the mechanism of surtseyan volcanism during eruption the vent of a surtseyan volcano is occupied by a highly mobile slurry of tephra hyaloclastite and water as magma ascends rapidly through the slurry mixing occurs due to velocity shear acceleration of the fluid fluid contacts and mass incorporation consequent expansion of the admixed water causes the jetting and continuous up rush activity that characterizes surtseyan volcanism there is no evidence of the widely accepted fuel coolant interaction mechanism

the relationship between the hawaiites and basalts of the itcha volcanic complex central british columbia volumetrically subordinate alkaline mafic lava flows form a late capping stage over the earlier felsic lavas that form the shield of the itcha volcanic complex ivc of the anahim volcanic belt avb in central british columbia b c the mafic capping stage of the ivc is dominated by hawaiites which are the earliest of the mafic lavas and are succeeded by alkali olivine basalts aob and then by basanites the alkali olivine basalts can be subdivided into high intermediate and low mgo aob groups all of which share similar hfse ratios e g nb zr with the hawaiites high al contents and sr zr ratios indicate that hawaiites and fe rich evolved aob were derived from primitive aob parental magmas by crystal fractionation of a wehrlitic assemblage at pressures on the order of 8 to 10 kbar high si and low fe contents indicate that the majority of the evolved aob lavas however do not represent an intermediate stage in the liquid line of descent to hawaiites but were most likely produced by gabbroic fractionation from primitive aob magmas at relatively low pressures the parental magmas of the majority of these lavas were distinct from those of the observed high mgo basalts having higher hfse contents and being more si under saturated the high al high sr zr signature of high pressure fractionation of a clinopyroxene dominated assemblage in the ivc is shared by hawaiites of other alkaline volcanic suites of the canadian cordillera such as the edziza volcanic complex in northern b c and appears to be a feature of hawaiites in many localities including hawaii and iceland viscosities calculated for both high and low pressure crystal fractionation models suggest that aphyric hawaiites are residual liquids escaped from a wehrlitic crystalline network at elevated pressures possibly at the base of the crust

structural characteristics of congelation and platelet ice and their role in the development of antarctic land fast sea ice ice cores were obtained in january 1990 from the land fast ice in mcmurdo sound for a study of variations in texture fabric sub structure composition and development two primary ice types were observed congelation and platelet with a minor amount of frazil ice congelation ice growth precedes platelet ice accretion congelation ice fabrics show frequent moderate to strong alignments a phenomenon believed to be due to water current control of selective ice crystal growth platelet ice originates at the base of the congelation ice initially as a porous latticework of tabular ice crystals which subsequently consolidate by congelation of the interstitial water interstitial congelation ice fabrics generally have little or no alignment indicating the reduced effect of currents within the platelet latticework prior to solidification platelet crystal textures range from small wavy edged forms to large blade like forms platelet crystal fabrics indicate that in addition to being randomly oriented the platelet latticeworks commonly include many crystals with their flat 0001 faces oriented both parallel and normal to the base of the overlying ice plate width data suggest that the interstitial congelation ice growth rates remain similar to those of the overlying congelation ice this effective increase in growth rates probably happens because the latticework of accumulating platelets ahead of the freezing interface ensures that the water within the platelet layer is at the freezing point and less heat has to be removed from platelet rich water than from platelet free water for a given thickness of congelation ice to grow the negative oceanic heat flux associated with platelet ice formation in mcmurdo sound explains why mcmurdo sound fast ice is thicker than ross sea pack ice and also why it reaches a greater thickness than arctic fast ice grown in a similar polar marine climate plate widths in the mcmurdo sound congelation ice suggest however that it grows no faster than arctic congelation ice

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

evaluation of possible gas microseepage mechanisms petroleum microseepage anomalies over petroleum accumulations are commonly explained by rapid vertical migration of colloidal gas bubbles through fracture networks this article is a theoretical anal ysis of this mechanism and of continuous gas phase flow in frac tures the gas bubble ascent mechanism is much slower than re ported microseepage velocities so it cannot account for observed microseepage in contrast continuous gas phase flow through frac tures can equal or exceed reported microseepage velocity while maintaining total flux low enough so that petroleum accumulations can exist for geological lengths of time fracture entry pressure for bubbles is more than twice that of a continuous gas phase so con tinuous gas phase migration also requires a lower pressure threshold before initiating seepage vertical microseepage is therefore best explained by the same mechanism interpreted for macroseepage although this article provides a theoretically justified mecha nism for microseepage it also shows why interpretation of surface microseepage signals is problematic fracture geometry controls seepage velocity and flux so geochemical anomalies may indicate an increase in fracture aperture as well as possible subsurface ac cumulations larger fractures require very low gas capillary entry pressures so in some settings surface seepage could result from fractures over stratal migration pathways as well as over petroleum accumulations

magnetotelluric measurements across the eastern neovolcanic zone in south iceland a magnetotelluric survey consisting of 18 separate soundings was performed in 1982 along a 125 km profile crossing the eastern neovolcanic zone in south iceland although the three dimensional ocean effect may be significant at the longest periods which previous workers have analyzed to study features in the upper mantle beneath iceland our studies suggest that this effect is minimal for periods less than 1000 s at distances greater than 50 km from the coast therefore we are confident that two dimensional interpretations are valid at periods suitable for interpreting crustal resistivities such a two dimensional interpretation presented here indicates the presence of a layer 5 km thick of low resistivity 5 ohm m at the crust mantle interface 10 20 km depth the depth to this layer increases from the center of the neovolcanic zone this feature has been seen elsewhere in iceland by other workers however the present study suggests that east of the neovolcanic zone the low resistivity layer is not present which we argue may be associated with a recent eastward shift of the axial rift zone to a new location within older crust a striking feature of our two dimensional crustal interpretation is that pockets of low resistivity material appear to be present at approximately 8 km depth at the base of the crust beneath the two volcanic centers that we have studied torfajokull and hekla these may be manifestations of local magma chambers

on small scale horizontal variations of salinity in first year sea ice measurements of salinity over horizontal distances of 38 to 76 cm in a thick first year ice sheet have revealed significant differences a maximum salinity difference of 2 was observed between ice core segments from the same depth the mean standard deviation for 10 cm thickness increments through the 2 0 m ice sheet was 0 39 between the five closely spaced cores the most likely mechanism for these significant differences in salinity over short distances is differential brine drainage in the ice sheet due to varying locations of brine drainage channels a simple one dimensional model which assumes a normally distributed arrangement of brine drainage channels provides results consistent with the horizontal differences observed

