Biomarker signatures of the Ediacaran–Early Cambrian origin petroleum from the central Sichuan Basin, South China: Implications for source rock characteristics

Accepted Manuscript Biomarker signatures of the Ediacaran–Early Cambrian origin petroleum from the central Sichuan Basin, South China: Implications for source rock characteristics Bin Cheng, Zhonghong Chen, Tong Chen, Chengyu Yang, T.-G. Wang PII:

S0264-8172(18)30209-5

DOI:

10.1016/j.marpetgeo.2018.05.012

Reference:

JMPG 3342

To appear in:

Marine and Petroleum Geology

Received Date: 3 February 2018 Revised Date:

30 April 2018

Accepted Date: 17 May 2018

Please cite this article as: Cheng, B., Chen, Z., Chen, T., Yang, C., Wang, T.-G., Biomarker signatures of the Ediacaran–Early Cambrian origin petroleum from the central Sichuan Basin, South China: Implications for source rock characteristics, Marine and Petroleum Geology (2018), doi: 10.1016/ j.marpetgeo.2018.05.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Biomarker signatures of the Ediacaran–Early Cambrian origin petroleum from the central Sichuan Basin, South China: Implications for source rock characteristics

School of Geosciences, China University of Petroleum, 66 Changjiang West Road,

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a

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Bin Cheng a, b, *, Zhonghong Chen a, *, Tong Chen c, Chengyu Yang c, T.-G. Wang c

Huangdao District, Qingdao, Shandong, 266580, China

Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary

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b

Minerals, Shandong University of Science and Technology, Qingdao, Shandong 266590, China. c

College of Geosciences, China University of Petroleum, 18 Fuxue Road, Changping

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District, Beijing, 102249, China

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* Corresponding author.

Tel.: +86 17660924009. E-mail address: [email protected]

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Tel.: +86 15966825759. E-mail address: [email protected]

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ACCEPTED MANUSCRIPT Abstract

Hydrocarbons with an Ediacaran–Early Cambrian origin from the central Sichuan

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Basin, South China were investigated for biomarker compositions. They were obtained by solvent extraction from the Upper Ediacaran reservoir core samples, which contain abundant solid bitumen. The solid bitumen is widely believed to be formed by thermal

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cracking of early-charged oils in the reservoirs, which have presumed source rocks of an

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Ediacaran–Early Cambrian age. Thermal maturity parameters based on phenanthrenes and aromatic steroids suggest a very high thermal maturity for the extracted hydrocarbons, which is consistent with the severe maturation undergone by their host reservoirs. Age-diagnostic biomarkers including C30 24-isopropylcholestanes and

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mid-chain monomethyl alkanes support source rocks of an Ediacaran–Early Cambrian age. The extracted hydrocarbons have very similar biomarker signatures, which include high relative abundance of acyclic isoprenoids and pristane/phytane < 1; high

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abundance of 2α-methylhopanes; high abundance of C27 to C29 steranes with a slight

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preference for either C27 or C29 homologues; high relative abundance of diasteranes; the concomitant

presence

of

24-n-propyl

and

24-isopropylcholestanes

and

24-isopropyl/24-n-propylcholestanes > 1; and the presence of C26 norcholestanes, methyl- and ethylsteranes. The presence of hopanoids including extended series and methylhopanes indicates contributions from prokaryotic bacteria while the presence of various steroids including regular steranes and methylsteranes indicates contributions from the eukaryotic algae. Parameters based on tricyclic terpane and hopane, 2

ACCEPTED MANUSCRIPT dibenzothiophene/phenanthrene ratios, and C27 diasteranes/ (diasteranes + regular steranes) ratios suggest a major shale source, but a mixed shale-carbonate source cannot be completely excluded. Presence of C30 demethylsteranes, low pristane/phytane ratios,

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high gammacerane index, and presence of 28,30-dinorhopane and 2-methyhopanes indicate that the source rocks were deposited in a stratified, anoxic marine depositional

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environment with enhanced salinity conditions.

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Keywords:

Biomarker; Ediacaran–Early Cambrian; Source rock characteristic; Sichuan Basin

1 Introduction

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Large amounts of crude oils, which are sourced by and/or reservoired in the Ediacaran–Early Cambrian sediment sequences, have been discovered at many localities

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of the world. For example, heavy oils were produced from the Jodhpur Formation (540– 640Ma) in the Bikaner-Nagaur Basin in India and were proposed to derive from the

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organic-rich laminated dolomites in the overlying Bilara Formation of this basin (Bhattacharya et al., 2017; Dutta et al., 2013; Peters et al., 1995). Additionally, large accumulations of oils with source rocks of Ediacaran–Early Cambrian age were found in South Oman (Grantham et al., 1990; Grosjean et al., 2009; Höld et al., 1999; Klomp, 1986) and on the Siberia Platform (Fowler and Douglas, 1987; Kelly et al., 2011; Summons and Powell, 1992). Discoveries of these oils have facilitated organic

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ACCEPTED MANUSCRIPT geochemical studies of the Late Precambrian–Early Cambrian sediments. Some of them have sought to cast light on the nature of early life, while the others have been concerned with the petroleum potential (Summons et al., 1988).

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The Anyue gas field is a newly discovered giant gas field in the central Sichuan Basin, South China (Wei et al., 2015; Zou et al., 2014). Very large amounts of natural

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gases have been discovered in the Upper Ediacaran and Lower Cambrian carbonate reservoirs (Wei et al., 2015). Abundant solid reservoir bitumen was also observed in

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these carbonate reservoirs. Several studies have claimed that the natural gases and solid bitumen were formed by thermal cracking of early-charged oils in these reservoirs (Wei et al., 2015; Zou et al., 2014). A recent study conducted by Yang et al. (2018) provided evidences from occurrence, anisotropy, and reflectance of the solid bitumen, which

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support a pyrolysis origin. However, source rocks for these early-charged oils have not been well defined. Four sets of potential source rocks were proposed to be present in the

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Ediacaran–Early Cambrian sedimentary sequences (Zou et al., 2014). Although the exact intervals have not been determined, the early-charged oils in the Upper Ediacaran

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and Lower Cambrian carbonate reservoirs were believed to originate from source rocks within the Ediacaran–Early Cambrian sedimentary sequences (Chen et al., 2017; Wei et al., 2015; Zou et al., 2014). However, to date, few data about source rock characteristics such as the type of organic matter input and depositional environment of these oils are available (Chen et al., 2017). In the present study, solvent extractable hydrocarbons obtained from reservoir core samples that contain abundant solid bitumen were

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ACCEPTED MANUSCRIPT thoroughly investigated for biomarker compositions. The objective is to identify characteristics of the source rocks that generated the early-charged oils in those

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reservoirs.

