[acs symposium series] geochemistry of sulfur in fossil fuels volume 429 || sulfur isotope data...

Chapter 30

Sulfur Isotope Data Analysis of Crude Oils from the Bolivar Coastal Fields (Venezuela)

B. Manowitz1, H. R. Krouse2, C. Barker3, and Ε. T. Premuzic1

1Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973

2Department of Physics, University of Calgary, Calgary, Alberta T2N 1N4, Canada

3Geosciences Department, University of Tulsa, Tulsa, OK 74104

Oils in the Bolivar Coastal Fields of Venezuela have been divided into five major oil classes believed to reflect largely variations caused by biodegradation in the reservoirs. Classes are based on variations in composition of hydrocarbons, NSO components (including asphaltenes) and API gravity, as well as other considerations including the geological framework and reservoir depths and temperatures. Based on sulfur isotope data reported here, the o i ls f a l l into two groups, a non- or little-biodegraded group with δ34S values averaging +7.56 ‰ and a heavily biodegraded group in which δ34S values average +5.1 ‰. Thermal alteration effects also are probable with reservoir temperatures ranging from 20°C to 209°C. Various possibilities for explaining the isotopic data are considered. The relatively

narrow range in δ34S values suggests reasonably uniform source-rock character and rather minor isotopic changes from all alteration processes.

The Bolivar Coastal Fields (BCF) of eastern Lake Maracaibo, Venezuela, contain f i v e classes of o i l as reflected by t h e i r API gra v i t i e s , C15+ saturates-and-aromatics contents as well as t h e i r t o t a l nitrogen, sulfur, and oxygen (NSO) compositions. Biodégradation appears to have had a major role i n co n t r o l l i n g the

0097-6156/90/0429-0592$06.25/0 © 1990 American Chemical Society

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In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

30. Μ Α Ν Ο W I T Z E T A L . Sulfur Isotope Data Analysis ofCrude OUs 593

compositions of the various classes of o i l . The geology and geochemistry of crude o i l s and source rocks i n the BCF have been described (1-3).

In the Cretaceous, the area was part of the platform of a large geosyncline, and by the Eocene i t was near a coast where a series of large sandy deltas was deposited, with t e r r e s t r i a l sediments on the south and thick marine shales to the north. At t h i s time, conditions for o i l generation i n the shales and migration to the sands may have been established, and the subsequent Oligocène faulting, u p l i f t , and erosion could have allowed meteoric water to penetrate into reservoirs. During the Miocene and Pliocene, the basin was t i l t e d f i r s t west and then south, and f i l l e d with continental sediments from the r i s i n g Andes. T i l t i n g i s s t i l l continuing and o i l i s moving up along the Oligocène unconformity, forming surface seeps. Most of the o i l f i e l d s are located i n sands above t h i s unconformity or i n f a u l t blocks immediately below i t .

Thirty crude o i l s from the BCF were collected (1) along two p a r a l l e l and generally southwest-northeast trends. The areal extent of the BCF showing locations of wells sampled i s shown i n Figure 1. These o i l s were characterized by t h e i r API gravity, percent saturates, aromatics, NSO and asphaltene compounds, gas chromatograms for whole o i l s , C4-C7 fractions, and aromatics. Concurrently, 24 associated waters were also sampled and analyzed for Ca + +, Mg**, Na+, HC03, C03

-, SOA", pH, and t o t a l dissolved solids (TDS) (1).

In the present work, twenty-seven of these o i l s were separately analyzed for sulfur content and sul f u r isotope r a t i o (<53AS) . The samples were oxidized i n a Parr Instrument Company bomb. Sulfate i n washings from the bomb were precipitated with Ba 2 +. The BaS04 p r e c i p i tate served for gravimetric determination of the S-content conversion to S02 for mass spectrometry (4). The 3 AS/ 3 2S abundance ratios are presented in the usual <534S notation.

O i l Classes

As described (1), the o i l s were divided into f i v e classes on the basis of t h e i r chemical compositions (Table I ) . These classes are consistent with reservoir assignment (Table I I ) .

The four o i l s i n Class 1, from Wells CL-20, CL-99, VLC-531, and VLC-642, are dark o l i v e green and are very l i g h t , with API grav i t i e s ranging from 36.6° to 42.2°.

