Compositional and geochemical characteristics of light hydrocarbons for typical marine oils and typi

Chin J Geochem. (2008)27: 407-411DOl:10.1007/s11631008-04075Compositional and geochemical characteristics oflight hydrocarbons for typical marine oils andtypical coal-generated oils in ChinaWANG Xiang"and ZHANG Min 2Department of Geochemistry, Yangtze University, Jingzhou 434023, ChinaThe Key Laboratory of oil Gas Resources and Exploration Technology, Ministry of Education, Yangtze University, Jinzhou 434023, chinaCarrespondingauthorE-mail:wanglang1980@163.comReceived November 7, 2007; accepted December 24, 2007Abstract Different types of crude oils have different light hydrocarbon compositional and geochemical characterstics. Based on the light hydrocarbon data from two kinds of oils, i. e, coal-generated oils and marine oils in China,ght hydrocarbons in marine oils in the Tazhong area are generally relatively enriched in n-heptane, ancoal-generated oils from the Turpan Basin are enriched in methylcyclohexane, The K, values, reported by mang1987), range from 0.97 to 1. 19 in marine oils, basically consistent with what was reported by Mango on light hy-drocarbons in terms of the majority of the crude oil data. But the KI values of coal-generated oils are particularlyhigh(1.35-1.66)and far greater than those of marine oils; heptane values in marine oils, ranging from 32. 3%to45.4%, and isoheptane values, ranging from 1.9 to 3.7, are respectively higher than those of coal-generated oils, in-dicating that the oils are in the high-maturity stage. In addition, expulsion temperatures of coal-generated cils fromthe Turpan Basin are obviously lower than those of marine oils from the Tazhong area.Key words marine oil; coal-generated oil; light hydrocarbon; Mango's light hydrocarbon parameter(K1);heptane value; iso-heptane valuewidely applied in natural oil and gas exploration1 Introduction(Mango, 1987, 1990, 1992, 1994; Zhang Min and LinRenzi, 1994; Lin Renzi and Zhang Min, 1996: TenLight hydrocarbons occurring predominantly in Haven, 1996). Based on the results of previous studiescondensates and light oils are the critical components focusing on light hydrocarbons by several researchers,of natural oils and gases. Schaefer et al. (1978a, b) typical marine oils from the Tazhong area in the Tarimfirstly established the capillary gas chromatography Basin and typical coal-generated oils from the Turpantechniques for analyzing the light fractions(Cz-Cs)of Basin were selected for the analysis and comparisonsource rocks and oils, which has provided the basisof differences in compositional characteristics of lightthe study of light hydrocarbons. Subsequently, from hydrocarbons between the two oil categories so as to1978 to 1982, Schaefer(1980), Leythaeuser(1983), provide new clues to the genetic classification of oilsHunt (1980, 1984)and Snowdon (1982)did muchresearch work on the generation, migration and dis- 2 Geological setting and samplestribution rules of light hydrocarbons, established manygeochemical parameters for light hydrocarbons whichThe Tarim Basin is the biggest sedimentary basincan be applied to directing oil and gas exploration, in China, its maximum residual thickness of sedimenand thus laid down the solid foundation for the study tary rocks is above 1.6x10 m, and the residual vol-of light hydrocarbons in various fields of oil and gas ume exceeds 400x10km'. It has been proved by theresearcexploration data available that the Tarim Basin is alsoIn recent ten years or so, many researchers have a zone with abundant marine oils in China. Thebeen studying the generating mechanisms of light hy- Tazhor中国煤化工 nong uplift of thedrocarbons, especially the light hydrocarbon Tarimdarysteady-state catalytic hypothesis proposed by Mango, unitsCNMHGcentral fault zonewhich has made the data on light hydrocarbons more and the southem slope zone of the Tazhong uplift.www.gyig.ac.cnwww.springerlink.comrIngerChin. Geochem. (2008)27: 407-411Research results showed that the Tazhong area is en- of n-alkanes range from 21.9%to 30. 