Carbon Isotope Ratios Of Methane Research Articles

Acid-Extracted Hydrocarbon Anomalies and Significance in the Chaoshan Depression of the Northern South China Sea

To predict the favorable zones and the types of reservoirs, acid extraction has been used in the Chaoshan depression to detect trace amounts of light hydrocarbons, heavy hydrocarbons, and the δ 13C (‰) of methane. As a result, two integration anomalous zones for exploration activity were blocked out in the northeastern and southwestern parts of the Chaoshan Depression, respectively. By analyzing the differentiation law and structural characteristics of hydrocarbon gases, as well as the stable carbon isotope ratio of methane, the underlying reservoirs were predicted to be gas reservoirs, and the seismically interpreted Dongsha-A (DS-A) structure was predicted to be a gas-rich structure. By correlating the seismic profile and geochemical anomalies, it was determined that fault planes and micro-fractures are the main controlling factors for the occurrence of the seabed’s geochemical anomalies. A composite formation mechanism of “lower generation, upper accumulation and micro fractures leaking” is proposed for the control of the underlying petroleum reservoirs, as well as for the micro-fracture control of permeability and surface adsorption control. Acid-extracted hydrocarbon anomalies have favorable indicating significance for exploration activity.

Deep-sea bottom-water environment change caused by sediment resuspension on the continental slope off Sanriku, Japan, before and after the 2011 Tohoku Earthquake

The 2011 off the Pacific coast of Tohoku Earthquake occurred off southern Sanriku, Japan, on March 11 and generated strong shaking and huge tsunami along the entire eastern coast of Tohoku. The mainshock and numerous large aftershocks caused mass-wasting sedimentation events on the continental shelf, slope, and trench bottom. To investigate the impacts of the 2011 Tohoku Earthquake on the deep-sea bottom-water environment off Sanriku, we conducted shipboard surveys up to ~ 2000 dbar during 2011–2018 and long-term monitoring of the seafloor on the continental slope using a deep-sea station (~ 1000 dbar) off Otsuchi Bay during 2012–2018. The high turbidity (maximum ~ 6%) was observed for the bottom water deeper than 500 dbar on the continental slope of the entire area off Sanriku during 2012–2018. This high turbidity was caused by sporadic sediment resuspension induced by frequent large aftershocks. Furthermore, dissolved oxygen concentrations in the bottom layer from 1000 to 1500 dbar dropped significantly by about 10% after the earthquake, while nutrients and dissolved inorganic carbon showed no significant changes but exhibited wide variations. The high turbidity was associated with the increase in the concentrations of phosphate, dissolved inorganic carbon, and methane, as well as the decrease in those of dissolved oxygen and nitrate. This suggests that remineralization of suspended organic matter resulting from the respiration and denitrification of microbial communities after the earthquake caused the chemical properties of the deep-sea bottom-water. The deep-sea bottom-water environment change was maintained by sporadic sediment resuspension due to continued large aftershocks and was likely caused by variations in dissolved inorganic carbon and phosphate. There are two peaks in the concentration and carbon isotope ratio of methane on the deeper slope from 1000 to 2000 dbar near the hypocenter, which were advected along isopycnal surfaces of 27.38σθ (1000 dbar) and 27.56σθ (1500 dbar). The source of the shallower peak of chemical input is considered to be the sediment resuspension from the shallow sediment on the continental slope induced by the mainshock and large aftershocks.Graphical abstract

Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water

Abstract. Isotope fractionation factors associated with various biogeochemical processes are important in ensuring the reliable use of isotope tracers in biogeosciences at large. Methane is a key component of the subsurface biosphere and a notable greenhouse gas, making the accurate evaluation of methane cycles, including microbial methanotrophy, imperative. Although the isotope fractionation factors associated with methanotrophy have been examined under various conditions, the dual-isotope fractionation factors of aerobic methanotrophy in oxic seawater remain unclear. Here, we investigated hydrogen and carbon isotope ratios of methane as well as the relevant biogeochemical parameters and microbial community compositions in hydrothermal plumes in the Okinawa Trough. Methanotrophs were found to be abundant in plumes above the Hatoma Knoll vent site, and we succeeded in simultaneously determining hydrogen and carbon isotope fractionation factors associated with the aerobic oxidation of methane (εH=49.4±5.0 ‰, εC=5.2±0.4 ‰) – the former being the first of its kind ever reported. This εH value is comparable with values reported from terrestrial ecosystems but clearly lower than those from aerobic and anaerobic methanotroph enrichment cultures, as well as incubations of methanotrophic isolates. The covariation factor between δ13CCH4 and δDCH4, Λ (9.4 or 8.8 determined using two different methods), was consistent with those from methanotrophic isolate incubations. These values are valuable for understanding dynamics of methane cycling in the marine realm, and future applications of the approach to other habitats with methanotrophic activity will help reveal whether the small εH value observed is a ubiquitous feature across all marine systems.

