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

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Journal of Petroleum Science and Engineering

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Hydrous of different 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 article info abstracts

Article history: Hydrous pyrolysis experiments were conducted on immature petroleum source rocks containing Received 3 December 2013 different types of kerogen (I and II) at a temperature range of 250–550 1C to investigate the role of Accepted 24 November 2014 kerogen type in petroleum formation at high temperature. At temperatures less than 350 1C, the bulk Available online 3 December 2014 results and biomarker compositions were quite similar and show negligible variations for different Keywords: kerogen types. However, at high-temperature, above 400 1C (400–550 1C), unexpected results were Hydrous pyrolysis observed. The initial and peak temperature of the hydrocarbon generation of type II sources rocks is High temperature lower than the type I. The tricyclic terpenes were common in samples above 400 1C. The Ts/Tm values did Source rock not show a linear relationship with the temperature increase. The C3122S/(SþR) values did not show Kerogen significant change with temperature increase. However, a good correlation between the sterane maturity GC–MS parameters and the thermal maturity were shown, especially for type I kerogen. Temperature increase Biomarkers 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.

1. Introduction based on the cognition that organic matter exposed to higher temperature for a shorter period of time has the same maturity as Because of their specific genesis and structural characteristics, organic matter exposed to lower temperature for a longer period of steranes and terpanes are widely used in petroleum exploration. time, suggesting that temperature and time compensate for each They also serve as a molecular fingerprint of sedimentary organic other as effects on organic matter maturity (Stach et al., 1982). matter (OM), indicating its origin of depositional environment, and Therefore, it seems to be valid to simulate the natural maturation thermal maturity (Volkman, 2006; Wang et al., 2009). Previous process in the laboratory by increasing temperature during a short studies have documented that slight enrichment in C27 steranes period of time (Han et al., 2001). Taking into account subcritical occurs with increasing thermal stress, and questioned usage of 20S to temperatures (Lewan, 1985), most of the hydrous pyrolysis

20R C29 sterane ratio as a maturity index (Lewan et al., 1986). experiments were performed at temperatures below 400 1C. There- Hydrous pyrolysis is used to determine petroleum generation fore, only a few thermal simulation experiments have been con- mechanisms and assess the petroleum generation potential of source ducted and reported at temperatures above 400 1C. In the present rocks (Lewan, 1979, 1983, 1985; Winters et al., 1983). Numerous study, two source rocks containing different types of kerogen (I and hydrous pyrolysis experiments have been done in both open and II) were used for the thermal simulation experiment. Attempts have closed systems, using different types of organic matter and condi- been made to observe the differences productivity of different tions, in order to understand the mechanisms of oil generation kerogen type and variations in the distributions of biomarkers (Huizinga et al., 1987a,b; Michels and Landais, 1994; Michels et al., released from the samples by pyrolysis. Especially, we conducted 1995; Béhar et al., 1997; Pan et al., 2009). These studies are mainly high temperature thermal simulation experiments (400–550 1C) among petroleum source rocks containing different types of kerogen. Andinvestigatedthetrendsandcomparethedistributionofthe

n þ þ classic biomarker compositions from different liquid phases at high Corresponding author: Tel.: 86 931 4960921; fax: 86 931 8278667. 1 E-mail address: [email protected] (Z. Wang). temperature (above 400 C). http://dx.doi.org/10.1016/j.petrol.2014.11.021 0920-4105/& 2014 Elsevier B.V. All rights reserved. 210 M. Liang et al. / Journal of Petroleum Science and Engineering 125 (2015) 209–217

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 Gray mudstone, Shahejie Fm, Liaohe Basin 2553.6–2558 0.59 427 0.28 10.75 2.37 453 21

