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1.
Hydro-thermo-chemo and mechanically coupled processes determine hydrate morphology and control gas production from hydrate-bearing sediments. Force balance, together with mass and energy conservation analyses anchored in published data provide robust asymptotic solutions that reflect governing processes in hydrate systems. Results demonstrate that hydrate segregation in clayey sediments results in a two-material system whereby hydrate lenses are surrounded by hydrate-free water-saturated clay. Hydrate saturation can reach ≈2% by concentrating the excess dissolved gas in the pore water and ≈20% from metabolizable carbon. Higher hydrate saturations are often found in natural sediments and imply methane transport by advection or diffusion processes. Hydrate dissociation is a strongly endothermic event; the available latent heat in a reservoir can sustain significant hydrate dissociation without triggering ice formation during depressurization. The volume of hydrate expands 2-to-4 times upon dissociation or CO2CH4 replacement. Volume expansion can be controlled to maintain lenses open and to create new open mode discontinuities that favor gas recovery. Pore size is the most critical sediment parameter for hydrate formation and gas recovery and is controlled by the smallest grains in a sediment. Therefore any characterization must carefully consider the amount of fines and their associated mineralogy.  相似文献   

2.
Hydrocarbon gases were determined in sediments from three mud volcanoes in the Sorokin Trough. In comparison to a reference station outside the mud volcano area, the deposits are characterized by an enrichment of high-molecular hydrocarbons (C2–C4), an absence of unsaturated homologues, a predominance of iso-butane in comparison with n-butane, and the presence of gas hydrate. The molecular composition of the hydrocarbon gases suggests their deep sources and thermogenic origin. In the pelagic sediments at the reference station, the methane concentration is relatively low (up to 49 ml/l); maximum concentrations are reached in deposits of the Dvurechenskii mud volcano (up to 400 ml/l). It was the first time that gas hydrate was sampled at the Dvurechenskii mud volcano. The gas extracted by dissociation of hydrate samples was dominated by methane (99.5%) with low amounts of ethane and propane (less than 0.5%). The isotopic composition of the methane varies between –62 and –66 PDB in 13C, and between –185 and –209 SMOW in D, indicating a mainly biogenic origin with an admixture of thermogenic gas.  相似文献   

3.
Geochemical properties of gas hydrate accumulation associated with an active gas vent on the continental slope offshore northeast Sakhalin Island in the Sea of Okhotsk have been investigated. The pore water chemistry data suggest that the gas hydrates (GHs) were formed in an environment of upward-migrating fluid combined with a mechanism of pore water segregation. The upward infiltration of water enriched mainly by Cl– and K+ species appears to occur on the background of earlier diagenesis processes within the gas vent sediments. The GHs were formed from water with chlorinity ranging from 530 to 570 mM. The 18O and D of GH water varied from –1.4 to –1.8 and from –13 to –18, respectively, representing a mix of seawater and infiltrating fluid water. A complex interaction of pore water, water of ascending fluid and segregated pore water during hydrate formation is also supported by water content measurements and observed gas hydrate structure. The direction of segregated water is opposite to upward fluid migration. Decreasing activity of the gas vent is inferred by comparing the present top of the recovered hydrate layer with previous observations.  相似文献   

4.
In seismic sections, the presence of a gas hydrate stability zone (GHSZ) is often marked by a bottom-simulating reflector, which has a negative polarity with respect to the seafloor. The present study reveals the response of seismic wave characteristics and amplitude versus offset (AVO) effects of large offset compressional (P) and converted (PS) waves for a GHSZ/free-gas configuration, using a two-dimensional elastic anisotropic modelling technique. The modelling results would provide a priori information, which allows unique determination of parameters of seismic models for the design of ocean bottom seismometer experiments over continental margins for the purpose of gas hydrate exploration. The AVO analyses on long offset P and PS waves based on synthetic data yield a typical gas hydrate/free-gas response, as the reflectivity increases with incidence angle.  相似文献   

