首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 622 毫秒
1.
《Applied Geochemistry》2001,16(3):323-338
Chemical and isotopic compositions are reported for water, and CO2 and noble gases in groundwater and soda springs from Bioko, Principé, São Tomé and Annobon, all islands located in the off-shore part of the Cameroon Volcanic Line in West Africa. The soda spring waters are of Ca–Mg–HCO3 type, with δD and δ18O values that range from −20 to −8‰ and −5.4 to −2.7‰ respectively, indicative of a meteoric origin. CO2 is the main gas species in the springs. δ13C–CO2 values vary from −2.8 to −5.0‰, overlapping the observed mantle C range (−3 to −8‰). CO2/3He ratios (3–9×109) suggest that most C (∼90%) in the samples is derived from the mantle. Neon has atmospheric isotopic compositions, while Ar is slightly enriched in radiogenic 40Ar. 3He/4He ratios (3.0 to 10.1×10−6 or 2.1 to 7.2Ra, where Ra is the atmospheric ratio of 1.4×10−6) are much higher than those for typical crustal fluids (∼10−8) but lower than those expected for fluids derived from ‘high-3He/4He’ hotspots like Loihi and Iceland. This precludes significant contributions of such fluids in the source regions of the gases, and by inference, in the magmatism of these oceanic islands. Alternatively, approximately 90% of the He in São Tomé gases is inferred to be derived from a source similar to the MORB source. The 3He/4He ratio for the Bioko gas (6.6×10−6) may be derived from a source with a higher time integrated (U+Th)/3He ratio than the MORB source.  相似文献   

2.
The paper presents data on the composition of a gas phase of underground fluids in the East Carpathian region, including 3He/4He, 40Ar/36Ar, and 4He/20Ne ratios. The argon isotope composition of these gases was used to estimate the fractions of atmospheric Aratm and radiogenic 40Arrad formed in the rocks, N2/Aratm ratio, and to reveal the admixture of nonatmogenic (“excess”) nitrogen in most samples. The CO2 content in gases positively correlates with the fraction of mantle component in fluid helium. At the same time, the CO2 content shows a negative correlation with the total helium (and light 3He enriching mantle derivatives), thus excluding the simultaneous influx of CO2 and helium from a common mantle source in the fluids. A wide spectrum of 3He/4He = R in gases of the region spanning three orders of magnitude confirms the concept of mixing of the crustal and mantle components in the helium. However, even gases with similar R values show a wide scatter of He concentrations. This is mainly caused by the additional influx of other gases: CH4 formed during OM transformation or CO2 released during the thermal metamorphism of carbonate sequences. Correlation of the CH4/3He ratio and the helium isotope composition in the Carpathian gases indicates the crustal origin of hydrocarbons, which formed economic gas pools in the Ciscarpathian Trough and the adjacent part of the Folded Carpathians. Lateral chemical and isotope variations revealed in the underground fluids are related to the tectonic zoning of the region. The helium isotope variations are also consistent with the geodynamic setting of the region (thinning of the crust and lithosphere towards the Pannonian Basin, growth of the background conductive heat flow and corresponding ascent of isotherms). In combination with geothermal data, they reflect specifics of the mantle heat-and-mass flow discharge.  相似文献   

3.
The chemistry and isotope ratios of He, C (δ13C) and H (δD) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing in central Italy. The C isotopic composition of CO2 (δ13C–CO2 −2.0‰ to −3.8‰) and He isotope ratios (R/RA 0.12–0.27) were used to calculate the fraction of CO2 originating from mantle degassing vs. sedimentary sources. The results show that CO2 predominantly (average of 75%) derives from the thermo-metamorphic reaction of limestone. Between 6% and 22% of the CO2 in the samples derives from organic-rich sedimentary sources. The mantle source accounts for 0–6% of the total CO2; however, in two samples, located in proximity to the most important faults of the plain, the mantle accounts for 24% and 42%. The presence of faults and fractures allows upward gas migration from a deep source to the Earth’s surface, not only in the peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, but also in the pre-Apennine and Apennine belts. Isotope ratios of CH4 (δ13C–CH4 −6.1‰ to −22.7‰; δD–CH4 −9‰ to −129‰) show that CH4 does not appear to be related to mantle or magma degassing, but it is the product of thermal degradation of organic matter (i.e. thermogenic origin) and/or the reduction of CO2 (i.e. geothermal origin). Most of the samples appear to be affected by secondary microbial oxidation processes.  相似文献   

