seismic and structural characterization of fluid escape pipes using 3d and partial stack seismic from the loyal field uk a multiphase and repeated intrusive mechanism acknowledgements we thank an anonymous reviewer for the several constructive comments the seismic interpretation and image processing was carried out in the seislab facility at the university of aberdeen sponsored by bg bp and chevron seismic imaging analysis was performed using geoteric ffa and analysis of seismic amplitudes was performed in petrel 2016 schlumberger we would like to thank the tuscany phd regional program and the erasmus exchange for funding the aberdeen permanence of one of us d m gazprom for supporting a j phd bp for the release of the loyal field seismic dataset utilized in this research paper and also n vanden beukel bp and m gorling bp and his colleagues for their assistance

automated feature extraction and spatial organization of seafloor pockmarks belfast bay maine usa article i nfo seafloor pockmarks occur worldwide and may represent millions of m 3 of continental shelf erosion but few numerical analyses of their morphology and spatial distribution of pockmarks exist we introduce a quantitative definition of pockmark morphology and based on this definition propose a three step geomorphometric method to identify and extract pockmarks from high resolution swath bathymetry we apply this gis implemented approach to 25 km 2 of bathymetry collected in the belfast bay maine usa pockmark field our model extracted 1767 pockmarks and found a linear pockmark depth to diameter ratio for pockmarks field wide mean pockmark depth is 7 6 m and mean diameter is 84 8 m pockmark distribution is non random and nearly half of the field s pockmarks occur in chains the most prominent chains are oriented semi normal to the steepest gradient in holocene sediment thickness a descriptive model yields field wide spatial statistics indicating that pockmarks are distributed in non random clusters results enable quantitative comparison of pockmarks in fields worldwide as well as similar concave features such as impact craters dolines or salt pools published by elsevier b v

simulating seismic chimney structures as potential vertical migration pathways for co2 in the snohvit area sw barents sea model challenges and outcomes carbon capture and storage ccs activities at the snohvit field barents sea will involve carrying out an analysis to determine which parameters affect the migration process of co2 from the gas reservoir to what degree they do so and how sensitive these parameters are to any changes this analysis will aim to evaluate the effects of applying a broad but realistic range of reservoir fault and gas chimney properties on potential co2 leakage at various depths throughout the subsurface fluid flow might take place through parts of or the entire extent of the overburden one of the aims of the analysis is assessing the potential of co2 reaching the seabed using the snohvit gas reservoir and overburden in the barents sea a series of geological models were built using seismic and well log data we then performed numerical simulations of co2 migration in focused fluid flow structures identification of potential migration pathways and their extent such as gas chimneys and faults and their incorporation into these models and simulations will provide a realistic insight into the migration potential of co2 in the simulations the co2 is injected over a 20 year period at a rate of 0 7 mt year and migration is allowed to take place over a 2000 year time frame for domains of approximately 21 km2 for the caprock fault models 24 km2 for the realistic gas chimney models and 35 km2 for the generic gas chimney models in a layered sedimentary succession the total mass of co2 injected in the reservoir during the 20 year injection period is 14 mt there is a strong interaction between the various parameters but the parameter that had the most influence on the co2 migration process was probably the permeability of the reservoirs especially the average permeability k also for the faulted caprock scenarios it should be noted that at near surface depths the permeability of 765 md is already adequate for a good co2 flow at the chimney top level 600 m however a further increase in permeability has an additional effect on improving co2 flow overall considering the slow upward migration velocity of the plume this geological setup can be regarded as a suitable storage site

discovery of a natural co2 seep in the german north sea implications for shallow dissolved gas and seep detection a natural carbon dioxide co2 seep was discovered during an expedition to the southern german north sea october 2008 elevated co2 levels of 10 20 times above background were detected in seawater above a natural salt dome 30 km north of the east frisian island juist a single elevated value 53 times higher than background was measured indicating a possible co2 point source from the seafloor measured ph values of around 6 8 support modeled ph values for the observed high co2 concentration these results are presented in the context of co2 seepage detection in light of proposed subsurface co2 sequestering and growing concern of ocean acidification we explore the boundary conditions of co2 bubble and plume seepage and potential flux paths to the atmosphere shallow bubble release experiments conducted in a lake combined with discrete bubble modeling suggest that shallow co2 outgassing will be difficult to detect as bubbles dissolve very rapidly within meters bubble plume modeling further shows that a co2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly co2 enriched water tends to sink toward the seabed results suggest that released co2 will tend to stay near the bottom in shallow systems 200 m and will vent to the atmosphere only during deep water convection water column turnover while isotope signatures point to a biogenic source the exact origin is inconclusive because of dilution this site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor

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

permeability evolution during the formation of gas hydrates in marine sediments 1 fluxes of gas energy and fluid water with dissolved gas are known to control the distribution of gas hydrate and free gas in gas hydrate reservoirs but theoretical studies have so far not focused on the effect of sediment properties e g permeability on these fluxes during the formation of gas hydrate deposits using a transient one dimensional two phase gas hydrate and fluid numerical model based on a coupled conservation approach we examine permeability clogging during gas hydrate formation in end member porous media and fractures initially lacking hydrate and determine the resulting changes in fluid and advective energy flux gas hydrate accumulates most rapidly at depth in both systems a result consistent with borehole logging and seismic studies on the blake ridge consideration of the ratio of surface area to fluid volume explains model results that imply more rapid permeability reduction in the porous media system and more uniform hydrate formation as a function of depth in the fracture system depending on fluid flux fracture aperture or pore sizes and other factors the timescale for permeability clogging due to the accumulation of gas hydrate ranges from thousands to millions of years regenerating gas hydrate to a concentration of 10 15 of pore space in marine settings with fluid fluxes characteristic of hydrate ridge or blake ridge requires 103 105 years meaning that hydrate deposits in these settings cannot be viewed as a naturally renewable resource these results also imply that significant time is required to reestablish gas hydrate deposits after large scale dissociation events associated with global scale climate change

morphology of pockmarks along the western continental margin of india employing multibeam bathymetry and backscatter data this study addresses the morphology of pockmarks along the western continental margin of india using multibeam bathymetry and backscatter data here for the first time we have utilized the application of arcgis geographical information system for understanding the morphology of pockmarks from the western continental margin of india the pockmarks observed in water depths of 145 330 m are circular elliptical or elongated in plan view with an average length and width of 157 72 m and 83 19 m respectively the average pockmark relief and perimeter are 1 9 0 9 m and 412 181 m respectively the pockmarks have average areas and volumes of 10 759 m and 15 315 m respectively spatial separation that coincides with 210 m isobath divides the pockmarks into two groups with differing distributions and morphologies these pockmarks originated from seepages of biogenic or thermogenic gas or from pore fluids from deeper sources migrated vertically along the faults besides a possible structural control the pockmark morphologies are also affected by bottom currents and or by submarine slumping the average acoustic backscatter strength from pockmark centre is higher 35 db than the average backscatter of the total area 40 db which suggests their possible linkage to the precipitation of diagenitic minerals from biodegradation of seepage material

