observed correlation between the depth to base and top of gas hydrate occurrence from review of global drilling data a global inventory of data from gas hydrate drilling expeditions is used to develop relationships between the base of structure i gas hydrate stability top of gas hydrate occurrence sulfate methane transition depth pressure water depth and geothermal gradients the motivation of this study is to provide first order estimates of the top of gas hydrate occurrence and associated thickness of the gas hydrate occurrence zone for climate change scenarios global carbon budget analyses or gas hydrate resource assessments results from publically available drilling campaigns 21 expeditions 52 drill sites off cascadia blake ridge india korea south china sea japan chile peru costa rica gulf of mexico and borneo reveal a first order linear relationship between the depth to the top and base of gas hydrate occurrence the reason for these nearly linear relationships is believed to be the strong pressure and temperature dependence of methane solubility in the absence of large difference in thermal gradients between the various sites assessed in addition a statistically robust relationship was defined between the thickness of the gas hydrate occurrence zone and the base of gas hydrate stability in meters below seafloor the relationship developed is able to predict the depth of the top of gas hydrate occurrence zone using observed depths of the base of gas hydrate stability within less than 50 meters at most locations examined in this study no clear correlation of the depth to the top and base of gas hydrate occurrences with geothermal gradient and sulfate methane transition depth were identified

constraints on mantle evolution from 187os 188os isotopic compositions of archean ultramafic rocks from southern west greenland 3 8 ga and western australia 3 46 ga initial 187os 188os isotopic compositions for geochronologically and geologically well constrained 3 8 ga spinel peridotites from the itsaq gneiss complex of southern west greenland and chromite separates from 3 46 ga komatiites from the pilbara region of western australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle the measured compositions of 187os 188os 0 0 10262 2 from an olivine separate and 0 10329 3 for a spinel separate from 3 8 ga peridotite g93 42 are the lowest yet reported from any terrestrial sample the corrections for in situ decay over 3 8 ga for these low re os phases are minimal and change the isotopic compositions by only 0 5 and 2 2 for the spinel and the olivine respectively resulting in 187os 188os 3 8 ga 0 1021 0 0002 and 0 1009 0 0002 respectively these data extend direct measurement of os isotopic compositions to much earlier periods of earth history than previously documented and provide the best constraints on the os isotopic composition of the early archean terrestrial mantle analyses of pilbara chromites yield 3 46 ga mantle compositions of 0 1042 0 0002 and 0 1051 0 0002 these new data combined with published initial os isotopic compositions from late archean and early proterozoic samples are compatible with the mantle or at least portions of it evolving from a solar system initially defined by meteorites to a modern composition of 187os 188os 0 0 1296 0 0008 as previously suggested from peridotite xenolith data meisel et al 2001 the associated 187re 188os 0 0 435 0 005 thus chondritic 187os 188os compositions were a feature of the upper mantle for at least 3 8 billion years requiring chondritic re os ratios to have been a characteristic of the very early terrestrial mantle in contrast nonchondritic initial compositions of some archean komatiites demonstrate that os isotopic heterogeneity is an ancient feature of plume materials reflecting the development of variable re os mantle sources early in earth history the lower average 187os 188os 0 1247 for abyssal peridotites snow and reisberg 1995 indicate that not all regions of the modern mantle have evolved with the same re os ratio the relative sizes of the various reservoirs are unknown although mass balance considerations can provide some general constraints for example if the unradiogenic 187os 188os modern abyssal peridotite compositions reflect the prevalent upper mantle composition then the complementary high re os basaltic reservoir must represent 20 to 40 by mass of the upper mantle taken here as 50 of the entire mantle depending on the mean storage age the difficulties associated with efficient long term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly re depleted rather abyssal peridotites sample anomalous mantle regions the existence of 3 8 ga mantle peridotites with chondritic 187os 188os compositions and with os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer these observations require that any highly siderophile element rich component must have been added to the earth and transported into and grossly homogenized within the mantle by 3 8 ga either large scale mixing of impact materials occurred on very short 0 100 myr timescales or the interpretation preferred here the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at 3 8 to 3 9 ga

eoarchean contrasting ultra high pressure to low pressure metamorphisms 1000 c gpa explained by tectonic plate convergence in deep time abstract greenland s itsaq gneiss complex igc shows eoarchean 3600 ma 250 400 c gpa low t p high pressure and 1000 c gpa high t p metamorphic regimes demonstrating a similarity of contrasting metamorphic t p regimes from the phanerozoic back to the start of earth s rock record low t p metamorphism produced i deep crustal eclogitised mafic rocks which upon partial melting formed the tonalites dominating the igc ii 550 c 2 6 gpa conditions 250 c gpa demonstrated by an olivine antigorite titano chondrodite titano clinohumite relict assemblage within mantle slivers showing geochemical and crystallographic features of a suprasubduction environment that were exhumed into the crust by 3712 ma iii rare vestiges of 3658 ma high pressure garnet clinopyroxene granulite and iv barrovian style kyanite staurolite assemblages high t p metamorphism is shown by 3669 ma crustal melts equilibrated with orthopyroxene this was coeval to the youngest juvenile tonalitic crust in the complex latter derived by anatexis under low t p conditions and a 3670 3570 ma history of deep crust migmatisation under low pressure garnet free conditions structural geology of the igc indicates its low t p regimes coincide with crustal imbrication by compression of arc like tholeiites boninite like lavas andesites felsic intermediate volcano sedimentary rocks and chemical sedimentary rocks whereas post 3660 ma high t p metamorphism was marked by late orogenic extension exhumation and deep crustal flow with mafic underplating and partial melting generating granites thus the diversity of earth s earliest recorded geodynamic settings resembles more those of modern geodynamics than the lithological and structural relationships expected from theoretical non uniformitarian scenarios like drip tectonics in a stagnant lid regime the recognition of an ultra high pressure 250 c gpa metamorphic regime at 3700 ma in the igc removes the last argument against a form of plate tectonics operating throughout the archean hence since the start of the rock record a mobile lid plate tectonic regime contributed to interior heat loss facilitating chemical communication and feedbacks between earth s surface and its deep interior

preferential mode of gas invasion in sediments grain scale mechanistic model of coupled multiphase fluid flow and sediment mechanics 1 we present a discrete element model for simulating at the grain scale gas migration in brine saturated deformable media we rigorously account for the presence of two fluids in the pore space by incorporating forces on grains due to pore fluid pressures and surface tension between fluids this model which couples multiphase fluid flow with sediment mechanics permits investigation of the upward migration of gas through a brine filled sediment column we elucidate the ways in which gas migration may take place 1 by capillary invasion in a rigid like medium and 2 by initiation and propagation of a fracture we find that grain size is the main factor controlling the mode of gas transport in the sediment and we show that coarse grain sediments favor capillary invasion whereas fracturing dominates in fine grain media the results have important implications for understanding vent sites and pockmarks in the ocean floor deep subseabed storage of carbon dioxide and gas hydrate accumulations in ocean sediments and permafrost regions our results predict that in fine sediments hydrate will likely form in veins following a fracture network pattern and the hydrate concentration will likely be quite low in coarse sediments the buoyant methane gas is likely to invade the pore space more uniformly in a process akin to invasion percolation and the overall pore occupancy is likely to be much higher than for a fracture dominated regime these implications are consistent with laboratory experiments and field observations of methane hydrates in natural systems

bottom simulating reflectors seismic velocities and avo effects we obtain the wave velocities of ice and gas hydrate bearing sediments as a function of concentration and temperature unlike previous theories based on simple slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids grain and ice or clathrate and a liquid water and a porous matrix containing gas and water for consolidated berea sandstone the theory underestimates the value of the compressional velocity below 0 c including grain ice interactions and grain cementation yields a good fit to the experimental data strictly speaking water proportion and temperature are closely related fitting the wave velocity at a given temperature allows the prediction of the velocity throughout the range of temperatures provided that the average pore radius and its standard deviation are known the reflection coefficients are computed with a viscoelastic single phase constitutive model the analysis is carried out for the top and botto

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

high temperature hydration of ultramafic granulites from the southern marginal zone of the limpopo belt by infiltration of co2 rich fluid abstract the peak metamorphic assemblage of olivine orthopyroxene calcic amphibole spinel is replaced by a variety of high temperature retrograde assemblages in ultramafic granulites from the southern marginal zone of the limpopo belt the observed replacement of olivine according to the reaction forsterite co2 enstatite magnesite is restricted to the zone of rehydration which bounds the granulite zone in the south this observation is consistent with the post peak metamorphic infiltration of an externally derived co2 rich fluid during the high temperature post peak metamorphic history of the limpopo belt this event established a retrograde orthoamphibole isograd conditions of the retrogression are estimated to be near 670 c 6 kbar and xco2 fluid 0 67 c isotopic compositions of co2 extracted from magnesite produced by the high temperature breakdown of olivine supports a deep seated mantle source for the co2 rich retrograde fluid

complex gas hydrate system in a gas chimney south china sea abstract gas hydrates and deep sea cold seeps are of interest to industry and academia for several reasons including their implications for offshore geohazards and unconventional resources and for global climate change and carbon cycling guangzhou marine geological survey gmgs implemented its fifth gas hydrate drilling expedition in the qiongdongnan sea area qdn sa of the northern south china sea scs from june to september 2018 multidisciplinary investigations including logging while drilling lwd pressure and non pressure coring gas composition analyses in situ temperature and permeability measurements pore water geochemical analyses and a regional seismic survey reveal a complex distribution of gas hydrate within a vertical chimney structure lwd profiles record high resistivity anomalies and high p wave velocity variations at 9 174 mbsf suggesting the coexistence of free gas and gas hydrate in the sediment column throughout the investigated gas chimney hydrate bound and void gas is an admixture of methane 70 1 98 3 and heavier hydrocarbons c2h6 0 16 17 59 c3h8 0 05 9 88 i c4h10

