Coalbed Methane Accumulation Research Articles

Applying 3D geological modeling to predict favorable areas for coalbed methane accumulation: a case study in the Qinshui Basin

Applying 3D geological modeling to predict favorable areas for coalbed methane accumulation: a case study in the Qinshui Basin

Porosity Characteristics of Coal Seams and the Control Mechanisms of Coal Petrology in the Xishanyao Formation in the Western Part of the Southern Junggar Basin

The porosity characteristics of coal seams serve as a pivotal factor in assessing the development potential of coalbed methane (CBM) resources, significantly influencing the adsorption and permeability capabilities of coal reservoirs, as well as the accumulation, entrapment, and preservation of CBM. In this study, we focused on the coal seams of the Xishanyao Formation in the western part of the southern Junggar Basin (NW China). By leveraging the complementarity of nuclear magnetic resonance (NMR), low-temperature liquid nitrogen experiments, and high-pressure mercury intrusion porosimetry (MIP) in spatial exploration range and precision, we conducted a comprehensive analysis to achieve a fine description of porosity characteristics. Furthermore, we explored the coal petrology factors controlling the pore characteristics of the Xishanyao Formation, aiming to provide geological evidence for the selection of favorable areas and the development potential evaluation of CBM in the study area. The results indicate the following: (1) The total pore volume of the coal samples is 6.318 × 10−3 cm3/g on average, and the micropore volume accounts for a relatively high proportion (averaging 44.17%), followed by the fine pores (averaging 39.41%). The average porosity is approximately 3.87%, indicating good gas storage and connectivity of the coal seams, albeit with some heterogeneity. The coal reservoir is dominated by micropores and fine pores with diameters less than 100 nm, and the pore structure is characterized by low pore volume and high pore area. (2) The pore structure is influenced by both the coalification degree and the coal maceral. Within the range of low coalification, porosity increases with the increase in coalification degree. Building upon this, an increase in the vitrinite content promotes the development of micropores and fine pores, while an increase in the inertinite content promotes the development of meso–macropores. The clay mineral content exhibits a negative correlation with the adsorption pore volume ratio and a positive correlation with the seepage pore volume ratio.

Genesis and Accumulation Mechanism of Low-Rank CBM: A Case Study in the Jiergalangtu Block of Erlian Basin, Northern China

Genesis and Accumulation Mechanism of Low-Rank CBM: A Case Study in the Jiergalangtu Block of Erlian Basin, Northern China

Main controlling factor of coalbed methane enrichment area in southern Qinshui Basin, China

Despite the significant progress made in coalbed methane (CBM) exploration and development in recent years, understanding of CBM enrichment mechanisms remains limited. This study aims to elucidate the CBM enrichment mechanism in the southern Qinshui Basin, China, by analyzing characteristics of global CBM basins and building a geological model of the study area. Field analyses are conducted to predict sweet spots of high CBM abundance and production potential. The findings reveal a high-yield model of CBM accumulation at relatively elevated structural positions within enriched areas. Compared to other global basins, low permeability poses the primary challenge for CBM development in China. Coal seam thickness shows minimal variation in southern Qinshui Basin, exerting negligible impact on CBM productivity. The shallow burial depth of coal seams in this region results in low stress, conferring high permeability conducive to high CBM yields. In situ stress conditions exert a primary control on the development of microfracture systems, which in turn govern reservoir permeability. This work provides new insights into CBM enrichment patterns in the southern Qinshui Basin. The proposed high-yield model enables better understanding of favorable conditions for CBM accumulation. Overall, this study represents a valuable contribution toward unlocking China’s CBM potential through improved geological characterization.

Characteristics of Deep Coal Reservoir and Key Control Factors of Coalbed Methane Accumulation in Linxing Area

