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Kinetics Of Natureal Gas Hydrate Formation/Dissolution And Bacteria Processes In The Gas Seeping System In Marine Environment

Posted on:2005-12-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:D F ChenFull Text:PDF
GTID:1100360125954451Subject:Institute of Geochemistry
Abstract/Summary:PDF Full Text Request
Duo Fu Chen (Geochemistry) Directed by Pingan PengBased on the geological analyses of the gas seeping system in the Bush Hill in GC185, Gulf of Mexico, a compositional kinetic model of hydrate crystallization and dissolution from a gas stream was constructed in the Bush Hill gas seeping system. The Bush Hill seeping system is fed by reservoir gas from the nearby Jolliet field. On average ~9% of the vent gas is precipitated as hydrate in the subsurface. Total About ~1.1 X 109m3 (STP) of gas may have accumulated as hydrate in the system during the system age of 10,000 yrs. A gas seeping channelway must shift position on a -600 yr timeframe because their plumbing will plug completely with hydrate in about this time interval. The compositional differences in the vent and hydrate gases in the system are controlled mainly by gas venting rate. If venting rates are fast (gas flux q>-20 kg/m2-a) in the earlier stage of the seeping system, the vent gas has almost the same composition as the source gas, and the heaviest possible hydrates are crystallized at the surface. If venting rates are slow (gas flux q <~0.5 kg/m2-a) in the later stage, hydrate crystallization will not reach the sea floor, seep carbonate will be occurred on the sea floor. Between these extremes both the vent and hydrate gas compositions depend strongly on venting rate, and gas hydrate and seep community is developed on the sea floor of the gas seeping system. Changes in vent and hydrate gas chemistry observed at submersible visits to the Bush Hill gas seeping system suggest the variations of venting rates between different submersible visiting time and different bubble streams (locations) over the last 10 years. The chemistry of hydrate and vent gases imply that the Bush Hill system has higher vent rate in the recent than in the past. The model shows that subsurface hydrates will dissolve when the local gas mass flux decrease. Temporarily raising the seafioor temperature can cause hydrates to dissolve, the dissolution kinetics must be fast compared to the crystallization kinetics, and the dissolution gases must be so rapidly removed thatthey do not inhibit the rate of dissolution.The microanalyses show that bacteria fossils mostly in nanometer scale were preserved greatly in their byproduct-seep carbonates collected in the gas seeping system in GC 238, the offshore Louisiana Gulf of Mexico. These microfossils displays the same form and aggregate structure that is characteristic of archaea/sulfate reducing bacteria colonies. These bacteria collaborate to oxidize venting methane to CO2 and reduce seawater sulfate to H2S by their metabolism in the seeping system. The abundance of COi and HaS, causes the seep carbonate to be deposited. The petrology, stable isotope, and REE in the carbonates from sea floor near Dongsha in South China Sea show that some of carbonates are the seep carbonate. The strongly negative carbon isotopic value and the preservation of bacteria fossils in the carbonates, which are resemble to the seep carbonate of Gulf of Mexico and differ from marine carbonate, indicate the occurrence of gas seeping system in the nearby Dongsha area, and these seep carbonate can be possibly as an indicator of gas hydrate occurrence.Geological characteristics of Qiongdongnan basin are comparatively propitious to hydrate occurrence. The gas hydrates occur probably in the subsurface where water depth is larger than about 450 m with a maximum thickness of ~300-400m of gas hydrate stable zone. About 1.65 X 1012m3 hydrate gas is estimated in the basin. In Tibet Plateau permafrost, the gas hydrate buried from ~ 40-~ 2200m. The resource potential of hydrate gas estimated as about 1.2xlOu-2.4xl014m3. Gas hydrate is propitious to occur where frozen layer is thicker and thermal gradient is lower. Seasonal change of air temperature in Tibet Plateau does not affect gas hydrate that is buried below 30m deep. On the global warming, gas hydrate will be unstable and finally disappeared in Tibet Plateau permafrost.
Keywords/Search Tags:marine gas seeping system, kinetics of gas hydrate formation and dissolution, bacteria and seep carbonate, Gulf of Mexico and South China Sea, Tibet Plateau permafrost
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