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Permafrost degradation and its environmental effects on the Tibetan Plateau: A review of recent research

Over the past century, fossil fuels have provided the majority of China's energy. However, their extensive utilization leads to a shortage and environmental pollution. Recently, submarine and permafrost gas hydrate deposits have been investigated as a possible clean and sustainable energy source by governmental institutions, research organizations, and energy industries in China. The primary objective of this paper is to review the potential studies pertaining to gas hydrate exploration and resource assessment, the safe and efficient exploitation of gas hydrates and the basic properties of gas hydrates. To date, there are over 20 institutions and organizations in China committed to gas hydrate investigation, among which the Guangzhou Marine Geological Survey (GMGS) and the Chinese Academy of Geological Sciences (CAGS) etc. primarily focus on gas hydrate exploration research, while the China National Offshore Oil Corporation (CNOOC) Research Center, Guangzhou Institute of Energy Conversion (GIEC) and China University of Petroleum-Beijing (CUPB) etc. concentrate on gas hydrate mining technologies. In this paper, the occurrence and exploration of gas hydrates in both permafrost regions and the continental slope of China have been determined from numerous research contributions and are presented. Moreover, the latest progress in gas hydrate fundamental studies, including hydrate phase equilibria, hydrate formation mechanisms, hydrate thermal physical properties and the acoustics and resistivity characteristics of gas hydrates are briefly reviewed, and relevant data are gathered and compared. Emphasis is also placed on gas hydrate mining technologies and gas production using depressurization methods, thermal stimulation methods or other methods. Furthermore, the security of natural gas hydrate-bearing sediments during gas production and the environmental impacts of gas hydrate are identified. With additional financial and political support and advanced research facilities, research on gas hydrates in China is progressing rapidly but is still in its early developing stage, thus, future work should be undertaken with greater diligence.

The status of natural gas hydrate research in China: A review

Seep carbonates and preserved methane oxidizing archaea and sulfate reducing bacteria fossils suggest recent gas venting on the seafloor in the Northeastern South China Sea

Qilian Mountain permafrost,with an area of about 10×104km2,is located in the north of Qinghai-Tibet plateau. It has of perfect conditions and great prospecting potential for gas hydrate. The Scientific Drilling Project of Gas Hydrate in the Qilian Mountain permafrost,which locates at the Jugenghu Mining Area in the Muli Coalfield,Tianjun County,Qinghai Province,has been implemented by China Geological Survey in 2008~2009. Four scientific drilling wells have been completed with a total footage of 2059.13m. Samples of gas hydrate were collected separately in the holes of DK-1,DK-2 and DK-3. Gas hydrate is hosted under permafrost zone at the 133~396m interval. The samples are white or ivory crystal and easily burning. Distinct low temperature anomaly has been identified by infrared camera. The gas hydrate-bearing cores strongly bubble in water. Gas-bubble and water-drop emit from the samples and then retain characteristic honeycombed structure. The typical spectrum curve of gas hydrate is detected using Raman spectrometry. Furthermore,the logging profile also indicates high electrical resistivity and sonic velocity. Gas hydrate in Qilian Mountain is found within a thinner permafrost zone with a shallower buried depth and characterized by more complex gas component and coalbed methane origin,suggesting a new type of hydrate.

Gas Hydrates in the Qilian Mountain Permafrost, Qinghai, Northwest China

Gas hydrate occurrences in the Qilian Mountain permafrost, Qinghai Province, China

Permafrost is one of major geological environments for gas hydrate occurrences. Mean altitude is over 4000m and the permafrost area is about 1.4 x 10(6)km(2) in Qinghai-Tibet Plateau. Based on the thickness of frozen layer and thermal gradient in Qinghai-Tibet Plateau permafrost, the occurrence and distribution of gas hydrate in the low-middle latitude and high altitude permafrost is predicted by using the thermodynamic method of natural gas hydrate stable temperature and pressure. The thermodynamic phase equilibrium of thermogenic and biogenic gas hydrate imply that gas hydrate is buried in depth from similar to 27 to similar to 2070m, and the resource potential of natural gases caged in hydrates are estimated as about 1.2 x 10(11) to 2.4 x 10(14)m(3) in Qinghai-Tibet Plateau permafrost. Gas hydrate is propitious to occur where the frozen layer is thicker and thermal gradient is lower in the permafrost. Seasonal change of air temperature in Qinghai-Tibet Plateau affects only the uppermost sediments 10m and does not affect gas hydrate that is buried below 30m. Under the global warming, gas hydrate will be unstable and degrade in the permafrost. Its distribution area will be gradually reduced and finally disappear in Qinghai-Tibet Plateau permafrost.

Formation condition and distribution prediction of gas hydrate in Qinghai-Tibet Plateau permafrost

Oil and gas prospecting indicates plenitudinous occurrence and promising perspective of both biogenic and thermogenic gas resource of the Qiongdongnan basin in the South China Sea Seismic sections show the occurrences of mud diapirs, gas-filled zones, and faults that serve to connect gas-filled zones and the seafloor The blur zone in bottom water on the seismic section may suggest gas plume in seawater These characteristics are comparatively propitious to hydrate generation By using the thermodynamic method, we determine the distribution features of gas hydrate stable zones (GHSZ) and predict the possible locations of gas hydrate occurring near the seafloor in the Qiongdongnan basin Biogenic methane hydrate is distributed in the subsurface where water depth is larger than ~600m, and has a GHSZ maximum thickness of ~314m, while thermogenic gas hydrate is distributed in the subsurface where water depth is generally larger than ~450m, and has a GHSZ maximum thickness of ~410m The biogenic and thermogenic gas hydrate has a total perspective volume of ~10×109m3 with a hydrate gas volume of ~ 16 × 1012m3 in the Qiongdongnan basin

Bin Xia

Papers

Formation condition and distribution prediction of gas hydrate in Qinghai-Tibet Plateau permafrost

Distribution Features of Gas Hydrate Stable Zones and Resource Prediction of the Qiongdongnan Basin in the South China Sea