bottom simulating reflectors seismic velocities and avo effects we obtain the wave velocities of ice and gas hydrate bearing sediments as a function of concentration and temperature unlike previous theories based on simple slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids grain and ice or clathrate and a liquid water and a porous matrix containing gas and water for consolidated berea sandstone the theory underestimates the value of the compressional velocity below 0 c including grain ice interactions and grain cementation yields a good fit to the experimental data strictly speaking water proportion and temperature are closely related fitting the wave velocity at a given temperature allows the prediction of the velocity throughout the range of temperatures provided that the average pore radius and its standard deviation are known the reflection coefficients are computed with a viscoelastic single phase constitutive model the analysis is carried out for the top and botto

preferential mode of gas invasion in sediments grain scale mechanistic model of coupled multiphase fluid flow and sediment mechanics 1 we present a discrete element model for simulating at the grain scale gas migration in brine saturated deformable media we rigorously account for the presence of two fluids in the pore space by incorporating forces on grains due to pore fluid pressures and surface tension between fluids this model which couples multiphase fluid flow with sediment mechanics permits investigation of the upward migration of gas through a brine filled sediment column we elucidate the ways in which gas migration may take place 1 by capillary invasion in a rigid like medium and 2 by initiation and propagation of a fracture we find that grain size is the main factor controlling the mode of gas transport in the sediment and we show that coarse grain sediments favor capillary invasion whereas fracturing dominates in fine grain media the results have important implications for understanding vent sites and pockmarks in the ocean floor deep subseabed storage of carbon dioxide and gas hydrate accumulations in ocean sediments and permafrost regions our results predict that in fine sediments hydrate will likely form in veins following a fracture network pattern and the hydrate concentration will likely be quite low in coarse sediments the buoyant methane gas is likely to invade the pore space more uniformly in a process akin to invasion percolation and the overall pore occupancy is likely to be much higher than for a fracture dominated regime these implications are consistent with laboratory experiments and field observations of methane hydrates in natural systems

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

pore water anomalies of submarine gas hydrate zones as tool to assess hydrate abundance and distribution in the subsurface what have we learned in the past decade abstract the most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the ocean drilling program odp notably legs 112 141 146 and 164 and to a lesser extent legs 131 160 170 and 186 leg 164 in particular was dedicated to gas hydrate drilling in the blake ridge gas hydrate field in the west atlantic in an unprecedented multidisciplinary research effort paull et al 1996 proc ocean drill program initial rep 164 623 pp paull c k matsumoto r wallace p j dillon w p eds 2000a proc ocean drill program sci results 164 ocean drilling program college station tx 459 pp most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones this started with qualitative evidence for advective flow from drilling the hydrate bearing peru and hydrate free barbados active margins continued with the nankai trough accretionary prism the japan trench slope and the cascadia and costa rica active margins and culminated in the quantitative advection diffusion model of egeberg and dickens chem geol 153 1999 53 for the passive margin setting of the blake ridge advective flow regimes are different at active and passive margins as there is a tendency for the flow at active margins to be focussed along landward dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step pattern of the pore water profiles for passive margins but also for some active margin sites advection is from sources below the drilled section either through subvertical faults passive margins or from the decollement zone active margins we have learned that the well known coupled pore water anomalies that are ascribed to hydrate dissociation downward chlorinity decrease combined with 18 o increase hesse and harrison 1981 earth planet sci lett 55 1981 453 need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low 18 o fluids however if the anomalies show up hydrates are present almost invariably with the exception of advected low cl high 18 o waters coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate affected pore water profiles at passive margins instrumental for modeling are chlorine isotopes which provide an effective tool to assess advection rates the egeberg and dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions modeling the more complex pore water profiles of active margins is a challenge for the future compared to geophysical methods to estimate hydrate concentration the geochemical method gives minimum amounts

a simple method of predicting s wave velocity prediction of shear wave velocity plays an important role in seismic modeling amplitude analysis with offset and other exploration applications this paper presents a method for predicting s wave velocity from the p wave velocity on the basis of the moduli of dry rock elastic velocities of water saturated sediments at low frequencies can be predicted from the moduli of dry rock by using gassmann s equation hence if the moduli of dry rock can be estimated from p wave velocities then s wave velocities easily can be predicted from the moduli dry rock bulk modulus can be related to the shear modulus through a compaction constant the numerical results indicate that the predicted s wave velocities for consolidated and unconsolidated sediments agree well with measured velocities if differential pressure is greater than approximately 5 mpa an advantage of this method is that there are no adjustable parameters to be chosen such as the pore aspect ratios required in some other methods the predicted s wave velocity depends only on the measured p wave velocity and porosity

