Methane Hydrate Deposits Research Articles

Widespread gas hydrate instability on the upper U.S. Beaufort margin

AbstractThe most climate‐sensitive methane hydrate deposits occur on upper continental slopes at depths close to the minimum pressure and maximum temperature for gas hydrate stability. At these water depths, small perturbations in intermediate ocean water temperatures can lead to gas hydrate dissociation. The Arctic Ocean has experienced more dramatic warming than lower latitudes, but observational data have not been used to study the interplay between upper slope gas hydrates and warming ocean waters. Here we use (a) legacy seismic data that constrain upper slope gas hydrate distributions on the U.S. Beaufort Sea margin, (b) Alaskan North Slope borehole data and offshore thermal gradients determined from gas hydrate stability zone thickness to infer regional heat flow, and (c) 1088 direct measurements to characterize multidecadal intermediate ocean warming in the U.S. Beaufort Sea. Combining these data with a three‐dimensional thermal model shows that the observed gas hydrate stability zone is too deep by 100 to 250 m. The disparity can be partially attributed to several processes, but the most important is the reequilibration (thinning) of gas hydrates in response to significant (~0.5°C at 2σ certainty) warming of intermediate ocean temperatures over 39 years in a depth range that brackets the upper slope extent of the gas hydrate stability zone. Even in the absence of additional ocean warming, 0.44 to 2.2 Gt of methane could be released from reequilibrating gas hydrates into the sediments underlying an area of ~5–7.5 × 103 km2 on the U.S. Beaufort Sea upper slope during the next century.

Draft genome sequence of Methanoculleus sp. MH98A, a novel methanogen isolated from sub-seafloor methane hydrate deposits in Krishna Godavari basin

Members of the genus Methanoculleus are among the most prevalent methanogens in biomethanation processes especially in marine and brackish environments. A methanogen, identified as a novel species of the genus Methanoculleus, was isolated from deep sub-seafloor sediment obtained from the Krishna Godavari Basin off the eastern coast of India. This methanogen is thought to be the supplier of the methane in the submarine methane hydrate deposits. Further study of this microorganism could possibly help to revolutionize the energy industry. The draft genome of Methanoculleus sp. MH98A is presented.

Breakdown of under-water gas hydrate deposits

At the floor and upper sediment layers of the submarine volcano Hakon Mosby gas (methane) hydrate deposits have been discovered. Measurements of the water-dissolved methane concentration at different depths near the volcano have shown that its level is higher than the average level in the Ocean. In order to understand the effects observed in concentration distribution, a model is proposed for describing the propagation of methane formed as a result of decomposition of depth and floor hydrate over the hydrosphere. Within the framework of physical modeling the kinetics of methane dissolution and its distribution in the hydrosphere are traced numerically with account for transfer anisotropy in different directions and undercurrents. The dissolution rate and the mass transfer directions are determined and the concentration fields in the floor layers are calculated. It is shown that methane is rapidly dispersed in the hydrosphere and, thus, significant volumes with high methane concentration cannot be formed. Practical recommendations are given for hunting hydrate deposits in water areas. The model proposed is applicable to describing the propagation of any admixture in different anisotropic media.

Taliks in relict submarine permafrost and methane hydrate deposits: Pathways for gas escape under present and future conditions

AbstractWe investigate the response of relict Arctic submarine permafrost and gas hydrate deposits to warming and make predictions of methane gas flux to the water column using a 2‐D multiphase fluid flow model. Exposure of the Arctic shelf during the last glacial cycle formed a thick layer of permafrost, protecting hydrate deposits below. However, talik formation below paleo‐river channels creates permeable pathways for gas migration from depth. An estimate of the maximum gas flux at the present time for conditions at the East Siberian Arctic Seas is 0.2047 kg yr−1 m−2, which produces a methane concentration of 142 nM in the overlying water column, consistent with several field observations. For conditions at the North American Beaufort Sea, the maximum gas flux at the present time is 0.1885 kg yr−1 m−2, which produces a methane concentration of 78 nM in the overlying water column. Shallow sediments are charged with residual methane gas after venting events. Sustained submergence into the future should increase gas venting rate roughly exponentially as sediments continue to warm. Studying permafrost‐associated gas hydrate reservoirs will allow us to better understand the Arctic's contribution to the global methane budget and global warming.

