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Biomarker Study of Depositional Paleoenvironments and Organic Matter Inputs for Permian Coalbearing Strata in the Huaibei Coalfield, East China Qingguang Li,†,‡ Yiwen Ju,*,‡,§ Ping Chen,∥ Yue Sun,‡ Min Wang,§ Xiaoshi Li,‡ and Jian Chen∥ †

College of Resource and Environmental Engineering, Guizhou University, Guiyang, Guizhou Province 550025, China Key Lab of Computational Geodynamics of Chinese Academy of Sciences, College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China § Research Institute of Unconventional Petroleum and Renewable Energy (RIUP&RE), China University of Petroleum (East China), Qingdao, Shandong Province 266580, China ∥ School of Earth and Environment, Anhui University of Science and Technology, Huainan, Anhui Province 232001, China ‡

ABSTRACT: This study presents detailed information regarding the organic geochemical and biomarker characteristics of the Permian coal-bearing strata in the Huaibei coalfield. The type of organic matter, sources input, depositional paleoenvironments, and thermal maturity are revealed by the organic molecular signatures study. These are of great significance to understand the coalification process and the generation of the coalbed methane. The extractable organic matter (EOM) in coal and coaly mudstone samples are 1.14% and 0.11% on average, respectively (polar compounds > aromatic hydrocarbons > saturated hydrocarbons). The midchain n-alkane (n-C20−n-C24) is the dominant fraction for coal (42.33% on average) and coaly mudstone (44.91% on average). Compared to the continental coal-bearing basins, the Pr/Ph ratios (mostly 1300 m in thickness) is the primary

Table 1. Basic Characteristics of Coal and Mudstone Samples Collected from the Huaibei Coalfield (Ro and TOC Data from Reference 3) macerals (%) sample ID

coal mine

colliery

seam no.

burial depth (m)

TOC* (%)

Ro* (%)

vitrinite (%)

inertinite (%)

liptinite (%)

Mud01 C01 C02 Mud02 Mud03 Mud04 C03 C04 C05 C06 Mud05 Mud06 C08 C09

Suixiao Suixiao Suixiao Linhuan Linhuan Linhuan Linhuan Linhuan Linhuan Linhuan Suzhou Suzhou Suzhou Suzhou

ST ST YZ LH HZ LH HZ LH LH HZ QN LL LL LL

no. 3 coal roof no. 3 coal no. 5 coal no. 7 coal roof no. 10 coal roof no.10 coal roof no. 10 coal no. 10 coal no. 7 coal no. 9 coal no. 7 coal roof no. 7 coal roof no. 8 coal no. 8 coal

554.0 553.0 422.0 377.0 551.0 601.0 317.0 353.0 554.0 722.0 354.0 354.0 280.0 404.0

15.0 71.0 88.0 8.0 2.2 0.5 92.1 84.5 87.7 91.4 5.1 1.5 86.5 99.7

1.20 1.36 1.69 0.97 0.97 1.38 0.96 1.30 1.15 2.54 1.50 0.78 0.96 0.98

76.8 97.8 97.7 82.0 79.6 96.1 79.0 94.9 96.3 96.3 93.7 71.4 86.4 82.7

19.3 1.8 2.0 14.3 10.8 2.9 14.5 3.5 3.2 3.7 5.8 12.2 13.6 17.3

3.9 0.4 0.3 3.7 9.6 1.0 6.5 1.5 0.5 0.0 0.5 16.4 0.0 0.0

B

DOI: 10.1021/acs.energyfuels.6b02602 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels A GCMS-QP2010PLUS gas chromatograph−mass spectrometer was used to analyze the compositional characteristics of the saturated hydrocarbons. The conditions of the GC-MS are as follows: the chromatographic column, HP-5MS (30 m × 0.32 mm × 0.25 μm); temperature program, 80 °C (1 min isothermal time), heating rate to 310 °C at 3 °C/min (5 min isothermal time), and finally to 320 °C at 10 °C/min (30 min isothermal time); hydrogen flame ionization detector (FID); pure helium as the carrier gas; constant current mode; spectrometer, operated in electron ionization mode (70 eV, full scan).The biomarker analysis was carried out in the National Research Centre for Geoanalysis, Chinese Academy of Geological Sciences.

