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Gas and water samples were collected from coalbed methane (CBM) wells, rivers, and springs in the southern Junggar Basin (SJB). These samples were analyzed for gas composition, stable isotopes, 16s ribosomal ribonucleic acid sequence, chemical compositions, and radioisotopes. The objective of this study was to understand CBM genesis in the Junggar Basin, the reason for abnormal CO2 accumulation, the development of microbial communities, the source of coalbed water, and the timing of methanogenesis. The CBM genesis is complex in the SJB, but it is closely related to microbial activities. The stagnant zone, which experiences limited groundwater recharge, may represent a relatively closed system where CO2 is easily trapped and the residual CO2 becomes progressively enriched in 13C. Only two families of methanogens (i.e., Methanobacteriaceae and Methanospirillaceae) are present in the coalbed waters, indicating that CO2 reduction is the main pathway for generating microbial gas. The coalbed water samples from the Houxia and Manasi–Hutubi regions plot around the local meteoric water line (LMWL), indicating recharge by modern meteoric water and rivers. However, the samples from the Miquan and Fukang regions plot below the LMWL, reflecting older snowmelt water recharge. Isotopic dating indicates that the age of coalbed water in the Miquan and Fukang regions is 43.5–2000 ka. Early coalification and later hydrological events collectively determined the regional variations in CBM genesis and gas composition in the SJB.

This paper analyzes regional hydrogeological conditions and divides the study area into three hydrogeological types and seven hydrogeological units, to investigate hydrogeology and its effect on coalbed methane (CBM) enrichment in the southern Junggar Basin, China. From this work, it is found that the groundwater flow paths in the study area are the joint effects of south-to-north and west-to-east flows. This study also shows that microbial gases are widely developed, although the depth limit of microbial gas occurrence is still unclear in the study area. Microbial CO2 reduction is the leading formation path in the study area, except for the Houxia region, where fermentation is the formation mechanism. The abnormally high CO2 in stagnant zones (i.e., water flow is slow and stagnant) is mainly associated with methanogenesis, whereas relatively low CO2 (microbial or thermogenic) is present where water flow is active. The average CBM content within the Xishanyao Formation changes within various hydrogeological units; moreover, the average CBM content within the Badaowan Formation of the same hydrogeological unit (e.g., Fukang) suggests that the hydrogeological and CBM enrichment conditions are different within various structural types. Overall, the hydrogeological conditions exert control on the gas content in the study area; that is, the gas content is high in stagnant zones. Finally, influenced by supplemental microbial gases, changes in the CBM oxidation zone are relatively complex in the study area, the depth of which has no obvious correlation with hydrogeological conditions and changes significantly from west to east.

Independent superposed coalbed methane (CBM)-bearing systems are widely distributed in coal-bearing strata with multiple coal seams in western Guizhou province, which is liable to cause interference among systems in the multilayer combination production process. Therefore, precisely defining the independent superposed CBM-bearing systems is important for CBM development in this area. However, the recognition of gas-bearing system at present was mostly based on the sedimentary cycle and the sequence stratigraphic framework, of which ignored the control of in-situ stress on physical properties of coal-bearing strata. In this work, the distribution characteristics of in-situ stress and its control on coal permeability was analyzed systematically based on well test parameters of 22-layer coal seams measured within depths from 352 to 1245 m in the Panguan area. Within depths shallower than 500 m, σH and σh tend to decrease with an increasing depth, and the stress field gradually transforms from σH>σh>σv to σv>σH>σh. For immediately coal seams (500–750 m), the σv>σH>σh type is dominant, revealing a normal faulting stress type. For coal seams with depth >750 m, σH and σh increases rapidly instead of continuously decreasing, and the stress field transformed into the type of σH> σv> σh when being deeper than 1000 m. Coal permeability shows a low→ high → low→ extremely low trend with the increase of depth, and it was verified with well test permeability measured in other areas of western Guizhou, the essence of which is the compression and deformation of the pore-fracture system under the control of in-situ stress. The correlation between pore pressure and depth was also illustrated, which showed five linear relationships in depth of 1000 m, separately. Those five depth ranges represent five independent pressure systems, and the depth boundary point between different pressure systems is precisely the conversion depth of stress fields and coal permeability. Within the same pressure system, the gas content increases as the buried depth increases. For CBM development in western Guizhou province, commingling production need carry out in the same pressure system to avoid interlayer interference, and it is recommended that middle CBM systems (500–750 m) with lower stress and higher permeability should be a priority for CBM development. At depths >750 m, CBM development conditions tend to deteriorate.

