Coalbed Methane Enrichment Characteristics and Exploration Target Selection in the Zhuozishan Coalfield of the Western Ordos Basin, China

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1. Introduction

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The commercial development of coalbed methane (CBM) resources is well established in a number of countries around the world, including the United States, Australia, China, India, and Canada. (1) The methane in coals occurs in several ways, including adsorbed gas on micropore (<2 nm in diameter) surfaces, trapped gas in pores of the coal matrix, free gas in fractures (cleat system), and dissolved gas in coal seam water. (2,3) CBM exploration and development have many advantages: (1) CBM is a type of clean energy, (2) extraction of CBM could greatly reduce hazards and thus strengthen mine safety, and (3) CBM utilization could decrease the emission of greenhouse gases. (4,5)

China is rich in CBM resources, with a geological resource of 30.05 × 1012 m3, (6) and the development and utilization of CBM can optimize China’s energy structure and reduce greenhouse gas emissions. (7,8) China has made great progress in the CBM accumulation mechanism and development methods, such as in the Ordos Basin, (9−11) Qinshui Basin, (12,13) and Jungar Basin. (14,15) However, to achieve China’s goal of carbon neutrality by 2060, the scale of CBM development in China still needs to be urgently upgraded. Some areas with a low priority for exploration become the current focus of the investigation, such as the Zhuozishan coalfield at the western margin of the Ordos Basin.

Although the CBM exploration and development are successful at the eastern margin of the Ordos Basin, the controlling factors of CBM formation and CBM resources at the western margin of the basin are not well understood. The prospect of CBM exploration in the western Ordos Basin is still not very clear. The Carboniferous–Permian coal seams of the Zhuozishan coalfield at the western margin of the Ordos Basin have a high degree of thermal evolution, a large coal seam thickness, and a moderate burial depth. (16) These conditions should favor the enrichment of the CBM. Therefore, the Zhuozishan coalfield may have high potential for CBM exploration. The study on the accumulation mechanisms of CBM is not only beneficial to the further development of CBM resources in this coalfield but also provides support for the development of the CBM industry in China.

The geological factors controlling the CBM enrichment include characteristics of the coal seams, (17,18) coal reservoir characteristics, (18−22) structure conditions, (23−25) and hydrogeological conditions. (26−29) The Zhuozishan coalfield is located in front of a large and structurally reworked complex fold-thrust belt. (30,31) The presence of a large number of faults and folds influences the distribution and quality of CBM resources. The recognition of the control factors of CBM enrichment in the Zhuozishan coalfield allows a selection of favorable target areas for CBM exploration and provides reference for CBM exploration in complex structural areas.

In this paper, the factors such as coal seam thickness, coal burial depth, coal petrology, coal quality, and coal ranks are investigated. The CBM enrichment models are established based on the depositional environments, structural features, and hydrogeological conditions. Furthermore, the exploration potential of CBM is evaluated, and the target areas for CBM exploration are optimized. We hope that the results of this study can provide support for further CBM exploration in the western Ordos Basin.

2. Geological Setting

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The Zhuozishan coalfield is located in Wuhai City, southwestern Inner Mongolia Autonomous Region, with a south–north length of approximately 100 km and a west–east width of 5–25 km, and a cover of approximately 1930 km2. Geologically, the Zhuozishan coalfield is situated in the northern segment of the western margin of the Ordos Basin (Figure 1a). The western boundary of the coalfield is the Gander–Xilaifeng thrust belt. The eastern boundary is the Eastern Zhuozishan thrust belt. The north and south sides of the coalfield are the Qianli Mountain and Zhengyiguan strike-slip fault zone, respectively (Figure 1b). The main coal-bearing strata of the Carboniferous–Permian in the Zhuozishan coalfield include the Taiyuan Formation and overlying Shanxi Formation (Figure 1c), which were formed in the intracratonic paralic setting of the North China Plate, (32) and are now preserved in a series of thrust-related folds (Figure 1d). Both formations were developed with a marine–continental transitional facies. (33,34) The Taiyuan Formation comprises interbedded sandstone, siltstone, mudstone, limestone, and coals, with No. 16 coal being the main mineable seam (Figure 2). The Shanxi Formation is composed of interbedded thick sandstone, siltstone, mudstone, and coals, with No. 9 coal being the main mineable seam (Figure 2).

Figure 1

Figure 1. (a) Outline of the Ordos Basin showing the location map of the Zhuozishan coalfield; (b) regional geological and structural map of the Zhuozishan coalfield (modified from Wang et al., 2014. Copyright [2014] [China Mining Magazine Co., Ltd.]); (39) (c) lithostratigraphic profile showing positions of major coal seams in the Zhuozishan coalfield (Ordo.: Ordovician; Carb.: Carboniferous); and (d) west to east geological cross section of the Zhuozishan coalfield (modified from Zhang et al., 2008. Copyright [2008] [Chinese Journal of Geology]). (40)

Figure 2

Figure 2. N–S trended cross section showing sequence correlation and depositional facies of the Carboniferous–Permian coal-bearing strata in the Zhuozishan coalfield.

After deposition of the coal-bearing strata, the western margin of the Ordos Basin went through the process of tectonic deformation. From the middle Permian to the middle Triassic, the western margin of the Ordos Basin underwent a stable subsidence along with the entire North China Craton, which resulted in the deposition of continental successions: Xiashihezi Formation, Shangshihezi Formation, Sunjiagou Formation, and Liujiagou Formation. (35) In the Late Triassic, due to the Indosinian movement, the sedimentary region of the North China Craton has shrunk largely, and the Ordos Basin was transformed into a separated continental lacustrine basin. (36) Since the Late Jurassic, the western margin of the Ordos Basin was influenced by the Yanshanian movement, resulting in the formation of east–west thrust belts. Subsequently, during the Neogene, this area was affected by the extrusion of the Tibetan Plateau, and the thrust nappe belts continued to develop. (37)

At present, the preservation of coals in the Zhuozishan coalfield is generally controlled by two fold-thrust belts. The Eastern Zhuozishan thrust belt was formed in the Late Jurassic–Early Cretaceous, and the Eastern Gander Mountain thrust belt was formed in the Late Cretaceous–Neogene (Figure 1d). Both thrust belts branch out southward into several relatively small faults. (38) The asymmetric anticline belt was developed along the eastern margin of the coalfield, with the Archean or Lower Paleozoic strata being exposed. The syncline belt was developed along the western margin of the coalfield, constituting the footwall of the western neighboring thrust fault (Figure 1d). The Carboniferous–Permian coal-bearing series are well preserved in the syncline belt.

3. Methods

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3.1. Sample Information and Experimental Method

A total of 10 coal samples were collected from two different coal mines in the Zhuozishan coalfield; 5 samples belong to the No. 9 coal seam of Baiyunwusu mine, and 5 samples were collected from the No. 16 coal seam of the Laoshidan mine.

The experiments performed in this study included the coal macrolithotype identification, maximum vitrinite reflectance (Ro,max, %), coal macerals, and coal proximate analysis. The coal macrolithotypes are classified according to the Chinese National Standard GB/T 18023–2000. A Leitz MPV-3 photometer was used to determine Ro,max, and coal macerals following the Chinese National Standards GB/T 6948–1998 and GB/T 8899–1998, respectively. The proximate analysis test was conducted following the Chinese National Standard GB/T 30732–2014. Proximate analysis includes determination of ash yield and contents of moisture, volatile matter, and fixed carbon, on an air-dried basis.

3.2. Resource Evaluation Method

The method for the resource evaluation of CBM is different from that of conventional natural gas because the CBM is mainly preserved in the adsorption state in the coal reservoirs. The generation and enrichment of CBM are controlled by more complex factors. Numerous methods have been used to estimate the CBM resources, including the volumetric method, numerical simulation method, GIS-based multilevel fuzzy mathematical method, and analogy forecast method. (41−44) This study uses the volumetric method to evaluate the CBM resources in key areas, and the calculating formula is as follows

Gi=∑j=1nCrj·Cj¯

(1)

where

n is the total number of subunits subdivided into the ith calculation units.

Gi is the geological resource of CBM in the ith calculation unit, in hundred million cubic meters (108 m3).

Crj is the coal resource of the jth subunit, in hundred million tonnes (108t).

Cj¯

the average gas content of the coal reservoir in an air-dried basis, in cubic meters per tonne (m3/t).

For calculation units lacking data on coal resources, the volumetric method can be used with the following formula

Gi=0.01Ai·Hi·Di·Ci

(2)

where

Gi is the geological resource of CBM in the ith calculation unit, in hundred million cubic meters (108 m3).

Ai is the gas-bearing area of the coal seam in the ith calculation unit, in square kilometers (km2).

Hi is the apparent thickness of the coal seam in the ith calculation unit, in meters (m).

Di is the density of coal in the ith calculation unit with air dry basis, in tonnes per cubic meter (t/m3).

Ci is the gas content of coal in the ith calculation unit with air-dried basis, in cubic meters per tonne (m3/t).

The resource abundance of CBM can reflect the distribution of the CBM resources, and it can be calculated by the total CBM resource divided by the coal distribution area.

The gas contents of the coal seam used in the CBM resource evaluation of this study were collected in the related geological reports and literature by the actual measurement method (Table S1).

4. Coal-Bearing Strata and Coal Seam Characteristics

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4.1. Coal Seam Distribution

The thickness of the Carboniferous–Permian coal-bearing strata ranges from 140 to 380 m in the Zhuozishan coalfield. The total thickness of the coal seams ranges from 10.5 to 20.78 m (Figure 3a). The coal seams in the Taiyuan Formation are thicker than those in the Shanxi Formation (Figure 2). The regional distribution of the total coal seam thickness is characterized by being thicker in the central parts and thinner in the northeast and southwest parts of the coalfield. The coal accumulation centers are mainly distributed in the Kulihuoshatu and Baiyunwusu mine areas, in which the Kulihuoshatu mine area has the largest coal seam thickness, reaching 24.6 m (Figure 3a). The burial depth of the coal seams varies greatly, ranging from 133.1 to 1232.1 m, with a general deepening trend from the north to the south (Figure 3b). The maximum burial depth of coal seams, which is based on the bottom surface of the deepest coal seam (No. 17 coal seam), is located in the Hongliushu mine area, followed by the Tiegaisumu mine area, with both areas having a burial depth greater than 600 m. The burial depth of coal seams in the Kulihuoshatu mine area is the shallowest (<300 m) (Figure 3b,c).

Figure 3

Figure 3. (a) Contours of the total coal seam thickness of Carboniferous–Permian coal-bearing strata in the Zhuozishan coalfield; (b) maximum burial depth of the coals of Carboniferous–Permian in the Zhuozishan coalfield (base of No. 17 coal); and (c) location map of mine areas and boreholes in the Zhuozishan coalfield.

4.2. Coal Proximate Analysis and Coal Petrology

The moisture content of the coals varies from 0.36 to 0.88%, the ash yield ranges from 14.21 to 57.93%, and the volatile matter content ranges between 16.09 and 24.13% (Table 1). The results of proximate analysis show that the coals in the study area are characterized by extra-low moisture, low to medium volatile matter, and medium to high ash yield, which are classified following the China Coal Industry Standards MT/T 850-2000, MT/T 849-2000, and GB/T 15224.1-2018, respectively.

Table 1. Results of Proximate Analysis of Coals from the Zhuozishan Coalfielda

sample IDcoal seamMad(wt %)Aad(wt %)Vad(wt %)FCad(wt %)WH-2No. 90.7257.9316.0925.26WH-4No. 90.8826.6919.4952.94WH-6No. 90.7814.2124.1360.88WH-8No. 90.7518.2522.2858.72WH-10No. 90.5448.6817.7733.01LSD 16-4-1No. 160.4543.3717.2638.91LSD 16-4-3No. 160.547.1317.6634.71LSD 16-4-5No. 160.4444.2016.5038.86LSD 16-4-7No. 160.3625.9520.2153.49LSD 16-4-9No. 160.4435.9118.0345.62

a

Mad, moisture on air-dried basis; Aad, ash yield on air-dried basis; Vad, volatile on air-dried basis; and FCad, fixed carbon on air-dried basis.

The coal macrolithotypes are predominantly semi-dull coal, followed by dull coal and semi-bright coal, with moderate hardness. The maceral compositions are dominated by vitrinite, with the content ranging between 59.1 and 77.3%, averaging 68.30%. The content of inertinite varies from 18.9 to 34.9%, averaging 27.36%. The content of the liptinite is generally low, being less than 2%, and its influence may be insignificant and can be neglected (Table 2).

Table 2. Results of Macrolithotypes, Maceral Compositions, Mineral Contents, and Vitrinite Reflectance of the Coals in the Zhuozishan Coalfielda

sample IDcoal seamcoal macrolithotypevitrinite (%)inertinite (%)liptinite (%)mineral (%)Ro,max (%)WH-2No. 9semi-dull60.234.81.83.21.09WH-4No. 9semi-dull64.230.11.93.81.07WH-6No. 9semi-dull6533.3–1.71.04WH-8No. 9semi-dull59.134.91.44.61.05WH-10No. 9semi-dull65.131.7–3.21.13LSD-16-4-1No. 16semi-dull71.923.2–4.91.18LSD-16-4-3No. 16semi-dull72.623.5–3.91.21LSD-16-4-5No. 16semi-dull73.121.2–5.71.35LSD-16-4-7No. 16semi-dull74.522–3.51.31LSD-16-4-9No. 16dull77.318.9–3.81.27

a

Ro,max, averaged maximum vitrinite reflectance under oil-immersed reflected light and “–”, no data.

The average vitrinite content of No. 16 coal is higher than that of No. 9 coal, and the average inertinite content of No. 16 coal is lower than that of No. 9 coal, which means that No. 16 coal is more conducive to CBM accumulation.

4.3. Coal Rank

The reflectance of vitrinite of coal samples in the Zhuozishan coalfield is relatively constant. The maximum vitrinite reflectance (Ro,max) ranges from 1.04 to 1.13% for No. 9 coal, and from 1.18 to 1.35% for No. 16 coal (Table 2). Both coal seams belong to medium-rank coal. Previous studies have shown that the gas content increases with the coal rank, (45,46) which can be attributed to the increase of micropore and pore specific surface area. (47) The No. 16 coal seam may have higher gas contents than the No. 9 coal seam due to the higher maturity.

4.4. Coal Reservoir Characteristics

4.4.1. Coal Structure

Coal structure is generally classified into four types, including primary coal, cataclastic coal, granulitic coal, and mylonitic coal. (48,49) Primary coal is beneficial to the preservation of CBM, but it also has a relatively weak adsorption capacity and permeability. (50) The tectonically deformed coals, represented by cataclastic coal, granulitic coal, and mylonitic coal, (48) have distinct effects on CBM migration. (51) As brittle-ductile and ductile tectonically deformed coals, granulitic coal and mylonitic coal have numerous micropores, which enhance methane adsorption. (52) However, the original fractures and cleat system are completely broken and blocked, which causes a considerable decrease of permeability in coals, making the coals not conducive to the development of coalbed methane. (53) By contrast, cataclastic coal has higher permeability resulting from abundant migration channels provided by fractures, which is most favorable for CBM development. (48,54) In the Zhuozishan coalfield, the coal structure of the No. 9 seam is dominated by primary coal, while that of the No. 16 seam is dominated by primary–cataclastic coal. The No. 16 coal seam should have better advantages than the No. 9 coal seam for the CBM development.

4.4.2. Permeability and Reservoir Pressure

Permeability is an important parameter in the evaluation of coal reservoirs. The permeability of No. 9 coal in the Zhuozishan coalfield is between 0.010 and 0.039 mD, with an average of 0.018 mD. (55) The permeability of No. 16 coal is between 0.047 and 0.078 mD, with an average of 0.056 mD. (55) According to the classification standard of the coal reservoir permeability proposed by Kang et al. (2017), (56) No. 9 and No. 16 coals in the Zhuozishan coalfield both belong to low permeability reservoirs (0.01–0.1 mD), with No. 16 coal having greater permeability than No. 9 coal.