5.
The BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well was drilled and cored from 606.5 to 760.1 m on the North Slope of Alaska, to evaluate the occurrence, distribution and formation of gas hydrate in sediments below the base of the ice-bearing permafrost. Both the dissolved chloride and the isotopic composition of the water co-vary in the gas hydrate-bearing zones, consistent with gas hydrate dissociation during core recovery, and they provide independent indicators to constrain the zone of gas hydrate occurrence. Analyses of chloride and water isotope data indicate that an observed increase in salinity towards the top of the cored section reflects the presence of residual fluids from ion exclusion during ice formation at the base of the permafrost layer. These salinity changes are the main factor controlling major and minor ion distributions in the Mount Elbert Well. The resulting background chloride can be simulated with a one-dimensional diffusion model, and the results suggest that the ion exclusion at the top of the cored section reflects deepening of the permafrost layer following the last glaciation (∼100 kyr), consistent with published thermal models. Gas hydrate saturation values estimated from dissolved chloride agree with estimates based on logging data when the gas hydrate occupies more than 20% of the pore space; the correlation is less robust at lower saturation values. The highest gas hydrate concentrations at the Mount Elbert Well are clearly associated with coarse-grained sedimentary sections, as expected from theoretical calculations and field observations in marine and other arctic sediment cores.  相似文献   

6.
程俊  王淑红  黄怡  颜文 《海洋科学》2019,43(5):110-122
综述了天然气水合物赋存区甲烷渗漏活动的地球化学响应指标的研究进展,分析了应用单一指标识别甲烷渗漏活动各自所存在的问题,包括浅表层沉积物孔隙水中CH_4、SO_4~(2–)、Cl~–等离子浓度随深度的变化;浅层沉积物全岩W_(TOC)(W表示质量分数,TOC表示总有机碳)和W_(TS)(TS表示总硫)之间的相关性及比值;自生碳酸盐岩δ~(13)C和δ~(18)O;自生矿物重晶石、黄铁矿、自生石膏的δ~(34)S;有孔虫壳体和生物标志化合物的δ~(13)C等。结果表明孔隙水中的CH_4、SO4_~(2–)浓度及溶解无机碳的碳同位素组成可以用来识别目前正在发生的甲烷渗漏活动;而沉积物中的WTS、自生矿物的δ~(34)S、钡含量及其异常峰值和生物标志化合物的δ~(13)C等指标的联合使用可以更真实准确地反映地质历史时期天然气水合物赋存区的甲烷渗漏活动。因此,在实际研究过程中,可将孔隙水和沉积物两种介质的多种指标相结合。随着非传统稳定同位素(Fe、Ca、Mg等)和沉积物氧化还原敏感元素(Mo、V、U等)等研究的发展,甲烷渗漏活动地球化学响应指标的研究也将得到拓展,而多种地球化学指标的联合使用将为天然气水合物勘探及其形成分解过程识别研究提供重要的科学依据。  相似文献   

7.
《Marine and Petroleum Geology》2012,29(10):1856-1869
We have implemented a 2-dimensional numerical model for simulating gas hydrate and free gas accumulation in marine sediments. The starting equations are those of the conservation of the transport of momentum, energy, and mass, as well as those of the thermodynamics of methane hydrate stability and methane solubility in the pore-fluid. These constitutive equations are then integrated into a finite element in space, finite-difference in time scheme. We are then able to examine the formation and distribution of methane hydrate and free gas in a simple geologic framework, with respect to the geothermal heat flow, fluid flow, the methane in-situ production and basal flux. Three simulations are performed, leading to the build up of hydrate emplacements largely linear through time. Models act primarily as free gas accumulators and are relatively inefficient with respect to hydrate emplacements: 26–33% of formed methane are converted to hydrate. Seepage of methane across the sea-floor is negligible for fluid flow below 2. 10−11 kg/m2/s. At 5.625 10−11 kg/m2/s however, 9.7% of the formed methane seeps out of the model. Moreover, along strike variation arising in the 2-dimensional model are outlined. In the absence of focused flow, the thermodynamics of hydrate accumulation are primarily one-dimensional. However, changes in free methane compressibility (density) and methane solubility (the intrinsic dissolved methane flux) subtlety impact on the formation of a free gas zone and the distribution of the hydrate emplacements in our 2-dimensional simulations.  相似文献   

8.
We have implemented a 2-dimensional numerical model for simulating gas hydrate and free gas accumulation in marine sediments. The starting equations are those of the conservation of the transport of momentum, energy, and mass, as well as those of the thermodynamics of methane hydrate stability and methane solubility in the pore-fluid. These constitutive equations are then integrated into a finite element in space, finite-difference in time scheme. We are then able to examine the formation and distribution of methane hydrate and free gas in a simple geologic framework, with respect to the geothermal heat flow, fluid flow, the methane in-situ production and basal flux. Three simulations are performed, leading to the build up of hydrate emplacements largely linear through time. Models act primarily as free gas accumulators and are relatively inefficient with respect to hydrate emplacements: 26–33% of formed methane are converted to hydrate. Seepage of methane across the sea-floor is negligible for fluid flow below 2. 10−11 kg/m2/s. At 5.625 10−11 kg/m2/s however, 9.7% of the formed methane seeps out of the model. Moreover, along strike variation arising in the 2-dimensional model are outlined. In the absence of focused flow, the thermodynamics of hydrate accumulation are primarily one-dimensional. However, changes in free methane compressibility (density) and methane solubility (the intrinsic dissolved methane flux) subtlety impact on the formation of a free gas zone and the distribution of the hydrate emplacements in our 2-dimensional simulations.  相似文献   

