implication of seismic attenuation for gas hydrate resource characterization mallik mackenzie delta canada wave attenuation is an important physical property of hydrate bearing sediments that is rarely taken into account in site characterization with seismic data we present a field example showing improved images of hydrate bearing sediments on seismic data after compensation of attenuation effects compressional quality factors estimated from zero offset vertical seismic profiling data acquired at mallik northwest territories canada demonstrate significant wave attenuation for hydrate bearing sediments these results are in agreement with previous attenuation estimates obtained from sonic logs and crosshole data at different frequency intervals the application of an inverse q filter to compensate attenuation effects of permafrost and hydrate bearing sediments improved the resolution of surface 3d seismic data and its correlation with log data particularly for the shallowest gas hydrate interval compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone our results show that characterization of the mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate bearing strata and hence the volume of hydrates in place

influence of recent depositional and tectonic controls on marine gas hydrates in trujillo basin peru margin abstract the development of bottom simulating reflectors bsrs and the gas hydrate stability zone ghsz in continental margins is related to recent depositional and tectonic processes this interrelation is important for understanding the potential resource of gas hydrate deposits the purpose of this study is to understand the influence of such activity on the marine gas hydrate system of the seaward extension of the trujillo basin peru margin here we analyze near seafloor heat flow probe data high resolution multichannel seismic mcs profiles swath bathymetry and ocean floor observation system ofos images based on our results we identify three main physiographical subregions in the study area 1 an area with turbidites continuous bsrs and low to moderate near seafloor heat flow 7 33 mw m 2 2 an area with sediment waves of turbidity origin a mix of continuous and patchy bsrs and moderate near seafloor heat flow 26 39 mw m 2 3 an area with extensional faulting patchy bsrs and moderate to high near seafloor heat flow 52 110 mw m 2 we conclude that sediment flow processes are the result of recent depositional controls and faulting is the result of recent tectonic activity near seafloor heat flow around chemoherms is moderate estimates of bsr derived heat flow show differences to near seafloor heat flow this difference points to advection of fluids occurring at the seafloor alive calyptogena and tube worms were observed in these venting zones based on our analysis we conclude that 1 recent depositional processes control the development of continuous bsrs and may restrict heat transfer through the ghsz in the form of fluid venting at the seafloor 2 recent tectonic processes control the development of patchy bsrs and allow heat transfer through the ghsz along faults which is manifested as fluid venting at the seafloor

attenuation measurements from sonic waveform logs in methane hydrate bearing sediments at the nankai trough exploratory well off tokai central japan 1 i have used full waveform logs from the nankai trough exploratory well off central japan to estimate both compressional and shear attenuation in sediments containing methane hydrate mh the attenuation estimates are based on a median frequency shift to the amplitude spectrum of the recorded waveforms this paper is concerned with attenuation at sonic frequencies of 10 20 khz for compressional waves and 500 1000 hz for shear waves i observed that the presence of mh increases the host sediments seismic attenuation and shear attenuation may be more helpful than compressional attenuation in detecting or characterizing mh bearing sediments as compressional attenuation is affected by the presence of gas moreover the ratio of compressional to shear attenuation is found to be a more sensitive indicator of the presence of low saturation gas than the corresponding velocity ratio

effect of thermal non equilibrium seafloor topography and fluid advection on bsr derived geothermal gradient abstract the seafloor and bottom simulating reflectors bsrs are interpreted from the 3d seismic data acquired in krishna godavari kg offshore basin in the vicinity of sites drilled cored during national gas hydrate program nghp expedition 01 the shallow structures such as inner toe thrust fault system regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles the geothermal gradient is estimated from the depths and temperatures of the seafloor and the bsr the temperature at the bsr depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the bsr represents the base of the gas hydrate stability zone the spatial variations in geothermal gradient gtg show a strong correlation with seafloor topography in the kg basin the gtg decreases by 13 30 over the topographic mounds formed due to inner toe thrust faults and recent mass transport deposits the gtg decreases by only 5 10 over the mounds likely due to defocusing of heat flux based on one dimensional topographic modeling hence the gtg perturbation due to topography alone cannot explain the observed gtg anomaly the temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe thrust faults or rapid deposition of sediments due to slumping sliding in contrast an increase in gtg by 10 15 is observed in the vicinity of major fault systems we presume that the likely mechanism for the increase in gtg is fluid advection from a deeper part of the basin a detailed thermal modeling involving the effect of surface topography high sedimentation rates fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in kg offshore basin

the structure of methane gas hydrate bearing sediments from the krishna godavari basin as seen from micro ct scanning abstract the indian national gas hydrate program nghp expedition 1 of 2006 cored through several methane gas hydrate deposits on the continental shelf around the coast of india the pressure coring techniques utilized during the expedition hyacinth and pcs enabled recovery of gas hydrate bearing fine grained sediment cores to the surface after initial characterization core sections were rapidly depressurized and submerged in liquid nitrogen preserving the structure and form of the hydrate within the host sediment once on shore high resolution x ray ct scanning was employed to obtain detailed three dimensional images of the internal structure of the gas hydrate using a resolution of 80 m the detailed structure of the hydrate veins present in each core could be observed and allowed for an in depth analysis of orientation width and persistence of each vein hydrate saturation estimates could also be made and saturations of 20 30 were found to be the average across the core section with some portions showing highs of almost 60 saturation the majority of hydrate veins in each core section were found to be orientated between 50 and 80 to the horizontal analysis of the strikes of the veins suggested a slight preferential orientation in individual sample sections although correlation between individual sections was not possible due to the initial orientation of the sections being lost during the sampling stage the preferred vein orientation within sample sections coupled with several geometric features identified in individual veins suggest that hydraulic fracturing by upward advecting pore fluids is the main formation mechanism for the veined hydrate deposits in the k g basin

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

synthesis of deep multichannel seismic and high resolution sparker data implications for the geological environment of the krishna godavari offshore eastern continental margin of india abstract proven hydrocarbon occurrence in the krishna godavari kg basin a part of the eastern continental margin of india initiated intense exploration activities to discern the potential hydrocarbon bearing structures payoff zones recent discovery of gas hydrates in its offshore triggered detailed exploration measures adopting advanced seismic data acquisition techniques supported by deep drilling to understand its genesis interpretation of multichannel seismic mcs sections highlighted several distinct horizons from lower cretaceous to recent in the sediment overburden and the nature and configuration of the underlying crust the sections also depicted the presence of growth faults shale bulging toe thrust faulting delta progradation active sliding slumping buried channels pinchout structures and reflection free zones indicating the different types of sedimentary processes varied geological environments and neotectonic activity in the kg offshore high resolution sparker hrs data revealed subsidence gas fluid expulsion pathways occasional diapiric intrusions and growth faulting in the upper slope and toe thrust faulting upthrust and diapiric intrusions in the lower slope the mid slope is characteried by a well stratified e w trending basin bounded by upthrust and diapirism the entire region is associated with gas escape features blanking zones columnar type gas vents fault controlled mud shale diapirs large dimension gas saturated zones and pockmarks the growth faults shale bulging and toe thrusting seen in the mcs data since oligocene time could be linked to shale diapirism upthrust gas fluid expulsion subsidence and mini basin formation observed on the shallow hrs records synthesis of these datasets suggests that the distribution of gas hydrate in the shallow sediments may be controlled by the deeper structural and tectonic configuration therefore knowledge about shallow sedimentary column and its connectivity with deeper strata through faults fractures and other structural features and the geological architecture are significantly important to evaluate the genesis of gas hydrates

