Estimation of the methane flux from mud volcanoes along Chishan Fault,southwestern Taiwan

Because of the increasing public awareness of the greenhouse effect, geological emissions of methane (GEM) have gained more attention. The Intergovernmental Panel on Climate Change included GEM in the natural sources of methane category in the Fourth Assessment Report in 2007. The methane flux from mud volcanoes (MVs) is the second leading source within GEM. However, given that gas flux from MVs has been measured directly only in Sicily and Azerbaijan, the global methane estimation is still a conservative assumption based on limited data. The behavior of MV activities is seldom reported. This study attempts to estimate annual methane emissions through MVs by using a video recorder to record MV activities along the Chishan Fault and determining the bubble size and flux rate of each MV. Observation results, after a 1 year observation period, indicate that the annual emissions of the 19 MVs along the Chishan Fault are 0.1–0.2% of the global methane flux from MVs, highlighting that the methane emission from these MVs should not be neglected. In addition, the continuous monitoring of a particular MV disclosed periodic characteristics of MV activities. These results manifest the spatial and temporal variations of MV activities along the Chishan Fault. The spatial variations are related to the extensional kinematics of the Chishan fault, while the temporal variation relates to the earth-tide-related MV activities. The estimated annual methane emissions from the study are far more than those of previous studies and are still considered conservative. Results of this study may provide valuable information for researchers attempting to estimate the quantity of GEM.     

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C₁–C₄, CO₂]). Molecular ratios of LMWHC (C₁/[C₂ + C₃] > 1000) and stable isotopic compositions of methane (δ¹³C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C₁/C₂₊ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C₁–C₄, CO₂]) relative to the other gas types and the virtual lack of ¹⁴C–CH₄ indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C₁–C₄, CO₂)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of ¹⁴C–CH₄ (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.     

A new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere

A new estimate of global methane emission into the atmosphere from mud volcanoes (MVs) on land and shallow seafloor is presented. The estimate, considered a lower limit, is based on 1) new direct measurements of flux, including both venting of methane and diffuse microseepage around craters and vents, and 2) a classification of MV sizes in terms of area (km²) based on a compilation of data from 120 MVs. The methane flux to the atmosphere is conservatively estimated between 6 and 9 Mt y^–1. This emission from MVs is 3–6% of the natural methane sources and is comparable with ocean and hydrate sources, officially considered in the atmospheric methane budget. The total geologic source, including MVs, seepage from seafloor, microseepage in hydrocarbon-prone areas and geothermal sources, would amount to 35–45 Mt y^–1. The authors believe it is time to add this parameter in the Intergovernmental Panel on Climate Change official tables of atmospheric methane sources.GEM     

An experimental comparison of gas generation from three oil fractions: Implications for the chemical and stable carbon isotopic signatures of oil cracking gas

Although oil cracking has been documented as one of the important sources of gas in many overmature marine sedimentary basins, the chemical and carbon isotopic signatures of gases of this origin are still open to question. In this study a Cambrian crude oil from the central Tarim basin, along with its main separated fractions (saturates, aromatics and asphaltenes), were pyrolyzed in sealed gold tubes to investigate how generated gases vary in chemical and carbon isotopic composition and how this variation would influence the genetic interpretation of oil cracking gas. The results indicate that the gases from cracking of aromatics and asphaltenes are much drier and more enriched in ¹³C than the gases from the cracking of saturates and crude oil at the same level of thermal maturity. In the experimental run of 20 °C/h, the dryness index of the gases (defined as the volume percentage of C₁ in C_1–5) from the cracking of saturates ranges from 26.2–90.6% with the methane carbon isotope change ranging from −54.8‰ to −35.5‰, whereas the dryness index is never lower than 60.6% for the gases from the cracking of aromatics with methane carbon isotope ranging from −39.9‰ to −32.2‰. Correspondingly, experimental data for the four samples plot in different areas in diagrams designed to distinguish oil cracking gas from kerogen cracking gas, such as ln(C₂/C₃) vs. δ¹³C₂–δ¹³C₃ and δ¹³C₁ vs. δ¹³C₂–δ¹³C₃, indicating compositional variability of crude oil could assert an important influence in these diagrams. Therefore it is prudent to bring other geological constraints into consideration to avoid misinterpretation.The kinetic parameters for the bulk generation of C_1–5 gas and the methane carbon isotope fractionation extrapolated to geological conditions of 2 °C/Ma and an initial temperature of 50 °C show that the temperatures of C_1–5 gas generation from the aromatics and asphaltenes are lower than those from the saturates and crude oil due to their lower activation energies and frequency factors. Generation of C_1–5 gases from the aromatics is modeled to be initiated about 122 °C whereas the initiation temperature for the saturates sample is 176 °C. Below 189 °C (EasyRo = 1.8%), the yields of C_1–5 gases follow the order: aromatics > asphaltenes > crude oil > saturates. At similar thermal maturity levels, the methane carbon isotopic compositions are significantly different for the four samples, with an order of ¹³C enrichment: aromatics > asphaltenes > crude oil > saturates, however the difference in methane carbon isotopes becomes smaller with increasing temperature. This indicates that methane carbon isotopic values can be significantly different for gases cracked from oils that are compositionally diverse, especially in the early stage of methane generation.     

