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Abstract Different micro-distribution patterns in gas hydrates may produce different acoustic effects on the deposition medium. The influence of hydrate distributions on the reservoir and the acoustic response of the hydrate during the dissociation process are very important. In this paper, ultrasonic velocity and hydrate saturation were measured by ultrasonic measurement and time domain reflection (TDR) techniques, respectively. The relationship between the wave velocities and saturation of the hydrate along with X-ray computed tomography (X-CT) scanning results were used to analyze hydrate distribution patterns and their acoustic response characteristics. The results show that the rate of decrease in hydrate saturation in the dissociation stage remains basically unchanged, and the hydrate dissociation rate remains constant. During the process of hydrate dissociation, the hydrate first disrupts the stable sedimentary skeleton structure, and then breaks down at the contacts with sediment particles. Then, the hydrate gradually decomposes into the pore space away from the particle contacts and the hydrate is mainly present in suspended mode. Finally, hydrate decomposes entirely and free bubbles are in contact with the hydrate. In the early stage of hydrate dissociation, compressional and shear wave velocities decreased rapidly by 453 m/s and 307 m/s, respectively. In the later stage of hydrate dissociation, compressional and shear wave velocities decreased by 271 m/s and 119 m/s, respectively, with smaller decreases in wave velocities than in the early hydrate dissociation stage. The results show that the measured changes in P-wave velocity are consistent with results obtained from a site in the Shenhu area. The micro-distribution modes during the hydrate dissociation process were inferred. At the initial stage of dissociation, the hydrate decomposed at the contacts with sediment particles, whereas the hydrate was mainly suspended in the pores in the later stage. Highlights • The ultrasonic detection and X-CT technology were used to measure elastic wave velocities and hydrate distribution. • The relationship between the velocity and the hydrate saturation in sands and sediments from South China Sea was established. • The micro-distribution of hydrate in sands and sediments from South China Sea was discussed during hydrate dissociation. • The acoustic characteristics were combined to infer microscopic distribution patterns in the South China Sea sediments. [ABSTRACT FROM AUTHOR]

Marine gas hydrates accumulate primarily in coarse-grained, high-permeability layers; however, highly saturated natural gas hydrates have been discovered in the fine-grained sediments of Shenhu area, South China Sea (SCS). This may be explained by key factors, such as the great abundance of foraminifera shells. In this paper, by analyzing the SCS foraminifera structure and performing hydrate formation experiments in the foraminifera shells, the contribution of foraminifera to hydrate accumulation in the SCS was investigated from a microscopic point of view. Simulations of hydrate formation were carried out in both pure SCS foraminifera shells and the host sediments. Pore structures in typical foraminifera were studied by use of micro-focus X-ray computed tomography (CT) and scanning electron microscopy (SEM). Hydrate growth and occurrence characteristics in the foraminifera shells were observed in-situ. The results showed that the presence of foraminifera significantly enhanced the effective porosity of the SCS sediments. Moreover, while the hydrates grew preferentially in the chambers of the coarse-grained foraminifera by adhering to the inner walls of the foraminifera shells, no apparent hydrate accumulation was observed in the fine-grained or argillaceous matrix. These findings provide a basis for further studies on the accumulation mechanism of hydrates and physical properties of hydrate reservoir in the South China Sea. [ABSTRACT FROM AUTHOR]

China carried out a drilling campaign in the eastern part of the Pearl River Mouth (PRM) basin and successfully obtained natural gas hydrate samples in 2013. Laboratory based studies for the characteristics of gas hydrate were carried out on one of the hydrate-bearing sediment sample. The gas hydrate sample shows nodular occurrence with host matrix predominantly composed of silty sand and clay. On the basis of the Raman spectroscopic and X-ray diffraction results, the gas hydrate demonstrates a typical structure I (sI) with cage occupancy of methane is more than 99.5% in large cage and 91.4% in small cage, respectively, corresponding to a hydration numbers of 5.90 by thermodynamic calculation. The molecular compositions of hydrate-bound gas suggest that the guest molecules are predominantly methane (>99.9%), with trace amount of ethane (0.04%) and propane (0.01%). The isotopic analysis indicates that the methane of hydrate-bound gas of the studied drilling well in PRM basin is a typical microbial origin. [ABSTRACT FROM AUTHOR]

