Triterpane and sterane biomarkers in the YA13-1 condensates from Qiongdongnan Basin, South China Sea

Chemical Geology 199 (2003) 343 – 359 www.elsevier.com/locate/chemgeo

Triterpane and sterane biomarkers in the YA13-1 condensates from Qiongdongnan Basin, South China Sea Yi Zhou a,b,*, Guoying Sheng a, Jiamo Fu a, Ansong Geng a, Junhong Chen a,1, Yongqiang Xiong a, Qiming Zhang c a

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China b Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, PR China c Institute of Petroleum Exploration and Development, Nanhai West Oil Corporation, Zhanjiang 524057, PR China Received 27 December 2001; accepted 28 March 2003

Abstract Triterpanes and steranes in condensates from the YA13-1 gas field, Qiongdongnan Basin, were monitored. The YA13-1 condensates have unusual biomarker distributions dominated by terpanes and steranes derived from higher plants. Anomalously abundant 18a-oleanane and remarkably abundant bicadinanes are present in the YA13-1 condensates, whereas the 17a-hopane contents are extremely low. Taraxastane and significantly abundant 17a-diahopanes occur in the condensates. In addition, a number of unknown C29 and C30 pentacyclic triterpanes including previously unreported compounds were detected in the condensates, some of which are significantly abundant. The unknown compounds may be terrestrial biomarkers. C29 homologues are relatively predominant among the regular and rearranged steranes. The diasterane concentrations are markedly higher than those of regular steranes. The maturity of the YA13-1 condensates is relatively high, at the peak to late oil generation stage (corresponding to 0.85 – 1.10% Ro), based on sterane and terpane and including bicadinane maturity parameters (i.e. T/(T1 + R) and T/R bicadinane ratios). The above maturity assessment result is different from that based on diamondoid maturity parameters (%Ro = 1.60 – 1.70) [Org. Geochem. 25 (1996) 179], which can be explained by a contribution of hydrocarbons from two sources at different depths. The YA13-1 condensates were probably generated from the Yacheng and Lingshui coal-bearing source rocks buried both in the Qiongdongnan Basin (3400 – 5000 m) and in the Yinggehai Basin (>5000 m). The possible contribution of lower maturity hydrocarbons from the Yacheng and Lingshui Formations (3400 – 4100 m) in the Qiongdongnan Basin to the YA13-1 gases and condensates should not be neglected. D 2003 Published by Elsevier Science B.V. Keywords: Triterpanes; Steranes; Terrestrial biomarkers; Unknown pentacyclic triterpanes; Molecular maturity; YA13-1 condensates; Qiongdongnan Basin; Yinggehai Basin; China

1. Introduction * Corresponding author. Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, PR China. E-mail address: [email protected] (Y. Zhou). 1 Present address: Geoscience Australia, GPO Box, Canberra, ACT 2601, Australia.

The composition of biomarker compounds, especially steroid and triterpenoid derivatives, in sedimentary rocks and oils, are of special interest because

0009-2541/03/$ - see front matter D 2003 Published by Elsevier Science B.V. doi:10.1016/S0009-2541(03)00123-2

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these compounds may reflect the depositional environment, origin and diagenetic/maturation history of geological organic matter (Peters and Moldowan, 1993). A detailed triterpane and sterane biomarker study of condensates from the YA13-1 gas field, the largest offshore gas field of China found in the Qiongdongnan and Yinggehai Basins in the South China Sea (Fig. 1), was performed in an effort to further understand the origin and the maturity of the condensates. The Qiongdongnan Basin and the Yinggehai Basin, two adjacent Tertiary sedimentary basins (together termed Ying –Qiong Basins), are separated by a major basin boundary fault system (the No. 1 fault system), which is oriented northwest to southwest (Figs. 1 and 2). The Qiongdongnan and Yinggehai Basins are located in the northwest and northeast, respectively, of the No. 1 fault system. The former basin trends east –northeast, and the latter northwest. The geology of the Qiongdongnan and Yinggehai Basins was reported in detail elsewhere (e.g. Zhang and Zhang, 1993; Hao et al., 1995; Zhang and Hao, 1997; Hao et al., 2000). Only a brief summary is presented here. The two basins show significant differences in their structural development. The Qiongdongnan Basin displays a characteristic

passive-margin basin development from rifting to regional subsidence, and is filled with Eocene and Oligocene rift sediments and Miocene – Quaternary post rift sediments. The formation and evolution of the Qiongdongnan Basin are believed to be closely related to the opening of the South China Sea (Chen et al., 1993; Zhang and Hao, 1997). In contrast, the Yinggehai Basin is a transform-extensional basin whose formation and development were controlled by lithosphere extension and strike slip movement along the Red River structure zone (Chen et al., 1993; Zhang et al., 1994). The Yinggehai Basin is filled with a thick wedge of Tertiary, mostly marine sediments interrupted between the Oligocene and Miocene by a pronounced angular unconformity. The Tertiary sediments in the Ying – Qiong Basins are up to 17 km thick. A complete stratigraphic column has not been revealed by drilling (Fig. 3). The YA13-1 gas field located in the footwall of the No. 1 fault system is developed in a drape anticline trap in the Qiongdongnan Basin (Fig. 2). The main producing interval are the fan-delta sandstones in the Lingshui Formation and Sanya Formation. The gas field is well sealed by shales in the Meishan Formation (Hao et al., 2000). The YA13-1 trap formed at the end of the middle Miocene (approximately 10.5 Ma), which is

Fig. 1. Map showing the location of the YA13-1 gas field. The Qiongdaongnan Basin and the Yinggehai Basin are defined as the areas northwest and northeast, respectively, of the No. 1 fault system. Cross section A – AVis shown in Fig. 2.

