Maturity Control on the Patterns of Hydrocarbon Regeneration from Coal

Vol. 74 NoACTA GEOLOGICA SINICAJune 2000Maturity Control on the patterns of HydrocarbonRegeneration from CoalzoU Yanrong,Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640YANG QL, LIU Dameng, KANG Xidong and TANG DazhenChina University of geosciences, Beijing 100083Abstract Rock-Eval pyrolysis and Py-GC experiments on naturally and artificially matured coal samples werecarried out. The results suggest that both depolymerization and defuctionalization exist during the maturation andevolution of coal. The patterns of hydrocarbon regeneration are diverse at different stages of the maturation and evolution.When the vitrinite reflectance(Ro)is 0.7%-0.9%, bitumen is the richest in coal while activation energy is theminimum, and the temperature of peak yield is lower than that of primary hydrocarbon generation. However, if re-flectance is greater than 0.9%, defunctionalization predominates and the temperature of peak regeneration is shown ilagging compared with the primary hydrocarbon generation. When reflectance is out of the oil window", the peaktemperature of hydrocarbon regeneration and that of the primary hydrocarbon generation seems to be continuousKey words: coal, maturity, pyrolysis, hydrocarbon regeneration1 Introduction2 Samples and ExperimentSome hydrocarbon regeneration experiments have The coal samples were collected from the coal-bearinbeen performed in recent years. The results are diverse formations-the Taiyuan Formation ( C3t), Shanxibecause of maturity variation of samples employed In Formation (P1s), Xiashihezi Formation (P2xcomparison with the primary peak hydrocarbon gen- Shangshihezi Formation(P2S), in the late Palaeoeration, the hydrocarbon regeneration peak has been sequence, North China. Their vitrinite reflectancesdescribed in lagging(Ran, 1995) or in continuance (Ro) are from 0.65%0-0.92%, i.e. within the"oil wn-(Liu et al., 1996). At present, however, it is lacking in dow. The geological characteristics of samples arestudy of hydrocarbon regeneration of samples with present in tareflectance Ro) ranging from 0.6%to 0.9%0. It is deThe coal samples were ground into powder. Theserved to further investigate the relationship between powdered sample Al was split into two groups. One isthe two peaks. The hydrocarbon regeneration peak of naturally matured samples and the other is artificiallycoal, collected from the late Palaeozoic coal-bearing matured samples after heated at a constant heating ratesequence in North China, is investigated in this paper. of 20C/min to give its residues at different tempera-turesTable 1 Geological characteristics and pyrolysis data of coal samplesT-peak(mg/g)(mg/g)(mg/g TOC) (mg/g TOC)(C)232.750.81P243714.6199.340.892B114365563l134中国煤化工CNMHGMaturity Control on the Patterns of Hydrocarbon Regeneration from Coal Zou Yanrong et al.Rock-Eval pyrolysis was used to measure the hypeak in Table 1) of some highly matured samples aredrocarbon yield of powdered samples and their resi- lower than those of naturally matured samples. Thedues at different temperatures with a constant heating hydrocarbon -yield curves of artificially matured samrate of 20C/min from 200t to 600T for all sam- ples against temperature are demonstrated in Fig.1ples in order to obtain curves of hydrocarbon yield vs. Obviously, the peak product temperatures of its resitemperature for naturally and artificially matured dues at 350t and 400t are lower than those ofcoals. In addition, both powdered coal Al and its resi- original sample Al. Further investigation of the residue at 350T were subjected to the analysis of Plues at 300C and 380C exhibits that their temperatures of peak product is between 415C and 430C, i.elower than those of sample Al. This is similar to the3 Results and discussionexperiment result of sample Sll. In other words, thepeak product temperatures of some highly maturedRock-Eval pyrolysis data for the naturally matured samples, no matter naturally or artificially, are rela-samples are given in Table 1. Some curves of hydro- tively low when they are reheated as long as theircarbon yield vs temperature are illustrated in Fig. la, reflectance is larger than 0.67-0.92%. The Py-GC datawhich shows that the peak product temperatures (i.e. T- of sample al show that low-carbon-number hydrocarbon piinly beCl and C20. when heated up to350C( Fig. 2a); whereas highcarbon-number hydrocarbon, fromMg1Ro=0.92%C20 to C30, predominates whenc13Ro=081%residue at 350C is heatedB1bRo=086%fromoom temperature to 340Cresult from another angle. as theat350℃ is heatedfrom350℃to400℃, the productis still prede200250350600carbon-number hydrocarbon due toTemperature(℃)further cracking of macromoleculehyThwithin a certain Ro range, a largeSample Atregenerated from coal at temperaResidue at350℃tures lower than the primary peakResidue at 400r itemperatures of hydrocarbon gen-Residue at450c!氵erationCoal is not a homogeneoussubstance and consists of variousconstituents-nacerals(Stahl1982).Thhydrocarbon generating capacity of0025050an important maceralmacerals and certain optical propig. 