Hydrous pyrolysis of different kerogen types of source rock at high temperature-bulk results and biomarkers

Journal of Petroleum Science and Engineering 125 (2015) 209–217

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Hydrous pyrolysis of different kerogen types of source rock at high temperature-bulk results and biomarkers Mingliang Liang a,b,d, Zuodong Wang a,n, Jianjing Zheng a, Xiaoguang Li c, Xiaofeng Wang a, Zhandong Gao a,d, Houyong Luo a,d, Zhongping Li a, Yu Qian a,d a Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou, Gansu Province 730000, China b The institute of Biology, Guizhou academy of Sciences, Guiyang 550000, China c Exploration and Development Research Institute, Liaohe Oilfield Company, PetroChina, Panjin 124010, China d University of Chinese Academy of Sciences, Beijing 100049, China

art ic l e i nf o

a b s t r a c t s

Article history: Received 3 December 2013 Accepted 24 November 2014 Available online 3 December 2014

Hydrous pyrolysis experiments were conducted on immature petroleum source rocks containing different types of kerogen (I and II) at a temperature range of 250–550 1C to investigate the role of kerogen type in petroleum formation at high temperature. At temperatures less than 350 1C, the bulk results and biomarker compositions were quite similar and show negligible variations for different kerogen types. However, at high-temperature, above 400 1C (400–550 1C), unexpected results were observed. The initial and peak temperature of the hydrocarbon generation of type II sources rocks is lower than the type I. The tricyclic terpenes were common in samples above 400 1C. The Ts/Tm values did not show a linear relationship with the temperature increase. The C3122S/(S þR) values did not show significant change with temperature increase. However, a good correlation between the sterane maturity parameters and the thermal maturity were shown, especially for type I kerogen. Temperature increase resulted in significant variation in the relative contents of C27, C28 and C29 steranes, suggesting that the distribution of steranes is greatly affected by thermal maturity. & 2014 Elsevier B.V. All rights reserved.

Keywords: Hydrous pyrolysis High temperature Source rock Kerogen GC–MS Biomarkers

1. Introduction Because of their specific genesis and structural characteristics, steranes and terpanes are widely used in petroleum exploration. They also serve as a molecular fingerprint of sedimentary organic matter (OM), indicating its origin of depositional environment, and thermal maturity (Volkman, 2006; Wang et al., 2009). Previous studies have documented that slight enrichment in C27 steranes occurs with increasing thermal stress, and questioned usage of 20S to 20R C29 sterane ratio as a maturity index (Lewan et al., 1986). Hydrous pyrolysis is used to determine petroleum generation mechanisms and assess the petroleum generation potential of source rocks (Lewan, 1979, 1983, 1985; Winters et al., 1983). Numerous hydrous pyrolysis experiments have been done in both open and closed systems, using different types of organic matter and conditions, in order to understand the mechanisms of oil generation (Huizinga et al., 1987a,b; Michels and Landais, 1994; Michels et al., 1995; Béhar et al., 1997; Pan et al., 2009). These studies are mainly

n

Corresponding author: Tel.: þ 86 931 4960921; fax: þ 86 931 8278667. E-mail address: [email protected] (Z. Wang).

http://dx.doi.org/10.1016/j.petrol.2014.11.021 0920-4105/& 2014 Elsevier B.V. All rights reserved.

based on the cognition that organic matter exposed to higher temperature for a shorter period of time has the same maturity as organic matter exposed to lower temperature for a longer period of time, suggesting that temperature and time compensate for each other as effects on organic matter maturity (Stach et al., 1982). Therefore, it seems to be valid to simulate the natural maturation process in the laboratory by increasing temperature during a short period of time (Han et al., 2001). Taking into account subcritical water temperatures (Lewan, 1985), most of the hydrous pyrolysis experiments were performed at temperatures below 400 1C. Therefore, only a few thermal simulation experiments have been conducted and reported at temperatures above 400 1C. In the present study, two source rocks containing different types of kerogen (I and II) were used for the thermal simulation experiment. Attempts have been made to observe the differences productivity of different kerogen type and variations in the distributions of biomarkers released from the samples by pyrolysis. Especially, we conducted high temperature thermal simulation experiments (400–550 1C) among petroleum source rocks containing different types of kerogen. And investigated the trends and compare the distribution of the classic biomarker compositions from different liquid phases at high temperature (above 400 1C).

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Fig. 1. Location of the Liaohe depression in China and the map of sampling sites.