9.
The Dongsha area is one of the most promising target areas for gas hydrate exploration in the South China Sea(SCS).The study of pore water geochemistry has played a key role in Chinese gas hydrate exploration.Br/Cl,I/Cl and δ37Cl in pore water were applied here in tracing gas hydrate occurrence,chemical evolution of pore fluids and water/rock interactions in low temperature sediment environments.The samples were collected from Sites HD255 PC and HD309 PC in the Dongsha area in 2004.At Site HD255 PC,we found the elevated Br/Cl,I/Cl and decreased SO_4/Cl at the depth of 4–5 m,suggestive of a laterally migrated fluid probably generated from the gas hydrate occurrence.The range of δ37Cl is –0.54‰ to +0.96‰,and positive δ~(37)Cl at 4–5 m interval should be related with different diffusion rates between ~(35)Cl and ~(37)Cl.At Site HD309 PC,a laterally migrated fluid was also found at the depth of 3–4 m,with the Br/Cl two times to that of the seawater and decreased I/Cl,indicating the fluid has no relationship with the gas hydrate.In this site,the chlorine isotopic composition varies from –0.7‰ to+1.9‰.Extra high Br/Cl might relate with the deep generated fluid.At higher temperature and pressure,the Br/Cl of the fluid is elevated during the hydrous silicate formation,while positive δ37Cl is also associated with the same mechanism.  相似文献   

10.
The chemical states of iron in near-shore and deep-sea sediments were investigated by means of57Fe Mössbauer spectroscopy in combination with selective and nonselective chemical leachings. As far as a limited number of the sediments analyzed are concerned, Mössbauer spectra of near-shore sediments consist of high-spin paramgnetic ferrous (=1.13 mm/s, Eq=2.65 mm/s) and paramagnetic ferric (=0.35 mm/s, Eq=0.64 mm/s) components, while those of deep-sea sediments are composed of high-spin paramagnetic ferrous, paramagnetic ferrous, paramagnetic ferric and magnetic ferric (0.4 mm/s,H510 KG) components. The Fe2+/Fe3+ ratios of deepsea sediments are much smaller than those in near-shore sediments, while the total contents of iron in the former are much higher than those in the latter. This is principally due to the high contents of authigenic ferric oxides in deep-sea sediments. Further, in the aluminosilicate fraction, the Fe2+/Fe3+ ratios of deep-sea sediments are also smaller than those of near-shore sediments. This is probably attributed to high contents of clay minerals and authigenic aluminosilicates in deep-sea sediments relative to near-shore ones. The magnetic components in deepsea sediments are attributable to hematite, magnetite and/or maghemite.  相似文献   

11.
Carbon and nitrogen stable isotope ratios (13C and 15N) of surface sediments were measured within Osaka Bay, in the Seto Inland Sea in Japan, in order to better understand the sedimentation processes operating on both terrestrial and marine organic matter in the Bay. The 13C and 15N of surface sediments in the estuary of the Yodo River were less than –23 and 5 respectively, but increased in the area up to about 10 km from the river mouth. At greater distances they became constant (giving 13C of about –20 and 15N about 6). It can be concluded that large amounts of terrestrial organic matter exist near the mouth of the Yodo River. Stable isotope ratios in the estuary of the Yodo River within 10 km of the river mouth were useful indicators allowing study of the movement of terrestrial organic matter. Deposition rates for total organic carbon (TOC) and total nitrogen (TN) over the whole of the Bay were estimated to be 63,100 ton C/year and 7,590 ton N/year, respectively. The deposition rate of terrestrial organic carbon was estimated to be 13,200 (range 2,000–21,500) ton C/year for the whole of Osaka Bay, and terrestrial organic carbon was estimated to be about 21% (range 3–34) of the TOC deposition rate. The ratio of the deposition rate of terrestrial organic carbon to the rate inflow of riverine TOC and particulate organic carbon (POC) were estimated to be 19% (range 3–31) and 76% (range 12–100), respectively.  相似文献   