Simulation of Subsea Gas Hydrate Exploitation

The recovery of methane from gas hydrate layers that have been detected in several subsea sediments and permafrost regions around the world is a promising perspective to overcome future shortages in natural gas supply. Being aware that conventional natural gas resources are limited, research is going on to develop required and sustainable technologies for the production of natural gas from such new sources since the early 1990s.In recent years, intensive research has focused on the capture and storage of CO2 from combustion processes to reduce climate impact. While different man-made or natural reservoirs like deep aquifers, exhausted oil and gas deposits or other geological formations are considered to store gaseous or liquid CO2, the storage of CO2 as hydrate in former methane hydrate deposits is another promising alternative. Due to beneficial stability conditions, methane recovery may be well combined with CO2 storage in the form of hydrates. Regarding technological implementation many problems have to be overcome.Within the scope of the German research project »SUGAR« different technological approaches for the optimized exploitation of gas hydrate deposits are evaluated and compared by means of dynamic system simulations and analysis. Detailed mathematical models for the most relevant chemical and physical processes are developed. The basic mechanisms of gas hydrate formation/ dissociation and heat and mass transport in porous media are considered and implemented into simulation programs.Simulations based on geological field data have been carried out. The effects occurring during gas production and CO2 storage within a hydrate deposit are identified and described for various scenarios. The behavior of relevant process parameters such as pressure, temperature and phase saturations is discussed for different strategies.Simulation results for different scenarios, e. g. simple depressurization and CO2 injection, reveal that both the production of natural gas and the CO2 storage are possible with acceptable rates. In case of simultaneous CO2 injection an increased production rate can be expected. The results strongly depend on deposit conditions, especially multiphase flow rates are dominated by permeability terms – in other words the reservoir permeability controls production and injection rates. Furthermore, the heat transport within heterogeneous layered deposits leads to enhanced hydrate decomposition and thus higher production rates.

Pore-water chemistry of sediment cores off Mahanadi Basin, Bay of Bengal: Possible link to deep seated methane hydrate deposit

Organoclastic degradation and anaerobic oxidation of methane (AOM) are known to influence sulfate, methane and bicarbonate concentrations of sediment pore-waters. In this work we have analyzed the pore-fluid composition of three piston cores (36–39 m long) collected from water depths ranging from 1429 to 1691 m. We attribute the observed linear sulfate concentration profiles and (ΔCa + ΔAlk)/ΔSO42− ratios dominantly to anaerobic oxidation of methane (AOM) induced sulfate consumption. The gas rich layers just below the base of hydrate stability zone (BGHSZ) is the possible source of the enhanced diffusive flux of biogenic methane (δ13CCH4 ranging from −99.7 to −106.3‰ VPDB). Occurrences of disseminated gas hydrate in the Mahanadi Basin within the hydrate stability zone (210–220 m thick) supports the link between linear sulfate profile, high rate of AOM and hydrate occurrence.

Effect of Domestic Power Plants to the Low Voltage Transformer Areas from the Nonlinear Distortion Point of View

A case study is presented in this work, where a 4 kW photovoltaic domestic power plant, located in Hungary, has been examined. Based on the measured voltage and current signals we examined the frequency domain behaviour of the power plant and the effect to the power quality, mainly the total harmonic distortion, of the low voltage transformer area. Evaluation of the measurement database suggests that this type of synchronous power injection has serious effect on the voltage distortion of the low voltage grid, and this effect highly depends on the ratio of the injected power magnitude related to the nominal power of the inverter part. The available hardware structure enables us not only to eliminate this phenomenon but also to improve the existing quality of the low voltage grid. It needs only a modified control structure that can be implemented in the inverter control device. It causes power loss reduction in the low voltage grid. This reduction helps lowering the power loss in the phase conductor, and more radically in the neutral conductor of the transportation line. In the case of electric power production from fossil sources, it lowers the greenhouse gas emission, to lower the environmental and social- welfare effects of the climate change. 1. Motivation and aim There is an ongoing discussion in the scientific and public society on the exhaustion of fossil fuel reserves and on how the climate change is affecting our planet. Current opinion says that the primarily emitted greenhouse gases (GHG) from natural and artificial sources are responsible for the effects of climate change. These effect the solar radiation, and thus act as key factors affecting the global weather. Nobody denies the constrained nature of fossil energy resources, but they use optimistic estimations of the quantity and availability of these energy resources, that may become available with the help of new scientific and technological developments. There is a huge literature about the prediction of fossil energy resources, and their effect on the CO2 concentration in the atmosphere, and as a result on further climate change impacts over the next 200 y period of time. The effects of fossil fuels depletion (Chiari and Zecca , 2011), economic growth (Nel and Cooper, 2009), the wind and solar energy production (Leggett and Ball, 2012) and the exploitation of methane hydrate deposits (Glasby, 2003) on global warming is predicted. The predicted CO2 concentration increases significantly, that result in rising average temperature values. The situation is getting worse by the global economic growth, and the extended use of newly available fossil energy sources (primarily deepwater oil and gas deposits) (Nel and Cooper, 2009). Without intervention, significant anthropogenic impacts should be expected in the 21st century. We must influence this process by all means in order to avoid the above described anthropogenic effects. We need to reduce the emissions by reducing the use of resources, which produce greenhouse gases. Surveys suggest that the economic sectors responsible for CO2 emission in the European Union are as follows (in descending order): energy and heat production (32 %), road transport (22 %), household

