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Carbon isotope geochemistry of Paleozoic oils from Big Horn Basin
Carbon isotope geochemistry of Paleozoic oils from Big Horn Basin H. Mta~s CHCNG. SHERIW. BRAND* and PATRICKL. GRIZZLE Bartlesville Energy Technology Center. U.S. Department of Energy. Bartlesville. OK 74003. U.S.A.
Abstract--Twenty crude oils from Paleozoic reservoirs in the Big Horn Basin. Wyoming were fractionated into light hydrocarbons. saturates. monoaromatics. diaromatics, polyaromatics-and-polars. and asphaltenes. Amounts and isotopic composition of each fraction were found to be internall) consistent with the degree of maturation of the oil. confirming the established single source origin for these oils. A variation of approximately three per mil in the carbon isotopic composition of the whole oil and individual fraction was explained as being caused by maturation. The isotope-type-curves for these oils, the variations in isotopic composition as a function of compound group-type. were not found to be as simple as commonly believed. Other alteration processes. such as migration and deasphalting. might have influenced the pattern of the isotope distributions among fractions. The conservative nature of mass balance and subsequent isotope flows among various fractions of crude oil showed that the maturation of crude oil consists of consecuGve processes leadmg from larger and more polar components into smaller and less polar components culminating in natural gas. Calculations were made to project the amount of condensates associated with the crude oils. and the amount of natural gas which had been generated from crude oils of a given maturit).
INTRODUfflON VARIATIONSin stable carbon
isotopic composition have been useful in understanding the origin of petroleum. and relating oil-to-oil and oil-to-source rock in petroleum geochemistry. Principles of these and other applications have been described by FUEX (1977) and STAHL (1977). The variations in the isotopic composition among petroleum molecules and fractions of varying boiling ranges have been important in the understanding of petroleum genesis (SILVERMAN.1967. 1971). The differences in isotopic compositions among organic materials from varying depositional environments. along with those among plants of varying photosynthetic mechanisms have been used in determining the original source materials in petroleum (KCI~NScr al., 1974; SUTTON,1979). The close relationship between the isotopic composition of petroleum and that of the source rock has been the underlying principle for oil-to-source rock correlations (WELTEet al., 1975: STAHL 1978). Finally. the similarity in isotopic composition for petroleums that have originated from the same source materials has been used in oil-to-oil correlations (ERDMAN and MORRIS. 1974: HAN-WEINHEIMER and HIRNER. 1980: CLAYTONand SWETLAND,1980). In oil-to-oil correlations, the number of oil types that are present in a given basin is often controversial. For example. JONESand SMITH (1965) reported five types of oils in the Permian basin, West Texas and * Present address: Union Oil Co.. Union Science and Technology Diviston. 376 S. Valencia Ave.. Brea. CA 92621. U.S.A.
New Mexico. by using the correlation index of petroleum fractions. whereas KVENVOLDEN and SQUIRES (1967) found only three types of oil by using the stable carbon isotopic composition of whole oils. This confusion on the number of oil types is often caused by the researcher’s objectives in defining oil types and by a lack of knowledge concerning the effects of maturation and other alteration processes on crude oil composition. In this paper. therefore, a single oil type is defined to include those which originated from the same source materials but might have undergone varying degrees of natural transformation processes, such as maturation. migration. biodegradation, etc. These secondary alteration processes have been described by WINTERS and WILLIAMS(1969). EVANSer al. (1971). BAILEYer al. ( 1974) and MILNER er al. (1977). For a given oil type. the internal variations in crude oil composition may drastically change the isotopic composition of a whole oil (FUEX, 1977). Therefore, it is a common practice to use a topped crude oil instead of a whole crude oil to obtain better comparative data. isotopic composition of a topped crude oil is largely based on the traditional use of the isotope techniques which yields an averaged and integrated isotopic information. Therefore. the final and conclusive answer may not be immediately possible from the isotope data alone, and further supporting evidence from other analytical methods are often necessary. Two recent developments in isotope geochemistry have helped to overcome this difficulty: one is the use of hydrogen isotopes in addition to those of carbon. and the other is the use of separated fraction. such as the aromatic fraction, which is more resistant to
1803
H. MOSESCHUNG er ul
1804
natural alteration processes and less sensitive to sample handling techniques WHOELL and MILNER, 1981). Recently. the principle of carbon-13 heterogeneity In natural organic materials has been used to under-
stand maturation of carbonaceous materials (CHUNG and SACKETT,1979) and the formation of natural gas (CHUNGand SACKETT, 1980). This principle may well be applied to understand the geochemistry of crude oil. The effects of internal variations of isotopic composition and varying oil composition are caused by the natural history of crude oil, and, therefore, should be used in understanding the geochemistry of crude oil. To demonstrate this thesis, twenty crude oils were selected from the Big Horn basin, which are believed to have a single source origin but to differ widely in maturity (ORR. 1974). We will demonstrate how the isotopic flow among fractions of varying polarities would proceed with increasing maturity of oil. We will then try to interpret the pattern of the so-called isotope type curve (STAHL. 1977. 1978). And, finally, we will show what kind of useful information can be obtained from the most sensitive fraction of oil, the gasoline-range hydrocarbons. The geochemistry of the Paleozoic oils in the Big Horn basin has been worked out in detail by previous workers (STONE.1967: ORR. 1974). For the purpose of this paper. a brief geological history is summarized here. Relatively immature oils which were derived from the Phosphoria shale of Permian age migrated by jurassic time into stratigraphic traps in areas that later were folded into the Big Horn and the Wind River Basins (SHELDON,1967). From Early Jurassic until Late Cretaceous, the oils were subjected to a similar thermal history. Secondary migration of the oils occurred during the Laramide orogeny which created the Big Horn Basin in Paleocene time. Subsequent to the Laramide folding, the oils were subjected to different depth and thermal histories. In short. the present compositional variations in the Paleozoic oils of the Big Horn Basin were largely caused by thermal maturation of primary oils which were originally derived from the Phosphoria Shale. PROCEDURES
A weighed amount of crude oil was placed m a vacuum oven at 60 C overnight to remove materials up to approximately n-decane The amounts of light hydrocarbons were calculated from the weight differences. Asphaltenes (ASPH) were obtained by precipitation with pentane (oil:pentane = 1: 50). Removal of the pentane yielded the asphaltenefree oil containing materials higher than approximately n-dodecane. The amounts of ASPH were calculated from the weight differences. The asphaltene-free oil was passed
through a silica cartridge Sep-Pak (Waters Associates) to remove polar components. No further work was carried out on these small amounts of polar materials. The resulting hydrocarbons were fractionated mto saturate (SAT), monoaromatic (MONO). diaromatic (DI). and polyaromatic-and-polar (PAP) fractions by preparative high perform-
ante liquid chromatography. The asphaltene-free 011 \\as loaded onto a Woelm alumina column ISS-2ooO-C.I.25 in. o.d. ‘x I2 m. length) in series with a silica .ge) column [SS-2000-C. 1.25 m. o.d. x 12 in.. length). The mrtlal eluent was 1.5”,, methylene chloride in pentane for elution of SAT fraction. 4 linear gradient from 1.5 to XI”,, methylene chloride m pentane followed to elute the MONO and DI fractions, and the PAP fraction was eluted using 75”,, methylene chloride in pentane durmg a backflush of the alumina column. Following solvent removal. the amounts of each fraction were calculated. Sepururlon und conrhurion (?t liyhr h!~drocurhons (LHCI
The separation and combustion of LHC \*ere carried out by fractlonal distillation similar to that of CHUNG and SACKETT(1979). The mildly heated oil was expanded mto two traps cooled with ice-water. The noncondensable gases were transferred by a Toepler pump into a combustion system via one additional trap cooled with ice-water. Transfer of LHC into the combustion system was monitored by the vacuum gauge. The carbon dioxide formed was purified and sealed in a Pyrex tube for isotopic analysis. A gas chromatogram of the residue of fractional distdlation showed that the amount of n-tridecane was approxlmately half of the combined quantities of n-dodecane and !I-tetradecane. Isotope onalysis The combustion of most samples and subsequent isotopic analyses of carbon dioxide were performed by Coastal Science Laboratory. Port Aransas. Texas and the Isotope Geochemistry Laboratory. University of South Florida. St Petersburg, Florida. To avoid the posstbie discrepancy m the isotopic composition of each laboratory standard. all samples used in specific studies were sent to the same place. Isotopic data in Table I were from the University of South Florida. and data in Table 3 were from the Coastal Science Laboratory. The mean differences in isotopic compositions of seven samples were found to be 0.22 per mil between these laboratories. The isotopic data are presented in the usual 6 notation
and related to the Chicago PDB standard
RESULTS AND DISCUSSION Internul
au%Uion
of isotopic
composition
and rurGxq
oil composition
SILVERMAN (1967. 1971) measured carbon isotopic compositions of n-alkanes and distillation fractions from several crude oils and found isotopic evidences concerning the genesis of petroleum. Sixteen fractions from the Bureau of Mines Hempel distillation were obtained from four crude oils reservoired in carbonate rocks and their isotopic compositions are plotted in Fig. 1 as a function of the average distillation temperature. The oil from the South Swan Hills (Devonian, Alberta) shows a rather smooth distribution in isotopic composition. whereas the other oils show large variations among fractions. The oil. 76048 (Ordovician, Michigan: The oil number is from the Crude Oil Data Bank at Bartlesville Energy Technology Center), shows the heaviest isotopic composition in the range of octane to octadecane (12@300 C). The oil. 75051 (Silurian. Michigan),
1805
Carbon isotope geochemistry of Paleozoic oils -25
I
1
Vacuum
-31 0
I 100 AVERAGE
I 200 DISTILLATION
1
dlsttllation
1 300 400 TEMPERATURE,‘C
Rerll
ml
Fig. 1. The carbon isotopic composition of the fraction vs the average distillation temperature. The sample number is from the Crude Oil Data Bank at the Bartlesville Energy Technology Center.
shows a continuous decrease in isotopic composition with increasing distillation temperature. The oil, 75061 (Devonian, Michigan). has generally isotopically lighter fractions, although fractions of middle distillation cuts are isotopically heavier. Trends in the figure suggest that different oil types may have different isotopic variations. It is also noted that topping a whole oil for isotopic measurement is not always necessary. as observed here for the South Swan Hills oil and more for the Big Horn oils of low maturity in the following section. In the past years. the Bartlesville Energy Technology Center under the API Project No. 60 characterized in detail several representative crude oils. The first step was to distill the crude oils into three boiling-range fractions. 37&535 . 535-675 and >675’C. Each fraction was further separated into six fractions. saturates (SAT). monoaromatics (MONO), diaromatics (DI)., polyaromatics (POLY). bases and acids. In addition. the resins in the 675’C residuums were separated into strong bases, weak bases, strong acids and weak acids. The separation procedure and detailed chemistry is beyond the scope of this paper. and can be found elsewhere (DOOLEYet al.. 1973; HIRSCHer ul.. 1974: GREENand HOFF, 1980). The isotopic compositions for these fractions are of interest to study the heterogeneous nature of carbon-l 3 distribution among fractions of varying polarities. Two oils were chosen for this study. the oil from South Swan Hills and the oil from Wilmington (Miocene. California). The gas chromatogram and correlation index of the South Swan Hills oil show it to be
a paraffinic oil (TISSOTand WELTE,1978). whereas the Wilmington oil shows sign of mild biodegradation (pristanejn-C,, = 1.58, phytanein-C,s = 1.32). The analytical information is given in Table 1. and the isotopic data are plotted in Fig. 2. Some important generalizations can be made and a few interesting research areas may also be suggested. First. the isotopic composition becomes heavier with increasing polarity or polarizability of the fraction in the order of SAT. MONO, DI, POLY. bases and acids. For the Wilmington crude. the isotopic compositions of acids and bases deviate from this trend. More noticeable is that the isotopic composition of acids is lighter than that of bases. This may be explained by the subsurface oxidation of the crude oil; more carboxylic functional groups are found as compared to most crude oils (GREENand HOFF, 1980). Or, it may be caused by the secondary asphaltenes formed during biodegradation (STAHL.1980). Second, the isotopic compositions of fractions in the lower boiling-range are heavier than those of fractions in the higher boiling-range. This behavior is obvious for the South Swan Hills because of the maturity of the crude oil. The SAT fraction from the Wilmington deviates from this trend, that is, smaller molecules are isotopically lighter. An explanation probably lies in the immature nature of this oil. or in the selectivity of biodegradation toward this fraction. The above explanations are preliminary; no generalizations can be made from only two oils. However. it is important to point out the heterogeneous nature of carbon-13 in crude oil fractions separated by polarity
1806 Table 1. Variation of compound classes by Ylgh Performance ilquid Chromatography and isotope balance. The positive numbers refer to weight distribution in percentage. The negative numbers refer to stable carbon isotopic composition in per mil.
