Journal of Earth Science  2019, Vol. 30 Issue (2): 376-386   PDF    
0
Molecular and Isotopic Characteristics of Mature Condensates from the East China Sea Shelf Basin Using GC×GC-TOFMS and GC-IRMS
Shan Chao 1,2, Ye Jiaren 1, Scarlett Alan 2, Grice Kliti 2     
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
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

Conventional molecular biomarkers (e.g., steranes, terpanes) from mature or high mature condensate usually cannot be accurately identified in conventional gas chromatography mass spectrometry (GC-MS) analysis due to low content and low signal-to-noise ratio (Chen et al., 1996). Moreover, lots of information in the light hydrocarbons of condensate cannot be effectively revealed using conventional GC-MS analysis due to co-elution (Li et al., 2012). Instrumental methods such as GC×GC-TOFMS and GC-IRMS as in structural investigations of organic geomacro-molecules, have provided a wealth of useful information on condensates. The application of GC×GC-TOFMS has been recognized as a powerful method for solving co-elution problems (Aguiar et al., 2010; Ventura et al., 2010), which can help identify novel compounds and geochemical indicators (Ventura et al., 2012; Aguiar et al., 2011).

The gas chromatography-isotopic ratio/mass spectrometry (GC-IRMS) analytical technique permits continuous flow determination of carbon isotopic values of individual components in complex mixtures of geochemical interest (Freeman et al., 1990; Hayes et al., 1990, 1987; Matthews and Hayes, 1978). This method has been successfully applied to investigate the characteristics of specific components in the saturate hydrocarbon fraction to establish oil-oil and oil-source rock correlations (Inaba and Suzuki, 2003; Li et al., 1997). Many researchers have applied this technique to evaluate the presence of unusual polycyclic alkanes in extracts of source rocks and crude oils where hopanoids and steranes were either absent or present in extremely low abundance samples (Zhu et al., 2003; Schaeffer et al., 1994).

The East China Sea Shelf Basin (ECSSB) is China's largest offshore basin with good exploration prospects. Previous biomarker parameter studies show that the condensates in this area are usually derived from terrestrial organic matter and most formed in the oxidation sedimentary environment (Zhu et al., 2012), however, this information is insufficient for the analysis of oil-source correlation as many source formations are present in similar sedimentary environments which make it hard to distinguish the characteristics of each formation. This study aims to examine the natural compositions of condensates and carbon and hydrogen isotopic values of n-alkanes from the ECSCB using GC×GC-TOFMS and GC-IRMS in order to effectively distinguish their depositional environments and source input. This study benifits further oil-oil correlations or oil-source rock correlations in this area.

1 BACKGROUND AND ANALYZING METHOD 1.1 Geological Setting

The East China Sea Shelf Basin is China's largest offshore basin with an area of about 2.4×105 km2. This basin can be further subdivided into several depressions that formed at different stages of tectonic development. Among them, Xihu depression and Lishui depression have been two focal points for intensive petroleum exploration in ECSSB (Dai et al., 2014; Cukur et al., 2012; Li et al., 2009; Yang et al., 2004). Developed on Pre-Cambrian and Paleozoic metamorphic basement, the sedimentary sequence, with approximately 10 000 m in maximum thickness, consists of fifteen formations (Fig. 1).

Download:
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.

So far, four hydrocarbon-bearing formations have been identified in the Xihu depression, including the Baoshi, Pinghu, Huagang and Longjing formations from bottom to top. Economic oil and gas reservoirs are mostly distributed in the Pinghu and Huagang formations, while gas-bearing intervals identified in Lishui depression are located in the Mingyuefeng Formation.

1.2 Sample Collection, Column Chromatography and 5Å Molecular Sieving

Seven typical condensate samples were collected from the Mingyuefeng, Pinghu and Huagang formations. The physical property characteristics of these condensates are shown in Table 1.

Table 1 Geochemical parameters for selected samples from the East China Sea Shelf Basin

Saturated hydrocarbon, aromatic hydrocarbon and non-hydrocarbon (polar/resin/NSO) fractions were chromatographically separated on a column (25 cm×1 cm i.d.) of neutral alumina over silica gel (120 ℃, 8 h). Saturated fractions were eluted with hexane (40 mL), the aromatic hydrocarbon fraction with a mixture of n-hexane and DCM (40 mL, 7 : 3, v : v) and the polar (NSO) fraction with a mixture of DCM and MeOH (40 mL, 1 : 1, v : v).

