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Geochemistry, grouping and origins of crude oils in the Western Pearl River Mouth Basin, offshore South China Sea
Organic Geochemistry 34 (2003) 993–1008 www.elsevier.com/locate/orggeochem
Geochemistry, grouping and origins of crude oils in the Western Pearl River Mouth Basin, offshore South China Sea Baojia Huanga,*, Xianming Xiaoa, Mingqiang Zhangb a
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China b CNOOC Nanhai West Research Institute, Zhanjiang 524057, Guangdong, China
Abstract Thirty-seven crude oils and 20 source rocks were selected for detailed molecular geochemical and isotopic analyses in order to establish the genetic relationships between the discovered oils and various petroleum source facies in the Western Pearl River Basin, offshore South China Sea. Four groups of oils were identified. The Group I oils, distributed in the southwestern WC19-1 field of Wenchang B sag, are characterized by high abundance of 4-methyl steranes relative to regular steranes, low abundance of bicadinanes and high d13C values, showing good correlation with the medium-deep lacustrine source facies in the Wenchang Formation. The Group II oils represent the majority of the discovered oil reserves in the study area, including those from the Qionghai uplift and the northeast block of the WC19-1 oil field. These oils have moderate concentrations of C30 4-methylsteranes, abundant bicadinanes and low d13C values, and correlate well with the shallow lake source facies of the Wenchang Formation in the Wenchang B and A sags. The group III oils, found in the Wenchang A sag and surrounding areas, are devoid of C30 4-methylsteranes, with abundant bicadinanes and similar d13C values to those of the Group II oils. These characteristics show close affinity with the coal-bearing sequence in the Enping Formation, thus suggesting a coaly source for these oils. The Group IV oils, found only in the WC8-3 field, display intermediate chemical compositions between the Group II and Group III oils, most likely from mixed sources in the Wenchang A sag. These oil–source genetic relationships suggest dominantly short distance oil migration and thus strong source facies controls on the geographic distribution of oil and gas fields within the Western Pearl River Mouth Basin. # 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction The Pearl River Mouth Basin is one of the four major Cenozoic sedimentary basins of the northern continental shelf offshore South China Sea (Fig. 1a). Great attention has been paid to this basin in recent years because of its large sedimentary volumes and petroleum potentials. This study covers the western segment of the Pearl River Mouth Basin (i.e., west of 113 100 E), also called as the Western Pearl River Mouth Basin (WPRM). More than 20 wells have been drilled in this area up to now, with significant discoveries. These include five oil fields, one gas field, and a number of oil-
* Corresponding author. Tel.: +86-759-390-0573. E-mail address: [email protected] (B. Huang).
gas shows in the upper Oligocene and the lower Miocene strata. Recent studies (Gong and Li, 1997; Zhu et al., 1999) demonstrated the potential occurrence of petroleum source rocks in the Eocene Wenchang Formation and the lower Oligocene Enping Formation and the compositional complexity of discovered oils. However, relatively little has been done to establish the compositional groupings of the oils and their genetic relationships with prospective source rock intervals. As part of a larger study of evaluating petroleum systems in the WPRM, the objectives of this manuscript are to document the geochemical characteristics of various oils and source rocks, to identify oil families, and correlate oils with appropriate source rocks through the use of molecular and stable carbon isotopic parameters. An improved understanding of the oil–source
0146-6380/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0146-6380(03)00035-4
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B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
Fig.1. Map showing the location (a) and structural divisions (b) of the Western Pearl River Mouth Basin: (1)=Qionghai Sag; (2)=Qionghai Uplift; (3)=Wenchang B Sag; (4)=Wenchang C Sag; (5)=Wenchang A Sag; (6)=Yangyiang Uplift; (7)=Yangjiang Sag.
relationships within the study area is important, as the area is still at the early stage of petroleum exploration.
Paleocene to early Oligocene time, and a series of structures, such as grabens, horsts, half-grabens and cuestas, were formed as the result of fault extension and rifting. During the late Oligocene, the basin subsided, forming a series of extensional-shear structures including anticlines and half anticlines. During the late middle Miocene, the basin underwent extensive uplifting, resulting in basinwide fluid migration and redistribution (Zhang and Huang, 1991; Gong and Li, 1997; Zhu et al., 1999). In response to the above structural evolution, the dominant environment for the sedimentary deposition in the WPRM changed from paleo-lakes in the early stage, through a bay, to an open-marine setting in the late stage (Fig. 2; Gong and Li, 1997; Zhu et al., 1999). The Shenhu Formation, occurring only locally, is dominated by 300–1000 m of coarse clastic sediments, with little hydrocarbon source potential. The major petroleum source rocks in the WPRM were developed during the deposition of the Wenchang and Enpping formations (Zhu et al., 1999). The Wenchang Formation, deposited during the peak stage of lake expansion, consists mainly of 800–1500 m of black shales, silty mudstones, interbedded with thin sandstones. In contrast, the Enping Formation is composed of shallow lacustrine sandstones interbeded with black shales and thin coal seams, with a total thickness of 1000–2100 m. Immediately above these source rocks are the tidal flat sandstones of the Zhuhai Formation. Because sandstones account for around 70% of the rock volume over a large area, these sandstones formed the most important reservoir rocks in the WPRM, as well as the carrier beds for hydrocarbons. The overlying open-marine sediments in the Miocene to Pleistocene strata, with a total thickness of 2000–3500 m, act as an excellent regional seal for hydrocarbon fluids (Fig. 2). All available geological and geochemical data indicate the presence of petroleum source rocks in the Wenchang and Enping formations (Zhang and Huang, 1991; Zhu et al., 1999). It was suggested that petroleum generated from these source rocks migrated through faults into the sandstone bodies in the upper part of the Zhuhai Formation–Zhujiang Formation (Gong and Li, 1997; Zhu et al., 1999), the major exploration targets in this Basin.
3. Samples and experimental conditions 2. Geological setting The WPRM contains up to 9000 m of Cenozoic sediments. It includes two structurally positive units (Qionghai and Yangjiang uplifts) and five structural sags (Qionghai, Wenchang A, Wenchang B, Wenchang C, and Yangjiang) (Fig. 1b). The structural evolution of the basin can be divided into an early faulting and a later subsiding stage, thus forming two distinctive structural layers. The faulting stage occurred during the
A total of 37 oil samples collected during drill stem test and repeat formation test were employed in this study. About 100 rock samples (including cores and cuttings from 23 wells in the WPRM) were also included for total organic carbon (TOC) and source potential determination using a Rock-Eval II instrument. Selected rock samples were extracted using dichloromethane in a Soxhlet apparatus (72 h). After asphaltene precipitation and fractionation using column chroma-
B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
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Fig. 2. Schematic stratigraphic column of the Western Pearl River Mouth Basin (after Zhu et al., 1999).
tography, saturate fractions obtained from both oils and rock extracts were analyzed using a Hewlett-Packard 5890II Gas Chromatograph (GC) fitted with a 50 m0.32 mm i.d. (0.25 mm film thickness) HP-5 fused silica capillary column. The carrier gas was helium at a flow rate of 1.5 ml/min. The samples were injected using a split-less injector maintained at 300 C. The GC oven temperature was programmed from 35 to 300 C at 5 C/ min and maintained at the final temperature for 30 min. Gas chromatography-mass spectrometric analyses (GC– MS) were carried out using an HP5890II-5792 gas chromatography-mass spectrometry-computer data system. GC conditions: HP-5 fused silica column (50 m0.32 mm i.d.; 0.25 mm film thickness); helium as carrier gas; oven temperature programmed from 50 to
300 C at 4 C/min. Samples were routinely analyzed in selective ion monitoring mode (SIM), occasionally in full scan mode (m/z 50–500). The mass spectrometer was operated using electron energy of 70 eV, EM voltage of 2000 v, and an ion source temperature of 250 C. Compound identifications were based on the comparison of mass spectra and GC retention times. A number of rock samples with reasonable hydrocarbon source potentials were chosen for kerogen isolation. The whole oil samples and kerogen isolates were then analyzed using a Finnigan MAT-251 mass spectrometer for stable carbon isotopes ( d13C), with an analytical precision of 0.02% based on the PDB standard. The laboratory reference used in this study was NBS-19.
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4. Results and discussion 4.1. Genetic grouping of the crude oils On the basis of several organic geochemical attributes including biomarker distributions and stable carbon isotopic data, oils and condensates from the WPRM can be classified into four compositional groups (Tables 1 and 2; Figs. 3 and 4). The geographical distribution for each of the different groups of oils is shown Fig. 1.
4.1.1. Group I oils Two oil samples from the WC19-1 oil field in the Wenchang B Sag belong to this group (Tables 1 and 2). The physical properties of the oils are listed in Table 1. They are black to brown in color, with medium to high density (0.8498–0.9200g/cm3). These oils are characterized by high wax (20.47–24.37%) and low sulfur contents, indicating a non-marine origin. In addition to the relatively high d13C values (24.22 to 25.75%), the Group I oils are differentiated from
Table 1 Oil samples and basic dataa Area
Sample
Depth (m)
Fm.
