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1.
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   

2.
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   

3.
《Organic Geochemistry》1987,11(2):115-119
This paper presents C and H isotope compositions of compounds involved in methane production by pure cultures of Methanobacterium formicicum. The C isotope compositions of the methane produced and of the residual CO2 are compared to data observed in natural conditions in marine sediments. This comparison leads to further evidence that CO2 reduction is an important mechanism for microbial generation of methane in deep marine sediments. The H isotope compositions show involvment of the water hydrogen into methane as well as hydrogen exchange between water and molecular hydrogen in the course of CO2 reduction. A mechanism is proposed as a possible explanation for the data obtained involving conjugated reactions of CO2-reduction and enzymatic reduction of water.  相似文献   

4.
Vertical profiles of concentration and C-isotopic composition of dissolved methane and carbon dioxide were observed over 26 months in the catotelm of a deep (6.5 m) peat bog in Switzerland. The dissolved concentrations of these gases increase with depth while CO2 predominates over CH4 (CO2 ca. 5 times CH4). This pattern can be reproduced by a reaction-advection-ebullition model, where CO2 and CH4 are formed in a ratio of 1:1. The less soluble methane is preferentially lost via outgassing (bubbles). The isotopic fractionation between CO2 and CH4 also increases with depth, with αC values ranging from 1.045 to 1.075. The isotopic composition of the gases traces the passage of respiration-derived CO2 (from the near surface) through a shallow zone with methanogenesis of low isotopic fractionation (splitting of fermentation-derived acetate). This solution then moves through the catotelm, where methanogenesis occurs by CO2 reduction (large isotopic fractionation). In the upper part of the catotelm the C-13-depleted respiration-derived CO2 pool buffers the isotopic composition of CO2; the δ13C of CO2 increases only slowly. At the same time strongly depleted CH4 is formed as CO2 reduction consumes the depleted CO2. In the lower part of the catotelm, the respiration-derived CO2 and shallow CH4 become less important and CO2 reduction is the dominant source of CO2 and CH4. Now, the δ13C values of both gases increase until equilibrium is reached with respect to the isotopic composition of the substrate. Thus, the δ13C values of methane reach a minimum at intermediate depth, and the deep methane has δ13C values comparable to shallow methane. A simple mixing model for the isotopic evolution is suggested. Only minor changes of the observed patterns of methanogenesis (in terms of concentration and isotopic composition) occur over the seasons. The most pronounced of these is a slightly higher rate of acetate splitting in spring.  相似文献   

5.
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6.
Carbon stable-isotope compositions of coexisting carbon dioxide and methane from geothermal springs across the Central Andes of northern Chile and Bolivia are reported. A total of 60 samples were analyzed for δ13CCO2 and, of these, 10 were selected for δ13CCH4 analyses. The Central Andes are characterized by an active volcanic arc and an unusually thick (up to 75 km) continental crust behind the arc, beneath the high plateau region of the Altiplano. Furthermore, helium-isotope evidence suggests active mantle degassing in a 350-km-wide zone beneath the thick continental crust in the Central Andes (Hoke et al., 1994).

The present results show a wide range of δ13CCO2 (-14.9 to -0.6‰) and a surprisingly heavy δ13CCH4 (?20.9 to ?12.3‰). The difference between δ13CCO2 and δ13CCH4 (δ13CCO2-CH4 ) for individual samples varies between 1.5‰ and 13.5‰. The δ13CCO2 results show wide and overlapping ranges in the samples collected from the Precordillera, the Volcanic Arc (or Western Cordillera), the Altiplano, and the Eastern Cordillera. The widest ranges occur in the Eastern Cordillera (?15.0 to ?4.8‰) and the Altiplano (?20 to ?6‰). The δ13CCO2 results for geothermal samples from the Volcanic Arc range between ?8.0‰ (Surire) and ?0.6‰ (Abra de Nappa), whereas δ13CCO2 measured in gases collected from geothermal springs in the Precordillera range from ?10 to ?5‰.

The relationships between 3He/4He, δ13CCO2 , and δ13CCH4 are used to distinguish between crustal and mantle origins. The wide (21‰) range in the is interpreted to reflect contributions from different CO2 sources that include organic and inorganic crustal and mantle carbon. Assuming isotopic equilibrium between coexisting methane and carbon dioxide, Δ13CCO2-CH4 suggests very high equilibrium temperatures, in excess of 530°C, for some geothermal systems that also are characterized by a high (up to 63%) mantle-derived helium component.

