Journal of Earth Science, Vol. 30, No. 2, p. 376–386, April 2019 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-018-1001-3
Molecular and Isotopic Characteristics of Mature Condensates from the East China Sea Shelf Basin Using GC×GC-TOFMS and GC-IRMS
Chao Shan 1, 2, Jiaren Ye *1, Alan Scarlett2, Kliti Grice2 1. Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China 2. WA Organic and Isotope Geochemistry Centre, and John de Laeter Centre, the Institute for Geoscience Research, Department of Applied Chemistry, Curtin University of Technology, Perth WA 6845, Australia Chao Shan: https://orcid.org/0000-0002-0846-0832; Jiaren Ye: https://orcid.org/0000-0001-5699-8074
ABSTRACT In this study, biomarkers, together with stable carbon (δ13C) and hydrogen (δD) isotopic compositions of n-alkanes have been examined in a suite of condensates collected from the East China Sea Shelf Basin (ECSSB) in order to delineate their source organic matter input, depositional conditions and evaluate their thermal maturity. Previously, GC-MS analyses have shown that all the condensates are formed in oxidizing environment with terrestrial plants as their main source input. No significant differences were apparent for biomarker parameters, likely due to the low biomarker content and high maturity of these condensates. Conventional GC-MS analysis however, may provides limited information on the sources and thermal maturity of complex mixtures due to insufficient component resolution. In the current study, we used comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) to increase the chromatographic resolution. Compounds such as alkyl cyclohexanes, alkyl cyclopentanes and diamondoids, which can be difficult to identify using conventional GC-MS analysis, were successfully identified using GC×GC-TOFMS. From our analyses we propose two possibly unreported indicators, including one maturity indicator (C5--cyclohexane/C5+-cyclohexane) and one oxidation-reduction environment indicator (alkyl-cyclohexane/alkyl-cyclopentane). Multiple petroleum charging events were proposed as an explanation for the maturity indicators indexes discrepancy between methyl-phenanthrene index (MPI) and methyl-adamantane index (MDI). In addition, the stable isotopic results show that condensates from the Paleogene have significantly higher positive δ13C values of individual n-alkanes than the Neogene samples. Based on δD values, the samples can be divided into two groups, the differences between which are likely to be attributed to different biosynthetic precursors. Variation within each group can likely be attributed to vaporization. KEY WORDS: condensate, biomarker characteristic, source information, GC×GC-TOFMS, GC-IRMS.
0 INTRODUCTION useful information on condensates. The application of GC×GC- Conventional molecular biomarkers (e.g., steranes, ter- TOFMS has been recognized as a powerful method for solving panes) from mature or high mature condensate usually cannot co-elution problems (Aguiar et al., 2010; Ventura et al., 2010), be accurately identified in conventional gas chromatography which can help identify novel compounds and geochemical in- mass spectrometry (GC-MS) analysis due to low content and dicators (Ventura et al., 2012; Aguiar et al., 2011). low signal-to-noise ratio (Chen et al., 1996). Moreover, lots of The gas chromatography-isotopic ratio/mass spectrometry information in the light hydrocarbons of condensate cannot be (GC-IRMS) analytical technique permits continuous flow deter- effectively revealed using conventional GC-MS analysis due to mination of carbon isotopic values of individual components in co-elution (Li et al., 2012). Instrumental methods such as complex mixtures of geochemical interest (Freeman et al., 1990; GC×GC-TOFMS and GC-IRMS as in structural investigations Hayes et al., 1990, 1987; Matthews and Hayes, 1978). This of organic geomacro-molecules, have provided a wealth of method has been successfully applied to investigate the character- istics of specific components in the saturate hydrocarbon fraction *Corresponding author: [email protected] to establish oil-oil and oil-source rock correlations (Inaba and Su- © China University of Geosciences (Wuhan) and Springer-Verlag zuki, 2003; Li et al., 1997). Many researchers have applied this GmbH Germany, Part of Springer Nature 2019 technique to evaluate the presence of unusual polycyclic alkanes in extracts of source rocks and crude oils where hopanoids and Manuscript received November 18, 2015. steranes were either absent or present in extremely low abundance Manuscript accepted March 17, 2016. samples (Zhu et al., 2003; Schaeffer et al., 1994).
Shan, C., Ye, J. R., Scarlett, A., et al., 2019. Molecular and Isotopic Characteristics of Mature Condensates from the East China Sea Shelf Basin Using GC×GC-TOFMS and GC-IRMS. Journal of Earth Science, 30(2): 376–386. https://doi.org/10.1007/s12583-018- 1001-3. http://en.earth-science.net Molecular and Isotopic Characteristics of Mature Condensates from the East China Sea Shelf Basin 377
The East China Sea Shelf Basin (ECSSB) is China’s largest 2012; Li et al., 2009; Yang et al., 2004). Developed on Pre-Cam- offshore basin with good exploration prospects. Previous bi- brian and Paleozoic metamorphic basement, the sedimentary se- omarker parameter studies show that the condensates in this area quence, with approximately 10 000 m in maximum thickness, are usually derived from terrestrial organic matter and most consists of fifteen formations (Fig. 1). formed in the oxidation sedimentary environment (Zhu et al., So far, four hydrocarbon-bearing formations have been iden- 2012), however, this information is insufficient for the analysis tified in the Xihu depression, including the Baoshi, Pinghu, of oil-source correlation as many source formations are present Huagang and Longjing formations from bottom to top. Economic in similar sedimentary environments which make it hard to dis- oil and gas reservoirs are mostly distributed in the Pinghu and tinguish the characteristics of each formation. This study aims to Huagang formations, while gas-bearing intervals identified in examine the natural compositions of condensates and carbon and Lishui depression are located in the Mingyuefeng Formation. hydrogen isotopic values of n-alkanes from the ECSCB using GC×GC-TOFMS and GC-IRMS in order to effectively distin- 1.2 Sample Collection, Column Chromatography and 5Å guish their depositional environments and source input. This Molecular Sieving study benifits further oil-oil correlations or oil-source rock cor- Seven typical condensate samples were collected from relations in this area. the Mingyuefeng, Pinghu and Huagang formations. The physical property characteristics of these condensates are 1 BACKGROUND AND ANALYZING METHOD shown in Table 1. 1.1 Geological Setting Saturated hydrocarbon, aromatic hydrocarbon and non- The East China Sea Shelf Basin is China’s largest offshore hydrocarbon (polar/resin/NSO) fractions were chromatographically basin with an area of about 2.4×105 km2. This basin can be fur- separated on a column (25 cm×1 cm i.d.) of neutral alumina over ther subdivided into several depressions that formed at different silica gel (120 °C, 8 h). Saturated fractions were eluted with hexane stages of tectonic development. Among them, Xihu depression (40 mL), the aromatic hydrocarbon fraction with a mixture of n- and Lishui depression have been two focal points for intensive hexane and DCM (40 mL, 7 : 3, v : v) and the polar (NSO) fraction petroleum exploration in ECSSB (Dai et al., 2014; Cukur et al., with a mixture of DCM and MeOH (40 mL, 1 : 1, v : v).
