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Multiply isotopically substituted molecules (‘clumped’ isotopologues) can be used as geothermometers because their proportions at isotopic equilibrium relative to a random distribution of isotopes amongst all isotopologues are functions of temperature. This has allowed measurements of clumped-isotope abundances to be used to constrain formation temperatures of several natural materials. However, kinetic processes during generation, modification, or transport of natural materials can also affect their clumped-isotope compositions. Herein, we show that methane generated experimentally by closed-system hydrous pyrolysis of shale or nonhydrous pyrolysis of coal yields clumped-isotope compositions consistent with an equilibrium distribution of isotopologues under some experimental conditions (temperature–time conditions corresponding to ‘low,’ ‘mature,’ and ‘over-mature’ stages of catagenesis), but can have non-equilibrium (i.e., kinetically controlled) distributions under other experimental conditions (‘high’ to ‘over-mature’ stages), particularly for pyrolysis of coal. Non-equilibrium compositions, when present, lead the measured proportions of clumped species to be lower than expected for equilibrium at the experimental temperature, and in some cases to be lower than a random distribution of isotopes (i.e., negative Δ18 values). We propose that the consistency with equilibrium for methane formed by relatively low temperature pyrolysis reflects local reversibility of isotope exchange reactions involving a reactant or transition state species during demethylation of one or more components of kerogen. Non-equilibrium clumped-isotope compositions occur under conditions where 'secondary’ cracking of retained oil in shale or wet gas hydrocarbons (C2-5, especially ethane) in coal is prominent. We suggest these non-equilibrium isotopic compositions are the result of the expression of kinetic isotope effects during the irreversible generation of methane from an alkyl precursor. Other interpretations are also explored. These findings provide new insights into the chemistry of thermogenic methane generation, and may provide an explanation of the elevated apparent temperatures recorded by the methane clumped-isotope thermometer in some natural gases. However, it remains unknown if the laboratory experiments capture the processes that occur at the longer time and lower temperatures of natural gas formation.

Carbon capture and storage (CCS) is a key technology to mitigate the environmental impact of carbon dioxide (CO2) emissions. An understanding of the potential trapping and storage mechanisms is required to provide confidence in safe and secure CO2 geological sequestration1,2. Depleted hydrocarbon reservoirs have substantial CO2 storage potential1,3, and numerous hydrocarbon reservoirs have undergone CO2 injection as a means of enhanced oil recovery (CO2-EOR), providing an opportunity to evaluate the (bio)geochemical behaviour of injected carbon. Here we present noble gas, stable isotope, clumped isotope and gene-sequencing analyses from a CO2-EOR project in the Olla Field (Louisiana, USA). We show that microbial methanogenesis converted as much as 13–19% of the injected CO2 to methane (CH4) and up to an additional 74% of CO2 was dissolved in the groundwater. We calculate an in situ microbial methanogenesis rate from within a natural system of 73–109 millimoles of CH4 per cubic metre (standard temperature and pressure) per year for the Olla Field. Similar geochemical trends in both injected and natural CO2 fields suggest that microbial methanogenesis may be an important subsurface sink of CO2 globally. For CO2 sequestration sites within the environmental window for microbial methanogenesis, conversion to CH4 should be considered in site selection.Microbial methanogenesis converts up to 19% of the carbon dioxide injected into an oil field to methane, suggesting that microbial methanogenesis may be a globally important subsurface process. [ABSTRACT FROM AUTHOR]

