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Based on an elaboration of the resource potential and annual production of tight sandstone gas and shale gas in the United States and China, this paper reviews the researches on the distribution of tight sandstone gas and shale gas reservoirs, and analyzes the distribution characteristics and genetic types of tight sandstone gas reservoirs. In the United States, the proportion of tight sandstone gas in the total gas production declined from 20%--35% in 2008 to about 8% in 2023, and the shale gas production was 8 310x108 m3 in 2023, about 80% of the total gas production, in contrast to the range of 5%--17% during 2000--2008. In China, the proportion of tight sandstone gas in the total gas production increased from 16% in 2010 to 28% or higher in 2023. China began to produce shale gas in 2012, with the production reaching 250x108 m3 in 2023, about 11% of the total gas production of the country. The distribution of shale gas reservoirs is continuous. According to the fault presence, fault displacement and gas layer thickness, the continuous shale gas reservoirs can be divided into two types: continuity and intermittency. Most previous studies believed that both tight sandstone gas reservoirs and shale gas reservoirs are continuous, but this paper holds that the distribution of tight sandstone gas reservoirs is not continuous. According to the trap types, tight sandstone gas reservoirs can be divided into lithologic, anticlinal, and synclinal reservoirs. The tight sandstone gas is coal-derived in typical basins in China and Egypt, but oil-type gas in typical basins in the United States and Oman. [ABSTRACT FROM AUTHOR]

Exploration and development of large gas fields is an important way for a country to rapidly develop its natural gas industry. From 1991 to 2020, China discovered 68 new large gas fields, boosting its annual gas output to 1 925×108 m³ in 2020, making it the fourth largest gas-producing country in the world. Based on 1696 molecular components and carbon isotopic composition data of alkane gas in 70 large gas fields in China, the characteristics of carbon isotopic composition of alkane gas in large gas fields in China were obtained. The lightest and average values of δ13C1, δ13C2, δ13C3 and δ13C4 become heavier with increasing carbon number, while the heaviest values of δ13C1, δ13C2, δ13C3 and δ13C4 become lighter with increasing carbon number. The δ13C1 values of large gas fields in China range from -71.2‰ to -11.4‰ (specifically, from -71.2‰ to -56.4‰ for bacterial gas, from -54.4‰ to -21.6‰ for oil-related gas, from -49.3‰ to -18.9‰ for coal-derived gas, and from -35.6‰ to -11.4‰ for abiogenic gas). Based on these data, the δ13C1 chart of large gas fields in China was plotted. Moreover, the δ13C1 values of natural gas in China range from -107.1‰ to -8.9‰, specifically, from -107.1‰ to -55.1‰ for bacterial gas, from -54.4‰ to -21.6‰ for oil-related gas, from -49.3‰ to -13.3‰ for coal-derived gas, and from -36.2‰ to -8.9‰ for abiogenic gas. Based on these data, the δ13C1 chart of natural gas in China was plotted. [ABSTRACT FROM AUTHOR]

The influence of water on gas generation from humic type organic matter at highly to over mature stage was investigated with thermal simulation experiments at high temperature and pressure. The result of the experiments indicates that the effect of water on gas generation was controlled by the thermal maturity of organic matter. Water could enhance gas generation and increase hydrocarbon gas yields significantly at over mature stage of humic type organic matter. Hydrogen isotopic compositions of coal-derived gases generated at highly to over mature stage were mainly controlled by thermal maturity of source rocks, but also affected by formation water. Highly and over mature coal measure source rocks are widely distributed in China. The hydrocarbon gas generation capacity of coal measure source rocks and resource potential of coal-derived gases in deep formations would be significantly enhanced assuming that formation water could be involved in the thermal cracking of highly to over mature organic matter in real geological settings.

Natural gases in China are mainly coal-derived gas, with assistance from oil-typed gas. At present, many genetic identification methods, from hydrocarbon composition and isotope to light hydrocarbon and biomarker indexes, have been formed, but combined methods from non-hydrocarbon gases are lacking. Based on compositions and isotopes geochemical characteristics and the differences of non-hydrocarbon nitrogen gas in coal-derived gas and oil-typed gas, and combining the isotopic geochemical characteristics of non-hydrocarbon helium, the comprehensive identification methods of coal-derived gas and oil-typed gas for hydrocarbon gases according to the associated non-hydrocarbon gases of nitrogen and helium are established and the preliminary applications have been engaged. The main recognitions are as follows:1)Coal-derived gas generally has relatively lower nitrogen abundance, mainly distributed from 0 to 31.2% with main frequency from 0 to 2%. Oil-typed gas, on the other hand, usually has relatively higher nitrogen abundance, mainly distributed from 1.1 to 57.1% with main frequency from 2 to 16%. 2) Coal-derived gas generally has relatively heavier nitrogen isotope values, mainly distributed from -8 to 19.3‰ with main frequency from -8 to 8‰.Oil-typed gas usually has relatively lower nitrogen isotope values, mainly distributed from -10.6 to 4.6‰ with main frequency from -8 to 4‰.3)The geochemical characteristic differences of coal-derived gas and oil-typed gas are mainly due to the fact that the sapropel parent material is relatively rich in nitrogen element and rich in light δ14N, while the humic parent material is relatively poor in nitrogen element and rich in heavy δ15N. The differences on thermal maturity of source rocks, the redox conditions of source rock sedimentary environment, and the salinity of water body are also important effective factors.4)Differences on nitrogen abundances and isotopes in coal-derived gas and oil-typed gas have great significance in genetic identification. The genetic identification chart of R/Ra-δ15N for organic and inorganic nitrogen in natural gas and the comprehensive joint identification charts of R/Ra-δ15N-N2 of nitrogen and helium for coal-derived gas and oil-typed gas have been established, and are of great reference in investigating the origins and sources of natural gas and guiding natural gas exploration in China.

The identification of the origin and source of natural gas is always a difficult and hot issue. Hereinto, the maturity identification is one of the most important scientific problems. Many empirical equations have been established to decipher the relationship between the maturity of gas source rocks and the carbon isotopic composition of natural gas. However, these equations proposed often fail to identify the maturity of the source rocks correctly, which in turn prevents the identification of genetic types and source rocks of the natural gas because the petroliferous sedimentary basins in China are complex and diverse, with multiple sets of source rocks and different thermal history. In this paper, the oil-associated gas from the Permian lacustrine source rocks and the coal-derived gas from the Jurassic source rocks in Junggar and Turpan-Hami basins have been investigated to decipher the relationship between the maturity (vitrinite reflectance) of gas source rocks and the carbon isotopic composition of methane. The equations established are δ13C1=25lgRo−42.5 for oil-associated gas, and δ13C1=25lgRo−37.5 for coal-derived gas. These new equations are suitable for the maturity identification of source rocks in most petroliferous basins, and favorable for the identification of the genetic type and source of natural gas, which is very important to improve the geological theory of natural gas. [ABSTRACT FROM AUTHOR]

