Abstract

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Twenty crude oil samples were obtained from the Gudong Oilfield and their organic geochemical characteristics were analyzed. The oil samples were classified into three families by hierarchical cluster analysis and principal component analysis based on 13 source-related and depositional environment-related biomarker parameters. Oils in family I have low ratios of C₁₉/C₂₃ tricyclic terpanes and C₂₄ tetracyclic terpane/C₂₆ tricyclic terpanes, and relatively high ratios of steranes/hopanes and C₃₀ 4-methylsteranes/ααα20R C₂₉ sterane, thus indicating that microalgae were the dominant organic matter input for the source rocks of family I. The gammacerane/C₃₀ hopane ratios are higher than that of family II and family III, whereas the C₃₅/C₃₄ homohopane ratios are lower, thus indicating a suboxic, brackish water environment for the source rocks. The inferred source rock is the first member of the Shahejie Formation in the Huanghekou Sag. Family II is characterized by high ratios of C₁₉/C₂₃ tricyclic terpanes and C₂₄ tetracyclic terpane/C₂₆ tricyclic terpanes but relatively low ratios of steranes/hopanes and C₂₇/C₂₉ αααR steranes. These findings suggest that the original organic matter of the source rocks had a greater contribution from terrigenous higher plants than from microalgal. The relatively low ratios of gammacerane/C₃₀ hopane and C₃₅/C₃₄ homohopane suggest that the source rocks were deposited in an oxic environment with a low salinity, thus corresponding to the Dongying Formation in the Huanghekou Sag. Family III oils have high C₂₇/C₂₉ ααααR steranes ratios and low C₃₀ 4-methylsteranes/ααα20R C₂₉ ratios, which indicate the contribution of microalgae (especially zooplankton algae) to the source rocks. The relatively high abundance of C₃₅ homohopane and low gammacerane/C₃₀ hopane ratios suggest a weakly reducing condition with low salinity, which is in accordance with the third member of the Shahejie Formation in the Gunan Sag. The C₃₁S/(S + R) homohopane ratios imply that oil samples in this study are in the mature stage, although the ratios of C₂₉20S/(20S + 20R) and C₂₉ββ/(αα + ββ) steranes suggest that the maturity of family II is higher than that of family I and family III.

1. Introduction

The Zhanhua Depression is one of the most petroliferous petroleum areas in the Bohai Bay Basin.¹ The Gudong Oilfield is a complex petroleum accumulation belt located in the northeast region of the Zhanhua Depression and in the south area of the Zhuangxi–Gudong buried–hill draping structural belt.² The Zhanhua Depression is a typical Tertiary rifted basin superimposed on a negative inverted basin developed during the Late Jurassic to Early Cretaceous.³ The Gudong Oilfield locates in the northeast region of the Zhanhua Depression and is surrounded by three sags: the Gunan Sag to the south, Wuhaozhuang Sag to the northwest, and Huanghekou Sag northeast (Figure ​Figure11c).⁴ However, source rocks in the Wuhaozhuang Sag are characterized by a shallow burial depth, low richness, and poor oil generation capacity.⁵ Previous geochemical studies have suggested that hydrocarbons in the Gudong Oilfield were derived from source rocks in the Gunan Sag and Huanghekou Sag.⁶ The Paleogene Shahejie (Es), Dongying (Ed), and Neogene Guantao (Ng) formations are the main oil-bearing strata that have been identified to date.² The oil-bearing area covers 16.39 km², with proven oil reserves of 4619 × 10⁴ t.⁷ The major sedimentary facies in the Gudong Oilfield are lacustrine and deltaic deposits.⁸

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Figure 1

Study area and sample locations. (a) Location of the Bohai Bay Basin in China, (b) location of the study area in the Zhanhua Depression, China, and (c) distribution of crude oil samples in the Gudong Oilfield.