Methane (CH4) is an important greenhouse gas and amplifier of climate change. However, the causes of atmospheric CH4 variations over glacial-interglacial cycles remain unresolved. We propose that microbial methanogenesis along the shallow margins of sedimentary basins provides a source of atmospheric CH4 temporally connected with both advance and retreat of continental ice sheets. Extensive biodegradation of hydrocarbons in the Antrim Shale Formation, Michigan, United States, is associated with an active subsurface consortium of fermentative and methanogenic microorganisms. This activity was initially stimulated when saline formation waters were diluted by meltwater derived from overriding Pleistocene ice sheets. During glaciation, CH4 produced by this community accumulated in the shale at a rate of 1 Tg CH4 per 1000 yr as a result of ice coverage and increased hydrostatic pressure. We estimate that at present the Antrim Shale contains only 12%-25% of the cumulative mass of CH4 generated in the shale over the Pleistocene, indicating that CH4 that had accumulated during glaciation was subsequently released following ice-sheet retreat. While release from the Antrim Shale represents only a small part of the global CH4 budget, when extended to other glaciated sedimentary basins, subsurface methanogenesis may generate a substantial, previously unrecognized source of atmospheric CH4 during deglaciation. [ABSTRACT FROM AUTHOR]

Natural gas is a key energy resource, and understanding how it forms is important for predicting where it forms in economically important volumes. However, the origin of dry thermogenic natural gas is one of the most controversial topics in petroleum geochemistry, with several differing hypotheses proposed, including kinetic processes (such as thermal cleavage, phase partitioning during migration, and demethylation of aromatic rings) and equilibrium processes (such as transition metal catalysis). The dominant paradigm is that it is a product of kinetically controlled cracking of long-chain hydrocarbons. Here we show that C 2+ n -alkane gases (ethane, propane, butane, and pentane) are initially produced by irreversible cracking chemistry, but, as thermal maturity increases, the isotopic distribution of these species approaches thermodynamic equilibrium, either at the conditions of gas formation or during reservoir storage, becoming indistinguishable from equilibrium in the most thermally mature gases. We also find that the pair of CO 2 and C 1 (methane) exhibit a separate pattern of mutual isotopic equilibrium (generally at reservoir conditions), suggesting that they form a second, quasi-equilibrated population, separate from the C 2 to C 5 compounds. This conclusion implies that new approaches should be taken to predicting the compositions of natural gases as functions of time, temperature, and source substrate. Additionally, an isotopically equilibrated state can serve as a reference frame for recognizing many secondary processes that may modify natural gases after their formation, such as biodegradation., Competing Interests: The authors declare no competing interest.

We present precise measurements of doubly deuterated methane ( 12 CH 2 D 2 ) in natural methane samples using tunable infrared laser direct absorption spectroscopy (TILDAS). Using a 413 m optical path length astigmatic Herriott cell and two quantum cascade lasers (QCLs) scanning the spectral regions of 1090.46 ± 0.1 and 1200.23 ± 0.1 cm -1 , the instrument simultaneously measures the five main isotopologues of methane. The ratios 13 CH 3 D/ 12 CH 4 and 12 CH 2 D 2 / 12 CH 4 are measured at 0.01‰ and 0.5‰ (1σ) instrumental precision, respectively. The instrumental accuracy was assessed by measuring a series of methane gases with a range of δ 13 C and δD values but with the abundances of all isotopologues driven to thermal equilibrium at 250 °C. The estimated accuracy of Δ 12 CH 2 D 2 is 1‰ (1σ) on the basis of the results of the heated methane samples. This new TILDAS instrument provides a simple and rapid technique to explore the sources of methane in the environment.

Molecular investigations of the sulfate reducing bacteria that target the dissimilatory sulfite-reductase subunit A gene (dsr A) are plagued by the nonspecific performance of conventional PCR primers. Here we describe the incorporation of the FailSafe PCR System to optimize environmental analysis of dsr A by PCR amplification and denaturing gradient gel electrophoresis. PCR-DGGE analysis of dsr A composition revealed that SRB diversity was greater and more variable throughout the vertical profile of a marine sediment core obtained from a gas hydrate site (GC234) in the Gulf of Mexico than in a sediment core collected from a nearby site devoid of gas hydrates (NBP). Depth profiled dsr B abundance corresponded with sulfate reduction rates at both sites, though measurements were higher at GC234. This study exemplifies the numerical and functional importance of sulfate reducing bacteria in deep-sea sedimentary environments, and incremental methodological advancements, as described herein, will continue to streamline the analysis of sulfate reducer communities in situ., (Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)