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1.
Previous studies of methane and higher hydrocarbon gases in Precambrian Shield rocks in Canada and the Witwatersrand Basin of South Africa identified two major gas types. Paleometeoric waters were dominated by hydrocarbon gases with compositional and isotopic characteristics consistent with production by methanogens utilizing the CO2 reduction pathway. In contrast the deepest, most saline fracture waters contained gases that did not resemble the products of microbial methanogenesis and were dominated by both high concentrations of H2 gas, and CH4 and higher hydrocarbon gases with isotopic signatures attributed to abiogenic processes of water-rock reaction in these high rock/water ratio, hydrogeologically-isolated fracture waters. Based on new data obtained for the higher hydrocarbon gases in particular, a model is proposed to account for carbon isotope variation between CH4 and the higher hydrocarbon gases (specifically ethane, propane, butane, and pentane) consistent with abiogenic polymerization. Values of δ13C for CH4 and the higher hydrocarbon gases predicted by the model are shown to match proposed abiogenic hydrocarbon gas end-members identified at five field sites (two in Canada and three in South Africa) suggesting that the carbon isotope patterns between the hydrocarbon homologs reflect the reaction mechanism. In addition, the δ2H isotope data for these gases are shown to be out of isotopic equilibrium, suggesting the consistent apparent fractionation observed between the hydrocarbon homologs may also reflect reaction mechanisms involved in the formation of the gases. Recent experimental and field studies of proposed abiogenic hydrocarbons such as those found at mid-ocean spreading centers and off-axis hydrothermal fields such as Lost City have begun to focus not only on the origin of CH4, but on the compositional and isotopic information contained in the higher hydrocarbon gases. The model explored in this paper suggests that while the extent of fractionation in the first step in the hydrocarbon synthesis reaction chain may vary as a function of different reaction parameters, δ13C values for the higher hydrocarbon gases may be predicted by a simple mass balance model from the δ13C values of the lower molecular weight precursors, consistent with abiogenic polymerization. Integration of isotopic data for the higher hydrocarbon gases in addition to CH4 may be critical for delineation of the origin of the hydrocarbons and investigation of formation mechanisms.  相似文献   

2.
A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH4 and CO2 in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in 13C by ∼10‰ in values of δ13C of CH4, and by ∼0.55‰ in values of δ13C of CO2, relative to the values of the high-temperature source fluid (δ13C of CH4 =−20.1 ± 1.2‰, δ13C of CO2 =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in 13C of both CH4 and CO2 at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and 13C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH4 and CO2 concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable 13C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis.  相似文献   

3.
The chemical and isotopic composition of fumarolic gases emitted from Nisyros Volcano, Greece, and of a single gas sample from Vesuvio, Italy, was investigated in order to determine the origin of methane (CH4) within two subduction-related magmatic-hydrothermal environments.Apparent temperatures derived from carbon isotope partitioning between CH4 and CO2 of around 340°C for Nisyros and 470°C for Vesuvio correlate well with aquifer temperatures as measured directly and/or inferred from compositional data using the H2O-H2-CO2-CO-CH4 geothermometer. Thermodynamic modeling reveals chemical equilibrium between CH4, CO2 and H2O implying that carbon isotope partitioning between CO2 and CH4 in both systems is controlled by aquifer temperature.N2/3He and CH4/3He ratios of Nisyros fumarolic gases are unusually low for subduction zone gases and correspond to those of midoceanic ridge environments. Accordingly, CH4 may have been primarily generated through the reduction of CO2 by H2 in the absence of any organic matter following a Fischer-Tropsch-type reaction. However, primary occurrence of minor amounts of thermogenic CH4 and subsequent re-equilibration with co-existing CO2 cannot be ruled out entirely. CO2/3He ratios and δ13CCO2 values imply that the evolved CO2 either derives from a metasomatized mantle or is a mixture between two components, one outgassing from an unaltered mantle and the other released by thermal breakdown of marine carbonates. The latter may contain traces of organic matter possibly decomposing to CH4 during thermometamorphism.  相似文献   

