Gas Hydrate Occurrence Research Articles

Methane hydrate formation in amino acids / sodium montmorillonite systems

To understand the occurrence of natural gas hydrates in seabed sediments, it is crucial to examine the mechanisms of methane (CH4) hydrate formation in sodium montmorillonite (Na-Mt) systems in the presence of amino acid. Accordingly, this study employed kinetics experiments and molecular dynamics simulations to investigate CH4 hydrate nucleation and growth in an Na-Mt system containing alanine (Ala), leucine (Leu), and phenylalanine (Phe), respectively. Kinetics and Raman experiments showed that, compared with Ala, Leu and Phe enhanced hydrogen bonding between water molecules surrounding Na-Mt. This enhancement was due to the long carbon chain of Leu and the phenyl ring of Phe and facilitated CH4 hydrate formation. Moreover, in the Na-Mt system, Ala reduced CH4 consumption, whereas Leu and Phe increased CH4 consumption. Molecular dynamics simulations revealed that the strength of electrostatic interactions between the negatively charged Na-Mt surface and the functional groups of amino acids affected the distribution of amino acids, thereby altering CH4 aggregation and CH4 hydrate nucleation processes. The strong interaction between Na-Mt and Ala significantly disrupted interfacial interactions between Na-Mt and water molecules. In contrast, the weaker interactions between Na-Mt and Leu and Phe, respectively, meant that these amino acids affected CH4 hydrate nucleation in the bulk-like solution by influencing the arrangement of water molecules. These findings indicate that interfacial interactions between Na-Mt and amino acids play a crucial role in CH4 hydrate formation. Overall, this study generated insights into the formation kinetics and nucleation properties of CH4 hydrates in clay mineral–amino acid complexes that may increase understanding about the occurrence of natural gas hydrates in marine sediments.

Characterization of the Structural–Stratigraphic and Reservoir Controls on the Occurrence of Gas Hydrates in the Eileen Gas Hydrate Trend, Alaska North Slope

One of the most studied permafrost-associated gas hydrate accumulations in Arctic Alaska is the Eileen Gas Hydrate Trend. This study provides a detailed re-examination of the Eileen Gas Hydrate Trend with a focus on the gas hydrate accumulation in the western part of the Prudhoe Bay Unit. This integrated analysis of downhole well log data and published geophysical data has provided new insight on structural, stratigraphic, and reservoir controls on the occurrence of gas hydrates in the Eileen Gas Hydrate Trend. This study revealed the relatively complex nature of the gas hydrate occurrences in the Eileen Gas Hydrate Trend, with gas hydrates present in a series of coarsening upward, laterally pervasive, mostly fine-grained sand beds exhibiting high gas hydrate saturations. Most of the gas hydrate-bearing reservoirs in the Eileen Gas Hydrate Trend are laterally segmented into distinct northwest- to southeast-trending fault blocks, occur in a combination of structural–stratigraphic traps, and are only partially hydrate filled with distinct down-dip water contacts. These findings suggest that the traditional parts of a petroleum system (i.e., reservoir, gas source, gas migration, and geologic timing of the system formation) also control the occurrence of gas hydrates in the Eileen Gas Hydrate Trend.

Microbial communities in the fluid migration zone in the sediments of the Krasny Yar methane seep (South Baikal)

The diversity and structure of microbial communities were investigated using 16S rRNA gene barcoding in the sediments of the Krasny Yar methane seep, in the zone of near-surface occurrence of gas hydrates and the presence of oxidized and restored channels. The diversity of both bacteria and archaea along the core depth was similar to the diversity found in sediments from other areas: methylotrophic methanogens and microorganisms involved in different stages of the organic matter fermentation were detected in the sediment strata of all depths investigated. Migration flows of oxygen-rich and aerobic bacteria-rich near-bottom water influenced greatly the diversity of microbial communities in oxidized channels. Fluids migrating from the deep zone to the bottom surface provided transport from anoxygenic sediments of anaerobic archaea involved in the AOM process. The data obtained are consistent with geochemical and geothermal indicators defining the zone of active migration of near-bottom waters.

