gas migration into gas hydrate bearing sediments on the southern hikurangi margin of new zealand we present multichannel seismic data from new zealand s hikurangi subduction margin that show widespread evidence for gas migration into the field of gas hydrate stability gas migration along stratigraphic layers into the hydrate system manifests itself as highly reflective segments of dipping strata that originate at the base of hydrate stability and extend some distance toward the seafloor the highly reflective segments exhibit the same polarity as the seafloor reflection indicating that localized gas hydrate precipitation from gas charged fluids within relatively permeable layers has occurred high density velocity analysis shows that these layer constrained gas hydrate accumulations are underlain by thick up to 500 m free gas zones which provide the source for focused gas migration into the hydrate layer in addition to gas being channeled along layers we also interpret gas migration through a fault zone into the field of hydrate stability in this case a low velocity layer within the hydrate stability zone extends laterally away from the fault which might indicate that gas charged fluids have also migrated away from the fault along strata at this site where both dipping strata and faulting seem to influence fluid migration we observe anomalously high velocities at the base of hydrate stability that we interpret as concentrated gas hydrates our results give insight into how shallow fluid flow responds to permeability contrasts between strata fault zones and perhaps also the gas hydrate system itself ultimately these relationships can lead to gas migration across the base of hydrate stability and the precipitation of concentrated hydrate deposits

estimates of heat flow derived from gas hydrates a gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data from the depth of the reflectors heat flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments this method is applied to the data in the nankai trough around central america and along the blake outer ridge the estimated values consistent with those measured by conventional means suggest that derivation of heat flow by this method is feasible on the landward slope of the nankai trough we find that heat flow increases downslope toward the floor of the trough

effects of bottom water warming and sea level rise on holocene hydrate dissociation and mass wasting along the norwegian barents continental margin 1 gas hydrate gh stability modeling results explain why some major holocene submarine landslides along the norwegian barents margin could have been triggered by gh dissociation during the early to middle holocene not during the lowest sea levels of the last glacial maximum lgm our model results show that subbottom depths of 170 260 m below the pre slide continental slope ca 350 475 m present water depth must have passed out of gas hydrate stability zone ghsz by 8 15 ka as the effect of warm bottom water inflow at 11 ka penetrated into the subbottom overcoming the effects of pressure increase due to sea level rise slr the component of local slr due to the isostatic response to fennoscandian deglaciation is shown to be relatively insignificant particularly for the part of the upper continental slope where the slide probably began the stability relations show that gh could have formed under the ice sheet before deglaciation and below deeper shelf areas after sea levels began to rise but before significant warming near the ghsz base to the extent water deeper than 800 m has remained cold 1 to 0 c since lgm times the ghsz continued to thicken in deep water and gh dissociation could not have triggered holocene failure in that regime the present distribution of gh stability is limited to water depths greater than about 400 m in the storegga slide area and the thickness of the ghsz increases with water depth

gas hydrate distribution and hydrocarbon maturation north of the knipovich ridge western svalbard margin a bottom simulating reflector bsr occurs west of svalbard in water depths exceeding 600 m indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern north atlantic margin regional bsr mapping shows the presence of hydrate and free gas in several areas with the largest area located north of the knipovich ridge a slow spreading ridge segment of the mid atlantic ridge system here heat flow is high up to 330 mw m 2 increasing towards the ridge axis the coinciding maxima in across margin bsr width and heat flow suggest that the knipovich ridge influenced methane generation in this area this is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination to evaluate the source rock potential on the western svalbard margin we applied 1d petroleum system modeling at three sites the modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons the bulk petroleum mass produced since the eocene is at least 5 kt and could be as high as 0 2 mt most likely source rocks are miocene organic rich sediments and a potential eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the knipovich ridge the presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the svalbard margin varies spatially

classification of sea floor features associated with methane seeps along the gulf of cadiz continental margin abstract based on recently gathered swath bathymetry high to ultra high resolution seismic and underwater camera data along with dredging and coring samples this paper examines the relationship between sea floor features and the nature of hydrocarbon enriched fluid and gas leaks from degassing of deeply buried sediments along the continental margin of the gulf of cadiz eastern central atlantic a classification into three main groups is proposed on the basis of the morphology and nature of deposits 1 mud volcanoes 2 methane derived authigenic carbonates mdac mounds and 3 crater like pockmarks mud volcanoes are topographically cone shaped sea floor edifices built up from catastrophic mud and fluid degassing intercalated with periods of inactivity so far more than 25 mud volcanoes have been discovered in the gulf of cadiz named in memory of deceased colleagues e g ginsburg and baraza or researchers birth places e g faro cibeles almazan san petersburgh yuma rabat bonjardim coruna gades these structures range from 800 to 2500 m in diameter and tower 150 300 m above the seabed the volcanoes consistently feature a well defined outer ring or circular terrace and an inner dome all mud volcanoes are built up of episodes of mud breccia flows intercalated with deep current deposits with evident indications of gas saturation degassing structures a strong h2s smell and chemosynthetic fauna such as pogonophora sp tube worms and calyptogena sp commonly observed carbonate crusts and slabs overlying some mud volcanoes are thought to have been formed by slow diffuse venting during periods of inactivity or slower rates of fluid venting following the ejection of mud a fermentation process the result of microbial mediated oxidation of hydrocarbon enriched fluids seems to play an important role in the growth of large deep water carbonate mounds and chimneys during periods of low methane seep fluid pressure more than 400 crater like pockmarks a common sea floor feature in the tasyo field have been observed their seabed diameter ranges from 500 to 3000 m with steep slopes ranging from 12 to 35 according to their internal structure pockmarks can be grouped into 1 pockmarks with low central mounds and 2 crater like pockmarks apparently collapse structures lacking central mounds with a flat to slightly undulating sea bottom surrounded by steep slopes up to 25

cessation reactivation of polygonal faulting and effects on fluid flow in the voring basin norwegian margin polygonal faults mainly oriented n50 n110 and n170 are abundant in the upper part of the mud dominated kai formation upper miocene lower pliocene of the voring basin a second less developed tier of polygonal faults oriented n20 n80 and n140 exists at the base of the overlying naust formation upper pliocene present the faults abruptly terminate upward below a thick interval of debris flows we propose a dynamic model in which 1 the development of polygonal faults discontinues temporarily as a result of a change in regional sedimentation leading to inactive polygonal faults 2 rapid emplacement of debris flows in the late pleistocene creates a new interval of polygonal faults in the lower part of the naust formation immediately beneath the debris flow and some faults penetrate into the underlying kai formation 3 some polygonal faults within the kai formation are reactivated and propagated upward into the base of the naust formation the high interconnectivity between faulted layers allows the fluids to reach shallower depths forming well expressed pipes and pockmarks on the sea floor the model of cessation reactivation of polygonal faults constrains the sealing capacity of sedimentary cover over the reservoirs and helps to reconstruct the fluid migration history through the sedimentary column

high resolution seismic stratigraphy sedimentary processes and the origin of seabed cracks and pockmarks at nyegga mid norwegian margin abstract densely spaced high resolution topas seismic profiles and em1002 bathymetric data reveal the presence of numerous pockmarks mound like structures and elongated seabed cracks at nyegga offshore mid norway the seabed cracks are located adjacent to the northern escarpment of the storegga slide appearing as graben like structures in the topas data unlike the cracks pockmarks and mound structures are largely associated with vertical zones of acoustic blanking at depth interpreted as pathways for vertically migrating gaseous fluids based on the topas data a new seismostratigraphic framework has been established and correlated to previously published age models of images cores md99 2291 and md99 2289 seismic facies interpretation suggests repeated and rapid deposition of up to 40 m thick glacigenic wedges in the eastern part of the study area around 18 2 14 c ka bp 21 8 cal ka 17 5 14 c ka bp 20 8 cal ka and 16 9 14 c ka bp 20 cal ka towards the west glacimarine deposition has prevailed characterized by progressively increasing sedimentation rates with peak values of 30 m ka during the period from 15 0 14 c ka bp 18 2 cal ka to 15 8 14 c ka bp 19 cal ka as the distribution of the nyegga pockmarks closely coincides with the main late weichselian sediment depocenters we suggest a relation between rapid and repeated sedimentation and periodic overpressure generation at depth ultimately leading to fluid expulsion at the seabed and the formation of the nyegga pockmark field in contrast seabed cracks at nyegga appear to have formed due to local extension which we relate to horizontal stress reduction as a consequence of the storegga slide event potentially this event has been accompanied by renewed vertical fluid migration and the most recent stage of pockmark development

pockmarks and methanogenic carbonates above the giant troll gas field in the norwegian north sea abstract acoustic imaging has revealed more than 7000 pockmarks on the seafloor above the troll east gas field in the norwegian north sea we present the first comprehensive study conducted on one of the world s largest pockmark fields complementing the acoustic data with extensive sampling geochemical and petrographical studies specifically we aimed at detecting possible active seepage still present over this vast area the pockmarks are present as isolated structures on average 35 m wide and up to 100 m in size in addition smaller satellite pockmarks surround some of the pockmarks in contrast to the muddy surroundings parts of the investigated pockmarks contain laterally extensive carbonate deposits or meter sized carbonate blocks these blocks provide shelter to abundant colonies of benthic megafauna the carbonate blocks are comprised of micritic mg calcite and calcite micritic aragonite and botryoidal aragonite framboidal pyrite is also commonly present carbon isotopic values of the carbonates are 13c depleted 13c as low as 59 7 and with 18o up to 4 5 indicating a methanogenic origin possibly linked to gas hydrate dissociation pore water extracted from shallow cores from the centre and the flanks of the pockmarks show similar cl and so4 profiles as the reference cores outside the pockmarks ruling out active methane seepage this conclusion is also supported by seafloor video observations that did not reveal any evidence of visual fluid seepage and by the absence of microbial mats and by the fact that the carbonate blocks are exposed on the seafloor and party oxidized on the surface we conclude that methane seepage formed this extensive gas field following to gas hydrate dissociation