Density (g/cm3)
Pour point ( C)
WC B Sag
W1912-4 W1915-RFT WC1911-2
1700–1712 1407 1269–1281
ZH2 ZJ2 ZJ2
0.8498 0.8689 0.9200
QH Uplift
W1311-5 W1311-4 W1311-3 W1311-2 W1311-1 W1312-2 W1312-1 W1321-5 W1321-4 W1321-3 W1321-2 W1321-1 W1322-2 W1322-1 Q1812-2 Q1812-1 Q1811-RFT W831-4 W831-3 W831-2
1235–1244 1296–1303 1385–1402 1412–1420 1465–1473 1230–1250 1400–1410 997–1004 1087–1105 1206–1216 1249–1256 1291–1312 1106–1124 1180–1190 1214–1219 1192–1202 1197.5 1747–1757 2614–2635 2690–2701
ZJ1 ZJ1 ZJ2 ZJ2 ZJ2 ZJ1 ZJ2 ZJ1 ZJ1 ZJ1 ZJ1 ZJ2 ZJ2 ZJ2 ZJ2 ZJ2 ZJ2 ZJ1 ZH1 ZH1
WC A Sag
W911-3 W911-2 W911-1 W921-5 W921-4 W921-2 W921-3 W1431-5 W1431-4 W1431-3 W1431-2 W1431-1
3230–3250 3390–3415 3490–3525 3344–3352 3661–3699 3770–3799 3968–4000 1785–1791 2223–2240 2285–2308 2343–2365 2378–2417
YJ Uplift
Y3311-RFT Y3211-RFT
1788.5 1593
Sulphur (%)
Wax (%)
Viscosity (50 C,mm2/s)
Asphaltene (%)
33 36 2
0.06 0.10 0.14
24.37 20.47 13.21
9.43 – 56.24
1.49 2.78 2.88
0.7963 0.9100 0.8205 0.8279 0.8255 0.7803 0.8315 0.8105 0.8024 0.8990 0.8965 0.8830 0.8016 0.7989 0.9138 0.9352 0.8876 0.8032 0.8009 0.7599
1 7 2 14 17 9 10 2 7 9 9 2 7 7 25 29 14 4 21 2
0.07 0.19 0.10 0.10 0.10 0.07 0.12 0.08 0.08 0.19 0.18 0.17 0.07 0.07 0.19 0.19 0.12 0.07 0.06 0.03
5.09 5.44 8.01 9.89 8.29 8.57 6.68 5.49 6.55 6.51 6.17 6.28 7.75 7.19 3.07 4.49 9.43 8.75 13.21 5.74
1.90 38.97 3.04 3.77 3.52 1.16 3.77 2.52 2.38 26.05 21.52 13.58 1.7 1.62 – – 14.57 2.44 2.46 1.06
1.12 7.62 2.32 3.74 3.15 0.56 2.32 1.45 1.09 4.75 5.55 3.92 1.59 1.75 4.79 5.07 1.64 1.04 0.59 0.00
ZH2 ZH2 ZH2 ZH1 ZH2 ZH2 ZH2 ZJ ZH ZH ZH ZH
0.7627 0.7651 0.7837 0.7552 0.7583 0.7652 0.7637 0.7560 0.7759 0.7460 0.7611 0.7500
1 5 8 6 6 4 5 22 0 <30 25 19
0.04 0.04 0.06 0.02 0.02 0.01 0.03 0.05 0.09 0.03 0.04 0.03
2.51 4.48 5.44 4.28 3.58 2.95 2.24 2.21 4.99 0.34 0.91 1.42
0.95 1.05 1.54 1.42 1.00 0.94 0.98 0.89 1.20 0.70 0.83 0.78
0.00 0.05 0.07 0.12 0.12 0.12 0.18 0.14 0.35 0.08 0.07 0
ZJ1 ZJ1
0.8004 0.8102
20 20
0.06 0.07
8.22 6.92
1.72 2.64
1.60 1.28
a Formation names: ZJ: Zhujiang Fm.; ZJ1: Upper part of the Zhujiang Fm.; ZJ2: Lower part of the Zhujiang Fm.; ZH: Zhuhai Fm.; ZH1: Upper part of the Zhuhai Fm.; ZH2: Lower part of the Zhuhai Fm.
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B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008 Table 2 Selected geochemical parameters for crude oils from the Western Pearl River Mouth Basina Sample
Depth(m) Fm. C27 C28 C29 4-MS/St Ol/Hop BCD/Hop 20S d13Coil (%) CPI Pr/Ph Pr/n-C17 Ph/n-C18 Group
Wenchang W1912-4 W1915-R W1911-2
B Sag 1700–1712 ZH2 32 1407 ZJ2 38 1277–1281 ZJ2 31
16 17 17
52 45 52
0.85 0.43 0.37
0.12 0.10 0.11
0.00 0.00 0.62
0.46 24.66 0.59 24.56 0.58 28.20
1.15 2.43 1.10 2.60 1.04 1.72
0.51 0.66 0.55
0.23 0.25 0.32
I I II
Qionghai Uplift W1311-5 1235–1244 W1311-4 1296–1303 W1311-3 1385–1402 W1311-2 1412–1420 W1311-1 1465–1473 W1312-2 1230–1250 W1312-1 1400–1410 W1321-5 997–1004 W1321-4 1087–1105 W1321-3 1206–1216 W1321-2 1249–1256 W1321-1 1291–1312 W1322-2 1106–1124 W1322-1 1180–1190 Q1811-R 1197.5 W831-4 1747–1757 W831-3 2614–2620 W831-2 2690–2701
ZJ1 ZJ1 ZJ2 ZJ2 ZJ2 ZJ1 ZJ2 ZJ1 ZJ1 ZJ1 ZJ1 ZJ2 ZJ2 ZJ2 ZJ2 ZJ1 ZH1 ZH1
29 25 26 31 23 23 31 28 26 29 19 27 32 26 32 24 22 21
26 18 20 21 23 26 21 16 22 14 13 12 15 18 20 16 17 25
45 56 53 48 55 52 48 56 52 57 68 61 53 56 48 60 61 54
0.37 0.38 0.38 0.39 0.39 0.36 0.41 0.41 0.52 0.38 0.43 0.41 0.41 0.41 0.40 0.30 0.34 Tr
0.14 0.10 0.11 0.10 0.12 0.11 0.10 0.16 0.16 0.16 0.16 0.15 0.12 0.13 0.10 0.12 0.09 0.23
0.76 0.48 0.42 0.53 0.47 0.48 0.39 0.73 0.76 0.52 0.33 0.45 0.84 0.71 0.55 1.16 0.68 3.18
0.61 0.65 0.63 0.68 0.61 0.64 0.72 0.69 0.65 0.70 0.70 0.68 0.60 0.57 0.67 0.59 0.62 0.54
30.23 30.24 29.07 28.97 29.02 29.48 28.92 28.70 28.08 27.47 27.88 29.96 29.48 29.04 28.32 30.19 29.15 30.37
1.10 1.11 1.08 1.04 1.04 1.10 1.12 1.04 1.14 1.15 1.07 1.18 1.13 1.18 1.12 1.11 1.15 1.10
2.41 2.10 2.38 2.94 2.77 2.86 2.25 2.66 2.74 1.54 1.82 1.88 2.92 2.52 2.51 3.02 2.88 3.60
0.28 0.75 0.27 0.27 0.27 0.28 0.30 0.31 0.36 0.45 0.42 0.39 0.30 0.28 0.49 0.32 0.35 0.29
0.14 0.35 0.14 0.12 0.12 0.10 0.13 0.11 0.15 0.32 0.27 0.24 0.13 0.11 0.21 0.13 0.15 0.10
II II II II II II II II II II II II II II II IV II III
Wenchang W911-3 W911-2 W921-5 W921-4 W1431-5 W1431-4 W1431-3 W1431-2 W1431-1
ZH2 ZH2 ZH1 ZH2 ZJ ZH ZH ZH ZH
33 34 32 33 31 29 40 32 35
26 26 21 19 23 24 27 21 31
41 40 47 43 45 47 33 47 34
Tr Tr Tr Tr Tr Tr Tr Tr Tr
0.13 0.16 0.20 0.05 0.25 0.18 0.12 0.19 0.26
1.56 2.00 2.00 6.50 2.08 1.96 2.90 3.33 7.25
0.47 0.57 0.56 0.41 0.43 0.46 0.50 – –
27.20 29.82 29.51 29.54 30.92 30.55 31.54 30.23 30.71
1.06 1.17 1.11 1.06 1.13 1.10 1.29 1.18 1.08
3.00 3.30 2.91 2.61 2.65 2.80 3.43 2.83 3.09
0.23 0.25 0.24 0.18 0.40 0.25 0.24 0.21 0.23
0.09 0.09 0.09 0.08 0.20 0.12 0.10 0.10 0.09
III III III III III III III III III
ZJ
15
28
57
Tr
0.22
5.00
0.50 27.78
1.07 4.90
0.54
0.14
III
A Sag 3230–3250 3390–3415 3344–3352 3661–3699 1785–1791 2223–2240 2285–2308 2343–2365 2378–2417
Yangjiang Uplift Y3211-R 1593
a C27, C28 and C29 (%): relative percentage of C27, C28 and C29 steranes within the C27–C29 steranes; 4MS/ST: ratio of C30 4-methylsteranes to C29 regular steranes, calculated from the m/z 217 mass fragmentograms; Ol/Hop: oleanane/C30 hopane ratio, calculated from the m/z 191 mass fragmentograms; BCD/Hop: ratio of bicadinanes ‘‘W+T’’ to C30 hopane, calculated from the m/z 412 mass chromatograms; 20S: 20S/(20S+20R) ratio for aaa-C29 steranes. Pr=pristane, Ph=phytane.
oils in other groups by their high abundance of C30 4methylsteranes (with the ratio of C30 4-methylsteranes to C29 regular steranes in the range of 0.78–0.85) and lack of bicadinane that is commonly found in other groups of oils. 4-Methylsteranes are common constituents of the Cenozoic lacustrine sediments in eastern China (Fu et al., 1990) and their biological precursors may be related to certain dinoflagellates thriving in freshwater lakes (Brassell et al., 1988; Goodwin et al., 1988). Results obtained from the Paleocene-Eocene oil
shales in the adjacent Maoming Basin onshore indicate that dinoflagellates with high hydrocarbon source potentials occur dominantly in deep, freshwater lacustrine facies (Fu et al., 1985), thus the Group I oils were most likely derived from source rocks deposited in a freshwater lacustrine setting. 4.1.2. Group II oils Oils in this group include those from the Qionghai Uplift and the northeast block of the WC19-1 oil field
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B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
Fig. 3. m/z 191, 217 and 369 Mass fragmentograms of the end member oils in the Western Pearl River Mouth Basin (‘‘O’’: oleanane; ‘‘W’’ & ‘‘T’’: bicadinanes; ‘‘X’’: structurally unknown C29 pentacyclic triterpane; C30-4MST: C30 4-methylsteranes). (A) Group I oils; (B) Group II oils; (C) Group III oils; (D) Group IV oil, likely as a mixture of Groups II/III oils.