4. RESULTS 4.1. Macerals of the Organic Matters in Coal and Coaly Mudstone Samples. The burial depth of all coal samples was between 288 and 722 m (Table 1). Ro of the coal samples were in the range from 0.96% to 1.69%, indicating the organic matter in the coal was at the mature to postmature stage. Sample C06 with a highly elevated Ro value (2.54%) is near the magmatic intrusion. The TOC contents in coal samples are normally >70.0% and the vitrinite is the main maceral (79.0−97.7%, 91.4% on average), followed by inertinite (1.8−17.3%, 7.5% on average) and liptinite (0.0−6.5%, 1.2% on average). The relationship between the macerals composition and the TOC contents is not obvious. The vitrinite contents and Ro values did not show strong relationship either (Table 1). The burial depth of the coaly mudstone samples ranged between 354 and 601 m. The organic matter was also at the mature to postmature stage (0.78−1.50%,Ro). Relatively lower TOC contents occur in the coaly mudstone samples (0.5− 15.0%), but the coaly mudstone in the study area still has good gas potential at gas generation peak according to the Rock-Eval pyrolysis experiments.3,10 The vitrinite contents are from 71.4% to 96.1%, 83.3% on average. The contents of the inertinite (2.9−19.3%, 7.5% on average) and the liptinite (0.5−16.4%, 1.2% on average) are higher than in coal samples. Along with the increase of Ro values, the contents of the vitrinite increase in coaly mudstone samples. 4.2. Extractable Organic Matters in Coal and Coaly Mudstone Samples. The contents of the extractable organic matter (EOM) varied from 0.03 to 1.44 wt % (1.14 wt % on average). The polar compounds were the main group component, and its relative amount was up to 57.4 wt % on average (Figure 2), followed by aromatic hydrocarbons (35.5 wt % on average) and saturated hydrocarbons (7.1 wt % on average). In coaly mudstone samples, the contents of the EOM was in the range from 0.03 to 0.36 wt % (0.11 wt % on average). There was no obvious difference in the group compositions between coal samples and coaly mudstone samples. The polar compounds were still the main group in composition (50.4 wt % on average), followed by aromatic hydrocarbons (41.1 wt % on average) and saturated hydrocarbons (8.5 wt % on average). The similar group compositions in both sample subsets are attribute to the similar biogenic sources of the organic matter.30 In addition, parameters related to maturity and sources input were calculated according to the previous studies,11,12,31 including carbon preference index (CPI), odd−even predominance (OEP), pristane/phytane (Pr/Ph), Ts/(Ts + Tm), 20S/ (20S + 20R) C27 diasterane ratios, 22S/(22S + 22R) C31 homohopane ratios, and so on. The results were presented in Tables 2 and 3 in detail. 4.3. Distribution of Saturated Hydrocarbons. The range of n-alkanes for coal and coaly mudstone samples was

Figure 2. Ternary plot showing the percentage of saturated hydrocarbons, aromatic hydrocarbons, and NSO compounds.