4.
《Applied Geochemistry》1999,14(1):119-131
The major source of methane (CH4) in subsurface sediments on the property of a former hazardous waste treatment facility was determined using isotopic analyses measured on CH4 and associated groundwater. The site, located on an earthen pier built into a shallow wetland lake, has had a history of waste disposal practices and is surrounded by landfills and other waste management facilities. Concentrations of CH4 up to 70% were found in the headspace gases of several piezometers screened at 3 different depths (ranging from 8 to 17 m) in lacustrine and glacial till deposits. Possible sources of the CH4 included a nearby landfill, organic wastes from previous impoundments and microbial gas derived from natural organic matter in the sediments.Isotopic analyses included δ13C, δD, 14C, and 3H on select CH4 samples and δD and δ18O on groundwater samples. Methane from the deepest glacial till and intermediate lacustrine deposits had δ13C values from −79 to −82‰, typical of natural “drift gas” generated by microbial CO2-reduction. The CH4 from the shallow lacustrine deposits had δ13C values from −63 to −76‰, interpreted as a mixture between CH4 generated by microbial fermentation and the CO2-reduction processes within the subsurface sediments. The δD values of all the CH4 samples were quite negative ranging from −272 to −299‰. Groundwater sampled from the deeper zones also showed quite negative δD values that explained the light δD observed for the CH4. Radiocarbon analyses of the CH4 showed decreasing 14C activity with depth, from a high of 58 pMC in the shallow sediments to 2 pMC in the deeper glacial till. The isotopic data indicated the majority of CH4 detected in the till deposits of this site was microbial CH4 generated from naturally buried organic matter within the subsurface sediments. However, the isotopic data of CH4 from the shallow piezometers was more variable and the possibility of some mixing with oxidized landfill CH4 could not be completely ruled out.  相似文献   

5.
We have developed a quantitative model of CO2 and H2O isotopic mixing between magmatic and hydrothermal gases for the fumarolic emissions of the La Fossa crater (Vulcano Island, Italy). On the basis of isotope balance equations, the model takes into account the isotope equilibrium between H2O and CO2 and extends the recent model of chemical and energy two-end-member mixing by Nuccio et al. (1999). As a result, the H2O and CO2 content and the δD, δ18O, and δ13C isotope compositions for both magmatic and hydrothermal end-members have been assessed. Low contributions of meteoric steam, added at a shallow depth, have been also recognized and quantified in the fumaroles throughout the period from 1988 to 1998. Nonequilibrium oxygen isotope exchange also seems to be occurring between ascending gases and wall rocks along some fumarolic conduits.The δ13CCO2 of the magmatic gases varies around −3 to 1‰ vs. Peedee belemnite (PDB), following a perfect synchronism with the variations of the CO2 concentration in the magmatic gases. This suggests a process of isotope fractionation because of vapor exsolution caused by magma depressurization. The hydrogen isotopes in the magmatic gases (−1 to −‰ vs. standard mean ocean water [SMOW]), as well as the above δ13CCO2 value, are coherent with a convergent tectonic setting of magma generation, where the local mantle is widely contaminated by fluids released from the subducted slab. Magma contamination in the crust probably amplifies this effect.The computed isotope composition of carbon and hydrogen in the hydrothermal vapors has been used to calculate the δD and δ13C of the entire hydrothermal system, including mixed H2O-CO2 vapor, liquid water, and dissolved carbon. We have computed values of about 10‰ vs. SMOW for water and −2 to −6.5‰ vs. PDB for CO2. On these grounds, we think that Mediterranean marine water (δDH2O ≈ 10‰) feeds the hydrothermal system. It infiltrates at depth throughout the local rocks, reaching oxygen isotope equilibrium at high temperatures. Interaction processes between magmatic gases and the evolving seawater also seem to occur, causing the dissolution of isotopically fractionated aqueous CO2 and providing the source for hydrothermal carbon. These results have important implications concerning fluid circulation beneath Vulcano and address the more convenient routine of geochemical surveillance.  相似文献   