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

vertical evolution of fluid venting structures in relation to gas flux in the neogene quaternary of the lower congo basin offshore angola abstract the processes preceding the genesis of pockmarks and their subsequent evolution has been rarely studied since pockmarks were discovered in 1970 a high resolution 3 d seismic survey from the continental slope of the lower congo basin allows us to study in detail the structures that developed before the formation of pockmarks as well as their subsequent geometric evolution this paper aims at reporting new type of fluid related structures that relate to pockmarks the objective of this research is to understand 1 the evolution of these structures and their genetic relationship with pockmarks and 2 how their morphologies may be influenced by changes in venting intensity over time pockmark and chimney structures in this survey are hosted within late miocene to quaternary hemipelagites above clastic reservoirs in a petroliferous basin they are all associated with positive high amplitude anomalies phaas on seismic data phaas are interpreted from their dramatically higher acoustic impedance relative to the background sedimentary values of the host units and by their association with seismic pull up effects from their planform and cross sectional geometry phaas are primarily interpreted as being due to the development of methane related carbonates but they may be associated with gas hydrates in one or two examples methanogenic carbonates form either by authigenic accumulation or by anaerobic methane oxidation amo once upwardly migrating methane enters the sulfate methane transition zone smtz typically within the first few tens of meters below the seabed therefore vertical accumulations of phaas are interpreted to reflect the temporal evolution of methane flux migrating upward to reach the paleo seabed or smtz timing of local scale fluid leakage can therefore be diagnosed by analyzing phaas at seep locations that occur in association with pockmark chimney structures a conceptual model for the evolution of different fluid venting structures is proposed here and is based on the analysis of venting structures that we expressed by 3 d seismic reflections in this model the main features are linked into a process based sequence culminating in the infill and termination of the pockmarks as fluid venting sites our model summarizes different rhythms of venting which are expressed by different types of phaas along with venting structures linear phaas and conduits inferred slow seeps they can be succeeded by sub circular phaas and shallow depressions which inferred slow to moderate seeps then the seep can become either a fast vent and terminated into a pockmark crater or it can become less active and be sealed afterwards subsurface sediment fluidizations and remobilizations are likely to happen in the last case and are probably caused by hydrate dissociations

geological and morphological setting of 2778 methane seeps in the dnepr paleo delta northwestern black sea abstract the dnepr paleo delta area in the nw black sea is characterized by an abundant presence of methane seeps during the expeditions of may june 2003 and 2004 within the eu funded crimea project detailed multibeam seismic and hydro acoustic water column investigations were carried out to study the relation between the spatial distribution of methane seeps sea floor morphology and sub surface structures 2778 new methane seeps were detected on echosounding records in an area of 1540 km 2 all seeps are located in the transition zone between the continental shelf and slope in water depths of 66 to 825 m the integration of the different geophysical datasets clearly indicates that methane seeps are not randomly distributed in this area but are concentrated in specific locations the depth limit for the majority of the detected seeps is 725 m water depth which corresponds more or less with the stability limit for pure methane hydrate at the ambient bottom temperature 8 9 c in this part of the black sea this suggests that where gas hydrates are stable they play the role of buffer for the upward migration of methane gas and thus prevent seepage of methane bubbles into the water column higher up on the margin gas seeps are widespread but accurate mapping illustrates that seeps occur preferentially in association with particular morphological and sub surface features on the shelf the highest concentration of seeps is found in elongated depressions pockmarks above the margins of filled channels on the continental slope where no pockmarks have been observed seepage occurs along crests of sedimentary ridges there seepage is focussed by a parallel stratified sediment cover that thins out towards the ridge crests on the slope seepage also appears in the vicinity of canyons bottom flanks and margins or near the scarps of submarine landslides where mass wasting breaches the fine grained sediment cover that acts as a stratigraphic seal the seismic data show the presence of a distinct gas front which has been used to map the depth of the free gas within the sea floor sediments the depth of this gas front is variable and locally domes up to the sea floor where the gas front approaches the seafloor gas bubbles were detected in the water column a regional map of the sub surface depth of the gas front emphasises this gas front versus seep relationship the integration of all data sets indicates that the spatial distribution of methane seeps in the dnepr paleo delta is mainly controlled by the gas hydrate stability zone as well as by stratigraphic and sedimentary factors

argon diffusion in pyroxenes implications for thermochronometry and mantle degassing abstract the rate at which argon diffuses in pyroxenes is relevant to several problems in the earth and planetary sciences pyroxenes are among the most abundant minerals in the earth s upper mantle and are important hosts for potassium and radiogenic 40 ar 40 ar additionally pyroxenes commonly contain 25 of the total 40 ar within meteorites and therefore hold great potential for quantifying the timing duration and thermal conditions of events experienced by meteorites and their parent bodies we conducted detailed feedback controlled laser heating diffusion experiments using synthetically produced 37 ar in gem quality clinopyroxene cpx and orthopyroxene opx crystals stepwise heating between 850 and 1350 c yielded linear arrhenius arrays from which activation energies e a of 379 2 4 1 and 371 0 6 0 kj mol and pre exponential factors d o of 1 36 10 4 6 15 10 5 1 12 10 4 and 5 73 10 2 2 25 10 2 3 72 10 2 were determined for cpx and opx respectively these e a s are an order of magnitude higher than previously reported values obtained with a different experimental approach watson et al 2007 and indicate that ar diffusion within pyroxenes is strongly temperature dependent at high temperatures ar diffusion in pyroxenes is rapid which indicates that diffusive 40 ar distributions observed within meteoritic pyroxenes may provide information about brief high temperature shock heating events in contrast subsequent low t conditions should minimally influence these concentration gradients the experimental ar diffusion kinetics in opx and cpx correspond to closure temperatures of 600 700 and 700 800 c respectively for 10 c ma cooling these results suggest that diffusive equilibration through 0 1 0 5 mm pyroxene grains occurs in minutes to hours at basaltic melt generation temperatures 1300 c and is therefore unlikely to inhibit mantle degassing

the early holocene sea level rise the causes anatomy and consequences of the early holocene sea level rise ehslr are reviewed the rise of ca 60m took place over most of the earth as the volume of the oceans increased during deglaciation and is dated at 11 650 7000 cal bp the ehslr was largely driven by meltwater release from decaying ice masses and the break up of coastal ice streams the patterns of ice sheet decay and the evidence for meltwater pulses are reviewed and it is argued that the ehslr was a factor in the ca 8470 bp flood from lake agassiz ojibway patterns of relative sea level changes are examined and it is argued that in addition to regional variations temporal changes are indicated the impact of the ehslr on climate is reviewed and it is maintained that the event was a factor in the 8200 bp cooling event as well as in changes in ocean current patterns and their resultant effects the ehslr may also have enhanced volcanic activity but no clear evidence of a causal link with submarine sliding on continental slopes and shelves can yet be demonstrated the rise probably influenced rates and patterns of human migrations and cultural changes it is concluded that the ehslr was a major event of global significance knowledge of which is relevant to an understanding of the impacts of global climate change in the future

mud volcanoes at the shelf margin of the east china sea abstract roughly circular sediment mounds varying from tens to hundreds of meters in diameter and from a few meters up to 40 m in height have been mapped for the first time at the shelf edge of the east china sea ecs high resolution seismic profiles together with swath bathymetric and acoustic data indicate that these features are common on the ecs outer shelf and on the upper western slope of the okinawa trough they are associated with unusually large pockmarks up to 40 m in depth and 600 m in diameter on seismic records bright spots phase inversions and other acoustic anomalies indicate that gas and or fluid escape plays an important role in the formation of these mounds they are often associated with normal faults that occur along the western slope of the okinawa trough these faults acting as conduits for migrating fluid based on regional stratigraphic correlation we propose that the source of mud and fluid is deeper older than 235 ka the seepage process is suspected to be very recent and probably still active based on the facts that 1 despite high sedimentation rate the mounds are not buried by recent sediments and 2 last glacial maximum deposits are reworked at the emplacement of the mounds carbonate cemented sediments and deep water reefs possibly associated with fluid seepage could be the origin of local patches of very high backscattering acoustic facies mapped with the swath bathymetric system

