Geochemical characteristics and significances of rearranged ... - NSFC

Journal of Petroleum Science and Engineering 131 (2015) 138–149

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Geochemical characteristics and significances of rearranged hopanes in hydrocarbon source rocks, Songliao Basin, NE China Lian Jiang a,b, Min Zhang a,b,n a b

Key Laboratory of Exploration Technology for Oil and Gas Research (Yangtze University), Ministry of Education, Wuhan 430100, China School of Earth Environment and Water Resources, Yangtze University, Wuhan 430100, Hubei, China

art ic l e i nf o

a b s t r a c t

Article history: Received 29 January 2015 Accepted 16 April 2015 Available online 5 May 2015

On the basis of GC/MS analysis, a suite of 56 core samples from the Songliao Basin in northeastern China was found to contain variable amounts of four series of rearranged hopanes: the 17α(H)-diahopane series (C29 diahopane and C30 diahopane), the 18α(H)-neohopane series (Ts and C29Ts), the early-eluting rearranged hopane series (C30 early-eluting rearranged hopane or 30E) and the 21-methyl-28-norhopane series (C29 28-nor-spergulane or 29Nsp). The results reveal that compounds in the same series of rearranged hopanes covary with each other closely, whereas linear correlations of four series of rearranged hopanes are diverse. On the whole, the 17α(H)-diahopane series shows an excellent correlation to the early-eluting rearranged hopane series. Both the early-eluting rearranged hopane series and the 17α(H)-diahopane series exhibit a general correlation with the 21-methyl-28-nor-hopane series. A general relationship between the 17α(H)-diahopane series and the 18α(H)-neohopane series is also noted. The 21-methyl-28-nor-hopane series and the early-eluting rearranged hopane series are not closely correlated with the 18α(H)-neohopane series. It seems that the rearranged hopanes show no covariance with some terpanes. Interestingly, rearranged hopanes display positive relations with C27–C29 αββ/ααα regular sterane or C27–C29 20S/20R regular sterane. Except for thermal maturity of hydrocarbon source rocks, this phenomenon is probably related to clay catalysis, mineral matrix or other factors. Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.

Keywords: rearranged hopanes biomarkers geochemical characteristics Songliao Basin

1. Introduction Rearranged hopanes refer to a class of biomarkers with carbon ring framework identical to that of regular hopanes, with the methane side chain carbon position being distinct from that of regular hopanes (Peters and Moldowan, 1993). Multiple homologs occur in hydrocarbon source rocks and crude oils. In recent years, they have received increasing attention as biological markers with applications for geochemical studies of petroleum source rocks and oils (Horstad et al., 1990; Moldowan et al., 1992; Telnæs et al., 1992; Farrimond et al., 1994; Wang et al., 2000; Zhao and Zhang, 2001; Huang et al., 2003; Zhang et al., 2009). 18α(H)-22,29,30-trisnorneohopane (Ts), which has a rearranged methyl group at C-17, is one of the first hopanoid hydrocarbons to be rigorously characterized by X-ray (Whitehead, 1974; Smith, 1975). Then, Trendel et al. (1990) isolated a rearranged and degraded hopanoid [18α (H)-25,30-norneohopane] from a biodegraded asphalt. It was presumably formed in the asphalt by microbiological degradation resulting in

n Corresponding author at: School of Earth Environment and Water Resources, Yangtze University, Wuhan 430100, Hubei, China. E-mail address: [email protected] (M. Zhang).

http://dx.doi.org/10.1016/j.petrol.2015.04.035 0920-4105/Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.

loss of a C-10 methyl group (Requejo and Halpern, 1989; Peters and Moldowan, 1991) suggesting the precursor 18α(H)-30-norneohopane (C29Ts) may exist in nonbiodegraded oils. Moldowan et al. (1991) identified by nuclear magnetic resonance (NMR) spectroscopy techniques a member of the 18α(H)-neohopane series (denoted “C29Ts”). They also determined (by X-ray crystallography) the structure of 17α15α-methyl-27-norhopane (C30 diahopane), a member of the 17α(H)diahopane series. In addition, they observed a pseudohomologous series of C29–C34 17α(H)-diahopanes using GC–MS–MS. These compounds were similar to a C29–C34 pseudohomologous series of triterpane compounds which were designated as “putative neohopanes” in Middle Proterozoic oils of the McArthur Basin in northern Australia (Summons et al., 1988a, 1988b). Killops and Howell (1991) and Telnæs et al. (1992) noted the occurrence of a further series of unidentified rearranged hopanes in oils. Compounds in this pseudohomologous series are notable in eluting approximately two carbon numbers earlier than the regular hopanes. Farrimond and Telnæs (1996) reported this early-eluting rearranged hopane series, which appears to extend from C27 to C35 (without C28 member). The C30 member (C30 earlyeluting rearranged hopane or 30E) of the series was synthesized by Nytoft et al. (2007) in the laboratory, while the structures of higher homologs were unclear. The C29 28-nor-spergulane (“X” in

L. Jiang, M. Zhang / Journal of Petroleum Science and Engineering 131 (2015) 138–149

Huang et al. (2003)), which is a member of the 21-methyl-28-norhopane series, was first found in some of the lacustrine oils from the western Pearl River Basin offshore South China. The abundance of C29 28-nor-spergulane (29Nsp) relative to bicadinanes W and T was used to distinguish the various petroleum source facies in this basin. Subsequently, Nytoft et al. (2006) indentified a new series of rearranged hopanes ranging from C29 to at least C34 as 28-norspergulanes (or 21-methyl-28-nor-hopanes) using NMR spectroscopy. Most commonly, 29Nsp is always the dominant member of the series and seems to be particularly abundant in some oils from lacustrine source rocks in South East Asia. Currently, many scholars have explored the formation conditions and geochemical attributes of rearranged hopanes (Corbett and Smith, 1969; Fowler and Brooks, 1990; Armanios et al., 1992; Dasgupta et al., 1995). The majority held that C30 diahopane may

139

be formed by clay-mediated acidic catalysis under oxic or suboxic environment (Philip and Gilbert, 1986; Moldowan et al., 1991; Peters and Moldowan, 1993; Farrimond and Telnæs, 1996), whereas some scholars believed that clay catalysis under moderately alkaline conditions is helpful for the formation of C30 diahopane (Xiao et al., 2004). C30 diahopane had been regarded as a terrestrial biomarker, because of its recognition in coals, terrigenous hydrocarbon source rocks and terrigenous crudes (Philip and Gilbert, 1986). However, considering isotopic similarity to the regular hopanes and their occurrence in an extended pseudohomologous series, the diahopanes were generally regarded as having a bacterial origin (Moldowan et al., 1991). Some scholars proposed that red algae could possibly be the biogenic source of diahopanes (Zhang et al., 2007). In addition, the algae in saline water environment might also be a source of C30

Fig. 1. Map showing the location and structural divisions of the Lishu Fault Depression in the Songliao Basin.

