Hydrate Accumulations Research Articles

Seismic velocity and reflectivity analysis of concentrated gas hydrate deposits on the southern Hikurangi Margin (New Zealand)

In the southern Hikurangi subduction margin, widespread gas hydrate accumulations are inferred based on the presence of bottom simulating reflections and recovered gas hydrate samples, mainly associated with thrust ridges. We present a detailed analysis of high- and medium-resolution seismic reflection data across Glendhu and Honeycomb ridges, two elongated four-way closure systems at the toe of the deformation wedge. High-amplitude reflections within the gas hydrate stability zone, coincident with high seismic velocities, suggest the presence of highly concentrated gas hydrate accumulations in the core regions of the anticlinal ridges. A novel method involving combined seismic velocity and reflectivity analysis and rock physics modelling is used to estimate hydrate saturations in localised areas. The effective medium model consistently predicts gas hydrate saturations of ~30% of the pore space at Glendhu Ridge and >60% at Honeycomb Ridge, whereas the empirical three-phases weighted equation likely underestimates the amount of gas hydrate present. We note that our estimates are dependent on the vertical resolution of the seismic data (5–14 m), and that the existence of thin layers hosting gas hydrate at higher concentrations is likely based on observations made elsewhere in similar depositional environments. A comparison between the two ridges provides insights into the evolution of thrust related anticlines at the toe of the accretionary wedge. We propose that the main driving mechanism for concentrated hydrate accumulation in the study area is along-strata gas migration. The vertical extent of these accumulations is a function of the steepness of the strata crossing the base of gas hydrate stability, and of the volume of sediments from which fluid flows into each structure. According to our interpretation, older structures situated further landward ofthe deformation front are more likely to host more extensive concentrated hydrate deposits than younger ridges situated at the deformation front and characterised by more gentle folding. The method introduced in this work is useful to retrieve quantitative estimates of gas hydrate saturations based on multi-channel seismic data.

Hydrocarbon migration pathway and methane budget for a Gulf of Mexico natural seep site: Green Canyon 600

Hydrocarbon seeps occur worldwide along continental margins and act as conduits for fluid discharge from the lithosphere to the overlying hydrosphere/atmosphere. The dynamics and rates of hydrocarbon release at cold seeps remain poorly constrained. Seepage enables a variety of processes that alter the seafloor morphology and affect the geochemistry of substrata, supporting diverse and important chemosynthetic communities. Here we merge complementary geochemical (oil fingerprinting), geophysical (deep seismic, subbottom, backscatter, multibeam) and video/imaging (Video Time Lapse Camera, DSV ALVIN video, ROV video, AUV photo surveys) data sets to constrain the pathways and magnitudes of methane fluxes from the source reservoir to the seafloor at a well-studied, prolific seep site, Green Canyon (GC) 600 in the Northern Gulf of Mexico. Oil samples from the reservoir, an active vent, and the sea-surface showed compositional similarities consistent with the plumbing system structure identified in seismic data. Spatial distribution of seep indicators such as bacterial mats, microbial communities, methane derived carbonates, and hydrate outcrops, were then used to quantify the total magnitude of methane potentially sequestered in the study domain. Using a systems approach, we combined published values for methane fluxes with data we collected across various scales and resolutions to compile a methane budget for GC600. The system considered for the methane budget is defined as the movement of methane from the salt ridge to the seafloor-water interface for each seep domain, Birthday Candles and Mega Plume. Total estimated input of methane was 2.8×108–2.2×109 mol/yr in the Birthday Candles domain, and 2.7×108–2.3×109 in the Mega Plume domain. The combined total output of the system ranged from 3.2×105–8.2×105 and 3.2×106–5.2×106 mol/yr respectively for Birthday Candles and Mega Plume domains, leaving a potential surplus (input minus output) of 2.6×108–2.3×109 mol/yr. Processes that could balance this budget include accumulation of gas hydrate and sediment free-gas, and the underestimated potential of biological sinks such as methane oxidation.