The pressure of the coal reservoir in the Zhuozishan coalfield ranges from 2.75 to 5.38 MPa, with a pressure gradient of 0.78 to 0. 91 MPa/100 m, (55) which mostly belongs to the normal pressure reservoir.

4.5. Characteristics of the Major Coal Seam

Through comparing the burial depth and thickness of the individual coal seam (Figure 2), maceral content, coal rank, coal structure, and permeability of No. 9 and No. 16 coal seams in the Zhuozishan coalfield, the No. 16 coal seam shows better potential than No. 9 coal seam for CBM accumulation. Therefore, No. 16 is selected as the major coal seam for the gas enrichment study in the Zhuozishan coalfield.

4.5.1. Distribution of the Major Coal Seam

The No. 16 coal seam in the Zhuozishan coalfield is developed at the bottom of the Taiyuan Formation, with a thickness of 1.0–6.0 m (Figure 4a). The largest coal seam thickness is located in the Laoshidan area, with a thickness greater than 6 m, followed by the Kabuqi mine area and the Shen-1 well area of Qipanjing, with a thickness greater than 5 m (Figure 4a). The coal seam thickness in the southern and eastern coalfields is smaller, which is less than 2 m. The burial depth of the No. 16 coal seam in the Zhuozishan coalfield ranges from 200 to 1100 m (Figure 4b). Overall, the burial depth gradually increases from the east and west flanks toward the central axis of synclines in the coalfield. The maximum burial depth of the No. 16 coal seam is located in the Hongliushu mine area, which is more than 1000 m (Figures 3c and 4b). The roof of the No. 16 coal seam is mainly composed of mudstone, with sandstone and siltstone locally present (Figures 3c and 4c). Except for the local sandstone roof with poor sealing capacity in the Qipanjing mine area, the roof in the rest of the coalfield is mostly composed of mudstones with better sealing capacity, which is conducive to CBM preservation.

Figure 4

Figure 4. (a) Contours of the coal seam thickness of No. 16 coal seam; (b) contours of the burial depth of No. 16 coal seam; (c) roof lithology distribution of No. 16 coal seam; and (d) contour map shows the gas content of the No. 16 coal seam and coalfield structure.

4.5.2. Distribution of the CBM content

In this study, the gas content data in published studies were collected and used in constructing the contours of the gas contents of No. 16 coal in the Zhuozishan coalfield. (16,55,57−61) The contour map shows that the gas content decreases from the northwest to the southeast in the Zhuozishan coalfield. The higher gas content occurs in the Kabuqi mine, the Muergou mine, and the Dilibangwusu mine, with the gas content generally greater than 10 m3/t. The gas contents of the coals in the Baiyunwusu and Burgastai mines are generally greater than 8 m3/t, while that of the residual area is less than 4 m3/t (Figures 3c and 4d).

5. CBM Accumulation

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5.1. Controlling Factors of Gas Accumulation

5.1.1. Coal Depositional Conditions

Sedimentary conditions determine the characteristics of coal accumulation, lithofacies assemblage of coal-bearing strata, spatial distribution of the coal reservoir, (62,63) and the physical properties of the coal reservoirs and caprocks. (64,65)

5.1.1.1. Control of the sedimentary environment on CBM reservoirs

The sedimentary environment is closely related to the development of the coal seams, controlling the coal seam thickness, coal quality, and coal physical properties. (65,66) The coal seam thickness and coal quality will determine the CBM generation, adsorption, and preservation. (17)

Coal seam thickness not only controls the CBM-generating potential of coal reservoirs but also plays an important role in CBM preservation, and the thicker the coal seam, the more favorable for CBM preservation. (64) The previous studies of Carboniferous–Permian in the North China Craton have shown that the most favorable coal-forming environments include the interdistributary bay of the delta plain, fluvial plain, and the tidal flat─lagoon. (33) As a part of the North China Craton, the Zhuozishan coalfield was developed with similar coal-forming environments.

In general, the thicker coal seam would be conducive to the enrichment of CBM. (17) Within the individual coal mine areas, such as Kabuqi and Gongwusu, a significant positive correlation exists between the coal seam thickness and the gas content (Figure 5). Furthermore, Spearman correlation analysis is used to analyze the relationship between the coal thickness and the gas content in the entire coalfield and individual mine area. The Spearman correlation coefficients (SCC) of the entire coalfield and Kabuqi and Gongwusu mine areas are −0.34 (n = 18, p = 0.168 > 0.05), 0.733 (n = 10, p = 0.16 < 0.05), and 0.881 (n = 8, p = 0.004 < 0.01), respectively. The SCC also reveals a significant positive correlation between coal thickness and gas content in Kabuqi and Gongwusu mine areas. However, for the entire Zhuozishan coalfield, the relationship between coal seam thickness and gas content is insignificant and even shows a negative correlation. This indicates that coal seam thickness has a positive effect on the gas content under similar geological conditions, but there are other more critical factors that obscure or diminish this positive effect in the entire coalfield.

Figure 5

Figure 5. Relationship between coal seam thickness and gas content of the No. 16 coal seam in different coal mines of the Zhuozishan coalfield (the black dotted line represents the trend of all data points, and data sources refer to Table S1).

The formation of macerals is closely related to the swamp depositional environment. (67) It is generally believed that vitrinite is mainly formed in a reducing environment, which is mostly related to peat swamps with higher water levels. (68) Inertinite is mostly formed in peat swamps with shallow water cover, periodic exposure, and wildfire events. (69,70) Liptinite is a group of macerals derived from nonhumifiable plant matter and relatively hydrogen-rich remains such as sporopollenin, resins, waxes, and fats, which are closely related to the oxygen-rich peat swamp environment where the lignocellular tissue of the plant remains can be oxidized and decomposed, and the stable components are relatively enriched. (71)

Peat swamps in tidal flats are mostly water-covered low-lying swamps with strong reducibility, and the coal formed in this environment has high vitrinite contents and beneficial reservoir physical properties. (72) In the Taiyuan Formation, the No. 16 coal seam was mainly developed in the tidal flat and has a vitrinite content greater than 70% (Table 2), which is conducive to CBM generation and adsorption. However, the alluvial plain would favor the hydrodynamic and oxidizing conditions. The coal formed in this environment would have a high content of inertinite. (73) The content of inertinite in coal seams is the key parameter controlling the formation of pores and fractures, and the unfilled inertinite also has better adsorption capacity. (74) Where micropores are not developed, inertinite can also be used as the main adsorption medium. (75) In low–medium-rank coals, the higher content of inertinite tends to be associated with a larger proportion of mesopores and macropores, which is conducive to the migration of methane and the development of CBM. (76) In the Shanxi Formation, No. 9 coal seam in the Zhuozishan coalfield was mainly formed in a fluvial swamp. (34) The fluvial swamp could not maintain a long time of stable water cover, which is driven by fluvial flooding. The periodic oxidation environment blocked the formation of vitrinite, while inertinite was relatively enriched, resulting in a high content of inertinite, being higher than 30% (Table 2). In comparison to the No. 9 coal seam, the inertinite content of the No. 16 coal seam is lower but still slightly higher than 20%, which is also conducive to gas migration.

The depositional environment has an obvious influence on the ash yield in coal, thus directly affecting the reservoir’s physical properties of the coal. (77) The ash yield is closely related to the terrigenous clastic input during peat accumulation, which is controlled by hydrodynamic conditions and fluctuating water levels in the peat swamp. (78) As for the same rank coals, the higher ash yields tend to be associated with the lower gas adsorption capacity of the coal reservoir. (47,79) During the deposition of the Shanxi Formation, the depositional environments of the coalfield were mainly the fluvial plain and delta plain, (34) where the active hydrodynamic conditions would favor the input of terrigenous siliciclastics. Therefore, the average ash yield in the No. 9 coal seam of the Shanxi Formation is 33.15%. In contrast, the No. 16 coal seam of the Taiyuan Formation, formed in the tidal flat, has a relatively low ash yield of 24.16% (Table 1).

As for the major coal seam, when the ash yield in coal is lower than 20%, the average value of the gas content is 5.38 m3/t, and when the ash yield is higher than 20%, the average value of the gas content is 3.61 m3/t (Figure 6). The SCC between the ash yield and the gas content of the coals for the entire coalfield and Kabuqi, Laoshidan, and Gongwusu mine areas, are −0.483 (n = 18, p = 0.042 < 0.05), 0.036 (n = 7, p = 0.939), −0.5 (n = 3, p = 0.667), and −0.881(n = 8, p = 0.004 < 0.01). It shows that the negative correlation between ash yield and gas content exists in the entire coalfield. However, within the individual coal mines, such as the Kabuqi and Laoshidan mines, the ash yield has insignificant correlations with the gas content, which are shown by the SCC or linear correlation coefficient (Figure 6). On the contrary, a significant negative linear correlation exists in the Gongwusu, which is consistent with the SCC. Overall, the gas content decreases with the ash yield in the Zhuozishan coalfield, but this trend is not obvious, which manifests that the ash yield is not the key controlling factor for CBM accumulation.

Figure 6

Figure 6. Relationship between coal ash yield and gas content of the No. 16 coal seam in different coal mines of the Zhuozishan coalfield (the black dotted line represents the trend of all data points, and data sources refer to Table S1).

5.1.1.2. Influence of the Sedimentary Rock Sequence on CBM Preservation

The caprock of the No. 16 coal seam in the Zhuozishan coalfield is dominated by mudstone, with locally developed sandstone and siltstone (Figure 4c). The areas with high gas contents are mostly developed with mudstone sealing rocks (Figure 4d). The caprock might play more important roles in CBM accumulation than other sedimentary factors. (15)

The sedimentary environment of the Taiyuan and Shanxi formations changed gradually from the tidal flat─lagoon and delta facies to fluvial facies. This variation creates distinguishable reservoir–caprock combinations, resulting in different sealing capabilities. Four lithological associations of the reservoirs and caprocks are identified in the Zhuozishan coalfield, which are characterized by different superposing patterns of coal and its surrounding rocks (Figure 7). The first pattern is represented by the upward succession of the medium-grained sandstone–mudstone–thin coal–thick-bedded coarse-grained sandstone, which was deposited in fluvial plain facies (Figure 7a). The second pattern is represented by the succession of the fine-grained sandstone–mudstone–thick coal–medium-grained sandstone, which was deposited in the upper delta plain facies (Figure 7b). These two patterns mainly occur in the Shanxi Formation. The third pattern is represented by the succession of the fine-grained sandstone–thick coal–interbedded fine-grained sandstone–mudstone, which was deposited in lower delta plain facies (Figure 7c). The fourth pattern is represented by the succession of the mudstone–thick coal–limestone–thick mudstone, which was deposited in the tidal flat─lagoon facies (Figure 7d). The latter two patterns mainly occur in the Taiyuan Formation. Comparing these lithological associations, it can be deduced that the coal seams deposited in the lower delta plain and tidal flat have relatively better sealing capacity resulting from the caprock of thick-bedded mudstone and interbedded thin-bedded fine-grained sandstone. On the contrary, the coal seams deposited in the fluvial plain and upper delta facies have a relatively poor sealing capacity due to the thick-bedded coarse-grained sandstone roof. The methane-capped condition of the coal seam can be estimated by identifying the lithological association. The reservoir–caprock combination occurring in the lower delta and tidal flat─lagoon facies is the most favorable for CBM preservation in the Zhuozishan coalfield.

Figure 7

Figure 7. CBM preservation ability of different sedimentary rock sequences in the Zhuozishan coalfield.

5.1.2. Burial History5.1.2.1. Thermal History

From the Early Paleozoic to the Middle Mesozoic, the average paleo–geothermal gradient in the Ordos Basin ranged from 2.2 °C/100 m to 3.0 °C/100 m. (80) In the late Mesozoic, the average paleotemperature gradient in the Ordos Basin reached 4.5 °C/100 m, (80) which greatly accelerated the maturation of the organic matter in the Upper Carboniferous–Lower Permian. The average Ro,max of coal reached 1.26%. The coal-bearing strata began to generate abundant gas in the Middle Jurassic. (81) Subsequently, the peak stage of gas generation occurred in the Early Cretaceous. Then, the gas generation ended and the gas gradually escaped in the Late Cretaceous. (82) The Early Cretaceous was an important stage for the CBM generation in the coals of the Taiyuan and Shanxi formations of the Zhuozishan coalfield.

5.1.2.2. Present-Day Burial Depth of Coal

For the entire coalfield, the correlation between the gas contents and the present-day burial depth of coal seams is not significant (Figure 8). For the individual mine areas, including the Kabuqi, Laoshidan, Gongwusu, and Lutian mine areas, it can be revealed that a relatively positive correlation exists between the coal seam burial depth and the gas content (Figure 8). However, the low coefficients also indicate that the variation in the gas content is not dominated by a single factor. Even though the linear correlation coefficients are low, the higher gas content still occurs in the deeper strata. For example, in the Kabuqi mine, the average gas content in shallow-buried coal (0–300 m) is 6.8 m3/t, while that in deep-buried coal (300–600 m) is 8.8 m3/t. To certify the relationship between buried depth and gas content, Spearman correlation analysis is carried out for the entire coalfield and individual coal mines. The SCC of the entire coalfield, Kabuqi, Laoshidan, Gongwusu, and Lutian mine areas are 0.683 (n = 67, p = 0 < 0.01), 0.85 (n = 46, p = 0 < 0.01), 0.576 (n = 9, p = 0.052 > 0.05), 0.857 (n = 8, p = 0.007 < 0.01), and 1 (n = 4, p = 0 < 0.01), respectively. The result demonstrates a significant positive correlation between the entire coalfield and individual coal mines. This means that within similar geological conditions, the gas content tends to increase with the present-day coal burial depth.

Figure 8

Figure 8. Relationship between burial depth and gas content of the No. 16 coal seam in different coal mines of the Zhuozishan coalfield (the black dotted line represents the trend of all data points, and data sources refer to Table S1).

5.1.2.3. Coal Rank

The Ro value of the No. 16 coal seam in the Zhuozishan coalfield is characterized by an overall trend of decreasing from the west to the east. The relationship between the variation of the gas content and Ro is not obvious for the entire coalfield. However, on the east side of the coalfield with a relatively low Ro value, the gas content is generally less than 2 m3/t. The Ro is greater than 1.0% in the area with high gas content. The eastern and western margins of the coalfield are located at the syncline margin, but the gas content of the western side is slightly higher than that of the eastern side (Figure 9). These phenomena indicate that coal rank may still have a positive effect on the CBM accumulation in No. 16 coal seam.

Figure 9

Figure 9. Contour map shows Ro and gas contents of the No. 16 coal seam in the Zhuozishan coalfield.

5.1.3. Coalfield Structure

Different geological structures can lead to differences in the occurrence, coal structure, physical properties, fracture development, and groundwater runoff conditions of coal reservoirs and surrounding rocks, which have an impact on the gas content. (83) The development of geological structures such as faults and folds had a substantial influence on the CBM enrichment in the Zhuozishan coalfield.

The Zhuozishan coalfield is structurally complex, which resulted from the development of numerous thrust faults and folds. The formation of the Kabuqi syncline and Zhuozishan anticline was influenced by thrusting in the northern part of the western margin of the Ordos Basin. The older strata in the study area, which mainly include Ordovician and Cambrian, are thrust over the coal-bearing strata, and some reverse faults are developed on the gentle flank of the syncline. The reverse faults tend to form the structural trap conditions which are favorable for the gas enrichment in coal seams. (45) The Kabuqi mine area is developed with this trap type (Figures 3c and 4d). The gas content increases gradually when it is closer to the axis of the Kabuqi syncline. Generally, synclines have better CBM preservation conditions than anticlines. (17,84) As for a single syncline, due to the compression at the syncline axis area, the gas permeability of surrounding rocks becomes lower, and the CBM can be better sealed in the axis area. As a result, the axes of the syncline have a relatively high gas content in the central parts of the Zhuozishan coalfield (Figure 4d), indicating that the syncline in the study area has a positive impact on the CBM enrichment. The large anticline is not conducive to CBM preservation, and the gas contents in the coals of the Zhuozishan anticline and the Gander anticline are lower.

