PETROLEUM EXPLORATION AND DEVELOPMENT, 2021, 48(5): 1077-1088 doi: 10.1016/S1876-3804(21)60092-7

Geochemical significances of 8,14-secohopanes in marine crude oils from the Tazhong area in the Tarim Basin, NW China

BAO Jianping,*, YANG Xi, ZHU Cuishan

Key Laboratory of Oil & Gas Resource and Exploration Technology, Yangtze University, Wuhan 430100, China

Corresponding authors: * E-mail: 101064@yangtzeu.edu.cn

Received: 2020-12-27   Revised: 2021-08-25  

Fund supported: national Natural Science Foundation of China(41772119)
national Natural Science Foundation of China(41272169)

Abstract

8,14-secohopanes in the marine oils from the Tazhong area in the Tarim Basin are detected by gas chromatography-mass spectrometry (GC-MS) and gas chromatography-mass spectrometry-mass spectrometry (GC-MS-MS), and their distributions and compositions are compared in order to study their potential significances in oil-source correlation. C35+ extended hopane series and three series of extended 8,14-secohopanes can be detected in two kinds of end-member oils in the Tazhong area in the Tarim Basin, and they are different in distribution, suggesting that they may have some special geochemical significance. The presence of 8,14-secohopanes in two kinds of end-member oils in the Tarim Basin suggests that these biomarkers are primary, and not related to biodegradation. The relative abundance of 8,14-secohopanes in the type-A oil is much less than that in the type-B oil, and the 8,14-secohopanes content in end-member oils is much less than that in the corresponding mixed oils. Based on the relative contents of 8,14-secohopanes and the compositions of common steranes and triterpanes, it is very effective to distinguish different crude oils from the Tazhong area. The great difference in the relative abundance of 8,14-secohopanes between the type-A oil and type-B oil suggests that their formation may require some specific geological- geochemical conditions.

Keywords: 8,14-secohopanes; biomarkers; marine end-member oils; mixed oils; oil-source correlation; Cambrian-Ordovician; Tazhong Uplift; Tarim Basin

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BAO Jianping, YANG Xi, ZHU Cuishan. Geochemical significances of 8,14-secohopanes in marine crude oils from the Tazhong area in the Tarim Basin, NW China. PETROLEUM EXPLORATION AND DEVELOPMENT, 2021, 48(5): 1077-1088 doi:10.1016/S1876-3804(21)60092-7

Introduction

Hopanoids are a kind of biomarkers indicating the bacterial contribution of prokaryotes and widely occur in various source rocks and fossil fuels in different geological ages and depositional environments[1,2,3,4]. The types of biomarkers with hopane molecular skeleton in the geological samples are very complicate and diverse, including hopanes, methylhopanes, demethylhopanes, secohopanes, hexcyclichopanes and benzohopanes etc. [5,6,7,8,9]. The geochemical information provided by their distributions and compositions is widely used to study depositional environment, organic matter source, thermal maturation and petroleum geochemistry[10,11,12].

The hopanoids commonly detected in source rock and crude oil are mainly between C27 and C35 in carbon number, and hopanoids beyond C35 are rare. Hopanes up to or beyond C40 have been found in biodegraded oil[13,14]. C45 hopanes and 3β-methylhopanes have been detected in the K2q source rock of Songliao basin[15]. As well known, C35 bacteriohopanetetrol in cell membrane can explain the origin and source of C27-35 hopanoids in the geological samples, but so far, we have little information on the biological precursor of hopanoids beyond C35. 8,14-secohopanes are usually detected in biodegraded oil in the previous study, with carbon numbers mainly between C27 and C30[16,17,18], sometimes up to C35[19,20]. They include six series, namely, 8(H), 14(H), 17(H), 21(H)-(series I), 8(H), 14(H), 17(H), 21(H)-(series II), 8(H), 14(H), 17(H), 21(H)-(series III), 8(H), 14(H), 17(H), 21(H)-(series IV), 8(H), 14(H), 17(H), 21(H)-(series V) and 8(H), 14(H), 17(H), 21(H)-(series VI)[19,20]. Their origin in crude oil could be related to biodegradation[9, 21], thermal degradation[5, 7, 22] and strong resistance to biodegradation[16]. In addition, 8,14-secohopanes have been detected in different source rocks at different maturity [23,24,25], suggesting that their generation isn’t necessarily related to biodegradation of crude oil. And the opening of C-ring for hopanes could happen in early diagenesis[23]. Previous studies on them focused mainly on their detection and report, and little attention was paid to their geochemical significances and potential practical values.

The Tarim Basin is the largest petroliferous basin in China, where there are two sets of marine source rocks in the platform, Cambrian-Lower Ordovician and Middle-Upper Ordovician[26,27,28,29,30,31]. Based on the distribution and compositions of biomarkers, the marine crude oil in the Tarim Basin can be divided into two types: type-A oil with higher contents of gammacerane and C28 steranes[32], and type-B oil with evidently lower contents of gammacerane and C28 steranes and C27R, C28R and C29R steranes in “V” shape[26, 32-35]. In the Tazhong area, the end-member oil from a single source is rare, and most of the crude oil samples are mixed oils of different natures[34,35], making oil-source study very difficult. Although in the previous studies, many different methods, such as various molecular parameters[26, 29-36], specific-compound carbon isotope[34,35], sulfur and carbon isotope composition[36,37,38,39,40] were used to correlate crude oil and source rock samples, the controversy over oil sources still remains. Recently, a great amount of crude oil and natural gas has been discovered in the Cambrian pre-salt reservoir of Tarim Basin, but their source is still controversial[41,42,43]. The main reason is that the great difference in maturity of organic matter between the two sets of marine source rocks hinders the direct oil-source correlation, especially, the Cambrian-Lower Ordovician source rock is at high-over mature stage and could not match in maturity with the discovered crude oils.

Compared with the common hopanes, 8,14-secohopanes has higher thermal stability and stronger resistance to biodegradation. In this paper, we tried to discriminate origins and sources of different crude oils, including different end-member oils and mixed oils, from the Tazhong uplift by examining the distribution and composition of 8,14-secohopanes in the oils. The study provides a new method for the oil source research of marine oil in the Tarim Basin.

