2	16.3	46.8	5.7	15.1	9.4	
R14–21	528	2.6	64.7	4.4	0.32	0.55	0.0010	69.9	5.3	57.3	4.7	16.8	12.1	
R14–22	552	3.1	80.5	1.43	0.084	0.051	0.0008	82.1	1.57	50.9	3.2	14.5	11.6	
R14–23	576	3.5	104.7	1.08	0.052	0.0034	0.0005	105.8	1.13	53.8	1.88	13.1	11.2	
R14–24	600	3.9	114.5	0.59	0.011	0.0021	0.0006	115.1	0.61	28.0	4.2	12.4	8.1	

Liquid Hydrocarbon Yield during Pyrolysis Experiment

The yield of C6–14 and C14+ first increases and then decreases with temperature (Figure 5a,b and Table 2). Under the heating rate of 2 °C/h, the maximum yields of C6–14, C14+, and C6+ (66.2, 132.0, and 195.0 mg/g TOC) were obtained at 384 °C (EasyRo = 1.1%), 360 °C (EasyRo = 0.9%), and 360 °C (EasyRo = 0.9%), respectively. The temperature at which peak yields were obtained for C14+ was lower than that for C6–14 due to the preferential generation of hydrocarbons with a higher molecular weight at lower temperatures. The decline of liquid hydrocarbon yield with increasing temperature is ascribed to a higher cracking rate than the rate of generation.

Figure 5 C6–14 (a) and C14+ (b) yields with increasing pyrolysis temperature in the gold-tube experiments for Benxi Formation mudstone in the Ordos Basin.

Stable Carbon Isotopes of CH4, C2H6, and C3H8

The δ13C values of CH4, C2H6, and C3H8 are shown in Figure 6 and Table 3. With increasing pyrolysis temperature under a heating rate of 20 °C/h, the δ13C1 value first decreases from −34.1‰ to −38.5‰ and then increases to −24.8‰. Under the heating rate of 2 °C/h, the δ13C1 value first decreases from −37.1 to −39.3‰ and then increases to −24.8‰ (Figure 6). The lowest δ13C1 value was obtained at 432 °C (Easy Ro = 1.36%) under a heating rate of 20 °C/h and at 384 °C under a heating rate of 2 °C/h. The δ13C values of C2H6 and C3H8 increase with temperature. Under a heating rate of 2 °C/h, δ13C2 increases from −31.0 to −8.7‰ (504 °C) and δ13C3 increases from −27.6 to −24.3‰ (456 °C). There is a trend of δ13C3 > δ13C2 > δ13C1 over the entire temperature range.

Figure 6 Variations of δ13C values of CH4, C2H6, and C3H8 with increasing temperature at the heating rates of 2 and 20 °C/h for the Benxi Formation mudstone in the Ordos Basin.

Table 3 Stable Carbon Isotopes of CH4, C2H6, and C3H8 from the Gold-Tube Pyrolysis of Benxi Formation Mudstone in the Ordos Basin

sample	temperature (°C)	EASY% Ro %	δ13C1 ‰	δ13C2 ‰	δ13C3 ‰	
2 °C/h	
R14–1	336	0.73	–37.1	 	 	
R14–2	360	0.86	–39.3	 	 	
R14–3	384	1.08	–39.3	–31.0	–27.6	
R14–4	408	1.36	–37.9	–30.1	–27.1	
R14–5	432	1.69	–35.6	–28.4	–26.2	
R14–6	456	2.09	–34.3	–26.5	–25.9	
R14–7	480	2.52	–30.8	–24.7	–25.3	
R14–8	504	2.99	–29.5	–23.0	–24.3	
R14–9	528	3.46	–26.8	–12.4	 	
R14–10	552	3.86	–26.4	–8.7	 	
R14–11	576	4.19	–26.1	 	 	
R14–12	600	4.45	–24.8	 	 	
20 °C/h	
R14–13	336	0.57	–34.1	–30.7	–26.2	
R14–14	360	0.68	–34.6	–30.2	–28.3	
R14–15	384	0.79	–37.7	–30.4	–27.9	
R14–16	408	0.96	–38.3	–28.7	–26.4	
R14–17	432	1.19	–38.5	–27.5	–26.2	
R14–18	456	1.47	–37.7	–26.6	–26.0	
R14–19	480	1.80	–36.8	–25.3	–26.3	
R14–20	504	2.19	–33.8	–20.6	–23.5	
R14–21	528	2.62	–30.0	–22.1	–22.8	
R14–22	552	3.06	–26.4	–17.7	 	
R14–23	576	3.49	–27.4	–14.5	 	
R14–24	600	3.87	–24.8	–12.3	 	

