The key stage and moment of coalbed gas reservior evolution in the Qinshui Basin, China

ARTICLESis different from the test temperature, it could have sig- Chinese Science Bulletin 2005 Vol. 50 Supp. 92-98nificant contribution to the research on the CBM storage The key stage and momenthistorical course, resource prediction, optimization of favorable block, reservoir characteristic description, etc, of coalbwhen combined with the coal seam buried history, geo-gas reserviortemperature variation course, coal metamorphosed process evolution in the qinshuiBasin ChinaAcknowledgements The authors would like to express thanks to thesupport to the research project"Adsorption characteristic and ZHAO Mengjun, SONG Yan, SU Xianbo"gas-storage mechanism of coal seam"(Grant No. 2002CB211703). The LIU Shaobo, QIN Shengfei, HONG Fengauthors would also like to acknowledge the support and assistance from LIN XiaoyingCoal Research Institute(CCRD)1. Research Institute of Petroleum Exploration and Development, Petro-ReferencesChina, Beijing 100083, China;2. Henan Polytechnic University, Jiaozuo 454000, ChinaCorrespondence should be addressed to Zhao Mengjun(email: zmj@L.Bustin,R.M.,ClarksonC.r,geOlogicalcontrolsoncoalbedpetrochina.com.cnmethane reservoir capacity and gas content, International Joumal ofAbstract The evolution of coalbed gas reservoir is charCoal Geology, 1998, 38: 3-26.acterized by coalbed gas geochemistry and gas content On2. Zhong, L. w, Zheng, Y. Z, Yuan, Z.R. et al, The adsorption capa- the basis of burial history and thermal history, the formingprocess of coalbed gas reservoir and the gas accumulativebility of coal under integrated influence of temperature and prehistory in the Qinshui Basin are discussed in this paper. Thesure and predicted of content quantity of coal bed gas, Journal of difference of the thermal history, geochemistry characteristic,China Coal Society(in Chinese), 2002, 27(6): 581-585and gas accumulative history between Yangcheng andHuozhou areas shows that the formation of coalbed gas res-3. Zhao, Z. G, Tang, X. Y, Zhang, G M, Experiment and signifi- ervoir in the Qinshui Basin is controlled by the geologicalcance of isothermal adsorption of methane on coal under higher process in the critical stage and the critical moment. Thetemperature Coal Geology and Exploration (in Chinese), 2001. components and isotopes of coalbed methane are determinedby the stage at which the coal maturation reaches its maxi-94):29-30mum rank. The coalbed methane accumulative history isrelated to the temperature and pressure of the coal burial4. Scott,A.R,Hydrogeologic factors affecting gas content distribu- history, because the coalbed gas is mainly in adsorptive state.tion in coal bed, International Joumal of Coal Geology, 2002, 50: It is stated that the gas content in the coal seam is controlled363-387.by the moment when the coal seam is uplifted to the shallow5. Hu, T, Ma, Z. F, Yao, H. Q, Prediction of adsorption isothermsest posKeywords: burial history, key stage and moment, gas accumulativeith isosteric heat of adsorption, Journal of Nanjing University of process, Qinshui BasinTechnology(in Chinese), 2002, 24(2): 34-38DOI:10.136098zk00196. Zhu, B. Y. Zhao, Z. G, Basis of Interface Chemistry (in Chinese1 IntroductionBeijing: Chemical Engineering Press, 1996, 336-342Duong, D. D, Adsorption Analysis: Equilibria and Kinetics, LonThe geochemistry characteristic of coalbed gas is thekey aspect of coalbed methane geology, and has been endon: Imperial College Press, 1998, 149-17deavored by many researchers for a long time!-.Up to8. Cui, Y. J, Yang, X. L, Method of volume correction fornow,the origin of coalbed gas has been well under-multi-component gas isothermal adsorption, Coal Geology and Ex- stood.9, but few investigations relate to the relationshipploration(in Chinese), 2001, 29(5): 25-27.between the geochemistry characteristics of coalbed gas9. Dubinin, M. M, The potential theory of adsorption of gases anand the formation and evolution of coalbed gas reservoirThe difference between coalbed gas composition andapors for adsorbents with energetically nonuniform surface, methane 8C value is very great. This is caused by coalChemical Review, 1960, 60(2): 235-241maceral, coal rank, gas generating process, burial depth,10. Amankwah, K. A. G. Schwarz, J.A. A modified approach for es. temperature. and pressure,. The