Carbon Isotopic Composition Of Methane Research Articles

Factors that control the stable carbon isotopic composition of methane produced in an anoxic marine sediment

The carbon isotopic composition of methane produced in anoxic marine sediment is controlled by four factors: (1) the pathway of methane formation, (2) the isotopic composition of the methanogenic precursors, (3) the isotope fractionation factors for methane production, and (4) the isotope fractionation associated with methane oxidation. The importance of each factor was evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Methane did not accumulate during the initial 42‐day period when sediment contained sulfate, indicating little methane production from “noncompetitive” substrates. Following sulfate depletion, methane accumulation proceeded in three distinct phases. First, CO2 reduction was the dominant methanogenic pathway and the isotopic composition of the methane produced ranged from −80 to −94‰. The acetate concentration increased during this phase, suggesting that acetoclastic methanogenic bacteria were unable to keep pace with acetate production. Second, acetate fermentation became the dominant methanogenic pathway as bacteria responded to elevated acetate concentrations. The methane produced during this phase was progressively enriched in 13C, reaching a maximum δ13C value of −42‰. Third, the acetate pool experienced a precipitous decline from >5 mM to <20 μM and methane production was again dominated by CO2 reduction. The δ13C of methane produced during this final phase ranged from −46 to −58‰. Methane oxidation concurrent with methane production was detected throughout the period of methane accumulation, at rates equivalent to 1 to 8% of the gross methane production rate. Thus methane oxidation was too slow to have significantly modified the isotopic signature of methane. A comparison of microcosm and field data suggests that similar microbial interactions may control seasonal variability in the isotopic composition of methane emitted from undisturbed Cape Lookout Bight sediment.

Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands

The stable carbon isotopic composition of methane (δ13C) emitted from two peatland sites in the Marcell Experimental Forest in northern Minnesota was investigated during the snow‐free season of 1989–1990. A seasonal range in δ13C values of 13‰ was seen for a forested bog with heavier (13C enriched) methane emitted during the wanner summer months. This shift was correlated with water table level suggesting control by microbial oxidation. Methane from a nearby poor fen transitional to bog dominated byCarex oligospermashowed a similar temporal trend but with a much smaller range of 5‰ during the same time period and with no water table level correlation. The methane emitted from the fen was consistently heavier than that emitted by the bog.

Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands

Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands

Determination of the isotopic composition of atmospheric methane and its application in the Antarctic

A procedure for the determination of the 13C/12C ratio and the 14C abundance in atmospheric methane is presented. The method is based on the collection of air samples in stainless steel tanks at a pressure of 7 bar. The air is processed in the laboratory by cryogenic removal of condensibles, followed by oxidation of the methane content after which the resulting CO2 is collected. Also CO is removed prior to oxidation. The 13C/12C ratio is determined on the CO2 sample by stable isotope ratio mass spectrometry. The 14C content is determined by means of accelerator mass spectrometry. The overall precision of the technique is 0.1‰ for δ13C and 1.5 pMC for 14C. The method has been used to determine the carbon isotopic composition of methane in air collected at Baring Head, New Zealand, and also in air collected on aircraft flights between New Zealand and Antarctica. No gradient in the carbon isotopic composition between Baring Head and South Pole station was detected. The annual mean δ13C value at Baring Head was −47.13±0.20‰ for 1989 which includes seasonal effects probably due to OH variations and local meteorology. The annual mean 14C value at Baring Head in 1989 was 118.3 percent modern (pMC).

Hydrogen and carbon isotopic compositions of methane in N2- and CO2-rich gases from Central Japan.

N2- and CO2-rich bubble gases collected from five springs and four wells in Paleozoic metamorphic rock and Cretaceous granite areas of Central Japan contain up to 22.9% methane. δD and δ13C values are between -100 and -180‰, and between -24 and -51‰, respectively, for the methane. A linear correlation between δD and δ13C values observed is almost identical with that for the TT(m) type thermogenic methane. The methane in the bubble gases is likely to be originated by thermal decomposition of organic matter in deep-seated metasedimentary rocks. The contribution of volcanic, magmatic and mantle-derived methanes to the bubble gases is probably insignificant, whereas 3He/4He values given by previous works indicate the presence of gas components derived from deep-seated sources, such as the magma, lower crust and mantle, in the gases.