Table 1 Geochemical characteristics of the samples used in the hydrous pyrolysis. Kerogen type

Source rock

Depth (m)

Rc (%)

Tmax ( 1C)

S1 (mg/g)

S2 (mg/g)

TOC (%)

HI (mg/g)

OI (mg/g)

II I

Gray mudstone, Shahejie Fm, Liaohe Basin Brown gray shale, Shahejie Fm, Liaohe Basin

2553.6–2558 1387.49–1390.19

0.59 0.54

427 434

0.28 1.86

10.75 69.2

2.37 8.73

453 792

21 23

Rc (%)—vitrinite reflectance was calculated using the methylphenanthrene index (MPI). Tmax—temperature corresponding to S2 peak maximum; S1—free hydrocarbons (mg/g rock); S2—pyrolysate hydrocarbons (mg/g rock);TOC—total organic carbon; HI—hydrogen index (S2  100/TOC) (mg/g TOC); OI—oxygen index (S3  100/TOC) (mg/g TOC).

2. Samples and experimental

350, 400, 450, 500 and 550 1C, which heating rates of 10 1C /h, and kept within 71 1C of the setting point for 24 h.

2.1. Samples 2.4. Expelled oil and bitumen Two source rocks of mudstone and shale containing different types of kerogen (I and II) were used for the thermal simulation experiment. Both of the samples were collected from Shahejie Formation, Liaohe Basin, North China (Fig. 1) (Mu et al., 2008). The Shahejie Formation source rocks are comprised of black gray-dark gray mudstone, gray mudstone and oil shale, with a total thickness up to 1000 m. The depths of initial samples are 2553.6–2558/m (type II) and 1387.49–1390.19/m (type I), respectively. The present temperatures are about 93and 51 1C, according to the average value of geothermal gradient, which reaches 36.5/km (Wang et al., 2003). The organic maturity of the two samples are relatively low, and the Rc values are generally less than 0.6%. For hydrous pyrolysis experiments, samples were first powdered to less than 100 mesh. 2.2. Rock-eval pyrolysis Rock-eval pyrolysis was performed using a Rock-eval II system (Delsi instrument, Suresnes, France). And organic-carbon-content (TOC) was determined using a Leco CS-344 analyzer. Group organic geochemical data for investigated samples are given in Table 1. These results indicate that source rocks contain kerogen type I and II (Huang et al., 1984). The Tmax values of the two samples are 434 1C and 427 1C, respectively, indicating low to marginal thermal maturity (Espitalie et al., 1985). 2.3. Hydrous pyrolysis Hydrous pyrolysis experiments were performed in a stainless steel reactor. 15–50 g of powdered sample and 50 ml of deionized water were put in the reactor, and sealed the chamber then flushed three times with pure nitrogen gas to remove the air, and then evacuated. A sensor and an external control device were used to control the temperature within the autoclave. The hydrous pyrolysis experiments were performed at temperatures 250, 300,

Two organic phases were collected from the reactor after hydrous pyrolysis: expelled oil and bitumen. Expelled oil occurs in the reactor as liquid products (Lewan, 1985). The rock chips were removed from the reactor and dried in a vacuum oven at 50 1C for 24 h, and then powdered to less than 100 mesh. Bitumen was extracted from the powdered samples in a Soxhlet apparatus for 72 h with a mixture of chloroform and methanol (97:3, v-v). After precipitation of the asphaltenes by n-hexane, maltenes were separated into three fractions by column chromatography over silica gel and aluminum oxide (3:1). The saturated hydrocarbons fraction was eluted with petroleum ether, the aromatic hydrocarbons with dichloromethane and the NSO fraction (polar fraction, which contains nitrogen, sulfur, and oxygen compounds) with methanol. 2.5. GC–MS analyses The biomarkers in the saturated hydrocarbons fraction of the expelled oils and bitumen obtained by hydrous pyrolysis were analyzed by gas chromatography–mass spectrometry (GC–MS). A 6890 N gas chromatograph/5973 N mass spectrometer equipped with a HP-5 column (30 m  0.32 mm i.d.  0.25/μm film thickness) was used. The GC oven temperature was raised from 80 1C to 300 1C (held 30 min) at 4 1C min  1. Helium was used as a carrier gas. MS conditions were: electron ionization (EI) at 70 eV with an ion source temperature of 250 1C. A full scan mode (m/z 20 to m/z 750) was applied.