Then, the total saturated hydrocarbon fraction was separated into normal alkane and branched/cyclicalkane fractions through 5Å molecular sieves before gas chromatography isotope ratio mass spectrometry (GC-IRMS) analysis to reduce problems with coelution of compounds and to enhance the precision of carbon isotopic analysis. Specifically, samples in cyclohexane was added to a 2 mL vial, 3/4 of it filled with activated 5Å molecular sieves. The vial was capped and placed into an oven (80 ℃, overnight). The resulting solution was then cooled and filtered through a small column of silica plugged with cotton wool (pre-rinsed with cyclohexane) and the sieves were rinsed thoroughly with cyclohexane yielding the branched/cyclic fraction (5Å excluded). The n-alkanes were recovered by dissolution of the sieve with HF (2 mL, 50% w/v), followed by neutralization with saturated sodium bicarbonate solution. The aqueous phase was extracted with n-pentane (1 mL×5). This fraction contained n-alkanes suitable for gas chromatography-isotope ratio mass spectrometry (GC-IRMS) analysis.

1.3 GC-MS Analysis

GC/MS analyses of hydrocarbons were carried out at Curtin University of Technology using an HP 6890 gas chromatograph interfaced to an Agilent 5975 mass-selective detector. Instrument analytical conditions were described in Grice et al. (2008). Biomarkers were identified by their retention times and comparison to published mass spectral data (Freeman et al., 1990).

1.4 GC×GC-TOFMS Analysis

Analyses were performed at Curtin University of Technology using a Pegasus 4D GC×GC-TOFMS instrument LECO (St. Joseph, Michigan, USA) consisting of an Agilent 7890A GC equipped with a liquid nitrogen-cooled pulse-jet modulator. The GC column system consisted of a primary Rxi-5MS (60 m×0.25 mm×0.25 μm) column and a secondary RXi-17MS (1.5 m×0.18 mm×0.18 μm) column. The GC was operated in splitless mode (1 μL of a 5 mg/mL deasphalt and depolar oil injected at 310 ℃) using He as a carrier gas at a constant flow of 1 mL/min. The primary oven temperature program was from 40 ℃ (1 min hold) to 320 ℃ at a rate of 2 ℃/min and the secondary oven temperature held 5 ℃ higher than the primary. The modulator temperature was 15 ℃ higher than the primary oven, and the modulation period was 3 s with a 0.5 s hot-pulse time. The TOF-MS detector signal was sampled at 100 spectra/s with a scan range of 45-550 amu. The transfer line was held at a constant temperature of 315 ℃ and the TOF source temperature was 250 ℃.

1.5 Compound Specific Isotope Analysis (CSIA)

Carbon isotope ratios of n-alkanes and aromatics, compound specific deuterium analysis of n-alkanes were measured using a HP 6890 GC coupled to a Micromass Iso Prime isotope ratio mass spectrometer coupled to a HP 6890 GC at Curtin University of Technology. Details of the instrument and analytical conditions are described in Grice et al. (2008). A standard mixture of n-alkanes (n-C12 to n-C32) was used daily to test the performance of the instrument. For the carbon analysis, average values of at least two analyses of each sample and with a standard deviation of less than 0.4‰ were reported and for the deuterium analysis, average values of at least three analyses of each sample and with a standard deviation of less than 4‰ were reported.

Carbon and hydrogen isotopic compositions are reported using the V-PDB (Vienna PeeDee Belemnite) for carbon and standard δ-notation relative to V-SMOW (Vienna Standard Mean Ocean Water) for hydrogen.

2 RESULTS AND DISCUSSION 2.1 Basic Geochemical Parameters

The basic geochemical parameters for the selected samples are reported in Table 1. The thin liguid chromatography flame ionization detector (TLC-FID) analysis shows that the saturated hydrocarbon accounted for a significant proportion (ca. 80%-89%) of the samples. Specifically, the samples from the Huagang Formation show higher concentrations of saturate hydrocarbons, followed by the samples from the Pinghu Formation, then the Mingyuefeng Formation. The aromatic hydrocarbon, NSO and asphaltene fractions had similar concentrations for these samples.

The total carbon preference index (CPI) from n-C15 to n-C35 (Bray and Evans, 1961) ranges between 1.08 and 1.50, reaching their thermal evolution equilibrium interval. The maturity indicators (i.e., C29 20S/(20S+20R), C29 ββ/(αα+ββ)) also reach their thermal evolution equilibrium intervals, which are 0.52-0.55 and 0.61-0.71 respectively (Seifert and Moldowan, 1986). The methyl-phenanthrene index (MPI) maturity indicator was homogeneous with a range between 0.7 and 1.0.

