Energy & Fuels 1987, I , 468-476
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Hydrocarbon Distributions in Crude Oil Asphaltene Pyrolyzates. 1. Aliphatic Compoundst D. M. Jones* and A. G . Douglas Organic Geochemistry Unit, Geology Department, The University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K.
J. Connan Elf-Aquitaine (Production), 64018 Pau Cedex, France Received July 7, 1987. Revised Manuscript Received September 16, 1987
The pyrolysis of asphaltenes has been proposed as a potentially useful method for oil-oil and oil-source rock correlations, particularly for oils that have been altered by biodegradation. In order to investigate the effects of the pyrolysis conditions on the amounts and distributions of the aliphatic hydrocarbons generated, asphaltenes separated from two crude oils were pyrolyzed in a closed system at 250,290, and 330 "C, under hydrous and anhydrous conditions. The two oils chosen were from the Douk Daka well, Congo Basin, West Africa, and the Boscan field, Maracaibo Basin, Venezuela. The results showed that the amounts of hydrocarbons generated at 250 "C were extremely small but increased by up to 2 orders of magnitude at 330 "C. The absolute and relative amounts of individual hydrocarbon classes (n-alkanes, steranes, and triterpanes) generated from the two asphaltenes, at a given temperature, were generally different and therefore could be used to differentiate the asphaltenes. These differences were most pronounced when the asphaltenes were pyrolyzed at 330 "C. The distributions of hydrocarbons produced from an asphaltene fraction were broadly similar at all three pyrolysis temperatures, as were the amounts and distributions produced by hydrous and anhydrous pyrolysis. Although there were some differences between the hydrocarbon distributions in the maltene fractions of the oils and those in their asphaltene pyrolyzates, several significant similarities existed, especially in the sterane distributions. Gammacerance was present in the asphaltene pyrolyzates of the oil that contained it as a free hydrocarbon, and this indicates that it may be a useful marker for correlation work when asphaltene pyrolysis is used. On the other hand 28,30-bisnorhopane was not present in the asphaltene pyrolyzate of the Boscan oil that did contain this hydrocarbon, suggesting some limitation of the use of this compound for correlation purposes. Values of the maturity parameters deduced from the triterpane and sterane distributions of the asphaltene pyrolyzates appeared to be related to the maturity of the oils from which they were derived. The differences in the distributions and amounts of n-alkanes, steranes, and triterpanes that are generated from different crude oil asphaltenes by using sealed-system pyrolysis support suggestions that the techniques will be useful for oil-oil correlation purposes.
Introduction Pyrolysis of asphaltenes isolated from crude oil yields an oil-like product that contains hydrocarbons. The distributions of these hydrocarbons have been reported to provide information on the source and maturity of the asphaltenes and therefore on the oils (or bitumens) that contain them.'+ Asphaltenes are the fraction of an oil (or bitumen) precipitated by the addition of an excess of a low molecular weight n-alkane (usually n-pentane or n-he~tane).'~They are thus a solubility class; that is, their composition and the amount recovered during their isolation are dependent on the method used to precipitate them.ag The amounts of asphaltenes in crude oils varies widely from less than 1 % in light oils to more than 20% in heavy oils and bitumens.'O In rock extracts, the amount can vary between 0 and 60%, with an average value of 20% being quoted for extracts of type I1 organic matter." Several generalized models have been postulated for the structures of asphaltenes.8J1J2 They are thought to consist ~
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+Presented at the 193rd National Meeting of the American Chemical Society, Symposium on Pyrolysis in Petroleum Exploration Geochemistry, Denver, CO, April 1987.
of large, central, polynuclear assemblages containing heteroatoms, alkyl chains, and hydroaromatic ring systems. Average molecular masses for petroleum asphaltenes have been estimated to be between 1000 and 20000 Da, but the tendency for asphaltenes to associate, even in dilute solutions of nonpolar solvents and the presence of adsorbed (1)Rubinstein, I.; Spyckerelle,C.; Strausz, 0. P. Geochim. Cosmochim. Acta 1979,43,1-6.
(2)Arefyev, 0 . A.; Makushina, V. M.; Petrov, A. A. Int. Geol. Rev.
1982,24,723-728.