Origins of methane in hydrothermal systems

Isotopic and compositional evidence indicates that the principal source of methane in most subaerial hydrothermal systems is from thermocatalysis of organic matter (thermogenesis). No firm evidence exists for abiogenic production at hydrothermal temperatures. However, abiogenesis represents the principal methane source in high-temperature rock-dominated mid-ocean-ridge hydrothermal systems, in which methane is derived from the rock itself. The importance of carbon-isotope equilibrium between CH 4 and CO 2 at hydrothermal temperatures (≤350°C) is in dispute, although in most systems studied, equilibrium at such temperatures appears unlikely. Carbon isotope compositions of methane in high-temperature systems vary from ∼ −50 to -15‰ vs. PDB, with the most negative values occurring in systems with high CH 4 abundances, i.e. where thermocatalysis is important. Carbon isotope compositions of coexisting CO 2 also decrease with increasing CH 4 abundance (or degree of thermogenic input). This coupled carbon isotope variation suggests that high-temperature oxidation and thermocatalytic breakdown of organic carbon compounds are important in many hydrothermal environments. The enrichment of 13C in methane from high-temperature mid-ocean ridge environments is consistent with isotopic equilibrium at very high temperatures (> 500°C) within the host rocks, and supports the concept of methane genesis within the rock prior to hydrothermal extraction. Unusual 13C-enriched methanes are also found in some subaerial geothermal systems, although a rock-derived origin remains speculative in these cases. In light of the existing evidence, it appears that the sources of methane (either thermogenesis from organic carbon or abiogenesis within rocks) are much more important in determining the relative abundance and carbon isotope composition of hydrothermal methane than any subsequent approach to chemical and/or isotopic equilibrium under hydrothermal conditions.

Carbon isotopic composition of methane in Florida Everglades soils and fractionation during its transport to the troposphere

The δ13C stable carbon isotopic composition of methane collected in bubbles from the submerged soils of specific environments within the Everglades wetland in southern Florida, United States, varied from −70‰ to −63‰ across the system while organic carbon in the soils and dominant plants varied from −28‰ to −25‰. A methane isotopic budget based upon the soil bubble isotope data and published methane flux measurements predicted a flux of isotopic composition −65‰, a value 5‐10‰ more depleted in 13C than the isotopic composition of methane emanating to the atmosphere. Emergent aquatic plants, which are known to be active methane transporters between soil and atmosphere in this ecosystem, were found to transport methane of δ13C content up to 12‰ different from the δ13C content of the soil methane bubble reservoir. Methane 14C content at one site was determined to be 108.6% modern (Δ14C = 83 ± 10‰).

Seasonal variations in ebullitive flux and carbon isotopic composition of methane in a tidal freshwater estuary

Methane carbon isotopic composition ranged from −76.9 to −62.6‰ in a tidal freshwater estuary (the White Oak River, North Carolina, United States) with site specific seasonal variations ranging from 6 to 10‰. During warmer months, tidally induced bubble ebullition actively transported this methane to the atmosphere. At two sites, these seasonally varying fluxes ranged from 1.2 ± 0.3 to 1.3 ± 0.3 mol CH4 m−2yr−1 (19.2 to 20.8 g CH4m−2yr−1), with flux‐weighted average isotopic compositions at two sites of −66.3 ± 0.4 and −69.5 ± 0.6‰. The carbon isotopic composition of naturally released bubbles was shown to be indistinguishable from the sedimentary methane bubble reservoir at three sites, leading to the conclusion that isotopic fractionation did not occur during the ebullition of methane. The hypothesis was developed that ebullitive methane fluxes are depleted in 13CH4 relative to fluxes transported via molecular diffusion or through plants, as zones of 13C enriching microbial methane oxidation are bypassed.