The light hydrocarbons are abundant in high mature condensate samples analyzed in this study. In general, the low molecular weight hydrocarbons (n-C11-n-C20, LMWH) are more abundant than the higher molecular weight hydrocarbon (> n-C25, HMWH) (Table 1). The samples show low or absent of gammacerane with its index ranging from 0.05 to 0.12, indicating low salinity and water stratification in depositional environment.

The pristane/phytane ratio has been extensively used in organic geochemistry. Pristane and phytane originate from the oxidation (and subsequent decarboxylation) and reduction of the phytol side chain of chlorophyll, respectively. The generation of pristane and phytane is significantly controlled by the oxic or anoxic conditions during sedimentation with aerobic degradation promoting pristane (Didyk et al., 1978; Powell and McKirdy, 1973). The ternary diagram of pristane/phytane (Pr/Ph), Pr/n-C17, Ph/n-C18 distribution has been widely used to investigate the organic facies of parent source. These biomarkers suggest that all the condensate samples were deposited in limnetic facies (Fig. 2a).

Download:
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).

The molecular composition of the sterane series was generally marked by a predominance of C29 sterane over C27 and C28 steranes. C27, C28 and C29 compounds (αααR), implying a strong contribution of higher land plants versus aquatic organisms. However, the sample x-7 is a special case with relatively high concentration of C27, indicate relatively higher aquatic organisms contribution.

2.2 Comprehensive GC×GC-TOFMS Characterization 2.2.1 Bulk composition of condensate

A total of eleven groups of compounds were classified (Fig. 3) including n-alkanes, iso-alkanes, cyclic alkanes and aromatic hydrocarbons. Normal alkanes were in the range of n-C3 to n-C31. Mono-, bi- and tri-cyclic aromatics were also present. Compounds (e.g., alkyl cyclohexane, alkyl cyclopentane and diamondoids) that cannot readily be identified using conventional chromatography were resolved by GC×GC-TOFMS.

Download:
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).
2.2.2 Long chain alkylated cyclic alkanes

As important components of gas condensate (Williams et al., 1988), naphthenic compounds accounted for a large proportion of the condensate samples. Among them, alkyl-cyclohexanes and alkyl-cyclopentanes are two types of biologically characteristic compounds which have been considered as possible indicators of as biosynthetic precursors, maturity, and postdepositional processes (Fowler et al., 1986; Rubinstein and Strausez, 1979). The origins of n-alkylcyclohexanes and methyl-n-alkylcyclohexanes were not able to be determined because both algal and bacterial sources are related to the occurrence of n-alkylcyclohexanes. Thermoacidophilic bacterium Bacillus acidocaldarius which has C17 and C19 n-cyclohexyl acids as dominant fatty acid components (Oshima and Ariga, 1975; de Rosa et al., 1972) was proposed to be responsible for the n-alkyl-cyclohexanes in some sediments. The possibility of n-alkyl-cyclohexanes derived from the cyclisation of straight-chain algal fatty acids has also been proposed (Johns et al., 1966) and demonstrated experimentally by finding both n-alkyl-cyclohexanes and methyl-n-alkyl-cyclohexanes among the products of saturated and unsaturated fatty acids following heating with a clay catalyst (Rubinstein and Strausez, 1979). Also, Spiro (1984) has suggested that alkyl-cyclohexanes may be derived from cracking of the kerogen at high temperatures under the influence of certain minerals (Fowler et al., 1986).

The long side chain substitution cyclohexane and cyclopentane studied previously by GC-MS usually concentrated in the carbon number < C8, with little attention given to > C9 due to their low abundance and vulnerability to the interference of alkane compounds which make tem difficult to be resolved. However, these compounds were much better resolved using GC×GC-TOFMS.

Characterisation by GC×GC-TOFMS analysis of the x-1 condensate (Fig. 3) shows peak markers for the mono alicyclic hydrocarbons are well resolved from the n- and iso-alkanes. The separation of the alkylcyclopentane and alkylcyclohexane series is revealed in Fig. 4 and identification provided in Table 2. Using GC×GC chromatography these two series of compounds can be qualitative and quantitative identified (Ventura et al., 2010).

Download:
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

The relative abundances of each of the alkyl-cyclohexanes are shown in Fig. 5. It can be seen that the relative abundances of alkyl-cyclohexanes decrease dramatically from C1-C5 branched chain alkyl-cyclohexane but much less so for the branched C6-C17 alkyl-cyclohexanes. The ∑(C1+…+C5)-cyclohexane/∑(C6+…+C16)-cyclohexane ratio versus the methyldiamantane index (4-methyldiamantane/(1-+3-+4-methyldiamantane)) (Fig. 6) show positive correlation indicating that the ratio is probably associated with maturity and might be used as an additional maturity parameter. As discussed above, the samples are from similar origin precursors, therefore origin precursors are ruled out as effect factors of the change in ∑(C1+…+C5)-cyclohexane/∑(C6+…+C16) ratio. However, if the ratio is affected by post depositional processes further investigation is required.

