Abnormal overpressure distribution and natural gas accumulation in foreland basins, Western China

Abnormal overpressurecoefficient of over 1. 8)is mainly caused by fluid thermo-activities and pressure release. On the other hand, over-distribution and natural gaspressures in the compress basins are seldom studied InChina, the phenomenon of overpressure is more obviousaccumulation in forelandand more complicated. But there are only a few studieswhich are concentrated on the area of nw sichuan Ba-basins. Western Chinasin. 8, Southwest China. The fundamental question aboutabnormal pressure, i. e. the causation of overpressure inSONG Yan. 2, XIA Xinyu, HONG Feng'this kind of basin, is not clear. Thus the relationship be-QIN Shengfei& FU guoyoutween overpressure and natural gas accumulation is difficult to determine1. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China;Based on the analysis of the relationship between the2. Institute of Geochemistry, Chinese Academy of Sciences, Guiyang generation of overpressure and the formation of natural550002. Chinagas reservoir in the main foreland basins in China, inondence should be addressed to Song Yan (e-mail: cluding the Kuga Depression in Tarim, the South Deprescn)sion in the Junggar Basin, and the NW Depression in theAbstract Abnormal overpressure occurs in the foreland Sichuan Basin, the influence of overpressure on the disbasins of Kuqa, South Junggar and West Sichuan in China. tribution of natural gas and the accumulation model ofThe pressure coefficients are high. Overpressure exists in natural gas accumulation in the abnormal high-pressurewide areas and various strata. The layers of overpressure area have been discussed in detail in this papoverpressure is controlled by the piedmont depression. The 1 Distribution of abnormal pressure in the forelandmechanisms of overpressure formation in the Kuga andbasinsSouth junggar Depression include disequilibrium compaction and tectonic compression; the importance of these two( 1 High pressure coefficient, wide distributive arefactors varies in different basins and in different stages of the and various strata. The Kuga, South Junggar and NWsame basin. Different models of gas accumulation are esta b-lished to explain the relationship between overpressure dis-sure dis- Sichuan Depressions are characteristic for widely laterallytribution and gas pool formation, and the influence of over- and vertically distributed overpressure with high pressurepressure on the gas pools. These models include:(j the coefficient. Drill stem tests and seismic data show thatviolent tectonic movement leads to the pool formation in overpressure in the above pressures is distributed mainlyoverpressure belt (Kela-2 gas field in Kuga); (i) the pres- in the central part of these depressions. For example,wellstime (Hutubi gas field in southern junggar Basin);(LIl)through the pressure transfer the gas migrates and accumu- pression commonly drilled high-pressure; seismic datalates (Xinchang gas field in Western Sichuan Basin)predict that overpressure also exists in the sags betweenKeywords: foreland basin, abnormal pressure, genesis, natural gasthe above two belts; abnormal high pressure in the SouthaccumulationJunggar Depression occurs in the second and third tectonic belts from drilling data, while seismic data show thatNatural gas resource is abundant in the foreland overpressure exists widely in this depression; abnormalbasins of China, recently where several giant andhigh pressure exists in the north part of the NW Sichuandium-size gas fields are discovered, including the Kela-2 Depressionfield in the Kuga overthrust belt in the tarim basinVertically, overpressure exists in Mesozoic and Ce-Abnormal overpressure is very common in these basins. nozoic strata in these depressions. In Kuqa,the strataFor example, high-pressure with pressure coefficient of beneath the Eogene gypsum rock in the Kuqa Depressionover 2.0 exists in the Kuqa Depression, the south margin are over-pressure strata. In southern Junggar, high presof the Junggar Basin and the NW Depression of the S- sure exists in Upper Cretaceous and Eogene, except thatchuan Basin. Therefore, the relationship between over- the gas-producing layer in the Hutubi gas field is normalpressure and natural gas accumulation is highly con- pressure strata; seismic data show that high pressure existscernein Lower Cretaceous and Jurassic, which are not reachedMuch work has been done on overpressure in China by drilling. In NW Sichuan, high pressure exists in Jurasand abroad, which is focused on the extension basins, sic