Carbon and hydrogen isotopic composition and generation pathway of biogenic gas in China

Chin JGeochem. (2009)28: 119-1DO:10.1007/sll631-00901195Carbon and hydrogen isotopic composition andgeneration pathway of biogenic gas in ChinaSHEN Ping, WANG Xiaofeng, XU Yin, SHI Baoguang, and XU Yongchang*The Key Laboratory of Gas Geochemistry, Chinese Academy of Sciences, Lanzhou 730000, ChinaReceived May 22, 2008; accepted August 10, 2008Abstract The carbon and hydrogen isotopic composition of biogenic gas is of great importance for the study of itsgeneration pathway and reservoiring characteristics. In this paper, the formation pathways and reservoiring charac-teristics of biogenic gas reservoirs in China are described in terms of the carbon and hydrogen isotopic compositionsof 31 gas samples from 10 biogenic gas reservoirs. The study shows that the hydrogen isotopic compositions of thesebiogenic gas reservoirs can be divided into three intervals: 8DCH>-200fc,250/<8DCH<-200%o and 8DcH<-250%The forerunners believed that the main generation pathway of biogenic gas under the condition of continental freshwater is acetic fermentation. Our research results showed that the generation pathway of biogenic gas under the con-dition of marine facies is typical CO2- reduction, the biogenic gas has heavy hydrogen isotopic composition: itsSDCH, values are higher than-200fo; that the biogenic gas under the condition of continental facies also was gener-ated by the same way, but its hydrogen isotopic composition is lighter than that of biogenetic gas generated undertypical marine facies condition: -250%o<8DCH<-200%c, the SDCH values may be related to the salinity of the watermedium in ancient lakes. From the relevant data of the Qaidam Basin, it can be seen that the hydrogen isotopicomposition of biogenic methane has the same variation trend with increasing salinity of water medium. There arebiogenic gas reservoirs formed in transitional regions under the condition of continental facies. These gas reservoirsresulted from both CO2-reduction and acetic fermentation, the formation of which may be related to the non-variantalinity of ancient water medium and the relatively high geothermal gradient, as is the case encountered in theBaoshan Basin. The biogenic gas generating in these regions has light hydrogen isotopic composition: 8DCH<-250/and relatively heavy carbon isotopic composition. There is a fairly strong negative corelation between the carbonisotopic composition and the hydrogen isotopic composition. The generation mechanism and pathway of carbon, anthe hydrogen isotopic composition of biogenic gas may be used to ascertain whether biogenic gas samples from thenatural world are of industrial utilization value. In general, the biogenic gas formed by way of acetic fermentation isnot propitious to the formation of gas reservoirs.Key words biogenic gas; isotope; generation pathwayand exploitation have attracted more and more atten-As an important and clean high quality energy, whicha e ed up in the Qaid aeid of 3000x10mhas1 Introductionbeen prows enormous resource prospect for this kindnatural gas has been widely used in all walks of lifeife of unconventional gas resources (Su Aiguo et aland by millions of households, which is propitious to 2006). Biogenic gas and conventional thermogenicproduction and is convenient for livelihood. The use gas are different in leading agent. Biogenic gas isof natural gas has improved the environment and the generated largely from the geochemical process ofquality of human life. Under the circumstance that the specific microbial communities in the natural worldWest-East Natural Gas Transmission Pipelines have That is to say, biogenic gas has its own hydrocar-been built, the importance and significance of proving bon-generating mechanism, reservoiring characteris-up more natural gas resources is self-evident. In this tics and distribution rules. Therefore, studying thesecase, unconventional natural gas has been given arulesrtant scientific in-great deal of attention in research and exploration forma中国煤化工In this(Zhang Shuichang et al, 2005). Biogenic gas