(Table 1; Fig. 1b). These oils generally have low to medium density (0.7599–0.8960 g/cm3), medium wax (5.09–9.89%) and low sulfur contents. Four samples with a relatively high density (0.9100–0.9418 g/cm3), high asphaltene content but low wax content (Table 1) were likely affected by biodegradation. The Group II oils are characterized by a moderate abundance of C30 4-methylsteranes (with the ratio of
C30 4-methylsteranes to C29 steranes in the range of 0.37–0.52), detectable but generally low contents of bicadinanes (labeled as ‘‘W’’ and ‘‘T’’ in Fig. 3; van Aarssen et al., 1992a,b), and relatively low d13C values (27.47 to 30.24%). The d13C values of the Group II oils are approximately 3–4% lower than those of the Group I oils, thus indicating that these two groups are genetically unre-
B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
Fig. 4. Cross plot of bicadinanes (W+T)/C30ab-hopane ratio vs. whole oil or source rock kerogen d13C values for samples collected from the Western Pearl River Mouth Basin, showing oil grouping and the correlation of oils in different genetic groups with their prospective source rocks. MWC: Mediumdeep lake facies in the Wenchang Formation, SWC: shallow lake facies in the Wenchang Formation; EP: coal-bearing sequence in the Enping Formation.
lated. The reduced abundance of 4-methylsteranes and the presence of bicadinanes, biomarkers for dammar resins of angiosperm plant precursors commonly occurring in southeastern Asia (van Aarssen et al., 1992a,b), indicate that the Group II oils were derived from source rocks with increased contribution of terrigenic organic matter. 4.1.3. Group III oils This group includes oils produced from the Wenchang 9-2 and Wenchang 14-3 condensate-gas fields, as well as oil-shows from several other wells (Table 2). These oils are low in density, pour point, viscosity, initial boiling point, and wax content (Table 1), but with high gas to oil ratios (GOR, up to 1000–5000 m3/m3). At the molecular level, these oils have relatively high benzene and toluene contents (8–10%) within the C6 and C7 hydrocarbons. As shown in Fig. 5, Group III oils are clearly separated from Group I and II oils. Group III oils also have high pristane/phytane ratios (3–5), and high abundance of bicadinanes and oleananes relative to hopanes, with the C30 4-methylsteranes being either absent or in low concentrations (Fig. 3). The high pristane/phytane ratios are similar to those of oils derived from terrestrial source rocks in the Tarim (Li et al., 1999; Hanson et al., 2000) and Turpan basins in NW China (Li et al., 2001), and Gippsland Basin in Australia (Philp and Gilbert, 1986). Bicadinanes, markers for specific angiosperm resins occur abundantly in the Tertiary strata of Southeastern Asia (Noble et al., 1985; Cox et al., 1986) and in the condensate oils of the Yacheng 13-1 field from the adjacent Qiongdongnan Basin (Fig. 1a) (Zhang and Huang, 1991; van Aarssen et
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Fig. 5. Plot of benzene/n-hexane vs. toluene/n-heptane ratios of oils, indicating the separation of Group III with Groups I and II oils.
al., 1992b; Zhang et al., 1994). These characteristics suggest that the source rocks for the Group III oils were most likely deposited in a freshwater lacustrine-deltaic swamp setting. Isotopically, the Group III oils have similar d13C values (27.70 to 31.05%) to those of Group II oils, but the two groups can be distinguished from each other by the abundance of C30 4-methylsteranes relative to regular steranes and the abundance of bicadinanes (‘‘W’’and ‘‘T’’) relative to a structurally unrecognized compound (‘‘X’’ in Fig. 3). This pentacyclic compound, with a base peak at the m/z 369 and a molecular ion at 398 in its mass spectrum (Fig. 6b), occurs widely in the South China Sea region. Group IV oils. Only one oil sample was assigned into this group. This oil (W831-4, Table 1) was produced from the sandstone reservoirs in the Zhuhai and Zhujiang formations of the Wenchang 8-3 oil field, located in the transitional area between the Qionghai Uplift and Wenchang A sag. It is black-brown in color, with relatively high wax content (8.75%) and low density (0.8 g/ cm3). We believe that this oil is a mixture of Groups II/ III oils as it shows intermediate physical properties and chemical compositions (Figs. 3 and 4). 4.2. Geochemistry of source rocks As indicated by the results of regional geological and geochemical studies (Zhu et al., 1999), the likely candidates for effective petroleum source rocks in the WPRM occur in the Wenchang and Enping formations. The key geochemical parameters of these source rocks are summarized in Table 3.
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B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
Fig. 6. Selected mass fragmentograms (a) of the saturated hydrocarbon fraction from a Group II oil (W1311-5) in the Western Pearl River Mouth Basin (‘‘O’’: oleanane; ‘‘W’’ and ‘‘T’’: bicadinanes. Compound ‘‘X’’ is a structurally unknown C29 pentacyclic triterpane, whose mass spectrum is shown in (b).
Table 3 Selected geochemical parameters for representative source rocks from the Western Pearl River Mouth Basina Area
Sample
QH Uplift
W821-r
WC A sag
W911-r W1411-r
40006 4610–4619
EP WC
WC B sag
W1912-r W1912-rd L1321-rd
3186–3189 3306–3309 3142.5
WC WC WC
a
Depth (m)
Fm
C28
C29
4-MS/ST
O/Hop
BCD/Hop
20S
d13Ckerogen (%)
Group
26
56
0.00
0.16
5.16
0.44
28.77
III
30 36
27 16
43 48
0.00 0.30
0.19 0.10
4.30 0.40
0.32 0.47
29.59 28.85
II II
18 28 46
18 13 17
65 59 37
0.43 0.41 2.10
0.10 0.06 0.05
0.16 0 0
0.32 0.65 0.67
28.49 25.10 –
II I I
C27
EP
See Table 2 for abbreviations.
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4.2.1. Wenchang Formation Black shales occur widely in the Wenchang Formation, with a total thickness of several hundred meters (Zhang and Huang, 1991; Zhu et al., 1999). Based on available data, two types of source rocks can be recognized (Figs. 7 and 8). The first type consists of black shales formed in the medium-deep water lake facies. These shales contain 0.65–5.22% TOC, with mainly type II1 and I kerogens. Another source rock type
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includes shales, carbonaceous shales and coals formed in the shallow lake facies, with variable TOC content (0.27–27.29%) and type III, II2 and III kerogens. Clear differences can be observed from the rock extracts between the two source facies. As shown in Fig. 9, the rock extracts of the medium-deep lake facies have relatively high ratios of C30 4-methylsteranes to C29 regular steranes (0.65 1.41), and low abundance of oleananes and bicadinanes derived from angiosperm
Fig. 7. Cross plots of TOC vs. Rock-Eval S1+S2 values for potential source rocks in the Wenchang (a) and Enping formations (b).
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Fig. 8. Modified Van Krevelen diagram showing the Hydrogen Indices–Tmax relationships for source rocks in the Wenchang (a) and Enping formations (b).
Fig. 9. m/z 191, 217 and 369 Mass fragmentograms of selected source rock extracts from the Western Pearl River Mouth Basin.
B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
plants (ten Haven et al., 1993). The rock extracts of the shallow lake facies are also low in oleanane/C30 hopane ratios (0.1–0.15) and in the abundances of bicadinanes (‘‘W’’ and ‘‘T’’) and an unknown compound ‘‘X’’ (Fig. 9), but they are differentiated from the mediumdeep water lake facies by their low ratios of C30 4methylsterane to C29 regular steranes (0.14–0.43). On the m/z 369 mass fragmentogram, they form a distinctive pattern of ‘‘W’’‘‘T’’ < ‘‘X’’ (Fig. 9). The two source facies also show clear difference in the stable carbon isotope compositions of isolated kerogens. Kerogens isolated from rocks formed in the mediumdeep lake facies show d13C values in the range of 22.13 to 25.82%, 3–6% higher than those of the kerogens from the shallow lake facies (Fig. 10). This data appears to be incompatible with the general isotopic compositions of kerogens from the Mesozoic-Cenozoic sedimentary basins in eastern China, where type III kerogens are usually enriched in 13C (with d13C values of 25 23%) compared to type I kerogens which typically have d13C values of 28 29.5% (Huang, 1993). Higher than typical land plant d13C values have been reported previously for certain aquatic, desert and salt-marsh plants and tropical grasses (6 19%;
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Smith and Epstein, 1971; Sutton, 1979; Sofer, 1984). Thus, the relatively high d13C values observed for the kerogens from the medium-deep water lake facies in the Wenchang Formation may be attributed to the significant incorporation of specific types of aquatic organisms into the kerogens. The Wenchang Formation entered the threshold of oil-generation at a burial depth of approximately 2600 m. As shown in Fig. 11, source rocks at the central Wemchang B Sag reached the main stage of oil generation during the Miocene to Pliocence time. 4.2.2. Enping Formation As described earlier, the Enping Formation is dominated by a coal-bearing sequence. The potential petroleum source rocks include black shales, carbonaceous shales, and coals. Due to the nature of the coaly strata, these rocks contain a wide range of TOC, with the Hydrogen Indices in the range of 80–150 mg/gTOC. Microscopic examination indicates that the kerogens contain over 80% of vitrinite, and thus are classified mainly into type III and II2 kerogens (Figs. 7 and 8). Distinctive biomarker features of the Enping Formation rock extracts include the high contents of bicadi-
Fig. 10. d13C values for source rock kerogens in various source rocks from the West Pearl River Mouth Basin.