characterization of in situ elastic properties of gas hydrate bearing sediments on the blake ridge during ocean drilling program leg 164 shear sonic velocity and other geophysical logs were acquired in gas hydrate bearing sediments on the blake ridge to characterize the very distinct seismic signature of such formations anomalous low amplitudes overlying a strong bottom simulating reflector bsr a comparison of the bulk moduli derived from the logs to standard elastic consolidation models shows that the sediments are overconsolidated above the bsr at 440 meters below seafloor mbsf because of the presence of hydrates below the bottom of the thermodynamic hydrate stability zone at 520 mbsf the high compressibility of the formation and the attenuation of the monopole sonic waveforms are typical of sediments partially saturated with free gas between these two depths gas hydrate and free gas seem to coexist within the gas hydrate stability zone we estimate the amount of gas hydrates using different models based on theories for wave scattering in multiphase media and for grain cementation in close agreement with measurements made on discrete in situ samples the latter describes most accurately the interactions between the matrix the pore fluids and the hydrates this model indicates that 5 to 10 of the pore space is occupied by hydrates deposited uniformly on the surface of the grains the comparison with gassmann s model also show that the amount of free gas below the bsr never exceeds 5 of the pore space but is high enough to generate the bsr the coexistence of free gas and gas hydrates below the bsr may be explained by capillary effects in the smaller pores or by remaining crystalline structures after partial hydrate decomposition

analysing sand dominated channel systems for potential gas hydrate reservoirs using an avo seismic inversion technique on the southern hikurangi margin new zealand abstract gas hydrates have recently been recognised as a class of unconventional petroleum resource and the economic viability of gas production from hydrates is now being viewed as a realistic possibility within the next decade therefore potential offshore hydrate accumulations in the world class endowed gas hydrate province the hikurangi margin new zealand represent a significant medium to long term opportunity to meet the country s future energy requirements in this paper we delineate a potential gas hydrate reservoir in the east coast basin new zealand and quantitatively estimate its gas hydrate concentrations from 2d seismic data with no well information available the target is interesting for exploration since it shows evidence for gas hydrate bearing sands in particular buried channel systems we use a combined analysis of high resolution velocity analysis amplitude versus offset avo attribute and avo inversion to investigate whether we can identify regions that are likely to contain highly concentrated gas hydrates and whether they are likely to be sand dominated to estimate hydrate concentrations we apply a rock physics model our results indicate the presence of several up to 200 m thick zones that are likely to host gas hydrates with one location predicted to consist of high permeable channel sands and an inferred gas hydrate saturation of 25 these findings suggest significant amounts of gas hydrates may be present in high quality reservoirs on this part of the margin

identifying patchy saturation from well logs gassmann s 1951 equations relate the elastic bulk ksat and shear gsat moduli of a fully saturated rock to the elastic bulk and shear moduli of the dry rock frame kdry and gdry respectively porosity and the bulk moduli of the mineral phase ks and pore fluid kf

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

gas hydrate saturations estimated from fractured reservoir at site nghp 01 10 krishna godavari basin india 1 during the indian national gas hydrate program expedition 01 nghp 01 one of the richest marine gas hydrate accumulations was discovered at site nghp 01 10 in the krishna godavari basin the occurrence of concentrated gas hydrate at this site is primarily controlled by the presence of fractures assuming the resistivity of gas hydrate bearing sediments is isotropic the conventional archie analysis using the logging while drilling resistivity log yields gas hydrate saturations greater than 50 as high as 80 of the pore space for the depth interval between 25 and 160 m below seafloor on the other hand gas hydrate saturations estimated from pressure cores from nearby wells were less than 26 of the pore space although intrasite variability may contribute to the difference the primary cause of the saturation difference is attributed to the anisotropic nature of the reservoir due to gas hydrate in high angle fractures archie s law can be used to estimate gas hydrate saturations in anisotropic reservoir with additional information such as elastic velocities to constrain archie cementation parameters m and the saturation exponent n theory indicates that m and n depend on the direction of the measurement relative to fracture orientation as well as depending on gas hydrate saturation by using higher values of m and n in the resistivity analysis for fractured reservoirs the difference between saturation estimates is significantly reduced although a sizable difference remains to better understand the nature of fractured reservoirs wireline p and s wave velocities were also incorporated into the analysis

