Hydrogen Isotopic Composition Of Methane Research Articles

Studying the isotopic composition of microbial methane with a genetically-tractable methanogen

Nearly all biogenic methane is produced by a group of microorganisms called methanogenic archaea (or methanogens). Methanogens can use a variety of substrates, such as H2 + CO2, acetate, and methylated compounds, for methanogenesis. Previous studies have shown that the stable carbon and hydrogen isotopic compositions of methane produced by methanogens can vary drastically depending on the substrate composition and concentration in the environment. For instance, the concentration of H2 in the environment has a substantial impact on the isotopic composition of methane derived from hydrogenotrophic methanogenesis (reduction of CO2 to methane using H2 as the electron donor) (Valentine et al. 2004, Penning et al. 2005). While there is substantial empirical data on isotopic signatures of methane from different substrates and under different conditions, the physiological and molecular features that control these values are not as well understood. To address this, we are using the metabolically diverse and genetically tractable methanogen, Methanosarcina acetivorans as a model system to uncover key cellular processes that control the stable bulk isotopic composition of methane (i.e., 13C/12C and D/H ratios), and the distributions of the “clumped” 13CH3D and 12CH2D2 isotopologues. The methanogen M. acetivorans grows on a wide variety of compounds such as acetate, methanol, methylamines, and methylsulfides. We found that the methylotrophic pathways (for methanol and trimethylamine) and the aceticlastic pathway have large and similar primary hydrogen isotopic effects (α of ~0.45). These data are in contrast to previous findings and imply a minor isotopic exchange between CH4 and H2O (Valentine et al. 2004, Gruen et al. 2018). Focusing first on the methylotrophic pathway, we generated mutants of two key enzymes in the methylotrophic pathway: a) methyl coenzyme M reductase (Mcr) that catalyzes the last step in methanogenesis and b) methyltransferases that catalyze the first step in methylotrophic methanogenesis from methanol (Mta). A mutant with reduced Mcr expression had no observable change in the hydrogen isotopic effect relative to the wild-type, validating the initial observation of minimal H2O-CH4 hydrogen isotopic exchange. One of the Mta mutants, which only expressed a specific methyltransferase isoform, had a smaller carbon isotopic effect relative to the other isoforms (α of ~1.074 vs. ~1.080). Since the isoforms are thought to be identical in structure, the different isotopic effects could result from differential expression of each isoform, or from different kinetic properties. By combining our genetic approaches with traditional and high-resolution isotopic analytical methods, we aim to develop a quantitative understanding of the mechanisms that control the isotopic compositions of biological methane. Our preliminary results show that M. acetivorans would be an ideal candidate for such research, which could help in understanding methanogens’ physiology in natural environments in past, present, and future Earth.

Experimental determination of hydrogen isotope exchange rates between methane and water under hydrothermal conditions

The hydrogen isotopic composition of methane (CH4) is used as a fingerprint of gas origins. Exchange of hydrogen isotopes between CH4 and liquid water has been proposed to occur in both low- and high-temperature settings. However, despite environmental evidence for hydrogen isotope exchange between CH4 and liquid water, there are few experimental constraints on the kinetics of this process. We present results from hydrothermal experiments conducted to constrain the kinetics of hydrogen isotope exchange between CH4 and supercritical water. Seven isothermal experiments were performed over a temperature range of 376–420 °C in which deuterium-enriched water and CH4 were reacted in flexible gold reaction cell systems. Rates of exchange were determined by measuring the change in the δD of CH4 over the time course of an experiment. Regression of derived second order rate constants (kr) vs. 1000/T (i.e., an Arrhenius plot) yields the following equation: ln(kr) = −17.32 (±4.08, 1 s.e.) × 1000/T + 3.19 (±6.01, 1 s.e.) (units of kr of sec−1 [mol/L]−1), equivalent to an activation energy of 144.0 ± 33.9 kJ/mol (1 s.e.). These results indicate that without catalysts, CH4 will not exchange hydrogen isotopes with liquid water on a timescale shorter than the age of the Earth (i.e., billions of years) at temperatures below 100–125 °C. Exchange at or below these temperatures is thought to occur due to the activity of life, and thus hydrogen isotopic equilibrium between methane and water may be a biosignature at low temperatures on Earth (in the present or the past) and on other planetary bodies. At temperatures ranging from 125 to 200 °C, hydrogen isotope exchange between CH4 and liquid water can occur on timescales of millions to hundreds of thousands of years, indicating that in thermogenic natural gas systems CH4 may isotopically equilibrate with water and achieve equilibrium isotopic compositions. Finally, the kinetics indicate that in deep-sea hydrothermal systems, the hydrogen (and thus clumped) isotopic composition of CH4 is likely set by formation and/or storage conditions isolated from the active flow regime. The determined kinetics indicate that once methane is entrained in circulating fluids, the expected time-temperature pathways are insufficient for measurable hydrogen isotope exchange between CH4 and water to occur.

