δ13C Values Of Methane Research Articles

Long‐range transport of littoral methane explains the metalimnetic methane peak in a large lake

AbstractIn large and stratified lakes, substantial methane stocks are often observed within the metalimnion. The origin of the methane (CH4) accumulated in the metalimnion during stratification, which can sustain significant emissions during convective mixing, is still widely debated. While commonly attributed to the transport of methane produced anaerobically ex situ, recent evidence suggests that oxic in situ methane production could also contribute to metalimnetic methane peaks. Here, we assessed the origin, that is, pelagic CH4 production or transport of sublittoral CH4 through the interflow, of metalimnetic methane in Lake Geneva, the largest lake in Western Europe. Microbial diversity data do not support the hypothesis of oxic methane production in the metalimnion. In contrast, both spatial and temporal surveys of methane show that maxima occur at depths and sites most affected by the Rhône River inflow. Methane δ13C values point to an anaerobic sublittoral methane source, within a biogeochemical hotspot close to the river delta region, and an efficient transport across several kilometers in a vertically well‐constrained metalimnion. Our current findings emphasize the indirect role of river interflows for the long‐range transport of CH4 produced in sediment biogeochemical hotspots, even for large lakes where sublittoral habitats represent a fairly limited fraction of the lake volume.

Clumped Isotope Signatures of Abiotic Methane: The Role of the Combinatorial Isotope Effect

AbstractMethane clumped isotope signatures of abiogenesis may be diagnostic of the origin of methane on Earth and other planetary bodies. We performed synthesis of abiogenic methane in hydrothermal conditions between 130 and 300°C and determined δ13C, δD, Δ13CH3D, and Δ12CH2D2. The experiments were performed by heating water in the presence of Fe0 powder and CO. The reduction of water on metallic iron led to the formation of H2. CO was reacted with both H2 and H2O, generating both CH4 and CO2. Methane δ13C values are isotopically depleted by ∼25‰ relative to the CO starting material. This is consistent with carbon isotopic equilibrium between methane, carbon monoxide and carbon dioxide in our experiments. In contrast, D/H ratios are inconsistent with equilibrium isotopic fractionation, as illustrated by δD values of methane fractionated by ∼500‰ relative to starting H2O. This suggests that under our experimental conditions, hydrogen additions to carbon may be governed by kinetics. Δ13CH3D values track experimental temperature, with values between +1.5‰ and +5.0‰ for most samples. In contrast, Δ12CH2D2 values are displaced from equilibrium. We find exclusively negative Δ12CH2D2 values, showing deficits down to 40‰ relative to thermodynamic equilibrium. We interpret the data as evidence for distinct, kinetically induced D/H pools contributing to methane assembly, that is, a combinatorial effect. The cumulative D/H fractionations associated with CO hydrogenation explain the direction and magnitude of Δ12CH2D2 values during abiotic methane formation. We suggest that near equilibrium Δ13CH3D with negative Δ12CH2D2 signatures will help identify methane formed abiotically in nature.

Archaeal lipid biomarkers in near-surface sediments at a giant colony of the bivalve Calyptogena: Molecular records of a massive methane release event associated with methane hydrate dissociation

To investigate the possibility that dissociation of subsurface methane hydrate (MH) in the eastern Nankai Trough, offshore of Japan, led to the formation of a giant colony of the bivalve Calyptogena (currently mostly dead), the carbon isotope ratios (δ13C) of archaeal lipids and methane were measured in near-surface core sediments at Daini-Tenryu Knoll. The irregular variation of porewater methane δ13C with depth (from −75 ‰ to −26 ‰) suggested that originally low δ13C microbial methane was degraded in different proportions by anaerobic methane oxidation. Consistent with this inference, biomarkers of anaerobic methanotrophic archaea (ANME), namely, crocetane (2,6,11,15-tetramethylhexadecane), PMI (2,6,10,15,19-pentamethylicosane), and diethers (archaeol and hydroxyarchaeols), were detected in lipid extracts. The low diether δ13C values (−121 ‰ to −104 ‰) were characteristic of ANME, but less variable than the methane δ13C values, and the relationships between diethers and methane δ13C values deviated from regression lines derived using worldwide data from modern methane seep sites. In contrast, δ13C values of the ANME source methane predicted from those regression lines and the diether δ13C values agreed well with methane δ13C values in MH samples obtained by nearby deep drilling. This result strongly suggests that most of the diethers were produced by ANME that proliferated during a past massive methane release event associated with MH dissociation. The crocetane δ13C value, measured in a mixture with phytane and estimated from the correlation of the δ13C of the mixture with the mole fraction of crocetane, was about −127 ‰. More than half of the PMI δ13C values were greater than −100 ‰, suggesting the background presence of fossil PMI from methanogens.