Download:
Figure 5. The relative abundance of alkyl-cyclohexanes for the studied gas condensate samples in ECSSB.
Download:
Figure 6. ∑(C1+…+C5)-cyclohexane/∑(C6+…+C16) ratio vs. the MDI for the studied gas condensate samples in ECSSB.

The isomerization of cyclohexane to methylcyclopentane has been discussed previously (Triwahyono et al., 2005) and show that the conversion of cyclohexane is drastically reduced when hydrogen is absence. Hence, compared with cyclopentane, cyclohexane is more stable in oxidation conditions. The Pr/Ph ratio is widely used as an oxidation-reduction environment indicator, and high value of Pr/Ph usually corresponds to strong oxidizing environment. The strong positive correlation between Pr/Ph and alkyl-cyclohexane/alkyl-cyclopentane (Fig. 7) is in agreement with the cyclohexane isomerization, suggesting that the alkyl-cyclohexane/alkyl-cyclopentane ratio can be used as an available oxidation-reduction environment indicator.

Download:
Figure 7. Alkyl-cyclohexane/alkyl-cyclopentane vs. Pr/Ph for the studied gas condensate samples in ECSSB.
2.2.3 Diamondoid hydrocarbons

The diamondoids are cagelike structures usually abundant in condensates, organic-rich rocks and coals (Wei et al., 2006; Dahl et al., 1999). They comprise adamantanes with one cage, diamantanes with two cages and triamantanes with three cages etc. Because of this highly stablecage structure, they resist thermal decomposition, and thus can be used to assess thermal destruction of oil (Wei et al., 2007). However, the diamondoids are typically present at relatively low concentrations are co-elute when analysed by conventional GC-MS (Li et al., 2012). By contrast, GC×GC-TOFMS improves the resolution and separation efficiency of the compounds (Li et al., 2014, 2012). Using GC×GC-TOFMS, 17 adamantanes and 8 diamantanes were detected in all analysed condensate samples (Fig. 8, Table 3).

Download:
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.
Download:
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.
Table 3 Diamondoid compounds (adamantanes and diamantanes) identified in the condensate by GC×GC-TOFMS analysis from sample x-1

Two diamondoid hydrocarbon ratios (MAI-methyl adamantane index (1-MA/(1-MA+2-MA)), and MDI-methyl adamantane index (1-MD/(1-MD+3-MD+4-MD)), were first proposed by Chen et al. (1996) as maturity parameters responsible for high-mature to over-mature crude oils and source rocks (Ro=0.9%-2.0%). The follow-upstudy by Li et al. (2012) suggested that adamantane and diamantane indices can be used for oil correlation and maturity determination of oils that have undergone evaporation (Li et al., 2014).

The maturity level of the oil samples estimated by MDI values (Table 4) ranges from 0.94% to 1.44%, higher than the maturity based on the conversion of MPI. The discrepancy between MPI and MDI maturity indicator might due to different phases of petroleum charging, which is commonly seen in this study area (Ye et al., 2006). Specifically, the methylphenanthrenes might be related to the low maturity petroleum charging in the early period and the methyldiamantanes might be associated with the high maturity petroleum charging in the late period.

Table 4 Diamondoid maturity parameters for oils from the East China Sea Shelf Basin
2.3 Compound-Specific Stable Carbon Isotopes (CSIA) of n-Alkanes

The δ13C and δD values of individual n-alkanes (n-C12 to n-C33) are summarized in Figs. 10, 11, respectively. The δ13C values of individual n-alkanes appear to be divided into two categories, whereas the δ13C values of Neogene condensate samples fall within a narrow range from -27‰ to -31‰. It is worth noting that one Paleogene sample x-7 is significantly more positive in δ13C with its value ranging from -26‰ to -27.5‰. As the maturities of these samples are similar, the δ13C value difference between the samples is likely to be the result of the different biosynthetic precursors. Previous studies have shown that n-alkanes from aquatic organisms have relatively positive δ13C n-alkanes from terrestrial plant (Hayes et al., 1987) which is consistent with the above results of C27, C28 and C29 compounds analysis that x-7 have higher aquatic organisms that other samples.

Download:
Figure 10. Stable carbon isotopic profiles of n-alkanes from ECSSB.
Download:
Figure 11. Stable hydrogen isotopic profiles of n-alkanes from ECSSB.