I Brown gray shale, Shahejie Fm, Liaohe Basin 1387.49–1390.19 0.54 434 1.86 69.2 8.73 792 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— 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 Two organic phases were collected from the reactor after experiment. Both of the samples were collected from Shahejie hydrous pyrolysis: expelled oil and bitumen. Expelled oil occurs Formation, Liaohe Basin, North China (Fig. 1)(Mu et al., 2008). The in the reactor as liquid products (Lewan, 1985). The rock chips Shahejie Formation source rocks are comprised of black gray-dark were removed from the reactor and dried in a vacuum oven at gray mudstone, gray mudstone and oil shale, with a total thickness 50 1C for 24 h, and then powdered to less than 100 mesh. Bitumen up to 1000 m. The depths of initial samples are 2553.6–2558/m was extracted from the powdered samples in a Soxhlet apparatus (type II) and 1387.49–1390.19/m (type I), respectively. The present for 72 h with a mixture of chloroform and methanol (97:3, v-v). temperatures are about 93and 51 1C, according to the average After precipitation of the asphaltenes by n-hexane, maltenes were value of geothermal gradient, which reaches 36.5/km (Wang et al., separated into three fractions by column chromatography over 2003). The organic maturity of the two samples are relatively low, silica gel and aluminum oxide (3:1). The saturated hydrocarbons and the Rc values are generally less than 0.6%. For hydrous fraction was eluted with petroleum ether, the aromatic hydro- pyrolysis experiments, samples were first powdered to less than carbons with dichloromethane and the NSO fraction (polar frac- 100 mesh. tion, which contains nitrogen, sulfur, and oxygen compounds) with methanol. 2.2. Rock-eval pyrolysis 2.5. GC–MS analyses Rock-eval pyrolysis was performed using a Rock-eval II system (Delsi instrument, Suresnes, France). And organic-carbon-content The biomarkers in the saturated hydrocarbons fraction of the (TOC) was determined using a Leco CS-344 analyzer. Group expelled oils and bitumen obtained by hydrous pyrolysis were – – organic geochemical data for investigated samples are given in analyzed by gas chromatography mass spectrometry (GC MS). A Table 1. These results indicate that source rocks contain kerogen 6890 N gas chromatograph/5973 N mass spectrometer equipped μ fi type I and II (Huang et al., 1984). The T values of the two with a HP-5 column (30 m 0.32 mm i.d. 0.25/ m lm thick- max 1 samples are 434 1C and 427 1C, respectively, indicating low to ness) was used. The GC oven temperature was raised from 80 Cto 1 1 1 marginal thermal maturity (Espitalie et al., 1985). 300 C (held 30 min) at 4 C min . 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 2.3. Hydrous pyrolysis 750) was applied.

Hydrous pyrolysis experiments were performed in a stainless steel reactor. 15–50 g of powdered sample and 50 ml of deionized 3. Results and discussion water were put in the reactor, and sealed the chamber then flushed three times with pure nitrogen gas to remove the air, 3.1. Amounts of expelled oil and bitumen and then evacuated. A sensor and an external control device were used to control the temperature within the autoclave. The hydrous Group composition data for the investigated samples of pyrolysis experiments were performed at temperatures 250, 300, expelled oil and bitumen are given in Table 2. The amount of M. Liang et al. / Journal of Petroleum Science and Engineering 125 (2015) 209–217 211

Table 2 Group composition for the samples of expelled oil and bitumen.

Sample T (1C) Sw (g) Expelled oil Relative content of expelled oil (%) Emg/g.TOC Rw (g) Bitumen Relative content of bitumen (%) Bmg/g TOC Amg/g TOC (mg) (mg) Sat Aro NSO Asp Sat Aro NSO Asp

Type II 250 50 6.42 40.81 5.14 45.64 8.41 5.42 50 50.17 37.47 17.00 39.92 5.60 42.34 47.76 300 30 3.63 41.87 4.41 42.15 11.57 5.11 29 19.4 37.42 12.06 44.02 6.49 28.23 33.33 350 30 29.76 19.59 4.40 56.96 19.05 41.86 29 61.09 31.20 25.60 35.27 7.94 88.88 130.74 400 30 26.76 32.40 17.56 42.30 7.74 37.64 29 111.01 32.84 25.90 32.93 8.33 161.52 199.15 450 20 6.14 24.92 18.89 42.35 13.84 12.95 19 22.55 13.75 14.06 48.69 23.50 50.08 63.03 500 20 5.58 32.08 19.35 35.13 13.44 11.77 18.5 7.02 42.17 17.52 34.62 5.70 16.01 27.78 550 15 3.68 36.68 7.34 46.20 9.78 10.35 9 1.64 54.27 18.90 2.44 42.07 7.69 18.04