geophysical and geochemical evidence for subsurface hydrocarbon leakage in the bering sea alaska abstract geophysical and geochemical surveys were conducted on the bering sea shelf to investigate whether hydrocarbons found in the surficial sediment particularly in association with certain geophysical events indicate the presence and nature of more deeply reservoired oil and gas deposits surface sediment samples were obtained at 1000 sites where high resolution seismic data indicated the presence of gas seepage and or surface faulting in addition sites for background measurements were occupied away from areas of gas seepage results of the geophysical survey show seismic wipeout zones to be present throughout the bering sea shelf sediment samples obtained within the shallow wipeout zones confirm that these features are caused by gas in addition compositional and isotopic analyses indicate that the gas is of biogenic and thermogenic origins the thermogenic leakage appears to be related to localized faulting and shallow structuring the high resolution seismic data therefore provide an excellent tool for locating leakage points thereby enhancing the opportunity of finding migrated thermogenic hydrocarbons in surficial sediments an examination of vertical variability shows the hydrocarbon concentrations to be strongly related to various processes occurring in the surficial sediments core samples must be taken from below the zone of maximum disturbance to obtain unaltered hydrocarbon samples

equations for determining the gas and brine volumes in sea ice samples on a etabli des equations utilisables pour determiner le taux de gaz inclus dans la glace de mer a partir de mesures de densite de salinite et de temperature dans des gammes de temperatures entre 2 c et 30 c inversement ces relations peuvent etre utilisees pour calculer la densite de la glace de mer en fonction de la temperature et de la salinite en prenant en compte a la fois la presence de gaz et de cristaux de sel dans la glace on donne aussi des equations qui permettent le calcul du volume de gaz et d eau salee dans la glace a des temperatures autres que celle pour laquelle la densite a ete determinee

a magmatic geohygrometer application to mount st helens and other dacitic magmas compositions of andesitic to dacitic melts in equilibrium with plagioclase orthopyroxene and clinopyroxene have been experimentally calibrated as a geohygrometer application to melson s data on melt inclusions and matrix glasses from the 1980 1982 cycle of eruptions of mount st helens reveals systematic dewatering trends ca 4 1980 to 1 1982 h 20 in subliquidus intratelluric melt in pyroclastics and 2 3 1980 to 1 1982 in subliquidus melt in dome building dacites before each pyroclastic event intratelluric crystallization achieved melt h 20 contents of ca 6 at a depth of at least 8 km a necessary condition for crystallization of amphibole dewatering trends indicate that individual magma blobs or ribbons then ascended to successively higher levels over the 3 yr cycle from a minimum depth of 4 km in 1980 to 0 35 km in 1982 and pyroclastically erupted comparison of mount st helens compositions to worldwide andesites and dacites suggests that near liquidus arc magmas that crystallize plagioclase and pyroxenes contain l 2 h 2 0 on average whereas magmas in continental margins contain appreciably more h2o

the influence of ocean flow on newly forming sea ice 1 the heat and mass balance of the arctic ocean is very sensitive to the growth and decay of sea ice and the interaction between the heat and salt fields in the oceanic boundary layer the hydraulic roughness of sea ice controls the detailed nature of turbulent fluxes in the boundary layer and hence is an important ingredient in model parameterizations we describe a novel mechanism for the generation of corrugations of the sea ice ocean interface present a mathematical analysis elucidating the mechanism and present numerical calculations for geophysically relevant conditions the mechanism relies on brine flows developing in the sea ice due to bernoulli suction by flow of ocean past the interface for oceanic shears at the ice interface of 0 2 s 1 we expect the corrugations to form with a wavelength dependent upon the permeability structure of the sea ice which is described herein the mechanism should be particularly important during sea ice formation in wind maintained coastal polynyas and in leads this paper applies our earlier analyses of the fundamental instability to field conditions and extends it to take account of the anisotropic and heterogeneous permeability of sea ice

hydrocarbons from near surface sediments of the barents sea north of svalbard indication of subsurface hydrocarbon generation abstract the barents sea is considered as an important target for oil and gas exploration but the petroleum potential of its shelf and slope regions is unknown here we present results of a research cruise to the northern hinlopen margin at the transition to the southern nansen basin and the eastern yermak plateau multichannel reflection seismic data acquisition heat flow measurements and geochemical analyses of near surface sediments obtained by gravity coring were conducted to study the northern barents sea shelf and the early opening of the nansen basin and decipher their petroleum potential seismic data indicate high thicknesses of up to 2000 m of cenozoic sediments heat flow density values in the study area range between 67 and 108 mw m 2 the sediment samples were analysed for bulk geochemistry and sorbed hydrocarbon gases and for two sites for extractable hydrocarbons data from extractable n alkanes n c 25 and bulk hi and oi from rock eval organic matter demonstrate predominantly terrigenous organic material most likely derived from ice transported allochthonous sediments none of the sediments revealed substantial amounts of methane in pore waters arguing against active hydrocarbon seepage in the studied areas however thermogenic gases sorbed to the sediment matrix clay minerals organic matter and or carbonates were found in concentrations of up to 600 ppb on sediment wet wt basis for the samples from the northern hinlopen margin and particularly from the adjacent nansen basin a paleo fluid flow of thermogenic gas is indicated and accompanied by higher n alkanes with a modal petroleum like distribution 13 c values of methane ethane and propane and gas compositions point at a mainly marine source rock origin of all studied gases with early oil window maturities of the associated rocks 0 6 0 9 vr from this data an admixture of type iii derived thermogenic gases is indicated for some of the yermak plateau sediments for which also the lowest abundances of sorbed gases 50 100 ppb were observed gas geochemical characteristics in the samples with low gas abundances can partially be explained by an input of gases through ice transport of allochthonous hydrocarbons which were bound to mature organic matter for a site on the northern hinlopen margin ne of svalbard right at the southern termination of the nansen basin a different situation is indicated in this area the highest concentrations of sorbed gases most likely derived from sediments with an early oil window maturity and a marine kerogen type ii typical isotopic distribution at this location a pseudo well was constructed from 2d seismic data for reconstruction of thermal and maturity evolution the simulation results indicate that an early to middle eocene source rock would be in the early oil window since the early miocene a possible source rock here and in the circum arctic region could have been formed by azolla algae and other flourishing primary producers