Wilmington -22.5
370-535'C 24.7, -22.0
South
SAT. YONO. DI. POLY.
Hydrocarbons -22.9
35.9,
-24.2
16.3, 11.4, 26.4, 4.6, 4.6.
-22.8 -22.7 -22.4 -21.2 -22.1
15.7, 9.2, 15.8, 47.6. 10.3; 5.4,
-23.0 -22.8 -22.5 -21.6 -21.6 -22.7
2.3, 1.6, 3.0, 38.3, 39.8, 8.7. 4.0; 4.2,
-23.4 -23.1 -22.7 -22.5 -22.1 -22.2 -23.2 -22.5
Bases Acids
535-675'C 14.5. -22.4
SAT. MONO. 01. POLY.
Hydrocarbons -22.5 Bases Acids
.675'C 22.1,
SAT. MONO. DI. POLY.
Hydrocarbons -22.5 -22.2 Strong Bases Weak Bases Strong Acids Weak Acids
SAT. MONO.
Hydrocarboos -28.1
370-535-c 18.4, -28.1
Swan -28.2
U‘
POLY. Bases Acids SAT. Hydrocarbons -28.1
535-675'C 6.0. -28.0
MONO. 01. POL".
Bases Acids SAT. MONO. 01. POLY.
Hydrocarbons -28.2
>675'C 3.0. -28.1
Strong Bases Weak Bases Strong Acids sieak Acids
Hills
64.7. 11.1. 5.1; 17.6, 1.4, 1.6,
-28.4 -27.5 -26.7 -26.8 -26.3 -26.6
46.5, 8.5, 9.4, 29.1, 4.4, 2.1,
-28.6 -28.2 -27.4 -27.3 -27.i -27.3
4.e. 9.5, 9.8, 49.0, 18.6. 2.5, 3.2, 2.4,
-29.1 -28.5 -28.6 -27.4 -27.6 -27.5 -27.5 -27.5
and boiling point. It is also important that the mode of heterogeneity is believed to be associated with the geochemical history of the crude oil.
ameter, the Z number. which was defined by ORR (1974) as:
Curbon
where Z is a maturity parameter. API is the API gravity of oil and S’N is the weight ratio of sulfur vs nitrogen. Other maturity parameters are possible but the Z number is found to be satisfactory and is used in this paper. It IS emphasized, however, that any maturity parameter deals with chemical changes occurring during the maturation of crude oil. Therefore, the compositional changes of the crude oil during maturation may not necessarily relate linearly to the given maturity parameter. The relationship of the isotopic composition of the whole oil versus the Z number in Fig. 3 shows that an overall isotopic enrichment occurs with increasing degree of matur-
isotope yeochernistry
the Big Horn
oj‘ the Ptrleozo~
oils jkorn
Basin
ORR (1974) has reported the geochemistry of the Paleozoic oils from the Big Horn basin. He concluded that the compositional changes of the Big Horn oils were mainly controlled by maturation effects and that other alteration processes such as water-washing and biodegradation were minor except for a few of the heaviest oils. To further verify these conclusions, twenty crude oil samples were obtained and selected information for each sample is shown in Table 2. The isotopic composition of a whole oil, not a topped oil. is shown in Fig. 3 as a function of the maturity par-
Z = O.W(API
) + 1.3J(S/N) + 15.1,
370-535=x ’ 535-675°C A B675”C
l
L
/ -29
S Swan
1
-28
Wllmlnaton
Hills
-27
,
/
-26
-25
6’3C Fig. 2. The carbon
1
1
-24
-23
-22
(%.)
isotopic composttion of compound group-t)pe of var! mg bodtng ranges separated from the South Swan Hills and Wilmington crude.
Carbon Table 2.
Sample No.
Mobil sample No.
isotope geochemistry
of Paleozoic
1807
oils
Information on Big Horn Paleozoic Oils, from 01-r(1974).
Reservoir formation and age
Field
M3723 M3145 M3728 M3727 M3144 M3918 M3722 M3464 M3146 M3726 M3463
Phosphoria-Permian Tensleep-Penn Tensleeo-Penn Tensleep-Penn Tensleep-Penn Phosphoria-Permian Tensleep-Penn Tensleep-Penn Tensleep-Penn Phosphoria-Permian Tensleep-Penn
M3729
Tensleep-Penn
H3730
Phosphoria-Permian
M3734 M3921 Ml44 M2B6 M3919 M3169 Ml66
Phosphoria-Permian Tensleep-Penn Phosphoria-Permian Tensleep-Penn Phosphoria-Permian Tensleep-Penn Phosphoria-Permian
Slick
API gravity
2 number
10580
30.0 34.0
54 53
27.2 25.0 25.0
:; 49
32.7 31.9
:;
38.3 25.4 21.7 21.0
:: :;
-28.1 -29.1 -28.4 -28.4 -28.4 -27.5 -28.3 -26.7 -28.7 -28.2 -29.9
23.1
44
-29.1
3392
23.5
44
-29.0
3744 2700 9970 2705 7133 3013 8520
21.2 .28.8 32.9
41 70 85
-29.0 :;;.;
Creek
Elk Basin Elk Basin Frannie Frannie Manderson Neiber Southeast Golden Eagle Elk Basin South Garland Oregon Basin South Oregon Basin North Oregon Basin North Hamilton Dome Bonanza Worland Bonanza Manderson Torchliqht Silvertip
The difference in isotopic composition between the least and most mature oil was 3.6 per mil as reported by ORR (1974. p. 2302). FUEX (1977. p. 163) stated “This wide range in isotopic composition may not be the result of maturation effect alone.” Whereas in reviewing KVENVOLDEN and SQUIRES (1967) work on the Ellenburger (Lower Devonian) oils in the Permian basin. FUEX (1977. p. 168) suggested that a normal oil and a condensate formed by thermal cracking of that oil may differ greatly in carbon isotopic composition. by saying “The result is that oils probably having a common source have isotopic values which differ by as much as 7 per mil.” How wide the range of isotopic composition should be for a given oil type is not well understood. The results of high performance liquid chroma-
ation.
-25
Depth in feet
I
5300 6494 3920 2565 7549 10320 9600 7000 4360 3650
3474
31.3 33.2
Es6 1:;
29.2 38.9
6i3C-oil("/,,)
-2714 -27.6 -27.6 -25.7
tographic separation of the asphaltene-free samples are shown in Table 3. The weight distribution for saturates (SAT), monoaromatics (MONO) and diaromatics (DI), polyaromatics-and-polars (PAP) and asphaltenes (ASPH) is shown in relation to the Z number in Fig. 4. The MONO and DI are combined in the figure, because the sum of these fractions is generally small and varies inconsistently. Very small changes are observed in the compositions between Z = 54 and Z = 68. However. it is apparent that the amounts of SAT increase at the expense of other fractions as the maturation of crude oil progresses. The SAT formed will eventually be the source of condensates and natural gas associated_ with the reservoired crude oil, if the trap condition is feasible. This poses the. question as to whether the maturation or reser-
I
I
I
I
-26/-
. w
I
.
.
l
. . 1
.
. .. .
. .
l
Fig. 3. The carbon
I 40
I 50
.
. . .
.
.
-30
.
I 60
I 70 2
Samples in this paper
I 80
-I
I 90
isotopic composition of the Paleozoic oils from the Big Horn parameter. Z number. All data are from ORR (1974).
Basin vs the maturity
1808 Table 3. Variation of compound classes Chromatography and isotope balance.
Isotopic Sample No.
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 i6 17 18 19 20
*Light
Topped crude -28.3 -28.2 -26.4 -28.4 -28.7 -27.6 -28.4 -26.: -28.7 -28.5 -29.1 -29.1 -29.1 -28.9 -27.8 -28.1 -28.0 -28.2 -28.0 -26.2
LHC*
SAT
-27.2 -27.7 -28.0 -27.8 -28.0 -26.5 -27.8 -27.2 -28.4
-28.4 -28.0 -28.3 -28.4 -28.6 -27.5 -28.6 -26.3 -28.8 -28.2 -29.4 -28.8 -29.! -29.0 -27.6 -28.2 -28.0 -28.2 -28.2 -26.4
-28:8 -28.5 -28.4 -28.6 -26.6 -26.2 -26.5 -26.7 -26.6 -25.7
hydrocarbons
composition
MONO
(LHC) generally
PAP
-27.9 -28.3 -28.2 -28.2 -28.7 -27.3 -27.0 -25.7 -28.3 -28.3 -29.2 -28.9 -29.0 -28.3 -28.0 -27.9 -27.7 -28.2 -27.4 -26.5
include
ASPH
-23.0 -28.4 -28.7 -28.2 -28.1 -28.1 -28.3 -26.3 -28.8 -28.6 -29.3 -28.9 -29.1 -28.9 -27.7 -28.0 -28.2 -28.4 -27.9 -27.1
compounds
-23.9 -28.6 -29.0 -28.6 -29.0 -27.6 -28.7 -26.4 -28.9 -28.9 -29.0 -29.1 -29.1 -28.9 -28.8 -28.2 -28.4 -28.6 -28.5 --
less
LHC’ 12.2 8.3 lo.? 10.4 8.4 15.3 i2.3 9.9 7.3 i.9 5.3 9.! 6.7 6.6 17.5 25.1) 24.7 21.: 4.5 11.8
SAT
47.7 48.6 50.1 39.2 40.7 53.1 50.4 65.2 48.5 42.i 34.9 35.5 33.3 37.3 45.9 53.6 46.3 48.0 51.6 58.2
bicw i5.3 !j.O IO.4 15.1 15.2 14.4 15.3 11.6 13.9 16.4 ‘6.2 i5.6 16.2 16.1 15.3 13.9 15.3 14.6 14.3 !0.7
(-,',j
01
PAP
14.6 i2.4 12.4 17.6 17.3 !4.2 13.4 8.9 i3.3 17.3 X.0 18.8 16.7 17.8 i6.5 14.8 14.4 13.9 12.5 3.5
22.4 24.0 27.1 28.2 26.E :8.3 20.9 14.3 24.3 24.2 30.3 30.1 28.2 28.9 22.3 17.8 24.2 22.5 21.6 12.6
-
50
60
-z-
70
80
90
120
z
Fig. 4. The tielght
ASPH !0.5 9.4 11.5 12.3 11.3 5.3 8.1 5.5 13.3 19.7 17.3 12.7 14.a 16.7 6.6 6.0 7.1 3.2 6.0 0 3
?ecovery yield
( )
91.9 92.8 39.5 91.' -39.5 -88.0 90.4 91.0 -32.9 35.3 86.4 86.2 ------
than r-C13
-
MONO + Dl
%----
Llquld
sitions of all fracttons do increase linearly with maturation. It should be noted that the relative degree of maturity estimated from the isotopic composition of one fraction for a given oil is consistent with the relative degrees estimated from other fractions. The isotopic composition of ASPH is plotted against the amounts of ASPH in Fig. 6. Here. the relationship is asymptotic. rather than inversely proportional as seen for the aromatic fractions. Not shown in the figure is the sample. No. 20. which is devoid of asphaltenes. This pseudo-first order tendency indicates the possibility of ASPH as being a source of other fractions. The isotopic composition of
voiring process of crude 011 IS carried out under a closed system or in an open system. because the sum of all fractions are calculated to be IOO~,,. In order to gain an insight into this question. isotopic information on each fraction is needed. The isotopic composition of SAT, MONO. DI and PAP are plotted as a function of their relative amounts in Fig. 5-A. B. C, D respectively. The isotopic composition of SAT increases with increasing amounts of the fraction, whereas opposite trends are found for all other fractions. Because the amounts of all other fractions decrease with increasing maturity of crude oil (increasing Z number. Fig. 4). the isotopic compo-
70-
Perfonndnce
k'eignt distribution
!'I,.,
DI
-28.4 -28.6 -28.3 -28.4 -28.8 -27.7 -28.4 -26.1 -28.8 -28.5 -29.1 -29.0 -29.0 -28.9 -27.6 -28.2 -27.9 -28.3 -28.1 -26.4
by Yigh
percentage of SAT. MONO.