1. Samples and experiments

Thirty marine crude oil samples from the Tazhong area in the Tarim Basin, including type-A and type-B oils, and the mixed oils of biodegraded oils and conventional oils, were colleted and studied in detail.

Normal hexane was used to extract asphaltene from the crude oil samples. The de-asphaltened oil samples were separated into saturated hydrocarbon, aromatic hydrocarbon, and non-hydrocarbon fractions using an alumina/silica gel column chromatography sequentially eluted with n-hexane, toluene, and dichloromethane (CH2Cl2), respectively. Based on the method suggested by Sun et al.[44], the saturated hydrocarbon fractions of typical marine end-member oil were further divided into normal alkanes and branched/cyclic hydrocarbon subfractions using urea adduction. The branched/cyclic hydrocarbons subfractions and saturated hydrocarbon fractions in the relevant oil samples were analyzed by GC-MS and GC-MS-MS, respectively.

Saturated hydrocarbon fractions were analyzed by an Agilent 6890 gas chromatograph coupled to a 5975 mass selective detector equipped with an HP-5MS capillary column (30 m × 0.25 mm × 0.25 μm) in GC-MS test. The temperature program: the temperature of 50 °C initially was kept for 2 min, increased to 100 °C at 20 °C/min, then from 100 °C to 315 °C at 3°C/min, and finally maintained at 315 °C for 16.83 min. The injector and ion source temperatures were 300 °C and 230 °C, respectively. Helium was injected at 1.04 mL/min as the carrier gas. The scan range was from 50 to 580 amu in full scan and multiple ion detection (MID) modes in the electron ionization (EI) mode at 70 eV. Androstane was used as an internal standard compound to determine the concentrations of steranes and terpanes in the crude oil samples.

Branched/cyclic hydrocarbon subfractions were analyzed by a Thermo Fisher Scientific TSQ Quantum-XLS in GC-MS-MS test. The column was a HP-5MS fused silica capillary column (30 m × 0.25 mm × 0.25 μm). The temperature program: the temperature was kept at 50 °C for 1 min first, and then increased from 50 °C to 100 °C at 20 °C/min, from 100 °C to 320 °C at 3 °C/min, and maintained at 320 °C for 15.17min. The injector and ion source temperatures were 310 °C and 230 °C, respectively. Helium was injected at 1.04 mL/min as carrier gas. The ionization electron energy was 30 eV. The GC-MS-MS analysis was run in parent ions (m/z 372+14n and m/z 370+14n, n = 0-13) → daughter ions (m/z 191 and m/z 123) modes for the extended hopanes and 8,14-secohopanes, respectively. Argon was used as collision gas, and collision energy was 20 eV. The peak areas of every member in C27-40 extended hopanes and 8,14-secohopanes were integrated in the corresponding chromatograms of GC-MS-MS.

2. Distribution and composition of biomarkers and origins of the crude oil samples

2.1. Distribution and composition of biomarkers

Chain alkanes, including normal alkanes and phytane series, in the saturated hydrocarbon fractions of the typical crude oil samples from the Tazhong area are abundant and complete (Fig. 1), and normal alkanes are much higher in abundance than pristane (Pr) and phytane (Ph). As normal alkanes in crude oil have the weakest resistance to biodegradation, the presence of complete and abundant normal alkanes shows the oil samples are unbiodegraded normal oils. Their Pr/Ph ratiosare similar and between 1.0 and 1.5, and Pr/nC17 and Ph/nC18 ratios are basically less than 0.5 (Table 1), showing that the marine oil samples are similar in compositions of chain alkanes and seem not to be biodegraded.

Fig. 1.

Fig. 1.   Total ion current of GC-MS of the saturated hydrocarbon fractions in the typical crude oil samples from the Tazhong uplift. (a) TZ 11, Silurian, 4301.00-4307.00 m; (b) TZ 113, Silurian, 4525.35-4671.42 m; (c) TZ 74, Ordovician, 4683.40-4699.05 m; (d) TZ 122, Ordovician, 4631.88-4733.92 m.


Table 1   Biomarker parameters and origin of crude oil samples from the Tazhong uplift.

Well No.Depth/mAgePr/PhPr/nC17Ph/nC18Ts/TmG/C31HC27R/
C29R
C28R/
C29R
C29 20S/
(20S+20R)
C29ββ/
(αα+ββ)
C23T/
C30H
C28NH/
C29H
C29NH/
C30H
Oil type
TZ114 301.00-
4 307.00
S1.220.430.490.410.910.430.710.460.410.08//Type-A
TZ304 997.00-
5 026.00
O1.370.320.310.391.040.430.730.450.400.02//Type-A
TZ104 227.00-
4 234.00
C1.110.310.330.620.120.510.260.500.560.88//Type-B
TZ354 946.00-
4 951.00
S1.330.400.370.810.090.640.370.490.540.28//Type-B
TZ354 320.00-
4 323.00
C31.190.340.340.780.130.590.350.490.540.41//Type-B
TZ1114 357.50-
4 364.00
S1.130.390.430.570.120.630.270.490.561.01//Type-B
TZ1134 525.35-
4 671.42
S1.100.380.410.640.130.720.270.500.550.85//Type-B
TZ1174 288.29-
4 304.17
S1.090.440.500.490.070.650.290.510.561.45//Type-B
TZ404 334.00-
4 340.00
C31.140.340.340.900.080.660.240.460.571.17//Type-B
TZ474 390.50-
4 402.00
C31.150.310.310.930.090.620.310.520.581.04//Type-B
TZ4014 408.50-
4 980.50
C31.150.330.340.900.070.720.260.490.571.16//Type-B
TZ63 647.00-
3 652.50
C1.570.600.660.400.350.450.580.440.380.200.100.05Type A mixed oil
TZ6214 851.10-
4 885.00
O2+32.070.210.111.110.340.920.670.480.410.060.350.12Type A mixed oil
TZ704 735.00-
4 770.00
O1.160.300.300.660.380.670.620.430.440.210.190.08Type A mixed oil
TZ8285 595.00-
5 603.00
O1.160.310.311.110.470.720.590.450.470.240.200.08Type A mixed oil
TZ856 313.00-
6 550.00
O1.130.440.460.610.350.450.550.520.510.130.140.09Type A mixed oil
TZ243 790.87-
3 807.21
C31.240.320.300.410.420.350.620.510.530.500.380.10Type A mixed oil
TZ545 747.00-
5 764.00
O1.340.340.290.800.560.680.580.450.410.130.090.03Type A mixed oil
TZ624 052.98-
4 073.58
S1.370.520.400.550.480.740.700.510.460.880.170.10Type A mixed oil
TZ744 683.40-
4 699.05
O1.150.250.241.030.700.760.600.470.410.090.420.14Type A mixed oil
TZ444 854.00-
4 877.00
O1.170.280.270.520.350.480.690.490.500.460.100.06Type A mixed oil
TZ4013 244.00-
3 308.00
C11.300.320.290.840.130.620.240.500.561.810.410.16Type B mixed oil
TZ4013 685.00-
3 703.00
C1.230.320.300.960.140.890.240.490.581.740.530.16Type B mixed oil
TZ4023 246.00-
3 280.00
C11.290.330.300.800.120.520.260.480.561.720.410.16Type B mixed oil
TZ4043 619.47-
3 681.81
C1.330.310.280.820.140.610.260.500.571.560.440.18Type B mixed oil
TZ4213 570.50-
3 575.00
C31.300.370.340.830.190.810.280.500.581.920.510.18Type B mixed oil
TZ1224 631.88-
4 733.92
O1.210.330.320.870.120.990.260.520.571.260.940.22Type B mixed oil
TZ43 532.00-
3 548.00
C21.360.360.330.820.160.500.220.450.512.320.420.13Type B mixed oil
TZ1694 241.09-
4 283.52
O1.110.430.440.970.130.820.320.530.591.070.220.09Type B mixed oil
TZ753 701.00-
3 715.00
C1.260.320.290.830.160.810.320.540.571.960.530.20Type B mixed oil