Influence mechanism of interfacial organic matter and salt system on carbon dioxide hydrate nucleation in porous media

CO2 carbon sequestration in marine gas hydrate occurrence areas is a potential way to mitigate the continued growth of CO2 in the atmosphere. The organic matters of marine sediment can affect the phase equilibrium conditions of CO2 hydrate in salt system, and the organic salt system in pore medium has effect on the nucleation growth of CO2 hydrate. The organic matters on the surface of the sediment is mainly composed of lipids, proteins, carbohydrates, and unsaturated hydrocarbon organic compounds, and these polar functional groups weaken the inhibitory effect of salt ions on hydrates through coordination bonds and Ca ion complexation. The nucleation kinetics of CO2 hydrate under organic salt system in pore medium in marine environment is an important parameter determining the carbon sequestration of hydrate. In addition, experimental results show that clay surface organic matters (CSOM) can reduce the surface tension of pore water, and thus pore water migration occurs during hydrate growth and expansion. The basic background in our findings will bring a much better recognition of phase equilibrium conditions among the offshore marine sediment CO2 sequestration process and further assess the location of potential marine CO2 storage sites.

Rock-physics model of a gas hydrate reservoir with mixed occurrence states

Seismic interpretation of gas hydrates requires the assistance of rock physics. Changes in gas hydrate saturation can alter the elastic properties of formations, and this relationship can be considerably influenced by the occurrence state of gas hydrates. Pore-filling, load-bearing, and cementing types are three single gas hydrate occurrence states commonly considered in rock-physics investigations. However, many gas hydrate-bearing formations are observed to have mixed occurrence states, and their rock-physics properties do not fully conform to models of single occurrence states. We develop a generalized rock-physics model for gas hydrate-bearing formations with three mixed occurrence states observed in the field or laboratory experiments: coexisting pore-filling-type and matrix-forming-type gas hydrates (case 1); pore-filling type when [Formula: see text] (gas hydrate saturation) < [Formula: see text] (critical saturation) and pore-filling + matrix-forming type when [Formula: see text] (case 2); and matrix-forming type when [Formula: see text] and matrix-forming + pore-filling type when [Formula: see text] (case 3). Instead of initial porosity, the apparent porosity (the volume fraction of an effective pore filler) [Formula: see text] represents the influence of occurrence states on the pore space. These three mixed occurrence states can be modeled using a unified workflow, in which the volume fractions of various gas hydrate types are expressed in general forms in terms of the apparent porosity. In addition, the model considers the effect of a pore filler on shear modulus. The developed model is validated through calibration with real well-log data and published experimental data corresponding to five gas hydrate-bearing formations. The model effectively interprets the influences of gas hydrate saturation and occurrence state on these formations. Thus, the generalized model provides a theoretical basis for the analysis of sensitive elastic parameters and quantitative interpretation for gas hydrate reservoirs.

Influence of Natural Gas Hydrate Distribution Patterns on the Macroscale–Mesoscale Mechanical Properties of Hydrate-Bearing Sediments

Studying the mechanical characteristics of hydrate-bearing sediments (HBS) contributes to the comprehensive understanding of the mechanical behavior in environments with natural gas hydrate (NGH) occurrences. Simultaneously, the distribution patterns of hydrates significantly influence the strength, deformation, and stability of HBS. Therefore, this paper employs particle flow code (PFC) to conduct biaxial discrete element simulations on specimens of HBS with different hydrate distribution patterns, revealing the macroscale–mesoscale mechanical properties, evolution patterns, and destructive mechanisms. The results indicate that the strain-softening behavior of HBS specimens strengthens with the increase in hydrate layer thickness, leading to higher peak strength and E50 values. During the gradual movement of the hydrate layer position (Ay) from both ends to the center of the specimen (Ay = 0.40 mm → Ay = 20 mm), the strain-softening behavior weakens. However, when Ay = 20 mm, the specimen exhibits evident strain-softening behavior again. Moreover, with an increase in the angle between the hydrate layer and the horizontal direction (α) greater than 20°, the peak strength of the specimen increases, while E50 shows an overall decreasing trend. The influence of axial loads on the hydrate layer in specimens varies with α, with larger contact forces and fewer cracks observed for higher α values.

Machine-learning application to assess occurrence and saturations of methane hydrate in marine deposits offshore India

Artificial neural networks (ANN) were used to assess methane hydrate occurrence and saturation in marine sediments offshore India. The ANN analysis classifies the gas hydrate occurrence into three types: methane hydrate in pore space, methane hydrate in fractures, or no methane hydrate. Further, predicted saturation characterizes the volume of gas hydrate with respect to the available void volume. Log data collected at six wells, which were drilled during the India National Gas Hydrate Program Expedition 02 (NGHP-02), provided a combination of well-log measurements that were used as input for machine-learning (ML) models. Well-log measurements included density, porosity, electrical resistivity, natural gamma radiation, and acoustic wave velocity. Combinations of well logs used in the ML models provide good overall balanced accuracy (0.79 to 0.86) for the prediction of the gas hydrate occurrence and good accuracy (0.68 to 0.92) for methane hydrate saturation prediction in the marine accumulations against reference data. The accuracy scores indicate that the ML models can successfully predict reservoir characteristics for marine methane hydrate deposits. The results indicate that the ML models can either augment physics-driven methods for assessing the occurrence and saturation of methane hydrate deposits or serve as an independent predictive tool for those characteristics.