China has made significant progress in the exploration and development of natural gas in the past 70 years, from the gas-poor country to the world's sixth largest gas production country. In 1949, the annual gas output in China was 1 117×104 m3, the proved gas reserves were 3.85×108 m3, and the average annual gas consumption and available reserves of per person were 0.020 6 m3 and 0.710 7 m3, respectively. By 2018, the average domestic annual gas production per person was 114.857 6 m3 and the reserves were 12 011.08 m3, and the average domestic annual gas production and reserves per person in the past 70 years increased by 5 575 times and 16 900 times, respectively. The exploration and development of large gas fields is the main way to rapidly develop the natural gas industry. 72 large gas fields have been discovered in China so far, mainly distributed in three basins, Sichuan (25), Ordos (13) and Tarim (10). In 2018, the total gas production of the large gas fields in these three basins was 1 039.26×108 m3, accounting for 65% of the total gas production in China. By the end of 2018, the cumulative proved gas reserves of the 72 large gas fields had amounted to 124504×108 m3, accounting for 75% of the total national gas reserves (16.7×1012 m3). New theories of natural gas have promoted the development of China's natural gas industry faster. Since 1979, the new theory of coal-derived gas has boosted the discovery of gas fields mainly from coal-measure source rocks in China. In 2018, the gas production of large coal-derived gas fields in China accounted for 50.93% and 75.47% of the total national gas production and total gas production of large gas fields, respectively. Guided by shale gas theories, shale gas fields such as Fuling, Changning, Weiyuan and Weirong have been discovered. In 2018, the total proved geological reserves of shale gas were 10 455.67× 108 m3, and the annual gas production was 108.8×108 m3, demonstrating a good prospect of shale gas in China. [ABSTRACT FROM AUTHOR]

The high pressure – high temperature Culzean field, UK Central North Sea, contains lean gas condensate in the Triassic Joanne sandstones and the Middle Jurassic Pentland sandstones. A comprehensive gas analysis programme was installed as an integrated part of field development in order to monitor gas composition, distribution and origin in the reservoirs and overburden pre‐ production start‐up. Isotube OUT and isotube IN gas samples were collected. The isotube IN data show that some gas is recycled, including alkenes representing contamination from the degradation of mud additives; but concentrations are minor, and do not seem to affect the isotope values derived from the C2 and C3 isotube OUT gases significantly. 13C‐enriched methane derived from drill‐bit metamorphism is recorded in the isotube IN gas, but likewise in low concentrations. Gas data were also acquired from a Continuous Isotope Logging Tool (CILT) which measures real‐time gas concentrations and isotope values of C1–C3 each foot through the entire drilled section. The CILT thus provides a continuous trend of methane isotope values versus depth, and this trend is useful in identifying changes in gas composition. However, concerns related to CILT include: (i) C1–C3 stable carbon isotope detection limits for isotube OUT gas analyses are considerable lower than for CILT; due to the lower isotube gas concentrations required for measurement of C3 isotopes, isotubes are able to map a shallower vertical thermogenic gas migration front in the overburden. (ii) Discrepancies between isotube OUT and CILT isotope values may be significant and cannot be assigned to analytical uncertainty; by contrast, test gas and isotube OUT isotope values are comparable. Hence, CILT isotope values from specific depths cannot stand in isolation but must be complemented by isotube OUT isotope measurements. Gas in the Pentland reservoir is the most coaly in composition due to self‐sourcing from the Pentland coals. The coals are the primary source rock for the gas encountered in the entire reservoir interval at Culzean, but the underlying Joanne sandstones contain contributions from highly mature marine shales in the Kimmeridge Clay Formation and/or Heather Formation. The Lower Cretaceous seal on top of the Pentland reservoir is relatively tight but some minor migration/leakage of thermogenic gas into the overburden is recorded by the detection of C3 isotopes. Thermogenic gas also occurs in high porosity intervals in the Upper Cretaceous succession but this gas is interpreted to have migrated laterally in porous carrier beds and did not enter these intervals at the well locations. [ABSTRACT FROM AUTHOR]

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Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas & Oil is the property of Editorial Department of Natural Gas & Oil and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The geochemical characteristics, origin, and source of natural gas in the Dongping–Niudong areas are researched based on the information of components and the stable carbon isotopes of natural gas. The main component of hydrocarbon gas is methane, and C2+ possesses a relatively low content that ranges from 0.5% to 6.5%. The dryness coefficient of the Dongping area is more than 0.97, which categorizes as dry gas. The dryness coefficient of the Niudong area ranges from 90.3% to 98.1%, whilst having the characteristics of dry gas coexisting with wet gas. The δ13C1 dryness coefficient varies from −17.58‰ to −36.4‰, and δ13C2 is greater than −29‰. The gas of the Dongping 3 block has the characteristics of an abiogenic gas such as the reversed carbon isotope series. The combination of the relationship between the component of CO2 and δCCO213 and the background of development of hydrocarbon-generating sag, suggests that it is coal-derived gas. The RO and fluid inclusion studies show that the gas in Niudong area is mainly derived from Early Jurassic coal-measure source rocks of Kunteyi Sag, and there are two periods of hydrocarbon fillings. The first filling is mainly oil, and the second is mainly high mature gas; gas in Dongping area is mainly derived from Early Jurassic coal-measure source rocks of Pingdong–Yiliping Sag, and there are four periods of high-over mature gas fillings. The phenomenon of reversed carbon isotope series in Dongping area resulted from the multiple gas charging.

Tight sandstone gas from coal-measure source rock is widespread in China, and it is represented by the Xujiahe Formation of the Sichuan Basin and the Upper Paleozoic of the Ordos Basin. It is affected by planar evaporative hydrocarbon expulsion of coal-measure source rock and the gentle structural background; hydrodynamics and buoyancy play a limited role in the gas migration-accumulation in tight sandstone. Under the conditions of low permeability and speed, non-Darcy flow is quite apparent, it gives rise to gas-water mixed gas zone. In the gas displacing water experiment, the shape of percolation flow curve is mainly influenced by core permeability. The lower the permeability, the higher the starting pressure gradient as well as the more evident the non-Darcy phenomenon will be. In the gas displacing water experiment of tight sandstone, the maximum gas saturation of the core is generally less than 50% (ranging from 30% to 40% and averaging at 38%); it is similar to the actual gas saturation of the gas zone in the subsurface core. The gas saturation and permeability of the core have a logarithm correlation with a correlation coefficient of 0.8915. In the single-phase flow of tight sandstone gas, low-velocity non-Darcy percolation is apparent; the initial flow velocity (Vd) exists due to the slippage effect of gas flow. The shape of percolation flow curve of a single-phase gas is primarily controlled by core permeability and confining pressure; the lower the permeability or the higher the confining pressure, the higher the starting pressure (0.02–0.08 MPa/cm), whereas, the higher the quasi-initial flow speed, the longer the nonlinear section and the more obvious the non-Darcy flow will be. The tight sandstone gas seepage mechanism study shows that the lower the reservoir permeability, the higher the starting pressure and the slower the flow velocity will be, this results in the low efficiency of natural gas migration and accumulation as well as low gas saturation. The laboratory modeling on gas migration accumulation mechanism in coal-measure tight sandstone can provide a theoretic foundation to reveal the tight sandstone gas enrichment regularity, evaluation of prospecting area, and the study of development mechanism.

