heat flux through an old 175 ma passive margin offshore southeastern united states new heat flow data on the southeastern united states passive margin show that measured uncorrected flux averages 49 11 8 mw m 2 through old 175 ma oceanic crust nonuniform thermal gradients were measured at about half of the 114 penetrations that comprise the data set and over the full range of water depths 1900 m to 4250 m at which data were collected with the simplifying assumption that the nonuniform gradients were caused by a step function change in bottom water temperatures at some time before the heat flow cruises concave down decreasing dt dz with depth and concave up increasing dt dz sediment thermal gradients can be explained by respective average temperature decreases and increases of 0 1 0 2 k amplitude occurring 28 35 days before the measurements thermal gradients throughout the entire region are strongly influenced by oceanographic phenomena and locally by the presence of subsurface diapiric structures while sediment thickness variations and lateral differences in sedimentation rate and sediment lithology appear to exercise relatively little control over thermal regimes mean reduced heat flow in the study area is estimated at 49 mw m 2 by decompacting and back stripping the 5 8 km of sediment deposited on the margin since the formation of the underlying oceanic crust this value agrees with previous measurements made on younger crust in the blake ridge area but is significantly higher than the reduced heat flow value in a similar passive margin setting at the baltimore canyon trough we use the calculated average thermal gradient value and an assumption of constant conductivity to estimate temperatures of 19 5 c to 24 5 c at the bottom simulating reflector bsr on the blake ridge slope

gas hydrate systems in petroleum provinces of the sw barents sea abstract 3d seismic data of the sw barents sea provided information about the existence of structure ii gas hydrates and shallow gas in marine sediments gas hydrates and shallow gas exist in the sw barents sea in conjunction with deep hydrocarbon reservoir leakage available conventional 3d seismic data show acoustic chimneys that extend to depths 1700 ms two way travel time below the seafloor the location of the chimneys coincides with structural boundaries and fault complexes cutting through mostly jurassic and triassic strata vertical fluid flow through these chimneys transports thermogenic gas to the gas hydrate stability zone causing the formation of a bottom simulating reflector bsr in addition variations of fluid flux across deep seated fault complexes cause changes in heat flow which explains the shoaling of the bsr and the formation of a tilted bsr tbsr at the border between the ringvassoya fault complex and the loppa high the stability model using thermogenic gas composition type i and ii shows gas hydrates are stable at 225 m 345 m depth corresponding to variation in the geothermal gradient for upper bsr 28 7 c km and 37 c km and lower bsr levels 23 4 c km and 28 3 c km these lateral changes in the temperature gradient can be reconciled with changes in upward fluid flow rates of 1 8 2 2 mm yr 1 such gas hydrate and fluid flow systems on arctic continental margins especially in shallow water depths are potential gas emitters due to their sensitivity to warming ocean temperatures

method of estimating the amount of in situ gas hydrates in deep marine sediments abstract the bulk volume of gas hydrates in marine sediments can be estimated by measuring interval velocities and amplitude blanking of hydrated zones from true amplitude processed multichannel seismic reflection data in general neither velocity nor amplitude information is adequate to independently estimate hydrate concentration a method is proposed that uses amplitude blanking calibrated by interval velocity information to quantify hydrate concentrations in the blake ridge area of the us atlantic continental margin on the blake ridge blanking occurs in conjunction with relatively low interval velocities the model that best explains this relation linearly mixes two end member sediments hydrated and unhydrated sediment hydrate concentration in the hydrate end member can be calculated from a weighted equation that uses velocity estimated from the seismic data known properties of the pure hydrate and porosity inferred from a velocity porosity relationship amplitude blanking can be predicted as the proportions of hydrated and unhydrated sediment change across a reflection boundary our analysis of a small area near dsdp 533 indicates that the amount of gas hydrates is about 6 in total volume when the interval velocity is used as a criterion and about 9 5 when amplitude information is used this compares with a calculated value of about 8 derived from the only available measurement in dsdp 533

the nature distribution and origin of gas hydrate in the chile triple junction region abstract a bottom simulating reflector bsr is regionally distributed throughout much of the chile triple junction ctj region downhole temperature and logging data collected during ocean drilling program odp leg 141 suggest that the seismic bsr is generated by low seismic velocities associated with the presence of a few percent free gas in a 10 m thick zone just beneath the hydrate bearing zone the data also indicate that the temperature and pressure at the bsr best corresponds to the seawater methane hydrate stability field the origin of the large amounts of methane required to generate the hydrates is however problematic low total organic carbon contents and low alkalinities argue against significant in situ biogenic methanogenesis but additional input from thermogenic sources also appears to be precluded increasing thermal gradients associated with the approach of the spreading ridge system may have caused the base of the hydrate stability field to migrate 300 m upwards in the sediments we propose that the upward migration of the base of the stability field has concentrated originally widely dispersed hydrate patches into the more continuous hydrate body we see today the methane can be concentrated if the gas hydrates can form from dissolved methane transported into the hydrate zone via diffusion or fluid advection a strong gradient may exist in dissolved methane concentration across the bsr leading to the steady reabsorbtion of the free gas zone during the upward migration of the bsr even in the absence of fluid advection

methane rich plumes on the carolina continental rise associations with gas hydrates seafloor venting of microbial gases occurs at 2167 m water depth over the blake ridge diapir gas rich plumes were identified acoustically in the water column up to 320 m above a pockmarked sea floor associated with active chemosynthetic biological communities plumes and venting fluids emanate from near a small fault that extends downward toward a dome in the bottom simulating reflector indicating that fluid and or gas migration is associated with gas hydrate bearing sediment below these plumes might be caused by gas bubbles or buoyant dumps of gas hydrate that float upward from the seafloor 18 refs 3 figs

gravity driven faults migration pathways for recycling gas after the dissociation of marine gas hydrates abstract three dimensional seismic data from offshore of mauritania west africa reveal tiers of normal and tear faults that are approximately parallel to the strike and dip of the slope respectively they formed as a result of multiple episodes of down slope creep and have propagated through 60 140 m of sediment some intersect the seabed while others are buried and formed during earlier down slope translation gas hydrates the base of which is evidenced by aligned amplitude terminations or a bottom simulating reflection form within the faulted succession there are spatial relationships among the fault locations breached gas accumulations and the location of shallower free gas sealed by hydrates these are consistent with the upward resetting of the base of the hydrates the migration of dissociated gas up the faults to either become trapped below its reset base in a new free gas zone or potentially recycled back into a hydrate phase there is little evidence for venting of dissociated gas at the seabed gravity driven faulting is common on continental and island slopes where gas hydrates are expected to occur therefore this model for the utilization of gravitationally driven faults for the migration and recycling of gas should be generally applicable

controls on gas hydrate system evolution in a region of active fluid flow in the sw barents sea abstract the location and stability of gas hydrates in the sw barents sea is poorly constrained due to complex geological geochemical and geophysical conditions including poor controls on regional heat flow and gas chemistry understanding the stability of gas hydrates in this region is important as recent studies suggest destabilizing hydrates may lead to methane discharge into the ocean and possibly in to the atmosphere here we use high resolution 3d p cable seismic data combined with 3d heat flow and fluid flow models to place new constraints on gas hydrate stability in this region the 3d p cable seismic data acquired in 2009 west of loppa high show cross cutting reverse polarity high amplitude reflectors interpreted as the base of gas hydrate stability to constrain heat flow fluid flow and gas hydrate stability within the 3d seismic volume we use a 3d steady state finite difference diffusive thermal model that incorporates regional bottom water temperature from ctd casts expected geothermal gradients and gas composition derived from well data in general modelled bottom simulating reflectors are deeper than observed bsrs our analysis weighs multiple factors that might explain the discrepancy between observed and modelled bottom simulating reflector depths from this analysis we propose that the most significant discrepancies in bsr depth are likely related to changes in regional fluid heat flow and fluid geochemistry the anomalously shallow bottom simulating reflectors can be explained via vertical fluid flow that might include ensuing potential effects on gas composition pore water salinity and temperature our estimate suggest that a maximum vertical fluid flux of approximately 12 mm y is necessary to explain the most significant anomalies our study provides new insight into regional heat flow geochemistry and end member vertical fluid flux rates in the barents sea moreover it documents that the fluid flow system is active and most likely very dynamic

pockmark formation and evolution in deep water nigeria rapid hydrate growth versus slow hydrate dissolution in previous works it has been suggested that dissolution of gas hydrate can be responsible for pockmark formation and evolution in deep water nigeria it was shown that those pockmarks which are at different stages of maturation are characterized by a common internal architecture associated to gas hydrate dynamics new results obtained by drilling into gas hydrate bearing sediments with the mebo seafloor drill rig in concert with geotechnical in situ measurements and pore water analyses indicate that pockmark formation and evolution in the study area are mainly controlled by rapid hydrate growth opposed to slow hydrate dissolution on one hand positive temperature anomalies free gas trapped in shallow microfractures near the seafloor and coexistence of free gas and gas hydrate indicate rapid hydrate growth on the other hand slow hydrate dissolution is evident by low methane concentrations and almost constant sulfate values 2 m above the gas hydrate occurrence zone