δ13CCH4 results suggest that methane has not formed by bacteriogenic processes or by thermal decomposition of organic matter, but rather abiogenically through the high-temperature reaction between H2 and CO2. The δ13CCH4 results for the samples from the Volcanic Arc and from two CO2-rich geothermal springs in the Altiplano (Coipasa-2 and Belen de Andamarca) are similar to those reported from hydrothermal fluids emitted from the East Pacific Rise (Welhan, 1988) and White Island, New Zealand (Hulston and McCabe, 1962), suggesting a mantle-derived carbon component in the methane.  相似文献   

7.
Niutuozhen geothermal field is located in the Jizhong graben, belonging to the northern part of Bohai Bay Basin in North China. Chemical and isotopic analyses were carried out on 14 samples of the geothermal fluids discharged from Neogene Minghuazhen (Nm), Guantao (Ng), and Jixianian Wumishan (Jxw) formations. The δ2H and δ18O in water, δ13C in CH4, δ13C in CO2, and 3He/4He ratio in the gases were analyzed in combination with chemical analyses on the fluids in the Niutuozhen geothermal field. The chemical and isotopic compositions indicate a meteoric origin of the thermal waters. The reservoir temperatures estimated by chemical geothermometry are in the range between 60 and 108 °C. The results show that the gases are made up mainly by N2 (18.20–97.42 vol%), CH4 (0.02–60.95 vol%), and CO2 (0.17–25.14 vol%), with relatively high He composition (up to 0.52 vol%). The chemical and isotopic compositions of the gas samples suggest the meteoric origin of N2, predominant crustal origins of CH4, CO2, and He. The mantle-derived He contributions are calculated to be from 5 to 8% based on a crust–mantle binary mixing model. The deep temperatures in the Jxw reservoir were evaluated based on gas isotope geothermometry to be in the range from 141 to 165 °C. The mantle-derived heat fraction in the surface heat flow is estimated to be in the range of 48–51% based on 3He/4He ratios.  相似文献   

8.
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   

9.
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   

10.
The application of chemical and isotopic geothermometry to geothermal systems is reviewed, pointing out the uses and limitations of specific reactions in estimating deep temperatures from well, hot-spring and fumarole discharges.At present the most reliable indicators are: the silica-water equilibria; the Na/K ratio; the isotopic distributions Δ2H(H2“H2O), Δ2H(H2“CH4), Δ18O(H2O“HSO?4); and the gas reactions CO2 + 4H2 ? CH4 + 2H2O, and 2NH3 ? N2 + 3H2. Many other qualitative chemical indicators exist.  相似文献   

11.
Hydrocarbon seepage is a surface expression where fluids mixed with sediments and hydrocarbons are expelled through fracture systems that potentially tap into gas–petroleum reservoirs. Hydrocarbons released from most seeps appear to be thermogenic on the basis of their relative abundance and isotopic composition. The potential for subsurface microbial processes modifying these geochemical fingerprints remains poorly constrained. In this study, microcosm incubations were conducted on mud slurries supplied with/without various methanogenic precursors at temperatures ranging from ambient conditions to 90 °C, in order to assess microbial CH4 formation in the subsurface beneath hydrocarbon seeps. The analyses indicated that CH4 production was positive at ?80 °C, regardless of whether or not or which precursors were added. However, the pattern of CH4 production rates varied with the precursor and temperature. In general, the optimum CH4 production from H2/CO2 and formate occurred over a wide range of temperatures (?40 °C), whereas that from acetate, methanol and methylamine was restricted to relatively lower temperatures (40–50 °C). The CH4 recoveries, together with the C isotopic compositions of CH4, further indicated that the quantities of CH4 produced could not completely account for the quantities of precursor consumed, suggesting that a complex metabolic network was involved in the transformation of the added precursor and organic C inherited from inoculated sediments. Microbial CH4 was estimated to constitute 7–61% of the CH4 observed using experimentally-derived apparent isotope fractionations as the end member compositions. This illustrates the possibility that microbial CH4 produced at shallower depths could quantitatively and isotopically alter deeply-sourced thermogenic CH4 in hydrocarbon seep environments.  相似文献   