Molecular and isotopic compositions of hydrocarbon gases generated from anomalously 13C-enriched sapropelic sedimentary organic matter in lacustrine basins

For lacustrine basins, the enrichment of 13C in sedimentary organic matter is closely related to the growth of plankton biological productivity. Consequently, there are many cases where the sapropelic organic matter is more 13C-enriched than the humic organic matter. However, the geochemical characteristics of the pyrolysis products produced by 13C-enriched lacustrine sapropelic organic matter are less discussed. Herein, two lacustrine sapropelic kerogen samples with remarkably different carbon isotopic ratios (–23.2‰ and –28.4‰) and a coal sample (–25.6‰) were performed by a closed and dry pyrolysis system at temperatures ranging from 300 °C to 600 °C. The results indicated that the carbon isotopic compositions of hydrocarbon gas products were significantly affected by the initial carbon isotopic composition of their sources. At similar simulated temperatures, the carbon isotopic compositions of methane generated from the 13C-enriched sapropelic sample and the coal sample were more consistent. However, owing to the difference in their gas-generation mechanism, the natural gas generated from the coal sample had a more 13C-enriched isotopic composition of ethane (δ13C2) and a higher dryness, particularly at higher maturity levels. The hydrogen isotopic composition of methane (δ2HC1) was markedly enriched in deuterium as the thermal maturity increased and may have been independent of the genetic type and salinity. Therefore, we used the “Dryness vs. δ2HC1” and “δ13C2 vs. δ2HC1” diagrams to confirm the genetic type of natural gas in lacustrine basins.

Constraints of molecular and stable isotopic compositions on the origin of natural gas from Middle Triassic reservoirs in the Chuanxi large gas field, Sichuan Basin, SW China

Abstract The Middle Triassic Leikoupo Formation in the Sichuan Basin is a gypsum-bearing stratum. The Chuanxi (CX) large gas field has been recently discovered in Middle Triassic reservoirs, with a controversial gas source. The gas geochemistry is analyzed in this study to investigate the gas origin and source. The H2S-bearing dry gas in the CX gas field has an H2S content ranging from 1.07% to 4.09%, with a gas souring index (GSI) of 0.0120–0.0438. The δ13C1, δ13C2, and δ2HCH4 values range from –31.8 to –30.4‰, from –32.9 to –29.6‰, and from –153 to –136‰, respectively. Genetic identification based on the geochemical characteristics suggests that the gas is an oil-type and derives from the secondary cracking of oil. According to the GSI and alkane carbon isotopes, the gas experienced weak TSR alteration without ethane. Potential high-quality source rocks include the Upper Permian mudstones and marlstones, as well as the Lower Cambrian mudstones, with a total organic carbon content generally higher than 2.0%. A comparison of the carbon isotopic compositions between ethane in the gas and kerogen and the hydrogen isotopic compositions of methane implies that natural gas has an affinity with the Upper Permian rather than the Lower Cambrian source rocks. The extremely low development of solid bitumen in the reservoirs suggests the inexistence of large-scale paleo-oil pools in the Leikoupo Formation. The Middle Triassic natural gas in the CX gas field originates from the Upper Permian Longtan Formation, where the gas pools are dominated by direct filling of secondary cracking gas.

