OS23A-1030
The simulation of Methane hydrate crystal growth by Monte-Carlo Method
Recently, methane hydrate has been expected as a new and promising energy resource, and it abundantly occurs under the deep-sea beds. However, there is still a considerable technical and economical problem for a practical exploration of methane hydrate as an energy resource. For example, it is considerable difficult to control the stability of the methane hydrate reservoirs. In this way, it is fundamental to know and predict the process that controls the methane hydrate formation in order to produce it safely and efficiently. In this study, we simulate the process of methane hydrate crystal growth with CO2 substitution. We have adopted the combination of cellular automata with Monte Carlo method in a similar way as suggested by Buenas, Kvamme and Svandal (J. of Crystal Growth 2006). The effects of CO2 substitution on methane hydrate growth will be investigated by considering diffusion of both species (CO2 and methane) and using the free energy of the systems that has been obtained by molecular dynamics calculations and thermodynamic data. The growth process is investigated under the relevant thermodynamic conditions (temperature and pressure) of methane hydrate reservoirs and takes in account three main physical effects: solidification of the liquid hydrate, the diffusion of the molecular species (CO2 and CH4) and the heat transport.
OS23A-1031
The particle size effect on Gas Hydrate Formation in powdered silica particles
Based on the investigations in the past years, it has been recognized that methane hydrates in Nankai Trough primarily occur in turbidite sediments (Fujii et al. 2005; Uchida et al., 2005). Turbidite is composed of a set of sediments, generally becoming finer upward in particle size, from coarse sand to clay (Bouma, 1962). In natural environment the formation of methane hydrate will be inevitably subject to the influence of sediments, so the modes of gas hydrate formation and occurrence might be different in the sediments with various particle sizes and mineral compositions. The elucidation of this issue, how sediments affect methane hydrate formation and occurrence will help in efficient hydrate exploration, accurate estimation of hydrate reserve, and the design of hydrate production method. In this research, we especially studied the particle size effect on the water conversion degree to hydrate using a set of powdered silica particles with the size from medium silt (<20 μm) to medium sand (250 ~ 500 μm). The test specimens were saturated with 3.5% NaCl solution, simulating the interstitial water of marine sediments, and reacted with methane gas at the pressure of ~ 10 MPa and temperature of 3° C. The water conversion degree to hydrate in a test specimen was estimated with the amount of gas that was clathrated in hydrate. The obtained results indicate a clear relationship between water conversion degree to hydrate and particle size: only 3.2 % when particle size is <20 μm, increasing dramatically from 5.7% to 82.8 % when particle size changes from ~30 μm (coarse silt) to ~200 μm (fine sand), and almost stable at ~ 80% when particle size is > 250 μm (medium sand). Because the test materials are all silica, the difference in water conversion degree to hydrate should be resulted from physical properties of silica particle, specific surface area, and/or the property confined by silica particle, pore size. This study was carried out as a part of Research Consortium for Methane Hydrate Resources in Japan (MH21).
OS23A-1032
Guest Chemical Composition as a Significant Parameter for Hydrate Interpretation
Each guest in compound hydrate has both independent and co-dependent links to formation and dissociation processes. At the simplest level, compound hydrate can be modeled as a single entity that grows at a fixed composition, for example, the case where an infinite reservoir of gas is exposed to a limited quantity of water. In real systems, the rate at which reactant guests are brought to the formation region will often be the limiting process and an infinite gas supply cannot be assumed. Under these conditions compositional changes that occur during formation lead to multiple hydrate formation processes, some of which are concurrent but others which are sequential. Longer time scales (but relatively short on the geologic time scale) can lead to redistribution of materials to their lowest energy states: it looks like an infinite supply of gas caused the hydrate to form, but one cannot accurately back predict the original gas phase based only on the hydrate composition due to selective uptake processes. One must understand the formation/composition history to understand the current state. Dissociation processes can also be complex. These processes include selective release of guests, and restabilization due to changes in the concentration of supporting solutions. For example, bleaching of select formers occurs when hydrate is exposed to an under saturated fluid stream. In some cases these composition changes can lead to azeotrope-like behavior, while in others complete loss of certain guests can occur.
OS23A-1033
Experimental Study in Laboratory on consolidation and gas production behavior during Dissociation of Methane Hydrate by Depressurization
Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because large amount of MH exists in marine sediments or in permafrost regions worldwide. Depressurization process is regarded as the most effective process for gas recovery from the viewpoint of gas productivity and economical efficiency, compared with the other in-situ dissociation processes of MH. However, increase of effective stress during depressurization causes consolidation of MH sediments and permeability reduction. As a result, decrease of gas productivity is also supposed. Therefore, it is very important to understand the behavior in MH reservoir, especially in developing the extraction system for MH, and when considering the environmental impacts due to the development. We conducted an experimental study on consolidation and gas production behavior during MH Dissociation by Depressurization. In a real MH field, it is supposed that vertical consolidation of sediments occurs whereas gas and water flow in horizontal direction. For the purpose of considering porosity change due to consolidation and MH dissociation, the special type of experimental apparatus was designed. To reproduce the real flow condition of gas and water, we used disc shape samples as simulated MH sediment. Horizontal radial flow in porous media during MH dissociation was constructed whereas vertical load system was used to simulate rock pressure conditions in real MH sediment. We changed initial temperature, dissociation pressure, MH saturation, and sand grain size as experimental parameter to clarify the effect of these parameters on MH dissociation, consolidation, and dissociated gas production.
OS23A-1034
Formation of a Structure II Hydrate by 1,4-Thioxane in Sea Water
We have show that a sII clathrate hydrate containing 1,4-thioxane (TO) will form under the appropriate pressure and temperature conditions. The molecular size of TO poised it at the boundary between sII hydrate formation either as the sole cage occupant, or where a second help gas molecule is required to stabilize the small hydrate cages. In addition to its size, TO is chemical similar to cyclic ethers, such as tetrahydrofuran, which readily form clathrate hydrates. For the experimental temperatures in this study (all above 273.15 K), a pure TO hydrate was not observed to form. However, binary hydrates of TO and either CH4 and N2 were readily formed under moderate pressure conditions (33-158 bar). Both Raman spectroscopy and visual observations were used to verify that a solid hydrate formed and that TO was trapped in the cages. For TO + CH4, a pressure-temperature phase equilibria diagram was created. This showed that the addition of TO increased hydrate stability versus a pure CH4 system by approximately 10 degC, indicating that should CH4 gas be present a TO hydrate would readily form at shallow depths under typical oceanic temperature regimes. TO is of environmental interest as a breakdown product of mustard gas (1,1'-thiobis[2-chloroethane] as a result of hydrolysis in sea water, and has been shown to occur in seafloor chemical weapon disposal sites. In the years after World War II large quantities of chemical weapons were disposed of in the ocean at sites off the US east and west coasts, off Japan, in the Baltic and Adriatic Seas, and in the Russian Arctic, until the signing of the London Convention in 1972. Mustard gas represents the largest tonnage of weapons materiel and while the general scheme of breakdown by hydrolysis is known there is little information on the actual behavior of these breakdown products in marine sediments. This work illuminates such gaps in knowledge, and formation of a hydrate profoundly alters molecular mobility and diffusion away from a site. Simple calculations show that other help gases, such as H2S, should also form a mixed hydrate with TO with remarkable ease; thus, extending the range of possible hydrate formation with TO in anoxic marine sediments to shallow depths, if a second hydrate former is present.
OS23A-1035
Constitutive Equation of Variable Compliance Type for Artificial Sediment Containing Methane Hydrate
Methane hydrate (MH) is anticipated to be a promising energy resource of natural gas. In order to evaluate productivity of methane gas from MH reservoirs, it is necessary to develop a gas production numeric simulator. For precise assessment of long-term productivity, it is important to predict mechanical behaviors of MH reservoirs in consideration of their time-dependency. In this study, loading rate dependency, one of important time-dependent properties, of artificial sediment containing MH was experimentally examined. Results of triaxial compression tests of water-saturated sand (saturated-sand specimen) and sand containing synthetic MH (hydrate-sand specimen), at conditions of confining pressure 9 MPa, pore water pressure 8 MPa and temperature 278 K, were presented. Strain rate was changed alternately between two speeds at a constant strain interval. Experimental results indicate that saturated-sand specimen shows negligible small strain rate dependency. Strain rate dependency of hydrate-sand specimen was considerably stronger than that of saturated-sand specimen. Referring to earlier works, the time-dependency of hydrate-sand specimen seemed to be weaker than that of ice or MH and comparable with that of frozen sand. Based on the experimental results, constitutive equation of variable compliance type, which focuses on time- dependent behaviors, was applied to sand specimen containing MH as the first step towards formulation of mechanical properties of MH reservoirs. As a result, it was found that the previously reported results obtained in constant strain rate tests can be explained by the proposed equation. Moreover, by a numerical analysis using the equation, the prediction of long-term mechanical behavior can be made.