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Fig. 2. Cross section showing the Qiongdaongnan Basin and the Yinggehai Basin, and the YA13-1 gas field. Ya – Ling FM, Yacheng and Lingshui Formations; Mei – San FM, Meishan and Sanya Formations.

much earlier than the beginning of gas migration and accumulation (approximately 5.8 Ma). The essential elements of trap formation and generation, migration, and accumulation of gas basically occurred together (Chen et al., 1993, 1998). The thick Tertiary sedimentary series (Fig. 3) in the Ying –Qiong Basins has several sets of source rocks providing rich material for the formation of gas (Zhang and Zhang, 1993; Sun, 1994). Potential source rocks in the Ying – Qiong Basins are suggested to comprise the Yacheng and Lingsui Formations (Oligocene coastal plain/neritic deposits), Meishan and Sanya Formations (Lower and Middle Miocene neritic/bathyal deposits, mainly developed in the Yinggehai Basin), Ying – Huang Formations (Pliocene neritic/bathyal deposits) and lacustrine Eocene (Zhang and Zhang, 1993; Sun, 1994; Zhou and Sheng, 1995; Chen et al., 1998; Geng et al., 1998). Condensates have been tested by drill-stem tests (DST) from 6 wells of the YA13-1 gas field, although gases are

dominant in the field with a gas/condensate ratio of 13,000 – 60,000 m3/m3. There are a variety of publications with organic geochemical information on the YA13-1 condensates. The condensates are generally considered to originate from the same source rocks, based on bulk geochemical characteristics and biomarker distribution characteristics of the condensates by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analyses (Li and Pan, 1990; Zhang and Zhang, 1993; Lu and Zhang, 1994; Dzou et al., 1995; Zhou and Sheng, 1995; Zhou, 1996; Chen et al., 1996; Hao et al., 1998). Hopane and sterane biomarker compounds in the condensates, however, cannot be routinely monitored by GC-MS analysis due to their low concentrations and the interference of bicadinanes and other triterpanes. The maturity of the condensates was estimated, based on paraffin index, methylphenanthrene index and Pr/Ph ratios (e.g. Lu and Zhang, 1994) and on diamondoid maturity param-

Fig. 3. Generalized stratigraphy and dominate lithology of the Ying – Qiong Basins.

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eters (Chen et al., 1996), but not on triterpane and sterane parameters. The maturity of the condensates remains controversial. In this paper, we discuss the distributions of steranes and triterpanes monitored in the YA13-1 condensates by utilizing effective analytical methods and techniques.

2. Samples and methods 2.1. Samples Condensates from five wells in the YA13-1 gas field were sampled for bulk chemical and GC analyses. Because of the similar bulk geochemical and biomarker distribution characteristics of the condensates by GC and GC-MS analyses, only two condensate samples (YA1311 and YA1314) from the Lingshui Formation and the Sanya Formation sandstone reservoir in the Qiongdongnan Basin gas field were analyzed for triterpane and sterane biomarkers. The two samples were selected to represent the YA131 condensates.

70 jC (1 min isothermal) at 4 jC/min to 290 jC for branched and cyclic alkane fractions, respectively. The GC-MS analyses of the whole condensates (only for sesquiterpane detection) and the branched/ cyclic alkane fractions were carried out in full scan mode on a Finnigan-MAT Model TSQ-70B mass spectrometer interfaced to a Varian 3400 gas chromatograph equipped with a DB-1 (J&W) fused silica column (60 m
0.25 mm, i.d.; 0.25 Am thick film). The column was operated in the splitless mode using helium as carrier gas. The oven temperature was programmed from 70 jC (2 min isothermal) to 220 jC at 3 jC/min, and then at 2 jC/min to 300 jC (30 min isothermal). The ionization energy was 70 eV. GC-MS/MS analysis was carried out on the same instrument. The mass spectrometer was operated in parent ion mode. Argon was used as the collision gas at a collision pressure of ca. 0.5 Torr, and the collision energy was  10 eV. The temperature of the GC oven was programmed in the same way as for gas chromatography-mass spectrometry analysis.

3. Results and discussion 2.2. Isolation of branched and cyclic alkane fraction 3.1. General Firstly, the condensates were distilled under vacuum and then the fraction boiling above 210 jC was separated into aliphatic hydrocarbon, aromatic hydrocarbon and polar (NSO) fractions by column chromatography over silica/alumina (6:1). The saturated hydrocarbon fraction was obtained by elution ˚ molecular sieve rewith petroleum ether. A 5 A moved the n-alkanes from the saturated hydrocarbon fraction to yield a branched and cyclic alkane fraction, which was further analyzed by GC-MS and GC-MS/MS. 2.3. GC, GC-MS and GC-MS/MS analysis The GC analysis of whole condensates and branched and cyclic alkane fractions was carried out using a Hewlett Packard 5880A gas chromatograph equipped with an HP-1 fused silica column (25 m
0.32 mm, i.d.). Hydrogen was used as carrier gas. The oven temperature was programmed from 40 jC (5 min isothermal) at 2 jC/min to 290 jC (30 min isothermal) for whole condensates samples and from

The condensates from the YA13-1 gas field do not show any signs of biodegradation. They have a low to intermediate density from 0.80 to 0.86 g/cm3 and wax contents from 1% to 21%. The hydrocarbon distribution characteristics of some condensates by GC and GC-MS analyses have already been reported by several researchers (e.g. Li and Pan, 1990; Zhou and Sheng, 1995). One of the marked characteristics of the YA13-1 condensates is their high aromaticity. Aromatic components account for 30 – 33% in the C15 + hydrocarbon fraction. The lower molecular weight aromatic compounds such as benzene, toluene and naphthalene are enriched in the condensates (e.g. Li and Pan, 1990; Zhou and Sheng, 1995). The n-alkanes (C9 – C39) of the YA13-1 condensates show a bimodal distribution dominated by long-chain components. Pr/ Ph ratios of the condensates range from 6 to 11, and Pr/n-C17 ratios are >1, with a mean of 1.5. In the m/z 123 mass chromatogram of the whole condensates, drimane sesquiterpanes are present in high concentrations, compared with the triterpanes. Moreover,

