Paleozoic oil±source rock correlations in theTarim basin, NW China

S.C. Zhang a, A.D. Hanson b,*, J.M. Moldowan b, S.A. Graham b, D.G. Liang a,E. Chang b, F. Fago b

aResearch Institute of Petroleum Exploration and Development (RIPED), China National Petroleum Corporation, Beijing,

People's Republic of ChinabDepartment of Geological and Environmental Sciences, Stanford University, Stanford, CA, USA

Received 16 November 1998; accepted 5 January 2000

(returned to author for revision 23 March 1999)

Abstract

We studied a suite of 40 oils and extracts of purported source rocks from the Tarim basin in NW China. The maingroup of oils comes from Tazhong and Tabei wells, which sample the largest known petroleum accumulations in thebasin. These oils can be statistically correlated with extracts of Ordovician rocks based upon high relative concentra-

tions of 24-isopropylcholestanes and low relative concentrations of dinosteranes, triaromatic dinosteroids, and 24-norcholestanes. In contrast, extracts from Cambrian rocks have low relative concentrations of 24-isopropylcholestaneswith high relative concentrations of dinosteranes, triaromatic dinosteroids, and 24-norcholestanes. Although someTarim basin Cambrian rocks yield high total organic carbon contents, we see little evidence in the analyzed oil samples

to suggest that they came from Cambrian source rocks. # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Tarim basin; Ordovician; Tazhong; Tabei; 24-isopropylcholestanes; Dinosteranes; Triaromatic dinosteroids; Norcholes-

tane; China; Cambrian; Carboniferous

1. Introduction

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

basin with numerous petroleum source rocks of di�erentages (Ulmishek, 1984; Lee, 1985; Fan et al., 1991; Gra-ham et al., 1990; Wang et al., 1992; Hendrix et al., 1995).

The basin has undergone several phases of structuraldeformation (Li et al., 1996), and may have undergonemultiple periods of hydrocarbon generation, accumula-

tion and migration (Xiao et al., 1996). The resultantcomplexity of the Tarim basin makes the study of pet-roleum geology di�cult. The age, type and maturation

of marine oils are some of the important unresolvedproblems in the basin.

Previously conducted oil±source rock correlationattempts [Graham et al., 1990; internal Research Insti-tute of Petroleum Exploration and Development

(RIPED), unpublished data] concluded that the mainsource rocks in the Tarim basin are within the Cambro±Ordovician marine sediments. However, it remained

unclear which strata within the 5±7 km thick Cambro±Ordovician section (Gu et al., 1994) contain the sourcerocks or how much the Cambrian and Ordovician

source rocks contributed individually to hydrocarbonsin the accumulations.Hanson et al. (2000) conducted an organic geochem-

ical study on forty oils and purported source rock sam-ples from wells scattered throughout the Tarim basin(Table 1, Fig. 1). They employed cluster analysis usingJMP1 (1995) statistical software and reported the pre-

sence of at least seven genetic groups of oils. Thesegenetic groups include three groups of oils derived fromdi�erent facies of marine Ordovician source rocks and

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0146-6380(00 )00003-6

Organic Geochemistry 31 (2000) 273±286

www.elsevier.nl/locate/orggeochem

* Corresponding author at current address: Texaco

Exploration Ð Nigeria, 4800 Fournace Place, Bellaire, TX

77401, USA. Tel.: +1-713-432-2207; fax: +1-713-432-2832.

E-mail address: hansoad@texaco.com (A.D. Hanson).

Fig. 1. Map of the Tarim basin including major structural elements [after Gu et al. (1994), as modi®ed with proprietary data from RIPED]. Well locations are shown as dots. Major

cities are shown as squares. Shading is indicative of Phanerozoic sediment thickness with darker areas indicating greater thickness.

274

S.C.Zhanget

al./

Organic

Geochem

istry31(2000)273±286

four groups of oils derived from di�erent non-marine

source rocks.In this paper, we focus on a subset of those oils

(identi®ed as Group 1 in Table 1) whose attributes sug-

gest they were derived from lower Paleozoic sourcerocks, and report the results of our oil±source rock cor-relation studies. By combining detailed geochemicalanalyses of selected oil and rock samples, especially the

correlation of certain biomarkers, with our knowledgeof the geology, we attempt to determine the age andtype of the e�ective source rocks.

We note that several proper names used in the texthave been spelled in di�erent ways in the past, due todi�erent transliterations of the original names into

English (e.g.``Tarim'' is also spelled ``Talimu'' by someauthors). Where this has arisen, the o�cial Pinyin form(Anon., 1990) has been used.

2. Paleozoic geologic setting

Nearly the entire Paleozoic section in the Tarim basinis comprised of marine strata (Gu et al., 1994). Cambrianand Lower Ordovician sections consist of dominantly

shallow-water carbonates, whereas Middle±Upper

Ordovician strata range from platformal carbonates inthe west to deep-water turbiditic sandstones and shalesin the east. Carboniferous±Lower Permian sequences

are stacked marine to non-marine transgressive±regressive successions, whereas the Upper Permian isnon-marine. Mesozoic and Cenozoic strata are almostexclusively non-marine.

Paleozoic source rocks are found in Cambrian,Ordovician and Carboniferous±Lower Permian sequen-ces (Wang et al., 1992). Silurian, Devonian and Upper

Permian strata contain no known source rock facies.Unpublished work at RIPED documented aspects

of Cambrian, Lower Ordovician and Middle±Upper

Ordovician source rocks in the subsurface of the Tarimbasin. Results and interpretations based on these data,and available published data, are summarized in the

following three sections.

2.1. Cambrian

In the eastern Tarim basin, purported Cambriansource rocks are marls, mudstones, and dolomitesdeposited in starved basins (Gu et al., 1994). Average

Table 1

List of Tarim oil samples and related data

Sample Area Field Depth (m) Res. Agea Density Sulfur (%) Asph. (%) Wax (%) 13C oil Groupb

37LN14 Tabei Lunnan 5274.2±5363 O1 0.846 0.09 1.44 12.9 ÿ31.6 1

3YM2 Luntai Yingmaili 5940-5953 O1 0.879 0.85 13.6 5.7 ÿ33.4 1

2YH5 Luntai Yaha 5801.5±5807 O 0.758 0.02 0 2.4 ÿ30.2 4

9TZ16 Tazhong Tazhong 4244.6±4259.5 O2 0.896 0.45 13.7 3.4 ÿ32.7 2

8TZ11 Tazhong Tazhong 4417-4435 S ± ± ± ± ÿ32.5 1

65TZ11 Tazhong Tazhong 4310 S ± ± ± ± ± ?1

22LN44 Tabei Lunnan 5084±5095 C 0.785 ± ± ± ÿ31.7 1

32Qun5 Bachu Qunkuqiake 4874.9±4884 C 0.796 ± 2.1 5.1 ÿ34.5 3

39Qu1 Bachu Qunkuqiake 4745.5±4731.4 C ± ± ± ± ÿ31 5

28JF124 Tabei Jiefangqu 5081±5095 C 0.787 ± ± ± ÿ31.1 1

10TZ401 Tazhong Tazhong 3244-3247.5 CI 0.863 0.55 24.7 4.7 ÿ33.2 1

11TZ411 Tazhong Tazhong 3703-3704.5 CIII 0.898 0.7 15.9 6.3 ÿ32.1 1

13TZ161 Tazhong Tazhong 3805.2±3821.5 CIII 0.869 0.35 11.7 2.6 ÿ31.6 1

6TZ6 Tazhong Tazhong 3710.9±3728.6 CIII 0.764 ± ± ± ÿ31.7 1

57TZ24 Tazhong Tazhong 3790.9±3807.2 CIII ± ± ± ± ± 2

25LN57 Tabei Lunnan 4341.8±4344 TII 0.747 ± ± ± ÿ32 1

33Yi603 Kuqa Yiqikelike 471.2±489.2 J2 ± ± ± ± ÿ26.1 6

36YH3 Luntai Yaha 5327.5±5401.8 K 0.790 ± 0.01 12 ÿ27.2 4

38YT2 Luntai Yangtake 5327.5±5401.8 K ± ± ± ± ÿ25.3 4

1YH1 Luntai Yaha 5459.5±5466 E 0.827 ± 6.09 6.2 ÿ29.3 4

40KS1 SW Keshen 6370±6388.1 E2 ± ± ± ± ± 7

30Ke2 SW Kekeya 3247.5±3298 N1 ± ± ± ± ÿ29.1 7

63KLT West Kelatou Surface-seep N1 ± ± ± ± ± ?4

a O1=Lower Ordovician, O=Ordovician (undi�erentiated, O2=Middle Ordovician, S=Silurian, C=Carboniferous (undi�er-

entiated), CI=Lower Carboniferous, CIII=Upper Carboniferous, TII=Middle Triassic, J2=Middle Jurassic, K=Cretaceous,

E=Paleogene, E2=Eocene, N1=Miocene.b Genetic group to which samples belong according to Hanson et al. (2000). Samples 65TZ11 and 63KLT are potentially, though

not de®nitively, members of Groups 1 and 4, respectively, and as such are marked with a ``?''.

