Geochemical characteristics and formation process of natural gas in Kela 2 gas field

fields, located in the frontal uplit zone and originate fromGeochemical characteristicsthe same source rocksl8!. Gas occurred in the Kelasustructural zone (such as in Kela 2 gas field) is relativelyand formation process ofdry in components and heavier in carbon isotope, but thenatural gas in Kela 2 gas fieldreasonable explanation to which has not been provided sofar. In this paper, by studying geochemical characteristicsZHAO Mengjun，LU Shuangfang，of the gas and source of Kela 2 gas field, we think that theWANG Tingdong & LI Jianabnormal character is related to source rock quality, bu1. Rscarch Lstiture of Perolem Exploraion and Development. Pet more imporantly, it has something to do with theroC hina, Beifing 100083, China;multistage gas pool formation process. We have alsoDaging Petroleum Institute. Anda 15 1400, China;discussed the possibility and process of the twice pool3. Southwest Petroleum Institule, Nanchong 637000，ChinaCorrespondence should be adressed to Zhao Mengjun (e mail: zmj@ formation of Kela 2 gas field.petrochina com.cn)IGeochemical characteristics of natural gas in KelaAbstract On the basis of a large amount of natural gas 2 gas fieldcomponents and the carbon isotope as well as some other( i ) Obviously drier in gas components. Theanalysis data in Kela 2 gas field, the geochemical character-component of natural gas in Kela 2 gas field iistics, source, origin, and formation process of natural gashave been dscussed The components of gas in the feld tend hydrocarbon (able D), and it contains litle non-bydro-to be“dry", and the drying eoffcient is close to 1.0. The carbon, for example, the content of CO2 is 0%- 1.24%，carbon isotope tends to be heavier, for instance, the average N2 is 0.6%- 2.84%; methane is the main componentof 8"C1 is -27.36%o and that of 8"C2 is -18.5%c Compre-among gaseous hydrocarbons, the content of which ishensive analysis shows that humic natural gas in the Kuqa 96.9%- 98.22%， whereas the content of ethane is as lowpetroleum systen comes mainly from Triassic and Jurassicas 0.31%- 0.53%，and there are little hydrocarbonsource rocks, and the contribution of Jurassic source rocks tocomponents heavier than ethane, so the drying coefficientl maybeimore than that of Triassic rocks. The maino poRC，es，， Feoto is close to 1.0. Compared with natural gas occurring incomposition lies in the fact that Kela 2 natural gas is the ac-Yiqikelike structural zone and frontal uplift in Kuqa de-Cumnulation of late production of humic souree Tocks, and it pression, Kela 2 gas tends obviousty to be dryer.is affected by the abnormal high pressure as well. Consider-Most of the gas in Yiqikelike structural zone aning the lhydrocarbon generating and structural history, we frontal uplift in Kuqa depression presents wet gascan regard the gas pool formation processes as twice fllingcharacteristic, for example, in Yinan 2 gas pool, the con-and twice adjusting (destroying),hat is,ne fllingd tent of C is 82.78%- -94.41 %, and averagely 89.03%; thedestroying process in the early Himnalayan movement and thecontent of C2+ is 2.15% -10.02%，on average 6.26%，illing and adjusting in the late Himalayan movement.Condensate gas pools occur in the north front of theKeywords: natural gas geochemlstry, source of natural gas, reservoir north uplift of the Tarim Basin,formation process, Kela 2 gas field.take, Yingmai 7, Yaha, Tiergen, etc., the content of C isKela 2 gas field lies in the west of the beeline anti-66%- 92.38%，averagely 71.8%- 88.31%; that of C2+cline zone of Kuqa depression, Tarim Basin, the south ofis 1.33%- 26.18%，on average 6.78% 17.25%. As forcondensate oil, its content is 31.8一185.6 g/m in Yangwhich is Baicheng sag, and the north is monocline zoneTriassic-Jurassic hydrocarbon source rocks devel-take and Yingmai 7 reservoir, whereas 547.6- 760.3 g/moped in Kuqa depession 5 Some scholars think that the in Yaha and Tiergen reservoir(in) Obviously heavier in carbon isotopecomposi-natural gas comes from Jurassic coal bearing source rockstion. Except for some gas in the Kelasu structural zoneaccording to the components and carbon isotope charac-teristics, and it is typical coal-derived ga[26]. kbut moreof Kuqa