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Fig. 11. Burial history curves for source rocks at the depocenters of the Wenchang B Sag (a) and Wenchang A Sag (b). The locations of the wells used for this modeling work are shown in Fig. 1b. Oil window is defined as Ro=0.61.35%. Formation names: ShWC=Shenhu and Wenchang, EP=Enping; ZH=Zhuhai; ZJ=Zhujiang; HJ=Hanjiang; YH=Yuehai; WS=Wanshan; Q=Quaternary (after Zhu et al., 1999).
nane (‘‘W’’ and ‘‘T’’), the high oleanane/C30 hopane ratios (0.2 0.25), a unique bicadinane distribution pattern (with ‘‘W’’< ‘‘T’’‘‘X’’ in the m/z 369 mass fragmentogram, and the near absence of C30 4methylsteranes (Fig. 9). The d13C values of the kerogens from the Enping Formation source rock range from 26.67 30.02%, much lower than those of the Wenchang Formation.
Of all of the formations examined, only the Enping Formation appears to have received significant inputs of dammar-type resins, although the occurrence of oleananes in other formations illustrates that angiosperm inputs were prevalent. This observation may be the reflection of a restricted climatic condition compared to angiosperms in general for the special source (Dipterocarpaceae trees) from
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which dammar resin was derived (Pearson and Mahaboob, 1993). The Enping Formation source rocks have developed well in all the sags, and reached 0.6% Ro in maturity at a burial depth of approximately 2700 m (Fig. 11). This formation, currently with burial depths exceeding 3300– 4000 m in the depocenters, is mostly within the conventional oil window, or even in the gas-generating stage (Zhang and Huang, 1991; Zhu et al., 1999).
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4.3. Oil–source correlation It is obvious from the above discussion that the four groups of oils produced from the WPRM were derived from different source rocks. We consider the relative abundance of C30 4-methylsteranes, bicadinanes and stable carbon isotopic data to be parameters for oil– source correlation here, because they are effective differentiators for different source rocks.
Fig. 12. m/z 191, 217 and 369 mass fragmentograms showing the correlation of three genetic groups of oils with potential source rocks.
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Fig. 12 shows the m/z 191, 217 and 369 mass fragmentograms for three end-member oils and their correlative source rocks: (1) both Group I oils and the medium-deep water lacustrine source rocks in the Wenchang Formation are high in C30 4-methylsteranes but low in bicadinanes, suggesting a clear genetic relationship; (2) the Group II oils show a strong affinity to the shallow lake source facies in the Wenchang Formation, because of their moderate C30 4-methylsteranes and the presence of detectable bicadinanes with ‘‘W’’< ‘‘T’’ <‘‘X’’; (3) the Group III oils are correlated to the deltaic swamp source facies in the Enping Formation, based on their near absence of C30 4-methylsteranes and relatively abundant bicadinanes with ‘‘W’’< ‘‘T’’‘‘X’’. This interpretation is further supported by the cross plots of the d13C values versus the ratio of C30 4-methyl steranes to C29 regular steranes (Fig. 13) and bicadinanes (W+T)/C30ab-hopane ratio (Fig. 4). Generally, non-biodegraded oils display d13C values similar to those of their source kerogens (Tissot and Welte, 1984; Softer, 1984). Very few oils in study area were biodegraded, thus the stable carbon isotopic compositions of these oils can provide valuable information on their source rocks. The d13C values of the oils from the WPRM Basin are 1–2% lower than those of the kerogens from their prospective source rocks (Figs. 4 and 13), indicating genetic relationships between the oils and their source rocks.
Based on the conventional ‘‘fingerprinting’’ correlation technique, the only oil in the Group IV could not be correlated with any known source rocks. This oil has a ratio of C30 4-methyl steranes relative to steranes similar to those of the Group II oils, but a distribution of bicadinanes similar to those of the Group III oil. None of the source rocks analyzed in this study is a suitable match for Group IV. Fig. 14 shows that mixing of oils derived from the shallow lake facies in the Wenchang Formation (Group II) with those from the Enping Formation (Group III) would yield an oil with such intermediate molecular and isotopic compositions. Thus, oil from the Wenchang 8-3 field (# W831-4, Table 1) could have originated from mixed sources. This correlation scheme is geologically quite feasible as only the two source rocks are known to occur in the adjacent Wenchang A sag.
5. Conclusions This study demonstrates that the Western Pearl River Mouth Basin contains three different petroleum source rock facies, including a medium-deep lake facies and a shallow lake facies in the Eocene Wenchang Formation and a deltaic swamp, coal-bearing facies in the Oligocene Enping Formation. Three of the four groups of oils can be correlated reasonably with each of these source facies by their relative distribution of 4-methylsteranes,
Fig. 13. Cross plot of C30 4-methylsterane to C29 sterane ratio (x-axis) vs. whole oil or source rock kerogen d13C values (y-axis) showing the correlation of oils in different genetic groups with their prospective source rocks. MWC: Medium-deep lake facies in the Wenchang formation, SWC: shallow lake facies in the Wenchang formation; EP: coal-bearing sequence in the Enping formation.
B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
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Fig. 14. m/z 191, 217 and 369 mass fragmentograms showing the intermediate composition of the Group IV oil (W831-4 from the WC8-3 field), likely as a mixture of Groups II/III oils.
bicadinanes and stable carbon isotopic values. The oils in the WC19-1 oil field were derived from a mediumdeep lake facies in the Wenchang Formation within the Wenchang B sag; the oils in the Qionghai Uplift were dominantly derived from a shallow lake facies in the Wenchang Formation within the Wenchang B sag; the oils in the Wenchang A sag and Yangjiang uplift were derived from the coal-bearing strata in the Enping Formation. The only oil in the fourth group was attributed to the mixing of two different source facies in the Wenchang A sag. The proposed oil–source relationships and implicated short lateral migration distances indicate strong controls of mature source kitchens on the oil and
gas distributions within the Western Pearl River Mouth Basin.
Acknowledgements Dr. Maowen Li of Geological Survey of Canada is thanked for improving the English writing of this manuscript and for providing several references cited in the text. Mr. Li Li and Yiwen Huang provided valuable assistance during the execution of the project. We are extremely grateful to Drs. Zhiwen Han and Chunqing Jiang for constructive comments on an early version of
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this manuscript. This paper has been supported by the Chinese Academy of Sciences (project no.: KZCX2-110) and Chinese State 973 project (project no.: G199043308). References Brassell, S.C., Sheng, G., Fu, J., Eglinton, G., 1988. Biological markers in lacustrine Chinese oil shales. In: Fleet, A.J., Kelts, K., Talbot, M.R. (Eds.), Lacustrine Petroleum Source Rocks. Blackwell, pp. 299–308. Cox, H.C., de Leeuw, J.W., Schenck, P.A., 1986. Bicadinane, a C30 pentacyclic isoprenoid hydrocarbon found in crude oil. Nature 319, 316–318. Fu, J., Xu, F., Chen, D., Liu, D., Hu, C., Jia, R., Xu, S., Brassell, S.C., Eglinton, G., 1985. Biological markers of organic input in Maoming oil shale. Geochimica 2, 119–127. (in Chinese). Fu, J., Sheng, G., Xu, J., Eglinton, G., Gower, A.P., Jia, R., Fan, S., Peng, P., 1990. Application of biological markers in the assessment of paleoenvironments of Chinese non-marine sediments. Organic Geochemistry 16, 769–779. Gong, Z., Li, S., 1997. Continental Margin Basin Analysis and Hydrocarbon Accumulation of the Northern South China Sea. Science Press, Beijing. (in Chinese). Goodwin, N.S., Mann, A.L., Patience, R.C., 1988. Structure and significance of C30 4-methyl steranes in lacustrine shales and oils. Organic Geochemistry 12, 495–506. Hanson, A.D., Zhang, S.C., Moldowan, J.M., Liang, D.G., Zhang, B.M., 2000. Molecular organic geochemistry of the Tarim Basin, Northwest China. Bulletin of American Association of Petroleum Geologists 84, 1109–1128. Huang, D., 1993. Distribution of carbon in organic matters of continental source rocks. China Offshore Oil & Gas 7 (4), 1– 5. (in Chinese). Li, M., Lin, R., Liao, Y., Snowdon, L.R., Wang, P., Li, P., 1999. Organic geochemistry of oils and condensates in the Kekeya field, Southwest Depression of the Tarim Basin (China). Organic Geochemistry 30, 15–37. Li, M., Bao, J., Lin, R., Stasiuk, L., Yuan, M., 2001. Revised models for hydrocarbon generation, migration and accumulation in Jurassic coal measures of the Turpan Basin, NW China. Organic Geochemistry 32, 1127–1152. Noble, R.A., Alexander, R., Kagi, R.J., Knox, J., 1985. Tetracyclic diterpenoid hydrocarbons in some Australian coals,
sediments and crude oils. Geochimica et Cosmochimica Acta 49, 1241–1247. Philp, R.P., Gilbert, T.D., 1986. Biomarker distribution in Australian oils predominantly derived from terrigenous source material. Organic Geochemistry 10, 73–84. Pearson, M.J., Mahaboob, A., 1993. Bicadinanes and other terrestrial terpenoids in immature Oligocene sedimentary rocks and a related oil from the Surma Basin, N. E. Bangladesh. Organic Geochemistry 20, 539–554. Sofer, Z., 1984. Stable carbon isotope compositions of crude oil, application to source depositional environments and petroleum alteration. Bulletin of American Association of Petroleum Geologists 68, 31–49. Smith, B.N., Epstein, S., 1971. Two categories of d13C/d12C ratios for higher plants. Plant Physiology 47, 380–384. Sutton, A., 1979. Depositional environments and their relation to chemical composition of Java Sea crude oils. In: Proceedings of the Seminar on Generation and Maturation of Hydrocarbons in Sedimentary Basin, Manila, 1977. UN/ESDCP, CCOP Technology Publication 6, Bangkok, pp. 163–179. ten Haven, H.L., Lafargue, E., Kotarba, M., 1993. Oil/oil and oil/source rock correlation in the Carpathian Foredeep and Overthrust, southeast Poland. Organic Geochemistry 20, 935–959. Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence, 2nd ed. Springer-Verlag, Berlin. van Aarssen, B.G.K., Hessels, J.K.C., Abbink, O.A., de Leeuw, J.W., 1992a. The occurrence of polycyclic sesqui-, di-, triand oligoterpenoids from a resinous polymeric cadinene in crude oils from southeast Asia. Geochimica et Cosmochimica Acta 56, 1231–1246. van Aarssen, B.G.K., Zhang, Q., de Leeuw, J.W., 1992b. An unusual distribution of bicadinanes, tricadinanes and oliocadinanes in sediments from the Yacheng Gasfield, China. Organic Geochemistry 18, 805–812. Zhang, Q.X., Li, L., Huang, B.J., 1994. Characteristics of hydrocarbon generation of Meishan formation in the Yinggehai Basin. Advances in Natural Sciences 4 (4), 442–456. (in Chinese). Zhang, Q.X., Huang, B.J., 1991. Genetic types and generation history of natural gases from major basins in the South China Sea. China Offshore Oil & Gas 4, 5–13. (in Chinese). Zhu, W., Li, M., Wu, P., 1999. Petroleum system of Zhu III subbasin, Pearl River Mouth Basin, South China Sea. Bulletin of American Association of Petroleum Geologists 83, 990–1003.
