Base Of Gas Hydrate Stability Research Articles

Possible link between weak bottom simulating reflections and gas hydrate systems in fractures and macropores of fine-grained sediments: Results from the Hikurangi Margin, New Zealand

Small amounts of free gas in interstitial sediment pores are known to significantly lower compressional (P-) wave velocity (Vp). This effect, combined with moderately elevated Vp from the presence of gas hydrates, is usually thought to be the cause for the often observed strong negative reflection coefficients of bottom simulating reflections (BSRs) at the base of gas hydrate stability (BGHS). At several locations however, weak BSRs have been observed, which are difficult to reconcile with a presence of gas in sediment pores. We here present a rock physics model for weak BSRs on the Hikurangi Margin east of New Zealand. Thin sections of a fine-grained mudstone sample from a submarine outcrop in the vicinity of a weak BSR show macroscopic porosity in the form of fractures and intrafossil macropores. We apply the Kuster-Toksöz theory to predict seismic velocities for a rock with water-saturated interstitial micropores and gas or hydrates in macroscopic pore space simulating fractures or compliant macropores. We match field observations of a weak BSR with a reflection coefficient of −0.016 with two end-member models; (1) rocks with gas hydrate-filled voids with a concentration of <4% of bulk sediment overlying water-filled voids, or (2) fully gas-saturated voids at a concentration of <2% beneath water-filled voids. A natural system is likely to consist of a combination of these end-members and of macroporosity filled with a mixture of water and gas or hydrate. Our results suggest weak BSRs may be caused by gas hydrate systems in fractures and macropores of fine-grained sediments with fully water-saturated interstitial pore space. Gas may be supplied into the macroscopic pore space by diffusion-driven short-range migration of methane generated within the gas hydrate stability field or, our favoured model based on additional geologic considerations, long-range advective migration from deeper sources along fractures.

Controls on gas hydrate system evolution in a region of active fluid flow in the SW Barents Sea

The location and stability of gas hydrates in the SW Barents Sea is poorly constrained due to complex geological, geochemical, and geophysical conditions, including poor controls on regional heat flow and gas chemistry. Understanding the stability of gas hydrates in this region is important, as recent studies suggest destabilizing hydrates may lead to methane discharge into the ocean and possibly in to the atmosphere. Here, we use high-resolution 3D P-Cable seismic data, combined with 3D heat flow and fluid flow models to place new constraints on gas hydrate stability in this region. The 3D P-Cable seismic data, acquired in 2009 west of Loppa High, show cross-cutting, reverse polarity, high-amplitude reflectors interpreted as the base of gas hydrate stability. To constrain heat flow, fluid flow, and gas hydrate stability within the 3D seismic volume, we use a 3D steady-state, finite difference diffusive thermal model that incorporates regional bottom water temperature from CTD casts, expected geothermal gradients, and gas composition derived from well data. In general, modelled bottom simulating reflectors are deeper than observed BSRs. Our analysis weighs multiple factors that might explain the discrepancy between observed and modelled bottom simulating reflector depths. From this analysis, we propose that the most significant discrepancies in BSR depth are likely related to changes in regional fluid/heat flow and fluid geochemistry. The anomalously shallow bottom simulating reflectors can be explained via vertical fluid flow that might include ensuing potential effects on gas composition, pore water salinity and temperature. Our estimate suggest that a maximum vertical fluid flux of approximately 12 mm/y is necessary to explain the most significant anomalies. Our study provides new insight into regional heat flow, geochemistry, and end-member vertical fluid flux rates in the Barents Sea. Moreover, it documents that the fluid flow system is active and most likely, very dynamic.