is just a paperCN MH Gtopic characteriskind of unconventional natural gas, and its research tics or aoiugeuc gas reservoirs in Chinaare described; on this basis, the hydrocar/wwgigSpringer120Chin J Geochem. (2009)28: 119-124bon-generating mechanisms and reservoiring path- standard samples during the seventh"Five-Year Planways of these biogenic gas reservoirs are discussed. period. A few previous work standard samples wereThese studies have provided helpful scientific infor- repeatedly measured by a new high-temperature pynation for the exploration of this kind of unconven- rolysis method, and the results showed that the twotional natural gas resources.testing methods are of good comparability for themethane isotope values of the same gas sample. The2 Samples and testing methodserrors of the two methods are within 10%%, which in-dicates that the relevant data can be comprehensivelyThe authors collected 31 gas samples from 10 applied according to the 10%o error of hydrogen iso-gas reservoirs in sequence from Songliao, Liaohe, tope values measured by the two methods. The 8DCH4Jinhu(in Jiangsu), Qidong, Sanshui, Baise, Qaidam, values of the previous 4 work standard samples, suchYinggehai, Baoshan (in Yunnan) and luliang (in as Well Wen-3l, measured by the two methods, areYunnan). These samples are distributed mainly in the listed in Table IQuaternary, Tertiary and Cretaceous systems at shallow depths.Table 1. Hydrogen isotopic values of work standard gasTwo methods were adopted in the separation andethane for reference measured by the two methodstesting of hydrocarbon gas. In order to deal with theWell No. Method I 8D aH (%o) Method 2 &DaH(%)relevant data systematically, it is necessary to contrastthe testing results. Only under comparable circum-243±1.7Zhuang-20200.l±2.1stances, can we make comprehensive use of such data.43.0±2.2(1)During the time from the seventh to the ninth Note: Method I is the hydrogen production method of zinc granule re-Five-Year Plan"periods: the separation of methane duction, the 8DoH, values are the results determined at Lanzhou Institutend its homologues was carried out by GC, then the of Geology Geological Scientific Research Institute of Shengli Oil-methane was put into the oxidation fumace and burntield Method 2 is the high temperature pyrolisis method(the mass spec-to generate COz and H2O. The CO2 was used to de- trometer DeltaPtXP), the &do values are the results measured at Lan-termine carbon isotopes and the generated H2O was zhou Institute of Geology in 2004reduced to H2 by the British zinc granules. MAT-251and MAT-252 mass spectrometers were used to de-In order to further attest the above results, thetermine CO2 and H2, respectively. Analysis of hydro- carbon and hydrogen isotopic values of a number ofgen isotopic composition was always offline when the gas samples in Lanzhou were determined by Method 2.measurement was carried out on a mat- 252 mass The relevant data obtained by the two methods arespectrometer in the online form of carbon isotope. The listed in Table 2 for comparison with our results de-measurement accuracy of carbon and hydrogen iso- termined by method I in the pasttopes were 0. 3%o and 5%, respectively.(2)The Delta plus XP GC/TC/IRMS ) mass Table 2. Carbon and hydrogen isotopic values determinedspectrometer has been introduced since this centuryby the two methods on the same saThe instrument employs high-temperature-pyrolysis,and C and H ions can be obtained directly to carry out Well No. Stratum Depth (m) (e)(%)(o)(ethe online measurement of carbon or hydrogen iso- Sezhong-1Q 638.9-650.965.4-23561.8-223topes Measurement condition: the al2O3 columnm×0.53mm×20um) was used. Warming up programSezhong9Q12564-136769234463225the start temperature was set at 30C, and kept constant for 5 minutes. Then the temperature was raisedListed in Table 2 are the results determined byto 200C at the rate of 15C/min and kept constant for the two different methods at different times at Lan-3 minutes, followed by taking helium as the carrier zhou Institute of Geology. The errors involved ingas. Under the temperature of 1450C, hydrocarbon