A GC-SSIM-CRDS system: Coupling a gas chromatograph with a Cavity Ring-Down Spectrometer for onboard Twofold analysis of molecular and isotopic compositions of natural gases during ocean-going research expeditions

Carbon dioxide (CO2) and methane (CH4) are two climate-sensitive components of gases migrating within sediments and emitted into the water column on continental margins. They are involved in several key biogeochemical processes entering into the global carbon cycle. In order to perform onboard measurements of both the molecular and stable carbon isotope ratios (δ13C) of CH4 and CO2 of natural gases during oceanic cruises, we have developed a novel approach coupling gas chromatography (GC) with cavity ring-down spectroscopy (CRDS). The coupled devices are connected to a small sample isotope module (SSIM) to form a system called GC-SSIM-CRDS. Small volumes of natural gas samples (<1 mL) are injected into the GC using a headspace autosampler or a gas-tight syringe to separate the chemical components using a Shincarbon ST packed column and for molecular quantification by thermal conductivity detection (TCD). Subsequently, CO2 from the sample is trapped in a 7 mL loop at 32 °C before being transferred to the CRDS analyzer for sequential determination of the stable carbon isotope ratios of CH4 and CO2 in 24 min. The loop is an open column (without stationary phase). This approach does not require the use of adsorbents or cooling for the trapping step. Optimization of the separation step prior to analysis was focused on the influence of two key separation factors 1) the flow of the carrier gas and 2) the temperature of the oven. Our analytical system and the measurement protocol were validated on samples collected from gas seeps in the Sea of Marmara (Turkey). Our results show that the GC-SSIM-CRDS system provides a reliable determination of the molecular identification of CH4 and CO2 in complex natural gases, followed by the stable carbon isotope ratios of methane and carbon dioxide.

Carbon isotope fractionation during shale gas transport: Mechanism, characterization and significance

The gas in-place (GIP) content and the ratio of adsorbed/free gas are two key parameters for the assessment of shale gas resources and have thus received extensive attention. A variety of methods have been proposed to solve these issues, however none have gained widespread acceptance. Carbon isotope fractionation during the methane transport process provides abundant information, serving as an effective method for differentiating the gas transport processes of adsorbed gas and free gas and ultimately evaluating the two key parameters. In this study, four stages of methane carbon isotope fractionation were documented during a laboratory experiment that simulated gas transport through shale. The four stages reflect different transport processes: the free gas seepage stage (I), transition stage (II), adsorbed gas desorption stage (III) and concentration diffusion stage (IV). Combined with the results of decoupling experiments, the isotope fractionation characteristics donated by the single effect (seepage, adsorption-desorption and diffusion) were clearly revealed. We further propose a technique integrating the Amoco curve fit (ACF) method and carbon isotope fractionation (CIF) to determine the dynamic change in adsorbed and free gas ratios during gas production. We find that the gases produced in stage I are primarily composed of free gas and that carbon isotope ratios of methane ( δ 13C1) are stable and equal to the ratios of source gas ( δ 13 C 1 0 ). In stage II, the contribution of free gas decreases, while the proportion of adsorbed gas increases, and the δ 13C1 gradually becomes lighter. With the depletion of free gas, the adsorbed gas contribution in stage III reaches 100%, and the δ 13C1 becomes heavier. Finally, in stage IV, the desorbed gas remaining in the pore spaces diffuses out under the concentration difference, and the δ 13C1 becomes lighter again and finally stabilizes. In addition, a kinetic model for the quantitative description of isotope fractionation during desorption and diffusion was established.