Deep coalbed methane (CBM, commonly accepted as >1500 m) has enormous exploration and development potential, whereas the commercial development of deep CBM exploration areas wordwide has been quite limited. The Linxing area, with coals buried approximately 2000 m deep, shows great development potential. Based on a basic geological analysis of structural and hydrodynamic conditions, combining field tests of reservoir temperature and pressure and indoor measurements of maceral composition, proximate analysis, thermal maturity, porosity and permeability, the factors controlling deep CBM accumulations were discussed. The results show that the present burial depth of the No. 8 + 9 coal seam, mainly between 1698 and 2158 m, exhibits a high reservoir temperature (45.0–64.0 °C) and pressure (15.6–18.8 MPa), except for the uplift area caused by the Zijinshan magma event (with coal depth approximately 1000 m). The maximum vitrinite reflectance (Ro,max) of the coal varies from 1.06% to 1.47%, while the magma-influenced areas reach 3.58% with a relatively high ash content of 31.3% (air-dry basis). The gas content calculated by field desorption tests shows a wide range from 7.18 to 21.64 m3/t. The key factors controlling methane accumulation are concluded from regional geological condition variations. The north area is mainly controlled by structural conditions and the high gas content area located in the syncline zones. The center area is dominated by the Zijinshan magma, with relatively high thermal maturity and a high gas content of as much as 14.5 m3/t. The south area is developed with gentle structural variations, and the gas content is mainly influenced by the regional faults. Furthermore, the groundwater activity in the eastern section is stronger than that in the west, and the hydrodynamic stagnant areas in the western are more beneficial for gas accumulation. The coals vary from 3.35% to 6.50% in porosity and 0.08 to 5.70 mD in permeability; thus, hydrofracturing considering high temperature and pressure should be applied carefully in future reservoir engineering, and the co-production of gas from adjacent tight sandstones also should be evaluated.

Genesis of Calcite Veins in 8# Coal Seam of the Upper Carboniferous Benxi Formation, Southeastern Margin of Ordos Basin

The 8# coal seam in the Benxi Formation of the southeastern margin of the Ordos Basin is a deep coal seam with abundant coalbed methane resources. Calcite veins are commonly developed within the 8# coal seam, and their formation processes and mechanisms have significant implications for the enrichment of deep coalbed methane. Genesis of the calcite veins was analyzed to reveal the impact of the calcite veins formation on coalbed methane accumulation, with an integrated application of petrographic study by thin section, cathodoluminescence analysis, carbon-oxygen isotope analysis, and homogeneous temperature measurements of fluid inclusions. The research findings indicate that the calcite veins in the 8# coal seam can be classified into three stages: C1, C2, and C3. The diagenetic fluids of C1 primarily originated from contemporaneous seawater. The fluids responsible for the formation of C2 primarily consist of organic fluids enriched in biogenic gas, whereas the fluids contributing to the formation of C3 are primarily associated with liquid hydrocarbons originated form decarboxylation of organic matter. Furthermore, the development of both C2 and C3 is influenced by deep hydrothermal fluids resulting from tectonic heating events during the Early Cretaceous. By combining analysis of the hydrocarbon accumulation history and burial history in the study area, it has been established that C2 formation occurred during the Late Triassic to Early Jurassic, while C3 formation took place during the Late Jurassic to Early Cretaceous. The exploration and production practices in the study area have firmly established the crucial significance of the formation and evolution of calcite veins within the 8# coal seam for the migration and accumulation of coalbed methane. The research outcomes provide valuable insights for the exploration of deep coalbed methane enrichment areas.

South Anze Structure and Its Control on Coalbed Methane Aggregation in the Qinshui Basin and the Mechanism of Syncline Gas Enrichment in the Qinshui Basin

The phenomenon of coalbed-methane synclinal accumulation in the Qinshui Basin has been widely reported, but it has mainly been observed in the core block of the Qinshui Syncline. The questions arise: does this phenomenon exist in the wing of the Qinshui Syncline and, if so, what is the mechanism behind it? Further study is required to answer these questions. This paper focuses on the South Anze No. 3 coal seam in the Qinshui Basin as an example. It conducts a systematic sorting of coalbed-methane geological characteristics and an analysis of the effects of structural assemblage characteristics, genetic mechanisms, and structural control on coalbed-methane accumulation. Additionally, it examines the basin structure and evolution during the critical period of the Qinshui Basin, as well as the gas geological characteristics of adjacent areas, in order to discuss the gas-rich mechanism of the syncline in the Qinshui Basin. Key insights obtained from the study include the following: (i) The whole South Anze is a nosing structure that plunges from west to east and superposes secondary folds and faults in different directions. Four deformation zones can be identified based on the characteristics of structural assemblage, including NEN-oriented compressive structures, ENE-trend shear fractures, EW-trend compressive fractures, and EW-trend compressive folds. The formation of structural assemblage in the study area is attributed to the compression in the Indosinian and Yanshanian, and the fault inversion in the Himalayan period. (ii) The ENE-trend shear fracture deformation area located in the nosing uplift is a low CBM (coalbed methane) content area due to gas diffusion during the Himalayan extension. The syncline in the combination of NEN-trend and EW-trend “ejective folds” in the west and south of the study area is a high-value area of coalbed-methane content. It is further verified that the law of syncline gas accumulation in the Qinshui Basin is also applicable to the wing of the Qinshui Syncline. (iii) Since the formation of the Qinshui Syncline, the main coal seam has been in an extensional environment below the neutral plane, resulting in the main dissipation of coalbed methane. During its geological history, surface water penetrated the aquifer above the main coal seam through two channels: the extensional area above the neutral plane of the adjacent anticline and the shear fracture. A hydrostatic pressure seal is formed in the Qinshui Syncline and the secondary syncline is superimposed upon it, which is the cause of gas enrichment in the syncline of the Qinshui Basin. (iv) Weak deformation in the syncline basin is the focus of global coalbed-methane exploration and development. The mechanism proposed in this paper can provide ideas and references for further understanding of coalbed-methane enrichment in this type of basin.

