Geochemical Journal, Vol. 38, pp. 461 to 471, 2004
Abundant non-hydrocarbons in crude oils from the western Qaidam Basin,
northwest China
YI DUAN,1* YAORONG QIAN,2 C HUANYUAN WANG1, ZHIPING WANG,3 XIAOBAO ZHANG,1 HUI ZHANG,1
BAOXIANG WU 1 and GUODONG ZHENG 4
1
Lanzhou Institute of Geology, Chinese Academy of Sciences, Lanzhou, Gansu Province 730000, People’s Republic of China
2
Analytical Chemistry Branch, BEAD/Office of Pesticide Programs, US Environmental Protection Agency,
701 Mapes Rd., Fort Meade, MD 20755-5350, U.S.A.
3
Institute of Botany, the Chinese Academy of Sciences, Beijing 100093, People’s Republic of China
4
Department of Earth and Planetary System Science, Graduate School of Science, Hiroshima University,
Higashi-Hiroshima 739-8526, Japan
(Received October 6, 2003; Accepted April 7, 2004)
The non-hydrocarbon fraction of immature-low maturity crude oils from the western Qaidam Basin which were formed
in the source beds deposited in highly saline and strongly reducing environments were analyzed by GC-MS to determine
their compositions. Abundant fatty acids, alkanols, fatty acid glycerol monoesters, and stenols were identified. Geochemical
analyses suggested that these compounds were largely originated from the oil source rocks. The linear compounds consist
primarily of the short-chain homologues (C10–C20). The C27–C 29 sterols detected were dominated by C27 stenols. The
molecular characteristics suggest that algae and bacteria were the major source organism. The presence of abundant fatty
acid glycerol monoesters suggests that the biological lipids, particularly bacterial phospholipids were important contributors to the formation of the crude oils studied. The existence of abundant unsaturated linear compounds, stenols and high
CPI values of linear non-hydrocarbon compounds is consistent with the low maturity of these crude oils.
Keywords: non-hydrocarbon, polar organic compound, fatty acid, alkanol, fatty acid glycerol monoester, crude oil
geochemical characteristics of the source material, they
can provide additional useful information about the source
material, maturity and genesis of crude oils. A few studies on the carboxylic acids in crude oils have suggested
that these compounds could be used as indicators of maturation, biodegradation and migration (Behar and Albrecht,
1984; Jaffé et al., 1988a, b; Jaffé and Gardinali, 1990;
Jaffé and Gallardo, 1993; Borgund and Barth, 1993;
Meredith et al., 2000). Nitrogen-containing compounds
and phenols in crude oils have also been reported as tracers of secondary oil migration (Yamamoto and Taguchi,
1991; Yamamoto, 1992; Ioppolo et al., 1992; Li et al.,
1992; Taylor et al., 2001). Comprehensive studies of nonhydrocarbons in crude oils are, therefore, important and
can provide valuable information for our understanding
about the nature and formation of oils.
Qaidam Basin is a Tertiary sedimentary basin of an
interior saline lake located in the northeastern corner of
the Tibetan plateau in northwest China (Fig. 1). During
the Cenozoic period, the climate of the Qaidam Basin area
was dry and the water supply to the lake was limited, resulting in the formation of saline and hypersaline lakes
(Huang et al., 1993). The dry palaeoclimate and high salinity of the water had a profound effect on the abundance,
nature and preservation of organic matter in the sediments.
INTRODUCTION
The organic geochemistry of hydrocarbon biomarkers
in crude oils has been studied extensively and many
chemical parameters have been developed to evaluate the
origins, genetic relationships, thermal maturity, migration and extent of biodegradation of oils (e.g., Seifert and
Moldowan, 1978; Mackenzie, 1984; Moldowan et al.,
1985; Philp, 1985; Peters and Moldowan, 1993 and references therein). However, few studies have been conducted on the geochemistry of non-hydrocarbons in crude
oils. Our understanding of non-hydrocarbon compounds
in petroleum is rather limited largely due to the complexity of non-hydrocarbons and the lack of adequate analytical techniques.
Non-hydrocarbons are important constituents of crude
oils and high concentrations of these compounds are found
in immature and low maturity crude oils (Wang et al.,
1995; Qiang et al., 1997). Because non-hydrocarbons
contain heteroatoms (such as O, N, S) and functional
groups which could be indicative of biochemical and
*Corresponding author (e-mail: [email protected])
Copyright © 2004 by The Geochemical Society of Japan.
461
Fig. 1. Study area showing the sampling location.
Lacustrine mudstones, marls and calcareous mudstones
of Oligocene and Miocene were widespread in the western Qaidam Basin. The Gancaigou Formation deposited
in a deep lacustrine environment is the main oil source
rocks for the western Qaidam Basin (Fig. 2). These source
rocks had an average TOC value of 0.56%, type II organic matter and a variable maturity (Qinghai Petroleum
Administration, unpublished data).
Investigations of the tectonics and distribution of reservoirs (Fig. 2) have suggested that the distance of oil
migration was generally less than 10 km and some oils
were accumulated in the source rocks (Qinghai Petroleum
Administration, unpublished data). The hydrocarbon
geochemistry of Qaidam Basin has been studied previously (Ritts et al., 1999; Philp et al., 1991; Huang et al.,
1991; Hanson et al., 2001). The non-hydrocarbons in
crude oils remain largely unknown. The main objectives
of this study were to investigate the composition and distribution of non-hydrocarbons in crude oils from this environment and to assess the geochemical significance of
these non-hydrocarbons.
