Gas Hydrates In Marine Sediments Research Articles

Quantification of mixed hydrate and free gas concentrations by the joint integration of velocity and conductivity information

Quantification of gas hydrates in marine sediments is crucial for understanding gas hydrate systems. By empirical relationships or effective medium modelling, gas hydrate concentrations can be derived from velocity and/or conductivity logs. However, these approaches do not take the co-occurrence of free gas and gas hydrate into account leading to large uncertainties in the calculated free gas and gas hydrate concentrations. To overcome this issue we adopt a joint elastic and electric self-consistent/differential effective medium model as the basis for a new joint inversion scheme that distinguishes between both phases. We apply this scheme to p-wave velocity and electric induction data measured by downhole-logging of boreholes at Formosa Ridge off Taiwan - a known hydrate province with an active gas conduit. Gaussian Mixture Modeling separates the background signal of the host medium from anomalies and allows to determine a background porosity as a probability density function of depth. We use this derived porosity to jointly invert electrical conductivity and velocity data for hydrate and free gas concentrations. At Formosa Ridge, we find two resistive anomalies, one in the shallow and another in the deep part of the borehole. Only the deep anomaly in conductivity coincides with a high-velocity anomaly. This is consistent with ∼30% hydrate with ∼1% free gas concentration. For the shallow anomaly, increased velocities due to hydrate concentrations of ∼15% are compensated by a decrease in velocity due to ∼1% of free gas. The method reconciles the different sensitivities of the two data types and yields hydrate and free gas concentrations that are largely consistent with geochemically derived values.

Molecular dynamics simulation on methane hydrate formation in clay nanopores of edge surfaces

Natural gas hydrates are predominantly buried in clay sediments in natural environments, where are some edge surfaces of clay particles directly contacting the hydrates. However, the exact nature of the interaction between these surfaces and the hydrates, as well as their influence on hydrate formation, remains elusive. Herein, microsecond molecular dynamics simulations have been performed to investigate CH4 hydrates formation in nanopores consisting of clay edge surfaces, to reveal the effects of clay edge surfaces and layer charges. The simulation results show that the clay edge surfaces affect CH4 hydrate formation by changing the distribution of water and CH4 molecules via surface adsorption, mainly ascribed to the different polarities of the groups on the edge surfaces of different clays. The greater the electronegativity of the clay, the stronger the inhibition of CH4 hydrate formation, thus, the electroneutral clays are more beneficial for CH4 hydrate formation than the electronegative clays. Moreover, in the early stage of the simulation, compared with the electronegative clays, the electroneutral clays are more favorable for the diffusion of CH4 molecules from the nanopores into the bulk solution and then promote CH4 hydrate formation. On the other hand, the ions in the solution gradually aggregate together and their distribution becomes denser and more ordered. The edge surfaces of electroneutral clay are more accessible to hydrate solids than electronegative clay. These molecular insights into the formation behavior of CH4 hydrates in clay nanopores consisting of edge surfaces help to understand the formation process of natural gas hydrates in marine sediments.

Effects of marine environments on methane hydrate formation in clay nanopores: A molecular dynamics study

In nature, CH4 hydrates are mainly buried in marine sediments. The complex marine environments on the seafloor continuously affect hydrate formation. Herein, systematic molecular simulations have been performed to investigate CH4 hydrate formation in clay nanopore, mainly affected by several marine environmental factors, including seawater salinity, pressure and temperature. Simulation results show that these factors exert different effects on hydrate formation in the nanopore and the outside bulk solutions by affecting the mass transfer and phase separation inside and outside of the nanopore. Specifically, high salinity hinders the diffusion of CH4 molecules from nanopores into the outside bulk solutions, promoting hydrate formation in nanopore and inhibiting hydrate formation in bulk solution; salinity has a dual effect on hydrate formation in the whole system by changing the local CH4 concentration via the formation of the hydration of salt ions. High pressure favors the diffusion of CH4 molecules from nanopore into outside bulk solutions, promoting hydrate formation in bulk solution and inhibiting hydrate formation in nanopore; high pressure promotes hydrate formation at the nanopore throats by increasing CH4 concentration and reducing ion concentration therein. In contrast, temperature significantly affects hydrate formation in the system by causing phase separation, i.e. high temperature promotes the aggregation of CH4 molecules to form nanobubbles and inhibits hydrate formation. Under high temperature conditions, the nanobubble in the nanopore gradually decomposes, while the nanobubble in the outside bulk solution grows an extra-large cylindrical nanobubble. These molecular insights into the formation behavior of CH4 hydrates in clay nanopores are helpful for understanding the formation process of natural gas hydrates in marine sediments and the development and utilization of CH4 hydrates.

