origin of natural gases and associated gas hydrates in the shenhu area northern south china sea results from the china gas hydrate drilling expeditions abstract geochemical data for hydrate gases acquired from the gmgs3 and gmgs4 gas hydrate drilling expeditions conducted by the guangzhou marine geological survey gmgs are used to explore the origin of hydrate gases and their relationship to deep hydrocarbon reservoirs and to evaluate the contribution of different genetic types of gases to the formation and accumulation of gas hydrates in the shenhu area of the northern part of the south china sea scs compositionally methane is the dominant gas 90 in the void gas and pressure core gas in addition as much as 3 of the gas is composed of c2 hydrocarbons including ethane propane iso butane butane iso pentane and n pentane the 13c ch4 and d ch4 values indicate a mixed biogenic thermogenic origin for the hydrate forming gas the methane isotope correlation indicates that the source of the hydrate gas is closely related to the deep conventional gas reservoirs discovered in the baiyun sag panyu low uplift area both the hydrate gases and the deep reservoir gases are sourced from the hydrocarbon kitchens in the baiyun sag revealing a paragenetic relationship within the same petroleum system the composition of the hydrocarbons and the isotopic variation of methane with depth suggest that the thermogenic gas was likely affected by compositional and isotopic fractionation during the long distance migration from the deep source rocks to the shallow gas hydrate stability zone ghsz the impact of biodegradation on a solely thermogenic gas could also affect the final composition of the hydrate forming gas analysis of the ghsz based on gas hydrate compositions suggests that the occurrence of thermogenic gas could also indicate the coexistence of structure i si and structure ii sii gas hydrates in the shenhu area with the sii hydrates accumulating in or below the lower part of the si ghsz the confirmed presence of sii hydrates in the shenhu area relocated the base of the ghsz deeper than was indicated by the bottom simulating reflector which warrants further study in future explorations for gas hydrates in the shenhu area

origin distribution and alteration of asphalts at chapopote knoll southern gulf of mexico abstract following the discovery of asphalt volcanism in the campeche knolls a research cruise was carried out in 2006 to unravel the nature of the asphalt deposits at chapopote the novel results support the concept that the asphalt deposits at the seafloor in 3000 m of water depth originate from the seepage of heavy petroleum with a density slightly greater than water the released petroleum forms characteristic flow structures at the seafloor with surfaces that are ropy or rough similar to magmatic lava flows the surface structures indicate that the viscosity of the heavy petroleum rapidly increases after extrusion due to loss of volatiles consequently the heavy petroleum forms the observed asphalt deposit and solidifies detailed survey with a remotely operated vehicle revealed that the asphalts are subject to sequential alterations e g volume reduction leading to the formation of visible cracks in the asphalt surface followed by fragmentation of the entire deposit while relatively fresh asphalt samples were gooey and sticky older fragmented pieces were found to be brittle without residual stickiness furthermore there is evidence for petroleum seepage from below the asphalt deposits leading to local up doming and sometimes to whip shaped extrusions extensive mapping by tv guided tools of chapopote asphalt volcano indicates that the main asphalt deposits occur at the south western rim that borders a central crater like depression the most recent asphalt deposit at chapopote is the main asphalt field covering an area of 2000 m2 asphalt volcanism is distinct from oil and gas seepage previously described in the gulf of mexico and elsewhere because it is characterized by episodic intrusions of semi solid hydrocarbons that spread laterally over a substantial area and produce structures with significant vertical relief as chapopote occurs at the crest of a salt structure it is inferred that asphalt volcanism is a secondary result of salt tectonism

distribution and characteristics of natural gas hydrates in the shenhu sea area south china sea abstract structure type gas composition and cage occupancy are important gas hydrate characteristics which play an important role in calculating gas hydrate stability and estimating the amount of global hydrate bound methane china conducted the fourth gas hydrate drilling campaign gmgs4 in the shenhu sea area ssa of the south china sea in 2016 during which comprehensive logging while drilling lwd data and natural gas hydrate samples were obtained the lwd data shows high electrical resistivity and high sound velocity at various depth indicating the occurrence of gas hydrate in this area coexistence of structure i si and structure ii sii gas hydrates in the south china sea was confirmed for the first time by using raman spectra analysis the methane cage occupancies of large cages and small cages in si hydrate are 0 95 and 0 96 respectively the hydration number of si hydrate is 6 03 gas composition analysis shows that methane 98 7 is the dominate gas with small amount of heavier hydrocarbon gases ethane 0 006 0 976 propane 0 033 0 216 isobutane 0 004 0 047 and other non hydrocarbon gases carbon dioxide