Kinetic Parameters for Gaseous Hydrocarbon Generation

Hydrocarbon generation can be described by a set of reactions with a single frequency factor and discrete activation energy.9,31 The Kinetics 2000 software developed by Braun and Burnham32 was used to calculate the kinetic parameters of C1–5 generation. The results suggest a good fit of the cumulative yields of the total gaseous hydrocarbons calculated from the kinetic parameters and experimental data (Figure 7a). The activation energy for the generation of C1–5 from the organic matter of the Benxi Formation mudstone sample ranges from 43 to 63 kcal/mol, with a frequency factor of 5.76E+11 s–1 (Figure 7b and Table 4). The weighted average and the main peak of activation energy are 57.4 and 63 kcal/mol, respectively.

Figure 7 Best-fit curves (a) and kinetic parameters (b) for the generation of C1–C5 gases from the Benxi Formation mudstone in the Ordos Basin.

Table 4 Kinetic Parameters for C1–5 Generation from Benxi Formation Mudstone in the Ordos Basin

frequency factor	5.76 × 1011 s–1	
activation energy (Ea; kcal/mol)	fraction (%)	activation energy (Ea; kcal/mol)	fraction (%)	
43	0.12	56	0	
44	0	57	0	
45	0.59	58	7.3	
46	0.23	59	25.1	
47	0	60	0	
48	2.3	61	0	
49	4.7	62	0	
50	0	63	27.1	
51	8.3	 	 	
52	0	 	 	
53	0	 	 	
54	10.7	 	 	
55	13.6	 	 	

Discussion

Late Gas Generation Potential of Coal-Measured Source Rocks

The late gas generation potential of coal-measured source rocks is their gas generation potential at a thermal maturity of around 2.0%Re (vitrinite reflectance equivalent from Easy%Ro). The recombination reactions of free liquid hydrocarbons and bitumen with kerogen are the major sources of methane at very high maturity.33−35 Li et al. demonstrated that the interaction between kerogen and oil components leads to the generation of hydrocarbon gases.36 For the Westphalian coals from the Ruhr Basin, the late gas generation potential (between 560 and 700 °C) ranges between 30 and 42 mg/g TOC.37 Huang et al.12 reported that at a thermal maturity of around 2.0%, coal samples from the Tarim Basin have a similar gas generation potential (50–70 mg/g TOC), even if the HI of the coal samples had a large range of 57–278 mg/g TOC. The Upper Paleozoic mudstone samples from the Ordos Basin have an HI range of 85–197 mg/g TOC, and gas generation potentials at a thermal maturity of about 2.0%Re range between 60 and 100 mg/g TOC (Figure 8a,b and Table 5). The gas generation potentials of Upper Paleozoic coals with an HI range of 103–364 mg/g TOC at a thermal maturity of about 2.0%Re range between 50 and 92 mg/g TOC (Figure 8a,b). Even though the HI values of coal samples are higher than those of mudstone samples, the difference in the late gas generation potential between mudstone samples and coal samples is small. For the Upper Paleozoic mudstone samples, there is a good linear relationship between the late gas generation potential and the HI or the (S1 + S2)/TOC ratio (Figure 8a,b). However, there is no relationship between the late gas generation potential and HI or (S1 + S2)/TOC for the Upper Paleozoic coal samples (Figure 8a,b). As late gas generation is closely related to the interaction between the liquid component or bitumen and kerogen,34,35,41 the composition and mass of liquid hydrocarbons should be the main factor that influences late gas generation potential. However, due to the complex chemical structure of coals, the liquid hydrocarbon yield is usually not related to the HI or (S1 + S2)/TOC values.42,43 The good linear relationship between the late gas generation potential and HI and (S1 + S2)/TOC for the mudstone samples suggests that both of these parameters are closely related to the amount or composition of liquid hydrocarbons. The distinct correlations between HI and late gas generation potential for mudstones and coals suggest that the compositions of liquid hydrocarbons and bitumens generated by coals and coal-measured mudstones are different.

Figure 8 Relationships between late gas generation potential (%Re < 2.0%) and HI (a) and (S1 + S2)/TOC (b), and relationships between total hydrocarbon gas C1–5 yields at 4.5 Re% and HI (c) and (S1 + S2)/TOC (d). See Table 1 for the definition of the Rock-eval parameters.