the interdisciplinary marine system of the amundsen sea southern ocean recent advances and the need for sustained observations the southern ocean exerts a profound influence on the functioning of the earth system in part because its location and unique bathymetric configuration enable direct linkages to the other major ocean basins ganachaud and wunsch 2000 and lumpkin and speer 2007 it is the site of the world s largest current system the antarctic circumpolar current acc which transfers waters and climatically ecologically important tracers between the atlantic indian and pacific oceans rintoul et al 2001 in addition to the strong horizontal connectivity the acc is also characterized by a vigorous overturning circulation which upwells warm nutrient rich waters from intermediate depth to the surface where they are modified by interactions with the atmosphere and cryosphere to form new water masses some of which are lighter and others more dense marshall and speer 2012 this overturning circulation structures the southern ocean both horizontally and vertically dictates the levels of its communication with the rest of the global ocean and is a fundamental control on the sequestration of carbon from the atmosphere into the ocean interior sallee et al 2012 in some locations the upwelled waters can intrude onto the antarctic shelves supplying heat and nutrients to the shallower regions this is believed to be especially effective in west antarctica where the southern edge of the acc moves close to the shelf break martinson 2011 orsi et al 1995 and thoma et al 2008

seismic imaging of migration pathways by advanced attribute analysis alaminos canyon 21 gulf of mexico abstract potential accumulations of gas hydrates in alaminos canyon block 21 ac21 in the gulf of mexico are thought to occur in a shallow sand rich interval stratigraphically separated from sources of free gas below the base of the gas hydrate stability zone bghsz by an intervening thick layer of clay and silt rich sediments availability of sufficient gas charge from depth in addition to local biogenic sourcing is considered key to the formation of gas hydrates in the ghsz implicitly a detailed understanding of geometries associated with fault and fracture networks in relation to potential gas migration pathways can provide additional confidence that seismic amplitude anomalies are related to gas hydrate accumulations delineation of fault and fracture systems from high resolution seismic data in and below the gas hydrates stability zone ghsz was performed using an automated algorithm ant tracking the capturing of small scale detail has particular significance at ac21 revealing a pervasive network of typically small extent discontinuities indicative of fracturing throughout this intervening clay and silt rich layer of mass transport deposits mtds ant tracking features appear to correlate to some extent with potential gas hydrate accumulations supporting the concept that fracturing possibly provides migration pathways albeit via a tortuous complex path this study demonstrates that the ant tracking attribute in conjunction with detailed seismic interpretation and analysis can provide valuable evidence of potential gas migration pathways

the impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two dimensional 2 d numerical model that accounts for mass transfer over geological timescales the model extends a previously documented one dimensional 1 d model such that lateral variations in permeability k become important various simulations quantitatively demonstrate how focused fluid flow through high permeability zones affects local hydrate accumulation and saturation simulations that approximate a vertical fracture network isolated in a lower permeability shale kfracture kshale show that focused fluid flow through the gas hydrate stability zone ghsz produces higher saturations of gas hydrate 25 70 and free gas 30 60 within the fracture network compared to surrounding shale simulations with a dipping high permeability sand layer also result in elevated saturations of gas hydrate 60 and free gas 40 within the sand because of focused fluid flow through the ghsz increased fluid flux a deep methane source or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high permeability zones permeability anisotropy with a vertical to horizontal permeability ratio on the order of 10 2 enhances transport of methane charged fluid to high permeability conduits as a result gas hydrate concentrations are enhanced within these high permeability zones the dip angle of these high permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects hydrate and free gas saturations can be characterized by a local peclet number localized vertical focused and advective flux relative to diffusion relative to the methane solubility gradient somewhat analogous to such characterization in 1 d systems even in lithologically complex systems local hydrate and free gas saturations might be characterized by basic parameters local flux and diffusivity

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

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

pore pressure prediction in gas hydrate bearing sediments of krishna godavari basin india abstract predrill estimate of pore pressure pp is important for well design especially in areas containing gas hydrate bearing sediments here we report the pressure distribution in the gas hydrate bearing sediments in krishna godavari k g basin predicted from the seismic data along two lines passing through the site 10 of expedition 01 exp 01 conducted by the indian national gas hydrate programme nghp a series of elastic parameters namely p wave velocity vp s wave velocity vs density vp vs p impedance zp and s impedance zs were derived from seismic data using pre stack inversion first the pp estimated from a sonic transit time log was calibrated with pressure core measurements in the depth interval of 1060 1280 m its magnitude is found to be ranging from 10 5 to 12 9 mpa where the vertical stress varies from 10 7 to 13 4 mpa within the same depth interval this estimated pp was next treated as a target log desired output and seismic attributes resulting from pre stack seismic inversion were used as input parameters in the training of a multi layered feedforward network mlfn finally we use the trained network to generate a subsurface pp distribution map along two 2d multichannel seismic profiles line x and line y passing through nghp01 10 within a time interval of 1420 1620 ms corresponding to a depth interval of 1060 1280 m this corresponds to sedimentary column from 22 to 242 mbsf our results demonstrate that the pressure remains hydrostatic within the gas hydrate bearing sediments and is mostly above hydrostatic below the gas hydrate bearing zone

spatial distribution of gas hydrate deposits in krishna godavari offshore basin bay of bengal abstract to delineate the occurrences of gas hydrates and free gas in the krishna godavari kg basin bay of bengal we used 3d seismic data to construct a velocity model of high spatial density we used an automated version of migration velocity analysis to estimate p wave velocities using kirchoff prestack time migration we manually checked all common image gathers cigs to assess the reliability of the auto picking algorithm the velocity model was validated against available downhole information from drilling sites nghp 01 02 nghp 01 03 and nghp 01 10 the model shows a anomalously high velocities at site nghp 01 10 where significant gas hydrates were reported and b background velocities at sites nghp 01 02 and nghp 01 03 where a negligible amount of hydrate was recovered a bottom simulating reflector bsr is identified in the 3d seismic data and an average velocity map is prepared above the bsr background velocities of marine sediments in the kg basin vary from 1 43 km s to 1 6 km s velocities higher than 1 65 km s indicate gas hydrate bearing sediments while velocities lower than 1 43 km s indicate free gas bearing sediments high velocities observed below seafloor mounds and ridges suggest the presence of gas hydrate in a structurally deformed area slope basins are generally devoid of gas hydrates free gas is observed at depths of up to several hundred meters below the bsr suggesting the migration of deeper fluid gas into the gas hydrate stability zone ghsz zones of gas hydrates and free gas are preferentially oriented along major faults indicating the role of faults in the transportation of free gas and formation of gas hydrates deposits we identify a free gas bearing horizon at depths 300 m below the bsr which might contribute to the lateral migration of free gas gas hydrate deposits in kg basin are classified into two categories i leaky deposits which occur in the areas of active seafloor faults and are characterized by heterogeneous distribution of gas hydrates and ii non leaky deposits which are formed in areas of subsurface faults and show a relatively homogeneous distribution of gas hydrates site nghp 01 10 is a good example of leaky gas hydrate deposits above a deep seated gas reservoir

