tions often have the analogues terpanes fingerprints[2]. So

Molecular fossils and far, among so many terpenoids compounds discovered,

except some compounds with specific structures in ter-

oil-source rock correlations in panes, such as oleanane (angiosperms-specific marker),

Tarim Basin, NW China biaddinanes (angiosperm dammar resins-specific marker)

and tetracyclic diterpanes (coniferophyte-specific marker)

having source-specific meaning, only few can act as mo-

1 1 2

ZHANG Shuichang, LIANG Digang, LI Maowen,

lecular fossils for taxon-specific source indicators. Ster-

3 1

XIAO Zhongyao & HE Zhonghua

anes, derived from hydrocarbon sterols, are indeed of a

1. Key Laboratory of Petroleum Geochemistry, Research Institute of very ancient lineage. They are sedimentary markers for

Petroleum Exploration and Development, China National Petroleum

eukayotes, including plants, algae and animals. Sterols

Corporation, Beijing 100083, China;

appear to be the chemical gatekeepers for higher life

2. Geological Survey of Canada, 3303-33 st., NW Calgary, Alberta T2L

forms. They impart a quasi-structure or shape to cell

2A7, Canada;

3. Research Institute of Petroleum Exploration and Development, Tarim membranes and form the basic structures for the sex hor-

Oilfield, Kuerle 841000, China

mones[3].

Correspondence should be addressed to Zhang Shuichang (e-mail:

However, not all the terpenoids and steroid com-

abpmt1@r.postjobfree.com)

pounds have fossil meaning. Being the indicators of

Abstract The distribution of molecular fossils (bio- oil-source rock correlation, they should satisfied the fol-

markers) of steroid compounds in extracts from some spe- lowing three conditions: (1) specific source; (2) little af-

cific geologic age in the Tarim Basin have been analyzed and fected or unaffected by maturation; and (3) little affected

are used as the fingerprints for the oil-source rock correla-

or unaffected by secondary alteration (such as deasphalt-

tion. Having been affected by maturation, migration, phase

ing, biodegradation, thermal alteration in reservoir, wa-

fractionation and biodegradation, not any molecular fossils

ter-washing, phase fractionation and geochromatography

related to source and environment can be used as the finger-

during migration and within-reservoir).

prints for oil-source rock correlation. Some special bio-

The Tarim Basin, NW China, is a large, composite

markers widely existed in the extracts from Cambrian and

basin with numerous ancient petroleum source rocks of

Ordovician rocks in the Tarim Basin and showed obvious

difference in each stratum, including dinosteranes (C30), different geological ages. The basin might have undergone

4-methyl-24-ethyl-cholestanes (C30) and their aromatized multiple periods of hydrocarbon generation, accumulation

steroids, C24-norcholestanes and C28 steranes originated from

and migration. The resultant complexity of the Tarim Ba-

dinoflagellates and diatom. Few oils such as the heavy oil

sin makes the study of oil-source rock correlation disput-

drilled in the Cambrian reservoir from Tadong 2 well (TD2)

able. Recent disputes are focused on whether the source

correlated well with the extracts from the Cambrian. The

rock is the Cambrian or the Ordovician. One of the un-

amazing similarity of the relative contents of these com-

derlying reasons is the inconsistency of parameters for

pounds between the marine oils produced in Tazhong and

oil-source rock correlation by different researchers and

Tabei uplifts and the extracts from the Upper Ordovician

neglecting the three factors as mentioned above. In this

suggests that the Middle-Upper Ordovician is the very likely

main source for the marine oils. paper, combining with the property of source rocks,

maturation difference, migration-fractionation during mi-

Keywords: molecular fossils, dinosterane, triaromatic dinosterane,

gration and accumulation and biodegradation, the analysis

24-norcholestane, dinoflagellate, Cambrian, Upper Ordovician,

Tarim Basin. of marine oil-source rocks correlations in the Tarim Basin

was carried out carefully by using algae-related steroids

Molecular fossils, as an independent chemical record compounds composed of higher carbon numbers with

and a mutually complementary for hard fossils, are pre- equal or near molecular weights. The result shows that