Tracing the sources and evolution processes of shale gas by coupling stable (C, H) and noble gas isotopic compositions: Cases from Weiyuan and Changning in Sichuan Basin, China

The source and thermal evolution history of organic matter for the Longmaxi shale are still debated. This study analyzed the molecular and stable carbon isotopic compositions of hydrocarbons (CH4, C2H6, and C3H8) and CO2 as well as the stable hydrogen isotopic compositions of methane, ethane, and noble gases (He, Ne, Ar, Kr, and Xe). Shale gases in the WY and CN areas show an extremely-low-wetness with CH4 concentrations range from 93.41% to 99.01%. Non-hydrocarbon gases are mainly N2 (0.22%–2.81%) and CO2 (0.03%–1.35%). H2S have not been detected. Different δ13C1 and δ13C2 values in WY and CN shale gases (WY: −37.3‰ to −35.0‰ and −40.3‰ to −38.3‰, CN: −29.8‰ to −26.3‰ and −35.3‰ to −32.7‰) and various carbon isotope-composition distribution patterns (δ13C1>δ13C2<δ13C3 and δ13C1>δ13C2>δ13C3) of hydrocarbons indicate a complex evolution process. WY shale gases include more oil-cracking gas than CN shale gases, suggesting WY shale gases more like come from Type I-II organic matter. In shale gas systems, methane content and δ13C1 ratios vary with the degree of thermal evolution, so the origin of shale gas cannot be determined using carbon isotope data alone. The wide range of δ13CCO2 values (−8.9‰ to −0.8‰) and N2/40Ar ratios (20.8–165.1) suggests multiple origins of the gases. Emeishan mantle plume provides the source of heat for some thermo-genic gas. Noble gas isotopic compositions (3He/4He: 0.001Ra to 0.019Ra) indicate air and crustal origins with no significant contribution from the mantle. 40Ar/36Ar ratios (1194.3–4604.5) are consistent with the age of Longmaxi strata calculated by accumulative effect of Ar isotope. The shale gas humidity, carbon isotope ratios, and the carbon isotope-composition distribution patterns may contain information indicating the shale gas sweet spot.

Recent, deep-sourced methane/mud discharge at the most active mud volcano in the western Mediterranean

Active mud volcanism in the West Alboran Basin (WAB) is closely associated with tectonically mobilized, overpressurized shales and shale-diapirism. This appears to control mud expulsion at Carmen mud volcano, a cone-shaped structure 65 m high and 1 km in basal diameter.The presence of gas-rich mud breccia, living chemosynthetic fauna, the absence of hemipelagic draping and the abrupt transition that occurs between high dissolved sulfate in the uppermost interval and low sulfate together with high methane concentrations in the lowermost sediment interval all point to a recent expulsion of mud breccia at the summit of Carmen MV.For the lowermost interval, the depletion of major elements (i.e., Ca2+ and Mg2+) and the enrichment of trace species (i.e., Li and B) in the pore water all indicate a deep fluid source. The δ18Opw (5.7‰ VSMOW) and δDpw (−10‰ VSMOW) of pore water in the lowermost interval correspond with smectite dehydration as the main pore-water freshening mechanism. Water-formation temperatures calculated with empirical geo-thermometers (K-Na, K-Mg; δ18Opw, δDpw, and dissolved B) reveal that fluids were generated at temperatures of ~140 ± 20 °C. Taking a regional geothermal gradient for the WAB of 25–27 °C/km, this points to a fluid source from ~5 ± 1 km sediment depth. This is not only consistent with the depth of overpressurized shales and megabreccia of Lower to Middle Miocene age, but it also fits nicely with the Upper/Middle-Miocene seawater value for the porewater 87Sr/86Sr derived from dissolving carbonates. The stable carbon and hydrogen isotopic composition of methane (δ13Cmethane ~ −59.4‰ VPDB and δDmethane −184‰ VSMOW) for the deepest samples of summit-core GP05PC is consistent with the mentioned deep origin.Mud breccia expulsion of overpressurized deep sedimentary units would be accompanied by rigorous degassing, leading to rapid, ‘instantaneous’ replacement of pore fluid by bottom water in the upper sediments. The absence of oxidized sediment draping, the seawater-like pore-water composition in the uppermost part of the mud breccia interval, and the abrupt methane to sulfate transition all provide evidence for a very recent mud expulsion.The distinctively kink-shaped pore-water Cl− profile in core GP05PC has been used in a numerical transport-reaction model to derive the timing for this event. This eruptive event appears to have taken place very recently, namely 12 ± 5 yrs prior to the 2012 coring, thus in the year 2000 CE.