visual observation of gas hydrate formation and dissociation in synthetic porous media by means of glass micromodels visual observation of gas hydrates at the microscopic scale in synthetic porous media provides unequivocal visual evidence that clathrates can form in systems without the presence of a free gas phase hydrates were formed from a soluble liquid hydrate former tetrahydrofuran c 4 h 8 o from free gas ch 4 and from dissolved gas co 2 clathrates were found to form within the center of pore spaces rather than on grain surfaces cementation of grains only occurred in regions of a small grain size or where a large proportion of pore space was filled with hydrate however even at high clathrate saturations a thin film of free water persisted on grain surfaces the results have important implications for the potential cementing effect of hydrates on sediments and thus for sediment permeability slope stability and seismic interpretation of hydrate bearing sediments

complex gas hydrate system in a gas chimney south china sea abstract gas hydrates and deep sea cold seeps are of interest to industry and academia for several reasons including their implications for offshore geohazards and unconventional resources and for global climate change and carbon cycling guangzhou marine geological survey gmgs implemented its fifth gas hydrate drilling expedition in the qiongdongnan sea area qdn sa of the northern south china sea scs from june to september 2018 multidisciplinary investigations including logging while drilling lwd pressure and non pressure coring gas composition analyses in situ temperature and permeability measurements pore water geochemical analyses and a regional seismic survey reveal a complex distribution of gas hydrate within a vertical chimney structure lwd profiles record high resistivity anomalies and high p wave velocity variations at 9 174 mbsf suggesting the coexistence of free gas and gas hydrate in the sediment column throughout the investigated gas chimney hydrate bound and void gas is an admixture of methane 70 1 98 3 and heavier hydrocarbons c2h6 0 16 17 59 c3h8 0 05 9 88 i c4h10

a conceptual model for hydrocarbon accumulation and seepage processes around chapopote asphalt site southern gulf of mexico from high resolution seismic point of view 1 as part of the german r v meteor m67 2 expedition in 2006 to the southern gulf of mexico a set of 2d high resolution seismic profiles was acquired across the chapopote knoll to study seafloor asphalt occurrences on the basis of regional seismostratigraphic studies and dsdp drilling a more highly reflective coarse grained sediment unit of late miocene age is identified as a potential shallow hydrocarbon reservoir although a unit of that age would typically be buried by pliocene and pleistocene sediment cover at chapopote local salt tectonism has highly elevated the structure since the late miocene and the miocene reservoir is locally above present day regional datum the elevation resulted in a thin 100 200 m fine grained sediment cover on the crest of the knoll above the reservoir because oil and gas production can be expected at depth in jurassic cretaceous and tertiary source rocks the presence of high amplitude reflector packages within the reservoir unit is interpreted as an evidence of hydrocarbons this is variously supported by observations of crosscutting reflectors polarity reversal and drops in instantaneous frequency the thin seal above the reservoir unit facilitates leakage of trapped hydrocarbons hydrocarbons apparently invaded into the seal sediments in the wider vicinity around the crest of the knoll even extending beyond the area where seafloor asphalt is known the asphalt site thus may be a currently active spot while the rest of the crest may be temporarily sealed by solid phase hydrocarbons we propose that a shallow large reservoir with deeply sourced relatively heavy petroleum is principally responsible for the formation of asphalts on the seafloor