6.
A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C1/C2+ > 100), high C2/C3 and i-C4/n-C4 ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ13CCH4 from −79.5‰ to −45.0‰, δ13CC2H6 from −53.7‰ to −32.2‰, δ13CC3H8 from −36.5‰ to −20.1‰, δ13CnC4H10 from −32.7‰ to −24.5‰, and δ13CCO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with 13C enriched methane and 13C depleted CO2, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, 13C enriched methane and variable δ13C of CO2, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with 13C depleted methane and 13C enriched CO2, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain 13C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.  相似文献   

7.
This paper reports the isotope effects in an open-system Fischer-Tropsch type (FTT) synthesis, with implications for the origin of natural abiogenic hydrocarbons. The starting form of carbon was CO2, with carbon and hydrogen isotopic compositions measured for products of catalytic hydrogenation of CO2 on iron and cobalt catalysts (FTCO2-Fe and FTCO2-Co) at 350 and 245 °C, respectively, and 10 MPa. The carbon isotopic composition of the resulting saturated hydrocarbons (alkanes) as a function of carbon number shows a positive trend for both FTCO2-Fe and FTCO2-Co, with a fractionation of 2-4‰ and 3-6‰ between CH4 and C2H6 over the Fe and Co catalysts, respectively. The unsaturated hydrocarbons (alkenes) do not show any trend. A strong kinetic isotope fractionation (>40‰) occurred between CO2 and CH4 in both experiments. The hydrogen isotope fractionation between alkanes appeared to be similar to that found in natural (thermogenic and biogenic) gases, with enrichment in deuterium of longer hydrocarbon chains; the dominant H/D fractionation occurred between CH4 and C2H6. Alkenes in the products of the FTCO2-Fe reaction are enriched in deuterium (∼50‰) and do not show any trend versus carbon number. We suggest that other than FTT reactions or a simple mixing are responsible for the occurrence of the inverse isotopic trends in both δ13C and δD found in light hydrocarbons in some terrestrial environments and meteorites.  相似文献   

8.
A series of laboratory studies were conducted to increase understanding of stable carbon (13C/12C) and hydrogen (D/H) isotope fractionation arising from methanogenesis by moderately thermophilic acetate- and hydrogen-consuming methanogens. Studies of the aceticlastic reaction were conducted with two closely related strains of Methanosaeta thermophila. Results demonstrate a carbon isotope fractionation of only 7‰ (α = 1.007) between the methyl position of acetate and the resulting methane. Methane formed by this process is enriched in 13C when compared with other natural sources of methane; the magnitude of this isotope effect raises the possibility that methane produced at elevated temperature by the aceticlastic reaction could be mistaken for thermogenic methane based on carbon isotopic content. Studies of H2/CO2 methanogenesis were conducted with Methanothermobacter marburgensis. The fractionation of carbon isotopes between CO2 and CH4 was found to range from 22 to 58‰ (1.023 ≤ α ≤ 1.064). Greater fractionation was associated with low levels of molecular hydrogen and steady-state metabolism. The fractionation of hydrogen isotopes between source H2O and CH4 was found to range from 127 to 275‰ (1.16 ≤ α ≤ 1.43). Fractionation was dependent on growth phase with greater fractionation associated with later growth stages. The maximum observed fractionation factor was 1.43, independent of the δD-H2 supplied to the culture. Fractionation was positively correlated with temperature and/or metabolic rate. Results demonstrate significant variability in both hydrogen and carbon isotope fractionation during methanogenesis from H2/CO2. The relatively small fractionation associated with deuterium during H2/CO2 methanogenesis provides an explanation for the relatively enriched deuterium content of biogenic natural gas originating from a variety of thermal environments. Results from these experiments are used to develop a hypothesis that differential reversibility in the enzymatic steps of the H2/CO2 pathway gives rise to variability in the observed carbon isotope fractionation. Results are further used to constrain the overall efficiency of electron consumption by way of the hydrogenase system in M. marburgensis, which is calculated to be less than 55%.  相似文献   