evidence of slumping sliding in krishna godavari offshore basin due to gas fluid movements abstract the krishna godavari kg offshore basin is one of the promising petroliferous basins of the eastern continental margin of india drilling in this basin proved the presence of gas hydrate deposits in the shallow marine sediments beyond 750 m water depths and provided lithologic and stratigraphic information we obtained multibeam swath bathymetry covering an area of about 4500 km 2 in water depths of 280 1800 m and about 1260 line km of high resolution seismic hrs records the general lithology of midslope deposits is comprised of nannofossil rich clay nannofossil bearing clay and foraminifera bearing clay the hrs records and bathymetry reveal evidence of slumping and sliding of the upper and midslope sediments which result in mass transport deposits mtd in the northwestern part of the study area these deposits exhibit 3 9 5 km widths and extend 10 13 km offshore the boundaries of the mtds are often demarcated by sharp truncation of finely layered sediments fls and the mtds are characterized by acoustically transparent zones in the hrs data average thickness of recent mtds varies with depth i e in the upper slope the thickness is about 45 m while in the lower slope it is about 60 m and in deeper offshore locations they attain a maximum thickness of about 90 m a direct indication for slumping and mass transportation of deposits is provided by the age reversal in 14 c ams dates observed in a sediment core located in the midslope region seismic profiling signatures provide indications of fluid gas movement we propose that the presence of steep topographic gradients high sedimentation rates a regional fault system diapirism fluid gas movement and neotectonic activity may have facilitated the slumping sliding of the upper slope sediments in the kg offshore basin

national gas hydrate program expedition 01 offshore india gas hydrate systems as revealed by hydrocarbon gas geochemistry abstract the national gas hydrate program expedition 01 nghp 01 targeted gas hydrate accumulations offshore of the indian peninsula and along the andaman convergent margin the primary objectives of coring were to understand the geologic and geochemical controls on the accumulation of methane hydrate and their linkages to underlying petroleum systems four areas were investigated 1 the kerala konkan basin in the eastern arabian sea 2 the mahanadi and 3 krishna godavari basins in the western bay of bengal and 4 the andaman forearc basin in the andaman sea upward flux of methane at three of the four of the sites cored during nghp 01 is apparent from the presence of seafloor mounds seismic evidence for upward gas migration shallow sub seafloor geochemical evidence of methane oxidation and near seafloor gas composition that resembles gas from depth the kerala konkan basin well contained only co2 with no detectable hydrocarbons suggesting there is no gas hydrate system here gas and gas hydrate from the krishna godavari basin is mainly microbial methane with 13c values ranging from 58 9 to 78 9 with small contributions from microbial ethane 52 1 and co2 gas from the mahanadi basin was mainly methane with lower concentrations of c2 c5 hydrocarbons c1 c2 ratios typically 1000 and co2 carbon isotopic compositions that ranged from 70 7 to 86 6 for methane and 62 9 to 63 7 for ethane are consistent with a microbial gas source however deeper cores contained higher molecular weight hydrocarbon gases suggesting a small contribution from a thermogenic gas source gas composition in the andaman basin was mainly methane with lower concentrations of ethane to isopentane and co2 c1 c2 ratios were mainly 1000 although deeper samples were

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

influence of total organic carbon deposition on the inventory of gas hydrate in the indian continental margins abstract total organic carbon toc content of marine sediments represents residual carbon originally derived from terrestrial and marine sources which has survived seafloor and shallow subseafloor diagenesis ultimately its preservation below the sulfate reduction zone in marine sediments drives methanogenesis within the gas hydrate stability zone ghsz methane production along continental margins can supersaturate pore fluids and lead to the formation of gas hydrate in this paper we examine the inventory and sources of toc in sediments collected from four regions within the ghsz along the indian continental margins the recovered sediments vary in age from oligocene to recent mean toc abundance is greatest in the krishna godavari k g basin and decreases progressively to the mahanadi basin andaman wedge and kerala konkan k k basin this decrease in toc is matched by a progressive increase in biogenic caco3 and increasing distance from terrestrial sources of organic matter and lithogenic materials organic carbon sources inferred from c n and 13ctoc range from terrestrial k g basin to mixed marine and terrestrial mahanadi basin to marine dominant andaman wedge and k k basin in the k g basin variation in the bulk 13ctoc is consistent with changes in c3 and c4 vegetation driven by monsoon variability on glacial interglacial timescales whereas in the mahanadi basin a shift in the 13ctoc likely reflects the onset of c4 plant deposition in the late miocene a large shift the 13ctoc in the k k basin is consistent with a change from c3 to c4 dominated plants during the middle miocene we observe a close relationship between toc content and gas hydrate saturation but consider the role of sedimentation rates on the preservation of toc in the zone of methanogenesis and advective flow of methane from depth although toc contents are sufficient for in situ methanogenesis at all the sites where gas hydrates were observed or inferred from proxy data seismic borehole log pressure core and gas composition data coupled with relatively high observed gas hydrate saturations suggest that advective gas transport may also play a role in the saturation of methane and the formation of gas hydrates in these regions although toc content may be a first order indicator for gas hydrate potential the structural and stratigraphic geologic environment along a margin will most likely dictate where the greatest gas hydrate saturations will occur

comparison of methane mass balance and x ray computed tomographic methods for calculation of gas hydrate content of pressure cores abstract gas hydrate saturation was calculated for twelve pressure cores taken during national gas hydrate program nghp expedition 01 in the krishna godavari basin bay of bengal at a location where thin gas hydrate veins were common one of two methods were used to calculate gas hydrate saturation for each core methane mass balance after depressurization and gas collection considered the gold standard for measurement of gas hydrate saturation or voxel intensity analysis rather than binary segmentation of x ray computed tomographic ct reconstructions gas hydrate saturation in cores measured by methane mass balance was calculated to be 17 8 10 9 11 9 13 6 9 5 1 4 and 0 by percent of core volume gas hydrate saturation in similar cores measured by intensity analysis of ct reconstructions was 17 2 9 1 6 7 7 8 and 3 1 by percent of core volume this quantitative ct intensity analysis contained systematic errors and therefore the calculated hydrate saturations are lower bounds the systematic errors can be removed from the quantitative ct analysis by converting the ct intensities to real densities though this was not possible for this study all pressure core gas hydrate saturations were similar in magnitude to each other as well as to independent estimates of gas hydrate saturation from porewater freshening and all variations in saturation could be explained by natural variation between samples ct intensity or preferably density analysis of pressure cores showed promise for calculation of the saturation of vein hydrate in natural samples allowing pressure cores to be used for further analyses under pressure after hydrate quantification theoretical examination of ct density analysis showed that this method would be unable to detect pore filling gas hydrate judicious examination of the results from ct density analysis versus other hydrate quantification methods on the same samples might allow quantification of pore filling hydrate