coalbed methane 8Ctimating pseudovapour pressure in the application of the Dubinin-中国煤化工 atural gas with similarmatuAstskhove Equation, Carbon, 1995, 33(9): 1313-1319CNMHGmaturity, the isotopicion-diffusion-migration,Received March 9, 2005:June 30, 2005) the generation of secondary biogenetic gas, and the iso-Chinese Science Bulletin Vol 50 Supp, December 2005ARTICLEStopic exchange reaction between CO2 and CH 6.8, 10-12The Shanxi Formation of Lower Permian: The Forma-The gas content is controlled by coal rank, burial depth, tion is 32.79-71.64 m thick, averagely 45 m, and consistthe lithology of coal roof and floor, and the fault 3-7.of sandstone, sandy mudstone, mudstone, siltstone, andBased on the study of burial and thermal history, the 3-4 coal seams, in which No. 3 coal seam is the main ingeochemistry characteristic and the gas content in critical Yangcheng district with thickness of 3.50-7.70 m andstage and moment during the coalbed gas reservoir evolu- 5.63 m on average. No. 2 coal seam is the main intion are discussed, and the forming process of coalbed gas Huozhou district. The thickness of No. 2 is 1.50-5.60 mreservoir and the history of gas accumulation in the Qin- averagely 3.26 mshui basin are shown in this paper. Furthermore, it is con-Up to now, more than 70 coalbed gas wells have beencluded that the critical period is one of the most important drilled in the south of Qinshui Basin. A coalbed methanefactors on the gas geochemistry and gas content in the reservoir with high permeability and gas content is discovered in this district. The previous investigations on2 Geological settingcoal distribution, coal rank, gas content, coal adsorptionreservoir permeability, sealing condition, and hydrodyThe Qinshui Basin lies in the southeast of Shanxi namics indicated that the south of Qinshui Basin is anProvince, and covers 23 5x10 km:, The sedimentaryrocks include the Ordovician, the Upper Carboniferous optimal district for coalbed gas deve\//17, 18.20Benxi and Taiyuan Formations, the Lower Permian Shanxi 3 Sample analysisand Xiashihezi Formations, the Upper Permian Shangshihezi and Shiqianfeng Formations, the Triassic, and the15 samples of vitrinite reflectance and 5 samples of gasQuaternary in the Qinshui Basin. The Carboniferous Tai-nalysis are completed in this study. vitrinite reflectivityormation and the Permian Shanxi formais measured under the Leitz MPV-SP microphotometermain coal-bearing sequences, in which No 15. No. 3. and The composition of coalbed gas is carried out underNo. 2 are the main coal seams for coalbed gas exploration HP-5880GC. The methane dC value is measured usingdevelopment6. 18. 9)Finngan MAT-252 mass spectroscope, the packed col-The Taiyuan Formation of Upper Carboniferous: The umn of coalbed gas fractionation is Porapak Q, 2mxl/8Formation is 68.28-140.64 m thick, averagely 97 m, and SS6, the maximum temperature is 200C, and the risitconsists of limestone, mudstone, sandy mudstone, silt- rate of temperature is 8C/min The analysis errors are lessstone, sandstone, and 7-16 coal seams, in which No. 15 than 0.3%o when the analytical standard is compared withcoal seam is the main. The thickness of No. 15 coal seGBW04405 referenced gas. The measured and collectedis about 0.80-5.92 m, averagely 2.77 m. No. 9 coal sdata are shown in Table 1. The methane 8c values areand No. 16 exist locallygreatly different between Yangcheng district in the southNo 2 coalNo. 3 coalNo. I5 coalsource of date32.1-33.563539Hu et al. 2001well-30.129.63302999--3367-28.96--293299-35.7Yangcheng-299--34.75Zhang et al, 2000-38.69well-266-27.6Sihe coal minecoal mineFZ002-34.2neasured in this inves.FZ003TL003中国煤化工oal mine476HuozhouCNMHGXinzhi coalChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESof Qinshui Basin and Huozhou district in the west, which(4) Stage Iv was another slow subsidence stage in theindicates that the coal seams experienced the different Middle Jurassic period. The average depositional rate wasthermal evolutionabout 16 m/Ma(52 ft/Ma)in this stage. Because of the4 Results and analysisYanshanian Orogeny, the Qinshui Basin rudiment formedThe maximum subsidence exceeded 400 m4. 