the free gas zone beneath gas hydrate bearing sediments and its link to fluid flow 3 d seismic imaging offshore mid norway abstract hydrate recycling and vertical migration of fluids from deep sources are processes evoked as controllers of the formation and stability of the free gas zone fgz beneath the base of the gas hydrate stability zone bghsz these processes have been often investigated through analytical and numerical modeling at some locations in continental margins however the seismic response of sediments deformed by such mechanisms has been poorly investigated due to the lack of reliable seismic resolution to clearly image anomalies confined to the fgz the present study is dedicated to the seismic characterization of the fgz beneath the bghsz at a hydrate site offshore mid norway using high resolution p cable 3d seismic data dim amplitude anomalies showing mainly lineations in a polygonal pattern of distribution 50 80 m vertical extension 150 300 m lateral extension and up to a few kilometers long are interpreted as evidence of sediment remobilization possibly aided by hydrate dissociation at focused fluid flow zones below former bases of the ghsz the exclusive occurrence of the anomalies within a depth range comprising estimated paleo bghszs during the last 200 kyr suggests that the anomalies are associated to paleo hydrate bearing sediment seals emplaced at depths controlled by overburden it also supports recently reported indications of main periods of fluid expulsion towards the seafloor occurring between 200 and 130 ka bp

australian offshore natural hydrocarbon seepage studies a review and re evaluation abstract surprisingly few natural hydrocarbon seeps have been identified in australia s offshore basins despite studies spanning thirty years early studies of natural hydrocarbon seepage around the australian margin were generally based on the geochemical analysis of stranded bitumens water column geochemical sniffer sampling synthetic aperture radar or airborne laser fluorosensor later studies involved the integration of these remote sensing and geochemical techniques with multi channel and shallow seismic a review of these earlier studies indicates that many seepage interpretations need to be re evaluated and that previous data sets when placed in a global context often represent normal background hydrocarbon levels low recent burial and subsidence rates are not favourable for high rates of seepage there are also difficulties in proving seepage on high energy shallow carbonate shelves where seabed features may be rapidly re worked and modern marine signatures are overprinted on authigenic seep carbonates thus the relatively few sites of proven natural hydrocarbon seepage in australia s offshore sedimentary basins can be reconciled relative to their geological occurrences and the dominantly passive margin setting active thermogenic methane seepage on the yampi shelf the only proven documented occurrence in australia is driven by deposition of a thick late tertiary carbonate succession and late miocene tectonic reactivation therefore to increase the success of detecting and correctly interpreting natural hydrocarbon seepage data need to be analysed and integrated within the context of the local geological setting and with an understanding of what is observed globally

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

the geochemical characteristics of the hydrate bound gases from the nyegga pockmark field norwegian sea the nyegga pockmark field located south east of the voring plateau norwegian sea was investigated on the uob cruises gs07 148 2007 and gs08 155 2008 remotely operated vehicle rov monitoring and successful recovery of gas hydrate give insights to the origin of the hydrocarbon gases in the complex pockmark feature named g11 isotope values of methane 13c ranging from 72 4 to 66 2 and d from 202 0 to 198 0 in the hydrate bound gas suggest that the methane is microbially produced and originates from co2 reduction it is proposed that the ethane from the gas hydrate has mixed thermogenic and microbial contributions which is supported by the fact that the gas hydrate also contains propane and isobutane the composition of the hydrate bound gas thus leads to small inclusions of structure ii sii in the hydrate while the major part is structure i si the isotope data and gas composition together with the geological setting suggest that the hydrate bound gas originated predominantly from a free gas system located beneath the gas hydrate stability zone and accumulated under glaciated debris formations the free gas system is mainly microbial methane from in situ production however a contribution of thermogenic gas from deeper polygonal faults to the free gas system seems evident from the composition of the gas the rov monitoring strengthens previous suggestions that the fauna in the complex pockmark g11 are supported by ongoing micro seepage

distribution of gas hydrates on continental margins by means of a mathematical envelope a method applied to the interpretation of 3d seismic data 1 a 3d seismic volume from the nankai trough accretionary wedge se japan is used to evaluate the subsurface distribution of gas hydrates as a function of structural and stratigraphic complexity variable heat flow patterns and the presence of subsurface fluid conduits eleven equations were modified for depth pressure and temperature modeled in 3d and compared with the distribution of bottom simulating reflections bsrs offshore nankai the results show that the equations produce overlapping and thus potentially consistent predictions for the distribution of bsrs leading us to propose the concept of a bsr stability envelope as a method to quantify the subsurface distribution of gas hydrates on continental margins in addition we show that the ratio r between shallow and deep bsrs of seven subenvelopes which are defined by bsr stability equations indicates local gas hydrate equilibrium conditions values of r 1 the majority of bsrs are located in warmer structural traps the method in this paper can be used to recognize any divergence between observed and theoretical depths of occurrence of bsrs on 3d or 4d time lapse seismic volumes in the nankai trough our results point out for equilibrium conditions in bsrs located away from the megasplay fault zone and major thrust faults this latter observation demonstrates the applicability of the method to a the recognition of subsurface fluid conduits and b the prediction of maximum and minimum depths of occurrence of gas hydrates on continental margins under distinct thermal and hydrologic conditions

elastic wave velocity characterization of gas hydrate bearing fractured reservoirs in a permafrost area of the qilian mountain northwest china abstract there are two types of gas hydrate bearing reservoirs in the permafrost area of qilian mountain most of the gas hydrates occur mainly in the fractured mudstone reservoirs and rarely in the pores of the sandstone reservoirs in this study for the acoustic velocity characterization of the fractured gas hydrate reservoirs of the qilian mountain permafrost area some mudstone core samples were collected for physical rock experiments such as the acoustic experiment and the porosity and permeability experiment an acoustic velocity numerical simulation of gas hydrate reservoirs was performed according to the biot theory and the differential effective medium theory with the conditions of multiple gas hydrate occurrence models including the suspension model the semi cementation model and the cementation model and considering both infinite and penny shaped cracks fracture porosity was added to the core samples that only contain matrix porosity with fracture porosity ranging from 0 01 to 5 the variation laws between acoustic velocity with fractured porosity and hydrate saturation are obtained 1 in the case of an infinite crack if the fractured porosity is 0 01 1 the p wave velocity decreases rapidly in the case of the three occurrence models if the fractured porosity is higher than 1 the acoustic velocity decreases gradually if the crack shape is a penny shaped crack the p wave velocity decreases almost linearly with increasing fracture porosity 2 if the hydrate occurrence model is the suspension model the p wave velocity increases slightly with increasing hydrate saturation if the occurrence model is the semi cementation model or the cementation model when the gas hydrate saturation of the infinite crack ranges from 0 to 80 the acoustic velocity increases approximately linearly whereas when the gas hydrate saturation ranges from 80 to 100 the velocity increases rapidly if the crack is a penny shaped crack the velocity increases almost linearly with increasing gas hydrate saturation from 0 to 100 3 it is found that the fractured gas hydrate reservoirs of the qilian mountain permafrost area contain both penny shaped and infinite cracks of which the infinite crack is the main crack shape the gas hydrate occurrence in the qilian mountain permafrost area mainly follows the suspension model this has significance for the seismic exploration and log evaluation of gas hydrate bearing fractured reservoirs in the permafrost area of the qilian mountain in studying the acoustic velocity characterization the crack shapes and occurrence models of gas hydrate reservoirs in the study area

complex pockmarks with carbonate ridges off mid norway products of sediment degassing abstract a new type of pockmark has been discovered off mid norway they contain up to 10 m high ridges of methane derived authigenic carbonate mdac rock the pockmarks were investigated using rovs remotely operated vehicles in 2003 and 2004 the complex pockmarks are located well within the gas hydrate pressure temperature stability field and are associated with a regional bsr bottom simulating reflector our high resolution seismic data demonstrate that the near surface sedimentary bedding dips into vertical pipe structures underlying the pockmarks and that the pipes are deeply rooted the pockmarks are circular in plan view and contain up to 190 m long and 40 m wide ridges formed by up to 4 m wide carbonate rocks piled on top of each other in irregular piles other key features of the complex pockmarks include 1 a distinct fauna with local bacterial mats small tubeworms stalked crinoids and pycnogonids sea spiders and 2 the presence of light hydrocarbon gases c 1 c 5 in clay rich sediments the methane 13 c values range from 54 to 69 pdb suggesting the presence of both bacterial and thermogenic gases during our two day observation periods only micro seepage occurred without ebullition or other visual fluid flow petrography and geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane derived aragonite within the sediments 13 c 52 to 58 pdb we suggest that the complex pockmarks formed by sudden catastrophic fluid flow and that the large carbonate ridges represent dislodged mdacs originally formed subsurface prior to the event subsequent micro seepage is still active and may indicate that the pipes represent fluid flow conduits which transport both bacterial and thermogenic hydrocarbons to the water column our data do not suggest pressure buildup and do not predict future eruptions from the complex pockmarks thus our data rather suggest that the catastrophic formation of the complex pockmarks may be a one time event