from n-C15 to n-C35 (Figure 3). According to Table 2, n-C20−nC24 was the predominant n-alkane fraction and the relative amounts of n-C31−n-C35 were lower than 10%. ∑C22−/∑C23+ was in the range from 0.71 to 3.65 for coal samples and from 0.58 to 2.27 for coaly mudstone samples, indicating a complicate composition for both coal and coaly mudstone samples. Though CPI approaches 1.0, it still demonstrates slight odd carbon preponderance. A distinct presence of odd−even predominance (OEP(2) > 1) in the long-chain n-alkanes was observed in all the coal and coaly mudstone samples (Table 3). Pristane (Pr) and phytane (Ph) are very important acyclic isoprenoids. The Pr/Ph ratio for most coal and coaly mudstone samples was lower than 1.0, except for three samples (C08, C09, and Mud05). This is not in accordance with the classic terrestrial coal petrography.15,22,25,26 The m/z 217 chromatograms show the distributions of steranes (C27−C29) in Figure 3. Except for C06 and Mud02, C29 steranes are the dominant regular steranes in all the coal and coaly mudstone samples, followed by C28 steranes and C27 steranes. Diasteranes were observed in all the samples, but the ∑diasteranes/∑steranes ratios are very low, ranging from 0.03 to 0.29 in coal samples and from 0.07 to 0.30 in coaly mudstone samples (one exception, Mud03). The m/z 191 chromatograms show the distribution of hopanes. 17α,-22,29,30-Trisnorhopane (Tm) predominates over 18α,-22,29,30-trisnorneohopane (Ts) in coal samples (Ts/(Ts +Tm) ⟨0.50), but three of the five coaly mudstone samples showed the opposite tendency. The relative abundance of C29 norhopane is generally lower than the C30 hopane in all the coal and coaly mudstone samples (one exception, Mud03) with C29/C30 ratios in the range of 0.13−0.95. The homohopanes are dominated by the C31 homohopanes and decreasing toward the C35 homohopanes. The C35 homohopanes and other higher molecular homohopanes are absent in C01, C05, C07, Mud01, Mud05, and Mud06. The ratios of C31/∑C31−35 homohopanes range from 0.38 to 1.00 in coal samples and from 0.38 to 0.58 in coaly mudstone samples. The ratios of C31 homohopanes 22S/(22S + 22R) range from 0.47 to 0.60 in coal samples and from 0.30 to 0.60 in mudstone samples (Table 3). C


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Table 2. Composition Characteristics for Biomarkers in Coal and Coaly Mudstone Samples Collected from the Huaibei Coalfield (a) Coal Samples for Given Coal Mine Area Suixiao parama,b EOM (%) Sat (%) Aro (%) NSO (%) Sat/Aro n-alkanes n-C15−n-C19 n-C20−n-C24 n-C25−n-C30 n-C31−n-C35

(%) (%) (%) (%)

∑C22−/∑C23+ steroids C27 (%) C28 (%) C29 (%)

Linhuan C02

C03

C04

C05

C06

C07

C08

C09

0.64 1.61 38.17 60.22 0.04

5.56 1.46 49.18 49.37 0.03

1.06 36.86 41.07 22.07 0.90

0.39 2.61 26.62 70.78 0.10

0.25 1.91 57.81 40.29 0.03

0.04 0.85 23.65 75.49 0.04

0.03 16.54 52.95 30.51 0.31

0.81 1.25 10.05 88.69 0.12

1.44 1.25 19.6 79.15 0.06

52.41 31.90 12.32 3.38

4.38 52.95 35.73 6.94

61.17 21.96 11.39 5.47

9.48 57.57 29.58 3.37

9.03 51.03 30.10 9.83

16.31 47.29 30.71 5.69

53.93 24.93 14.41 6.72

14.71 47.98 33.90 3.41

20.46 45.32 32.16 2.06

3.18

1.00

3.65

0.81

0.87

0.88

2.75

0.74

0.71

16.40 36.75 46.84

15.10 32.55 52.35

28.66 25.9 45.44

16.80 30.95 52.24 (b) Coaly Mudstone

31.34 22.62 46.04

15.45 24.91 59.64

16.01 40.56 16.42 34.80 20.28 35.36 49.19 39.16 48.22 Samples for Given Coal Mine Area

Suixiao param

a,b

EOM (%) Sat (%) Aro (%) NSO(%) Sat/Aro n-alkanes n-C15−n-C19 n-C20−n-C24 n-C25−n-C30 n-C31−n-C35

(%) (%) (%) (%)

∑C22−/∑C23+ steroids C27 (%) C28 (%) C29 (%)

