Full text of DOI:10.1007/BF02901173

of Geology, and the analytical error is less than 0.3‰. The
isotopes of saturated monomer hydrocarbons were mainly
measured on Mat-252 at the State Key Laboratory of
Mineralization, Nanjing University, the hydrogen isotopes
were measured on MAT-251 with the analytical error less
than 5‰.
Isotopic composition charac-
teristics and identification of
immature and low-mature oils
The carbon isotopes of crude oils and corresponding
Pr/Ph ratios are listed in table 1.
XU Yongchang1, SHEN Ping1, LIU Wenhui1,
GUAN Ping2 & HUANG Difan3
1
oils
Distribution frequency of carbon isotopes of whole
1. State Key Laboratory of Gas Geochemistry, Lanzhou Institute of Ge-
ology, Chinese Academy of Sciences, Lanzhou 730000, China;
2. Department of Geology, Peking University, Beijing 100871, China;
3. Institution of Exploration & Development, China Petroleum & Natural
Gas Corporation Limited, Beijing 100083, China
The G13C frequency distribution of whole oils of
immature and low-mature oils from different areas in
China is shown in fig. 1, from which one can see that the
frequency of the G13C values shows a doublet distribution,
the first peak group is the main peak group, ranging from
ꢁ34‰ to ꢁ28‰ and with ꢁ30‰ to ꢁ29‰ as the main
peak values, the second peak group locates between
ꢁ28‰ and ꢁ25‰ with ꢁ27‰ to ꢁ26‰ as the main peak
values.
The immature and low-mature oils of the first peak
group have rather light carbon isotopes and the deposi-
tional environment of their hydrocarbon-forming parent
materials is related to lacustrine facies[5]. The parent mate-
Abstract
Isotopic composition characteristics and the
significance of immature and low-mature oils are first sys-
tematically discussed. The carbon isotopes of the whole oil
can be divided into two groups, one has Gꢀ13C main peak val-
ues ranging from ꢁ30‰ to ꢁ29 and the other from ꢁ27‰ to
ꢁ25‰, they are related to lacustrine and salt-lake facies or
swamp facies, respectively. The carbon isotopic fractionation
among different group components is relatively small, usu-
ally less than 2‰ and the biggest difference in fractionation
often occurs between saturated and aromatic fractions. Their
GꢀD values vary between ꢁ180‰ and ꢁ130‰. The main peak
of their GꢀD values concentrates between ꢁ170‰ and ꢁ150‰,
suggesting a domination of lacustrine facies. However, the
secondary peak ranges from ꢁ160‰ to ꢁ150‰, showing a
frequent salinization of paleo-water bodies. The average Gꢀ13C
values of the methane vary between ꢁ50‰ and ꢁ52‰, about
10‰ lighter than those of mature oils. There is a relatively
good correlationship between immature and low-mature oils
and their source rocks in carbon isotopic compositions of
group fractions and monohydrocarbons; moreover, com-
pared with the source rocks of mature oils, that of immature
oils is often relatively depleted in 13C, which is one of the
characteristics of immature oils, differing from those of ma-
ture oils.
rials are comparatively good, usually Type-ĉ or Type-Ċ
kerogen, constituting the major part of immature and
low-mature oils.
The immature and low-mature oils of the second
peak group have relatively heavier carbon isotopic com-
positions and they are mainly related to two depositional
environments of oil-forming parent materials, namely
salt-lake and swamp facies. However, the Type-ċ kero-
gen is not the only parent material of these immature and
low-mature oils. Since oils and kerogens from salt-lake
facies often have heavier carbon isotopic compositions,
their original organic matter may be Type-ċ or even
Type-ĉ kerogen, which characterizes the salt-lake kero-
gen.
Keywords: immature and low-mature oil, isotope, associated gas.
The study on the hydrocarbon formation, migration
and accumulation of organic matter at the low evolution-
ary stage to form an immature and low-mature petroleum
reservoir is one of the frontiers and hot points in the cur-
ü
rent oil-gas geochemistry[1 3]. Relatively speaking, the
isotopic study on immature oils and gases, especially on
immature oils, falls behind and few data are available.