9.
The δ13C values of dissolved HCO3? in 75 water samples from 15 oil and gas fields (San Joaquin Valley, Calif., and the Houston-Galveston and Corpus Christi areas of Texas) were determined to study the sources of CO2 of the dissolved species and carbonate cements that modify the porosity and permeability of many petroleum reservoir rocks. The reservoir rocks are sandstones which range in age from Eocene through Miocene. The δ13C values of total HCO3? indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin.The range of δ13C values for the HCO3? of these waters is ?20–28 per mil relative to PDB. This wide range of δ13C values is explained by three mechanisms. Microbiological degradation of organic matter appears to be the dominant process controlling the extremely low and high δ13C values of HCO3? in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low δ13C values (< ?10 per mil) are obtained in waters where concentrations of SO42? are more than 25 mg/l and probably result from the degradation of organic acid anions by sulfate-reducing bacteria (SO42? + CH3COO? → 2HCO3? + HS?). The high δ13C values probably result from the degradation of these anions by methanogenic bacteria (CH3COO? + H2OaiHCO3? + CH4).Thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO2 and CH4 is probably the major source of CO2 for production zones with subsurface temperatures greater than 80°C. The δ13C values of HCO3? for waters from zones with temperatures greater than 100°C result from isotopic equilibration between CO2 and CH4. At these high temperatures, δ13C values of HCO3? decrease with increasing temperatures and decreasing concentrations of these acid anions.  相似文献   

10.
New isotopic and chemical data on the sodium bicarbonate water and associated gases from the Razdolnoe Spa located in the coastal zone of Primorsky Kray of the Russian Far East, together with previous stable isotope data (δ18O, δD, δ13C), allow elucidation of the origin and evolution of the groundwater and gases from the spa. The water is characterized by low temperature (12 °C), TDS – 2.5–6.0 g/L, high contents of B (∼5 mg/L) and F (4.5 mg/L) and low contents of Cl and SO4. Water isotopic composition indicates its essentially meteoric origin which may comply with an older groundwater that was recharged under different (colder) climatic conditions. Major components of bubbling gases are CH4 (68 vol%), N2 (28%) and CO2 (4%). The obtained values δ13C and δD for CO2 and CH4 definitely indicate the marine microbial origin of methane. Thus the high methane content in the waters relates to the biochemical processes and presence of a dispersed organic matter in the host rocks. Based on the regional hydrogeology and the geological structure of the Razdolnoe Spa, Mesozoic fractured rocks containing Na–HCO3 mineral water and gases are reservoir rocks, a chemical composition of water and gases originates in different environmental conditions.  相似文献   

11.
Carbon isotope fractionation factors reported for aerobic bacterial oxidation of CH4(αCH4-CO2) range from 1.003 to 1.039. In a series of experiments designed to monitor changes in the carbon isotopic fractionation of CH4 by Type I and Type II methanotrophic bacteria, we found that the magnitude of fractionation was largely due to the first oxidation step catalyzed by methane monooxygenase (MMO). The most important factor that modulates the (αCH4-CH3OH) is the fraction of the total CH4 oxidized per unit time, which strongly correlates to the cell density of the growth cultures under constant flow conditions. At cell densities of less than 0.1 g/L, fractionation factors greater than 1.03 were observed, whereas at cell densities greater than 0.5 g/L the fractionation factors decreased to as low as 1.002. At low cell densities, low concentrations of MMO limit the amount of CH4 oxidized, while at higher cell densities, the overall rates of CH4 oxidation increase sufficiently that diffusion of CH4 from the gaseous to dissolved state and into the cells is likely the rate-determining step. Thus, the residual CH4 is more fractionated at low cell densities, when only a small fraction of the total CH4 has been oxidized, than at high cell densities, when up to 40% of the influent CH4 has been utilized. Therefore, since Rayleigh distillation behavior is not observed, δ13C values of the residual CH4 cannot be used to infer the amount oxidized in either laboratory or field-studies. The measured (αCH4-CH3OH) was the same for both Type I and Type II methanotrophs expressing particulate or soluble MMO. However, large differences in the δ13C values of biomass produced by the two types of methanotrophs were observed. Methylosinus trichosporium OB3b (Type II) produced biomass with δ13C values about 15‰ higher than the dissimilated CO2, whereas Methylomonas methanica (Type I) produced biomass with δ13C values only about 6‰ higher than the CO2. These effects were independent of the magnitude of the initial carbon isotope fractionation caused by MMO and were relatively constant despite changing ratios of assimilatory to dissimilatory carbon transformation by the organisms. This suggests that the difference in biomass carbon isotopes is primarily due to differences in the fractionation effect at the formaldehyde branch point in the metabolic pathway, rather than assimilation of CO2 by Type II methanotrophs.  相似文献   

