high resolution resistivity imaging of marine gas hydrate structures by combined inversion of csem towed and ocean bottom receiver data we present high resolution resistivity imaging of gas hydrate pipe like structures as derived from marine controlled source electromagnetic csem inversions that combine towed and ocean bottom electric field receiver data acquired from the nyegga region offshore norway two dimensional csem inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures located in proximity to a major gas hydrate pipe like structure known as the cne03 pockmark the resistivity model resulting from the csem data inversion resolved the cne03 hydrate structure in high resolution as inferred by comparison to seismically constrained inversions our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean bottom and towed receiver csem data simultaneously the approach applied here can be utilised to map and monitor seafloor mineralisation freshwater reservoirs co2 sequestration sites and near surface geothermal systems

on the resource evaluation of marine gas hydrate deposits using sea floor transient electric dipole dipole methods methane hydrates are solid nonstoichiometric mixtures of water and the gas methane they occur worldwide in sediment beneath the sea floor and estimates of the total mass available there exceed 1016kg since each volume of hydrate can yield up to 164 volumes of gas offshore methane hydrate is recognized as a very important natural energy resource the depth extent and stability of the hydrate zone is governed by the phase diagram for mixtures of methane and hydrate and determined by ambient pressures and temperatures in sea depths greater than about 300 m the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor the fraction of hydrate in the sediment usually increases with increasing depth the base of the hydrate zone is a phase boundary between solid hydrate and free gas and water its depth is determined principally by the value of the geothermal gradient it stands out on seismic sections as a bright reflection the diffuse upper boundary is not as well mar

late anhydrite cements mark basin inversion isotopic and formation water evidence rotliegend sandstone north sea the lower permian rotliegend sandstone in the leman field today contains formation waters which are extremely saline 165 530 238 531 mg l with cation chloride ratios indicating mixing of zechstein marine evaporite brines with meteoric water ancient rotliegend formation waters inferred from the analyses of cements were similarly a mixture of marine brine and meteoric water 18o 0 3 smow d 16 smow the strontium isotopic compositions of present day formation waters 87sr 86sr 0 7107 are also consistent with mixing between late permian marine brines 87sr 86sr 0 707 and radiogenic strontium from a detrital source 87sr 86sr 0 720 0 730 isotopic analyses of texturally late intergranular and fracture filling anhydrite suggest waters very similar to the present day formation waters 34s values of anhydrite cements 10 cdt indicate that sulphate in the rotliegend originated from the stratigraphically overlying zechstein evaporites the 87sr 86sr 150 ratios of anhydrites 0 7080 0 7113 however suggest variable mixing of strontium sourced from the overlying zechstein evaporites with more radiogenic strontium derived from diagenetic reactions in the underlying carboniferous shales the oxygen isotope compositions of anhydrite reflect precipitation temperatures between 120 and 140 c from waters with a 18o composition similar to the present day formation waters approximately 0 smow these are the maximum inferred burial temperatures for the rotliegend sandstone in the leman field and would have occurred at maximum burial depths of 3 5 4 km 11 500 13 100 ft during the late jurassic early cretaceous at the start of basin inversion these data indicate an unusual short lived catastrophic event of cross formational flow through fractures which were tectonically induced by basin inversion pore fluids may have escaped from the overlying and underlying formations into the lower pressure rotliegend sandstone regional aquifer to drain this part of the basin

methane released from groundwater the source of natural gas accumulations in northern west siberia the west siberian basin is the largest continental sedimentary basin on earth and contains about one third of the world s conventional reserves of natural gas from gas compositional and isotope data this gas can be described as a mixture of thermogenic gas from lower cretaceous rocks 13c1 36 c2 10 vol and almost pure methane 98 vol with the relatively light stable carbon isotope signature of 51 13c1 on average the proportion of the isotopically light methane increases from the lower cretaceous to the top of the cretaceous hydrocarbon system top of the pokur formation where the main reservoirs are situated the composition and the huge volume of gas is difficult to explain with the established models of gas generation in sediments the hydrogeology of the basin is determined by thick aquifer systems with an almost basin wide lateral continuity the largest hydrodynamic system is the cretaceous aquifer in which 98 of the natural gas is trapped the groundwater within the cretaceous aquifer is saturated with the same isotopically light methane as that found in the reservoirs of the pokur formation based on petrophysieal data a detailed hydrogeological model of the cretaceous aquifer beneath the largest gas accumulation the urengoy gas field has been developed and the contribution of gas release from groundwater was compared with the volume of gas in the reservoirs the potential methane release from groundwater has been evaluated as a result of 1 cooling of sediments in quaternary times 2 the lateral gradient of the hydraulic pressure under the gas field and 3 the pressure and temperature decrease due to cenozoic uplift the role of quaternary cooling is insignificant and according to our calculations the contribution of the effect by groundwater flow is small the volume of isotopically light methane released from groundwater due to a neogene uplift is very large and can explain the entire gas in place the source of this gas must be located south of the gas field province

geophysical signatures for low porosity can mimic natural gas hydrate an example from alaminos canyon gulf of mexico natural gas hydrate in sand sediments can increase both the measured compressional velocity and resistivity the same geophysical signatures occur however in low porosity sand we investigate the possible occurrence of natural gas hydrate in a sand interval in alaminos canyon block 21 ac 21 in the gulf of mexico drilled by the u s gas hydrate joint industry project the sand interval has an increase in resistivity 2 2 m and a strong peak and trough at the top and bottom of the sand on exploration seismic which has been interpreted as natural gas hydrate we reexamine the logging data and construct a new reservoir model that matches the measured resistivity the high density sublayers in the sand and the surface seismic trace our model shows that the sand interval in ac 21 is most likely water saturated and the slight increase in resistivity higher measured density and the seismic amplitudes are caused by a reduction in porosity to 30 in the sand interval relative to a porosity of 42 in the surrounding marine muds more broadly we show that the average depth where the porosity of marine muds becomes lower than sand sediment is 900 mbsf though it could be as shallow as 600 mbsf for high porosity sands in any case the similar geophysical signatures for water saturated sand and low saturations of natural gas hydrate in sand probably occur throughout the gas hydrate stability zone at most sites worldwide

mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs abstract secondary porosity created by the post depositional leaching of detrital grains and cements can be of great economic importance the term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced in reality there are two end member volumetric outcomes to the leaching process firstly the mineral can be completely dissolved and its components transported out of the system in such an instance the porosity of the rock will be enhanced secondly it is possible that a mineral will decompose and by product minerals will be precipitated which will occupy approximately the same volume as the original mineral the latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock this will occur at elevated temperatures and or where the pore waters have been in contact with the rock for a long time these conditions are commonly met during intermediate to deep burial many metastable minerals such as high temperature feldspars probably decompose in this way a consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity the mass transport equation may be used to examine the physico chemical environment where enhanced porosities most likely occur during leaching by using a simple non equilibrium mass transport model based on the silica system it is possible to demonstrate the relative importance of flow rate reaction rates specific surfaces and temperature in controlling the areal extent of leaching wide leaching zones are related to high flow rates which in turn promote a high throughput of undersaturated fluid and dispersion leaching over a wide area is also favoured by low reaction rates which will occur at low temperatures although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than for instance meteoric water thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface the lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower more intense leaching zones as a result providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance in order to transport undersaturated pore waters appreciable distances in the deep subsurface it is necessary to minimize their interaction with the host rock this could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network

coast effect distortion of marine magnetotelluric data insights from a pilot study offshore northeastern japan abstract we report on strong coast effect distortions observed for broadband marine magnetotelluric mt data collected on the forearc offshore northeastern japan eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple station algorithm yielding good mt responses and inter station transfer functions at periods of 7 10 000 s transverse electric te mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s while the transverse magnetic tm mode responses are galvanically depressed below the te responses an analysis of inter station transfer functions confirms that the apparent resistivity cusps are a magnetic field rather than electric field phenomenon consisting of an amplitude minimum and rapid phase change around a characteristic frequency poynting vectors for a te coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin we show that the characteristic frequency and position of the te mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor the depth of the ocean and the distance from the coastline by including coastlines and bathymetry in 2d inversion we recover the seafloor conductivity structure along the forearc demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere