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

control of quaternary sea level changes on gas seeps gas seeping to the seafloor through structures such as pockmarks may contribute significantly to the enrichment of atmospheric greenhouse gases and global warming gas seeps in the gulf of lions western mediterranean are cyclical and pockmark life is governed both by sediment accumulation on the continental margin and quaternary climate changes three dimensional seismic data correlated to multi proxy analysis of a deep borehole have shown that these pockmarks are associated with oblique chimneys the prograding chimney geometry demonstrates the syn sedimentary and long lasting functioning of the gas seeps gas chimneys have reworked chronologically constrained stratigraphic units and have functioned episodically with maximum activity around sea level lowstands therefore we argue that one of the main driving mechanisms responsible for their formation is the variation in hydrostatic pressure driven by relative sea level changes

gas migration pathways and slope failures in the danube fan black sea abstract a large geophysical dataset including bathymetry and 2d and 3d p cable seismic data revealed evidence of numerous gas flares near the s2 canyon in the danube fan northwestern black sea this dataset allows us to investigate potential relationships between gas migration pathways gas vents observed at the seafloor and submarine slope failures vertical gas migration structures as revealed in the seismics appear to be concentrated near submarine slope failure structures where these seismically defined features extend upwards to the seafloor they correlate with the location of gas flares however not all these structures reach the seafloor in some cases because they are capped by overlying sediments a strong correlation is inferred between gas migration pathways heterogeneous mass transport deposits and contacts between adjacent units of contrasting lithology although missing age constrains prevent a final judgement we discuss the potential relationship between submarine slope failures and gas migration in order to determine if gas migration is a precursor to failure or if the presence of slope failures and associated mass transport deposits facilitates the migration of gas our observations indicate that lithological heterogeneity mass transport deposits and minor sediment deformation control gas migration pathways and the formation of gas chimney like features gas migration is focused and gradual resulting in gas flares where the chimney like features extend to the seafloor with no evidence of erosive features such as pockmarks

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

flow velocity measurements in vegetated channels understanding of the hydraulics of flow over vegetation is very important to support the management of fluvial processes in this paper the flow over flexible bottom vegetation is experimentally studied a two dimensional acoustic doppler velocimeter is used to measure the local flow velocities for different vegetation concentrations discharges and flume slopes the influence of vegetation concentration and the depth vegetation height ratio on the measured velocity profiles is analyzed all measured velocity distributions are s shaped and exhibit a three zone profile the relationship between the velocity distribution and the turbulence intensity distribution is also analyzed the characteristics inflection point maximum value asymptotes of the measured velocity distributions and the results of the previous investigators are used to select an analytical expression for the shape of the velocity profile a theoretical velocity profile is deduced using the classical prandtl s mixing length approach with a new expression for the mixing length the deduced four coefficient profile allows description of the flow both inside and above the vegetation the physical and geometrical meaning of the four coefficients are also shown doi 10 1061 asce 0733 9429 2002 128 7 664 ce database keywords velocity profile vegetation channel flow

comment on evidence for the biotic origin of seabed pockmarks on the australian continental shelf by r j mueller marine and petroleum geology 2015 doi 10 1016 j marpetgeo 2014 12 016 abstract the study of seabed depressions that may be related to fluid seepage requires a multi layer approach the application of a one hypothesis fits all approach to understanding seabed characteristics fails to critically assess information necessary to undertake a robust assessment here we contend that the broad brush approach assigning pockmark formation on the northwest shelf of australia to a single organism is insufficiently robust to assign a reliable interpretation of pockmark formation in this setting

shallow gas in the holocene mud wedge along the inner east china sea shelf abstract high resolution seismic surveys were carried out in the inner shelf of the east china sea to investigate the distribution of shallow gas in holocene sediments shallow gas is distributed extensively in the holocene strata of the shelf which is dominated by the yangtze river organic rich fine grain sediments deposited throughout the holocene are essential for gas generation and accumulation the thickness of the holocene sediment is the main controlling factor determining the distribution of shallow gas gas seepages were found mostly at a water depth up to 20 m where sediment erosion mainly occurs coincidence between shallow gas seepage and high methane concentration in the water column suggests that gas emission from these sediments is a potentially significant source of greenhouse gas the interaction between shallow gas migration and coastal erosion would therefore potentially lead to increased greenhouse gas emission and accelerated sediment erosion such results can be applied to other costal locations around the world

acoustic and biological characteristics of seafloor depressions in the north yellow sea basin of china active fluid seepage in shallow water seafloor abstract high resolution multibeam bathymetry revealed the occurrence of numerous craterlike depressions so called pockmarks in the sea floor of the north yellow sea basin nysb to investigate whether these pockmarks are related to any type of fluid flow or gas seepage from subsurface stratum acoustic survey and box samplings were conducted in a selected area containing several typical pockmarks this allowed for an integrated analysis of the morphology distribution and subsurface strata as well as a direct comparison of seafloor characteristics among different areas inside and outside of a pockmark additionally surface sediments were analyzed for particle structure and grain size based on the combination of multibeam bathymetry and backscatter intensity data 282 seafloor depressions were accurately identified moreover the formation of pockmark can be attributed to existence of gas chimneys and other shallow gases revealed by shallow seismic profile records in the subsurface strata sediments inside and outside the pockmark showed remarkable discrepancies in terms of grain size and benthic fauna additionally strong internal reflection area was observed inside the depressions where benthic habitats are abundant and sediments contain strawberry pyrite and carbonate cementation side scan sonar also recorded sediment plumes in the water column which indicates that the pockmarks are still active several pearl string like accumulations of pockmark were found in the study area which well agree with the distribution of buried channels and lakes these results thus provide new insights in interpreting shallow water fluid seepage with implications for our current knowledge of overall sedimentation process in offshore continental shelves

constraints on the dynamics of pockmarks in the sw barents sea evidence from gravity coring and high resolution shallow seismic profiles abstract newly acquired sediment cores and high resolution seismic lines were studied in order to constrain the timing and mechanism of pockmark formation in the loppa high area in the southwestern barents sea the evolution of the pockmarks after their formation was also investigated together with the possibility that they are currently active features the cores retrieved within the pockmarks lack a layer of ice rafted debris and an overlying sequence of laminated glaciomarine sediments that were deposited on the undisturbed seafloor at the inception of the bolling interstadial 15 cal kyr bp the pockmarks are suggested to have formed in that period probably due to climate change induced destabilisation of methane hydrates and the subsequent release of free gas the depressions deepened as a result of reduced sedimentation above active seeps the pockmark cores contain a series of gravel rich layers which are attributed to the winnowing of sediment during episodic fluid venting activity the seismic images reveal little disturbance of the most ancient sediments underneath the pockmarks indicating that the pockmark chimneys are localised features and were not imaged the holocene marine sediment cover in the pockmarks is thinner than on the surrounding seafloor indicating relatively low sedimentation rates inside the depressions the gas seepage may thus have been active until the recent past although no indications of present day methane flux were found

cold seeps at the salt front in the lower congo basin i current methane accumulation and active seepage abstract active high intensity gas seepage is documented for the first time at the seaward edge of the salt occurrence in the southern lower congo basin microbial methane release from the seafloor occurs on the crests of two 800 m high ridges formed by fault propagation folding intense uplift is documented since the end of the miocene by distinct onlapping reflections on the landward flank of these ridges a paleo pockmark structure suggests an onset of seepage coincident with this deformation period high resolution seismic imaging reveals methane migration along strata from oligocene miocene fan deposits towards the ridge crests where large gas accumulations form beneath a discontinuous bottom simulating reflection bsr detailed mapping revealed that free gas and gas hydrate occurrences below and above the base of the gas hydrate stability zone are closely linked to sedimentary strata in the flanks of topographic ridges gas transport through the gas hydrate stability zone originates from the shallowest area of the bsr directly beneath the seafloor seep sites suggesting pressure controlled venting these sites represent the most seaward salt related gas seepage features documented in the area and illustrate the initiation of long lasting seepage at the front of an area of compressional tectonics at a passive continental margin

