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. 2022 Feb 22;7(9):7715–7724. doi: 10.1021/acsomega.1c06496

Biogeochemical Assessment of the Coalbed Methane Source, Migration, and Fate: A Case Study of the Shizhuangnan Block, Southern Qinshui Basin

Yang Li †,‡, Jian Chen †,‡,*, Shuheng Tang §,∥,⊥, Songhang Zhang §,∥,⊥, Zhaodong Xi §,∥,⊥
PMCID: PMC8908359  PMID: 35284701

Abstract

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The exploration and exploitation of coalbed methane (CBM), an essential unconventional gas resource, have received much attention. In terms of shallow groundwater assessment during CBM production, biogenic methane natural formation in situ and methane migration from deep sources into shallow aquifers need to be of most concern. This study analyzes geochemical surveys including ions, isotopes, and dissolved methane concentrations in 75 CBM coproduced water samples in the southern Qinshui Basin. Most of these water samples are weakly alkaline. Some samples’ negative oxidation/reduction potential (ORP) values reveal that the CBM reservoir water samples are mainly produced from reductive groundwater environments. Cl–, Na+, and HCO3– are the dominant ionic constituents of the water samples, which are usually associated with dissolved methane concentrations. The biogeochemical parameters and isotopic features provide an opportunity to assess the origin, migration, and oxidation of biogenic or thermogenic methane. Some water samples suggest biogenic methane formation in situ characterized by negligible SO42– and NO3– concentrations and low δ13CCH4. Only a few water samples indicate the migration of biogenic methane into shallow aquifers without oxidation based on elevated SO42–, NO3–, and δ13CDIC and low δ13CCH4. A few cases characterized by elevated δ13CCH4, negative δ13CDIC values, and negligible SO42– and methane concentrations suggest the oxidation of biogenic methane rather than the migration of thermogenic methane. A significant number of cases mean methane migration to shallow aquifers. Partial oxidation of thermogenic or mixed methane is evaluated by negligible SO42–, NO3–, and methane concentrations and elevated δ13CCH4. Dissolved methane isotopic compositions and aqueous biogeochemical features help study methane formation and potential migration in shallow groundwater.

1. Introduction

The rapid development of drilling and hydraulic fracturing technologies has promoted the large-scale exploitation of coalbed methane (CBM).1,2 Groundwater migration is the main reason for methane migration and accumulation.3 The direct impact of groundwater transport on methane has attracted widespread attention, and methane transport significantly impacts water quality caused by CBM exploitation.4 In general, elevated methane concentrations are mainly found in the Na–Cl and Na–HCO3 groundwater types.5 However, the analysis of groundwater biogeochemical characteristics to evaluate redox conditions can reveal whether methane is thermally or biologically formed and whether it is formed in situ or migrated from deep areas.

Determining the CBM source by its isotopic characteristics is essential, but isotopic fractionations caused by methane oxidation may affect CBM source identification.6 Therefore, it is necessary to develop reliable methods to distinguish true thermogenic methane and pseudothermic methane and demonstrate methane migration from deep geological formations to shallow ones. Biogenic methane formation requires a high degree of reductive environment, meaning that methanogens produce methane after oxygen, nitrate, and sulfate are consumed. The analysis of the coupling relationship within biogeochemical parameters can provide a significant basis for the possibility of biogenic methane formation in situ. Furthermore, the carbon and hydrogen isotopic characteristics of biogenic methane are different from those of thermogenic methane,7 and isotopic analyses of dissolved inorganic carbon (δ13CDIC), nitrate (δ15NNO3, δ18ONO3), and sulfate (δ34SSO4, δ18OSO4) provide a vital complement to the study of aquifer redox environments for methane formation and migration.8 Therefore, a comprehensive analysis of groundwater biogeochemistry and isotopes can make it possible to determine the origin and migration of methane.

The Qinshui Basin is one of China’s earliest CBM commercial development zones, which has considerable coal and CBM resources. The Shanxi Formation and Taiyuan Formation are the primary sources of CBM exploration and development. In the mid-1990s, most coalbed methane exploration took off with the advance of mining technology and infrastructure. The Shizhuangnan block is an essential commercial CBM development block in the Qinshui Basin.9 This study aims to determine the source or migration of coalbed methane by describing redox processes in shallow groundwater environments of the Shizhuangnan block. This method takes advantage of the general laws governing the methane source, migration, and fate and can be transferred to other CBM reservoir studies.

