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(A) Map of the structural outline in the Junggar Basin, China. (B) Coal seam burial history of the Xishanyao and Badaowan Formations in the southern Junggar Basin, modified from Wang et al. (2009). Pink, white, and purple shaded areas in (B) represent the burial-unlift process of Badaowan, Sangonghe, and Xishanyao Formations, respectively. (C) Composite stratigraphic column showing the coal-bearing formations in the southern Junggar Basin and the five aquifers. E = Paleogene; J 1 = Lower Jurassic; J 2 = Middle Jurassic; J 3 = Upper Jurassic; K 1 = Lower Cretaceous; K 2 = Upper Cretaceous; N = Neogene; Q = Quaternary.
Contexts in source publication
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... Ma), Indosinian (257-205 Ma), Yanshan (205-65 Ma), and Himalayan (65 Ma-present day) tectonic movements from the late Paleozoic to the Quaternary ( Scheltens et al., 2015). The Piedmont thrust belt is a superimposed multiphase structural belt that formed during the late Hercynian foreland basin stage and can be divided into five secondary structural units: the Sikeshu sag, the Qigu fault-fold belt, the Huomatu anticlinal zone, the Huan anticlinal zone, and the Fukang fault zone ( Fu et al., 2019) ( Figure 1A). Previous studies have shown that the SJB has been influenced by uplift of the northern Tianshan and Bogda Mountains since the middle Miocene ( Ma et al., 2015). ...
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... Xishanyao and Badaowan Formations are the main coal-bearing strata in the SJB ( Sun et al., 2012). The Sangonghe Formation, which separates the two units, contains only a few minor coalbeds ( Figure 1C). The sedimentary environments of the Badaowan Formation are fluvial and swamp facies, whereas the Xishanyao Formation includes delta, lacustrine, and swamp facies sediments ( Tian and Yang, 2011). ...
Context 3
... by multiple ice ages, the snowmelt in the northern Tianshan Mountains easily formed surface runoff and entered the surface water system. Several rivers are in the region, which flow from south to north ( Figure 1A), and their discharges change with the seasonal variation in snowmelt. The isotopic data indicate that these rivers are recharged by the snowmelt in the SJB, and the rivers, snowmelt, and underground water are closely related ( Yao et al., 2016). ...
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... isotopic data indicate that these rivers are recharged by the snowmelt in the SJB, and the rivers, snowmelt, and underground water are closely related ( Yao et al., 2016). Snowmelt actively recharges the five independent aquifers in the SJB, including the (1) Quaternary sandy conglomerate, (2) Jurassic sandstone in the lower part of the Toutunhe Formation, (3) Jurassic sandstone in the upper part of the Xishanyao Formation, (4) Jurassic sandstone in the lower part of the Xishanyao Formation, and (5) Jurassic sandstone in the upper part of the Badaowan Formation ( Figure 1C). The aquitards between the aquifers are composed of mudstone and silty mudstone. ...
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... the various ion concentrations exhibit differential changes along the flow paths, reflecting the water-rock reactions. As can be seen in Figure 10, HCO 3 -, Cl -, Na + , and K + concentrations and the TDS value all increase with increasing burial depth in Fukang and Miquan, indicating that water-rock reactions should have occurred between the coalbed water and the rock; however, SO 4 2-, Ca 2+ , and Mg 2+ do not correlate with burial depth in these regions because of the low ion concentrations in stagnant water environments. ...
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... both are similar to the test results reported by Yao et al. (2016) (Table 4). Figure 11 shows the stable isotopic compositions of all of the water samples with respect to the GMWL established by Craig (1961): ...
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... general, the GMWL and LMWL are similar, and they both characterize the isotopic composition of the meteoric water in the SJB. As can be seen in Figure 11, the river water samples from Houxia and Manasi-Hutubi plot around the LMWL, indicating that the river water is closely associated with modern meteoric water. In addition, the river water samples plot in the topmost position on the LMWL, with heavier hydrogen and oxygen isotopes. ...
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... can be seen in Figure 11, the CBM-coproduced water samples from Houxia and Manasi-Hutubi also plot around the LMWL, indicating that the coalbed Table 1 for the locations of the water samples. ...
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... is, the hydrodynamic field is more active in these regions, and the coal measure strata can receive supplies from modern water. In addition to this, the coalbed water samples from Fukang and Miquan plot below the LMWL (i.e., the 18 O drift), with a more evident shift in Fukang (Figure 11). However, hydrogen isotope exchange commonly occurs between coalbed water and coal, CH 4 , and H 2 S in a sealed water environment, which produces D drift (i.e., shift to the left) ( Chen et al., 2008). ...
Context 10
... this study, the Fukang and Miquan regions belong to the hydrodynamic stagnant zone, and the high TDS values (similar to basinal brines) should exhibit a significant shift to the right. As can be seen in Figure 11, the modern snowmelt water line is located to the lower right of the LWML ( Yao et al., 2016). Therefore, the coalbed water in these regions may be mainly supplied by much older snowmelt, which probably plots to the lower right. ...
Context 11
... have shown that soluble hydrogen-bearing and oxygen-bearing minerals continuously exchange isotopic compositions with groundwater during groundwater migration ( Li et al., 2015). As shown in Figure 12A, B, the TDS value in Fukang and Miquan shows a significant negative correlation with burial depth (i.e., the TDS value tends to increase along the direction of groundwater migration). Moreover, the dD and d 18 O values of the coalbed water in Fukang and Miquan show a significant negative correlation with the TDS values ( Figure 12B-D). ...
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... shown in Figure 12A, B, the TDS value in Fukang and Miquan shows a significant negative correlation with burial depth (i.e., the TDS value tends to increase along the direction of groundwater migration). Moreover, the dD and d 18 O values of the coalbed water in Fukang and Miquan show a significant negative correlation with the TDS values ( Figure 12B-D). As discussed above, the TDS value of the coalbed water increases along the flow paths, and the deeper areas are thought to be less affected by evaporation (i.e., the lighter isotopic compositions). ...
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... can be seen in Figure 2, one spring water sample collected from the Fukang region had dD and d 18 O values of -87.67‰ and -11.85‰, respectively (Table 3). The tectonic positions of CBM well 17 (i.e., -90.33‰ and -12.54‰) in Fukang and CBM well 32 (i.e., -85.73‰ and -12.39‰) in Miquan are considered to be closer to the recharge zone of the surface water ( Figure 12; Table 3) since their hydrogen and oxygen isotopes are similar to those of the spring water sample. In addition, the burial depth increases, whereas the TDS value decreases from Houxia (south) to Manasi-Hutubi (north) ( Figure 12A, B), further indicating that these two hydrogeological units are completely independent of each other. ...
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... tectonic positions of CBM well 17 (i.e., -90.33‰ and -12.54‰) in Fukang and CBM well 32 (i.e., -85.73‰ and -12.39‰) in Miquan are considered to be closer to the recharge zone of the surface water ( Figure 12; Table 3) since their hydrogen and oxygen isotopes are similar to those of the spring water sample. In addition, the burial depth increases, whereas the TDS value decreases from Houxia (south) to Manasi-Hutubi (north) ( Figure 12A, B), further indicating that these two hydrogeological units are completely independent of each other. ...
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... CBM in the Xishanyao Formation in Miquan is identified as a typical PMG (CR dominate), and it shows almost no signs of thermogenic gas even when its depth reaches 1165 m (Table 2). Several other lines of evidence include the following: (1) no linear relationship exists between d 13 C-C 1 and burial depth ( Figure 13B), whereas a linear relationship is characteristic of thermogenic gas ( Figure 13A); and (2) the CBM in Miquan has only C 1 , C 2 , and C 3 as hydrocarbon gas (C 2 and C 3 are present in much smaller amounts than C 1 ) (Table 4), and microbes can only generate these three hydrocarbon compounds (Oremland et al., 1988;Hinrichs et al., 2006). Moreover, abnormally high CO 2 (up to 40%) is another characteristic of the CBM in Miquan, and it shows an increasing trend with increasing burial depth ( Figure 14B). ...
