天然气水合物成因探讨

天然气水合物是未来的能源资源。其分布于极地地区、深海地区及深水湖泊中。在海洋里,天然气水合物主要分布于外大陆边缘和洋岛的周围,其分布与近代火山的分布范围具有一致性。同位素组成表明天然气水合物甲烷主要是由自养产甲烷菌还原CO₂形成的。典型的大陆边缘沉积物有机碳含量低（＜0．5％～1．0％）,不足以产生天然气水合物带高含量的甲烷。赋存天然气水合物的沉积物时代主要为晚中新世-晚上新世,具有一定的时限性,并且天然气水合物与火山灰或火山砂共存,表明其形成与火山-热液体系有一定联系。火山与天然气水合物空间上的一致性表明,天然气水合物甲烷的底物可能主要是由洋底火山喷发带来的CO2。由前人研究结果推断 HCO^－₃在脱去两个O原子的同时,可能发生了亲核重排,羟基 H原子迁移到 C原子上,形成了甲酰基（HCO^－）,使甲烷的第一个 H原子来源于水。探讨了甲烷及其水合物的形成机制,提出了天然气水合物成因模型。

DISCUSSION OF THE ORIGIN OF GAS HYDRATES

Gas hydrates, distributed in the sediments of the deep seas and lakes and the permafrost of the polar region, are potential sources of energy in the future. Gas hydrates mainly occur in the outer continental and insular margins, which are in agreement with the distribution of recent volcanoes. Carbon isotopic compositions indicate that methane of gas hydrates in marine sediments is generated by autotrophic methanogens, which reduce CO₂ to CH₄. There is not enough organic carbon (< 0.5%~1.0%) in the typical continental margins to produce high content of methane in the gas hydrate zone. The age of the sediments hosting gas hydrates primarily ranges from late Miocene to late Pliocene. The coexistences of gas hydrates with volcanic ash or sands to some extent demonstrate that their formation is associated with volcano-hydrothermal system. The spatial consistency between volcanoes and gas hydrates indicates that the methane in hydrates may be derived from the reduction of large amounts of CO₂ provided by submarine volcano-hydrothermal system. From the fact that the hydrogens of microbial methane originate in water, it may be deduced that with the reduction of HCO^-₃ a transfer of H atom occurs from the O atom to the C atom within HCO^-₃ anions formed by the disassociation of dissolved CO₂ in water, and furthermore, the formyl group (HCO^-) forms. Thus one of H atoms in CH₄ generated by autotrophic methanogens by the reduction of HCO^-₃ anions arises from water, but how the other three come from water remains to be explored next. According to some literatures, authigenic carbonates can present within the gas hydrate zone, which demonstrates that the precipitation of authigenic carbonates is associated with methane production and gas hydrate formation. Their precipitation results from the dissolution of CO₂ supplied by submarine volcano-hydrothermal system, not from the oxidation of methane in gas hydrate zones. Methane production consumes bicarbonate (HCO^-₃), which restrains the further dissociation of HCO^-₃ to CO^2-₃ an H⁺, and this may explain the decrease of carbonate mineral content within gas hydrate zones. The submarine volcano hydrothermal system is favorable to methane production by thermophilic methanogens, meanwhile, gigantic pressure produced by seawater column prevents methane escaping, and gas hydrates were formed and preserved with the drop of temperature. Ocean undercurrent may result in the migration of CO₂ and methane. A preliminary model of origin for gas hydrates has been proposed from our discussions. On the other hand, autotrophic methanogens present us the prospects that methane may be artificially produced from CO₂ and H₂O.