Combining reactive transport modeling with geochemical observations to estimate the natural gas hydrate accumulation

Predicting the distribution and resource of gas hydrates and understanding gas hydrate forming mechanisms are critical for assessing natural gas hydrate exploration potential, as well as exploiting hydrates. This study aims to provide a portable solution for evaluating resource of natural gas hydrate and quantifying contribution of methane sources via numerical simulations constrained by site-specific data. To numerically describe the complex process of biogenic methane production, an integrated simulation package, TOUGH + Hydrate + React (TOUGH + HR), was developed by coupling reactive transport, biodegradation and deposition of organic matter with behavior of hydrate-bearing system. Based on observed data from site SH2 in the South China Sea, a growing one-dimensional column model was constructed, and simulated via the developed TOUGH + HR tool. The results showed that when considering biogenic methane was the only source for hydrate, simulated maximum saturation of hydrate reached ~ 0.19, which is much lower than the observed value (~0.46), suggesting that the in-situ biogenic methane is not enough to form the high-saturation hydrate. When the upward flux of methane (considered as thermogenic methane) increased to 1.00 × 10−11kg·m-2·s-1, both simulated saturation and distribution of hydrates matched the observed data well, including the profile of remained total organic carbon (TOC), the location of interface between dissolved methane and sulfate (SMI), and the derived chlorinity. Simulation results suggest that the ratio of biogenic methane to thermogenic methane forming hydrates was about 1:3. Predicted amount of methane hydrate using the column model was 3258.33 kg, very close to the estimated based on field observation (3112.82 kg).

Hydrate-induced clogging of sand-control screen and its implication on hydrate production operation

Blockage of sand-control media is one of the main obstacles that affect gas production efficiency from gas hydrate-bearing sediments. Hydrate reformation is a potential plugging inducer of sand-control media. In this study, we conduct a series of experiments using steel-wired screen mesh to examine the hydrate-induced clogging of sand-control screen. The screen mesh sample was installed into a closed-circuit circulating system under gas-water two-phase flow condition to simulate bottomhole multiphase production processes. The results indicate that hydrate formation within the screen would causes permeability loss of the screen up to 98%. The pseudo-permeability of the screen sample shows dual-gradient decreasing characteristics during hydrate clogging. Therefore, we speculate that hydrate accumulating and hydrate particle bridging are the two main mechanisms causing screen plugging. The hydrate accumulating sub-process is mainly controlled by the degree of subcooling, while the hydrate particle-bridging sub-process is affected by fluid flow rate. Furthermore, the “ J -shape” coiling of pressure-temperature relationship can be used as an indicator in diagnosing possible bottomhole screen plug during hydrate exploitation. Artificial interference of downhole temperature is strongly recommended to mitigate screen plugging induced by hydrate reformation. • Hydrate clogging causes permeability loss of sand-control screen up to 98%. • Hydrate accumulation causes dual-gradient decrease in the permeability of sand-control screen. • The “ J -shape” coiling of P - T curve is a indicator to diagnose downhole hydrate-induced screen clogging. • Artificial temperature interference is recommended to mitigate hydrate induced screen clogging.

Hydrate slurry flow characteristics influenced by formation, agglomeration and deposition in a fully visual flow loop

Gas hydrates pose impeded flow risks and serious safety hazards in oil and gas transportation pipelines. This makes it imperative to look for appropriate hydrate control strategies. In this study, the hydrate slurry flow characteristics were investigated under multiphase flow conditions using a high-pressure fully visual flow loop. At different liquid loadings and mixture velocities, pressure drop variations were monitored from the initial hydrate formation to deposition and bedding, and the flow patterns were observed throughout the experiments. It was found that the hydrate slurry flow may involve four stages of hydrate formation, agglomeration, deposition and bedding prior to pipeline blockage. The effective volume and water conversion fractions were also obtained at different stages. The results showed that the lower liquid loading causes quicker and more severe hydrate blockage problem mainly due to more mass transfer of gas into water hence higher water conversion rate. Moreover, lower mixture velocity resulted in a higher hydrate bedding tendency. The results also confirmed further formation and deposition of gas hydrates due to a drastic temperature drop immediately after the liquid flow stopped in the system. In addition, the flow characteristics in sloped pipe sections (both upslope and downslope) were studied to investigate the effect of driving forces on the hydrate deposition. Slug flow was observed in the upslope while stratified flow in the downslope. Furthermore, hydrate accumulation tends to occur in the transition position between the horizontal pipes and the sloped pipes due to the effect of gravity.

