asphalt volcanoes as a potential source of methane to late pleistocene coastal waters natural petroleum seepage emits large volumes of oil and methane to the oceans every year accompanied by the formation of asphalt volcanoes on the sea floor the discovery of seven asphalt volcanoes off the coast of southern california may help to explain high methane emissions recorded during the late pleistocene

methane fluxes and carbonate deposits at a cold seep area of the central nile deep sea fan eastern mediterranean sea abstract high acoustic seafloor backscatter signals characterize hundreds of patches of methane derived authigenic carbonates and chemosynthetic communities associated with hydrocarbon seepage on the nile deep sea fan ndsf in the eastern mediterranean sea during a high resolution ship based multibeam survey covering a 225 km2 large seafloor area in the central province of the ndsf we identified 163 high backscatter patches at water depths between 1500 and 1800 m and investigated the source composition turnover flux and fate of emitted hydrocarbons systematic parasound single beam echosounder surveys of the water column showed hydroacoustic anomalies flares indicative of gas bubble streams above 8 of the high backscatter patches in echosounder records flares disappeared in the water column close to the upper limit of the gas hydrate stability zone located at about 1350 m water depth due to decomposition of gas hydrate skins and subsequent gas dissolution visual inspection of three high backscatter patches demonstrated that sediment cementation has led to the formation of continuous flat pavements of authigenic carbonates typically 100 to 300 m in diameter volume estimates considering results from high resolution autonomous underwater vehicle auv based multibeam mapping were used to calculate the amount of carbonate bound carbon stored in these slabs additionally the flux of methane bubbles emitted at one high backscatter patch was estimated 0 23 to 2 3 106 mol a 1 by combined auv flare mapping with visual observations by remotely operated vehicle rov another high backscatter patch characterized by single carbonate pieces which were widely distributed and interspaced with sediments inhabited by thiotrophic chemosynthetic organisms was investigated using in situ measurements with a benthic chamber and ex situ sediment core incubation and allowed for estimates of the methane consumption 0 1 to 1 106 mol a 1 and dissolved methane flux 2 to 48 106 mol a 1 our comparison of dissolved and gaseous methane fluxes as well as methane derived carbonate reservoirs demonstrates the need for quantitative assessment of these different methane escape routes and their interaction with the geo bio and hydrosphere at cold seeps

hydrocarbon plumbing systems of salt minibasins offshore angola revealed by three dimensional seismic analysis analysis of three dimensional seismic data from the lower congo basin offshore angola reveals numerous fluid flow features in the miocene to holocene succession and the potential for large shielded traps underneath basinward overhanging salt structures the fluid flow evidence includes present day sea floor pockmarks clustered above salt structures pliocene pleistocene stacked paleopockmarks and miocene pockmark fields other fluid flow features include high amplitude cylindrical pipe structures 60 to 300 m 197 984 ft wide and 25 to 300 m 82 984 ft high within lower and middle miocene strata thick 150 m 492 ft high reflectivity zones within the pliocene succession associated with bottom simulating reflections and subvertical low amplitude chimneys originating from the deeper section 1 km 0 6 mi beneath the sea floor the miocene pockmark fields occur at a specific horizon suggesting a regional fluid expulsion event at ca 12 ma and the miocene fluid flow regime is interpreted to be dominated by thermogenic fluids supplied via carrier beds and leaking vertically above structural highs the pliocene pleistocene fluid flow regime was dominated by short distance vertical fluid migration and expulsion related to early stage diagenetic processes involving biogenic methane and pore water the present day fluid flow regime is inferred to be dominated by thermogenic fluids primarily controlled by kilometer scale salt flank controlled migration the study emphasizes the use of seismically imaged fluid flow features in hydrocarbon systems analysis by documenting the evolution of an overburden plumbing system through time involving several fluid types and flow regimes depending on the spatiotemporal availability of thermogenic and diagenetic fluids and the tectonostratigraphic occurrence of aquifers traps and seals

early oligocene stratigraphic turnover on the west africa continental margin a signature of the tertiary greenhouse to icehouse transition the post rift stratigraphy on the west african margin is characterized by aggradation of a carbonate ramp during late cretaceous to eocene epochs and progradation of a terrigenous wedge from oligocene to the present such first order structure has been attributed in the past to geodynamic forcing however comparison of the stratigraphic record of the margin with eustasy 18o and 87sr 86sr curves shows a close temporal relationship with the tertiary climate cooling an increase of continental weathering and a long term lowering of sea level we suggest that the transition from low amplitude high frequency sea level changes during the greenhouse period to high amplitude high frequency sea level changes during the icehouse period may account for i the switch from an aggrading carbonate ramp to a prograding clastic wedge and ii the enhanced continental weathering and increased terrigenous influx to the margin

cold seeps at the salt front in the lower congo basin ii the impact of spatial and temporal evolution of salt tectonics on hydrocarbon seepage abstract this study investigates the distribution and evolution of seafloor seepage in the vicinity of the salt front i e the seaward boundary of salt induced deformation in the lower congo basin lcb seafloor topography backscatter data and tv sled observations indicate active fluid seepage from the seafloor directly at the salt front whereas suspected seepage sites appear to be inactive at a distance of 10 km landward of the deformation front high resolution multichannel seismic data give detailed information on the structural development of the area and its influence on the activity of individual seeps during the geologic evolution of the salt front region the unimpeded migration of gas from fan deposits along sedimentary strata towards the base of the gas hydrate stability zone within topographic ridges associated with relatively young salt tectonic deformation facilitates seafloor seepage at the salt front bright and flat spots within sedimentary successions suggest geological trapping of gas on the flanks of mature salt structures in the eastern part of the study area onlap structures associated with fan deposits which were formed after the onset of salt tectonic deformation represent potential traps for gas which may hinder gas migration towards seafloor seeps faults related to the thrusting of salt bodies seawards also disrupt along strata gas migration pathways additionally the development of an effective gas hydrate seal after the cessation of active salt induced uplift and the near surface location of salt bodies may hamper or prohibit seafloor seepage in areas of advanced salt tectonic deformation this process of seaward shifting active seafloor seepage may propagate as seaward migrating deformation affects congo fan deposits on the abyssal plain these observations of the influence of the geologic evolution of the salt front area on seafloor seepage allows for a characterization of the large variety of hydrocarbon seepage activity throughout this compressional tectonic setting

transience and persistence of natural hydrocarbon seepage in mississippi canyon gulf of mexico analysis of the magnitude of oil discharged from natural hydrocarbon seeps can improve understanding of the carbon cycle and the gulf of mexico gom ecosystem with use of a large archive of remote sensing data in combination with geophysical and multibeam data we identified mapped and characterized natural hydrocarbon seeps in the macondo prospect region near the wreck site of the drill rig deepwater horizon dwh satellite image processing and the cluster analysis revealed locations of previously undetected seep zones including duplicate detections a total of 562 individual gas plumes were also observed in multibeam surveys in total sar imagery confirmed 52 oil producing seep zones in the study area in almost all cases gas plumes were associated with oil producing seep zones the cluster of seeps in the vicinity of lease block mc302 appeared to host the most persistent and prolific oil vents oil slicks and gas plumes observed over the dwh site were consistent with discharges of residual oil from the wreckage in contrast with highly persistent oil seeps observed in the green canyon and garden banks lease areas the seeps in the vicinity of macondo prospect were intermittent the difference in the number of seeps and the quantity of surface oil detected in green canyon was almost two orders of magnitude greater than in mississippi canyon