S AMPLES AND METHODS
Samples of crude oils from four oil fields in the western Qaidam Basin were collected with ground-glass stoppered brown flasks in 2001 (Fig. 1). The samples were
kept frozen after collection. The oil reservoir sands are
located within the Eocene to Pliocene strata (Fig. 2).
Each oil sample (40–100 mg) was initially treated with
n-hexane and then filtered to remove asphaltenes. The
deasphalted crude oils were further fractioned by column
chromatography using a 50 × 1 cm i.d. column packed
with 6 g alumina (70–230 mesh, activated for 12 h at
450°C) and 9 g silica gel (80–120 mesh, activated for 12
h at 150°C). Aliphatic hydrocarbons, aromatic hydrocarbons and non-hydrocarbons were obtained by eluting with
200 ml of hexane, 200 ml of benzene and 50 ml of methanol sequentially. The weight of each fraction was deter462 Y. Duan et al.
Fig. 2. Tertiary stratigraphy, depositional systems, potential
source rock and reservoir intervals and source rock parameters
of western Qaidam Basin. MSP: Mean C29 sterane 20S/(20R +
20S) ratio; Ro: Vitrinite reflectances in Oligocene source rocks
of Lucan 1 well.
mined gravimetrically after solvent removal. Prior to instrumental analysis the non-hydrocarbon fraction was
silylated with bis(trimethylsilyl)trifluoroacetamide
(BSTFA, 80°C, 30 min).
The derivatized non-hydrocarbon fraction was
analyzed by a Hewlett-Packard 5890II gas chromatography-5989A mass spectrometry (GC-MS) equipped with
a HP-5 capillary column (50 m × 0.25 mm i.d., 0.37 µm
film thickness). The temperature was programmed from
80 to 300°C at 4°C min–1 and held at 300°C for 15 min.
The temperatures of the injector, transfer line and ion
source of the GC-MS were 300°C, 300°C and 250°C, respectively. The samples were analyzed in the scan mode
from m/z 20 to 800.
Silica gel, alumina as well as filters and cottons were
extracted with dichloromethane/methanol (2:1, v/v) in a
Soxhlet apparatus for 36 h before using. All the solvents
used were distilled twice. All flasks and glass columns
were washed with distilled water followed by methanol
rinse. Method blanks were processed and analyzed using
the identical procedure described above and showed no
traces of target compounds.
RESULTS
Previous studies have shown that these oils all have
low pristane to phytane ratios, an even n-alkane preference, high gammacerane content, significant levels of
>C30 homohopanes, and low isomer ratios of C29 steranes
(Table 1, Duan et al., 2004). These data are consistent
with the conclusion that the oils were generated from
Tertiary hypersaline lacustrine source rocks and have low
maturity levels (Duan et al., 2004).
Composition of non-hydrocarbons
The crude oils studied contain abundant nonhydrocarbons with total concentrations in the range of
Table 1. Composition of hydrocarbons in the crude oils (Duan et al., 2004, submitted)
γ /C30—gammacerane/C 30 hopane.
Table 2. Background information and bulk composition of the crude oils
Sample No.
Background information
Oil field
Qai-1
Qai-2
Qai-3
Qai-4
Qai-5
Qai-6
Huatugou
Shizigou
Youshashan
Youshashan
Gasikule
Gasikule
Age and horizon
Composition
Depth (m)
Asph
Ali
Aro
Non-hydro (mg/g)
489
1134
242
889
1499
3401
29
32
16
47
128
50
323
269
607
499
444
501
98
141
350
151
79
81
550
558
270
303
349
369
Late Pliocene (N2 )
Miocene (N1 )
Early Pliocene (N2 )
Miocene (N1 )
Early Pliocene (N2 )
Early Oligcene (E3 )
Asph—Asphaltene; Ali—Aliphatic fraction; Aro—Aromatic fraction; Non-hydro—Non-hydrocarbon.
Table 3. Relative abundances of non-hydrocarbon compounds in the crude oils
Sample No.
Qai-1
Qai-2
Qai-3
Qai-4
Qai-5
Qai-6
n-Alkanols (%)
Fatty acids (%)
Glycerol esters (%)
Stenols (%)
C1 8 ∆9 Amide (%)
35.1
33.3
39.1
37.2
36.9
29.6
29.6
13.1
12.2
42.3
13.4
21.3
32.7
46.6
45.8
18.8
48.1
47.6
1.5
1.6
1.9
0.8
0.8
1.3
1.1
5.4
1.0
0.9
0.8
0.2
Percent concentrations of these compounds were calculated on the basis of their abundances on total ion current chromatogram.
270–550 mg/g (Table 2). The non-hydrocarbon concentration is higher in Huatugou and Shizigou oil fields than
in Gasikule oil field. The non-hydrocarbon fraction appears to be composed mainly of fatty acids, alkanols and
fatty acid glycerol monoesters (Fig. 3). The relative abundance of these non-hydrocarbons is listed in Table 3. Fatty
acids, alkanols, and fatty acid glycerol monoesters are
the predominant components in the samples (Table 3).