Shallow methane hydrate system controls ongoing, downslope sediment transport in a low‐velocity active submarine landslide complex, Hikurangi Margin, New Zealand

AbstractMorphological and seismic data from a submarine landslide complex east of New Zealand indicate flow‐like deformation within gas hydrate‐bearing sediment. This “creeping” deformation occurs immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. (1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. (2) Overpressure below low‐permeability gas hydrate‐bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. (3) Gas hydrate‐bearing sediment could exhibit time‐dependent plastic deformation enabling glacial‐style deformation. We favor the final hypothesis that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time‐dependent deformation behavior of gas hydrate‐bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and intersediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a reevaluation of slope stability following future climate‐forced variation in bottom‐water temperature.

The impact of fluid advection on gas hydrate stability: Investigations at sites of methane seepage offshore Costa Rica

Fluid flow through marine sediments drives a wide range of processes, from gas hydrate formation and dissociation, to seafloor methane seepage including the development of chemosynthetic ecosystems, and ocean acidification. Here, we present new seismic data that reveal the 3D nature of focused fluid flow beneath two mound structures on the seafloor offshore Costa Rica. These mounds have formed as a result of ongoing seepage of methane-rich fluids. We show the spatial impact of advective heat flow on gas hydrate stability due to the channelled ascent of warm fluids towards the seafloor. The base of gas hydrate stability (BGHS) imaged in the seismic data constrains peak heat flow values to ∼60 mWm−2 and ∼70 mWm−2 beneath two separate seep sites known as Mound 11 and Mound 12, respectively. The initiation of pronounced fluid flow towards these structures was likely controlled by fault networks that acted as efficient pathways for warm fluids ascending from depth. Through the gas hydrate stability zone, fluid flow has been focused through vertical conduits that we suggest developed as migrating fluids generated their own secondary permeability by fracturing strata as they forced their way upwards towards the seafloor. We show that Mound 11 and Mound 12 (about 1 km apart on the seafloor) are sustained by independent fluid flow systems through the hydrate system, and that fluid flow rates across the BGHS are probably similar beneath both mounds. 2D seismic data suggest that these two flow systems might merge at approximately 1 km depth, i.e. much deeper than the BGHS. This study provides a new level of detail and understanding of how channelled, anomalously-high fluid flow towards the seafloor influences gas hydrate stability. Thus, gas hydrate systems have good potential for quantifying the upward flow of subduction system fluids to seafloor seep sites, since the fluids have to interact with and leave their mark on the hydrate system before reaching the seafloor.

Gas hydrate identified in sand-rich inferred sedimentary section using downhole logging and seismic data in Shenhu area, South China Sea

Downhole wireline log (DWL) data was acquired from eight drill sites during China's first gas hydrate drilling expedition (GMGS-1) in 2007. Initial analyses of the acquired well log data suggested that there were no significant gas hydrate occurrences at Site SH4. However, the re-examination of the DWL data from Site SH4 indicated that there are two intervals of high resistivity, which could be indicative of gas hydrate. One interval of high resistivity at depth of 171–175 m below seafloor (mbsf) is associated with a high compressional- wave (P-wave) velocities and low gamma ray log values, which suggests the presence of gas hydrate in a potentially sand-rich (low clay content) sedimentary section. The second high resistivity interval at depth of 175–180 mbsf is associated with low P-wave velocities and low gamma values, which suggests the presence of free gas in a potentially sand-rich (low clay content) sedimentary section. Because the occurrence of free gas is much shallower than the expected from the regional depth of the bottom simulating reflector (BSR), the free gas could be from the dissociation of gas hydrate during drilling or there may be a local anomaly in the depth to the base of the gas hydrate stability zone. In order to determine whether the low P-wave velocity with high resistivity is caused by in-situ free gas or dissociated free gas from the gas hydrate, the surface seismic data were also used in this analysis. The log analysis incorporating the surface seismic data through the construction of synthetic seismograms using various models indicated the presence of free gas directly in contact with an overlying gas hydrate-bearing section. The occurrence of the anomalous base of gas hydrate stability at Site SH4 could be caused by a local heat flow conditions. This paper documents the first observation of gas hydrate in what is believed to be a sand-rich sediment in Shenhu area of the South China Sea.