Geochemical comparison between gas in fluid inclusions and gas produced from the Upper Triassic Xujiahe Formation,Sichuan Basin,SW China

Natural gas in the Xujiahe Formation of the Sichuan Basin is dominated by hydrocarbon (HC) gas, with 78–79% methane and 2–19% C₂₊ HC. Its dryness coefficient (C₁/C_1–5) is mostly < 0.95. The gas in fluid inclusions, which has low contents of CH₄ and heavy hydrocarbons (C₂₊) and higher contents of non-hydrocarbons (e.g. CO₂), is a typical wet gas produced by thermal degradation of kerogen. Gas produced from the Upper Triassic Xujiahe Formation (here denoted field gas) has light carbon isotope values for methane (δ¹³C₁: −45‰ to −36‰) and heavier values for ethane (δ¹³C₂: −30‰ to −25‰). The case is similar for gas in fluid inclusions, but δ¹³C₁ = −36‰ to −45‰ and δ¹³C₂ = −24.8‰ to −28.1‰, suggesting that the gas experienced weak isotopic fractionation due to migration and water washing. The field gas has δ¹³C_CO2 values of −15.6‰ to −5.6‰, while the gas in fluid inclusions has δ¹³C_CO2 values of −16.6‰ to −9‰, indicating its organic origin. Geochemical comparison shows that CO₂ captured in fluid inclusions mainly originated from source rock organic matter, with little contribution from abiogenic CO₂. Fluid inclusions originate in a relatively closed system without fluid exchange with the outside following the gas capture process, so that there is no isotopic fractionation. They thus present the original state of gas generated from the source rocks. These research results can provide a theoretical basis for gas generation, evolution, migration and accumulation in the basin.     

Origin,distribution and hydrogeochemical controls on methane occurrences in shallow aquifers,southwestern Ontario,Canada