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

methane related carbonate cements in pockmarks of the north sea abstract some pockmarks on the norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms detailed examination of a sandstone sample from a pockmark in norwegian block 25 7 in the north sea has revealed the presence of mg calcite and aragonite cements some of the latter forming botryoids all cement types are characterized by extremely light carbon isotopic compositions with a mean 13c value of 56 1 pdb which shows that the cements contain carbonate produced by oxidation of biogenic methane oxidation occurred in both the oxic and anoxic diagenetic zones trace amounts of interstitial methane mean 13c 40 8 and higher hydrocarbon gases up to c5 with a c1 cn ratio of 0 855 0 874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas

observation and tentative interpretation of a double bsr on the nankai slope seismic data collected during the french japanese kaiko tokai cruise of r v l atalante on the upper slope of the eastern nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector bsr type reflectors the upper bsr is traced as a continuous reflector over about 10 km as water depth decreases from 850 m to 550 m its depth below seafloor decreases from 200 m to 40 m the lower bsr is traced at 50 100 m below the upper one the two bsrs end abruptly near the summit of the daichii tenryu knoll into an area where the 3 5 khz record suggests active gas expulsion through the seabed the observed depth of the upper bsr fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments thus we interpret the upper bsr as an active methane hydrate bsr we further suggest that the lower bsr is a residual hydrate related bsr this could have followed a recent migration of the base of the methane hydrate stability zone from the lower bsr to the upper one as possible causes for this migration we discuss sea bottom warming and tectonic uplift the bsr migration could have occurred as a response to a 1 2 c sea bottom warming or with an equivalent effect an event of fast uplift of the seafloor by about 90 m we do not discard other interpretations of the lower bsr such as an active hydrate related bsr formed from a mixture of gases

identification and numerical modelling of hydrocarbon leakage in the lower congo basin implications on the genesis of km wide seafloor mounded structures abstract we present a combined approach of interpretation of 2d seismic reflection data and numerical modelling of hydrocarbon generation and migration across the southern slope of the lower congo basin in order to investigate the factors controlling timing and distribution of hydrocarbon leakage in this area we identified three main families of past and present day leakage features 1 mid upper miocene seismic chimneys concentrated basinwards and ending up on buried pockmarks 2 plio pleistocene chimneys rather clustered to the east of the study area and ending up in seafloor pockmarks and 3 fewer scattered chimneys identified within the miocene sequences ending up in shallow enhanced reflectors flat spots stratigraphic and structural elements seem to control the distribution of these features at least two major events of leakage occurred during the middle late miocene and intermittently during the pliocene present external factors as sediment supply are associated to the miocene leakage event whilst internal structural elements probably triggered the pliocene to present day leakage a major seabed morphological feature represented by a margin paralleled belt of more than 1 km wide mounds was identified above growth faults to the east of the study area data constrained 2d hc generation and migration modelling suggests a genetic link between these structures and vertical migration leakage of thermogenic methane sourced from either currently mature oligo miocene source rocks or secondary cracking and further expulsion from over mature upper cretaceous source rocks hence the mounds are likely to represent a lineation of methane derived carbonate build ups despite the natural limitations of a 2d migration model when combined and calibrated with observations from seismic data it can be used as a valid tool to assess petroleum migration routes in sedimentary basins to the best of our knowledge this is the first integrated approach combining both seismic observations and numerical modelling carried out in the angola basin

remote sensing evaluation of geophysical anomaly sites in the outer continental slope northern gulf of mexico abstract synthetic aperture radar sar images obtained from satellites are a reliable tool for localizing natural hydrocarbon seeps for this study we used the texture classifier neural network algorithm tcnna to interpret sar data from the radarsat satellite and a geostatistical clustering analysis to compare seeps detected in 579 sar images covering the northern gulf of mexico gom geostatistical analysis results indicate that in a typical active seep formation oil vents would be found within a seep formation 2 5 km in diameter repeated observations of slicks at a given seep formation indicate that advection of rising oil in the water column causes an offset from the vent depending on water depth at 500 m the radial offset is up to 1400 m at 2000 m it is up to 3270 m observations with submersibles showed that in 100 of the cases the calculated seep formations that are matched with active oil seeps correspond to anomalies interpreted from surface amplitude maps and migration pathways in the seismic data however episodically larger releases from persistent seeps occurred and also some other seep formations showed intermittent releases our analysis indicates that active oil seeps detected with sar represent a subset of the total array of geophysical features generated by hydrocarbon migration on the northern continental slope of the gulf of mexico

neogene sedimentation history of the congo fan abstract high resolution seismic reflection data were interpreted to develop a model for the neogene sedimentation in the congo fan area identification of two reflectors representing marine isotope stages 6 and 12 allows a distinction between the early and the late quaternary a single sediment source system during the late paleogene early neogene gradually changes into a two sediment source system in the late neogene since the late quaternary the congo river has been the prevailing sediment source the evolving benguela current further modified the system and led to a high biogenic productivity evidence for gas in the sediments can be found in the whole area of investigation additionally indications for a tectonic control of the congo canyon have been found in the distribution of sedimentary units and unit thicknesses