3. Results and discussion 3.1. Amounts of expelled oil and bitumen Group composition data for the investigated samples of expelled oil and bitumen are given in Table 2. The amount of

M. Liang et al. / Journal of Petroleum Science and Engineering 125 (2015) 209–217

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Table 2 Group composition for the samples of expelled oil and bitumen. Sample T (1C) Sw (g) Expelled oil Relative content of expelled oil (%) (mg) Sat Aro NSO Asp

Emg/g.TOC Rw (g) Bitumen Relative content of bitumen (%) (mg) Sat Aro NSO Asp

Bmg/g TOC Amg/g TOC

Type II

250 300 350 400 450 500 550

50 30 30 30 20 20 15

6.42 3.63 29.76 26.76 6.14 5.58 3.68

40.81 41.87 19.59 32.40 24.92 32.08 36.68

5.14 4.41 4.40 17.56 18.89 19.35 7.34

45.64 42.15 56.96 42.30 42.35 35.13 46.20

8.41 11.57 19.05 7.74 13.84 13.44 9.78

5.42 5.11 41.86 37.64 12.95 11.77 10.35

50 29 29 29 19 18.5 9

50.17 19.4 61.09 111.01 22.55 7.02 1.64

37.47 37.42 31.20 32.84 13.75 42.17 54.27

17.00 12.06 25.60 25.90 14.06 17.52 18.90

39.92 44.02 35.27 32.93 48.69 34.62 2.44

5.60 6.49 7.94 8.33 23.50 5.70 42.07

42.34 28.23 88.88 161.52 50.08 16.01 7.69

47.76 33.33 130.74 199.15 63.03 27.78 18.04

Type I

250 300 350 400 450 500 550

50 30 30 20 20 15 15

2.37 5.96 74.25 106.46 152.00 22.06 6.07

17.30 27.52 25.09 46.35 40.01 34.36 40.03

0.84 5.37 7.23 12.28 14.66 21.85 10.71

60.76 51.01 50.25 35.33 39.84 38.17 41.52

21.10 16.11 17.43 6.05 5.49 5.62 7.74

0.54 2.28 28.35 60.97 87.06 16.85 4.64

48.5 29 28 18 16 13 8

364.36 293.49 939.35 838.25 147.67 40.67 16.56

13.97 12.02 12.38 21.02 37.51 22.79 13.77

13.62 6.01 18.72 19.75 31.50 26.04 24.94

67.99 76.90 49.83 40.83 25.70 49.42 57.43

4.42 5.07 19.70 18.40 5.29 1.75 3.86

86.05 115.93 384.29 533.44 105.72 35.84 23.71

86.60 118.20 412.64 594.42 192.78 52.68 28.35

Sw—sample weight; Rw—residue weight; Sat—saturated hydrocarbons; Aro–aromatic hydrocarbons; NSO—nitrogen–sulphur–oxygen containing compounds (polar compounds); Asp—asphaltenes. Emg/g  TOC ¼ Expelled oil/Sw  TOC(mg/g  TOC); Bmg/g  TOC ¼Bitumen/Rw  TOC(mg/g  TOC); Amg/g  TOC ¼Emg/g  TOC þ Bmg/g  TOC.

Fig. 2. Trend of productivity (mg/g  TOC) of expelled oil and bitumen with temperature.

expelled oil and bitumen were calculated to compare TOC conversion rates of different kerogen types during hydrous pyrolysis. For both kerogen pyrolysis experiments, the amounts of expelled oil and bitumen produced (Amg/g TOC) in all experiments increases consistently from 47.76 mg/g to 86.60 mg/g  TOC at 250 1C to the highest 199.15–594.42 mg/g  TOC at 400 1C, and then decrease to 18.04–28.35 mg/g  TOC at 550 1C (Table 2, Fig. 2a). The trend of the bitumen productivity (Bmg/g TOC) and amounts productivity (Amg/g TOC) with temperature was quite similar for both kerogen types, rising to 400 1C, followed by a decrease (Table 2, Fig. 2a). At the temperature of 400 1C, samples reached the maximum bitumen productivity and total of the expelled oil and bitumen productivity. However, the total of the expelled oil and bitumen generation of type I kerogen was always higher than the type II. This result indicates that the petroleum generation potential of type I organic matter is higher in comparison to type II kerogen, which can be attributed to the differences in TOC content and kerogen structure of the initial samples. Although the type I kerogen contains higher TOC and amount of products productivity than the type II, the productivity of expelled oil (Emg/g TOC) of the latter is higher than the former at temperatures

below 350 1C (Table 2; Fig. 2b). At the temperature of 450 1C, type I kerogen attained the expelled oil generation maximum, while maximum of expelled oil productivity for type II kerogen corresponds to 350–400 1C. This result indicates that at low thermal maturity, the content of expelled oil products of type II kerogen is higher in comparison to type I (Wang et al., 1997; Wang et al., 2008). The adsorption of organic matter on the indigenous mineral components and catalytic effect of the mineral matrix on kerogen cracking during the hydrous pyrolysis of is very low, because of the influence of water (Koopmans et al., 1998; Jovančićević et al., 1993; Huizinga et al., 1987b). TOC (total organic carbon) and the kerogen type determine the oil-generating capacity of source rocks, yet the kerogen type has more significant influence to the expelled oil productivity of source rocks. Especially at the lower thermal maturity, the type of kerogen has significant impact on the expelled oil productivity. The activation energy for conversion of OM to oil products has significant impact on the oil-generation from the source rocks at low thermal maturity (Lu et al., 2001; Huang, 1996). The observed results showed that activation energy required for hydrocarbons generation from type II kerogen is lower than for type I.