Effects of surface property of mixed clays on methane hydrate formation in nanopores: A molecular dynamics study

HypothesisMixed clays (e.g. montmorillonite, illite and kaolinite) are ubiquitous in hydrate-bearing sediments under seafloor, and their surfaces inevitably affect the formation of natural gas hydrates therein. Nevertheless, the actual effects of clay surfaces on hydrate formation remain elusive. ExperimentsSystematic molecular dynamics simulations have been performed to investigate CH4 hydrate formation in mixed clay nanopores of montmorillonite, illite and kaolinite, to examine the effects of surface property and layer charges of mixed clays. FindingsSimulation results indicate that the surfaces of mixed clays affect CH4 hydrate formation in the nanopores by changing the CH4 concentration (xCH4) and ion concentration (xions) in the middle region of the nanopores via surface adsorption for CH4, H2O and ions. Specifically, the surfaces of montmorillonite and illite, the siloxane and gibbsite surfaces of kaolinite show different affinities for adsorbing CH4, H2O and ions, which can significantly affect the xCH4 and xions in the interfacial and middle regions of the nanopores. Moreover, hydrate growth shows certain surface preference. These molecular insights into the effect of mixed clay surfaces on CH4 hydrate formation can help to understand the formation mechanism of natural gas hydrate in marine sediments.

Primary deposition and early diagenetic effects on the high saturation accumulation of gas hydrate in a silt dominated reservoir in the Gulf of Mexico

On continental margins, high saturation gas hydrate systems (>60% pore volume) are common in canyon and channel environments within the gas hydrate stability zone, where reservoirs are dominated by coarse-grained, high porosity sand deposits. Recent studies, including the results presented here, suggest that rapidly deposited, silt-dominated channel-levee environments can also host high saturation gas hydrate accumulations. Here we present several sedimentological data sets, including sediment composition, biostratigraphic age from calcareous nannofossils, grain size, total organic carbon (TOC), C/N elemental ratio, δ13C-TOC, CaCO3, total sulfur (TS), and δ34S-TS from sediments collected with pressure cores from a gas hydrate rich, turbidite channel-levee system in the Gulf of Mexico during the 2017 UT-GOM2-1 Hydrate Pressure Coring Expedition. Our results indicate the reservoir is composed of three main lithofacies, which have distinct sediment grain size distributions (type A-silty clay to clayey silt, type B-clayey silt, and type C-sandy silt to silty sand) that are characteristic of variable turbidity current energy regimes within a Pleistocene (< 0.91 Ma) channel-levee environment. We document that the TOC in the sediments of the reservoir is terrestrial in origin and contained within the fine fraction of each lithofacies, while the CaCO3 fraction is composed of primarily reworked grains, including Cretaceous calcareous nannofossils, and part of the detrital load. The lack of biogenic grains within the finest grained sediment intervals throughout the reservoir suggests interevent hemipelagic sediments are not preserved, resulting in a reservoir sequence of silt dominated, stacked turbidites. We observe two zones of enhanced TS at the top and bottom of the reservoir that correspond with enriched bulk sediment δ34S, indicating stalled or slowly advancing paleo-sulfate-methane transition zone (SMTZ) positions likely driven by relative decreases in sedimentation rate. Despite these two diagenetic zones, the low abundance of diagenetic precipitates throughout the reservoir allowed the primary porosity to remain largely intact, thus better preserving primary porosity for subsequent pore-filling gas hydrate. In canyon, channel, and levee environments, early diagenesis may be regulated via sedimentation rates, where high rates result in rapid progression through the SMTZ and minimal diagenetic mineralization and low rates result in the stalling of the SMTZ, enhancing diagenetic mineralization. Here, we observed some enhanced pyritization to implicate potential sedimentation rate changes, but not enough to consume primary porosity, resulting in a high saturation gas hydrate reservoir. These results emphasize the important implications of sedimentary processes, sedimentation rates, and early diagenesis on the distribution of gas hydrate in marine sediments along continental margins.

