Stable Carbon Isotopes Of Methane Research Articles

Improved global wetland carbon isotopic signatures support post-2006 microbial methane emission increase

Atmospheric concentrations of methane, a powerful greenhouse gas, have strongly increased since 2007. Measurements of stable carbon isotopes of methane can constrain emissions if the isotopic compositions are known; however, isotopic compositions of methane emissions from wetlands are poorly constrained despite their importance. Here, we use a process-based biogeochemistry model to calculate the stable carbon isotopic composition of global wetland methane emissions. We estimate a mean global signature of −61.3 ± 0.7‰ and find that tropical wetland emissions are enriched by ~11‰ relative to boreal wetlands. Our model shows improved resolution of regional, latitudinal and global variations in isotopic composition of wetland emissions. Atmospheric simulation scenarios with the improved wetland isotopic composition suggest that increases in atmospheric methane since 2007 are attributable to rising microbial emissions. Our findings substantially reduce uncertainty in the stable carbon isotopic composition of methane emissions from wetlands and improve understanding of the global methane budget.

New approaches to distinguish shale-sourced and coal-sourced gases in petroleum systems

Current approaches relating thermogenic gases to either shale source rocks (predominantly type II kerogen) or coal source rocks (predominantly type III kerogen) are not reliable and not globally applicable. This is because these mostly empirical approaches were developed using small poorly-constrained datasets from limited locations. The evaluation of a large global dataset of molecular and isotopic properties of gases from unconventional shale and coal reservoirs suggests that two genetic diagrams based on stable carbon isotopes of methane and ethane, δ13C-C2H6versus δ13C-CH4 and δ13C-CH4versus Δ(δ13C-C2H6 – δ13C-CH4), provide the best separation of shale-sourced and coal-sourced gases. Newly designated genetic fields and shale/coal separation lines on these diagrams were tested and validated using data from five petroleum systems with, likely, only shale (class B and A organofacies) source rocks (the Maracaibo Basin in Venezuela, the Guajira Basin in Colombia and the Rub Al Khali Basin in Iran) and only coal (class F organofacies) source rocks (the Southern Permian Basin in Germany and the Sichuan Basin in China). The practical usefulness of this new approach to gas-source correlations was demonstrated in two case studies from petroleum systems with debated source rock organofacies (the Mozambique Basin in Mozambique and the Indus Basin in Pakistan). These better constrained and more reliable diagrams with genetic fields and shale/coal separation lines represent a new tool for the evaluation of petroleum systems.

Assessment of shallow subsea hydrocarbons as a proxy for leakage at offshore geologic CO2 storage sites

This study is part of a multi-phase effort to identify and characterize offshore geological carbon storage (GCS) potential along the Texas Gulf Coast. Previous efforts acquired a high-resolution 3D seismic dataset (P-cable™) and interpreted a seismically discontinuous zone as shallow gas pockets (<100 m below seafloor) associated with a vertical chimney structure. Our approach was to measure hydrocarbon concentrations and stable carbon isotopes near the seafloor to assess if gas migrated vertically from seismic anomalies. Deep-sourced thermogenic hydrocarbons would indicate that the structure is transmissive which could indicate an unacceptable risk for GCS at the site. Alternatively, hydrocarbons formed in-situ from biogenic processes would not preclude transmissivity of the structure but would add information to the risk assessment. Gases were extracted from 23 piston core samples recovered between 2.56 and 3.50 m subseafloor. Light hydrocarbons and stable carbon isotopes of methane were used for source attribution. The result was that geochemical signatures were consistent with typical background values observed within the first few meters of subsea sediment and therefore did not indicate leakage from depth. From our analysis, we offer insights into the use of hydrocarbon molecular and isotopic compositions at the seafloor for signal attribution as part of environmental assessments at geological CO2 storage sites.