Based on 232 sets of well-test results measured at depths from 135.9 to 1407.19 m in 15 coalbed methane (CBM) development blocks in China, correlations between parameters including reservoir pressure (P o ), shut-in pressure (P c ), breakdown pressure (P f ), in-situ stress, coal permeability, and depth ( D ) were determined. The distribution of in-situ stress in the eastern margin of the Ordos Basin (EMOB), southern Qinshui Basin (SQB), and western Guizhou province (WGP) were analyzed systematically, and its control on the coal reservoir permeability was also addressed. The minimum and maximum envelope curves of the stress ratio (including k (( σ H + σ h )/2 σ v )), K H ( σ H / σ v ) and K h ( σ h /σ v )) variability with depth was obtained using the Brown and Hoek (1978) method, and the average envelope curves were established by mathematical derivation on the condition that horizontal principal stresses increase linearly with depth. The results show that the maximum principal stress ( σ H ), minimum principal stress (σ h ) and vertical stress (σ v ) vary with depth and location. Generally, there are two main stress regimes in major coalfields in China: σ H > σ h > σ v (59.48%) and σ v > σ H > σ h (34.52%). The σ H > σ h > σ v stress regime has the smallest proportion (6%), which appears mainly in relatively shallower coal seams ( k , K H and K h , were plotted between two envelopes, which can be used to estimate the range of σ H and σ h at a certain depth. With increasing depths, three ratios decrease toward a fixed value, e.g., K H → 0.86, k → 0.74, and K h → 0.6. For coal seams with a depth > 1000 m, k is generally σ v gradually becomes the dominant stress. Coal reservoir permeability decreases exponentially with the increase in effective in-situ stress (EIS, ( σ H + σ h + σ v )/3 − P o ). Vertically, four distinct vertical bands of coal permeability can be found in the EMOB, SQB and WGP, defined by the compression and deformation of the coal-matrix under certain stress regimes: in shallow coal seams (EMOB, σ H > σ h > σ v ) where σ H and σ h tend to increase, and coal permeability decreases exponentially with an increasing depth; in intermediate coal seams (EMOB, 800–1000 m; SQB, 650–825 m; WGP, 500–750 m), the σ v > σ H > σ h stress regime plays a major role, with relatively high coal permeability. For deep coal seams (EMOB, > 950 m; SQB, > 825 m; WGP, > 750 m), CBM development conditions deteriorate due to a state of “extremely low permeability and extremely high in-situ stress”.

In this study, a series of laboratory experiments were performed to firstly analyze the characteristics of the pore structure in low-rank coal (R o 1000 nm), with a poor development of mesopores (100–1000 nm). The fractal features of adsorbed pores and seepage pores were defined and calculated using fractal dimensions D 2 (2.563–2.926) and D 4 (2.683–3.263), respectively. D 2 has a positive correlation with moisture content, while D 4 has a positive correlation with R o and fixed carbon and a negative correlation with the volatile content. Both D 2 and D 4 have a weaker association with the ash yield and no apparent relationship with coal macerals, indicating that coal-forming materials and environments have little influence on the pore structure fractal characteristics. D 2 has a strongly correlation with the BET surface area (R 2 = 0.9026) and the average pore diameter (R 2 = 0.9841), again proving that D 2 can be used to characterize the pore structure fractal dimension of adsorbed pores. D 2 shows a negative correlation with the Langmuir volume (V L ), but no apparent relationship with the Langmuir pressure (P L ), indicating that the pore structure of absorbed pores has little effect on methane desorption during the process of CBM exploitation. D 4 has weak positive correlation with permeability. Overall, fractal analysis is beneficial for better understanding the pore structure, methane adsorbability and seepage ability of low-rank coal.