Based on the isotopic data of 39 immature and low-
mature oils systematically analysed in recent years and of
4 mature oils used for the comparison, the present note
discusses the isotopic composition characteristics of im-
mature and low-mature oils and their identifying indica-
tors, meanwhile some data of previous studies are em-
ployed. The isotopic measurement of whole oils and dif-
ferent group components was performed in the Shengli
Petroleum Geology Institution and the Lanzhou Institute
Fig. 1. Frequency distribution of G13C values of immature and mature
oils.
2
Correlation between G13C values and Pr/Ph ratios
of whole oils
Two dotted lines, referring herein to G13C = ꢁ28‰,
according to the demarcation in fig. 1 and Pr/Ph=1.5, re-
Chinese Science Bulletin Vol. 46 No. 22 November 2001
1923

NOTES
spectively divide all of symbols in fig. 2 into four zones
representing four different depositional environments of
oil-forming parent materials.
are less than or equal to ꢁ28‰ but Pr/Ph ratios greater
than 1.5, i.e. most of them are 2f. Compared with the
oil-forming environment of zone a, it has mildly reducing
conditions and all of samples here come from the Jinggu
Basin that is known to be of lacustrine fresh-water source
rocks, above and beneath which there are coal beds and no
wonder their reducing conditions are relatively weak. Or-
ganic petrological study has shown that in the Jinggu Ba-
sin exinite and sapropelinite account for 70% ü95% of
the maceral in the source rocks, being advantageous to oil
(ν) Zone a. More samples concentrate in this zone
whose G13C values are less than or equal to ꢁ28‰ and
Pr/Ph ratios less than 1.5, suggesting a lacustrine envi-
ronment with relatively strong reducing conditions and a
better kerogen type.
(ξ) Zone b. Samples in this zone are of similar
oil-forming conditions to those in zone a. Their G13C values
Table 1 Carbon isotopic compositions of immature and low-mature crude oils in China
G13C (‰)
aromatics
Location
Sample No.
Well No.
Pr/Ph
whole oil
saturated
nonhydrocarbon
asphaltene
1
2
Gao1-6-14
Gao81
2.1
ꢁ31.1
ꢁ29.8
ꢁ28.8
ꢁ28.4
ꢁ29.8
ꢁ28.4
ꢁ29.4
ꢁ29.2
ꢁ29.6
ꢁ29.9
1.06
3
Gao3-0-041
ꢁ31.0
ꢁ29.3
ꢁ29.0
ꢁ29.2
Liaohe
4
5
6
7
8
9
Tuo12
1.02
0.6
ꢁ29.9
ꢁ30.4
ꢁ29.6
ꢁ29.0
ꢁ28.1
ꢁ27.8
ꢁ29.0
ꢁ30.9
ꢁ29.6
ꢁ29.0
ꢁ27.9
ꢁ27.1
ꢁ28.1
ꢁ29.0
ꢁ28.4
ꢁ28.1
ꢁ26.5
ꢁ26.3
ꢁ28.2
ꢁ29.4
ꢁ28.4
ꢁ28.0
ꢁ27.3
ꢁ26.0
ꢁ28.3
ꢁ29.4
ꢁ28.0
ꢁ26.6
ꢁ26.0
ꢁ26.5
Lei11
Du126
1.18
Guangbei6-Xie2
Gunan24-2
Chengbei36
1.02
2.7
Jiyang
10
11
12
13
14
15
16
17
18
19
Tan342
0.4
ꢁ25.6
ꢁ26.5
ꢁ27.5
ꢁ26.1
ꢁ27.1
ꢁ28.7
ꢁ26.9
ꢁ34.2
ꢁ33.2
ꢁ26.2
ꢁ25.9
ꢁ24.0
ꢁ26.8
ꢁ27.4
ꢁ25.5
ꢁ29.9
ꢁ30.9
ꢁ25.7
ꢁ25.7
ꢁ25.8
ꢁ27.2
ꢁ27.4
ꢁ26.7
ꢁ27.3
ꢁ30.0
ꢁ26.3
ꢁ25.7
ꢁ25.8
ꢁ28.0
ꢁ26.8
ꢁ26.7
