gas hydrates clathrates causing pore water freshening and oxygen isotope fractionation in deep water sedimentary sections of terrigenous continental margins the occurrence of gas hydrates in deep water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep sea drilling results on the pacific continental slope off guatemala gas hydrates were brought up for the first time from two holes 497 498a drilled during leg 67 of the dsdp in water depths of 2360 and 5500 m respectively the hydrates occur in organic matter rich pleistocene to miocene terrigenous sediments in the hydrate bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10 20 m subbottom depth pore waters at the bottom of the holes near 400 m subbottom have as little as half the chlorinity of seawater i e 9 similar but less pronounced trends were observed during previous legs of the dsdp in other hydrate prone segments of the continental margins where recharge of fresh water from the continent can be excluded e g leg 11 the crystallization of hydrates like ice excludes salt ions from the crystal structure during burial the dissolved salts are separated from the solids subsidence results in a downward motion of the solids including hydrates relative to the pore fluids thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation the volume of the latter decreases downhole thus causing downward decreasing salinity chlorinity hydrate formation is responsible for oxygen isotope fractionation with18o enrichment in the hydrate explaining increasingly more positive 18o values in the pore fluids recovered after hydrate dissociation with depth

in situ gas hydrate hydrate saturation estimated from various well logs at the mount elbert gas hydrate stratigraphic test well alaska north slope abstract in 2006 the u s geological survey usgs completed detailed analysis and interpretation of available 2 d and 3 d seismic data and proposed a viable method for identifying sub permafrost gas hydrate prospects within the gas hydrate stability zone in the milne point area of northern alaska to validate the predictions of the usgs and to acquire critical reservoir data needed to develop a long term production testing program a well was drilled at the mount elbert prospect in february 2007 numerous well log data and cores were acquired to estimate in situ gas hydrate saturations and reservoir properties gas hydrate saturations were estimated from various well logs such as nuclear magnetic resonance nmr p and s wave velocity and electrical resistivity logs along with pore water salinity gas hydrate saturations from the nmr log agree well with those estimated from p and s wave velocity data because of the low salinity of the connate water and the low formation temperature the resistivity of connate water is comparable to that of shale therefore the effect of clay should be accounted for to accurately estimate gas hydrate saturations from the resistivity data two highly gas hydrate saturated intervals are identified an upper 43 ft zone with an average gas hydrate saturation of 54 and a lower 53 ft zone with an average gas hydrate saturation of 50 both zones reach a maximum of about 75 saturation

in situ thermal conductivity of gas hydrate bearing sediments of the mallik 5l 38 well 1 detailed knowledge about thermal properties of rocks containing gas hydrate is required in order to quantify processes involving gas hydrate formation and decomposition in nature in the framework of the mallik 2002 program three wells penetrating a continental gas hydrate occurrence under permafrost were successfully equipped with permanent fiber optic distributed temperature sensing cables temperature data were collected over a 21 month period after completing the wells thermal conductivity profiles were calculated from the geothermal data as well as from a petrophysical model derived from the available logging data and application of mixing law models results indicate that thermal conductivity variations are mainly lithologically controlled with a minor influence from hydrate saturation average thermal conductivity values of the hydrate bearing sediments range between 2 35 and 2 77 w m 1 k 1 maximum gas hydrate saturations can reach up to about 90 at an average porosity of 0 3

physical properties and sedimentological features of hydrate bearing samples recovered from the first gas hydrate production test site on daini atsumi knoll around eastern nankai trough abstract before producing gas from gas hydrate it is important to clarify the physical properties of the methane hydrate reservoir and its sediments during the 2012 pressure coring campaign pressure core samples were retrieved from the northwest slope of daini atsumi knoll one of the outer ridges of fore arc basins along the northeast the nankai trough the pressure cores were sampled continuously throughout the turbidite sequences in the methane hydrate concentrated zone mhcz the cores were subjected to onboard nondestructive property analyses and x ray computed tomography x ray ct images of the cores were collected internal structures of the cores were observed in the x ray images which were used to judge core quality results for p wave velocities and bulk densities which were also measured on the pressure cores aboard the ship were compared with data from logging while drilling lwd p wave velocities of cores that were retrieved by pressure corer were compared with methane hydrate saturations calculated from several methods in general p wave velocities from logging while drilling lwd measurements corresponded to gas hydrate saturation calculated from lwd after compensating for the different vertical resolutions of lwd tools and pressure core analysis p wave velocities from the pressure cores corresponded well to methane hydrate saturation calculated from logging a unique interval at 290 300 m below seafloor was identified where methane hydrate saturations computed from lwd data did not correspond to p wave anomalies measured in cores from the same interval this difference could be due to lateral inhomogeneity in lithology between the logging and coring wells with distinct local hydrate crystallization precipitation environments

geophysical signatures for low porosity can mimic natural gas hydrate an example from alaminos canyon gulf of mexico natural gas hydrate in sand sediments can increase both the measured compressional velocity and resistivity the same geophysical signatures occur however in low porosity sand we investigate the possible occurrence of natural gas hydrate in a sand interval in alaminos canyon block 21 ac 21 in the gulf of mexico drilled by the u s gas hydrate joint industry project the sand interval has an increase in resistivity 2 2 m and a strong peak and trough at the top and bottom of the sand on exploration seismic which has been interpreted as natural gas hydrate we reexamine the logging data and construct a new reservoir model that matches the measured resistivity the high density sublayers in the sand and the surface seismic trace our model shows that the sand interval in ac 21 is most likely water saturated and the slight increase in resistivity higher measured density and the seismic amplitudes are caused by a reduction in porosity to 30 in the sand interval relative to a porosity of 42 in the surrounding marine muds more broadly we show that the average depth where the porosity of marine muds becomes lower than sand sediment is 900 mbsf though it could be as shallow as 600 mbsf for high porosity sands in any case the similar geophysical signatures for water saturated sand and low saturations of natural gas hydrate in sand probably occur throughout the gas hydrate stability zone at most sites worldwide