sedimentation stratigraphy and petroleum potential of krishna godavari basin east coast of india the krishna godavari basin is located in the central part of the eastern passive continental margin of india the structural grain of the basin is northeast southwest exposures of upper cretaceous sedimentary rocks demarcate the basin margin toward the north west where the northwest southeast trending pranahita godavari graben abuts the basin the basin contains thick sequences of sediments with several cycles of deposition ranging in age from late carboniferous to holocene a major delta with a thick argillaceous facies that has prograded seaward since the late cretaceous is a hydrocarbon exploration target magnetic and gravity data predicted the basin architecture which was subsequently confirmed by a multichannel seismic sur vey the basin is divided into subbasins by fault controlled ridges sediments accumulated in subbasins more than 5 km thick above the basement ridges thin sediments are found until the jurassic period sediments were deposited in the rift valley and in topo graphic lows this sequence is completely overlain by a lower cretaceous transgressive sedimentary wedge later continued delta progradation characterized basin sedimentation with an areal extent of approximately 45 000 km2 this proven petroliferous basin has potential reservoirs ranging in age from the permian to the pliocene exploratory drilling of more than 350 wells in more than 160 structures has resulted in the discovery of 42 oil and gas bearing structures good source rocks are known from sequences ranging in age from permian carboniferous to early miocene because the reservoir sand bodies have limited lateral variation understanding the stratigraphy and depositional sub environments in different sequences is essential to decipher the favorable locales for reservoir sands tilted fault blocks growth faults and related rollover anticlines provide the structural traps begin page 1624

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

seismic evidence of free gas migration through the gas hydrate stability zone ghsz and active methane seep in krishna godavari offshore basin abstract seismic amplitude versus offset avo analysis was performed on the multichannel seismic data in the vicinity of site nghp 01 10 where thick fracture filled gas hydrate is reported to understand the fluid migration in fracture dominated gas hydrate system amplitude preserved common image gathers cig are used in extracting the seismic amplitude with offset for seafloor bottom simulating reflectors bsr and different horizons within the gas hydrate stability zone ghsz the extracted amplitudes are fitted with three term shuey s avo approximation to estimate the intercept and gradient values based on the intercept and gradient values two distinct classes of seismic avo responses are observed class iv avo decreasing negative amplitudes with increasing offsets and class iii avo increasing negative amplitudes with increasing offsets for the bsr and other horizons above the bsr the large negative intercept and gradient values which indicate class iii avo ranges from 0 2 to 1 8 for the inline seismic profile and from 0 01 to 0 3 for the crossline profile the presence of class iii avo anomaly within the gas hydrate stability zone ghsz provides evidence of existence of free gas which may be migrating along the fault system some of the active faults are exposed on the surface and show gas venting signatures such as gas flares and pockmarks in the multibeam data

epr study of kerogens from three alaskan north slope wells abstract whole rock and isolated kerogens from the seabee inigok and ikpikpuk wells of the alaskan north slope have been analyzed by electron paramagnetic resonance epr spectroscopy the sample suite includes representatives of the torok kingak lisburne sadlerochit and endicott formations power saturation studies show a definite correlation with the downhole maturation the extent of aromaticity of the sample as defined by 13 c nmr and with the beginning of the oil window for the kingak formation strong correlations within formations across the depositional basin are seen abrupt changes in epr parameters indicate a change in the nature of the organic matter present and may reflect a facies change peak widths and spin densities correlate with maturity as indicated by vitrinite reflectance and sample aromaticity as measured in solid state 13 c nmr spectra

transient hydraulic fracturing and gas release in methane hydrate settings a case study from southern hydrate ridge episodic seafloor methane venting is associated with focused fluid flow through fracture systems at many sites worldwide we investigate the relationship between hydraulic fracturing and transient gas pressures at southern hydrate ridge offshore oregon usa two colocated seismic surveys acquired 8 years apart at hydrate ridge show seismic amplitude variations interpreted as migration of free gas in a permeable conduit horizon a feeding an active methane hydrate province the geophysical surveys also reveal transients in gas venting to the water column we propose that episodic gas migration and pressure fluctuations in the reservoir underlying the regional hydrate stability zone rhsz at southern hydrate ridge influence methane supply to the rhsz and are linked with periodic fracturing and seafloor methane venting we model the effect of pore pressure variations within the deep methane source on fracturing behavior with a 1d model that couples multiphase flow hydrate accumulation and pore pressure buildup as the reservoir pressure increases fractures open when the pore pressure exceeds the hydrostatic vertical effective stress gas then flows through the fractures and vents at the seafloor while hydrate precipitates in the fracture system we show that active seafloor gas venting occurs for approximately 30 years and that the available methane reservoir is exhausted 30 to 55 years after the onset of pressure buildup this provides important constraints on the time scale of transient fluid flow at southern hydrate ridge and illustrates how pore pressure pulses affect fluid flow and fracturing behavior in active methane hydrate provinces

geochemical and geological constraints on the composition of marine sediment pore fluid possible link to gas hydrate deposits abstract pore water sulfate consumption in marine sediments is controlled by microbially driven sulfate reduction via organo clastic and methane oxidation processes in this work we present sediment pore fluid compositions of 10 long sediment cores and high resolution seismic data from the krishna godavari k g basin bay of bengal our results show occurrence of transient s and kink types and steady state quasi linear sulfate concentration profiles which are attributed partly to the anaerobic oxidation of methane 13 c ch 4 84 8 to 100 1 vpdb and organo clastic sulfate reduction influence of aom on alkalinity is evident from the presence of authigenic carbonate layers with highly depleted carbon isotope ratios in core md161 8 the authigenic carbonates represent the paleo smtzs and suggest marked fluctuation in vertical methane flux our geophysical data show the acoustic signatures of upward fluid migration from shallow sub surface whereas coring during nghp expedition 01 confirms the presence of sub surface gas hydrate deposits in k g basin which can be linked to deep methane sources the geochemical analysis suggests that shallow methane source can be attributed to high burial flux of labile organic matter due to high sedimentation rate sampling sites with high methane flux from the shallow gas source are characterized by quasi linear sulfate concentration profile and a shallow sulfate methane transition zone smtz and may not be necessarily linked to deeper gas hydrate deposits in contrast the deep methane source results in a transient kink type sulfate profile and a deeper smtz we have observed a close link between the occurrence of gas hydrate and the s kink type sulfate profile we interpret the short lived kink in the sulfate profiles as a result of recent enhancement in vertical methane flux possibly driven by reactivation of fault fractures systems which provide the conduits for fluid flow