detection of gas hydrates in faults using azimuthal seismic velocity analysis vestnesa ridge w svalbard margin accepted for publication in journal of geophysical research solid earth copyright 2020 american geophysical union further reproduction or electronic distribution is not permitted

characterisation and correlation of active hydrocarbon seepage using geophysical data sets an example from the tropical carbonate yampi shelf northwest australia abstract for the first time in australia active present day hydrocarbon seepage has been imaged on the tropical carbonate yampi shelf in 50 and 90 m water depth seepage is evidenced by gas plumes in the water column that are associated with seabed features such as clusters of reflective blocks hard grounds pockmark fields and mounds seepage activity and intensity seems to vary with changes in pressure related to macro tidal cycles the seabed features coincide with sub surface features such as areas of seismic signal attenuation under high amplitude reflectors seismic discontinuities and bright spots hydrocarbon migration seepage pathways appear to be controlled by the reactivation of pre existing fractures and dykes within the basement the types of seabed features and their preservation on a tropical carbonate shelf are strongly influenced by the coarse bioclastic nature of sediments and the high energy of macro tidal currents and storm reworking

hydrocarbon leakage through focused fluid flow systems in continental margins transport of liquid and gaseous hydrocarbons through focused fluid flow systems is a widespread process in continental margins and sedimentary basins which is gaining increased attention in the assessment of geohazards environment conservation and securing fossil energy resources studying the abundance distribution and drivers for this process is crucial for understanding its role in 1 the dynamics of gas hydrate accumulation and destabilization 2 submarine slope stability and related tsunamis 3 the plethora of chemosynthetic benthic ecosystems that develop in deep seep sites and 4 the input of greenhouse gases e g methane into the ocean atmosphere system which may influence the atmospheric carbon budget and earth s paleo and present climate new ocean exploration tools provide ever more data and improve our understanding of these systems however the subject still suffers from a lack of interdisciplinary knowledge dissemination the ongoing international debate about the timing and the processes that control fluid expulsion in sedimentary basins is fuelled by their implications for structural and petroleum geology because fluids expelled at cold seeps originate at depth they represent open windows into the underlying petroleum systems and are valuable indicators for the reservoir systems they may also help in deciphering past and predicting future climate change because worldwide release of large amounts of fluids may have an impact on the chemistry of the ocean and atmosphere

a double bottom simulating reflector in the western ross sea antarctica 1 we present seismic evidence of the presence of gas hydrates and free gas in victoria land basin western ross sea antarctica the occurrence of gas hydrates is inferred from a bottom simulating reflector bsr the first identified in the ross sea area the gas presence was analyzed through targeted reprocessing of the multichannel seismic reflection data 3000 m streamer 120 channels 60 fold acquired in 1990 by the italian r v ogs explora the seismic data were reprocessed in order to increase the signal to noise ratio by adopting a true amplitude approach allowing a subsequent amplitude variations with offset avo analysis to extract information on both p and s wave reflectivity along a profile from the deeper basin to an intrabasin structural high the bsr evolves into crosscutting reflectors ccrs and enhanced amplitude reflectors ears locally another seismic event is interpreted to be a second bsr bsr0 identified about 150 m below the first bsr and is possibly related to the presence of a mixture of gases comprising methane and hydrocarbons with heavier molecular weight and therefore associated with a deeper hydrate stability zone the presence of faults controls the upward gas migration connecting the free gas zone below the bsr and the mud volcanoes and pockmarks on the seafloor the modeling of the hydrate equilibrium zone shows that the observed bsr bsr0 could be stable at the present pressure temperature conditions

water segregation in the course of gas hydrate formation and accumulation in submarine gas seepage fields abstract gas hydrate accumulations associated with submarine gas seepage fields have been detected in the okhotsk sea and the gulf of mexico it is possible that gas hydrates known from the continental margin offshore northern california oregon and nigeria are also connected with gas seepage fields gas hydrated sediments in the accumulations 1 occur at very low subbottom depths sometimes directly at the sea bottom 2 have specific structures resembling cryogenic structures caused by hydrates subhorizontal lenticular bedded and porphyraceous structures 3 the sediments can be distinguished from overlying non hydrated sediments by a higher water content 4 an association of gas hydrates and calcium carbonate concretions occurs the behaviour of gas and water during gas hydrate formation under bottom gas seepage conditions is considered gas hydrate formation occurs at the upper part of the sedimentary cover within the limits of the diffusion halo around the up going flow of gas which saturates the pore water water migrating to the gas hydrate accumulation sites from adjacent sediments is involved in hydrate formation as well the mechanism of gas transportation to the gas hydrate accumulation sites is diffusion the way of water transport during hydrate accumulation is similar to ice segregation during permafrost formation the water segregation is responsible for the formation of specific structure caused by hydrates and for higher than usual water content of hydrate bearing sediments in submarine gas seepage fields

distribution and origin of seismic chimneys associated with gas hydrate using 2d multi channel seismic reflection and well log data in the ulleung basin east sea abstract analysis of multi channel seismic reflection and well log data from the ulleung basin east sea reveals that two distinct types of seismic chimneys have been identified in the basin based on their geometries seismic reflection patterns well log response and lithology type i is a mound like feature with a transparent or chaotic reflection pattern well log interpretation and lithology of this type indicate that type i consists of mobilized homogeneous mud including fracture filling gas hydrate this type is mainly located on the structural highs and is mostly restricted to the pliocene sedimentary succession in contrast type ii is a pipe like feature with vertically stacked distorted reflectors based on well log data and lithology type ii filled with fracture filling gas hydrate without breaking up primary sedimentary structure this type appears along the deep seated fault and mainly developed within the quaternary deposit our primary conclusion is that type i resulted from the subsurface sedimentary remobilization in response to regional uplift in the pliocene whereas type ii resulted from the focused vertical fluid flow through fractures triggered by reactivation of deep seated fault during the quaternary

seafloor facies related to upward methane flux within a giant pockmark of the lower congo basin the origin of the cold fluid venting from a giant pockmark within the lower congo basin has been elucidated based upon results of precise mapping submersible dives gravity coring and isotopic analyses realized under a total ifremer cooperative project zaiango and biozaire projects during four dives of the ifremer rov victor 6000 the bottom was filmed hard and soft samples were lifted from the sediment and water samples were collected with a ctd rosette system the detailed dip map shows that the 800 m wide giant pockmark is a composite feature due to the coalescence of multiple 100 m wide depressions that displays a broad range of biological mineralogical and chemical features on the seafloor leading to a seafloor anomaly recorded on the multibeam imagery methane rich fluids migrating through the sedimentary column from a buried palaeochannel clearly react with the sulphate rich circulating sea water to produce hydrogen sulphide and bicarbonate ions this situation leads to a concentrical arrangement of the sedimentary facies with methane related features in the centre and sulphide related features at the periphery this organization is correlated with high levels of methane up to 20 mol l measured in the centre of the giant pockmark responsible for the crystallization of gas hydrates at the bottom in this model the concentrical organization of mineralogical and biological features reflects a geochemical partitioning related to the peripheral progressive mixing of the methane flux

geophysical characterisation of active thermogenic oil seeps in the salt province of the lower congo basin part ii a regional validation abstract the lower congo basin is known to discharge a substantial volume of oil towards the sea surface from more than one hundred seafloor seep sites distributed throughout the deep province of the lower congo basin a large geochemical coring survey confirmed the presence of oil on the seabed the combination with the seismic data considerably improved the identification of the origin of the oil slicks on the seabed multiple specific geophysical characteristics of thermogenic hydrocarbon seep sites were highlighted in a previous detailed analysis of seismic datasets this study aims to test the characteristics previously identified at regional scale the active discharge zone is limited to the distal province of the basin it is characterised by strong compression shortening due to the sliding of the post salt super sequence resulting in numerous salt diapirs that control the location of seafloor oil seep sites the paper describes the bathymetric reflectivity amplitude and sub bottom profiler characteristics of a group of thermogenic seep sites they correspond essentially to submarine mounds or pockmarks of complex and irregular shape surrounded by hummocky mounds active oil pockmarks are systematically associated with positive amplitude anomalies on the seabed and are linked to vertical high amplitude columns rooted in the seismic reflector and associated with the base of the gas hydrate stability zone the sub bottom profiler data shows that the hummocky mounds are connected by a network of faults to high amplitude bodies buried under a consistent sediment thickness based on the definition of specific geophysical features seafloor mounds complex shape and irregular pockmarks positive anomalies of seafloor amplitude high amplitude vertical pipes we identified a series of potential oil seep sites at basin scale the mounds are particularly recognizable using the seismic curvature attribute we have identified 2946 individual hummocks that are grouped in 50 zones with a density of 35 240 per km2 they are believed to be associated with asphalt storage on the seafloor and related to the biodegradation of heavy oils during hydrocarbon dysmigration through the sedimentary pile unusual double bsrs occur over the study area these are also a specific feature of thermogenic seep sites barely half of the potential sites identified on the seismic datasets are associated with recurrent oil slicks at the sea surface the proportion of remaining anomalies may be associated with inactive seep sites over the period of satellite based monitoring or gas dominated seep sites