2 Geological setting

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The Sichuan Basin located in southwest China (Fig. 1a) is a very large intracratonic sedimentary basin covering an area of 1.8 × 105 km2 (Wei et al., 2008). It

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is a very important natural gas base because of the discovery and development of two giant gas fields, the Weiyuan and Anyue gas fields. The present-day Sichuan Basin is bounded by the Longmenshan fracture belt in the northwest, Wanyuan fracture belt in the northeast, Hanyuan fracture belt in the southwest, and Qiyao fracture belt in the

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southeast (Liu et al., 2016; Wei et al., 2008). It can be divided into five broad structural zones, namely, the western steep structural zone, the northern gentle structural zone, the

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eastern steep structural zone, the southeastern gentle structural zone, and the central gentle structural zone (Liu et al., 2016). The Weiyuan gas field and the Anyue gas field

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are both located in the central zone (Fig. 1b). They were developed on the Leshan-Longnvsi Paleozoic paleo-uplift, but they occur in two different structural units (Wei et al., 2015). The Anyue gas field was developed on the Gaoshiti-Moxi structural unit, which was initially formed during the Late Ediacaran–Early Cambrian due to the Tongwan tectonic movement (Wei et al., 2015). The Ediacaran (named Sinian according to Chinese stratigraphic nomenclature)

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ACCEPTED MANUSCRIPT strata in the central Sichuan Basin are composed of the Lower Doushantuo (DST) Formation and the Upper Dengying (DY) Formation (Fig. 1c). The DST Formation consists of shales, calcareous shales, and intercalated dolomite. The DY Formation can

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be subdivided into four members from bottom to top, namely, the D-1, 2, 3, 4 members (Zou et al., 2014). The D-1 member is composed of micritic–microcrystalline dolomite

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intercalated with algal-laminated dolomite, the D-2 member is dominated by algal mat dolomites, the D-3 member is composed of laminated dolomite in the upper part and

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dark shales in the lower part, and the D-4 member is dominated by algal dolomite (Wei et al., 2015; Zou et al., 2014). The Lower Cambrian strata are composed of the Qiongzhusi

(QZS)

Formation,

the

Canglangpu

(CLP)

Formation,

and

the

Longwangmiao (LWM) Formation, from bottom to top (Fig. 1c). They are dominated

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by black shales, muddy siltstones and dolomites, respectively (Zou et al., 2014). Potential source rocks in the Anyue gas field occur in the Lower Ediacaran DST

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Formation, the Lower Cambrian QZS Formation, the D-3 member of the Upper Ediacaran DY Formation, and the D-2 and D-4 members of the DY Formation (Zou et

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al., 2014). The main gas reservoirs occur in the LWM Formation, and the D-2 and D-4 members of the DY Formation.

3 Samples and methods

3.1 Sample preparation

Twenty reservoir core samples containing variable amounts of solid bitumen from 6

ACCEPTED MANUSCRIPT seventeen wells in the Anyue gas field were investigated (Fig. 1d, Table 1). All the core samples were from the Upper Ediacaran DY Formation, of which ten was collected from the D-2 member and the remainder were collected from the D-4 member. The

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surface (ca. 5mm) layer of the core samples was removed by grinding prior to crushing. The interior of the samples was powdered and exhaustively extracted for 48 hours using

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a Soxhlet apparatus and a mixture of dichloromethane with methanol (93:7 v/v). The extracted bitumen samples were treated with n-hexane to precipitate the asphaltene

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fractions. The resultant maltenes were further fractionated into a saturated hydrocarbon fraction, an aromatic hydrocarbon fraction and a polar fraction by column chromatography using activated silica gel/alumina (3:2, w/w). Saturated and aromatic hydrocarbons were eluted with n-hexane and a mixture of dichloromethane/n-hexane

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(2:1, v/v), respectively. Selected saturated fraction samples were further separated into n-alkane and branched/cyclic sub-fractions using 5 Å molecular sieves (Table 1).

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3.2 Gas chromatography-mass spectrometry (GC–MS)

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GC–MS analyses of the saturated hydrocarbon fractions were carried out on an Agilent 5975i mass spectrometer coupled to an Agilent 6890 gas chromatograph equipped with an HP-5MS capillary column (60 m × 0.25 mm × 0.25 µm film thickness). The mass spectrometer was operated in both full-scan and selected ion monitoring (SIM) modes with an electron ionization mode at 70 eV. The GC oven temperature was initially set at 50 °C with a holding time of 1 min and programmed to

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ACCEPTED MANUSCRIPT 120 °C at 20 °C/min, then to 310 °C at 3 °C/min with a final holding time of 25 min. The aromatic hydrocarbon fractions were analyzed using the same equipment and column but with a different temperature program. For analyses of the aromatic fractions,

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the GC oven temperature was initially set at 80 °C for 1 min and programmed to 310 °C at 3 °C/min with a final holding time of 16 min. The mass spectrometer was operated in

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the full scan mode with an electron ionization mode at 70 eV. Helium was used as the

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carrier gas.

3.3 Metastable reaction monitoring GC–MS (MRM GC–MS)

Analyses of the branched/cyclic sub-fractions were carried out on an Agilent 6890 gas chromatography interfaced to a Quatro II mass spectrometer equipped with a DB-5

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fused silica capillary column (60 m × 0.25 mm × 0.25 µm film thickness). Helium was used as the carrier gas at a constant flow (1 ml/min). The GC oven was initially set at

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100 °C with a holding time of 1 min and programmed to 320 °C at 4 °C/min with a final holding time of 20 min. The mass spectrometer was operated in the metastable reaction

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monitoring (MRM) mode to observe dozens of parent-daughter transitions representing various biomarker compounds. The dwell time was 100 ms for each transition, with a 20 ms interchannel delay. The source was operated in the EI mode and the ionization energy was 70 eV. Argon was used as the collision gas at 2 × 104 mbar with a collision energy of 20 eV.

