252
Energy & Fuels 1988,2, 252-258
pore volumes from 0.25 to 0.55 cm3/g, and benzene-accessible micropore volumes from 0.07 to 0.17 cm3/g. 4. The development of microporosity in the gasified chars is favored by the presence of humotelinite in the initial c d s ,while a greater content of humodetrinite seems to be related to the increase of mesoporosity.
Acknowledgment. This research was sponsored by POLTEGOR in Wroclaw and by the Polish Academy of Sciences (Scientific Program CPBP 01.16). Acknowledgement is made to the donors of the Petroleum Research
Fund, administered by the American Chemical Society, for partial support of this activity. We feel indebted to Dr. 0. P. Mahajan, Amoco Corp., Naperville, IL,for great help in the presentation of this work a t the Symposium on the Surface Chemistry of Coals. Thanks are also due to Dr. M. Stolarski,Dr.E. Sliwka and Dr.J. Surygala from the Technical University of Wroclaw for providing the samples of extract residues and to Dr. J. Szwed-Lorenzfor the petrographical analyses of brown coals. Ragistry No. C, 7440-44-0.
Characterization of Sedimentary Organic Matter by Preparative Pyrolysis: Comparison with Rock-Eva1 Pyrolysis and Pyrolysis-Gas Chromatography Techniquest M. Vandenbroucke,* F. Behar, and J. Espitalig Institut Frangais du Pgtrole, BP 31 1, 92506 Rueil Malmaison Cedex, France Received May 5, 1987. Revised Manuscript Received November 16, 1987
Estimation of petroleum potential (amount and quality) of sedimentary organic matter is now often done by pyrolyzing the organic matter and analyzing effluents. A new technique, preparative pyrolysis followed by separation and detailed analysis of saturates and unsaturates, is compared with Rock-Eval pyrolysis and pyrolysis-gas chromatography. Examples are given for the three main types of organic matter, through samples taken a t the beginning of the diagenesis zone and a t the beginning of the catagenesis zone. They show that preparative pyrolysis is more time-consuming than Rock-Eval or Py-GC techniques but gives a very clear differentiation between types of organic matter, even when mixtures of organic inputs or alteration problems occur, and this occurs as early as the diagenesis stage.
Introduction Organic geochemistry applied to petroleum exploration aims to analyze the organic mattter from sediments to predict the amount and quality of crude oils in reservoirs. The main problem there is to characterize both the origin and the thermal evolutionary level of this organic matter, by studying either the extract or the kerogen. For extracts, some phenomena such as migration or contamination can modify or even prevent such reconstruction. These phenomena do not affect kerogen. Its macromolecular structure can be studied either by bulk analyses or by destructive techniques (thermal or chemical). This paper focuses on three pyrolysis techniques developed a t the Institut Franqais du PBtrole (IFP): a bulk one, the wellknown Rock-Eval pyrolysis, and two analytical ones, pyrolysis-GC and preparative pyrolysis, with particular emphasis on this latter, which is the moat recent and for which few results are published. Rock-Eva1Pyrolysis A. Description. This pyrolysis technique is largely used among organic geochemistry labs, and detailed characteristics have been published elsewhere.'-, Therefore only a brief review of the parameters obtained by this method will be presented. 'Presented at the Symposium on Pyrolysis in Petroleum Exploration Geochemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-6, 1987.
Figure 1shows a simplified diagram of the device and of the resulting data in the most frequent configuration (Rock-Eval I1 with a carbon module for analysis of sediments in the oil window zone). The sample (bulk sediment or pure organic fraction) is placed in a porous crucible, swept by helium, and then heated a t 300 "C for 3 min. Free hydrocarbons are thermovaporized, and the resulting SIpeak is recorded on the flame ionization detector (FID). The temperature is then raised to 600 "C with a 25 "C/min ramp. The pyrolysis effluents are measured by the S, peak on the FID. The maximum of this peak occurs at a temperature (T-) that is characteristic of the natural maturation stage of the organic matter in the sediment. Meanwhile, the carbon dioxide released during the pyrolysis up to 390 "C is trapped (S,peak) and then analyzed by a thermal conductivity detector (TCD). The sample is then transferred to an oxidation oven (600 "C, air or oxygen sweeping) so as to burn the residual carbon in the sediment. The resulting carbon dioxide (S, peak) is measured by a TCD. The weight of sediment to by pyrolyzed is measured, and the detector responses are calibrated according to a standard sediment. The whole set of data can thus be related to the unit weight of sediment (1)EspitaliB, J.; Laporte, J. L.; Madec, M.; Marquis, F.; Leplat, P.; Paulet, J. Reu. Inst. Fr. Pet. 1977,32, 1, 23-45. (2) Orr, W. L. In Aduames in Organic Geochmistry 1981, Bjoroy,M., et al., E&.; Wiley: Chichester, England, 1983; pp 775-787. (3) EspitaliC, J.; Deroo, G.; Marquis, F. Rock-Eual Pyrolysis and its Applications; Editions Technip: Paris, 1987.