140


diahopane (Liu et al., 2014). The complexity of geological conditions and diverse study objects result in the difference in the influencing factors of rearranged hopanes, thus the origin and diagenesis of rearranged hopanes remain a puzzle (Smith and Bend, 2004). This is one of scientific problems to be solved in organic geochemical field. It is also a breakthrough of new theories and applications in oil and gas exploration. Although the factors (e.g. depositional environments, diagenetic conditions and parent material sources) related to the formation of rearranged hopanes have been researched widely (Moldowan et al., 1991; Farrimond and Telnæs, 1996; Liu et al., 2014), the relationships

between the different rearranged hopane series and other biomarkers have rarely been studied. The present paper comprises a detailed study of four series of rearranged hopanes, and records the occurrence of different rearranged hopane series in 56 hydrocarbon source rock samples from the Late Jurassic to Early Cretaceous of the Songliao Basin in northeastern China. According to the relative concentration of C30 diahopane, the rearranged hopanes are divided into three groups. The correlations between four series of rearranged hopanes are reported, and their geochemical significances are discussed. This research should improve the understanding of rearranged hopanes and their relationships within the study area. In

Fig. 2. Late Jurassic–Cenozoic stratigraphy of the Lishu Fault Depression in the Songliao Basin. Note: 1 ¼mudstone; 2 ¼pelitic siltstone; 3¼ oil shale; 4¼carbonaceous mudstone; 5 ¼siltstone; 6¼ medium sandstone; 7 ¼conglomerate; 8¼ coarse sandstone–conglomerate; 9¼ pebbly sandstone; 10 ¼volcanic rock; 11¼ metamorphic rock; 12¼ coal; 13¼stratigraphical break. Modified from Chen (2012), Song et al. (2014) and Wang et al. (2015).


addition, it is significative for oil-source correlation and the identification of primary hydrocarbon source rocks in the Songliao Basin.

2. Geological background Situated in northeastern China, the Songliao Basin is one of the largest continental petroliferous basins, with a 750 km length, 330–370 km width and a total area of about 260,000 km2 (Chen, 2012; Zhang et al., 2013; Li et al., 2013; Song et al., 2014). There

141

were four major tectonic episodes during the Triassic of Mesozoic to Cenozoic in the Songliao Basin, which controlled the tectonic evolution and sedimentary filling of the basin. The Late Triassic to Middle Jurassic event resulted in a group of small fault basins. The Late Jurassic to Early Cretaceous was the main period of rifting, responsible for the formation of the Huoshiling Formation (J3h), Shahezi Formation (K1sh) and Yingcheng Formation (K1yc). During this period, organic-rich mudstones were deposited, forming the most important hydrocarbon source rocks in the Lishu Fault Depression of the Songliao Basin. The Early Cretaceous to Late

Table 1 Geochemical parameters of representative hydrocarbon source rocks in the Songliao Basin. Well

Depth (m)

Formation

Lithology

1

2

3

4

5

6

7

8

9

10

11

Content

SN92 SN17 SN148 SN152 SN65 SN65 SN203 SN145 Shuang101 Shuang101 Shuang101 Shuang101 DB32 DB32 DB32 SN10 SN92 SN75 SN105 SN106 SN146 SN148 SN17 SN18 SN18 SN18 SN52 SN52 SN52 SN52 SN64 SN65 SN118 SN166 SN202 SN203 SN203 SW6 SW8 SW8 SN22 SN22 SN55 SN140 SN140 SN153 SN153 SN145 SN162 SN162 SN79 SN80 SN80 Shuang101 SN17 SN52

2081.43 2258.8 1840.5 2276.76 1684.7 1701.5 2037.5 2888.4 2120 2212 2390.15 2397.5 2886.63 2904.5 3106 1704.96 2713.8 2004.6 2093.56 1940.8 1778.56 2023.82 2328 1375 1896 2232 2509.4 2871.3 3207.9 3234.2 1862.48 1556.05 1303.8 1727.3 1650.5 1496 2191.5 1987.46 1697.2 1946.2 1048.3 1333.8 1309.8 1218.4 1320.44 1520.6 1619.46 2686 2128.3 2591 1834.88 2429.7 2681.3 2116.92 2455 3444.5

K1yc K1sh K1yc K1sh K1yc K1yc K1sh K1sh J3h J3h J3h J3h K1d K1d K1yc K1d K1sh K1d K1sh K1yc K1sh K1sh K1sh K1d K1yc K1yc K1yc K1yc K1sh K1sh K1yc K1yc K1yc K1q K1yc K1yc K1sh K1yc K1yc K1sh K1d K1yc K1yc K1yc K1sh K1sh K1sh K1yc K1d K1yc K1d K1d K1yc J3h K1sh K1sh

Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Mudstone Carbonaceous mudstone Carbonaceous mudstone

0.815 0.835 0.803 0.825 0.747 0.785 0.834 1.206 – – – 0.667 1.087 1.190 1.275 0.743 1.058 0.824 0.820 0.838 0.761 0.844 0.915 0.717 0.843 0.845 0.967 1.271 1.439 1.274 0.812 0.739 0.650 0.797 – 0.735 0.866 0.821 – 0.855 0.615 0.700 0.662 0.648 0.654 0.749 0.766 1.023 0.859 1.204 0.816 1.365 1.474 0.541 0.953 1.695