organic maturation thermal history and hydrocarbon generation in the orcadian basin scotland the devonian orcadian basin contains thick sequences of organic rich lacustrine sediments of interest as hydrocarbon source rocks throughout the basin the range of organic maturation is large spore colours vary from yellow orange to black and vitrinite reflectance for type iii kerogens ranges from 0 7 to 10 5 maximum palaeotemperatures are estimated to range from those typical of moderate burial diagenesis 100 c to those characteristic of greenschist facies metamorphism 400 c the highest maturities are related to contact metamorphism by devonian plutons and it is postulated that a large pluton underlies much of caithness high and low maturity regions are often juxtaposed across major faults which were active during variscan inversion regionally the lowest maturity is about 0 8 r o implying uplift and erosion of 2 3 km of cover basin wide this cover was probably a sequence of late devonian and carboniferous rocks removed largely during variscan uplift geothermal gradients were high but variable during devonian extension this together with the contact metamorphic affects of the devonian plutons resulted in early maturation metamorphism of large areas of the basin most of the onshore sequence entered the oil window during devonian and carboniferous times significant generation of hydrocarbons from orcadian lacustrine facies offshore during subsequent mesozoic burial will have occurred only in those areas which escaped maturation during the palaeozoic

numerical models on the influence of partial melt on elastic anelastic and electrical properties of rocks part ii electrical conductivity abstract partial melting influences both the elastic and electrical properties of mantle rocks significantly but in a different manner a set of theoretical models of the electrical conductivity of partially molten rocks is developed for different geometries of the melt distribution the models correspond to a similar set on elasticity and anelasticity presented in a preceding paper part i both model sets allow combined interpretations of seismic and geoelectric observations from anomalous hot mantle regions by a systematic variation of the melt geometry the following melt geometries are assumed in the conductivity models melt films tubes isolated ellipsoidal inclusions films and ellipsoidal inclusions with a variable degree of interconnection distributed geometries and superpositions of spheres with films and with tubes unexplored conduction mechanisms near the melting temperature film thicknesses of molecular sizes or effects of volatiles are not considered resistor network theory is utilized to account for a variable degree of interconnection and distributed geometries thereby a new possible interpretation of archie s law is found in terms of varying interconnection furthermore it is found that a reduced degree of interconnection may have an important influence on the conductivity at melt fractions characteristic for a weak to moderate decrease in seismic velocity the model results are compiled together with the elastic models of part i in a set of diagrams which may be applied systematically to observed electrical and seismic data to find constraints on the state of melting

mapping the resistivity structure of walker ridge 313 in the gulf of mexico using the marine csem method abstract a marine controlled source electromagnetic csem campaign was carried out in the gulf of mexico to further develop marine electromagnetic techniques in order to aid the detection and mapping of gas hydrate deposits marine csem methods are used to obtain an electrical resistivity structure of the subsurface which can indicate the type of substance filling the pore space such as gas hydrates which are more resistive results from the walker ridge 313 study wr 313 are presented in this paper and compared with the gulf of mexico gas hydrate joint industry project ii jip2 logging while drilling lwd results and available seismic data the hydrate known to exist within sheeted sand deposits is mapped as a resistive region in the two dimensional 2d csem inversion models this is consistent with the jip2 lwd resistivity results csem inversions that use seismic horizons provide more realistic results compared to the unconstrained inversions by providing sharp boundaries and architectural control on the location of the resistive and conductive regions in the csem model the seismic horizons include 1 the base of the gas hydrate stability zone bghsz 2 the top of salt and 3 the top and bottom of a fine grained marine mud interval with near vertical hydrate filled fractures to constrain the csem inversion model the top of salt provides improved location for brines water saturated salt and resistive salt inversions of the csem data map the occurrence of a halo of conductive brines above salt the use of the bghsz as a constraint on the inversion helps distinguish between free gas and gas hydrate as well as gas hydrate and water saturated sediments

marine controlled source electromagnetic method used for the gas hydrate investigation in the offshore area of sw taiwan abstract bottom simulating reflectors bsrs high methane flux shallow sulfide methane interfaces fluids venting from the seafloor authigenic carbonates within sediments methane reefs and self biomes are common seafloor features in the area off southwest taiwan the geophysical and geochemical signatures of these features suggest a high potential for gas hydrate gh reservoirs in the region the bsrs are typically interpreted as the boundary between free gases and solid hydrate whereas the upper reaches of the hydrate stability zone and the distribution of gas hydrate in shallow sediments are not well understood this study shows the first results of a marine controlled source electromagnetic survey conducted in the offshore area of sw taiwan in 2010 the survey aimed to provide electrical resistivity information of the shallow sediments three target areas were surveyed 1 an area to the southeast of the xiaoliuchiu island gas seep g96 2 an area in the west of the yung an ridge yar and 3 an area in the northwest of the good weather ridge gwr in total fourteen survey lines with a total length of 72 km were completed our preliminary results show that relatively high resistivity anomalies occur within pockmarks and at gas seepage sites the apparent resistivity is estimated to be about 1 ohm m higher than background in g96 and yar sites while an anomaly up to 2 ohm m is found in the gwr at gas seep site g96 the high resistivity anomaly may be due to the existence of authigenic carbonates whereas the high resistivity anomaly in the nw of the gwr site may also be due to the existence of gas hydrate in the shallow seabed based on the resistivity anomaly the gas hydrate saturation is about 16 in the shallow sediments below the pockmark area in the northwest side of the gwr site

on the importance of offshore data for magnetotelluric studies of ocean continent subduction systems 1 the presence of water in the mantle wedge overlying a subduction zone is expected to significantly enhance electrical conductivity raising it above that of the cold subducting slab or surrounding regions of dehydrated mantle this suggests that magnetotelluric mt transects across subduction systems measuring regional electrical conductivity structure might be able to indirectly trace the pathways of water migration into the mantle for ocean continent subduction it is logistically simpler to collect mt transects on the continental side of the system however we show that such land data are relatively insensitive to details of the electrical connections between the ocean and mantle in contrast seafloor measurements on the landward and seaward side of the trench are very sensitive to these electrical connections and are essential to understanding the electrical structure of the entire subduction system in particular the conductivity structure of the hydrated mantle wedge overlying the slab can only be studied using offshore mt data we demonstrate this result using a model of an ocean continent system although our results can be generalised to other subduction geometries

using csem techniques to map the shallow section of seafloor from the coastline to the edges of the continental slope many important processes occur within the shallow section of the seafloor on the continental shelf and slope yet conventional geophysical constraints on the physical properties within this critical boundary layer are limited some of the key constraints involve quantification of fluids within the seafloor which can be provided by electrical methods this paper reviews the application of a towed em system to map the uppermost 20 m of seafloor in a variety of settings ranging from nearshore regions in water depths of approximately 10 m on the continental shelf out to water depths of 1300 m the system is a mapping tool that provides areal maps of seafloor resistivity and has been used for a variety of purposes including sedimentary characterization and facies mapping evaluation of groundwater discharge and mapping seafloor mounds in the gulf of mexico thought to contain massive deposits of gas hydrate