In terms of controls of the fault types, several major reverse faults were developed in the Zhuozishan coalfield, and these faults have disrupted the continuity of coal seams and their surrounding strata to varying degrees. The reverse faults, formed by tectonic compressive stress, are mostly closed faults. These faults have an obstructive effect on the CBM escaping and thus favor gas preservation. Due to the uplift of the hanging wall of the reverse fault, the burial depth of coals becomes shallow, the pressure of the overlying strata becomes lower, and the adsorption of the coal reservoir is weakened, which leads to a large amount of coalbed methane dissipation. The Laoshidan mine and Gongwusu mine are both located southwest of the Gander–Xilaifeng fault, where the coal in the hanging wall of the reverse fault has a shallow burial depth and a low gas content.

5.1.4. Hydrological Factors

The coal-bearing strata in the Zhuozishan coalfield can be subdivided into four aquifers and four aquicludes. The water-yield property of all aquifers is relatively low, and the confining capacity of the aquicludes is uneven. (16) The aquiclude at the bottom of the Taiyuan Formation is distributed in the entire area and has the best confining capacity, while the aquiclude at the bottom of the Shanxi Formation is less continuous laterally and its confining capacity is limited to some extent.

Hydrogeological conditions have a great influence on the preservation and transport of CBM, and also on the exploration and extraction of CBM. (29) The gas-controlling role of hydrogeological conditions in the Zhuozishan coalfield is mainly reflected in two aspects, including the hydraulic migration and the hydraulic plugging. The hydraulic plugging effect is conducive to CBM enrichment.

Hydraulic migration and diffusion are common in fault-developed areas. (85,86) In the Zhuozishan coalfield, the faults destabilize each aquifer, leading to water migration, and the gas escapes with the flow of water. The Qipanjing mine area has extremely developed fault structures, especially normal faults, and these faults may lead to obvious hydraulic migration and diffusion, which may explain the low gas contents in the coal seams.

Hydraulic plugging commonly exists in asymmetric synclines. (87) For example, in the Kabuqi syncline, the east side is developed with the thrust fault dominated by compression, and the west side is developed with the Gander–Xilaifeng thrust fault, both of which are dominated by thrust faults, which should have plugging effects on the CBM. Each aquifer in the Kabuqi syncline has no hydraulic connection, and the aquifer has less water yield and slow groundwater runoff, which must have reduced CBM escaping. Otherwise, the coal seam would be exposed in some areas along the fold margin. Under this condition, CBM is also enriched in the syncline axis by hydraulic plugging of atmospheric precipitation, which can keep CBM from escaping.

5.2. CBM Enrichment Models

According to the geological conditions and the characteristics of CBM generation and preservation, two types of CBM enrichment models in the Zhuozishan coalfield are proposed, namely, the CBM enrichment in syncline─hydraulic plugging below the thrust nappe and the CBM enrichment by fault─confined aquifer plugging.

5.2.1. CBM Enrichment in Syncline─Hydraulic Plugging below the Thrust Nappe

The relationship between the main fold styles and the gas contents in the study area has shown that the maximum gas contents are located in the axial area of the Kabuqi syncline (Figure 10). Combined with the previous research results, the reasons for the higher gas contents in the Kabuqi syncline can be examined in more detail. The depositional environments were dominated by the delta-tidal flat, which favored the deposition of a thick coal reservoir, and a thick regional mudstone sealing cover. The Kabuqi syncline is located at the fold-thrust belt. (31) Due to the compression stress, the axial area of the syncline trapped by thrust faults has better reservoir physical conditions, higher gas adsorption capacity, and favorable gas-bearing conditions. (88−90) The groundwater runoff is slow, which has a better hydraulic plugging effect on the CBM. As a result of these conditions, the Kabuqi mine has relatively better CBM generation and preservation conditions, and thus has higher gas contents, making it a beneficial area for CBM exploration and development. This model can provide an example of CBM exploration and development in the fold-thrust zone, and it is also developed in the northern area of the Zhuozishan coalfield.

Figure 10

Figure 10. Sketch model showing the CBM enrichment in syncline─hydraulic plugging below the thrust nappe as of a case from the Kabuqi syncline.

5.2.2. CBM Enrichment by Fault─Confined Aquifer Plugging

In the Zhuozishan coalfield, a number of reverse faults with compression stress are developed. The maximum gas contents are located in the central area between the Xilaifeng reverse fault and the Heilonggui reverse fault (Figure 11). The adjacent reverse faults compress the coal reservoir, increasing the burial depth of coals in the central part. The compression stress from faults, together with the pressure of the confined aquifer, could improve the adsorption capacity of the coal seam and seal the upward-escaping CBM in the central part. The CBM near the reverse faults on both sides is blocked from escaping, which protects the central gas preservation.

Figure 11

Figure 11. Sketch model showing the CBM enrichment by fault─confined aquifer plugging.

The CBM enrichment in the southern Zhuozishan coalfield, such as Baiyunwusu and Laoshidan mines, is in agreement with this model. The CBM enrichment by fault and confined aquifer plugging will be also conducive to future CBM exploration in complex structural areas with abundant faults.

6. CBM Resource Evaluation and Target Area Selection

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6.1. CBM Resource Evaluation

6.1.1. Selection of Key Parameters6.1.1.1. Coal-Bearing Area

Divided by the boundary between mine areas, the CBM resource calculation area in the Zhuozishan area has been subdivided into 14 calculation units, including the Muergou mine area, Kabuqi mine area, Dilibangwusu mine area, Baiyunwusu mine area, Kulihuoshatu mine area, Qipanjing Shen-1 area, Qipanjing west mine area, Hongliushu mine area, Burgastai mine area, Tegaisumu mine area, Laoshidan mine area, Gongwusu mine area, Lutian (open-pit) mine area, and Sidaoquan mine area.

6.1.1.2. Gas Content

The gas contents of the coal seam in the CBM resource evaluation of the Zhuozishan coalfield are mainly obtained in the related geological reports and literature by the actual measurement method (Table S1), and in the area without the values of gas contents, the gas contents are obtained by the analogy method.

6.1.2. Results of CBM Resource Evaluation

After determining the calculation units and calculation parameters, the CBM resource of each calculation unit of the Zhuozishan coalfield is calculated separately, based on formulas (1) and (2) mentioned previously. The resource abundance of the CBM of each calculation unit is calculated by the total CBM resource divided by the coal distribution area. The results show that the total coal-bearing area of the 14 evaluation units in the entire Zhuozishan coalfield is 578.4 km2, the total gas-bearing volume is 428.78 × 108 m3, and the total resource abundance is 0.74 × 108 m3/km2. The resource abundances of CBM in the Hongliushu mine and the Dili Gongwusu mine are the largest, both being greater than 0.7 × 108 m3/km2. The resource abundances in the rest of the area are relatively small (Table 3).

Table 3. Calculation Results of the CBM Resource in the Zhuozishan Coalfield

mineburial depth (m)coal resource (104t)gas content (m3/t)coal-bearing area (km2)CBM resource (108 m3)CBM resource abundance (108 m3/km2)Kabuqi0–600193 166.48.45114.98163.231.42Hongliushu0–6007218.544.6557.293.360.06600–100033 858.396.5757.2922.40.391000–150026 382.057.8157.2920.60.36total67 458.986.3457.2946.360.81Dilibangwusu0–600134 020.315.38100.2572.090.72Burgastai0–60029 469.125.3523.0715.770.68Baiyunwusu0–60090 247.698.05114.0472.650.64Laoshidan0–60012 401.123.488.924.320.48Qipanjing west0–60011 804.131.1215.931.320.08600–100016 559.243.1115.935.150.321000–15003034.673.5815.931.090.07total31 398.032.615.937.560.47Gongwusu0–6007713.423.185.742.450.43Kulihuoshatu0–60047 763.871.4717.987.020.39Muergou0–60024 260.038.1651.4519.80.38Qipanjing Shen-10–60058 924.621.4729.678.660.29Tegaisumu0–6001777.231.7830.380.320.01600–100019595.13.4730.386.80.221000–15003053.194.1230.381.260.04total24425.523.1230.388.380.27Lutian0–6002249.320.155.740.340.06Sidaoquan0–600738.232.052.960.150.05total 724236.664.23578.4428.780.74

6.2. Selection of Favorable CBM Target Areas

6.2.1. Principle of Preferential Selection of Favorable CBM Target Areas

In consideration of the CBM geological characteristics in the Zhuozishan coalfield, the principles of preferential selection of favorable areas with priority for CBM exploration and development are comprehensively determined, which include: ① the thickness of the main coal seam >3 m; ② the burial depth of the coal floor of the main coal seam is 200–600 m; and ③ the average gas content of the main coal seam >6 m3/t.

6.2.2. Results of Favorable Area Selection

The contours of thickness, burial depth, gas content, caprock sealing capacity, and fault of the main coal seam in the Zhuozishan coalfield are superimposed, and then the favorable CBM areas of the main coal seam in the study area are preferentially selected (Figure 12a). According to the above selection principles, two favorable areas for CBM exploration and development were selected in the No. 16 coal seam (Figure 12b), including the Kabuqi favorable area, with a CBM resource of 96.84 × 108 m3 and a resource abundance of 0.61 × 108 m3/km2, and the Baiyunwusu favorable area, with a CBM resource of 17.78 × 108 m3 and a resource abundance of 0.45 × 108 m3/km2 (Table 4).

Figure 12

Figure 12. (a) Map showing the selection of favorable CBM target areas of the No. 16 coal seam in the Zhuozishan coalfield and (b) map showing the resource evaluation results of the favorable CBM target areas of the No. 16 coal seam in the Zhuozishan coalfield.

Table 4. Resource Evaluation of the CBM favorable area of No. 16 Coal in the Zhuozishan Coalfield

favorable areaaverage coal seam thickness (m)average gas content (m3/t)area (km2)resource (108 m3)resource abundance (108 m3/km2)Kabuqi4.59.05158.5396.840.61Baiyunwusu3.58.5539.6117.780.45

7. Conclusions

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(1)

The CBM geological conditions in the Zhuozishan coalfield are characterized by a greater total coal seam thickness (10.5–20.78 m), various coal burial depths (133.1–1232.1 m), medium coal rank (Ro, max, 1.04–1.35%), relatively high vitrinite contents (59.1–77.3%), and relatively high ash yields in coal (14.21–57.93%). The coal structure is dominated by primary and cataclastic coals, and the coal seam has low permeability and normal reservoir pressure. No. 16 seam has a greater thickness (1–6 m), an appropriate burial depth (200–1100 m), and mudstone-dominated caprocks. The vitrinite content of No. 16 coal (71.9–77.3%) is higher than that of No. 9 coal (59.1–65.1%), and the inertinite content of No. 16 coal (18.9–23.5%) is lower than that of No. 9 coal (30.1–34.9%). No. 16 coal is the favorable target seam for CBM exploration and exploitation in the Zhuozishan coalfield.

(2)

Coal seam thickness in the study area is positively correlated with the gas content, and the ash yield is negatively correlated with the gas content. The coal rank has little effect on the variation of the gas content, and the coal burial depth has a better correlation with the gas content within each individual mine area. Mudstone-dominated caprocks are beneficial to the preservation of coalbed methane, and the reservoir–caprock combination developed in the lower delta plain and the tidal flat─lagoon is conducive to CBM accumulation. In terms of geological structures, the gas contents are higher in the syncline below the thrust nappe, while the gas contents are lower in the anticline. Combining the depositional environment, structure features, and hydrogeological conditions, two types of CBM enrichment models are proposed, namely, the syncline─hydraulic plugging below the thrust nappe model and the fault─confined aquifer plugging model.

(3)

The volumetric method is applied to evaluate the CBM resources in the Zhuozishan coalfield. The total CBM resource is 428.78 × 108 m3, with a total resource abundance of 0.74 × 108 m3/km2. Two favorable CBM areas are preferentially selected, namely, the Kabuqi favorable area and the Baiyunwusu favorable area, which will be the future exploration target areas.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c04141.

Gas content parameters collected from references in the Zhuozishan coalfield (PDF)

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Author Information

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Bin Sun - Department of Unconventional Resources, Research Institute of Petroleum Exploration and Development, PetroChina, Beijing100083, China;  Email: [email protected]

Zhenghui Gao - College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing100083, China

Jian’an Li - College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing100083, China

Beilei Sun - Department of Earth Science and Engineering, Taiyuan University of Technology, Taiyuan030024, Shanxi, China

Minfang Yang - Department of Unconventional Resources, Research Institute of Petroleum Exploration and Development, PetroChina, Beijing100083, China

Jiamin Zhou - College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing100083, China

Haipeng Yao - Inner Mongolia Coal Exploration Unconventional Energy CO., LTD, Hohhot010010, Inner Mongolia, China

Fenjin Sun - Department of Unconventional Resources, Research Institute of Petroleum Exploration and Development, PetroChina, Beijing100083, China

Longyi Shao - College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing100083, China;  https://orcid.org/0000-0001-9975-6091

The authors declare no competing financial interest.

Acknowledgments

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This study is supported by the “14th Five-Year Plan” Forward-Looking Basic and Important Science and Technology Project of PetroChina Co., LTD. (2021DJ2306), the Shanxi Province Science and Technology Major Project (20191102001), and the Fundamental Research Funds for the Central Universities (2022YJSDC05).

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Li, Yong; Pan, Songqi; Ning, Shuzheng; Shao, Longyi; Jing, Zhenhua; Wang, Zhuangsen

Science China: Earth Sciences (2022), 65 (7), 1211-1228CODEN: SCESCI; ISSN:1869-1897. (Springer)

A review. Coal, coal measure gas, coal conversion to oil and gas, and coal-based new materials are reliable guarantees for stable energy supply and economic and social development in China. The coal-dominated resource endowment and the economic and social development stage det. the irreplaceable position of coal resources in the energy system. Coal measure resources, including aggregated or dispersed solids, liq. and gaseous multitype energies, and metal as well as nonmetallic minerals, are the products of multisphere interaction and metallogenetic materials generation, migration, and accumulation. Coal measures record rich deep-time geol. information of transitional and terrestrial peat bogs, which is a crucial carrier to reveal ecosystem evolution, significant org. carbon sequestration, atm. O2/CO2 variation, and wildfire events. Coal measure evolution is accompanied by the migration and transformation of various materials during diagenesis-metamorphism, forming differentiated coal compns. besides properties and various mineral resources in its adjacent strata. The enrichment condition, occurrence state, and sepn. potential are the premise for level-by-level use and efficient development of coal measure resources. Coal measure metallogeny is based on the metallogenic system of multiple energy and mineral resources in coal measures and their environmental effects.

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Chen, Y.; Tang, D. Z.; Xu, H.; Li, Y.; Meng, Y. J. Structural controls on coalbed methane accumulation and high production models in the eastern margin of Ordos Basin, China. J. Nat. Gas Sci. Eng. 2015, 23, 524– 537,  DOI: 10.1016/j.jngse.2015.02.018

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Structural controls on coalbed methane accumulation and high production models in the eastern margin of Ordos Basin, China

Chen, Yue; Tang, Dazhen; Xu, Hao; Li, Yong; Meng, Yanjun

Journal of Natural Gas Science and Engineering (2015), 23 (), 524-537CODEN: JNGSA4; ISSN:1875-5100. (Elsevier B.V.)