0887-0624188/2502-0252$01.50/0 0 1988 American Chemical Society
Energy & Fuels, Vol. 2, No. 3, 1988 253
Characterization of Sedimentary Organic Matter
CRUDE ROCK
T.C.D.
am. /
/'
700.
Figure 1. Simplified diagram of the Rock-Eval I1 device.
or organic carbon, this latter being taken as the s u m of carbon in S1, S2,and S,. The carbon in the S3 peak is negligible in the oil and gas window; however, this procedure leads to an underestimation of the total organic carbon in immature sediments. The hydrogen index, HI, is defined as the amount of pyrolyzable organic matter (S,) by unit weight of organic carbon; the oxygen index, 01, is the amount of carbon dioxide (S3) by unit weight of organic carbon; the production index PI is S1/(S1 + S2). B. Recording and Interpreting the Rock-Eval Data. The size of samples to be pyrolyzed is smalh weights are 100 mg for bulk sediment and 10 mg for pure organic matter. The whole set of analyses takes around 20 min, and the device is equipped with an automatic sampler. Theae characteristics permit the analyses of a large number of samples, thus providing a statistical approach of important parameters for source-rock evaluation. The Rock-Eval parameters are generally compared to those of three reference types of organic matter by means (Figure 3) diaof the HI/OI (Figure 2) and HI/Tgrams."' However, an interpretation of data from bulk sediments in terms of organic matter types is sometimes confusing, particularly when the carbon content is low: HI can be severely decreased by the mineral matrix effect. This problem can be at least partly overcome by correction charta or pyrolysis of the corresponding isolated kerogens. C. Application to Exploration Problems. 1. Geochemical Logging and Mapping. The Oil Show Analyzer, the version of Rock-Eval for use on well sites, permits the analysis of cuttings as soon as they are recovered. Measured parameters are organic carbon content, oil and gas amounts, remaining generative potential, production index, and 5"-. Figure 4 shows an example of geochemical logging.6 Furthermore, evaluation maps of source rocks in a basin area can be built and used as basic data for mathematical modeling.e (4) Tissot, B.; Durand, B.; Espitalib, J.; Combaz, A. AAPG Bull. 1974, 58,4!39-606.
(5) Espitalib, J.; Marquis, F.; Barsony, I. In Analytical Pyrolysis; Voorheea, K. J., Ed.;Butterworth: London, 1984; pp 276-304. (6) Ungerer, P.; Pelet, R. Nature (London) 1987,327, 52-64.
f
1
1 0
50
100
150
-OXYGEN
200
250
300
I N D E X ( m q C o t / q TOC 1
400 __*
Figure 2. Classification of reference organic matters in the HI/OI
diagram. Reproducedwith permission from ref 3. Copyright 1987 Editions Technip.
.
a
Immature-oil limit : 0 FM types 1,nadn
.
380 900 420 443 460 480 500 520 540 56 Tmx('C1
___*
Figure 3. Classification of reference organic matters in the HI/T,, diagram. Reproduced with permission from ref 3. Copyright 1987 Editions Technip.
2. Source-RockEvaluation. The diagramsHI/OI and HI/?"- obtained from Rock-Eval pyrolysis are used, with the restrictions discussed above. 3. Screening Method for Detailed Studies. RockEval pyrolysis is often used for selecting samples for further geochemical analyses.
Pyrolysis-Gas Chromatography A. Description. This technique allows the chromatographic separation of hydrocarbons issued from thermovaporization or pyrolysis (peaks S1or S2of Rock-Eval), but it should be used only with pure organic matter, i.e. asphaltenes, kerogens, coals, humic acids, etc., as the
Vandenbroucke et al.
