Paleozoic oil±source rock correlations in the
Tarim 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
baResearch Institute of Petroleum Exploration and Development (RIPED), China National Petroleum Corporation, Beijing, People's Republic of China
bDepartment 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 main group of oils comes from Tazhong and Tabei wells, which sample the largest known petroleum accumulations in the basin. These oils can be statistically correlated with extracts of Ordovician rocks based upon high relative concentra-tions of isopropylcholestanes and low relative concentraconcentra-tions of dinosteranes, triaromatic dinosteroids, and 24-norcholestanes. In contrast, extracts from Cambrian rocks have low relative concentrations of 24-isopropylcholestanes with high relative concentrations of dinosteranes, triaromatic dinosteroids, and 24-norcholestanes. Although some Tarim 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 dierent ages (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 structural deformation (Li et al., 1996), and may have undergone multiple periods of hydrocarbon generation, accumula-tion and migraaccumula-tion (Xiao et al., 1996). The resultant complexity of the Tarim basin makes the study of pet-roleum geology dicult. The age, type and maturation of marine oils are some of the important unresolved problems in the basin.
Previously conducted oil±source rock correlation attempts [Graham et al., 1990; internal Research Insti-tute of Petroleum Exploration and Development (RIPED), unpublished data] concluded that the main source rocks in the Tarim basin are within the Cambro± Ordovician marine sediments.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 source rocks or how much the Cambrian and Ordovician source rocks contributed individually to hydrocarbons in 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 using JMP1(1995) statistical software and reported the
pre-sence of at least seven genetic groups of oils. These genetic groups include three groups of oils derived from dierent facies of marine Ordovician source rocks and
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 0 0 3 - 6
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.
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.
S.C.
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four groups of oils derived from dierent 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 source rocks, and report the results of our oil±source rock cor-relation studies. By combining detailed geochemical analyses of selected oil and rock samples, especially the correlation of certain biomarkers, with our knowledge of the geology, we attempt to determine the age and type of the eective source rocks.
We note that several proper names used in the text have been spelled in dierent ways in the past, due to dierent transliterations of the original names into English (e.g.``Tarim'' is also spelled ``Talimu'' by some authors). Where this has arisen, the ocial Pinyin form (Anon., 1990) has been used.
2. Paleozoic geologic setting
Nearly the entire Paleozoic section in the Tarim basin is comprised of marine strata (Gu et al., 1994). Cambrian and Lower Ordovician sections consist of dominantly
shallow-water carbonates, whereas Middle±Upper Ordovician strata range from platformal carbonates in the west to deep-water turbiditic sandstones and shales in the east. Carboniferous±Lower Permian sequences are stacked marine to non-marine transgressive± regressive successions, whereas the Upper Permian is non-marine. Mesozoic and Cenozoic strata are almost exclusively 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 Tarim basin. 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 Cambrian source rocks are marls, mudstones, and dolomites deposited 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 (undierentiated, O2=Middle Ordovician, S=Silurian, C=Carboniferous
(undier-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
total organic carbon (TOC) values within individual wells generally range from 1.24 to 2.28 wt% but reach a maximum of 5.52 wt% (see Table 2 for a partial listing of the data for source rocks included in this study). Strata with TOC >1.0 wt% occupy 60±70% of the sequence 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 widely used in China (Chen et al., 1996; as derived from Liu et al., 1994), but which is not calibrated to methods employed outside China. Based on VRE data, the sug-gested remaining generative potential of these source rocks must be low.
To the west on the Bachu uplift, Cambrian evaporites of lagoonal facies are present in the He4 well drilled along the Hotan River (Fig. 1). Lithologies include marl and muddy dolomite which have relatively high abun-dances of organic matter with a maximum TOC of 2.14 wt%. VRE data suggest that the source rocks are in the highly mature condensate stage (VRE values of 1.65 to 1.70%).
2.2. Lower Ordovician
The best documented purported source rocks of the Lower Ordovician are similar to those described for the Cambrian. The average TOC in the TD1 well in eastern Manjaer is 1.93 wt%. Marine slope facies should occur in the central and western parts of the Tarim basin, but source 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 rocks described 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 mainly muddy limestones and marls deposited in shelf-edge and slope settings (RIPED, unpublished data). Purported source rocks of Middle±Late Ordovician age are widely distributed along the northern slope and the crest of the Tazhong uplift with a thickness of about 80±100 m. Similar rocks exist in some Lunnan (LN) wells in North Tarim, 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, unpublished data).