2. Geological Background

Qinshui Basin, located in the southeastern Shanxi province, is a large complex synclinal basin formed on the late Paleozoic basement (Figure 1a). The coal resources in the Qinshui Basin are mostly bituminous and anthracite from Carboniferous to Permian. Hence, Qinshui Basin has excellent conditions to exploit coalbed methane resources, which is the first and largest coalbed methane commercial development basin in China.10,11 The south of Qinshui Basin is a high investment and research area of CBM exploration and development. The CBM production of the southern Qinshui Basin accounts for more than 90% of the total yield of the Qinshui Basin. The Shizhuangnan block is located in the northwest dipping slope belt in the southern Qinshui Basin (Figure 1b). The tectonic movement has a significant influence on the tectonic morphology of this area. The overall structural characteristics of the Shizhuangnan block are relatively simple, and the overall topography gradually tilts from southeast to northwest.12 The most significant Sitou fault in the northwest area is a normally closed fault extending from northeast to southwest (Figure 1c). In the southern part of the Sitou fault, the distance and dip angle gradually become smaller, and there are some hidden minor faults around. The Sitou fault has poor water connectivity, which is significant to CBM reservoir formation in the study area.13

Figure 1.

Figure 1

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(a) Location of the Qinshui Basin in China; (b) location of the Shizhuangnan block in the southern Qinshui Basin of China (FZ: the Fanzhuang block; MB: the Mabi block; PZ: the Panzhuang block; SZN: the Shizhuangnan block; ZZ: the Zhengzhuang block); and (c) main structure diagram and CBM wells for the study in the Shizhuangnan block.

The exploration and development of CBM in the Shizhuangnan block is the No. 3 coal seam of Shanxi Formation and No. 15 coal seam of Taiyuan Formation, which are large and stable minable seams. The No. 3 coal seam roof is mainly composed of mudstone and sandy mudstone, while the floor is mainly composed of siltstone and mudstone. The No. 3 coal seam in the Shizhuangnan block has a wide range and stable distribution, located in the lower part of the Shanxi Formation. The average thickness of the No. 3 coal seam used in the study area is 6 m, and the total buried depth is about 450–900 m. The overall coal seam shape shows a trend of shallowness in the southeast and depth in the northwest.8,14 In this study, the No. 3 coal seam is the primary source of samples.

Due to the difference in the occurrence form and storage space, each type of aquifer is different in hydraulic connection and dynamic change. According to the difference of reservoir space, the aquifer in the south of the Qinshui Basin is divided into aquifer types. The sandstone fissured confined water aquifer of the Shanxi Formation is deeply buried in the lower Permian strata, which is the primary source of the No. 3 coal seam in the south of the Qinshui Basin (Figure 2). The No. 3 coal seam is a weakly confined aquifer between sandstone water layers, belonging to a solid water-rich aquifer. On the regional scale, the hydrogeological conditions of the Shizhuangnan block are relatively simple. Overall, the Shizhuangnan block can be approximated as a west-dipping monoclinal structure. Stable aquifers are formed between each aquifer, and there is no vertical hydraulic connection between these aquifers, creating an independent coal aquifer system. The groundwater in the Shizhuangnan block is deeply buried, and its runoff is slow under gravity action. The exposed part of the Jinhuo fault zone on the basin’s eastern edge is high, and the coal reservoir is replenished after receiving atmospheric precipitation and surface runoff. Sitou fault on the west forms a natural barrier of the underground reservoir. CBM is blocked under hydrostatic pressure, leading to high reservoir pressure. The retention area has good sealing and gas-bearing properties, which is conducive to the storage of CBM.

Figure 2.

Figure 2

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Schematic diagram of the aquifer and strata of the southern Qinshui Basin.