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... CBM in the Xishanyao Formation in Miquan is identified as a typical PMG (CR dominate), and it shows almost no signs of thermogenic gas even when its depth reaches 1165 m (Table 2). Several other lines of evidence include the following: (1) no linear relationship exists between d 13 C-C 1 and burial depth ( Figure 13B), whereas a linear relationship is characteristic of thermogenic gas ( Figure 13A); and (2) the CBM in Miquan has only C 1 , C 2 , and C 3 as hydrocarbon gas (C 2 and C 3 are present in much smaller amounts than C 1 ) (Table 4), and microbes can only generate these three hydrocarbon compounds (Oremland et al., 1988;Hinrichs et al., 2006). Moreover, abnormally high CO 2 (up to 40%) is another characteristic of the CBM in Miquan, and it shows an increasing trend with increasing burial depth ( Figure 14B). ...
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... other lines of evidence include the following: (1) no linear relationship exists between d 13 C-C 1 and burial depth ( Figure 13B), whereas a linear relationship is characteristic of thermogenic gas ( Figure 13A); and (2) the CBM in Miquan has only C 1 , C 2 , and C 3 as hydrocarbon gas (C 2 and C 3 are present in much smaller amounts than C 1 ) (Table 4), and microbes can only generate these three hydrocarbon compounds (Oremland et al., 1988;Hinrichs et al., 2006). Moreover, abnormally high CO 2 (up to 40%) is another characteristic of the CBM in Miquan, and it shows an increasing trend with increasing burial depth ( Figure 14B). A recent study performed by Fu et al. (2019) on the changes in the regional hydrodynamic field indicates that Miquan is the catchment egion in the SJB. ...
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... The high TDS values (up to 45,000 mg/L) suggest that present-day coalbed water is not good for the survival of methanogens and their methanogenesis ( Fu et al., 2019). (3) The low gas content of the shallow coal seam (much <2 m 3 /t) indicates that the Miquan region received little or no supply of modern microbial gas ( Figure 14B). (4) The abnormally high CO 2 (up to 40%) suggests that microbial gas generation should have stopped in the present stage because CR is the main formation pathways of microbial gas in Miquan (i.e., conversion of CO 2 to CH 4 ). ...
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... the SJB, hydrocarbon generation (i.e., thermogenic gas) in the coal seams of the Badaowan and Xishanyao Formations commenced in the Early Cretaceous (R o ranging from 0.5% to 0.8%), with maximum burial occurring during the Late Cretaceous ( Wang et al., 2009) and the coal seams having been continuously uplifted since the Cenozoic (Figure 1B). At a suitable burial depth with suitable hydrogeologic conditions, the coal measure strata can receive a supply of surface water carrying microbes, and PMGs or biodegraded thermogenic gases are generated by the reaction of the microbes with the coal or thermogenic gases. ...
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... a suitable burial depth with suitable hydrogeologic conditions, the coal measure strata can receive a supply of surface water carrying microbes, and PMGs or biodegraded thermogenic gases are generated by the reaction of the microbes with the coal or thermogenic gases. However, the analysis suggests that the Xishanyao coal in Miquan may not have reached the temperature and pressure necessary for the massive formation of thermogenic gas during subsidence (step 1, Figure 14A). As shown in Table 6, the R o of the Xishanyao coal is low in Miquan (from 0.50% to 0.70%), and the R o value of CBM well MQ-1 is only 0.68%, even when its burial depth reaches 1565 m. ...
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... this, the Xishanyao coal was continuously uplifted and eroded until the Quaternary when it began to receive a supply of microbe-carrying surface snowmelt water (step 2, Figure 14A), and PMGs began to actively generate because of shallow microbial activities (step 3, Figure 14A). As the groundwater flowed basinward, the dissolved microbial gases migrated progressively downward, which resulted in changes in the gas composition and the isotopic composition (i.e., migration fractionation). ...
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... this, the Xishanyao coal was continuously uplifted and eroded until the Quaternary when it began to receive a supply of microbe-carrying surface snowmelt water (step 2, Figure 14A), and PMGs began to actively generate because of shallow microbial activities (step 3, Figure 14A). As the groundwater flowed basinward, the dissolved microbial gases migrated progressively downward, which resulted in changes in the gas composition and the isotopic composition (i.e., migration fractionation). ...
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... under the sealing effects of the hydrodynamic field, PMGs can be effectively trapped in Miquan (step 4, Figure 14A). Vertical variations in the gas content and gas components of the CBM exploratory wells in Miquan (e.g., well-I shown in Figure 14B) indicate that PMGs can be more effectively sealed in deep coal reservoirs (i.e., high CH 4 and high CO 2 ), but shallow microbial gases tend to escape to some extent (i.e., low CH 4 and low CO 2 ) because of poor sealing conditions. ...
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... under the sealing effects of the hydrodynamic field, PMGs can be effectively trapped in Miquan (step 4, Figure 14A). Vertical variations in the gas content and gas components of the CBM exploratory wells in Miquan (e.g., well-I shown in Figure 14B) indicate that PMGs can be more effectively sealed in deep coal reservoirs (i.e., high CH 4 and high CO 2 ), but shallow microbial gases tend to escape to some extent (i.e., low CH 4 and low CO 2 ) because of poor sealing conditions. In addition to this, microbial methanogenesis may have stopped at this stage, and the lack of present-day microbial gas supply could be another reason for the low gas content in the shallow coal seams. ...
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... lines of evidence include the following. (1) An evident linear relationship exists between d 13 C-C 1 and burial depth ( Figure 13A) (i.e., as burial depth increases the d 13 C becomes heavier). (2) Butane (C 4 ) has been identified in Fukang, which, to the best of our knowledge, cannot be produced by microbes (Table 4). ...
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... Butane (C 4 ) has been identified in Fukang, which, to the best of our knowledge, cannot be produced by microbes (Table 4). (3) The d 13 C of the C 1 -C 4 plots as a semistraight line on the Chung plot in Miquan ( Chung et al., 1988), suggesting the limited mixing of gases of different origins ( Figure 15; Table 7). Moreover, the thermogenic CBM also exhibits evident characteristics of microbial degradation in Fukang. ...
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... shown in Table 5, the R o values of the Badaowan coal (0.64%-0.94%) in Fukang are significantly greater than those of the Xishanyao coal (0.50%-0.70%) in Miquan. That is, the Badaowan coal in Fukang should have reached the stage in which the massive formation of thermogenic gas occurs (step 1, Figure 16A), whereas the Xishanyao coal in Miquan may not have reached the hydrocarbon generation threshold yet. Next, the Xishanyao coal has almost been removed as a result of tectonic movements (e.g., Indosinian and Yanshan) in Fukang, whereas the Badaowan coal has been gradually uplifted to near the surface and has received a supply of surface snowmelt water containing microbes (step 2, Figure 16A). ...
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... is, the Badaowan coal in Fukang should have reached the stage in which the massive formation of thermogenic gas occurs (step 1, Figure 16A), whereas the Xishanyao coal in Miquan may not have reached the hydrocarbon generation threshold yet. Next, the Xishanyao coal has almost been removed as a result of tectonic movements (e.g., Indosinian and Yanshan) in Fukang, whereas the Badaowan coal has been gradually uplifted to near the surface and has received a supply of surface snowmelt water containing microbes (step 2, Figure 16A). In addition, the microbes began to metabolize the thermogenic wet gases that were generated during coalification, or they directly reacted with the Badaowan coal, generating a massive amount of biodegraded thermogenic gases and less PMGs (step 3, Figure 16A). ...
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... the Xishanyao coal has almost been removed as a result of tectonic movements (e.g., Indosinian and Yanshan) in Fukang, whereas the Badaowan coal has been gradually uplifted to near the surface and has received a supply of surface snowmelt water containing microbes (step 2, Figure 16A). In addition, the microbes began to metabolize the thermogenic wet gases that were generated during coalification, or they directly reacted with the Badaowan coal, generating a massive amount of biodegraded thermogenic gases and less PMGs (step 3, Figure 16A). Soon after this, climate change interrupted the recharge from surface snowmelt water and gradually formed a relatively closed system in Fukang. ...
