Migration and accumulation of natural gas in Kela-2 gas field

Migration and accumulationhydrocarbon source rock which migrated and accumulatedin the reservoir directly ). There are researchers who thinkof natural gas in Kela-2 gasthat, by paying attention to the dynamic process of naturalgas accumulation, the Kela-2 gas reservoir was formedfieldlate by the deep-basin gas mixing with a small number ofearly natural gas accumulated in the gas pocket zoneBut they still lay importance on the opinion that naturalWANG Zhaoming, WANG TingdongXIAO Zhongyao, XU Zhiming?, LI Mei& LIN Feng then migrated and accumulated directly into the reser-1. Research Institute of Exploration and Development, Tarim Oil Field voirsny, PetroChina, Korla 841000, Chi2. Southwest Petroleum Institute, Nanchong 637001, Chinan essence, all the above opinions focus on the hugeCorrespondence should be addressed to Wang Zhaoming (e-mailgeneration strength of hydrocarbon source rock andwangzhaoming@tz.cnpc.com.cn)the abundant gas source but have not explained intrinsiAbstract with the guidance of petroleum system theory, cally the reason why only Kela-2 gas field has the richestthe dynamic filling history of natural gas in the Kela-2 gas natural gas accumulation while all kinds of favorablefield is analyzed by using a large suite of oil and gas geo. conditions are at the same level. Oil and gas are mobilechemistry evidence in combination with the tectonic evolu-with the typical characteristic of flow ability. Therefore,tion history and reservoir evolution history. It concludes that they must be studied dynamically. This paper has reme-the Kela-2 gas field was formed by capturing the gas gener- died the shortcomings of the above opinions and put for-ated during the main gas generation period, while the latekerogen cracking gas contributed a little to the gas field. It ward that the Kela-2 gas field can be the largest gas resersuggests that the gas generated during the main gas genervoir in the mainland of china for the reason that it hastion accumulated in the early- formed wide -gentle anticline captured the natural gas generated during the main gaswhich is the necessary condition for natural gas to re-migrate generation stage of hydrocarbon source rock whereas theand enrich late to form the large-scale gas reservoir. The filling of late kerogen cracking gas is not so meaningful tonewest research shows that the filling history of gas in the the occurrence of the Kela-2 gas field. And we haveed with the natural gas accumulation in the early wide-gen. shown that the natural gas accumulation during the maintle anticline as well as late re-migration and enrichment of gas generation stage in the early wide -gentle anticline isthe essential condition for the re-migration and enrichKeywords: Kela-2 gas field, migration and accumulation of natural ment and the formation of the large-scale gas field.Thegas, carbon isotope, gas reservoir geochemistrypresentation of the opinion has made the theoretic modelfor natural gas to accumulate more perfectlThe Kela-2 gas field is located in Baicheng, xolhat 1 Geological backgroundAnticline Kela-2 is located in the east of strlies in Kuqa Depression, northern Tarim Basin. The pre- Kelasu, which can be subdivided into two structurasent exploration shows that the Kela-2 gas field, with the tions. that is. lower tertiary above-salt section andreserves of 2503.- >10 m and the recoverable reserves salt section in the vertical direction. The above-salt strucof 1877.79.108m, is the biggest one in the mainland of ture is a kind of prociding-off fault, which inclines gener-ral gas field is, of course, the most richly accumulatedally towards the north. The under-salt structure is a kindis really significant both theoretically and practically to of overlapping double structure which lies under the slid-the natural gas exploration of Kuqa Depression and even ing-off fault, and it is a kind of fault-anticline structurethe whole country to study the distribution regularity of formed by reversing along a strike fault of north-incliningnatural gas accumulation in this gas field. The value to and nearly nee trend, and it is gentle on the south partstudy the Kela-2 