comparison and implications from strikingly different authigenic carbonates in a nyegga complex pockmark g11 norwegian sea authigenic carbonates sampled in a complex pockmark g11 from the nyegga area south western voring basin reveal strikingly different morphologies a comparative study of macro and micro features of these neighbouring samples was approached in order to attempt a reconstruction of the pockmark dynamics the carbonate morphologies include thin 1 cm platy carbonate crusts blocky and massive carbonate ridges up to several metres in size and irregularly shaped carbonate deposits consisting of interconnected tubular and uneven intervals displaying high porosity our observations indicate that three distinct mechanisms of fluid seepage and carbonate precipitation occurred within the pockmark petrography studies reveal that the samples contain three generations of predominantly aragonitic cements in addition zoned crystals with coexisting mg calcite and dolomite are abundant in one sample suggesting a trend of gradual increase of the sulphate concentrations in the pore fluids during carbonate precipitation isotope measurements on bulk carbonate cements show consistent values with 13 c as low as 52 1 18 o as high as 6 4 and 87 86 sr of 0 7092 anaerobic oxidation of methane close to seafloor conditions can explain these isotope data the carbonates have a common carbon source despite the differences in morphology methane probably originated from the dissolution of gas hydrates as supported from calculated 18 o enrichment in the pore fluids and the methanogenic carbon we propose a model where the differences in bulk carbonate morphology are related to local sediment properties depth of carbonate precipitation and the flow rate of the seeping fluids

gas hydrate formation and its accumulation potential in mohe permafrost china abstract the mohe region is an area of continuous permafrost in northernmost china with strong similarities to other known gas hydrate bearing regions permafrost thickness is typically 20 80 m average surface temperature ranges from 0 5 c to 3 0 c and the geothermal gradient is roughly 1 6 c 100 m we estimate that 204 66 1012 m3 of hydrocarbon gases have been generated in the mohe basin from nearly 1000 m middle jurassic dark mudstones providing ample gas source for gas hydrate formation numerous folds in the shallow section provide opportunities to trap gas within sandstones and siltstones reservoirs bounded by competent mudstone seals gas migration to the shallow section is enabled via fault fracture zones and fracture systems based on core description and observations of gas releases from drilled wells we infer that the mohe region could hold large quantities of natural gas in the form of gas hydrate

evidence from three dimensional seismic tomography for a substantial accumulation of gas hydrate in a fluid escape chimney in the nyegga pockmark field offshore norway in recent years it has become evident that features commonly called gas chimneys provide major routes for methane to pass through the methane hydrate stability zone in continental margins and escape to the ocean one of many such chimneys lying beneath pockmarks in the southeastern voring plateau off norway was investigated with a high resolution seismic experiment employing a 2 d array of sixteen 4 component ocean bottom seismic recorders at approximately 100 m separation and a dense network of shots to define the 3 d variation of the chimney s structure and seismic properties the tomographic model derived from p wave travel times shows that p wave velocity inside the chimney is up to 300 m s higher than in the surrounding strata within the methane hydrate stability zone the zone of anomalously high velocity about 500 m wide near its base narrowing to about 200 m near the seabed extends to a depth of 250 m below the seafloor the depth extent of this zone and absence of high velocity beneath the base of the methane hydrate stability field make it more likely that it contains hydrate rather than carbonate if a predominantly fracture filling model is appropriate for the formation of hydrate in low permeability sediment the maximum hydrate concentration in the chimney is estimated to be 14 27 by total volume depending on how host sediment properties are affected by hydrate formation doming of the strata penetrated by the chimney appears to be associated with the emplacement of hydrate accompanying the invasion of the gas hydrate stability zone by free gas

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

hydrate occurrence in europe a review of available evidence abstract large national programs in the united states and several asian countries have defined and characterised their marine methane hydrate occurrences in some detail but european hydrate occurrence has received less attention the european union funded project marine gas hydrate an indigenous resource of natural gas for europe migrate aimed to determine the european potential inventory of exploitable gas hydrate to assess current technologies for their production and to evaluate the associated risks we present a synthesis of results from a migrate working group that focused on the definition and assessment of hydrate in europe our review includes the western and eastern margins of greenland the barents sea and onshore and offshore svalbard the atlantic margin of europe extending south to the northwestern margin of morocco the mediterranean sea the sea of marmara and the western and southern margins of the black sea we have not attempted to cover the high arctic the russian ukrainian and georgian sectors of the black sea or overseas territories of european nations following a formalised process we defined a range of indicators of hydrate presence based on geophysical geochemical and geological data our study was framed by the constraint of the hydrate stability field in european seas direct hydrate indicators included sampling of hydrate the presence of bottom simulating reflectors in seismic reflection profiles gas seepage into the ocean and chlorinity anomalies in sediment cores indirect indicators included geophysical survey evidence for seismic velocity and or resistivity anomalies seismic reflectivity anomalies or subsurface gas escape structures various seabed features associated with gas escape and the presence of an underlying conventional petroleum system we used these indicators to develop a database of hydrate occurrence across europe we identified a series of regions where there is substantial evidence for hydrate occurrence some areas offshore greenland offshore west svalbard the barents sea the mid norwegian margin the gulf of cadiz parts of the eastern mediterranean the sea of marmara and the black sea and regions where the evidence is more tenuous other areas offshore greenland and of the eastern mediterranean onshore svalbard offshore ireland and offshore northwest iberia we provide an overview of the evidence for hydrate occurrence in each of these regions we conclude that around europe areas with strong evidence for the presence of hydrate commonly coincide with conventional thermogenic hydrocarbon provinces

integrated petrographic and geochemical record of hydrocarbon seepage on the voring plateau authigenic carbonate crusts nodules and chemoherms were sampled from pockmarks and mud diapirs on the southern part of the voring plateau during the ttr 8 and ttr 10 marine expeditions a petrographic and geochemical study was carried out to investigate their possible relationship with the seepage of hydrocarbon fluids all authigenic carbonates are depleted in 13 c 31 6 13 c 18 o 18 o 4 6 18 o 18 o rich fluid the occurrence of different carbonate mineral phases aragonite calcite dolomite is possibly related to varying dissolved sulphate concentrations in the diagenetic environment fluid inclusion microthermometry and raman spectroscopy indicate the presence of an aqueous hydrocarbon mixture inside the inclusions this seepage mixture was almost certainly immiscible resulting in heterogeneous trapping

significance of mud volcanism 1 mud volcanism and diapirism have puzzled geoscientists for 2 centuries they have been described onshore and offshore in many places on earth and although they occur in various tectonic settings the majority of the features known to date are located in compressional tectonic scenarios this paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described mud volcanoes show variable geometry up to tens of kilometers in diameter and several hundred meters in height and a great diversity regarding the origin of the fluid and solid phases gas predominantly methane water and mud may be mobilized at subbottom depth of only a few meters but in places can originate from several kilometers depth with minor crustal or mantle input the possible contribution of mud extrusion to global budgets both from quiescent fluid emission and from the extrusive processes themselves is important in regions where mud volcanoes are abundant such as the collision zones between africa and eurasia fluid flux through mud extrusion exceeds the compaction driven pore fluid expulsion of the accretionary wedge also quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and hence to greenhouse climate

mud volcanoes and gas hydrates in the anaximander mountains eastern mediterranean sea abstract detailed multibeam sedimentological and geophysical surveys provide ample new data to confirm that the anaximander mountains eastern mediterranean are an important area for active mud volcanism and gas hydrate formation more than 3000 km of multibeam track length was acquired during two recent missions and 80 gravity and box cores were recovered morphology and backscatter data of the study area have better resolution than previous surveys and very detailed morphology maps have been made of the known targeted mud volcanoes amsterdam kazan and kula especially the amsterdam crater and the related mud breccia flows gas hydrates collected repeatedly from a large area of amsterdam mud volcano at a sub bottom depth of around 0 3 1 5 m resemble compacted snow and have a rather flaky form new gas hydrate sites were found at amsterdam mud volcano including the mud flow sloping off to the south gas hydrates sampled for the first time at kazan mud volcano are dispersed throughout the core samples deeper than 0 3 m and display a rice like appearance relative chronology and ams dating of interbedded pelagic sediments late holocene hemipelagic sapropel layer s1 and ash layers within the mud flows indicate that successive eruptions of kula mud volcano have a periodicity of about 5 10 kyrs new mud volcanoes identified on the basis of multibeam backscatter intensity were sampled documented as active and named athina and thessaloniki gas hydrates were sampled also in thessaloniki mud volcano the shallowest 1264 m among all the active mediterranean sites at the boundary of the gas hydrate stability zone biostratigraphical analyses of mud breccia clasts indicated that the source of the subsurface sedimentary sequences consists of late cretaceous limestones paleocene siliciclastic rocks eocene biogenic limestones and miocene mudstones rough estimations of the total capacity of the anaximander mud volcanoes in methane gas are 2 56 6 40 km3

estimating the time of pockmark formation in the sw xisha uplift south china sea using reaction transport modeling carbon cycling and fluid seepage in marine sediments over the late quaternary were investigated at a now extinct pockmark located in a mega pockmark field in the sw xisha uplift nw south china sea measured particulate organic carbon poc content and porewater sulfate so4 2 dissolved inorganic carbon dic concentrations and 34s so4 2 distributions were used to constrain a non steady state reaction transport model and quantify poc mineralization rates as well as estimate the time when fluid flow ceased at the investigated pockmark an increase in poc content and 34s so4 2 and a decrease in sulfate concentrations in the upper ca 2m at the pockmark and a reference core implied an increase in the flux and reactivity of organic matter during the early holocene around 10kyrb p caused by enhanced primary productivity during the strengthened southwestern summer monsoon these features were simulated with the model assuming a holocene increase in poc flux and reactivity subsequently starting from an initial condition reminiscent of a modern active cold seep hydrate ridge hindcast simulations showed that fluid seepage at the pockmark ceased ca 39kyr ago this corresponds to a relative sea level high stand which is believed to be associated with gas hydrate stabilization the non steady state model presented in this contribution can also be used to constrain the time when fluid seepage ceased at other presently extinct cold seeps when suitable sediment and porewater data are available