The hydrogen isotopic composition of alkanes in crude oils is controlled by three factors: isotopic compositions of biosynthetic precursors, source water δD values, and postdepositional processes (Wang and Huang, 2003). A large variation was observed in the δD of individual n-alkanes from the samples analyzed. In general, two types of hydrogen distribution were apparent: Group Ⅱ includes sample x-5 and x-7, and the remaining samples belong to Group Ⅰ. The isotopic profile shape of Group Ⅰ appears to be flat in the whole range except for a slight increasing trend in carbon chain length from n-C13 to n-C15. By contrast, the isotopic profile shape of Group Ⅱ show noteworthy unimodal distribution with the most positive δD value at n-C15.

δD composition of n-alkane from mature oils or sediments usually remain constant with slight increasing trend with increasing chain length (Dawson et al., 2007, 2005; Pedentchouk et al., 2006; Radke et al., 2005; Schimmelmann et al., 2004). This differential fractionation effect is speculated to be due to the combined effect of the greater extent of thermal cracking of higher molecular weight n-alkanes compared to lower molecular weight homologues, and the generation of isotopically lighter, lower molecular weight compounds. This differential fractionation proceeds significantly at higher maturity (Ro > 1.0%) (Tang et al., 2005). The depleted δD values with carbon chain length showed in x-5 and x-7 is likely to be attributed to the biosynthetic precursors effect. The n-alkane δD values from C3 angiosperm tree and C3 angiosperm herb plants are reported to have decreasing trend with increasing carbon number (Chikaraishi and Naraoka, 2007). The x-5 and x-7 samples are therefore likely to have more δD depleted biosynthetic precursors.

Previous studies show that the thermally induced hydrogen fractionation of individual n-alkanes in crude oil are less than 10‰ 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 variation within Group Ⅰ. In addition, the similar sampling location and depth of x-1 and x-2 (Table 1) rule out the source water δD values as an influence factor. Therefore, the variation within the group is more likely to be affected by postdepositional processes, e.g., evaporation and biodegradation are reported to be the main postdepositional processes which affect the δD values in n-alkanes. As there is no sign of biodegradation in these samples, the effect of vaporization was explored.

A vaporization experiment was carried out in a laboratory with temperature (24±1 ℃) and humidity (22%±1%) control. The samples were allowed to evaporate in a fume hood with an air velocity of 81 ft/min without any agitation for 12 h. Sample x-5 was used for the evaporization experiments 81% the starting compounds were left in the vial after evaporite for 12 h. The δD values of the residual n-alkanes became more negative by 4‰-30‰. Therefore, we suggest that the variation of δD values within Group Ⅰ samples might be result from different degrees of evaporation.

3 CONCLUSION

Conventional GC-MS analysis of condensate samples from ECSSB was not able to provide sufficient information on depositional environments and source input, and failed to distinguish these condensates. The combination of GC×GC-TOFMS and CSIA analyses provided additional information concerning the maturity, environment and postdepositional processes involved in the production of these condensates and enabled them to be distinguished from each other. These observations indicate that GC×GC-TOFMS and CSIA analyses could be regarded as effective methods to distinguish condensates with relatively low content of biomarkers and shows potential for further oil-oil correlation or oil-source rock correlation.

The long side chain substitution cyclohexane and cyclopentane, and diamandoids, that can be difficult to resolve and identify using the conventional gas chromatography can be more readily identified by GC×GC-TOFMS. A new maturity indicator, C5--cyclohexane/C5+-cyclohexane, and a new oxidation-reduction environment indicator, alkyl-cyclohexane/alkyl-cyclopentane, was proposed, but further verification is required.

Discrepancies were observed for the maturity indicators, MPI and MDI. One explanation for this is that the relatively high molecular weight methylphenanthrene was derived mainly from an early petroleum charging with low maturity, and the methyldiamantane derived mainly from a late petroleum charging with high maturity.

The Paleogene samples have significantly more positive δ13C values of individual n-alkanes than the Neogene samples. δD values of n-alkane could divide samples into two groups; the variation between the two groups is likely to be attributed to different biosynthetic precursors and the variation within the group is more likely to be affected by vaporization.

ACKNOWLEDGMENTS

This study was sponsored by the National Science and Technology Major Project of China (Nos. 2016ZX05024-002-003, 2016ZX05027-001-005). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-1001-3.