OS23A-1036
Experimental study on steam and inhibitor injection into methane hydrate bearing sediments
Natural gas hydrate that exists in the ocean sediment is thought to constitute a large methane gas reservoir and is expected to be an energy resource in the future. In order to make recovery of natural gas from hydrates commercially viable, hydrates must be dissociated in-situ. Inhibitor injection method is thought to be one of the effective dissociation method as well as depressurization and thermal stimulation. Meanwhile, steam injection method is practically used for oil sand to recover heavy oil and recognized as a means that is commercially successful. In this study, the inhibitor injection method and the steam injection method for methane hydrate bearing sediments have been examined and discussed on an experimental basis. New experimental apparatuses have been designed and constructed. Using these apparatuses, inhibitor and steam were successfully injected into artificial methane hydrate bearing sediments that were simulated in laboratory scale. In the case of inhibitor injection, characteristic temperature drop during dissociation was observed. And decreases of permeability that is caused by the reformation of methane hydrate were prevented effectively. In the case of steam injection, the phase transition from vapor water to liquid water in methane hydrate bearing sediments was observed. It can be concluded that roughly 44 % of total hydrate origin gas was produced after steam injection. From these approaches, the applicability of these methods as enhanced gas recovery methods are discussed.
OS23A-1037
Numerical simulation of hydrate formation morphology in cylindrical experimental sand columns
Using X-ray computed tomography (CT), formation of methane hydrate in columns of moist sand was observed under various conditions of initial water saturation, porosity, pressure, and temperature. Contrary to many studies in porous media that assumed methane hydrate forms uniformly throughout the media, x-ray CT observations of laboratory studies involving hydrate formation showed nonuniform patterns that included circular bands, centered around the core axis, locally concentrations, or uniformly disseminations. Nucleation and accumulation of methane hydrate in porous media can be affected by many factors such as porosity, water saturation, relative permeability, heat conductivity, and inhibiting salts. However, there is no clear explanation on how or why these various patterns of heterogeneous distribution of hydrate formation take place. We conducted numerical simulations using TOUGH+Hydrate to investigate the nonuniform hydrate formation in moist sand, and compared the numerical predictions to the experiment results. The simulations were able to generate the various patterns of hydrate formation and to provide an understanding of the dynamic processes governing the hydrate formation. We conducted sensitivity analyses with respect to the initial water saturation, inhibitor concentration, and porosity, and we evaluated the impact of those parameters on the hydrate formation patterns.
OS23A-1038
A seismic reflectivity study of the methane hydrate layer in the offshore area, southwestern Taiwan
In this study we present the result of an investigation of the regional variation behavior in bottom-simulating reflector (BSR) reflectivity of the gas hydrate-bearing sediments in the offshore, southwestern Taiwan. We focus on the analysis and interpretation of airgun-array signals recorded by ocean bottom seismographs (OBSs) during 2004 and 2006. Eight profiles of seismic reflection/refraction with a total length of about 140 km and recorded by 50 recovered OBSs were acquired on the active and passive margins in offshore southwestern Taiwan. The profiles of the 2004 and 2006 profiles are oriented in E-W and N-S direction and the 2005 survey is trending in NW-SE perpendicular to the continental margin. For all the OBS lines, amplitudes of the direct water arrival, the multiple, and the BSR were picked interactively. A quantitative representation of reflector strength is provided by calculation of reflection coefficients. In general, the seafloor reflection coefficients for the active and passive margins are estimated as 0.1-0.25. Maximum of up to 0.47 were observed over the topographic high in the active margin. Those high amplitudes of seafloor reflection could be associated with widespread carbonate pavement observed with TowCam in the active and passive margins, southwest Taiwan. For the BSR the largest reflection coefficients were observed in the passive margin. This area has average values of about 0.12 with localized highs between 0.15 and 0.18, while the active margin area exhibited relatively lower values of about 0.015-0.05. If we assumed that the velocity below the BSR has a constant value. Variations in BSR reflection coefficients are attributed to velocity variations of the gas hydrate above the BSR. Estimates of the hydrate concentration have been made from BSR reflection coefficients obtained in this study. Mean concentration are abut 20~25% of the total sediment volume for the passive margin, and are about 5~10% for the active margin. The results of calculated reflection coefficient of the BSR in offshore SW Taiwan suggest that inferred hydrate concentration for the passive margin profiles is relatively higher than that for the active margin profiles.
OS23A-1039
Dynamic Cycling of Barium in Marine Sediments: A Case study From a Gas Hydrate Potential Region Offshore Southwestern Taiwan
The dissociation and dissolution of gas hydrate in marine sediments affects importantly of Ba and methane cycle. Three piston cores, MD052911, MD052912 and MD052913, collected during Marion Dufresne cruise from a potential gas hydrate area offshore southwestern Taiwan, were used for dissolved SO42- and Ba2+ analyses in pore waters, as well as exchangeable Ba2+ in sediments, to study hydrate gas venting history. The formation of barite front was identified in each core at shallow depth and theirs respective accumulation ages were estimated using a simple diffusion model. Dissolved SO42- and Ba2+ indicate that the sulfate hydrate transition (SHT) depth is located at 900, 1100 and 760 cm in MD052911, MD052912 and MD052913, respectively. Dissolved Ba2+ increased largely below the depth where sulfate depletion occurred and reached a maximum concentration of 14.8, 12.4 and 6.2 £gM at 660, 1800 and 760 cm, respectively. The detected sedimentary barite front coincides with the modern boundary of upward diffusion of pore water Ba2+, occurring right above the SHT boundary in all three cores and reaches a Ba concentration of 23, 54 and 40 ppm, respectively. Estimated upwardly diffusive Ba2+ flux is 1.93*10-6, 8.86*10-7 and 6.55*10-7 mmol/cm2/yr, which may take 25,000, 110,000 and 213,000 years respectively to accumulate such barite front. Sulfate reduction rate (SRR) in the study area, average ~70 £gM/yr, falls in a similar range as those of regions with intermediately low methane flux and SHT depth of 10-40m. The calculated downward sulfate fluxes, 1.78*10- 3 - 3.2*10-3 mmol/cm2/yr, agree with observations at methane-rich margins where methane were dominantly consumed by oxidation. The authigenic barite fronts formation above the SHT serves as a useful tool to assess the flux variation of upward methane at present and in the past. The unique low concentration of detritus barite in sediments offshore Taiwan cause rather low Ba2+ in pore waters and low exchangeable Ba2+ (23-54 ppm) in sediments compared with Blake Ridge and Gulf of Mexico. This, in turn, offers a sensitive opportunity to apply a diffusion estimation for studying gas venting history using barite front formation in sediment column.
OS23A-1040
Deep-sea floor instability to cause of deep-water cable fault, off East Taiwan
In 2002 to 2003 years, many deep-water cable faults were taken place in the sites deeper than 4,700 m of water depth, off east Taiwan to the Okinawa Trench. Many commercial base cables were seriousely brocken in the events. To investgate the cause of the cable fault in the deep water environment we examined the location and timing of the cable faults and compared them with data/records of the bathymetry, sesimic records and precipitation of the coast range of Taiwan. Because the events were most likely to be caused by turbidite flows that run and developed along the submarine channels such as Taitung and Hualien Channels. Velocity of the turbidites reached 10 to 12 m/s on an average in the setting. The turbidtes were not hyperpicnal flow being caused by onland food event but seismoturbidite. The events were taken place just after earthqaukes ranging from 5.0 to 6.0, and the depths of the EQ sources were shallower than 23 km in and around the coast range of Taiwan. What we learn from this case is that the cable fault may be happened if the condition (or rule) is satisfied. If it is true, the cable fault in the deep water environment in the region can be predicted.