Suzhou

C01

Linhuan

Suzhou

Mud01

Mud02

Mud03

Mud04

Mud05

Mud06

0.10 1.75 35.86 62.40 0.05

0.36 23.50 28.51 47.98 0.82

0.04 6.01 20.69 73.31 0.29

0.03 14.77 54.60 30.63 0.27

0.03 4.16 40.22 55.62 0.10

0.09 0.80 66.60 32.61 0.01

10.66 57.83 28.84 2.67

44.45 30.28 19.00 6.27

19.86 47.51 29.96 2.67

39.78 38.7 16.36 5.15

30.99 44.05 19.62 5.34

23.42 51.09 18.45 7.03

0.58

2.27

0.95

2.15

1.50

1.56

20.14 33.08 46.78

49.42 18.47 32.11

17.3 27.07 55.63

36.09 20.47 43.44

17.17 35.72 47.11

32.76 26.12 41.12

EOM, extractable organic matter; Sat, saturated hydrocarbons; Aro, aromatic hydrocarbons; NSO, polars. b∑C21−/∑C22+: [∑(from n-C15 to nC22)]/[∑(from n-C23 to n-C35)].

a

5. DISCUSSION

for the sources input, sedimentary environment, and maturity analysis, if the samples suffered from biodegradation. 5.1. Thermal Maturity. A series of indicators are often used to evaluate the thermal maturity level of the organic matters, including vitrinite reflectance (Ro, %), pyrolysis Tmax data, and the kind of biomarker parameters such as 20S/(20S + 20R) and αββ/(ααα + αββ) C29 steranes ratios, 20S/(20S + 20R) C27 diasteranes ratios, Ts/(Ts + Tm) and 22S/(22S + 22R) C31 homohopane ratios (Table 3). However, these ratios are often affected by water-washing, oxidation, and biodegradation.12,15,25,30,35,36 So, the combination of the geological background is very important, and the strict screening of these parameters is indispensable during the thermal maturity analysis.

The significant unresolved complex mixtures (UCMs) were observed in some of the coal and coaly mudstone samples (Figure 3), indicating some degree of biodegradation. This would influence the composition and the distribution of the saturated hydrocarbon fractions.28,32−34 In mature coals as well as in crude oils, biodegradation would start from more easily digestible, lighter compounds. When suffered from severe biodegradation, the n-alkanes would be totally disappeared and some steranes or hopanes would also be degraded to some extent.12,16 So, comprehensive analysis of the geological evolution and the previous research of the study area are needed. Otherwise, some of the parameters might be misused D


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Table 3. Basic Geochemical Parameters for Biomarkers in Coal Extracts of Samples Collected from Huaibei Coalfield (a) Coal Extracts of Samples for Given Coal Mine Area Suixiao param Ro (%) Ts/(Ts + Tm) Pr/Ph n-alkanes CPI(1) OEP(1) OEP(2) alkylcyclohexanes ∑C20−/∑C21+ C27 diasterane 20S/(20S + 20R) C29 steranes αααS/(αααS + αααR) αββ/(ααα + αββ) 20S/(20S + 20R) ∑diasteranes /∑steranes ∑hopances/ ∑steranes C29 norhopane/C30 hopane C31 homohopanes 22S/(22S + 22R) C32 homohopanes 22S/(22S + 22R) C33 homohopanes 22S/(22S + 22R) C34 homohopanes 22S/(22S + 22R) C35 homohopanes 22S/(22S + 22R) GI ∑tricyclic terpanes/17α(H)-hopance homohopanes ∑C31/∑C31−35 tricyclic terpanes C19/C20 C20/C21 C21/C23