Critical tectonic events and their geological controls on deep buried coalbed methane accumulation in Daning-Jixian Block, eastern Ordos Basin

Critical tectonic events and their geological controls on deep buried coalbed methane accumulation in Daning-Jixian Block, eastern Ordos Basin

Coalbed methane accumulation, in-situ stress, and permeability of coal reservoirs in a complex structural region (Fukang area) of the southern Junggar Basin, China

The enrichment of coalbed methane (CBM), in-situ stress field, and permeability are three key factors that are decisive to effective CBM exploration. The southern Junggar Basin is the third large CBM basin in China but is also known for the occurrence of complex geological structures. In this study, we take the Fukang area of the southern Junggar Basin as an example, coalbed methane accumulation and permeability, and their geological controls were analyzed based on the determination of geological structures, in-situ stress, gas content, permeability, hydrology and coal properties. The results indicate that gas contents of the Fukang coal reservoirs are controlled by structural framework and burial depth, and high-to-ultra-high thickness of coals has a slightly positive effect on gas contents. Perennial water flow (e.g., the Baiyanghe River) favors gas accumulation by forming a hydraulic stagnant zone in deep reservoirs, but can also draw down gas contents by persistent transportation of dissolved gases to ground surfaces. Widely developed burnt rocks and sufficient groundwater recharge make microbial gases an important gas source in addition to thermogenic gases. The in-situ stress field of the Fukang area (700–1,500 m) is dominated by a normal stress regime, characterized by vertical stress > maximum horizontal stress > minor horizontal stress. Stress ratios, including lateral stress coefficient, natural stress ratios, and horizontal principal stress ratio are all included in the stress envelopes of China. Permeability in the Fukang area is prominently partitioned into two distinct groups, one group of low permeability (0.001–0.350 mD) and the other group of high permeability (0.988–16.640 mD). The low group of permeability is significantly formulated by depth-dependent stress variations, and the high group of permeability is controlled by the relatively high structural curvatures in the core parts of synclines and the distance to the syncline core. Meanwhile, coal deformation and varying dip angles intensify the heterogeneity and anisotropy of permeability in the Fukang area. These findings will promote the CBM recovery process in China and improve our understanding of the interaction between geological conditions and reservoir parameters and in complex structural regions.