The evolution of chemical groups and isotopic fractionation at different maturation stages during lignite pyrolysis

In this study, the evolution of different chemical groups and isotopic fractionation at different maturation stages of coals were investigated by gold-tube pyrolysis, detailed geochemical analysis and theoretical calculations based on Density Function Theory (DFT). The results from non-isothermal pyrolysis of lignite revealed that the conversion and hydrocarbon generation of lignite can be divided into four maturation stages in the range of Ro from 0.35 to 4.66%. Fourier-transform infrared spectra (FTIR) results of residual coals showed that the evolution of different functional groups dominates the generation of oil and gas products at different maturation stages. Correspondingly, the activation energy for methane generation was fitted into four Gaussian distributions, which elucidated different formation mechanisms or precursors of methane. Theoretical calculations indicated that the difference of isotopic fractionation kinetics for the cleavage of different chemical groups dominates carbon and hydrogen isotopic compositions of methane during lignite pyrolysis. By calculation of the isotopic fractionation factor (α=k∗/k) for the cleavage of ethyl and methyl radical, a simplified model was established to predict the carbon isotopic ratios of ethane (δ13C2) with and without cracking. Moreover, thermodynamic calculations confirmed that the occurrence of recombination between ethane and polycyclic aromatics is available at temperatures between 25 and 650°C. It is also demonstrated that the isotopic exchange between ethane and methyl aromatics can occur and will result in the depletion of D for residual coals and 13C for ethane during this process. Hence, the recombination reactions should be responsible for the rollover of δ13C2 at extremely high maturity both in experimental and geological conditions.

Assessing Methane in Shallow Groundwater in Unconventional Oil and Gas Play Areas, Eastern Kentucky.

The expanding use of horizontal drilling and hydraulic fracturing technology to produce oil and gas from tight rock formations has increased public concern about potential impacts on the environment, especially on shallow drinking water aquifers. In eastern Kentucky, horizontal drilling and hydraulic fracturing have been used to develop the Berea Sandstone and the Rogersville Shale. To assess baseline groundwater chemistry and evaluate methane detected in groundwater overlying the Berea and Rogersville plays, we sampled 51 water wells and analyzed the samples for concentrations of major cations and anions, metals, dissolved methane, and other light hydrocarbon gases. In addition, the stable carbon and hydrogen isotopic composition of methane (δ13 C-CH4 and δ2 H-CH4 ) was analyzed for samples with methane concentration exceeding 1 mg/L. Our study indicates that methane is a relatively common constituent in shallow groundwater in eastern Kentucky, where methane was detected in 78% of the sampled wells (40 of 51 wells) with 51% of wells (26 of 51 wells) exhibiting methane concentrations above 1 mg/L. The δ13 C-CH4 and δ2 H-CH4 ranged from -84.0‰ to -58.3‰ and from -246.5‰ to -146.0‰, respectively. Isotopic analysis indicated that dissolved methane was primarily microbial in origin formed through CO2 reduction pathway. Results from this study provide a first assessment of methane in the shallow aquifers in the Berea and Rogersville play areas and can be used as a reference to evaluate potential impacts of future horizontal drilling and hydraulic fracturing activities on groundwater quality in the region.

Hydrogen isotope characteristics of thermogenic methane in Chinese sedimentary basins

The hydrogen isotope composition of methane is an important parameter in natural gas research and provides complementary information to that provided by carbon isotopes. The stable hydrogen isotope ratios of 313 natural gas samples from six of China’s sedimentary basins are used to evaluate the factors that influence the stable hydrogen isotopic composition of methane. An important factor is the δD of organic matter in hydrocarbon source rocks, which is influenced by the sedimentary environment and type of organic matter. Natural gases generated from sapropelic organic matter have relatively less negative δDCH4, while natural gases generated from humic organic matter have relatively more negative δDCH4. The other factor is thermal maturity. Increased thermal maturity leads to δDCH4 becoming less negative and there exists a two stage linear relationship between the δDCH4 and the logarithm of the vitrinite reflectance (Ro) value for natural gases generated from type III kerogen. The third factor is the environmental conditions of the aqueous medium in the original depositional environment and after sedimentation. Elemental hydrogen from water participates in biochemical processes as organisms grow and is exchanged during the sedimentation and diagenesis of organic matter as well as maturity process of kerogen to generate gases. The influence of the aqueous medium on δDCH4 after formation of natural gases can be ignored. Among these factors, the aqueous medium is the key constraining factor. The δDCH4 in natural gases can be combined with the use of stable carbon isotopic composition to identify the origins of natural gases and to aid gas-source correlations.

