Search

Prior studies have shown an ambiguous relationship between gas-hydrate saturation and seismic attenuation in different regions, but the effect of gas-hydrate morphology on seismic attenuation of hydrate-bearing sediments was often overlooked. We have combined seismic data with rock-physics modeling to elucidate how gas-hydrate saturation and morphology may control seismic attenuation. To extract P-wave attenuation, we process the vertical seismic profile data within a frequency range of 30–150 Hz and the sonic logging data within 10–15 kHz from three wells in the south Hydrate Ridge, offshore Oregon (USA), collected during Ocean Drilling Program Leg 204 in 2000. We calculate the P-wave attenuation using spectral matching and centroid frequency shift methods, and we use Archie’s relationship to derive gas-hydrate saturation from the resistivity data above the bottom-simulating reflection at the same wells. To interpret the observed seismic attenuation in terms of the effects of the gas-hydrate saturation and the morphology, we use the hydrate-bearing effective sediment rock-physics model. By comparing the observed and model-predicted attenuation values, we infer that (1) seismic attenuation appears to not be dominated by any single factor — instead, its variation is likely governed by the gas-hydrate saturation and the morphology; (2) the relationship between seismic attenuation and gas-hydrate saturation varies with different hydrate morphologies; (3) the squirt flow, occurring at different compliances of adjacent pores driven by pressure gradients, may be responsible for the significantly large or small attenuation over a broad frequency range.

Gypsum often occurs in marine sediments as an authigenic mineral, but it may also be produced artificially through oxidation of sulfide during sample storage and preparation. Therefore, the origin of the gypsum should be carefully checked, especially for those DSDP, ODP and IODP samples that have stored for a long time. It is nearly impossible to distinguish between gypsum formed by these two processes based on the mineral morphology or the sulfur and oxygen isotope methods currently used. In this study, we proposed a simple, quantitative scheme based on chemical extractions of iron and sulfur to distinguish whether the gypsum in marine sediment samples is an authigenic mineral or an artificial product formed after sample collection. The new method was tested on two suites of deep-water sediment cores sampled from Hydrate Ridge and Kaoping Slope, in which the origins of gypsum were known beforehand, and was successfully validated. Finally, the potential influence of artificial productions of elemental sulfur and jarosite on the application of this new method was evaluated and the corresponding solutions were given.

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

Gas hydrate is regarded as a kind of important energy resource in the recent decades. The identification of gas hydrate is a hot issue for researchers all over the world in order to exploration and exploitation. Previous research shows some methods to identify gas hydrate, but these methods also have shortcomings in different actual conditions. It is still a difficult problem to distinguish gas hydrate accurately from marine sediments. Here we propose a cross plot analysis of velocity to identify gas hydrate. Firstly, we simulate different minerals bearing sediments by rock physics models to obtain different physical properties. After a series of experiments, the results show that there are obvious differences between gas hydrate bearing sediments and other minerals bearing sediments in P-wave and S-wave velocity. Therefore, the cross plot of velocity can be used to distinguish gas hydrate qualitatively. Finally, we verify the cross plot method using the data in the Ocean Drilling Program (ODP) Leg 204 expedition of the Hydrate Ridge along the Oregon continental margin, where gas hydrate is confirmed via logging. The hydrate identification results of actual velocity data by cross plot analysis are generally in keeping with the results of core samples. Moreover, using the cross plot method also can estimate gas hydrate saturation range, which is in accordance with the measurement of chloride (Cl−) concentration in most cases.

The formation of gas hydrate reservoir in marine sediments is mainly controlled by methane supply and sedimentary burial. Based on the mass conservation of methane in gas hydrate system, a numerical model of gas hydrate formation was established considering the methane supplied by dissolved methane diffusion, pore water advection, and in situ methanogenesis. A case study of ODP site 1247 at the Hydrate Ridge, offshore Oregon shows that dissolved methane transported by molecular diffusion and pore water advection is the major supply of methane for gas hydrate formation, while in situ methanogenesis contributes little to the gas hydrate reservoir. The gas hydrate reservoir was also evaluated considering the changes of sedimentation rate since 1.67 Ma. Our model results show that the variations of sedimentation rate lead to little change in the size of gas hydrate reservoir at ODP 1247. The calculated hydrate saturation amounts to ∼0%∼3%, which is consistent with the measured values using pressure coring.

Summary Reservoir parameters of gas hydrate bearing sediments are significant to reservoir characterization. Typically, these parameters are evaluated separately based on single well log measurements with simple linear or empirical rock physics models. This paper proposes a scheme for joint stochastic inversion of elastic and electrical data to predict reservoir parameters with non-linear rock physics models. This scheme first establishes the elastic-electrical rock physics templates to generate a certain range of hydrate saturation, porosity and clay content, and then considers these ranges as the prior information for a subsequent inversion. Furthermore, a stochastic inversion method is used to achieve different realizations, where mean values are treated as the interpretable reservoir parameters and the different realizations quantify the inversion uncertainty. Finally, we implement this joint inversion approach to the well logs measured at site 1250F, Hydrate Ridge. Results show that hydrate saturation, porosity and clay content have a good agreement with core samples. Meanwhile, the quantitative analysis of these realizations show that the maximum standard deviation is no more than 8%. Based on these results, we conclude that our joint inversion scheme is feasible and reliable when integrating elastic and electrical data for reservoir parameter estimation.

Authigenic carbonate precipitation occurs within marine sediments where sulfate-dependent anaerobic methane oxidation occurs. Geochemical and isotopic analyses of authigenic carbonates are commonly used as indicators of carbon sources and environmental conditions present during carbonate formation, but burial diagenesis and recrystallization can overprint these signals. Plane polarized light (PPL) and cathodoluminescent (CL) petrography allows for detailed characterization of carbonate phases and their subsequent alteration. Petrographic, isotopic, and geochemical characteristics of modern offshore authigenic carbonates from central and northern Cascadia are compared with Oligocene–Pliocene fossil seep carbonates uplifted on the Olympic Peninsula. Coupled analyses show the value and complexity of separating primary vs. secondary signals with relevance to understanding fluid-burial history in methane seep provinces on tectonically active convergent margins. The modern, offshore, near-seafloor diagenetic environment (S. Hydrate Ridge and Barkley Canyon) is dominated by acicular and microcrystalline aragonite and high-Mg calcite (HMC, > 12 mol % Mg). PPL and CL data illustrate that aragonite and HMC phases recrystallize to intermediate-Mg calcite (IMC, 5–12 mol% Mg) during burial and diagenesis and eventually to low-Mg calcite (LMC

We develop a kinetic model for hydrate crystallization from methane gas venting through shallow sediments at Hydrate Ridge on the Cascadia margin of Oregon that predicts how pore water chlorinity, temperature, and crystallized hydrate evolve after the onset of steady venting. Predictions are compared to observations at Ocean Drilling Program Site 1249. In the preferred model, calculated gas hydrate saturation and chloride concentrations reach those observed at depths less than 20m below seafloor (bsf) under the southern summit of Hydrate Ridge in similar to 650years, and the vertical water flux must be less than 50kg/m(2)/yr. Hydrate accumulates more slowly between 20m bsf and the base of the hydrate stability zone where there is no free gas, accumulating to observed levels of a few volume percent of hydrate in 10(5) to 10(6)years, depending on the water flux that is assumed through this zone. This dichotomy means that the presently observed gas venting must have been diverted to this area similar to 650years ago, or be episodic and infrequent. If the gas venting for the last 650years has been as observed today, the latent heat of hydrate precipitation in the upper 20m of sediments would have caused the temperature to increase similar to 0.8 degrees C at similar to 20m bsf and similar to 0.2 degrees C at similar to 100m bsf. This would have caused a similar to 5m rise in the elevation of the base of hydrate stability zone, and decreased the rate of hydrate crystallization from 1.5kg CH4/m(2)/yr 650years ago to 0.7kg CH4/m(2)/yr today.