architecture of the crust and uppermost mantle in the northern canadian cordillera from receiver functions the northern canadian cordillera ncc is an active orogenic belt in northwestern canada characterized by deformed autochtonous and allochtonous structures that were emplaced in successive episodes of convergence since the late cretaceous seismicity and crustal deformation are concentrated along corridors located far 200 to 800 km from the convergent plate margin proposed geodynamic models require information on crust and mantle structure and strain history which are poorly constrained we calculate receiver functions using 66 broadband seismic stations within and around the ncc and process them to estimate moho depth and p to s velocity ratio vp vs of the cordilleran crust we also perform a harmonic decomposition to determine the anisotropy of the subsurface layers from these results we construct simple seismic velocity models at selected stations and simulate receiver function data to constrain crust and uppermost mantle structure and anisotropy our results indicate a relatively flat and sharp moho at 32 2 km depth and crustal vp vs of 1 75 0 05 seismic anisotropy is pervasive in the upper crust and within a thin 10 15 km thick sub moho layer the modeled plunging slow axis of hexagonal symmetry of the upper crustal anisotropic layer may reflect the presence of fractures or mica rich mylonites the subhorizontal fast axis of hexagonal anisotropy within the sub moho layer is generally consistent with the se nw orientation of large scale tectonic structures these results allow us to revise the geodynamic models proposed to explain active deformation within the ncc

230 th 238 u disequilibrium in island arcs evidence from the aleutians and the marianas complex models for magma generation in island arcs have recently been proposed which require varying contributions from three possible sources 1 the mantle 2 subducted altered oceanic crust and 3 sediments see for example refs 1 6 these models involve partial melting of metasomatized mantle the metasomatizing liquids derive from the subducted slab and are large ion lithophile lil element bearing fluids or are themselves partial melts further modification of the chemical composition of the lavas may occur by processes such as shallow level fractional crystallization because the th u ratio is quite different for each of the three possible magma sources mentioned above and as th and u may behave differently during fractionation processes particularly if volatile transport is involved studies of 230 th 238 u disequilibrium have great potential in investigations of arc lavas our data given here imply that the th u ratio in the sources of mariana and aleutian island arc lavas is similar to ocean ridge basalt sources although there is a hint of a somewhat lower th u component in the aleutian source the data also confirm and extend the observations by gill and allegre and condomines that oceanic arc lavas are distinguished from magmas erupted in virtually all other volcanic environments by activity ratios 230 th 238 u 1 implying relative enrichment of uranium

marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate marine pore water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the carolina rise and blake ridge u s atlantic continental margin here sulfate gradients are linear implying that sulfate depletion is driven by methane flux from below rather than by the flux of sedimentary organic matter from above thus these linear sulfate gradients can be used to quantify and assess in situ methane flux which is a function of the methane inventory below

a kinetic model for the pattern and amounts of hydrate precipitated from a gas steam application to the bush hill vent site green canyon block 185 gulf of mexico 1 we construct a linear kinetic model of hydrate crystallization from a gas stream we use this model to predict the fraction of gas that crystallizes as hydrate in the subsurface of bush hill and the depth profile of subsurface hydrate accumulation this is possible because the bush hill vent is fed by reservoir gas from the nearby jolliet field whose composition is known on the average 9 of the vent gas is precipitated as hydrate in the subsurface although other explanations are possible the observed vent gas compositions and the greater range of hydrate gas compositions are consistent with a single source gas whose venting rate varies by a factor of at least 3 over periods of a few years or less the predicted depth profile of hydrate accumulation and the hydrate content of the bush hill mound suggest that between 1 1 109 and 2 8 109 m3 stp of gas may have accumulated as hydrate between the seafloor and 614 m depth for the radiometrically and geologically suggested system age of 10 000 years the time average venting rate is 106 m3 yr 0 7 106 kg yr if distributed evenly across the 600 m diameter mound as suggested by echo sounder images the methane flux is 3 2 kg m2 yr this is 103 times that inferred for hydrates associated with bottom simulating seismic reflectors the subsurface hydrate accumulation and the cumulative methane venting are related we show how both may be estimated from measurements of vent gas composition bottom water temperature and geothermal gradient

natural gas hydrate investigations by synchrotron radiation x ray cryo tomographic microscopy srxctm 1 we report the 3d microstructure analyses of natural gas hydrates sampled from gulf of mexico the samples were characterized by synchrotron radiation x ray cryo tomographic microscopy srxctm using the tomcat beam line at the swiss light source sls the srxctm demonstrates its applicability to unlock some microscopic features of the marine hydrates in particular of crystallite size and grain boundary network the gas hydrate domains are surrounded by a network of pores of typically a few micrometers which are largely due to decomposition out of the srxctm data the porosity total volume of the voids the void surface area and number of the total gas filled voids have been calculated the results reveal the capability of srxctm to access the 3d microstructure which is of fundamental importance to model the petrophysical properties of natural gas hydrates