Significant progress has been made in coalbed methane (CBM) exploration and development in the eastern margin of the Ordos Basin where nearly 2000 CBM wells have been drilled, achieving a max. gas prodn. rate of 16000 m3/d by 2013. The geol. evolution of the eastern Ordos Basin plays an important role in the CBM formation. This study is focused on the interrelationship between structural geol., gas accumulation and prodn. characteristics of the No. 4 + 5 coal in the Permian Shanxi Formation as well as the No. 8 + 9 coal in the Carboniferous Taiyuan Formation. This research is based on the data collected from CBM prodn. wells and coal samples from coalmines and exploration wells. The results show that thermogenic gas is the dominant CBM source in the study area and there are two significant generation periods, the coalification in the Triassic and the magmatic thermometamorphism during the Yanshan movement. Combining the structure background and hydrogeol. conditions, the monoclinic-hydraulic sealing model was proposed as the representative CBM enrichment model. Different types of structures are also classified, and their influence on the CBM accumulation is discussed. Compressional structures formed during the Yanshan movement are conducive to CBM enrichment and retention; however, the tensional structures formed during the Himalaya movement may have led to CBM dissipation. Combining the structural effect on the CBM prodn. with CBM exploration and development practices in the study area, the following three types of high gas prodn. models are summarized: updip of the monocline, the axial part of the anticline or nose structure, and the structural high far from the normal fault.

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Shen, J.; Li, K. X.; Zhang, H. W.; Shabbiri, K.; Hu, Q. J.; Zhang, C. The geochemical characteristics, origin, migration and accumulation modes of deep coal-measure gas in the west of Linxing block at the eastern margin of Ordos Basin. J. Nat. Gas Sci. Eng. 2021, 91, 103965  DOI: 10.1016/j.jngse.2021.103965

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The geochemical characteristics, origin, migration and accumulation modes of deep coal-measure gas in the west of Linxing block at the eastern margin of Ordos Basin

Shen, Jian; Li, Kexin; Zhang, Hewei; Shabbiri, Khadija; Hu, Qiujia; Zhang, Cong

Journal of Natural Gas Science and Engineering (2021), 91 (), 103965CODEN: JNGSA4; ISSN:1875-5100. (Elsevier B.V.)

The deep coal-measure gas, including coalbed methane (CBM), tight gas (TG) and shale gas (SG), has attracted widely attention because of its great potential for natural gas development and multiple benefits. To det. the gas origin, migration and evolution in the west of Linxing block, 56 coal-measure gas samples from different horizons and depth were collected. The gas components and stable isotope compns. were measured, and the results show that methane is the main compn. of coal measure gas, distributed between 78.83% and 96.67%. The av. carbon dioxide and nitrogen concns. are 1.99 and 1.56 vol %, resp. Coal-measure gas has apparent dry and wet gas coexist characteristics with drying coeff. values ranging from 0.821 to 0.989. The δ13C(CH4) and δD(CH4) are between -41.81 and -26.65 and -224.1 to -180.5 , with an av. value of -36.23 and -206.5, resp. The coal-measure gas in the study area is mainly coal-type gas mixed with a small amt. of oil-type gas. The gas origin is mainly thermogenic and in Benxi Formation and Taiyuan Formation there is inorg. CO2 due to the magma intrusion. The vertical and lateral migration processes of natural gas were obsd. controlled by dynamic isotope fractionation effect and migration fractionation effect.

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Li, Y.; Yang, J. H.; Pan, Z. J.; Meng, S. Z.; Wang, K.; Niu, X. L. Unconventional Natural Gas Accumulations in Stacked Deposits: A Discussion of Upper Paleozoic Coal-Bearing Strata in the East Margin of the Ordos Basin, China. Acta Geol. Sin. 2019, 93, 111– 129,  DOI: 10.1111/1755-6724.13767

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Unconventional Natural Gas Accumulations in Stacked Deposits: A Discussion of Upper Paleozoic Coal-Bearing Strata in the East Margin of the Ordos Basin, China

Li, Yong; Yang, Jianghao; Pan, Zhejun; Meng, Shangzhi; Wang, Kai; Niu, Xinlei

Acta Geologica Sinica (English Edition) (2019), 93 (1), 111-129CODEN: AGSIDP; ISSN:1000-9515. (John Wiley & Sons, Inc.)

Different strata combinations are identified with coal deposition and favor for continuous gas accumulations, including the tidal flat, deltaic and fluvial systems distributed in most of the study areas. Methane was not only generated from the thick coal seams in the Taiyuan and Shanxi formations, but also from shale and dark mudstones. The coal, shale and tight sandstones are proved of remarkable gas content and hydrocarbon indications, and the gas satn. of tight sandstones decreases upward. The stacked deposit combinations vary isochronally in different areas, while the coal seams were developed stably showing good gas sources. Two key stages control the hydrocarbon enrichment, the continuous subsidence from coal forming to Late Triassic and the anomalous paleo-geothermal event happened in Early Cretaceous, as indicated by the fluid inclusions evidence. Extensive areas show good hydrocarbon development potential presently, and more works should be focused on the evaluation and selection of good reservoir combinations.

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Hou, Y. H.; Liu, D. N.; Zhao, F. H.; Zhong, L. H.; Emmanuel, N. N.; Zhang, Q. Geological Characteristics Affecting Coalbed Methane: A Case Study in the Anze Area, Southern Qinshui Basin. Nat. Resour. Res. 2022, 31, 1425– 1442,  DOI: 10.1007/s11053-022-10032-z

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Geological Characteristics Affecting Coalbed Methane: A Case Study in the Anze Area, Southern Qinshui Basin

Hou, Yuehua; Liu, Dongna; Zhao, Fenghua; Zhong, Linhua; Emmanuel, N. Nnachi; Zhang, Qi

Natural Resources Research (Dordrecht, Netherlands) (2022), 31 (3), 1425-1442CODEN: NRREFQ; ISSN:1520-7439. (Springer)

The main purpose of this study was to investigate the geol. characteristics affecting coalbed methane (CBM) enrichment in the Anze area in China for the exploitation of methane and predicting optimum CBM enrichment. To study methane enrichment, we developed maps of coal structure via correspondence anal., used rescaled-range (R/S) anal. to assess the degree of development of fractures, measured adsorption capacities of methane in coal, and detd. present-day methane contents and vitrinite-reflectance values at depths of ∼ 800-1200 m in the southeastern Qinshui Basin. The results showed the following. (1) Coals in the Anze coalfield exhibited high adsorption capacities, which affect coal rank, but adsorption capacities in the eastern and southern parts were greater than those in the western and northern parts. (2) Both the upper and lower parts of the coalfield were characterized by primary coal and detrital coal, whereas the middle part mostly contained granular coal. This is a typical interbedded structure that is divided clearly into three types. Reservoir conditions in the northern and southern parts near well A1, which revealed distinctly thick beds of primary and cataclastic coal, were more favorable compared with other southern areas and south-central parts. (3) Per the R/S anal., the reservoir fractal dimension was large in the fault-development zone in the middle part, esp. near the major faults F1 and F2 in the south-central parts. Reservoir conditions in the northern parts and near wells A1, A10, and A19 in the southern parts were more favorable compared than those in the area near faults F1 and F2.

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Guo, Z. Q.; Cao, Y. X.; Zhang, Z.; Dong, S. Geological controls on the gas content and permeability of coal reservoirs in the Daning Block, southern Qinshui Basin. ACS Omega 2022, 7, 17063– 17074,  DOI: 10.1021/acsomega.2c00371

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Geological Controls on the Gas Content and Permeability of Coal Reservoirs in the Daning Block, Southern Qinshui Basin

Guo, Zhiqi; Cao, Yunxing; Zhang, Zheng; Dong, Shi

ACS Omega (2022), 7 (20), 17063-17074CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)

The gas content and permeability of the coal reservoir are the key factors affecting coalbed methane (CBM) productivity. To investigate the geol. controls on the permeability and gas content of coal reservoirs in the Daning block, southern Qinshui Basin, geol. surveys combined with lab. expts., including coal petrol. anal., proximate anal., and methane isothermal adsorption expts., were carried out. The gas content of coals in the Daning block ranges from 5.56 to 17.57 (avg. 12.83) m3/t, and the coal permeability is generally above 0.1 mD, averaging 0.96 mD. The gas content of coal reservoirs shows decreasing trends with the increase in ash yield and moisture content, while tends to increase with the increase of vitrinite content; however, the correlation coeffs. are all extremely low. The gas content presents a strong pos. correlation with the burial depth of coal seams, but overall poorly correlates with the coal thickness. The CBM-rich areas are generally located at the hinge zones of secondary synclines, while the lower gas content areas commonly occur at the hinge zones of secondary anticlines. The normal faults are developed in the Daning block, and as expected, the gas content of coal seams that are near the normal faults is commonly lower. The well testing permeability of coal reservoirs in the Daning block decreases exponentially with the increase of the min. horizontal stress (σh) and the max. horizontal principal stress (σH). With the increase of the burial depth, the coal permeability also decreases exponentially. The primary and cataclastic structure coals generally have a higher hydro-fracturing permeability than the granulitic and mylonitic structure coals. This work can serve as a guide for the target area selections of CBM enrichment and high prodn. in the Daning block.

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Li, Y.; Zhang, C.; Tang, D. Z.; Gan, Q.; Niu, X. L.; Wang, K.; Shen, R. Y. Coal pore size distributions controlled by the coalification process: An experimental study of coals from the Junggar, Ordos and Qinshui basins in China. Fuel 2017, 206, 352– 363,  DOI: 10.1016/j.fuel.2017.06.028

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Coal pore size distributions controlled by the coalification process: An experimental study of coals from the Junggar, Ordos and Qinshui basins in China

Li, Yong; Zhang, Cheng; Tang, Dazhen; Gan, Quan; Niu, Xinlei; Wang, Kai; Shen, Ruiyang

Fuel (2017), 206 (), 352-363CODEN: FUELAC; ISSN:0016-2361. (Elsevier Ltd.)

Various sizes of pores in coal, which are generally formed by org. matter during the coalification process, have a direct influence on coalbed methane extn. However, few studies have investigated the pore size distributions across the thermal evolution of coal from peat to anthracite. In this project, three series of coal samples collected from three key CBM development basins with graded vitrinite reflectance values (Ro), the eastern Junggar basin (Ro of approx. 0.5%), eastern Ordos basin (Ro of approx. 2.2%) and southern Qinshui basin (Ro of approx. 3.0%), were systematically characterized by optical observations, low-temp. nitrogen adsorption/desorption, and NMR (NMR) methods. The av. pore radius calcd. by the Brunauer-Emmett-Teller (BET) method shows that the low-rank (L) series (averaging 14.17 nm) has values higher than either the middle-rank (M, 12.70 nm) or high-rank (H, 12.66 nm) samples. Bright and semi-bright coals (detd. by the overall relative lustre and percentage of bright components) are generally distributed with relatively higher pore radii (averaging 16.86 nm for all 3 series) than the semi-dull and dull coals (9.50 nm). The range of pore sizes decreases as the coal rank increases, and the NMR transverse relaxation (T2) spectrum decreases from bi-modal and tri-modal (M and L series) to unimodal curves (H series). However, the pore surfaces and complexity inside the coal increase with the coal rank, with the fractal results showing a three-stage fitting slope of the H series compared with the M (two-stage) and L (one-stage) coals. The observations are generally caused by the L coals, which mainly include plant tissue pores, while the M series coals are characterized by circle-shaped tissue pores and gas pores. The H series of flattened tissue pores and more diverse gas pores are identified in the higher-rank coals. Combined with the thermogenic gas generation process of coal, three key transition points were recognized: (1) Ro of approx. 0.5%, transition of dehydration to bituminization with coals being much more compacted, shown as the >100 nm range pores decreasing sharply; (2) Ro of approx. 1.2%, the beginning of the debituminization stage with the intensive generation of thermogenic gas, with pores ranging between 10 and 50 nm increasing quickly; and (3) Ro of approx. 1.9%, coal being transformed into anthracite, becoming much more compacted with the induction of cleats/fractures, shown as another decrease in >100 nm range pores but an increase in 50-100 nm range pores. These observations could deepen the understanding of the complex pore size distribution differences between different coal ranks and the impact of the thermal evolution on the coal heterogeneity and its reservoir characteristics.

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Hou, H. H.; Liang, G. D.; Shao, L. Y.; Tang, Y.; Mu, G. Y. Coalbed methane enrichment model of low-rank coals in multi-coals superimposed regions: a case study in the middle section of southern Junggar Basin. Front. Earth Sci. 2021, 15, 256– 271,  DOI: 10.1007/s11707-021-0917-6

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Coalbed methane enrichment model of low-rank coals in multi-coals superimposed regions and a case study in the middle section of southern Junggar Basin

Hou, Haihai; Liang, Guodong; Shao, Longyi; Tang, Yue; Mu, Guangyuan

Frontiers of Earth Science (2021), 15 (2), 256-271CODEN: FESRBN; ISSN:2095-0209. (Springer GmbH)

Abstr.: The Middle Jurassic Xishanyao Formation in the central section of the southern Junggar Basin has substantial amts. of low-ranked coalbed methane (CBM) recourses and is typically characterized by multi superimposed coal seams. To establish the CBM enrichment model, a series of exptl. and testing methods were adopted, including coal maceral observation, proximate anal., low temp. nitrogen adsorption (LTNA), methane carbon isotope detn., porosity/permeability simulation caused by overburden, and gas content testing. The controlling effect of sedimentary environment, geol. tectonic, and hydrogeol. condition on gas content was analyzed in detail. The results demonstrate that the areas with higher gas content (an av. of 8.57 m3/t) are mainly located in the Urumqi River-Santun River (eastern study area), whereas gas content (an av. of 3.92 m3/t) in the Manasi River-Taxi River (western study area) is relatively low. Because of the combined effects of strata temp. and pressure, the gas content in coal seam first increases and then decreases with increasing buried depth, and the crit. depth of the inflection point ranges from 600 m to 850 m. Affected by the changes in topog. and water head height, the direction of groundwater migration is predicted from south to north and from west to east. Based on the gas content variation, the lower and middle parts of the Xishanyao Formation can be divided into three independent coal-bearing gas systems. Within a single gas-bearing system, there is a pos. correlation between gas content and strata pressure, and the key mudstone layers sepg. each gas-bearing system are usually developed at the end of each highstand system tract. The new CBM accumulation model of the multi-coals mixed genetic gas shows that both biol. and thermal origins are found in a buried depth interval between 600 m and 850 m, suggesting that the coals with those depths are the CBM enrichment horizons and favorable exploration regions in the middle section of the southern Junggar Basin. An in-depth discussion of the low-rank CBM enrichment model with multi-coal seams in the study region can provide a basis for the optimization of CBM well locations and favorable exploration horizons.

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Qin, R. F.; Cao, D. Y.; Wang, A. M.; Li, H.; Bai, C. G. CBM reservoir-forming model of Zhuozishan mining area in western margin of Ordos basin. Coal Geol. Explor. 2018, 46, 54– 58

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Fu, H. J.; Tang, D. Z.; Xu, T.; Xu, H.; Tao, S.; Zhao, J. L.; Chen, B. L.; Yin, Z. Y. Preliminary research on CBM enrichment models of low-rank coal and its geological controls: A case study in the middle of the southern Junggar Basin, NW China. Mar. Pet. Geol. 2017, 83, 97– 110,  DOI: 10.1016/j.marpetgeo.2017.03.007

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Warwick, P. D. Coal systems analysis: A new approach to the understanding of coal formation, coal quality and environmental considerations, and coal as a source rock for hydrocarbons. In Coal Systems Analysis. Geological Society of America Special Paper; Warwick, P. D., Ed.; Boulder, CO, USA, 2005; pp 1– 8.