Isotopic difference between hydrate-bound and sediment gases retrieved at Lake Baikal

The objective of our study is to clarify the formation process of natural gas hydrates in the sublacustrine sediments of Lake Baikal, and we are now compiling data on stable isotope ratios of hydrocarbons in hydrate-bound and sediment gases, which have been collected in the framework of collaboration between Russia, Belgium and Japan for 15 years. The results obtained in the VER19-03 cruise at the southern and central Baikal basins were summarized in this report. Comparison of stable isotope ratios of hydrate-bound and sediment gases at the same core and depth was performed. The methane carbon isotope ratio of the hydrate-bound gas was 1 to 2 smaller than the sediment gas. The methane hydrogen isotope ratio was similarly about 5 smaller. These results can be explained generally by the isotopic fractionation of methane. In contrast, some sediment cores from pockmarks in the center of the mud volcano showed small or reversed differences in methane carbon isotope ratios, as well as small differences in methane hydrogen isotope ratios, suggesting that dissociation of gas hydrate is occurring at that point.

The contribution of zooplankton to methane supersaturation in the oxygenated upper waters of the central Baltic Sea

AbstractWe report on methane enrichments that were observed during summer in the upper water column of the Gotland Basin, central Baltic Sea. In the eastern part of the basin, methane concentrations just below the thermocline varied between 15 nM and 77 nM, in contrast to the western part where no methane enrichments could be detected. Stable carbon isotope ratios of methane (δ13C‐CH4 of −67.6‰) indicated its in situ biogenic origin from CO2 reduction, which was supported by clonal sequences that clustered with Methanomicrobiaceae, a family of methanogenic Archaea. Incubation experiments with a Temora longicornis dominated seston fraction obtained from the relevant depth showed a positive correlation between seston concentration and methane production rates. Our results, in combination with previous literature outcomes, suggest that the methane enrichment in the eastern basin might be sustained by a diet‐consumer relationship between the dinoflagellate Dinophysis norvegica and the copepod T. longicornis. However, our mass balance indicates that a local methane production of 110 pmol L−1 d−1 was needed to maintain the methane enrichment, and that the estimated production rate from our incubation experiments of 0.3 pmol CH4 d−1 per adult T. longicornis (about 1 pmol L−1 d−1) was too low to maintain the methane enrichment by zooplankton associated methane production only. These calculations also showed that methane was consumed below the thermocline and not transported into the upper‐ocean, suggesting that other sources in the mixed layer in the range of 95 pmol L−1 d−1 are needed to maintain the observed methane air–sea flux.

The gas isotope interpretation tool: A novel method to better predict production decline

Production decline prediction is important to understand the performance and life span of oil and gas wells. The most common prediction method is decline curve fitting based on available production rate data. Such data are fit with different equations that extrapolate to future time. However, the parameters are commonly poorly constrained, especially when the production rate data are limited. In this study, we establish a novel gas isotope interpretation tool to better predict the resource quantity and life span of producing gas wells. This tool is based on the evolution of methane carbon isotope ratios (δ13C1) caused by different gas-releasing processes during production. It requires (1) real-time methane carbon isotope ratio data, (2) continuous gas production rate data for a certain period of time, and (3) basic geological and engineering conditions. We successfully applied the production decline prediction tool to a producing shale gas well in the Barnett Shale. We obtained real-time δ13C1 data for approximately 1 yr using our proprietary, field-deployable gas chromatography–infrared isotope ratio analyzer. The prediction in this well from the isotope method showed a total reserve of up to 7.34–7.75 BCF (2.07–2.19 × 108 m3), which was used to constrain the production decline trend of the study well. The measured production rate data were first fit using the Arps equation, which then joined to an exponential decline curve smoothly at approximately 10 yr, such that the cumulative production calculation from integration of the product rate curve equaled to the total reserve predicted by the isotope method. The novel production decline prediction method thus provided important constraint on the future well production and expected ultimate recoverable reserves.