drogen isotope measurements are basically less thanand hydrogen isotopes are <0.3%0 and <3%o(for thements. For the carbon isotope, the geochemical dis-hydrogen in CHa and C2H6), respectivelycrimination of natural gas is not affected. So it can beThe standard hydrogen used to testbelieved that the data obtained by the two methods areof relativmparability. now the carbon andisotope was the steel-cylinder-containedcalibrated according to the international中国煤化工r3 I biogenic gasstandard sample SMOW and the work standard sam-CNMHGple of natural gas, such as Well Wen-3l, calibratedaccording to the intemational carbon and hydrogen 3 Results and discussionChinJGeochem. (2009)28: 119-1243. 1 Distribution characteristics of carbon and hy- biogenic gas methane, 8DCH4 <-200% as the limit ofrogen isotopesbiogenic gas formed in the environment of continentalfacies and 8DcHa-200%0 as the limit of biogenic gasAccording to the principle of 8C1<-60%0, -60%0 formed in the environment of marine faciesis taken as the carbon isotopic upper limit value andthe carbon isotopic values of the biogenic gas samplesare all within this range. The carbon isotopic values of31 methane samples are within the range of-60.5%0--73.6%0, with the difference of 13%o. The majority ofthe 8Ct values are within the range of more than-70%, and the 8 Ci values of only 7 samples are lessthan-%0, accounting for about 20% of the total. Thehydrogen isotopic composition of biogenic gas methane has been measured by the authors. The 8DcH4values of 31 gas samples are within the range287%0-152%0, with the difference of 135%o Its his-togram is shown in Fig. 1170.180-190200-210220230-240250-260+270·2Table 3. Carbon and hydrogen isotopic composition dataoDo, (SMow.elof 31 gas samples from10 biogenic gas reservoirs in ChinaFig. I. The histogram of hydrogen isotopes of biogenic gas fields inWell No(PDB, %e)China, I. Baoshan; 2. Luliang: 3. Qaidam; 4. Yinggehai; 5. Baise; 6Sanshui; 7. Jinhu; 8. Qidong: 9. Liaohe: 10. SongliaoFigure I shows the histogram of the 8DCH4 valuesof 31 biogenic gas samples in China. The 8DCH4 val-Jinhuues of only 4 gas samples are more than -200%0,ascan be seen from this figure. According to the authorsclassification, these gas samples should designated toYinggehai Ledong-28-Ibiogenic gas formed in the environment of marineLedong-8-1-1 Qfacies or salty lake facies. Specific analysis of the 4gas samples showed that two of the samples are related to the Quatemary system, the Pliocene of theezhong-23Yinggehai marine facies, and the other two are the gassamples of Qidong, North Jiangsu from the shallowlayer of the Quaternary system. It has been known thatthere occurred seawater transgression in Qidong2一North Jiangsu during the Quaternary. That is to say,aliang (injudging from the information available, it is basicallyYunnan)N2721-235feasible to take 8DCH4-200% as the limit to identifyLu.3that the ancient sedimentary medium is of continentalfresh water facies or marine facies and salty lake faBaoshan(in3.2 Carbon and hydrogen isotopic composition ofbiogenic gas and its generation mechanismIt is generally believed that the hydrogen isotopiccomposition of biogenic gas methane is related mainlyThe carbon and hydrogen isotopic compositionto that of the ancient medium water body in the hy- of biogenic gas methane is related to the water medrocarbon-generating environment, which is the main alum in the period of its generation and the generationfactor controlling the &DCHA values of biogenic gas pathway. The carbon and hydrogen isotopic composi-(Liao Y.S. et al, 1997; Rice and Claypool, 1981; tions of the 3I gas samples in China are shown inSchoell, 1980; Sessions et aL, 1999; Shen Ping andgXu Yongchang, 1991, 1993; Woltemate et al, 1984). ton中国煤化工smFg2 can beBased on the studies of domestic and foreign scholars, divideCN MH Oect to their distriShen Ping et al.