Study of Pyrolysis Kinetics of Asphaltenes in Silurian Oilsands in the Tarim Basin

680Vol. 74 No. 3ACTA GEOLOGICA SINICASept. 2000Study of Pyrolysis Kinetics of Asphaltenes inSilurian Oilsands in the Tarim BasinLIAO Zewen and GENG AnsongThe State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640, GuangdongAbstractIt is a special petroleum geological phenomenon that Silurian oilsands are extensively distributed inthe central and northeast Tarim basin. Some geochemical studies of the oilsands have been carried out, but there isstill great controversy over the hydrocarbon-regenerating potential of oilsands and the possibility of Silurian oilsandsas hydrocarbon source rocks. In this study, the kinetics of asphaltenes pyrolysis was directly used to simulate the po-tential of Silurian oilsands for regenerating hydrocarbons. According to the experimental results, combined with otherrelated organic geochemical analysis, it is considered that Silurian oilsands in the Tarim basin have a high hydrocar-bon- regenerating potential and are latent hydrocarbon source rocks.Key words: asphaltene, pyrolysis, kinetics, Tarim basin, Silurian1 Introductionthose from the northern Tarim, are latent hydrocarbonsource rocks.Silurian oilsands in the Tarim basin are mainly dis-tributed in the Central depression and the central and2 Samples and Experimental Proceduresnorthern Tarim areas and regionally are controlled bypalaeo-uplift. They are characterized by great thic k-2.1 Samplesness and extensive distribution (Lin and Zhang, 1996;A total of 12 Silurian oilsand samples were collectedYu, 1996; Zhang Min and Zhu, 1996; Liu, 1998; from the Tarim basin, including those from hole HalZhang Guangya and Song, 1998; Wang Tonghe, 1999;in the northern Tarim area and hole Tz11 and holeWang Yi et al, 1999). Recent studies (Gu et al., 1998)Tz12 in the central Tarim area.indicate that Silurian oilsands came from the under-The basic geochemical data of all the samples arelying Cambrian-Ordovician source rocks, and thatlisted in Table 1. The fractional compositions of theafter the formation of the oilsands late-formed hydro-oilsands were determined using the TLC-FID methodcarbons were injected into them. Then the study of the (Karlsen and Larter, 1991). All the samples were fea-hydrocarbon-regenerating potential of Silurian oil-tured by a high content of non-hydrocarbons and asands and their possibility as hydrocarbon source low content of saturated hydrocarbons. The total or-rocks becomes an important subject. However, recentganic carbon (TOC) contents of all the samples areresearch results show that unamity in this issue has notbove 0.5%，of which half of them are even abovebeen reached (Gu et al, 1998; Zhang et al, 1997), and1%.further work is necessary.In this study the kinetics of asphaltenes (ASP) py-2.2 Experimental proceduresrolysis was directly used to simulate the hydrocarbon-The oilsands were extracted with chloroform in orderregenerating potential of the oilsands. According toto remove and prepare asphatenes from the oilsands.the experimental results, it is considered that the Silu-Two grams of the extracts were put into a 500-ml flat-rian oilsands from the Tarim basin have a high hydro-bottomed flask. About 1.5 grams of dichloromethanecarbon-regenerating potential. The oilsands, especially中国煤化工YHCNMH G.Pyrolysis Kinetics of Asphaltenes in Silurian Oilsands in the Tarim Basin. Liao et al.681Table 1 Basic geochemical parameters of Siturian oilsandsFriedman frst-order model or called the series-in the Tarim basinSampieSample DepthTOC SATARO RES ASPwound first order model (Fu and Qin, 1995). ThisNO.