Linhuan

Suzhou

C01

C02

C03

C04

C05

C06

C07

C08

C09

1.36 0.45 0.62

1.69 0.15 0.33

0.96 0.48 0.89

1.30 0.38 0.19

1.15 0.24 0.26

2.54 0.50 0.74

− 0.36 0.73

0.98 0.19 1.40

0.98 0.10 1.41

1.14 0.83 1.75 13.40 0.42

1.25 0.95 3.61 2.28 0.44

0.98 0.54 1.53 6.8 0.52

1.00 0.95 1.08 3.82 0.61

1.06 0.70 1.29 4.19 0.49

1.02 0.96 1.03 5.97 0.51

1.03 0.48 1.58 11.87 0.54

1.09 0.97 1.24 3.64 0.54

1.04 0.96 1.15 3.10 0.43

0.07 0.20 0.09 0.10 0.58 0.13 0.47 − − − − 0.23 0.60 −

0.15 0.27 0.20 0.05 0.58 0.55 0.59 0.62 0.60 0.61 0.39 0.23 0.12 0.44

0.25 0.32 0.30 0.03 1.09 0.40 0.59 0.57 0.60 0.60 0.57 0.09 2.82 0.43

0.23 0.33 0.27 0.19 1.27 0.38 0.58 − 0.52 0.52 0.44 0.13 0.18 0.58

0.10 0.23 0.14 0.12 1.11 0.25 0.52 0.61 0.51 − − 0.07 0.29 0.38

0.42 0.37 0.42 0.07 0.76 0.45 0.60 0.58 0.60 0.62 0.66 0.15 0.39 0.46

0.10 0.21 0.14 0.29 2.07 0.30 0.54 0.62 0.56 − − 0.13 1.10 0.51

0.39 0.39 0.39 0.08 0.79 0.58 0.59 0.60 0.58 0.65 0.61 0.17 0.20 0.45

0.36 0.38 0.36 0.26 3.50 0.95 0.58 0.59 0.59 0.72 0.41 0.05 0.15 0.44

0.50 0.70 0.80

0.40 2.50 0.40

0.20 1.10 1.30

0.50 0.40 0.60

− 0.50 0.90

0.30 0.60 0.60

0.30 1.50 1.20

0.70 6.10 0.60

1.40 6.10 1.60

(b) Coaly Mudstone Extracts of Samples for Given Coal Mine Area Suixiao param Ro (%) Ts/(Ts + Tm) Pr/Ph n-alkanes CPI(1) OEP(1) OEP(2) alkylcyclohexanes ∑C20−/∑C21+ C27 diasterane 20S/(20S + 20R) C29 steranes αααS/(αααS + αααR) αββ/(ααα + αββ) 20S/(20S + 20R) ∑diasteranes/∑steranes ∑hopances/ ∑steranes C29 norhopane/C30 hopane C31 homohopanes 22S/(22S + 22R) C32 homohopanes 22S/(22S + 22R) C33 homohopanes 22S/(22S + 22R) C34 homohopanes 22S/(22S + 22R) C35 homohopanes 22S/(22S + 22R) GI ∑tricyclic terpanes/17α(H)-hopances homohopanes ∑C31/∑C31−35 tricyclic terpanes

Lihuan

Suzhou

Mud01

Mud02

Mud03

Mud04

Mud05

Mud06

1.20 0.45 0.67

0.97 0.50 0.29

1.38 0.55 0.96

0.97 0.42 0.43

1.50 0.50 1.27

0.78 0.56 0.29

0.79 0.55 1.11 4.80 0.58

1.13 1.03 1.25 6.91 0.54

1.00 0.70 1.29 4.63 0.59

1.21 0.89 1.62 6.93 0.56

1.04 0.96 1.07 7.36 0.51

0.80 0.70 1.01 7.63 0.44

0.10 0.23 0.15 0.09 0.62 0.25 0.54 0.57 0.62 − − 0.20 0.60 0.53

0.39 0.40 0.41 1.30 0.92 0.61 0.30 0.22 0.89 0.26 0.72 0.19 4.58 0.39

0.20 0.28 0.24 0.11 3.01 1.08 0.58 0.60 0.58 0.62 0.42 0.04 0.34 0.55

0.33 0.37 0.36 0.30 1.32 0.58 0.60 0.58 0.50 0.49 0.94 0.12 2.23 0.39

0.09 0.22 0.11 0.07 0.55 0.14 0.53 − − − − 0.20 0.79 −

0.24 0.38 0.31 0.37 0.93 0.46 0.54 0.53 − − − 0.19 1.14 0.58

E


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Energy & Fuels Table 3. continued (b) Coaly Mudstone Extracts of Samples for Given Coal Mine Area Suixiao param C19/C20 C20/C21 C21/C23