Genetic types and origins of natural gases from eastern Fukang Sub-depression of the Junggar Basin, NW China: Implication for low-mature coal-derived gases

Although the Fukang Sub-depression is the largest hydrocarbon generation depression in the Junggar Basin and promises favorable potential for oil and gas exploration, only several small–medium oil/gas accumulations have been discovered so far. After analyzing the molecular and stable carbon isotopic compositions of natural gases and its associated low molecular weight hydrocarbons (LMWHs) from eastern Fukang Sub-depression, their genetic types and origins are discussed in this study. Natural gases are wet gases with an average dryness coefficient of 0.84. Stable carbon isotopes of methane and ethane are well correlated with the thermal evolution trend of coal-derived gases. The C5–7 LMWHs are enriched in methyl cyclohexane but they lack n-alkanes, indicating a predominance of higher plant input. Gases have maturities ranging 0.76%–0.93%Ro and are well consistent with their low n-heptane (H) and iso-heptane (I) contents. The geochemical characteristics of natural gases from eastern Fukang Sub-depression are quite similar with those from the Turpan–Hami Basin. Their gases are considered to be low-mature gases derived from the Lower–Middle Jurassic coal measures. In contrast, they show remarkable differences with those from the Wucaiwan gas field; they are considered to have high-mature gases derived from the Carboniferous coal measures.

Variations of Stable Carbon Isotopes of CH4 Emission from Three Typical Rice Fields in China

Little is known about the stable carbon isotopes of methane (CH4) emitted (δ13CH4emitte4) from permanently flooded rice fields and double rice-cropping fields. The CH4 emission and corresponding δ13CH4emitte4 under various field managements (mulching, water regime, tillage, and nitrogen (N) fertilization) were simultaneously measured in three typical Chinese rice fields, a permanently flooded rice field in Ziyang City, Sichuan Province, Southwest China, a double-rice cropping field in Yingtan City, Jiangxi Province, Southeast China, and a rice-wheat rotation field in Jurong City, Jiangsu Province, East China, from 2010 to 2012. Results showed different seasonal variations of δ13CH4emitte4 among the three fields during the rice-growing season. The values of δ13CH4emitte4 were negatively correlated with corresponding CH4 emissions in seasonal variation and mean, indicating the importance of CH4 production, oxidation, and transport associated with isotopic fractionation effects to the δ13CH4emitte4. Seasonal variations of δ13CH4emitte4 were slightly impacted by mulching cultivation, tillage, and N application, but highly controlled by drainage. Meanwhile, tillage, N application, and especially mulching cultivation had important effects on seasonal mean CH4 emissions and corresponding δ13CH4emitte4, with low emissions accompanied by high values of δ13CH4emitte4. Seasonal mean values of δ13CH4emitte4 from the three fields were similar, mostly ranging from −60‰ to −50‰, which are well in agreement with previously published data. These demonstrated that seasonal variations of δ13CH4emitte4 mainly depended on the changes in CH4 emission from rice fields and further indicated the important effects of methanogenic pathways, CH4 oxidation, and CH4 transport associated with isotope fractionation effects influenced by field managements on δ13CH4emitte4.

Online monitoring of stable carbon isotopes of methane in anaerobic digestion as a new tool for early warning of process instability

Effective control of anaerobic digestion in biogas plants requires the monitoring of process sensitive and rapid response parameters in order to ensure efficient biogas production and to prevent potential process failure. In this study, stable carbon isotopes of methane (δ13CCH4) produced in a full-scale continuous stirred-tank reactor were investigated as a potential new monitoring tool for this purpose. Over a six-month period with variable organic loading rates, δ13CCH4-values were measured online by a portable high-precision laser absorption spectrometer. During a stress period of consecutive high organic loading, δ13CCH4-values early indicated process changes in contrast to traditionally monitored parameters where a change was observed some five to ten days later. Comparison of the stable isotope values with data from microbial analyses showed a distinct relationship between the quantity of potentially acetoclastic methanogens and δ13CCH4-values. This finding indicates an association between dominant methanogenic pathways and carbon isotope values.