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

The Zhuozishan coalfield at the western margin of the Ordos Basin is one of the main coal-mining areas in China, and recent explorations have revealed the great potential for coalbed methane (CBM) resources in its Carboniferous and Permian strata. In this paper, the controlling factors of CBM enrichment of the major coals are studied in this coalfield and the CBM resources are estimated based on the analysis of the coal petrology and compilation of literature data on the gas content. The result of the coal petrology analysis of 10 samples shows that 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%). The R o,max value of No. 16 coal (1.18-1.35%) is higher than that of No. 9 coal (1.04-1.13%), and both coals are of medium rank. Due to greater thickness, deeper burial depth, and better coal petrology characteristics, the No. 16 coal seam of the Taiyuan Formation is selected as the major coal seam for CBM resource estimation, which has a thickness of 1-6 m and a present-day burial depth of 200-1100 m. The gas content of this coal seam varies mostly between 4 and 10 m3/t. Positive correlation between the coal seam thickness as well as present-day burial depth and the gas content suggests that the thick and deeply buried coal seams are favorable for CBM preservation. The ash yield shows an insignificant negative correlation with the gas content, indicating that ash yield is not an important factor for CBM enrichment. The syncline hinges located below the thrust zones show higher gas content due to greater burial depths. In contrast, the anticline hinges at shallower depths tend to have lower gas contents. Based on the combined information about sedimentary environments, structural patterns, and hydrogeology, two CBM accumulation models are put forward in the study area that include syncline-hydraulic plugging below thrust nappe and fault-confined aquifer plugging. The volumetric method is used to estimate the CBM resources, and results indicate that the CBM resource in the whole coalfield is 428.78 × 108 m3, and the total resource abundance is 0.74 × 108 m3/km2. Two favorable areas for the CBM exploration are optimized based on the resource amount and resource abundance. One of the favorable areas is the Kabuqi area in the northern part of the coalfield, and another is the Baiyunwusu area in the central part of the coalfield. These two areas will be the CBM priority exploration areas at the western margin of the Ordos Basin.

A new insight into coalbed methane occurrence and accumulation in the Qinshui Basin, China

Coalbed methane (CBM) recovery from coal seams can benefit low-carbon energy utilization and reduce greenhouse gas emissions. The Qinshui Basin, which evolved from the North China Craton Basin in the Late Paleozoic, provides abundant coal-bearing rocks and relatively stable geological conditions for CBM occurrence. To better understand CBM accumulation characteristics, various aspects of geological structure, magmatic activity, hydrodynamics, coal seam thickness, burial depth, and sedimentary characteristics of coal measures within the basin were comprehensively investigated, along with the CBM reservoir aspects of coal components, mechanical properties, pore-fracture characteristics, and gas contents. The results show that the coal-forming environment in the northern Qinshui Basin is dominated by river flood and delta plains, while the central Qinshui Basin mainly consists of delta front, inter-delta bay, and lacustrine facies. The southern part is primarily lacustrine facies with scarce delta front subfacies. In terms of ash yield, that of coal seam 3# in the center of the study area is significantly higher than that in the north and south because the center is closer to the provenance and carries a large amount of detritus during the flood season. The primary coal structure is well preserved, and the facies cleats are largely filled with dolomite and kaolinite, suggesting mineralized fluid activity after the development of the major fracture. The gas content is mainly controlled by geological structure and hydrogeological conditions. Interestingly, for the coal seams > 1000 m away from the fault, the nature of the fault has no significant effect on the gas content. This study provides insights into the efficient exploration and exploitation of CBM in Qinshui Basin.

Structural compartmentalization and its relationships with gas accumulation and gas production in the Zhengzhuang Field, southern Qinshui Basin

The Zhengzhuang Field is a new developing coalbed methane (CBM) field in the southern Qinshui Basin. To disclose the high interwell heterogeneity in production, we provide a detailed investigation of local structural controls on the gas accumulation in the Zhengzhuang Field. The Zhengzhuang Field is a northwest-dipping homocline that contains abundant normal faults, and several secondary folds, karst collapse columns, and thrust faults. These structures divided the study area into three regions (fault-trough, fault-fold, and sub-sag) and 11 sub-regions. The gas content of the No. 3 coal seam is 0 to 35 m3/t, and the gas content exhibits a distinctly positive correlation with the methane δ13C isotopic ratio. Both gas content and methane δ13C isotopic composition are compartmentalized by structural regions or sub-regions, indicating significant effects from structural effects. The general gas-content distribution was separated by low-permeability boundaries created by F1 and F2 faults, resulting in several gas reservoir compartmentalizations. Locally, secondary faulting and small-scale folding resulted in redistributions of gas contents, i.e., relatively high gas content on the downthrow side of secondary faults and in the structural-trough zones of small-scale folds. As a continuous-type gas reservoir, the CBM accumulation mechanism is complex and differs from conventional gas reservoirs. Because coal has an extremely high adsorption capacity, the gravity separation of free gas and water is notably subordinate to sorption on micropore surfaces in CBM reservoirs. It is known that the No. 3 coal seam has extremely low permeability and low–medium gas saturation; thus, the gas isotopic fraction caused by hydrodynamic flushing probably enhanced the gas desorption and resulted in the migration of δ13C-enriched gas within the coal seam and into adjacent locations with high reservoir pressures for gas re-adsorption. This mechanism explains the relatively high gas content and enriched methane isotopic composition in the local structural lows in the Zhengzhuang Field. Structural controls on gas production indicate that the most productive CBM wells are commonly completed in the structural highs of the fault-nose-structure (i.e., the sweet spots), where there is high reservoir permeability resulting from stress relief, and also relatively high gas content induced by the secondary migration of gas from the structural lows. The source driving force for gas migration within the coal seam is the result of gas diffusion fractionation which commonly occurs simultaneously with the slow seepage of groundwater from low to high spots in the Zhengzhuang Field.