Fatty acid
n-Fatty acids, unsaturated and iso-fatty acids and
phytanic acid are all detected in the oil samples. The identifications of fatty acids are based on the diagnostic fragment ions of m/z 117 (Fig. 4). The mass spectra of n- and
iso-fatty acids have intense M +, M+-15 and M+-43 frag-
ments (Fig. 5A) consistent with that of the standard compound (Fig. 5A′, #24459 in NIST98 Mass Spectrum Library, U.S.A.). Unsaturated acids and phytanic acid (Figs.
5B and C) have the same mass spectra compared to that
of the standard compounds (Figs. 5B′ and C′, #25980 and
124090 in NIST98 Mass Spectrum Library, U.S.A., respectively). Analytical results suggest that the n-fatty
acids are comprised of C10–C26 components and dominated by C16 and C18 fatty acids (Fig. 4 and Table 4). The
C20–/C20+ ratios (<C 20 components/⭓C 20 components)
range from 13.1 to 20.9 (Table 2). The distribution pattern is consistent with those in marine oils reported by
Jaffé and Gallardo (1993), suggesting that these fatty acids are probably derived from algae and bacteria (Chuecas
and Riley, 1969; Cranwell, 1974; Gaskell et al., 1975;
Abundant non-hydrocarbons in crude oils from the western Qaidam Basin, northwest China 463
Table 4. Composition of n-alkanols, stenols and n-fatty acids in the crude oils
U/S—C 16 + C18 unsaturated acids/C16 + C18 saturated acids; ++—high abundance; +—low abundance; C20–/C 20+—<C 20 components/⭌C 20
components.
Fig. 3. Representative total ion current chromatogram of the
non-hydrocarbon fraction (TMS ester) in the samples.
Fig. 4. Mass chromatogram for m/z 117 of fatty acids in sample Qai-6.
Riley, 1969; Rezanka et al., 1983; Piorreck et al., 1984;
Grimalt et al., 1992; Russell et al., 1997).
Matsuda and Koyama, 1977; Duan, 2000). The high even
over odd carbon preference indices (CPI) of fatty acids
(CPI10–26) of 4.3–9.8 are similar to those reported in recent sediments (Venkatesan, 1988; Duan et al., 1998;
Duan and Ma, 2001).
The unsaturated fatty acids of C16 and C18 are also
present in the samples and the ratios of unsaturated fatty
acids to saturated acids of same carbon number range from
0.20 to 0.56, reflecting a high relative proportion of unsaturated fatty acids. The abundance of iso-fatty acids of
C14 and C15 is low in these oil samples. These unsaturated and iso-fatty acids are generally thought to be originated from bacteria (Matsuda and Koyama, 1977; Perry
et al., 1979; Volkman et al., 1980; Duan et al., 1997a).
Abundant unsaturated fatty acids are also indicative of
low maturity organic matter. The detected phytanic acid,
which could derive from the phytyl side chain of chlorophylls, in these oil samples is a potential marker of algae,
cyanobacteria or purple sulfur bacteria (Chuecas and
464 Y. Duan et al.
Alcohols
The n-alkanols are present in the crude oils in relatively high abundance. The identification of alkanols is
based on their mass spectra, particularly the base peak
(m/z 75) and characteristic ion M +-15 (Fig. 6A) which
has the same mass spectrum as standard compound (Fig.
6A′, #79424 in NIST98 Mass Spectrum Library, U.S.A.).
The alkanols are comprised of C14–C 30 components with
C18 being the most abundant alkanol for all the samples
(Fig. 7 and Table 4). The C20–/C20+ ratios (<C20 components/⭓C20 components) are from 2.3 to 3.6 for samples
of Qai-1, Qai-2, Qai-3, and Qai-4, reflecting the predominance of the short-chain n-alkanols. However, the nalkanols in samples Qai-5 and Qai-6 consist primarily of
the long-chain homologues with the C20–/C20+ ratios of
0.7 and 0.8, respectively. Short-chain n-alkanols are generally thought to be derived from aquatic organisms, such
as planktonic algae or submerged macrophytes (Ogura et
Fig. 5. Mass spectra of TMS-derivatives of C 18 fatty acid (A), C 18∆9 fatty acid (B) and phytanic acid (C) in the samples Qai-6
examined and TMS-derivatives of authentic standard C18 fatty acid (A′ ), C18∆9 fatty acid (B′) and phytanic acid (C ′).
al., 1989). Long-chain n-alkanols are considered to be
originated from terrigenous plants (Eglinton and
Hamilton, 1967; Cranwell, 1981; Duan et al., 1997b),
although carbon isotopic data indicate that they can also
come from algae and bacteria (Duan et al., 1997c). The
CPI14–30 values range from 3.8–9.1, showing the features
of low maturity organic matter.
Sterols are composed mainly of the unsaturated homologues (Figs. 3 and 7 and Table 4). Their identifications
are based on the chromatographic retention times (Fig.
7), mass spectra (Fig. 6B) and the comparison of their
mass spectra with published MS data (Fig. 6B′, #121811
in NIST98 Mass Spectrum Library, U.S.A.; John et al.,
1979). The dominant component in the samples is cholest5-en-3β-ol which is thought to be originated primarily
from zooplankton, although it is reported to be common
in algae as well (Gagosian et al., 1983a, b; Volkman,
1986). Both of the 24-methylcholest-5-en-3β-ol and 24ethylcholest-5-en-3 β-ol are low. These two compounds
are thought to be originated from higher plants (Rieley et
al., 1991; Goad and Goodwin, 1972), but may also derive from diverse algae species (Volkman, 1986). This
distribution of sterols is similar to that of steranes in the
samples, although abundance of C27 stenol is higher than
that of C27 steranes.