1. Hydrocarbon yield variation of Al with temperature:(a) for naturallyerties of vitrinite and sporinitematured samples and(b) for artificially matured samples中国煤化工CNMHG372Vol. 74 No. 2ACTA GEOLOGICA SINICAJune 2000Oil and gas formation may bedescribed by two types of reactionpolymerization anddedefunctionalization (Ungerer,1990). The former considersheavy compounds like asphalteor resin as intermediates in the30formation of oil, which maydecompose into extractable products. and this reaction schemeseems more suitable for kerogenof type I; while the latter is more520representative of coal pyrolysis, inCarbon Numberwhich oil and gas are released asthe bonds are rupture from looserto tighter ones. As stated abovehowever, coal is heterogeneouscals. Thehydrogen-rich macerals in coal areof type Iduring hydrocarbon generationand extractable organic matter,uch as asphaltene and resins arefirst generated. Hydrocarbon andresin+aspaltene are degraded andCarbon Numberkerogen decomposition (DelvauxFig. 2. Carbon number distribution of pyrolysis:(a) coal Al heated from roomet aL., 1990; Wu et al., 1986),temperature to 350C;(b)residue at 350C heated from room temperature to 340/Cwhich causes a T-max shift to-wards low temperrelationship between coal properties and stages ofal., 1986). In addition, the activation energy ofgeneration (M. Teichmuller, 1986) have been thebecomes lower because of bitumen(Shen et al., 1996)oughly studied by previous researchers. Heavy hydroHydrogen-poor macerals in coal, however, appear closecarbon (Murchison, 1987) and coal extracts (M. to defunctionalization, and petroleum is released asTeichmuller, 1986; Lin et al., 1987)reach the maxr tighter bonds are ruptured In other words, both depo-mum yield when the vitrinite reflectancelymerization and defunctionalization exist during the0.8-0.9%, which coincides with the second coalific a-whole evolution and maturation of coal. but which onetion jump(M. Teichmuller, 1986). The results suggestis predominant depends on the maturity and maceralsthat the hydrocarbon generation is related to the matuof coalrity and maceral of coal. The fact that so many changesThe hydrogen-rich macerals of coal are decomposedtook place at the same stage of rank reflects an impor- first into heavy compounds under naturally and artifi-tant change of the chemical reaction type, i. e. a shiftcially maturation conditions. When the reflectancefrom predominantly hydrocarbon-generating to pre- about 0.9%, depolymerization of hydrogen-rich macerdominately fragmentation reaction (M. Teichmullerals finishes and bitumen and the generation of heavyand R. Teichmuller, 1982.)hydrocarbonTYno to the mini中国煤化工CNMHGMaturity Control on the Patterns of Hydrocarbon Regeneration from Coalal Zou Yanrong et al.mum of activation energy. After that, bitumen and the primary hydrocarbon generation. As a result, theheavy hydrocarbon become less and less as they crack peak temperature of regeneration from coal will appearfurther into gas, and the defunctionalization of hydro- earlier. When Ro is greater than 0.9%0, the peak temgen-poor macerals becomes more and more important perature peak regeneration appears later in comparisonin the formation mechanism of hydrocarbons. Along with the primary one, while the peaks for the primarywith the transform of oil and gas generation mechanism, and regeneration might be continuous as the reflectancethe activation energy gets higher again. While the is less than 0.6%. The experiments in this study togethreflectance is greater than 0.9%0, the defunctionalization er with the results obtained by Q.G. Ran(1995)and lipredominates. As a result, the activation energy of coal et al (1995, 1996) presented a dynamic series ofdefunctionalization becomes higher and higher(Fig 3). hydrocarbon regeneration patterns, which demonstratesIt should be noted that the activation energy increases the control of maturity control over the patterns oflocally when Ro is between 0.6-0.7%. This may be hydrocarbon regeneration from coalcaused by the condensation within this Ro range(Wang Soluble organic matter, no matter it is regarded as aet al., 1997; Bahr et aL., 1991). Therefore, hydrocarbon primary product or an intermediate in the formation ofregeneration patterns of the coal with Ro ranging from petroleum, is an important part of source rocks for0.6-0.7% seem to be rather complicated and await hydrocarbon regeneration and contributed to the regenfurther investigationeration, especially as Ro ranges from 0.7-0.9%, whichshould be noticed in the evaluation of hydrocarbon3 ConclusionsregenerationThe experiment results in this study demonstrate that Acknowledgementsthe pattern of hydrocarbon regeneration from coal iscontrolled by maturity. The patterns are diverse at This project was granted financial support from thedifferent stages of maturation and evolution. when the China Postdoctoral Science Foundation and Chinesevitrinite reflectance Ro) is between 0.7%0 and 0.9%o, National Key Science and Technology Project (96-110the bitumen is the most abundant in coal, and the acti- 01-02)vation energy is relatively low. The temperature of thek product will become lower compared with that ofChinese manuscript received May 1999epted Nov 1990edited by Zhou JiaEnglish manuscript edited by500O- E10Liu XinzhuReferencesBahr. M. Akiyama. M. and Sanadia.Y.. 1991. In situ high temperature ESR measurements forkerogen maturation. Org. GeDelvaux, D, Martin H, Leplat P.Eval pyrolysis as an improved toolfor sedimentary organic matteranalysis. Org. Geochem, 16(4-6)Fig 3. Activation energy varying with maturity within the oil windowE10 and E90 denote the activation energy when hydrocarbon yields are 10% and 90%0LinR Davis A, and Bensley D Frespectively1987. The Chemistry of vitrine中国煤化工CNMHG374Vol. 74 No. 2ACTA GEOLOGICA SINICAJune 200011(5):393-399ry and state of the art. Org. Geochem, 10: 581-599Liu Luofu, Wang Weihua and Li Suyuan, 1995. The simulation Ungerer, Pstudy on secondary generation of hydrocarbon. Acta Sediing of oil formation and expulsion. Org. 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