seafloor instabilities and sediment deformation processes the need for integrated multi disciplinary investigations abstract in this paper we present the current practice of investigations of seafloor instabilities and deformation processes based on extensive research conducted over the last years which sets the scene for future research activities in this field the mapping of the continental margins and coastal areas with ever increasing resolution systematically reveals evidence of instabilities and deformation processes both active and palaeo features in order to properly assess the hazards and risks related to these features an integrated and multi disciplinary approach is essential but challenging such an approach consists of combining field data geophysics geology sedimentology geochemistry and geotechnical data with numerical simulations constrained by results from laboratory data as such it is of paramount importance to build a common knowledge base and understanding that unify these disciplines into more complete and conceptual models constrained by all the data we review the status of this integrated approach adapted to palaeo landslides e g storegga ana vesteralen and recent deformations finneidfjord nice gulf of guinea allowing to identify gaps in our knowledge at these sites by reviewing these case studies one can conclude that each case remains highly site specific in which both the regional and local geological tectonic settings have a distinct effect of the type of instability or deformation taking place or that can take place our knowledge on the actual triggers remains poorly constrained and there is even ambiguity for historic landslides e g finneidfjord also our knowledge of the preconditioning factors is incomplete there is a general lack of geotechnical data both in situ and from laboratory and therefore modelling the dynamics e g rheology of the instabilities relies on a number of assumptions rather than facts in addition excess pore pressure and its evolution is one of the key parameters driving instabilities despite this fact in situ excess pore pressure is rarely measured or monitored much work remains to be done to relate and integrate geotechnical data with geophysics e g through inversion and rock physical models in order to obtain additional quantitative information from the sub surface but also with respect to partial saturation free gas hydrate and pore pressure behaviour or lithologies it is of critical importance to be able to identify the different processes which can lead to hazardous situations which include establishing recurrence intervals timing and frequencies through event recognition and age control and magnitudes so that proper mitigation measures can be developed in this perspective the smaller scale instabilities deserve much attention as there are many instances where such features had far reaching consequences for society e g nice finneidfjord in that perspective human interferences e g exploitation drilling blasting loading must be one of the factors that should be taken into consideration

clay minerals in sediments of portuguese reservoirs and their significance as weathering products from over eroded soils a comparative study of the maranhao monte novo and divor reservoirs south portugal the southern region of portugal is subjected to several forms of over erosion most leached products mainly composed of fine particles containing nutrients metals or pesticides are easily transported by river flows when these are hindered by a physical barrier such as a dam the particulate load accumulates on the bottom of the reservoirs often leading to a pronounced decrease of water quality bottom sediments from three reservoirs were subjected to grain size analysis and a study of clay minerals by x ray diffraction most sediments contain a diverse set of clay minerals mostly illites smectites chlorites and kaolinites the nature of the clay minerals reflects the nature of the parent rocks during the cycles of transport and temporary deposition they may undergo significant chemical and physical transformations which lead to an increase of expandable properties and therefore to a higher cationic exchange capacity determining its important role as vehicles of environmental pollutants

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

the electrical properties of a 2d conductive zone under the juan de fuca ridge vertical gradient electromagnetic sounding vgs on the endeavour segment of juan de fuca mid ocean ridge reveals the presence of a 2d ridge parallel conductivity anomaly if the anomaly is caused mainly by melt in a conventional upper mantle upwelling zone alone then the conductivity of the zone is about 0 6 s m the corresponding archie s law melt fraction exceeds 0 10 a significantly lower melt fraction requires a sheet like well interconnected melt upwelling zone conductivity can be reduced by a third if the anomaly is broadened and a crustal conductor is added to the model

temporal and spatial evolution of a gas hydrate bearing accretionary ridge on the oregon continental margin a seismic reflection survey on the oregon continental margin conducted in 1989 indicates the widespread presence of gas hydrate beneath the middle and lower slope of this accretionary margin the seismic signature of gas hydrate a bottom simulating reflector bsr with negative polarity that locally cuts across stratigraphic horizons is especially well developed beneath hydrate ridge this anomalously shallow accretionary ridge was drilled during ocean drilling program leg 146 to study fluid venting in this paper we focus on the seismic data from the southern part of hydrate ridge where little evidence of active venting has previously been reported but where the seismic data indicate a complicated subsurface plumbing system apparent disruptions of the bsr beneath the western ridge flank suggest dissociation of gas hydrate in response to slumping a double bsr beneath the southern crest suggests hydrate destabilization in response to tectonic uplift and folding on the basis of these and other observations we propose a qualitative model for the evolution of a hydrate bearing ridge in an active accretionary complex in which gas hydrate initially stabilizes the sea floor permitting construction of large ridges that are then eaten away by slumps along their margins the north to south variation in sea floor venting and subsurface seismic structure along hydrate ridge may reflect different stages in the temporal evolution of one of these ridges

electromagnetic surveying of seafloor mounds in the northern gulf of mexico seafloor controlled source electromagnetic data probing the uppermost 30 m of seafloor sediments have been collected with a towed magnetic dipole dipole system across two seafloor mounds at approximately 1300 m water depth in the northern gulf of mexico one of these mounds was the focus of a recent gas hydrate research drilling program rather than the highly resistive response expected of massive gas hydrate within the confines of the mounds the em data are dominated by the effects of raised temperatures and pore fluid salinities that result in an electrically conductive seafloor this structure suggests that fluid advection towards the seafloor is taking place beneath both mounds similar responses are seen at discrete locations away from the mounds in areas that might be associated with faults further suggesting substantial shallow fluid circulation raised temperatures and salinities may inhibit gas hydrate formation at depth as has been suggested at other similar locations in the gulf of mexico

direct seismic detection of methane hydrate on the blake ridge seismic detection of methane hydrate often relies on indirect or equivocal methods new multichannel seismic reflection data from the blake ridge located approximately 450 km east of savannah georgia show three direct seismic indicators of methane hydrate 1 a paleo bottom simulating reflector bsr formed when methane gas froze into methane hydrate on the eroding eastern flank of the blake ridge 2 a lens of reduced amplitudes and high p wave velocities found between the paleo bsr and bsr and 3 bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas velocities within the lens 1910 m s are significantly higher than velocities in immediately adjacent strata 1820 and 1849 m s conservative estimates show that the hydrate lens contains at least 13 bulk methane hydrate within a 2 km3 volume yielding 3 2 1010kg 1 5 tcf 4 2 1010 m3 of methane low seismic amplitudes coupled with high interval velocities with

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

biodegradation of oil in uplifted basins prevented by deep burial sterilization biodegradation of crude oil by bacterial activity which has occurred in the majority of the earth s oil reserves is known to reduce greatly the quality of petroleum in reservoirs for economically successful prospecting for oil it is therefore important to understand the processes and conditions in geological formations that lead to oil biodegradation although recent studies speculate that bacterial activity can potentially occur up to temperatures as high as 150 degrees c refs 3 4 it is generally accepted that effective petroleum biodegradation over geological timescales generally occurs in reservoirs with temperatures below 80 degrees c ref 2 this appears however to be at odds with the observation that non degraded oils can still be found in reservoirs below this temperature here we compile data regarding the extent of oil biodegradation in several oil reservoirs and find that the extensive occurrence of non biodegraded oil in shallow cool basins is restricted to those that have been uplifted from deeper hotter regions of the earth we suggest that these petroleum reservoirs were sterilized by heating to a temperature around 80 90 degrees c during deep burial inactivating hydrocarbon degrading organisms that occur in the deep biosphere even when such reservoirs are subsequently uplifted to much cooler regions and filled with oil degradation does not occur implying that the sterilized sediments are not recolonized by hydrocarbon degrading bacteria