Fatty acid glycerol monoesters
A series of compounds eluting in the range similar to
C18–C 25 n-alkanols with the m/z 129, 147 and 218 as diagnostic ions have been detected in the oil samples. The
distribution of these compounds in the m/z 147 mass chromatogram (Fig. 8) closely matches that in the total ion
current (Fig. 3), apparently representing the same series
of compounds. The mass spectra features (Figs. 9A and
B) are consistent with fatty acid glycerol monoester structures (Figs. 9A′ and B′, #129019 and #58284 in NIST98
Mass Spectrum Library, U.S.A., respectively; Heller and
Milne, 1978).
Mass spectra containing m/z 129, 147 and 218 fragment ions are indicative of the presence of TMS group in
the glycerol moiety of molecules. Although the molecular ion is very small, the M+-103 and M+-15 ions are apparent and can be used to determine the molecular weight
of related compounds. Other diagnostic ions generated
Abundant non-hydrocarbons in crude oils from the western Qaidam Basin, northwest China 465
Fig. 6. Mass spectra of TMS-derivatives of C 16 n-alkanol (A) and cholest-5-en-3β -ol (B) in the samples Qai-5 examined and
TMS-derivatives of authentic standard C16 n-alkanol (A′ ) and cholest-5-en-3β-ol (B ′ ).
Fig. 7. Mass chromatogram for m/z 75 of alcohols in sample
Qai-4.
466 Y. Duan et al.
Fig. 8. Mass chromatogram for m/z 147 of fatty acid glycerol
monoesters in sample Qai-4. (I—Series I in Fig. 10; II—Series
II in Fig. 10).
Fig. 9. Mass spectra of TMS-derivatives of fatty acid glycerol monoesters I 11 (A) and II3 (B) in the samples examined and TMSderivatives of authentic standard fatty acid glycerol monoesters I11 (A′ ) and II3 (B′).
Table 5. Composition of fatty acid glycerol monoesters in the crude oils
Sample No.
Qai-1
Qai-2
Qai-3
Qai-4
Qai-5
Qai-6
1-Fatty acid glycerol monoesters
2-Fatty acid glycerol monoesters
Cran g e
Cmax
C1 6 /C1 8
Cran g e
Cmax
C1 6 /C1 8
12–18
12–18
12–18
12–18
12–18
12–18
16
18
16
16
18
18
1.02
0.94
1.16
1.27
0.94
0.86
14–18
14–18
14–18
14–18
14–18
14–18
16
18
16
16
16
16
1.64
0.70
1.67
2.20
1.55
1.15
during electron impact ionization mass spectrometry are
m/z 183 + n14 (where n = 0 to 6), which arises from cleavage of the O-O bond of the fatty acid moiety (MCnH2n–1O, n = 12 to 18). It appears that there are two
different groups of compounds with their mass spectra
matching the fatty acid glycerol monoesters. The first
series have base peaks at m/z 315 + n14 (n = 0 to 6; Fig.
10) in their mass spectra and are tentatively identified as
1-fatty acid glycerol monoesters. The second series of the
compounds are characterized by base peaks at m/z 129
and strong diagnostic ions at m/z 285 + n14 (n = 0, 2, 4)
in their mass spectra and are tentatively identified as 2-
1-Isomer (C1 6 + C1 8 )/
2-isomer (C1 6 + C1 8 )
5.75
10.29
7.75
6.43
5.89
6.04
fatty acid glycerol monoesters. These compounds have
not been reported previously in crude oils.
The distribution feature of fatty acid glycerol
monoesters (Fig. 8) is similar to that of fatty acids in the
samples (Fig. 4). 1-Saturated fatty acid glycerol
monoesters range from C12 to C18 with a maximum at C16
or C18. The abundance of C16 and C18 1-fatty acid glycerol monoethers differs in the different samples, C16/C18
ratios ranging from 0.49 to 1.27 (Table 5). 1-Unsaturated
fatty acid glycerol monoesters (C16, C 18) and 1-iso fatty
acid glycerol monoesters (C14, C 15) are present in low
abundance. The abundance of 2-fatty acid glycerol
Abundant non-hydrocarbons in crude oils from the western Qaidam Basin, northwest China 467
Fig. 10. Mass fragmentation pattern of the two series of fatty
acid glycerol monoesters in the samples examined.
monoesters is far lower than that of 1-fatty acid glycerol
monoesters. 2-Unsaturated fatty acid glycerol monoesters
and 2-iso-fatty acid glycerol monoesters are not detected
in the samples. Fatty acid glycerol monoesters are probably derived from the natural hydrolysis of glycerol esters, which are the major components of oil and fat of
organisms and bacteria cell walls.
DISCUSSION
Oil source material
Previous investigations on the origin of oil and the
source organic material are largely based on the compositions of hydrocarbons. However, these compounds are
the diagenetic products of precursor, and some of the information with regard to the nature of the source organic
matter might have been lost during diagenesis. Nonhydrocarbon compounds in crude oils might have retained
some of the original structures of their precursor (Duan,
2001) and can provide more genetic evidence than
diagenetic hydrocarbons.