Open bay or gulf facies occur in the Kalpin area and the Awati depression. Purported source rocks include the Sargan shale or Yingan shale, whose TOC is 0.05± 2.25 wt% (average 0.89 wt%). Similar TOC values were reported by Graham et al. (1990) for outcrop samples of the Saergan Formation from the Kalpin area (TOC values of 0.21±2.75 wt%).
In contrast, Ordovician strata of the Manjaer and Tangguzibasi depressions can be over 10 km thick. The sediments 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 T
max 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
contourites. Lithologies are volcanic lithic rich sand-stones and dark massive mudstone. Thick shales and sandstones in the Manjaer depression have very low TOC 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 source rocks were completed using dichloromethane for 72 h in a Soxhlet apparatus at the RIPED experimental center in Beijing. Extracts and oil samples were then sent to Stanford University where analyses described below were completed.
Extracts and whole oils were analyzed via standard (n-C12and higher) gas chromatography (GC) on an HP
5890A gas chromatograph.
All samples were separated using high performance liquid chromatography as described in Peters and Moldowan (1993). Saturate fractions were spiked with a known quantity of 5b-cholane and then treated with a high Si/Al ZSM-5 zeolite (``silicalite'') preparation to remove normal alkanes. All saturate fractions were 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 Micromass Autospec Q in an MRM±GC±MS (metastable reaction monitoring) mode. We used a 60 m DB-1 (fused silicone) (J&W Scienti®c Column), with a 0.25 mm i.d., and 25 mm ®lm thickness. H2 was the carrier
gas. GC±MS analyses of aromatic fractions were completed at the Houston Advanced Research Center using an HP 5890 with a 5970 Mass Selective Detector. The GC column was an HP-1 column (100% poly-dimethylsiloxane), with 0.25mm ®lm thickness, 0.25 mm i.d., and 60 m length. The temperature program was isothermal for 4 min at 70C, then increased
3C/min to 145C, followed by 2C/min to 320C.
Injector temperature was 300C, and the carrier gas was
helium.
Organic geochemical analyses consisting of sulfur, wax, asphaltene, and density measurements, as well as bulk stable carbon isotope analyses, carried out on selected oils by RIPED in China (Table 1) are included. The ®eld, depth of the producing zone, and the reservoir ages are also listed for oil samples in Table 1.
4. Results
4.1. Sample populations
Oils in this study (Table 1) include samples from all known producing areas of the basin. Source rocks
sam-ples were selected by use of preliminary screening methods, including TOC and Rock-Eval analyses. In some cases, the TOC values for our samples are low compared to the maximum TOC values reported above. However, with Tmax values of 440±450C (Table 2),
these source rocks have already generated oil and the original TOC values, prior to generation, would have been signi®cantly higher. We report limited data for Cambrian rocks, but note that despite having samples from widely separated areas (the KN1 well in north-eastern Tabei uplift and the He4 well on the Bachu uplift), the geochemical signature of our Cambrian samples 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±Late Ordovician and Carboniferous aged source rock sam-ples are from the Tazhong (TZ) and TaC1 area. Our limited Carboniferous source rock samples re¯ect the general lack of good quality source rocks found to date within Carboniferous strata. Source rock sample names, ®eld names, depth of core samples, and geologic ages are listed in Table 2. TOC, Rock-Eval pyrolysis results, and lithological descriptions from RIPED are also included for most samples in Table 2. The position of major structural elements in the basin and well locations are shown in Fig. 1.
4.2. Characteristics of the Tazhong±southern Tabei oil group
This single genetic group (indicated as Group 1 in Table 1) consists of oil samples from two geographic locations: Tazhong (TZ) and the southern part of the Tabei 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. They have whole oil carbon isotope values of ±31 toÿ33%
PDB, pristane/phytane (Pr/Ph) ratios of 0.83±1.4, low Pr/n-C17 ratios, high C23 tricyclic/(C23 tricyclic+C30
hopane) ratios, and well-preserved homohopanes, and they lackb-carotane (Hanson, 1999; Hanson et al., 2000). There are only minor dierences between the Tazhong and Tabei oils based on these features.
