【作者】 陈多福；

【导师】 彭平安；

【作者基本信息】 中国科学院研究生院（广州地球化学研究所） ， 地球化学， 2004， 博士

【摘要】 通过美国墨西哥湾GC185区BUSH HILL海底天然气渗漏系统的地质特征分析，建立了海底渗漏天然气沉淀水合物及分解的动力学模型。对Bush HILL渗漏系统内水合物形成和分解动力学过程和控制因素的研究表明，渗漏系统的天然气来源于邻近的Jolliet油气藏，渗漏天然气中约有9％在海底沉积物中形成了水合物，单个天然气渗漏通道在～600年后将被沉淀的水合物堵塞。在渗漏系统活动的1万年中，海底沉淀了～1.1×10⁹m³的水合物天然气。渗漏系统的天然气渗漏速度是影响海底渗漏和水合物天然气化学组成的最主要控制因素。在海底天然气渗漏系统演化的早期（高速渗漏，q＞～20 kg／m²-a），海底渗漏天然气几乎具有与气源天然气一致的天然气组成、形成的水合物具有最重的天然气组成。在渗漏系统的晚期（低速渗漏，q＜～0.5 kg／m²-a），在海底附近没有水合物沉淀。介于二者之间的中期是海底水合物和自养生物群的发育阶段，海底水合物和渗漏天然气的化学组成主要受渗漏速度的控制。10年来渗漏系统的海底直接观测表明，海底水合物天然气、渗漏天然气、天然气渗漏速度在时空上是多变的，并且，过去的渗漏速度比现今要慢。在系统渗漏速度降低的演化过程中，单个通道内化学组成上不稳定的水合物将分解。如果海底水温的短期突变，水合物分解不仅在动力学上是非常快的，而且，化学组成上不稳定的水合物分解所产生的天然气必须快速移出，否则分解天然气将阻碍水合物的进一步分解。 海底天然气渗漏系统甲烷氧化细菌特征及其产物-冷泉碳酸盐岩的微观分析表明，冷泉碳酸盐岩保存了大量的以纳米为主的细菌化石。这些化石形态和集合体结构表明可能是石化的甲烷氧化细菌和硫酸盐还原细菌。这些微生物通过生命代谢过程，使渗漏系统中的CH₄被氧化转变为CO₂，同时孔隙水中的SO₄^2-还原成H₂S，与Ca和Fe离子结合沉淀冷泉碳酸盐岩。南海东沙附近海底碳酸盐岩的岩石学、稳定同位素和稀土元素特征表明存在冷泉碳酸盐岩，碳酸盐岩特别负的碳同位素、及其保存的细菌化石是海底天然气渗漏活动的直接标志，可能是南海东沙附近海域水合物存在的另一个重要标志。 对海底渗漏系统中热成因天然气水合物资源预测方法进行了研究，应用热力学预测方法对我国南海琼东南盆地、青藏高原冻土带水合物分布有利区和资源远景进行了探讨。琼东南盆地具有天然气水合物形成的地质条件，琼东南盆地水合物分布于水深大于约450m的海底，稳定带最大厚度约300一400m，盆地内水合物天然气远景为¹.65xl0‘丫。青藏高原多年冻土带水合物埋藏于⁴0一220枷，计算表明青藏高原冻土带水合物天然气资源约1 .2 x10，’一2.4 xlo，丫。在冻土层越厚·冻土层及冻土层之下沉积层的地温梯度越小的地区，最有利于水合物发育。气温季节性变化对水合物影响不大。在全球气温快速上升的背景下，青藏高原水合物将失稳，最终完全消失。更多还原

【Abstract】 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.更多还原