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

cold water coral banks and submarine landslides a review this paper aims to review the relation between cold water coral bank development and submarine landslides both are common features on continental margins but so far it has not been reviewed which effect if at all they may have upon each other indirect and direct relations between coral banks and landslides are evaluated here based on four case studies the magellan mound province in the porcupine seabight where fossil coral banks appear partly on top of a buried slide deposit the sula ridge reef complex and the storegga landslide both off mid norway and the mauritania coral bank province associated with the mauritanian slide complex for each of these locations positive and negative relationships between both features are discussed based on available datasets locally submarine landslides might directly favour coral bank development by creating substratum where corals can settle on enhancing turbulence due to abrupt seabed morphological variations and in some cases causing fluid seepage in turn some of these processes may contribute to increased food availability and lower sedimentation rates landslides can also affect coral bank development by direct erosion of the coral banks and by the instantaneous increase of turbidity which may smother the corals on the other hand coral banks might have a stabilising function and delay or stop the headwall retrogradation of submarine landslides although local relationships can be deduced from these case studies no general and direct relationship exists between submarine landslides and cold water coral banks

seismic velocity structure at a gas hydrate reflector offshore western colombia from full waveform inversion seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs which have been interpreted as being formed at the base of a methane hydrate stability field such reflectors might arise either from an impedance contrast between high velocity partially hydrated sediments and water saturated sediments or from a contrast with partially gas saturated sediments these alternatives may be hard to distinguish by conventional amplitude versus offset or waveform modeling approaches here we investigate the origin of a high amplitude bsr in the accretionary wedge offshore of western colombia by seismic waveform inversion the inversion procedure consists of three steps firstly determination of root mean square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time slowness domain secondly determination of accurate interval velocities between these reflectors by a monte carlo search for maximum energy and thirdly a waveform fit in the frequency slowness domain using differential reflectivity seismograms and a conjugate gradient optimization algorithm to minimize the sample by sample waveform misfit between data and synthetic at two locations near a structural high we find a 30 m thick low velocity zone beneath the bsr with the properties of a partially gas saturated zone while at a third location where the bsr amplitude is lower we find no evidence for anomalously low velocities the preferential development of the bsr in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the bsr indicate a mechanism of bsr formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water

submarine pingoes indicators of shallow gas hydrates in a pockmark at nyegga norwegian sea complex pockmarks up to 300 m wide and 12 m deep are located in the nyegga area in the norwegian sea bathymetric data and direct visual documentation and sampling with rovs remotely operated vehicles have shown that these pockmarks contain abundant methane derived authigenic carbonate rocks furthermore geochemical results and the finding of seep associated organisms including tubeworms and bacteria shows that the pockmarks are still active fluid flow locations hovland m svensen h forsberg c f johansen h fichler c fossa j h jonsson r rueslatten h 2005 complex pockmarks with carbonateridges off mid norway products of sediment degassing marine geology 218 191 206 here we report the discovery of localized pingo like sediment mounds up to 1 m high and 4 m wide they occur inside one of the nyegga complex pockmarks g11 all of the seven structures we investigated have four characteristics in common 1 they have a positive topography rounded mounds and cones 2 they are partly covered in bacterial mats indicating ongoing fluid flow 3 they are partly covered in a carpet of small living tubeworms polychaetes which utilize methane 4 they have distinct corrosion pits on their surfaces indicating fluidization and point source corrosion of the covering sediments probably caused by localized sub surface hydrate dissociation we interpret the features as true submarine pingoes formed by the local accumulation of hydrate ice below the sediment surface it is inferred that the pingoes are formed as documented hydrocarbon gases methane ethane propane and butane migrate upwards through distinctive sub surface channels or conduits inside the pockmark we suggest that these submarine hydrate pingoes manifest the exact locations where fluid flow through the seafloor is currently active and that they can therefore be used as small scale indicators of active seepage

split strata bounded gas hydrate bsr below deposits of the storegga slide and at the southern edge of the voring plateau abstract following the investigations of the 8th training through research ttr8 cruise 1998 the voring plateau storegga slide area was revisited during the ttr10 expedition of rv professor logachev in august 2000 using a single channel seismic system bottom simulating reflections bsrs were observed in undisturbed sediments of the voring plateau and below the deposits of the storegga slide the bsr is expressed on the seismic records either as a strong reflection often crosscutting local stratigraphy or as a facies change between high amplitude enhanced reflections below and normal amplitude reflections above it analysis of the seismic signature of the bsrs and adjacent enhanced reflections showed that the anomalous reflections demonstrate reversed polarity and cause amplitude and frequency shadows below indicating the presence of free gas trapped below hydrate bearing sediments for each of the lines where bsr was observed the theoretical base of the local gas hydrate stability field ghsf was calculated the resulting calculated depths of the theoretical ghsf fit well with observed bsrs and confirm the gas hydrate nature of the reflections on most of the profiles the bsr is split into several isolated zones the reflection appears at specific stratigraphic layers between strata where the bsr is not observed four bsr demonstrating layers were identified continuing upslope below the ghsf as enhanced reflections these layers must possess some physical or petrographical property that makes them most favourable for gas hydrate formation and or accumulation of free gas all these layers lie within the thick 1 s twt upper pliocene quaternary glacial interglacial sedimentary sequence it is assumed that they correspond to some strata of coarser terrigenic material which would feature higher permeability relative to surrounding sediments and or to undercompacted layers with increased water content

mass transport deposits and gas hydrate occurrences in the ulleung basin east sea part 1 mapping sedimentation patterns using seismic coherency abstract seismic coherency measures such as similarity and dip of maximum similarity were used to characterize mass transport deposits mtds in the ulleung basin east sea offshore korea using 2 d and 3 d seismic data several slope failure masses have been identified near drill site ubgh1 4 the mtds have a distinct seismic character and exhibit physical properties similar to gas hydrate bearing sediment elevated electrical resistivity and p wave velocity sediments recovered from within the mtds show a reworked nature with chaotic assemblage of mud clasts additionally the reflection at the base of mtds is polarity reversed relative to the seafloor similarly to the bottom simulating reflector commonly used to infer the presence of gas hydrates the mtds further show regional seismic blanking absence of internal reflectivity which is yet another signature often attributed to gas hydrate bearing sediments at the drill site ubgh1 4 no gas hydrate was recovered in sediment cores from inside a prominent mtd unit instead pore filling gas hydrate was recovered only within thin turbidite sand layers near the base of the gas hydrate stability zone with the analysis of seismic attributes the seismic character of the prominent mtd unit 3 was investigated the base of the mtd unit exhibits deep grooves interpreted as gliding tracks from either outrunner blocks or large clasts that were dragged along the paleo seafloor similar seismic features were identified on the seafloor although the length of the gliding tracks on the seafloor is much shorter a few hundred meters to 1 km compared to over 10 km long tracks at the base of the mtd the seismic coherency attributes allowed to estimate the volume of the failed sediment as well as the direction of the flow of sediment tracking the mtd and extrapolating its spatial extent from the 3 d seismic volume to adjacent 2 d seismic profiles a possible source region of this mass failure was defined 50 km upslope of site ubgh1 4

atypical heat flow near gas hydrate irregularities and cold seeps in the baikal rift zone abstract in this paper we address the irregular behaviour and geometry of the gas hydrate stability zone hsz inferred from reflection seismic data in relation to heat flow measurements the study area lies in the hanging wall of the posolsky fault in the southern baikal basin sbb side scan sonar imagery already revealed an undulating antithetic active fault structure and several isolated active vent structures remarkably these fluid discharge structures occur only where the base of the hydrate stability zone bhsz as inferred from seismic reflection profiles is fluctuating and discontinuous independent of lake floor morphology the correlation between the interpreted bhsz and heat flow data across the malenki seep is reasonable on a seismic profile south of the fluid escape features the bhsz is expressed as an oscillatory but continuous reflection and shows poor correlation with heat flow measurements in nearly all cases measured heat flow exceeds inferred heat flow additionally the local inferred minima are anomalously low compared to the expected background values in the sbb these observations suggest that the present day hydrate accumulation and its meta stability are more complicated than originally suspected the limited area of these anomalies their amplitudes and their occurrence in the immediate vicinity of faults and fluid escape features suggest that fluid convection cells disturb local gas hydrate stability conditions

norwegian margin outer shelf cracking a consequence of climate induced gas hydrate dissociation a series of en echelon cracks run nearly parallel to the outer shelf edge of the mid norwegian margin the features can be followed in a 60 km long and 5 km wide zone in which up to 10 m deep cracks developed in the seabed at 400 550 m water depth the time of the seabed cracking has been dated to 7350 14c years bp 8180 cal years bp which corresponds with the main storegga slide event 8100 250 cal years bp reflection seismic data suggest that the cracks do not appear to result from deep seated faults but it cannot be ruled out completely that tension crevices were created in relation to past movements on the headwall of the storegga slide the cracking zone corresponds well to the zone where the base of the hydrate stability zone bhsz outcrops evidence of fluid release in the bhsz outcrop zone comes from an extensive pockmark field we suggest that post glacial ocean warming triggered the dissociation of gas hydrates while the interplay between dissociation overpressure and sediment fracturing on the outer shelf remains to be understood