elastic impedance inversion of multichannel seismic data from unconsolidated sediments containing gas hydrate and free gas the elastic properties of hydrated sediments are not well known which leads to inaccuracy in the evaluation of the amount of gas hydrate worldwide elastic impedance inversion is useful in estimating the elastic properties of sediments containing gas hydrate or free gas trapped beneath the gas hydrate from angle dependent p wave reflections we reprocess the multichannel u s geological survey seismic line bt 1 from the blake ridge off the east coast of north america to obtain migrated common angle aperture data sets which are then inverted for elastic impedance two new algorithms to estimate p impedance and s impedance from the elastic impedance are developed and evaluated using well log data from ocean drilling program odp leg 164 these new algorithms are stable even in the presence of modest noise in the data the vs vp ratio poisson s ratio and lame parameter terms and are estimated from the p impedance and s impedance the hydrated sediments have high elastic impedance high p impedan

scale dependent gas hydrate saturation estimates in sand reservoirs in the ulleung basin east sea of korea abstract through the use of 2 d and 3 d seismic data several gas hydrate prospects were identified in the ulleung basin east sea of korea and thirteen drill sites were established and logging while drilling lwd data were acquired from each site in 2010 sites ubgh2 6 and ubgh2 10 were selected to test a series of high amplitude seismic reflections possibly from sand reservoirs lwd logs from the ubgh2 6 well indicate that there are three significant sand reservoirs with varying thickness two upper sand reservoirs are water saturated and the lower thinly bedded sand reservoir contains gas hydrate with an average saturation of 13 as estimated from the p wave velocity the well logs at the ubgh2 6 well clearly demonstrated the effect of scale dependency on gas hydrate saturation estimates gas hydrate saturations estimated from the high resolution lwd acquired ring resistivity vertical resolution of about 5 8 cm reaches about 90 with an average saturation of 28 whereas gas hydrate saturations estimated from the low resolution a40l resistivity vertical resolution of about 120 cm reaches about 25 with an average saturation of 11 however in the ubgh2 10 well gas hydrate occupies a 5 m thick sand reservoir near 135 mbsf with a maximum saturation of about 60 in the ubgh2 10 well the average and a maximum saturation estimated from various well logging tools are comparable because the bed thickness is larger than the vertical resolution of the various logging tools high resolution wireline log data further document the role of scale dependency on gas hydrate calculations

origin and evolution of fracture hosted methane hydrate deposits 1 fracture hosted methane hydrate deposits exist at many sites worldwide these sites often have hydrate present as vein and fracture fill as well as disseminated through the pore space we estimate that thousands to millions of years are required to form fracture systems by hydraulic fracturing driven by occlusion of the pore system by hydrate this time scale is a function of rates of fluid flow and permeability loss low permeability layers in a sedimentary column can reduce this time if the permeability contrast with respect to the surrounding sediments is of order 10 or greater additionally we find that tensile fracturing produced by hydrate heave around hydrate lenses is a viable fracture mechanism over all but the lowermost part of the hydrate stability zone with our coupled fluid flow hydrate formation model we assess fracture formation at four well studied hydrate provinces blake ridge offshore south carolina hydrate ridge offshore oregon keathley canyon block 151 offshore louisiana and the krishna godavari basin offshore india we conclude that hydraulic fracturing due to pore pressure buildup is reasonable only at hydrate ridge and the krishna godavari basin owing to sediment age constraints and that hydrate filled fractures observed at blake ridge and keathley canyon block 151 are formed either by hydrate heave or in preexisting fractures our findings offer new insight into the processes and time scales associated with fracture hosted hydrate deposits which help further our understanding of hydrate systems