enhanced methane flux event and sediment dispersal pattern in the krishna godavari offshore basin evidences from rock magnetic techniques abstract detailed rock magnetic measurements such as susceptibility susceptibility of anhysteric remanent magnetization arm saturation isothermal remanent magnetization sirm soft isothermal remanent magnetization soft irm and hard irm hirm were carried out on the topmost sediment samples of 73 gravity cores collected in the krishna godavari kg offshore basin to understand the distribution of magnetic parameters and the associated diagenetic signatures related to upward methane flux the variation in magnetic parameters in the kg offshore basin fluctuates around the average value 105 10 8 m3 kg 1 arm 1 22 10 5 m3 kg 1 sirm 2040 10 5 am2 kg 1 soft irm 1807 and hirm 82 except at the location gc 07 15 767 n 81 814 e where the magnetic parameters arm and sirm drop to 1 6th 1 14th and 1 8th of the average near surface value indicating substantial loss in the concentration of primary magnetic iron oxide minerals anomalous low value of arm and arm sirm at gc 07 suggests that the magnetic remanence property is governed by coarser pseudo single domain psd or multi domain md grains to understand the nature of the anomalous magnetic parameters at gc 07 complete downcore variation of magnetic parameters was also studied the magnetic parameters were found to be even more depleted at all depths with susceptibility dropping at 5 2 mbsf to 1 200th of its average near surface value interestingly this core shows a rapid increase in the concentration of dissolved methane from 0 nm at surface to 153 nm at around 5 2 mbsf where the sulfate concentration is also reduced to zero the anaerobic oxidation of methane aom and the sulfate methane transition zone smtz occur between 4 2 and 5 2 mbsf based on the present day sulfate concentration profile the close association between the dissolution of magnetic iron oxide minerals and aom at gc 07 suggests that the reduced magnetization is primarily due to the release of hs by aom the magnetic iron oxide minerals are dissolved not only at smtz but also above the current depth of the smtz it is likely that the methane flux was higher in the past leading to upward shift of the smtz and the hs released by aom facilitated the dissolution of magnetic iron oxide minerals at shallower depths alternatively the hs diffusing upward from the current depth of smtz might have lead to dissolution at shallower depth therefore additional geological geochemical investigations are warranted at gc 07 to establish the dissolution mechanism of magnetic iron oxide minerals the variability in different magnetic parameters within the slope basin and channels shows systematic decrease in the concentration of magnetic minerals and increase in the abundance of stable single domain ssd magnetic grains with water depth the observed strong correlation between the silt fraction and the magnetic susceptibility for the slope basin and channels suggests that the variation in magnetic parameters is predominantly controlled by the sediment sorting transport process as finer particles are deposited in deeper water

tectonostratigraphy and depositional patterns in krishna offshore basin bay of bengal the krishna godavari basin along the east coast of india covers the deltaic and interdeltaic areas of the krishna and godavari rivers and extends into the offshore figure 1 this basin is situated along the shoulder of a rifted passive continental margin which developed during the separation of india from antarctica in the late jurassic this rifting episode is superimposed on an earlier northwest southeast gondwana graben trend such as the pranhita godavari graben figure 2a which extends from onshore into the krishna godavari basin this is an earlier prerift depositional episode onto precambrian crystalline basement which commenced in the early permian and continued through the middle jurassic the late jurassic rift episode caused mechanical extension along the eastern margin leading to the formation of northeast southwest horst and graben features based on bouguer gravity data murty and ramakrishna 1980 recognized different tectonic elements figure 2b such as krishna graben bapatla horst west godavari subbasin including the median kaza kaikalur horst tanuku horst and east godavari subbasin including narasapur yanam high

evidence of paleo cold seep activity from the bay of bengal offshore india 1 we report evidence of paleo cold seep associated activities preserved in methane derived carbonates in association with chemosynthetic clams calyptogena sp from a sediment core in the krishna godavari basin bay of bengal visual observations and calculations based on high resolution wet bulk density profile of a core collected on board r v marion dufresne may 2007 show zones of sharp increase in carbonate content 10 55 vol within 16 20 meters below seafloor mbsf the presence of calyptogena clam shells chimneys shell breccias with high mg calcite cement and pyrite within this zone suggest seepage of methane and sulfide bearing fluid to the seafloor in the past highly depleted carbon isotopic values 13c ranges from 41 to 52 vpdb from these carbonates indicate carbon derived via anaerobic oxidation of methane extrapolated mean calendar age 58 7 ka b p of the clastic sediments at a depth of 16 mbsf is close to the upper limit of the u th based depositional age 46 2 3 7 and 53 0 1 6 ka of authigenic carbonates sampled from this level thereby constraining the younger age limit of the carbonate deposition methane expulsion events the observed carbonate deposition might have resulted from the flow of methane enriched fluids through the fracture network formed because of shale diapirism

basin architecture and petroleum system of krishna godavari basin east coast of india krishna godavari basin is a peri cratonic passive margin basin on the east coast of india figure 1 the onland part consists of 28 000 km2 and is mostly alluvium covered krishna and godavari are the two major river systems which drain the area and discharge in the bay of bengal the offshore basinal area covers 24 000 km2 to the isobath of 200 m however the basin extends into deeper water and covers a much larger area the basin s characteristic feature is its en echelon horst and graben system which is filled with a thick pile of sediments of permian to recent age and emerging as one of india s most promising petroliferous areas commercial accumulation of hydrocarbons occurs in sediments from the permian to as young as the pliocene

esr assessment of kerogen maturation and its relation with petroleum genesis abstract petroleum hydrocarbons are formed by breakdown of kerogen preserved in source rocks throughout the process of catagenesis this process is accompanied by free radical generation in kerogen the availability of measuring free radicals in kerogen using esr to deduce a maturation estimation of potential source rocks has been hindered due to the presence of the solvent extractable organic molecules seom trapped within the kerogen matrix spin concentration ns of the kerogen treated with pyridine k py represents the paramagnetic centers of the kerogen matrix itself and provides a potential parameter to evaluate kerogen maturation

