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Influence of maturity on carbazole and benzocarbazole distributions in crude oils and source rocks from the Sonda de Campeche, Gulf of Mexico
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Pergamon
Org. Geochem. Vol. 29, No. 1 3, pp. 183-194, 1998 © 1998ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S0146-6380(98)00181-8 0146-6380/98/$ - see front matter
Influence of maturity on carbazole and benzocarbazole distributions in crude oils and source rocks from the Sonda de Campeche, Gulf of Mexico H E A T H E R CLEGG *~, HEINZ WILKES ~, THOMAS OLDENBURG ~, DEMETRIO SANTAMARIA-OROZCO''2 and BRIAN HORSFIELD"~ 'Institute for Petroleum and Organic Geochemistry, Research Centre Jtilich, 52425 Julich, Germany and qnstituto Mexicano del Petrdleo, Mexico City, Mexico Abstraet--Carbazole geochemistry has been studied for a petroleum system in which vertical migration is prominent and where reservoirs are locally sourced. An essentially uniform organofacies of organicrich Tithonian source rocks from the Sonda de Campeche (Gulf of Mexico), covering a well defined maturity sequence (0.36-1.29% Rr) and associated crude oils (0.49-0.92% Re) from Palaeocene reservoirs formed the sampling base. Comparison of the distribution of alkylcarbazoles and benzocarbazoles in rock bitumens and oils revealed that fractionation due to primary expulsion had no effect on the distribution of shielded/exposed carbazoles within crude oils. Perhaps more importantly, the benzocarbazole ratio in both rock extracts and crude oils increased with maturity, indicating that this parameter cannot be directly used as a migration indicator in petroleum systems where vertical migration through faults and fissures represents the main avenues of oil migration. © 1998 Elsevier Science Ltd. All rights reserved Key words~arbazole, benzocarbazole distribution, crude oils, Sonda de Campeche
INTRODUCTION
(1997) observed a similar feature for the Hekkingen Formation, Norway and Westphalian coals, Germany (0.4-1.0% Rr) and Li et al. (1997) documented a strong maturation influence for the Duvernay Formation, Canada. The important question as to whether this effect is carried over into crude oils has until now remained unanswered. Indeed, it can be argued that even if carbazole distributions in source rocks are affected by maturity the effect on generated crude oils could still be minor because intense petroleum generation occurs over a rather narrow maturity range (e.g. 0.6-0.8% Rr). This study documents the influence of maturity on carbazoles in crude oils: preliminary results have already been published (Horsfield et al., 1998). Care was taken to select a petroleum system displaying a wide range in source rock maturity coupled with short (1-3 km), essentially vertical, migration pathways via fissures and faults into reservoirs and in which long range lateral migration was ruled out. The Tithonian-Palaeocene petroleum system of the Sonda de Campeche (Holguin, 1987; Gonzfilez and Holguin, 1991) was chosen because of these factors.
Carbazoles and benzocarbazoles are common constituents of rock extracts and crude oils. Until now, research has mainly concentrated on the use of their distribution as secondary migration markers. Larter et al. (1996) proposed the benzocarbazole parameter (benzo[a]carbazole/benzo[a]carbazole + benzo[c]carbazole) as an indicator of migration distance and charging direction. Other investigations have used the distribution of carbazoles to illustrate the effect of primary migration as well as secondary migration, e.g. Li et al. (1995) reported that in crude oils there is an enrichment of alkylcarbazoles relative to alkylbenzocarbazoles, enrichment of so-called nitrogen shielded isomers to nitrogen exposed isomers and finally an enrichment of higher to lower homologues with increasing migration distance. For the carbazoles to function as indicators of migration the effects of source rock maturity must be minor by comparison. Clegg et al. (1997) recently reported that the benzocarbazole a/c ratio was initially low and then rose rapidly in the classical Posidonia Shale maturation series from the Hils Syncline, Germany (0.48-1.45% Rr). Harrison et al.
SAMPLESANDMETHODS *Present address: Robertson Research International Limited, Llanrhos, Llandudno, North Wales LL30 lSA, U.K. #To whom correspondence should be addressed. Tel.: +49-2461-613-670; Fax: +49-2461-612-484; E-mail: [email protected].
Study area
The Sonda de Campeche is part of the offshore portion of the Sureste Basin (Fig. 1) and represents one of the most prolific oil-producing areas of
183
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Fig. 1. Location and stratigraphy of the Sonda de Campeche, Gulf of Mexico (from Santamaria et al., 1998). Wells analysed in this study are listed in Table 1. Mexico. The stratigraphic sequence consists of Jurassic to Recent sediments (Gonz~llez and Holguin, 1991). The tectonic style is extensional, leading to the development of half grabens and horst blocks. The Sonda de Campeche is a text book example of a petroleum system in which vertical migration has taken place, being described as a "supercharged, high impedance petroleum system" by Demaison and Huizinga (1994). The major source rock is Tithonian, consisting of massive black shales and dark grey clayey laminated mudstones deposited in an anoxic marine carbonate shelf (Angeles-Aquino, 1987). While some variability in organofacies is evident from organic petrological studies (Santamaria et al., 1995), this is manifested only to a small extent in bulk chemical terms. Indeed, maturity plays an overiding control, as documented by progressively decreasing hydrogen indices, a classical "oil window" defined by yields of soluble organic matter and decreasing yields of thiophenes in pyrolysis gas chromatograms (Santamaria et al., 1998 and references therein). The strong influence of maturation is also registered on a molecular level, as documented for the alkylbenzo- and alkyldibenzothiophenes (Santamaria et al., 1998). The aforementioned studies revealed that source rock maturity increases from north to south:
in the northern part of the basin, vitrinite reflectance values are <0.45% Rr whereas the central area is mature (>0.5 < 1.0% Rr), coinciding with the highest oil production. The southern area is overmature as reflected by vitrinite reflectance values greater than 1.0% Rr and only light oils, gas and condensates are found. The crude oils generated from these source rocks accumulated throughout the Kimmeridgian to Pliocene mainly in the calcareous breccias of the Lower Paleocene (Santiago and Baro, 1990). API gravities and molecular maturity parameters based on sulphur compounds show the same NE SW zonation as do source rock properties (Santamaria et al., 1998). Samples
Out of a total of 35 Tithonian core samples and 19 produced oils and DSTs from wells available for study, the 9 source rock cores and 8 crude oils reported here are considered to be representative of the regional geochemical framework (Fig. 1; Table 1). The source rocks cover the entire hydrogen index-oxygen index evolution path for Type II kerogens and the vitrinite reflectance range is 0.361.29%. Additionally, analytical pyrolysates show a depletion in alkylthiophenes and a relative enrich-
185
Carbazole and benzocarbazole distributions in crude oils and source rocks
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ment in n-hydrocarbons with increasing maturity; rock extracts are increasingly enriched in saturated hydrocarbons, and molecular maturity parameters (e.g. C3-benzothiophenes, dibenzothiophenes) show excellent correlations with maturity (Santamaria et al., 1995, 1998). Crude oils display sulphur contents in the range 0.2 to 4%, API gravities cover the range 10 to 38 ° and saturated hydrocarbon contents from 950-428 #g/mg oil, while a maturity range of 0.49-0.92% Rc was calculated from molecular maturity parameters (Santamaria et al., 1998). Briefly, maurity of the oils was calculated with reference to the C3-benthiophene index, showing a good correlation to vitrinite reflectance values (R2~0.96, n = 10). Geochemical and biomarker data for source rocks and crude oils are shown in Table 1.
to appropriate standards as reported by Clegg et al. (1997). The same procedure was used to identify and quantify 1,4-dimethylcarbazole, 1,5-dimethylcarbazole, 1,2-dimethylcarbazole, 2,7dimethylcarbazole, 2,4-dimethylcarbazole and 2,5dimethylcarbazole, whose elution order (cf. Li et al., 1992) has recently been revised by Bowler et al. (1997). Table 2 documents the full list of selected characteristic ions and response factors used for analysis of carbazole derivatives. Assignment of 1,3dimethylcarbazole, 1,6-dimethylcarbazole and 1,7dimethylcarbazole is based on published retention time data (Nakazawa et al., 1981; Ignatiadis et al., 1985).
Methods
Low polarity NSO compounds
Rock samples were cleaned and then crushed in a disc mill. Rock powder was extracted with dichloromethane containing 1% methanol using a modified flow blending extraction method (Radke et al., 1978). The low polarity nitrogen, sulphur and oxygen (NSO) compounds fraction containing the carbazole derivatives was isolated according to the polarity-affinity liquid chromatography method of Willsch et al. (1997). G C - M S analyses of the fractions was performed using a Finnigan Mat Magnum instrument. The gas chromatograph was equipped with a temperature programmable injection system (KAS 3, Gerstel). A SGE fused silica capillary column (50m by 0.25ram i.d.) coated with BPX-5 (film thickness 0.22/~m) was used with helium as carrier gas. The oven temperature was programmed from 110 to 340°C at 3°C/min. The mass spectrometer was operated in EI mode at 70 eV. Full scan mass spectra were recorded over the mass range m/z = 100-490 with 4 scans/s. Carbazole, methylcarbazoles, 1,8-dimethylcarbazole, 1-ethylcarbazole, benzo[a]carbazole and benzo[c]carbazole were identified and quantified with reference
The percentage of the mildly polar NSO fraction for both source rocks and oils is presented in Table 3. For both source rock extracts and crude oils, there is an overall decrease with increasing maturity. In addition, source rock extracts contain larger amounts of this fraction than seen for the crude oils at any given maturity within the range 0.36-0.91% Rr. This is presumably due to preferential sorption by kerogen, in the source rock during primary migration (Sandvik et al., 1992).