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cadinanes and homo-cadinanes also have a significantly high abundance; especially one of the C15 cadinanes is in abnormally high concentration, nearly as abundant as the C15 drimane. Three to four isomers of cadinanes and two to three homo-cadinane isomers occur in the condensates. C24 terrigenous tetracyclic terpanes are more abundant than 17a-hopanes. All these features of the YA13-1 condensates described here are indicative of a terrestrial plant origin. 3.2. Triterpanes 3.2.1. 17a-Hopanes The two condensate samples have very similar 17a-hopane distributions. The m/z 191 mass chromatogram (Fig. 4A) shows that 17a-hopanes are present in very low concentration, compared with other triterpanes such as 18a-oleanane, bicadinanes (T and W), and unknown triterpanes (Fig. 5 showing mass spectra of two unknown triterpanes). 22,29,30-trisnorneo-18a-hopane (Ts), 22,29, 30-trisnor-17ahopane (Tm) and C31 + hopanes could not be identified in this study. GC-MS/MS parent ion analysis (Fig. 6) indicates that both 30-nor-17a-hopane and 30-norneo-18a-hopane (C29Ts) have very low concentrations and that the former coelutes with a relatively abundant unknown C30 triterpane (peak ‘b’ in Fig. 4A). The 17a-diahopane in the studied samples is present in appreciable amount, and was tentatively identified as g-lupane in earlier work (Li and Pan, 1990; Lu and Zhang, 1994); the compound has a hopane retention index of 29.25, i.e. very close to the value of 29.26 reported in the literature (Killops and Howell, 1991). It was initially thought that rearranged hopanes are terrigenous markers because of their presence in coals (e.g. Philp and Gilbert, 1986). Peters and Moldowan (1993 and references therein) suggested that these compounds originate from bacteria hopanoid precursors in oxic to sub-oxic (dysaerobic) clay-rich depositional environments. 3.2.2. 18a-Oleanane, 24-nor-lupane and taraxastane 18a-Oleanane is unusually abundant in all YA13-1 condensates. In this study, the 18a-oleanane/17ahopane ratios in the YA1311 and YA1314 condensates are 2.5 and 2.3, respectively (Table 1), which, as far as we know, are the highest ratios ever obtained in sediments and oils unaltered by biodegradation. The

Fig. 4. Partial m/z 191 (A) and m/z 217 (B) mass chromatograms of the branched/cyclic hydrocarbon fraction of the YA1314 condensate (A and B, respectively). Most of the peaks in the m/z 217 mass chromatogram give mass spectra which do not correspond to steranes, but rather to bicadinane resin compounds. Peak assignments are given in Table 2. Peaks 1, 3, 5, a and b are unknown compounds. Spectra of peaks a and b are shown in Fig. 5. Note— The m/z 191 and m/z 217 mass chromatograms of the branched/ cyclic hydrocarbon fraction of the YA1311 condensate (Zhou and Sheng, 1995) can also be found in literature (Hao et al., 1998; their Fig. 9).

18a-oleanane/17a-hopane ratio, which is commonly used to calculate the relative concentration of 18aoleanane in sediments and oils, is generally lower than 1.5 in geological samples. Abundant 18a-oleanane has first been found in sediments and oils from the Niger Delta in Nigeria, with 18a-oleanane/17ahopane ratios as high as ca. 2 (Ekweozor et al., 1979; Rullko¨tter et al., 1994). 18a-Oleanane has become a well-known biomarker derived from diage-

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Fig. 5. Mass spectra of two unknown triterpanes detected in YA13-1 condensates (A, Peak a in Figs. 4 and 6; B, Peak b in Figs. 4 and 6).

netic and catagenetic transformation of angiosperm triterpanoids such as h-amyrin (olea-12-en-3-h-ol) and lupeol (e.g. Ekweozor and Udo, 1988; Rullko¨tter et al., 1994). Therefore, it provides unequivocal evidence, at the molecular level, of terrigenous organic matter in sediments and oils. No other terrigenous triterpane has a widespread molecular fossil record which is close to that of 18a-oleanane. It is now known that 18a-oleanane occurs in sediments and oils younger than the Early Cretaceous (Peters and Moldowan, 1993, and references therein), although its presence is not a common feature of all terrestrially influenced sediments deposited since that time. Exceptions have been reported in sediments

older than Early Cretaceous, i.e. Middle Jurassic (Moldowan et al., 1994). Moldowan et al. (1994) reported on the relationship between source rock age, abundance of angiosperm fossils and the oleanane/hopane ratio. They suggested that an oil with oleanane/hopane ratio of >0.25 was probably derived from Tertiary source rocks. The occurrence of 18aoleanane in high relative abundance may indicate a marine/continental transitional environment such as deltaic and coastal swamp environments rather than a freshwater coal swamp or other freshwater environments. The preservation of the saturated oleanane skeleton is less common in Tertiary coaly sediments from freshwater environments (e.g. Philp and Gilbert,

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Fig. 6. GC-MS/MS parent ion analysis of the m/z 191 daughter ion in the branched/cyclic hydrocarbon fraction of the YA1311 condensate, showing the distribution of triterpanes. Peak assignments are given in Table 2.

1986; Strachan et al., 1988; Sosrowidjojo et al., 1994), but rather is more commonly associated with deltaic conditions (e.g. Hoffman et al., 1984; Ekweozor and Udo, 1988; Sosrowidjojo et al., 1994; Wan Hasiah and Abolins, 1998) and coastal swamps (Zhou and Sheng, 1995; Zhou, 1996). Murray et al. (1997) suggested that contact of land plant matter with sea-

water during early diagenesis enhances the expression of oleanane in a mature sediment or oil and the oleanane precursors can be altered to aromatic oleanoids in freshwater environments such as fresh coal swamp and fluvio-deltaic environments. The enrichment of 18a-oleanane in the YA13-1 condensates apparently indicates a significant contri-

Table 1 Biomarker parameters and ratios of the YA1311 and YA1314 condensates Samples

Depth (m)

Terpane ratios Pr/Ph

OL/H

(W + T)/H

DH/H

YA1311 YA1314

3573 – 3586 3898 – 3922

8.78 7.28

2.29 2.51

4.41 6.34

0.68 0.36

Steranes

Diasteranes(%)

NT/H

C29/C27

C27

C28

C29

2.46 1.02

H1 ND

19 ND

21 ND

60 ND

H, 17a-hopane; OL, oleanane; W, trans – trans – trans-bicadinane; T, cis – cis – cis-bicadinane; DH, diahopane; NT, unknown triterpane (peak 5 in Fig. 4A); Steranes C29/C27, SC29 steranes /SC27 steranes; Diasteranes (%), relative percentage of C27, C28 and C29 diasteranes; ND, not detected.