12.
We investigated the effect of microbial activity on the chemistry of hydrothermal fluids related to the Vicano–Cimino system, central Italy. The database included the composition and δ13C CO2 and δ13C CH4 values for soil gas from an area characterized by intense degassing of fluids having a deep origin. The δ13C CH4 values along vertical profiles in the soil indicated that CH4 was controlled by microbial oxidation occurring at shallow (< 50 cm) depth, where free O2 was available. This was consistent with the vertical gradients of CH4, H2S and O2 concentrations. The δ13C CO2 values in soil gas, characterized by a composition similar to that of the hydrothermal fluids, were not significantly influenced by biodegradation. On the contrary, gas strongly affected by air contamination showed a significant δ13C CO2 fractionation. Microbial activity caused strong consumption of hydrothermal alkanes, alkenes, cyclics and hydrogenated halocarbons, whereas benzene was recalcitrant. Oxygenated compounds from hydrocarbon degradation consisted of alcohols, with minor aldehydes, ketones and carboxylic acids. A predominance of alcohols at a high rate of degassing flux, corresponding to a short residence time of hydrothermal gas within the soil, indicated incomplete oxidation. N-bearing compounds were likely produced by humic substances in the soil and/or related to contamination by pesticides, whereas α-pinene traced air entering the soil. The study demonstrates that microbial communities in the soil play an important role for mitigating the release to the atmosphere of C-bearing gases, especially CH4, through diffuse soil degassing, a mechanism that in central Italy significantly contributes to the discharge of CO2-rich gas from deep sources.  相似文献   

13.
Rice fields are an important source for the greenhouse gas methane produced by acetoclastic and hydrogenotrophic methanogenesis. Fractionation of 13C/12C can in principle be used to quantify the relative contribution of these pathways, but our knowledge of isotopic fractionation during reduction of CO2 and turnover of acetate in different methanogenic environments is still scarce. We therefore measured δ13C signatures in two types of anoxic Italian rice field soils, one with high and one with low degradable organic matter (OM) content. Both soils were incubated in the presence and absence of methyl fluoride, a specific inhibitor of acetoclastic methanogenesis. Optimization of methyl fluoride concentration resulted in complete inhibition of acetoclastic methanogenesis. CH4 was then exclusively produced by hydrogenotrophic methanogenesis, allowing determination of the isotopic signatures and fractionation factors specific for this methanogenic pathway. Acetate, which was then no longer consumed, accumulated and was used for determination of the isotopic signature of the fermentatively produced acetate (both total acetate and methyl carbon of acetate). Hence, all isotopic signatures, including fractionation factors were determined for the methanogenic soil. These data, were then used for computation of the relative contribution of the two methanogenic pathways. In the high OM soil, the contribution of acetoclastic methanogenesis to total CH4 production increased simultaneously with decreasing acetate concentration. In the low OM soil, methanogenesis from H2/CO2 was clearly greater than theoretically expected. Furthermore, isotope fractionation of hydrogenotrophic methanogenesis indicated that the in situ energy status of methanogens strongly depended on the availability of organic carbon in the rice field soil system. Collectively, our data show that the study of isotopic fractionation in methanogenic environments allows a deeper insight into the ongoing processes, which may be quite different in the same ecosystem with different content of degradable OM.  相似文献   