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
aA=C
4.3. Age and oil±source correlation relevant parameters
Several biomarkers have been shown to be related to speci®c modern taxa, and molecular paleontological studies 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). Age diagnostic biomarkers employed in this study include 24-isopropylcholestanes, dinosteranes (4a ,23,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-Isopropylcholestane
The modern precursors of 24-isopropylcholestane are found almost exclusively in marine porifera (sponges) referred to as Demospongiae (McCarey et al., 1994). 24-n-propylcholestanes are related to 24-n -propyl-cholesterols which are biosynthesized by marine Chrysophyte algae (Moldowan et al., 1990) of the order Sarcinochrysidales. They have been linked to a marine prasinophyte (Volkman et al., 1994) and are common in marine invertebrates, presumably via a dietary origin. Trace amounts of 24-isopropylcholestane typically occur in Phanerozoic marine rock extracts. However, in Vendian±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 (McCarey et al., 1994).
The ratios 24-isopropylcholestane/24-n -propylcholes-tane of Tarim Paleozoic rock extracts and marine oils are plotted along the x-axis of Fig. 2. We found low relative 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 Cambrian values to one of two possibilities. The ®rst possibility is that depositional environments during the Ordovician were more suitable for growth of sponges and they are an important contribution. The second possibility is that sponges were abundant during the Cambrian, but the organic matter was not well preserved in the shallow water depositional environments. The low Cambrian ratios 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 amounts seen 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 the Tazhong and Tabei oils (Fig. 2). Cambrian extracts are signi®cantly dierent from Tazhong and Tabei oils, as well as from Ordovician extracts, based on these bio-markers.
4.3.2. Dinosteranes
Precursors 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 petroleums that originated from Triassic or younger source rocks (Summons et al., 1987) with a few examples in Pre-cambrian extracts. A larger survey including numerous Paleozoic 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 for dino¯agellates during the Triassic, although occurrences of possible dino¯agellate fossils have been reported in the Paleozoic with the oldest suspected species coming from rocks of Silurian age (Tappan, 1980). More recently, Early Cambrian dino¯agellate ancestors con-taining dinosterane have been identi®ed (Moldowan and Talyzina, 1998).
We found dierent concentrations of dinosteranes in Tarim oils and extracts of dierent ages. In all Cambrian
Fig. 2. Crossplot of 24-isopropylcholestanes/24-n -propylcho-lestanes versus methyl sterane (C30) ratios showing separation
extracts, the dinosterane relative concentration is high, whereas in Ordovician extracts, the dinosterane relative concentration varies from high to low. In Carboniferous extracts, the concentration is low in mudstone [TZ6(MS)] and high in limestone [TZ6(LS)]. Tazhong and 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 no overlap of the 95% con®dence interval for the Cam-brian source rocks relative to the Tazhong and Tabei oils (Fig. 2). Similar to the results mentioned above, comparison of mean values, standard deviation and 95% con®dence interval calculations indicate that Cambrian extracts are signi®cantly dierent 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 derived through bacterial oxidation or thermally induced clea-vage and loss of a methyl group from larger steroids (>C26). Paleozoic and older oils usually show little
or no 24-norcholestanes (Moldowan et al., 1991; Holba et al., 1998a,b). The associated rock record data are less systematic, but in the Tarim basin we see signi®cant amounts of 24-norcholestanes in some of our Ordovician and 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 dierent from Ordovician extracts.
4.3.4. Triaromatic dinosteroids
Triaromatic dinosteroid data also support a correla-tion between Tazhong and southern Tabei oils and Ordovician, not Cambrian, source rocks. Statistically dierent high ratios of dinosteroids/(dinosteroids+3b -methyl-24-ethyl-cholesteroid) were detected in Cambrian extracts whereas Ordovician and Carboniferous extracts have low ratios (Fig. 3). Ordovician and Cambrian extracts are widely separated, and oils from Tazhong and Tabei uniformly show triaromatic dinosteroids distributions similar to Ordovician extracts (Fig. 3). Moldowan et al. (1999) suggested that abundant triaromatic-dinosteroids in some Paleozoic rocks might be related to the con-centration of marine acritarchs (phytoplanktonic algal cysts of uncertain anity) (e.g. Moldowan and Taly-zina, 1998) potentially derived from dino¯agellates which may have been primary producers in Paleozoic oceans (Tasch, 1980). The underlying cause of diering amounts of triaromatic dinosteroids in the Cambrian versus the Ordovician of the Tarim basin is therefore presumably related to changing paleo-oceanographic conditions between Cambrian and Ordovician time. Whatever the underlying cause of dierent levels of triaromatic dinosteroids, the dierences again provide a basis for distinguishing Cambrian from Ordovician samples.
