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The effect of maturation on the isotopic composition of fossil fuels
Oroamc Geochemistry Vol
3, pp. 29 to 36, 1981
0146-6380/81/010029-08102.00/'0 P e r g a m o n Press Lid
Prmted m Great Britain
The effect of maturation on the isotopic composition of fossil fuels D. RIGBYI, B. D. BATTS2 and J. W. SMITHI ~C.S.I.R.O. Division of Fossil Fuels, P.O. Box 136, North Ryde, Australia, 2113 2School of Chemistry, Macquarie University, North Ryde, Australia, 2113 (Received 25 September 1980; accepted in revised form 12 December 1980)
Abstract--Few measurements of the D/H ratios in liquid and solid fossil fuels have been made. Additional isotopic data are reported here in the form of D/H and ~3C/t2C ratios for crude oils and brown coals which occur together in the Bass Strait region of Australia. Studies of petrographic fractions of these coals and of the changes m the isotopic composition of coals and coal extracts with increasing maturity show that the original D/I-I ratios characteristic of contributing plant materials are more readily erased and homogenized during early diagenesis than the corresponding ~3C/~2C ratios. INTRODUCTION THE DISTRIBUTIONof carbon isotopes in coal and oil has been well studied in the past. It has been shown that there is little change in 13C/12C ratios of coal with either geological age (Degens, 1969) or degree of coalification up to the anthracite stage (Wickman, 1953; Rankama, 1963). However, contrary to an earlier report (Landergren, 1955), increases in c513C values* have been observed in thermally altered sedimentary organic matter (McKirdy and Powell, 1974; Simoneit et al., 1978). Recently Chung (1976) has also shown that the lipids extractable from a wide range of coals are depleted in 13C relative to the parent coal. The 613C values of crude oil are largely determined by the ~3Cfl2C ratios of the source organic matter (Fuex, 1977). In addition 613C values of crude oil fractions vary with boiling range (Silverman, 1967; Fuex, 1977) and increase through the series saturated hydrocarbons, aromatics and N, S, and O compounds (Silverman and Epstein, 1958; Stahl, 1978). Small increases in 13C/~2C ratios have been reported to accompany the biodegradation (Stahl, 1978) and-maturation (Welte et al., 1975; Stahl, 1976) of crude oil fractions. The distribution of deuterium in coal and crude oils is less well established and only the following isotopic data appear to be available. Schiegl and Vogel (1970) have reported tSD values* from - 127 to - 140%o for 19 samples of South African coals and from - 9 2 to -112%o for 6 German coals. More recently Redding (1978) has recorded a wider range of 6D values from -65.2 to -153.8~/oo for 24 coals of different origins and different ranks. The published data on D/I-I ratios in crude oils are restricted to those of Schiegl and Vogel (1970), Estep and Hoering (1978) and Schoell and Redding (1978). The examination by these last authors of 26 crudes of Late Tertiary to Ordovician * ~3C/~2C and D/H ratios are reported using the usual t5 notations relative to the Poe Dee Belemnite (PDB) and standard mean ocean water (SMOW), respectively.
age gave tSD values from - 9 0 to -160%o. They also found the D/I-I ratio in these oils to increase through the series, saturated hydrocarbons, aromatics and N, S and O compounds (Schoell and Redding, 1978). Although Estep and Hoeting (1978) have suggested that large-scale hydrogen exchange reactions accompany the maturation of crude oils, presumably such reactions were not sufficient to erase totally the isotopic characteristics of contributing biological materials. Alternatively, on the present evidence the observed trend may reflect differences in hydrogen isotope distribution introduced during maturation and formation of these compounds. Inhomogeneity in deuterium distribution has also been noted in coals, with significant variations, presumably within seams, occurring over short distances (Redding, 1978). The broad distribution of 13C and deuterium in plants (a primary source of fossil fuels) is largely controlled by climate and environment, i.e. temperate or tropical, terrigenous or marine (Degens, 1969; Epstein and Yapp, 1976). Other factors include variations in the pathways for the photosynthetic fixation of carbon dioxide (Smith and Epstein, 1970, 1971), and even the availability of carbon dioxide may play a significant role (Deuser et al., 1968). Within plants, isotopic differences have been reported between leaves, bark, wood, sap, etc. (Schiegl and Vogel, 1970), and between the whole plant and the solvent extractable lipids (Smith and Epstein, 1970). On this evidence it appeared reasonable to relate the variations seen in the isotopic composition of immature fossil fuels to corresponding variations in the general composition, and concentration of particular components, in the contributing preserved source materials. In the Gippsland Basin (Fig. 1), commercial quantities of brown coal are found onshore and crude oil offshore. Evidence for a terrestrial origin of this oil has been advanced (Brooks, 1970; Powell and McKirdy, 1975). In the adjacent offshore Bass Basin, both coal and oil have been found, although there are no viable commercial ventures at present. Local coal 29
30
D. RIGBY,B. D. BATTSand J. W, SMITH
F
(,~ 0~~'~
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HALIBUT MACKERAL KINGFISH HAPUKU CORMORANT
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~
'='l Fig. I. Sample locations, Bass and Gippsland Basins.
measures have been suggested as the source of the oil in the Cormorant 1 well (Brown, 1976). It has been shown that many of the original constituents of the Yallourn brown coal (which outcrops in the Gippsland Basin) remain optically and chemically distinguishable after more than 15 Myr (Brooks and Smith, 1967). Accordingly, because of this high degree of preservation, it was expected that the characteristic isotopic compositions of the original plant constituents might similarly be preserved or reflected in the coals and coal extracts from this Basin. Since excellent suites of well-documented coal and oil samples from the Bass Strait were already available from a parallel investigation (Rigby, 1980), these materials were considered to be ideally suited for study of the isotopic changes which accompany fossil fuel maturation. SAMPLE LOCATIONS The geographical locations of the samples of crude oil and coal from the Gippsland and Bass Basins are shown in Fig. I. The Gippsland Basin is bounded to the north by the Victorian Ranges and to the south and southwest by the Bassian Rise. The Basin is wedge shaped, widening to approximately 120 km and deepening to the east. Four fifths of the Basin lies off-shore where commercial quantities of coal and gas are found in Tertiary sediments under 30-120 m of water. Simpli-
fled cross sections along AA' and BB' of Fig. 1 are shown in Fig. 2. Onshore, comznercial quantities of brown coal occur where the Latrobe Group rises steeply and outcrops in the Yallourn and neighbouring coal fields. Duplicate suites of maeroscopically recognizable lithotypes were hand picked from a single hand specimen of Yallourn brown coal. These comprised fossil resins, a 'woody coal' derived from the lignin-rich wood of higher land plants, a 'pollen coal' rich in the exinitic macerals derived from hydrogen-rich plant materials (sporopollenin, cutin, waxes and other plant lipids), a 'gellite' thought to be formed via humic gels derived from lignin or cellulose, and a black/brown macrolithotype containing less of the recognizable plant tissue and more carbonaceous detrital material. A small sample of leaf coal (also from Yallourn), a material apparently comprised entirely of preserved leaves, was also included for comparison. Six coals from the offshore portion of the Gippsland Basin were obtained at depths of 10710 1527, 1752, 2045, 2466 and 2649m from cores and cuttings from wells in the Marlin field. These subbituminous coals represent a maturation series in which rank increases with depth (Kantsler et al., 1978). The oil suite comprised a total of 12 samples representing each of the gas/oil and oil provinces of the Gippsland Basin as shown in Fig. 1. A second suite of samples has been obtained from the adjacent Bass Basin. This Basin lies wholly off-
Maturation and the isotopic composition of fossil fuels
A
31
A j
DOLPHIN SEA LEVEL ~
BREAM KINGFISH
[
I .
