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The acoustic properties of hydrate deposits are important parameters for hydrate geophysical exploration, and the gas leakage model plays a very important role in hydrate accumulation systems. In order to reflect the gas supply environment during hydrate formation, a high-pressure device with a simulated leakage system was designed to achieve different methane flux supplies. The effects of different methane fluxes on the hydrate formation rate and the maximum hydrate saturation were obtained. The results in this study indicate that similar hydrate formation rates occur in systems with different methane fluxes. However, when the methane flux is large, it takes longer to reach the maximum hydrate saturation, and the larger the methane flux, the larger the hydrate saturation formed. In each methane flux system, the elastic velocity increased slowly with increasing hydrate saturation at the beginning of hydrate formation, but velocity increased quickly when the hydrate saturation reached 50–60%. In order to take into account the effect of the gas, the calculated values of the elastic velocity model were compared with the experimental data, which indicated that the BGTL theory and the EMT model are more adaptable and can be used to deduce hydrate morphology. In the large methane flux system, the hydrate mainly forms at grain contacts when the hydrate saturation is 10–60%. As the hydrate saturation reaches 60–70%, hydrate forms first in the pore fluid, and then the hydrates contact sediment particles. [ABSTRACT FROM AUTHOR]

Anaerobic oxidation of methane and sulfate reduction was studied in the pore waters of four cores at two stations of the middle Okinawa Trough. Pore water vertical distributions of sulfate, methane, sulfide, total alkalinity, ammonium, and phosphate were determined in this study. Our results show strong linear sulfate concentration gradients of 6.83 mmol m in Core A and 5.96 mmol m in Core C, which were collected from two stations. Concurrent variations of methane, total alkalinity and hydrogen sulfide all exhibit steep increases with depth at both cores, which indicate active methane seep activities around two stations. Pore water ammonium and phosphate concentrations reveal minor influences of organic matter degradation on sulfate reduction at two stations. Sulfate methane interface (SMI) was extrapolated from linear sulfate profiles in methane seep cores. Shallower SMI depths (A: 4.9 mbsf; C: 5.4 mbsf) indicate strong methane fluxes and active anaerobic oxidation of methane in the underlying sediments. [ABSTRACT FROM AUTHOR]

Abstract: Dry water (DW) has been recently demonstrated to be an effective medium for methane storage in a hydrated form. Here, a series of experiments have been carried out on dry water methane hydrates (DW–MH) to investigate their formation and dissociation rates, storage capacity and structural characteristics. The result shows that the storage capacity of MH increases at least 10% by using DW relative to using surfactants like sodium dodecyl sulfate (SDS) solution. Also, it is found that controls on pressure–temperature (P–T) condition have influences on the induction and reaction time of DW–MH formation, i. e. the induction and reaction time are much shorter when the reaction cell is cooled to ~3°C first. On the basis of Raman spectra, the hydration number is calculated as 5.934±0.06 at different positions of the DW–MH, which suggests that the sample is very homogeneous. The dissociation process of the DW–MH sample exhibits a rapid release of methane gas at the first stage of dissociation. Although hydrate dissociation is prevented by the effect of self preservation, most methane gas has released from the hydrate, however, before the self preservation occur. [Copyright &y& Elsevier]

The elastic wave velocities of hydrate-bearing sediments (HBS) are considerably affected by the content of free gas and hydrate. Although several existing models relate the free gas and hydrate saturation to acoustic velocities, the accuracy of these models is still uncertain because of the difficulty in determining the gas content. In this study, we acquired the gas volume fraction and acoustic velocity data of the hydrate-bearing sands through a X-ray computed micro-tomography (CT) and an ultrasonic apparatus, respectively. The acoustic velocities increase slowly at low hydrate saturation (S h < 33%), whereas they increase rapidly when hydrate saturation exceeds 33%. Using the measured data, we verified three commonly used velocity models, namely, the Biot-Gassmann theory by Lee (BGTL), effective media theory (EMT), and simplified three-phase equation (STPE). The results obtained using the BGTL are consistent with the experimental results when hydrate saturation exceeds 45%, while the EMT-B model is more suitable for predicting P-wave velocity (V p) when the hydrate saturation varies from 15% to 55%. The value of V p calculated by the STPE model, without considering the effects of gas on velocity, is higher than the experimental value. We further applied a new method combining the Wood and Domenico equations to calculate the bulk modulus of HBS containing methane gas, which can improve the precision and applicability of the STPE model. Compared with the EMT and BGTL models, the modified STPE model is more suitable for predicting V p of unconsolidated reservoirs with high porosity and permeability. These results provide a new method for further experimental research and a theoretical reference for accurately estimating hydrate saturation through logging data during natural gas exploration and development. • A CT-ultrasonic detection apparatus was used to measure the micro distribution and velocity of hydrate bearing sediments simultaneously. • The accuracy of different velocity models and equations was verified. • A new bulk modulus calculation method is applied to improve the accuracy and adaptability of the velocity model. [ABSTRACT FROM AUTHOR]

The article presents a study of accumulation procedure of biogenic gas hydrates at topographic highs in the Shenhu Area of Northern South China Sea. The Shenhu area reveals high geothermal fluxes at topographic low and low geothermal fluxes at topographic highs. The rates of fluid migration at fractures were higher compared to other positions that affected the accumulation of gas hydrates and vertical transport of methane.

The sulfate–methane transition zone (SMTZ) is an important diagenetic redox boundary within marine sediments where the anaerobic oxidation of methane (AOM), coupled with bacterial sulfate reduction, can promote sulfur isotopic enrichments in several solid phase minerals including pyrite (FeS 2 ). Authigenic pyrite can form in concentrated abundances within the SMTZ and as such, can be used as a proxy to identify paleo-SMTZs. This study uses enrichments in 34 S and anomalously high abundances of authigenic pyrites in 287 samples from the northern South China Sea (SCS) to determine the paleo-SMTZ. The pyrite samples were collected from sediment cores acquired at three sites, each of which are known to be located in natural gas hydrate-bearing regions. We assess the relative abundances of authigenic pyrites, the types of pyrite morphologies recovered in the cored sediments, and the sulfur isotopic values of recovered pyrite samples using two methods: (1) handpicked sample analysis using a binocular microscope, and (2) the chromium reduction method. Our results show that pyrite concentrations and sulfur isotopic compositions exhibit synchronous fluctuations, particularly from 6.8 m below seafloor (mbsf) to 8.4 mbsf at all three study sites. There is a significant increase in the occurrence of rod-like pyrite morphology within this key interval. We define the position of the paleo-SMTZ by the presence of anomalously high accumulations of pyrites at greater than 5.0 wt.% using the handpicking method or greater than 0.5 wt.% via the chromium reduction method, along with positive Δδ 34 S excursions greater than 10.0‰ VCDT. We discovered a regional paleo-SMTZ that is shallower than the modern SMTZ, suggesting a previous period of elevated methane flux from depth, possibly related to widespread gas hydrate dissociation. [ABSTRACT FROM AUTHOR]