ten years of marine csem for hydrocarbon exploration marine controlled source electromagnetic csem surveying has been in commercial use for predrill reservoir appraisalandhydrocarbonexplorationfor10 years althougha recent decrease has occurred in the number of surveys and publications associated with this technique the method has become firmly established as an important geophysical tool intheoffshoreenvironment thisisaconsequenceoftwoimportant aspects associated with the physics of the method first it is sensitive to high electrical resistivity which although not an unambiguous indicator of hydrocarbons is an important property of economically viable reservoirs second although the method lacks the resolution of seismic wave propagation it has a much better intrinsic resolution than potential field methods such as gravity and magnetic surveying which until now have been the primary nonseismicdatasetsusedinoffshoreexploration althoughbymany measures marine csem is still in its infancy the reliability and noise floors of the instrument systems have improved significantly over the last decade and interpretation methodology has progressed from simple anomaly detection to 3d anisotropic inversion of multicomponent data using some of theworld sfastestsupercomputers researchdirectionspresently include tackling the airwave problem in shallow water by applying time domain methodology continuous profiling tools and the use of csem for reservoir monitoring during production

marine controlled source electromagnetic sounding 2 the pegasus experiment the marine controlled source electromagnetic sounding method developed over the past 15 years at scripps institution of oceanography employs a towed seafloor electric dipole transmitter of moment 4 104 am and multiple free vehicle seafloor electric field recorders a survey of 40 ma normal oceanic lithosphere in the northeast pacific using frequencies of 0 25 to 24 hz and synchronous stacking of 0 25 to 12 hour duration detected signals at transmitter receiver ranges between 5 and 95 km one dimensional electrical conductivity structure is recovered from the data using the occam process of nonlinear regularized inversion repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km well into the upper mantle with bounds placed on conductivity to depths of 60 km structure shallower than about 1 km is comparable to that obtained by a similar experiment on the east pacific rise and by borehole logging with a sharp increase in resistivity at depths of 600 800 m although strictly our experiment is sensitive only to integrated square root of conductivity or total attenuation in the surface layers the lower crust and upper mantle has a resistivity between 2 and 7 104 m and a transverse resistance of at least 109 m2 suggesting at most 0 3 volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle although resolution is weak below 30 km our data are compatible with a dry olivine model of mantle conductivity temperature

a marine controlled source electromagnetic survey to detect gas hydrates in the qiongdongnan basin south china sea abstract the qiongdongnan basin qb is an important prospecting area for gas hydrates in the south china sea scs however it is difficult to infer gas hydrates using a bottom simulating reflector bsr under the influence of flat seabed multiples and bubble effects in the qb the upper limit of the gas hydrate stability zone ghsz and the distribution of gas hydrates in the shallow seabed are still not well understood a marine controlled source electromagnetic mcsem survey was completed in order to collect electric resistivity information along a 4 5 km profile electromagnetic data were acquired and processed at nine sites the seafloor receiver orientations were evaluated using the orthogonal procrust rotation analysis opra method as a result of isotropic vertical transverse isotropic vti and triaxial anisotropic inversion of electric field pmax data a 2 d resistivity image of the seabed was presented preliminary results show that the shallow seabed can be divided into four electric layers according to thermodynamic conditions in the qb the estimated thickness of ghsz is approximately 320 m for biogenic methane and 272 m for thermogenic gas in the study area the bsr and hydrates were interpreted by integrating the resistivity model along with thermodynamic calculation and seismic reflection data the mcsem result indicates there are abundant natural gas originating or partly originating from a microbial source beneath the ghsz

marine em techniques for gas hydrate detection and hazard mitigation marine controlled source electromagnetic csem sounding is a new tool available to geophysicists for offshore hydrocarbon exploration although the technique has been developed for the detection of deep hydrocarbon reservoirs with relatively high resistivities it also has the potential to be a useful tool for geohazard mitigation via gas hydrate detection the hydrate target occurs in the shallow section hundreds of meters in depth and is manifested by subtle resistivity contrasts a few m this requires modifications to the csem technique to extend its capability of imaging the shallower hydrate section

first results from a marine controlled source electromagnetic survey to detect gas hydrates offshore oregon 1 submarine gas hydrate is a hazard to drilling a potential hydrocarbon resource and has been implicated as a factor in both submarine slope stability and climate change bulk in situ electrical resistivities evaluated from electromagnetic surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data we conducted a marine controlled source electromagnetic sounding at hydrate ridge oregon usa in august 2004 electromagnetic fields transmitted by a deep towed horizontal electric dipole source were measured by a linear array of 25 seafloor electromagnetic receivers positioned 600 m apart to produce a dense coverage in the recorded electric field data results are presented in simple form by apparent resistivity pseudosections which produce an approximate image of lateral resistivity variations across the study region resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors archie s law based on pseudosection apparent resistivities predicts volumetric hydrate concentrations vary from 0 30 across the ridge

modeling of the influence of gas hydrate content on the electrical properties of porous sediments the geophysical investigation of gas hydrate accumulations within marine sediments and the sediments of permafrost regions suffer from a lack of information on the influence of the hydrate content on the physical sediment properties the estimation of the hydrate content using downhole electrical measurements based on archie s law requires the knowledge of the saturation exponent the saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape connectivity and constrictivity of the pore network and the distribution of the conducting phase based on different models that account for the different morphological forms of gas hydrates found during gas hydrate sampling in various research wells the influence of gas hydrate content on the electrical properties of the hydrate bearing sediment was investigated for all studied forms of hydrate occurrence disseminated in the pore space nodular and layered the saturation exponent depends on the sediment properties and on saturation itself the growth of gas hydrate nodules lenses and layers is a process that is assumed to result in the displacement and compaction of the surrounding sediment because of this change of sediment properties during hydrate generation the saturation exponent for these forms of hydrate occurrence depends strongly on the relationship between porosity and formation resistivity factor expressed in the form of archie s cementation exponent for the case that hydrate occurs disseminated in the pore space and the assumption that capillary effects are important for hydrate generation the saturation exponent depends on grain size and grain size sorting for the parameters chosen for these model calculations the saturation exponent varies between 0 5 and 4 the use of a constant mean value for the saturation exponent of approximately 2 can result in both underestimation and overestimation of the hydrate content

marine electromagnetic methods a new tool for offshore exploration electromagnetic survey methods used for decades mainly for mining exploration on land have burst upon the offshore hydrocarbon exploration scene over the past five years this move driven by the costs and challenges associated with the deepwater exploration environment has left many companies scrambling to understand new and unfamiliar technologies and has resulted in a sudden expansion of the portfolios of the few nonseismic geophysicists left in the oil industry of the two main methods magnetotelluric mt surveys are at least somewhat familiar from their use on land as a reconnaissance tool and for problem solving in areas of poor seismic performance and to a large extent can be applied to the marine environment in a similar way to their terrestrial counterparts the other dominant technology controlled source electromagnetic csem sounding has featured much less in land exploration but more importantly behaves so very differently in the deepwater marine environment that it might as well

the assessment of marine gas hydrates through electrical remote sounding hydrate without a bsr marine gas hydrates prevalent in offshore sediments in canada and japan are a possible hydrocarbon resource a hazard to drilling and a source of a major greenhouse gas quantitative estimates of hydrate concentrations in deep sea sediment are difficult to obtain by conventional methods we have sought novel techniques specifically designed for assessment such as transient electric dipole dipole electromagnetics the latter method is based on the assumption of a reduced electrical conductivity in hydrate rich zones field trails of new apparatus on the cascadia margin have proven successful excellent data were collected and analysed using a differential phase method to reduce systematic error apparent resistivities collected on three lines demonstrate that the resistivity of the seafloor is remarkably uniform over the whole survey area the average hydrate concentration deduced with the aid of a reference model based on the electrical logs of odp holes 888 and 889 is about 17 26 of pore space 9 13 of sediment volume in the 100 m interval above the bsr the values are consistent with those obtained by other analyses further the presence of hydrate is predicted in a region to the east of 889b where there is no visible bsr