Geochemistry, grouping and origins of crude oils in the Western Pearl River Mouth Basin, offshore South China Sea Baojia Huanga,*, Xianming Xiaoa, Mingqiang Zhangb a
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China b CNOOC Nanhai West Research Institute, Zhanjiang 524057, Guangdong, China
Abstract Thirty-seven crude oils and 20 source rocks were selected for detailed molecular geochemical and isotopic analyses in order to establish the genetic relationships between the discovered oils and various petroleum source facies in the Western Pearl River Basin, offshore South China Sea. Four groups of oils were identified. The Group I oils, distributed in the southwestern WC19-1 field of Wenchang B sag, are characterized by high abundance of 4-methyl steranes relative to regular steranes, low abundance of bicadinanes and high d13C values, showing good correlation with the medium-deep lacustrine source facies in the Wenchang Formation. The Group II oils represent the majority of the discovered oil reserves in the study area, including those from the Qionghai uplift and the northeast block of the WC19-1 oil field. These oils have moderate concentrations of C30 4-methylsteranes, abundant bicadinanes and low d13C values, and correlate well with the shallow lake source facies of the Wenchang Formation in the Wenchang B and A sags. The group III oils, found in the Wenchang A sag and surrounding areas, are devoid of C30 4-methylsteranes, with abundant bicadinanes and similar d13C values to those of the Group II oils. These characteristics show close affinity with the coal-bearing sequence in the Enping Formation, thus suggesting a coaly source for these oils. The Group IV oils, found only in the WC8-3 field, display intermediate chemical compositions between the Group II and Group III oils, most likely from mixed sources in the Wenchang A sag. These oil–source genetic relationships suggest dominantly short distance oil migration and thus strong source facies controls on the geographic distribution of oil and gas fields within the Western Pearl River Mouth Basin. # 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction The Pearl River Mouth Basin is one of the four major Cenozoic sedimentary basins of the northern continental shelf offshore South China Sea (Fig. 1a). Great attention has been paid to this basin in recent years because of its large sedimentary volumes and petroleum potentials. This study covers the western segment of the Pearl River Mouth Basin (i.e., west of 113 100 E), also called as the Western Pearl River Mouth Basin (WPRM). More than 20 wells have been drilled in this area up to now, with significant discoveries. These include five oil fields, one gas field, and a number of oil-
* Corresponding author. Tel.: +86-759-390-0573. E-mail address: [email protected] (B. Huang).
gas shows in the upper Oligocene and the lower Miocene strata. Recent studies (Gong and Li, 1997; Zhu et al., 1999) demonstrated the potential occurrence of petroleum source rocks in the Eocene Wenchang Formation and the lower Oligocene Enping Formation and the compositional complexity of discovered oils. However, relatively little has been done to establish the compositional groupings of the oils and their genetic relationships with prospective source rock intervals. As part of a larger study of evaluating petroleum systems in the WPRM, the objectives of this manuscript are to document the geochemical characteristics of various oils and source rocks, to identify oil families, and correlate oils with appropriate source rocks through the use of molecular and stable carbon isotopic parameters. An improved understanding of the oil–source
0146-6380/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0146-6380(03)00035-4
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Fig.1. Map showing the location (a) and structural divisions (b) of the Western Pearl River Mouth Basin: (1)=Qionghai Sag; (2)=Qionghai Uplift; (3)=Wenchang B Sag; (4)=Wenchang C Sag; (5)=Wenchang A Sag; (6)=Yangyiang Uplift; (7)=Yangjiang Sag.
relationships within the study area is important, as the area is still at the early stage of petroleum exploration.
Paleocene to early Oligocene time, and a series of structures, such as grabens, horsts, half-grabens and cuestas, were formed as the result of fault extension and rifting. During the late Oligocene, the basin subsided, forming a series of extensional-shear structures including anticlines and half anticlines. During the late middle Miocene, the basin underwent extensive uplifting, resulting in basinwide fluid migration and redistribution (Zhang and Huang, 1991; Gong and Li, 1997; Zhu et al., 1999). In response to the above structural evolution, the dominant environment for the sedimentary deposition in the WPRM changed from paleo-lakes in the early stage, through a bay, to an open-marine setting in the late stage (Fig. 2; Gong and Li, 1997; Zhu et al., 1999). The Shenhu Formation, occurring only locally, is dominated by 300–1000 m of coarse clastic sediments, with little hydrocarbon source potential. The major petroleum source rocks in the WPRM were developed during the deposition of the Wenchang and Enpping formations (Zhu et al., 1999). The Wenchang Formation, deposited during the peak stage of lake expansion, consists mainly of 800–1500 m of black shales, silty mudstones, interbedded with thin sandstones. In contrast, the Enping Formation is composed of shallow lacustrine sandstones interbeded with black shales and thin coal seams, with a total thickness of 1000–2100 m. Immediately above these source rocks are the tidal flat sandstones of the Zhuhai Formation. Because sandstones account for around 70% of the rock volume over a large area, these sandstones formed the most important reservoir rocks in the WPRM, as well as the carrier beds for hydrocarbons. The overlying open-marine sediments in the Miocene to Pleistocene strata, with a total thickness of 2000–3500 m, act as an excellent regional seal for hydrocarbon fluids (Fig. 2). All available geological and geochemical data indicate the presence of petroleum source rocks in the Wenchang and Enping formations (Zhang and Huang, 1991; Zhu et al., 1999). It was suggested that petroleum generated from these source rocks migrated through faults into the sandstone bodies in the upper part of the Zhuhai Formation–Zhujiang Formation (Gong and Li, 1997; Zhu et al., 1999), the major exploration targets in this Basin.
3. Samples and experimental conditions 2. Geological setting The WPRM contains up to 9000 m of Cenozoic sediments. It includes two structurally positive units (Qionghai and Yangjiang uplifts) and five structural sags (Qionghai, Wenchang A, Wenchang B, Wenchang C, and Yangjiang) (Fig. 1b). The structural evolution of the basin can be divided into an early faulting and a later subsiding stage, thus forming two distinctive structural layers. The faulting stage occurred during the
A total of 37 oil samples collected during drill stem test and repeat formation test were employed in this study. About 100 rock samples (including cores and cuttings from 23 wells in the WPRM) were also included for total organic carbon (TOC) and source potential determination using a Rock-Eval II instrument. Selected rock samples were extracted using dichloromethane in a Soxhlet apparatus (72 h). After asphaltene precipitation and fractionation using column chroma-
B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
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Fig. 2. Schematic stratigraphic column of the Western Pearl River Mouth Basin (after Zhu et al., 1999).
tography, saturate fractions obtained from both oils and rock extracts were analyzed using a Hewlett-Packard 5890II Gas Chromatograph (GC) fitted with a 50 m0.32 mm i.d. (0.25 mm film thickness) HP-5 fused silica capillary column. The carrier gas was helium at a flow rate of 1.5 ml/min. The samples were injected using a split-less injector maintained at 300 C. The GC oven temperature was programmed from 35 to 300 C at 5 C/ min and maintained at the final temperature for 30 min. Gas chromatography-mass spectrometric analyses (GC– MS) were carried out using an HP5890II-5792 gas chromatography-mass spectrometry-computer data system. GC conditions: HP-5 fused silica column (50 m0.32 mm i.d.; 0.25 mm film thickness); helium as carrier gas; oven temperature programmed from 50 to
300 C at 4 C/min. Samples were routinely analyzed in selective ion monitoring mode (SIM), occasionally in full scan mode (m/z 50–500). The mass spectrometer was operated using electron energy of 70 eV, EM voltage of 2000 v, and an ion source temperature of 250 C. Compound identifications were based on the comparison of mass spectra and GC retention times. A number of rock samples with reasonable hydrocarbon source potentials were chosen for kerogen isolation. The whole oil samples and kerogen isolates were then analyzed using a Finnigan MAT-251 mass spectrometer for stable carbon isotopes ( d13C), with an analytical precision of 0.02% based on the PDB standard. The laboratory reference used in this study was NBS-19.