variation of system openness and geochemical features in overpressured sandstones of the yinggehai basin offshore south china sea abstract hydrocarbon discoveries in recent years in the upper miocene of the yinggehai basin show differences in overpressure magnitude gas and mineral components overpressure close to or reaching the leakage threshold has induced high system openness for one field but the overpressure of another field remains to be safely closed a new vision to investigate the interplay between the system openness and the gas chemical and mineralogical features were conducted the gas components in the high openness system are rich in high maturity hydrocarbons and inorganic co2 which both have heavier carbon isotopes low maturity gases with lighter carbon isotopes are dominant in the low openness system the abnormal carbon isotope ratio between iso butane and n butane in the high openness system confirms that early low maturity leakage has generated abnormal compositional and isotopic fractionation although the sedimentation process was uniform in the two systems the high openness system has rare k feldspar grains and chlorite but it is rich in illite smectite mixed layers i s precipitation is characterized by ankerite and kaolinite in the high openness system whereas the low openness system primarily consists of calcite the strong dissolution of k feldspar grains and chlorite should have released abundant potassium and silicon the high abundances of i s and kaolinite which are both sensitive to potassium and the rarity of quartz overgrowths in the high openness system imply that the leakage of potassium silicon from the sandstones has induced a mass imbalance between dissolution and precipitation even the sandstone porosity was slightly increased by the mass imbalance in the high openness system the risk of hydrocarbon expulsion during brine leakage cannot be ignored

geological controls on fluid flow and gas hydrate pingo development on the barents sea margin vista a basic research program in collaboration between the norwegian academy of science and letters and equinor

gas bubble cavities in deltaic muds lake powell delta glen canyon national recreation area hite utah abstract cavities produced by migrating gas mainly methane emanating from the near subsurface are a rarely reported sedimentary structure from either modern or ancient clastic sediments the delta of the colorado river in lake powell glen canyon national recreation area provides an excellent recently accessible locality for examining soft sediment deformation structures generated by escaping gases and fluids sub meter scale to tens of meter scale structures including craters salses and sand and mud volcanoes are present on the modern lake powell delta abundant sub centimeter scale cavities are found in excavations through crater sediments and exposed along cut bank exposures of the colorado river in cross section the cavities are subvertical irregular to sigmoidal to lenticular shaped 1 3 mm short axes up to 3 0 cm long axes and 1 cm intermediate axes cavities are developed in cm scale laminated silty clay and clay graded beds the tops of the cavities are wider than their bases the cavities observed in the lake powell delta are interpreted as mode 1 fractures and probably represent a clastic morphologic analog to molar tooth carbonate structures the characteristics of the cavities and the geometries and deformation of the surrounding delta mud host are consistent with bubbles and fracturing generated by gas migration experiments and theoretical calculations but vary because of inherent heterogeneity of the sediments fractures developed as gas migrated into pre existing cavities causing overpressure in cavities and leading to subvertical mode 1 fracture propagation recent sudden decompression related to rapid lake level drops of lake powell may have enhanced the release of gas primarily bacterially produced leading to the development of these soft sediment deformation structures

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

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

u th isotope constraints on gas hydrate and pockmark dynamics at the niger delta margin abstract the application of uranium thorium dating methods to authigenic carbonates provides unique constraints on the temporal evolution of methane seeps at ocean margins in this study we report u th isotope measurements for carbonate breccias collected from within a hydrate bearing pockmark located at the niger delta margin these concretions were extracted from a carbonate rich layer in the upper two meters of a sediment core n2 ks 44 1200 m water depth well above the present day sulphate methane transition zone about 3 m depth and the presence of gas hydrates in the sediment the stratigraphy of core n2 ks 44 was established by tuning its downcore al ti profile to a well dated nearby reference core and carbonate 230 th u ages were calculated using isochron methods our results indicate that a major event of aragonite precipitation occurred between about 13 and 2 5 ka at the studied location comparison of sediment accumulation rates at both core n2 ks 44 and the nearby reference site suggests that the initial stage of carbonate precipitation between 13 and 10 ka was associated with sediment winnowing probably related to intense fluid seepage in contrast our data indicate that sedimentation rates rapidly increased within the pockmark after 7 ka in agreement with the presence of carbonate breccias exhibiting u th ages older than their corresponding stratigraphic age this observation would suggest that sediment reworking took place after that time possibly caused by erosion of the surrounding sediment within the pockmark we hypothesize that the period of carbonate formation between 13 and 2 5 ka was related to an upward migration of gas hydrate reservoirs to the near seafloor environment after this pulse of enhanced fluid flow the diminution of methane fluxes at the base of the local gas hydrate occurrence zone would have led to hydrate dissolution in sub surface sediments and pockmark formation thereby explaining the progressive increase in sedimentation rates the absence of recent carbonate concretions and the deepening of the sulphate methane transition zone at site n2 ks 44 inferred from pore water data overall these results provide further constraints about the relationship between gas hydrate dynamics and the evolution of pockmarks at ocean margins through time

giant pockmarks in a carbonate platform maldives indian ocean abstract circular structures and depressions in carbonate platforms are known to represent karst chimneys or sinkholes which form as a response to rock solution this formation mechanism is plausible for shallow water carbonates which lie in the reach of meteoric diagenesis or fresh water lenses circular structures which occur in deeper waters however need an alternative interpretation such an example of sea floor depressions in more than 300 m deep waters occurs in the inner sea of the maldives carbonate platform in the indian ocean the structures were mapped with multibeam and parasound multi channel seismics were used to link the depressions with structures at depth the circular depressions have diameters of up to 3000 m and depths of up to 180 m the craters are interpreted as pockmarks formed through the venting of gas and fluids gas and fluid lenses below the pockmarks are reflected by bright spots in the seismic sections as well as a reduction of the instantaneous frequency these areas at depth are linked to chimneys connected to faults and drowned oligocene carbonate banks a model is presented that relates the different forms and sizes of the structures to distinct development stages of sea floor deformation to one process early stages of gas and fluid migration into the shallow part of the sedimentary succession induce formation of dome shaped bodies initial gas and fluid escape to the sea floor is reflected by the formation of sand volcanoes and aligned small pockmarks active pockmarks are the deepest and have the shape of truncated cones in cross section mature pockmarks are characterized by erosion of the flanks of the structure by bottom currents late stage pockmarks are bowl shaped in cross section and are to different degrees filled by drift sediments packages of strata revealing high reflection amplitudes and high interval velocities interpreted as microbially mediated carbonate precipitates underlie some of the pockmarks the pockmarks in the maldives show that circular structures other than solution related features can be abundant in carbonate platform deposits and that such structures may be more abundant in the geological record of carbonate platforms as previously thought pockmarks in the maldives indicate that the archipelago is an example of a hydrocarbon system which consists of an isolated oceanic carbonate platform overlying a volcanic basement and lacustrine source rocks

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

dispersion model for varying vertical shear in vegetated channels a dispersion model for a wide range of depthwise vertical shear is derived by using perturbation analysis and power m law velocity profile for m 1 the velocity profile provides linear shear whereas m 1 provides nonlinear shear and for m 20 the velocity profile resembles the flow through emergent vegetation the power law represented parametrically simulates well the complex shear profiles involved in emergent and submerged vegetated flows the proposed model shows reasonable agreement with past data on vegetated flows for a wide range of nonlinear shear velocities