The coal measure gas sources of coal-derived gas fields in the Late Paleozoic China are the Lower Carboniferous Dishuiquan Formation, the Upper Carboniferous Batamayineishan Formation and Benxi Formation, the Lower Permian Taiyuan Formation and Shanxi Formation, and the Upper Permian Longtan Formation. The coal-derived gas accumulates in Ordovician, Carboniferous, Permian, and Paleocene reservoirs and are distributed in Ordos Basin, Bohai Bay Basin, Junggar Basin, and Sichuan Basin. There are 16 gas fields and 12 of them are large gas fields such as the Sulige large gas field which is China's largest reserve with the highest annual output. According to component and alkane carbon isotope data of 99 gas samples, they are distinguished to be coal-derived gas from coal-derived gas with δ13C2 > −28.5‰ and δ13C1 -δ13C2 -δ13C3 identification chart. The Late Paleozoic relative gas fields of coal measure are significant for the Chinese natural gas industry: proven natural gas geological reserves and annual output of them account for 1/3 in China, and the gas source of three significant large gas fields is coal-derived, which of five significant large gas fields supporting China to be a great gas producing country. The average reserves of the gas fields and the large gas fields formed from the late Paleozoic coal measure are 5.3 and 1.7 times that of the gas fields and the large gas fields in China.

The carbon isotopic series of alkane gases were divided into three types: (1) positive carbon isotopic series: δ13C values increased with increasing carbon numbers among the C1–C4 alkanes, which is a typical characteristic for primary alkane gases; (2) negative carbon isotopic series: δ13C values decreased with increasing carbon numbers among the C1–C4 alkanes; and (3) partial carbon isotopic reversal, which had no increasing or decreasing relationship between the δ13C values and carbon numbers. Negative carbon isotopic series were further divided into primary and secondary origins. The primary is a typical characteristic of abiogenic gases, while the secondary is a result of the secondary alteration imposed on biogenic gases usually observed in over-mature shale gas and coal-derived gas. Previous research has proposed several possible explanations for negative carbon isotopic series of secondary origin, such as secondary cracking, diffusion, and the Rayleigh fractionation of ethane and propane through redox reaction with the participation of transition metal and water at 250–300 °C. After a comparative study, the authors found that the negative carbon isotopic series of secondary origin for both shale gas and coal-derived gas appeared in areas where source rocks (shales) were at an over-mature stage, but not in areas where source rocks (shales) were only at a high-maturity stage. As a result, high maturity (>200 °C) was the main controlling factor for the occurrence of negative carbon isotopic series of secondary origin in thermogenic gases. Within this maturity interval, secondary cracking, diffusion, and Rayleigh fractionation of ethane and propane could happen separately or together.

Fifty-one large gas fields had been proved in China until 2013. Specifically, exploration characteristics of those discovered since 2000 are as follows: (1) Large gas fields are only found in basins with sedimentary area larger than 10 × 104 km2; (2) Large gas fields have been proved in 9 basins, with total proved reserves of 27085.88 × 108 m3 before 2005, much less than that after 2005, which reached 81683.77 × 108 m3 by the end of 2013; (3) The reserve abundance of large gas fields varies a lot. The Kela2 gas field has the largest reserve abundance of 59.05 × 108 m3/km2, which is 86 times that of the smallest reserve abundance, i.e. 0.684 × 108 m3/km2 of the Jingbian gas field; and (4) The reservoirs of large gas fields between 3000 m and 4500 m share a large proportion of proved reserves, accounting for 46.11% of the total. Development characteristics of the large gas fields in China are as follows: (1) The yield of large gas fields is essential to the natural gas industry of China. In 2013, the total yield was 922.72 × 108 m3, accounting for 76.3% of the nation’s total natural gas yield; (2) The yield is dominated by coal-derived gas, which reached 710.13 × 108 m3 in 2013, accounting for 77.0% of the total yield of large gas fields in China; and (3) The yield of key large gas fields (Sulige, Jingbian, Daniudi, Puguang, and Kela2) is fundamental in making China a major gas producer.

Recent studies on both conventional (thermogenic gas) and unconventional (shale gas) gases show comparable carbon/hydrogen isotope rollover, i.e., before the rollover point, carbon/hydrogen isotope increases with increasing thermal maturity (or decreasing gas wetness), while after the rollover point, the gas component (i.e., ethane, propane, etc.) becomes more depleted in 13 C or 2 H with increasing thermal maturity. In order to further investigate potential mechanism of the isotope rollover, combined with previous researches on isotope rollover, this study carried out two batches of pyrolysis experiments on a low mature coal sample from the Songliao Basin, China at constant temperatures for about 72 h under the condition with added water and without added water. The results demonstrate that carbon and hydrogen isotope rollover of ethane and propane started at 500 °C, no matter with added water or without added water. Combined with previous studies on gases of various origins, it is found that such isotopic anomalies were a common phenomenon at high thermal mature stage, despite of what kind of the source rocks and whether adding water or not under the laboratory condition. This is likely caused by the mixing of gases directly from the decomposition of macromolecule of kerogen and those from the tightly entrapped straight-chain species in kerogen at relatively high thermal mature stage. These straight-chain species are normally entrapped in the kerogen, and will probably be released at high thermal maturation stage. Marine facies source rocks from southern China also demonstrate the existence of such tightly entrapped hydrocarbons at very high thermal maturity. The results may have important significance on the gas exploration at relatively high mature stage and have a guidance on the gas production changes during the exploration. [ABSTRACT FROM AUTHOR]

In order to determine the source of the light hydrocarbons in the Kuqa Depression of the Tarim Basin, Northwest China, their geochemical characteristics are documented, and possible influencing factors are discussed in a broad context on the basis of geochemical analyses of natural gas samples from 19 wells in combination with previously reported natural gas components and carbon isotope values from the same depression. The biomarker analytical results show that (a) methyl cyclohexane (37.5–60.3%) of natural gas predominates the C7 light hydrocarbon series (methyl cyclohexane, heptanes, and dimethyl cyclopentane), and (b) the heptane content (25.0–54.8%) is also relatively high. The C6–7 light hydrocarbon series (aromatics, normal alkanes, and cycloalkanes) have abnormally high aromatics (benzene and toluene) content (26.2–83.8%). Heptane value (13.31–37.77%) and isoheptane value (2.10–7.64%) indicate that the Kuqa natural gases are of high maturity and typical of coal‐derived gas (gas generated by sapropelic source rock). Their light hydrocarbons, however, appear to have some characteristics of mixed‐source organic matter. Specifically, the light hydrocarbons in the Dina 2 gas field show characteristics of the coal‐derived gas; those in the Dabei gas field show some features of the oil‐associated gas (gas generated by humic source rock), whereas those in the Kela 2 and Keshen 2 gas fields and Well Yangtake‐101 display characteristics of the mixed source organic matter. The abnormally high content of the aromatics in the Kuqa Depression may have resulted mainly from their high productivity during the thermal maturity of humic type organic matters and the gases accumulation of late stage. The different relative contents of the aromatics in various oil and gas fields may have been accounted by different organic matter types, maturity, and distribution patterns of source rocks. [ABSTRACT FROM AUTHOR]