tracing seafloor methane emissions with benthic foraminifera results from the ana submarine landslide eivissa channel western mediterranean sea article the hypothesis that benthic foraminifera are useful proxies of local methane emissions from the seafloor has been verified on sediment core ks16 from the headwall of the ana submarine landslide in the eivissa channel western mediterranean sea the core ms312 from a nearby location with no known methane emis sions is utilised as control the core was analysed for biostratigraphy benthic foraminiferal assemblages hyalinea balthica and uvigerina peregrina carbon and oxygen stable isotope composition and sedimentary structures the upper part of the core records post landslide deglacial and holocene normal marine hemipe lagic sediments with highly abundant benthic foraminifera species that are typical of outer neritic to upper bathyal environment in this interval the 13 c composition of benthic foraminifera indicates normal marine environment analogous to those found in the control core below the sedimentary hiatus caused by the em placement of the slide the foraminiferal assemblages are characterised by lower density and higher shannon index markedly negative 13 c shifts in benthic foraminifera are attributed to the release of methane through the seabed the mean values of the 13 c anomaly in u peregrina are 0 951 0 208 in the pre landslide sediments and 0 269 0 152 in post slide reworked sediments deposited immediately above the hiatus the 13

ocean warming and gas hydrate stability on the mid norwegian margin at the storegga slide abstract the sensitivity of oceanic gas hydrates and submarine slope stability to the combined forcing of sea level changes and bottom water perturbation is a critical issue for risk assessment in the storegga slide area on the mid norwegian margin evidence for the existence of gas hydrates both inside and outside the storegga slide complex comes from reflection seismic profiles where a bottom simulating reflection marks today s base of the gas hydrate stability zone paleo bottom water temperatures show a relatively fast increase at approximately 12 5 10 ka calendar years following the younger dryas while stable warm water conditions have prevailed since then despite sea level rise this warm water inflow caused a major reduction in the thickness of the gas hydrate stability zone along the upper slope of the mid norwegian margin modelling results indicate that critical hydrate stability conditions and consequently the maximal potential pore pressure build up occur at the location of the storegga slide headwall although the major phase of hydrate melting predates the storegga slide event dated at 8 2 ka calendar years reduced hydrate stability conditions could have facilitated or contributed to sub marine slope failure additionally the bottom simulating reflections within the slide complex seem to have nearly adjusted to new equilibrium conditions highlighting the dynamics of hydrate stability in continental margin sediments under environmental changes climate change geohazards

a study of submarine steep slope failures triggered by thermal dissociation of methane hydrates using a coupled cfd dem approach this paper presents an extended numerical study on submarine landsliding induced by instantaneous thermal dissociation of a methane hydrate layer by considering different dissociation scenarios in terms of position and volume of dissociated gas hydrates a novel scheme of coupling computational fluid dynamics and distinct element method cfd dem is implemented the behavior of fluid is simulated by cfd incorporating an empirical equation of state for slightly compressible liquid whereas that of sediment skeleton with methane hydrates by dem through a two dimensional thermo hydro mechanical bond contact model the scenario of whole hydrate dissociation identifies four typical stages of submarine landsliding the initiation of landslide after hydrate dissociation the onset of the submarine landslide the sliding process and the final deposition and stabilization which is characterized by some microscopic variables e g particle displacement fluid velocity excess hydrostatic pressure particle scale energy input and dissipation so to attain a complete analysis of the landslide process reasonable representation of the slope instability phenomenon as triggered by thermal dissociation of gas hydrates is obtained besides slope failure scenarios with different positions and volumes of the occurring mh dissociation are presented to better understand their influence on the evolution of the instability four landslide types are generated by the different dissociation scenarios fall slump flow and a combination of slump and flow which are further analyzed by outputting some microscopic information e g force chain averaged pure rotation rate and bond distributions

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

effect of gas hydrates melting on seafloor slope instability we present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature pressure pore water chemistry and the mean pore size distribution the model uses a new formulation based on the enthalpy form of the law of conservation of energy the developed model shows that due to a temperature and pressure increase hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water this original result confirms what has been already shown through experiments the second part of the paper presents an application of the model through a back analysis of the giant storegga slide on the norwegian margin two of the most important changes during and since the last deglaciation hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature were considered in the calculation simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the storegga slope once again the developed model leads to predictions which are supported by laboratory experiment results but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone

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

geophysical evidence of gas hydrates in shallow submarine mud volcanoes on the moroccan margin 1 gas hydrates inside mud volcanoes have been observed in several locations but are generally found at water depths of 1000 m and deeper we present the first observation of the base of a gas hydrate stability zone within a shallow mud volcano in the el arraiche mud volcano field on the moroccan atlantic margin the mud volcano base is located at about 475 m and is over 125 m high on high resolution seismics we observed an anomalous but coherent reflection under the slopes of the mud volcano the event was interpreted as the base of a gas hydrate stability zone because of its inverse polarity and its morphology far from the crater the event is nearly parallel to the seafloor closer toward the crater the event shallows inside the mud volcano crater no event is observed a stability model using thermogenic gas compositions is applied to local p t conditions indicating that thermogenic gas hydrates can be stable at this depth the high modeled heat flow in the crater of the mud volcano indicates a focused flow of warm fluids below the slopes of the mud volcano the inferred heat flow is also elevated but less high in areas of thermogenic gas production gas hydrates can occur at shallow water depths even in areas with high heat flow this also suggests that dewatering of the accretionary wedge complex is mainly focused along fault surfaces and through seafloor structures such as mud volcanoes

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

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

a seismic study of methane hydrate marine bottom simulating reflectors multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors bsrs on the northern cascadia subduction zone margin off vancouver island the reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate the shallow water depth of 1300 m and the 3600 m long hydrophone array have allowed bsr amplitude versus offset and high resolution velocity analysis as well as modelling of vertical incidence data the results of all three types of analysis can be best explained by a 10 to 30 m thick high velocity layer located immediately above the bsr about 300 m below the seafloor having a sharp base and transitional top in the layer about one third of the sediment pore spaces must be filled with hydrate ice there is no seismically detectable free gas beneath the bsrs these results put important constraints on models for the distribution and formation of bsr hydrate

linking continental slope failures and climate change testing the clathrate gun hypothesis it has been suggested that the release of clathrates rather than expansion of wetlands is the primary cause of the rapid increases observed in the ice core atmospheric methane record during the pleistocene because submarine sediment failures can involve as much as 5000 gt of sediment and have the capacity to release vast quantities of methane hydrates one of the major tests of the clathrate gun hypothesis is determining whether the periods of enhanced continental slope failure and atmospheric methane correlate to test the clathrate gun hypothesis we have collated published dates for submarine sediment failures in the north atlantic sector and correlated them with climatic change for the past 45 k y more than 70 by volume of continental slope failures during the past 45 k y was displaced in two periods between 15 and 13 ka and between 11 and 8 ka both these intervals correlate with rising sea level and peaks in the methane record during the bolling allerod and preboreal periods these data support the clathrate gun hypothesis for glacial interglacial transitions the data do not however support the clathrate gun hypothesis for glacial millennial scale climate cycles because the occurrence of sediment failures correlates with heinrich events i e lows in sea level and atmospheric methane a secondary use of this data set is the insight into the possible cause of continental slope failures glacial period slope failures occur mainly in the low latitudes and are associated with lowering sea level this finding suggests that reduced hydrostatic pressure and the associated destabilization of gas hydrates may be the primary cause the bolling allerod sediment failures were predominantly low latitude suggesting an early tropical response to deglaciation e g enhanced precipitation and sediment load to the continental shelf or warming of intermediate waters in contrast sediment failures during the preboreal period and the majority of the holocene occurred in the high latitudes suggesting either isostatic rebound related earthquake activity or reduced hydrostatic pressure caused by isostatic rebound causing destabilization of gas hydrates

volcanogenic mass flow deposits and seafloor diapirism following the largest insular quaternary eruption of the eastern mediterranean at nisyros island aegean volcanic arc abstract nisyros volcanic island at the eastern end of the aegean volcanic arc was mainly constructed after the ultra plinian kos plateau tuff kpt eruption at 161 ka the surrounding submarine area consists mostly of marginal mass transport deposits that commonly characterize insular stratovolcanoes this study aims at providing insights into the deposition and volume of such deposits by elucidating the products of a mega eruption and subsequent paroxysmal volcanicity typical of subduction zone island arc systems seismic reflection and multibeam bathymetry around nisyros identify pyroclastic and debris flow deposits covering areas several orders of magnitude greater than the island the entire seabed between the basins of east kos and tilos is underlain by a 7 km3 volcaniclastic unit that defines a ubiquitous basal unconformity at an average depth of 30 m below the seafloor southwest of nisyros two groups of avalanche deposits from the same unit are bathymetrically identified on the slopes of the northernmost karpathos basin apron seismic stratigraphic markers that trace the unit on the shelf of tilos suggest it is concomitant with the kpt eruption this formerly unknown submarine expansion of the kpt deposits into three neighboring basins when added to the volume of this massive unit to the rest of the nisyros surroundings gives it an explosivity index vei 6 as such it represents the largest quaternary subaqueous volcanogenic mass flow deposit of the eastern mediterranean from this dataset it can be inferred that the submarine advance of the erupted material occurred mainly through erosive gravity flows envisioned as the main dispersion mechanism of large scale subaqueous pyroclastic deposits of plinian eruptions east of nisyros unique small scale diapiric domes and seafloor mounds are attributed to loading by the kpt in combination with regional faulting subsequent nisyros effusive activity produced pyroclastic units around the northeast as well as debris avalanche and lava flows south of the island these have been correlated to known onshore formations the present study documents the intensity and impacts of a volcanic island mega eruption and paroxysmal epilogue over a cycle of 100 kyr