phosphorus sequestration in fe rich sediments from two brazilian tropical reservoirs abstract this study investigates the important role of fe oxide hydroxide rich bottom sediments from two brazilian tropical reservoirs on p sequestration from the overlying waters which decreases the risk of eutrophication data on p fractionation indicates that p adsorption capacity of sediments is predominantly associated with fe followed by orthosphosphate adsorbed onto al hydr oxides and onto different forms of apatite ca p the soluble and loosely bound p forms are low to have a better understanding of the role of fe oxides on the adsorption and sequestration of p in the reservoirs data obtained by sequential extraction of p have been related to concentrations of total fe and of various fe fractions extracted through an optimized fractionation scheme these analyses indicated high levels of fe bound to oxides hydroxides these mineral oxides and the preponderance of ph dependent charged clay minerals kaolinite in the clay fraction induce the sequestration of the p entering the lakes mainly in particulate form by strong retention of po 4 3 through surface adsorption and precipitation phenomena phosphorus retention provided by the replacement of oh groups on the basal plane of the minerals for p anions decreases the negative impact of p inputs in these water systems there is significant correlation between the concentrations of soluble reactive p in the water srp and major chemical components of sediments sio2 fe2o3 al2o3 k2o p2o5 which are related to the dominant bedrock geology the interaction of srp with bed sediments and the chemical conditions of the environment decrease of redox potential with depth and in the dry period a low o2 abundance can explain fluctuations of srp and tp total p in the water column due to the dynamic equilibrium in the sediment water interface in the dry season reducing conditions in hypoliminia enhance the reduction of fe iii to the more soluble fe ii with subsequent release of the strongly retained p this mechanism increases the soluble p fraction in sediments and the soluble reactive p in the water column in this period the straight linkage between geochemical properties of sediments and p concentration in the water column reported in this article suggests that study of the mineralogical and geochemical composition of bottom sediments could help to improve model predictions of p concentrations in surface waters

remote sensing of hydrocarbon layers by seabed logging sbl results from a cruise offshore angola detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry seismic methods have traditionally been used in this context but the results can be ambiguous another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids modeling presented by eidesmo et al 2002 demonstrates that by using seabed logging sbl a special application of frequency domain controlled source electromagnetic csem sounding the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying

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

the holocene storegga slide tsunami in the united kingdom all currently known sites in the united kingdom with evidence for the holocene storegga slide tsunami are described information on the altitude distribution stratigraphical context age particle size profile and microfossil characteristics of the deposits is presented the tsunami involved a greater area than previously described reaching a coastline over 600 km long the ubiquitous sand layer which forms the main deposit associated with the event is shown to exhibit a consistent morphology and a particle size profile marked by fining upwards sequences an analysis of new and previously published radiocarbon dates indicates that from evidence in the united kingdom the event took place sometime around 7100 radiocarbon years bp 7900 calibrated years bp a new isobase model for mainland scotland and adjacent areas providing a preliminary estimate of land uplift since the tsunami is presented the model estimates contemporary sea surface level offshore at 14 m below the present day mean high water spring tides tsunami sediment run up is greatest in inlets where it reaches at least 25 m on shetland and at least 5 m along the mainland coastline to the south and run up of the tsunami would have exceeded these values the tsunami sediments identified here are considered particularly valuable as a synchronous marker horizon

bayesian inversion of marine controlled source electromagnetic data offshore vancouver island canada this paper applies nonlinear bayesian inversion to marine controlled source electromagnetic csem data collected near two sites of the integrated ocean drilling program iodp expedition 311 on the northern cascadia margin to investigate subseafloor resistivity structure related to gas hydrate deposits and cold vents the cascadia margin off the west coast of vancouver island canada has a large accretionary prism where sediments are under pressure due to convergent plate boundary tectonics gas hydrate deposits and cold vent structures have previously been investigated by various geophysical methods and seabed drilling here we invert time domain csem data collected at sites u1328 and u1329 of iodp expedition 311 using bayesian methods to derive subsurface resistivity model parameters and uncertainties the bayesian information criterion is applied to determine the amount of structure number of layers in a depth dependent model that can be resolved by the data the parameter space is sampled with the metropolis hastings algorithm in principal component space utilizing parallel tempering to ensure wider and efficient sampling and convergence nonlinear inversion allows analysis of uncertain acquisition parameters such as time delays between receiver and transmitter clocks as well as input electrical current amplitude marginalizing over these instrument parameters in the inversion accounts for their contribution to the geophysical model uncertainties one dimensional inversion of time domain csem data collected at measurement sites along a survey line allows interpretation of the subsurface resistivity structure the data sets can be generally explained by models with 1 to 3 layers inversion results at u1329 at the landward edge of the gas hydrate stability zone indicate a sediment unconformity as well as potential cold vents which were previously unknown the resistivities generally increase upslope due to sediment erosion along the slope inversion results at u1328 on the middle slope suggest several vent systems close to bullseye vent in agreement with ongoing interdisciplinary observations

status of sedimentation in king talal dam case study from jordan this paper focuses on characterizing the current status of physical and chemical characteristics of king talal dam ktd sediments the quality of sediments was investigated based on several international sediment quality guidelines the volume of sediment and sedimentation rate in ktd was also estimated five types of granulometric textural facies were observed for the sediments of ktd reservoir bed with an average grain size distribution of gravel 1 38 sand 32 38 silt 32 35 and clay 32 38 the fraction of pb zn co cr ni fe cu sr mn and cd zn pb co cr ni cu sr mn respectively based on the sonic survey of ktd reservoir the annual sedimentation rate was about 0 4061 mcm year in which about 18 66 of the reservoir initial storage capacity was lost to sediments and a projected lifespan of about 198 years

application of fluid flow systems analysis to reconstruct the post carboniferous hydrogeohistory of the onshore and offshore netherlands abstract the reconstruction of the post carboniferous hydrogeohistory of the onshore and offshore netherlands is part of an ongoing tno project on fluid flow systems analysis on geological timescales the scope of the project is to assess the present day hydrogeological and fluid flow conditions of the sedimentary basins in the onshore and offshore netherlands and to analyse and model the evolution of fluid flow systems in these basins in order to increase the process based knowledge of the causes of present day physico chemical characteristics of rocks and fluids in the sedimentary fill of the dutch part of the north sea basin this article concerns the first phase of the project and provides an overview of the characteristic features of the post carboniferous evolution of external and internal processes acting on the sedimentary fill and their role in shaping the hydrogeological and hydrodynamical setting of the netherlands the post carboniferous evolution of the fluid flow conditions is described for five geological time periods during each time period different forces have acted simultaneously on the fluids and different fluid flow systems have coexisted and interacted the present day active driving forces and processes influencing fluid flow conditions include sedimentary loading gas generation tectonic forces topographic relief of the water table fluid density differences and human activities the article presents a brief outline of the resulting fluid flow systems observed direct and indirect indicators of fluid flow pressure temperature salinity sediment diagenetic minerals are consistent with the reconstructed hydrogeological and hydrodynamic setting of the netherlands

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

preliminary interpretation of electromagnetic heat flow seismic and geochemical data for gas hydrate distribution across the porangahau ridge new zealand porangahau ridge located offshore the wairarapa on the hikurangi margin is an active ocean continent collision region in northeastern new zealand coastal waters bottom simulating reflections bsrs in seismic data indicate the potential for significant gas hydrate deposits across this part of the margin beneath porangahau ridge a prominent high amplitude reflection band has been observed to extend from a deep bsr towards the seafloor review of the seismic data suggest that this high amplitude band is caused by local shoaling of the base of gas hydrate stability due to advective heat flow and it may constitute the location of elevated gas hydrate concentrations during r v tangaroa cruise tan0607 in 2006 heat flow probing for measurements of vertical fluid migration sediment coring for methane concentrations and additional seismic profiles were obtained across the ridge in a subsequent 2007 expedition on r v sonne cruise so191 a controlled source electromagnetic csem experiment was conducted along the same seismic geochemical and heat flow transect to reveal the electrical resistivity distribution csem data highlight a remarkable coincidence of anomalously high resistivity along the western landward flank of the ridge which point to locally higher gas hydrate concentration above the high amplitude reflection band measured sediment temperature profiles also along the western flank consistently show non linear and concave geothermal gradients typical of advective heat flow geochemical data reveal elevated methane concentrations in surface sediments concomitant with a rapid decline in sulfate concentrations indicating elevated methane flux and oxidation of methane in conjunction with sulfate reduction at the landward ridge base together these data sets suggest that the western rim of porangahau ridge is a tectonically driven zone of rising fluids that transport methane and cause an upward inflection of the base of gas hydrate stability and the formation of locally enriched gas hydrate above the reflective zone