Type I 250 50 2.37 17.30 0.84 60.76 21.10 0.54 48.5 364.36 13.97 13.62 67.99 4.42 86.05 86.60 300 30 5.96 27.52 5.37 51.01 16.11 2.28 29 293.49 12.02 6.01 76.90 5.07 115.93 118.20 350 30 74.25 25.09 7.23 50.25 17.43 28.35 28 939.35 12.38 18.72 49.83 19.70 384.29 412.64 400 20 106.46 46.35 12.28 35.33 6.05 60.97 18 838.25 21.02 19.75 40.83 18.40 533.44 594.42 450 20 152.00 40.01 14.66 39.84 5.49 87.06 16 147.67 37.51 31.50 25.70 5.29 105.72 192.78 500 15 22.06 34.36 21.85 38.17 5.62 16.85 13 40.67 22.79 26.04 49.42 1.75 35.84 52.68 550 15 6.07 40.03 10.71 41.52 7.74 4.64 8 16.56 13.77 24.94 57.43 3.86 23.71 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 TOC¼Expelled oil/Sw TOC(mg/g TOC); Bmg/g TOC¼Bitumen/Rw TOC(mg/g TOC); Amg/g TOC¼Emg/g TOCþBmg/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 con- below 350 1C(Table 2; Fig. 2b). At the temperature of 450 1C, type I version rates of different kerogen types during hydrous pyrolysis. kerogen attained the expelled oil generation maximum, while For both kerogen pyrolysis experiments, the amounts of expelled maximum of expelled oil productivity for type II kerogen corre- oil and bitumen produced (Amg/g TOC) in all experiments sponds to 350–400 1C. This result indicates that at low thermal increases consistently from 47.76 mg/g to 86.60 mg/g TOC at maturity, the content of expelled oil products of type II kerogen is 250 1C to the highest 199.15–594.42 mg/g TOC at 400 1C, and higher in comparison to type I (Wang et al., 1997; Wang et al., 2008). then decrease to 18.04–28.35 mg/g TOC at 550 1C(Table 2, The adsorption of organic matter on the indigenous mineral Fig. 2a). The trend of the bitumen productivity (Bmg/g TOC) and components and catalytic effect of the mineral matrix on kerogen amounts productivity (Amg/g TOC) with temperature was quite during the hydrous pyrolysis of is very low, because of the similar for both kerogen types, rising to 400 1C, followed by a influence of water (Koopmans et al., 1998; Jovančićević et al., 1993; decrease (Table 2, Fig. 2a). At the temperature of 400 1C, samples Huizinga et al., 1987b). TOC (total organic carbon) and the kerogen reached the maximum bitumen productivity and total of the type determine the oil-generating capacity of source rocks, yet the expelled oil and bitumen productivity. However, the total of the kerogen type has more significant influence to the expelled oil expelled oil and bitumen generation of type I kerogen was always productivity of source rocks. Especially at the lower thermal higher than the type II. This result indicates that the petroleum maturity, the type of kerogen has significant impact on the generation potential of type I organic matter is higher in compar- expelled oil productivity. The activation energy for conversion of ison to type II kerogen, which can be attributed to the differences OM to oil products has significant impact on the oil-generation in TOC content and kerogen structure of the initial samples. from the source rocks at low thermal maturity (Lu et al., 2001; Although the type I kerogen contains higher TOC and amount of Huang, 1996). The observed results showed that activation energy products productivity than the type II, the productivity of expelled oil required for hydrocarbons generation from type II kerogen is (Emg/g TOC) of the latter is higher than the former at temperatures lower than for type I. 212 M. Liang et al. / Journal of Petroleum Science and Engineering 125 (2015) 209–217