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25-norhopanes are a reliable indicator to determine severe biodegradation of crude oil[9], but they show very different distributions in the crude oil samples from the Tazhong uplift. As shown in Fig. 2, 25-norhopanes are absent in the crude oil samples from well TZ 11 and TZ 113, combined with the abundant and whole chain alkanes in them, it is inferred that they haven’t been biodegraded and are normal oil (Fig. 1). This is also the main basis for determining the original nature of marine oil in Tarim Basin. In contrast, the crude oil samples from well TZ 74 and TZ 122 have abundant and complete chain alkanes (Fig. 1), but also have relatively abundant 25-norhopanes detected (Fig. 2), suggesting that they are mixed oils of severely biodegraded oil and normal oil. The phenomenon mentioned above shows that the accumulation history of reservoirs in the study area is complicate, that is, the crude oil filled into the reservoirs early has suffered severe biodegradation, but later, fresh oil filled again into the reservoirs, resulting in the coexistence of two kinds of oils. It is difficult to determine the real nature of the crude oil samples only based on the total ion current of GC-MS of saturated hydrocarbon fractions or the gas chromatography of whole oil. The assemblage of 25-norhopanes and normal alkanes in crude oils is a relatively reliable evidence to discriminate end-member oil and mixed oil. Based on this criterion, the 30 crude oil samples from the study area are divided into normal oil (end-member oil) and mixed oil of biodegraded oil and normal oil (Table 1).

Fig. 2.

Fig. 2.   Distribution of 25-norhopanes (m/z 177) in typical crude oil samples from the Tazhong uplift (C29H-C35H refer to hopanes; C29Ts refers to C29 18α(H)-norneohopane; TsN, TmN C28NH-C32NH refer to 25-norhopanes). (a) TZ11, S, 4301.00- 4307.00 m; (b) TZ113, S, 4525.35-4671.42 m; (c) TZ 74, O, 4683.40-4699.05 m; (d) TZ 122, O, 4631.88-4733.92 m.


2.2. Types of crude oil origins

Based on the distribution and composition of steranes and terpanes in the crude oil samples from the Tazhong uplift, there are relatively gammacerane and C28 steranes, almost no diasteranes, and very low C19-26 tricyclic terpanes and C21-22 short side chain steranes in the crude oil samples from well TZ 11 and TZ 30 (Fig. 3a, Table 1). But this kind of crude oil has higher concentrations of biomarkers such as steranes and terpanes, especially steranes and hopanes (Table 2), representing typical crude oil derived from the Cambrian-Lower Ordovician source rocks[32] and the so-called type-A oil. In the crude oil from Well TZ 113, gammacerane is very low, C28 steranes in C27-29 steranes is much lower than that in the type-A oil, and C27R, C28R and C29R steranes form in “V” shape, C19-26 tricyclic terpanes and C21-22 short side chain steranes are very abundant, diasteranes are moderate but lower than the content of regular steranes (Fig. 3b, Table 1), and the concentrations of biomarkers of steranes and terpanes are much lower than that in the type-A oil (Table 2). This kind of oil is very similar to type-B oil in biomarker features, but its exact source remains controversial. Based on the data from Zhu et al.[40], the carbon isotope values of different kerogens in Cambrian and Middle and Upper Ordovician source rocks are partly similar, so it is not surprising that type-A and type-B oils have similar carbon isotope composition.

Fig. 3.

Fig. 3.   Distribution of terpanes (m/z 191) and steranes (m/z 217) in the typical crude oil samples from the Tazhong uplift (C21T, C23T and C29T and C30T refer to C21, C23, C29 and C30 tricyclic terpanes; C21-22 refers to short side chain steranes, C29H-C35H represent hopanes; Ts and Tm refer to 18α(H) and 17α(H)-22,29,30-trisnorhopanes; C27R, C28R and C29R refer to C27-29 5α(H), 14α(H), 17α(H)-20R steranes; G refers to gammacerane). (a) TZ11, S, 4301.00-4307.00 m; (b) TZ113, S, 4525.35-4671.42 m; (c) TZ 74, O, 4683.40-4699.05 m; (d) TZ 122, O, 4631.88-4733.92 m.


Table 2   Concentrations of main biomarkers and carbon isotope values of the crude oil samples from Tazhong uplift.