a natural gas hydrate system on the exmouth plateau nw shelf of australia sourced by thermogenic hydrocarbon leakage abstract the identification of natural gas hydrates and shallow free gas in sedimentary basins is critical for understanding the organic carbon cycle dynamics in the shallow geosphere as well as for geohazard studies related to the development of commercial hydrocarbon fields in this study we report for the first time the evidence for the potential occurrence of a natural gas hydrate system along the continental margin of australia on the exmouth plateau nw shelf by the use of high quality 3d seismic data calibrated by downhole data from odp and industry boreholes we interpret a series of shallow high amplitude anomalies including patchy bottom simulating reflections as the seismic expression of localised accumulations of free gas beneath the base of hydrate stability and overlying high saturation hydrates or authigenic carbonates the hydrate bearing reservoir is constituted by neogene and quaternary fine grained carbonate nannofossil and foraminifera rich oozes the patchy distribution of the shallow free gas and hydrate accumulations which reflects the geometry of deeper fault blocks is hypothesised to result from the leakage of thermogenic gas from deeper reservoirs the interpretation of a thermogenic origin for the hydrate forming gases is supported by 1 the existence in the study area of several thermogenic gas discoveries within late triassic reservoirs 2 seismic evidence of cross stratal and stratal pathways for migrating gases in the overburden 3 the presence of free gas accumulations at depths intermediate between the deep reservoirs and shallow gas hydrate systems and 4 geochemical and lithological evidence that the conditions which favour the generation of shallow microbial gas are not present in the area the acquisition of downhole log and sample data through the observed seismic amplitude anomalies is necessary to test the interpretations and hypotheses presented in this manuscript

the physics of gas chimney and pockmark formation with implications for assessment of seafloor hazards and gas sequestration pockmarks form where fluids discharge through seafloor sediments rapidly enough to make them quick and are common where gas is present in near seafloor sediments this paper investigates how gas might lead to pockmark formation the process is envisioned as follows a capillary seal traps gas beneath a fine grained sediment layer or layers perhaps layers whose pores have been reduced in size by hydrate crystallization gas accumulates until its pressure is sufficient for gas to invade the seal the seal then fails completely a unique aspect of capillary seals releasing a large fraction of the accumulated gas into an upward propagating gas chimney which displaces water like a piston as it rises near the seafloor the water flow causes the sediments to become quick i e liquefied in the sense that grain to grain contact is lost and the grains are suspended dynamically by the upward flow the quickened sediment is removed by ocean bottom currents and a pockmark is formed equations that approximately describe this gas piston water drive show that deformation of the sediments above the chimney and water flow fast enough to quicken the sediments begins when the gas chimney reaches half way from the base of its source gas pocket to the seafloor for uniform near surface sediment permeability this is a buoyancy control not a permeability control the rate the gas chimney grows depends on sediment permeability and the ratio of the depth below seafloor of the top of the gas pocket to the thickness of the gas pocket at the time of seal failure plausible estimates of these parameters suggest gas chimneys at blake ridge could reach the seafloor in less than a decade or more than a century depending mainly on the permeability of the deforming near surface sediments since these become quick before gas is expelled gas venting will not provide a useful warning of the seafloor instabilities that are related to pockmark formation however detecting gas chimney growth might be a useful risk predictor any area underlain by a gas chimney that extends half way or more to the surface should be avoided c 2009 elsevier ltd all rights reserved

high intensity gas seepage causes rafting of shallow gas hydrates in the southeastern black sea abstract submarine gas hydrates are a major global reservoir of the potent greenhouse gas methane since current assessments of worldwide hydrate bound carbon vary by one order of magnitude new technical efforts are required for improved and accurate hydrate quantifications here we present hydrate abundances determined for surface sediments at the high flux batumi seep area in the southeastern black sea at 840 m water depth using state of the art autoclave technology pressure sediment cores of up to 2 65 m in length were recovered with an autoclave piston corer backed by conventional gravity cores quantitative core degassing yielded volumetric gas bulk sediment ratios of up to 20 3 proving hydrate presence the cores represented late glacial to holocene hemipelagic sediments with the shallowest hydrates found at 90 cmbsf calculated methane concentrations in the different cores surpassed methane equilibrium concentrations in the two lowermost lithological black sea units sampled the results indicated hydrate fractions of 5 2 of pore volume in the sapropelic unit 2 and mean values of 21 pore volume in the lacustrine unit 3 we calculate that the studied area of 0 5 km2 currently contains about 11 3 kt of methane bound in shallow hydrates episodic detachment and rafting of such hydrates is suggested by a rugged seafloor topography along with variable thicknesses in lithologies we propose that sealing by hydrate precipitation in coarse grained deposits and gas accumulation beneath induces detachment of hydrate sediment chunks floating hydrates will rapidly transport methane into shallower waters and potentially to the sea atmosphere boundary in contrast persistent in situ dissociation of shallow hydrates appears unlikely in the near future as deep water warming by about 1 6 c and or decrease in hydrostatic pressure corresponding to a sea level drop of about 130 m would be required because hydrate detachment should be primarily controlled by internal factors in this area and in similar hydrated settings it serves as source of methane in shallow waters and the atmosphere which is mainly decoupled from external forcing

multi frequency seismic study of gas hydrate bearing sediments in lake baikal siberia abstract in this paper we present and discuss the frequency dependent behaviour of the acoustic characteristics of methane hydrate bearing sediments in lake baikal siberia five different types of seismic sources airgun array two types of single airguns watergun and sparker are used encompassing a frequency bandwidth from 10 up to 1000 hz on low frequency airgun array data the base of the hydrate stability zone hsz is observed as a high amplitude bottom simulating reflection bsr with reversed polarity the amplitude and continuity of the bsr decrease or even disappear on medium to high frequency data a feature explained in terms of vertical and horizontal resolution the increasing reflection amplitude of the bsr with increasing offset the calculated reflection coefficient of the bsr and the occurrence of enhanced reflections below the bsr suggest the presence of free gas below the hsz the observation of some enhanced reflections extending above the bsr may be interpreted as an indication for free gas co existing with hydrates within the hsz amplitude blanking above the bsr is highly variable while the bsr itself appears to act as a low pass frequency filter for medium to high frequency data new single channel airgun profiles provide the first seismic information across the baikal drilling project bdp 97 deep drilling site at which hydrate bearing sediments were retrieved at about 200 m above the base of the local hsz at the drilling site there are no seismic characteristics indicative of the presence of hydrates combination of the drilling and seismic information has allowed us to make a rough estimation of the volume of hydrates and carbon stored in the sediments of lake baikal which lead us to conclude that the lake baikal gas hydrate reservoirs do not form a prospective energy resource

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

influence of depth and mixing regime on sedimentation in a small fluctuating tropical soda lake the historical sediment record of lake sonachi kenya was used to study the influence of lake depth and mixing regime on patterns of sedimentation in a small fluctuating tropical soda lake lake sonachi last desiccated completely in the early nineteenth century and has fluctuated between 3 and 18 m lake depth over the past 115 yr a freeze core of offshore sediments describes recent lake history as a succession of meromictic episodes represented by varved or subannually laminated muds and holomictic episodes represented by more coarsely layered muds two interbedded horizons of colloidal amorphous silica dated to a period of rising lake level after a prolonged lowstand were deposited by abiogenic ph driven precipitation from the water column and represent the instantaneous sequestering of an estimated 63 mg liter 21 or possibly 50 of the lake s dissolved silica reservoir changes in offshore sedimentation and inferred bottom dynamics over time indicate that sediment resuspension and focusing in lake sonachi occur mostly during infrequent events of deep circulation between the average mud deposition boundary depth at 2 m and the chemocline depth at 4 5 m wind driven sediment redistribution across the lake floor is important only at lake depths of 3 m dry sediment accumulation has varied between 86 and 620 g m 22 yr 21 over the past 175 yr with no relationship to lake depth but on average lower rates during meromixis 199 6 90 g m 22 yr 21 than during holomixis 349 6 152 gm 22 yr 21 net organic carbon accumulation offshore varied between 0 and 92 g m 22 yr 21 with no significant relationship to either lake depth or mixing regime at the time of deposition sedimentary organic carbon content 5 7 26 9 is negatively correlated with bulk sediment accumulation 73 of this variation is accounted for by the clastic dilution of sedimented planktonic algal production with low organic littoral sediments redeposited offshore

