Characterization of gas accumulation at a venting gas hydrate system in Shenhu area, south china sea

2021 ◽  
pp. 1-45
Author(s):  
JInqiang Liang ◽  
Zijian Zhang ◽  
Jingan Lu ◽  
Guo Yiqun ◽  
Zhibin Sha ◽  
...  
Keyword(s):  
South China Sea ◽  
Wave Velocity ◽  
South China ◽  
Gas Hydrates ◽  
Gas Hydrate ◽  
Stability Zone ◽  
Shenhu Area ◽  
Free Gas ◽  
Seismic Amplitude ◽  
Hydrate Stability

Bottom-simulating reflections (BSR) in seismic data have been widely accepted to indicate the base of methane gas hydrate stability zone (MGHSZ) and free gas was thought to exist only below it. However, real geologic systems are far more complex. Here, we presented the results of three-dimensional seismic, logging while drilling (LWD), in situ and coring measurements at a venting gas hydrate system in the Shenhu area of the South China Sea. Our studies reveal that free gas has migrated upward through the thermogenic gas hydrate stability zone (TGHSZ) into the MGHSZ and become a part of the gas hydrate system. Seismic amplitude anomalies and core results suggest the presence of free gas above the base of MHSZ at 165 mbsf and the presence of thermogenic gas hydrates below it in the well SC-W01. Analyses of P-wave velocity, S-wave velocity, density, and porosity logs reveal free gas occurs above and below the MGHSZ as well. Integrating log and core analysis with seismic interpretation suggests that the variation in seismic amplitude within chaotic zone is associated with variable gas saturations, and a large amount of methane and thermogenic gases accumulate near the complex BSRs. We propose that relative permeability likely plays a significant role in the free gas distribution and formation of gas hydrates within a venting gas hydrate system, while the effect of dissolved-gas short migration is not ignored. Our results have important implications for understanding the accumulation and distribution of gas hydrates and free gas in the venting gas hydrate system and seeps at the seafloor.

Evaluation of gas hydrate-bearing sediments below the conventional bottom-simulating reflection on the northern slope of the South China Sea

2017 ◽  
Vol 5 (3) ◽  
pp. SM61-SM74 ◽  
Author(s):  
Jinqiang Liang ◽  
Zijian Zhang ◽  
Pibo Su ◽  
Zhibin Sha ◽  
Shengxiong Yang
Keyword(s):  
South China Sea ◽  
South China ◽  
Gas Hydrate ◽  
The South China Sea ◽  
Northern Slope ◽  
Stability Zone ◽  
Shenhu Area ◽  
Free Gas ◽  
Bottom Simulating Reflection ◽  
Hydrate Stability

The continuous bottom-simulating reflection (BSR) is commonly considered to mark the base of gas hydrate stability zone. Below this depth, gas hydrate gives away to free gas or water filling with pore spaces of sediments. We integrated and analyzed seismic data collected in 2008, and logging-while-drilling (LWD) data and coring results acquired by the Fugro Voyager in 2015 in the Shenhu area on the northern slope of the South China Sea. Based on seismic and well-log correlation, a BSR with typical characteristics of gas hydrates and free gas was identified at 237 m, below the mudline (BML). However, LWD data reveal a 63 m thick hydrate layer from 205 to 268 m BML. Increases in resistivity and velocity at 262 m BML indicate that gas hydrate is likely presented below the BSR. The observed pore-water freshening with depth and infrared image of core samples are consistent with geophysical interpretation. Seismic and well interpretations reveal continuous, discontinuous, and pluming BSRs in the Shenhu area. The continuous BSR indicates the base of the methane gas hydrate stability zone, and structure II gas hydrate is likely presented below the BSR. Deep thermogenic fluid locally entrapped within shallow-buried sediments may reinforce gas-hydrate accumulations near the discontinuous and pluming BSRs. We conclude that seismic responses of structure II gas hydrate can be distinct from structure I gas hydrate. Understanding the seismic characterizations of structures I and II will help in the evaluation of gas-hydrate reservoirs and inferring the presence of deep thermogenic reservoirs.

scholarly journals Acoustic Velocity Log Numerical Simulation and Saturation Estimation of Gas Hydrate Reservoir in Shenhu Area, South China Sea

2013 ◽  
Vol 2013 ◽  
pp. 1-13 ◽  
Author(s):  
Kun Xiao ◽  
Changchun Zou ◽  
Biao Xiang ◽  
Jieqiong Liu
Keyword(s):  
Numerical Simulation ◽  
South China Sea ◽  
Wave Velocity ◽  
South China ◽  
Gas Hydrate ◽  
P Wave ◽  
Shenhu Area ◽  
Free Gas ◽  
P Wave Velocity ◽  
Hydrate Saturation