RESULTSAND DISCUSSION
Concentration o f alkylcarbazoles and benzocarbazoles Alkylcarbazoles and benzocarbazoles are constituents of the mildly polar NSO fraction. The maximum concentration of carbazole, total methylcarbazoles, total C2 carbazoles and the sum of benzo[a]carbazole and benzo[c]carbazole in source rocks are, respectively, 45, 333, 385 and 49/~g/g bitumen at 1.09% Rr. For crude oils the maximum yields are 13 (0.78% Re), 70 (0.78% Re), 81 (0.92% Re) and 10 #g/g oil (0.92% Re), respectively. This represents an approximate four fold depletion in
Table 2. Relativeresponsefactors determinedfor selectedcarbazoleswith referenceto N-phenylcarbazole Compound Carbazole l-Methylcarbazole 3-Methylcarbazole 2-Methylcarbazole 4-Methylcarbazole 1,8-Dimethylcarbazole 1-Ethylcarbazole 1,4-Dimethylcarbazole 1,5-Dimethylcarbazole 1,2-Dimethylcarbazole 2,7-Dimethylcarbazole 2,4-Dimethylcarbazole 2,5-Dimethylcarbazole Benzo[a]carbazole Benzo[c]carbazole
Peak label
MW
Target ion
RRF
C 1 3 2 4 1,8 IE 1,4 1,5 1,2 2,7 2,4 2,5 a c
167 181 181 181 181 195 195 195 195 195 195 195 195 217 217
167 181 181 181 181 195 195 195 195 195 195 195 195 217 217
1.1 0.74 0.70 0.71 0.70 0.81 0.43 0.57 1.01 0.82 0.71 0.83 0.86 0.68 0.72
RRF = relativeresponsefactor. MW = molecularweight.
Carbazole and benzocarbazole distributions in crude oils and source rocks
187
Table 3. Yields (#g/g bitumen or oil) for selected carbazoles for Tithonian source rocks and reservoired crude oils from the Sonda de Campeche. For identification of abbreviated carbazole names see Table 2 Wells
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0.36 0.55 0.57 0.65 0.71 0.81 0.91 1.09 1.29
27.5 33.8 22.7 29.8 33.5 21.7 14.7 12.0 2.7
ll.0 4.5 4.1 2.8 2.8 1.8 29.5 45.5 12.7
1
3
2
4
1,8
13.9 6.3 8.0 9.0 4.2 8.7 4.4 5.7 5.4 4.2 14.2 3.8 5.8 7 5.7 9.4 3.9 4.8 5.3 9 7.2 3.6 4.2 4.4 6.4 5.2 2.9 3.8 3.5 3.3 81.9 18.1 35.5 51.5 33.8 129.5 3 7 . 7 8 0 . 9 8 5 . 4 58.0 13.8 3,8 11.7 9.3 2.6
Oils(R¢insteadofR~r2ndcolumn) N 0.49 16.8 3.3 5.1 M 0.55 11.0 2.4 4.3 O 0.66 14.5 3.5 3.9 R 0.73 7.1 3.8 10.8 P 0.73 8.7 4.3 9.6 T 0.78 6.5 12.7 26.6 S 0.82 4.5 6.5 16.2 V 0.92 2.5 12.3 23.1
2.5 2.6 3.3 5.1 4.7 11.8 7.2 9.2
3,0 2,9 3,7 6,1 5.8 15,2 9.5 12.3
3.7 2.6 3.8 5.0 5.3 16.4 8.9 14.9
3.3 3.1 3.7 8.7 5.5 14.2 11.2 13.1
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1,5
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10.5 8.8 13.7 13.5 11.2 5.6 53.9 82.3 4.9
4.4 3.6 8.4 5.7 3.9 1.4 50.1 80.1 4.0
5.5 2.5 4.2 4.3 3.2 0.73 21.1 43.4 1.9
4.1 2.4 2.2 3.5 3.4 1.8 ll.l 25.6 2.0
4.3 4.2 2.2 2.3 3.3 3.4 2.8 3.0 2.3 2.5 0.9 0.8 25.1 21.9 4 1 . 7 43.5 3.9 2.3
2.5 2.3 2.0 2.6 3.0 5.4 3.0 4.1
4.4 6.6 7.8 13.0 11.2 18.0 18.7 17.3
2.6 1.3 3.3 6.4 6.5 11.4 13.0 13.2
7.8 1.2 2.9 5.8 4.6 8.6 9.2 8.2
2.3 2.3 3.4 6.6 6.3 9.6 11.1 10.4
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2,5
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5,5 2,1 3.3 3.5 2.7 3.0 26.8 33.1 13.8
6.6 2.6 3.3 3.5 3.2 2.6 14.7 16.0 7.4
2.5 3.7 2.3 2.7 2.9 2.6 4.0 6.2
3.0 3.6 1.2 2.2 2.3 2.3 2.8 3.8
LPNSO = proportion of low polar nitrogen, sulphur, oxygen fraction as a percentage of total bitumen. c r u d e oils relative to source r o c k b i t u m e n s for these c o m p o u n d s , respectively. C a r b a z o l e a n d total m e t h y l c a r b a z o l e c o n c e n t r a t i o n [Fig. 2(a) a n d (b)] for M e x i c a n s o u r c e r o c k s b e t w e e n 0 . 3 6 - 0 . 8 1 % Rr display a s t e a d y p r o g r e s s i v e d e c r e a s e to 0.81% Rr, after w h i c h t h e r e is a n a b r u p t s w i t c h to h i g h e r values, the m a x i m u m o c c u r r i n g at 1.09% Rr. F o r c r u d e oils the c o n c e n t r a t i o n s h o w s a s t e a d y increase o v e r the m a t u r i t y r a n g e studied. B e t w e e n the m a t u r i t y r a n g e o f a p p r o x i m a t e l y 0 . 6 0 . 8 2 % Rc t h e c o n c e n t r a t i o n o f c a r b a z o l e , a n d total m e t h y l c a r b a z o l e s ( 0 . 7 0 - 0 . 8 2 % Re) is g r e a t e r in t h e expelled oil t h a n t h e b i t u m e n [Fig. 2(a) a n d (b)]. T h e diverging t r e n d s seen for c a r b a z o l e s a n d m e t h y l c a r b a z o l e s f o r oils a n d source r o c k s m a y ind i c a t e the e x p u l s i o n o f these c o m p o u n d s w i t h i n this b r o a d m a t u r i t y r a n g e , after w h i c h w i t h i n c r e a s i n g m a t u r i t y b e t w e e n 0 . 8 1 - 1 . 0 9 % Rr, s e c o n d a r y cracking o f m o r e resistant k e r o g e n m i g h t p r o d u c e a n e w s o u r c e o f these c o m p o u n d s in s o u r c e rocks. It s h o u l d also be k e p t in m i n d t h a t t h e d e c r e a s i n g c o n c e n t r a t i o n in b i t u m e n a n d i n c r e a s i n g c o n c e n t r a t i o n in c r u d e oils m a y also be influenced by a preferential increase/decrease of other components, e.g. w i t h i n this m a t u r i t y r a n g e f o r s o u r c e r o c k e x t r a c t s the total b i t u m e n yield increases r e a c h i n g a m a x i m u m at 0 . 7 1 % Rr ( S a n t a m a r i a et al., 1998). F o r all source r o c k s a n d oils, irrespective o f m a t u r i t y , 1 - m e t h y l c a r b a z o l e is the m o s t a b u n d a n t m e t h y l c a r b a z o l e a n d 3 - m e t h y l c a r b a z o l e t h e least a b u n d a n t (Table 3). This is in c o n t r a s t to the w o r k o f D o r b o n et al. (1984) o n c r u d e oils a n d Li et al. (1995) for source rocks, w h o r e p o r t e d 4 - m e t h y l c a r b a z o l e as t h e m o s t a b u n d a n t a n d 1 - m e t h y l c a r b a z o l e as t h e least a b u n d a n t m e t h y l c a r b a z o l e . I n f e r e n c e s f r o m these findings are discussed u n d e r t h e h e a d i n g f r a c t i o n a t i o n effects.
F o r t h e C2 c a r b a z o l e s in b o t h source r o c k s a n d c r u d e oils t h e r e is a d o m i n a n c e o f c a r b a z o l e s c o n t a i n i n g i s o m e r s with m e t h y l g r o u p s at t h e 1,8-positions a n d 1-position, e.g. 1 , 8 - d i m e t h y l c a r b a z o l e , 1 , 3 - d i m e t h y l c a r b a z o l e , 1 , 6 - d i m e t h y l c a r b a z o l e , 1,7d i m e t h y l c a r b a z o l e , 1 , 4 - d i m e t h y l c a r b a z o l e a n d 1,5d i m e t h y l c a r b a z o l e . 1 , 4 - D i m e t h y l c a r b a z o l e o c c u r s in Q
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Fig. 2. (a) Concentration of carbazole (#g/g bitumen) for Tithonian source rocks (filled squares) and reservoired crude oils (open circles) as a function of maturity. (b) Concentration of total methylcarbazoles (#g/g bitumen)~ for Tithonian source rocks (filled squares) and reservoired crude oils (open circles) as a function of maturity.