(3)Behar, F.;Pelet, R.; Roucache, J. Org. Geochem. 1984,6,587-595. (4) Behar, F.;Pelet, R. J. Anal. Appl. Pyrolysis 1986,8, 173-187. (5)Cassani, F.;Eglinton, G. Chem. Geol. 1986,56,167-183. (6) Van Graas, G.Org. Geochem. 1986,I O , 1127-1135. (7) Ignasiak, T.; Kemp-Jones, A. V.; Strausz, 0. P. J . O g . Chem. 1977, 42,312-320. (8) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1980; pp 118-129,189-252. (9)Speight, J. G.In Proceedings of the International Symposium on Characterization of Heauy Crude Oils and Petroleum Residues; Editions Technip: Paris, 1984; pp 32-41. (10)Huc, A,-Y.; Behar, F.; Roussel, J . 4 . In Proceedings of the International Symposium on Characterization of Heavy Crude Oils and Petroleum Residues; Editions Technip: Paris, 1984;pp 99-103. (11)Pelet, R.; Behar, F.;Monin, J. C. Org. Geochem. 1986, I O ,
481-498. (12)Yen, T.F.Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1972, 17, F102-F114.
0887-0624/87/2501-0468$01.50/0 0 1987 American Chemical Society
Crude Oil Asphaltene Pyrolyzates
Energy & Fuels, Vol. 1, No. 6,1987 469
a OIL
ASPHALTENES (anhydrous pyrolysis1
d
ASPHALTENES (hydrous pyrolysis) C 250°C
250S
''I
I
I I
I
1
j I
h 33ooc
,liI I I 1
Figure 1. Gas chromatograms of aliphatic hydrocarbon fractions from the Boscan oil (a and b) and its asphaltene hydrous and anhydrous pyrolyzates (e-h). Numbered peaks are n-alkanes of corresponding carbon number, Pr and Ph are pristane and phytane, x and y are prist-1-ene and prist-2-ene, respectively.
resins can cause wide variations in these mea~urementa.~,'~ The origin of crude oil asphaltenes has not been conclusively determined but there is evidence that they are genetically related to the kerogen of the source rock, and it has been suggested that they are small parts of the kerogen released during thermal mat~ration.",~~ Previous studies of petroleum asphaltenes have indicated that the hydrocarbons generated from them by using a variety of pyrolysis methods are related to those in the maltene fractions of the oils and could therefore be useful for oil-oil and oil-source rock correlation purpose^.'^*'^ Since asphaltenes appear to be resistant to biodegradation, this potential use for correlation purposes may be especially valuable when the distributions of free hydrocarbons in the maltene fraction of oils, which are normally used for correlation purposes, have been altered by biodegradation. 1-5~14 Most currently used laboratory methods of thermally degrading geopolymers into lower molecular weight fragments result in mixtures that, to varying extents, reflect the pyrolysis c o n d i t i ~ n s . ~ ~ Recently, J~J~ a method of artificial maturation that involves heating rock chips in
a sealed vessel in the presence of excess water (hydrous pyrolysis) has been reported to produce an expelled oil that more closely resembles a naturally produced oil than that obtained by using other pyrolysis methods.17-19 In the present study, hydrous and anhydrous pyrolysis of crude oil asphaltenes has been undertaken, and the distributions of aliphatic hydrocarbons (particularly the biological markers) in the pyrolyzates determined. The aims of this work were, first, to determine the effects of the pyrolysis conditions (temperature, hydrous versus anhydrous methods) on the distributions of the generated hydrocarbons, second, to assess the similarity of the pyrolyzate hydrocarbon distributions with those in the maltene fractions of the oils, and, third, to determine whether the asphaltene pyrolyzates from different oils were sufficiently different for them to have potential use for correlation purposes.
(13) Bandurski, E. Energy Sources 1982, 6 , 47-66. (14) Telnaes, N.; Speers, G. C.;Steen, A.; Douglas, A. G. In Petroleum Geochemistry in Exploration of the Norwegian Shelf.; Thomas, B. M. et al., Eds.; Graham & Trotman: London, 1985; pp 287-292. (15) Seifert, W. K. Geochim. Cosmochim. Acta 1978, 42, 473-484. (16) Rowland, S. J.;Aareskjold, K.; Xuemin, G.; Douglas, A. G.Org. Geochem. 1986,10, 1033-1040.
