青藏高原冻土温度与厚度

Assessing the permafrost temperature and thickness conditions favorable for the occurrence of gas hydrate in the Qinghai–Tibet PlateauWu Qingbai *,Jiang Guanli,Zhang PengState Key Laboratory of Frozen Soil Engineering,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou 730000,Chinaa r t i c l e i n f o Article history:Received 28March 2009Accepted 30October 2009Available online 16December 2009Keywords:Qinghai–Xizang Plateau PermafrostNatural gas hydrate Geothermal gradientsa b s t r a c tPermafrost accounts for about 52%of the total area of the Qinghai–Tibet Plateau,and the permafrost area is about 140Â104km 2.The mean annual ground temperature of permafrost ranges from À0.1to À5°C,and lower than À5°C at extreme high-mountains.Permafrost thickness ranges from 10to 139.4m by borehole data,and more than 200m by geothermal gradients.The permafrost geothermal gradient ranges from 1.1°C/100m to 8.0°C/100m with an average of 2.9°C/100m,and the geothermal gradient of the soil beneath permafrost is about 2.8–8.5°C/100m with an average of 6.0°C/100m in the Qinghai–Tibet Plateau.For a minimum of permafrost geothermal gradients of 1.1°C/100m,the areas of the potential occur-rence of methane hydrate (sI)is approximately estimated to be about 27.5%of the total area of perma-frost regions in the Qinghai–Tibet Plateau.For an average of permafrost geothermal gradients of 2.9°C/100m,the areas of the potential occurrence of methane hydrate (sI)is approximately estimated about 14%of the total area of permafrost regions in the Qinghai–Tibet Plateau.For the sII hydrate,the areas of the potential occurrence of sII hydrate are more than that of sI methane hydrate.Ó2009Elsevier Ltd.All rights reserved.1.IntroductionGas hydrates are ice-like crystalline solids composed of water and gas in which water molecules trap gas molecules in a cage-like structure known as a clathrate,which can be formed when gas and water mixtures are subjected to high pressure or low temperature conditions [15].Gas hydrates widespread in permafrost regions and beneath the sea in sediments of outer continental margins.Although estimates of its occurrence vary widely,the carbon re-serves of gas hydrate in the world may reach 2Â1016m 3,twice of explored conventional sources of energy [10].Gas hydrates are a new kind of potential and clean energy resource.And gas hy-drates,especially methane hydrate,may easily to dissociate with variations of temperature and pressure.And the dissociation of hy-drates may play a great role in climate change due to their strong greenhouse effect [12–14].Now,the direct confirmation of gas hydrates was obtained in permafrost regions in northern hemisphere,for example,in the North Slope of Alaska,the Mackenzie Delta–Beaufort Sea area,Can-ada,and the west Siberian basin,Russia [4].All available data show that the methane-hydrate stability zone locates generally 300–600m beneath the permafrost.The sample of intra-perma-frost gas hydrates was obtained in the Mackenzie Delta,NorthwestTerritories,Canada [5].Although the sample of the intra-perma-frost gas hydrate cannot be obtained in west Siberian,amounts of evidences collected on gas release indicated that the intra-per-mafrost gas hydrates may exist [20].Gas hydrates in permafrost re-gions are nearly related to permafrost conditions [2,3,6,7].Two primary factors affecting the distribution of the gas hydrates stabil-ity zone:the geothermal gradient and the gas composition deter-mine the temperature and pressure conditions for the gas hydrate formation and its accumulations,and some other factors,such as the pore-fluid salinity and the pore-pressure,have a little effect,too [4].The potential occurrence of gas hydrates in permafrost regions in the Qinghai–Tibet Plateau has a significant impact to the utilization of new energy resource,the climate change and the environment.Amounts of conductive works were developed to study the potential occurrence of gas hydrates in permafrost regions [1,9,18,19,22,25].It is generally believed that geological conditions are conducive to gas hydrates formation.Gas hydrate,containing H 2S,C 2H 6and C 3H 8and CH 4may exist beneath permafrost in the Qinghai–Xizang Plateau [9,19,22].Chen et al.