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Fig. 3. Concentration of benzo[a]carbazole (/~g/g bitumen) for Tithonian source rocks (filled squares) and reservoired crude oils (open circles) as a function of maturity. (b) Concentration of benzo[c]carbazole (/~g/g bitumen) for Tithonian source rocks (filled squares) and reservoired crude oils (open circles) as a function of maturity. o
the highest concentration for both source rock extracts and crude oils. For crude oils the concentration of C2 carbazoles increases over the entire maturity range, meanwhile in source rock bitumen the maximum concentration of 82.3/~g/g bitumen occurs at 1.09% Rr, thereafter displaying a drastic decrease to 4.9/~g/g bitumen at 1.29% Rr. The yield of benzo[a]carbazole and benzo[c]carbazole [Fig. 3(a) and (b)] in source rocks varies between 2 and 33/~g/g bitumen and in oils the variation is significantly less being between 2.3 to 6.2/~g/g bitumen. F o r source rocks between 0.360.81% R~, benzo[c]carbazole occurs in higher concentration than benzo[a]carbazole, while at higher maturity > 0.81% Rr, the converse applies. Data on molecular mechanics of these two benzocarbazoles indicate that benzo[a]carbazole is less stable than benzo[c]carbazole (Harrison et al., 1997), thereby suggesting that the prominence of benzo[a]carbazole is brought about from specific precursors within the kerogen. Benzocarbazoles occur in smaller amounts than alkylcarbazoles (Table 3) for both source rocks and crude oils. This corroborates work by Frolov et al. (1989) and contrasts work by Dorbon et al. (1984) and Frakman et al. (1987) who reported that for crude oils and Athabasca asphaltenes respectively, alkylbenzocarbazoles occurred in largely greater amounts over alkylcarbazoles. Similarly, for the clastic Posidonia Shales, higher concentrations of
benzo[a]carbazole and benzo[c]carbazole were found in the maturity range 0.48-0.88% Rr relative to carbazole concentration (Clegg et al., 1997)• The concentration of carbazole, methylcarbazoles, C2 alkylcarbazoles and benzo[a]carbazole and benzo[c]carbazole point to intense generation and for selective retention occurring between 0.9-1.09% Rr. The strong difference between these yield profiles and that for the CI~+ saturated hydrocarbons and aromatics which display a maximum at 0.71% Rr (Santamaria et al., 1998), might suggest that carbazoles are derived from a thermally more stable fraction of kerogen than the saturates and aromatics and that different processes other than random cracking as occurs for petroleum formation. The lateness of intense carbazole generation for these rocks is similar to the generation of dibenzothiophenes (Santamaria et al., 1998) although maximum generation for this compound class occurred at 0.91% Rr, slightly earlier than for the carbazoles (1.09% R0. Composition o f alkylcarbazoles
C0-C5 carbazoles are found in both crude oils and source rocks. The distribution varies for both according with maturity• A summed ion chromatogram ( m / z 167, 181, 195, 209, 223, 237) for one of the reservoired crude oils (0.92% Re) is shown in Fig. 4 to illustrate the isomeric complexity of the Co-C5 carbazoles. All four methylcarbazole isomers are present. Dimethylcarbazoles, in contrast to ethylcarbazoles constitute the major fraction of the C2 carbazoles. C3-C5 carbazoles display a high degree of complexity and individual compounds are not identified. C4 and C5 carbazoles for immature Tithonian samples are present in only low concentrations only. Such small amounts of C4 and C5 carbazoles have been reported before by Dorbon et al. (1984) and Ignatiadis et al. (1985). This contrasts to high abundances noted for carbonate rich Lower Keg River source rocks (0.50-0.60% Rr) from Canada (Clegg et al., 1997a) and crude oils reported by Frolov et al. (1989), suggesting that the original type of the organic matter is important in determining overall carbazole distributions. A l k y l c a r b a z o l e ratios
A detailed comparison of the methylcarbazole ratios for the Posidonia Shale as a function of maturity was given in Clegg et al. (1997). Briefly, a systematic increase was noted for the following ratios: 1-methylcarbazole/1-methylcarbazole + 2methylcarbazole, 1-methylcarbazole/1-methylcarbazole + 3-methylcarbazole, 1-methylcarbazole/1methylcarbazole + 4methylcarbazole, 2-methylcarbazole/2-methylcarbazole + 3-methylcarbazole, 1,8dimethylcarbazole/1,8-dimethylcarbazole + 1-ethylcarbazole in the range 0.48-0.88% Rr. A single sample at 1.45% again displayed lower values,
Carbazole and benzocarbazole distributions in crude oils and source rocks
m/z 167 l
E+05 1.907
m/z 181
E+05 2.628
m/z 195
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E+0S 136
Fig. 4. Isomeric distribution of Co-C5 carbazoles (m/z 167, 181, 195, 209, 223, 237) for a selected Tithonian sourced reservoired crude oil (0.92% Re). giving the overall maturity trend a bell shaped appearance• For the Tithonian source rocks and crude oils the distribution of these ratios is highly variable. The 1,8-dimethylcarbazole/1,8-dimethylcarbazole + 1-ethylcarbazole ratio is shown here, as it displays a clear bell shaped trend with maturity with an outlier at 0.65% Rr [Fig. 5(a)]. For both the Tithonian source rocks and crude oils and the Posidonia Shale there is a systematic increase of this ratio with maturity although absolute values differ. The systematic increase of this ratio could be due to dealkylation of the 1-ethylcarbazole as maturity proceeds, a mechanism equivalent to that proposed for 4-ethyldibenzothiophene (Radke and Willsch, 1994). Because, the dealkylation of 1-ethylcarbazole would lead to the formation of 1-methylcarbazole, a plot of 1-methylcarbazole/1methylcarbazole + 1-ethylcarbazole is shown in Fig. 5(b). If dealkylation occurs, it is reasonable to expect an increase in this ratio• As seen from Fig. 5(b) this occurs for both the Posidonia Shales
and Tithonian reservoired crude oils. However for the Tithonian source rocks this ratio decreases between 0.36 to 0.81% Rr suggesting that for these source rocks dealkylation of 1-ethylcarbazole to 1methylcarbazole is not a dominant process in this maturity range• Fractionation effects The larger amounts of the mildly polar fraction in the source rock extracts, in comparison to crude oils, illustrates that bitumen is enriched in this fraction due to the effects of primary expulsion as discussed for the Posidonia Shale (Leythaeuser et al., 1988). Similarly, Li et al. (1995) suggested that fractionation effects could be discerned on a molecular level by plots of methylcarbazole and C2 carbazoles, the concept being that alkyl substituents adjacent to the N - H functional group would shield it preventing sorption by hydrogen bonding. Thus crude oils should contain an enrichment of shielded isomers, conversely, exposed isomers should be retained in
190
Heather Clegg et al.
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1.3
A
Vitrinite Reflectance(%) Fig. 5. (a) The ratio 1,8-dimethylcarbazole/1,8-dimethylcarbazole + 1-ethylcarbazole vs. maturity for both source rocks and crude oils. Solid triangles denote Posidonia Shale (after Clegg et al., 1997), open circles (Tithonian reservoired crude oils), filled squares (Tithonian source rocks). (b) The ratio 1-methylcarbazole/1-methylcarbazole + 1-ethylcarbazole vs. maturity for both source rocks and crude oils. See (a) for identification of symbols.
the source rock (Li et al., 1995). A ternary diagram showing the distribution of 1-methylcarbazole, 4methylcarbazole and 3 - + 2-methylcarbazole for source rocks (B) and oils (A) from Li et al., 1995 shown in Fig. 6, illustrates this point. The right hand part of Fig. 6 reveals that oils and sources rocks from the Sonda de Campeche have essentially identical compositions in the middle of the ternary diagram. Shown in the middle ternary diagram are data for the clastic Posidonia Shales showing the same phenomena and because interlab comparisons have shown that differences are not due to an artifact of work up procedures (Bowler, personal communication), we conclude that methylcarbazoles have limited use as migration indices. Likewise a plot of the distribution of shielded dimethylcarbazole/exposed dimethylcarbazoles vs. C3 alkylcarbazoles/C2 alkylcarbazoles reveals no enrichment of higher homologues and shielded dimethylcarbazoles in oils relative to the bitumens (Fig. 7). This is in contrast to Y a m a m o t o (1992) who found bitumens were enriched in exposed dimethylquinolines and C2 alkylquinolines relative to oils. Thus, for the crude oils studied here, fractionations occurring as a result of primary migration have no effect on the distribution of carbazoles. The ternary plot in the righthand part of Fig. 6 displaying data for genetically related carbonate source rocks and crude oils, illustrates that maturity has no effect on the distribution of methylcarbazoles. Hence the lack of primary migration effects and maturity in carbonate source rocks and oils suggests that methylcarbazoles show promise as a useful lateral migration marker in other carbonate
Fig. 6. Ternary diagrams (format after Li et al., 1995) showing the relative proportions of 1-methylcarbazole, 4-methylcarbazole and 3- + 2-methylcarbazole in source rocks and crude oils. In the foreground are compositions for Tithonian source rocks and reservoired crude oils from the Sonda de Campeche. In the middle triangle are compositions for the Posidonia Shale (1 = 0.48%, 2 = 0.53%~ 3 = 0.68%, 4 = 0.73%, 5 = 0.88% and 6 = 1.45% Rr). In the background is comparative published data on petroleum (A) and source rocks (B) (Li et al., 1995).
Carbazole and benzocarbazole distributions in crude oils and source rocks
191
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Shielded isomers/exposed isomers in C2 alkylcarbazoles Fig, 7. Plots of the ratio of shielded dimethylcarbazoles (1,3-dimethylcarbazole, 1,6-dimethylcarbazole, 1,7-dimethylcarbazole, 1,4-dimethylcarbazole, 1,5-dimethylcarbazole and 1,2-dimethylcarbazole) to exposed dimethylcarbazoles (2,7-dimethylcarbazole, 2,4-dimethylcarbazole and 2,5-dimethylcarbazole) and the ratios of C3-alkylcarbazoles to Cz-alkylcarbazoles in Tithonian source rocks (filled squares) and reservoired crude oils (circles). source rock/high sulphur crude oil provinces where long distance lateral migration is feasible (Horsfield et al., 1998).
C2 alkylcarbazoles
For the C2 carbazoles there is a dominance of those containing isomers with methyl groups at the 1,8-position (fully shielded) and 1,3-dimethylcarbazole, 1,6-dimethylcarbazole, 1,7-dimethylcarbazole, 1,4-dimethylcarbazole and 1,5-dimethylcarbazole (all partially shielded). Isomers in which alkyl substitution is in any position other than 1 or 8 are termed exposed, e.g. 2,4-dimethylcarbazole. Previous studies concerning the respective solubilities of these C2 carbazoles in hydrocarbon solvents indicates that shielded carbazoles display a greater solubility than exposed carbazoles. No partitioning between crude oils and bitumens was seen here. The bitumens do not show an enrichment of exposed carbazoles and in crude oils there is no enrichment of shielded isomers. Such an effect has been reported by Li et al, (1995) for shales and corresponding crude oils from the Liaohe Basin, China.
Distribution o f benzocarbazoles
The distribution of C0-C3 benzocarbazole isomers for a reservoired crude oil (0.92% Re) is shown in Fig. 8. For both crude oils and bitumen, benzo[b]carbazole is present but in only small amounts (Fig. 8). This is similar to previous work on crude oils and shales (Dorbon et al., 1984; Li et al., 1995), although Harrison et al. (1997) reported that coals contain large amounts of benzo[b]carbazole.