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... this stage, the existing CO 2 was progressively converted to CH 4 by CO 2 -reducing methanogenesis, and the residual CO 2 became progressively enriched in 13 C, leading to abnormally high positive d 13 C-CO 2 values. Ultimately, the hydrodynamic field became stagnant in Fukang, resulting in high TDS values, which provide effectively hydrodynamic sealing for the biodegraded thermogenic gases (step 4, Figure 16A). Biodegraded thermogenic gases can be effectively preserved in a shallow coal reservoir (e.g., well-II shown in Figure 16B), resulting in a relatively shallow CBM oxidation zone (<468 m [<1535 ft]). ...
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... the hydrodynamic field became stagnant in Fukang, resulting in high TDS values, which provide effectively hydrodynamic sealing for the biodegraded thermogenic gases (step 4, Figure 16A). Biodegraded thermogenic gases can be effectively preserved in a shallow coal reservoir (e.g., well-II shown in Figure 16B), resulting in a relatively shallow CBM oxidation zone (<468 m [<1535 ft]). ...
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... the Xishanyao CBM (<1000 m [<3281 ft]) is considered to be the PMG (CR dominate) in Manasi-Hutubi, whereas the deeper coal reservoir exhibits the characteristics of thermogenic gas. Given this, in the Manasi-Hutubi region, we propose a CBM accumulation mode for the PMG supply in shallow areas and thermogenic gas escape in deeper areas within the hydrodynamic active zone (Figure 17). Our analysis suggests that the Xishanyao coal is the main coal seam in ManasiHutubi, and the R o values (0.60%-0.79%) are significantly greater than those of the Xishanyao coal in Miquan (0.50%-0.70%). ...
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... analysis suggests that the Xishanyao coal is the main coal seam in ManasiHutubi, and the R o values (0.60%-0.79%) are significantly greater than those of the Xishanyao coal in Miquan (0.50%-0.70%). That is, during burial and coalification, the Xishanyao coal in Manasi-Hutubi reached the massive formation of thermogenic gas stage (step 1, Figure 17A), whereas the Xishanyao coal in Miquan did not. Subsequently, the Xishanyao coal has been gradually uplifted to near the surface under the effects of tectonic movements, and the existing thermogenic gases began to continually escape. ...
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... the Xishanyao coal has been gradually uplifted to near the surface under the effects of tectonic movements, and the existing thermogenic gases began to continually escape. Meanwhile, surface snowmelt water carried microbes into the shallow coal seams caused by the active hydrodynamic conditions (step 2, Figure 17A). Soon after this, the microbes progressively degraded the coal OM to generate PMGs (including CH 4 and CO 2 ) (step 3, Figure 17A). ...
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... surface snowmelt water carried microbes into the shallow coal seams caused by the active hydrodynamic conditions (step 2, Figure 17A). Soon after this, the microbes progressively degraded the coal OM to generate PMGs (including CH 4 and CO 2 ) (step 3, Figure 17A). Because it is located close to the snowmelt zone, recharge by surface snowmelt water continued until the present day in Manasi-Hutubi, developing a relatively open or semiopen system. ...
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... microbial methanogenesis was able to continue to the present day in Manasi-Hutubi, whereas it was not in Miquan and Fukang. Ultimately, the microbial gases dissolved in the groundwater moved continually downward, and then they mixed with the escaped thermogenic gases in the relatively deep coal reservoir (step 4, Figure 17A). Taking 1000 m as the boundary, the Xishanyao CBM is dominated by PMG (CR) (<1000 m), including a certain amount of thermogenic gas derived from a deep coal reservoir. ...
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... the CO 2 concentration and an increase in the N 2 concentration ( Figure 17B). Based on our analyses, the Lower-Middle Jurassic strata (i.e., Xishanyao and Badaowan Formations) experienced simple subsidence and uplift, but the extent of the subsidence (reflecting the degree of coalification) may have differed in the various regions of the SJB. ...
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... 1b = Jurassic Badaowan Formation; P = Permian strata; T = Triassic strata. Figure 17. (A) The formation steps of the primary microbial gas supply (shallow) and thermogenic gas escape (deep) within the hydrodynamically active zone in the Manasi-Hutubi region. ...
Context 39
... Ma), Indosinian (257-205 Ma), Yanshan (205-65 Ma), and Himalayan (65 Ma-present day) tectonic movements from the late Paleozoic to the Quaternary ( Scheltens et al., 2015). The Piedmont thrust belt is a superimposed multiphase structural belt that formed during the late Hercynian foreland basin stage and can be divided into five secondary structural units: the Sikeshu sag, the Qigu fault-fold belt, the Huomatu anticlinal zone, the Huan anticlinal zone, and the Fukang fault zone ( Fu et al., 2019) ( Figure 1A). Previous studies have shown that the SJB has been influenced by uplift of the northern Tianshan and Bogda Mountains since the middle Miocene ( Ma et al., 2015). ...
Context 40
... Xishanyao and Badaowan Formations are the main coal-bearing strata in the SJB ( Sun et al., 2012). The Sangonghe Formation, which separates the two units, contains only a few minor coalbeds ( Figure 1C). The sedimentary environments of the Badaowan Formation are fluvial and swamp facies, whereas the Xishanyao Formation includes delta, lacustrine, and swamp facies sediments ( Tian and Yang, 2011). ...
Context 41
... by multiple ice ages, the snowmelt in the northern Tianshan Mountains easily formed surface runoff and entered the surface water system. Several rivers are in the region, which flow from south to north ( Figure 1A), and their discharges change with the seasonal variation in snowmelt. The isotopic data indicate that these rivers are recharged by the snowmelt in the SJB, and the rivers, snowmelt, and underground water are closely related ( Yao et al., 2016). ...
Context 42
... isotopic data indicate that these rivers are recharged by the snowmelt in the SJB, and the rivers, snowmelt, and underground water are closely related ( Yao et al., 2016). Snowmelt actively recharges the five independent aquifers in the SJB, including the (1) Quaternary sandy conglomerate, (2) Jurassic sandstone in the lower part of the Toutunhe Formation, (3) Jurassic sandstone in the upper part of the Xishanyao Formation, (4) Jurassic sandstone in the lower part of the Xishanyao Formation, and (5) Jurassic sandstone in the upper part of the Badaowan Formation ( Figure 1C). The aquitards between the aquifers are composed of mudstone and silty mudstone. ...
Context 43
... the various ion concentrations exhibit differential changes along the flow paths, reflecting the water-rock reactions. As can be seen in Figure 10, HCO 3 -, Cl -, Na + , and K + concentrations and the TDS value all increase with increasing burial depth in Fukang and Miquan, indicating that water-rock reactions should have occurred between the coalbed water and the rock; however, SO 4 2-, Ca 2+ , and Mg 2+ do not correlate with burial depth in these regions because of the low ion concentrations in stagnant water environments. ...
Context 44
... both are similar to the test results reported by Yao et al. (2016) (Table 4). Figure 11 shows the stable isotopic compositions of all of the water samples with respect to the GMWL established by Craig (1961): ...
Context 45
... general, the GMWL and LMWL are similar, and they both characterize the isotopic composition of the meteoric water in the SJB. As can be seen in Figure 11, the river water samples from Houxia and Manasi-Hutubi plot around the LMWL, indicating that the river water is closely associated with modern meteoric water. In addition, the river water samples plot in the topmost position on the LMWL, with heavier hydrogen and oxygen isotopes. ...
Context 46
... can be seen in Figure 11, the CBM-coproduced water samples from Houxia and Manasi-Hutubi also plot around the LMWL, indicating that the coalbed Table 1 for the locations of the water samples. ...
Context 47
... is, the hydrodynamic field is more active in these regions, and the coal measure strata can receive supplies from modern water. In addition to this, the coalbed water samples from Fukang and Miquan plot below the LMWL (i.e., the 18 O drift), with a more evident shift in Fukang (Figure 11). However, hydrogen isotope exchange commonly occurs between coalbed water and coal, CH 4 , and H 2 S in a sealed water environment, which produces D drift (i.e., shift to the left) ( Chen et al., 2008). ...
Context 48
... this study, the Fukang and Miquan regions belong to the hydrodynamic stagnant zone, and the high TDS values (similar to basinal brines) should exhibit a significant shift to the right. As can be seen in Figure 11, the modern snowmelt water line is located to the lower right of the LWML ( Yao et al., 2016). Therefore, the coalbed water in these regions may be mainly supplied by much older snowmelt, which probably plots to the lower right. ...