Natural gas reservoirs in organic-rich shales in the Appalachian and Michigan basins in the United States are currently being produced via hydraulic fracturing. Stratigraphically-equivalent shales occur in the Canadian portion of the basins in southwestern Ontario with anecdotal evidence of gas shows, yet there has been no commercial shale gas production to date. To provide baseline data in the case of future environmental issues related to hydraulic fracturing and shale gas production, such as leakage of natural gas, saline water, and/or hydraulic fracturing fluids, and to evaluate hydrogeochemical controls on natural gas accumulations in shallow groundwater in general, this study investigates the origin and distribution of natural gas and brine in shallow aquifers across southwestern Ontario. An extensive geochemical database of major ion and trace metal chemistry and methane concentrations of 1010 groundwater samples from shallow, domestic wells in bedrock and overburden aquifers throughout southwestern Ontario was utilized. In addition, select wells (n = 36) were resampled for detailed dissolved gas composition, δ¹³C of CH₄, C₂, C₃, and CO₂, and δD of CH₄. Dissolved gases in groundwater from bedrock and overburden wells were composed primarily of CH₄ (29.7–98.6 mol% of total gas volume), N₂ (0.8–66.2 mol%), Ar + O₂ (0.2–3.4 mol%), and CO₂ (0–1.2 mol%). Ethane was detected, but only in low concentrations (<0.041 mol%), and no other higher chain hydrocarbons were present, except for one well in overburden overlying the Dundee Formation, which contained 0.81 mol% ethane and 0.21 mol% propane. The highest methane concentrations (30 to >100 in situ % saturation) were found in bedrock wells completed in the Upper Devonian Kettle Point Formation, Middle Devonian Hamilton Group and Dundee Formation, and in surficial aquifers overlying these organic-rich shale-bearing formations, indicating that bedrock geology is the primary control on methane occurrences. A few (n = 40) samples showed Na–Cl–Br evidence of brine mixing with dilute groundwater, however only one of these samples contained high (>60 in situ % saturation) CH₄. The relatively low δ¹³C values of CH₄ (−89.9‰ to −57.3‰), covariance of δD values of CH₄ and H₂O, positive correlation between δ¹³C values of CH₄ and CO₂, and lack of higher chain hydrocarbons (C₃₊) in all but one dissolved gas sample indicates that the methane in groundwater throughout the study area is primarily microbial in origin. The presence or absence of alternative electron acceptors (e.g. dissolved oxygen, Fe, NO₃, SO₄), in addition to organic substrates, controls the occurrence of microbial CH₄ in shallow aquifers. Microbial methane has likely been accumulating in the study area, since at least the Late Pleistocene to the present, as indicated by the co-variance and range of δD values of CH₄ (−314‰ to −263‰) and associated groundwater (−19‰ to −6‰ δD-H₂O).     

Estimation of carbon dioxide flux degassing from percolating waters in a karst cave: Case study from Bijambare cave,Bosnia and Herzegovina

Drip water, collected above three actively forming stalagmites, has been analyzed and the results discussed using a calcite saturation index versus equilibrium carbon dioxide partial pressure theoretical relationship. Percolating water appears to have been originated from a parent solution in equilibrium with a carbon dioxide rich gas phase having a minimum concentration varying between 15,000 ppmv and 26,000 ppmv while large part of the variability recorded in the cave can be explained by different stage of degassing. Similarly, analyses performed at several cave pools confirm that drip water rapidly achieves equilibrium with the cave atmosphere after impact on the stalagmite apex, while oversaturation is retained longer. Using these boundary conditions, the changes in inorganic carbon concentration in the percolating water have been calculated and converted into fluxes using an average effective infiltration flow estimated from the annual water balance.The predicted flux of carbon dioxide degassing from drip water is in the range of 0.03–0.06 μmol m⁻² s⁻¹. This flux has been found to be one of the major sources of carbon dioxide in the cave atmosphere during low ventilation periods.     

Extreme methane deuterium,nitrogen and helium enrichment in natural gas from the Homorod seep (Romania)