distribution of subsurface hydrocarbon seepage in near surface marine sediments hydrocarbon seeps in surficial marine sediments are of two types active and passive active seeps occur where gas bubbles pockmarks or bright spots are visible on seismic profiles and where chemosynthetic communities are present in conjunction with large concentrations of migrated hydrocarbons macroseeps these generally occur where generation and migration of hydrocarbons from source rocks are ongoing today at maximum burial or where significant migration pathways have developed from recent tectonic activity passive seeps occur where concentrations of migrated hydrocarbons are usually low microseeps with few or no geophysical anomalies these occur typically in areas where generation and expulsion is relict no longer at maximum burial or regional seals prevent significant vertical migration the type of seepage controls the distribution of migrated hydrocarbons in the near surface sediments and should dictate the sampling equipment and approach used to detect seeps active seeps are usually detected near the water sediment interface in the water column or at the sea surface and at relatively large distances from major leak points most conventional sediment and water samplers can capture active seeps the gulf of mexico santa barbara channel and parts of the north sea have active hydrocarbon seeps passive seeps can only be detected relatively far below the water sediment interface and require samples to be collected near leak points sampling equipment must penetrate the zone of maximum disturbance or any shallow migration barriers in areas where surficial sediments are coarse grained or compacted conventional gravity corers will not work other options for subsurface sampling include vibracores jet cores and rotary cores precise location of samples site specific using seismic profiles to locate leak points is critical to detect passive hydrocarbon seeps the beaufort and bering seas offshore alaska and parts of the north sea contain passive seeps

development and properties of sea ice in the coastal regime of the southeastern weddell sea from october to december 1986 a program consisting of sea ice core analysis in combination with sea ice observations was carried out from the icebreaker r v polarstern as part of the winter weddell sea project the ship operated in the central and southeastern weddell sea with interests focusing on the ice shelf front between 70 s and 77 s where a system of coastal polynyas extended for several tens to hundreds of kilometers opened and maintained despite temperatures well below freezing by offshore wind forcing three wind controlled ice formation regimes may be distinguished 1 for wind speeds 10 m s the ice cover is heavily ridged and rafted 50 and formation of granular ice predominates of the total length of cores sampled in the coastal regime 32 consisted of columnar and 35 of granular ice in addition we observed two unique ice classes along the coast platelet ice formed through consolidation of underwater ice platelets underneath the solid ice cover contributed 11 to our core samples another 22 consisted of intermediate columnar granular ice which originates under the dynamic hydrographic conditions of the near ice shelf region

solidification of leads theory experiment and field observations thin sea ice plays a central role in the surface heat and mass balance of the arctic ocean in order to develop understanding of these balances we describe and analyze highly resolved temperature data taken through the air sea ice interface during the transition from an ice free to an ice covered arctic ocean surface the data were taken to observe the thermodynamic evolution of a lead a process that has previously only been accessible to measurement techniques confined to the lead edge our detailed analysis of the field data is guided by recent theoretical and experimental advances in understanding the phase dynamics of directionally solidified alloys because of the dearth of direct observations we also present time series of the relevant heat fluxes inferred from our data and demonstrate the controlling influence that the internal phase evolution has on these quantities we have previously examined the stability of the brine trapped in a growing sea ice matrix both theoretically and experimentally and now find that haline convection driven from within the growing layer is consistent with this previous work and with the nature of direct turbulence measurements the importance of this process is that although ice growth is continuous the local brine flux commences abruptly only after some time in contrast to what had previously been supposed hence the ice growth process itself is a source of intermittency in oceanic boundary layer turbulence furthermore we find that in this particular situation the sea ice growth is not simply a square root function of time in contrast to the model typically used in numerical simulations by far the most practical methods of studying lead convection are numerical simulations and laboratory models and a strong conclusion of this study is the importance of the proper treatment of the boundary conditions describing the buoyancy flux

evolution of the iceland hotspot mantle geochemical enrichment beneath the iceland faeroes hotspot has been episodic suggesting that blobs rather than a continuous plume may be rising in the region recurring rift propagation southward from central iceland causing progressive melting of the lower crust followed by the upper mantle best explains the geochemical variations found along the south west and south east neovolcanic zones and their degree of thermal maturity

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

high and low pressure phase equilibria of a mildly alkalic lava from the 1965 surtsey eruption implications for the evolution of mildly alkalic and transitional basalts in the south eastern propagating rift zone of iceland abstract the south eastern propagating rift zone of iceland shows a progression from tholeiitic to transitional and mildly alkalic basalts going toward the front of the propagator a petrogenetic model has been formulated based on low and high pressure melting experiments the evolution of the magmas behind the front of the propagating rift is dominated by near surface processes e g katla and hekla volcanic systems compared with the one atmosphere liquid line of descent the vestmannaeyjar lavas erupted at the front of the propagating rift have systematically higher al2o3 and na2o contents consistent with the experimentally determined effects of high pressure labradoritic plagioclase megacrysts which occur in the early phase of the surtsey eruption at the front of the propagator are consistent with the predicted effect of high pressure on plagioclase composition on the other hand augites similar to those of the high pressure experiments are unknown among the phenocryst and xenolith assemblages of the vestmannaeyjar lavas and other icelandic lavas petrographic evidence points toward a high water content in the evolved lavas of the vestmannaeyjar a relatively high water activity and high pressure in the magma chambers at the front of the propagator could have caused a significant suppression of the liquids temperature in particular for plagioclase seismic and magnetotelluric evidence suggest that magma chambers behind the propagating front occur at the depth equivalent to 2 3 kbar of pressure at the front magma chambers have been located by geophysical evidence at significantly greater depths equivalent to 3 8 kbar the lavas erupted at the front of the propagator are located to the alkalic side of the thermal divide and therefore can be expected to evolve toward nepheline saturation under slightly hydrous conditions the most evolved of these lavas are of ferrobasaltic compositions and may be saturated with augite the lavas erupted behind the front are located to the transitional side of the divide and evolve toward quartz saturation under essentially anhydrous conditions in contrast to the vestmannaeyjar lavas the lavas behind the propagator often contain augite as a phenocryst it is concluded that the chemical variation observed along the south eastern propagating rift of iceland is an effect of primary chemical features and that water plays a significant role on the liquid lines of descent at the front of the propagator