14.
On the basis of recently reported data on the kinetics of carbon-13 exchange between CO2 and CH4 at temperatures above 500°C, first order rate constants log k = 11.16?10,190/T were derived allowing variations in Δ, the difference in the isotopic composition of coexisting CO2 and CH4, to be evaluated as a function of initial composition and cooling rate of the rising geothermal fluid. Observed Δ-values in geothermal discharges are likely to represent frozen in compositions attained after minimum residence times of 20 ka at 400°C or 10 Ma at 300°C. The carbon-13 contents of any biogenic gases are unlikely to have been affected by thermal re-equilibration at temperatures below 200°C. The chemical equilibrium involving CO2 and CH4 can be expected to proceed about a hundred times faster than isotopic equilibration.  相似文献   

15.
In this study, compositions and δ13C and δ2H isotopic values of hydrocarbon gases from 5 mines in the Witwatersrand basin, South Africa, support the widespread occurrence of microbially produced methane in millions of years-old fissure waters. The presence of microbial methane is, to a large extent, controlled by the geologic formations in which the gases are found. Samples from the Witwatersand Supergroup have the largest microbial component based on δ13C and δ2H signatures and CH4/C2+ values. Based on mixing between a microbial CH4 component and a more 13C-enriched and 2H-depleted C2+-rich end member, conservative estimates of the % contribution of microbial CH4 to the gas samples range from >90% microbial CH4 at Beatrix, Masimong, and Merriespruit, to between 5 and 80% microbial CH4 at Evander, and <18% microbial CH4 at Kloof. The Witwatersrand basin’s history of thermal alteration of organic-rich ancient sedimentary units suggests a thermogenic origin for this 13C-enriched end member. Alternatively, the potential for an abiogenic origin similar to hydrocarbon gases produced by water-rock interaction at other Precambrian Shield mines is discussed. Microbial methane is predominantly found in paleo-meteoric fissure waters with δ18O and δ2H values that fall on the meteoric waterline, and have temperatures between 30 to 40°C. In contrast, fissure waters with a larger component of nonmicrobial hydrocarbon gases show a trend towards more enriched δ18O and δ2H values that fall well above the meteoric waterline, and temperatures of 45 to 60°C. The enrichment in 18O and 2H in these samples, and their high salinity, are similar to the isotopic and compositional characteristics of saline groundwaters and brines produced by water-rock interaction at Precambrian Shield sites elsewhere. The reported 100 Ma ages of fissure waters from the Witwatersrand and Ventersdorp formations suggest that these microbial hydrocarbon gases are the product of in situ methanogenic communities in the deep subsurface of the Witswaterand basin. Small subunit ribosomal RNA genes were amplified using archaeal-specific primer sets from DNA extracts derived from several of these waters. Fissure waters with a high proportion of microbial methane also contained sequences resembling those of known methanogens.  相似文献   

16.
According to the adsorption-desorption characteristics of coalbed gas and analysis of various experimental data, this paper proposes that the generation of secondary biogenic gas (SBG) and its mixing of with the residual thermogenic gas at an early stage inevitably lead to secondary changes of the thermogenic gas and various geochemical additive effects. Experimental results also show that the fractionation of the carbon isotope of methane of coal core desorption gas changes very little; the δ13C1 value of the mixed gas of biogenic and thermogenic gases is between the δ13C1 values of the two “original” gases, and the value is determined by the carbon isotopic compositions and mixing proportions of the two “original” methanes. Therefore this paper proposes that the study on the secondary changes of the thermogenic gas and various additive effects is a new effective way to study and identify SBG. Herein, a systematic example of research on the coalbed gas (Huainan coalbed gas) is further conducted, revealing a series of secondary changes and additive effects, the main characteristics and markers of which are: (1) the contents of CO2 and heavy-hydrocarbons decrease significantly; (2) the content of CH4 increases and the gas becomes drier; (3) the δ13C and δD values of methane decrease significantly and tend to have biogenetic characteristics; and (4) the values of δ13C2 and δ13CCO2 grow higher. These isotopic values also change with the degradation degrees by microbes and mixing proportions of the two kinds of gases in different locations. There exists a negative correlation between the δ13C1 vs δ13CCO2 values. The △δ13CC2–C1 values obviously become higher. The distributions of the △δ13CCO2–C1 values are within certain limits and show regularity. There exist a positive correlation between the N2 versus Ar contents, and a negative correlation between the N2 versus CH4 contents, indicating the down forward infiltration of the surface water containing air. These are important markers of the generation and existence of SBG.  相似文献   