old marine seismic and new satellite radar data petroleum exploration of north west labrador sea canada abstract this paper presents some new concepts in the petroleum systems of the northern labrador and southern baffin island offshore region of eastern canada the focus of this work is the region of the hekja o 71 gas discovery of 1979 by aquitaine one of only five wells drilled between 1976 and 1980 within an area covering some 166 000 square kilometers within the saglek basin this study emerged from a broad scale re examination of the petroleum potential of this area from the crust up using regional reflection refraction and potential field data an opportunity to use sar synthetic aperture radar data from the radarsat 1 earth observation satellite was taken to incorporate alternative data sources to support this exploratory review and resource assessment examination of the final map of interpreted slick like features on the sea surface revealed a close correlation of some of these occurrences to several bathymetric features which have underlying seismic signatures similar to previously identified gas hydrate pipes or chimney anomalies as seen in data from offshore nova scotia and in the irish sea globally many active marine hydrocarbon seeps appear on the seafloor as pockmarks in this case no seafloor depressions were seen on conventional multi channel 2 d seismic data associated with active seepage instead very distinct mound like structures are seen associated with seeps at two locations east of hudson strait the presence of significant cold water coral in close proximity to the active seepage sites may show a link between seafloor petroleum seepage and coral reef development similar to that observed offshore norway close examination of the near surface seismic character led to the interpretation of what was originally seen as seismic noise in vintage data as likely being fluid escape chimneys or gas pipes along the eastern edge of the hekja structural complex where favourable hydrocarbon trapping conditions are present no active seeps were seen there this may be due to other factors such as the nature of the fluids that are escaping or the possibility of self sealing activity by biological agents such as bacterial matting with associated carbonate hard ground formation one seep anomaly was noted relatively close to the major basaltic eruptive complex near the gjoa g 37 well this may be due to the presence of volcaniclastic hosted hydrocarbons similar in style to those recently discovered at the rosebank field in paleocene volcanics in the faroe shetland basin the occurrence of an active seep in a volcanically dominated terrain raises the potential for a radical shift in accepted types of petroleum leads and where they may occur the presence of apparent oil seepage versus the past history of finding only gas prone accumulations is a strong indicator that a second oil prone petroleum system exists in the saglek basin the signs of active petroleum systems as shown by oil seep anomalies with confirming seismic evidence of discrete sea floor structures in close proximity makes a compelling argument for re examining these areas with an eye towards stratigraphic instead of structural plays

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

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

recurrent frontal slicks of a coastal ocean upwelling shadow 1 marine ecosystems are greatly influenced by the structure and dynamics of fronts in coastal upwelling systems frontogenesis occurs frequently by upwelling and transport of cold water and warming in sheltered upwelling shadow retention sites monterey bay in the california current upwelling system hosts a dynamic upwelling shadow environment using a decade of satellite synthetic aperture radar sar observations with ancillary remote sensing and in situ data we describe recurrent surface slicks that develop along the seaward periphery of the monterey bay upwelling shadow and we examine their relationships with fronts slick median dimensions 17 5 km long and 0 8 km wide describe their elongated structure although the typical pattern is a single slick multiple slicks may concurrently develop in association with different types of fronts repeated volume surveys through a front underlying a slick revealed lateral mixing and interleaving of regional water types velocity fields from coastal hf radar show that slicks may coincide with a variety of surface circulation patterns that they may extend contiguously across regions having very different surface velocity and that they may be separated from the shear front of upwelling filaments by 5 10 km slicks occur in all seasons and may coincide with both upwelling and downwelling wind forcing surfactant accumulation in small scale convergence zones is indicated as the primary mechanism of slick formation not ocean current shear or small scale air sea coupling the results of this study emphasize the role of upwelling system fronts in creating small scale structure and dynamics that influence plankton ecology

on the origin of multiple bsrs in the danube deep sea fan black sea highlights a stack of four bsrs were identified in levee deposits of the danube deep sea fan the multiple bsrs are not caused by overpressure compartments the multiple bsrs reflect stages of stable sealevel lowstands during glacial times gas underneath the previous ghsz does not start to migrate for thousands of years abstract high resolution 2d seismic data reveal the character and distribution of up to four stacked bottom simulating reflectors bsr within the channel levee systems of the danube deep sea fan the theoretical base of the gas hydrate stability zone ghsz calculated from regional geothermal gradients and salinity data is in agreement with the shallowest bsr for the deeper bsrs bsr formation due to overpressure compartments can be excluded because the necessary gas column would exceed the vertical distance between two overlying bsrs we show instead that the deeper bsrs are likely paleo bsrs caused by a change in pressure and temperature conditions during different limnic phases of the black sea this is supported by the observation that the bsrs correspond to paleo seafloor horizons located in a layer between a buried channel levee system and the levee deposits of the danube channel the good match of the observed bsrs and the bsrs predicted from deposition of these sediment layers indicates that the multiple bsrs reflect stages of stable sealevel lowstands possibly during glacial times the observation of sharp bsrs several 10 000 of years but possibly up to 300 000 yr after they have left the ghsz demonstrates that either hydrate dissociation does not take place within this time frame or that only small amounts of gas are released that can be transported by diffusion the gas underneath the previous ghsz does not start to migrate for several thousands of years

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

monitoring of natural oil seepage in the lower congo basin using sar observations abstract synthetic aperture radar sar is a spaceborne tool allowing near real time imagery over large ground footprints of areas hundreds of kilometres wide sar also allows persistent observations of the process of oil discharges benefiting from 1 day and night observations 2 independence of cloud cover and 3 high revisiting frequencies the interpretation of 82 sar scenes over the lower congo basin for the 1994 2012 period allowed the recognition of 1400 natural seepage slicks associated with 102 individual seep sites we acquired an additional sar data set with a short revisit time over a selected prolific area the data set consisted of 22 sar scenes acquired over a 10 day period meaning a maximum revisiting period of 12 h between subsequent sar acquisitions the short term approach shows that seepage slicks were detected with wind speeds between 1 5 and 6 5 m s 2 91 to 12 63 knots both long term and short term monitoring evidences that seepage events are intermittent with the occurrence rate of oil release ranging from 5 to 80 short term monitoring shows that the seepage pace is independent between seep sites suggesting contrasted controlling factors it also shows that the residence time of seepage slicks is systematically shorter than 12 h which is far below reported values understanding the seepage slick residence time is essential to provide an accurate estimation of the fluid flow and to compute the volumetric outflow the integration of in situ mooring points measuring the current velocity 10 m below the sea surface in addition to local wind field and slick length allowed us to more accurately estimate the residence time of oil slicks at the sea surface before vanishing the results show that the sea surface residence time of oil slicks is limited to a few hours with a median value of 3 h 15 min these new residence time estimations allowed us to propose a quantification of the regional oil output in the lower congo basin estimated at 4380 m3 year this area may therefore be considered as the world s third biggest oil supplying province from natural leakages