evolution of fluid expulsion and concentrated hydrate zones across the southern hikurangi subduction margin new zealand an analysis from depth migrated seismic data identification of methane sources controlling hydrate distribution and concentrations in continental margins remains a major challenge in gas hydrate research lack of deep fluid samples and high quality regional scale seismic reflection data may lead to underestimation of the significance of fluid escape from subducting and compacting sediments in the global inventory of methane reaching the hydrate zone the water column and the atmosphere the distribution of concentrated hydrate zones in relation to focused fluid flow across the southern hikurangi subduction margin was investigated using high quality long offset 10 km streamer pre stack depth migrated multichannel seismic data analysis of low p wave velocity zones bright reverse polarity reflections and dim amplitude anomalies reveals pathways for gas escape and zones of gas accumulation the study shows the structural and stratigraphic settings of three main areas of concentrated hydrates 1 the opouawe bank dominated by focused periodic fluid input along thrust faults sustaining dynamic hydrate concentrations and gas chimneys development 2 the frontal anticline with a basal set of protothrusts controlling permeability for fluids from deeply buried and subducted sediments sustaining hydrate concentrations at the crest of the anticline and 3 the hikurangi channel with buried sand dominated channels hosting significant amounts of gas beneath the base of the hydrate zone in sand dominated channels gas injection into the hydrate zone favors highly concentrated hydrate accumulations the evolution of fluid expulsion controlling hydrate formation offshore southern hikurangi is described in stages during which different methane sources in situ buried and thermogenic have been dominant

high heat flow anomalies over salt structures on the texas continental slope gulf of mexico we made 74 closely spaced 2 km apart heat flow measurements around and over two salt structures on the texas continental slope gulf of mexico the values outlined the shape of the heat flow anomalies over both structures based on a preceding high resolution seismic survey we interpreted these structures to be a cylindrical plug and a salt tongue extending from the crest of a wall shaped feeder the heat flow observations clearly reflect differences between the two features and are consistent with the prior structural interpretation the values over the salt plug are nearly all greater than 70 mw m the measurements over the salt tongue have a sharp heat flow peak of 90 mw m associated with the presumed feeder and rather uniform values around 60 mw m over the remainder the variation of heat flow over both structures is smooth and shows no apparent scatter heat flow values off these features are uniformly low around 30 mw m thermal effects from bottom water temperature fluctuation slope sedimentation diapiric movement of the salt body and pore fluid migration appear unable to provide a satisfactory explanation for the observations however thickness variations of a highly conductive salt body can easily account for the heat flow anomalies we suggest that modeling of the conductive anomaly should provide substantial constraints on the bottom geometry of the salt

benthic foraminifera of the blake ridge hydrate mound western north atlantic ocean abstract in a 240 m 70 m area at the top of the blake ridge diapir water depth 2150 m the benthic foraminiferal assemblage in surface sediment 0 1 cm consists of about 130 species hydrocarbon seepage is indicated here by the presence of methanotrophic bivalves and white bacterial mats and by present day formation of gas hydrate the foraminiferal assemblage density is generally high indicating an abundance of food bolivina lowmani and globocassidulina subglobosa dominate the assemblage in the silt clay substrate whether at the edge of bathymodiolid mussel beds or under bacterial mats the only notable difference between these two sample groups is in the higher variation of foraminiferal density in the mussel bed samples a control sample at 32 29 654 n and 76 11 324 w taken from bioturbated sediment light grey silt and clay with no visible bacterial or molluscan colonization is comparable to the seep samples in the distribution of calcareous species but shows a much higher diversity and abundance of agglutinated species no segregation of foraminiferal species by substrate geochemistry can be inferred for the seep samples and the possible effect of variable sulfide or methane fluxes on the community cannot be demonstrated by our data

gas hydrate and associated free gas in the dongsha area of northern south china sea abstract a large ongoing cold seep named jiulong methane reef was discovered in the dongsha area of northern south china sea scs in 2004 to understand links among seafloor methane seepage subsurface gas hydrate origin of gas source and structural styles controlling the formation and accumulation of gas hydrate high resolution multi channel seismic data were interpreted in this region bottom simulating reflectors bsrs known as the base of gas hydrate stability zone were identified and mapped additionally seismic attributes interval velocity model and acoustic p wave impedance were obtained to better delineate the spatial distribution of gas hydrate hydrate bearing sediments are characterized by weak reflection i e blanking zone above bsrs high velocities and relatively small acoustic impedance differences while enhanced reflection low frequency and low velocities beneath bsrs suggest the presence of underlying free gas beneath solid hydrate a number of faults mud diapirs and a possible submarine landslide were observed in seismic profiles both faults and mud diapirs are useful conduits allowing methane gas to migrate upward and linking free gas zones beneath bsrs hydrate zones and seafloor methane seepage a submarine landslide with a length scale of 10 km is observed in a seismic profile the landslide may trigger dissociation of gas hydrate accounting for the occurrence of jiulong methane seepage seismic observations and estimated heat flow coupled with geochemical analyses suggest that gas source of methane in this region probably originates from mixture of biogenic and thermogenic methane this study would not only help to understand gas hydrate system in the dongsha area but also provide insights to geological hazards associated with the dissociation of gas hydrate

characteristics of gas hydrate occurrences associated with mud diapirism and gas escape structures in the northwestern sea of okhotsk abstract an interpretation of 8450 line km of seismic reflection data acquired during four cruises in the northwestern sea of okhotsk shows widespread occurrence of free gas or gas hydrates in the sediment this occurrence is documented seismically by gas escape structures acoustic blanking and by a bottom simulating reflector bsr gas escape structures and vents are concentrated on the northern sakhalin continental margin here they are associated with local tectonic movements along a n s trending dextral shear system the inessa shear zone mud diapirism is common in the derugin basin where compression dominates this diapirism is probably closely related to the gas accumulations in the sediment below the base of the gas hydrate stability zone bghsz conductive heat flow derived from the bsr depth distribution averages about 30 8 mw m 2 only adjacent to basement highs and around the inessa shear zone do values higher than 80 22 mw m 2 occur these high values are attributed to an elevated geothermal gradient in the tectonically active zone where fluid venting takes place areas of drift sedimentation and mass wasting where sedimentation rates are high are characterised by computed heat flow values below 30 8 mw m 2 however these values must be corrected upwards by 3 20 depending on the sedimentation rate applicable 3 8 100 cm kyr the total amount of methane preserved in the hydrate stability zone hsz and trapped as free gas beneath the bsr is estimated at 17 14 10 12 m 3 for the northwestern sea of okhotsk and 15 12 10 13 m 3 for the entire sea of okhotsk the latter figure represents about 0 8 of the global reservoir of methane gas from hydrates our study documents that the semi enclosed sea of okhotsk offers favourable conditions for the accumulation of gas in its sediments on account of its subarctic climate and the prevailing hydrologic regime these conditions include high primary productivity low bottom water temperatures and high sedimentation rates

seismic attenuation normal moveout stretch and low frequency shadows underlying bottom simulating reflector events abstract in many cases the seismic response of bottom simulating reflectors is characterised by low frequencies called low frequency shadow generally this phenomenon is interpreted as attenuation due to partial saturation with free gas actually this frequency loss may have multiple causes with a normal moveout stretch as a possible candidate to analyse this phenomenon we compute synthetic seismograms by assuming a lossy bottom simulating layer with varying quality factor and thickness bounded by the upper hydrate brine gas brine and lower gas brine brine interfaces first we estimate the shift of the centroid frequency of the power spectrum as a function of the travelled distance of the seismic pulse then we perform one dimensional numerical experiments to quantify the loss of frequency of the seismic event below the bottom simulating reflector as a function of the quality factor of the bottom simulating layer and its thickness due to wave interference then we compute shot gathers to obtain the stacked section with and without the normal moveout stretch correction and with and without the presence of wave attenuation in the bottom simulating layer the results indicate that the low frequency shadow due to the normal moveout stretch is stronger than that due to attenuation and may constitute a false indicator of the presence of gas in fact often the low frequency shadow overlies events with higher frequencies in contradiction with the physics of wave propagation this is particularly evident when the low frequency shadow is so extensive that the presence of high frequencies below cannot be justified by the acquisition geometry

the physics of gas chimney and pockmark formation with implications for assessment of seafloor hazards and gas sequestration vol 27 pg 82 2010