Abstract: This work extensively investigated global tight sandstone gas, and geologically and geochemically analyzed the tight sandstone gas in China's Ordos, Sichuan, and Tarim basins. We compared typical tight sandstone gas in China with that in North America. We proposed six conditions for the formation of China's tight sandstone gas, and illustrated the geological characteristics of tight sandstone gas. In China, gas‐bearing tight sandstones were mainly deposited in continental lake deltas and marine‐terrigenous facies basin environments, associated with coal‐measure strata, and were mostly buried deeper than 2000 m under a formation pressure of 20–30 MPa, with pressure coefficients varying from overpressure to negative pressure. In other countries, tight gas bearing sandstones were dominantly deposited in marine to marine‐terrigenous facies environments, occurred in coal‐measure strata, and were mostly buried shallower than 2000 m in low‐pressure systems. We systematically analyzed tight sandstone gas in the Ordos, Sichuan, and Tarim basins in terms of chemical compositions, geochemical characteristics of carbon isotopes, origins, and sources. Tight sandstone gas in China usually has a hydrocarbon content of >95%, with CH41 content >90%, and a generally higher dry coefficient. In the three above‐mentioned large tight sandstone gas regions, δ13C1 and δ13C2 mainly ranges from –42‰ to –28‰ and from –28‰ to –21‰, respectively. Type III coal‐measure source rocks that closely coexist with tight reservoirs are developed extensively in these gas regions. The organic petrology of source rocks and the carbon isotope compositions of gas indicate that tight sandstone gas in China is dominantly coal‐derived gas generated by coal‐measure strata. Our analysis of carbon isotope series shows that local isotope reversals are mainly caused by the mixing of gases of different maturities and that were generated at different stages. With increasing maturity, the reversal tendency becomes more apparent. Moreover, natural gas with medium‐low maturity (e.g., Xujiahe Formation natural gas in the Sichuan Basin) presents an apparent reversal at a low‐maturity stage, a normal series at a medium ‐maturity stage, and a reversal tendency again at a high‐maturity stage. Finally, we proposed four conditions for preferred tight sandstone gas “sweep spots,” and illustrated the recoverable reserves, proven reserves, production, and exploration prospects of tight sandstone gas. The geological and geochemical characteristics, origins, sources, and exploration potential of tight sandstone gas in China from our research will be instructive for the future evaluation, prediction, and exploration of tight sandstone gas in China and abroad. [ABSTRACT FROM AUTHOR]

The natural gas components and geochemistry of 38 ultra-deep gas wells (burial depth greater than 6 000 m) in the Sichuan Basin were analyzed to determine the genesis of ultra-deep natural gas in the basin. The ultra-deep natural gas components of the basin have the following characteristics: Methane has an absolute advantage, which can be up to 99.56% with an average of 86.6%; ethane is low, with an average of 0.13%; there is nearly no propane and butane. So it is dry gas at over-mature thermal stage. The content of H 2 S can be up to 25.21%, with an average of 5.45%. The alkane gas isotopes are: the carbon isotope varies from −32.3‰ to −26.7‰ for methane and from −32.9‰ to −22.1‰ for ethane. There is nearly no carbon isotopic reversal among methane and its homologues. Hydrogen isotope varies from −156‰ to −114‰ for methane, and from −103‰ to −89‰ for some ethane. The carbon isotope of CO 2 varies from −17.2‰ to 1.9‰ and most of them fall within the range of 0±3‰. According to the δ 13 C 1 - δ 13 C 2 - δ 13 C 3 plot, except some wells, all other ultra-deep gas wells are dominated by coal-derived gas. Based on the CO 2 origin distinguishing plot and δ 13 C CO2 , except some individual wells, most of the ultra-deep CO 2 are of carbonate metamorphic origin. H 2 S in the ultra-deep layer of Longgang and Yuanba gas fields belongs to thermochemical sulfate reduction (TSR), while H 2 S from Well Shuangtan belongs to thermal Geochemical characteristics of ultra-deep natural gas in the decomposition of sulfides (TDS). [ABSTRACT FROM AUTHOR]

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There are oil and gas showing in the coal exploration, shaft building and exploitation in the southern Ordos basin, associated with oil type gas, which is different from authigenic coal reservoir. In order to verify this speculation, 220 gas samples are collected from coal seam,roof,floor and the gob and tested, including heavy hydrocarbon content C2+, methane carbon isotope δ13C1, methane hydrogen isotope δD and ethane carbon isotope δ13C2. The result shows that the origins of coalbed methane are mixed by coal-derived gas and oil type gas. By Schoell plate analysis,oil type mixed gas is composed by pyrolysis condensate gas, biogenic gas and oil-associated gas in coal seams and its surrounding rocks. The oil type gas migrated from Yanchang formation shale in Triassic system by faults and fissures,and which added the gas content in the coal reservoir. [ABSTRACT FROM AUTHOR]

The source of the Lower Paleozoic natural gas in the Ordos Basin has been a highly controversial issue. Using data obtained from newly drilled wells, we present the geochemical characteristics of Lower Paleozoic natural gas in various regions of the Ordos Basin. The Lower Paleozoic natural gas is dominated by hydrocarbons, of which methane accounts for 40.17%–97.24%, and the heavier gaseous hydrocarbon (C 2+ ) ranges from 0.01% to 9.72%. Non-hydrocarbon gases in the Lower Paleozoic gas reservoir are mainly CO 2 and N 2 , 2%–10% for CO 2 and 0.04%–48.56% for N 2 , higher than those in the Upper Paleozoic gas reservoir. This observation showed that the high CO 2 and N 2 content was related to the Ordovician marine carbonate rock deposits. The carbon and hydrogen isotopes of the methane, ethane, and propane in the Lower Paleozoic natural gas of the Ordos Basin showed various degrees of reversal, indicating the mixing origin of natural gas. In the central–eastern Ordos Basin, coal-derived gas from the Carboniferous–Permian (C-P) coal measure and oil-associated from the Lower Ordovician Majiagou Formation marine source rocks gas coexist, as well as mixtures of these two gases. For the Lower Paleozoic natural gas from the western margin of the basin, coal-derived gas coexist with oil-associated gas, but no mixing gas, in which coal-derived gas originated mainly from the C–P source rocks and oil-associated gas from the Lower Ordovician Kelimoli Formation and Middle Ordovician Wulalike Formation source rocks in the western margin of the basin. The Lower Paleozoic natural gas in the southern part of the Ordos Basin was mixing gas, and the oil-associated gas is the dominant one. The natural gas was originated from the Lower Ordovician Majiagou Formation source rocks in the central–eastern Ordos Basin and the Pingliang Formation mudstone and argillaceous limestone at the southern margin of the basin. [ABSTRACT FROM AUTHOR]

In order to determine the origin of Carboniferous-Permian (C-P) coal measure gas and Ordovician weathered crust gas, and their gas-filling modes, the chemical and isotopic compositions of the natural gas along with the co-produced groundwater from Paleozoic in the Yanchuannan (YCN) gas field of the Ordos Basin, China, were analyzed. It was found in this work that the gas in the upper gas group of weathered crust (O 1 m 5 1+2 ) reservoirs and the C-P gas had similar δ 13 C 1 and δ 13 C 3 compositions and they were less negative than those of the gas in the lower gas group (O 1 m 5 4 ). While all weathered crust gases had δ 13 C 2 values that are more negative than the C-P gases. The O 1 m 5 4 gas and C-P gas had a very clear boundary in δDCH 4 value, while the O 1 m 5 1+2 gas had the δDCH 4 ranges intersecting with the two others. The C-P gas is inferred derived from coal measures. The O 1 m 5 1+2 gas near erosional valley-form is a mixture of coal-derived gas and marine sapropelic gas, and the remaining Ordovician gases are inferred to originate from marine sapropelic source rock. Both gases were characterized by secondary cracking gas, and thermochemical sulfate reduction (TSR) altered the chemical and isotopic composition of weathered crust gas to a lesser extent due to short TSR reaction time. The changes in water type and total dissolved solids from the Permian to the Ordovician and the D shift phenomenon of O 1 m 5 1+2 water, reveals the fluid vertical migration between the Upper and Lower Paleozoic. Coupled with geological evolution analysis, the ancient erosional valley-form and the surrounding solution cavities and fractures are inferred the main migration channels for coal-derived gas into weathered crust reservoir. Moreover, three vertical gas-filling modes are proposed, i.e., gas seepage in free phase, diffusion phase and water-solution phase, with the importance in descending order. [ABSTRACT FROM AUTHOR]