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Fig. 3. The TIC mass chromatograms of saturated hydrocarbons from initial rocks and hydrous pyrolysates produced at temperature range from 250 1C to 550 1C.

3.2. Molecular composition of hydropyrolysates The TIC, m/z 191 and m/z 217 mass chromatograms of the saturated hydrocarbons from the initial source rocks and hydrous pyrolysis products at temperatures range from 250 1C to 550 1C are shown in Figs. 3 and 6 and 7. The values of the molecular parameters calculated from compositions of biomarkers are given in Table 3. As shown in Fig. 3, distribution of the saturated hydrocarbons in the bitumen isolated from initial source rocks showed similar characteristics: Both samples are characterized by a low proportion of n-alkanes. The relative abundance of isoprenoids (pristane and phytane) was higher than n-alkanes. The relative abundance of steranes and hopanes are also more abundant than n-alkanes too. These results suggest that they were immature organic matter (Tuo et al., 2007), coinciding with the maturely parameters Tmax and Rc.

3.2.1. General characteristics At the low temperature of 250 1C, the distributions of saturated hydrocarbons of expelled oil and bitumen obtained by hydrous pyrolysis were quite similar to the original samples: lower relative abundance of n-alkanes than isoprenoids, steranes and terpanes, and a strong odd over even n-alkane predominance above C23 (Fig. 3; Table 3). At the temperatures of 300 1C to 450 1C, the relative abundance of n-alkanes increased gradually, and predominance of odd n-alkane homologues notably decreased (parameter OEP25–29; Table 3). Above the temperature of 450 1C, a significant difference in biomarker compositions between expelled oil and bitumen is observed (Fig. 3). The cracking of long-chain nalkanes resulted in narrowing of the n-alkanes range in bitumen. The temperature range of maximum hydrocarbon generation for type I kerogen corresponds to 350–450 1C. Below this range, the organic matter is in the immature state, above this temperature,

the cracking of long-chain n-alkanes might occur to produce gas of short-chain alkanes and methane. Main hydrocarbon generation for type II kerogen corresponds to the temperature range 300– 450 1C. Those temperature ranges of hydrocarbon generation might mean the oil window’s diversity and gas window’s consistency in those two type’s kerogen. The difference of temperature range of main hydrocarbon generation of two samples, suggests that the initial temperature and the peak temperature of hydrocarbon generation of type II sources rocks is lower than type I sources rocks. (Wang et al., 1997; Wang et al., 2008). For the all of these kerogen pyrolysis experiments, the relative aboundance n-alkanes in expelled oil is always higher than in bitumen, regardless to the kerogen type. Especially at the temperature of 500 1C to 550 1C, the higher relative abundance of n-alkanes of expelled oil happened. Differences between the biomarker compositions of expelled oils and bitumens in these experiments patterned those caused by natural petroleum migration (Peters et al., 1990). Heavier, more polar compounds are preferentially retained in the bitumen, and the lighter hydrocarbons released from kerogen and expelled by hydrous pyrolysis. At the low temperature of 250 1C and 300 1C, the type of kerogen can be considered, the nalkanes relative abundances of expelled oil for type II were higher than for type I. This result indicates that at low thermal maturity, not only the expelled oil productivity of type II kerogen is higher in comparison to type I, but also the concentrations of lighter hydrocarbons. This result confirms that at low thermal maturity the type II kerogen activation energy required to hydrocarbon generating is lower than for type I.

3.2.2. Isoprenoids, pristane and phytane Phytane and pristane are primarily derived from the phytol side-chain on the chlorophyll skeleton in phototrophic organisms, such as cyanobacteria, most algae and higher plants (Peters et al.,

Table 3 Basic biomarker parameters of the initial samples and hydrous pyrolysates produced at different temperatures produced at different temperatures. Sample code n-Alkane n-Alkane Pr/Ph Pr/nC17 Ph/nC18 ∑C22-/∑C23 þ OEP25–29 Ts/Tm G/C30H C29/C30H C3122S/(Sþ R) C3222S/(S þR) C29ββ/(αα þ ββ) C2920S/R(S þR) Relative content of regular αα(R)-steranes(%) range maximum C28 C29 C27 11-38 13-31 13-35 14-33 13-33 14-32 14-30 14-29 13-33 13-35 13-36 13-36 14-28 14-25 14-31 12-35 15-29 14-31 13-32 12-34 12-32 13-33 14-26 14-31 15-33 13-33 13-35 13-31 14-28 14-25