( 1991)and Schoell 1980)put for-n. oCHAsection B: -250%-200foo, which is consis-tent with Section A in Fig. 2. The point groups of thegas samples are in the field of the biogenic gas formedby way of CO2-reduction in the marine facies envi-ronment. The hydrogen of methane comes mainlyfrom water medium, namely, related to seawater. TheSD values of seawater are obviously larger than thoseof fresh water, so it is highly reasonable that the bio-genic methane in this region has the heaviest hydro-gen isotopic composition.6D (SMOW.%)(2)On the left of the figure the point groups rep-Fig 3 Origin classification of carbon and hydrogen isotopes of bio resent mainly the biogenic gas of the Cretaceous sys-genic gas in China. 1. Jinhu; 2. Qidong 3. Songliao: 4. Liaohe; 5.tem in the Songliao Basin and of the Tertiary systemSanshui; 6. Baise; 7. Yinggehai: 8. Qaidam: 9. Luliang: 10. Baoshanin the Baoshan Basin, Yunnan. The gas-forming environments of the two basins are both of continentalStudies showed that the generation mechanism of fresh water lake facies. Organic matter is mainly ofBiogenic methane has a vital influence on the carbon and hydrogen isotopic characteristics of these gasThere are two main methane-generating pathways samples are: the carbon isotopic composition is relaassociated with biological processes in the natural tively heavy and the hydrogen isotopic composition isworld, namely acetic fermentation and CO2-reductionmarked by 8DCHA <-250fo, which is equivalent to theWhiticar (1999) believed that acetic fermentationcase3. The pormainly took place in the environment of continentalgroup中国煤化工 nly of transitionalCNMH Ge acetic fermentawater,3/4 from the methyl groups of acetic acid; the tonsclassificationenvironment in which the biogenic gas was formed by8DCH4<-250%0 indicates that the gas-forming envi-Chin J Geochem.(2009)28: 119-124ronment should undoubtedly be fresh water medium, or represented by point groups on the right of Fig. 3.so the biogenic gas in the two areas should be formed For the biogenic gas of continental facies origin, whenby way of acetic fermentation. But, the points are dis- its salinity cannot reach the salinity of salty lake facies,tributed mainly in the transitional zone, which is a acetic fermentation usually is not the generationquestion worthy of consideration. This phenomenon pathway of this kind of biogenic gas, and the majoritywill be elaborated in the following text.of it still belongs to the CO2-reduction type. But this(3)The points in the central part of Fig 3 mainly kind of biogenic gas can be distinguished from theepresent the biogenic gas samples from the Quater- biogenic gas of typical marine facies(including saltynary system of the Qaidam Basin and the Tertiary lake facies)origin by hydrogen isotopic compositionsystem of the Luliang Basin. There were also col- The SDCH values are within the range of -250%lected gas samples from Liaohe, Sanshui, Baise and -200%0. The biogenic gas samples are distributed inJinhu(in North Jiangsu). The dD values of these gas Section B of Fig. 2 and the central part of Fig 3, andsamples are within the range of -250%0-200%0 and its carbon isotopic composition is similar to that ofthe 8CI values vary over a relatively wide range, biogeneric gas of marine facies origin. Section C inwhich is equivalent to the case encountered in Section Fig. 2 is equivalent to the transitional zone in Fig. 3B in Fig. 2. The isotopic characteristics of theseThe carbon and hydrogen isotopic compositions ofsamples are: their hydrogen isotopic composition is biogegeric gas in Section C and the transitional zoneheavier than that of the transitional biogenic gas but have unique characteristics:(a) the hydrogen isotopictheir carbon isotopic composition is lighter than that composition is lightest, 8DCH4 <-250%0, and the car-of the transitional biogenic gas on the whole. Com- bon isotopic composition is relatively heavy;(b) therepared with the biogenic gas of marine facies, these is a fairly strong negative correlation between thebiogenic gas samples have similar carbon isotopic carbon and hydrogen isotopic compositions, and thecomposition but relatively light hydrogen isotopic coefficient, R, in the regression equation of gas sam-mposition. As to the salinity for hydrocarbon gen- ples from the Baoshan Basin (in Yunnan)anderation, these biogenic gas samples lie