served in sediments. Due to the stability of their carbon only some residud paleo-reservoirs such as the heavy oil

skeletons and complex fingerprints character even in found in Tadong 2 well (TD2) from the Cambrian reser-

diagenesis and catagenesis, they have been extensively voir and natural gas reservoirs in the platform basin are

used in the petroleum geochemistry. Correlations obtained contributed by the Cambrian, whereas the commercial

from biomarkers fingerprints enable people to understand reservoirs discovered in Tazhong and Tabei uplifts

the origin of oils and their migration from source rocks to up-to-date are mainly contributed by marl that deposited

reservoirs. on the slope of the marginal platform in the age of the

In various biomarkers, terpanes and steranes are two Middle-Upper Ordovician.

kinds of compounds which are the most widely used for

1 Samples and experiments

correlating. Terpanes come mainly from prokaryote (e.g.

bacteria)[1]. Almost all oils contain terparnes because bac- The rock samples were selected from the core of

teria exist widely in the sediments. The oils generated Cambrian and Ordovician strata in the Tarim Basin. After

from different source rocks deposited under similar condi- careful core observation and strict TOC and Rock-eval

20 Chinese Science Bulletin Vol. 47 Supp. December 2002

screening, only marl, limestone and mud/shale samples TD2 are developed in the under-compensated basin. The

are rich in organic matters, and samples with TOC more Lianglitage Formation (O3l), Upper Ordovician are de-

than 0.4% were chosen for the next stage of analysis. veloped in a slope of the marginal platform. The analyzed

These samples are distributed in different sedimentary oil samples are selected from almost all kinds of crude

facies. The Cambrian in wells in the eastern part of the oils produced in marine commercial reservoirs from

basin, such as Tadong 1 well (TD1), TD2 well and Kunan

Tazhong and Tabei uplifts and some oils produced in

1 well (KN1) are under-compensated basin sediments.

non-commercial reservoirs such as the Cambrian heavy

The Cambrian in Tacan 1 well (TAC1) in the middle of

oil in TD2. The brief introduction of all studied samples is

the basin is marginal facies of the evaporated lagoon. The

listed in table 1.

Cambrian from He 4 well (H4) in the western of the basin

For the analysis methods and procedure of sample

is the evaporated lagoon and Fang 1 well (F1) is the mar-

extraction and separation, and biomarker analytical

ginal platform to the slope of the marginal platform. The

Heituwa Formation (O1h), Lower Ordovician in TD1 and methods see refs. [4, 5].

Table 1 List of all samples in this study

Rock samples

Age Representative wells TOC Depositional environment

Upper Sinian eastern part of the basin: TD1 0.90 under-compensated deep-water basin

eastern part of the basin: TD2, KN1 deep-water basin

0.47 4.88

Cambrian center of the basin: TaC1 0.70 marginal platform-marginal basin

western part of the basin: He4, F1, Kang2 0.31 0.57 evaporated lagoon

Lower Ordovician eastern of basin: TD1, TD2 under-compensated deep-water basin

0.40 7.62

northern slope of Tazhong (9 wells): TZ6, 12, 53, etc. 0.40 12.82 slope of marginal platform

Upper Ordovician southern slope of Tabei: LN46, X3 0.86 slope of marginal platform

Crude oil samples

Heavy oils ( d 4 0 >0.92)

2

Tadong: TD2(Cam); West of Lunnan faults: LN1(O), LN11(O), LG9(O); Tahe oilfield: TK401(O), 402(O), S66(O)

Tazhong oilfield: TZ1(3755 m), TZ11(4314 m, 4411.2 m, 4417 4435 m, S), TZ15(O3), TZ35(CIII)

Normal oils ( d 4 0 0.82 0.92):

2

Lunnan oilfield: LN2(T), JF100(T), LN44(T), LN32(C); Tahe oilfield: TK301(O), TK303(O), S47(O);

Hadexun oilfield: HD1(C), HD4(C); Donghetang oilfield: DH4(C), HD1(C); Yingmaili: YM2(O)