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594 G E O C H E M I S T R Y O F S U L F U R IN F O S S I L F U E L S

Figure 1. Areal extent of Bolivar Coastal Fields showing locations of wells sampled (Reprinted with permission from Réf. 1. Copyright 1983 Bockmeulen et al.)

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30. ΜΑΝΟWITZ ET AL. Sulfur Isotope Data Analysis ofCrude Oils 595

Table I. O i l Classes and Their Compositional Ranges

Benzene • f O i l C15+ Saturates Aromatics NSOs Toluene

Class (%) (%) (%) (%) (%) C7/MCH 1 55-65 69-74 14-19 7-13 12-17 1.18-1.29 2A 67-76 47-55 26-32 15-21 4.8-8.7 1.05-1.55 2B 75-83 39-46 30-33 16-21 3.7-7.6 0.68-0.93 2C 84.6 34.5 34.6 18.4 1.5 0.40 3A 86-88 27-34 33-35 22-25 2.5-2.7 0.30-0.35 3B 81-8 24-33 35-38 21-28 1.1-3.5 0.16-0.34 3C 91-95 21-27 38-41 23-26 0.8-3.7 0.05-0.07 4 79-82 35-44 31-33 16-23 3.2-4.3 0.67-0.78 5 84.0 51.1 30.1 15.5 5.8 0.25

Table II. O i l Classes and Their Reservoirs

Class Oils Reservoirs

1 CL-20.CL-99 Cretaceous Cogollo VLC-531,VLC-642 Eocene C

2A VLA-488S,VLE-357,SVS-12U Eocene C SVS-158 Miocene Santa Barbara

2B SVS-54,SVS-9L,SVS-124, Eocene Β -6 VLB-446,LL-1199,LL-1932

2C PB-230,TJ-742 Eocene Β -6 3A B-345,B-750,B-1307 Miocene Bachaquero

3B B-484,B-650 Miocene Bachaquero PB-315,R-335 Miocene Lower Lagunillas

3C LB-678 Miocene Bachaquero LB-1122 Miocene Lagunillas and LaRosa

4 LL-405,LL-752,LL-1107 Miocene Lower Lagunillas and LaRosa

5 TJ-210 Miocene Lower Lagunillas and LaRosa

The o i l s i n t h i s Class have a lower content of aromatics i n the C15+ fraction, and a higher percentage of benzene and toluene i n the CA-C7 range compared with the rest of the o i l s . The η-heptane (C7) -to-methylcyclohexane (MCH) r a t i o i s greater than one.

The Class 2 o i l s have fewer saturates than those i n Class 1, more aromatics and NSOs, and about half as much benzene and toluene i n the CA-C7 l i g h t f r a c t i o n (Table I ) . They can be subdivided into those o i l s with C7/MCH rati o s greater than one (Class 2A), and those for which

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596 G E O C H E M I S T R Y O F S U L F U R I N F O S S I L F U E L S

the r a t i o i s less than unity (Class 2B). API g r a v i t i e s for Class 2A range from 27° to 33°.

The Class 2B o i l s , i n which methylcyclohexane i s the predominant l i g h t hydrocarbon, extend the compositional trends that distinguish Classes 1 and 2A, and the ranges of values for the distinguishing c h a r a c t e r i s t i c s show very l i t t l e overlap (Table I ) .

O i l s TJ-742 and PB-230 f a l l within the range of the Class 2B o i l s i n gross characteristics, but show lower pristane/phytane r a t i o s . In TJ-742, the normal d i s t r i b u t i o n of n-alkanes i s absent and none are prominent above C2A, suggesting a moderately to severely bacte-r i a l l y degraded o i l . The r e l a t i v e l y high isoprenoid-to-n-alkane abundance i s also consistent with i n c i p i e n t or continuing b a c t e r i a l a c t i v i t y , which i s further supported by the lower C10-C14 n-alkane content. The o i l , however, i s currently from a reservoir at a depth of about 6,000 f t (1,829 m), and the temperature i s above 176°F (80°C) , which would seem to exclude or l i m i t continuing b a c t e r i a l a c t i v i t y . I t i s probably also affected by "water-washing," as shown by the very low content of benzene and toluene.