DI. PAP and ASPH
LS Z number
Carbon isotope geochemistry of Paleozoic oils
1809
-26 /
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201
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-27;
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a s -
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~
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,’
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.3
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30
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IX)
50
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25
70
‘\
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1
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-\
‘\ .15\ 10 *I-’ 7 l’16
16 lt.6 \ \\*
-,,k
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‘2
l
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\
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\
.3
‘1
.I4
.12’\
\ \
*I3 .II
\
L-
IO
15
20
pAp
3O
,,:25
Fig. 5. The carbon isotopic composition vs the weight percentage for SAT (A). MONO (B). DI (C) and
PAP(D). ASPH becomes heavier with decreasing amounts of ASPH which in turn decreases with increasing maturity of the crude oil. The consistent internal variations of oil compositions and their isotopic values support that the Paleozoic oils from the Big Horn Basin are derived
from the single source origin. and also indicate that the isotopic flow follows a pattern of consecutive reactions starting from ASPH through various aromatics down to SAT. Thus. the isotope and mass flows will eventually culminate in natural gas. Therefore. the 3.6 per mil difference between the
-26,-
1 I I
-27 3 g E z -28cl, n C0 -29
I \ \ \ \ \ \
8.’ I I \ \ \ \ \ \
.6 \
\
‘\
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\ ‘\.I6 ‘\ 17 ‘1 \ I+*: ‘8 l 2 ‘+ . . \* -. 7y5 I -1 ,* 5’9 .3:_.12 .I3 AA --c I
1
‘O I
15
IO
I$---& *II
20
ASPH (%I Fig. 6. The carbon isotopic composition
of ASPH
vs the weight distribution.
25
35
1810
H. MOSESCHUNGet ui. I
-26
I
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3 S I5 bz
-27
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1
1’ ’ l2
,I’
/
. 1 -28
-29
-27
6’3C-S~T
I 1
-26
(x.)
Fig. 7. The calculated carbon isotopic composition of the combmed fracuon of MONO and DI rs that of SAT. after Fuex 11977).
most and least mature oils from the Big Horn Basin appears to be caused by the maturation effects alone. and more changes in isotopic composition are likely, since the most mature oil. No. 20, has an API gravity of only 38.9 (Table 2). The separation method employed here differs from the conventional ASTM method. Although some polar compounds in the crude oil stay in the SepPak column, some are coeluted with polyaromatics to form PAP. To find the isotopic composition of the aromatic fraction, calculations were made based on the weight distribution and isotopic composition of
MONO and DI. The calculated isotopic composition of the aromatic fraction is shown in Fig. 7 against that of SAT. About a 3 per mil variation occurs in the isotopic compositions of both fractions. and these variations are directly proportional to the maturity of the crude oil. Techniques used for grouping oils in this manner may be misleading without clear evidences that prove the variations of isotopic composition for the given oil type are relatively small (KOONS rr al.. 1974: RCE and POLITO.1979). Asphaltenes, according to definition. include two major classes of compounds. One class is the large
1
lWHOLE
i
TOPPED SAT
L-
MONO DI PAP ASPH
i -30
1
-29
-28
-27
-26
-
s13c (%.a) Fig. 8. The isotope-type-curve
for the selected Paleozox (1977).
011s from the BIG Horn Basin. after
STAHL
Carbon Isotope geochemistry of Paleozoic oils
non-polar molecules whose solubilities are significantly less than those of small molecules; the other is very polar molecules of medium size. Based on the traditional concept of the generation of petroleum. the degradation of kerogen. these two classes of compounds would show the closest similarity to the original precursors. Also. fractions of decreasing polarity would have progressively lighter isotopic compositions. The saturate fraction is considered to be products enriched with the lighter isotope the aromatic fraction as by-products enriched with the heavier isotope. and the polar and asphaltene fractions as remnants of starting materials. Therefore. the isotope-type-curve developed originally by STAHL (1977. 1978) is logical. However. the question is whether the mass flow and subsequent changes in isotopic composition among various fractions are fully understood not only during the genesis of petroleum but also during maturation. migration and many other alteration processes. Information on variations of compound grouptypes and their isotopic compositions for all the Big Horn oils are shown in Table 3 and only representative samples are shown in Fig. 8 in terms of the isotope-type-curve. The isotope-type-curves are not as simple as those previously reported by STAHL(1977. 1978, 1980). First, grouping oil types based on the similarities of isotope-type-curves which have a rather small variation of isotopic composition is difficult. Second, the isotopic composition of ASPH may not necessarily predict the isotopic composition of a source rock. This technique of oil-to-source rock correlation is more useful with low maturity crude oils.
Oil number
whose asphahenes show a better proximity to source rocks. Finally. the lighter isotopic composition of ASPH may not necessarily be explained by the secondary formation of asphaltenes. such as biodegradation. The gas chromatograms for oils Nos. 15 and 20 are shown in Fig. 9-A and B. They show no evidence of biodegradation although their isotope-typecurves are the most likely to be explained due to the biodegradation effects according to STAHL(1980). Natural processes, such as migration, maturation and other alteration processes affecting each fraction at varying rates. are thought to be involved in shaping the individual isotype-type-curve. For the Big Horn oils, definitive answers are not yet possible; only preliminary interpretations are made here. The isotopic composition of DI is lighter than that of SAT for samples 2, 5, IO. 12. 15 and 20 (Table 3), probably caused by isotopic fractionation of DI during the migration of crude oils. However, no geographic and stratigraphic generalities are found for these oils. Also, the isotopic composition of ASPH is heavier than that of SAT for samples 8, 1I and 14 (Table 3), probably caused by deasphalting process in the reservoir. The final and significant factor in shaping the isotope-type-curves may be the heterogeneous nature of the Phosphoria Shale itself. ORR (1974, p. 2301) reported that the mean isotopic composition of the Phosphoria Formation in Western Wyoming, Idaho and Montana as -28.3 per mil with a two-sigma range of 1.4 per mil. Research in these areas is currently in progress at the Bartlesville Energy Technology Center. The gross compositional variations and their iso-
20
RETENTION
1811
TIME
-
Fig. 9. The glass capillary gas chromatogram of a whole oil for oil No. 15(A) and 20(B). lsoprenoid alkanes are noted as ip in (A) and normal alkanes as numerals in (B).
H. Moses CHUNG
1812 I
/
I
et ui. /
I
1
-25
0 0
40
50
60
Fig. 10. The
carbon
70
80 z
90
100
Isotope grochemistrr of light hydrocarbons (LHC) The LHC associated with crude oil are generally believed to be the most sensitive to sample handling techniques and probably the least informative among the crude oil fractions. LHC are lost during production. shipping storing and handling of samples. Also.
the vapor pressure difference among LHC constttuents distort the compositional variation. Therefore, some researchers ignore the LHC components in characterizing crude oils (TISSOT and ROUCACHE, 1980), whereas others use them qualitatively (PHILIPPI, 1975: THOMPSON.1979) and quantitatively (YOUNGet al.. 1977). The detailed characterization of LHC in the Big Horn oils is found to be less informative than other oils from the Michigan basin. the Java Sea, and the Los Angeles basin (CHUNG and GRIZZLE. 1981). The preferential loss of LHC during degassing of oil has already affected even higher n-alkanes and isoprenoids. although effects of biodegradation and waterwashing may be locally operative. The LHC from the Big Horn oils were separated
/’ Sample smeller
which than
shows nC6
nC5
30
/’
Ftg. I I.
,
40
’
/’
/’
401
al
1’
60
70
2
00
90
:oo
,
1’ I
._>
/’
A__
50
120
isotopic composition of LHC vs 2 number.
topic values indicate that the Paleozoic oils in the Big Horn Basin are mainly influenced by maturation effects. However. the isotope-type-curves for these oils are found to be different from the commonly accepted idea. Variations of the isotope-type-curve may be influenced by the local characteristics of reservoirs and are believed to be a useful tool in understanding the natural transformation processes of petroleum.
$,
110
~_.~_
II3
2’)
130
The amount of LHC vs Z number. The amounts of LHC associated with the oil No. 20 IS assumed to be W’,, of the whole oil under the reservoir condition.