Tazhong oilfield: TZ4(C), TZ401(C), TZ111(C), TZ24(C), TZ44(O), TZ161(C), TZ16(O), TZ11(S)

Waxy oils: JF123(O), LN14(O), LN8(O)

Condensates ( d 4 0 0.82):

2

Lunnan oilfield: LN44(O), JF124(C), LN57(T); Tazhong oilfield: TZ6(C), TZ1(C), TZ45(O)

Others (oil sands and samples without industrial value)

TZ24(O3l), TZ12(O1), TZ4(oil sand, S), X3(C, oil stained; O, heavy oil with water), CH2(C, oil stained), YW1(O, oil sand)

24-norcholestanes decreases (fig. 1). Besides, triaromatic

2 Results

dinosteranes in the extracts show regular changes. Al- Biomarkers distribution in source rocks of dif- though the reasons for these changes are not clear enough

ferent age at present, these differences can be used as the good indi-

Distribution of C30 methylsteranes (m/z 414 231), cators for distinguishing the source rocks among the

C27 C29 steranes (m/z M+ 217) and C26 norcholestanes Cambrian-Lower Ordovician from the Upper Ordovician.

(m/z 358 217) in the extracts from three sets of Low Oil-source rock correlation

Paleozoic stratas which are rich in organic matter in the The selection of parameters used for oil-source rock

Tarim Basin are shown in constract (fig. 1). The relative correlation is very important in the Paleozoic basin where

content of these compounds shows obvious and system- multiple source rocks deposit and multiple hydrocarbon

atic change with the geologic age. It is clear that the ex- accumulations are real clearly. Due to the distinct maturity

tracts from the Cambrian and Lower Ordovician show a difference of the source rocks between the Cambrian and

similar distribution, different from that of the Upper Or- Upper Ordovician, maturation will distort the oil-source

dovician. From the Cambrian-Lower Ordovician to the rock correlation. Meanwhile, hydrocarbon may have a

Upper Ordovician, the intensities of dinosteranes (4, 23, multiple filling history and has undergone obvious migra-

24-trimethyl-cholstane) and 4 -methyl-24-ethylcholes- tion-fractionation (resulting in the oil rich in the light

tanes of C30 steranes decrease dramatically to the degree component) and biodegradation (resulting in the oil rich in

the heavy component and the decrease of the concentra-

of trace. Meanwhile, the concentration of C27 C29 diast-

ernaes increases and the abundance of C28 steranes and tion of the component) during the migration, accumulation

Chinese Science Bulletin Vol. 47 Supp. December 2002 21

Fig. 1. The distribution of C30 methylsteranes (m/z 414 231), C27 C29 steranes (m/z M+ 217) and C26 norcholestanes (m/z 358 217) in the ex-

tracts from Cambrian and Ordovician source rocks in the Tarim Basin, showing regular changes with the geologic age.

and adjustment. The parameters, composed of the com- oil-source rock correlation in the Tarim Basin. Here we

pounds with obviously different molecular weights such suggest that the algae-related steroid compounds with

as tricyclic terpane/hopane, C23 tricyclic terpane/C30 ho- equal or near molecular weight and higher carbon number

may be more suitable for marine oil-source rock correla-

pane, C24 tetracyclic terpane/C30 hopane, C19 C21 tri-

tions in the Tarim Basin. These compounds include di-

cyclic terpane/C23 C24 tricyclic terpane and dri-

nosteranes derived from dinoflagellate, 4-methyl-24-

mane/homodr- imane, have the biological origin to some

ethylcholestanes and their aromatized steroids[6],

extent. But comparisons among the data should have mul-

24-norcholestanes representing the records and behaviours

tiple meaning. Therefore, they should not be used for

22 Chinese Science Bulletin Vol. 47 Supp. December 2002

of diatoms[7,8] and C28 steranes originated from diatom, in the same well, with high abundance of dinosteranes and

coccolithophores and dinoflagellates[9]. As shown in fig. 1, 4 -methyl-24-ethylcholestanes (fig. 2(a)). Whereas the

it is clear that these parameters show an obvious differ- oils from the Tabei uplift represented by the oils from

ence in the extracts between the Cambrian and Ordovician. LG9 (whether the heavy oils, or the normal oils, or con-