(%)_ (%)(%)(%)___ (%)model considers that the kinetic parameters, theHal-01 6074.4 2.86 5.56 15.1345.30 34.01Ha1-02 6092.71.52 7.09 13.2355.10 24.59activation energy and the frequency factor, are vari-Ha1-03 .6158.1 1.22 5.55 10.73Ha1-04 6233.2 1.70 3.93 10.9752.20 31.51able in the pyrolysis process. In other words, there57.22 27.88Hal-05 6313.70.62 6.51 8.7759.6825.04are different kinetic parameters at different x (as-phaltene inversion fractions). The reglarity of theT211-01 4310.2 3.86 3.333 18.5951.02 27.06variation of the kinetic parameters with the inversion1Z1- 024426.4 0.71 7.90 9.571211-0S4441.5 0.51 9.26 10.0467.04 13.66fractions can reflect the features of the asphalteneTz11-04 455.5 0.82 13.68 14.91 63.487.9structure and composition. The basic equation of thismodel isTz12-0240178 1.39 1.6 21.53 6098 6.22 .1z12-034253.0 1.58 1.46 15.31 5738 15.26dx;/dt=k; (1-x)(1)- Tz12-06 400.2 0.92 11.10 13.17_ 60.70 15.03where与is the asphaltene inversion fraction at theAbbreviations: TOC=total organic carbon;SAT-saturated hydrocarbons;ARO=aromaties; RES=resin; ASP- -asphaltenes.time t and k; is the reaction rate constant when theasphaltene inversion fraction is x The parameter k,well dispersed. About 150 -200 ml of heptane were .can be expressed by the Arrhenius lawadded in the flask. It was fully agitated with a mag-k =Aexp(-E;/RT)2)netostirter for 12 hours and then left overmight in awhere A and E are the kinetic parameters when thedark place. The asphaltene deposits were gained byinversion fraction is x, R is the gas constant, and T iscentrifugal handling (4000 r/min for 8 10 minutes.the absolute temperature.The deposits were shifted to filter paper for furtherAccording to the above two equations, the kineticextraction with heptane for another 12 hours to wash parameters, the activation energy E; and the frequencythe impurities such as resins wrapped in the asphalte-factor A at diferent 与can be gained throughnes until the solution in the extractor was colourless.chemico-dynamic handling (Fu and Qin, 1995).At last, the asphaltenes were gained by extraction withIn our work, the geochemical significance of the .benzene. The prepared asphaltenes are conchoidal andparameter, frequency factor A, is discussed. The fre-brittle in most cases, with metal luster. The solventsquency factor A; increases exponentially with in-used were all of analytical grade.creasing activation energy E, which can be accountedThe contents of the elementsC, H, 0 and N of thefor through the micro-evolution features of asphalteneasphaltenes were measured with the instrument vario .pyrolysis. The related kinetic model discussed in小EL (execuive standard SY5122-86), and the organictails can be found in the authors' previous work (Liaosulphur was analyzed with the instrument LECO-and Geng, in press).CS400 (standard SY5116-86). The experiment on the4 Results and Discussionpyrolysis kineis of asphaltenes was crried out by theOGEII Work Station System in Beijing (execuive4.1 Potential of Silurian oilsands for regeneratingstandard SY5117-86). According to Fredman's first-hydrocarbonsorder model (Fu and Qin, 1995), the kinetic parameters,The potential of oilsands for regenerating hydrocar-the activaion energy E and the frequency factor A,bons can be directly simulated through the pyrolysiswere gained through chemico-dynamic handling.kinetics of asphaltenes.3 Kinetic ModelsThe pyrolysis results of asphaltenes are listed in Ta-ble 2. The distribution of activation energies is shownTissot and Pelet (1987) considered that the degradationin Figure 1. All the samples in this study have a highof oil constituents conforms with the kinetic first-orderpotential for generating pyrolytic hydrocartbons (Tablemodels. Regarding the micro-evolution features of2). S2 are all above 300 mg/g except Ha1-02. For all theasphaltene pyrolysis, the selected kinetic model is thesamples, S are small and. the average activation ener-中国煤化工YHCNMHG ..682Vol. 74 No. 3ACTA GEOLOGICA SINICASept. 2000ergy is above 450 kJ/mol. From theTable 2 Rock-Eval data and kinetic parametersvalues of frequency factor A in Ta-AverageMax. pyro.ble 2， we may find that theSampleFree HCPyro. HCactivationfrequency CoffcientNo.S (mg/g)S2 (mg/g) energy EfactorAparameter A is favourable for hy-T max(C)(kJ/ mol)_(1sc)drocarbon gener ation.Ha1-014341.14374.002551.63E+1609965Ha1-024415.23287.14276.83E+170.9952From the above discussion, theHa1-0319.09313.93321.02E+220.9982Silurian oilsands in this study anHa1-0425.11389.53451.16E+310 9961Ha1-05 .14.50326.00297.98E+18believed to be of high potential forregenerating hydrocarbons. TheTz11-0116.82471.214.35E+170.9836Tz11-02 .31 73471 732663 79E+17). 