Occurrence and origin of methane in groundwater in Alberta (Canada): Gas geochemical and isotopic approaches

To assess potential future impacts on shallow aquifers by leakage of natural gas from unconventional energy resource development it is essential to establish a reliable baseline. Occurrence of methane in shallow groundwater in Alberta between 2006 and 2014 was assessed and was ubiquitous in 186 sampled monitoring wells. Free and dissolved gas sampling and measurement approaches yielded comparable results with low methane concentrations in shallow groundwater, but in 28 samples from 21 wells methane exceeded 10mg/L in dissolved gas and 300,000ppmv in free gas. Methane concentrations in free and dissolved gas samples were found to increase with well depth and were especially elevated in groundwater obtained from aquifers containing coal seams and shale units. Carbon isotope ratios of methane averaged −69.7±11.1‰ (n=63) in free gas and −65.6±8.9‰ (n=26) in dissolved gas. δ13C values were not found to vary with well depth or lithology indicating that methane in Alberta groundwater was derived from a similar source. The low δ13C values in concert with average δ2HCH4 values of −289±44‰ (n=45) suggest that most methane was of biogenic origin predominantly generated via CO2 reduction. This interpretation is confirmed by dryness parameters typically >500 due to only small amounts of ethane and a lack of propane in most samples. Comparison with mud gas profile carbon isotope data revealed that methane in the investigated shallow groundwater in Alberta is isotopically similar to hydrocarbon gases found in 100–250meter depths in the WCSB and is currently not sourced from thermogenic hydrocarbon occurrences in deeper portions of the basin. The chemical and isotopic data for methane gas samples obtained from Alberta groundwater provide an excellent baseline against which potential future impact of deeper stray gases on shallow aquifers can be assessed.

Carbon isotope ratios suggest no additional methane from boreal wetlands during the rapid Greenland Interstadial 21.2

AbstractSamples from two Greenland ice cores (NEEM and NGRIP) have been measured for methane carbon isotope ratios (δ13C‐CH4) to investigate the CH4 mixing ratio anomaly during Greenland Interstadial (GI) 21.2 (85,000 years before present). This extraordinarily rapid event occurred within 150 years, comprising a CH4 mixing ratio pulse of 150 ppb (∼25%). Our new measurements disclose a concomitant shift in δ13C‐CH4 of 1‰. Keeling plot analyses reveal the δ13C of the additional CH4 source constituting the CH4 anomaly as −56.8 ± 2.8‰, which we confirm by means of a previously published box model. We propose tropical wetlands as the most probable additional CH4 source during GI‐21.2 and present independent evidence that suggests that tropical wetlands in South America and Asia have played a key role. We find no evidence that boreal CH4 sources, such as permafrost degradation, contributed significantly to the atmospheric CH4 increase, despite the pronounced warming in the Northern Hemisphere during GI‐21.2.

Genetic types and geochemical characteristics of natural gases in the Jiyang Depression, China

Natural gases were widely distributed in the Jiyang Depression with complicated component composition, and it is difficult to identify their genesis. Based on investigation of gas composition, carbon isotope ratios, light hydrocarbon properties, as well as geological analysis, natural gases in the Jiyang Depression are classified into two types, one is organic gas and the other is abiogenic gas. Abiogenic gas is mainly magmatogenic or mantle-derived CO2. Organic gases are further divided into coal-type gas, oil-type gas, and biogas according to their kerogen types and formation mechanisms. The oil-type gases are divided into mature oil-type gas (oil-associated gas) and highly mature oil-type gas. The highly mature oil-type gases can be subdivided into oil-cracking gas and kerogen thermal degradation gas. Identification factors for each kind of hydrocarbon gas were summarized. Based on genesis analysis results, the genetic types of gases buried in different depths were discussed. Results showed that shallow gases (<1,500 m) are mainly mature oil-type gases, biogas, or secondary gases. Secondary gases are rich in methane because of chromatographic separation during migration and secondary biodegradation. Secondary biodegradation leads to richness of heavy carbon isotope ratios in methane and propane. Genesis of middle depth gases (1,500–3,500 m) is dominated by mature oil-type gases. Deep gases (3,500–5,500 m) are mainly kerogen thermal degradation gas, oil-cracking gas, and coal-type gas.