WellDepth/mAgeConcentration of biomarkers/(mg•g-1)δ13C/‰
C19-31 tricyclic terpaneshopanessteranes
TZ114 301.00-4 307.00S3.630 319.289 58.827 0-31.7
TZ304 997.00-5 026.00O2.175 625.161 311.755 4-32.0
TZ104 227.00-4 234.00C1.378 21.545 00.839 0-32.2
TZ354 946.00-4 951.00S1.642 04.578 91.453 7-32.5
TZ354 320.00-4 323.00C31.095 82.273 80.938 7-32.1
TZ1114 357.50-4 364.00S1.887 01.943 31.022 3-32.2
TZ1134 525.35-4 671.42S2.758 93.324 01.727 1-32.4
TZ1174 288.29-4 304.17S3.334 32.433 71.586 8-32.3
TZ404 334.00-4 340.00C30.795 70.656 50.464 9-31.9
TZ474 390.50-4 402.00C31.819 11.663 71.042 4-32.2
TZ4014 408.50-4 980.50C30.827 90.703 20.499 7-32.2
TZ63 647.00-3 652.50C1.205 33.761 33.066 8-29.8
TZ6214 851.10-4 885.00O2+35.169 124.036 46.992 8-29.8
TZ704 735.00-4 770.00O1.015 72.720 71.681 0-31.9
TZ8285 595.00-5 603.00O0.811 91.948 52.794 6-32.3
TZ856 313.00-6 550.00O3.429 611.361 26.355 9-33.4
TZ243 790.87-3 807.21C31.347 41.718 42.078 4-31.8
TZ545 747.00-5 764.00O0.959 63.553 21.796 0-31.8
TZ624 052.98-4 073.58S2.939 72.267 91.746 0-32.4
TZ744 683.40-4 699.05O0.935 63.451 01.261 1-31.4
TZ444 854.00-4 877.00O1.008 01.496 11.565 2-31.9
TZ4013 244.00-3 308.00C10.827 90.708 10.575 0-33.5
TZ4013 685.00-3 703.00C0.701 70.433 00.438 4-32.2
TZ4023 246.00-3 280.00C10.809 30.632 90.494 7-33.4
TZ4043 619.47-3 681.81C0.795 20.642 60.488 5-33.1
TZ4213 570.50-3 575.00C30.911 40.545 20.459 2-32.5
TZ1224 631.88-4 733.92O0.906 70.564 80.701 3-31.8
TZ43 532.00-3 548.00C20.740 20.419 30.369 8-32.8
TZ1694 241.09-4 283.52O1.735 01.583 31.248 7-31.8
TZ753 701.00-3 715.00C0.883 40.579 40.516 4-32.9

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In the crude oil from well TZ 74, C19-26 tricyclic terpanes and C21-22 short side chain steranes are lower, gammacerane and C28 steranes are relatively high but lower than that in the type-A oil (Fig. 3c, Table 1), showing that this oil is similar to type-A oil. However, 25-norhopanes and complete normal alkanes commonly occur in this oil (Table 2), suggesting that it is a mixed oil of severe biodegraded oil and unbiodegraded oil. In this mixed oil, the severely biodegraded oil must be type-A oil, and the fresh oil filling later is type-B oil. The crude oil from well TZ122 shows coexistence of 25-norhopanes and complete normal alkanes (Figs. 1d and 2d), consistent with mixed oil. In this kind of oil, the distribution and compo-sition of steranes and terpanes are similar to type-B oil, with higher contents of C19-26 tricyclic terpanes and C21-22 short side chain steranes, obviously lower contents of gammacerane and C28 steranes (Fig. 3d, Table 1), lower concentrations of steranes and terpanes than that in type-B oil (Table 2), suggesting that this oil is similar to type-B oil in origin. It is inferred that the severe biodegraded oil in this mixed oil is type-B oil, while the origin of fresh oil filling later is difficult to be determined due to different distributions of C19-26 tricyclic terpanes.

In conclusion, based on the coexistence of 25-norhopanes and whole normal alkanes, the crude oil samples can be divided into end-member oil and mixed oil (Table 1), and then, combined with the compositions of steranes and terpanes, the end member oil samples can be divided into type-A and type-B, and the mixed oil samples can be classified into the type A mixed oil and the type B mixed oil (Fig. 4). The type A mixed oil is the mixture of severely biodegraded type-A oil and unbiodegraded type-B oil, and the type B mixed oil is the mixture of severely biode-graded type-B oil and normal oil with unknown origin. Because of similar low contents of gammacerane and C28 steranes, the type-B oil and the type B mixed oil fall in same areas in Fig. 4 and are difficult to be discriminated.

Fig. 4.

Fig. 4.   Composition of steranes and triterpanes and origins of the crude oil samples from the Tazhong uplift.


3. Extended hopanes and extended 8,14-secohopanes

3.1. Extended hopanes

C35+ extended hopanes are usually detected in biodegraded oil in previous studies[13,14]. But in the GC-MS-MS analysis of branched/cyclic hydrocarbon fractions in this study, C35+ extended 17α(H) and 21β(H)-hopanes were detected in two kinds of end-member marine oil from the Tazhong uplift (Fig. 5), suggesting that these biomarkers may be common in geological samples, only they are low in content and difficult to be detected in crude oil by conventional test means. Their presences in the end-member oil shows that they should be a kind of primary biomarkers derived directly from some special biological precursors and are not related to biodegradation of crude oil. This is supported by the K2q source rock samples from Songliao basin[15].

Fig. 5.

Fig. 5.   Chromatograms of parent ion (m/z 370+14n, n is 0-11)-daughter ion (m/z 191) of C27-38 extended hopanes in the crude oil from Well TZ 30 (C29Ts represents C29 18α(H)-norneohopane).


It is noted that the two kinds of end-member oils from the Tazhong uplift have some differences in carbon numbers of extended hopanes. The extended hopanes in the type-B oil are from C27 to C40 in carbon number, while in the type-A oil, the extended hopanes are C27-38, and C39-40 extended hopanes were not detected probably because of low content (Fig. 6). Moreover, the extended hopanes in two kinds of end-member oils differ considerably in carbon number components. In the type-A oil, all components in the extended hopanes differ widely in relative content, and neighboring components vary greatly in content, for example, the C30 hopane accounts for up to about 50% of the whole extended hopane series, while other components are obviously low in content (Fig. 6a). In contrast, in the type-B oil, the components of extended hopanes have small difference in relative content, and neighboring components (especially C27—35) vary small in content (Fig. 6b). This feature is related to the origin of marine oil in the Tazhong area needs to be studied further.