Context 49
... have shown that soluble hydrogen-bearing and oxygen-bearing minerals continuously exchange isotopic compositions with groundwater during groundwater migration ( Li et al., 2015). As shown in Figure 12A, B, the TDS value in Fukang and Miquan shows a significant negative correlation with burial depth (i.e., the TDS value tends to increase along the direction of groundwater migration). Moreover, the dD and d 18 O values of the coalbed water in Fukang and Miquan show a significant negative correlation with the TDS values ( Figure 12B-D). ...
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... shown in Figure 12A, B, the TDS value in Fukang and Miquan shows a significant negative correlation with burial depth (i.e., the TDS value tends to increase along the direction of groundwater migration). Moreover, the dD and d 18 O values of the coalbed water in Fukang and Miquan show a significant negative correlation with the TDS values ( Figure 12B-D). As discussed above, the TDS value of the coalbed water increases along the flow paths, and the deeper areas are thought to be less affected by evaporation (i.e., the lighter isotopic compositions). ...
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... can be seen in Figure 2, one spring water sample collected from the Fukang region had dD and d 18 O values of -87.67‰ and -11.85‰, respectively (Table 3). The tectonic positions of CBM well 17 (i.e., -90.33‰ and -12.54‰) in Fukang and CBM well 32 (i.e., -85.73‰ and -12.39‰) in Miquan are considered to be closer to the recharge zone of the surface water ( Figure 12; Table 3) since their hydrogen and oxygen isotopes are similar to those of the spring water sample. In addition, the burial depth increases, whereas the TDS value decreases from Houxia (south) to Manasi-Hutubi (north) ( Figure 12A, B), further indicating that these two hydrogeological units are completely independent of each other. ...
Context 52
... tectonic positions of CBM well 17 (i.e., -90.33‰ and -12.54‰) in Fukang and CBM well 32 (i.e., -85.73‰ and -12.39‰) in Miquan are considered to be closer to the recharge zone of the surface water ( Figure 12; Table 3) since their hydrogen and oxygen isotopes are similar to those of the spring water sample. In addition, the burial depth increases, whereas the TDS value decreases from Houxia (south) to Manasi-Hutubi (north) ( Figure 12A, B), further indicating that these two hydrogeological units are completely independent of each other. ...
Context 53
... CBM in the Xishanyao Formation in Miquan is identified as a typical PMG (CR dominate), and it shows almost no signs of thermogenic gas even when its depth reaches 1165 m (Table 2). Several other lines of evidence include the following: (1) no linear relationship exists between d 13 C-C 1 and burial depth ( Figure 13B), whereas a linear relationship is characteristic of thermogenic gas ( Figure 13A); and (2) the CBM in Miquan has only C 1 , C 2 , and C 3 as hydrocarbon gas (C 2 and C 3 are present in much smaller amounts than C 1 ) (Table 4), and microbes can only generate these three hydrocarbon compounds (Oremland et al., 1988;Hinrichs et al., 2006). Moreover, abnormally high CO 2 (up to 40%) is another characteristic of the CBM in Miquan, and it shows an increasing trend with increasing burial depth ( Figure 14B). ...
Context 54
... CBM in the Xishanyao Formation in Miquan is identified as a typical PMG (CR dominate), and it shows almost no signs of thermogenic gas even when its depth reaches 1165 m (Table 2). Several other lines of evidence include the following: (1) no linear relationship exists between d 13 C-C 1 and burial depth ( Figure 13B), whereas a linear relationship is characteristic of thermogenic gas ( Figure 13A); and (2) the CBM in Miquan has only C 1 , C 2 , and C 3 as hydrocarbon gas (C 2 and C 3 are present in much smaller amounts than C 1 ) (Table 4), and microbes can only generate these three hydrocarbon compounds (Oremland et al., 1988;Hinrichs et al., 2006). Moreover, abnormally high CO 2 (up to 40%) is another characteristic of the CBM in Miquan, and it shows an increasing trend with increasing burial depth ( Figure 14B). ...
Context 55
... other lines of evidence include the following: (1) no linear relationship exists between d 13 C-C 1 and burial depth ( Figure 13B), whereas a linear relationship is characteristic of thermogenic gas ( Figure 13A); and (2) the CBM in Miquan has only C 1 , C 2 , and C 3 as hydrocarbon gas (C 2 and C 3 are present in much smaller amounts than C 1 ) (Table 4), and microbes can only generate these three hydrocarbon compounds (Oremland et al., 1988;Hinrichs et al., 2006). Moreover, abnormally high CO 2 (up to 40%) is another characteristic of the CBM in Miquan, and it shows an increasing trend with increasing burial depth ( Figure 14B). A recent study performed by Fu et al. (2019) on the changes in the regional hydrodynamic field indicates that Miquan is the catchment egion in the SJB. ...
Context 56
... The high TDS values (up to 45,000 mg/L) suggest that present-day coalbed water is not good for the survival of methanogens and their methanogenesis ( Fu et al., 2019). (3) The low gas content of the shallow coal seam (much <2 m 3 /t) indicates that the Miquan region received little or no supply of modern microbial gas ( Figure 14B). (4) The abnormally high CO 2 (up to 40%) suggests that microbial gas generation should have stopped in the present stage because CR is the main formation pathways of microbial gas in Miquan (i.e., conversion of CO 2 to CH 4 ). ...
Context 57
... the SJB, hydrocarbon generation (i.e., thermogenic gas) in the coal seams of the Badaowan and Xishanyao Formations commenced in the Early Cretaceous (R o ranging from 0.5% to 0.8%), with maximum burial occurring during the Late Cretaceous ( Wang et al., 2009) and the coal seams having been continuously uplifted since the Cenozoic (Figure 1B). At a suitable burial depth with suitable hydrogeologic conditions, the coal measure strata can receive a supply of surface water carrying microbes, and PMGs or biodegraded thermogenic gases are generated by the reaction of the microbes with the coal or thermogenic gases. ...
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... a suitable burial depth with suitable hydrogeologic conditions, the coal measure strata can receive a supply of surface water carrying microbes, and PMGs or biodegraded thermogenic gases are generated by the reaction of the microbes with the coal or thermogenic gases. However, the analysis suggests that the Xishanyao coal in Miquan may not have reached the temperature and pressure necessary for the massive formation of thermogenic gas during subsidence (step 1, Figure 14A). As shown in Table 6, the R o of the Xishanyao coal is low in Miquan (from 0.50% to 0.70%), and the R o value of CBM well MQ-1 is only 0.68%, even when its burial depth reaches 1565 m. ...
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... this, the Xishanyao coal was continuously uplifted and eroded until the Quaternary when it began to receive a supply of microbe-carrying surface snowmelt water (step 2, Figure 14A), and PMGs began to actively generate because of shallow microbial activities (step 3, Figure 14A). As the groundwater flowed basinward, the dissolved microbial gases migrated progressively downward, which resulted in changes in the gas composition and the isotopic composition (i.e., migration fractionation). ...
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... this, the Xishanyao coal was continuously uplifted and eroded until the Quaternary when it began to receive a supply of microbe-carrying surface snowmelt water (step 2, Figure 14A), and PMGs began to actively generate because of shallow microbial activities (step 3, Figure 14A). As the groundwater flowed basinward, the dissolved microbial gases migrated progressively downward, which resulted in changes in the gas composition and the isotopic composition (i.e., migration fractionation). ...
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... under the sealing effects of the hydrodynamic field, PMGs can be effectively trapped in Miquan (step 4, Figure 14A). Vertical variations in the gas content and gas components of the CBM exploratory wells in Miquan (e.g., well-I shown in Figure 14B) indicate that PMGs can be more effectively sealed in deep coal reservoirs (i.e., high CH 4 and high CO 2 ), but shallow microbial gases tend to escape to some extent (i.e., low CH 4 and low CO 2 ) because of poor sealing conditions. ...
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... under the sealing effects of the hydrodynamic field, PMGs can be effectively trapped in Miquan (step 4, Figure 14A). Vertical variations in the gas content and gas components of the CBM exploratory wells in Miquan (e.g., well-I shown in Figure 14B) indicate that PMGs can be more effectively sealed in deep coal reservoirs (i.e., high CH 4 and high CO 2 ), but shallow microbial gases tend to escape to some extent (i.e., low CH 4 and low CO 2 ) because of poor sealing conditions. In addition to this, microbial methanogenesis may have stopped at this stage, and the lack of present-day microbial gas supply could be another reason for the low gas content in the shallow coal seams. ...