Lihuan

Suzhou

Mud01

Mud02

Mud03

Mud04

Mud05

0.40 0.50 0.50

0.40 0.70 0.50

0.80 1.10 0.70

0.30 0.60 0.90

0.40 0.60 0.50

Mud06 − − −

a

CPI(1) = 2[(n-C23 + n-C25 + n-C27 + n-C29)]/[(n-C22 + n-C24 + n-C26 + n-C28) + (n-C24 + n-C26 + n-C28 + n-C30)]. OEP(1) = [n-C21 + 6(n-C23) + n-C25]/[4(n-C22) + 4(n-C24)]. OEP(2) = [n-C25 + 6(n-C27) + n-C29]/[4(n-C26) + 4(n-C28)]. Gammacerane index (GI), gammacerane/C30 17α(H),21β(H)-hopane. −, compounds absent or compounds present but concentrations too low to calculate a parameter value.

most of the coal and coaly mudstone samples (apart from C08, C09, and Mud05). Microbial activity should be one of the possible reasons for the abnormal Pr/Ph and CPI ratios. However, n-alkanes are more easily biodegraded than acyclic isoprenoids. This assumption is contradictory with the low Pr/n-C17 and Ph/nC18 values. Another likely reason for this is water-washing, because Pr is slightly better soluble than Ph. Due to branched compounds being more soluble, this phenomenon is always accompanied by decreasing values of Pr/n-C17 and Ph/n-C18. Obviously, in samples Mud01 and C01, it shows a characteristic cut out chromatogram in Figure 3. In addition, thermal maturity should be one of the very important factors which will change n-alkane distributions and Pr/n-C17 and Ph/n-C18 ratios significantly. Since the studied coal samples have Ro ranging from 0.96% to 1.69%, the dominance of middle-chained nalkanes (n-C20−n-C24) can be largely formed from thermal cracking of long-chained n-alkanes. The n-alkane distribution pattern and Pr/n-C17 and Ph/n-C18 ratios are very likely to lose their original geochemical meaning to differentiated sources input and depositional environment. However, these factors seem to still have the function for indicating a mixed organic matter composition according to Figure 4. In fact, the steranes could provide further evidence for the mixed organic matter sources input. According to the previous studies, C27 regular steranes are related to marine phytoplankton, C28 regular steranes may reflect the lacustrine algae sources input, and higher plants are indicated by the C29 regular steranes.45,46 All the coal and coaly mudstone samples are located in the overlapping region of terrestrial, open marine, and estuarine environments, indicating a mixed input from biological sources according to the ternary classification (Figure 5). Moreover, the higher relative amounts of C29 regular steranes reflect that higher plants input should be the dominated organic matter sources.22,28,42,47 It is proved that some of the C19 and C20 tricyclic terpanes are from higher plants.12 In fact, both tricyclic terpanes and pentacyclic terpanes could also derive from bacteria. Commonly, C21/C23 ratios of tricyclic terpanes higher than 1.0 indicate continental deposition and C21/C23 < 1.0 reflects marine sedimentary inputs.35,48,49 So, a mixture of marine and terrestrial organic matter sources can also be confirmed in the coal and coaly mudstone samples (Table 3). ∑Tricyclic terpanes/17α(H)-hopance could also be used to study the microbial input and evaluate the biodegradation impact.47 According to Figure 6, the signal of microbial input in coaly mudstone samples is stronger than in the coal samples. On the whole, higher plants dominate the sources input, but the marine sources of the organic matter in coal and coaly mudstone strata cannot be ignored. This is attributed to the