9831high content of the element sulphurTz11-0327.14458 57303.77E+200.9954.in Table 4 reflects a marine source32.852718.99E+170.9779of oilsands; there are relatively lowTz12-0231.90464.282577.94E+160.9933contents of the elements oxygenTz12-0343823.33513.842831.89E+180.9972and nitrogen and a high H/C ratio515.551.82E+19but a low O/C ratio. According tc* R indicates the correlation coefficient between activation energy E; and frequency factor logA; when thethe related study results for kerotransition rate of asphaltenes isx.gens, it is considered that the preTable 3 Distribution of the hydrocarbon yields (HC%) in dferent activation energysent of plenty of aliphatic segmentsintervals (k.Jmo)in the structure of asphaltenes isActivation energy interval (kJ/ mol)possible due to the fact that abun-<150150- -250 250- -300300-350350- -450dant hydrocarbons may still have5.4171.478.593.892.831.306.522.8037.2835.6410.456.507.33been generated by breakdown S-C8.20.2727.2416.5637.986.74and C-C bonds in the asphaltene1.595.5840.17.4123.00Ha1-051.840.7830.578.146.982.689.02molecules. The results of elementalanalyses further confirm that theT211-0180.756.263.052.401.166.34asphaltenes from the Silurian oiTz11-025.169.2578.876.71Tz1-033.1812.1247.0624.475.191.346.62sands have a high potential for 1e-Tz11-044.0710.9774.6710.30generating hydrocarbons.4.8637.2344.926.256.7628.1053.457.424.17.594.2 Silurian oilsands as potentialTz12-064.048.3035.7244.442source rocksIs it possible for Silurian oilsands togies E are large, which shows that the oilsands are ma-be hydrocarbon source rocks? Theture. There a good correlation between activation ener-present research results of this problem are still contro-gy E and frequency factor A (correlation coefficneit R=versial. Some authors don' t think that Silurian oilsands0.9779 -.9982), which indicates that the frequency can serve as efctive hydrocarbon source rocks (Liu etfactor A is advantageous to the reaction rate parameteral., 1997), while others take Silurian oilsands for subor-k; (Eq. (2)). It is especially important at the stage ofdinate hydrocarbon source rocks in the Tarim basinhigh activation energies because it can warrant the high(Zhang et al., 1997).hydrocarbon yield for asphaltenes. This can also beThe above discussion shows that Silurian oilsands inrealized from Figure 1.the Tarim basin have a high potential for hydrocarbonThe distributions of hydrocarbon yields (HC%) inregeneration and in addition are characterized by adifferent activation energy intervals (kJ/mol) are listedgreat thickness, widespread distribution and a highin Table 3. The major hydrocarbon generation intervalsTOC content (Table 1). Furthermore, they are mature.lie between 150 and 350 kJ/mol. For hole Ha#1, there isAll these indicate that the Silurian oilsands in the Tarimstill a good hydrocarbon yield when the activation an-basin have a F中国煤化工ns as sourceTYHCNMH G.Pyrolysis Kinetics of Asphaltenes in Silurian Oilsands in the Tarim BasinLiao et al.683Ha1-03Ha1-01Ha1-021s-呈圣10.w200.. 400 o00500 600E (kJ/mol)°70ηHa1-05Ha1-04Tz11-01200400600D00 1000。0200Tz11-02Tz11-03Tz11-04o叶20.山300 350°]Tz12-02 .Tz12-06 .Tz12-0330色2010400500100 20040E(k/mol)E(kJ/mo)FHg. 1. Rlains beween the yroearbo yied of apatenes and he dsribution of sivaion erges for al thesamples.中国煤化工MYHCNMH G .684Vol. 74 No.3ACTA GEOLOGICA SINICASept. 