Around the World

In science news around the world this week, a Japanese group extracted natural gas from a deep-sea methane hydrate deposit; a White House commission says that the U.S. government could consider testing the anthrax vaccine in children; a man died after being infected with rabies through a kidney transplant; and more.

Underwater Gas Tornado and its Possible Occurrence in Nature

An underwater gas tornado is a hydrodynamic phenomenon inverse to the well-known sucking whirlpool. Because it occurs only under special conditions, it has not received sufficient attention for possible applications and has not been studied theo- retically. After demonstrating a simple experiment, we study the hydrodynamic solutions of the gas flow in the tornado core interacting with the surrounding rotating liquid. The main question we try to answer here is whether this phenomenon can oc- cur under natural conditions. We assume that methane-hydrate deposits in the sea bottom are a mighty underwater gas source. There are a few geological examples of bottom traces that seem quite similar to traces of atmospheric tornadoes. These so- called pockmarks are located over methane-hydrate deposits and are possibly traces of underwater tornados. We suggest an experiment to test this hypothesis.

Simulations of long term methane hydrate dissociation by pressure reduction using an extended RetrasoCodeBright simulator

Methane hydrates in sediments are generally not in thermodynamic equilibrium. This implies that there may be several competing hydrate phase transitions in naturally existing hydrate reservoirs. In addition to dissociation due to instability imposed by changes in temperature and/or pressure which can bring hydrate outside stability also gradients in chemical potential caused by concentration gradients may lead to dissociation or formation of hydrate. Mineral surfaces bring additional thermodynamic phases of impact for hydrate phase transitions. The limited numbers of reservoir simulators which have incorporated hydrate are normally simplified by considering only pressure and temperature as criteria for hydrate stability region. In cases where kinetic description is used it is normally based on oversimplified models, typically models derived from experiments in pressure, temperature volume controlled laboratory cells. In the case of hydrate production through pressure reduction heat transport might dominate the kinetics and simplified heat transport kinetic models are frequently in use for this purpose. In lack of reliable data from full scale hydrate production the reservoir simulators are the only tools which can be used to evaluate efficiency of different production scenarios. Several research groups have been recently working on this subject. The approaches for inclusion of hydrates as a phase, and corresponding production simulation results from different simulators vary. In this work we have applied a fundamentally different approach, in which we have reworked a reactive transport reservoir simulator, namely RetrasoCodeBright into a hydrate simulator. This has been accomplished by adding hydrates as pseudo-mineral components. This opens up for non equilibrium thermodynamic description since kinetic models for different competing hydrate phase transitions can be included through their respective kinetic models. The main theoretical tool for generating these kinetic models has been phase field theory simulations, with thermodynamic properties derived from molecular modeling. The detailed results from these types of simulations provide information on the relative impact of mass transport, heat transport and thermodynamics of the phase transition, which enable qualified simplifications for implementation into RetrasoCodeBright. Details of the simulator, and numerical algorithms, are discussed in detail and some relevant examples are shown. In particular we applied the reservoir simulator to data from Mount Elbert methane hydrate deposits from North Slope, Alaska. Pressure reduction is used as gas production method.

Tracing the Paleo sulfate‐methane transition zones and H2S seepage events in marine sediments: An application of C‐S‐Mo systematics

Microbially mediated anaebic oxidation of methane (AOM) coupled with sulfate consumption within the sulfate methane transition zone (SMTZ) in marine sediments is a widely recorded biogeochemical reaction and has profound influence on the atmospheric CH4budget, marine carbon cycle and composition of sediment pore fluids. Recognizing the paleo‐SMTZs in the marine sediments/rock records can throw light on the variation of paleo‐methane fluxes and occurrences of cold seep (H2S + CH4) events through geologic time. Here, we present results from carbonate carbon, pyrite sulfur and molybdenum analyses for two sediment cores overlying the methane hydrate deposits in the Bay of Bengal. The results show intimate association of isotopically depleted carbonate carbon and enriched pyrite sulfur, constraining the paleo SMTZ within the sediment column. In addition, anomalous enrichments of Mo concentrations indicate hydrogen sulfide seepage events. Here, we propose a geochemical tool using C‐S‐Mo sytematics to decipher the paleo‐SMTZs in marine sediments and rocks.