anisotropic amplitude variation of the bottom simulating reflector beneath fracture filled gas hydrate deposit 1 for the first time we report the amplitude variation with angle ava pattern of bottom simulating reflectors bsrs beneath fracture filled gas hydrate deposits when the effective medium is anisotropic the common depth point cdp gathers of two mutually perpendicular multichannel seismic profiles located in the vicinity of site nghp 01 10 are appropriately processed such that they are fit for ava analysis ava analysis of the bsr shows normal incidence reflection coefficients of 0 04 to 0 11 with positive gradients of 0 04 to 0 31 indicating class iv pattern the acoustic properties from isotropic rock physics model predict class iii ava pattern which cannot explain the observed class iv ava pattern in krishna godavari basin due to the anisotropic nature of fracture filled gas hydrate deposits we modeled the observed class iv ava of the bsr by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic hti medium after accounting for anisotropic wave propagation effects on bsr amplitudes the effective medium properties are estimated using backus averaging technique and the ava pattern of bsrs is modeled using the properties of overlying hti and underlying isotropy hti media with or without free gas anisotropic ava analysis of the bsr from the inline seismic profile shows 5 30 gas hydrate concentration equivalent to fracture density and the azimuth of fracture system fracture orientation with respect to the seismic profile is close to 45 free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system f1

multi channel 2d seismic constraints on pore pressure and vertical stress related gas hydrate in the deep offshore of the mahanadi basin india abstract estimation of pore pressure and in situ vertical stress magnitude is essential for understanding the geomechanical behavior of the gas hydrate sediments in deep offshore of the mahanadi basin the basin located at northern side of eastern continental margin of india ecmi contains gas hydrate in clay silt sediments the pore pressure and vertical stress are mapped on two 2d multi channel seismic data of lines mh 38a and mh 38b with aid of information of three wells namely nghp 01 19 nghp 01 09 and nghp 01 08 initially the coefficients of best fit curve have been computed from velocity effective stress plot for the individual wells and applied on the seismic velocity to transform into the effective stress the vertical stress has been computed from both the seismic and log data therefore the pore pressure is predicted by subtracting the effective stress from the vertical stress the pore pressure have been mapped in gas hydrate stability zone ghsz and sediments below bottom simulating reflector bsr the pore pressure and vertical stress gradient are 10 11 mpa km and 10 67 mpa km respectively the pore pressure and vertical stress from seismic data are closely matched at well location with excellent goodness of fit r2 varying from 0 82 to 0 95 normal pressure is observed in the gas hydrate bearing sediments but slightly high pressures are noticed below the bsr indicating presence of free gas the pore pressure from seismic data will guide drillers for choosing the mud weight during well drilling and casing optimizing in other part of deep offshore in the mahanadi basin

extraterrestrial organic matter preserved in 3 33 ga sediments from barberton south africa electron paramagnetic resonance epr analysis of carbonaceous volcanic tidal sediments from the 3 33 ga old josefsdal chert kromberg formation barberton greenstone belt documents the presence of two types of insoluble organic matter iom 1 iom similar to that previously found in archean cherts from numerous other sedimentary rocks in the world and of purported biogenic origin 2 anomalous iom localized in a 2 mm thick sedimentary horizon detailed analysis by continuous wave epr and pulse epr reveals that iom in this layer is similar to the insoluble component of the hydrogenated organic matter in carbonaceous chondrites suggesting that this narrow sedimentary horizon has preserved organic matter of extraterrestrial origin this conclusion is supported by the presence in this thin layer of another anomalous epr signal at g 3 attributed to ni cr al ferrite spinel nanoparticles which are known to form during atmospheric entry of cosmic objects from this epr analysis it was deduced that the anomalous sedimentary layer originates from deposition in a nearshore environment of a cloud of tiny dust particles originating from a flux of micrometeorites falling through the oxygen poor archean atmosphere

seismic imaging of a fractured gas hydrate system in the krishna godavari basin offshore india gas hydrate was discovered in the krishna godavari kg basin during the india national gas hydrate program nghp expedition 1 at site nghp 01 10 within a fractured clay dominated sedimentary system logging while drilling lwd coring and wire line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500 m transect three dimensional 3d seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures a system of two fault sets was identified part of a typical passive margin tectonic setting the lwd derived fracture network at hole nghp 01 10a is to some extent seen in the seismic data and was mapped using seismic coherency attributes the fractured system around site nghp 01 10 extends over a triangular shaped area of 2 5 km2 defined using seismic attributes of the seafloor reflection as well as seismic sweetness at the base of the gas hydrate occurrence zone the triangular shaped area is also showing a polygonal nearly hexagonal fault pattern distinct from other more rectangular fault patterns observed in the study area the occurrence of gas hydrate at site nghp 01 10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source the triangular shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits additionally the fault associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone it is proposed that there are additional locations in the kg basin with possible gas hydrate accumulation of similar tectonic conditions and one such location was identified from the 3d seismic data 6 km nw of site nghp 01 10

gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the krishna godavari basin offshore india abstract the national gas hydrate program expedition 01 cored ten sites in the krishna godavari basin located on the southeastern margin of india a comprehensive suite of pore water solute concentrations and isotope ratios were analyzed to investigate the distribution and concentration of gas hydrate along the margin in situ diagenetic and metabolic reactions and fluid migration and flow pathways gas hydrate was present at all of the sites cored and in situ microbial methanogenesis leads to estimates of depth integrated average gas hydrate saturations that are typically

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

vulcan a deep towed csem receiver we have developed a three axis electric field receiver designed to be towed behind a marine electromagnetic transmitter for the purpose of mapping the electrical resistivity in the upper 1000 m of seafloor geology by careful adjustment of buoyancy and the use of real time monitoring of depth and altitude we are able to deep tow multiple receivers on arrays up to 1200 m long within 50 m of the seafloor thereby obtaining good coupling to geology the rigid body of the receiver is designed to reduce noise associated with lateral motion of flexible antennas during towing and allows the measurement of the vertical electric field component which modeling shows to be particularly sensitive to near seafloor resistivity variations the positions and orientations of the receivers are continuously measured and realistic estimates of positioning errors can be used to build an error model for the data during a test in the san diego trough offshore california inversions of the data were able to fit amplitude and phase of horizontal electric fields at three frequencies on three receivers to about 1 in amplitude and 1 in phase and vertical fields to about 5 in amplitude and 5 in phase the geological target of the tests was a known cold seep and methane vent in 1000 m water depth which inversions show to be associated with a 1 km wide resistor at a depth between 50 and 150 m below seafloor given the high resistivity 30 m and position within the gas hydrate stability field we interpret this to be massive methane hydrate

seismic wave attenuation in methane hydrate bearing sediments vertical seismic profiling data from the nankai trough exploratory well offshore tokai central japan 1 data from two vertical seismic profiles vsps from the nankai trough exploratory well offshore tokai central japan are used to estimate compressional attenuation in methane hydrate mh bearing sediments at seismic frequencies of 30 110 hz we compare spectral ratio and centroid frequency shift methods for measuring attenuation to isolate intrinsic attenuation from total attenuation attenuation is computed from multiples using one dimensional synthetic vsp data from sonic and density logs the use of two different measurement methods and two vsps recorded at just 100 m separation provides an opportunity to validate the attenuation measurements no significant compressional attenuation was observed in mh bearing sediments at seismic frequencies macroscopically the peaks of highest attenuation in the seismic frequency range correspond to low saturation gas zones in contrast high compressional attenuation zones in the sonic frequency range 10 20 khz are affected by the presence of methane hydrates in the same well locations thus this study demonstrated the frequency dependence of attenuation in mh bearing sediments mh bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range