Stable carbon isotopes of methane for real‐time process monitoring in anaerobic digesters

Efficient operation and stability of biogas plants requires continuous monitoring of the digester content. Traditional laboratory analysis of digester sludge is often complex and time‐consuming and shows a delayed response to disruptions within the fermentation process. As a new approach, we applied an online measurement technique (laser absorption spectroscopy) for real‐time monitoring of stable carbon isotopes of methane () in a pilot‐scale biogas digester (3500 L) regularly fed with maize silage. Generally, isotopic composition of methane gives information about specific substrate degradation, that is, methanogenic pathways that reflect the actual digester state. First results of a 2‐wk monitoring experiment show that stable carbon isotopes of methane respond promptly and highly dynamic to changes in the process state of the digester. In combination with other monitoring parameters (methane production rate, concentration of volatile fatty acids, and pH) the fluctuations in can be interpreted as a change in methanogenic pathways due to a high organic loading rate. In this context, might be used as a new parameter tool for monitoring and characterization of the process state of the digester.

Detection of stable carbon isotopes of methane with a 2.4-μm distributed feedback laser

Direct absorption spectra of methane (12CH4 and 13CH4) at room temperature have been recorded using a 2.4-μm distributed feedback laser. Integrated cross-sections of 13CH4 were measured and compared with those listed in the HITRAN 2008 and 2012 databases, indicating that there are some discrepancies between our results and both editions of the HITRAN database. The pressure-broadening parameters of those 13CH4 lines in synthetic air were also examined experimentally. For the purpose of determining the carbon isotope ratio (13CH4/12CH4) through the spectral measurements, candidate line pairs of 12CH4 and 13CH4 within the range 4147–4154cm−1 were explored. Resultantly, the well-isolated line pair of 12CH4 at 4151.843cm−1 and 13CH4 at 4151.966cm−1 was suggested to be a candidate for determining the carbon isotope ratio of CH4 in this wavenumber region.

Geochemical characteristics of the shallow soil above the Muli gas hydrate reservoir in the permafrost region of the Qilian Mountains, China

In this study, the Muli gas hydrate reservoir in the Qilian Mountains was chosen as a test area for the geochemical exploration of gas hydrates in mid-latitude regions. Soil headspace gases, acid-extracted hydrocarbons and stable carbon isotopes of methane, and soil magnetic susceptibility as well as microbes were tested. The results show that the distribution of geochemical anomalies can be well correlated with the underlying gas hydrate reservoir. Acid-extracted hydrocarbons, soil headspace gases, and the stable carbon isotopes of methane can be considered as major indicators for geochemical exploration of gas hydrates, whereas magnetic susceptibility and microbes served as complementary indicators. The stable carbon isotopes of methane and the hydrocarbon composition of the surface geochemical anomalies indicated a thermogenic origin, which shows that the gas source of the potential gas hydrate reservoir in this area may be contributed by deep oil and coal-formed gases. An accumulation model for the gas hydrate reservoirs was also developed and an integrated exploration project for gas hydrate, oil and coal bed methane is also proposed for the Muli area.

Nationwide oil and gas geochemical exploration program in China

A new nationwide oil and gas exploration program (NOGEP) was planned. Currently ongoing oil and gas geochemical mapping pilot studies are being conducted in the Songliao Basin. Demonstration basins in pilot studies mainly include Songliao Basin, Sichuan Basin, Tarim Basin, and the Qiangtang Basin. A nationwide oil and gas geochemical exploration that will start in 2016 using two sampling densities of 1/25km2 and 1/100km2 were considered. Oil and gas geochemical indicators include acid-released hydrocarbon, headspace gas, soil microorganisms, altered carbonates of soil, fluorescence, soil heat-released mercury, iodine and stable carbon isotopes of methane. In the nationwide oil and gas geochemical program, oil and gas basins with areas greater than 1×104km2 will be covered rapidly at a low sampling density to provide an evaluation of oil and gas potential at basin and region scales. Abnormalities indicative of oil and gas will be screened utilizing geological, seismic, and other data, allowing exploration targets to be selected for discovering large- and medium-sized oil and gas fields.