pore water anomalies of submarine gas hydrate zones as tool to assess hydrate abundance and distribution in the subsurface what have we learned in the past decade abstract the most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the ocean drilling program odp notably legs 112 141 146 and 164 and to a lesser extent legs 131 160 170 and 186 leg 164 in particular was dedicated to gas hydrate drilling in the blake ridge gas hydrate field in the west atlantic in an unprecedented multidisciplinary research effort paull et al 1996 proc ocean drill program initial rep 164 623 pp paull c k matsumoto r wallace p j dillon w p eds 2000a proc ocean drill program sci results 164 ocean drilling program college station tx 459 pp most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones this started with qualitative evidence for advective flow from drilling the hydrate bearing peru and hydrate free barbados active margins continued with the nankai trough accretionary prism the japan trench slope and the cascadia and costa rica active margins and culminated in the quantitative advection diffusion model of egeberg and dickens chem geol 153 1999 53 for the passive margin setting of the blake ridge advective flow regimes are different at active and passive margins as there is a tendency for the flow at active margins to be focussed along landward dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step pattern of the pore water profiles for passive margins but also for some active margin sites advection is from sources below the drilled section either through subvertical faults passive margins or from the decollement zone active margins we have learned that the well known coupled pore water anomalies that are ascribed to hydrate dissociation downward chlorinity decrease combined with 18 o increase hesse and harrison 1981 earth planet sci lett 55 1981 453 need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low 18 o fluids however if the anomalies show up hydrates are present almost invariably with the exception of advected low cl high 18 o waters coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate affected pore water profiles at passive margins instrumental for modeling are chlorine isotopes which provide an effective tool to assess advection rates the egeberg and dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions modeling the more complex pore water profiles of active margins is a challenge for the future compared to geophysical methods to estimate hydrate concentration the geochemical method gives minimum amounts

observations related to tetrahydrofuran and methane hydrates for laboratory studies of hydrate bearing sediments the interaction among water molecules guest gas molecules salts and mineral particles determines the nucleation and growth behavior of gas hydrates in natural sediments hydrate of tetrahydrofuran thf has long been used for laboratory studies of gas hydrate bearing sediments to provide close control on hydrate concentrations and to overcome the long formation history of methane hydrate from aqueous phase methane in sediments yet differences in the polarizability of thf polar molecule compared to methane nonpolar molecule raise questions about the suitability of thf as a proxy for methane in the study of hydrate bearing sediments from existing data and simple macroscale experiments we show that despite its polar nature thf s large molecular size results in low permittivity prevents it from dissolving precipitated salts and hinders the solvation of ions on dry mineral surfaces in addition the interfacial tension between water and thf hydrate is similar to that between water and methane hydrate the processes that researchers choose for forming hydrate in sediments in laboratory settings e g from gas liquid or ice and the pore scale distribution of the hydrate that is produced by each of these processes likely have a more pronounced effect on the measured macroscale properties of hydrate bearing sediments than do differences between thf and methane hydrates themselves

a simple method of predicting s wave velocity prediction of shear wave velocity plays an important role in seismic modeling amplitude analysis with offset and other exploration applications this paper presents a method for predicting s wave velocity from the p wave velocity on the basis of the moduli of dry rock elastic velocities of water saturated sediments at low frequencies can be predicted from the moduli of dry rock by using gassmann s equation hence if the moduli of dry rock can be estimated from p wave velocities then s wave velocities easily can be predicted from the moduli dry rock bulk modulus can be related to the shear modulus through a compaction constant the numerical results indicate that the predicted s wave velocities for consolidated and unconsolidated sediments agree well with measured velocities if differential pressure is greater than approximately 5 mpa an advantage of this method is that there are no adjustable parameters to be chosen such as the pore aspect ratios required in some other methods the predicted s wave velocity depends only on the measured p wave velocity and porosity

characterization of in situ elastic properties of gas hydrate bearing sediments on the blake ridge during ocean drilling program leg 164 shear sonic velocity and other geophysical logs were acquired in gas hydrate bearing sediments on the blake ridge to characterize the very distinct seismic signature of such formations anomalous low amplitudes overlying a strong bottom simulating reflector bsr a comparison of the bulk moduli derived from the logs to standard elastic consolidation models shows that the sediments are overconsolidated above the bsr at 440 meters below seafloor mbsf because of the presence of hydrates below the bottom of the thermodynamic hydrate stability zone at 520 mbsf the high compressibility of the formation and the attenuation of the monopole sonic waveforms are typical of sediments partially saturated with free gas between these two depths gas hydrate and free gas seem to coexist within the gas hydrate stability zone we estimate the amount of gas hydrates using different models based on theories for wave scattering in multiphase media and for grain cementation in close agreement with measurements made on discrete in situ samples the latter describes most accurately the interactions between the matrix the pore fluids and the hydrates this model indicates that 5 to 10 of the pore space is occupied by hydrates deposited uniformly on the surface of the grains the comparison with gassmann s model also show that the amount of free gas below the bsr never exceeds 5 of the pore space but is high enough to generate the bsr the coexistence of free gas and gas hydrates below the bsr may be explained by capillary effects in the smaller pores or by remaining crystalline structures after partial hydrate decomposition

early archean serpentine mud volcanoes at isua greenland as a niche for early life the isua supracrustal belt greenland of early archean age 3 81 3 70 ga represents the oldest crustal segment on earth its complex lithology comprises an ophiolite like unit and volcanic rocks reminiscent of boninites which tie isua supracrustals to an island arc environment we here present zinc zn isotope compositions measured on serpentinites and other rocks from the isua supracrustal sequence and on serpentinites from modern ophiolites midocean ridges and the mariana forearc in stark contrast to modern midocean ridge and ophiolite serpentinites zn in isua and mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average based on recent results of zn isotope fractionation between coexisting species in solution the isua serpentinites were permeated by carbonate rich high ph hydrothermal solutions at medium temperature 100 300 c zinc isotopes therefore stand out as a ph meter for fossil hydrothermal solutions the geochemical features of the isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the mariana forearc the reduced character and the high ph inferred for these fluids make archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids

analysing sand dominated channel systems for potential gas hydrate reservoirs using an avo seismic inversion technique on the southern hikurangi margin new zealand abstract gas hydrates have recently been recognised as a class of unconventional petroleum resource and the economic viability of gas production from hydrates is now being viewed as a realistic possibility within the next decade therefore potential offshore hydrate accumulations in the world class endowed gas hydrate province the hikurangi margin new zealand represent a significant medium to long term opportunity to meet the country s future energy requirements in this paper we delineate a potential gas hydrate reservoir in the east coast basin new zealand and quantitatively estimate its gas hydrate concentrations from 2d seismic data with no well information available the target is interesting for exploration since it shows evidence for gas hydrate bearing sands in particular buried channel systems we use a combined analysis of high resolution velocity analysis amplitude versus offset avo attribute and avo inversion to investigate whether we can identify regions that are likely to contain highly concentrated gas hydrates and whether they are likely to be sand dominated to estimate hydrate concentrations we apply a rock physics model our results indicate the presence of several up to 200 m thick zones that are likely to host gas hydrates with one location predicted to consist of high permeable channel sands and an inferred gas hydrate saturation of 25 these findings suggest significant amounts of gas hydrates may be present in high quality reservoirs on this part of the margin

identifying patchy saturation from well logs gassmann s 1951 equations relate the elastic bulk ksat and shear gsat moduli of a fully saturated rock to the elastic bulk and shear moduli of the dry rock frame kdry and gdry respectively porosity and the bulk moduli of the mineral phase ks and pore fluid kf

trace metal deposition and mobility in the sediments of two lakes near sudbury ontario the accumulation and mobility of fe mn al cu ni and pb in the sediments of two lakes clearwater ph 4 5 and mcfarlane ph 7 5 near sudbury ontario have been investigated the al cu and ni concentrations are expectedly relatively high in the overlying waters of clearwater lake and much lower for al and cu in mcfarlane lake the low trace metal concentrations found in the anoxic porewaters of clearwater lake could be explained by a sharp increase in porewater ph concomitant with so42 reduction and h2s production within the first 1 2 cm of the sediments which has conceivably led to the precipitation of mineral phases such as al oh 3 nis and cus in both lakes fe concentrations in anoxic porewaters appear to be controlled by fes and or feco3 formation solubility calculations also indicate mnco3 precipitation in mcfarlane lake in clearwater lake however both porewater and total mn were relatively low a possible result of the continuous loss of mn ii through the acidic interface it is suggested that upwardly decreasing total mn profiles resulting from the removal of mn from the top sediment layers under acidic conditions may constitute a reliable symptom of recent lake acidification the downward diffusion of ai cu and ni from the overlying water to the sediments has been estimated from their concentration gradients at the interface and compared to their total accumulation rates in the sediments in both lakes the diffusion of al is negligible compared to its accumulation rate however diffusion accounts for 24 52 of the accumulation of cu in the sediments of clearwater lake but appears negligible in mcfarlane lake the downward diffusive flux of ni is important and may explain 76 161 of the estimated ni accumulation rate in clearwater lake and 59 in mcfarlane lake the porewater cu and ni profiles suggest that the subsurface sedimentary trace metal peaks observed in clearwater lake as in other acid lakes may not be caused by sediment leaching or by a recent reduction in sedimentation but may have a diagenetic origin instead diffusion to the sediments thus appears to be an important and previously overlooked trace metal deposition mechanism particularly in acid lakes