The P-T stability conditions of gas hydrate in different systems (i.e., solution, silica sand, and marine sediment) were studied using multi-step decomposition method with our experimental equipment. The effects of different ions with various concentrations and sediment grains on the P-T stability conditions of gas hydrate were investigated. The results show that different ions have different influences on the phase equilibrium of gas hydrate. However, the influence of ions is in a similar trend: the larger the concentration, the bigger the P-T curve shifts to the left. For the silica sand, the influence of pore capillarity of coarse particles (> 460 μm) can be negligible. The P-T curve measured in coarse silica is in agreement with that in pure water. However, the influence of pore capillarity of fine particles (< 35 μm) is significant. The maximum reduction value of temperature is 1.5 K for methane hydrate under stable state. The sediment from the South China Sea significantly affects the P-T stability conditions of methane hydrate, with an average reduction value of 1.9 K within the experimental conditions. This is mainly the result of both the pore water salinity and the pore capillarity of sediment. Because the pore water salinity is keeping diluted by the fresh water released from hydrate dissociation, the measured P-T stability points fall on different P-T curves with the decreasing salinity. [ABSTRACT FROM AUTHOR]

Abstract: For the first time gas hydrate samples were recovered in the South China Sea during a drilling campaign implemented in Shenhu area in June 2007. Laboratory based studies for the characteristics of gas hydrate were carried out on two hydrate-bearing sediment samples. Observations found that gas hydrates occurred in the pores of fine grained sediments. The hydrates were structure I based on the Raman spectroscopic results. The guest gas molecules in hydrate cages were predominantly methane (>99.3%) and trace amount of ethane (0.4%) and propane (0.04%) as determined with the method of gas chromatography. As estimated from the intensities of Raman peaks, the cage occupancy rates were more than 99% in large cage, and~86% in small cage, respectively, corresponding to a hydration number of 5.9. [Copyright &y& Elsevier]

Abstract The Shenhu area, located in the Pearl River Mouth Basin, South China Sea (SCS), is currently one of the most promising exploration areas for gas hydrates. In this study, the lithological characteristics, as well as the molecular and isotopic composition of hydrocarbon gases are systematically reported for the first time for core sediments obtained from two new drilling sites (referred to as W01B and W02B) in the southeast Shenhu area. Both gas hydrate-bearing and gas hydrate-free samples are characterized by fine-grained sediments (dominantly coarse silt and to a lesser extent, fine and medium silt) and similar median grain-sizes (5.94Φ to 6.49Φ), sorting (2.32 to 2.59), kurtosis (0.82 to 0.97), skewness (−0.12 to 0.03) and mineral compositions. Abundant authigenic pyrites (including euhedral and framboidal pyrites) also occur in these sediments. All these characteristics collectively indicate that the core sediments from the southeast Shenhu area formed in a relatively stable, low-energy and anoxic sedimentary environment. Nevertheless, the hydrate-bearing layers in this study are characterized by lower sand contents and more foraminifera than gas hydrate-free layers. The molecular and isotopic composition of hydrocarbon gases hosted by secondary minerals in the studied sediments indicates that they are of thermogenic origin. Combined with previously published data, we suggest that the abundant foraminifera within the sediments of the Shenhu area play a significant role in controlling the formation of gas hydrates by increasing the porosities of sediments, and that the thermogenic gas is an important source for the hydrocarbon gases of hydrates in the SCS. Highlights • The gas hydrate-bearing and barren samples show similar grain size distribution. • These sediments formed in a stable, low-energy and anoxic sedimentary environment. • Foraminifera within the sediments are favorable for the formation of gas hydrates. [ABSTRACT FROM AUTHOR]

Gas hydrate has been recognized as a potential energy resource in South China Sea (SCS). Understanding the acoustic response of gas hydrate formation in the SCS sediments is essential for regional gas hydrate investigation and quantification. The sediments were obtained from gravity core sampling at E 115°12.52363′ N 19°48.40299′. Gas hydrate was formed within a “gas + water-saturated SCS sediments” system. Combination of a new bender element technique and coated time domain reflectometry (TDR) was carried out to study the acoustic response of hydrate occurrence in SCS sediments. The results show the acoustic signal becomes weak when hydrate saturation (S h ) is lower than 14%. The acoustic velocities (Vp, Vs) of the sediments increase with S h during hydrate formation, and Vs increases relatively faster when S h is higher than 14%. These results indicate that tiny hydrate particles may firstly float in the pore fluid, which causes a significant acoustic attenuation, but has little influence on shear modulus. As time lapses and S h approaches 14%, numerous particles coalesce together and contact with sediment particles. As a result, Vs has a sharp increase when hydrate saturation exceeds 14%. Several velocity models were validated with the experimental data, which suggests a combination of the BGTL (Biot–Gassmann Theory modified by Lee) model and the Weighted Equation is suitable to estimate S h in SCS. [ABSTRACT FROM AUTHOR]