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... lines of evidence include the following. (1) An evident linear relationship exists between d 13 C-C 1 and burial depth ( Figure 13A) (i.e., as burial depth increases the d 13 C becomes heavier). (2) Butane (C 4 ) has been identified in Fukang, which, to the best of our knowledge, cannot be produced by microbes (Table 4). ...
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... Butane (C 4 ) has been identified in Fukang, which, to the best of our knowledge, cannot be produced by microbes (Table 4). (3) The d 13 C of the C 1 -C 4 plots as a semistraight line on the Chung plot in Miquan ( Chung et al., 1988), suggesting the limited mixing of gases of different origins ( Figure 15; Table 7). Moreover, the thermogenic CBM also exhibits evident characteristics of microbial degradation in Fukang. ...
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... shown in Table 5, the R o values of the Badaowan coal (0.64%-0.94%) in Fukang are significantly greater than those of the Xishanyao coal (0.50%-0.70%) in Miquan. That is, the Badaowan coal in Fukang should have reached the stage in which the massive formation of thermogenic gas occurs (step 1, Figure 16A), whereas the Xishanyao coal in Miquan may not have reached the hydrocarbon generation threshold yet. Next, the Xishanyao coal has almost been removed as a result of tectonic movements (e.g., Indosinian and Yanshan) in Fukang, whereas the Badaowan coal has been gradually uplifted to near the surface and has received a supply of surface snowmelt water containing microbes (step 2, Figure 16A). ...
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... is, the Badaowan coal in Fukang should have reached the stage in which the massive formation of thermogenic gas occurs (step 1, Figure 16A), whereas the Xishanyao coal in Miquan may not have reached the hydrocarbon generation threshold yet. Next, the Xishanyao coal has almost been removed as a result of tectonic movements (e.g., Indosinian and Yanshan) in Fukang, whereas the Badaowan coal has been gradually uplifted to near the surface and has received a supply of surface snowmelt water containing microbes (step 2, Figure 16A). In addition, the microbes began to metabolize the thermogenic wet gases that were generated during coalification, or they directly reacted with the Badaowan coal, generating a massive amount of biodegraded thermogenic gases and less PMGs (step 3, Figure 16A). ...
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... the Xishanyao coal has almost been removed as a result of tectonic movements (e.g., Indosinian and Yanshan) in Fukang, whereas the Badaowan coal has been gradually uplifted to near the surface and has received a supply of surface snowmelt water containing microbes (step 2, Figure 16A). In addition, the microbes began to metabolize the thermogenic wet gases that were generated during coalification, or they directly reacted with the Badaowan coal, generating a massive amount of biodegraded thermogenic gases and less PMGs (step 3, Figure 16A). Soon after this, climate change interrupted the recharge from surface snowmelt water and gradually formed a relatively closed system in Fukang. ...
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... this stage, the existing CO 2 was progressively converted to CH 4 by CO 2 -reducing methanogenesis, and the residual CO 2 became progressively enriched in 13 C, leading to abnormally high positive d 13 C-CO 2 values. Ultimately, the hydrodynamic field became stagnant in Fukang, resulting in high TDS values, which provide effectively hydrodynamic sealing for the biodegraded thermogenic gases (step 4, Figure 16A). Biodegraded thermogenic gases can be effectively preserved in a shallow coal reservoir (e.g., well-II shown in Figure 16B), resulting in a relatively shallow CBM oxidation zone (<468 m [<1535 ft]). ...
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... the hydrodynamic field became stagnant in Fukang, resulting in high TDS values, which provide effectively hydrodynamic sealing for the biodegraded thermogenic gases (step 4, Figure 16A). Biodegraded thermogenic gases can be effectively preserved in a shallow coal reservoir (e.g., well-II shown in Figure 16B), resulting in a relatively shallow CBM oxidation zone (<468 m [<1535 ft]). ...
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... the Xishanyao CBM (<1000 m [<3281 ft]) is considered to be the PMG (CR dominate) in Manasi-Hutubi, whereas the deeper coal reservoir exhibits the characteristics of thermogenic gas. Given this, in the Manasi-Hutubi region, we propose a CBM accumulation mode for the PMG supply in shallow areas and thermogenic gas escape in deeper areas within the hydrodynamic active zone (Figure 17). Our analysis suggests that the Xishanyao coal is the main coal seam in ManasiHutubi, and the R o values (0.60%-0.79%) are significantly greater than those of the Xishanyao coal in Miquan (0.50%-0.70%). ...
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... analysis suggests that the Xishanyao coal is the main coal seam in ManasiHutubi, and the R o values (0.60%-0.79%) are significantly greater than those of the Xishanyao coal in Miquan (0.50%-0.70%). That is, during burial and coalification, the Xishanyao coal in Manasi-Hutubi reached the massive formation of thermogenic gas stage (step 1, Figure 17A), whereas the Xishanyao coal in Miquan did not. Subsequently, the Xishanyao coal has been gradually uplifted to near the surface under the effects of tectonic movements, and the existing thermogenic gases began to continually escape. ...
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... the Xishanyao coal has been gradually uplifted to near the surface under the effects of tectonic movements, and the existing thermogenic gases began to continually escape. Meanwhile, surface snowmelt water carried microbes into the shallow coal seams caused by the active hydrodynamic conditions (step 2, Figure 17A). Soon after this, the microbes progressively degraded the coal OM to generate PMGs (including CH 4 and CO 2 ) (step 3, Figure 17A). ...
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... surface snowmelt water carried microbes into the shallow coal seams caused by the active hydrodynamic conditions (step 2, Figure 17A). Soon after this, the microbes progressively degraded the coal OM to generate PMGs (including CH 4 and CO 2 ) (step 3, Figure 17A). Because it is located close to the snowmelt zone, recharge by surface snowmelt water continued until the present day in Manasi-Hutubi, developing a relatively open or semiopen system. ...
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... microbial methanogenesis was able to continue to the present day in Manasi-Hutubi, whereas it was not in Miquan and Fukang. Ultimately, the microbial gases dissolved in the groundwater moved continually downward, and then they mixed with the escaped thermogenic gases in the relatively deep coal reservoir (step 4, Figure 17A). Taking 1000 m as the boundary, the Xishanyao CBM is dominated by PMG (CR) (<1000 m), including a certain amount of thermogenic gas derived from a deep coal reservoir. ...
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... the CO 2 concentration and an increase in the N 2 concentration ( Figure 17B). Based on our analyses, the Lower-Middle Jurassic strata (i.e., Xishanyao and Badaowan Formations) experienced simple subsidence and uplift, but the extent of the subsidence (reflecting the degree of coalification) may have differed in the various regions of the SJB. ...
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Citations
... The combined use of 129 I/ 127 I ratio, 3 H and 14 C is often utilized to determine boundary values for periods of formation water [42][43][44]. 3 H, 14 C as well as 129 I/ 127 I ratio in the southern Junggar Basin were tested to determine the periods of coalbed water older than 0.0435 Ma and younger than 2 Ma [44]. Combined with the hydrogen and oxygen isotopes drift signature of coalbed water samples (Fig. 6), it is determined that the formation water component of the closed hydrogeological unit (i.e., Miquan and Fukang) is predominantly snowmelt water of 0.045 to 2 Ma. ...
... The combined use of 129 I/ 127 I ratio, 3 H and 14 C is often utilized to determine boundary values for periods of formation water [42][43][44]. 3 H, 14 C as well as 129 I/ 127 I ratio in the southern Junggar Basin were tested to determine the periods of coalbed water older than 0.0435 Ma and younger than 2 Ma [44]. Combined with the hydrogen and oxygen isotopes drift signature of coalbed water samples (Fig. 6), it is determined that the formation water component of the closed hydrogeological unit (i.e., Miquan and Fukang) is predominantly snowmelt water of 0.045 to 2 Ma. ...