23 18 23 23 23 23 23 18 23 29 23 22 17 17 23 27 20 20 17 17 16 20 17 20 20 23 23 18 14 17

1.25 1.00 0.52 0.51 1.03 0.71 0.79 0.71 0.91 0.93 0.76 1.36 1.67 1.50 0.52 0.24 0.05 0.31 0.64 0.78 0.68 0.47 0.16 0.22 0.21 0.77 0.93 1.55 0.50 0.19

1.52 1.06 0.73 0.72 0.95 0.47 1.06 0.73 2.04 1.29 1.36 0.61 0.17 0.72 0.39 1.20 0.83 1.31 1.32 0.90 0.47 0.55 0.53 1.08 1.11 1.53 0.84 0.29 0.11 0.60

1.47 1.13 0.56 0.20 1.03 0.59 1.29 1.02 3.08 1.93 2.05 0.44 0.12 0.80 0.59 25.50 18.33 – 4.83 1.34 0.82 1.28 4.00 9.00 7.83 2.61 0.94 0.19 0.47 4.57

1.08 1.80 0.92 0.86 1.06 1.13 1.06 3.69 1.05 0.25 1.02 1.27 8.04 25.50 1.29 0.32 6.69 – 2.22 1.64 3.70 1.37 4.86 1.36 0.88 1.22 1.33 2.36 6.32 16.43

1.58 1.45 1.51 1.25 1.09 1.11 1.23 1.25 2.77 1.95 1.29 1.08 1.03 1.38 1.36 4.19 1.95 1.82 1.41 1.22 1.24 1.05 0.38 5.25 3.32 1.58 1.15 1.22 1.13 1.18

0.57 0.67 0.58 0.29 0.17 0.19 0.45 0.71 0.56 0.54 0.31 0.13 0.29 0.57 0.50 0.40 1.33 1.17 0.80 1.38 1.38 0.57 0.89 0.33 0.13 0.71 0.05 0.07 1.13 1.27

0.08 0.06 0.04 0.21 0.03 0.05 0.03 0.10 0.03 0.06 0.17 0.03 0.04 0.02 0.04 0.42 0.25 0.40 0.40 0.52 0.50 0.53 0.06 0.34 0.41 0.49 0.60 0.43 0.29 0.14

0.37 0.39 0.30 0.31 0.73 0.86 0.62 0.52 0.34 0.45 0.54 1.04 0.89 0.45 0.38 0.04 0.12 0.15 0.14 0.36 0.36 0.42 0.55 0.06 0.09 0.18 0.49 0.40 – 0.56

0.53 0.46 0.43 0.39 0.65 0.45 0.42 0.60 0.44 0.44 0.42 0.45 0.44 0.50 0.44 0.61 0.57 0.50 0.25 0.25 0.29 0.42 0.62 0.81 0.83 0.33 0.30 0.27 – –

0.5 0.5 0.44 0.40 0.37 0.38 0.38 0.59 0.40 0.40 0.33 0.38 – – – 0.58 – – 0.29 – – – – – – 0.44 0.20 – – –

0.33 0.30 0.30 0.25 0.23 0.24 0.20 0.40 0.32 0.31 0.31 0.40 0.45 0.37 0.37 0.30 0.26 0.20 0.18 0.27 0.33 0.36 0.40 0.27 0.27 0.30 0.34 0.37 0.40 0.41

0.24 0.20 0.17 0.15 0.15 0.14 0.13 0.36 0.18 0.16 0.20 0.26 0.34 0.24 0.23 0.23 0.20 0.13 0.10 0.20 0.33 0.33 0.36 0.20 0.20 0.21 0.25 0.29 0.36 0.36

37.14 42.74 42.31 44.90 45.83 51.22 46.43 31.82 49.59 41.10 42.25 55.05 54.35 57.84 48.15 22.28 24.07 20.00 45.00 48.78 64.71 59.09 39.29 20.00 19.05 23.19 30.46 51.92 51.28 46.43

18.92 20.97 21.15 26.53 16.67 14.63 14.29 22.73 18.18 19.86 21.13 17.43 19.57 19.61 20.37 42.76 37.04 36.00 30.00 29.27 23.53 27.27 25.00 38.46 38.10 37.68 35.10 28.85 22.22 25.00

43.94 36.29 36.54 28.57 37.50 34.15 39.28 45.45 32.23 39.04 36.62 27.52 26.09 22.55 31.48 34.96 38.89 44.00 25.00 21.95 11.76 13.64 35.71 41.54 42.86 39.13 34.44 19.23 26.50 28.57