and Upper Triassic, which are the main strata in thissuch as the Yingqiong Basin[I-31in the South China Sea, depr中国煤化工Mexico Gulf Coast and the North Seal bl. The factor conbeneath thick low-pertrolling overpressure in this kind of basins is commonly mealCNMH Gressure in Kuga changesdisequilibrium compress, with also hydrocarbon genera- abruptly from static water pressure to high overpressuretion and clay mineral transformation. Studies show that in just in the Eogene gypsum layer, showing that this layer ofthis kind of basin, strong overpressure(with pressure gypsum related closely to the formation of overpressureChinese S序嫂 etin VoTable 1 Measured pressure data of wells in the foreland basinsHorizonPressure/MPaDepth of producing/mPressure coefficient60.5178Wenxingchang613.0Guan-6TTTTEEJEE4107.058.713093.5Kela-3Kela-346977Yinan -21.70Dongqiu-594.284325,51785,49Anjihai85.964159.0cheLo Yh-lLn-13oYt.Jurassic (mof Cretaceous(MPa)o Hu-2o Qi-8,(m)30001800chODa-l01000pinkness ofEL. jotFig 1. Abnormal pressure distribution in the depressions in foreland basins, West China.(a) Kuqa; (b)South Junggar; (c)NW Sichuan.In the South Junggar Depression, overpressure generally in a layer with high mud content. Such an instance alsooccurs beneath the thick Anjihaihe Fm. mudstone(Eo- app中国煤化工 I well of the same depresgene); although abnormal overpressure may occur aboveAnjihaihe Fm. in some places, the upper boundary ofCNMHGbe seen that the existenceoverpressure is also in thick mudstone. For instance, the of regional-distributed low-permeability strata is benefiupper boundary of overpressure in the Dunan-l well oc- cial to the formation and preservation of abnormal prescurs in the internal under 3530 m(Neogene), which is just sure, especially in the area of violent tectonic movement,Chinese Science Bulletin Vol 47 Supp. December 2002An-6with the area of piedmont depression in Kuqa, SouthInterval transit time/us Mud content (Junggar and Nw Sichuan(fig. 1). The first tectonic belt in1000the piedmont is generally free of overpressure. From thedepression to the slope that is away from the orogenic belt,overpressure diminished gradually. The reason is that thedepression in foreland basins is near to the orogenic belt,and the severe tectonic belt can bring about abnormalhigh-pressure. Besides, the high deposition rate in depression can bring about disequilibrium compaction then200Noverpressure. The mudstone developed in the thick sediments in the piedmont depressions is beneficial to thepreservation of overpressure. On the other hand, in thepiedmont overthrust belts, owing to the well-developedfault system and the obvious uplift, even if overpressuremight have occurred in geological history, and it cannot bopreserved. From the above, it can be seen that both thebeneficial causing geological factor and the beneficial00preservation condition are needed for overpressure.50002 Formation and evolution of overpressure in fore-land basinsKela-1ifferent kinds of mechanisms have been put for-Interval transit timeμsMud content (%ward to explain the formation of overpressure. Accordinto fluid status equations, the change of pressure of porefluids in strata is caused by the change of volume, temperature and amount of fluid Owing to the flux of porefluids, the above changes do not always bring about ab-normal pressure; only with the existence of low-permeability layers, which delayed the flux of fluids, can abnormal pressure occur. The mechanism of pore volumereduction to cause overpressure includes disequilibriumcompaction 9, tectonic compaction! o) and mineral deposition. the mechanism of fluid volume increase to causeoverpressure includes hydrocarbon generation 2. Themechanism of temperature increase to cause overpressureincludes the fluid pressure increasing with formation tem-perature, but in a normal subsiding process, the effect of2500temperature increasing to cause overpressure is just a fewpercents of disliquilibrium compression 3, 141, so unlessthere are abnormal thermo-fluid activities such a mecha3000nism is not important. The foreland basins in China arequickly subsided compress basins with high gas-genFig. 2. The relationship between abnormal pressure and lithology in erating potential, thus the main factors bring about over-pressure in these basins include disequilibrium compaction, tectonic compression and gas generationwhere the plasticity of low-permeability layer can preventDf the above different mechanisms, the importancethe formation of fracture thus it is good for the preserva- varies in different basins and in different stages of thetion of abnormal pressure. For example, the Ku- same basin and pressure coefficient also varies obviouslylugeliemu Fm.