1 Burial history(S)Stage V was mainly an uplifting stage from the Late4.1.1 Geothermal gradient. Before the Jurassic to Cre- Jurassic to the present. The whole basin is uplifted fromtaceous Yanshanian Orogeny, the Qinshui Basin, same as the Late Jurassic to the Paleocene. The coal-bearingthe Ordos basin is not an absolute subsidence basin but aquences and their overburden were severely eroded. Thepart of the North China Craton Basin. The Middle Yan- secondary faulted basin developed in this stage owing toshanian Orogeny from the Late Jurassic to the Early Cretaceous is an important tectonic movement in North China the Himalayan Orogeny. The local district was filled byThe movement not only made an end of the evolutive histhe Neogene and Quaternary deposits and the maximumtory of North China Craton Basin, but also formed two subsidence exceeded 1000 m in the northwest basininterior basins, the Ordos Basin and the Qinshui Basin,Based on the burial history of the Qinshui Basin andwhich have different tectonic styles i9. The thermal his- the measured Ro of Taiyuan and Shanxi Formations, thetory and burial history controlled coalification, gas gen- burial history diagrams in Yangcheng and Huozhou dis-eration, and gas accumulation in the Qinshui Basin. The tricts are matching by BaSIN- MOD software. The stratathermal event in the Middle Yanshanian Orogeny from the in the two districts went through different sedimentary andlate Jurassic to the early Cretaceous is the most important burial process. The strata in the Yangcheng district were infactor that influenced coalbed gas generation and accu- a slow uplifting and erosion stage from the Late JurassicmulationThe geothermal gradient in the Qinshui to the Early Cretaceous and in a rapid uplifting and ero-Basin is given in Table 2.sion stage from the Late Cretaceous to the Neogene(Fig4.1.2 The burial history in Yangcheng and Huozhou 1). The strata in the Huozhou district were in a rapid upDistrict. The Taiyuan and Shanxi Formations in the lifting and erosion stage from the Late Jurassic to theQinshui Basin have experienced five evolution stages by Early Cretaceous and in a normal uplifting and erosionSang Shuxun et al. 24stage from the Late Cretaceous to the Neogene( Fig. 2)(1) Stage I was a slow subsidence stage from the LateCarboniferous to the Early Permian period. The averageD(■斷■depositional rate was commonly not more than 25 m/Ma(82 ft/Ma). At the end of the Early Permian, the maximburial depth of No. 15 coal seam was nearly 300 m1000(2) Stage II was a rapid subsidence stage from the LatePermian to the end of Late Triassic period. The averagedepositional rate was 80-100 m/Ma(263-328 ft/Ma)The burial depth of coal seams was rapidly increased. Thesedimentation center was in Yangcheng-Jincheng-Houma3000where the maximum burial depth of No 15 reached 4800m at the end of Later Triassic(3)Stage Ill was an uplifting and erosion stage in the4000Early Jurassic period. Because of the Yanshanian Orogeny,0=0the coal-bearing sequence was uplifted and eroded extensively with the maximum uplift exceeding 1000 m(3281Fig. 1. The burial and thermal history in Yangcheng districtTable 2 The history of geothermal gradient in the Qinshui BasinErGeothermal gradient(C/100 m)E-Qrapid subsidence locallyK2-NJ3-K1中国煤化工3CNMHG 3P2-T3C2-Pslow subsidence3Chinese Science Bulletin Vol 50 Supper2005ARTICLESD CPTJKEINOto 3. 73%, and the methane isotopes are heavier with 8cvalues from -29.%o to-367%, showing over mature gasThe No. 15 coalbed gas also has an over-mature feature,with methane from 95.7% to 99. 15%, N2 from 0.21%to3.85%. The methane 8C value is from -20.8%0 to3869%.At the end of the Triassic, No 2 coal seam was basi.cally on mature phase(Ro =0.8%0-1. 2%)in Huozhoudistrict. From Jurassic to present, No 2 coal seam was notaffected by the abnormal thermal event because HuoMountain Uplift uplifted greatly. The highest maturationreached at the end of the Triassic in Huozhou district,4000which is the critical period. The coalbed gas reservoir ispe/MEformed earlier than that in Yangcheng. The coalbed gascomposition of No. 2 is CH4 68.35%-99.35%, N2Fig. 2. The burial and thermal history in Huozhou district4.63%-30.87% and the methane 5c value is47.6%--51.7%4.2 Geothermal history and coalbed gas geochemicalThe differences of thermal history and coalbed gascharacteristicsgeochemistry characteristics between Yangcheng district4.2.1 Geothermal history. Hydrocarbon generation and Huozhou district indicate that the geological processexperienced two critical periods in the Yangcheng distriin the critical period controls the gas compositionof the south of Qinshui Basin( Fig. 1): The first was at the methane 8C value. For example, the Permian coal exend of the Triassic, in which the first peak of hydrocarbon perienced the highest maturation(Ro=0.8%-1. 