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bution of angiosperm organic matter to the source rocks of the condensates. GC-MS/MS parent ion analysis (Fig. 6) shows the occurrence of 24-norlupane and taraxastane (peaks 6 and 11 in Fig. 6, respectively) based on the relative retention times and characteristics of their mass spectra. These two triterpanes are also of angiosperm origin (Kimble et al., 1974; Peakman et al., 1991; Rullko¨tter et al., 1994). In the Ying – Qiong Basins, taraxastane also occurs widespread in coals and mudstones containing abundant terrigenous organic matter (Zhou and Sheng, 1995; Zhou, 1996). In the m/z 191 mass chromatogram, it elutes just before (22S)-homo-17a-hopane. The compound has previously been identified by several authors as ursane, which is known to be erroneous (ten Haven et al., 1993). Although Kimble et al. (1974) reported Kovats indices for taraxastane [19h(H)] and its diastereoisomer lupane-I [19a(H)] termed 19a(H)-taraxastane by Perkins et al. (1995), there has been no unambiguous identification of these compounds in oils and sediments. Compared with most other C30 triterpanes such as 17a-hopane and oleanane, these compounds have long GC retention times (Kimble et al., 1974). This has led to speculation about the possible occurrence of one of these compounds in crude oils derived from terrigenous organic matter which contain an unknown C30 triterpane eluting before the (22S)-17a-homo-hopane (Pearson and Alam, 1993; Rullko¨tter et al., 1994; Sosrowidjojo et al., 1994; Perkins et al., 1995). Taraxastane and its diastereoisomer are postulated to be derived from diagenetic alteration of taraxer-14ene or lup-20(29)-ene derivatives commonly occurring in higher plants (Rullko¨tter et al., 1994; Perkins et al., 1995). Compared with oleanane, taraxastane has a narrow range of triterpene precursors and the above two triterpene derivatives are included in the precursors of oleanane (Rullko¨tter et al., 1994). This can explain that taraxastane is usually present in lower concentration in the sedimentary record than oleanane, and the occurrence of oleanane in geological samples is much more widespread than that of taraxastane. 3.2.3. Bicadinanes The two condensates contain very abundant bicadinane resin biomarkers, such as bicadinanes T, W and R. Compound T has been assigned as trans – trans –

trans-bicadinane (Cox et al., 1986) and W as cis –cis – trans-bicadinane (van Aarssen et al., 1990b). The m/z 191 mass chromatogram (Fig. 4A) shows that both W and T in the YA13-1 condensates have higher concentrations than 17a-hopane. The ratios of (W + T)/ 17a-hopane in the two studied samples are 4.41 and 6.43, respectively (Table 1). It is notable that these bicadinanes dominate the m/z 217 mass chromatogram (Fig. 4B) and that no steranes and diasteranes can be identified. The distributions of bicadinane resin biomarkers including homo-bicadinanes and secobicadinanes as well as W, T and their isomers are shown in Fig. 7. The structures of compounds R, T1 and other bicadinane isomers are unknown. Compound Z (Fig. 4A) reported in earlier work (Li and Pan, 1990; Lu and Zhang, 1994) is actually comprised of two co-eluting compounds, one is bicadinane R and the other one is homo-bicadinane (peak HB2 in Fig. 6). In the condensates, bicadinanes W and T and homo-bicadinanes are present in high abundance, whereas seco-bicadinanes have low abundance. The

Fig. 7. Partial m/z 151, m/z 369 and m/z 383 mass chromatograms (A, B and C, respectively) of the branched/cyclic hydrocarbon fraction of the YA1311 condensate, showing the distribution of seco-bicadinanes (A), bicadinanes (B) and homo-bicadinanes (C). W, cis – cis – trans-bicadinane; T, trans – trans – trans-bicadinane; T1 and R, bicadinane isomers.

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seco-bicadinanes may be less stable at higher maturity. High concentrations of bicadinanes hitherto have been reported to occur in Tertiary higher plant-derived oils and sediments from Southeast Asia, such as Brunei, Malaysia, Indonesia, Bangladesh and southern China (Grantham et al., 1983; van Aarssen et al., 1990a; Alam and Pearson, 1990; Pearson and Alam, 1993; Sosrowidjojo et al., 1994; Zhou and Sheng, 1995) and minor amounts of bicadinanes have also been found in oils and sediments outside Southeast Asia (e.g. New Zealand, Australia, Philippines and Papua New Guinea) and in samples of pre-Tertiary age (Murray et al., 1994b; van Aarssen et al., 1994; Armanios et al., 1995). Although the tropical angiosperm family Dipterocarpaceae is not the only angiosperm from which sedimentary bicadinanes can be derived (Murray et al., 1994a; van Aarssen et al., 1994; Armanios et al., 1995), it is suggested that the bicadinanes enriched in the Tertiary oils of Southeast Asia are associated with dammar resins from Dipterocarpaceae plants (van Aarssen et al., 1990a; Murray et al., 1994a). Remnants of these plants only occur in rocks of Oligocene or younger age (e.g. Bande and Prakash, 1986; Lakhanpal and Guleria, 1986). It has been verified that the bicadinanes are derived from thermal degradation of the polycadinene macromolecule present in dammar resins and that similar processes lead to a mixture of cadinanes (van Aarssen et al., 1990a; 1992a). Therefore, the occurrence of these abundant compounds in the fossil fuels is a good indicator for the presence of the polycadinene producers in the catchment area of depositional environment and hence for angiosperm plants that produce dammar-type resins. Accordingly, the YA13-1 condensates, in which cadinanes, bicadinanes and 18aoleanane are present in high abundance, were generated from a source rock containing a great deal of mainly angiosperm-derived terrestrial organic material, which was similar to the organic matter that generated the terrestrial oils in Indonesia (Hoffman et al., 1984; van Aarssen et al., 1992a). It was observed that bicadinane W, compared with other bicadinane isomers, is much more abundant in oils than in bicadinane-rich rock extracts (van Aarssen et al., 1992a,b; Pearson and Alam, 1993). The proposed interpretation is that isomerization at higher maturity and preferential migration relative to other