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 C29sterane stereoisomer ratios
show-ing diershow-ing trends for Ordovician and Cambrian extracts. Regression lines (solid) with 95% con®dence intervals (dashed lines) are shown for Cambrian and Ordovician extracts. R2
4.4. Parameters related to maturity, lithology, and depositional environment
Biomarker maturity parameters are useful in deter-mining the thermal history of petroleum and related source rocks. However, several studies have shown that many biomarker maturity parameters are also depen-dent on the character of the source rock itself (Peters et al., 1990; Moldowan et al., 1994). For example, McKirdy et al. (1983) suggested that sterane maturation parameters may be aected by the mineral matrix of the source rocks. They observed that oils from a probable carbonate 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). Similar observations were made by Huang et al. (1990) for oils derived from gypsum-, salt-, and carbonate-rich rocks.
Maturity eects on two dierent ratios of C29steranes
in Paleozoic rocks from the Tarim basin re¯ect dier-ences in ages and lithologies (Fig. 4). For Cambrian limestone extracts, the C29 aaa 20S/(20S+20R) ratio
increases more rapidly than the abb/(abb+aaa) ratio, producing a trend with a slope that is steeper than that of the C29steranes of Ordovician extracts (mainly from
marls). Although the two Carboniferous extracts have almost the same maturity (Roaround 0.7%) and come
from depths separated by only 90 m, the isomerization ratios of the two samples show considerable dierence. 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 the
limestones. Although both Cambrian and Ordovician samples are carbonates, the Ordovician samples contain clays which might accelerate isomerization of the C14
and C17 positions. In highly mature source rocks, an
eventual decrease in the abb/(abb+aaa) ratio occurs (Peters et al., 1990). As stated earlier, vitrinite re¯ec-tance equivalence (VRE) data suggest that Cambrian source rocks in exploration wells (e.g. TD1 and KN1 in eastern Manjaer) are overmature (VRE>2.0%). The high VRE of the Cambrian samples may account for a decrease in the abb/(abb+aaa) ratios as described above. Although the Cambrian extracts have mature biomarker parameters (see Table 3), and elevatedTmax
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 eect that the lithology of the source rocks imparts to the isomerization rate of C29steranes,
the trends of Tabei and Tazhong oils and Ordovician rocks are consistent (Fig. 4) and thus support their close anity.
Diasterane/regular sterane ratios are aected by both thermal maturity and inorganic characteristics of the source rock or the depositional environment. Catalysis by acidic sites on clays has been proposed as the mechanism 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 source rocks and oils derived from them (Peters and
Moldo-Fig. 5. Crossplot of C27 diasteranes/steranes versus C29
tri-cyclic/(C29tricyclic+C30hopane) 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, andR2values 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
wan, 1993) which has further been shown to be depen-dent on clay/TOC ratios (Kaam-Peters et al., 1998). In the 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, the samples 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 also carbonates, but with varying percentages of clay, and as a result, the diasterane/sterane ratios in their extracts are widely distributed. Tazhong and Tabei oils contain moderate amounts of diasteranes. The geochemical sig-ni®cance of C29 tricyclic terpane/(C29 tricyclic terpane
+C30 hopane) ratio, also plotted on Fig. 5, is not
fully understood but re¯ects both organic matter input and thermal maturity. Tricyclic terpanes are known to be more stable and related to dierent precursors than hopanes and as such this ratio represents a source and maturity 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 indistinguishable Tazhong and Tabei oils. The trend of Tazhong oils projects back to the sterane axis farther to the left than the trend of the Tabei oils. Tabei oils project back closer to more shale-rich marl extracts. We have seen a version of this ®gure (RIPED, unpublished data) that contained signi®cantly more samples than what we have access to and the Tazhong oil data project back to a position coincident with the Cambrian source rocks. Our inter-pretation of these results is that Tazhong oils were derived from carbonate-rich source rocks. We are not suggesting that Tazhong oils are derived from Cambrian carbonate source rocks; rather, we believe that the source rocks that generated Tazhong and Tabei oils are Ordovician, but the Tazhong source rocks contain more carbonate content, whereas the source rocks of Tabei oils contained more shale. The lack of elevated C29
tri-cyclics in the source rocks may be further support for our 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 lowTs/(Ts+Tm) compared to oils gener-ated from shales (McKirdy et al., 1983, 1984). Bitumens of anoxic and acidic hypersaline source rocks generally show highTs/(Ts+Tm) (RullkoÈtter and Marzi, 1988).