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ACCEPTED MANUSCRIPT 4. Results

4.1. Bulk composition of extracted hydrocarbons

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The total yields of extracted hydrocarbons are generally low, which is consistent with the severe maturation of the reservoirs (Zou et al., 2014). Total amounts of the

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solvent extractable hydrocarbons vary from 10 to 104.2 ppm (ug/g rock) (Table 1). Variations in the total yields of the samples are attributed to different abundances of

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solid bitumen occurring in the reservoir rocks (Chen et al., 2017). Concentrations of total saturated hydrocarbons are in the range of 1.7 to 69.6 ppm, while those of the aromatic hydrocarbons are in the range of 0.4 to 38.1 ppm (Table 1). The ratios of saturated to aromatic hydrocarbon concentrations are in the range of 0.7–11.5, with an

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average of 3.0 (Table 1).

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4.2 Saturated hydrocarbon assemblage

4.2.1 Acyclic compounds

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Solvent extractable bitumen from the D-4 and D-2 reservoir core samples shows a

remarkable similarity in saturated hydrocarbon compositions. Total ion chromatograms (TIC) of saturated hydrocarbon fractions of representative samples are presented in Fig. 2. A series of C14–C34 n-alkanes was observed. They show a very similar unimodal pattern and display a predominance of C18–C23 n-alkanes. Most extracted bitumen samples contain lower amounts of mid-chain monomethyl alkanes relative to n-alkanes

9

ACCEPTED MANUSCRIPT (Fig. 2). These compounds were identified by comparing their retention positions and mass spectra with those reported by Klomp (1986) and Fowler and Douglas (1987). The mid-chain monomethyl alkanes were also found in the Neoproterozoic oils of Oman

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(Grosjean et al., 2009) and Siberian (Kelly et al., 2011) and considered a characteristic of Precambrian source rocks. The major regular acyclic isoprenoids identified are those

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in the C18–C20 range, and a C25 isoprenoid alkane (2,6,10,14,18-pentamethyleicosane) (Fig. 2). All the samples have higher concentrations of phytane than pristane under GC–

(Table 2).

4.2.2 Cyclohexylalkanes

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MS full-scan analyses, with the pristane/phytane ratio (Pr/Ph) ranging from 0.22 to 0.74

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n-Alkylcyclohexanes and methyl-n-alkylcyclohexanes were detected in lower concentrations relative to n-alkanes (Fig. 2). The n-alkylcyclohexanes extend from C16

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to at least C27, with a predominance of C20 isomer in most samples (Fig. 2). The methyl-n-alkylcyclohexanes range from C17 to at least C27, with the possible

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co-occurrence of three series of methyl-n-alkylcyclohexanes (1,2-, 1,3- and 1,4-substituted isomers) indicated by the presence of homologous triplets in the m/z 97 mass chromatograms (Fig. 2).

4.2.3 Terpanes

Metastable reaction monitoring GC–MS (MRM GC–MS) analyses of tricyclic terpanes revealed the presence of the 13β(H),14α(H)-tricyclic terpanes extending from 10

ACCEPTED MANUSCRIPT C19 to at least C35, although the C32+ homologues are present in very low concentrations. MRM GC–MS analyses of hopanes revealed the presence of the series of 17α(H),21β(H)-hopanes. As shown in Fig. 3, the most prominent peak in the hopane

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series is that of the 17α(H),21β(H)-C30 hopane. The 17α(H),21β(H)-30-norhopane (C29) is the second most abundant hopane. The 18α(H),21β(H)-30-norneohopane (C29Ts)

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elutes on the trailing edge of the C29 hopane and presents in concentrations approximately 30% of the abundance of the C29 hopane (Fig. 3). Components with 28

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carbon atoms have a low abundance, and the highest peak was assigned as 17α(H),21β(H)-29,30-dinorhopane based on its elution position relative to the same compound identified in oils from the Siberian Platform (Summons and Powell, 1992). The 17α(H),21β(H)-28,30-dinorhopane was also detected in these samples (Fig. 3). It is

low

concentrations.

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a common constituent of many marine bitumen and crude oils and typically occurs in The

C27

hopanes

are

represented

by

the

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17α(H)-22,29,30-trinorhopane (Tm) and the 18α(H)-22,29,30-trinorneohopane (Ts). Extended 17α(H),21β(H)-hopanes with 31 to 35 carbon atoms are present in most of the

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studied samples and decrease in concentrations with increasing carbon numbers. 17β(H),21α(H)-Hopanes (moretanes) were also detected, although they were present in much lower concentrations relative to their corresponding hopanes. Gammacerane was identified in the 412→191 metastable reaction chromatograms, showing much lower concentrations compared to the C30 hopane. The presence of a series of methylhopanes was revealed by the MRM M+→205 chromatograms (Fig. 4). According to their elution

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ACCEPTED MANUSCRIPT positions reported by previous studies (Eigenbrode et al., 2008; Summons and Jahnke, 1990),

they

were

assigned

to

2α-methyl-17α(H),21β(H)-hopanes.

3β-Methyl-17α(H),21β(H)-hopanes may also be present in the current samples,

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although they are in much lower abundances relative to the 2α-methylhopanes (Fig. 4).

4.2.4 Steranes

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MRM GC–MS analyses of steranes revealed the predominance of the C27 to C29

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regular steranes (Fig. 5). Distributions of the C27 to C29 regular steranes show only slight preference for either C27 or C29 steranes, which is generally characteristic of normal marine shale oils (Logan et al., 1997). 13β(H),17α(H)-Diasteranes are also present and they are in lower concentrations relative to the C27–C29 regular steranes.

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Values of the C27 diasteranes/ (diasteranes + regular steranes) ratios are in the range of 0.16 to 0.29 (Table 2). The C30 desmethylsterane distributions revealed by the 414→217

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metastable reaction chromatograms are characterized by the concomitant presence of the 24-n-propyl and 24-isopropylcholestanes (Fig. 5). They were identified based on

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their elution positions and behaviors reported by previous studies (McCaffrey et al., 1994; Moldowan et al., 1990). These compounds are present in much lower amounts relative

to

the

C27–C29

regular

steranes.