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bicadinanes due to different molecular geometries may both be responsible for the difference (Philp and Gilbert, 1986). However, neither explanation has hitherto been proven. Although bicadinane W is abundant in the YA13-1 condensates, a statistical study of bicadinane W contents in rock extracts in the Ying – Qiong Basin indicates that the W/T ratio does not depend on the maturity of organic matter in the rocks; i.e. there is no close relationship between the relative content of W and the maturity of the organic matter (Zhou and Sheng, 1995; Zhou, 1996). The high relative abundance of bicadinane W is also observed in some rocks containing organic matter of low maturity (Zhou and Sheng, 1995; Zhou, 1996), which is not consistent with the above suggestion that the W/T ratio is related to the maturity of organic matter. In contrast, the ratio of compound T to compounds T1 and R or the T/R ratio has been found to depend on the organic matter maturity measured in rocks (Murray et al., 1994b; Zhou and Sheng, 1995; Zhou, 1996; Sosrowidjojo et al., 1996; discussed below). 3.2.4. Unknown pentacyclic triterpanes A number of unknown C29 and C30 non-hopanoid compounds with molecular weights of 398 and 412, respectively (Table 2), were detected in the two analyzed condensate samples. These components are significantly more abundant than 17a-hopane (Fig. 4A; Table 1). Mass spectral characteristics of these unknown compounds (Fig. 5) indicate that they are pentacyclic triterpanes. Compound ‘a’ (Fig. 5) besides the molecular ion of m/z 398 has a base peak at m/z 177, corresponding to a nor-triterpane with a chemical composition of C29H50, whereas triterpane ‘b’ (Fig. 5) has a molecular weight of 412 and a base peak at m/z 191, corresponding to a C30H52 compound. The mass spectra of the above two compounds both have intense molecular ion and lack an [M-43]+ fragment associated with the loss of an isopropyl moiety in hopanoids, lupanoid and bicadinanoid compounds. GC-MS/MS parent ion analysis (Fig. 6) clearly shows the isomeric distribution of the unknown C29 and C30 compounds. At least three C29 and four C30 unknown triterpanes occur in the YA13 condensates. A C29 (peak a) and a C30 compound coelute, and a C30 compound (peak b) coelutes with nor-17a-hopane. The precise structures of these pentacyclic triterpanes

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Table 2 Peak assignments for triterpanes in Figs. 4A and 6 Peaks

MW Base Assignments peak (MS)

Ref.

W T R T1 HB1 HB2 Z

412 412 412 412 426 426

95 95 95 95 95 95

a,b,c b,c,d b,c e b,c b,c c

1

398

177

a

398

177

3

412

191

4

412

191

5

412

191

C29H C29Ts 6 b

398 398 398 412

191 191 177 191

8 9 OLE C30H 10 11 C31H

412 412 412 412 426 412 426

191 191 191 191 191 191 191

cis – cis – trans-bicadinane trans – trans – trans-bicadinane bicadinane isomer bicadinane isomer homo-bicadinane isomer homo-bicadinane isomer bicadinane R coeluting with homo-bicadinane HB2 unknown C29 pentacyclic triterpanea unknown C29 pentacyclic triterpanea unknown C30 pentacyclic triterpane unknown C30 pentacyclic triterpanea unknown C30 pentacyclic triterpane 30-nor-17a-hopane 30-norneo-18a-hopane 24-nor-lupane unknown C30 pentacyclic triterpanea 17a-diahopane g-lupane-I 18a-oleanane 17a-hopane homo-17a-diahopane(22S + 22R) taraxastane homo-17a-hopanes(22S + 22R)

b,e,f,g,h

b,e,f,g,h i i j,k

l,i m n i l,i m,k n

(a) van Aarssen et al. (1990b); (b) van Aarssen et al. (1992b); (c) Pearson and Alam (1993); (d) Cox et al. (1986); (e) Sosrowidjojo et al. (1994); (f) Ekweozor et al. (1979); (g) Armanios et al. (1994); (h) Czochanska et al. (1988); (i) Moldowan et al. (1994); (j) Peakman et al. (1991); (k) Rullko¨tter et al. (1994); (l) Killops and Howell (1991); (m) Kimble et al. (1974); (n) Peters and Moldowan (1993). a Compounds reported for the first time.

are unknown. However, close inspection of the published m/z 191 mass chromatograms of crude oils suggests that two of the unknown C30 triterpanes, peaks 3 and 5 in Fig. 4, have the same GC retention behavior and similar spectral characteristics as those previously reported in quite a few oils derived from a source containing large amounts of higher plant material, such as Nigerian crude oils (Ekweozor et al., 1979), Southeast Asian crude oils (van Aarssen et al., 1992a; Sosrowidjojo et al., 1994; Armanios

et al., 1994) and crude oils from New Zealand (Czochanska et al., 1988; Norgate et al., 1999). The two C29 triterpanes (peaks 1 and a in Fig. 6) and the other C30 triterpanes observed in this study, however, have not been reported previously. It is inferred that the unknown pentacyclic triterpanes may be terrigenous biomarkers. To find further supporting evidence for this inference from the distribution of these compounds in various rocks with different organic matter origin, numerous rock extracts were analyzed. It is interesting that the unknown triterpanes are also ample in coals and mudstones from Yacheng and Lingsui Formations containing abundant terrigenous biomarker compounds such as bicadinanes and oleanane (Li and Pan, 1990; Zhou and Sheng, 1995; Zhou, 1996). In contrast, Ying – Huang Formation mudstones, in which the biomarker distribution indicates mainly an aquatic organic matter source, do not contain a considerable quantity of such triterpanes (Zhou and Sheng, 1995; Zhou, 1996). It seems that the unknown compounds originate from higher-plant precursors deposited in a clay-rich sub-oxic depositional environment. 3.3. Steranes The m/z 217 mass chromatogram (Fig. 4B) does not show a clear sterane and diasterane distribution due to interference of bicadinane resin biomarkers, as discussed before. GC-MS/MS parent ion analysis was carried out in order to obtain information on source, depositional environment and maturation. One of the analytical results shows that steranes have a lower concentration than hopanes. With a 17a-hopane/steranes ratio of 7.5, the condensate resembles coal-originated Indonesian oils (17ahopane/steranes of 6 – 10; Hoffman et al., 1984). Fig. 8 shows that regular steranes are present in very low concentration, compared with diasteranes (Table 1). It is also indicated that, among the regular steranes, C29 steranes are more abundant than C27 and C28 steranes. The same carbon number distribution applies to the diasteranes (Fig. 8; Table 1). The predominance of C29 steranes and diasteranes further suggests a higher-plant origin for the condensates, even though the use of sterane predominance alone for predicting source material type has been demonstrated to be unreliable (Volkman, 1986).