The Gudong Oilfield is a complex hydrocarbon accumulation zone. The regional tectonic characteristics, the controlling factors of reservoir formation, and the type of reservoir have been well studied.^2,7,9 However, the organic geochemical characteristics of crude oils in the Gudong Oilfield have not yet been investigated, and the differences between oils are unknown. Some oils exhibit different degrees of biodegradation, and the relationship between the distribution of biodegraded oils and their locations is unclear. This makes it more difficult to analyze the organic geochemical characteristics and identify the oil sources. In this study, the biomarker characteristics and affinities of crude oil samples collected from the Gudong Oilfield were examined. The oil–oil correlation is a common application in petroleum geochemistry,¹⁰ and it is known that more appropriate parameters make the research results more reliable.¹¹ Chemometrics have a unique advantage for the comprehensive consideration of the influence of multiple parameters on the oil–oil correlation.¹² In this study, chemometric methods, including hierarchical cluster analysis (HCA) and principal component analysis (PCA), were employed to classify and identify the affinities of crude oil samples based on selected biomarker parameters.

2. Geological Setting

The Zhanhua Depression has undergone multicyclic tectonic movement and evolution, which was characterized by multistage hydrocarbon generation and multistage petroleum accumulation.^13,14 The Gudong Oilfield possesses abundant oil and gas resources and has complex oil source relationships due to the characteristics of multiple oil sources and a variety of reservoir types.¹³ In the Gunan Sag, the major source rocks are the third member of the Shahejie (Es₃) Formation and the first member of the Shahejie (Es₁) Formation.² The Es₃ Formation is a high-quality source rock consisting of gray and dark-gray mudstone, oil shale, and calcilutite, which were deposited in a freshwater–brackish environment.^6,8 The Es₁ Formation comprises mudstone and oil shale that were deposited in a brackish–saline lacustrine environment.⁶ The total organic carbon (TOC) contents of the Es₃ and Es₁ members have been found to range from 0.85 to 6.08% and 2.8 to 11.6%, respectively.² In the Huanghekou Sag, the primary source rocks are the Es₃, Es₁, and Ed₃ members.^15,16 The Es₃ member consists of dark-gray mudstone and calcareous shale, which were deposited in reducing and freshwater environments.¹⁵ The Es₁ member was deposited in a saline reducing environment and consists of dark-gray mudstone, calcareous shale, dolomite limestone, and bioclastics.¹⁶ The third member of the Dongying (Ed₃) Formation was deposited in a weakly oxic to weakly reducing environment.¹⁶ The TOC contents of the Es₃, Es₁, and Ed₃ members have been found to range from 0.33 to 9.19%, 0.11 to 2.67%, and 0.63 to 1.62%, respectively.¹⁶ The major organic input for the source rock in the Gunan Sag was microalgae.⁸ Source rocks in the Huanghekou Sag contain a large amount of terrestrial bio-inputs.¹⁶

The oils in the Gudong Oilfield have been charged at least twice.¹⁷ The first stage was in the Ed Formation; the petroleum accumulated during this stage underwent severe biodegradation.¹⁸ The reservoirs were severely destroyed by later tectonic movements, with only biodegradation products, resin, and asphaltene remaining.¹⁸ The late accumulation stage in the Minhuazhen Formation was important, from which the petroleum reserves mainly originated.^7,18 During this stage, the tectonic setting was relatively stable and provided good geologic conditions for petroleum preservation.¹⁷ Most oils accumulated during this stage were buried at a shallow depth and underwent slight biodegradation.^17,18