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

investigation on the geochemical dynamics of a hydrate bearing pockmark in the niger delta a joint industrial project with ifremer and total provides insights into the tectonic setting and the geochemistry of a large active pockmark offshore w africa in the niger delta the study combines both geophysical seismic and geochemical data to infer the dynamics of a 300 m wide pockmark located at 667 m water depth two calypso cores one at the centre and one outside the pockmark along with three box cores two inside the pockmark and one outside were collected to study the geochemistry of gas hydrates carbonates and pore waters two seismic lines a 3d high resolution random line and a 2d high resolution seismic line provide a good description of the plumbing of the system the integrated results depict a very active pockmark characterized by several gas charged bodies a well identified bsr marking a hydrate accumulation zone charged from deeper sources via a network of faults hydrates and carbonate concretions have been detected inside the pockmark while only the latter has been found outside the hydrate analyses show that the hydrate forming gases are of thermogenic origin albeit overwhelmingly composed of methane pore water analyses reveal the occurrence of anaerobic oxidation of methane coupled with sulphate reduction at shallow depth 200 cm the chloride concentration profile exhibits both values lower than that of seawater which is inherent to hydrate dissociation after core recovery and positive anomalies values 7 higher than that of seawater background at the lowermost part of the core the latter evidence suggests current or recent hydrate formation at this area the carbonate dating provides additional temporal information and indicates that this pockmark has been active since 21 kyr

shallow gas accumulations in the german north sea abstract shallow gas here defined as free gas that is trapped in unconsolidated deltaic and shallow marine siliciclastic sediments of plio pleistocene age is found within the topmost 1000 m of sediment in the southern north sea shallow amplitude anomalies in seismic reflection data are likely due to the presence of gas the most prominent and easy to recognize indicators are high amplitude anomalies or bright spots that are widespread within the southern north sea gas from shallow reservoirs is currently produced offshore the netherlands in this study we determine whether there are analogous shallow gas accumulations within the german north sea therefore we screened 2d and 3d multichannel seismic data for shallow amplitude anomalies several clusters of bright spots are identified above salt domes that closely resemble the economic deposits known in the dutch sector in both size and their characteristic multilayered shape three of these potential gas accumulations occurring in combination with additional hydrocarbon indicators such as seismic attenuation and velocity push downs are investigated in detail and compared to shallow gas fields from offshore the netherlands amplitude anomalies indicate gas seepage on the seafloor that may contribute to the atmospheric methane budget and may have an impact on offshore infrastructure

focused fluid flow and the sub seabed storage of co2 evaluating the leakage potential of seismic chimney structures for the sleipner co2 storage operation abstract the integrity of the caprock of a storage formation is the most crucial parameter for the long term performance of a geological co 2 storage site the sleipner area in the southern viking graben hosts the first and longest operating industrial scale co 2 storage project where co 2 is injected in a saline aquifer of the utsira formation time lapse seismic monitoring shows neither that co 2 has left the utsira formation nor indications for fracturing of the caprock by the co 2 injection activity which is in agreement with previous numerical simulations however large chimney structures as close as 7 km from the injection point indicate that the caprock has been breached in the geological past which may raise questions about the integrity of the caprock above the sleipner co 2 storage site here we present seismically constrained numerical fluid flow simulations that evaluate the influence of chimney structures on the long term performance of the co 2 storage operation at sleipner the simulation could reproduce the spreading of the sleipner co 2 plume which is controlled by the anisotropic permeability field of the utsira formation and the regional dip of the formation top we have performed long term plume evolution simulations which show that the injected co 2 will not reach the existing chimney structures assuming a realistic injection duration of 30 years our simulations indicate that an unrealistically long injection period between 92 and 140 years would be required for the co 2 to reach the existing chimney structures in this case a comparably low chimney permeability of 10 md may be sufficient to facilitate co 2 migration from the storage formation to the seafloor once the co 2 has reached a chimney structure however the simulations indicate that it is very unlikely that the co 2 may migrate along existing chimney structures at sleipner our results highlight that the reconstruction of palaeo fluid flow systems and the identification of focused fluid conduits should be considered in the assessment of co 2 storage sites

new evidence of perfect overlapping of haploops and pockmarks field is it a coincidence abstract in the shallow waters of the bay of concarneau south brittany france previous studies reported a dense settlement of the species haploops tube dwelling bioengineer amphipod that perfectly overlaps a field of pockmarks a possible mechanistic link may therefore exist between the haploops colonies and the shallow pockmarks to test this hypothesis interferometry sonar and sub bottom profiler chirp data were acquired in two new areas located in the marine estuary of the loire river la lambarde and le croisic the new sites share the same sedimentary and geological characteristics as those found in the bay of concarneau the three sites are similarly located in shallow and muddy environments above a faulted middle eocene formation that is incised by paleo valleys despite different hydrodynamic conditions and estuarine influence in the two new investigated areas the haploops also settle within the pockmarks field limits to better constrain the association pockmarks haploops we compared the sedimentary features seismic signatures and total organic carbon toc content of the sediment active pockmark chimneys displaying highly reworked profiles and slightly higher toc contents suggest a local deeper source of organic matter which might be an important source of nutrient for the haploops in southern brittany the pockmarks haploops spatial overlap is not a simple coincidence and may therefore present a general pattern for identification of settlement preferences of the specie

velocity and sediment concentration profiles in river flows an equation for the exponent in the power law velocity distribution is formulated by relating the rate of energy dissipation due to turbulent shear of the sediment bed layer to the increased rate of fluid shear energy dissipation produced by the moving sediment the suspended sediment concentration distribution corresponding to the power law velocity profile is calculated and simplified the relation derived for the power law exponent is found to be in good agreement with data from a large group of flume and river flows the powerlaw velocity profile and the corresponding concentration distribution are shown to be in excellent conformity with the distributions measured by vanoni and brooks

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

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

the limited growth of vegetated shear layers 1 in contrast to free shear layers which grow continuously downstream shear layers generated by submerged vegetation grow only to a finite thickness because these shear layers are characterized by coherent vortex structures and rapid vertical mixing their thickness controls exchange between the vegetation and the overlying water experiments conducted in a laboratory flume show that the growth of these obstructed shear layers is arrested once the production of shear layer scale turbulent kinetic energy ske is balanced by dissipation of ske within the canopy this equilibrium condition along with a mixing length closure scheme was used in a one dimensional numerical model to predict the mean velocity profiles of the experimental shear layers the agreement between model and experiment is very good but field application of the model is limited by a lack of description of the drag coefficient in a submerged canopy

evidence for the biotic origin of seabed pockmarks on the australian continental shelf abstract the generally accepted formation mechanism of pockmarks worldwide is the expulsion of fluid at the seafloor but such a mechanism does not explain the close association between pockmarks and seabed infrastructure such as pipelines and wellheads within the stag oil field on the north west shelf of australia furthermore certain characteristics of the pockmarks such as conical mounds of sediment positioned around their perimeters are strongly suggestive of a biotic origin pockmarks in this case are typically 5 m in diameter and 1 m deep excavated within a sandy seabed in 45 m water depth inspection of rov footage acquired during oilfield operations within the stag field supports but does not entirely confirm without doubt the proposition that the pockmarks are created by fish of the genus epinephelus having determined the characteristic features of pockmarks within the stag field which mark them as biotic excavations data from commercial seabed surveys at 11 other sites on the north west shelf all of which reveal numerous pockmarks was reviewed for evidence of similar pockmark characteristics based on the review it appears likely that the majority of pockmarks on the shallow north west shelf between 40 m and 130 m water depth are representative of biological rather than geological activity the probability that pockmarks less than approximately 10 m in diameter throughout the remainder of the australian continental shelf are also the result of purely biological activity is high as demonstrated by the analysis of data from three further sites close inspection of seabed survey data from further afield could extend the findings of this paper throughout not only the tropical indo west pacific the range of the particular fish species implicated on the north west shelf but potentially worldwide if other species can be shown to display similar behaviour

source rock characteristics and rock eval based hydrocarbon generation kinetic models of the lacustrine chang 7 shale of triassic yanchang formation ordos basin china abstract highly variable depositional environment and multiple provenances caused rapid changes in facies laterally and vertically resulting in contrasting hydrocarbon generation potential and kinetics within the chang 7 shale of the upper triassic yanchang formation of ordos basin china assessing the shale oil resource potential requires several geochemical models that provide adequate information for characterizing the variations as traditional generation kinetics is derived from a single immature sample the kinetic model will not be representative for such variations this paper discusses the source rock heterogeneity and develops hydrocarbon generation kinetic models for the chang 7 shale using kinetics inversion techniques from routine rock eval data data collected from various sources for the chang 7 shale were analyzed and subdivided into groups based on their bulk geochemical properties and linked to petrophysical facies to describe the facies variation hydrocarbon generation kinetic parameters were then estimated for each of the source rock groups this study suggests that multiple kinetic models better handle spatial variation in evaluating source rock kinetics and resource potential in a lacustrine basin with highly variable lateral organofacies