ꢁ29.1
ꢁ31.0
Guang33
Wang8-1
Hong3-2
Wangsi4-1
Bansheng56
Guan128
Guan143
Zhang18-28
Ban25
0.35
0.33
0.87
0.37
ꢁ25.6
ꢁ26.6
ꢁ26.8
ꢁ27.2
Jianghan
0.43
0.53
0.31
1.71
ꢁ32.6
ꢁ28.0
ꢁ25.1
Huanghua
ꢁ25.2
ꢁ23.9
ꢁ23.0
ꢁ24.1
20
21
Zhao27
0.62
0.21
ꢁ25.6
ꢁ28.5
ꢁ26.7
ꢁ29.0
ꢁ25.0
ꢁ27.7
ꢁ27.2
ꢁ27.8
ꢁ25.8
ꢁ28.5
Jizhong
Erlian
Xiliu101
22
23
Ba1
Ba9
0.39
0.78
ꢁ33.5
ꢁ34.5
ꢁ31.5
ꢁ29.8
ꢁ31.4
ꢁ30.6
ꢁ30.4
ꢁ32.8
ꢁ30.7
24
25
Ta5
0.21
0.25
ꢁ29.7
ꢁ29.7
ꢁ29.7
ꢁ31.9
ꢁ28.8
ꢁ29.8
ꢁ29.2
ꢁ30.1
ꢁ29.5
ꢁ29.3
Subei
Anhui
Lin1
26
Wang16
0.46
ꢁ29.3
ꢁ29.4
ꢁ28.3
ꢁ28.9
ꢁ28.9
27
28
29
30
Mi103
Yun1
0.29
0.52
0.48
0.69
ꢁ29.8
ꢁ28.1
ꢁ29.4
ꢁ29.9
ꢁ30.1
ꢁ27.5
ꢁ28.3
ꢁ28.3
ꢁ29.7
ꢁ26.5
ꢁ29.9
ꢁ27.9
ꢁ28.7
ꢁ27.8
ꢁ29.0
ꢁ29.8
ꢁ28.1
ꢁ28.9
Henan
Zhao1
Wei135
ꢁ27.2
31
32
33
Lai64
0.82
0.88
0.93
ꢁ29.9
ꢁ31.8
ꢁ30.0
ꢁ30.2
ꢁ31.6
ꢁ30.2
ꢁ29.9
ꢁ30.4
ꢁ29.1
ꢁ29.0
ꢁ27.4
ꢁ29.9
ꢁ27.2
ꢁ30.5
ꢁ30.0
Chang23
Da407
Songliao
34
35
36
37
38
Niu2
1.9
ꢁ32.1
ꢁ30.0
ꢁ29.3
ꢁ31.4
ꢁ29.1
ꢁ32.4
ꢁ31.1
ꢁ30.2
ꢁ28.7
Niu2-1
Niu2-6
Niu5-1
Niu7
2.0
1.89
1.95
2.85
Jinggu
ꢁ31.4
ꢁ30.9
ꢁ29.6
ꢁ31.0
39
40
41
42
43
HuaS1-103
Yao319
Qidong1
Qi6-3
0.58
0.33
0.38
0.74
0.51
ꢁ24.9
ꢁ25.7
ꢁ25.3
ꢁ25.2
ꢁ26.3
Qaidam
Yao24
1924
Chinese Science Bulletin Vol. 46 No. 22 November 2001

Fig. 2. G13C-Pr/Ph relationship of immature and low-mature oils.
formation.
(ο) Zone c. In this zone G13C values should be
greater than ꢁ28‰ but Pr/Ph ratios less than 1.5. In fact,
the samples distributed currently in this zone have G13C
values ranging from ꢁ27‰ to ꢁ24‰ and Pr/Ph ratios less
than 1. The depositional environment of their parent ma-
terials is very strongly reducing, the type of organic matter
oils of the same source usually possess the similar type
curves because of the isotopic inherited effect of parent
materials and a certain point of the tendency line is the
G13C value of kerogens of source rocks. Based on the data
shown in table 1 the following conclusions can be drawn.
(1) In the data of group components of 34 samples
there are 6 samples whose 13C increases basically with the
increase of polarity, namely their group components have
a normal type curve of G13C values. They, amounting to
17% of the total samples, are Nos. 7, 22, 25, 29, 31 and 34.
Moreover, such a tendency can also be found in other 5
samples, which, accounting for 15% of the total samples,
are Nos. 8, 16, 17, 32 and 38. The samples whose carbon
isotopic compositions of the aromatics are heavier than
and similar to those of the non-hydrocarbons and asphal-
tenes are 14 and 17, respectively, i.e. about 21 samples,
amounting to 62% of the total samples, have relatively
heavier carbon isotopic compositions of aromatics, which
may be regarded as one of the characteristics of immature
and low-mature oils.