OS23A-1041
Discovery of "Hydrothermal" Chemosynthetic Community in a Cold Seep Environment, Formosa Ridge: Seafloor Observation Results from First ROV Cruise, off Southwestern Taiwan
First ROV diving survey has been conducted around the selected three sites (Formosa Ridge, Sites C and G) in offshore SW Taiwan, where gas hydrate broadly occurs within the thick Quaternary marine sediments. The Formosa Ridge and Site G are located in the South China Sea Continental Slope, whereas Site C is located in the frontal portion of a fault-bend fold in the accretionary wedge off SW Taiwan. Large, dense chemosynthetic communities were discovered at water depth of about 1120 - 1140 m on the top of the Formosa Ridge by direct observation using ROV Hyper-Dolphin of JAMSTEC. Colonies are distributed in depressions or fissures of large, carbon-depleted authigenic carbonates. The community is dominated by deep- sea mussel, Bathymodiolus platifrons, and galatheid crab, Shinkaia crosnieri. Most of species corresponds to those of hydrothermal vent fields in the Okinawa Trough, such as the Hatoma Knoll and the Iheya Ridge. Methane gas bubbles were found within the colony, and methane hydrate was also found beneath the aggregation of B. platifrons, which is harboured endosymbiont methanotroph in their gill in the Okinawa Trough. On the other hand, S. crosnieri from the Okinawa Trough culture numerous epibiont sulfur-oxidizing bacteria on their carapace. Therefore, they require high concentration of H2S from the subseafloor fluids. Single channel seismic profiles show the presence of clear BSR underneath the Formosa Ridge, and suggest the possible upward migration of gassy fluids beneath the top of the ridge. Therefore, the ghydrothermalh chemosynthetic communities are supported by unusual high concentrations of methane and H2S fluids from the subseafloor. This is first discovery of "hydrothermal" chemosynthetic communities in cold seep environments of the world. No active seeps and the related phenomena are found around the Site G. Large seep carbonates around the summit are often covered by mud veneer and organisms. Therefore, this mud volcano is inferred to be terminated its activity. Swath bathymetry around Site C probably shows the presence of a mud volcano on the seafloor. Unfortunately, bad sea state didnft permit to finish our mission for ROV survey.
OS23A-1042
Physical and bio-chemical mass-balance model around seafloor cold seepages
Natural cold seepages are characterized as rapid upward transports of methane from deeper part of geological structures to the seafloors. Prior to reach the seafloors, when methane meets downwards diffusing seawater sulfate, it is oxidized anaerobically by a consortium of microorganisms that use sulfate as an oxidant, producing sulfide. The anaerobic oxidation of methane and anaerobic sulfate reduction are clarified as a coupled biological activity. A significant portion of the bicarbonate produced after the sulfate reduction as authigenic carbonate, mainly aragonite and high-Mg calcite, near the seafloor. Where the methane fluxes are much, these anaerobic reactions occur just beneath the seafloor. There, usually sulfur oxidizing microorganisms are visible on the seafloor just above the coupled consortium of microorganisms. They are called bacterial mats. When the fluxes too much, direct methane bubbling occurs and chemosynthesis-immobilization communities such as tubeworms and clams distribute around the bubbling locations with the bacterial mats. The physical and bio-chemical mass-balance model around cold seepages on seafloor and in water column has been studied by the authors and some preliminary results were reported (Yamazaki et al., 2005 and 2006; Takeuchi et al., 2007). The approach is to analyze the existing field observation and numerical modeling studies of cold seepages and to create a new physical and bio-chemical mass-balance model in the environment. The model is separated into three parts. They are methane supply, seafloor ecosystem, and water column units. The seafloor ecosystem unit has been improved to analyze the unsteady formation processes of the ecosystem. The time dependencies of formations of the consortium of microorganisms (AOM), the chemosynthetic community, and bicarbonates examined with the improved model are introduced. After the bubbling from seafloor, the methane bubble jet blows up in the water column due to the buoyancy. Then the dissolution and dispersion process of the methane plume with the oxidation occurs in the water column. These are the functions included in the water column unit. Because no systematic observation of methane plume behavior was found in literatures, a combined field observation with ADCP mooring and CTD-casting with seawater sampling was conducted during the R/V Tansei-maru cruise KT-06-26 at the Umitaka Spur off Joetsu region, eastern Japan Sea. Applying the simultaneous measurement data of methane concentration distribution and current profile in the water column obtained from the field observation into the water column unit, the methane flux bubbling into the water column was estimated. The result is also introduced. When we use the natural methane hydrates in deep-ocean as energy resources, the reactions of ecosystem around the exploitation sites must be assessed in advance. Unsteady and short term solutions and expectations are necessary in the assessment.
OS23A-1043
BSR 3D Architecture and Heat Flow Derivation on the Kumano Forearc Basin - Nankai Margin
3D seismic reflection data acquired on the Kumano forearc basin to guide the NanTroSEIZE drilling project on the Nankai accretionary prism reveal a high amplitude bottom simulating reflector (BSR) interpreted as marking a phase transition between methane hydrate and free gas in the pore space of both accreted and forearc sediments. Detailed 3D mapping of the BSR shows lateral variations of the reflection pattern in terms of continuity and amplitude correlated to the properties of the cross-cut sediments suggesting the greater role taken by the permeable layers in the migration of free gas produced within the column until the Base of the Hydrate Stability Zone (BHSZ). The BSR is locally underlined by a lower bright reflection with the same polarity as the seafloor. Its context of appearance above possible deep fluid drains better suggest a gas-water contact reflection rather than a remnant BSR from a former position. We also determine the depth of the hydrate-gas phase boundary in order to estimate the geothermal gradient and hence the regional heat-flow in 3D. It offers information about active processes occurring in this area over the downgoing plate. In sub-surface, erosion-sedimentation and shallow faulting take an important part of the calculated BSR derived heat flow. However, we also evidenced the impact of deeper processes related with fluid migration and initiation of mud diapirism. The results suggest a model of the Kumano Basin with free gas produced by biodegradation within the shallow sedimentary column and then stacked under the BHSZ after migration along permeable layers. In addition, possible important amount of gas-charged fluids migrates from deeper depths within accreted sediments and along deep faults to reach the BHSZ with generation of a lower brilliant reflector at the base of the gas-charged fluids pure water interface.
OS23A-1044
Dissolved Methane in seawater and seafloor mud around the gas plumes, in off Naoetsu, eastern margin of Japan Sea
The giant gas plumes were observed by acoustic echo sounder in off Naoetsu, eastern margin of Japan Sea. Methane hydrates have been collected by piston corers during UT04 cruise in 2004, and hydrate coated methane bubble during NT06-19 cruise in 2006 have been observed. In order to evaluate the impact of methane seepage on the ocean and atmosphere systems, the concentration and isotopic compositon of methane in the seawater columns and interstitial waters were measured. Methane concentrations of the seawater above methane plumes were extermely high as much as 100nmol/L.Furthermore, the concentration was reached to 15umol/L at just above the gas plume. The seawater are still undersaturated, but they are anomoulously high compared to background seawater (5nmol/L). Hence observations suggests that the plumes are responsible for high concentrations of methane in this area. High metahne concentration in seawater column were also obserbved in area far from the plume. The anomalies are explained as the result of lateral migration due to currents which were clearly detected by ADCP (Acoustic Doppler Current Plofiler). Vertical profiles of methane concentrations in seawater column showed characteristic features.Shallow high concentration anomalies wereobserved at 200m to 300m below sea level (mbsl). This depth is related to hydrostatic pressure boundary of methane hydrate stability as well as the boundary of the water mass called Japan Sea Proper Water (JSPW). We concluded that the methane hydrate particles floated up to the depths, and were dissociated and dissolved around this depths. Dissolved methane laterally migrated along the boundary of the water masses. Another higher concentration zones are at 100m to 200m above the seafloor regardless of the methane plume sites. Such high anomalies are thought to be caused by back-ground diffusion of methane emitted from the seafloor. The depth of SMI of the area indicate high methane fluxes. Carbon isotopic compositions of methane dissolved in sediment interstitial water showed regional features. Heavier values of delta carbon 13(-29 to -55permil PDB) were found around the gas plumes. Heaviest delta carbon 13 values were seen in the pockmarks and gas hydrate bearing samples. While those obtained far from the plumes were lighter (-70 to -90permil PDB). Methane of the plume sites are thought to be composed of thermogenic decomposition of organic matters. Methane in sediments are most depleted in C-13 at around the SMI depth, and tends to increases upward and downward. Fractionations observed above SMI were explained as the result of microbial methane oxidation, whereas the profile below the SMI is likely to reflect increasing amount of thermogenic methane and/or differential upward diffusion-advection of isotopically light methane.