The origin of low molecular weight hydrocarbons associated with biogenic gas from the Eastern Depression in Qaidam Basin, China

Abstract The Eastern Depression of Qaidam Basin in China is the largest Quaternary gas producing area in the world. The chemical compositions, C1–C2 carbon and hydrogen isotopic compositions, low molecular weight hydrocarbons (C6–C7LMWHs) in 10 natural gas samples from three large gas fields (Tainan, Sebei 1 and Sebei 2) in Eastern Depression of Qaidam Basin were analyzed. The carbon and hydrogen isotopic composition of methane in the three gas fields is relatively enriched in 12C and 1H with δ13C1 values ranging from −69.6 ‰ to −65.5 ‰ (average −67.4 ‰) and δ1H from −251 ‰ to −231 ‰ (average −244 ‰). The data shows that the gases are defined as microbial in origin and generated primarily by bacterial CO2 reduction. The composition distribution of C6–C7LMWHs differs between the fields. LMWHs associated with the gas in the Sebei 1 gas field are different from those in the Tainan and Sebei 2 gas fields. The relative content of cycloalkane among C6–C7LMWHs in the Tainan and Sebei 2 gas fields is very high, ranging from 37.02% to 43.36% and 33.99% to 43.71% respectively. However, in the Sebei 1 gas field, of the total C6–C7LMWHs, the relative content of isoalkane is the highest, ranging from 37.17% to 45.92% with an average of 42.48. These traits indicate that the origins of C6–C7LMWHs are probably different from the bacterial action that is predominant in the Sebei 1 gas field. We infer that catalytic reaction affects the C6–C7LMWHs compositions in the Tainan and Sebei 2 gas fields.

Isotopic patterns of hydrothermal hydrocarbons emitted from Mediterranean volcanoes

We have analyzed the carbon isotopic composition of CO2, methane, ethane, propane and n-butane, the hydrogen isotopic composition of methane as well as total concentrations of gas constituents contained in the Mediterranean volcanic–hydrothermal discharges of Nisyros (Greece), Vesuvio, La Solfatara, Ischia and Pantelleria (all Italy) to determine the origin of the hydrocarbons. Isotopic criteria conventionally used for hydrocarbon classification suggest thermogenic origins, except for Pantelleria, for which an abiogenic origin is indicated. These findings would imply that thermogenic sources can provide methane/(ethane+propane) concentration ratios as high as those usually observed for microbial hydrocarbons. However, additional consideration of gas concentration data challenges the suitability of conventional criteria for the classification of hydrocarbons emanating from hydrothermal environments. Methane seems to be in close equilibrium with co-occurring CO2, whereas its higher chain homologues are not. Therefore, it cannot be excluded that methane on the one hand and ethane, propane and n-butane on the other hand have distinct origins. The carbon isotopic composition of methane might be controlled by the carbon isotopic composition of co-occurring inorganic CO2 and by hydrothermal temperatures whereas the carbon isotopic composition of the higher n-alkanes could correspond to the maturity of organic matter and/or to the residence time of the gasses in the source system.

Real-time drilling mud gas monitoring for qualitative evaluation of hydrocarbon gas composition during deep sea drilling in the Nankai Trough Kumano Basin.