Methane (CH₄) in terrestrial environments, whether microbial, thermogenic, or abiogenic, exhibits a large variance in C and H stable isotope ratios due to primary processes of formation. Isotopic variability can be broadened through secondary, post-genetic processes, such as mixing and isotopic fractionation by oxidation. The highest and lowest ¹³C and ²H (or D, deuterium) concentrations in CH₄ found in various geologic environments to date, are defined as “natural” terrestrial extremes. We have discovered a new extreme in a natural gas seep with values of deuterium concentrations, δD_CH4, up to + 124‰ that far exceed those reported for any terrestrial gas. The gas, seeping from the small Homorod mud volcano in Transylvania (Romania), also has extremely high concentrations of nitrogen (> 92 vol.%) and helium (up to 1.4 vol.%). Carbon isotopes in CH₄, C₂H₆ and CO₂, and nitrogen isotopes in N₂ indicate a primary organic sedimentary origin for the gas (a minor mantle component is suggested by the ³He/⁴He ratio, R/Ra ~ 0.39). Both thermogenic gas formation modeling and Rayleigh fractionation modeling suggest that the extreme deuterium enrichment could be explained by an oxidation process characterised by a δD_CH4 and δ¹³C_CH4 enrichment ratio (ΔH/ΔC) of about 20, and may be accounted for by abiogenic oxidation mediated by metal oxides. All favourable conditions for such a process exist in the Homorod area, where increased heat flow during Pliocene–Quaternary volcanism may have played a key role. Finally we observed rapid variations (within 1 h) in C and H isotope ratios of CH₄, and in the H₂S concentrations which are likely caused by mixing of the deep oxidized CH₄–N₂–H₂S–He rich gas with a microbial methane generated in the mud pool of one of the seeps.We hypothesize that the unusual features of Homorod gas can be the result of a rare combination of factors induced by the proximity of sedimentary organic matter, mafic, metal-rich volcanic rocks and salt diapirs, leading to the following processes: a) primary thermogenic generation of gas at temperatures between 130 and 175 °C; b) secondary alteration through abiogenic oxidation, likely triggered by the Neogene–Quaternary volcanism of the eastern Transylvanian margin; and c) mixing at the surface with microbial methane that formed through fermentation in the mud volcano water pool. The Homorod gas seep is a rare example that demonstrates how post-genetic processes can produce extreme gas isotope signatures (thus far only theorized), and that extremely positive δD_CH4 values cannot be used to unambiguously distinguish between biotic and abiotic origin.     

Biodegradation of CO2, CH4 and volatile organic compounds (VOCs) in soil gas from the Vicano–Cimino hydrothermal system (central Italy)

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.     

Mineral chemistry and geochemical behavior of hydrothermal alterations associated with mafic intrusive-related Au deposits at the Atud area,Central Eastern Desert,Egypt

Vein-type gold deposits in the Atud area are related to the metagabbro–diorite complex that occurred in Gabal Atud in the Central Eastern Desert of Egypt. This gold mineralization is located within quartz veins and intense hydrothermal alteration haloes along the NW–SE brittle–ductile shear zone, as well as along the contacts between them. By using the mass balance calculations, this work is to determine the mass/volume gains and losses of the chemical components during the hydrothermal alteration processes in the studied deposits. In addition, we report new data on the mineral chemistry of the alteration minerals to define the condition of the gold deposition and the mineralizing fluid based on the convenient geothermometers. Two generations of quartz veins include the mineralized grayish-to-white old vein (trending NW–SE), and the younger, non-mineralized milky white vein (trending NE–SW). The ore minerals associated with gold are essentially arsenopyrite and pyrite, with chalcopyrite, sphalerite, enargite, and goethite forming during three phases of mineralization; first, second (main ore), and third (supergene) phases. Three main hydrothermal alteration zones of mineral assemblages were identified (zones 1–3), placed around mineralized and non-mineralized quartz veins in the underground levels. The concentrations of Au, Ag, and Cu are different from zone to zone having 25–790 ppb, 0.7–69.6 ppm, and 6–93.8 ppm; 48.6–176.1 ppb, 0.9–12.3 ppm, and 39.6–118.2 ppm; and 53.9–155.4 ppb, 0.7–3.4 ppm, and 0.2–79 ppm for zones 1, 2, and 3, respectively.The mass balance calculations and isocon diagrams (calculated using the GEOISO-Windows program) revealed the gold to be highly associated with the main mineralized zone as well as sericitization/kaolinitization and muscovitization in zone 1 more than in zones 2 and 3. The sericite had a higher muscovite component in all analyzed flakes (average X_Ms = 0.89), with 0.10%–0.55% phengite content in wall rocks and 0.13%–0.29% phengite content in mineralized quartz veins. Wall rocks had higher calcite (CaCO₃) contents and lower MgCO₃ and FeCO₃ contents than the quartz veins. The chlorite flakes in the altered wall rocks were composed of pycnochlorite and ripidolite, with estimated formation temperatures of 289–295 °C and 301–312 °C, respectively. Albite has higher albite content (95.08%–99.20%) which occurs with chlorite in zone 3.     