Stable carbon isotopic composition of methane from some natural and anthropogenic sources

The results of δ13C measurements of several types of major sources of atmospheric methane are as follows: rice paddies, −67‰; the peat bogs of the Lake Agassiz region of northern Minnesota, −67±5‰; swamps of the Florida Everglades, −55±3‰ and biomass burning, −24 to −32‰. In addition, results are presented of a study of the δ13C of CH4 released from a slough, compared to the CH4 in the bottom sediment. These isotopic values are used, together with previously published data, to make up a tentative budget of the fluxes of the major sources for atmospheric methane with an average isotopic composition matching the measured value for atmospheric CH4, taking into account the fractionation effect of the sink processes. This budget requires the existence of a significant flux from an anthropogenic source of heavy CH4, calculated to be 45±15 Tg yr−1 if attributed to CH4 from biomass burning, with δC = −25‰.

Carbon isotopic composition of methane in Florida Everglades soils and fractionation during its transport to the troposphere

Carbon isotopic composition of methane in Florida Everglades soils and fractionation during its transport to the troposphere

Seasonal variations in ebullitive flux and carbon isotopic composition of methane in a tidal freshwater estuary

Seasonal variations in ebullitive flux and carbon isotopic composition of methane in a tidal freshwater estuary

A preliminary study on the geochemieal characteristics of the coal-type gas in the Zhongyuan (central China)-Huabei (north China) oil-gas area

Based on the first-hand information on the Zhongyuan-Huabei oil-gas area, this paper gives a description of the geochemical characteristics of the coal-type gas in this area. In terms of the carbon isotopic composition of methane, argon isotopic characteristics of natural gases, the carbon isotopic composition of condensate and the abundance of mercury in natural gases, the paper discusses the differences in chemical and isotopic compositions between the coal-type gas and the oil-type gas, and presents the indices for comprehensively distinguishing the coal-type gas from the oil-type gas. A systematic and scientific dissetation of some typical examples of gas reservoirs, such as the Wenliu, Suqiao and Shenze gas reservoirs, gives evidence suggesting that the gas reservoirs are of coal type. This indicates the existence of natural gas reservoirs of commercial value related to gas-source rocks of the Carboniferous-Permian coal series. This study provides important clues to the exploration and development of the coaltype gas reservoirs in China.

Interpretation of Surface Sediment Methane Carbon Isotopes for Detection of Subsurface Hydrocarbons: ABSTRACT

In surface prospecting for hydrocarbons, it is often assumed that the carbon isotopic composition of methane can provide information on the origin of the gas. Unfortunately, this assumption may not be valid for many methane isotopes collected in recent surface sediments due to (1) mixing of gases having multiple origins and (2) alteration due to secondary processes. Examination of a relatively large data set demonstrates the strong correlation of total methane gas volume to the corresponding isotopic composition. Based upon a comparison of gas volume versus methane isotopes, three types of isotopic compositions have been defined: Type I, small volumes of methane gas with very heavy isotopic compositions; Type II, large volumes of methane gas with very light isotopic compositions; and Type A, average to above-average volumes of methane with moderate isotopic composition. In order to assess the reliability of methane isotope values for surface prospecting, a large data set from deep core samples must be collected. A volume versus isotopic analysis used in combination with the hydrocarbon composition data can aid in determining the significance of the isotopic composition and subsequent interpretation of origin. Those samples which contain both Type A isotopic compositions and anomalous higher molecular weight hydrocarbonsmore » will provide the most reliable information on the origin of migrated gas.« less

Comparison of marine gas hydrates in sediments of an active and passive continental margin

Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3191 m water depth on a spit-like extension of the continental rise on a passive margin in the Atlantic Ocean. Site 568, at 2031 m water depth, is in upper slope sediment of an active accretionary margin in the Pacific Ocean. Both sites are characterized by high rates of sedimentation, and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that transgress sedimentary structures and parallel the seafloor, suggested the presence of gas hydrates at both sites, and, during coring, small samples of gas hydrate were recovered at subbottom depths of 238m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates wer methane, ethane, and CO 2. Residual methane in sediments at both sites usually exceeded 10 mll −1 of wet sediment. Carbon isotopic compositions of methane, CO 2, and ΣCO 2 followed parallel trends with depth, suggesting that methane formed mainly as a result of biological reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. These geochemical characteristics define some of the conditions associated with the occurrence of gas hydrates formed by in situ processes in continental margin sediments.