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4. Results and discussion 4.1. Genetic grouping of the crude oils On the basis of several organic geochemical attributes including biomarker distributions and stable carbon isotopic data, oils and condensates from the WPRM can be classified into four compositional groups (Tables 1 and 2; Figs. 3 and 4). The geographical distribution for each of the different groups of oils is shown Fig. 1.
4.1.1. Group I oils Two oil samples from the WC19-1 oil field in the Wenchang B Sag belong to this group (Tables 1 and 2). The physical properties of the oils are listed in Table 1. They are black to brown in color, with medium to high density (0.8498–0.9200g/cm3). These oils are characterized by high wax (20.47–24.37%) and low sulfur contents, indicating a non-marine origin. In addition to the relatively high d13C values (24.22 to 25.75%), the Group I oils are differentiated from
Table 1 Oil samples and basic dataa Area
Sample
Depth (m)
Fm.
Density (g/cm3)
Pour point ( C)
WC B Sag
W1912-4 W1915-RFT WC1911-2
1700–1712 1407 1269–1281
ZH2 ZJ2 ZJ2
0.8498 0.8689 0.9200
QH Uplift
W1311-5 W1311-4 W1311-3 W1311-2 W1311-1 W1312-2 W1312-1 W1321-5 W1321-4 W1321-3 W1321-2 W1321-1 W1322-2 W1322-1 Q1812-2 Q1812-1 Q1811-RFT W831-4 W831-3 W831-2
1235–1244 1296–1303 1385–1402 1412–1420 1465–1473 1230–1250 1400–1410 997–1004 1087–1105 1206–1216 1249–1256 1291–1312 1106–1124 1180–1190 1214–1219 1192–1202 1197.5 1747–1757 2614–2635 2690–2701
ZJ1 ZJ1 ZJ2 ZJ2 ZJ2 ZJ1 ZJ2 ZJ1 ZJ1 ZJ1 ZJ1 ZJ2 ZJ2 ZJ2 ZJ2 ZJ2 ZJ2 ZJ1 ZH1 ZH1
WC A Sag
W911-3 W911-2 W911-1 W921-5 W921-4 W921-2 W921-3 W1431-5 W1431-4 W1431-3 W1431-2 W1431-1
3230–3250 3390–3415 3490–3525 3344–3352 3661–3699 3770–3799 3968–4000 1785–1791 2223–2240 2285–2308 2343–2365 2378–2417
YJ Uplift
Y3311-RFT Y3211-RFT
1788.5 1593
Sulphur (%)
Wax (%)
Viscosity (50 C,mm2/s)
Asphaltene (%)
33 36 2
0.06 0.10 0.14
24.37 20.47 13.21
9.43 – 56.24
1.49 2.78 2.88
0.7963 0.9100 0.8205 0.8279 0.8255 0.7803 0.8315 0.8105 0.8024 0.8990 0.8965 0.8830 0.8016 0.7989 0.9138 0.9352 0.8876 0.8032 0.8009 0.7599
1 7 2 14 17 9 10 2 7 9 9 2 7 7 25 29 14 4 21 2
0.07 0.19 0.10 0.10 0.10 0.07 0.12 0.08 0.08 0.19 0.18 0.17 0.07 0.07 0.19 0.19 0.12 0.07 0.06 0.03
5.09 5.44 8.01 9.89 8.29 8.57 6.68 5.49 6.55 6.51 6.17 6.28 7.75 7.19 3.07 4.49 9.43 8.75 13.21 5.74
1.90 38.97 3.04 3.77 3.52 1.16 3.77 2.52 2.38 26.05 21.52 13.58 1.7 1.62 – – 14.57 2.44 2.46 1.06
1.12 7.62 2.32 3.74 3.15 0.56 2.32 1.45 1.09 4.75 5.55 3.92 1.59 1.75 4.79 5.07 1.64 1.04 0.59 0.00
ZH2 ZH2 ZH2 ZH1 ZH2 ZH2 ZH2 ZJ ZH ZH ZH ZH
0.7627 0.7651 0.7837 0.7552 0.7583 0.7652 0.7637 0.7560 0.7759 0.7460 0.7611 0.7500
1 5 8 6 6 4 5 22 0 <30 25 19
0.04 0.04 0.06 0.02 0.02 0.01 0.03 0.05 0.09 0.03 0.04 0.03
2.51 4.48 5.44 4.28 3.58 2.95 2.24 2.21 4.99 0.34 0.91 1.42
0.95 1.05 1.54 1.42 1.00 0.94 0.98 0.89 1.20 0.70 0.83 0.78
0.00 0.05 0.07 0.12 0.12 0.12 0.18 0.14 0.35 0.08 0.07 0
ZJ1 ZJ1
0.8004 0.8102
20 20
0.06 0.07
8.22 6.92
1.72 2.64
1.60 1.28
a Formation names: ZJ: Zhujiang Fm.; ZJ1: Upper part of the Zhujiang Fm.; ZJ2: Lower part of the Zhujiang Fm.; ZH: Zhuhai Fm.; ZH1: Upper part of the Zhuhai Fm.; ZH2: Lower part of the Zhuhai Fm.
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B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008 Table 2 Selected geochemical parameters for crude oils from the Western Pearl River Mouth Basina Sample
Depth(m) Fm. C27 C28 C29 4-MS/St Ol/Hop BCD/Hop 20S d13Coil (%) CPI Pr/Ph Pr/n-C17 Ph/n-C18 Group
Wenchang W1912-4 W1915-R W1911-2
B Sag 1700–1712 ZH2 32 1407 ZJ2 38 1277–1281 ZJ2 31
16 17 17
52 45 52
0.85 0.43 0.37
0.12 0.10 0.11
0.00 0.00 0.62
0.46 24.66 0.59 24.56 0.58 28.20
1.15 2.43 1.10 2.60 1.04 1.72
0.51 0.66 0.55
0.23 0.25 0.32
I I II
Qionghai Uplift W1311-5 1235–1244 W1311-4 1296–1303 W1311-3 1385–1402 W1311-2 1412–1420 W1311-1 1465–1473 W1312-2 1230–1250 W1312-1 1400–1410 W1321-5 997–1004 W1321-4 1087–1105 W1321-3 1206–1216 W1321-2 1249–1256 W1321-1 1291–1312 W1322-2 1106–1124 W1322-1 1180–1190 Q1811-R 1197.5 W831-4 1747–1757 W831-3 2614–2620 W831-2 2690–2701
ZJ1 ZJ1 ZJ2 ZJ2 ZJ2 ZJ1 ZJ2 ZJ1 ZJ1 ZJ1 ZJ1 ZJ2 ZJ2 ZJ2 ZJ2 ZJ1 ZH1 ZH1
29 25 26 31 23 23 31 28 26 29 19 27 32 26 32 24 22 21
26 18 20 21 23 26 21 16 22 14 13 12 15 18 20 16 17 25
45 56 53 48 55 52 48 56 52 57 68 61 53 56 48 60 61 54
0.37 0.38 0.38 0.39 0.39 0.36 0.41 0.41 0.52 0.38 0.43 0.41 0.41 0.41 0.40 0.30 0.34 Tr
0.14 0.10 0.11 0.10 0.12 0.11 0.10 0.16 0.16 0.16 0.16 0.15 0.12 0.13 0.10 0.12 0.09 0.23
0.76 0.48 0.42 0.53 0.47 0.48 0.39 0.73 0.76 0.52 0.33 0.45 0.84 0.71 0.55 1.16 0.68 3.18
0.61 0.65 0.63 0.68 0.61 0.64 0.72 0.69 0.65 0.70 0.70 0.68 0.60 0.57 0.67 0.59 0.62 0.54
30.23 30.24 29.07 28.97 29.02 29.48 28.92 28.70 28.08 27.47 27.88 29.96 29.48 29.04 28.32 30.19 29.15 30.37
1.10 1.11 1.08 1.04 1.04 1.10 1.12 1.04 1.14 1.15 1.07 1.18 1.13 1.18 1.12 1.11 1.15 1.10
2.41 2.10 2.38 2.94 2.77 2.86 2.25 2.66 2.74 1.54 1.82 1.88 2.92 2.52 2.51 3.02 2.88 3.60
0.28 0.75 0.27 0.27 0.27 0.28 0.30 0.31 0.36 0.45 0.42 0.39 0.30 0.28 0.49 0.32 0.35 0.29
0.14 0.35 0.14 0.12 0.12 0.10 0.13 0.11 0.15 0.32 0.27 0.24 0.13 0.11 0.21 0.13 0.15 0.10
II II II II II II II II II II II II II II II IV II III
Wenchang W911-3 W911-2 W921-5 W921-4 W1431-5 W1431-4 W1431-3 W1431-2 W1431-1
ZH2 ZH2 ZH1 ZH2 ZJ ZH ZH ZH ZH
33 34 32 33 31 29 40 32 35
26 26 21 19 23 24 27 21 31
41 40 47 43 45 47 33 47 34
Tr Tr Tr Tr Tr Tr Tr Tr Tr
0.13 0.16 0.20 0.05 0.25 0.18 0.12 0.19 0.26
1.56 2.00 2.00 6.50 2.08 1.96 2.90 3.33 7.25
0.47 0.57 0.56 0.41 0.43 0.46 0.50 – –
27.20 29.82 29.51 29.54 30.92 30.55 31.54 30.23 30.71
1.06 1.17 1.11 1.06 1.13 1.10 1.29 1.18 1.08
3.00 3.30 2.91 2.61 2.65 2.80 3.43 2.83 3.09
0.23 0.25 0.24 0.18 0.40 0.25 0.24 0.21 0.23
0.09 0.09 0.09 0.08 0.20 0.12 0.10 0.10 0.09
III III III III III III III III III
ZJ
15
28
57
Tr
0.22
5.00
0.50 27.78
1.07 4.90
0.54
0.14
III
A Sag 3230–3250 3390–3415 3344–3352 3661–3699 1785–1791 2223–2240 2285–2308 2343–2365 2378–2417
Yangjiang Uplift Y3211-R 1593
a C27, C28 and C29 (%): relative percentage of C27, C28 and C29 steranes within the C27–C29 steranes; 4MS/ST: ratio of C30 4-methylsteranes to C29 regular steranes, calculated from the m/z 217 mass fragmentograms; Ol/Hop: oleanane/C30 hopane ratio, calculated from the m/z 191 mass fragmentograms; BCD/Hop: ratio of bicadinanes ‘‘W+T’’ to C30 hopane, calculated from the m/z 412 mass chromatograms; 20S: 20S/(20S+20R) ratio for aaa-C29 steranes. Pr=pristane, Ph=phytane.
