Secondary biogenic coalbed gas in some coal fields of China

ARTICLESChinese Science Bulletin 2005 Vol 50 Supp 24-29ground and used as a new and clean energy source as wellSecondary biogenic coalbedas a high-quality chemical raw material. It is also believedthat the coalbed gas is a non-conventional gas. As a result,gas in some coal fields ofthe coalbed gas investigation has become a new branch ofacademic disciplinesChinaThe study of the genetic types of coalbed gas is one ofthehe important researches. Generally, the coalbed gas canTAO Mingxin", WANG Wanchun, XIE Guangxinbe divided into(microbial) biogenic gas and thermogenicLI Jinying, WANG Yanlong, ZHANG Xiaojungas3 Biogenic gas is formed through metabolic activiZHANG Hong, SHI Baoguang& GAO Bties of bacteria at the early depositional stage of the or-ganic matter, i.e. under thermal evolution degree of1.Key Laboratory of Gas Geochemistry, Institute of Geology and Geo- Ro C1 value of thermogenic methane according to the thermal compaction and coalification".evolution degree of the coal rocks(with Ro value from 0.87%In the study of the coalbed gas in the San Juan Basin,to 1.43%o), showing the characteristics of the secondary biAmerica, Scott et al. 7 have found that there exists a secgenic gas;(ii) the D value of methane ranges from -244%6 ondary biogenic gas in the basin, which is produced byto-196%;(iv)8C2 value ranges from -267%c to -15.9% bacteria metabolizing wet gas compunents, n-alkanes andand8C3 value ranges from -10.8% to-25.3%, indicating other organic compounds during coalification after coalthat the heavier hydrocarbons have a thermogenic origin; (V) formation. Geochemical compositions of the secondarythe content of COz is very low, and 8 Ccoz value changes biogenic gas are similar to those of the primary biogenicgreatly, reflecting a characteristic of secondary change; (vi)8 SN2 value ranges mainly from-1% to +1%, indicating N2 gas except for the difference in the thermal evolution de-derived significantly from air. The negative linear correlation grees and burial depth of coal( source)rocks, namely thebetween the contents of N2 and CH4 reflects the activity of vitrinite reflectance(Ro) value of coal rocks ranges frombacteria bearing surface water infiltrating into coal beds. 0.30% to 1.50%+ for the secondary biogenic gas, over theThe comprehensive tracing indices show that the coalbed gas typical Ro value for the primary biogenic gas formationin the studied areas is the mixed gas of primarily secondary (R<0.30%). The formation conditions for the secondarybiogenic gas and a part of remnant thermogenic gas. The biogenic gas are coal beds uplift to shallow subsurface anduplift of coal beds and the development of faults in the stud- metabolic activity of bacteria introduced by surface watersied areas create favorable conditions for the formation of the moving through permeable coal bedsSmith and Pallasser 8l and Ahmed and Smith( % studiedKeywords: coalbed gas, secondary biogenic gas, thermogenic gas, the coalbed gas in the Sydney and the bowen basins ofdry gas, 8 C, 8D, 8N, China.Australia and proposed that the coalbed gas was formedDOI:10.1360/98zk0001through CO2 reduction pathway of the metabolic activityof bacteria, and was part of the secondary biogenic gasThe coalbed gas refers to an in-situ generated and re- Kotarba0 thought that the coalbed gas in Upper Silesianservoired gas dominated by methane in coal beds, also and Lublin basins of Poland was partly formed throughcalled gas in the coalmine, Because of the great disaster CO2 reduction pathway, and possibly also belonged to thecaused by gas in coalmine production, the coalbed gas has seccenic gasbeen regarded as harmful gas for a long time. In 1980s, it中国煤化工 s indicate that the sec-was in America that the ground exploration and develop- ondsCNMHGCtype of the coalbedment of coalbed gas were first carried out successfullye……ym… energy strategiesThis forms a new rising coalbed gas industry and proves The identification of genetic types of coalbed gas, es-that the coalbed gas could be recovered on a large scale on pecially shallow strata coalbed gas is complicated by theChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESbiodegradation of thermogenic wet gas components al- 0. 