2.13 3.09 0.37 2.01 1.93 2.23 2.82 0.33 5.52 0.96 1.10 1.42 2.39 1.94 0.19 0.30 0.42 0.46 0.68 0.30 1.84 2.87 1.50 1.08 0.85 0.82 0.39 0.76 1.04 1.21 878.00 1.32 2.58 0.50 0.71 0.62 2.09 0.55 0.59 0.33 1.86 5.03 3.01 3.69 0.08 0.83 1.92 0.34 0.25 0.43 0.47 0.18 0.26 2.20 23.69 24.54

1.55 2.92 0.04 0.44 2.32 2.05 2.00 0.05 1.79 0.86 1.18 3.29 0.15 0.12 0.04 0.14 0.06 0.06 0.11 0.06 0.11 0.48 0.15 0.10 0.08 0.14 0.02 0.08 0.06 0.05 0.07 0.16 1.39 0.04 0.05 0.10 0.27 0.03 0.03 0.05 0.35 1.15 0.78 0.02 0.03 0.03 0.17 0.04 0.02 0.05 0.04 0.03 0.04 0.92 0.47 0.34

1.09 2.01 0.22 0.26 0.41 0.24 0.73 0.21 0.22 0.25 0.56 0.39 0.11 0.10 0.10 0.10 0.08 0.09 0.12 0.14 0.07 0.17 0.11 0.11 0.14 0.11 0.09 0.13 0.11 0.09 0.11 0.05 0.11 0.14 0.07 0.19 0.13 0.12 0.08 0.12 0.07 0.06 0.06 0.11 0.09 0.09 0.08 0.10 0.11 0.10 0.12 0.09 0.08 0.11 0.12 0.12

0.66 0.58 0.25 0.24 0.79 0.53 0.88 0.22 0.30 0.38 1.02 0.29 0.12 0.13 0.12 0.12 0.11 0.10 0.12 0.18 0.05 0.14 0.14 0.11 0.16 0.14 0.09 0.15 0.12 0.09 0.10 0.03 0.16 0.15 0.06 0.17 0.13 0.12 0.07 0.15 0.01 0.02 0.03 0.11 0.09 0.11 0.05 0.12 0.12 0.11 0.13 0.09 0.09 0.12 0.13 0.13

1.29 0.67 0.70 0.23 1.75 1.48 1.60 0.17 0.74 0.76 1.57 1.04 0.27 0.27 0.36 0.38 0.35 0.40 0.34 0.52 0.10 0.15 0.42 0.35 0.49 0.42 0.28 0.40 0.40 0.27 0.30 0.07 0.74 0.49 0.24 0.70 0.43 0.53 0.20 0.58 0.09 0.03 0.04 0.38 0.25 0.37 0.06 0.40 0.44 0.40 0.45 0.27 0.35 0.36 0.35 0.42

2.17 1.64 1.76 0.37 6.96 6.38 7.17 0.78 2.80 2.91 5.29 4.60 0.52 0.53 0.65 1.10 0.77 0.99 0.56 1.23 0.18 0.18 1.07 0.84 1.29 1.16 0.57 0.60 0.95 0.58 0.54 0.08 1.95 1.71 0.38 3.92 1.25 1.77 0.40 1.28 0.08 0.03 0.05 0.95 0.59 0.81 0.09 0.94 0.80 0.98 1.37 0.46 0.84 1.29 0.85 1.10

– – 0.05 – 0.12 0.11 0.27 – 0.08 0.36 0.32 0.20 – – – 0.14 – – – 0.14 – – – – – – – – – – – – – 0.03 – 0.05 0.04 0.03 – – – – – 0.14 0.09 – – – – – – – – 0.03 – 0.04

0.85 0.94 0.06 0.09 0.09 0.06 0.20 0.05 – – 0.16 0.08 – – – – – – – – – 0.06 – – – – – – – – – – 0.03 – – – – – – – – – – – – – – – – – – – – – – –

1.97 2.29 0.80 1.22 1.42 1.46 1.53 0.83 – – 1.56 1.48 0.90 0.91 0.88 0.75 0.81 0.70 0.89 0.91 0.75 1.31 – – – – – – – – – – 0.95 0.80 0.81 0.95 0.86 0.78 0.79 0.93 0.92 0.66 1.05 0.89 0.85 0.87 0.86 0.84 0.86 0.81 0.75 0.83 0.82 1.02 0.96 0.80

0.88 0.87 0.75 0.80 0.89 0.87 1.01 0.88 0.98 0.91 1.01 0.95 0.78 0.79 0.81 0.75 0.73 0.71 0.77 0.80 0.72 0.72 0.73 0.78 0.76 0.77 0.79 0.78 0.80 0.79 0.83 0.77 0.81 0.72 0.72 0.94 0.76 0.73 0.75 0.72 0.56 0.71 0.62 0.75 0.78 0.79 0.81 0.74 0.75 0.74 0.75 0.75 0.74 0.88 0.83 0.76

Abnormal High High

Low

Note: 1¼ vitrinite reflectance (%); 2 ¼total organic carbon (TOC; wt%); 3 ¼bitumen “A” in source rock (%); 4¼ C30 diahopane/C30 hopane; 5¼ C29 diahopane/C29 hopane; 6¼ C29Ts/C29 hopane; 7¼ Ts/Tm; 8 ¼29Nsp/C29 hopane; 9 ¼30E/C30 hopane; 10¼ C27–C29 αββ/ααα regular sterane; 11¼ C27–C29 20R/20S regular sterane.