22S/(22S + 22R) C31 homohopanes and 20S/(20S + 20R) C27 diasteranes ratios are almost approaching equilibrium (0.57−0.60), showing the organic matter in coals and coaly mudstones are mainly at the mature to postmature stage.13,22 This is consistent with the Ro (%) values and the Rock-Eval pyrolysis results from the previous study.3 Usually, Pr/Ph would decrease with the increasing maturity. However, the relationship between Pr/Ph and Ro (%) is not strong in this study. In addition, 20S/(20S + 20R) and αββ/(ααα + αββ) C29 steranes ratios lose the function of indicating the thermal maturity. This is likely attributable to the complicated sources input and the biodegradation. 5.2. Sources Input Composition. Most of the samples showed a (fronted) unimodal type of n-alkanes distribution in the m/z 85 chromatograms in Figure 3. There are no obviously dominant n-alkanes group in all the coal and coaly mudstone samples (Table 2). Overall, the midchain n-alkanes (n-C19−nC24) demonstrate slightly higher contents (40% in average) than the rest of the two groups. Especially in C02, C04, C05, Mud01, and Mud06, the (n-C19−n-C24) relative amounts are all >50%. According to the previous studies,30,37,38 the short-chain nalkanes (n-C 13 −n-C 18 ) come from phytoplankton and zooplankton, the midchain n-alkanes (n-C19−n-C24) are from bacteria, and the long-chain n-alkanes (n-C25−n-C35) are from higher terrestrial plants. In addition, the cone and shoots of some fossil conifer species might contain high contents of midchain n-alkanes (n-C21−n-C 25).39 So, the microbial organisms might contribute a certain amount of organic material and the sources input for the coal-bearing strata might be multiple. The large ranges of ∑C22−/∑C23+ for both coal (0.71−3.65) and coaly mudstone samples (0.58−2.27) also indicate complicated sources input compositions.28 This is in accordance with the interactive deposition of marine and terrestrial sediments.29 The higher plants input is supported by the OEP(2) > 1.0.12,40 However, the low CPI ratio is inconsistent with the common coals composed by higher plants in continental coalbearing basins (Table 3). Coals with low CPI ratios were also found by some previous studies.20,38,41 The validity of Pr/Ph is an important issue and considerable circumspection is required,25,30 because they might be affected by many factors such as maturity, the oxicity of the depositional environment, and different sources of the precursors.12,38 To some extent, Pr/Ph can reflect the sources input of organic matter.22,26,42,48 High Pr/Ph ratios reflect high contribution of terrigenous organic matter deposited in relatively oxidizing environment.12 Pr/Ph may decrease with increasing thermal maturity in high rank coals,43 but it cannot explain those values less than 1.0 in F


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Figure 3. Representative total ion current [TIC] chromatograms of saturated and aromatic hydrocarbon fractions for coal samples (C01, C05, C08, and C09) and mudstone samples (Mud01, Mud03, and Mud06) collected from Huaibei coalfield.

transitional environment and the alternate deposition of marine and terrestrial sediments. Biodegradation, water-washing, and thermal maturation are all important factors which would change the distribution of saturated hydrocarbons significantly.

However, the transitional sedimentary environments and mixed sources input could also be reflected according to the analysis above. G


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Figure 4. log plot of the ratios of Ph/n-C18 versus Pr/n-C17 (modified after conclusions in refs 30 and 44 using data from this work).

Figure 6. Cross-plots of (a) C27/C29 steranes versus Pr/Ph and (b) tricyclic terpane/sterane versus hopance/sterane showing the sources input (drawn from conclusions given in refs 28, 35, and 47 using data from this work).

terrestrial organic sources under oxidizing condition. Moreover, the lower Pr/n-C17 and Ph/n-C18 are not powerful evidence for an oxic depositional environment, because the locations of the data in Figure 4 basically indicate a transitional sedimentary environment.26,42 This is in accordance with previous studies in that the coal-bearing strata are a set of sedimentary rocks deposited interactively by marine and terrestrial sediments.5,29,50 Under acid conditions or oxic environments, it is favorable for sterols to convert into diasteranes. The low ∑diasteranes/ ∑steranes ratios exclude the possibility that the coal-bearing stratum formed in a strong oxic environment (Table 3). As a salinity biomarker, gammacerane was identified with low content in all the samples. This also disagrees with a strong oxic sedimentary environment for the coal-bearing strata according to the previous studies elsewhere.12,25 High contents of C35 homohopanes indicate a strong reducing marine sedimentary environment, whereas, under oxic conditions, they are usually in low amounts.22The domination of C31 homohopanes, decreasing toward the C35 homohopanes, in all the coal and coaly mudstone samples suggest it should not be a strong reducing marine sedimentary environment for the coal-bearing strata. The amounts of hydrocarbon molecules such as the nalkanes, Pr, and Ph are only valid for some depositional environments with distinct features. However, the cross-plot of the gammacerane index (GI) and Pr/Ph can further provide very useful information for distinguishing some transitional sedimentary environments.22,25,51 Obviously, the coal-bearing

Figure 5. Ternary diagram of regular steranes (C27−C28−C29) showing the relationship between sterane composition, sources input, and depositional environment (based on conclusions from refs 26, 28, and 42 using data from this work).