A cold vent is an area where methane-rich fluid seepage occurs. This seepage may alter the local temperature, salinity, and subsequent accumulation of the gas hydrate. Using a kinetic gas hydrate formation model and in situ measurement of temperature, salinity and fluid flux at the southern summit of Hydrate Ridge, we simulate the gas hydrate accumulation at three distinct fluid sites: clam, bacterial mat, and gas discharge sites. At the clam sites (pore water flux 0.8 mol kg−1) by the formation of gas hydrate causing the base of the hydrate stability zone to move gradually from ∼115 to ∼70 meters below seafloor (mbsf). The gas hydrate saturation at the clam sites is relatively high. The water flux at the bacterial mat sites ranges from 100 to 2500 kg m−2 yr−1. The water flow suppresses the increase in salinity resulting in a salinity close to or slightly higher than that of seawater (< 0.65 mol kg−1). Heat advection by water flow increases temperature significantly, shifting the base of the hydrate stability zone to above 50 or even 3 mbsf. The gas hydrate saturation is relatively low at the bacterial mat site. At the gas discharge sites, the pore water flux could reach 1010 kg m−2 yr−1, and the temperature could reach that of the source area in 9 min. There is no gas hydrate formation at the gas discharge sites. Our simulative analysis therefore reveals that a lower pore water flux would result in lower salinity, higher temperature, and a shallower base of the hydrate stability zone. This in turn induces a lower gas hydrate formation rate, lower hydrate saturation, and eventually less gas hydrate resources.

An Oligocene cold-seep limestone (Lincoln Creek Formation, Washington State, USA) was studied for its lipid biomarker inventory. Biomarker analysis on minute amounts of sample (tens of mg) and complementary 13Ccarbonate analyses allowed us to link biogeochemical processes with individual, closely intertwined carbonate phases. The ancient seep deposit exhibits four major carbonate phases, according to the paragenetic sequence of (I) micrite, (II) yellow aragonite, (III) clear aragonite and (IV) equant calcite spar. For the micrite, varying but significant amounts of archaea-derived isoprenoids clearly indicate that the precipitation of this phase was induced by the microbial anaerobic oxidation of methane (AOM). However, water column-derived lipids present in this carbonate phase reflect the incorporation of organic matter from background sediment cemented by authigenic micrite. Yellow aragonite made up only a minor rock component (

Porosity is a key parameter in calculating the velocity of gas hydrate bearing sediments and quantifying the amount of gas hydrate. The variation of porosity is affected by many factors. The influences of different factors on porosity are distinct. The purpose of this paper is to analyze the main factors that affect the overall and local change of porosity in marine sediments where gas hydrate was sampled. Porosity logs were collected from ODP Leg 164, Blake Ridge, ODP Leg 204, Hydrate Ridge, and IODP expedition 311, Cascadia Margin. Based on the characteristic of porosity variation in depth, porosity was divided into three components: low frequency component, middle frequency component, and high frequency component. The factors influencing each component were discussed. From the analysis, we observed that the porosity of unconsolidated sediment was very high, and the decreasing trend of low frequency component versus depth was affected by compaction. In addition, the initial porosity and slope of low frequency component variation were affected by the content of fine grain and geothermal gradient respectively. The middle component could reflect the variation of lithology, which was affected by the content variation of different sized grains and gas hydrate. The high frequency component was affected by the frequent change of grain size. The existence of volcanic ash-rich sand caused a high value to the high frequency component. The results are applicable to porosity evaluation in gas hydrate bearing sediments.

Methane hydrates, ice-like compounds that consist of water and methane, represent a potentially enormous unconventional methane resource that may play a critical role in climate change and ocean acidification; however, it remains unclear how much hydrate exists. Here, using a newly developed three-dimensional (3-D) thermal technique, we reveal a novel method for detecting and quantifying methane hydrate. The analysis reveals where fluids migrate in three dimensions across a continental margin and is used to quantify hydrate with meter-scale horizontal resolution. Our study, located at Hydrate Ridge, offshore Oregon (United States), suggests that heat flow and hydrate concentrations are coupled and that 3-D thermal analysis can be used to constrain hydrate and fluid flow in 3-D seismic data. Hydrate estimates using this technique are consistent with 1-D drilling results, but reveal large, previously unrecognized swaths of hydrate-rich sediments that have gone undetected due to spatially limited drilling and sampling techniques used in past studies. The 3-D analysis suggests that previous hydrate estimates based on drilling at this site are low by a factor of approximately three.

There are at present no validated methods for reliably finding economically significant accumulations of natural gas hydrate in marine environments. The seismic bottom simulating reflector (BSR) has been regarded as a primary indicator of hydrate presence in marine environments, but the presence of a BSR conveys no information about the abundance of hydrate in the sediments above it. Seafloor features such as gas seeps, pockmarks or hydrate outcrops may be qualitative markers of deeper hydrate presence, but cannot be interpreted quantitatively. Another approach to exploration geophysics is required to find exploitable gas hydrate reservoirs with high reliability. It is known that in many cases gas is supplied to the gas hydrate stability zone primarily through faults or fractures. In a certain range of gas flux, these fissures should become mineralized with gas hydrate and form vertical or subvertical dykes. The dip and strike of these dykes are controlled by the principal stress directions, which can be predetermined. Thus multiple hydrate dykes are expected to be parallel. Even if the greatest volume of gas hydrate is to be found in sub-horizontal permeable beds, the steeply dipping mineralized conduits that fed gas to them may be the most reliable marker of substantial subsurface hydrate presence. Geological and geophysical survey methods sensitive to parallel arrays of vertical and subvertical hydrate dykes are presented. Gas hydrate is thought to be an important component of deep water marine sediments, and is thus of interest to those looking for economically significant sources of fossil fuels. Indeed, seismic evidence of offshore gas hydrate deposits is widespread. A compilation (Kvenvolden & Lorenson 2001) documents more than one-hundred occurrences of hydrate on continental margins and in inland seas. However, drilling campaigns in some promising offshore areas, such as Blake Ridge offshore South Carolina (Paull et al. 2000), and Hydrate Ridge offshore Oregon (Trehu et al. 2004a), have shown gas hydrate to be generally dilute throughout the gas hydrate stability zone. Higher concentrations can be found in limited depth intervals, sometimes coincident with beds of relatively coarse permeable sand within otherwise fine-grained marine sediments. If marine gas hydrate is abundant, the mechanisms by which it accumulates below the seafloor may make it difficult or impossible to find using the conventional techniques of exploration geophysics as presently applied. This chapter presents a new exploration paradigm based on a probable mechanism of marine hydrate deposit accumulation. Present exploration methods The most important technique for probing deep water geological formations is the marine seismic survey. Modern seismic surveys efficiently generate three-dimensional images of the subsurface over large areas. Gas hydrate has a distinctive seismic signature, the bottom simulating reflector (BSR) (Shipley et al. 1979). The BSR is seen in marine seismic images, running parallel to, and several hundred metres below, the seafloor, approximately coincident with the base of the gas hydrate stability zone (GHSZ). As such its presence has been a primary driver of many hydrate exploration campaigns. Several major offshore drilling campaigns have been directed primarily by the presence of the BSR, including ODP Leg 164 at Blake Ridge (Paull et al. 2000) and the 32-well campaign in the Nankai Trough, offshore Japan, in 2004 (Takahashi & Tsuji 2005). The US Department of Energy/ Chevron Joint Industry Project (JIP), which drilled at two sites in the Gulf of Mexico, selected one site based on BSR occurrence and the other based on geological evidence (Claypool et al. undated). BSR maps were combined with geological, geochemical and microbiological information to choose drilling sites offshore India in 2006 (Ramana et al. 2006). Unfortunately, the BSR is not always a good predictor of abundant hydrate occurrence. For example, at Blake Ridge, little hydrate was found in a well drilled to a strong BSR, whereas there were hydrate shows in a well drilled in a locale where the BSR was absent (Paull et al. 2000). From: LONG, D., LOVELL, M. A., REES, J. G. & ROCHELLE, C. A. (eds) Sediment-Hosted Gas Hydrates: New Insights on Natural and Synthetic Systems. The Geological Society, London, Special Publications, 319, 21–28. DOI: 10.1144/SP319.3 0305-8719/09/$15.00 # The Geological Society of London 2009. When interpreting the significance of the BSR, three principles should be kept in mind. Firstly, very little gas is required to produce a strong seismic reflector (Domenico 1977). Secondly, very little hydrate is needed to elevate the capillary pressure of a finegrained marine sediment to the point where it resists the entry of gas. Thirdly, an apparently continuous seismic reflector does not imply a continuous gas-saturated stratum. High-resolution seismic acquisition and processing show that a strong BSR, which appears continuous at low resolution, can be produced by small discontinuous pockets of gas (Wood et al. 2002; Dai et al. 2004). Seismic velocity anomalies have been used to detect and/or quantify gas hydrate presence in the subsurface (Barth et al. 2004; Dai et al. 2004). Seismic and well log estimates of hydrate saturation have been compared at Hydrate Ridge (Kumar et al. 2007) and at the JIP Atwater Valley site (Dai et al. 2008). However, the ability of seismic velocity analysis to accurately predict hydrate saturation depends on an accurate transform between hydrate saturation and acoustic velocity, and a good model for the velocity profile of the sediment in the absence of hydrate. Another characteristic of the seismic response to gas hydrate is amplitude blanking within the GHSZ, noted by early investigators of marine hydrate deposits (Shipley et al. 1979). Various theories have been proposed to explain blanking. One holds that hydrate, which increases the acoustic velocity of unconsolidated sediments, is most likely to form in high porosity, i.e. low velocity, strata, thus reducing the acoustic contrast with neighbouring strata (Lee & Dillon 2001; Hornbach et al. 2003). Blanking has also been explained by the disruption of sedimentary stratigraphy in marine environments thought to harbour hydrate (Chapman et al. 2002). A third explanation is destructive interference from vertically displaced reflectors within the Fresnel zone (Wood & Ruppel 2000). A fourth explanation attributes blanking to the presence of liquid and gas migrating upwards through conduits (Chapman et al. 2002). Still other theories remain current (Hornbach et al. 2007). Regardless of the cause, amplitude blanking has not been been correlated with economically significant hydrate deposits. Mechanisms of gas hydrate reservoir