(2) The biggest fractionation of carbon isotopes gen-
erally exists between saturated hydrocarbons and asphal-
tenes for a normal crude oil, if the carbon isotopic compo-
sitions are relatively light, this fractionation is often
greater than 2‰. However, in the case of immature and
low-mature oils the situation is more complicated and the
total fractionation, in general, is smaller than that of nor-
mal crude oils. The fractonations of 23 samples among the
43 samples listed in table 1 are smaller than 2‰ and this
proportion is about 68%; morever, quite a number of the
samples possess the greatest fractionation between satu-
rated hydrocarbons and aromatics, which, to some extent,
can be regarded as one of the characteristics of immature
and low-mature oils.
is not Type ċ though their kerogens and crude oils pos-
sess heavy carbon isotopic compositions. In general, the
G13C value of kerogens cannot be used to identify the
types of organic matter of salt-lake facies.
(π) Zone d. The division line for this zone is G13C
values greater than ꢁ28‰ and Pr/Ph ratios greater than 1.5.
Samples in this zone are of poor types of organic matter
and their depositional environments are usually weakly
reducing or oxydizing. Typical coal-formed oils from the
Turpan-Hami Basin are distributed in this zone and their
Pr/Ph ratios are generally greater than 3. A distinct differ-
ence from common immature and low-mature oils is that
those oils from the Turpan-Hami Basin are light oils,
which is one of the important indexes of the immature and
low-mature coal-formed oil. Sample No.19 from Well
Ban-25 is located in this zone and its source rock is
formed in a proximal open lake basin that receives plenty
of terrestrial organic debris, such as pine and cypress input.
After biological decomposition and reworking, light oils
characterized by bicyclic sesquiterpanes are formed.
Maybe, this kind of light oils can be regarded as a tran-
sient product from the immature and low-mature oil of
lacustrine facies to that of swamp facies.
3 Carbon isotopic characteristics of group compo-
nents of crude oils
The carbon isotopic compositions of group compo-
nents for the same crude oil show a tendency of the 12C
depletion with the increase of polarity, and the type curve
drawn according to G13C values of saturated hydrocarbons,
aromatics, non-hydrocarbons and asphatenes is often em-
ployed in the oil-oil and oil-source correlations[4,5]. Crude
4
Corretationship of Gꢀ13Csat -Gꢀ13Caro
It can be seen in fig. 3 that most of the points are
basically distributed along a band with 45e gradient and
can be roughly divided into two zones.
Chinese Science Bulletin Vol. 46 No. 22 November 2001
1925

NOTES
Fig. 3. G13CSat-G13CAro relationship of immature and low-mature oils.
Fig. 4. G13Coil-GDoilǂ relationship of immature and low-mature oils.
(1) Zone A basically includes all of oils formed in
lacustrine facies and equals zone a in fig. 2.
(2) Zone B mainly consists of oils formed in
salt-lake and swamp facies and corresponds, on the whole,
to zones c and d in fig. 2.
oils as a boundary between fresh, brackish water facies
and saline water facies.
Fig. 4 shows a 13C - D correlationship of immature
G
G
and low-mature oils. A more distinct concept can be
drawn out from fig. 4 due to the concrete indication of oil
production areas. For example, the GD values of the crude
oil samples from the Baise, Nanxiang and Liaohe Basins
approach ꢁ150‰, suggesting a certain salinization of the
oil-forming environment; however, the oils from the
Songliao Basin are mainly formed under a freshwater
lacustrine facies. Moreover, the oils from the Tur-
pan-Hami Basin seem to be formed in a freshwater facies
as well, in combination with their carbon isotope data and
other information it can be drawn that their oil-forming
environment should be a freshwater swamp facies.
(3) The 45e gradient line indicates two directions of
lake evolution. As for immature and low-mature oils, it
principally means salinization, and oils from salt-lake
facies of the Jianghan and Qaidam Basins are distributed
in zone B. The other direction of the tendency line implies
swampiness to form coal-type immature and low-mature
oils, such as oils from the Turpan-Hami Basin.