14%. A good negative liner correlation exists betweenready in the strata, which changes the contents and carbon the contents of methane and nitrogen of the desorbed gasisotopic compositions of some components of the ther- from the XS-02 well (Fig. 1)mogenic coalbed gas, and by the formation of secondarybiogenic gas, which alters the carbon isotopic compositionof coalbed methane. The genetic types of coalbed gas di-Enhongrectly affects the evaluation of coalbed gas resources.Therefore, it is not only theoretically significant, but espe-Linearity (Xinji)cially has important practical value for the identificationof genetic types and the evaluation of resources of coalbedgas to study the conditions of the secondary biogenic gasformation and to expound the geochemical characteristicsR=0.998and differentiation basis of the secondary biogenic gasThis paper will discuss the basic characteristics of theecondary biogenic coalbed gas found in some coal fieldof chinaCH4(%)The analyses of gas samples were made in the Key Fig. 1. N, and CHa contents variation correlation of the coalbed gasLaboratory of Gas Geochemistry, CAS. Carbon isotopiccompositions were measured on the MAT-252 GC-C-MS The main hydrocarbon component of the coalbed gasS-01 well is also methane, ranging fromthe international standard PDB with the precision better 67.40% to 94.29%, and the heavier hydrocarbons arethan #0. 25%o. Hydrogen isotopic compositions were mainly ethane, ranging from 0.04% to 0.37%. The drynessthe Delta Plus XP GC-C-MS system and index(C /C1-s)of hydrocarbons is from 0.995 to 0.999,hydrogen isotopic values were given relative to the inter- indicating that the gas also belongs to dry gas. The mainnational standard SMow with the precision better than non-hydrocarbon component is also N2, ranging fromt1. 5%. The components of the gas samples were analyzed 3. 62%to 30.60%. The content of CO2 is in the range ofon a MAT-271 trace gas mass spectrometer.1. 12% to 1.93% and that of Ar is from 0.06% to 0.43%1 Coalbed gas of the Xinji area of Anhui ProvinceThe content of Co is very low, from 0.01% to 0.05%. AThe Xinji coal mine, located in the middle part of the negative liner correlation also exists between the contentssouth margin of the Huainan Coalfield, Anhui Province, of methane and nitrogen from the XS-01 well(Fig. 1)The 8C, value of the coalbed gas from the XS-02south margin of the north China Platform The main coal well ranges from -54.7%0 to.3%6, with the averagebeds occur in the Taiyuan Formation(C3t) of the Carbon- value of -574%o, and that of the XS-01 well ranges fromIferous system, the Shanxi Formation(Pish), the Lower -50.7%0 to-58.7%, with the average value of-54.9%0Shihezi Formation(P x)and the Upper Shihezi Formation For gas of the XS-02 well,8C2 value ranges from(P2s)of the Permian system, and there are totally 45 coal16.1%0 to -26.7%, averagely -22.1%0; 8C3valuebeds. Gas samples were collected from desorption of coal ranges from -10.8%0 to -25 3%0. For gas of the XS-01well,8C2 value ranges from-15.9%0 to-23.6%0,avercores from the XS-02 well and the XS-01 well of the Xinji agely-20.7%o; and only one sample has 83C3 value ofmine24.2%0. It shows that the carbon isotopic compositions ofLI Characters of contents and isotopic compositions of wet gas components are heavier and have the characteris-coalbedtics of the thermogenic coalbed gas.The results of the content analysis show that the main The Od value of the coalbed methane in the Xinji areadrocarbon component of the coalbed gas from the is in the range of -243%0 to%0. The OD value of theXS-02 well is methane, ranging from 55.11% to 95.75%, biogenic coalbed methane in the Sydney and Bowen Bamostly from 60% to 90%. The contents of heavier hydro- sins of Australia is in the range of -152% to-255%0,av-carbons are very low, with ethane ranging from 0% to eragely (-21717)%00. To compare, the OD value of the42%0, and propane ranging from 0 to O. 18%. The dryness coalbed methane of the Xinji area is in the range of that ofindex(C,/C -s)of hydrocarbons is from 0.993 to 1.0, in- the Sydney and Bowen Basins and lighter on a wholedicating that the gas belongs to dry gas. The main Therefonon-hydrocarbon component is N2, ranging from 2.86%roger中国煤化吗 area has the hy-ristic of the biogenic42. 