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

significance of hydrocarbon seepage relative to petroleum generation and entrapment near surface indications of migrating hydrocarbons provide the petroleum systems analyst critical information about source organic matter type maturation organic maturity migration migration pathway delineation and in selected geologic settings specific prospect hydrocarbon charge all petroliferous basins exhibit some type of near surface signal but the hydrocarbon leakage to surface is not always detectable with conventional seep detection methods understanding the petroleum seepage system hence petroleum dynamics of a basin is key to understanding and using near surface geochemical methods for basin assessment and prospect evaluation the relationships between near surface hydrocarbon seepage and subsurface petroleum generation and entrapment are often complex the petroleum seepage system contain four key elements seepage activity qualitative expressions of relative leakage rates active vs passive and episodic vs continuous seepage type concentration of migrated thermogenic hydrocarbon relative to in situ material macro vs micro migration focus major direction of bulk flow leakage relative to the subsurface hydrocarbon generation and or entrapment and near surface seep disturbances near surface processes which can greatly alter or block seepage signals the rate and volume of hydrocarbon seepage to the surface greatly control near surface geological and biological responses and thus are the best method of sampling and analysis to detect hydrocarbon leakage effectively to properly predict subsurface petroleum properties interpretation of near surface geochemical data must recognize many potential problems including recent organic matter input transported hydrocarbons bacterial alteration mixing contamination and fractionation effects surface geochemical data should always be integrated with other geological data calibration datasets to determine the utility of near surface geochemical techniques within particular basinal settings are essential when evaluating prospects for hydrocarbon charge

gas hydrate fluid flow and free gas formation of the bottom simulating reflector abstract gas hydrate in continental margins is commonly indicated by a prominent bottom simulating seismic reflector bsr that occurs a few hundred metres below the seabed the bsr marks the boundary between sediments containing gas hydrate above and free gas below most of the reflection amplitude is caused by the underlying free gas gas hydrate can occur without a bsr however and the controls on its formation are not well understood here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone ghsz the first is the well recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the ghsz moves upward relative to hydrate bearing sediment the second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the ghsz when hydrate is present near its base this mechanism for gas production called the solubility curvature mechanism is possible in systems where the influence of diffusion becomes important relative to the influence of advection and where the gas water solubility decreases to a minimum several hundred metres below the ghsz we investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility curvature mechanism and where it is dominated by hydrate recycling we show that the former is dominant in areas with low rates of upward fluid flow such as old rifted continental margins low rates of seafloor uplift and high geothermal gradient and or pressure conversely free gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift such as in subduction zone accretionary wedges using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins we are able to resolve a number of problems these include why sub bsr free gas zones are generally thin in convergent margins and thick in passive margins and why areas such as the gulf of mexico and mackenzie delta do not have regional bsrs finally we use our understanding of how bsrs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable bsr

the long term evolution of the congo deep sea fan a basin wide view of the interaction between a giant submarine fan and a mature passive margin zaiango project abstract we have integrated the relatively unknown distal domains of the lower congo basin where the main depocenters of the congo submarine fan are located with the better constrained successions on the shelf and upper slope through the analysis of thousands of km of 2d seismic reflection profiles off shore the congo angola passive margin the basin architecture is depicted by two ca 800 km long regional cross sections through the northern congo and southern angola margin a large unit deposited basinward of the aptian salt limit is likely to be the abyssal plain equivalent of the upper cretaceous carbonate shelf that characterized the first post rift deposits in west equatorial african margins a latest turonian shelf deepening event is recorded in the abyssal plain as a long period coniacian eocene of condensed sedimentation and basin starvation the onset of the giant tertiary congo deep sea fan in early oligocene following this event reactivates the abyssal plain as the main depocenter of the basin the time space partitioning of sedimentation within the deep sea fan results from the interplay among increasing sediment supply margin uplift rise of the angola salt ridge and canyon incision throughout the neogene oligocene early miocene turbidite sedimentation occurs mainly in nw se grabens and ponded inter diapir basins on the southern margin angola seaward tilting of the margin and downslope salt withdrawal activates the up building of the angola escarpment which leads to a northward congo shift of the transfer zones during late miocene around the miocene pliocene boundary the incision of the congo submarine canyon confines the turbidite flows and drives a general basinward progradation of the submarine fan into the abyssal plain the slope deposition is dominated by fine grained hemipelagic deposits ever since results from this work contribute to better understand the signature in the ultra deep deposits of processes acting on the continental margin as well as the basin wide sediment redistribution in areas of high river input

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

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

seismic chimneys in the southern viking graben implications for palaeo fluid migration and overpressure evolution highlights 3d seismic interpretation reveals 46 large chimney structures in the svg chimneys can be subdivided in three categories based on their seismic appearance seismic appearance of chimneys enables reconstructing formation mechanisms overpressure and seal weakening by deformation control chimney formation fluid flow along chimneys represents an efficient pressure transfer mechanism abstract detailed understanding of natural fluid migration systems is essential to minimize risks during hydrocarbon exploration and to evaluate the long term efficiency of the subsurface storage of waste water and gas from hydrocarbon production as well as co2 the southern viking graben svg hosts numerous focused fluid flow structures in the shallow 1000 m subsurface the seismic expressions of vertical fluid conduits are variously known as seismic chimneys or pipes seismic pipes are known to form large clusters seismic chimneys have so far been described as solitary structures here we show that the study area in the svg hosts more than 46 large scale vertical chimney structures which can be divided in three categories implying different formation processes our analysis reveals that seal weakening formation wide overpressure and the presence of free gas are required to initiate the formation of vertical fluid conduits in the svg the presence of numerous vertical fluid conduits implies inter stratigraphic hydraulic connectivity which significantly affects the migration of fluids in the subsurface chimney structures are important for understanding the transfer of pore pressure anomalies to the shallow parts of the basin

seafloor oxygen consumption fuelled by methane from cold seeps the leakage of cold methane rich fluids from subsurface reservoirs to the sea floor sustains some of the richest ecosystems on the sea bed these cold seep communities consume around two orders of magnitude more oxygen than the surrounding sea floor as a result of the microbial consumption of seep methane

shallow plumbing systems inferred from spatial analysis of pockmark arrays abstract this study describes and analyses an extraordinary array of pockmarks at the modern seabed of the lower congo basin offshore angola in order to understand the fluid migration routes and shallow plumbing system of the area the 3d seismic visualization of feeding conduits pipes allowed the identification of the source interval for the fluids expelled during pockmark formation spatial statistics are used to show the relationship between the underlying polarised polygonal fault ppfs patterns and seabed pockmarks distributions our results show ppfs control the linear arrangement of pockmarks and feeder pipes along fault strike but faults do not act as conduits spatial statistics also revealed pockmark occurrence is not considered to be random especially at short distances to nearest neighbours