Environmental conditions and mechanisms restricting microbial methanogenesis in the Miquan region of the southern Junggar Basin, NW China

AbstractEarly microbial gas has been sealed in reservoirs in the Miquan region of NW China, with little or no supply of current microbial gas. To date, the environmental conditions and mechanisms restricting microbial methanogenesis are still unclear in the Miquan region. Thus, in this study, a series of gas and water samples from coalbed methane (CBM) exploitation wells and in situ coal samples were collected and analyzed to determine the potential for current microbial gas generation, the methanogenic pathways, the source of nutrients, the influence of the environmental conditions on in situ microbial communities and their methanogenesis, and the mechanisms restricting microbial methanogenesis. The gas-production simulation experiments revealed that the existing microbes in the coalbed water were less efficient at converting coal into methane under the approximate in situ conditions, which further verified that there was little or no supply of current microbial gas. The stable isotope compositions of the gas samples suggested that carbon dioxide (CO2) reduction was the dominant metabolic pathway for generating CBM, whereas the methanogenic communities contained a mixture of acetoclastic and methylotrophic methanogens in local areas. The nutrients available for the microbes mainly included the total dissolved nitrogen (TDN) and total dissolved carbon (TDOC), and the in situ dissolution of the coals was a significant source of the TDN, whereas the TDOC was mainly supplied by surface water. The microbes in the different tectonic settings were significantly controlled by different combinations of environmental factors, and there was no single environmental factor that completely dominated the spatial variability of the microbial communities. The gradual stagnation of the water environment led to an increase in salinity and a decrease in nutrients, which were likely the main factors restricting microbial methanogenesis under in situ conditions. Combined with the results of the rate-limiting stages of the anaerobic fermentation, the mechanisms restricting microbial methanogenesis can be finally determined in the Miquan region. These discoveries presented in this case study provide a significant supplement to the geological theory of CBM accumulation, and have a guiding significance for CBM development in the Miquan region.

Biogenic Methane Accumulation and Production in the Jurassic Low-Rank Coal, Southwestern Ordos Basin

Geological conditions are the key for coalbed methane (CBM) accumulation and production. However, the geological feature of CBM accumulation and production in the Jurassic of Ordos Basin lacks systematic and detailed evaluation, resulting in poor CBM production in this area. This study has determined the genetic types of gas according to geochemistry characteristics of the gas, the geological factors to control CBM accumulation and production performance were revealed, and a comprehensive method was established to evaluate favorable areas based on 32 sets of CBM well production data from Jurassic Yan’an Formation. The results show the coal macerals are rich in inertinite (41.13~91.12%), and the maximum reflectance of vitrinite (Ro,max) in coal is 0.56~0.65%. According to gas compositions and carbon isotopes analysis, the δ13C(CH4) is less than −55‰, and the content of heavy hydrocarbon is less than 0.05%. The value of C1/(C2 + C3) is 6800~98,000, that is, the CBM is a typical biogenic gas of low-rank coal. The CBM accumulation model is the secondary biogenic on the gentle slope of the basin margin, in which gas content is closely related to buried depth and hydrodynamic environment, i.e., the high gas content areas are mainly located in the groundwater weak runoff zone at the burial depth of 450 m~650 m, especially in the syncline. Meanwhile, gas production mainly depends on the location of the structure. The high gas production areas of vertical wells were distributed on the gentle slope with high gas content between anticline and syncline, and the horizontal wells with good performance were located near the core of the syncline. According to the above analysis combined with the random forest model, the study area was divided into different production favorable areas, which will provide a scientific basis for the CBM production wells.