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 cracking of long-chain n-alkanes might occur to produce gas of short-chain alkanes and methane. Main hydrocarbon generation The TIC, m/z 191 and m/z 217 mass chromatograms of the for type II kerogen corresponds to the temperature range 300– saturated hydrocarbons from the initial source rocks and hydrous 450 1C. Those temperature ranges of hydrocarbon generation pyrolysis products at temperatures range from 250 1C to 550 1Care might mean the oil window’s diversity and gas window’s consis- shown in Figs. 3 and 6 and 7. The values of the molecular tency in those two type’s kerogen. The difference of temperature parameters calculated from compositions of biomarkers are given range of main hydrocarbon generation of two samples, suggests in Table 3. As shown in Fig. 3, distribution of the saturated that the initial temperature and the peak temperature of hydro- hydrocarbons in the bitumen isolated from initial source rocks carbon generation of type II sources rocks is lower than type I showed similar characteristics: Both samples are characterized by sources rocks. (Wang et al., 1997; Wang et al., 2008). a low proportion of n-alkanes. The relative abundance of isopre- For the all of these kerogen pyrolysis experiments, the relative noids (pristane and phytane) was higher than n-alkanes. The aboundance n-alkanes in expelled oil is always higher than in relative abundance of steranes and hopanes are also more abun- bitumen, regardless to the kerogen type. Especially at the tempera- dant than n-alkanes too. These results suggest that they were ture of 500 1C to 550 1C,thehigherrelativeabundanceofn-alkanes immature organic matter (Tuo et al., 2007), coinciding with the of expelled oil happened. Differences between the biomarker com- maturely parameters Tmax and Rc. positions 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 3.2.1. General characteristics retained in the bitumen, and the lighter hydrocarbons released from At the low temperature of 250 1C, the distributions of saturated kerogen and expelled by hydrous pyrolysis. At the low temperature hydrocarbons of expelled oil and bitumen obtained by hydrous of 250 1Cand3001C,thetypeofkerogencanbeconsidered,then- pyrolysis were quite similar to the original samples: lower relative alkanesrelativeabundancesofexpelledoilfortypeIIwerehigher abundance of n-alkanes than isoprenoids, steranes and terpanes, than for type I. This result indicates that at low thermal maturity, not and a strong odd over even n-alkane predominance above C23 only the expelled oil productivity of type II kerogen is higher in (Fig. 3; Table 3). At the temperatures of 300 1C to 450 1C, the comparison to type I, but also the concentrations of lighter hydro- relative abundance of n-alkanes increased gradually, and predo- carbons. This result confirmsthatatlowthermalmaturitythetypeII minance of odd n-alkane homologues notably decreased (para- kerogen activation energy required to hydrocarbon generating is meter OEP25–29; Table 3). Above the temperature of 450 1C, a lower than for type I. significant difference in biomarker compositions between expelled oil and bitumen is observed (Fig. 3). The cracking of long-chain n- alkanes resulted in narrowing of the n-alkanes range in bitumen. 3.2.2. Isoprenoids, pristane and phytane The temperature range of maximum hydrocarbon generation for Phytane and pristane are primarily derived from the phytol type I kerogen corresponds to 350–450 1C. Below this range, the side-chain on the chlorophyll skeleton in phototrophic organisms, organic matter is in the immature state, above this temperature, 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/nC 17 Ph/nC 18 ∑C22- /∑C23þ OEP25–29 Ts/Tm G/C30 HC29 /C30 HC31 22S/(SþR)C32 22S/(SþR)C29 ββ/(ααþββ)C29 20S/R(SþR) Relative content of regular αα(R)-steranes(%) range maximum C 27 C28 C29