1000 m long gas blow out pipes abstract this study presents seismic observation of pipe anomalies from offshore nigeria outcrops of blow out pipes from rhodes greece and geophysical modelling of an acoustic pipe the studies give insight into how pipes form their internal structure the seismic image and geophysical artefacts related to the pipes over one hundred seafloor craters 100 m 700 m wide and up to 30 m deep have been observed on the seafloor offshore nigeria they are underlain by interpreted cones and seismic pipe anomalies that can be traced down to reservoir zones at 1000 m 1300 m below the seafloor the seismic pipe anomalies are 50 m 150 m wide and almost vertical they are interpreted as up scaled pipes found in outcrops on rhodes greece the outcrops show pipe related structures at three levels lowest the reservoir rock contains metre sized cavities which are filled with a mixture of clay derived from the overlying cap rock in the middle several circular to oval structures in plane view of pipes are observed in the lowest part of the cap rock highest 15 m into the clay cap rock strongly sheared country rock forms circular structures with a core of structureless clay based on outcrop observation on rhodes we constructed an acoustic model of a 50 m wide and 1000 m long pipe seismic modelling proves that such pipes would be expressed in seismic data that they are similar to the seismic pipe anomalies offshore nigeria but this study also revealed that prominent intra pipe reflections are artefacts a formation model for the pipes is suggested high fluid overpressure in the reservoir generated hydro fractures from the reservoir to seafloor where a mixture of gas and fluid flowed at high speed to form pipes cones and seafloor craters after hours to weeks of gas and fluid flow through the pipe the pore pressure in the reservoir dropped and the blow out terminated muddy slurry fell back and plugged the cavity in the reservoir and the pipe

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

asphalt volcanism and chemosynthetic life in the campeche knolls gulf of mexico in the campeche knolls in the southern gulf of mexico lava like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt which chilled and contracted after discharge the site also includes oil seeps gas hydrate deposits locally anoxic sediments and slabs of authigenic carbonate asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the gulf of mexico

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

establishing criteria to distinguish oil seep from methane seep carbonates hydrocarbon seeps harbor copious chemosynthesis dependent life the traces of which are preserved in the fossil record within authigenic carbonates these environments are mostly characterized by seepage of methane rich fluids yet numerous crude oil dominated seeps have been discovered in recent years oil seepage has a profound influence on the local fauna but recognizing such seeps in the rock record remains elusive this study presents new geochemical data that will allow for a more confident identification of ancient oil seep deposits geochemical data from modern and ancient seep limestones reveal that oil dominated seep carbonates are enriched in rare earth elements and uranium compared to their methane dominated counterparts these trace element patterns have the potential to serve as a basis for an improved understanding of the adaptation of chemosynthetic life to oil seepage and to better constrain the marine carbon cycle in the geologic past

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

the controls on the composition of biodegraded oils in the deep subsurface part 1 biodegradation rates in petroleum reservoirs abstract biodegradation rates in oilfields have been assessed conservatively using whole oil column minimum rate estimates diffusion controlled oil column compositional gradient modelling and mixed oil kinetic models biodegradation rate constants first order are around 10 6 10 7 yr 1 for hydrocarbons in the degradation zones these corresponding well with zero order field wide minimum rate estimates of about 10 8 kg hydrocarbons kg oil year for the whole oil column with biodegradation induction times of around 1 2 ma to perturb an entire oil column for light oil reservoirs and 10 20 ma for heavy oil reservoir degradation the results indicate that where we see continuous gradients in the oil columns degradation must have been occurring episodically for many millions of years to remove the n alkanes from an oil i e about 10 of an oil around ca 15 ma is needed for a heavy oil ca 5 ma for a light n sea oil the timescales of oilfield degradation and filling are thus very similar and consequently the degree of biodegradation will be substantially controlled by oilfield charge history assessment of mixed degraded non degraded oil occurrence provides an independent confirmation that these rates are realistic and that timescales of degradation and field charging are similar the maximum effective rate constant of degradation ultimately controlled by the limiting effect of diffusion of alkanes to the oil water contact owc ca 10 4 yr 1 for a 130 m thick oil column first order rate constant is well above the estimated rate constants indicating oil biodegradation rate is not limited by electron donor supply i e hydrocarbons but by supply of nutrients or oxidants this suggests that diffusive transport of nutrients and electron acceptors in the aquifer to the site of biodegradation may be adequate to maintain the low rate biosphere

fluid flow features in hydrocarbon plumbing systems what do they tell us about the basin evolution abstract fluid flow features such as sandstone intrusions mud volcano systems and pockmarks represent well known geological structures generated due to flow of fluids in the subsurface however the integration of fluid flow features in quantitative and qualitative basin analysis is still in its infancy this study presents a review and a qualitative integration of fluid flow features in basin analysis of the danish north sea area and of salt mini basins offshore angola based on results from six case studies focusing on the detection mapping and evaluation of features related to focused fluid flow in these areas in the study the diversity of seismically imaged fluid flow features in hydrocarbon plumbing systems is reviewed and their utility is demonstrated this is done by focusing on the aspects of fluid flow related features that can be integrated into basin analysis studies and how these may help us increase the understanding of basin evolution and to achieve the major goals of basin analysis such as understanding the depositional and tectonic context of the basin and the presence and extent of hydrocarbons particularly concerning the determination of fluid flow timing triggers sources and routes fluid flow features represent a major but underused tool in basin analysis

methane and minor oil macro seep systems their complexity and environmental significance abstract the two main observations characterising marine and lacustrine methane macro seeps are ebullition through holes in the sea or lake bed and hydroacoustic flares in the water column this paper reviews multi year multi scale and multi discipline results from three seep locations in the north sea and combines the knowledge with recent seafloor and water column results from seeps in the santa barbara basin california a seep off west africa seeps in the gulf of mexico and in lake baikal russia we have identified a total of 12 characteristics of methane and minor oil macro seeps that are not only geological in nature but also biological and geochemical these are shown to impact the marine environment in different ways not least in benefactory manners as primary producers mainly bacteria and archaea tend to bloom during seepage therefore the seepage is inferred to have a fertilizing effect on both the seafloor and the water column which may be of broad ecological and biological significance the study concludes with a holistic conceptual seep model which is expected to be of interest to a broad range of researchers in the fields of oceanography and limnology

major controlling factors on hydrocarbon generation and leakage in south atlantic conjugate margins a comparative study of colorado orange campos and lower congo basins abstract we present a supra regional comparative study of the major internal and external factors controlling source rock sr maturation and hydrocarbon hc generation and leakage in two pairs of conjugate margins across the south atlantic the brazil campos basin angola lower congo basin margins located in the central segment and the argentina colorado basin south africa orange basin in the southern segment our approach is based on the analysis and integration of borehole data 1d numerical modeling 2d seismic reflection data and published reports coupling of modeling results sedimentation rate calculation and seal bypass system analysis reveal that 1 oil window is reached by syn rift srs in the southern segment during the early to late cretaceous when thermal subsidence is still active while in the central segment they reach it in late cretaceous neogene during a salt remobilization phase and 2 early hc generation from post rift srs in the southern segment and from all srs in the central segment appears to be controlled mainly by episodes of increased sedimentation rates the latter seems to be associated with the andes uplift history for the western south atlantic basins campos and colorado and to a possibly climate driven response for the eastern south atlantic basins orange and lower congo additionally we observe that the effect of volcanism on sr maturation in the southern segment is very local the comparison of cretaceous mass transport deposit mtd episodes with hc peak of generation and paleo leakage indicators in the southern segment revealed the possible causal effect that hc generation and leakage have over mtd development interestingly paleogene leakage indicators which were identified in the argentina south africa conjugate margins occur contemporaneously to low sedimentation rate periods nonetheless present day leakage indicators which were also identified in both pairs of conjugate margins might be related to seal failure events linked to eustatic sea level drops