142


Cretaceous was a major episode of subsidence. During this stage, hydrocarbon source rocks of Quantou Formation (K1q), Qingshankou Formation (K2qn) and Nengjiang Formation (K2n) were deposited in a series of well-developed lakes. The final structural inversion and contraction phase of the Songliao Basin occurred from the Nengjiang Formation (K2n) of Late Cretaceous to Cenozoic (Chen, 2012; Wang et al., 2015). Most of samples in the study area were collected from the Lishu Fault Depression in the Songliao Basin. Only individual samples came from the Changling Sag, adjacent to the Lishu Fault Depression. The Lishu Fault Depression is located in the southeast uplifted zone of the Songliao Basin, which developed during the Late Jurassic to Early Cretaceous, belonging to be an independent half-graben basin with an area of about 2350 km2 (Fig. 1). It is confirmed that the Lishu Fault Depression is a mature and independent haft-garben fault basin. The area can be divided into four structural belts, that is, a central structural belt, a northern slope belt, a southeastern slope belt and a Sangshutai sag belt. The sedimentary environment changes result in the diversity of the sedimentary facies in the Lishu Fault Depression. Previous research reported that the sedimentary facies of the Lishu Fault Depression changed from shore-shallow lake to shallow lake, to semi-deep and deep lake, and finally to shallow lake with water depth from shallow to deep to shallow (Chen, 2012; Li et al., 2013). With the diversification of water depth, the thicknesses of source rocks differ with each in the formations (Fig. 2). Major sedimentary facies in the western steep zone include alluvial fan and fan delta. The sedimentary facies are semi-deep and deep lake face in the center of depression, braided river delta face in the eastern and northern gentle slope zone. As we can see from Fig. 2, it develops many sets of source rocks vertically, including J3h, K1sh, K1yc, K1d, K1q and K2qn. The Sifangtai Formation (K2s) and Mingshui Formation (K2m) of the Late Cretaceous, Paleogene and Neogene are missing in the Lishu Fault Depression. The K1yc is in the unconformable contact with K1d (Chen, 2012; Li et al., 2013; Zhang et al., 2013).

3. Experimental 3.1. Samples A total of 56 core samples were collected from the J3h (5), K1sh (18), K1yc (23), K1d (9) and K1q (1) of Late Jurassic to Early Cretaceous in the Songliao Basin. Fifty-one source rocks are from 28 wells (DB32, SN10, SN92, SN75, SN105, SN106, SN146, SN148, SN152, SN17, SN18, SN52, SN64, SN65, SN118, SN166, SN202, SN203, SW6, SW8, SN22, SN55, SN140, SN153, SN145, SN162, SN79, SN80) in the Lishu Fault Depression; only five source rocks come from Shuang 101 well in the Changling Sag. The locations of the wells sampled (without Shuang 101 well) are shown in Fig. 1. 3.2. Analytical methods Fifty samples were selected for vitrinite reflectance (VR) analysis especially for this research. The measurement of reflectance was made on polished resin-embedded whole rock blocks with a Leica MPV3 photomicroscope. The vitrinite reflectance values of samples range from 0.541% to 1.695% with an average of 0.906% (Table 1). All core samples were crushed into fine powder and analysed for their contents of total organic carbon (TOC; wt%) by combustion in a LECO CS-200 induction furnace. Before TOC determinations, carbonate carbon was removed by HCl treatment. TOC values of the most samples are generally higher than 0.4% (Table 1).

Bitumen extractions were preformed on 56 samples using a Soxhlet apparatus for 72 h with a dichloromethane/methanol mixture (93:7 v/v). The bitumen “A” contents are in a range of 0.02– 3.29%, with a mean of 0.49% (Table 1). Then, the extracted bitumens were fractionated into saturated, aromatic hydrocarbons, NSOs (nitrogen, sulfur and oxygen) and asphaltenes using open column chromatography. Only the saturated fraction was analysed in this study. Saturated hydrocarbons were analyzed using gas chromatography-mass spectrometry (GC–MS) in full scan mode. GC–MS was carried out with a HP 5973 mass spectrometer, coupled to a HP 6890 GC equipped with a HP-5MS fused silica capillary column (30 m 0.25 mm i.d., film thicknesses 0.25 μm). The GC temperature was programmed to start at 50 1C for 1 min, increase to 100 1C at a rate of 20 1C/min, and from 100 to 310 1C at a rate of 3 1C/ min with a final hold of 18 min. Helium was used as the carrier gas with a rate of 1.0 ml/min and the ionization source operated at 70 eV. For the analysis of biomarkers, the fragmentograms for triterpanes (m/z 191) and steranes (m/z 217) were recorded.

4. Results and discussion 4.1. Distribution of rearranged hopanes The m/z 191 mass chromatograms of hydrocarbon source rocks show abundant occurrence of the rearranged hopanes in the study area. These mainly include four series of rearranged hopanes: the 17α(H)-diahopane series, the 18α(H)-neohopane series, the earlyeluting rearranged hopane series and the 21-methyl-28-norhopane series (Fig. 3). The 17α(H)-diahopane series appears to extend from C27 to C35 (although the C27 member is not distinct), but with no C28 member; the C31–C35 members elute as pairs of 22S- and 22R-isomers in m/z 191 mass chromatogram (Moldowan et al., 1991). Both C29 diahopane and C30 diahopane are detected in all hydrocarbon source rocks in the study area, whereas their homologs (ZC31) are seldom detected. Compounds in the 18α(H)-neohopane series include C27 and C29–C30 members; the C27 member (Ts) and C29 member (C29Ts) are widely recognized now, while the existence of higher homologs (above C31) is still unclear. Farrimond and Telnæs (1996) identified the peak eluted after C30 regular hopane as the C30 member (C30Ts) of the 18α (H)-neohopane series in m/z 191 mass chromatogram. In addition to Ts and C29Ts, a small peak also appears in the same region in individual samples of hydrocarbon source rocks from the Songliao Basin (Fig. 2). Whether it is C30Ts has not been determined yet. The early-eluting rearranged hopane series, just like the 17α(H)-diahopane series, extends from C27 to C35, but with no C28 (Moldowan et al., 1991; Farrimond and Telnæs, 1996). The C30 member (30E) of the series is commonly abundant in continental oils of China. For instance, 30E is abundant in oils from the Yingmaili and Yaha region of the Tarim Basin in western China (Zhu et al., 1997). The 30E peak elutes after Ts and slightly in front of Tm (Fig. 3). It has been identified in only 12 hydrocarbon source rocks in the Songliao Basin. The carbon distribution of the 21-methyl-28-nor-hopanes series ranges from C27 to C34; the C29 member (29Nsp) is always the dominant member of the series and can be detected in most crude oils or mature sediments using GC–MS. The 29Nsp peak elutes midway between C30 diahopane and 17β(H),21α(H)-30-norhopane (C29 moretane) in m/z 191 mass chromatogram (Nytoft et al., 2006). In the study area, 22 hydrocarbon source rocks are found to contain certain contents of 29Nsp. Hydrocarbon source rocks of the Songliao Basin show variable C30 diahopane/C30 hopane ratios, ranging from 0.05 to 2.01. Fortyfour hydrocarbon source rocks display relatively low abundance of C30 diahopane (C30 diahopane/C30 hopane o0.2). They are mainly from the K1sh and K1yc; just two samples come from the K1q and


C30 H

C29H

143

SN65 1556.05m mudstone K1yc m/z 191

Tm C30 M C31 H C32 H

C29 M

C33 H

Ts C30 H

C34 H

C35 H

G SN203 2037.5m mudstone K1sh m/z 191

Ts C30 Dia C29Ts C29H Tm

C31 H

C30Ts ?