Carbon isotope effects of methane transport through Anahuac Shale — A core gas study

Compositional data and stable isotope ratios are critical datasets for the study of hydrocarbon generation, expulsion, and migration. These geochemical parameters are also important for studies of leakage detection for geological carbon sequestration. Diffusion, gas–liquid partitioning and adsorption, the three most crucial processes to affect gas transport through shale and other low-permeability formations, are known to cause stable isotope fractionation. A considerable number of studies have been conducted on stable isotope fractionation associated with diffusion and gas–liquid partitioning. However, significantly fewer data are reported in the literature that specifically addresses adsorption carbon isotope effects. Moreover, adsorption isotope effects are rarely verified or seldom demonstrated in geologic systems. In this study, a shale core taken from the Anahuac Formation in Texas Gulf Coast, which overlies several Frio reservoirs, was analyzed for composition and stable isotope ratios of the residual gas. These geochemical parameters are used to identify gas migration and associate transport mechanisms in the caprock formation. Gas samples extracted from the core contain alkane gas components (C1–C5) and show a systematic variation in carbon isotope ratios of methane along depth. An analytical model is proposed to interpret the observed carbon isotope trend on the basis of coupled processes of diffusion and adsorption. At a ratio of retardation of 0.99 between 12CH4 and 13CH4 (with 12CH4 having a preferential bonding with the clay mineral) and a diffusion time of 1.5Ma, the modeled curve fits well with observed upward increasing δ13CCH4 values. This study demonstrates the isotope effect of adsorption in a natural system and its importance in quantifying gas-migration rate and distance in low-permeability systems.

Geochemical Characteristics and Generation Process of Mixed Biogenic and Thermogenic Coalbed Methane in Luling Coalfield, China

The exploration and development of coalbed methane (CBM) is often associated with the thermogenic gas. Because the mixed CBM derived from thermogenic and secondary biogenic gases was discovered in many coal-bearing basins of the world, secondary biogenic CBM which makes a significant contribution to the gas content is becoming a hot topic. In the present study, the origin of the gas in the Luling coalfield of China was first identified through molecular and stable isotope testing. Then, based on the basin evolution history, the generation process of thermogenic CBM and mixed CBM from biogenic and thermogenic gases was analyzed using two calculations. The results show that the carbon isotopic ratios of methane and carbon dioxide in Luling coalfield range from −67.6‰ to −50.5‰ and from −12.6‰ to −8.7‰, respectively, and the hydrogen isotopic ratios of methane range from −228‰ to −206‰. The isotope data indicate that the CBM in the Luling coalfield consists of both biogenic and thermogenic gases. Moreover, the generation process of mixed CBM can be divided into three stages: primary biogenic gas, thermogenic gas, and secondary biogenic gas. Cap outburst dissipation was determined to be the main migration mechanisms of hybrid CBM in Luling coalfiled, whereas diffusion was along with the whole process of CBM generation. Various factors, including maturity, temperature, and the time required for allochthonous methanogenic bacteria to move through the coal bed, were discussed in affecting the generation of secondary biogenic gas. These factors control the quantity, rate, and start and end of secondary biogenic CBM generation, respectively.

Gases in Southern Junggar Basin mud volcanoes: Chemical composition, stable carbon isotopes, and gas origin

Mud volcanoes are a common geological phenomenon in tectonically compressed areas on land and offshore. Mud volcano eruptions hold great significance for research on tectonic activity, the sedimentary environment and oil and gas accumulation. Methane emitted from mud volcanoes is also a source of greenhouse gas. Many mud volcanoes have developed in the southern Junggar Basin, Northwest China, but they have been studied very little. In this study, the chemical composition, stable carbon isotopes and gas origin of these mud volcanoes are analysed. The major gas component from the mud volcanoes in the southern Junggar Basin is methane, with an average value of 92.81%. The other gas components are ethane (4.8–2.93%), propane (0.01–0.05%), CO2 (0.11–5.36%) and N2 (0–3.63%). The methane carbon isotope ratios (δ13C1) are between −38.92‰ and −42.82‰, and ethane carbon isotope ratios (δ13C2) are −20.50‰ to −22.95‰. All these data have similar characteristics to other mud volcanoes around the world. Based on the C1 (methane)/(C2 (ethane) + C3 (propane)) and δ13C1, δ13C2 results, the released gas is a coal-type thermogenic gas. The gas is from a middle-low Jurassic coal-measure source.