depression, 8"C1 value of the gas in condensate一39%，, and 8'C2 is -22%o I 25%oinvestigators think that the natural gas is related to both gas pooTriassic and Jurassic rockslL37.8, and the contribution t(fig. 1), which represents the typical humic gas character.the gas pool of Jurassie rocks is more than that of TriassicThe 8"C and 8'C2 values in Kela 2 gas field are obvi-rocks'8!. Gas in the Kelasu structural zone also comesously more positive, 8"C1 is -27.3%o- 31.1%o, andmainly from Triassic Jurassic source rocks, but it is quite8'C2 is -16.8%0- -19.4%00 (table 2), the difference be-different from that in Yangtake, Yaha, Yingmai 7th gastween 8"C2 and δ°C is as highas 7.9- 14.3.Chinese Science Bulltin Vol. 47 Supp. December 2002113中国煤化工MHCNMHGTable 1 Characteristics of natural gas components in Kela 2 and Kela 3 gas fieldsWell No.Depth/mFormation C1CC3iC4售nC4iCsnCsCO2C2+C/C-KL2 3499.87- 3534.696.90.3091.24 1.55 0.31 0.997KL2 3499.87 3534.698.05 0.40.94 0.60.40.996KL23888 389598.22 0.53 0.04 0.01 0.01 0.010.55 0.6.60.9943803- -380998.08 0.42 0.04 0.01 0.014 0.56KL33104.58 -3198.70.091.240.9933185- 319596.06 0.780.5 2.66 0.78 0.9923347- 3348.596.36 0.620.18 2.840.62 0.9943347- -336796.36 2.60.15 0.892.60.9743455- -35594.360.733.3 1.620.9923472- -347998.370.310.12 1.20.997Table2 Carbon isotope characteristics of components of natural gas in Kela 2 gas fieldHorizonDepth/n8"Cco, (%8'C (%e)8'C2 (%o)8'C; (%o)8'Cs (%c)Kela 201Kb3936- 3938-15.83-26.16-19.06-2214K2b3630 -3640-27.07-18.48-19.08- 20.313770- 3795-22.57-27.19-17.87-19.14- 20.55K,b4016- 4021-18.58-27.3219.00-19.5420.90Kela 2021472-1481-15.3728.24-18.86-19.1520.91Kela 2n.d.-27.-193803- 38027 8-1873498.87-3534.66-27一19.4n.d,3544 3550n.一30.8-17.717.1of organic matter and maturity9.10. The statistical results158*provided by Xu Yongchang (1994) and Dai Jinxing (2000)on large to middle sized coal formed gas fields show th:the ocurrence of 8C is -32%一- 38%, and that of是108C2 is -22%28%d9.10. On the contrary, the valuesof 8"C and 8"C2 are obviously heavier in Kela 2 gasBfield, averagely -27.36%o and -18.50%o respectively. Sthe gas in Kela 2 is regarded as over-mature coal-formedgaso, though such a heavy carbon isotope composition isAery rare even in high- to over-mature coal-formed gas.From the direct correlation of carbon isotopeposition of benzeneiamroaten Detreon istopes som:between pyrolysis simu--40-35-30-25lation production of source rocks and the natural gas, wcan further explain the relationship between them. Wes'C, PDB (% )have carried out a series of pyrolysis simulation experi-■Dabeil Ficld● Kela2 Ficld▲Kela3 Field 0 Yinan2 Fieldments on source rock samples of different types, such as* Yudong2 Field。 Yingmai7 Field + Yaha Fieldcoal, carbonaceous mudstone and mudstone, and withdifferent maturation levels in Kuqa depression, and have< Tiergen Field 。Tai2 Field o Tibei Fieldmeasured the δ ' C values of light hydrocarbon compo-S thatFig. I，Genetic classification of natural gas in the Tarim Basin (afternents in experimentalproduction. The result showsZhao et al. 2001). Al. Marine mature gas; A2. marine high toover-mature gas; B, marine humic-prone mature gas; cI, terrestrial wetcan the 8gas; C2. terrestrial dry gas.tion products be close to that of gas in the Kelasu struc-tural zone, such as Kela 2 (fig. 2). This suggests indirectly2 Source of natural gas in Kela 2 gas fieldthat Kala 2 natural gas comes from Triassic source rocks,but more directly, it shows that the gas in Kela 2 has a( i ) The gas in Kelasu structural zone related to the close relationship between high-over mature coal andcoal-bearing strata. The carbon isotope values of meth- carbonaceous mudstone. Can we think, however, that Kelaane and its homologue are mainly controlled both by type 2 natural gas comes only or mainly from Jurassic coal and114Chinese Science Bulletin Vol 472 Sunn Iecember2002中国煤化工THCNMHGcarbonaceous mudstone on the basis of the above analy-mation process(3) The differencee between 8'C and 8"C2 insis?As we have known, the source rocks of Triassic andKela 2 natural gas can reach as high as 10%0m (tableJurassic in Kuqa depression contain humic-prone andwhich suggests the mixed characteristics or the combina-humic oric matter respectively, and coal strata, marshtion