thermal evolution of permeability and microstructure in sea ice 1 the fluid permeability k of sea ice constrains a broad range of processes such as the growth and decay of seasonal ice the evolution of summer ice albedo and biomass build up such processes are critical to how sea ice and associated ecosystems respond to climate change however studies of k and its dependence on brine porosity and microstructure are sparse here we present a multifaceted theory for k which closely captures laboratory and field data x ray computed tomography provides an unprecedented look at the brine phase and its connectivity we find that sea ice displays universal transport properties remarkably similar to crustal rocks yet over a much narrower temperature range our results yield simple parameterizations for fluid transport in terms of temperature and salinity and permit more realistic representations of sea ice in global climate and biological models

modeling the transport of passive tracers in sea ice 1 a model for the vertical transport of passive tracers in sea ice is presented two reynolds flux closure approximations are proposed for the tracer velocity dispersion term in the new ice tracer model icet the schemes enhanced molecular diffusion emd and mixing length diffusion mld are suggested parameterizations of gravity drainage the dominant mechanism of desalination in growing sea ice the performance of icet using each parameterization is tested against two experimental studies simulations i compare profiles of brine salinity with passive tracer concentrations during sea ice growth and melt and simulations ii compare bottom boundary salt and brine volume fluxes with passive tracer fluxes and bottom diffusivities icet behavior is evaluated under three salinity forcing prescriptions both schemes successfully reproduce expected tracer concentrations in simulations i when the salinity forcing evolves logarithmically however in simulations ii the mld scheme outperforms emd in two significant ways 1 growth rate trends in the mld diffusivity parallel measurements of brine volume flux while emd bottom diffusivities remain almost constant and 2 mld bottom tracer concentrations decrease with ice growth rate similar to excluded brine salinity while emd trends are contrary hence enhanced molecular diffusion is highly questionable for implementation in models of reactive passive tracers the icet mixing length diffusivity is a viable choice in sea ice biogeochemical modeling it remains a challenge to assess whether mld can provide quantitative estimates of desalination at each time step for use in forcing icet or for implementation in a model of sea ice salinity

surface sediment hydrocarbons as indicators of subsurface hydrocarbons field calibration of existing and new surface geochemistry methods in the marco polo area gulf of mexico multiple methods are currently used to collect prepare extract and analyze near surface migrated hydrocarbons from marine sediments to evaluate subsurface petroleum generation and entrapment few have been rigorously tested to evaluate their effectiveness a gulf of mexico field calibration survey over the marco polo field was undertaken as part of an industry funded research project to better understand previously published and unpublished seabed geochemical results and determine which gas and liquid hydrocarbon extraction methods best characterize migrated hydrocarbons in near surface sediments the marco polo calibration data set demonstrates the importance of targeted coring and sampling depth to improve the detection of seabed migrated thermogenic hydrocarbons core samples should be collected along major migration pathways cross stratal leakage features identified by conventional deep seismic and high resolution sea floor imaging not all targeted cores hit the designated feature and thus collecting replicates along key migration features is critical collecting sediment samples below the near surface transition zone known as the zone of maximum disturbance is also important to avoid possible alteration effects and interference by recent organic matter geochemical analysis should include a full range of hydrocarbon types light hydrocarbon gases c1 c5 gasoline range c5 c10 and high molecular weight hmw hydrocarbons c15 the interstitial sediment gas data should be plotted on a total hydrocarbon gas c1 c5 versus wet gas fraction c2 c5 c1 c5 chart to identify background fractionated and anomalous populations compound specific isotopic analysis on selected anomalous samples is critical to correctly identify migrated subsurface gases from near surface generated microbial gases microdesorption bound gases did not provide gas compositions or compound specific isotope ratios similar to the marco polo reservoir gases and thus the bound gas extraction is not recommended a gasoline range analysis provides a new range of hydrocarbons rarely examined in surface geochemical studies that assist in identifying thermogenic hydrocarbons extraction gas chromatography and total scanning fluorescence tsf maximum fluorescence intensity provided information on the presence of thermogenic hmw hydrocarbons but did not work as well with the low level microseepage samples the tsf fluorogram signature was similar for both seep and regional reference background samples and did not help to identify migrated thermogenic hydrocarbons the marco polo calibration study provides a framework to better understand how best to collect targeted deep cores and extract migrated hydrocarbons from near surface marine sediments and to evaluate the results

data processing and recognition of seepage and microseepage anomalies of acid extractable hydrocarbons in the south slope of the dongying depression eastern china abstract the successful use of near surface anomalous signals exhibited in all petroliferous basins must lead to great development in petroleum exploration the elementary equations and methods for objective and accurate separation of anomalous signals from background signals are the most critical requirements in these surface geochemical surveys therefore we have worked up a series of fundamental equations and methods for eliminating surface interference and influences of geological conditions on anomaly intensities and recognizing multi type anomalies of uni and multi variates by using statistics fractal geometry wavelet analysis and artificial neural networks these equations illustrate that the threshold i e the boundary between background and micro seepage anomalies is determined by means standard deviations and prior probabilities of background and micro seepage anomalies when multi normality is met and that the traditional equation for the threshold background mean plus one or two standard deviations is not correct in the face of many new equations and methods it is important how to comprehensively use them to resolve the problems in geochemical surveys a geochemical survey in the south slope of the dongying depression eastern china provides a good opportunity to address this issue the anomalies obtained with the traditional equation do not reflect oil gas fields this study reveals three problems in the previous data processing and anomaly recognition 1 the surface interference caused by the variation of soil composition was not identified and removed 2 the influence of caprock thickness on anomaly intensities was not revealed and eliminated and 3 the micro seepage related to oil gas pools and seepage related to faults anomalies were not separated and correctly recognized furthermore the interaction of these problems makes the data processing and anomaly recognition more difficult to ensure effective elimination of the surface interference and caprock thickness influence and correct recognition of uni variate anomalies we iteratively applied the wavelet analysis based methods to eliminate the surface interference and caprock thickness influence and statistical methods with new fundamental equations to recognize uni variate anomalies of seepage and microseepage and then multi variate anomalies of seepage and microseepage were recognized with our fundamental equations and the corresponding methods in the results the seepage anomalies display a string bead shaped pattern and are distributed along faults and the micro seepage anomalies are ring shaped and coincide with oil gas pools sandbodies or traps therefore reprocessing of geochemical data with our fundamental equations and methods combined in an iterative manner can resolve the complicated problems in data processing and anomaly recognition and thus greatly improve the predictive capability of surface geochemical survey