3. Results and Discussion

3.1. Geochemical Characteristics of the CBM Reservoir Water of the Shizhuangnan Block

The geochemical properties of the CBM coproduced water samples in the Shizhuangnan block include pH, ORP, main ionic parameters, and isotopic characteristics. Main ionic parameters, pH, and ORP were tested for all 75 water samples. The pH values of these collected water samples range from 7.2 to 8.9, suggesting that the reservoir water is alkaline in the Shizhuangnan block. The ORP values vary from −141 to 184 and differ among various areas in the study block, affected by the reservoir redox environments. In total, 50 CBM coproduced water samples have ORP positive values, while 25 water samples have negative ORP values. Negative ORP values generally mean reductive reservoir conditions, while positive values suggest oxidized reservoir environments. Dissolved methane concentrations of all water samples were determined from the Shizhuangnan block. These methane concentrations were measured above the detection limit, but it has a wide value range from 0.0001 to 1 mmol/L. Elevated dissolved methane concentrations related to negative ORP reveal that the majority of the highest concentrations of methane were saved predominantly under the reductive reservoir conditions (Figure 3).

Figure 3.

Figure 3

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Plot of ORP and pH versus dissolved methane concentrations indicated by the size and color of symbols in the CBM coproduced water samples from the No. 3 coal seam.

Ion compositional characteristics reveal that Na+, Cl–, and HCO3– account for a vast proportion of the significant anions in the Shizhuangnan block. The average concentrations of Na+, Cl–, and HCO3– are 15.71, 8.50, and 6.03 mmol/L, respectively. The majority of SO42– concentrations are commonly lower than other ions, while NO3– concentrations are relatively high with a range from 0.10 to 1.60 compared with SO42–. With the flow of coal seam water, water–rock interactions and microbial action change the concentrations of some ions, resulting in the depletion of SO42– and NO3– and the increase in Na+, Cl–, and HCO3–.15,16

Dissolved methane concentrations can be correlated with different geochemical data from the CBM coproduced water samples (Figure 4a,b). The first type with low Na+, Cl–, and HCO3– concentrations meaning freshwater near the original point has low methane concentrations. The second groundwater type with low Cl– and high Na+ and HCO3– concentrations has low methane concentrations. The samples of this type seem to be affected by cation exchange, resulting in increased Na+ and HCO3–.17 These samples are mainly located in the central and western parts of the Shizhuangnan block. The third type of groundwater samples with high Na+, Cl–, and HCO3– concentrations has elevated methane concentrations following other studies in other CBM reservoirs.5,7,10 Therefore, to further identify the occurrence environments of methane in CBM reservoir water, the following geochemical indicators are used to characterize it.

Figure 4.

Figure 4

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(a) Plot of Cl– and Na+ versus dissolved methane concentrations and (b) plot of Na+ and HCO32– versus dissolved methane concentrations indicated by the size and color of symbols in the CBM coproduced water samples from the No. 3 coal seam.

3.2. Biogeochemical Parameters and Redox Environments of Methane Occurrence

Several biogeochemical parameters are used to characterize the metabolic activities of different microorganisms to study the CBM reservoir redox conditions for methane production, migration, and storage.18 Methane oxidative reactions may occur when methane migrates to groundwater with better oxidative conditions. A series of redox reactions, including denitrification, manganese and iron reduction, reduction of bacterial sulfate, and methane formation, occur successively as the groundwater environments transition from oxidative conditions to reductive conditions. For example, when the primary oxidant in groundwater is sulfate, the methane oxidation process is accompanied by bacterial sulfate reduction. Microorganisms mediate the process, called anaerobic oxidation methane (AOM) and bacterial sulfate reduction (BSR). With the consumption of oxidants, the relative reductive environments are conducive to methanogen metabolism and biogenic methane production in situ.19,20

Most samples with ORP more than 0 contain negligible dissolved methane concentrations (Figure 3). It is impossible to generate biogenic methane in situ in a relatively oxidized groundwater environment. Only a few samples with high ORP values dissolve a certain amount of methane, resulting from methane migration from other areas.21 There is a negative correlation between NO3– contents and methane concentrations in groundwater samples because groundwater environments containing high NO3– are not suitable for methane preservation (Figure 5). Similarly, there is a negative correlation between SO42– contents and methane concentrations. It can be inferred that the presence of elevated methane in groundwater is not apparent until SO42– concentrations are less than 0.01 mmol/L. When methane migrates into aquifers containing O2, SO42–, or NO3–, relatively reductive conditions may be formed after these oxidizers are consumed.22 The redox ladder concept further explains these analysis results (Figure 5).