3. Samples and Methods

4. Results

4.2. GC Fingerprints

N-alkanes and isoprenoids are widely used in petroleum geochemistry to investigate the depositional environment and origin of deposited organic matter.^23,24 However, n-alkanes are the first compound class consumed by microbes during biodegradation.²³ In most investigated oils from the Gudong Oilfield, n-alkanes exhibited unique distributional and compositional characteristics due to biodegradation. Some oils in this study contain a full suite of n-alkanes in the carbon number range of C₁₀–C₃₅₊ (maximum at C₂₁–C₂₃) and no obvious even/odd predominance (Figure ​Figure22a). The Pr/Ph ratios range from 0.67 to 2.52 (Table 1), which suggests significant differences between the depositional environment and organic matter input of the source rocks for the crude oil samples.^25,26 In contrast, varying degrees of biodegradation are present in other oils, and n-alkanes are destroyed and contain significant unresolved complex mixture. The biodegradation extent is between level 0 and level 4;²⁷ the hopanes, steranes, and aromatic fractions are well preserved and could be used to reveal the paleoenvironment, material sources of organic matter, and maturity of oils.

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Figure 2

Representative gas chromatograms of whole oil (GC) showing the distributions of paraffins (a), and representative m/z 191 (b) and m/z 217 (c) mass chromatograms showing the sterane and triaromatic steroid distributions, respectively. Note: Pr = pristane; Ph = phytane; IS = internal standard; TT = tricyclic terpane; C₂₄Te = C₂₄ tetracyclic terpane; H = hopane or homohopane; Ts = C₂₇18α(H) 22,29,30-trisnorhopane; Tm = C₂₇17α(H) 22,29,30-trisnorhopane; C₂₉Ts = 18α(H)-30-norneohopane; C₂₉H = C₂₉17α(H),21β(H)-30-norhopane; C₃₀H = 17α (H),21β (H)-hopane.

4.3. Biomarker Composition and Distribution

4.3.1. Terpenoids

Terpenoid hydrocarbons were measured using m/z 191 mass chromatograms, and the terpane biomarker distributions for representative samples are shown in Figure ​Figure22b. The relative abundance of tricyclic terpanes (TT) is low, with C₂₃TT being dominant, followed by C₁₉TT or C₂₁TT in the oil samples (Figure ​Figure22b). The ratios of C₁₉/C₂₃TT and C₂₆/C₂₅TT range from 0.21 to 0.58 and 1.02 to 1.36 (Table 2), respectively. The extended tricyclic terpane ratios (ETR) range from 0.12 to 0.29. Pentacyclic triterpane chromatograms occur in significant abundance and are dominated by 17α(H),21β(H) hopanes (C₃₀H) in all samples; the C₃₁R/C₃₀H ratios vary from 0.21 to 0.35. In most samples, the relative abundance of C₃₁–C₃₅ homohopane generally decreases with increasing carbon member, while the relative intensities of C₃₄H are slightly lower than those of C₃₅H in samples GDN27 and GDN28. Gammacerane (Ga) is present in all samples, although the concentrations vary significantly. The ratios of C₃₄H/C₃₅H and Ga/C₃₀H range from 0.53 to 1.05 and 0.04 to 0.32, respectively. 17α(H)-trinorhopane (Tm) and 18α(H)-trinorneohopane (Ts) show low relative abundances, and the Ts/Tm ratios range from 0.57 to 1.61 (Table 1, Figure ​Figure66b), thus indicating that there were significant differences among different source rock members in terms of the depositional environment and maturation.²⁸

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Figure 6

Correlations of the maturation-related ratios of (a) C₂₉ααα20S/(20S + 20R) vs C₂₉ββ/(αα + ββ), (b) Ts/Tm vs C₂₉Ts/C₂₉H, and (c) MDR vs MPI 1, showing the variations in maturity of the crude oils in different families. Compound symbols are described in the footnotes of Tables 2 and 3 and in Figures ​Figures22 and ​and33.