Free or hydrate-bound gas in the seafloor has been of scientific, ecologic and economic interest for many years because it predominantly contains high concentrations of low-molecular-weight hydrocarbons. A prerequisite of accurate quantifications of gases in sediments is to preserve pressure and temperature close to the in situ conditions during recovery. Here we introduce two new sediment coring devices that allow for the recovery of near-surface gas- and gas-hydrate-bearing sediments and subsequent investigations using several different techniques such as visualisation by computerized tomography, quantitative degassing, and sediment and porewater analyses. The first coring tool, the Multiple Autoclave Corer (MAC), resembles a standard multiple corer in terms of applications, size and core length of about 55 cm. The second tool, the Dynamic Autoclave Piston Corer (DAPC), is similar to a piston corer in application and size and enables one to take cores of up to 2.5 m length. Both focus on the investigation of near-surface sediments, which are most strongly affected by changes in bottom-water temperature and hydrostatic pressure, which in turn influence continental slope stability. Some results from recent offshore applications show the potential of these tools.

The phase behavior of multi-component hydrate systems formed in the ocean has been studied using a sea-going in situ Raman spectrometer. Results from a field study using a synthetic hydrate sample and preliminary measurements of naturally occurring thermogenic hydrates at Barkley Canyon are reported and compared. This work follows early field trials using simple and binary synthetic hydrates and an expedition to Hydrate Ridge, studying natural hydrates primarily composed of methane. This refined Raman technique was combined with visual observations utilizing remotely operated vehicles as experimental platforms in an oceanic laboratory. Traditional laboratory Raman analysis was also performed on hydrate samples following recovery. A synthetic hydrate was first studied in Monterey Bay to characterize the formation, evolution, and dissociation of multi-component gas hydrates formed in the deep ocean as a guide to understanding complex behavior of natural hydrate systems. The gas mixture (92.5% CH 4 , 3.8% C 2 H 6 , 1.9% C 3 H 8 , 0.4% i -C 4 H 10 , 0.4% n -C 4 H 10 , 0.1% i -C 5 H 12 , 0.1% n -C 5 H 12 , and 0.1% neo-C 5 H 12 ) was chosen to simulate a typical natural gas composition. Regions of sII and mixed sI–sII hydrate were measured in situ after 65 days of formation. The presence of a heterogeneous inter-mixed sI–sII hydrate suggests a formation mechanism where hydrate growth proceeds from localized pockets of isolated gas, such as gas bubbles separated from the bulk vapor phase by hydrate membranes, where sI formation follows the depletion of sII-forming gases within these individual hydrate reactors. Unstable conditions during recovery led to significant hydrate dissociation, leaving only ice and a compositionally homogenous sII hydrate phase, with no evidence of the sI that had been observed at the seafloor. To investigate whether this process may operate in natural systems, a Raman investigation of the natural thermogenic hydrates at Barkley Canyon was carried out. Massive seafloor hydrate outcrops varied in appearance from yellow, due to oil staining, to white. While the yellow hydrate had significant interference due to fluorescence, detailed analysis of the in situ Raman data was possible for the white hydrate. This white hydrate was mainly sII, but the presence of sI was also detected. The heterogeneity in both structure and composition of these natural hydrates was similar to the synthetic hydrate formed in Monterey Bay. These similarities suggest that a controlled seafloor experiment was able to simulate a natural thermogenic hydrate with reasonable accuracy. Further development of theory and testing is needed to determine if the ‘individual hydrate reactor’ mechanism was indeed the pathway to growth in this natural system.

The state of preservation of natural gas hydrate samples, recovered from 6 sites drilled during ODP Leg 204 at southern summit of Hydrate Ridge, Oregon Margin, has been investigated by X-ray diffraction (XRD) and cryo-scanning-electron-microscopy (cryo-SEM) techniques. A detailed characterization of the state of decomposition of gas hydrates is necessary since no pressurized autoclave tools were used for sampling and partial dissociation must have occurred during recovery prior to the quench and storage in liquid nitrogen. Samples from 16 distinct horizons have been investigated by synchrotron X-ray diffraction measurements at HASYLAB/ Hamburg. A full profile fitting analysis (“Rietveld method”) of synchrotron XRD data provides quantitative phase determinations of the major sample constituents such as gas hydrate structure I (sI), hexagonal ice (Ih) and quartz. The ice content (Ih) in each sample is related to frozen water composed of both original existing pore water and the water from decomposed hydrates. Hydrate contents as measured by diffraction vary between 0 and 68 wt.% in the samples we measured. Samples with low hydrate content usually show micro-structural features in cryo-SEM ascribed to extensive decomposition. Comparing the appearance of hydrates at different scales, the grade of preservation seems to be primarily correlated with the contiguous volume of the original existing hydrate; the dissociation front appears to be indicated by micrometer-sized pores in a dense ice matrix.

Oceanic gas hydrates have been measured near the seafloor for the first time using a seagoing Raman spectrometer at Hydrate Ridge, Oregon, where extensive layers of hydrates have been found to occur near the seafloor. All of the hydrates analyzed were liberated from the upper meter of the sediment column near active gas venting sites in water depths of 770–780 m. Hydrate properties, such as structure and composition, were measured with significantly less disturbance to the sample than would be realized with core recovery. The natural hydrates measured were sI, with methane as the predominant guest component, and minor/trace amounts of hydrogen sulfide present in three of the twelve samples measured. Methane large-to-small cage occupancy ratios of the hydrates varied from 1.01 to 1.30, in good agreement with measurements of laboratory synthesized and recovered natural hydrates. Although the samples visually appeared to be solid, varying quantities of free methane gas were detected, indicating the possible presence of occluded gas in a hydrate bubble fabric.