geophysical and geochemical survey of a large marine pockmark on the malin shelf ireland 1 marine pockmarks are a specific type of seabed geological setting resembling craters or pits and are considered seabed surface expressions of fluid flow in the subsurface a large composite pockmark on the malin shelf off the northern coast of ireland was surveyed and ground truthed to assess its activity and investigate fluid related processes in the subsurface geophysical including acoustic and electromagnetic data confirmed the subsurface presence of signatures typical of fluids within the sediment shallow seismic profiling revealed a large shallow gas pocket and typical gas related indicators such as acoustic blanking and enhanced reflectors present underneath and around the large pockmark sulphate profiles indicate that gas from the shallow reservoir has been migrating upwards at least recently however there are no chimney structures observed in the sub bottom data and the migration pathways are not apparent electromagnetic data show slightly elevated electrical conductivity on the edges of the pockmarks and a drop below regional levels within the confines of the pockmark suggesting changes in physical properties of the sediment nuclear magnetic resonance nmr experiments were employed to characterize the organic component of sediments from selected depths very strong microbial signatures were evident in all nmr spectra but microbes outside the pockmark appear to be much more active than inside these observations coincide with spikes in conductivity and the lateral gas bearing body suggesting that there is an increase in microbial activity and biomass when gas is present

transverse bed slope in alluvial channel bends the study has reviewed existing analytical models for the prediction of the steady state transverse bed profile in channel bends formulated an alternative model and compared their predictive capability by laboratory and field data the alternative model which is based on a theory for the critical conditions for sediment entrainment gives good agreement between measured and predicted bed slopes the study shows that the key parameter in all available models is grain size the prediction of the complete bed topography of a given bend for a given flow data requires knowledge of the spatial variation of the grain size

methane cold seeps as biological oases in the high arctic deep sea cold seeps can support unique faunal communities via chemosynthetic interactions fueled by seabed emissions of hydrocarbons additionally cold seeps can enhance habitat complexity at the deep seafloor through the accretion of methane derived authigenic carbonates mdac we examined infaunal and megafaunal community structure at high arctic cold seeps through analyses of benthic samples and seafloor photographs from pockmarks exhibiting highly elevated methane concentrations in sediments and the water column at vestnesa ridge vr svalbard 79 n infaunal biomass and abundance were five times higher species richness was 2 5 times higher and diversity was 1 5 times higher at methane rich vestnesa compared to a nearby control region seabed photos reveal different faunal associations inside at the edge and outside vestnesa pockmarks brittle stars were the most common megafauna occurring on the soft bottom plains outside pockmarks microbial mats chemosymbiotic siboglinid worms and carbonate outcrops were prominent features inside the pockmarks and high trophic level predators aggregated around these features our faunal data visual observations and measurements of sediment characteristics indicate that methane is a key environmental driver of the biological system at vr we suggest that chemoautotrophic production enhances infaunal diversity abundance and biomass at the seep while mdac create a heterogeneous deep sea habitat leading to aggregation of heterotrophic conventional megafauna through this combination of rich infaunal and megafaunal associations the cold seeps of vr are benthic oases compared to the surrounding high arctic deep sea

the spatial pattern and drainage cell characteristics of a pockmark field nile deep sea fan over 25 300 seabed pockmarks were mapped from the rosetta channel region of the western niledeepseafan ndsf using concurrent high resolution 2d chirp profiler and multibeam bathymetry data which spans the holocene pleistocene period within the region apockmarkfield containing 13 800 pockmarks was analysed using spatial statistics to determine the distribution of pockmarks within the field pockmarks within the field are small 16 m diameter shallow 0 5 m deep circular depressions which formed within the last 6500 years the fluid source for the field is identified as an accumulation generation of gas beneath a hemipelagic seal c 20 40 ms beneath the seabed the position of the pockmarks is shown to be unrelated to the depth to the fluid source and an irregular high amplitude acoustic anomaly which is tentatively interpreted as a possible carbonate precipitate of biogenic microbial activity statistical spatial analysis of the field confirms the distribution of pockmarks is not random an exclusion zone surrounding each individual pockmark is identified the exclusion zone is a unique minimum radius around each pockmark which is not penetrated by any other pockmark the exclusion zone works in unison with self organised criticality soc in determining the spatial distribution of pockmarks within the field the exclusion zone is interpreted as apockmark drainagecell a conceptual model for apockmarkdrainagecell is proposed whereby pockmark formation dissipates a radius area of fluid and overpressure thereby preventing the formation of another pockmark within that cell consequently pockmarks are observed to separate or produce anti clustering tendencies within the field

evidence of a widespread paleo pockmarked field in the orange basin an indication of an early eocene massive fluid escape event offshore south africa abstract the detailed interpretation and seismic attribute analysis of 2d and 3d seismic reflection data sets revealed the presence of a distinct seismic horizon displaying circular and elongated depressions in early eocene sediments along the northwestern slope of the orange basin on the southwest african margin their occurrence within a single seismic horizon indicates a contemporaneous formation process we interpret the depressions as paleo pockmarks which occur as single pockmarks predominantly nne ssw aligned pockmark trains and coalescing elongated pockmarks the paleo pockmarked surface covers an area of 2800 km 2 and has been mapped by integrating the 3d dataset with industrial 2d seismic grids the spatial and temporal occurrence of the paleo pockmarks is associated to two episodes of rapid sedimentation during the late maastrichtian to paleocene comprising fine grained shelf eroded material we propose that they were formed during the early eocene by the seepage of both 1 overpressured pore fluids from disequilibrium compaction of upper maastrichtian sediments and 2 thermogenic gas from underlying cretaceous mature source rocks we show that the location of the pockmarks is controlled by the underlying faults system of the paleo slope the paleocene eocene transition is known for its series of hyperthermal events characterized by negative excursions of the carbon isotope curve the early eocene widespread event of fluid and thermogenic gas escape identified in this work can be an example of the type of carbon release processes occurring in hydrocarbon prone sedimentary basins during this time

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

hydrocarbon plumbing systems above the snohvit gas field structural control and implications for thermogenic methane leakage in the hammerfest basin sw barents sea abstract based on the analysis of the high resolution 3d seismic data from the sw barents sea we study the hydrocarbon plumbing system above the snohvit and albatross gas field to investigate the geo morphological manifestation and the dynamics of leakage from the reservoir fluid and gas escape to the seafloor is manifested in this area as mega pockmarks 1 2 km wide large pockmarks

buried pockmarks on the top chalk surface of the danish north sea and their potential significance for interpreting palaeocirculation patterns three dimensional 3d seismic data from the southern danish north sea were used to analyse the morphology and spatial distribution of depressions in the danian chalk deposit previously these depressions were either interpreted as karst structures or pockmarks the observed depressions occur in an interval from 25 ms below to 12 ms above the top chalk surface three types of depressions were differentiated based on their plan view geometry and their degree of symmetry type 1 comprising sub circular and symmetrical depressions is the dominant group ca 70 type 2 elongated and symmetrical depressions represents only a small fraction ca 5 the elongated and asymmetrical depressions of type 3 compose ca 25 in cross section each depression type can be either characterised by a v or a u shape the maximum size of the depressions ranges from 50 to 580 m with an average internal depth of 10 m we interpret the depressions as pockmarks formed by the expulsion of biogenic or thermogenic fluids at the danian seafloor likely the initial form of the pockmarks has been circular type 1 and was subsequently modified for types 2 and 3 to an elongated form by currents the long axis of the pockmarks is interpreted to represent the effective current direction the inferred direction is sub parallel to the palaeobathymetric contours the 3d seismic interpretation of pockmarks presented in this paper contributes to the understanding of fluid migration and palaeocirculation patterns during the sedimentation of the terminal chalk group in the southern danish north sea