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Chalmers, G. R. L.; Bustin, R. M. On the effects of petrographic composition on coalbed methane sorption. Int. J. Coal Geol. 2007, 69, 288– 304,  DOI: 10.1016/j.coal.2006.06.002

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On the effects of petrographic composition on coalbed methane sorption

Chalmers, Gareth R. L.; Bustin, R. Marc

International Journal of Coal Geology (2007), 69 (4), 288-304CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)

The effect of petrog. compn. on the methane sorption capacity has been detd. for a suite of coals and org.-rich shales. Subbituminous and bituminous coals were sepd. into bright and dull lithotypes by hand picking. The methane sorption capacities range between 0.5 and 23.9 cm3/g at a pressure of 6 MPa. The low volatile bituminous Canmore coal and the anthracite sample have the highest capacities with the "natural coke" having the lowest. For low-rank coals there is no significant difference between bright and dull samples except for one coal with the dull sample having a greater sorption capacity than its bright equiv. For higher-rank coals, the bright samples have a greater methane capacity than the dull samples and the difference between sample pairs increases with rank. The boghead coal samples have the highest sorption capacities in the liptinite-rich coals suite and are higher than subbituminous to medium volatile bituminous samples. Pore size distribution indicates that methane is held as soln. gas in liptinite-rich coals and by phys. sorption in micropores in liptinite-poor coals. These contrasting processes illustrate that liptinite-rich samples need to be independently assessed. The pos. relationship between reactive inertinite content and methane sorption capacity occurs within the subbituminous to medium volatile bituminous coals because the reactive inertinite is structurally similar to vitrinite and have a higher microporosity than non-reactive inertinite. Reactivity of inertinite should be assessed in coalbed methane studies of dull coals to provide a better understanding of petrog. compn. effects on methane capacity.

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Keshavarz, A.; Sakurovs, R.; Grigore, M.; Sayyafzadeh, M. Effect of maceral composition and coal rank on gas diffusion in Australian coals. Int. J. Coal Geol. 2017, 173, 65– 75,  DOI: 10.1016/j.coal.2017.02.005

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Effect of maceral composition and coal rank on gas diffusion in Australian coals

Keshavarz, Alireza; Sakurovs, Richard; Grigore, Mihaela; Sayyafzadeh, Mohammad

International Journal of Coal Geology (2017), 173 (), 65-75CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)

Gas diffusion within the coal matrix plays a key role in detg. the rate of natural gas depletion and enhanced coal bed methane prodn. via CO2 sequestration (CO2-ECBM) in coal seam gas reservoirs. In this work, we investigated the influence of maceral compn. and coal rank on CO2 and CH4 diffusion rates of 18 bituminous and sub-bituminous Australian coals. We obtained measures of the gas diffusion rate and the spread of diffusion times. Gas diffusion rate through coal pores was found to vary over 6 orders of magnitude depending on the coal rank and maceral compn. This diffusion rate was independent of pressure in the range 1-5 bar. It increased substantially with inertinite content of the coal in the lower rank and medium rank coals examd. In the high rank coals, the diffusion rate was less sensitive to maceral compn., but alternatively, this may reflect regional variations in the dependence of diffusion rate with maceral compn. The CO2 diffusion rate was faster than the CH4 diffusion rate. The factor describing the spread of diffusion times generally increased with increasing vitrinite content but for a given coal was similar for both CH4 and CO2. This suggests the gases penetrate the same parts of the coal structure. Based on the exptl. data, different synthetic coalbed simulation models were constructed to analyze the impact of CH4 and CO2 diffusion coeffs. on ECBM and CO2 sequestration performance. The numerical simulation results showed that CH4 prodn. rate is inversely proportional to the sorption time, if the bulk flow in the cleats does not create any restriction. The results also indicated that CO2 breakthrough time is a function of the CO2 sorption time - if the CO2 adsorption is not fast enough, the injected CO2 will be spread into the seam, resulting in an early breakthrough.

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Scott, S.; Anderson, B.; Crosdale, P.; Dingwall, J.; Leblang, G. Coal petrology and coal seam gas contents of the Walloon Subgroup Surat Basin, Queensland, Australia. Int. J. Coal Geol. 2007, 70, 209– 222,  DOI: 10.1016/j.coal.2006.04.010

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Coal petrology and coal seam gas contents of the Walloon Subgroup - Surat Basin, Queensland, Australia

Scott, Steven; Anderson, Bruce; Crosdale, Peter; Dingwall, Julie; Leblang, Garry

International Journal of Coal Geology (2007), 70 (1-3), 209-222CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)

Core, exploration and appraisal drilling over the last four years have targeted the Juandah (upper) and Taroom (lower) Coal Measures of the Middle Jurassic Walloon Subgroup of the Injune Creek Group. These wells have shown that the high-volatile bituminous, perhydrous coals of the Walloon Subgroup have gas contents of between 1 and 14 m3/t and some wells have encountered gas flows at rates of over 30,000 m3/day. This recent work has confirmed the initial supposition that the coals within the Walloon Subgroup contain sufficient quantities of gas to be economically viable as a coal seam gas source. Coal vols., depth and structure have also been confirmed from the initial work. What has become apparent during this work is the variability in gas content across each of the seams in the Juandah and Taroom Coal Measures and the variability in coal petrog. compn. of these two units are controlled by many factors. What has also become apparent is that the Taroom Coal Measures have a lower av. gas content than the overlying Juandah Coal Measures. The obvious conclusion is that the difference in gas content is related to a difference in coal petrol. Not only do the gas contents of these coal intervals differ, their gas adsorption capacities also differ with the Taroom Coal Measures coals displaying, on av., lower gas content and lower gas adsorption capabilities. Again the obvious cause would be coal petrol. differences, but the Juandah and Taroom Coal Measures coals have similar maceral percentages. An understanding of the relationship between gas desorption, gas adsorption and coal compn. is vital in detg. which areas in the basin offers the most economically viable targets for the commercialization of coal seam gas in the Surat Basin.

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Teng, J.; Mastalerz, M.; Hampton, L. Maceral controls on porosity characteristics of lithotypes of Pennsylvanian high volatile bituminous coal: Example from the Illinois Basin. Int. J. Coal Geol. 2017, 172, 80– 94,  DOI: 10.1016/j.coal.2017.02.001

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Maceral controls on porosity characteristics of lithotypes of Pennsylvanian high volatile bituminous coal: Example from the Illinois Basin

Teng, Juan; Mastalerz, Maria; Hampton, LaBraun

International Journal of Coal Geology (2017), 172 (), 80-94CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)

Porosity characteristics of vitrain, clarain, durain, and fusain lithotypes of the Springfield Coal Member of the Petersburg Formation and the Danville and Hymera Coal Members of the Dugger Formation from the Illinois Basin were investigated with a special emphasis on the control of coal macerals on pore-size distribution. These Pennsylvanian coals are of high volatile bituminous rank and have vitrinite reflectance ranging from 0.51 to 0.60%. The lithotypes studied show decreasing values of Brunauer-Emmett-Teller surface area, and micro- and mesopore vols. from vitrain through clarain and durain to fusain. Within the mesopore size range, vitrain and clarain are dominated by pore-size widths of 4 to 10 nm, whereas durain and fusain have more vol. for pores larger than 20 nm. In contrast to mesopores, micropore sizes are very similar for all lithotypes, averaging 1.37 to 1.39 nm. In addn. to differences among lithotypes, there are significant differences in pore characteristics among the three coals studied, with largest surface areas and pore vols. documented for the Hymera, followed by the Danville and the Springfield. A strong relationship exists between surface area, mesoporosity, and microporosity and maceral compn., with vitrinite having a very strong pos. correlation, liptinite having a weak pos. correlation, and inertinite having a strong neg. correlation. Neg. correlations of total porosity with vitrinite and liptinite and pos. correlations with inertinite suggest that among the maceral groups, pores in inertinite contribute most to the total porosity. The Fourier transform IR spectrometry technique demonstrates that fusains from the three coals studied have higher aromaticity and a higher degree of arom. ring condensation and lower hydrocarbon potential than the other lithotypes, whereas chem. differences between vitrain, clarain, and durain are less distinct. In addn., there is a relationship between aromaticity of the lithotypes and surface area and mesopore and micropore vols.

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Groshong, R. H.; Pashin, J. C.; Mclntyre, M. R. Structural controls on fractured coal reservoirs in the southern Appalachian Black Warrior foreland basin. J. Struct. Geol. 2009, 31, 874– 886,  DOI: 10.1016/j.jsg.2008.02.017

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Hou, Q. L.; Li, H. J.; Fan, J. J.; Ju, Y. W.; Wang, T. K.; Li, X. S.; Wu, Y. D. Structure and coalbed methane occurrence in tectonically deformed coals. Sci. China Earth Sci. 2012, 55, 1755– 1763,  DOI: 10.1007/s11430-012-4493-1

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Structure and coalbed methane occurrence in tectonically deformed coals

Hou, Quan Lin; Li, Hui Jun; Fan, Jun Jia; Ju, Yi Wen; Wang, Tian Kai; Li, Xiao Shi; Wu, Yu Dong

Science China: Earth Sciences (2012), 55 (11), 1755-1763CODEN: SCESCI; ISSN:1869-1897. (Springer)

A review. Research on structure of tectonically deformed coals (TDC) is a key issue in coal and gas outburst prevention and coalbed methane (CBM) exploitation. This paper presents a summary on the research progress in TDC's structural-genetic classification, tectonic strain influence on coal microstructure, coal porosity system, coal chem. structure and constituents, and their relationship with the excess coalbed methane. Previous studies suggested that tectonic deformation had significant influence on coal microstructure, coal super microstructure, and even chem. macromol. structure. The main mechanisms of coal deformation are the tectonic stress degrdn. and polycondensation metamorphism (dynamical metamorphism). Besides, under different deformation mechanisms, the ultra- and micro-structure and chem. constituents of TDC presented distinct characteristics. Based on these achievements, we propose one possible evolutionary trend of TDC with different deformation mechanisms, and suggest that the coal and gas outburst in the TDC, esp. in the mylonitic coals, may be not only controlled by geol. structure, but also influenced by the tectonic stress degrdn. of ductile deformation. Therefore, further study on TDC should be focused on the controlling mechanism of deformation on structure and compn. of coal, generation conditions and occurrence state of excess coalbed methane from deformation mechanism of coal.

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Pashin, J. C.; Groshong, R. H. Structural control of coalbed methane production in Alabama. Int. J. Coal Geol. 1998, 38, 89– 113,  DOI: 10.1016/S0166-5162(98)00034-2

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Structural control of coalbed methane production in Alabama

Pashin, Jack C.; Groshong, Richard H., Jr.

International Journal of Coal Geology (1998), 38 (1-2), 89-113CODEN: IJCGDE; ISSN:0166-5162. (Elsevier Science B.V.)

Thin-skinned structures are distributed throughout the Alabama coalbed methane fields, and these structures affect the prodn. of gas and water from coal-bearing strata. Extensional structures in Deerlick Creek and Cedar Cove fields include normal faults and hanging-wall rollovers, and area balancing indicates that these structures are detached in the Pottsville Formation. Compressional folds in Gurnee and Oak Grove fields, by comparison, are interpreted to be detachment folds formed above decollements at different stratigraphic levels. Patterns of gas and water prodn. reflect the structural style of each field and further indicate that folding and faulting have affected the distribution of permeability and the overall success of coalbed methane operations. Area balancing can be an effective way to characterize coalbed methane reservoirs in structurally complex regions because it constrains structural geometry and can be used to det. the distribution of layer-parallel strain. Comparison of calcd. requisite strain and borehole expansion data from caliper logs suggests that strain in coalbed methane reservoirs is predictable and can be expressed as fracturing and small-scale faulting. However, refined methodol. is needed to analyze heterogeneous strain distributions in discrete bed segments. Understanding temporal variation of prodn. patterns in areas where gas and water prodn. are influenced by map-scale structure will further facilitate effective management of coalbed methane fields.

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Lamarre, R. A. Hydrodynamic and stratigraphic controls for a large coalbed methane accumulation in Ferron coals of east-central Utah. Int. J. Coal Geol. 2003, 56, 97– 110,  DOI: 10.1016/S0166-5162(03)00078-8

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Hydrodynamic and stratigraphic controls for a large coalbed methane accumulation in Ferron coals of east-central Utah

Lamarre, Robert A.

International Journal of Coal Geology (2003), 56 (1-2), 97-110CODEN: IJCGDE; ISSN:0166-5162. (Elsevier Science B.V.)

Upper Cretaceous coals within the Ferron Sandstone Member of the Mancos Shale contain large vols. of coalbed methane. Coals at the northern end of the 80-mi-long (129 km) Ferron trend have produced more than 200 billion cubic feet (BCF) (5663×106 m3) of coalbed methane, with daily prodn. of 260 million cubic feet per day (MMCFD) (7.36×106 m3/day) from 470 wells during Nov. 2000. Core data from exploratory wells drilled along trend to the south indicate that the gas content of the coals decreases dramatically from north to south, even though the rank and maturity of the coals decreases only slightly. The area with lowest gas content correlates with the outcrop of the Ferron coals. Hydrodynamic studies have shown that the Ferron coals are aquifers that are recharged from the Wasatch Plateau to the west. Regional mapping indicates that the productive fields are large stratigraphic traps where the coals pinchout updip into tight marine shales to the east. Much of the produced gas is secondary biogenic gas and migrated thermogenic gas that has moved from the Wasatch Plateau and south margin of the Uinta Basin, resp. Further south, near the town of Emery, coals are present at the surface, but most of the gas has been flushed out of the coals due to redn. of reservoir pressure and active water flow from the west. Therefore, the entire Ferron trend probably contained tremendous vols. of stratigraphically trapped coalbed methane before uplift and erosion exposed the southern coals to the atm.

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Li, Y.; Tang, D. Z.; Xu, H.; Elsworth, D.; Meng, Y. J. Geological and hydrological controls on water coproduced with coalbed methane in Liulin, eastern Ordos basin, China. AAPG Bull. 2015, 99, 207– 229,  DOI: 10.1306/07211413147

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Scott, A. R. Hydrogeologic factors affecting gas content distribution in coal beds. Int. J. Coal Geol. 2002, 50, 363– 387,  DOI: 10.1016/S0166-5162(02)00135-0

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Hydrogeologic factors affecting gas content distribution in coal beds

Scott, Andrew R.

International Journal of Coal Geology (2002), 50 (1-4), 363-387CODEN: IJCGDE; ISSN:0166-5162. (Elsevier Science B.V.)

Gas content in coal is not fixed but changes when equil. conditions within the reservoir are disrupted. Therefore, gas content distribution in coal varies laterally within individual coal beds, vertically among coal beds in a single well, and within thicker coal beds. Major hydrogeol. factors affecting gas content variability include gas generation, coal properties, and reservoir conditions. Gas generation affects gas content variability on a regional scale, whereas coal properties influence gas content distribution on a regional and local scale. Reservoir conditions affect gas content more locally within specific fields or individual wells. The potential for high gas content is controlled directly by the amt. of thermogenic and secondary biogenic gases generated from the coal which in turn are controlled by burial history, maceral compn., and basin hydrodynamics. Variability in mineral matter (ash) and moisture content, sorption behavior among macerals, diffusion coeffs., and permeability result in heterogeneous gas content distribution. Gas content decreases with decreasing pressure and temp., and coal beds become undersatd. with respect to methane during basin uplift and cooling. Gas content generally increases where conventional and hydrodynamic trapping of coal gases occur and may decrease in areas of active recharge with downward flow potential and/or convergent flow where there is no mechanism for entrapment.