The crude oils from western Qaidam Basin contain
abundant non-hydrocarbon components, which are composed mainly of fatty acids, alkanols, fatty acid glycerol
monoesters, and stenols (Table 3). Previous studies have
shown that fatty acids in crude oils can be formed by the
biodegradation of reservoir oil (Jaffé et al., 1988a; Jaffé
and Gallardo, 1993). Meanwhile, molecular distributions
of fatty acids in crude oils have also been reported to be
severely affected by oil migration (Jaffé et al., 1988a, b).
However, predominance of n-alkanes in the samples indicates that the crude oils studied herein are nonbiodegraded oils (Fig. 11; Duan et al., 2004).
Previous studies have shown that the oils in the studied reservoirs have not undergone long distance migration (Huang et al., 1993; Qinghai Petroleum Administration, unpublished data). In addition, organic-rich
sediments, such as coal, are also absent in the western
Qaidam Basin. Therefore, abundant non-hydrocarbon
components in the samples show that extraction of the
lipids from carrier beds by migrating oil along the path
may not be significant. Thus, the fatty acids in the oil
samples likely originated from oil source rocks. These
468 Y. Duan et al.
Fig. 11. Representative chromatogram of n-alkanes in the samples.
fatty acids could be released from the loosely bound acids of biogenetic origin in the kerogen matrix at the stage
of low maturity (Ro = 0.30–0.55), in combination with
the biogenic free fatty acids (Jaffé and Gardinali, 1990).
This is consistent with the low maturity of the samples as
discussed in following paragraphs.
The n-alkanols in crude oils are little studied previously, but they have the similar genetic pathway as the
fatty acids. In the crude oils of western Qaidam Basin,
the predominance of short-chain fatty acids, short-chain
n-alkanols and C27 stenol, and the presence of abundant
unsaturated fatty acids, iso-fatty acids, and phytanic acid
indicate that the oil-formation organic material consists
mostly of algae and bacteria-typed organic matter, while
the contribution of land plants to the crude oils is limited. The results of oil-source correlations (Qinghai Petroleum Administration, unpublished data) have suggested
that the crude oils in the Gasikule oil field were accumulated mainly from the surrounding area near the field. The
oils in the Shizigou and Huatugou oil fields are derived
from Shizigou area. Investigations of sedimentary facies
and paleogeography have suggested that the Tertiary river
systems of western Qaidam Basin are primarily located
in the southern part of Gasikule area (Huang et al., 1993).
Thus, the terrigenous organic matter derived by the river
systems was likely limited to the Gasikule oil field.
The studied samples contain abundant fatty acid glycerol monoesters. Fatty acid glycerol monoesters can be
generated during the biochemical process of glyceride
synthesis and during the decomposition organic matter.
These fatty acid glycerol monoesters were probably preserved in the highly saline and strongly reducing environment of the region. Hydrolysis of the oil and fat from
the organisms as well as bacterial phospholipids could
produce fatty acid glycerol monoesters during diagenesis.
The highly saline and alkaline environment of the region
further facilitates the generation and preservation of these
compounds (Xu, 1984). Therefore, the biological lipids
are important constituents of the organic material for the
formation of the low maturity crude oils in the western
Qaidam Basin.
It is generally believed that plant and bacterial lipids
consist primarily of the unsaturated fatty acid glycerol
esters, while animals have high abundance of the saturated fatty acid glycerol esters (Wang, 1983). Although
the saturated fatty acid glycerol monoesters dominate in
the studied samples, hydrocarbon biomarker analyses
suggest that the crude oils were generated largely from
algae (e.g., diatoms and dinoflagellates) and bacteriatyped organic material (Table 1, Duan et al., 2004). This
implies that the saturated fatty acid glycerol monoesters
in the samples were perhaps derived from the reduction
of the unsaturated homologues. In the early diagenetic
stage, it is possible that unsaturated compounds convert
to the saturated compounds by the addition of hydrogen
in reducing environments.
Oil thermal maturity
The non-hydrocarbon compounds detected in the samples are likely biogenic and relatively unstable. Their presence in large amounts indicates that the maturity of the
crude oils is low. The carbon preference index (CPI) of
n-alkanes in crude oils has been used as an indicator of
the maturity for oils. Similarly, if n-fatty acids and nalkanols in crude oils are derived primarily from oil source
rocks, their CPI values might also reflect the oil maturity. It has been reported that the CPI values of n-fatty
acids and n-alkanols are in the range of 3.2–13.8 and 11.5–
34.2 in living plant leaf waxes, respectively (Rieley et
al., 1991), and of 4.2–18.2 and 1.3–17.4 in recent marine
sediments, respectively (Keswani et al., 1984). With the
increasing thermal maturity, the molecular distribution of
n-fatty acids and n-alkanols with a high CPI is presumed
to be transformed into a distribution with a CPI value of
approximately one (Kvenvolden, 1966, 1967; Jaffé and
Gardinali, 1990). The similarity of the CPI values of nfatty acids and n-alkanols in the crude oils to those in
living plant leaves and recent marine sediments, together
with the presence of abundant unsaturated homologues
and stenols, agrees with the low maturity of the crude
oils. This conclusion is in agreement with other studies.
For example, the crude oils have low C29 sterane 20S/
(20R + 20S) ratio of 0.25–0.42 and low C29 sterane ββ/
(ββ + αα) ratio of 0.25–0.43 (Table 1), which show that
these oils studied are immature-low mature (Huang et al.,
1991; Huang et al., 1993; Wang et al., 1995; Qiang et al.,
1997). On the other hand, study of the source rocks indicates that the majority of the Tmax values in Tertiary source
rocks are less than 435°C. Vitrinite reflectance in
Oligocene potential source rocks of Lucan 1 well ranges
from 0.30 to 0.55 (Fig. 2). The average of 5 α , 14 α ,
17α(H)-C29 20S/(20S + 20R) ratio for Oligocene potential source rocks is 0.29 (ranging from 0.1 to 0.5; Qinghai
Petroleum Administration, unpublished data).