Table 5 Basic Geochemical Parameters and Maximum C1–5 Generation Potential of Coal-Measured Source Rocks from the Ordos Basin and Tarim Basin

sample	Fm.	S1 mg/g	S2 mg/g	TOC %	(S1 + S2)/TOC mg/g TOC	HI mg/g TOC	late gas generation potential mg/g TOC	C1–5 yield mg/g TOC	data source	
coal	 	 	 	 	 	 	 	 	 	
JKC1	J2k	2.9	101.1	71.9	144.6	141	 	90.7	ref (12)	
JKC2	J2k	4.5	136.2	74.4	189.1	183	 	95.0	 	
JKC3	J2k	0.3	41.6	72.8	57.5	57	 	70.1	 	
TTC1	T3t	4.2	208.1	75.3	282.0	276	 	120.9	 	
TTC4	T3t	4.8	172	77.3	228.9	223	 	107.2	 	
TTC11	T3t	4.2	196.4	70.5	284.4	278	 	113.0	 	
TTC18	T3t	9.6	200.4	79.3	264.9	253	 	115.6	 	
S-7a	P1s	9.18	226.2	62.2	378.3	363	70.8	173.6	ref (8)	
19GE-04	P1t	0.84	49.76	48.1	105.2	103	63.2	109.7	ref (38)	
Le-1a	P1t	2.56	150.2	60.5	252.4	248	91.9	137.8	ref (3)	
1–38	P1s	9.66	173.8	66.3	276.8	262	49.6	129.5	ref (23)	
mudstone	 	 	 	 	 	 	 	 	 	
Le-1b	P1s	0.2	3.5	4.1	90.3	85	65.6	100.0	ref (3)	
S-7b	P1s	0.34	1.04	1.17	118.0	89	60.7	99.0	ref (8)	
19GE-01	P1t	0.08	12.8	6.5	197.9	197	92.7	157.8	ref (5)	
P01	P1s	0.39	15.2	7.3	213.7	208	100	166.1	ref (39)	
P02	P1s	1.55	9.6	8.5	131.3	113	59.6	120.3	ref (40)	

Elevated thermal maturities (Re > 2.0%) are crucial to account for the coal-measured source rocks generating and expelling sufficient gaseous hydrocarbons for the formation of gas fields in the southern Ordos Basin. In this area, there is a greater thickness of mudstones in the Shanxi, Taiyuan, and Benxi formations than of coal seams. Previous studies have shown that a marine influence could increase the hydrogen content of coal-measured source rocks.44−47 Marine-influenced coal-measured source rocks are widely distributed in the southern Ordos Basin. In addition, marine-influenced coal-measured source rocks from the Taiyuan and Benxi formations may have greater late gas generation potential than the nonmarine-influenced ones.

Gas Generation Potential of Coal-Measured Source Rocks

Usually, the gas generation potential of coal-measured source rocks is lower than the HI or (S1 + S2)/TOC values of initial coal-measured source rocks.12 It seems likely that the HI and (S1 + S2)/TOC cannot be used to assess the gas generation potential of coal source rocks. For coal samples within the Upper Triassic Taliqike Formation and Middle Jurassic Kezilenuer Formation,12 there is a good linear relationship between the HI values and C1–5 yields at 4.5%Re (Figure 8c and Table 5). The HI values of Benxi Formation mudstones of low-medium thermal maturity from the Ordos Basin vary from 22 to 268 mg/g TOC (Figure 3). It seems likely that the Benxi Formation mudstones have heterogeneous hydrocarbon generation potentials. Similar variations in HI values for the Shanxi and Taiyuan formations have been reported.3,5,8 It is likely that the heterogeneous hydrocarbon generation potential of the mudstones is related to changes in the sedimentary environment. The C1–5 hydrocarbon gas generation potentials of mudstones from the Upper Paleozoic strata in the Ordos Basin with different HI values have been investigated and reported by previous studies3,5,8,39,40 (Table 5). For the samples chosen by previous studies to represent the Upper Paleozoic coal-measured source rocks in the Ordos Basin (the Shanxi, Taiyuan, and Benxi formations), there are good linear relationships between HI values and C1–5 yields at 4.5%Re (Figure 8c and Table 5). There are also good linear correlations between the (S1 + S2)/TOC values and C1–5 yields at 4.5%Re for the coal samples from the Tarim Basin and coal-measured source rocks from the Ordos Basin (Figure 8d). It should be noted that for samples with similar HI and (S1 + S2)/TOC values, the gas generation potential of mudstone samples from the Upper Paleozoic strata of the Ordos Basin is higher than that of the coal samples from the Tarim and Ordos basins (Figure 8c,d). This might be related to the different sedimentary environments of the mudstone and coal samples. Previous studies showed that mudstone samples from the Upper Paleozoic are characterized by pristane/phytane (Pr/Ph) ratios < 1.0, whereas Pr/Ph ratios of coal samples from the Upper Paleozoic are > 2.0. The low Pr/Ph ratios of the Upper Paleozoic mudstone are related to high paleo-salinity48 and a reducing environment, which is beneficial for the preservation of sedimentary lipids.49,50 Therefore, for samples with similar HI values, the coal samples might be richer in aromatic hydrocarbons and phenols than the mudstone samples.