0 i
I i
I
2 KM i
B 1
B
YALLOURN
METRES
500
MORWELL
MERRIMAN CK
STRZELECKI GROUP ;" 0
I0
20
i
I
I
KM
Fig. 2. Simplified cross-section of the offshore and onshore Gippsland Basin as shown in Fig. 1. shore; it is partially enclosed by the Australian mainland to the north, Tasmania to the south and the Flinders Island-Kent Group and King Island on the northeast and west flanks, respectively. The Basin contains sediments ranging from early Cretaceous to Late Tertiary in age and is covered by water 30-90 m in depth. Five coal samples were obtained from the Cormorant 1 well at depths of 1521, 1684, 1832, 2233 and 2756 m, respectively. These coals occur in the Upper Eastern View coal measures of the Bass Basin which is regarded, in the lithological and stratigraphic senses, as equivalent to the Latrobe Valley Group in the adjacent Gippsland Basin (Brown, 1976). At the present time, the only show of oil (non commercial) occurs in the Cormorant 1 well; this has been analysed for comparison.
separated by chromatography on Florisil as described above. A flesh suite of coal samples was washed with 10% hydrochloric acid to remove any carbonates before determination of the 613C values of the coals. Acid-washed coals, and crude oil samples and extract fractions (sample size permitting) were quantitatively combusted to carbon dioxide and water using a modified version of the apparatus described by Kaplan et al. (1970). The combustion products were freed from impurities arising from the sulphur or nitrogen compounds and collected. The 13C content of the carbon dioxide was determined directly on a Micromass 602B mass spectrometer. The water was reduced over zinc at 420°C (Lyon and Cox, 1975) and the D/H ratios of the resultant hydrogen determined using a modified AEI MS20 spectrometer.
ANALYTICAL PROCEDURES
RESULTS AND DISCUSSION
Coal samples from the Marlin 1 and Cormorant 1 wells were ground to pass a - 7 2 BS sieve and extracted in a Soxhlet thimble with chloroform/methanol (90:10) to yield a crude extract. After removal of solvent in a stream of nitrogen, the crude extract was dispersed in hexane to give soluble and insoluble fractions. Both of these fractions were freed from solvent as before and the alkanes were then separated from the hexane-soluble fraction by chromatography on Florisil using hexane as the eluant. Ultraviolet light was used to monitor the commencement of elution of the aromatic fractions. The alkane fraction was analyzed by gas-liquid chromatography (Rigby, 1980). The alkane fractions of the crude oils were similarly
Variations in the isotopic composition of the macroscopically recognizable lithotypes from the Yallourn brown coal are shown graphically in Fig. 3, together with the isotopic composition of the leaf coal specimen. As might be expected from the isotopic fractionation of the carbon isotopes in modern plants (Park and Epstein, 1960; Smith and Epstein, 1970), the pollen coal, derived chiefly from plant lipids, is relatively more depleted in 13C than either the woody coal derived from the plant lignin, or the gellite, which is though to be formed from both lignin and cellulose via humic gels. The black/brown coal, which comprises more carbonaceous detrital material, has an intermediate carbon isotope composition.
OG 3 / 1 - 2 - c
32
D. RIGBY, B. D.
BATTS
and J. W. SMITH
-23 j
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LEAF COAL
/ RESINS
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-26 POLLEN COAL I
!
-200
I
-180
I
-160
D %.
-140
I
I
-120
--I00
SM.O.W.
Fig. 3. Isotopic composition of petrographic constituents of Yallourn brown coal. PERIOD
~J3 C%. P.D.B. -30 -28 -26 -24
,/
/ O~
~ D */,,. S . M . O . W . -f40 -120 -ioo - 8 o
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N - A L K A N E DISTRIBUTION I N DEPTI" COAL EXTRACTS OIL OCCURRENCES M %
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eme
COAL
e - - e ALKANE EXTRACTS
20 25 3 0 CARBON NUMBER
CARBON
20 25 30 NUMBER
Fig. 4. n-Alkane distribution and isotopic analysis of coal and oil samples from the Marlin field.
-
Maturation and the isotopic composition of fossd fuels The deuterium contents of the macrotithotypes, with the exception of the resins, are relatively constant with 6D values from - 100 to - 120~/oo.The high lipid content of the pollen coal suggests that it might be relatively depleted in deuterium. That this is not so IS probably best attributed to isotopic exchange reactions during early maturation processes as suggested by Estep and Hoering (1978). Only in the resin samples is ct, the mean isotopic fractionation of hydrogen relative to the woody coals (1.078), similar to that observed in the lipid extracts of modern plants (1.092, Smith and Epstein, 1970). It has been suggested (G. Eglinton, personal communication, 1979) that the polymeric structure of these resins shields them against hydrogen exchange reactions during the maturation of organic matter. A further contributory factor is that the larger particle size of these resins results in a relatively pure lipid fraction after hand-picking. The other hand-picked macrolithotypes can only be regarded as concentrates of particular land plant residues. These macrolithotypes have co-existed for more than 15 Myr and, from the isotopic data in Fig. 3, it seems evident that there has been little isotopic equilibration of carbon between the species although maturation processes may have had a greater influence on the hydrogen isotope ratios. The effect of maturation on the carbon and hydro-
PERIOC
Bf3 C*/,., P.D.B. -30
-28
,
-26
i
gen isotope distributions in the alkane extracts of coals from both the Marlin 1 well and the Cormorant 1 well can be observed in Figs 4 and 5, respectively. In both these wells the rank of the coals increases with depth (vitrinite reflectance increases from 0.45 to 0.68~ and from 0.47 to 1.04~/~ respectively). In all samples the extracts are more enriched in the lighter isotopes than the parent coals; the magnitude of this isotopic fractionation is relatively constant for carbon but decreases for hydrogen with increasing maturation of the alkanes. To test whether the coal extracts from the Marlin field were contaminated with oil from that Basin, the distribution of n-alkanes in the coal extracts and the crude oil were determined and compared as shown in Fig. 4. Indication of contamination by crude oil was found only in the sample of coal from 1071 m in the Marlin field. This sample yielded an alkane extract (0.33~) that is greater than the average (0.18~) and an n-alkane distribution pattern significantly more mature than those found for extracts of coals from much greater depths (Fig. 4). In the Marlin field, no linear correlation between isotopic composition and either depth, age or rank of the coal was observed. However, with increasing maturity of the alkane extracts of the coal (as evidenced by the increase in the shorter chain n-alkanes,
D %. S.M.O.W.