Gas hydrate model and free gas model are established, and two-phase theory (TPT) for numerical simulation of elastic wave velocity is adopted to investigate the unconsolidated deep-water sedimentary strata in Shenhu area, South China Sea. The relationships between compression wave (P wave) velocity and gas hydrate saturation, free gas saturation, and sediment porosity at site SH2 are studied, respectively, and gas hydrate saturation of research area is estimated by gas hydrate model. In depth of 50 to 245 m below seafloor (mbsf), as sediment porosity decreases, P wave velocity increases gradually; as gas hydrate saturation increases, P wave velocity increases gradually; as free gas saturation increases, P wave velocity decreases. This rule is almost consistent with the previous research result. In depth of 195 to 220 mbsf, the actual measurement of P wave velocity increases significantly relative to the P wave velocity of saturated water modeling, and this layer is determined to be rich in gas hydrate. The average value of gas hydrate saturation estimated from the TPT model is 23.2%, and the maximum saturation is 31.5%, which is basically in accordance with simplified three-phase equation (STPE), effective medium theory (EMT), resistivity log (Rt), and chloride anomaly method.

Downward shift of gas hydrate stability zone due to seafloor erosion in the eastern Dongsha Island, South China Sea

2020 ◽  
Vol 38 (4) ◽  
pp. 1188-1200
Author(s):  
Jiapeng Jin ◽  
Xiujuan Wang ◽  
Min He ◽  
Jie Li ◽  
Chengzhi Yan ◽  
...  
Keyword(s):  
South China Sea ◽  
South China ◽  
Gas Hydrate ◽  
Stability Zone ◽  
China Sea ◽  
Downward Shift ◽  
Hydrate Stability

scholarly journals Numerical Modeling of Gas Migration and Hydrate Formation in Heterogeneous Marine Sediments

2019 ◽  
Vol 7 (10) ◽  
pp. 348 ◽  
Author(s):  
Keqi Bei ◽  
Tianfu Xu ◽  
Songhua Shang ◽  
Zilin Wei ◽  
Yilong Yuan ◽  
...  
Keyword(s):  
South China Sea ◽  
South China ◽  
Hydrate Formation ◽  
Stable Region ◽  
Gas Migration ◽  
Stability Zone ◽  
Shenhu Area ◽  
China Sea ◽  
Porosity And Permeability ◽  
Hydrate Stability

The formation of marine gas hydrates is controlled by gas migration and accumulation from lower sediments and by the conditions of the hydrate stability zone. Permeability and porosity are important factors to evaluate the gas migration capacity and reservoir sealing capacity, and to determine the distribution of hydrates in the stable region. Based on currently available geological data from field measurements in the Shenhu area of Baiyun Sag in the northern South China Sea, numerical simulations were conducted to estimate the influence of heterogeneities in porosity and permeability on the processes of hydrate formation and accumulation. The simulation results show that: (1) The heterogeneity of the hydrate stability zone will affect the methane migration within it and influence the formation and accumulation of hydrates. This is one of the reasons for the formation of heterogeneous hydrates. (2) When the reservoir is layered heterogeneously, stratified differences in gas lateral migration and hydrate formation will occur in the sediment, and the horizontal distribution range of the hydrate in a high porosity and permeability reservoir is wider. (3) To determine the dominant enrichment area of hydrate in a reservoir, we should consider both vertical and lateral conditions of the sedimentary layer, and the spatial coupling configuration relationships among the hydrate stability region, reservoir space and gas migration and drainage conditions should be considered comprehensively. The results are helpful to further understand the rules of hydrate accumulation in the Shenhu area on the northern slope of the South China Sea, and provide some references for future hydrate exploration and the estimation of reserves.

Geothermal investigation of the thickness of gas hydrate stability zone in the north continental margin of the South China Sea

2017 ◽  
Vol 36 (4) ◽  
pp. 72-79 ◽  
Author(s):  
Yanmin Wang ◽  
Shaowen Liu ◽  
Feifei Hao ◽  
Yunlong Zhao ◽  
Chunyan Hao
Keyword(s):  
South China Sea ◽  
Continental Margin ◽  
South China ◽  
Gas Hydrate ◽  
The South China Sea ◽  
Stability Zone ◽  
The South ◽  
China Sea ◽  
The North ◽  
Hydrate Stability
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