II-E-250 1C: Type II-Expelled oil-250 1C; II-B-250 1C: Type II-Bitumen-250 1C. Pr—pristane; Ph—phytane; OEP—Odd-Even Predominance, OEP ¼1/4 [(n-C25 þ 6n-C27 þn-C29)/(n-C26 þ n-C28)]; Ts—C2718α(H)-22,29,30-trisnorneohopane; Tm—C2717α(H)-22,29,30-trisnorhopane; G—gammacerane; C29H–C2917α (H)-norhopane; C30H—C3017α(H)21β(H)-hopane; C3122(S)—C3117α(H)21β(H)22(S)-hopane; C3122(R)—C3117α(H)21β(H)22(R)-hopane; C32(S)—C3217α(H)21β(H)22(S)-hopane; C32(R)–C3217α(H)21β(H)22(R)-hopane; C29ββ—C2914β(H) 17β(H)20(R)-sterane; C29αα and C2920(R)—C2914α(H)17α(H)20(R)-sterane; C2920(S)—C2914α(H)17α(H)20(S)-sterane.

M. Liang et al. / Journal of Petroleum Science and Engineering 125 (2015) 209–217

II-Initial II-E-250 1C II-E-300 1C II-E-350 1C II-E-400 1C II-E-450 1C II-E-500 1C II-E-550 1C II-B-250 1C II-B-300 1C II-B-350 1C II-B-400 1C II-B-450 1C II-B-500 1C II-B-550 1C I-Initial I-E-250 1C I-E-300 1C I-E-350 1C I-E-400 1C I-E-450 1C I-E-500 1C I-E-550 1C I-B-250 1C I-B-300 1C I-B-350 1C I-B-400 1C I-B-450 1C I-B-500 1C I-B-550 1C

213

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M. Liang et al. / Journal of Petroleum Science and Engineering 125 (2015) 209–217

Fig. 4. Values of the Pr/Ph ratio of hydrous pyrolysates produced at temperature range from 250 1C to 550 1C.

2005). The relative abundances of pristane (Pr), phytane (Ph) and other isoprenoids are commonly used as biomarkers of photosynthetic organisms, and to estimate the redox conditions of depositional environment of organic matter (Peters et al., 2005; Brooks et al., 1969; Powell and McKirdy, 1973). However, the origins of pristane and phytane is much more complex than simply the reduction or oxidation of the phytol side chain in chlorophylls (Goossens et al., 1984). Previous investigations suggest that most of the pristane in crude oils originates by thermal cleavage of isoprenoid moieties bound by nonhydrolyzable C–C and/or C–O bonds within the kerogen matrix (Larter et al., 1979, 1981). In our study, for the both kerogen types, Pr/Ph values of expelled oil and bitumen produced in all pyrolysis experiments increase to 1.03 (0.78) at 400 1C and 1.67 (1.55) at 450 1C, followed by a decrease (Table 3, Fig. 4.). Increase in the relative abundances of n-alkanes with temperature, resulted in linear decrease of the Pr/nC17 and Ph/nC18 parameters (Table 3). This result seems to support previous views, for source rock samples within the oil-generative window (about 450 1C), the mixing of pristane from kerogen by thermal cleavage caused the Pr/Ph value increased with increasing temperature, and in the over-mature stage, kerogen condense as pyrobitumen and degenerating into high mature light oils (Fig. 3.) and oils cracking into gases by thermal cleavage (Lu et al., 2001).

3.2.3. Terpanes Terpanes are ubiquitous in source rocks and crude oils, and they are primarily derived from microbial (prokaryotic) membrane lipids (Ourisson et al., 1982). Numerous terpane parameters have been used to estimate the conditions of depositional environment of organic matter and assess the level of thermal maturity, such as Gammacerane/C30hopane, Ts/Tm, C29 hopane /C30 hopane, C3122S/ (S þR), etc. These parameters were calculated from m/z 191 mass chromatograms (Fig. 5) and values are given in Table 3. Gammacerane to C30 hopane ratio (G/C30H) was used to assess salinity of the depositional environment. Values of this parameter (G/C30H¼0.02–0.06) suggest that the type II sample was formed in freshwater environment whereas type I sample was deposited in