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Fig. 8. GC-MS/MS parent ion analysis of the branched/cyclic hydrocarbon fraction of the YA1311 condensate, showing the distribution of steranes and diasteranes.

It is generally considered that diasterane formation is mainly controlled by three factors, i.e. clay mineral catalysis, thermal maturation and oxic/anoxic depositional conditions. Low diasterane/sterane ratios are associated with anoxic, clay-poor, carbonate source rocks, whilst high ratios are generally found in oils derived from source rocks that contain abundant clay minerals (Peters and Moldowan, 1993). Van KaamPeters et al. (1998), however, have shown that relative diasterane abundance does not correlate with clay content, but rather with the amount of clay relative to organic carbon (clay/TOC ratio). Their results may explain the anomalously high diasterane abundance in some carbonate-derived oils and bitumens. In contrast, Inaba et al. (2001) observed that the relative diasterane abundance in mudstone sediments with similar maturity levels increases with increasing clay mineral content and the oxic/anoxic depositional conditions of the sediments are less

influential for diasterane formation in a case of the Yabase oil field. They also pointed out that the relative diasterane abundance may correlate with the clay/TOC ratio, or with the clay content due to the comparatively uniform organic carbon contents in their samples. In this study, the very high relative concentration of diasteranes present in the YA13-1 condensates probably suggests that the condensates were generated from clay-rich source rocks (i.e. shales). The strong diasterane predominance could also have been caused by the effects of oil migration over long distances. However, such an explanation seems not to apply here, since the YA13-1 condensates are trapped relatively close to the proposed source rocks, i.e. the Yacheng and Lingsui Formations (discussed below). Alternatively, the high concentration of diasteranes may be attributed to the higher maturity of the organic matter (Peters and Moldowan, 1993).

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3.4. Maturity of the YA13-1 condensates The maturity of organic matter can be assessed by utilizing sterane and triterpane parameters (Peters and Moldowan, 1993). As discussed earlier, hopanes and steranes in the YA13-1 condensates cannot be routinely monitored by GC-MS due to interference of bicadinanes and other triterpanes, whereas GC-MS/ MS parent ion analysis of the branched/cyclic hydrocarbon fraction resolves the problem (Table 3). The 22S/(22S + 22R) ratio of the C31 17a-hopanes in the YA1311 condensate is 0.59. The value corresponds to the equilibrium end point of about 0.57– 0.60 (Peters and Moldowan, 1993 and references therein), indicating that the condensate is at an advanced stage of thermal maturity (%Ro>0.6 – 0.7). The 20S/ (20S + 20R) and the 5a,14h,17h/SC29 ratios of C29 steranes are 0.52 and 0.60, respectively. The former ratio has reached the equilibrium end point of isomerization, the latter has not (equilibrium value about 0.67– 0.71; Peters and Moldowan, 1993 and references therein), indicating that the condensate has a maturity corresponding to the peak oil generation stage. In addition, the ratios of diasteranes/(diasteranes +steranes), C29Ts/(C29Ts + nor-17a-hopane) and tricyclic terpanes/tricyclic terpanes + 17a-hopanes) are 0.90, 0.42 and 0.89, respectively, also suggesting a relatively high maturity of the condensate. Because steranes and hopanes are present in very low concentrations in the YA13-1 condensates, they may not represent the bulk of the fluid and thus not reflect the actual maturity of the condensates, even of the high-carbon-number components. Since resin

Table 3 Maturity parameters and ratios of the YA1311 and YA1314 condensates Samples Hopanes

Steranes

C31S* C29Ts* Tri* C29S* C29ahh* YA1311 0.59 YA1314 ND

0.42 ND

0.89 0.52 0.87 ND

0.60 ND

Dia* T/ T/R (T1 + R) 0.90 4.2 ND 4.0

8.3 8.0

C31S*, 22S/(22S + 22R) of the C31 17a-hopanes; C29TS*, C29Ts/ (C29Ts +30-nor-17a-hopane); Tri*, tricyclic terpanes/(tricyclic terpanes +17a-hopanes); C29S*, 20S/(20S + 20R) of C29 steranes; C29ahh*, 5a, 14h, 17h/SC29 of C29 steranes; Dia*, diasteranes/ (regular steranes + diasteranes); T/(T1 + R), bicadiane T/(T1 + R) ratio; T/R, bicadiane T/R ratio; ND, not detected.