12.
The apparent stability constants for chloride and sulfate ions with Co (II) at ionic strength of 0.67 were determined by the cation exchange method. The value of the stability constant 1 for chloride ion with Co (II) ion was 0.79±0.055. The stability constants 1, 2 and 3 for sulfate ion with Co (II) ion were 12.0±0.27, 91.5±11.4 and l,110±250, respectively. The chemical species of Co (II) in seawater was estimated at the pH of 8.0 to be present as Co2+ (63%), CoCl+ (27%) and CoSO4 0 (8.6%) using the known value of dissociation constant of Co (II), and under the assumption that only major inorganic anions are responsible for the chemical equilibrium of Co (II).  相似文献   

13.
Ten gas-vent fields were discovered in the Okhotsk Sea on the northeast continental slope offshore from Sakhalin Island in water depths of 620—1040 m. At one vent field, estimated to be more than 250 m across, gas hydrates, containing mainly microbial methane (13C = –64.3), were recovered from subbottom depths of 0.3–1.2 m. The sediment, having lenses and bedded layers of gas hydrate, contained 30–40% hydrate per volume of wet sediment. Although gas hydrates were not recovered at other fields, geochemical and thermal measurements suggest that gas hydrates are present.  相似文献   

14.
Microbial Diversity in Nankai Trough Sediments at a Depth of 3,843 m   总被引：6，自引：0，他引：6  
Dense populations of bivalves, primarily Calyptogena sp., were observed at cold seeps of the Nankai Trough. Bacterial input to the sediment was estimated through determination of phospholipid ester-linked fatty acid (PLFA) and DNA profiles. Results indicated a bacterial biomass of 109 cells (g dry wt)-1 while individual fatty acid profiles revealed a predominance of monounsaturated fatty acids, mainly 18:1 isomers. The presence of these fatty acids can be interpreted to reflect a response to low temperature and a predominance of psychrophilic bacteria. DNA fragments encoding bacterial ribosomal RNA small-subunit sequences (16S rDNA) were amplified by the polymerase chain reaction method using DNA extracted directly from the sediment samples. From the sequencing results, at least 19 kinds of bacterial 16S rDNAs related to mostly the Proteobacteria and a few gram-positive bacteria were identified. These results suggest that the bacterial community in the Nankai Trough sediments consists of mainly bacteria belonging to the Proteobacteria , , and subdivisions. Bacteria belonging to the and subdivisions, which are known to include epibiont and sulfate reducing bacteria, respectively, were mostly detected in the sediment obtained from inside the area of the Calyptogena community, and the -Proteobacteria may function to supply reduced sulfur to bacterial endosymbionts of Calyptogena.  相似文献   

15.
Fossil methane from the large and dynamic marine gas hydrate reservoir has the potential to influence oceanic and atmospheric carbon pools. However, natural radiocarbon (14C) measurements of gas hydrate methane have been extremely limited, and their use as a source and process indicator has not yet been systematically established. In this study, gas hydrate-bound and dissolved methane recovered from six geologically and geographically distinct high-gas-flux cold seeps was found to be 98 to 100% fossil based on its 14C content. Given this prevalence of fossil methane and the small contribution of gas hydrate (≤ 1%) to the present-day atmospheric methane flux, non-fossil contributions of gas hydrate methane to the atmosphere are not likely to be quantitatively significant. This conclusion is consistent with contemporary atmospheric methane budget calculations.In combination with δ13C- and δD-methane measurements, we also determine the extent to which the low, but detectable, amounts of 14C (~ 1–2% modern carbon, pMC) in methane from two cold seeps might reflect in situ production from near-seafloor sediment organic carbon (SOC). A 14C mass balance approach using fossil methane and 14C-enriched SOC suggests that as much as 8 to 29% of hydrate-associated methane carbon may originate from SOC contained within the upper 6 m of sediment. These findings validate the assumption of a predominantly fossil carbon source for marine gas hydrate, but also indicate that structural gas hydrate from at least certain cold seeps contains a component of methane produced during decomposition of non-fossil organic matter in near-surface sediment.  相似文献   