gas field is so important that it has at- and steep on the north part(fig. 1). The research on thetracted general attentionduring Xisan Orogeny. The tectonic movements of Kugainterpret the regularity of gas filling in this field. And all Depression were extremely strong at that time. Most m-the models were proposed statically to study the accumulation regularity of natural gas. They think that the Kela-2 profile recovery and is supported by the geochemicalstructure lies in the center of Mesozoic hydrocarbon sour- evidence of oil and gas. The first period occurred afterce rock of Kuqa and the strength of gas generation is great Lower Tertiary deposited, during which the rudimentarywith sufficient gas source so that a large-scale gas field, struaraceous and Lower tertiaryKela-2 gas field, occurs. Furthermore, they also empha-中国煤化工size that the natural gas is late kerogen cracking gas ofCNMHGcte 1)Zhao Jingzhou, The distribution regularity of oil and gas in Tarim Basin, the 9th Five-Year State Key Science and Technology Research Proy2) Liang Digang, The study on the oil and gas source and reservoir formation, the 9th Five-Year State Key Science and Technology ResearchProject, 2000Chinese S序嫂 letin Vo.47Supp. December2002107sediments of gypsum and mud rock formed the seal of the sics in vertical direction. Jurassic hydrocarbon sourcetrap. The reservoir consists mainly of the Tertiary dolos- rocks are mainly distributed in the Qiakemake Formationtone reservoir and the Lower Tertiary sandy con-glomer- J2 Q, the Kezileluer Formation ,K,)and the Yangxiaate reservoir as well as the clastic reservoir of Cretaceous Formation J2y). Among those, the Yangxia FormationBashijiqike and Baxigai Formations, which are the best and the Kezilenuer Formation can be classified into goodreservoirs in Kuqa Depression and the nearby area so far. hydrocarbon source rock because they have the characThe second period was the Kangcun Period during which teristics of bigger thickness and higher organic materiala wide-gentle anticline belt was formed. The Kela-2 abundance. Despite big thickness, the Triassic Kelamayistructure was located near the anticline top. The third Formation (T,k) is classified into poor hydrocarbon sourperiod was from the Kuqa Period to the present during ce rock because most samples have low organic materialwhich the south-north compressing movement of Xisan abundance that belongs to the scope of non-hydrocarbonOrogeny was strengthened further, the fault-propagation source rock. Compared with the Kelamayi Formation, thefold in the structure was developed further and the present Huangshanjie Formation (T h)and the taliqike FormationKela-2 structure was formed in the end(T;t)are the middle-good hydrocarbon source rock be-Kela-203 Kela-2012 Kela-204cause they have a higher organic carbon content and hydrocarbon generation potential(table 1)The hydrocarbon source rocks of coal-bearing strata2200E1水kmin Kuqa Depression are mainly distributed in the Jurassic2400Kezilenuer Formation and the yangxia formation Thecontent of organic carbon varies from 55% to 70%0; chlo-roform bitumen "A from 0.161% to 4.413%0.1 on av.2800Horizontal scale 0 1 23 kmerage, the total hydrocarbon content from 0.0419%to1. 1349%0,0.4167% on average. Pyrolysis hydrocarbonFig 1. Crosssection of east-west trend of Kela-2 gas resegeneration potential mainly varies from 20 to 40 mg/g,It must be pointed out that(-1)after Kangcun Fwith the highest value above 100 mg/gmation deposition, the compaction improved some mud- bon source rock, on with element analysis of hyocarbon isotope and pyrolysis hydrogen index analysis, it iswhich makes the natural gas accumulate efficiently; (1) known that H/C is mostly lower than 0.8, and the macergypsum deposit in the Jidike Formation improved sealproperty of Lower Tertiary and Cretaceous traps and als are mainly vitrinite and inertinite, liptinite is mostlylower than 10%, carbon isotope value of kerogen is oaccumulation; and (iii) for Jurassic has been buried lower than 100, and the one of mudstone and carbargillitdeeply, it is not exploited further. However, from the is relatively high. All these indicate that the kerogen tyviewpoint of exploration