( Eogene)thick gypsum layer in Kuqa and In the following, the formation and evolution of overpres-Anjihaihe Fm. ( Eogene)thick plastic mudstone in South sure in the Kuga and South Junggar Depressions will beJunggar are the key factor for the forming and preservat- analyzeding of abnormal high-pressure in these two areas. On theFormation and evolution of abnormal pressurecontrathe nw Sichuan Basin, such a thick plathe中国煤化工layer does not exist, so that overpressure disappearedgradually from lower to upper strata, until normal geo- andCNMHGnain factor of generatinThe regional distribution of overpressure fits well time abnormality(which mainly reflects the abnormalityChinese S序嫂 letin Vo.47Supp. December2002of porosity, thus reflects disequilibrium compaction) of of Kuqa Depression (including Kelasu structural beltEogene mudstone occurred in a certain depth. Although occurred violent uplift. In the structure uplift area, that thethe depth varies in different wells, they are in the same fluid temperature became lower and the pore rebound inlayer. For example, the interval transit time of mudstone stratum can lead to fluid pressure in stratum became lowerin the Kela-I well leaves away from normal compression This can easily make ancient pressure disappear ( if therecurve from 1800 m depth; the anomaly of density also was not any other mechanisms that cause high pressure)occurs in this depth(fig. 3). The interval transit time The gas generation ceased as the stratum has been upliftedanomaly also occurs in the Kela-2, Kela-201 and Kela-3 and the temperature fell 4 16-19). From the end of the Neowells with the depths of 3200, 2800 and 3100 m in Eo- gene to now, the other mechanisms that cause overpresgene respectively, reflecting that disequilibrium compac- sure had ceased, so the main controlling factor should betion once in geological time in this area. In Eogene, the tectonic compressionstructure background of Kuqa Depression is of extensionThe tectonic uplift and compression have changedkind,and the overpressure cannot be formed by tectonic the former overpressure longitudinal distribution patternscompressionalThe overpressure in upper Tertiary disappeared in mostpart of the depression as the structure uplifted. The overInterval transit time/usDensity/g·cmpressure was mainly distributed in Lower Tertiary, Creta-ceous and Jurassic with big depth and high compressionThe Cretaceous sand body can form small traps and keephigh pressure due to the fault incision and its transfigura500Nktion which made the sand body disconnect in large areaanymore(1 )The origin and evolution of overpressure inSouthern Junggar Basin. The overpressure characteristics in Southern Junggar Basin are similar to those of1505NKuga Depression. That a well drilled in the north Tianshan showing a high interval transit time demonstratedthat Southern Junggar Basin existed disequilibrium com-paction. The difference lies in the fact that the connectivity in the Southern Junggar Basin is poorer than that ofKuqa and shows no obvious tectonic uplift. This helps toform and keep overpressure caused by disequilibriumimpaction in the Southern Junggar Basin. Calculation bythe equilibrium depth method showed that Tertiary hadFig. 3. The relationships between interval transit time, density and little difference between the calculated and measureddepth.pressure coefficients, while the Cretaceous had big differOt The overpressure produced according to the above ence. This proved that the overpressure in shallow partnechanism in distributed as follows: there is super over- mainly caused by disequilibrium, and the deep part ispressure in the lower permeable layer of Tertiary stratum, mainly of tectonic compression originoverpressure in Jurassic stratum, and little overpressure3 Relationship between overpressure and gas accuCretaceous sandstone because of its nice connectivity.mulation(2)Since Neogene, structure compression graduallybecame the main reason of producing overpressure in1) Controlling of abnormal pressure on gas poolKuqa Depression. Since Neogene, under the effect of formation. There is a close relationship between overHimalaya orogeny, the southern Tianshan thrust violently pressure and gas accumulation Overpressure has eithertowards Tarim Basin, and then Kuga Depression endured advantageous side or disadvantageous side for gas accustrongly compression in geological history. From the Ke- mulating. The former includes: (1)Overpressure layer canlasu structural