1%, inferiorriched in marine oils. Eleven typical marine crude oil iso-alkanes. In addition, the contents of cycloalkanessamples were collected from the Ordovician and Car- are generally equal to those of iso-alkanes, reflectingboniferous strata in the Tazhong area for geochemical the characteristics of terrigenous precursor input.analysis. Sample information is giTable 1The Turpan Basin lies in the eastern part of theXinjiang Uygur Autonomous Region, measured atabout 60-130 km in length, covering an area of5.28x10- from east to west. As is well known. theTurpan Basin is a famous coal- and oil/gas-bearingbasin which is divided into three first-ordered structural units, including the Turpan Basin Depression inthe central-western region, the Liaodun uplift in thecentral-eastern region and the Hami Depression in theeastern region. The available results of successful oiland gas exploration indicated that the Turpan Basin isa typical coal-generated oil-enriched zone. Thirteencoal-generated oil samples analyzed in this study werecollected from the Xiaocaohu Sag, the Shengbei SFig. I. Triangle diagram of Cs-7 series of typical marine oils from theand the Qiudong Sag, and the samples are distributedTarim Basin and typical coal-generated oils from the Turpan Basin.formations3 ExperimentalThe instrument used was a varian HP6890 GasChromatograph interfaced to a pyrolysis oven. Thereleased hydrocarbons were flushed onto a fused silicacolumn(oV-I capillary column; 25 mx0. 25 mmx025um). The temperature programming of the gas chro-matograph started at 30C(hold time: 6 minutes), witha temperature gradient of 6C/min to 100C/min. TheFig. 2. Triangle diagram of c, series of typical marine oils from thecolumn was linked to a flame ionization detector, us-Tarim Basin and typical coal-generated oils from the Turpan Basin.ing nitrogen gas as the carrier gas, and gas flow ratio4.2 Compositional characteristics of C, light hydrocarbon fractions4 Results and discussionC, light hydrocarbon fractions include three cate4. 1 Compositional characteristics of Cs-7 fractionsgories: (1)methylcyclohexane (MCyC6)derivedmainly from lignin, cellulose and carbohydrate ofhigher plants, has a relatively stable thermodynamicsLeythzeuser et al.(1979)suggested that character and is a better parameter for constraining then-alkanes are abundant in the light hydrocarbon fractypes of terrigenous precursors. Furthermore, its greattions derived from sapropelic precursors, and the light abundance is characteristic of the light hydrocarbonshydrocarbon fractions derived from humic precursors of coal-generated oils;(2)dimethylcyclopentaneare enriched in iso-alkanes and aromatic hydrocarbons, (DMCyCs) with different structures, derived chieflycharacteristic of terrigenous precursors. Cs, Cs and Cu by thermal maturity, but its extensive occurrence is acoal-generated oils also show the above characteris- rine oils;(3)n-heptane(nC) primarily originatingtIcsAs is shown in Fig. 1, n-alkanes are relativelyfrom algae and bacteria, is excessively sensitiverich in marine oils, ranging from 43.3%to 49. 4%, and中国煤化工 be used as a go1989). In the oiliso-alkanes are relatively low(25.3%35. 1%); ForsamCNMHGontents of C,lightcoal-generated oils, iso-alkanes are relatively abun- hydrocarbon fractions for marine oils are significantlyChin J Geochem. (2008)27: 407-411different from those of coal-generated oils. It is ob- methylcyclohexane is 32.6%(lower than that ofserved in marine oils that n-heptane is high in abun rine oils) and 57.2%(greater than that of marinedance, with an average of 58.4%, but methylcyclo- respectively. As is shown in Fig. 2, particularly obvihexane is much lower, averaging 29. 7%. In contrast to ous differences are noticed in distribution pattern formarine oils, the average abundance of n-heptane and marine and coal-generated oils.Oil type KL380520-3821.501070914166202006150.00Marine oil 1.07 0.89TZ4516050.006297624854004877.01.I0.90358220.295XCH HTII37500-140020246Js52410202424401.611.05764.00-281900.0815.I0074434Coal0069TurpanJx generated 1.58 1..7290.0392748.70-275840J33430一3547-35503630.000047Note: XCH. Xiaocaohu: QD. Qiudong: SB. Shengbei; PI. isoheptane value; Pl]. heptane value; KI.(2-methylhexane+2, 3-dimethylpentane)A3-methy lhexane+ 2, 4dimethylpemtanc) R 2-methylhexane/3-methy lhexane: R,. 2, 4-dimethyipenmtane/, 3-dimethylpentane.Based on elementary analysis and carbon isotope is the aromatic curve representing humic precursorsata, Hu Tilin et al. (1990)proposed the The zone between the two curves represents the hy-methyl-cyclohexane index(MCH-f) and applied this ride type.index to classifying organic matter: a)MCH-1<35+2for type I; b)MCH-]=(35+2) (50+2)for type ll; c)MCH=(50±2)(65±2) for typeⅢ;andd)● Marine oil50F 8Coal-generated oilMCH-I>65+2 for type [l2 or coal type. Depending onthe index proposed by Hu Tilin et aL.