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Thermal Evolution of Adsorbed/Occluded Hydrocarbons inside Kerogens and its Significance as Exemplified by One Low Matured Kerogen from Santanghu Basin, NW China Bin Cheng, Junyan Du, Yankuan Tian, Hu Liu, and Zewen Liao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00218 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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Thermal Evolution of Adsorbed/Occluded Hydrocarbons inside Kerogens and its Significance as Exemplified by One Low Matured Kerogen from Santanghu Basin, NW China

Bin Cheng,† Junyan Du,†,‡ Yankuan Tian,† Hu Liu,†,§ Zewen Liao,†,* †

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,

Guangzhou 510640, China ‡

Guangxi Institute of Oceanology, Nanning 530022, China

§

Sichuan Key Laboratory of Shale Gas Evaluation and Exploitation, Chengdu 600091, China

ABSTRACT: Organic compounds could be adsorbed and even occluded within the macromolecular structures of kerogen. Studies concerning thermal evolution of adsorbed/occluded hydrocarbons inside kerogens will be helpful in understanding its structural characteristics and evolution features and estimating the effectiveness of adsorbed and occluded fractions. In this present work, the adsorbed and occluded hydrocarbons have been released from a low matured kerogen from the Upper Permian Lucaogou Formation of Santanghu Basin, NW China, and from its pyrolysis residues by solvent extraction method and oxidative treatment respectively. The results showed that some n-alkanes, terpanes and steranes were detected from both the adsorbed and occluded fractions. Series of even-carbon-numbered n-alk-(1)-enes were also determined from the occluded components. The early stage of thermal evolution showed similarities in the biomarker features for both adsorbed and occluded hydrocarbons, however, variations were noted in the biomarker compositions with increasing thermal levels. The obvious mutations of C21-22 Pregnanes/C27-29 Steranes ratios from adsorbed to 1

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occluded fractions at high thermal evolution suggested that high concentration pregnane (containing homopregnane) of soluble components in high or over mature stage may be mainly derived from the corresponding components covalently bound within kerogen. The occluded components exhibited a stronger thermal stability than the adsorbed components. This study showed that the evaluation of the thermal evolution characteristics of kerogens/source rocks based on biomarkers compositions from the adsorbed components at the high to over matured stages was difficult, while this kind assessment of kerogens/source rocks can be hopefully achieved from the occluded components.

1. INTRODUCTION With large molecular weight and complex structure, kerogen has previously been studied based on its average molecular representations. Its basic structural units are polycyclic aromatic rings connected to several other aliphatic chains and some hetero atom groups on the aromatic rings.1,2 In the complex three-dimensional structures, some micropore units are commonly present3 which can accommodate some small molecules4 that are referred to as mobile phase.5 Extraction of kerogen using different solvents such as chloroform, mixture of methanol-acetone-chloroform and mixture of CS2 and N-methyl-2-pyrrolidinone have shown that the yields of extraction often increase with increasing polarity of the organic solvent, indicating the abundant of non-covalent bond interactions among the organic matters within the kerogen structure.6,7 Kerogen structure consists of both the adsorbed and occluded components which coexisted as non-covalent bonded moieties in the pore units.8 The potential locations for adsorbed and occluded hydrocarbons inside geomacromolecule can refer to those inside asphaltene directly described by Snowdon et al.9 Generally, the components that are easily extracted from the periphery of the 2


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macromolecules by the conventional organic solvents can be referred to as the adsorbed fraction, while components at the core portion of the macromolecular structure which are difficult to extract using the conventional organic solvents are considered as occluded fraction. The occluded component can be well preserved by the macromolecular structure and thus reducing the transformative effects of later geochemical processes and thereby preserving the earlier organic geochemical information.10-12 Researches have shown that mild oxidative treatment of asphalt and solid bitumen with H2O2 can properly release the occluded compounds which have been found useful in the study of organic matter with depleted soluble fractions,13 oil (bitumen)-source correlation,14,15 identification of mixed-source reservoirs,16-18 and hydrocarbons accumulation and evolution.19 Considering kerogen as an important geological macromolecule, studies concerning the adsorbed and occluded fractions of kerogen will be helpful in understanding its structural characteristics and evolution features. Lynch and Webster20 found that the rigid and mobile phases of kerogen are controlled by temperature based on in-situ 1H-NMR method. In this present work, the authors investigated the geochemical features of adsorbed/occluded components of kerogen at different thermal evolution stages. The study is expected to discuss the effectiveness of adsorbed and occluded fractions and provide a better understanding of the varying characteristics of kerogen during formation and evolution of oil and gas.