4.
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   

5.
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   

6.
Chemical and isotopic compositions have been measured for N2-He-rich bubbling gases discharging from hot springs in the Hainan Island, Southern China. Observed 3He/4He ratios (0.1–1.3 RA) indicate the occurrence of a mantle component throughout the island, which has been highly diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium is high in the northern island (12%–16% of total He) and gradually decreases towards southern coast (1%–3% of total He). Such a distribution pattern is most likely controlled by the Pleocene-Quaternary volcanic activities in the northern island and groundwater circulation along the deep major faults. The 40Ar/36Ar and N2/Ar ratios suggest that N2 and Ar of the hot spring gases are mostly meteoric. Although δ13C values of CO2 (–20‰ to –27‰) with low concentrations are consistent with the biogenic origin, the combination of 3He/4He and d13CCO2 suggests a two end-member mixing of mantle and crustal components with CO2/3He ratios of 2×109 and 8×1011, respectively. However, the low CO2/3He ratios (1–22×106) can not be ascribed in terms of the simple mixing but has to be explained by the addition of radiogenic 4He and loss of CO2 by calcite precipitation in the hydrothermal system, which is most likely controlled by the degree of gas-water-rock interaction.  相似文献   

7.
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   

8.
Carbon and noble gas isotope analyses are reported for bubbling gas samples from the Tengchong volcanic geothermal area near the Indo-Eurasian suture zone. All samples contain a resolvable component of mantle-derived 3He. Occurrence of mantle-derived 3He coincides with surface volcanism. However, 3He occurs over a larger geographic areathan do surface volcanics. δ13C values for CO2 and CH4 vary from -33.4‰ to 1.6 ‰ and from -52.8‰ to -2.8‰, respectively. He and C isotope systematics indicate that CO2 and CH4 in the CO2-rich gases originated predominantly from magmatic component mixed with crustal CO2 produced from carbonate. However, breakdown of organic matter and near-surface processes accounts for the CH4 and CO2 in N2-rich gases. 3He/4He ratio distribution pattern suggests that mantle-derived He and heat sources of high-temperature system in central Tengchong originate from a hidden magma reservoir at subsurface. CO2-rich gases with the highest 3He/4He ratio (5.2 Ra) may be representative of the  相似文献   

9.
Chemical and isotopic compositions have been measured for CO2-rich bubbling gases discharging from cold springs in Wudalianchi intra-plate volcanic area, NE China. Observed 3He/4He ratios (2–3 RA) and δ13C values of CO2 (−5‰ to −3‰) indicate the occurrence of a mantle component released and transferred to the surface by the Cenozoic extension-related magmatic activities. The CO2/3He ratios are in wide range of (0.4–97 × 109). Based on the apparent mixing trend in a 3He/4He and δ13C of CO2 diagram from all published data, the extracted magmatic end-member in the Wudalianchi Volcano has 3He/4He, δ13C and CO2/3He value of ∼3.2 RA, ∼−4.6‰ and ∼6 × 1010, respectively. These values suggest that the volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events, as revealed by geothermal He and CO2 from the adjacent Changbaishan volcanic area.  相似文献   