10 pages, 6 figures, 1 table., Soupy and mousse-like fabrics are disturbance sedimentary features that result from the dissociation of gas hydrate, a process that releases water. During the core retrieval process, soupy and mousse-like fabrics are produced in the gas hydrate-bearing sediments due to changes in pressure and temperature conditions. Therefore, the identification of soupy and mousse-like fabrics can be used as a proxy for the presence of gas hydrate in addition to other evidence, such as pore water freshening or anomalously cool temperature. We present here grain-size results, mineralogical composition and magnetic susceptibility data of soupy and mousse-like samples from the southern Hydrate Ridge (Cascadia accretionary complex) acquired during Leg 204 of the Ocean Drilling Program. In order to study the relationship between sedimentary texture and the presence of gas hydrates, we have compared these results with the main textural and compositional data available from the same area. Most of the disturbed analyzed samples from the summit and the western flank of southern Hydrate Ridge show a mean grain size coarser than the average mean grain size of the hemipelagic samples from the same area. The depositional features of the sediments are not recognised due to disturbance. However, their granulometric statistical parameters and distribution curves, and magnetic susceptibility logs indicate that they correspond to a turbidite facies. These results suggest that gas hydrates in the southern Hydrate Ridge could form preferentially in coarser grain-size layers that could act as conduits feeding gas from below the BSR. Two samples from the uppermost metres near the seafloor at the summit of the southern Hydrate Ridge show a finer mean grain-size value than the average of hemipelagic samples. They were located where the highest amount of gas hydrates was detected, suggesting that in this area the availability of methane gas was high enough to generate gas hydrates, even within low-permeability layers. The mineralogical composition of the soupy and mousse-like sediments does not show any specific characteristic with respect to the other samples from the southern Hydrate Ridge., The samples for this study were provided from the Ocean Drilling Program, sponsored by the US National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. in which Spain is included as part of the European Consortium for Ocean Research rilling (ECORD). This work was supported by the Spanish Ministry of Education and Science (Ministerio de Educación y Ciencia, MEC) through Acciones Especiales: REN 2001-5262-E and BTE 2002-11698-E to E. Gràcia, National Project IMPULS REN2003-05996- MAR, EVENT CGL2006-12861-C02-02, and the FPU PhD fellowship 2003-2872 to E. Piñero. The authors thank Alan Judd for the English revision of the manuscript, and F. Abegg and D. Orange for their valuable reviews that greatly improved this manuscript.

Drilling on Hydrate Ridge, offshore Oregon, during ODP Leg 204 enabled us to investigate fabrics of gas hydrate samples in a wide depth range of the gas hydrate stability zone (GHSZ). X-ray computerized tomographic imaging on whole-round samples, frozen in liquid nitrogen, revealed that layered gas hydrate structures are related to variable processes occurring at different sediment depths. Shallow gas hydrates often form layers parallel or sub-parallel to bedding and also crosscut sedimentary strata and other gas hydrate layers, destroying the original depositional fabric. The dynamic processes interacting with this complicated plumbing system in this shallow environment are responsible for such highly variable gas hydrate fabrics. Gas hydrate layers deeper in the sediments are most often dipping with various angles, and are interpreted as gas hydrate precipitates filling tectonic fractures. These originally open fractures are potential candidates for free gas transportation, and might explain why free gas can rapidly emanate from below the bottom-simulating reflector through the GHSZ to the seafloor.

High-resolution multi-frequency and multi-component seismic data were acquired at northern Hydrate Ridge in the accretionary complex of the Cascadia subduction zone to quantify the amount of hydrate and free gas in the sediment. We present a detailed local analysis of four component (4C) ocean bottom seismometer (OBS) data and show the importance of multi-frequency and shear wave data for determining hydrate reservoir properties. A detailed model of the elastic parameters at the bottom simulating reflector (BSR) is developed by using synthetic seismogram modelling. The main focus in this study is an amplitude-versus-offset (AVO) analysis of shear waves, which originate by PS-conversion at the BSR in 73 m below the seafloor (bsf). The AVO analysis enables the determination of the shear wave velocity above the BSR. A velocity of 400 m/s indicates that the presence of gas hydrate in the pore space significantly increases the shear modulus of the sediment above the BSR. Information about the attenuation and the shape of the BSR transition zone is obtained from the frequency-dependent reflection amplitudes of the BSR. The BSR is shown to be a gradual type transition zone of 1.5–2.5 m thickness. Average Q factors of Qp = 150 for P-waves and of Qs = 35 for S-waves are determined within the gas hydrate stability zone (GHSZ). The low Qs factor points to a pronounced attenuation of S-waves in the uppermost sediments. From rock physics modelling, the hydrate concentration is estimated to vary locally between 3–12% of the pore space. Below the BSR, free gas concentrations of 0.5% and 8% are determined for homogeneous and patchy distributed gas, respectively.

Anaerobic oxidation of methane (AOM) impacts carbon cycling by acting as a methane sink and by sequestering inorganic carbon via AOM-induced carbonate precipitation. These precipitates commonly take the form of carbonate nodules that form within methane seep sediments. The timing and sequence of nodule formation within methane seep sediments are not well understood. Further, the microbial diversity associated with sediment-hosted nodules has not been well characterized and the degree to which nodules reflect the microbial assemblage in surrounding sediments is unknown. Here, we conducted a comparative study of microbial assemblages in methane-derived authigenic carbonate nodules and their host sediments using molecular, mineralogical, and geochemical methods. Analysis of 16S rRNA gene diversity from paired carbonate nodules and sediments revealed that both sample types contained methanotrophic archaea (ANME-1 and ANME-2) and syntrophic sulfate-reducing bacteria (Desulfobacteraceae and Desulfobulbaceae), as well as other microbial community members. The combination of geochemical and molecular data from Eel River Basin and Hydrate Ridge suggested that some nodules formed in situ and captured the local sediment-hosted microbial community, while other nodules may have been translocated or may represent a record of conditions prior to the contemporary environment. Taken together, this comparative analysis offers clues to the formation regimes and mechanisms of sediment-hosted carbonate nodules.

54 sediments were sampled at 5 sites, 1244, 1245, 1246, 1250 and 1251 from ODP Leg 204, Hydrate Ridge, Cascadia Margin. The stable carbon and nitrogen isotopic composition, δ13C of organic matter and δ15N, have been used to characterize the depositional variation of sediments with gas hydrate bearing. Values of δ13C of bulk organic matter range between −22.37‰ and −24.72‰ PDB, with a mean value of −23.54±1.17‰ PDB; δ15N of the same sedimentary samples range from 3.30‰ to 5.68‰, with a mean value of 4.49±1.19’; Values of C/N ratio range from 4.53 to 9.26, with a mean value of 6.90±2.37. The occurrence of gas hydrate changed the diagenetic evolution trend of the carbon and nitrogen in organic matter. The δ15N and C/N ratio depletion suggest that the nitrogen fixation related with the bacteria/archaea maybe occurred in the sediments near the hydrate. The stable carbon and nitrogen isotopic composition could be used to evaluate the depositional environment in the study area.

Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules (Sloan, 1998). Gas hydrates are widely present in permafrost and deep oceanic environments around the world. Hydrate Ridge, offshore Oregon, is one of the areas where hydrates are found in relatively high concentration. In marine environments, methane hydrates are usually stable at temperatures of 0–20° C, water depths greater than 500 m, and in sediments up to 300 m below the seafloor. Small amounts of free gas are often present below the gas-hydrate stability zone (GHSZ). The hydrate-to-free-gas contact gives a strong acoustic impedance contrast, which is evident on seismic sections as a bottom-simulating reflection (BSR). Offshore gas hydrate systems have received attention from the scientific community because of their potential to be

Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly 13C-depleted carbon isotope values of the samples (as low as − 48.1 ‰ PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The δ18O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 ‰ PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 ‰ PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place.