17.
In this study, the geochemistry and origin of natural gas and formation waters in Devonian age organic-rich shales and reservoir sandstones across the northern Appalachian Basin margin (western New York, eastern Ohio, northwestern Pennsylvania, and eastern Kentucky) were investigated. Additional samples were collected from Mississippian Berea Sandstone, Silurian Medina Sandstone and Ordovician Trenton/Black River Group oil and gas wells for comparison. Dissolved gases in shallow groundwaters in Devonian organic-rich shales along Lake Erie contain detectable CH4 (0.01–50.55 mol%) with low δ13C–CH4 values (−74.68 to −57.86‰) and no higher chain hydrocarbons, characteristics typical of microbial gas. Nevertheless, these groundwaters have only moderate alkalinity (1.14–8.72 meq/kg) and relatively low δ13C values of dissolved inorganic C (DIC) (−24.8 to −0.6‰), suggesting that microbial methanogenesis is limited. The majority of natural gases in Devonian organic-rich shales and sandstones at depth (>168 m) in the northern Appalachian Basin have a low CH4 to ethane and propane ratios (3–35 mol%; C1/C2 + C3) and high δ13C and δD values of CH4 (−53.35 to −40.24‰, and −315.0 to −174.6‰, respectively), which increase in depth, reservoir age and thermal maturity; the molecular and isotopic signature of these gases show that CH4 was generated via thermogenic processes. Despite this, the geochemistry of co-produced brines shows evidence for microbial activity. High δ13C values of DIC (>+10‰), slightly elevated alkalinity (up to 12.01 meq/kg) and low SO4 values (<1 mmole/L) in select Devonian organic-rich shale and sandstone formation water samples suggest the presence of methanogenesis, while low δ13C–DIC values (<−22‰) and relatively high SO4 concentrations (up to 12.31 mmole/L) in many brine samples point to SO4 reduction, which likely limits microbial CH4 generation in the Appalachian Basin. Together the formation water and gas results suggest that the vast majority of CH4 in the Devonian organic-rich shales and sandstones across the northern Appalachian Basin margin is thermogenic in origin. Small accumulations of microbial CH4 are present at shallow depths along Lake Erie and in western NY.  相似文献   

18.
Permian Khuff reservoirs along the east coast of Saudi Arabia and in the Arabian Gulf produce dry sour gas with highly variable nitrogen concentrations. Rough correlations between N2/CH4, CO2/CH4 and H2S/CH4 suggest that non-hydrocarbon gas abundances are controlled by thermochemical sulfate reduction (TSR). In Khuff gases judged to be unaltered by TSR, methane δ13C generally falls between −40‰ and −35‰ VPDB and carbon dioxide δ13C between −3‰ and 0‰ VPDB. As H2S/CH4 increases, methane δ13C increases to as much as −3‰ and carbon dioxide δ13C decreases to as little as −28‰. These changes are interpreted to reflect the oxidation of methane to carbon dioxide.Khuff reservoir temperatures, which locally exceed 150 °C, appear high enough to drive the reduction of sulfate by methane. Anhydrite is abundant in the Khuff and fine grained nodules are commonly rimmed with secondary calcite cement. Some cores contain abundant pyrite, sphalerite and galena. Assuming that nitrogen is inert, loss of methane by TSR should increase N2/CH4 of the residual gas and leave δ15N unaltered. δ15N of Paleozoic gases in Saudi Arabia varies from −7‰ to 1‰ vs. air and supports the TSR hypothesis. N2/CH4 in gases from stacked Khuff reservoirs varies by a factor of 19 yet the variation in δ15N (0.3–0.5‰) is trivial.Because the relative abundance of hydrogen sulfide is not a fully reliable extent of reaction parameter, we have attempted to assess the extent of TSR using plots of methane δ13C versus log(N2/CH4). Observed variations in these parameters can be fitted using simple Rayleigh models with kinetic carbon isotope fractionation factors between 0.98 and 0.99. We calculate that TSR may have destroyed more than 90% of the original methane charge in the most extreme instance. The possibility that methane may be completely destroyed by TSR has important implications for deep gas exploration and the origin of gases rich in nitrogen as well as hydrogen sulfide.  相似文献   