2. MATERIALS AND METHODS 2.1. Sample and thermal simulation experiment. The calcareous mudstone was selected from the Upper Permian Lucaogou Formation of Santanghu Basin, NW China. The previously reported Tmax, production index (PI) and vitrinite reflectance (VRo) values for the same calcareous mudstione 3


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used in this present study were ~436°C, ~0.01 and 0.52%,21 indicating low maturity of the sample. The powdered mudstone samples were pressed into five separate cylinders which was introduced into the heater for pyrolysis, details about this device can be referred by Liao et al.22 The samples were subjected to varying temperatures of 280°C, 380°C, 420°C, 480°C, and 560°C for 72h and at a constant pressure of 80MPa. The five pyrolysis residues and the raw mudstone were iteratively treated with HCl and HF to obtain six kerogen samples, details about this treatment process can be referred by Fu and Qin5. Elemental C, H and O of the residual kerogens were detected by Du et al.21 and she found that their H/C values decreased with the increasing simulation temperatures and their equivalent Ro values were 0.49% (280°C kerogen), 1.21% (380°C kerogen), 1.67% (420°C kerogen), 2.41% (480°C kerogen), 3.60% (560°C kerogen), respectively.

2.2. Acquisition of the adsorbed/occluded compounds from kerogen. Kerogens were Soxhlet extracted with n-hexane, acetone and dichloromethane consecutively for 120h, in order to obtain the adsorbed components and preclude interference from these components into the occluded ones. For the kerogens prepared from pyrolysis residues, copper were added to the extraction solvent to remove elemental sulfur from kerogens. The extracted residues were then used in oxidative degradation treatment. Benzene (~28 ml) served as the solvent, while H2O2/CH3COOH (about 12 ml/12 ml) served as the oxidation reagents. The reaction was allowed to take place for 48h under magnetic stirring at room temperature. The products were filtered through a Büchner funnel and washed down with dichloromethane. The obtained liquid phase was then transferred into a 250 ml separating funnel (made of PTFE) followed by the addition of ultrapure water to obtain separate layers of organic and inorganic phases,16 and the organic phase was collected and concentrated. 4


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The solvent extracts (adsorbed components) and the organic phase of the oxidative degradation products (containing the occluded components) were further separated by column chromatography using 18 cm column (silica gel:aluminum oxide = 2:1) and eluted with n-hexane and toluene to yield saturated hydrocarbon and aromatic hydrocarbon components, respectively.

2.3. Gas Chromatography–Mass Spectrometry analysis. Analysis of n-hexane eluates was done on a DSQ II and Trace GC Ultra combined system from Thermo Fisher. GC conditions were: HP-1MS chromatographic column (60 m × 0.32 mm × 0.25 µm); helium as carrier gas with a constant flow mode at 1.2 mL/min. MS conditions were: EI mode; ion source electron energy, 70eV; ion source temperature, 260°C; mass scan range, 50-650 dalton. The temperature ramp-up procedure for the n-hexane eluate was as follows: 80°C initial temperature held for 4 min then raised to 295°C at a rate of 4°C /min and held for 20 min.

3. RESULTS AND DISCUSSION In this study, solvent extract was defined as the adsorbed component inside kerogen and the saturated hydrocarbon derived from oxidative degradation reaction belonged to the occluded component, which has been discussed by Liao et al.23-26 The geochemical characteristics of n-hexane eluates from the column separation will be discussed in details in this work.

3.1. Thermal evolution characterization of adsorbed components. 3.1.1. Components from total ion chromatogram (TIC). Sulfurs determined in the n-hexane extract of the raw kerogen, include S6, S7 and S8 molecules, with S8 been the dominant on the TIC chromatogram (Figure 1). The C14-C27 normal alkanes are predominated by n-C23, as well as 25-norhopane and C29-C30 hopanes are also detected. For the extracts from pyrolysed kerogen residues, the brown color of the 5