OS23A-1045
Relation between methane hydrate-bearing formations and geological phenomena on the seafloor in the eastern Nankai Trough, Japan
In 2002, a series of high-resolution 3D seismic surveys was conducted in the Tokai-Oki, the Daini-Atsumi Knoll, the Kumano-nada in the eastern Nankai Trough, Japan. Research Consortium for Methane Hydrate Resources in Japan (MH21) conducted resource assessment of methane hydrate in the eastern Nankai Trough by various seismic data analyses combining results of the exploratory wells conducted in 2005. By these analyses, occurrence of methane hydrate in the eastern Nankai Trough is coming to light. The MH21 has also interpreted the relation between methane hydrate-bearing formations and various geological phenomena on the seafloor, such as pockmarks and carbonate outcrops, using the 3D seismic data in the three survey areas. Bathymetric maps and seafloor amplitude maps constructed by the high-resolution 3D data provided lots of information on the seafloor. Some areas show very high intensity on the seafloor amplitude maps. It is expected that the areas showing strong amplitude correspond to the distribution of carbonate outcrops which are likely precipitated by methane seep activities. By checking the seafloor amplitude maps, seismic sections and methane seep sites observed by the previous submersible dives, some significant correlations are recognized between methane hydrate-bearing formations and various phenomena on the seafloor. It may be likely that the occurrence of methane hydrate and the geological phenomena on the seafloor have a strong implication with some typical geologic structures, e.g. shallow fault, highly-permeable sediments and hydraulic fractures, which may control the fluid migration. Besides, in this study we learnt that bathymetric map and seafloor amplitude map constructed by the high- resolution 3D seismic data are very useful not only for interpretation of relation between methane hydrate-bearing formation and various phenomena on the seafloor but also for designing the following seafloor investigations. This study is conducted by the MH21.
OS23A-1046
Detailed analysis of methane hydrate concentrated zone of lobe type
Japan Oil, Gas and Metals National Corporation (hereinafter called JOGMEC), as a member of MH21 Research Consortium, takes charge of a study of the Research for Resources Assessment, and is pursuing a possibility that methane hydrate, which is presumed to be distributed around ocean area of Japan, will be energy resources. JOGMEC is currently conducting analysis of seismic data which was acquired by 3D seismic survey conducted from Tokai-Oki to Kumano-nada in the eastern Nankai Trough by METI (Ministry of Economy, Trade and Industry) in 2002 under the national program of assessment for methane hydrates as energy resources. It was understood that methane hydrate was correlated to high resistivity and high velocity based on the results of drilling surveys and velocity analysis, and that methane hydrate concentrated zones can be roughly classified into the channels and lobes in seismic geomorphology because they were characterized with reserves consisting turbidite sand bodies. In this study, the detailed analysis of the inner structure of the methane hydrate concentrated zone of lobe type was conducted to understand the occurrence configurations of methane hydrates. The reflected waves that construct the methane hydrate concentrated zones in the seismic data were extracted and those reflected waves were classified into some groups every one reflector. As the result, some reflectors that construct the methane hydrate concentrated zones were revealed. Those reflectors show the layers including methane hydrates, and the detailed distribution of the methane hydrates in those layers was revealed by the intensity distribution of the amplitude. This time, we introduce the example of the detailed analysis of the methane hydrate concentrated zone in the lobe of submarine fan.
OS23A-1047
Numerical Simulation of Generation and Migration of Methane and Accumulation of Methane Hydrate in the Eastern Nankai Trough
We have developed 2-D numerical simulator "SIGMA-MH" for methane hydrate (MH) accumulation in order to simulate geological/physicochemical phenomena related to MH deposits formation in deepwater sediment and to clarify controlling factors for MH accumulation. SIGMA-MH is a modified version of "SIGMA-2D", basin simulator for petroleum system developed by JOGMEC. SIGMA-MH has models such as microbial methane generation model and MH formation/dissociation model, with which MH accumulation can be simulated. An empirical model is used for microbial methane generation model. In this model, maximum gas generation rate is estimated from organic accumulation rate, which are derived from ODP gas sample analysis. Kinetic reaction model is applied for MH formation/dissociation. MH formation/dissociation rate is expressed as a function of the difference between the fugacity of methane at gas phase and the fugacity of methane at the three- phase equilibrium for MH-water-methane gas. Using SIGMA-MH, case studies were conducted for two sections including boreholes at Tokai-Oki and Daini- Atsumi area where MH concentration is detected. Geological structure used in the simulation was estimated by seismic survey. Rock facies were determined by logging and core sample. Heat flow was calibrated by present temperature measured by geothermometer. Base case, shallow case, and deep case were conducted as a sensitivity analysis for paleo-water depth. As a result, a series of recycling process, that is, methane gas generation, MH formation, MH dissociation, gas migration, and MH formation, was simulated. MH accumulation estimated by simulation nearly corresponds to that of actual borehole. The phenomena that methane gas migrates through permeable sand layers and MH accumulates in them suggests that permeability are a key of MH accumulation. It was found that paleo-water depth plays an important role because it affects on present MH accumulation. For future work, we will conduct sensitivity analyses of quantitative parameters and paleo-water depth and improve SIGMA-MH as a tool for understanding controlling factors of MH accumulation. This study was carried out as a part of Research Consortium for Methane Hydrate Resources in Japan (MH21).
OS23A-1048
Methane-derived carbonates from the Joetsu basin, eastern margin Japan Sea: possible evidence for strong methane seepage
A number of gas plumes, sea floor gas hydrate, carbonate nodules, pockmarks and mounds are observed at Umitaka spur (UT) and Joetsu knoll (JK) off Joestu, eastern margin of Japan Sea. Methane derived carbonate nodules were recovered by piston cores, dredges and ROV hyper dolphin from plume sites on the mounds and around pockmarks. Three types of occurrence of carbonate nodules have been identified; (i) those associated with gas hydrates nearby plumes, (ii) exposed on the sea floor, (iii) existing in sediments without gas hydrates far away from gas plumes. Type (iii) carbonates indicate that strong methane seepage occurred in the area in the past. Mineral composition, carbon and oxygen isotopic composition of carbonates have been analyzed and U-Th ages have been determined to examine the past active methane seep events related to the dissociation of gas hydrates in the study area. Carbonate nodules are grouped into (i) aragonite dominant type (70-100 percent in peak high ratio), (ii) calcite dominant type (almost 100 percent in peak high ratio) and (iii) mixture of calcite and aragonite. Dolomite occurs as minor constituents. Carbon isotopic composition of these three types of nodules from JK are between -27 and -32 permil (VPDB) and between -5 and -25 permil (VPDB) from UT. The difference is likely to reflect the different contribution of methane derived bicarbonates. Gas hydrate dissociation events in JK is considered to have been larger than UT. The oxygen isotopic compositions of aragonite dominant nodules from both JK and UT are mainly between +3.5 and +5.5 permil (VPDB). Assuming that the oxygen isotope value of sea water was +0.3 permil (VSMOW), paleo temperatures are calcurated to be very low, -2.2 to -6.6 degree. This strongly suggest that the oxygen isotope value of aragonite precipitating water was +3.3 to +3.4 permil (VPDB), much heavier than sea water value. Heavy oxygen water have been possibly derived from the dissociation of gas hydrates, so aragonite dominant nodules may indicate past gas hydrate dissociation events. To the contrary, the oxygen isotopic compositions of calcite dominant nodules are observed to be +1.5 to +4.5 permil (VPDB), suggesting that these nodules precipitate either under higher temperature (more than 3 degree) or heavy-oxygen- depleted waters. Considering that the bottom water temperature is constantly low at around 0.3 degree throughout the last glacial-interglacial periods, calcite dominant nodules are explained to have been formed in light-oxygen waters, perhaps in the residual waters of gas hydrate formation. Preliminary results of the U-Th dating for carbonate nodules from UT (Watanabe et al, 2006) indicate that nodules were precipitated between 12 and 35 ka, centered around 20 ka. More U-Th dating needs to clarify the relationship between climate change and gas hydrates formation / disossiation. Reference Watanabe et al. (2006) ISC 2006 FUKUOKA, JAPAN, (abstract)
OS23A-1049
Methane hydrate reservoir model around the Eastern Nankai Trough area offshore Japan
The Research Consortium for Methane Hydrate Resources in Japan (hereafter the MH21 Research Consortium) was established to undertake research in accordance with Japan's Methane Hydrate Exploitation Program. In 1996 and 2001, 2D seismic surveys were conducted and delineated the BSR distribution. The 3D seismic survey was conducted in this area in 2002. Bottom Simulating Reflectors (BSRs) were widely found on the marine seismic data acquired in the shelf-slope in the Eastern Nankai Trough area. BSRs indicate the existence of methane hydrates. However, we cannot estimate detail reservoir information from distribution of BSRs. The gas hydrate-bearing sediments are heterogeneous and complex both vertically and horizontally, because methane hydrate-bearing layers are mainly turbidite sands - channel-levee or lobe sediments. The occurrence of methane hydrates was first confirmed by corings and borehole logging data around the Eastern Nankai Trough offshore Japan in 2000. METI conducted two drilling campaigns around the eastern Nankai Trough area. Total 38 boreholes were drilled, and recorded many logging data by wireline or LWD. We separate three types of methane hydrate reservoir among methane hydrate-bearing zone from reviewing the well logging data mainly the resistivity and P or S wave velocity. One is low saturation-type, the other is uncontinuous high saturation-type, the last is the continuous high saturation-type, and also we call the methane hydrate- concentrated zone as this type. If we will explore and exploit the methane hydrate, our main target is the continuous high saturation-type, as the methane hydrate-concentrated zone. We delineated the methane hydrate reservoir by the picking the reflector as the methane hydrate sub reservoir body for the turbidite channel-levee or lobe system around the eastern Nankai Trough area. On the other hand, we reviewed the P or S wave velocity and the methane hydrate saturation at the wells in the methane hydrate-concentrated zone. We estimated the matrix-support or pore-filling model from the relationship between the P or S wave velocity and the methane hydrate saturation.
