木里地区永久冻土层的形成及对天然气水合物的成藏影响

Formation of permafrost and its influence on natural gas hydrate accumulation in Muli Region,Qinghai Province

以青海木里天然气水合物成藏地区为研究对象,总结分析了现有木里地区永久冻土和天然气水合物的特点;基于青藏高原第四纪的气候调查相关证据和现有地质勘探资料,利用FLAC;模拟计算永久冻土的形成过程,计算结果表明,现有的永久冻土可能形成于晚全新世新冰期(4000~3000至1000 a BP),约经历170 a的降温后形成稳定的永久冻土层底部边界,深度为130 m,永久冻土层中的温度梯度为1.64℃/hm,计算结果与现场勘探结果相当吻合。此外,基于永久冻土层的形成过程、天然气水合物A/B类形成时的温压曲线及天然气水合物的成藏现状,分析木里地区天然气水合物分布相对分散的原因,一是由于地质构造的原因,烃类气体仅迁移至140 m深度左右,天然水合物在原地发生相变;也可能是烃类气体迁移至更浅层的地层中,但由于多年冻土的反复演化,浅层天然气水合物动态分解后以气态形式进入大气。相关成果可为木里地区天然气水合物的勘探和开采提供思路。

The characteristics of the existing permafrost and natural gas hydrates were summarized and analyzed by studying the natural gas hydrate formation in Muli Region,Qinghai Province.On the basis of the evidence of the Quaternary climate survey and the available geological exploration data on the Qinghai-Tibet Plateau,FLAC;was used to simulate and calculate the formation of permafrost.The calculation results show that the existing permafrost probably formed during the late Holocene Neoglacial(4000-3000 to 1000 a BP),and a stable boundary at bottom of the permafrost layer formed after about 170 a of cooling at a depth of 122 m.Furthermore,the temperature gradient in the permafrost layer is 1.64℃/hm.The calculated results are quite consistent with the results obtained in the field exploration.In addition,on the basis of the formation of the permafrost layer,temperature and pressure curves of the formation of A/B class natural gas hydrates and the current status of natural gas hydrate accumulation,the reasons for the relatively scattered distribution of natural gas hydrates in Muli Region were analyzed.First,due to the geological structure,the hydrocarbon gas only migrated to the depth of around 140 m,and the natural hydrate underwent a phase change in situ.Second,it is possible that the hydrocarbon gas migrated to the shallower strata.However,as the repeated evolution of permafrost over many years,the gas hydrate entered the atmosphere in gaseous form after being decomposed dynamically.The related results can provide some ideas for the exploration and exploitation of the natural gas hydrate in Muli Region.