establishing hydrocarbon charge to the ashmore platform bonaparte basin australia a natural seeps study abstract to date there is one proven hydrocarbon accumulation on the ashmore platform bonaparte basin australia with hydrocarbon charge remaining a key exploration risk to the south the neighbouring browse basin has proven lateral migration of generated hydrocarbons to the basin bounding highs as evidenced by seeps located on the yampi shelf this paper describes the findings of a natural seeps study carried out to establish if migrating subsurface hydrocarbons reach the southern flanks of the ashmore platform basement high the integrated study combined remote sensing geophysical acoustic photographic and geochemical techniques and has identified three areas of seepage one area characteristic of persistent seepage and two areas of interpreted episodic leakage geochemical data collected from samples at one of these sites demonstrates the presence of thermogenic liquid hydrocarbons with isotopic compositions falling within the range of values exhibited by oils sourced by the lower cretaceous echuca shoals formation the identification of active natural seepage along the southern flank of the ashmore platform provides evidence that hydrocarbons generated within the caswell sub basin are able to laterally migrate onto the flanks of the ashmore platform structural high as such these findings reduce charge risk for the ashmore platform and regional exploration risks in the northern browse basin

yampi shelf browse basin north west shelf australia a test bed for constraining hydrocarbon migration and seepage rates using combinations of 2d and 3d seismic data and multiple independent remote sensing technologies abstract the yampi shelf on australia s north west shelf is highly prospective with two discrete hydrocarbon sources producing dry gas and oil to reduce exploration uncertainty relating to gas flushing and poor top seal capacity a study was undertaken to characterise hydrocarbon migration in the area it used a combination of seismic amplitude and structural data integrated with shipboard water column geochemical sniffer wasi data satellite synthetic aperture radar or sar data and aircraft acquired airborne laser fluorosensor alf data data were acquired synchronously and in staged programs to allow both direct comparison and time series analysis of results massive natural dry gas and oil seepage was detected though the relative abilities of wasi sar and alf to detect and characterise this seepage were markedly different the spatial distribution concentration and relative composition of the detected seepage were controlled principally by the regional seal s thickness and capacity rather than by the inherent composition and flux of the migrating hydrocarbons wasi preferentially identified gas seepage often in basin ward locations because the high relative permeability of gas favoured its early leakage even through thick seals sar preferentially identified oil seepage which was episodic and largely restricted to the basin margin at the regional zero edge of seal reflecting the low relative permeability of oil even through thin seals it leaked late alf principally detected low level oil seepage from charged traps and was hence most useful for trap ranking the ability of these remote sensing tools as well as that of seismic data itself to detect hydrocarbons appears critically dependant upon interplays between the relative sensitivity of the assorted tools to detect various hydrocarbon phases and the capacity of the top seal itself the study has demonstrated that the interactions between geology and hydrocarbon charge are predictable and that understanding these interactions is crucial for the reliable interpretation of remote sensing data

temporal and spatial patterns of biological community development at nascent deep sea hydrothermal vents 9 50 n east pacific rise the april 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9 45 n and 9 52 n on the east pacific rise provided a unique opportunity to follow temporal changes in biological community structure from the birth of numerous deep sea hydrothermal vents in march l992 dsv alvin was used to deploy an on bottom observatory the biologic geologic transect to monitor faunal succession along a 1 37 km segment of the axial summit caldera between 9 49 61 n and 9 50 36 n depth 2520 m photo and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the transect were performed in march 1992 december 1993 october 1994 and november 1995 photographic and chemical time series analyses revealed the following sequence of events in low temperature venting areas 1 immediately following the 1991 eruption hydrogen sulfide and iron concentrations in diffuse fluids were extremely high 1 mmol kg 1 and microbially derived material blanketed active areas of venting in the form of thick microbial mats 2 mobile vent fauna e g amphipods copepods octopods and galatheid and brachyuran crabs and non vent fauna e g nematocarcinid shrimp proliferated in response to this increased biological production 3 within 1 yr of the eruption areal coverage of microbial mat was reduced by 60 and individuals of the vestimentiferan tube worm tevnia jerichonana settled gregariously in areas where diffuse flow was most intense 4 two years after the eruption maximum levels of h2s decreased by almost half from 1 90 to 0 97 mmol kg 1 and dense thickets of the vestimentiferan riftia pachyptila dominated vent openings previously inhabited by tevnia jerichonana 5 three years after the eruption maximum hydrogen sulfide levels declined further to 0 88 mmol kg 1 and mussels bathymodiolus thermophilus were observed on basaltic substrates 6 four years after the eruption galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0 72 mmol kg 1 also by this time mussels had colonized on to tubes of riftia pachyptila 7 between 3 and 5 yr after the eruption there was a 2 to 3 fold increase in the number of species in the faunal assemblages in the absence of additional volcanic tectonic disturbance we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5 10 yr following the eruption we also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase we hypothesize that the above series of events 1 7 represents a general sequence of biological successional changes that will occur at newly formed low temperature deep sea hydrothermal vents along the northern east pacific rise and contiguous ridge axes megafaunal colonization at deep sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life history strategies of individual species physical oceanographic processes and the dynamic hydrothermal environment our observations indicate that the successful sequential colonization of dominant megafaunal vent species from tevnia jerichonana to riftia pachyptila to bathymodiolus thermophilus also may be strongly influenced by temporal changes in geochemical conditions additional evidence demonstrating the close link between diffuse vent flux fluid geochemistry and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow these correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast spreading mid ocean ridges should consider not only the interplay of species specific life history strategies community productivity and physical oceanographic processes but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species

velocity independent τ p moveout in a horizontally layered vti medium local slopes of seismic events carry complete information about the structure of the subsurface this information is sufficient for accomplishing all time domain imaging tasks without the need to estimate or know the seismic velocity model a velocity independent p imaging approach has been developed to perform moveout correction in horizontally layered vertical transverse isotropy vti media the effective and interval anisotropic parameters are transformed into data attributes through the use of slopes and become directly mappable to the zero slope traveltime the p transform is the natural domain for anisotropy parameter estimation in layered media because the phase velocity is given explicitly in terms of p therefore the p transform permits reflection traveltime modeling and inversion that are simpler than traditional methods which are based on taylor series expansions of traveltime in the t x domain synthetic and field data tests demonstrate the practical effectiveness of the p