Assessment of the response of subaqueous methane hydrate deposits to possible climate change in the twenty-first century

Large reserves of methane are trapped in oceanic hydrate deposits. Increase in temperature of the oceans makes a contribution to the dissociation of oceanic hydrate accumulations and release of poten� tially large amounts of methane into the atmosphere. Such releases into the atmosphere lead to an increase in the greenhouse effect and, accordingly, serious climate changes and to accelerate the gas hydrate dissociation. In this work we evaluated the sta� bility of the existing reserves of subaqueous (underwa� ter) gas hydrates and possible methane release with the dissociation of methane hydrates in the twentyfirst century (1). Methane hydrates are compounds in which meth� ane molecules are in cells formed by water molecules. They are widely distributed in the permafrost zones and the oceanic bottom sediments along the continental slopes, where they are stable at the current PTvalues. Methane hydrates are a potentially large source of energy in comparison with other known sources of hydrocarbons. The total carbon in hydrates is esti� mated as 10 4 Gton C (2), which is a significant amount in comparison with the carbon content (3.8 ×10 4 Gton C) dissolved in the oceanic water and in soil and plants (2 × 10 3 Gton C) and the atmosphere (7.3 ×10 2 Gton C) (3). The total fossil fuel reserves, including coal, are about 5 ×10 3 Gton C (4); i.e., they are consistent with gas hydrate reserves. Methane is the third (after water vapor and carbon dioxide) greenhouse gas, which has a significant effect on the radiation balance of Earth's climate system and can be released into the atmosphere as a result of min� ing and use of hydrates as an energy source. Sudden releases of methane into the atmosphere may occur due to massive underwater shifts of the Earth's crust and an increase in temperature in the oceanic bottom sediments. According to the model estimates, in case of an increase in the oceanic water temperature by a few degrees, methane hydrate reserves should be much smaller (5). Releases of methane in decomposition of methane hydrates could have been a cause of abrupt climate change in the past (6, 7). The Paleocene- Eocene temperature maximum is a wellknown exam� ple of a period of abrupt climate change that was likely associated with massive release of methane from hydrates 55 Ma ago. In some areas (including the Car� ibbean Sea, North Atlantic, the Weddell Sea, and tropical Pacific Ocean), a shift of 2.5- δ 13 С in biogenic carbonate and organic matter was noted. This may be associated with the release of 1500-2000 Gton of methane over several thousand years (6). Such a large release of methane could influence cli�

Diffusive counter dispersion of mass in bubbly media

We consider a liquid bearing gas bubbles in a porous medium. When gas bubbles are immovably trapped in a porous matrix by surface-tension forces, the dominant mechanism of transfer of gas mass becomes the diffusion of gas molecules through the liquid. Essentially, the gas solution is in local thermodynamic equilibrium with vapor phase all over the system, i.e., the solute concentration equals the solubility. When temperature and/or pressure gradients are applied, diffusion fluxes appear and these fluxes are faithfully determined by the temperature and pressure fields, not by the local solute concentration, which is enslaved by the former. We derive the equations governing such systems, accounting for thermodiffusion and gravitational segregation effects, which are shown not to be neglected for geological systems-marine sediments, terrestrial aquifers, etc. The results are applied for the treatment of non-high-pressure systems and real geological systems bearing methane or carbon dioxide, where we find a potential possibility of the formation of gaseous horizons deep below a porous medium surface. The reported effects are of particular importance for natural methane hydrate deposits and the problem of burial of industrial production of carbon dioxide in deep aquifers.