Origins of sediments and fluids in submarine mud volcanoes off Tanegashima Island, northern Ryukyu Trench, Japan

Mud volcanoes are topographic features through which over-pressurized sediments are intruded upward and erupted, transporting fluids and sediments from the deep subsurface to the surface/seafloor. To understand material and biogeochemical cycling through mud volcanoes, we investigated the origins of sediments and fluids erupted by four submarine mud volcanoes off Tanegashima Island, along the northern Ryukyu Trench: MV#1–3 and MV#14. We estimated the ages of the source sequences of the mud volcano sediments to be middle to late Miocene based on nannofossils and 10Be ages. The sediments were characteristically enriched in illite. The similar mineral compositions and vitrinite reflectance values (0.42%–0.45%) of sediments erupted by the mud volcanoes strongly suggest that each is rooted in the same source sequence. Pore waters had Cl− concentrations ∼40% that of seawater and were proportionally enriched in 18O and depleted in D, indicating the addition of freshwater from the dehydration of clay minerals. However, the smectite and illite contents (<40%) in the clay size fraction ruled out in situ smectite dewatering as the cause of the pore water Cl− dilution. Thus, fluids derived from clay dewatering must have originated from deeper than the source sequence of the mud volcano sediments. Vertical Cl− profiles indicate that the upward fluid advection rate and eruption frequency decrease from MV#3 to MV#2, MV#1, and MV#14 (MV#14 being dormant). At the active mud volcanoes, the C1/C2 ratios (<100) and methane δ13C values (−56‰ to −42‰) indicate that hydrocarbon gases are mostly derived from the thermal decomposition of organic matter in deep sediments where the in situ temperature exceeds 80°C. At the dormant MV#14, high C1/C2 ratios (700–4,000) and low methane δ13C values (ca. −75‰) suggest the limited supply of thermogenic methane and the subsequent shallow methanogenesis. Because vitrinite reflectance values indicate that the source sediments are too immature to produce thermogenic hydrocarbons, the hydrocarbon gases, like the fluids derived from clay dewatering, were probably supplied from deeper than the source strata. The supply of deep fluids into the source sequence might be a universal phenomenon in subduction zones, and may play an important role in mud volcanism and associated biogeochemical cycling.

Insights into gas mixing: evidence from hydrocarbon geochemical characteristics

The study of mixed natural gas can clarify the specific sources of natural gas accumulation and provide the basis for the exploitation of natural gas resources. Analysis of gas compositions and carbon isotopic ratios of different components of mixed natural gas shows that the carbon isotopic fractionation of methane and ethane from different sources has a small difference after mixing. This conclusion is derived from the experimental data in Table 1, 0.88 is the average of all Ro values. Well DN8 and DN9 produced light oil with a density between 0.75 and 0.82 g/cm3 and viscosity between 1.63 and 3.3 MPa. Generally, in the high maturity stage of organic matter, the carbon isotopic fractionation values of methane and ethane of natural gas from a single source should display a difference of at least 4.0% to 5.0%. The δ13C value of methane and ethane in well DN8 are only 1.6%. It is considered that the mixing of natural gas from different sources causes this phenomenon.

Stable Isotope Evaluation of Geothermal Gases from the Kızıldere and Tekke Hamam Geothermal Fields, Western Anatolia, Turkey