The Class 3 o i l s are a l l degraded, with API g r a v i t i e s ranging from 14° to 18°. This Class has been subdivided into three sub-classes for convenience i n discussing various degrees of degradation. The heavy C15+ f r a c t i o n increases i n content down the Classes (3A to 3C), as does the aromatic content of the o i l . With increasing degradation, the C7/MCH r a t i o decreases steadily. The amount of benzene and toluene decreases, but the NSO and asphaltene contents are variable. The Class 3A o i l s show the presence of most of the n-alkanes, although they are depleted i n the C9-C13 range. In the Class 3B o i l s , most of the n-alkanes beyond C 1 2

have been degraded, pristane and phytane are absent, and the content of the other l i g h t isoprenoids i s reduced. The Class 3C o i l s were the most degraded, with the C15+ fr a c t i o n accounting for up to 95% of the o i l s .

The Class 3 o i l s (B-345, B-650, B-760, B-1307, LB-678, and LB-335) are a l l found i n Miocene reservoirs i n the South Bachaquero area. In t h i s area, Class 3A o i l s are deeper and further south than 3B o i l s while the most degraded Class 3C o i l s are further north and shallower.

The Class 4 o i l s are distinguished by t h e i r unusual n-alkane d i s t r i b u t i o n , which i s unlike that for any of the other classes; these o i l s contain no n-alkanes heavier than about C15, are not condensates and have API g r a v i t i e s that range from 22.9° to 24.7° (Table I I I ) . The compounds i n the l i g h t fractions, including the

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30. MANOWITZ ET AL. Sulfur Isotope Data Analysis of Crude Oils 597

Table III. Correlation of Sulfur Data with Other Parameters-BCF Oils

(TOL Age of 53*S Reserv. Grav. Asphal. +BZ) Reservoir

< % o ) %S T°C API ο % % Rock

CLASS 1 CL20 +8. ,95 0. 38 205 36. 6 1. .7 14. 5 Cretaceous

Cogollo CL99 +9. ,49 0. ,31 209 40. 2 2. .1 Cretaceous

Cogollo VLC531 +6. ,97 0. ,16 177 40. 0 3. 4 Eocene C VLC642 +6, ,89 0, ,05 1 1 1 40, 0 1.3

8. ,07Av 0. ,22Av 19lAv 39. 2Av 2. .1 Av CLASS 2A VLA488 +8, .80 1. ,38 116 27. 5 4. 7 6. 75 Eocene C SUS9L +8. ,57 0. ,80 117 31. 6 3. 5 Eocene B-6 SUS 54 +6. ,24 1. ,09 130 30. 8 2. 6 Eocene B-6 SUS158 +7. ,82 0. ,89 119 32. 6 1. 4 Miocene

Santa Barb. VLE357 +7. ,74 1. ,91 137 28, 9 6.5

7. ,83Av 1. ,21Av 124Av 30. 3Av 3. 7Av CLASS 2B TJ742 +8. .01 2. .57 97 16. ,9 12. 5 5. 65 Eocene B-6 LL1932 +6. .03 1. .42 96 26. 9 7. 9 Eocene B-6 LL1199 +5. .31 1. .59 104 23. 2 11. 9 Eocene B-6 VLB446 +5. .79 0. .88 113 22. ,1 9. 6 Eocene B-6 SUS12U +9. .30 1. .32 113 30, 5 5, 9 Eocene C

6. ,88Av 1. .56Av 105Av 23. ,9Av 9. ,6Av CLASS 3A B750 +4. .30 2. .39 84 17. ,0 13. 3 2. 6 Miocene Β B1307 P 35 2. .46 .87 16, ,9 13. 3 Miocene Β

4, .32Av 2. .42Av 86Av 17. OAv 13. ,3Av CLASS 3B B650 +4, .81 2. .50 66 15. ,7 12. ,9 2. 3 Miocene Β B484 +4, .29 2. .47 71 14. ,1 12. ,3 Miocene Β B345 +4, .61 2. .27 87 16. ,8 11. ,5 Miocene Β PB315 +6, .71 2, .08 63 17, ,9 9. ,4 Miocene LL

5. .lOAv 2, .33Av 72Av 16. .lAv 11. ,5Av CLASS 3C LB678 +4, .25 2, .90 42 10. .6 12. ,6 2. 25 Miocene Β LB112 +6, .66 1, .69 68 11. .6 11. .0 Miocene LL SEEP +4, .08 2, ,71 20 11, .7 16. ,8

5, .OAv 2, ,40Av 43Av 11. .3Av 13. .5Av CLASS 4 LL405 +3, .99 2, .00 68 24. .7 8. ,5 3. ,75 Miocene LL LL752 +6 .66 2, .17 70 23. .2 8. .0 Miocene LL LL1107 +6 f 44 1 ,74 _71 22. .9 8. .8 Miocene LL

5 .7Av 1. .97Av 70Av 23. .6Av 8. .4Av CLASS 5 TJ210 +7 .61 1 .47 63 24. .4 3. .3 5. ,8 Miocene LL

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l i g h t aromatics, appear to be similar to those i n the other o i l s . The properties of the bulk fractions might otherwise be considered very simi l a r to those of Class 2B. The Class 4 o i l s are a l l found i n the Miocene L-5 reservoir i n the Lagunillas f i e l d , nearshore i n the northern part of the lake.