5.3. Depositional Environment. The Huaibei coalfield is situated in the southeastern edge of the North China Plate. The Carboniferous and Permian coal-bearing strata in this coalfield are a set of sedimentary rocks composed by transitional facies sequences, deposited interactively by marine and terrestrial sediments.5 The coal-bearing strata were developed on a shallow-sea continental shelf and the carbonate platform sedimentary system. The barrier system was first formed at the bottom. Then, the constructive deltaic depositional system was formed under the fluvial action in the littoral area. In the late Permian, it transformed into a destructive deltaic depositional system and then became an alluvial plain.29,50 In spite of the detailed research on sedimentology and tectonic evolution, the organic geochemistry characteristics about the organic matter sources input and the depositional paleoenvironments in the study area have been insufficient up to now. Pristane (Pr) and phytane (Ph), as very important biomarkers, are often used to reflect the environment features.18,44 In the study area, the lower values of Pr/Ph in coal and coaly mudstone samples are different from the classic H


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Energy & Fuels strata were formed in a weak oxidizing−reducing environment according to Figure 7.

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C27S: C27-steranes C28S: C28-steranes C29Nor: 17α,21β-30-norhopane C29S: C29-steranes C30H: 17α(H),21β(H)-hopane C30Mor: 17β(H),21(H)-moretane C31Hom: C31-homhopanes C32Hom: C32-homhopanes CPI: carbon preference index Ga: gammacerane OEP: odd−even predominance Ph: phytane Pr: pristane Tm: C27-17α,-22,29,30-trisnorneohopane Ts: C27-18α,-22,29,30-trisnorneohopane

AUTHOR INFORMATION

Corresponding Author

Figure 7. Cross-plots of pristane/phytane (Pr/Ph) versus gammacerane index: (1) strong reducing environment; (2) weak oxidizing− reducing environment; (3) weak oxidizing environment (using data from refs 12, 25, and 47).

*E-mail: E-mail:[email protected]. ORCID

Yiwen Ju: 0000-0003-1574-9279 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the National Natural Science Foundation of China (Grant Nos. 41530315, 41372213, and 41402139) and the National Key Basic Research Program of China (Grant No. 2014CB238901). We gratefully acknowledge the help from the engineering technicians in the collieries of the Huaibei mining group during sampling.

On the whole, there seems to be no severe variation for the sedimentary environment and sources input between the coal and coaly mudstone strata, reflecting stability and continuity during the coal-bearing stratum deposition. This is beneficial for accumulation of the coal organic matter.

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6. CONCLUSION The main conclusions of the investigations are as follows: (1) Maturity estimates based on biomarker maturity parameters, vitrinite reflectance (Ro, %), and pyrolysis Tmax data demonstrate that the organic matter in coals and coaly mudstones from the Huaibei coalfield are at the mature to postmature stage. However, 20S/(20S + 20R) and αββ/(ααα + αββ) C29 steranes ratios are unsuitable parameters for thermal maturity analysis in the study area. (2) The various biomarker parameters such as CPI, Pr/n-C17, Ph/n-C18, and ∑diasteranes/∑steranes have been selected to elucidate its depositional environment conditions, indicating that the coal-bearing strata in the Huaibei coalfield were formed in a transitional zone of open marine and estuarine. Under weak oxidizing−reducing environment, the interactive and stable marine and terrestrial deposition is a benefit for forming multiple coal seams. (3) The molecular distribution of saturated hydrocarbons in the Huaibei coalfield reflects a mixed organic matter sources input. This is related to the depositional paleoenvironments. Higher plants are the main provenance, the marine sources cannot be ignored, and the signal of microbial inputs in coaly mudstone is stronger than in the coal samples.

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APPENDIX

Compound and other abbreviations for peak assignments in the gas chromatograms of saturated fractions in the m/z 85, 191, and 217 mass chromatograms. I


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