(17) Lewan, M. D.; Winters, J. C.; McDonald, J. H. Science (Washington, D.C.)1979,203,897-899. (18). Winters, J. C.;Williams, J. A.; Lewan, M. D. In Adoances in Organic Geochemistry 1981; Bjoroy, M., et al., Eds.; Wiley: Chichester, U.K., 1983; pp 524-533. (19) Lewan, M.D.;Bjoroy, M.; Dolcater, D. L. Geochim. Cosmochin. Acta 1986,50, 1977-1987.
Experimental Section Two oils were used in this study. One was from the Douk Daka offshore well situated in the Congo Basin, West Africa. The source
Jones et al.
470 Energy & F u e l s , Vol. 1, No. 6,1987 a OIL (unhwtedl
ASPHALTENES (anhydrous pyrolysis)
ASPHALTENES (hydmus pyrolysis)
d 2500C C 250oC
-TIME
-
-
TIME
-
Figure 2. Gas chromatograms of aliphatic hydrocarbon fractions from the Douk Daka oil (a and b) and ita asphaltene hydrous and anhydrous pyrolyzates (c-h). Peak assignments are as for Figure 1. each bomb were transferred to a separating funnel and extracted rock, a lacustrine deposit of Barremian age containing type 1/11 kerogen, is the same as that for the Emeraude O ~ I The S .second ~ ~ ~ with dichloromethane (4 x 20 mL) and light petroleum (4 X 50 oil is from the Boscan field in the Maracaibo Basin, Venezuela. mL): on this small scale no separated oil phase was present. The This marine oil is sourced from predominantly type I1 kerogen combined extracts were concentrated and then separated into aliphatic and aromatic hydrocarbon fractions, by thin-layer in the La Luna formation of Turonian/Coniacian age.22 chromatography (Kieselgel 60 G) using light petroleum as deAsphaltenes were obtained from the heavy oils by a method similar to that previously described.'+ Briefly, a 40-fold excess veloper. Aliquots of the unheated oils were similarly separated. of n-heptane was stirred into the heavy oil (1-4 g) dissolved in Gas chromatographic analyses were performed on a Carlo Erba dichloromethane (1-3 9). The resulting mixture was allowed to (5160-Mega) instrument fitted with a flame ionization detector, using on-column injection onto a fused silica capillary column (50 settle (8 h or overnight) before vacuum filtration through a sintered glass (porosity 4) funnel. The asphaltenea were washed twice m X 0.32 mm i.d.) coated with OV-1 in an oven temperature with chilled (4 "C) n-heptane (2 X 50 mL) and then further programmed from 50 to 310 "C at 4 "C min-' (final temperature purified by reprecipitation (X3) with n-heptane from solution in for 25 min): hydrogen was used as carrier gas. Gas chromatogdichloromethane as described above. raphy-mass spectrometry (GC-MS) was carried out on a HewPyrolysis experiments were performed in purpose-built stainlett-Packard 5890A-5970B MSD system. Analyses were by less-steel ''bombs" (grade 316, capacity 35 mL). Bombs containing splitless injection onto a fused silica capillary column (25 m X 0.2 mm) coated with a cross-linked methylsilicone phase in an the asphaltenes (typically 70-150 mg) were flushed with nitrogen oven programmed from 40 "C to 150 "C at 10 "C m i d and then (5-10 min) prior to being sealed with soft copper gaskets. Hydrous pyrolysis experiments were carried out as above, except that from 150 to 300 "C at 4 "C min-', where the final temperature distilled water (11mL) was also added. Even heating was achieved was held for 20 min. The instrument was operated in scan mode by placing the bombs in a large pressure vessel (1.0 L capacity, (m/z50-500) with a cycle time of 1.1s or in the selected ion mode, Parr Instrument Co.) partly filled with water (ca. 300 mL) and monitoring six ions each with a dwell time of 40 ms. fitted with a pressure gauge and rupture disk. The vessel was The hopanes (Cn-Csl) and regular steranes (C,-C,) in the heated rapidly to the