2%)ingeneration formed with the R, value of the coal from 0.9% Huozhou district in the Late Triassic. Therefore, theto 13%: the second was from the late jurassic to the methane 8 c value is light and N2 content is high.WhileEarly Cretaceous, in which the second peak of hydrocar- the coal in Yangcheng district experienced not only thebon generation formed owing to an abnormal geothermaevent in the Yanshanian Orogeny with the Ro value of the coalification in the Middle Yanshanian Orogeny, theoal from 2.4% to 4.2%. The first peak of hydrocarbon maturation is high (Ro=2.4%6-4.2%). Therefore, thegeneration was at the end of the Triassic with the Ro value methane 8C value is high and N2 content is lowof the coal from 0.8% to 1.2%. There was no more coal 4.3 The history of coalbed gas accumulationmaturation or the second peak of hydrocarbon generation The history of gas accumulation in the middle andfrom the Late Jurassic to the Early Cretaceous in HuozhouDistrict, because Huo Mountain Uplift was continuously past/251. Because the coalbed gas is mainly adsorbed onuplifted in this stage and the regional thermal event didnot affect the coal seams in this district(Fig. 2)coal surface, only the adsorbed gas is discussed in this4.2.2 The critical period. The geological period inwhich the coal maturation reached the maximum rank is 4.3.1 The temperature and pressure at the critical mo-called critical period, which controls the gas content and ment. The critical moments include the end of Triassicfor the first peak of hydrocarbon generation when thethe geochemical characteristics of coallbed gas. Contrarily, burial depth of the Shanxi and Taiyuan Fcthe coalbed gas composition and the 8C of methane can reached the maximum depth, the end of Early Creindicate the thermal history. The coalbed gas reservoirthe south of Qinshui Basin experienced the abnormalfor the second peak of hydrocarbon generation when thethermal event in Middle Yanshanian Orogeny and reached temperature is highest, the moment when the burial depththe maximum maturation of r. value from 2, 4 to 4. 2%of the coal seams is shallowest, and present (Table 3). TheTherefore, the Middle Yanshanian Orogeny is the critical temperature and pressure at these critical moments controlperiod of coalbed gas reservoir formation. The geochem-the coal adsorption capacityistry characteristic of coalbed gas in this district is con- 4.3.2 The history of coalbed gas accumulationtrolled by the geological process in this period Compared (1)R于V凵中国煤化工 on capacity and temwith Huozhou coalbed gas reservoir, the coalbed gas res- peratu-orptionapacity Iservoir in Yangcheng district is lately formed. Therefore, mainlyCNMHNo. 3 coalbed gas in Yangcheng is mainly made up of adsorption capacity is controlled by the coal properties,methane from 94.49% to 99.28%, a little N2 from 0.25% temperature, and pressure,4. The adsorption capacity isChinese Science Bulletin Vol 50 Supp. December 200595ARTICLESTable 3 The temperature and pressure at the critical rDistrictCritical momentNo, 2 coalNo. 15 coalend of triassic141℃end of Early Cretaceous256℃/3774MPa262℃876MFend of Neocene30℃/5.5MPa32℃/60MPal15℃1.62MPaetaceous140℃/357MPa25℃/15only controlled by temperature and pressure when coal(2)The history of coalbed gas accumulation. Theformed and reached certain maturity. The adsorption ca- history of No. 3 coalbed gas accumulation in Yangchengpacity is lower with temperature being higher and pressure district is shown in Fig. 4. The burial depth of No. 3 coalbeing lower. Under the normal geothermal gradient, the seam was about 4 000 m at the end of Trassic. Because ofcoal adsorption capacity reaches its maximum at about the temperature influence, the adsorption capacity is low1500 m burial depth due to the influence of temperature and about 15 m/t. The middle Yanshanian Orogeny is theand pressure, and has a decline trend with the burial depth geological period when the coal experienced the highestover 1500 m2/. According to Zhao et al., the isothermal temperature, but the absorption capacity is also low.Theadsorption parameters of Well. JSI in Yangcheng district adsorption capacity reaches its maximum when the burialare: PL of No. 3 is 3.034 MPa and VL 39.91 m/t; PL of No. depth of coal was about 1500 m. Therefore, the current15 is 3. 