S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286 275

total organic carbon (TOC) values within individualwells generally range from 1.24 to 2.28 wt% but reach a

maximum of 5.52 wt% (see Table 2 for a partial listingof the data for source rocks included in this study).Strata with TOC >1.0 wt% occupy 60±70% of thesequence and net source rock thickness ranges from 120

to 415 m. Cambrian source rocks are overmature (VRE> 2.0%) in exploration wells (TD1 well and KN1 well)in the eastern Manjaer depression (Fig. 1) based upon

vitrinite re¯ectance equivalence (VRE), which is widelyused in China (Chen et al., 1996; as derived from Liu etal., 1994), but which is not calibrated to methods

employed outside China. Based on VRE data, the sug-gested remaining generative potential of these sourcerocks must be low.To the west on the Bachu uplift, Cambrian evaporites

of lagoonal facies are present in the He4 well drilledalong the Hotan River (Fig. 1). Lithologies include marland muddy dolomite which have relatively high abun-

dances of organic matter with a maximum TOC of 2.14wt%. VRE data suggest that the source rocks are in thehighly mature condensate stage (VRE values of 1.65 to

1.70%).

2.2. Lower Ordovician

The best documented purported source rocks of theLower Ordovician are similar to those described for theCambrian. The average TOC in the TD1 well in eastern

Manjaer is 1.93 wt%. Marine slope facies should occurin the central and western parts of the Tarim basin, butsource rocks with high amounts of organic carbon have

not been penetrated to date. The purported source rocks,based on unpublished VRE data, appear to be mature,

to potentially overmature, and their hydrocarbon-generating history is similar to the Cambrian rocksdescribed above.

2.3. Middle±Upper Ordovician

Organic geochemical studies have identi®ed high

TOC rocks in the Middle±Upper Ordovician (RIPED,unpublished data; Fan et al., 1991). These are mainlymuddy limestones and marls deposited in shelf-edge and

slope settings (RIPED, unpublished data). Purportedsource rocks of Middle±Late Ordovician age are widelydistributed along the northern slope and the crest of theTazhong uplift with a thickness of about 80±100 m.

Similar rocks exist in some Lunnan (LN) wells in NorthTarim, as well as in some wells near the Hotan River(He) on the Bachu uplift (Fig. 1). Based on measure-

ments of 298 samples, the average TOC is 0.43 wt%with a maximum of 6 wt% (RIPED, unpublisheddata).

Open bay or gulf facies occur in the Kalpin area andthe Awati depression. Purported source rocks includethe Sargan shale or Yingan shale, whose TOC is 0.05±

2.25 wt% (average 0.89 wt%). Similar TOC values werereported by Graham et al. (1990) for outcrop samples ofthe Saergan Formation from the Kalpin area (TOCvalues of 0.21±2.75 wt%).

In contrast, Ordovician strata of the Manjaer andTangguzibasi depressions can be over 10 km thick. Thesediments are characterized by turbiditic ¯ysch and

Table 2

List of Tarim source rock extracts and related data

Sample Area Field Depth (m) Agea TOC S1 S2 S1+S2 Tmax Lithology

7KN1 NE Tabei Kunan 5183.3 Cam 0.9 0.04 0.08 446 Marl w/calcite veins

9He4 Hetianhe Hetianhe 5079.9 Cam 0.47 352

24KN1 NE Tabei Kunan 5505 Cam 0.82 0.05 0.08 436 Muddy ls

35TZ6 Tazhong Tazhong 3886.1 O2 1.2 0.53 1.42 433 Black ls

20TZ12 Tazhong Tazhong 4805.2 O2 0.43 1.44 1.11 424 Grey ls w/black ms interbeds

2LN46 S Tabei Lunnan 6161 O2+3 86.0 0.18 0.90 434 Dark grey ls

10TaC1 Tazhong Tazhong 4029.7 O2+3 0.98 0.15 0.65 442 Black±grey marl

11TZ12 Tazhong Tazhong 5074.6 O2+3 0.78 0.34 0.73 451 Grey±black muddy micrite

12TZ30 Tazhong Tazhong 4916 O2+3 2.23 0.27 0.68 443 Dark grey micrite

51TZ7 Tazhong Tazhong 4298.2 O2+3 0.31 0.02 0.04 450

33TZ35 Tazhong Tazhong 5392 O2+3 0.4 0.04 0.17 434 Grey muddy ls

37TZ201 Tazhong Tazhong 5137.6 O2+3 1.33 0.67 2.09 440 Grey±black micrite (algal ls)

38TaC1 Tazhong Tazhong 4920.9 O2+3 0.7 0.49 1.57 440 Grey±black dolomite

14TZ6 Tazhong Tazhong 3469 C 4.17 1.10 2.31 449 Grey biocalcilutite

17TZ6 Tazhong Tazhong 3556.1 C 0.8 0.07 0.18 437 Dark grey ms

40Yang1 Kuche Yangxia 6429 J 22.98 80.27 Carbonaceous ms

41Yang1 Kuche Yangxia 6433 J 73.83 100.26 Coal

a Cam=Cambrian, O2=Middle Ordovician, O2+3=Middle±Late Ordovician, C=Carboniferous, J=Jurassic.

276 S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286

contourites. Lithologies are volcanic lithic rich sand-stones and dark massive mudstone. Thick shales andsandstones in the Manjaer depression have very lowTOC values (commonly <0.2 wt%), and good source

rocks have not been documented within these sections(RIPED, unpublished data; Gu et al., 1994).

3. Methods

Organic matter extractions from purported sourcerocks were completed using dichloromethane for 72 h ina Soxhlet apparatus at the RIPED experimental center

in Beijing. Extracts and oil samples were then sent toStanford University where analyses described belowwere completed.Extracts and whole oils were analyzed via standard

(n-C12 and higher) gas chromatography (GC) on an HP5890A gas chromatograph.All samples were separated using high performance

liquid chromatography as described in Peters andMoldowan (1993). Saturate fractions were spiked witha known quantity of 5b-cholane and then treated with

a high Si/Al ZSM-5 zeolite (``silicalite'') preparationto remove normal alkanes. All saturate fractionswere analyzed on an HP 5890 Series II GC and Trio 1

VG Masslab gas chromatograph±mass spectrometer(GC±MS) and on an HP 5890-II GC-VG MicromassAutospec Q in an MRM±GC±MS (metastable reactionmonitoring) mode. We used a 60 m DB-1 (fused

silicone) (J&W Scienti®c Column), with a 0.25 mmi.d., and 25 mm ®lm thickness. H2 was the carriergas. GC±MS analyses of aromatic fractions were

completed at the Houston Advanced Research Centerusing an HP 5890 with a 5970 Mass Selective Detector.The GC column was an HP-1 column (100% poly-

dimethylsiloxane), with 0.25 mm ®lm thickness, 0.25 mmi.d., and 60 m length. The temperature programwas isothermal for 4 min at 70�C, then increased3�C/min to 145�C, followed by 2�C/min to 320�C.Injector temperature was 300�C, and the carrier gas washelium.Organic geochemical analyses consisting of sulfur,

wax, asphaltene, and density measurements, as well asbulk stable carbon isotope analyses, carried out onselected oils by RIPED in China (Table 1) are included.

The ®eld, depth of the producing zone, and the reservoirages are also listed for oil samples in Table 1.