desired temperature (250-330 "C) and held pyrolyzates were quantified by comparing the peak heights in the there isothermally (*5 "C)for 72 h. When cool, the contents of m / z 191 and m/z 217 mass chromatograms, respectively, with those of added internal standards. Mixtures of known quantities of dodecylperhydroanthracene(prominent m/z 191 fragment ion) and deuteriated cholestane (3-D2Cnaaa 20R, prominent m/z 219 (20)Claret, J.; Tchikaya, J. B.; Tiesot, B.; Deroo, G.; Van Dorsselaer, fragment ion) were used as internal standards for the hopanes A. In Advances in Organic Geochemistry 1975; C a m p , R., Goni, J., E&.; and steranes, respectively. The C1s-C32n-alkanea were quantified Enadimsa: Madrid, 1977; pp 509-522. by comparison of their gas chromatographic peak areas with that (21)Connan, 3.;Coustau, H. AAPG Bull., in press. of an added internal standard (1-chlorotetradecane) by using a (22) Talukdar, S.; Gallango, 0.;Chin-A-Lien,M. Org.Geochem. 1986, 10, 261-279. Hewlett-Packard 3390A integrator. Relative response factor
Crude Oil Asphaltene Pyrolyzates
C,5-C32
Energy & Fuels, Vol. 1, No. 6,1987 471
i
n-ALKANES
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I I I
10000
J I
I
I
I
I,
290
-
-
I
130
TEMPERATURE OC
TEMPERATURE OC .--I
OOUK 0AKA.CONGO IHYOROUS)
A--A
DOUK 0AKA.CONGO IANHYOROUS)
0-0
BOSCAN IHYOROUS)
e--+
BOSCAN IANHYOROUS)
-
TEMPERATURE OC
Figure 3. Amounts of n-alkanes (a), hopanes (b), and steranes (c) produced by hydrous and anhydrous pyrolysis of the Boscan and Douk Daka asphaltenes at different temperatures. differences were not taken into account since errors due to these have been shown to be minor.23-25
Results and Discussion
A prominent suite of n-alkanes in the gas chromatograms of the aliphatic hydrocarbons isolated from the Boscan oil (Figure la) suggests that it has not been extensively biodegraded. However, when compared with their abundance in other crude oils, the low abundance of n-alkanes relative to branched alkanes below Czoand relative to the Cm-CS5n-alkanes indicates that mild biodegradation of this oil has occurred. The absence of nalkanes in the Douk Daka oil (Figure 2a) indicates that this oil has been biodegraded, but the presence of acyclic isoprenoid alkanes suggests that the extent of biodegradation is only moderate.26 Gas chromatograms of the hydrocarbon fractions of the hydrous and "anhydrous" pyrolyzates (250 "C) of the Boscan asphaltene showed a distribution of n-alkanes (Figure lc,d) extending from about n-Cg to above n-Cd6 (truncated a t n-C39 in the figures shown). The term "anhydrous" is used for convenience and signifies that there was no water added to the asphaltenes in the heating experiments. The long-chain (C30+) n-alkanes present in (23) Rullkotter, J.; Mackenzie, A. S.; Welte, D. H.; Leythaeuser, D.; Radke, M. Org. Geochem. 1984,6, 817-827. (24) Dahl,B.;Speers, G. C.; Steen, A,; Telnaes, N.; Johansen, J. E. In Petroleum Geochemistry in Erploration of the Norwegian Shelf; Thomas, B. M., et al., Eds.; Graham & Trotman: London, 1985; pp 303-307. (25) Giger, W.; Schaffner, C. Anal. Chem. 1978, 50, 243-249. (26) Volkman, J. K.;Alexander, R.; Kagi,R. I.; Rowland, S. J.; Sheppard, P. N. Org. Geochem. 1984,6,619-632.
these pyrolyzates are thought to be derived from the maltene fractions (i.e. they exist as free hydrocarbons) in the Boscan ail and may have been adducted into the asphaltene fraction. GC-MSdata (discussed later) indicated that there was no significant adduction of cyclic hydrocarbons from the maltene into the asphaltene fractions. The concentration of the adducted n-alkanes is extremely low, but the even smaller amounts of hydrocarbons produced by pyrolysis of the asphaltene at 250 "C (