marine seepage variability and its impact on evaluating the surface migrated hydrocarbon seep signal abstract detection of chemically identifiable hydrocarbon oil and gas seepage is an important tool in offshore exploration generated and or trapped oil and gas can leak to near surface sediments and ocean in detectable concentrations the seepage surface expression is controlled by hydrocarbon generation regional pressure regimes and migration pathways the migrated hydrocarbon concentration and composition varies due to seepage rates and near surface physical and biological processes that can alter the signal a seepage continuum exists from large volume oil and gas seeps known as macro seepage to lower volume seeps known as micro seepage only detectible with special sampling and analysis understanding the impact of migrated hydrocarbon concentration relative to in situ organic material and locally derived microbial hydrocarbons is critical this paper describes factors that control the near surface geochemical signal as well as the challenges in deciphering the migrated hydrocarbon signature to evaluate subsurface hydrocarbon generation and entrapment

surface sediment gases as indicators of subsurface hydrocarbons examining the record in laboratory and field studies abstract near surface sediment gases are commonly examined in surface geochemistry surveys to evaluate subsurface hydrocarbon generation and entrapment sediment gases reside in interstitial spaces bound to mineral or organic surfaces and or entrapped in carbonate inclusions characterizing migrated sediment gases with minimal compositional and isotopic fractionation is challenging and different methods are currently in use to extract and analyze surface sediment gases these methods have not been compared in controlled laboratory experiments the key to calibration experiments is to know the charge gas composition and thus have a basis to evaluate the sediment gas extraction procedure for this laboratory study marine sediments were homogenized and cleansed of in situ light hydrocarbons and bacteria replicate sediment samples were prepared and charged with two different hydrocarbon gas compositions and two concentration levels along with the requisite blank control samples the sediment gases were characterized by four laboratories using a variety of gas extraction procedures three headspace interstitial two adsorbed acid extraction and one ball mill occluded the results have been compared to the charge gases and each method evaluated the laboratory calibration experiments demonstrated that selected interstitial gas extraction methods examined do provide sediment gas compositions and compound specific isotopes similar to the charge gases however some of the results indicate that systematic sampling and or laboratory procedures associated with the adsorbed and ball mill gas extraction methods have altered the original charge gas compositions thus it is our recommendation to use a headspace interstitial gas extraction method in conjunction with a tight sealing storage container and water preparation procedure to best extract and analyze migrated sediment gases

numerical simulations of the profile properties of undeformed first year sea ice during the growth season a simulation scheme is developed that estimates salinity profiles for first year sea ice during the growth season as a function of the growth history of the ice the model considers the dependence of the initial ice salinity on ice growth velocity and sea water salinity and also the subsequent drainage of brine from the ice the equation for ice growth assumes a linear temperature profile within the ice and is driven by surface heat balance equations that are based on smoothed climatic data for the central arctic basin the estimated salinity profiles are in good agreement with natural profiles although temperature and salinity profiles depend upon the time of the year when ice growth is initiated the brine volume profiles which they specify are essentially a unique function of ice thickness a conclusion that holds even when the insulative effects of snow are considered the temperature and brine volume profiles are then utilized to calculate the ice strength and elastic modulus profiles which in turn specify the composite mechanical properties of the ice sheets through the application of a theory developed by assur 1967 significant differences which are largest for thin ice sheets are observed between ice sheet properties as calculated using composite plate theory and properties calculated from uniform plate theory and average ice properties these results provide a justification for the practice common within the ice modeling community of parameterizing the mechanical behavior of pack ice on the basis of the ice thickness distribution

the petrology of basalts from surtla surtsey iceland abstract surtla is the entirely submarine volcanic pile of a satellite vent which was active shortly after initiation of the main surtsey eruption blocks of lava and glassy elastic deposits collected from surtla are of mildly alkaline olivine basalt and are compositionally the most evolved material determined from the surtsey volcano petrographical and chemical analyses confirm the previously postulated continuum between the evolved derivative and more primitive parental compositions widely occurring in the vestmannaeyjar volcanic system but not hitherto found in a single eruption a compositionally zoned magma chamber is invoked

geology of the heimaey volcanic centre south iceland early evolution of a central volcano in a propagating rift geology of the heimaey volcanic centre south iceland early evolution of a central volcano in a propagating rift

crustal xenoliths in the 6220 bp saefell tuff cone south iceland evidence for a deep diatreme forming surtseyan eruption abstract three types of xenoliths were collected from a 80 m high vertical section of the saefell tuff cone on heimaey iceland these types are alkali basaltic type i sedimentary type ii and cpx bearing basalts type iii they are distinct in hand specimen thin section and geochemistry near the base of the eruptive succession only juvenile material and type i xenoliths are present the middle section has type ii xenoliths in addition to type i xenoliths of type iii first appear in the uppermost xenolith rich horizons where they are found together with all other types juvenile saefell clasts are present in various amounts throughout the investigated section a downward migration of explosion foci during the eruption is established based on the distribution of the different types of xenoliths in the tuff cone i e the appearance of type i type ii type iii xenoliths with increasing stratigraphic height in combination with the observed relations in basement stratigraphy beneath heimaey in the beginning of the eruption a combination of high eruption rate and abundant seawater resulted in shallow seated explosions depositing only juvenile material and type i xenoliths the downward migration of explosion foci is suggested to be the result of lowered influx of seawater due to the formation of a cone that reached 50 75 m a s l in combination with lowered eruption rate as the basement stratigraphy beneath heimaey is well documented by a 1565 m deep drill hole the depth of the explosions can be constrained to vary between less than 170 m in the beginning of the eruption and exceeding 820 m in the final phases this is much deeper than previously reported for any surtseyan tuff cone forming eruption and consistent with the downward penetration of a diatreme