geologic implications of gas hydrates in the offshore of india krishna godavari basin mahanadi basin andaman sea kerala konkan basin gas hydrate resource assessments that indicate enormous global volumes of gas present within hydrate accumulations have been one of the primary driving forces behind the growing interest in gas hydrates gas hydrate volumetric estimates in recent years have focused on documenting the geologic parameters in the gas hydrate petroleum system that control the occurrence of gas hydrates in nature the primary goals of this report are to review our present understanding of the geologic controls on the occurrence of gas hydrate in the offshore of india and to document the application of the petroleum system approach to the study of gas hydrates national gas hydrate program of india executed the national gas hydrate program expedition 01 nghp 01 in 2006 in four areas located on the eastern and western margins of the indian peninsula and in the andaman sea these areas have experienced very different tectonic and depositional histories the peninsular margins are passive continental margins resulting from a series of rifting episodes during the breakup and dispersion of gondwanaland to form the present indian ocean the andaman sea is bounded on its western side by a convergent margin where the indian plate lithosphere is being subducted beneath southeast asia nghp 01 drilled logged and or cored 15 sites 31 holes in the krishna godavari basin 4 sites 5 holes in the mahanadi basin 1 site 2 holes in the andaman sea and 1 site 1 hole in the kerala konkan basin holes were drilled using standard drilling methods for the purpose of logging while drilling and dedicated wireline logging as well as through the use of a variety of standard coring systems and specialized pressure coring systems nghp 01 yielded evidence of gas hydrate from downhole log and core data obtained from all the sites in the krishna godavari basin the mahanadi basin and in the andaman sea the site drilled in the kerala konkan basin during nghp 01 did not yield any evidence of gas hydrate most of the downhole log inferred gas hydrate and core recovered gas hydrate were characterized as either fracture filling in clay dominated sediments or as pore filling or grain displacement particles disseminated in both fine and coarse grained sediments geochemical analyses of gases obtained from sediment cores recovered during nghp 01 indicated that the gas in most all of the hydrates in the offshore of india is derived from microbial sources only one site in the andaman sea exhibited limited evidence of a thermogenic gas source the gas hydrate petroleum system concept has been used to effectively characterize the geologic controls on the occurrence of gas hydrates in the offshore of india

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

numerical simulations of sand migration during gas production in hydrate bearing sands interbedded with thin mud layers at site nghp 02 16 abstract the geomechanical behavior of gas hydrate bearing sands during gas production is complex involving changes in hydrate dependent mechanical characteristics and also redistribution of stresses carried by the hydrate when an interbedded mud layer is present the complex response of the hydrate bearing sediments is more pronounced because hydrate dissociation tends to occur preferentially in the sands adjacent to the hydrate free mud layer which can act as a heat source this leads to non uniform sediment deformation and potentially to shearing deformation along the sand mud layers in addition since the hydrate bearing sand permeability becomes far greater than the mud permeability with hydrate dissociation a greater hydraulic gradient may exist along the interface between the sand mud layers these two behaviors may contribute to sand migration and thus a thorough study on the effect of a thin interbedded mud layer on the extent of sand migration is necessary this paper presents numerical sand migration modeling of sand mud interbedded hydrate bearing sediments found at site nghp 02 16 one of 25 sites drilled as a part of national gas hydrate program expedition 02 nghp 02 in the offshore of india subjected to depressurization for a comparison and better understanding simulation of homogeneous hydrate bearing sediments is also conducted it is found that greater sand migration occurs when the interbedded mud layer is present because of greater shearing deformation and greater pressure gradients at sand mud interface

inferred gas hydrates and clay diapirs near the storegga slide on the southern edge of the voring plateau offshore norway this paper presents a new data set including single channel airgun seismic lines okean long range side scan sonar data and gravity cores acquired on the edge of the voring plateau and in the region of the storrega slide the data acquisition was part of the 8th training through research ttr 8 expedition with r v professor logachev the acoustic profiles clearly show two laterally separated zones characterised by the presence of bottom simulating reflectors bsrs at about 350 ms twt subbottom depth the lower bsr zone is located on the slope of the voring plateau in the immediate vicinity of the northern headwall of the storegga slide and in some places below the slide deposits this zone runs parallel to the general trend of the continental slope the spatial distribution of the upper bsr zone located upslope in an area where diapir like structures are found does not demonstrate any topographic control interpretation of high backscatter patches on the okean sonographs associates the observed structures with fluid escape features on the seabed most of them are pockmarks but in a few places diapirs are cropping out and form dome like elevations after analysing the behaviour of the bsr sometimes crosscutting and its acoustic characteristics reversed polarity and after applying seismic inversion processing to estimate the acoustic velocity change across the bsr this reflector is interpreted to represent the base of the local gas hydrate stability field ghsf this information was used to derive the regional geothermal gradient spatial variations of the inferred geothermal field appeared to be negligible the observation of two separated bsr zones suggests different environmental controls for the growth of the hydrates enhanced reflectors observed in the intermediate zone can be explained by the presence of strata bound free gas accumulations and migration combined with overlying permeability barriers therefore a model for gas hydrate formation in relation to directional fluid migration processes fluid escape features and the presence of diapiric structures is proposed in this paper our model is supported by sedimentological analyses of seafloor samples which demonstrate the presence of stiff clays with evidence of gas migration and by paleontological studies of the cores retrieved from the pockmarks the presence of a bsr in the sedimentary section within the slide scar area implies the repositioning of hydrates to newly established equilibrium conditions after the slide event this observation allows us to simply estimate an upper limit of the fluid migration velocity in the order of several cm year finally the effect of hydrate dissociation on slope instability is considered assuming that the in situ decomposition of hydrates due to instantaneous depressurisation is slow enough to permit the excess volume of released gas and water to be re distributed through the whole sedimentary section above the paleo bsr with consuming heat and increasing pore pressure dissociation tends to shift the pt conditions back to equilibrium values it appears that the dissociation of hydrates due to sliding would cause only 0 2 increase of the pore pressure which would hardly contribute to further slope instability