REFERENCES CITED
Aguiar, A., Aguiar, H. G. M., Azevedo, D. A., et al., 2011. Identification of Methylhopane and Methylmoretane Series in Ceará Basin Oils, Brazil, Using Comprehensive Two-Dimensional Gas Chromatography Coupled to Time-of-Flight Mass Spectrometry. Energy & Fuels, 25(3): 1060-1065. DOI:10.1021/ef1013659
Aguiar, A., Silva, A. I., Azevedo, D. A., et al., 2010. Application of Comprehensive Two-Dimensional Gas Chromatography Coupled to Time-of-Flight Mass Spectrometry to Biomarker Characterization in Brazilian Oils. Fuel, 89(10): 2760-2768. DOI:10.1016/j.fuel.2010.05.022
Bray, E. E., Evans, E. D., 1961. Distribution of N-Paraffins as a Clue to Recognition of Source Beds. Geochimica et Cosmochimica Acta, 22(1): 2-15. DOI:10.1016/0016-7037(61)90069-2
Chen, J. H., Fu, J. M., Sheng, G. Y., et al., 1996. Diamondoid Hydrocarbon Ratios: Novel Maturity Indices for Highly Mature Crude Oils. Organic Geochemistry, 25(3/4): 179-190. DOI:10.1016/s0146-6380(96)00125-8
Chikaraishi, Y., Naraoka, H., 2007. Δ13C and ΔD Relationships among Three n-Alkyl Compound Classes (n-Alkanoic Acid, n-Alkane and n-Alkanol) of Terrestrial Higher Plants. Organic Geochemistry, 38(2): 198-215. DOI:10.1016/j.orggeochem.2006.10.003
Cukur, D., Horozal, S., Lee, G. H., et al., 2012. Timing of Trap Formation and Petroleum Generation in the Northern East China Sea Shelf Basin. Marine and Petroleum Geology, 36(1): 154-163. DOI:10.1016/j.marpetgeo.2012.04.009
Dahl, J. E., Moldowan, J. M., Peters, K. E., et al., 1999. Diamondoid Hydrocarbons as Indicators of Natural Oil Cracking. Nature, 399(6731): 54-57. DOI:10.1038/19953
Dai, L. M., Li, S. Z., Lou, D., et al., 2014. Numerical Modeling of Late Miocene Tectonic Inversion in the Xihu Sag, East China Sea Shelf Basin, China. Journal of Asian Earth Sciences, 86: 25-37. DOI:10.1016/j.jseaes.2013.09.033
Dawson, D., Grice, K., Alexander, R., 2005. Effect on Maturation on the Indigenous ΔD Signatures of Individual Hydrocarbons in Sediments and Crude Oils from the Perth Basin (Western Australia). Organic Geochemistry, 36(1): 95-104. DOI:10.1016/j.orggeochem.2004.06.020
Dawson, D., Grice, K., Alexander, R., et al., 2007. The Effect of Source and Maturity on the Stable Isotopic Compositions of Individual Hydrocarbons in Sediments and Crude Oils from the Vulcan Sub-Basin, Timor Sea, Northern Australia. Organic Geochemistry, 38(7): 1015-1038. DOI:10.1016/j.orggeochem.2007.02.018
de Rosa, M., Gambacorta, A., Minale, L., et al., 1972. The Formation of Ω-Cyclohexyl-Fatty Acids from Shikimate in an Acidophilic Thermophilic Bacillus. A New Biosynthetic Pathway. Biochemical Journal, 128(4): 751-754. DOI:10.1042/bj1280751
Didyk, B. M., Simoneit, B. R. T., Brassell, S. C., et al., 1978. Organic Geochemical Indicators of Paleoenvironmental Conditions of Sedimentation. Nature, 272(5650): 216-222. DOI:10.1038/272216a0
Fowler, M. G., Abolins, P., Douglas, A. G., 1986. Monocyclic Alkanes in Ordovician Organic Matter. Organic Geochemistry, 10(4-6): 815-823. DOI:10.1016/s0146-6380(86)80018-3
Freeman, K. H., Hayes, J. M., Trendel, J. M., et al., 1990. Evidence from Carbon Isotope Measurements for Diverse Origins of Sedimentary Hydrocarbons. Nature, 343(6255): 254-256. DOI:10.1038/343254a0
Grice, K., Mesmay, R. D., Glucina, A., et al., 2008. An Improved and Rapid 5A Molecular Sieve Method for Gas Chromatography Isotope Ratio Mass Spectrometry of n-Alkanes (C8-C30+). Organic Geochemistry, 39(3): 284-288. DOI:10.1016/j.orggeochem.2007.12.009