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copper changed into black, indicating that abundant of sulfur has been removed. On the TIC chromatogram of the n-hexane eluates, n-alkanes generally show higher carbon number with the increasing pyrolysis temperatures. Based on this observation, n-alkanes have been classified in this present study into two superimposed unimodal groups of A and B (Figure 1). N-alkanes in group A ranged from n-C15 to n-C20 with predominant of n-C18, while those in the group B range from n-C15 to n-C31 (or C32) and dominated by n-C25 (or n-C26) alkanes. From 280°C to 420°C, the C20-/C21+ (group A/group B) ratios ranged from 0.11 to 0.36 (Figure 1). The concentration of n-alkanes from group A is obviously lower than that of group B, and even could not be detected at 480°C. At the 560°C, the n-alkanes are almost disappeared and only a few branched alkanes are present. The abundance of element sulfur may suggest an anoxic deposition environment of the source rocks. The 25-norhopane is mainly detected from severely biodegradation oils and has been considered as a biomarker for oil biodegradation,27,28 but 25-norhopane is also identified from Permian source rocks of Santanghu basin.29,30 Du et al.30 suggested that the 25-norhopane detected from the Lucaogou Formation mudstone might have come from the original hopanoid compounds because this set of source rock have been exposed to the microbe degradation during the period of deposition or early diagenesis process. The shorter chain n-alkanes (C17-C21) have been attributed to originate from marine phytoplankton and microbes31-33 and the medium chain n-alkanes (C23-C27) have been considered to be generally derived from macroscopic algaes34,35 while the long chain (C27-C35) are mainly from higher-plants.36-38 The bimodal distributions of n-alkanes from pyrolysis kerogen may indicate that its parent biomass was derived from both lower aquatic organisms and higher-plants.

3.1.2. Terpane compound distributions. Tricyclic terpanes of C20-C26, hopanes of C29-C32, and 25-norhopane are determined from extract of the raw kerogen on the m/z191 chromatogram (Figure 2). 6


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The relative concentration of tricyclic terpanes show an increasing trend whiles the hopanes indicate a decreasing pattern with increasing pyrolysis temperature. The ratios of C29-hopane/ C30-hopane reach equilibrium between 0.5-0.6 in spite of increasing simulation temperature, except at 560°C. The tricyclic terpane/pentacyclic terpane (Tri/Penta) ratio was calculated based on C20, C21, C23, and C24 tricyclic terpanes and C29 and C30 for pentacyclic terpane. The results show that this ratio increased with increasing pyrolysis temperature (Table 2). The Tri/Penta ratio reached a maximum value of 20.92 under the pyrolysis temperature of 560°C, which may suggest a large number of hopanes being cracked. The raw kerogen has a low C31 hopane-22S/(22S+22R) ratio of 0.44, indicating low maturity of sample that is consistent with the vitrinite reflectance (Ro) value of 0.52%. Its gammacerane index (gammacerane/αβ-hopane) is 0.33, suggesting a deposition environment with some salinity. Wang et al.39,40 have previously reported β-carotene, 8β(H)-drimane and 8β(H)-homodrimane in this set of source rock, which were closely related to saline-lake environment.

3.1.3. Sterane compound distributions. The C27-C29 regular steranes from the n-hexane extract of the raw kerogen show that C27 < C28 < C29 on the m/z 217 chromatogram (Figure 3). A few of C23-C25 steranes are also detected in the n-hexane extract from the raw kerogen. The relative concentrations of pregnane and homo-pregnane show an increasing trend while that of regular steranes indicate a decreasing pattern with increasing pyrolysis temperatures. This is quite obvious in the ratios of C21-22P/C27-29S (Table 2). The ratios of C29S-20S/(20S+20R) and C29S-ββ/(ββ+αα) reach equilibrium after the temperature of 420°C. Owing to the isomerization and cracking of regular steranes, the distribution model of C27-C29 steranes changed gradually, firstly as straight up → asymmetrical inverted "V"(380°C) → asymmetrical "V"(480°C) → inverted "V"(560°C) (Figure 3). The ratios of C27S/C27-29S 7


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from adsorbed fractions are increasing while the C28S/C27-29S and C29S/C27-29S ratios are decreasing until the pyrolysis temperature increases to 480°C (Table 2). The regular steranes C27 and C28 are mainly originated from lower aquatic organisms41 while the C29 can be from both lower organisms and terrestrial higher plants.42 Yin et al.43 have reported the dominance of Gymnospermous Polle grains and a small amount of Pteridophyte spores in mudstone from Lucaogou Formation. Based on their reports and the two superimposed unimodal distributions of n-alkanes observed in this present study after kerogen pyrolysis (Figure 1), it is therefore established that the main source of the parent biomass were derived from both the lower aquatic organisms and terrestrial higher plants. Although the C23-C25 steranes are homologues of regular steranes, their presence in this low matured samples could not be from the branch cracking of long-chain sterane. Pan et al.44 held that abundant of short-chain steranes can be produced from biodegradation of some precursor molecules under high salinity and reducing conditions. (22E)-27-norcholesta-5, 7, 22-trien-3β-ol has been the only known natural compound with a carbon skeleton that could serve as precursor for short-chain 20-n-alkylpregnanes and their aromatic analogs.45 Therefore, the presence of C23-C25 steranes might have been from the degradation of (22E)-27-norcholesta-5, 7, 22-trien-3β-ol.