of natural gas from different souresto. We think thatgante nanetcspeatcat.mudstone and lacustrine and swamp mudstone occur inKela 2 natural gas has the contribution of Triassic sourcethe source rock sequences with huge thickness, whichrocks. Because the OM type of Triassic is better than thatrepresents the "mixed" character to some extents. Thus,of Jurassic, and the average 8 c value of kerogensince the content of light hydrocarbon in natural gas from-25%c, and that of Jurassic is -23.5%5. As a result, thedifferent sources is different, it cannot reflect the actualbig difference of δ"C and 8°C2 is causcd by mixedsource of natural gas, the most component of whichaccumulation.methane. Therefore, the origin and source of gas deter-(4) The carbon isotopes corrclation of light-hydro-mined by geochemical indexes should follow the practicalcarbon shows that Kela 2 natural gas is related to MesO-geological situation.zoic coal and carbonaceous mudstone of Kuqa depression(fig. 2). We have known that Kela 2 natural gas almost20 Cdoes not contain the component greater than C2 (table 1),●Land the natural gasaccumulation process is obviously21twice pool formation and late gas accumulation (under-neath), so in the late gas accumulation, light-hydrocarbon22carbon isotope maybe have only reflected the characteris-tics of Jurassic coal which has relatively low maturity.Based on the above analysis, we can get the result-23that the natural gas in Kela 2 gas field is from Triassic andJurassic source rocks, which is consistent with the conclu--24◆Mudestonesion that humic natural gas in Kuqa depression comes■Carbagllitefrom Triassic and Jurassic source rocks8. On account ofthe distribution range, thickness, abundance and total gas25Natural gasgeneration intensity of Jurassic being larger than that ofTriassic， especiallyal gas pool formned in Eo20gene in Kuga depression maybe have been destroyed, and"Cxnxm(% )it is more effective for natural gas pool formation of thegas generated since Neogene, the maximum gas genera-Fig.2. Correlative diagram of carbon isotope composition betweenion intensity of Triassic since Neogene is only 40X 10( ii) Comprehensive analysis-Kela 2 natural gas be-m3/km?，much less than that of Jurassic, 75 x 108m/km2FS]. As a result, we think that the contribution ofing from Triassic-Jurassic source rocks.According tothe following comprehensive analysis, we can see thatJurassic source rocks to Kela 2 gas field is more than thatKela 2 natural gas comes mainly from Triassic and Juras-of Triassic source rocks.sic source rocks.3 Origin of natural gas in Kela 2(1) The maximum thickness of coal layer of Triassic( i ) Significance of geochemical characteristics ofand Jurassic in Kuga depression is about 50 m, whereasnatural gas in Kela 2. As stated above, the componentsthe dark mudstone source rocks can be as thick as moreof Kela 2 natural gas are quite “dry", and the drying coef-than 1200 m, so, to Kuga depression, the contribution officient is close to 1 .0; the carbon isotope is obviously high,coal layers should not be emphasized too much.(2) The values of 8"C2 and 8"C3 of Kela 2 gas areof other condensate gas in Kuqa depression are generallyboth more than -20%0, So the gas is considered as-33%o to一39%, and the difference between them iscoal-formed gas coming from the over mature Jurassiccoal layer. But in fact, the source rocks of lacustrine4.5%o -10.5%; the average of δ C2 is -18.5%o, -22%marsh mudstone of Triassic and Jurassic are mainly humic,25% in other condensate gases, and the difference isand it is normal to generale natural gas with heavier 8"C2,3.5% 6.5%0.So wshould not ignore the contribution of Jurassic andIf we discriminate the natural gas according to dry-udstone to the gas pool. As a,ing cofficient and methane carbon isotope, Kela 2 naturalresult, it is more proper to call the Kela 2 natural gas hu-gas is over--mature.ure. By means of the formula provided bymic gas. As for the heavy carbon isotope composition inDai Jinxingl, R。of Kela 2 natural gas is 2.90% -3.85%c.he Kelasu structural zone, we think it related to pool for-But this is not coincident with the value measured in out-Chinese Science Bulletin Vol. 47 Supp. December 2002115中国煤化工MHCNMHGcrop section as well as that determined on the basis of hydrocarbons generated before R, is less than 0.99buried and thermal history. The present R。