Fig. 6.

Fig. 6.   Histogram of relative abundance of components in extended hopanes of the type-A oil (a) and type-B oil (b) from the Tazhong uplift.


The hopanes detected by conventional GC-MS in the crude oil are from C27 to C35 in general, C35+ extended hopanes were detected by GC-MS-MS in the two kinds of marine end-member oils from the Tazhong area, because they are very low in relative abundance compared with C31-35 homohopanes. For example, the C36-40 extended hopanes account for only about 0.03% to 0.70% in the type-B oil, and the C36-38 extended hopanes in the type-A oil vary from 0.01% to 0.14% in content, much less than C31-35 homohopanes. Therefore, C35+ extended hopanes can hardly be observed in Fig. 6a. It is noted that C31 to C35 and C36 to C40 in the extended hopanes gradually decrease in relative abundance with the increase of carbon number, but the two kinds of oils both show abrupt drop in relative abundance from C35 to C36, with the extent up to 4.5-6.0 times, suggesting that C31-35 homohopanes and C36-40 extended hopanes in the marine crude oils from the Tazhong uplift are probably derived from different biological precursors.

3.2. Extended 8,14-secohopanes

In the previous studies, 8,14-secohopanes have been detected in crude oil samples[5, 16-22] and source rocks of different natures[23,24,25], with carbon numbers mainly between C27 and C31[5, 14-18, 21-25], and up to C35 in a few biodegraded oil samples from individual areas. They can be divided into 6 series[19,20]. In this study, three series of extended 8,14-secohopanes up to C40 were detected in the two kinds of marine end-member oils from the Tazhong uplift, they are 8α(H), 14α(H), 17α(H), 21β(H)-; 8α(H), 14α(H), 17β(H), 21α(H)-; and 8α(H), 14β(H), 17α(H), 21β(H)-, respectively (Fig. 7), corresponding to series II, IV and VI[20]. Their stereo configurations at C-17 and C-21 are inherited from hopane and moretane. At least five isomers of C27 8,14-secohopanes can be detected in different types of crude oils, but their stereo configurations for five isomers of C27 8,14-secohopanes are unknown. In order to compare with extended hopanes, the relative abundances of C27-40 extended 8,14-secohopanes were calculated by normalization of their peak area in three series of extended 8,14-secohopanes. Afterwards, the distribution and composition of extended C27-40 8,14-secohopanes in different end-member oils will be discussed.

Fig. 7.

Fig. 7.   Chromatograms of parent ion (m/z 372+14n, n is 0-11)-daughter ion (m/z 123) of C27-38 extended 8,14-secohopanes in the crude oil from Well TZ 30 (symbols “×”, “+” and “*” refer to series II, IV and VI of 8,14-secohopanes, respectively; 22S and 22R are two isomers of 8,14-secohopanes at C-22).


In the type-A oils from the Tazhong area, the range of carbon number for the extended 8,14-secohopanes is from C27 to C38, C39-40 extended 8,14-secohopanes couldn’t be detected due to too low content (Fig. 8a), consistent with the distribution of the extended hopanes. In contrast, the extended 8,14-secohopanes in the type-B oil are complete in carbon number composition, with carbon number up to C40 (Fig. 8b), comparable to the carbon number composition of the extended hopanes in this kind of crude oil. It is found that the carbon numbers in the extended 8,14-secohopanes in the two kinds of end-member oils show some regularity in relative abundance. For example, carbon numbers in the same kind of end-member oil samples show similar variation features, but those in different types of end-member oil samples show different variation trends. Moreover, there is a clear correspondence in the variations of their relative abundances in extended hopanes and extended 8,14-secohopanes in the same type of end-member oil (Fig. 6). Clearly, the extended hopanes and 8,14-secohopanes in the two kinds of end-member oils from the Tazhong uplift are similar in distribution and composition features, suggesting that they could be derived from the same biological precursor.

Fig. 8.

Fig. 8.   Histograms of relative abundances of carbon numbers in the extended 8,14-secohopanes in type-A (a) and Type-B (b) oils from the Tazhong uplift.


It is noted that just like the extended hopanes, the relative abundances from C31 to C35 and from C36 to C40 in the extended 8,14-secohopanes gradually decrease with the increase of carbon number, but the decrease of abundance from C35 to C36 is abrupt, with a decreasing extent of up to 3-5 times (Fig. 8), suggesting that there is a close relationship in the origin and biological input of extended hopanes and extended 8,14-secohopanes in the crude oil samples from the Tazhong uplift.

3.3. Relationship between 8,14-secohopanes and hopanes in different types of crude oils

The extended hopanes and extended 8,14-secohopanes have been detected in the two kinds of end-member oils from the Tazhong uplift, and each carbon number of extended 8,14-secohopanes consist of at least three series and six isomers. As C29 and C30 8,14-secohopanes in series Ⅱ are early eluted, higher in abundance, they are easier to identify in the m/z 123 mass chromatograms (Fig. 9). Once the peak areas of C29 and C30 of hopanes and 8,14-secohopanes in series II are integrated in the m/z 191 and m/z 123 mass chromatograms, respectively, their relative compositions in different crude oils in the study area can be obtained to study origins and sources of the oils.

Fig. 9.

Fig. 9.   m/z 123 mass chromatograms of the two kinds of end-member oils and related mixed oils from the Tazhong uplift (scC27H and scC29H-scC32H refer to C27 and C29-32 8,14-secohopanes). (a) TZ 11, Silurian, 4301.00-4307.00 m; (b) TZ 30, Ordovician, 4997.00-5026.00 m; (c) TZ 74, Ordovician, 4683.40-4699.05 m; (d) TZ 113, Silurian, 4525.35-4671.42 m; (e) TZ 35, Silurian, 4946.00-4951.00 m; (f) TZ 122, Ordovician, 4631.88-4733.92 m.