19.
Production of CH4 and CO2 was quantified in anoxically incubated soil samples taken from an Italian rice field. The rates increased with temperature between 10 and 37°C. The δ13C of the accumulated CO2, CH4 and acetate changed with time in a systematic way. The data were used in mass balance equations to constrain isotopic fractionation factors and pathways of CH4 production. The calculations were further constrained by the determination of 14CH4 production from 14CO2 at steady state. At 50°C, CH4 was exclusively produced from CO2, indicating a fractionation factor of αCO2/CH4 = 1.073. Between 10 and 37°C, the results showed a temporal change in the methanogenic pathway. A relatively high (40-60%) CO2-derived fraction of CH4 production in the beginning was followed by a phase in which contribution of CO2-derived CH4 decreased to low (<15%) values, and ultimately by the steady state phase in which values increased to <40% (the theoretically expected value). The rate of change from one phase to the next increased with temperature. Incubation temperature had a strong effect on the overall fractionation of 13C during the formation and consumption of acetate, with stronger fractionation at low than at high temperature. The results further showed that, especially at low temperatures, fractionation occurred during acetate turnover and acetoclastic methanogenesis, despite the fact that steady-state conditions caused (apparent) substrate-limitation.  相似文献   

20.
Molecular transport (diffusion) of methane in water-saturated sedimentary rocks results in carbon isotope fractionation. In order to quantify the diffusive isotope fractionation effect and its dependence on total organic carbon (TOC) content, experimental measurements have been performed on three natural shale samples with TOC values ranging from 0.3 to 5.74%. The experiments were conducted at 90°C and fluid pressures of 9 MPa (90 bar). Based on the instantaneous and cumulative composition of the diffused methane, effective diffusion coefficients of the 12CH4 and 13CH4 species, respectively, have been calculated.Compared with the carbon isotopic composition of the source methane (δ13C1 = −39.1‰), a significant depletion of the heavier carbon isotope (13C) in the diffused methane was observed for all three shales. The degree of depletion is highest during the initial non-steady state of the diffusion process. It then gradually decreases and reaches a constant difference (Δ δ = δ13Cdiff −δ13Csource) when approaching the steady-state. The degree of the isotopic fractionation of methane due to molecular diffusion increases with the TOC content of the shales. The carbon isotope fractionation of methane during molecular migration results practically exclusively from differences in molecular mobility (effective diffusion coefficients) of the 12CH4 and 13CH4 entities. No measurable solubility fractionation was observed.The experimental isotope-specific diffusion data were used in two hypothetical scenarios to illustrate the extent of isotopic fractionation to be expected as a result of molecular transport in geological systems with shales of different TOC contents. The first scenario considers the progression of a diffusion front from a constant source (gas reservoir) into a homogeneous “semi-infinite” shale caprock over a period of 10 Ma.In the second example, gas diffusion across a 100 m caprock sequence is analyzed in terms of absolute quantities and isotope fractionation effects. The examples demonstrate that methane losses by molecular diffusion are small in comparison with the contents of commercial size gas accumulations. The degree of isotopic fractionation is related inversely to the quantity of diffused gas so that strong fractionation effects are only observed for relatively small portions of gas.The experimental data can be readily used in numerical basin analysis to examine the effects of diffusion-related isotopic fractionation on the composition of natural gas reservoirs.  相似文献   

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