Fig. 6 shows theTs/(Ts+Tm) ratios of our Paleozoic marine source rocks plotted against the C27diasterane/
regular sterane ratios. It is important to note that the regression lines for the dierent groups of source rocks were forced through the origin on the crossplot and the reportedR2values re¯ect this condition. Our reason for
forcing the regression lines through the origin is that both the Ts/(Ts+Tm) and the C27 diasterane/regular
sterane ratios are known to increase with increasing thermal maturity and thus these values will move away from 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. The Carboniferous mudstone [TZ6(MS)] and some Ordovi-cian marls show slightly higher dia/(dia+reg) sterane ratios with low Ts/(Ts+Tm). On this type of plot, Eh eects 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 fromG. prisca.
samples move related to thermal maturity eects (Mol-dowan et al., 1994). Eects related to pH cause shifts in the same direction as that of thermal maturity (Moldo-wan et al., 1994) and thus pH eects cannot be dis-tinguished from thermal maturity eects by this analysis alone. These eects are related to rearrangement of the original molecular compounds (Moldowan et al., 1994) and thus are not controlled by the original organic input. In Fig. 6, Cambrian limestone bitumen shows lower dia/(dia+reg) C27 sterane ratio and moderately
low Ts/(Ts+Tm) ratio compared to Ordovician marl bitumen. The ratios of the marine oils plot in the same area 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-alkane distributions with a second mode in then-C23ton-C30
range are usually associated with terrestrial higher plant waxes (Tissot and Welte, 1984). Many of the Ordovician extracts from Tazhong display a bimodal distribution of
n-alkanes (Fig. 7A). However, in these samples which are taken from strata that were deposited prior to the evolution of land plants, the bimodality is not related to higher plant waxes of terrestrial facies. Instead, we observe an odd carbon number predominance in the
n-C15 to n-C19 range which is uncommon except in
Ordovician source rocks. Reed et al. (1986) and Jacob-son et al. (1988) documented Gloeocapsamorpha prisca
bearing Middle Ordovician rocks and oils that generally have dominant C15 to C19 n-alkanes, low amounts of
heavier n-alkanes, a virtual absence of isoprenoids including Pr and Ph, and an odd carbon number pre-dominance. The Ordovician extracts from the Tarim basin appear to have the G. priscain¯uence in the C15
to C19n-alkanes plus a second mode maximizing in the
n-C23ton-C25range (Fig. 7A). Clearly, this second mode
is not the same as that seen in bimodal samples derived from younger strata with higher plant waxes which occurs in the n-C29 to n-C31 range. The observed
bimodality in these samples implies that there was more than one type of organic matter input. Based on organic petrological analyses, the rock samples contain not only planktonic algae such asTasmanceae, G. prisca, Leio-sphaeridiaandNostocaceae,but also benthic algae such as Macroalgae (brown algae), acritarchs, cryptospores and arthropods.
The GC traces of Cambrian extracts (Fig. 7B) gen-erally have a single mode in then-C15 to n-C20 range
with a slight odd carbon predominance and low con-centrations aboven-C21. It is possible that G. priscais
the dominant source of organic matter input, although it is unlikely that it is the only source due to the high pristane and phytane (Fig. 7B) and high sterane concentrations. We are unaware of any reports in the literature 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 these peaks, the presence of them in Cambrian extracts and the lack of them in any of the Ordovician rock extracts or in any of the Tazhong or Tabei oils may be further evidence against 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 in the extracts did not contribute to the oil generation or the second mode has been removed due to the high thermal maturity of the oils. Group 1 oils exhibit an odd carbon predominance and have low levels of isoprenoids (Fig. 7C). The paleoecological conditions associated with G. priscaare presumed to be anoxic depositional environments (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 stems with verticillate leaves and is regarded as the oldest bryophyte fossil in the world (L.Z. Bian, 1997 pers. comm.,). Additionally, cryptospores were found in the kerogen, which is also unique among Ordovician strata of the world. Any potential connection between these ®ndings is speculative, and further work is needed to determine whether or not the unusual normal alkane distributions are related to these unusual paleontologic ®ndings.