3.2. Thermal evolution characteristics of occluded components. 3.2.1. Components from total ion chromatogram (TIC). A series of even-carbon-numbered n-alk-(1)-enes, n-alkanes and element sulfur (S6, S7, S8) are detected from the occluded fraction of the raw kerogen, with n-alk-(1)-enes as the main compounds (Figure 4). The even-carbon-numbered n-alk-1-enes are mostly from C16 to C32 with predominant of C22 and C24. The n-alkanes range from C16-C30 indicating similar distribution to that of adsorbed fraction. The ratio of n-alk-1-ene/n-alkane (-ene/-ane for short) with the 8


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same carbon number is >1. Within the temperature range of 280°C and 380°C of the pyrolysed samples, only C18 and C20 n-alk-1-enes are detected from the trapped fraction of kerogen residues, with the ratios of -ene/-ane 1.67). The occluded components exhibited a stronger thermal stability than the adsorbed components. The obvious mutations of C21-22P/C27-29S ratios from adsorbed to occluded fractions in high thermal evolution suggested that high concentration pregnane (containing homopregnane) of soluble components in high or over mature stage might have been derived mainly from the corresponding components covalently bound within kerogen. In conclusion, the evaluation of the thermal evolution characteristics of kerogens/source rocks based on biomarkers compositions from the adsorbed components at the high or over mature stages was difficult while proper assessment can be achieved from the occluded components.

AUTHOR INFORMATION *Corresponding Author 13


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*Tel: +86 20 85290190; Fax: +86 20 85290706; E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has been financially supported by the National Natural Science Foundation of China (Grant No. 41502128 and 41502121). We thank Dr. Lekan Faboya from Nigeria for improving the English writing of the original manuscript.

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(43) Yin, F.; Liu, H.; Hua, H., 2002.Late Permian sporopollen assemblage from Lucaogou Formation in Santanghu basin. Oil Gas Geology 2002, 23, 392-397. (44) Pan, Z.; Huang, D.; Lin, R. Identification of a complete series of short side chain steroids (C20-C26) in crude oil and source rock and its geochemical significance. Geochim. Cosmochim. Ac. 1991, 9, 106-113. (45) Li, M.; Jiang, C. Bakken/Madison petroleum systems in the Canadian Williston Basin. Part 1: C21-C26 20-n-alkylpregnanes and their triaromatic analogs as indicators for Upper Devonian-Mississippian epicontinental black shale derived oils? Org. Geochem. 2001, 32, 667-675. (46) Alexander, R.; Kralert, P. G.; Kagi, R. I. Kinetics and mechanism of the thermal decomposition of esters in sediments. Org. Geochem. 1992, 19, 133-140. (47) Alexander, R.; Kralert, P. G.; Sosrowidjojo, I. B.; Kagi, R. I. Kinetics and mechanism of the thermal elimination of alkanes from secondary stanyl and triterpenyl esters: implications for sedimentary processes. Org. Geochem. 1997, 26, 391-398. (48) De Leeuw, J. W.; Bass, M. Early diagenesis of steroids, In: Biological Markers in the Sedimentary Record; Johns, R.B. Ed.; Elsevier, Amsterdam, 1986. (49) Wingert, S.; Pomerantz, M. Structure and significance of some twenty-one and twenty-two carbon petroleum steranes. Geochim. Cosmochim. Ac. 1986, 50, 2763-2769. (50) Abbott, G. D.; Bennett, B.; Petch, G. S. The thermal degradation of 5α(H)-cholestane during closed-system pyrolysis. Geochim. Cosmochim. Ac. 1995, 59, 2259-2264. (51) Huang, D.; Zhang, D.; Li, J. The origin of 4-methyl steranes and pregnanes from tertiary strata in the Qaidam basin, China. Org. Geochem. 1994, 22, 343-348. (52) Liu, H. Ph. D. Thesis, Guangzhou Institute of Geochemistry, CAS, Guangzhou, China, 2015. 19


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Figure 1. Chromatograms of the n-hexane eluents from the soluble fractions of kerogens. The biomarker assignments are listed in Table 1. Form (a) the n-hexane extracts of the raw kerogen; (b)-(f): the n-hexane extracts of the pyrolysis residual kerogens under different simulation temperatures.