264 Energy & Fuels, Vol. 2, No. 3, 1988
WELL N o . 2
1 7 PRODUCTION ,NDEX (GAS)
PRODUCTION PRODUCTION PyRoLYslS INDEX INDEX TEMPERATURE (OIL) (0IL + GAS1 (TI
--PETROLEUM POTENTIAL (kg HC/t rock)
r
t
c
L
=IFigure 4. Example of geochemical logging. Reproduced with permission from ref 5. Copyright 1984 Butterworth.
mineral matrix effect lowers especially the CI5+ fraction of the effluent. The IFP device consists of a microfurnace, shown in Figure 5, which replaces the injector in a classical gas chromatograph.' The furnace is directly connected to the capillary column. Other devices using Curie point fila(7) Behar, F.; Pelet, R.; RoucachB, J. Org. Geochem. 1984,6,587-595.
ments are often used,8v9but with a microfurnace, any vaporization or pyrolysis temperature can be selected. The carrier gas, helium, flows through the furnace. The sample (25-100 pg) is introduced into the system inside a small (8) Meuzelaar, H. L. C.; Ficke, H. G.; den Harinck, H. C. J.Chromatogr. Sci. 1976, 13,12-17. (9) van de Meent, D.; Brown, S. C.; Philp, R. P.; Simoneit, B. R. Geochim. Cosmochim. Acta 1980,44, 999-1013.
-
Energy & Fuels, Vol. 2, No. 3, 1988 255
Characterization of Sedimentary Organic Matter GC DEVICE
PYROLYSIS DEVICE 1
. et
od
5mm
Figure 5. Microfurnace device for Py-GCstudies.
boat carved a t the end of a gold rod. It is first placed in the cold part of the device to sweep out any air or residual solvent. Then it is pushed into the hot part, the temperature of which is regulated a t the chosen value (maximum 550 "C), left there for 30 s, and then withdrawn. During pyrolysis, the chromatograph oven is kept a t 0 "C. The pyrolysis effluents are thus trapped on the column head except Cl-C5hydrocarbons. Separation is then done according to the following conditions: capillary column coated with a CPSil 5 film 0.45 pm thick, with an inner diameter of 0.32 mm, and a length of 25 m; temperature program 0-300 "C with a 6 "C/min ramp. A quantitative evaluation versus time or carbon range can be performed by means of a paraffin oil standard. B. Developments of the Method. Only one part of the pyrolysis effluents is eluted through the apolar capillary column, and as hydrocarbons and polar compounds are injected together, there is often a high unresolved background and the numerous peaks obtained are not easily identifiable. This problem occurs particularly for immature samples where volatile oxygenated compounds are abundant. Many authors have tried to overcome the difficulty with Py-GC-MS or direct Py-MS.loJ1 However when detection is done by measuring the total ionic current in place of using a FID, the nonlinear response due to the ion multiplier prevents quantitative analysis. In some cases, the lack of molecular peaks or characteristic fragments in some compounds does not allow their identification. Additional problems arise from the complexity of the mixture, which decreases the contribution of each compound. Nevertheless, this technique offers an elegant way to recognize some specific biological compounds by characteristic pyrolysis products such as furans from sugars or cellulose or vanillyl, syringyl, and guaiacyl derivatives from lignin. Pyrolysates of samples in the oil window maturation level are easier to analyze, as they contain a greater amount of simpler hydrocarbons. The use of a flame ionization detector allows, in that case, a quantitative estimation. However, only effluents able to go through the chromatographic column are monitored, and this can prevent comparative studies because of the variations of the nature of pyrolysates trapped in the column head. C. Application to Exploration Problems. Py-GC techniques are used a t the IFP for oil and source-rock correlations based on pyrolysis of asphaltenes from oils and chloroform e x t r a ~ t s . ~ JThe ~ example shown in Figure 6 illustrates the identification facilities in the case of biod~~~
(10) Wilson, M. A.; PhiIp, R. P.; Gillam, A. H.; Gilbert, T.D.; Tate, K.R. Geochim. Cosmochim. Acta 1983,47,497-502. (11) Nip, M.; de Leeuw, J. W.; Schenck, P. A.; Meuzelaar, H. L. C.; Stout, S. A.; Given, P. H.; Boon J. J. J. Anal. Appl. Pyrolysis 1985,8, 221-239. (12) Pelet, R.; Behar, F.; Monin, J. C. In Advances in Organic Geochemistry 1985, Part I; Pergamon: Oxford, England, 1986, pp 481-498.
egraded oils. It shows clearly that before biodegradation oil C had an n-alkane distribution similar to that of the type B oils. A similar technique can be used for correlations between oils and source rocks, as it has been shown that pyrolysis of asphaltenes and kerogens from the same source-rock releases the same hydrocarbon fragments.12 An advantage of this method is that only asphaltenes need to be prepared and not kerogens. An evident requirement is that the maturation level of the source rock that is to be compared corresponds to the equivalent maturation level of the oil.