india national gas hydrate program expedition 02 operational and technical summary abstract the india national gas hydrate program is being steered by the government of india s ministry of petroleum and natural gas mopng with participation of directorate general of hydrocarbons dgh oil and natural gas corporation limited ongc and the national oil companies and research institutes of india the india national gas hydrate program expedition 01 nghp 01 established the presence of gas hydrate in the krishna godavari kg and mahanadi basins and in the offshore area of the andaman sea basin however the gas hydrates discovered during nghp 01 were mainly distributed as fracture filling material in fine grained clay rich sediments the india national gas hydrate program expedition 02 nghp 02 was carried out with an objective to discover gas hydrate in sand rich sediment along the eastern offshore margin of india ongc planned and executed nghp 02 on the behalf of the mopng nghp 02 started on march 3 2015 and was completed on july 28 2015 total 147 days using the japanese scientific drilling vessel chikyu d v chikyu during nghp 02 42 holes at 25 sites were drilled cored and or surveyed with downhole logging tools these sites were located in four areas along the eastern margin of india and formally named area a mahanadi basin three sites area b northern part of the kg basin twelve sites area c central part of the kg basin six sites and area e southern part to the kg basin four sites all 25 sites established during nghp 02 were first drilled and logged with logging while drilling lwd tools and an additional 17 holes were then drilled and or cored with conventional coring tools hpcs escs or pressure coring tools pctb wireline logging was conducted in 10 holes and formation tests using a dual packer modular formation dynamics tester mdt tool were carried out in two holes the onboard science team used the laboratory facilities on the d v chikyu to examine and analyse the physical properties geochemistry and sedimentology of all the cores collected during the expedition core samples were also analysed in additional post expedition shore based studies conducted in numerous domestic and international gas hydrate research laboratories the nghp 02 sediment cores were archived at the national gas hydrate core repository in mumbai india which is associated with the ongc gas hydrate research and technology centre ghrtc the necessary data for characterizing the occurrence of gas hydrate such as interstitial water chlorinities core derived gas chemistry physical and sedimentological properties thermal images of the recovered cores pressure core and downhole measured logging data lwd and or conventional wireline log data were obtained from most of the drill sites established during nghp 02 almost all the drill sites yielded evidence for the occurrence of gas hydrate however the inferred in situ concentration of gas hydrate varied substantially from site to site for the most part the interpretation of downhole logging data core thermal images interstitial water analyses and pressure core images from the sites established during nghp 02 indicate that the occurrence of concentrated gas hydrate is mostly associated with coarser grained sand rich sediments this paper presents the operational and technical summary of nghp 02

origin of gas in gas hydrates as interpreted from geochemistry data obtained during the national gas hydrate program expedition 02 krishna godavari basin offshore india abstract this report presents the result of geochemical studies carried out on cores recovered from krishna godavari basin k g basin during the national gas hydrate program expedition 02 nghp 02 nghp 02 targeted gas hydrate accumulations in sand facies in the k g basin off the east coast of the indian peninsula three areas area b area c and area e in the k g basin were investigated during this expedition 10 research sites in k g basin were cored and samples for geochemistry analysis were collected high toc tn ratios indicate an input from terrestrial organic matter analysis of gas samples from pressure cores also acquired during nghp 02 were composed of mostly methane carbon isotopic studies show that methane gas sampled from recovered gas hydrate samples were derived from biogenic microbial sources this list of factors appears to indicate that the gas associated with the recovered hydrate bearing core samples were derived from a microbial source however the biogeochemical processes responsible for generating the microbial gases found with the recovered gas hydrates may not have been limited to current limits off the gas hydrate stability zone it is important to note here that microbial process likely extend to more than 1 km deep into the basin so gas coming from below the gh stability zone can still be from a microbial source in addition the microbial gas within the stability zone was likely in part recycled from hydrate accumulations that has passed through the base of the gas hydrate stability zone as a product of sedimentation with the dissociated microbial gas migration again vertically in the section entering the gas hydrate stability zone and reforming gas hydrates although the organic carbon required for in situ methanogenesis and gas hydrate formation is abundant in the shallow sedimentary section of the kg basin the faulted fractured and deformed stratigraphy observed in the kg basin coupled with high pore filling gas hydrate saturations at many sites suggests that advective processes may be contributing to the supply of methane to the gas hydrates in the kg basin the possible migration of gas from greater depths is also supported by seismic evidence of through going migration pathways that can connect the gas hydrate system with deeper petroleum systems it is important to note here that the deep conventional gas accumulations in the basin also contain microbially sourced gas