Volatiles transported from the Earth’s interior to the surface through permeable faults provide insights on the gas composition of deep reservoirs, mixing and migration processes, and can also be applied as gas-geothermometer. Here, we present carbon (δ13C), hydrogen (δ2H) and nitrogen (δ15N) isotopic data of CO2, CH4, and N2 from gas samples collected from the Kızıldere and Tekke Hamam geothermal fields, located along the eastern segment of the Büyük Menderes Graben, Turkey. The stable isotopic composition of carbon (δ13C) ranges from +0.30 to +0.99‰ (PDB) for CO2 from Kızıldere and is slightly more variable (−0.95 to +1.3‰) in samples from Tekke Hamam. Carbon isotope data in combination with CO2/3He data reveal that ~97% (Tekke Hamam) to ~99% (Kızıldere) of CO2 derives from limestone sources, with the residual CO2 being magmatic in origin with no evidence for CO2 from organic sources. The slightly higher contribution of limestone-derived CO2 in Kızıldere, compared to Tekke Hamam can be attributed to the higher temperatures of the Kızıldere reservoir and resulting amplified fluid–limestone interaction, as well as helium depletion during phase separation for Kızıldere samples. In contrast to the carbon isotopic composition of CO2, the δ13C values of methane from Kızıldere and Tekke Hamam are clearly distinct and vary between −23.6 and −20.8‰ for Kızıldere and −34.4 and −31.7‰ for Tekke Hamam, respectively. The δ2H-CH4 composition is also distinct, measured as −126.7‰ for Kızıldere and −143.3‰ for Tekke Hamam. CO2-CH4 carbon isotope geothermometry calculations based on the isotopic fractionation of δ13C between the dominant component CO2 and the minor component CH4 reveals temperatures 20–40 °C and 100–160 °C higher than the bottom–hole temperatures measured for Tekke Hamam and Kızıldere, respectively. Based on the CO2-CH4 carbon isotope disequilibrium, unusual high methane concentrations of ~0.3 to 0.4 vol.-% and CH4/3He-δ13C-CH4 relationships we suggest thermal decomposition of late (Tekke Hamam) to over-mature (Kızıldere) organic matter and, to some extent, also abiogenic processes as principal source of methane. The N2/36Ar ratios of most samples reveal the existence of a non–atmospheric nitrogen component within the gas mixture issuing from both fields, in addition to a constant contribution of atmospheric derived nitrogen accompanied into the system via the meteoric recharge of the geothermal system. Based on the δ15N isotopic ratios (varying between −4.44‰ and 4.54‰), the non–atmospheric component seems to be a mixture of both sedimentary (crustal organic) and mantle nitrogen. The thick Pliocene sedimentary sequence covering the metamorphic basement is the likely major source for the thermogenic content of CH4 and crustal N2 gas content in the samples.

The effect of thermochemical sulfate reduction of n-pentane and condensate on the chemical and isotopic compositions of gases: Implications for the organic reactant and reaction extent

Evaluating the composition of organic reactants initially involved in thermochemical sulfate reduction (TSR) reaction and better understanding the chemical and isotopic compositions of gases are helpful to minimize the risks involved in exploration of natural sour gas reservoirs. Variations in the chemical and carbon isotopic compositions of the gas produced during high pressure (50Mpa), isothermal (360 °C) hydro-pyrolysis experiments of the TSR reaction and thermal cracking involving n-pentane (n-C5) and condensate were investigated using a high-pressure gold tube system. The results show that the yields of CH4, CO2 and H2S increased with reaction time, and that the yields of C2–C4 firstly increased and then decreased. The δ13C value of alkane gases increased with reaction time. The δ13C value of CO2 produced from the TSR reaction involving n-C5 became less negative. The yields of alkane gas, CO2 and H2S from the TSR reaction of n-C5 were up to two times higher than that those from the TSR reaction of condensate. This indicates that lighter liquid hydrocarbons with higher hydrogen content produce more hydrocarbon and non-hydrocarbon gases. The relationship between the dryness, δ13C values of alkane gases and sourness could help to estimate whether the organic reactant was liquid or gaseous hydrocarbons. The value of ln(C2/C3) at the early stage of TSR reaction can be used to roughly estimate the composition of the liquid hydrocarbons involved in the TSR reaction. Our analysis also shows that the value of ln(C2/C3) firstly increased and then decreased with increasing gas dryness during the TSR reaction, which could be an effective index to assess the TSR reaction extent and provide new clues for identifying gas origin. The δ13C values of methane and ethane produced in the TSR reaction involving n-C5 reversed to δ13C1 > δ13C2 during the 48–60h period, which could be related to the oxidation of alkanes by HSO4−. This could help explain the δ13C1 > δ13C2 in TSR influenced gas reservoirs with low H2S content.

Biomarker insights into a methane-enriched Holocene peat-setting from “Doggerland” (central North Sea)