a non destructive method for measuring the salinity and solid fraction of growing sea ice in situ we describe an instrument developed to make in situ measurements of salinity and solid fraction profiles in growing sea ice the vertical resolution of the measurements is up to a few millimeters with a temporal resolution of up to fractions of a second the technique is based on impedance measurements between platinum wires around which sea ice grows data obtained using this instrument in laboratory experiments are in good agreement with theoretical predictions in a field test in the arctic the bulk salinity of growing sea ice has been measured in situ throughout the whole depth of the ice layer the data are compared with bulk salinities obtained from ice cores and confirm the general understanding that the bulk salinity in ice core studies is significantly underestimated in the lower parts of the cores the approach can also be used in other glaciological applications and for general studies of two phase two component porous media

thermal conductivity of landfast antarctic and arctic sea ice 1 we present final results from a program to measure the thermal conductivity of sea ice with in situ thermistor arrays using an amended analysis of new and previously reported ice temperatures results from landfast first year fy ice near barrow alaska and mcmurdo sound antarctica are consistent with predictions from effective medium models but 10 15 higher than values from the parameterization currently used in most sea ice models we observe no previously reported anomalous near surface reduction which is now understood to have been an artifact nor a convective enhancement to the heat flow although our analysis is limited to temperatures below 5c at which brine percolation is restricted results for landfast multiyear my ice in mcmurdo sound are also consistent with effective medium predictions and emphasize the density dependence we compare these and historical measurements with effective medium predictions and the representation commonly used in sea ice models developed originally for my arctic ice we propose an alternative expression derived from effective medium models appropriate for both my and fy ice that is consistent with

sea ice salinity and structure a winter time series of salinity and its distribution 1 we present a winter time series of antarctic sea ice salinity from eastern mcmurdo sound an area close to an ice shelf where a subice platelet layer forms below the sea ice late in winter this dramatically changes the sea ice structure as the sea ice grows into the subice platelet layer every 2 weeks during the 5 months of sea ice formation salinity profiles were measured along with detailed measurements of ice structure and growth rates once the influence of growth rate on sea ice bulk salinity is removed the data from 69 cores and the results of a basic parameterization demonstrate that bulk salinity for platelet ice is higher than that for columnar sea ice we also present measurements of the salinity profile close to the ice water interface and use these to investigate the expected regime of fluid flow within the permeable portions of the sea ice with particular reference to mushy layer and percolation theory finally we provide a new distribution of sea ice salinity from 740 measurements which can be interpreted as the sum of two spatial fields that we attribute to sea ice samples with and without brine channels and which should be reproduced by any realistic sea ice models this distribution suggests that two measurements of quantities linearly linked to sea ice salinity must differ by 29 if they are to be considered different with 90 confidence

observations of platelet ice growth and oceanographic conditions during the winter of 2003 in mcmurdo sound antarctica 1 platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth or loosely attached to the ice water interface related to the proximity of ice shelves it is known to form in supercooled seawater this study details the conditions for its growth during the austral winter of 2003 in mcmurdo sound antarctica a key finding is that the presence of platelet ice in the sea ice cover is conclusively linked to the time history of the appearance of ice crystals in the water column monitored using the strength of the backscattered signal from an acoustic doppler current profiler as a proxy generally these crystals appeared from mid may as water near the interface became supercooled this near surface supercooling appears to be related to a simultaneous abrupt change in the structure of the upper 250 m of the water column from one that behaved dynamically and contained both the warmest and coolest measured temperatures to one in which the water was essentially isothermal near its surface freezing temperature near surface ocean temperature was also affected by tidal mixing and by an increase in the thickness and density over the course of winter of the surface mixed layer created by salt rejection during ice growth these processes allowed cold water trapped by buoyancy in a band at the base of the mixed layer in early winter to gain access to the ice water interface by midwinter this band of cold water probably had its origin beneath the mcmurdo ice shelf

phase relations in transitional and alkali basaltic glasses from iceland basaltic glasses from the three alkalic areas of iceland snaefellsnes volcanic zone sudurland volcanic zone and vestmannaeyjar volcanic area contain plagioclase olivine clinopyroxene chromian spinel and titanomagnetite as phenocryst phases the glasses are hypersthene to nepheline normative alkali basaltic with feo mgo ratios between 1 4 4 7 olivine ranges in composition from fo90 to fo55 plagioclase from an90 to an50 and clinopyroxene from en45fs10wo45 to en40fs17wo43 clinopyroxene reveals systematic ti al metastable crystallization trends related to the composition of the enclosing glass two types of phenocryst are present in most glasses and show a bimodality in size and composition microphenocryst phases are those most likely to have crystallized from the enclosing glass while macrophenocrysts may have crystallized from a liquid of slightly less evolved composition the glasses show complex phenocryst glass relations which can be related to a polybaric effect the normative glass compositions are related to 2 phase cotectic surfaces in the basalt tetrahedron and define the position of the 3 phase cotectic line in general with increasing feo mgo in the glass the phenocryst assemblages vary from clinopyroxene olivine and plagioclase along a clinopyroxene olivine surface to olivine and plagioclase along an olivine plagioclase surface the normative glass compositions show a deflection from clinopyroxene bearing to clinopyroxene free glasses the appearance of plagioclase together with clinopyroxene and olivine can be explained in the light of experimental investigations of the effect of pressure on phase relations the major element variation of the glasses is interpreted as representing mantle derived magma batches of primary liquids modified to some degree by high 6 kbar and intermediate to low pressure below 3 kbar crystal fractionation towards equilibrium phase relations during ascent and residence in crustal magma chambers the observed deflection in normative compositions of the glasses marks the position of the high pressure 3 phase cotectic line the bimodality in size and composition of plagioclase and olivine phenocrysts can be related to high pressure crystal fractionation in the melt the fe ti basalt glasses from sudurland are believed to be quenched high pressure compositions

desalination processes of sea ice revisited 1 we reexamine five processes that have been suggested to be important for the loss of salt from sea ice these processes are the initial fractionation of salt at the ice ocean interface brine diffusion brine expulsion gravity drainage and flushing with surface meltwater we present results from analytical and numerical studies as well as from laboratory and field experiments that show that among these processes only gravity drainage and flushing contribute to any measurable net loss of salt we show that during ice growth the salinity field is continuous across the ice ocean interface hence there is no immediate segregation of salt at the advancing front