Figure 1. General structure and the strata of the East China Sea Shelf Basin. Inset map shows the locality of the basin and the tectonic units (modified after Dai et al., 2014): (1) Min-Zhe uplift belt; (2) East China Sea Shelf Basin; (3) Diaoyudao folded uplift belt; (4) the frontal basin of continental shelf; (5) Okinawa trough; (6) Ryukyu Island and (7) Ryukyu Trench. 378 Chao Shan, Jiaren Ye, Alan Scarlett and Kliti Grice
Then, the total saturated hydrocarbon fraction was sepa- analysis, average values of at least three analyses of each sample rated into normal alkane and branched/cyclicalkane fractions and with a standard deviation of less than 4‰ were reported. through 5Å molecular sieves before gas chromatography isotope Carbon and hydrogen isotopic compositions are reported ratio mass spectrometry (GC-IRMS) analysis to reduce problems using the V-PDB (Vienna PeeDee Belemnite) for carbon and with coelution of compounds and to enhance the precision of standard δ-notation relative to V-SMOW (Vienna Standard carbon isotopic analysis. Specifically, samples in cyclohexane Mean Ocean Water) for hydrogen. was added to a 2 mL vial, 3/4 of it filled with activated 5Å mo- lecular sieves. The vial was capped and placed into an oven 2 RESULTS AND DISCUSSION (80 °C, overnight). The resulting solution was then cooled and 2.1 Basic Geochemical Parameters filtered through a small column of silica plugged with cotton The basic geochemical parameters for the selected samples wool (pre-rinsed with cyclohexane) and the sieves were rinsed are reported in Table 1. The thin liguid chromatography flame thoroughly with cyclohexane yielding the branched/cyclic frac- ionization detector (TLC-FID) analysis shows that the saturated tion (5Å excluded). The n-alkanes were recovered by dissolution hydrocarbon accounted for a significant proportion (ca. 80%–89%) of the sieve with HF (2 mL, 50% w/v), followed by neutraliza- of the samples. Specifically, the samples from the Huagang For- tion with saturated sodium bicarbonate solution. The aqueous mation show higher concentrations of saturate hydrocarbons, fol- phase was extracted with n-pentane (1 mL×5). This fraction con- lowed by the samples from the Pinghu Formation, then the tained n-alkanes suitable for gas chromatography-isotope ratio Mingyuefeng Formation. The aromatic hydrocarbon, NSO and as- mass spectrometry (GC-IRMS) analysis. phaltene fractions had similar concentrations for these samples. The total carbon preference index (CPI) from n-C15 to n-C35 1.3 GC-MS Analysis (Bray and Evans, 1961) ranges between 1.08 and 1.50, reaching GC/MS analyses of hydrocarbons were carried out at Cur- their thermal evolution equilibrium interval. The maturity indi- tin University of Technology using an HP 6890 gas chromato- cators (i.e., C29 20S/(20S+20R), C29 ββ/(αα+ββ)) also reach their graph interfaced to an Agilent 5975 mass-selective detector. In- thermal evolution equilibrium intervals, which are 0.52–0.55 strument analytical conditions were described in Grice et al. and 0.61–0.71 respectively (Seifert and Moldowan, 1986). The (2008). Biomarkers were identified by their retention times and methyl-phenanthrene index (MPI) maturity indicator was homo- comparison to published mass spectral data (Freeman et al., geneous with a range between 0.7 and 1.0. 1990). The light hydrocarbons are abundant in high mature con- densate samples analyzed in this study. In general, the low mo- 1.4 GC×GC-TOFMS Analysis lecular weight hydrocarbons (n-C11–n-C20, LMWH) are more Analyses were performed at Curtin University of Technol- abundant than the higher molecular weight hydrocarbon (>n-C25, ogy using a Pegasus 4D GC×GC-TOFMS instrument LECO (St. HMWH) (Table 1). The samples show low or absent of gammac- Joseph, Michigan, USA) consisting of an Agilent 7890A GC erane with its index ranging from 0.05 to 0.12, indicating low equipped with a liquid nitrogen-cooled pulse-jet modulator. The salinity and water stratification in depositional environment. GC column system consisted of a primary Rxi-5MS (60 m×0.25 The pristane/phytane ratio has been extensively used in or- mm×0.25 μm) column and a secondary RXi-17MS (1.5 m×0.18 ganic geochemistry. Pristane and phytane originate from the ox- mm×0.18 μm) column. The GC was operated in splitless mode idation (and subsequent decarboxylation) and reduction of the (1 μL of a 5 mg/mL deasphalt and depolar oil injected at 310 °C) phytol side chain of chlorophyll, respectively. The generation of using He as a carrier gas at a constant flow of 1 mL/min. The pristane and phytane is significantly controlled by the oxic or primary oven temperature program was from 40 °C (1 min hold) anoxic conditions during sedimentation with aerobic degrada- to 320 °C at a rate of 2 °C/min and the secondary oven tempera- tion promoting pristane (Didyk et al., 1978; Powell and McKirdy, ture held 5 °C higher than the primary. The modulator tempera- 1973). The ternary diagram of pristane/phytane (Pr/Ph), Pr/n-C17, ture was 15 °C higher than the primary oven, and the modulation Ph/n-C18 distribution has been widely used to investigate the or- period was 3 s with a 0.5 s hot-pulse time. The TOF-MS detector ganic facies of parent source. These biomarkers suggest that all signal was sampled at 100 spectra/s with a scan range of 45–550 the condensate samples were deposited in limnetic facies (Fig. amu. The transfer line was held at a constant temperature of 2a). 315 °C and the TOF source temperature was 250 °C. The molecular composition of the sterane series was gener- ally marked by a predominance of C29 sterane over C27 and C28 1.5 Compound Specific Isotope Analysis (CSIA) steranes. C27, C28 and C29 compounds (αααR), implying a strong Carbon isotope ratios of n-alkanes and aromatics, com- contribution of higher land plants versus aquatic organisms. pound specific deuterium analysis of n-alkanes were measured However, the sample x-7 is a special case with relatively high using a HP 6890 GC coupled to a Micromass Iso Prime isotope concentration of C27, indicate relatively higher aquatic organ- ratio mass spectrometer coupled to a HP 6890 GC at Curtin Uni- isms contribution. versity of Technology. Details of the instrument and analytical conditions are described in Grice et al. (2008). A standard mix- 2.2 Comprehensive GC×GC-TOFMS Characterization ture of n-alkanes (n-C12 to n-C32) was used daily to test the per- 2.2.1 Bulk composition of condensate formance of the instrument. For the carbon analysis, average val- A total of eleven groups of compounds were classified (Fig. 3) ues of at least two analyses of each sample and with a standard including n-alkanes, iso-alkanes, cyclic alkanes and aromatic hy- deviation of less than 0.4‰ were reported and for the deuterium drocarbons. Normal alkanes were in the range of n-C3 to n-C31. Molecular and Isotopic Characteristics of Mature Condensates from the East China Sea Shelf Basin 379
Mono-, bi- and tri-cyclic aromatics were also present. Compounds cannot readily be identified using conventional chromatography (e.g., alkyl cyclohexane, alkyl cyclopentane and diamondoids) that were resolved by GC×GC-TOFMS.