Stable carbon isotope geochemistry of adsorbed alkane gases in near-surface soils of the Saurashtra Basin,India

The stable carbon isotopic compositions of light hydrocarbon gases adsorbed in near-surface soil and sediments from the Saurashtra basin were characterized for their origin and maturity. Saurashtra is considered geologically prospective for oil and gas reserves; however, a major part of the basin is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. Surface geochemical prospecting, based on micro-seepage of hydrocarbons from subsurface accumulations, could be advantageous in such areas. In light of this, 150 soil samples were collected from the northwestern part of Saurashtra, around the Jamnagar area, where a thick sedimentary sequence of about 2–3 km exists under 1–1.5 km of Deccan basalt. The concentration of acid desorbed alkane gases from soil samples was found to vary (in ppb) as: methane (C₁) = 3–518; ethane (C₂) = 0–430; propane (C₃) = 0–331; i-butane (iC₄) = 0–297; n-butane (nC₄) = 2–116; i-pentane (iC₅) = 0–31 and n-pentane (nC₅) = 0–23, respectively.Fifteen samples with high concentrations of alkane gases were measured for their δ¹³C₁; δ¹³C₂ and δ¹³C₃ compositions using gas chromatography–combustion-isotope ratio mass spectrometry (GC–C-IRMS). The values for methane varied from ? 27 to ? 45.4‰, ethane from ? 20.9 to ? 27.6‰, and propane from ? 20.4 to ? 29.1‰ versus the Vienna PeeDee Belemnite (VPDB). The carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gases derived from thermogenic sources. Comparisons of carbon isotopic signatures and compositional variations with the standard carbon isotopic models suggest that hydrocarbon gases found in the shallow depths of the study area are not of bacterial origin but are formed thermally from deeply buried organic matter, likely to be mainly a terrestrial source rock with a partial contribution from a marine source. These gases may have migrated to the near-surface environment, where they represent an admixture of thermally generated hydrocarbon gases from mixed sources and maturity. The maturity scale (δ¹³C versus Log Ro %) applied to the surface sediment samples of the Jamnagar area indicated the source material to be capable of generating oil and gas. The detection of thermogenic alkane gases in near-surface sediments offers the possibility of hydrocarbons at depth in Saurashtra.     

Authigenesis of vivianite as influenced by methane-induced sulfidization in cold-seep sediments off southwestern Taiwan

Authigenesis of iron-rich phosphate nodules occurs in iron-rich cold-seep sediments (MD052911 core) at Yung-An Ridge offshore southwestern Taiwan. Raman, FTIR, and quantitative X-ray energy-dispersive spectroscopic analyses indicate that the phosphate mineral is vivianite (or barićite) and shows Fe/Mg molar ratios spanning from ca. 0.6 to 4.0 and a general down core trend of increasing Fe/Mg ratios. The formation of vivianite is limited to a depth interval of 13–17 mbsf (meters below seafloor) and is most prominent at ∼16 mbsf in association with high dissolved iron concentrations and depleted dissolved sulfide below a peak sulfidization zone (enriched in mackinawite and greigite). Alternate growths of vivianite and iron monosulfides and compositional zoning with Mg enriched towards the peripheries of individual nodules occur in the transition from the zone of vivianite mineralization to the sulfidization zone. The crystallization of vivianite below the sulfidization front could have been favored by scavenging of downward diffusive dissolved sulfide from pore waters in the sulfidization zone. Alternate growths and overlapping of the zones of iron monosulfides and vivianite can be attributed to fluctuations of the sulfidization front and methane flux. The discovery of vivianite in the Yung-An Ridge sediments implies that authigenic vivianite can be an important sink for phosphorus burial in cold-seep sediments that have high reactive-iron contents and high sedimentation rates.     