Values

of

the

24-isopropyl/24-n-propylcholestanes ratio determined in representative samples are all higher than 1.0 (Table 2). Moreover, C26 desmethylsteranes were also detected by using the 358→217 MRM traces (Fig. 5). They are typically present in much lower

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ACCEPTED MANUSCRIPT concentrations relative to the C27–C29 regular steranes. Distributions of the C26 desmethylsteranes show high relative abundances of 27- and 24-norcholestanes. 21-Norcholestanes are also present, showing lower concentrations relative to the 24-

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and 27-norcholestanes. In addition, three series of methylsteranes, i.e., 2α-, 3β-, and 4α-methylsteranes were also detected. The distributions of methylsteranes are largely

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dominated by the 3β-methylsteranes (Fig. 6). Predominance of the 3β-methylsteranes over methylsteranes alkylated at C-2 and C-4 is characteristic of the Late Proterozoic

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and Paleozoic geological samples (Summons and Powell, 1992). The C30 4α-methylsteranes identified on the 414→217 metastable reaction chromatograms include 4α,23,24-trimethylcholestanes and 4-methyl-24-ethylcholestanes (Fig. 6). The 4α,23,24-trimethylcholestanes (dinosteranes) were also detected by using 414→98

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metastable reaction chromatograms (not shown in the present study). These compounds were also observed in the South Oman oils, which have been proven to be associated

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with source rocks of the Neoproterozoic to Cambrian Huqf Supergroup (Grosjean et al., 2009). Moreover, the transitions from molecular ions to m/z 245 daughter ions revealed

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the presence of 3β-ethylsteranes (Fig. 6). They were also observed in the Late Proterozoic oils from the Siberian Platform (Summons and Powell, 1992).

4.3 Aromatic fraction hydrocarbons

Fig. 7 shows GC–MS TIC of the aromatic hydrocarbon fractions of representative samples. Phenanthrene and its alkylated homologues including methyl- and

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ACCEPTED MANUSCRIPT dimethyl-phenanthrenes are major compounds detected. Several pyrolytically derived polycyclic aromatic hydrocarbons, such as fluoranthene and pyrene were found to be present in relatively high abundances (Fig. 7). Distribution patterns of phenanthrene and

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its alkylated homologues show great differences. Some samples show a predominance of the parent compound phenanthrene while the others are dominated by the alkylated

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homologues. Triaromatic steroids with 21 to 22 and 26 to 28 carbon atoms were identified in m/z 231 mass chromatograms (Fig. 7). They show variations in the relative

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abundances between the C21–C22 and C26–C28 compounds.

5. Discussion

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5.1 Organic matter input of source rock

The n-alkane distributions suggest a marine origin for the organic matter that is probably derived from bacteria and/or algae (Fowler and Douglas, 1987). Although

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specific precursors for min-chain monomethyl alkanes have not been defined, these

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compounds are most likely derived from prokaryotic organisms (Höld et al., 1999; Summons and Powell, 1992; Summons et al., 1988). The presence of regular acyclic isoprenoids including extended homologs indicates contributions from photosynthetic bacteria and archaebacteria such as methanogens and extreme halophiles (Moldowan and Seifert, 1979; Volkman and Maxwell, 1986). The n-alkylcyclohexanes and methyl-n-alkylcyclohexanes probably have the same precursor as the n-alkanes suggested by their similar distribution patterns, although the n-alkylcyclohexanes could 14

ACCEPTED MANUSCRIPT also have a direct biological source (Fowler and Douglas, 1987). The hopanoids have been widely proposed to derive from the bacteriohopanols found in eubacteria (Ourisson et al., 1979; Peters et al., 2005; Summons and Walter,

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1990). However, there are no universal rules governing the occurrence of hopanoids within the eubacteria, except for the extended bacteriohopane polyols. Thus, it is now

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generally accepted that hopanes, especially the extended series of C31–C35 compounds, can be indicative of bacterial input. A full series of 17α(H),21β(H)-hopanes (C27–C35)

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present in relatively high abundances have been detected in all studied samples, indicating significant bacterial input to the source organic matter. In addition, the rearranged hopanes and neohopanes were also detected. Despite that their direct biologic precursors have not been identified, they are probably formed by diagenetic

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rearrangement of regular hopanoids (Moldowan et al., 1991; Seifert and Moldowan, 1978). In addition, the presence of methylated hopanes is consistent with the

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contribution of organic matter from bacteria. Hopanoids bearing a 2β- or 3β-methyl group at ring-A have been identified in the lipids of extant bacteria. The fossil

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2α-methylhopanes are believed to be derived from cyanobacteria (Summons et al., 1999) while the 3β-methylhopanes are potentially indicative of contributions from methanotrophic proteobacteria (Collister et al., 1992; Farrimond et al., 2004). Relatively high abundances of C27–C29 regular steranes have been detected in the present samples. They are diagenetic and catagenetic alteration products of sterols, which are membrane components abundant in most eukaryotes. Normally, their

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ACCEPTED MANUSCRIPT presence can indicate contributions of organic matter from eukaryotes, although there are no applicable rules that would permit assignment of particular steranes to specific eukaryotes. Cyanobacteria have been proposed as potential sources of the C29 steranes

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found in some sediments (Fowler and Douglas, 1987), but evidence that cyanobacteria actually biosynthesize sterols is equivocal (Ourisson et al., 1987). Some bacteria have

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been reported to synthesize several sterol compounds, but their sterol biosynthesis is indeed limited to a few distinctive structural isomers (Brocks et al., 2003; Summons and

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Powell, 1992). It is the eukaryotic algal that have been widely accepted as the most likely source for these steranes. The C27–C29 sterane distributions observed in the studied samples can be attributed to eukaryotic algal input to the source organic matter. The presence of C26 desmethylsteranes is consistent with previous observations of such

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components in the Late Proterozoic sediments (Summons et al., 1988). Although the source organisms contributing such components are not known with certainty, they were

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proposed to be derived from eukaryotic algae (Summons et al., 1988). The presence of dinosteranes agrees with earlier reports of their occurrences in the Precambrian

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sediments (Pratt et al., 1991; Summons and Walter, 1990). These compounds found in the Triassic and younger sediments have been believed to be indicative of contributions from dinoflagellates, while for older sediments, they were believed to be derived from some eukaryotic organisms that may have biochemical affinities with the dinoflagellates (Moldowan

and

Talyzina,

1998;

Summons

and

Walter,

1990).