values of Ju-23% of the total, and the amount of hydrocarbons gener-rassic source rocks in Baicheng depression should beated before R is less than 1.3% is 52% of the total. And1. 2%-2 3%. that of Triassic should be 1 8%- -2 6%[5] itaccording to Dai Jinxing' the 8'C is -35.05% whenis obvious that the maturity of the natural gas calculated isR。is0.9% and it is -32.79%o when R。is 1.3% So if themuch higher than the actual one of the source rocks,early hydrocarbon with light isotope has scattered, it willOn the other hand, the content of N2 in natural gas isresult in the enrichment of heavy isotope in natural gas inquite low (table 1), which suggests that the maturity ofthe late stage.source rocks could not be very high, because in mudstoneHere we use the simulation experiments on lacus-and coal strata, the content of N2 increases with the in-trine source rocks for reference to discuss further thiscreasing maturity, which presents a kind of situation thatproblem. Table 3 shows the evolvement characteristics ofthe content of N2 increases with the increasing values ofmethane carbon isotope in natural gas. If R。is more thander gas generating and expulsing at the same time, when3.0%，the N2 content should be more than 10.0%[121. Thenthe source rocks are under the pressure system. We canhow can we explain the dryer component and higher iso-see that the higher the evolvement, the more obvious thetope values of Kela 2 gas?gas carbon isotope fractionation, which can be as high as(ii ) Origin of the natural gas containing dryer con-25.77%o at the temperature step of 450- 500C". Thou-tent and heavier isotope.Kela 2 natural gas comesgh Kela2 natural gas mainly comes from humicmainly from Jurassic source rocks, secondly from Triassicrocks,n the other hand, the results of simulation experi-rocks. The discrimination of kerogen type of source rocksment explain that Kela 2 gas is the production of step ac-shows that the Triassic are type 1- I，and Jurassic arecumulation, that is to say, it is the production of higmainly I，secondly, are 0,5] The 8'C values ofevolvement stage .kerogen of these two sets of source rocks are very high,Table 3 Carbon isotope of natural gas whose generation and expulsionand the arrange of which for Triassic is 28% 22%o,oured at the same time under presse system”averagely -25%e; that of Jurassic rocks is - 26%o一- 2 1%o0TemperatureEYoNementnd most of them occur in -25%0 to about - 22%e aver8"C(%0) 8"C,(%) 8"'C, (%)R。(%)-23.5%e. So it is understandablenatural ggenerated during the high to over-mature stage will have a350- 40047.0840.0736.02 0.8 1.0high value of 8 3C, which is corresponding with the car--31.09-26.25-26.93 1.0 -1.5bon isotope fractionation principle of natural gas genera-450- 500-25.77-22.22-24.62 L.5- 2.0tion processU3.4The problem lies in the fact that the component car-(2) Influence of dissolving under abnormal pressureon isotope composition of Kela 2 natural gas is obvi-n natural gas components. As we have known, thously heavier than others in pools of Kuga depressionproduction of source rocks which are whether type II orwith the same source. On the other hand, even though R。typeI kerogen with R value being more than 2.0% stillof Triassic Jurassic source rocks is 1.8%- 2.5%, 8'C ofdoes not contain propane, this is mainly related to the wa-contains some ,pane. Butgenerative natural gas should be -29%- -32%o. As aresult, we think that the cause of dry component and higher dissolution occurring at deep buried depth when hy-isotope in Kela 2 gas lies not only in the high-overmaturedrocarbon generates, the maximum of the burial depth hashumic hydrocarbon source rocks, but mainly in the latereached over 7000 m. With the pressure increasing, theaccumulmulation, and the abnormal high pressuree as well.solubility increases, and the propane scatters. At the same(1) Kela 2 natural gas is the result of late accumula-ime, in the lateral compression basin, structural compres-tion. Considering the geological evolvement history of sion causes rock strain and porosity to decrease, and if thesource rocks in Baicheng sag and the pool formation his- pore fluicannot flow smoothly, it willtory of Kela 2 gas ficeld, we think that Kela 2 gas pool has crease of pore pressure and abnormally high pressure icaught the production of Triassic source rocks after R。