12.
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   

13.
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   

14.
Three hundred and thirty new 13C analyses of diamonds are presented, indicating, in conjunction with earlier published work, a range of about 30%. in the carbon isotopic composition of diamonds. The frequency distribution of diamond δ13C analyses shows a very pronounced mode at ?5 to ?6%.vs PDB, a large negative skewness, and a sharp boundary at about ?1%.. Analyses of diamonds from the Premier and Dan Carl mines, South Africa, demonstrate that: (1) differences in 13C content that can be related to diamond color and shape are smaller than 1%.; (2) the mean 13C content of kimberlite carbonates is 1–2%. lower than that of associated diamonds; (3) significant differences in 13C content exist between the mean isotopic compositions of diamonds from these two pipes; (4) the variability in δ13C differs from one mine to the other.Computations were carried out evaluating the effect on the 13C content of diamonds of: (i) various precipitation processes; (ii) the abundance of the species H2, H2O, CH4, CO, CO2 and O2 in the vapor; (iii) the initial isotopic composition variability of the source carbon; (iv) variations of the carbon isotope effects resulting from changes in pressure and temperature and (v) reservoir effects (Rayleigh fractionation). Fifty-eight genetic models were investigated for compatibility with the 13C distribution in diamonds and associated carbonate. The modeling does not permit an unambiguous answer to the question whether or not a vapor participated in diamond formation, although the presence of methane during diamond formation is compatible with the carbon isotopic composition data, possible oxygen fugacities in the mantle and with the composition of gases liberated from diamonds. In all probability carbon isotope effects in the diamond formation process were small, and the very large range in δ13C observed was inherited from the source carbon.  相似文献   

15.
Natural gases and associated condensate oils from the Zhongba gas field in the western Sichuan Basin, China were investigated for gas genetic types and origin of H2S by integrating gaseous and light hydrocarbon geochemistry, formation water compositions, S isotopes (δ34S) and geological data. There are two types of natural gas accumulations in the studied area. Gases from the third member of the Middle Triassic Leikoupo Formation (T2l3) are reservoired in a marine carbonate sequence and are characterized by high gas dryness, high H2S and CO2 contents, slightly heavy C isotopic values of CH4 and widely variable C isotopic values of wet gases. They are highly mature thermogenic gases mainly derived from the Permian type II kerogens mixed with a small proportion of the Triassic coal-type gases. Gases from the second member of the Upper Triassic Xujiahe Formation (T3x2) are reservoired in continental sandstones and characterized by low gas dryness, free of H2S, slightly light C isotopic values of CH4, and heavy and less variable C isotopic values of wet gases. They are coal-type gases derived from coal in the Triassic Xujiahe Formation.The H2S from the Leikoupo Formation is most likely formed by thermochemical SO4 reduction (TSR) even though other possibilities cannot be fully ruled out. The proposed TSR origin of H2S is supported by geochemical compositions and geological interpretations. The reservoir in the Leikoupo Formation is dolomite dominated carbonate that contains gypsum and anhydrite. Petroleum compounds dissolved in water react with aqueous SO4 species, which are derived from the dissolution of anhydrite. Burial history analysis reveals that from the temperature at which TSR occurred it was in the Late Jurassic to Early Cretaceous and TSR ceased due to uplift and cooling thereafter. TSR alteration is incomplete and mainly occurs in wet gas components as indicated by near constant CH4 δ13C values, wide range variations of ethane, propane and butane δ13C values, and moderately high gas dryness. The δ34S values in SO4, elemental S and H2S fall within the fractionation scope of TSR-derived H2S. High organo-S compound concentrations together with the occurrence of 2-thiaadamantanes in the T2l reservoir provide supplementary evidence for TSR related alteration.  相似文献   

16.
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   

17.
Carbon dioxide (CO2) is considered to be the most important greenhouse gas in terms of overall effect. CO2 geological storage in coal beds is of academic and industrial interest because of economic synergies between greenhouse gas sequestration and coal bed methane (CH4) recovery by displacement/adsorption. Previously, most work focused on either theoretical analyses and mathematical simulations or gas adsorption?Cdesorption experiments using coal particles of millimeter size or smaller. Those studies provided basic understanding of CH4 recovery by CO2 displacement in coal fragments, but more relevant and realistic investigations are still rare. To study the processes more realistically, we conducted experimental CH4 displacement by CO2 and CO2 sequestration with intact 100?×?100?×?200?mm coal specimens. The coal specimen permeability was measured first, and results show that the permeability of the specimen is different for CH4 and CO2; the CO2 permeability was found to be at least two orders of magnitude greater than that for CH4. Simultaneously, a negative exponential relationship between the permeability and the applied mean stress on the specimen was found. Under the experimental stress conditions, 17.5?C28.0 volumes CO2 can be stored in one volume of coal, and the displacement ratio CO2?CCH4 is as much as 7.0?C13.9. The process of injection, adsorption and desorption, displacement, and output of gases proceeds smoothly under an applied constant pressure differential, and the CH4 content in the output gas amounted to 20?C50% at early stages, persisting to 10?C16% during the last stage of the experiments. Production rate and CH4 fraction are governed by complex factors including initial CH4 content, the pore and fissure fabric of the coal, the changes in this fabric as the result of differential adsorption of CO2, the applied stress, and so on. During CO2 injection and CH4 displacement, the coal can swell from effects of gas adsorption and desorption, leading to changes in the microstructure of the coal itself. Artificial stimulation (e.g. hydraulic fracturing) to improve coalbed transport properties for either CO2 sequestration or enhanced coal bed methane recovery will be necessary. The interactions of large-scale induced fractures with the fabric at the scale of observable fissures and fractures in the laboratory specimens, as well as to the pore scale processes associated with adsorption and desorption, remain of profound interest and a great challenge.  相似文献   