42%0; the content of CO2 is in the range of 0.51% to gasCNMHG1.91% and that of ar is from 0.05% to 0.68%. CO is the The 8" N value otnitrogen trom the XS-01 well coalmaleficent gas but in low content, only from 0.02% to core desorbed gas is in the range of% to 4%, and threChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESof the samples have 8N values of -1%G, 0%o and 1%c,which is in accord with atmospheric dn value(O%oo)within the range of analytical error(=1%o)lysis of the genetic types of coalbedBiogenic gasesThe carbon isotopic composition of methane is an important index for the identification of natural gas generaThermos◆ Liyazhuangtion. It is considered that the carbon isotopic compositiongasesof biogenic methane is very light, and 8CI value beinggenerally lower than%0. As for the biogenic coalbedmethane, it is also thought 8Civalue to be lower than55%01-31. The studies of Smith and Pallasser 8), and Ahmed and Smith showed that 8CI value could be6Ccm,(%)high as%0. As stated above, the 8C I value ofFig. 2. Genetic type characteristics of 8C and SD values of the coalcoalbed gas from the XS-02 well of the Xinji area ranges bed methanefrom-54.7%0 to.3%0, with the average value of57.4%;that of the XS-01 well ranges from-507% to XS-0I wells range from -26 2%0 to -16 1%o and.6%058. 7%6.with the average value of_54.9%. All this indi- to-15.9%o, respectively ;8C value of propane rangescates that methane has the basic characteristics of the car. from -25. 3%o to-108%0. Both ethane and propane arebon isotopic composition of the biogenic originenriched in or abnormally enriched inC, reflecting theThe primary biogenic gas generated during the early thermogenic origin of heavier hydrocarbons and the variacolification stage of peat, generally under 0.5%Ro value tion characteristics of carbon isotopic compositions ofof coal rock-6l, or under 0.3%R, value!?l. The Ro value heavier hydrocarbons are altered by bacteria. The exis-of coal rocks of the coal beds from which gas samplestence of a small amount of heavier hydrocarbons and theirwere collected in the Xinji area is from 0.88% to 0.91% notable heavier carbon isotopic compositions also supportwith average value of 0.90%. According to Scott et al. "), it the idea that biogenic methane in this area is of secondaryis in the thermal stages after maximum wet gas generationbiogenic origin, and it contains the remnant of ther-(Ro value from 0.60% to 0.80%)and onset of the intense mogenic gas at the same timeAccording to the classification plot of the conventionalthermogenic methane generation(Ro value from 0.80%to natural gas with the correlation of carbon and hydrogen1.00%). As a result, the gas could not have the characterisitics of the primary biogenic gas. Based on the analysis Isotopes of methane(Schoell), all samples of the coal-of the thermal evolutionary degree of the coal rocks, the bed gas of the Xinji area distribute in the range of mixedbiogenicmethaneinthisareashouldbelongtothesecongasofbiogenicandthermogenicorigins(fig.2).com-bined with the above data and characteristics it could beDai et al. assume that coalbed gases from the coal proposed that the coalbed gas of the Xinji area is part ofmine do not accord with the relationship of 8"Cr-Rothe secondary biogenic gas and there also exists a smallcoal type(formed) gas, while the coalbed gases from mid- amount of thermogenic gas in itstatistic relationship betweend& d with it. Based on theAs mentioned above, a negative liner correlation existsbetween the contents of methane and nitrogen of coalbed8C1=40. 51xlog Ro-344 (0.3%