(1990), it isnoted that MCH-I values for the Tazhong typical ma-rine oils range from 23.3 to 34.1, averaging 29.7, justfalling into the type I range; compared with typicalmarine oils, MCH-I values for the Turpanoal generated oils range from 56.9 to 60.1, averaging57.3, also falling into the type Ill range, which is inagreement with the results of genetic analysis of organic matter for marine oils and coal-generated oilsFig. 3. Heptane value vs. isoheptane value plot of typical marine oils4.3 Heptane and isoheptane values of crude oilsAccording to the fact that the degree of alkylaIt can be seen in our study that heptane values fortion becomes higher with increasing oil thermal ma- marine oils range from 32.3%to 45.4%, averagingturity, Thompson (1979) proposed two parameters 40. 2%, and isoheptane values range from 1.9 to 3. 7,reflecting thermal maturity, heptane value and isohep- averaging 2.9, showing that marine oils basically havetane value, in order to distinguish oil thermal maturity, entstage. Except mabut he soon found that these two parameters were中国煤化工controlled by different precursors. In this case, coalThompson established two curves: one is the aliphatic ingCNMHG2420e时curve representing sapropelic precursors and the other suggesting that coal-generated oil samples are of410Chin J Geochem. (2008)27: 407-411thermal maturity. As is shown in Fig. 3, marine oils and are basically consistent with the characteristics ofand coal-generated oils fall onto the aliphatic and Mango's light hydrocarbon parameter(K1), belongingaromatic curves, respectively, and it is obvious that to the normal range, but oil samples from the Turpanthere is a positive correlation between thermal matur- Basin exhibit an outlier character with respect to theirity and the two parametersKI values ranging from 1.35 to 1.66(averaging 1.52)for the ratio (2-methylhexane+2, 3-dimethylpen4. 4 Characteristics of Mango's light hydrocarbon tane)( 3-methylhexane+2, 4-dimethylpentane), theparameter (Kl)factor leadithe particularly high KI values maybe related to the sedimentary environment, especiallyMango(1987)reported, for the first time, theacidic clay minerals which played an important role inremarkable invariance of four isoheptane in petro. this aspect( Zhang Min and Zhang Jun, 1998). It canleum: regardless of the absolute concentrations, the be seen from Fig. 4 that the Ki values for marine oilsratio of sums of concentrations of (2-methylhexane+ 2, are obviously lower than those for coal-generated oils,3-dimethylpentane)(3-methylhexane+2, 4-dimethylp- and there is a good correlation between KI andentane)remains constant, the K, value is around unity. 2-MH/3-MH for different types of oils(Zhang Min,The authors found that the K values for marine 2005)oil samples range from 0.97-1 19(averaging 1.07)10[Coal-generated▲ Marine oi■Coal- generated oilMarine oil的工N(3-M+2,4DMP)/(C4C)(%)Fig. 4.Plots of the concentrations of 3 methy lhexane+ 2, 4-dimethylpentane vs. the concentrations of 2-methylhexane+, 3-dimethylpentane andKiratio vs 2-MH/3-MH for typical marine oils from the Tarim Basin and typical coal-generated oils from the Turpan Basin.temperatures for marine oils are higher than those for4.5 The correlation of oil expulsion temperaturescoal-generated oils(As is shown in Fig. 5), which is inaccord with the temperatures we calculated previously,The expulsion temperatures were evaluated by depending on Ro (vitrinite reflectance)Mango( 1987, 1990)and modified by Bement( 1995)These authors suggested that the ratio of 2,Isoheptane4-dimethylpentane to 2, 3-dimethylpentane( 2, 4-DMP/2, 3-DMP)is a function of temperature,independent of time and kerogen type. The ratio wasCoal-generated oflsuggested to reflect the temperature the oil has beenexposed to in the source rocks, therefore, reflectingthe temperature condition at the moment of hydrocar-bon expulsion. In view of this concept, the followingempirically-derived formula was published to calcu-117late the expulsion temperature of oil using light hy-drocarbons in oil samples(Mango, 1997)Cm(℃)=140+15[n(2,4DMP2,3DMPApplying this approach, we found that the stud-ied marine oils from the Tarim Basin were expelled attemperatures ranging from 115.3 to 1294C(averag-135ing 1244C), while coal-generated oils expelled from中国煤化工 cs for typical methe Turpan Basin at temperatures varying betweenherated oils for the968℃ C and 1022℃, averaging100.4℃. Based on theYHANMHGcalculated results, it should be noted that the expulsionChin J Geochem. (2008)27: 407-4115 ConclusionsLeythaeuser D. (1979)Generation and migration of light hydrocarbons insedimentary basins [). Ong Geochem. 