Unlike high-rank coalbed gas (CBG), which is dominated by thermogenic gas with relatively single components, the low to medium rank CBG has complex components (N 2 and CO 2 rich CBG blocks are frequently found) and diverse sources (a mixture of thermogenic and biogenic gas, or even dominated by biogenic gas), and the enrichment model of CBG reservoirs is not clear yet. The present study was carried out in the southern Junggar Basin. The field desorption data were utilized to analyze the composition. CH 4 was found to be the main component of CBG in the southern Junggar Basin, and high N 2 and CO 2 were found to exist in the Manasi-Hutubi and Miquan areas, respectively. Gas composition, gas and water stable isotopes were tested to clarify the source and fate of each gas (CH 4 , CO 2 and N 2). The results shows that biogenic CH 4 is absolutely dominant in the low-medium rank CBG in the southern Junggar Basin, with microbial CH 4 accounting for more than 90% of the total in the Liuhuanggou and Miquan areas in particular. Thermogenic, biogenic, and crustal CO 2 are all detected in the southern Junggar Baisn. Manasi-Hutubi, Liuhuanggou, Fukang, and Jimushaer CO 2 exhibit biogenicity. The intrusion of CO 2 from middle to late Miocene crustal sources mainly triggered the anomalously high CO 2 in the Miquan area. Excess N 2 in the Manasi-Hutubi area originated from the atmosphere and was a consequence of hydrodynamic destruction of CBG reservoirs. In addition, the enrichment pattern of low to medium rank CBG reservoirs with biogenicity as the main source was revealed. Low to medium rank CBG reservoirs could be classified from shallow to deep: (1) Gas weathering zone; (2) Microbial gas zone; (3) Mixed gas zone; (4) Thermogenic gas zone. Microbial gas zone and the mixed gas zone are the enrichment zones of low to medium rank CBG (coal seams from 600 m to 1000 m in the southern Junggar Basin).
... HCO 3 − is the product of SO 4 2− desulfurization. Low SO 4 2− , high HCO 3 − , and TDS can be used as evidence of good sealing or relative reduction of the underground environment [31]. Proper temperature, pH, and trace elements are critical to improving metabolic efficiency. ...
... HCO3 − is the product of SO4 2− desulfurization. Low SO4 2− , high HCO3 − , and TDS can be used as evidence of good sealing or relative reduction of the underground environment [31]. Proper temperature, pH, and trace elements are critical to improving metabolic efficiency. ...
... These effects can be ignored due to the high flow rate of water in the pipeline and the short sampling time. In other coal reservoirs, there are cases of ANME (Anaerobic Methanototropicarchaea) discovery [6,31]. ANME was not found in the study area, but sulfate-reducing, nitrifying, and other microorganisms exist. ...
The rise of coalbed methane bioengineering enables the conversion and utilization of carbon dioxide through microbial action and the carbon cycle. The environment of underground coal reservoirs is the result of a comprehensive effort by microorganisms. Some studies on reservoir microorganisms have progressed in laboratory conditions. However, it does not replicate the interaction between microorganisms and the environment on site. Hydraulic fracturing is an engineering technology to improve the natural permeability of tight reservoirs and is also a prerequisite for increasing biomethane production. In addition to expanding the pore and fracture systems of coal reservoirs, hydraulic fracturing also improves the living conditions of microbial communities in underground space. The characteristics of microbial communities in the reservoir after hydraulic fracturing are unclear. To this end, we applied the 16S rRNA sequencing technique to coalbed methane production water after hydraulic fracturing south of the Qinshui Basin to analyze the microbial response of the hydraulic fracturing process in the coal reservoir. The diversity of microbial communities associated with organic degradation was improved after hydraulic fracturing in the coal reservoir. The proportion of Actinobacteria in the reservoir water of the study area increased significantly, and the abundance of Aminicenantes and Planctomycetes increased, which do not exist in non-fracturing coalbed methane wells or exist at very low abundance. There are different types of methanogens in the study area, especially in fracturing wells. Ecological factors also determine the metabolic pathway of methanogens in coal seams. After hydraulic fracturing, the impact on the reservoir’s microbial communities remains within months. Hydraulic fracturing can strengthen the carbon circulation process, thereby enhancing the block’s methane and carbon dioxide circulation. The study provides a unique theoretical basis for microbially enhanced coalbed methane.
... Coalbed methane (CBM) has the characteristics of high heat and low pollution as a kind of green and clean associated product of coal metamorphism (Lin et al. 2019 the discovered CBM is thought to be formed via biogenic and thermogenic pathways (Park and Liang 2016) . Biogenic CBM has been found in the coalfields of many countries, including China, Australia, and the USA (Fu et al. 2021;Guo et al. 2020b;Midgley et al. 2010), and even been recognized as the primary source of CBM in some areas (Strąpoć et al. 2011). Biomethane can be considered as a ''renewable" resource because it is generated by the biodegradation of organics in coal via diverse microorganisms or single methanogen (Mayumi et al. 2016, Park and Liang 2016, Scott 1999, Vick et al. 2019. ...
Biomethane generation by coal degradation not only can increase coalbed methane (CBM) reserves, namely, microbially enhanced coalbed methane (MECBM), but also has a significant effect on the pore structure of coal which is the key factor in CBM extraction. The transformation and migration of organics in coal are essential to pore development under the action of microorganisms. Here, the biodegradation of bituminous coal and lignite to produce methane and the cultivation with inhibition of methanogenic activity by 2-bromoethanesulfonate (BES) were performed to analyze the effect of biodegradation on coal pore development by determining the changes of the pore structure and the organics in culture solution and coal. The results showed that the maximum methane productions from bituminous coal and lignite were 117.69 μmol/g and 166.55 μmol/g, respectively. Biodegradation mainly affected the development of micropore whose specific surface area (SSA) and pore volume (PV) decreased while the fractal dimension increased. After biodegradation, various organics were generated which were partly released into culture solution while a large number of them remained in residual coal. The content of newly generated heterocyclic organics and oxygen-containing aromatics in bituminous coal was 11.21% and 20.21%. And the content of heterocyclic organics in bituminous coal was negatively correlated with SSA and PV but positively correlated with the fractal dimension which suggested that the retention of organics contributed greatly to the decrease of pore development. But the retention effect on pore structure was relatively poor in lignite. Besides, microorganisms were observed around fissures in both coal samples after biodegradation which would not be conducive to the porosity of coal on the micron scale. These results revealed that the effect of biodegradation on pore development of coal was governed by the combined action of organics degradation to produce methane and organics retention in coal whose contributions were antagonistic and determined by coal rank and pore aperture. The better development of MECBM needs to enhance organics biodegradation and reduce organics retention in coal.
... The Lower Jurassic Badaowan Formation (J 1 b) in the Fukang area and Middle Jurassic Xishanyao Formation (J 2 x) in the Miquan area are target layers for this study (Fig. 1e). Excessive CO 2 concentrations in CBG of the Miquan area have been found [12], however the sources of CO 2 have not been systematically explained. Certainly, the effect of exogenous CO 2 on the gas production pathways of microorganisms has not been discussed. ...
... The combined use of 129 I/ 127 I ratio, 3 H and 14 C is an effective method for evaluating the end-member values of formation water residence time [52][53][54]. 14 C, and 129 I/ 127 I ratio were detected by Fu et al. (2021) to speculate the ages of the coalbed water of the J 1 b and J 2 x in Fukang and Miquan areas [12]. The 129 I/ 127 I ratio of the water samples were measured at the State Key Laboratory of Loess and Quaternary Geology, Xi'an Accelerator Mass Spectrometer Center, Institute of Earth Environment, Chinese Academy of Sciences. ...
... The combined use of 129 I/ 127 I ratio, 3 H and 14 C is an effective method for evaluating the end-member values of formation water residence time [52][53][54]. 14 C, and 129 I/ 127 I ratio were detected by Fu et al. (2021) to speculate the ages of the coalbed water of the J 1 b and J 2 x in Fukang and Miquan areas [12]. The 129 I/ 127 I ratio of the water samples were measured at the State Key Laboratory of Loess and Quaternary Geology, Xi'an Accelerator Mass Spectrometer Center, Institute of Earth Environment, Chinese Academy of Sciences. ...