multi measurement approach for establishing the base of gas hydrate occurrence in the krishna godavari basin for sites cored during expedition nghp 02 in the offshore of india abstract the 2015 national gas hydrate program of india s second expedition nghp 02 acquired logging and coring datasets for constraining the base of the gas hydrate occurrence zone deepest gh and the theoretical base of gas hydrate stability zone bghs these data are used here for two primary goals to constrain the deepest occurrence of gas hydrate relative to predicted stability limits and the observed bsr and to characterize the nature of the contact between gas hydrate bearing sediment and the underlying gas hydrate free sediment a consensus depth for the deepest gh is derived for each nghp 02 coring site from downhole indicators of gas hydrate occurrence obtained from well log electrical resistivity and acoustic data pressure core compressional wave velocity measurements and conventional core measurements of anomalously low temperatures to establish the theoretical bghs models of gas hydrate phase stability with depth are compared with downhole temperature profiles derived from 1 assuming a constant geothermal gradient consistent with downhole temperature measurements and 2 assuming constant heat flow using a geotherm through the downhole temperature measurements and incorporating thermal conductivity calculated from borehole logging data although the deepest nghp 02 gh occurrences are controlled at several sites by a lithologic boundary most sites have deepest gh occurrences within a single coarse grained lithology cutoffs within a single coarse grained lithology which occur for the primary nghp 02 area b gas hydrate reservoir will inhibit pore pressure drawdowns used to extract methane from gas hydrate as an energy resource

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

comparison of the physical and geotechnical properties of gas hydrate bearing sediments from offshore india and other gas hydrate reservoir systems abstract the sediment characteristics of hydrate bearing reservoirs profoundly affect the formation distribution and morphology of gas hydrate the presence and type of gas porewater chemistry fluid migration and subbottom temperature may govern the hydrate formation process but it is the host sediment that commonly dictates final hydrate habit and whether hydrate may be economically developed in this paper the physical properties of hydrate bearing regions offshore eastern india krishna godavari and mahanadi basins and the andaman islands determined from expedition nghp 01 cores are compared to each other well logs and published results of other hydrate reservoirs properties from the hydrate free kerala konkan basin off the west coast of india are also presented coarser grained reservoirs permafrost related and marine may contain high gas hydrate pore saturations while finer grained reservoirs may contain low saturation disseminated or more complex gas hydrates including nodules layers and high angle planar and rotational veins however even in these fine grained sediments gas hydrate preferentially forms in coarser sediment or fractures when present the presence of hydrate in conjunction with other geologic processes may be responsible for sediment porosity being nearly uniform for almost 500 m off the andaman islands properties of individual nghp 01 wells and regional trends are discussed in detail however comparison of marine and permafrost related arctic reservoirs provides insight into the inter relationships and common traits between physical properties and the morphology of gas hydrate reservoirs regardless of location extrapolation of properties from one location to another also enhances our understanding of gas hydrate reservoir systems grain size and porosity effects on permeability are critical both locally to trap gas and regionally to provide fluid flow to hydrate reservoirs index properties corroborate more advanced consolidation and triaxial strength test results and can be used for predicting behavior in other nghp 01 regions pseudo overconsolidation is present near the seafloor and is underlain by underconsolidation at depth at some nghp 01 locations

d 18o enriched waters of the coast range mountains northern california connate and ore forming fluids abstract d and 18o enriched waters of several weight percent salinity issue from hot and cold springs in franciscan complex and great valley sequence rocks in the northern coast range mountains of california although these waters have been described separately in the past and called metamorphic connate and serpentinizing fluids this study shows that the hot and cold springs are all fed by one source of water trapped cretaceous seawater modified by a variety of reactions the district is an example of the complexities of subsurface waters and their importance for our understanding of metamorphic fluid sources oil field waters and ore transport problems the subsurface waters have a range of 34s values including some near marine sulfate values 20 and 13c near marine calcite values 0 strontium isotopic signatures in the spring waters are derived from the great valley sequence 87 sr 86 sr values near 0 705 the most saline spring pool is lined with serpentine phases found together as a fine white precipitate and has a ph of 11 one portion of the subsurface water is heated and reduced it transports au ag sb as and hg to the surface gold grades of the hot spring precipitates range from less than 1 ppm to over 10 ppm these heated waters are chemically and isotopically similar to the fluids which formed au mineralization at the nearby mclaughlin gold mine the hot springs precipitate pyrite and native sulfur and are supersaturated with respect to calcite other springs are less saline and are oxidized cooler and diluted versions of the hot spring fluid they do not carry ore metals but they are supersaturated with respect to silica and calcite 3 he 4 he ratios of the springs gases are all above atmospheric values and indicate a possible magmatic component consistent with the presence of quaternary clear lake volcanics 129i data yield a minimum age of 60 80 ma for the source of the iodide in the waters consistent with the connate nature of the source fluid fehn et al 1992