BackgroundIntegrated Ocean Drilling Program Expedition 338 was the second scientific expedition with D/V Chikyu during which riser drilling was conducted as part of the Nankai Trough Seismogenic Zone Experiment. Riser drilling enabled sampling and real-time monitoring of drilling mud gas with an onboard scientific drilling mud gas monitoring system (“SciGas”). A second, independent system was provided by Geoservices, a commercial mud logging service. Both systems allowed the determination of (non-) hydrocarbon gas, while the SciGas system also monitored the methane carbon isotope ratio (δ13CCH4). The hydrocarbon gas composition was predominated by methane (> 1%), while ethane and propane were up to two orders of magnitude lower. δ13CCH4 values suggested an onset of thermogenic gas not earlier than 1600 meter below seafloor. This study aims on evaluating the onboard data and subsequent geological interpretations by conducting shorebased analyses of drilling mud gas samples.ResultsDuring shipboard monitoring of drilling mud gas the SciGas and Geoservices systems recorded up to 8.64% and 16.4% methane, respectively. Ethane and propane concentrations reached up to 0.03 and 0.013%, respectively, in the SciGas system, but 0.09% and 0.23% in the Geoservices data. Shorebased analyses of discrete samples by gas chromatography showed a gas composition with ~0.01 to 1.04% methane, 2 – 18 ppmv ethane, and 2 – 4 ppmv propane. Quadruple mass spectrometry yielded similar results for methane (0.04 to 4.98%). With δD values between -171‰ and -164‰, the stable hydrogen isotopic composition of methane showed little downhole variability.ConclusionsAlthough the two independent mud gas monitoring systems and shorebased analysis of discrete gas sample yielded different absolute concentrations they all agree well with respect to downhole variations of hydrocarbon gases. The data point to predominantly biogenic methane sources but suggest some contribution from thermogenic sources at depth, probably due to mixing. In situ thermogenic gas production at depths shallower 2000 mbsf is unlikely based on in situ temperature estimations between 81°C and 85°C and a cumulative time-temperature index of 0.23. In conclusion, the onboard SciGas data acquisition helps to provide a preliminary, qualitative evaluation of the gas composition, the in situ temperature and the possibility of gas migration.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-014-0015-8) contains supplementary material, which is available to authorized users.

Improving accuracy and precision of ice core δD(CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;) analyses using methane pre-pyrolysis and hydrogen post-pyrolysis trapping and subsequent chromatographic separation

Abstract. Firn and polar ice cores offer the only direct palaeoatmospheric archive. Analyses of past greenhouse gas concentrations and their isotopic compositions in air bubbles in the ice can help to constrain changes in global biogeochemical cycles in the past. For the analysis of the hydrogen isotopic composition of methane (δD(CH4) or δ2H(CH4)) 0.5 to 1.5 kg of ice was hitherto used. Here we present a method to improve precision and reduce the sample amount for δD(CH4) measurements in (ice core) air. Pre-concentrated methane is focused in front of a high temperature oven (pre-pyrolysis trapping), and molecular hydrogen formed by pyrolysis is trapped afterwards (post-pyrolysis trapping), both on a carbon-PLOT capillary at −196 °C. Argon, oxygen, nitrogen, carbon monoxide, unpyrolysed methane and krypton are trapped together with H2 and must be separated using a second short, cooled chromatographic column to ensure accurate results. Pre- and post-pyrolysis trapping largely removes the isotopic fractionation induced during chromatographic separation and results in a narrow peak in the mass spectrometer. Air standards can be measured with a precision better than 1‰. For polar ice samples from glacial periods, we estimate a precision of 2.3‰ for 350 g of ice (or roughly 30 mL – at standard temperature and pressure (STP) – of air) with 350 ppb of methane. This corresponds to recent tropospheric air samples (about 1900 ppb CH4) of about 6 mL (STP) or about 500 pmol of pure CH4.

Comparative study of stable carbon and hydrogen isotopes of alkane gases sourced from the Longtan and Xujiahe coal-bearing measures in the Sichuan Basin, China