Benzocarbazole ratio
The benzocarbazole ratio has been proposed as a maturity independent secondary migration parameter (Larter et al., 1996). In order for this ratio to be used in all petroleum settings a detailed understanding of the effect of maturity in source rocks is required. Figure 9 shows the distribution of this parameter for both Tithonian source rocks and crude oils as well as for the Posidonia Shale (Clegg et aL, 1997a). A systematic increase with maturity is seen up to 1.09% R~, for both source rocks and crude oils as well as the Posidonia Shale (Fig. 9). Likewise, hydrous pyrolysis experiments conducted on the Ghareb Fro. (Cretateous, Jordan) also showed an increase of the ratio over the temperature range (200-360°C) (in preparation). In contrast to the 1,8-dimethylcarbazole/1-ethylcarbazole ratio discussed earlier, both the Tithonian and Posidonia Shale samples data lie on the same slope, illustrating that the generation of benzocarbazoles is similar in both carbonate and clastic rocks, while the differing profiles for the C2 carbazoles suggests that generation varies in clastic and carbonate source rocks, while in the Posidonia Shales, the overall percentages of carbazoles, benzocarbazoles and dibenzocarbazoles varied with maturity suggesting these compounds have different precursors and behave differently during maturation. Both benzo[a]carbazole and benzo[c]carbazole concentrations in crude oils and source rocks increase with maturity. The similarity of the benzocarbazole parameter in crude oils and their Tithonian source rocks at a given maturity indicates that maturity is the prime controlling factor determining this ratio in source rocks and, most importantly, crude oils in this petroleum province. Thus, at a given maturity of the source rock, oil will be
192
Heather Clegg et al. a
~/z217~
c
E+04
E+04 5.874
m/z2
4
~
m/z259
E+04
I
E+04
Fig. 8. Isomeric distribution of Co C3 benzocarbazoles (m/z 217, 231,245, 259) for a selected Tithonian reservoired crude oil (0.92% Ro). expelled with a benzocarbazole ratio equivalent to the benzocarbazole ratio in the source rock. Hence, in the Sonda de Campeche, a single average benzocarbazole ratio in expelled oils, previously suggested by Larter et al. (1996) cannot be assumed. This is not to say that benzocarbazoles ratios cannot be ruled out as migration indicators. The effect of long distance vertical migration through clay enriched rocks may lead to a geochromatographic fractionation as has been proposed for other nitrogen compounds (Yamamoto et al., 1991; Yamamoto, 1992). A similar effect has been noted for oils which have
0.65
~
0.55
,.~0.45 0.3
A
I
0.8 VitriniteReflectance(%)
L
1.3
Fig. 9. The ratio benzo[a]carbazole/benzo[a]carbazole + benzo[c]carbazole vs. maturity for both source rocks and crude oils. See Fig. 5(a) for identification of symbols.
migrated long distances through the pore systems of mudstones rather than via faults whereby the sorbed nitrogen compounds are removed to a greater extent (Larter et al., 1997). However, in areas of predominantly faults, fissures maturity effects appear to overshadow migration effects
CONCLUSIONS
The Tithonian source rock sequence representing an essentially uniform organo-facies spanning a wide maturity range (0.36-1.29% Rr) and its associated crude oils (0.49-0.92% Re), provide a unique opportunity to study the effects of maturation in both source rocks and crude oils, as well as fractionation effects caused by primary expulsion. Quantitative determination of carbazole derivatives from carbonate Tithonian source rocks reveals that peak generation and/or retention occurs at a relatively late stage of maturity (maximum yield at 1.09% Rr). The depletion of carbazole and methylcarbazoles in source rocks between 0.36-0.81% R r and an increase of these compounds in crude oils (0.49-0.92% R~) indicates that these compounds are expelled from source rocks in this maturity range. However, an examination of the distribution of the so-called shielded, partially shielded and exposed carbazoles reveals that primary expulsion does not effect the individual distribution of these compounds based on shielding effects during pri-
Carbazole and benzocarbazole distributions in crude oils and source rocks mary migration. N o differences were seen in the distribution of these c o m p o u n d s in either source rocks a n d oils. Finally, the systematic increase of the benzocarbazole ratio in b o t h source rocks a n d crude oils with m a t u r i t y indicates this ratio is strongly controlled by m a t u r i t y of the source rock. Thus, in petroleum systems where s h o r t distance vertical oil migration through faults a n d fractures has occurred, this ratio should n o t be used to estimate m i g r a t i o n distance a n d charge directions. In fact the ratio in these oils indicates the m a t u r i t y o f the source rock f r o m which it has been expelled. Acknowledgements--This work was performed under the auspices of the ENOG (European Network of Organic Geochemistry). ENOG comprises Laboratoire de G6ochemie, I.FP., Rueil Malmaison, France; Institute for Petroleum and Organic Geochemistry, Research Center Jfilich (KFA), Germany; Laboratoire de Chimie Organique des Substances Naturelles associe au C.N.R.S., Universit6 Louis Pasteur, Strasbourg, France; Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research, Den Burg, Netherlands; Organic Geochemistry Unit, University of Bristol, U.K.; Department of Environmental Chemistry (CID-CSIC), Barcelona, Spain and Geological Institute, University of Cologne, Cologne, Germany. We are grateful to Steve Larter and Bernie Bowler (NRG, University of Newcastle) for supplying standard carbazole compounds. The technical assistance of A. Ropertz, H. Willsch and U. Disko is gratefully acknowledged. We thank the reviewers, Maowen Li and Barry Bennett, for their insightful comments and suggestions. REFERENCES
Angeles-Aquino, F. L. (1987) Estudio EstratigrfificoSedimentol6gico del Jurfisico Superior en la Sonda de Campeche. Ingenieria Petrolera A I P M 27, 45-55. Bowler, B. F. L., Larter, S. R., Clegg, H., Wilkes, H., Horsfield, B. and Li, M. (1997) Dimethylcarbazoles in crude oils: comment on liquid chromatographic separation schemes for pyrrole and pyridine nitrogen aromatic heterocycle fractions from crude oils suitable for rapid characterisation of geochemical samples. Analytical Chemistry 69, 3128-3129. Clegg, H., Wilkes, H. and Horsfield, B. (1997) Carbazole distributions in carbonate and clastic source rocks. Geochimica et Cosmochimica Acta 61, 5335 5345. Demaison, G. and Huizinga, B. J. (1994) Genetic classification of petroleum systems using three factors: charge, migration and entrapment. In The Petroleum System.from Source to Trap, eds. L. B. Magoon and W. G. Dow, AAPG Memoir 60. Dorbon, M., Garrigues, P., Ignatiadis, I., Edward, M., Arpino, P. and Guiochon, G. (1984) Distribution of carbazole derivatives in petroleum. Organic Geochemistry 7, 111-120. Frakman, Z., Ignasiak, T. M., Montgomery, D. S. and Strausz, O. P. (1987) Nitrogen compounds in Athabasca asphaltene: the carbazoles. AOSTA Journal of Research 3, 131-138. Frolov, Y. B., Smirnov, M. B., Vanyukova, N. A. and Sanin, P. I. (1989) Carbazoles of crude oil. Petroleum Chemistry U.S.S.R. 29, 87 102. Gonz~lez, G. R. and Holguin, Q. N. (1991) Geology of the source rocks of Mexico, Source-Rock Geology. In XIII Worm Petroleum Congress, Topic 2, Forum with Posters. Buenos Aires, Argentina, 1 10.
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Harrison, E., Telnaes, N., Wilhelms, A., Horsfield, B., Van Duin, A., Bennet, B. and Larter, S. R. (1997) Maturity controls on carbazole distributions in coals and source rocks. In Organic Geochemistry, Poster Sessions J~'om the 18th International Meeting on Organic Geochemistry, Maastricht, The Netherlands, 22-26 September, ed. B. Horsfield et al. pp. 235-236. Holguin, Q. N. (1987) Evaluaci6n del Sureste de M6xico. BOL Asoc. Mex. Geol. Petrol. 37, 3 48. Horsfield, B., Clegg, H., Wilkes, H. and SantamariaOrozco, D. (1998) Maturity control of carbazole distributions in petroleum systems. Naturwissenschqften 85, 233-237. Ignatiadis, I., Schmitter, J. M. and Arpino, P. (1985) Separation et identification par chromatographie en phase gazeuse et chromatographie en phase gazeusespectrometrie de masse de composes azotes d'une huile lourde deasphaltee. Evolution de leur distribution apres un hydrotraitement catalytique. Journal of Chromatography 17, 87 111. Larter, S. R., Bowler, B. F. L., Li, M., Chen, M., Brincat, D., Bennet, B., Noke, K., Donohoe, P., Simmons, D., Kohnen, M., Allan, J., Telnaes, N. and Horstad, I. (1996) Molecular indicators of secondary oil migration distances. Nature 383, 593-597. Larter, S. R., Aplin, A. C., Bennett, B., Bowler, B., Noke, K., Harrison, E., Telnaes, N., Wilhelms, A., Eglinton, L. and Whelan, J. (1997) An assessment of vertical migration mechanisms in mudstone rich basins using organic geochemistry. In Organic Geochemistry, Poster Sessions from the 18th International Meeting on Organic Geochemistry, Maastricht, The Netherlands, 22-26 September, ed. B. Horsfield et al. pp. 13-14. Leythaeuser, D., Littke, R., Radke, M. and Schaefer, R. G. (1988) Geochemical effects of petroleum migration and expulsion from Toarcian source rocks in the Hils syncline area, NW-Germany. Advances in Organic Geochemistry 1987, eds. L. Mattavelli and L. Novelli. Pergamon, Oxford. Organic Geochemistry 13, 489 502. Li, M., Larter, S., Stoddart, D. and Bjoroy, M. (1992) Practical liquid chromatographic separation schemes for pyrrolic and pyridinic nitrogen aromatic heterocycle fractions from crude oils suitable for rapid characterisation of geochemical samples. Analytical ChemisOT 64, 1337-1344. Li, M., Larter, S. R., Stoddart, D. and Bjoroy, M. (1995) Fractionation of pyrrolic nitrogen compounds in petroleum during migration: derivation of migration-related geochemical parameters. In The Geochemistry of Reservoirs, eds. J. M. Cubitt and W. A. England, Geol. Soc. Spec. Pub. No. 86. pp. 103-123. Li, M., Yao, H., Stasiuk, L. D., Fowler, M, G. and Larter, S. R. (1997) Effect of maturity and petroleum expulsion on pyrrolic nitrogen compound yields and distributions in Duvernay Formation petroleum source rocks in central Alberta, Canada. Organic Geochemistry 26, 731-744. Nakazawa, T., Kuroki, M. and Tsunashima, Y. (1981) The chemistry of carbazoles IX. Substituent effect in the gas liquid chromatography of methylcarbazoles. Journal of Chromatography 211, 388 391. Radke, M. and Willsch, H. (1994) Extractable alkyldibenzothiophenes in Posidonia Shale (Toarcian) source rocks: Relationship of yields to petroleum formation and expulsion. Geochimica et Cosmochimica Acta 46, 5223 5244. Radke, M., Sittardt, H. G. and Welte, D. H. (1978) Removal of soluble organic matter from rock samples with a flow through extraction cell. Analytical Chemistry 50, 663.