5
Carbon isotopic characteristics of hydrogen iso-
topes and associated gases in immature and low-ma-
ture oils
From the data of refs. [6, 7], it can be seen that ex-
cept for the Turpan-Hami Basin, the G13C values of meth-
ane associated with immature and low-mature oils range
from ꢁ46‰ to ꢁ58‰; for example, the G13C1 values of 12
samples from the Liaohe Basin vary from ꢁ47.2‰ to
The hydrogen isotopic composition of crude oils is
mainly related with the salinity of depositional medium
when oil-forming parent materials are formed. By study-
ing hydrogen isotopes of crude oils from terrestrial basins
in China, Shen Ping[5] delimited the GD=ꢁ150‰ of whole
1926
Chinese Science Bulletin Vol. 46 No. 22 November 2001

compositions of source rocks are heavier than those of
crude oils, which is considered as the results of the mass
discrimination effect happening during the hydrocarbon
migration to enable lighter isotopes to be expelled out
preferentially and heavier ones as well as of the isotopic
fractionation left behind during the hydrocarbon forma-
tion. Among six combinations between the immature and
low-mature source rock and oil there are three combina-
tions that have such a reverse correlation, accounting for
50%. If counting in Well Ba 9 of the Erlian Basin (fig.
6(d)) and Well Tou 12 of the Liaohe Basin (fig. 6(e)), in
which three G13C values of the source rocks are heavier
than their counterparts of the crude oils, one can rea-
sonablly draw a conclusion that it is very common for the
correlation between immature and low-mature source
rocks and their corresponding oils that the G13C values of
the soluble organic matter extracted from source rocks are
less than those of crude oils and their group components.
Another feature of the type of lightness is that the ten-
dency of the type curve towards the 13C enrichment be-
comes clear with increasing the polarity of group compo-
nents.
ꢁ52.4‰, averaging to ꢁ50‰, those of 6 samples from the
Jiyang Basin range from ꢁ47.1‰ǂto ꢁ55.4‰, averaging
to ꢁ51.9‰, 17 samples from the Subei Basin possess the
G13C1 values of ꢁ46.2‰ toꢀꢁ57.6‰ with an average of
ꢁ52‰, and 4 samples from the Jianghan Basin have G13C1
values range from ꢁ46.9‰ to ꢁ55.5‰ with ꢁ49.7‰ as the
mean value. Fig. 5 is a G13C1 frequency distribution of
typical gases associated with immature and low-mature
oils, from which it can be seen that their G13C1 values
range from ꢁ46‰ to ꢁ58‰ while those of gases associ-
ated with normal crude oils vary generally from ꢁ40‰ to
ꢁ48‰[6,7]. Therefore, in the case of carbon isotopes, the
G13C1 value of an associated gas can be regarded as one of
the indicators to its associated immature and low-mature
oils if the value ranges from ꢁ50‰ to ꢁ54‰.
(ξ) Type of crossing. The two combinations in fig.
6(d) and fig. 6(e) belong to this type. Isotopic type curves
of source rocks and oils are mutually across; on an aver-
age, the type of lightness is still dominant, i.e. among the
five values those of source rocks with 12C enrichment ac-
count for 60%. One of the characteristics for the type of
crossing is that the carbon isotopic composition of aro-
matics is depleted in 12C compared with the counterparts
of non-hydrocarbons and asphaltenes.
Fig. 5. G13C1 frequency distribution of gases associated with immature
and low-mature oils.
6 Correlation of oil-source rock and monomer hy-
drocarbons by carbon isotopes
(ο) Type of heaviness in source rocks. Four out of
the five components in this type possess a feature that the
G13C value of source rocks is greater than that of oils. As
mentioned above, this is the main type for normal oils.
The 13C enrichment of source rocks is identical with the
fractionation mechanism caused by the fractionation effect
of thermo-evolutionary dynamics and hydrocarbon migra-
tion. However, among the comparison diagrams of type
curves for immature and low-mature oils and source rocks
there is only one typical group, namely that shown by fig.
6(f), in which the two type curves of the Well Lei36
source rock and Well Lei11 oil from the Liaohe Basin
demonstrate that all the components of the crude oil are
rich in 12C compared with their counterparts of the source
rock except for the G13C value of the saturated hydrocar-
bon of the oil that is similar to but slightly heavier than its
counterpart of the source rock. In addition, a similar
relationship exists between the Well Gudong 23-1 source
rock and the Well Guangbei-6-Xie2 oil of the Jiyang Sag
(fig. 6(b)), but their fractionation value is much smaller
than that in fig. 6(f).