capillary controls on methane hydrate distribution and fracturing in advective systems 1 we simulate 1 d steady advective flow through a layered porous medium to investigate how capillary controls on solubility including the gibbs thomson effect in fine grained sediments affect methane hydrate distribution in marine sediments we compute the increase in pore fluid pressure that results from hydrate occluding the pore space and allow fractures to form if the pore fluid pressure exceeds a fracture criterion we apply this model to hydrate ridge and northern cascadia two field sites where hydrates have been observed preferentially filling cm scale coarse grained layers we find that at hydrate ridge hydrate forms in the coarse grained layers reaching saturation of 90 creating fractures through intervening fine grained layers after 2000 years at northern cascadia hydrate forms preferentially in the coarse grained layers but 2 105 years are required to develop the observed hydrate saturations 20 60 suggesting that hydrate formation rates may be enhanced by an additional source of methane such as in situ methanogenesis we develop expressions to determine the combinations of sediment physical properties and methane supply rates that will result in hydrate filled coarse grained layers separated by hydrate filled fine grained layers the conditions necessary to fracture the fine grained layers and the conditions that will lead to complete inhibition of hydrate formation as pore space is constricted this work illustrates how sediment physical properties control hydrate distribution at the pore scale and how hydrate distribution affects fracturing behavior in marine sediments

characteristics and interpretation of fracture filled gas hydrate an example from the ulleung basin east sea of korea abstract through the use of 2 d and 3 d seismic data a total of thirteen sites were selected and drilled in the east sea of korea in 2010 a suite of logging while drilling lwd logs was acquired at each site lwd logs from the ubgh2 3a well indicate significant gas hydrate in clay bearing sediments including several zones with massive gas hydrate with a bulk density less than 1 0 g m 3 for depths between 5 and 103 m below the sea floor the ubgh2 3a well was drilled on a seismically identified chimney structure with a mound feature at the sea floor average gas hydrate saturations estimated from the isotropic analysis of ring resistivity and p wave velocity logs are 80 13 and 47 16 respectively whereas they are 46 17 and 45 16 respectively from the anisotropic analysis modeling indicates that the upper part of chimney between 5 and 45 m below sea floor mbsf is characterized by gas hydrate filling near horizontal fractures 7 dip and the lower part of chimney between 45 and 103 mbsf is characterized by gas hydrate filling high angle fractures on the basis of ring resistivity and p wave velocity the anisotropic analysis using p40h resistivity phase shift resistivity at 32 mhz with 40 inch spacing and the p wave velocity yields a gas hydrate saturation of 46 15 and 46 15 respectively similar to those estimated using ring resistivity and p wave velocity but with quite different fracture dip angles differences in vertical resolution depth of investigation and a finite fracture dimension relative to the tool separation appear to contribute to this discrepancy forward modeling of anisotropic resistivity and velocity are essential to identify gas hydrate in fractures and to estimate accurate gas hydrate amounts

estimating pore space gas hydrate saturations from well log acoustic data relating pore space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment in the present study sonic velocities of gas hydrate bearing sands are modeled using a three phase biot type theory in which sand gas hydrate and pore fluid form three homogeneous interwoven frameworks this theory is developed using well log compressional and shear wave velocity data from the mallik 5l 38 permafrost gas hydrate research well in canada and applied to well log data from hydrate bearing sands in the alaskan permafrost gulf of mexico and northern cascadia margin velocity based gas hydrate saturation estimates are in good agreement with nuclear magneto resonance and resistivity log estimates over the complete range of observed gas hydrate saturations

downhole log evidence for the coexistence of structure ii gas hydrate and free gas below the bottom simulating reflector in the south china sea abstract stratigraphic layered pore filling gas hydrates are identified above the bottom simulating reflector bsr using the well log and core data acquired at sites w11 and w17 during the third gas hydrate drilling expedition conducted by china s geological survey guangzhou marine geological survey gmgs3 in the south china sea a seismic profile near site w17 reveal the presence of two bsrs i e double bsr which we show to relate to zones of structure i gas hydrate i bsr and structure ii gas hydrate ii bsr well log data from site w17 between the i bsr projected depth of 250 mbsf and ii bsr projected depth of 330 mbsf showed anomalous responses for gas hydrate bearing sediments with high resistivity high s wave velocity and alternating high and low p wave velocities pressure core data support the interpretation that structure ii gas hydrate occurs at a depth of 263 mbsf at site w17 the cross plot between log derived neutron and density porosities reveals a free gas bearing layer at a depth of 258 270 mbsf suggesting gas hydrate coexists with free gas between the i bsr and the ii bsr synthetic seismograms generated from the p wave velocity and density logs further support the presence of free gas in this section based on the coexistence of hydrate free gas and water the simplified three phase equation stpe was modified to simultaneously estimate free gas and hydrate saturations beneath the i bsr from p wave and s wave velocity logs assuming uniform or patchy distributions of free gas the estimated free gas and hydrate saturations together with gas compositions from pressure core samples collectively indicate that structure ii gas hydrate and free gas are interbedded and coexist below the i bsr at site w17 our study of the coexistence of gas hydrate and free gas between the double bsr at site w17 provides new insights into gas hydrate systems in nature that contain more complex gas chemistries