compounds, such as cadinanes and bicadinanes, are enriched in the condensates (discussed above), related maturity parameters may be reliable for maturity assessment in the condensates. Besides the above conventional sterane and triterpane maturity parameters, the recently proposed bicadinane maturity parameters, i.e. T/(T1 + R) (Sosrowidjojo et al., 1996) and T/ R ratios (Zhou and Sheng, 1995; Zhou, 1996; Zhou et al., unpublished data), were also calculated in this study (Table 3). The T/(T1 + R) bicadinane ratios of 4.0 and 4.2 of the two analyzed condensate samples also indicate that the condensates are at an advanced stage of thermal maturity (corresponding to 0.85– 1.10% Ro), according to the relationship of the T/ (T1 + R) ratio and organic matter maturity described by Sosrowidjojo et al. (1996; their Figs. 2 and 7). Sosrowidjojo et al. (1996) found that the T/(T1 + R) ratio increases with increasing maturity of organic matter in rocks in the South Sumatra Basin, Indonesia. They concluded that the T/(T1 + R) ratio may be a useful maturity parameter for ranking the relative maturity of sediments and oils. Investigation of the T/(T1 + R) ratio variation with vitrinite reflectance measured in sediments in the Ying – Qiong Basins also indicates that the T/(T1 + R) bicadinane ratio may be a good maturity parameter (Zhou et al., unpublished data). The bicadinanes T1 and R are presumed to have trans –trans – trans ring configuration and to differ from compound T in the ring position or axial/equatorial location of substituent groups, since their mass spectra differ only slightly from that of compound T (Murray et al., 1994b; Sosrowidjojo et al., 1996). Compound T1, however, is difficult to be detected in some sediments in the Ying – Qiong Basins, because interfering peaks prevent accurate measurement of the small T1 peak in the m/z 369 mass chromatograms. It is notable that the T/ R bicadinane ratios in rock extracts in the Ying – Qiong Basins, calculated from the m/z 369 mass chromatogram, seem to better depend on the maturity of the organic matter in the rocks (Zhou and Sheng, 1995; Zhou, 1996; Zhou et al., unpublished results). When %Ro is 0.68, for instance, T/R is 2.5; when %Ro = 0.92, T/R is 5.0; when %Ro = 1.10, T/R is 9.8. The variation of T/R ratios in relation to %Ro can be expressed as follows: %Ro ¼ 0:3666lnðT=RÞ þ 0:2798 ðR2 ¼ 0:93; n ¼ 11Þ

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It is inferred that the variation in T/R ratios closed reflects variation in maturity. Hence, the T/R ratios of 8.0 and 8.3 (Table 3) of the analyzed two samples indicate a relatively high maturity of the YA13-1 condensates (%Ro = 1.04 –1.06). The above bicadinane maturity parameters have two advantages over the sterane and hopane maturity parameters; firstly they continue to change into the oil window and beyond the point where the sterane maturity parameter has maximized, as was also pointed out by Sosrowidjojo et al. (1996); secondly they are less affected by diagenetic facies effects because the relevant compounds are bound in a polymer during diagenesis (Sosrowidjojo et al., 1996). Based on the above-described sterane and terpane and including bicadinane maturity parameters, the YA13-1 condensates are at the peak to late oil generation stage (corresponding to 0.85 – 1.10% Ro), which is slightly different from other studies suggesting a low to moderate maturity (0.70 –1.00% Ro) of the YA13-1 condensates, based on paraffin index, methylphenanthrene index and Pr/Ph ratios (Lu and Zhang, 1994). These three parameters, which are affected by organic matter type and depositional environment (Peters and Moldowan, 1993 and references therein), are generally utilized as auxiliary parameters to assess maturation of organic matter in sediments and oils. Pr/Ph is usually applied as an indicator of palaeoenvironment (e.g. ten Haven et al., 1987; Fu et al., 1990). As to the YA13-1 condensates, the high Pr/Ph ratios probably mainly reflect the suboxic environment with abundant terrestrial organic matter supply. It is noted that the result of our present condensate maturity assessment based on sterane and terpane maturity parameters is markedly different from that based on diamondoid parameters by Chen et al. (1996). Chen et al. (1996) identified diamondoids in oils and sediments from the Tarim, Yinggehai, Qiongdongnan and other Chinese basins. Based on their findings, two diamondoid maturity parameters were introduced and used to determine the thermal maturity of thermogenic gas and condensates. They concluded that the YA13-1 condensates have higher maturities, equivalent to %Ro values of 1.60 – 1.70. Dzou et al. (1995) made a similar maturity estimation based on carbon isotopes in the C2 –C3 hydrocarbons of the YA13-1 gases. Since

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the n-alkanes (f C39) of the YA13-1 condensates show a bimodal distribution with long-chain components accounting for a large portion, the difference between Chen et al.’s (1996) and our maturity assessment can be explained by contributions of hydrocarbons from two sources at different depths. The condensates may represent the combination of overmature hydrocarbons, dominated by short-chain components without steranes and triterpanes from deeper source rocks, and lower maturity gas and condensates with long-chain components accounting for a considerable portion from shallower source rocks and hence different maturity. Actually, the above presumption is consistent with that of Chen et al. (1996). The contribution of lower maturity hydrocarbons to the condensates, however, might be underestimated by the previous researches (Chen et al., 1996, 1998). The main reason is that they believed that the extremely low concentration of steranes resulted from hydrocarbon absorption along the migration pathway of the overmature hydrocarbons. Thus the maturity levels (corresponding to the peak oil generation stage) based on sterane isomerization ratios could not reflect the actual maturity of the YA13-1 gases and condensates (Chen et al., 1996, 1998). In this study, however, the distribution of resin compounds including cadinanes and the recently proposed effective bicadinane maturity parameters suggest that at least the longchain components of the YA13-1 condensates have a lower maturity (0.85 –1.10% Ro). This means that the contribution of lower maturity hydrocarbons to the YA13-1 gases and condensates may have played a more important role than that proposed in the previous studies (Chen et al., 1996, 1998) and hence should be reassessed. 3.5. Source of the YA13-1 condensates The biomarker data indicate that the maturity of the YA13-1 condensates should not be lower than that of the organic matter in the source rocks corresponding to the reservoir depth range of 3570 to 3940 m. The corresponding %Ro ratios of coals and mudstones in this depth range in the YA13-1 structural area are around 0.75 –1.10% (e.g. Zhang and Zhang, 1993). The distribution characteristics of biomarkers, especially of terrigenous triterpane and sterane biomarkers,