Table 1 Geochemical parameters for selected samples from the East China Sea Shelf Basin
Sample No. x-1 x-2 x-3 x-4 x-5 x-6 x-7 Well A BC C D D E Formation Huagang Huagang Huagang Pinghu Huagang Pinghu Mingyuefeng Age Oligocene Oligocene Oligocene Eocene Oligocene Eocene Palaeogene Depth (m) 2 984.50 3 031.50 2 512.50 3 085.00 2 703.20 3 541.20 2 238.00 Density (g/cm3) 0.81 0.80 0.75 0.77 0.75 0.81 0.78 Saturates (%) 88.24 88.59 87.56 84.59 88.79 87.92 83.25 Aromatics (%) 5.70 4.52 4.43 4.04 5.30 5.31 4.82 NSO (%) 3.14 3.95 2.14 7.70 3.43 4.24 8.98 Asphaltenes (%) 2.92 2.94 5.87 3.67 2.48 2.53 2.95
Major peak C19 C20 C15 C15 C14 C20 C13 LMWH/HMWH 18.08 13.91 18.10 20.88 110.83 5.68 3.91 CPI 1.50 1.08 1.22 1.18 1.23 1.11 1.22 Pr/Ph 8.90 8.86 7.81 5.18 3.45 6.30 4.45
Pr/n-C17 1.31 1.60 1.11 1.46 0.11 0.13 1.42
Ph/n-C18 0.15 0.17 0.15 0.34 0.32 0.21 0.36 Ts/(Ts+Tm) N/A 0.18 0.19 0.19 N/A 0.38 0.46 Gammacerane index N/A 0.06 N/A 0.09 N/A 0.05 0.12 DBT/P 0.12 0.12 0.03 0.07 0.13 0.12 0.06
C31 isomerization index N/A 0.58 0.59 0.60 N/A 0.55 0.54
C27–C29 αα20R Steranes (%) N/A 14 : 10 : 77 19 : 14 : 66 19 : 14 : 66 N/A 15 : 18 : 67 51 : 16 : 33
C2920S/(20S+20R) 0.80 0.91 0.55 0.65 0.90 0.95 0.85
C29ββ/(αα+ββ) 0.80 1.05 0.87 0.64 0.70 1.30 0.81 MPI 1.08 0.50 0.70 0.71 0.65 0.97 0.89 Ro 1.02 0.79 0.87 0.87 0.85 0.98 0.95
LMWH/HMWH. Low/high molecular weight hydrocarbon in the saturated fraction (GC/MS); Pr/Ph. pristane/phytane (GC/MS); CPI.
carbon preference index from n-C23 to n-C29 (GC/MS); Ts/Ts+Tm. 18α(H)-22, 29, 30-trisnorneohopane/[18α(H)-22, 29, 30-trisnorneoho-
pane+17α(H)-22, 29, 30-trisnorhopane] (m/z 191); oleanane index. oleanane/C30-17α(H), 21β(H)-hopane (m/z 191); gammacerane index.
gammacerane/C30 17α(H), 21β(H)-hopane (m/z 191); DBT/P. dibenzothiophene/phenanthrene; C31 isomerization index. C31-17α(H),
21β(H)-29-homohopane-22S/[C31-17α(H), 21β(H)-29-homohopane-22S+C31-17α(H), 21β(H)-29-homohopane-22R] (m/z 191); C27–C29
ααα20R-steranes. 100%(C27 5α(H), 14α(H), 17α(H)-20R-sterane/[C27 5α(H), 14α(H), 17α(H)-20R-sterane+C28 5α(H), 14α(H), 17α(H)-
20R-sterane+C29 5α(H), 14α(H), 17α(H)-20R-sterane] (m/z 217); C2920S/(20S+20R). C29 5α(H), 14α(H), 17α(H)20S-sterane/[C29 5α(H),
14α(H), 17α(H)-20S-sterane+C29 5α(H), 14α(H), 17α(H)-20R-sterane] (m/z 217); C29 (ββ/αα+ββ). C29 5α(H), 14β(H), 17β(H)(20S+20R)-
sterane/[C29 5α(H), 14α(H), 17α(H)(20S+20R)-sterane+C29 5α(H), 14β(H), 17β(H)(20S+20R)-sterane] (m/z 217).