Figure 5.

Figure 5

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Plot of NO3– and SO42– versus dissolved methane concentrations indicated by the size and color of symbols in the CBM coproduced water samples from the No. 3 coal seam.

Figure 5 reveals the coupling relationship between NO3– and SO42– concentrations and dissolved methane concentrations. The first groundwater sample type with high NO3– concentrations (>1 mmol/L) and moderate sulfate concentrations (0.001–0.1 mmol/L) contains only a small amount of dissolved methane characterized by a light gray circle. The elevated NO3– and SO42– concentrations represent that neither complete denitrification nor complete bacterial sulfate reduction took place in the groundwater conditions. Accordingly, the formation of biogenic methane in situ is restricted. In this type of groundwater sample, only one sample has elevated methane concentrations (0.1 < CH4 < 1.0 mmol/L), which is associated with the migration of methane to aquifers containing elevated NO3– and SO42–. The second groundwater sample type has high SO42– (>0.01 mmol/L), low NO3– (<1 mmol/L), and negligible dissolved methane concentrations, except for one groundwater sample (0.1 < CH4 < 1.0 mmol/L), which is characterized by the light yellow circle. It may be due to methane migration into these aquifers and partial oxidation through denitrification or BSR.23 The last groundwater sample type contains negligible NO3– and SO42– concentrations corresponding to these samples with the highest methane concentrations (light red circle). Therefore, the reservoir environments with the complete occurrence of BSR and denitrification are beneficial to methane preservation.22

3.3. Isotopic Characteristics in the CBM Water Reservoir of the Shizhuangnan Block

3.3.1. Nitrogen and Oxygen Isotope Ratios of Nitrate in the CBM Water Reservoir of the Shizhuangnan Block

Isotopes of NO3–, SO42–, DIC, and methane are used to explain redox processes such as denitrification. Just 20 water samples have been measured for nitrate isotopes. The δ15NNO3 values range from −3.9 to 29.1‰, while δ18ONO3 values range from −18.4 to 22.8‰. The water source can be obtained by nitrogen and oxygen isotope ratios of nitrate.24,25 A water sample kind characterized by relative low δ18ONO3 values (<0‰) and low δ15NNO3 (<10‰) with low NO3– concentrations demonstrates that NO3– is thought to result from the nitrification of organic matter (Figure 6a,b). This kind of groundwater is dominated by the lowest methane concentrations, except for one sample with methane concentrations between 0.001 and 0.01 mmol/L. Another kind of groundwater with relatively high δ15NNO3 (>10‰) and low δ18ONO3 (−10‰ < δ18ONO3 < 10‰) accompanied by relatively high NO3– concentrations likely results from manure spreading. These samples almost have negligible methane (<0.001 mmol/L) associated with different groundwater sources. The last kind of water is characterized by high δ18ONO3 (>10‰) and δ15NNO3 (>20‰) with negligible NO3– and methane concentrations. Theoretically, this isotopic signature could be derived from mineral fertilizers. In the process of denitrification, with the decrease of NO3– concentration, the residual NO3– gradually enriched in 15N and 18O. It can be seen that these samples may be affected by denitrification (Figure 6b).26,27 Thus, nitrate isotopes reveal the various nitrate origins accompanied by little sign of denitrification depending on these isotopic analyses.

Figure 6.

Figure 6

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(a) Plot of δ18ONO3 and δ15NNO3 versus dissolved methane concentrations and (b) plot of NO3– and δ15NNO3 versus dissolved methane concentrations indicated by the size and color of symbols in the CBM coproduced water samples from the No. 3 coal seam.