oils in other groups by their high abundance of C30 4methylsteranes (with the ratio of C30 4-methylsteranes to C29 regular steranes in the range of 0.78–0.85) and lack of bicadinane that is commonly found in other groups of oils. 4-Methylsteranes are common constituents of the Cenozoic lacustrine sediments in eastern China (Fu et al., 1990) and their biological precursors may be related to certain dinoflagellates thriving in freshwater lakes (Brassell et al., 1988; Goodwin et al., 1988). Results obtained from the Paleocene-Eocene oil
shales in the adjacent Maoming Basin onshore indicate that dinoflagellates with high hydrocarbon source potentials occur dominantly in deep, freshwater lacustrine facies (Fu et al., 1985), thus the Group I oils were most likely derived from source rocks deposited in a freshwater lacustrine setting. 4.1.2. Group II oils Oils in this group include those from the Qionghai Uplift and the northeast block of the WC19-1 oil field
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Fig. 3. m/z 191, 217 and 369 Mass fragmentograms of the end member oils in the Western Pearl River Mouth Basin (‘‘O’’: oleanane; ‘‘W’’ & ‘‘T’’: bicadinanes; ‘‘X’’: structurally unknown C29 pentacyclic triterpane; C30-4MST: C30 4-methylsteranes). (A) Group I oils; (B) Group II oils; (C) Group III oils; (D) Group IV oil, likely as a mixture of Groups II/III oils.
(Table 1; Fig. 1b). These oils generally have low to medium density (0.7599–0.8960 g/cm3), medium wax (5.09–9.89%) and low sulfur contents. Four samples with a relatively high density (0.9100–0.9418 g/cm3), high asphaltene content but low wax content (Table 1) were likely affected by biodegradation. The Group II oils are characterized by a moderate abundance of C30 4-methylsteranes (with the ratio of
C30 4-methylsteranes to C29 steranes in the range of 0.37–0.52), detectable but generally low contents of bicadinanes (labeled as ‘‘W’’ and ‘‘T’’ in Fig. 3; van Aarssen et al., 1992a,b), and relatively low d13C values (27.47 to 30.24%). The d13C values of the Group II oils are approximately 3–4% lower than those of the Group I oils, thus indicating that these two groups are genetically unre-
B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
Fig. 4. Cross plot of bicadinanes (W+T)/C30ab-hopane ratio vs. whole oil or source rock kerogen d13C values for samples collected from the Western Pearl River Mouth Basin, showing oil grouping and the correlation of oils in different genetic groups with their prospective source rocks. MWC: Mediumdeep lake facies in the Wenchang Formation, SWC: shallow lake facies in the Wenchang Formation; EP: coal-bearing sequence in the Enping Formation.
lated. The reduced abundance of 4-methylsteranes and the presence of bicadinanes, biomarkers for dammar resins of angiosperm plant precursors commonly occurring in southeastern Asia (van Aarssen et al., 1992a,b), indicate that the Group II oils were derived from source rocks with increased contribution of terrigenic organic matter. 4.1.3. Group III oils This group includes oils produced from the Wenchang 9-2 and Wenchang 14-3 condensate-gas fields, as well as oil-shows from several other wells (Table 2). These oils are low in density, pour point, viscosity, initial boiling point, and wax content (Table 1), but with high gas to oil ratios (GOR, up to 1000–5000 m3/m3). At the molecular level, these oils have relatively high benzene and toluene contents (8–10%) within the C6 and C7 hydrocarbons. As shown in Fig. 5, Group III oils are clearly separated from Group I and II oils. Group III oils also have high pristane/phytane ratios (3–5), and high abundance of bicadinanes and oleananes relative to hopanes, with the C30 4-methylsteranes being either absent or in low concentrations (Fig. 3). The high pristane/phytane ratios are similar to those of oils derived from terrestrial source rocks in the Tarim (Li et al., 1999; Hanson et al., 2000) and Turpan basins in NW China (Li et al., 2001), and Gippsland Basin in Australia (Philp and Gilbert, 1986). Bicadinanes, markers for specific angiosperm resins occur abundantly in the Tertiary strata of Southeastern Asia (Noble et al., 1985; Cox et al., 1986) and in the condensate oils of the Yacheng 13-1 field from the adjacent Qiongdongnan Basin (Fig. 1a) (Zhang and Huang, 1991; van Aarssen et
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Fig. 5. Plot of benzene/n-hexane vs. toluene/n-heptane ratios of oils, indicating the separation of Group III with Groups I and II oils.
al., 1992b; Zhang et al., 1994). These characteristics suggest that the source rocks for the Group III oils were most likely deposited in a freshwater lacustrine-deltaic swamp setting. Isotopically, the Group III oils have similar d13C values (27.70 to 31.05%) to those of Group II oils, but the two groups can be distinguished from each other by the abundance of C30 4-methylsteranes relative to regular steranes and the abundance of bicadinanes (‘‘W’’and ‘‘T’’) relative to a structurally unrecognized compound (‘‘X’’ in Fig. 3). This pentacyclic compound, with a base peak at the m/z 369 and a molecular ion at 398 in its mass spectrum (Fig. 6b), occurs widely in the South China Sea region. Group IV oils. Only one oil sample was assigned into this group. This oil (W831-4, Table 1) was produced from the sandstone reservoirs in the Zhuhai and Zhujiang formations of the Wenchang 8-3 oil field, located in the transitional area between the Qionghai Uplift and Wenchang A sag. It is black-brown in color, with relatively high wax content (8.75%) and low density (0.8 g/ cm3). We believe that this oil is a mixture of Groups II/ III oils as it shows intermediate physical properties and chemical compositions (Figs. 3 and 4). 4.2. Geochemistry of source rocks As indicated by the results of regional geological and geochemical studies (Zhu et al., 1999), the likely candidates for effective petroleum source rocks in the WPRM occur in the Wenchang and Enping formations. The key geochemical parameters of these source rocks are summarized in Table 3.
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Fig. 6. Selected mass fragmentograms (a) of the saturated hydrocarbon fraction from a Group II oil (W1311-5) in the Western Pearl River Mouth Basin (‘‘O’’: oleanane; ‘‘W’’ and ‘‘T’’: bicadinanes. Compound ‘‘X’’ is a structurally unknown C29 pentacyclic triterpane, whose mass spectrum is shown in (b).
Table 3 Selected geochemical parameters for representative source rocks from the Western Pearl River Mouth Basina Area
Sample
QH Uplift
W821-r
WC A sag
W911-r W1411-r
40006 4610–4619
EP WC
WC B sag
W1912-r W1912-rd L1321-rd
3186–3189 3306–3309 3142.5
WC WC WC
a
Depth (m)
Fm
C28
C29
4-MS/ST
O/Hop
BCD/Hop
20S
d13Ckerogen (%)
Group
26
56
0.00
0.16
5.16
0.44
28.77
III
30 36
27 16
43 48
0.00 0.30
0.19 0.10
4.30 0.40
0.32 0.47
29.59 28.85
II II
18 28 46
18 13 17
65 59 37
0.43 0.41 2.10
0.10 0.06 0.05
0.16 0 0
0.32 0.65 0.67
28.49 25.10 –
II I I
C27
EP
See Table 2 for abbreviations.