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

constraints on gas hydrate distribution and morphology in vestnesa ridge western svalbard margin using multicomponent ocean bottom seismic data source at https doi org 10 1029 2018jb016574 2019 american geophysical union all rights reserved

resistivity image beneath an area of active methane seeps in the west svalbard continental slope the arctic continental margin contains large amounts of methane in the form of methane hydrates the west svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone the presence of bottom simulating reflectors bsrs on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area resistivity obtained from marine controlled source electromagnetic csem data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments we acquired two csem lines in the west svalbard continental slope extending from the edge of the continental shelf 250 m water depth to water depths of around 800 m high resistivities 5 12 m observed above the bsr support the presence of gas hydrate in water depths greater than 600 m high resistivities 3 4 m at 390 600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone ghsz of the continental slope in addition high resistivities 4 8 m landward of the ghsz are coincident with high amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas pore space saturation estimates using a connectivity equation suggest 20 50 per cent hydrate within the lower slope sediments and less than 12 per cent within the upper slope sediments a free gas zone beneath the ghsz 10 20 per cent gas saturation is connected to the high free gas saturated 10 45 per cent area at the edge of the continental shelf where most of the seeps are observed this evidence supports the presence of lateral free gas migration beneath the ghsz towards the continental shelf

marine magnetotellurics for petroleum exploration part i a sea floor equipment system induction in electrically conductive seawater attenuates the magnetotelluric mt fields and coupled with a minimum around 1 hz in the natural magnetic field spectrum leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s for this reason the marine mt method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure rarely does a sea floor mt response extend to a 100 s period to be useful for mapping continental shelf structure at depths relevant to petroleum exploration however mt measurements need to be made at periods between 1 and 1000 s this can be accomplished using ac coupled sensors induction coils for the magnetic field and an electric field amplifier developed for marine controlled source applications the electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording in deep 1 km water motional noise in magnetic field sensors appears not to be a proble

pore habit of methane hydrate and its evolution in sediment matrix laboratory visualization with phase contrast micro ct abstract describing the pore habit of methane hydrate in sediment matrices is essential for understanding natural distribution of methane hydrate methane trace transport and solidification in the hydrate stability zone physical properties of hydrate bearing sediments and the associated influence on potential gas production pore habit visualization in natural media at pore scale even with laboratory synthesized cores has been challenging due to the similar densities of methane hydrate from pore liquid in this work we used phase contrast assisted micro ct with potassium iodine doped brine to visualize four phases sand particles pore fluid methane hydrate and methane gas this study visualizes the pore habit of methane hydrate at various stages including during hydrate formation in excess gas systems its evolution after brine injection to replace pore fluid and hydrate formation in excess water systems hydrate tends to adopt round and smooth surfaces when in contact with water while exhibits relatively angular interfaces when in contact with methane gas hydrate formation in excess gas systems results in a partial cementing and partial mineral coating pore habit while hydrate in excess water systems develops mainly as pore filling and locally cementing or mineral coating where big gas pockets exist at the initial state pore liquid replacement from methane gas to brine triggers a shift of hydrate pore habit towards pore filling methane hydrate evolution over time produces bigger hydrate particles but with less contact area with sand particles the effects of hydrate pore habit become less important as hydrate particle size exceeds the pore size additionally hydrate formation could trap residual methane gas and brine as inclusions

relationships between seepage tectonics and subsurface petroleum reserves abstract the relationship between visible petroleum seepage and commercial subsurface reserves has been investigated by a study of the distribution of documented seeps in south east asia this is accompanied and integrated with a broader global study the intensity of flowing seepage is controlled largely by present day tectonics and by the timing of petroleum migration when such influences are reduced by isolating sets of tectonically similar basins correlations are found between numbers of seeps and basin reserves at a sub basinal scale seeps are observed to concentrate over tectonic features such as active diapirs active faults and uplifted basin margins it is relatively rare for seeps to directly overlie major fields although many seeps may be linked to downdip accumulations by long tortuous paths the best seep accumulation relationships are observed in active compressional and diapiric settings particularly those involving shallow reservoirs seeps are rare where traps are buried beneath thick undisturbed overburdens the prime value of visible seeps in most frontier basins lies at the regional level in the clues they give to the nature extent and quantitative hydrocarbon potential of a basin s source system

marine controlled source electromagnetic study of methane seeps and gas hydrates at opouawe bank hikurangi margin new zealand marine controlled source electromagnetic csem data have been collected to investigate methane seep sites and associated gas hydrate deposits at opouawe bank on the southern tip of the hikurangi margin new zealand the bank is located in about 1000 m water depth within the gas hydrate stability field the seep sites are characterized by active venting and typical methane seep fauna accompanied with patchy carbonate outcrops at the seafloor below the seeps gas migration pathways reach from below the bottom simulating reflector at around 380 m sediment depth toward the seafloor indicating free gas transport into the shallow hydrate stability field the csem data have been acquired with a seafloor towed electric multi dipole system measuring the inline component of the electric field csem data from three profiles have been analyzed by using 1 d and 2 d inversion techniques high resolution 2 d and 3 d multichannel seismic data have been collected in the same area the electrical resistivity models show several zones of highly anomalous resistivities 50 m which correlate with high amplitude reflections located on top of narrow vertical gas conduits indicating the coexistence of free gas and gas hydrates within the hydrate stability zone away from the seeps the csem models show normal background resistivities between 1 and 2 m archie s law has been applied to estimate gas gas hydrate saturations below the seeps at intermediate depths between 50 and 200 m below seafloor saturations are between 40 and 80 and gas hydrate may be the dominating pore filling constituent at shallow depths from 10 m to the seafloor free gas dominates as seismic data and gas plumes suggest

quantitative characterization of gas hydrate bearing sediment using elastic electrical rock physics models abstract successful evaluation of reservoir resources in gas hydrate bearing sediments depends on the accurate estimation of reservoir parameters such as hydrate saturation porosity and clay content elastic and electrical measurements are affected by several rock and fluid properties and their joint interpretation can provide complementary information and improve the accuracy of reservoir characterization results in this work we propose a new rock physics inversion workflow based on 3d rock physics template of elastic and electrical attributes for the direct estimation of reservoir parameters we first conduct a quantitative analysis of the available well log data and core data to understand the morphology of gas hydrate within the marine sediment and choose the appropriate elastic rock physics model then we construct the 3d rock physics template by discretizing the 3d space of reservoir parameters and implementing the suitable elastic and electrical models to calculate the poisson s ratio p wave impedance and electrical resistivity for each combination of values of rock and fluid properties finally we superimpose the measurements on the templates and use a grid searching method to find the nearest gridded node to the data point and minimize the objective function of the mismatch between data and model predictions to investigate the feasibility of our proposed inversion scheme we test a rock physics model based on effective medium theory and a modified archie s equation and invert for reservoir parameters of gas hydrate bearing sediments at well 1250f and 1247b in hydrate ridge the good match between inversion results and core data suggests that the proposed inversion scheme is applicable to real data and allows combining elastic and electrical data for reservoir parameter estimation of gas hydrate bearing sediments