4. Results

4.1. Sample populations

Oils in this study (Table 1) include samples from allknown producing areas of the basin. Source rocks sam-

ples were selected by use of preliminary screeningmethods, including TOC and Rock-Eval analyses. Insome cases, the TOC values for our samples are lowcompared to the maximum TOC values reported above.

However, with Tmax values of 440±450�C (Table 2),these source rocks have already generated oil and theoriginal TOC values, prior to generation, would have

been signi®cantly higher. We report limited data forCambrian rocks, but note that despite having samplesfrom widely separated areas (the KN1 well in north-

eastern Tabei uplift and the He4 well on the Bachuuplift), the geochemical signature of our Cambriansamples is very similar, and thus may be representative

of the Cambrian in general.Samples of Early Ordovician age are from the Taz-

hong (TZ) and Lunnan (well LN46) areas. Middle±LateOrdovician and Carboniferous aged source rock sam-

ples are from the Tazhong (TZ) and TaC1 area. Ourlimited Carboniferous source rock samples re¯ect thegeneral lack of good quality source rocks found to date

within Carboniferous strata. Source rock sample names,®eld names, depth of core samples, and geologic agesare listed in Table 2. TOC, Rock-Eval pyrolysis results,

and lithological descriptions from RIPED are alsoincluded for most samples in Table 2. The position ofmajor structural elements in the basin and well locations

are shown in Fig. 1.

4.2. Characteristics of the Tazhong±southern Tabei oilgroup

This single genetic group (indicated as Group 1 inTable 1) consists of oil samples from two geographic

locations: Tazhong (TZ) and the southern part of theTabei uplift (Fig. 1) (Hanson, 1999; Hanson et al.,2000). Oils from southern Tabei come from Lunnan

(LN), Yingmaili (YM), and Jiefangqu (JF) wells (Table 1).Oils in this genetic group are easily distinguished from

other genetic groups of oil in the Tarim basin. Theyhave whole oil carbon isotope values of ±31 to ÿ33%PDB, pristane/phytane (Pr/Ph) ratios of 0.83±1.4, lowPr/n-C17 ratios, high C23 tricyclic/(C23 tricyclic+C30

hopane) ratios, and well-preserved homohopanes, and

they lack b-carotane (Hanson, 1999; Hanson et al., 2000).There are only minor di�erences between the Tazhongand Tabei oils based on these features.

Sources for the two geographic groups appear to besimilar. However, Tazhong oils generally have higherconcentrations of sulfur than Tabei oils. High concentra-

tions of sulfur-bearing compounds (dibenzothiophenes)have been found in the Tazhong 4 ®eld (RIPED,unpublished data). These features indicate that somesource rocks in the Tazhong area might have been

deposited in strongly reducing environments, or that thesource rocks have a higher carbonate content (Hugheset al., 1995).

S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286 277

Table 3

Key GC, GC±MS, and MRM±GC±MS data from oils and source rocks from the Tarim basin generated during this studya

Samples Pr/Ph Pr/n-C17 Ts/Ts+Tm A B C D E F G H I J K L M

Oils

1YH1 3.07 0.24 0.31 0.71 0.10 0.03 0.40 0.22 0.84 0.25 0.04 0.48 0.48 0.58 0.54 0.36

2YH5 3.50 0.22 0.56 0.55 0.10 0.05 0.51 0.34 0.61 0.31 0.04 0.55 0.55 0.59 0.61 0.10

3YM2 0.93 0.30 0.36 0.53 0.40 0.23 0.35 0.39 0.10 0.23 0.08 0.60 0.51 0.60 0.58 0.03

6TZ6 1.00 0.10 0.56 0.37 0.81 0.35 0.57 0.24 0.13 0.52 0.02 0.53 0.50 0.43 0.57 0.15

8TZ11 0.89 0.27 0.38 0.42 0.40 0.30 0.30 0.37 0.10 0.26 0.10 0.58 0.49 0.60 0.60 0.05

9TZ16 1.04 0.23 0.40 0.41 0.29 0.18 0.38 0.42 0.25 0.29 0.09 0.54 0.48 0.60 0.59 0.05

10TZ401 0.95 0.19 0.50 0.45 0.68 0.36 0.48 0.35 0.11 0.30 0.08 0.57 0.51 0.58 0.60 0.05

11TZ411 0.96 0.17 0.53 0.42 0.69 0.40 0.55 0.40 0.13 0.32 0.10 0.59 0.46 0.55 0.61 0.07

13TZ161 1.00 0.19 0.63 0.34 0.79 0.43 0.55 0.37 0.21 0.31 0.09 0.56 0.51 0.58 0.61 0.08

22LN44 1.44 0.24 0.61 0.45 0.64 0.37 0.42 0.31 0.16 0.34 0.08 0.58 0.49 0.54 0.62 0.09

25LN57 1.06 0.30 0.42 0.46 0.65 0.39 0.40 0.33 0.21 0.30 0.07 0.59 0.52 0.59 0.61 0.03

28JF124 1.10 0.21 0.67 0.45 0.60 0.48 0.43 0.36 0.11 0.25 0.07 0.62 0.53 0.59 0.61 0.04

30Ke2 1.20 0.09 0.64 0.64 0.68 0.00 0.52 0.24 0.46 0.21 0.00 0.62 0.53 0.45 0.66 0.08

32Qun5 1.50 0.16 0.67 0.44 0.74 0.51 0.61 0.32 n.p. 0.32 0.00 0.60 0.50 0.51 0.60 0.08

33Yl603 4.38 8.18 0.21 0.87 0.02 0.04 0.56 0.32 0.48 0.24 0.02 0.49 0.46 0.61 0.56 0.06

36Yh3 2.50 0.15 0.58 0.64 0.13 0.04 0.45 0.22 0.85 0.25 0.00 0.49 0.45 0.61 0.61 0.38

37LN14 0.98 0.17 0.71 0.50 0.63 0.71 0.52 0.48 0.09 0.28 0.10 0.62 0.51 0.43 0.47 0.05

38YT2 2.17 0.10 0.63 0.60 0.11 0.04 0.48 0.25 0.77 0.39 0.03 0.51 0.46 0.60 0.62 0.36

39Qu1 2.00 0.25 0.54 0.72 0.15 0.11 0.42 0.34 0.64 0.25 0.05 0.55 0.49 0.60 0.60 0.05

40KS1 1.43 0.08 0.63 0.58 0.58 0.42 0.47 0.21 n.p. 0.20 0.00 0.63 0.53 0.52 0.61 0.10

57TZ24 0.95 0.20 0.23 0.36 0.49 0.27 0.41 0.57 0.19 0.31 0.07 0.55 0.50 0.52 0.54 n.d.

63KLT 1.38 bio 0.49 0.58 0.04 0.05 0.53 0.13 0.66 0.22 0.03 0.43 0.33 0.56 0.60 n.d.

65TZ11 1.11 bio 0.47 0.63 0.41 0.13 0.30 0.39 0.19 0.23 0.06 0.26 0.25 0.52 0.55 n.d.

Source rocks

2LN46 1.26 0.41 0.58 0.69 0.18 0.05 0.42 0.34 0.12 0.25 0.04 0.51 0.48 0.60 0.60 0.19

7KN1 0.69 0.48 0.34 0.59 0.39 0.06 0.12 0.20 0.68 0.75 0.04 0.39 0.46 0.59 0.59 0.33

9He4 0.33 0.42 0.30 0.64 0.03 0.01 0.17 0.19 0.75 0.55 0.08 0.36 0.42 0.60 0.57 0.38

10TaC1 1.18 0.33 0.10 0.92 0.03 0.01 0.59 0.22 0.20 0.27 0.04 0.33 0.41 0.52 0.57 0.06