The

24-n-propylcholestanes are proposed to be derived from marine Pelagophyceae (Love et

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ACCEPTED MANUSCRIPT al., 2009; Moldowan et al., 1990). The predominance of 24-isopropylcholestanes over 24-n-propylcholestanes in Late Proterozoic and Early Cambrian sediments and oils has been suggested to reflect the radiation of sponges (Love et al., 2009; McCaffrey et al.,

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1994). The presence of both compounds may reflect the contribution of organic matter from Pelagophyceae and sponges, although they are present in much lower amounts

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relative to regular steranes in our studied samples. It has been noticed that the relative abundance of steranes is lower than that of hopanes in the studied samples. The ratio of

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steranes to hopanes is primarily used as an indicator of input from eukaryotes (mainly algae) relative to bacteria (Moldowan et al., 1985). Values of the steranes/hopanes ratio for our studied samples are in the range of 0.24–0.71 (Table 2), suggesting that a higher number of prokaryotes (bacteria) relative to eukaryotes has been incorporated into

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kerogens responsible for the extracted hydrocarbons.

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5.2 Lithology and depositional environment of source rock

Several parameters based on various tricyclic terpanes and hopanes have been

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commonly used to decipher source rock lithology (Al-Ameri and Zumberge, 2012; Dutta et al., 2013; Peters et al., 2005). Cross-plots of C22/C21 versus C24/C23 tricyclic terpane, C26/C25 tricyclic terpane versus C31R/C30 hopane, and C29/C30 versus C35S/C34S hopane (Fig. 8, Table 2) suggest a marine shale source for the majority of the extracted bitumen samples in this study (Al-Ameri and Zumberge, 2012; Dutta et al., 2013; Peters et al., 2005). Values of the dibenzothiophene to phenanthrene (DBT/P) ratio and the C27

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ACCEPTED MANUSCRIPT diasteranes/ (diasteranes + regular steranes) ratio for most of the studied samples are below 0.1 and higher than 0.2, respectively (Table 2), which is consistent with shale as the source rocks (Hughes et al., 1995; Peters et al., 2005). However, it is also noticed

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that several samples have lower C27 diasteranes/ (diasteranes + regular steranes) ratios (< 0.2), suggesting possible contributions from marine carbonate source rocks (Peters et

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al., 2005). Potential source rocks including shale and carbonate within the Ediacaran– Lower Cambrian sedimentary intervals have been observed in the Gaoshiti–Moxi

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paleo-uplift and adjacent areas (Wei et al., 2015; Zou et al., 2014). Therefore, a major shale source can be inferred for petroleum in this area, while a mixed shale-carbonate source cannot be completely excluded.

The presence of C30 demethylsteranes indicates a marine depositional environment

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for the source rocks (Moldowan, 1984; Moldowan et al., 1990). The pristane/phytane (Pr/Ph) ratio has been widely applied to interpret redox conditions of the source rock

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depositional environment (Didyk et al., 1978). Low values of Pr/Ph ratio (< 0.8) have been suggested to typify anoxic, commonly hypersaline conditions of the depositional

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environment (ten Haven et al., 1987; Peters et al., 2005). As shown in Table 2, values of the Pr/Ph ratio for all the studied bitumen samples are below 0.8 (0.22–0.74), suggesting that they were generated from marine source rocks deposited under an anoxic environment. The presence of 28,30-DNH is consistent with a highly reducing depositional environment (Brocks et al., 2003; Mello et al., 1989). The gammacerane index, expressed as 100 × gammacerane/17α(H), 21β(H)-hopane, varies from 5.4 to

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ACCEPTED MANUSCRIPT 20.0 with an average of 14.1 (Table 2). These exceedingly high values may signal hypersaline episodes of source rock deposition (Moldowan et al., 1985). The presence of 2α-methylhopanes could also be indicative of a hypersaline environment since these

and Powell, 1987).

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5.3 Probable source for extracted hydrocarbons

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hydrocarbons have been reported to be often associated with that condition (Summons

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A burial history study of the central Sichuan Basin suggested that the Cambrian and Upper Neoproterozoic formations in this region have reached a maximum temperature of >200 °C (Zou et al., 2014). Natural gases in the Lower Cambrian and Upper Ediacaran gas reservoirs are commonly considered to be formed by thermal

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cracking of crude oils (Chen et al., 2017; Liu et al., 2016; Wei et al., 2015; Yang et al., 2018). The once-existed crude oils in these reservoirs were subjected to extensive

EP

thermal alteration, forming natural gases and residual solid bitumen through disproportionation reactions. On the other hand, it has been suggested that C15+

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hydrocarbons can survive for geological periods of time at temperatures clearly above 200 °C (Brocks et al., 2003; Brocks and Pearson, 2005; Price, 1993). Several examples of high-temperature petroleum fields have been reported in the previous studies (McNeil and Bement, 1996; Sajgó, 2000). Moreover, series of saturated and aromatic compounds have also been reported to be present in the extracts of ancient and/or highly mature sediments (Brocks et al., 2003; George and Jardine, 1994). Price (1993) has

19

ACCEPTED MANUSCRIPT proposed that several factors in addition to burial temperature and geological time may control the organic-matter metamorphic reactions which involve hydrocarbon generation, maturation, and thermal destruction. Many laboratory experiments have

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strongly suggested that the presence of water, increasing fluid pressures, and closed systems all may suppress thermal destruction of the C15+ hydrocarbon (Price, 1993;

SC

Price and Wenger, 1992). The presence of water and abnormally high pressure have been both encountered in the gas reservoirs studied (Wei et al., 2015), which could have

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suppressed the oil destruction reactions and preserved considerable amounts of C15+ hydrocarbons.

The high thermal maturity of the extracted bitumen samples is consistent with the extensive thermal maturation undergone by their host reservoirs. Phenanthrenes and

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aromatic steroids have been frequently used to evaluate petroleum thermal maturity at highly mature stages (Brassell et al., 1984; Peters et al., 1995; Radke, 1988; Seifert and

EP

Moldowan, 1978). It has been observed that the Methylphenanthrene Index (MPI-1) reaches its maximum value at a vitrinite reflectance equivalent of 1.7% and then

AC C

decreases with higher maturities (Boreham et al., 1988). Low MPI-1 values can be indicative of immature or highly mature bitumen. Its reversal at advanced stages of thermal maturation is caused by demethylation of methylphenanthrenes to phenanthrene and can be recognized by high values of the phenanthrene to the sum of methylphenanthrenes ratios (P/MP) and/or low values of 1-MP/P ratios (Brocks et al., 2003; Radke, 1988). The methylphenanthrene distribution factor (MPDF) has also been

20

ACCEPTED MANUSCRIPT used to evaluate thermal maturity of highly mature bitumen, although it could be influenced by methylation of phenanthrene (Brocks et al., 2003). In contrast to the MPI-1 and MPDF the methylphenanthrene ratio (MPR) is less affected by

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demethylation and methylation reactions and could be applied as the most reliable phenanthrene parameter in the high maturity range (Brocks et al., 2003; Radke, 1988).

