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Organic geochemistry of Venezuelan extra-heavy oils
Chemical Geology, 56 (1986) 167--183
167
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
[21
ORGANIC GEOCHEMISTRY OF VENEZUELAN EXTRA-HEAVY OILS 1. Pyrolysis of Asphaltenes: A Technique for the Correlation and Maturity Evaluation of Crude Oils F. C A S S A N I a n d G. E G L I N T O N
Organic Geochemistry Unit, University of Bristol, School of Chemistry, Bristol BS8 1TS (Great Britain) (Received February 26, 1986; revised and accepted March 4, 1986) Abstract Cassani, F. and Eglinton, G., 1986. Organic geochemistry of Venezuelan extra-heavy crude oils, 1. Pyrolysis of asphaltenes: a technique for the correlation and maturity evaluation of crude oils. Chem. Geol., 56: 167--183. The asphaltene fraction from Cerro Negro (Venezuela) oil has been used to test the effects of pyrolysis temperature on the distribution of alkanes generated by this technique. Particular attention has been paid to the biological marker compounds (steranes, triterpanes) released in these experiments and to the correlation and maturity parameters as normally applied in organic geochemical exploration. Pyrolysis temperatures up to 330°C have little effect on the distribution of these compounds, whereas higher temperatures cause thermal destruction of hopanes and steranes, thereby affecting the values of such parameters. The distribution of these compounds in the pyrolyzates was less mature than those normally found in crude oils. Four Venezuelan marine oils, from different regions and of different levels of biomarker maturity, but of generally similar depositional origin as identified by their triterpane and presumed sterane (in the case of those heavily biodegraded) distributions have been studied. These oils were readily distinguished as their respective asphaltene pyrolyzates on the basis of the hopane/sterane ratio and the distribution of C~7--C29 5a (H), 14/3(H), 17~(H)-steranes. The distribution patterns of steranes and hopanes obtained in the asphaltene pyrolyzates have been compared with those reported for pyrolysis of sediments. The data are compatible with an origin for the asphaltenes involving catagenic degradation of the kerogen in the source rock. The pyrolysis technique applied to asphaltenes is suited to the characterization of heavy biodegraded oils or weathered source rocks where most of the molecular information has been destroyed. 1. I n t r o d u c t i o n A s p h a l t e n e s are t h e n o n - h y d r o c a r b o n fractions precipitated from petroleums or bitum e n s b y the a d d i t i o n of an excess of an na l k a n e ( f r o m p e n t a n e t o h e p t a n e ) . T h e y are 0009-2541/86/$03.50
normally soluble in benzene. The content of t h i s f r a c t i o n varies f r o m s m a l l a m o u n t s ( ~ 0 . 1 - - 1 . 0 % ) i n l i g h t oils a n d t h e e x t r a c t s o f mature source rocks to high quantities (~ 10-25%) in " i m m a t u r e " o r b i o d e g r a d e d oils. T h e s t r u c t u r e o f a s p h a l t e n e s h a s b e e n in-
© 1986 Elsevier Science Publishers B.V.
168 vestigated in numerous studies and average structures postulated. Thus, a proposed asphaltene framework consists of condensed polynuclear aromatic ring systems bearing heteroatoms (nitrogen, oxygen and sulphur), alkyl chains and h y d r o c a r b o n rings (e.g., Speight, 1984). Evidence from spectrometric and degradative analysis (Rubinstein and Strausz, 1979; Rubinstein et al., 1979; Bandursky, 1982; Behar et al., 1984) suggests that kerogens and petroleum asphaltenes could have similar structures, since oil-like material, containing n-alkanes or n-alkane--n-alkane pairs from C3 to C40 (depending on the technique used), can be generated from them b y pyrolysis. Therefore, the asphaltenes could be fragments of the original kerogen from which oil is derived, presumably by heteroatomic cleavage (Tissot and Welte, 1978; Bandurski, 1982). Rubinstein and Strausz (1979) and Rubinstein et al. (1979) have heated asphaltenes (from Prudhoe Bay oil, Alaska and Athabasca oil, Alberta, Canada) and released biological markers such as 17a(H),21~(H)-terpanes (from C27 to Css) with distributions similar to that of the original oil. E k w e o z o r and Strausz (1983) studied the tricyclic terpanes between C,9 and C30 released by pyrolysis of the Athabasca asphaltenes, which contain high concentrations of the precursors of these compounds. No analysis has been reported involving the use of biological markers (such as steranes and triterpanes) generated from asphaltenes to assess maturity and correlation by the procedures c o m m o n l y used in sediments and oil samples (e.g., Shi Ji-Yang et al., 1982). The release of such biomarkers by pyrolysis or chemical degradation techniques would provide new information which will help solve geochemical problems, especially in the case of biodegraded oils. Thus, pyrolysis experiments with sediments (Gallegos, 1975; Seifert, 1978; Seifert and Moldowan, 1980; Mackenzie et al., 1983; Moldowan and Seifert, 1984; Noble et al., 1985) have shown some differences between the sterane and triterpane distributions of the pyrolyzates and
those of their respective rock extracts. For example, the degree of "isomerization" of these c o m p o u n d s in the pyrolyzate can be less than in the solvent extract of the same rock. However, the pyrolysis of sediments has several problems that do n o t exist in the case of asphaltene pyrolysis: (1) there are polar compounds associated with the mineral matrix and kerogen which on pyrolysis may generate hydrocarbons additional to those released by the kerogen; (2) hydrocarbons trapped or adsorbed in the mineral matrix and kerogen m a y be released; and (3) a catalytic effect of the mineral matter (which may be different for different sediments) may be operating. Nevertheless, the distribution of polycyclic alkanes can still be used as a correlation and maturity aid, as suggested by Seifert (1978). In this study, pyrolysis experiments have been performed on a Cerro Negro (heavy oil from the Orinoco Oil Belt, eastern Venezuelan Basin) asphaltene sample at different temperatures (from 300 ° to 370 ° C) for 3 days to investigate the distribution of the biomarkers generated. We have sought to release the biomarkers with a minimum of thermal alteration of the original configurations in the asphaltene structure. The distributions of several specific types o f alkane have been assessed in several Venezuelan marine oils from different regions and different levels of biomarker maturity by using combined gas chromatography- mass spectrometry (GC--MS) of the pyrolyzates of asphaltenes. This is an a t t e m p t at better characterization of these otis, where the information embodied within structures could be affected by the operation of several processes, such as maturation, migration, addition of fresh petroleums and biodegradation. 2. Experimental (analytical procedure in Fig. 1)
2.1. Asphaltenes The asphaltene fraction was separated from the oil by a standard method (Ignasiak et al., 1977), but using n-heptane instead of n-
169
SOLUBLE
iNSOLUBLE
I
LUBLE
WITH n - C 7
INSOLUBLE SOLUBLE
Fig. 1. Analytical procedure for the precipitation of asphaltenes from a crude oil and fraetionation of asphaltene pyrolysis products. pentane for the precipitation. A mixture of oil and toluene (1 : 1) was diluted with 60 volumes of u-heptane and kept for a minim u m of 16 hr. with stirring under nitrogen. The asphaltenes were filtered through a thimble and Soxhlet extracted with n-heptane for 48 hr. and recovered from the thimble with dichloromethane (DCM). The solvent was removed on a rotary evaporator and the asphaltenes dried using a stream o f dry nitrogen and then under vacuum overnight at 60°C.
2.2. Pyrolysis Samples for pyrolysis (between 0.5 and 1.0 g) were placed in thick-walled Pyrex ®
tubes (25 ml), evacuated (0.1 Torr), flushed with nitrogen (this procedure was repeated 3 times), and sealed under vacuum with a glassblowing torch. The tubes were protected during heating by wrapping in aluminium foil and placing in individual stainless-steel tubes. An old gas chromatograph oven was used for the heating experiments (temperature + 3 ° C). The samples were heated at temperatures between 300 ° and 370°C for 72 hr. The tubes were opened after cooling in liquid nitrogen (to reduce pressure due to gaseous products). The pyrolysis products were pulverized, transferred to a thimble and Soxhlet extracted with n-heptane for 48 hr. The solvent was then removed on a rotary evaporator and the pyrolysis extract separated by thin-layer chromatography (TLC) (silica gel; 0.4 mm thick; n-hexane as eluent). The plates were visualized (Rhodamine e 6G; UV light) and four fractions obtained: Alkane hydrocarbons (top band; Rf 0.8--0.9); aromatic hydrocarbons (Rf > phenanthrene); polyaromatic compounds (Rf < phenanthrene) and polar compounds (at the b o t t o m o f the plate). Each band was scraped off the plate and the respective fractions eluted with DCM (50 ml). The solvent was finally removed in a rotary evaporator and each fraction evaporated under a stream of dry nitrogen. Further separation of the alkane fraction with silica gel impregnated with silver nitrate (5%) did not give appreciable amounts (<5% of the total alkanes) of unsaturated compounds. Higher amounts of unsaturated compounds (~30%) were obtained in a resin fraction pyrolyzate of the Cerro Negro oil sample.
2. 3. Gas chromatography (GC) and computerized gas chromatography--mass spectrometry
(C--GC--MS) Gas chromatography of the alkane fractions from the pyrolysis extracts was performed by split/splitless injection on a Carlo Erba ® series FTV 2150 gas chromatograph fitted with an OV-I coated fused silica capillary column (~20 m × 0.25 mm i.d.). A tem-
170 perature programme of 50--280°C at 6°C m i n . -~ w a s e m p l o y e d a n d t h e c a r r i e r gas ( H e ) h a d a f l o w r a t e o f ~ 1 m l m i n . -~. C--GC--MS was performed on a Carlo E r b a ® ( M e g a s e r i e s ) gas c h r o m a t o g r a p h i n t e r faced with a Finnigan ® 4000 mass spectrome t e r . T h e gas c h r o m a t o g r a p h was equipped
with on-column injection and fitted with a fused silica capillary column (OV-1, 25 m × 0.3 mm i.d.) and programmed from 50--280 ° C a t 6 ° C m i n . -~. H e l i u m w a s u s e d as t h e c a r r i e r gas. T h e m a s s s p e c t r o m e t e r w a s o p e r a t e d in the EI mode (ionizing energy 35 eV, source temperature 250°C) with multiple ion detec-
TABLEI Tricyclic terpane (I), hopane (II) and sterane (III) assignments and abbreviated names (for reference to Figs. 4, 5 and 7 ) Peak
Basic structure*
Component
Abbreviated name
Tricyclic terpanes and hopanes: A-I J K L M
I I + II II II II
N
II
O
II
P
II
Q R
II II
S
II
T
II
U V
II II
C20--C29 tricyclics C29 tricyclic + C27 18a(H)-22,29,30-trisnorneohopane C27 1 7a(H)-22,29,30-trisnorhopane C:s 17a(H),18a(H),21~(H)-28,30-bisnorhopane C29 demethylated hopane C29 1 7a(H),21#(H)-30-norhopane C29 17[3(H),21a(H)-30-norhopane C30 17~(H),21~(H)-hopane C30 17[3(H),21~(H)-hopane C31 17a(H),21~(H)-homohopane, (22S + 22R) C32 1 7a(H),21~(H)-bishomohopane, (22S + 22R) C33 1 7a(H),2113(H)-trishomohopane, (22S + 22R) C3~ 17a(H),21~(H)-tetrakishomohopane, (22S + 22R) C35 1 7a(H),21~(H)-pentakishomohopane, (22S + 22R)
C2o-3 to C29-3 C:9-3 + Ts Tm C28 bisnorhopane C29 ~,# C29 ~,~ C30 ~,;~ C30 ~,~ C3~ ~,~ (22S + 22R) C3: ~,~ (22S + 22R) C33 ~,~ (22S + 22R) C3, ~,~ (22S + 22R) C35 ~,~ (22S + 22R)
Steranes: a
III
b
III
c d e /" g h i j k
III III III III III III III III III
C2~ sterane C2: sterane C~7 5a(H),14~(H),17a(H),20S-cholestane C~ 5a(H),14~(H),1 7~(H),(20R + 20S)-cholestane C~ 5a(H),14a(H),1 7a(H),2OR-cholestane C~ 5a(H),14a(H),l 7a(H) 20S-24-methyl-cholestane C~s 5a(H),14~(H),1 7~(H) (20R + 20S)-24-methyl-cholestane C~s 5a(H),14a(H),l 7~(H) 20R-24-methyl-cholestane C~9 5~(H),14~(H),1 7a(H) 20S-24-ethyl-cholestane C2~ 5~(H),14~(H),1 7~(H) (20R + 20S)-24-ethyl-cholestane C~ 5~(H),14~(H),l 7a(H) 20R-24-ethyl-chotestane
*Basic structures of tricyclic terpanes (I), hopanes (II) and steranes (III):
:lII)
C~ a,~,a,20S C~ a,~,~ (20R + 20S) C~ ~ , a , a , 2 0 R C2~ a,a,a,20S C~s ~,~,~ (20R + 20S) C~8 a,a,~ 20R C~ a,a.~ 20S C~9 ~,~,~ (20R + 20S) C:9 ~,~,~ 20R
171
ti0n. Data were acquired and processed using an INCOS ® 2300 data system. Steranes and triterpanes were identified by comparison of their retention times and mass spectra with those of known samples: Paris Basin (France) rock extracts (Mackenzie et al., 1980a) and Athabascan oil (Ekweozor and Strausz, 1983). Demethylated hopanes, 28bisnorhopane and 25,28,30-trisnorhopane were identified by comparison of their mass spectra and retention pattern with literature data (Seifert and Moldowan, 1979; RullkStter and Wendisch, 1982; Volkman et al., 1983).