mixed gas hydrate structures at the chapopote knoll southern gulf of mexico abstract in underwater hydrocarbon seepage environments gas hydrates are considered to play a significant role as shallow gas reservoirs and buffers for light hydrocarbon expulsion here we report on mixed hydrate structures from the chapopote knoll in the southern gulf of mexico and discuss several options on how a mixture of structure i si and structure ii sii gas hydrate may occur in nature locally resolving microscopic methods are needed to characterize the coexistence of different hydrate structures at geological hydrate deposits we used raman spectroscopy x ray diffraction and gas chromatography for our investigations gas hydrates were found within the matrix and pores of the asphalts extruded at the seafloor two of the three hydrate pieces investigated comprised only si formed mostly from methane in contrast one piece comprised an intimate mixture of both si and sii with sii representing ca 25 wt and si ca 75 wt of the hydrate present the two structures were closely associated within individual grain agglomerates the crystallites of sii were significantly larger than of si suggesting differences in the nucleation density or different crystallite ages the structural coexistence may be a result of one or more processes i de mixing into two hydrate structures during the growth from the gas phase which provides an additional degree of freedom for lowering the free energy in the system ii fractionated crystallization with a subsequently changing molecular composition iii crystallization from separated gas bubbles with different hydrocarbon compositions and water and iv partial transformation from sii to si after hydrate nucleation ceasing when a thermodynamically stable state was reached the presented work will affect future assessments of natural hydrate deposits at thermogenic hydrocarbon systems as it shows that both hydrate types i and ii can be present at a certain geological site and may provide a lingering strength to the system if conditions fall below the si stability limit this suggests that present worldwide hydrate occurrences are likely to be underestimated

chemical and structural characteristics of gas hydrates from the haima cold seeps in the qiongdongnan basin of the south china sea abstract crystalline structure cage occupancy and gas composition are important gas hydrate characteristics which can be used to calculate the gas hydrate s stability and to estimate the energy potential in may and october of 2015 a guangzhou marine geological survey team investigated two sites in the haima cold seeps in the qiongdongnan basin northern south china sea they recovered massive gas hydrate samples by conventional gravity coring after the shenhu sea area and dongsha sea area the qiongdongnan basin became a new area on the northern slope of the south china sea where gas hydrates have been found in order to reveal the structural and geochemical characteristics of the natural gas hydrates samples were analyzed by micro raman spectroscopy and x ray diffraction under ambient pressure and low temperature conditions the results indicate that the gas hydrate samples from the haima cold seeps are typical structure i hydrates with a hydration number of 6 12 6 19 in addition trace amounts of h2s trapped in the hydrate were identified based on its characteristic vibrational signature the gas composition and 13c ch4 of the hydrate bound gas samples were analyzed for gas source correlation all of the gas samples are dominated by methane with small amounts of ethane and propane and had relatively light 13c ch4 indicating that all of the hydrate bound gases are mixtures of biogenic and thermogenic gas the thermogenic gas is inferred to be mainly derived from the coal layers of the late miocene pliocene yacheng formation in the northern lingshui sag

characterization of natural gas hydrate recovered from pearl river mouth basin in south china sea abstract china carried out a drilling campaign in the eastern part of the pearl river mouth prm basin and successfully obtained natural gas hydrate samples in 2013 laboratory based studies for the characteristics of gas hydrate were carried out on one of the hydrate bearing sediment sample the gas hydrate sample shows nodular occurrence with host matrix predominantly composed of silty sand and clay on the basis of the raman spectroscopic and x ray diffraction results the gas hydrate demonstrates a typical structure i si with cage occupancy of methane is more than 99 5 in large cage and 91 4 in small cage respectively corresponding to a hydration numbers of 5 90 by thermodynamic calculation the molecular compositions of hydrate bound gas suggest that the guest molecules are predominantly methane 99 9 with trace amount of ethane 0 04 and propane 0 01 the isotopic analysis indicates that the methane of hydrate bound gas of the studied drilling well in prm basin is a typical microbial origin