between typical Songliao Basin reaches 0.92. The carbon and hydro-marine facies sediments and continental fresh water gen isotopic characteristics of this group of gas sam-facies sediments. According to the authors' previous ples indicate that this kind of biogenic gas is similar toclassification, the gas-generating conditions are char- the biogenic gas formed by way of acetic fermentationacterized mainly by continental fresh water, and in According to the geological background of thesome cases, by the lacustrine water salinization, but Baoshan Basin, we can get two basic understandingsthe fluid cannot reach the salinity of seawater. In the (a)the water medium is fresh water without saliniza-mode of Whiticar, the biogenic gas of the sedimentary tion;(b)high temperature is propitious to the generabasins of continental fresh water facies shouldtion of biogenic gas by acetic fermentation(valentinegenerated mainly by way of acetic fermentation. et al., 2004), and in the Baoshan Basin there is a rela-However, as stated above, only a few biogenic gas tively high geothermal gradient(7-8C/100 m).Fromreservoirs don' t belong to the acetic fermentation type the data available, although there are two pathwaysin the case of SDCH4<-250oo, they are in the transi- acetic fermentation and COz-reduction for the genera-tional zone. That means the biogenic gas reservoirs in tion of biogenic gas, some scholars believe that aceticSection B are mainly in the gas formation pathway of fermentation is the main generation pathway of bio-COx-reduction. Of course, it can also be seen from this genic gas under continental fresh water environment.figure that these points are very close to the transi- According to the available data of China,s biogenictional zone, but there is a certain difference in hydro- gas reservoirs, it has almost never been found that thegen isotopic composition as compared with the bio. reservoired biogenic gas belongs to the acetic fermengenic gas formed in a typical marine facies environ- tation type. Meanwhile the relevant research showedmentthat on the vertical section of the strata, the generationCombining Figs. 2 and 3, we can get the followof biogenic gas turns from being dominated by theing understanding: the generation pathway of biogenic acetic fermentation type to the CO2-reduction typegas in a typical marine facies environment is with increasing depth(Whiticar, 1999). According toCO2-reduction. The characteristics of this kind of the results of research on the biogenic gas-generatingbiogenic gas are: the hydrogen isotopic composition is pathway represented by the carbon and hydrogen iso-heavy, 8DcHA>-200%0; the carbon isotopic composi- topic composition of 31 gas samples from 10 gas res-tion varies over a relatively wide range; there is no ervoirs in China. it can be seen that for the reservoirobvious correlation between the carbon and hydrogen ing中国煤化工 ration mechanismtopic compositions, but for the variation trend, andthere is a slightly positive correlation, This kind of bonCNMHGit to say, the cabiogenic gas is equivalent to that in Section A(Fig. 2) gas samples can be used to ascertain whether the gasChin J, Geochem. (2009)28: 119-124samples are related to the reservoired biogenic gas or Acknowledgements The research was supponot in a certain sense.by the National Natural Science Foundation of( Grant No.40703001)5 ConclusionsReferences(1)The carbon and hydrogen isotopic compostion of biogenic gas is an important discrimination Liao Yongsheng. Chen Wenzheng, and Duan Yucheng(i997) Isotope geo-sign to study its generation mechanism and pathway,chemistry of organic mineral resources. In Isotope Geochemistry ofespecially for the hydrogen isotope, which is moreChina(eds. Yu J.S.and Li Y S )[M]. pp. 467-593 Science Press, Beijing (in Chinese)( 2)The forerunners believed that the main gRice DD. and Claypool G.E.(1981)Generation, accumulation and resourceeration pathway of biogenic gas in the continentalpotential of biogenic gas [] Bull. AAPG. 65, 5-25.fresh water environment is acetic fermentation. Ourchoell M. 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