Energy Return on Energy Invested (EROI) for the Electrical Heating of Methane Hydrate Reservoirs

We model the low frequency electrical heating of submarine methane hydrate deposits located at depths between 1000 and 1500 m, and determine the energy return on energy invested (EROI) for this process. By means of the enthalpy method, we calculate the time-dependent heating of these deposits under applied electrical power supplied to a cylindrical heater located at the center of the reservoir and at variable depths. The conversion of the produced water to steam is avoided by limiting the heater temperature. We calculate the volume of methane hydrate that will melt and the energy equivalent of the gas thus generated. The partial energy efficiency of this heating process is obtained as the ratio of the gas equivalent energy to the applied electrical energy. We obtain EROI values in the range of 4 to 5, depending on the location of the heater. If the methane gas is used to generate the electrical energy required in the heating (in processes with a 33% efficiency), the effective EROI of the process falls in the range of 4/3 to 5/3.

Natural Gas Production from Methane Hydrate Deposits Using Clathrate Sequestration: State-of-the-Art Review and New Technical Approaches

This paper focuses on reviewing the currently available solutions for natural gas production from methane hydrate deposits using CO2 sequestration. Methane hydrates are ice-like materials, which form at low temperature and high pressure and are located in permafrost areas and oceanic environments. They represent a huge hydrocarbon resource, which could supply the entire world for centuries. Fossil-fuel-based energy is still a major source of carbon dioxide emissions which contribute greatly to the issue of global warming and climate change. Geological sequestration of carbon dioxide appears as the safest and most stable way to reduce such emissions for it involves the trapping of CO2 into hydrocarbon reservoirs and aquifers. Indeed, CO2 can also be sequestered as hydrates while helping dissociate the in situ methane hydrates. The studies presented here investigate the molecular exchange between CO2 and CH4 that occurs when methane hydrates are exposed to CO2, thus generating the release of natural gas and the trapping of carbon dioxide as gas clathrate. These projects include laboratory studies on the synthesis, thermodynamics, phase equilibrium, kinetics, cage occupancy, and the methane recovery potential of the mixed CO2–CH4 hydrate. An experimental and numerical evaluation of the effect of porous media on the gas exchange is described. Finally, a few field studies on the potential of this new gas hydrate recovery technique are presented.

メタンハイドレート開発と環境影響

Methane hydrate (MH) is expected as a new energy resource. Meanwhile it is sometimes said in general public that the MH development has an adverse affect on global environment. However, at present, it is speculated that the environmental risks of the MH development are not thought to be any more serious than those posed by the conventional oil and gas production if the MH production will be conducted by the use of the depressurization method for the pore-filling MH deposits in sandy sediments.Phase 2 of the Japan's Methane Hydrate R&D Program began in JFY2009. From this phase, the Research Consortium for Methane Hydrate Resources in Japan (MH21) began to conduct a comprehensive environmental study along the rules of the environmental risk assessment to establish an environmental management plan for future MH development. It was identified that the environmental risk factors on the MH development are almost same as the ones on the conventional oil and gas production. Meanwhile the MH21 recognized that the following events might be the environmental risk factors for future MH development ; (1) methane leakages from the seafloor, (2) discharge of production water into the ocean, (3) seafloor subsidence, and (4) submarine landslides. To predict the severities of these events, the MH21 is currently implementing to conduct the baseline surveys, develop the numerical models, conduct the ecotoxicity tests on marine organisms and construct the environmental monitoring systems for offshore MH production test planned in JFY2012.

Origin and evolution of fracture‐hosted methane hydrate deposits

Fracture‐hosted methane hydrate deposits exist at many sites worldwide. These sites often have hydrate present as vein and fracture fill, as well as disseminated through the pore space. We estimate that thousands to millions of years are required to form fracture systems by hydraulic fracturing driven by occlusion of the pore system by hydrate. This time scale is a function of rates of fluid flow and permeability loss. Low‐permeability layers in a sedimentary column can reduce this time if the permeability contrast with respect to the surrounding sediments is of order 10 or greater. Additionally, we find that tensile fracturing produced by hydrate heave around hydrate lenses is a viable fracture mechanism over all but the lowermost part of the hydrate stability zone. With our coupled fluid flow‐hydrate formation model we assess fracture formation at four well‐studied hydrate provinces: Blake Ridge offshore South Carolina, Hydrate Ridge offshore Oregon, Keathley Canyon Block 151 offshore Louisiana, and the Krishna‐Godavari Basin offshore India. We conclude that hydraulic fracturing due to pore pressure buildup is reasonable only at Hydrate Ridge and the Krishna‐Godavari Basin owing to sediment age constraints, and that hydrate‐filled fractures observed at Blake Ridge and Keathley Canyon Block 151 are formed either by hydrate heave or in preexisting fractures. Our findings offer new insight into the processes and time scales associated with fracture‐hosted hydrate deposits, which help further our understanding of hydrate systems.