gas hydrates in ocean bottom sediments gas hydrates belong to a special category of chemical substances known as inclusion compounds an inclusion compound is a physical combination of molecules in which one component becomes trapped inside the other in gas hydrates gas molecules are physically trapped inside an expanded lattice of water molecules the pressures and temperatures beneath arctic water depths greater than 1 100 ft 335 m and subtropical water depths greater than 2 000 ft 610 m are suitable for the formation of methane hydrate theoretical depths to the base of a gas hydrate layer in ocean bottom sediments are determined by assuming 1 a constant hydrostatic pressure gradient 2 two typical hydrothermal gradients 3 variable geothermal gradients and 4 pure methane hydrated with conna e seawater in addition to pressure and geothermal gradient other variables affecting the stability of gas hydrate are examined these variables are hydrothermal gradient sediment thermal conductivity heat flow hydrate velocity gas composition and connate water salinity if these variables are constant in a lateral direction and the above assumptions are valid a local geothermal gradient can be determined if the depth to the base of a gas hydrate is known the base of the gas hydrate layer is seen on seismic profiles as an anomalous reflection nearly parallel to the ocean bottom cross cutting geologic bedding plane reflections and generally increasing in sub ocean bottom time with increasing water depth the acoustic impedance is a result of the relatively fast velocity hydrate layer overlying slower velocity sediments in addition free gas may be trapped beneath the hydrate thereby enhancing the reflection

evaluation of the performance of the oceanic hydrate accumulation at site nghp 02 09 in the krishna godavari basin during a production test and during single and multi well production scenarios author s moridis gj reagan mt queiruga af boswell r abstract 2018 elsevier ltd the objective of this study is to quantify by means of numerical simulation the response of the complex system of gas hydrate accumulations at site nghp 02 09 krishna godavari basin indian ocean to different production conditions and to determine the technical feasibility of gas production through depressurization induced dissociation the study assesses the suitability of the site for a long term production test involving a single vertical well and the long term potential of the deposit under full field production using a system of multiple vertical wells we simulate gas and water flow estimate the production performance of the accumulation and separately investigate the corresponding geomechanical response of the system results indicate that production from site nghp 02 09 under the conditions of a long term field test involving a single vertical well is technically feasible and can yield high gas production rates however an inability to fully isolate the water bearing zones results in production that is largely from dissolved gas rather than hydrate dissociation and is thus burdened by excessive water production given the estimated physical properties of the reservoir system site nghp 02 09 does not appear to be a promising location for a single well field test of gas production but may be a promising production target for full field production operations using a multi well system in which exterior wells can mitigate water inflows to allow interior wells to more effectively depressurize the formation and capture methane from gas hydrate dissociation geomechanical issues need to be carefully considered as significant displacements are possible which can be challenging to well construction and stability

gas hydrate saturation and reservoir characterization at sites nghp 02 17 and nghp 02 19 krishna godavari basin eastern margin of india abstract we have characterized the gas hydrate reservoir at sites nghp 02 17 and nghp 02 19 based on isotropic and anisotropic velocity modeling by utilizing both the wireline wl and logging while drilling lwd data independently the isotropic study is carried out using the simplified three phase biot equation stpbe with density derived porosity and calculated bulk density of sediments as inputs the anisotropic study uses the transverse isotropic theory for laminated media by considering a layered reservoir model composed of two components i fractures filled with 100 gas hydrates and ii isotropic medium composed of water saturated sediments in the anisotropic velocity modeling the reservoir is modeled by incorporating fractures with varying dip angles from 0 to 90 we estimated gas hydrate saturations using isotropic and anisotropic velocity analysis at both sites nghp 02 17 and nghp 02 19 in both the cases the lwd data derived isotropic model yielded what appears to be the most accurate values an average saturation of 45 8 in the high velocity interval between 269 and 285 5 mbsf at site nghp 02 17 and 33 8 in the high velocity interval between 306 and 312 mbsf in nghp 02 19

modeling of the influence of gas hydrate content on the electrical properties of porous sediments the geophysical investigation of gas hydrate accumulations within marine sediments and the sediments of permafrost regions suffer from a lack of information on the influence of the hydrate content on the physical sediment properties the estimation of the hydrate content using downhole electrical measurements based on archie s law requires the knowledge of the saturation exponent the saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape connectivity and constrictivity of the pore network and the distribution of the conducting phase based on different models that account for the different morphological forms of gas hydrates found during gas hydrate sampling in various research wells the influence of gas hydrate content on the electrical properties of the hydrate bearing sediment was investigated for all studied forms of hydrate occurrence disseminated in the pore space nodular and layered the saturation exponent depends on the sediment properties and on saturation itself the growth of gas hydrate nodules lenses and layers is a process that is assumed to result in the displacement and compaction of the surrounding sediment because of this change of sediment properties during hydrate generation the saturation exponent for these forms of hydrate occurrence depends strongly on the relationship between porosity and formation resistivity factor expressed in the form of archie s cementation exponent for the case that hydrate occurs disseminated in the pore space and the assumption that capillary effects are important for hydrate generation the saturation exponent depends on grain size and grain size sorting for the parameters chosen for these model calculations the saturation exponent varies between 0 5 and 4 the use of a constant mean value for the saturation exponent of approximately 2 can result in both underestimation and overestimation of the hydrate content