Molecular insights into the effects of surface property and pore size of non-swelling clay on methane hydrate formation

Systematic molecular dynamics simulations have been performed to elucidate the effects of basal surface and pore-size on CH4 hydrate formation from the homogeneous CH4 solution in the slit-nanopores of non-swelling clay, i.e. kaolinite. The results indicate that the basal surfaces of kaolinite clay affect hydrate formation mainly by changing the aqueous CH4 concentration via surface adsorption for water and CH4 molecules. Specifically, the highly hydrophilic gibbsite surface strongly adsorbs water molecules and repels CH4 molecules from the interfacial region to the bulk phase, thus, slightly increasing the aqueous CH4 concentration in the bulk phase and is beneficial for hydrate formation. The hydrophobic siloxane surface adsorbs CH4 molecules from the solution and lowers the aqueous CH4 concentration, and then obviously prolongs the induction time and slows down hydrate growth kinetics. Larger pore size is found to be more favorable for hydrate formation, while the extreme narrow nanopores could inhibit hydrate formation. Direct contact of CH4 hydrate solids with the gibbsite surface is almost separated by the strongly adsorbed interfacial water, which is very difficult to be converted into hydrates. In contrast, CH4 hydrate solids can directly interact with the siloxane surface by forming hydrogen bonds or via semi-cages, as the interfacial water is not so strongly adsorbed and can easily form hydrates. These molecular insights into the formation process of CH4 hydrate in the nanopores of kaolinite clay are helpful to understand the formation of natural gas hydrates in marine sediments.

A new approach for the identification of gas hydrate in marine sediments

It has always been the focus of researchers to accurately identify gas hydrate location. Geophysical prospecting is a widely used method for gas hydrate exploration, which has high credibility, especially seismic exploration technology is most generally used. In our study, we analyze the different physical properties of gas hydrate and other minerals bearing in unconsolidated marine sediments based on the effective medium theory. Then, a new attribute is put forward to discriminate gas hydrate. The logging data in the Dongsha area of the South China Sea and the Hydrate Ridge along the Oregon continental margin are applied to validate this method. Our test results are basically in line with actual situation, which provides a new approach in hydrate identification.

An experimental investigation on methane hydrate morphologies and pore habits in sandy sediment using synchrotron X-ray computed tomography

As pore-scale morphologies and spatial distribution (pore habits) of natural gas hydrates in marine sediments considerably affect their physical/mechanical properties, they have extensively been investigated by X-ray computed tomography (XRCT) and especially synchrotron X-Ray computed tomography (SXRCT). While both image spatial and scan temporal resolutions are being improved over time, it is still challenging to distinguish water from methane hydrate in an image due to their low absorption contrast. In this study, methane hydrate formation and growth in wet sand were observed at submicron/micron scale using SXRCT. Saline water (Potassium iodide - KI) was used in order to improve the image contrast. Evolutions of methane hydrate morphologies and distribution at both the pore and sample scales were observed. Results are discussed based on various mechanisms related to material behaviors and experimental conditions, e.g. suction variation during methane hydrate formation, local temperature gradient in the sample, and particularly the interaction of X-rays with the sample. Methane hydrate formation is interpreted as a dynamic process, favoring the Ostwald ripening. Furthermore, morphologies and pore habits of methane hydrates under excess-gas and excess-water conditions are discussed. Some recommendations are finally given for further studies on methane hydrates-bearing sediments via XRCT or SXRCT.