C32 H

29Nsp

C29 Dia

C33 H C34 H

G

SN92 2081.43m mudstone K1yc m/z 191

C29Ts 30E Ts

C29H C29 Dia

C30 Dia C30 H C30Ts ?

Tm

Fig. 3. Mass chromatograms (m/z 191) showing the distribution of rearranged hopanes in representative samples, Songliao Basin (SN65 well, 1556.05 m; SN203 well, 2037.5 m; SN92 well, 2081.43 m). Note: H ¼regular hopane; Dia ¼ diahopane; M ¼moretane.

J3h. A total of 10 samples, mostly from the K1sh, K1yc and J3h, show the ratios of C30 diahopane/C30 hopane between 0.2 and 1.0. The C30 diahopane/C30 hopane ratios of two samples that come from the K1sh and K1yc are higher than 1.0. According to the relative abundance of C30 diahopane, the rearranged hopanes in the study area are divided into three groups: abnormal high (C30 diahopane/ C30 hopane4 1.0), high (C30 diahopane/C30 hopane ¼0.2–1.0) and low (C30 diahopane/C30 hopane o0.2), which is consistent with previous studies (Zhang et al., 2009). Geochemical parameters regarding the representative hydrocarbon source rocks are provided in Table 1. 4.2. Relationship between rearranged hopanes 4.2.1. Relationship between rearranged hopanes of same series Previous studies have illustrated that the relative amounts of rearranged hopanes depend on lithology, original organic matter, depositional environment (i.e. oxic or anoxic conditions), sedimentary and diagenetic media conditions, thermal maturity of hydrocarbon source rocks, etc. (Sieskind et al., 1979; Moldowan et al., 1991; Wang et al., 2000; Zhang et al., 2009). The formation mechanisms and key influencing factors of rearranged hopanes in geological bodies are unknown. However, the rearranged hopanes

of same series seem to have a similar genetic mechanism and display identical geochemical attributes. Although hydrocarbon source rocks in the Songliao Basin are found to contain four series of rearranged hopanes, one compound of rearranged hopanes is detected in both the early-eluting rearranged hopane series and the 21-methyl-28-nor-hopane series. Therefore, typical compounds of the other two rearranged hopane series are separately chosen to discuss the relationships between the same series of rearranged hopanes. As shown in Fig. 4, a good linear relationship exists between C30 diahopane/C30 hopane and C29 diahopane/C29 hopane (r2 ¼0.8640). Similarly, the C29Ts/C29 hopane also exhibits a good positive correlation with Ts/Tm (r2 ¼0.8919). The good linearities between them illustrate that the same series of rearranged hopanes may form in a similar environment of deposition or be derived from the same parent material sources.

4.2.2. Relationship between rearranged hopanes of different series Representative compounds in four series of rearranged hopanes are selected for drawing analysis in order to explore their connections. The results indicate that linear correlations exist between the 17α(H)-diahopane series, the 18α(H)-neohopane


2.5

2.0

2.0

1.6

C29Ts/C29 hopane

C30 diahopane/C30 hopane

144

1.5 1.0 0.5 0.0 0.0

0.5 1.0 C29 diahopane/C29 hopane

1.2 0.8 0.4 0.0 0.0

1.5

2.0

4.0 Ts/Tm

6.0

8.0

Fig. 4. Biomarker ratio cross-plots: (a) C30 diahopane/C30 hopane vs. C29 diahopane/C29 hopane (r2 ¼0.8640); (b) C29Ts/C29 hopane vs. Ts/Tm (r2 ¼ 0.8918). Note: ◆ ¼abnormal high; □ ¼ high; ○ ¼low.

2.0

2.0

C29Ts/C29 hopane


2.5

1.5 1.0 0.5 0.0 0.0

0.5

1.0

1.5

2.0

1.5 1.0 0.5 0.0 0.0

2.5

0.3

C29Ts/C29 hopane

2.0 1.5 1.0 0.5 0.0 0.0

0.2

0.4 0.6 0.8 30E/C30 hopane

1.0 0.5

0.1

0.2 0.3 0.4 29Nsp/C29 hopane

0.5

0.4

0.8

30E/C30 hopane


1.5

0.0 0.0

1.0

1.0

0.6 0.4 0.2 0.0 0.0

1.5

2.0

C29Ts/C29 hopane


2.5

0.6 0.9 1.2 30E/C30 hopane

0.1 0.2 0.3 0.4 29Nsp/C29 hopane

0.5

0.3 0.2 0.1 0.0 0.0

0.1 0.2 0.3 29Nsp/C29 hopane

0.4

Fig. 5. Biomarker ratio cross-plots: (a) C30 diahopane/C30 hopane vs. C29Ts/C29 hopane (r2 ¼ 0.6976); (b) C29Ts/C29 hopane vs. 30E/C30 hopane (r2 ¼ 0.3270); (c) C30 diahopane/ C30 hopane vs. 30E/C30 hopane (r2 ¼ 0.9161); (d) C29Ts/C29 hopane/vs. 29Nsp/C29 hopane (r2 ¼ 0.3232); (e) C30 diahopane/C30 hopane vs. 29Nsp/C29 hopane (r2 ¼ 0.6626); (f) 30E/C30 hopane vs. 29Nsp/C29 hopane (r2 ¼ 0.6968). Note: ◆ ¼abnormal high; □ ¼high; ○¼ low.