The Dongpu Depression, located in the southwest of the Bohai Bay Basin, east China, is enriched in natural gas with proven reserves of about 480 billion m 3 . However, their genesis and distribution are not completely clear. On the basis of molecular, stable carbon isotope and noble gas isotope (Ar) composition of a large amount of gas samples and combined with the geological background, this paper analyzed the genesis and migration features of Paleogene gas from major gas fields in the Dongpu depression. The results show that Paleogene gases in the Dongpu Depression are of thermogenic origin, and can be divided into coal-derived gas, oil-associated gas and their mixture. Furthermore, Natural gases from the E 2 s 3 L and E 2 s 3 M are predominantly oil-associated gas generated from E 2 s 3 source rocks, while those from E 2 s 4 , and E 2 s 3 U and E 2 s 2 in the Baimiao area are derived from the Carboniferous-Permian coaly source rocks. In addition, the coal-derived gas can also mix with the oil-associated gas, which can be called mixed coal-derived gas and oil-associated gas. Carbon isotope data suggest that gases of E 2 s 3 and most E 2 s 2 are predominately unaltered, while those of E 2 s 4 and several E 2 s 2 mostly from Baimiao area are altered and undergoing strong dissipation. The geographical variation in the gas composition can be attributed to the geochemical fraction caused by vertical migration through the basement faults for the coal-derived gas in E 2 s 4 and several E 2 s 3 U and E 2 s 2 in Baimiao area. While the oil-associated gas in E 2 s 3 L and E 2 s 3 M are resulted from the near-source or even in situ accumulation. [ABSTRACT FROM AUTHOR]

Volcanic reservoirs are extensive in the Songliao Basin and mainly include intermediate-basic rocks in the northern part, intermediate-acidic rocks in Xujiaweizi in the southern part, and acidic rocks in the Jinglin block. The natural gas in the volcanic reservoirs of the Songliao Basin has a wide range of compositions, with alkanes being dominant in most cases, although carbon dioxide is dominant in some wells. Generally, the gas in the volcanic rocks near deep faults has high contents of carbon dioxide, whereas the natural gas in volcanic rocks far from faults has low carbon dioxide contents. The gas in the volcanic reservoirs is of multiple origins, including abiogenic gas of probable mantle origin (generally found in wells with high carbon dioxide contents) and organic gas mainly derived from organic matter in the basin. The abiogenic alkanes have δ3C values in the order of δ13C1>δ13C2>δ13C3>δ13C4, which is opposite that of alkanes of organic origin. The 3He/4He ratios of the fluid inclusions from the volcanic reservoirs range from 0.286x10-6 to 7.33x10-6, with an average of 2.48x10-6, and the R/Ra ratios range from 0.26 to 5.24, with most values being greater than 1.0, indicating mixed origins of noble gases from the crust and the mantle. The gas in fluid inclusions from the volcanic reservoirs has δ13C1 values ranging from -17.1 to -28.7% (PDB), δ13C2 values ranging from -23.4 to -32.4% (mostly approximately -25%), and δ13Cco2 values ranging from -10.97 to -21.73%, which are significantly different from the isotopic compositions of the gas in the present reservoirs, suggesting that some abiogenic alkanes may have been charged into the reservoirs during the geologic history of the basin. The early charged CO2 is mainly organic in origin, while the abiogenic CO2 was charged during the main accumulation period, producing a mix of origins for the gas in the volcanic reservoirs of the Songliao Basin. The abiogenic alkanes, He, and CO2 in the natural gas indicate the addition of some abiogenic gas to the gas. According to the relationship between the distribution and attitude of volcanic rocks and faults, we found that the abiogenic gas reservoirs are located near fault zones, whereas the organic and mixed gas reservoirs are located far from fault zones. The geochemical study of natural gas is helpful in determining the origin and spatial distribution patterns of gas in deep volcanic reservoirs and for directing further gas exploration in the Songliao Basin. [ABSTRACT FROM AUTHOR]

This work discussed the origins, alteration and accumulation processes of the oil and gas in the Kekeya gas condensate field based on molecular compositions, stable carbon isotopes, light hydrocarbons, diamondoid hydrocarbons and biomarker fingerprints. A comparison study is also made between the geochemical characteristics of the Kekeya hydrocarbons and typical marine and terrigenous hydrocarbons of the Tarim Basin. Natural gas from the Kekeya gas condensate field is derived from Middle-Lower Jurassic coal measures while the condensates are derived from Carboniferous-Permian marine source rocks with a higher maturity. In the study area, both natural gas and condensates have experienced severe water washing. A large amount of methane was dissolved into the water, resulting in a decrease in the dryness coefficient. Water washing also makes the carbon isotopic compositions of the natural gas more negative and partially reverse. Considering that the gas maturities are higher than once expected, gas generation intensity in the study area should be much stronger and the gas related to the Jurassic coal measures could promise a greater prospecting potential. As a result of evaporative fractionation, the Kekeya condensates are enriched in saturates and lack aromatics. Evaporative fractionation disguises the original terrigenous characteristics of the light hydrocarbons associated with the natural gas, making it appear marine-sourced. Thus, alteration processes should be fully taken into consideration when gas-source correlations are carried out based on light hydrocarbons. With the condensates discovered in the study area all being 'migration phase', the pre-salt Cretaceous and Jurassic reservoirs may promise great exploration potential for the 'residual phase' hydrocarbons. This research not only is of significance for oil and gas exploration in the southwest Tarim Basin, but also sheds light on the oil/ gas-source correlations in general. [ABSTRACT FROM AUTHOR]

Abiotic synthesis through carbon dioxide (CO 2) reduction may provide the prebiotic organic compounds needed for the emergence of life, whereas petroleum geologists pay more attention to whether abiotic gas generated by purely chemical reactions plays a prominent role in the formation of industrial gas pools like biotic gas. The Cretaceous reservoir associated with volcanic rocks of the Songliao Basin in northeastern China is suspected of possessing abiotic gas due to some abnormal geochemical characteristics, such as abundant 13C-enriched CO 2 , 13C-enriched CH 4 , 13C-depleted C 2 H 6 , and an isotopic trend of decreasing 13C content with growing carbon number. To understand further the genesis of deep natural gas, we examined the molecular and isotopic compositions of C 1 –C 4 alkanes, CO 2 , and light hydrocarbons (LHs) of equal number (four) of gas samples collected from the Yingcheng (K 1 yc) volcanic and Denglouku (K 1 d) clastic reservoirs in the Changling (CL-I) gas field. Compared with the products of abiotic polymerization composed of 13C-depleted straight-chain alkanes, various 13C-enriched LH isomers that are highly compatible with the molecular and isotopic compositions of coal-derived thermogenic gas are confirmed in our samples, such as 2-methylhexane (2-MH), 2,3-dimethylpentane (2,3-DMP), cyclohexane (CH), methylcyclohexane (MCH), and other branched-chain alkanes or cycloalkanes. The striking invariable K 1 ratio and resemblance of isotopic curves of LHs further indicate that the natural gas in different reservoirs is derived from underlying homologous source rocks, regardless of the variation of CO 2 content (K 1 yc > 15% and K 1 d < 1%). Combining geological conditions, we propose that C 1 –C 4 and LHs that have no inherent correlation with the concentration of CO 2 , which is predominantly derived from mantle degassing, are generated from the thermal decomposition of 13C-enriched type III organic matter during burial and diagenesis rather than sourced from mantle degassing, abiotic polymerization, or mixing of thermogenic gas. In addition, we also suggest that the geochemical anomalies consisting of 13C-enriched methane and 13C-depleted ethane are the products of the chemical mechanisms controlled by high temperature, and mantle-derived non-hydrocarbon gas (e.g., CO 2 and He) are simply added to thermogenic gas. • Light hydrocarbons associated with natural gas are used to elucidate the gas origin. • CO 2 predominantly sourced from mantle degassing is simply added to thermogenic gas. • LHs are derived from homologous humic organic matter rather than abiotic processes. • Isotopic anomaly of natural gas is caused by coal-derived gas in over-mature stage. [ABSTRACT FROM AUTHOR]