gas hydrate saturation in the krishna godavari basin from p wave velocity and electrical resistivity logs abstract during the indian national gas hydrate program nghp expedition 01 a series of well logs were acquired at several sites across the krishna godavari kg basin electrical resistivity logs were used for gas hydrate saturation estimates using archie s method the measured in situ pore water salinity seafloor temperature and geothermal gradients were used to determine the baseline pore water resistivity in the absence of core data arp s law was used to estimate in situ pore water resistivity uncertainties in the archie s approach are related to the calibration of archie coefficient a cementation factor m and saturation exponent n values we also have estimated gas hydrate saturation from sonic p wave velocity logs considering the gas hydrate in frame effective medium rock physics model uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model in both methods we assume that gas hydrate forms in sediment pore space combined observations from these analyses show that gas hydrate saturations are relatively low s h 15 20 in two zones between 168 and 198 mbsf site nghp 01 05 s h 35 38 in two discrete zone between 70 and 90 mbsf and site nghp 01 07 shows the gas hydrate saturation more than 25 in two zones between 75 and 155 mbsf a total of 10 drill sites and associated log data regional occurrences of bottom simulating reflectors from 2d and 3d seismic data and thermal modeling of the gas hydrate stability zone were used to estimate the total amount of gas hydrate within the kg basin average gas hydrate saturations for the entire gas hydrate stability zone seafloor to base of gas hydrate stability sediment porosities and statistically derived extreme values for these parameters were defined from the logs the total area considered based on the bsr seismic data covers 720 km 2 using the statistical ranges in all parameters involved in the calculation the total amount of gas from gas hydrate in the kg basin study area varies from a minimum of 5 7 trillion cubic feet tcf to 32 1 tcf

origin of low chloride fluid in sediments from the eastern continental margin of india results from the national gas hydrate program expedition 02 abstract national gas hydrate program expedition 02 was conducted in 2015 by d v chikyu in the western bay of bengal india analyses of interstitial water in sediment from four drilling sites along the eastern and western transect in the krishna godavari basin show that dissolved chloride cl concentrations decreased with depth from seawater values 550 mm near the sediment surface to 300 400 mm at 350 400 m below seafloor mbsf suggesting the upward advection of low cl fluid excursions of cl concentrations toward low values in gas hydrate zones were attributed to the presence of fresh water released by dissociation of gas hydrate during core recovery at sites in the western transect cl concentrations below the gas hydrate reservoir around 280 mbsf were ca 60 mm and ca 45 mm lower than those from above the reservoir and 18o and d values were 1 2 and 0 7 1 3 higher than those from above the reservoir respectively these results suggest that a fraction of migrating low cl fluid is trapped in the gas hydrate zone causing decreases of 18o and d in the residual water due to isotopic fractionation during gas hydrate formation from mass balance calculations we estimated that 10 20 of the advected fluid is trapped at the gas hydrate zone at all sites the 18o and d values below the gas hydrate zone representing relatively unaltered low cl fluid were 0 5 1 higher and ca 13 15 lower than those of seawater respectively this trend is consistent with water derived from the dehydration of clay minerals which generally occurs in deeply buried sediments the contribution of clay mineral dehydration is relatively large at the western transect compared to that in the eastern transect our results imply that fluid advection contributes to the accumulation of gas hydrate in the study area

numerical analysis of gas production potential from a gas hydrate reservoir at site nghp 02 16 the krishna godavari basin offshore india feasibility of depressurization method for ultra deepwater environment abstract sandy gas hydrate reservoirs are considered to be promising deposits for gas recovery but the feasibility of the depressurization method in ultra deepwater environments remains unclear because high pressure gradient induced by depressurization may increase the water production and reduce the gas water ratio of produced fluid in numerical simulations this study predicts the gas production behavior of a hydrate reservoir site nghp 02 16 in the krishna godavari basin offshore india located in ultra deepwater depth 2546 5 m the reservoir properties such as the hydrate saturation and initial effective in situ permeability were determined from log data and pressure core analysis by comparing the hydrate saturations calculated from resistivity log with the estimations from p wave velocities of pressure cores the model parameters a m and n of archie s equation were determined as 1 8 2 0 and 2 9 respectively the initial effective permeability of the hydrate bearing layers with up to 78 hydrate saturation was estimated to range from approximately 0 01 to 1 0 millidarcy md at day 180 after the start of depressurization in a single vertical well the gas production rates ranged from 320 to 9500 sm3 d depending on the bottom hole pressure lowering the bottom hole pressure increased the gas production rate and gas water ratio of the produced fluid the bottom hole pressure of 10 mpa or less is recommended for the production test condition in this site however aquifer below the gas hydrate bearing zone and poor hydraulic sealing of the over and under burden increased the water production hydraulic isolation of the gas hydrate bearing layers is essential when applying depressurization in an ultra deepwater environment

electrical anisotropy due to gas hydrate filled fractures in 2006 the indian national gas hydrate program expedition01 ornghp 01 discoveredgashydrateasfillinnearvertical fractures in unconsolidated sediments at several drillingsitesontheindiancontinentalmargins thesegashydrate filled fractures were identified on logging while drilling resistivity images the gas hydrate filled fracture intervals coincide with high measured resistivity at the nghp 01 sites high measured resistivity translates into high hydrate saturationsviaarchie sequation however thesehighsaturations contradict lower gas hydrate saturations determined from pressure core and chlorinity measurements also in intervalswithnear verticalgashydrate filledfractures thereis considerable separation between phase shift and attenuation resistivity logs with 2 mhz resistivity measurements being significantly higher than 400 khz resistivity measurements we modeled the sensitivity of the propagation resistivity measurements in the gas hydrate filled fracture intervals at nghp 01 sites 5 and 10 near vertical hydrate filled fracturescancausetheabnormallyhighresistivitymeasurements in vertical holes due to electrical anisotropy the model suggests the gas hydrate saturations in situ are usually significantlylowerthanthosecalculatedfromarchie sequation in addition these modeled gas hydrate saturations generally agree with the lower gas hydrate saturations obtained from pressure core and chlorinity measurements at nghp 01 sites5and10