reconstruction of a 3700 ma transgressive marine environment from isua greenland sedimentology stratigraphy and geochemical signatures abstract the rare preservation of a stratigraphy in the northwest of the eoarchean isua supracrustal belt greenland demonstrates the world s earliest recognised marine transgression stratigraphically lowest is the 3710 ma solvang volcanic formation of picrites basalts and basaltic andesites with arc like geochemical signatures close to its erosional top this formation s volcanic rocks show increase in k2o with depletion of na2o depletion in sr with no alteration in eu abundance and 18ovsmow increase to 16 interpreted as subaerial weathering at 3700 ma the sedimentary snowpatch formation was deposited unconformably on the solvang volcanic formation locally the unconformity is marked by the laterally discontinuous conglomerate member dominated by rounded quartz clasts but also with clasts of basalt and andesite up to 10 cm across layered chert and ultramafic rocks the conglomerate member carries sparse detrital zircons the youngest of which are 3710 ma the conglomerates are succeeded by the sandstone and dolostone member of layered quartz and dolomite clastic sandstones that locally preserve relict shallow marine cross bedding with scoured layers tempestite breccia beds and very rare stromatolite horizons the sandstone and dolostone member is succeeded by the 3695 ma bif member the snowpatch formation quartz dolomite sandstones and interlayered stromatolites have seawater like ree y trace element patterns variably modified by a negligible to significant influx of a terrigenous component trace element signatures such as ti v ratios and normalising ree y data to a 3710 ma weathered volcanic rock rather than post archean average shale suggest this component was derived from a juvenile arc source resembling the basement solvang volcanic formation the detailed field and geochemical evidence such as positive correlation between fe mg and pr yb in the snowpatch formation dolostones and bif banded iron formation indicates that the dolomitic lithologies developed in a shallow water setting with clastic sedimentation during a marine transgression over a weathered 3710 ma volcanic arc basement followed by bif deposition in deeper water thus by 3700 ma shallow water exposed settings had already been established which were the site of stromatolite development

microscale chemical heterogeneity in groundwater ronen d magaritz m gvirtzman h and garner w 1987 microscale chemical hetereogeneity in groundwater j hydrol 92 173 178 large variations in the concentration of cl no a and so42 in the upper water layers of a polluted aquifer were found between samples that were vertically separated by as little as 3 cm this was disclosed when a multilayer dialysis cell device was used to obtain undisturbed real time samples from test wells this first time presentation of chemical fine structure provides new insights into mechanisms of contamination of an aquifer by surface inputs and raises the question of the validity of groundwater quality data based on single samples however obtained

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

the link between bottom simulating reflections and methane flux into the gas hydrate stability zone new evidence from lima basin peru margin bottom simulating reflections bsrs are probably the most commonly used indicators for gas hydrates in marine sediments it is now widely accepted that bsrs are primarily caused by free gas beneath gas hydrate bearing sediments however our insight into bsr formation to date is mostly limited to theoretical studies two endmember processes have been suggested to supply free gas for bsr formation i dissociation of gas hydrates and ii migration of methane from below during a recent campaign of the german research vessel sonne off the shore of peru we detected bsrs at locations undergoing both tectonic subsidence and non sedimentation or seafloor erosion tectonic subsidence and additionally perhaps seafloor erosion causes the base of gas hydrate stability to migrate downward with respect to gas hydrate bearing sediments this process rules out dissociation of gas hydrates as a source of free gas for bsrs at these locations instead free gas at bsrs is predicted to be absorbed into the gas hydrate stability zone bsrs appear to be confined to locations where the subsurface structure suggests focusing of fluid flow we investigated the seafloor at one of these locations with a tv sled and observed fields of rounded boulders and slab like rocks which we interpreted as authigenic carbonates authigenic carbonates are precipitations typically found at cold vents with methane expulsion we retrieved a small carbonate cemented sediment sample from the seafloor above a bsr about 20 km away this supported our interpretation that the observed slabs and boulders were carbonates all these observations suggest that bsrs in lima basin are maintained predominantly by gas that is supplied from below demonstrating that this endmember process for bsr formation exists in nature results from ocean drilling program leg 112 showed that methane for gas hydrate formation on the peru lower slope and the methane in hydrocarbon gases on the upper slope is mostly of biogenic origin the 13c composition of the recovered carbonate cement was consistent with biologic methane production below the seafloor although possibly above the bsr we speculate that the gas for bsr formation in lima basin also is mainly biogenic methane this would suggest the biologic productivity beneath the gas hydrate zone in lima basin to be relatively high in order to supply enough methane to maintain bsrs

characterization of suspended particles collected in groundwater under natural gradient flow conditions microscale eulerian variations in the flux mineralogical composition and size of suspended particles have been found in a contaminated sandy aquifer under natural gradient flow conditions during an 8 month study period particle variability has been detected along a 16 m saturated section of the aquifer at a scale of centimeters and meters in the vertical and horizontal dimensions respectively the average concentration of particles in groundwater varied between 1 and 40 mg l but high concentrations of up to 5000 mg l were determined in specific 3 cm vertical intervals of the aquifer the particles were primarily composed of caco3 11 57 quartz 7 39 and clays 8 43 most of the particles were within the 140 3000 nm size range with size modes varying from 310 to 660 nm the large amounts of suspended particles are considered to be related to high inputs of dissolved organic carbon into groundwater from sewage effluents which have been used for agricultural irrigation since the early 1960s as a result of organic matter biodegradation in the saturated zone anoxic conditions developed and the pco2 content of groundwater increased dramatically pcaco2 10 1 8 to 10 1 3 atm it is postulated that part of the carbonate cement of the rocks dissolved and detrital caco3 quartz and clay were released as colloidal particles part of the clay particles could have also been transported through the unsaturated zone into groundwater after mobilization from surface layers as a result of the intermittent input of water of different chemical quality in the prevailing anoxic conditions of groundwater at the study site dissolved oxygen concentrations of 1 mg l colloidal stability is enhanced by organic matter coating of particles

three dimensional seismic studies of deep water hazard related features on the northern slope of south china sea mass transport deposits and geological features related to fluid flow such as gas chimneys mud diapirs and volcanos pockmarks and gas hydrates are pervasive on the canyon dominated northern slope of the pearl river mouth basin of the south china sea these deposits and structures are linked to serious geohazards and are considered risk factors for seabed installations based on high resolution three dimensional seismic surveys seismic characteristics distributions and origins of these features are analyzed a distribution map is presented and geometrical parameters and spatial distribution patterns are summarized results show that various groups of the mapped features are closely tied to local or regional tectonism and sedimentary processes mass transport complexes are classified as slides near the shelf break initially deformed slumps on the flanks of canyons and highly deformed slumps on the lower slope downslope of the mouth of canyons we propose them to be preconditioned by pore pressure changes related to sea level fluctuations steep topography and fluid and fault activities gas chimneys are mainly located in the vicinity of gas reservoirs while bottom simulating reflectors are observed within the gas chimney regions suggesting gas chimneys serve as conduits for thermogenic gas mud diapirs volcanos and pockmarks are observed in small numbers and the formation of pockmarks is related to underlying gas chimneys and faults this study aims at reducing risks for deep water engineering on the northern slope of south china sea c 2016 elsevier ltd all rights reserved

conditions for hydrofracture and the fluid permeability of accretionary wedges abstract the dewatering of fine grained sediment piles in accretionary wedges is controlled by zones of active deformation episodic hydrofracture in these zones enhances permeability enabling fluid flow to be faster than the speed of underthrusting here i investigate the mechanics of simultaneous shear failure and hydrofracture in muddy rocks as a function of the faulting mode the physical properties of model rock types are chosen to represent uncemented mud or cherty marly mudstones that have undergone significant diagenesis during thrust faulting hydrofracture always requires pore fluid pressures in excess of the lithostatic load whereas shallow level wrenching or normal faulting permits hydrofracture at hydrostatic pressures significantly lower than lithostatic load all three faulting modes are expected to occur in an accretionary wedge attempting to dynamically maintain its critical taper the model computations indicate that in the upper 80 120 m of a section of soft mudstone hydrofracture inevitably accompanies wrench or normal faulting hard mudstones inevitably hydrofracture to depths of up to 1400 m during normal faulting these differences in mechanical behaviour have profound consequences for fluid venting capabilities of accretionary wedges they influence hydrocarbon trapping and play a role in the localisation of mud diapirs and mud volcanism

extending nmr data for permeability estimation in fine grained sediments abstract we combine nuclear magnetic resonance nmr transverse relaxation time data and gamma ray data to estimate lithology dependent permeability in silt and clay rich sediments this approach extends the utility of the schlumberger doll research sdr permeability equation from reservoirs to aquicludes and seals and thus improves the value and robustness of nmr data data from keathley canyon northern gulf of mexico show that nmr data can be used to define permeability from 10 18 to 10 14 m 2 0 001 10 millidarcies as calibrated and tested by direct measurements on core samples we performed uniaxial constant rate of strain consolidation experiments on sediments from keathley canyon to determine core scale permeability permeabilities from these experiments were compared to permeabilities calculated from logging while drilling data a better fit between log derived permeability and laboratory measured permeability was obtained using the sdr equation with a variable coefficient a rather than a constant a as is typically used we show how a is a function of lithology and can be modeled from gamma ray data the relationship between a and gamma ray values suggests that variations in a are caused by platy clay minerals and the effect they have on the pore system our results provide improved means for permeability estimation for application in basin flow modeling hydrocarbon migration modeling and well completion design

interaction of metals with mineral surfaces in a natural groundwater environment abstract sorption characteristics of four major constituents of sedimentary rocks quartz calcite kaolinite and bentonite are studied under undisturbed field conditions in the coastal plain aquifer of israel dynamic interaction of minerals with groundwater is indicated by simultaneous enrichment of minerals with respect to certain metals and depletion with respect to others the distribution coefficient d is calculated for ni cu cr co fe cd mn and pb over a 2 m depth interval in the upper part of the aquifer the distribution of fe pb and ni is governed by the surface properties of the minerals cd and mn show a strong affinity for calcite and may be incorporated into calcite grains cu and cr are most effectively sorbed by kaolinite the suggested experimental approach may be useful for the study of site specific contamination with hazardous metals