Two sets of coal-bearing measures, the Upper Triassic Xujiahe Formation (T3x) and the Upper Permian Longtan Formation (P2l), have been extensively developed in the Sichuan Basin, SW China and some large gas fields related to these coal-bearing measures have been discovered. The stable carbon and hydrogen isotopic composition of alkane gases sourced from the two sets of coal-bearing measures (T3x and P2l gases) have been investigated in this study. The main controls on the gas composition and isotopic abundances of gases in the Sichuan Basin have been identified and distinguished on the basis of their molecular and isotopic composition. The carbon and hydrogen isotopic compositions of methane (δ13CC1: −42.5‰ to −26.8‰; δDC1: −173‰ to −107‰) vary widely and show a positive correlation with the thermal maturity of the source rocks. The δ13CC2 value of the T3x and P2l gases in the Sichuan Basin ranged from −28.2‰ to −21.3‰, and showed less variations with increasing maturity from Ro values of 1.2% to 2.8%. However, these δ13CC2 values differ greatly from that of oil-associated gas in the Sichuan Basin. Thus we provide further evidence that the value of δ13CC2 is a useful parameter to distinguish coal-derived and oil associated gases. The hydrocarbon gases in the Xujiahe and Jurassic reservoirs in contrast showed little isotopic difference indicating that secondary migration has little influence on their carbon and hydrogen isotope compositions. In addition, the δ13C values of P2l CO2 (−2.6− to 3.3‰) may reflect an origin from dissolution of the carbonates in the reservoir by acidic fluids containing H2S.

Carbon and Hydrogen Isotopic Composition of Methane and C2+ Alkanes in Electrical Spark Discharge: Implications for Identifying Sources of Hydrocarbons in Terrestrial and Extraterrestrial Settings

The low-molecular-weight alkanes--methane, ethane, propane, and butane--are found in a wide range of terrestrial and extraterrestrial settings. The development of robust criteria for distinguishing abiogenic from biogenic alkanes is essential for current investigations of Mars' atmosphere and for future exobiology missions to other planets and moons. Here, we show that alkanes synthesized during gas-phase radical recombination reactions in electrical discharge experiments have values of δ(2)H(methane)>δ(2)H(ethane)>δ(2)H(propane), similar to those of the carbon isotopes. The distribution of hydrogen isotopes in gas-phase radical reactions is likely due to kinetic fractionations either (i) from the preferential incorporation of (1)H into longer-chain alkanes due to the more rapid rate of collisions of the smaller (1)H-containing molecules or (ii) by secondary ion effects. Similar δ(13)C(C1-C2+) and δ(2)H(C1-C2+) patterns may be expected in a range of extraterrestrial environments where gas-phase radical reactions dominate, including interstellar space, the atmosphere and liquid hydrocarbon lakes of Saturn's moon Titan, and the outer atmospheres of Jupiter, Saturn, Neptune, and Uranus. Radical recombination reactions at high temperatures and pressures may provide an explanation for the combined reversed δ(13)C(C1-C2+) and δ(2)H(C1-C2+) patterns of terrestrial alkanes documented at a number of high-temperature/pressure crustal sites.

メタン循環に関わる海底下生命圏

Methane is one of the major end products from photosynthetic organic matter. Based on stable carbon and hydrogen isotopic compositions of methane, biological pathways mainly consisting of carbon dioxide reduction coupled to molecular hydrogen oxidation and acetate fermentation and abiological pathways such as thermal degradation of organic matter have been systematically classified. However, recent advances in subseafloor biosphere research have unveiled the complexity of processes involved in the transformation, migration and fate of methane. Particularly, it has been recognized that marine sediments with high flux of methane harbor novel lineages of microorganisms, the physiological traits of which are largely unknown due to their resistance to cultivation. In this review article, microbiological investigations that shed light on DNA-based microbial community structures and metabolic diversity and activity related to methane production and consumption in organic-rich marine sediments are briefly summarized.

Pyrolysis experiments of forest marsh peat samples with different maturities: An attempt to understand the isotopic fractionation of coalbed methane during staged accumulation

In order to understand the isotopic features of coalbed gas formed in staged accumulation processes, five starting materials with different maturity were prepared from a peat at temperatures of 250°C, 300°C, 350°C, 400°C and 450°C, respectively. Closed-system isothermal pyrolysis experiments were performed on these samples at various temperature intervals. The carbon and hydrogen isotopic compositions of methane and ethane generated during the pyrolysis of the samples with different thermal maturities were determined. The results showed that the higher the initial thermal maturity of the starting sample, the heavier the carbon and hydrogen isotopic compositions of the generated methane and ethane. It was observed that the carbon isotope compositions of the generated methane differed significantly depending on initial thermal maturity of the samples, whereas the hydrogen isotopic compositions did not differ as much. The relationships of the δ13C and δD values of gases generated from the samples with maturity intervals of 0.8–4.8%, 1.1–4.8%, 1.9–4.8%, 2.3–4.8%, 3.1–4.8% Ro, respectively, were established. These results may provide a basis for studying on the isotopic geochemistry of coalbed gas formed in staged accumulation processes.