Drivers of focused fluid flow and methane seepage at south Hydrate Ridge, offshore Oregon, USA

Methane seepage at south Hydrate Ridge (offshore Oregon, United States), one of the best-studied examples of gas venting through gas hydrates, is the seafloor expression of a vigorous fluid flow system at depth. The seeps host chemosynthetic ecosystems and release significant amounts of carbon into the ocean. With new three-dimensional seismic data, we image strata and structures beneath the ridge in unprecedented detail to determine the geological processes controlling the style of focused fluid flow. Numerical fluid flow simulations reveal the influence of free gas within a stratigraphic unit known as Horizon A, beneath the base of gas hydrate stability (BGHS). Free gas within Horizon A increases the total mobility of the composite water-gas fluid, resulting in high fluid flux that accumulates at the intersection between Horizon A and the BGHS. This intersection controls the development of fluid overpressure at the BGHS, and together with a well-defined network of faults, reveals the link between the gas hydrate system at depth and methane seepage at the surface.

Occurrence and nature of “bottom simulating reflectors” in the northern Gulf of Mexico

Subsurface interpretation, utilizing a database of more than 450,000 km2 (175,000 mi2) of three-dimensional (3-D) seismic in the northern Gulf of Mexico (GoM), reveals 145 discrete areas, totaling 4450 km2 (1.1 million acres) where the base of gas hydrate stability (BGHS) can be confidently inferred from seismic data. Unlike many other areas of the world, the majority of these features are not Bottom Simulating Reflectors (BSRs) in the “classic” sense, meaning continuous coherent events that cross-cut primary stratigraphy. Those typical, or continuous BSRs, are noted in only 24% of the features identified within this study. In contrast, the most common seismic manifestation of the BGHS in the GoM (59%) is the discontinuous “BSR”, delineated by widely separated anomalous seismic events that align in general conformance with seafloor bathymetry. A third type of seismic feature, pluming “BSRs”, are continuous events that are not bottom-simulating, but are bowed toward the seafloor and represent areas where large, but areally-limited increases in heat flow (linked to strong vertical fluid flux), perturb the BGHS. The limited nature of continuous BSRs and the relative abundance of discontinuous and pluming forms are attributed to the strong lithologic and structural heterogeneity of the northern GoM basin. This lithologic and structural complexity has served to disrupt and localize regionally pervasive and homogeneous gas flux that is consistent with the formation of large, continuous BSRs noted across other less complex continental margins. The various BSR forms identified in this study are shown to be closely associated (125 of 145) with the occurrence of seafloor amplitude anomalies, which are in turn usually associated with the flanks and crests of salt-cored ridges. These associations are interpreted to reflect the co-dependence of BSRs and seafloor reflectivity along the migration pathways that typify this geologic setting.

Gas hydrate resource potential in the Terrebonne Basin, Northern Gulf of Mexico

The Terrebonne Basin is a salt-withdrawal mini-basin within the northeast portion of the Walker Ridge protraction area in northern Gulf of Mexico continental slope that contains a thick sequence of upper Pliocene and Pleistocene clastic sediment. Data acquired during the 2009 Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) logging-while-drilling (LWD) program confirmed the presence of gas hydrate within a variety of sand and clay units. Integration of the Leg II LWD data with regional seismic mapping allows for the identification of various facies assemblages within the sand units and an initial estimation of the gas hydrate in-place resources throughout the Terrebonne basin. A total of ∼4.4×109m3 (1.55×1011ft3) of gas occurs within highly saturated gas hydrate accumulations within channel, proximal levee, and distal levee facies of four primary Lower Pleistocene sand reservoirs. These sand accumulations occur at the base of gas hydrate stability and locally trap additional, unquantified accumulations of free gas. A number of additional thin hydrate-bearing sand units are also observed to occur at shallower depths. Potential recoverable volumes from this accumulation compare favorably with those realized from conventional deepwater gas reservoirs in the vicinity. In addition, Leg II LWD data delineated the occurrence of a stratal-bound occurrence of gas hydrate-filled fractures at low bulk volume saturations within a thick, shallow, and predominantly fine-grained unit. This unit is estimated to contain roughly 17.0×109m3 (5.87×1011ft3) of gas. The areal gas hydrate resource density within the Terrebonne basin is calculated at 1.183×109m3 per km2 where delineated sand reservoirs are present and 0.32×109m3 per km2 where sands are thought to be absent.