capillary effects on hydrate stability in marine sediments 1 we study the three phase liquid gas hydrate stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases the capillary pressure a measure of the pressure difference across a curved phase interface exerts a key control on the methane solubility in liquid hydrate l h and liquid gas l g systems by calculating the l h and l g solubilities as a function of water depth pressure and pore size interface curvature we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three phase zone the top of the three phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases the thickness of the three phase zone increases as the distribution of pore sizes widens the top of the three phase zone can overlie or underlie the bulk three phase equilibrium depth at blake ridge we predict that the three phase zone is 27 7 m thick and that the top of the three phase zone lies 13 m above the predicted bulk equilibrium depth this reconciles the observation of the bottom simulating reflector bsr at blake ridge that is shallower than the predicted bulk equilibrium depth in contrast at hydrate ridge where water depth is shallower we predict that the three phase zone is 20 4 m thick and that the top of the three phase zone lies 0 7 m below the predicted bulk equilibrium depth our model which predicts an upward shift in the top of free gas occurrence with increasing water depth pressure is compatible with worldwide observations that the bsr is systematically shifted upward relative to the bulk equilibrium depth as water depth pressure is increased

avo study of a gas hydrate deposit offshore costa rica amplitude variation with offset avo analysis and waveform inversion are techniques used to determine qualitative or quantitative information on gas hydrates and free gas in sediments however the quantitative contribution of gas hydrates to the acoustic impedance contrast observed at the bottom simulating reflector and the reliability of quantitative avo analyses are still topics of discussion in this study common midpoint gathers from multichannel wide angle reflection seismic data acquired offshore costa rica have been processed to preserve true amplitude information at the bottom simulating reflector for a quantitative avo analysis incorporating angles of incidence of up to 60 corrections were applied for effects that significantly alter the observed amplitudes such as the source directivity avo and rock physics modelling indicate that free gas immediately beneath the gas hydrate stability zone can be detected and low concentrations can be quantified from avo analysis whereas the offset dependent reflectivity is not sensitive to gas hydrate concentrations of less than about 10 at the base of the gas hydrate stability zone bulk free gas saturations up to 5 have been determined from the reflection seismic data assuming a homogeneous distribution of free gas in the sediment assuming a patchy distribution of free gas increases the estimated concentrations up to 14 there is a patchy occurrence of bottom simulating reflectors south east of the nicoya peninsula on the continental margin offshore costa rica avo analysis indicates that this phenomenon is related to the local presence of free gas beneath the gas hydrate stability zone probably related to a focused vertical fluid flow in areas without bottom simulating reflectors the results indicate that no free gas is present

depositional characteristics and accumulation model of gas hydrates in northern south china sea abstract the south china sea scs shows favorable conditions for gas hydrate accumulation and exploration prospects bottom simulating reflectors bsrs are widely distributed in the scs using seismic and sequence stratigraphy the spatial distribution of bsrs has been determined in three sequences deposited since the late miocene the features of gas hydrate accumulations in northern scs were systematically analyzed by an integrated analysis of gas source conditions migration pathways heat flow values occurrence characteristics and depositional conditions including depositional facies rates of deposition sand content and lithological features as well as some depositional bodies structural slopes slump blocks and sediment waves this research shows that particular geological controls are important for the presence of bsrs in the scs not so much the basic thermodynamic controls such as temperature pressure and a gas source based on this a typical depositional accumulation model has been established this model summarizes the distribution of each depositional system in the continental shelf continental slope and continental rise and also shows the typical elements of gas hydrate accumulations bsrs appear to commonly occur more in slope break zones deep water gravity flows and contourites the gas hydrate bearing sediments in the shenhu drilling area mostly contain silt or clay with a silt content of about 70 in the continental shelf bsrs are laterally continuous and the key to gas hydrate formation and accumulation lies in gas transportation and migration conditions in the continental slope a majority of the bsrs are associated with zones of steep and rough relief with long term alternation of uplift and subsidence rapid sediment unloading can provide a favorable sedimentary reservoir for gas hydrates in the continental rise bsrs occur in the sediments of submarine fans turbidity currents