2. 4. Determination o f molecular weight The molecular weights were determined on a Perkin-Elmer ® model 115 vapour pressure osmometer (VPO) at three different concentrations (1%, 2% and 4% w/v and extrapolated to infinity) in pyridine at 60°C.
L
B BO~CAN M ~AOMETE CERR@ NEGBC: C~ 0RBZLAL -
-
-
- -
BASN BOUNDARY ORINOCO 01L BEL ~ BOUNDARY AND AREAS
Fig. 2. Locations of oil samples (-) from Maracaibo Basin (Boscan oil) and Eastern Venezuelan Basin: Orocual oil at the north of the basin; Cerro Negro oil in the eastern part of the Orinoco Oil Belt and Machete oil in the western part of this same area in the south of the basin. 3. Results and discussion
3.1. Samples 2.5. Quantitation and measurement o f parameters All hydrocarbon parameters were quantitated using peak areas in the corresponding mass fragmentograms obtained by GC--MS (see Table I for sterane and triterpane assignments). The % 20S/(20R + 20S)ratio was calculated from m/z 217 (C29 a,a,a-steranes) mass fragmentograms; the %/3,~(20R + 20S)/ C2~ a,~,/3(20R + 20S) + C29 a,a,a(20S + 2OR) ratio was calculated from m/z 217 mass fragmentogram and the relative proportions of the C33 (22S) and (22R) 17a(H),21[3(H) isomers (m/z 191) were used to calculate the 22S/(22S + 22R) hopane ratio (Mackenzie et al., 1980a). The relative abundances of the 5a(H),14~(H),17~(H) 20R and 20S components for different carbon numbers (C27 to C~9) were measured from m/z 218 fragmentograms and the ratio hopanes/steranes (hop/st) was obtained from the relative abundances of C30 17a(H),21~(H)-hopane (measured from m/z 191 mass fragmentograms) to C29 a,a,a(20S + 2OR) + a43,/3(20R + 20S)-steranes (Mackenzie and Maxwell, 1981).
Fig. 2 shows the locations of the oil samples studied in this work (see Table II for reservoir depths and ages). Boscan oil (Maracaibo Basin, west Venezuela) is an "immat u r e " heavy oil, slightly biodegraded and probably originated from La Luna shale (Cretaceous). Cerro Negro oil (Orinoco Oil Belt, eastern Venezuelan Basin) is a very heavy, middle mature oil located in the eastern part of the Orinoco Oil Belt, which is one of the largest heavy-oil accumulations in the world. The origin of this oil is probably from the Cretaceous source rocks at the north of the basin (Querecual Formation). Demaison (1977) concluded that this oil migrated to the edge of the basin over a distance between 90 and 190 km and was biodegraded in shallow reservoirs by bacteria introduced by meteoric waters. The Machete oil is located in the western part of the Orinoco Oil Belt and is less mature than Cerro Negro oil. This oil is also heavy and very biodegraded and probably has a different origin from Cerro Negro oil. The mature oil from the Orocual field at the north of the basin is t h o u g h t to
172
T A B L E II GC--MS c o r r e l a t i o n a n d m a t u r i t y p a r a m e t e r s for t h e V e n e z u e l a n oils e x a m i n e d *~ Samples
Age .2
Boscan M a c h e t e .3 Cerro Negro .3 Orocual
Depth (m)
E M M M
1,650 495 820 1,576
Correlation parameters
Maturity parameters
hop/st
% C2~
% C2s
% C29
% T / ( T + M) % DPEP
2.1 1.6 1.1 1.1
35 32 30 33
28 32 32 34
37 36 38 34
50 63 67 93
70 52 41 17
* ~ F o r details o f m e a s u r e m e n t s see T a b l e III a n d S e c t i o n 2. ,5 Ages: M = M i o c e n e ; E = E o c e n e (for oil reservoir age). .3 T h e c o r r e l a t i o n p a r a m e t e r s for t h e s e oils c o r r e s p o n d t o a less b i o d e g r a d e d oil f r o m t h e same area; in these particular samples t h e s t e r a n e s a n d h o p a n e s have b e e n a f f e c t e d b y b i o d e g r a d a t i o n . T A B L E III M o l e c u l a r p a r a m e t e r s e m p l o y e d (see Tables II, V a n d V I I I ) Parameter
Ratio
Significance
% 20S
C=9 e , e ,~-steranes:
i s o m e r i z a t i o n at s t e r a n e C-20(S); ratio rises f r o m 0 to - 5 0 % w i t h increasing m a t u r i t y ; oil g e n e r a t i o n c o u l d s t a r t at ~ 4 0 % ( M a c k e n z i e et al., 1 9 8 2 )
20S 20S + 20R
% ~,~
C29 regular steranes: ~,~,~(R+S) total steranes
% 22S
C33 e , B - ( H ) - t r i s h o m o h o p a n e s : 22S (22S + 22R
% T / ( T + M)
steroid hydrocarbon: C~s-triaromatic C=9 m o n o + C28-triaromatic
% DPEP
porphyrins: DPEP DPEP + A E T I O
i s o m e r i z a t i o n at ~terane C-14 a n d C-17; ratio rises f r o m 0-50% to ~ 7 0 - - 7 5 % w i t h increasing m a t u r i t y ; i s o m e r i z a t i o n n o r m a l l y c o m p l e t e b e f o r e m a j o r g e n e r a t i o n of h y d r o c a r b o n s ( M a c k e n z i e a n d Maxwell, 1 9 8 1 ) i s o m e r i z a t i o n at C-22(S); r a t i o rises f r o m 0 t o 60% ( E n s m i n g e r et al., 1 9 7 4 ) ; i s o m e r i z a t i o n n o r m a l l y c o m p l e t e b e f o r e m a j o r g e n e r a t i o n of h y d r o c a r b o n s ( M a c k e n z i e a n d Maxwell,
1981) a r o m a t i z a t i o n o f C ring a r o m a t i c h y d r o c a r b o n ; r a t i o rises f r o m 0 t o 100% w i t h increasing m a t u r i t y ; this ratio is m o r e temp e r a t u r e d e p e n d e n t t h a n % 2 0 S s t e r a n e r a t i o (Mackenzie et al., 1 9 8 2 ) ; oil g e n e r a t i o n c o u l d s t a r t at - 4 0 - - 6 0 % a p p a r e n t C--C b o n d cleavage of isocyclic ring in the DPEP porp h y r i n s (e.g., Didyk et al., 1 9 7 5 ) or t h e r m a l d e s t r u c t i o n of DPEP p o r p h y r i n s (Barwise a n d Park, 1 9 8 3 ) , r a t i o falls f r o m ~ 1 0 0 % to 0; oil g e n e r a t i o n c o u l d start at ~ 8 0 - - 6 0 % (Barwise a n d Park, 1 9 8 3 )
hop/st
C3o c~, ~-hopane/C29 regular steranes
this ratio is sensitive to changes in the a m o u n t of algal material ( l o w e r values) i n c o r p o r a t e d i n t o f o r m i n g s e d i m e n t s , relative t o h i g h e r - p l a n t organic m a t t e r (higher values, ~ 5 ) (Mackenzie et al., 1 9 8 2 )
% C2~ % C2.
relative a b u n d a n c e s of C~7 , C2s a n d C~9 a , ~ , ~ - s t e r a n e s
these d i s t r i b u t i o n s d e p e n d o n the n a t u r e of the organisms w h i c h c o n t r i b u t e to t h e organic m a t t e r o f t h e f o r m i n g sediments (Huang and Meinschein, 1979)
173
have originated from the same source rock as Cerro Negro oil but was generated at a higher level of maturity. Table II shows the correlation and maturity parameters for the different samples studied (for details of these parameters see Table III). Hop/st ratio is the same for the Cerro Negro and Orocual oils, slightly higher for Machete oil and higher still for Boscan oil. The C~7 to C29 a,~,t3-sterane distribution is similar for Orocual, Machete and Cerro Negro oils and slightly different for Boscan oil. The maturity parameters normally used for sediments (% 20S; % ~,~ in the steranes and % 22S in the hopanes) cannot be applied in the case of these samples because they are affected by severe biodegradation (e.g., Seifert and Moldowan, 1979). Therefore, other parameters, i.e. % of triaromatic steroid hydrocarbons (e.g., Mackenzie et al., 1981) and % DPEP(*) porphyrins (e.g., Mackenzie et al., 1980b), have been used to determine the maturity of these oils; these parameters are apparently only affected by biodegradation in extreme cases (Hajibrahim, 1978; Wardroper et al., 1984).
*DPEP = deoxophylloerythroetioporphyrin.
3.2. Pyrolysis o f Cerro Negro asphaltenes at 300 °, 330 °, 350 ° and 370°C. The Cerro Negro asphaltene was pyrolyzed at four different temperatures (300 ° , 330 ° , 350 ° and 370°C) for 3 days to investigate the effect of increasing temperature on biological markers and also to determine h o w closely the changes parallel those in sediments suffering increasing burial depths over geological time. Table IV shows the composition of the pyrolyzate with increasing pyrolysis temperature. The yields of the heptane extracts and their fractions are relatively low at temperatures of 300 ° or 330°C b u t increase at higher temperatures at the expense of the asphaltene residue which generates these fractions. At 370°C a decrease in the yield of the resin-I fraction is observed;this p h e n o m e n o n is interpreted as a preferential degradation of this fraction to form hydrocarbons. In contrast with the results reported in similar experiments by Rubinstein and Strausz (1979), low amounts of unsaturated c o m p o u n d s (<5%) were separated by TLC (5% silver nitrate). In the same table the elemental analysis results for the asphaltene residue fractions show a decrease of the H/C ratio and sulphur content with increasing temperature b u t the nitrogen content appears to be stable.
T A B L E IV C o m p o s i t i o n o f the n - h e p t a n e - s o l u b l e p r o d u c t s and e l e m e n t a l c o m p o s i t i o n of the residual a s p h a l t e n e s f r o m pyrolysis o f Cerro Negro a s p h a l t e n e s at four d i f f e r e n t t e m p e r a t u r e s (300 °, 330 °, 350 ° and 370°C, for 3 days) Temperature (° C)
300 330 350 370
Yield, % w e i g h t a s p h a l t e n e
E l e m e n t a l c o m p o s i t i o n o f residue
n-heptane-soluble fraction
residue
Tot. ext.
Alk.
Aro.
P. aro.
Resins-I
5.1 9.9 16.0 18.4
0.3 0.7 2.2 4.2
I.i 1.7 3.6 6.5
0.4 1.5 2.7 3.4
3.3 6.0 7.5 4.3
94.9 90.1 84.0 81.6
H/C
1.08 1.04 1.00 0.93
S
N
(%)
(~)
5.15 5.00 4.80 4.30
1.5 1.1 1.4 1.3
Tot. ext. = t o t a l e x t r a c t ; Alk. = Alkanes (Rf = 0 . 8 - - 0 . 9 ) ; Aro. = a r o m a t i c s ( R f > p h e n a n t h r e n e ) ; P. aro. = polya r o m a t i c s (Rf < p h e n a n t h r e n e ) ; Resins-I = resin-like material o f the n - h e p t a n e e x t r a c t ; residue = a s p h a l t e n e residue after n - h e p t a n e e x t r a c t i o n .
174
PYROLYZATE
PYROLYZATE
3oo°c
350°C
1 Z
Z PYROLYZATE 330°C X
I= Retention
t 4me
Retent
ion
t ime
Fig. 3. Gas c h r o m a t o g r a m s for t h e alkane f r a c t i o n s o f Cerro Negro a s p h a l t e n e p y r o l y z a t e s at d i f f e r e n t temp e r a t u r e s o f pyrolysis ( 3 0 0 ° , 330 ° , 3 5 0 ° a n d 3 7 0 ° C ) ( X = p r i s t a n e ; Y = p h y t a n e a n d Z = C~7 n=alkane).
(a)
Retention
(b)
(c)
m/z 191
F
j
H, \
Retention
mlz
217
T
I ,K B
time
(
t,me
U
V
Retention
t4me
Fig. 4. Gas c h r o m a t o g r a m (a) a n d mass f r a g m e n t o g r a m s (b a n d c) o f t h e a l k a n e f r a c t i o n o f Cerro Negro oil s h o w i n g (b) tricyclic t e r p a n e a n d h o p a n e (m/z 1 9 1 ) a n d (c) s t e r a n e (m/z 2 1 7 ) d i s t r i b u t i o n s (see T a b l e I for s t e r a n e a n d tricyclic t e r p a n e a n d h o p a n e a s s i g n m e n t s ) .
175
(a)
m/z
191
PYROLYZA1 e
(b)
m/z
217
300CC a
d c
g 13
R B
E
b
K F G
H(J '
h~
PYROLYZATE 330°C
PYROLYZATE 350~C
PYROLYZATE 370°C
Retention
time
Retention
time
Fig. 5. Mass fragmentograms of the alkane fractions of a Cerro Negro asphaltene p y r o l y z a t e at 300 °, 330 ° , 350 ° and 370°C showing: (a) tricyclic terpane and h o p a n e (m/z 191); and (b) sterane (m/z 217) distributions (see Table I for sterane and tricyclic terpane and h o p a n e assignments).