18.
Gold mineralization of the Seolhwa mine occurs in a single stage of massive quartz veins which filled the north‐east‐trending fault shear zones in the Jurassic granitoid of 161 Ma within the Gyeonggi Massif. The vein quartz contains three main types of fluid inclusions at 25°C: (i) aqueous type I inclusions (0–15 wt.% NaCl) containing small amounts of CO2; (ii) gas‐rich (more than 70 vol. %), vapor‐homogenizing, aqueous type II inclusions; and (iii) low‐salinity (less than 5 wt.% NaCl), liquid CO2‐bearing, type III inclusions. The H2O‐CO2‐CH4‐N2‐NaCl inclusions represent immiscible fluids trapped earlier along the solvus curve in the temperature range 250–430°C at pressures of ～1 kb. Detailed fluid inclusion chronologies suggest a progressive decrease in pressure during the mineralization. Aqueous inclusion fluids represent either later fluids evolved through extensive fluid unmixing from a homogeneous H2O‐CO2‐CH4‐N2‐NaCl fluid due to decreases in temperature and pressure, or the influence of deep circulated meteoric waters. Initial fluids were homogeneous H2O‐CO2‐CH4‐N2‐NaCl fluids as follows: 250° to 430°C, 16–62 mol% CO2, 5–14 mol% CH4, 0.06–0.31 mol% N2 and salinities of 0.4–4.9 wt.% NaCl. The T‐X data for the Seolhwa mine suggest that the hydrothermal system has been probably located nearer to the granitic melt, which facilitated the CH4 formation and resulted in a reduced fluid state indicated by the predominance of pyrrhotite. Measured and calculated isotopic compositions of the hydrothermal fluids [δ18O = 5.3–6.5‰; δD =?69 to ?84‰] provide evidence of the CH4‐H2O equilibria and further indicate that the auriferous fluids were magmatically derived. Both the dominance of δ34S values of sulfides close to the meteoric reference (?0.6–1.4‰; δ34SΣS values of 0.3–1.1‰) and the available δ13C data (?4‰) are consistent with their deep igneous source. The Seolhwa mine was probably formed by extensive fracturing and veining due to the thermal expansion of water derived from the Jurassic granitoid melt.  相似文献   

19.
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   

20.
A geochemical survey, in shallow aquifers and soils, has been carried out to evaluate the feasibility of natural gas (CH4) storage in a deep saline aquifer at Rivara (MO), Northern Italy. This paper discusses the areal distribution of CO2 and CH4 fluxes and CO2, CH4, Rn, He, H2 concentrations both in soils and shallow aquifers above the proposed storage reservoir. The distribution of pathfinder elements such as 222Rn, He and H2 has been studied in order to identify potential faults and/or fractures related to preferential migration pathways and the possible interactions between the reservoir and surface. A geochemical and isotopic characterization of the ground waters circulating in the first 200 m has allowed to investigation of (i) the origin of the circulating fluids, (ii) the gas–water–rock interaction processes, (iii) the amount of dissolved gases and/or their saturation status. In the first 200 m, the presence of CH4-rich reducing waters are probably related to organic matter (peat) bearing strata which generate shallow-derived CH4, as elsewhere in the Po Plain. On the basis of isotopic analysis, no hints of thermogenic CH4 gas leakage from a deeper reservoir have been shown. The δ13C(CO2) both in ground waters and free gases suggests a prevalent shallow origin of CO2 (i.e. organic and/or soil-derived). The acquisition of pre-injection data is strategic for the natural gas storage development project and as a baseline for future monitoring during the gas injection/withdrawing period. Such a geochemical approach is considered as a methodological reference model for future CO2/CH4 storage projects.  相似文献   

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