-24
33
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N-ALKANE DEPTH DISTRIBUTION L~ IN COAL EXTRACTSI"' %
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EXTRACTS
20 25 30 CARBON NUMBER
SANDSTONES AND VOLCANICS IGNEOUS INTRUSIVE Fig. 5. n-Alkane distribution and isotopic analysis of coal and oil from the Cormorant well (Bass Basin).
34
D. RIGBY,B. D. BATTSand J. W. SMITH Table 1. Isotopic analyses of crude oil samples from the Glppsland Basra Well
Depth (m)
Crude oil 6 13C(°/oo)
Alkane fraction
6D(°/oo)
613C(°/oo) 6D(°/oo)
Turrum 2
1578
-26.3
-112
-26.2
-112
Mackerel
2390
-25.2
-118
-26.3
-106
Kingfish
2311
-25.6
-125
-25.8
-111
Hapuku 1
2836
-26.0
-115
-26.2
-107
Halibut
2377
-25.7
-115
-26.3
-120
Marlin AI4
1710
-26.3
-112
-26.6
-110
Marlin A24
3259
-26.6
-110
-26.8
-106
Bream 2
1934
-25.8
-122
-26.1
-115
Flounder 4
2532
-26.1
-119
-26.6
-110
Tuna 3
1410
-25.9
-121
-26.1
-121
Dolphin
1219
-25.2
-125
-25.6
-116
305
-24.7
-120
-24.9
-123
-25.8
-118
-26.1
-113
0.54
5
0.51
6
Lakes Entrance No. 8
Mean Standard deviation
Fig. 4), a decrease in the fractionation of hydrogen isotopes between the alkane extracts and the parent coal was seen. Similarly, coal samples from the Cormorant 1 well in the Bass Basin exhibited a decreasing hydrogen isotope fractionation between the coal and the alkane extracts with increasing maturity of the aikanes (Fig. 5). Also, as in the Marlin 1 well, no correlation between isotopic composition and depth or rank of coals was evident. The isotopic compositions of the crude oils and the alkane fractions of these oils are shown in Table 1. Despite the variation in the chemical composition of these crude oils, which range from essentially naphthenic (Tuna 3, Dolphin and Lakes Entrance No. 8) to paraffinic, the isotopic compositions are relatively uniform. The established increase in the 13C content of crude oil fractions with increasing polarity or polarizability (Silverman and Epstein, 1958; Silverman, 1971; Stahl 1978) explains the lower laC contents of the alkane fractions relative to the whole crude oil. A corresponding decrease in the deuterium content of the alkane fraction was not observed. In fact, the deuter-
ium content of these alkane fractions increased slightly relative to the whole oil. This trend is in accord with the suggestion (Estep and Hoering, 1978) that large-scale hydrogen exchange reactions occur during the maturation of hydrocarbons; exchange with water can only result in an increase in the deuterium content of the alkane fractions. A lack of positive correlation between the distribution of carbon and hydrogen isotopes in crude oils has also been reported by Schoell and Redding (1978) who have also found that the saturated fractions were more enriched in deuterium than the whole crude oil. CONCLUSION The carbon isotope composition of Yallourn coal is similar to that of the offshore coals which occur at a greater depth in the Marlin well, however, the deuterium contents of the latter are significantly greater. Since all these coals appear to have been formed from a similar organic source, the variation in the distribution of deuterium probably largely reflects differences in the environment of maturation. Variations in the isotopic composition of Yallourn
Maturation and the isotopic composition of fossil fuels brown coal can be explained in terms of the relative concentration of particular precursors. The range of 613C values observed in the macrolithotypes separated from a single hand specimen stress a need for caution when sampling coal seams for isotopic analysis. Indeed, the range observed (2.9%0) is similar to that reported for all Tertiary coals (Degens, 1969). In both the Marlin 1 well and the Cormorant 1 well (apart from the sample at 1832 m), the fractionation of carbon isotopes between the whole coal and the alkane extract remained relatively constant as the maturity of the coal increased. The fractionation of hydrogen isotopes decreased with increasing maturation. A higher deuterium content has also been observed in the alkane extracts of crude oils. In the Cormorant 1 well, where some evidence for bacterial actxvity has been found (Rigby, 1980) such activity has not significantly affected the distribution of hydrogen isotopes. Maturation processes appear to have had a greater effect on the isotopic distribution of hydrogen than carbon. Hydrogen exchange reactions either internally within the coal or externally with the environment would result in an increase in the D/I-I ratios of the coal extracts. It could be expected that a marine environment (with a higher D/I-I ratio) would have a greater effect on the D / H ratios of maturing coal than a terrestrial freshwater environment. This may explain the higher D / H ratios found in the coal and coal extracts from the Marlin 1 well compared with the onshore coals from Yallourn (see Figs 3 and 4). Indeed the Paleocene and early Eocene sedimentary patterns (Threlfall et al., 1976) for the Gippsland Basin would suggest a marine to marginal marine environment during the deposition of the Marlin 1 coals. A similar argument will equally well explain the increased deuterium contents of mature crude oils. The carbon isotope compositions of the onshore and offshore coals in the Gippsland Basin are similar to those of the offshore crude oils, and would suggest a similarity of precursors. However, these coals have not reached a degree of maturity consistent with the generation of such significant quantities of liquid hydrocarbons. Acknowledgements--We thank Esso Australia and Dr G. C. SMXTHof the State Electricity Commission of Victoria for providing many of the samples used in this investigation; Dr G. C. SM1TH for measuring the vitrinite reflectances of the Tertiary coal samples; and Mrs M. SMYTn and Dr L. HAt,lmTON for hand-picking the duplicate suites of Yallourn macrolithotypes.