isformed. During the late Himalayan movement, huge scalegreater than 1.3% and that of Jurassic source rocks after lateral compression has formed abnormally high pressureR。is greater than 0.9%. The production prior to these pe-in Kuqa depression. For example, Kela 2 gas field is SU-totnese.pe-beendestroyed completely except that a ltte per-pressure with the pressuree of 1.95--2.20 MPa, so weTriassic crude oil remains because of the destroying andcannot ignore the influence of theabnormally high pres-adjusting of oil and gas pool. According to the hydrocar- sure on the unusual content of component more than pro-bon generation model of type I kerogen, the amount of pane.1) Sun YOongge, Fan Sanfa, The hydrocarbon generating process of main source rocks and crude oil ternostabilitit in Tarim Basin, Researchreport under the National Key Science and Technology Programs (Grant No.99-111 -01-03-02). 2000.116Chinese Science Bulltin Vol. 47 Supp. December 2002中国煤化工MHCNMHGNeogene disposition, R。of Jurassic souree rocks is 1.8%-2.2%， that of tiassic source rocks is 2.2%- 2.4%. Allthese have established the foundation for the twice pool51 MPformation of Kela 2 gas field.(2) The oil-source rock correlation shows that 8Heptanesmall amount of oil in Kela 2 gasas field is the mixed oil1 HeptaneJrassic2. Methylcyclopentanefrom Triassic lacustrine marsh mudstone andcoal-bearing mudstone, for example, the，S"c ofcoal-formed oil is generally heavier Lhan -26%17.8, thatof Kela 3 oil from Jurassic is -29.9%, but that of mixedsource oil in Kela 2 is -27.3%; microbe degradationcompound in Kela 2 oil has been detected, which reflectsthat early oil from Triassic has been destroyed and thA人pool is flled lately with Jurassic oil mainly The reasonwhy the dry component and heavy isotope lies in thatKela 2 natural gas is the proi Late accumulation.(3) The characteristics of terpane show(breservoir bitumen in Cretaceous to Eogene reservoir arI MPrelated to Triassic source rocks, and we have detected theexistence of 25-nothopane (fig. 4); the values of Pr/nC17and Pr/nC18 of oil are no more than 0.35 and 0.2 respec-tively in Kuqa depression, but the reservoir bitumen inKela 2 reservoir are higher obviously, and the maximumsare 0.72 and 0.47, which present some biodegradationfeatures. We think that it is caused by biodegradation ofoil generated in Triassic source rocks and accumulatedduring the eaarly Himalayan movement. It should be no-ticed that the maturity of Kela 3 oil coming from Triassicis similar to that of the reservoir bitumen in Kela 3 andKela 2. The S/(S+R) ratio of C29 sterane is 0.41 in Kela 3oil, 0.42- 0.43 in Kela 3 reservoir bitumen and 0.40-Fig. 3. The infuence of pressure on light hydrocarbon of natural gas.0.522 reservoir bitumen. These reflect that theyAiming at this problem, we have caried Qut the gasare products from the same source at the same stage anddissolving simulation experiments on the high pressure,undergone biodegradation.hat is, increasing pressure of the normal gas to some(4) It can be divided into 3 groups for actually meas-value by high pressure I, keeping for some time, thenured inclusion uniform temperature, one group occurs atsampling respectively under different pressure conditions,n itions60 90'C, which accords to the depth of early hydrocar-and analyzing the influence of pressure on the natural gasbon accumulation from Triassic source rocks; anothercomponents. The experimental results are shown in fig. 3occurs at 100- 150C, which is corresponding to thewhich shows that prior to increasing pressure, the contentdepth of late hydrocarbon accumulation from Triassic andof normal alkane is relatively rich; and after increasingJurassic source rocks; the group whose temperature ispressure, the C alk ane component dses in the natural200C or so reflects probably theI thermal inci-gas when the pressure is high enough to change the facies.dent. Inclusion of carbene often occurs in early calciteSo we think that under abnormal high pressure, the actioncement, and line shape asphalt inclusion occurs in quartzthat causes the heavy hydrocarbon