Figure 2. Ternary diagrams showing the proportions of (a) relative abundances of pristane/phytane (Pr/Ph), Pr/n-C17, Ph/n-C18 (I. limnetic facies; II. fresh water lake facies; III. brackish_saline; IV. salt-lake facies)(Wang et al., 1997); and (b) relative abundances of C27, C28, C29 5a(H), 14a(H), 17a(H)-steranes (20R) (I. terrestrial plant; II. phytoplankton; III. algea; IV. mixed; V. mainly terrestrial plant; VI. mainly phytoplankton; VII. mainly algea) (modified after Kikuchi et al., 2010; Moldowan et al., 1985). 380 Chao Shan, Jiaren Ye, Alan Scarlett and Kliti Grice
2.2.2 Long chain alkylated cyclic alkanes hydrocarbons are well resolved from the n- and iso-alkanes. The As important components of gas condensate (Williams et al., separation of the alkylcyclopentane and alkylcyclohexane se- 1988), naphthenic compounds accounted for a large proportion of ries is revealed in Fig. 4 and identification provided in Table 2. the condensate samples. Among them, alkyl-cyclohexanes and Using GC×GC chromatography these two series of compounds alkyl-cyclopentanes are two types of biologically characteristic can be qualitative and quantitative identified (Ventura et al., compounds which have been considered as possible indicators of 2010). as biosynthetic precursors, maturity, and postdepositional pro- The relative abundances of each of the alkyl-cyclohexanes cesses (Fowler et al., 1986; Rubinstein and Strausez, 1979). The are shown in Fig. 5. It can be seen that the relative abundances origins of n-alkylcyclohexanes and methyl-n-alkylcyclohexanes of alkyl-cyclohexanes decrease dramatically from C1–C5 were not able to be determined because both algal and bacterial branched chain alkyl-cyclohexane but much less so for the sources are related to the occurrence of n-alkylcyclohexanes. branched C6–C17 alkyl-cyclohexanes. The ∑(C1+…+C5)- Thermoacidophilic bacterium Bacillus acidocaldarius which has cyclohexane/∑(C6+…+C16)-cyclohexane ratio versus the me- C17 and C19 n-cyclohexyl acids as dominant fatty acid components thyldiamantane index (4-methyldiamantane/(1-+3-+4- (Oshima and Ariga, 1975; de Rosa et al., 1972) was proposed to methyldiamantane)) (Fig. 6) show positive correlation indicating be responsible for the n-alkyl-cyclohexanes in some sediments. that the ratio is probably associated with maturity and might be The possibility of n-alkyl-cyclohexanes derived from the cyclisa- used as an additional maturity parameter. As discussed above, tion of straight-chain algal fatty acids has also been proposed the samples are from similar origin precursors, therefore origin (Johns et al., 1966) and demonstrated experimentally by finding precursors are ruled out as effect factors of the change in both n-alkyl-cyclohexanes and methyl-n-alkyl-cyclohexanes ∑(C1+…+C5)-cyclohexane/∑(C6+…+C16) ratio. However, if the among the products of saturated and unsaturated fatty acids fol- ratio is affected by post depositional processes further investiga- lowing heating with a clay catalyst (Rubinstein and Strausez, tion is required. 1979). Also, Spiro (1984) has suggested that alkyl-cyclohexanes The isomerization of cyclohexane to methylcyclopentane may be derived from cracking of the kerogen at high temperatures has been discussed previously (Triwahyono et al., 2005) and under the influence of certain minerals (Fowler et al., 1986). show that the conversion of cyclohexane is drastically reduced The long side chain substitution cyclohexane and cyclo- when hydrogen is absence. Hence, compared with cyclopentane, pentane studied previously by GC-MS usually concentrated in cyclohexane is more stable in oxidation conditions. The Pr/Ph the carbon number
Figure 3. Hydrocarbon classes present in x-1 condensate oil characterized by GC×GC-TOFMS. Dots represent peak markers of individual compounds eluting according to volatility (1D) and polarity (2D). Molecular and Isotopic Characteristics of Mature Condensates from the East China Sea Shelf Basin 381
Figure 4. GC×GC-TOFMS peak marker plot of long chain alkylated cyclopentane and cyclohexane from x-1 gas condensate.
Table 2 Alkyl-cyclohexane and alkyl-cyclopentane compounds identified by GC×GC-TOFMS analysis from x-1 gas condensate
Compound abbreviation m/z Molecular weight First-dimensional Second-dimensional Molecular formula retention time (s) retention time (s)
Methyl-cyclohexane 83 98 540 1.15 C7H14
Ethyl-cyclohexane 83 112 835 1.37 C8H16
Propyl-cyclohexane 83 126 1 195 1.49 C9H18
Butyl-cyclohexane 83 140 1 640 1.57 C10H20
Pentyl-cyclohexane 83 154 1 925 1.57 C11H22
Hexyl-cyclohexane 83 168 2 545 1.64 C12H24
Heptyl-cyclohexane 83 182 2 970 1.67 C13H26
Octyl-cyclohexane 83 196 3 375 1.68 C14H28
Nontyl-cyclohexane 83 210 3 755 1.7 C15H30
Decyl-cyclohexane 83 224 4 115 1.71 C16H32
Undecyl-cyclohexane 83 238 4 455 1.73 C17H34
Dodecane-cyclohexane 83 252 4 775 1.75 C18H36
Tridecyl-cyclohexane 83 266 5 080 1.77 C19H38
Tetradecyl-cyclohexane 83 280 5 370 1.79 C20H40
Pentadecyl-cyclohexane 83 294 5 650 1.8 C21H42
Hextadecyl-cyclohexane 83 308 5 915 1.83 C22H44
Heptadecyl-cyclohexane 83 322 6 170 1.85 C23H46
Ethyl-cyclopentane 69 98 410 1.14 C7H14
Propyl-cyclopentane 69 112 830 1.27 C8H16
Butyl-cyclopentane 69 126 1 205 1.46 C9H18
Pentyl-cyclopentane 69 140 1 650 1.54 C10H20
Hexyl-cyclopentane 69 154 2 970 1.67 C11H22
Nonyl-cyclopentane 69 168 3 375 1.68 C12H24
Decyl-cyclopentane 69 182 3 750 1.67 C13H26 382 Chao Shan, Jiaren Ye, Alan Scarlett and Kliti Grice
Figure 5. The relative abundance of alkyl-cyclohexanes for the studied gas condensate samples in ECSSB.
Figure 6. ∑(C1+…+C5)-cyclohexane/∑(C6+…+C16) ratio vs. the MDI for the Figure 7. Alkyl-cyclohexane/alkyl-cyclopentane vs. Pr/Ph for the studied gas studied gas condensate samples in ECSSB. condensate samples in ECSSB.