Six groups of type curves are drawn in fig. 6 ac-
cording to the carbon isotopic data of oils and corre-
sponding source rocks, and three types of correlations are
delineated as follows:
(ν) Type of lightness in source rocks. There are
five G13C values of each sample, namely for saturated
hydrocarbon, whole oil or asphaltene A, aromatics,
non-hydrocarbon and asphaltene. If four values of a
source rock show that it is rich in 12C, this source rock
belongs to the type of lightness. For example, the G13C
values of the source rock from Well Guan 996 of the
Huanghua Sag are lighter than those of the crude oil from
Well Guan 143 (fig. 6(a)), so does the source rock from
Well Gudong 23-1 of the Jiyang Sag compared with the
crude oil from Well Guan 24 (fig. 6(b)). Except for the
asphaltene of the crude oil from Well Yun 1 of the Miyang
Sag, the G13C values of other components are heavier than
those of the source rock from the same well (fig. 6(c)), so
its source rock can also be regarded as belonging to the
type of lightness. Generally speaking, carbon isotopic
Chinese Science Bulletin Vol. 46 No. 22 November 2001
1927

NOTES
Fig. 6. Comparison of carbon isotopic type curves of oils and source rocks.
Fig. 7. Carbon isotopic correlation of monomer hydrocarbons between crude oils and source rocks.
Carbon isotopic correlation of monomer hydrocar-
is more depleted in 13C than that of crude oils. As men-
tioned above, the type of lightness in source rocks is the
characteristic of carbon isotopic compositions of mono-
mer hydrocarbons from immature and low-mature oils,
like what is shown in fig. 7(a, b); however, fig. 7(c, d)
may belong to the above-mentioned type of crossing. Ex-
cept fig. 7(e), all of them can also be regarded as taking
on the character of the type of lightness in source rocks,
speaking precisely, before C24 they all possess this char-
acter except the case as shown by fig. 7(e).
bons among the five groups of source rocks and crude oils
is demonstrated in fig. 7, from which the following can be
deduced:
(1) Source rocks and crude oils possess a similar
shape of carbon isotopic curves of monomer hydrocarbons,
approximately before C16 the G13C values show a tendency
to enrich 13C, at C16 they are of the 13C peak, and after-
ward they have a tendency of 12C enrichment with the
increase of carbon numbers. The similarity of G13C curve
shapes of monomer hydrocarbons indicates a constituent
likeness of precursors, namely there is a comparability
between oils and source rocks.
7
Conclusions
The G13C frequency histogram of whole oils shows a
doublet distribution, indicating that immature and
low-mature oils consist of two groups of crude oils. One
(2) Like the G13C type curve of oils and source rocks,
the G13C value of monomer hydrocarbons of source rocks
1928
Chinese Science Bulletin Vol. 46 No. 22 November 2001

constitutes the major part of immature and low-mature
oils and its G13C values of the whole oil mainly range from
ꢁ34‰ to ꢁ28‰ with the main peak values from ꢁ30‰ to
ꢁ29‰. Combined with Pr/Ph ratios and other data, this
type of crude oils is thought to form mainly in reducing
environments related with lacustrine facies and to have
13C values between the two different oil-associated
G
methanes is usually as big as 8‰ü10‰, thus, it can be a
powerful identification indicator.
Correlations by the G13C type curves of group com-
ponents from source rocks and oils as well as by the G13C
curves of monomer hydrocarbons from source rocks and
oils have shown that a relatively good comparability ex-
ists between immature and low-mature oils and their pos-
sible source rocks. On the basis of the comparable G13C
features between immature and low-mature oils and
source rocks, the present note divides them into three dif-
ferent types, i.e. the type of lightness in source rocks, the
type of crossing and the type of heaviness in source rocks.
