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Early 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.

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.

The ability to accurately analyze nitrogen (N) isotopes of coal is important for evaluating its contribution to NOX emissions to the atmosphere via coal combustion. Although an elemental analyzer/isotope ratio mass spectrometer is highly efficient for analyzing the N isotopes of coal, it requires large amounts of organic matter (>3 mg) to produce enough N2 to enable N isotope measurements due to the high organic carbon contents of coal with relatively low N contents. Therefore, to completely oxidize the organic matter, more oxidant matter is required to enhance coal combustion in a reactor tube in an elemental analyzer through injecting O2 during the combustion process. One reference material (RM) (GBW11104) was used to determine the effect of amounts of O2 on the accuracy of nitrogen isotope values obtained. Our results show that small amounts of O2 injected into the EA resulted in incomplete coal combustion, which yielded a lower N content and more positive N isotope ratio for RM (GBW11104) than those obtained by complete combustion. Thus, to ensure the complete combustion of coal, large O2 injection amounts are required. The refined method was applied to provide the N isotope values of coals imported from Southeast Asia, Russia, and Australia. Combined with the carbon isotope values, the imported coals from three coal-producing regions (Southeast Asia, Russia, and Australia) display distinct characteristics.

Shales from the Lower Cambrian Niutitang Formation of Yangtze Platform have been widely investigated due to its shale gas potential. To better illustrate the pore structure and fractal characteristics of shale, a series of experiments were conducted on outcrop samples from the Lower Cambrian Niutitang Formation on Yangtze Platform, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and low-temperature nitrogen adsorption. Frenkel-Halsey-Hill (FHH) model was adopted to calculate the fractal dimensions. Furthermore, the relationships between fractal dimensions and pore structure parameters and mineral composition are discussed. FE-SEM observation results show that interparticle pores are most developed in shale, followed by intraparticle pores. This study identified the fractal dimensions D1 (ranging from 2.558 0 to 2.710 2) and D2 (ranging from 2.541 5 to 2.765 2). The pore structure of the Niutitang Formation shale is primarily controlled by quartz and clay content. Fractal dimensions are able to characterize the pore structure complexity of Niutitang Formation shale because D1 and D2 correlate well with average pore diameter and quartz content.

Based on 54 sets of well test data of 29 coalbed methane (CBM) wells, the distribution characteristic of in situ stress in the eastern part of the southern Junggar Basin and its control on permeability (K), reservoir pressure (Po), and gas content (G) were discussed systematically. The results show that three types of in situ stress regime exist and are converted corresponding to a certain depth, (1) σv > σh); (2) 600‐1050 m is the stress transition zone (σH ≈ σv > σh); and (3) >1050 m is the normal fault regime (σv > σH>σh). Regionally, with depths σv > σh type in the western Miquan, σH ≈ σv > σh type in the middle Fukang, and σv > σH > σh type in the eastern Jimushaer, respectively. Controlled by stress regime and vertical belting, coal K shows a trend of “remarkably decreased, rebounded increase and greatly decreased,” and two decreasing stages ( 1050 m) are mainly influenced by horizontal stress and vertical stress, respectively. Taking a burial depth of 1000‐1150 m as a boundary, the relationship between G and depth converts from “continually increasing” to “gradually decreasing,” which is in good agreement with the converted interface of stress regime from σH ≈ σv > σh type to σv > σH > σh type. Taking the converted interfaces of G (1000‐1150 m), K (800 m), and the prediction depth of the weathered zone (400 m) into consideration, CBM development potential in the study area can be divided into three grades, that is, (1) 400‐800 m (high K and medium G), (2) 800‐1150 m in Miquan and 800‐1000 m in Fukang (medium K and high G), and (3) >1150 m in Miquan and >1000 m in Fukang (low K and poor G). Overall, a key CBM development breakthrough will most likely be made in the study area within the scope of 600‐800 m due to the better G and higher K.