three dimensional seismic investigations of the sevastopol mud volcano in correlation to gas fluid migration pathways and indications for gas hydrate occurrences in the sorokin trough black sea new 3 d seismic investigations carried out across the sevastopol mud volcano in the sorokin trough present 3 d seismic data of a mud volcano in the black sea for the first time the studies allow us to image the complex three dimensional morphology of a collapse structured mud volcano and to propose an evolution model the sevastopol mud volcano is located above a buried diapiric structure with two ridges and controlled by fluid migration along a deep fault system which developed during the growth of the diapirs in a compressional tectonic system overpressured fluids initiated an explosive eruption generating the collapse depression of the sevastopol mud volcano several cones were formed within the depression by subsequent quiet mud extrusions although gas hydrates have been recovered at various mud volcanoes in the sorokin trough no gas hydrates were sampled at the sevastopol mud volcano a bsr bottom simulating reflector is missing in the seismic data however high amplitude reflections bright spots observed above the diapiric ridge near the mud volcano at a relatively constant depth correspond to the approximate depth of the base of the gas hydrate stability zone bghsz thus we suggest that gas hydrates are present locally where gas fluid flow occurs related to mud volcanism i e above the diapir and close to the feeder channel of the mud volcano depth variations of the bright spots of up to 200 ms twt might be caused by temperature variations produced by variable fluid flow

high resolution deep tow multichannel seismic and sidescan sonar survey of the submarine mounds and associated bsr off nicaragua pacific margin bathymetric and conventional multichannel seismic surveys offshore nicaragua and costa rica have revealed numerous mud mounds beneath which the generally widespread bsr is not well imaged however many of the mounds are partially capped by patches of authigenic carbonate crusts so it was not clear if the semitransparent seismic facies and the apparent gaps in the bsr beneath the mounds are real or due to poor normal incidence seismic penetration through the cap rocks to address these problems a high resolution seismic survey was carried out over the continental slope of the nicaraguan pacific margin using a deep towed multichannel seismic streamer dtmcs along with a sidescan sonar system dts to image submarine mud mounds and the associated bsr the proximity of the very short 39 m active length but high resolution 17 channel streamer to the seafloor of the deep towed system allows greatly improved lateral resolution whereas the relatively large source receiver offset allows the undershooting of the cap rocks for the first time our data show that the bsr in many cases continues but rises beneath the mounds this is consistent with the advection of deep warm fluids and thus increased heat flow through the mounds the occurrence of mud mounds seems to be controlled by the locations of faults

effects of dissociation on the shear strength and deformation behavior of methane hydrate bearing sediments abstract a series of tests were conducted in order to investigate the shear strength and deformation behavior of methane hydrate bearing sediments during dissociation using the thermal recovery method or depressurization method an innovative temperature controlled high pressure triaxial apparatus which can reproduce the in situ conditions of hydrate reservoirs was used the results indicate that 1 the failure strength of isotropically consolidated methane hydrate bearing sediments which dissociated completely using the thermal recovery method is less than that of pure toyoura sand however the initial stiffness and volumetric strain are higher than that of pure toyoura sand 2 the thermal recovery method will cause the failure of methane hydrate bearing sediments when the axial load is higher than the strength of methane hydrate bearing sediments after dissociation 3 the depressurization method will not cause collapse of methane hydrate bearing sediments during depressurization however water pressure recovery will lead to failure when the axial load is larger than the strength of the methane hydrate bearing sediments after dissociation 4 the depressurization rate shows little effect on the ultimate deformation of methane hydrate bearing sediments while the initial deformation rate increases with increasing depressurization rate 5 the larger the reduction of pore pressure the larger axial strain and volumetric strain

sediment instability related to fluid venting in miocene authigenic carbonate deposits of the northern apennines italy chemosynthetic carbonates identified by isotopic palaeoecological and sedimentological features are concentrated in middle late miocene satellite and foredeep deposits of the northern apennines chemoherms in the foredeep are hosted in thick pelitic intervals probably deposited in intrabasinal structural highs which are entirely or partly involved in large slumps in many cases associated with extrabasinal slides sediment textures in carbonates and in the enclosing foredeep pelitic sediments indicate a link between hydrocarbon fluid venting sediment deformation and mobilisation and tectonics the intensity and style of fluid release phases directly influenced chemoherm typology and also determined overpressure conditions in low shear strength pelitic sediments favouring sediment mobilisation and influencing slope instability which widely affected the apennine foredeep chemosynthetic carbonates are associated with sites of tectonically fractured and compressed sediments in the apennine foredeep thrust belt system thus indicating a relation with the tectonic loading of the apennine thrust sheets which favoured fluid expulsion along forerunner faults possible gas hydrate contributions to fluid expulsion processes are discussed based on sediment textures compared with modern vent areas finally sediment instability may have facilitated a large amount of fluid escape thus stopping carbonate precipitation

heat flow and gas hydrate saturation estimates from andaman sea india gas hydrate was recovered in the andaman sea along the eastern coast of the andaman islands during the india national gas hydrate program nghp expedition 01 at site nghp 01 17 coring confirmed gas hydrate occurs predominantly in discrete volcanic ash layers pore water chemistry electrical resistivity and p wave velocity logs are used to estimate gas hydrate saturations at site nghp 01 17 gas hydrate saturation estimated from chloride concentrations shows values up to 85 of the pore space for distinct ash layers from 270 m below seafloor to the base of gas hydrate stability zone bghsz gas hydrate saturations estimated from the electrical resistivity and acoustic velocity logs using standard empirical relations and modeling approaches are comparable to each other but saturations are only 20 of the pore space on average this much lower gas hydrate saturation estimate from the log data is a result of overall reduced resolution of the logging tools relative to the typically 20 30 cm thick hydrate bearing ash layers available 2d multi channel seismic data were also analyzed and a bottom simulating reflector bsr was imaged along several seismic profiles the depth of the bsr is more than 600 m along the seismic line crossing site nghp 01 17 which makes this one of the deepest bsrs observed worldwide to understand the unusual depth of the bsr we mapped its depth and estimated heat flow from the bsr depth using a simple conductive model bsr derived heat flow values range from 12 to 41 5 mw m2 from the study area and follow the bathymetry trend of dominant north south ridges and can be explained with the east ward trending increase in heat flow toward the current seafloor spreading center we also modeled the bghsz to analyze the linkage between gas hydrate occurrences in the andaman sea and its relation to the tectonic activity our analysis suggests an extensively variable bghsz in the andaman sea controlled mainly by overall low geothermal gradients consistent local minor variations were observed with lower heat flow values over prominent topographic highs and higher values in valleys troughs due to focusing and defocusing effects of the topography

methane migration within the submarine gas hydrate stability zone under deep water conditions abstract possible pathways of methane migration and associated gas hydrate accumulation are considered gas hydrates precipitate in water filtration routes when the methane concentration in water exceeds its solubility which significantly decreases within the hydrate stability zone towards the sea floor in the vicinity of the ascending free gas flows gas hydrates are formed within a diffusion aureole resulting from the difference between chemical potentials of gaseous and water dissolved methane the role of diffusion is also essential in the vicinity of gas saturated water flows as well as within and above the sections where biochemical methane is intensively generated a chemical potential gradient of dissolved methane which is a necessary condition for gas hydrate accumulation is created due to inhomogeneity of the geological medium manifested as temperature and water salinity gradients or changes of permeability the thermobaric gas hydrate stability zone is a geochemical barrier for hydrocarbon gas emigrating from sediments into the water column however methane is not completely preserved in a hydrate state it is unknown yet which fraction of migrating methane is detained on this barrier

three dimensional structure of fluid conduits sustaining an active deep marine cold seep 1 cold seeps in deep marine settings emit fluids to the overlying ocean and are often associated with such seafloor flux indicators as chemosynthetic biota pockmarks and authigenic carbonate rocks despite evidence for spatiotemporal variability in the rate locus and composition of cold seep fluid emissions the shallow subseafloor plumbing systems have never been clearly imaged in three dimensions using a novel high resolution approach we produce the first three dimensional image of possible fluid conduits beneath a cold seep at a study site within the blake ridge gas hydrate province complex dendritic features diverge upward toward the seafloor from feeder conduits at depth and could potentially draw flow laterally by up to 103 m from the known seafloor seep a pattern similar to that suggested for some hydrothermal vents the biodiversity community structure and succession dynamics of chemosynthetic communities at cold seeps may largely reflect these complexities of subseafloor fluid flow

seismic character of bottom simulating reflectors examples from the mid norwegian margin three classes of bottom simulating reflectors bsr cross cut the post breakup sediments of the mid norwegian margin the first class is caused by free gas at the base of the pressure and temperature dependent gas hydrate stability zone the second class of bsr is caused by the diagenetic transition from opal a to opal ct the third class of bsr is always observed underneath the opal a opal ct transition but heat flow data and the amplitude characteristics of this arrival exclude one of the known silicate diagenetic transitions or gas hydrates as the explanation for this reflector odp site 643 drilling results from the voring plateau suggest two possible processes as the reason for this third bsr a smectite illite conversion or b a sudden increase in the abundance of authigenic carbonates the genesis of both is pressure and temperature dependent and could potentially result in a cross cutting seismic reflector the data are not conclusive as to which process is causing the third class of observed bsr