fate of rising methane bubbles in stratified waters how much methane reaches the atmosphere there is growing concern about the transfer of methane originating from water bodies to the atmosphere methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport this work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere the bubble model predicts the evolving bubble size gas composition and rise distance and is suitable for almost all aquatic environments the model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep oxic saline water with excellent results methane bubbles from within the hydrate stability zone typically below 500 m water depth in the ocean are believed to form an outer hydrate rim to explain the subsequent slow dissolution a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone the calibrated model explains the impressively tall flares 1300 m observed in the hydrate stability zone of the black sea this study suggests that only a small amount of methane reaches the surface at active seep sites in the black sea and this only from very shallow water areas 100 m clearly the black sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere although substantial amounts of methane may reach the surface in shallow lakes and reservoirs

grain size measurements of natural gas hydrates abstract methane hydrates are present in marine seep systems and occur within the gas hydrate stability zone very little is known about their crystallite sizes and size distributions because they are notoriously difficult to measure crystal size distributions are usually considered as one of the key petrophysical parameters because they influence mechanical properties and possible compositional changes which may occur with changing environmental conditions variations in grain size are relevant for gas substitution in natural hydrates by replacing ch 4 with co 2 for the purpose of carbon dioxide sequestration here we show that crystallite sizes of gas hydrates from some locations in the indian ocean gulf of mexico and black sea are in the range of 200 400 m larger values were obtained for deeper buried samples from odp leg 204 the crystallite sizes show generally a log normal distribution and appear to vary sometimes rapidly with location

asphalt volcanism and chemosynthetic life in the campeche knolls gulf of mexico in the campeche knolls in the southern gulf of mexico lava like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt which chilled and contracted after discharge the site also includes oil seeps gas hydrate deposits locally anoxic sediments and slabs of authigenic carbonate asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the gulf of mexico

biogeochemical controls on authigenic carbonate formation at the chapopote asphalt volcano bay of campeche abstract the campeche knolls in the bay of campeche southern gulf of mexico were investigated through detailed seafloor mapping rov surveys and sediment and pore water sampling the knolls are elongated submarine hills created by salt tectonics with a positive relief of up to 800 m above the surrounding seafloor several of the knolls are associated with sea surface oil slicks identified from satellite data indicating the presence of hydrocarbon seeps on the seafloor one of the knolls named chapopote was studied in detail during two international research expeditions so174 2 and m67 2 and is characterized by extensive hydrocarbon seepage including large asphalt flows oil and gas seeps and seafloor gas hydrate deposits chemosymbiotic biological communities and authigenic carbonate deposits are associated with the seeps and are the result of both biogeochemical turnover and the interaction between downward diffusing seawater and hydrocarbon rich pore fluids at shallow sediment depth authigenic carbonates are characterized by aragonite exhibit a porous texture and are cemented by a matrix of microsparitic to sparitic aragonite macropores of the carbonates were completely filled with liquid oil carbonate microfabrics include peloidal or clotted fabrics that may indicate the existence of microenvironments resulting from microbial metabolism banded botryoidal aragonite cements line the intra and bioclasts and incompletely fill the pore spaces the stable carbon isotopic composition of authigenic aragonite varies between 28 6 and 17 9 pdb identifying oil oxidation as the primary source of carbon to the dic pool while lipid biomarker data demonstrate the concurrent existence of microbial communities responsible for anaerobic oxidation of methane aom these observations indicate the presence of additional aom independent reactions responsible for carbon sequestration at hydrocarbon seeps and demonstrate the complexity of biogeochemical processes at seep sites in the gulf of mexico basin oxygen isotope data of authigenic aragonite vary from 2 5 to 3 8 pdb indicating carbonate precipitation in slight disequilibrium with the surrounding pore fluids