Carbon Isotope Variation of Coal-Measured Source Rocks

For pyrolysis data with a temperature gradient of 2 °C/h, the variation of δ13C1 with thermal maturity (%Re) shows that when thermal maturity is < 1.1%, the δ13C1 value for the Benxi Formation mudstone is higher than that of the Shanxi Formation mudstone (Figure 9). In contrast, when the %Re is > 1.1, the δ13C1 value for the Benxi Formation mudstone is lower than that of the Shanxi Formation mudstone because the samples have entered the main oil cracking stage. The oil generation potential of the Benxi Formation mudstone is higher than that of the Shanxi Formation mudstone. Zhao et al.8 reported that the δ13C1 of the Shanxi Formation mudstone is lower than that of the Shanxi Formation coal when the %Re is < 1.7 and that at high maturities the reverse trend appeared. The Shanxi Formation coal has a higher oil generation potential than the Shanxi Formation mudstone.8 When the %Re is higher than 1.7%, the lower δ13C1 value for the Shanxi Formation coal compared with the Shanxi Formation mudstone may be related to the cracking of C6+. The δ13C value of methane generated from coal-measured source rocks at an early mature stage varies from −45 to −37‰ (Figure 8). Based on a comparison among Benxi Formation mudstone, Shanxi Formation mudstone, and Shanxi Formation coal, the sample with a lower oil generation potential has a lower δ13C1 in the early mature stage. Yu et al. proposed that the gas produced at an initial temperature of about 334 °C under a heating rate of 20 °C/h (corresponding to 0.56% Re) is not just the original product released from kerogen but has been altered by gas incorporation into kerogen.51 Once free oil and wet gas molecules are incorporated into kerogen, the bound molecules can easily decompose into smaller molecules due to a substantial reduction of activation energy for carbon–carbon bond rupture. Larger oil molecules are more competitive in being incorporated into kerogen compared with wet gases, which leads to a negative correlation between the gas dryness ratio and the yields of liquid components.51 Therefore, for samples with a lower oil generation potential, more wet gas or small petroleum molecules could be incorporated into the kerogen, leading to the generation of methane and a greater dryness at low thermal maturities compared with samples with higher oil generation potentials. The wet gas or small petroleum molecules incorporated into the kerogen structure in the low maturity stage might have lower δ13C values than the components in the kerogen that could be cracked to generate methane.52,53 The methane generated from the cracking of these wet gas and small petroleum molecules might have lower δ13C values compared with the methane from kerogen primary cracking.

Figure 9 Evolution of stable carbon isotopes of methane with increasing EASY%Ro (%Re) for the Benxi Formation mudstone and Shanxi Formation coal and mudstone (data from ref (8)).

Generation of Gaseous Hydrocarbons under Geological Conditions

The oil and gas generation observed in the simulation experiments cannot be directly applied to geological conditions because the gas generation from high-temperature pyrolysis is significantly lower than that of gaseous hydrocarbons under geological conditions under long-term low temperatures, even though they have the same Ro.28 Geologists model and evaluate hydrocarbon generation by applying hydrocarbon generation dynamics software to experimental data and extrapolating to geological conditions.54 Based on the closed gold-tube pyrolysis data of the Benxi Formation mudstone, the frequency factor and activation energy distribution of C1–C5 were obtained under two heating rates (Figure 7 and Table 4). Wells Shen-27, M-115, and Y-88 (Figure 1) were selected to simulate the hydrocarbon gas generation histories of Benxi Formation mudstones in the Ordos Basin by combining the activation energy and frequency factors obtained by experimentation.

A transformation ratio of 0.2–0.8 is usually taken as the main gas production stage of C1–5.29 The Benxi Formation mudstone in Y-88 entered the main gas production stage in the Late Jurassic (∼156 Ma) and reached the final transformation ratio (73.8%) in the Early Cretaceous (Figure 10a). The Benxi Formation mudstone in M-115 entered the primary gas production stage in the Early Cretaceous (∼128 Ma), achieving a final transformation ratio (32.8%) near the Late Cretaceous. The Benxi Formation mudstone in Shen-27 exhibits a notably low final transformation ratio of 18.1% in the Late Cretaceous. The thermal maturity of the basin gradually increases from north to south, with the specific thermal maturity of the Benxi Formation in the three wells being Shen-27 = 1.28% Ro, M-115 = 1.75% Ro, and Y-88 = 3.1% Ro (Figure 10b). This trend was further corroborated by the observed gradual increase in the final conversion rate, which also increases from north to south.