Carbon isotope geochemistq
and combusted in a vacuum system. The LHC fraction contains gaseous and light gasoline materials up through rl-C12 and n-C,,. The isotopic composition of the LHC fraction is plotted against the Z number in Fig. 10. Sample No. 10 is missing in the figure, because no carbon dioxide from this oil was sent for isotope analysis. Surprisingly. the isotopic composition of the LHC fraction increases with increasing maturity of crude oil. This is possible if the LHC associated with the crude oil are the same in quantity and isotopic composition as the in-place LHC in the reservoir, or if negligible isotopic fractionation occurs during degassing and volatilization. Which factor is dominant is believed to be affected by the maturity of oil and reservoir conditions. The maturity information obtained from the isotopic composition of the LHC fraction in the Big Horn oils suggests that the topping procedure of crude oil for oil-to-oil correlation may neglect one of the possible maturity parameters of the crude oil. The amount of LHC (Table 3) is shown in Fig. 11 as a function of Z number. Oils, of which gas chromatograms show the n-pentane peak smaller than the n-hexane peak. are marked with circles in the figure. Because the evidence for degassing and volatilization of LHC is strong. temporary linear lines are drawn to indicate the ranges of LHC contents for oils of a given maturity. This linear relationship of the amounts of LHC versus the maturity of crude oil may be an artifact, although oils of less maturity seem to fit the linear trend. For the oil No. 20. the amount of LHC is projected to be 48”,, of the whole oil. The large amount of LHC in the most mature oil is sufficient to change the physical properties of the whole oil. Such deasphalting processes may explain the quantity and isotopic composition of asphaltenes associated with the oil of the highest maturity. The deasphalting process is believed to precipitate the isotopically lighter asphaltenes preferentially. Assuming the oil. No. 20, has 48’; of LHC of -25.7 per mil in isotopic composition, the weight distributions of other fractions of this oil are recalculated from Table 3. Using the same isotopic values for each fraction. the isotopic composition of the in-situ reservoired oil will be. 48( - 25.7) + 35.5( - 26.4) +lO.O(-26.4) x-
+ 6.6(-27.1) = 100X
- 26.1
-26.1 per mil. which falls into a tolerable range in Fig. 3. Similar calculations for other oils, No. 6, 8 and 19. show similar results. Another exercise is to predict the amounts of methane formed during the maturation from the oil of Z number 40 to the oil of Z number 120. The isotopic composition of methane is assumed to be lighter than that of the whole oil of Z number 120 by 17 per mil (CHUNGand SACKETT. 1979: SILVERMAN, 1971). If the
of
1813
Pnleozoic oils
mean isotopic compositions for oils of Z number 40 and 120 are taken as -28.7 and -26.1 per mil respectively (Fig. 3). the amount of carbon converted into methane is expressed as lOO(-28.7) = X(-45.7)+-
(100 - X)(-26.1)
x = 13.3 Then 13.3”” of the total carbon has been converted into methane during the maturation from Z = 40 to Z = 120. Because one gram of oil gives 1500cc of carbon dioxide after a complete combustion (S,C = 80.4), 13.3”; of one gram of oil is equivalent lo 200 cc of methane per gram of oil, which in turn is equal lo 1120ft3 of natural gas per barrel of oil. This figure is reasonable for a well which will produce a mature oil. The above calculations are based on many assumptions; most importantly, a closed system is assumed in the reservoir. However, mass balances and subsequent isotope flows starting from ASPH lo natural gas are found to be ititernally consistent. The readjusted weight percentages of SAT to possible amounts of natural gas and LHC are found to be no longer proportional lo the Z number, indicating SAT as a source of natural gas and LHC. SUMMARY
Crude oil components from several representative oils separated in terms of boiling ranges by distillation and in terms of polarities by high performance liquid chromatography show various patterns of carbon isotope distributions. This carbon- 13 heterogeneous nature among components is believed lo be controlled by the geochemical history of a crude oil. Therefore, this distribution has a potential use in grouping and characterizing crude oil types. Twenty Paleozoic oils from the Big Horn Basin were fractionated into SAT, MONO, DI. PAP and ASPH. The amounts of SAT increase with increasing maturity. whereas the amounts of other fractions decrease. However. the isotopic compositions of all fractions become heavier with increasing maturity. The consistencies in internal variations of isotopic composition and varying oil composition for the Big Horn oils show that these oils have a single source origin and that maturation is the most important factor affecting the crude oil composition. Several observations are made: (1) Approximately 3 per mil difference in isotopic composition of the whole oil can be attributed to the maturation of a single type oil. (2) The isotopic composition of ASPH in the crude oil does not always show a proximity to that of the source rock. (3) The classification of crude oil types according to the isotope-type-curve is found to be not as simple as commonly believed. Other factors during maturation and natural transformation of crude oils control the pattern of the isotope-type-curve. (4) The continuous variation in the isotopic composition of
1814
H. MOSESCHUNGer
the least sensitive fraction. such as the aromatic fraction. argues against the general use of this fraction to classify oil types.
The isotopic composition of LHC associated with the Big Horn oils becomes proportionally heavier with Increasing maturity of the crude oil. Apparently no or very little isotope fractionation occurs during volatilization and degassing of LHC. This indicates that topping the whole oil to obtain more consistent isotopic information for oil-to-oil correlation may result in neglecting one maturity parameter of the crude oil. For some oils, the whole oil gas chromatograms show effects of degassing by having the n-C, peak smaller than the n-C, peak. Assuming the amount of LHC is directly proportional to the maturity of oil, calculations are made to predict the amounts of LHC and natural gas associated with crude oil of a given maturity in the reservoir. The conservative nature of mass balance and subsequent isotope flows among various fractions of crude oil show that the maturation of crude oil consists of consecutive processes leading from larger and more polar components into smaller and less polar components, culminating in natural gas. .-l~~rto~~/rdyr~~~rr~r.s-~ The authors would like to thank Dr Corp.. Field Research Lab.. Dallas for the crude oil samples and for many stimulating discussions on the crude oil geochemistry. Special thanks are due KAY PAYNE for her patience In typing this article. WILSON L. ORR. Mobtl Research and Development
REFERENCES BAILEYN. J. L.. ECANSC. R. and MILNERC. W. D. (197Ji .Applying petroleum geochemistry to search for oil: Examples from Western Canada Basin. Bull. ,4m. 4~s. Petrol. Geol. 58, 2284-2294. CHIJN~IH. M. and SACXETTW. %I. 11979) Use of stable carbon isotope composltions of pyrolytically derived methane as maturity indices for carbonaceous materials. Gu<,c~/rirn. C<~smochim..4cttr 43. I979- 1988. CHUS~; H. MM. and SATKETT W. M. (1980) Carbon Isotope effects during the pyrolytic formation of early methane from carbonaceous materials. Pray. Phw Chum. Eurth. in press. CHL.S(; H. M. and GRIZZLE P. L. 11981) Composittonal parameters of crude oil as studied by glass capillary gas chromatography. in preparation. CLAYTOS J. L. and SWETLANDP. J. ( 1980) Petroleum generation and migration in Denver basin. Bull. ‘401. A.+.\. Perrd. Gtd. 64; I61 3-1633. DAILEY J. E.. HIRXYHD. E.. COLEMAN H. J. and THOMPSON C. J. (1973) Compound-type separation and characterlzation studies for a 370 to 535 C distillate of Swan Hills (South Field). Alberta. Canada. crude 011. L S. Eur. .Miw\ Rep. Imust. 782 I. 30 pp. ERDMAUJ. G. and MORRtSD. A. 11974) Geochemical correlation of petroleum. Bull. Am. Ass. Perrol. Geol. 58, 28262837. EVANSC. R.. RO(;ERSM. A. and BAILEYN. J. L. (1971) Evolution and alteration of petroleum m Western Canada. Ckem. Grol. 8. I J7- 170. Fr EY ‘4. N. 119771The use of stable carbon Isotopes in
al.
hydrocarbon exploration. J. Grochem. E.rp/or. 7. 155-188. GREESJ. B. and HOFF R. J. (19801 Ltquid chromiitography on silica using mobile phases containing_ aliphatic car. boxylic acids 2. Application in fossil fuel characterization. J. Chromutoyr. 209. 231-250. HAHN-WEINHEIMER @. and HIRNER A. (1980) Chemical composition and stable carbon tsotopic ratios of crude 011sfrom Southern Germany. Org. Geochem. 2. 45-53 HIRSCHD. E., DOOLEYJ. E.. COLEMAXH. J. and THOMPSON C. J. t 1974) Compound-type separation and characterization studies for a 370 to 535 C distillate of Wilmington. Calif.. crude oil. U.S. Eur. .Mines Rrp. /west. 7893, 28 pp. J~UES T. S. and SMITHH. M. (1965) Relationships of oil composition and stratigraphy in the Permian basm of W. Texas and New Mexico. In Fluids in Subsurface Endronments-a 4 . 101-224.
Symposium.
Am. .4s.s. Petrol.
Geol. Mem.
KHANS C. B.. BO%DJ. G. and PIERCEF. L. (19741 Effects of
depositional composition
and postdepositional history on chemical of Lower Tuscaloosa oils. Bull. .-lm. Ass. Perrol. Geol. 58. 1272-1280. KVENVOLDEN K. A. and SQUIRESR. M. (1967) Carbon isotopic composition of crude oils from Ellenburger group (Lower Ordovician). Permian basin. West Texas and Eastern New Mexico. Bull. 4m. Ass. Petrol. Geol. 51. 1293-1303. MILSER C. W. D.. ROGERSM. A. and EVANSC. R. (19771 Petroleum transformatton in reservoirs. J. Geochem. E.up/or. 1, 101-153. ORR W. L. (19741 Changes m sulfur content and Isotopic ratios of sulfur durmg petroleum maturation-study of BIG Horn basin Paleozoic oils. Bull. Am. 4~s. Petrol. Gee/. 58, 2295-23 18. Pt-nLtppt G. T. (1975) The deep subsurface temperature controlled origin of the gases and gasoline-range hydrocarbons of petroleum. Grochim. Cosmochim.. Acta 39, 1353-1373. ROE G. D. and POLITOL. J. (1979) Source rocks for oils m the Ardjuna subbasin of the northwest Java basin. Indonesta. c..V.. ESC.4P.. CCOP 7&h. Pub/. No. 6. 180-194. SVHOELL.M. and MILNERC. W. D. I198 1I Isotopic crude oil correlation using ‘3C,1zC and D/H variations in compound classes of petroleum. Bull. .4tn. Ass.. Petrol. Geol.. in press. SHELDONR. P (1967) Long-distance migratron of 011 in Wyoming. .Mr. Geol. 4. 53-65. SLLVERMAN S. R. (19671 Carbon isotope evidence for the role of lipids in petroleum formation. J. .4m. Oil Chem. SOC. 44, 691-695. SILVERMAY S. R. (19711 Influence of petroleum origm and transformation on Its dtstribution in sedimentary rocks. Proc. 8th World
Petrol.
Conyr.
2. 47-54.
STAHLW. J. (1977) Carbon and mtrogen rsotopes m hydrocarbon research and exploration. Chem. Geol. 20, 121-~149. STAHLW. J. (1978) Source rock-crude oil correlatron by isotope typ-curves. Grochim Cosmochirn. .4cru 43, 1573-1577. ST..\HLW. J (19801L’be of “C “C analyses for the tdentlticatIon of the degradation of hydrocarbons by aerobic bacteria. Geockim. C/,\moc,/!irn. .4cru 44, 1903-1907. STONE D. S. (19671 Theory of Paleozoic oil and gas accumulation in Big Horn basm. Wyoming. Bull. Am. .4ss. Perrd
G~w/ 51, 205621
14.
SL TTO%C. (1979) Deposnlonal envtronments and their relation to chemical composition of Java Sea crude oils. L,‘..V. ESC.4 P. CCOP 7&h. Puhl. No. 6. l6FI 79. THOMPSONK. F M. (1979) Light hydrocarbons of petroleum: Internal evtdence of thermal history. Bull. .4m. .-l.s.~.Perrd. GM/. 63, 540. TISSOT B. P. and WELTED. H. ( 1978) Perrdwm Formtrriorr irntl 0~ (‘urrr)ict’. Springer.
Carbon TISS~T B. P. and ROL~CATHEJ. (1980) Principles
isotope
geochemistry
of a genetic
classification of crude oils. In F~rcrs and Principles cf. Uiwld Pcwolrtrr~~ O~~~rence (ed. A. D. Miall). Can. Sot.
Petrol. Gcol. Ment. 6. 209-217. WELTE D. H.. HAGEMAS H. W.. HOLLERBACH A.. LEYTHAEI’SER D. and STAHL W. (1975) Correlation between petroleum and source rock. Proc. Yrh World Perrol. Conqr. 2, I79- 19 1.
of Paleozoic
oils
1815
WINTERS J. C. and WILLIAMS J. A. (1969) Microbial alteration of crude oil in the reservorr. Srrnp. Pvtro/c,rrrt~ Transfiwn~ariorts it1 Geologic Enr.irorl,ivrlr.\. .4rn. C/tern. Sot. Mrrr. New York. Sept 7-12. 1969. E22 E31. YOCNG A.. MONAGHAK P. H. and STH\VI.ISRFRFT.R R. T. (1977) Calculation of ages of hydrocarbons m oils-phyca! chemistry applied to petroleum eeochemistq I. Bull. .4rn. 4.0. Perrol. Gcol. 61. 573-600.