O i l from Well TJ-210 i s put i n a class by i t s e l f and i s d i f f e r e n t from the other classes. I t i s produced from the same sands of the L-5 reservoir as Class 4 o i l s , although a short distance away. I t i s depleted i n n-alkanes i n the C9-C13 range, but shows an abundant d i s t r i b u t i o n from C u-C 3 5. Pristane and phytane are both prominent. I t does not appear to be unduly affected by water washing as i t contains an appreciable amount of benzene and toluene. The d i s t r i b u t i o n of components i n the C4-C7 range shows a larger proportion of naphthenic hydrocarbons compared to the rest of the o i l s . Its gross chemical composition would otherwise suggest a Class 2A-type o i l . The o i l contains many more saturates and n-alkanes than any other of the Miocene-reservoir crudes analyzed.

Relationships Among O i l Classes

Grouping of the analyzed o i l s into f i v e categories was done for convenience, and although i t was based on gross chemical composition i t does not necessarily have any genetic implications. Typical whole o i l gas chroma-tograms representative of each class of o i l s are given i n Figure 2. Differences i n o i l composition can can be due not only to d i f f e r e n t source rocks, but also the effects of maturation and a l t e r a t i o n that occur i n the reservoirs (or during migration to them). The most obvious differences among the o i l s i n the BCF are i n t h e i r normal alkane d i s t r i b u t i o n s . Since these compounds are the most susceptible to biodégradation, i t suggests that bacteria may have been one factor i n developing the observed o i l chemical compositions.

Support for a common source for the Class 1 and Class 2 o i l s i s provided by the s i m i l a r i t y i n the r e l a t i v e abundance of the isoprenoids, s i m i l a r i t i e s i n the minor peaks i n the t o t a l o i l gas chromatograms, the nature of the aromatic compounds, and GCMS data for bio-markers (5) . The nC9 to nC n sections of the gas chromatographic traces for t y p i c a l o i l s from Classes 1, 2A, and 2Β are given i n Figure 3 which shows that the patterns of the minor peaks are almost i d e n t i c a l . The aromatic traces ("fingerprints") for these three o i l s (Figure 4) show close s i m i l a r i t i e s , although the

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CLASS 1

VLC-642

; ! I 173 eC

! ! I I

CLASS 3A

B-1307 87 e C

CLASS 2A

VLE-357 137 eC

J

CLASS 3B

PB-315 63°C

CLASS 2B

LL-1199 104 eC

CLASS 5 TJ-210

CLASS 3C

LB-1122 68 e C

CLASS 4

Figure 2. Whole o i l gas chromatograms for representative o i l s from each of d i f f e r e n t classes (Reprinted with permission from Réf. 1. Copyright 1983 Bockmeulen et al.)

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600 GEOCHEMISTRY OF SULFUR IN FOSSIL FUELS

104eC

Figure 3. Details of gas chromatograms i n range from nC9 to nC n for representative o i l s from Classes 1, 4, and 5 (Reprinted with permission from Réf. 1. Copyright 1983 Bockmeulen et al.)

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Figure 4. Gas chromatograms for aromatic fractions of representative o i l s from Classes 1, 2A, and 2B (Reprinted with permission from Réf. 1. Copyright 1983 Bockmeulen et al.)

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aromatic response for the deep Cretaceous o i l s i s somewhat higher, probably r e f l e c t i n g a s l i g h t l y greater maturity. Because aromatic compounds r e s i s t biodégradati o n more than n-alkanes, t h e i r d i s t r i b u t i o n i n the C 1 5

f r a c t i o n w i l l be either unaffected or considerably less affected as degradation proceeds. Further, i n the l i g h t fractions, the small aromatic molecules can be removed by the water washing that accompanies biodégradation, and t h i s leads to a reduced concentration of benzene and toluene. Multiringed c y c l i c compounds and many highly branched saturates are also resistant to b a c t e r i a l degradation and remain unchanged in r e l a t i v e amounts during the early stages of degradation, so that the s i m i l a r i t y i n the minor peaks (such as those shown i n Figure 3) suggests a common ancestor for the o i l s now distinguished as Classes 1 and 2 on the basis of normal alkane d i s t r i b u t i o n .