4.3.2. Steroids

The sterane mass chromatograms (m/z 217) for representative samples are shown in Figure ​Figure22c. In most samples, the concentrations of pregnanes and diasteranes are low relative to regular steranes. The distributions of C₂₇ααα(20R), C₂₈ααα(20R), and C₂₉ααα(20R) regular steranes presented an asymmetric “V” type (Figure ​Figure22c). The relative abundance of C₂₇–C₂₉ regular steranes is calculated using the peak areas. The normalized relative abundances of %C₂₇, %C₂₈, and %C₂₉ ααα20R are 26–37, 10–32, and 35–53%, respectively. The C₂₇/C₂₉ ααα(20R) ratios range from 0.57 to 1.25. C₃₀ 4α-methyl-24-ethylcholestanes (C₃₀-4M steranes) were found in all samples with varying abundance. The ratios of C₃₀-4M to C₂₉ regular steranes (C₃₀-4M/ααα20R C₂₉) range from 0.21 to 0.99 (Table 2), reflecting a varying contribution of microalgae.²⁹ As maturity-related parameters, the C₂₉20S/(20S + 20R) and C₂₉ββ/(αα + ββ) ratios range from 0.38 to 0.55 and 0.32 to 0.49, respectively, thereby indicating that the oil samples were generated during the early-mature–mature stage.²⁷

4.4. Oil–Oil Correlation Based on Chemometrics

The geochemical characteristics of the oil samples exhibited considerable variation because they originated from different source rocks. To improve our understanding of the genetic relationships among the crude oil samples, chemometric methods were applied to classify them and extract the leading factors.¹⁹ Chemometric methods can improve the calculation accuracy of genetic relationships by extracting useful information from a large amount of geochemical data.³⁰

HCA of 13 source-related and depositional environment-related parameters produced a dendrogram that roughly classified the crude oil samples. An oil family is defined as a group of oils from source rocks deposited in a similar environment and with similar organic matter input.¹⁹ The oils were divided into three families with a similarity coefficient of 0.55 (Figure ​Figure33). The scores plot of PCA represents the best classification of samples from n-dimensional space into two-dimensional or three-dimensional space by extracting the relationship between parameters.^10,21 Based on the score plot for the samples, we obtained their classification. In addition, based on the correlation coefficient between the principal component (PC) and each variable parameter, we determined the main controlling variables.³¹ As shown in Figure ​Figure44, the PCA scores and loading plots were employed to select the biomarker parameters obtained from all oil biomarker data from the Gudong Oilfield. The results showed that PC1, PC2, and PC3 accounted for 81.1, 8.4, and 4.0% of the total variance in the original dataset, respectively. The oil samples from the Gudong Oilfield were classified into three families on the scores plot of PCA, which was consistent with the HCA results. As shown in Figure ​Figure55, the loading on PC1 is dominated by a positive correlation with the ratios of S/H and C₃₀H-4M/ααα20R C₂₉ sterane. These parameters are mainly related to the organic matter input of source rocks. The loadings on PC2 mainly exhibit a positive correlation with the ETR and C₃₅/C₃₄H ratios, which are associated with the depositional environment of source rocks.

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Figure 3

Separation of the Gudong oils into three different genetic oil families based on the hierarchical cluster analysis of 13 source- and deposition environment-related biomarker ratios.

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Figure 4

Three-dimensional view of principal component analysis identifying oil genetics of the Gudong Oilfield. PC 1, PC2, and PC3 are principal components accounting for 81.1, 8.4, and 4.0% of the variance in the data, respectively.

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Figure 5

Factor loading of the selected biomarker variables.