The Yan’an gas field in the Ordos Basin, China, was discovered in 2014 and contains reserves of up to 100 billion m 3 . Its occurrence in the basin contradicts the distribution model of “gas in the north, oil in the south”. Upper Paleozoic natural gas in the Yan’an area is dominated by methane, with an average dryness coefficient (C 1 /C 1–4 , %) of 99.5% (δ 13 C 1 = −30.8 to −27.5‰), with minor amounts of CO 2 = 3.8%, N 2 = 0.5%, and ethane (δ 13 C 2 = −37.2 to −30.5‰) and traces of higher hydrocarbons. The gas is predominantly coal-derived originating from Carboniferous–Permian humic hydrocarbon source rocks and minor oil-type gas from sapropelic source rocks. The carbon isotope series includes reversals, with δ 13 C 2 < δ 13 C 1 and rare δ 13 C 3 < δ 13 C 2 < δ 13 C 1 . The enriched δ 13 C 1 value is a result of the extremely high thermal maturity, while the depleted δ 13 C 2 value cannot be explained solely by the simple addition of oil-type gas. The complex carbon isotope reversals result from the combined effects of several secondary reactions under extremely high thermal maturity. [ABSTRACT FROM AUTHOR]

Natural gases from the Qiudong and Hongtai gas fields in the Taibei sag in the Turpan-Hami Basin of China were analyzed for stable hydrogen and carbon isotopic characteristics, to investigate the origin of the gas and the gas-source correlation. Natural gases from the two gas fields are dominated by alkane gases with methane content varying from 81.61% to 92.57%. Dryness ranges from 0.76 to 0.93, indicating an abundance of wet gas. The carbon and hydrogen isotopic compositions of the methane vary from − 39.8‰ to − 42.6‰ and from − 263‰ to − 279‰ in the Qiudong gas field, respectively, and from − 37.7‰ to − 39.0‰ and from − 250‰ to − 257‰ in the Hongtai gas field, respectively. All gases have a characteristic carbon and hydrogen isotopic distribution pattern among the C 1 –C 5 alkanes, in which δ 13 C CH 4 < δ 13 C C 2 H 6 < δ 13 C C 3 H 8 < δ 13 C C 4 H 10 < δ 13 C C 5 H 12 and δ 2 H CH 4 < δ 2 H C 2 H 6 < δ 2 H C 3 H 8 . Both the stable carbon and hydrogen isotopic compositions of the gases and the hydrogen isotopic value of the formation water suggest that gases in the two gas fields are coal-derived without admixtures with gases of other origin. According to the calculation with empirical δ 13 C CH 4 –R o % relationship, the thermal maturity (vitrinite reflectance R o %) of the source rocks is up to 0.76% in the Qiudong area and up to 0.86% in the Hongtai area, which are in the oil window and approaching the start of major thermogenesis at the top of the maturity range. Since the gases in the Hongtai gas field are relatively more enriched in 13 C and 2 H, combined with the local geological thermal evolution history, it is concluded that the gases in the two gas fields were derived from the Lower Jurassic Badaowan source rocks. The heavier hydrogen and carbon isotopes of the gases from the Hongtai gas field are interpreted as a product of higher thermal maturity of the source rocks. This is in consistent with the thermal maturity of the local source rocks. The modern thermal maturity of the top of the Badaowan Formation is around 0.8% in the Qiudong area and around 1.0% in the Hongtai area. The combined use of both carbon and hydrogen isotopes is of high value in providing information on gas origin and gas-source correlation, which will be very useful for future gas exploration. [ABSTRACT FROM AUTHOR]

The Zizhou gas field is a large, tight sandstone gas field in the Ordos Basin, China. Based on the analysis of the geological background, gas components, light hydrocarbon composition, carbon and hydrogen isotope values of alkane gases, and the geochemical correlation between gases of the Zizhou gas field and analogs from other fields in the basin, we conclude that : (a) most natural gases in the Zizhou gas field are dry with a small content of wet gas; (b) the gases have similar carbon isotopic and light hydrocarbon compositions to the analogs from other fields, as well as much more positive δ 13 C values and high contents of C 5–7 i -alkanes and methylcyclohexanes, indicating typical coal-derived gas; (c) carbon isotopic reversal in some gas samples is due to mixing of coal-derived gases of different maturities; (d) the gases are sourced from mature to high-mature Carboniferous–Permian coal-measures; and (e) the gas reservoirs represent continuous accumulations. [ABSTRACT FROM AUTHOR]

Coal-derived, large scale gas fields derived from the Mesozoic coal measures in China are mainly distributed in the Middle-Lower Jurassic coal measures in the Tarim, Junggar and Turpan-Hami basins in northwest China, and the Upper Triassic Xujiahe Formation coal measure in Sichuan Basin, central China. In 2011, the annual production was 21.6 × 109 m³ and the proved geological reserves were 2485 × 109 m³, accounting for 21% and 30% of the total in China, respectively. Based on analyses of gas composition and stable carbon isotopes ratios of 203 samples and stable carbon isotopes of 102 CO2 samples, the following conclusions were made. (a) Based on diagnostic plots using the stable carbon isotopic and molecular composition of gas samples, alkane gas from the Mesozoic coal measures in China is shown to be coal-derived. (b) According to the δ13C2 > vs. C2H6 plot of a great number of oil-derived and coal-derived gases in China, it is concluded that gases with δ13C2 > > -28.5‰ are coal-derived and those with δ13C2 > < -28.5‰ are oil-derived in most cases. (c) Among the natural gases from the Mesozoic coal measures in China, primary coal-derived gases with normal carbon isotopic distribution pattern among the C1-C4 alkanes (i.e. δ13C1 < δ13C2 > < δ13C3 < δ13C4) are dominant. (d) Carbon isotopic pattern reversal mainly results from the mixing of coal-derived gases having different maturities but the same source and secondly from microbial oxidation of propane (e.g. Mu 3 and Mu 4 wells in the Gumudi gas field, Junggar Basin). (e) CO2 in the coal-derived gases from the Mesozoic coal measures in China has both biogenic and abiogenic origins. The biogenic origin is dominant and the abiogenic CO2 is mainly found in the Kuqa Depression in the Tarim Basin and western Sichuan Basin. (f) Isotopic differences between heavy hydrocarbon gases and methane become less with increasing maturity. [ABSTRACT FROM AUTHOR]