initiation of gas hydrate pockmark in deep water nigeria geo mechanical analysis and modelling abstract a review of recent literature shows that two geomorphologically different types of pockmarks contribute to gas seepage at the seafloor type 1 pockmarks are defined as seafloor craters associated to fluid seepage and are the most classical type referred to as pockmarks in the literature in contrast type 2 pockmarks reveal a complex seafloor morphology that may result from the formation decomposition of gas hydrates in underlying sedimentary layers interpretation of very high resolution seismic data sedimentological analyses and geotechnical measurements acquired from the eastern niger submarine delta reveal that type 2 pockmarks are associated to the presence at depth of a conical body of massive gas hydrates based on acquired data theoretical analysis and numerical modelling it was possible to propose a novel geo mechanical mechanism controlling the irregular seafloor deformations associated to type 2 pockmark and to show that pockmark shapes and sizes are directly linked to the initial growth and distribution of sub seafloor gas hydrates the study illustrates the role of gas hydrates formation in the fracturation deformation of the subsurface sediment and the formation of type 2 pockmarks

topographic constraints on the evolution and connectivity of titan s lacustrine basins the topography provided by altimetry sar topo and stereo radargrammetry has opened new doors for titan research by allowing for quantitative analysis of morphologic form using altimetry measurements we show that titan s maria are consistent with an equipotential surface but that several filled lakes are found to be hundreds of meters above this sea level suggesting that they exist in isolated or perched basins within a given drainage basin empty lake floors are typically higher than the liquid elevation of nearby lakes seas suggesting local subsurface connectivity the majority of titan s lakes reside in topographically closed sharp edged depressions whose planform curvature suggests lateral expansion through uniform scarp retreat many but not all empty lake basins exhibit flat floors and hectometer scale raised rims that present a challenge to formation models we conclude that dissolution erosion can best match the observed constraints but that challenges remain in the interpretation of formation processes and materials

identification and characterization of fluid escape structures pockmarks in the estremadura spur west iberian margin abstract located on the west iberian margin between cabo carvoeiro and cabo da roca the estremadura spur is a trapezoidal promontory elongated in an east west direction extending until the tore seamount recently a field with more than 70 pockmarks was discovered in the nw region of the estremadura spur outer shelf lourinha monocline pockmarks are the seabed culminations of fluid migration through the sedimentary column and their characteristic seabed morphologies correspond to cone shaped circular or elliptical depressions the characterization of these features and the understanding of the associated fluid escape process are the main objectives of this work here we characterize these structures to understand their structural and stratigraphic control based on 1 seismic processing and interpretation of the high resolution 2d single channel sparker seismic dataset 2 bathymetric and backscatter interpretation and 3 rov direct observation of the seafloor the analysis of the seismic profiles allowed the identification of six seismic units disturbed by the migration and accumulation of fluids the estremadura spur outer shelf has been affected by several episodes of fluid migration and fluid escape during the pliocene quaternary that are expressed by a vast number of seabed and buried pockmarks at present the pockmarks are mainly inactive as the seabed pockmarks are covered by recent sediments it is concluded that the migration of fluids to the seabed occurred over the pliocene quaternary as indicated by the buried pockmarks at different depths below the seabed the vertical stacking of various pockmarks suggests a cyclical fluid flow activity that can possibly be the result of the eustatic sea level variations and the subsequent changes of the hydrostatic pressure

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

the holocene gas system of the ria de vigo nw spain factors controlling the location of gas accumulations seeps and pockmarks abstract high resolution seismic profiles cored sediment gravity cores and vibro cores and radiocarbon data from suitable shell material were used to establish the stratigraphic context of gassy sediments in the ria de vigo mapping of seeps and pockmarks were compared with the stratigraphy of gassy sediment to ascertain the relationships in the ria de vigo three holocene seismic units were identified the first two uh 1 and uh 2 correspond to the transgressive systems tract tst and the third uh 3 corresponds to the highstand systems tract hst the presence of acoustic turbidity within the three seismic units indicates that generation of gas took place during the relative sea level rise that occurred after the younger dryas event i e during the tst and mainly during the hst the relative sea level variations are shown to be a major factor controlling the type fine vs coarse grained and distribution of facies lateral and vertical that generate and or allow gas accumulation or escape acoustic turbidity due to presence of free gas was recognized in the three holocene units with a total of 12 acoustic turbidity fields mapped in the ria de vigo no significant variations in the spatial distribution or extent of the gas fields or in their vertical location below the present sea bed have been observed during the seismic surveys carried out in the ria during the last decade furthermore gas escape from the sediment and active pockmark has been observed therefore we conclude that gas escape seeps pockmark generation and maintenance and methane formation are in equilibrium the combined analysis of high resolution seismic profiles sediment temperature pore water salinity and hydrostatic pressure revealed 1 a direct relationship between depth of free gas and hydrostatic pressure and 2 a direct link between sediment temperature and number of acoustic plumes from seabed to water column more acoustic plumes were detected in summer when the temperature of the sediment was higher than in winter indicating a seasonal variation of seeps within the ria salinity showed no significant differences between sea bottom water and sediment pore water and was therefore not a major factor controlling gas front variations

modern analogues to middle ordovician sedimentary mounds and washout depressions abstract mounds and depressions containing dense fossil accumulations occur in a bed of middle ordovician age in the upper ottawa valley canada the appearance of these relict seabed features is similar to modern features containing dense accumulations of live species in the north sea it is suggested that the middle ordovician fossil accumulations relied on a similar local upward flux of minerals and nourishment as north sea communities which live in areas where sediments contain light hydrocarbon gases if further petrological and sedimentological studies on the middle ordovician deposits could confirm such a relationship then it would be the first time relict or fossil pockmarks were found in the sedimentary column

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

acoustic evidence of shallow gas accumulations and active pockmarks in the izmir gulf aegean sea abstract based on high resolution chirp seismic multibeam bathymetry and side scan sonar data collected in the izmir gulf aegean sea in 2008 and 2010 gas related structures have been identified which can be classified into three categories 1 shallow gas accumulations and gas chimneys 2 mud diapirs and 3 active and inactive pockmarks on the chirp profiles shallow gas accumulations were observed along the northern coastline of the outer izmir gulf at 3 20 m below the seabed they appear as acoustic turbidity zones and are interpreted as biogenic gas accumulations produced in organic rich highstand fan sediments from the gediz river the diapiric structures are interpreted as shale or mud diapirs formed under lateral compression due to regional counter clockwise rotation of anatolian microplate furthermore the sedimentary structure at the flanks suggests a continuous upward movement of the diapirs several pockmarks exist close to fault traces to the east of hekim island most of them were associated with acoustic plumes indicating active degassing during the survey period in 2008 another chirp survey was carried out just over these plumes in 2010 to demonstrate if the gas seeps were still active the surveys indicate that the gas seep is an ongoing process in the gulf based on the chirp data we proposed that the pockmark formation in the area can be explained by protracted seep model whereby sediment erosion and re distribution along pockmark walls result from ongoing or long lasting seepage of fluids over long periods of time the existence of inactive pockmarks in the vicinity however implies that gas seepage may eventually cease or that it is periodic most of the active pockmarks are located over the fault planes likely indicating that the gas seepage is controlled by active faulting