176 The alkanes generated from the pyrolysis were analyzed by GC and C--GC--MS. The GC traces (Fig. 3) reveal the generation of nalkanes from C14 to C40 and isoprenoids such as pristane (C19) and phytane (C20). These hydrocarbons were not present in the original oil due to their prior removal by biodegradation (Fig. 4). Not many differences are observed among the distributions of n-alkanes (Fig. 3) at 300 ° , 330 ° and 350°C, but at 370 ° C a small increase in the lower-molecularweight n-alkanes is observed, probably due to the cracking of higher-molecular-weight compounds. The pristane/phytane ratio is close to one at 300°C and increases slightly at higher temperatures. The C--GC--MS results for the steranes (m/z 217) and triterpanes (m/z 191) present in the asphaltene sample heated at 300°C, indicate (Fig. 5) a new generation of compounds in the pyrolyzate (see Table I for compound assignments) very similar to those of the original oil in the case of the triterpanes. However, in the case of the steranes these compounds have been altered by biodegradation and the normal steranes are almost absent in the oil (compare Figs. 4 and 5). The "triterpanes" in the pyrolyzate have a pattern of trieyclic compounds similar to that of the oil but there are some differences in the hopanes, where the C29 and C30 ~,~-hopanes (moretanes) are present in higher concentrations than in the oil. The C2a bisnorhopane and the C29-demethylated hopane were also
a b s e n t in t h e p y r o l y z a t e (as w e r e t h e series o f d e m e t h y l a t e d h o p a n e s d e t e c t e d in t h e m/z 177 m a s s f r a g m e n t o g r a m s o f t h e oil s a m p l e ) . T h e d e m e t h y l a t e d h o p a n e s are a p p a r e n t l y f o r m e d as an e f f e c t o f b i o d e g r a d a t i o n ( R e e d , 1 9 7 7 ; Seifert a n d M o l d o w a n , 1 9 7 9 ) o f t h e oil and cannot be produced from the asphaltene. T h e m/z 177 mass f r a g m e n t o g r a m in t h e p y r o l y z a t e also e x h i b i t e d t h e a b s e n c e o f t h e 25,28,30-trisnorhopane, w h i c h is p r e s e n t in t h e original oil. T h e l a c k o f t h e C28 b i s n o r h o p a n e and t h e 25,28,30-trisnorhopane in t h e p y r o l y z a t e suggests t h a t t h e s e c o m p o u n d s do not form part of the asphaltene structure but are free lipids, as suggested b y M o l d o w a n a n d Seifert ( 1 9 8 4 ) f o r s e d i m e n t p y r o l y z a t e s . V e r y small a m o u n t s o f d i a s t e r a n e s and a r o m a t i c steroids are released b y this t y p e o f p y r o l y s i s , in parallel w i t h t h e results o f M a c k e n z i e et al. (1983) for sediment pyrolyzates. T h e steranes g e n e r a t e d b y p y r o l y s i s p r e s e n t an i m m a t u r e d i s t r i b u t i o n , with a p r e d o m i n a n c e o f t h e 5a(H),14~(H),17~(H)-configurat i o n o v e r t h e 5a(H),14~(H),17~(H)-configurat i o n and o f t h e 2 0 R o v e r 20S in t h e 5 a ( H ) , 1 4 a ( H ) , I 7a (H)-steranes. T h e m a t u r i t y p a r a m eters n o r m a l l y used in s e d i m e n t s : % 20S a n d % ~,~ o f t h e steranes and % 22S o f t h e h o p a n e s (Tables I I I and V) are l o w e r in t h e p y r o l y z a t e t h a n in t h e oil (oil f r o m t h e s a m e area b u t w i t h u n b i o d e g r a d e d steranes a n d h o p a n e s ) . T h e h o p / s t ratio is higher in t h e p y r o l y z a t e t h a n in t h e u n b i o d e g r a d e d oil ( c o m p a r e Tables II a n d V). T h e higher value
TABLE V GC--MS results of pyrolysis performed on Cerro Negro asphaltenes at four different temperatures (300 ° , 330 °, 350 ° and 370°C, for 3 days) (Figs. 5 and 8)* Temperature
Correlation parameters
(°c)
hop/st
% C~7
% C28
% C29
% 20S
% ~,[3
% 22S
300 330 350 370
1.8 1.7 2.7 3.3
35 37 40 46
32 33 32 32
33 30 28 22
27 31 42 47
44 42 48 50
57 55 56 52
Maturity parameters
*For details of measurements see Table III and Experimental.
177
of this ratio has been found by Seifert (1978) in sediment pyrolyzates. At pyrolysis temperatures higher than 300°C, changes in the steranes and terpanes of the pyrolyzates can be observed. The ratio hop/st (Table V) does not differ significantly at 300 ° and 330°C, but at higher temperatures there is a dramatic increase (from 1.7 at 330°C to 3.3 at 370°C), while the proportion of the C29 to the other normal steranes decreases. A preferential thermal degradation of steranes would seem to be the explanation. This effect can be seen more clearly in Fig. 5 where there is a decrease of the regular steranes (from C=~ to C29) compared to the C21 and C22 steranes (peaks a and b) with increasing temperature. A thermal destruction of the hopanes has also been noticed, with an increase in the low-molecular-weight tricyclic terpanes as well as an enhancement of the 17a(H)-trisnorhopane (Tm). This last phenomenon was also observed by Aquino Neto (1981) during heating of a Grenade oil and attributed to a release of this component from the asphaltenes. The maturity parameters, such as % 20S and % /3,~-steranes, increase steadily with pyrolysis temperature. These changes are interpreted in terms of a preferential destruction of the 2 0 R a,a,a components and of the (20S + 20R)a,a,a over the ( 2 0 R + 20S)a,~,~-steranes. Little change occurs in the parameter % 22S of the C33 hopanes. An important question relative to the pyrolysis of asphaltenes is whether or not the compounds released are actually part of the as-
phaltene structure or are only trapped within it. One mechanism for trapping may result from the secondary structure network resulting from hydrogen bonding associations of this polymeric fraction (e.g., Speight, 1984). We have partially resolved this question by methylation of the Cerro Negro asphaltenes. The asphaltenes were methylated with CH3I [using the method applied to coal samples by Liotta (1979) and Liotta et al. (1981)] which limits these hydrogen bonding associations (detected by infrared spectra) by selective alkylation of the acidic hydroxyl groups. This methylation procedure gives a methylated asphaltene which is largely free of this secondary structure and probably exhibits lower molecular weights (see Table VI; MW 5000--6000 falls to ~ 2 2 0 0 in the methylated asphaltene; molecular weight measured by VPO in pyridine). This methylated product was Soxhlet extracted with n-heptane and ~1% of extractable material was obtained. The TLC silica gel separation of this extract did not show any hydrocarbons. This extract was comprised mainly of a resinous material which was probably associated to the asphaltene by hydrogen bonds. Hence, we conclude that precipitation of asphaltenes with n-heptane and purification of this fraction through hot washing with n-heptane (Soxhlet extraction), gives a product free of trapped hydrocarbons. Therefore, the hydrocarbons released in pyrolysis experiments must come almost entirely from cleavage of the asphaltene framework. These results agree with those of Behar et al. (1984)
TABLE VI Variation of molecular weight of Cerro Negro asphaltene after methylation (determined by vapor pressure osmometry at three different concentrations and extrapolated to ~) in pyridine at 60°C Sample
Molecular weights Concentration* :
Cerro Negro asphaltene Cerro Negro asphaltene (methylated)
4%
2%
1%
5,080 2,220
5,340 2,210
5,360 2,270
*Concentration in weight of asphaltene/volume of pyridine.
6,500 2,290
178
w h o checked the purity of the asphaltene (precipitated with n-heptane) by liquid chromatography and thermovaporization at 300°C. In summary, pyrolysis o f Cerro Negro asphaltenes at 300 ° or 330°C, as suggested by Rubinstein et al. (1979), releases molecules whose structures and configurations are little affected by thermal degradation. Higher temperatures adversely affect the distributions of biological markers such as steranes and terpanes. The similarities between the pyrolysis results of Cerro Negro asphaltenes and those performed with sediments are in accord with the suggestion that asphaltenes of crude oils and source-rock bitumens originate from the kerogen by natural thermal degradation during catagenesis. This last characteristic is very important in the case of crude oils where many factors can affect the distribution of the biomarkers during their generation from the source rock and their migration and accumulation in the reservoir. 3.3. Pyrolysis o f asphaltenes o f four Venezuelan oils at 300°C The asphaltene fractions of three other Venezuelan oils of different origin and maturity were pyrolyzed at 300 ° C for 3 days. These conditions, as stated above, provide molecular
information in the form of evolved hydrocarbons which have been only minimally affected b y thermal alteration. Table VII shows the composition of the heptane extracts of the pyrolyzates. The yields of heptane extracts and resins I decrease with the increasing maturity (Table II) of the respective oils. In the case of the hydrocarbon fractions, the alkanes show similar yields in the four samples examined. The GC results (Fig. 6) for the different pyrolyzates are in general very similar, with some minor differences in the Machete pyrolyzate. The GC--MS results (Table VIII) show that the maturity parameters (see Table III), such as % 20S and % ~,~ of the steranes, are characteristic o f an immature sample;however, their values do not vary much among the samples. The parameter % 22S of the hopanes shows, in contrast with the previous ratios, values characteristic of a mature oil sample. The isomerization processes appear to be retarded in the asphaltenes due to a possible protection effect that these polymeric molecules produce over the biological markers attached to them. Most of the characteristics observed in the distribution o f steranes and triterpanes in the case o f the Cerro Negro pyrolyzate at 300 ° C, are also found in the other three samples examined. However, relative concentrations
T A B L E VII C o m p o s i t i o n o f t h e n - h e p t a n e - s o l u b l e f r a c t i o n f r o m p y r o l y s i s ( 3 0 0 ° C, 3 d a y s ) o f a s p h a l t e n e [¥actions o f f o u r oils f r o m d i f f e r e n t areas o f M a r a c a i b o a n d e a s t e r n V e n e z u e l a n basins Sample
Yield, % w e i g h t a s p h a l t e n e n-heptane-soluble fraction
Boscan Machete Cerro N e g r o Orocual
residue
T o t . ext.
Alk.
Aro.
P. aro.
Resins-I
12.8 9.2 5.1 3.3
0.3 0.4 0.3 0.3
1.8 1.6 1.1 1.0
1.0 1.8 0.4 0.6
9.7 5.4 3.3 0.9
87.2 90.8 94.9 97.2
T o t . ext. = t o t a l e x t r a c t ; Alk. = a l k a n e s ( R f = 0 . 8 - - 0 . 9 ) ; Aro. = a r o m a t i c s ( R f > p h e n a n t h r e n e ) ; P. aro. = polya r o m a t i c s ( R f < p h e n a n t h r e n e ) ; R e s i n s - I = resin-like m a t e r i a l o f t h e n - h e p t a n e e x t r a c t ; r e s i d u e = a s p h a l t e n e r e s i d u e after n-heptane extraction.
179 PYROLYZATE
PYROLYZATE
ERRO
8OSCAN
NEGRO
PYROLYZATE MACHETE PYROLY ZATE OROCUAL
l, Retent,on
t,rne
Retentfon
t,rne
Fig. 6. Gas c h r o m a t o g r a m s for t h e a l k a n e f r a c t i o n s of Boscan, M a c h e t e , Cerro Negro a n d Orocual a s p h a l t e n e p y r o l y z a t e s at 3 0 0 ° C ( X = p r i s t a n e ; Y = p h y t a n e a n d Z = C17 n-alkane).
T A B L E VIII G C - - M S p y r o l y s i s results ( 3 0 0 ° C , 3 d a y s ) o f a s p h a l t e n e f r a c t i o n s o f four oils f r o m d i f f e r e n t areas of M a r a c a i b o a n d E a s t e r n V e n e z u e l a n basins (see Figs. 7 a n d 8)* Sample
Boscan Machete Cerro Negro Orocual
Correlation parameters
Maturity parameters
hop/st
% C:7
% C:8
% C29
% 20S
% ~,~
% 22S
3.0 2.7 1.8 1.9
40 44 35 37
27 28 32 31
33 28 33 32
33 26 27 26
46 43 44 46
56 55 54 55
* F o r details of m e a s u r e m e n t s see Table III a n d S e c t i o n 2.
of tricyclic terpanes are higher in the pyrolyzates o f the asphaltenes of the more mature otis, while the concentration of the C27 17a(H)-trisnorhopane (Tm) is less (Fig. 7). Probably the most important finding is that pyrolysis of the asphaltenes can be used to characterize oils -- even when they are extre-
mely biodegraded. The correlation parameter hop/st is higher in the pyrolyzates than in the original unbiodegraded oils; however, there is a direct correlation between the value of this ratio in the different oils and in their asphaltene pyrolyzates (cf. Tables II and VIII). For instance, the hop/st value for the Cerro
180
PI
(a)
m/z
191
BOSCAN N
e
(b)
m/z
217
dl
I
k
R ,t
A S
c '
E F G
Hf J
f(
~1t f il,i
u
v
AqR L ' *
~J~'LJ/'
MACHETE
CERRO
NEGRO
j2x OROCUAL
P
Retention
time
Retention
t~me
Fig. 7. Mass fragmentograms of the alkane fractions derived from pyrolysis (30O°C) of the asphaltene fractions of Boscan, Machete, Cerro Negro and Orocual oils showing: (a) tricyclic terpane and hopane (m/z 191); and (b) sterane (m/z 217 ) distributions (see Table I for sterane and tricyclic terpane and hopane assignments).