The type-A oil and the type-B oil from the Tazhong area differ significantly in the contents of 8,14-secohopanes, and the two kinds of mixed oils generally inherit those features of their end-member oils (Fig. 9). But it is noted that the contents of 8,14-secohopanes in both the type A mixed oil and the type B mixed oil are much higher than those in their own end-member oils. For example, on the m/z 123 mass chromatogram of the type A mixed oil, the relative abundances of C29 and C30 8,14-secohopanes are higher than those of C29 and C30 hopanes, but in the type-A oil, the relative abundances of C29 and C30 8,14- secohopanes are much lower than those of C29 and C30 hopanes (Fig. 9a-9c). 8,14-secohopanes in type-B oil is rich but higher in content in type B mixed oil. For example, in the type B mixed oil, C29 and C30 8,14-secohopanes are higher in relative abundant, while the corresponding C29 and C30 hopanes almost can not be identified in the m/z 123 mass chromatogram because of low contents (Fig. 9d-9f).

The content of 8,14-secohopanes is lower in the type-A oil, the scC29H/C29H and scC30H/C30H ratios are less than 0.20, but C28R/C29R and G/C31H ratios higher. In contrast, 8,14-secohopanes are abundant in the type-B oil, their scC29H/C29H and scC30H/C30H ratios are about 0.60-1.2 and 0.4-1.0, respectively, but the C28R/C29R and G/C31H ratios are lower. Therefore, it is very easy to discriminate the two kinds of end-member oils in Fig. 10.

Fig. 10.

Fig. 10.   Relationships beween C28R/C29R-G/C31H and scC29H/C29H-scC30H/C30H in different types of crude oils from the Tazhong uplift. (a) scC29H/C29H vs G/C31H; (b) scC30H/C30H vs G/C31H; (c) scC29H/C29H vs C28R/C29R; (d) scC30H/C30H vs C28R/C29R.


The A mixed oil A is a kind of mixture of severely biodegraded type-A oil and type-B oil with different maturity, so it shows the variation patterns of mixed oil in the related biomarker parameters. For example, the type A mixed oil inherits the higher abundances of C28 steranes and gammacerane from the type-A oil, but has higher content of 8,14-secohopanes. The type B mixed oil is a kind of a mixture of severely biodegraded type-B oil and unbiodegraded type B oil with different maturity, and it has much higher content of 8,14-secohopanes than the type-B oil, and their scC29H/C29H and scC30H/C30H ratios of 1.5-2.3 and 1.3-1.6, respectively. They can be distinguished from each other in Fig. 10. Therefore, based on the relative abundances of 8,14-secohopanes and other biomarker parameters, the different end-member oils and different mixed oils in the study area can be easily discriminated from each other, showing that this method is practical in oil-source correlation. As mentioned above, 8,14-secohopanes are present in normal oils, and more abundant in the mixed oils of biodegraded oil and normal oil, suggesting that this kind of biomarkers have stronger resistance to biodegradation than hopanes, so they can enrich in very severely biodegraded oil. This is also why the mixed oil has much higher abundance of 8,14-secohopanes. Therefore, this method could be more suitable for oil-source study of severely biodegraded oils.

In the previous study, it was thought that 8,14-seco-hopanes were the products of biodegradation of crude oil[21]. But they were detected in unbiodegraded oils from the Tazhong uplift in this study, suggesting that their presence isn’t necessarily related to biodegradation of crude oil. Moreover, C27-32 8,14-secohopanes were also detected in extracts from immature oil shale[24] and marginal mature boghead coal samples[23], showing that the generation of 8,14-secohopanes in the geological samples may occur in the diagenesis stage. Their presence in high to over mature marine source rocks[25] and severely biodegraded oil[5, 16, 19-20] suggests that they have not only high thermal stability, but also strong biodegradation capacity. The wide distribution of 8,14-secohopanes in different end-member oils and mixed oils from the Tazhong uplift should be related to their strong resistance to biodegradation, this feature can provide a way to oil-source study for severely biodegraded oil in which other biomarkers such as common steranes and terpanes have been altered in distribution and composition or disappear.

So far, most studies on 8,14-secohopanes in the geological samples stay at the detection and report of these compounds, and there is little information about their formation mechanism and probable influencing factors. The content difference in 8,14-secohopanes from two kinds of marine end-member oils from the Tazhong uplift shows that depositional environment of source rock such as salinity and stratification of water column may be important factors affecting their generation.

4. Conclusions

The C35+ extended hopanes and extended 8,14-secohopanes coexist in the two kinds of marine end-member oils from the Tazhong uplift, suggesting that they are primary biomarkers, and not related to biodegradation of crude oil. The analysis results show that in the type-A oil rich in gammacerane and C28 steranes, these two kinds of biomarkers have a distribution of carbon number from C27 to C38, but in the type-B oil with lower contents of gammacerane and C28 steranes, their carbon numbers can be up to C40. C35 bacteriohopanetetrol can explain the origin and source of C31-35 hopanes and 8,14-secohopanes, but can not explain the origin and source of the C35+ extended hopanes and extended 8,14-secohopanes in the marine oil samples. It is inferred therefore that the C35+ extended hopanes and extended 8,14-secohopanes in the geological samples may be derived from an unknown C40 biological precursor with hopane molecular skeleton in the prokaryotes.

There is some obvious difference in the relative abundance of 8,14-secohopanes in different types of crude oils from the Tazhong uplift, and their content in the type-A oil and the type A mixed oil is much less than those in the type-B oil and the type B mixed oil, respectively. Based on the relative composition between steranes, gammacerane, hopanes and 8,14-secohopanes in different crude oils from the Tazhong uplift, the two kinds of end-member oils and related mixed oils in the study area can be effectively discriminated from each other, showing that this method has good prospect and wide application in marine oil-source study in the Tarim Basin.

Reference

OURISSON G, ALBRECHT P, ROHMER M.

The hopanoids: Palaeochemistry and biochemistry of a group natural products

Pure and Applied Chemistry, 1979, 51(4):709-729.

DOI:10.1351/pac197951040709      URL     [Cited within: 1]

OURISSON G, ROHMER M, PORALLA K.

Prokaryotic hopanoids and other polyterpenoid sterol surrogates

Annual Review of microbiology, 1987, 41:301-333.

DOI:10.1146/micro.1987.41.issue-1      URL     [Cited within: 1]

OURISSON G, ALBRECHT P, HOPANOIDS L.