A Numerical Model to Estimate the Effects of Variable Sedimentation Rates on Methane Hydrate Formation—Application to the ODP Site 997 on Blake Ridge, Southeastern North American Continental Margin

AbstractSedimentation and burial rate can vary considerably at continental slopes, due to constantly changing tectonic processes on the geological timescale. This variation may affect the transport of methane in sediment, as manifested in the change of pore water flux, methanogenesis, methane diffusion and hydrate removal from hydrate stability zone (HSZ) and further affect the accumulation of hydrate in submarine sediments. Most of previous models assumed a constant sedimentation rate and thus may result in inaccurate estimates of total amount of hydrate within the HSZ. In this study, we developed a hydrate accumulation model that is capable of handling multiple sedimentation stages. Site 997 of ODP Leg 164, with data indicating its recent history of four sedimentation stages of different rates, is chosen to investigate the significance of sedimentation rate variation experienced at this location. We examined the effect of varying sedimentation rates on hydrate accumulation by taking in consideration sediment compaction, in situ methanogenesis, and composition and component transport processes. Simulation results suggest that the history of hydrate accumulation at Site 997 is characterized by a sequence of increase, decrease, and then increase till the present day. At present, the hydrate deposit has accumulated to 8.30 × 104 mol/m2, and the average hydrate saturation near the base of the HSZ is 6.3%, which is in general agreement with published estimates in the literature. The accumulation of hydrate at Site 997 was significantly affected by variable sedimentation rates, and nearly one half of the hydrate deposit was accumulated during the last 2.5 Myr.

Estimation of the thermal conductivity of a heterogeneous CH4-hydrate bearing sample based on particle swarm optimization

Knowledge of the thermophysical properties of methane hydrate-bearing sediments (MHBS) plays a critical role in the evaluation of fluid production potential from gas hydrate accumulations in geologic media. The process of estimating the values of key flow and transport parameters related to MHBS is of great importance. In this paper, we developed an integrated optimization workflow by integrating a global optimization algorithm, specifically particle swarm optimization (PSO) with the TOUGH+Hydrate v1.5 hydrate simulator. The optimization technique was employed to estimate the composite thermal conductivity of MHBS (λMHBS) through history-matching the simulated results with the experimental observation over time automatically in a process of stepwise heating an aqueous-rich heterogeneous MH-bearing core sample (SH = 0.40 and SA = 0.57) at P = 7.0 MPa. Two different composite models were tested and compared for their performance in predicting the temperature response. A sensitivity study was conducted to provide guidance on the selection of PSO parameters to accelerate the convergence speed. The optimal λMHBS obtained from the Bejan linear model is 2.16 W/mK with an absolute average deviation (AAD) of 2.66%, which is superior than the Somerton nonlinear model. Based on the results from the sensitivity study, the coupling of PSO algorithm with T+H v1.5 was proved to be robust and reliable. It is found that a particle size over 20, a maximum velocity coefficient around 0.5 and a well-defined parameter range guided by literature improves the efficiency in searching the optimal solution, achieving a global best cost with fewer iterations. The optimization procedure developed in this paper can be adopted in estimation of other groups of parameters related to MH-bearing systems (i.e. thermal, flow, geomechanical properties and the kinetic rate parameters of MH reactions) and in the optimization of production strategies from MH reservoirs (e.g. well positioning, pressure drawdown level and rate, etc.) to improve the energy efficiency of the production process.