The sources of CO2 in coalbed gas (CBG), and their geological significance have been widely discussed. However, the influence of exogenous CO2 on the pathway of microbial degradation of coal to CH4 has not been identified and carefully discussed. To investigate the modification of exogenous CO2 on microbial degradation coal to CH4 pathways, 22 CBG and coproduced water samples from the Miquan and Fukang areas of the southern Junggar Basin were sampled in this study and tested for stable isotopes. The results show that there are significant differences in CBG composition and genesis between Miquan and Fukang areas. The CBG in the Fukang area is mainly CH4 , and the thermogenic gas and biogenic gas account for 32.8% − 34.1% and 65.9% − 67.2%, respectively. Almost all the biogenic gas is produced by the CO2 reduction pathway. Unlike the Fukang area, the CBG in the Miquan area has a high CO2 concentration (2.0% to 36.9%). He and Ne isotopes indicate that CO2 in the Miquan area are mainly of crustal origin and have undergone at least 72.5% loss and dilution. Almost all CBG in Miquan area is of biological origin (92.3% to 92.9%). However, the production pathway of biogas is very different from that of Fukang. The CO2 reduction pathway and acetic acid/methyl fermentation pathway account for 65.2% − 79.3% and 20.7% − 34.8%, respectively. The thermogenic gas and biogenic gas in the two areas are from Early Cretaceous and Pleistocene, respectively. The only difference is that CBG in Miquan area was intruded by crustal-derived CO2 during middle to late Miocene. Crust-derived CO2 was utilized by microorganisms to produce enough CH3COOH, which provided sufficient substrate for microorganisms to produce CH4 using
CH3COOH. Disturbance by exogenous CO2 explains differences in microbial methanogenic pathways in two areas. This study not only clarifies the fate of CO2 in the Miquan area, but also raises the principle of modification of microbial methanogenic pathways by exogenous CO2.
... High natural abundances of CO 2 can occasionally be found in closed settings. For example, Fu et al. (2021) reported CO 2 contents as high as >40 vol. % in coalbed methane from the southern Junggar Basin due to a stagnant hydrodynamic condition. ...
... % in coalbed methane from the southern Junggar Basin due to a stagnant hydrodynamic condition. Furthermore, CO 2 reduction via microbial methanogenesis can deplete CO 2 in natural gases, such as reported in the southern Junggar Basin and northern Tarim Basin, China (Fu et al., 2021(Fu et al., , 2022. ...
... In terms of gas enrichment and origin, Fu et al. (2016), Ou et al. (2018), and Yu and Wang (2020) evaluated the CBM potential of the southern Junggar Basin. Zhi et al. (2013), Fu et al. (2019), Fu et al. (2021) and Wang et al. (2022) investigated the origin and distribution of coalbed gas in the Jurassic coal reservoirs from different parts of the southern Junggar Basin by isotopic and compositional analysis. Several enrichment patterns were further proposed by Li et al. (2018) and Hou et al. (2021). ...
... In general, thermogenic methane has δ 13 C values higher than −55‰ and biogenic methane has values lower than −60‰ because of preferential depleted carbon consumption by methanogens (Whiticar, 1996(Whiticar, , 1999Rice et al., 2008). The Intermediate carbon isotopic value has been interpreted to be a mixture of coexisting biogenic and thermogenic gases (Hu et al., 2010;Fu et al., 2021;Zhang B et al., 2021;Wang et al., 2022). Biogenic gases in the study area have been demonstrated to be dominated by second-stage biogenic gases and a combined microbial pathway of methane generation via both CO 2 reduction and acetate fermentation was proposed by Wang et al. (2022). ...
... Biogenic gases in the study area have been demonstrated to be dominated by second-stage biogenic gases and a combined microbial pathway of methane generation via both CO 2 reduction and acetate fermentation was proposed by Wang et al. (2022). Fu et al. (2021) pointed out that CO 2 reduction is the main pathway for generating microbial gas based on the identification of bacterial and archaeal16s rRNA genes in formation water . What is worth noting is that burnt rocks are widely developed around coal outcrops in the Fukang area and other places of the southern Junggar Basin. ...
The enrichment of coalbed methane (CBM), in-situ stress field, and permeability are three key factors that are decisive to effective CBM exploration. The southern Junggar Basin is the third large CBM basin in China but is also known for the occurrence of complex geological structures. In this study, we take the Fukang area of the southern Junggar Basin as an example, coalbed methane accumulation and permeability, and their geological controls were analyzed based on the determination of geological structures, in-situ stress, gas content, permeability, hydrology and coal properties. The results indicate that gas contents of the Fukang coal reservoirs are controlled by structural framework and burial depth, and high-to-ultra-high thickness of coals has a slightly positive effect on gas contents. Perennial water flow (e.g., the Baiyanghe River) favors gas accumulation by forming a hydraulic stagnant zone in deep reservoirs, but can also draw down gas contents by persistent transportation of dissolved gases to ground surfaces. Widely developed burnt rocks and sufficient groundwater recharge make microbial gases an important gas source in addition to thermogenic gases. The in-situ stress field of the Fukang area (700–1,500 m) is dominated by a normal stress regime, characterized by vertical stress > maximum horizontal stress > minor horizontal stress. Stress ratios, including lateral stress coefficient, natural stress ratios, and horizontal principal stress ratio are all included in the stress envelopes of China. Permeability in the Fukang area is prominently partitioned into two distinct groups, one group of low permeability (0.001–0.350 mD) and the other group of high permeability (0.988–16.640 mD). The low group of permeability is significantly formulated by depth-dependent stress variations, and the high group of permeability is controlled by the relatively high structural curvatures in the core parts of synclines and the distance to the syncline core. Meanwhile, coal deformation and varying dip angles intensify the heterogeneity and anisotropy of permeability in the Fukang area. These findings will promote the CBM recovery process in China and improve our understanding of the interaction between geological conditions and reservoir parameters and in complex structural regions.
... This can provide a basis for identifying the sources of water produced from the CBM wells and then determining the effect of the water production on the pressure reduction, as well as the gas desorption and production in the reservoirs. Moreover, the isotopic composition of the surface water can be heavier, due to the evaporative fractionation effect, which is particularly evident in arid and semiarid regions [42]. ...
The quantitative identification of water sources is an important prerequisite for objectively evaluating the degree of aquifer interference and predicting the production potential of coalbed methane (CBM) wells. However, this issue has not been solved yet, and water sources are far from being completely understood. Stable water isotopes are important carriers of water source information, which can be used to identify the water sources for CBM wells. Taking the Zhijin block in the Western Guizhou Province as an example, the produced water samples were collected from CBM wells. The relationships between the stable isotopic compositions of the produced water samples and the production data were quantitatively analyzed. The following main conclusions were obtained. (1) The δD and δ18O values of the produced water samples were between −73.37‰ and −27.56‰ (average −56.30‰) and between −11.04‰ and −5.93‰ (average −9.23‰), respectively. The water samples have D-drift characteristics, showing the dual properties of atmospheric precipitation genesis and water–rock interaction modification of the produced water. An index d was constructed to enable the quantitative characterization of the degree of D-drift of the produced water. (2) The stable isotopic compositions of produced water showed the control of the water sources on the CBM productivity. The probability of being susceptible to aquifer interference increased with the increasing span of the producing seam combination, reflected in the lowering δD and δ18O values and the decreasing gas productivity. (3) Three types of water, namely, static water, dynamic water, and mixed water, were identified. The characteristic values of the isotopic compositions of the static and dynamic water were determined. Accordingly, a quantitative identification method for the produced water sources was constructed, based on their stable isotopic compositions. The identification results have a clear correlation with the gas production, and the output of the static water contributes to the efficient CBM production. The method for the quantitative identification of the water sources proposed in this study, can help to improve the CBM development efficiency and optimize the drainage technology.
... The increasing attention to clean energy and the greenhouse effect has promoted the efficient development and exploration of China's coalbed methane (CBM) industry [1][2][3][4][5][6][7]. Conventionally, CBM was exploited by pumping out reservoir water (i.e., the reservoir depressurization method) to accelerate adsorbed methane desorption from insitu coal seams. ...