CO2 emissions and heat flow through soil,fumaroles, and steam heated mud pools at the Reykjanes geothermal area,SW Iceland

Carbon dioxide emissions and heat flow through soil, steam vents and fractures, and steam heated mud pools were determined in the Reykjanes geothermal area, SW Iceland. Soil diffuse degassing of CO₂ was quantified by soil flux measurements on a 600 m by 375 m rectangular grid using a portable closed chamber soil flux meter and the resulting data were analyzed by both a graphical statistical method and sequential Gaussian simulations. The soil temperature was measured in each node of the grid and used to evaluate the heat flow. The heat flow data were also analyzed by sequential Gaussian simulations. Heat flow from steam vents and fractures was determined by quantifying the amount of steam emitted from the vents by direct measurements of steam flow rate. The heat loss from the steam heated mud pools was determined by quantifying the rate of heat loss from the pools by evaporation, convection, and radiation. The steam flow rate into the pools was calculated from the observed heat loss from the pools, assuming that steam flow was the only mechanism of heat transport into the pool. The CO₂ emissions from the steam vents and mud pools were determined by multiplying the steam flow rate from the respective sources by the representative CO₂ concentration of steam in the Reykjanes area. The observed rates of CO₂ emissions through soil, steam vents, and steam heated mud pools amounted to 13.5 ± 1.7, 0.23 ± 0.05, and 0.13 ± 0.03 tons per day, respectively. The heat flow through soil, steam vents, and mud pools was 16.9 ± 1.4, 2.2 ± 0.4, and 1.2  ± 0.1 MW, respectively. Heat loss from the geothermal reservoir, inferred from the CO₂ emissions through the soil amounts to 130 ± 16 MW of thermal energy. The discrepancy between the observed heat loss and the heat loss inferred from the CO₂ emissions is attributed to steam condensation in the subsurface due to interactions with cold ground water. These results demonstrate that soil diffuse degassing can be a more reliable proxy for heat loss from geothermal systems than soil temperatures. The soil diffuse degassing at Reykjanes appears to be strongly controlled by the local tectonics. The observed diffuse degassing defines 3–5 elongated N–S trending zones (000–020°). The orientation of the diffuse degassing structures at Reykjanes is consistent with reported trends of right lateral strike slip faults in the area. The natural CO₂ emissions from Reykjanes under the current low-production conditions are about 16% of the expected emissions from a 100 MWe power plant, which has recently been commissioned at Reykjanes.     

Origin and age of pore waters in an actively venting gas hydrate field near Sado Island,Japan Sea: Interpretation of halogen and 129I distributions

A gas hydrate field with highly active venting of methane was recently found near Sado Island in the eastern Japan Sea. Piston cores were collected from active venting sites and nearby locations in the Umitaka Spur–Joetsu Knoll area during two cruises in 2004 (UT04) and 2005 (KY05-08). We report here halogen concentrations and ¹²⁹I/I ratios in pore waters associated with gas hydrates from these expeditions. The strongly biophilic behavior of I and, to a lesser degree, of Br together with the presence of the long-lived iodine radioisotope (¹²⁹I) allow evaluation of potential source materials for methane in gas hydrate systems. Depth profiles of all three halogens, particularly the very rapid downward increases of Br and I concentrations, strongly suggest input of deep fluids enriched in Br and I, but the profiles also display the effects of gas hydrate formation and dissociation. Although the ¹²⁹I/I ratios are modified by ¹²⁹I from seawater and sediments at shallow depth, likely ratios of the deep fluids are estimated to be between 400 × 10^− 15 and 600 × 10^− 15, equivalent to a Late Oligocene to Early Miocene age. Ages in the active methane venting sites typically are closer to the old end of this range than those in the reference sites. This age range suggests that the methane associated with venting and gas hydrate formation in this area is derived from organic materials accumulated during the initial opening of the Japan Sea. The Umitaka Spur–Joetsu Knoll gas hydrate field demonstrates the movement of deep fluids associated with the release of significant amounts of methane from the seafloor, processes which might be important components of mass transfer and carbon cycle in the shallow geosphere.     

Distribution of glycerol dialkyl glycerol tetraether lipids in the water column of Lake Tanganyika

We studied the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in suspended particulate matter from the water column of Lake Tanganyika (East Africa), where sediment studies had shown the applicability of the TEX₈₆ proxy for reconstructing surface lake water temperature. GDGTs, in particular crenarchaeol, showed maximum abundance within the suboxic zone (100–180 m), suggesting that this is the preferred niche of ammonia-oxidizing Thaumarchaeota. Despite evidence for anaerobic methane oxidation in deep anoxic water (300–1200 m) no unambiguous evidence for an imprint of methanotrophic archaea on GDGT distribution was found. Comparison of TEX₈₆ and BIT indices with those of surface sediments suggests that the sedimentary GDGTs are derived predominantly from the oxic zone and suboxic zone of the lake.     