eruption reconstruction formation of flow lobe tumuli and eruption duration in the 5900 bp helgafell lava field heimaey south iceland abstract the 5900 bp helgafell eruption started with an intense fire fountaining phase that deposited scoria and spatter near the vent spatter accumulated agglutinated and was subsequently emplaced as clastogenic flows the intense initial phase was followed by effusive emplacement of tube fed pahoehoe this type of lava emplacement makes up the bulk of the lava field and contains abundant inflation features such as tumuli and pressure ridges we measured the depths of 358 inflation clefts in tumuli and the thicknesses of the upper vesicular crust in flow lobes from the helgafell lava field measurements of altitude between a point source of lava tubes and the depth of inflation clefts in tumuli suggest that the magmastatic pressure affects initial tumuli formation the overall growth of tumuli is however more dependent on effusion rate and collapse clogging of individual tubes than on the magmastatic head within tubes three areas with abundant tumuli are recognized in the helgafell lava field each of which was active for at least 47 days the average time for tumuli formation is 32 days using the crustal measurements of inflation features we test the hypothesis that the total eruption duration can be estimated for a pre historic volcanic eruption we chose the helgafell lava field to test this hypothesis because it has 1 abundant inflation features that are easily measured and 2 the majority of lava flows emplaced during the eruption are well exposed our result shows that the duration of the helgafell eruption can be estimated to 11 12 months yielding an average volumetric effusion rate for the helgafell eruption of 0 05 0 06 km3 month this result is very similar to the effusion rates for two known eruptions in the same volcanic system i e the 1963 1967 surtsey and the 1973 eldfell eruptions with effusion rates of 0 02 and 0 04 km3 month respectively given that all three eruptions occurred within the same volcanic system and that they have similar volumetric effusion rates we suggest that our estimated eruption duration for helgafell is reasonable our results building on the suggestion by hon et al 1994 hon k kauhikaua j denlinger r mackay k 1994 emplacement and inflation of pahoehoe sheet flows observations and measurements of active lava flows on kilauea volcano hawaii geol soc amer bull 106 351 370 that the duration of inflation can be estimated for individual flows by measuring inflation features show that by combining abundant and measureable inflation features with a well established stratigraphy the duration of an entire pre historic eruption can be estimated

formation and structure of refrozen cracks in land fast first year sea ice 1 this study characterizes the healing process and structure of undeformed linear parallel sided flooded cracks in land fast sea ice field investigations and refreezing experiments were performed in mcmurdo sound ross sea antarctica between 1998 and 2002 data from a two dimensional thermistor array are used to show that the ice water interface of freezing cracks is arch shaped due to bidirectional heat flow to the surrounding ice cover and to the atmosphere ice growing laterally into the crack is found to desalinate over a prolonged period of time until the isotherms are approximately horizontal superposition of heat flow to the atmosphere and to the host sea ice sheet allows the refreezing progress to be modeled analytically close to the ice air interface the salinity is higher at the sides of wide refrozen cracks than it is at the center however deeper down and in narrower cracks in general the salinity is higher at the center than at the sides a finite volume computational fluid dynamics cfd model reproduces the generally arch shaped alignment of brine pockets this pattern is attributed to convection in the mushy layer crystals are found to grow upstream into the crack due to a salinity gradient in the buoyant convective flow

iceland and the ocean floor comparison of chemical characteristics of the magmatic rocks and some volcanic features the volcanism in iceland occurs on both rift zones and non rifting zones the rift zone volcanism produces rock suites of the tholeiitic series ranging from primitive tholeiites morb to highly silicic rocks the non rifting volcanic zones produce rock suites of transitional to mildly alkaline or even calc alkaline composition the basalts typically being feti basalts over 50 per cent of the rift zone production in iceland is of the primitive morb type the relative amount of the evolved rock types as well as the total volcanic production increases inland along the rift zones the rock types of the icelandic rift zones are identical to those of the submerged oceanic rift zones but the icelandic production is somewhat offset towards a more voluminous evolved end the skewness in volume relations of rock types as compared to the oceanic rifts in general is mainly caused by the products of the non rifting volcanic zones of iceland and only to a small degree by the rift zone products

annual cycle of landfast sea ice in prydz bay east antarctica 1 under the chinese national antarctic research expedition program in 2006 the annual thermal mass balance of landfast ice in the vicinity of zhongshan station prydz bay east antarctica was investigated sea ice formed from mid february onward and maximum ice thickness occurred in late november snow cover remained thin and blowing snow caused frequent redistribution of the snow the vertical ice salinity showed a question mark shaped profile for most of the ice growth season which only turned into an i shaped profile after the onset of ice melt the oceanic heat flux as estimated from a flux balance at ice ocean interface using internal ice temperatures decreased from 11 8 3 5 w m 2 in april to an annual minimum of 1 9 2 4 w m 2 in september it remained low through late november in mid december it increased sharply to about 20 0 w m 2 simulations applying the modified versions of stefan s law taking account the oceanic heat flux and ice atmosphere coupling compare well with observed ice growth there was no obvious seasonal cycle for the thermal conductivity of snow cover which was also derived from internal ice temperatures its annual mean was 0 20 0 04 w m 1 c 1

growth rate and salinity profile of first year sea ice in the high arctic this paper describes the growth of sea ice and the salinity profiles observed in eclipse sound near pond inlet baffin island canada during the winter of 1977 78 a numerical method of calculation has been developed to incorporate the variations in snow conditions and physical properties of ice and snow during the growth season it is shown that the growth rate can be predicted reasonably well it is also shown that the vertical salinity profile in the ice towards the end of the season provides a record of previous climatological conditions a dependence has been shown between the predicted growth rate and the measured salinity

structure and eruptive mechanisms at surtsey volcano iceland glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of surtsey volcano despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island the tuff includes abundant accretionary lapilli and tuff vesicles indicating that it was all deposited subaerially during the growth of the tuff rings repeated hydromagmatic explosion cycles began with a series of intermittent tephra finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours this nozzle like continuous activity produced eruption columns 100 250 m in diameter and 500 2000 m in height which probably quarried several hundred metres below the ground surface the continuous uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down during construction of the tuff rings concentric faults repeatedly downdropped a funnel like structure 400 800 m in diameter several hundred metres thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level ring dykes later intruded upward along these faults and fed small lava flows heat in the surface tephra probably originated primarily from these shallow intrusions