184 MPa and VL 46.84 m/t28. According to Li et coalbed gas content is mainly controlled by the adsorptional., isothermal adsorption parameters of No. 2 in Xishan capacity at the shallowest burial depth in the geologicalarea of the western Qinshui Basin are: PL in Huozhou dis- history. The range of the calculated current adsorptiontrict is 2.76 MPa: Vi is 22.85 m/")capacity is 24-27 m/, averagely 26.68 m/t. The actualBased on a great deal of data, two curve lines showing No. 3 coalbed gas content is: Well. JSI is 21.97-27.17he relationship between the adsorption capacity and the m/t at 522.4-519.5 m depth, averagely 25.29 m/t;Wellburial depth for two different rank coals are established in JS2 is 20.16-26.70 m/t at 514.42-519.5 m depth,av-Fig. 3. In the figure, the solid part of the lines less than1000 m in depth is supported by the exploration data, and eragely 22.48 m/t; Well. JS3 is 15.2-21.91 m/t atthe dotted part more than 1000 m in depth is mainly hy- 510.0-518.0 m depth, averagely 17. I m/t; Well. JS4 ispothetical. The coal rank with Ro= 1.0% corresponds to 21.97-27 17 m/t at 524.6-529.56 m depth, averagelythe maturity at the end of the Triassic in the south of Qin- 25 25 m/t. The saturation of No. 3 coalbed gas reservoirshui Basin, and the coal rank with Ro=3.5% corresponds is high with 68%-98.2%in this district. The current ac-south of Qinshui Basin. Therefore, the gas accumulative due to the gas escaping by the diffusion e calculated valueto the maturity at the end of the Early Cretaceous in the tual coalbed gas content is lower than thehistory in Yangcheng and Huozhou can be conformed using the temperature and pressure at the critical momente Theoretical adsoption content in key periodo Gas content of actual measurement at present a.Gas content/m.tL Buried depth in key periode Key geological period2500%Geologic age/MaR。=35%Fig. 4. The gas accumulation history of No. 3 coal seam in YangchengTYHaf No. 15 coal is: WellFig 3. The relationship between No. 3 coal adsorption capacity and the JSI中国煤化工-60976 m depth,,ayCNMHG20.81-2359 m'/ at1)Li Jianwu, Zhang Xiaowen, Li Jing et al., The evaluation of coalbed gas exploration target in Xishan and Luan areas, Research Report, 199696Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLES609.95-613. 45 m depth, averagely 21.55 m/t; Well. JS3 calculated adsorption capacity is 8.05 m /t. The thicknessis 10.52-15.44 m/t at 603.42-605.6 m depth, averagely of the Neogene and the Quaternary is 350 m. The pressure12.70 m /t: Well. JS4 is 15.45-32.32 m/t at 615.5- has great effects on adsorption capacity because the pres620. 20 m depth, averagely 23.64 m /t. The saturation of sure at the end of the Neogene is not as much as that ofNo. 15 coalbed gas reservoir is relatively low in this dis- nowadays. Therefore, the end of the Eocene is the criticaltrict, only 38.7%-72.1%, because the sealing capability moment to decide the coalbed gas content(Figs. 5 and 6)of No. 15 roof is weaker than that of no. 3The current actual gas content is lower than the adsorptionThe depositional setting and coal maceral are similar capacity when the burial depth is shallowest because ofbetween No. 2 coal seam in Huozhou district and Nocoalbed gas escaping(Fipal seam in Yangcheng district. Therefore, the accumulaive history of No. 2 coal seam in Huozhou district can bediscussed according to the relationship between temperaHuozhou Areature/pressure and No. 3 coal adsorption capacity in Yangcheng district when Ro is 1.0%(Fig. 5). The burial depthof No. 2 coal seam is nearly 4000 m at the end of Triassiche adsorption capacity is relatively low with gas contentc Key period determininggas contenbeing about 17 m/. The current maturity of coal consistswith that at the end of the triassic. because there is nothermal event in the Middle Yanshanian Orogeny inHuozhou district. The adsorption capacity is highest with60gas content being about 20 m/t when the burial depth ofcoal seam is about 1500 m. The calculated adsorption capacity is only 7-8 m /t when the shallowest burial deptAge/Mais only 300 m in this district. The calculated gas content ofNo. 2 coal seam is relatively high with 12-17 m/t, aver-Jincheng Areaagely 14.83 m/t, because of the Quaternary deposits. Thecurrent actual gas content is 2. 