2.2.3 Diamondoid hydrocarbons for oil correlation and maturity determination of oils that have The diamondoids are cagelike structures usually abundant undergone evaporation (Li et al., 2014). in condensates, organic-rich rocks and coals (Wei et al., 2006; The maturity level of the oil samples estimated by MDI Dahl et al., 1999). They comprise adamantanes with one cage, values (Table 4) ranges from 0.94% to 1.44%, higher than the diamantanes with two cages and triamantanes with three cages maturity based on the conversion of MPI. The discrepancy be- etc. Because of this highly stablecage structure, they resist ther- tween MPI and MDI maturity indicator might due to different mal decomposition, and thus can be used to assess thermal de- phases of petroleum charging, which is commonly seen in this struction of oil (Wei et al., 2007). However, the diamondoids are study area (Ye et al., 2006). Specifically, the methylphenan- typically present at relatively low concentrations are co-elute threnes might be related to the low maturity petroleum charging when analysed by conventional GC-MS (Li et al., 2012). By con- in the early period and the methyldiamantanes might be associ- trast, GC×GC-TOFMS improves the resolution and separation ated with the high maturity petroleum charging in the late period. efficiency of the compounds (Li et al., 2014, 2012). Using GC×GC-TOFMS, 17 adamantanes and 8 diamantanes were de- 2.3 Compound-Specific Stable Carbon Isotopes (CSIA) of tected in all analysed condensate samples (Fig. 8, Table 3). n-Alkanes 13 Two diamondoid hydrocarbon ratios (MAI-methyl adaman- The δ C and δD values of individual n-alkanes (n-C12 to n- 13 tane index (1-MA/(1-MA+2-MA)), and MDI-methyl adaman- C33) are summarized in Figs. 10, 11, respectively. The δ C values tane index (1-MD/(1-MD+3-MD+4-MD)), were first proposed of individual n-alkanes appear to be divided into two categories, by Chen et al. (1996) as maturity parameters responsible for whereas the δ13C values of Neogene condensate samples fall high-mature to over-mature crude oils and source rocks within a narrow range from -27‰ to -31‰. It is worth noting that (Ro=0.9%–2.0%). The follow-upstudy by Li et al. (2012) one Paleogene sample x-7 is significantly more positive in δ13C suggested that adamantane and diamantane indices can be used with its value ranging from -26‰ to -27.5‰. As the maturities of Molecular and Isotopic Characteristics of Mature Condensates from the East China Sea Shelf Basin 383 these samples are similar, the δ13C value difference between the postdepositional processes (Wang and Huang, 2003). A large samples is likely to be the result of the different biosynthetic pre- variation was observed in the δD of individual n-alkanes from cursors. Previous studies have shown that n-alkanes from aquatic the samples analyzed. In general, two types of hydrogen distri- organisms have relatively positive δ13C n-alkanes from terrestrial bution were apparent: Group II includes sample x-5 and x-7, plant (Hayes et al., 1987) which is consistent with the above results and the remaining samples belong to Group I. The isotopic pro- of C27, C28 and C29 compounds analysis that x-7 have higher file shape of Group I appears to be flat in the whole range ex- aquatic organisms that other samples. cept for a slight increasing trend in carbon chain length from The hydrogen isotopic composition of alkanes in crude n-C13 to n-C15. By contrast, the isotopic profile shape of Group oils is controlled by three factors: isotopic compositions of II show noteworthy unimodal distribution with the most posi- biosynthetic precursors, source water δD values, and tive δD value at n-C15.
Figure 8. GC×GC-TOFMS chromatogram of adamantanes identified in the condensate from Well x-3. Numbers refer to compounds are listed in Table 3.
Figure 9. GC×GC-TOFMS chromatogram of diamantanes identified in the condensate from Well x-1. Numbers refer to compounds are listed in Table 3.
Figure 10. Stable carbon isotopic profiles of n-alkanes from ECSSB. Figure 11. Stable hydrogen isotopic profiles of n-alkanes from ECSSB. 384 Chao Shan, Jiaren Ye, Alan Scarlett and Kliti Grice
Table 3 Diamondoid compounds (adamantanes and diamantanes) identified in the condensate by GC×GC-TOFMS analysis from sample x-1
Peak Compound m/z Molecular 1-D retention time (s) 2-D retention Molecular Full name of compound No. abbreviation Weight time (s) formula
1 Admantane 136 136 1 890 2 C10H16 Admantane
2 1-MA 135 150 1 975 1.88 C11H18 1-Methyl-adamantane
3 1, 3-DMA 149 164 2 040 1.77 C12H20 1, 3-Dimethyl-adamantane
4 1, 3, 5-TMA 163 178 2 970 1.89 C13H22 1, 3, 5-Trimethyl-adamantane
5 2-MA 135 150 2 255 2.05 C11H18 2-Methyl-adamantane
6 1, 4-DMA (Z) 149 164 2 300 1.9 C12H20 Cis-1, 4-dimethyl-adamantane
7 1, 4-DMA (E) 149 164 2 330 1.91 C12H20 Trans-1, 4-dimethyl-adamantane
8 1, 3, 6-TMA 163 178 2 990 1.89 C13H22 1, 3, 6-Trimethyl-adamantane
9 1, 2-DMA 149 164 2 440 1.99 C12H20 1, 2-Dimethyl-adamantane
10 1, 3, 4-TMA (Z) 163 178 2 570 1.84 C13H22 Cis-1, 3, 4-trimethyl-adamantane
11 1, 3, 4-TMA (E) 163 178 2 595 1.88 C13H23 Trans-1, 3, 4-trimethyl-adamantane
12 1, 2, 5, 7-TeTMA 177 192 2 925 1.95 C14H24 1, 2, 5, 7-Tetramethyl-adamantane
13 1-EA 135 164 2 545 2 C12H19 1-Ethyl-adamantane
14 1-M-3-EA 149 164 2 610 1.89 C13H22 1-Methyl-3-ethyl-adamantane
15 TMA 163 178 2 625 1.91 C13H23 Trimethyl-adamantane
16 1-E-3, 5-DMA 163 178 2 630 1.78 C13H24 1-Methyl-3, 5-dimethyl-adamantane
17 2-EA 135 164 2 660 2.07 C12H20 2-Ethyl-adamantane
18 Diamantane 188 188 3 655 2.58 C14H20 Diamantane