The Ordovician–Silurian organic-rich shales in the Wufeng and Longmaxi Formations are widespread in southern China and are the most important shale gas producers in China. The shales display varying total organic carbon content ranging from 0.2% to 6% and are composed of black siliceous mudstone, shelly limestone, siliceous clay-rich mudstone, gray silty–shaly interlaminated mudstone, gray argillaceous mudstone, and calcareous or dolomitic mudstone. Detailed stratigraphic correlation of these formations was achieved, and a high-resolution sequence stratigraphy was constructed based on the integration of well-log data, mineralogical, geochemical, and petrophysical analyses. The Wufeng Formation and the Guanyinqiao Bed were interpreted to be a condensed section with four cycles of sea-level fluctuation. The Longmaxi Formation consists of three members. The Long-1 Member was interpreted to be a third-order depositional sequence that can be subdivided into a transgressive systems tract, an early highstand systems tract, and a late highstand systems tract. Within the constructed isochronal stratigraphic framework, the characteristics and genetic mechanism of different shale lithofacies were studied. The deposition of black siliceous mudstone of high total organic carbon in the Wufeng Formation and the Long-1 transgressive systems tract was likely influenced by volcanic activity. The Guanyinqiao shelly limestone or calcareous mudstone was associated with the Ordovician–Silurian transition when glaciation and mass extinction took place. The gray silty–shaly interlaminated mudstone of moderate total organic carbon in the Long-1 early highstand systems tract was the product of bottom flow intrusion. The deposition of gray argillaceous mudstone and calcareous or dolomitic mudstone with low total organic carbon in the Long-1 late highstand systems tract resulted from enhanced terrigenous input from peripheral uplifts. The distribution pattern of different lithofacies varies greatly in the Upper Yangtze area, and this difference is a result of dissimilar sediment provenances and variable hydraulic restriction levels that were controlled by basin geometry. A better understanding of the distribution pattern and depositional history of the Wufeng and Longmaxi Formations might provide guidance for future profitable shale gas exploration.

The lower Cambrian Niutitang shales, as one of target intervals with the greatest potential for shale gas exploration and development, have attracted much attention. Nevertheless, the organic matter enrichment mechanisms of the lower Cambrian Niutitang shales need further study, especially in the hydrothermal active zone. In this study, samples from ND1 well in western Hubei Province, middle Yangtze region, South China, were investigated for the controlling factors of organic matter accumulation of Lower Cambrian Niutitang shales by detailed petrographic, mineralogic, and geochemical proxies. The results show that hydrothermal activity and sea level fluctuation controlled the redox conditions and paleoproductivity of seawater and ultimately controlled the organic matter accumulation of Niutitang formation. In the Niu-1 member, the intense hydrothermal events lead to a suboxic to anoxic environment, which is conducive to the organic matter preservation. However, low sea level strengthens the restriction of water mass and reduced nutrient upwelling into the shelf, leading to decreased marine primary productivity, which was ultimately responsible for depleted organic matter accumulation in the Niu-1 member. In the Niu-2 member, the anoxic-euxinic environment and high paleoproductivity, driven by continuous hydrothermal activity and rising sea level, were the main factors controlling the enrichment of organic matter. In the Niu-3 member, the dysoxic to oxic condition plus low primary productivity, caused by the disappearance of hydrothermal activities and sea-level fall, resulted in the unfavorable organic matter accumulation. The results of this paper enrich the model of organic matter enrichment in the lower Cambrian black shale in the middle Yangtze region.

The Ordovician–Silurian (O–S) transition was a critical interval in geological history. Multiple geochemical methods are used to explore the changes in oceanic environment. The Nd isotopic compositions in the Yangtze Sea are controlled by two sources: the continental erosion and the Panthalassa Ocean. HighεNd(t) values during the Katian, late Hirnantian and Rhuddanian intervals are associated with the high sea level, which resulted in less terrestrial input based on the low Ti/Al and Zr/Al ratios. In contrast, lowεNd(t) values during the early Hirnantian interval are related to the sea-level fall; in this case, the exposure of submarine highs and the growth of Yangtze Oldlands could lead to more continental materials being transported into the Yangtze Sea based on high Ti/Al and Zr/Al ratios. In addition, the negativeεNd(t) excursion can also be attributed to the weak circulation between the Yangtze Sea and Panthalassa Ocean when sea level was low. Furthermore, the sea-level eustacy plays a significant role in the changes in redox water conditions. The redox indices, mainly UEF, Ce/Ce* and Corg/PT, across the O–S transition show a predominance of anoxic ocean over the Yangtze Sea during the Katian, late Hirnantian and Rhuddanian intervals, and an oxygenated episode was briefly introduced during the early Hirnantian period because of the fall in sea level. The Late Ordovician biotic crisis was marked by two-phase extinction events, and the change in sea level and redox chemistry may be the important kill mechanisms.