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Sandvik, E. I., Young, W. A. and Curry, D. J. (1992) Expulsion from hydrocarbon sources; the role of organic absorption. Organic Geochemistry 19, 77 87. Santamaria, O. D., di Primio, R., Pickel, W., Holguin, Q. N. and Horsfield, B. (1995) Organic facies and maturity of Tithonian source rocks from the Sonda de Campeche, Mexico. In Organic Geochemistry:
Developments and Applications to Energy, Climate, Environment and Human History, eds. J. O. Grimalt and C. Dorrosoro. Donostia-San Sebastian, pp. 152-154. Santamaria, O. D., di Primio, R., Holguln, Q. N.. and Horsfield, B. (1998) Influence of maturity on distributions of benzo- and dibenzothiophenes in Tithonian source rocks and crude oils, Sonda de Campeche, Mexico. Organic Geochemistry 28, 423-439.
Santiago, A. J. and Baro, A. D. (1990) Mexico's giant fields, 1978-1988 decade: giant oil and gas fields of decade 1978 1988. American Association of Petroleum Geology Memoir 54, 73-99. Willsch, H., Clegg, H., Horsfield, B., Radke, M. and Wilkes, H. (1997) Liquid chromatographic separation of sediment, rock and coal extracts and crude oil into compound classes. Analytical Chemistry 69, 4203-4209. Yamamoto, M., Taguchi, K. and Sasaki, K. (1991) Basic nitrogen compounds in bitumen and crude oils. Chemical Geology 93, 193-206. Yamamoto, M. (1992) Fractionation of azaarenes during oil migration. Organic Geochemistry 19, 389~402.
Pergamon
Org. Geochem. Vol. 29, No. 1 3, pp. 183-194, 1998 © 1998ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S0146-6380(98)00181-8 0146-6380/98/$ - see front matter
Influence of maturity on carbazole and benzocarbazole distributions in crude oils and source rocks from the Sonda de Campeche, Gulf of Mexico H E A T H E R CLEGG *~, HEINZ WILKES ~, THOMAS OLDENBURG ~, DEMETRIO SANTAMARIA-OROZCO''2 and BRIAN HORSFIELD"~ 'Institute for Petroleum and Organic Geochemistry, Research Centre Jtilich, 52425 Julich, Germany and qnstituto Mexicano del Petrdleo, Mexico City, Mexico Abstraet--Carbazole geochemistry has been studied for a petroleum system in which vertical migration is prominent and where reservoirs are locally sourced. An essentially uniform organofacies of organicrich Tithonian source rocks from the Sonda de Campeche (Gulf of Mexico), covering a well defined maturity sequence (0.36-1.29% Rr) and associated crude oils (0.49-0.92% Re) from Palaeocene reservoirs formed the sampling base. Comparison of the distribution of alkylcarbazoles and benzocarbazoles in rock bitumens and oils revealed that fractionation due to primary expulsion had no effect on the distribution of shielded/exposed carbazoles within crude oils. Perhaps more importantly, the benzocarbazole ratio in both rock extracts and crude oils increased with maturity, indicating that this parameter cannot be directly used as a migration indicator in petroleum systems where vertical migration through faults and fissures represents the main avenues of oil migration. © 1998 Elsevier Science Ltd. All rights reserved Key words~arbazole, benzocarbazole distribution, crude oils, Sonda de Campeche
INTRODUCTION
(1997) observed a similar feature for the Hekkingen Formation, Norway and Westphalian coals, Germany (0.4-1.0% Rr) and Li et al. (1997) documented a strong maturation influence for the Duvernay Formation, Canada. The important question as to whether this effect is carried over into crude oils has until now remained unanswered. Indeed, it can be argued that even if carbazole distributions in source rocks are affected by maturity the effect on generated crude oils could still be minor because intense petroleum generation occurs over a rather narrow maturity range (e.g. 0.6-0.8% Rr). This study documents the influence of maturity on carbazoles in crude oils: preliminary results have already been published (Horsfield et al., 1998). Care was taken to select a petroleum system displaying a wide range in source rock maturity coupled with short (1-3 km), essentially vertical, migration pathways via fissures and faults into reservoirs and in which long range lateral migration was ruled out. The Tithonian-Palaeocene petroleum system of the Sonda de Campeche (Holguin, 1987; Gonzfilez and Holguin, 1991) was chosen because of these factors.
Carbazoles and benzocarbazoles are common constituents of rock extracts and crude oils. Until now, research has mainly concentrated on the use of their distribution as secondary migration markers. Larter et al. (1996) proposed the benzocarbazole parameter (benzo[a]carbazole/benzo[a]carbazole + benzo[c]carbazole) as an indicator of migration distance and charging direction. Other investigations have used the distribution of carbazoles to illustrate the effect of primary migration as well as secondary migration, e.g. Li et al. (1995) reported that in crude oils there is an enrichment of alkylcarbazoles relative to alkylbenzocarbazoles, enrichment of so-called nitrogen shielded isomers to nitrogen exposed isomers and finally an enrichment of higher to lower homologues with increasing migration distance. For the carbazoles to function as indicators of migration the effects of source rock maturity must be minor by comparison. Clegg et al. (1997) recently reported that the benzocarbazole a/c ratio was initially low and then rose rapidly in the classical Posidonia Shale maturation series from the Hils Syncline, Germany (0.48-1.45% Rr). Harrison et al.
SAMPLESANDMETHODS *Present address: Robertson Research International Limited, Llanrhos, Llandudno, North Wales LL30 lSA, U.K. #To whom correspondence should be addressed. Tel.: +49-2461-613-670; Fax: +49-2461-612-484; E-mail: [email protected].
Study area
The Sonda de Campeche is part of the offshore portion of the Sureste Basin (Fig. 1) and represents one of the most prolific oil-producing areas of
183
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i.,#,lm r ' , - L , n ( , - ~ . ¢
Fig. 1. Location and stratigraphy of the Sonda de Campeche, Gulf of Mexico (from Santamaria et al., 1998). Wells analysed in this study are listed in Table 1. Mexico. The stratigraphic sequence consists of Jurassic to Recent sediments (Gonz~llez and Holguin, 1991). The tectonic style is extensional, leading to the development of half grabens and horst blocks. The Sonda de Campeche is a text book example of a petroleum system in which vertical migration has taken place, being described as a "supercharged, high impedance petroleum system" by Demaison and Huizinga (1994). The major source rock is Tithonian, consisting of massive black shales and dark grey clayey laminated mudstones deposited in an anoxic marine carbonate shelf (Angeles-Aquino, 1987). While some variability in organofacies is evident from organic petrological studies (Santamaria et al., 1995), this is manifested only to a small extent in bulk chemical terms. Indeed, maturity plays an overiding control, as documented by progressively decreasing hydrogen indices, a classical "oil window" defined by yields of soluble organic matter and decreasing yields of thiophenes in pyrolysis gas chromatograms (Santamaria et al., 1998 and references therein). The strong influence of maturation is also registered on a molecular level, as documented for the alkylbenzo- and alkyldibenzothiophenes (Santamaria et al., 1998). The aforementioned studies revealed that source rock maturity increases from north to south:
in the northern part of the basin, vitrinite reflectance values are <0.45% Rr whereas the central area is mature (>0.5 < 1.0% Rr), coinciding with the highest oil production. The southern area is overmature as reflected by vitrinite reflectance values greater than 1.0% Rr and only light oils, gas and condensates are found. The crude oils generated from these source rocks accumulated throughout the Kimmeridgian to Pliocene mainly in the calcareous breccias of the Lower Paleocene (Santiago and Baro, 1990). API gravities and molecular maturity parameters based on sulphur compounds show the same NE SW zonation as do source rock properties (Santamaria et al., 1998). Samples
Out of a total of 35 Tithonian core samples and 19 produced oils and DSTs from wells available for study, the 9 source rock cores and 8 crude oils reported here are considered to be representative of the regional geochemical framework (Fig. 1; Table 1). The source rocks cover the entire hydrogen index-oxygen index evolution path for Type II kerogens and the vitrinite reflectance range is 0.361.29%. Additionally, analytical pyrolysates show a depletion in alkylthiophenes and a relative enrich-
185
Carbazole and benzocarbazole distributions in crude oils and source rocks
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ment in n-hydrocarbons with increasing maturity; rock extracts are increasingly enriched in saturated hydrocarbons, and molecular maturity parameters (e.g. C3-benzothiophenes, dibenzothiophenes) show excellent correlations with maturity (Santamaria et al., 1995, 1998). Crude oils display sulphur contents in the range 0.2 to 4%, API gravities cover the range 10 to 38 ° and saturated hydrocarbon contents from 950-428 #g/mg oil, while a maturity range of 0.49-0.92% Rc was calculated from molecular maturity parameters (Santamaria et al., 1998). Briefly, maurity of the oils was calculated with reference to the C3-benthiophene index, showing a good correlation to vitrinite reflectance values (R2~0.96, n = 10). Geochemical and biomarker data for source rocks and crude oils are shown in Table 1.
to appropriate standards as reported by Clegg et al. (1997). The same procedure was used to identify and quantify 1,4-dimethylcarbazole, 1,5-dimethylcarbazole, 1,2-dimethylcarbazole, 2,7dimethylcarbazole, 2,4-dimethylcarbazole and 2,5dimethylcarbazole, whose elution order (cf. Li et al., 1992) has recently been revised by Bowler et al. (1997). Table 2 documents the full list of selected characteristic ions and response factors used for analysis of carbazole derivatives. Assignment of 1,3dimethylcarbazole, 1,6-dimethylcarbazole and 1,7dimethylcarbazole is based on published retention time data (Nakazawa et al., 1981; Ignatiadis et al., 1985).