[1]estimated the amount of gas hydrates in per-mafrost regions of the Qinghai–Tibet Plateau according to the natu-ral gas component in Xingjiang and Caidamu areas and permafrost conditions,ranged from about 1.2Â1011–2.4Â1014m 3.Permafrost conditions are critical to the potential occurrence of gas hydrates in the Qinghai–Tibet Plateau,especially permafrost thickness and temperature.In this work,permafrost conditions,0196-8904/$-see front matter Ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.enconman.2009.10.035*Corresponding author.E-mail address:qbwu@ (W.Qingbai).Energy Conversion and Management 51(2010)783–787Contents lists available at ScienceDirectEnergy Conversion and Managementj ou r na l h om e pa ge :w w w.e lse vi e r.c om /lo c at e /en c on mansuch as thermal gradient,permafrost thickness,and permafrost tem-perature data are collected to study and assess the temperature con-ditions favorable for the potential occurrence of gas hydrates and permafrost in the Qinghai–Tibet Plateau.The permafrost thickness and the thermal gradient within and beneath the permafrost are used to analyze the gas hydrate stability conditions.2.Regional distribution of permafrostPermafrost distribution in the Qinghai–Tibet Plateau is affected not only by altitude and latitude,but also by terrain slope,vegeta-tion and snow cover,geological structures and underground water. For every latitude increase of1°,the temperature of permafrost de-creases about1°C and the thickness of permafrost increases about 20–30m.For every100m the altitude rises,the temperature of permafrost decreases about0.8–0.9°C and the thickness of perma-frost increases about20m[24].Permafrost accounts for about52%of the total area of the Qing-hai–Tibet Plateau,which is about140Â104km2[23].Regionally, permafrost in the Qinghai–Tibet Plateau can be divided into four parts(Fig.1):(1)high-mountain permafrost areas of the Alerkin–Qilian Mountains,(2)continuous permafrost areas of the Qiang-tang Plateau,(3)discontinuous permafrost areas in the east of the Qinghai–Tibet Plateau,and(4)high-altitude sporadic perma-frost zone of the Nianqing–Tanggula–Himalayas Mountains.The high-mountain permafrost area locates in the areas of Aler-kin–Qilian Mountain whose area is about9.5Â104km2.In this area,the mean annual air temperature ranges fromÀ3.0to À5.7°C.The mean annual ground temperature of permafrost ranges fromÀ0.1toÀ2.3°C,the thermal gradient of permafrost ranges from 2.0to 4.5°C/100m,and the permafrost thickness ranges from10to140m[8].Permafrost features are dominantly controlled by altitude.The continuous permafrost area locates in the Qiangtang Pla-teau,and its area is about60.7Â104km2.In this area,the altitude ranges from4500to5000m and its annual mean air temperature ranges fromÀ3.6toÀ9.8°C[24].According to the measured data [24],the mean annual ground temperature of permafrost ranges fromÀ0.2toÀ4.0°C,and lower thanÀ5°C at the extreme high-mountains.The thermal gradient of permafrost ranges from1.0 to6.0°C/100m,and the permafrost thickness ranges from10to 140m.In the western of the Qinghai–Tibet Plateau,the annual mean temperature is lower thanÀ6.0°C,and maybe the perma-frost temperature there is the lowest one among the whole Plateau.The discontinuous permafrost area locates in the east of the Qinghai–Tibet Plateau,and its area is about14Â104km2.In this area,the altitude ranges from3500to4500m and its annual mean air temperature ranges fromÀ3.5toÀ5.5°C[21].According to the measured data,the mean annual ground temperature of perma-frost ranges fromÀ0.1toÀ1.0°C[21].The permafrost thickness ranges from12to60m[17,21].The permafrost in this area is in extensive degradation.The high-altitude sporadic permafrost zone of the Nianqing–Tanggula–Himalayas Mountains locates in the south of the Qing-hai–Tibet Plateau.In this area,the altitude ranges from3500to 7000m and its annual mean air temperature ranges from0to À3°C,and lower thanÀ3°C in extreme high-mountains[24]. There has no measured data about this area,however,the latitude of permafrost distribution boundary ranges from5200to5600m,Fig.1.The permafrost distribution in the Qinghai–Xizang Plateau,1is the high-mountain permafrost areas in Qilian Mountains,2is the continuous permafrost areas in Qitang Plateau,3is the discontinuous permafrost areas in eastern of Qinghai–Tibet Plateau,and4is the sporadic permafrost areas in NianQing