Preparative Pyrolysis A. Description. Pyrolyses are performed in an originally designed oven (Figure 7). Again the pure organic matter, extracted by chloroform if necessary, is introduced in a small boat carved a t the end of a gold rod. The rod is first pushed through a leak-proof nut and gasket into the cooler (ca. 60 "C) front part of the oven. In this device the carrier gas is argon instead of helium. A preliminary heating a t 320 "C (150 "C for immature organic fractions) for 2 min vaporizes residual hydrocarbons, if any. The oven temperature is then raised to 550 "C (balistic ramp, around 60 "C/min) and allowed to stand a t 550 "C for 5 min. Outside of the pyrolysis zone, argon, carrying the compounds released by the. sample, liquefies in the trap that is cooled by liquid nitrogen. When pyrolysis is completed, the trap is disconnected, removed from the liquid nitrogen and allowed to heat slowly. Argon vaporizes and flows out of the trap. The pyrolysate is then dissolved in chloroform and recovered by evaporation. Consequently, low-molecular-weight compounds are lost up to molecules containing ca. 14 carbon atoms. The pyrolysate is finally fractionated by microcolumn liquid chromatography into saturates and unsaturates, aromatics and slightly polar compounds, and NSO compounds (Figure 7). Each step of the fractionation of saturates and unsaturates is analyzed by GC with a FID. Saturates are then fractionated into n-alkanes and iso/ cycloalkanes by insertion in 5-A molecular sieves. The n-alkanes, which are easily analyzed as the major peaks of the preceding GC step, are not recovered. The iso/cyclo fraction is analyzed by GC-MS. The unsaturated fraction dissolved in n-heptane is saturated by hydrogenation under 40 bars of H2at 120 "C for 1/2 h with a commercial catalyst (rhodium fixed on alumina). The mass balance and GC of the hydrogenated unsaturates ensure that the reaction is quantitative. The same fractionation procedure using 5-A molecular sieves is then applied to the hydrogenated unsaturates. The hydrogenated n-alkenes are discarded as previously, and the hydrogenated iso/cycloalkenes are analyzed by GC-MS. B. Application to Exploration Problems. This method can be used for research of new parameters, which could be a guide for petroleum exploration. Some questions have been partly solved with this technique, as shown by the following examples. 1. Comparison of Carbon Structures in Organic Fractions from Immature and Mature Sediments. The organic content of recent sediments, rich in oxygenated functional groups, is hardly soluble in organic solvents. On the contrary, it can be partly extracted by acidic or basic aqueous solutions. A widely used fractionation procedure consists of 0.1 M NaOH extraction, followed by precipitation of the solution by HC1 at pH 2. Fulvic acids are defined as the fraction soluble both in acidic and basic solutions, and humic acids are the fraction soluble only in basic solutions. The insoluble part of the organic matter can be obtained after destruction of minerals and is termed
Vandenbroucke et al.
256 Energy & Fuels, Vol. 2, No. 3, 1988
I
TYPE A
TYPE B
CRUDE OIL
CRUDE OIL
I
HCS
I
BIODEGRADED HCS
OIL
1
I ASPHALTENE 450.C.3OS
I
ASPHALTENE 402.301
~
ASPHALTENE
ASPHALTENE 550.C.30~
550.C-30s
i 20
10
10
XI
XI
10
Figure 6. Correlation of oils by Py-GC of asphaltenes. Reproduced with permission from ref 7. Copyright 1984 Pergamon. PYROLYSIS DEVICE
TRAPPING DEVICE -
>
ORGANIC
IS@C-SS@C 6O"Clnm
Mdirvlir 5 A
SI*#
Rho6vnIL0, 15%) PHs = 40 b, l2oCC
Figure 7. Preparative pyrolysis: device and analytical procedure.