181 20
60
30
50
\ 5o/
oK
" "
\3o
\
60
2o
~ 20
36
~ C29
~
~o
Boscan pyrolyzate distribution was different from those of the other three oils, as expected since this oil is situated in a different basin (Maracaibo Basin). The asphaltenes apparently protect biological marker moieties such as steranes and triterpanes, from maturity effects occurring in both the source rock and the reservoir. The yields of alkanes released by pyrolysis of asphaltenes at 300°C are relatively low (Table VII), but the content of steranes and triterpanes in the pyrolyzates is high enough to permit their analysis by GC--MS (Fig. 7) even without the prior removal of the abundant n-alkanes.
I*
4. Conclusions Fig. 8. Triangular diagram of the relative abundances (peak areas in m / z 218 fragmentograms) of 5a(H), 14~(H),17~(H)-steranes (C~7 to C29) for different oil samples (.) and their asphaltene pyrolyzates (A; 300°C, 3 days) (B = Boscan, M = Machete; CN = Cerro Negro; O = Orocual).
Negro and Orocual oils (these two oils are t h o u g h t to have originated from the same source rock but at different levels of maturity) is the same (1.1) and almost the same for their asphaltene pyrolyzates (1.8 and 1.9, respec.). In the other oils, the hop/st value is higher in the pyrolyzates but relates to the value of this ratio in the original oil. The distributions of the C27, C28 and C29 a,~,~-steranes, which are almost constant for the different oils (see Table II and Fig. 8) are now better differentiated in the case o f the asphaltene pyrolyzates (see Table VII and Fig. 8). This distribution is almost the same for the Orocual and Cerro Negro pyrolyzates, as might be expected, since both these oils are probably derived from the same source rock (Querecual Formation). The distribution of the Machete pyrolyzate, however, was very different, indicating that the oil, although located in the same basin as Orocual and Cerro Negro, was derived from a different source rock, probably located at the north of the Machete area (see Fig. 2). Again, the
(1) Methylation of Cerro Negro asphaltene diminishes the e x t e n t of association via hydrogen bonding which presumably maintains part of the secondary structure of thispolymeric material. This intermolecular H-bonded portion of the secondary structure evidently does not contain trapped hydrocarbons. (2) Pyrolysis of Cerro Negro asphaltene at different temperatures from 300 ° to 370°C reveals that heating at 300°C releases good yields of biomarker hydrocarbons. Pyrolysis at higher temperatures releases larger quantities but unfortunately the sterane distributions are progressively altered, thereby diminishing their value in correlation procedures. (3) Pyrolysis of Cerro Negro asphaltene at the preferred temperature of 300°C releases steranes with distributions less mature than those normally found in crude oils. The pyrolyzates show a predominance of a,a,a over a,~,~ isomers and of the 2 0 R over 2 0 S isomers in the a,a,a-steranes. The hopanes released have a distribution similar to that of the parent oil, though lacking the C2s bisnorhopane, the 2 5 , 2 8 , 3 0 - t r i s n o r h o p a n e and the demethylated hopanes. (4) Pyrolysis of asphaltenes (at 300°C) of three additional Venezuelan marine oils, released pyrolyzates similar to that obtained from the Cerro Negro oil. The sterane and
182
triterpane distributions in the pyrolyzates permitted an improved characterization of the parent otis, based on the hop/st ratio and the distributions of the C27 to C29 a,j3,j3steranes. From these data the Machete and Cerro Negro oils appear to have different origins. (5) The biomarker patterns observed in the asphaltene pyrolyzates obtained in the present work compare well with those described in the literature (Seifert, 1978; Moldowan and Seifert, 1984; Noble et al., 1985) for the pyrolyzates of sediments containing kerogens. These results lend support to the view (Tissot and Welte, 1978) that the asphaltenes represent portions of kerogens liberated during the oil generation process. The molecular components of asphaltenes in oils appear to escape the biodegradation modifications experienced by the free hydrocarbon components and hence pyrolysis of asphaltenes represents a valuable method for obtaining molecular information important in oil--oil and oil-source rock correlation.
Acknowledgements The authors thank INTEVEP, S.A. for permission to publish and f o r providing the samples examined. They wish to thank Mrs. A.P. Gowar and Mr. C. Saunders (Bristol University) for technical support with GC--MS. They are also grateful to the analytical service of School of Chemistry (Bristol University) for the elemental analysis and molecular weight determinations. Further they are indebted to the Natural Environment Research Council for their support of the mass spectrometry and computing facilities (GR3/2951 and GR3/3758).
References Aquino Neto, F.R., Trendel, J.M., Restle, A., Connan, J. and Albrecht, P.A., 1983. Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleums. In: M. Bjor~by, P. Albrecht, C. Cornford, K. de Groot, G. Eglinton, E. Galimov, D. Leythaeuser, R. Pelet, J. RullkStter
and G. Speers (Editors), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 659--667. Bandurski, E., 1982. Structural similarities between oil-generating kerogens and petroleum asphaltenes. Energy Sources, 6: 47--66. Barwise, A.J.G. and Park, P.J.D., 1983. Petroporphyrin fingerprinting as a geochemical marker. In: M. Bjor~by, P. Albrecht, C. Cornford, K. de Groot, G. Eglinton, E. Galimov, D. Leythaeuser, R. Pelet, J. Rullk6tter and G. Speers (Editors), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 668--674. Barwise, A.J.G. and Roberts, I., 1984. Diagenetic and catagenetic pathways for porphyrins in sediments. Org. Geochem., 6: 167--176. Behar, F., Pelet, R. and Roucache, J., 1984. Geochemistry of asphaltenes. Org. Geochem., 6: 587-595. Demaison, G.J., 1977. Tar sands and supergiant oil fields. Bull. Am. Assoc. Pet. Geol., 61: 1950-1961. Didyk, B.M., Alturki, Y.I.A., Pillinger, C.T. and Eglinton, G., 1975. Petroporphyrins as indicators of geothermal maturation. Nature (London), 256: 563--575. Ekweozor, C.M. and Strausz, O.P., 1983. Tricyclic terpanes in the Athabasca oil sands: their geochemistry. In: M. BjorCy, P. Albrecht, C. Cornford, K. de Groot, G. Eglinton, E. Galimov, D. Leythaeuser, R. Pelet, J. RullkStter and G. Speers (Editors), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 746--766. Ensminger, A., Van Dorsselaer, A., Spyckerelle, C., Albrecht, P. and Ourisson, G., 1974. Pentacyclic triterpanes of the hopane type as ubiquitous geochemical markers: origin and significance. In: B. Tissot and F. Bienner (Editors), Advances in Organic Geochemistry 1973. Editions Technip, Paris, pp. 245--260. Gallegos, E.J., 1975. Terpane--sterane release from kerogen by pyrolysis gas chromatography--mass spectrometry. Anal. Chem., 47: 1524--1528. Hajibrahim, S.K., 1978. Application of petroporphyrins to the maturation migration and origin of crude oils. Ph.D. Thesis, University of Bristol, Bristol (unpublished). Huang, W. and Meinschein, W.G., 1979. Sterols as ecological indicators. Geochim. Cosmochim. Acta, 43: 739--745. Ignasiak, T., Strausz, O.P. and Montgomery, D.S., 1977. Oxygen distribution and hydrogen bonding in Athabasca asphaltene. Fuel, 56: 359--365. Liotta, R., 1979. Selective alkylation of acidic hydroxyl groups in coal. Fuel, 58: 724--728. Liotta, R., Rose, K. and Hippo, E., 1981. O-alkylation of coal and its implications for the chemical and physical structure of coal. J. Org. Chem., 46: 277--283.
183 Mackenzie, A.S. and Maxwell, J.R., 1981. Assessment of thermal maturation in sedimentary rocks by molecular measurements. In: J. Brooks (Editor), Organic Maturation Studies and Fossil Fuel Exploration. Academic Press, London, pp. 239--254. Mackenzie, A.S., Patience, R.L., Maxwell, J.R., Vandenbroucke, M. and Durand, B., 1980a. Molecular parameters of maturation in the Toarcian shales, Paris Basin, France, 1. Changes in the configurations of acyclic isoprenoids alkanes, steranes and triterpanes. Geochim. Cosmochim. Acta, 44: 1709--1721. Mackenzie, A.S., Quirke, J.M.E. and Maxwell, J.R., 1980b. Molecular parameters of maturation in the Toarcian shales, Paris Basin, France, II. Evolution of metalloporphyrins. In: A.G. Douglas and J.R. Maxwell (Editors), Advances in Organic Geochemistry 1979. Pergamon, Oxford, pp. 239--248. Mackenzie, A.S., Hoffman, C.F. and Maxwell, J.R., 1981. Molecular parameters of maturation in the Toarcian shales, Paris Basin, France, III. Changes in the aromatic steroid hydrocarbons. Geochim. Cosmochim. Acta, 45: 1345--1355. Mackenzie, A.S., Brassell, S.C., Eglinton, G. and Maxwell, J.R., 1982. Chemical fossils: the geological fate of steroids. Science, 217: 491--504. Mackenzie, A.S., Li Renwei, Maxwell, J.R., Moldowan, J.M. and Seifert, W.K., 1983. Molecular measurements of thermal maturation of Cretaceous shales from the overthrust belt, Wyoming, U.S.A. In: M. Bjor~by, P. Albrecht, C. Cornford, K. de Groot, G. Eglinton, E. Galimov, D. Leythaeuser, R. Pelet, J. Rullk6tter and G. Speers (Editors), Advances in Organic Geochemistry 1981. Wiley Chichester, pp. 496--503. Mackenzie, A.S., Maxwell, J.R., Coleman, M.L. and Deegan, C.E., 1984. Biological marker and isotope studies of North Sea crude oils and sediments. Proc. l l t h World Pet. Cong., 2: 45--56. Moldowan, J.M. and Seifert, W.K., 1984. Structure proof and significance of stereoisomeric 28,30bisnorhopane in petroleum and petroleum source rocks. Geochim. Cosmochim. Acta, 48: 1651-1661. Noble, R., Alexander, R. and Kagi, R.I., 1985. The occurrence of bisnorhopane, trisnorhopane and 25-norhopanes as free hydrocarbons in some Australian shales. Org. Geochem., 8: 171--176. Reed, W.E., 1977. Molecular composition of weathered petroleum and comparison with its possible source. Geochim. Cosmochim. Acta, 41: 237-247.
Rubinstein, I. and Strausz, O.P., 1979. Thermal treatment of the Athabasca oil sand bitumen and its component parts. Geochim. Cosmochim. Acta, 43: 1887--1893. Rubinstein, I., Spyckerelle, C. and Strausz, O.P., 1979. Pyrolysis of asphaltenes: a source of geochemical information. Geochim. Cosmochim. Acta, 43: 1--6. RullkStter, J. and Wendisch, D., 1982. Microbial alteration of 17a(H)-hopanes in Madagascar asphalts: removal of C-10 methyl group and ring opening. Geochim. Cosmochim Acta, 46: 15451553. Seifert, W.K., 1978. Steranes and terpanes in kerogen pyrolysis for correlation of oils and source rocks. Geochim. Cosmochim. Acta, 42: 473--484. Seifert, W.K. and Moldowan, J.M., 1978. Application of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochim. Cosmochim. Acta, 42: 77--92. Seifert, W.K. and Moldowan, J.M., 1979. The effect of hiodegradation on steranes and terpanes in crude oils. Geochim. Cosmochim. Acta, 43: 111126. Seifert, W.K. and Moldowan, J.M., 1980. The effect of thermal stress on source rock quality as measured by hopane stereochemistry. In: A.G. Douglas and J.R. Maxwell (Editors), Advances in Organic Geochemistry 1979. Pergamon, Oxford, pp. 229-237. Shi JiYang, Mackenzie, A.S., Alexander, R., Eglinton, G., Gowar, A.P., Wolff, G.A. and Maxwell, J.R., 1982. A biological marker investigation of petroleums and shales from the Shengli oilfield, the People's Republic of China. Chem. Geol., 35: 1-31. Speight, J.G., 1984. The chemical nature of petroleum asphaltenes. In: Characterization of Heavy Crude Oils and Petroleum Residues. Editions Technip, Paris, pp. 32--41. Tissot, E.P. and Welte, D.H., 1978. Petroleum Formation and Occurrence. Springer, Berlin, 538 pp. Volkman, J.K., Alexander, R., Kagi, R.I. and RullkStter, J., 1983. GC--MS characterization of C~ and C2s triterpanes in sediments and petroleum. Geochim. Cosmochim. Acta, 47: 1033--1040. Wardroper, A.M.K., Hoffmann, C.F., Maxwell, J.R., Barwise, A.J.G., Goodwin, N.S. and Park, P.J.D., 1984. Crude oil biodegradation under simulated and natural conditions, II. Aromatic steroid hydrocarbons. Org. Geochem., 6: 605--617.