16.
The downward short- and long-wave radiation fluxes at the sea surface (S, L) were measured aboard the R/VHakuho Maru, University of Tokyo, for the period of 117 days on six cruises from 1981 to 1985 in the western North Pacific near Japan. The upward fluxes of short- and long-wave radiation (S, L) were calculated by Payne's (1972) table and the Stefan-Boltzmann's law, respectively. The sensible and laten heat fluxes (Q h ,Q e ) were also estimated from an aerodynamic bulk method.From April to August, the daily mean value ofS varied with the amplitude of 100200 Wm–2. The value ofS was estimated approximately 6% ofS in all seasons. The difference betweenL andL was so small that the net radiation flux (Q n ) was dominated byS. In addition, the net heat flux at the sea surface was also dominated byS due to small values ofQ h andQ e , and then the ocean was warmed at the rate of 111 Wm–2 in April and 63 Wm–2 in August in the Oyashio Area, and 132 Wm–2 in May and 164 Wm–2 in June in the Kuroshio Area, respectively.From September to March, a remarkable negative correlation between the day to day variation ofS and that ofL was observed except when an intense cold air outbreak occurred. It was found that the correlation was caused by the cloud climatological feature of the western North Pacific in this period.S was not a dominant factor in the net heat flux. The value ofQ h +Q e in the Kuroshio Area ranged from 260 Wm–2 to 630 Wm–2, much larger thanQ n which ranged from –8 Wm–2 to 92 Wm–2 in the leg mean values (each leg period was about 10 days). Then the ocean was cooled at the rate of –160–620 Wm–2 during this period. The net heat flux in the Kuroshio Area averaged over five legs from late November to February was –473 Wm–2. This value is 50100% larger than the climatological values reported so far.The temporal and spatial variability of radiation fluxes and heat fluxes during each leg was also discussed.  相似文献   

17.
Methane investigations carried out in the Okhotsk Sea show that the methane flux from the earths interior into the water column increased during periods of seismo-tectonic activity between 1988 and 2002. In this case, methane gas hydrates found on the northeast Sakhalin slope may have decomposed due to a reactivation of fault zones. Methane emissions in the Okhotsk Sea generally can be divided into two forms. Firstly, methane vents from decomposing gas hydrates and/or free gas exist below gas hydrate saturated sediments via fault zones, venting into the water column with high bubble concentrations that were recorded by echosounding. These hydro-acoustic anomalies were named flares. Methane concentration inside these flares reached 10,000–20,000 nl/l (background methane concentrations in the Okhotsk Sea are less than 90–100 nl/l). Secondly, methane migrates as seepage into the water column from oil- and gas-bearing sedimentary source rocks on the eastern Sakhalin shelf, without showing acoustic anomalies in the water column, probably by filtration and diffusion processes. In these areas methane concentration reached 500–3,000 nl/l. In seismo-tectonically active regions, like the northwestern part of the Okhotsk Sea, many new flares were observed. Their distribution and orientation are usually controlled by fault zones (East Sakhalin Shear Zone in the Okhotsk Sea).  相似文献   

18.
Distributions of dissolved nutrients and Chl. a were investigated in the Sangga Besar River Estuary in the well-managed Matang Mangrove Forest in West Malaysia. In the estuary, spring tide concentrations of ammonium, silicate and phosphate were higher than those in the neap tide, which suggests that these nutrients are flushed from the mangrove area by the inundation and tidal mixing of the spring tide. Ammonium comprised over 50% of the dissolved inorganic nitrogen in the spring tide, while nitrite tended to dominate in the neap tide, indicating the predominance of nitrification inside the estuary in neap tides. Nutrient concentrations in the creek water were higher than those of estuarine water, indicating the nutrient outwelling from the mangrove swamp and ammonium regeneration from mangrove litter in the creek sediments. The maximum concentration of Chl. a in spring tides reached 80 g/l while it was below 20 g/l in the neap tides. These variations in the phytoplankton biomass and nutrients probably reflect the greater nutrient availability in the spring tide due to outwelling from the mangrove swamp and creek.  相似文献   

19.
 On the southwestern Barents Sea shelf, sediments containing gas hydrates that overlie free gas have been inferred from multichannel seismic data. The volume of suspected gas hydrate is tentatively estimated to about 1.9×108 m3. The gas hydrate zone probably formed from thermogenic gas leaking from a deeper source. The hydrate zone may have thickened during the Neogene by including gas originally trapped as free gas below the hydrate following a significant downward migration of the isotherms caused by erosion and/or subsidence. Within the present oceanographic conditions, gas hydrate is suspected to be stable or slowly decomposing. Received: 20 December 1996 / Revision received: 20 August 1997  相似文献   

20.
Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C1/C2+ from all the seepage sites were in the range of 1,500–50,000, while δ13C and δD values of methane ranged from ?66.0 to ?63.2‰ VPDB and ?204.6 to ?196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. δ13C values of ethane and propane (i.e., ?40.8 to ?27.4‰ VPDB and ?41.3 to ?30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ13C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.  相似文献   

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