10.
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   

11.
《Applied Geochemistry》2001,16(7-8):895-910
Coalbed gases in the Lower Silesian Coal Basin (LSCB) of Poland are highly variable in both their molecular and stable isotope compositions. Geochemical indices and stable isotope ratios vary within the following ranges: hydrocarbon (CHC) index CHC=CH4/(C2H6+ C3H8) from 1.1 to 5825, wet gas (C2+) index C2+=(C2H6+ C3H8+ C4H10+ C5H12) / (CH4+ C2H6+ C3H8+ C4H10+ C5H12) 100 (%) from 0.0 to 48.3%, CO2–CH4 (CDMI) index CDMI=CO2/(CO2+ CH4) 100 (%) from 0.1 to 99.9%, δ13C(CH4) from −66.1 to −24.6‰, δD(CH4) from −266 to −117‰, δ13C(C2H6) from −27.8 to −22.8‰, and δ13C(CO2) from −26.6 to 16.8‰. Isotopic studies reveal the presence of 3 genetic types of natural gases: thermogenic (CH4, higher gaseous hydrocarbons, and CO2), endogenic CO2, and microbial CH4 and CO2. Thermogenic gases resulted from coalification processes, which were probably completed by Late Carboniferous and Early Permian time. Endogenic CO2 migrated along the deep-seated faults from upper mantle and/or magma chambers. Minor volumes of microbial CH4 and CO2 occur at shallow depths close to the abandoned mine workings. “Late-stage” microbial processes have commenced in the Upper Cretaceous and are probably active at present. However, depth-related isotopic fractionation which has resulted from physical and physicochemical (e.g. diffusion and adsorption/desorption) processes during gas migration cannot be neglected. The strongest rock and gas outbursts occur only in those parts of coal deposits of the LSCB which are dominated by large amounts of endogenic CO2.  相似文献   

12.
In the Czech-German border region of the Vogtland and NW Bohemia (western Eger rift, Central Europe), chemical and isotopic compositions (C, N, He, Ar) of free gas from a thermal water escape (fluorite mine, Schönbrunn), two mineral springs (“Eisenquelle,” Bad Brambach; “Sprudel III,” Bad Elster) and a mofette (Bublak) located along an ∼40-km long traverse are reported. The gases of Bublak and Bad Brambach are CO2-rich (>99 vol.%) and have δ13C values of −1.95 and −4.29‰, respectively. With distance from the center of CO2 degassing (Bublak) the δ13C values decrease, most likely due to physico-chemical fractionation of CO2 between gaseous and aqueous phases rather than to admixture of organic/biogenic CO2. The δ15N values range between −3.2 and −0.6‰, compared to an upper mantle value of −4.0 ± 1.0‰. The four locations are characterized by 3He/4He ratios decreasing from 5.9 Ra in the center (Bublak) to 0.8 Ra in the periphery (Schönbrunn) and give evidence for mixing of He from a deep-seated magmatic source with a crustal source. The location with the highest 3He/4He ratio (5.9 Ra) is accompanied by the highest 40Ar/36Ar (550). We argue that the nitrogen of the Bublak mofette gas is a mixture of predominantly atmospheric and mantle-derived components, whereas at the other three locations crustal nitrogen may also be present. The Bublak δ15N value of ≈−4.5 ± 1.0‰ represents the first free gas δ15N reference from the European subcontinental mantle (ESCM) and indicates that, in contrast to the 3He/4He ratios, the δ15N values are equal for ESCM and MORB, respectively.  相似文献   

13.
Isotopic-geochemical study revealed the presence of mantle He (3He/4He up to 223 × 10?8) in gases from mud volcanoes of eastern Georgia. This fact confirms that the Middle Kura basin fill encloses an intrusive body previously distinguished from geophysical data. Wide variations in the carbon isotopic composition δ13C of CH4 and CO2 and the chemical composition of gas and water at a temporally constant 3He/4He ratio indicate their relation with crustal processes. Unusual direct correlations of the 3He/4He ratio with the contents of He and CH4 and the 40Ar/36Ar ratio can be explained by the generation of gas in the Cenozoic sequence of the Middle Kura basin.  相似文献   