Methods
Low polarity NSO compounds
Rock samples were cleaned and then crushed in a disc mill. Rock powder was extracted with dichloromethane containing 1% methanol using a modified flow blending extraction method (Radke et al., 1978). The low polarity nitrogen, sulphur and oxygen (NSO) compounds fraction containing the carbazole derivatives was isolated according to the polarity-affinity liquid chromatography method of Willsch et al. (1997). G C - M S analyses of the fractions was performed using a Finnigan Mat Magnum instrument. The gas chromatograph was equipped with a temperature programmable injection system (KAS 3, Gerstel). A SGE fused silica capillary column (50m by 0.25ram i.d.) coated with BPX-5 (film thickness 0.22/~m) was used with helium as carrier gas. The oven temperature was programmed from 110 to 340°C at 3°C/min. The mass spectrometer was operated in EI mode at 70 eV. Full scan mass spectra were recorded over the mass range m/z = 100-490 with 4 scans/s. Carbazole, methylcarbazoles, 1,8-dimethylcarbazole, 1-ethylcarbazole, benzo[a]carbazole and benzo[c]carbazole were identified and quantified with reference
The percentage of the mildly polar NSO fraction for both source rocks and oils is presented in Table 3. For both source rock extracts and crude oils, there is an overall decrease with increasing maturity. In addition, source rock extracts contain larger amounts of this fraction than seen for the crude oils at any given maturity within the range 0.36-0.91% Rr. This is presumably due to preferential sorption by kerogen, in the source rock during primary migration (Sandvik et al., 1992).
RESULTSAND DISCUSSION
Concentration o f alkylcarbazoles and benzocarbazoles Alkylcarbazoles and benzocarbazoles are constituents of the mildly polar NSO fraction. The maximum concentration of carbazole, total methylcarbazoles, total C2 carbazoles and the sum of benzo[a]carbazole and benzo[c]carbazole in source rocks are, respectively, 45, 333, 385 and 49/~g/g bitumen at 1.09% Rr. For crude oils the maximum yields are 13 (0.78% Re), 70 (0.78% Re), 81 (0.92% Re) and 10 #g/g oil (0.92% Re), respectively. This represents an approximate four fold depletion in
Table 2. Relativeresponsefactors determinedfor selectedcarbazoleswith referenceto N-phenylcarbazole Compound Carbazole l-Methylcarbazole 3-Methylcarbazole 2-Methylcarbazole 4-Methylcarbazole 1,8-Dimethylcarbazole 1-Ethylcarbazole 1,4-Dimethylcarbazole 1,5-Dimethylcarbazole 1,2-Dimethylcarbazole 2,7-Dimethylcarbazole 2,4-Dimethylcarbazole 2,5-Dimethylcarbazole Benzo[a]carbazole Benzo[c]carbazole
Peak label
MW
Target ion
RRF
C 1 3 2 4 1,8 IE 1,4 1,5 1,2 2,7 2,4 2,5 a c
167 181 181 181 181 195 195 195 195 195 195 195 195 217 217
167 181 181 181 181 195 195 195 195 195 195 195 195 217 217
1.1 0.74 0.70 0.71 0.70 0.81 0.43 0.57 1.01 0.82 0.71 0.83 0.86 0.68 0.72
RRF = relativeresponsefactor. MW = molecularweight.
Carbazole and benzocarbazole distributions in crude oils and source rocks
187
Table 3. Yields (#g/g bitumen or oil) for selected carbazoles for Tithonian source rocks and reservoired crude oils from the Sonda de Campeche. For identification of abbreviated carbazole names see Table 2 Wells
R~
LPNSO
C
Rocks B D E F G H I J K
0.36 0.55 0.57 0.65 0.71 0.81 0.91 1.09 1.29
27.5 33.8 22.7 29.8 33.5 21.7 14.7 12.0 2.7
ll.0 4.5 4.1 2.8 2.8 1.8 29.5 45.5 12.7
1
3
2
4
1,8
13.9 6.3 8.0 9.0 4.2 8.7 4.4 5.7 5.4 4.2 14.2 3.8 5.8 7 5.7 9.4 3.9 4.8 5.3 9 7.2 3.6 4.2 4.4 6.4 5.2 2.9 3.8 3.5 3.3 81.9 18.1 35.5 51.5 33.8 129.5 3 7 . 7 8 0 . 9 8 5 . 4 58.0 13.8 3,8 11.7 9.3 2.6
Oils(R¢insteadofR~r2ndcolumn) N 0.49 16.8 3.3 5.1 M 0.55 11.0 2.4 4.3 O 0.66 14.5 3.5 3.9 R 0.73 7.1 3.8 10.8 P 0.73 8.7 4.3 9.6 T 0.78 6.5 12.7 26.6 S 0.82 4.5 6.5 16.2 V 0.92 2.5 12.3 23.1
2.5 2.6 3.3 5.1 4.7 11.8 7.2 9.2
3,0 2,9 3,7 6,1 5.8 15,2 9.5 12.3
3.7 2.6 3.8 5.0 5.3 16.4 8.9 14.9
3.3 3.1 3.7 8.7 5.5 14.2 11.2 13.1
lE
1,4
1,5
1,2
2,7
2,4
3.5 2.7 6.6 3.6 3.0 2.6 7.6 11.1 1.7
10.5 8.8 13.7 13.5 11.2 5.6 53.9 82.3 4.9
4.4 3.6 8.4 5.7 3.9 1.4 50.1 80.1 4.0
5.5 2.5 4.2 4.3 3.2 0.73 21.1 43.4 1.9
4.1 2.4 2.2 3.5 3.4 1.8 ll.l 25.6 2.0
4.3 4.2 2.2 2.3 3.3 3.4 2.8 3.0 2.3 2.5 0.9 0.8 25.1 21.9 4 1 . 7 43.5 3.9 2.3
2.5 2.3 2.0 2.6 3.0 5.4 3.0 4.1
4.4 6.6 7.8 13.0 11.2 18.0 18.7 17.3
2.6 1.3 3.3 6.4 6.5 11.4 13.0 13.2
7.8 1.2 2.9 5.8 4.6 8.6 9.2 8.2
2.3 2.3 3.4 6.6 6.3 9.6 11.1 10.4
2.0 1.0 2.0 3.8 3.6 6.4 7.0 7.8
2,5
1.3 0.8 1.8 3.3 3.5 6.0 6.5 6.8
a
c
5,5 2,1 3.3 3.5 2.7 3.0 26.8 33.1 13.8
6.6 2.6 3.3 3.5 3.2 2.6 14.7 16.0 7.4
2.5 3.7 2.3 2.7 2.9 2.6 4.0 6.2
3.0 3.6 1.2 2.2 2.3 2.3 2.8 3.8
LPNSO = proportion of low polar nitrogen, sulphur, oxygen fraction as a percentage of total bitumen. c r u d e oils relative to source r o c k b i t u m e n s for these c o m p o u n d s , respectively. C a r b a z o l e a n d total m e t h y l c a r b a z o l e c o n c e n t r a t i o n [Fig. 2(a) a n d (b)] for M e x i c a n s o u r c e r o c k s b e t w e e n 0 . 3 6 - 0 . 8 1 % Rr display a s t e a d y p r o g r e s s i v e d e c r e a s e to 0.81% Rr, after w h i c h t h e r e is a n a b r u p t s w i t c h to h i g h e r values, the m a x i m u m o c c u r r i n g at 1.09% Rr. F o r c r u d e oils the c o n c e n t r a t i o n s h o w s a s t e a d y increase o v e r the m a t u r i t y r a n g e studied. B e t w e e n the m a t u r i t y r a n g e o f a p p r o x i m a t e l y 0 . 6 0 . 8 2 % Rc t h e c o n c e n t r a t i o n o f c a r b a z o l e , a n d total m e t h y l c a r b a z o l e s ( 0 . 7 0 - 0 . 8 2 % Re) is g r e a t e r in t h e expelled oil t h a n t h e b i t u m e n [Fig. 2(a) a n d (b)]. T h e diverging t r e n d s seen for c a r b a z o l e s a n d m e t h y l c a r b a z o l e s f o r oils a n d source r o c k s m a y ind i c a t e the e x p u l s i o n o f these c o m p o u n d s w i t h i n this b r o a d m a t u r i t y r a n g e , after w h i c h w i t h i n c r e a s i n g m a t u r i t y b e t w e e n 0 . 8 1 - 1 . 0 9 % Rr, s e c o n d a r y cracking o f m o r e resistant k e r o g e n m i g h t p r o d u c e a n e w s o u r c e o f these c o m p o u n d s in s o u r c e rocks. It s h o u l d also be k e p t in m i n d t h a t t h e d e c r e a s i n g c o n c e n t r a t i o n in b i t u m e n a n d i n c r e a s i n g c o n c e n t r a t i o n in c r u d e oils m a y also be influenced by a preferential increase/decrease of other components, e.g. w i t h i n this m a t u r i t y r a n g e f o r s o u r c e r o c k e x t r a c t s the total b i t u m e n yield increases r e a c h i n g a m a x i m u m at 0 . 7 1 % Rr ( S a n t a m a r i a et al., 1998). F o r all source r o c k s a n d oils, irrespective o f m a t u r i t y , 1 - m e t h y l c a r b a z o l e is the m o s t a b u n d a n t m e t h y l c a r b a z o l e a n d 3 - m e t h y l c a r b a z o l e t h e least a b u n d a n t (Table 3). This is in c o n t r a s t to the w o r k o f D o r b o n et al. (1984) o n c r u d e oils a n d Li et al. (1995) for source rocks, w h o r e p o r t e d 4 - m e t h y l c a r b a z o l e as t h e m o s t a b u n d a n t a n d 1 - m e t h y l c a r b a z o l e as t h e least a b u n d a n t m e t h y l c a r b a z o l e . I n f e r e n c e s f r o m these findings are discussed u n d e r t h e h e a d i n g f r a c t i o n a t i o n effects.