Formation, evolution and destruction of natural gas hydrate systems in marine environments have been extensively discussed since the first gas hydrate samples were recovered N30 yr ago [1]. The origin and sources of hydrate-bound gases [2] and waters [3], the distribution of phases within and below the gas hydrate stability zone (GHSZ) [4], mechanisms of crystal growth [5], and gas hydrate concentration in pore space [6] are among the most debated issues. Recent field studies facilitated by Ocean Drilling Program (ODP) [7,8] and dedicated government programs [9,10] provided a wealth of geological, geophysical, and geochemical information from hydrate-bearing locations. Integrated interpretation of various datasets holds a great potential to advance our understanding of gas hydrate systems and to constrain speculations about the role of gas hydrates as a potential energy source [11] and a component of the global carbon cycle [12].

Anaerobic methane-oxidizing microbial communities in sediments at cold methane seeps are important factors in controlling methane emission to the ocean and atmosphere. Here, we investigated the distribution and carbon isotopic signature of specific biomarkers derived from anaerobic methanotrophic archaea (ANME groups) and sulphate-reducing bacteria (SRB) responsible for the anaerobic oxidation of methane (AOM) at different cold seep provinces of Hydrate Ridge, Cascadia margin. The special focus was on their relation to in situ cell abundances and methane turnover. In general, maxima in biomarker abundances and minima in carbon isotope signatures correlated with maxima in AOM and sulphate reduction as well as with consortium biomass. We found ANME-2a/DSS aggregates associated with high abundances of sn-2,3-di-O-isoprenoidal glycerol ethers (archaeol, sn-2-hydroxyarchaeol) and specific bacterial fatty acids (C16:1ω5c, cyC17:0ω5,6) as well as with high methane fluxes (Beggiatoa site). The low to medium flux site (Calyptogena field) was dominated by ANME-2c/DSS aggregates and contained less of both compound classes but more of AOM-related glycerol dialkyl glycerol tetraethers (GDGTs). ANME-1 archaea dominated deeper sediment horizons at the Calyptogena field where sn-1,2-di-O-alkyl glycerol ethers (DAGEs), archaeol, methyl-branched fatty acids (ai-C15:0, i-C16:0, ai-C17:0), and diagnostic GDGTs were prevailing. AOM-specific bacterial and archaeal biomarkers in these sediment strata generally revealed very similar δ13C-values of around −100. In ANME-2-dominated sediment sections, archaeal biomarkers were even more 13C-depleted (down to −120), whereas bacterial biomarkers were found to be likewise 13C-depleted as in ANME-1-dominated sediment layers (δ13C: −100). The zero flux site (Acharax field), containing only a few numbers of ANME-2/DSS aggregates, however, provided no specific biomarker pattern. Deeper sediment sections (below 20 cm sediment depth) from Beggiatoa covered areas which included solid layers of methane gas hydrates contained ANME-2/DSS typical biomarkers showing subsurface peaks combined with negative shifts in carbon isotopic compositions. The maxima were detected just above the hydrate layers, indicating that methane stored in the hydrates may be available for the microbial community. The observed variations in biomarker abundances and 13C-depletions are indicative of multiple environmental and physiological factors selecting for different AOM consortia (ANME-2a/DSS, ANME-2c/DSS, ANME-1) along horizontal and vertical gradients of cold seep settings.

Fluid flow at cold vent sites is usually driven by the ascent of overpressured fluids from subsurface reservoirs. Porosity changes in surface sediments due to precipitation and dissolution of authigenic carbonates affects fluid flow and biogeochemical turnover. From observations, it is known that carbonate precipitates often occur in distinct layers in high concentrations, surrounded by layers with low carbonate content. Using a non-steady state model, we simulate aragonite and calcite precipitation and dissolution in a 2-m long sediment column, located under a bacterial mat at Hydrate Ridge, Cascadia accretionary margin. Assuming a constant pressure gradient over 7000 years, fluid flow, anaerobic oxidation of methane (AOM) rates, and carbonate precipitation and dissolution rates show strong oscillations evoked by changes in permeability and fluid flow over time. The porosity reaches values below 0.35 in the carbonate layers that reduce the fluid flow velocity from an initial value of 30 cm a−1 to a minimum value of about 2 cm a−1. These significant changes in the fluid flow system displace the depth of sulfate penetration. The simulation predicts cycles of carbonate crust formation and dissolution with a duration of 2000–2700 years resulting in several distinct carbonate layers. During periods of high fluid flow, AOM reaches rates over 1000 μmol cm−1 a−1 and methane fluxes out of the sediment reach 200 μmol cm−1 a−1. During periods of low fluid flow, AOM is about 450 μmol cm−1 a−1 and the methane flux into the bottom water vanishes completely. The oscillations are dampened so that fluid flow and biogeochemical turnover slowly approach steady state after about 7000 years towards the end of the simulation period, showing a 1-m-thick area at the surface with carbonate concentrations of about 25 wt.%. Flow oscillations may also impact the colonization of chemoautotrophic larva and bacteria in and on the sediment. The frequency of precipitation–dissolution cycles of 2000–2700 years is long enough for vent biota to react to changes in the sediments, for example explaining the occurrence of buried Calyptogena and Acharax mussel shells at former vent sites that are not presently active. The evaluation of side scan sonar data reveals large areas of cemented sediments and carbonate pavements north of the investigated site. The situation found in the investigated core may be characterized as intermediate stage of carbonate cementation. This ongoing process will also form a solid carbonate pavement at this site in the future.

We report and discuss molecular and isotopic properties of hydrate-bound gases from 55 samples and void gases from 494 samples collected during Ocean Drilling Program (ODP) Leg 204 at Hydrate Ridge offshore Oregon. Gas hydrates appear to crystallize in sediments from two end-member gas sources (deep allochthonous and in situ) as mixtures of different proportions. In an area of high gas flux at the Southern Summit of the ridge (Sites 1248–1250), shallow (0–40 m below the seafloor [mbsf]) gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic C2+ gases, which migrated vertically and laterally from as deep as 2- to 2.5-km depths. In contrast, deep (50–105 mbsf) gas hydrates at the Southern Summit (Sites 1248 and 1250) and on the flanks of the ridge (Sites 1244–1247) crystallize mainly from microbial methane and ethane generated dominantly in situ. A small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth (e.g., Sites 1244 and 1245). Non-hydrocarbon gases such as CO2 and H2S are not abundant in sampled hydrates. The new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at Hydrate Ridge formed from gas derived from decomposition of deeper and older gas hydrates. Gas hydrate formation at the Southern Summit is explained by a model in which gas migrated from deep sediments, and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone (GHSZ). Free gas migrated into the GHSZ when the overpressure in gas column exceeded sealing capacity of overlaying sediments, and precipitated as gas hydrate mainly within shallow sediments. The mushroom-like 3D shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit, the decrease of gas solubility in shallow sediment, and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure.

Extensive methane hydrate layers are formed in the near-surface sediments of the Cascadia margin. An undissociated section of such a layer was recovered at the base of a gravity core (i.e. at a sediment depth of 120 cm) at the southern summit of Hydrate Ridge. As a result of salt exclusion during methane hydrate formation, the associated pore waters show a highly elevated chloride concentration of 809 mM. In comparison, the average background value is 543 mM. A simple transport-reaction model was developed to reproduce the Cl- observations and quantify processes such as hydrate formation, methane demand, and fluid flow. From this first field observation of a positive Cl- anomaly, high hydrate formation rates (0.15–1.08 mol cm-2 a-1) were calculated. Our model results also suggest that the fluid flow rate at the Cascadia accretionary margin is constrained to 45–300 cm a-1. The amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water. Thus, most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from CH4 dissolved in the pore water.