The difference in quartz types in shales not only affects the porosity and permeability of the rocks, but also reflects the difference in the sedimentary environments. We established the formation mechanism and numerical model of quartz in shales of Wufeng and Longmaxi formations in the Wangjiawan Section, South China, based on thin-section studies using SEM (scanning electron microscope), SEM-CL (cathodoluminescence), XRD (X-ray diffraction) and geochemical analyses. There are two types of quartz in the shales: detrital quartz and authigenic quartz. Detrital quartz is mostly silt-size, typically ranging from 10 to 60 μm in size and subangular to angular monocrystal in shape, and brighter than authigenic quartz by CL intensity; authigenic quartz is present in two phases in shape: grain overgrowths and crystallite grains. Overgrowth surfaces are subhedral. Crystallite grains are typically less than 10 μm in size, euhedral or subhedral monocrystal in shape. Authigenic quartz can be subdivided into biogenic quartz and clay mineral transformed quartz according to the source of silicon. In the numerical model, the content of detrital quartz is relatively consistent (20%); the content of biogenic quartz ranges from 40% to 70%, with a sharp fall (0–30%) in the Guanyinqiao mudstone. During the Katian, a lower anoxic and dense water column make the dissolution of biogenic silica well preserved. Biogenic quartz is the major contributor to the sediment. During the early Hirnantian interval, due to the drop of sea level and the oxygenation of seafloor, the sediment is mainly composed of clay transformed quartz and detrital quartz. During the latest Hirnatian and Rhuddanian, rapid sea level rise and anoxic ocean enhance the preservation of the biogenic silica, thereby biogenic quartz re-emerges as the major contributors to the sediment. Authigenic crystallite grains and grain overgrowths have filled in primary pore space and have decreased the interparticle porosity, however, as a rigid framework, they can suppress compaction and maintain the internal pore structure. The formation of authigenic quartz results in the increase of total quartz, which fortifies the brittleness of rocks and is beneficial to the development of shale gas.

Based on numerous microfossils in Mesozoic of North Carnarvon Basin in Australia, the Mesozoic paleoclimate and paleoenvironment were analyzed, the paleogeographic background and development model of Mesozoic high-quality source rocks were investigated and discussed. Variation of microfossil species and their relative contents in Mesozoic different intervals indicates changes of paleoclimate and paleoenvironment. The paleoclimate shows a cycle variation of arid–humid–arid–humid, the regression occurred in the Early Triassic–Late Triassic and the transgression occurred in the Late Triassic–Late Cretaceous. Four sedimentary facieses including delta facies, littoral facies, restricted sea facies and open sea facies were developed in the Mesozoic. The open sea was mainly developed in the Early Triassic, the delta was distributed in the Middle–Late Triassic, the restricted sea was especially well developed in the Jurassic, and the restricted sea and open sea were mainly distributed in the Cretaceous. Characteristics of microfossil assemblage in these four sedimentary facies are dramatically different. From the delta facies, littoral facies, restricted sea facies to open sea facies, content of pollen gradually decreases, but content of the dinoflagellate + acritarch gradually increases. The delta facies and littoral facies are dominated by the pollen. In the restricted sea facies, content of the pollen is equivalent to that of the dinoflagellate + acritarch. The open sea facies is dominated by the dinoflagellate and acritarch. Supply of sediment and formation of organic matter are influenced by the paleoclimate and paleoenvironment, and type of organic matter is controlled by the microfossil assemblage. Based on the palaeogeographic background, paleoclimatic condition and microfossil assemblage, two developmental models of the Mesozoic source rocks such as the development model of terrestrial organic matter under the background of large delta and the development model of mixed organic matter under the background of the restricted sea, were proposed. Keywords: Microfossil assemblage, Sporopollen, Dinoflagellate and acritarchs, Paleoclimate reconstruction, Evolution of sedimentary environment, Development model of source rocks, Mesozoic, North Carnarvon basin

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.