Hayes, J. M., Freeman, K. H., Popp, B. N., et al., 1990. Compound-Specific Isotopic Analysis: A Novel Tool for Reconstruction of Ancient Biogeochemical Processes. Organic Geochemistry, 16(4-6): 1115-1128. DOI:10.1016/0146-6380(90)90147-r
Hayes, J. M., Takigiku, R., Ocampo, R., et al., 1987. Isotopic Composition and Probable Origins of Organic Molecules in the Eocene Messel Shale. Nature, 329(6134): 48-51. DOI:10.1038/329048a0
Inaba, T., Suzuki, N., 2003. Gel Permeation Chromatography for Fractionation and Isotope Ratio Analysis of Steranes and Triterpanes in Oils. Organic Geochemistry, 34(4): 635-641. DOI:10.1016/s0146-6380(03)00017-2
Johns, R. B., Belsky, T., McCarthy, E. D., et al., 1966. The Organic Geochemistry of Ancient Sediments Ⅱ. Geochimica et Cosmochimica Acta, 30(12): 1191-1222. DOI:10.1016/0016-7037(66)90120-7
Kikuchi, T., Suzuki, N., Saito, H., 2010. Change in Hydrogen Isotope Composition of N-Alkanes, Pristane, Phytane, and Aromatic Hydrocarbons in Miocene Siliceous Mudstones with Increasing Maturity. Organic Geochemistry, 41(9): 940-946. DOI:10.1016/j.orggeochem.2010.05.004
Li, C. F., Zhou, Z., Ge, H., et al., 2009. Rifting Process of the Xihu Depression, East China Sea Basin. Tectonophysics, 472(1-4): 135-147. DOI:10.1016/j.tecto.2008.04.026
Li, M. W., Riediger, C. L., Fowler, M. G., et al., 1997. Unusual Polycyclic Aromatic Hydrocarbons in the Lower Cretaceous Ostracode Zone Sedimentary and Related Oils of the Western Canada Sedimentary Basin. Organic Geochemistry, 27(7/8): 439-448. DOI:10.1016/s0146-6380(97)00026-0
Li, S. F., Hu, S. Z., Cao, J., et al., 2012. Diamondoid Characterization in Condensate by Comprehensive Two-Dimensional Gas Chromatography with Time-of-Flight Mass Spectrometry: The Junggar Basin of Northwest China. International Journal of Molecular Sciences, 13(9): 11399-11410. DOI:10.3390/ijms130911399
Li, Y., Xiong, Y., Chen, Y., et al., 2014. The Effect of Evaporation on the Concentration and Distribution of Diamondoids in Oils. Organic Geochemistry, 69: 88-97. DOI:10.1016/j.orggeochem.2014.02.007
Matthews, D. E., Hayes, J. M., 1978. Isotope-Ratio-Monitoring Gas Chromatography Mass Spectrometry. Analytical Chemistry, 50(11): 1465-1473. DOI:10.1021/ac50033a022
Moldowan, J. M., Seifer, W. K., Gallegos, E. J., 1985. Relationship between Petroleum Composition and Depositional Environment of Petroleum Source Rocks. American Association of Petroleum Geologists Bulletin, 69: 1255-1268.
Oshima, M., Ariga, T., 1975. Cyclohexy1 Fatty Acids in Acidophilic Thermophihc Bacteria. Journal of Biology Chemistry, 250: 6963-6968.
Pedentchouk, N., Freeman, K. H., Harris, N. B., 2006. Different Response of δD Values of n-Alkanes, Isoprenoids, and Kerogen during Thermal Maturation. Geochimica et Cosmochimica Acta, 70(8): 2063-2072. DOI:10.1016/j.gca.2006.01.013
Powell, T. G., McKirdy, D. M., 1973. Relationship between Ratio of Pristane to Phytane, Crude Oil Composition and Geological Environment in Australia. Nature, 243(124): 37-39. DOI:10.1038/physci243037a0
Radke, J., Bechtel, A., Gaupp, R., et al., 2005. Correlation between Hydrogen Isotope Ratios of Lipid Biomarkers and Sediment Maturity. Geochimica et Cosmochimica Acta, 69(23): 5517-5530. DOI:10.1016/j.gca.2005.07.014
Rubinstein, I., Strausz, O. P., 1979. Geochemistry of the Thiourea Adduct Fraction from an Alberta Petroleum. Geochimica et Cosmochimica Acta, 43(8): 1387-1392. DOI:10.1016/0016-7037(79)90129-7
Schaeffer, P., Poinsot, J., Hauke, V., et al., 1994. Novel Optically Active Hydrocarbons in Sediments: Evidence for an Extensive Biological Cyclization of Higher Regular Polyphenols. Angewandte Chemie, 33(11): 1166-1169. DOI:10.1002/anie.199411661