Technical Feasibility of Using Frac-Packed Wells for Producing Natural Gas from Offshore Gas Hydrate Reservoirs

This paper presents a feasibility analysis of using frac-packing completion techniques to produce natural gas from offshore gas hydrate reservoirs. A case study was carried out for the gas hydrate accumulations in the northern South China Sea. The feasibility analysis covers the requirements of proppant size, fluid injection rate, fracturing pressure, and well productivity. For the median grain size of sediments in the studied formation from 2.60 to 28.96 μm with an average value of 8.49 μm, the required range of proppant size is between 333 mesh to 748 mesh (0.001 inch ~ 0.003 inch). Since the proppants in this size range are not commercially available, it would be economical to use screened natural sands as proppants in frac-packing operations. The minimum flow rate of fracturing fluid required to carry the 0.003 inch proppant/sand into the fracture tip at 510 ft is 3.64 bpm, which is much lower than the practical values ranging from 20 bpm to 100 bpm. Therefore proppant/sand transport during frac-packing is not a concern. To create a horizontal fracture of 510 ft radius with a fracturing fluid injection rate of 72 bpm, the maximum bottom hole injection pressure is predicted to be 2,378 psi, which is only 334 psi above the reservoir pressure and can be handled by most pumps used in frac-packing operations. Well productivity forecast with a simplified mathematical model shows that a commercial gas production rate of 16 MMscf/day is achievable with the fracture radius of 510 ft. However, the model requires further validation.

One-Dimensional Study on Hydrate Formation from Migrating Dissolved Gas in Sandy Sediments

Upward migration of gas-dissolved pore fluid is an important mechanism for many naturally occurring hydrate reservoirs. However, there is limited understanding in this scenario of hydrate formation in sediments. In this preliminary work, hydrate formation and accumulation from dissolved gas in sandy sediments along the migration direction of brine was investigated using a visual hydrate simulator. Visual observation was employed to capture the morphology of hydrates in pores through three sapphire tubes. Meanwhile, the resistivity evolution of sediments was detected to characterize hydrate distribution in sediments. It was observed that hydrates initially formed as a thin film or dispersed crystals and then became a turbid colloidal solution. With hydrate growth, the colloidal solution converted to massive solid hydrates. Electrical resistivity experienced a three-stage evolution process corresponding to the three observed hydrate morphologies. The results of resistivity analysis also indicated that the bottom–up direction of hydrate growth was consistent with the flow direction of brine, and two hydrate accumulation centers successively appeared in the sediments. Hydrates preferentially formed and accumulated in certain depths of the sediments, resulting in heterogeneous hydrate distribution. Even under low saturation, the occurrence of heterogeneous hydrates led to the sharp reduction of sediment permeability.

Dynamic accumulation of gas hydrates associated with the channel-levee system in the Shenhu area, northern South China Sea

Abstract Research on the formation and distribution of submarine channel systems and associated gas-bearing fluids is of great significance for gas hydrate exploration. Disseminated gas hydrates with high saturation up to 65% were recovered from a submarine ridge, equivalent to the levee of the channel–levee system in the Shenhu area, northern South China Sea. Sedimentary deposits in the submarine ridge were dominated by fine-grained silt and clay-rich silt; gas hydrates with relatively high saturation preferentially accumulated in coarser sediments with less clay content. Although abundant foraminifera fossils may have increased reservoir pore space, their presence was not a necessary condition for high-saturation hydrates. Higher levels of pyrite appeared in the reservoirs corresponding to high-saturation hydrates, which suggests that the reducing environment caused by sufficient methane provided adequate gas to form higher-saturation hydrates. Because of the migration of the channel–levee system, different channels formed their respective depositional systems composed of channel-filling, buried channel-filling, erosion grooves, and slumped turbidities. Relatively coarse-grained deposits were identified in the channel fillings and levees, and the accumulation of hydrates was affected by the lithological features of the sediments and their spatial coupling with the gas hydrate stability zone (GHSZ). GHSZ modeling based on in situ measurements indicated that erosion and sedimentation, as well as variations of the geothermal gradient, resulted in the upward/downward migration of bottom simulating reflectors (BSRs). On the erosion flank of the channel, the strata thinned, and rapid erosion was likely to destroy the shallower BSR, causing gas hydrate decomposition and methane release, and may have caused turbidite slumping and seepage, whereas the strata thickened on the deposition flank of the channel. The BSR in the channel–levee system would gradually move toward the new GHSZ, eventually forming a new BSR; parts of the BSR that formed under the original P–T conditions have remained, and double BSRs occurred in the seismic profile. The thermal fluid that moved upward through a gas chimney may also have caused the migration of the GHSZ, resulting in the emergence of double BSRs. During the lateral migration of the channel and the vertical migration of the gas-bearing fluid, there was a dynamic adjustment relationship between the GHSZ and the erosion–deposition process of the channel, resulting in the dynamic accumulation of hydrates in the Shenhu area. A model to demonstrate the relationship between channel migration and variation of the BSR was established, which is of great significance for understanding the formation and accumulation mechanisms of gas hydrates.