migration of methane gas through the hydrate stability zone in a low flux hydrate province new high resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable low methane flux blake ridge crest this movement of gaseous methane through a thermo dynamic regime where it should be trapped as hydrate suggests that dynamic migrations of gas play an important role in the interaction of subseafloor methane with the ocean in the study area none of the seismic amplitude anomalies that provide evidence for gas migration reaches the seafloor instead they terminate at the base of a highly reflective unfaulted capping layer seismic inversions of anomalous regions show 1 increased velocities beneath the hydrate stability zone suggesting less gas and 2 increased velocities within the hydrate stability zone associated with observed low amplitude chimneys and bright spots indicating increased hydrate concentrations these observations and analyses indicate that methane migrates upward as free gas hundreds of meters into the hydrate stability zone before forming hydrate the observations strongly imply that given appropriate permeable pathways free gas can escape into the ocean even in a low flux environment the hydrate stability zone is not an impermeable barrier to free gas migration

the practical application of 2d inversion to marine controlled source electromagnetic data an algorithm is presented for the inversion of marine controlled source electromagnetic csem data that uses a 2d finite difference fd forward driver this code is demonstrated by inverting a csem data set collected at hydrate ridge oregon consisting of 25 seafloor sites recording a 5 hz transmission frequency the sites are located across a bathymetric high with variations in water depth of 300 m along the 16 km profile to model this complex seafloor bathymetry accurately the fd grid was designed by careful benchmarking using a different 2d finite element fe forward code a comparison of the fe and fd forward model solutions verifies that no features in the inversion are due to inaccuracies of the fd grid the inversion includes the local seawater conductivity depth profile as recorded by the transmitter s conductivity temperature depth gauge because seawater conductivity is known to have a significant effect on the csem responses an apparent resistivity pseudosection of the csem data resemb

methane hydrates in controlled source electromagnetic surveys analysis of a recent data example data from a recently acquired sea bed logging deep water survey are analysed for resistive bodies at depths below mudline shallower than about 300 m a model consistent with known methane hydrate properties is found to explain near offset structures over an offset scale of a few hundred metres observed in the data the lateral near seabed resolution of the sea bed logging method was determined to less than 100 m for source frequencies of up to 10 hz the importance of accurate hydrate maps to improve data processing is demonstrated by placing synthetic reservoirs below hydrates and observing their effects on reference model processing the phase is shown to be less perturbed by shallow resistors than the amplitude which is an important quality control of standard anomaly maps while patchy shallow resistors can generally be mapped with simple normalized magnitude versus offset and phase versus offset difference analyses large area distributions of hydrates over kilometres are hard to distinguish from deeper structures using controlled source electromagnetic data only short of conducting a full 3d inversion of a sufficiently large survey beyond the study confirms the applicability of controlled source electromagnetic techniques in general to map shallow resistive structures for drilling hazards and possible future exploration of methane hydrates as an energy source

changes in extent of phosphorus release in a shallow lake lake groβer muggelsee germany berlin due to climatic factors and load abstract based upon biweekly investigations of interstitial and pelagial water parameters as well as sediment phosphorus p speciation the mechanisms important for the summer p release in eutrophic lake gro er muggelsee germany are described microbial processes were the driving force of mainly anaerobic and also aerobic p release from iron reductant soluble and organic p rich sediments for 1992 and 1993 two years of very different runoff characteristics the extent of p release was indirectly controlled by external factors including the supply of dissolved oxygen and nitrate via inflow high runoff changes in land use e g decreasing nitrogen fertilization and climatic factors and it was facilitated by wind induced rapid changes of stratification and mixing events

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

the continental margin off oregon from seismic investigations in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was carried out aboard the german rv sonne as a cooperative experiment between geomar the usgs and coas offshore central oregon which is the subject of this study the experiment involved the collection of wide angle refraction and reflection data along three profiles across the continental margin using ocean bottom seismometers obs and hydrophones obh as well as land recorders two dimensional modelling of the travel times provides a detailed velocity structure beneath these profiles the subducting oceanic crust of the juan de fuca plate can be traced from the trench to its position some 10 km landward of the coastline at the coastline the moho has a depth of 30 km the dip of the plate changes from 1 5 westward of the trench to about 6 5 below the accretionary complex and to about 16 further eastward below the coast the backstop forming western edge of the siletz terrane an oceanic plateau that was accreted to north america about 50 ma ago is well defined by the observations it is located about 60 km to the east of the deformation front and has a seaward dip of 40 at its seaward edge the base of the siletz terrane seems to be in contact with the subducting oceanic crust implying that sediments are unlikely to be subducted to greater depths the upper oceanic crust is thinner to the east of this contact than to the west at depths greater than 18 km the top of the oceanic crust is the origin of pre critical reflections observable in several land recordings and in the data of one ocean bottom instrument these reflections are most likely caused by fluids that are released from the oceanic crust by metamorphic facies transition

an introduction to marine controlled source electromagnetic methods for hydrocarbon exploration early development of marine electromagnetic methods dating back about 80 years was driven largely by defense military applications and use for these purposes continues to this day deepwater frequency domain electric dipole dipole controlled source electromagnetic csem methods arose from academic studies of the oceanic lithosphere in the 1980s and although the hydrocarbon exploration industry was aware of this work the shallow water environments being explored at that time were not ideally suited for its use low oil prices and increasingly successful results from 3d seismic methods further discouraged investment in costly alternative geophysical data streams these circumstances changed in the late 1990s when both statoil and exxonmobil began modeling studies and fieldtrials of csem surveying in deep water around 1000 m or deeper specifically for characterizing the resistivity of previously identified drilling targets trials offshore angola in 2000 2002 by both these companies showed that

assessing methane release from the colossal storegga submarine landslide 1 marine slope failure involving methane gas hydrate bearing sediments is one mechanism for releasing enormous quantities of methane to the ocean and atmosphere the storegga slide on the norwegian margin is the largest known holocene aged continental margin slope failure complex and is believed to have occurred in sediments that may have initially contained gas hydrate here we report pore water sulfate gradient measurements that are used as a proxy for the relative amounts of methane that exist in continental margin sediments associated with the colossal storegga slide these measurements suggest that a considerable inventory of methane occurs in sediments adjacent to and unaffected by the storegga slide events but indicate that methane is notably absent from sediments on the sole of the slide and distal deposits created by the slide events either methane was lost during previous pleistocene failure events or was never present in significant concentrations within the sediments that failed

chemistry of oil field brines in relation to diagenesis of reservoirs 2 reconstruction of palaeo water composition for modelling illite diagenesis in the greater alwyn area north sea abstract input data for water composition are required in order to numerically simulate diagenetic episodes in rock dominated closed system modelling the composition of water initially present in the pores has at most only a minor influence on the stability of the system and is controlled by the mineral composition in open system modelling the composition of infiltrating water can play an important role brent water salinities of the greater alwyn area exhibit a regional trend from the north to the south the k cl ratio of these waters varies along two trends it is higher in the north than in the south alkalinity and published pco2 values are used to calculate ph under subsurface conditions very few silica measurements are available aluminium analyses of pore water are not available some reservoirs of the greater alwyn area experienced a major reduction in permeability due to illite and quartz precipitation coeval with kaolinite and k feldspar dissolution saturation of present day brent waters with respect to these minerals is examined the method proposed in part 1 is applied to reconstruct a range of possible compositions for ancient waters in brent reservoirs between several extreme conditions one of these extremes is provided by buffering water with a set of minerals chosen from petrographic considerations the other extremes correspond to reasonable states of over saturation with respect to particular minerals e g quartz and concern the values of dissolved si and al which are the least well characterised concentrations from the routine analyses of formation waters