Methane and carbon dioxide sorption on a set of coals from India

Complete sorption isotherm characteristics of methane and CO₂ were studied on fourteen sub-bituminous to high-volatile bituminous Indian Gondwana coals. The mean vitrinite reflectance values of the coal samples are within the range of 0.64% to 1.30% with varying maceral composition. All isotherms were conducted at 30 °C on dry, powdered coal samples up to a maximum experimental pressure of ~ 7.8 MPa and 5.8 MPa for methane and CO₂, respectively.The nature of the isotherms varied widely within the experimental pressure range with some of the samples remained under-saturated while the others attained saturation. The CO₂ to methane adsorption ratios decreased with the increase in experimental pressure and the overall variation was between 4:1 and 1.5:1 for most of the coals. For both methane and CO₂, the lower-ranked coal samples generally exhibited higher sorption affinity compared to the higher-ranked coals. However, sorption capacity indicates a U-shaped trend with rank. Significant hysteresis was observed between the ad/desorption isotherms for CO₂. However, with methane, hysteresis was either absent or insignificant. It was also observed that the coal maceral compositions had a significant impact on the sorption capacities for both methane and CO₂. Coals with higher vitrinite contents showed higher capacities while internite content indicated a negative impact on the sorption capacity.     

Geochemistry of mineral water and gases of the Razdolnoe Spa (Primorye,Far East of Russia)

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.     

Effect of organic-matter type and thermal maturity on methane adsorption in shale-gas systems

A series of methane (CH₄) adsorption experiments on bulk organic rich shales and their isolated kerogens were conducted at 35 °C, 50 °C and 65 °C and CH₄ pressure of up to 15 MPa under dry conditions. Samples from the Eocene Green River Formation, Devonian–Mississippian Woodford Shale and Upper Cretaceous Cameo coal were studied to examine how differences in organic matter type affect natural gas adsorption. Vitrinite reflectance values of these samples ranged from 0.56–0.58 %R_o. In addition, thermal maturity effects were determined on three Mississippian Barnett Shale samples with measured vitrinite reflectance values of 0.58, 0.81 and 2.01 %R_o.For all bulk and isolated kerogen samples, the total amount of methane adsorbed was directly proportional to the total organic carbon (TOC) content of the sample and the average maximum amount of gas sorption was 1.36 mmol of methane per gram of TOC. These results indicate that sorption on organic matter plays a critical role in shale-gas storage. Under the experimental conditions, differences in thermal maturity showed no significant effect on the total amount of gas sorbed. Experimental sorption isotherms could be fitted with good accuracy by the Langmuir function by adjusting the Langmuir pressure (P_L) and maximum sorption capacity (Γ_max). The lowest maturity sample (%R_o = 0.56) displayed a Langmuir pressure (P_L) of 5.15 MPa, significantly larger than the 2.33 MPa observed for the highest maturity (%R_o > 2.01) sample at 50 °C.The value of the Langmuir pressure (P_L) changes with kerogen type in the following sequence: type I > type II > type III. The thermodynamic parameters of CH₄ adsorption on organic rich shales were determined based on the experimental CH₄ isotherms. For the adsorption of CH₄ on organic rich shales and their isolated kerogen, the heat of adsorption (q) and the standard entropy (Δs⁰) range from 7.3–28.0 kJ/mol and from −36.2 to −92.2 J/mol/K, respectively.     