By the end of the year 2010, a total of 15 large tight gas fields have been found in China, located in the Ordos, Sichuan, and Tarim basins. The annual production and total reserves of these fields in 2010 were 222.5×10 and 28657×10 m, respectively, accounting for 23.5% and 37.3%, respectively, of the total annual production and reserves of natural gases in China. They took a major part of all natural gas production and reserves in China. According to the analyses of 81 gas samples, the stable carbon and hydrogen isotopic compositions of tight gases in China have following characteristics: (1) Plots of δC- δC- δC, δC-C/C and δC- δC demonstrate the coal-derived origin of tight gases in China; (2) For the primary alkane gases, both carbon and hydrogen isotopic values increase with increasing molecular mass, i.e., δC< δC< δC< δC and δH< δH< δH; (3) The isotopic differences of δC- δC, δC- δC, δH- δH and δH- δH decrease with increasing R (%) and C/C; (4) There are seven causes for the carbon and hydrogen isotopic reversal, however, the carbon and hydrogen isotopic reversal of tight gases in China is caused mainly by multiple stages of gas charge and accumulation. [ABSTRACT FROM AUTHOR]

Abstract: Natural gases from the Kuqa depression, Lunnan and Tazhong low uplifts in the Tarim basin of China were analyzed for stable hydrogen and carbon isotopic characteristics, to compare the isotopic differences, particularly of hydrogen, between coal-derived and oil-derived gases. Gases from the Kuqa depression are coal-derived and sourced from Middle–Lower Jurassic coal measures, possibly with a minor contribution from Triassic lacustrine mudstones. Gases from the Lunnan and Tazhong low uplifts are thought to be sourced from Cambrian–Lower Ordovician marine sapropelic organic matter. Natural gases from the Lunnan low uplift and Ordovician strata in the Tazhong low uplift are oil-cracked, and gases from the Carboniferous and Silurian strata in the Tazhong low uplift are oil-associated. Most gases have expected hydrogen isotopic distributions among C1–C3 alkanes, i.e., δ2HCH4 <δ2HC2H6 <δ2HC3H8. However, nearly all samples that show carbon isotopic reversal between C1 and C2 alkanes (δ13CCH4 >δ13CC2H6) in the Lunnan low uplift also have corresponding hydrogen isotopic reversal (δ2HCH4 >δ2HC2H6) between these alkanes. In the Tazhong low uplift, except for the Silurian gases, nearly all the Carboniferous and Ordovician gases have regular carbon isotopic distribution pattern among C1–C2 alkanes (δ13CCH4 <δ13CC2H6) but with hydrogen isotopic reversal between C1 and C2 alkanes (δ2HCH4 >δ2HC2H6). This probably indicates that except the influences of mixing with gases at different maturity, hydrogen isotopes of natural gases are also sensitive to other processes such as hydrogen exchange between water and organic matter during thermal maturation. No analytical differences in δ2HCH4 have been found between coal-derived gases sourced from continental humic organic matter in the Kuqa depression and the oil-derived gases sourced from the Carboniferous and Silurian marine sapropelic organic matter in the Tazhong low uplift. All coal-derived gases in the Kuqa depression have δ2HCH4 values less than −150‰, while the oil-cracked gases in the Lunnan low uplift and the Ordovician oil-cracked gases in the Tazhong low uplift have slightly greater δ2HCH4 values (>−150‰). However, the Silurian and Carboniferous oil-associated gases in the Tazhong low uplift also have quite low δ2HCH4 values (around −160‰). The clearly different hydrogen isotopic values of gases from different strata in the Tazhong low uplift and the close hydrogen isotopic values between gases from the Kuqa depression and Silurian and Carboniferous strata in the Tazhong low uplift, all imply that hydrogen isotopes of alkane gases are influenced by factors more than thermal maturity and depositional environments. Great care needs to be paid when using the hydrogen isotopes for gas genetic study, and a combination with local geological settings and other geochemical features is suggested. This is because hydrogen isotopes of alkane gases in one basin may be not applicable in the other basins. [Copyright &y& Elsevier]

Abstract: Tight sandstone gas belongs to unconventional natural gas. There is no unanimous agreement about the source of tight sandstone gas source. Both the Yuanba and Tongnanba gas fields in the Sichuan Basin produce tight sandstone gas from Xujiahe Formation gas reservoirs. The geochemical data such as the carbon and helium isotopes of the 14 gas samples from the Xujiahe Formation of 9 wells in the Yuanba gas field show that the gas mainly consists of coal-derived gas from the self-source type, oil-type gas from other source, and a small amount of abiogenic gas, suggesting combination gas sources. The similar geochemical data of 5 gas samples from the Xujiahe Formation of 5 wells in the Tongnanba gas field indicate that the gas consists of oil-type gas from other source, accompanied by a small amount of abiogenic gas, also being of mixed origins. Therefore the Xujiahe Formation tight sandstone gases of the Yuanba and Tongnanba gas fields do not come from coal measure alone. They have a variety of sources, mainly the coal-derived gas or oil-type gas of organic origin or the mixture of them. [Copyright &y& Elsevier]

Abstract: To investigate the ability of exogenous bacteria to degrade brown coal, methanogens were enriched from anaerobic sludge and domesticated using brown coal as carbon source. After domestication, the lag time of initial gas production is shortened from 12 to 6 days and the CH4 production increased by 29.2% in 30 days. The generated biogas is composed of CH4 and a little CO2, no heavy hydrocarbons are detected. Experiments on gas production influencing factors demonstrate that the best initial pH for the culture medium is 7.0 and the maximum gas production is 1.9 times and 2.4 times higher than that at pH 6.4 and pH 7.4, respectively. The particle size of coal is one of factors influencing the gas production: the general trend is the smaller the particle size, the bigger the gas production, but the variation of gas production is not significant with decreasing particle size. Gas produced by the culture medium accounts for around 50% of the total gas production and it is likely caused by the addition of L-cysteine (0.5 g/L) and yeast extract (1 g/L) to the medium. [Copyright &y& Elsevier]

Abstract: Tight gas reservoirs refer to the tight sandstone fields or traps accumulating natural gas of commercial values. According to reservoir characteristics, reserves, and structural height, they can be divided into two types, continuous-type and trap-type: the former are located at the lower part of the structure and have indistinct trap boundaries, inconsistent gas-water boundaries and reversal of gas and water, and their reservoirs are the same as or near the source; the latter are located at the higher part of the structure, with gas above water in traps, low reserves, and relatively high production. Tight gas in China is all coal-derived, dominantly alkane gases (C1–4), in which the amount of methane is greatest and the alkane gases have positive carbon isotopic series. The content of non-hydrocarbon gases (mainly CO2 and N2) is low. At the end of 2010, the reserves and annual production of tight gas in China accounted for 39.2% and 24.6% of the total natural gas, respectively, and the proportions are expected to increase. Compared to the shale gas and coalbed gas, tight gas should be considered in priority in the exploration and exploitation of unconventional gas in China. [Copyright &y& Elsevier]