result of the Kuqa Depression of Triassic and Jurassic hydrocarbon source rock in Kuqaand other areas, Tertiary and Jurassic sandstones are alleservoirs of commercial value. such as the jurassDepression is of type LlYangxia formation and the ahe formation all these must(1. Thermal evolution history of hydrocarbon soube taken into account in the study of the natural gas mie rock. The hydrocarbon source rocks of Triassicgration and accumulationHuangshanjie Formation entered the threshold of oil genck evaluation and thermal eration during Early Tertiary, entered the medium-matureevolution historystage during the early period of Late Tertiary and enteredthe high-mature stage during the late period of Late Terti( 1) Hydrocarbon source rock evaluation. The ary. At present, the hydrocarbon source rocks of thehydrocarbon source rocks of Kuqa Depression include the Huangshanjie Formation have mostly reached the overcoal-bearing formation and mudstones that deposited in mature stage. The evolution characteristics of the Taliqikethe lake facies, which are mainly distributed in Triassic- Formation are similar to that of the huangshanjie FormaJurassic, and Jurassic abundance is higher than Trias-Table 1 Pyrolysis data of five sets of main dark hydrocarbon source rocks of mudstone in Kuqa Depressiondrocarbon source rockThickness of hydrocar- Organic carbon Pyrolysis potentialHydrogen indexbon source rock/mQiakemake Formation( q)11.54Kezilenuer Formation(T kz)中国煤化工423-45432-3202.15437-528Yangxia Formation(Ty)37-238HCNMHG443-557Taliqike Formation(T;t)6-210463-555Huangshanjie Fcormation(T:h)38-4466ere carried out from the data whose organic carbon is higher than 0.4 %, And all parameters are mean value. Tmas data comingfrom the mean value of each geological profileChinese Science Bulletin Vol 47 Supp. December 2002During the early period of Late Tertiary, main hydro- drocarbon generation stage. Moreover, Triassic and Crearbon source rocks in Jurassic did not enter the threshold taceous reservoirs have low maturity in diagenesis. Theof oil generation on the south of Kela-l-Kela-2-Dongqiu-originally accumulated gas is also difficult for further5-Yinan-2 area, while they entered the threshold of oil synchronous evolution in the reservoir. There are manygeneration in the north of those areas. During Miocene of factors that will influence the hydrocarbon component ofnatural gas. Therefore, although maturity and parent mateLate Tertiary hydrocarbon source rocks in most areas rial are decisive factors in many cases, it is inappropriateentered the mature to high-mature stage. At present, Ju- to only consider the two factors. However, there are tworassic hydrocarbon source rocks in Baicheng sub-De- problems to be paid attention to in the Kelasu area. Thepression enters the high mature stage. Three sets of hy- first one is that the products of C?* were mainly generateddrocarbon source rocks in Jurassic are slightly different in during the oil generation. If the oil and gas of this periodwere destructed (bad seal, including biodegradation andpresent maturity, with Ro 2.0%-2.2% in the Yangxia diffusive loss), the captured hydrocarbon is only the oneFormation, 1.8%-2.0% in the Kezilenuer Formation, and generated beyond the oil generation stage. But in respect1.4%-1.8% in the Qiakemake Formation. But in the east with the type three-parent-material, the quantity of gasthey are in the low mature to mature stage(table 2)generated beyond the oil generation stage is more thanthat of the previous period. Therefore, they can only capTable 2 R (%)of Kela-2 area during different geological periodsture dry gas. The second one is that deep burial can alsoHydrocarbon At the end of At the end ofchange the composition of hydrocarbon. For example,ource rockCretaceous Early Tertiaryformation water dissolves not only light hydrocarbons 4J, q1.8but also natural gas 5.6, which will result in the change ingas components. There is no propane to be measured inthe Sinian natural gas of Weiyuan and Ziyang, SichuanThe reason is that in the high evolution region, propaneTh0.722.6had experienced large overpressure and deep burial ingeological history in which the gas was washed inten3 Origin of