Peats formed widespread early after the Last Glacial Maximum around the coast of the Dogger Bank, the central high of the now flooded “Doggerland” (central North Sea). Here we present biomarker and gas geochemical insights into near-surface peat cored at 43 m water depth. Based on their probable age of ~10 ka before present (BP), the peats are likely part of the so-called basal Holocene peats. Pore waters in the samples document slight methane enrichments in peat layers (up to 250 nM methane vs 60 nM above the peat layer) and δ13C-values of methane ranging between ca. −46‰ and −60‰ PDB. Ethane, propane, and n-butane were also found and may either document upward migration of thermogenic hydrocarbons in the past or, more likely, peat-specific gases, which is indicated by accompanying abundant unsaturated analogs (i.e. ethene and propene). High concentrations of elemental sulfur (up to 19 wt.%) and sulfur-specific transformation products of plant triterpenoids (Des-A-triterpenoids) point to microbial generation during and/or slightly after peat-formation as hydrocarbon source. Biomarkers indicated that peats were formed in a variable environment with high contributions from higher plant wax material. However, a C20 highly branched isoprenoid (HBI) likely records abundant diatoms typical of an oligotrophic lake environment. In addition, all samples contained a suite of bacterial hopenes and fernenes (hop-17(21)-ene, diploptene, fern-7(8)-ene). A strongly fluctuating methane-flux and/or -turnover during peat formation is recorded in partially high 13C-depletions of <−60‰ PDB of these biomarkers. This underlines the importance of peat layers for methane generation and consumption on “Doggerland” during the Holocene.

Cryptic Methane-Cycling by Methanogens During Multi-Year Incubation of Estuarine Sediment

As marine sediments are buried, microbial communities transition from sulfate-reduction to methane-production after sulfate is depleted. When this biogenic methane diffuses into the overlying sulfate-rich sediments, it forms a sulfate-methane transition zone (SMTZ) because sulfate reducers deplete hydrogen concentrations and make hydrogenotrophic methanogenesis exergonic in the reverse direction, a process called the anaerobic oxidation of methane (AOM). Microbial participation in these processes is often inferred from geochemistry, genes, and gene expression changes with sediment depth, using sedimentation rates to convert depth to time. Less is known about how natural sediments transition through these geochemical states transition in real-time. We examined 16S rRNA gene amplicon libraries and metatranscriptomes in microcosms of anoxic sediment from the White Oak River estuary, NC, with three destructively sampled replicates with methane added (586-day incubations) and three re-sampled un-amended replicates (895-day incubations). Sulfate dropped to a low value (∼0.3 mM) on similar days for both experiments (312 and 320 days, respectively), followed by a peak in hydrogen, intermittent increases in methane-cycling archaea starting on days 375 and 362 (mostly Methanolinea spp. and Methanosaeta spp., and Methanococcoides sp. ANME-3), and a methane peak 1 month later. However, methane δ13C values only show net methanogenesis 6 months after methane-cycling archaea increase and 4 months after the methane peak, when sulfate is consistently below 0.1 mM and hydrogen increases to a stable 0.61 ± 0.13 nM (days 553–586, n = 9). Sulfate-reducing bacteria (mostly Desulfatiglans spp. and Desulfosarcina sp. SEEP-SRB1) increase in relative abundance only during this period of net methane production, suggesting syntrophy with methanogens in the absence of sulfate. The transition from sulfate reduction to methane production in marine sediments occurs through a prolonged period of methane-cycling by methanogens at low sulfate concentrations, and steady growth of sulfate reducers along with methanogens after sulfate is depleted.

High δ13C values of methane in residual gas from a secondary biogenic methane reservoir as a possible indicator of thermogenic gas preserved in closed pores: A case study of the Kushiro Coal Mine, Japan

In a secondary biogenic methane reservoir, closed pores may enclose thermogenic methane formed during diagenesis, whereas open pores carry biogenic methane. To assess this possibility, the isotopic compositions of desorbed and residual gas from the Eocene Harutori (coals and sedimentary rocks) and Paleocene Shiomi (sedimentary rocks) formations in the Kushiro Coal Mine were analyzed assuming that the desorbed and residual gas were derived mainly from open and closed pores, respectively. Values of δ13C(CH4) for residual gas from the Shiomi (−54.6‰ to −39.9‰) and Harutori (−77.9‰ to −60.5‰) formations were higher than those for desorbed gas (−80.9‰ to −71.9‰ and − 81.2‰ to −68.9‰, respectively), and ∆13C(CH4) values (δ13C(CH4)residual gas − δ13C(CH4)desorbed gas) were greater for the Shiomi Formation (18.5‰ to 37.5‰) than the Harutori Formation (3.3‰ to 14.4‰). The relatively high δ13C(CH4) values for residual gas and the large ∆13C(CH4) value for the Shiomi Formation imply preservation of thermogenic gas in some closed pores in this formation. The bulk diffusion coefficient of methane in minerals is lower than that of kerogen by more than two orders of magnitude. Therefore, methane in the closed pores of minerals should be preserved for longer periods than that in the closed pores of kerogen. The closed pores enclosing thermogenic methane in the Shiomi Formation are likely developed in the mineral matrix of this formation.