The Class 3 o i l s show various stages of normal alkane loss that suggest progressive b a c t e r i a l degradat i o n . The gas chromatograms for the aromatic fractions of the 3A and 3B classes generally resemble those for the 2A and 2B o i l s , but show some minor differences from Class 1 (Figure 6). The l i g h t e r ends are d i f f i c u l t to correlate, but minor peaks between the normals show very simil a r d i s t r i b u t i o n s . Thus, i t appears that the Class 1, 2, and 3 o i l s could be members of the same family, but with d i f f e r i n g degrees of degradation. If t h i s i s true, then presumably they share a common source rock (1,2) ·

Class 4 o i l s present a somewhat d i f f e r e n t pattern. In terms of C g-C n and corresponding range of aromatic compounds, there i s a s i m i l a r i t y to Class 1 o i l s (Figures 4 and 5), an observation which would be consistent with a common source. However, there i s a difference i n the normal alkane d i s t r i b u t i o n when Class 4 o i l s are compared to Class 1 or Class 2 o i l s . Class 4 o i l s have a lower benzene and toluene content and appear more biodegraded r e l a t i v e to Class 1 o i l s (see Tables I and I I I ) . The Class 4 o i l s are a l l i n a Miocene reservoir, the L-5, which i s i n permeable contact with the Eocene Β sands below. Other o i l s i n the area do not show the same normal alkane d i s t r i b u t i o n , but these differences are i n s u f f i c i e n t to invoke a separate source rock. The TJ-210 o i l (Class 5) i s also reservoired i n an L-5 sand and i s unusual i n that i t i s depleted i n normal alkanes between Cg and C 1 3 (Figure 2). I t i s poss i b l e that the Class 5 and the Class 4 o i l s are related by some process of natural d i s t i l l a t i o n as reported (6) for Trinidad o i l s . In t h i s process the l i g h t ends (C15+)

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30. M A N O W I T Z E T A L . Sulfur Isotope Data Analysis of Crude Oils 603

CL-99 (Class 1)

LL-1107 (Class 4)

Figure 5. Details of gas chromatograms i n range from nC9 to nC n for representative o i l s from Classes 1 and 4 (Reprinted with permission from Réf. 1. Copyright 1983 Bockmeulen et al.)

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604 G E O C H E M I S T R Y O F S U L F U R IN F O S S I L F U E L S

Figure 6. Gas chromatograms for aromatic fractions of representative o i l s from Classes 1, 2A, 2B, 3A and 3B (Reprinted with permission from Réf. 1. Copyright 1983 Bockmeulen et al.)

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that characterize the Class 4 o i l s would be separated from a Class 1 parent to leave a residue resembling TJ-210. This type of process has been demonstrated experimentally by heating a sample of the CL-99 or VLC-642 Class 1 o i l at 100°C i n a flask f i t t e d with a vert i c a l sandpacked column. After several days, the mater i a l d i s t i l l e d up into the sand column resembled the Class 4 o i l s while the residue i n the flask had a composition very close to that of o i l TJ-210.

In summary, a l l the BCF o i l s analyzed showed very si m i l a r patterns for minor components and aromatics, and appeared to be related. Major compositional trends r e f l e c t variations i n the amounts of normal alkanes. The sequence of t h e i r removal i s the same as that observed i n many other areas and attributed to b a c t e r i a l degradation. A few o i l s show unusual normal alkane d i s tributions that are not well explained by currently understood degradation or migration processes.

In order to c l a r i f y some of the above discussed trends we have analyzed o i l s from the BCF for £ 3 4S and S contents and compared the results with other known parameters.