gas hydrate saturations estimated from fractured reservoir at site nghp 01 10 krishna godavari basin india 1 during the indian national gas hydrate program expedition 01 nghp 01 one of the richest marine gas hydrate accumulations was discovered at site nghp 01 10 in the krishna godavari basin the occurrence of concentrated gas hydrate at this site is primarily controlled by the presence of fractures assuming the resistivity of gas hydrate bearing sediments is isotropic the conventional archie analysis using the logging while drilling resistivity log yields gas hydrate saturations greater than 50 as high as 80 of the pore space for the depth interval between 25 and 160 m below seafloor on the other hand gas hydrate saturations estimated from pressure cores from nearby wells were less than 26 of the pore space although intrasite variability may contribute to the difference the primary cause of the saturation difference is attributed to the anisotropic nature of the reservoir due to gas hydrate in high angle fractures archie s law can be used to estimate gas hydrate saturations in anisotropic reservoir with additional information such as elastic velocities to constrain archie cementation parameters m and the saturation exponent n theory indicates that m and n depend on the direction of the measurement relative to fracture orientation as well as depending on gas hydrate saturation by using higher values of m and n in the resistivity analysis for fractured reservoirs the difference between saturation estimates is significantly reduced although a sizable difference remains to better understand the nature of fractured reservoirs wireline p and s wave velocities were also incorporated into the analysis

elastic impedance inversion of multichannel seismic data from unconsolidated sediments containing gas hydrate and free gas the elastic properties of hydrated sediments are not well known which leads to inaccuracy in the evaluation of the amount of gas hydrate worldwide elastic impedance inversion is useful in estimating the elastic properties of sediments containing gas hydrate or free gas trapped beneath the gas hydrate from angle dependent p wave reflections we reprocess the multichannel u s geological survey seismic line bt 1 from the blake ridge off the east coast of north america to obtain migrated common angle aperture data sets which are then inverted for elastic impedance two new algorithms to estimate p impedance and s impedance from the elastic impedance are developed and evaluated using well log data from ocean drilling program odp leg 164 these new algorithms are stable even in the presence of modest noise in the data the vs vp ratio poisson s ratio and lame parameter terms and are estimated from the p impedance and s impedance the hydrated sediments have high elastic impedance high p impedan

geology age and origin of supracrustal rocks at akilia west greenland new mapping geochronology and geochemistry of the supracrustal enclave and associated orthogneisses at akilia west greenland support a 3825 6 ma age for the sequence and a chemical sedimentary origin for the controversial fe rich quartz pyroxene gneisses lithologies of the enclave comprise laterally continuous mappable units of mafic amphibolite ultramafic rocks and two fe rich quartz pyroxene units a minor anthophyllite garnet rock with chemical characteristics suggesting a sedimentary protolith was identified the earliest foliation parallels lithologic contacts this foliation was isoclinally folded about a steep hinge surface and then tightly refolded about a steep ns hinge plane the latest folding produced limbs exhibiting contrasting strains the full deformation history is shared by all lithologies of the enclave two previously unidentified orthogneiss sheets preserve deformed magmatic crosscutting relations with amphibolite and ultramafic units in the low strain limb the supracrustal anthophyllite garnet rock experienced zircon growth at 3600 and 2700 ma in common with amphibolites these ages are interpreted to reflect metamorphism at granulite and amphibolite facies respectively consistent with independent regional evidence the crosscutting orthogneiss sheets preserve a complex history of zircon growth but only earliest cores indicate th u exchange equilibrium with the bulk rock at magmatic conditions weighted mean 207 pb 206 pb ages of cores of 3730 7 ma and 3825 6 ma date the crystallization ages of the sheets indicating that the akilia supracrustal rocks were deposited 3819 ma quartz pyroxene rocks on akilia have been interpreted as metamorphosed chemical sediments that may preserve carbon isotope evidence for life at the time of their formation an alternative proposal is an origin by metasomatic alteration our results show that these rocks experienced a history of deformation and zircon growth identical to other lithologies of the supracrustal body metamorphic zircons give a minimum 207 pb 206 pb age of 3589 13 ma regardless of origin the two models of formation of quartz pyroxene rocks were evaluated using field petrologic and geochemical tests the distribution of the units the absence of field or petrologic indicators of metasomatism and geochemistry oxygen and nontraditional stable isotopes immobile elements and ree together support a sedimentary but not a metasomatic origin because their protolith was sedimentary the quartz pyroxene rocks are a primary part of the supracrustal sequence and have the same minimum age despite strong deformation and polymetamorphism the akilia supracrustal enclave contains information about terrestrial surficial processes at 3819 ma

scale dependent gas hydrate saturation estimates in sand reservoirs in the ulleung basin east sea of korea abstract through the use of 2 d and 3 d seismic data several gas hydrate prospects were identified in the ulleung basin east sea of korea and thirteen drill sites were established and logging while drilling lwd data were acquired from each site in 2010 sites ubgh2 6 and ubgh2 10 were selected to test a series of high amplitude seismic reflections possibly from sand reservoirs lwd logs from the ubgh2 6 well indicate that there are three significant sand reservoirs with varying thickness two upper sand reservoirs are water saturated and the lower thinly bedded sand reservoir contains gas hydrate with an average saturation of 13 as estimated from the p wave velocity the well logs at the ubgh2 6 well clearly demonstrated the effect of scale dependency on gas hydrate saturation estimates gas hydrate saturations estimated from the high resolution lwd acquired ring resistivity vertical resolution of about 5 8 cm reaches about 90 with an average saturation of 28 whereas gas hydrate saturations estimated from the low resolution a40l resistivity vertical resolution of about 120 cm reaches about 25 with an average saturation of 11 however in the ubgh2 10 well gas hydrate occupies a 5 m thick sand reservoir near 135 mbsf with a maximum saturation of about 60 in the ubgh2 10 well the average and a maximum saturation estimated from various well logging tools are comparable because the bed thickness is larger than the vertical resolution of the various logging tools high resolution wireline log data further document the role of scale dependency on gas hydrate calculations

mobile sediment in an urbanizing karst aquifer implications for contaminant transport here we investigate geochemical characteristics of sediment in different compartments of a karst aquifer and demonstrate that mobile sediments in a karst aquifer can exhibit a wide range of properties affecting their contaminant transport potential sediment samples were collected from surface streams sinkholes caves wells and springs of a karst aquifer the barton springs portion of the edwards balcones fault zone aquifer central texas and their mineralogy grain size distribution organic carbon content and specific surface area analyzed statistical analysis of the sediments separated the sampling sites into three distinct groups 1 streambeds sinkholes and small springs 2 wells and 3 caves sediments from the primary discharge spring were a mix of these three groups high organic carbon content and high specific surface area gives some sediments an increased potential to transport contaminants the volume of these sediments is likely to increase with continued urbanization of the watershed

origin and evolution of fracture hosted methane hydrate deposits 1 fracture hosted methane hydrate deposits exist at many sites worldwide these sites often have hydrate present as vein and fracture fill as well as disseminated through the pore space we estimate that thousands to millions of years are required to form fracture systems by hydraulic fracturing driven by occlusion of the pore system by hydrate this time scale is a function of rates of fluid flow and permeability loss low permeability layers in a sedimentary column can reduce this time if the permeability contrast with respect to the surrounding sediments is of order 10 or greater additionally we find that tensile fracturing produced by hydrate heave around hydrate lenses is a viable fracture mechanism over all but the lowermost part of the hydrate stability zone with our coupled fluid flow hydrate formation model we assess fracture formation at four well studied hydrate provinces blake ridge offshore south carolina hydrate ridge offshore oregon keathley canyon block 151 offshore louisiana and the krishna godavari basin offshore india we conclude that hydraulic fracturing due to pore pressure buildup is reasonable only at hydrate ridge and the krishna godavari basin owing to sediment age constraints and that hydrate filled fractures observed at blake ridge and keathley canyon block 151 are formed either by hydrate heave or in preexisting fractures our findings offer new insight into the processes and time scales associated with fracture hosted hydrate deposits which help further our understanding of hydrate systems

clumped isotope thermometry of magnesium carbonates in ultramafic rocks abstract magnesium carbonate minerals produced by reaction of h 2 o co 2 with ultramafic rocks occur in a wide range of paragenetic and tectonic settings and can thus provide insights into a variety of geologic processes including 1 deposition of ore grade massive vein cryptocrystalline magnesite 2 formation of hydrous magnesium carbonates in weathering environments and 3 metamorphic carbonate alteration of ultramafic rocks however the application of traditional geochemical and isotopic methods to infer temperatures of mineralization the nature of mineralizing fluids and the mechanisms controlling the transformation of dissolved co 2 into magnesium carbonates in these settings is difficult because the fluids are usually not preserved clumped isotope compositions of magnesium carbonates provide a means to determine primary mineralization or re equilibration temperature which permits the reconstruction of geologic processes that govern magnesium carbonate formation we first provide an evaluation of the acid fractionation correction for magnesium carbonates using synthetic magnesite and hydromagnesite along with natural metamorphic magnesite and low temperature hydromagnesite precipitated within a mine adit we show that the acid fractionation correction for magnesium carbonates is virtually indistinguishable from other carbonate acid fractionation corrections given current mass spectrometer resolution and error in addition we employ carbonate clumped isotope thermometry on natural magnesium carbonates from various geologic environments and tectonic settings cryptocrystalline magnesite vein deposits from california red mountain magnesite mine austria kraubath locality turkey tutluca mine eskisehir district and iran derakht senjed deposit exhibit broadly uniform 47 compositions that yield apparent clumped isotope temperatures that average 23 7 5 0 c based on oxygen isotope thermometry these clumped isotope temperatures suggest mineralization at shallow crustal depths in the presence of meteoric water hydrous magnesium carbonates from a 400 km latitudinal transect along the serpentinized peridotite bodies of the california coast ranges record clumped isotope temperatures between 14 2 and 22 7 2 8 c in agreement with historical maximum temperatures during the rainy season for california talc carbonate alteration of ultramafic rocks in greenland isua supracrustal belt and vermont ludlow yields clumped isotope alteration temperatures of magnesite and dolomite between 326 and 490 c broadly consistent with paragenesis and thermodynamic analysis for co 2 metasomatism of ultramafic rocks these metamorphic carbonates extend the applicability of clumped isotope thermometry to high temperature magnesium carbonate systems and indicate equilibrium blocking temperatures for magnesite of 490 c our study demonstrates the applicability of the clumped isotope approach to provide information on the formation of magnesium carbonates as ore resources surface records of climate and metamorphic assemblages