F o r t h e C2 c a r b a z o l e s in b o t h source r o c k s a n d c r u d e oils t h e r e is a d o m i n a n c e o f c a r b a z o l e s c o n t a i n i n g i s o m e r s with m e t h y l g r o u p s at t h e 1,8-positions a n d 1-position, e.g. 1 , 8 - d i m e t h y l c a r b a z o l e , 1 , 3 - d i m e t h y l c a r b a z o l e , 1 , 6 - d i m e t h y l c a r b a z o l e , 1,7d i m e t h y l c a r b a z o l e , 1 , 4 - d i m e t h y l c a r b a z o l e a n d 1,5d i m e t h y l c a r b a z o l e . 1 , 4 - D i m e t h y l c a r b a z o l e o c c u r s in Q
100 ~'~
.• ~ ,~ t~
l0
0
•
0
1 0.3
.~ ,,~
0
0.8
1.3
1000
b
,,O
_~ ~
100
• ~" ~
10
0.3
0 0 0
oot,,
•
0.8 Vitrinite Reflectance (%)
I 1.3
Fig. 2. (a) Concentration of carbazole (#g/g bitumen) for Tithonian source rocks (filled squares) and reservoired crude oils (open circles) as a function of maturity. (b) Concentration of total methylcarbazoles (#g/g bitumen)~ for Tithonian source rocks (filled squares) and reservoired crude oils (open circles) as a function of maturity.
188
Heather Clegg et al.
~
100
•
•
lO 0 7~
~
I
0.8
1.3
lOO
lO
e~
I
0.3
,,D
•
-•
• •
I I 0.8 1.3 Vitrinite Reflectance (%)
0.3
Fig. 3. Concentration of benzo[a]carbazole (/~g/g bitumen) for Tithonian source rocks (filled squares) and reservoired crude oils (open circles) as a function of maturity. (b) Concentration of benzo[c]carbazole (/~g/g bitumen) for Tithonian source rocks (filled squares) and reservoired crude oils (open circles) as a function of maturity. o
the highest concentration for both source rock extracts and crude oils. For crude oils the concentration of C2 carbazoles increases over the entire maturity range, meanwhile in source rock bitumen the maximum concentration of 82.3/~g/g bitumen occurs at 1.09% Rr, thereafter displaying a drastic decrease to 4.9/~g/g bitumen at 1.29% Rr. The yield of benzo[a]carbazole and benzo[c]carbazole [Fig. 3(a) and (b)] in source rocks varies between 2 and 33/~g/g bitumen and in oils the variation is significantly less being between 2.3 to 6.2/~g/g bitumen. F o r source rocks between 0.360.81% R~, benzo[c]carbazole occurs in higher concentration than benzo[a]carbazole, while at higher maturity > 0.81% Rr, the converse applies. Data on molecular mechanics of these two benzocarbazoles indicate that benzo[a]carbazole is less stable than benzo[c]carbazole (Harrison et al., 1997), thereby suggesting that the prominence of benzo[a]carbazole is brought about from specific precursors within the kerogen. Benzocarbazoles occur in smaller amounts than alkylcarbazoles (Table 3) for both source rocks and crude oils. This corroborates work by Frolov et al. (1989) and contrasts work by Dorbon et al. (1984) and Frakman et al. (1987) who reported that for crude oils and Athabasca asphaltenes respectively, alkylbenzocarbazoles occurred in largely greater amounts over alkylcarbazoles. Similarly, for the clastic Posidonia Shales, higher concentrations of
benzo[a]carbazole and benzo[c]carbazole were found in the maturity range 0.48-0.88% Rr relative to carbazole concentration (Clegg et al., 1997)• The concentration of carbazole, methylcarbazoles, C2 alkylcarbazoles and benzo[a]carbazole and benzo[c]carbazole point to intense generation and for selective retention occurring between 0.9-1.09% Rr. The strong difference between these yield profiles and that for the CI~+ saturated hydrocarbons and aromatics which display a maximum at 0.71% Rr (Santamaria et al., 1998), might suggest that carbazoles are derived from a thermally more stable fraction of kerogen than the saturates and aromatics and that different processes other than random cracking as occurs for petroleum formation. The lateness of intense carbazole generation for these rocks is similar to the generation of dibenzothiophenes (Santamaria et al., 1998) although maximum generation for this compound class occurred at 0.91% Rr, slightly earlier than for the carbazoles (1.09% R0. Composition o f alkylcarbazoles
C0-C5 carbazoles are found in both crude oils and source rocks. The distribution varies for both according with maturity• A summed ion chromatogram ( m / z 167, 181, 195, 209, 223, 237) for one of the reservoired crude oils (0.92% Re) is shown in Fig. 4 to illustrate the isomeric complexity of the Co-C5 carbazoles. All four methylcarbazole isomers are present. Dimethylcarbazoles, in contrast to ethylcarbazoles constitute the major fraction of the C2 carbazoles. C3-C5 carbazoles display a high degree of complexity and individual compounds are not identified. C4 and C5 carbazoles for immature Tithonian samples are present in only low concentrations only. Such small amounts of C4 and C5 carbazoles have been reported before by Dorbon et al. (1984) and Ignatiadis et al. (1985). This contrasts to high abundances noted for carbonate rich Lower Keg River source rocks (0.50-0.60% Rr) from Canada (Clegg et al., 1997a) and crude oils reported by Frolov et al. (1989), suggesting that the original type of the organic matter is important in determining overall carbazole distributions. A l k y l c a r b a z o l e ratios
A detailed comparison of the methylcarbazole ratios for the Posidonia Shale as a function of maturity was given in Clegg et al. (1997). Briefly, a systematic increase was noted for the following ratios: 1-methylcarbazole/1-methylcarbazole + 2methylcarbazole, 1-methylcarbazole/1-methylcarbazole + 3-methylcarbazole, 1-methylcarbazole/1methylcarbazole + 4methylcarbazole, 2-methylcarbazole/2-methylcarbazole + 3-methylcarbazole, 1,8dimethylcarbazole/1,8-dimethylcarbazole + 1-ethylcarbazole in the range 0.48-0.88% Rr. A single sample at 1.45% again displayed lower values,
Carbazole and benzocarbazole distributions in crude oils and source rocks
m/z 167 l
E+05 1.907
m/z 181
E+05 2.628
m/z 195
,-~" ,v,u
e~
..~
209
t~
¢q~e~~
"
189
E+05 2.097
t~
¢~
1
E+0S
m/z2 2 3 ~ ~ . ~ [
m/z 237
_.=
E+05 1.298
E+0S 136
Fig. 4. Isomeric distribution of Co-C5 carbazoles (m/z 167, 181, 195, 209, 223, 237) for a selected Tithonian sourced reservoired crude oil (0.92% Re). giving the overall maturity trend a bell shaped appearance• For the Tithonian source rocks and crude oils the distribution of these ratios is highly variable. The 1,8-dimethylcarbazole/1,8-dimethylcarbazole + 1-ethylcarbazole ratio is shown here, as it displays a clear bell shaped trend with maturity with an outlier at 0.65% Rr [Fig. 5(a)]. For both the Tithonian source rocks and crude oils and the Posidonia Shale there is a systematic increase of this ratio with maturity although absolute values differ. The systematic increase of this ratio could be due to dealkylation of the 1-ethylcarbazole as maturity proceeds, a mechanism equivalent to that proposed for 4-ethyldibenzothiophene (Radke and Willsch, 1994). Because, the dealkylation of 1-ethylcarbazole would lead to the formation of 1-methylcarbazole, a plot of 1-methylcarbazole/1methylcarbazole + 1-ethylcarbazole is shown in Fig. 5(b). If dealkylation occurs, it is reasonable to expect an increase in this ratio• As seen from Fig. 5(b) this occurs for both the Posidonia Shales
and Tithonian reservoired crude oils. However for the Tithonian source rocks this ratio decreases between 0.36 to 0.81% Rr suggesting that for these source rocks dealkylation of 1-ethylcarbazole to 1methylcarbazole is not a dominant process in this maturity range• Fractionation effects The larger amounts of the mildly polar fraction in the source rock extracts, in comparison to crude oils, illustrates that bitumen is enriched in this fraction due to the effects of primary expulsion as discussed for the Posidonia Shale (Leythaeuser et al., 1988). Similarly, Li et al. (1995) suggested that fractionation effects could be discerned on a molecular level by plots of methylcarbazole and C2 carbazoles, the concept being that alkyl substituents adjacent to the N - H functional group would shield it preventing sorption by hydrogen bonding. Thus crude oils should contain an enrichment of shielded isomers, conversely, exposed isomers should be retained in
190
Heather Clegg et al.