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All the extracted bitumen samples studied, except MX10 and MX12, have relatively low 1-MP/P values, indicating that their low MP1-1 values are due to dealkylation of

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methylphenanthrenes at very high thermal maturities (Radke, 1988). Their calculated vitrinite reflectance (Rc%) according to Boreham et al. (1988) are in the range of 2.3– 2.9% (Table 2). This advanced stage of thermal maturation is also supported by their high MPR and MPDF values (Table 2). Several samples (e.g. GS20 and MX8) show

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extremely high P/MP and low 1-MP/P ratios, suggesting probable pyrolytic alteration (Brocks et al., 2003). Their very low, inverted MPI-1 and very high Rc% values are in

EP

agreement with this explanation. Indeed, these samples have been found to have enhanced concentrations of several polycyclic aromatic hydrocarbons including (Fl),

pyrene

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fluoranthene

(Py),

benz[a]anthracene

(BaA),

chrysene

(Chy),

benzofluoranthenes (BFl), and benzopyrenes (BPy) and have abnormally high values of the Fl/(Fl + Py), BaA/(BaA + Chy), and BFl/(BFl + BPy) ratios, suggesting thermal alteration of abnormal heating (Cheng et al., 2018). This is consistent with the reports by Liu et al. (2016) and Yang et al. (2018) which proposed thermal alteration of hydrothermal activity. Samples MX10 and MX12 have much higher 1-MP/P and lower

21

ACCEPTED MANUSCRIPT P/MP values compared to the remainder samples, suggesting that their MPI-1 values may have not reached the maximum. Their Rc% values according to Boreham et al. (1988) are much lower than the remaining samples, which may suggest that they were

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not influenced by the hydrothermal activity. With increasing thermal maturity, monoaromatic steroids (MA) were converted to triaromatic steroids (TA) with the ratio

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of TA/(MA+TA) increasing from 0 to 100% (Peters et al., 2005). In addition, apparent carbon-carbon bond cracking in the side chains with increasing thermal maturity was

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also documented for both MA and TA (Mackenzie et al., 1981; Seifert and Moldowan, 1978). The ratios of short-chain to long-chain steroid hydrocarbons such as the MA(I)/MA(I+II) and TA(I)/TA(I+II) also increase from relatively low to very high values during thermal maturation (Peters et al., 2005). Monoaromatic steroid

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hydrocarbons were found to be absent or present in extremely low concentrations in aromatic hydrocarbon fractions of the extracted bitumen samples, while triaromatic

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steroid hydrocarbons were present in all studied samples with variable amounts. It can be deduced that the extracted hydrocarbons from the reservoir rock samples have

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experienced severe thermal maturation, leading to the significant conversion of monoaromatic to triaromatic steroids (Mackenzie et al., 1981). However, the TA(I)/TA(I+II) ratios show variable but relatively low values which seems inconsistent with the thermal history of the host rock. This can be explained by severely biodegradation and multiple oil charge in these reservoirs, which is evidenced by the co-occurrence of 25-norhopanes (not shown in the present study) and abundant

22

ACCEPTED MANUSCRIPT n-alkanes. Because C26 to C28 triaromatic steroids are much more resistant to biodegradation compared to C20 and C21 triaromatic steroids (Peters et al., 2005), they could have survived from the severe biodegradation of the earlier oil charge and were

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incorporated into the later oil charge, leading to relatively low TA(I)/TA(I+II) values.

Age-diagnostic biomarkers suggested that the extracted hydrocarbons originate

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from an Infracambrian source rock. They contain 24-isopropylcholestanes and show 24-isopropyl/24-n-propylcholestane ratios typical of petroleum derived from Upper

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Precambrian–Lower Cambrian source rocks (Bhattacharya et al., 2017; McCaffrey et al., 1994). Late Proterozoic and Early Cambrian oils and rock extracts from India, Oman, and Siberian all have high values of the 24-isopropyl/24-n-propylcholestane ratio (> 1), whereas younger and older samples have lower values (Bhattacharya et al., 2017;

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Grosjean et al., 2009; Kelly et al., 2011). In addition, the presence of mid-chain monomethyl alkanes supports an Infracambrian source origin. They have been found in

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Neoproterozoic oils of Oman and Siberian and are considered characteristic of Precambrian–Early Cambrian source rocks (Grosjean et al., 2009; Kelly et al., 2011). In

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summary, the extracted hydrocarbons from the Upper Ediacaran Dengying reservoir core samples probably consist of biomarkers that have survived severe thermal alteration. These biomarkers obtained from the D-4 and D-2 samples have very similar distribution patterns, suggesting that they were derived from a same source. Their composition supports source rocks of an Ediacaran–Early Cambrian age and provides useful information on the source rock characteristics including organic matter input,

23

ACCEPTED MANUSCRIPT lithology and depositional environment.

6. Conclusion

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Twenty reservoir core samples containing abundant solid bitumen from the Upper Ediacaran Dengying Formation of the central Sichuan Basin, South China were

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investigated for biomarker compositions. Although the total yields of extracted hydrocarbons are generally low, they probably contain biomarkers survived from the

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severely thermal alteration of early-charged oils in these reservoirs, which are presumed to be derived from the Ediacaran to Early Cambrian source rocks. The high thermal maturity of extracted hydrocarbons is consistent with the thermal maturation of their host reservoirs, and age-diagnostic biomarkers including 24-isopropylcholestanes

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support an Ediacaran–Early Cambrian origin. Source-related biomarkers indicate an organic matter input mainly from prokaryotic bacteria and eukaryotic algae. A major

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shale source deposited in a stratified, anoxic marine depositional environment with enhanced salinity conditions was proposed, although a mixed shale-carbonate source

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cannot be completely excluded.

Acknowledgement

This work was supported by the China Postdoctoral Science Foundation (Grants No. 2016M602217), the National Natural Science Foundation of China (Grants No. 41272140 and 41702140), the Shandong Provincial Natural Science Foundation of

24

ACCEPTED MANUSCRIPT China (Grants No. ZR2017LD005), and the Fundamental Research Funds for the Central Universities (Grand No. 18CX02183A). Dr. Qinhong Hu, Dr. Richard Patience, and three anonymous reviewers are gratefully acknowledged for their constructive

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comments and suggestions which have significantly improved the quality of the manuscript.