A novel isotopic approach to distinguish primary microbial and thermogenic gases in shallow subsurface environments

The stable carbon isotopic compositions of methane and ethane have been used for several decades to characterize petroleum gas in the subsurface. However, difficulties have frequently remained in determining the exact origin of natural gas samples derived from microbial activity, thermal degradation of organic matter (e.g. thermogenic gas), and mixtures of both components. To further refine this approach, we represent the carbon isotope difference between methane and ethane, or Δ13CC1–C2, plotted versus the individual δ13C values of methane and ethane as a new graphical tool for a refined assessment of natural gas origins and their mixtures. In addition, a new mathematical notation using the δ13C values of methane and ethane is also proposed for discrimination of the origin of natural gas samples. This parameter has been termed isotope factor O(C1/C2) and is defined as the ratio [(δ13CC1*1000)/(δ13CC2)3]. When this factor is represented against the δ13C values of methane and ethane, an improved distinction between primary microbial and thermogenic gases is achieved. Our study does not propose mechanisms for microbial generation of ethane, but the data suggest that the isotopic composition of ethane should be taken into account for the characterization of the origin of natural gas samples from shallow subsurface environments. For primary microbial and thermogenic gases with similar δ13C values of methane, the primary microbial gas will be characterized by lower O(C1/C2) values (<1.5) compared to the thermogenic gas (>2), and will have a Δ13CC1–C2 value 10–20‰ more positive than that of thermogenic gas. Gas mixtures have intermediate O(C1/C2) and Δ13CC1–C2 values and are graphically identifiable using the here presented classification tools. The effectiveness of the new graphical tools is demonstrated using natural gas samples from Canadian groundwater wells, low-permeability petroleum reservoirs (including coalbed methane plays) in Canada and the USA, Quaternary primary microbial gas accumulations in China, and secondary microbial gases from Northern Africa and China.

Gas composition and carbon isotopic variation during shale gas desorption: Implication from the Ordovician Wufeng Formation - Silurian Longmaxi Formation in west Hubei, China

This paper focuses on the gas composition and carbon isotopic variation during gas desorption from the shale in Ordovician Wufeng Formation-Silurian Longmaxi Formation. The results show that the desorbed shale gases are predominately comprised of methane (content: 98.36–98.76%) and thus are typical dry gas. As the desorption proceeded, the content of methane gradually decreased, and the drying coefficient (C1/C1-5) of the desorbed shale gases also gradually decreased. The drying coefficient decreased from 98.58 to 99.04% (98.81% on average) of the initial desorbed samples to 88.72–96.78% (94.15% on average) of the final desorbed samples. Based on the variation patterns of gas component ratios during the shale gas desorption, the order of the shale's absorption capacity for different gas components is determined as CO2 > H2 > C3H8 > C2H6 > CH4 > He. The drying coefficient of the shale gas is negatively correlated with the total shale gas content due to the different adsorption capability of shale for gaseous hydrocarbons. The δ13C values tended to become heavier during desorption in general. The δ13C1 values of the initial desorbed samples were the lowest, ranging from −30.71‰ to −25.63‰. In detail, the δ13C values of methane increased by 5.41–25.93‰, while those of ethane increased within a relatively smaller range. The carbon isotopic fractionation is mainly ascribed to diffusion and adsorption/desorption. Moreover, the carbon isotopic fractionation degree of methane was correlated with the shale gas content during the field desorption, suggesting that the carbon isotopic fractionation degree of methane is a potential tool for the identification and selection of favorable sweet spots of shale gas. In addition, the shale gas from Wufeng-Longmaxi Formation in west Hubei consists of highly mature oil-cracking gas and kerogen-cracking gas. All of the desorbed shale gas is characterized by isotopic reversal and the extent of the isotopic reversal increased as desorption continued. This is primarily caused by the mixing of the gas from thermal degradation of kerogen with the gas from residual oil cracking and wet gas as well as the diffusion of the shale gas. The carbon isotope reversal of methane and ethane is not directly related to shale gas content.