5. Discussion

5.1. Geochemical Characterization of Oil Families

5.1.1. Thermal Maturity

The thermal maturity of these oils was assessed by the saturate and aromatic biomarker parameters. Previous studies demonstrated that the C₃₁S/(S + R) homohopane ratio is highly specific for immature to early oil generation.^10,23 The values are within the range of 0.56–0.60, which are close to the equilibrium values (0.57–0.62), thus suggesting that the oil samples are mature. However, the commonly used maturity parameters, i.e., C₂₉ 20S/(20S + 20R) and C₂₉ ββ/(αα + ββ) steranes,²⁷ indicate that the maturity of family II is higher than that of family I and family III. This is supported by the fact that the ratios of C₂₉ 20S/(20S + 20R) and C₂₉ ββ/(αα + ββ) of family II (0.49–0.55 and 0.43–0.49, respectively, Figure ​Figure66a) are higher than those of family I (0.38–0.45 and 0.32–0.41, respectively) and family III (0.40–0.41 and 0.37–0.39, respectively). The aromatic biomarker parameters of dibenzothiophene (DBT) isomers and alkyl phenanthrenes (MP) can also be used to evaluate maturity.³² As shown in Figure ​Figure66c, the MDR (4-MDBT/1-MDBT) and methyl phenanthrene indicates (MPI 1) values range from 3.01 to 4.64 and from 0.63 to 0.90, respectively. These exceeded those of family I and III, thus agreeing with variation trend of the ratios of C₂₉20S/(20S + 20R) and C₂₉ββ/(αα + ββ) steranes. The equivalent vitrinite reflectance (%Rc = 0.6 × MPI 1 + 0.4) values are between 0.53 and 0.94, which indicates that all of the oil samples are at the mature stage.^23,32 However, the maturity of family II (%Rc range from 0.78 to 0.85) is higher than that of family I (%Rc range from 0.53 to 0.79) and family III (%Rc range from 0.72 to 0.76). The maturity of oil can also be reflected by hopane parameters, for example, the Ts/Tm and C₂₉Ts/C₂₉ hopane ratios both increase with maturity.^23,33 Family II has higher Ts/Tm and C₂₉Ts/C₂₉ hopane ratios (1.35–1.61 and 0.47–0.59, respectively, Figure ​Figure66b) in comparison to family I. However, for family III, the Ts/Tm and C₂₉Ts/C₂₉ hopane ratios are abnormally low (0.57–0.61 and 0.19–0.25, respectively). The reversal of the Ts/Tm and C₂₉Ts/C₂₉ hopane ratios might be associated with the depositional environment and some specific lithologies, for instance, carbonates or evaporitic source rocks.³³

5.1.2. Sources of Organic Matter

The relative amounts of C₂₇–C₂₉ regular steranes are commonly employed to provide information on organic facies and organic matter input.³⁴ In general, C₂₇, C₂₈, and C₂₉ sterols are associated with zooplankton, phytoplankton, and higher plants, respectively;^23,35,36 however, brown algae and many species of green algae can also generate C₂₉ sterols.³⁷ The average C₂₇/C₂₉ ααααR sterane ratios are low in family I and family II (average at 0.79 and 0.70, respectively). For family III, the C₂₇/C₂₉ αααR sterane ratios are relatively high (0.88–1.01). The high abundance of C₁₉TT and C₂₄ tetracyclic terpane (C₂₄Tet) can indicate the input of terrigenous organic matter.^23,38 The oil samples in family I exhibited low ratios of C₁₉/C₂₃TT (0.09–0.24) and C₂₄Te/C₂₆TT (0.56–1.03, average of 0.69, Figure ​Figure77a). Oils in family II are characterized by relatively high ratios of C₁₉/C₂₃TT (0.42–0.58) and C₂₄Tet/C₂₆TT (0.89–1.15). Further, C₃₀-4M steranes are prominent in some special microalgae such as prymnesiophyte algae.²⁹ A low S/H ratio is more indicative of terrigenous and/or microbially reworked organic matter.³⁸⁻⁴⁰ As shown in Figure ​Figure77b, the S/H and C₃₀-4M/ααα20R C₂₉ ratios in family I are relatively high, in the ranges of 0.38–0.45 and 0.66–0.99, respectively. The S/H values of family II and family III are lower than that of family I (average of 0.25 and 0.26, respectively). The C₃₀-4M/ααα20R C₂₉ ratios of family II (0.21–0.65) are lower than those of family I and family III (0.73–0.88). Collectively, these parameters suggest that microalgae are the dominant organic matter source for the source rock of oils in family I, and that C₂₉ αααR steranes are possibly from brown algae or special types of green algae. For family II, parameters suggest a large contribution of terrigenous organic matter input and lower contribution of microalgae to the source rock. For family III, microalgae (especially zooplankton algae) make a large contribution to the organic matter of the source rock.