capillary controls on methane hydrate distribution and fracturing in advective systems 1 we simulate 1 d steady advective flow through a layered porous medium to investigate how capillary controls on solubility including the gibbs thomson effect in fine grained sediments affect methane hydrate distribution in marine sediments we compute the increase in pore fluid pressure that results from hydrate occluding the pore space and allow fractures to form if the pore fluid pressure exceeds a fracture criterion we apply this model to hydrate ridge and northern cascadia two field sites where hydrates have been observed preferentially filling cm scale coarse grained layers we find that at hydrate ridge hydrate forms in the coarse grained layers reaching saturation of 90 creating fractures through intervening fine grained layers after 2000 years at northern cascadia hydrate forms preferentially in the coarse grained layers but 2 105 years are required to develop the observed hydrate saturations 20 60 suggesting that hydrate formation rates may be enhanced by an additional source of methane such as in situ methanogenesis we develop expressions to determine the combinations of sediment physical properties and methane supply rates that will result in hydrate filled coarse grained layers separated by hydrate filled fine grained layers the conditions necessary to fracture the fine grained layers and the conditions that will lead to complete inhibition of hydrate formation as pore space is constricted this work illustrates how sediment physical properties control hydrate distribution at the pore scale and how hydrate distribution affects fracturing behavior in marine sediments

increasing the accuracy of estimated porosity and saturation for gas hydrate reservoir by integrating geostatistical inversion and lithofacies constraints abstract porosity saturation thickness of gas hydrate bearing layers and their spatial distributions are key factors for production tests the gas hydrate bearing layers ranging from a few to tens of meters thickness are confirmed by logging while drilling lwd data and core samples at sites w11 and sc 03 in the shenhu area south china sea gas hydrate bearing layers of two wells about 200 m show that not all layers exhibit good lateral continuity several thin layers 4 m with low gas hydrate saturations 15 20 of the pore space or non gas hydrate are laterally discontinuous the interbedding of gas hydrate bearing and non hydrate layers with different saturations thickness and random distribution cause the heterogeneities in the reservoir the gas hydrate saturation and distribution inverted from deterministic and traditional geostatistical inversion have high uncertainties due to the limitations of the empirical relationship and probability density functions pdfs a better understanding of spatial distribution and saturation for each gas hydrate layer is useful to build a realistic net hydrate pay and resource estimation in this study we first divide gas hydrate saturations porosity acoustic impedance into low and high values based on the estimated lwd results which is call as lithofacies then we propose to create multivariate pdfs by lithofacies constraints the acoustic impedance saturation thickness porosity of gas hydrate bearing layers and non gas hydrate layers are inverted by displaying geostatistical inversion with lithofacies constraint we found the spatial variations and heterogeneous distributions with a thickness ranging from 4 to 35 m and the laminated gas hydrate layers with saturations ranging from 10 to 40 are clearly shown from seismic data which are useful for the horizontal well development method of a hydrate production test

dynamic accumulation of gas hydrates associated with the channel levee system in the shenhu area northern south china sea abstract research on the formation and distribution of submarine channel systems and associated gas bearing fluids is of great significance for gas hydrate exploration disseminated gas hydrates with high saturation up to 65 were recovered from a submarine ridge equivalent to the levee of the channel levee system in the shenhu area northern south china sea sedimentary deposits in the submarine ridge were dominated by fine grained silt and clay rich silt gas hydrates with relatively high saturation preferentially accumulated in coarser sediments with less clay content although abundant foraminifera fossils may have increased reservoir pore space their presence was not a necessary condition for high saturation hydrates higher levels of pyrite appeared in the reservoirs corresponding to high saturation hydrates which suggests that the reducing environment caused by sufficient methane provided adequate gas to form higher saturation hydrates because of the migration of the channel levee system different channels formed their respective depositional systems composed of channel filling buried channel filling erosion grooves and slumped turbidities relatively coarse grained deposits were identified in the channel fillings and levees and the accumulation of hydrates was affected by the lithological features of the sediments and their spatial coupling with the gas hydrate stability zone ghsz ghsz modeling based on in situ measurements indicated that erosion and sedimentation as well as variations of the geothermal gradient resulted in the upward downward migration of bottom simulating reflectors bsrs on the erosion flank of the channel the strata thinned and rapid erosion was likely to destroy the shallower bsr causing gas hydrate decomposition and methane release and may have caused turbidite slumping and seepage whereas the strata thickened on the deposition flank of the channel the bsr in the channel levee system would gradually move toward the new ghsz eventually forming a new bsr parts of the bsr that formed under the original p t conditions have remained and double bsrs occurred in the seismic profile the thermal fluid that moved upward through a gas chimney may also have caused the migration of the ghsz resulting in the emergence of double bsrs during the lateral migration of the channel and the vertical migration of the gas bearing fluid there was a dynamic adjustment relationship between the ghsz and the erosion deposition process of the channel resulting in the dynamic accumulation of hydrates in the shenhu area a model to demonstrate the relationship between channel migration and variation of the bsr was established which is of great significance for understanding the formation and accumulation mechanisms of gas hydrates

decomposition of methane hydrates in sand sandstone clays and glass beads 1 decomposition conditions of methane hydrates in sediments were measured during formation decomposition cycles as test sediments we used silica sand sandstone and clays kaoline and bentonite which are typical natural materials known as hydrate bearing sediments and the range of samples cover a range of water saturating abilities to better understand the results we also used uniformly sized glass beads pore effects on decomposition of these materials were investigated by analyzing the pore space distributions of the materials and by varying the initial water content of the samples the results obtained for sand and sandstone samples indicated that the final decomposition temperatures were shifted lower than those for bulk hydrates at the same pressure temperature shifts were more negative for smaller initial water contents with the maximum shift being approximately 0 5 k the results were consistent with those measured for glass beads with nearly the same particle size for kaoline clays the shift was at most 1 5 k we conclude that the decomposition conditions are mainly affected by the pore sizes the surface textures and mineral components had less influence on the results we confirmed that glass beads mimic the effect of sediments for sand sandstone and kaoline clays which have little to no swelling when put in contact with water on the other hand for bentonite particles the results indicated that methane hydrates formed not only between the particles but also in the interlayers a thermodynamic promoting effect was found for dilute bentonite solutions although the positive decomposition temperature shift was at most 0 5 k

petrogenesis of the contact metamorphic rocks beneath the stillwater complex montana the stillwater gabbroic complex was emplaced at about 2700 ma in a metasedimentary section consisting of pelitic schist iron formation and quartzite contact metamorphism resulted in the development of a 500 1000 m hypersthene hornfels zone at the contact and a 500 1500 m cordierite cummingtonite hornfels zone farther from the contact weakly foliated biotite schist occurs beyond the hornfels pelitic rocks are poor in potassium generally lack muscovite and typically contain the assemblage garnet staurolite gedrite chlorite cordierite biotite plagioclase ilmenite quartz the diagnostic hornfels assemblage is cordierite cummingtonite biotite plagioclase ilmenite quartz cummingtonite is replaced by hypersthene near the contact the assemblages in the biotite schist formed during a low pressure regional metamorphism temperature ranged from 450 625 c in biotite schist to 650 800 c in hypersthene hornfels pressure estimates range from 100 250 mpa in biotite schist to 50 300 mpa in hypersthene hornfels the alpha h 2 o in the hypersthene zone is estimated to have been 0 27 orthoamphibole in the biotite schist has compositions that fall on a well defined linear trend from fe 3 5 mg 3 5 si 8 o 22 oh 2 to na 0 7 fe 3 5 mg 1 4 al 2 0 al 2 5 si 5 5 o 22 oh 2 there is no evidence for any immiscibility between anthophyllite and gedrite at the pressure of metamorphism orthoamphibole was replaced by clinoamphibole with increasing grade the petrogenetic grid calculated from estimates of thermodynamic data for the observed compositions of minerals indicates that most reactions in biotite schist occur in the temperature range 525 550 c at 200 mpa the formation of the assemblage cordierite cummingtonite occurs at 600 c and hypersthene is stable above 700 c the net reaction during formation of cordierite cummingtonite hornfels from biotite schist indicates little loss of h 2 o and a small increase in solid volume the pressure and temperature of metamorphism are consistent with subvolcanic emplacement of the complex at a depth of 6 7 km in metasedimentary country rock following a period of regional metamorphism

characteristics and interpretation of fracture filled gas hydrate an example from the ulleung basin east sea of korea abstract through the use of 2 d and 3 d seismic data a total of thirteen sites were selected and drilled in the east sea of korea in 2010 a suite of logging while drilling lwd logs was acquired at each site lwd logs from the ubgh2 3a well indicate significant gas hydrate in clay bearing sediments including several zones with massive gas hydrate with a bulk density less than 1 0 g m 3 for depths between 5 and 103 m below the sea floor the ubgh2 3a well was drilled on a seismically identified chimney structure with a mound feature at the sea floor average gas hydrate saturations estimated from the isotropic analysis of ring resistivity and p wave velocity logs are 80 13 and 47 16 respectively whereas they are 46 17 and 45 16 respectively from the anisotropic analysis modeling indicates that the upper part of chimney between 5 and 45 m below sea floor mbsf is characterized by gas hydrate filling near horizontal fractures 7 dip and the lower part of chimney between 45 and 103 mbsf is characterized by gas hydrate filling high angle fractures on the basis of ring resistivity and p wave velocity the anisotropic analysis using p40h resistivity phase shift resistivity at 32 mhz with 40 inch spacing and the p wave velocity yields a gas hydrate saturation of 46 15 and 46 15 respectively similar to those estimated using ring resistivity and p wave velocity but with quite different fracture dip angles differences in vertical resolution depth of investigation and a finite fracture dimension relative to the tool separation appear to contribute to this discrepancy forward modeling of anisotropic resistivity and velocity are essential to identify gas hydrate in fractures and to estimate accurate gas hydrate amounts