The Effect of Bacterial Sulfate Reduction Inhibition on the Production and Stable Isotopic Composition of Methane in Hypersaline Environments

The aim of this research was to investigate the competition between methanogens and sulfate-reducing bacteria in hypersaline environments. Samples of photosynthetic microbial mats, both soft mats (salinities of 55–126 ppt) and gypsum-hosted endoevaporite mats (salinities of 77–320 ppt), were obtained from hypersaline environments in California, USA, Mexico and Chile. Methane production was determined from the increase in headspace methane concentration within incubation vials containing mat samples. At the end of the incubation period, the δ13C values of produced methane were measured. Soft microbial mat vials containing molybdate, a specific inhibitor of bacterial sulfate reduction, exhibited dramatically higher methane production rates and higher (enriched in 13C) methane δ13C values than the controls. This suggests that the inhibition of sulfate reduction allowed the methanogens at these sites to use the competitive substrates (H2 and/or acetate) made available. Further, the higher δ13C values of the produced methane suggest that substrates (both competitive and non-competitive) were used to near completion. At the endoevaporite sites, which have much higher salinities than the soft mat sites, methane production was not significantly different and the methane δ13C values either remained the same or decreased (depleted in 13C) with added molybdate. We suggest that substrate availability increased enough to allow for somewhat greater isotopic fractionation resulting in the lower methane δ13C values that were observed, but not enough to significantly increase measured production rates. Where no changes in either methane production rates or δ13C values occurred, we hypothesize that salinity itself was inhibiting sulfate reduction and thus controlling microbe populations and rates of metabolism.

Archaeal lipid biomarker as a tool to constrain the origin of methane at ancient methane seeps: Insight into subsurface fluid flow in the geological past

The origin of methane discharged at ancient methane seeps is a key to understand subsurface fluid-flow processes in sedimentary basins. The carbon isotopic composition of methane-seep carbonates can be used to estimate the origin of methane, but mixing of multiple carbon sources results in ambiguous interpretation. We here constrained the origin of methane at ancient seeps by using the carbon isotopic composition and the isotope fractionation of lipid biomarkers of anaerobic methane-oxidizing archaea. We estimated the carbon-isotope fractionation between methane and biomarkers as the offset between their δ13C values at modern seep sites in the Japan Sea and other regions of the world. The biomarker pentamethylicosane (PMI) extracted from a modern seep carbonate of the Japan Sea shows a 44‰ offset from the seep methane. On the basis of literature data, we calculated the carbon-isotope offset as mostly ~20–60‰ for PMI at modern seeps across the world. We also assessed the relationship of the δ13C values between methane and biomarkers by regression analysis on the data taken at the modern seeps, obtaining regression formulas to estimate the δ13C values of methane from those of biomarkers. For ancient seeps, we examined Miocene to Pleistocene methane-seep carbonate rocks collected from Japan Sea sediments. PMI extracted from these ancient carbonates have δ13C values ranging from −137‰ to −93‰. Using the δ13C values of PMI and the carbon-isotope offset and regression formulas, we estimated that the methane at the ancient seeps was mainly of biogenic origin and migrated from the shallow subsurface.

Assessing effects of sulfate minerals on petroleum generation in sedimentary basins using hydrous pyrolysis: I. Light alkanes

Abstract A series of hydrous pyrolysis experiments were conducted to assess the role of sulfate minerals on the chemical and isotopic compositions of light hydrocarbons generated during thermal degradation of kerogen. The shale source rock with low maturity (Tmax = 425 °C) from Eagle Ford formation in southeast Texas was used in the experiments at 330 °C for the duration of 72 h, while the abundance of gypsum in each experiment varies from 0, 0.5, to 1 wt%. Under experimental conditions, the positive effect of gypsum on the yields of gaseous alkanes (C1 - C4) and CO2 has been explicitly observed, with an exponential relationship between the yield of each gas and the amount of gypsum. While the relative abundance of C2 - C4 alkanes has shown an increasing trend with gypsum, the gas dryness (C1/∑C1-4), however, decreased from 73.1% with no gypsum to 49.6% with gypsum of 1 wt%. It was attributed to lower overall yields and corresponding higher increased generation rates of C2 - C4 alkanes than methane through thermal cracking of kerogen and other organic compounds. The carbon isotope value of each alkane reached the highest in the experiment with 1 wt% gypsum. Compared to the experiment without gypsum, there was an increase of 11.5‰ for the δ13C value of methane with 1 wt% gypsum. The extent of this 13C enrichment between experiments with different amount of gypsum became less for C2 to C4. The δ13C value of CO2 was the highest among generated gases, ranging from −4.7 to −2.1‰. Combined with chemical compositions of gases, it suggested that enhanced thermal cracking reactions were predominant in generation of light alkanes with the presence of gypsum, whereas dissolution of carbonates was the main source of CO2. The reactions involving thermochemical sulfate reduction (TSR) were minor (if present) under experimental conditions, most likely due to the small quantity of gypsum (up to 1 wt%) and neutralization of acidity by carbonates. Nonetheless, the changes in chemical and isotopic compositions of light alkanes, which were ultimately caused by the presence of gypsum, need to be incorporated into geochemical proxies for assessing source rocks (maturation in particular) and their generation potential.