Coalbed methane geology and exploration potential in large, thick, low-rank seams in the Bayanhua Sag of the Erlian Basin, northern China

The Erlian Basin may be one of the most promising areas for low-rank coalbed methane (CBM) in China. The Cretaceous coal-bearing strata in the Bayanhua sag contain abundant low-rank CBM resources with suitable depths and large coal thicknesses (1.00–78.85 m, avg.22.53 m). However, the research on CBM geology and exploration potential is still lacking, which severely hinders its exploration and development. This paper presents an integrated study of CBM geology in terms of gas reservoir properties, gas-bearing characteristics and controlling factors. The results of the analysis show that lignite and subbituminous coals are the main coal types present in the study area, with medium porosity (7.3–27.8%, avg.20.77%) and low permeability (0.05–21.8 mD, avg.5.54 mD) being dominant. The pores are dominantly transition pores with inkpot shapes, which is beneficial to the adsorption of CBM. The gas content of the coal seams is 1.66–4.45 m3/t and most coal reservoirs are oversaturated. The CBM is primarily of secondary microbial origin, mostly carbon dioxide reduction. The accumulation of CBM is mainly influenced by the structure type, roof lithology and hydrogeology in the study area. According to the integrated analysis of the vertical changes in the physical properties and gas-bearing characteristics, the dominant horizon tends to be the No. 6-7 coal seam, followed by the No. 6-4 coal seam. Based upon the CBM geology and the factors controlling enrichment, the prospecting potential in the Bayanhua sag was appraised, and two favorable areas are proposed for CBM exploitation.

Coalbed methane gas content and its geological controls: Research based on seismic-geological integrated method

Gas content is a crucial indicator in the process of coalbed methane (CBM) exploration and development. However, the strong heterogeneity of CBM reservoirs makes it difficult to predict gas content by conventional geological methods. In this paper, we proposed a seismic-geological integrated method that is based on main controlling factors to predict coalbed gas content. The first step is to construct parameters such as deformation residual, fault density, net-to-gross ratio, and coal thickness to quantitatively analyze the effect of geological factors on gas content. Results show that tectonic deformation, fault systems, burial depth, and the sandstone content of the sequence after coal accumulation are key factors controlling the gas content, whereas coal thickness is weakly correlated with the gas content. Then, the degree of tectonic deformation and displacement were quantitatively calculated on the basis of 3D seismic interpretation, and the spatial distribution of sedimentary bodies and coal thickness was acquired by seismic sedimentology and facies-controlled inversion. Finally, a weighted fusion method was proposed to construct a multivariate linear model to quantitatively predict the gas content. The relative error of gas content predicted by the integrated method is 10.2%–28.4%, indicating that this new method is applicable and feasible. In addition, according to the combination of gas controlling factors, the CBM enrichment scenarios in the study area were divided into three types, in which the deep-buried, local concave, muddy roof regions are most favorable for CBM accumulation (Type Ⅲ, located in the southeast and north parts of the study area). This study provides a new way for block-scale CBM enrichment prediction under complex geological conditions.

CO2-enriched CBM accumulation mechanism for low-rank coal in the southern Junggar Basin, China

This paper mainly focused on the genesis of coalbed methane (CBM), CO2 origin and the CBM accumulation mechanism in the southern Junggar Basin using a combination of geochemical and geological analysis methods. From this work, it is found that the degree of early coalification and the current structural and hydrogeological conditions jointly determine the regional differences in CBM genesis, accumulation and gas-bearing capacity. Microbial (biogenic) gases are widely developed in the whole study area, while their generation types and contributions vary greatly in different areas and structural belts. The CBM in the eastern Urumqi (ER) and Jimusaer (JMSE) areas is primary microbial (biogenic gas) that have been generated through CO2 reduction pathway and mixed with a small amount of early mature thermogenic gas. The Baiyang River (BYR) area has a mixed gas source of early mature thermogenic gas, primary microbial gas and secondary microbial gas. The CBM in other areas is mixed gas of thermogenic and secondary microbial origins. Abnormally high CO2 concentration and extremely positive δ13C-CO2 are all related to microbial methanogenesis. The CO2 in the ER and JMSE areas and partial CO2 in the BYR area are produced by the microorganism-mediated degradation of sedimentary organic matter. The selective consumption by microbial reduction and differential dissolution of groundwater result in an increase in CO2 concentration with increasing burial depth in the ER and BYR areas. The CO2 in other areas is originally derived from the deoxidation reaction of early coalification which then undergoes secondary biotransformation. The CO2 intervention commonly runs through the whole process of CBM accumulation in low-rank coal in the study area. Due to isotope fractionation caused by groundwater flow and differential modification of microorganisms, the vertical variations of δ13C-CH4 and δ13C-CO2 are positively correlated with burial depth, and the genetic types and gas-bearing characteristics of CBM have obvious vertical zoning under the condition of high-dip structures.