Effects of the Solidification of Capillary Bridges on the Interaction Forces between Hydrate Particles

The presence of unconverted water drops in bulk hydrocarbon is likely to bridge hydrate particles and cause hydrate agglomeration, leading to hydrate accumulation or bedding in oil and gas pipeline...

Effect of Salinity Stress on Phenylpropanoid Genes Expression and Related Gene Expression in Wheat Sprout

The effect of salinity (NaCl treatment) on the nutritive value of wheat sprouts was investigated by analyzing the expression of phenylpropanoid biosynthetic pathway genes and the levels of phenylpropanoid compounds. Treatment with various concentrations of NaCl (50, 100, and 200 mM) resulted in increased epicatechin levels but decreased accumulation of catechin hydrate, benzoic acid, and quercetin compounds in the sprouts compared with the control (0 mM). The trans-cinnamic acid, 4-hydroxybenzoic acid, ferulic acid, epicatechin, and total phenylpropanoid level in wheat sprout was the highest at 50 mM of NaCl treatment. Six-day-old wheat plantlets exposed to 50 mM NaCl for 6, 12, 24, 48, and 72 h, showed that the total phenylpropanoids accumulation was the highest at 48 h after the treatment and most of the treatments showed higher phenylpropanoid content than the control at the same time points. Although the shoot and root length and the fresh weight of wheat sprouts decreased with NaCl treatment, these results suggest that treatment of 50 mM NaCl improves the nutritional quality of wheat sprouts, due to increased phenylpropanoid concentrations.

Investigating the role of faults in fluid migration and gas hydrate formation along the southern Hikurangi Margin, New Zealand

The Hikurangi Margin off the east coast of the North Island (Te Ika-a-Māui) is a tectonically active subduction zone and the location of New Zealand’s largest gas hydrate province. Faults are internally complex volumetric zones that may play a significant role in the migration of fluids beneath the seafloor. The combined processes of deformation and fluid migration result in the formation of concentrated hydrate accumulations along accretionary ridges. It is not fully understood to what extent faults control fluid migration along the Hikurangi Margin, and whether deep-seated thrust faults provide a pathway for thermogenic gas to migrate up from sources at depth. Using 2D models based on seismic data from the region we investigated the role of thrust faults in facilitating fluid migration and contributing to the formation of concentrated gas hydrates. By altering permeability properties of the fault zones in these transient state models we can determine whether faults are required to act as fluid flow pathways. In this study we focus on two study sites offshore southern Wairarapa, using realistic yet simplified fault geometries derived from 2D seismic lines. The results of these models allow us to start to disentangle the complex relationship between fault zone structure, permeability, geometry, fluid migration and gas hydrate formation. Based on the model outputs we propose that faults act as primary pathways facilitating fluid migration and are critical in the formation of concentrated gas hydrate deposits.

Distributions of gas hydrate and free gas accumulations associated with upward fluid flow in the Sanriku-Oki forearc basin, northeast Japan

Abstract We applied automated velocity analysis to a 3D seismic data volume to obtain a high-resolution seismic velocity model that we used to investigate the influence of fluid behavior on the subsurface distribution of gas in the Sanriku-Oki forearc basin, northeast Japan. We identified free gas accumulations as zones of low P-wave velocity separated from overlying high-velocity gas hydrates by the clear seismic boundary of the bottom-simulating reflection (BSR) between them. We then used conductive modeling to map upward heat flow in our study area from the depth of the BSR derived from the velocity model. The estimated heat flow demonstrates that upward fluid flux has considerable influences on the distributions of both gas hydrate and free gas. The areas of high heat flow (representing high fluid flux) correspond to underlying permeable geological features such as gas chimneys, faults, and the edges of porous slumps, suggesting that these features provide fluid migration pathways from Eocene to Oligocene source rocks to free gas and hydrate accumulations in overlying sedimentary rocks. The techniques we have employed can contribute to the high-resolution mapping of gas hydrate and free gas accumulations in similar deep-water reservoirs off the eastern continental margin of Japan and at tectonically similar plate subduction margins in other regions.