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Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
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ORGANIC GEOCHEMISTRY OF VENEZUELAN EXTRA-HEAVY OILS 1. Pyrolysis of Asphaltenes: A Technique for the Correlation and Maturity Evaluation of Crude Oils F. C A S S A N I a n d G. E G L I N T O N
Organic Geochemistry Unit, University of Bristol, School of Chemistry, Bristol BS8 1TS (Great Britain) (Received February 26, 1986; revised and accepted March 4, 1986) Abstract Cassani, F. and Eglinton, G., 1986. Organic geochemistry of Venezuelan extra-heavy crude oils, 1. Pyrolysis of asphaltenes: a technique for the correlation and maturity evaluation of crude oils. Chem. Geol., 56: 167--183. The asphaltene fraction from Cerro Negro (Venezuela) oil has been used to test the effects of pyrolysis temperature on the distribution of alkanes generated by this technique. Particular attention has been paid to the biological marker compounds (steranes, triterpanes) released in these experiments and to the correlation and maturity parameters as normally applied in organic geochemical exploration. Pyrolysis temperatures up to 330°C have little effect on the distribution of these compounds, whereas higher temperatures cause thermal destruction of hopanes and steranes, thereby affecting the values of such parameters. The distribution of these compounds in the pyrolyzates was less mature than those normally found in crude oils. Four Venezuelan marine oils, from different regions and of different levels of biomarker maturity, but of generally similar depositional origin as identified by their triterpane and presumed sterane (in the case of those heavily biodegraded) distributions have been studied. These oils were readily distinguished as their respective asphaltene pyrolyzates on the basis of the hopane/sterane ratio and the distribution of C~7--C29 5a (H), 14/3(H), 17~(H)-steranes. The distribution patterns of steranes and hopanes obtained in the asphaltene pyrolyzates have been compared with those reported for pyrolysis of sediments. The data are compatible with an origin for the asphaltenes involving catagenic degradation of the kerogen in the source rock. The pyrolysis technique applied to asphaltenes is suited to the characterization of heavy biodegraded oils or weathered source rocks where most of the molecular information has been destroyed. 1. I n t r o d u c t i o n A s p h a l t e n e s are t h e n o n - h y d r o c a r b o n fractions precipitated from petroleums or bitum e n s b y the a d d i t i o n of an excess of an na l k a n e ( f r o m p e n t a n e t o h e p t a n e ) . T h e y are 0009-2541/86/$03.50
normally soluble in benzene. The content of t h i s f r a c t i o n varies f r o m s m a l l a m o u n t s ( ~ 0 . 1 - - 1 . 0 % ) i n l i g h t oils a n d t h e e x t r a c t s o f mature source rocks to high quantities (~ 10-25%) in " i m m a t u r e " o r b i o d e g r a d e d oils. T h e s t r u c t u r e o f a s p h a l t e n e s h a s b e e n in-
© 1986 Elsevier Science Publishers B.V.
168 vestigated in numerous studies and average structures postulated. Thus, a proposed asphaltene framework consists of condensed polynuclear aromatic ring systems bearing heteroatoms (nitrogen, oxygen and sulphur), alkyl chains and h y d r o c a r b o n rings (e.g., Speight, 1984). Evidence from spectrometric and degradative analysis (Rubinstein and Strausz, 1979; Rubinstein et al., 1979; Bandursky, 1982; Behar et al., 1984) suggests that kerogens and petroleum asphaltenes could have similar structures, since oil-like material, containing n-alkanes or n-alkane--n-alkane pairs from C3 to C40 (depending on the technique used), can be generated from them b y pyrolysis. Therefore, the asphaltenes could be fragments of the original kerogen from which oil is derived, presumably by heteroatomic cleavage (Tissot and Welte, 1978; Bandurski, 1982). Rubinstein and Strausz (1979) and Rubinstein et al. (1979) have heated asphaltenes (from Prudhoe Bay oil, Alaska and Athabasca oil, Alberta, Canada) and released biological markers such as 17a(H),21~(H)-terpanes (from C27 to Css) with distributions similar to that of the original oil. E k w e o z o r and Strausz (1983) studied the tricyclic terpanes between C,9 and C30 released by pyrolysis of the Athabasca asphaltenes, which contain high concentrations of the precursors of these compounds. No analysis has been reported involving the use of biological markers (such as steranes and triterpanes) generated from asphaltenes to assess maturity and correlation by the procedures c o m m o n l y used in sediments and oil samples (e.g., Shi Ji-Yang et al., 1982). The release of such biomarkers by pyrolysis or chemical degradation techniques would provide new information which will help solve geochemical problems, especially in the case of biodegraded oils. Thus, pyrolysis experiments with sediments (Gallegos, 1975; Seifert, 1978; Seifert and Moldowan, 1980; Mackenzie et al., 1983; Moldowan and Seifert, 1984; Noble et al., 1985) have shown some differences between the sterane and triterpane distributions of the pyrolyzates and
those of their respective rock extracts. For example, the degree of "isomerization" of these c o m p o u n d s in the pyrolyzate can be less than in the solvent extract of the same rock. However, the pyrolysis of sediments has several problems that do n o t exist in the case of asphaltene pyrolysis: (1) there are polar compounds associated with the mineral matrix and kerogen which on pyrolysis may generate hydrocarbons additional to those released by the kerogen; (2) hydrocarbons trapped or adsorbed in the mineral matrix and kerogen m a y be released; and (3) a catalytic effect of the mineral matter (which may be different for different sediments) may be operating. Nevertheless, the distribution of polycyclic alkanes can still be used as a correlation and maturity aid, as suggested by Seifert (1978). In this study, pyrolysis experiments have been performed on a Cerro Negro (heavy oil from the Orinoco Oil Belt, eastern Venezuelan Basin) asphaltene sample at different temperatures (from 300 ° to 370 ° C) for 3 days to investigate the distribution of the biomarkers generated. We have sought to release the biomarkers with a minimum of thermal alteration of the original configurations in the asphaltene structure. The distributions of several specific types o f alkane have been assessed in several Venezuelan marine oils from different regions and different levels of biomarker maturity by using combined gas chromatography- mass spectrometry (GC--MS) of the pyrolyzates of asphaltenes. This is an a t t e m p t at better characterization of these otis, where the information embodied within structures could be affected by the operation of several processes, such as maturation, migration, addition of fresh petroleums and biodegradation. 2. Experimental (analytical procedure in Fig. 1)
2.1. Asphaltenes The asphaltene fraction was separated from the oil by a standard method (Ignasiak et al., 1977), but using n-heptane instead of n-
169
SOLUBLE
iNSOLUBLE
I
LUBLE
WITH n - C 7
INSOLUBLE SOLUBLE
Fig. 1. Analytical procedure for the precipitation of asphaltenes from a crude oil and fraetionation of asphaltene pyrolysis products. pentane for the precipitation. A mixture of oil and toluene (1 : 1) was diluted with 60 volumes of u-heptane and kept for a minim u m of 16 hr. with stirring under nitrogen. The asphaltenes were filtered through a thimble and Soxhlet extracted with n-heptane for 48 hr. and recovered from the thimble with dichloromethane (DCM). The solvent was removed on a rotary evaporator and the asphaltenes dried using a stream o f dry nitrogen and then under vacuum overnight at 60°C.
2.2. Pyrolysis Samples for pyrolysis (between 0.5 and 1.0 g) were placed in thick-walled Pyrex ®
tubes (25 ml), evacuated (0.1 Torr), flushed with nitrogen (this procedure was repeated 3 times), and sealed under vacuum with a glassblowing torch. The tubes were protected during heating by wrapping in aluminium foil and placing in individual stainless-steel tubes. An old gas chromatograph oven was used for the heating experiments (temperature + 3 ° C). The samples were heated at temperatures between 300 ° and 370°C for 72 hr. The tubes were opened after cooling in liquid nitrogen (to reduce pressure due to gaseous products). The pyrolysis products were pulverized, transferred to a thimble and Soxhlet extracted with n-heptane for 48 hr. The solvent was then removed on a rotary evaporator and the pyrolysis extract separated by thin-layer chromatography (TLC) (silica gel; 0.4 mm thick; n-hexane as eluent). The plates were visualized (Rhodamine e 6G; UV light) and four fractions obtained: Alkane hydrocarbons (top band; Rf 0.8--0.9); aromatic hydrocarbons (Rf > phenanthrene); polyaromatic compounds (Rf < phenanthrene) and polar compounds (at the b o t t o m o f the plate). Each band was scraped off the plate and the respective fractions eluted with DCM (50 ml). The solvent was finally removed in a rotary evaporator and each fraction evaporated under a stream of dry nitrogen. Further separation of the alkane fraction with silica gel impregnated with silver nitrate (5%) did not give appreciable amounts (<5% of the total alkanes) of unsaturated compounds. Higher amounts of unsaturated compounds (~30%) were obtained in a resin fraction pyrolyzate of the Cerro Negro oil sample.
2. 3. Gas chromatography (GC) and computerized gas chromatography--mass spectrometry
(C--GC--MS) Gas chromatography of the alkane fractions from the pyrolysis extracts was performed by split/splitless injection on a Carlo Erba ® series FTV 2150 gas chromatograph fitted with an OV-I coated fused silica capillary column (~20 m × 0.25 mm i.d.). A tem-
170 perature programme of 50--280°C at 6°C m i n . -~ w a s e m p l o y e d a n d t h e c a r r i e r gas ( H e ) h a d a f l o w r a t e o f ~ 1 m l m i n . -~. C--GC--MS was performed on a Carlo E r b a ® ( M e g a s e r i e s ) gas c h r o m a t o g r a p h i n t e r faced with a Finnigan ® 4000 mass spectrome t e r . T h e gas c h r o m a t o g r a p h was equipped
with on-column injection and fitted with a fused silica capillary column (OV-1, 25 m × 0.3 mm i.d.) and programmed from 50--280 ° C a t 6 ° C m i n . -~. H e l i u m w a s u s e d as t h e c a r r i e r gas. T h e m a s s s p e c t r o m e t e r w a s o p e r a t e d in the EI mode (ionizing energy 35 eV, source temperature 250°C) with multiple ion detec-
TABLEI Tricyclic terpane (I), hopane (II) and sterane (III) assignments and abbreviated names (for reference to Figs. 4, 5 and 7 ) Peak
Basic structure*
Component
Abbreviated name
Tricyclic terpanes and hopanes: A-I J K L M
I I + II II II II
N
II
O
II
P
II
Q R
II II
S
II
T
II
U V
II II
C20--C29 tricyclics C29 tricyclic + C27 18a(H)-22,29,30-trisnorneohopane C27 1 7a(H)-22,29,30-trisnorhopane C:s 17a(H),18a(H),21~(H)-28,30-bisnorhopane C29 demethylated hopane C29 1 7a(H),21#(H)-30-norhopane C29 17[3(H),21a(H)-30-norhopane C30 17~(H),21~(H)-hopane C30 17[3(H),21~(H)-hopane C31 17a(H),21~(H)-homohopane, (22S + 22R) C32 1 7a(H),21~(H)-bishomohopane, (22S + 22R) C33 1 7a(H),2113(H)-trishomohopane, (22S + 22R) C3~ 17a(H),21~(H)-tetrakishomohopane, (22S + 22R) C35 1 7a(H),21~(H)-pentakishomohopane, (22S + 22R)
C2o-3 to C29-3 C:9-3 + Ts Tm C28 bisnorhopane C29 ~,# C29 ~,~ C30 ~,;~ C30 ~,~ C3~ ~,~ (22S + 22R) C3: ~,~ (22S + 22R) C33 ~,~ (22S + 22R) C3, ~,~ (22S + 22R) C35 ~,~ (22S + 22R)
Steranes: a
III
b
III
c d e /" g h i j k
III III III III III III III III III
C2~ sterane C2: sterane C~7 5a(H),14~(H),17a(H),20S-cholestane C~ 5a(H),14~(H),1 7~(H),(20R + 20S)-cholestane C~ 5a(H),14a(H),1 7a(H),2OR-cholestane C~ 5a(H),14a(H),l 7a(H) 20S-24-methyl-cholestane C~s 5a(H),14~(H),1 7~(H) (20R + 20S)-24-methyl-cholestane C~s 5a(H),14a(H),l 7~(H) 20R-24-methyl-cholestane C~9 5~(H),14~(H),1 7a(H) 20S-24-ethyl-cholestane C2~ 5~(H),14~(H),1 7~(H) (20R + 20S)-24-ethyl-cholestane C~ 5~(H),14~(H),l 7a(H) 20R-24-ethyl-chotestane
*Basic structures of tricyclic terpanes (I), hopanes (II) and steranes (III):
:lII)
C~ a,~,a,20S C~ a,~,~ (20R + 20S) C~ ~ , a , a , 2 0 R C2~ a,a,a,20S C~s ~,~,~ (20R + 20S) C~8 a,a,~ 20R C~ a,a.~ 20S C~9 ~,~,~ (20R + 20S) C:9 ~,~,~ 20R
171
ti0n. Data were acquired and processed using an INCOS ® 2300 data system. Steranes and triterpanes were identified by comparison of their retention times and mass spectra with those of known samples: Paris Basin (France) rock extracts (Mackenzie et al., 1980a) and Athabascan oil (Ekweozor and Strausz, 1983). Demethylated hopanes, 28bisnorhopane and 25,28,30-trisnorhopane were identified by comparison of their mass spectra and retention pattern with literature data (Seifert and Moldowan, 1979; RullkStter and Wendisch, 1982; Volkman et al., 1983).