dissolve in water hassecondary increasing edge inner slot. All these asphaltaffected the natural gas components in some degree.inclusions are gray,rown or dark, with deep brown or4 Tracing of Kela 2 gas field formation processwithout fluorescence; a Large amount of gas inclusions( i ) Geochemical evidence of twice pool formationoccur in late dolomite and anhydrite vein, and xeno-morphic gas inclusion in dolomite is gray in color, withof Kela 21) The hydrocarbon generation history of sourcegray or without fluorescence; in anhydrite, the shape ofgas inclusion is pipe and gray in color, which suggests thatrocks in Baicheng sag suggests that at the end of Eogene,the flling process of oil and gas takes place after the ce-R。values of Triassic source rocks could reach 1.3%，thatf Jurassic reached 0.9%; due to the huge thickness ofment course.117Chinese Science Bulletin Vol. 47 Supp. December 2002中国煤化工MHCNMHGSandstone. KIbs ,3741 mm/z 191Ju M LJwJwhmnSandstone, Krbs; 3741 mm/z 177Sandstone, E, 3568 mFig 4. The characters of terpanes and norhopane of reservoir bitumen in well Kela 2.(ii ) Matching relationship between trap formationmines that such trap as Kela 2 mainly accumulates gas.and oil and gas generration. Kuqa depression structural(ili) The process of gas accumulation in Kela 2 field.traps had been formed in the piedmont front of mountainAlthough people have been cognizant of theimpor-under the compression in late Cretaceous to the end oftance of late gas accumulation to the gas pool formation inEogene, and the raps mainly collect oil and gas from Tri-Kuga depression, it cannot be used alone to explain thesic hydrocarbon source rocks. The existence of a lttlecharacteristics of quite dry components and heavy carbonamount of early Triassic oil in Kela 2 gas field shows thatisotope composition in Kela 2. Based on the oil andthe magnitude of Kela 2 trap was very smallthen.Thegas- source rocks correlation and reservoir geochemistrystructural distortion has been the most intensive since thestudy, combiningwith the hydrocarbon generation historyMiocene, and has formed frontal unfold upthrust andof source rocks and structural development history, wemany types of fold related to faultage in tum from north tocan generalize the process of gas poo! formation intosouth. This was the forrmation stage of main structures intwice filling and twice regulating (destroying) process,he Kuga foreland basin and Kela 2 trap20.2. On onethat is, the early Himalayan oil and gas filling and dehand, the late structuraldistortiondestroy and rearrangestroying processes and the late Himalayan flling and ad-the early oil and gas accumulation, and on the other hand,justing processes.it matches the gas generation and expulsion, which deter-(1I) Early Himalayan petroleum accumulation andChinese Science Bulle中国煤化工cember2002TYHCNMHGdestroying. In early Himalayan, that is, at the end ofof late Himalayan movement.Eogene, the maturity of Triassic source rocks near KelasuAcknowledgements This work was supported by the Project ofhad been quite high and R was about 1.3%; and that of"Sources of Oil and Gas and the Formation Function in Tarim Basin"Jurassic source rocks was rather low with R being aboutTechnaloey Proeram The etnloration o oil and natural Casin the Tarim0.9%. At that time, the structural rudiment had formed inBasinCretaceous and Eogene strata, and in Eogene, gypsumoccurred, which could serve as cap rocks. The oil and gasReferencespool accumulated in this period was mainly distributed in1. Zhao Mengjun, Zhou Xingxi, Lu Shuangfang, Tarim Basin rich inCretaceous and Eogene strata. Afterwards, affected by theJia Chenezao Oin Shenefei. 1 Oiming. Formation and distribu-early Himalayan movement, the faultage had beention of coal-formed petroleum in the Kuga foreland depression ofunlocked and surface water infiltrated, and the pool hadCoal (eds. Dai Jinxing, Fu Chengde. Xia XinyU) (in Chinese)，been destroyed by degradation.