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Yao, Y. B.; Liu, D. M.; Yan, T. T. Geological and hydrogeological controls on the accumulation of coalbed methane in the Weibei field, southeastern Ordos Basin. Int. J. Coal Geol. 2014, 121, 148– 159,  DOI: 10.1016/j.coal.2013.11.006

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Geological and hydrogeological controls on the accumulation of coalbed methane in the Weibei field, southeastern Ordos Basin

Yao, Yanbin; Liu, Dameng; Yan, Taotao

International Journal of Coal Geology (2014), 121 (), 148-159CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)

Com. exploration and prodn. of coalbed methane (CBM) in the Weibei field, Ordos Basin, China has rapidly increased since 2010. The Weibei field has become one of the most productive CBM areas in China. However, relatively few studies have investigated the migration of gas and water in the coal reservoir and their controls on the gas accumulation. This study conducts stable isotope analyses and quality tests for groundwater samples, discusses the relationships between the fluid flow pathways and tectonics, and concludes by discussing the geol. and hydrol. controls on potential gas accumulation in the Weibei field. The coalbed groundwaters contain primarily sodium and bicarbonate and are effectively devoid of sulfate, calcium and magnesium. The groundwaters are typically freshwater, with total dissolved solids (TDS) values ranging from 814 to 2657 mg/L. Differences in hydrogeol. and structural geol. divide the study area into four gas domains. In the northern Hancheng area, the predominant northwest flow of groundwater has resulted in higher gas content in the west (> 12 m3/t) than in the east (8-12 m3/t), even though the coals in the east have high thermal maturity (2.1%-2.3% Ro). The area with the highest gas content (> 16 m3/t) is in the region near the downthrown side of the Xuefeng-Nan Thrust Fault in the northern Hancheng area, and the fault forms a barrier to the northwestward flow of groundwater. The area with the lowest gas content (< 4 m3/t) is in the southwest Heyang area, where there is no entrapment mechanism and the gas has been flushed out of the coals due to a redn. of hydrostatic pressure and active groundwater flow from the east. Structural and hydrodynamic mechanisms, esp. the intensity of the hydrodynamic activity and the groundwater flow pathways, are important for gas accumulation in the Weibei field.

30

Wang, S. M. Coal Accumulation and Coal Resource Evaluation of Ordos Basin; Coal Industry Publishing House: Beijing, 1996; p 437.

(in Chinese).

31

Darby, B. J.; Ritts, B. D. Mesozoic contractional deformation in the middle of the Asian tectonic collage: the intraplate Western Ordos fold-thrust belt, China. Earth Planet. Sci. Lett. 2002, 205, 13– 24,  DOI: 10.1016/S0012-821X(02)01026-9

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Mesozoic contractional deformation in the middle of the Asian tectonic collage: the intraplate Western Ordos fold-thrust belt, China

Darby, Brian J.; Ritts, Bradley D.

Earth and Planetary Science Letters (2002), 205 (1-2), 13-24CODEN: EPSLA2; ISSN:0012-821X. (Elsevier Science B.V.)

Intraplate deformation can occur at great distances from synchronous plate boundaries and is most likely the result of strain concns. into weak zones. These weak zones may be pre-existing crustal heterogeneities such as faults, thermally weakened zones due to magmatism and/or the effects of a thick sedimentary cover, or mech. contrasts between adjacent crustal blocks. Intraplate deformation is not exclusively related to continent-continent collision; it can be the result of a change in subduction dynamics or the interaction of two convergent plate boundaries. Asia has many examples of active intraplate deformation that are thought to be related to the Cenozoic Indo-Asian collision. Although much attention is placed on active examples, discerning controls on pre-Cenozoic Asian intraplate deformation will lead to not only a better understanding of the processes involved in intraplate orogens but also the tectonic evolution of Asia. Situated on either side of the Yellow River along the western margin of the Ordos Plateau, the intraplate Western Ordos fold-thrust belt involves units as old as Archean basement and as young as Late Jurassic(). The fold-thrust belt is spectacularly exposed in two ranges, the Helan Shan and the Zhuozi Shan. Cross-sections drawn through the north-trending fold-thrust belt suggest 30% min. shortening. A paleocurrent reversal within the Lower-Middle Jurassic section is interpreted to mark the onset of contraction and mountain building. A Late Jurassic() synorogenic boulder conglomerate records the final deformation in the fold-thrust belt. Synchronous with contraction, and cutting across the fold-thrust belt, is an ∼E-W-striking strike-slip fault that displays right-lateral drag and sub-horizontal striae. We interpret this fault as an accommodation structure in the thrust belt. Following ∼E-W contraction, the thrust belt was dismembered by an ∼N15°W-trending left-lateral strike-slip fault. The left-lateral fault displaces the frontal portion of the thrust belt (the Zhuozi Shan) 62 km to the north, relative to a more internal portion (the Helan Shan). We propose that the anomalous orientation and intraplate location of the Western Ordos fold-thrust belt is a function of mech. contrasts between a previously deformed area and a stable crustal block (Ordos) to the east. Pre-existing crustal weakness or anisotropy along the western margin of the Ordos block is most likely related to either a Late Proterozoic-Early Paleozoic aulacogen, a Triassic fault system, or to both. Although its intraplate location and the complexities of Mesozoic Asian plate interactions make it difficult to relate deformation in the Western Ordos fold-thrust belt to a specific contemporaneous plate boundary, its N-S trend may suggest a link with paleo-Pacific subduction along the eastern margin of Asia.

32

Shang, G. X. Late Paleozoic Coal Geology of North China Platform; Shanxi Science and Technology Press: Taiyuan, 1997; p 405.

(in Chinese with English abstract).

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Shao, L. Y.; Dong, D. X.; Li, M. P.; Wang, H. S.; Wang, D. D.; Lu, J.; Zheng, M. Q.; Cheng, A. G. Sequence-paleogeography and coal accumulation of the Carboniferous - Permian in the North China Basin. J. China Coal Soc. 2014, 39, 1725– 1734,  DOI: 10.13225/j.cnki.jccs.2013.9033

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Shao, L. Y.; Wang, D. D.; Dong, D. X.; Li, M. P.; Li, L. Depositional Environment and Coal Accumulation Regularity of Coal-bearing Series in Ordos Basin; Geological Publishing House: Beijing, 2020; p 229.

(in Chinese)

35

Zhai, M. G. Ordos Block (Basin) is a key to understand early continental evolution and tectonic regime of the North China Craton. Chin. Sci. Bull. 2021, 66, 3441– 3461,  DOI: 10.1360/TB-2021-0113

36

Zhang, B. H.; Zhang, J.; Zhao, H.; Nie, F. J.; Wan, Y. N.; Zhang, Y. P. Tectonic evolution of the western Ordos Basin during the Palaeozoic-Mesozoic time as constrained by detrital zircon ages. Int. Geol. Rev. 2019, 61, 461– 480,  DOI: 10.1080/00206814.2018.1431963

37

Li, B.; Meng, Z. F.; Song, Y.; Li, X. B. Tectonic-sedimentary response of foreland basin in western margin of Ordos Basin. J. Jilin Univ. Earth Sci. Ed. 2007, 37, 703– 709

38

Li, T. B. The characteristics and evolution of thrust nappe structure in the west margin of Ordos basin. Dissertation, China University of Geoscience: Beijing, 2006.

39

Wang, Y.; Zhu, Y. M.; Yao, N. W.; Xie, K. Z.; Song, Y. Sedimentology characteristics and coal-forming models debating in the coal-containing strata of Zhuozishan coalfield. China Mineral Mag. 2014, 23, 100– 104

40

Zhang, J. S.; He, Z. X.; Fei, A. Q.; Li, T. B.; Huang, X. N. Epicontinental mega thrust and nappe system at north segment of the western rim of the Ordos Block. Sci. Geol. Sin. 2008, 43, 251– 281,  DOI: 10.2298/SOS0803251S

41

Geng, M.; Chen, H.; Chen, Y. P.; Zeng, L. J.; Chen, S. S.; Jiang, X. C. Methods and results of the fourth round national CBM resources evaluation. Coal Sci. Technol. 2018, 46, 64– 68

42

Cai, Y. D.; Liu, D. M.; Yao, Y. B.; Li, J. Q.; Guo, X. Q.; Zhang, B. R. Geological controlling factors and prospective areas of coalbed methane in Jixi Basin. J. Jilin Univ., Earth Sci. Ed. 2014, 44, 1779– 1788

43

Hu, H. T.; Lu, S. F.; Xue, H. T.; Wang, S. H.; Xu, C. W.; Wang, W. M. Multilevel fuzzy comprehensive assessment method and its application in the abundance of the CBM resources potential. Math. Pract. Theory 2014, 44, 126– 132

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Li, L. Z.; Cai, Z. H. Evaluate proved reserves of CBM in China by new model. Sci. Technol. Eng. 2014, 14, 191– 194

45

Qin, Y. Advances and reviews on research of coalbed gas geology in China. Geol. J. China Univ. 2003, 9, 339– 358

46

Su, X. B.; Zhang, L. P.; Lin, X. Y. Influence of coal rank on coal adsorption capacity. Nat. Gas Ind. 2005, 25, 19– 21

47

Mu, G. Y.; Hou, H. H.; Zhang, J. Q.; Tang, Y.; Li, Y. N.; Sun, B.; Li, Y.; Jones, T.; Yuan, Y.; Shao, L. Y. Fractal characterization of pore structure and its influence on CH4 adsorption and seepage capacity of low-rank coals. Front. Earth Sci. 2022,  DOI: 10.1007/s11707-022-0969-2

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Li, L. J.; Liu, D. M.; Cai, Y. D.; Wang, Y. J.; Jia, Q. F. Coal structure and its implications for coalbed methane exploitation: A review. Energy Fuels 2021, 35, 86– 110,  DOI: 10.1021/acs.energyfuels.0c03309

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Coal Structure and Its Implications for Coalbed Methane Exploitation: A Review

Li, Lijing; Liu, Dameng; Cai, Yidong; Wang, Yingjin; Jia, Qifeng

Energy & Fuels (2021), 35 (1), 86-110CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)

A review. Coal structure refers to the internal structural characteristics of coals including the degree of macroscopic and microscopic deformation, pore structure, and mech. properties after various geothermal and geol. stresses, which have substantial impacts on coal mining safety and coalbed methane (CBM) exploitation. The coal structure is an indicator of coal mech. strength, which dets. the petrophys. properties of CBM reservoirs and gas extn. or CBM prodn. efficiency from coals. Coals with different structures are divided into primary coals and tectonically deformed coals (TDCs). To explore the macro- and microscopic petrophysics of TDCs, a wide range of techniques including geophys. logging, seismic inversion, amplitude variation with offset (AVO), SEM (SEM), NMR (NMR), and Fourier transform IR spectroscopy (FTIR) have been employed. From a macroscopic perspective, the geol. and geophys. methods are generally used to distinguish the degree of deformation of whole coal seams. Microscopic methods including SEM, NMR, and FTIR, are normally used to investigate the internal petrophysics (e.g., pore and fracture, macromol. structure, and fluid performance) of TDCs. Herein, the macro- and micro-petrophys. properties of TDCs and their impacts on CBM exploration and exploitation are systematically reviewed and potential opportunities of future directions are recommended. Extensive studies have shown that the development zone of TDCs in the brittle series, notably cataclastic coals, has high potential for CBM exploration and exploitation, while that of TDCs in the ductile series is a dangerous region for coal mining and subject to gas outbursts. This review aims to provide background on the coal structure, summarize evaluation methods, highlight its geol. drivers, and explain its significance in CBM exploitation. We will end with proposals for future research directions.

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Hou, Q. L.; Li, H. J.; Fan, J. J.; Ju, Y. W.; Wang, T. K.; Li, X. S.; Wu, Y. D. Structure and coalbed methane occurrence in tectonically deformed coals. Sci. China Earth Sci. 2012, 55, 1755– 1763,  DOI: 10.1007/s11430-012-4493-1

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49

Structure and coalbed methane occurrence in tectonically deformed coals

Hou, Quan Lin; Li, Hui Jun; Fan, Jun Jia; Ju, Yi Wen; Wang, Tian Kai; Li, Xiao Shi; Wu, Yu Dong

Science China: Earth Sciences (2012), 55 (11), 1755-1763CODEN: SCESCI; ISSN:1869-1897. (Springer)

A review. Research on structure of tectonically deformed coals (TDC) is a key issue in coal and gas outburst prevention and coalbed methane (CBM) exploitation. This paper presents a summary on the research progress in TDC's structural-genetic classification, tectonic strain influence on coal microstructure, coal porosity system, coal chem. structure and constituents, and their relationship with the excess coalbed methane. Previous studies suggested that tectonic deformation had significant influence on coal microstructure, coal super microstructure, and even chem. macromol. structure. The main mechanisms of coal deformation are the tectonic stress degrdn. and polycondensation metamorphism (dynamical metamorphism). Besides, under different deformation mechanisms, the ultra- and micro-structure and chem. constituents of TDC presented distinct characteristics. Based on these achievements, we propose one possible evolutionary trend of TDC with different deformation mechanisms, and suggest that the coal and gas outburst in the TDC, esp. in the mylonitic coals, may be not only controlled by geol. structure, but also influenced by the tectonic stress degrdn. of ductile deformation. Therefore, further study on TDC should be focused on the controlling mechanism of deformation on structure and compn. of coal, generation conditions and occurrence state of excess coalbed methane from deformation mechanism of coal.

50

Wang, H.; Yao, Y. B.; Liu, D. M.; Cai, Y. D.; Yang, Y. H.; Zhou, S. Q. Determination of the degree of coal deformation and its effects on gas production in the southern Qinshui Basin, North China. J. Pet. Sci. Eng. 2022, 216, 110746  DOI: 10.1016/j.petrol.2022.110746

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50

Determination of the degree of coal deformation and its effects on gas production in the southern Qinshui Basin, North China

Wang, Hui; Yao, Yanbin; Liu, Dameng; Cai, Yidong; Yang, Yanhui; Zhou, Shengqiang

Journal of Petroleum Science & Engineering (2022), 216 (), 110746CODEN: JPSEE6; ISSN:0920-4105. (Elsevier B.V.)

Coal deformation significantly impacts coalbed methane (CBM) productivity. However, existing methods are incapable of quant. evaluating coal deformation on a high accuracy when there is limited core recovery from a CBM well. This study proposes a well logging coal structure index (WCSI) to quantify the degree of coal deformation. The WCSI of targets is calcd. based on the data from well logs and coal cores, and it can also be estd. from well logs when no coal cores are recovered from wells. Thus, the WCSI can be applied to the regional-scale evaluation of coal deformation degree, and its effectiveness has been validated by data for 245 samples from the No. 3 coal seam in 55 exploration wells in the southern Qinshui Basin (SQB). Results indicate that the reservoir permeability and hydraulic fracturing effectiveness depend on the degree of coal seam deformation, which further affects the unique distribution of gas prodn. in the known prodn. regions in the SQB. Generally, the lowly deformed coal seam (WCSI (avg.) < 40) occurs mainly in the tectonically 'stable' regions (no faults or only small-scale faults development) where the micro-fractures/cleats in coal are not well developed. Low WCSI (avg.) commonly indicates low permeability (<0.06 mD) and poor hydraulic fracturing of the reservoir, resulting in relatively low gas prodn. rate of wells in these regions. Secondly, the moderately deformed coal seam (40 < WCSI (avg.) < 60) is mainly found in areas close to moderate-scale faults, where coal seams have relatively high reservoir permeability (>0.06 mD) and hydraulic fracturing is easiest. This causes high gas prodn. rate from wells in these regions. Thirdly, the highly deformed coal seam (WCSI (avg.) > 60) is generally found close to large-scale faults, indicating very low reservoir permeability (<0.03 mD) and the worst effectiveness from hydraulic fracturing of the reservoir, resulting in the lowest gas prodn. rate of wells in these regions. In summary, the prodn. efficiency from CBM wells in the SQB can thus be described in terms of the WCSI (avg.) value, and hence this parameter can help with preliminary forecasting gas prodn.