Key Factors for Depressurization-Induced Gas Production from Oceanic Methane Hydrates

Oceanic methane hydrate (MH) deposits have been found at high saturations within reservoir-quality sands in the Eastern Nankai Trough and the Gulf of Mexico. This study investigates the key factors for the success of depressurization-induced gas production from such oceanic MH deposits. A numerical simulator (MH21-HYDRES: MH21 Hydrate Reservoir Simulator) was used to study the performance of gas production from MH deposits. We calculated the hydrate dissociation behavior and gas/water production performance during depressurization for a hypothetical MH well. Simulation runs were conducted under various initial reservoir conditions of MH saturation, temperature, and absolute permeability. A productivity function (PF) was introduced as an indicator of gas productivity, which is a function of gas production rate, water production rate, and discount rate. The simulations showed that recovery factors over 36% and maximum gas production rates over 450 000 Sm3/d were expected for the most suitable conditions of a class 3 deposit (i.e., an isolated MH deposit that is not in contact with any hydrate-free zone of mobile fluids). However, gas productivity was affected by formation temperature and initial effective permeability. The values of PF increased with increasing formation temperature when the initial permeability of the deposit was higher than a threshold value (the threshold permeability); however, it decreased for the deposit below the threshold permeability. The threshold permeability was estimated to be between 1 and 10 mD in the class 3 deposit. These results suggest that key factors for the success of depressurization-induced gas production from oceanic MH are as follows: (1) The initial effective permeability of the MH deposit is higher than the threshold value, and (2) the temperature of the MH deposit is as high as possible.

Gas Production System From Methane Hydrate Layers by Hot Water Injection Using Dual Horizontal Wells

Abstract In this study, we investigate a system of gas production from methane hydrate layers involving hot water injection using dual horizontal wells. Physical and numerical models simulating the gas production process from methane hydrate layers within a hot water chamber are proposed. Experiments with scaled two-dimensional physical models using an imitated hydrate layer (NaHCO3 ice formation) were performed to investigate fluid flow characteristics and production performance. The thermal simulator was used to simulate experimental chamber growth and field production. Numerical simulations for the processes were successfully performed with a two-component (water and oil or methane hydrates), three-phase (water, methane hydrates and methane gas) and three-dimensional model, matching the physical model. Results of the history-matched numerical simulations were in good agreement with data on production and chamber shapes obtained using the Intermediate3-Stone1 wettability model. Simulations of field production using dual horizontal wells 500 m in length were performed to evaluate cumulative gas production over 3 years of injection with 500 × 103 kg/day of hot water, which varied from 5 × 106 to 9 × 106 std m3. The production process appears economical, in view of the expected convective heat transfer from the chamber boundary and buoyancy force on dissociated methane gas. Introduction Recent studies confirm that conserved methane hydrate deposits in sedimentation layers at a depth of more than 200 to 300 m from the bottom of the sea floor can be utilized as novel natural gas resources. In-situ hydrate decomposition into water and gas is required to produce methane gas economically from these layers, since methane hydrate is a type of non-mobile solid energy resource. To trigger methane hydrate decomposition, decompression or temperature increase out of the equilibrium zone is necessary, while heat should be supplied for continuous dissociation. Accordingly, new gas production systems that supply heat continuously into the methane hydrate layers have been reported. The conventional methods of gas production to date include depressurization, inhibitor injection and thermal recovery.

Dominance of putative marine benthic Archaea in Qinghai Lake, north‐western China

Recent studies have revealed important and versatile roles that Archaea play in a wide variety of environmental processes on Earth. In this study, we investigated the abundance and diversity of archaeal communities in lake water and a 5 m sediment core collected from Qinghai Lake on the Tibetan Plateau, north-western China. An integrated approach was employed including geochemistry, quantitative polymerase chain reaction (Q-PCR) and 16S rRNA gene analysis. Here, we show that Archaea dominated the prokaryotic community in the lake sediments. Members of putative marine benthic groups [Marine Benthic Group (MBG)-B, -C and -D] and Miscellaneous Crenarchaeotic Group (MCG) were dominant, many of which were previously reported to be predominantly present in deep-sea environments. These results demonstrate that these groups are not limited to marine sediments. Despite their ubiquitous presence in aquatic environments, metabolic functions of these important groups largely remain unknown. Whereas many of these groups (such as MBG-B and -D) have typically been found in methane-hydrate deposits in marine environments, our carbon isotopic and molecular results from Qinghai Lake sediments indicate a lacustrine origin.