11TZ12 1.44 0.21 0.51 0.88 0.05 0.03 0.41 0.29 0.07 0.33 0.05 0.53 0.46 0.58 0.59 0.17

12TZ30 1.99 0.54 0.38 0.68 0.21 0.07 0.32 0.37 0.12 0.58 0.07 0.49 0.45 0.58 0.57 0.32

14TZ6 1.12 0.51 0.13 0.82 0.08 0.02 0.14 0.22 0.26 0.30 0.02 0.36 0.40 0.55 0.60 0.17

17TZ6 1.69 1.03 0.07 0.91 0.03 0.00 0.63 0.31 0.13 0.28 0.09 0.30 0.45 0.56 0.56 0.13

20TZ12 1.18 0.14 0.44 0.88 0.06 0.05 0.32 0.30 0.25 0.47 0.04 0.44 0.45 0.58 0.55 0.31

24KN1 1.51 0.62 0.41 0.62 0.40 0.05 0.12 0.18 0.72 0.63 0.03 0.43 0.50 0.59 0.61 0.29

33TZ35 0.80 0.27 0.52 0.92 0.06 0.06 0.31 0.38 0.12 0.23 0.06 0.59 0.48 0.60 0.61 0.10

35TZ6 1.02 0.14 0.61 0.88 0.08 0.17 0.54 0.55 0.10 0.23 0.05 0.54 0.49 0.56 0.60 0.02

37TZ201 5.08 0.61 0.42 0.94 0.04 0.03 0.58 0.36 0.05 0.25 0.06 0.50 0.47 0.58 0.58 0.05

38TaC1 0.24 0.22 0.67 0.88 0.10 0.08 0.36 0.36 0.05 0.29 0.07 0.55 0.50 0.56 0.64 0.13

40Yang1 2.00 0.27 0.33 0.84 0.17 0.04 0.34 0.26 0.81 0.35 0.02 0.49 0.46 0.56 0.58 0.17

41Yang1 1.96 0.33 0.31 0.87 0.10 0.04 0.25 0.31 0.84 0.44 0.03 0.49 0.50 0.55 0.53 0.15

51TZ7 0.78 0.11 0.51 0.97 0.03 0.01 0.31 0.31 0.14 0.29 0.05 0.57 0.50 0.57 0.54 0.06

TGMN10 2.21 1.47 0.83 0.50 0.03 0.01 0.48 n.d. n.d. 0.32 0.00 0.25 0.27 0.62 0.62 0.04

KRN2 2.13 0.29 0.62 0.65 0.08 0.02 0.42 n.d. n.d. 0.22 0.01 0.18 0.21 0.67 0.58 0.03

KRN15 2.50 0.48 0.93 0.75 0.14 0.04 0.41 n.d. n.d. 0.27 0.00 0.36 0.50 0.63 0.62 0.05

KRN19 1.42 0.49 0.18 0.63 0.13 0.03 0.50 n.d. n.d. 0.36 0.01 0.41 0.49 0.63 0.59 0.06

a A=C24 tetra/(C24 tetra+C26 tricyclics); B=C23 tricyclic/(C23 tricyclic+C30 hopane); C=C29 tricyclic/(C29 tricyclic+C30

hopane); D=C27 diasteranes/(dia+regular); E=24-iso-/24-n-propylcholestane; F=dinosteroids/(dinosteroids + 3-methyl-24-

ethyl-cholesteroid); G=24/(24+27) norcholestanes; H=C35 homohopane/(C31-C35) homohopanes; I=C29 abb/(abb+aaa)steranes; J=C29 20S/(20S+20R); K=C31 homohopane isomerization ratio; L=C32 homohopane isomerization ratio;

M=dinosteranes/(dinosteranes +3b-methyl-24-ethyl-cholestanes); bio=biodegraded; n.d.=no data; n.p.=no peaks detected.

278 S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286

4.3. Age and oil±source correlation relevant parameters

Several biomarkers have been shown to be related tospeci®c modern taxa, and molecular paleontologicalstudies have revealed correlations which allow for the

use of certain biomarkers as indicators of geologic age(Moldowan et al., 1996; Holba et al., 1998a,b). Agediagnostic biomarkers employed in this study include

24-isopropylcholestanes, dinosteranes (4a,23,24-tri-methylcholestanes), triaromatic dinosteroids, and 24-norcholestanes. The source of these data is given in the

Appendix and key results are listed in Table 3.

4.3.1. 24-IsopropylcholestaneThe modern precursors of 24-isopropylcholestane are

found almost exclusively in marine porifera (sponges)referred to as Demospongiae (McCa�rey et al., 1994).24-n-propylcholestanes are related to 24-n-propyl-

cholesterols which are biosynthesized by marineChrysophyte algae (Moldowan et al., 1990) of the orderSarcinochrysidales. They have been linked to a marine

prasinophyte (Volkman et al., 1994) and are common inmarine invertebrates, presumably via a dietary origin.Trace amounts of 24-isopropylcholestane typically

occur in Phanerozoic marine rock extracts. However, inVendian±Cambrian rock extracts and oils high con-centrations (relative to 24-n-propylcholestane) are char-acteristic, implying that sponges or related organisms

were relatively more abundant in some Vendian±Cam-brian paleoenvironments than at other times (McCa�reyet al., 1994).

The ratios 24-isopropylcholestane/24-n-propylcholes-tane of Tarim Paleozoic rock extracts and marine oilsare plotted along the x-axis of Fig. 2. We found lowrelative concentrations of 24-isopropylcholestane in

Cambrian extracts with ratios of 24-isopropylcholestane/24-n-propylcholestane around 20%. However, in Ordo-vician extracts the ratio is generally greater than 25%

with an average value of 35%. This ®nding is not sur-prising since sponges are commonly seen in Middle±Upper Ordovician Tarim core samples. We attribute the

higher Ordovician values relative to the Cambrianvalues to one of two possibilities. The ®rst possibility isthat depositional environments during the Ordovician

were more suitable for growth of sponges and they arean important contribution. The second possibility isthat sponges were abundant during the Cambrian, butthe organic matter was not well preserved in the shallow

water depositional environments. The low Cambrianratios might also be due to a relatively abundant popu-lation of algae that contributed 24-n-propylcholestanes.

No matter what caused the higher relative concentra-tions of 24-isopropylcholestanes, the elevated amountsseen in the oil samples are support for a marine source

rock origin for the oils and provide a basis for the dis-tinction between Cambrian and Ordovician samples.Based on their 24-isopropylcholestane/24-n-propyl-

cholestane ratios, there is no statistical overlap (mean,standard deviation, and 95% con®dence interval calcu-lations) between the Cambrian source rocks and theTazhong and Tabei oils (Fig. 2). Cambrian extracts are

signi®cantly di�erent from Tazhong and Tabei oils, aswell as from Ordovician extracts, based on these bio-markers.

4.3.2. DinosteranesPrecursors of dinosteranes are dinosterols, which

occur abundantly (Withers, 1987), and almost exclu-sively (Volkman et al., 1993), in modern dino¯agellates.Dinosterane has been mainly reported in petroleumsthat originated from Triassic or younger source rocks

(Summons et al., 1987) with a few examples in Pre-cambrian extracts. A larger survey including numerousPaleozoic extracts shows a continuous dinosterane

record from the Proterozoic to the Tertiary, but typi-cally with lower relative abundances in the Paleozoic(Moldowan et al., 1999). This age relationship corre-

sponds with the earliest widespread fossil evidence fordino¯agellates during the Triassic, although occurrencesof possible dino¯agellate fossils have been reported in

the Paleozoic with the oldest suspected species comingfrom rocks of Silurian age (Tappan, 1980). Morerecently, Early Cambrian dino¯agellate ancestors con-taining dinosterane have been identi®ed (Moldowan and

Talyzina, 1998).We found di�erent concentrations of dinosteranes in

Tarim oils and extracts of di�erent ages. In all Cambrian

Fig. 2. Crossplot of 24-isopropylcholestanes/24-n-propylcho-

lestanes versus methyl sterane (C30) ratios showing separation

of Cambrian and Ordovician extracts. Indications of mean

values, standard error, one standard deviation, and 95% con-

®dence intervals are given in the key on the right and are shown

for Cambrian extracts (upper left) and Tazhong and Tabei oils

(lower center-right). Tazhong and Tabei oils plot with some of

the Ordovician extracts.

S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286 279

extracts, the dinosterane relative concentration is high,

whereas in Ordovician extracts, the dinosterane relativeconcentration varies from high to low. In Carboniferousextracts, the concentration is low in mudstone

[TZ6(MS)] and high in limestone [TZ6(LS)]. Tazhongand Tabei oils contain low concentrations of dinoster-ane and thus do not correlate with Cambrian extracts

but can be correlated with some Ordovician extracts(Fig. 2). Based on their dinosterane/(dinosterane+3b-methyl-24-ethylcholestane) ratios, there is again nooverlap of the 95% con®dence interval for the Cam-

brian source rocks relative to the Tazhong and Tabeioils (Fig. 2). Similar to the results mentioned above,comparison of mean values, standard deviation and

95% con®dence interval calculations indicate thatCambrian extracts are signi®cantly di�erent from Taz-hong and Tabei oils, as well as from Ordovician extracts

based on the dinosterane biomarkers.