Bottom simulating reflectors in the Manila Trench forearc and its implications on the occurrence of gas hydrates in the region

Gas hydrate occurrence and stability in active plate margins have been reported in various localities, with investigations focusing on its potential as an alternative energy resource or the threat it poses due to dissociation and methane release into the atmosphere. The Manila Trench forearc, proximal to active margins and probable methane-rich sediments, can be an analog for gas hydrate formation and geologic preconditions constrained by tectonics and sedimentation processes. The possible occurrence of gas hydrates in offshore western Luzon Island is reported for the first time based on bottom simulating reflectors (BSRs) in multi-channel seismic reflection datasets. Results indicate a prevalence of BSRs on the frontal wedge seaward of the North Luzon Trough (NLT) and in the West Luzon Trough (WLT) basin fill. Continuous, discontinuous, double, and pluming BSRs in the forearc encompass a total area of approximately 15,400 km2 at depths of 223–553 mbsf in the frontal wedge and 486 to 1428 mbsf within the basins. Enhanced amplitude reflectors (EARs) above and below the BSRs indicate possible gas hydrate and free gas accumulations, respectively. Gas chimneys associated with the upward migration of methane-rich fluids appear to be controlled by deep structural features. The accretion of continent-derived sediments along the northern segment of the frontal wedge promotes the formation of fluid migration structures, enhancing fluid migration and gas hydrate formation at shallower depths. Estimated geothermal gradient values for the frontal wedge range from 28 to 92 °C/km, consistent with previously reported in-situ measurements from offshore SW Taiwan. In contrast, arc-derived sediments within the NLT and west-ward fluid migration along the landward-tilted sequences limit gas hydrate accumulation within the basin. To the south, the preferential formation of gas hydrates within the WLT basin points to deep gas sources within the forearc basin sediments, with upward movement of methane-rich fluids along normal faults. Estimated geothermal gradient values in the basin are lower, ranging from 12 to 45 °C/km, reflecting lower thermal conditions. The absence of BSRs seaward of the WLT signifies widespread gas hydrate dissociation in the frontal wedge slope.

Reservoir classification and log prediction of gas hydrate occurrence in the Qiongdongnan Basin, South China Sea

Classifying natural gas hydrate reservoirs effectively and carrying out reservoir classification modelling is crucial, but to date, research on building artificial intelligence-assisted logging curve reservoir classification models is not abundant. As exploration and development have progressed, an increasing number of fine-grained reservoirs are being discovered, and their strong heterogeneity makes correct reservoir classification even more important. Two wells used for detecting hydrates in the Qiongdongnan (QDN) Basin are used to explore the relationship between logging response parameters and reservoir quality, as well as the method of building a logging-based reservoir classification model. Through K-means clustering and Adaboost methods, the K-means method is considered to be able to correspond to the hydrate enrichment degree, while the random forest method can establish an effective reservoir classification model (the recognition accuracy is 95%). In the different categories of reservoirs, the physical properties of the reservoirs are obviously poor, and the corresponding hydrate saturation is also low, which indicates that heterogeneity has indeed affected the enrichment of hydrates in fine-grained reservoirs. This reservoir classification research method can effectively recognize reservoirs.

Revealing the Extent of Submarine Permafrost and Gas Hydrates in the Canadian Arctic Beaufort Sea Using Seismic Reflection Indicators

AbstractThe Canadian Arctic Southern Beaufort Sea is characterized by prominent relict submarine permafrost and gas hydrate occurrences formed by subaerial exposure during extensive glaciations in Pliocene and Pleistocene. Submarine permafrost is still responding to the thermal change as a consequence of the marine transgression that followed the last glaciation. Submarine permafrost is still underexplored and is currently the focus of several research projects as its degradation releases greenhouse gases that contribute to climate change. In this study, seismic reflection indicators are used to investigate the presence of submarine permafrost and gas hydrates on the outer continental shelf where the base of permafrost is expected to cross‐cut geological layers. To address the challenges of marine seismic data collected in shallow water environments, we utilize a representative synthetic model to assess the data processing and the detection of submarine permafrost and gas hydrate by seismic data. The synthetic model allows us to minimize the misinterpretation of acquisition and processing artifacts. In the field data, we identify features along with characteristics arising from the top and base of submarine permafrost and the base of the gas hydrate stability zone. This work shows the distribution of the present submarine permafrost along the southern Canadian Beaufort Sea region and confirms its extension to the outer continental shelf. It supports the general shape suggested by previous works and previously published numerical models.