OS23A-1050
Turbidite channel as methane hydrate concentrated zone - study results of 3D seismic data interpretation -
3D seismic data were acquired in the eastern Nankai Trough area, offshore Japan by METI in 2002 in order to evaluate resource potential of methane hydrates. Through the studies on these seismic data and the results of multi-well drilling surveys carried out in 2004, more than 10 methane hydrate concentrated zones were delineated in this area. The methane hydrate concentrated zones were characterized by turbidite sand bodies which can be roughly classified into channels and lobes in the seismic geomorphology point of view. We focused our attempt to reveal internal structure of a channel type methane hydrate concentrated zone using high resolution 3D seismic data. Basically, channel complex can be recognized by reflectors suggesting erosion surfaces and internal bodies of sand layers. The whole channel is vertically divided to the upper and lower parts bounded by BSR. As it is difficult to interpret the internal structure in the lower part due to the less continuous reflectors, only the upper part was geomorphologically analyzed by picking the high amplitude reflectors which suggest the hydrate concentrated sand dominated bodies. The group of the picked reflectors suggests the 3D feature of the channel development. The channel was bended during its sediment deposition. The original channel flowed from northeast to south. And later, the flow was from northeast to west-southwest. Thus, we can find the detailed shape of each sand dominated body within channel, and therefore, it is expected that detailed analysis of many reflector patches in the channel can teach us the properties of methane hydrate bearing thin sand dominated bodies by classifying their shapes. Further, such properties will contribute to the future precise volume estimation of methane hydrates, and also to the model construction for production simulation. This study is carried out for Research Consortium for Methane Hydrate Resources in Japan (MH21).
OS23A-1051
The Microstructure of Nankai Trough Methane hydrate sediments by Scanning Electron Microscope
The microstructure of Nankai Trough Methane hydrate sediments was observed by Field Emission Scanning Electron Microscope (FE-SEM; JEOL JSM7000F) with an energy dispersive X-ray detector (EDX-detector; JEOL JED2300F). The samples were obtained using Pressure-Temperature Core Sampler (PTCS) that can prevent hydrate dissociation during coring by keeping pressure insitu conditions. Sample that was taken by conventional tools for coring was kept shape by self-prevention nature itself, nevertheless many excellent observations had been reported. Therefore, it was expected that the PTCS core sample was suitable for observing microstructure of methane hydrate occupied in sediments, even though samples were exposed to atmosphere very short time on vessel before storage in liquid nitrogen. In addition, a low-vacuum system and a cryostat sample stage was installed in FE-SEM for observing methane hydrate bearing sediments. The former can make observation until sample chamber pressure is less than 30Pa, and the latter can control temperature from room temperature to 80K. Each sample was trimmed using cryo- system (Gatan Alto2500) at liquid nitrogen temperature and was settled on cryostat sample stage for observation. To compare with images before and after methane hydrate dissociation at the same region, we had made the freezed-dried sample within chamber of FE-SEM under controlling temperature of cryostat sample stage. Sample had been kept observing during drying by a low-vacuum system's work so as to confirm no breakage had been occur. Result of observation, the following were described as microstructural feature of Nankai Trough Methane hydrate bearing sediments, which had been freezed and dried, (1) There are a number of pyrite crystals existing among sand grains, which crystal size are less than 1 micron. (2) Framboidal pyrite can be seen among sand grains. (3) many vacant space can be seen among sand grains ; there are no pyrite and no clay minerals. (4)Pyrite and clay minerals make thin membrane structure around the vacant space. The space occupied by methane hydrate must be blank when hydrate dissociation occur without disturbing microstructure of sediments. This estimation is supported by EDX analysis that carbon detected regions would become vacant spaces by drying. Thus, we concluded that the methane hydrate exists in the vacant space of pore space and is covered with pyrite and clay minerals. These results will become help to understand methane hydrate formation processes in sediments.
OS23A-1052
Gas Hydrate Accumulations Where and how Much? Can Seismic Reflection Intensities Provide the Answers?
Location of gas hydrate accumulations on seismic reflection records provides qualitative overview of distributions. This data analysis is supported with shallow pore water geochemical profiles and vertical fluid migration measured with heatflow probes. These indirect measurements of acoustic signal intensities need further evaluation to provide quantitative estimates of the hydrate loading between sites and within sites. Geophysical and geochemical data are compared for three different continental margin locations -two passive (Atwater Valley and Alaminos Canyon, Gulf of Mexico) and one active (Hikurangi, New Zealand). In order to provide quantitative estimates of subbottom hydrate distributions we have interpreted the following features on records from these three locations. BSR; seismic blanking zones above the BSR which are adjacent to vertical seismic blanking zones; strong reflectors below the BSR and regions tentatively identified as perched BSRs. Seismic interpretations are integrated with pore water geochemical and vertical fluid flux profiles. While the Alaminos Canyon profile is in deeper water than the Hikurangi Margin profile, the same interpretation of seismic features applies. Both areas have variable BSR signal strength along this transect and clearly exhibit well-developed seismic blanking areas, regions of vertical flux and strong reflectors below the BSR. In addition, both records show the existence of perched BSRs. Our recognition of the perched BSR follows the location of faults that cross the lower BSR and terminate at the base of an overlying free-gas pocket indicated by the presence of a very strong acoustic reflector. Beneath the gas accumulation, the seismic blanking indicates that gas hydrate is present. The difference between these two sites is primarily in the strength and continuity of the reflective horizons, which are initially interpreted as a function of the water depth and the presence of avenues for vertical migration. Atwater Valley provides further seismic reflection data calibration for comparing Hikurangi Margin and Alaminos Canyon interpretations. In this region, salt diapirs are present below the hydrate stability zone. Geochemical pore water and vertical fluid migration profiles indicate high vertical gas fluxes from deep hydrate deposits. However, high pore water chloride concentrations in these profiles, lead to the conclusion that salt diapirs cause lower hydrate stability and rapid vertical gas migration. Notably absence on the seismic reflection record for Atwater Valley is a perched BSR. This is explained by the upward migration of the underlying salt dome and the associated tensional faulting which provides the necessary pathways for rapid vertical gas flux.