REFERENCES Brooks, J. D., 1970, The use of coals as indicators of the occurrence of oil and gas: A P E A J., v. 10, p. 35-50. Brooks, J. D., and Smith, J. W., 1967, The diagenesis of plant lipids during the formation of coal, petroleum and natural gas--l. Changes in the n-paraffin hydrocarbons: Geochim. Cosmochira. Acta v. 31, p. 2389-2397. Brown, B. R., 1976, Bass basin. Some aspects of the pet-
35
roleum geology, m Leslie, R. B., Evans, H. J., and Knight, C. L., eds, Economic Geology of Australia and Papua New Guinea, v. 3, Petroleum, Australasian Institute of Mining and Metallurgy, p. 67-82. Chung, H. M., 1976, Isotope fractionation during the maturation of organic matter. Ph.D. Thesis submatted to Texas A&M Universaty. Degens, E. T., 1969, Biogeochemistry of stable carbon isotopes, in Eglmton, G., and Murphy, M. T. J., eds, Orgame Geochemistry, Springer-Verlag, p. 304-329. Deuser, W. G., Degens, E. T., and Guillard, R. R. L., 1968, Carbon isotope relationships between plankton and sea water: Geochim. Cosmochim. Acta, v. 32, p. 632-660. Epstein, S., and Yapp, C. R., 1976, Chmatic implications of the D/H ratio of hydrogen in C-H groups in tree cellulose: Earth Planet. Sci. Lett., v. 30, p. 252-261. Estep, M. F., and HoerinG, T. C., 1978, The orgamc geochemistry of the stable hydrogen isotopes, in Zartmann, R. E., ed., Short Papers of the 4th International Conference on Geochronology, Cosmochronology, Isotope Geology 1978, U.S. Geological Survey Open File Report, 78-701, p. 108-109. Fuex, A. N., 1977, The use of stable asotopes in hydrocarbon exploration: J. Geochem. Explor., v. 7, p. 155-188. Kantsler, A. J., Smith, G. C., and Cook, A. C., 1978, Lateral and vertical rank variation: implications for hydrocarbon exploration: A P E A d., v. 18, p. 143-156. Kaplan, I. R., Smith, J. W., and Ruth, E., 1970, Carbon and sulphur concentration and isotopic compositaon in Apollo 11 lunar samples: Proceedings of the Apollo 11 Lunar Science Conference, v. 2, p. 1317-1329. Landergren, S., 1955, A note on 12C/13C in metamorphosed alum shale: Geochim. Cosmochim. Acta, v. 7, p. 240--241. Lyon, G. L., and Cox, M. A., 1975, The reduction of water to hydrogen for D/H ratio analysis, using zinc in a matrix of sand: Inst. of Nuclear Science, New Zealand. Report No. 50/108/GLL. McKirdy, D. M., and Powell, T. G., 1974, Metamorphic alteration of carbon isotope composition an ancient sedimentary organic matter. New evidence from Australia and South Africa: Geolooy, v. 12, p. 591-595. Park, R., and Epstein, S., 1960, Carbon isotope fractionation during photosynthesis: Geochim. Cosmocham. Acta, v. 21, p. 110-126. Powell, T. G., and McKirdy, D. M., 1975, Geochemical character of crude oils from Australia and Papua New Guinea, m Leslie, R. B., Evans, H. J., and Knight, C. L., eds, Economic Geology of Australia and Papua New Guinea, v. 3, Petroleum, Australasian Instatute of Mining and Metallurgy, p. 18-29. Rankama, K., 1963, Progress in Isotope Geology, Interscience. Redding, C., 1978, Hydrogen and carbon isotopes in coals and kerogens, in Zartmann, R.E., ed., Short Papers of the 4th International Conference on Geochronology, Cosmochronology, Isotope Geology 1978, U.S. Geological Survey Open File Report, 78-701, p. 348-349. Rigby, D., 1980, The measurement of carbon and deuterium isotopes as a guide to the genesis of fossil fuels. M.Sc. (Hons) Thesis, Macquarie University. Schiegl, W. E., and Vogel, J. C., 1970, Deuterium content of organic matter: Earth Planet. Sci. Lett., v. 7, p. 307-313. Sehoell, M., and Redding, C., 1978, Hydrogen isotope composition of selected crude oils and their fractions, in Zartmann, R. E., ed., Short Papers of the 4th International Conference on Geochronology, Cosmochronology, Isotope Geology, 1978, U.S. Geological Survey Open File Report 78-701, p. 384-385. Silverman, S. R., 1967, Carbon isotopic evidence for the role of lipids in petroleum formation: J. Am. Od Chem. Soc., v. 44, p. 691-695.
36
D RIGBV, B. D. BATrs and J. W. SMrrH
Silverman, S. R., 1971, Influence of petroleum origin and transformation on its distribution and distribution in sedimentary rocks: Proceedings of the 8th World Petroleum Congress, v. 2, p. 47-54. Sdverman, S. R., and Epstein, S., 1958, Carbon ~sotopic compositions of petroleums and other sedimentary organic materials" Bull. Am. Assoc. Pet. Geol., v. 42, p. 998-1012. Simoneit, R. T., Brenner, S., Peters, K. E., and Kaplan, I. R., 1978, Thermal alteration of Cretaceous black shale by basaltic intrusions in the Eastern Atlantic: Nature, v. 273, p. 501-504. Smith, B. N., and Epstein, S., 1970, Blogeochem~stry of the stable isotopes of hydrogen and carbon in salt marsh biota: Plant Physiol., v. 46, p. 738-742. Smith, B. N., and Epstein S., 1971, Two categories of 13C/12C ratios for higher plants: Plant Physiol., v. 47, p. 380-383.
Stahl, W. J., 1976, Economically important application of carbon isotope data of natural gases and crude oil, m Nuclear Techniques in Geochemzstry and Geophysics, IAEA, p. 213-222. Stahl, W. J., 1978, Source rock-crude oil correlation by isotopic type-curves: Geochim. Cosmochim. Acta, v. 22, p. 1573-1577. Threlfall, W. F., Brown, B. R., and Griffith, B. R., 1976, Gippsland basin, off-shore, in Leslie, R. B., Evans, H. J., and Knight, C. L., eds, Economic Geology of Austraha and Papua New Guinea, v. 3, Petroleum, Australasian Institute of Mining and Metallurgy, p. 41-67. Welte, D. H., Kalkreuth, W., and Hoefs, J., 1975, Age trend in carbon-isotopic composition in Paleozoic sediments: Naturwissenschaften, v. 62, p. 482-483. Wickman, F. E., 1953, Wird das haufigkeitsverhaltnis der kohlenstoffisotopen bei der inkohlung verandert?: Geochim. Cosmochim. Acta, v. 3, p. 214-252.