Geomechanical constitutive modelling of gas hydrate-bearing sediments by a state-dependent multishear bounding surface model

Natural gas hydrate in marine sediments and permafrost areas is considered as an important potential energy source. Since hydrate dissociation will reduce the stability of gas hydrate-bearing sediments (GHBS) and may cause wellbore failures and geological disasters during gas production, it is necessary to reveal the mechanical behavior of GHBS for the safe exploitation of natural gas hydrate. This paper proposes a geomechanical constitutive model of GHBS within the multishear bounding surface framework. Following the slip theory of plasticity, a constitutive formulation is obtained by splitting the macro constitutive response of sediments into a macro volume response and a series of micro shear responses in spatial distributions related to virtual microshear structures. Each microshear structure describes micro shear and dilatancy responses in three orthogonal orientations. A micro stress–strain relationship and a micro stress–dilatancy relationship are established for each orientation of the microshear structure. The model comprehensively describes the consolidation, hardening, softening, dilatation, collapse, and non-coaxial characteristics of gas hydrate-bearing sediments by introducing the multishear concept, state parameter, evolution law of hydrate bonding and debonding, and collapse strain caused by hydrate dissociation. The effectiveness of the model is confirmed by simulating the available published laboratory tests on the samples of synthetic and natural GHBS under different pore pressures, temperatures, initial void ratios, hydrate saturations, and initial effective confining stresses.

Effects of Salt Ions on the Methane Hydrate Formation and Dissociation in the Clay Pore Water and Bulk Water

Gas hydrates in marine sediments are promising energy resources, while an effective recovery of methane from clay pores relies on a comprehensive appreciation of the hydrate evolution inside and outside the pore especially at a saline environment. Molecular dynamics simulations were conducted to investigate the methane hydrate formation and dissociation in the sodium montmorillonite (Na-MMT) interlayer with fresh water and saline water, respectively, by characterizing the distribution and transportation of methane and ions (K+, Na+, and Ca2+), the overall and local four-body structural order parameter, and the radial distribution functions. Results indicated that it was much easier to form methane hydrates in the bulk water than in the pore water, while the hydrates in the pore region were more readily dissociated than in the bulk region. The effects of salt ions on the hydrate formation were opposite in these two regions, which highlighted the role of the salting-out effect and the ion exchange between b...

Hydromechanical Properties of Gas Hydrate‐Bearing Fine Sediments From In Situ Testing

AbstractThe hydromechanical properties of gas hydrate‐bearing sediment are key in assessing offshore geohazards and the resource potential of gas hydrates. For sandy materials, such properties were proved highly dependent on hydrate content (Sh) as well as on their distribution and morphology. Owing to difficulties in testing gas hydrate‐bearing clayey sediments, the impact of hydrates on the behavior of such materials remains poorly understood. Hence, to provide insight into the characterization of clayey sediments containing hydrate, this study relies on a unique database of in situ acoustic, piezocone, and pore pressure dissipation measurements collected in a high gas flux system offshore Nigeria. Compressional wave velocity measurements were used as means of both detecting and quantifying gas hydrate in marine sediments. The analysis of piezocone data in normalized soil classification charts suggested that contrary to hydrate‐bearing sands, the behavior of gas hydrate‐bearing clays tends to be contractive. Correlations of acoustic and geotechnical data have shown that the stiffness and strength tend to increase with increasing Sh. However, several sediment intervals sharing the same Sh have revealed different features of mechanical behavior; suggesting that stiffness and strength of gas hydrate‐bearing clays are influenced by the distribution/morphology of gas hydrate. Pore pressure dissipation data confirmed the contractive behavior of gas hydrate‐bearing clays and showed that at low hydrate content, the hydraulic diffusivity (Ch) decreases when Sh increases. However, for Sh exceeding 20%, it was shown that an increase of Ch with Sh could be linked to the presence of fractures in the hydrate‐sediment system.