51

Liu, G. Influence of pore structure characteristics of primary coal on coalbed methane adsorption. Adv. Mater. Sci. Eng. 2022, 2022, 3087252  DOI: 10.1155/2022/3087252

52

Cao, Z. D.; Lin, B. Q.; Liu, T. The impact of depositional environment and tectonic evolution on coalbed methane occurrence in West Henan, China. Int. J. Min. Sci. Technol. 2019, 29, 297– 305,  DOI: 10.1016/j.ijmst.2019.01.006

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The impact of depositional environment and tectonic evolution on coalbed methane occurrence in West Henan, China

Cao, Zhaodan; Lin, Baiquan; Liu, Ting

International Journal of Mining Science and Technology (2019), 29 (2), 297-305CODEN: IJMSL6; ISSN:2095-2686. (Elsevier B.V.)

A deeper understanding of the mechanisms by which geol. factors (depositional environment and tectonic evolution) control the occurrence of coalbed methane (CBM) is important for the utilization of CBM resources via surface-drilled wells and the elimination of coal-methane outbursts, the latter of which is a key issue for coal mine safety. Based on drill core data, high-pressure isothermal adsorption expts., SEM expts., mercury intrusion porosimetry, and X-ray diffraction expts. Results showed that the depositional environment led to the epigenetic erosion of tidal flat coal-accumulating structures by shallow-delta distributary channel strata. Epigenetic erosion by the depositional environment also increased coal body ash content (from 6.9% to 21.4%) and mineral content, filling the cleat system and reducing porosity, reducing methane storage capacity. The max. methane adsorption capacity of the coal body reduced from 35.7 cm3/g to 30.30 cm3/g, and Langmuir pressure decreased from 1.39 MPa to 0.909 MPa. Hence, the methane adsorption capacity of the coal body decreased while its capacity for methane desorption increased. Owing to the tectonic evolution of West Henan, tectonically deformed coal is common; as it evolves from primary cataclastic coal to granulitic coal, the angle of the diffraction peak increases, d002 decreases, and La, Lc, and Nc increase; these traits are generally consistent with dynamic metamorphism.

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Cheng, Y. P.; Pan, Z. J. Reservoir properties of Chinese tectonic coal: A review. Fuel 2020, 260, 116350  DOI: 10.1016/j.fuel.2019.116350

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Reservoir properties of Chinese tectonic coal: A review

Cheng, Yuanping; Pan, Zhejun

Fuel (2020), 260 (), 116350CODEN: FUELAC; ISSN:0016-2361. (Elsevier Ltd.)

A review. Tectonic coal, formed after the intact coal being subjected to long-term intense squeezing, shearing and deformation, is characterized by brittle or ductile damaged coal body, with the characteristics of low cohesion, low strength and low permeability. Most of the outburst accidents in China occurred in tectonic coal seams due to the difficulties in gas drainage. In this review article, reservoir properties, including pore structure, adsorption, diffusion, permeability and geomech. properties of the tectonic coal are reviewed in detail and compared with those of the intact coal, as these properties are important for gas drainage. It was found that tectonic coal in general shows larger total pore vol. and sp. surface area than intact coal for larger pores due to tectonism, however, no significant difference is obsd. in smaller pores due to the combined opposing effects of metamorphism and tectonism. Diffusion coeff. of tectonic coal is generally higher than that of intact coal, and tectonic coal typical has higher adsorption capacity than intact coal. Compressive strength and elasticity modulus are smaller for tectonic coal than intact coal. Field permeability of tectonic coal is obviously lower than that of intact coal, which is on the contrary to the exptl. results from lab. It was found that using reconstituted samples for tectonic coal in the lab. is the main cause for this discrepancy between field and lab. observations. It is suggested that more work is required on tectonic coal and a few research areas are proposed for future research.

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Li, H. Y.; Ogawa, Y.; Shimada, S. Mechanism of methane flow through sheared coals and its role on methane recovery. Fuel 2003, 82, 1271– 1279,  DOI: 10.1016/S0016-2361(03)00020-6

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Mechanism of methane flow through sheared coals and its role on methane recovery

Li, Huoyin; Ogawa, Yujiro; Shimada, Sohei

Fuel (2003), 82 (10), 1271-1279CODEN: FUELAC; ISSN:0016-2361. (Elsevier Science Ltd.)

Tectonically sheared coals are closely related with coal-bed methane exploitation, and are classified into cataclastic coals and mylonitic coals in view of their deformation mechanism, either brittle or ductile. SEM and reflected light microscopy observations showed that cataclastic coals possess a hierarchy of open, continuous and connecting fractures, whereas mylonitic coals always display tightly compressed and collapsed fractures. Mercury porosimetry studies also indicated that compared with normal coals, cataclastic coals possess greater porosity, more sp. surface area and wider av. fracture aperture; whereas mylonitic coals are characterized by narrower av. fracture aperture and a great deal of sp. surface area. Based on exptl. data and actual site experiences, a new model of methane flow within coals was proposed. Gas flow through sheared coals, unlike normal coals-a simple dual porosity system comprising a matrix of micropores that are surrounded by cleats, also contain addnl. steps because of overwhelming sheared fractures and different deformation mechanisms. This model can explain why gas outbursts are always accompanied by small-scale compressively geol. structures where mylonitic coals often occur, and why in such area the rate of gas extn. is unexpectedly lower, despite the presence of a great vol. of methane.

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Zhao, Y.; Wang, Y. L.; Jiang, P. S. Analysis on the content of coalbed methane and its influencing factors in Bohai Bay of Wuhai City in Inner Mongolia. China Coalbed Methane 2013, 10, 12– 16

56

Kang, Y. S.; Sun, L. Z.; Zhang, B.; Gu, J. Y.; Mao, D. L. Discussion on classification of coalbed reservoir permeability in China. J. China Coal Soc. 2017, 45, 186– 194

57

Wang, W. F. The gas-geology law and gas forecasting of No. 16 coal seam in Haibowan mining area. Dissertation, Henan Polytechnic University: Henan, 2012.

58

Wang, X. Q. Prediction of gas geology and research on gas drainage technology based on 16# coal seam of Pinggou Coal Mine; Inner Mongolia University of Science and Technology: Inner Mongolia, 2013.

59

Chen, P.; Qi, R. Study on the geological pattern of gas in 16# coal seam of Gongwusu Coal Mine. Henan Sci. Technol. 2013, 6, 183– 184

60

Hao, T. X.; Song, W. Y. Research on gas occurrence law of No. 6 coal seam in Haibowan mining area. China Coal 2013, 39, 94– 97

117.

61

Li, H.; Yao, H. P.; Li, F. C. CBM resource distribution features in Inner Mongolia. Coal Geol. China 2016, 28, 43– 48,  DOI: 10.1016/j.chemgeo.2016.06.007

62

Bohacs, K. M.; Suter, J. R. Sequence stratigraphic distribution of coaly rocks: fundamental controls and paralic examples. AAPG Bull. 1997, 81, 1612– 1639

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Sequence stratigraphic distribution of coaly rocks: fundamental controls and paralic examples

Bohacs, Kevin; Suter, John

AAPG Bulletin (1997), 81 (10), 1612-1639CODEN: AABUD2; ISSN:0149-1423. (American Association of Petroleum Geologists)

Significant vols. of terrigenous org. matter can be preserved to form coals only when and where the overall increase in accommodation approx. equals the prodn. rate of peat. Accommodation is a function of subsidence and base level. For mires, base level is very specifically the groundwater table. In paralic settings, the groundwater table is strongly controlled by sea level and the pptn./evapn. ratio. Peat accumulates over a range of rates, but always with a definite max. rate set by original org. productivity and space available below depositional base level (groundwater table). Below a threshold accommodation rate (nonzero), no continuous peats accumulate, due to falling or low groundwater table, sedimentary bypass, and extensive erosion by fluvial channels. This is typical of upper highstand, lowstand fan, and basal lowstand-wedge systems tracts. Higher accommodation rates provide relatively stable conditions with rising groundwater tables. Mires initiate and thrive, quickly filling local accommodation vertically and expanding laterally, favoring accumulation of laterally continuous coals in paralic zones within both middle lowstand and middle highstand systems tracts. If the accommodation increase balances or slightly exceeds org. productivity, mires accumulate peat vertically, yielding thicker, more isolated coals most likely during of late lowstand-early transgressive and late transgressive-early highstand periods. At very large accommodation increases, mires are stressed and eventually inundated by clastics or standing water (as in middle transgressive systems tracts). These relations should be valid for mires in all settings, including alluvial, lake plain, and paralic. The tie to sea level in paralic zones depends on local subsidence, sediment supply, and groundwater regimes. These concepts are also useful for investigating the distribution of seal and reservoir facies in nonmarine settings.

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Shao, L. Y.; Zhang, P. F.; Gayer, R. A.; Chen, J. L.; Dai, S. F. Coal in a carbonate sequence stratigraphy framework: the Upper Permian Heshan Formation in central Guangxi, southern China. J. Geol. Soc. 2003, 160, 285– 298,  DOI: 10.1144/0016-764901-108

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Coal in a carbonate sequence stratigraphic framework: the Upper Permian Heshan Formation in central Guangxi, southern China

Shao, Longyi; Zhang, Pengfei; Gayer, R. A.; Chen, Jialiang; Dai, Shifeng

Journal of the Geological Society (London, United Kingdom) (2003), 160 (2), 285-298CODEN: JGSLAS; ISSN:0016-7649. (Geological Society Publishing House)

Microfacies and sedimentol. analyses of the Upper Permian coal-bearing Heshan Formation in the Heshan coalfield of central Guangxi suggest that these coal measures formed in a marine carbonate platform setting. Low-volatile bituminous coals with very high org. sulfur overlie either paleosols or locally developed cherts. The petrog. and geochem. of the coals suggest formation in low-lying mires, in which marine influence increased upwards. In the Heshan and overlying Dalong Formations, four third-order sequences are described based on the recognition of four sequence boundaries. Higher-order sequences within each of these third-order sequences are also documented and, within Sequence III, coal seams are developed above higher-order transgressive surfaces, representing the deposits formed during the lag time between initial flooding of the platform and the onset of carbonate prodn. In the Heshan Formation, the coals with greatest thickness occur immediately above third-order transgressive surfaces. It is argued that, in some coal-bearing siliciclastic-free marine carbonate-platform settings, accommodation creation rates and peat accumulation rates are balanced, hence greater coal accumulation can be achieved at the transgressive surface rather than at the max. flooding surface.

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Hou, H. H.; Shao, L. Y.; Tang, Y.; Li, Y. N.; Liang, G. D.; Xin, Y. L.; Zhang, J. Q. Coal seam correlation in terrestrial basins by sequence stratigraphy and its implications for palaeoclimate and palaeoenvironment evolution. J. Earth Sci. 2020, 1– 24,  DOI: 10.15625/0866-7187/15571

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Hou, H. H.; Shao, L. Y.; Wang, S.; Xiao, Z. H.; Wang, X. T.; Li, Z.; Mu, G. Y. Influence of depositional environment on coalbed methane accumulation in the Carboniferous-Permian coal of the Qinshui Basin, northern China. Front. Earth Sci. 2019, 13, 535– 550,  DOI: 10.1007/s11707-018-0742-8

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Influence of depositional environment on coalbed methane accumulation in the Carboniferous-Permian coal of the Qinshui Basin, northern China

Hou, Haihai; Shao, Longyi; Wang, Shuai; Xiao, Zhenghui; Wang, Xuetian; Li, Zhen; Mu, Guangyuan

Frontiers of Earth Science (2019), 13 (3), 535-550CODEN: FESRBN; ISSN:2095-0209. (Springer GmbH)

Based on analyses of the lithofacies palaeogeog. of the Taiyuan and the Shanxi Formations in the Qinshui Basin, the spatial variations of the coal seam thickness, coal maceral compn., coal quality, and gas content, together with the lithofacies of the surrounding rocks in each palaeogeog. unit were investigated. The results show that the thick coals of the Taiyuan Formation are mainly distributed in delta and barrier island depositional units in the Yangquan area in the northern part of the basin and the Zhangzi area in the southeastern part of the basin. The thick coals of the Shanxi Formation are located within transitional areas between delta plain and delta front depositional units in the central southern part of the basin. The Taiyuan Formation generally includes mudstone in its lower part, thick, continuous coal seams and limestones in its middle part, and thin, discontinuous coal seams and limestone and sand-mud interbeds in its top part. The Shanxi Formation consists of thick, continuous sandstones in its lower part, thick coal seams in its middle part, and thin coal seams, sandstone, and thick mudstone in its upper part. From the perspective of coal-bearing sedimentol. and coalbed methane (CBM) geol., the lithol. and thickness of the surrounding rocks of coal seams play more significant roles in controlling gas content variation than other factors such as coal thickness, coal macerals, and coal quality. Furthermore, it is found that the key factors influencing the gas content variation are the thicknesses of mudstone and limestone overlying a coal seam. At similar burial depths, the gas content of the Taiyuan coal seams decreases gradually in the lower delta plain, barrier-lagoon, delta front, barrier-tidal flat, and carbonate platform depositional units. The CBM enrichment areas tend to be located in zones of poorly developed limestone and well-developed mudstone. In addn., the gas content of the Shanxi Formation is higher in the coals of the delta front facies compared to those in the lower delta plain. The CBM enrichment areas tend to be assocd. with the thicker mudstones. Therefore, based on the lithol. distribution and thickness of the rocks overlying the coal seam in each palaeogeog. unit of the Taiyuan and Shanxi Formations, the areas with higher gas content are located in the north-central basin for the Taiyuan coals and in the southern basin for the Shanxi coals. Both of these areas should be favorable for CBM exploration in the Qinshui Basin.

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Zhao, L.; Qin, Y.; Cai, C. F.; Xie, Y. W.; Wang, G.; Huang, B.; Xu, C. L. Control of coal facies to adsorption-desorption divergence of coals: A case from the Xiqu Drainage Area, Gujiao CBM Block, North China. Int. J. Coal Geol. 2017, 171, 169– 184,  DOI: 10.1016/j.coal.2017.01.006

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Control of coal facies to adsorption-desorption divergence of coals: A case from the Xiqu Drainage Area, Gujiao CBM Block, North China

Zhao, Long; Qin, Yong; Cai, Chunfang; Xie, Yiwei; Wang, Gang; Huang, Bo; Xu, Chenlu

International Journal of Coal Geology (2017), 171 (), 169-184CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)

Adsorption and desorption of coal to methane are substantially related to coal facies, referring to the prodn. potential of a coalbed methane (CBM) well. Based on analyses of the stratified channel samples from coal seams No. 2, No. 4 and No. 9 in the Xiqu Drainage Area, Gujiao CBM Block, North China, the relationship between coal facies and adsorption-desorption behaviors was first proposed and discussed. Using the coal facies diagrams suggested by previous investigators, four types of coal facies were distinguished in the coal seams, including the upper delta plain wet forest swamp (I), the lower delta plain fen (II), the lower delta plain marsh (III) and the back barrier low moor (IV). The upper delta plain wet forest swamp occurred dominantly, developing at the bottom and top of all the coal seams. Both the ash yield and groundwater index (GWI) tend to increase from coal facies I to IV, whereas the volatile compds., fixed carbon, org. sulfur, total sulfur, micropore vol. and sp. surface area, tissue preservation index (TPI) and vegetation index (VI) decreased, which indicates changes in the vegetation types, water supplements and disturbance, acid-base properties and redox conditions in the paleomires. Paleomire evolution accompanied the rise and drop of the sea level, with a transgression or regression process that could be represented by the coal facies sequence. Coal seams No. 2, No. 4 and No. 9 were discriminated into two, three and two evolving stages, resp., and these stages were characterized by complete cycles, except stage 4-I from the lower delta plain marsh. Mudstone partings in the coal seams deposited during the highest sea level. Collectively, paleomire environments, esp. the variation of vegetation types and mire water condition, result in adsorption and desorption divergences among different coal facies. Arborescent plants decrease from coal facies I to IV as the hydrodynamic intensity increases, leading to corresponding changes in botanical tissue preservation, medium oxidizability, mineral content and, ultimately, adsorption and desorption behaviors. In view of the gas content and desorption capacity of the coal layers in the same coal seam, the layers from the upper delta plain wet forest swamp may have the best potential for CBM drainage, followed by those from the lower delta plain fen and marsh, while those from the back barrier low moor may have the worst potential.