the unusual summer of 1994 in east asia iod teleconnections 1 an extremely hot and dry summer of 1994 was reported in east asian countries using observational data we have demonstrated that the indian ocean dipole iod is at least one possible cause of the abnormal east asian summer climate an anomalous cyclonic circulation over the western pacific and the southern china weakened the monsoonal northward flow in the lower troposphere an anomalous anticyclonic circulation with the equivalent barotropic structure around japan korea and the northeastern part of china caused the hot and dry summer of 1994 this accumulation of the lower potential vorticity in the far east is related to the wave activity from the mediterranean sahara region the monsoon desert mechanism connects a rossby wave source with the iod induced diabatic heating around the bay of bengal another rossby wave train pattern was generated in the upper troposphere and propagates northeastward from the southern china both the rossby wave patterns influenced the circulation changes over east asia

potential impact on climate of the exploitation of methane hydrate deposits offshore abstract considerable attention has focussed on methane hydrates as a potential energy resource but much less on the potential environmental problems of exploiting these deposits in fact methane hydrate deposits represent a dynamic system formed as a result of the continuous migration of methane gas along fracture systems any exploitation of methane hydrates offshore will perforce take place in these unstable systems and could lead to the large scale release of ch 4 under unfavourable circumstances however most of this ch 4 would be oxidized to co 2 within the water column under steady state conditions only under exceptional circumstances would large scale release of ch 4 into the atmosphere take place the impact of exploiting these deposits offshore on global climate may therefore be limited provided serious efforts are made to minimize environmental impacts at present prospects for the commercial exploitation of methane hydrates are in their infancy considerable ingenuity will be required to satisfy the requirements of safe operating conditions mitigation of environmental hazards and commercial viability the need to exercise caution before attempting to exploit these deposits is emphasized

geological controls on focused fluid flow associated with seafloor seeps in the lower congo basin a synthesis of backscatter imagery coupled with a large 3d seismic dataset in the lower congo basin lcb reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth with the exception of one region of anomalous backscatter positive relief mounds all inferred seep sites occur in negative relief pockmarks the extensive 3d seismic dataset in the lcb offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems and in general to reconstruct the relationship between tectonics and fluid flow in continental margins the fluid seeps in the lcb are associated with morphologically stratigraphically or tectonically controlled focused fluid flow the integration of the geophysical datasets backscatter imagery coupled to 3d seismic clearly indicates that fluid seeps are not randomly distributed but their seabed organization reflects 1 the location of the underlying structure reservoir or trap where the fluids are coming from 2 the geometry and morphology of the reservoir trap and 3 the discontinuities in the sedimentary column along which fluids have migrated in the lcb seafloor pockmarks are always associated with underlying tectonic structures fault zones salt diapirs polygonal faults or buried sedimentary bodies turbiditic channels erosional surfaces whereas they never occur above sub horizontal parallel stratified fine grained sediments even if triggering processes can not be clearly defined here we propose a model of seafloor fluid seep organization which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from this qualitative 3d model provides insight into the geohydrologic processes of continental margins

an intensity index for the east asian winter monsoon the thermal contrast betweenthe asian continentand the adjacent oceans is the primary aspect of the east asian winter monsoon eawm that can be well represented in the sea level pressure slp field based on this consideration a new slp based index measuring the intensity of the eawm is proposed by explicitly taking into account both the east west and the north south pressure gradients around east asia the new index can delineate the eawm related circulation anomalies well including the deepened shallow midtropospheric east asian trough sharpened and accelerated widened and decelerated upper tropospheric east asian jet stream and enhanced weakened lower tropospheric northerly winds in strong weak eawm winters compared with previous indices the new index has a very good performance describing the winter mean surface air temperature variations over east asia especially for the extreme warm or cold winters the index is strongly correlated with several atmospheric teleconnections including the arctic oscillation theeurasianpattern andthenorthpacificoscillation westernpacificpattern implyingthepossible internal dynamics of the eawm variability meanwhile the indexis significantly linked to el ni oscillation enso and the sea surface temperature sst over the tropical indian ocean moreover the sst anomalies over the tropical indian ocean are more closely related to the index than enso as an independent predictor this adds further knowledge to the prediction potentials of the eawm apart from enso the predictability of the index is high in the hindcasts of the centre national de recherches m cnrm model from development of a european multimodel ensemble system for seasonal to interannual prediction demeter hence it would be a good choice to use this index for the monitoring prediction and research of the eawm

fault system and thermal regime in the vicinity of site nghp 01 10 krishna godavari basin bay of bengal abstract drilling coring activities onboard joides resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of krishna godavari kg basin bay of bengal and the expedition recovered fracture filled gas hydrate at the site nghp 01 10 in this paper we analyze high resolution multi channel seismic mcs high resolution sparker hrs bathymetry and sub bottom profiler data in the vicinity of site nghp 01 10 to understand the fault system and thermal regime we interpreted the large scale fault system 5 km predominantly oriented in nnw sse direction near nghp 01 10 site which plays an important role in gas hydrate formation and its distribution the increase in interval velocity from the baseline velocity of 1600 m s to 1750 1800 m s within the gas hydrate stability zone ghsz is considered as a proxy for the gas hydrate occurrence whereas the drop in interval velocity to 1400 m s suggest the presence of free gas below the ghsz the analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large scale fault system we conclude that the gas hydrate concentration near site nghp 01 10 and likely in the entire kg basin is controlled primarily by the faults and therefore has high spatial variability we also estimated the heat flow and geothermal gradient gtg in the vicinity of nghp 01 10 site using depth and temperature of the seafloor and the bsr we observed an abnormal gtg increase from 38 c km to 45 c km at the top of the mound which remarkably agrees with the measured temperature gradient at the mound nghp 01 10 and away from the mound nghp 01 03 we analyze various geological scenarios such as topography salinity thermal non equilibrium of bsr and fluid gas advection along the fault system to explain the observed increase in gtg the geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in gtg the approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm yr 0 37 0 6 mm yr

heat flow regimes and the hydrate stability zone of a transient thermogenic fault controlled hydrate system woolsey mound northern gulf of mexico abstract this study aims to constrain the base of the hydrates stability field in structurally complexsites using the case of woolsey mound a fault controlled transient thermogenic hydrates system in mississippi canyon block 118 northern gulf of mexico we have computed the base of the hydrates stability field integrating results from a recent heat flow survey designed to investigate geothermal anomalies along fault zones which exhibit different fluid flux regimes an advanced compositional simulator was used to model hydrate formation and dissociation at woolsey mound and addresses the following hypotheses 1 migrating thermogenic fluids alter thermal conditions of the hydrate stability zone hsz so heat flow reflects fault activity 2 gas hydrate formation and dissociation vary temporally at active faults temporarily sealing conduits for migration of thermogenic fluids 3 high salinity and inclusion of thermogenic gases with higher molecular weight than methane produce opposite effects on the depth to the bottom of the hydrate stability zone applications of results include identifying and quantifying hydrate deposits in shallow sediments using an interdisciplinary approach that includes multiple resolution seismic data evaluation geological and geochemical groundtruthing and heat flow analyses as a proxy for activity along faults

gas hydrates associated with gas chimneys in fine grained sediments of the northern south china sea drilling for gas hydrates at the northern continental margin of the south china sea scs provides a unique insight into the formation of the gas hydrate system in fine grained sediments even though most high concentrations of hydrates are found in coarse sediments detailed studies of 3d seismic data from a deep water basin baiyun depression of this region reveal the presence of overpressured fluid flow mostly manifested in gas chimneys gas chimneys are commonly characterized by disrupted reflections dr dim amplitude anomalies da and enhanced reflections er on conventional seismic profiles gas chimneys are also characterized by chaotic low continuity and low frequency on instantaneous frequency and instantaneous amplitude profiles gas hydrates and a prominent bsr were discovered in the region of gas chimney occurrence logging data and sample analyses from drill holes over the chimney structures indicate close relationships between active fluid flow and the formation and accumulation of gas hydrates our results document that gas chimneys consisting of a connected network of fractures provide a passageway along which fluids ascend beneath the gas hydrate stability zone ghsz for hydrate formation in fine grained sediments of the northern south china sea fluids accumulate in uppermost part of the slope where they are trapped beneath the prograding submarine delta sequence which is not permeable enough for fluids to migrate through gas charged fluids may originate from deep seated hydrocarbon reservoirs which are indicated by the molecular and isotopic signatures of gases in gas hydrates occurrence zone we suppose that the fractures extend into the ghsz creating a space for hydrate condensation in the fine grained sediments of the slope gas chimneys may account for the overpressured fluid activity and most likely the gas hydrate enrichment in the northern scs c 2012 elsevier b v all rights reserved