high resolution heat flow density in the lower congo basin 1 heat flow density hfd has been derived in the lower congo basin from 293 conventional probe measurements 424 oil exploration data and about 1 000 000 bottom simulating reflector bsr values each set of data provides independent estimates of temperature gradients associated with specific uncertainties and geographic distributions but a statistical comparison in overlapping domains shows a relatively good consistency in situ conductivity has been measured in conventional surveys and estimated from rock composition and porosity in oil exploration boreholes in the deep offshore measurements and estimates are consistently low and increase significantly toward the shelf domain thermal conductivity does not change significantly in the interval between the water bottom and the bsr and can be estimated from the surface measurements to provide an inference of hfd the combined sets of hfd in the lower congo basin cover the overall domain of the continental margin and a part of the oceanic domain high resolution imaging of the bsr by three dimensional 3 d seismic studies provides in addition good quality nominal resolution of 500 m in the deep offshore at short length scales on the order of kilometers large variations of hfd from 30 to 140 mwm 2 exist above salt diapirs and canyons related to heat refraction on the other hand no small scale effect related to fluid flow has been detected although active venting is known at larger length scale hfd in the deep offshore is different on both sides of the congo submarine channel 54 8 mwm 2 to the north and 48 12 mwm 2 to the south which are also characterized by different basin tectonic styles at the margin scale hfd is significantly different in the oceanic domain 42 3 mwm 2 the deep continental offshore 52 10 mwm 2 and the continental shelf 65 15 mwm 2 a progressive reduction of the radiogenic heat contribution in the extended continental crust is the more likely the cause of these differences mantle heat flow below the continental margin is consequently high compared to that expected 15 mwm 2 for old shields such as congo

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

imaging of biogenic and anthropogenic ocean surface films by the multifrequency multipolarization sir c x sar results from the analyses of several spaceborne imaging radar c x band synthetic aperture radar sir c x sar images are presented which were acquired during the two sir c x sar missions in april and october 1994 by the l c and x band multipolarization sar aboard the space shuttle endeavour the images showing natural biogenic surface slicks as well as man made anthropogenic mineral oil spills were analyzed with the aim to study whether or not active radar techniques can be applied to discriminating between these two kinds of surface films controlled slick experiments were carried out during both shuttle missions in the german bight of the north sea as well as in the northern part of the sea of japan and the kuroshio stream region where surface films of different viscoelastic properties were deployed within the swath of the shuttle radars the results show that the damping behavior of the same substance is strongly dependent on wind speed at high wind speed 8 12 m s the ratio of the radar backscatter from a slick free and a slick covered water surface damping ratio is smaller than at low to moderate wind speeds 4 7 m s at 12 m s only slight differences in the damping behavior of different substances were measured by sir c x sar furthermore several sar scenes from various parts of the world s oceans showing radar signatures of biogenic as well as anthropogenic surface films at low to moderate wind speeds are analyzed the damping behavior of these different kinds of oceanic surface films varies particularly at lband where the biogenic surface films exhibit larger damping characteristics results of polarimetric studies from multipolarization sar images showing various surface films are presented it can be delineated from these results that bragg scattering as well as specular reflection contribute to the backscattered radar signal at low incidence angles up to 30 it is concluded that at low to moderate wind speeds multifrequency radar techniques seem to be capable of discriminating between the different surface films whereas at high wind conditions a discrimination seems to be difficult

geophysical signatures of mud mounds at hydrocarbon seeps on the louisiana continental slope northern gulf of mexico abstract mud mounds are common features at cold seeps in many areas of the world they are abundant in the northern gulf of mexico where they occur at the numerous hydrocarbon seeps on the continental margin although the mounds are widely known their geophysical signatures and geology have not been widely studied in detail in this study we examined mounds in two areas of the upper louisiana slope using geophysical data of several types 11 12 khz side scan sonar 3 5 khz echo sounder profiles near bottom 2 12 khz chirp sonar profiles and three dimensional 3d multichannel seismic data in side scan images the mud mounds appear as ovoid high backscatter spots associated with larger irregular high backscatter zones along faults and fault complexes three different sonar responses were noted 1 a more or less uniform appearance 2 a central higher backscatter spot surrounded with a lower return halo bulls eye and 3 a more or less uniform high backscatter area with an acoustic dead spot near its center near bottom chirp sonar records indicate high lateral variability in seafloor reflection characteristics implying that the long range acoustic signature of a given mound results from the integration of seafloor characteristics in these records anomalous seafloor echo types include strong seafloor reflectors hardbottom or buried hardbottom acoustic wipe out attenuation and acoustic turbidity prolonged echoes cores from acoustically anomalous seafloor areas indicate that a variety of seep related material has been introduced into the near seafloor sediments the prevalence of strong bottom return signatures implies that the formation of hard grounds or crusts is an integral part of hydrocarbon seep evolution and suggests that much of the ovoid high backscatter signatures may result from such features multichannel seismic reflection data from several of the active mounds show columnar subsurface chaotic reflection zones and strong reflections in sediments directly underneath seafloor seismic amplitude maps from these mounds are either generally uniform with a positive or negative anomaly or display a ring of higher reflectivity around the mound periphery positive or negative anomalies may depend on the degree of seafloor lithification and sediment gas charging the ring feature is thought to result from the preferential concentration of seep debris in the mound periphery relative to the center some mounds which show no seismic reflection anomaly from the seafloor appear inactive altogether observations from this study are consistent with the mud mounds being formed by sedimentary volcanism the data imply they are holocene features that have a life cycle that includes hardbottom formation which progressively crusts over the fluid vent ending its activity

tectonic geomorphology and hydrocarbon induced topography of the mid channel anticline santa barbara basin california the geomorphology of the western sector of the mid channel anticline mca santa barbara southern california suggests the actively growing fold is laterally propagating to the west the presence of fold scarps and cross faults that segment the structure suggests that buried faults that are producing the folding are present at shallow depths the summit area of the anticline at the last glacial maximum 22 to 19 ka was probably a small late pleistocene island evidence for presence of the island includes what appears to be terrestrial erosion and is supported by assumption of sea level change and rates of uplift and subsidence pockmarks and domes ranging in diameter from 10 to 100 m and several meters deep are present along the crest and flanks of the mca these features appear to be the result of hydrocarbon emission their formation has significantly modified the surface features producing simple to complex erosional and or constructional topography a large pockmark near the anticline crest dated by two calibrated ams radiocarbon dates of 25 3 and 36 9 ka continues to emit hydrocarbon gases we term the topography produced by hydrocarbon emission as hydrocarbon induced topography hit 2007 elsevier b v all rights reserved