II-Initial 11-38 23 1.25 1.52 1.47 1.08 1.58 0.57 0.08 0.37 0.53 0.5 0.33 0.24 37.14 18.92 43.94 II-E-250 1C 13-31 18 1.00 1.06 1.13 1.80 1.45 0.67 0.06 0.39 0.46 0.5 0.30 0.20 42.74 20.97 36.29 II-E-300 1C 13-35 23 0.52 0.73 0.56 0.92 1.51 0.58 0.04 0.30 0.43 0.44 0.30 0.17 42.31 21.15 36.54 II-E-350 1C 14-33 23 0.51 0.72 0.20 0.86 1.25 0.29 0.21 0.31 0.39 0.40 0.25 0.15 44.90 26.53 28.57 II-E-400 1C 13-33 23 1.03 0.95 1.03 1.06 1.09 0.17 0.03 0.73 0.65 0.37 0.23 0.15 45.83 16.67 37.50 II-E-450 1C 14-32 23 0.71 0.47 0.59 1.13 1.11 0.19 0.05 0.86 0.45 0.38 0.24 0.14 51.22 14.63 34.15 II-E-500 1C 14-30 23 0.79 1.06 1.29 1.06 1.23 0.45 0.03 0.62 0.42 0.38 0.20 0.13 46.43 14.29 39.28 II-E-550 1C 14-29 18 0.71 0.73 1.02 3.69 1.25 0.71 0.10 0.52 0.60 0.59 0.40 0.36 31.82 22.73 45.45 .Lage l ora fPtoemSineadEgneig15(05 209 (2015) 125 Engineering and Science Petroleum of Journal / al. et Liang M. II-B-250 1C 13-33 23 0.91 2.04 3.08 1.05 2.77 0.56 0.03 0.34 0.44 0.40 0.32 0.18 49.59 18.18 32.23 II-B-300 1C 13-35 29 0.93 1.29 1.93 0.25 1.95 0.54 0.06 0.45 0.44 0.40 0.31 0.16 41.10 19.86 39.04 II-B-350 1C 13-36 23 0.76 1.36 2.05 1.02 1.29 0.31 0.17 0.54 0.42 0.33 0.31 0.20 42.25 21.13 36.62 II-B-400 1C 13-36 22 1.36 0.61 0.44 1.27 1.08 0.13 0.03 1.04 0.45 0.38 0.40 0.26 55.05 17.43 27.52 II-B-450 1C 14-28 17 1.67 0.17 0.12 8.04 1.03 0.29 0.04 0.89 0.44 – 0.45 0.34 54.35 19.57 26.09 II-B-500 1C 14-25 17 1.50 0.72 0.80 25.50 1.38 0.57 0.02 0.45 0.50 – 0.37 0.24 57.84 19.61 22.55 II-B-550 1C 14-31 23 0.52 0.39 0.59 1.29 1.36 0.50 0.04 0.38 0.44 – 0.37 0.23 48.15 20.37 31.48 I-Initial 12-35 27 0.24 1.20 25.50 0.32 4.19 0.40 0.42 0.04 0.61 0.58 0.30 0.23 22.28 42.76 34.96 I-E-250 1C 15-29 20 0.05 0.83 18.33 6.69 1.95 1.33 0.25 0.12 0.57 – 0.26 0.20 24.07 37.04 38.89 I-E-300 1C 14-31 20 0.31 1.31 –– 1.82 1.17 0.40 0.15 0.50 – 0.20 0.13 20.00 36.00 44.00 I-E-350 1C 13-32 17 0.64 1.32 4.83 2.22 1.41 0.80 0.40 0.14 0.25 0.29 0.18 0.10 45.00 30.00 25.00 I-E-400 1C 12-34 17 0.78 0.90 1.34 1.64 1.22 1.38 0.52 0.36 0.25 – 0.27 0.20 48.78 29.27 21.95 I-E-450 1C 12-32 16 0.68 0.47 0.82 3.70 1.24 1.38 0.50 0.36 0.29 – 0.33 0.33 64.71 23.53 11.76 I-E-500 1C 13-33 20 0.47 0.55 1.28 1.37 1.05 0.57 0.53 0.42 0.42 – 0.36 0.33 59.09 27.27 13.64 I-E-550 1C 14-26 17 0.16 0.53 4.00 4.86 0.38 0.89 0.06 0.55 0.62 – 0.40 0.36 39.29 25.00 35.71 I-B-250 1C 14-31 20 0.22 1.08 9.00 1.36 5.25 0.33 0.34 0.06 0.81 – 0.27 0.20 20.00 38.46 41.54 I-B-300 1C 15-33 20 0.21 1.11 7.83 0.88 3.32 0.13 0.41 0.09 0.83 – 0.27 0.20 19.05 38.10 42.86 I-B-350 1C 13-33 23 0.77 1.53 2.61 1.22 1.58 0.71 0.49 0.18 0.33 0.44 0.30 0.21 23.19 37.68 39.13 I-B-400 11CC 13-35 13-31 23 18 0.93 1.55 0.84 0.29 0.94 0.19 1.33 2.36 1.15 1.22 0.05 0.07 0.60 0.43 0.49 0.40 0.30 0.27 0.20– 0.340.37 0.25 0.29 30.46 51.92 35.10 28.85 34.44 19.23 I-B-450I-B-500 1C 14-28 14 0.50 0.11 0.47 6.32 1.13 1.13 0.29 –– – 0.40 0.36 51.28 22.22 26.50 I-B-550 1C 14-25 17 0.19 0.60 4.57 16.43 1.18 1.27 0.14 0.56 ––0.41 0.36 46.43 25.00 28.57