series, the early-eluting rearranged hopane series and the 21methyl-28-nor-hopane series except for two samples with abnormally high abundance of C30 diahopane. Fig. 5a illustrates the relationship of C30 diahopane/C30 hopane and C29Ts/C29 hopane. Apart from two samples with C30 diahopane/C30 hopane4 1.0, the C30 diahopane/C30 hopane of all other samples shows a good linear correlation with the C29Ts/C29 hopane. Two hydrocarbon source rocks with abnormal high C30 diahopane abundance are from 2258.8 m in SN17 well (C30 diahopane/C30 hopane ¼2.01) and 2081.43 m in SN92 well (C30 diahopane/C30 hopane ¼1.09), respectively (Table 1). Fig. 5b shows the relationship of C29Ts/C29 hopane and 30E/C30 hopane. Because of the 18α(H)-neohopane series has a limited carbon number distribution, the precursor is probably diplopterol, diploptene, another C30 hopene, or possibly a C29 hopanoid rather than the extended hopenes (Moldowan et al., 1991; Wang et al., 2000; Zhu et al., 2007). Nevertheless, the origin and genesis of the earlyeluting rearranged hopanes are unclear (Nytoft et al., 2007). The C29Ts/C29 hopane exhibits a general positive correlation with 30E/ C30 hopane, while the correlation coefficient is only 0.3270. It suggests that the early-eluting rearranged hopane series and the 18α(H)-neohopane series differ in parent material sources or form in different depositional environments. Suitable precursors for the 17α(H)-diahopane series may derive from hop-17(21)-ene intermediates via an allylic oxidation step at C-16, Δ15,16 double bond formation, and rearrangement involving the methyl group at C-14 during diagenesis (Seifert and Moldowan, 1978; Moldowan et al., 1991). As shown in Fig. 5c, a distinct positive correlation exists between C30 diahopane/C30 hopane and 30E/C30 hopane, which is similar to previous research (Farrimond and Telnæs, 1996; Zhang, unpublished results). It indicates a related diagenetic mechanism for the formation of C30 diahopane and 30E. Moreover, the close relationship between the carbon number distributions of the 17α(H)-diahopane series and the early-eluting rearranged hopane series suggest a related origin. The precursors of the 21-methyl-28-nor-hopane series are hop-17(21)-enes which are oxidized to hopadienes, rearranged to 28-nor-spergula-12,17-dienes and finally reduced, forming 28-norspergulanes. Similar to the 17α(H)-diahopane series, sub-oxidizing environment is helpful to the formation of the 21-methyl-28-norhopane series. The 29Nsp is often positively correlated with the other C29 hopanes, i.e. neohopanes and diahopanes (Nytoft et al., 2006). The positive correlations of C30 diahopane/C30 hopane and 30E/C30 hopane with 29Nsp/C29 hopane are illustrated in Fig. 5e and f, corresponding to the r2 values of 0.6626 and 0.6968, respectively. Nevertheless, Fig. 5d indicates that the 29Nsp/C29 hopane shows a relatively poor relationship with C29Ts/C29 hopane (r2 ¼0.3232). The close correlation between the 17α(H)-diahopane series and the early-eluting rearranged hopane series argues strongly for a common origin. A bacterial source for the hopanoid precursors of all four series of rearranged hopanes is inferred from the widespread geological occurrence of rearranged hopanes (Ageta et al., 1987; Moldowan et al., 1991; Farrimond and Telnæs, 1996; Nytoft et al., 2006 and references therein), and from the isotopic evidence (for neohopanes and diahopanes) of Moldowan et al. (1991). Nevertheless, it is clear that the 18α(H)-neohopane series has the potential for more diverse origins, involving the rearrangement of a greater range of biological precursors. Moreover, the 18α (H)-neohopane series is distinct in its restricted carbon number distribution (rC30), and must be considered to have at least partly different origins to the other three series of rearranged hopanes. These reasons may be account for the relatively general or poor linearities between the 18α(H)-neohopane series and the other three series of rearranged hopanes. Laboratory experiments indicate that the precursors of the 21-methyl-28-nor-hopane series

145

could be 28-nor-spergula-12,17-dienes, which are easily formed from hop-17(21)-enes (Nytoft et al., 2006). The general covariances between the 17α(H)-diahopane series, the early-eluting rearranged hopane series and the 21-methyl-28-nor-hopane series in the samples studied here lead one to consider a similar formation mechanism or depositional environment for these three series of rearranged hopanes. In conclusion, compounds of the different rearranged hopane series in the study area exhibit diverse linear correlations with each other. Overall, the 17α(H)-diahopane series is closely correlated with the early-eluting rearranged hopane series. Both the 21methyl-28-nor-hopane series and the early-eluting rearranged hopane series exhibit a relatively poor relationship with the 18α (H)-neohopane series, while the 17α(H)-diahopane series displays a general correlation with the 18α(H)-neohopane series. Moreover, a general correlation also exists between the 21-methyl-28-norhopane series and the 17α(H)-diahopane series or the earlyeluting rearranged hopane series. 4.3. Geochemical significances of rearranged hopanes 4.3.1. Relationship between rearranged hopanes and biomarker parameters Factor analysis is a multivariate statistical algorithm that is usually used to analyze geological data. It is an important means of system and genetic classification, including R factor analysis, Q factor analysis and correspondence analysis. They are all based on principal component analysis, no matter which analysis method is (Liu et al., 2011). Principal component analysis (PCA) is a statistical procedure that uses an orthogonal transformation to convert a set of observations of possibly correlated variables into a set of values of linearly uncorrelated variables called principal components. The number of principal components is less than or equal to the number of original variables. This transformation is defined in such a way that the first principal component has the largest possible variance (that is, accounts for as much of the variability in the data as possible). And each succeeding component, in turn, has the highest variance possible under the constraint that it is orthogonal to the preceding components. It not only reduces the dimensionality of complex data sets but also retains original data information through the PCA (Jolliffe, 1986; Meglen, 1992; Liu et al., 2011). Thirteen parameter variables are chosen to carry on the PCA by the SPSS statistical software in order to investigate the connections between the different series of rearranged hopanes and other biomarkers in hydrocarbon source rocks effectively and efficiently. The variables mainly include C30 diahopane/C30 hopane, C29 diahopane/C29 hopane, C29Ts/C29 hopane, Ts/Tm, C27–C29 αββ/ ααα regular sterane, C27-C2920R/20S regular sterane, C29 diasterane/C29 sterane, C24 tetracyclic diterpene/C26 triterpane (C24Te/ C26TT) and extended tricyclic terpane ratio (ETR). In SPSS we can obtain the statistics by selecting the dimension reduction and then placing the aforementioned variables in the variables dialog box and then selecting the descriptives button and selecting the antiimage and the Kaiser–Meyer–Olkin (KMO) and Bartlett's test of sphericity options. In this study, the Kaiser–Meyer–Olkin measure of sampling adequacy is a bit low at 0.768, however Bartlett's test of sphericity has an associated P value of o0.001 as by default SPSS reports P values of less than 0.001 as 0.000. It is clear from above results that we can continue and perform a valid factor analysis. Subsequently, the principal components factoring extraction methods and the varimax method of rotation are chosen to get the loading plot. Furthermore, we can obtain the factor scores for individual and then compare them by selecting the regression analysis method and then obtain the factor score coefficient matrix by checking the display factor score coefficient matrix