Geohopanoids: The most abundant natural products on Earth?

Cheminform, 1993, 24(9):398-402.

[Cited within: 1]

ROHMER M, BISSERET P, NRUNLIST S.

The hopanoids, prokaryotic triterpenoids and precursors of ubiquitous molecular fossils

New Jersey: Prentice Hall, 1992: 1-17.

[Cited within: 1]

SCHMITTER J M, SUCROW W, ARPINO P J.

Occurrence of novel tetracyclic geochemical markers: 8,14-seco-hopanes in the Nigerian crude oil

Geochimica Cosmochimica Acta, 1982, 46(11):2345-2350.

DOI:10.1016/0016-7037(82)90206-X      URL     [Cited within: 5]

HUSSLER G, ALBRECHT P, OURISSON G.

Benzohopanes: A novel family of hexacyclic geomarkers in sediments and petroleums

Tetrahedron Letters, 1984, 25:1179-1182.

DOI:10.1016/S0040-4039(01)91554-0      URL     [Cited within: 1]

CONNAN J, DESSORT D.

Novel family of hexacyclic hopanoid alkanes (C32-C35) occurring in sediments and oils from anoxic paleoenvironments

Organic Geochemistry, 1987, 11(2):103-113.

DOI:10.1016/0146-6380(87)90032-5      URL     [Cited within: 2]

MOLDOWAN J M, FAGO F J, CARLSON R M, et al.

Rearranged hopanes in sediments and petroleum

Geochimica Cosmochimica Acta, 1991, 55(11):3333-3353.

DOI:10.1016/0016-7037(91)90492-N      URL     [Cited within: 1]

RULLKÖTTER J, WENDISCH D.

Mircobial alteration of 17α(H)-hopanes in Madagascar asphalts: Removal of C-10 methyl group and ring opening

Geochimica Cosmochimica Acta, 1982, 46(9):1545-1553.

DOI:10.1016/0016-7037(82)90313-1      URL     [Cited within: 3]

FU J, SHENG G, PENG P, et al.

Peculiarities of salt lake sediments as potential source rocks in China

Organic Geochemistry, 1986, 10(1):119-126.

DOI:10.1016/0146-6380(86)90015-X      URL     [Cited within: 1]

CLARK J P, PHILP R P.

Geochemical characteristics of evaporate and carbonate depositional environments and correlation of associated crude oils in the Black Creek Basin, Alberta

Bulletin of Canadian Petroleum Geology, 1989, 37:401-416.

[Cited within: 1]

CONNAN J, BOUROULLEC J, DESSORT D, et al.

The microbial input in carbonate-anhydrite facies of a sabkha palaeoenvironment from Guatemala: A molecular approach

Organic Geochemistry, 1986, 10(1):29-50.

DOI:10.1016/0146-6380(86)90007-0      URL     [Cited within: 1]

RULLKÖTTER J, PHILP R P.

Extended hopanes up to C40 in Thornton bitumen

Nature, 1981, 292(5824):616-618.

DOI:10.1038/292616a0      URL     [Cited within: 2]

WANG P R, LI M W, LARTER S R.

Extended hopanes beyond C40 in crude oils and source rock extracts from the Liaohe Basin, N.E. China

Organic Geochemistry, 1996, 24(5):547-551.

DOI:10.1016/0146-6380(96)00037-X      URL     [Cited within: 3]

ZHU Changfeng, CUI Xingqian, HE Yuxin, et al.

Extended 3β-methylhopanes up to C45 in source rocks from the Upper Cretaceous Qingshankou Formation, Songliao Basin, northeast China

Organic Geochemistry, 2020, 142(1):1-8.

[Cited within: 2]

JIANG Z S, FOWLER M G, LEWIS C A, et al.

Polycyclic alkanes in a biodegraded oil from the Kelamayi Oilfield, northwestern China

Organic Geochemistry, 1990, 15(1):35-46.

DOI:10.1016/0146-6380(90)90183-Z      URL     [Cited within: 4]

ROBISON N, EGLINTON G, BRASSELL S C, et al.

Hydrocarbon compositions of bitumens associated with igneous intrusions and hydrothermal deposits in Britain

Organic Geochemistry, 1986, 10(1):145-162.

DOI:10.1016/0146-6380(86)90018-5      URL     [Cited within: 1]

SCARLETT A G, DESPAIGNE-DIAZ A I, WILDE S A, et al.

An examination by GC×GC-TOFMS of organic molecules present in highly degraded oils emerging from Caribbean terrestrial seeps of Cretaceous age

Geoscience Frontiers, 2019, 10(1):5-15.

DOI:10.1016/j.gsf.2018.03.011      URL     [Cited within: 2]

FAZEELAT T, ALEXANDER R, KAGI R I.

Extended 8,14-secohopanes in some seep oils from Pakistan

Organic Geochemistry, 1994, 21(3):257-264.

DOI:10.1016/0146-6380(94)90189-9      URL     [Cited within: 4]

FAZEELAT T, ALEXANDER R, KAGI R I.

Molecular structures of sedimentary 8,14-secohopanes inferred from their gas chromatographic retention behaviour

Organic Geochemistry, 1995, 23(7):641-646.

DOI:10.1016/0146-6380(95)00044-F      URL     [Cited within: 5]

WENGER L M, ISAKSEN G H.

Control of hydrocarbon seepage intensity on level of biodegradation in sea bottom sediments

Organic Geochemistry, 2002, 33(12):1277-1292.

DOI:10.1016/S0146-6380(02)00116-X      URL     [Cited within: 3]

VILLAR H J, PÜTTMANN W.

Geochemical characteristics of crude oils from the Cuyo Basin, Argentina

Organic Geochemistry, 1990, 16(3):511-519.

DOI:10.1016/0146-6380(90)90066-9      URL     [Cited within: 2]

WANG T G, SIMONEIT B R T, PHILP R P, et al.

Extended 8β(H)-drimane and 8,14-secohopanes series in Chinese Boghead coal

Energy Fuels, 1990, 4(2):177-183.

DOI:10.1021/ef00020a009      URL     [Cited within: 4]

DELRIO J C, GARCIAMOLLA J, GONZALEZVILA F J, et al.