1, 191-204.(1)Light hydrocarbons of marine oils in the Liao Yongsheng(1989)Application of light hydrocarbons and carbon iso-Tazhong area are generally enriched in nC7, andtope from canned cuttings in petroleum exploration In Collection ofoal-generated oils from the Turpan Basin are en-Thesis on Natral Gas [C]. pp 58-64. Petroleum Industry Press, Bei-riched in MCyC6(2) The K values range from 0.97 to 1. 19 for Lin Renzi and Zhang Min(1996)Composition characteristics and originmarine oils, their distribution is basically consistenttype of light hydrocarbons or crude oils, Tarim Basin U). Geologywith the conclusion Mango drew on light hydrocarbons based on the majority of the crude oil data. But Mango F.D. (1987) An invariance in the isoheptane of petroleum []. seb-the K, values of coal-geoils are particularlyence.237,514517high(1.35-1.66)and far greater than those of marine Mango F.D. (1990)The origin of light hydrocarbon in petroleum: A kineticoilstest of the steady state catalytic hypothesis U). Geochim. e Cosmo-(3) Heptane values for marine oils, from 32.3%im, acta.541215-1323to 45.4%, and isoheptane values, from 1.9 to 3.7, are Mango FD(1992)Transition metal catalysis in the generation of petrolrespectively higher than those of coal-generated oils,and natural gas [] Geochim ef Cosmochim. Acta. 56, 553-555which indicates that the oils are in the high maturity Mango F.D.(1994)The origin of light hydrocarbons in petroleum: ringpreference in the closure of carbocyclic rings []. Geochim. ef Cos.(4)The expulsion temperatures for coal-gener-mochim Acta. 58, 895-901.ated oils(96. 8-102.2 C)are lower than those for ma- Schaefer RG, Leythaeuser D, and Weiner B (1978)Single-step capillaryrine oils(1153-1294C)collumn gas chromatographic methoed for extraction and analysis ofsub-parts per billion(10E9) amounts of hydrocarbons(Cr-Co) fromAcknowledgements This study was financiallyrock and crude oil samples and its application in petroleum geochem-supported by the National Natural Science Foundationistry [] Chromatography. 167, 355-363of China( Grant No. 40573030)Schaefer RG(1980)Analysis of trace amounts of hydrocarbons(Cr-C)from rocks and crude oil samples and its applicationReferencesgeochemistry []. Physics and Chemistry of the Earth. 12, 149-156Bement W.O.( 1995) The temperature of oil generation as defined with Crcarbon generation model for terrestrial organic matter [] AAPG Bull.chemistry maturity parameter(2, 4-DM P/2, 3-DMP ratio). In Organi66,775-788Geochemistry: Development and Application to Energy, Climate Ten Haven H..(1996)Application and limitations of Mango's light hydro-Emvironment and Human History(eds Grimalt J.O. and Dorronsorocarbon parameters in petroleum correlation studies []. Org GeochemC )(C]. pp 505-507. European Association of Organic Geochemists,24,957976Donostian-San SebastianThompson KF..(1979) Light hydrocarbons in subsurface sediments )Hu Tilin, Ge Baoxiong Chang Yigang, and Liu Bin(1990) The develop-Geochimica et Casmochimica Acta 43, 657-672ment and application of fingerprint parameters for hydrocarbons ab- Zhang Min(2005)Compositional characteristics and geochemical signifi-sorbed by source rocks and light hydrocarbons in natural gas p]cance of light hydrocarbons for crude oils in Tarim Basin, China [].Petroleum Geology d Erperiment. 4, 378-379.Chinese ournal of Geochemistry. 24, 228-23Hunt JM(1980)Formation of C-C, hydrocarbons from bacterial degrada- Zhang Min and Lin Renzi(1994)On the catalysis of transition metals in thetion of naturally occurring terpenoids []. Nature. 288, 577-588process of light hydrocarbons generation [J]. Geological Science andHunt J M(1984)Generation and migration of light hydrocarbon [ Sek-Technology information. 13, 75-80ence.226,1265-1270Zhang Min and Zhang Jun(1998)The development and application ofLeythseuser D. (1983)Diffusion of light hydrocarbons on subsurface sango's light hydrocabons parameters U ]. Petroleum Exploration andmentary rocks P]. AAPG Bull. 67, 186-203中国煤化工CNMHG