At the summit of Hydrate Ridge (ODP Sites 1249 and 1250), pore fluids are highly enriched in dissolved chloride (up to 1370 mM) in a zone that extends from near the sediment surface (∼1 mbsf) to depths of 25±5 mbsf. Below this depth, brines give way to chloride values approaching seawater concentrations with lower chloride anomalies superimposed on baseline values. We developed a one dimensional, non-steady state, transport reaction model to simulate the observed chloride enrichment at Site 1249. Our model shows that in order to reach the observed high chloride values, methane must be transported in the gas phase from the depth of the BSR to the seafloor. Methane transport exclusively in the dissolved phase is not enough to form methane hydrate at the rates needed to generate the observed chloride enrichment. Methane transport in the gas phase is consistent with geophysical and logging data, estimates of gas pressure beneath the BSR, and observations of bubble plumes at the seafloor. In order to reproduce the observed chloride and gas hydrate distributions, the model requires an enhanced rate of hydrate formation in near surface sediments, which we implement through depth-dependent kinetic constants. We argue that this is justified by changes in geomechanical properties of the sediment. At depths shallower than 25 mbsf the force of crystallization can overcome effective overburden stress, and hydrate growth proceeds by particle displacement, thus minimizing capillary inhibition effects. Our calculations indicate the hydrates in the upper sediments of the ridge summit are probably younger than 1500 years, although the age is difficult to constrain. Independent estimates based on seafloor observations at this site yield gas hydrate formation rates at the ridge crest on the order of 102 mol m−2 year−1. These rates are several orders of magnitude higher than those estimated for Site 997 on the Blake Ridge

The recognition of finely disseminated gas hydrate in deep marine sediments heavily depends on various indirect techniques because this mineral quickly decomposes upon recovery from in situ pressure and temperature conditions. Here, we discuss molecular properties of closely spaced gas voids (formed as a result of core recovery) and gas hydrates from an area of relatively low gas flux at the flanks of the southern Hydrate Ridge offshore Oregon (ODP Sites 1244, 1245 and 1247). Within the gas hydrate occurrence zone (GHOZ), the concentration of ethane (C 2 ) and propane (C 3 ) in adjacent gas voids shows large variability. Sampled gas hydrates are enriched in C 2 relative to void gases but do not contain C 3 . We suggest that the observed variations in the composition of void gases is a result of molecular fractionation during crystallization of structure I gas hydrate that contains C 2 but excludes C 3 from its crystal lattice. This hypothesis is used to identify discrete intervals of finely disseminated gas hydrate in cored sediments. Variations in gas composition help better constrain gas hydrate distribution near the top of the GHOZ along with variations in pore water chemistry and core temperature. Sediments near the base of the gas hydrate stability zone are relatively enriched in C 2+ hydrocarbon gases. Complex and poorly understood geological and geochemical processes in these deeper sediments make the identification of gas hydrate based on molecular properties of void gases more ambiguous. The proposed technique appears to be a useful tool to better understand the distribution of gas hydrate in marine sediments and ultimately the role of gas hydrate in the global carbon cycle.

Uranium (U) concentrations and activity ratios (δ 234U) of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon-rich fluids ("cold seeps") at Hydrate Ridge, off the coast of Oregon, USA. The low U concentrations (mean: 1.3 ± 0.4 μg/g) and high δ 234U values (165-317‰) of gas hydrate carbonates reflect the influence of sedimentary pore water indicating that these carbonates were formed under reducing conditions below or at the seafloor. Their 230Th/ 234U ages span a time interval from 0.8 to 6.4 ka and cluster around 1.2 and 4.7 ka. In contrast, chemoherm carbonates precipitate from marine bottom water marked by relatively high U concentrations (mean: 5.2 ± 0.8 μg/g) and a mean δ 234U ratio of 166 ± 3‰. Their U isotopes reflect the δ 234U ratios of the bottom water being enriched in 234U relative to normal seawater. Simple mass balance calculations based on U concentrations and their corresponding δ 234U ratios reveal a contribution of about 11% of sedimentary pore water to the bottom water. From the U pore water flux and the reconstructed U pore water concentration a mean flow rate of about 147 ± 68 cm/a can be estimated. 230Th/ 234U ages of chemoherm carbonates range from 7.3 to 267.6 ka. 230Th/ 234U ages of two chemoherms (Alvin and SE-Knoll chemoherm) correspond to time intervals of low sealevel stands in marine isotope stages (MIS) 2, 4, 5, 6, 7 and 8. This observation indicates that fluid flow at cold seep sites sensitively reflects pressure changes of the hydraulic head in the sediments. The δ 18O PDB ratios of the chemoherm carbonates support the hypothesis of precipitation during glacial times. Deviations of the chemoherm δ 18O values from the marine δ 18O record can be interpreted as to reflect temporally and spatially varying bottom water and/or vent fluid temperatures during carbonate precipitation between 2.6 and 8.6°C.

A numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate-bearing surface sediments from a cold vent site situated at Hydrate Ridge, an accretionary structure located in the Cascadia Margin subduction zone. Steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below. The model results underline the dominance of advective fluid flow that forces a large inflow of methane from below (869 mumol cm(-2) a(-1)) inducing high oxidation rates in the surface layers. Anaerobic methane oxidation is the major process, proceeding at a depth-integrated rate of 870 mumol cm(-2) a(-1). A significant fraction (14%) of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite. The total rate of carbonate precipitation (120 mumol cm(-2) a(-1)) allows for the build-up of a massive carbonate layer with a thickness of I m over a period of 20,000 years. Aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane-charge fluids is high enough ( greater than or equal to10 cm a(-1)) to maintain super-saturation with respect to this highly soluble carbonate phase. It precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments, suggesting microbial catalysis. The investigated station is characterized by high carbon and oxygen turnover rates (approximate to1000 mumol cm(-2) a(-1)) that are well beyond the rates observed at other continental slope sites not affected by fluid venting. This underlines the strong impact of fluid venting on the benthic system, even though the flow velocity of 10 cm a(-1) derived by the model is relative low compared to fluid flow rates found at other cold vent sites. Non-steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow. Moreover, they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities (

Deep subsurface sediments from the Nankai Trough, Japan Sea, ODP Leg 190, sites 1173, 1174, 1177, and near-surface sediments from Hydrate Ridge, NE-Pacific have been analysed by high performance liquid chromatography (HPLC)–electrospray ionisation (ESI)-mass spectrometry (MS). The main objective was to utilize the presence of intact phospholipids as a direct indicator of viable microorganisms. The extracts of Nankai Trough sediments were found to contain a variety of phospholipid (PL) structures, well-known to stem from microorganisms, to depths as great as 745 mbsf and in situ temperatures as high as 85 °C. In addition, high relative amounts of lysophospholipids (e.g. lysophosphatidylcholines) exceeding those of the regular phospholipids were detected. Diglyceride mass fragments of various PLs have been assigned to fatty acyl side-chains of typical chain length (C14, C16, C18, C20) and degree of unsaturation (zero, one or two double bonds). Similar results were obtained for the phospholipid distribution in extracts of organic matter-rich Hydrate Ridge sediments. To date, the enhanced occurrence of lysophospholipids cannot be explained completely but a response to increasing thermal and ecological stress seems probable.

The economic potential of well-studied offshore gas hydrate accumulations and provinces is assessed qualitatively based on consideration of geological, technological, and economic factors. Three types of gas hydrate accumulations are suggested. Structural accumulations occur where thermogenic, bacterial, or mixed gases are rapidly transported from the subsurface petroleum system to the gas hydrate stability zone along faults, mud volcanoes, and other structures (e.g. northwestern Gulf of Mexico, Hydrate Ridge, and Haakon Mosby mud volcano). These accumulations are generally characterized by high gas hydrate concentration in sediment, high resource density, high recovery factors, as well as low development and production costs. It is likely that structural accumulations provide marginal or economic gas hydrate reserves if they represent significant volumes of hydrate-bound gas. Stratigraphic accumulations occur in relatively permeable sediments and form largely from bacterial methane generated in situ or slowly migrated from depth in the section (e.g. Blake Ridge, Gulf of Mexico minibasins). These accumulations are generally characterized by low gas hydrate concentration in sediments and low recovery factor, as well as high development and production costs. Stratigraphic accumulations mainly provide a subeconomic gas hydrate resource. However, in cases such as the Nankai Trough province, high gas hydrate concentration occurs in permeable sand layers and may represent a viable exploration and exploitation target. Less geological data are available on the combination gas hydrate accumulations controlled both by structures and stratigraphy. On the global scale, gas hydrate reserves are likely to represent only a small fraction of the gas hydrate resource because the largest volume of gas hydrate is in subeconomic stratigraphic accumulations. However, some concentrated gas hydrate accumulations may be exploited profitably, and those should be subjected to detailed quantitative economic analysis.