radical concentration of kerogen as a maturation parameter a good correlation was found between spin concentration and burial depth for kerogen extracted with benzene methanol followed by pyridine or dioxane differences in slopes of the regression lines of spin concentration vs burial depth are dependent upon the geothermal gradient of the studied areas free radical concentration might be a good maturation parameter for kerogens of the same type

diagenesis of magnetic minerals in a gas hydrate cold seep environment off the krishna godavari basin bay of bengal we carried out detailed magnetic measurements of the core md161 8 located in the vicinity of site nghp 01 10 where 128 m of hydrate is confirmed by drilling coring to understand the diagenesis of magnetic minerals in a gas hydrates cold seep environment the rock magnetic measurements along with sem eds and xrd analyses show a zone of reduced magnetic susceptibility zone 2 where most of the magnetic minerals are dissolved the enhanced concentration of chromium reducible sulfur crs in this zone suggests an intense pyritization process while isotopically depleted authigenic carbonates indicate sulfate reduction via anaerobic oxidation of methane aom therefore the dissolution of magnetic minerals is attributed to the hs released during aom that has resulted in the reduction in the magnitude of magnetic parameters within zone 2 a zone of enhanced susceptibility zone 2a is observed between 17 68 and 23 6 mbsf and is located beneath the present day sulfate methane transition zone smtz the frequencydependent magnetic susceptibility and low temperature magnetic measurements suggest the abundance of fine grained superparamagnetic sp sized ferrimagnetic particles the sem eds and xrd analyses show the presence of greigite which occurs in interstices between the pyrite crystals such occurrence of greigite in sediments has important implications in the interpretation of paleomagnetic records we evaluated the likely mechanism for the greigite formation in kg offshore basin and our data suggest that the formation of greigite may be related to either paleo smtz or anaerobic oxidation of pyrite it is unlikely that the formation of greigite can be explained by the downward diffusion of sulfide below the current depth of smtz however further investigations are required to ascertain the mechanism for the

estimation of seismic attenuation of gas hydrate bearing sediments from multi channel seismic data a case study from krishna godavari offshore basin seismic attenuation of gas hydrate bearing sediments is estimated from the multi channel seismic data acquired in krishna godavari kg offshore basin in the vicinity of site nghp 01 10 where 128 m of fracture filled hydrate is confirmed by drilling coring to understand the effect of gas hydrate on the seismic attenuation the seismic quality factor q which is the inverse of seismic attenuation is estimated from two methods first the spectral ratio method which utilizes the logarithmic ratio of the seafloor and bottom simulating reflector bsr spectra and second the peak frequency method which depends upon the variations of the peak frequency of the bsr spectrum as a function of traveltime in the study area the complex fault system has perturbed the seafloor resulting in noisy seafloor spectrum as a result the logarithm of the spectral ratio was also noisy and the q values obtained from spectral ratio method were found to be unstable on the other hand the peak frequency method is found to be relatively stable as it depends only on the peak frequency of the bsr which can be reliably obtained by conventional processing this method yields an effective q value of bsr which represents a weighted average of the interval q values of the water column and the gas hydrate bearing sediments since the q value of water column can be treated as a constant the variations in effective q value can be interpreted in terms of the variations of q in the gas hydrate bearing sediments for a constant thickness of water column the estimated effective q values along the inline and crossline seismic profiles depend on several factors such as gas hydrate free gas and the complex fault system the combined interpretation of the quality factor and the interval velocity model helps to understand these effects in different zones of the seismic data the attenuation of the background marine sediments is estimated from the analysis of crossline seismic profile the gas hydrate bearing sediments as inferred from the velocity model show low attenuation as compared to that of the background sediments in contrast active migration of free gas across the fault system leads to increase in seismic attenuation within the gas hydrate stability zone the complex fault zone may also lead to an increase in seismic attenuation the blanked seismic zone shows a marginal decrease in seismic attenuation probably related to loadbearing form of gas hydrate our attenuation results are compared against the results obtained from waveform inversion albeit with a higher resolution the relative variation of q estimated from the proposed method is close to the average q values estimated from full waveform inversion for most of the cdps the proposed methodology can be a useful technique to estimate seismic attenuation from multi channel seismic data

seismic characterization of hydrates in faulted fine grained sediments of krishna godavari basin unified imaging 1 a combination of diffusion and advection in fine grained sediments can create a patchy bottom simulating reflector bsr which has little to no apparent correlation with the overlying hydrate distribution using 2d seismic data from faulted clay rich sediments in the krishna godavari kg basin we show both the hydrate distribution and the bsr structure are fault controlled our demonstration hinges upon a kinematically accurate p wave velocity vp model which is estimated using a composite traveltime inversion depth migration method in an iterative manner the flexibility of the method allows simultaneous usage of traveltimes from multiple discontinuous reflectors the application begins with a simple vp model from time processing which is reflective of a diffusive continuous hydrate and free gas bearing system the application converges in three iterations yielding a final vpmodel which is suggestive of a patchy distribution of hydrates and free gas possibly developing through a combined diffusive advective system the depth image corresponding to the final vp model can be interpreted for faults that suggest ongoing tectonism the bsr appears to be truncated at active faults zones both the final vp model and the corresponding depth image can be reconciled with the hydrate distribution and bsr depth at logging coring sites located 250 m away from the line by projecting the sites along the strike direction of the regional faults

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

possible link between weak bottom simulating reflections and gas hydrate systems in fractures and macropores of fine grained sediments results from the hikurangi margin new zealand abstract small amounts of free gas in interstitial sediment pores are known to significantly lower compressional p wave velocity vp this effect combined with moderately elevated vp from the presence of gas hydrates is usually thought to be the cause for the often observed strong negative reflection coefficients of bottom simulating reflections bsrs at the base of gas hydrate stability bghs at several locations however weak bsrs have been observed which are difficult to reconcile with a presence of gas in sediment pores we here present a rock physics model for weak bsrs on the hikurangi margin east of new zealand thin sections of a fine grained mudstone sample from a submarine outcrop in the vicinity of a weak bsr show macroscopic porosity in the form of fractures and intrafossil macropores we apply the kuster toksoz theory to predict seismic velocities for a rock with water saturated interstitial micropores and gas or hydrates in macroscopic pore space simulating fractures or compliant macropores we match field observations of a weak bsr with a reflection coefficient of 0 016 with two end member models 1 rocks with gas hydrate filled voids with a concentration of