stable residue (or sometimes humin). Knowledge about the structural relationship between these fractions allows understanding of their behavior, their relation to the kerogen of ancient sediments, and their contribution to the source-rock oil potential. By this technique we have compared recent organic matter from lacustrine, marine, and terrestrial environments.13 Figure 8 shows the total alkanes and alkenes of the CI3+ fractions from pyrolysates of stable residues, humic acids, and fulvic acids in these environments. The
following points are observed (a) For immature stable residues, the carbon distributions of n-alkaneslalkenes in the CI3+ pyrolysates are typical of the organic environment and show the same pattem as n-alkanes in oils or extracts from mature sediments in similar pale~environments:'~ a flat distribution for the lacustrine sample, a decrease beyond Cz0for the marine sample, and odd/even predominance in the range C,-C, for the terrestrial sample. Thus stable residues and kerogens are related structures. (b) The carbon distributions are fairly similar in pyrolysates of stable residues and humic acids, although the latter shows a shift of carbon numbers towards lighter molecules and an enrichment in polycyclic structures. On the contrary, carbon distributions from fulvic acids are atypical. Thus carbon structures and the oil-generating potential are similar in stable residues and humic acids, whereas fulvic acids have different structures. 2. Organic Sedimentology. The source of the organic input in sediments can be characterized according to hydrocarbon distributions in insoluble fraction pyrolysates. This allows, for instance, a distinction to be made between alteration of an initial algal organic matter and a mixture of this matter with terrestrial input, which in both cases lowers the atomic H/C ratio and the Rock-Eval hydrogen index. It will be illustrated here on an example from one of the African rift lakes. Recent sediments sampled in this lake, in zones where there is only autochthonous algal input and in others where an allochthonous input by rivers is possible and where sediment redistribution by bottom streams occurs, have been compared. Two sediments sampled along the side (Ll) and in the bottom (L2)of the subaquatic channel prolongating a river flowing into the lake were analyzed. As shown in Table Ia, geochemical data on L1 kerogen indicate a typical algal organic matter whereas L2 kerogen could either be a mixture of algal and terrestrial organic matters or algal organic matter only but altered during transportation processes. The yields and composition of pyrolysates are given in Table Ib, and gas chromatograms of fractionated alkanes and alkenes are shown in Figure 9. It can be observed that whereas the hydrogen index is lowered by a factor of more than 2 in L2 kerogen, the GC traces of saturates and unsaturates are similar and typical of lacustrine algal organic matter, as shown by comparison with ~
(13) Vandenbroucke, M.; Behar,F. In Lacutrine Petroleum Source
Rocks; Geological Society Special Publication; Blackwell: London, 1987; in press.
~~
(14) Tissot, B.;Pelet R.; Roucach6, J.; Combaz, A. In Advances in Organic Geochemiatry 1975; Campos, R., Goni, J., Eds.; Enadimsa: Madrid, 1977; pp 117-154.
Characterization of Sedimentary Organic Matter
Energy & Fuels, Vol. 2, No. 3, 1988 257
.
SATURATES + UNSATURATES
/ STABLE RESIDUES
\
HUMIC ACIDS
FULVIC ACIDS
i d Y
a E
3
I
I
a0
I
20
YI
50
20
CARBON NVYBER of n- A L W N S
B b
Figure 8. GC traces of total alkanes/alkenes in the CIS+ pyrolysates of stable residues, humic acids, and fulvic acids from lacustrine, marine, and terrestrial recent sediments. Table I. Lacustrine Samples
sample
L1 L2
water depth, m 355 243
(a) Main Geochemical Parameters for Initial Samples % organic C sediment kerogen atomic H/C atomic O/C 6.8 60 1.31 0.21 3.1 56 0.89 0.31
HI, mg/g of C 539 204
(b) Pyrolysis Data SamDle ~
L1 L2
Dvrolvsate vield. mile of C 551 230
'Weight percent of the pyrolysate.
alkanes + alkenes' 14.3 11.6
*Weight percent of total alkanes
the GC traces of Figure 8. The decrease of the hydrogen index can thus be related without ambiguity in this sample to alteration of the same initial algal organic matter. Several observations concerning specific biomarkers in the pyrolysis effluents show the possible uses of this method for resolution of various problems in geochemistry. For instance, we can observe the following: (a) The compounds leading later to pristane, phytane, and steranes are obtained for the major part in the form of alkenes. This could signify that they are linked in the organic matter of the sediment by ether, ester, or amide links. (b) Hopanes deriving from bacterial attack during sedimentation are found in the pyrolysates both in the alkene
aromatics' 10.1 9.7
saturatesb 27 36
unsaturatesb 73 64
+ alkenes. and alkane fractions. In the iso/cycloalkane fraction, 178(H)isomers, thermally unstable, are the major constituents. This is a proof that secondary isomerization reactions are not important and that carbon structures in kerogens can be determined by preparative pyrolysis. In the alkene fraction, it cannot be proved as 5-A molecular sieve insertion requires previous catalytic hydrogenation, which induces isomerization. As the less stable triterpane isomers are found in recent sediment pyrolysates, the different isomer ratios can be used as a maturation parameter, even when the isomers cannot be measured in solvent extracts. (c) An advantage of preparative pyrolysis over classical Py-GC or Py-GC-MS is the possibility to concentrate some
Vandenbroucke et al.