19 4-MD 187 202 3 705 2.4 C15H22 4-Methyl-diamantane
20 4, 9-DMD 201 216 3 750 2.25 C16H24 4, 9-Dimethyl-diamantane
21 1-MD 187 202 3 845 2.52 C15H22 1-Methyl-diamantane
22 1, 2+2, 4-DMD 201 216 3 850 2.33 C16H24 1, 2-+2, 4-Dimethyl-diamantane
23 4, 8-DMD 21 216 3 865 2.36 C16H24 4, 8-Dimethyl-diamantane
24 3-MD 187 202 3 925 2.57 C15H22 3-Methyl-diamantane
25 3, 4-DMD 201 216 3 965 2.39 C16H24 3, 4-Dimethyl-diamantane
Table 4 Diamondoid maturity parameters for oils from the East China Sea fractionation of individual n-alkanes in crude oil are less than 10‰ Shelf Basin when their Ro are under 1.3% (Tang et al., 2005). The similar maturity of Group I samples rule out maturity as a factor for the Sample MAI MDI Ro (%)=2.438 9MDI+0.436 4 variation within Group I. In addition, the similar sampling loca- x-1 0.49 0.40 1.420 441 549 tion and depth of x-1 and x-2 (Table 1) rule out the source water x-2 0.33 0.39 1.376 343 551 δD values as an influence factor. Therefore, the variation within x-3 0.38 0.33 1.236 447 820 the group is more likely to be affected by postdepositional pro- x-4 0.39 0.41 1.443 653 745 cesses, e.g., evaporation and biodegradation are reported to be x-5 0.41 0.31 1.201 668 844 the main postdepositional processes which affect the δD values x-6 0.43 0.28 1.128 533 917 in n-alkanes. As there is no sign of biodegradation in these sam- x-7 0.37 0.21 0.937 322 104 ples, the effect of vaporization was explored. A vaporization experiment was carried out in a laboratory δD composition of n-alkane from mature oils or sediments with temperature (24±1 °C) and humidity (22%±1%) control. usually remain constant with slight increasing trend with in- The samples were allowed to evaporate in a fume hood with an creasing chain length (Dawson et al., 2007, 2005; Pedentchouk air velocity of 81 ft/min without any agitation for 12 h. Sample et al., 2006; Radke et al., 2005; Schimmelmann et al., 2004). x-5 was used for the evaporization experiments 81% the starting This differential fractionation effect is speculated to be due to compounds were left in the vial after evaporite for 12 h. The δD the combined effect of the greater extent of thermal cracking of values of the residual n-alkanes became more negative by 4‰– higher molecular weight n-alkanes compared to lower molecular 30‰. Therefore, we suggest that the variation of δD values weight homologues, and the generation of isotopically lighter, within Group I samples might be result from different degrees of lower molecular weight compounds. This differential fractiona- evaporation. tion proceeds significantly at higher maturity (Ro>1.0%) (Tang et al., 2005). The depleted δD values with carbon chain length 3 CONCLUSION showed in x-5 and x-7 is likely to be attributed to the biosynthetic Conventional GC-MS analysis of condensate samples from precursors effect. The n-alkane δD values from C3 angiosperm ECSSB was not able to provide sufficient information on depo- tree and C3 angiosperm herb plants are reported to have decreas- sitional environments and source input, and failed to distinguish ing trend with increasing carbon number (Chikaraishi and Nara- these condensates. The combination of GC×GC-TOFMS and oka, 2007). The x-5 and x-7 samples are therefore likely to have CSIA analyses provided additional information concerning the more δD depleted biosynthetic precursors. maturity, environment and postdepositional processes involved Previous studies show that the thermally induced hydrogen in the production of these condensates and enabled them to be Molecular and Isotopic Characteristics of Mature Condensates from the East China Sea Shelf Basin 385 distinguished from each other. These observations indicate that 57. https://doi.org/10.1038/19953 GC×GC-TOFMS and CSIA analyses could be regarded as effec- Dai, L. M., Li, S. Z., Lou, D., et al., 2014. Numerical Modeling of Late tive methods to distinguish condensates with relatively low con- Miocene Tectonic Inversion in the Xihu Sag, East China Sea Shelf Basin, tent of biomarkers and shows potential for further oil-oil corre- China. Journal of Asian Earth Sciences, 86: 25–37. lation or oil-source rock correlation. https://doi.org/10.1016/j.jseaes.2013.09.033 The long side chain substitution cyclohexane and cyclo- Dawson, D., Grice, K., Alexander, R., 2005. Effect on Maturation on the In- pentane, and diamandoids, that can be difficult to resolve and digenous ΔD Signatures of Individual Hydrocarbons in Sediments and identify using the conventional gas chromatography can be more Crude Oils from the Perth Basin (Western Australia). Organic Geochem- readily identified by GC×GC-TOFMS. A new maturity indicator, istry, 36(1): 95–104. https://doi.org/10.1016/j.orggeochem.2004.06.020 C5--cyclohexane/C5+-cyclohexane, and a new oxidation- Dawson, D., Grice, K., Alexander, R., et al., 2007. The Effect of Source and reduction environment indicator, alkyl-cyclohexane/alkyl- Maturity on the Stable Isotopic Compositions of Individual Hydrocar- cyclopentane, was proposed, but further verification is required. bons in Sediments and Crude Oils from the Vulcan Sub-Basin, Timor Discrepancies were observed for the maturity indicators, Sea, Northern Australia. Organic Geochemistry, 38(7): 1015–1038. MPI and MDI. One explanation for this is that the relatively high https://doi.org/10.1016/j.orggeochem.2007.02.018 molecular weight methylphenanthrene was derived mainly from de Rosa, M., Gambacorta, A., Minale, L., et al., 1972. The Formation of Ω- an early petroleum charging with low maturity, and the methyl- Cyclohexyl-Fatty Acids from Shikimate in an Acidophilic Thermophilic diamantane derived mainly from a late petroleum charging with Bacillus. A New Biosynthetic Pathway. Biochemical Journal, 128(4): high maturity. 