4.3.3. 21- And 24-norcholestanes (C26)Traces of 24-norcholesterols occur in living marine

algae and invertebrates, suggesting an origin in eukar-yotes which may, or may not, have prokaryotic sym-bionts. However, the 21- and 27-norcholestanes appear

to have no direct sterol precursors, but may be derivedthrough bacterial oxidation or thermally induced clea-vage and loss of a methyl group from larger steroids

(>C26). Paleozoic and older oils usually show littleor no 24-norcholestanes (Moldowan et al., 1991; Holbaet al., 1998a,b). The associated rock record data are less

systematic, but in the Tarim basin we see signi®cantamounts of 24-norcholestanes in some of our Ordovicianand Carboniferous rocks, in all our Cambrian rocks,and in one Jurassic coal. Tabei and Tazhong oils can be

statistically distinguished from Cambrian and Carboni-ferous extracts and cluster with Ordovician extracts(Fig. 3). Statistical methods using norcholestane ratios

indicate that Tazhong and Tabei oils are not sig-ni®cantly di�erent from Ordovician extracts.

4.3.4. Triaromatic dinosteroidsTriaromatic dinosteroid data also support a correla-

tion between Tazhong and southern Tabei oils and

Ordovician, not Cambrian, source rocks. Statisticallydi�erent high ratios of dinosteroids/(dinosteroids+3b-methyl-24-ethyl-cholesteroid) were detected in Cambrian

extracts whereas Ordovician and Carboniferous extractshave low ratios (Fig. 3). Ordovician andCambrian extractsare widely separated, and oils from Tazhong and Tabeiuniformly show triaromatic dinosteroids distributions

similar to Ordovician extracts (Fig. 3). Moldowan et al.(1999) suggested that abundant triaromatic-dinosteroidsin some Paleozoic rocks might be related to the con-

centration of marine acritarchs (phytoplanktonic algalcysts of uncertain a�nity) (e.g. Moldowan and Taly-zina, 1998) potentially derived from dino¯agellates

which may have been primary producers in Paleozoicoceans (Tasch, 1980). The underlying cause of di�eringamounts of triaromatic dinosteroids in the Cambrian

versus the Ordovician of the Tarim basin is thereforepresumably related to changing paleo-oceanographicconditions between Cambrian and Ordovician time.Whatever the underlying cause of di�erent levels of

triaromatic dinosteroids, the di�erences again provide abasis for distinguishing Cambrian from Ordoviciansamples.

Fig. 3. Crossplot of nordiacholestane and triaromatic methyl

steroids ratios, showing clear separation of Cambrian and

Ordovician extracts. Indicators of statistical measures are

shown in the same manner as in Fig. 2. Marine oils plot with

the Ordovician extracts and do not match Cambrian extract

data.Fig. 4. Crossplot of two C29 sterane stereoisomer ratios show-

ing di�ering trends for Ordovician and Cambrian extracts.

Regression lines (solid) with 95% con®dence intervals (dashed

lines) are shown for Cambrian and Ordovician extracts. R2

values indicate correlation coe�cients for extract data. Taz-

hong and Tabei oils plot along the trend de®ned by Ordovician

extracts.

280 S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286

4.4. Parameters related to maturity, lithology, anddepositional environment

Biomarker maturity parameters are useful in deter-

mining the thermal history of petroleum and relatedsource rocks. However, several studies have shown thatmany biomarker maturity parameters are also depen-

dent on the character of the source rock itself (Peterset al., 1990; Moldowan et al., 1994). For example,McKirdy et al. (1983) suggested that sterane maturation

parameters may be a�ected by the mineral matrix of thesource rocks. They observed that oils from a probablecarbonate source plot on the left of the Seifert and

Moldowan (1981) empirical trend on the C29 diagram(see Fig. 3.56 of Peters and Moldowan, 1993). Similarobservations were made by Huang et al. (1990) for oilsderived from gypsum-, salt-, and carbonate-rich rocks.

Maturity e�ects on two di�erent ratios of C29 steranesin Paleozoic rocks from the Tarim basin re¯ect di�er-ences in ages and lithologies (Fig. 4). For Cambrian

limestone extracts, the C29 aaa 20S/(20S+20R) ratioincreases more rapidly than the abb/(abb+aaa) ratio,producing a trend with a slope that is steeper than that

of the C29 steranes of Ordovician extracts (mainly frommarls). Although the two Carboniferous extracts havealmost the same maturity (Ro around 0.7%) and come

from depths separated by only 90 m, the isomerizationratios of the two samples show considerable di�erence.The limestone sample [TZ6(LS)] is closer to the Cam-brian trend, whereas the mudstone sample [TZ6(MS)]

shows relatively high 20S/(20S+20R) and low abb/(abb+aaa) ratios. This may imply that the isomeriza-tion rate of the C20 position is faster than the iso-

merization rate of the C14 and C17 positions for thelimestones. Although both Cambrian and Ordoviciansamples are carbonates, the Ordovician samples containclays which might accelerate isomerization of the C14

and C17 positions. In highly mature source rocks, aneventual decrease in the abb/(abb+aaa) ratio occurs(Peters et al., 1990). As stated earlier, vitrinite re¯ec-

tance equivalence (VRE) data suggest that Cambriansource rocks in exploration wells (e.g. TD1 and KN1 ineastern Manjaer) are overmature (VRE>2.0%). The

high VRE of the Cambrian samples may account for adecrease in the abb/(abb+aaa) ratios as describedabove. Although the Cambrian extracts have mature

biomarker parameters (see Table 3), and elevated Tmax

values (Table 2), we suspect, based on the high abun-dance of biomarkers in the Cambrian rock extracts (e.g.C29 aaa 20R+20S steranes=498±704 ppm), that the

maturity is not as high as the VRE values suggest.Regardless of the e�ect that the lithology of the sourcerocks imparts to the isomerization rate of C29 steranes,

the trends of Tabei and Tazhong oils and Ordovicianrocks are consistent (Fig. 4) and thus support their closea�nity.

Diasterane/regular sterane ratios are a�ected by boththermal maturity and inorganic characteristics of thesource rock or the depositional environment. Catalysis

by acidic sites on clays has been proposed as themechanism by which diasteranes are produced in sedi-ments (Rubinstein et al, 1975). Acidic catalysis is neces-sary for the conversion of sterenes to diasterenes before

eventual conversion to diasteranes. Thus, diasterane/sterane ratios are typically low in carbonate sourcerocks and oils derived from them (Peters and Moldo-

Fig. 5. Crossplot of C27 diasteranes/steranes versus C29 tri-

cyclic/(C29 tricyclic+C30 hopane) showing the subtle variation

between Tazhong and Tabei oils. The trend of Tazhong oils

projects back to a point farther to the left suggesting a carbonate-

rich source rock, whereas Tabei oils were derived from more

clay-rich source rocks. Regression lines, 95% con®dence lines,

and R2 values are based on Tazhong, and Tabei oils, respec-

tively. Dashed lines indicate the position of the Tazhong

regression line based on additional unpublished data.

Fig. 6. Crossplot of C27 diasteranes/steranes versus Ts/

(Ts+Tm). With increasing maturity, samples plot further

toward the upper right. However, Cambrian extracts which are

of presumed higher maturity than Ordovician extracts plot

closer to the lower left, eliminating them as possible source

rocks for the oils. Likewise, extracts in the lower right could

not have generated the oils. Regression lines were forced

through the origin and R2 values re¯ect this condition.