5. Discussion
Purported hydrocarbon source rocks have been penetrated along the northern and southern slopes of
the Manjaer Depression. This study of the molecular organic geochemistry of Tazhong and Tabei oils and the resultant oil±rock extract correlation con®rms that Ordovician shaley carbonates along the Tazhong and Tabei uplifts are the source for marine oils in the Tarim basin. Relatively TOC-rich rocks have been discovered along the slope facies extending from the Tazhong uplift to the Bachu uplift. There is no evidence that black marine turbidites (¯ysch sandstone and mudstone) of the Manjaer Depression have generated oil. Marls loca-ted in the slope areas appear to be the only eective source rocks. These are analogous to the Middle±Upper Ordovician black shales of the New York±Ontario region of North America, where muddy marls in slope settings adjacent to a starved foreland basin are the source rocks (Lehmann et al., 1995).
Although it seems likely that Cambrian sources were capable of generating hydrocarbons in the past, we see little evidence to support a Cambrian source for the marine oils discovered to date. This conclusion is drawn from the distributions of dinosteranes, triaromatic dinosteroids, 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. CamCam-brian 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 He4 well), although these data are questionable based on the extracts we studied.
A hypothesis that has often been repeated related to tar sands found in Silurian and Carboniferous strata such as TZ33, TZ12, and TZ201 suggests that they might have been derived from Cambrian source rocks (e.g. Yang, 1991). We analyzed one such Silurian tar sand, 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 and Tabei oils, as well as the tar sand, belong to the same group (Hanson, 1999; Hanson et al., 2000). The only indication of a possible Cambrian charge in the tar sand was the presence of signi®cant amounts of 25-norho-panes. These compounds were also found in other Tarim marine oil samples from both Tazhong and Tabei (Fig. 8). The existence of those compounds has been shown to re¯ect microbially induced demethylation of hopanes within the reservoir (Moldowan and McCaf-frey, 1995). This might imply paleobiodegradation, sug-gesting an earlier charge from another (presumably Cambrian) 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 entirely representative of the Cambrian and Ordovician source rocks that may exist in the Tarim basin. Despite this
potential limitation of our data, the observed correla-tions we document between produced oils and our source 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 geochemically distinguished for the ®rst time. Cambrian strata are characterized by high relative concentrations of triaro-matic dinosteroid, dinosterane, and 24-norcholestanes. These compounds are at trace levels to absent in Ordovician 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 highlight
dierences in the depositional environments and can separate 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 minimum zone, permitting deposition and preservation of some rocks as anoxic marls. The source for Tazhong oils contained greater amounts of carbonate than did the source for Tabei oils. Virtually all geochemical para-meters indicate that oils from Tazhong and Tabei match some 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 Aliates and the Stanford Molecular Organic Geochemistry Industrial Aliates Programs. J.M.M. acknowledges partial research sup-port from ACS-PRF Grant No.30245-AC2. Aromatic GC±MS data were collected by Mike Darnell at the Geotechnology Research Institute at HARC with expenses paid by Texaco's International Exploration Division. Statistical advice was provided by Paul Switzer and Tom Hickson. Critical reviews by Brad Ritts, Albert Holba, Ger van Graas, and Jaap Sinninghe Damste greatly improved this article.
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Measured ratios Data source Chromatogram
C23Tricyclic/C23tricyclic+C30hopane GC±MS m/z191
24-Isopropylcholestane/24-n-propylcholestane MRM±GC±MS m/z414!217
Dinosterane/dinosterane+3b-methyl-24-ethylcholestane MRM±GC±MS m/z414!231
24/(24+27) Nordiacholestane MRM±GC±MS m/z358!217
Dinosteroids/(dinosteroids +3methyl-24-ethyl-cholesteroid) GC±MS m/z245
C29aaa20S/(20S+20R) MRM±GC±MS m/z400!217
C29abb/(abb+aaa) MRM±GC±MS m/z400!217
C29Ticyclics/(C29tricyclic +C30hopane) GC±MS m/z191
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