Possible gas hydrates in the Northland and northern Taranaki Basins: Indirect evidence from seismic data

We present indirect evidence from seismic data for gas hydrates in the deepwater Northland and northern Taranaki Basins west of New Zealand's North Island. We observe high-amplitude reflections in the regional gas-hydrate stability zone that we interpret as gas pockets and infer that the gas is likely to be surrounded by gas hydrate. We also observe a reflection that mimics the shape of the base of gas-hydrate stability and is possibly a bottom simulating reflection. These preliminary findings, which need to be corroborated by further analyses, may constitute the first evidence for gas hydrates in these basins close to New Zealand's primary petroleum province.

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient ( a), cementation factor ( m) and saturation exponent ( n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 ( S h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 ( S h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km 2. Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.

Geothermal modeling of the gas hydrate stability zone along the Krishna Godavari Basin

A wide-spread bottom simulating reflector (BSR), interpreted to mark the thermally controlled base of the gas hydrate stability zone, is observed over a close grid of multichannel seismic profiles in the Krishna Godavari Basin of the eastern continental margin of India. The seismic data reveal that gas hydrate occurs in the Krishna Godavari Basin at places where water depths exceed 850 m. The thickness of the gas hydrate stability zone inferred from the BSR ranges up to 250 m. A conductive model was used to determine geothermal gradients and heat flow. Ground truth for the assessment and constraints on the model were provided by downhole measurements obtained during the National Gas Hydrate Program Expedition 01 of India at various sites in the Krishna Godavari Basin. Measured downhole temperature gradients and seafloor-temperatures, sediment thermal conductivities, and seismic velocity are utilized to generate regression functions for these parameters as function of overall water depth. In the first approach the base of gas hydrate stability is predicted from seafloor bathymetry using these regression functions and heat flow and geothermal gradient are calculated. In a second approach the observed BSR depth from the seismic profiles (measured in two-way travel time) is converted into heat flow and geothermal gradient using the same ground-truth data. The geothermal gradient estimated from the BSR varies from 27 to 67°C/km. Corresponding heat flow values range from 24 to 60 mW/m2. The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths.

Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope: Coring operations, core sedimentology, and lithostratigraphy

In February 2007, BP Exploration (Alaska), the U.S. Department of Energy, and the U.S. Geological Survey completed the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well (Mount Elbert well) in the Milne Point Unit on the Alaska North Slope. The program achieved its primary goals of validating the pre-drill estimates of gas hydrate occurrence and thickness based on 3-D seismic interpretations and wireline log correlations and collecting a comprehensive suite of logging, coring, and pressure testing data. The upper section of the Mount Elbert well was drilled through the base of ice-bearing permafrost to a casing point of 594 m (1950 ft), approximately 15 m (50 ft) above the top of the targeted reservoir interval. The lower portion of the well was continuously cored from 606 m (1987 ft) to 760 m (2494 ft) and drilled to a total depth of 914 m. Ice-bearing permafrost extends to a depth of roughly 536 m and the base of gas hydrate stability is interpreted to extend to a depth of 870 m. Coring through the targeted gas hydrate bearing reservoirs was completed using a wireline-retrievable system. The coring program achieved 85% recovery of 7.6 cm (3 in) diameter core through 154 m (504 ft) of the hole. An onsite team processed the cores, collecting and preserving approximately 250 sub-samples for analyses of pore water geochemistry, microbiology, gas chemistry, petrophysical analysis, and thermal and physical properties. Eleven samples were immediately transferred to either methane-charged pressure vessels or liquid nitrogen for future study of the preserved gas hydrate. Additional offsite sampling, analyses, and detailed description of the cores were also conducted. Based on this work, one lithostratigraphic unit with eight subunits was identified across the cored interval. Subunits II and Va comprise the majority of the reservoir facies and are dominantly very fine to fine, moderately sorted, quartz, feldspar, and lithic fragment-bearing to -rich sands. Lithostratigraphic and palynologic data indicate that this section is most likely early Eocene to late Paleocene in age. The examined units contain evidence for both marine and non-marine lithofacies, and indications that the depositional environment for the reservoir facies may have been shallower marine than originally interpreted based on pre-drill wireline log interpretations. There is also evidence of reduced salinity marine conditions during deposition that may be related to the paleo-climate and depositional conditions during the early Eocene.