Coalbed Methane Accumulation Indicated by Geochemical Evidences from Fracture-filling Minerals in Huaibei coalfield, East China

Coalbed Methane Accumulation Indicated by Geochemical Evidences from Fracture-filling Minerals in Huaibei coalfield, East China

Mesozoic Tectono-Thermal Event of the Qinshui Basin, Central North China Craton: Insights From Illite Crystallinity and Vitrinite Reflectance

The Qinshui Basin is in the Central Orogenic Belt of the North China Craton (NCC), and the production of coalbed methane accounts for 70% of China’s total coalbed methane output. Aiming at the unclear dynamic causes of large-scale coalbed methane accumulation in the basin and the unclear response relationship with the destruction of the NCC, we present joint illite crystallinity and vitrinite reflectance study across the Zijin Mountain and Qincan 1 wells of the Qinshui Basin, respectively. Inverse modeling suggested that tectono-thermal events occurred during the Early Cretaceous, associated with the maximum burial depth and heat flow. The maximum paleo-geothermal temperature and gradient reconstruction results recorded at the Carboniferous strata are 180–190°C and 6.5°C/100 m. The denudation thicknesses recorded by illite crystallinity of Zijinshan and the vitrinite reflectance of the Qincan 1 Well are 3,180.63 m and 3,269.32 m, respectively. We propose that the Qinshui Basin was affected by the extensional environment of the NCC, which caused deep lithospheric thinning and magma upwelling, and a tectono-thermal event occurred during the Early Cretaceous in Qinshui Basin. In addition, the accumulation of coalbed methane, triggered by a tectono-thermal event during the Early Cretaceous, is consistent with the early Cretaceous accumulation and mineralization events in the NCC. Overall, our results reflect the subduction event influence of the western Pacific plate into the East Asian continental plate on the tectono-thermal history of the Central Orogenic Belt of the NCC, which is theoretically significant for clarifying the thermal lithospheric thickness and rheological structure of the basin, as well as the evolutionary history of coalbed methane, and the basin response relationship to the destruction of the NCC.

Multiple Influences of Paleogeography, Sequence Stratigraphy, and Fold Structure on Coalbed Methane Accumulation in the Lopingian (Late Permian) of Junlian Coalfield, SW China

Junlian coalfield is one of the main targets for coalbed methane (CBM) exploration and development in the southwest China. Based on field geological survey, core observation, gas content statistics, coal maceral composition, vitrinite reflectance ( R O ), proximate analysis and trace element test, lithological types, lithofacies, sedimentary environment, and structural analysis, this research established the sequence stratigraphy frame, revealed the plane distribution characteristics of sedimentary facies, and defined CBM accumulation mode. The results show that six rock types were identified and further subdivided into twenty lithofacies types. Four types of sedimentary systems such as alluvial plains, delta, lagoon-tidal flat, and carbonate platform were summarized according to their combination characteristics. Additionally, 12-14 fourth-order sequences and three third-order sequences CSI, CSII, and CSIII were divided, and a sequence stratigraphic framework of the Lopingian coal-bearing series was established. Among them, the features of third-order sequence CSIII paleogeography from west to east are alluvial plains, deltas, lagoon-tidal flats, and limited carbonate platforms. Thick coal seams are mainly developed in the sedimentary environment of tidal flats, delta plains, and floodplains behind banks. Closely related to coal seam thickness, gas contents of Lopingian coal seams are generally higher than 8 m3/t, except the low level in northwest and partial denudation areas. CBM accumulation is significantly controlled by the fold structure, and the hydraulic plugging effect makes the syncline core favorable for CBM accumulation. Furthermore, favorable geostress conditions enable the secondary anticline to become a favorable area for CBM accumulation when the sealing conditions are better. This research will provide a theoretical guide for the exploration and development of CBM in the study area.