Temporal variability of in situ methane concentrations in gas hydrate‐bearing sediments near Bullseye Vent, Northern Cascadia Margin

To assess the temporal variability in the methane fluxes from marine sediments that overly gas hydrate bearing sediments and the factors that might control its rate, in situ methane concentrations were measured near Bullseye Vent on the Northern Cascadia continental margin. A long‐term sampling device collected overlying water and pore‐fluid samples from 25 cm above seafloor, at the sediment‐water interface (SWI), and 7 cmbsf (centimeters below seafloor) over a 9 month period (August 2009–May 2010). These samples provide a record at ∼4 day resolution of in situ methane, ethane, propane, sulfate, and chloride concentrations, as well as stable carbon isotope ratios of methane (δ13C‐CH4) and dissolved inorganic carbon (δ13C‐DIC). We show that pore fluids near the SWI are saturated or supersaturated with respect to methane (∼80 mM) and the methane flux from the seabed is variable over time. We hypothesized that regional seismic activity controlled this variable CH4 flux in the Northern Cascadia continental margin setting. However, we found no direct correlation between earthquakes and CH4 flux. We also posited alternative controls on CH4 flux variability, such as storms, regional oceanography and microbial activity. Again, no direct correlation was seen. This study takes first steps toward exploring which physical factors play a role in methane flux from hydrate‐bearing sediments.

Aerobic methanotrophy within the pelagic redox-zone of the Gotland Deep (central Baltic Sea)

Abstract. Water column samples taken in summer 2008 from the stratified Gotland Deep (central Baltic Sea) showed a strong gradient in dissolved methane concentrations from high values in the saline deep water (max. 504 nM) to low concentrations in the less dense, brackish surface water (about 4 nM). The steep methane-gradient (between 115 and 135 m water depth) within the redox-zone, which separates the anoxic deep part from the oxygenated surface water (oxygen concentration 0–0.8 mL L−1), implies a methane consumption rate of 0.28 nM d−1. The process of microbial methane oxidation within this zone was evident by a shift of the stable carbon isotope ratio of methane between the bottom water (δ13C CH4 = −82.4‰ and the redox-zone (δ13C CH4 = −38.7‰. Water column samples between 80 and 119 m were studied to identify the microorganisms responsible for the methane turnover in that depth interval. Notably, methane monooxygenase gene expression analyses for water depths covering the whole redox-zone demonstrated that accordant methanotrophic activity was probably due to only one phylotype of the aerobic type I methanotrophic bacteria. An imprint of these organisms on the particular organic matter was revealed by distinctive lipid biomarkers showing bacteriohopanepolyols and lipid fatty acids characteristic for aerobic type I methanotrophs (e.g., 35-aminobacteriohopane-30,31,32,33,34-pentol), corroborating their role in aerobic methane oxidation in the redox-zone of the central Baltic Sea.

Geochemical characteristics and possible origin of natural gas in the Taibei Depression, Turpan-Hami Basin, China

292 chemical composition data and 82 isotopic composition data of gas samples collected from the Taibei Depression of the Turpan-Hami Basin, West China, were used in the study of their origin. Non-hydrocarbon gas is poor in most samples whereas abundant nitrogen in some samples is positively correlated with δ13C1. Although methane is the main constituent, higher molecular gaseous hydrocarbons, from ethane to pentane, are detected in most samples, in accordance with the distribution of oil reservoirs. The stable carbon isotope ratios of methane, ethane and propane are defined as δ13C1: −45.5‰ to −33.5‰, δ13C2: −30.2‰ to −10.5‰, and δ13C3: 27.6‰ to −11 respectively. According to the distribution of carbon isotope ratios, 2 families of gas can be grouped, most showing normal distribution of carbon isotopes, and others having obvious heavier carbon isotopes and being of abnormal distribution. Based on the isotopic composition, the disagreement between the relationship of Δ(δ13C1-δ13C2) and δ13C2 and that of δ(δ13C1-δ13C2) and δ13C2, and the calculated Ro, there are oil-associated gas, coal-derived gas and mixture of them. Other samples with obviously heavier isotopic compositions from the Yanmuxi oilfield of the Taibei Depression have been degraded by organisms.