The potential influence of shallow gas and gas hydrates on sea floor erosion of Rock Garden, an uplifted ridge offshore of New Zealand

Regional erosion of the Rock Garden ridge top, a bathymetric high within New Zealand’s Hikurangi Subduction Margin, is likely associated with its gas hydrate system. Seismic data reveal gas pockets that appear partially trapped beneath the shallow base of gas hydrate stability. Steady-state fluid flow simulations, conducted on detailed two-dimensional geological models, reveal that anomalous fluid pressure can develop close to the sea floor in response to lower-permeability hydrate-bearing sediments and underlying gas pockets. Transient simulations indicate that large-scale cycling of fluid overpressure may occur on time scales of a few to tens of years. We predict intense regions of hydro-fracturing to preferentially develop beneath the ridge top rather than beneath the flanks, due to more pronounced overpressure generation and gas migration through hydrate-bearing sediments. Results suggest that sediment weakening and erosion of the ridge top by hydro-fracturing could be owed to fluid dynamics of the shallow gas hydrate system.

Preliminary interpretation of electromagnetic, heat flow, seismic, and geochemical data for gas hydrate distribution across the Porangahau Ridge, New Zealand

Porangahau Ridge, located offshore the Wairarapa on the Hikurangi Margin, is an active ocean-continent collision region in northeastern New Zealand coastal waters. Bottom simulating reflections (BSRs) in seismic data indicate the potential for significant gas hydrate deposits across this part of the margin. Beneath Porangahau Ridge a prominent high-amplitude reflection band has been observed to extend from a deep BSR towards the seafloor. Review of the seismic data suggest that this high-amplitude band is caused by local shoaling of the base of gas hydrate stability due to advective heat flow and it may constitute the location of elevated gas hydrate concentrations. During R/V Tangaroa cruise TAN0607 in 2006 heat flow probing for measurements of vertical fluid migration, sediment coring for methane concentrations, and additional seismic profiles were obtained across the ridge. In a subsequent 2007 expedition, on R/V Sonne cruise SO191, a controlled source electromagnetic (CSEM) experiment was conducted along the same seismic, geochemical, and heat flow transect to reveal the electrical resistivity distribution. CSEM data highlight a remarkable coincidence of anomalously high resistivity along the western, landward flank of the ridge which point to locally higher gas hydrate concentration above the high amplitude reflection band. Measured sediment temperature profiles, also along the western flank, consistently show non-linear and concave geothermal gradients typical of advective heat flow. Geochemical data reveal elevated methane concentrations in surface sediments concomitant with a rapid decline in sulfate concentrations indicating elevated methane flux and oxidation of methane in conjunction with sulfate reduction at the landward ridge base. Together, these data sets suggest that the western rim of Porangahau Ridge is a tectonically driven zone of rising fluids that transport methane and cause an upward inflection of the base of gas hydrate stability and the formation of locally enriched gas hydrate above the reflective zone.