brine fluxes from growing sea ice it is well known that brine drainage from growing sea ice has a controlling influence on its mechanical electromagnetic biological and transport properties and hence upon the buoyancy forcing and ecology in the polar oceans when the ice has exceeded a critical thickness the drainage process is dominated by brine channels liquid conduits extending through the ice we describe a theoretical model for the drainage process using mushy layer theory which demonstrates that the brine channel spacing is governed by a selection mechanism that maximizes the rate of removal of stored potential energy and hence the brine flux from the system the fluid transport through the sea ice and hence the scaling laws for brine fluxes and brine channel spacings are predicted importantly the resulting brine flux scaling is consistent with experimental data for growth from a fixed temperature surface allowing all parameters in the scaling law to be determined this provides an experimentally tested first principles derivation of a parameterization for brine fluxes from growing sea ice

structural and stratigraphic controls on subsurface fluid flow at the veslemoy high sw barents sea abstract subsurface and seafloor fluid flow anomalies are gaining large interest after the finding of five new hydrocarbon discoveries and observation of large gas flares in the sw barents sea in the present study we have analysed structural and stratigraphic controls on fluid flow towards the seafloor using gravity cores selected based on subsurface gas anomalies observed on seismic data from the veslemoy high sw barents sea the subsurface fluid flow at the veslemoy high is observed to be controlled by 1 the morphology and orientation of regional faults structural highs and sedimentary basins 2 the presence of paleocene silica ooze deposits that changes microstructure with temperature thereby controlling fluid flow and 3 the location of regional and local open faults formed by glacial loading and unloading analysis of extractable organic matter in subsurface holocene sediments corroborates the active migration pathways inferred from seismic data micropalaeontological studies on benthic foraminifera reveal methane seep associated assemblages that confirm the interpretation of subsurface gas anomalies in seismic data we ultimately link these new results to the geological evolution history of the region to give a comprehensive model for the fluid flow system within the study area

vacuum desorption of light hydrocarbons adsorbed on soil particles a new method in geochemical exploration for petroleum light hydrocarbons in soils are the most important indicators in geochemical hydrocarbon exploration however the methods actually used to detect light hydrocarbons in soil sometimes provide data with a low signal to noise ratio and or high level of interference this research explores vacuum desorption a new method for light hydrocarbon analysis vacuum desorption is a partial extraction technique designed to identify light hydrocarbons that are tightly adsorbed on soil particles this method was used in the huimin sag basin of east china microseepage anomalies are developed over oil pool s105 of the shanghe field in this basin and larger seepage anomalies are developed over faults in contrast to the method of acid extraction of soil hydrocarbons the vacuum desorption method provides a low noise background that enables a more effective identification of anomalies the new method is also cost and time effective and is capable of covering large areas

the origin and accumulation of multi phase reservoirs in the east tabei uplift tarim basin china abstract a giant deep strata oil gas field with complex fluid characteristics was recently discovered in the hade yuke area of the east tabei uplift tarim basin nw china the deep fluids show a lateral co existence of several gas condensate and oil phase reservoirs characterized through the integration of complimentary geochemical e g bulk composition biomarker and stable carbon isotope and geological data typical features of the gas and condensate reservoirs which were mainly distributed in the eastern parts of the study site included high gor gas to oil ratio hydrocarbon gases with relatively heavy 13c values and crude oils with high wax content in contrast oil reservoirs which mostly occurred in more westerly locations had lower gor values lighter 13c values and lower wax content indicating gradual change of fluid characters the petroleum accumulations derive from two hydrocarbon charge events an early oil charge in the late hercynian and a later gas charge in the late himalayan which migrated from east to west leading to a gradual variation in fluid phase and characteristics alteration of paleo oil reservoirs by this secondary gas charge was therefore primarily responsible for the complex fluid character and multi phase reservoirs presently in place secondary condensates were formed from the retrograde condensation of paleo reservoirs our re construction of the hydrocarbon accumulation process encourages future exploration endeavours to target commercially viable gas accumulations in the deep east strata and oil reservoirs in the west

origin of near surface hydrocarbon gases bound in northern barents sea sediments abstract in the southern part of the norwegian barents sea extensive hydrocarbon exploration and drilling has led to several oil and gas discoveries yet little is known on the petroleum systems and potential hydrocarbon reservoirs in the northern norwegian barents sea if hydrocarbons generated by thermogenic processes deep in the subsurface migrate to the surface traces of these hydrocarbons can remain in the near surface sediments and provide indirect information on potential petroleum systems near surface hydrocarbon prospecting is a method frequently used in hydrocarbon exploration of frontier areas where no direct geochemical information from drilling is available to improve knowledge of subsurface structures and the evolution of potential petroleum systems in the northern barents sea seismic data and near surface sediment samples were collected analysis of bound hydrocarbon gases extracted from these sediments revealed concentrations significantly above background in areas along the hornsund knollega fault complex as well as near margins of the olga basin generally the compositional and stable carbon isotope signatures of bound gases indicate thermogenic origins from source rocks of oil window maturity for near surface gases with anomalous high concentrations whereas for sediments with low concentration of bound gases mixture of thermogenic and microbial gas is indicated amount and composition of bound gas extracted from source rock samples from spitsbergen indicate that contribution of transported material may have influenced the bound gas in near surface marine sediment of the barents sea in areas where presence of mature organic matter is indicated in the knolegga fault complex near the western barents sea margin high concentrations of thermogenic gas in near surface sediments are associated with fault bound basins and most likely originate from the paleocene eocene torsk formation in the olga basin higher bound gas concentrations occur near the southern border of the basin corresponding to sub cropping late jurassic early cretaceous shales whereas elevated concentrations in the northern olga basin are associated with reactivated faults reaching close to the surface sediments above the center of the basin show significantly lower bound gas concentrations these observations indicate that the jurassic shales act as a regional seal for hydrocarbons and that reactivated faults at the basin margin represent pathways for migration to the surface 1d basin and petroleum systems modeling for the olga basin indicates that early to middle triassic sediments reached oil window maturity and represent the most likely source for thermogenic near surface gas in that area