3, pp. 29 to 36, 1981
0146-6380/81/010029-08102.00/'0 P e r g a m o n Press Lid
Prmted m Great Britain
The effect of maturation on the isotopic composition of fossil fuels D. RIGBYI, B. D. BATTS2 and J. W. SMITHI ~C.S.I.R.O. Division of Fossil Fuels, P.O. Box 136, North Ryde, Australia, 2113 2School of Chemistry, Macquarie University, North Ryde, Australia, 2113 (Received 25 September 1980; accepted in revised form 12 December 1980)
Abstract--Few measurements of the D/H ratios in liquid and solid fossil fuels have been made. Additional isotopic data are reported here in the form of D/H and ~3C/t2C ratios for crude oils and brown coals which occur together in the Bass Strait region of Australia. Studies of petrographic fractions of these coals and of the changes m the isotopic composition of coals and coal extracts with increasing maturity show that the original D/I-I ratios characteristic of contributing plant materials are more readily erased and homogenized during early diagenesis than the corresponding ~3C/~2C ratios. INTRODUCTION THE DISTRIBUTIONof carbon isotopes in coal and oil has been well studied in the past. It has been shown that there is little change in 13C/12C ratios of coal with either geological age (Degens, 1969) or degree of coalification up to the anthracite stage (Wickman, 1953; Rankama, 1963). However, contrary to an earlier report (Landergren, 1955), increases in c513C values* have been observed in thermally altered sedimentary organic matter (McKirdy and Powell, 1974; Simoneit et al., 1978). Recently Chung (1976) has also shown that the lipids extractable from a wide range of coals are depleted in 13C relative to the parent coal. The 613C values of crude oil are largely determined by the ~3Cfl2C ratios of the source organic matter (Fuex, 1977). In addition 613C values of crude oil fractions vary with boiling range (Silverman, 1967; Fuex, 1977) and increase through the series saturated hydrocarbons, aromatics and N, S, and O compounds (Silverman and Epstein, 1958; Stahl, 1978). Small increases in 13C/~2C ratios have been reported to accompany the biodegradation (Stahl, 1978) and-maturation (Welte et al., 1975; Stahl, 1976) of crude oil fractions. The distribution of deuterium in coal and crude oils is less well established and only the following isotopic data appear to be available. Schiegl and Vogel (1970) have reported tSD values* from - 127 to - 140%o for 19 samples of South African coals and from - 9 2 to -112%o for 6 German coals. More recently Redding (1978) has recorded a wider range of 6D values from -65.2 to -153.8~/oo for 24 coals of different origins and different ranks. The published data on D/I-I ratios in crude oils are restricted to those of Schiegl and Vogel (1970), Estep and Hoering (1978) and Schoell and Redding (1978). The examination by these last authors of 26 crudes of Late Tertiary to Ordovician * ~3C/~2C and D/H ratios are reported using the usual t5 notations relative to the Poe Dee Belemnite (PDB) and standard mean ocean water (SMOW), respectively.
age gave tSD values from - 9 0 to -160%o. They also found the D/I-I ratio in these oils to increase through the series, saturated hydrocarbons, aromatics and N, S and O compounds (Schoell and Redding, 1978). Although Estep and Hoeting (1978) have suggested that large-scale hydrogen exchange reactions accompany the maturation of crude oils, presumably such reactions were not sufficient to erase totally the isotopic characteristics of contributing biological materials. Alternatively, on the present evidence the observed trend may reflect differences in hydrogen isotope distribution introduced during maturation and formation of these compounds. Inhomogeneity in deuterium distribution has also been noted in coals, with significant variations, presumably within seams, occurring over short distances (Redding, 1978). The broad distribution of 13C and deuterium in plants (a primary source of fossil fuels) is largely controlled by climate and environment, i.e. temperate or tropical, terrigenous or marine (Degens, 1969; Epstein and Yapp, 1976). Other factors include variations in the pathways for the photosynthetic fixation of carbon dioxide (Smith and Epstein, 1970, 1971), and even the availability of carbon dioxide may play a significant role (Deuser et al., 1968). Within plants, isotopic differences have been reported between leaves, bark, wood, sap, etc. (Schiegl and Vogel, 1970), and between the whole plant and the solvent extractable lipids (Smith and Epstein, 1970). On this evidence it appeared reasonable to relate the variations seen in the isotopic composition of immature fossil fuels to corresponding variations in the general composition, and concentration of particular components, in the contributing preserved source materials. In the Gippsland Basin (Fig. 1), commercial quantities of brown coal are found onshore and crude oil offshore. Evidence for a terrestrial origin of this oil has been advanced (Brooks, 1970; Powell and McKirdy, 1975). In the adjacent offshore Bass Basin, both coal and oil have been found, although there are no viable commercial ventures at present. Local coal 29
30
D. RIGBY,B. D. BATTSand J. W, SMITH
F
(,~ 0~~'~
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-.
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3 MARLIN 4 TURRUM 5 TUNA 6 FLOUNDER 7 e 9 Io tJ
HALIBUT MACKERAL KINGFISH HAPUKU CORMORANT
0I,.=.,=.===d40KM MAP AREA
~
'='l Fig. I. Sample locations, Bass and Gippsland Basins.
measures have been suggested as the source of the oil in the Cormorant 1 well (Brown, 1976). It has been shown that many of the original constituents of the Yallourn brown coal (which outcrops in the Gippsland Basin) remain optically and chemically distinguishable after more than 15 Myr (Brooks and Smith, 1967). Accordingly, because of this high degree of preservation, it was expected that the characteristic isotopic compositions of the original plant constituents might similarly be preserved or reflected in the coals and coal extracts from this Basin. Since excellent suites of well-documented coal and oil samples from the Bass Strait were already available from a parallel investigation (Rigby, 1980), these materials were considered to be ideally suited for study of the isotopic changes which accompany fossil fuel maturation. SAMPLE LOCATIONS The geographical locations of the samples of crude oil and coal from the Gippsland and Bass Basins are shown in Fig. I. The Gippsland Basin is bounded to the north by the Victorian Ranges and to the south and southwest by the Bassian Rise. The Basin is wedge shaped, widening to approximately 120 km and deepening to the east. Four fifths of the Basin lies off-shore where commercial quantities of coal and gas are found in Tertiary sediments under 30-120 m of water. Simpli-
fled cross sections along AA' and BB' of Fig. 1 are shown in Fig. 2. Onshore, comznercial quantities of brown coal occur where the Latrobe Group rises steeply and outcrops in the Yallourn and neighbouring coal fields. Duplicate suites of maeroscopically recognizable lithotypes were hand picked from a single hand specimen of Yallourn brown coal. These comprised fossil resins, a 'woody coal' derived from the lignin-rich wood of higher land plants, a 'pollen coal' rich in the exinitic macerals derived from hydrogen-rich plant materials (sporopollenin, cutin, waxes and other plant lipids), a 'gellite' thought to be formed via humic gels derived from lignin or cellulose, and a black/brown macrolithotype containing less of the recognizable plant tissue and more carbonaceous detrital material. A small sample of leaf coal (also from Yallourn), a material apparently comprised entirely of preserved leaves, was also included for comparison. Six coals from the offshore portion of the Gippsland Basin were obtained at depths of 10710 1527, 1752, 2045, 2466 and 2649m from cores and cuttings from wells in the Marlin field. These subbituminous coals represent a maturation series in which rank increases with depth (Kantsler et al., 1978). The oil suite comprised a total of 12 samples representing each of the gas/oil and oil provinces of the Gippsland Basin as shown in Fig. 1. A second suite of samples has been obtained from the adjacent Bass Basin. This Basin lies wholly off-
Maturation and the isotopic composition of fossil fuels
A
31
A j
DOLPHIN SEA LEVEL ~
BREAM KINGFISH
[
I .