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Figure 7

Correlations of the source-related ratios of (a) C₁₉/C₂₃TT vs C₂₄Te/C₂₆TT and (b) C₃₀-4M/ααα 20R C₂₉ sterane vs S/H, showing the interpreted differences in organic matter input of the source rocks for the crude oils in different families. Compound symbols are described in the footnote of Table 2 and in Figure ​Figure22.

5.1.3. Depositional Environment

All three families can be inferred to be derived from lacustrine source rocks, as supported by high C₂₆/C₂₅TT (1.02–1.36) and low C₃₁R/C₃₀H (0.21–0.28) ratios.²³ High abundance of gammacerane is usually associated with a high salinity or strong water column stratification.^41,42 The Ga/C₃₀H ratios are between 0.04 and 0.32 (Table 2, Figure ​Figure88a), which suggests variable water conditions of the source rocks. The salinity (or water column stratification) of the source rock for family I (Ga/C₃₀H ratios range from 0.18 to 0.32) is higher than those of family II and family III (Ga/C₃₀H ratios are in the ranges of 0.04–0.13 and 0.09–0.17, respectively). The ETR can be used to reflect the salinity during sediment deposition.^23,43 The ETR values of the three families were 0.22–0.29, 0.14–0.18, and 0.12–0.15, respectively, showing the same variation tendency as the Ga/C₃₀H ratios, indicating that the salinity of the source rock for family I is higher than those for family II and family III. The relative abundance of C₃₄ and C₃₅ homohopanes is commonly used to evaluate redox conditions, and high C₃₅/C₃₄H values are commonly interpreted as an indicator of highly reducing depositional environment.²⁴ The C₃₅/C₃₄H ratios of family I range from 0.59 to 0.80 (Table 2, Figure ​Figure88), which imply a suboxic depositional setting of the corresponding source rocks. The oil samples in family II come from source rocks that were deposited in a weak oxidizing environment, as supported by the average C₃₅/C₃₄H ratio of 0.57. For oil samples in family III, the C₃₅/C₃₄H ratios are between 0.97 and 1.05, thus suggesting a reducing depositional environment of the source rocks.

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Figure 8

Correlations of the depositional environment-related ratios of (a) C₃₅/C₃₄H vs Ga/C₃₀H and (b) C₃₅/C₃₄H vs ETR, showing the interpreted differences in depositional redox conditions of the source rocks for the crude oils in different families. Compound symbols are described in the footnote of Table 2 and in Figure ​Figure22.

6. Conclusions

Chemometric analysis of 13 source-related and depositional environment-related parameters for 20 crude oil samples from the Gudong Oilfield revealed that they generally fell into three families. The organic geochemical characteristics of the oil families indicate that the oils all originated from lacustrine source rocks, although the thermal maturity, organic matter sources, and depositional environment of their source rocks differ. Family I was from the source rock deposited under suboxic conditions in a brackish water environment, with microalgae having been the dominant source of organic matter. The inferred source rock is the Es₁ source rock in the Huanghekou Sag, and a substantial portion of the oils in the Gudong Oilfield originated from the source rock. The source rock of family II was deposited in a weakly oxidizing environment with a large amount of terrigenous organic input. It is probable that the source rock is Ed member in the Huanghekou Sag. For family III, the Es₃ member in the Gunan Sag is the most likely source. Oils in family III were deposited under a weakly reducing environment with microalgae (especially zooplankton algae) providing the organic matter input. All of the crude oil samples are at the mature stage; however, the maturity of family II is higher than those of family I and family III.

Acknowledgments

Notes

The authors declare no competing financial interest.

References