Tanggula Mountains and Himalaya Mountains.784W.Qingbai et al./Energy Conversion and Management51(2010)783–787where the annual mean air temperature ranges fromÀ2.5to À3.0°C[24].3.Permafrost conditions3.1.Geothermal gradientsGeothermal gradients within permafrost and thawed soil be-neath permafrost are very important to the formation of gas hy-drate.The analysis of ten boreholes temperature along the Qinghai–Tibet Highway which penetrated through the base of the permafrost showed that the geothermal gradients within per-mafrost ranges from 1.8to 6.6°C/100m with an average of 4.6°C/100m,and the geothermal gradients in six of these bore-holes were higher than5.0°C/100m[16].In recent years,the geothermal gradient within permafrost can be obtained by the measurements of permafrost temperatures in deep borehole(Fig.2).The geothermal gradient within permafrost along the Qinghai–Tibet Highway ranges from1.1°C/100to8.0°C/100m with an average of2.9°C/100m.Except Kunlun Mountains,the geo-thermal gradients within permafrost along Qinghai–Tibet Highway are lower than4.0°C/100m with an average of2.3°C/100m.Actually,the geothermal gradient in permafrost is influenced by climate,soil and water features,etc.During the geological time,the geothermal gradient in permafrost is variable.However,the geo-thermal gradient of the soil beneath the base of permafrost is rel-ative steady.Generally,the geothermal gradient of the soil beneath permafrost is about2.8–8.5°C/100m with an average of6.0°C/ 100m in the Qinghai–Tibet Plateau by the analysis of ten bore-holes temperatures,and the geothermal gradients of eight bore-holes were higher than5.0°C/100m.Meanwhile,the geothermal gradient in seasonally frozen ground in the Qinghai–Tibet Plateau is about1.5–6.5°C/100m with an average of3.9°C/100m[24].Compared with permafrost in other places of the world(Table 1)[4],the geothermal gradients within and beneath permafrost in the Qinghai–Tibet Plateau are conducive for the formation and the preservation of gas hydrate.3.2.Permafrost thicknessPermafrost thickness,which is restricted by the thermal gradi-ent of thawed soil under the base of permafrost,is a very important thermodynamic condition related to the stability of natural gas hy-drate.The geothermal gradient within permafrost controls the var-iation of permafrost thickness,and the permafrost thickness increases exponentially with thermal gradient decreasing in the Qinghai–Tibet Plateau[24].Recent drilling in the Qinghai–Tibet Plateau revealed that the permafrost thickness is greater than128m in the Wa-Li-Xi-Li-Tang Basin,and up to139.4m in Qilian Mountains[24].However,de-duced from the ground temperature data of a100m deep borehole in the Fenghuo Mountains(Fig.2),the permafrost thickness there could be up to about220m.The continuous permafrost zone of the Qiangtang Basin has the potential to form much thicker permafrost layer.In this article,the result of38statistical samples of permafrost thickness shows that permafrost thickness is linear with the mean annual ground temperature of permafrost.The formulation of H=14.6–34.74T cp(T cp–the mean annual ground temperature at the depth of zero annual amplitude,H–permafrost thickness) can be used to simplify the estimate of the permafrost thickness. And then,based on the statistical relationships among the mean annual ground temperature,altitude,latitude and continentality [11],we can approximately indicate the spatial pattern of perma-frost thickness over the Qinghai–Tibet Plateau(Fig.3).Fig.3indi-cates that permafrost thickness ranges from100to more than 200m in Qiangtang basin,and from50to more than200in the areas of Alerkin–Qilian Mountain,and from0to50m in the east of the Qinghai–Tibet Plateau,indicating that permafrost thickness conditions is favorable for the potential occurrence of gas hydrate over the Qinghai–Tibet Plateau.4.Potential occurrence of gas hydrate in permafrost regionsThere has no data on pore-pressure from borehole in Qinghai–Tibet Plateau,so we can approximately estimate the average hydrostatic pore-pressure gradients of15kPa/m to20kPa/m with-in permafrost layer and10kPa/m within the soil beneath perma-frost layer[1].Fig.4show gas hydrate stability phase diagram to indicate the relationships between permafrost and gas hydrate occurrence.Fig.4show that