tsunami simulations for regional sources in the south china and adjoining seas we present 14 scenarios of potential tsunamis in the south china sea and its adjoining basins the sulu and sulawezi seas the sources consist of earthquake dislocations inspired by the the study of historical events either recorded since 1900 or described in historical documents going back to 1604 we consider worst case scenarios where the size of the earthquake is not limited by the largest known event but merely by the dimension of the basin over which a coherent fault may propagate while such scenarios are arguably improbable they may not be impossible and as such must be examined for each scenario we present a simulation of the tsunami s propagation in the marine basin exclusive of its interaction with the coastline our results show that the south china sulu and sulawezi seas make up three largely independent basins where tsunamis generated in one basin do not leak into another similarly the sunda arc provides an efficient barrier to tsunamis originating in the indian ocean furthermore the shallow continental shelves in the java sea the gulf of thailand and the western part of the south china sea significantly dampen the amplitude of the waves the eastern shores of the malay peninsula are threatened only by the greatest and most improbable of our sources a mega earthquake rupturing all of the luzon trench we also consider two models of underwater landslides which can be triggered by smaller events even in an intraplate setting these sources for which there is both historical and geological evidence could pose a significant threat to all shorelines in the region including the malay peninsula

assessment of gas hydrate stability zone and geothermal modeling of bsr in the andaman sea abstract wide spread bottom simulating reflectors bsrs are observed along available multichannel seismic profiles covering an area of about 290 km 2 in the andaman sea the seismic data shows that the bsr occurs at places where water depth exceeds 1000 m and is identified by cross cutting relationships with the dipping reflectors the bsr that represents the base of gas hydrate stability field can be used to infer the gas hydrate stability thickness which ranges between 518 m to 861 m depending on water depths in situ measurement at site 17 during the indian national gas hydrate program nghp expedition 01 shows very low geothermal gradient 19 2 c km a conductive model was used to determine geothermal gradients from bsrs which is calculated and varying between 10 c km to 40 c km the low geothermal gradient is responsible for the deepest bsr or gas hydrate stability zone ghsz in the andaman region and in the world the geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed bsr depths

defining the updip extent of the gas hydrate stability zone on continental margins with low geothermal gradients 1 the distribution of gas hydrate on a continental slope is often characterized as a wedge that pinches out on the seafloor this part of the hydrate stability zone is particularly relevant for studies of the dynamics of hydrate accumulations such as processes related to slope stability or hydrate dissociation leading to methane release into the overlying ocean for regions with very low geothermal gradients we have produced a series of thermobaric models of the shallow hydrate stability zone that contain an unexpected geometrical distribution of hydrated sediments in these models the shallowest part of the stability field thickens and bulges landward such a feature is more likely to happen in regions where low geothermal gradients are further lowered by high sedimentation rates also the effect is greater beneath colder oceans although a hydrate stability zone bulge would be difficult to image with conventional seismic methods there are numerous locations around the world where such a system could develop

sediments with gas hydrates internal structure from seismic avo we interpret amplitude variation with offset avo data from a bottom simulating reflector bsr offshore florida by using rock physics based synthetic seismic models a previously conducted velocity and avo analysis of the in situ seismic data showed that the bsr separates hydrate bearing sediments from sediments containing free methane the amplitudes at the bsr are increasingly negative with increasing offset this behavior was explained by p wave velocity above the bsr being larger than that below the bsr and s wave velocity above the bsr being smaller than that below the bsr we use these avo and velocity results to infer the internal structure of the hydrated sediment to do so we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space 1 the hydrate cements grain contacts and strongly reinforces the sediment and 2 the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame only the second model can qualitatively reproduce the observed avo response thus inferred internal structure of the hydrate bearing sediment means that 1 the sediment above the bsr is uncemented and thereby mechanically weak and 2 its permeability is very low because the hydrate clogs large pore space conduits the latter explains why free gas is trapped underneath the bsr the seismic data also indicate the absence of strong reflections at the top of the hydrate layer this fact suggests that the high concentration of hydrates in the sediment just above the bsr gradually decreases with decreasing depth this effect is consistent with the fact that the low permeability hydrated sediments above the bsr prevent free methane from migrating upwards

a large methane plume east of bear island barents sea implications for the marine methane cycle a pockmark field extending over 35 km2 at 74 54 n 27 3 e described by solheim and elverhoi 1993 was re surveyed and found to be covered with more than 30 steep sided craters between 300 and 700 m in diameter and up to 28 m deep the craters are thought to have been formed by an explosive gas eruption anomalously high concentrations of methane in the shelf waters around the craters suggest that a strong methane source near this area is still active today methane enrichment more than 10 km away from the crater field indicates the large dimensions of a plume and the amount of gas released from sources below the seafloor of the barents sea shelf from the characteristic vertical decrease of methane towards the sea surface it is concluded that biota are extensively using this energy pool and reducing the methane concentration within the water column by about 98 between 300 m depth and the sea surface degassing to the atmosphere is minimal based on the shape of the methane concentration gradient nevertheless the net flux of methane from this area of the barents sea is about 2 9 104 g ch4 km 2 yr 1 and thus in the upper range of the presently estimated global marine methane release this flux is a minimum estimate and is likely to increase seasonally when rough weather leads to more effective vertical mixing during autumn and winter the amount of methane consumed in the water column however is about 50 times greater and hence should significantly contribute to the marine carbon inventory

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

the place of tubular concretions in hydrocarbon cold seep systems late miocene urenui formation taranaki basin new zealand the late miocene urenui formation cropping out on the eastern margin of taranaki basin currently new zealand s only commercially exploited hydrocarbon basin is dominated by siliciclastic slope mudstone 5 carbonate that locally includes a variety of tubular concretions 50 85 carbonate dominated by commonly large pipe and bulbous varieties as much as 0 5 to more than 10 m 1 6 to 33 ft long and 5 to 70 cm 2 to 28 in across these concretions formed from the precipitation of fine grained calcite or dolomite cement within siliciclastic mud in the shallow subsurface likely 100 m burial most support a central conduit that may be empty or filled with sediment or multiple generations of post shallow burial carbonate cements depleted 13c values of the concretionary calcite cements 40 to 25 pdb indicate that their carbon was sourced from oxidized methane in contrast enriched 13c values of the concretionary dolomite cements 10 to 11 pdb indicate that their carbon was derived from methanogenic co2 and or extensive methane oxidation the range of 18o values for the concretionary cement types 2 to 5 pdb supports precipitation from fluids both depleted and enriched with respect to new zealand miocene marine waters which are suggested to reflect episodes of gas hydrate formation and dissociation respectively the tubular carbonate concretions are interpreted to mark the subsea floor pathways of focused methane infused upward fluid escape and as such they represent a part of the shallow subsurface plumbing system of a cold seep system developed on the continental margin bounding the taranaki basin in the late miocene this margin has been strongly influenced by periodic movements along the major taranaki fault thrust system which coincidentally directly underlies the urenui concretion occurrences and was probably the most likely avenue for active fluid migration within the hydrocarbon seep system the tubular concretions experienced periodic intervals of exhumation and burial caused by margin failure because of greater slope instability from injected fluids and resulted in conglomeratic channel fill deposits within the urenui formation that include mass transported tubular concretions

seafloor depressions on the nigerian margin seabed morphology and sub seabed hydrate distribution abstract gas hydrate quantification using acoustic data requires proper knowledge of the mineralogy of their host sediment in this paper a petrophysical model allowing gh quantification at sites where mineralogy profiles are absent is proposed this approach is applied to a high gas flux pockmark system in the gulf of guinea where in situ acoustic and geotechnical measurements together with core measurements could have been correlated and tied to seismic data projections of the in situ measurements on seismic profiles have shown that the study area not only accommodates zones of shallow and dense gh but also zones where solid hydrate and free gas coexist as well as pockets of free gas further analysis of several seismic profiles has allowed illustrating the detailed gh occurrence zone within the study area estimate its volume and its occupancy ratio of the pockmark correlations between gh content and 3d bathymetry sections have allowed to draw a link between different gh contents and the morphology of the pockmark which also shares similarities with the morphology of the gh occurrence zone it accommodates