Figure 10 Models of the transformation ratios (a), Ro (b), and mass of generated C1–5 hydrocarbon gases (c) for the Benxi Formation mudstone in wells Shen-27, M-115, and Y-88 under geological conditions in the Ordos Basin. The burial history and thermal history involved in the modeling of the three wells were reproduced with permissions from ref (23) Copyright 2024 American Chemical Society.

The thickness and transformation ratio of source rocks are the main factors that could affect the mass of the generated hydrocarbon gases. Based on the Benxi Formation mudstone thickness of strata around Shen-27 (15 m), M-115 (42 m), and Y-88 (30 m) in the Ordos Basin, and using the kinetic parameters for C1–5 generation and the HI values of the source rocks, the mass of generated hydrocarbon gases was calculated (Figure 10c). The final masses of hydrocarbon gases generated from the Benxi Formation mudstone in wells Y-88, M-115, and Shen-27 are 0.91 × 105 t, 0.56 × 105 t, and 0.16 × 105, respectively. It is evident that the thickness and transformation ratio of the Benxi Formation mudstone increase southward, and the mass of the generated hydrocarbon gases also increases southward. Therefore, for unconventional gas exploration of the Benxi Formation in the southeastern Ordos Basin, the significant contribution of the Benxi Formation mudstone to natural gas production should be valued. As fluid pressure varies during the hydrocarbon generation and expulsion process, and pressure could influence the yield and composition of generated hydrocarbons,55 the variation of pressure with increasing thermal maturity should be considered in future work.

Conclusions

Confined pyrolysis experiments were performed to determine the yields and kinetic parameters for gaseous hydrocarbon formation for a representative Benxi Formation mudstone with an HI value of 137 mg/g TOC and a Tmax value of 434 °C from the Ordos Basin. By comparing the yields and carbon isotopes of hydrocarbon gases generated from the Benxi Formation mudstone and other coal-measured source rocks from the Ordos and Tarim basins, some evidence is provided for gas potential and source evaluations of the Upper Paleozoic strata in the Ordos Basin.1. The Benxi Formation mudstone finally yielded a total gaseous hydrocarbon (C1–C5) content of 143.0 mg/g TOC at a heating rate of 2 °C/h. The peak yields for C6–14, C14+, and C6+ were 66.2, 132.0, and 195.0 mg/g TOC, respectively, at the same heating rate.

2. Despite higher HI values in coal samples compared to mudstone samples, Upper Paleozoic coal-measured source rocks exhibit a comparable late gas generation potential (%Re > 2.0). The Benxi Formation mudstone, with increasing thickness and thermal maturity southward, holds significant promise for unconventional gas production in the southeastern Ordos Basin.

3. The late gas generation potential of Upper Paleozoic mudstone samples correlates strongly with HI and the (S1 + S2)/TOC ratio, whereas no such correlation exists for coal samples. These contrasting relationships likely stem from distinct kerogen chemistries between the two lithologies.

4. For Upper Paleozoic coal-measured source rocks with similar HI, mudstone samples have a higher total gas generation potential than coal samples. This disparity may be attributed to differing depositional environments. Additionally, for samples with similar HI values, coals might be richer in aromatic hydrocarbons and phenols relative to mudstones.

5. A comparison of three Upper Paleozoic coal-measured source rocks reveals that samples with a lower oil generation potential exhibit lower δ13C1 values during the early maturity stage. This correlation may be related to the lower δ13C values of wet gas or small hydrocarbon molecules that could be incorporated into the kerogen structure, compared to the components that contribute to the primary cracking of kerogen to form methane. The reason for this phenomenon requires further study.

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China program (Grant Nos. 42473032, 41803043), the Foundation for Basic and Applied Basic Research of Guangdong Province (Grant No. 2021A1515012562), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA14010103), and the State Key Laboratory of Organic Geochemistry, GIGCAS (Grant No. SKLOG2020-1). This is a contribution to the project of Theory of Hydrocarbon Enrichment under Multi-Spheric Interactions of the Earth (THEMSIE04010104). The authors thank Prof. Simon George for help with the English. The authors greatly appreciate editor Sarbajit Banerjee and two reviewers for their valuable comments.
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