_•
•
0.8 •
i 0.6
0
AA
A
p
~ 0.4
A
0.2
~ 0.95 "~ 0.85
io
"~ 0.65 ~ 0.55 0.45 0.3
I
I
0.8
1.3
A
Vitrinite Reflectance(%) Fig. 5. (a) The ratio 1,8-dimethylcarbazole/1,8-dimethylcarbazole + 1-ethylcarbazole vs. maturity for both source rocks and crude oils. Solid triangles denote Posidonia Shale (after Clegg et al., 1997), open circles (Tithonian reservoired crude oils), filled squares (Tithonian source rocks). (b) The ratio 1-methylcarbazole/1-methylcarbazole + 1-ethylcarbazole vs. maturity for both source rocks and crude oils. See (a) for identification of symbols.
the source rock (Li et al., 1995). A ternary diagram showing the distribution of 1-methylcarbazole, 4methylcarbazole and 3 - + 2-methylcarbazole for source rocks (B) and oils (A) from Li et al., 1995 shown in Fig. 6, illustrates this point. The right hand part of Fig. 6 reveals that oils and sources rocks from the Sonda de Campeche have essentially identical compositions in the middle of the ternary diagram. Shown in the middle ternary diagram are data for the clastic Posidonia Shales showing the same phenomena and because interlab comparisons have shown that differences are not due to an artifact of work up procedures (Bowler, personal communication), we conclude that methylcarbazoles have limited use as migration indices. Likewise a plot of the distribution of shielded dimethylcarbazole/exposed dimethylcarbazoles vs. C3 alkylcarbazoles/C2 alkylcarbazoles reveals no enrichment of higher homologues and shielded dimethylcarbazoles in oils relative to the bitumens (Fig. 7). This is in contrast to Y a m a m o t o (1992) who found bitumens were enriched in exposed dimethylquinolines and C2 alkylquinolines relative to oils. Thus, for the crude oils studied here, fractionations occurring as a result of primary migration have no effect on the distribution of carbazoles. The ternary plot in the righthand part of Fig. 6 displaying data for genetically related carbonate source rocks and crude oils, illustrates that maturity has no effect on the distribution of methylcarbazoles. Hence the lack of primary migration effects and maturity in carbonate source rocks and oils suggests that methylcarbazoles show promise as a useful lateral migration marker in other carbonate
Fig. 6. Ternary diagrams (format after Li et al., 1995) showing the relative proportions of 1-methylcarbazole, 4-methylcarbazole and 3- + 2-methylcarbazole in source rocks and crude oils. In the foreground are compositions for Tithonian source rocks and reservoired crude oils from the Sonda de Campeche. In the middle triangle are compositions for the Posidonia Shale (1 = 0.48%, 2 = 0.53%~ 3 = 0.68%, 4 = 0.73%, 5 = 0.88% and 6 = 1.45% Rr). In the background is comparative published data on petroleum (A) and source rocks (B) (Li et al., 1995).
Carbazole and benzocarbazole distributions in crude oils and source rocks
191
3.0
mm
2.5 ¢J
.~
2.0
..~
1.5 1.o
,.~ 0.5
¢3
o 0
I
I
2
4
6
Shielded isomers/exposed isomers in C2 alkylcarbazoles Fig, 7. Plots of the ratio of shielded dimethylcarbazoles (1,3-dimethylcarbazole, 1,6-dimethylcarbazole, 1,7-dimethylcarbazole, 1,4-dimethylcarbazole, 1,5-dimethylcarbazole and 1,2-dimethylcarbazole) to exposed dimethylcarbazoles (2,7-dimethylcarbazole, 2,4-dimethylcarbazole and 2,5-dimethylcarbazole) and the ratios of C3-alkylcarbazoles to Cz-alkylcarbazoles in Tithonian source rocks (filled squares) and reservoired crude oils (circles). source rock/high sulphur crude oil provinces where long distance lateral migration is feasible (Horsfield et al., 1998).
C2 alkylcarbazoles
For the C2 carbazoles there is a dominance of those containing isomers with methyl groups at the 1,8-position (fully shielded) and 1,3-dimethylcarbazole, 1,6-dimethylcarbazole, 1,7-dimethylcarbazole, 1,4-dimethylcarbazole and 1,5-dimethylcarbazole (all partially shielded). Isomers in which alkyl substitution is in any position other than 1 or 8 are termed exposed, e.g. 2,4-dimethylcarbazole. Previous studies concerning the respective solubilities of these C2 carbazoles in hydrocarbon solvents indicates that shielded carbazoles display a greater solubility than exposed carbazoles. No partitioning between crude oils and bitumens was seen here. The bitumens do not show an enrichment of exposed carbazoles and in crude oils there is no enrichment of shielded isomers. Such an effect has been reported by Li et al, (1995) for shales and corresponding crude oils from the Liaohe Basin, China.
Distribution o f benzocarbazoles
The distribution of C0-C3 benzocarbazole isomers for a reservoired crude oil (0.92% Re) is shown in Fig. 8. For both crude oils and bitumen, benzo[b]carbazole is present but in only small amounts (Fig. 8). This is similar to previous work on crude oils and shales (Dorbon et al., 1984; Li et al., 1995), although Harrison et al. (1997) reported that coals contain large amounts of benzo[b]carbazole.
Benzocarbazole ratio
The benzocarbazole ratio has been proposed as a maturity independent secondary migration parameter (Larter et al., 1996). In order for this ratio to be used in all petroleum settings a detailed understanding of the effect of maturity in source rocks is required. Figure 9 shows the distribution of this parameter for both Tithonian source rocks and crude oils as well as for the Posidonia Shale (Clegg et aL, 1997a). A systematic increase with maturity is seen up to 1.09% R~, for both source rocks and crude oils as well as the Posidonia Shale (Fig. 9). Likewise, hydrous pyrolysis experiments conducted on the Ghareb Fro. (Cretateous, Jordan) also showed an increase of the ratio over the temperature range (200-360°C) (in preparation). In contrast to the 1,8-dimethylcarbazole/1-ethylcarbazole ratio discussed earlier, both the Tithonian and Posidonia Shale samples data lie on the same slope, illustrating that the generation of benzocarbazoles is similar in both carbonate and clastic rocks, while the differing profiles for the C2 carbazoles suggests that generation varies in clastic and carbonate source rocks, while in the Posidonia Shales, the overall percentages of carbazoles, benzocarbazoles and dibenzocarbazoles varied with maturity suggesting these compounds have different precursors and behave differently during maturation. Both benzo[a]carbazole and benzo[c]carbazole concentrations in crude oils and source rocks increase with maturity. The similarity of the benzocarbazole parameter in crude oils and their Tithonian source rocks at a given maturity indicates that maturity is the prime controlling factor determining this ratio in source rocks and, most importantly, crude oils in this petroleum province. Thus, at a given maturity of the source rock, oil will be
192
Heather Clegg et al. a
~/z217~
c
E+04
E+04 5.874
m/z2
4
~
m/z259
E+04
I
E+04
Fig. 8. Isomeric distribution of Co C3 benzocarbazoles (m/z 217, 231,245, 259) for a selected Tithonian reservoired crude oil (0.92% Ro). expelled with a benzocarbazole ratio equivalent to the benzocarbazole ratio in the source rock. Hence, in the Sonda de Campeche, a single average benzocarbazole ratio in expelled oils, previously suggested by Larter et al. (1996) cannot be assumed. This is not to say that benzocarbazoles ratios cannot be ruled out as migration indicators. The effect of long distance vertical migration through clay enriched rocks may lead to a geochromatographic fractionation as has been proposed for other nitrogen compounds (Yamamoto et al., 1991; Yamamoto, 1992). A similar effect has been noted for oils which have
0.65
~
0.55
,.~0.45 0.3
A
I
0.8 VitriniteReflectance(%)
L
1.3
Fig. 9. The ratio benzo[a]carbazole/benzo[a]carbazole + benzo[c]carbazole vs. maturity for both source rocks and crude oils. See Fig. 5(a) for identification of symbols.
migrated long distances through the pore systems of mudstones rather than via faults whereby the sorbed nitrogen compounds are removed to a greater extent (Larter et al., 1997). However, in areas of predominantly faults, fissures maturity effects appear to overshadow migration effects
CONCLUSIONS
The Tithonian source rock sequence representing an essentially uniform organo-facies spanning a wide maturity range (0.36-1.29% Rr) and its associated crude oils (0.49-0.92% Re), provide a unique opportunity to study the effects of maturation in both source rocks and crude oils, as well as fractionation effects caused by primary expulsion. Quantitative determination of carbazole derivatives from carbonate Tithonian source rocks reveals that peak generation and/or retention occurs at a relatively late stage of maturity (maximum yield at 1.09% Rr). The depletion of carbazole and methylcarbazoles in source rocks between 0.36-0.81% R r and an increase of these compounds in crude oils (0.49-0.92% R~) indicates that these compounds are expelled from source rocks in this maturity range. However, an examination of the distribution of the so-called shielded, partially shielded and exposed carbazoles reveals that primary expulsion does not effect the individual distribution of these compounds based on shielding effects during pri-
Carbazole and benzocarbazole distributions in crude oils and source rocks mary migration. N o differences were seen in the distribution of these c o m p o u n d s in either source rocks a n d oils. Finally, the systematic increase of the benzocarbazole ratio in b o t h source rocks a n d crude oils with m a t u r i t y indicates this ratio is strongly controlled by m a t u r i t y of the source rock. Thus, in petroleum systems where s h o r t distance vertical oil migration through faults a n d fractures has occurred, this ratio should n o t be used to estimate m i g r a t i o n distance a n d charge directions. In fact the ratio in these oils indicates the m a t u r i t y o f the source rock f r o m which it has been expelled. Acknowledgements--This work was performed under the auspices of the ENOG (European Network of Organic Geochemistry). ENOG comprises Laboratoire de G6ochemie, I.FP., Rueil Malmaison, France; Institute for Petroleum and Organic Geochemistry, Research Center Jfilich (KFA), Germany; Laboratoire de Chimie Organique des Substances Naturelles associe au C.N.R.S., Universit6 Louis Pasteur, Strasbourg, France; Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research, Den Burg, Netherlands; Organic Geochemistry Unit, University of Bristol, U.K.; Department of Environmental Chemistry (CID-CSIC), Barcelona, Spain and Geological Institute, University of Cologne, Cologne, Germany. We are grateful to Steve Larter and Bernie Bowler (NRG, University of Newcastle) for supplying standard carbazole compounds. The technical assistance of A. Ropertz, H. Willsch and U. Disko is gratefully acknowledged. We thank the reviewers, Maowen Li and Barry Bennett, for their insightful comments and suggestions. REFERENCES
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