751–754. https://doi.org/10.1042/bj1280751 The Paleogene samples have significantly more positive Didyk, B. M., Simoneit, B. R. T., Brassell, S. C., et al., 1978. Organic Geo- δ13C values of individual n-alkanes than the Neogene samples. chemical Indicators of Paleoenvironmental Conditions of Sedimentation. δD values of n-alkane could divide samples into two groups; the Nature, 272(5650): 216–222. https://doi.org/10.1038/272216a0 variation between the two groups is likely to be attributed to dif- Fowler, M. G., Abolins, P., Douglas, A. G., 1986. Monocyclic Alkanes in Or- ferent biosynthetic precursors and the variation within the group dovician Organic Matter. Organic Geochemistry, 10(4–6): 815–823. is more likely to be affected by vaporization. https://doi.org/10.1016/s0146-6380(86)80018-3 Freeman, K. H., Hayes, J. M., Trendel, J. M., et al., 1990. Evidence from Carbon ACKNOWLEDGMENTS Isotope Measurements for Diverse Origins of Sedimentary Hydrocarbons. This study was sponsored by the National Science and Nature, 343(6255): 254–256. https://doi.org/10.1038/343254a0 Technology Major Project of China (Nos. 2016ZX05024-002- Grice, K., Mesmay, R. D., Glucina, A., et al., 2008. An Improved and Rapid 003, 2016ZX05027-001-005). The final publication is available 5A Molecular Sieve Method for Gas Chromatography Isotope Ratio + at Springer via https://doi.org/10.1007/s12583-018-1001-3. Mass Spectrometry of n-Alkanes (C8–C30 ). Organic Geochemistry, 39(3): 284–288. https://doi.org/10.1016/j.orggeochem.2007.12.009 REFERENCES CITED Hayes, J. M., Freeman, K. H., Popp, B. N., et al., 1990. Compound-Specific Aguiar, A., Aguiar, H. G. M., Azevedo, D. A., et al., 2011. Identification of Isotopic Analysis: A Novel Tool for Reconstruction of Ancient Biogeo- Methylhopane and Methylmoretane Series in Ceará Basin Oils, Brazil, chemical Processes. Organic Geochemistry, 16(4–6): 1115–1128. Using Comprehensive Two-Dimensional Gas Chromatography Coupled https://doi.org/10.1016/0146-6380(90)90147-r to Time-of-Flight Mass Spectrometry. Energy & Fuels, 25(3): 1060– Hayes, J. M., Takigiku, R., Ocampo, R., et al., 1987. Isotopic Composition 1065. https://doi.org/10.1021/ef1013659 and Probable Origins of Organic Molecules in the Eocene Messel Shale. Aguiar, A., Silva, A. I., Azevedo, D. A., et al., 2010. Application of Comprehen- Nature, 329(6134): 48–51. https://doi.org/10.1038/329048a0 sive Two-Dimensional Gas Chromatography Coupled to Time-of-Flight Inaba, T., Suzuki, N., 2003. Gel Permeation Chromatography for Fractiona- Mass Spectrometry to Biomarker Characterization in Brazilian Oils. Fuel, tion and Isotope Ratio Analysis of Steranes and Triterpanes in Oils. Or- 89(10): 2760–2768. https://doi.org/10.1016/j.fuel.2010.05.022 ganic Geochemistry, 34(4): 635–641. https://doi.org/10.1016/s0146- Bray, E. E., Evans, E. D., 1961. Distribution of N-Paraffins as a Clue to 6380(03)00017-2 Recognition of Source Beds. Geochimica et Cosmochimica Acta, 22(1): Johns, R. B., Belsky, T., McCarthy, E. D., et al., 1966. The Organic Geochem- 2–15. https://doi.org/10.1016/0016-7037(61)90069-2 istry of Ancient Sediments II. Geochimica et Cosmochimica Acta, 30(12): Chen, J. H., Fu, J. M., Sheng, G. Y., et al., 1996. Diamondoid Hydrocarbon 1191–1222. https://doi.org/10.1016/0016-7037(66)90120-7 Ratios: Novel Maturity Indices for Highly Mature Crude Oils. Organic Kikuchi, T., Suzuki, N., Saito, H., 2010. Change in Hydrogen Isotope Com- Geochemistry, 25(3/4): 179–190. https://doi.org/10.1016/s0146- position of N-Alkanes, Pristane, Phytane, and Aromatic Hydrocarbons 6380(96)00125-8 in Miocene Siliceous Mudstones with Increasing Maturity. Organic Ge- Chikaraishi, Y., Naraoka, H., 2007. Δ13C and ΔD Relationships among Three ochemistry, 41(9): 940–946. https://doi.org/10.1016/j.orggeo- n-Alkyl Compound Classes (n-Alkanoic Acid, n-Alkane and n-Alkanol) chem.2010.05.004 of Terrestrial Higher Plants. Organic Geochemistry, 38(2): 198–215. Li, C. F., Zhou, Z., Ge, H., et al., 2009. Rifting Process of the Xihu Depression, https://doi.org/10.1016/j.orggeochem.2006.10.003 East China Sea Basin. Tectonophysics, 472(1–4): 135–147. Cukur, D., Horozal, S., Lee, G. H., et al., 2012. Timing of Trap Formation and https://doi.org/10.1016/j.tecto.2008.04.026 Petroleum Generation in the Northern East China Sea Shelf Basin. Li, M. W., Riediger, C. L., Fowler, M. G., et al., 1997. Unusual Polycyclic Marine and Petroleum Geology, 36(1): 154–163. Aromatic Hydrocarbons in the Lower Cretaceous Ostracode Zone Sedi- https://doi.org/10.1016/j.marpetgeo.2012.04.009 mentary and Related Oils of the Western Canada Sedimentary Basin. Dahl, J. E., Moldowan, J. M., Peters, K. E., et al., 1999. Diamondoid Hydro- Organic Geochemistry, 27(7/8): 439–448. carbons as Indicators of Natural Oil Cracking. Nature, 399(6731): 54– https://doi.org/10.1016/s0146-6380(97)00026-0 386 Chao Shan, Jiaren Ye, Alan Scarlett and Kliti Grice
Li, S. F., Hu, S. Z., Cao, J., et al., 2012. Diamondoid Characterization in Con- 69(18): 4505–4520. https://doi.org/10.1016/j.gca.2004.12.026 densate by Comprehensive Two-Dimensional Gas Chromatography with Triwahyono, S., Abdul, J. A., Shamsuddin, M., et al., 2005. Isomerization of