Methane hydrate-bearing sediments: Pore habit and implications

Abstract Hydrate-bearing sediments are relevant to the organic carbon cycle, seafloor instability, and as a potential energy resource. Sediment characteristics affect hydrate formation, gas migration and recovery strategies. We combine the physics of granular materials with robust compaction models to estimate effective stress and capillary pressure in order to anticipate the pore habit of methane hydrates as a function of the sediment characteristics and depth. Then, we compare these results to an extensive database of worldwide hydrate accumulations compiled from published studies. Results highlight the critical role of fines on sediments mechanical and flow properties, hydrate pore habit and potential production strategies. The vast majority of hydrate accumulations (92% of the sites) are found in fines-controlled sediments at a vertical effective stress between σ′z = 400 kPa and 4 MPa, where grain-displacive hydrate pore habit prevails in the form of segregated lenses and nodules. While permeation-based gas recovery by depressurization is favored in clean-coarse sediments, gas recovery from fines-controlled sediments could benefit from enhanced transmissivity along gas-driven fractures created by thermal stimulation.

Geological controls on the occurrence of recently formed highly concentrated gas hydrate accumulations in the Shenhu area, South China Sea

Abstract The Shenhu area is an important gas hydrate exploration and production test zone in the northern slope of South China Sea. A sediment layer with high gas hydrate saturations and abnormally high pore-water chlorinity concentrations has been identified from recently acquired logging-while-drilling (LWD) data and core samples. Data from four sites (W19, W18, SC-01 and SC-02) indicates that most gas hydrate-bearing sediments (GHBSs) form preferentially in low gamma ray log inferred coarser-grained units, except at SC-02. At this site, a striking increase in pore-water chlorinity values, with a maximum value exceeding 816 mM at a depth of 158 mbsf, indicates an active or recently-active system. A one-dimensional (1D) diffusion model is used to estimate the time when the gas hydrate formed based on the saturation, thickness, and porosity of gas hydrate-bearing units at each site. The results show that gas hydrates at sites SC-02 and W18 respectively formed 19–29 thousand and 26–28 thousand years ago (assuming a closed system). To further investigate the apparently young age of these highly saturated gas hydrate accumulations, three-dimensional (3D) seismic variance and frequency attributes extracted along different sedimentary layers are shown to provide evidence for vertical and lateral gas migration along normal faults and gas chimneys. The high pore-water chlorinities and evidence for fluid migration from deeper sedimentary sections suggest that upward migration of methane gas into the hydrate stability zone may have contributed to the recent formation of the highly concentrated gas hydrate occurrences identified in the Shenhu area.

Numerical modeling of methane hydrate accumulation with mixed sources in marine sediments: Case study of Shenhu Area, South China Sea

The natural gas hydrates detected in Shenhu Area, South China Sea are considered to have great potential for exploitation and to become a research hotspot. However, details of the dynamic accumulation and evolution processes of gas hydrate over a long time in this area remain unclear. In this study, we developed a numerical model to predict the accumulation of gas hydrates in the marine sediments by introducing the biogenic methanogenesis module to the framework of the existing simulator TOUGH+HYDRATE. A one-dimensional dynamic model was applied to reproduce the formation of gas hydrate at site SH2 in Shenhu Area. The burial of sediments and associated phenomena (e.g., evolution of temperature, sediment compaction and consequent reduction in sediment porosity and permeability, fluid expulsion) are taken into account. Modeling results indicate that with the conventional conversion coefficient (CC) of organic carbon to methane of 0.15, the amount of in-situ biogenic methane is not sufficient to form the highly saturated hydrate detected at this location and the remaining methane (91.4%) is supplied by the upward flow of fluids. The tectonic movement that took place about 1.5 Ma created pathways for the fluids to migrate vertically. The pore water and methane gas fluxes in the upward fluids were determined as 2 × 10−10 kg/s·m2 and 1 × 10−11 kg/s·m2, respectively, using the proved hydrate saturation profile and measured pore water chlorinity from recovered core samples. The distribution of hydrate in the sediments is significantly influenced by pore water salinity. The inhibitory effect of salt on hydrate formation, which shifts the pressure of three-phase (i.e., solid hydrate, methane gas and pore water) equilibrium to actual pore water pressure, could be the major reason for the appearance of a three-phase co-existence zone in the hydrate concentrated zone. Additionally, the water flux and diffusivity of salt have a profound effect on the formation of gas hydrate by affecting the salinity of pore water in the gas hydrate concentrated zone. The results of this study are of great significance for guiding marine gas hydrate exploration and resource evaluation.