Influences of water media on the hydrogen isotopic composition of natural gas/methane in the processes of gaseous hydrocarbon generation and evolution

The influences of water media on the hydrogen isotopic composition of organic-thermogenic natural gas were tested in three series of experiments on coal pyrolysis, with no water, deionized water (\(\delta D_{H_2 O} \)=−58‰), and seawater (\(\delta D_{H_2 O} \)=−4.8‰) added, respectively. The experimental results show that the productivities of H2 and CO2 obviously increased under hydrous conditions and that the productivity of CH4 also remarkably increased in the high-evolution phase of hydrous experiments. Water was involved in the chemical reaction of hydrocarbon generation, and then the hydrogen isotopic composition of methane was affected. There is a linear correlation between the hydrogen isotopic composition of methane and its productivity, as reflected in the three series of experiments. In the case of the same CH4 productivity, the hydrogen isotopic composition of the methane produced in anhydrous experiments was the heaviest, that of the methane produced in seawater-adding experiments came second, and that of the methane produced in deionized water-adding experiments was the lightest. The hydrogen isotopic composition of natural gas/methane is affected by the following factors: 1) the characteristics of hydrogen isotopic composition of organic matter in source rocks, 2) the thermal evolution extent of organic matter, and 3) fossil-water media in the natural gas-generation period. The experimental results show that the influence of the fossil-water medium in the natural gas-generation period was lower than that of the other factors.

Carbon and hydrogen isotopic compositions and their evolutions of gases generated by herbaceous swamp peat at different thermal maturity stages

Coalbed methane (CBM) accumulation models include continuous gas accumulation and staged gas accumulation. However, studied on the geochemical characteristics and indices to evaluate staged accumulation CBM are lacking. This study for the first time obtained the carbon and hydrogen isotopic compositions of methane and ethane generated at different evolution stages using thermal simulation of samples prepared by treating an herbaceous swamp peat at different temperatures. The results showed that the carbon isotopic compositions of methane and ethane were obviously affected by the thermal evolution level of the starting sample, while the hydrogen isotopic compositions were closely related to the maturity of gases. The carbon and hydrogen isotopic compositions of gases generated by coal-forming organic matter with R o values from 1.2%, 1.7%, 2.4%, 3.2% and 3.7% to 5.2% were determinated. The relationship between R o values and the carbon and hydrogen isotopic compositions of gases generated by coal-forming organic matter at different evolution stages as well as the carbon or hydrogen isotopic relationships between methane and ethane were established. The results provide a scientific basis for studying the genesis of CBM generated at different maturity intervals and understanding the geochemical characteristics of staged accumulation CBM. These results were applied to a case study on CBM from the southern Qinshui basin, and it was found that the CBM accumulated after the Middle Jurassic and was characteristic of staged gas accumulation. This is consistent with the result of geological studies, and further showed that the results of thermal simulation experiments are very important for evaluating the genesis of natural CBM.

Hydrogen Isotopes Preclude Marine Hydrate CH 4 Emissions at the Onset of Dansgaard-Oeschger Events

The causes of past changes in the global methane cycle and especially the role of marine methane hydrate (clathrate) destabilization events are a matter of debate. Here we present evidence from the North Greenland Ice Core Project ice core based on the hydrogen isotopic composition of methane [deltaD(CH4)] that clathrates did not cause atmospheric methane concentration to rise at the onset of Dansgaard-Oeschger (DO) events 7 and 8. Box modeling supports boreal wetland emissions as the most likely explanation for the interstadial increase. Moreover, our data show that deltaD(CH4) dropped 500 years before the onset of DO 8, with CH4 concentration rising only slightly. This can be explained by an early climate response of boreal wetlands, which carry the strongly depleted isotopic signature of high-latitude precipitation at that time.