Cruise CAGE-17-3

The overall goal of cruise CAGE 17-3 is to collect seismic, multibeam and water column data in an area along the northern flank of the Bear island trough and on the W-Svalbard margin. The objectives associated with the overall goal are to better understand the occurrence of gas hydrates and fluid leakage, the source of the gas in these systems, and seafloor expressions of fluid seepage. In order to address these objectives we plan to carry out:
 2D and 3D P-Cable seismic and ocean-bottom seismic acquisition over a complex leakage and source-to-sink plumbing system in the Leirdjupet Fault complex in the SW Barents Sea;Recovery of one of CAGE’s ocean floor observatory from the crater area at the northern flank of the Bear Island Trough;Repeat of P-Cable 3D seismic acquisition on the eastern segment of the Vestnesa Ridge for time-lapse seismic studies of fluid flow and gas hydrates dynamics;2D multi-channel seismic for reconnaissance and stratigraphic correlation in the Molloy Ridge area;Multibeam mapping to fill in gaps and improve resolution of existing data using the upgraded EM302 system on R/V Helmer Hanssen
 The cruise may be known as: CAGE17_3

CAGE16-6 Cruise Report: Cruise CAGE16-6

The overall goal of cruise CAGE 16-6 is to collect seismic, multibeam and water column data on the W-Svalbard margin and in the outer parts of the Storfjord Trough. The objectives associated with the overall goal are to better understand the occurrence of gas hydrates and fluid leakage, the source of the gas in these systems, and seafloor expressions of fluid seepage. In order to address these objectives we plan to carry out: P-Cable 3D seismic acquisition over gas hydrate and fluid flow areas with potential abiogenic gas sources;P-Cable 3D seismic acquisition over gas-hydrated pingo structures in the Storfjorden Trough;Recovery of 5 OBS stations that were deployed at the Lunde pockmark in October last year for long-term monitoring of microseismicity;Acquisition of additional OBS seismic data to fill gaps in existing data on the eastern segment of the Vestnesa Ridge;2D multi-channel seismic for reconnaissance and stratigraphic correlation;Multibeam mapping to fill in gaps and improve resolution of existing data using the upgraded EM302 system on R/V Helmer Hanssen;Water column mapping of gas flaring on the Vestnesa Ridge using the upgraded EM302 system on R/V Helmer Hanssen The cruise may be known as: CAGE16_6

Gas hydrate reservoir identification based on rock physics modelling and sensitive elastic parameters

Abstract Seismic bottom simulating reflections (BSR) analysis and seismic inversion are commonly used for gas hydrate reservoir interpretation. The relationship between gas hydrate saturation and elastic parameters can be influenced by gas hydrate occurrence state (e.g. pore-filling type gas hydrate or load-bearing type gas hydrate), and this may cause inaccurate interpretation. We first used the simplified three-phase Biot equation (STPBE) to model a formation containing two types of gas hydrate at the same time. Then the effects of occurrence state and authigenic minerals on the relationship between saturation and varied elastic parameters are analysed. Results show that bulk modulus (K), shear modulus ($\mu $), P-wave velocity (${V}_p$), S-wave velocity (${V}_s$), velocity ratio (${V}_p/{V}_s$), Poisson's ratio (v) and $\mu \rho $ increase at different rates with gas hydrate saturation, ${V}_p/{V}_s$ and v show relative higher sensitivity to occurrence state. Ratios of elastic parameter increments are further used to highlight the anomalies caused by gas hydrate. Four attributes ($\Delta K/\Delta \mu $, $\Delta {V}_p/\Delta {V}_s$, $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $) show good sensitivity to both gas hydrate saturation and occurrence state. $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $ can be used to distinguish gas hydrate with varied occurrence states from authigenic minerals (limestone, opal, pyrite and others). Two selected sensitive attributes $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $ are applied to well logs, four gas hydrate-bearing intervals in well 2L-38 from Mallik permafrost area in Canada and one gas hydrate-bearing interval in well A from Shenhu area in South China Sea are identified. These results are consistent with the interpreted result from the resistivity log using Archie's formula. This investigation may provide effective tools for the seismic interpretation of gas hydrate reservoirs.