attachment formation during partitioning of oblique convergence in the ketilidian orogen south greenland subhorizontal attachment zones provide coupling between lithospheric layers in orogenic belts a mid crustal attachment zone is exposed in the palaeoproterozoic ketilidian orogen south greenland which formed as a result of north directed oblique convergence at a cordilleran type margin rifting c 2 1 ga and compressional deformation and magmatism 1850 ma on the continental margin was followed by an extended sinistral transpression from 1850 to 1730 ma now separated into three episodes or peaks of activity the first episode was focused on the back arc region and was followed by the main arc construction phase during which transpression was partitioned into strike slip and contraction components despite the longevity of this active margin system individual tectonic events took place rapidly e g development of fore arc d1 d3 and accompanying high temperature low pressure metamorphism took place over c 12 ma we explain the fore arc and batholith evolution by the upward migration of an underlying attachment structure through the upper crustal partitioned blocks this migration may be attributed to an increase in the geothermal gradient accompanied by or followed by exhumation of the mid crust the partially molten hence weak attachment zone solidified and strengthened during cooling before emplacement of the post orogenic rapakivi suite during the third distinct phase of mild sinistral transpression

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

elastic velocities of partially gas saturated unconsolidated sediments abstract fluid in sediments significantly affects elastic properties of sediments and gas in the pore space can be identified by a marked reduction of p wave velocity or a decrease of poisson s ratio the elastic properties of gas saturated sediments can be predicted by the classical biot gassmann theory bgt however parameters for the bgt such as the biot coefficient or moduli of dry frame of unconsolidated and high porosity sediments are not readily available dependence of velocities on differential pressure or porosity for partially gas saturated sediments is formulated using properties derived from velocities of water saturated sediments laboratory samples for unconsolidated and consolidated sediments and well log data acquired for unconsolidated marine sediments agree well with the predictions however because the p wave velocity depends highly on how the gas is saturated in the pore space such as uniform or patch the amounts of gas estimated from the p wave velocity contains high uncertainty the modeled vp vs ratio of partially gas saturated sediment using the patch distribution is usually greater than 1 6 whereas the ratio modeled assuming a uniform distribution is about 1 6 thus poisson s ratio or vp vs ratio may be used to differentiate patch from uniform saturation but differences between various models of patch saturation cannot be easily identified

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

the nature and distribution of bottom simulating reflectors at the costa rican convergent margin summary bottom simulating reflectors bsrs at the base of the hydrate stability zone are often observed in marine seismic reflection data the compressional p wave velocity structure of a bsr offshore costa rica was analysed by applying a full waveform inversion technique the resulting velocity profile indicates that the bsr is caused by a thin low velocity layer suggesting the presence of at least a small amount of free gas in the sediment pore space in undisturbed sediment sections bsrs offshore costa rica are observed over much of the continental margin they frequently occur at shallow depths beneath the seafloor usually 100 400 m at some locations bsrs appear to intersect the seafloor this was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards in the study area the vertical movement of the hydrate stability zone relative to a given point in the sediment column one of the potential factors leading to bsr formation or suppression is controlled by both sedimentation and vertical tectonism both processes may hence play a role in controlling bsr distribution bsrs are absent in areas which have been affected by slumping except where the sedimentary section above the bsr remained intact during slumping this indicates that slope failure can cause the destruction of bsrs

early proterozoic tectonic styles and associated mineral deposits of the north china platform abstract the remarkable change in tectonic style from archean granite greenstone belts and high grade terrains to linear sedimentary basins marks the beginning of the early proterozoic in china the different evolutionary histories of and accumulations in the early proterozoic basins result in different archean early proterozoic boundaries two time limits 2 8 and 2 5 ga are significant for the boundary in eastern china early proterozoic basins displaying different tectonic styles contain different mineral deposits the liaodong peninsula type of early proterozoic basin contains the most abundant mineral resources and deposit types including stratabound b fe and pb zn deposits in the eugeosynclinal facies of the liaojitite suite and pyrite p and magnesite deposits in miogeosynclinal facies rocks the magnesite deposits in liaodong peninsula are the largest in the world and were formed in a unique geological setting