belt in northern part of Kuqa Depression to act as a sealing layer. The overpressure layer can stop thethe depth of 4000 m in Tabei uplift, the horizontal com- upward migration of natural gas, thus gas pools form bepression is 80--120 MPa at large". The gypsum mudstone neath the overpressure layer (2)Owing to the pressurevas distributed all over the depression. The fluid dis- increase due to the hydrocarbon generation, the hydrocarcharged difficultly and inevitably caused abnormal high bon laver often accompanies with the overpressure belt.pressure. So, the structural pressure plays important roles Whe中国煤化工 cracking strength of thein forming and maintaining abnormal high pressurerockihen the pressure will reIn addition, since the end of Neogene, the most part 1CNMH GpS hydrocarbon migrate1)Song Yan, Wang Yi, Wang Zhenliang et al., The dynamics of natural gas migration and accumulation and its relationship to the formation ofnatural gas pool, The restricted report made by Research Institute of Petroleum Exploration and Development, PetroChina, 2000Chinese Science Bulletin Vol 47 Supp. December 2002from source rock into reservoir. (3)If the sandstone reser- basins. As shown in fig 4, the vertical distributions ofvoir had high pressure in its early stage, the porosity of abnormal pressure belt and gas layer are different in Kuqathe rock would not diminish under compaction. In other Southern Junggar Basin and Western Sichuan Basinwords, under the same pressure, the porosity of sandstoneIn Kuqa Depression, taking Kela-2 gas field as anith abnormal high pressure is higher than that of normal example, the gas layers are composed mainly of LowerpressureCretaceous, hydrocarbon source rock is middle-lowerThe disadvantageous influence of overpressure on Jurassic coal measures, and the regional cap rock is Lowergas accumulation includes: (I)If the pressure in the reser- Tertiary big set of gypsum and salt layer. The abnormalvoir is higher than that in source rock, it is difficult for high pressure occurs widely in the underlying strata ofnatural gas to enter the reservoir. That is, the abnormalhigh pressure blocks the migration and accumulation of gypsum and salt layer with a unique pressure systemnatural gas.(2) When the reservoir has abnormal high Junggar Basin is Ziniquanzi Formation in Eogene. Thecan easily happen. Theoretically, when the fluid pressurein strata is higher than the cracking pressure of the rock,ap rock is the Anjihai Formation thick plastic mudstonethe gas will leak. Apart from salt rock, gypsum and mud- in Eogene. The gas pool has normal pressure, while thestone, the cracking pressure of common rocks is equivaoverlying strata have high pressure, and the underlylent to the pressure coefficient of 1.96 91. In fact, when strata are expected to be high pressure. In the other wellsshowing no gas, the strata from Eogene to Cretaceous allfluid pressure corresponds with 70%-90% of its cracking have overpressurepressure, the natural gas will begin to leak. It is difficult tokeep and cap the gasMany gas layers have been discovered in WesternSo, to study the relationship between abnormal pres- Jurassic, in which the source rock is Upper Triassic coalSichuan Basin. The strata range from Upper Triassic tosure and the formation of gas pool, the key problems arto analyze the time and space matches between abnormal measures. It is short of a big set of plastic layers to bepressure belt and naturalgeneration, migration and regional caprock, which is different from Kuqa and Southaccumulation, and relative positions between abnormal ern Junggar Basin. In the north part of Sichuan Deprespressure belt and hydrocarbon source rock, reservoir and sion, the gas layers from Upper Triassic to Jurassic allthe route of natural gas migration, the relationship be- have abnormal high pressure. The pressurefficienttween peak time of natural gas generation and the over- reduces gradually from lower part to upperpressure distribution and evolution, the changes of ab-(11) Accumulation modelsnormal pressure during gas pool formation,etc(1) Model of Kela-2--- Accumulation with formai) Distribution of abnormal pressure and the rela- tion of overpressure in reservoir by severe tectonictionship between the pressure and gas pool in key foreland movements. Based on a research on the history of res0152.0250501.520(c)jakKtgSource中国煤化工CNMHGYiqikelikeFig 4. Relations between abnormal pressure and gas pool. (a) Kuqa Basin;(b)Southern Junggar Basin; (c)Western Sichuan Basin.Chinese S序嫂弻 etin Vo.47Supp. December2002ervoir -forming. it can be inferred that the