Focussed fluid flow on the Hikurangi Margin, New Zealand — Evidence from possible local upwarping of the base of gas hydrate stability

The southern Hikurangi Subduction Margin is characterized by significant accretion with predicted high rates of fluid expulsion. Bottom simulating reflections (BSRs) are widespread on this margin, predominantly occurring beneath thrust ridges. We present seismic data across the Porangahau Ridge on the outer accretionary wedge. The data show high-amplitude reflections above the regional BSR level. Based on polarity and reflection strength, we interpret these reflections as being caused by free gas. We propose that the presence of gas above the regional level of BSRs indicates local upwarping of the base of gas hydrate stability caused by advective heatflow from upward migrating fluids, although we cannot entirely rule out alternative processes. Simplified modelling of the increase of the thermal gradient associated with fluid flow suggests that funnelling of upward migrating fluids beneath low-permeability slope basins into the Porangahau Ridge would not lead to the pronounced thermal anomaly inferred from upwarping of the base of gas hydrate stability. Focussing of fluid flow is predicted to take place deep in the accretionary wedge and/or the underthrust sediments. Above the high-amplitude reflections, sediment reflectivity is low. A lack of lateral continuity of reflections suggests that reflectivity is lost because of a destruction of sediment layering from deformation rather than gas-hydrate-related amplitude blanking. Structural permeability from fracturing of sediments during deformation may facilitate fluid expulsion on the ridge. A gap in the BSR in the southern part of the study area may be caused by a loss of gas during fluid expulsion. We speculate that gaps in otherwise continuous BSRs that are observed beneath some thrusts on the Hikurangi Margin may be characteristic of other locations experiencing focussed fluid expulsion.

Testing proposed mechanisms for seafloor weakening at the top of gas hydrate stability on an uplifted submarine ridge (Rock Garden), New Zealand

We evaluate different hypotheses concerning the formation of a peculiar, flat-topped ridge at Rock Garden, offshore of the North Island of New Zealand. The coincidence of the ridge bathymetry with the depth at which gas hydrate stability intersects the seafloor has been previously used to propose that processes at the top of gas hydrate stability may cause seafloor erosion, giving rise to the flat ridge morphology. Two mechanisms that lead to increased fluid pressure (and sediment weakening) have previously been proposed: (1) periodic formation (association) and dissociation of gas hydrates during seafloor temperature fluctuations; and (2) dissociation of gas hydrates at the base of gas hydrate stability during ridge uplift. We use numerical models to test these hypotheses, as well as to evaluate whether the ridge morphology can develop by tectonic deformation during subduction of a seamount, without any involvement from gas hydrates. We apply a commonly-used 1D approach to model gas hydrate formation and dissociation, and develop a 2D mechanical model to evaluate tectonic deformation. Our results indicate that: (1) Tectonics (subduction of a seamount) may cause a temporary flat ridge morphology to develop, but this evolves over time and is unlikely to provide the main explanation for the ridge morphology; (2) Where high methane flux overwhelms the anaerobic oxidation of methane via sulphate reduction near the seafloor, short-period temperature fluctuations (but on timescales of years, not months as proposed originally) in the bottom water can lead to periodic association and dissociation of a small percentage of gas hydrate in the top of the sediment column. However, the effect of this on sediment strength is likely to be small, as evidenced by the negligible change in computed effective pressure; (3) The most likely mechanism to cause sediment weakening, leading to seafloor erosion, results from the interaction of gas hydrate stability with tectonic uplift of the ridge, provided bulk permeability strongly decreases with increasing hydrate content. Rather than overpressure developing from dissociation of hydrates at the base of gas hydrate stability (as previously thought), we found that the weakening is caused by focusing of gas hydrate formation at shallow sediment levels. This creates large fluid pressures and can lead to negative effective pressures near the seafloor, reducing the sediment strength.

日本海東縁，上越海盆の高メタンフラックス域におけるメタンハイドレートの成長と崩壊

A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface. High heat flow, ∼100 mK/m, deposition of organic-rich strata, ∼1.0 to 1.5%TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface (SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. δ13C of methane hydrate and seep gases are mostly within -30 to -50‰, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with δ13C of -50 to -100‰. Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys. The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600-1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters, (ii) degassing from ascending fluids, (iii) bound water effect and deficiency of free-waters, and (iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur.(View PDF for the rest of the abstract.)