S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286 281

wan, 1993) which has further been shown to be depen-dent on clay/TOC ratios (Kaam-Peters et al., 1998). Inthe Tarim basin, the abundance of diasteranes in bitu-mens varies with the age and lithology of rocks. Because

all the Cambrian rocks in this study are carbonates, thesamples plot close to the origin on the crossplot of C27

dia/(dia+regular) steranes diagram (Fig. 5). The Car-

boniferous limestone [TZ6(LS)] also plots in this area.The Ordovician rock samples in this study are alsocarbonates, but with varying percentages of clay, and as

a result, the diasterane/sterane ratios in their extractsare widely distributed. Tazhong and Tabei oils containmoderate amounts of diasteranes. The geochemical sig-

ni®cance of C29 tricyclic terpane/(C29 tricyclic terpane+C30 hopane) ratio, also plotted on Fig. 5, is notfully understood but re¯ects both organic matter inputand thermal maturity. Tricyclic terpanes are known to

be more stable and related to di�erent precursors thanhopanes and as such this ratio represents a source andmaturity parameter (Seifert and Moldowan, 1978;

Aquino Neto et al., 1983; Peters et al., 1990). The C29

tricyclic terpane/(C29 tricyclic terpane+C30 hopane)ratio plotted versus the dia/(dia+reg) C27 steranes

(Fig. 5) reveals trends within otherwise indistinguishableTazhong and Tabei oils. The trend of Tazhong oilsprojects back to the sterane axis farther to the left than

the trend of the Tabei oils. Tabei oils project back closerto more shale-rich marl extracts. We have seen a versionof this ®gure (RIPED, unpublished data) that containedsigni®cantly more samples than what we have access to

and the Tazhong oil data project back to a positioncoincident with the Cambrian source rocks. Our inter-pretation of these results is that Tazhong oils were

derived from carbonate-rich source rocks. We are notsuggesting that Tazhong oils are derived from Cambriancarbonate source rocks; rather, we believe that the

source rocks that generated Tazhong and Tabei oils areOrdovician, but the Tazhong source rocks contain morecarbonate content, whereas the source rocks of Tabeioils contained more shale. The lack of elevated C29 tri-

cyclics in the source rocks may be further support forour inferences regarding lower levels of thermal matur-ity than what reported VRE data suggest.

Ts/(Ts+Tm) ratios are not only related to maturity,but also to organic facies and depositional environments(Moldowan et al., 1986). Oils derived from carbonates

usually show low Ts/(Ts+Tm) compared to oils gener-ated from shales (McKirdy et al., 1983, 1984). Bitumensof anoxic and acidic hypersaline source rocks generally

show high Ts/(Ts+Tm) (RullkoÈ tter and Marzi, 1988).Fig. 6 shows the Ts/(Ts+Tm) ratios of our Paleozoic

marine source rocks plotted against the C27 diasterane/regular sterane ratios. It is important to note that the

regression lines for the di�erent groups of source rockswere forced through the origin on the crossplot and thereported R2 values re¯ect this condition. Our reason for

forcing the regression lines through the origin is thatboth the Ts/(Ts+Tm) and the C27 diasterane/regular

sterane ratios are known to increase with increasingthermal maturity and thus these values will move awayfrom the origin with increasing maturity.

On Fig. 6, our Cambrian and Carboniferous lime-stones have lower Ts/(Ts+Tm) and dia/(dia+reg)sterane ratios relative to most Ordovician marls. TheCarboniferous mudstone [TZ6(MS)] and some Ordovi-

cian marls show slightly higher dia/(dia+reg) steraneratios with low Ts/(Ts+Tm). On this type of plot, Ehe�ects shift data orthogonal to the direction in which

Fig. 7. GC±FID traces of a Tazhong extract (A) showing a

bimodal n-alkane distribution and a Cambrian extract (B)

showing characteristics suggesting derivation from G. prisca.

Unidenti®ed peaks that consistently appear in Cambrian

extracts and which are absent in Ordovician extracts and oils

are identi®ed with an asterisk. Group 1 oils typically display

slight odd carbon predominance and have low levels of iso-

prenoids (C).

282 S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286

samples move related to thermal maturity e�ects (Mol-dowan et al., 1994). E�ects related to pH cause shifts inthe same direction as that of thermal maturity (Moldo-

wan et al., 1994) and thus pH e�ects cannot be dis-tinguished from thermal maturity e�ects by this analysisalone. These e�ects are related to rearrangement of the

original molecular compounds (Moldowan et al., 1994)and thus are not controlled by the original organicinput. In Fig. 6, Cambrian limestone bitumen shows

lower dia/(dia+reg) C27 sterane ratio and moderatelylow Ts/(Ts+Tm) ratio compared to Ordovician marlbitumen. The ratios of the marine oils plot in the samearea as some of the Ordovician bitumens and away from

the Cambrian and Carboniferous data points.

4.5. Attributes of the Ordovician rock extract organic

matter

Some GC ®ngerprints are indicative of particular

organic matter input. For example, bimodal n-alkanedistributions with a second mode in the n-C23 to n-C30

range are usually associated with terrestrial higher plantwaxes (Tissot and Welte, 1984). Many of the Ordovician

extracts from Tazhong display a bimodal distribution ofn-alkanes (Fig. 7A). However, in these samples whichare taken from strata that were deposited prior to the

evolution of land plants, the bimodality is not related tohigher plant waxes of terrestrial facies. Instead, weobserve an odd carbon number predominance in the

n-C15 to n-C19 range which is uncommon except inOrdovician source rocks. Reed et al. (1986) and Jacob-son et al. (1988) documented Gloeocapsamorpha prisca

bearing Middle Ordovician rocks and oils that generallyhave dominant C15 to C19 n-alkanes, low amounts ofheavier n-alkanes, a virtual absence of isoprenoidsincluding Pr and Ph, and an odd carbon number pre-

dominance. The Ordovician extracts from the Tarimbasin appear to have the G. prisca in¯uence in the C15

to C19 n-alkanes plus a second mode maximizing in the

n-C23 to n-C25 range (Fig. 7A). Clearly, this second modeis not the same as that seen in bimodal samples derivedfrom younger strata with higher plant waxes whichoccurs in the n-C29 to n-C31 range. The observed

bimodality in these samples implies that there was morethan one type of organic matter input. Based on organicpetrological analyses, the rock samples contain not only

planktonic algae such as Tasmanceae, G. prisca, Leio-sphaeridia and Nostocaceae, but also benthic algae suchas Macroalgae (brown algae), acritarchs, cryptospores

and arthropods.The GC traces of Cambrian extracts (Fig. 7B) gen-

erally have a single mode in the n-C15 to n-C20 range

with a slight odd carbon predominance and low con-centrations above n-C21. It is possible that G. prisca isthe dominant source of organic matter input, althoughit is unlikely that it is the only source due to the high

pristane and phytane (Fig. 7B) and high steraneconcentrations. We are unaware of any reports in theliterature that describe G. prisca in Cambrian rocks,

although such occurrences apparently are not unknown(K. Peters, 1997, pers. comm.). Four unidenti®ed peaks(Fig. 7B) occur in the GC data from all of our Cam-

brian extracts. Although we have not identi®ed thesepeaks, the presence of them in Cambrian extracts and thelack of them in any of the Ordovician rock extracts or in

any of the Tazhong or Tabei oils may be further evidenceagainst a Cambrian rock correlation with the oils.On most GC traces of oils from Tabei and Tazhong

only this ®rst mode is observed (Fig. 7C). Either the

organic matter that generated the second mode seen inthe extracts did not contribute to the oil generation orthe second mode has been removed due to the high

thermal maturity of the oils. Group 1 oils exhibit an oddcarbon predominance and have low levels of isoprenoids(Fig. 7C). The paleoecological conditions associated

with G. prisca are presumed to be anoxic depositionalenvironments (Reed et al., 1986).We note that in the study of Ordovician organic

matter in Tarim core samples, an intact bryophyte fossil

was found in the TZ35 well. This fossil has binary stemswith verticillate leaves and is regarded as the oldestbryophyte fossil in the world (L.Z. Bian, 1997 pers.

comm.,). Additionally, cryptospores were found in thekerogen, which is also unique among Ordovician strataof the world. Any potential connection between these

®ndings is speculative, and further work is needed todetermine whether or not the unusual normal alkanedistributions are related to these unusual paleontologic

®ndings.

5. Discussion

Purported hydrocarbon source rocks have beenpenetrated along the northern and southern slopes of

Fig. 8. M/z 191 and 177 chromatograms for sample 25LN57

showing the presence of a series of 25-norhopanes (shaded

peaks) potentially indicative of paleobiodegradation.