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ACCEPTED MANUSCRIPT Figure captions

Fig. 1. (a) Map showing location of the Sichuan Basin in China, (b) the Anyue gas field

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in the basin, (c) the generalized stratigraphic column for the Ediacaran–Early Cambrian succession in central Sichuan Basin and (d) the sampled wells in the gas

Qiongzhusi; DY: Dengying; DST: Doushantuo.

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field. Formation names: LMW: Longwangmiao; CLP: Canglangpu; QZS:

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Fig. 2. GC–MS total ion chromatograms (TIC) showing distributions of n-alkanes and mid-chain monomethyl alkanes and mass chromatograms showing distributions of acyclic

isoprenoids

(m/z

183),

n-alkylcyclohexanes

(m/z

83)

and

methyl-n-alkylcyclohexanes (m/z 97) in representative samples. Numbers above

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major peaks refer to the carbon numbers; X: mid-chain monomethyl alkanes; Pr: pristane; Ph: phytane.

Fig. 3. MRM chromatograms showing distributions of hopanes in representative Ts:

18α(H)-trisnorneohopane;

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samples.

Tm:

17α(H)-trisnorhopane;

DNH:

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dinorhopane; H: αβ-hopane; G: gammacerane; S and R define the stereochemistry at C-22. The trace identifiers show reaction transition, carbon number and the relative height of the most intense peak in each trace normalized to the highest peak in all traces.

Fig. 4. MRM chromatograms showing distributions of methylhopanes in representative samples. MeH: methylhopane; H: αβ-hopane; S and R define the stereochemistry at C-22. The trace identifiers see Fig. 3. 33

ACCEPTED MANUSCRIPT Fig. 5. MRM chromatograms showing distributions of steranes in representative samples. 21-nor: 21-norsterane; 24-nor: 24-norsterane; 27-nor: 27-norsterane; βα, ααα, αββ denote 13β(H), 17α(H)-diasteranes, 5α(H), 14α(H), 17α(H)- and 5α(H),

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14β(H), 17β(H)-steranes, respectively; n- and i- indicate C30 24-n-propyl and 24-isopropylcholestanes, respectively; S and R define the stereochemistry at C-20.

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The trace identifiers see Fig. 3.

Fig. 6. MRM chromatograms showing distributions of methylsteranes and ethylsteranes

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in representative samples. 2α, 3β and 4α denote 2α-methyl, 3β-methyl and 4α-methylsterane, respectively; ααα and αββ denote 5α(H), 14α(H), 17α(H)- and 5α(H), 14β(H), 17β(H)-steranes, respectively; S and R define the stereochemistry at C-20. The trace identifiers see Fig. 3.

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Fig. 7. GC–MS total ion chromatograms (TIC) showing distributions of aromatic hydrocarbons and mass chromatograms showing distributions of phenanthrene and

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its alkylated homologues (m/z 178 + 192 + 206) and aromatic steroids (m/z 231). P: phenanthrene, MP: methylphenanthrene; DMP: dimethylphenanthrene; Fl:

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fluoranthene; Py: pyrene; S and R define the stereochemistry at C-20.

Fig. 8. Cross plots of the C22/C21 versus C24/C23 tricyclic terpane ratios, the C26/C25 tricyclic terpane versus C31R/C30 hopane ratios and the C29/C30 versus C35S/C34S hopane ratios. The arrows indicate the zones representing the lithology, which are based on the reference diagrams presented in Dutta et al. (2013) and Peters et al. (2005).

34

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Fig. 1

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35

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Fig .2

36

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Fig. 3

37

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Fig. 4

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38

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Fig. 5

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39

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Fig. 6

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40

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Fig. 7

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41

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Fig. 8

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ACCEPTED MANUSCRIPT Table 1 Basic information, extracted yields and bulk composition for the studied samples Sat/ppm

Aro/ppm

Resin/ppm

Asphaltene/ppm

(µg/g rock)

(µg/g ock)

(µg/g rock)

(µg/g rock)

(µg/g rock)

4960

39.4

15.0

4.5

12.2

5.3

3.4

D-4

5015

67.4

31.6

12.9

8.4

9.1

2.4

GS10

D-4

5078

59.4

31.6

11.4

6.1

4.2

2.8

GS16*

D-4

5457

44.6

24.7

6.9

GS20*

D-4

5183

47.8

33.2

8.1

MX8

D-4

5108

11.5

2.4

0.4

MX11*

D-4

5077

17.8

11.1

1.0

MX17*

D-4

5075

46.8

25.9

MX21

D-4

5139

21.1

MX51*

D-4

5397

104.2

GS6

D-2

5373

10.0

GS11

D-2

5376

23.3

GS28*

D-2

5393

44.3

MX8*

D-2

5425

41.7

MX9-S

D-2

5424

MX9-D*

D-2

5434

MX10

D-2

5471

MX11

D-2

5496

MX12

D-2

5430

GS2*

AC C

MX19

D-2

5438

Sat/Aro

4.8

5.2

3.6

3.2

1.6

4.1

2.9

3.5

5.5

1.4

0.5

11.5

10.0

4.0

6.3

2.6

6.0

6.0

0.0

4.9

1.0

69.6

11.5

8.7

8.2

6.0

1.7

0.9

3.3

2.0

1.8

6.3

7.6

3.4

4.2

0.8

18.6

12.8

7.1

4.0

1.4

18.3

8.7

9.6

0.9

2.1

SC

D-4

M AN U

GS1

TE D

Depth/m

64.1

31.1

8.3

19.0

2.3

3.7

98.8

46.0

38.1

9,5

1.1

1.2

11.6

1.9

2.8

0.5

3.7

0.7

19.4

7.3

9.4

1.0

0.8

0.8

26.1

1.9

1.3

2.8

12.9

1.5

27.8

10.4

3.7

5.3

7.9

2.8

EP

Formation

RI PT

EOM/ppm

Name

*: samples further analyzed by MRM GC–MS; D-4: fourth member of the Upper Ediacaran Dengying Formation; D-2: second member of the Upper Ediacaran Dengying Formation; EOM: extracted organic matter; Sat: saturated hydrocarbon fraction; Aro: aromatic hydrocarbon fraction