formation timeof Kela-2 gas field should be the end Kuga episode ofNeogen when there was strong tectonic movement anduplift in Kuqa DepressionBefore Neogene, although there is overpressurethe Jurassic source rock strata while no overpressure inCretaceous reservoir, the mudstone in Upper Jurassic andhe bottom of Lower Cretaceous is able to prevent themigration of petroleum generated from Jurassic. AroundIIithe end of Neogene, with the strong tectonic compression,the formation fluid pressure increases quickly and fromIITTTthe jurassic strata to Neogene strata both become overpressure zones. The two following factors should bereached in case that the gases generated by source rocksbelow the zones could be accumulated within these overpressure zones:. 1) strong tectonic movement;.1. plas-tic rocks such as gypsolith or salt in overlying strTaking the Kela-2 gas field as an example, the strontectonic movement not only results in many large fractures that become the favorable channel for petroleum mition but also becomes the major driving force fordrocarbonBecause the strong tectonic uplift breaks the fluidpressure equibalance and acts as a"suction pumping", thegas accumulated in underlying strata migrates upwardsand accumulates again under the gypsolith cap rocks. Theexistence of plastic rocks such as gypsolith results in thefollowing facts:(I)the fractures made by strong tectoniccompression disappear in the flowing rock formation,providing the accumulation conditions for gas migratingby faults; (1) the fracture pressure of gypsolith is farer than that of sandstone and mudstone angeopressured fluid can hardly break though, whichfavorable to the preservation of gas pools2) Model of the Hutubi gas field in South JunggarOverpressure decreased then gas accumulated. Theanalysis on the Jurassic petroleum system in South Jungar shows that the forming time of Hutubi gas field is themend of Neogene and the gas generated by Jurassic sourcerocks migrates up by faults and then accumulates 20. It isemphasized that in north Junggar, only the ZiniquanziEntrape more, formed over-pressure gas poolformation pressure of Neogene in Hutubi gas fields isEntrape less, formed normal pressure gas poolnormal pressure, while the corresponding strata in neighboring well are overpressure. It is because the partial release of pressure in the background of regional overpresure results in the pressure difference between sandstoneSource bed Gas poolFaultreservoir and underlying source rock strata, which is advantageous to gas accumulation from source rock strata toreservoir. There is also overpressure between normal人[圈pressure reservoir and overlying mudstone strata, whichdirection Overpressurecauses the gas, during the process of migrating upv中国煤化工8asystemto be prevented under overlying overpressure mudstonecap rocks and then be accumulated. These conditions areCNMHGthe crucial factors for the formation of Hutubi gas field. If Fig. 5. Accumulation model of the Hutubi gas field in South Junggarthe closed condition in the relief channel of overlying (a)overpressure system in the early stage: (b)relief of shallow strata; (c)mudstone cap rocks is excellent, then the seal condition in discharge hydrocarbon of deep drataChinese Science Bulletin Vol 47 Supp. December 2002cap rocks is favorable and the hydrocarbon expulsion Referencessibe that enorousng s uae to ks itS muga ed in is pai . Mia nid h ocuzrhenie oh oi amigas in Chinese se ing:pressure zones and large-scale gas fields are formed. ConGeological Publishing house. 2000. 156-215versely, if the closed time of relief channel is late, the2. Hao Fang, Zou Huayao, Jiang Jianqun, The dynamics of oil andgas formation and its research progress, Earth Science Frontiersclosed condition or the leakproofness is bad and the hyGin Chinese), 2000. 7(3): 11drocarbon expulsion intensity of underlying source rocks 3. Zhang Qiming, Liu Funing, Yang Jihai, The overpressure systemshould be low, and only a few gas can be accumulated inand oil and gas migration in Yingehai Sea Basin (in Chinese),these normal pressure zones and small gas fields can beChina offshore Oil and gas(Geology), 1996, 10(2): 65Law, B. E, Spencer, C. w, Abnormal pressure in hydrocarbonformed. Correspondingly, when there was overpressure innvironments, in Abnormal Pressures in Hydrocarbon Environ-source rock strata, and the strata catch abundant gasments: AAPG Memoir (eds Law, B E, Ulmishek, G. F, Slavinoverpressure would appear again and high pressure gasV.L),1998,70:l-1pools could be formed in shallow layers, otherwise normal 5.Leach, W G, Distribution of hydrocarbons in abnormal pressurepressure gas pools could be formed, such as Hutubi gasvelopments in Petroleum Science, 38(eds Fertl, W.H., Chapmanfield(fig. 5)R E, Hotz, R. F), New York: Elsevier, 1993, 391"-428Compared with the Kuqa foreland basin, the traps in6. Cayley, G. T, Hydrocarbon migration in central North Sea, inSouth Junggar did not experience severe tectonic move-Petroleum Geology in Northwest Europe(eds. Brooks, J., Glennie,ments as Kuqa did in the end of Neogene, thus the condiK. W.. London: Graham and Trotman, 1987. 