146


option in the factor scores dialog box. More details about the operation steps of the SPSS statistical software for PCA are introduced by Xue (2010). Two principal components (PCs) were calculated, and PCs 1 and 2 comprise 42.9% and 30.5% of the variance within the scaled data set, respectively. As shown in Fig. 6, the loadings indicate some degree of covariance between the 17α(H)-diahopane series and the 18α(H)-neohopane series (positive PC1); the diasterane plot closes to PC1 ¼0, and thus shows no such correlation. This statistical interpretation is confirmed by the cross-plot of C30 diahopane/C30 hopane and C29Ts/ C29 hopane ratios, in which they show a general correlation with each other (Fig. 5a). It seems that no relationships exist between the rearranged hopanes and some terpanes or diasteranes (scatter plots are omitted). However, the rearranged hopanes exhibit close relationships with steranes (positive PC1, Fig. 6), with further discussion is presented later.

Fig. 6. Loadings plot (PC1 vs. PC2) from the principal component analysis of the biomarker data set (9 variables; PC1 and PC2 comprise 42.9% and 30.5% of the variance in the scaled data set, respectively). Note: □¼ C29 diasterane/C29 sterane, ◇¼C24Te/ C26TT; ◆¼ ETR; ■¼ C27–C29 αββ/ααα sterane and C27–C29 20R/20S sterane; ▼¼ C29Ts/C29 hopane and Ts/Tm; ▲¼ C30 diahopane/C30 hopane and C29 diahopane/C29 hopane.

4.3.2. Relationship between rearranged hopanes and steranes Steranes are thought to be derived from the natural product of sterols during early diagenesis. The C21–C22 pregnanes, C27–C29 regular steranes and diasteranes are usually detected in hydrocarbon source rocks and crude oils. Because of the stable structure and strong ability of resistance to biodegradation, steranes can be

A3

f SN75 2004.6m K1d Ro = 0.824%

m/z 191

a

A2

cd A4

C1 A1 B1

n

m/z 217

C2 B2D1

D2

E

m/z 191

A5

e

b A6

A7

h

i

jk

l m

g

A8

SN152 2276.76m K1sh Ro = 0.825%

m/z 217

F

m/z 191

SN65 1684.7m K1yc Ro = 0.747%

m/z 217

G

m/z 191

SN153 1520.6m K1sh Ro = 0.749%

m/z 217

Fig. 7. Mass chromatograms (m/z 191 and m/z 217) of four hydrocarbon source rocks in the Songliao Basin (SN75 well, 2004.6 m; SN152 well, 2276.76 m; SN65 well, 1684.7 m; SN153 well, 1520.6 m). Note: A1–A8 ¼ Tm, C29–C35 hopane; B1–B2 ¼C29–C30 diahopane; C1–C2 ¼ Ts, C29Ts; D1–D2 ¼ C29–C30 moretane; E ¼ gammacerane; F ¼ 30E; G ¼29Nsp; a ¼C21 pregnane; b¼ C22 pregnane; c ¼ C27 ααα-(20S) sterane; d¼ C27 αββ-(20R) sterane; e¼ C27 αββ-(20S) sterane; f ¼ C27 ααα-(20R) sterane; g¼ C28 ααα-(20S) sterane; h ¼C28 αββ-(20R) sterane; i ¼C28 αββ-(20S) sterane; j ¼ C28 ααα-(20R) sterane; k ¼C29 ααα-(20S) sterane; l ¼C29 αββ-(20R) sterane; m ¼C29 αββ-(20S) sterane; n ¼ C29 ααα-(20R) sterane; Ro ¼ vitrinite reflectance.


well preserved in sediments (Curiale et al., 1983; Peters and Moldowan, 1993). Moreover, they contain a great deal of information about maturity, organic matter source, sedimentary depositional environment, etc. (Fowler and Douglas, 1987; Lu et al., 2008). Diasteranes may be derived from steranes via rearrangement involving the migration from C-10 and C-13 to C-5 and C-14 of the methyl group. The formation is dominated by the thermal stresses in the sediments or related to rearrangements of bacterial hopanoids with catalysis by acidic clay during the early diagenesis process (Rubinstein et al., 1975; Sieskind et al., 1979; De Leeuw et al., 1989). Farrimond and Telnæs (1996) observed that the diasterane/sterane shows a good relationship with the diahopane/hopane or early-eluting series/hopane. However, the C29Ts/ norhopane shows no covariance with the diasterane/sterane. In contrast to previous studies, there is no correlation between rearranged hopanes and diasteranes in this research. Interestingly, on the premise of similar maturity, the abundances of ββ regular steranes of hydrocarbon source rocks with relatively high content of C30 diahopane are larger than that of samples with relatively low content of C30 diahopane. For example, the ion peaks of ββ regular steranes of the SN152 well (C30 diahopane/C30 hopane¼ 0.26; vitrinite reflectance¼ 0.824%) are apparently higher than that of the SN75 well (C30 diahopane/C30 hopane¼0.09; vitrinite reflectance¼ 0.825%) in m/z 217 mass chromatogram (Fig. 7). Likewise, this phenomenon occurs in the samples of SN65 well (vitrinite reflectance¼ 0.747%, C30 diahopane/C30 hopane¼0.41) and SN153 well (vitrinite reflectance¼0.749%, C30 diahopane/C30 hopane¼0.09). Fig. 8 illustrates the relationships of different rearranged hopanes and C27–C29 αββ/ααα regular sterane. It is clear that C30 diahopane/ C30 hopane, C29Ts/C29 hopane, 30E/C30 hopane and 29Nsp/C29 hopane correlate with C27–C29 αββ/ααα regular sterane in general. In addition, these compounds in the different rearranged hopane