And from the point of thermodynamics, they are not af- densates) show similarity with the extracts from the Upper

fected by maturation or secondary alteration. Ordovician in TZ6, both with a low content of dinoster-

(1) Dinosteranes and 4 -methyl-24-ethylcholestanes. anes and 4 -methyl-24- ethylcholstanes and a high con-

Dinosteranes and 4 -methyl-24-ethylcholestanes are tent of 3a-methyl-24-ethylcholestanes (fig. 2(b)).

regarded as the dinoflagellate-specific biological mark- From the relationship between the relative content of

ers[10,11], and dinoflagellate is known to originate from the dinosteranes and 4 -methyl-24-ethylcholestanes in all

Middle Triassic. Due to the identification of the abun- extracts and oils (fig. 3(a)), it can be seen that there are

distinct differences between the extracts from the Cam-

dance of dinosteranes and 4 -methyl-24-ethylcholestanes

from the sediments which are rich in organic matter from brian and most oils and the extracts from the Upper Ordo-

the pre-Cambrian and Silurian, the origin of dinoflagellate vician. Except the oil from TD2, about 95% confidence

may be much earlier than our imagination before[6,12]. As interval of the Cambrian source rocks does not overlap

discussed above, there are obvious differences in the rela- those of other oils, which overlap with most Upper Ordo-

vician extracts.

tive content of dinosteranes and 4 -methyl-24-ethylcho-

(2) Triaromatic dinosteranes and other triaromatized

lestanes in extracts from two sets of source rocks. All

steroids. The mass chromatogram of triaromatic dinos-

extracts from t he Cambrian h ave relatively h igh

terane (m/z 245) shows a consistent result with dinoster-

dinosteranes content, whereas the extracts from the

Middle-Upper Ordovician have a lower content of anes and 4 -methyl-24-ethylcholestanes. The oil from

dinosteranes. This difference should be the foundation of TD2 shows a good correlation with extracts from the

the oil-source correlation. From the distribution of C30 Cambrian in the same well, with triaromatic dinosteranes

methylsteranes from two groups of extracts and oils (fig. (peak 7) as the most abundant compounds, 4-methyl-

2), it can be seen that the heavy oil from TD2 shows ex- 24-ethyltriaromatic cholestanes (peaks 8 and 11) as the

cellent similarity with the extract at intervals of 4674.2 m second and 3 - methyl-24-ethyltriaromatic cholestane

Fig. 2. The distribution of methylsteranes (m/z 231) in extracts and oils. (a) The heavy oil in TD2 and extract from interval of 4647.2 m, mudstone,

Cambrian in TD2; (b) the heavy oil of LG9 and extract from interval of 3886.1 m, marls of Upper Ordovician from TZ6. Peak 1,

23,24-trimethylcholestanes (dinosteranes); peak 2, 4-methyl-24-ethylcholestane; peak 4, 3-methyl-24-ethylcholestanes.