methane and minor oil macro seep systems their complexity and environmental significance abstract the two main observations characterising marine and lacustrine methane macro seeps are ebullition through holes in the sea or lake bed and hydroacoustic flares in the water column this paper reviews multi year multi scale and multi discipline results from three seep locations in the north sea and combines the knowledge with recent seafloor and water column results from seeps in the santa barbara basin california a seep off west africa seeps in the gulf of mexico and in lake baikal russia we have identified a total of 12 characteristics of methane and minor oil macro seeps that are not only geological in nature but also biological and geochemical these are shown to impact the marine environment in different ways not least in benefactory manners as primary producers mainly bacteria and archaea tend to bloom during seepage therefore the seepage is inferred to have a fertilizing effect on both the seafloor and the water column which may be of broad ecological and biological significance the study concludes with a holistic conceptual seep model which is expected to be of interest to a broad range of researchers in the fields of oceanography and limnology

impacts of the global sea surface temperature anomaly on the evolution of circulation and precipitation in east asia on a quasi quadrennial cycle based on multi source observational data from 1979 to 2013 we investigate the evolution of the global sea surface temperature anomaly ssta on a quasi quadrennial cycle and its impact on the seasonal evolution of circulation and precipitation in east asia we do this by using principal oscillation pattern analysis and associated correlation pattern analysis together with numerical experiments from an atmospheric general circulation model the results indicate that the global ssta exerts profound impacts on the east asian climate anomaly while distinct responses exist between the ssta developing and decaying stages from winter to autumn in the developing year southern china is typically wetter than normal in the decaying year however the precipitation over most parts of east asia is above normal from winter to spring and the main rainfall band meandering from the yangtze river to south japan is enhanced in summer during the developing and decaying years an upper level anomalous cyclone moves from northeast asia to the tibetan plateau and then retreats back a low level anomalous cyclone occurs over northeast asia in spring but weakens in summer and then strengthens gradually from autumn to the following summer an anomalous philippine sea anticyclone occurs in winter and spring but is replaced by an anomalous cyclone in summer before reappearing and persisting from autumn to the decaying year the abrupt change in circulation patterns in summer of the developing year might be related to the anomalous weakening of the tibetan plateau heating these anomalies can be attributed to the combined effects of the global large scale heating regional scale oceanic forcing and thermal feedback of the land

seismic characterization of hydrates in faulted fine grained sediments of krishna godavari basin full waveform inversion in fine grained faulted sediments both stratigraphic and fault induced structural variations can simultaneously determine the gas hydrate distribution insights into hydrate distribution can be obtained from p wave velocity vp and attenuation qp 1 character of the gas hydrate stability zone ghsz in this paper we apply frequency domain full waveform inversion fwi to surface towed 2d multichannel seismic data from the krishna godavari kg basin india to image the fine scale 100 x 30 m vp and qp 1 variations within the ghsz we validate the inverted vp model by reconciling it with a sonic log from a nearby 250 m well the vp model shows a patchy distribution of hydrate away from the faults dominated parts of the profile hydrates demonstrate stratigraphic control which appears to be permeability driven the qp 1 model suggests that attenuation is relatively suppressed in hydratesbearing sediments elevated attenuation in non hydrate bearing sediments could be driven by the apparent pore fluid immiscibility at seismic wavelengths the vp and the qp 1 models also suggest that fault zones within the ghsz can be hydrate or free gas rich depending on the relative supply of free gas and water from below the ghsz

seismic characteristics of fluid escape pipes in sedimentary basins implications for pipe genesis abstract fluid escape pipes were first documented from 3d seismic data over a decade ago and have subsequently been identified in many petroliferous basins worldwide they are characterized on seismic data by vertical to sub vertical zones of reduced reflection continuity that have a columnar geometry in three dimensions the upper terminations of these pipes commonly coincide with pockmarks or palaeo pockmarks signifying a close connection of pipe formation with a high flux fluid expulsion process dimensions range from tens to hundreds of metres in diameter and hundreds to over a thousand metres in height and the slenderness ratio defined as height diameter ranges from 0 8 to over 20 pipes are frequently associated with sub vertical clustering of amplitude anomalies on seismic data related either to the presence of free gas or to cementation linked to the passage of hydrocarbons three mechanisms have been suggested to explain pipe genesis 1 hydraulic fracturing 2 erosional fluidisation and 3 capillary invasion we suggest a further two possible mechanisms in the form of localised collapse by volume loss and synsedimentary flow localisation we review all five mechanisms and conclude that it is unlikely that a single mechanism applies but that combinations of these processes may all occur in particular contexts fluid escape pipes may be far more widespread that currently appreciated and they may play a critical role in secondary hydrocarbon migration and in providing leakage pathways for trapped hydrocarbons through overlying seals

evidence for extensive methane venting on the southeastern u s atlantic margin we present the first evidence for widespread seabed methane venting along the southeastern united states atlantic margin beyond the well known blake ridge diapir seep recent ship and autonomous underwater vehicle auv collected data resolve multiple water column anomalies 1000 m height and extensive new chemosynthetic seep communities at the blake ridge and cape fear diapirs these results indicate that multiple highly localized fluid conduits punctuate the areally extensive blake ridge gas hydrate province and enable the delivery of significant amounts of methane to the water column thus there appears to be an abundance of seabed fluid flux not previously ascribed to the atlantic margin of the united states

gas hydrates free gas distribution and fault pattern on the west svalbard continental margin summary a 3 d tomographic analysis of seismic velocity and attenuation fields is presented with the purpose to analyse the intimate relation of gas hydrates and free gas distribution with the fault pattern the 3 d four component seismic data have been acquired offshore western svalbard the analysis of the subbottom topography of the base of the stability field of gas hydrates indicated by the bottom simulating reflector and the thickness of the underlying free gas bearing zone suggest a fault induced compartmentalization of the gas reservoir the proposed fluid and gas circulation scheme assumes deep sourced warm fluids moving upwards mainly along high permeability faults and fractures below the gas hydrate stability zone faults change from being a pathway of fluid flow to being barriers to fluid flow in the hydrate stability zone consequently below the hydrate stability zone free gas is transferred from the permeable faults to non faulted but fault bounded sediments where it accumulates below the base of the hydrate stability zone producing a significantly thicker free gas layer the tomographic approach is therefore proposed as an effective procedure to provide detailed information on 3 d p and s wave velocity and attenuation distribution in marine sediments and specifically to examine the details of the distribution of the gas hydrates and of the free gas for which the dependence on the vp vs and qp is now fairly well known

migration of methane at the diapiric structure of the western continental margin of india insights from seismic data investigations are being carried out to locate the indirect evidences of gas occurrences in the indian continental margins by identifying features like pockmarks shale diapirs venting as well as bottom simulating reflectors bsr we investigate one such case of a shale diapirs occurrence of free gas and gas hydrates is reported using a complex trace analysis to study the variation of reflection strength and instantaneous frequency along individual reflections a frequency analysis over the flank of the shale diapir indicates attenuation of high frequencies possibly representing the migration of fluids probably methane along the fault pathways similarly a plot of reflection strength along an individual reflector shows the migration pathways and significant blanking

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

multiple bottom simulating reflections in the black sea potential proxies of past climate conditions abstract a previously unknown pattern of multiple bottom simulating reflections bsrs occurs on high resolution reflection seismic data in the danube deep sea fan associated with acoustic features indicating free gas our study provides evidence that this pattern is developed in relation with the architecture of distinct channel levee systems of the danube fan channel levee systems hosting multiple bsrs act as relatively sealed gas bearing systems whose top is situated above the base of the gas hydrate stability zone bghsz inside these systems free gas accumulates below the bghsz under a combined lithological structural and stratigraphical control the uppermost bsr marks the current equilibrium bghsz for a gas composition of more than 99 methane model derived depths of the bghsz for different gas compositions and pressure temperature conditions show that multiple bsrs would correspond to the bghsz either for 1 layers of gas hydrates with high contents of heavy hydrocarbons or hydrogen sulphide or 2 stable climatic episodes with temperatures between glacial values and the present day conditions as the gas hydrate compositions required by hypothesis 1 are in sharp contradiction with the general background of the gas composition in the study area we suggest that multiple bsrs are most probably relics of former positions of the bghsz corresponding to successive steps of climate warming in this case they can provide sea bottom paleotemperature values for these episodes and hence they are potential new proxies for deciphering past climate conditions