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Dai, S. F.; Bechtel, A.; Eble, C. F.; Flores, R. M.; Franch, D.; Graham, I. T.; Hood, M. M.; Hower, J. C.; Korasidis, V. A.; Moore, T. A.; Püttmann, W.; Wei, Q.; Zhao, L.; O’Keefe, J. M. K. Recognition of peat depositional environments in coal: A review. Int. J. Coal Geol. 2020, 219, 103383  DOI: 10.1016/j.coal.2019.103383

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Recognition of peat depositional environments in coal: A review

Dai, Shifeng; Bechtel, Achim; Eble, Cortland F.; Flores, Romeo M.; French, David; Graham, Ian T.; Hood, Madison M.; Hower, James C.; Korasidis, Vera A.; Moore, Tim A.; Puttmann, Wilhelm; Wei, Qiang; Zhao, Lei; O'Keefe, Jennifer M. K.

International Journal of Coal Geology (2020), 219 (), 103383CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)

A review. Peat depositional environments, the sites where and conditions under which peat accumulates, significantly influence a resultant coal's phys. properties, chem. compn., and coal utilization behavior. Recognition of peat depositional environments for coal is a challenging endeavor because coal's obsd. compositional properties not only result from a variety of geol. processes operating during peat accumulation, but also reflect the influence of adjoining or external depositional sedimentary environments and alteration during later diagenesis and/or epigenesis. The maceral or microlithotype compn. of any one layer of peat can be the product of years or decades of plant growth, death, decay, and post-burial infiltration by roots in addn. to the symbiotic, mutualistic, parasitic, and saprophytic relationships with non-plant biota, such as arthropods, fungi, and bacteria. The overprint of increasing thermal maturation and fluid migration through time on the resulting coal can make these relationships difficult to recognize. Therefore, published models based on maceral compn. alone must be used with great caution. Lipid compns., even from lipid-poor low-rank coals, can provide important information about depositional environments and paleoclimate, esp. if combined with the results of org. petrog. and paleontol. studies. Just as sulfur derived from seawater provides environmental clues, the ratios of two particularly relevant trace elements rather than a single trace element can be used to interpret peat depositional environments. Epigenetic minerals, as well as their corresponding chem. compns. should not be used for such a purpose; similarly, resistant terrigenous minerals deposited during peat accumulation in many cases should be used with considerable caution. The interactions of the biota present in the peat-forming ecosystem, often detd. using palynol. and geochem. proxies, and their interpretation in the context of geog. and paleoclimate are important means for deciphering peat depositional environments. Overall, a combination of evidence from geochem., mineralogy, palynol., and petrol. of coal and from stratigraphy, sedimentol., and sedimentary facies of related rocks is necessary for accurate and comprehensive detn. of depositional environments. The need for interdisciplinary studies is underscored by peat compositional properties, which have been greatly affected by various processes during the syngenetic, diagenetic or epigenetic stages of coal formation.

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The genesis of coal from the viewpoint of coal petrology

Teichmueller, Marlies

International Journal of Coal Geology (1989), 12 (1-4), 1-87CODEN: IJCGDE; ISSN:0166-5162.

A review with many refs.

69

Pocknall, D.; Thomas, M.; Melville, E.; Demchuk, T. Palynology and organic petrography of the Tyler Formation (lower Pennsylvanian), Williston Basin, North Dakota, USA. Palynology 2021, 45, 321– 335,  DOI: 10.1080/01916122.2020.1817168

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Xu, X. T.; Shao, L. Y.; Eriksson, K. A.; Zhou, J. M.; Wang, D. D.; Hou, H. H.; Hilton, J.; Wang, S.; Lu, J.; Jones, T. P. Widespread wildfires linked to early Albian Ocean Anoxic Event 1b: Evidence from the Fuxin lacustrine basin, NE China. Global Planet. Change 2022, 215, 103858,  DOI: 10.1016/j.gloplacha.2022.103858

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Shen, W. C.; Shao, L. Y.; Tian, W. G.; Chen, G.; Chen, F.; Hou, H. H.; Li, Z.; Sun, B.; Lu, J. Study on geological controls and enrichment models of coalbed methane in the Wuwei Basin in eastern North Qilian, northwestern China. Energy Explor. Exploit. 2019, 37, 429– 452,  DOI: 10.1177/0144598718803693

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Study on geological controls and enrichment models of coalbed methane in the Wuwei Basin in eastern North Qilian, northwestern China

Shen, Wenchao; Shao, Longyi; Tian, Wenguang; Chen, Gang; Chen, Fei; Hou, Haihai; Li, Zhen; Sun, Bin; Lu, Jing

Energy Exploration & Exploitation (2019), 37 (1), 429-452CODEN: EEEXDU; ISSN:2048-4054. (Sage Publications Ltd.)

The Wuwei Basin is located in the Gansu Corridor, which has abundant coalbed methane resources of 2.75 × 1011 m3. However, a low degree of coalbed methane exploration exists, and only a few wells have been drilled in local regions due to insufficient understanding of coalbed methane enrichment and its main controlling factors. This study analyzed the controlling factors of coalbed methane enrichment, including coal reservoir characteristics, hydrogeol. conditions, and the original sedimentary environment of the coal-bearing strata. The results showed that the main coal seams were developed in the Taiyuan Formation, and were mostly concd. in the Yingpan Sag in the south and the Ermahu Sag in the north of the study area. The macrolithotype of the coals in this basin was mainly semi-bright coal with a medium to high rank. Coal macerals were mainly vitrinite, ranging between 65.1% and 91.6% (averaged 81.70%), followed by liptinite, ranging between 1.9% and 29.5% (averaged 8.82%), and inertinite, ranging between 0.2% and 16.5% (averaged 3.66%). Mineral contents varied from 2.5% to 15.1% (averaged 6.16%). The macrolithotype and microlithotype of the Taiyuan Formation coals were favorable for coalbed methane formation. Through comparative anal. of moisture content, ash yield, gas content, and coal-forming sedimentary environments, it was found that the coal formed in the lagoon environment had a higher gas content and lower ash yield than that of the coal formed in the tidal flat environment. The high contents of total dissolved solids in aquifers around coal seams (1.75-16.70 g/L) reflected the closed hydrodynamic environment and were favorable for the preservation of coalbed methane in the Yingpan Sag. Considering various controlling factors (i.e., structure, sedimentation and hydrogeol.), three coalbed methane enrichment models were proposed. The model of coalbed methane enrichment in the synclinorium was the most favorable for the enrichment of coalbed methane in the Yingpan Sag.

73

Han, D. X.; Ren, D. Y.; Wang, Y. B.; Jin, K. L.; Mao, H. L.; Qin, Y. Coal petrology of China; China University of Mining and Technology Press: Xuzhou, 1996; 593 p.

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Hou, H. H.; Shao, L. Y.; Li, Y. H.; Li, Z.; Wang, S.; Zhang, W. L.; Wang, X. T. Influence of coal petrology on methane adsorption capacity of the Middle Jurassic coal in the Yuqia Coalfield, northern Qaidam Basin, China. J. Pet. Sci. Eng. 2017, 149, 218– 227,  DOI: 10.1016/j.petrol.2016.10.026

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Influence of coal petrology on methane adsorption capacity of the Middle Jurassic coal in the Yuqia Coalfield, northern Qaidam Basin, China

Hou, Haihai; Shao, Longyi; Li, Yonghong; Li, Zhen; Wang, Shuai; Zhang, Wenlong; Wang, Xuetian

Journal of Petroleum Science & Engineering (2017), 149 (), 218-227CODEN: JPSEE6; ISSN:0920-4105. (Elsevier B.V.)

The lower coalbed methane (CBM) adsorption capacity of the low rank coals is not only related to its lower maturity, but also detd. by the coal maceral compns. In this study, a total of 13 samples including 10 coals and 3 carbonaceous mudstones, were collected from the Middle Jurassic Dameigou Formation in the borehole YQ-1 of the Yuqia Coalfield, northern Qaidam Basin, NW China. Coal lithotypes, maceral compns., coal ranks, coal facies and methane adsorption characteristics of these samples were investigated using microscopic observation, proximate anal., porosity anal., and isothermal adsorption expts. The results show that the maceral compn. has a great influence on the methane adsorption capacity, and the influence of vitrinite on the methane adsorption is generally stronger than that of inertinite at a similar coal rank. For most samples, there is a pos. correlation between the vitrinite contents and the Langmuir vol. (VL), a neg. correlation between the inertinite contents and the VL, and no obvious correlation between the exinite contents and the VL. Furthermore, the vitrinite/inertinite (V/I) ratio also shows a pos. relationship with the VL. However, some samples contg. large amts. of unfilled fusinite and/or semifusinite have more pore spaces favoring methane adsorption and can also adsorb significant quantities of methane. Consequently, coals with higher vitrinite contents, in assocn. with some unfilled fusinites or semifusinites, should have the greatest adsorption capacity. Coal maceral compns. vary with the types of coal facies, and thus the methane adsorption capacity of coals may be closely related to coal facies. It is found that the methane adsorption capacities of the coals in a wet forest swamp (Type I) and an intergradation forest swamp (Type II) are greater than those in a drained forest swamp (Type III) and an open water peat swamp (Type IV). It is suggested that the area developed with the wet forest swamp and in the intergradation forest swamp with high porosity should be the best target areas for the CBM exploration in the Yuqia Coalfield.

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Ettinger, J.; Eremin, I.; Zimakov, B.; Yanovskaya, M. Natural factors influencing coal sorption properties: I. Petrography and the sorption properties of coals. Fuel 1966, 45, 267– 275

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Natural factors influencing coal sorption properties. I. Petrography and the sorption properties of coals

Ettinger, I. L.; Eremin, I. V.; Zimakov, B. M.; Yanovskaya, M.

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Sun, B.; Shao, L. Y.; Lu, X.; Gao, Y.; Zhao, Y. H.; Chen, G. Evaluation of CBM pool forming conditions, Panguan area, Guizhou. Nat. Gas Geosci. 2008, 19, 427– 432

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Shao, L. Y.; Xu, X. T.; Wang, S.; Wang, D. D.; Gao, D.; Wang, X. T.; Lu, J. Research progress of palaeogeography and palaeoenviornmental evolution of coal-bearing series in China. J. Palaeogeogr (Chinese edition). 2021, 23(01), 19– 38.

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Lu, J.; Shao, L. Y.; Yang, M. F.; Li, Y. H.; Zhang, Z. F.; Wang, S.; Yun, Q. C. Coal facies evolution, sequence stratigraphy and palaeoenviornment of swamp in terrestrial basin. J. China Coal Soc. 2014, 39, 2473– 2481

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Laxminarayana, Chikatamarla; Crosdale, Peter J.

International Journal of Coal Geology (1999), 40 (4), 309-325CODEN: IJCGDE; ISSN:0166-5162. (Elsevier Science B.V.)

The effect of coal compn., particularly the org. fraction, on gas sorption has been investigated for Bowen Basin and Sydney Basin, Australia. Maceral compn. influences on gas retention and release were investigated by using iso-rank pairs of hand-picked bright and dull coal in the rank range of high volatile bituminous (0.78% Ro max) to anthracite (3.01% Ro max). Adsorption isotherm results of dry coals indicated that Langmuir vol. (VL) for bright and dull coal types followed discrete, second-order polynomial trends with increasing rank. Bright coals had a min. VL at 1.72% Ro max and dull coals had a min. VL at 1.17% Ro max. At low rank, VL was greater in bright coal by about 10 cm3/g, but as rank increased, the bright and dull trends converged and crossed at 1.65% Ro max. At ranks higher than 1.65% Ro max, both bright and dull coals followed similar trends. These competing trends mean that the importance of maceral compn. on VL varies according to rank. In high volatile bituminous coals, increases in vitrinite content are assocd. with increases in adsorption capacity. At ranks higher than medium to low volatile bituminous, changes in maceral compn. may exert relatively little influence on adsorption capacity. The Langmuir pressure (PL) showed a strong relationship of decreasing PL with increasing rank, which was not related to coal type. It is suggested that the obsd. trend is related to a decrease in the heterogeneity of the pore surfaces, and subsequent increased coverage by the adsorbate, as coal rank increases. Desorption rate studies on crushed samples show that dull coals desorb more rapidly than bright coals and that desorption rate is also a function of rank. Coals of lower rank have higher effective diffusivities. Mineral matter has no influence on desorption rate of these finely crushed samples. The evolution of the coal pore structure with changing rank is implicated in diffusion rate differences.

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Zhang, J. Y.; Liu, D. M.; Cai, Y. D.; Pan, Z. J.; Yao, Y. B.; Wang, Y. J. Geological and hydrological controls on the accumulation of coalbed methane within the No. 3 coal seam of the southern Qinshui Basin. Int. J. Coal Geol. 2017, 182, 94– 111,  DOI: 10.1016/j.coal.2017.09.008

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Geological and hydrological controls on the accumulation of coalbed methane within the No. 3 coal seam of the southern Qinshui Basin

Zhang, Junyan; Liu, Dameng; Cai, Yidong; Pan, Zhejun; Yao, Yanbin; Wang, Yingjin

International Journal of Coal Geology (2017), 182 (), 94-111CODEN: IJCGDE; ISSN:0166-5162. (Elsevier B.V.)

Since 2006, the Zhengzhuang (ZZ) block has been part of a com. coalbed methane (CBM) prodn. area in the southern Qinshui Basin, which has become one of the most productive CBM areas in China. However, hydrogeol. study of the migration gas and groundwater of the ZZ block has not been systematically launched. This study focuses on geochem. anal., including quality anal. and stable isotope analyses for the water from the CBM prodn. wells, discusses the groundwater flow pathways and their influencing factors, and evaluates the geol. and hydrol. controls on gas accumulation in the study area. The results show that the produced waters produced with total dissolved solids (TDS) values of 152.8-5389.49 mg/L are typically fresh water and brackish water. The water type is primarily bicarbonate and the dissolved ions are effectively devoid of sulfate, calcium and magnesium. With the low desulfurization coeffs. of 0.24-5.63, the hydrochem. condition is conductive to CBM preservation. The predominant flow of groundwater originates from the southwestern source, which flows to the north basin center and then shifts to the east deep graben due to the drastic structural variation. Four hydrodynamic zones were distinguished through the differences in hydrodynamic conditions. In the southwest Zone I and the central Zone II, the gas contents are relatively low (13.17-24.53 m3/t) despite the high thermal maturity (Ro,max = 3.31%-3.97%). However, the stagnant Zone III in the north and the deep stagnant Zone IV in the southeast show comparatively high gas content (16.5-31.44 m3/t). The complex geostructures act synergistically with the intense hydrodynamic activities for gas accumulation. Three CBM enrichment modes were distinguished: hydrodynamic trapping at structure lows with high hydrostatic pressure; conventional trap of impermeable boundary; and hydraulic flushing and gas loss through the fault. These investigations may serve for CBM exploration and development in the study area.

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网址：Coalbed Methane Enrichment Characteristics and Exploration Target Selection in the Zhuozishan Coalfield of the Western Ordos Basin, China https://klqsh.com/news/view/264947

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