Analysis of <S3*S and S Data. Table III l i s t s the sulf u r data along with other parameters. The <S34S values are self-consistent with the classes of o i l i d e n t i f i e d by Bockmeulen et a l . (1). <S3AS values correlate well with other class parameters i n that they are consistent with 1) chemical composition as indicated by API° and percent asphaltenes, 2) age of producing formation, and 3) e v i dence of biodégradation. Separating the data into two groups, non- or slightly-biodegraded (Class 1, 2A, and 2B) and heavily biodegraded (Class 3A, 3B, 3C, and 4), the 634S values for the non- or slightly-biodegraded group average +7.56. On the assumption that the v a r i a b i l i t y i n sampling can be about 2 del units, these data are remarkably consistent and are a strong indication that a l l of the o i l s i n Classes 1, 2A, and 2B are from the same source. The <S3*S values for o i l s i n Classes 3A, 3B, 3C, and 4 average +5.1. The average differences i n <53AS are s t i l l not large, only 2.5 °/ o o r providing e v i dence that a l l of the o i l s are from the same source. Class 3 o i l s , however, do seem to have lower 3*S values i n that only two of the nine samples cause the large range difference. Class 5 seems to be an intermediary o i l , f a l l i n g between Classes 2 and 4 (see also Figure 10) .

There are several ways of interpreting the d i f f e r ences between the del values of o i l s i n Classes 1 and 2

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and Class 3. The lower values i n Class 3 could be interpreted as representing biodégradation of the o i l s . Biodégradation generally does not involve major sulfur bond changes. The sulfur compounds p r e f e r e n t i a l l y survive, and the degraded o i l s should retain the i s o topic value of the undegraded precursor o i l . I f organic sul f u r bonds are ruptured, normal k i n e t i c isotope effects favoring the l i g h t e r 32S would increase the £ 3 4S value of the unreacted material rather than the decrease observed. An alternative explanation i s that the observed change i n del value i s due to microbial action on the sulfate i n associated waters resul t i n g i n l i g h t e r s u l f u r being added to the o i l . Sulfate tends to have s i g n i f i c a n t l y positive <S3AS values, and the natural i s o tope s e l e c t i v i t y during i t s reduction ranges from negl i g i b l e to very large. Hence, o i l that has incorporated products of reduction may acquire 3AS enrichments or depletions. Orr (7) presented data where high temperature maturation and sulfate interactions enriched the o i l i n 3 AS. Thus, there i s another explanation for the differences i n the data. The formation of S° and H2S by thermochemical sulfate reduction and sulfur introduction at higher temperatures i s possible for Class 1 and 2 o i l s i f adequate sulfate i s available. This could make Class 1 and 2 o i l s i s o t o p i c a l l y heavier. In addition, differences i n thermal maturity can res u l t i n variations of one or two 634S units.

The sulfur content of the o i l s i s self-consistent within the classes of o i l s and consistent with anticipated thermal a l t e r a t i o n e f f e c t s . One would expect biodégradation to slow down or end at o i l temperatures of about 70°-80°C. In general, the data follow these trends. With r i s i n g temperature, there i s a tendency for crude o i l s i n reservoirs to become s p e c i f i c a l l y l i g h t e r and contain an increasing amount of low molecular weight hydrocarbons at the expense of high molecular weight constituents. This could be due simply to thermal a l t e r a t i o n or due to the p r e c i p i t a t i o n of asphaltene out of the o i l s by gas and/or low b o i l i n g hydrocarbon s o l u b i l i t y changes.

From the data, the average sulfur content (%S) of each class of o i l increases with increasing asphaltene content (Figure 8), whereas both sulfur and asphaltene content decrease l i n e a r l y with increasing temperature (Figure 7). This implies that both asphaltenes and sulfur were removed from the o i l during thermal a l t e r a t i o n . Similarly, the average sulfur content of each class of o i l decreases l i n e a r l y with increasing API gravity (Figure 8), suggesting that sulfur compounds are

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MANOWITZ ET AL. Sulfur Isotope Data Analysis of Crude Oils

Ο 2 0 4 0 6 0 8 0 100 120 140 1 6 0 180 2 0 0

Temperature, °C

Figure 7. Variation of Sulfur Content of BCF O i l s with Temperature.

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20 API° 4 0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

% Asphal tenes

Figure 8 . Change in API Gravity and in Percent Asphaltenes of BCF O i l s as a Function of Percent Sulfur.

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30. MANOWITZ ET AL. Sulfur Isotope Data Analysis ofCrude Oils 609

not metabolized by microbial action and tend to accumulat e so that degraded crudes have an increased s u l f u r content.