natural gassively. The solubility of gas in formation water will increase with the increasing pressure. At the same time( 1) Hydrocarbon component characteristics of natualthough the solubility of methane and ethane also in-ral gas. Among the natural gas of Kela-2 structure, the creases, the loss by dissolution is relatively small due tocontent of methane is more than 96.9%, the natural gas their high contents in natural gas. The loss of chemicalcomponents are so dry that their dry coefficient is up to compound above propane in natural gas is relatively large0.99, and the content of hydrocarbon gas above ethane because of its low contents in original gas, which makes itand non-hydrocarbon gas is very low. The content of pro- difficult to be measured. It was found that there are manpane is less than 0 I% and in most samples, the content is compounds above propane to be measured in undergroundtoo low to be measured (table 3). Some researchers con- water. The situation of kala-2 field is similar to that of thetribute this phenomenon to coal-bearing strata or its Weiyuan and Ziyang area. Recently, many people thinkhigher maturity of Ro above 3%0. The authors have differ- that the overpressure in the Kela-2 field is concerned withon it. It is impossible for a large-scale gas deepreservoir formed which only have captured late gas, butnot have captured the gas generated during the main hyTable 3 Natural gas components in Kela-2 gas fieldWellFormationDepth/mC1(%)C2G%)C3(%)C4(%)C5(%)CO2(%)N2(%)C1C1C5Kela-2013600-36079762Kela-201E3630-364097861210.99Kela-2010.99Kela-203712-37149821Kela-203 K bs963-397598.190.650.99Kela-2E3499-35349690Kela-2E3499-353498.06Kela-23567-3572el3590-359197.14Kela-23711-371397.390.52Kela-2K0.420.02000Kela-23888-389Kela-2043925-393098.290.50I中国煤化工50.61Kela-3CNMHG0.99burial. Therefore, water washing is a very important factor water could not be measured. The component of naturalfor making the composition of natural gas very dry. The gas in the Kela-2 field is very dry, which is not fully dueonly shortcoming is that the content of hydrocarbon in to high mature Jurassic coal-formed gasChinese S序嫂弻 etin Vo.47Supp. December2002(1) The research on nitrogen. ZhanglI and Du9 1.8% to 2.2% according to thermal and burial history. Thehave stated the general origin of nitrogen. And foreign actual maturity Ro of Triassic hydrocarbon source rock isauthors have also suggested a new model to interpret the from 2.2% to 2. 4% and the calculated maturity Ro isorigin of nitrogen. Kroosslol and Littkelll reported that beyond it. By applying James'difference method of C,the content of nitrogen in natural gas increases with the and C, carbon isotopell4, the calculated maturity of naturalncreasing maturity in the argillaceous and coal-bearing gas is low in general, with Ro from 1.0% to 1.5%. And thehydrocarbon source rock. It was shown that the content of calculated maturity Ro is from 1.06% to 2. 17% accordinnitrogen in natural gas increased with the methane carbon to the Stahls coal-formed gas equation 5. According toisotope becoming heavier Nitrogen content of natural gas all the calculation results above, it can be concluded thatin the Kela-2 gas field is very low, less than 2.2% and the the geological condition of this region is different fromcontent of most samples is less than 1%o. According to the the geological situations where the above three equationsobservation result of high evolution basins in China, when are founded. Therefore, these equations should be appliedRo is from 2.5%to 3.0%0, nitrogen content is about from with cautions. However, in the meantime, the un-match8%to 17%0, such as Ziyang and Weiyuan areas; when Ro between actual maturity and calculated one shows that theis from 2% to 2.2%, the nitrogen content is 5%0, such as origin of natural gas in this area is complicatedPaleozoic coal-bearing hydrocarbon source rock in theTheoretically, there are many reasons for the biggerChangqing gas field. These observation results also sup- difference between C and C carbon isotope. The heavierport the conclusion made by Krooss and Littke that the methane carbon isotope confirms that the higher differencontent of nitrogen in natural gas