Oil genesis
The distribution of n-alkanes in the studied crude oils
exhibits an apparently even carbon number predominance
with CPI values in the range of 0.88–0.98 and a maximum at C16 (C18 or C22). Such distributions of n-alkanes
have some similarity to those of n-fatty acids, n-alkanols
and glycerol monoesters in the samples, showing that the
n-alkanes may come in part from the reduced and
defunctionalized products of these compounds. The oilforming parent material is mainly algae and bacteria,
which contain abundant lipids. The low Pr/Ph ratio and
high γ/C30 hopane ratio in the studied crude oils (Table 1)
indicate the oil-forming environment having high salinity and under strong reduction condition (Duan et al.,
2004), so that these lipids are well preserved. At the stage
of low maturity most of the compounds containing functional groups and double bonds can be reducted to their
hydrocarbon counterparts. These hydrocarbons together
with the non-hydrocarbons not transformed contribute to
the formation of low mature crude oils of the studied region. This tentative interpretation of low maturity crude
oil genesis under study further supports the assumption
that some of crude oils were originated directly from the
transformation of soluble organic matter at low temperature (Huang et al., 1991).
CONCLUSIONS
Crude oils from the western Qaidam Basin contain
abundant non-hydrocarbons in the range of 270–550 mg/
g. They are mainly composed of fatty acids, alkanols and
two series of fatty acid glycerol monoesters. The fatty
acids and alkanols consist mainly of the short-chain homologues, with C16 and C18 being the most abundant components, respectively. The unsaturated and isocomponents are also present in low abundance. The distributions of fatty acid glycerol monoesters in the range
of C14–C 18 are similar to those of fatty acids. Stenols are
dominated by C27 homologues. The compositional characteristics of these non-hydrocarbons, together with the
low maturity of both oil and source rocks, and the highly
saline and strongly reducing environment, suggest that
the non-hydrocarbons are derived mainly from oil source
rocks.
The distribution profiles of these non-hydrocarbon
compounds show that algal and bacterial sourced material is the major oil-forming organic matter. The presence
of abundant fatty acid glycerol monoesters suggests that
the biological lipids, including bacterial phospholipids,
Abundant non-hydrocarbons in crude oils from the western Qaidam Basin, northwest China 469
are an important contributors to the formation of the crude
oils. The existence of unsaturated compounds (fatty acids, alkanols and sterols) as well as high CPI value of nfatty acids and n-alkanols reflects the low maturity of the
crude oils, which is consistent with the results of the studies of hydrocarbon biomarker of the crude oil and source
rocks.
Acknowledgments—The authors thank Dr. John Volkman for
his careful review and constructive comments which lead to a
great improvement to this manuscript. This work was supported
by Important Direction Project of Knowledge Innovation in
Resource and Environment Field, Chinese Academy of Sciences
(Grant No. KZCX3-SW-128).
REFERENCES
Behar, F. and Albrecht, P. (1984) Correlations between carboxylic acids and hydrocarbons in several crude oils. Alteration
by biodegradation. Organic Geochemistry 6, 597–604.
Borgund, A. E. and Barth, T. (1993) Migration behavior of petroleum associated short chain organic acids. Organic
Geochemistry 20, 1019–1025.
Chuecas, L. and Riley, J. P. (1969) Component fatty acids of
the total lipid of some marine phytoplankton. Journal of
the Marine Biological Association, UK 49, 97–116.
Cranwell, P. A. (1974) Monocarboxylic acids in lake sediments:
Indicators derived from terrestrial and aquatic biota of
paleoenvironmental trophic levels. Chemical Geology 14,
1–4.
Cranwell, P. A. (1981) Diagenesis of free and bound lipids in
terrestrial detritus deposited in a lacustrine sediment. Organic Geochemistry 3, 79–89.
Duan, Y. (2000) Organic geochemistry of recent marine
sediments from the Nansha Sea, China. Organic
Geochemistry 31, 159–167.
Duan, Y. (2001) Pentacyclic triterpenoid ketones in peat from
Gannan Marsh, China. Chinese Science Bulletin 46, 1433–
1435.
Duan, Y. and Ma, L. H. (2001) Lipid geochemistry in a sediment core from Ruoergai Marsh deposit (Eastern QinghaiTibet plateau, China). Organic Geochemistry 32, 1429–
1442.
Duan, Y., Wen, Q. B. and Luo, B. J. (1997a) Isotopic composition and probable origin of individual fatty acids in modern
sediments from Ruoergai Marsh and Nansha Sea, China.
Organic Geochemistry 27, 583–589.
Duan, Y., Cui, M. Z., Ma, L. H., Song, J. M., Zhou, S. X. and
Luo, B. J. (1997b) Organic geochemical studies of sinking
particulate material in China Sea area II: Geochemical significance of compositional features of ketone, aldehyde and
alcohol compounds. Chinese Science Bulletin 42, 1894–
1989.
Duan, Y., Wen, Q. B., Zheng, G. D. and Luo, B. J. (1997c) The
carbon isotopic study of individual alcohol compounds in
modern sediments from Nansha Sea, China. Science in
China (Series D) 40, 491–495.