Schimmelmann, A., Sessions, A., Boreham, C. J., et al., 2004. D/H Ratios in Terrestrially Sourced Petroleum Systems. Organic Geochemistry, 35(10): 1169-1195. DOI:10.1016/j.orggeochem.2004.05.006
Seifert, W. K., Moldowan, J. M., 1986. Use of Biological Markers in Petroleum Exploration. Methods in Geochemistry and Geophysics, 24: 261-290.
Spiro, B., 1984. Effects of the Mineral Matrix on the Distribution of Geochemical Markers in Thermally Affected Sedimentary Sequences. Organic Geochemistry, 6: 543-559. DOI:10.1016/0146-6380(84)90077-9
Tang, Y. C., Huang, Y. S., Ellis, G. S., et al., 2005. A Kinetic Model for Thermally Induced Hydrogen and Carbon Isotope Fractionation of Individual n-Alkanes in Crude Oil. Geochimica et Cosmochimica Acta, 69(18): 4505-4520. DOI:10.1016/j.gca.2004.12.026
Triwahyono, S., Abdul, J. A., Shamsuddin, M., et al., 2005. Isomerization of Cyclohexane to Methylcyclopentane over Pt/sulfate-ZrO2 Catalyst. 2nd International Conference on Chemical and Bioprocess Engineering, Sabah
Ventura, G. T., Raghuraman, B., Nelson, R. K., et al., 2010. Compound Class Oil Fingerprinting Techniques Using Comprehensive Two-Dimensional Gas Chromatography (GC×GC). Organic Geochemistry, 41(9): 1026-1035. DOI:10.1016/j.orggeochem.2010.02.014
Ventura, G. T., Simoneit, B. R. T., Nelson, R. K., et al., 2012. The Composition, Origin and Fate of Complex Mixtures in the Maltene Fractions of Hydrothermal Petroleum Assessed by Comprehensive Two-Dimensional Gas Chromatography. Organic Geochemistry, 45: 48-65. DOI:10.1016/j.orggeochem.2012.01.002
Wang, T. G., Zhong, N. N., Huo, D. J., et al., 1997. Several Genetic Mechanisms of Immature Crude Oils in China. Acta Sedimentologica Sinica, 2: 75-83.
Wang, Y., Huang, Y., 2003. Hydrogen Isotopic Fractionation of Petroleum Hydrocarbons during Vaporization: Implications for Assessing Artificial and Natural Remediation of Petroleum Contamination. Applied Geochemistry, 18(10): 1641-1651. DOI:10.1016/s0883-2927(03)00076-3
Wei, Z. B, Moldowan, J. M., Jarvie, D. M., et al., 2006. The Fate of Diamondoids in Coals and Sedimentary Rocks. Geology, 34(12): 1013-1023. DOI:10.1130/g22840a.1
Wei, Z. B., Moldowan, J. M., Zhang, S. C., et al., 2007. Diamondoid Hydrocarbons as a Molecular Proxy for Thermal Maturity and Oil Cracking: Geochemical Models from Hydrous Pyrolysis. Organic Geochemistry, 38(2): 227-249. DOI:10.1016/j.orggeochem.2006.09.011
Williams, J. A., Dolcater, D. L., Torkelson, B. E., et al., 1988. Anomalous Concentrations of Specific Alkylaromatic and Alkylcycloparaff in Components in West Texas and Michigan Crude Oils. Organic Geochemistry, 13(1-3): 47-60. DOI:10.1016/0146-6380(88)90024-1
Yang, S. C., Hu, S. B., Cai, D. S., et al., 2004. Present-Day Heat Flow, Thermal History and Tectonic Subsidence of the East China Sea Basin. Marine and Petroleum Geology, 21(9): 1095-1105. DOI:10.1016/j.marpetgeo.2004.05.007
Ye, J. R., Chen, H. H., Chen, J. Y., et al., 2006. Fluid History Analysis in the Xihu Depression, East China Sea. Natural Gas Industry, 26(9): 40-43.
Zhu, C. S., Zhao, H., Wang, P. R., et al., 2003. The Distribution and Carbon Isotopic Composition of Unusual Polycyclic Alkanes in the Cretaceous Lengshuiwu Formation, China. Organic Geochemistry, 34(7): 1027-1035. DOI:10.1016/s0146-6380(03)00037-8
Zhu, Y., Li, Y., Zhou, J., et al., 2012. Geochemical Characteristics of Tertiary Coal-Bearing Source Rocks in Xihu Depression, East China Sea Basin. Marine and Petroleum Geology, 35(1): 154-165. DOI:10.1016/j.marpetgeo.2012.01.005