549--5557. Xu Guosheng, The paloe-pressure control during the formation oftion was not benefit to the fluid movement in overpressurethe secondary gas pool"in central part of Western Sichuan Basinbelt and the upward migration of natural gas. That could(in Chinese), China Offshore Oil and Gas( Geology), 1995, 9(5):be the key factor of why the accumulation condition inSouth Junggar is not as good as that of Kuqa8. Xu Guosheng, Liu Zhongping, The formation and numericalsimulation on strata ancient pressure in Western Sichuan Basin (i(3)Model of the Xinchang gas field in the Nw SChinese), Experimental Petroleum Geology, 1996, 18(1): 117chuan---The transfer of overpressure in deep strata with 9. Osborne, M.J., Swarbrick, R E, Mechanisms for generatingnatural gas accumulation. Overpressure exists fromUpper Triassic source rock to Jurassic producing beds in 10. Sleep, N H, Blanpied, M. L, Creep, compaction and the weakthe nw Sichuan basin, but the pressure coefficient dropsfrom 2.0 to 1.2 gradually, indicating a process of over-Tigert, V. Alshaieb, Z, Pressure seals"-"- Their diageneticpressure transfer. The source of gas and the source of highanding-patterns, Earth Science Review, 1990, 29: 463pressure coexist in Upper Triassic, and the tectonic12. Barker. C. Calculated volume andthermal cracking of oil to gas in reservoirs, AAPG Bull, 1990, 74movement formed faults that reached the gas source andhigh- pressure fluids in the deep strata. As natural gas 13. Luo, X.R., Vasseur, G, Contributions of compaction andmigrates upwards, overpressure also occurs in the shallowquathermal pressuring to geopressure and the influence of envistrata. If the preservation conditions were good in theronmental conditions. AAPG Bulletin. 1992.76: 155014. Xia Xinyu, Song Yan, The influence of temperature on fluid presstrata, both the gas pool and overpressure could be formedsure during subsidence and uplift, Petroleum Exploration and Deotherwise, natural gas escaped thoroughly or accumulatedvelopment (in Chinese), 2001, 28(3): 8on a small scale, and overpressure was also difficult to be 15. Jia Chengzao, He Dengfa, Lei zhenyu et al. Petroleum Explora-preserved.dustry Press. 2000. 79-834 Conclusions16. Zhenbingqinci, Geological Modeling of Oil Traps(translated byAs mentioned above. the common features of over-Tong Xiaoguang, Jia Chengzao ), Beijing: China Petroleum Presspressure and natural gas accumulation in the foreland1991.112asins in the middle and west China are as follows:(l17. Hunt, J M, Petroleum Geochemistry and Geology, 2nd ed, NewOverpressure in the foreland basins occurs widely in bothYork: Freeman. 1995. 296"-298lateral scale and vertical strata, it laterally appears beneath 8. Swarbrick,R.E,Osborne,M.J,Mechanisat generate abthe thick low-permeability strata, especially in the centernormal pressure: An overview. inof depression and super overpressure easily appears inarbon Environments, AAPG Memoir(eds Law, B E, UImishekthese areas. (1) In the episode of the subsiding stage ofG.F.,Slaⅶin,v.L),1998,70:13-34.foreland basin, the disequilibrium compaction is the mostlyu, Song Yan, Fang Dequan, The influence on stratumimportant factor causing overpressure; the tectonicpressure during tectonic uplift and the origin of abnormal highpression is the most important causation at present.ressure of Kela-2 gas field (in Chinese), Natural Gas Industry,foreland basins, naturaf gas commonly accumulates2001,21(1:30overpressure system. The overpressure and faults caused20. Song Yan, Wang Zhenliang, Wang Yi et al, The Formation Coby tectonic compression determined the migration and latedition of Natural Gas Pool in Junggar Basin(in Chinese), Beijingaccumulation of natural gas. The plastic low-permeability中国煤化工cap rock is the key factor for the preservation of gas poolReceived August 20, 2002)nd overpressureCNMHGAcknowledgements This work was supported by the State Key Scietific Project on Natural Gas( Grant No. 99-110-01-02)and the NationalNatural Science Foundation of China( Grant No. 40102011)Chinese S序嫂弻 etin Vo.47Supp. December2002