estimating pore space gas hydrate saturations from well log acoustic data relating pore space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment in the present study sonic velocities of gas hydrate bearing sands are modeled using a three phase biot type theory in which sand gas hydrate and pore fluid form three homogeneous interwoven frameworks this theory is developed using well log compressional and shear wave velocity data from the mallik 5l 38 permafrost gas hydrate research well in canada and applied to well log data from hydrate bearing sands in the alaskan permafrost gulf of mexico and northern cascadia margin velocity based gas hydrate saturation estimates are in good agreement with nuclear magneto resonance and resistivity log estimates over the complete range of observed gas hydrate saturations

characteristics and dynamics of gas hydrate systems in the northwestern south china sea results of the fifth gas hydrate drilling expedition abstract in 2018 guangzhou marine geological survey conducted the fifth gas hydrate drilling expedition gmgs5 in the northwestern continental slope of south china sea we use pore water geochemistry pressure core ct scanning degassing quantification seismic data and in situ temperature test at four drill sites w01 w07 w08 and w09 to investigate the gas hydrate characteristics and discuss the dynamics of the gas hydrate system based on the ct scanning and pressure core degassing we demonstrate the coexistence of pore filling and vein like gas hydrates gas hydrate saturation estimated based on the chloride concentration 0 92 is higher than that based on the degassing quantification 0 52 9 the saturation discrepancy is mainly resulted from the sampling discrimination based on the average gas hydrate saturation estimated using chloride concentration and degassing the methane gas volume contained in the gas hydrate of one cubic meter sediment at drill sites w07 w08 and w09 are calculated to be 13 7 m3 26 82 m3 and 22 24 m3 and 3 03 m3 14 30 m3 and 16 25 m3 the geothermal gradients estimated based on the in situ temperature measurements at sites w07 w08 and w09 102 111 c km are more than 50 higher than that at the background site w01 65 c km it is inferred that the fractures and faults in the gas chimney at sites w07 w08 and w09 are the major contributors for the efficient fluid pathways and high geothermal gradients the pore water sulfate profiles and inferred sulfate reduction methane oxidation interface smi depths indicate that methane flux is lowest at site w01 and highest at site w08 the smi depths and top of gas hydrate occurrence is linearly correlated which provides an experienced function for understanding the local gas hydrate distribution kick type sulfate profiles at sites w07 and w09 indicate an ongoing increase of methane flux and corresponding shoaling smi depths in addition chloride enrichment in the shallow sediment at site w08 indicates a recent fast gas hydrate formation process the kick type sulfate profiles positive chloride anomalies and authigenic carbonates at multiple depths suggest that the system experienced dynamic changes in fluid flux and multi stage gas hydrate evolution on time scale from months to thousands of years understanding the controlling mechanism and periodicity of the composite pressure system is important for evaluating the gas hydrate resource and carbon cycling

the eoarchean legacy of isua greenland worth preserving for future generations abstract the eoarchean 3600 ma or millions of years ago folded and metamorphosed isua supracrustal belt and the adjacent orthogneiss exposures of greenland contain rare low deformation lenses that display some uniquely preserved components of earth s oldest rock record these include world s oldest but contested stromatolites in dolomitic carbonates conglomerates pillow basalts demonstrating submarine eruption slivers of upper mantle rocks formation of earliest continental crust by multistage tonalite diorite emplacement followed by intracrustal granite production all these diverse occurrences are keys to establish early earth s processes at the start of the geological record although some of these features are preserved at several localities other critical ones are exposed on only a few m2 of rock at single localities or are of historical significance none of these sites are currently protected and there is a reliance on responsible sampling to keep them intact for future generations given the high interest in the archean eon combined with the increased ease of fieldwork in remote localities many significant deep time localities in not only greenland but worldwide are in danger of eradication here five key isua area geological sites are presented with an explanation of their significance and worthiness for initially reliance on already collected samples but hopefully ultimately government protection this highlights an increasing problem of destruction of in situ evidence of earth s unique early geological heritage and the need for collaboration in protecting and archiving of these key scientific resources

time dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing 1 buffett and archer 2004 showed a model of the steady state global ocean clathrate reservoir that reproduces drill data and predicts a global methane inventory of about 5000 gton c the model inventory is very sensitive to ocean temperature with factor of two changes in methane inventory in response to temperature changes of 1 5 c here we explore the potential time dependent behavior of the clathrate reservoir assuming simple first order rate constants for methane accumulation and meltdown in response to changes in the predicted steady state reservoir for melting we divide the reservoir into two parts one melting catastrophically and the other slowly evolution of clathrates through geologic time is very sensitive to the choice of these rate constants a positive feedback exists during a transient meltdown event stronger if the catastrophic melting fraction is high and its melting timescale is fast a clathrate reservoir governed by such kinetics would melt down periodically on a multimillion year timescale determined by the rate constant for clathrate accumulation ideally we would like a model that predicts runaway melting in the paleocene and stability in the pleistocene however the model is most unstable when climate is cold rather than warm opting for stability in the pleistocene constrains the melting kinetic parameters leaving one degree of freedom the grow in timescale must be 5 myr or longer to get the deglacial 13c right but shorter than 10 myr in order for clathrates to have built up through geologically recent cooling the model thus constrained predicts methane fluxes of 200 gton c or less on deglaciations but eventual releases of 2000 4000 gton c in response to a 2000 gton c anthropogenic carbon release anthropogenic climate change differs from a deglaciation in that it warms the ocean to temperatures not seen in millions of years

measured acoustic wave velocities of r11 ccl3f hydrate samples with and without sand as a function of hydrate concentration we report acoustic laboratory measurements on initially unconsolidated sand packs with hydrate formation taking place in the pore space both p and s wave velocities were measured hydrate was formed from water and the refrigerant r11 ccl3f at 2 c and atmospheric pressure measurements were performed on two types of initially unconsolidated sand packs with average grain size 100 m and 280 m respectively p wave velocities varied from about 1700 m s at low hydrate content to 3810 m s at a calculated hydrate fraction in the pore space equal to 0 52 s wave velocities were only detectable at hydrate fractions larger than 0 35 where s wave velocities up to 2230 m s were measured our measurements indicate that acoustic measurements are not sensitive to small amounts of hydrate in a system of initially unconsolidated sand the data show a very distinct change in behavior when the hydrate fraction exceeds 0 35 the concentration at which hydrate cementation of the sand grains is believed to occur measurements for samples without sand samples containing water r11 and hydrate are also reported for hydrate without sand p wave velocities ranged from about 1400 m s at very low hydrate content to about 2500 m s at a measured hydrate fraction equal to 0 68 for these samples reliable s wave data were not detected

downhole log evidence for the coexistence of structure ii gas hydrate and free gas below the bottom simulating reflector in the south china sea abstract stratigraphic layered pore filling gas hydrates are identified above the bottom simulating reflector bsr using the well log and core data acquired at sites w11 and w17 during the third gas hydrate drilling expedition conducted by china s geological survey guangzhou marine geological survey gmgs3 in the south china sea a seismic profile near site w17 reveal the presence of two bsrs i e double bsr which we show to relate to zones of structure i gas hydrate i bsr and structure ii gas hydrate ii bsr well log data from site w17 between the i bsr projected depth of 250 mbsf and ii bsr projected depth of 330 mbsf showed anomalous responses for gas hydrate bearing sediments with high resistivity high s wave velocity and alternating high and low p wave velocities pressure core data support the interpretation that structure ii gas hydrate occurs at a depth of 263 mbsf at site w17 the cross plot between log derived neutron and density porosities reveals a free gas bearing layer at a depth of 258 270 mbsf suggesting gas hydrate coexists with free gas between the i bsr and the ii bsr synthetic seismograms generated from the p wave velocity and density logs further support the presence of free gas in this section based on the coexistence of hydrate free gas and water the simplified three phase equation stpe was modified to simultaneously estimate free gas and hydrate saturations beneath the i bsr from p wave and s wave velocity logs assuming uniform or patchy distributions of free gas the estimated free gas and hydrate saturations together with gas compositions from pressure core samples collectively indicate that structure ii gas hydrate and free gas are interbedded and coexist below the i bsr at site w17 our study of the coexistence of gas hydrate and free gas between the double bsr at site w17 provides new insights into gas hydrate systems in nature that contain more complex gas chemistries

capillary effects on hydrate stability in marine sediments 1 we study the three phase liquid gas hydrate stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases the capillary pressure a measure of the pressure difference across a curved phase interface exerts a key control on the methane solubility in liquid hydrate l h and liquid gas l g systems by calculating the l h and l g solubilities as a function of water depth pressure and pore size interface curvature we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three phase zone the top of the three phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases the thickness of the three phase zone increases as the distribution of pore sizes widens the top of the three phase zone can overlie or underlie the bulk three phase equilibrium depth at blake ridge we predict that the three phase zone is 27 7 m thick and that the top of the three phase zone lies 13 m above the predicted bulk equilibrium depth this reconciles the observation of the bottom simulating reflector bsr at blake ridge that is shallower than the predicted bulk equilibrium depth in contrast at hydrate ridge where water depth is shallower we predict that the three phase zone is 20 4 m thick and that the top of the three phase zone lies 0 7 m below the predicted bulk equilibrium depth our model which predicts an upward shift in the top of free gas occurrence with increasing water depth pressure is compatible with worldwide observations that the bsr is systematically shifted upward relative to the bulk equilibrium depth as water depth pressure is increased