increases with the in- ce is not due to the lower maturity. According to the comcreasing maturity in the argillaceous and coal-bearing prehensive analysis, it is considered that the mixture ofhydrocarbon source rock. This indicates that the natural nature gas from different sources contributed to this difgas captured in the studied area is mainly the one with ference. In combination with the geological condition, itmaturity Ro less than 2%. Therefore, the gas accumulated can be concluded that the methane and ethane in the natuhere is mainly the one generated during the main stage of ral gas have different sources. Ethane mainly comes fromgas generationJurassic hydrocarbon source rock and it is its heavy carbon(ii) The research on the gas carbon isotope. Gas isotope that reflects the Jurassic source. However,carbon isotope is relatively heavy in the Kela-2 gas field, methane comes from both Triassic and Jurassic hydrocarwith methane carbon isotope from.8 %co-25. 1 %o bon source rocks. The carbon isotope value of the presentand ethane carbon isotope from.4 %o-18.7%table methane is a mixing reflection of methane coming from4). Ethane carbon isotope of gas in the Kela-2 gas field is both the Jurassic and Triassic hydrocarbon source rocks. Itheavier and beyond the boundary between humic and must be pointed out that the natural gas does not includesapropelic gas. It seems that the gas in the region is relat- that generated by Jurassic hydrocarbon source rock withed to Jurassic coal-bearing hydrocarbon source rock. Ro less than 0.9% and by Triassic hydrocarbon source rockHowever, the gas maturity calculated according to with Ro less than 1.3%. The natural gas with light carbonmethane carbon isotope does not match with the actual isotope was lost, while that with heavier carbon isotopematurity of hydrocarbon source rock(table 5 ). For exam- generated during the late stage of Triassic and Jurassiby hydrocarbon source rocks is conserved and mixed. There.applying the Daijingxing's equation!213, the maturity Rfore, the natural gas shows a heavy carbon isotope and theof hydrocarbon source rock in the Kelasu area is frodifference of carbon isotope between C, and C2 is bi2.93%to 4.5%, but the actual maturity Ro should be fromFormationm四1(2=(c(=c(N(3499.87—3534.66Kela-2K3888-3895-2780.6Kela-2K0.56Kela-3310458-319879-188Table 5 Kela-2 and Yinan-2 natural gas maturity R (%o)calculated in different wayFormationDepth/mC,(% Daijinxing's equKela-2499-3534-273K8.8中国煤化工1.20KCNMHG63104-3198a)Daijinxing's equation: 83C= 14. 1 lgR-344: b)Stahls equation: 8 3C=8.6lgR-28Chinese Science Bulletin Vol 47 Supp. December 20024 Research on the formation of natural gas reservoirFrom the end of Kuqa Period to present(1 Homogenization temperature of inclusionHomogenization temperature of inclusion can be usedo judge the time of gas reservoir formation!].Thehomogenization temperature of inclusion shows twogroups of data, the first group ranging from 85 to 105(and the second group from 150 to 180_C Less analyticaldata may be up to 230.( The first group temperaturerepresents the Kangcun Period (the Middle XishanOrogeny), and the second group temperature representsRedistribution of natural gas accumulated earlythe time from the Kuqa Period(the Late Xishan Orogeny)and addition of natural gas of late high-maturityto the present. However, the second group temperature isAt the end of Kangcun Periodhigher than the present formation temperature, showing Fthat there is thermal liquid activity in the deep part ofcrust. The thermal liquid in the deep part could migrate upinto the reservoir. which could not exclude the fact thatthe gas generated from the Triassic could migrate up intothe reservoir. Although the inclusion formed during theNatural gas accumulation in wide.gentle anticlineEarly Xishan Orogeny could not be found in the reservoir,the biodegraded asphalt could only be formed during theEarly Xishan Orogeny according to the geological analy-At the end of Early Tertiarysis. showing that there was accumulation and destructioof oil and gas in this area during the Early XishanOrogeny.asphalt(i) Gas reservoir formation model. In combina-tion with the trap evolution history, it is concluded com-Oil and gas migrating along faultOil and gas destructionOil and gas migrating along strataduring the early periodprehensively that the formation history could be dividedinto three periods, that is, the accumulation and destrucFig. 