Injecting CO2 into coal reservoirs is essential both for enhancing coalbed methane recovery (CO2-ECBM) and CO2 geological sequestration. However, due to the difficulty in simultaneously monitoring multiphase methane (adsorbed and free phases) by conventional testing methods, the real-time variations in different phases of methane throughout the whole process of CO2-ECBM still remains to be clarified. In this study, with the introduction of self-designed nuclear magnetic resonance (NMR) measurement, three subbituminous and anthracite coals collected from deep-well drilling were used to investigate the dynamic characteristics of multiphase methane in CO2-ECBM. The results show that the adsorbed methane desorption efficiency could be additionally improved by ∼ 14%–26% with the injection of CO2 after conventional reservoir depressurization. In the process of CO2-ECBM, three different CO2-CH4 displacement rates emerged: the rate rapidly decreased during the initial CO2 injection time period and then began to slowly decrease until reaching CO2-CH4 competitive-adsorption equilibrium. Comparison of the NMR transverse relaxation (T2) characteristics of CO2-ECBM after natural and in-situ methane adsorption suggests that a low concentration ratio of CH4/CO2 could significantly improve the methane recovery. The promising results from the CO2-CH4 displacement experiments demonstrate the great significance of the CO2-ECBM method in enhancing methane recovery and CO2 geological sequestration for coals.
... For example, the carbon isotope composition of the methane (δ 13 C-C 1 ) becomes lighter (less than − 55‰), the carbon isotope composition (δ 13 C) of the CO 2 and the dissolved inorganic carbon (DIC) becomes abnormally high (more than +10‰), and the δ 13 C of the residual C 2+ components becomes heavier (Faiz and Hendry, 2006;Jones et al., 2008;Mikov, 2011). Based on this, the genesis of the CBG in different coal-bearing basins has been studied using the geochemical indicators of GBG and co-produced water samples (Tao et al., 2007;Bates et al., 2011;Hamilton et al., 2014;Fu et al., 2021). However, several studies have also reported that CBG from different sources might have similar geochemical characteristics, e.g., the δ 13 C-C 1 values of early thermogenic gas and microbial gas are relatively close (i.e., − 55‰ to − 73‰), resulting in some limitations in determining the origin of CBG (Strapoc et al., 2010). ...
... However, some evidence also indicates that some limitations or contradictions should exist in analyzing the formation pathways of microbial gas using the above methods. For instance, the isotopic data indicate that most microbial gases are generated via the CO 2 reduction pathway in coal-bearing basins, whereas the composition of the microbial community reveal that acetoclastic and methylotrophic methanogenesis also play an important role in some basins (Bates et al., 2011;Guo et al., 2012;Susilawati et al., 2015;Fu et al., 2021). Overall, the geochemical characteristics of gas and water samples should be combined with the compositions of microbial community to determine the genesis of CBG and the formation pathways of microbial gas in a certain region. ...
... bearing basins, e.g., the Junggar Basin (− 20.7‰ to +22.4‰), Powder River Basin (− 24.6‰ to +22.4‰), San Juan Basin (− 12.7‰ to +18.2‰), and Surat Basin (− 2.5‰ to +7.7‰) (Flores et al., 2008;Hamilton et al., 2014;Fu et al., 2021). Previous studies have reported that these abnormally high positive δ 13 C-CO 2 values are closely related to the conversion of a large amount of CO 2 to CH 4 in a close system (Fu et al., 2021). ...
Some important progress has been made in coalbed gas (CBG) exploration in the Kuqa-Bay coalfield in the northern Tarim Basin, where a demonstration base of CBG development and utilization has been established in the Tieliekedong (TD) region. Currently, there has been no detailed study of the formation mechanism of the CBG in the TD region, which has restricted the next phase of the CBG exploration. In this study, gas and water samples in the TD region were collected and analyzed to determine the genesis of the CBG, the formation pathways of the microbial gas, the source of the coalbed water, the composition of the microbial community, and the accumulation process of the CBG. The results indicate that the CBG in the TD region is a mixture of biodegraded thermogenic gas and secondary microbial gas. The geochemical characteristics of the water samples indicate that the coalbed water actively receives recharge from surface water, and it exhibits the characteristics of a weak-runoff water environment (i.e., a relatively closed system). Based on the stable isotope compositions of the gas-water samples, CO2 reduction is the main metabolic pathway for generating the microbial gas. That is, in the relatively closed system, initial CO2 from the different sources was converted to CH4 by hydrogenotrophic methanogens (e.g., Methanobacterium), and the residual CO2 became progressively enriched in ¹³C (i.e., high positive δ¹³C-CO2 and δ¹³C-dissolved inorganic carbon values). However, the microbial community composition indicates that acetogenic bacteria (e.g., Acetobacterium, and Lentimicrobium) and acetoclastic methanogens (i.e., Methanosaeta) are widely developed in the coalbed water, so acetoclastic methanogenesis should also take place. According to the priority of the biodegradation, the microbial oxidation of thermogenic C2+ components (e.g., ethane and propane) mainly occurred in the early stage and lasted for a short period of time, primarily producing CH4 enriched in ¹²C. After these components were consumed, the bacteria began to degrade the organic matter in the coal, and the methanogens continued to generate secondary microbial gas, finally forming a ¹³C-enriched pool of carbon species. Based on these results, the CBG accumulation in the TD region was determined. The results of this case study provide an obvious supplement to the geological theory of CBG accumulation, and have a guiding significance for CBG exploration in the TD region.
... With regard to continuous gas accumulation, determining the accumulation mechanism of the CBM reservoir has been a difficult challenge to date. It is generally thought that structural geology and hydrogeology are two key determinants of gas generation , gas migration (Bachu and Michael, 2003;Karacan and Goodman, 2012), and eventually gas-content distribution and gas accumulation (Scott, 2002;Van Voast, 2003;Pashin, 2020) in depositional coal basins, e.g., the San Juan Basin (Ayers Jr., 2002;Ambrose and Ayers Jr., 2007), the Black Warrior basin (Pashin, 1998(Pashin, , 2010, the Powder River Basin Quillinan and Frost, 2014), the east-central Utah (Lamarre, 2003), the northeastern Greater Green River Basin (Scott, 2002), the Soma Basin (Esen et al., 2020), the southeastern Ordos Basin (Yao et al., 2013, and the southern Junggar Basin (Fu et al., 2021). More importantly, CBM sweet spots are distributed in limited regions of a depositional basin. ...
The Zhengzhuang Field is a new developing coalbed methane (CBM) field in the southern Qinshui Basin. To disclose the high interwell heterogeneity in production, we provide a detailed investigation of local structural controls on the gas accumulation in the Zhengzhuang Field. The Zhengzhuang Field is a northwest-dipping homocline that contains abundant normal faults, and several secondary folds, karst collapse columns, and thrust faults. These structures divided the study area into three regions (fault-trough, fault-fold, and sub-sag) and 11 sub-regions. The gas content of the No. 3 coal seam is 0 to 35 m³/t, and the gas content exhibits a distinctly positive correlation with the methane δ¹³C isotopic ratio. Both gas content and methane δ¹³C isotopic composition are compartmentalized by structural regions or sub-regions, indicating significant effects from structural effects. The general gas-content distribution was separated by low-permeability boundaries created by F1 and F2 faults, resulting in several gas reservoir compartmentalizations. Locally, secondary faulting and small-scale folding resulted in redistributions of gas contents, i.e., relatively high gas content on the downthrow side of secondary faults and in the structural-trough zones of small-scale folds. As a continuous-type gas reservoir, the CBM accumulation mechanism is complex and differs from conventional gas reservoirs. Because coal has an extremely high adsorption capacity, the gravity separation of free gas and water is notably subordinate to sorption on micropore surfaces in CBM reservoirs. It is known that the No. 3 coal seam has extremely low permeability and low–medium gas saturation; thus, the gas isotopic fraction caused by hydrodynamic flushing probably enhanced the gas desorption and resulted in the migration of δ¹³C-enriched gas within the coal seam and into adjacent locations with high reservoir pressures for gas re-adsorption. This mechanism explains the relatively high gas content and enriched methane isotopic composition in the local structural lows in the Zhengzhuang Field. Structural controls on gas production indicate that the most productive CBM wells are commonly completed in the structural highs of the fault-nose-structure (i.e., the sweet spots), where there is high reservoir permeability resulting from stress relief, and also relatively high gas content induced by the secondary migration of gas from the structural lows. The source driving force for gas migration within the coal seam is the result of gas diffusion fractionation which commonly occurs simultaneously with the slow seepage of groundwater from low to high spots in the Zhengzhuang Field.