2. 4. Determination o f molecular weight The molecular weights were determined on a Perkin-Elmer ® model 115 vapour pressure osmometer (VPO) at three different concentrations (1%, 2% and 4% w/v and extrapolated to infinity) in pyridine at 60°C.
L
B BO~CAN M ~AOMETE CERR@ NEGBC: C~ 0RBZLAL -
-
-
- -
BASN BOUNDARY ORINOCO 01L BEL ~ BOUNDARY AND AREAS
Fig. 2. Locations of oil samples (-) from Maracaibo Basin (Boscan oil) and Eastern Venezuelan Basin: Orocual oil at the north of the basin; Cerro Negro oil in the eastern part of the Orinoco Oil Belt and Machete oil in the western part of this same area in the south of the basin. 3. Results and discussion
3.1. Samples 2.5. Quantitation and measurement o f parameters All hydrocarbon parameters were quantitated using peak areas in the corresponding mass fragmentograms obtained by GC--MS (see Table I for sterane and triterpane assignments). The % 20S/(20R + 20S)ratio was calculated from m/z 217 (C29 a,a,a-steranes) mass fragmentograms; the %/3,~(20R + 20S)/ C2~ a,~,/3(20R + 20S) + C29 a,a,a(20S + 2OR) ratio was calculated from m/z 217 mass fragmentogram and the relative proportions of the C33 (22S) and (22R) 17a(H),21[3(H) isomers (m/z 191) were used to calculate the 22S/(22S + 22R) hopane ratio (Mackenzie et al., 1980a). The relative abundances of the 5a(H),14~(H),17~(H) 20R and 20S components for different carbon numbers (C27 to C~9) were measured from m/z 218 fragmentograms and the ratio hopanes/steranes (hop/st) was obtained from the relative abundances of C30 17a(H),21~(H)-hopane (measured from m/z 191 mass fragmentograms) to C29 a,a,a(20S + 2OR) + a43,/3(20R + 20S)-steranes (Mackenzie and Maxwell, 1981).
Fig. 2 shows the locations of the oil samples studied in this work (see Table II for reservoir depths and ages). Boscan oil (Maracaibo Basin, west Venezuela) is an "immat u r e " heavy oil, slightly biodegraded and probably originated from La Luna shale (Cretaceous). Cerro Negro oil (Orinoco Oil Belt, eastern Venezuelan Basin) is a very heavy, middle mature oil located in the eastern part of the Orinoco Oil Belt, which is one of the largest heavy-oil accumulations in the world. The origin of this oil is probably from the Cretaceous source rocks at the north of the basin (Querecual Formation). Demaison (1977) concluded that this oil migrated to the edge of the basin over a distance between 90 and 190 km and was biodegraded in shallow reservoirs by bacteria introduced by meteoric waters. The Machete oil is located in the western part of the Orinoco Oil Belt and is less mature than Cerro Negro oil. This oil is also heavy and very biodegraded and probably has a different origin from Cerro Negro oil. The mature oil from the Orocual field at the north of the basin is t h o u g h t to
172
T A B L E II GC--MS c o r r e l a t i o n a n d m a t u r i t y p a r a m e t e r s for t h e V e n e z u e l a n oils e x a m i n e d *~ Samples
Age .2
Boscan M a c h e t e .3 Cerro Negro .3 Orocual
Depth (m)
E M M M
1,650 495 820 1,576
Correlation parameters
Maturity parameters
hop/st
% C2~
% C2s
% C29
% T / ( T + M) % DPEP
2.1 1.6 1.1 1.1
35 32 30 33
28 32 32 34
37 36 38 34
50 63 67 93
70 52 41 17
* ~ F o r details o f m e a s u r e m e n t s see T a b l e III a n d S e c t i o n 2. ,5 Ages: M = M i o c e n e ; E = E o c e n e (for oil reservoir age). .3 T h e c o r r e l a t i o n p a r a m e t e r s for t h e s e oils c o r r e s p o n d t o a less b i o d e g r a d e d oil f r o m t h e same area; in these particular samples t h e s t e r a n e s a n d h o p a n e s have b e e n a f f e c t e d b y b i o d e g r a d a t i o n . T A B L E III M o l e c u l a r p a r a m e t e r s e m p l o y e d (see Tables II, V a n d V I I I ) Parameter
Ratio
Significance
% 20S
C=9 e , e ,~-steranes:
i s o m e r i z a t i o n at s t e r a n e C-20(S); ratio rises f r o m 0 to - 5 0 % w i t h increasing m a t u r i t y ; oil g e n e r a t i o n c o u l d s t a r t at ~ 4 0 % ( M a c k e n z i e et al., 1 9 8 2 )
20S 20S + 20R
% ~,~
C29 regular steranes: ~,~,~(R+S) total steranes
% 22S
C33 e , B - ( H ) - t r i s h o m o h o p a n e s : 22S (22S + 22R
% T / ( T + M)
steroid hydrocarbon: C~s-triaromatic C=9 m o n o + C28-triaromatic
% DPEP
porphyrins: DPEP DPEP + A E T I O
i s o m e r i z a t i o n at ~terane C-14 a n d C-17; ratio rises f r o m 0-50% to ~ 7 0 - - 7 5 % w i t h increasing m a t u r i t y ; i s o m e r i z a t i o n n o r m a l l y c o m p l e t e b e f o r e m a j o r g e n e r a t i o n of h y d r o c a r b o n s ( M a c k e n z i e a n d Maxwell, 1 9 8 1 ) i s o m e r i z a t i o n at C-22(S); r a t i o rises f r o m 0 t o 60% ( E n s m i n g e r et al., 1 9 7 4 ) ; i s o m e r i z a t i o n n o r m a l l y c o m p l e t e b e f o r e m a j o r g e n e r a t i o n of h y d r o c a r b o n s ( M a c k e n z i e a n d Maxwell,
1981) a r o m a t i z a t i o n o f C ring a r o m a t i c h y d r o c a r b o n ; r a t i o rises f r o m 0 t o 100% w i t h increasing m a t u r i t y ; this ratio is m o r e temp e r a t u r e d e p e n d e n t t h a n % 2 0 S s t e r a n e r a t i o (Mackenzie et al., 1 9 8 2 ) ; oil g e n e r a t i o n c o u l d s t a r t at - 4 0 - - 6 0 % a p p a r e n t C--C b o n d cleavage of isocyclic ring in the DPEP porp h y r i n s (e.g., Didyk et al., 1 9 7 5 ) or t h e r m a l d e s t r u c t i o n of DPEP p o r p h y r i n s (Barwise a n d Park, 1 9 8 3 ) , r a t i o falls f r o m ~ 1 0 0 % to 0; oil g e n e r a t i o n c o u l d start at ~ 8 0 - - 6 0 % (Barwise a n d Park, 1 9 8 3 )
hop/st
C3o c~, ~-hopane/C29 regular steranes
this ratio is sensitive to changes in the a m o u n t of algal material ( l o w e r values) i n c o r p o r a t e d i n t o f o r m i n g s e d i m e n t s , relative t o h i g h e r - p l a n t organic m a t t e r (higher values, ~ 5 ) (Mackenzie et al., 1 9 8 2 )
% C2~ % C2.
relative a b u n d a n c e s of C~7 , C2s a n d C~9 a , ~ , ~ - s t e r a n e s
these d i s t r i b u t i o n s d e p e n d o n the n a t u r e of the organisms w h i c h c o n t r i b u t e to t h e organic m a t t e r o f t h e f o r m i n g sediments (Huang and Meinschein, 1979)
173
have originated from the same source rock as Cerro Negro oil but was generated at a higher level of maturity. Table II shows the correlation and maturity parameters for the different samples studied (for details of these parameters see Table III). Hop/st ratio is the same for the Cerro Negro and Orocual oils, slightly higher for Machete oil and higher still for Boscan oil. The C~7 to C29 a,~,t3-sterane distribution is similar for Orocual, Machete and Cerro Negro oils and slightly different for Boscan oil. The maturity parameters normally used for sediments (% 20S; % ~,~ in the steranes and % 22S in the hopanes) cannot be applied in the case of these samples because they are affected by severe biodegradation (e.g., Seifert and Moldowan, 1979). Therefore, other parameters, i.e. % of triaromatic steroid hydrocarbons (e.g., Mackenzie et al., 1981) and % DPEP(*) porphyrins (e.g., Mackenzie et al., 1980b), have been used to determine the maturity of these oils; these parameters are apparently only affected by biodegradation in extreme cases (Hajibrahim, 1978; Wardroper et al., 1984).
*DPEP = deoxophylloerythroetioporphyrin.
3.2. Pyrolysis o f Cerro Negro asphaltenes at 300 °, 330 °, 350 ° and 370°C. The Cerro Negro asphaltene was pyrolyzed at four different temperatures (300 ° , 330 ° , 350 ° and 370°C) for 3 days to investigate the effect of increasing temperature on biological markers and also to determine h o w closely the changes parallel those in sediments suffering increasing burial depths over geological time. Table IV shows the composition of the pyrolyzate with increasing pyrolysis temperature. The yields of the heptane extracts and their fractions are relatively low at temperatures of 300 ° or 330°C b u t increase at higher temperatures at the expense of the asphaltene residue which generates these fractions. At 370°C a decrease in the yield of the resin-I fraction is observed;this p h e n o m e n o n is interpreted as a preferential degradation of this fraction to form hydrocarbons. In contrast with the results reported in similar experiments by Rubinstein and Strausz (1979), low amounts of unsaturated c o m p o u n d s (<5%) were separated by TLC (5% silver nitrate). In the same table the elemental analysis results for the asphaltene residue fractions show a decrease of the H/C ratio and sulphur content with increasing temperature b u t the nitrogen content appears to be stable.
T A B L E IV C o m p o s i t i o n o f the n - h e p t a n e - s o l u b l e p r o d u c t s and e l e m e n t a l c o m p o s i t i o n of the residual a s p h a l t e n e s f r o m pyrolysis o f Cerro Negro a s p h a l t e n e s at four d i f f e r e n t t e m p e r a t u r e s (300 °, 330 °, 350 ° and 370°C, for 3 days) Temperature (° C)
300 330 350 370
Yield, % w e i g h t a s p h a l t e n e
E l e m e n t a l c o m p o s i t i o n o f residue
n-heptane-soluble fraction
residue
Tot. ext.
Alk.
Aro.
P. aro.
Resins-I
5.1 9.9 16.0 18.4
0.3 0.7 2.2 4.2
I.i 1.7 3.6 6.5
0.4 1.5 2.7 3.4
3.3 6.0 7.5 4.3
94.9 90.1 84.0 81.6
H/C
1.08 1.04 1.00 0.93
S
N
(%)
(~)
5.15 5.00 4.80 4.30
1.5 1.1 1.4 1.3
Tot. ext. = t o t a l e x t r a c t ; Alk. = Alkanes (Rf = 0 . 8 - - 0 . 9 ) ; Aro. = a r o m a t i c s ( R f > p h e n a n t h r e n e ) ; P. aro. = polya r o m a t i c s (Rf < p h e n a n t h r e n e ) ; Resins-I = resin-like material o f the n - h e p t a n e e x t r a c t ; residue = a s p h a l t e n e residue after n - h e p t a n e e x t r a c t i o n .
174
PYROLYZATE
PYROLYZATE
3oo°c
350°C
1 Z
Z PYROLYZATE 330°C X
I= Retention
t 4me
Retent
ion
t ime
Fig. 3. Gas c h r o m a t o g r a m s for t h e alkane f r a c t i o n s o f Cerro Negro a s p h a l t e n e p y r o l y z a t e s at d i f f e r e n t temp e r a t u r e s o f pyrolysis ( 3 0 0 ° , 330 ° , 3 5 0 ° a n d 3 7 0 ° C ) ( X = p r i s t a n e ; Y = p h y t a n e a n d Z = C~7 n=alkane).
(a)
Retention
(b)
(c)
m/z 191
F
j
H, \
Retention
mlz
217
T
I ,K B
time
(
t,me
U
V
Retention
t4me
Fig. 4. Gas c h r o m a t o g r a m (a) a n d mass f r a g m e n t o g r a m s (b a n d c) o f t h e a l k a n e f r a c t i o n o f Cerro Negro oil s h o w i n g (b) tricyclic t e r p a n e a n d h o p a n e (m/z 1 9 1 ) a n d (c) s t e r a n e (m/z 2 1 7 ) d i s t r i b u t i o n s (see T a b l e I for s t e r a n e a n d tricyclic t e r p a n e a n d h o p a n e a s s i g n m e n t s ) .
175
(a)
m/z
191
PYROLYZA1 e
(b)
m/z
217
300CC a
d c
g 13
R B
E
b
K F G
H(J '
h~
PYROLYZATE 330°C
PYROLYZATE 350~C
PYROLYZATE 370°C
Retention
time
Retention
time
Fig. 5. Mass fragmentograms of the alkane fractions of a Cerro Negro asphaltene p y r o l y z a t e at 300 °, 330 ° , 350 ° and 370°C showing: (a) tricyclic terpane and h o p a n e (m/z 191); and (b) sterane (m/z 217) distributions (see Table I for sterane and tricyclic terpane and h o p a n e assignments).