Junggar basin is one of the most typical low-rank coal-bearing basins in china. The Xishanyao coal developed in the southern Junggar basin contains plenty of coalbed methane (CBM) resources, with suitable burial depth and great coal thickness. To date, the researches about CBM geological characteristics, controlling factors and exploration potential are still lacking in the western of Urumchi River, which severely obstacle for further development of low-rank CBM. Firstly, CBM reservoir-forming conditions of Xishanyao coal in the middle of the southern Junggar basin are investigated in this paper. The results show that total coal thickness varies greatly from 18.58 to 66.34 m with some thick seams (>5 m). Coal rank ranges from 0.50 to 0.88%, with an average value of 0.65%. Coal macerals are dominated by vitrinite (45.6–66.4%, avg. 55.6), followed by inertinite (29.7–55.7%, avg. 42.8) and liptinite (0.5–4.7%, avg. 1.7). Coal reservoir property is relatively poor, with permeability ranging from 0.002 to 1.220mD (avg. 0.193) and porosity changing from 0.56 to 9.73% (avg. 5.99). Controlled by coal rank and coal macerals, pore type is dominated by micropore, followed by mesopore-macropore and little microfracture. Micro-fractures are mostly developed in bright coal and semi-bright coal. Methane adsorption isothermal measurement reveals that the maximum adsorption capacity (VL) varies from 6.73 to 14.14 m3/t. The in-place gas content ranges from 0.29 to 7.72 m3/t (avg. 5.1) and increases with the increasing of burial depth. Secondly, geological controls about CBM accumulation are analyzed in this paper, i.e., structure, sedimentation and hydrogeology play “the most important”, “the most basic” and “a supporting” role in the process of CBM accumulation, respectively. Finally, analytical hierarchy (AHP) fuzzy prediction method and optimal segmentation method are used to carve out the grade of exploration potential, and the best prospective target areas for low-rank CBM production are forecasted.

The coalbed methane (CBM) resources are abundant in low-rank coal (Ro < 0.70%), which has raised widespread concerns within domestic geologists. In brief, low-rank CBM resources are still at an early stage of exploration in China, and the study on adsorption mechanisms and control factors of low-rank CBM is not systematic. As an important geological event of dividing brown coal and bituminous coal, “first coalification jump” has an obvious control action to the coal composition and pore structure of coal reservoirs and the occurrence and migration modes of methane gas, and the targeted research remains lacking. In this paper, the control function of coal rank to reservoir properties was analyzed first and the role of “first coalification jump” was emphasized in the discussion. In addition, in combination with methane adsorption isotherm experiments under equilibrium moisture (30 °C), internal factors (e.g., coal rank, specific surface, macerals, volatiles, and moisture) that control methane adsorbability ...

This paper focusses on sequence stratigraphy of coal-bearing strata, coal petrography, coal chemistry, and the CH4 adsorption capacity of the Badaowan Formation in Santanghu Basin, China. The comprehensive coal characteristics of 12 coal samples were determined, coupled with maceral composition, proximate analysis, elemental analysis, reflectance (Ro), and high-pressure CH4 adsorption. The results indicated that maceral composition and chemical properties of coal were controlled by the sequence stratigraphic framework. During the TST, the content of vitrinite, the ratio of vitrinite to inertinite (V/I) and the content of carbon in coal increased gradually from bottom to top. While during HST, an opposite trend was observed. Furthermore, due to the intrusion of the lake water, the ash yield in the TST is higher than that in HST. The CH4 adsorption capacity is positively correlated with the content of vitrinite and V/I. There is a moderate positive correlation between fixed carbon contents and VLad. Ash content was strongly correlated to Langmuir volume under the air-dry basis, with increasing Ad associated with the reduction of the VLad. It was observed that the CH4 adsorption capacity generally gradually increased during the TST, and gradually decreased in the HST. Gas content also shows a similar change rule. CH4 adsorption capacity is functionally dependent on coal characteristics, which again are dependent on sequence stratigraphy and thus the coal-forming environment. The base level and cycles of the sequence establish a theoretical and practical framework for CBM extraction from low-rank coal reservoirs.

Large scale hydraulic fracturing is the primary industrial technique for obtaining shale gas. Previous studies indicated that research on the fracture network forming principles, geological controls, and fracability of the shale reservoir should be performed before hydraulic fracturing. This paper reports a detailed study of the controlling factors of continental shale reservoirs from the Triassic Yanchang Formation, Ordos Basin, China. First, the fracture network formation mechanisms of hydraulic fracturing are discussed systematically with related schematic diagrams. Second, mineralogical factors, lithomechanics factors, in-situ stress and natural fractures are discussed as the controlling factors of the formation of a complex fracture network according to previous research and field fracturing experience; further analysis is based on these factors. Third, the Young's modulus, Poisson ratio (PR), brittle mineral content, quartz content, clay content, differential horizontal stress, fracture development condition and other factors are normalized and given corresponding weights based on a judgment matrix. Then, a quantitative method (fracability index) is used to characterize the fracability of continental shale based on analytic hierarchy process (AHP) mathematical modeling. Finally, under the control of the same evaluation parameters and weight values, different evaluation indices of continental shale in the Ordos Basin and marine shale in the Sichuan Basin are calculated, and with the help of two indices, the fracability of continental shale is intuitively analyzed. The results show that the continental shale in the Yanchang Formation in the Ordos Basin has good fracability and is suitable for hydraulic fracturing to form a complex fracture network.