gas hydrate pingoes deep seafloor evidence of focused fluid flow on continental margins gas hydrates in the shallow subsurface form one of the largest reservoirs of methane in the global organic carbon cycle seafloor seeps and associated features represent the venting points of methane released from the shallow lithosphere to the hydrosphere and atmosphere here we document the discovery of seep related seafloor mounds in the kwanza basin offshore angola and employ high resolution three dimensional seismic analysis to unravel the subsurface plumbing system and the origin of mounds mounds with distinct morphologies and geophysical signatures illustrate different development stages associated with the formation and dissociation of shallow gas hydrate linked to thermogenic fluid migration along salt diapir flanks draining deeply buried salt minibasins the mounds are more than an order of magnitude larger than previously described submarine hydrate pingoes and comparable to hydraulic pingoes commonly found in terrestrial periglacial environments suggesting hydrate volumes of individual mounds up to 1 1 106 m3 equivalent to 2 0 108 m3 of methane gas the interpretation of seismically well defined seep related seafloor mounds brings new insight to the occurrence and development of concentrated near surface gas hydrate accumulations and their relationship with thermogenic fluid migration and host sediment properties along continental margins

pockmarks in the northern congo fan area sw africa complex seafloor features shaped by fluid flow abstract three pockmarks named hydrate hole black hole and worm hole were studied in the northern congo fan area at water depths around 3100 m the cross disciplinary investigations include seafloor observations by tv sled sampling by tv guided grab and multicorer as well as gravity coring in addition to hydroacoustic mapping by a swath system a parametric sediment echosounder and a deep towed sidescan sonar the pockmarks are morphologically complex features consisting of one or more up to 1000 m wide and 10 15 m deep depressions revealed by swath mapping high reflection amplitudes in the sediment echosounder records indicate the presence of a 25 30 m thick shallow sediment section with gas hydrates which have been recovered by gravity corer hydrates chemosynthetic communities and authigenic carbonates clearly indicate fluid flow from depths which we propose to be mainly in the form of ascending gas bubbles rather than advection of methane rich porewater evidence for seepage at the seafloor is confined to small areas within the seafloor depressions and was revealed by characteristic backscatter facies small meter scale sized depressions signified as pits exist in or close to the pockmarks but seafloor observations did not reveal evidence for the presence of typical seep organisms or authigenic carbonates areas of intermediate backscatter were inhabited by vesicomyid clams in soft sediments high backscatter was associated with vestimentiferan tubeworms siboglinidae and authigenic carbonates we discuss the three different environments pits vesicomyid clams vestimentifera carbonate in the light of differences in the geochemical setting pits are probably formed by escaping gas bubbles but seepage is too transient to sustain chemosynthetic life vesicomyid clams are present in sediments with gas hydrate deposits however the hydrates occur several meters below the surface indicating a lower flux compared to the vestimentifera carbonate environment in the latter environment accumulated carbonates and clam shells indicate that fine grained particles have been eroded away gas hydrates were found in this environment at depths below about 50 cm suggesting the highest supply with methane compared to the other environments

load transfer in drilled piles for concrete rock interface with similar triangular asperities abstract for drilled piles in rock stratum predictions of axial load deformation response offer guidance for the preliminary design in this development the profiles of socket wall would be idealized as similar triangular asperities and the expansion of the shaft is laterally confined across the socket wall under constant normal stiffness conditions cns the side shear resistance contributes from the overriding dilation and degradation shearing of asperities and the degradation of asperities develops asynchronously in particular a correlation is derived between asperity geometry properties and the global strength by including a simplified degradation criterion of the asperity the proposed model can be considered as a generalization of patton s model extending the case of regular triangular asperities to the similar ones on this basis the predicted shear mobilization relative to displacement can then be employed to update the load transfer function semi analytical solutions to this problem have been conducted by a finite difference scheme based on the validation of a case study the proposed prediction exhibits general accuracy with reference to the field observations to investigate the effect of asperity inclination on the ultimate and residual strengths a parametric study has also been conducted afterwards comparing with an existing method the prediction from the proposed model captures a ductile response and depicts a notable nonlinear behavior

the controls on the composition of biodegraded oils in the deep subsurface part ii geological controls on subsurface biodegradation fluxes and constraints on reservoir fluid property prediction the principal controls on the fluid properties of biodegraded oil systems have been determined by a combination of petroleum geochemistry numerical modeling of oil biodegradation in reservoirs and analysis of oil property data sets from a variety of geological settings petroleum biodegradation proceeds under anaerobic conditions in any reservoir that has a water leg and has not been heated to temperatures more than 80c in most reservoirs with low concentrations of aqueous sulfate methanogenic degradation is a primary mechanism of petroleum degradation whereas in waters containing abundant sulfate sulfate reduction and sulfide production may dominate net degradation of petroleum fractions in reservoirs is primarily controlled by the reservoir temperature the chemical compounds being degraded and relationships between the oil water contact owc area and oil volume the relative volumes of water leg to oil leg prior level of oil biodegradation and reservoir water salinity act as second order controls on the process typically degradation fluxes kilograms of petroleum destroyed per square meter of oil water contact area per year or kg petroleum m2 owc yr1 for fresh petroleum in clastic reservoirs are in the range of 103 104 kg petroleum m2 owc yr1 and increase with decreasing reservoir temperature from zero near 80c to a maximum flux at the owc of less than 103 kg petroleum m2 owc yr1 at a temperature less than 40c at very low reservoir temperatures and with severely degraded oils such as are seen in the near surface canadian tar sands at the present day the net degradation fluxes are much less than maximum values nutrient supply from the aquifer and adjacent shales mostly buffered by mineral dissolution probably provides the ultimate control on the range of degradation flux values oil compositional gradients and resulting oil viscosity variations are common on both reservoir thickness and field scales in biodegraded oil reservoirs and are a defining characteristic of heavy oil fields produced by crude oil biodegradation continuous vertical gradients in the oil columns document episodic degradation for many millions of years suggesting that the time scales of oil field degradation and petroleum charging are similar the flux temperature relationship we have derived coupled with typical reservoir charge histories defines the range of variation of fluid properties seen in many biodegraded oil provinces and identifies oil charge mixing of biodegraded and fresh oils and reservoir temperature history as the primary controls on fluid properties these flux temperature relationships are easily integrated into prospect charge modeling procedures sensitivity analyses show that the limiting factor in fluid property predictions using even this first level approach are ultimately constrained by the accuracy of current oil charge modeling estimates the absence today of any functional geochemical proxies for assessing oil residence time in oil fields and the substantial uncertainty in petroleum charging times estimated by forward basin modeling is a major obstacle to more accurate fluid property predictions that needs to be addressed