(2) Accumulation and rearrangement of natural gasZhao, M. J. Zhang, S. C The genetic groups of natural gas andconditionsformingas resvoir in Tarim Basio, China, Sympo-during the middle to late Himalayan movement. WithSum onPetroleum Geology of are Basinin China and Siberanthe upper strata depositing, the burial depth increases, andthe oil and gas has started to accumulate,，which istroleum Industry Press, 2001Qin Shengfei, Jia Chengzao, Tao Shizhen, Some characteristics ofated from Jurassic rocks with more than 0.9% Ro, as welloil and gasaccumulation in the Kudepression, Tarim Basin,as the natural gas which is generated from Triassic sourceCeologyn China RChinese) 2002.90 104rocks with more than 13% R。Because the main produc-gas- accumulating area in the Kuga foreland depression, Geologytion of Jurassic source rocks is gas, a lttle amount of oilScience (in Chinese), 2002Qin Shengfei, The genesis of abnormally natural gas in Kuga de-generated was mixed with biodegradation lacustrine oil,pression, Tarin Basin Explorer, 999 43):21.from Triassic hydrocarbon source rocks remaining athe major gas Teservoir in Tarim Basin, Natural Gas Industry (inearly stage, and shows the present biomarkers features ofChinese), 1999, 19(2): 38oil in Kela 2. Whereas the natural gas is mainly the prod-Zhao Mengjun, L Jian, Lu Shuangfang, The geochemical featuresucts of late generation, which caused the dry componentsof natural gas in Kuga depression and the:ussion on the gassource, PetroleumExploration and Development (in Chinese),nd heavy carbon isotope composition. Because the lateHimalayan structure movement at the end of N2k was veryXu Yongchang Genetic Theory of Natural Gas and Its Applicationintensive, Kela 2 structural traps uplifted, then the oil and0. Dai Jinxing. Zhong Ningning. Liu Dehan et al., Geological Basisgas accumulated in cretaceous anticline migrated again,and Key Controlling Factors of Large to Middle-sized Gas Fieldand adjusted into Kela 2 trap, thus Kela 2 gas fieldHouse, 2000, 11--2forrmed.1. Dai Jinxing. Qi Houfa, The relationship between C and R. incoal formed gas China, Chinese Science Bulletin (in Chinese),5 ConclusionsKela 2natural gas mainly contains hydrocarbonKroos. B. M.Linke, R.. Muller.B.etal.and methane from sedimentary organic matter: implications on thecomponents, the content of methane is 96.9%-98.22%,and it almost does not contain hydrocarbon with molecu-126: 219natural gas accumulations, Chemical Geology, 1995,lar size greater than ethane, the drying cofficient is close3. Prinzhofer, A.. Huc, A. Y, Genetic and post- genetic molecular andnations in natural gases,Chemical Geology,to 1.0; the 8CI in natural gas is -27.3%o- 31.1%0,8'3C2 is -16.8% 19.4%, so the Kela 2 natural gas4. Rooney, M. A., Cloypool, G. E., Chung, H. M. Modeling ther-bears the characteristics of dry components and heavyhydrocarbons, Chemical Geology, 1995. 126: 219.carbon isotope.Osborne, M.. Swarbrick, R. E. Mechanwasms that generatingAccording to the characteristics of components and1997. 81(6): 1023light hydrocarbon carbon isotope of Kela 2 natural gas,Song Yan, Can tectonic uplift bring about oerpressure?and combining with the comprehensive analysis of sourceOil &Gasogy (in Chinese). 20rocks distribution and potential, we think that the Kela 217. Cheng Japin ZAhap Wenzhtin YongealaPetroleuo form:natural gas comes mainly from Jurassic source rocks,troleum Exploration and Development 1998. 2544): 3secondly, from Triassic source rock18. Zhu Yangming. The geochemical characteristic of nonmarinehe reason whycomponents are dry and carbon iso-crude oils innese), 1997. 15(2): 26n Tarim Basin, Acta Sedimentologica Sinica (in Chi-tope is heavy in Kela 2 natural gas not only related to highZhangNai, The charactenistics of organic inclusions of Kela 2 andto over-mature humic source rocks, but related to Kela 2pool-forming in Kuga depression, Petroleum Exploration and De-natural gas being the product of late accumulation; in addition, the abnormally high pressure has affected heavy2Jia Dong, Lu Huafu, Cai Dongsheng ct al.,. Structureal analyses ofKuga oreland told-thrust belt along the northerm marginof Tarimhydrocarbon components at the same time.Kela 2 gas field was filled and adjusted (destrpyed) 21Tan Zuoji, Song Jianguo, Tertiary structure characteristicstwice, that is, the flling and destroying process in theevolution of Kuga foreland basin, Acta Petrolei Sinica, 1999，，that1s,"20(4) 7.early Himalayan movement and the flling and adjusting(Received August 9. 2002)Chinese Science Bulletin Vol. 47 Supp. December 200219中国煤化工HCNMH G