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

controls on evolution of gas hydrate system in the krishna godavari basin offshore india in this study we integrate environmental magnetic sedimentological and geochemical records of sediment core of hole nghp 01 10d overlying methane hydrate deposits to decipher the controls on the evolution of fracture filled gas hydrate system in the krishna godavari k g basin four distinct sedimentary units have been identified based on the sediment magnetic signatures an anomalous zone of enhanced magnetic susceptibility unit iii 51 9 160 4 mbsf coinciding with the gas hydrate bearing intervals is due to the presence of magnetite rich detrital minerals brought in by the river systems as a result of higher sedimentation events in k g basin and has no influence over hydrate formation a strong to moderate correlation between magnetite concentration and chromium reducible sulfur crs content indicates significant influence of sulfidization on the magnetic record and could be further exploited as a proxy to decipher paleo h2s seepage events analysis of high resolution seismic bathymetry and sub bottom profiler data reveals the existence of a regional fault system in k g basin the opening and closing dynamics of the faults facilitated the migration and trapping of required gas concentrations resulting in accumulation of gas hydrates at the studied site the seismic data provides support to the rock magnetic interpretations the observed variations in magnetic and geochemical properties have resulted from the episodic flow of methane and sulfide enriched fluids through the fracture filled network formed as a result of shale tectonism our study demonstrated the potential of using an enviro magnetic approach in combination with other proxies to constrain the evolution of gas hydrate system in marine environments

epr study of kerogens from middle valley northern juan de fuca ridge odp leg 139 abstract isolated kerogens from four sites in the middle valley hydrothermal region of the juan de fuca ridge ocean drilling project odp leg 139 were analysed by electron paramagnetic resonance epr spectroscopy measurements of peak width spin density and power saturation for site 857 kerogens which increased regularly in maturity downhole show correlation with vitrinite reflectance values from 0 61 to 2 5 indicating the start of the oil window at depths from 200 to 400 m spin density increases to 1 56 10 17 spins per gram and peak width decreases to 3 45 g gauss with increasing depth the tendency to power saturate also decreases with increasing maturity and increasing vitrinite reflectance within the oil window these trends are consistent with a model in which exchange processes are occurring and cause changes in the epr behavior of samples from this site sediments from other middle valley sites 855 856 and 858 contain large quantities of pyrite with mn 2 impurities which interact with the carbon radical to distort the epr measurements

seismic facies analyses as aid in regional gas hydrate assessments part i classification analyses the identification of areas of natural gas hydrate occurrence defining its concentration and regional distribution as well as understanding the processes that control gas hydrate formation is a crucial component of regional gas hydrate assessments in this study we incorporate an additional element into a regional assessment strategy by including the depositional environment defined through seismic facies classes the seismic facies classification is attempted using regional 2d seismic data and a 3d seismic volume as well as core and log data from two gas hydrate drilling expeditions carried out in the ulleung basin east sea to conduct a fully integrated gas hydrate assessment the facies classification described in this study is part of a procedure to conduct an assessment including 1 lithologic description of the recovered core and describe the deposition character of the sediment facies identified 2 seismic facies classification using 2d and 3d seismic data and their derived various seismic attributes 3 linking seismic facies classes to core derived sedimentological descriptions to define the host strata most favourable for gas hydrate occurrences 4 defining reservoir physical properties and linking these to seismic facies classes for reservoir property description 5 calculating regional elements of the gas hydrate petroleum system top of gas hydrate occurrence and base of gas hydrate stability 6 merging seismic facies and reservoir properties definition of the top and base of the gas hydrate stability zone to calculate gas hydrate saturations in each seismic facies and the total volume of gas hydrate present in the study area 7 defining probabilistic elements of the predicted volume of gas hydrate by combining statistical information of the individual input parameters through exhaustive monte carlo simulations the seismic facies classification algorithm used in this study is based on identifying clusters classes that show similar properties in the seismic data and derived attribute characteristics our classification scheme uses seven seismic attributes amplitude instantaneous frequency and amplitude also referred to as envelope sweetness acoustic impedance thin bed and parallel bedding indicator and incorporates the use of principle component analysis for data reduction the achieved classifications are overall robust and yield identical facies classes for the co located 2d and 3d data linking the seismic facies classes to the recovered core and sedimentological descriptions results in a sedimentological interpretation of the seismic facies cross plots of individual components as well as analysing the eigen vectors and values from the principle component analysis pca helps understand the physical meaning of these seismic facies classes and their seismic character the five facies classes assigned to the 3d volume and 2d seismic line after pca represent the mass transport deposit mtd units class 1 hemipelagic mud with some sandy turbidite class 2 hemipelagic mud with many sandy turbidites class 3 as well as water bearing mud with few sand layers and occurring mostly at shallow depth near the seafloor or beneath the base of the gas hydrate stability zone classes 4 and 5 within the study area and seismic data sets used gas hydrate occurrences are only linked to facies classes 2 and 3

gas hydrate destabilization and methane release events in the krishna godavari basin bay of bengal abstract methane release events have been linked to global warming alteration of the carbon cycle and influence on biota however unequivocal evidence of paleomethane release events are limited we report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core md161 8 from the krishna godavari k g basin bay of bengal the most negative 13 c spikes are recorded during the marine isotope stages mis 4 and at the transition of mis 5 to 4 occurrence of highly 13 c depleted average 13 c 48 2 4 vpdb authigenic high magnesian calcite are also reported within this time window from the core md161 8 in the present work an unequivocal explanation for the observed 13 c depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical electron microscopy or c o stable isotope ratio analyses due to possible influence of diagenetic alteration we attribute the observed episodic methane expulsion events as inferred from the negative 13 c excursions and earlier reports on the occurrence chemosynthetic bivalves and mo concentration anomaly to the destabilization of the base of gas hydrate stability zone bghsz sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage shale and salt tectonics in the passive continental margins being a globally observed phenomenon its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time additional evidence of methane emission from site md161 15 further supports the link between shale tectonics and methane emission

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

episodic basin wide fluid expulsion from geopressured shale sequences in the north sea basin a proposed general model for the episodic dewatering of thick shale successions is based on the recognition of a pervasive polygonal extensional fault network developed in the dominantly fine grained lower tertiary of the north sea basin seismic data show that the faults are arranged in stratigraphically bound structural units tiers that are delimited vertically by almost undeformed condensed sections and are restricted in distribution to the lowest permeability slope and basin floor facies i propose an episodic three stage mechanism to explain the fault genesis involving 1 the development of basin wide overpressured compartments 2 a density inversion between the overpres sured units and the overlying seal and 3 natural hydraulic fracturing pressure bleed off and resealing of the pressure compartment leading to a repeat of the cycle

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

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

physical properties and gas hydrate at a near seafloor thrust fault hikurangi margin new zealand the p paku fault zone drilled at iodp site u1518 is an active splay fault in the frontal accretionary wedge of the hikurangi margin in logging while drilling data the 33 m thick fault zone exhi