176 The alkanes generated from the pyrolysis were analyzed by GC and C--GC--MS. The GC traces (Fig. 3) reveal the generation of nalkanes from C14 to C40 and isoprenoids such as pristane (C19) and phytane (C20). These hydrocarbons were not present in the original oil due to their prior removal by biodegradation (Fig. 4). Not many differences are observed among the distributions of n-alkanes (Fig. 3) at 300 ° , 330 ° and 350°C, but at 370 ° C a small increase in the lower-molecularweight n-alkanes is observed, probably due to the cracking of higher-molecular-weight compounds. The pristane/phytane ratio is close to one at 300°C and increases slightly at higher temperatures. The C--GC--MS results for the steranes (m/z 217) and triterpanes (m/z 191) present in the asphaltene sample heated at 300°C, indicate (Fig. 5) a new generation of compounds in the pyrolyzate (see Table I for compound assignments) very similar to those of the original oil in the case of the triterpanes. However, in the case of the steranes these compounds have been altered by biodegradation and the normal steranes are almost absent in the oil (compare Figs. 4 and 5). The "triterpanes" in the pyrolyzate have a pattern of trieyclic compounds similar to that of the oil but there are some differences in the hopanes, where the C29 and C30 ~,~-hopanes (moretanes) are present in higher concentrations than in the oil. The C2a bisnorhopane and the C29-demethylated hopane were also
a b s e n t in t h e p y r o l y z a t e (as w e r e t h e series o f d e m e t h y l a t e d h o p a n e s d e t e c t e d in t h e m/z 177 m a s s f r a g m e n t o g r a m s o f t h e oil s a m p l e ) . T h e d e m e t h y l a t e d h o p a n e s are a p p a r e n t l y f o r m e d as an e f f e c t o f b i o d e g r a d a t i o n ( R e e d , 1 9 7 7 ; Seifert a n d M o l d o w a n , 1 9 7 9 ) o f t h e oil and cannot be produced from the asphaltene. T h e m/z 177 mass f r a g m e n t o g r a m in t h e p y r o l y z a t e also e x h i b i t e d t h e a b s e n c e o f t h e 25,28,30-trisnorhopane, w h i c h is p r e s e n t in t h e original oil. T h e l a c k o f t h e C28 b i s n o r h o p a n e and t h e 25,28,30-trisnorhopane in t h e p y r o l y z a t e suggests t h a t t h e s e c o m p o u n d s do not form part of the asphaltene structure but are free lipids, as suggested b y M o l d o w a n a n d Seifert ( 1 9 8 4 ) f o r s e d i m e n t p y r o l y z a t e s . V e r y small a m o u n t s o f d i a s t e r a n e s and a r o m a t i c steroids are released b y this t y p e o f p y r o l y s i s , in parallel w i t h t h e results o f M a c k e n z i e et al. (1983) for sediment pyrolyzates. T h e steranes g e n e r a t e d b y p y r o l y s i s p r e s e n t an i m m a t u r e d i s t r i b u t i o n , with a p r e d o m i n a n c e o f t h e 5a(H),14~(H),17~(H)-configurat i o n o v e r t h e 5a(H),14~(H),17~(H)-configurat i o n and o f t h e 2 0 R o v e r 20S in t h e 5 a ( H ) , 1 4 a ( H ) , I 7a (H)-steranes. T h e m a t u r i t y p a r a m eters n o r m a l l y used in s e d i m e n t s : % 20S a n d % ~,~ o f t h e steranes and % 22S o f t h e h o p a n e s (Tables I I I and V) are l o w e r in t h e p y r o l y z a t e t h a n in t h e oil (oil f r o m t h e s a m e area b u t w i t h u n b i o d e g r a d e d steranes a n d h o p a n e s ) . T h e h o p / s t ratio is higher in t h e p y r o l y z a t e t h a n in t h e u n b i o d e g r a d e d oil ( c o m p a r e Tables II a n d V). T h e higher value
TABLE V GC--MS results of pyrolysis performed on Cerro Negro asphaltenes at four different temperatures (300 ° , 330 °, 350 ° and 370°C, for 3 days) (Figs. 5 and 8)* Temperature
Correlation parameters
(°c)
hop/st
% C~7
% C28
% C29
% 20S
% ~,[3
% 22S
300 330 350 370
1.8 1.7 2.7 3.3
35 37 40 46
32 33 32 32
33 30 28 22
27 31 42 47
44 42 48 50
57 55 56 52
Maturity parameters
*For details of measurements see Table III and Experimental.
177
of this ratio has been found by Seifert (1978) in sediment pyrolyzates. At pyrolysis temperatures higher than 300°C, changes in the steranes and terpanes of the pyrolyzates can be observed. The ratio hop/st (Table V) does not differ significantly at 300 ° and 330°C, but at higher temperatures there is a dramatic increase (from 1.7 at 330°C to 3.3 at 370°C), while the proportion of the C29 to the other normal steranes decreases. A preferential thermal degradation of steranes would seem to be the explanation. This effect can be seen more clearly in Fig. 5 where there is a decrease of the regular steranes (from C=~ to C29) compared to the C21 and C22 steranes (peaks a and b) with increasing temperature. A thermal destruction of the hopanes has also been noticed, with an increase in the low-molecular-weight tricyclic terpanes as well as an enhancement of the 17a(H)-trisnorhopane (Tm). This last phenomenon was also observed by Aquino Neto (1981) during heating of a Grenade oil and attributed to a release of this component from the asphaltenes. The maturity parameters, such as % 20S and % /3,~-steranes, increase steadily with pyrolysis temperature. These changes are interpreted in terms of a preferential destruction of the 2 0 R a,a,a components and of the (20S + 20R)a,a,a over the ( 2 0 R + 20S)a,~,~-steranes. Little change occurs in the parameter % 22S of the C33 hopanes. An important question relative to the pyrolysis of asphaltenes is whether or not the compounds released are actually part of the as-
phaltene structure or are only trapped within it. One mechanism for trapping may result from the secondary structure network resulting from hydrogen bonding associations of this polymeric fraction (e.g., Speight, 1984). We have partially resolved this question by methylation of the Cerro Negro asphaltenes. The asphaltenes were methylated with CH3I [using the method applied to coal samples by Liotta (1979) and Liotta et al. (1981)] which limits these hydrogen bonding associations (detected by infrared spectra) by selective alkylation of the acidic hydroxyl groups. This methylation procedure gives a methylated asphaltene which is largely free of this secondary structure and probably exhibits lower molecular weights (see Table VI; MW 5000--6000 falls to ~ 2 2 0 0 in the methylated asphaltene; molecular weight measured by VPO in pyridine). This methylated product was Soxhlet extracted with n-heptane and ~1% of extractable material was obtained. The TLC silica gel separation of this extract did not show any hydrocarbons. This extract was comprised mainly of a resinous material which was probably associated to the asphaltene by hydrogen bonds. Hence, we conclude that precipitation of asphaltenes with n-heptane and purification of this fraction through hot washing with n-heptane (Soxhlet extraction), gives a product free of trapped hydrocarbons. Therefore, the hydrocarbons released in pyrolysis experiments must come almost entirely from cleavage of the asphaltene framework. These results agree with those of Behar et al. (1984)
TABLE VI Variation of molecular weight of Cerro Negro asphaltene after methylation (determined by vapor pressure osmometry at three different concentrations and extrapolated to ~) in pyridine at 60°C Sample
Molecular weights Concentration* :
Cerro Negro asphaltene Cerro Negro asphaltene (methylated)
4%
2%
1%
5,080 2,220
5,340 2,210
5,360 2,270
*Concentration in weight of asphaltene/volume of pyridine.
6,500 2,290
178
w h o checked the purity of the asphaltene (precipitated with n-heptane) by liquid chromatography and thermovaporization at 300°C. In summary, pyrolysis o f Cerro Negro asphaltenes at 300 ° or 330°C, as suggested by Rubinstein et al. (1979), releases molecules whose structures and configurations are little affected by thermal degradation. Higher temperatures adversely affect the distributions of biological markers such as steranes and terpanes. The similarities between the pyrolysis results of Cerro Negro asphaltenes and those performed with sediments are in accord with the suggestion that asphaltenes of crude oils and source-rock bitumens originate from the kerogen by natural thermal degradation during catagenesis. This last characteristic is very important in the case of crude oils where many factors can affect the distribution of the biomarkers during their generation from the source rock and their migration and accumulation in the reservoir. 3.3. Pyrolysis o f asphaltenes o f four Venezuelan oils at 300°C The asphaltene fractions of three other Venezuelan oils of different origin and maturity were pyrolyzed at 300 ° C for 3 days. These conditions, as stated above, provide molecular
information in the form of evolved hydrocarbons which have been only minimally affected b y thermal alteration. Table VII shows the composition of the heptane extracts of the pyrolyzates. The yields of heptane extracts and resins I decrease with the increasing maturity (Table II) of the respective oils. In the case of the hydrocarbon fractions, the alkanes show similar yields in the four samples examined. The GC results (Fig. 6) for the different pyrolyzates are in general very similar, with some minor differences in the Machete pyrolyzate. The GC--MS results (Table VIII) show that the maturity parameters (see Table III), such as % 20S and % ~,~ of the steranes, are characteristic o f an immature sample;however, their values do not vary much among the samples. The parameter % 22S of the hopanes shows, in contrast with the previous ratios, values characteristic of a mature oil sample. The isomerization processes appear to be retarded in the asphaltenes due to a possible protection effect that these polymeric molecules produce over the biological markers attached to them. Most of the characteristics observed in the distribution o f steranes and triterpanes in the case o f the Cerro Negro pyrolyzate at 300 ° C, are also found in the other three samples examined. However, relative concentrations
T A B L E VII C o m p o s i t i o n o f t h e n - h e p t a n e - s o l u b l e f r a c t i o n f r o m p y r o l y s i s ( 3 0 0 ° C, 3 d a y s ) o f a s p h a l t e n e [¥actions o f f o u r oils f r o m d i f f e r e n t areas o f M a r a c a i b o a n d e a s t e r n V e n e z u e l a n basins Sample
Yield, % w e i g h t a s p h a l t e n e n-heptane-soluble fraction
Boscan Machete Cerro N e g r o Orocual
residue
T o t . ext.
Alk.
Aro.
P. aro.
Resins-I
12.8 9.2 5.1 3.3
0.3 0.4 0.3 0.3
1.8 1.6 1.1 1.0
1.0 1.8 0.4 0.6
9.7 5.4 3.3 0.9
87.2 90.8 94.9 97.2
T o t . ext. = t o t a l e x t r a c t ; Alk. = a l k a n e s ( R f = 0 . 8 - - 0 . 9 ) ; Aro. = a r o m a t i c s ( R f > p h e n a n t h r e n e ) ; P. aro. = polya r o m a t i c s ( R f < p h e n a n t h r e n e ) ; R e s i n s - I = resin-like m a t e r i a l o f t h e n - h e p t a n e e x t r a c t ; r e s i d u e = a s p h a l t e n e r e s i d u e after n-heptane extraction.
179 PYROLYZATE
PYROLYZATE
ERRO
8OSCAN
NEGRO
PYROLYZATE MACHETE PYROLY ZATE OROCUAL
l, Retent,on
t,rne
Retentfon
t,rne
Fig. 6. Gas c h r o m a t o g r a m s for t h e a l k a n e f r a c t i o n s of Boscan, M a c h e t e , Cerro Negro a n d Orocual a s p h a l t e n e p y r o l y z a t e s at 3 0 0 ° C ( X = p r i s t a n e ; Y = p h y t a n e a n d Z = C17 n-alkane).
T A B L E VIII G C - - M S p y r o l y s i s results ( 3 0 0 ° C , 3 d a y s ) o f a s p h a l t e n e f r a c t i o n s o f four oils f r o m d i f f e r e n t areas of M a r a c a i b o a n d E a s t e r n V e n e z u e l a n basins (see Figs. 7 a n d 8)* Sample
Boscan Machete Cerro Negro Orocual
Correlation parameters
Maturity parameters
hop/st
% C:7
% C:8
% C29
% 20S
% ~,~
% 22S
3.0 2.7 1.8 1.9
40 44 35 37
27 28 32 31
33 28 33 32
33 26 27 26
46 43 44 46
56 55 54 55
* F o r details of m e a s u r e m e n t s see Table III a n d S e c t i o n 2.
of tricyclic terpanes are higher in the pyrolyzates o f the asphaltenes of the more mature otis, while the concentration of the C27 17a(H)-trisnorhopane (Tm) is less (Fig. 7). Probably the most important finding is that pyrolysis of the asphaltenes can be used to characterize oils -- even when they are extre-
mely biodegraded. The correlation parameter hop/st is higher in the pyrolyzates than in the original unbiodegraded oils; however, there is a direct correlation between the value of this ratio in the different oils and in their asphaltene pyrolyzates (cf. Tables II and VIII). For instance, the hop/st value for the Cerro
180
PI
(a)
m/z
191
BOSCAN N
e
(b)
m/z
217
dl
I
k
R ,t
A S
c '
E F G
Hf J
f(
~1t f il,i
u
v
AqR L ' *
~J~'LJ/'
MACHETE
CERRO
NEGRO
j2x OROCUAL
P
Retention
time
Retention
t~me
Fig. 7. Mass fragmentograms of the alkane fractions derived from pyrolysis (30O°C) of the asphaltene fractions of Boscan, Machete, Cerro Negro and Orocual oils showing: (a) tricyclic terpane and hopane (m/z 191); and (b) sterane (m/z 217 ) distributions (see Table I for sterane and tricyclic terpane and hopane assignments).