series also tend to exhibit good correlations with C27–C29 20S/20R regular sterane (Fig. 9). Generally speaking, steranes transform from the biological configuration to the geological configuration with the increase of thermal maturity (Peters and Moldowan, 1993; Lu et al., 2008). As hydrocarbon source rocks show similar maturity, it is unlikely that thermal maturity contributes to sterane isomerization. The relative high concentration of C30 diahopane in hydrocarbon source rocks may be related to sterane isomers. Spiro (1984) found sterane isomerization is not just controlled by time and temperature. The high degree of isomerization in the bound fraction of the black shales from upper Cretaceous of west Greenland can possibly be attributed to the high kaolinite content, which acts as proton donor catalyst. Ten Haven et al. (1986) observed that ancient hypersaline environments are characterized by relatively high amounts of 20R- and 20S-5a(H),l4β (H),l7β(H)-steranes, providing the immature samples with a “mature appearance”. They explained this by variations in the composition of the steroid precursors. To what extent the mineral matrix (e.g. gypsum) also plays an important role in sterane isomerization in a hypersaline environment of deposition. Tannenbaum et al. (1986) and Jovančićević et al. (1993) reported that silicates had the most pronounced effect on the stereochemical changes in the sidechains of the steranes (20R-20S) and triterpanes (22R-22S), and smaller effects on the changes in the triterpane rings (moretanehopane) and sterane rings [14α(H),17α(H)-14β(H),17β(H)] as well as on the formation of diasteranes. On the other hand, Alexander et al. (1984) investigated the mechanism of sterane epimerization catalyzed by clay. An alkyl hydrogen exchange mechanism was suggested, predicting a decreasing reactivity in the range of C24 4 C20 4 C14 EC17. These findings of Alexander et al. (1984) also corroborated with analysis results of

29Nsp/C29 hopane

2.0 C29Ts/C29 hopane

2.0 1.5 1.0 0.5

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0.5

1.0

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0.8

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147

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0.6 0.4 0.2

0.5 1.0 1.5 2.0 C27-C29 αββ/ααα regular sterane

0.0

Fig. 8. Biomarker ratio cross-plots: (a) C30 diahopane/C30 hopane vs. C27–C29 αββ/ααα regular sterane; (b) C29Ts/C29 hopane vs. C27–C29 αββ/ααα regular sterane; (c) 29Nsp/C29 hopane vs. C27–C29 αββ/ααα regular sterane; (d) 30E/C30 hopane vs. C27–C29 αββ/ααα regular sterane. Note: ◆ ¼abnormal high; □¼ high; ○ ¼ low.

148


2.0

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29Nsp/C29 hopane

C29Ts/C29 hopane

2.0

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1.0

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0.5

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0.4

0.8

30E/C30 hopane


2.5

0.3 0.2

0.6 0.4 0.2

0.1 0.0 0.0



0.0 0.0


Fig. 9. Biomarker ratio cross-plots: (a) C30 diahopane/C30 hopane vs. C27–C29 20S/20R regular sterane; (b) C29Ts/C29 hopane vs. C27–C29 20S/20R regular sterane; (c) 29Nsp/ C29 hopane vs. C27–C29 20S/20R regular sterane; (d) 30E/C30 hopane vs. C27–C29 20S/20R regular sterane. Note: ◆ ¼abnormal high; □¼ high; ○ ¼ low.

Tannenbaum et al. (1986) and Jovančićević et al. (1993). Previous research has reported that thermal maturity contributes to the formation of rearranged hopanes (Peters and Moldowan, 1993; Farrimond and Telnæs, 1996; Liu et al., 2014). However, the vitrinite reflectance values of 56 core samples within the study area do not covary with different rearranged hopanes. Combined with above analysis, it can be inferred that the good relationships of different rearranged hopanes and C27–C29 αββ/ααα regular sterane or C27–C29 20S/20R regular sterane are presumably influenced by catalysis of active clays, mineral matrix or other factors except for thermal maturity. In addition, further research is required to investigate the effects of illite, montmorillonite and kaolinite on sterane isomerization during the process of catalysis.

5. Conclusions (1) Four series of rearranged hopanes are observed in variable contents in a total of 56 hydrocarbon source rocks from the Songliao Basin in northeastern China: the 17α(H)-diahopane series (C29 diahopane and C30 diahopane), the 18α(H)-neohopane series (Ts and C29Ts), the early-eluting rearranged hopane series (C30 early-eluting rearranged hopane or 30E) and the 21methyl-28-nor-hopane series (C29 28-nor-spergulane or 29Nsp). On the basis of the relative concentration of C30 diahopane, the rearranged hopanes in the study area are divided into three groups: abnormal high (C30 diahopane/C30 hopane41.0), high (C30 diahopane/C30 hopane¼0.2–1.0) and low (C30 diahopane/ C30 hopaneo0.2). (2) Compounds in the same rearranged hopane series display close relationships with each other, whilst diverse linear correlations exist between the rearranged hopanes of different

series. On the whole, the 17α(H)-diahopane series shows a good correlation with the early-eluting rearranged hopane series. Both the 21-methyl-28-nor-hopane series and the early-eluting rearranged hopane series exhibit a relatively poor relationship with the 18α(H)-neohopane series, while the 17α(H)-diahopane series displays a general correlation with the 18α(H)-neohopane series. Moreover, a general correlation also exists between the 21-methyl-28-nor-hopane series and the 17α(H)-diahopane series or the early-eluting rearranged hopane series. (3) Rearranged hopanes do not covary with some terpanes. However, they exhibit general positive relations with C27–C29 αββ/ ααα regular sterane or C27–C29 20S/20R regular sterane. It is not only influenced by thermal maturity of hydrocarbon source rocks. Clay catalysis, mineral matrix or other factors also account for this phenomenon.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Grant no. 41272170). We are grateful to Dr. Rouhi Farajzadeh and three other anonymous reviewers for their helpful comments, suggestions, scientific and linguistic revisions of the manuscript. References Ageta, H., Shiojima, K., Arai, Y., 1987. Acid-induced rearrangement of triterpeniod hydrocarbons belonging to the hopane and migrated hopane series. Chem. Pharm. Bull. 35, 2705–2716.


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