Abstract: Hydrocarbon-generation kinetics is of applicable significance in the study of coal-derived gas pool formation. Its research progress in recent years includes: in combination with geothermal history, it is used as a tool to evaluate source rocks dynamically and depict the change of source kitchen with geological time; according to the carbon isotope fractionation kinetics, a more precise model of identifying coal-derived gas is developed, which contains the kinetic relationship among gas compounds; the combination of compositional kinetics and carbon isotopic kinetics leads to the study method of accumulation processes of coal-derived gases, which is constrained precisely by multi kinetic indices; with the reservoir records, generation kinetics is used to study the whole gas life including generation, charge and alteration. The problems that need investigation are generation kinetics of coal-derived gas at low maturity, pressure effects on gas generation, minerals and water influences, and kinetics of residual soluble organic matter at high maturity. [Copyright &y& Elsevier]

Abstract: Since its formation in China 30 years ago, the theory of coal-derived gas has not only opened a new field of coal-derived gas exploration, but also increased the proved reserves of coal-derived gas from 1/10 to 7/10 in the total gas of China; the nation''s proved gas reserves rise sharply from 2 264×108 m3 to 6.4×1012 m3; the annual production of gas rises from 137.34×108 m3 to 760×108 m3; and the nation''s coal-derived gas fields (reservoirs) increases from less than 10 (excluding Taiwan) to 124. After briefing the major developments of China''s coal-derived gas exploration, this paper concentrates on the exploration development of coal-derived gas in Ordos Basin, Tarim Basin and Sichuan Basin where large numbers of coal-derived gas fields have been discovered. With carbon isotopic data of alkane gases, it also demonstrates that some gas fields of controversial origin (Jingbian, Kekeya and Akemomu gas fields) are of, or predominantly of, coal-derived gas, and that a few gas reservoirs (Wolonghe, Naxi, Hejiang gas fields) in the Xujiahe Formation coals of Sichuan Basin are oil-associated gas. [Copyright &y& Elsevier]

Abstract: This study focuses on the direct sulfur recovery process (DSRP), in which SO2 can be directly converted into elemental sulfur using a variety of reducing agents over Ce1−x Zr x O2 catalysts. Ce1−x Zr x O2 catalysts (where x =0.2, 0.5, and 0.8) were prepared by a citric complexation method. The experimental conditions used for SO2 reduction were as follow: the space velocity (GHSV) was 30,000ml/g-cat h and the ratio of [CO (or H2, H2 +CO)]/[SO2] was 2.0. It was found that the catalyst and reducing agent providing the best performance were the Ce0.5Zr0.5O2 catalyst and CO, respectively. In this case, the SO2 conversion was about 92% and the sulfur yield was about 90% at 550°C. Also, a higher efficiency of SO2 removal and elemental sulfur recovery was achieved in the reduction of SO2 with CO as a reducing agent than that with H2. In the reduction of SO2 by H2 over the Ce0.5Zr0.5O2 catalyst, SO2 conversion and sulfur yield were about 92.7% and 73%, respectively, at 800°C. Also, the reduction of SO2 using synthetic gas with various [CO]/[H2] molar ratios over the Ce0.5Zr0.5O2 catalyst was performed, in order to investigate the possibility of using coal-derived gas as a reducing agent in the DSRP. It was found that the reactivity of the SO2 reduction using the synthetic gas with various [CO]/[H2] molar ratios was increased with increasing CO content of the synthetic gas. Therefore, it was found that the Ce1−x Zr x O2 catalysts are applicable to the DSRP using coal-derived gas, which contains a larger percentage of CO than H2. [Copyright &y& Elsevier]

Abstract: The hydrogen chloride removal characteristics of molded sodium-based sorbents containing sodium aluminate (NaAlO2) were examined in simulated hot coal-derived gas. The reaction rate and absorbing capacity of the molded sorbent were lower in comparison to the particle sorbent, and the ability to remove hydrogen chloride to below 1 ppm lasted only a short time. For improved reactivity of molded sorbent, γ-alumina powder was added as supporting material in the process of NaAlO2 formation. This sorbent had a higher surface area, reaction rate and absorbing capacity, and the removal ability to below 1 ppm was improved. Reaction modeling of the sodium-based sorbents indicated that the cross-flow type reactor was suitable for continuous removal of hydrogen chloride in large-scale cleanup systems for coal-derived gas. [Copyright &y& Elsevier]

Abstract: A new method is described for on-line extraction of complex mixtures of fluid inclusion volatiles for compound specific carbon- and hydrogen-isotope determination by gas-chromatography–continuous flow mass-spectrometry. Reproducibility for multiple aliquots of gas released from single samples and for duplicate samples is generally ≤±0.7‰ for δ 13C measurements of hydrocarbon and CO2 gases to concentrations as low as 20 nmol and ≤±7‰ for δD measurements of hydrocarbon gases to concentrations as low as 100 nmol. This approach significantly improves upon the detection limits for trace gas analysis in fluid inclusions and enables rapid stable isotope measurements of multiple gas species in a single run. Two examples are evaluated and discussed in their geological context. First, stable isotope results for CH4, CO2 and C2–C5 fluid inclusion gases hosted in the undersaturated peralkaline igneous complex of Khibina in northwest Russia indicate that CH4 was the juvenile magmatic gas phase (δ 13C=−13 to −8‰, δD=−120 to −50‰). The δ 13C values of the C2–C5 hydrocarbons are lower relative to CH4 (by up to 12‰), confirming their abiogenic origin. The δ 13C values of CO2 are consistently lower than CH4 (by up to 9‰) and the δD values of C2H6 are 70–100‰ lower than the δDCH4 values. This pattern indicates that the CO2 and higher hydrocarbons were generated during sub-solidus, kinetic CH4-oxidation fractionation and polymerisation reactions. Second, CH4, CO2 and C2H6 fluid inclusion gases were analysed from quartz veins associated with anthracite from the Nant Helen mine in the south Wales coalfields. The δ 13C values of CH4 (−27.9±0.1‰), CO2 (−3.1±0.8‰) and C2H6 (−20.7±0.1‰) are typical of mature thermogenic coal-associated natural gases, as is the δDCH4 values (−148±1‰). Assuming equilibrium, the Δ13CCO2–CH4 value (+24.8‰) suggests that temperatures may have reached ∼300 °C in the anthracite zone. [Copyright &y& Elsevier]

Zn–Mn based sorbents supported on SiO2, γ-Al2O3 and ZrO2, prepared by the incipient wetness impregnation method with calcination at 973K were investigated for the removal of H2S from coal derived gas at the temperature ranges of 773–973K. Results reveal that the SiO2 and ZrO2 supports exhibit the better performance because better removal efficiency. The addition of manganese effectually improves the vaporization of zinc. In addition, some operating parameters were also considered in order to understand as well as screen the suitable conditions for the development of Zn–Mn based sorbents on the removal of H2S. Over 98% sorbent utilization was established for the use of SiO2 at 873K. On the other hand, within the 5–15wt% of Zn–Mn oxides, no significant change in the sorbent utilization was observed. Up to 30wt% the sorbent utilization decreased slightly compared to lower contents, which may be attributed to the deficient dispersion. With increasing the H2 concentration, the sorbent utilization decreases and an adverse result is observed in the case of increasing CO concentration. The relationship between CO and H2 could be explained via the water–gas shift reaction. Moreover, the apparent activation energy and frequency factor as well as the predicted results were studied with a deactivation model. The results of regression fitting reveal the accurate prediction breakthrough behaviors for the removal of H2S. [Copyright &y& Elsevier]