Geochemical Conditions in Sediment Containing Gas Hydrates of Active and Passive Continental Margins: ABSTRACT

Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3,191 m water depth on a spitlike extension of the continental rise (Blake Outer Ridge) on a passive margin in the Atlantic Ocean. Site 568, at 2,031 m water depth, is in upper-slope sediment of an active accretionary margin (Middle America Trench) in the Pacific Ocean. Both sites are characterized by high rates of sedimentation (greater than 30 m/m.y.), and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that crosscut reflections from sedimentary layers and parallel reflections from the sea floo suggested the presence of gas hydrates at both sites. During coring, small samples of gas End_Page 276------------------------------ hydrate were recovered at subbottom depths of 238 m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates were methane, ethane, and CO2. Residual methane in sediments at both sites usually exceeded 10 ml per liter of wet sediment. Carbon isotopic compositions of methane, CO2, and ^SgrCO2 followed parallel trends with depth, suggesting that methane formed mainly as a result of biologic reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. The small samples of gas hydrates observed visually in cores confirm that gas hydrates are present at these sites, but much of the direct evidence for gas hydrates may be destroyed during the coring and recovery process. End_of_Article - Last_Page 277------------

Applications of Geochemistry to Production, Storage, and Use of Natural Gas: ABSTRACT

Geochemistry has become a standard tool in the exploration for oil and gas. Many of the concepts and techniques developed for exploration can be used with equal effectiveness in identifying environmental problems related to the production, storage, and use of natural gas. Contamination of shallow aquifers as a result of improperly completed gas or oil wells is a problem in some area. Similarly, gas which has migrated from underground gas-storage reservoirs also can contaminate shallow aquifers. Many shallow aquifers contain relatively high concentrations of microbially generated methane, and therefore detection of hydrocarbons is not sufficient to determine the source of the gas. Although microbial gas can frequently be distinguished from thermogenic gases by the absence of ethane and heavier hydrocarbons, migration through hundreds or thousands of feet of porous sediments can result in changes in the chemical composition of the gas, analogous to the changes that occur as gas passes through a chromatographic column. Therefore, the absence of heavier hy rocarbons is not always an indicator of source. Carbon isotopic composition of methane, however, appears to be relatively unaffected by migrational changes and can generally be used to distinguish between microbial and thermogenic methane. Questions also frequently arise as to the source of gas from gas and oil wells around the margins of gas-storage reservoirs. Although chemical analysis can sometimes be useful in distinguishing between storage gas and native gas, these gases are sometimes chemically quite similar. In the event that the gases cannot be distinguished chemically, determination of the carbon and/or the hydrogen isotopic composition of the methane may still provide positive identification. Gases generated in sanitary landfills or marshy areas sometimes can be interpreted as being the result of leakage from pipelines. In addition to the techniques already mentioned, radiocarbon dating of methane can be used to identify gases from these sources. End_of_Article - Last_Page 245------------

Geochemical prospecting in Mexico

In samples of semiconsolidated sediments at depths of 3, 15 and 30 m, C 1–C 7 headspace hydrocarbon gases were studied above a great number of structures in a swamp zone, of which some were known as oil- and/or gas-producing areas, whereas many others were barren. Of these, on oil-producing and one barren structure are selected to illustrate this paper. The data obtained have shown remarkable differences among the gasometric manifestations encountered in producing and barren areas. It was considered important to take samples at different levels, and it was found that beyond a depth of 15 m, biogenic gases differ from thermogenic ones, and gases of vertical migration from a possible horizontal migration. The success of distinction between hydrocarbon-producing and barren structures based upon this analysis is greater than 80%. Analysis of the carbon isotopic composition of methane in the adsorbed form from near-surface samples taking above one oil-producing structure, the data of which are presented here, yielded δ 13C values corresponding to a thermogenic origin of the gas.