Chinese Science Bulletin Vol. 47 Supp. December 2002 23

steranes) (fig. 3(b)), it can be seen that the ratios

of Dino-TAS/(Dino-TAS+3 -methyl-24-ethyl TAS) and

4-methyl-24-ethyl TAS/(4-methyl-24-ethyl-+3-methyl-24-

ethyl-)TAS in the extracts from the Cambrian and Lower

Ordovician exceed 60%, while the two values in extracts

of the Middle-Upper Ordovician are only 5% 25% and

30% 60%, respectively. The distinct differences of

triaromatic dinosteranes and 4-methyl-24-ethyl-TAS be-

tween two sets of source rocks should be a very interest-

ing case. Moldowan et al. thought that the abundance of

triaromatic dinosteranes in the Paleozoic extracts might

relate to the marine acritarch (phototrophs phytoplankton

of uncertain affinity)[12], and the latter might originate

from dinoflagellate which act as the primary production in

Paleozoic oceans[3]. Bian s recent research suggested that

the dinosteranes in the extracts from the Middle-Upper

Cambrian could be an origin of Paleogloeodinium which

should be widely spreaded in the Tarim ocean at that

time[14]. The ultimate reason of the difference in the con-

tents of dinosteranes in extracts from the Cambrian and

Ordovician in the Tarim Basin is supposed to relate to the

changing of paleo-oceanographic conditions between the

Cambrian and the Ordovician time[5]. No matter what

reason it is, this difference provides the additional proof

for distinguishing the source rocks of the Cambrian and

Ordovician. Most of the analyzed oils overlap with the

area of the Upper Ordovician, except a few oils such as

the heavy oil from TD2, oil from TZ45 (another oil from

TZ45 overlapping with the Upper Ordovician), oil from

TZ12 (a small amount of oil without commercial value

produced in the sandy dolomite reservoir from the Lower

Ordovician) and oil stain from CH2 (well Cao2 in Caohu

Sag) overlapping with the Cambrian.

Fig. 3. Crossplot of dinosteranes/(dinosteranes+ 20R C29 steranes)

Analyzing the distribution of triaromatic steranes on

and 4 -methyl-24-ethylcholestanes/(4 + 3 )-methyl-24-ethylcholest-

the m/z 231 mass chromatogram, we found that there was

anes (a) and crossplot of triaromatic dinosteranes/(triaromatic dinoster-

also great difference between two group samples. The

anes +3 -methyl-24-ethyltriaromatic cholestanes) and (4 -methyl-24-

concentrations of peaks 24, 25 and 27 are rather high in

ethyl+4-methyl)/(4-methyl+4-methyl-24-ethyl +3-methyl-24-ethyl) triar-

omatic cholestanes ratios (b) in Tarim oils and rock extracts, showing extracts of the Cambrian and their corresponding oils, but

clearly separation of the Cambrian and Upper Ordovician. Most marine

very low in the extracts from the Middle-Upper Ordovi-

oils overlap with the Upper Ordovician extracts. Dino-TAS: Triarmotic

cian and their corresponding oils in LG9 which are domi-

dinosteranes; 4-M-24-E-TAS: 4-methyl-24-ethyl triaromatic dinosteranes;

nated by peaks 26 and 28. Although the reason is not to-

4-M-TAS: 4-methyl triaromatic dinosteranes; 3-M-24-E-TAS: 3-methyl-

24-ethyl triaromatic dinosteranes. tally clear, these compounds used for oil-source rock cor-

relation have the appied effect as other compounds as dis-

(peak 9) as the least abundant compounds. The same to cussed above.

the oil from LG9 and extracts form the Middle-Upper (3) 24-norcholestanes. 24-norchloesterols are

Ordovician, which were characterized by 3 -methyl- known to be produced by eukaryotes. Whereas 21- and

24-ethyltriaromatic cholestane dominance, low concentra- 27-norcholestanes seem to have no direct sterol precursors.

tion of other compounds and almost undetectable triaro- Paleozoic and even older oils usually show little or no

matic dinosteranes, and compounds labeled * are sup- 24-norcholestanes [7,8,15] . Holba et al. found that 24-

posed to be 2-methyl-24-ethylatriaromatic cholestanes (fig. norcholestane and its derivatives appear most abundantly

4(b), m/z 245). in the modern or Paleozoic diatomaceous sediment

From the crossplot of triaromatic dinosteranes (Dino- and oils generated from siliceous source rock, suggesting

TAS)/(triaromatic dinosteranes+4 -methyl-24-ethyl triaro- that diatoms are the direct or indirect origin of

24-norcholestane[7,8]. Since the earliest appearing of dia-

matic steranes (TAS)) and 4-methyl-24-ethyl triaromatic

sternes/(4-methyl-24-ethyl-+3-methyl-24-ethyltriaromatic tom was in the Jurassic and then it was flourishing in the

24 Chinese Science Bulletin Vol. 47 Supp. December 2002

Fig. 4. The distribution of methyltriaromatic steranes (m/z 245) and triaromatic steranes (m/z 231) in Tarim oils and rock extracts. (a) Heavy oil from

TD2 and extract of Cambrian mudstone from the depth of 4674.2 m in the same well; (b) heavy oil from LG9 and extract of Upper Ordovician muddy

limestone from the depth of 3886.1 m in TZ6. Peak 7: 4,23,24-trimethyltriaromatic steranes (C29 triaromatic steranes); peak 8: 4-methyl-

24-ethyltriaromatic steranes (C29); peak 9: 3-methyl-24-ethyltriaromatic steranes; peak 11: 4-methyltriaromatic steranes (C27); peak 12:

3-methyltriaromatic steranes (C27); peak 13: 3-methyl-24-methyltriaromatic steranes (C28).