258 Energy & Fuels, Vol. 2, No. 3, 1988
ASPHALTENES WROIYSIS 550'c
KEROGEN PYROLYSIS 550T 1
SAMPLE 11 IHI=5391
SAMPLE L2 IHI=2041
II
I
I
I *.I..".
ALKeNES
YPE A .CRUDEOIL
lSO* CYCLO ALKANES
cg
YPE A .CRUDEOIL
/:2i
I'
I S 0 +CYCLO ALKENESlHZ
TYPE B .CRUDE OIL 15
25
h
CARBONNUMBEROFn ALKANES
is
1
-
Figure 9. Comparison of GC traces of alkanes and alkenes from kerogen pyrolysis in recent lacustrine samples. Names of some hopane isomers are abbreviated according to the following: C2, trisnorhopanes, 17a(H) = H27a and 17j3(H) = H27B; C29 norhopanes, 17P(H)21a(H)= H29Pa; C30 hopanes, 17fl(H)21S(H) = H30j3P; C31 homohopanea, 17@(H)21P(H) = H31,@. Key 527, 5a-cholestane;Pr, pristane; Ph, phytane; Pr-1-ene,pristene-1. specific biomarkers by a fractionation procedure, thus making their identification easier. Moreover, GC of the iso/cyclo fraction with a flame ionization detector allows absolute quantitative determination of isomers. This is an advantage over mass spectrometry, in which the size of peaks in fragmentograms depends on the spectrometer calibration and on the total composition of the hydrocarbon fraction. Thus, different samples can be compared quantitatively. 3. Use of Preparative Pyrolysis for Correlations. This pyrolysis technique applied to asphaltenes or kerogens allows the use of biomarkers for correlations. Figure 10 shows fragmentograms of m / e 191 and 217 for the total alkanes and hydrogenated alkenes in the pyrolysates of Venezuelan oils already seen by Py-GC. The biodegraded oil relates unambiguously to the type B oil by the steranes issued from pyrolysis; in contrast, as pentacyclic triterpanes do not show characteristic distributions, only tricyclic molecules of the 191 fragmentogram can be used for correlations. 4. Use for Maturation Assessment. Although hopanes from pyrolysates do not allow an identification of the organic matter type, the delay of their isomerization in the insoluble fraction relative to that in the extracts allows quantification of the maturation stage in advanced steps, even when equilibrium is reached for extracts. Therefore, hopanes from pyrolysates could be used for maturation determination when the amount of chloroform extract is too low to permit the identification of hopanes in extracts or when hopanes in extracts have already reached an isomeric equilibrium ratio.
Conclusions The pyrolysis techniques described in this paper have different specific applications in the exploration problems. Rock-Eval pyrolysis allows a rapid evaluation of the organic carbon content and petroleum potential on a great number of samples. It is the only way to establish geochemical maps and logs. Its statistical value permits a judicious choice of samples to be examined by more detailed analyses. The determination of origin and thermal
GC TIME DIRECTION
.
Figure 10. Correlation of oils by polycyclic biomarkers in asphaltene pyrolysates: left, fragmentograms for m / e 191 showing tricyclic molecules between Cm and CZsand pentacyclic hopanes between Cn and C,; right, fragmentograms for m / e 217 showing the C2,, Cza, and Cze steranes. evolution stage of the organic matter by this technique must be ascertained by other analyses. Pyrolysis-GC is a fairly rapid method, but a clear differentiation of organic matter types is obtained only in the catagenesis zone. An ideal application is the correlation of oils and rock extracts by pyrolyzing asphaltenes. Its advantage is the possibility to quantify the CI3- fraction of the pyrolysate. Preparative pyrolysis is a time-consuming but powerful technique that is more or less dedicated to laboratory rather than well site applications. The detailed study of hydrocarbons in pyrolysates has the potential to examine various problems: organic sedimentology, biomarkers, and structural study of macromolecules. However, the present technique does not allow the recovery of the CI3- pyrolysate. These different techniques are not exclusive. Rock-Eval pyrolysis including TOC analyses should always be the first geochemical analysis done on any sample. The other techniques are supplementary and provide much more detailed information on the nature of the organic matter.