0 i
I i
I
2 KM i
B 1
B
YALLOURN
METRES
500
MORWELL
MERRIMAN CK
STRZELECKI GROUP ;" 0
I0
20
i
I
I
KM
Fig. 2. Simplified cross-section of the offshore and onshore Gippsland Basin as shown in Fig. 1. shore; it is partially enclosed by the Australian mainland to the north, Tasmania to the south and the Flinders Island-Kent Group and King Island on the northeast and west flanks, respectively. The Basin contains sediments ranging from early Cretaceous to Late Tertiary in age and is covered by water 30-90 m in depth. Five coal samples were obtained from the Cormorant 1 well at depths of 1521, 1684, 1832, 2233 and 2756 m, respectively. These coals occur in the Upper Eastern View coal measures of the Bass Basin which is regarded, in the lithological and stratigraphic senses, as equivalent to the Latrobe Valley Group in the adjacent Gippsland Basin (Brown, 1976). At the present time, the only show of oil (non commercial) occurs in the Cormorant 1 well; this has been analysed for comparison.
separated by chromatography on Florisil as described above. A flesh suite of coal samples was washed with 10% hydrochloric acid to remove any carbonates before determination of the 613C values of the coals. Acid-washed coals, and crude oil samples and extract fractions (sample size permitting) were quantitatively combusted to carbon dioxide and water using a modified version of the apparatus described by Kaplan et al. (1970). The combustion products were freed from impurities arising from the sulphur or nitrogen compounds and collected. The 13C content of the carbon dioxide was determined directly on a Micromass 602B mass spectrometer. The water was reduced over zinc at 420°C (Lyon and Cox, 1975) and the D/H ratios of the resultant hydrogen determined using a modified AEI MS20 spectrometer.
ANALYTICAL PROCEDURES
RESULTS AND DISCUSSION
Coal samples from the Marlin 1 and Cormorant 1 wells were ground to pass a - 7 2 BS sieve and extracted in a Soxhlet thimble with chloroform/methanol (90:10) to yield a crude extract. After removal of solvent in a stream of nitrogen, the crude extract was dispersed in hexane to give soluble and insoluble fractions. Both of these fractions were freed from solvent as before and the alkanes were then separated from the hexane-soluble fraction by chromatography on Florisil using hexane as the eluant. Ultraviolet light was used to monitor the commencement of elution of the aromatic fractions. The alkane fraction was analyzed by gas-liquid chromatography (Rigby, 1980). The alkane fractions of the crude oils were similarly
Variations in the isotopic composition of the macroscopically recognizable lithotypes from the Yallourn brown coal are shown graphically in Fig. 3, together with the isotopic composition of the leaf coal specimen. As might be expected from the isotopic fractionation of the carbon isotopes in modern plants (Park and Epstein, 1960; Smith and Epstein, 1970), the pollen coal, derived chiefly from plant lipids, is relatively more depleted in 13C than either the woody coal derived from the plant lignin, or the gellite, which is though to be formed from both lignin and cellulose via humic gels. The black/brown coal, which comprises more carbonaceous detrital material, has an intermediate carbon isotope composition.
OG 3 / 1 - 2 - c
32
D. RIGBY, B. D.
BATTS
and J. W. SMITH
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-26 POLLEN COAL I
!
-200
I
-180
I
-160
D %.
-140
I
I
-120
--I00
SM.O.W.
Fig. 3. Isotopic composition of petrographic constituents of Yallourn brown coal. PERIOD
~J3 C%. P.D.B. -30 -28 -26 -24
,/
/ O~
~ D */,,. S . M . O . W . -f40 -120 -ioo - 8 o
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N - A L K A N E DISTRIBUTION I N DEPTI" COAL EXTRACTS OIL OCCURRENCES M %
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eme
COAL
e - - e ALKANE EXTRACTS
20 25 3 0 CARBON NUMBER
CARBON
20 25 30 NUMBER
Fig. 4. n-Alkane distribution and isotopic analysis of coal and oil samples from the Marlin field.
-
Maturation and the isotopic composition of fossd fuels The deuterium contents of the macrotithotypes, with the exception of the resins, are relatively constant with 6D values from - 100 to - 120~/oo.The high lipid content of the pollen coal suggests that it might be relatively depleted in deuterium. That this is not so IS probably best attributed to isotopic exchange reactions during early maturation processes as suggested by Estep and Hoering (1978). Only in the resin samples is ct, the mean isotopic fractionation of hydrogen relative to the woody coals (1.078), similar to that observed in the lipid extracts of modern plants (1.092, Smith and Epstein, 1970). It has been suggested (G. Eglinton, personal communication, 1979) that the polymeric structure of these resins shields them against hydrogen exchange reactions during the maturation of organic matter. A further contributory factor is that the larger particle size of these resins results in a relatively pure lipid fraction after hand-picking. The other hand-picked macrolithotypes can only be regarded as concentrates of particular land plant residues. These macrolithotypes have co-existed for more than 15 Myr and, from the isotopic data in Fig. 3, it seems evident that there has been little isotopic equilibration of carbon between the species although maturation processes may have had a greater influence on the hydrogen isotope ratios. The effect of maturation on the carbon and hydro-
PERIOC
Bf3 C*/,., P.D.B. -30
-28
,
-26
i
gen isotope distributions in the alkane extracts of coals from both the Marlin 1 well and the Cormorant 1 well can be observed in Figs 4 and 5, respectively. In both these wells the rank of the coals increases with depth (vitrinite reflectance increases from 0.45 to 0.68~ and from 0.47 to 1.04~/~ respectively). In all samples the extracts are more enriched in the lighter isotopes than the parent coals; the magnitude of this isotopic fractionation is relatively constant for carbon but decreases for hydrogen with increasing maturation of the alkanes. To test whether the coal extracts from the Marlin field were contaminated with oil from that Basin, the distribution of n-alkanes in the coal extracts and the crude oil were determined and compared as shown in Fig. 4. Indication of contamination by crude oil was found only in the sample of coal from 1071 m in the Marlin field. This sample yielded an alkane extract (0.33~) that is greater than the average (0.18~) and an n-alkane distribution pattern significantly more mature than those found for extracts of coals from much greater depths (Fig. 4). In the Marlin field, no linear correlation between isotopic composition and either depth, age or rank of the coal was observed. However, with increasing maturity of the alkane extracts of the coal (as evidenced by the increase in the shorter chain n-alkanes,
D %. S.M.O.W.