more saline environment (G/C30H ¼0.14–0.60). These results indicate that the gammacerane index has high specific for watercolumn stratification, and it’s unaffected by thermal effect. The Ts/Tm ratio and C3122S/(Sþ R) values were used to evaluate the level of thermal maturity. In present study, the Ts/Tm ratio did not show a linear relationship with the temperature increase. The C3122S/(S þR) values did not show a notable change with the temperature increase. There was a significant change of hopane composition at 400 1C and 450 1C. The relative abundance of C29hopane increased greatly, even beyond the content of C30-hopane (C29/C30H¼ 1.04) in the bitumen of type II at 400 1C, which can be related to the hopane demethylation resulted from thermal stress (Pan et al., 2006; Philip and Gilbert, 1984; van Graas, 1986). Besides, the previous investigations also demonstrated that C29 hopane is more stable than C30 hopane at high levels of thermal maturity (Peters et al., 2005). Thus, C29/C30 hopane ratio can be used as maturity indicator. The tricyclic terpanes were considered to originate from a regular C30 isoprenoid, such as tricyclohexaprenol, and could be constituents of prokaryote membranes (Aquino Neto et al., 1981). Also, there was an increase in tricyclic terpanes relative to hopanes in rock samples that contained elevated prasinophyte (Simoneit et al., 1986). Moreover, abundant in tricyclic terpanes commonly correlate with high paleolatitude Tasmanites-rich rocks, suggesting an origin from these algae (Peters et al., 2005). The m/z 191 mass chromatograms of saturated hydrocarbons (Fig. 5) showed that there was a trace tricyclic terpane generation below 400 1C. However, they were commonly present in the bitumen obtained at temperature above 450 1C. This data suggests that the tricyclic terpanes might be originated from regular C30 isoprenoid by thermal cleavage. It should be noticed that at temperatures above 500 1C, contents of the terpenoid biomarkers in hydropyrolysates were very low, particularly in those obtained from type I kerogen.

3.2.4. Steroids Previous studies proposed that the distributions of C27, C28, and C29 steranes might be used to differentiate ecosystems (Huang and Meinshein, 1979). The distribution and relative abundance of

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steranes are widely used to determine the source, type, and degree of thermal evolution of organic matter (Volkman, 2006; Farrimond et al., 1998). Regular C27–C29 steranes have detected in all samples and their distributions are comparable as shown by the mass chromatogram of m/z 217 in (Fig. 6). There were two notable observations of the steranes: (a) maturity parameters C29ββ/(αα þ ββ) and C2920S/

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(SþR) showed a linear relationship with the temperature raised, especially for the type I kerogen. However, both of the parameters showed a moderate variation. These data seem as a result of the balance between the relative rates of generation and thermal degradation of the different isomers (Farrimond et al., 1998). The type II source rock contains more C29 steranes and its precursors’ contributions from higher—plant organic matter within the kerogen

Fig. 5. The m/z 191 mass chromatograms of saturated hydrocarbons from initial rocks and hydrous pyrolysates produced at temperature range from 250 1C to 550 1C.

Fig. 6. The m/z 217 mass chromatograms of saturated hydrocarbons from hydrous pyrolysates produced at temperature range from 250 1C to 550 1C.

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matrix, as a typical of type II kerogen. Therefore the formation of secondary C29 steranes occurs from the kerogen by thermal cleavage, and complicates the sterane maturity parameters, C29ββ/ (αα þ ββ) and C2920S/(SþR)) of type II. (b) Significant variations in the relative contents of C27αα20R, C28αα20R and C29αα20R steranes with the temperature increase were observed (Fig. 6). The sterane distributions of Type II samples at temperatures 250–550 1C (expelled oil) and 250–550 1C (bitumen) are characterized by the predominance of biological configurations which show the follow order to abundance: C27 ZC29 4C28, indicating a typical mixture of terrigenous and aquatic OM, coincide to the type of kerogen. As the temperature increased, the relative abundance of C28 and C29 steranes gradually decreased, while C27 remained relatively constant increased. The line showing distribution of regular steranes of type II kerogen changed from irregular “V” to “L” (Table 3, Fig. 6). Similarly, the sterane distributions of Type I samples showed a significant variation from “anti-L” to “L”. After entering the oilgeneration window, the sterane distributions are characterized by the predominance of biological configurations which are showed the following order of abundance: C27 4C28 4C29, suggesting a typical contribution from marine OM-rich sediments. These results indicate that the distribution and relative abundance of regular steranes are effectively to determine the sediments OM source in the mature stage. For all of the pyrolysis experiments, the trend of the C27αα20R relative content with temperature was quite similar for both

kerogen types, rising to temperature about 450 1C, followed by a decrease (Fig. 7). At the temperature of 450 1C, samples reached the maximum relative abundance of C27αα20R. This result might indicate that formation of secondary steranes occurs by cracking the side chain and the early loss of C29 steranes from thermal cleavage (Lu and Kaplan 1992). When taking into account the difference of the type, we can see that there was more significant variation of C27αα20R relative content for type I than for type II kerogen (Fig. 7). This difference is supposedly due to that the type I kerogen contains more C27 steranes and its precursors within the kerogen matrix. The variation of C27αα20R relative content for type II mostly related to the early loss of C29 steranes by the demethylation from thermal pyrolysis. However, at the temperature of 500 and 550 1C, kerogen condenses as pyrobitumen and degenerating into high mature light oils by thermal cleavage.