3.3.2. Sulfur and Oxygen Isotope Ratios of Sulfate in the CBM Water Reservoir of the Shizhuangnan Block

In the process of BSR, SO42– concentrations are expected to decline, while 34S and 18O gradually accumulate in the residual SO42–. The water samples with the highest SO42– are companied with δ34SSO4 between 20 and −20‰ (Figure 7a). Furthermore, δ18OSO4 values (<0‰) indicate that the sulfate in these samples mainly comes from pyrite oxidation (Figure 7b), and some water samples associated with relatively low SO42– concentrations usually have low δ34S and δ18O, suggesting that sulfate comes from sulfide mineral oxidation.28 The elevated δ34SSO4 trend with gradually decreasing SO42– indicates that BSR has occurred under the groundwater conditions (Figure 7a). Some samples with BSR evidence contain high dissolved methane concentrations.

Figure 7.

Figure 7

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(a) Plot of SO42– and δ34SSO4 versus dissolved methane concentrations and (b) plot of δ18OSO4 and δ34SSO4 versus dissolved methane concentrations indicated by the size and color of symbols in the CBM coproduced water samples from the No. 3 coal seam.

3.3.3. Carbon Isotopic Compositions of Inorganic Carbon in the CBM Water Reservoir of the Shizhuangnan Block

The δ13CDIC values vary from −36.0 to 38.8 in the groundwater samples. δ13CDIC is a critical evaluation parameter for methanogenesis and dissolved methane concentrations. Elevated methane concentrations are usually associated with high δ13CDIC values. The isotopic characteristics of dissolved inorganic carbon (DIC) indicate the carbon source of DIC and the process of producing or influencing DIC. The negative δ13C values reveal that DIC comes from carbonate dissolution and organic matter oxidation.29 These δ13C values are associated with low methane concentrations (Figure 8a). The positive δ13C values mean the occurrence of methanogenesis in groundwater environments because methanogens preferentially utilize 12C, resulting in the remaining 13C enriched in DIC. These samples have relatively high dissolved methane concentrations.30

Figure 8.

Figure 8

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(a) Plot of DIC and δ13CDIC versus dissolved methane concentrations and (b) plot of lg SO42– and δ13CDIC versus dissolved methane concentrations indicated by the size and color of symbols in the CBM coproduced water samples from the No. 3 coal seam.

The water samples of high SO42– concentrations are associated with low δ13CDIC values (Figure 8b). Moreover, the water samples with the lowest SO42– concentrations are associated with the highest δ13CDIC values and elevated methane concentrations. This assumption confirms that BSR needs to be completed before methanogenesis in the coal reservoir water environments.31,32

3.4. Methane Formation, Migration, and Oxidation in Coal Reservoir Water

Isotopic distributions of methane and carbon dioxide often reflect the effects of methane formation, fractionation, and oxidation33 (Figure 9). These isotopic characteristics of methane and carbon dioxide differ among different blocks in the study area.10 Furthermore, the redox parameters are sufficient in the study area, including ORP, pH, main ions, and DIC, sulfate, and nitrate isotopic characteristics. Combined with the abovementioned analysis, the biogeochemical conditions for methane formation, migration, and oxidation can be evaluated.3,10,18

Figure 9.

Figure 9

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Plot of δ13CCH4 and δ13CCO2 represent methane formation and consumption containing four groups of the CBM coproduced water samples in the No. 3 coal seam.

3.4.1. Biogenic Methane Generation (Group No. 1)

As shown in Table 1, group No. 1 with characteristics of δ13CCH4 (<−55‰) suggests the biogenic methane source. Negligible NO3– and SO42– concentrations of geochemical signatures indicate that 12 of 75 methane in group No. 1 originated from methanogenesis. The other 63 water samples may have undergone migration and/or oxidation.

Table 1. Categories Classification, Methane Type, and Biogeochemical Characteristicsa.
category group no. 1 group no. 2 group no. 3 group no. 4
N 12 19 4 40
δ13CCH4 (‰) < –55 < –55 > –55 > –55
methane high relatively high low relatively low
NO3– negligible relatively high low negligible
SO42– negligible relatively high low negligible
δ13CDIC positive negative negative  
CH4 type A B C D
redox conditions reductive oxidized oxidized relatively reductive
proportion (%) 18 25 5 52
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a

A: biogenic methane in situ. B: biogenic methane migrated into more oxidized environments without oxidation. C: biogenic methane migrated into more oxidized environments. D: thermogenic methane or mixed methane origin.