OS23A-1053
A Reassessment of Gas Hydrate Occurrences in Lake Baikal
Lake Baikal is the only fresh-water lake with gas hydrates in its sedimentary fill (i.e., in a non-marine, non- permafrost setting). The hydrates have been sampled in deep boreholes and in short cores, and consist in most cases of nearly-pure methane sI hydrate of biogenic origin, although locally in specific settings sII hydrates have been observed as well. The extent of the deep hydrates was mapped by tracing the Bottom-Simulating Reflector on MCS profiles: they are present in large parts of the southern and central Baikal basins, at water depths of > 360 m. In several places, clusters of mud volcanoes occur in this hydrate province. They all occur in within the Gas- Hydrate Stability Zone and in the immediate vicinity of major, active faults. The mud volcanoes have dimensions of 250-1600 m in diameter and of 15-220 m in height; they are characterized by mud breccias and the extensive occurrence of near-bottom hydrates. Methane, fluid and mud release from these edifices is not continuous and most are dormant at present. The methane and associated fluids in some of them are believed to originate from destabilizing gas hydrates at 200-300 m sub-bottom depth under the influence of a tectonically controlled geothermal fluid pulse along adjacent faults, but this appears not to be the case for all. In addition, several active methane vents have recently been discovered, both in deep (> 800 m) and in shallow (< 230 m) water. Deep-water seeps occur outside the mud-volcano regions, and most of them outside the hydrate province; shallow-water seeps occur on the upper slopes of the basin, mostly in the vicinity of deltas, slope canyons or faults, and bubbles escaping from them often reach the lake surface. Methane release appears to be more continuous and bubbles can often be observed to reach the lake surface. INTAS Projects 1945 and 2309 have investigated these gas hydrates and gas seeps, as well as the consequences of the methane expulsion from mud volcanoes and shallow seeps for the waters of Lake Baikal, and has been monitoring mud volcano and seep activity over a period of 2 years.
OS23A-1054
Pseudo 3-D analysis of seismic data at seep locations in the Wairarapa area offshore New Zealand
Gas hydrates constitute a large reservoir for hydrocarbon gases, forming at continental slopes at high pressures and low temperatures. They may play a role in slope stability although the influence of changes in the hydrate stability conditions on slope failures are not yet fully explored. During the cruise SO 191-1 on RV Sonne, a number of multichannel seismic profiles have been recorded together with wide-angle observations to analyze gas and gas hydrate deposits in the area of Wairarapa, south-east of the Northern Island of New Zealand. This area is dominated by a compressional/transpressional tectonic setting related to the subduction of the Pacific Plate under the Australian Plate. Bottom simulating reflections (BSR) have been observed along the entire margin and also on our profiles. Where the base of gas hydrate stability (BGHSZ) follows the stratigraphy, a bundle of high-amplitude reflections appears instead of the BSR. Considerable signal attenuation beneath the BGHSZ reduces the signal penetration from up to 1.5 s TWT beneath the seafloor to 0.5s TWT. Several slumps and apparent sediment wave formations are found where the slope steepens. While the present seafloor bathymetry shows an elongated high, the seismic sections reveal a second anticlinal structure in the subsurface and a depression in between indicating a change of tectonic regime presumably in the past 1 or 2 million years. This corresponds to a rotation of the Hikurangi forearc of 50° from WNW in early Miocene to ENE today. The stratigraphy above the BSR seems to be dominated by relatively rapid deposition from mass wasting processes such as turbidites and channelized sediment transport deposits. Other than some minor fault displacements and fractures the structural deformation imprints appear to have ceased in the upper stratigraphic record. The position of a paleo-canyon can be clearly traced in deeper strata in between the structural highs. Heat flow values estimated from the depth of the BSR are constant in the working area except beneath the slumps. This indicates that the BSR has not yet adapted to the new seafloor level, i.e. the slumping has happened within the last few thousand years. http://www.ifm- geomar.de/index.php?id=newvents&L=1
OS23A-1055
Analysis of Wireline Acoustic Logs, India National Gas Hydrate Program (NGHP) Expedition 1
The Indian National Gas Hydrate Program (NGHP) Expedition 1 was designed to study the occurrence of gas hydrate along the east and west coast of India and near the Andaman Islands. In the spring and summer 2006, the expedition discovered gas hydrate in sand, silt, and clay dominated sediments. One of the best recognized influences of gas hydrate on the host sediment is a change in mechanical and elastic properties, typically an increase in sonic velocity, but also a measurable increase in attenuation. Recognizing the strong influence of gas hydrate and free gas on the propagation and amplitude of acoustic waves, sonic waveforms were recorded in multiple modes and frequencies in the ten sites where wireline logs were acquired. To complete the characterization and integrate the drilling data with the regional seismic surveys, vertical seismic profiles (VSP) were acquired successfully in six holes. These data, recorded with a wide range of frequency and scales provide an extensive survey of the acoustic properties in very diverse gas hydrate systems. Because of the poorly consolidated nature of the sediments in some east coast sites, automatic picking of velocity was only partially successful during the expedition, and a complete post cruise reprocessing of the sonic waveforms was necessary to draw accurate compressional (Vp) and shear velocity (Vs) logs in these holes. Synthetic seismograms generated with the Vp and density logs confirm the depth and nature of the main reflectors in the seismic surveys that were used to select the sites, in particular the BSR marking the deepest occurrence of gas hydrate. Despite heterogeneous distributions, the sonic logs clearly identify the presence of gas hydrate in very distinct intervals, and the eventual occurrence of free gas underneath. In addition to providing Vp and Vs logs, the amplitude of the waveforms offers a complete insight into the distribution of gas hydrate in a rich and deformed lithology. The dissipative influence of gas hydrate on acoustic waves is still poorly constrained and strongly dependent on the host sediment, but empirical relationships derived from sonic attenuation, hydrate saturation and gamma ray logs provide independent measures of the diversity of gas hydrate distribution in the sites visited.
OS23A-1056
Gas hydrate exploration of Porangahau Ridge, East Coast, North Island, New Zealand
During June and July 2006 the R/V Tangaroa collected high-resolution seismic profiles, EM 300 swath bathymetry, 3.5 sub-bottom, as well as water column echosounder data across Porangahau Ridge east of the North Island. Piston cores were recovered for pore water chemistry, microbiology, and paleoceanographic analyses. We also acquired heatflow data, CTDs, and seawater samples for water-column chemistry. The seismic data show amplitude anomalies beneath the ridge. The anomalies develop along a prominent N-S fault-propagation anticline. We analyzed reflection coefficients and conclude that the anomalies are most likely caused by free gas within the regional gas hydrate stability field as defined by the depth of bottom simulating reflections. We suggest that local warming associated with fluid expulsion through faults keeps the temperature at the anomalies outside of the gas hydrate stability field. Based on the seismic amplitudes, we predict at least ~7% of the pore space to be saturated with gas if gas is evenly distributed. Gas saturation is predicted to be almost 70% for "patchy'' gas distribution. For the pressure-temperature conditions beneath the ridge, gas at a saturation of 7% would form gas hydrate at a saturation of ~10% of pore space. Should the localized heat flow anomaly weaken, e.g., because of sealing of the faults, the ridge could become an area with significant hydrate deposits. We speculate that the Porangahau Ridge constitutes a gas hydrate "sweet spot" in the process of formation. Pore water chemistry shows a shoaling of the base of the sulfate reduction zone across this feature, indicative of elevated methane flux through the hydrate stability field. There is a distinct thermal anomaly across the Porangahau Ridge, albeit with a complex signature. On the other hand, there are no indications of methane expulsion into the water column, neither in the echosounder records nor in the water chemistry profiles from CTDs.
OS23A-1057
Cold Vents: A Study From Wairarapa Offshore New Zealand
The role of methane in the global bio-geo-system is one of the most important issues of present-day research. Cold seeps, where methane leaves the seafloor and enters the water column, provide valuable evidence of subsurface methane paths. The cruise SO 191-1 (Jan. 2007) on RV Sonne was dedicated to the investigation of local and regional transport processes of methane and gas hydrate deposits east of New Zealand. The eastern coast of New Zealand is shaped by intensive compressional tectonics caused by the subduction of the Pacific Plate under the Australian Plate. In the area of Wairarapa, offshore the southern tip of the north island, a number of extremely active seeps have been discovered. High resolution seismic sections show a variety of seep appearances. We see seismic chimneys either characterized by high amplitude reflections or by acoustic turbidity and faults acting as fluid paths. The bathymetric expression of the seeps also varies, we see seeps beneath a flat seafloor as well as pockmarks and small mounds. The images of the 3.5kHz Parasound system and of the deep-towed subbottom profiler system reveal the near- surface structure of the vent sites. While high amplitude spots within the uppermost 50mbsf are observed at the majority of the seeps, indicating carbonate concretions, a few seep sites are characterized by the complete absence of reflections, indicating a high gas content. Five ocean bottom methane sensors had been deployed, which continuously measured the methane concentration and water temperature over a period of several days. The results are most intriguing: One instrument near a seep measured a peak concentration of several microMol/l, while another sensor about 300m further recorded values by a factor of 10 less. This demonstrates the spatial confinement of methane expulsion at seeps. The temporal methane concentration varies over several orders of magnitude without any correlation between the five instrument locations or the tidal signal, in contrast to the temperature variation.