Detection of Gas Hydrates in Faults Using Azimuthal Seismic Velocity Analysis, Vestnesa Ridge, W‐Svalbard Margin

AbstractJoint analysis of electrical resistivity and seismic velocity data is primarily used to detect the presence of gas hydrate‐filled faults and fractures. In this study, we present a novel approach to infer the occurrence of structurally controlled gas hydrate accumulations using azimuthal seismic velocity analysis. We perform this analysis using ocean‐bottom seismic data at two sites on Vestnesa Ridge, W‐Svalbard Margin. Previous geophysical studies inferred the presence of gas hydrates at shallow depths (up to ~190–195 m below the seafloor) in marine sediments of Vestnesa Ridge. We analyze azimuthal P‐wave seismic velocities in relation with steeply dipping near‐surface faults to study structural controls on gas hydrate distribution. This unique analysis documents directional changes in seismic velocities along and across faults. P‐wave velocities are elevated and reduced by ~0.06–0.08 km/s in azimuths where the raypath plane lies along the fault plane in the gas hydrate stability zone (GHSZ) and below the base of the GHSZ, respectively. The resulting velocities can be explained with the presence of gas hydrate‐ and free gas‐filled faults above and below the base of the GHSZ, respectively. Moreover, the occurrence of elevated and reduced (>0.05 km/s) seismic velocities in groups of azimuths bounded by faults suggests compartmentalization of gas hydrates and free gas by fault planes. Results from gas hydrate saturation modeling suggest that these observed changes in seismic velocities with azimuth can be due to gas hydrate saturated faults of thickness greater than 20 cm and considerably smaller than 300 cm.

Gas hydrate quantification in Walker Ridge block 313, Gulf of Mexico, from full-waveform inversion of ocean-bottom seismic data

The Gulf of Mexico (GOM) Joint Industry Project Leg 2 logging-while-drilling data in Walker Ridge lease block 313 (WR313) in the GOM detected gas hydrate in coarse- and fine-grained sediments at sites WR313-G and WR313-H. The coarse-grained units are thin ([Formula: see text]) and highly saturated, whereas the fine-grained unit is thick (approximately 200 m) with low saturation and fracture-filling gas hydrate. Unlike its coarse-grained counterpart, the seismic character of the fine-grained unit does not clearly indicate the presence of gas hydrate, which would likely have remained undiscovered in the absence of drilling. In this paper, through frequency-domain acoustic full-waveform inversion (FWI) of ocean-bottom seismometer data along a 2D multichannel seismic transect near sites WR313-G and WR313-H, we detect and quantify gas hydrate in the fine-grained unit. Key results are as follows: First, the base of the gas hydrate stability zone, which is not obvious in the reflection profile, can be discerned in the FWI results. Second, the gas hydrate in the fine-grained unit is mainly confined to the area between two sets of opposite-dipping normal faults implying that the fault architecture may be partially responsible for this gas hydrate accumulation and distribution.

Results of 20 years of searching and studying gas hydrates in Lake Baikal

The 20-year history of studying the near-surface accumulations of gas hydrates in Lake Baikal revealed the same pattern of their formation and distribution in both marine environments and a freshwater body. However, Lake Baikal has its own features of the coexistence of gas hydrates with different structures and the causes of occurrence of hydrate-bearing mud volcanoes, which have been found and described nowhere in the world.