2000Table 4 Summary of elemental analyses of Silurian asphaltenes in the Tarim basinSample NC(%)H(%)0(%)N(%)S(%)_H/C (atom ratio) OIC (atom ratio)Ha1-0184.758.271.780.383.611.1710.0158Ha1-0286.34 .7.971.120.36 .3.561.1080.0097Ha1-0385.6 .1.360.321.150.0119Ha1-0486.26 .7.690.491.070.0106Ha1-0587.47 .7.891.040.510820.008984.700.28.1550.01698.172.030.631490.01792.180.601.180.01928.42.420.593.181950.0213Tz12-0282.178.342.680.57.2180.0245Tz12-038.72110.0149Tz12-0684.942.330.731800.0206rocks and should be a potential source rock. Accord-pyrolysis and their preliminary applications. Progress inNatural Science (in press) (in Chinese).ing to the results of the kinetic study of asphaltenes, theLin Renzi and Zhang Min, 1996. Compositional characteristicsoilsands can generate abundant hydrocarbons at the lateof light hydrocarbons and their genetic types of crude oils instage of pyrolysis due to the contribution of the fre-the Tarim basin. Geological Review (Supp), 42: 26- 30 (inChinese with English abstract).quency factor. This further indicates that the SilurianLiu Luofu, Xiao Zhongyao and Fang Jiahu, 1997. The Genesis ofoilsands in the Tarim basin have a great potential asTarim Silurian Oilsands and the Possibility as HydrocarbonSource Rocks (unpublished) (in Chinese).source rocks.Liu Luofu, 1998. Distribution and Significance of CarbazoleCompounds in Palaeozoic Oils from the Tazgong Uplift,5 ConclusionsTarim. Acta Geologica Sinica (Eng. ed.), 72(1): 87-93.Tissot, B.P., and Pelet, R., 1987. Thermal history of sedimen-The results of the pyrolysis kinetics of asphaltenes inary basins, maturation indices, and kinetics of oil and gasthe Silurian oilsands of the Tarim basin indicate that thegeneration. The American Association of Petroleum Geolo-gists Bulletin, 71(12): 1445- 1466Silurian oilsands possess a high potential for regen-Wang Tonghe, 1999. Structural Styles of Fronts of Thrust-erating hydrocarbons. They are mature and should beDetachment Faults in Petroleum-bearing Areas of Westernlatent hydrocarbon source rocks. They are important inChina. Acta Geologica Sinica (Eng. ed.), 73(4): 371-3exploration for petroleum reservoirs related to the Silu-Wang Yi, Zhang Yiwei, Jin Zhijun and Lu Kezheng,Analysis on the Tectonic- Stratigrahic Sequences in therian and especially in exploration for gas reservoirs inTarim Basin. Geological Review, 45(5): 504- -513 (in Chine-se with English abstract).related structures in the northern Tarim area.'u Bingsong, 1996. The Palaeozoic sequence stratigraphicsystem in northern Tarim basin. Geological Review, 42(1):Acknowledgements14- 21 (in Chinese with English abstrac).The authors are grateful to Fu Jiamo and Sheng GuoyingZhang Guangya and Song Jianguo, 1998. Control of BasicReworking on Hydrocarbon Accumulation and Preservationfor their helpful discussion and suggestions. This rein the Tarim Cratonic Basin, Northwest China. Geologicalsearch was a part of the key Project“Resource and Envi-Review, 44(5): 511- -521 (in Chinese with En glish abstract).Zhang Jiliang, Liu Yi, Bai Senshu et al, 1997. The Oil-Gas andronment Research” (KZ952-S1-435) supported by theResources in Tarim Basin. Beijjing: Geological PublishingChinese Academy of Sciences.House, 119 (in Chinese).Zhang Min and Zhu Y angming, 1996. Geochemical character-Manuscript received Nov. 1999istics of crude oils of the Kuche petroleum system in theTarim basin. Geological Review (Supp.), 42: 229-235 (inedited by Fei Zhenbi and Zhou JianReferencesFu Jiamo and Qin Kuangzong (eds.), 1995. The Geochemistry ofAbout the first authoriao ZewenBorn in 1969; graduated from theKerogens. Guangzhou: Guangdong Science and TechnologyPress, 541-552 (in Chinese).Department of Chemistry, Sichuan University, in 1991.Gu Yi, Luo Hong, Shao Zhibing, et al., 1998. The Genesis andNow he is studying in the Guangzhou Institute ofPreservation of Oil-Gas from North Tarim. Beijing:Geochemistry, Chinese Academy of Sciences for aGeological Publishing House, 126 (in Chinese)Karlsen, D.A., and Larter, S.R, 1991. Analysis of petroleumPhD degree. His major interest is petroleum gecfractions by TLC-FID: applications to petroleum reservoirchemistry. Telephone: 020-85290193; E-mail: liaozwdescription. Org. Geochem., 17(5): 603-617.Liao Zewen and Geng Ansong. Kinetic studies on asphaltenes@ gig.ac.cn中国煤化工MHCNMH G.