Duan, Y., Song, J. M., Cui, M. Z. and Luo, B. J. (1998) Organic
470 Y. Duan et al.
geochemical studies of sinking particulate material in China
Sea area I: Organic matter fluxes and distributional features
of hydrocarbon compounds and fatty acids. Science in China
(Series D) 41, 208–214.
Duan, Y., Wang, Z. P., Zhang, H. and Zhang, X. B. (2004)
Biomarker geochemistry of crude oils from Qaidam basin,
northwest China. Journal of Petroleum Geology (submitted).
Eglinton, G. and Hamilton, R. J. (1967) Leaf epicuticular waxes.
Science 156, 1322.
Gagosian, R. B., Nigrelli, G. E. and Volkman, J. K. (1983a)
Vertical transport and transformation of biogenic organic
compounds from a sediment trap experiment off the coast
of Peru. Coastal Upwelling: Its Sediment Record. Part A:
Responses of the Sedimentary Regime to Present Coastal
Upwelling (Suess, E. and Thiede, J., eds.), 241–272,
Plenum Press, New York.
Gagosian, R. B., Volkman, J. K. and Nigrelli, G. E. (1983b)
The use of sediment traps to determine sterol sources in
coastal sediments off Peru. Advances in Organic
Geochemistry (Bjoroy, M. et al., eds.), 369–379, Wiley,
Chichester.
Gaskell, S. J., Morris, R. J., Eglinton, G. and Calvert, S. E.
(1975) The geochemistry of recent marine sediment off
northwest Africa. An assessment of source of input and early
diagenesis. Deep-Sea Research 22, 777–789.
Goad, L. J. and Goodwin, T. W. (1972) The biosynthesis of
plant sterols. Progress in Phytochemistry 3, 113–198.
Grimalt, J. O., de Wit, R., Teixidor, P. and Albaiges, J. (1992)
Lipid biogeochemistry of Pormidium and Microcoleus mats.
Organic Geochemistry 26, 509–530.
Hanson, A. D., Ritts, B. D., Zinniker, D., Moldowan, J. M. and
Biffi, U. (2001) Upper Oligocene lacustrine source rocks
and petroleum systems of the northern Qaidam basin, northwest China. American Association of Petroleum Geologists
Bulletin 85, 601–619.
Heller, S. R. and Milne, G. P. A. (1978) EPA/NIH mass spectral data base. V4 U.S. Government Printing Office,
Washington, D.C., 3589–3590 pp.
Huang, D. F., Li, J. C., Zhang, D. J., Huang, X. M. and Zhou,
Z. H. (1991) Maturation sequence of Tertiary crude oils in
the Qaidam Basin and its significance in petroleum resource
assessment. Journal of Southeast Asian Earth Sciences 5,
359–366.
Huang, X. Z., Shao, H. S. and Gu, S. S. (1993) Formation and
Exploration Direction of Petroleum and Natural Gas in the
Qaidam Basin. Scientific and Technological Publishing
House of Gansu, China, 363–370 pp. (in Chinese).
Ioppolo, M., Alexander, R. and Kagi, R. I. (1992) Identification and analysis of C0-C3 phenols in some Australian crude
oils. Organic Geochemistry 18, 603–609.
Jaffé, R. and Gardinali, P. R. (1990) Generation and maturation of carboxylic acids in ancient sediments of the
Maracaibo Basin, Venezuela. Organic Geochemistry 16,
211–218.
Jaffé, R. and Gallardo, M. T. (1993) Application of carboxylic
acid biomarkers as indicators of biodegradation and migration of crude oils from the Maracaibo Basin, Western
Venezuela. Organic Geochemistry 20, 973–984.
Jaffé, R., Albrecht, P. and Oudin, J. L. (1988a) Carboxylic acids as indicators of oil migration—I. Occurrence and
geochemical significance of C-22 diastereoisomers of the
17βH, 21 βH C 30 hopanoic acid in geological samples. Organic Geochemistry 13, 483–488.
Jaffé, R., Albrecht, P. and Oudin, J. L. (1988b) Carboxylic acids as indicators of oil migration—II. The case of the
Mahakam Delta, Indonesia. Geochim. Cosmochim. Acta 52,
2599–2607.
John, G. L., Farrington, W. and Gagosian, R. B. (1979) Sterol
geochemistry of sediments from the western North Atlantic
Ocean and adjacent coastal areas. Geochim. Cosmochim.
Acta 43, 35–46.
Keswani, S. R., Dunham, K. W. and Meyers, P. A. (1984) Organic geochemistry of Late Cenozoic sediments from the
subtropical South Atlantic Ocean. Marine Geology 61, 25–
42.
Kvenvolden, K. (1966) Molecular distribution of normal fatty
acids and paraffins in some Cretaceous sediments. Nature
209, 573–577.
Kvenvolden, K. (1967) Normal fatty acids in sediments. Journal of American Oil Chemistry Society 44, 628–636.
Li, M., Larter, S. R. and Stoddart, D. (1992) Practical liquid
chromatographic separation schemes for pyrrolic and
pyridinic nitrogen aromatic heterocycle fractions from crude
oils suitable for rapid characterisation of geochemical samples. Anal. Chem. 64, 1337–1344.