Stable isotope reversal and evolution of gas during the hydrous pyrolysis of continental kerogen in source rocks under supercritical conditions

Semi-closed hydrous pyrolysis experiments were conducted to investigate the isotopic evolution of shale gas produced from continental organic-rich shales with increasing thermal maturity and prospecting potential. The δ13C values of methane, ethane, propane and n-butane became heavier with increasing thermal maturity and showed good relationships with vitrinite reflectance (VR). Gases expelled from type-I, type-II, and type-III kerogens followed the normal carbon sequence (δ13CC1 < δ13CC2 < δ13CC3 < δ13CnC4) and the hydrogen isotopic sequence (δ2HC1 < δ2HC2 < δ2HC3 < δ2HnC4 < δ2HiC4) among the alkanes at VRs lower than 2.14% Ro, 2.42% Ro, and 1.87% Ro respectively. The isotopically reversed gases (δ13CC1 < δ13CC2 < δ13CnC4 < δ13CC3) were observed in type-I kerogen with a VRs above 2.14% Ro and in type-II kerogen with a VR above 2.42% Ro. In type-III kerogen, isotopically reversed gases (δ13CC1 < δ13CnC4 < δ13CC3 < δ13CC2) were observed with a VR above 2.94% Ro. These results suggest four stages in the stable carbon isotope reversal of ethane, propane, and n-butane with increasing maturity. The isotopically reversed gases (δ13CC1 < δ13CC2 < δ13CnC4 < δ13CC3) may represent lower maturity and higher productivity shale gas than the isotopically reversed gases (δ13CC1 < δ13CnC4 < δ13CC3 < δ13CC2), indicating that continental type-I kerogen has the highest potential productivity for shale gas, and continental type-III kerogen has the lowest. Stable hydrogen isotope composition did not respond to the stable carbon isotope reversal in isotopically reversed gases. We suggest that indigenous generation and mixing may be the dominant mechanisms responsible for the stable carbon isotope reversal, and adsorption/desorption during hydrocarbon expulsion may promote the stable isotope reversal under semi-closed conditions. Overall, these results suggest that this study is an important contribution to continental shale gas exploration.

Geochemical and Isotope Study of Trichloroethene Degradation in a Zero-Valent Iron Permeable Reactive Barrier: A Twenty-Two-Year Performance Evaluation.

This study provides a twenty-two-year record of in situ degradation of chlorinated organic compounds by a granular iron permeable reactive barrier (PRB). Groundwater concentrations of trichloroethene (TCE) entering the PRB were as high as 10670 μg/L. Treatment efficiency ranged from 81 to >99%, and TCE concentrations from <1 μg/L to 165 μg/L were detected within and hydraulically down-gradient of the PRB. After 18 years, effluent TCE concentrations were above the maximum contaminant level (MCL) along segments of the PRB exhibiting upward trending influent TCE. Degradation products included cis-dichloroethene ( cis-DCE), vinyl chloride (VC), ethene, ethane, >C4 compounds, and possibly CO2(aq) and methane. Abiotic patterns of TCE degradation were indicated by compound-specific stable isotope data and the distribution of degradation products. δ13C values of methane within and down-gradient of the PRB varied widely from -94‰ to -16‰; these values cover most of the isotopic range encountered in natural methanogenic systems. Methanogenesis is a sink for inorganic carbon in zerovalent iron PRBs that competes with carbonate mineralization, and this process is important for understanding pore-space clogging and longevity of iron-based PRBs. The carbon isotope signatures of methane and inorganic carbon were consistent with open-system behavior and 22% molar conversion of CO2(aq) to methane.