Gas hydrate estimates in muddy sediments from the oxygen isotope of water fraction

Gas hydrate in deep-sea sediments is a potential energy resource, and has been the focus of extensive drilling research. However, direct evaluation of the amount of the gas hydrate in marine sediments has been difficult because the gas hydrate in recovered sediment cores is at least partly dissociated due to the drop in pressure and increase in temperature during onboard recovery. In this study, we apply a new method based on oxygen isotopic composition of the H2O fraction of both hydrate and mud sub-samples (δ18OH and δ18OM, respectively) in order to evaluate the volume percentage of gas hydrate in core sections collected from the Japan Sea off Joetsu and Oki, which contain different fabrics of the hydrate within hemipelagic mud. We measured isotopic composition of CO2 equilibrated with H2O of the sub-samples of a small size (typically 0.3cm3) carefully separated from the core sediments and sealed in glass vials. The volume percentage of gas hydrate (H in %) was determined using porosity of the mud sub-samples and oxygen isotopic composition of the bulk pore water (δ18OPW) squeezed from a certain length of core sediment (typically 20cm) including dissociated hydrate. 28 out of the 29 examined core sections indicate the relation in the isotopic values of the three components, δ18OH>δ18OPW>δ18OM, as expected from isotopic fractionation that enriches 18O in the hydrate component. Evaluated H-values of the 28 sections ranged from 1.0% to 95.4% and, for most of the section, the H-value was clearly larger than the value estimated by the hydrate distribution on core images. Our new method can, in a simple manner, correct for the underestimation of hydrate amount caused as a result of dissociation during core handling. Our oxygen isotopic data of the hydrate and mud sub-samples fits poorly with the isotopic evolutional curve that assumes Rayleigh fractionation in a closed system. This implies that the pore water isotopic composition may have been homogenized by diffusion and advection of less 18O-depleted pore water from the surrounding sediments. Presence of micro-scale hydrate in the mud matrix was suspected for some sections from the Joetsu site, which present a small difference between δ18OH and δ18OM as well as high CH2/CO2 ratios in headspace gas. We suggest that this method, if carried out with careful and quick onboard sampling, is appropriate for the estimation of gas hydrate as an energy resource based on the amount of hydrate present in marine mud.

Microsecond Molecular Dynamics Simulation of Methane Hydrate Formation in Humic-Acid-Amended Sodium Montmorillonite

Natural gas hydrate in marine sediments is a promising energy resource, while the atomistic level understanding of its formation on the organomineral complex remains limited. Microsecond molecular dynamics simulations were performed to investigate the methane hydrate growth in the sodium montmorillonite interlayer in the presence of natural sediment organic matter [leonardite humic acid (LHA)] at mass concentrations of 2 and 11%. The hydrate growth was characterized by the global and local four-body order parameter, surface distribution function, snapshots of molecular configurations, and face-saturated incomplete cage analysis. It clearly demonstrated the kinetic inhibition effects of LHA on hydrate formation on clay minerals, especially when the self-aggregation of LHA took place at a high concentration. Overall results highlighted the role of methane adsorption on LHA aggregates on the observed inhibition phenomenon, which changed the pathway of gas molecules by complex dynamic processes, such as aggre...

The connection between natural gas hydrate and bottom‐simulating reflectors

AbstractBottom‐simulating reflectors (BSRs) on marine seismic data are commonly used to identify the presence of natural gas hydrate in marine sediments, although the exact relationship between gas hydrate and BSRs is undefined. To clarify this relationship we compile a data set of probable gas hydrate occurrence as appraised from well logs of 788 industry wells in the northern Gulf of Mexico. We combine the well log data set with a data set of BSR distribution in the same area identified from 3‐D seismic data. We find that a BSR increases the chances of finding gas hydrate by 2.6 times as opposed to drilling outside a BSR and that the wells within a BSR also contain thicker and higher resistivity hydrate accumulations. Even so, over half of the wells drilled through BSRs have no detectable gas hydrate accumulations and gas hydrate occurrences and BSRs do not coincide in most cases.