Mackenzie, A. S. (1984) Applications of biological markers in
petroleum geochemistry. Advances in Petroleum
Geochemistry (Brooks, J. and Welte, D. H., eds.), 115–214,
Academic Press, London.
Matsuda, H. and Koyama, T. (1977) Early diagenesis of fatty
acids in lacustrine sediments—I. Identification and distribution of fatty acids in recent sediment from a freshwater
lake. Geochim. Cosmochim. Acta 41, 777–783.
Meredith, W., Kelland, S. J. and Jones, D. M. (2000) Influence
of biodegradation on crude oil acidity and carboxylic acid
composition. Organic Geochemistry 31, 1059–1073.
Moldowan, J. M., Seifert, W. K. and Gallegos, E. J. (1985)
Relationship between petroleum composition and
depositional environment of petroleum source rocks. AAPG
Bulletin 69, 1255–1268.
Ogura, K., Machihara, T. and Takada, H. (1989) Diagenesis of
biomarkers in Biwa Lake sediments over l million years.
Organic Geochemistry 16, 805–813.
Perry, G. J., Volkman, J. K., Johnson, R. B. and Bavor, H. J.
(1979) Fatty acids of bacterial origin in contemporary marine sediment. Geochim. Cosmochim. Acta 43, 1715–1725.
Peters, K. E. and Moldowan, J. M. (1993) The Biomarker Guide:
Interpreting Molecular Fossils in Petroleum and Ancient
Sediments. Englewood Cliffs, New Jersey, Prentice-hall, 363
pp.
Philp, R. P. (1985) Fossil Fuel Biomarkers: Applications and
Spectra. Elsevier, Amsterdam, 294 pp.
Philp, R. P., Fan, P., Lewis, C. A., Li, J., Zhu, H. and Wang, H.
(1991) Geochemical characteristics of oils from the
Chaidamu, Shanganning and Jianghan Basins, China. Journal of Southeast Asian Earth Sciences 5, 351–358.
Piorreck, M., Baasch, K. H. and Pohl, P. (1984) Biomass production total protein, chlorophylls, lipids and fatty acids of
freshwater green and blue-green algae under different nitrogen regimes. Phytochemistry 23, 207–216.
Qiang, J., Wang, W., Xu, H., Yang, S. and Qian, J. (1997) North
Jiangsu basin, eastern China: Accumulation of immature oils
and diagenesis of Palaeogene reservoir rocks. Journal of
Petroleum Geology 20, 287–306.
Rezanka, T., Vokoun, J., Slavicek, J. and Podojil, M. (1983)
Determination of fatty acids in algae by capillary gas chromatography-mass spectrometry. Journal of Chromatography 268, 71–78.
Rieley, G., Collier, R. J., Jones, D. M. and Eglinton, G. (1991)
The biogeochemistry of Ellesmere Lake, UK—I: Source
correlation of leaf wax inputs to the sedimentary lipid
record. Organic Geochemistry 17, 901–912.
Ritts, B. D., Hanson, A. D., Zinniker, D. and Moldowan, J. M.
(1999) Lower-Middle Jurassic nonmarine source rocks and
petroleum systems of the northern Qaidam basin, northwest
China. American Association of Petroleum Geologists Bulletin 83, 1980–2005.
Russell, M., Grimalt, J. O., Hartgers, W. A., Taberner, C. and
Rouchy, J. M. (1997) Bacterial and algal markers in sedimentary organic matter deposited under natural
sulphurization conditions (Lorca Basin, Murcia, Spain).
Organic Geochemistry 26, 605–625.
Seifert, W. K. and Moldowan, J. M. (1978) Applications of
steranes, terpanes and monoaromatics to the maturation,
migration and source of crude oils. Geochim. Cosmochim.
Acta 42, 77–95.
Taylor, P., Bennett, B., Jones, M. and Larter, S. (2001) The effect of biodegradation and water washing on the occurrence
of alkylphenols in crude oils. Organic Geochemistry 32,
341–358.
Venkatesan, M. I. (1988) Organic geochemistry of marine
sediments in Antarctic region: Marine lipids in McMurdo
Sound. Organic Geochemistry 12, 13–27.
Volkman, J. K. (1986) A review sterol markers for marine and
terrigenous organic matter. Organic Geochemistry 9, 83–
99.
Volkman, J. K., Johns, R. B., Gillan, F. T., Perry, G. T. and
Bavor, H. J. (1980) Microbial lipids of intertidal sediment—
I. Fatty acids and hydrocarbons. Geochim. Cosmochim. Acta
44, 1133–1143.
Wang, T. G., Zhong, N. N., Hou, D. J., Huang, G. H., Bao, J. P.
and Li, X. Q. (1995) Formational Mechanism and Occurrence of Low Mature Oils and Gases. Publishing House of
Petroleum Industry, Beijing, 128–151 pp. (in Chinese).
Wang, X. L. (1983) Organic Chemistry. People’s Education
Press, Beijing, 205–212 pp. (in Chinese).
Xu, J. D. (1984) Organic Chemistry. People Hygiene Publishing House of Beijing, Beijing, 237–254 pp. (in Chinese).
Yamamoto, M. (1992) Fractionation of azaarenes during oil
migration. Organic Geochemistry 19, 389–402.
Yamamoto, M. and Taguchi, K. (1991) Basic nitrogen compounds in bitumen and crude oils. Chemical Geology 93,
193–206.
Abundant non-hydrocarbons in crude oils from the western Qaidam Basin, northwest China 471