75-8.16 m/t, averagely5.07 m/t. Thereby, the saturation of No. 2 is very low,only19%-55%easurement at presentO Buried depth in key pericKey perioKey geological period5450501000=Age/MFig. 6. The map showing the key period that determines the gas contentat present in Houzhou area and in Jincheng area of Qinshui Basin.Geologic age/Ma5 ConclusionFig. 5. The gas accumulation history of No. 2 coal seam in Huozhou() The coal seams in Yangcheng district in the south ofQinshui Basin and Huozhou district in the west experi4.3.3 The critical moment controlling coalbed gas con- enced the different burial history. There were two hydrotent. The burial depth of coal seam is shallowest at the carbon generating phases at the end of Triassic and theend of the Neogene in Yangcheng district, only 550 m, and Late Jurassic to Early Cretaceous in Yangcheng district,the Quaternary deposits is only 50 m. The pressure has a and the coal maturity reached the highest level because oflight effect on adsorption capacity because the pressure at the abnormal thermal event in Middle Yanshanianthe end of the Neogene is as much as that of nowadays. Orogeny. There only went through one hydrocarbon genThis shows that the current gas content had been decided erating phase at the end of Triassic and there were no ab-at the end of Neogene. Therefore, the end of the Neogene norma中国煤化工 u districtis the critical moment when the coalbed gas reservoir(2)hich coal maturationformed(Figs. 4 and 6)C Gcal period to controlThe burial depth of No. 2 coal seam is shallowest at the the geochemical characteristics of coalbed gas. The com-end of the Eocene in Huozhou district, only 150 m. The position of coalbed gas and the methane 8C value canChinese Science Bulletin Vol. 50 Supp. December 2005ARTICLESalso reflect the process of coalbed gas reservoir formation. 12. Qin Yong, Tang Xiuy Ye Jianping et al, Discussion on theThe geochemical characteristics of coalbed gas inbution and genesis of stable carbon isotope composition ofHuozhou district indicate that the coalbed gas reservomethane in China, Journal of China University of Mining Tech-as formed in the early stage; while in the south of Qnology(in Chinese), 2000, 29(2): 113-119hui basin it was formed in the late stage13. Zhang Shengli, Chen Xiaodong, Geological controls on content and() There is a direct relation between the coalbed gasproductability of coal seam gas, Natural Gas Industry (in Chinese),accumulative history and the coal seam burial history and1997,17(4):15-19he forming process of coalbed gas reservoir. Coalbed gas 14. Zhao Qingbo, Potential evaluation parameters and exploration dicontent depends on the critical moment of gas accumularection in a coalbed gas region, Petroleum Exploration Devel-tive history. That is to say, it is the moment that the burialopment(in Chinese), 1997, 24(1): 6-10depth of coal seam is shallowest. The end of Neogene is 15. Sang Shuxun, Fan Bingheng, Qin Yong et al, Conditions of sealingthe critical moment when coalbedreservoir in thesouth of Qinshui Basin formed, and the end of Eocene isand accumulation in coalbed gas, Oil Gas Geology(in Chinese),the critical moment to form coalbed gas reservoir inlogical analysis, Natural Gas Industry (in Chinese), 2000, 20(4)Acknowledgements This work was supported by the National 973Coalbed Methane Project( Grant No. 2002CB211705) and InternationalTechnology Cooperation Project( Grant No. 2004CB720504)17. Fan Shengli, Analysis of coalbed gas in the south part of QinshuiBasin, Natural Gas Industry (in Chinese), 2001, 21(4): 35-38Referencesgas geology1. Rice, D. D, Claypool, G E, Generation, accumulation and re-Jincheng new mineral area, Coal Geology of China(in Chinese),source potential of biogenic gas, AAPG 1981, 65: 5-25.1999,11(3):28-342. Whiticar, M.J., Faber, E,, Schoell, M, Biogenic methane forma- 19. 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Qin Yong, Tang Xiuyi, Ye Jianping, Stable carbon isotope composi-中国煤化工 Pres20.5tion and desorption-diffusion effect of the Upper Paleozoic coalbedCNMHG in China(in Chinese),Bei-methane in North China, Geological Journal of China UniversitiesPetroleum Industry Press, 2001, 70-92.in Chinese),1998,4(2):127-13Received March 15, 2005; accepted July 25, 2005)chinese Science Bulletin Vol 50 Supp. December 2005