model of formation of double structure gas hydrates in lake baikal based on isotopic data 1 we measured the isotopic compositions of methane c1 and ethane c2 of hydrate bound gas and of dissolved gas in pore water retrieved from bottom sediments in lake baikal both structure i si 3 c2 and ii sii 14 c2 gas hydrates are observed in the same sediment cores in kukuy k 2 mud volcano we found that c2 d of si gas hydrate is larger than that of sii whereas c1 13c c1 d and c2 13c values are practically the same in both hydrate structures 13c of c1 and c2 of hydrate bound gas are several permil smaller than those in pore water showing that the current pore water is not the source of gas hydrates these findings lead to a new model where the sii gas hydrates were formed prior to the si hydrates

seismic evidence and formation mechanism of gas hydrates in the zhongjiannan basin western margin of the south china sea abstract an analysis of 3d seismic data from the zhongjiannan basin in the western margin of the south china sea scs reveals seismic evidence of gas hydrates and associated gases including pockmarks a bottom simulating reflector bsr enhanced reflection er reverse polarity reflection rpr and a dim amplitude zone daz the bsr mainly surrounds zhongjian island covering an area of 350 km2 in this 3d survey area the bsr area and pockmark area do not match each other where there is a pockmark developed there is no bsr the gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m a mature pockmark usually consists of an outside trough a middle ridge and one or more central pits with a diameter of several kilometers and a depth of several hundreds of meters the process of pockmark creation entails methane consumption dense faults in the study area efficiently transport fluid from large depths to the shallow layer supporting the formation of gas hydrate and ultimately the pockmark

microstructures of structure i and ii gas hydrates from the gulf of mexico gas hydrate samples from various locations in the gulf of mexico gom differ considerably in their microstructure distinct microstructure characteristics coincide with discrete crystallographic structures gas compositions and calculated thermodynamic stabilities the crystallographic structures were established by x ray diffraction using both conventional x ray sources and high energy synchrotron radiation the microstructures were examined by cryo stage field emission scanning electron microscopy fe sem good sample preservation was warranted by the low ice fractions shown from quantitative phase analyses gas hydrate structure ii samples from the green canyon in the northern gom had methane concentrations of 70 80 and up to 30 of c2 c5 of measured hydrocarbons hydrocarbons in the crystallographic structure i hydrate from the chapopote asphalt volcano in the southern gom was comprised of more than 98 methane fairly different microstructures were identified for those different hydrates pores measuring 200 400 nm in diameter were present in structure i gas hydrate samples no such pores but dense crystal surfaces instead were discovered in structure ii gas hydrate the stability of the hydrate samples is discussed regarding gas composition crystallographic structure and microstructure electron microscopic observations showed evidence of gas hydrate and liquid oil co occurrence on a micrometer scale that demonstrates that oil has direct contact to gas hydrates when it diffuses through a hydrate matrix

appearance and preservation of natural gas hydrate from hydrate ridge sampled during odp leg 204 drilling abstract the state of preservation of natural gas hydrate samples recovered from 6 sites drilled during odp leg 204 at southern summit of hydrate ridge oregon margin has been investigated by x ray diffraction xrd and cryo scanning electron microscopy cryo sem techniques a detailed characterization of the state of decomposition of gas hydrates is necessary since no pressurized autoclave tools were used for sampling and partial dissociation must have occurred during recovery prior to the quench and storage in liquid nitrogen samples from 16 distinct horizons have been investigated by synchrotron x ray diffraction measurements at hasylab hamburg a full profile fitting analysis rietveld method of synchrotron xrd data provides quantitative phase determinations of the major sample constituents such as gas hydrate structure i si hexagonal ice ih and quartz the ice content ih in each sample is related to frozen water composed of both original existing pore water and the water from decomposed hydrates hydrate contents as measured by diffraction vary between 0 and 68 wt in the samples we measured samples with low hydrate content usually show micro structural features in cryo sem ascribed to extensive decomposition comparing the appearance of hydrates at different scales the grade of preservation seems to be primarily correlated with the contiguous volume of the original existing hydrate the dissociation front appears to be indicated by micrometer sized pores in a dense ice matrix