Chinese Science Bulletin Vol. 47 Supp. December 2002 25

Late Cretaceous[8], the ratios of 24-/27-norchloestane and only a few oils such as TD2 drop in the areas of Cambrian

24-/27-nordiacholestane show two peak values in these extracts (fig. 5(a)).

two periods. But in the Tarim Basin, significant amounts (4) C28 steranes. Studies of this kind of compound

of 24-norcholestane in all Cambrian rocks and some Or- have shown that the increasing in C28 steranes among C27

dovician rocks have been identified. The difference be- C29 steranes in the Jurassic and Cretaceous oils may be

tween the Cambrian to Lower Ordovician rocks and Up- related to the increasing diversification of phytoplankton

per Ordovician rocks is that the former is rich in assemblages, including diatom, coccolithophores and

dinoflagellates[9]. Interestingly, the Cambrian extracts

24-norcholestane, while the latter is rich in 27-cholestane

(fig. 1). The 24-/27-norcholestane values of the Cambrian have higher content of C28 steranes among C27 C29 ster-

to Lower Ordovician extract are up to 0.3, while the val- anes (about 20% 26%), which are higher than the values

ues of Upper Ordovician extracts are less than 0.25 (fig.

of the early Paleozoic and even more ancient strata (3%

5(a)). The crossplot of the 24-/27-norcholestane and

17%) reported by Grantham and Wakefield[9], and aslo

triaromatic dinosteranes/(triaromatic dinosteranes + 3 -

higher than that of Middle-Upper Ordovician extracts. The

methyl-24-ethyl-triaromatic dinosteranes) can be clearly

C28 steranes distribution shows that most oils except oil in

separated into two sets of source rocks (fig. 5(a)). Most

TD2 are located in the area of the Upper Ordovician,

oils drop in the areas of Upper Ordovician extracts, and

which is almost the same as the parameters mentioned

above.

3 Discussion

As the result mentioned above, it will be reasonable

that the ratios of dinosteranes/(dinostranes + 3 -methyl-

24-ethylcholestanes), triaromatic dinosteranes/(triaromatic

dinosteranes + 3 -methyl-24-ethyltriaromatic dinoster-

anes) and 24-/27-norcholestanes can be used as effective

parameters for marine oil-source correlations in the Tarim

Basin. Based on the correlations, we suggest that the dis-

covered commercial reservoirs in Tazhong and Tabei up-

lifts at present mainly originate from the Upper Ordovi-

cian source rocks deposited in the slope of the marginal

platform and the heavy oil from TD2 is a typical oil origi-

nated from Cambrian source rocks. In other words, if the

heavy oil in TD2 is regarded as a type of oil having a

close genetic origin with Cambrian, then the oils in Lun-

nan-Tahe-Hadexun oilfields and Tazhong area, whose

molecular fossils are absolutely different from that of TD2,

should be regarded as another type of oil, which means

that their origin should be the Upper Ordovician only.

However, some researchers proposed that most of the oil

characterized by the molecular fossils of Upper Ordovi-

cian were the result of the pollution by the Middle-Upper

Ordovician with lower maturity and abundant biomarkers

during the oil migration. This leads to the oils generated

from the Cambrian showing the Middle-Upper Ordovician

character (Wang Tingdong et al., internal report). But this

inference is lack of supporting experimental data.