the intersection zone of temperature and gas hydrate stability conditions is gas hydrate stability zone. Permafrost thickness and thermal gradient of soil beneath perma-frost layer control gas hydrate stability zone,and permafrost thick-ness larger and thermal gradient smaller,gas hydrate stability zone thicker(Fig.4).Gas component controls gas hydrate stability zone, and when gas component consist of98%methane,1.5%ethane and 0.5%propane,the thickness of gas hydrate stability zone will in-crease and relative control condition of temperature and perma-frost thickness of gas hydrate occurrence will produce great changes(Fig.4).As depicted in standard text by Sloan and Koh[15],for the structure I of methane hydrate at0°C,approximately2.5MPa of pressure is required to keep it stable.Thus for normally consolida-tion sedimentary strata with fresh pore water,a minimum of about 133m of permafrost is required to keep stable of the methane hy-drate.For a minimum of permafrost geothermal gradients of 1.1°C/100m,structure I of methane hydrate is possible to form in the permafrost areas where the mean annual ground tempera-ture is lower thanÀ1.5°C.The total areas of the potential occur-rence of methane hydrate(sI)is approximately estimated to be about27.5%of the whole area of permafrost regions in the Qing-hai–Tibet Plateau.For an average of permafrost geothermal gradients of2.9°C/100m by the data of recent years,structure I of methane hydrate can possibly form in the permafrost areas where the mean annual ground temperature is lower than about À3°C,and the areas of the potential occurrence ofmethane Fig.2.The temperature gradients within permafrost in the Qinghai–Xizang Plateau.W.Qingbai et al./Energy Conversion and Management51(2010)783–787785hydrate (sI)is approximately estimated to be about 14%of the total area of permafrost regions in the Qinghai–Tibet Plateau.However,for the sII hydrate at 0°C,approximately 1.5MPa of pressure is required to have stable hydrate,thus for normally con-sideration sediment strata with fresh pore water,only about 100m of permafrost is required to have any stable sII hydrate.For a min-imum of permafrost geothermal gradients of 1.1°C/100m,sII hy-drate is possible to form in the permafrost areas where the mean annual ground temperature is lower than À1.0°C,and the areas of the potential occurrence of sII hydrate is approximately esti-mated to be about 35%of the total area of permafrost regions in the Qinghai–Tibet Plateau.5.ConclusionsBefore 1990,the permafrost geothermal gradient ranges from 1.8to 6.6°C/100m with an average of 4.6°C/100m.In recent years,the permafrost geothermal gradient ranges from 1.1°C/100m to 8.0°C/100m with an average of 2.9°C/100m.The geo-thermal gradient of the soil beneath permafrost is about 2.8–8.5°C/100m with an average of 6.0°C/100m in the Qinghai–Tibet Plateau,and the geothermal gradient in seasonally frozen ground in the Qinghai–Tibet Plateau is about 1.5–6.5°C/100m with an average of 3.9°C/100m.Permafrost thickness ranging from 50to 200m is predominant over the Qinghai–Tibet Plateau.The maximum permafrost thick-Table 1The characteristics of permafrost regions and gas hydrate.LocationPermafrost thickness (m)G p (°C/m)G t (°C/m)GHBS (m)Messoyakha 71°N,86°E3200.61.8500–1500Alaska69–71°N,146–160°E 174–630 1.5–4.5 1.6–5.2320–700Mackenzie Delta69.5°N,134–135°E 510–740 1.82.7800–3000Qinghai–Tibet Plateau32–37°N,92–95°E>1301.1–8.02.5–8.5unknownG p is the geothermal gradients within permafrost,G t is the geothermal gradient beneath permafrost,and GHBS is the stability thickness of gashydrate.Fig.3.Spatial pattern of permafrost thickness over the Qinghai–TibetPlateau.Fig.4.Gas hydrate stability phase diagram.786W.Qingbai et al./Energy Conversion and Management 51(2010)783–787ness by borehole is about140m,and by permafrost temperature date is up to220m.Permafrost thickness conditions are favorable for gas hydrate occurrence.For a minimum of permafrost geothermal gradients of1.1°C/ 100m,the areas of the potential occurrence of methane hydrate (sI)is approximately estimated to be about27.5%of the total area of permafrost regions in the Qinghai–Tibet Plateau.For an average of permafrost geothermal gradients of2.9°C/100m,the areas of the potential occurrence of methane hydrate(sI)is approximately estimated about14%of the total area of permafrost regions in the Qinghai–Tibet Plateau.For the sII hydrate,the areas of the poten-tial occurrence of sII hydrate seem to be more than that of sI meth-ane hydrate.AcknowledgementThis research was supported in part by the Outstanding Youth Foundation Project,Natural Science Foundation of China(Grant No.40625004)to Qingbai Wu,and the CAS Knowledge Innovation Key Project(Grant No.KZCX2-YW-330).References[1]Chen DF,Wang MC,Xia B.Formation condition and distribution prediction ofgas hydrate in Qinghai–Tibet Plateau permafrost.Chinese J Geophys 2005;48(1):179–87.