first determination of gas hydrate crystallite size distributions using high energy synchrotron radiation 1 due to experimental difficulties grain size distributions of gas hydrate crystallites are largely unknown in natural samples for the first time we were able to determine grain size distributions of six natural gas hydrates for samples retrieved from the gulf of mexico and from hydrate ridge offshore oregon from varying depths high energy synchrotron radiation provides high photon fluxes as well as high penetration depth and thus allows for investigation of bulk sediment samples the gas hydrate crystallites appear to be log normally distributed in the natural samples and to be of roughly globular shape the mean grain sizes are in the range from 300 600 m with a tendency for bigger grains to occur in greater depth possibly indicating a difference in the formation age laboratory produced methane hydrate starting from ice and aged for 3 weeks shows half a log normal curve with a mean value of 40 m this one order of magnitude smaller grain sizes suggests that care must be taken when transposing grain size sensitive petro physical data from laboratory made gas hydrates to natural settings

the origin of thermogenic gas hydrates on the northern cascadia margin as inferred from isotopic 13c 12c and d h and molecular composition of hydrate and vent gas the isotopic 13c and d and hydrocarbon compositions of hydrate bound and vent gas collected from the seafloor of barkley canyon northern cascadia margin offshore vancouver island canada were evaluated to characterize the gas and infer the type and maturity of the source rock kerogen the hydrate gas contained methane having 13c values from 43 4 to 42 6 d values from 143 to 138 and a large percentage 14 9 to 31 9 of c2 to c5 hydrocarbons these data are consistent with a thermogenic gas source the data from barkley canyon are interpreted within the context of similar data from the gulf of mexico and caspian sea thermogenic hydrates which occur in regions where the petroleum systems supporting gas and oil generation are better understood a stable carbon isotope based natural gas plot model and d h data from the hydrate gas indicate that the source rock for the barkley canyon hydrate and vent gas had primarily type iii kerogen mixed with a small fraction of type ii kerogen oil from a seep having a pristane phytane ratio of 3 2 was identified as a gas condensate which supports the conclusions drawn from the gas data a mechanism for explaining how fluids are conducted from the deep petroleum reservoir to the seafloor of barkley canyon is proposed

molecular and isotopic partitioning of low molecular weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high flux seepage site detailed knowledge of the extent of post genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties we investigated gases from a submarine high flux seepage site in the anoxic eastern black sea in order to elucidate molecular and isotopic alterations of low molecular weight hydrocarbons lmwhc associated with upward migration through the sediment and precipitation of shallow gas hydrates for this near surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the batumi seep area at 845 m water depth within the gas hydrate stability zone vent gas gas from pressure core degassing and from hydrate dissociation were strongly dominated by methane 99 85 mol of c1 c4 co2 molecular ratios of lmwhc c1 c2 c3 1000 and stable isotopic compositions of methane 13c 53 5 v pdb d h around 175 smow indicated predominant microbial methane formation c1 c2 ratios and stable isotopic compositions of lmwhc distinguished three gas types prevailing in the seepage area vent gas discharged into bottom waters was depleted in methane by 0 03 mol c1 c4 co2 relative to the other gas types and the virtual lack of 14c ch4 indicated a negligible input of methane from degradation of fresh organic matter of all gas types analyzed vent gas was least affected by molecular fractionation thus its origin from the deep subsurface rather than from decomposing hydrates in near surface sediments is likely as a result of the anaerobic oxidation of methane lmwhc in pressure cores in top sediments included smaller methane fractions 0 03 mol c1 c4 co2 than gas released from pressure cores of more deeply buried sediments where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices no indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found enrichments of 14c ch4 1 4 pmc in short cores relative to lower abundances max 0 6 pmc in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high flux hydrocarbon seep area

rising methane gas bubbles form massive hydrate layers at the seafloor extensive methane hydrate layers are formed in the near surface sediments of the cascadia margin an undissociated section of such a layer was recovered at the base of a gravity core i e at a sediment depth of 120 cm at the southern summit of hydrate ridge as a result of salt exclusion during methane hydrate formation the associated pore waters show a highly elevated chloride concentration of 809 mm in comparison the average background value is 543 mm a simple transport reaction model was developed to reproduce the cl observations and quantify processes such as hydrate formation methane demand and fluid flow from this first field observation of a positive cl anomaly high hydrate formation rates 0 15 1 08 mol cm 2 a 1 were calculated our model results also suggest that the fluid flow rate at the cascadia accretionary margin is constrained to 45 300 cm a 1 the amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water thus most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from ch4 dissolved in the pore water