Geochemistry of a naturally occurring massive marine gas hydrate

During Deep Sea Drilling Project (DSDP) Leg 84 a core 1 m long and 6 cm in diameter of massive gas hydrate was unexpectedly recovered at Site 570 in upper slope sediment of the Middle America Trench offshore of Guatemala. This core contained only 5–7% sediment, the remainder being the solid hydrate composed of gas and water. Samples of the gas hydrate were decomposed under controlled conditions in a closed container maintained at 4°C. Gas pressure increased and asymptotically approached the equilibrium decomposition pressure for an ideal methane hydrate, CH 4.5-3/4H 2O, of 3930 kPa and approached to this pressure after each time gas was released, until the gas hydrate was completely decomposed. The gas evolved during hydrate decomposition was 99.4% methane, ∼0.2% ethane, and ∼0.4% CO 2. Hydrocarbons from propane to heptane were also present, but in concentrations of less than 100 p.p.m. The carbon-isotopic composition of methane was −41 to −44 permil(( 0 00 ), relative to PDB standard. The observed volumetric methane/water ratio was 64 or 67, which indicates that before it was stored and analyzed, the gas hydrate probably had lost methane. The sample material used in the experiments was likely a mixture of methane hydrate and water ice. Formation of this massive gas hydrate probably involved the following processes: (i) upward migration of gas and its accumulation in a zone where conditions favored the growth of gas hydrates, (ii) continued, unusually rapid biological generation of methane, and (iii) release of gas from water solution as pressure decreased due to sea level lowering and tectonic uplift.

Genetic Characterization of Natural Gases

Natural gases can be characterized genetically using four properties: C2+ concentration, carbon and hydrogen isotope variations in methane, and carbon isotope variation in ethane. Three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters. In these diagrams, compositional fields have been defined for primary gases (biogenic, thermogenic associated, and thermogenic nonassociated) and for gases which result from mixing of these gases. These fields are strictly empirical and comprise compositional variations found in about 500 natural gases. The isotopic and compositional variations in natural gases can be described in terms of (1) processes during formation of the gases such as bacterial fermentation or maturation of organic matter, and (2) processes during secondary migration. Mixing of primary gases is an important and common process. Migration of gases predominantly affects the C2+ concentration, whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged, allowing an assessment of the origin of migrated gases and properties of their source rocks. The formation of gas from humic organic matter and coals is not yet clear from published data. The diagrams use data from various basins and areas. Interstitial gases from the Gulf of California are entirely of bacterial origin: traces of thermogenic gases are formed only in the vicinity of dolerite sills; gases in the south German Molasse basin and in the Vienna basin are of bacterial, mixed, and thermogenic origins. Data from the north Italian Po basin provide examples for genetic characterization of migrated gases.

Fractionation of carbon and hydrogen isotopes by methane-oxidizing bacteria

Carbon isotopic analysis of methane has become a popular technique in the exploration for oil and gas because it can be used to differentiate between thermogenic and microbial gas and can sometimes be used for gas-source rock correlations. Methane-oxidizing bacteria, however, can significantly change the carbon isotopic composition of methane; the origin of gas that has been partially oxidized by these bacteria could therefore be misinterpreted. We cultured methane-oxidizing bacteria at two different temperatures and monitored the carbon and hydrogen isotopic compositions of the residual methane. The residual methane was enriched in both 13C and D. For both isotopic species, the enrichment at equivalent levels of conversion was greater at 26°C than at 11.5°C. The change in δD relative to the change in δ 13C was independent of temperature within the range studied. One culture exhibited a change in the fractionation pattern for carbon (but not for hydrogen) midway through the experiment, suggesting that bacterial oxidation of methane may occur via more than one pathway. The change in the δD value for the residual methane was from 8 to 14 times greater than the change in the δ 13C value, indicating that combined carbon and hydrogen isotopic analysis may be an effective way of identifying methane which has been subjected to partial oxidation by bacteria.