nd and sr isotope systematics of clastic metasediments from isua west greenland identification of pre 3 8 ga differentiated crustal components recent field work on the isua metasediments has established that there are two lithological sequences a and b which are divided into a number of distinct formations each clastic metasedimentary unit has a relatively unique trace element signature the depleted mantle sm nd model ages of samples from all major units range from 3 59 to 3 98 ga whereas rb sr model ages cover a very wide range 1 9 4 6 ga the samples from the sequence a felsic gneiss formation a6 yield a relatively good rb sr isochron age of t 3 71 0 07 ga the sequence b samples do not yield a rb sr isochron however some of the samples plot close to the a6 rb sr isochron and sequences a and b appear to have essentially the same deposition age the sm nd data for both sequences yield initial end values in the range 1 to 3 for a deposition age tstrat 3 64 ga if tstrat 3 81 ga the range is 0 4 this range appears to reflect real variability of initial end values corresponding to the time of deposition data trends in end versus 1 nd diagrams suggest at least three distinct components in each of sequences a and b model age calculations suggest that some of these components must be derived from continental crustal sources that predate the time of deposition of these sediments by as much as 0 4 b y other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition the very high initial end values implied for early depleted mantle sources 2 to 4 suggest a very large pre 3 8 ga continental crust alternatively these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a 4 5 ga magma ocean

avo study of a gas hydrate deposit offshore costa rica amplitude variation with offset avo analysis and waveform inversion are techniques used to determine qualitative or quantitative information on gas hydrates and free gas in sediments however the quantitative contribution of gas hydrates to the acoustic impedance contrast observed at the bottom simulating reflector and the reliability of quantitative avo analyses are still topics of discussion in this study common midpoint gathers from multichannel wide angle reflection seismic data acquired offshore costa rica have been processed to preserve true amplitude information at the bottom simulating reflector for a quantitative avo analysis incorporating angles of incidence of up to 60 corrections were applied for effects that significantly alter the observed amplitudes such as the source directivity avo and rock physics modelling indicate that free gas immediately beneath the gas hydrate stability zone can be detected and low concentrations can be quantified from avo analysis whereas the offset dependent reflectivity is not sensitive to gas hydrate concentrations of less than about 10 at the base of the gas hydrate stability zone bulk free gas saturations up to 5 have been determined from the reflection seismic data assuming a homogeneous distribution of free gas in the sediment assuming a patchy distribution of free gas increases the estimated concentrations up to 14 there is a patchy occurrence of bottom simulating reflectors south east of the nicoya peninsula on the continental margin offshore costa rica avo analysis indicates that this phenomenon is related to the local presence of free gas beneath the gas hydrate stability zone probably related to a focused vertical fluid flow in areas without bottom simulating reflectors the results indicate that no free gas is present

depositional characteristics and accumulation model of gas hydrates in northern south china sea abstract the south china sea scs shows favorable conditions for gas hydrate accumulation and exploration prospects bottom simulating reflectors bsrs are widely distributed in the scs using seismic and sequence stratigraphy the spatial distribution of bsrs has been determined in three sequences deposited since the late miocene the features of gas hydrate accumulations in northern scs were systematically analyzed by an integrated analysis of gas source conditions migration pathways heat flow values occurrence characteristics and depositional conditions including depositional facies rates of deposition sand content and lithological features as well as some depositional bodies structural slopes slump blocks and sediment waves this research shows that particular geological controls are important for the presence of bsrs in the scs not so much the basic thermodynamic controls such as temperature pressure and a gas source based on this a typical depositional accumulation model has been established this model summarizes the distribution of each depositional system in the continental shelf continental slope and continental rise and also shows the typical elements of gas hydrate accumulations bsrs appear to commonly occur more in slope break zones deep water gravity flows and contourites the gas hydrate bearing sediments in the shenhu drilling area mostly contain silt or clay with a silt content of about 70 in the continental shelf bsrs are laterally continuous and the key to gas hydrate formation and accumulation lies in gas transportation and migration conditions in the continental slope a majority of the bsrs are associated with zones of steep and rough relief with long term alternation of uplift and subsidence rapid sediment unloading can provide a favorable sedimentary reservoir for gas hydrates in the continental rise bsrs occur in the sediments of submarine fans turbidity currents

in situ iron isotope analysis of pyrites in 3 7ga sedimentary protoliths from the isua supracrustal belt southern west greenland abstract the timing of the emergence of life remains one of the principal unresolved questions in the earth sciences putative relicts of microorganisms in the eoarchean ca 3 6 3 85 ga high grade metamorphic terranes do not preserve morphological evidence for early life but some relicts can be identified by their geochemical signatures created by metabolic processes among the oldest rocks of sedimentary origin ca 3 8 ga occur in the isua supracrustal belt isb southern west greenland these have undergone metamorphism up to the amphibolite facies despite intense metamorphism the carbon isotope compositions of graphite clots from the isua metasedimentary rocks suggest biological carbon fixation and provide the oldest evidence for this biological process microbial dissimilatory iron reduction dir is considered to be an early form of metabolism the microbial dir produced fe 2 aq with a lower 56 fe value from a precursor fe 3 bearing iron mineral however 56 fe values lower than 1 are not found in sedimentary rocks prior to about 2 9 ga here we report in situ iron isotope analysis of pyrites in sedimentary rocks from the isb using a near infrared femtosecond laser ablation multicollector icp ms nir fs la mc icp ms a large variation of 56 fe values from 2 41 to 2 35 was documented from 190 points within pyrite grains from 11 rock specimens including those interpreted to be banded iron formations bifs chert amphibole rich chert quartz rich clastic sedimentary rocks mafic clastic sedimentary rocks carbonate rocks and conglomerates we found that the distribution of 56 fe values depends on the lithology whereas there is no correlation between their 56 fe values and the metamorphic grade the 56 fe values of pyrites in bifs range from 0 25 to 2 35 indicating partial oxidation in the deep ocean especially the high 56 fe values up to 2 35 suggest that the bif was formed through interaction of ferruginous seawater with a highly alkaline hydrothermal fluid under anoxic conditions pyrite grains in a conglomerate carbonate rocks mafic clastic sedimentary rocks and amphibole rich cherts show negative 56 fe values around 1 5 down to 2 41 pointing to microbial dir in the eoarchean shallow sea in addition the relatively low 56 fe values of pyrites in the shallow water sediments suggest anoxic anoxygenic photoautotrophic iron oxidation in the photic zone

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

formation history and physical properties of sediments from the mount elbert gas hydrate stratigraphic test well alaska north slope abstract the synthesis of available geological information and surface temperature evolution in the alaska north slope region suggests that biogenic and deeper thermogenic gases migrated through fault networks and preferentially invaded coarse grained layers that have relatively high hydraulic conductivity and low gas entry pressures hydrate started forming before the beginning of the permafrost eventually the permafrost deepened and any remaining free water froze so that ice and hydrate may coexist at some elevations the single tested specimen depth 620 47 620 62 m from the d unit consists of uncemented quartzitic fine sand with a high fraction of fines 56 by mass finer than sieve 200 the as received specimen shows no evidence of gas present the surface texture of sediment grains is compatible with a fluvial deltaic sedimentation environment and shows no signs of glacial entrainment tests conducted on sediments with and without thf hydrates show that effective stress porosity and hydrate saturation are the major controls on the mechanical and geophysical properties previously derived relationships between these variables and mechanical geophysical parameters properly fit the measurements gathered with mount elbert specimens at different hydrate saturations and effective stress levels we show that these measurements can be combined with index properties and empirical geomechanical relationships to estimate engineering design parameters volumetric strains measured during hydrate dissociation vanish at 2 4 mpa therefore minimal volumetric strains are anticipated during gas production at the mount elbert well however volume changes could increase if extensive grain crushing takes place during depressurization driven production strategies if the sediment has unexpectedly high in situ porosity associated to the formation history or if fines migration and clogging cause a situation of sustained sand production

elastic velocities of partially gas saturated unconsolidated sediments abstract fluid in sediments significantly affects elastic properties of sediments and gas in the pore space can be identified by a marked reduction of p wave velocity or a decrease of poisson s ratio the elastic properties of gas saturated sediments can be predicted by the classical biot gassmann theory bgt however parameters for the bgt such as the biot coefficient or moduli of dry frame of unconsolidated and high porosity sediments are not readily available dependence of velocities on differential pressure or porosity for partially gas saturated sediments is formulated using properties derived from velocities of water saturated sediments laboratory samples for unconsolidated and consolidated sediments and well log data acquired for unconsolidated marine sediments agree well with the predictions however because the p wave velocity depends highly on how the gas is saturated in the pore space such as uniform or patch the amounts of gas estimated from the p wave velocity contains high uncertainty the modeled vp vs ratio of partially gas saturated sediment using the patch distribution is usually greater than 1 6 whereas the ratio modeled assuming a uniform distribution is about 1 6 thus poisson s ratio or vp vs ratio may be used to differentiate patch from uniform saturation but differences between various models of patch saturation cannot be easily identified