Abstract--Twenty crude oils from Paleozoic reservoirs in the Big Horn Basin. Wyoming were fractionated into light hydrocarbons. saturates. monoaromatics. diaromatics, polyaromatics-and-polars. and asphaltenes. Amounts and isotopic composition of each fraction were found to be internall) consistent with the degree of maturation of the oil. confirming the established single source origin for these oils. A variation of approximately three per mil in the carbon isotopic composition of the whole oil and individual fraction was explained as being caused by maturation. The isotope-type-curves for these oils, the variations in isotopic composition as a function of compound group-type. were not found to be as simple as commonly believed. Other alteration processes. such as migration and deasphalting. might have influenced the pattern of the isotope distributions among fractions. The conservative nature of mass balance and subsequent isotope flows among various fractions of crude oil showed that the maturation of crude oil consists of consecuGve processes leadmg from larger and more polar components into smaller and less polar components culminating in natural gas. Calculations were made to project the amount of condensates associated with the crude oils. and the amount of natural gas which had been generated from crude oils of a given maturit).
INTRODUfflON VARIATIONSin stable carbon
isotopic composition have been useful in understanding the origin of petroleum. and relating oil-to-oil and oil-to-source rock in petroleum geochemistry. Principles of these and other applications have been described by FUEX (1977) and STAHL (1977). The variations in the isotopic composition among petroleum molecules and fractions of varying boiling ranges have been important in the understanding of petroleum genesis (SILVERMAN.1967. 1971). The differences in isotopic compositions among organic materials from varying depositional environments. along with those among plants of varying photosynthetic mechanisms have been used in determining the original source materials in petroleum (KCI~NScr al., 1974; SUTTON,1979). The close relationship between the isotopic composition of petroleum and that of the source rock has been the underlying principle for oil-to-source rock correlations (WELTEet al., 1975: STAHL 1978). Finally. the similarity in isotopic composition for petroleums that have originated from the same source materials has been used in oil-to-oil correlations (ERDMAN and MORRIS. 1974: HAN-WEINHEIMER and HIRNER. 1980: CLAYTONand SWETLAND,1980). In oil-to-oil correlations, the number of oil types that are present in a given basin is often controversial. For example. JONESand SMITH (1965) reported five types of oils in the Permian basin, West Texas and * Present address: Union Oil Co.. Union Science and Technology Diviston. 376 S. Valencia Ave.. Brea. CA 92621. U.S.A.
New Mexico. by using the correlation index of petroleum fractions. whereas KVENVOLDEN and SQUIRES (1967) found only three types of oil by using the stable carbon isotopic composition of whole oils. This confusion on the number of oil types is often caused by the researcher’s objectives in defining oil types and by a lack of knowledge concerning the effects of maturation and other alteration processes on crude oil composition. In this paper. therefore, a single oil type is defined to include those which originated from the same source materials but might have undergone varying degrees of natural transformation processes, such as maturation. migration. biodegradation, etc. These secondary alteration processes have been described by WINTERS and WILLIAMS(1969). EVANSer al. (1971). BAILEYer al. ( 1974) and MILNER er al. (1977). For a given oil type. the internal variations in crude oil composition may drastically change the isotopic composition of a whole oil (FUEX, 1977). Therefore, it is a common practice to use a topped crude oil instead of a whole crude oil to obtain better comparative data. isotopic composition of a topped crude oil is largely based on the traditional use of the isotope techniques which yields an averaged and integrated isotopic information. Therefore. the final and conclusive answer may not be immediately possible from the isotope data alone, and further supporting evidence from other analytical methods are often necessary. Two recent developments in isotope geochemistry have helped to overcome this difficulty: one is the use of hydrogen isotopes in addition to those of carbon. and the other is the use of separated fraction. such as the aromatic fraction, which is more resistant to
1803
H. MOSESCHUNG er ul
1804
natural alteration processes and less sensitive to sample handling techniques WHOELL and MILNER, 1981). Recently. the principle of carbon-13 heterogeneity In natural organic materials has been used to under-
stand maturation of carbonaceous materials (CHUNG and SACKETT,1979) and the formation of natural gas (CHUNGand SACKETT, 1980). This principle may well be applied to understand the geochemistry of crude oil. The effects of internal variations of isotopic composition and varying oil composition are caused by the natural history of crude oil, and, therefore, should be used in understanding the geochemistry of crude oil. To demonstrate this thesis, twenty crude oils were selected from the Big Horn basin, which are believed to have a single source origin but to differ widely in maturity (ORR. 1974). We will demonstrate how the isotopic flow among fractions of varying polarities would proceed with increasing maturity of oil. We will then try to interpret the pattern of the so-called isotope type curve (STAHL. 1977. 1978). And, finally, we will show what kind of useful information can be obtained from the most sensitive fraction of oil, the gasoline-range hydrocarbons. The geochemistry of the Paleozoic oils in the Big Horn basin has been worked out in detail by previous workers (STONE.1967: ORR. 1974). For the purpose of this paper. a brief geological history is summarized here. Relatively immature oils which were derived from the Phosphoria shale of Permian age migrated by jurassic time into stratigraphic traps in areas that later were folded into the Big Horn and the Wind River Basins (SHELDON,1967). From Early Jurassic until Late Cretaceous, the oils were subjected to a similar thermal history. Secondary migration of the oils occurred during the Laramide orogeny which created the Big Horn Basin in Paleocene time. Subsequent to the Laramide folding, the oils were subjected to different depth and thermal histories. In short. the present compositional variations in the Paleozoic oils of the Big Horn Basin were largely caused by thermal maturation of primary oils which were originally derived from the Phosphoria Shale. PROCEDURES
A weighed amount of crude oil was placed m a vacuum oven at 60 C overnight to remove materials up to approximately n-decane The amounts of light hydrocarbons were calculated from the weight differences. Asphaltenes (ASPH) were obtained by precipitation with pentane (oil:pentane = 1: 50). Removal of the pentane yielded the asphaltenefree oil containing materials higher than approximately n-dodecane. The amounts of ASPH were calculated from the weight differences. The asphaltene-free oil was passed
through a silica cartridge Sep-Pak (Waters Associates) to remove polar components. No further work was carried out on these small amounts of polar materials. The resulting hydrocarbons were fractionated mto saturate (SAT), monoaromatic (MONO). diaromatic (DI). and polyaromatic-and-polar (PAP) fractions by preparative high perform-
ante liquid chromatography. The asphaltene-free 011 \\as loaded onto a Woelm alumina column ISS-2ooO-C.I.25 in. o.d. ‘x I2 m. length) in series with a silica .ge) column [SS-2000-C. 1.25 m. o.d. x 12 in.. length). The mrtlal eluent was 1.5”,, methylene chloride in pentane for elution of SAT fraction. 4 linear gradient from 1.5 to XI”,, methylene chloride m pentane followed to elute the MONO and DI fractions, and the PAP fraction was eluted using 75”,, methylene chloride in pentane durmg a backflush of the alumina column. Following solvent removal. the amounts of each fraction were calculated. Sepururlon und conrhurion (?t liyhr h!~drocurhons (LHCI
The separation and combustion of LHC \*ere carried out by fractlonal distillation similar to that of CHUNG and SACKETT(1979). The mildly heated oil was expanded mto two traps cooled with ice-water. The noncondensable gases were transferred by a Toepler pump into a combustion system via one additional trap cooled with ice-water. Transfer of LHC into the combustion system was monitored by the vacuum gauge. The carbon dioxide formed was purified and sealed in a Pyrex tube for isotopic analysis. A gas chromatogram of the residue of fractional distdlation showed that the amount of n-tridecane was approxlmately half of the combined quantities of n-dodecane and !I-tetradecane. Isotope onalysis The combustion of most samples and subsequent isotopic analyses of carbon dioxide were performed by Coastal Science Laboratory. Port Aransas. Texas and the Isotope Geochemistry Laboratory. University of South Florida. St Petersburg, Florida. To avoid the posstbie discrepancy m the isotopic composition of each laboratory standard. all samples used in specific studies were sent to the same place. Isotopic data in Table I were from the University of South Florida. and data in Table 3 were from the Coastal Science Laboratory. The mean differences in isotopic compositions of seven samples were found to be 0.22 per mil between these laboratories. The isotopic data are presented in the usual 6 notation
and related to the Chicago PDB standard
RESULTS AND DISCUSSION Internul
au%Uion
of isotopic
composition
and rurGxq
oil composition
SILVERMAN (1967. 1971) measured carbon isotopic compositions of n-alkanes and distillation fractions from several crude oils and found isotopic evidences concerning the genesis of petroleum. Sixteen fractions from the Bureau of Mines Hempel distillation were obtained from four crude oils reservoired in carbonate rocks and their isotopic compositions are plotted in Fig. 1 as a function of the average distillation temperature. The oil from the South Swan Hills (Devonian, Alberta) shows a rather smooth distribution in isotopic composition. whereas the other oils show large variations among fractions. The oil. 76048 (Ordovician, Michigan: The oil number is from the Crude Oil Data Bank at Bartlesville Energy Technology Center), shows the heaviest isotopic composition in the range of octane to octadecane (12@300 C). The oil. 75051 (Silurian. Michigan),
1805
Carbon isotope geochemistry of Paleozoic oils -25
I
1
Vacuum
-31 0
I 100 AVERAGE
I 200 DISTILLATION
1
dlsttllation
1 300 400 TEMPERATURE,‘C
Rerll
ml
Fig. 1. The carbon isotopic composition of the fraction vs the average distillation temperature. The sample number is from the Crude Oil Data Bank at the Bartlesville Energy Technology Center.
shows a continuous decrease in isotopic composition with increasing distillation temperature. The oil, 75061 (Devonian, Michigan). has generally isotopically lighter fractions, although fractions of middle distillation cuts are isotopically heavier. Trends in the figure suggest that different oil types may have different isotopic variations. It is also noted that topping a whole oil for isotopic measurement is not always necessary. as observed here for the South Swan Hills oil and more for the Big Horn oils of low maturity in the following section. In the past years. the Bartlesville Energy Technology Center under the API Project No. 60 characterized in detail several representative crude oils. The first step was to distill the crude oils into three boiling-range fractions. 37&535 . 535-675 and >675’C. Each fraction was further separated into six fractions. saturates (SAT). monoaromatics (MONO), diaromatics (DI)., polyaromatics (POLY). bases and acids. In addition. the resins in the 675’C residuums were separated into strong bases, weak bases, strong acids and weak acids. The separation procedure and detailed chemistry is beyond the scope of this paper. and can be found elsewhere (DOOLEYet al.. 1973; HIRSCHer ul.. 1974: GREENand HOFF, 1980). The isotopic compositions for these fractions are of interest to study the heterogeneous nature of carbon-l 3 distribution among fractions of varying polarities. Two oils were chosen for this study. the oil from South Swan Hills and the oil from Wilmington (Miocene. California). The gas chromatogram and correlation index of the South Swan Hills oil show it to be
a paraffinic oil (TISSOTand WELTE,1978). whereas the Wilmington oil shows sign of mild biodegradation (pristanejn-C,, = 1.58, phytanein-C,s = 1.32). The analytical information is given in Table 1. and the isotopic data are plotted in Fig. 2. Some important generalizations can be made and a few interesting research areas may also be suggested. First. the isotopic composition becomes heavier with increasing polarity or polarizability of the fraction in the order of SAT. MONO, DI, POLY. bases and acids. For the Wilmington crude. the isotopic compositions of acids and bases deviate from this trend. More noticeable is that the isotopic composition of acids is lighter than that of bases. This may be explained by the subsurface oxidation of the crude oil; more carboxylic functional groups are found as compared to most crude oils (GREENand HOFF, 1980). Or, it may be caused by the secondary asphaltenes formed during biodegradation (STAHL.1980). Second, the isotopic compositions of fractions in the lower boiling-range are heavier than those of fractions in the higher boiling-range. This behavior is obvious for the South Swan Hills because of the maturity of the crude oil. The SAT fraction from the Wilmington deviates from this trend, that is, smaller molecules are isotopically lighter. An explanation probably lies in the immature nature of this oil. or in the selectivity of biodegradation toward this fraction. The above explanations are preliminary; no generalizations can be made from only two oils. However. it is important to point out the heterogeneous nature of carbon-13 in crude oil fractions separated by polarity
1806 Table 1. Variation of compound classes by Ylgh Performance ilquid Chromatography and isotope balance. The positive numbers refer to weight distribution in percentage. The negative numbers refer to stable carbon isotopic composition in per mil.