Based on the quantification data of partial extracts

and oils, it can be seen that the absolute concentration of

terpanes and steranes in the extracts from the Middle-

Upper Ordovician and Cambrian has not shown a

consistent increasing or decreasing. The hopanes concen-

tration in the extracts from the Middle-Upper Ordovician

Fig. 5. Crossplot of 24-/27-norcholestanes and triaromatic dinoster-

anes/(triaromatic dinosteranes+3 -methyl-24-ethylcholestanes) and 24-/ (6369.55 g/g) is about 2.3 times higher than that in the

27-norcholestanes and C28 steranes, showing clear seperation of Cam-

Cambrian (2782.18 g/g). Whereas the steranes concen-

brian and Upper Ordovician extracts. Most Tarim marine oils overlap

tration in the extracts from the Cambrian (2553.78 g/g)

with the Upper Ordovician extracts.

26 Chinese Science Bulletin Vol. 47 Supp. December 2002

is about 1.4 times higher than that in the Middle-Upper slope of the marginal plateform. More importantly, we

have discovered the oil in TD2, a typical oil generated

Ordovician (1834.44 g/g). There is no such guestion that

from Cambrian source rocks. This discovery not only

the concentrations in the extracts from the Middle-Upper

proves the existence of two source oils in the Tarim Basin

Ordovician with medium maturity are always higher than

thus expanding the exploration scope, but aslo verifies the

those of the Cambrian with higher maturity. The differ-

effectiveness of the chosen parameters for distinguishing

ence should relate to various lithofacies. If the oils origi-

two sets of source rocks and their corresponding oils from

nated from Cambrian strata, firstly, they should have the

the research point.

Cambrian characteristics (rich in dinosteranes, 4-methyl-

steranes and their aromatized steriods and 24-nor-

Acknowledgements Partial biomarkers analysis and quantification

cholestanes), rather than the characteristics of the Mid- was finished in the Molecular Organic Geochemistry Laboratory of

dle-Upper Ordovician (poor in dinosteranes, 4-methyl- Standford University. Guidance and assistance were provided by Prof. J.

M. Moldowan and Dr. A. D. Hanson. Reviews by Prof. Wang Tieguan

steranes and their aromatized steriods and 24-norcho-

and Prof. Wang Peirong greatly improved this article. This work was

lestanes). In fact, the distribution of many molecular fos-

supported by the National Key Programs of Science and Technology of

sils in crude oils resembles well with that of Mid- China Petroleum and Natural Gas Exploration in the Tarim Basin

dle-Upper Ordovician rocks and even no trace of the during the 9th Five-Year Plan and the Standford China Industrial Affili-

Cambrian could be found. On the other hand, because the ates.

abundance of steranes is not dominant in the extracts from

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4 Conclusion

phyte microalgae of the genus Pavlova, Organic Geochemistry,

Depending on the correlation results above, two 1990, 15: 489.

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(ed. Taylor, F. J. R.), Oxford: Blackwell Scientific, 1987, 316.

parameters selected here reflect the obvious differences

12. Moldowan, J. M., Talyzina, N. M., Biogeochemical evidence for

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cian and Middle-Upper Ordovician. No matter what is the 1168.

reason of the differences (biological and environmental, 13. Tasch, P., Paleobiology of the Invertebrates: Data Retrieval from

the Fossil Record, New York: Wiley & Sons, 1980.

just like the discussion in ref. [4]), these differences

14. Bian Lizeng, Zhang Shuichang, Zhang Baoming et al., A kind of

formed the solid foundations of oil-source rock correla- coccoid dinoflagellates-like fossils gives a new exploration of

tions. Secondly, the commercial oil reservoirs with vari- source of dinosterane in the Early-Middle Cambrian, Chinese

ous properties (whether heavy oils, normal black oils, Science Bulletin, 2001, 46 (5), 420.

15. Moldowan, J. M., Lee. C. Y., Watt, D. C. et al., Analysis and oc-

condensates or waxy oils) discovered in Tabei and

currence of C26-sterane in petroleum and source rocks, Geo-

Tazhong uplifts at present have a close genetic relation- chimica et Cosmochimica Acta, 1991, 55: 1065.

ship with Upper Ordovician source rocks deposited in the (Received August 12, 2002)

Chinese Science Bulletin Vol. 47 Supp. December 2002 27

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