S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286 283

the Manjaer Depression. This study of the molecularorganic geochemistry of Tazhong and Tabei oils and theresultant oil±rock extract correlation con®rms thatOrdovician shaley carbonates along the Tazhong and

Tabei uplifts are the source for marine oils in the Tarimbasin. Relatively TOC-rich rocks have been discoveredalong the slope facies extending from the Tazhong uplift

to the Bachu uplift. There is no evidence that blackmarine turbidites (¯ysch sandstone and mudstone) ofthe Manjaer Depression have generated oil. Marls loca-

ted in the slope areas appear to be the only e�ectivesource rocks. These are analogous to the Middle±UpperOrdovician black shales of the New York±Ontario

region of North America, where muddy marls in slopesettings adjacent to a starved foreland basin are thesource rocks (Lehmann et al., 1995).Although it seems likely that Cambrian sources were

capable of generating hydrocarbons in the past, we seelittle evidence to support a Cambrian source for themarine oils discovered to date. This conclusion is drawn

from the distributions of dinosteranes, triaromaticdinosteroids, 24-isopropylcholestane and 24-norcholes-tanes. The absence of oils derived from Cambrian

source rocks may be related to the maturity of Cam-brian source rocks. Cambrian source rocks are poten-tially overmature (as suggested by the VRE>2.0%

from KN1 well, and VRE of 1.6±1.8% from the He4well), although these data are questionable based on theextracts we studied.A hypothesis that has often been repeated related to

tar sands found in Silurian and Carboniferous stratasuch as TZ33, TZ12, and TZ201 suggests that theymight have been derived from Cambrian source rocks

(e.g. Yang, 1991). We analyzed one such Silurian tarsand, and despite apparent biodegradation (loss of n-alkanes >C18), we found that it shared nearly all the

attributes of the Ordovician oils described above. Clus-ter analysis of the entire suite of Tarim oils using JMP1

(1995) statistical software showed that Tazhong andTabei oils, as well as the tar sand, belong to the same

group (Hanson, 1999; Hanson et al., 2000). The onlyindication of a possible Cambrian charge in the tar sandwas the presence of signi®cant amounts of 25-norho-

panes. These compounds were also found in otherTarim marine oil samples from both Tazhong and Tabei(Fig. 8). The existence of those compounds has been

shown to re¯ect microbially induced demethylation ofhopanes within the reservoir (Moldowan and McCaf-frey, 1995). This might imply paleobiodegradation, sug-

gesting an earlier charge from another (presumablyCambrian) source.The number of wells that have penetrated the deeply

buried lower Paleozoic section remain relatively limited

and thus our source rock samples may not be entirelyrepresentative of the Cambrian and Ordovician sourcerocks that may exist in the Tarim basin. Despite this

potential limitation of our data, the observed correla-tions we document between produced oils and oursource rock extracts suggest that our source rock sam-ples re¯ect important lower Paleozoic source rocks in

the Tarim basin.

6. Conclusions

Based on this biomarker study, Cambrian and Ordo-

vician strata in the Tarim basin can be geochemicallydistinguished for the ®rst time. Cambrian strata arecharacterized by high relative concentrations of triaro-

matic dinosteroid, dinosterane, and 24-norcholestanes.These compounds are at trace levels to absent inOrdovician strata.Organic matter in Ordovician source rocks shows the

signature of Porifera based on high relative concentra-tions of 24-isopropyl-cholestanes and G. prisca, evi-denced by the n-alkane patterns. Crossplots of Ts/

(Ts+Tm) and C27 dia/(dia+reg) steranes highlightdi�erences in the depositional environments and canseparate Cambrian and Carboniferous extracts from

Ordovician extracts.Apparent Ordovician source rocks were deposited in

slope environments adjacent to a widespread carbonate

platform upon which impinged the oxygen minimumzone, permitting deposition and preservation of somerocks as anoxic marls. The source for Tazhong oilscontained greater amounts of carbonate than did the

source for Tabei oils. Virtually all geochemical para-meters indicate that oils from Tazhong and Tabei matchsome extracts from Ordovician rocks and lack any sig-

ni®cant connection to potential Cambrian or Carboni-ferous source rocks.

Acknowledgements

Financial support for this study was provided by the

Stanford China Industrial A�liates and the StanfordMolecular Organic Geochemistry Industrial A�liatesPrograms. J.M.M. acknowledges partial research sup-

port from ACS-PRF Grant No.30245-AC2. AromaticGC±MS data were collected by Mike Darnell at theGeotechnology Research Institute at HARC with

expenses paid by Texaco's International ExplorationDivision. Statistical advice was provided by PaulSwitzer and Tom Hickson. Critical reviews by BradRitts, Albert Holba, Ger van Graas, and Jaap Sinninghe

Damste greatly improved this article.

Associate EditorÐJ. Sinninghe DamsteÂ

284 S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286

References

Anon, 1990. Map of the People's Republic of China, scale

1:4,000,000. China Cartographic Publishing House, Beijing.

Aquino Neto, F.R., Trendel, J.M., Restle, A., Connan, J.,

Albrecht, P., 1983. Occurrence and formation of tricyclic and

tetracyclic terpanes in sediments and petroleums. In: Bjoroy,

M. et al. (Eds.), Advances in Organic Geochemistry, 1981. J.

Wiley & Sons, New York, pp. 659±667.

Chen, J., Fu, J., Sheng, G., Liu, D., Zhang, J., 1996. Dia-

mondoid hydrocarbon ratios: novel maturity indices for

highly mature crude oils. Organic Geochemistry 25, 179±190.

Fan, P., Zhang, B.S., Wang, Y.X., Ying, G.G., Zhang, J.,

1991. Oil and Gas Geochemistry, Vol. 7. In: Fan, P., Ma, B.

(Eds.), Petroleum Geology of Tarim. Science Press, Beijing,

pp. 1±72.

Graham, S.A., Brassell, S., Carroll, A.R. et al., 1990. Char-

acteristics of selected petroleum source rocks, Xianjiang

Uygur autonomous region, Northwest China. American

Association of Petroleum Geologists Bulletin 74, 493±512.

Gu, J., Chou, J., Yan, H. et al., 1994. Sedimentary Facies

and Petroleum Accumulations: The Petroleum Exploration

in the Tarim basin, Vol. 3. Petroleum Industry Press, Beijing.

Hanson, A.D., 1999. Organic Geochemistry and Petroleum

Geology, Tectonics and Basin Analysis of Southern Tarim

and Northern Qaidam Basins, Northwest China. PhD dis-

sertation.

Hanson, A.D., Zhang, S.C., Moldowan J.M., Liang, D.G.,

Zhang, B.M., in press. Molecular organic geochemistry of

the Tarim basin, NW China. American Association of Pet-

roleum Geologists Bulletin.

Hendrix, M.S., Brassell, A.C., Carroll, A.R., Graham, S.A.,

1995. Sedimentology, organic geochemistry, and petroleum

potential of Jurassic coal measures: Tarim, Junggar, and

Turpan basins, northwest China. American Association of

Petroleum Geologists Bulletin 79, 929±959.

Holba, A.G., Dzou, L.I.P., Masterson, W.D. et al., 1998a.

Application of 24-norcholestanes for constraining the age of

petroleum. Organic Geochemistry 29, 1269±1283.

Holba, A.G., Tegelaar, E.W., Huizinga, B.J. et al., 1998b.

24-norcholestanes as age-sensitive molecular fossils. Geology

26, 783±786.

Huang, D., Li, J., Zhang, D., 1990. Maturation sequence of

continental crude oils in hydrocarbon basins in China and its

signi®cance. Organic Geochemistry 16, 521±529.

Hughes, W.B., Holba, A.G., Dzou, L.I., 1995. The ratios of

dibenzothiophene to phenanthrene and pristane to phytane

as indicators of depositional environment and lithology of

petroleum sources rocks. Geochimica et Cosmochimica Acta

59, 3581±3598.

Jacobson, S.R., Hatch, J.R., Teerman, S.C., Askin, R., 1988.

Middle Ordovician organic matter assemblages and their

e�ect on Ordovician-derived oils. American Association of

Petroleum Geologists Bulletin 72, 1090±1100.

JMP1, 1995. Statistical Discovery Software, Version 3.1. SAS

Institute, Inc.

Kaam-Peters, H.M.E. von, KoÈ ster, J., Gaast, S.J. van der,

Dekker, M., de Leeuw, J.W., Sinninghe Damste , J.S., 1998.

The e�ect of clay minerals on diasterane/sterane ratios.