Comparison of natural gases accumulated in Oligocene strata with hydrous pyrolysis gases from Menilite Shales of the Polish Outer Carpathians

This study examined the molecular and isotopic compositions of gases generated from different kerogen types (i.e., Types I/II, II, IIS and III) in Menilite Shales by sequential hydrous pyrolysis experiments. The experiments were designed to simulate gas generation from source rocks at pre-oil-cracking thermal maturities. Initially, rock samples were heated in the presence of liquid water at 330 °C for 72 h to simulate early gas generation dominated by the overall reaction of kerogen decomposition to bitumen. Generated gas and oil were quantitatively collected at the completion of the experiments and the reactor with its rock and water was resealed and heated at 355 °C for 72 h. This condition simulates late petroleum generation in which the dominant overall reaction is bitumen decomposition to oil. This final heating equates to a cumulative thermal maturity of 1.6% R_r, which represents pre-oil-cracking conditions. In addition to the generated gases from these two experiments being characterized individually, they are also summed to characterize a cumulative gas product. These results are compared with natural gases produced from sandstone reservoirs within or directly overlying the Menilite Shales. The experimentally generated gases show no molecular compositions that are distinct for the different kerogen types, but on a total organic carbon (TOC) basis, oil prone kerogens (i.e., Types I/II, II and IIS) generate more hydrocarbon gas than gas prone Type III kerogen. Although the proportionality of methane to ethane in the experimental gases is lower than that observed in the natural gases, the proportionality of ethane to propane and i-butane to n-butane are similar to those observed for the natural gases. δ¹³C values of the experimentally generated methane, ethane and propane show distinctions among the kerogen types. This distinction is related to the δ¹³C of the original kerogen, with ¹³C enriched kerogen generating more ¹³C enriched hydrocarbon gases than kerogen less enriched in ¹³C. The typically assumed linear trend for δ¹³C of methane, ethane and propane versus their reciprocal carbon number for a single sourced natural gas is not observed in the experimental gases. Instead, the so-called “dogleg” trend, exemplified by relatively ¹³C depleted methane and enriched propane as compared to ethane, is observed for all the kerogen types and at both experimental conditions. Three of the natural gases from the same thrust unit had similar “dogleg” trends indicative of Menilite source rocks with Type III kerogen. These natural gases also contained varying amounts of a microbial gas component that was approximated using the Δδ¹³C for methane and propane determined from the experiments. These approximations gave microbial methane components that ranged from 13–84%. The high input of microbial gas was reflected in the higher gas:oil ratios for Outer Carpathian production (115–1568 Nm³/t) compared with those determined from the experiments (65–302 Nm³/t). Two natural gas samples in the far western part of the study area had more linear trends that suggest a different organic facies of the Menilite Shales or a completely different source. This situation emphasizes the importance of conducting hydrous pyrolysis on samples representing the complete stratigraphic and lateral extent of potential source rocks in determining specific genetic gas correlations.     

Postdated melting of subcontinental lithospheric mantle by the Emeishan mantle plume: Evidence from the Anyi intrusion,Yunnan, SW China

The Anyi intrusion is located in the central zone of Emeishan large igneous province (ELIP), SW China. It outcrops in an area of about 0.65 km² and ~ 1 km thick and dips to the southwest. The Anyi intrusion consists of a lower clinopyroxenite zone, middle gabbro zone, and an upper monzonite–syenite zone. Up to 400 m thick stratiform disseminated Fe–Ti oxide layer with grades of 16–18 wt.% total Fe is hosted in the lower clinopyroxenite zone. Zircon SHRIMP U–Pb age (247 ± 3 Ma) indicates that the Anyi intrusion represents postdated mafic magmatism resulting from the ~ 260 Ma Emeishan mantle plume. Compared with the typical oxide-bearing intrusions (such as Panzhihua and Baima) formed at ~ 260 Ma in the ELIP, the Anyi intrusion is characterized by high alkaline contents and LREE/HREE ratios, extremely low εNd values (− 6.2 to − 7.6) and moderate high (⁸⁷Sr/⁸⁶Sr)_i values (0.7072 to 0.7086). These characteristics of the Anyi intrusion cannot be explained by fractional crystallization or crustal contamination, but may reflect a unique enriched continental lithospheric mantle source (a mantle source mixed between garnet pyroxenite and spinel peridotite). We propose that the postdated mafic magmatism associated with the formation of the Anyi intrusion and its Fe–Ti oxide ore may be the product of melting of a mantle source mixed between garnet pyroxenite and spinel peridotite in the shallow lithosphere caused by conductive heating combined with lithosphere thinning due to plume–lithosphere interaction.