Wilmington -22.5
370-535'C 24.7, -22.0
South
SAT. YONO. DI. POLY.
Hydrocarbons -22.9
35.9,
-24.2
16.3, 11.4, 26.4, 4.6, 4.6.
-22.8 -22.7 -22.4 -21.2 -22.1
15.7, 9.2, 15.8, 47.6. 10.3; 5.4,
-23.0 -22.8 -22.5 -21.6 -21.6 -22.7
2.3, 1.6, 3.0, 38.3, 39.8, 8.7. 4.0; 4.2,
-23.4 -23.1 -22.7 -22.5 -22.1 -22.2 -23.2 -22.5
Bases Acids
535-675'C 14.5. -22.4
SAT. MONO. 01. POLY.
Hydrocarbons -22.5 Bases Acids
.675'C 22.1,
SAT. MONO. DI. POLY.
Hydrocarbons -22.5 -22.2 Strong Bases Weak Bases Strong Acids Weak Acids
SAT. MONO.
Hydrocarboos -28.1
370-535-c 18.4, -28.1
Swan -28.2
U‘
POLY. Bases Acids SAT. Hydrocarbons -28.1
535-675'C 6.0. -28.0
MONO. 01. POL".
Bases Acids SAT. MONO. 01. POLY.
Hydrocarbons -28.2
>675'C 3.0. -28.1
Strong Bases Weak Bases Strong Acids sieak Acids
Hills
64.7. 11.1. 5.1; 17.6, 1.4, 1.6,
-28.4 -27.5 -26.7 -26.8 -26.3 -26.6
46.5, 8.5, 9.4, 29.1, 4.4, 2.1,
-28.6 -28.2 -27.4 -27.3 -27.i -27.3
4.e. 9.5, 9.8, 49.0, 18.6. 2.5, 3.2, 2.4,
-29.1 -28.5 -28.6 -27.4 -27.6 -27.5 -27.5 -27.5
and boiling point. It is also important that the mode of heterogeneity is believed to be associated with the geochemical history of the crude oil.
ameter, the Z number. which was defined by ORR (1974) as:
Curbon
where Z is a maturity parameter. API is the API gravity of oil and S’N is the weight ratio of sulfur vs nitrogen. Other maturity parameters are possible but the Z number is found to be satisfactory and is used in this paper. It IS emphasized, however, that any maturity parameter deals with chemical changes occurring during the maturation of crude oil. Therefore, the compositional changes of the crude oil during maturation may not necessarily relate linearly to the given maturity parameter. The relationship of the isotopic composition of the whole oil versus the Z number in Fig. 3 shows that an overall isotopic enrichment occurs with increasing degree of matur-
isotope yeochernistry
the Big Horn
oj‘ the Ptrleozo~
oils jkorn
Basin
ORR (1974) has reported the geochemistry of the Paleozoic oils from the Big Horn basin. He concluded that the compositional changes of the Big Horn oils were mainly controlled by maturation effects and that other alteration processes such as water-washing and biodegradation were minor except for a few of the heaviest oils. To further verify these conclusions, twenty crude oil samples were obtained and selected information for each sample is shown in Table 2. The isotopic composition of a whole oil, not a topped oil. is shown in Fig. 3 as a function of the maturity par-
Z = O.W(API
) + 1.3J(S/N) + 15.1,
370-535=x ’ 535-675°C A B675”C
l
L
/ -29
S Swan
1
-28
Wllmlnaton
Hills
-27
,
/
-26
-25
6’3C Fig. 2. The carbon
1
1
-24
-23
-22
(%.)
isotopic composttion of compound group-t)pe of var! mg bodtng ranges separated from the South Swan Hills and Wilmington crude.
Carbon Table 2.
Sample No.
Mobil sample No.
isotope geochemistry
of Paleozoic
1807
oils
Information on Big Horn Paleozoic Oils, from 01-r(1974).
Reservoir formation and age
Field
M3723 M3145 M3728 M3727 M3144 M3918 M3722 M3464 M3146 M3726 M3463
Phosphoria-Permian Tensleep-Penn Tensleeo-Penn Tensleep-Penn Tensleep-Penn Phosphoria-Permian Tensleep-Penn Tensleep-Penn Tensleep-Penn Phosphoria-Permian Tensleep-Penn
M3729
Tensleep-Penn
H3730
Phosphoria-Permian
M3734 M3921 Ml44 M2B6 M3919 M3169 Ml66
Phosphoria-Permian Tensleep-Penn Phosphoria-Permian Tensleep-Penn Phosphoria-Permian Tensleep-Penn Phosphoria-Permian
Slick
API gravity
2 number
10580
30.0 34.0
54 53
27.2 25.0 25.0
:; 49
32.7 31.9
:;
38.3 25.4 21.7 21.0
:: :;
-28.1 -29.1 -28.4 -28.4 -28.4 -27.5 -28.3 -26.7 -28.7 -28.2 -29.9
23.1
44
-29.1
3392
23.5
44
-29.0
3744 2700 9970 2705 7133 3013 8520
21.2 .28.8 32.9
41 70 85
-29.0 :;;.;
Creek
Elk Basin Elk Basin Frannie Frannie Manderson Neiber Southeast Golden Eagle Elk Basin South Garland Oregon Basin South Oregon Basin North Oregon Basin North Hamilton Dome Bonanza Worland Bonanza Manderson Torchliqht Silvertip
The difference in isotopic composition between the least and most mature oil was 3.6 per mil as reported by ORR (1974. p. 2302). FUEX (1977. p. 163) stated “This wide range in isotopic composition may not be the result of maturation effect alone.” Whereas in reviewing KVENVOLDEN and SQUIRES (1967) work on the Ellenburger (Lower Devonian) oils in the Permian basin. FUEX (1977. p. 168) suggested that a normal oil and a condensate formed by thermal cracking of that oil may differ greatly in carbon isotopic composition. by saying “The result is that oils probably having a common source have isotopic values which differ by as much as 7 per mil.” How wide the range of isotopic composition should be for a given oil type is not well understood. The results of high performance liquid chroma-
ation.
-25
Depth in feet
I
5300 6494 3920 2565 7549 10320 9600 7000 4360 3650
3474
31.3 33.2
Es6 1:;
29.2 38.9
6i3C-oil("/,,)
-2714 -27.6 -27.6 -25.7
tographic separation of the asphaltene-free samples are shown in Table 3. The weight distribution for saturates (SAT), monoaromatics (MONO) and diaromatics (DI), polyaromatics-and-polars (PAP) and asphaltenes (ASPH) is shown in relation to the Z number in Fig. 4. The MONO and DI are combined in the figure, because the sum of these fractions is generally small and varies inconsistently. Very small changes are observed in the compositions between Z = 54 and Z = 68. However. it is apparent that the amounts of SAT increase at the expense of other fractions as the maturation of crude oil progresses. The SAT formed will eventually be the source of condensates and natural gas associated_ with the reservoired crude oil, if the trap condition is feasible. This poses the. question as to whether the maturation or reser-
I
I
I
I
-26/-
. w
I
.
.
l
. . 1
.
. .. .
. .
l
Fig. 3. The carbon
I 40
I 50
.
. . .
.
.
-30
.
I 60
I 70 2
Samples in this paper
I 80
-I
I 90
isotopic composition of the Paleozoic oils from the Big Horn parameter. Z number. All data are from ORR (1974).
Basin vs the maturity
1808 Table 3. Variation of compound classes Chromatography and isotope balance.
Isotopic Sample No.
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 i6 17 18 19 20
*Light
Topped crude -28.3 -28.2 -26.4 -28.4 -28.7 -27.6 -28.4 -26.: -28.7 -28.5 -29.1 -29.1 -29.1 -28.9 -27.8 -28.1 -28.0 -28.2 -28.0 -26.2
LHC*
SAT
-27.2 -27.7 -28.0 -27.8 -28.0 -26.5 -27.8 -27.2 -28.4
-28.4 -28.0 -28.3 -28.4 -28.6 -27.5 -28.6 -26.3 -28.8 -28.2 -29.4 -28.8 -29.! -29.0 -27.6 -28.2 -28.0 -28.2 -28.2 -26.4
-28:8 -28.5 -28.4 -28.6 -26.6 -26.2 -26.5 -26.7 -26.6 -25.7
hydrocarbons
composition
MONO
(LHC) generally
PAP
-27.9 -28.3 -28.2 -28.2 -28.7 -27.3 -27.0 -25.7 -28.3 -28.3 -29.2 -28.9 -29.0 -28.3 -28.0 -27.9 -27.7 -28.2 -27.4 -26.5
include
ASPH
-23.0 -28.4 -28.7 -28.2 -28.1 -28.1 -28.3 -26.3 -28.8 -28.6 -29.3 -28.9 -29.1 -28.9 -27.7 -28.0 -28.2 -28.4 -27.9 -27.1
compounds
-23.9 -28.6 -29.0 -28.6 -29.0 -27.6 -28.7 -26.4 -28.9 -28.9 -29.0 -29.1 -29.1 -28.9 -28.8 -28.2 -28.4 -28.6 -28.5 --
less
LHC’ 12.2 8.3 lo.? 10.4 8.4 15.3 i2.3 9.9 7.3 i.9 5.3 9.! 6.7 6.6 17.5 25.1) 24.7 21.: 4.5 11.8
SAT
47.7 48.6 50.1 39.2 40.7 53.1 50.4 65.2 48.5 42.i 34.9 35.5 33.3 37.3 45.9 53.6 46.3 48.0 51.6 58.2
bicw i5.3 !j.O IO.4 15.1 15.2 14.4 15.3 11.6 13.9 16.4 ‘6.2 i5.6 16.2 16.1 15.3 13.9 15.3 14.6 14.3 !0.7
(-,',j
01
PAP
14.6 i2.4 12.4 17.6 17.3 !4.2 13.4 8.9 i3.3 17.3 X.0 18.8 16.7 17.8 i6.5 14.8 14.4 13.9 12.5 3.5
22.4 24.0 27.1 28.2 26.E :8.3 20.9 14.3 24.3 24.2 30.3 30.1 28.2 28.9 22.3 17.8 24.2 22.5 21.6 12.6
-
50
60
-z-
70
80
90
120
z
Fig. 4. The tielght
ASPH !0.5 9.4 11.5 12.3 11.3 5.3 8.1 5.5 13.3 19.7 17.3 12.7 14.a 16.7 6.6 6.0 7.1 3.2 6.0 0 3
?ecovery yield
( )
91.9 92.8 39.5 91.' -39.5 -88.0 90.4 91.0 -32.9 35.3 86.4 86.2 ------
than r-C13
-
MONO + Dl
%----
Llquld
sitions of all fracttons do increase linearly with maturation. It should be noted that the relative degree of maturity estimated from the isotopic composition of one fraction for a given oil is consistent with the relative degrees estimated from other fractions. The isotopic composition of ASPH is plotted against the amounts of ASPH in Fig. 6. Here. the relationship is asymptotic. rather than inversely proportional as seen for the aromatic fractions. Not shown in the figure is the sample. No. 20. which is devoid of asphaltenes. This pseudo-first order tendency indicates the possibility of ASPH as being a source of other fractions. The isotopic composition of
voiring process of crude 011 IS carried out under a closed system or in an open system. because the sum of all fractions are calculated to be IOO~,,. In order to gain an insight into this question. isotopic information on each fraction is needed. The isotopic composition of SAT, MONO. DI and PAP are plotted as a function of their relative amounts in Fig. 5-A. B. C, D respectively. The isotopic composition of SAT increases with increasing amounts of the fraction, whereas opposite trends are found for all other fractions. Because the amounts of all other fractions decrease with increasing maturity of crude oil (increasing Z number. Fig. 4). the isotopic compo-
70-
Perfonndnce
k'eignt distribution
!'I,.,
DI
-28.4 -28.6 -28.3 -28.4 -28.8 -27.7 -28.4 -26.1 -28.8 -28.5 -29.1 -29.0 -29.0 -28.9 -27.6 -28.2 -27.9 -28.3 -28.1 -26.4
by Yigh
percentage of SAT. MONO.