II-E-250 1C: Type II-Expelled oil-250 1C; II-B-250 1C: Type II-Bitumen-250 1C. þ þ þ — α — α — – α Pr—pristane; Ph—phytane; OEP—Odd-Even Predominance, OEP¼1/4 [(n-C 25 6n-C 27 n-C 29 )/(n-C 26 n-C 28 )]; Ts C27 18 (H)-22,29,30-trisnorneohopane; T m C27 17 (H)-22,29,30-trisnorhopane; G gammacerane; C 29 H C29 17 (H)-norhopane;30 H—C 30 C 17α(H)21β(H)-hopane; C 31 22(S)—C 31 17α(H)21β(H)22(S)-hopane; C 31 22(R)—C 31 17α(H)21β(H)22(R)-hopane; C 32 (S)—C 32 17α(H)21β(H)22(S)-hopane; C 32 (R)–C 32 17α(H)21β(H)22(R)-hopane; C 29 ββ—C 29 14β(H) –

17β(H)20(R)-sterane;29 αα and C C 29 20(R)—C 29 14α(H)17α(H)20(R)-sterane; C 29 20(S)—C 29 14α(H)17α(H)20(S)-sterane. 217 213 214 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 more saline environment (G/C30H¼0.14–0.60). These results indi- other isoprenoids are commonly used as biomarkers of photosyn- cate that the gammacerane index has high specific for water- thetic organisms, and to estimate the redox conditions of deposi- column stratification, and it’s unaffected by thermal effect. The tional environment of organic matter (Peters et al., 2005; Brooks Ts/Tm ratio and C3122S/(SþR) values were used to evaluate the et al., 1969; Powell and McKirdy, 1973). level of thermal maturity. In present study, the Ts/Tm ratio did not However, the origins of pristane and phytane is much more show a linear relationship with the temperature increase. The complex than simply the reduction or oxidation of the phytol side C3122S/(SþR) values did not show a notable change with the chain in chlorophylls (Goossens et al., 1984). Previous investiga- temperature increase. There was a significant change of hopane tions suggest that most of the pristane in crude oils originates composition at 400 1C and 450 1C. The relative abundance of C29- by thermal cleavage of isoprenoid moieties bound by non- hopane increased greatly, even beyond the content of C30-hopane hydrolyzable C–C and/or C–O bonds within the kerogen matrix (C29/C30H¼1.04) in the bitumen of type II at 400 1C, which can be (Larter et al., 1979, 1981). In our study, for the both kerogen types, related to the hopane demethylation resulted from thermal stress Pr/Ph values of expelled oil and bitumen produced in all pyrolysis (Pan et al., 2006; Philip and Gilbert, 1984; van Graas, 1986). experiments increase to 1.03 (0.78) at 400 1C and 1.67 (1.55) Besides, the previous investigations also demonstrated that at 450 1C, followed by a decrease (Table 3, Fig. 4.). Increase in C29 hopane is more stable than C30 hopane at high levels of the relative abundances of n-alkanes with temperature, resulted in thermal maturity (Peters et al., 2005). Thus, C29/C30 hopane ratio linear decrease of the Pr/nC17 and Ph/nC18 parameters (Table 3). can be used as maturity indicator. The tricyclic terpanes were This result seems to support previous views, for source rock considered to originate from a regular C30 isoprenoid, such as samples within the oil-generative window (about 450 1C), the tricyclohexaprenol, and could be constituents of prokaryote mem- mixing of pristane from kerogen by thermal cleavage caused the branes (Aquino Neto et al., 1981). Also, there was an increase in Pr/Ph value increased with increasing temperature, and in the tricyclic terpanes relative to hopanes in rock samples that con- over-mature stage, kerogen condense as pyrobitumen and degen- tained elevated prasinophyte (Simoneit et al., 1986). Moreover, erating into high mature light oils (Fig. 3.) and oils cracking into abundant in tricyclic terpanes commonly correlate with high gases by thermal cleavage (Lu et al., 2001). 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 3.2.3. Terpanes tricyclic terpane generation below 400 1C. However, they were Terpanes are ubiquitous in source rocks and crude oils, and commonly present in the bitumen obtained at temperature above they are primarily derived from microbial (prokaryotic) membrane 450 1C. This data suggests that the tricyclic terpanes might be lipids (Ourisson et al., 1982). Numerous terpane parameters have originated from regular C30 isoprenoid by thermal cleavage. It been used to estimate the conditions of depositional environment should be noticed that at temperatures above 500 1C, contents of of organic matter and assess the level of thermal maturity, such as the terpenoid biomarkers in hydropyrolysates were very low,