Reservoir solid bitumen is pervasive throughout the Sinian Dengying Formation dolomites of the Sichuan Basin and neighboring areas, yet its origin remains controversial. Optical and scanning electron microscope (SEM) studies of the Dengying Formation were carried out to elucidate the nature and origin of the solid bitumen. Solid bitumen in dissolution pores, intercrystal pores, and stylolites appears to have formed in association with regional hydrocarbon charging processes. Reservoir solid bitumen displays a variety of morphologies, each of which appears to reflect different precursor hydrocarbons. Massive/crust-like solid bitumen was probably derived from solid precursors whereas finger-like solid bitumen likely evolved from liquid precursors. Sheet-like and thin skinned globular solid bitumen appeared to have precipitated from wet gas precursors. Analyzed solid bitumen is characterized by high random reflectance (average = 3.4%) and bireflectance values (4.2–6.9%), exhibits strong optical anisotropy, and displays different coke structures, including granular mosaic, domain, and fibrous. The coke textures, which appear to reflect the nature of bitumen precursors, formed by physical coalescence of mesophase spheres and parallel arrangement of laminated aromatic sheets in association with a rapid heating event. The inferred thermal episode, which is evidenced by the presence of hydrothermal minerals, including saddle dolomite, quartz, authigenic illite, and barite, may have been associated with the Late Permian eruption of the Emeishan Traps. Thus, widespread solid bitumen in the Dengying Formation reflects the effects of hydrothermal alteration rather than the impact of normal burial-related thermal maturation. Further, the close association of sold bitumen and authigenic kaolinite indicates that natural deasphalting plays a role in the formation of reservoir solid bitumen.

The mineral characteristics (occurrence, type, and content) of low-rank coal and their influence on coalbed methane (CBM) reservoirs are investigated at the micro- and nanoscales. Six coal samples of three representative coalmines were used to demonstrate the uniform tectonization from the Zhundong coalfield, NW China. Based on optical microscopy and scanning electron microscopyenergy dispersive spectrum (SEM-EDS) analysis, the mineral composition and occurrence characteristics were discussed. The micro- and nanoscale reservoir characteristics in low-rank coal (pore size distribution and adsorption capability) were studied by diverse methods, including lowtemperature N2 adsorption/desorption, mercury intrusion porosimetry and CH4 isotherm adsorption analysis. The coal reservoir nuclear magnetic T2 spectra of porosity and movable fluid were obtained by combining low-field nuclear magnetic resonance (NMR) analysis, which has an advantage of determining pore fluid technology. The mineral content is highly variable (4˜16 vol.%) in the Xi Heishan prospecting area of the Qitai region. Kaolinite, goyazite, ankerite and anorthosite were microscopically observed to be filling in coal pores and microfractures, and the minerals are given priority to silicate minerals. There is a greater content of mesopores (100–1000 nm) and transition pores (10–100 nm), and they are well connected. The micropores (0–10 nm) are dominated by parallel plate, closed or wedge-shaped pores. Furthermore, the microfractures are mainly observed for types B (width ≥ 5 μm and length≤ 10 mm) and D (width

Co-accumulation of carbon dioxide and hydrocarbon gas is common and widely existing in oil and gas basins. Such two natural pools are located in Lishui sag (East China Sea Shelf Basin), as a favorable prospecting area for exploration. CO2 gas derived from inorganic mantle sources and CO2 accumulation times have been investigated. In this study, petrographic observation shows pure CH4, CH4-rich, H2O-rich and hybrid CH4-CO2 fluid inclusions exist in healed micro-fractures and overgrowths of quartz mineral in Palaeocene reservoirs of Lishui sag. The pressure–temperature–time–composition (PVT–x) properties of CH4-bearing inclusions were obtained by Raman quantitative analysis and thermodynamic models. These results provide important constraints on the P-T entrapment conditions and predict relative accumulation time of CH4 fluid. Combination of carbon isotope analyses, inclusion petrography, microthermometry, PT entrapment conditions of CH4-bearing inclusions, hydrocarbon generation and simulation of burial history permit to assign periods of CH4 fluid activity and its sequence with CO2 fluid. Methane charging time occurs during two stages in LS-A: low density CH4 fluid (~0.10 g/cm3 or lower) were trapped during late Oligocene (30-20 Ma) and high density (>0.115 g/cm3) during a Pliocene (8-1 Ma) CH4 accumulation event. CO2 fluid accumulation (Miocene, 21-4 Ma) displaced the methane gas trapped in the early stage. Pliocene CH4 fluid partial displaced Miocene CO2 fluid. In well B, only high density (0.144–0.153 g/cm3) recorded a late Oligocene to early Miocene (36-19 Ma) CH4 accumulation event, Pliocene (5-3 Ma) CO2 fluid displaced early CH4 fluid. Micro-thermometry, composition of fluid inclusion, features of reservoir, thermal maturity reflect hydrothermal fluid vertical migration and the existence of Supercritical CO2 extraction.