Sources of natural gases in Middle Cambrian reservoirs in Polish and Lithuanian Baltic Basin as determined by stable isotopes and hydrous pyrolysis of Lower Palaeozoic source rocks

Origin of natural gases associated with oil and condensate accumulations within the Middle Cambrian sandstone reservoir of the Polish and Lithuanian Baltic Basin was characterised by means of molecular composition, stable carbon isotopes of methane, ethane, propane, butanes, pentanes and carbon dioxide, stable hydrogen isotopes of methane and stable nitrogen isotopes of gaseous nitrogen. Generated gas from potential Upper Cambrian, Tremadocian, and Llandovery source rocks by hydrous pyrolysis at 330°C and 355°C for 72h was used to characterise thermogenic gas to evaluate correlation parameters based on molecular composition and stable isotopes. The pyrolysis conditions represent gas generation during oil generation, which appears to be the conditions represented by the natural gas accumulations and their low GORs (gas:oil ratios). The dryness of the pyrolysis and natural hydrocarbon compositions compare well, but do not provide a means of distinguishing the contributions of each source rock to the natural gas accumulations. The average δ13C value of the natural methane is 6.9‰ depleted in 13C compared to methane generated in the hydrous pyrolysis experiments. This difference is less for ethane and essentially nonexistent for propane, butanes, and pentanes. Tentatively, this diminishing difference with increasing carbon number is attributed to kinetic effects resulting from higher experimental temperatures. Although the δ13C values of methane and ethane from the hydrous pyrolysis experiments are not useful in direct correlations with natural gas accumulations, δ13C of propane, butanes, and pentanes is useful, and indicates that the Upper Cambrian and Tremadocian source rocks are the main contributors and that the Llandovery source rocks are not significant contributors to the Polish and Lithuanian Baltic natural gases. Polish natural gases with relatively higher methane and ethane are attributed to the mixing of drier, more mature gases from deeper parts of the basin to the west. Carbon dioxide of natural gases was generated during thermogenic processes and gaseous nitrogen was generated from NH4-rich illites of the clayey facies and from thermal transformation of organic matter of the Lower Palaeozoic strata. Hydrous pyrolysis gases have higher concentrations of CO2, H2S and H2 than the natural gases. This difference is attributed to reduction or loss of these highly reactive and soluble gases during migration and entrapment of the natural gases. Although CO2 concentrations between pyrolysis and natural gases are different, the δ13C values of the former fall within the range of the latter.

Isotopic Composition of Methane and Inferred Methanogenic Substrates Along a Salinity Gradient in a Hypersaline Microbial Mat System

The importance of hypersaline environments over geological time, the discovery of similar habitats on Mars, and the importance of methane as a biosignature gas combine to compel an understanding of the factors important in controlling methane released from hypersaline microbial mat environments. To further this understanding, changes in stable carbon isotopes of methane and possible methanogenic substrates in microbial mat communities were investigated as a function of salinity here on Earth. Microbial mats were sampled from four different field sites located within salterns in Baja California Sur, Mexico. Salinities ranged from 50 to 106 parts per thousand (ppt). Pore water and microbial mat samples were analyzed for the carbon isotopic composition of dissolved methane, dissolved inorganic carbon (DIC), and mat material (particulate organic carbon or POC). The POC delta(13)C values ranged from -6.7 to -13.5 per thousand, and DIC delta(13)C values ranged from -1.4 to -9.6 per thousand. These values were similar to previously reported values. The delta(13)C values of methane ranged from -49.6 to -74.1 per thousand; the methane most enriched in (13)C was obtained from the highest salinity area. The apparent fractionation factors between methane and DIC, and between methane and POC, within the mats were also determined and were found to change with salinity. The apparent fractionation factors ranged from 1.042 to 1.077 when calculated using DIC and from 1.038 to 1.068 when calculated using POC. The highest-salinity area showed the least fractionation, the moderate-salinity area showed the highest fractionation, and the lower-salinity sites showed fractionations that were intermediate. These differences in fractionation are most likely due to changes in the dominant methanogenic pathways and substrates used at the different sites because of salinity differences.