reservoir leakage along concentric faults in the southern north sea implications for the deployment of ccs and eor techniques high quality 3d seismic and borehole data in the broad fourteens basin southern north sea is used to investigate newly recognised concentric faults formed in salt withdrawal basins flanking reactivated salt structures throw depth and throw distance plots were used to understand the growth histories of individual faults as a result three families of concentric faults are identified a intra seal faults within a salt withdrawal basin b faults connecting the seal and the reservoir on the crest of an inverted anticline c raft bounding faults propagating into reservoir units they have moved obliquely and show normal throws even though they formed during a period of regional compression faults in the salt withdrawal basin and on the inverted anticline are highly segmented increasing the chances of compartmentalisation or localised fluid flow through fault linkages slip tendency analysis was carried out on the distinct fault families to compare the likelihood of slip along a fault at different pore fluid pressures and within different lithologies our results show that sections of the faults are optimally oriented with regards to maximum horizontal stresses hmax increasing the slip tendency the identified faults cut through a variety of lithologies allowing different values of pore fluid pressures to build up before faults reactivate within the vlieland sandstones pore fluid pressures of 30 mpa are not sufficient to reactivate pre existing faults whereas in the deeper posidonia shales faults might reactivate at pore fluid pressures of 25 mpa fluid flow features preferentially occur near fault segments close to failure heterogeneity in slip tendency along concentric faults and high degrees of fault segmentation present serious hazards when injecting co2 into the subsurface this study stresses the importance of high quality 3d seismic data and the need to evaluate individual fault systems when investigating potential reservoirs for carbon capture and storage and enhanced oil recovery

longitudinal dispersion coefficient in single channel streams using a new channel shape equation for straight channels and a more versatile channel shape or local flow depth equation for natural streams a method is developed for prediction of the longitudinal dispersion coefficient in single channel natural streams including straight and meandering ones the method involves derivation of a new triple integral expression for the longitudinal dispersion coefficient and development of an analytical method for prediction of this coefficient in natural streams the proposed method is verified using 70 sets of field data collected from 30 streams in the united states ranging from straight manmade canals to sinuous natural rivers the new method predicts the longitudinal dispersion coefficient where more than 90 calculated values range from 0 5 to 2 times the observed values the advantage of the new method is that it is capable of accurately predicting the longitudinal dispersion coefficient in single channel natural streams without using detailed dye concentration test data a comparison between the new method and the existing methods shows that the new method significantly improves the prediction of the longitudinal dispersion coefficient

the physics of bubbles in surficial soft cohesive sediments abstract this paper reviews current understanding of the physics of bubbles in soft unlithified cohesive sediments certainly in the top 100 m from the sediment water interface but likely much deeper experimental evidence primarily ct scans and internal pressure records indicate that near surface bubbles grow by elastically compressing and fracturing these sediments which results in thin irregular disks of gas these data do not suggest either plastic or fluid behavior on the part of the bulk sediment in addition there is no hint of capillary invasion of gas into the pores of fine grained sediments the growth rate of an elastic fracturing disk shaped bubble can vastly exceed that of a spherical bubble depending on the eccentricity of the bubble this effect results from the far more favorable surface to volume ratio of flattened bubbles the initial rate of rise of bubbles in cohesive sediments also appears to be governed by a fracture process which is driven by pseudo buoyancy the rate of bubble rise could be controlled by various possible mechanisms but only the predictions from a visco elastic fracture propagation model seem to produce rise velocities that are not ballistic experimental evidence demonstrates that a rising bubble can deviate from its path to the surface if it encounters a zone of sediment with lower lateral fracture toughness such a bubble will begin to move in a lateral direction but stall because pseudo buoyancy is no longer operative subsequent bubbles will do the same and accumulate in the low fracture toughness zone this explains the occurrence of shallow sub seafloor domes of gas and modeling of this process predicts only modest over pressures in such structures

distribution morphology and formation of pockmarks in lake constance germany pockmarks are well known in the marine environment but almost unreported from limnic systems in lake constance we investigated pockmarks near the mouth of the main tributary alpine rhine river using sediment cores and hydroacoustics sidescan sonar subbottom profiler horizontal scanning sonar multibeam echo sounder the maximum dimension of an individual pockmark is about 16 m in diameter and 3 4 m deep often with a round shape most pockmarks are located on top of morphological high points such as sediment waves or shoulders of old channels in front of the alpine rhine river they are collapse structures formed when a reservoir of biogenic methane rapidly empties into the water column ebullition of gas very often was observed indicating that biogenic methane still migrates toward pockmarks

subsurface methane sources and migration pathways within a gas hydrate mound system gulf of mexico 1 geochemical profiles were coupled with seismic information to examine subsurface hydrocarbon source migration and fate at a gulf of mexico carbonate gas hydrate mound woolsey mound three seafloor features were investigated in detail 1 major faults resulting from a rising salt body 2 an acoustic backscatter anomaly and 3 a pockmark associated with a major fault we analyzed sulfate chloride dissolved inorganic carbon and hydrocarbon concentrations and carbon isotopes in pore water extracted from 20 m piston cores to characterize gas source and calculate methane flux dissolved biogenic methane dominated the off fault sites while the contribution of thermogenic methane increased near a major fault where thermogenic gas hydrates were recovered within the pockmark methane concentrations were low and isotopes indicated a biogenic source since pockmarks are typically formed from expulsive fluid flow this suggests that either the pockmark is the legacy of a conduit that has become plugged or that the expulsed fluid is confined within the fault walls at the acoustic anomaly nonsteady state sulfate profiles suggested temporal variability in methane flux estimates from 75 gravity cores collected across woolsey mound since 2002 were mapped to display the spatial variability in methane flux relative to the faults methane flux to the seafloor was generally low but increased several fold near the faults suggesting that the faults may provide conduits for hydrocarbons to bypass the microbial biofilter and cross the sediment water interface

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

new sedimentary structures in seismites from sw tanzania evaluating gas vs water escape mechanisms of soft sediment deformation seismite horizons are abundant in cretaceous sandstones of the rukwa rift basin southwestern tanzania diverse morphologies of soft sediment deformation are preserved including two new unusual sedimentary structures herein referred to as 1 balloon shaped inflation structures and 2 surface fractures with linked sandstone splays this original description of new sedimentary structures contributes to the growing catalogue of seismically induced deformation features their unusual morphologies bring to the forefront an important though seldom touched upon question of how to differentiate between gas and water escape in soft sediment deformation features the recognition of the spectrum of soft sediment deformation structures contained in strata and their deformational mechanisms is important because it permits the differentiation between triggering mechanisms particularly seismic activity and can constrain such events spatially and temporally we interpret the surface fractures and linked sandstone splays to reflect a high gas effusion rate formed by the release of high pressure gas followed by a limited expulsion of water the balloon shaped inflation structures reflect lower gas effusion rates due to expulsion of lower pressure gas that did not breach the cretaceous surface seal when these gas pulses did breach the paleosurface they formed gas discharge pits these seismogenic structures are consistent with deposition of cretaceous strata in the rukwa rift during periods of active carbonatite volcanism seismicity and possibly hot spring activity this documentation serves as an important data point for the re examination of the classification scheme of soft sediment deformation structures as primarily water escape structures to accommodate for the genesis of some secondary sedimentary features by gas escape

focused hydrocarbon migration in shallow sediments of a pockmark cluster in the niger delta off nigeria the niger delta is one of the largest hydrocarbon basin offshore africa and it is well known for the presence of active pockmarks on the seabed during the guineco mebo cruise in 2011 long cores were taken from a pockmark cluster in order to investigate the state of its current activity gas hydrates oil and pore water were sampled for geochemical studies the resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks there is a clear tendency for gas migration within the hydrate bearing pockmarks and oil migration within the carbonate rich one this trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks we also demonstrate that anaerobic oxidation of methane aom is the main process responsible for the depletion of pore water sulfate with depths of the sulfate methane transition zone smtz ranging between 1 8 and 33 4 m in addition a numerical transport reaction model was used to estimate the age of hydrate layer formation from the present day sulfate profiles the results show that the sampled hydrate layers were formed between 21 and 3750 years before present overall this work shows the importance of fluid flow on the dynamics of pockmarks and the investigated cluster offers new opportunities for future cross site comparison studies our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades this article is protected by copyright all rights reserved