the effect of salinization and freshening events in coastal aquifers on nutrient characteristics as deduced from column experiments under aerobic and anaerobic conditions summary this study experimentally quantified the effect of seawater intrusion salinization and freshening events in coastal aquifers on nutrient n p and dsi dynamics across the fresh saline groundwater interface laboratory column experiments were conducted under aerobic and anaerobic conditions in order to simulate the processes occurring in the fresh saline interface they were performed with aquifer sediments simulating the natural conditions during alterations of natural fresh groundwater to seawater and vice versa the salinization and freshening experiments showed that nh4 and po43 and dsi were affected mainly by ion exchange processes while microbial activity controlled the nitrogen species no3 and no2 due to the cation exchange salinization generated enrichment above the expected conservative behavior of nh4 up to 80 mol l 1 an order of magnitude higher than in seawater or fresh groundwater under anaerobic conditions no3 was removed by denitrification as demonstrated by the decrease in no3 concentrations the increase in no2 concentrations and the increase in 15n by 15 25 clear evidence was shown for anion exchange of po43 which competes with hco3 and boron on adsorption sites dsi seems to take part in the exchange process similar to po43

the concentration of deep sea gas hydrates from downhole electrical resistivity logs and laboratory data abstract the concentration of gas hydrate at an ocean drilling program site on the continental slope off the coast of vancouver island has been estimated using a combination of downhole electrical resistivity logs resistivity and porosity of recovered core and core pore fluid salinity from a depth of 100 m to the base of gas hydrate stability bottom simulating reflector or bsr at about 225 m the log resistivities have an average of about 2 0 ohmm compared to 1 0 ohmm at a nearby deep sea reference site where no gas hydrate is present the downhole high resistivities result from a combination of 1 high resistivity hydrate filling part of the pore space and 2 low salinity in situ pore water the low salinity fluids are probably produced at greater depths by hydrate dissociation as the base of the stability field has moved upward with time both the hydrate concentration and the in situ salinity can be calculated if the hydrate concentration vs resistivity relation is known assuming that the effect of hydrate may be approximated by porosity reduction the relation is given by archie s law with exponent about two the hydrate affects the core pore fluid salinity through the dilution of the pure water produced by hydrate dissociation upon core recovery the core fluid salinity results from a combination of this dilution and of in situ freshening the in situ pore fluid salinity in the hydrate zone above the bsr is calculated to be about 80 of seawater whereas the measured salinity in the recovered cores is 60 of seawater the computed hydrate concentration in the 100 m interval above the bsr is 25 30 of pore space 12 15 of sediment volume in general agreement with the 20 estimated from the velocity data

episodic melt transport at mid ocean ridges inferred from magnetotelluric sounding oceanic crust is generated at mid ocean ridges by decompression melting of upwelling mantle at depths of between 50 and 120 km geodynamic and geochemical models of upwelling melt extraction and melt emplacement into crustal magma reservoirs present a variety of possible migration geometries most of which assume steady state or near steady state processes here we present results from marine magnetotelluric mt measurements carried out as part of the ramesses experiment on the slow spreading reykjanes ridge which support a model of melt extraction and migration that is episodic rather than steady state

detecting hydrocarbon reservoirs from csem data in complex settings application to deepwater sabah malaysia controlled source electromagnetic csem field surveys offer a geophysical method to discriminate between high and low hydrocarbon saturations in a potential reservoir however the same geological processes that create the possible hydrocarbon reservoir may also create topography and near surface variations of resistivity e g shallow gas or hydrates that can complicate the interpretation of csem data in this paper we discuss the interpretation of such data over a thrust belt prospect in deepwater sabah malaysia we show that detailed modeling of the key scenarios can help us understand the contributions of topography near surface hydrates and possible hydrocarbons at reservoir depth complexity at the surface and at depth requires a 3d electromagnetic modeling code that can handle realistic ten million cell models this has been achieved by using an iterative solver based on a multigrid preconditioner finite difference approach with frequency dependent grid adaptation

a marine electromagnetic survey to detect gas hydrate at hydrate ridge oregon gas hydrates are a potential energy resource and hazard for drilling and infrastructure yet estimates of global volume vary by over three orders of magnitude hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations a marine electromagnetic survey was carried out at hydrate ridge oregon usa with the aim of testing the use of controlled source electromagnetic csem and magnetotelluric mt methods to map gas hydrate and free gas below the gas hydrate stability zone a 2 d csem inversion supports the scenario deduced from previous seismic and drilling results which indicate two mechanisms of hydrate emplacement a transport dominated and reaction dominated regime a prominent resistive region of 2 5 4 m at a depth of about 130 mbsf near the seismic bottom simulating reflector bsr suggests that 27 to 46 per cent of the bulk volume is filled with hydrate depending on whether archie s law or the hashin strikman bounds are used this is representative of a reaction dominated regime for hydrate emplacement and where a significant low velocity zone exists based on a seismic tomography inversion suggests large quantities of free gas below the bsr electrical resistivity logging while drilling lwd data show general agreement with the csem inversion model except for a csem derived resistive region at seismic horizon a known to transport free gas into the gas hydrate stability zone inversion of mt data collected simultaneously during the csem survey provides a complimentary low resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model

erratum invasion of drilling mud into gas hydrate bearing sediments part i effect of drilling mud properties fulong ning 1 2 keni zhang 3 4 nengyou wu 2 ling zhang 1 gang li 2 guosheng jiang 1 yibing yu 1 li liu1 and yinghong qin1 1faculty of engineering china university of geosciences wuhan hubei 430074 china e mail nflzx cug edu cn 2guangzhou center for gas hydrate research chinese academy of sciences guangzhou 510640 china 3earth system division lawrence berkeley national laboratory berkeley ca 94720 usa 4college of water sciences beijing normal university beijing 100875 china

fracture networks and hydrate distribution at hydrate ridge oregon abstract analysis of resistivity at the bit rab images acquired during ocean drilling program odp leg 204 to southern hydrate ridge oregon reveals changes in the distribution and orientation of resistive fractures between the different hydrologic provinces sampled fractures on the ridge crest at sites 1247 1248 and 1250 are typically high angle 60 with isotropically distributed strikes while those on the ridge flanks at sites 1244 1246 have lower dips 20 60 and strike parallel to the trend of the ridge the eastern slope basin site 1251 lacks significant fracturing these differences indicate that the minimum principal stress 3 direction rotates to horizontal at the ridge crest as a result of topographically driven extension and gravitational collapse of the ridge chaotic fracturing on the ridge crest results from the interaction of the stress state and free gas migration which is confined to high permeability sediments and fracture zones by capillary pressure constraints in contrast on the ridge flanks well ordered fractures that agree with stress orientations determined from borehole breakouts result from an obliquely dipping 3 and the predominance of aqueous rather than gaseous flow

geochemical evolution of pore waters in the rotliegend early permian of northern germany abstract the distribution of early cements within permian rotliegend sandstones closely resembles cementation patterns within modern playa lake basins the isotopic compositions of eogenetic cements in the thick rotliegend sequence of germany show continental signatures typical of minerals precipitated in such an environment carbon sulphur and strontium isotopic ratios of many rotliegend cements which formed during burial suggest formation from a mixture of rotliegend fluids and waters from the overlying zechstein evaporites a two component mixing model for the evolution of rotliegend pore waters indicates that the isotopic signatures in these cements may result from the introduction of only minor amounts of zechstein fluid

seismic anisotropy at hydrate ridge 1 p wave velocity increases in the presence of gas hydrates and decreases in the presence of free gas in the sediments making it an excellent means to investigate gas hydrate systems however seismic velocity is typically derived from surface seismic data without consideration of seismic anisotropy the presence of anisotropy in the hydrate bearing sediments adds an additional complexity in data analysis however anisotropy can help reveal the distribution of hydrates here we report on the evidence of seismic anisotropy at hydrate ridge along the cascadia convergent margin we find that the south summit is anisotropic while the basin side east of south summit is isotropic anisotropy is likely caused by the hydrate veins we interpret the anisotropy parameters in terms of the distribution and fabric of gas hydrates