14.
《Applied Geochemistry》2002,17(10):1329-1342
Chemical (major and trace elements) and isotopic compositions (δD and δ18O in waters and δ13C in CO2 and 3He/4He in gases) of natural thermal (11) and cold (39) fluids (spring waters and gases) discharging from a tectonic window of Mesozoic limestones in central Italy, have proved to be the result of mixing processes inside the limestone formations. The limestones provide a preferential route for subsurface fluid migration and they gather both descending cold, Ca-HCO3, B-depleted groundwaters and rising convective Ca-SO4(HCO3), CO2-saturated, B-rich thermal waters. Atmospherically-derived descending gas components (N2, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N2, Ne, He-depleted hot rising waters. Boron in the liquid phase and N2 and Ne in the gas phase are the most useful elements to trace the mixing process. The deeper gas samples recognised in the area are associated with the hotter waters emerging in the area. In spite of being depleted in Ne and He and light hydrocarbons they have the higher measured 3He/4He ratios, suggesting a contribution of mantle 3He to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of 4He in the deeper gases. Paradoxically, there is more relative concentration of 4He in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the δ13C of CO2 in the deeper gas samples is considered. The shallower air-contaminated gas samples, although they should be affected by the addition of soil-13C depleted organic C, have δ13C in CO2 more positive than the deeper gas samples recognized. Since any deep hydrothermal source of CO2 should generate CO2 with more positive values of δ13C than those measured at surface, a multiple (single) calcite precipitation process from hydrothermal solutions, with C isotopic fractionation along the rising path inside the Mesozoic limestone formations, is proposed.  相似文献   

15.
We report in this paper a systematic investigation of the chemical and isotopic composition of groundwaters flowing in the volcanic aquifer of Mt. Vesuvius during its current phase of dormancy, including the first data on dissolved helium isotope composition and tritium content. The relevant results on dissolved He and C presented in this paper reveal that an extensive interaction between rising magmatic volatiles and groundwaters currently takes place at Vesuvius.Vesuvius groundwaters are dilute (mean TDS ∼ 2800 mg/L) hypothermal fluids ( mean T = 17.7°C) with a prevalent alkaline-bicarbonate composition. Calcium-bicarbonate groundwaters normally occur on the surrounding Campanian Plain, likely recharged from the Apennines. δD and δ18O data evidence an essentially meteoric origin of Vesuvius groundwaters, the contribution from either Tyrrhenian seawater or 18O-enriched thermal water appearing to be small or negligible. However, the dissolution of CO2-rich gases at depth promotes acid alteration and isochemical leaching of the permeable volcanic rocks, which explains the generally low pH and high total carbon content of waters. Attainment of chemical equilibrium between the rock and the weathering solutions is prevented by commonly low temperature (10 to 28°C) and acid-reducing conditions.The chemical and isotope (C and He) composition of dissolved gases highlights the magmatic origin of the gas phase feeding the aquifer. We show that although the pristine magmatic composition may vary upon gas ascent because of either dilution by a soil-atmospheric component or fractionation processes during interaction with the aquifer, both 13C/12C and 3He/4He measurements indicate the contribution of a magmatic component with a δ13C ∼ 0‰ and R/Ra of ∼2.7, which is consistent with data from Vesuvius fumaroles and phenocryst melt inclusions in olivine phenocrysts.A main control of tectonics on gas ascent is revealed by data presented in this paper. For example, two areas of high CO2 release and enhanced rock leaching are recognized on the western (Torre del Greco) and southwestern (Torre Annunziata-Pompeii) flanks of Vesuvius, where important NE-SW and NW-SE tectonic structures are recognized. In contrast, waters flowing through the northern sector of the volcano are generally colder, less saline, and CO2 depleted, despite in some cases containing significant concentrations of magma-derived helium. The remarkable differences among the various sectors of the volcano are reconciled in a geochemical interpretative model, which is consistent with recent structural and geophysical evidences on the structure of Somma-Vesuvius volcanic complex.  相似文献   

16.
New volatile data (CO2, H2O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO2 and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ13C values of dissolved CO2 or coupled CO2-Ar variations. This suggests that δ13C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H2O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB.Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO2 with δ13C of −6‰, 0.1 to 0.35 wt.% H2O, 3He/4He ∼11 RA, and CO2/3He of ∼2 × 109. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the 40Ar/36Ar may be low. The 3He/4He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 RA) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of 3He/4He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone.  相似文献   

17.
《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   

18.
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   

19.
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   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号