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Table 1. Biomarker assignments Peak C29DH C29H C30H C20T ~ C26T C31H ~ C32H C30G C21P C22P C23S ~ C29S

Compound C29 17α,21β(H)-25-norhopane C29 17α,21β(H)-30-norhopane C30 17α,21β(H) hopane C20 ~ C26 tricyclic terpane C31 ~ C32 17α,21β(H) 22S and 22R homo-hopane Gammacerane Pregnane Homo-pregnane C23 ~ C29 αααR Sterane

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Figure 2. Chromatograms of terpanes from the soluble fractions of kerogens. The biomarker assignments are listed in Table 1. Form (a) the n-hexane extracts of the raw kerogen; (b)-(f): the n-hexane extracts of the pyrolysis residual kerogens from different simulated temperatures.

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Table 2. Distribution of biomarker parameters in the adsorbed and occluded fractions of kerogens Samples

Tri/

C29H/ C21P/

C21-22P/

C27S/

C28S/

C29S/

C29S-20S/

C29S-ββ/

Penta

C30H

C22P

C27-29S

C27-29S

C27-29S

C27-29S

(20S+20R)

(αα+ββ)

K-H-Raw

0.30

0.50

2.66

0.05

0.17

0.38

0.44

0.15

0.21

K-H-280°C

0.37

0.62

2.57

0.06

0.17

0.38

0.45

0.18

0.18

K-H-380°C

0.78

0.59

2.29

0.35

0.19

0.40

0.40

0.32

0.26

K-H-420°C

1.15

0.53

3.19

0.33

0.28

0.34

0.38

0.45

0.36

K-H-480°C

0.88

0.46

3.27

0.15

0.29

0.32

0.39

0.46

0.39

K-H-560°C

20.92

0.86

4.08

0.54

0.36

0.34

0.31

0.26

0.37

K-O-Raw

0.26

0.51

1.81

0.06

0.22

0.36

0.42

0.25

0.22

K-O-280°C

0.28

0.62

2.44

0.06

0.18

0.38

0.44

0.17

0.15

K-O-380°C

0.27

0.66

2.32

0.14

0.27

0.35

0.38

0.33

0.26

K-O-420°C

0.31

0.62

3.10

0.13

0.33

0.30

0.37

0.47

0.41

K-O-480°C

0.26

0.60

2.78

0.08

0.33

0.28

0.38

0.49

0.41

K-O-560°C

0.20

0.80

2.53

0.11

0.33

0.28

0.39

0.51

0.43

Note: Tri/Penta: tricyclic terpane/pentacyclic terpane; C29H/C30H: C29 hopane/C30 hopane; C21P/C22P: pregnane/homo-pregnane; C21-22P/C27-29S: (pregnane+homo-pregnane)/C27-29 regular sterane; C27S/C27-29S: C27 regular sterane/C27-29 regular steranes; C28S/C27-29S: C28 regular sterane/ C27-29 regular steranes; C29S/C27-29S: C29 regular sterane/C27-29 regular steranes; C29S-20S/(20S+20R): αααC29 regular sterane-20S/(20S+20R); C29S-ββ/(ββ+αα): C29 regular sterane-ββ/(ββ+αα). K-H-Raw~K-H-560oC represent the adsorbed compounds and K-O-Raw~K-O-560oC represent the occluded ones from different temperature experiments.

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Figure 3. Chromatograms of steranes from the soluble fractions of kerogens. The biomarker assignments are listed in Table 1. Form (a) the n-hexane extracts of the raw kerogen; (b)-(f): the n-hexane extracts of the pyrolysis residual kerogens from different simulated temperatures.

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Figure 4. Chromatograms of the n-hexane eluents from the occluded fractions of kerogens. From (a) the n-hexane extracts of the raw kerogen; (b)-(f): the n-hexane extracts of the pyrolysis residual kerogens from different simulated temperatures.

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Figure 5. Chromatograms of terpanes from the occluded fractions of kerogens. The biomarker assignments are listed in Table 1. From (a) the n-hexane extracts of the raw kerogen; (b)-(f): the n-hexane extracts of the pyrolysis residual kerogens from different simulated temperatures.

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Figure 6. Chromatograms of steranes from the occluded fractions of kerogens. The biomarker assignments are listed in Table 1. From (a) the n-hexane extracts of the raw kerogen; (b)-(f): the n-hexane extracts of the pyrolysis residual kerogens from different simulated temperatures.

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Figure 7. The evolution of C27-C29 regular sterane from the adsorbed and occluded fractions under different thermal stress simulation experiments.

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Figure 8. The trend of (pregnane + homo-pregnane)/regular sterane ratios from the adsorbed/occluded fractions under different thermal stress simulation experiments.

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