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4.2.1. Wenchang Formation Black shales occur widely in the Wenchang Formation, with a total thickness of several hundred meters (Zhang and Huang, 1991; Zhu et al., 1999). Based on available data, two types of source rocks can be recognized (Figs. 7 and 8). The first type consists of black shales formed in the medium-deep water lake facies. These shales contain 0.65–5.22% TOC, with mainly type II1 and I kerogens. Another source rock type
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includes shales, carbonaceous shales and coals formed in the shallow lake facies, with variable TOC content (0.27–27.29%) and type III, II2 and III kerogens. Clear differences can be observed from the rock extracts between the two source facies. As shown in Fig. 9, the rock extracts of the medium-deep lake facies have relatively high ratios of C30 4-methylsteranes to C29 regular steranes (0.65 1.41), and low abundance of oleananes and bicadinanes derived from angiosperm
Fig. 7. Cross plots of TOC vs. Rock-Eval S1+S2 values for potential source rocks in the Wenchang (a) and Enping formations (b).
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Fig. 8. Modified Van Krevelen diagram showing the Hydrogen Indices–Tmax relationships for source rocks in the Wenchang (a) and Enping formations (b).
Fig. 9. m/z 191, 217 and 369 Mass fragmentograms of selected source rock extracts from the Western Pearl River Mouth Basin.
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plants (ten Haven et al., 1993). The rock extracts of the shallow lake facies are also low in oleanane/C30 hopane ratios (0.1–0.15) and in the abundances of bicadinanes (‘‘W’’ and ‘‘T’’) and an unknown compound ‘‘X’’ (Fig. 9), but they are differentiated from the mediumdeep water lake facies by their low ratios of C30 4methylsterane to C29 regular steranes (0.14–0.43). On the m/z 369 mass fragmentogram, they form a distinctive pattern of ‘‘W’’‘‘T’’ < ‘‘X’’ (Fig. 9). The two source facies also show clear difference in the stable carbon isotope compositions of isolated kerogens. Kerogens isolated from rocks formed in the mediumdeep lake facies show d13C values in the range of 22.13 to 25.82%, 3–6% higher than those of the kerogens from the shallow lake facies (Fig. 10). This data appears to be incompatible with the general isotopic compositions of kerogens from the Mesozoic-Cenozoic sedimentary basins in eastern China, where type III kerogens are usually enriched in 13C (with d13C values of 25 23%) compared to type I kerogens which typically have d13C values of 28 29.5% (Huang, 1993). Higher than typical land plant d13C values have been reported previously for certain aquatic, desert and salt-marsh plants and tropical grasses (6 19%;
1003
Smith and Epstein, 1971; Sutton, 1979; Sofer, 1984). Thus, the relatively high d13C values observed for the kerogens from the medium-deep water lake facies in the Wenchang Formation may be attributed to the significant incorporation of specific types of aquatic organisms into the kerogens. The Wenchang Formation entered the threshold of oil-generation at a burial depth of approximately 2600 m. As shown in Fig. 11, source rocks at the central Wemchang B Sag reached the main stage of oil generation during the Miocene to Pliocence time. 4.2.2. Enping Formation As described earlier, the Enping Formation is dominated by a coal-bearing sequence. The potential petroleum source rocks include black shales, carbonaceous shales, and coals. Due to the nature of the coaly strata, these rocks contain a wide range of TOC, with the Hydrogen Indices in the range of 80–150 mg/gTOC. Microscopic examination indicates that the kerogens contain over 80% of vitrinite, and thus are classified mainly into type III and II2 kerogens (Figs. 7 and 8). Distinctive biomarker features of the Enping Formation rock extracts include the high contents of bicadi-
Fig. 10. d13C values for source rock kerogens in various source rocks from the West Pearl River Mouth Basin.
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Fig. 11. Burial history curves for source rocks at the depocenters of the Wenchang B Sag (a) and Wenchang A Sag (b). The locations of the wells used for this modeling work are shown in Fig. 1b. Oil window is defined as Ro=0.61.35%. Formation names: ShWC=Shenhu and Wenchang, EP=Enping; ZH=Zhuhai; ZJ=Zhujiang; HJ=Hanjiang; YH=Yuehai; WS=Wanshan; Q=Quaternary (after Zhu et al., 1999).
nane (‘‘W’’ and ‘‘T’’), the high oleanane/C30 hopane ratios (0.2 0.25), a unique bicadinane distribution pattern (with ‘‘W’’< ‘‘T’’‘‘X’’ in the m/z 369 mass fragmentogram, and the near absence of C30 4methylsteranes (Fig. 9). The d13C values of the kerogens from the Enping Formation source rock range from 26.67 30.02%, much lower than those of the Wenchang Formation.
Of all of the formations examined, only the Enping Formation appears to have received significant inputs of dammar-type resins, although the occurrence of oleananes in other formations illustrates that angiosperm inputs were prevalent. This observation may be the reflection of a restricted climatic condition compared to angiosperms in general for the special source (Dipterocarpaceae trees) from
B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
which dammar resin was derived (Pearson and Mahaboob, 1993). The Enping Formation source rocks have developed well in all the sags, and reached 0.6% Ro in maturity at a burial depth of approximately 2700 m (Fig. 11). This formation, currently with burial depths exceeding 3300– 4000 m in the depocenters, is mostly within the conventional oil window, or even in the gas-generating stage (Zhang and Huang, 1991; Zhu et al., 1999).
1005
4.3. Oil–source correlation It is obvious from the above discussion that the four groups of oils produced from the WPRM were derived from different source rocks. We consider the relative abundance of C30 4-methylsteranes, bicadinanes and stable carbon isotopic data to be parameters for oil– source correlation here, because they are effective differentiators for different source rocks.
Fig. 12. m/z 191, 217 and 369 mass fragmentograms showing the correlation of three genetic groups of oils with potential source rocks.
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Fig. 12 shows the m/z 191, 217 and 369 mass fragmentograms for three end-member oils and their correlative source rocks: (1) both Group I oils and the medium-deep water lacustrine source rocks in the Wenchang Formation are high in C30 4-methylsteranes but low in bicadinanes, suggesting a clear genetic relationship; (2) the Group II oils show a strong affinity to the shallow lake source facies in the Wenchang Formation, because of their moderate C30 4-methylsteranes and the presence of detectable bicadinanes with ‘‘W’’< ‘‘T’’ <‘‘X’’; (3) the Group III oils are correlated to the deltaic swamp source facies in the Enping Formation, based on their near absence of C30 4-methylsteranes and relatively abundant bicadinanes with ‘‘W’’< ‘‘T’’‘‘X’’. This interpretation is further supported by the cross plots of the d13C values versus the ratio of C30 4-methyl steranes to C29 regular steranes (Fig. 13) and bicadinanes (W+T)/C30ab-hopane ratio (Fig. 4). Generally, non-biodegraded oils display d13C values similar to those of their source kerogens (Tissot and Welte, 1984; Softer, 1984). Very few oils in study area were biodegraded, thus the stable carbon isotopic compositions of these oils can provide valuable information on their source rocks. The d13C values of the oils from the WPRM Basin are 1–2% lower than those of the kerogens from their prospective source rocks (Figs. 4 and 13), indicating genetic relationships between the oils and their source rocks.
Based on the conventional ‘‘fingerprinting’’ correlation technique, the only oil in the Group IV could not be correlated with any known source rocks. This oil has a ratio of C30 4-methyl steranes relative to steranes similar to those of the Group II oils, but a distribution of bicadinanes similar to those of the Group III oil. None of the source rocks analyzed in this study is a suitable match for Group IV. Fig. 14 shows that mixing of oils derived from the shallow lake facies in the Wenchang Formation (Group II) with those from the Enping Formation (Group III) would yield an oil with such intermediate molecular and isotopic compositions. Thus, oil from the Wenchang 8-3 field (# W831-4, Table 1) could have originated from mixed sources. This correlation scheme is geologically quite feasible as only the two source rocks are known to occur in the adjacent Wenchang A sag.
5. Conclusions This study demonstrates that the Western Pearl River Mouth Basin contains three different petroleum source rock facies, including a medium-deep lake facies and a shallow lake facies in the Eocene Wenchang Formation and a deltaic swamp, coal-bearing facies in the Oligocene Enping Formation. Three of the four groups of oils can be correlated reasonably with each of these source facies by their relative distribution of 4-methylsteranes,
Fig. 13. Cross plot of C30 4-methylsterane to C29 sterane ratio (x-axis) vs. whole oil or source rock kerogen d13C values (y-axis) showing the correlation of oils in different genetic groups with their prospective source rocks. MWC: Medium-deep lake facies in the Wenchang formation, SWC: shallow lake facies in the Wenchang formation; EP: coal-bearing sequence in the Enping formation.
B. Huang et al. / Organic Geochemistry 34 (2003) 993–1008
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Fig. 14. m/z 191, 217 and 369 mass fragmentograms showing the intermediate composition of the Group IV oil (W831-4 from the WC8-3 field), likely as a mixture of Groups II/III oils.
bicadinanes and stable carbon isotopic values. The oils in the WC19-1 oil field were derived from a mediumdeep lake facies in the Wenchang Formation within the Wenchang B sag; the oils in the Qionghai Uplift were dominantly derived from a shallow lake facies in the Wenchang Formation within the Wenchang B sag; the oils in the Wenchang A sag and Yangjiang uplift were derived from the coal-bearing strata in the Enping Formation. The only oil in the fourth group was attributed to the mixing of two different source facies in the Wenchang A sag. The proposed oil–source relationships and implicated short lateral migration distances indicate strong controls of mature source kitchens on the oil and
gas distributions within the Western Pearl River Mouth Basin.
Acknowledgements Dr. Maowen Li of Geological Survey of Canada is thanked for improving the English writing of this manuscript and for providing several references cited in the text. Mr. Li Li and Yiwen Huang provided valuable assistance during the execution of the project. We are extremely grateful to Drs. Zhiwen Han and Chunqing Jiang for constructive comments on an early version of
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