Methane dynamics in a microbial community of the Black Sea traced by stable carbon isotopes in vitro

Microbial mats thriving at gas seeps within anoxic bottom water were sampled during a Black Sea expedition in summer 2001. These mats contain associations of archaea which belong to the ANME-1 and, to a lesser extent, to the ANME-2 cluster, both accompanied by sulphate-reducing bacteria of the Desulfosarcina/ Desulfococcus-group and perform the anaerobic oxidation of methane (AOM). Transferring living mat into the laboratory allowed for an in vitro investigation of its methane turnover. We incubated aliquots of these AOM performing mats over a time period of 242 days tracking concentrations and 13C/ 12C ratios of the methane. The data obtained showed a decrease in concentration accompanied by a relative enrichment of 12C in the residual methane. These results indicate that the mats performed, in the presence of sulphate, both methane oxidation and methane production (methanogenesis). Rate calculations using information from additional experiments and field data gave a ratio between methane oxidation and methanogenesis of about 2:1. The parallel investigation of concentrations and signatures of stable carbon isotopes of methane revealed methane dynamics within these mats exceeding considerably what becomes apparent from only recognising the methane concentrations.

Origin of the Neogene shallow gas accumulations in the Jiyang Superdepression, Bohai Bay Basin

Natural gas resources occur extensively along the east coast of China, with a number of large and medium-sized gas fields being discovered in recent years. Gas reservoirs include Neogene, Paleogene and the underlying Mesozoic and Paleozoic basement. Of the total proven natural gas reserves in the Jiyang Superdepression, Bohai Bay Basin, almost 89.7% is present in the shallow Neogene gas pools, in traps formed on top of the paleotopographic highs and along the margin of the secondary depressions. These gases are closely associated with heavy oils, occurring as gas caps or associated gases within the heavy oil pools, or in separate gas pools above, or updip from, the heavy oil pools. The gases contain over 95% methane and small quantities of C 2 + alkanes, nitrogen and carbon dioxide. The stable carbon isotopes of methane in these gases are up to 10‰ more positive than those of the thermogenic gases in the deep Paleogene reservoirs, with propane more enriched in 13C than butane. This study demonstrated that the majority of the petroleum source rocks in the Jiyang Superdepression tend to be oil-prone, and are currently within or shallower than the conventional oil window (0.45–1.0% Ro). The chemical and carbon isotopic compositions of the gases, together with the moderate to severe biodegradation of the associated heavy oils in the shallow Neogene strata, clearly suggest that the formation of the shallow natural gases in the Jiyang Superdepression result from the anaerobic degradation of accumulated oils in reservoir.

Methane in Red Sea brines

Concentrations of hydrocarbon gases and stable carbon isotope ratios of methane from the water column of Shaban, Kebrit, Atlantis II and Discovery deeps, Red Sea, have been determined. Methane concentrations (yield C 1) range from < 50 nL/L (Red Sea deep water) to ca. 22 × 10 −3 L/L (Kebrit brine). Stable carbon isotopes of methane are between −30 and 43‰. Hydrocarbon gases in the brines are originally of thermogenetic origin (Kebrit: C 1/C 2∼ 57; δ 13C 1 ∼ −30‰). Methane concentrations in the transition zones between brines and Red Sea deep water decreased, especially in the Atlantis II/Discovery deeps, associated with a strong shift of δ 13C 1 to positive values. This shift is related to bacterial oxidation of methane in the transition zone between brine layers and overlying Red Sea deep water. Oxidized methane mixes with Red Sea deep water methane. A connection between the Atlantis II and the Discovery brine is postulated on the basis of the geochemical data.

Carbon kinetic isotope effect in the oxidation of methane by the hydroxyl radical

The reaction of the hydroxyl radical (HO) with the stable carbon isotopes of methane has been studied as a function of temperature from 273 to 353 K. The measured ratio of the rate coefficients for reaction with 12CH4 relative to 13CH4 (k12/k13) was 1.0054 (±0.0009 at the 95% confidence interval), independent of temperature within the precision of the measurement, over the range studied. The precision of the present value is much improved over that of previous studies, and this result provides important constraints on the current understanding of the cycling of methane through the atmosphere through the use of carbon isotope measurements.