2. Model of reservoir formation of Kela-2 gas fieldtion of oil and gas during the Early Xishan Orogeny, the mined. The reservoir asphalt is rich in gammaceranewhichXishan Orogeny (Kangcun Period) and re-migration and similar to the biomarker characteristics of Triassic sourceenrichment of natural gas duing the late Xishan Orogeny rock(fig. 3), showing that the asphalt is from Triass(Kuqa Period)(fig. 2)source rock. In addition, 25-nor-hopane is found in theDuring the Early Xishan Orogeny, that is, at the end asphalt, which shows that the asphalt has undergonearound the Kelasu- Yiqikelike structure belt was very high, at that time was destructed soon, therefore, it belongs towith Ro being about 1.2%0-1.3%, while the maturity of invalid accumulation of oil and gasAccording to the distribution of reservoir asphalt, itAt this time, the rudimentary structure was formed in the could not be formed until the Lower Tertiary had depositCretaceous and Lower Tertiary, and the gypsum in the ed and the gypsum in the tertiary had seal property.If theLower Tertiary could act as seal. The oil and gas accu- cap rock had not seal property, the trap could not accu-mulated during this period were mainly distributed in the mulate oil and there was no oil to be biodegraded to formCretaceous and Lower Tertiary. Owing to the influence of asphalt. The destruction during this time has a great influEarly Xishan Orogeny, the faults were opened and surface ence on the component of oil and gas accumulated latewater penetrated into the reservoir, therefore the oil and For instance, the oil and gas generated by Jurassic at Ro upgas were biodegraded and destructed. At the end of Lower to 0.9% and Triassic at R, up to 1.2%-1.3% have beenTertiary, the burial depth of Cretaceous and Lower Terti- destructed. The oil and gas accumulated late were theing gravel in Cretaceous was only 1139 m deep, and the compound of products generated by Jurassic at R. aboveformation temperature was lower, about 40. C whichdepo中国煤化工ion, the burial depthmost beneficial for bacteria to thrive. Therefore the biodegradation was intense. The destruction of oil reservoir temCNMHGImproVed, and the geo-Creased. When the geoat this period left behind some asphalt in Cretaceous and temperature exceeded 70., the thriving of bacteria wasLower Tertiary. According to the biomarker in the reser- inhibited and the biodegradation was debated gradually. Itvoir asphalt, the oil and gas source could be deter- was only at this period that the oil and gas began to accuChinese S序嫂 letin Vo.47Supp. December2002111mulate efficiently. Because the maturity of Triassic source med very late. It has captured the gas generated fromrock has exceeded Ro 1.2 %o-1.3 %, the mobile oil accu- Triassic and Jurassic sources during the main gas genera-mulated in both Cretaceous and Tertiary shows the char- tion period(Ro from 1.0% to 2.0%0). In particular, theacteristics of Jurassic source rock. The mobile oil is char- wide- gentle anticlines formed during Middle Xishanacterized by poor gammacerane and C28 sterane and rich Orogeny are beneficial to the enrichment of natural gasdiasterane, which is similar to Jurassic source rock, Gas isSecondly, the Kela-2 gas field could become a largecomposed of gases generated by Triassic and Jurassic scale gas accumulation because it has good Lower Tertisource rock. During this period, oil and gas mainly accuary Ek reservoir and Cretaceous k bs reservoir. Moreover,mulated in Cretaceous and Lower Tertiary sandstone re- this set of reservoirs could form a good reservoir-sealervoir of wide-gentle anticline as well as in Jurassiccombination combined with the upper seal. Owing to thedeep burial depth of most of this kind of reservoirs, italternative candidate for exploration in the coming futurewhere the burial depth of reservoir is shallower50References0.4090.30Basin, China, Beijing: Petroleum Industry Press, 19973. 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