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

physical property characteristics of gas hydrate bearing reservoir and associated seal sediments collected during nghp 02 in the krishna godavari basin in the offshore of india abstract india s national gas hydrate program expedition 02 nghp 02 was conducted to better understand geologic controls on gas hydrate occurrence and morphology targeting coarse grained sediments along the lower continental slope offshore eastern india this study combines seismic logging while drilling data and a petroleum system approach to provide a regional geologic and lithologic context for 1 gas hydrate morphology and distribution and 2 effects of fine grained sediments clays and other grains smaller than 63 m on gas production in nghp 02 area b in the krishna godavari basin area b seismic data show a buried anticline syncline structure with strong reflectors r1 and r2 that delineate two of the five lithologic units unit i shallowest ii r1 iii iv r2 and v below the bas of gas hydrate stability throughout area b gas hydrate morphology depends on its placement within these units specifically core and grain scale measurements indicate fines content exerts a primary control on the gas hydrate distribution and morphology units i ii and iii are generally fine grained on the anticline crest these units host primarily grain displacing gas hydrate veins linked to pore occupying gas hydrate in thin localized coarser grained deposits diatoms in unit iii increase porosity with depth reaching 70 where it contacts unit iv the gas hydrate reservoir associated with reflector r2 the unit iii lithology and porosity allow fluid and dissolved phase methane to escape unit iv and form gas hydrate in the fine grained overburden within unit iv fine grained layers are interbedded with coarser grained gas hydrate reservoir sands and the fines content even in the sands is high enough to impact hydraulic and mechanical properties during gas production fluid motion during gas production can mobilize fines which can then clog pore throats limiting production rates pore water freshening during gas hydrate dissociation can increase fines mobilization particularly given the smectite identified in the fine grained interbeds

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

gas hydrate occurrence and distribution controlled by regional geological structure off eastern india estimates from logging while drilling in area b national gas hydrate program expedition 02 nghp 02 abstract the national gas hydrate program expedition 02 nghp 02 was designed to examine gas hydrate occurrences off the eastern margin of the indian peninsula twelve holes were logged with logging while drilling lwd tools and nine holes at five sites were cored in area b of nghp 02 five logging units were defined based on the characteristics in logging data responses distinctive logging units r1 and r2 characterized by high gamma ray log values and composed of quartz and biotite rich medium sized sand grain units that act as gas hydrate resevoirs and correlated to strong seismic reflectors hydrate bearing intervals can be identified by resistivity and sonic velocity logs and classified into pore filling type fracture filling type and a mixture of pore and fracture filling types based on borehole image characteristics pore filling type gas hydrate is typically developed in the case where the depths of r1 and or r2 is close to the seismic inferred bottom simulating reflector bsr fracture filling type hydrates associated the r1 section occur above the bsr associated with the anticline in area b the occurrence of pore filling and fracture filling gas hydrate represent different processes of hydrate formation related to the regional geological structure the stratigraphic layered pore filling gas hydrate occurrences are generated along often dipping sand layers which are also conduits for methane bearing fluid migrating from below the bsr in many settings fracture filling type gas hydrate is an alternative mechanism for the occurrence of gas hydrate in mudstone rich sediments above the bsr and found associated with the anticline in area b this structural constraint of the hydrate bearing fractures in the area b anticline suggest that the fracture dominated reservoirs possibly formed by anticlinal flexure and the upward flow of methane bearing fluids along porous sedimentary layers and the subsequent formation of pore filling gas hydrate occurrences account for the range of gas hydrate accumulations discovered during nghp 02

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

assessing the geomechanical stability of interbedded hydrate bearing sediments under gas production by depressurization at nghp 02 site 16 abstract establishing the geomechanical stability of marine sediments in the vicinity of a production well is one of the key design considerations in planning offshore gas production from marine hydrate reservoirs this paper presents an assessment of the sediment stability at india s national gas hydrate program expedition 2 nghp 02 site 16 area b offshore eastern india for which gas production is to be carried out by depressurization one important feature of the study is that extensive calibration of constitutive model parameters has been conducted based on laboratory test data from pressured core samples from analysis perspective the site is challenging because the hydrate reservoir consists of thin layers of hydrate bearing sands interbedded with mud moreover depressurization at the depth of a reservoir more than 2750 m below sea surface will lead to a pore pressure drop and accordingly an effective confining stress increase as high as 25 mpa in dealing with thin interbedded hydrate bearing strata meshing requirements for flow and geomechanical analysis are quite different from those for reservoirs with thicker massive layers an axisymmetric model and one way coupling simulations were thus adopted for this study in which the geomechanical study utilizes pore pressure and hydrate saturation output from the flow study but the flow study does not takes the porosity changes from the geomechanical analysis instead the reduction of porosity due to sediment deformation in the flow study is based on a pressure dependent pore compressibility relationship derived from geomechanical modeling the rationality is validated through back computing the pore compressibility from the geomechanical deformation results the study shows that large compression in the reservoir will result in movement of the sediments from above and below as well as laterally in smaller magnitudes and the sediment is deemed stable during the gas production period