OS23A-1058
Variable Fluid Flow Along The Nova Scotian Slope - A Study Of Gas Hydrates With Ocean- Bottom Seismic Data
Bottom-simulating seismic reflectors (BSRs) as indicators of gas hydrate have been observed in the Scotian Basin (eastern continental margin of Canada), although the presence of hydrate is comparatively rare. Former studies near the Mohican Channel (100 miles offshore Halifax) show a BSR in that area approx. 350m below seafloor with an underlying low-velocity zone of generally 150 m thickness. Models of hydrate and gas as part of the sediment frame give concentrations of 2 to 6 % hydrate and less than 1 % free gas (LeBlanc et al., 2007). Where a BSR is present, these values are considered representative of the hydrate and gas concentrations in this passive-margin environment. In 2006, a joint project by the Geological Survey of Canada and Dalhousie University targeted gas hydrate near the Mohican Channel. In this area, a clear BSR near the side-wall of the channel disappears in a direction away from the channel and parallel to slope. However, there are no obvious changes in geology that would cause the distributions of the BSR to vary strongly in this direction. To try and detect lateral changes in physical properties (particularly those controlling upward fluid flow), and to determine why the BSR is so localised in this environment, 19 ocean-bottom-seismometers (OBS) were deployed along a 20 km line from the area of the BSR to the area with no BSR. Data from a 3D seismic cube (EnCana Ltd.) over the OBS locations and the nearby Torbrook geotechnical borehole show that the BSR dissipates where the density of near-vertical polygonal faulting increases. Lateral variation in hydrate and gas distributions implied by the change in the BSR, and the coincident change in polygonal faulting, points to a variable upward fluid flow along slope. With the wide-angle data we attempt to ascertain velocities above and below the BSR at different distances from the Mohican Channel, and to use the inferred hydrate and gas concentrations to calculate the variation in upward fluid flow along the slope. This variation is linked to shear-wave splitting estimated from the OBS data by relating changes in shear-wave splitting to changes in the density of cracks.
OS23A-1059
Gas Hydrates on the Norway-Barents Sea-Svalbard margin(GANS)
The main objective of this Norwegian national initiative is to quantify gas accumulations in the form of hydrates in sediments on the Norway-Barents Sea-Svalbard margins, including an assessment of their dynamics and impacts on the seabed to provide knowledge; vital for a safe exploitation in oil and gas production. This overall objective is an initiative by five research institutions and the Norwegian Deepwater Programme, SEABED III (consortium of nine petroleum companies), to make a coordinated effort on a national level to achieve the main objective to make a coordinated effort on a national level to achieve the main objective by the following sub-goals: a) Geophysical characterisation of gas hydrates, b) Geological and geochemical setting of gas hydrate reservoirs and seeps, c) Gas hydrate dissociation and its effects on geomechanical properties, d) Theoretical and experimental evaluation of gas hydrate dynamics. Three contrasting target areas are of particular interest for field studies and experiments: (1) the mid-Norwegian margin at Nyegga, a national laboratory for gas hydrate research, (2) the Svalbard margin frontier area; important for understanding the geological controls on gas hydrates and fluids, and (3) the Barents Sea, a prolific area with possible occurrence of gas hydrates and clear evidence for active cold seeps. Our initiative allows for the establishment of an acknowledged Norwegian academia-industry network on gas hydrates where education of a new generation of interdisciplinarily trained scientists will be a central task. Our aims are to be achieved by integrating detailed geophysical studies of zones of gas hydrates and associated free gas in cooperation with geotechnical laboratory experiments, theoretical and experimental gas hydrate dynamic studies, geological studies, and geochemical studies of the fluids. The project is financed through the Norwegian Research Council - Petromaks (40 percent) and the SEABED III industry consortium (60 percent) over a period of 4 years starting in October 2006.
OS23A-1060
Analysis of High-Resolution OBS/H and MCS Data Across the Termination of a BSR in Lima Basin, Peru Margin Velocity Structure and Paleo-BSRs
Bottom simulating reflections (BSRs) in Lima Basin are confined to areas where the sub-surface structure is predicted to focus fluid flow. Most BSRs seem to be caused by free gas at the base of gas hydrate stability (BGHS). An increase of fluid flow and hence, methane flux, across the BGHS is thought to "trigger" formation of BSRs once methane concentration exceeds its solubility in pore water. Enhanced methane flux should also lead to elevated gas hydrate concentration above the BGHS. A BSR in the north of the basin coincides with outcropping layers, which often enhance fluid flow. During the GEOPECO campaign of the R/V Sonne in 2000, we deployed nine ocean bottom seismometers/hydrophones (OBS/Hs) at 1.85-km spacing across the termination of this BSR for a high-resolution OBS/H and short-streamer multichannel seismic transect. In order to investigate whether high gas hydrate concentrations are present in the hydrate zone, we compared velocities above the BSR to those in the same hydrate-free layer further landward (sequence L5 in an earlier classification). The OBS/H records were repositioned to sealevel in the τ-p domain followed by semblance analysis for determination of P-wave velocity (Vp). Vp in sequence L5 above the BSR appears lower than that at the hydrate-free site. This finding indicates that highly concentrated gas hydrates, if present at all, are confined to thin layers that do not significantly affect Vp of the entire interval. We have also identified at least two paleo-BSRs above the present BSR level. These paleo-BSRs most likely mark previous levels of the BGHS. Changes of the pressure/temperature regime must have happened sufficiently rapidly in order to "freeze" BSRs. We propose that the most likely mechanism for such rapid changes is sudden exhumation after mass wasting. The presence of at least two paleo-BSRs suggests that mass wasting has occurred repeatedly.
OS23A-1061
Frontal Ridge Slope Failure at the Northern Cascadia Margin: Fault Control and Possible Relation with Gas Hydrate Dissociation
A collapse structure is observed in newly acquired multi-beam bathymetry data and in seismic reflection data at the frontal ridge at the base of the slope off Vancouver Island, just landward of the deformation front. The head wall of the Frontal Ridge slide is ~250 m high and the slump has eroded a ~2.5 km long section into the ridge. The region is characterized by a wide distribution of gas hydrates based on the observation of the `Bottom Simulating Reflector' (BSR), that is assumed to coincide with the base of the methane hydrate stability zone. Hydrate was also observed in drilling by Integrated Ocean Drilling Program (IODP) Expedition 311. Since hydrates prevent sediment compaction, their dissociation in sediment pores is thought to decrease seafloor strength, potentially facilitating submarine landslides on continental slopes. Migrated seismic reflection data image a set of normal faults that clearly outcrop at the seafloor and can be traced from the surface through the sedimentary section to depths below the BSR in some locations. Seafloors scarps show that seafloor displacements of ~25 m to 75 m are generated. The faults strike in the NE-SW direction, perpendicular to the margin and parallel to the direction of convergence. Extensional motion on the faults is oriented NW-SE, perpendicular to the direction of compression on the margin. Disturbed sediments are confined to the vicinity of the normal faults. The two faults with the largest seafloor scarps clearly bound the region of slope failure on the frontal ridge, dramatically indicating that the lateral extent of slumping is fault-controlled. No BSR is observed within the slide region, but it is present in the surrounding region. The causal relationship between slope failure and gas hydrate dissociation is difficult to document, but a number of observations support a potential connection.