the morphologies and genesis of mega pockmarks near the xisha uplift south china sea pockmarks are normally regarded to be manifestations of fluids escape through the seabed kilometer wide depressions here called mega pockmarks occur as widespread features near the xisha uplift northern south china sea most of the pockmarks observed in this area are multi kilometers and much larger than normal pockmarks reported worldwide the maximum diameter observed is 3210 m and the maximum depth observed is 165 2 m based on multibeam data and 2d seismic data the pockmarks are circular elliptical and crescentic in plan view seismic profiles show that the genesis of pockmarks is related to fluid gas and or pore water escape according to the fluids pathways the pockmarks fall into four types 1 gas chimney related 2 depositional boundary related 3 gas chimney and inclined structure fault related 4 inclined structure related bottom currents are strong and complex in the study area the multibeam data and seismic profiles indicate that they may play an important role on extension maintenance and shaping of pockmarks the research of the study area is in its initial stage and the identification of these features as indicators of fluid flow is probably useful for hydrocarbon exploration c 2011 elsevier ltd all rights reserved

characterization of microbial activity in pockmark fields of the sw barents sea abstract multibeam bathymetry revealed the occurrence of numerous craterlike depressions so called pockmarks on the sea floor of the hammerfest basin and the loppa high south western barents sea to investigate whether these pockmarks are related to ongoing gas seepage microbial processes associated with methane metabolism were analyzed using geochemical biogeochemical and microbiological techniques gravity cores were collected along transects crossing individual pockmarks allowing a direct comparison between different locations inside assumed activity center on the rim and outside of a pockmark reference sites concentrations of hydrocarbons in the sediment particularly methane were measured as headspace free gas and in the occluded and adsorbed gas fraction down to a depth of 2 6 m below sea floor mbsf sulfate reduction rates were quantified by radiotracer incubations concentrations of dissolved sulfate in the porewater were determined as well neither the sulfate profiles nor the gas measurements show any evidence of microbial activity or active fluid venting methane concentrations and sulfate reduction rates were extremely low or even below the detection limit the results show that the observed sediment structures are most likely paleo pockmarks their formation probably occurred during the last deglaciation

submarine gas seepage in a mixed contractional and shear deformation regime cases from the hikurangi oblique subduction margin gas seepage from marine sediments has implications for understanding feedbacks between the global carbon reservoir seabed ecology and climate change although the relationship between hydrates gas chimneys and seafloor seepage is well established the nature of fluid sources and plumbing mechanisms controlling fluid escape into the hydrate zone and up to the seafloor remain one of the least understood components of fluid migration systems in this study we present the analysis of new three dimensional high resolution seismic data acquired to investigate fluid migration systems sustaining active seafloor seepage at omakere ridge on the hikurangi subduction margin new zealand the analysis reveals at high resolution complex overprinting fault structures i e protothrusts normal faults from flexural extension and shallow 1 km arrays of oblique shear structures implicated in fluid migration within the gas hydrate stability zone in an area of 2x7 km in addition to fluid migration systems sustaining seafloor seepage on both sides of a central thrust fault the data show seismic evidence for sub seafloor gas rich fluid accumulation associated with proto thrusts and extensional faults in these latter systems fluid pressure dissipation through time has been favored hindering the development of gas chimneys we discuss the elements of the distinct fluid migration systems and the influence that a complex partitioning of stress may have on the evolution of fluid flow systems in active subduction margins

anomalous depressions in the northern yellow sea basin evidences for their evolution processes abstract depressions of both known and unknown origin are observed in continental margin basins and ocean floors worldwide a field of anomalous depressions with dome structures within the centers are documented for the first time using sub bottom seismic profiles multibeam bathymetry data and well data in the northern yellow sea basin they have complex seabed morphologies circular elliptical polygonal linear and crescent shapes with kilometer scale diameters and shallow depths 10 s decimeters to meters focused fluid flow pathways such as gas chimneys faults and pipes and free gas enhanced amplitude anomalies are observed within the shallow strata the depressions are interpreted as immature pockmarks possibly caused by insufficient fluid supply compared to fully developed normal pockmarks they are suspected to still be in the middle stage of pockmark evolution their formation processes are suspected to be controlled by sub seabed plumbing fluid flux and bottom currents this study is important for geologic hazard assessments in the study area and it also contributes to the full understanding of pockmark evolution

formation of subsurface shallow gas accumulations in amurskiy bay peter the great bay sea of japan as a result of postglacial sea level change paleoceanographic conditions and hydrological activity abstract we present new data on the study of subsurface gas accumulations in the sediments of amurskiy bay the sea of japan geophysical research single channel continuous seismic profiling with airguns and high resolution acoustic investigations with a high frequency geopulse profiler shows that the bay depression is filled by pre quaternary sediments that are at least 0 8 s twt thick and overlaying late pleistocene holocene sediments that are up to 30 m thick the pre quaternary deposits are possibly represented by coal containing mesozoic and cenozoic rocks gas related acoustic anomalies were found in the late pleistocene holocene silty sediments in the northern part of the bay these gas related anomalies have been divided into blanket column turbidity and pillar anomalies a single pockmark was also observed in the bottom relief of the bay the gas accumulation area in the sediments is 19 km long and 12 km wide the degassing of the pre quaternary coal containing deposits is assumed to be the major cause of gas the formation of most of the gas anomalies in the late pleistocene holocene sediments and the trigger mechanism of gas migration may be associated with sea level changes paleoceanographic conditions sedimentation features and the hydrodynamic regime in the bay increases in the number of diatoms and the presence of aberrant forms of some silicoflagellates have been observed in areas with high gas contents areas with numerous small underwater elevations or mounds were distinguished in the bay in the northern part of the bay the mounds correspond to ostraceous reefs or other local shellfish colonies in the southern part of the bay the elevations could be related to methane derived authigenic carbonates which formed pavements for the fixation of shellfish colonies and construction of reefs

morphology and shallow structure of seafloor mounds in the canary basin eastern central atlantic ocean abstract the increasing volume of high resolution multibeam bathymetry data collected along continental margins and adjacent deep seafloor regions is providing further opportunities to study new morphological seafloor features in deep water environments in this paper seafloor mounds have been imaged in detail with multibeam echosounders and parametric sub bottom profilers in the deep central area of the canary basin 350 550 km west off el hierro island between 4800 and 5200 mbsl these features have circular to elongated shapes with heights of 10 to 250 m diameters of 2 24 km and with flank slopes of 2 50 based on their morphological features and the subsurface structures these mounds have been classified into five different types of mounds that follow a linear correlation between height and slope but not between height and size the first second subgroup a and third mound types show heights lower than 80 m and maximum slopes of 35 with extension ranging from 2 to 400 km2 and correspond to domes formed at the surface created by intrusions located at depth that have not outcropped yet the second subgroup b fourth and fifth mound types show higher heights up to 250 m high maximum slopes of 47 and sizes between 10 and 20 km2 and are related to the expulsion of hot and hydrothermal fluids and or volcanics from extrusive deep seated systems based on the constraints on their morphological and structural analyses we suggest that morphostructural types of mounds are intimately linked to a specific origin that leaves its footprint in the morphology of the mounds we propose a growth model for the five morphostructural types of mounds where different intrusive and extrusive phenomena represent the dominant mechanisms for mound growth evolution these structures are also affected by tectonics bulge like structures clearly deformed by faulting and mass movements slide scars and mass transport deposits in this work we report how intrusive and extrusive processes may affect the seafloor morphology identifying a new type of geomorphological feature as intrusive domes that have to date only been reported in fossil environments but might extend to other oceanic areas