constraints on the fluid supply rate into and through gas hydrate reservoir systems as inferred from pore water chloride and in situ temperature profiles krishna godavari basin india abstract we estimate the rate of upward pore fluid flow and chlorinity cl concentrations at depth through a joint analysis of cl concentration and temperature versus depth profiles obtained from gas hydrate related test sites drilled and cored in the krishna godavari basin off the eastern coast of india cl measured on conventional core samples decreases with depth at all sites but some of the cl profiles show a prominent convex shape whereas in situ temperature profiles obtained by the advanced piston coring temperature probe are mostly linear at all sites established during the national gas hydrate program 02 expedition nghp 02 assuming a one dimensional time dependent model for the advection of pore fluid including the sedimentation effect and depth dependent diffusivity we estimate darcy velocity and the cl concentrations at depth the best fit darcy velocity of 1 2 1 6 10 11 m s was estimated for the sites along the crest of the regional anticlinal structure in the nghp 02 area b which was significantly faster than those on the flanks of the anticline because the thermal diffusion coefficient is much larger than the chloride ion diffusion coefficient estimated darcy velocities are not great enough to generate nonlinear temperature profiles with depth which is consistent with observed linear thermal profiles

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

seismic and heat flow constraints from the gas hydrate system in the krishna godavari basin india abstract high resolution multi channel seismic reflection data acquired in the offshore domain of the krishna godavari kg basin provide seismic indicators of gas hydrate including bottom simulating reflectors bsr and high reflectivity zones below the bsr as an indication of free gas the recent recovery of gas hydrate by drilling across various sites in the kg basin together with the seismic indicators suggests overall widespread favourable gas hydrate stability conditions the bsrs provide indirect estimates of geothermal gradients and heat flow a simple conductive model is used to calculate heat flow from the depth of the bsr bsr derived heat flow values range from about 30 to 102 mw m2 across the kg basin we also modeled the base of gas hydrate stability zone bghsz to analyse the linkage between gas hydrate occurrences in the basin and its relation to the fault systems in the basin the influence of bottom water temperature pore water salinity and geothermal gradient variation on gas hydrate stability zone ghsz thickness is analysed in relation to both geological formations and tectonics our analysis suggests a highly variable ghsz in the kg basin controlled by local variations in the parameters of stability condition and faulting the bsr derived heat flow increases towards deeper water depths across the basin consistent local variations were also observed with low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs this variation may be due to focusing and defocusing effects of the topography however local bsr depths patterns may also be due to dynamic effects including the displacement of isotherms by thrust faulting additionally recent mass wasting events and normal faulting resulted in the bghsz being too shallow relative to the predicted model which assumed equilibrium conditions

live bed scour at submerged weirs abstractweirs or bed sills are low head hydraulic structures used for bed stabilization raising upstream water level and reducing flow velocity during high flow events the weir is fully submerged in the river and scouring occurs both upstream and downstream of the weir for a fully submerged weir the scour mechanism around the weir is dependent on approach flow intensity clear water scour conditions or live bed scour conditions and flow regimes surface flow regime or impinging jet regime over the weir the fast evolution of underwater mobile topographies and propagating bedforms increase the complexities of the scour process and the difficulties for scour measurement at the submerged weir under live bed scour conditions this paper develops a measurement and data processing technique for the study of scour at submerged weirs under extreme measurement environments and investigates the scour process both upstream and downstream of submerged weirs under live bed scour conditions the experiments are

lithofacies characterization of gas hydrate prospects discovered during the national gas hydrate program expedition 02 offshore krishna godavari basin india abstract lithofacies of host sediments associated with gas hydrate reservoirs may have a significant influence on the formation of hydrates the type of sediments and the depositional processes responsible for the gas hydrate bearing sediments along the eastern margin of the india have been studied by integrating core data and logging while drilling downhole logs borehole data correlated with seismic data were used for the interpretation of geological process responsible for gas hydrate formation the acquired seismic profiles were correlated with the downhole logs by identifying the shape of the well logs curves that indicate finning up and blocky patterns turbidite sands and debris flows were also identified from the log and core sediment studies this study identified two prominent types of depositional systems 1 turbidity currents and 2 debris flows were mainly responsible for the deposition of gas hydrate bearing reservoirs in the study area based on these studies two well developed gas hydrate systems have been identified in the krishna godavari basin and hydrates were determined to be distributed mostly in sand dominated facies fine grained sand in the identified turbidite deposits were shown to be a good hydrate reservoir the hydrate bearing sediments in the central part of the krishna godavari basin area c in d6 block consist of mostly debris flows which is consistent with scattered gravel and sand silt sized grains in channel levee systems

seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the beaufort sea the upper continental margin of the beaufort sea north of alaska is underlain by a strong bottom simulating reflection bsr that lies 300 to 700 m beneath the seafloor and corresponds to the phase boundary between interstitial water and natural gas below and solid gas hydrate above bsrs of similar origin are common worldwide where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment with or without underlying free gas in the sediment surprisingly little is known about the origin of these strong reflections in this paper we analyze the contrasting physical properties which produce bsrs by comparing synthetic bsr amplitudes and waveforms for varying source receiver offsets with multichannel seismic reflection data across the well developed bsr of the beaufort sea in order to discriminate whether free gas is present under the bsr or not it was necessary to supplement near vertical incidence data with prestack offset data the amplitude versus offset avo analysis indicates that the bsr is produced mainly by the existence of free gas in the clastic sediments beneath the bsr the zone of free gas is based on vertical incidence synthetics estimated to be thinner than 11 16 m it is possibly thicker than 16 m if the gas concentration decreases with depth saturation of gas hydrate in the sediment above the bsr is tentatively estimated from the avo modeling to be less than 10 of the pore volume