Time-of-Flight Mass Spectrometry: The Junggar Basin of Northwest Cyclohexane to Methylcyclopentane over Pt/sulfate-ZrO2 Catalyst. 2nd China. International Journal of Molecular Sciences, 13(9): 11399– International Conference on Chemical and Bioprocess Engineering, Sa- 11410. https://doi.org/10.3390/ijms130911399 bah Li, Y., Xiong, Y., Chen, Y., et al., 2014. The Effect of Evaporation on the Con- Ventura, G. T., Raghuraman, B., Nelson, R. K., et al., 2010. Compound Class centration and Distribution of Diamondoids in Oils. Organic Geochemistry, Oil Fingerprinting Techniques Using Comprehensive Two-Dimensional 69: 88–97. https://doi.org/10.1016/j.orggeochem.2014.02.007 Gas Chromatography (GC×GC). Organic Geochemistry, 41(9): 1026– Matthews, D. E., Hayes, J. M., 1978. Isotope-Ratio-Monitoring Gas Chroma- 1035. https://doi.org/10.1016/j.orggeochem.2010.02.014 tography Mass Spectrometry. Analytical Chemistry, 50(11): 1465–1473. Ventura, G. T., Simoneit, B. R. T., Nelson, R. K., et al., 2012. The Composition, https://doi.org/10.1021/ac50033a022 Origin and Fate of Complex Mixtures in the Maltene Fractions of Hydro- Moldowan, J. M., Seifer, W. K., Gallegos, E. J., 1985. Relationship between thermal Petroleum Assessed by Comprehensive Two-Dimensional Gas Petroleum Composition and Depositional Environment of Petroleum Chromatography. Organic Geochemistry, 45: 48–65. Source Rocks. American Association of Petroleum Geologists Bulletin, https://doi.org/10.1016/j.orggeochem.2012.01.002 69: 1255–1268 Wang, T. G., Zhong, N. N., Huo, D. J., et al., 1997. Several Genetic Mecha- Oshima, M., Ariga, T., 1975. Cyclohexy1 Fatty Acids in Acidophilic Ther- nisms of Immature Crude Oils in China. Acta Sedimentologica Sinica, 2: mophihc Bacteria. Journal of Biology Chemistry, 250: 6963–6968 75–83 (in Chinese with English Abstract) Pedentchouk, N., Freeman, K. H., Harris, N. B., 2006. Different Response of Wang, Y., Huang, Y., 2003. Hydrogen Isotopic Fractionation of Petroleum δD Values of n-Alkanes, Isoprenoids, and Kerogen during Thermal Mat- Hydrocarbons during Vaporization: Implications for Assessing Artificial uration. Geochimica et Cosmochimica Acta, 70(8): 2063–2072. and Natural Remediation of Petroleum Contamination. Applied Geo- https://doi.org/10.1016/j.gca.2006.01.013 chemistry, 18(10): 1641–1651. https://doi.org/10.1016/s0883- Powell, T. G., McKirdy, D. M., 1973. Relationship between Ratio of Pristane to 2927(03)00076-3 Phytane, Crude Oil Composition and Geological Environment in Australia. Wei, Z. B, Moldowan, J. M., Jarvie, D. M., et al., 2006. The Fate of Diamon- Nature, 243(124): 37–39. https://doi.org/10.1038/physci243037a0 doids in Coals and Sedimentary Rocks. Geology, 34(12): 1013–1023. Radke, J., Bechtel, A., Gaupp, R., et al., 2005. Correlation between Hydrogen https://doi.org/10.1130/g22840a.1 Isotope Ratios of Lipid Biomarkers and Sediment Maturity. Geochimica Wei, Z. B., Moldowan, J. M., Zhang, S. C., et al., 2007. Diamondoid Hydro- et Cosmochimica Acta, 69(23): 5517–5530. carbons as a Molecular Proxy for Thermal Maturity and Oil Cracking: https://doi.org/10.1016/j.gca.2005.07.014 Geochemical Models from Hydrous Pyrolysis. Organic Geochemistry, Rubinstein, I., Strausz, O. P., 1979. Geochemistry of the Thiourea Adduct 38(2): 227–249. https://doi.org/10.1016/j.orggeochem.2006.09.011 Fraction from an Alberta Petroleum. Geochimica et Cosmochimica Acta, Williams, J. A., Dolcater, D. L., Torkelson, B. E., et al., 1988. Anomalous 43(8): 1387–1392. https://doi.org/10.1016/0016-7037(79)90129-7 Concentrations of Specific Alkylaromatic and Alkylcycloparaff in Com- Schaeffer, P., Poinsot, J., Hauke, V., et al., 1994. Novel Optically Active Hy- ponents in West Texas and Michigan Crude Oils. Organic Geochemistry, drocarbons in Sediments: Evidence for an Extensive Biological Cycliza- 13(1–3): 47–60. https://doi.org/10.1016/0146-6380(88)90024-1 tion of Higher Regular Polyphenols. Angewandte Chemie, 33(11): 1166– Yang, S. C., Hu, S. B., Cai, D. S., et al., 2004. Present-Day Heat Flow, Ther- 1169. https://doi.org/10.1002/anie.199411661 mal History and Tectonic Subsidence of the East China Sea Basin. Ma- Schimmelmann, A., Sessions, A., Boreham, C. J., et al., 2004. D/H Ratios in rine and Petroleum Geology, 21(9): 1095–1105. Terrestrially Sourced Petroleum Systems. Organic Geochemistry, https://doi.org/10.1016/j.marpetgeo.2004.05.007 35(10): 1169–1195. https://doi.org/10.1016/j.orggeochem.2004.05.006 Ye, J. R., Chen, H. H., Chen, J. Y., et al., 2006. Fluid History Analysis in the Seifert, W. K., Moldowan, J. M., 1986. Use of Biological Markers in Petro- Xihu Depression, East China Sea. Natural Gas Industry, 26(9): 40–43 leum Exploration. Methods in Geochemistry and Geophysics, 24: 261– (in Chinese with English Abstract) 290 Zhu, C. S., Zhao, H., Wang, P. R., et al., 2003. The Distribution and Carbon Spiro, B., 1984. Effects of the Mineral Matrix on the Distribution of Geo- Isotopic Composition of Unusual Polycyclic Alkanes in the Cretaceous chemical Markers in Thermally Affected Sedimentary Sequences. Or- Lengshuiwu Formation, China. Organic Geochemistry, 34(7): 1027– ganic Geochemistry, 6: 543–559. https://doi.org/10.1016/0146- 1035. https://doi.org/10.1016/s0146-6380(03)00037-8 6380(84)90077-9 Zhu, Y., Li, Y., Zhou, J., et al., 2012. Geochemical Characteristics of Tertiary Tang, Y. C., Huang, Y. S., Ellis, G. S., et al., 2005. A Kinetic Model for Coal-Bearing Source Rocks in Xihu Depression, East China Sea Basin. Thermally Induced Hydrogen and Carbon Isotope Fractionation of Marine and Petroleum Geology, 35(1): 154–165. Individual n-Alkanes in Crude Oil. Geochimica et Cosmochimica Acta, https://doi.org/10.1016/j.marpetgeo.2012.01.005