Boron contents and boron, carbon and oxygen stable isotopes were determined for authigenic carbonates recovered from Ocean Drilling Program Leg 146, Oregon margin. Carbonate precipitates are the most widespread authigenic phase in the shallow accretionary wedge and carry chemical information about long-term variations in pore fluid origin and flow paths in the Cascadia subduction zone. Drilling the first ridge (toe area including the frontal thrust) and the second ridge (or Hydrate Ridge) of the prism demonstrated different fluid regimes, with higher B contents in the authigenic precipitates at the toe. The δ11B of 18 authigenic precipitates analysed ranges from 13.9‰ to as high as 39.8‰, extending the upper range of previously reported carbonate δ11B values considerably. When related to the δ11B ratio of their parent solutions, these data are characteristic of fluid-related processes in accretionary prisms. Together with δ13C and δ18O, δ11B ratios of the carbonate concretions, nodules and crusts allow one to distinguish between precipitation influenced by (i) seawater, (ii) fluid reservoirs at different depth levels within the accretionary prism and (iii) cage water from dissociated gas hydrates, the latter possibly indicating a fluctuation of the bottom simulating reflector during most recent Earth’s history. From this first systematic boron study on authigenic precipitates from an accretionary prism it is suggested that B contents of such carbonate crusts and concretions exceed those reported for other marine carbonates. Given the abundance of such precipitates at convergent margins, they represent a significant B sink in geochemical cycling. Isotopic compositions of the parent fluids to the carbonates mirror B chemistry of modern pore waters from convergent margins. The precipitates carry information of different subduction-related fluid processes over a certain period of time, and hence are a crucial tracer in the investigation of palaeo-fluid flow.

Naturally occurring gas hydrates forming in the ocean floor sediments have, ever since the 1970s, been heralded as the next century's viable energy source. Is this really so? What has the Ocean Drilling Program (ODP) found out about the nature of the Bottom Simulating Reflector (BSR) as an indicator of large volumes of gas hydrates and free natural gas? The findings so far from ODP's legs 146 and 164 have been rather discouraging with respect to the in situ amounts of both gas hydrates and free gas. In the sediments drilled on Leg 146, on the Cascadia accretionary wedge off Oregon, USA, the average amount of gas hydrates above a prominent BSR was estimated at 1 – 2 percent by sediment volume. The free gas amount below the BSR was 3–4 percent by sediment volume. On Leg 164 in the Western Atlantic Ocean (Blake Plateau) some higher amounts, averaging up to 4 – 6 percent and 5 – 7 percent by volume of sediment, respectively, were found. However, on the Hydrate Ridge, off Oregon (Cascadia, just south of Leg 146), there has recently been discovered apparently large amounts of sediment-mixed gas hydrates, which may be exploitable.Although the general percentages found so far are much too low for commercial extraction (even a high-grade copper-ore on land must average more than 6 percent by volume), there may exist some high-grade regions worth exploring. But, the question remains: Where shall we seek the high-grade gas hydrate deposits in ocean sediments? Besides the obvious deep-water high porosity conventional type of reservoirs, and judging from recent findings in the Gulf of Mexico and the Caspian Sea, perhaps the most promising locations are those where mud volcanoes occur on the ocean floor.

Although the presence of extensive gas hydrate on the Cascadia margin, offshore from the western U.S. and Canada, has been inferred from marine seismic records and pore water chemistry, solid gas hydrate has only been found at one location. At Ocean Drilling Program (ODP) Site 892, offshore from central Oregon, gas hydrate was recovered close to the sediment-water interface at 2–19 m below the seafloor (mbsf) at 670 m water depth. The gas hydrate occurs as elongated platy crystals or crystal aggregates, mostly disseminated irregularly, with higher concentrations occurring in discrete zones, thin layers, and/or veinlets parallel or oblique to the bedding. A 2- to 3-cm thick massive gas hydrate layer, parallel to bedding, was recovered at ∼ 17 mbsf. Gas from a sample of this layer was composed of both CH4 and H2S. This sample is the first mixed-gas hydrate of CH4 H2S documented in ODP; it also contains ethane and minor amounts of CO2. Measured temperatures of the recovered core ranged from 2 to −1.8°C and are 6 to 8 degrees lower than in-situ temperatures. These temperature anomalies were caused by the partial dissociation of the CH4 H2S hydrate during recovery without a pressure core sampler.During this dissociation, toxic levels of H2S (δ34S, +27.4‰) were released. The δ13C values of the CH4 in the gas hydrate, −64.5 to −67.5‰(PDB), together with δD values of −197 to −199‰(SMOW) indicate a primarily microbial source for the CH4. The δ18O value of the hydrate H2O is +2.9‰(SMOW), comparable with the experimental fractionation factor for sea-ice. The unusual composition (CH4 H2S) and depth distribution (2–19 mbsf) of this gas hydrate indicate mixing between a methane-rich fluid with a pore fluid enriched in sulfide; at this site the former is advecting along an inclined fault into the active sulfate reduction zone. The facts that the CH4 H2S hydrate is primarily confined to the present day active sulfate reduction zone (2–19 mbsf), and that from here down to the BSR depth (19–68 mbsf) the gas hydrate inferred to exist is a ≥99% CH4 hydrate, suggest that the mixing of CH4 and H2S is a geologically young process. Because the existence of a mixed CH4 H2S hydrate is indicative of moderate to intense advection of a methane-rich fluid into a near surface active sulfate reduction zone, tectonically active (faulted) margins with organic-rich sediments and moderate to high sedimentation rates are the most likely regions of occurrence. The extension of such a mixed hydrate below the sulfate reduction zone should reflect the time-span of methane advection into the sulfate reduction zone.

The crest of Hydrate Ridge harbors a variety of gas hydrates in near-surface sediments. Hydrate formation and destruction continuously shape the ridge topography. Interstitial Cl-anomaly patterns in conjunction with video-guided sampling have established that the uppermost sediment column contains several distinct layers of gas hydrate which are exposed at the sea floor. A methane-oxidizing bacterial consortium populates the exposures of hydrate; colonies of vent macro-fauna are abundant as well. Discharge of methane from destabilized hydrate at the seafloor stimulates high rates of benthic oxygen consumption. These rates, however, vary by many orders of magnitude spatially and temporally, highlighting the need for implementing seafloor observatories at gas hydrate sites. Two types of hydrate fabrics were observed: A highly porous fabric with an estimated pore space of approx. 60 vol.-% and a massive type, with no visible pore space. Both types contain varying amounts of chloride, which need to be taken into account when estimating hydrate volumes from Cl-depletion of pore waters. The porous hydrate has low bulk density, which may cause periodic release of large chunks of hydrate from the sea floor. They float to the surface and leave behind a chaotic topography of mounds and depressions. These pieces of floating hydrates constitute an important transport mechanism for methane from the seafloor directly to the atmosphere.

The microbial community inhabiting anaerobic sediments of Hydrate Ridge, an area characterized by extensive methane-venting and gas hydrate occurrence on the Cascadia accretionary prism, is dominated by a complex consortium of archaea and bacteria which performs anaerobic methane oxidation (AMO). The process is indicated by the dominance of archaeal-derived biomarkers such as crocetane, pentamethylicosane, and archaeol in anaerobic sediments and authigenic carbonates. The biomarkers are highly depleted in 13 C relative to their methane carbon source, which results in carbon isotopic compositions as low as -128%o PDB. The essentially limitless reservoir of methane available to the archaea in this gas hydrate environment allows the kinetic carbon isotope effect (e CH4-biomarker ) associated with AMO to be fully developed (e max = 72%o). No substrate depletion effects were detected. Varying isotope effects and concentrations of the biomarkers on and off Hydrate Ridge indicate that the abundance of these compounds is essentially dependent on the supply of methane from this special environmental setting.