Among six groups of G13C data, the type of lightness in
source rocks amounts to 50%. The feature of the type of
crossing is that three out of the five components, such as
asphaltene A, saturated hydrocarbon, aromatics, non-hy-
drocarbon and asphaltene, belong to the type of lightness
in source rocks, namely the relative 12C enrichment still
dominates the G13C values of source rocks and this type of
oil and source rock combination accounts for 33% of the
total data. There is only one, typical type of heaviness in
source rocks, among the six groups of data and its propor-
tion makes up 16% of the total data. The lighter G13C
value for the various components of the soluble organic
matter from source rocks in comparison with that from
oils is probably one of the carbon isotopic characteristics
of immature and low-mature oils.
chiefly typesĉ and Ċ kerogens. The other group has
G13C values varying from ꢁ28‰ to ꢁ25‰ with main peak
values from ꢁ27‰ to ꢁ26‰. According to Pr/Ph ratios
and other data, this kind of crude oils is deduced to form
in two depositional environments, i.e. salt-lake and
swamp facies. Oils formed under both of the conditions
are of G13C values of whole heavier oils, the salt-lake oil is
formed in a strongly reducing environment and often pos-
sesses a Pr/Ph ratio less than 0.5, usually its kerogen types
are good though their G13C values are of heavier oils.
However, the coal-type immature and low-mature oils of
swamp facies is formed under a comparatively oxydizing
condition, its Pr/Ph ratio is often greater than 3 and its
kerogen types are relatively poor. It is a light oil.
In crude oils G13C values of group components, such
as saturated hydrocarbons, aromatics non-hydrocarbons
and asphaltenes, generally show a little difference, namely
the carbon isotopic fractionations among various compo-
nents are small with the increase of polarity, their mutual
difference is usually less the 0.3‰; this may be one of the
characteristics of immature and low-mature oils. With the
increase of maturity, group components gradually show a
tendency of 13C enrichment as the polarity increases;
however, their type curves sometimes are not regular due
to the heavier carbon isotopic composition of aromatics.
The hydrogen isotopic composition of immature and
low-mature oils ranges from ꢁ180‰ to ꢁ130‰ and often
shows a normal distribution, indicating the variety of their
paleo-salinity, such as freshwater, brackish and salt-lake
Acknowledgements This work was supported by the China Petroleum
& National Gas Limited Corp. (Project No. 960006) and the National
Natural Science Foundation of China (Grant No. 49973010).
References
1. Huang Difan, Immature crude oil and its geochemical significance,
Acta Petroleum Sinica (in Chinese), 1987, 8(1): 1.
facies. The main peak groups of their GD values vary from
ꢀ
2. Wang Tieguan, Zhong Ningning, Hou Dujie et al., Formation
Mechanism and Distribution of Low-mature Oils and Gases (in
Chinese), Beijing: Petroleum Industry Press, 1995.
ꢁ170‰ to ꢁ160‰, suggesting the domination of
lacustrine facies for the formation of immature and
low-mature oils in China. The GꢀD values ranging from
ꢁ160‰ to ꢁ150‰ as the secondary peak indicate a
frequent salinization of paleowater bodies during the
formation of lacustrine immature and low-mature oils.
The carbon isotopic value of whole oils can hardly
be an indicator to the identification of immature and
low-mature oils; however, the G13C value of the methane
associated with oils is able to play an important role in
distinguishing immature and low-mature oils from normal
ones. In general, the G13C value of the methane associated
with mature oils ranges from ꢁꢀ48‰ to ꢁꢀ40‰ with an
average from ꢁꢀ41‰ to ꢁꢀ42‰; however, the methane as-
sociated with immature and low-mature oils mainly has a
G13C value varying from ꢁꢀ48‰ to ꢁꢀ58‰ and a medium
value ranging from ꢁ50‰ to ꢁ51‰. The difference in
3. Huang Difan, Development of the hydrocarbon-forming theory
(ĉ) üüHydrocarbon-forming evolutionary model of the im-
mature oil and organic matter, Advance in Earth Sciences (in
Chinese), 1996, 11(4): 327.
4. Huang Difan, Distribution characteristics of carbon isotopic com-
positions of the organic matter from terrestrial source rocks, China
Offshore Oil and Gas (Geology) (in Chinese), 1993, 7(4): 1.
5. Shen Ping, Compositional characteristics of carbon and hydrogen
isotopes in light hydrocarbons, Science in China, Ser. B, 1995,
38(2): 221.
6. Xu Yongchang (ed.), Genetic Theory on Natural Gas and Its Ap-
plication (in Chinese), Beijing: Science Press, 1993, pp. 1ü414.
7. Liu Wenhui, Xu Yongchang, Shi Jiyang et al., Bio- and Thermo-
Catalyzed Transitional Zone Gas (in Chinese), Beijing: Science
Press, 1998, pp. 1ü232.
(Received February 19, 2001; revised May 8, 2001)
Chinese Science Bulletin Vol. 46 No. 22 November 2001
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