3.4.2. Biogenic Methane Migrated to More Oxidized Aquifers (Group No. 2)

Group No. 2 includes 19 water samples characterized by low δ13CCH4 values (<−55‰) and relatively high SO42– concentrations (>0.01 mmol/L) and NO3– concentration (>1 mmol/L). The methane isotopic characteristics do not agree with geochemical parameters, suggesting that in situ biogenic methane migrated from reductive aquifers to oxidized aquifers with no oxidation.3 The δ13CDIC values for methane oxidation are consistent with the expected values because there is no evidence of methane oxidation, which may be due to the short residence time, the microbial anaerobic methane oxidation, and the relatively slow turnover of methane oxidation (Table 1).

3.4.3. Shallow Methane Aquifer of Apparent or Pseudothermogenic Methane (Group No. 3)

The four water samples of group No. 3 characterized by δ13CCH4 values (>−55‰) suggest the thermogenic methane source or methane oxidation. The elevated δ13CCH4 values are usually associated with low δ13CDIC, SO42–, and elevated δ34SSO4 values. It is supposed that biogenic methane oxidation by BSR is the primary cause rather than thermogenic methane from deeper aquifers. The 13C in the remaining methane is enriched in the process, resulting in a relatively high δ13C value, which may be misunderstood as a characteristic of thermogenic methane.18,34 As shown in Table 1, the carbon isotopes of methane and DIC, δ34SSO4, and low methane concentrations further prove methane oxidation. Therefore, it is presumed that the water samples in group No. 3 are either influenced by methane oxidation or analytical uncertainty resulting from low methane concentrations rather than thermogenic methane migration from deeper aquifers.

3.4.4. Biogenic–Thermogenic Mixed Methane (Group No. 4)

Group No. 4 includes 40 water samples characterized by negligible SO42– and NO3–, and these samples are associated with relatively low methane concentrations and high δ13CCH4 (>−55‰) (Table 1). It is supposed that thermogenic methane or mixed biogenic and thermogenic methane has migrated to shallow aquifers and has partially oxidized (Figure 9).

3.5. Carbon Isotope Variation in Methane and Dissolved Inorganic Carbon

The isotopic characteristics of methane and carbon dioxide provide a tool for studying microbial oxidation of methane.35,36 The biological oxidation of methane causes significant changes in methane and carbon dioxide isotopic characteristics. It is because microorganisms preferentially utilize 12C in methane, resulting in 13C enrichment in residual methane and 13C enrichment in residual carbon dioxide.37,38 The C and H isotopic patterns of dissolved methane in four sites affected by methane oxidation show a continuous and increasing enrichment with a slope of the oxidation trend (Figure 10a). The slope is 7‰, suggesting that every permil δ13CCH4 value variation approximately causes 7‰ δ2HCH4 variation.

Figure 10.

Figure 10

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(a) Characteristics 13C and 2H of dissolved methane over time and (b) characteristics 13C of dissolved CH4 and CO2 reveal oxidation trends in several wells from the No. 3 coal seam.

There is a clear relationship in carbon isotope separation between methane and the coexistence of carbon dioxide during the biogenic methane oxidation process. Carbon isotope fractionation factors are written as α13CCO2–CH4 = (δ13CCO2 + 1000)/(δ13CCH4 + 1000) in the biogenic methane oxidation process. The carbon isotopes δ13CCH4 and δ13CCO2 were collected from the abovementioned four wells (Figure 10b). The three lines represent isotopic fractionations of 0.985, 1.005, and 1.030, respectively.39,40 The relatively low α13CCO2–CH4 results and depleted δ13CCO2 values of these wells in the study area result from extensive methane oxidation and production of 12C in DIC.41,42