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clearly indicate that the YA13-1 condensates were generated from predominantly higher plant organic matter. The condensates are geologically charged in the YA13-1 drape anticline trap in the Qiongdongnan Basin, with the Linsui (Upper Oligocene) and Sanya Formation (Upper Miocene) sandstones as reservoir rocks, and the Meishan mudstone (Lower Miocene) as seal. So the source organic matter of the YA13-1 condensates should have been deposited earlier than the Early Miocene in the Qiongdongnan Basin. Among the Potential source rocks in the Qiongdongnan Basin, including the Yacheng and Lingsui Formations and the lacustrine Eocene, only the Yacheng and Lingsui Formations contain abundant terrestrial biomarker compounds of the same type, such as bicadinane resin compounds, oleanane, taraxastane, and C29 steranes (Li and Pan, 1990; Zhou and Sheng, 1995; Zhou, 1996). In the Yanan depression of the Qiongdongnan Basin (see Fig. 1), the thickness of the Yacheng and Lingsui Formations is ca. 1000 m, with %Ro ratios from 0.7 to >2.0 (Zhang and Zhang, 1993). Accordingly, the Yacheng and Lingsui Formations in the Qiongdongnan Basin probably are an important source of the YA13-1 condensates. The conclusion is consistent with that of Li and Pan (1990). Other studies suggest that the Yacheng and Lingsui Formations in the Yinggehai Basin west of the No. 1 fault (Fig. 2) can also contribute to the YA13-1 gases and condensates (especially the light hydrocarbon and other short-chain components; e.g. Sun, 1994; Chen et al., 1998). Fig. 2 shows that the thick Yacheng and Lingsui Formations (ca. 2000 m; undrilled) in the Yinggehai Basin were buried deeper than 5000 m (Fig. 2), with average %Ro of ca. 3.5. It is presumed that the current maturity levels of light hydrocarbons and other short-chain components in the YA13-1 gases and condensates, i.e. %Ro = 1.60– 1.70, may result from the combination of the overmature natural gas (%Ro>2.0) from the Yacheng and Lingsui Formations in the Yinggehai Basin and lower maturity gas and condensate (%Ro ranging from 0.75 to 1.20, mean 0.85 – 1.10) from the Qiongdongnan Basin (3400 – 4100 m), as well as contribution of higher maturity hydrocarbons without steranes and triterpanes (%Ro ranging from 1.20 to 2.10, mean 1.60 –1.70) from the Yacheng and Lingsui Formations (4100 –4800 m) in the Qiongdongnan

Basin. That is to say, almost the whole Yacheng and Lingsui Formations with burial depth between 3400 and 5000 m in the Qiongdongnan Basin and the Yacheng and Lingsui Formations deeply buried in the Yinggehai Basin may both contribute to the YA13-1 gases and condensates. In this case, the contribution of lower maturity hydrocarbons to the YA13-1 gases and condensates may account for a considerable portion.

4. Conclusions Triterpanes and steranes in theYA13-1 condensates from the Qiongdongnan Basin were analyzed by GC-MS and GC-MS/MS after branched and cyclic alkane fractions were separated from the condensates. The main conclusions of this work are summarized below: (1) The distribution characteristics of biomarkers, especially of terrigenous triterpane and sterane biomarkers, indicate that the YA13-1 condensates were generated from predominantly higher plant organic matter. (2) The condensates contain a number of unknown C29 and C30 pentacyclic triterpanes, some of which are significantly abundant. The unknown compounds may be terrestrial biomarkers indicating a higher-plant (angiosperm) origin. (3) High Pr/Ph ratios ranging from 6 to 11 in the YA13-1 condensates probably mainly reflect the sub-oxic depositional environment with terrestrial organic matter supply, but not maturity information. High diasteranes/steranes ratios in the YA13-1 condensates may both reflect a claydominated source rock and higher maturity of the condensates. (4) Sterane and triterpane maturity parameters indicate that the YA13-1 condensates have relatively high maturity, i.e. in the peak to late oil generation stage (corresponding to 0.85– 1.10% Ro). The difference between this maturity assessment result and that based on diamondoid maturity parameters (%Ro = 1.60 –1.70; Chen et al., 1996) can be explained by a contribution of hydrocarbons from two sources at different depths. Based on the publications on the

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biomarker distribution characteristics of potential source rocks in the Ying – Qiong Basins and the geology of the basins, the YA13-1 condensates possibly have been derived from the Oligocene coal-bearing Yacheng and Lingshui Formations developed in the Ying – Qiong Basins. The Yacheng and Lingsui Formations with burial depth between 3400 and 5000 m in the Qiongdongnan Basin and the Yacheng and Lingsui Formations deeply buried in the Yinggehai Basin may both contribute to the YA13-1 gases and condensates. In this case, the contribution of lower maturity hydrocarbons from the Yacheng and Lingsui Formations (3400 –4100 m) in the Qiongdongnan Basin to the YA13-1 gases and condensates may account for a considerable portion. Acknowledgements This work was financed by the National Planning Committee of China (Grant No. 85-102-10-2). Ms. Zhichun Liu, Mr. Zhengyue Li and Mr. Tongshou Xiang from SKLOG are acknowledged for their assistance with GC-MS analysis. We wish to thank J. Rullko¨tter, and Lynn Walter for the critical and constructive reviews that significantly improved the manuscript. [LW] References Alam, M., Pearson, M.J., 1990. Bicadinanes in oils from the Surma Basin, Bangladesh. Org. Geochem. 15, 461 – 464. Armanios, C.A., Alexander, R., Sosrowidjojo, I.B., 1994. Fractionation of sedimentary higher-plant derived pentacyclic triterpanes using molecular sieves. Org. Geochem. 21, 531 – 543. Armanios, C., Alexander, R., Sosrowidjojo, I.B., Kagi, R.I., 1995. Identification of bicadinanes in Jurassic organic matter from the Eromanga Basin, Australia. Org. Geochem. 23, 837 – 843. Bande, M.B., Prakash, U., 1986. The Tertiary flora of South East Asia with remarks in its palaeoenvironment and phytogeography of the Indo – Malayan region. Rev. Palaeobot. Palynol. 49, 203 – 233. Chen, P.H., Chen, Z., Zhang, Q., 1993. Sequence stratigraphy and continental margin development of the northwestern shelf of the South China Sea. AAPG Bull. 77, 842 – 862. Chen, J., Fu, J., Sheng, G., Liu, D., Zhang, J., 1996. Diamondoid hydrocarbon ratios: novel maturity indices for highly mature crude oils. Org. Geochem. 25, 179 – 190.

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