Hydrate Ridge is part of the accretionary complex at the Cascadia margin and is an area of widespread carbonate precipitation induced by the expulsion of methane-rich fluids. All carbonates on Hydrate Ridge are related to the methanecarbon pool either through anaerobic methanotrophy or through methanogenesis. Several petrographically distinct lithologies occur in boulder fields or in massive autochtonous chemoherm complexes which include methane-associated diagenetic mudstones and venting-induced breccias. The mudstones result from methane diagenesis in different sediment horizons and geochemical environments related to very slow methane venting. Cemented bioturbation casts occur as fragments, complex framework or as clasts together with bivalve shells as part of intraformational breccias, which are restricted to chemoherm complexes. Here, fluids ascend from the sub-seafloor and support aragonite-dominated carbonate precipitation near or at the sediment surface. Voids within mudclast breccias are either aragonite-rich indicating a formation near the surface at vent sites or are cemented by dolomite, which indicates formation in deeper parts of the sediment column. Brecciation is caused by tectonic or slump processes. In addition, we recognized a direct relationship between gas hydrates and sediment fracturing as well as the oxygen isotope composition of carbonate lithologies. Such gas hydrate-associated carbonates either show layered megapores and veins as relics of the original gas hydrate fabric or consist of aragonite-cemented intraclast breccias formed by growing and decomposing gas hydrate near the sediment surface. Both rock fabrics and the enrichment of 180 in high Mg-calcite demonstrate carbonate-forming mechanisms of gas hydrate.

The driving force of complex biogeochemical reactions in sedimentary environments during early diagenesis is the irreversible redox-conversion of organic matter. Our modeling approach incorporates interdependent diagenetic reactions evolving into a diffusive multi-component and multi-phase system by means of thermodynamic equilibrium calculations of species distribution. The modeling tool is the PHREEQC (version 2) computer code. Reaction kinetics of organic carbon conversion are integrated into the set of equilibrium reactions by defining the type and amount of converted organic matter at a certain time step. The model has been applied so far in different environmental settings: the gas hydrate dominated sediments of Hydrate Ridge (ODP Leg 204), the high accumulation area of the Amazon Fan, and the upwelling area off Peru.

[1] Both the concentration and the carbon isotope composition of dissolved inorganic carbon (DIC) vary considerably across the sulfate-methane transition (SMT) in shallow marine sediment at locations with gas hydrate. This variability has led to different interpretations for how carbon, including CH4, cycles within gas-charged sediment sequences over time. We extend a one-dimensional model for the formation of gas hydrate to account for downhole changes in dissolved CH4, SO42−, DIC, and Ca2+, and the δ13C of DIC. The model includes advection, diffusion, and two reactions that consume SO42−: degradation of particulate organic carbon (POC) and anaerobic oxidation of methane (AOM). Using our model and site-specific parameters, steady state pore water profiles are simulated for two sites containing gas hydrate but different carbon chemistry across the SMT: Site 1244 (Hydrate Ridge; DIC = 38 mM, δ13C of DIC = –22.5‰ PDB) and Site Keathley Canyon (KC) 151–3 (Gulf of Mexico; DIC = 16 mM, δ13C of DIC = −49.6‰ PDB). The simulated profiles for CH4, SO42−, DIC, Ca2+, and δ13C of DIC resemble those measured at the sites, and the model explains the similarities and differences in pore water chemistry. At both sites, an upward flux of CH4 consumes most net SO42− at a shallow SMT, and calcium carbonate removes a portion of DIC at this horizon. However, a large flux of 13C-enriched HCO3− enters the SMT from depth at Site 1244 but not at Site KC151–3. This leads to a high concentration of DIC with a δ13C much greater than that of CH4, even though AOM causes the SMT. The addition of HCO3− from depth impacts the slope of certain concentration crossplots. Crucially, neither the DIC concentration nor its carbon isotope composition at the SMT can be used to discriminate between sulfate reduction pathways.

[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.

Using the well developed Deep Ocean Raman In Situ Spectrometer (DORISS), we have developed an in situ deep-sea marine sediment pore water probe for geochemical studies. This novel device allows detailed measurement of pore water profiles. Our particular interest is in dissolved CH 4 , SO 4 =, H 2 S, and pH in situ without incurring artifacts due to the degassing that typically occurs with core recovery. A second generation DORISS instrument was developed at the Monterey Bay Aquarium Research Institute (MBARI) by engineering and science staff in 2005. The instrument is a plugin module for either of remotely operated vehicle (ROV) platforms at MBARI. DORISS has successfully performed in situ measurements on targets of scientific interest including high-temperature hydrothermal vent fluids, complex gas hydrates, and numerous other targets. Previously Raman techniques had been avoided to analyze sediment pore water geochemistry because of experience with the pore-waters strongly fluorescing. Simple experiments conducted with extracted pore-waters had previously demonstrated that fluorescence overwhelmed the relatively weak laser Raman signal. The in situ Raman pore water probe has worked around the fluorescence problem. The Raman pore water probe has been deployed a number of times at gas hydrate sites such as Hydrate Ridge, Barkley Canyon, and in the Santa Monica basin over the last two years. This paper will discuss the technique details and the exploration of expanding these same techniques into longer probes and looking at other difficult to obtain chemistries.

[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.

A quantitative simulation of interdependent diagenetic reactions in marine sediments using the PHREEQC (version 2) computer code is presented. The model recreates observed diagenetic signals (distribution of dissolved, mineral, and gaseous species) at ODP Leg 204, Site 1246 (Hydrate Ridge), including methane hydrate formation. The approach relies on thermodynamic calculations for equilibrium reactions in aqueous solutions that are constrained by the irreversible remineralisation of organic carbon. The model scenario describes a growing sediment column in which solids as well as aqueous species are progressively buried according to the observed sedimentation rate. Further, one-dimensional molecular diffusion of aqueous species is considered. Reaction kinetics of organic carbon remineralisation is integrated into the set of equilibrium reactions by defining the type and amount of remineralised organic carbon at each time step. The match between modelled and measured solid phase contents and pore water concentration profiles (pH, ammonia, sulphate, calcium, chloride, sodium, magnesium, and potassium) confirms the model validity. Sediments of ODP Leg 204, Site 1246 are located within the gas hydrate stability zone and methane is solely of biogenic origin. The modelled amount of in situ-generated biogenic methane exceeds the methane solubility with respect to methane hydrate. Carbon mass balance calculations reveal that the largest carbon sink is diagenetic carbonate, highlighting the influence of organic matter degradation on diagenetic mineral formation. The generated methane (as methane hydrate and dissolved methane) is the second most important carbon species within the sediments. Model calibration based on measured alkalinity and ammonia concentration profiles provides insight on the amount and type (marine or terrestrial) of organic matter that was initially deposited and subsequently remineralised. This information cannot be obtained solely from a characterisation of the relict sedimentary organic carbon. Consequently, geochemical models, such as the one presented in this study lead to a better understanding of paleoceanographic conditions such as marine productivity and terrigenous organic matter input due to prevailing paleocurrents.

[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.

[1] We simulate methane hydrate formation with multiphase flow and free gas within the regional hydrate stability zone (RHSZ). We find that hydrate distribution and fracture behavior are largely determined by the phase of the methane supply. We allow free gas to enter the RHSZ when porewater salinity increases to the value required for three-phase equilibrium. Fractures nucleate when the excess pore pressure exceeds the vertical hydrostatic effective stress. At Hydrate Ridge, where methane supply is dominantly free gas, hydrate saturation increases upwards and fractures nucleate high within the RHSZ, eventually allowing gas to vent to the seafloor. At Blake Ridge, where methane supply is dominantly in the dissolved phase, hydrate saturation is greatest at the base of the RHSZ; fractures nucleate here and in some cases could propagate through the RHSZ, allowing methane-charged water to vent to the seafloor.