[2]Collett TS.Natural gas hydrates of the Prudhoe Bay and Kuparuk river area,North Slope,Alaska.AAPG Bull1993;77(5):793–815.[3]Collett TS,Ginburg GD.Gas hydrates in the Messoyakha gasfield of the westSiberian basic–a extermination of the geologic evidence.In:Proceedings of7th international offshore and polar engineering conference,vol.96;1997.p.103.[4]Collett TS,Dallimore SR.Permafrost-associated gas Hydrate.In:Max MD,editor.Natural gas hydrate in oceanic and permafrost environment;2003.p.43–60. [5]Dallimore SR,Collett TS.Intrapermafrost gas hydrate from a deep corehole inthe Mackenzie Delta Northwest Territories,Canada.Geology1995;23:527–30.[6]Dallimore SR,Uchida T,Collett TS.Scientific Results from JAPEX/JNOC/GSCMallik2L238Gas Hydrate Research Well,Mackenzie Delta,Northwest Territories Canada.Bull Geological Survey Canada,vol.544;2002[7]Devidson DW,El-Defrawy MK,Fuglem MO.Natural gas hydrates in NorthernCanada.In:Proceedings of3th international conference on permafrost,vol.1.Edmonton,Alberta,Canada;1978.p.938–43.[8]Guo Pengfei.Permafrost in Qilian Shan area.In:Proceedings of2nd nationalconference on permafrost,Lanzhou,Gansu People’s Publishing House;1983.p.395–400[in Chinese].[9]Huang P,Pan GL,Wang LQ.Prospect evaluation of natural gas hydrateresources on the Qinghai–Tibet Plateau.Geol Bull China2002;21(11):794–8.[10]Kvenvolden KA.Potential effects of gas hydrate on human welfare,In:Proceeding of national academy of science colloquium,A;1999.p.3420–26.[11]Ku Xinbo,Wu Qingbai,Jiang Guanli.Potential distribution of natural gashydrate in the permafrost regions of Qinghai–Tibet Plateau.Nat Gas Geosci 2007;18(4):588–92.[12]MacDonald GJ.Role of methane clathrates in past and future climates.ClimChange1990;16(2):247–81.[13]Neue H.Methabe emission from ricefields:wetland ricefields may make amajor contribution to global warming.Bioscience1993;43(7):466–73. [14]Paull CK,Usseler III W,Dillion WP.Is the extent of glaciation limited by marinegas hydrate?Geophys Res Lett1991;18(4):432–4.[15]Sloan ED,Koh CA.Clathrate hydrates of natural gases.3rd ed.New York:CRCPress,Taylor and Francis Group,Publishers;2008.p.721.[16]Wang Jiachen,Li Shude.Analysis of geothermal conditions near permafrostbase along the Qinghai–Tibet Highway.Professional papers on permafrost studies of Qinghai–Xizang Plateau.Beijing:Scientific Press;1983.p.38–43.[17]Wang Shaolin,Luo Xiangrui,Guo Pengfei.The distributive characteristics offrozen ground in the east of Qinghai–Xizang Plateau.J Glaciol Geocryol 1991;13(2):131–40[in Chinese with English abstract].[18]Wu Qingbai,Jiang Guanli,Pu Yibin,Deng Yousheng.Relationships betweenpermafrost and gas hydrate on Qinghai–Xizang Plateau.Geol Bull China 2006;25(1–2):29–33.[19]Xu X,Cheng GD,Yu QH.Gas hydrates in permafrost regions on the Qinghai–Xizang Plateau.Advance in earth sciences1999;14(2):201–4[in Chinese]. [20]Yakushev VS,Chuvilin EM.Natural gas and gas hydrate accumulations withinpermafrost in Russia.Cold Reg Sci Technol2000;31(2000):189–97.[21]Zang Enmu,Wu Ziwang.The degradation of permafrost and highwaynzhou:Lanzhou University Press;1999.[22]Zhang LX,Xu XZ,Ma W.Permafrost in Qinghai–Xizang Plateau and natural gashydrate.Nat Gas Geosci2001;12(1):22–6[in Chinese].[23]Zhao L.The Freezing–Thawing Processes of Active Layer and Changes ofSeasonally Frozen Ground on the Tibetan Plateau.A Ph.D.Dissertation of Chinese Academy of Sciences;2004.[24]Zhou YW,Qiu GQ,Guo DX,Cheng GD,Li SD.Chinesegeocryology.Beijing:Science Press;2002[in Chinese].[25]Zhu YH,Liu YL,Zhang YQ.Formation conditions of gas hydrates in permafrostof the Qilian Mountain,Northwest China.Geol Bull China2006;25(1–2): 58–63.W.Qingbai et al./Energy Conversion and Management51(2010)783–787787。