Снижение гидратообразования при дросселировании газа с помощью дроссельной камеры

The analysis of the gas temperature change with a large drop on the shut-off valve forms a conclusion:the complete saturation of the gas with moisture present in it is the main condition for the occurrence of gas hydrates. In the process of gas preparation, the temperature of its dew point changes. A set of graphs of hydrate formation and dew points for specific parameters of the working medium form a range of values in which gas hydrates are formed. The proposed design of the throttle chamber allows you to reduce the pumping values to conditions that exclude hydrate formation. The calculation of the basic geometric and physical quantities confirming the effective distribution of pressure along the elliptical shape of the flow section of the throttle chamber is displayed. The use of a throttle chamber will ensure an increase in the service life of the control valves and increase the reliability of the functioning of gas production and transport elements.

The Species Composition and Distribution of Free-Living Nematodes (Nematoda) in the Area of the Methane Seep Posolskaya Bank of Lake Baikal

The Posolskaya Bank methane seep (southern Baikal, depths of ~300–500 m) is characterized by weak metha-ne discharge and the deep occurrence of gas hydrates. The species composition and distribution of nematodes from 44 samples of meiozoobenthos on the seeps (gas unloading point and gas hydrate) and background stations were analyzed. 31 species of nematodes from 12 genera, 8 families and 6 orders were identified; 94% of them were found at background stations and/or in other areas of the lake. The ratio of Baikal endemic and Siberian-Palearctic species was 6 : 1. Endemic species of the genera Paratrilobus and Tripyla dominated occurrence frequency and population density. The data obtained on the heterogeneous distribution of nematodes, the relationship between the density of their population and the presence of filaments of sulfur bacteria, copepod detritus, and ferromanganese crusts in the bottom sediments, as well as the habitation of worms with different types of food. These data are discussed.

Mechanisms for upward migration of methane in marine sediments

Methane, a non-negligible component of the global carbon budget, could be discharged upward through marine sediments to ocean floor by certain migration mechanisms. Although quite some studies have been conducted, the mechanisms for methane migration have not been well reviewed yet, especially in hydrate-bearing sediments. In this study, methane migration mechanisms are classified into diffusion and advection processes which include water movement, free gas flow, sediment failures, and recently developed gas migration through hydrate channels. The occurrence of natural gas hydrate might affect methane migration in three ways: (1) reducing the permeability of marine sediments and consequently hindering the upward movement of methane either in gas or liquid phase, (2) enhancing the geomechanical strength of marine sediments, which prevents the creation of new pathways for methane escape by sediment failures, and (3) benefiting upward methane migration by constructing hydrate channels at the interface of continuous gas columns. Generally, dissolved methane could hardly break through the gas hydrate stability zone and sulfate-methane transition zone because of the high consumption rate for methane in these two zones. For free methane gas, the capillary force is a strong resistance to free gas flow in porous sediments. However, whether for dissolved methane or free methane gas, discharge along pre-existing fractures or failure surfaces might be considerable. In addition, methane discharge by gas flow through hydrate channels is still hard to constrain. Finally, based on current research uncertainties in constraining the methane flux to the ocean, the research outlook is also addressed. It is suggested that more investigations should be conducted in three aspects: the flow characteristic of high-permeability conduits, the quantitative correlations of geomechanical properties and hydrate distribution, and the occurrence conditions of hydrate channels.

Effect of residual guest concentration in aqueous solution on hydrate reformation kinetics

Since the occurrence of gas hydrate mainly takes place at water–gas interface where having higher gas concentration, the gas concentration in the aqueous solution plays an important role in affecting the hydrate nucleation. Meanwhile, the aqueous solutions with dissolved gas are commonly found in oil / natural gas reservoirs, production wells and multiphase transportation pipelines, since these systems are always pressurized. This work investigated the reformation characteristics of gas hydrate in the aqueous solution with residual guest concentration, which was achieved by setting the dissociation (hold) pressure above the atmospheric pressure. The experimental results demonstrate that the induction time of hydrate reformation would significantly decrease with the increase of hold pressure, while barely change with duration time and reformation time. As the hold pressure can accelerate hydrate nucleation, an equivalent driving force model was established. This model exhibits more accurate and universal to represent the actual driving force of hydrate formation compared with using the subcooling. Based on the relationship between the induction time and the equivalent driving force, another model was established to predict the induction time of hydrate formation. When the difference of gas concentration in aqueous solution was eliminated by keeping the same hold pressure, the induction times of fresh water are higher than those of the decomposed water under all the higher driving forces. In addition, even though the hold pressures were always kept the same, a heat treatment in the hold pressure stage would cause the increase of the induction time of the subsequent hydrate reformation. All these findings prove the existence of some residual water structures in the solution after hydrate decomposition, resulting in the memory effect.