-24
33
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N-ALKANE DEPTH DISTRIBUTION L~ IN COAL EXTRACTSI"' %
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EXTRACTS
20 25 30 CARBON NUMBER
SANDSTONES AND VOLCANICS IGNEOUS INTRUSIVE Fig. 5. n-Alkane distribution and isotopic analysis of coal and oil from the Cormorant well (Bass Basin).
34
D. RIGBY,B. D. BATTSand J. W. SMITH Table 1. Isotopic analyses of crude oil samples from the Glppsland Basra Well
Depth (m)
Crude oil 6 13C(°/oo)
Alkane fraction
6D(°/oo)
613C(°/oo) 6D(°/oo)
Turrum 2
1578
-26.3
-112
-26.2
-112
Mackerel
2390
-25.2
-118
-26.3
-106
Kingfish
2311
-25.6
-125
-25.8
-111
Hapuku 1
2836
-26.0
-115
-26.2
-107
Halibut
2377
-25.7
-115
-26.3
-120
Marlin AI4
1710
-26.3
-112
-26.6
-110
Marlin A24
3259
-26.6
-110
-26.8
-106
Bream 2
1934
-25.8
-122
-26.1
-115
Flounder 4
2532
-26.1
-119
-26.6
-110
Tuna 3
1410
-25.9
-121
-26.1
-121
Dolphin
1219
-25.2
-125
-25.6
-116
305
-24.7
-120
-24.9
-123
-25.8
-118
-26.1
-113
0.54
5
0.51
6
Lakes Entrance No. 8
Mean Standard deviation
Fig. 4), a decrease in the fractionation of hydrogen isotopes between the alkane extracts and the parent coal was seen. Similarly, coal samples from the Cormorant 1 well in the Bass Basin exhibited a decreasing hydrogen isotope fractionation between the coal and the alkane extracts with increasing maturity of the aikanes (Fig. 5). Also, as in the Marlin 1 well, no correlation between isotopic composition and depth or rank of coals was evident. The isotopic compositions of the crude oils and the alkane fractions of these oils are shown in Table 1. Despite the variation in the chemical composition of these crude oils, which range from essentially naphthenic (Tuna 3, Dolphin and Lakes Entrance No. 8) to paraffinic, the isotopic compositions are relatively uniform. The established increase in the 13C content of crude oil fractions with increasing polarity or polarizability (Silverman and Epstein, 1958; Silverman, 1971; Stahl 1978) explains the lower laC contents of the alkane fractions relative to the whole crude oil. A corresponding decrease in the deuterium content of the alkane fraction was not observed. In fact, the deuter-
ium content of these alkane fractions increased slightly relative to the whole oil. This trend is in accord with the suggestion (Estep and Hoering, 1978) that large-scale hydrogen exchange reactions occur during the maturation of hydrocarbons; exchange with water can only result in an increase in the deuterium content of the alkane fractions. A lack of positive correlation between the distribution of carbon and hydrogen isotopes in crude oils has also been reported by Schoell and Redding (1978) who have also found that the saturated fractions were more enriched in deuterium than the whole crude oil. CONCLUSION The carbon isotope composition of Yallourn coal is similar to that of the offshore coals which occur at a greater depth in the Marlin well, however, the deuterium contents of the latter are significantly greater. Since all these coals appear to have been formed from a similar organic source, the variation in the distribution of deuterium probably largely reflects differences in the environment of maturation. Variations in the isotopic composition of Yallourn
Maturation and the isotopic composition of fossil fuels brown coal can be explained in terms of the relative concentration of particular precursors. The range of 613C values observed in the macrolithotypes separated from a single hand specimen stress a need for caution when sampling coal seams for isotopic analysis. Indeed, the range observed (2.9%0) is similar to that reported for all Tertiary coals (Degens, 1969). In both the Marlin 1 well and the Cormorant 1 well (apart from the sample at 1832 m), the fractionation of carbon isotopes between the whole coal and the alkane extract remained relatively constant as the maturity of the coal increased. The fractionation of hydrogen isotopes decreased with increasing maturation. A higher deuterium content has also been observed in the alkane extracts of crude oils. In the Cormorant 1 well, where some evidence for bacterial actxvity has been found (Rigby, 1980) such activity has not significantly affected the distribution of hydrogen isotopes. Maturation processes appear to have had a greater effect on the isotopic distribution of hydrogen than carbon. Hydrogen exchange reactions either internally within the coal or externally with the environment would result in an increase in the D/I-I ratios of the coal extracts. It could be expected that a marine environment (with a higher D/I-I ratio) would have a greater effect on the D / H ratios of maturing coal than a terrestrial freshwater environment. This may explain the higher D / H ratios found in the coal and coal extracts from the Marlin 1 well compared with the onshore coals from Yallourn (see Figs 3 and 4). Indeed the Paleocene and early Eocene sedimentary patterns (Threlfall et al., 1976) for the Gippsland Basin would suggest a marine to marginal marine environment during the deposition of the Marlin 1 coals. A similar argument will equally well explain the increased deuterium contents of mature crude oils. The carbon isotope compositions of the onshore and offshore coals in the Gippsland Basin are similar to those of the offshore crude oils, and would suggest a similarity of precursors. However, these coals have not reached a degree of maturity consistent with the generation of such significant quantities of liquid hydrocarbons. Acknowledgements--We thank Esso Australia and Dr G. C. SMXTHof the State Electricity Commission of Victoria for providing many of the samples used in this investigation; Dr G. C. SM1TH for measuring the vitrinite reflectances of the Tertiary coal samples; and Mrs M. SMYTn and Dr L. HAt,lmTON for hand-picking the duplicate suites of Yallourn macrolithotypes.