43

ACCEPTED MANUSCRIPT

Table 2 Selected geochemical parameters of saturated and aromatic hydrocarbon fraction for the extracted bitumen samples studied. 24Formation

Pr/Ph

C22/

C24/

C26/

C31R/

C29/

C35S/

C27 Dia/

G

S/H

1-MP P/MP

i/n

C21 TT

C23 TT

C25 TT

C30 H

C30 H

C34S H

(Dia + S)

D-4

0.50

0.39

nd

0.31

0.59

1.23

0.07

0.58

nd

0.22

GS2

D-4

0.58

0.28

1.5

0.18

0.63

1.38

0.19

0.54

0.58

0.22

GS10

D-4

0.71

0.36

nd

0.27

0.55

1.33

0.18

0.58

nd

GS16

D-4

0.63

0.24

1.6

0.26

0.57

1.22

0.20

0.56

0.58

GS20

D-4

0.73

0.29

1.9

0.25

0.54

1.30

0.19

0.59

MX8

D-4

0.49

0.39

nd

0.47

0.37

1.25

0.18

0.59

MX11

D-4

0.61

0.35

1.7

0.32

0.70

1.33

0.16

0.54

MX17

D-4

0.69

0.47

1.7

0.39

0.50

1.31

0.15

0.57

MX21

D-4

0.19

0.36

nd

0.27

0.55

1.19

0.18

MX51

D-4

0.65

0.45

1.2

0.25

0.45

0.95

0.17

GS6

D-2

0.42

0.34

nd

0.28

0.60

1.28

0.18

GS11

D-2

0.61

0.33

nd

0.26

0.60

1.21

GS28

D-2

0.64

0.39

1.6

0.34

0.58

MX8

D-2

0.74

0.64

1.8

0.24

0.47

MX9-S

D-2

0.63

0.39

nd

0.30

0.59

MX9-D

D-2

0.73

0.38

1.9

0.27

0.57

index

Rc%

MPDF

TA(I)/

DBT/

TA(I + II)

P

MPR

/P

7.3

1.51

0.12

0.48

2.7

0.61

1.95

0.31

0.06

14.1

1.05

0.16

0.69

2.6

0.65

2.22

0.15

0.08

0.21

19.0

0.52

0.29

1.27

2.3

0.70

2.77

0.23

0.07

0.19

16.6

1.21

0.12

0.66

2.6

0.67

2.41

0.19

0.08

M AN U

SC

GS1

MPI-1

RI PT

Name

0.22

18.3

5.53

0.03

0.17

2.9

0.66

2.28

0.42

0.07

0.53

0.26

15.4

0.61

0.29

0.89

2.5

0.60

1.99

0.25

0.04

0.44

0.22

15.4

0.47

0.36

1.08

2.4

0.61

1.97

0.46

0.06

0.40

0.22

13.7

0.70

0.23

0.81

2.6

0.60

2.18

0.21

0.08

TE D

0.53

0.48

0.23

18.2

0.30

0.59

1.36

2.3

0.62

2.01

0.51

0.02

0.60

0.49

0.29

15.0

0.73

0.21

0.92

2.5

0.66

2.30

0.19

0.06

0.57

0.46

0.23

11.3

0.41

0.39

1.35

2.3

0.67

2.38

0.65

0.05

EP

0.56

0.54

0.52

0.21

20.0

0.90

0.20

0.71

2.6

0.61

1.93

0.17

0.02

1.27

0.14

0.60

0.40

0.20

5.4

1.41

0.12

0.54

2.7

0.64

2.17

0.10

0.07

1.11

0.17

0.59

nd

0.20

12.0

4.29

0.03

0.23

2.9

0.69

2.64

0.33

0.06

1.21

0.18

0.52

0.57

0.18

12.4

0.84

0.20

0.75

2.6

0.62

2.06

0.18

0.07

1.26

0.18

0.52

0.58

0.16

19.3

1.07

0.17

0.62

2.7

0.61

1.96

0.15

0.05

AC C

0.18

44

ACCEPTED MANUSCRIPT

D-2

0.20

0.71

nd

0.32

0.45

1.11

0.10

0.43

nd

0.16

13.2

0.13

1.55

1.49

1.3*

0.56

1.67

0.08

0.04

MX11

D-2

0.72

0.40

nd

0.12

0.50

1.02

0.17

0.72

nd

0.26

17.2

3.34

0.06

0.24

2.9

0.60

1.77

0.50

0.03

MX12

D-2

0.22

0.33

nd

0.31

0.62

1.18

0.18

0.56

0.64

0.22

8.7

0.11

1.79

1.69

1.4*

0.59

1.82

0.35

0.04

MX19

D-2

0.38

0.31

nd

0.29

0.56

1.15

0.18

0.53

0.64

0.22

9.1

0.39

0.44

1.23

2.3

0.63

2.09

0.48

0.07

RI PT

MX10

SC

Pr/Ph: pristane/phytane; S/H: steranes/hopanes; 24-i/n: C30 24-isopropyl-/24-n-propylcholestanes; TT: tricyclic terpane; H: hopane; C27 Dia/(Dia + S): C27 diasteranes/ (diasteranes + steranes); G index: 100 × (gammacerane/C30 hopane); P: phenanthrene; MP: methylphenanthrenes; MPI-1: 1.5 × (2-methyl +

M AN U

3-methylphenanthrene)/(phenanthrene + 1-methyl + 9-methylphenanthrene); Rc%: calculated vitrinite reflectance, Rc=-0.55MPI-1 + 3.0, *Rc=0.7MPI-1 + 0.22 (Boreham et al., 1988); MPDF: methylphenanthrene distribution factor, (2-methyl + 3-methylphenanthrene)/( 2-methyl + 3-methyl + 1-methyl + 9-methylphenanthrene); MPR: 2-methyl/1-methylphenanthrene; TA(I)/TA(I + II): (C20 + C21)/( C20 + C21 + C26–C28 (20S + 20R)) triaromatic steroids; DBT/P:

AC C

EP

TE D

dibenzothiophene/phenanthrene.

45

ACCEPTED MANUSCRIPT

Highlights Biomarker signature of the Ediacaran–Early Cambrian origin petroleum is complex. Organic matter input of source rock is mainly from prokaryote and eukaryotic algae.

AC C

EP

TE D

M AN U

SC

RI PT

Source rock is a major shale source and deposits in a marine anoxic environment.