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

We have estimated the seismic attenuation in gas hydrate and free-gas-bearing sediments from high-resolution P-cable 3D seismic data from the Vestnesa Ridge on the Arctic continental margin of Svalbard. P-cable data have a broad bandwidth (20–300 Hz), which is extremely advantageous in estimating seismic attenuation in a medium. The seismic quality factor (Q), the inverse of seismic attenuation, is estimated from the seismic data set using the centroid frequency shift and spectral ratio (SR) methods. The centroid frequency shift method establishes a relationship between the change in the centroid frequency of an amplitude spectrum and the Q value of a medium. The SR method estimates the Q value of a medium by studying the differential decay of different frequencies. The broad bandwidth and short offset characteristics of the P-cable data set are useful to continuously map the Q for different layers throughout the 3D seismic volume. The centroid frequency shift method is found to be relatively more stable than the SR method. Q values estimated using these two methods are in concordance with each other. The Q data document attenuation anomalies in the layers in the gas hydrate stability zone above the bottom-simulating reflection (BSR) and in the free gas zone below. Changes in the attenuation anomalies correlate with small-scale fault systems in the Vestnesa Ridge suggesting a strong structural control on the distribution of free gas and gas hydrates in the region. We argued that high and spatially limited Q anomalies in the layer above the BSR indicate the presence of gas hydrates in marine sediments in this setting. Hence, our workflow to analyze Q using high-resolution P-cable 3D seismic data with a large bandwidth could be a potential technique to detect and directly map the distribution of gas hydrates in marine sediments.

Testing short-range migration of microbial methane as a hydrate formation mechanism: Results from Andaman Sea and Kumano Basin drill sites and global implications

Methane gas hydrates in marine sediments often concentrate in coarse-grained layers surrounded by fine-grained marine muds that are hydrate-free. Methane in these hydrate deposits is typically microbial, and must have migrated from its source as the coarse-grained sediments contain little or no organic matter. In “long-range” migration, fluid flow through permeable layers transports methane from deeper sources into the gas hydrate stability zone (GHSZ). In “short-range” migration, microbial methane is generated within the GHSZ in fine-grained sediments, where small pore sizes inhibit hydrate formation. Dissolved methane can then diffuse into adjacent sand layers, where pore size does not restrict hydrate formation and hydrates can accumulate. Short-range migration has been used to explain hydrate accumulations in sand layers observed in drill sites on the northern Cascadia margin and in the Gulf of Mexico. Here we test the feasibility of short-range migration in two additional locations, where gas hydrates have been found in coarse-grained volcanic ash layers (Site NGHP-01-17, Andaman Sea, Indian Ocean) and turbidite sand beds (Site IODP-C0002, Kumano forearc basin, Nankai Trough, western Pacific). We apply reaction–transport modeling to calculate dissolved methane concentration and gas hydrate amounts resulting from microbial methane generated within the GHSZ. Model results show that short-range migration of microbial methane can explain the overall amounts of methane hydrate observed at the two sites. Short-range migration has been shown to be feasible in diverse margin environments and is likely to be a widespread methane transport mechanism in gas hydrate systems. It only requires a small amount of organic carbon and sediment sequences consisting of thin coarse-grained layers that can concentrate microbial methane generated within thick fine-grained sediment beds; these conditions are common along continental margins around the globe.

Estimation of the global inventory of methane hydrates in marine sediments using transfer functions

Abstract. The accumulation of gas hydrates in marine sediments is essentially controlled by the accumulation of particulate organic carbon (POC) which is microbially converted into methane, the thickness of the gas hydrate stability zone (GHSZ) where methane can be trapped, the sedimentation rate (SR) that controls the time that POC and the generated methane stays within the GHSZ, and the delivery of methane from deep-seated sediments by ascending pore fluids and gas into the GHSZ. Recently, Wallmann et al. (2012) presented transfer functions to predict the gas hydrate inventory in diffusion-controlled geological systems based on SR, POC and GHSZ thickness for two different scenarios: normal and full compacting sediments. We apply these functions to global data sets of bathymetry, heat flow, seafloor temperature, POC input and SR, estimating a global mass of carbon stored in marine methane hydrates from 3 to 455 Gt of carbon (GtC) depending on the sedimentation and compaction conditions. The global sediment volume of the GHSZ in continental margins is estimated to be 60–67 × 1015 m3, with a total of 7 × 1015 m3 of pore volume (available for GH accumulation). However, seepage of methane-rich fluids is known to have a pronounced effect on gas hydrate accumulation. Therefore, we carried out a set of systematic model runs with the transport-reaction code in order to derive an extended transfer function explicitly considering upward fluid advection. Using averaged fluid velocities for active margins, which were derived from mass balance considerations, this extended transfer function predicts the enhanced gas hydrate accumulation along the continental margins worldwide. Different scenarios were investigated resulting in a global mass of sub-seafloor gas hydrates of ~ 550 GtC. Overall, our systematic approach allows to clearly and quantitatively distinguish between the effect of biogenic methane generation from POC and fluid advection on the accumulation of gas hydrate, and hence, provides a simple prognostic tool for the estimation of large-scale and global gas hydrate inventories in marine sediments.