Mantle origin of methane in the Black Sea

A new distribution map of gas seeps and mud volcanoes has been compiled of the Black Sea. It has been derived from the published coordinates for ca. 5000 gas seeps and 80 mud volcanoes. An analysis of the stable isotopic composition of methane has been performed for authigenic carbonates and sediments. The δ13C values of carbonates and sediments form 2 distinct tight groups depending of the geological environments. The diagrams of values of δ13C vs. δD and δ13C vs. C1/(C2 + C3) have been applied to assess earlier classifications of methane. The origin of methane from most of samples has turned out to be uncertainly determined. Based on seismic reflection data, the feeder channels of gas releases and mud volcanoes penetrate to the Pre-Mesozoic basement beneath the Polshkov High, Andrusov and Tetyaev Ridges and Sorokin Trough at a depth of up to 12 km. In the Central Black Sea the feeder channel of mud volcano reaches the mantle surface where a depth is 19 km. The highest concentration of gas seeps distribution is observed at the triple junction of the mantle faults in the NW Black Sea. Assessments of different mechanisms have been made for their ability to produce an unprecedented methane concentration in the Black Sea water column. There occurs thermogenic methane in the sediments from gas seepage and mud volcanoes areas. The thermogenic methane results from post genetic alternation of biogenic methane of the Paleogene—Neogene sediments. Biological methane produced from organic matter plays negligibly small role in accumulating the world’s largest quantities of anaerobic methane in the Black Sea. Nonorganic methane is most likely to form the tremendous reservoir of dissolved gas in the Black Sea water column below 150—200 m. The seeming scarcity of abi-otic methane is accounted for its recycling by microbial activity, misclassificating origin and producing by chemical syntheses, with its biological methane δ13C values.

Origin of methane and heavier hydrocarbons entrapped within Miocene methane-seep carbonates from central Japan

We examined the carbon isotopic and molecular compositions of residual gases within Miocene methane-derived carbonates collected in Japan. Methane, ethane, and propane were extracted by acid digestion of powdered carbonates. The isotopic and molecular compositions of the extracted hydrocarbons are inconsistent with conventional thermogenic and microbial gases. Despite a range of δ13C values from −67‰ to −38‰ (relative to Vienna Pee Dee Belemnite (VPDB)), the liberated hydrocarbons yielded consistently low methane to ethane + propane ratios (2–30). The extracted ethane and propane yielded anomalous δ13C values as low as −84‰, lower than those of the coexisting methane. The ethane and propane were most likely produced through thermal cracking of organic compounds preserved within the seep carbonates during burial. The observed unusual isotopic trends may be explained by the mixing of two thermogenic gas components with different carbon isotopic and molecular compositions. Nevertheless, a positive correlation between δ13C values of methane and relatively immature carbonates at one study site (Nakanomata) indicates that while methane was oxidized to bicarbonate from which carbonates precipitated, it was preserved within the host carbonate cements. Such a scenario indicates that the residual methane at least partly originates from the Miocene seep fluid. Smaller amounts of methane were also released during heating and crushing of chipped samples. The results suggest that methane was entrapped mainly within intracrystal inclusions, which is supported by the observation of abundant nanometer-scale voids in an individual crystal.

Fluid sources and stable isotope signatures in authigenic carbonates from the Northern Apennines, Italy

Authigenic carbonates are frequently associated with methane cold-seep systems, which extensively occur in various geologic settings worldwide. Of interest is the relation between the fluids involved in their formation and the isotopic signals recorded in the carbonate cements. Along the Northern Apennines foothills (Italy), hydrocarbons and connate waters still seeping nowadays are believed to be the primary sources for the formation of fossil authigenic carbonate found in Plio-Pleistocene marine sediments. Four selected outcrops of dolomitic authigenic carbonates were analysed to compare signature of seeping fluids with fractionation of stable carbon and oxygen isotopes recorded in the carbonate.Along the foothills, deep methane-rich fluids spontaneously rise to the surface through mud volcanoes or are exploited in wells drilled nearby to the fossil Plio-Pleistocene authigenic carbonates. The plumbing system providing fluids to present-day cold seeps was structurally achieved in Late Miocene and Plio-Pleistocene. δ13C values of methane, which vary from −51.9 to −43.0‰ VPDB, indicate that gas composition from the deep hydrocarbon reservoirs is relatively uniform along the foothills. On the contrary, δ13C in fossil authigenic carbonates strongly varies among different areas and also within the same outcrop.The different carbon sources that fed the investigated carbonates were identified and include: thermogenic methane from the deep Miocene reservoirs, 13C-enriched CO2 derived from secondary methanogenesis and microbial methane from Pliocene successions buried in the Po Plain. The δ13C variability documented among samples from a single outcrop testifies that the authigenic carbonates might represent a record of varying biogeochemical processes in the hydrocarbon reservoirs. The sources of stable oxygen isotopes in authigenic carbonates are often ascribed to marine water. Oxygen isotopic fractionation in the dolomite cements indicates that marine pore water couldn't be the sole source of oxygen. δ18O values provide a preliminary evidence that connate waters had a role in the carbonates precipitation. The concomitant occurrence of active cold seepages and fossil record of former plumbing systems suggests that generation and migration of hydrocarbons are long-lasting and very effective processes along the Northern Apennines foothills.