DI. PAP and ASPH
LS Z number
Carbon isotope geochemistry of Paleozoic oils
1809
-26 /
/A/
/’
201
/’
-27;
t
a s -
/
/
,,.’
l
,:‘4”
~
/’
-29;
,’
.I2 .I4
I’
I’ . 20
,
19 0,:
.3
/
/
/
/
/
1’
q
i
1
16””
* “6.7’
.5
/’
~
.’
17..2
)
.8
/’
,,*I5
t ln 0 -2t3+
Ico
/
I
I
l
1’
I /
.9/’ *I3 /
-1
/’
,/
II
/’ J
30
40
SAT
IX)
50
60
25
70
‘\
i
\
\
1
I \
\
\
\
\
-\
‘\ .15\ 10 *I-’ 7 l’16
16 lt.6 \ \\*
-,,k
‘, 1.7 . 5 .4
‘2
l
. IO ‘\‘9
1.
\
\
I ‘\
\
.3
‘1
.I4
.12’\
\ \
*I3 .II
\
L-
IO
15
20
pAp
3O
,,:25
Fig. 5. The carbon isotopic composition vs the weight percentage for SAT (A). MONO (B). DI (C) and
PAP(D). ASPH becomes heavier with decreasing amounts of ASPH which in turn decreases with increasing maturity of the crude oil. The consistent internal variations of oil compositions and their isotopic values support that the Paleozoic oils from the Big Horn Basin are derived
from the single source origin. and also indicate that the isotopic flow follows a pattern of consecutive reactions starting from ASPH through various aromatics down to SAT. Thus. the isotope and mass flows will eventually culminate in natural gas. Therefore. the 3.6 per mil difference between the
-26,-
1 I I
-27 3 g E z -28cl, n C0 -29
I \ \ \ \ \ \
8.’ I I \ \ \ \ \ \
.6 \
\
‘\
-1
\
\
\ ‘\.I6 ‘\ 17 ‘1 \ I+*: ‘8 l 2 ‘+ . . \* -. 7y5 I -1 ,* 5’9 .3:_.12 .I3 AA --c I
1
‘O I
15
IO
I$---& *II
20
ASPH (%I Fig. 6. The carbon isotopic composition
of ASPH
vs the weight distribution.
25
35
1810
H. MOSESCHUNGet ui. I
-26
I
I
-
3 S I5 bz
-27
/
-
? /
5 = 0 + g -26tii 0 ” M -29
-/
/
/
/
/
/
/
/
/ /7 / .
/:9
/
I 3. ’ .I1
.
.
/
912 /
L
.6
I 4:*31E? :I0
l= /
/
’
17
.
15
/
/
/
/
/
I
/‘8
.
/
.‘6
.9 14
/
/
/
/’
/
/
/
/
/
/
/
/
/
/
/
20,/’ /
/
/’
I !
1
1’ ’ l2
,I’
/
. 1 -28
-29
-27
6’3C-S~T
I 1
-26
(x.)
Fig. 7. The calculated carbon isotopic composition of the combmed fracuon of MONO and DI rs that of SAT. after Fuex 11977).
most and least mature oils from the Big Horn Basin appears to be caused by the maturation effects alone. and more changes in isotopic composition are likely, since the most mature oil. No. 20, has an API gravity of only 38.9 (Table 2). The separation method employed here differs from the conventional ASTM method. Although some polar compounds in the crude oil stay in the SepPak column, some are coeluted with polyaromatics to form PAP. To find the isotopic composition of the aromatic fraction, calculations were made based on the weight distribution and isotopic composition of
MONO and DI. The calculated isotopic composition of the aromatic fraction is shown in Fig. 7 against that of SAT. About a 3 per mil variation occurs in the isotopic compositions of both fractions. and these variations are directly proportional to the maturity of the crude oil. Techniques used for grouping oils in this manner may be misleading without clear evidences that prove the variations of isotopic composition for the given oil type are relatively small (KOONS rr al.. 1974: RCE and POLITO.1979). Asphaltenes, according to definition. include two major classes of compounds. One class is the large
1
lWHOLE
i
TOPPED SAT
L-
MONO DI PAP ASPH
i -30
1
-29
-28
-27
-26
-
s13c (%.a) Fig. 8. The isotope-type-curve
for the selected Paleozox (1977).
011s from the BIG Horn Basin. after
STAHL
Carbon Isotope geochemistry of Paleozoic oils
non-polar molecules whose solubilities are significantly less than those of small molecules; the other is very polar molecules of medium size. Based on the traditional concept of the generation of petroleum. the degradation of kerogen. these two classes of compounds would show the closest similarity to the original precursors. Also. fractions of decreasing polarity would have progressively lighter isotopic compositions. The saturate fraction is considered to be products enriched with the lighter isotope the aromatic fraction as by-products enriched with the heavier isotope. and the polar and asphaltene fractions as remnants of starting materials. Therefore. the isotope-type-curve developed originally by STAHL (1977. 1978) is logical. However. the question is whether the mass flow and subsequent changes in isotopic composition among various fractions are fully understood not only during the genesis of petroleum but also during maturation. migration and many other alteration processes. Information on variations of compound grouptypes and their isotopic compositions for all the Big Horn oils are shown in Table 3 and only representative samples are shown in Fig. 8 in terms of the isotope-type-curve. The isotope-type-curves are not as simple as those previously reported by STAHL(1977. 1978, 1980). First, grouping oil types based on the similarities of isotope-type-curves which have a rather small variation of isotopic composition is difficult. Second, the isotopic composition of ASPH may not necessarily predict the isotopic composition of a source rock. This technique of oil-to-source rock correlation is more useful with low maturity crude oils.
Oil number
whose asphahenes show a better proximity to source rocks. Finally. the lighter isotopic composition of ASPH may not necessarily be explained by the secondary formation of asphaltenes. such as biodegradation. The gas chromatograms for oils Nos. 15 and 20 are shown in Fig. 9-A and B. They show no evidence of biodegradation although their isotope-typecurves are the most likely to be explained due to the biodegradation effects according to STAHL(1980). Natural processes, such as migration, maturation and other alteration processes affecting each fraction at varying rates. are thought to be involved in shaping the individual isotype-type-curve. For the Big Horn oils, definitive answers are not yet possible; only preliminary interpretations are made here. The isotopic composition of DI is lighter than that of SAT for samples 2, 5, IO. 12. 15 and 20 (Table 3), probably caused by isotopic fractionation of DI during the migration of crude oils. However, no geographic and stratigraphic generalities are found for these oils. Also, the isotopic composition of ASPH is heavier than that of SAT for samples 8, 1I and 14 (Table 3), probably caused by deasphalting process in the reservoir. The final and significant factor in shaping the isotope-type-curves may be the heterogeneous nature of the Phosphoria Shale itself. ORR (1974, p. 2301) reported that the mean isotopic composition of the Phosphoria Formation in Western Wyoming, Idaho and Montana as -28.3 per mil with a two-sigma range of 1.4 per mil. Research in these areas is currently in progress at the Bartlesville Energy Technology Center. The gross compositional variations and their iso-
20
RETENTION
1811
TIME
-
Fig. 9. The glass capillary gas chromatogram of a whole oil for oil No. 15(A) and 20(B). lsoprenoid alkanes are noted as ip in (A) and normal alkanes as numerals in (B).
H. Moses CHUNG
1812 I
/
I
et ui. /
I
1
-25
0 0
40
50
60
Fig. 10. The
carbon
70
80 z
90
100
Isotope grochemistrr of light hydrocarbons (LHC) The LHC associated with crude oil are generally believed to be the most sensitive to sample handling techniques and probably the least informative among the crude oil fractions. LHC are lost during production. shipping storing and handling of samples. Also.
the vapor pressure difference among LHC constttuents distort the compositional variation. Therefore, some researchers ignore the LHC components in characterizing crude oils (TISSOT and ROUCACHE, 1980), whereas others use them qualitatively (PHILIPPI, 1975: THOMPSON.1979) and quantitatively (YOUNGet al.. 1977). The detailed characterization of LHC in the Big Horn oils is found to be less informative than other oils from the Michigan basin. the Java Sea, and the Los Angeles basin (CHUNG and GRIZZLE. 1981). The preferential loss of LHC during degassing of oil has already affected even higher n-alkanes and isoprenoids. although effects of biodegradation and waterwashing may be locally operative. The LHC from the Big Horn oils were separated
/’ Sample smeller
which than
shows nC6
nC5
30
/’
Ftg. I I.
,
40
’
/’
/’
401
al
1’
60
70
2
00
90
:oo
,
1’ I
._>
/’
A__
50
120
isotopic composition of LHC vs 2 number.
topic values indicate that the Paleozoic oils in the Big Horn Basin are mainly influenced by maturation effects. However. the isotope-type-curves for these oils are found to be different from the commonly accepted idea. Variations of the isotope-type-curve may be influenced by the local characteristics of reservoirs and are believed to be a useful tool in understanding the natural transformation processes of petroleum.
$,
110
~_.~_
II3
2’)
130
The amount of LHC vs Z number. The amounts of LHC associated with the oil No. 20 IS assumed to be W’,, of the whole oil under the reservoir condition.