2-D velocity structure in Kerala-Konkan basin using traveltime inversion of seismic data

The existence of gas-hydrates in marine sediments increases the seismic velocity, whereas even a small amount of underlying free-gas reduces the velocity considerably. The change in velocities against the background (without gas-hydrates and free-gas) velocity can be used for identification and assessment of gas-hydrates. Traveltime inversion of identifiable reflections from large offset multi channel seismic (MCS) experiment is an effective method to derive the 2-D velocity structure in an area. We apply this method along a seismic line in the Kerala-Konkan (KK) offshore basin for delineating the gas-hydrates and free-gas bearing sediments across a bottom simulating reflector (BSR). The result reveals a four layer 2-D shallow velocity model with the topmost sedimentary layer having velocity of 1,680–1,740 m/s and thickness of 140–190 m. The velocity of the second layer of uniform thickness (110 m) varies from 1,890 to 1,950 m/s. The third layer, exhibiting higher velocity of 2,100–2,180 m/s, is interpreted as the gas-hydrates bearing sediment, the thickness of which is estimated as 100 to 150 m. The underlying sedimentary layer shows a reduction in seismic velocity between 1,620 to 1,720 m/s. This low-velocity layer with 160–200 m thickness may be due to the presence of free-gas below the gas-hydrates layer.

Microbial community structure in methane hydrate-bearing sediments of freshwater Lake Baikal

Gas hydrates in marine sediments have been known for many years but recently hydrates were found in the sediments of Lake Baikal, the largest freshwater basin in the world. Marine gas hydrates are associated with complex microbial communities involved in methanogenesis, methane oxidation, sulfate reduction and other biotransformations. However, the contribution of microorganisms to the formation of gas hydrates remains poorly understood. We examined the microbial communities in the hydrate-bearing sediments and water column of Lake Baikal using pyrosequencing of 16S rRNA genes. Aerobic methanotrophic bacteria dominated the water sample collected at the lake floor in the hydrate-bearing site. The shallow sediments were dominated by Archaea. Methanogens of the orders Methanomicrobiales and Methanosarcinales were abundant, whereas representatives of archaeal lineages known to perform anaerobic oxidation of methane, as well as sulfate-reducing bacteria, were not found. Affiliation of archaea to methanogenic rather than methane-oxidizing lineages was supported by analysis of the sequences of the methyl coenzyme M reductase gene. The deeper sediments located at 85-90 cm depth close to the hydrate were dominated by Bacteria, mostly assigned to Chloroflexi, candidate division JS1 and Caldiserica. Overall, our results are consistent with the biological origin of methane hydrates in Lake Baikal.

Phase Equilibria and Thermodynamic Modeling of Ethane and Propane Hydrates in Porous Silica Gels

In the present study, we examined the active role of porous silica gels when used as natural gas storage and transportation media. We adopted the dispersed water in silica gel pores to substantially enhance active surface for contacting and encaging gas molecules. We measured the three-phase hydrate (H)-water-rich liquid (L(W))-vapor (V) equilibria of C(2)H(6) and C(3)H(8) hydrates in 6.0, 15.0, 30.0, and 100.0 nm silica gel pores to investigate the effect of geometrical constraints on gas hydrate phase equilibria. At specified temperatures, the hydrate stability region is shifted to a higher pressure region depending on pore size when compared with those of bulk hydrates. Through application of the Gibbs-Thomson relationship to the experimental data, we determined the values for the C(2)H(6) hydrate-water and C(3)H(8) hydrate-water interfacial tensions to be 39 +/- 2 and 45 +/- 1 mJ/m(2), respectively. By using these values, the calculation values were in good agreement with the experimental ones. The overall results given in this study could also be quite useful in various fields, such as exploitation of natural gas hydrate in marine sediments and sequestration of carbon dioxide into the deep ocean.