Geochimica et Cosmochimica Acta 62, 2923±2929.

Lee, K.Y., 1985. Geology of the Tarim basin with special empha-

sis on petroleum deposits, Xinjiang Uygur Zishiqu, Northwest

China. US Geological Survey Open-File Report 85-0616.

Lehmann, D., Brett, C.E., Cole, R., Baird, G., 1995. Distal

sedimentation in a peripheral foreland basin; Ordovician

black shales and associated ¯ysch of the western Taconic

Foreland, New York State and Ontario. Geological Society

of America Bulletin 107, 708±724.

Li, D., Liang, D., Jia, C., Wang, G., Wu, Q., He, D., 1996.

Hydrocarbon accumulations in the Tarim basin, China.

American Association of Petroleum Geologists Bulletin 80,

1587±1603.

Liu, R., Shi, J.Y., Zheng, X.M., 1994. A discussion on the

geochemical characteristics and unconventional evaluation

method of high altered carbonate rocks. Natural Gas Indus-

try 14, 62±66.

McCa�rey, M.A., Moldowan, J.M., Lipton, P.A. et al., 1994.

Paleoenvironmental implications of novel C30 steranes in

Precambrian to Cenozoic age petroleum and bitumen. Geo-

chimica et Cosmochimica Acta 58, 529±532.

McKirdy, D.M., Aldridge, A.K., Ypma, P.J.M., 1983. A geo-

chemical comparison of some crude oils from pre-Ordovician

carbonate rocks. Proceedings of the International Meeting

on Organic Geochemistry 10, 99±107.

Appendix. Sources of data

Measured ratios Data source Chromatogram

C23 Tricyclic/C23 tricyclic+C30 hopane GC±MS m/z 191

24-Isopropylcholestane/24-n-propylcholestane MRM±GC±MS m/z 414!217

Dinosterane/dinosterane+3b-methyl-24-ethylcholestane MRM±GC±MS m/z 414!231

24/(24+27) Nordiacholestane MRM±GC±MS m/z 358!217

Dinosteroids/(dinosteroids +3methyl-24-ethyl-cholesteroid) GC±MS m/z 245

C29 aaa 20S/(20S+20R) MRM±GC±MS m/z 400!217

C29 abb/(abb+aaa) MRM±GC±MS m/z 400!217

C29 Ticyclics/(C29 tricyclic +C30 hopane) GC±MS m/z 191

Dia/(dia+reg) C27 steranes MRM±GC±MS m/z 372!217

S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286 285

McKirdy, D.M., Kantsler, A.J., Emmett, J.K., Aldridge, A.K.,

1984. Hydrocarbon genesis and organic facies in Cambrian

carbonates of the eastern O�cer Basin, South Australia.

American Association of Petroleum Geologists Studies in

Geology 18, 13±31.

Moldowan, J.M., Sundararaman, P., Schoell, M., 1986. Sensi-

tivity of biomarker properties to depositional environment

and/or source input in the lower Toarcian of SW Germany.

Organic Geochemistry 10, 915±926.

Moldowan, J.M., Fago, F.J., Lee, C.Y. et al., 1990. Sedimen-

tary 24-n-propylcholestanes, molecular fossils diagnostic of

marine algae. Science 247, 309±312.

Moldowan, J.M., Lee, C.Y., Watt, D.S., Jeganathan, A., Slou-

gui, N., Gallegos, E.J., 1991. Analysis and occurrence of C26-

steranes in petroleum and source rocks. Geochimica et Cos-

mochimica Acta 55, 1065±1081.

Moldowan, J.M., Peters, K.E., Carlson, R.M.K., Schoell, M.,

Abu-Ali, M.A., 1994. Diverse applications of petroleum

biomarker maturity parameters. Arabian Journal for Science

and Engineering 19, 273±298.

Moldowan, J.M., McCa�rey, M.A., 1995. A novel hydro-

carbon degradation pathway revealed by hopane demethyla-

tion in a petroleum reservoir. Geochimica et Cosmochimica

Acta 59, 1891±1894.

Moldowan, J.M., Dahl, J., Jacobson, S.R. et al., 1996. Che-

mostratigraphic reconstruction of biofacies; molecular evi-

dence linking cyst-forming dino¯agellates with pre-Triassic

ancestors. Geology 24, 159±162.

Moldowan, J.M., Jacobson, S.R., Dahl, J., Al-Hajji, A., Huizinga,

B.J., Fago, F.J., 1999. Molecular fossils demonstrate Pre-

cambrian origin of dino¯agellates. In Zhuralev, A., Riding, R.

(Eds.), Ecology of the Cambrian Radiation. Cambridge Press.

Moldowan, J.M., Talyzina, N.M., 1998. Biogeochemical evi-

dence for dino¯agellate ancestors in the Early Cambrian.

Science 281, 1168±1170.

Peters, K.E.,Moldowan, J.M., Sundararaman, P., 1990. E�ects of

hydrous pyrolysis on biomarker thermal maturity parameters;

Monterey phosphatic and siliceous members. Organic Geo-

chemistry 15, 249±265.

Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide;

Interpreting Molecular Fossils in Petroleum and Ancient

Sediments. Prentice Hall, Englewood Cli�s, NJ, USA.

Reed, J.D., Illich, H.A., Hors®eld, B., 1986. Biochemical evo-

lutionary signi®cance of Ordovician oils and their sources.

Organic Geochemistry 10, 347±358.

Rubinstein I., Sieskind, O., Albrecht, P., 1975. Rearranged

steranes in a shale: occurrence and simulated formation.

Journal of the Chemical Society, Perkin Transaction I, 1833±

1836.

RullkoÈ tter, J., Marzi, L., 1988. Natural and arti®cial matura-

tion of biological markers in a Toarcian shale from northern

Germany. Organic Geochemistry 13, 639±645.

Seifert, W.K., Moldowan, J.M., 1978. Applications of steranes,

terpanes, and monoaromatics to the maturation, migration,

and source of crude oils. Geochimica et Cosmochimica Acta

42, 77±95.

Seifert, W.K., Moldowan, J.M., 1981. Paleoreconstruction by

biological markers. Geochimica et Cosmochimica Acta 45,

783±794.

Summons, R.E., Volkman, J.K., Boreham, C.J., 1987. Dinos-

terane and other steroidal hydrocarbons of dino¯agellate

origin in sediments and petroleum. Geochimica et Cosmo-

chimica Acta 51, 3075±3082.

Tappan, H., 1980. The Paleobiology of Plant Protists. W.H.

Freeman and Co., San Francisco, CA, USA.

Tasch, P., 1980. Paleobiology of the Invertebrates: Data

Retrieval from the Fossil Record. Wiley & Sons, New York,

USA.

Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and

Occurrence. Springer±Verlag, New York, USA.

Ulmishek, G., 1984. Geology and petroleum resources of

basins in western China. Argonne National Laboratory,

ANL/ES-146.

Volkman, J.K., Barrett, S.M., Dunstan, G.A., Je�rey, S.W.,

1993. Geochemical signi®cance of the occurrence of dinos-

terol and other 4-methyl sterols in a marine diatom. Organic

Geochemistry 20, 7±15.

Volkman, J.K., Barrett, S.M., Dunstan, G.A., Je�rey, S.W.,

1994. Sterol biomarkers for microalgae from the green

algal class Prasinophyceae. Organic Geochemistry 21, 1211±

1218.

Wang, Q., Nishidai, T., Coward, M.P., 1992. The Tarim basin,

NW China; formation and aspects of petroleum geology.

Journal of Petroleum Geology 15, 5±34.

Withers, N., 1987. Dino¯agellate sterols. In: Taylor, F.J.R.

(Ed.), The Biology of Dino¯agellates, Botanical Mono-

graphs, Vol. 21. Blackwell Scienti®c, Oxford, pp. 316±359.

Yang, B., 1991. Geochemical characteristics of oil from well

Shacan 2 in the Tarim basin. Journal of Southeast Asian

Earth Sciences 5, 401±406.

Xiao, X., Liu, D., Fu, J., 1996. Multiple phases of hydrocarbon

generation and migration in the Tazhong petroleum system

of the Tarim basin, People's Republic of China. Organic

Geochemistry 25, 191±197.

286 S.C. Zhang et al. / Organic Geochemistry 31 (2000) 273±286