Methane hydrate formation in turbidite sediments of northern Cascadia, IODP Expedition 311

Abstract Expedition 311 of the Integrated Ocean Drilling Program (IODP) to northern Cascadia recovered gas-hydrate bearing sediments along a SW–NE transect from the first ridge of the accretionary margin to the eastward limit of gas-hydrate stability. In this study we contrast the gas gas-hydrate distribution from two sites drilled ~ 8 km apart in different tectonic settings. At Site U1325, drilled on a depositional basin with nearly horizontal sedimentary sequences, the gas-hydrate distribution shows a trend of increasing saturation toward the base of gas-hydrate stability, consistent with several model simulations in the literature. Site U1326 was drilled on an uplifted ridge characterized by faulting, which has likely experienced some mass wasting events. Here the gas hydrate does not show a clear depth-distribution trend, the highest gas-hydrate saturation occurs well within the gas-hydrate stability zone at the shallow depth of ~ 49 mbsf. Sediments at both sites are characterized by abundant coarse-grained (sand) layers up to 23 cm in thickness, and are interspaced within fine-grained (clay and silty clay) detrital sediments. The gas-hydrate distribution is punctuated by localized depth intervals of high gas-hydrate saturation, which preferentially occur in the coarse-grained horizons and occupy up to 60% of the pore space at Site U1325 and > 80% at Site U1326. Detailed analyses of contiguous samples of different lithologies show that when enough methane is present, about 90% of the variance in gas-hydrate saturation can be explained by the sand (> 63 μm) content of the sediments. The variability in gas-hydrate occupancy of sandy horizons at Site U1326 reflects an insufficient methane supply to the sediment section between 190 and 245 mbsf.

Investigation of the role of gas hydrates in continental slope stability west of Fiordland, New Zealand

Sediment weakening due to increased local pore fluid pressure is interpreted to be the cause of a submarine landslide that has been seismically imaged off the southwest coast of New Zealand. Data show a distinct and continuous bottom‐simulating reflection (BSR)—a seismic phenomena indicative of the presence of marine gas hydrate—below the continental shelf from water depths of c. 2400 m to c. 750 m, where it intersects the seafloor. Excess pore fluid pressure (EPP) generated in a free gas zone below the base of gas hydrate stability is interpreted as being a major factor in the slope's destabilisation. Representative sediment strength characteristics have been applied to limit‐equilibrium methods of slope stability analysis with respect to the Mohr‐Coulomb failure criterion to develop an understanding of the feature's sensitivity to EPP. EPP has been modelled with representative material properties (internal angle of friction, bulk soil unit weight and cohesion) to show the considerable effect it has on stability. The best estimate of average EPP being solely responsible for failure is 1700 kPa, assuming a perfectly elastic body above a pre‐defined failure surface in a static environment.

Rise of the base of the gas hydrate zone since the last glacial recorded by rock magnetism

Gas hydrate, a clathrate of methane and water widespread on continental margins, has been implicated as a trigger of climate change and submarine slides as a result of methane release when the base of its stability zone moves upward rapidly. Direct tests of these hypotheses are made difficult by the ephemeral record of gas hydrate in sediment. In places, a seismic reflector (double bottom simulating reflector, BSR) appears to mark the old base of the gas hydrate layer, but the occurrence of this feature is patchy and its interpretation is controversial. Microbial activity is stimulated in the presence of gas hydrate, and results in the production of magnetic iron sulfides; the base of the gas hydrate interval is marked by a sharp reduction in the magnetic hysteresis parameter D J H . At Hydrate Ridge on the Cascadia margin, sampled during Ocean Drilling Program Leg 204, this signature occurs between 20 and 65 m below the present-day base of the gas hydrate zone, at a depth consistent with predictions for the base of gas hydrate stability given water depths and bottom-water temperatures appropriate for the last glacial maximum. Seismic evidence for a double BSR over part of Hydrate Ridge corroborates the rock magnetic interpretation.