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

the origin of early archean banded iron formations and of continental crust isua southern west greenland abstract a long standing geological debate is whether the fe component of precambrian banded iron formations bifs was derived from continental sources or had its origin in hydrothermal alteration of the oceanic crust recent ree and nd and fe isotope studies of iron rich layers in bifs have provided strong supporting evidence for the hydrothermal origin of the fe component pillow basalts from the 3800 3700 ma isua greenstone belt west greenland underwent significant enrichment of fe up to 21 wt fe2o3 and some chalcophile elements e g pb 5 4 7 2 ppm prior to a late archean tectonothermal metamorphic event recorded in the region published and new pb pb isotopic data from the isua pillow basalts and the spatially associated bifs suggest that the enrichment of fe and pb and deposition of the bifs took place between 3800 and 3600 ma similar pb isotopic compositions in pillow basalts and bifs are consistent with a common source of pb for both supracrustal rocks iron and pb were probably scavenged from the oceanic crust and were re deposited on the ocean floor by hydrothermal fluids suggesting that high temperature hydrothermal alteration of early archean oceanic crust played an important role in the deposition of the isua bifs geochemical lithological and structural data from the isua greenstone belt are all collectively consistent with a convergent margin geodynamic setting the isua bifs are spatially and temporally associated with boninitic and island arc picritic pillow basalts given the observations that cenozoic boninites and picrites tend to form in an intra oceanic arc forearc setting we suggest that the isua bif boninite and bif picrite associations were also deposited in a similar geodynamic setting the original pillow basalt bif stratigraphy was dismembered and transposed throughout the belt during early to late archean tectonothermal events the high temperature hydrothermal alteration of the oceanic crust produced significant hydrothermal discharge with large quantities of fe necessary to contribute to the deposition of the isua bifs the hydrothermal alteration may have resulted from the opening of an asthenospheric window developed as a consequence of ridge subduction beneath an early archean arc forearc region the ridge subduction model can also explain the origin of the contemporaneous tonalite trondhjemite granodiorite ttg intrusions in the isua region partial melting of laterally accreted and thickened oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of asthenospheric windows may have produced ttg melts the diapirically upwelling ttg melts formed the early archean continental crust and the complementary eclogitic residues contributed to the sub continental lithospheric mantle

anomalously cold temperatures observed at the base of the gas hydrate stability zone on the u s atlantic passive margin in situ measurements to depths of 415 m below sea floor in methane hydrate bearing sediments on the u s atlantic passive margin indicate that temperatures at the bottom simulating reflector bsr are anomalously low by 0 5 2 9 c if the bsr marks the base of gas hydrate stability bghs several hypotheses may explain the occurrence of the bsr at inappropriate pressure temperature p t conditions 1 if the bsr does not mark the bghs then p t conditions need not be sufficient to dissociate gas hydrate at this depth 2 the bsr may lie at nonequilibrium p t conditions due to incomplete readjustment in response to upper pleistocene holocene climate change however the occurrence of the blake ridge bsr at an overly shallow depth cannot be easily explained by realistic combinations of pressure driven deepening sea level rise and temperature driven shoaling bottom water temperature changes 3 the properties of sediments or pore fluids may inhibit the stability of gas hydrate in particular capillary forces arising in the fine grained montmorillonite rich sediments of the blake ridge may lead to shoaling of the bsr in this setting

the nature and distribution of bottom simulating reflectors at the costa rican convergent margin summary bottom simulating reflectors bsrs at the base of the hydrate stability zone are often observed in marine seismic reflection data the compressional p wave velocity structure of a bsr offshore costa rica was analysed by applying a full waveform inversion technique the resulting velocity profile indicates that the bsr is caused by a thin low velocity layer suggesting the presence of at least a small amount of free gas in the sediment pore space in undisturbed sediment sections bsrs offshore costa rica are observed over much of the continental margin they frequently occur at shallow depths beneath the seafloor usually 100 400 m at some locations bsrs appear to intersect the seafloor this was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards in the study area the vertical movement of the hydrate stability zone relative to a given point in the sediment column one of the potential factors leading to bsr formation or suppression is controlled by both sedimentation and vertical tectonism both processes may hence play a role in controlling bsr distribution bsrs are absent in areas which have been affected by slumping except where the sedimentary section above the bsr remained intact during slumping this indicates that slope failure can cause the destruction of bsrs

thermogenic gas controls high saturation gas hydrate distribution in the pearl river mouth basin evidence from numerical modeling and seismic anomalies abstract we integrate three dimensional 3d seismic data well log and two dimensional 2d modeling to understand the accumulation and saturation of gas hydrates and their relationships with biogenic and thermogenic gases in the pearl river mouth basin south china sea sites sh2 sh5 w17 and w19 the model based on physical geochemical and geological parameters from shallow gas hydrate sites deep oil sites and seismic interpretation and it is tied to well log and seismic quantitative interpretation to predict the main controlling factors for gas hydrate distribution the modeling results show that the biogenic methane generation is distributed within the strata shallower than 1500 mbsf meters below the seafloor whereas thermogenic gas generation ranges from 2300 mbsf to 6000 mbsf the erosion surfaces of migrating canyons represent important pathways for upward migration of biogenic gas resulting in average hydrate saturation 40 at the base of gas hydrate stability zone bghsz is related to the focused migration of thermogenic gas along faults and throughout permeable carrier beds although gas chimneys are widely identified from the coherence attribute extracted from 3d seismic data their impact on the obviously higher locally up to 60 gas hydrate saturations in the fine grained sediments appears site dependent and associated with fault activity by comparing well log derived gas hydrate saturations our modeling shows that about 80 of the methane forming hydrate is thermogenically sourced at sites w19 and w17 the integrated modeling effort presented in this work based on provides a better understanding of the different contributions from different gas sources for gas hydrate accumulation along the northern slope of the south china sea

geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas hydrate production test region in the shenhu area northern south china sea abstract integrated three dimensional seismic logging sediment cores and geochemical testing data were collected from guangzhou marine geological survey 3 and 4 hydrate drilling expeditions and used in this study for a comprehensive investigation of the geological and geophysical features and accumulation mechanism of hydrates in the first offshore gas hydrate production test region ghptr in the shenhu area of the south china sea seismic signatures indicative of disseminated hydrates and free gas include the bottom simulating reflector bsr gas chimney and mud diapir associated with enhanced seismic reflections acoustic blanking masking and chaotic appearance have been observed the acoustic travel time responses density and compensated neutron three porosity log analysis high precision grid tomography inversion analysis and constrained sparse spike inversion confirm the presence of free gas below the gas hydrate bearing zone ghbz free gas bearing zones have significantly different p wave impedances and low velocity anomalies than the overlying ghbz and surrounding strata these anomalous zones are controlled by the structural attitude of the reservoir strata which are characterized as inter bedded stratigraphic units variations in the type and geological characteristics of the hydrocarbon migration pathways were observed between sites w18 and w19 on the western ridge and sites w11 and w17 on the eastern ridge in the gmgs study area the efficiency of gas migration in the western ridge may be higher than that in the eastern ridge resulting in variations in hydrate gas types thickness of the ghbz and gas migration flux and accumulation except for site w11 hydrates were recovered below the structure i inferred bsr at sites w17 w18 and w19 the gas hydrate stability zone calculations reveal that the structure i hydrate stability zone differs from the bsr depth and is generally shallower than the base of the logging anomaly indicating the coexistence of structure i and ii hydrates the bsr is not indicative of the bghsz it is rather regarded as a transitional indicator of structure i and ii gas hydrates in the ghptr the appearance of free gas and hydrates below the structure i inferred bsr indicates that the shenhu area is characterized by a complex hydrate formation and accumulation system resulting from the supply of biogenic and thermogenic gases despite fine grained host sediments predominating the ghptr the coupling of favorable conditions including efficient hydrocarbon generation sufficient gas supply multiple pathways for gas migration and relatively high reservoir porosity have led to the development of highly saturated gas hydrate accumulations within relatively thick sedimentary sections which demonstrates a significant resource potential

the role of ultramafic rocks in regulating the concentrations of volatile and non volatile components during deep crustal metamorphism the common preservation of h2o and co2 bearing phases in high grade ultramafic rocks indicates that they represent reservoirs of volatile components in the deep crust the upper thermal stabilities of volatile bearing mg silicates and magnesite exceed those of volatile bearing phases in most other crustal lithologies the solubility of many rock forming components e g co2 al2o3 cao sio2 is considerably lower in fluids coexisting with ultramafic rocks than in adjacent quartzo feldspathic units metamorphism of crustal sections that include ultramafic rocks is accompanied by metasomatic reactions that regulate the chemistry of both volatile and non volatile components in deep crustal metamorphic fluids during amphibolite facies metamorphism ultramafic rocks can absorb and store volatile and non volatile components introduced from the country rock under conditions of low activities or fugacities an example is the early fixation of co2 h2o and aluminium by reaction of dunite to form chlorite magnesite and progressively more siliceous mg silicates along an early low xco2 reaction path one dimensional continuum models of fluid transport and reaction indicate that infiltration of fluids derived from quarzo feldspathic country rocks can cause enrichment of aluminium in ultramafic rocks up to the level of wt internal reaction between the metasomatic minerals within ultramafic units release fluids during prograde metamorphism fluids will be released in two pulses one with intermediate xco2 and aluminium activity at the level of aluminosilicate or corundum saturation and a later pulse of pure co2

high resolution tem study of jimthompsonite chesterite and chain width disorder in archean ultramafic rocks from isua west greenland jimthompsonite and chesterite occur as intergrowths within anthophyllite up to a few millimeters across in talc magnesite schist from the 3800 ma isua supracrustal belt west greenland pyribole lamellae span the entire intergrowth width and have their own crystal faces the schist is strongly foliated and pyribole intergrowths are aligned with this foliation indicating broad synchronicity among metamorphic deformation recrystallization and pyribole formation chain width errors polytypic disorder zipper terminations displacive planar faults replacement products by sheet silicates polygon shaped cavities cavity filling materials and low angle grain boundaries were revealed by hrtem based on cross cutting relationships the order of the formation of these defects was determined pyribole intergrowths formed prior to displacive planar faults and cavity filling materials fine sheet silicates replaced pyriboles or filled cracks after the pyribole intergrowth formation no obvious reaction or replacement products by talc or chlorite were confirmed at the interface between pyriboles and optically recognizable large sheet silicates suggesting that fine sheet silicates replacing pyriboles formed later than talc and chlorite polysomatic and polytypic lamellae may have grown simultaneously because the lamellae cross cut each other and one does not affect the formation of the other the possible reaction fronts that separate jimthompsonite or chesterite from anthophyllite were not found in 001 hrtem images of our samples instead narrow jimthompsonite and chesterite lamellae that break termination rules were commonly found these textural features can be better understood as a simultaneous growth of jimthompsonite chesterite and anthophyllite and not as a retrograde reaction product of anthophyllite