Gammacerane/C30hopane, Ts/Tm,C29 hopane /C30 hopane, C3122S/ particularly in those obtained from type I kerogen. (SþR), 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 3.2.4. Steroids salinity of the depositional environment. Values of this parameter Previous studies proposed that the distributions of C27,C28, and (G/C30H¼0.02–0.06) suggest that the type II sample was formed in C29 steranes might be used to differentiate ecosystems (Huang and freshwater environment whereas type I sample was deposited in Meinshein, 1979). The distribution and relative abundance of M. Liang et al. / Journal of Petroleum Science and Engineering 125 (2015) 209–217 215 steranes are widely used to determine the source, type, and degree (SþR) showed a linear relationship with the temperature raised, of thermal evolution of organic matter (Volkman, 2006; Farrimond especially for the type I kerogen. However, both of the parameters et al., 1998). showed a moderate variation. These data seem as a result of the

Regular C27–C29 steranes have detected in all samples and their balance between the relative rates of generation and thermal distributions are comparable as shown by the mass chromatogram degradation of the different isomers (Farrimond et al., 1998). The of m/z 217 in (Fig. 6). There were two notable observations of the type II source rock contains more C29 steranes and its precursors’ steranes: (a) maturity parameters C29ββ/(ααþββ)andC2920S/ 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. 216 M. Liang et al. / Journal of Petroleum Science and Engineering 125 (2015) 209–217 matrix, as a typical of type II kerogen. Therefore the formation of kerogen types, rising to temperature about 450 1C, followed by a secondary C29 steranes occurs from the kerogen by thermal decrease (Fig. 7). At the temperature of 450 1C, samples reached cleavage, and complicates the sterane maturity parameters, C29ββ/ the maximum relative abundance of C27αα20R. This result might (ααþββ)andC2920S/(SþR)) of type II. (b) Significant variations in indicate that formation of secondary steranes occurs by cracking the relative contents of C27αα20R,C28αα20R and C29αα20R ster- the side chain and the early loss of C29 steranes from thermal anes with the temperature increase were observed (Fig. 6). The cleavage (Lu and Kaplan 1992). sterane distributions of Type II samples at temperatures 250–550 1C When taking into account the difference of the type, we can see

(expelled oil) and 250–550 1C (bitumen) are characterized by the that there was more significant variation of C27αα20R relative predominance of biological configurations which show the follow content for type I than for type II kerogen (Fig. 7). This difference is order to abundance: C27 ZC294C28, indicating a typical mixture of supposedly due to that the type I kerogen contains more C27 terrigenous and aquatic OM, coincide to the type of kerogen. As the steranes and its precursors within the kerogen matrix. The varia- temperature increased, the relative abundance of C28 and C29 tion of C27αα20R relative content for type II mostly related to the steranes gradually decreased, while C27 remained relatively con- early loss of C29 steranes by the demethylation from thermal stant increased. The line showing distribution of regular steranes of pyrolysis. However, at the temperature of 500 and 550 1C, kerogen type II kerogen changed from irregular “V” to “L” (Table 3, Fig. 6). condenses as pyrobitumen and degenerating into high mature Similarly, the sterane distributions of Type I samples showed a light oils by thermal cleavage. significant variation from “anti-L” to “L”. After entering the oil- generation window, the sterane distributions are characterized by the predominance of biological configurations which are showed 4. Conclusions the following order of abundance: C27 4C284C29,suggestinga typical contribution from marine OM-rich sediments. These results Hydrous pyrolysis experiments were conducted on immature indicate that the distribution and relative abundance of regular petroleum source rocks containing different types of kerogen steranes are effectively to determine the sediments OM source in (I and II) at a temperature range of 250–550 1C to investigate the the mature stage. role of type in petroleum formation at high temperature (above

For all of the pyrolysis experiments, the trend of the C27αα20R 400 1C). For this purpose, the group composition, productivity of relative content with temperature was quite similar for both expelled oil and bitumen, and biomarker compositions (n-alkanes,

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