Characterisation of likelihood of gas hydrates occurrence in the South China Sea based on Bonferroni mean-based TOPSIS and fuzzy set theory

The efficiency of gas hydrate production depends on the success of gas exploration and occurrence evaluation. The existing evaluation models are generally univariate and only applicable to certain geological settings. This study presents a holistic approach to evaluate the likelihood of gas hydrate occurrence by supplying an index for mapping gas hydrate levels with depth. The approach integrates a generalised TOPSIS method with the fuzzy set theory. An expedition of gas hydrate conducted in the Shenhu area of the South China Sea was adopted as a case study to assess the reliability of the proposed index. As a multivariate model, the proposed approach enables the capture of non-linearity associated with gas hydrates in its entirety. The magnitude of the strength of the influential factor varies substantially from one site to another across the Shenhu area. The results also show that no site achieves the highest likelihood ‘Level V’. These results are consistent with the gas saturation values obtained using Archie’s relationship. For example, at SH4 and SH7, the values of the likelihood index are the highest between 170–185 m and 150–165 m, respectively, and the observed saturation at these locations varies from 20% (SH4) to 43% (SH7). The proposed likelihood index yields a prominent ability to quantify the level of occurrence of gas hydrates with depth at different sites. It appears to be an efficient multicriteria system bound to improve the management of the gas production trial stage.

Microbial communities associated with thermogenic gas hydrate-bearing marine sediments in Qiongdongnan Basin, South China Sea

Biogenic and thermogenic gas are two major contributors to gas hydrate formation. Methane hydrates from both origins may have critical impacts on the ecological properties of marine sediments. However, research on microbial diversity in thermogenic hydrate-containing sediments is limited. This study examined the prokaryotic diversity and distributions along a sediment core with a vertical distribution of thermogenic gas hydrates with different occurrences obtained from the Qiongdongnan Basin by Illumina sequencing of 16S rRNA genes as well as molecular and geochemical techniques. Here, we show that gas hydrate occurrence has substantial impacts on both microbial diversity and community composition. Compared to the hydrate-free zone, distinct microbiomes with significantly higher abundance and lower diversity were observed within the gas hydrate-containing layers. Gammaproteobacteria and Actinobacterota dominated the bacterial taxa in all collected samples, while archaeal communities shifted sharply along the vertical profile of sediment layers. A notable stratified distribution of anaerobic methanotrophs shaped by both geophysical and geochemical parameters was also determined. In addition, the hydrate-free zone hosted a large number of rare taxa that might perform a fermentative breakdown of proteins in the deep biosphere and probably respond to the hydrate formation.

Characterizing Gas Hydrate-Bearing Marine Sediments Using Elastic Properties—Part 2: Seismic Inversion Based on a Pore-Filling–Solid Matrix Decoupling Scheme

Characterizing gas hydrate-bearing marine sediments using seismic methods is essential for locating potential hydrate resources. However, most existing pre-stack seismic inversion methods estimate the properties of sediments containing gas hydrates without considering specific characteristics associated with gas hydrate occurrences. In the present study, a pore-filling–solid matrix decoupling amplitude variation with offset (AVO) formula is proposed to represent seismic reflectivity in terms of properties associated with gas hydrates. Based on the rock physics relationships of solid substitution, the parameters introduced into the decoupling AVO equation estimate the concentration of gas hydrates with different occurrences, including pore fillings mixed with water and solid components forming part of the dry sediment frame. A theoretical model test indicates that seismic attributes obtained with the decoupling AVO inversion are superior to the conventional wave velocities-related properties in predicting gas hydrate saturations. A realistic model test further validates the applicability of the proposed method in characterizing a gas hydrate system with varying concentrations and layer thickness. By adjusting the tuning parameters, the configurations and concentrations of the gas hydrate system can be identified using the obtained attributes. Therefore, the presented method provides a useful tool for the characterization of gas hydrate-bearing sediments.