4. Conclusions Hydrous pyrolysis experiments were conducted on immature petroleum source rocks containing different types of kerogen (I and II) at a temperature range of 250–550 1C to investigate the role of type in petroleum formation at high temperature (above 400 1C). For this purpose, the group composition, productivity of expelled oil and bitumen, and biomarker compositions (n-alkanes,

Fig. 7. The relative content of C27αα20R sterane in hydrous pyrolysates produced at temperature range from 250 1C to 550 1C.

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isoprenoids, terpanes and steranes) were used. The following conclusions can be drawn: 1. The trend of amounts of productivity of oil-like products (mg/ g  TOC) with temperature was quite similar for both types, and at temperature of 400 1C, samples reached the maximum productivity. However, there were significant differences in the yield which attributed to the differences of TOC and kerogen stricture of the original samples. 2. Because of the differences of the TOC and kerogen structure, the productivity of type I kerogen is higher than type II. Nevertheless, the yield of the expelled oil of the type II kerogen was higher than the type I below 350 1C, although the type I TOC was higher than the type II. This result suggests that the type of source rock is probably more important than TOC content for determining productivity at the early maturation. 3. Tricyclic terpanes were commonly present in the samples above 400 1C. Ts/Tm values did not show a linear relationship with the temperature increase. The C3122S/(Sþ R) values did not show significant change with the temperature increase too. These results suggest that the hopane maturity parameters are not reliable for thermal maturity in hydrous pyrolysis. However, the gammacerane to C30 hopane ratio (G/C30H) indicate that the gammacerane index has high specific for watercolumn stratification, and it’s unaffected by thermal maturity. 4. As the temperature increased, significant variations in the relative contents of C27αα20R, C28αα20R and C29αα20R steranes were occurred, showing the distribution of steranes is greatly affected by thermal maturity. However, the distribution and relative abundance of regular steranes are effective to determine the sediments OM source in the mature stage. Besides, a good correlation between the sterane maturity parameters and the thermal maturity was observed. Acknowledgements We are grateful to Prof. Ksenija A. Stojanović (University of Belgrade) and other anonymous reviewers for all of them helpful comments and suggestions that significantly improved the earlier version of this paper. The authors wish to thank Dr. John Volkman for him valuable help to improve the manuscript. This work has been performed with the approval of National Natural Science Foundation of China (Grant nos. 41072106 and 40972099). References Aquino Neto, F.R., Trendel, J.M., Restle, A., Connan, J., Albrecht, P.A., 1981. In: Bjorøy, M., et al. (Eds.), Advances in Organic Geochemistry. Wiley, Chichester, pp. 659–676 (1983). Béhar, F., Vandenbroucke, M., Tang, M., Marquis, F., Espitalié, J., 1997. Thermal cracking of kerogen in open and closed systems: determination of kinetic parameters and stoichiometric coefficients for oil and gas generation. Org. Geochem. 26, 321–339. Brooks, J.D., Gould, K., Smith, J.W., 1969. Isoprenoid hydrocarbons in coal and petroleum. Nature 222, 257–259. Espitalie, J., Derco, G., Marquis, F., 1985. La pyrolyse Rock-Eval et ses applications, Partie II. Rev. Inst. Franc. Pet 40 (6), 755–784. Farrimond, P., Taylor, A., TelnÆs, N., 1998. Biomarker maturity parameters: the role of generation and thermal degradation. Org. Geochem. 29, 1181–1197. Goossens, H., de Leeuw, J.W., Schenck, P.A., Brassell, S.C., 1984. Tocopherols as likely precursors of pristane in ancient sediments and crude oils. Nature 312, 440–442. Michels, R., Landais, P., Torkelson, B.E., Philp, R.P., 1995. Effects of effluents and water pressure on oil generation during confined pyrolysis and high-pressure hydrous pyrolysis. Geochim. Cosmochim. Acta 59, 1589–1604. Michels, R., Landais, P., 1994. Artificial coalification: comparison of confined pyrolysis and hydrous pyrolysis. Fuel 73 (1691-6). Mu, G.Y., Zhong, N.N., Liu, B., 2008. The geochemical characteristic and genetic type of crude oil in the western sag of the Liaohe Basin. Pet. Geol. Exp. 30, 611–616.

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