Carbon isotope geochemistq
and combusted in a vacuum system. The LHC fraction contains gaseous and light gasoline materials up through rl-C12 and n-C,,. The isotopic composition of the LHC fraction is plotted against the Z number in Fig. 10. Sample No. 10 is missing in the figure, because no carbon dioxide from this oil was sent for isotope analysis. Surprisingly. the isotopic composition of the LHC fraction increases with increasing maturity of crude oil. This is possible if the LHC associated with the crude oil are the same in quantity and isotopic composition as the in-place LHC in the reservoir, or if negligible isotopic fractionation occurs during degassing and volatilization. Which factor is dominant is believed to be affected by the maturity of oil and reservoir conditions. The maturity information obtained from the isotopic composition of the LHC fraction in the Big Horn oils suggests that the topping procedure of crude oil for oil-to-oil correlation may neglect one of the possible maturity parameters of the crude oil. The amount of LHC (Table 3) is shown in Fig. 11 as a function of Z number. Oils, of which gas chromatograms show the n-pentane peak smaller than the n-hexane peak. are marked with circles in the figure. Because the evidence for degassing and volatilization of LHC is strong. temporary linear lines are drawn to indicate the ranges of LHC contents for oils of a given maturity. This linear relationship of the amounts of LHC versus the maturity of crude oil may be an artifact, although oils of less maturity seem to fit the linear trend. For the oil No. 20. the amount of LHC is projected to be 48”,, of the whole oil. The large amount of LHC in the most mature oil is sufficient to change the physical properties of the whole oil. Such deasphalting processes may explain the quantity and isotopic composition of asphaltenes associated with the oil of the highest maturity. The deasphalting process is believed to precipitate the isotopically lighter asphaltenes preferentially. Assuming the oil. No. 20, has 48’; of LHC of -25.7 per mil in isotopic composition, the weight distributions of other fractions of this oil are recalculated from Table 3. Using the same isotopic values for each fraction. the isotopic composition of the in-situ reservoired oil will be. 48( - 25.7) + 35.5( - 26.4) +lO.O(-26.4) x-
+ 6.6(-27.1) = 100X
- 26.1
-26.1 per mil. which falls into a tolerable range in Fig. 3. Similar calculations for other oils, No. 6, 8 and 19. show similar results. Another exercise is to predict the amounts of methane formed during the maturation from the oil of Z number 40 to the oil of Z number 120. The isotopic composition of methane is assumed to be lighter than that of the whole oil of Z number 120 by 17 per mil (CHUNGand SACKETT. 1979: SILVERMAN, 1971). If the
of
1813
Pnleozoic oils
mean isotopic compositions for oils of Z number 40 and 120 are taken as -28.7 and -26.1 per mil respectively (Fig. 3). the amount of carbon converted into methane is expressed as lOO(-28.7) = X(-45.7)+-
(100 - X)(-26.1)
x = 13.3 Then 13.3”” of the total carbon has been converted into methane during the maturation from Z = 40 to Z = 120. Because one gram of oil gives 1500cc of carbon dioxide after a complete combustion (S,C = 80.4), 13.3”; of one gram of oil is equivalent lo 200 cc of methane per gram of oil, which in turn is equal lo 1120ft3 of natural gas per barrel of oil. This figure is reasonable for a well which will produce a mature oil. The above calculations are based on many assumptions; most importantly, a closed system is assumed in the reservoir. However, mass balances and subsequent isotope flows starting from ASPH lo natural gas are found to be ititernally consistent. The readjusted weight percentages of SAT to possible amounts of natural gas and LHC are found to be no longer proportional lo the Z number, indicating SAT as a source of natural gas and LHC. SUMMARY
Crude oil components from several representative oils separated in terms of boiling ranges by distillation and in terms of polarities by high performance liquid chromatography show various patterns of carbon isotope distributions. This carbon- 13 heterogeneous nature among components is believed lo be controlled by the geochemical history of a crude oil. Therefore, this distribution has a potential use in grouping and characterizing crude oil types. Twenty Paleozoic oils from the Big Horn Basin were fractionated into SAT, MONO, DI. PAP and ASPH. The amounts of SAT increase with increasing maturity. whereas the amounts of other fractions decrease. However. the isotopic compositions of all fractions become heavier with increasing maturity. The consistencies in internal variations of isotopic composition and varying oil composition for the Big Horn oils show that these oils have a single source origin and that maturation is the most important factor affecting the crude oil composition. Several observations are made: (1) Approximately 3 per mil difference in isotopic composition of the whole oil can be attributed to the maturation of a single type oil. (2) The isotopic composition of ASPH in the crude oil does not always show a proximity to that of the source rock. (3) The classification of crude oil types according to the isotope-type-curve is found to be not as simple as commonly believed. Other factors during maturation and natural transformation of crude oils control the pattern of the isotope-type-curve. (4) The continuous variation in the isotopic composition of
1814
H. MOSESCHUNGer
the least sensitive fraction. such as the aromatic fraction. argues against the general use of this fraction to classify oil types.
The isotopic composition of LHC associated with the Big Horn oils becomes proportionally heavier with Increasing maturity of the crude oil. Apparently no or very little isotope fractionation occurs during volatilization and degassing of LHC. This indicates that topping the whole oil to obtain more consistent isotopic information for oil-to-oil correlation may result in neglecting one maturity parameter of the crude oil. For some oils, the whole oil gas chromatograms show effects of degassing by having the n-C, peak smaller than the n-C, peak. Assuming the amount of LHC is directly proportional to the maturity of oil, calculations are made to predict the amounts of LHC and natural gas associated with crude oil of a given maturity in the reservoir. The conservative nature of mass balance and subsequent isotope flows among various fractions of crude oil show that the maturation of crude oil consists of consecutive processes leading from larger and more polar components into smaller and less polar components, culminating in natural gas. .-l~~rto~~/rdyr~~~rr~r.s-~ The authors would like to thank Dr Corp.. Field Research Lab.. Dallas for the crude oil samples and for many stimulating discussions on the crude oil geochemistry. Special thanks are due KAY PAYNE for her patience In typing this article. WILSON L. ORR. Mobtl Research and Development
REFERENCES BAILEYN. J. L.. ECANSC. R. and MILNERC. W. D. (197Ji .Applying petroleum geochemistry to search for oil: Examples from Western Canada Basin. Bull. ,4m. 4~s. Petrol. Geol. 58, 2284-2294. CHIJN~IH. M. and SACXETTW. %I. 11979) Use of stable carbon isotope composltions of pyrolytically derived methane as maturity indices for carbonaceous materials. Gu<,c~/rirn. C<~smochim..4cttr 43. I979- 1988. CHUS~; H. MM. and SATKETT W. M. (1980) Carbon Isotope effects during the pyrolytic formation of early methane from carbonaceous materials. Pray. Phw Chum. Eurth. in press. CHL.S(; H. M. and GRIZZLE P. L. 11981) Composittonal parameters of crude oil as studied by glass capillary gas chromatography. in preparation. CLAYTOS J. L. and SWETLANDP. J. ( 1980) Petroleum generation and migration in Denver basin. Bull. ‘401. A.+.\. Perrd. Gtd. 64; I61 3-1633. DAILEY J. E.. HIRXYHD. E.. COLEMAN H. J. and THOMPSON C. J. (1973) Compound-type separation and characterlzation studies for a 370 to 535 C distillate of Swan Hills (South Field). Alberta. Canada. crude 011. L S. Eur. .Miw\ Rep. Imust. 782 I. 30 pp. ERDMAUJ. G. and MORRtSD. A. 11974) Geochemical correlation of petroleum. Bull. Am. Ass. Perrol. Geol. 58, 28262837. EVANSC. R.. RO(;ERSM. A. and BAILEYN. J. L. (1971) Evolution and alteration of petroleum m Western Canada. Ckem. Grol. 8. I J7- 170. Fr EY ‘4. N. 119771The use of stable carbon Isotopes in
al.
hydrocarbon exploration. J. Grochem. E.rp/or. 7. 155-188. GREESJ. B. and HOFF R. J. (19801 Ltquid chromiitography on silica using mobile phases containing_ aliphatic car. boxylic acids 2. Application in fossil fuel characterization. J. Chromutoyr. 209. 231-250. HAHN-WEINHEIMER @. and HIRNER A. (1980) Chemical composition and stable carbon tsotopic ratios of crude 011sfrom Southern Germany. Org. Geochem. 2. 45-53 HIRSCHD. E., DOOLEYJ. E.. COLEMAXH. J. and THOMPSON C. J. t 1974) Compound-type separation and characterization studies for a 370 to 535 C distillate of Wilmington. Calif.. crude oil. U.S. Eur. .Mines Rrp. /west. 7893, 28 pp. J~UES T. S. and SMITHH. M. (1965) Relationships of oil composition and stratigraphy in the Permian basm of W. Texas and New Mexico. In Fluids in Subsurface Endronments-a 4 . 101-224.
Symposium.
Am. .4s.s. Petrol.
Geol. Mem.
KHANS C. B.. BO%DJ. G. and PIERCEF. L. (19741 Effects of
depositional composition
and postdepositional history on chemical of Lower Tuscaloosa oils. Bull. .-lm. Ass. Perrol. Geol. 58. 1272-1280. KVENVOLDEN K. A. and SQUIRESR. M. (1967) Carbon isotopic composition of crude oils from Ellenburger group (Lower Ordovician). Permian basin. West Texas and Eastern New Mexico. Bull. 4m. Ass. Petrol. Geol. 51. 1293-1303. MILSER C. W. D.. ROGERSM. A. and EVANSC. R. (19771 Petroleum transformatton in reservoirs. J. Geochem. E.up/or. 1, 101-153. ORR W. L. (19741 Changes m sulfur content and Isotopic ratios of sulfur durmg petroleum maturation-study of BIG Horn basin Paleozoic oils. Bull. Am. 4~s. Petrol. Gee/. 58, 2295-23 18. Pt-nLtppt G. T. (1975) The deep subsurface temperature controlled origin of the gases and gasoline-range hydrocarbons of petroleum. Grochim. Cosmochim.. Acta 39, 1353-1373. ROE G. D. and POLITOL. J. (1979) Source rocks for oils m the Ardjuna subbasin of the northwest Java basin. Indonesta. c..V.. ESC.4P.. CCOP 7&h. Pub/. No. 6. 180-194. SVHOELL.M. and MILNERC. W. D. I198 1I Isotopic crude oil correlation using ‘3C,1zC and D/H variations in compound classes of petroleum. Bull. .4tn. Ass.. Petrol. Geol.. in press. SHELDONR. P (1967) Long-distance migratron of 011 in Wyoming. .Mr. Geol. 4. 53-65. SLLVERMAN S. R. (19671 Carbon isotope evidence for the role of lipids in petroleum formation. J. .4m. Oil Chem. SOC. 44, 691-695. SILVERMAY S. R. (19711 Influence of petroleum origm and transformation on Its dtstribution in sedimentary rocks. Proc. 8th World
Petrol.
Conyr.
2. 47-54.
STAHLW. J. (1977) Carbon and mtrogen rsotopes m hydrocarbon research and exploration. Chem. Geol. 20, 121-~149. STAHLW. J. (1978) Source rock-crude oil correlatron by isotope typ-curves. Grochim Cosmochirn. .4cru 43, 1573-1577. ST..\HLW. J (19801L’be of “C “C analyses for the tdentlticatIon of the degradation of hydrocarbons by aerobic bacteria. Geockim. C/,\moc,/!irn. .4cru 44, 1903-1907. STONE D. S. (19671 Theory of Paleozoic oil and gas accumulation in Big Horn basm. Wyoming. Bull. Am. .4ss. Perrd
G~w/ 51, 205621
14.
SL TTO%C. (1979) Deposnlonal envtronments and their relation to chemical composition of Java Sea crude oils. L,‘..V. ESC.4 P. CCOP 7&h. Puhl. No. 6. l6FI 79. THOMPSONK. F M. (1979) Light hydrocarbons of petroleum: Internal evtdence of thermal history. Bull. .4m. .-l.s.~.Perrd. GM/. 63, 540. TISSOT B. P. and WELTED. H. ( 1978) Perrdwm Formtrriorr irntl 0~ (‘urrr)ict’. Springer.
Carbon TISS~T B. P. and ROL~CATHEJ. (1980) Principles
isotope
geochemistry
of a genetic
classification of crude oils. In F~rcrs and Principles cf. Uiwld Pcwolrtrr~~ O~~~rence (ed. A. D. Miall). Can. Sot.
Petrol. Gcol. Ment. 6. 209-217. WELTE D. H.. HAGEMAS H. W.. HOLLERBACH A.. LEYTHAEI’SER D. and STAHL W. (1975) Correlation between petroleum and source rock. Proc. Yrh World Perrol. Conqr. 2, I79- 19 1.
of Paleozoic
oils
1815
WINTERS J. C. and WILLIAMS J. A. (1969) Microbial alteration of crude oil in the reservorr. Srrnp. Pvtro/c,rrrt~ Transfiwn~ariorts it1 Geologic Enr.irorl,ivrlr.\. .4rn. C/tern. Sot. Mrrr. New York. Sept 7-12. 1969. E22 E31. YOCNG A.. MONAGHAK P. H. and STH\VI.ISRFRFT.R R. T. (1977) Calculation of ages of hydrocarbons m oils-phyca! chemistry applied to petroleum eeochemistq I. Bull. .4rn. 4.0. Perrol. Gcol. 61. 573-600.