trapping and migration of methane associated with the gas hydrate stability zone at the blake ridge diapir new insights from seismic data abstract the blake ridge diapir is the southernmost of a line of salt diapirs along the carolina trough diapirs cause faulting of the superjacent sediments creating pathways for migration of fluids and gas to the seafloor we analyzed reflection seismic data from the blake ridge diapir which is located in a region with known abundant gas hydrate occurrence a striking feature in these data is a significant shallowing of the base of gas hydrate stability bghs over the center of the diapir the seafloor is warped up by about 100 m above the diapir from about 2300 m to about 2200 m the bghs as indicated by a bottom simulating reflection bsr is about 4 5 s off the flanks of the diapir rising to about 4 15 s at the center above the diapir a fault system appears to rise vertically from the bghs to about 0 05 s below the seafloor 40 50 m it then diverges into several steeply dipping faults that breach the seafloor and cover an area 700 m in diameter other secondary faults diverge from the main fault or emerge directly from the bghs near the crest of the diapir gas and other fluids may migrate upward through the faults we performed complex trace analysis to compare the reflection strength and instantaneous frequency along individual reflections a low frequency anomaly over the center of the diapir indicates high seismic attenuation this is interpreted to be caused by migration of fluids probably methane along fault pathways the migration of gas i e probably mainly methane through the gas hydrate stability zone is not yet understood we speculate that pore fluids in the faults may be too warm and too salty to form gas hydrate even at depths where gas hydrate is stable away from the diapir alternatively gas hydrates may seal the fault walls such that water supply is too low to transform all the gas into gas hydrates the shallowing of the bsr may reflect increased heatflow above the diapir either caused by the high thermal conductivity of the underlying salt or by advective heat transport along with fluids high pore water salinity shifts the gas hydrate stability to lower temperatures and may also play a significant role in bsr shallowing we therefore investigated the possible effect of pore water salinity on shallowing of the bsr we found that bsr shallowing may theoretically be entirely caused by increased salinity over the diapir although geologically this would not be reasonable this observation demonstrates the potential importance of pore water salinity for lateral variations of bsr depths in particular above salt structures

late holocene foraminifera of blake ridge diapir assemblage variation and stable isotope record in gas hydrate bearing sediments abstract the presence of gas hydrates on the blake ridge diapir northeastern atlantic ocean offers an opportunity to study the impact of methane seepage on the ecology and geochemistry of benthic foraminifera in the late holocene three push cores covering a time span of 1000 yrs were retrieved from three distinct microhabitats at the top of the diapir at a water depth of 2150 m i sediments away from seepage control core ii sediments overlain by clusters of methanotrophic and thiotrophic bivalves and iii chemoautotrophic microbial mats the foraminiferal assemblages at the two seep sites are marked by a reduction in benthic foraminiferal species diversity coupled with a near absence of agglutinated species however an opportunistic population rise in ch4 or h2s tolerant calcareous species e g globocassidulina subglobosa and cassidulina laevigata that utilize the abundant trophic resources at the seeps has led to an increase in the overall assemblage density there the 18o and 13c values of three species of benthic foraminifera gyroidinoides laevigatus globocassidulina subglobosa and uvigerina peregrina and the planktonic species globorotalia menardii were acquired from all three cores the benthic species from methane seeps yield 13c values of 0 1 to 4 2 vpdb that are distinctly more 13c depleted relative to the 13c of 0 4 to 1 0 vpdb at the control off seep site the species from a mussel bed site exhibit more negative 13c values than those from microbial mats possibly reflecting different food sources and higher rate of anaerobic oxidation of methane the positive 13c values in the paired planktonic species suggest that authigenic carbonate precipitation did not overprint the observed 13c depletions hence the probable cause of negative 13c of benthic foraminifera is primary calcification from dissolved inorganic carbon dic containing mixed carbon fractions from a highly 13c depleted microbially oxidized methane and b a seawater source

anomalously cold temperatures observed at the base of the gas hydrate stability zone on the u s atlantic passive margin in situ measurements to depths of 415 m below sea floor in methane hydrate bearing sediments on the u s atlantic passive margin indicate that temperatures at the bottom simulating reflector bsr are anomalously low by 0 5 2 9 c if the bsr marks the base of gas hydrate stability bghs several hypotheses may explain the occurrence of the bsr at inappropriate pressure temperature p t conditions 1 if the bsr does not mark the bghs then p t conditions need not be sufficient to dissociate gas hydrate at this depth 2 the bsr may lie at nonequilibrium p t conditions due to incomplete readjustment in response to upper pleistocene holocene climate change however the occurrence of the blake ridge bsr at an overly shallow depth cannot be easily explained by realistic combinations of pressure driven deepening sea level rise and temperature driven shoaling bottom water temperature changes 3 the properties of sediments or pore fluids may inhibit the stability of gas hydrate in particular capillary forces arising in the fine grained montmorillonite rich sediments of the blake ridge may lead to shoaling of the bsr in this setting

a review of methane and gas hydrates in the dynamic stratified system of the blake ridge region offshore southeastern north america abstract the blake ridge region lies on the passive margin of southeastern north america and contains a large amount of methane gas hydrate the methane and methane gas hydrate are predominantly biogenic apparently produced by co2 reduction reflection seismics indicate that bottom simulating reflectors bsrs enclose 55 000 km2 with high amplitude bsrs covering 26 000 km2 ocean drilling program odp leg 164 drilled three deep holes on a 10 km long transect sites 994 995 and 997 water depth 2770 2798 m based on sampling and geochemical thermal seismic and borehole geophysical measurements gas hydrates are most likely present between 190 and 450 m in sediment column gas hydrate is most often disseminated throughout the sediment column although concentrations occur within specific sedimentary horizons within supposed fault zones and at the base of gas hydrate stability bghs where methane recycling produces more pervasive concentrations of gas hydrate estimates of gas hydrate inventory are based on a variety of methods including geochemical proxies vertical seismic profiling electric logging and measurements of in situ methane over the entire sediment column at least 2 4 of pore space volume 1 2 sediment volume is occupied by methane gas hydrates but average and maximum estimates are 5 4 and 12 respectively extrapolation of vertical gas hydrate and methane inventory over the area containing high amplitude bsrs yields estimates of 67 406 gt gigatons 1015 g of methane gas hydrate or 9 52 gt of methane and 2 6 27 gt of methane occurring as gas bubbles below the bghs average values are 185 gt of gas hydrate and 24 gt of methane as gas hydrate any gas hydrate occurring outside the area underlain by bsrs as suggested by geochemical evidence or that associated with low amplitude bsrs may increase these estimates by an unknown factor various data give conflicting pictures of mass transport with blake ridge sediments the data can be reconciled by viewing the upper sedimentary section accumulation of gas hydrate in the blake ridge sediments depends on the amount of methane leaving the system versus the amount of methane entering the gas hydrate stability zone ghsz over geologic time although there are some point sources of methane loss from the sediments e g seafloor seeps odp site 996 of unknown magnitude most of the documented methane loss occurs through diffusion and consumption at the sulfate methane interface smi by anaerobic methane oxidation amo 2 108 mol year 1 methane entering the ghsz at a rate of 1 3 109 mol year 1 indicates a methane trapping efficiency of 85 129i measurements suggest that the blake ridge system has accumulated gas hydrate over as much as 55 million years

assessment of gas hydrate saturation in marine sediments from resistivity and compressional wave velocity log measurements in the mahanadi basin india abstract gas hydrate was recovered in the mahanadi basin along the eastern continental margin of india during the india national gas hydrate program nghp expedition 01 in 2006 infrared imaging of recovered core confirmed gas hydrate occurs predominantly in discrete layers pore water chemistry electrical resistivity and acoustic velocity down hole logs were used to estimate gas hydrate saturations at three of the sites in the mahanadi basin sites nghp 01 08 09 and 19 gas hydrate saturation estimated from pore water chloride concentrations shows values up to 10 of the pore space at 200 m below seafloor just above the base of the gas hydrate stability zone bghsz gas hydrate saturations estimated from electrical resistivity and acoustic velocity logs using standard relations and modeling approaches are comparable to each other and saturations are 10 15 of the pore space seismic reflection data were also analyzed for the evidence of gas hydrate and a bottom simulating reflector bsr was imaged along the seismic profiles in the study area the depth of the bsr is varying from 200 m to 300 m below seafloor depending on water depth in the mahanadi basin the occurrence of gas hydrate was observed to be associated with deep water channel and levee complexes especially at site nghp 01 19 based on the regional seismic data but the cored logged section at each site lacked any significant sand fraction which does not allow for higher gas hydrate saturations as identified from seismic time slice data all sites drilled in the mahanadi basin are within the steeper slope region of the channel system and any sand bypassed this region significant sand deposition would occur further down slope where typical fan type deposits can be inferred from the seismic data and thus higher accumulations of gas hydrate would be expected