Measurements of 13C/12C Methane from Anaerobic Digesters: Comparison of Optical Spectrometry with Continuous-Flow Isotope Ratio Mass Spectrometry

Methane production by anaerobic digestion of biomass has recently become more attractive because of its potential for renewable energy production. Analytical tools are needed to study and optimize the ongoing processes in biogas reactors. It is considered that optical methods providing continuous measurements at high temporal resolution of carbon isotope ratios of methane (delta(13)C(CH4)) might be of great help for this purpose. In this study we have tested near-infrared laser optical spectrometry and compared it with conventional continuous-flow isotope ratio mass spectrometry (CF-IRMS) using several methane carbon isotope standards and a large number of biogas samples from batch anaerobic reactors. Results from measurements on these samples were used to determine and compare the precision of the two techniques and to quantify for systematic offsets. With pure standards analytical precision of measurements for delta(13)C(CH4) was found to be in the range of 0.33 and 0.48 per thousand, and 0.09 and 0.27 per thousand for the optical method and CF-IRMS, respectively. Biogas samples showed an average mean deviation of delta(13)C(CH4) of 0.38 per thousand and 0.08 per thousand for the optical method and CF-IRMS, respectively. Although the tested laser optical spectrometer showed a dependence of delta(13)C(CH4) on CH(4) mixing ratio in the range of 500 to 8000 ppm this could be easily corrected. After correction, the delta(13)C(CH4) values usually varied within 0.7 per thousand from those measured by conventional CF-IRMS and thus results from both methods agreed within the given analytical uncertainties. Although the precision of the conventional CF-IRMS is higher than the tested optical system, both instruments were well within the acceptable delta(13)C(CH4) precision required for biogas methane measurements. The advantages of the optical system are its simplicity of operation, speed of analysis, good precision, reduced costs in comparison to IRMS, and the potential for field applications.

Geochemical characteristics and play targets of gas in eastern Qaidam Basin, NW China

Abstract Natural gas exists extensively in the Sanhu region of eastern Qaidam Basin, NW China, where five gas fields and two gas-bearing structures have been discovered and where lots of gas shows have been found. The gas was identified as typical biogenic gas with geochemical characteristics of depleted methane carbon isotope ratios and low abundance of heavy hydrocarbon gas components. The generation of methane is mainly related to the CO 2 reduction pathway. The existence of C 5 -C 8 hydrocarbons in gas samples and enriched carbon isotope values of ethane and propane in a few gas samples indicate that natural gas in this region may contain some thermogenic gas. It can be concluded that favorable source rock and conservation conditions exist for the formation of biogenic gas and its accumulation in reservoirs. Expanding this exploration domain, especially paying more attention to reservoirs of shallow and ultra shallow biogenic gas, deep biogenic gas, and deep thermogenic hydrocarbon generation has been suggested for future exploration activities.

Chemical and isotopic analysis of hydrocarbon gas at trace levels: Methodology and results

Isotopic mass spectrometry coupled online with gas chromatography (GC-C-IRMS) permits measurement of relative proportions of gaseous hydrocarbon (CH 4 to C 4H 10) and CO 2, and determination of carbon isotope ratio of hydrocarbon molecules. Access to these parameters provides valuable information about the source and the genesis of naturally-occurring gas, as well as on post-formation physico-chemical processes which might have taken place in the geological environment. In particular, it is possible to distinguish hydrocarbon gas of bacterial origin from that of thermogenic origin based on proportion and carbon isotope ratio of methane as measured by GC-C-IRMS. However, in samples containing very low amounts of hydrocarbons (from 1 ppm to 1000 ppm), accurate measurement of isotope ratios is often impossible due to the limitations of conventional GC-C-IRMS techniques using direct sample introduction. A technique was developed to overcome this limitation. It is based on a novel approach allowing pre-concentration of hydrocarbons prior to GC-C-IRMS analysis. The pre-concentration step consists in selective trapping of hydrocarbon molecules on a cold adsorbent phase, and removal of non-adsorbed gases (N 2, O 2, Ar,…). In a second step, pre-concentrated alkanes are desorbed, and released in an inert carrier gas, focused through a capillary and introduced into the GC-C-IRMS for chromatographic separation and measurement of concentration and carbon isotope composition of each individual carbon molecule. In order to achieve sufficient accuracy, several operating conditions are of prime importance, including sufficient signal intensity, well defined peak shape and low signal/noise ratio. Accurate measurements can be performed on samples as small as 10 cm 3 of bulk gas in standard conditions, with concentrations as low as 1 ppm of methane, 0.5 ppm of ethane and 0.3 ppm of propane and butane. Total analytical uncertainty on δ 13C measurements ranges from ± 0.2‰ to ± 1.5‰, depending on the hydrocarbon molecule.