4. Conclusions

This study aims to determine the methane source, migration, and oxidation in the Shizhuangnan block by integrating isotopic parameters and biogeochemical data of dissolved methane and aqueous species. Identifying redox processes such as AOM and BSR is essential for determining methane occurrence in subsurface aquifers. The oxidation of biogenic methane in groundwater usually results in pseudothermogenic carbon isotopes, which may be misinterpreted as thermogenic methane intrusion from deep reservoirs. Therefore, it is inaccurate to differentiate the methane formation solely based on methane isotopic features. This study uses the concentrations and isotopes of dissolved methane and isotopic compositions of DIC, sulfate, and nitrogen to resolve potential ambiguities of the thermogenic methane source or biogenic methane oxidation. Low δ13CCH4 values (<−55‰) accompanied with negligible NO3– and SO42– concentrations provide clear evidence for methanogenesis in situ. Low δ13CCH4 values (<−55‰) combined with high NO3– and SO42– concentrations and high δ13CDIC reveal that biogenic methane migrated into more oxidized aquifers without oxidation. The samples with high δ13CCH4 values (>−55‰) and low δ13CDIC values (<5) coexisting with negligible SO42– and methane concentrations suggest oxidation rather than pseudothermogenic methane. Elevated δ13CCH4 values (>−55‰) and low methane concentrations combined with negligible NO3– and SO42– concentrations can result from mixed methane oxidation. Therefore, quantification of the extent of methane oxidation determines the methane origin in groundwater environments. The method developed in this study can be extended to other unconventional natural gas development areas worldwide.

5. Materials and Methods

This study selected 75 coalbed methane wells with regular long-term drainage in the No. 3 coal seam from the Shizhuangnan block. For sustainable and effective CBM exploitation, all wells are located away from collapse columns and faults. Before CBM coproduced water sampling, sterilized polyethylene containers with sufficient 5 L volumes were made available for all testing and flushed more than three times with the coproduced water samples. During the water sampling process, the samples were directly collected from the outlets of the CBM wells with filter paper to remove solid residues. There is no contact between these outlets and the sampling bottles. The entire bottle was filled with water and closed using a lid immediately during the collection process. To avoid the influence of residual water in the drainage pipe, water samples were collected from CBM wells with stable water flow. Before being transferred to the laboratory, the water samples were stored in an incubator within 1–5 °C.

A total of 91 samples from these 75 wells were determined for main ions and isotopic parameters containing Cl–, HCO32–, Na+, NO3–, SO42–, dissolved inorganic carbon (DIC), dissolved methane, and isotopic compositions for carbon, hydrogen, oxygen, and nitrogen. Some wells were sampled repeatedly at regular intervals to assess water quality change over time. The sampling well locations used for this study are distributed in the central and western part of the Shizhuangnan block (Figure 1c). To collect representative data of aquifer environments, pH and oxidation/reduction potential (ORP) were measured in the field.

The filtered samples were chemically analyzed for main ions in the laboratory, and the samples were acidified to pH < 2 for cationic analysis. The analytical instruments used for the concentrations of cations and anions are inductively coupled plasma atomic emission spectroscopy (ICP-AES) and ion chromatography (IC), respectively. Furthermore, the compositions of dissolved gas were determined by gas chromatography. The isotopes of DIC (δ13CDIC), NO3– (δ15NNO3, δ18ONO3), SO42– (δ34SSO4, δ18OSO4), dissolved methane (δ13CCH4, δ2HCH4), and (δ13CCO2, δ18OCO2) were analyzed on a ThermoFisher MAT 253 isotope ratio mass spectrometer coupled to a Trace GC Ultra and GC Isolink. Stable isotopes were recorded in the internationally accepted delta notation (‰) relative to VPDB for δ13C values, VSMOW for δ2H and δ18O values, VCDT for 34S values, and N2 in the air for δ15N values. The reliability of test results was determined by repeated analysis of select samples from some wells.

Acknowledgments

This work was supported by the Foundation of Anhui University of Science and Technology (No. xjzd2020-05), the National Natural Science Foundation of China (No. 42102216), the NSFC-Shanxi Coal-based Low Carbon Joint Fund of China (No. U1910205), the Natural Science Foundation for Distinguished Young Scholars of Anhui Province (No. 1908085J14), and the National Natural Science Foundation of China (No. 41972173). We would like to acknowledge China United Coalbed Methane Corporation for providing the production well date. Special thanks are given to anonymous reviewers for their comments and suggestions.

The authors declare no competing financial interest.

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