181 20
60
30
50
\ 5o/
oK
" "
\3o
\
60
2o
~ 20
36
~ C29
~
~o
Boscan pyrolyzate distribution was different from those of the other three oils, as expected since this oil is situated in a different basin (Maracaibo Basin). The asphaltenes apparently protect biological marker moieties such as steranes and triterpanes, from maturity effects occurring in both the source rock and the reservoir. The yields of alkanes released by pyrolysis of asphaltenes at 300°C are relatively low (Table VII), but the content of steranes and triterpanes in the pyrolyzates is high enough to permit their analysis by GC--MS (Fig. 7) even without the prior removal of the abundant n-alkanes.
I*
4. Conclusions Fig. 8. Triangular diagram of the relative abundances (peak areas in m / z 218 fragmentograms) of 5a(H), 14~(H),17~(H)-steranes (C~7 to C29) for different oil samples (.) and their asphaltene pyrolyzates (A; 300°C, 3 days) (B = Boscan, M = Machete; CN = Cerro Negro; O = Orocual).
Negro and Orocual oils (these two oils are t h o u g h t to have originated from the same source rock but at different levels of maturity) is the same (1.1) and almost the same for their asphaltene pyrolyzates (1.8 and 1.9, respec.). In the other oils, the hop/st value is higher in the pyrolyzates but relates to the value of this ratio in the original oil. The distributions of the C27, C28 and C29 a,~,~-steranes, which are almost constant for the different oils (see Table II and Fig. 8) are now better differentiated in the case o f the asphaltene pyrolyzates (see Table VII and Fig. 8). This distribution is almost the same for the Orocual and Cerro Negro pyrolyzates, as might be expected, since both these oils are probably derived from the same source rock (Querecual Formation). The distribution of the Machete pyrolyzate, however, was very different, indicating that the oil, although located in the same basin as Orocual and Cerro Negro, was derived from a different source rock, probably located at the north of the Machete area (see Fig. 2). Again, the
(1) Methylation of Cerro Negro asphaltene diminishes the e x t e n t of association via hydrogen bonding which presumably maintains part of the secondary structure of thispolymeric material. This intermolecular H-bonded portion of the secondary structure evidently does not contain trapped hydrocarbons. (2) Pyrolysis of Cerro Negro asphaltene at different temperatures from 300 ° to 370°C reveals that heating at 300°C releases good yields of biomarker hydrocarbons. Pyrolysis at higher temperatures releases larger quantities but unfortunately the sterane distributions are progressively altered, thereby diminishing their value in correlation procedures. (3) Pyrolysis of Cerro Negro asphaltene at the preferred temperature of 300°C releases steranes with distributions less mature than those normally found in crude oils. The pyrolyzates show a predominance of a,a,a over a,~,~ isomers and of the 2 0 R over 2 0 S isomers in the a,a,a-steranes. The hopanes released have a distribution similar to that of the parent oil, though lacking the C2s bisnorhopane, the 2 5 , 2 8 , 3 0 - t r i s n o r h o p a n e and the demethylated hopanes. (4) Pyrolysis of asphaltenes (at 300°C) of three additional Venezuelan marine oils, released pyrolyzates similar to that obtained from the Cerro Negro oil. The sterane and
182
triterpane distributions in the pyrolyzates permitted an improved characterization of the parent otis, based on the hop/st ratio and the distributions of the C27 to C29 a,j3,j3steranes. From these data the Machete and Cerro Negro oils appear to have different origins. (5) The biomarker patterns observed in the asphaltene pyrolyzates obtained in the present work compare well with those described in the literature (Seifert, 1978; Moldowan and Seifert, 1984; Noble et al., 1985) for the pyrolyzates of sediments containing kerogens. These results lend support to the view (Tissot and Welte, 1978) that the asphaltenes represent portions of kerogens liberated during the oil generation process. The molecular components of asphaltenes in oils appear to escape the biodegradation modifications experienced by the free hydrocarbon components and hence pyrolysis of asphaltenes represents a valuable method for obtaining molecular information important in oil--oil and oil-source rock correlation.
Acknowledgements The authors thank INTEVEP, S.A. for permission to publish and f o r providing the samples examined. They wish to thank Mrs. A.P. Gowar and Mr. C. Saunders (Bristol University) for technical support with GC--MS. They are also grateful to the analytical service of School of Chemistry (Bristol University) for the elemental analysis and molecular weight determinations. Further they are indebted to the Natural Environment Research Council for their support of the mass spectrometry and computing facilities (GR3/2951 and GR3/3758).
References Aquino Neto, F.R., Trendel, J.M., Restle, A., Connan, J. and Albrecht, P.A., 1983. Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleums. In: M. Bjor~by, P. Albrecht, C. Cornford, K. de Groot, G. Eglinton, E. Galimov, D. Leythaeuser, R. Pelet, J. RullkStter
and G. Speers (Editors), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 659--667. Bandurski, E., 1982. Structural similarities between oil-generating kerogens and petroleum asphaltenes. Energy Sources, 6: 47--66. Barwise, A.J.G. and Park, P.J.D., 1983. Petroporphyrin fingerprinting as a geochemical marker. In: M. Bjor~by, P. Albrecht, C. Cornford, K. de Groot, G. Eglinton, E. Galimov, D. Leythaeuser, R. Pelet, J. Rullk6tter and G. Speers (Editors), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 668--674. Barwise, A.J.G. and Roberts, I., 1984. Diagenetic and catagenetic pathways for porphyrins in sediments. Org. Geochem., 6: 167--176. Behar, F., Pelet, R. and Roucache, J., 1984. Geochemistry of asphaltenes. Org. Geochem., 6: 587-595. Demaison, G.J., 1977. Tar sands and supergiant oil fields. Bull. Am. Assoc. Pet. Geol., 61: 1950-1961. Didyk, B.M., Alturki, Y.I.A., Pillinger, C.T. and Eglinton, G., 1975. Petroporphyrins as indicators of geothermal maturation. Nature (London), 256: 563--575. Ekweozor, C.M. and Strausz, O.P., 1983. Tricyclic terpanes in the Athabasca oil sands: their geochemistry. In: M. BjorCy, P. Albrecht, C. Cornford, K. de Groot, G. Eglinton, E. Galimov, D. Leythaeuser, R. Pelet, J. RullkStter and G. Speers (Editors), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 746--766. Ensminger, A., Van Dorsselaer, A., Spyckerelle, C., Albrecht, P. and Ourisson, G., 1974. Pentacyclic triterpanes of the hopane type as ubiquitous geochemical markers: origin and significance. In: B. Tissot and F. Bienner (Editors), Advances in Organic Geochemistry 1973. Editions Technip, Paris, pp. 245--260. Gallegos, E.J., 1975. Terpane--sterane release from kerogen by pyrolysis gas chromatography--mass spectrometry. Anal. Chem., 47: 1524--1528. Hajibrahim, S.K., 1978. Application of petroporphyrins to the maturation migration and origin of crude oils. Ph.D. Thesis, University of Bristol, Bristol (unpublished). Huang, W. and Meinschein, W.G., 1979. Sterols as ecological indicators. Geochim. Cosmochim. Acta, 43: 739--745. Ignasiak, T., Strausz, O.P. and Montgomery, D.S., 1977. Oxygen distribution and hydrogen bonding in Athabasca asphaltene. Fuel, 56: 359--365. Liotta, R., 1979. Selective alkylation of acidic hydroxyl groups in coal. Fuel, 58: 724--728. Liotta, R., Rose, K. and Hippo, E., 1981. O-alkylation of coal and its implications for the chemical and physical structure of coal. J. Org. Chem., 46: 277--283.
183 Mackenzie, A.S. and Maxwell, J.R., 1981. Assessment of thermal maturation in sedimentary rocks by molecular measurements. In: J. Brooks (Editor), Organic Maturation Studies and Fossil Fuel Exploration. Academic Press, London, pp. 239--254. Mackenzie, A.S., Patience, R.L., Maxwell, J.R., Vandenbroucke, M. and Durand, B., 1980a. Molecular parameters of maturation in the Toarcian shales, Paris Basin, France, 1. Changes in the configurations of acyclic isoprenoids alkanes, steranes and triterpanes. Geochim. Cosmochim. Acta, 44: 1709--1721. Mackenzie, A.S., Quirke, J.M.E. and Maxwell, J.R., 1980b. Molecular parameters of maturation in the Toarcian shales, Paris Basin, France, II. Evolution of metalloporphyrins. In: A.G. Douglas and J.R. Maxwell (Editors), Advances in Organic Geochemistry 1979. Pergamon, Oxford, pp. 239--248. Mackenzie, A.S., Hoffman, C.F. and Maxwell, J.R., 1981. Molecular parameters of maturation in the Toarcian shales, Paris Basin, France, III. Changes in the aromatic steroid hydrocarbons. Geochim. Cosmochim. Acta, 45: 1345--1355. Mackenzie, A.S., Brassell, S.C., Eglinton, G. and Maxwell, J.R., 1982. Chemical fossils: the geological fate of steroids. Science, 217: 491--504. Mackenzie, A.S., Li Renwei, Maxwell, J.R., Moldowan, J.M. and Seifert, W.K., 1983. Molecular measurements of thermal maturation of Cretaceous shales from the overthrust belt, Wyoming, U.S.A. In: M. Bjor~by, P. Albrecht, C. Cornford, K. de Groot, G. Eglinton, E. Galimov, D. Leythaeuser, R. Pelet, J. Rullk6tter and G. Speers (Editors), Advances in Organic Geochemistry 1981. Wiley Chichester, pp. 496--503. Mackenzie, A.S., Maxwell, J.R., Coleman, M.L. and Deegan, C.E., 1984. Biological marker and isotope studies of North Sea crude oils and sediments. Proc. l l t h World Pet. Cong., 2: 45--56. Moldowan, J.M. and Seifert, W.K., 1984. Structure proof and significance of stereoisomeric 28,30bisnorhopane in petroleum and petroleum source rocks. Geochim. Cosmochim. Acta, 48: 1651-1661. Noble, R., Alexander, R. and Kagi, R.I., 1985. The occurrence of bisnorhopane, trisnorhopane and 25-norhopanes as free hydrocarbons in some Australian shales. Org. Geochem., 8: 171--176. Reed, W.E., 1977. Molecular composition of weathered petroleum and comparison with its possible source. Geochim. Cosmochim. Acta, 41: 237-247.
Rubinstein, I. and Strausz, O.P., 1979. Thermal treatment of the Athabasca oil sand bitumen and its component parts. Geochim. Cosmochim. Acta, 43: 1887--1893. Rubinstein, I., Spyckerelle, C. and Strausz, O.P., 1979. Pyrolysis of asphaltenes: a source of geochemical information. Geochim. Cosmochim. Acta, 43: 1--6. RullkStter, J. and Wendisch, D., 1982. Microbial alteration of 17a(H)-hopanes in Madagascar asphalts: removal of C-10 methyl group and ring opening. Geochim. Cosmochim Acta, 46: 15451553. Seifert, W.K., 1978. Steranes and terpanes in kerogen pyrolysis for correlation of oils and source rocks. Geochim. Cosmochim. Acta, 42: 473--484. Seifert, W.K. and Moldowan, J.M., 1978. Application of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochim. Cosmochim. Acta, 42: 77--92. Seifert, W.K. and Moldowan, J.M., 1979. The effect of hiodegradation on steranes and terpanes in crude oils. Geochim. Cosmochim. Acta, 43: 111126. Seifert, W.K. and Moldowan, J.M., 1980. The effect of thermal stress on source rock quality as measured by hopane stereochemistry. In: A.G. Douglas and J.R. Maxwell (Editors), Advances in Organic Geochemistry 1979. Pergamon, Oxford, pp. 229-237. Shi JiYang, Mackenzie, A.S., Alexander, R., Eglinton, G., Gowar, A.P., Wolff, G.A. and Maxwell, J.R., 1982. A biological marker investigation of petroleums and shales from the Shengli oilfield, the People's Republic of China. Chem. Geol., 35: 1-31. Speight, J.G., 1984. The chemical nature of petroleum asphaltenes. In: Characterization of Heavy Crude Oils and Petroleum Residues. Editions Technip, Paris, pp. 32--41. Tissot, E.P. and Welte, D.H., 1978. Petroleum Formation and Occurrence. Springer, Berlin, 538 pp. Volkman, J.K., Alexander, R., Kagi, R.I. and RullkStter, J., 1983. GC--MS characterization of C~ and C2s triterpanes in sediments and petroleum. Geochim. Cosmochim. Acta, 47: 1033--1040. Wardroper, A.M.K., Hoffmann, C.F., Maxwell, J.R., Barwise, A.J.G., Goodwin, N.S. and Park, P.J.D., 1984. Crude oil biodegradation under simulated and natural conditions, II. Aromatic steroid hydrocarbons. Org. Geochem., 6: 605--617.