OS23A-1062
Potential methane production in sediments from the Cascadia Margin, IODP Expedition 311
In many cases methane in natural gas hydrates was interpreted to have been produced through microbial methanogenesis. Some fundamental questions, for example, where microbial methane was produced or how to form the hydrates, still remain unsolved. Expedition 311 of Integrated Ocean Drilling Program (IODP) investigated gas hydrate in the accretionary prism of the Cascadia margin. We participated the Expedition 311 and obtained core samples from 5 drilling sites. The four sites transected across the Cascadia Margin and represented different stages in the evolution of gas hydrate across the margin from the earliest occurrence on the westernmost, first-accreted ridge (U1326) to its final stage at the eastward limit of gas-hydrate occurrence on the margin in shallow water (U1326). The fifth site (U1328) was at a nearby cold vent with active fluid and gas flow. We conducted culture experiments of the sediment cores under near in-situ temperature and tracer experiments using 14-C labeled bicarbonate and acetate in order to estimate potential methane production in the region and to elucidate which methanogenesis pathways were employed for methane production. Results of the culture experiments showed methane production would have occurred in the samples from gas hydrate zone and the deeper sediments below the BSRs. Sediments from near surface sediments did not show methane production. At the site of U1328, where gas hydrate was observed from surface to 50-m below sediment, culture experiments showed no methane production in the shallower sediments, but some methane production was detected in the deeper sediments. Results of the tracer experiments showed both carbonate reduction and acetate fermentation pathway would be employed for methane production. We detected activity of methanogenesis in the sediments from the shallower sediments, as well as the samples from gas hydrate zone and the deeper sediments. These results would provide an important clue for constraining formation model for gas hydrate.
OS23A-1063
Geologic Control on Porewater Geochemistry in Gas Hydrate Bearing Sediments in the Gulf of Mexico
Sedimentary geologic features such as faults, fissures and salt diapirs impact vertical and lateral fluid flow, and hence, methane advection in gas hydrate bearing marine sediments. High resolution, geochemical characterization, (piston cores) and geophysical (seismic) surveys were conducted in the Atwater Valley and Alaminos Canyon regions of the Gulf of Mexico in 2005 and 2007 respectively, in order to constrain fluid and gas flux in these areas. Porewater sulfate, chloride, methane and DIC concentrations as well as stable carbon isotope ratios of DIC confirm that at both sites geochemistry is largely influenced by local geology. Specifically, at locations in Alaminos Canyon where faults or fissures are evident in seismic data, porewater geochemistry appears to be controlled by advective processes. In the case of Atwater Valley, high methane vertical flux rates calculated from concentration data plotted against depth are significantly correlated to increasing porewater salinity likely resulting from salt diapirs which reduce gas hydrate stability. By contrast to these examples, sedimentary geochemical properties (for example) of cores collected distal to the faults, fissures or diapirs described above appear to be diffusion-dominated. Seismic data from the two study areas shows a large difference the scale, frequency and occurrence of geologic features that could influence fluid flow and gas fluxes. Therefore, accurate, spatially-normalized methane flux estimates for comparison between the sites depend largely on adequate representation of local geology. Results from these studies suggest that targeted, high- resolution geophysical and geochemical characterization of the local geologic environment is critical to understanding fluid and methane flux and estimating methane hydrate distributions in gas hydrate bearing marine sediments.
OS23A-1064
Trace Element Analysis as an Exploration Tool for Unconfined Class 2 Hydrate Deposits
Hydrate system analysis, analogous to hydrocarbon system analysis, is based on confirming significant gas flux, suitable thickness of hydrate stability zone, and suitably porous and permeable hydrate trap' beds in which economically significant hydrate crystallization may take place. Unconfined Class 2 hydrate deposits, which form by crystallization of dissolved natural gas in more porous and permeable sediments whose pore fluids may vent from the seafloor carry the dissolved rejected material. Analysis of superficial pore or vent water may provide a sensitive means of suggesting whether the water was once associated with hydrate crystallization. The rejection of dissolved ions and compounds during carbon dioxide hydrate precipitation on a chilled surface was determined to quantify the reject rate of specific materials from the hydrate matrix. Seawater and seawater doped with boric acid (H3BO3, MW 61.83 g/mol) and sodium borate decahydrate (Na2B4O7*10H2O, MW 381.37 g/mol) were used as crystallization solvents. Initial boron concentrations ranged from 4.1 to 27mg/L. Hydrate formation occurred with significant rejection of boron from the hydrate matrix, increasing the concentration in the crystallizing fluid. The initial boron concentration level did not affect the rejection efficiency, determined by comparing the initial concentration of boron to the concentration in the hydrate melt water. In addition to boron, naturally occurring calcium and magnesium concentration levels were also studied and showed a similar concentration reduction. Our experiments have shown that non-ionic dissolved species that occur in sediment pore water, in which hydrate may form, are rejected and concentrated during hydrate formation. The measurement of NaCl may provide a clue to the water having once been involved in subjacent hydrate formation, but other species that do not have additional crystallization opportunities in the sediment may be better clues to the extent of a hydrate formation. Identification of characteristic indicators of hydrate system participation would allow rapid sampling and analysis of seafloor water samples to be used as an exploration tool.
OS23A-1065
Exploitation of methane hydrate using exothermic heat of CO2 hydrate formation - Verification for performance of CO2 as warming material for marine sediments -
Methane hydrate is found in marine sediments and permafrost. But methane hydrate is solid and it doesnft flow. Then some dissociation methods of methane hydrate are proposed such as heating and depressurization. If carbon dioxide (CO2) can be used for dissolving methane hydrate in marine sediments, we could obtain natural gases with less impact for the global warming. Thus we focus on exothermic heat of CO2 hydrate formation to warm marine sediments. Then we proposed an injection method to disperse CO2 homogeneously into the sediments using CO2/water emulsion for controlling quantity of heat per unit volume. Using the emulsion, the performance of CO2 as warming material was about 9 degree centigrade to rise the temperature of marine sediments by a laboratory test and its numerical estimation. This performance of CO2 is effective to dissolve methane hydrate in marine sediments.
OS23A-1066
Calibration and analysis of methane hydrate beneath Alaska's North Slope using spectral decomposition of 3-D seismic reflection data
A primary difficulty for seismic interpretation of methane hydrates in regions with permafrost is that permafrost has similar acoustic properties to gas hydrate and is observed intermingling with hydrate-bearing strata in well logs and core samples. Post-stack processing of 3-D onshore seismic reflection data from the area near Milne Point, Alaska included predictive deconvolution to remove mostly short-period multiples and wavelet deconvolution to enhance frequency content; together these processes significantly enhanced data quality. However, spectral analysis of this enhanced data reveals a dominant frequency around 50 Hz and average tuning thicknesses for the survey ranging from 8-15 m, which may compromise interpretation of thin (1-8 m) hydrate intervals and small-offset faults. No direct hydrate indicators, such as polarity reversals, are evident in the data, which may be due to resolution issues. Although amplitude anomalies do not appear to directly correlate with hydrate occurrence in the data area, lateral variations in the waveform (from automated waveform analysis) appears to show a better correlation. Recent drilling, coring and logging within the survey area confirmed the existence of methane hydrate, and provided an opportunity to calibrate hydrate occurrence with the seismic response. In this analysis, in addition to conventional correlations and seismic interpretations, spectral decomposition was used to evaluate the effects of methane hydrate and permafrost zones on tuning responses at different frequencies, and provides a new tool for evaluation of methane hydrate accumulations.
OS23A-1067
Effects of Seafloor Temperature on the Distribution of Methane Hydrate
Deep ocean temperatures were 10-15 deg C warmer than present-day during the Early Cretaceous and Early Paleogene. Such temperatures would impact the distribution of gas hydrate in marine sediment. Clearly, the vertical extent of the Gas Hydrate Stability Zone (GHSZ) and the overall volume of sediment hosting gas hydrates at shallow water depths would be smaller than at present-day. Several authors have taken this to mean that overall amounts of gas hydrate and methane in marine sediments were much smaller in ancient warm oceans. However, this inference may be incorrect. In any case, it has not been appropriately evaluated. We have developed a one-dimensional numerical model that describes the formation and distribution of methane hydrate in marine sediment on geological time scales. Here we modify this model to examine the effect of changing seafloor temperature from 3 to 18 deg C in cases where microbial activity supplies most of the methane. Predictably, the temperature increase shifts the methane solubility curve in marine sediment and decreases the depth of the GHSZ. Less obvious but more important are temperature effects on the flux of seafloor organic carbon and the rate of methanogenesis. In some cases, increased seafloor temperature results in decreased amounts of methane hydrate. However, in other simulations, when seafloor organic fluxes and biogenic reaction rates increase significantly, amounts of methane hydrate can be higher than modeled for the present-day. It is possible that, during times of warm oceans, greater amounts of organic carbon enter the seafloor, microbes make methane from this carbon at much faster rates, and gas hydrate quantities exceed those at present-day. These somewhat counter-intuitive results may help to explain certain observations during warm climates.