Further, the i n i t i a l attack of aerobic bacteria i s on the saturate paraffins and straight chain substitu-ents of polycyclics (C15+). This i s consistent with the increase i n C15+ and a decrease i n saturates components in a l l of the o i l s analyzed at lower temperatures due to higher b a c t e r i a l a c t i v i t y . The fact that long chain substituents on the polycyclics, e.g., steranes, are affected precludes the p o s s i b i l i t y that changes i n the o i l , including sulfur compounds composition, are due sole l y to water washing (8). Thus, the correlations between a l l the parameters considered indicates a p o s s i b i l i t y of a combined eff e c t of biodégradation and water washing. Increased water washing decreases the toluene and benzene contents (along with other l i g h t ends), thereby increasing the sulfur content of the undissolved o i l as shown in Figure 9.

Another explanation for the lower <534S values i n the degraded o i l i s that "water washing" s e l e c t i v e l y removed thiophene and dibenzothiophene type compounds soluble i n the alcohol, acid, or phenol solutions present i n the biodegraded oil-water interface. This mechanism requires that these aromatics have higher 53AS values so that t h e i r removal w i l l deplete the average 634S values of the o i l . This should be observed as an increased r a t i o of saturated sulfur compounds over aromatic i n the biodegraded o i l .

A lternatively, i f reduction of sulfate i n associated waters occurred, the active sulfur (H2S) would have p r e f e r e n t i a l l y reacted with the saturates producing t h i o l s and sulf i d e s . This mechanism, too, should r e s u l t i n an increased r a t i o of a l i p h a t i c sulfur compounds over aromatic i n the biodegraded o i l .

The fact that a l l of the isotopic r a t i o s are r e l a t i v e l y close suggests reasonably uniform source rocks and r e l a t i v e l y minor isotopic changes by a l l of the interrelated a l t e r a t i o n processes involved (9). An additional useful oil-source rock correlation i s the pristane/phytane r a t i o . Except i n the most severely degraded o i l s , the isoprenoids pristane (pr) and phytane (ph) are present and pr/ph rati o s range from 0.75 to 1.48. For example, a plot of pr/ph versus 63AS (Figure 10) shows that the Class 1 o i l s overlap the range of the Class 2 (A and B) o i l s , while the reservoir ages show a clear separation, with the Miocene and Cretaceous o i l s being quite d i s t i n c t from those i n Eocene reservoirs.

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CO

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Toluene*Benzene, % Figure 9 . Total Toluene and Benzene vs. Sulfur 34 and vs. Percent Sulfur.

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30. MANOWITZ ET AL. Sulfur Isotope Data Analysis ofCrude Oils 611

en 7

ιο CO

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0.7 0.8 OS 1.0 I.I 1.2 1.3 1.4 1.5 Pristane/Phytane

Figure 10. Variation of <53AS with Pristane/Phytane. There are only data for three Miocene reservoirs, because the o i l s i n other Miocene reservoirs are highly degraded, therefore pristane and phytane are absent.

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Acknowl edcrment s

This research was supported in part by the U.S. Department of Energy under Contract No. DE-AC02-76CH00016. The Stable Isotope Laboratory at the University of Calgary is supported by the National Science and Engineering Research Council of Canada.

Literature Cited

1. Bockmeulen, H.; Barker, C.; Dickey, P. AAPG Bull. 1983, 67(2), 242-70.

2. Blaser, R.; White, C. AAPG Mem. 1984, 35, 229-52.

3. Talukdar, S.; Gallango, O.; Chin-A-Lien, M. In Advances in Geochemistry; Leythaeuser, D.; Rullkotter, J., Eds.; Pergamon Press, Inc.: New York, 1985; pp. 261-79.

4. Yanagisawa, F . ; Sakai, H. Anal. Chem. 1983, 55, 985-87.

5. Flory, D.; Lichenstein, H.; Bieman, K.; Biller, J.E. and Barker, C. Oil and Gas Journal 1983, 91-98.

6. Ross, L . ; Ames, R. Oil and Gas Journal 1988, 86(39), 72-6.

7. Orr, W. AAPG Bull. 1974, 50(11), 2295-2318. 8. Lafarge, E . ; Barker, C. AAPG Bull. 1988, 72(3),

263-78. 9. Krouse, R.H. J. Chemical Exploration 1987, 7,

189-211. RECEIVED March 5, 1990

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[acs symposium series] geochemistry of sulfur in fossil fuels volume 429 || sulfur isotope data...

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