Composition and origin of the aliphatic extractable hydrocarbon in the Puertollano (Spain) oil shale

Organic Geochemistry, 1994, 21(8):897-909.

DOI:10.1016/0146-6380(94)90049-3      URL     [Cited within: 3]

BAO Jianping, WANG Tieguan, WANG Jinyu. Organic Geochemistry of Mesozoic and Paleozoic marine source rocks in the Lower Yangtze area. Chongqing: Chongqing University Press, 1996: 26-40.

[Cited within: 4]

ZHANG Shuichang, LIANG Digang, ZHANG Baomin, et al. Marine petroleum formation in Tarim Basin. Beijing: Petroleum Industry Press, 2004.

[Cited within: 3]

YUN Jinbiao, JIN Zhijun, XIE Guojun.

Distribution of major hydrocarbon source rocks in the Lower Paleozoic, Tarim Basin

Oil & Gas Geology, 2014, 35(6):827-838.

[Cited within: 1]

ZHU Guangyou, CHEN Feiran, CHEN Zhiyong, et al.

Discovery and basic characteristics of the high-quality source rocks of the Cambrian Yuertusi Formation in Tarim Basin

Natural Gas Geoscience, 2016, 27(1):8-21.

[Cited within: 1]

YANG Fulin, YUN Lu, WANG Tieguan, et al.

Geochemical characteristics of the Cambrian source rocks in the Tarim Basin and oil-source correlation with typical marine crude oil

Oil & Gas Geology, 2017, 38(5):851-861.

[Cited within: 2]

JIANG Tongwen, HAN Jianfa, WU Guanghui, et al.

Differences and controlling factors of composite hydrocarbon accumulations in the Tazhong Uplift, Tarim Basin, NW China

Petroleum Exploration and Development, 2020, 47(2):213-224.

[Cited within: 1]

GUAN Shuwei, ZHANG Chunyu, REN Rong, et al.

Early Cambrian syndepositional structure of the northern Tarim Basin and a discussion of Cambrian subsalt and deep exploration

Petroleum Exploration and Development, 2019, 46(6):1075-1086.

[Cited within: 1]

BAO Jianping, ZHU Cuishan, WANG Zhifeng.

Typical end- member oil derived from Cambrian-Lower Ordovician source rocks in the Tarim Basin, NW China

Petroleum Exploration and Development, 2018, 45(6):1103-1114.

DOI:10.1016/S1876-3804(18)30113-7      URL     [Cited within: 3]

ZHANG Shuichang, HUANG Haiping.

Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China: Oil family classification

Organic Geochemistry, 2005, 36(8):1204-1214.

DOI:10.1016/j.orggeochem.2005.01.013      URL    

LI Sumei, PANG Xiongqi, JIN Zhijun, et al.

Petroleum source in the Tazhong Uplift, Tarim Basin: New insights from geochemical and fluid inclusion data

Organic Geochemistry, 2010, 41(6):531-553.

DOI:10.1016/j.orggeochem.2010.02.018      URL     [Cited within: 2]

LI Sumei, AMRANI A, PANG Xiongqi, et al.

Origin and quantitative source assessment of deep oils in the Tazhong Uplift, Tarim Basin

Organic Geochemistry, 2015, 78(1):1-22.

DOI:10.1016/j.orggeochem.2014.10.004      URL     [Cited within: 3]

SUN Yongge, XU Shiping, LU Hong, et al.

Source facies of the Paleozoic petroleum systems in the Tabei Uplift, Tarim Basin, NW China: Implications from aryl isoprenoids in crude oils

Organic Geochemistry, 2003, 34(4):629-634.

DOI:10.1016/S0146-6380(03)00063-9      URL     [Cited within: 2]

CAI Chunfang, LI Kaikai, MA Anlai, et al.

Distinguishing Cambrian from Upper Ordovician source rocks: Evidence from sulfur isotopes and biomarkers in the Tarim Basin

Organic Geochemistry, 2009, 40(7):755-768.

DOI:10.1016/j.orggeochem.2009.04.008      URL     [Cited within: 1]

CAI Chunfang, ZHANG Chunming, RICHARD H W, et al.

Application of sulfur and carbon isotopes to oil-source rock correlation: A case study from the Tazhong area, Tarim Basin, China

Organic Geochemistry, 2015, 83(6):140-152.

[Cited within: 1]

ZHENG Bing, GAO Renxiang.

Characteristics of carbon and sulfur isotopes in crude oils and oil-source correlation in the Tarim Basin

Petroleum Geology & Experiment, 2006, 28(3):281-285.

[Cited within: 1]

ZHU Xinjian, CHEN Jianfa, WU Jianjun, et al.

Carbon isotopic compositions and origin of Paleozoic crude oil in the platform region of Tarim Basin, NW China

Petroleum Exploration and Development, 2017, 44(6):997-1004.

[Cited within: 2]

ZHANG Jizhi, WANG Zhaoming, YANG Haijun, et al.

Origin and differential accumulation of hydrocarbons in Cambrian sub-salt dolomite reservoirs in Zhongshen Area, Tarim Basin, NW China

Petroleum Exploration and Development, 2017, 44(1):40-47.

DOI:10.1016/S1876-3804(17)30006-X      URL     [Cited within: 1]

WANG Zhaoming, XIE Huiwen, CHEN Yongquan, et al.

Discovery and exploration of Cambrian subsalt dolomite original hydrocarbon reservoir at Zhongshen-1 Well in Tarim Basin

China Petroleum Exploration, 2014, 19(2):1-13.

[Cited within: 1]

SONG Daofu, WANG Tieguan, LI Meijun.

Geochemistry and possible origin of the hydrocarbons from Wells Zhongshen1 and Zhongshen1C, Tazhong Uplift

SCIENCE CHINA Earth Sciences, 2016, 59(4):840-850.

DOI:10.1007/s11430-015-5226-z      URL     [Cited within: 1]

SUN Y G, CHEN Z Y, XU S P, et al.

Stable carbon and hydrogen isotopic fractionation of individual n-alkanes accompanying biodegradation: Evidence from a group of progressively biodegraded oils

Organic Geochemistry, 2005, 36(2):225-238.

DOI:10.1016/j.orggeochem.2004.09.002      URL     [Cited within: 1]

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