slide structure and role of gas hydrate at the northern boundary of the storegga slide offshore norway the storegga slide off the coast of norway is one of the largest underwater slide complexes known and has been proposed as a significant source of past methane release into the atmosphere we present pre stack depth migrated images from a new multichannel seismic data set in the storegga slide the northern scarp of the storegga slide has previously been interpreted as a single large recent slope failure however our images show strong evidence for a composite structure consisting of a much older event and recent slumping we observe onlapping features onto slide deposit highs and layer thickening as post slide sediments fill in accommodation space created at the slide scarp both of which support this conclusion displaced fault blocks are overlain by undeformed flat lying sediments also indicating considerable time between slide events according to dating of the base of the naust at this location this older slide event occurred at a minimum of 250 ka the causes of submarine slope failure are poorly understood but previous studies have proposed both earthquakes and dissociation of gas hydrates as triggering mechanisms pressure temperature modeling shows that assuming steady state conditions the bottom simulating reflector bsr would have been deeper than the glide plane at the time of slope failure the base of the gas hydrate stability zone and any gas that may have been present likely played only a minor role if any in slide initiation at this locale

recent depositional patterns of the nile deep sea fan from echo character mapping the nile deep sea fan was surveyed in 1998 using swath multibeam bathymetry backscatter imagery and 3 5 khz and seismic profiling on the basis of this new data set the fan has been divided into three main provinces western central and eastern recent sedimentary patterns as deduced from echo character mapping show that gravity induced sedimentary deposits are predominant and are expressed through either slides debris flows or turbidites turbidity processes are particularly active in the westernmost province through a well developed network of deep sea channels this network controls turbiditic flows that subsequently overflow levees mass wasting processes are extremely efficient all over the nile deep sea fan in the upper central province a broad allochthonous tongue of mass flow deposits generated by slope destabilizations may have led to channel avulsions and migrations in the eastern province a northwest southeast deformed belt resulting from combined deep seated and salt related tectonics contains large proportions of destabilized sediments i e originating from destabilization processes such as mass movements probably settled by recent tectonic activity this article highlights 1 the influence of both thin skinned and thick skinned tectonics on sedimentary distribution and 2 the predominance of gravity induced sediments turbidites slides and mass flows in such submarine fans the distribution of debris flows and sedimentary deposits settled by turbidity currents appears fundamental in predicting reservoir geometry and economic potential

evidence for long term instability in the storegga slide region off western norway abstract shallow seismic surveys around the storegga slide off western norway have allowed greater understanding of the development of this part of the european margin the northern flank of the scarp is formed of seismically well layered hemipelagic and distal glaciomarine deposits in which a variety of luid escape structures probably due to gas are locally abundant there is evidence of slides that substantially pre date the earliest slide previously recognized surveying on the north sea fan to the southwest of the storegga slide shows the markedly different nature of the autochthonous sediments on the southern flank of the storegga slide there is a predominance of glacigenic debris flows in the upper part of the sequence lesser maximum slopes and an apparent absence of interstitial gas and or hydrates this contrast has had considerable effect on slope stability and has influenced the position of the southwestern storegga slide boundary the north sea fan succession includes at least three major buried slides termed the vigra more and tampen slides all of which substantially pre date the storegga event and probably pre date predominantly glacigenic margin sedimentation post late weichselian slope failure is locally significant the region has a long but as yet chronologically poorly defined history of instability in which seismic triggering is considered to have been important

methane gas release from the storegga submarine landslide linked to earlyholocene climate change a speculative hypothesis the 3000 km3 storegga submarine landslide occurred c 8150 years ago from an area rich in gas hydrates off the western coast of norway a synchronous increase of 80 100 ppb in atmospheric methane c

progress in stability analysis of submarine slopes considering dissociation of gas hydrates gas hydrates have the potential to be a new energy source and a submarine geohazard though researchers generally agree about the association between gas hydrate dissociation and submarine slope failures the processes and mechanism of submarine slope failure caused by gas hydrate dissociation are not clearly understood in the last few years some authors have tried to analyse submarine slope stability by considering the existence and dissociation of gas hydrate and a few researchers have presented quantitative models this paper presents a review of the various causes of submarine slope failures associated with gas hydrate dissociation also analysis models of submarine slope stability associated with gas hydrate dissociation that are documented from the literatures including the infinite slope model wedge model slump and retrogressive failure model are interpreted and illustrated respectively

high resolution 3d seismic data indicate focussed fluid migration pathways above polygonal fault systems of the mid norwegian margin seismic attribute analysis and interpretation of high resolution 3d and 2d seismic data reveal focussed fluid flow processes through the gas hydrate stability zone ghsz at the northern flank of the giant storegga slide diffusive fluid migration predominantly starts from a widespread polygonal fault system in fine grained sediments of the miocene kai formation the overlying 600 700 m thick plio pleistocene naust formation shows spatially related soft sediment deformation and overlying fluid conduits a low relief antiform structure connects to an overlying 250 m high 300 m wide and 3 km elongated columnar zone where seismic signatures suggest self enhanced permeability i e natural hydraulic fracturing push down effects create an elongated depression caused by increased gas accumulations where a cluster of vertical cylindrical acoustic pipe structures originates these pipe clusters pierce the ghsz and indicate focussed fluid flow pathways originating from potentially overpressured sediments high seismic reflection amplitudes at the seafloor above the pipe structures may indicate pockmarks with authigenic carbonates and or gas hydrates the observed objects and seismic features presented are not stand alone indicators for fluid flow but a joint perspective illustrates that they are vertically tied together providing new insights to the effects of focussed fluid flow

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

patterns of subsurface fluid flow at cold seeps the hikurangi margin offshore new zealand based on multichannel seismic geoacoustic and methane sensor data four different areas along the hikurangi margin show multiple indications for seep activity including bright spots transparent zones vertical chimneys and the occurrence and distribution of bottom simulating reflectors locations where these features reach the seafloor are characterised by high backscatter intensity on sidescan sonar images and transparent zones in sediment echosounder profiles while methane sensors show episodic elevated methane concentrations near the seep sites methane discharge is facilitated by reduced hydrostatic pressure during low tides the greatest number of seeps at opouawe bank correlates with the highest methane activity along the hikurangi margin high heat flow values on flanks of ridges and low heat flow values on anticlines reflect a topographic effect on subsurface temperatures elevated heat flow occurs in the vicinity of seeps on opouawe bank we propose that there are two drivers behind methane seepage with respect to the migration pathways of methane through the gas hydrate stability zone ghsz to the seafloor 1 structurally controlled and 2 stratigraphically controlled in the structural model vertical chimneys are the major pathways for methane through the ghsz part of the upwardly migrating methane forms gas hydrate within the chimney in the stratigraphic model methane migration is stratigraphically controlled beneath seeps that are located on bathymetric highs and or where subsurface anticlines occur beneath seeps the structurally controlled seeps produce higher methane escape at the seafloor than those that are stratigraphically controlled a combination of both driving mechanisms results in the highest methane seepage rates at the tui seep on opouawe bank

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

late quaternary seismic stratigraphy and geological development of the south voring margin norwegian sea abstract high resolution seismic data and sediment cores show that an up to 280 m thick sedimentary sequence has been deposited on the south voring margin off mid norway the last ca 250 ka the sedimentary succession has been divided into six seismic units dominated by hemipelagic sediments five wedge shaped massive sequences of marine isotope stages 8 6 and 2 interfinger the hemipelagic deposits on the upper slope the wedge shaped sequences represent glacigenic debris flows that have been fed by till transported to the shelf edge by grounded ice sheets during maximum glaciations the hemipelagic units show well defined depocentres of various thicknesses on the upper continental slope seismic facies interpretation indicates that the sediment distribution locally has been controlled by currents commonly the hemipelagic units are characterised by parallel and continuous reflectors however the second youngest unit identified deposited between 15 7 and 15 0 14 c ka bp is acoustic transparent we suggest that this unit has been sourced by along slope transported meltwater plume deposits released during the initial stage of the last deglaciation of the norwegian channel the hemipelagic sedimentation rates have varied considerably throughout the studied time period until ca 21 14 c ka bp the rates did not exceed 1 4 m kyr whereas during the last glacial maximum the rates increased and reached values of about 36 m kyr before decreasing again at ca 15 14 c ka bp observation of iceberg scourings of mis 8 age about 800 m below the present day sea level suggest that the south voring margin has subsided by a rate of 1 2 m kyr in the late quaternary

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

a comprehensive avo classification in this paper a new classification for avo responses is proposed this classification covers all possible avo responses and is independent of the fluid content of the beds in 1989 rutherford and williams introduced a three fold classification of avo amplitude versus offset characteristics for seismic reflections from the interface between shales and underlying gas sands the classification scheme they proposed has become the industry standard and has proven its validity and usefulness in countless exploration efforts the rutherford and williams classification is primarily based on the sign and magnitude of the reflection coefficient r at the top of the sand these authors went on to observe that owing to the high poisson s ratio values for the typical encasing shales relative to the gas sands the avo response of all of these gas sand classes would be characterized by an increasingly negative reflection coefficient with increasing offset their classification is explicitly defined for gas sands in 1997 castagna and swan proposed avo crossplotting wherein an estimate of the normal incidence reflectivity is plotted against a measure of the offset dependent reflectivity using this approach castagna and swan graphically illustrated the continuum between the classes and defined the characteristics of the classes using what they termed avo intercept and avo gradient they also added a class 4 which they believed was originally included in rutherford and williams class 3 definition the authors of the present paper propose to expand and partly redefine the currently used classifications in order to distinguish among the schemes the term types as opposed to classes is used to name the different categories and in order to preserve congruity where the schemes overlap the sands which rutherford and williams would call class 1 2 or 3 are referred to as types 1 2 or 3 type 4 of the proposed