REFERENCES Brooks, J. D., 1970, The use of coals as indicators of the occurrence of oil and gas: A P E A J., v. 10, p. 35-50. Brooks, J. D., and Smith, J. W., 1967, The diagenesis of plant lipids during the formation of coal, petroleum and natural gas--l. Changes in the n-paraffin hydrocarbons: Geochim. Cosmochira. Acta v. 31, p. 2389-2397. Brown, B. R., 1976, Bass basin. Some aspects of the pet-
35
roleum geology, m Leslie, R. B., Evans, H. J., and Knight, C. L., eds, Economic Geology of Australia and Papua New Guinea, v. 3, Petroleum, Australasian Institute of Mining and Metallurgy, p. 67-82. Chung, H. M., 1976, Isotope fractionation during the maturation of organic matter. Ph.D. Thesis submatted to Texas A&M Universaty. Degens, E. T., 1969, Biogeochemistry of stable carbon isotopes, in Eglmton, G., and Murphy, M. T. J., eds, Orgame Geochemistry, Springer-Verlag, p. 304-329. Deuser, W. G., Degens, E. T., and Guillard, R. R. L., 1968, Carbon isotope relationships between plankton and sea water: Geochim. Cosmochim. Acta, v. 32, p. 632-660. Epstein, S., and Yapp, C. R., 1976, Chmatic implications of the D/H ratio of hydrogen in C-H groups in tree cellulose: Earth Planet. Sci. Lett., v. 30, p. 252-261. Estep, M. F., and HoerinG, T. C., 1978, The orgamc geochemistry of the stable hydrogen isotopes, in Zartmann, R. E., ed., Short Papers of the 4th International Conference on Geochronology, Cosmochronology, Isotope Geology 1978, U.S. Geological Survey Open File Report, 78-701, p. 108-109. Fuex, A. N., 1977, The use of stable asotopes in hydrocarbon exploration: J. Geochem. Explor., v. 7, p. 155-188. Kantsler, A. J., Smith, G. C., and Cook, A. C., 1978, Lateral and vertical rank variation: implications for hydrocarbon exploration: A P E A d., v. 18, p. 143-156. Kaplan, I. R., Smith, J. W., and Ruth, E., 1970, Carbon and sulphur concentration and isotopic compositaon in Apollo 11 lunar samples: Proceedings of the Apollo 11 Lunar Science Conference, v. 2, p. 1317-1329. Landergren, S., 1955, A note on 12C/13C in metamorphosed alum shale: Geochim. Cosmochim. Acta, v. 7, p. 240--241. Lyon, G. L., and Cox, M. A., 1975, The reduction of water to hydrogen for D/H ratio analysis, using zinc in a matrix of sand: Inst. of Nuclear Science, New Zealand. Report No. 50/108/GLL. McKirdy, D. M., and Powell, T. G., 1974, Metamorphic alteration of carbon isotope composition an ancient sedimentary organic matter. New evidence from Australia and South Africa: Geolooy, v. 12, p. 591-595. Park, R., and Epstein, S., 1960, Carbon isotope fractionation during photosynthesis: Geochim. Cosmocham. Acta, v. 21, p. 110-126. Powell, T. G., and McKirdy, D. M., 1975, Geochemical character of crude oils from Australia and Papua New Guinea, m Leslie, R. B., Evans, H. J., and Knight, C. L., eds, Economic Geology of Australia and Papua New Guinea, v. 3, Petroleum, Australasian Instatute of Mining and Metallurgy, p. 18-29. Rankama, K., 1963, Progress in Isotope Geology, Interscience. Redding, C., 1978, Hydrogen and carbon isotopes in coals and kerogens, in Zartmann, R.E., ed., Short Papers of the 4th International Conference on Geochronology, Cosmochronology, Isotope Geology 1978, U.S. Geological Survey Open File Report, 78-701, p. 348-349. Rigby, D., 1980, The measurement of carbon and deuterium isotopes as a guide to the genesis of fossil fuels. M.Sc. (Hons) Thesis, Macquarie University. Schiegl, W. E., and Vogel, J. C., 1970, Deuterium content of organic matter: Earth Planet. Sci. Lett., v. 7, p. 307-313. Sehoell, M., and Redding, C., 1978, Hydrogen isotope composition of selected crude oils and their fractions, in Zartmann, R. E., ed., Short Papers of the 4th International Conference on Geochronology, Cosmochronology, Isotope Geology, 1978, U.S. Geological Survey Open File Report 78-701, p. 384-385. Silverman, S. R., 1967, Carbon isotopic evidence for the role of lipids in petroleum formation: J. Am. Od Chem. Soc., v. 44, p. 691-695.
36
D RIGBV, B. D. BATrs and J. W. SMrrH
Silverman, S. R., 1971, Influence of petroleum origin and transformation on its distribution and distribution in sedimentary rocks: Proceedings of the 8th World Petroleum Congress, v. 2, p. 47-54. Sdverman, S. R., and Epstein, S., 1958, Carbon ~sotopic compositions of petroleums and other sedimentary organic materials" Bull. Am. Assoc. Pet. Geol., v. 42, p. 998-1012. Simoneit, R. T., Brenner, S., Peters, K. E., and Kaplan, I. R., 1978, Thermal alteration of Cretaceous black shale by basaltic intrusions in the Eastern Atlantic: Nature, v. 273, p. 501-504. Smith, B. N., and Epstein, S., 1970, Blogeochem~stry of the stable isotopes of hydrogen and carbon in salt marsh biota: Plant Physiol., v. 46, p. 738-742. Smith, B. N., and Epstein S., 1971, Two categories of 13C/12C ratios for higher plants: Plant Physiol., v. 47, p. 380-383.
Stahl, W. J., 1976, Economically important application of carbon isotope data of natural gases and crude oil, m Nuclear Techniques in Geochemzstry and Geophysics, IAEA, p. 213-222. Stahl, W. J., 1978, Source rock-crude oil correlation by isotopic type-curves: Geochim. Cosmochim. Acta, v. 22, p. 1573-1577. Threlfall, W. F., Brown, B. R., and Griffith, B. R., 1976, Gippsland basin, off-shore, in Leslie, R. B., Evans, H. J., and Knight, C. L., eds, Economic Geology of Austraha and Papua New Guinea, v. 3, Petroleum, Australasian Institute of Mining and Metallurgy, p. 41-67. Welte, D. H., Kalkreuth, W., and Hoefs, J., 1975, Age trend in carbon-isotopic composition in Paleozoic sediments: Naturwissenschaften, v. 62, p. 482-483. Wickman, F. E., 1953, Wird das haufigkeitsverhaltnis der kohlenstoffisotopen bei der inkohlung verandert?: Geochim. Cosmochim. Acta, v. 3, p. 214-252.