Gas evolution during kerogen pyrolysis of Estonian Kukersite shale in confined gold tube system

Organic Geochemistry 65 (2013) 74–82

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Gas evolution during kerogen pyrolysis of Estonian Kukersite shale in confined gold tube system Qingtao Wang a,b, Hong Lu a,⇑, Paul Greenwood c, Chenchen Shen a,b, Jinzhong Liu a, Ping’an Peng a a

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China University of Chinese Academic of Sciences, Beijing 100049, PR China c Centre for Exploration Targeting and WA Biogeochemistry Centre, University of Western Australia, Crawley, 6009 WA, Australia b

a r t i c l e

i n f o

Article history: Received 2 June 2013 Received in revised form 26 August 2013 Accepted 8 October 2013 Available online 17 October 2013

a b s t r a c t Pyrolysis of Kukersite kerogen was conducted in gold capsules, with the yield and stable carbon isotopic (d13C) values of selected gas components (C1, C2, C3, i-C4, n-C4, i-C5, n-C5, CO2) and liquid hydrocarbons (C6–C14) separately measured to investigate the primary versus secondary mechanisms of gas hydrocarbon generation from overmature source rocks. With increasing pyrolysis temperature over the range 336–600 °C (and especially > 430 °C) the progressive cracking of hydrocarbons led to increasing yields of low molecular weight gases, particularly CH4 and CO2. The increase determined for each of the C1–C5 hydrocarbons was in the order C5 > C4 > C3 > C2 > C1 below 408 °C, but showed the inverse order of C1 > C2 > C3 > C4 > C5 at > 420 °C. The yields (well reflected by traditional ln C2/C3 versus ln C1/C2 relationships) and stable isotopic profiles (e.g., d13C2–d13C3 versus ln C2/C3 plots) showed four distinct stages to the thermal evolution of the gas hydrocarbons: (1) During the first stage (final temperatures of 336– 360 °C and with heating rate of 2 °C/min) kerogen cracked mostly into C3þ , with just a small amount of C2 and minimal C1; (2) the second stage (360–408 °C) showed an increased production of lower molecular weight gases, particularly methane but also ethane and propane and the consistency of corresponding d13C2 and d13C3 values suggests they were produced in similar abundances; (3) the third stage (432–528 °C) was attributed to oil cracking as there were significant increases in the yields of both ethane and methane (cf. propane) and greater differences between d13C2 and d13C3; (4) a continued increase in methane during the fourth stage (552–600 °C) was attributed to cracking of C2, since no C3þ precursors survived to these pyrolysis temperatures. Methane (304 mg/g OC) was detected in much higher abundance than all other gases including CO2 at the final pyrolysis temperature of 600 °C, with initial kerogen cracking, secondary oil cracking and even the cracking of C2–C3 gases all contributing to its production. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Natural gas sources include biogenic gas, wet gas, gas condensate, dry gas, as well as shale gas which have recently attracted significant interest (North, 1985; Espitalié et al., 1988; Ungerer et al., 1988). They are mostly produced by thermal cracking of kerogen or oil substrates (Ungerer et al., 1988; Behar et al., 1992; Prinzhofer and Huc, 1995; Lorant et al., 1998; Tian et al., 2010), although exceptions include biogenic and earth mantle sources. The generation of gases from kerogen cracking as occurs in source rocks and oil cracking, which can occur in both source and reservoir rock, can overlap in geological

⇑ Corresponding author. Address: Room 307, Library Building, No. 511 Kehua Street, Wushan, Tianhe, Guangzhou, Guangdong, PR China. Tel.: +86 20 85290191; fax: +86 20 85290706. E-mail address: [email protected] (H. Lu). 0146-6380/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.orggeochem.2013.10.006

time and reservoir space (Tian et al., 2010). It is therefore important for exploration and economic considerations to distinguish the respective contribution of these two processes to the gases in hydrocarbon reserves. Most oil and gas fields in the Tarim Basin, one of China’s major gas and oil provinces, are thought to originate from mature to overmature Cambrian and Ordovician source rocks (Chen et al., 2000; Zhang et al., 2000, 2006; Wang et al., 2013). Much of the natural gas in the Tarim Basin (Fig. 1) has been attributed to secondary oil cracking (Qin et al., 2005). However, the exact origin of several large gas fields, including the Tazhong (Huang et al., 1999), Hetianhe (Qin et al., 2005; Wang et al., 2008) and YN2 gas fields (Zhao et al., 2005), continues to be debated. The Hetianhe gas field was thought to mainly derive from primary kerogen cracking (Qin et al., 2005) based on the classic d13C2–d13C3 versus ln C2/C3 plot which showed sharp increase in ln C2/C3 with small isotopic changes in d13C2–d13C3. By contrast, d13C1 values in the eastern

Q. Wang et al. / Organic Geochemistry 65 (2013) 74–82

part of Hetianhe were 1–3‰ lighter than d13C1 of kerogen cracked gas in the Tabei area. The more 13C depleted methane of Hetianhe was hence attributed to oil cracking (Wang et al., 2008). In the YN2 gas field, d13C1 values of 38.6‰ to 36.2‰ and d13C2 values of 30.9‰ to 34.7‰ as well as high concentrations of diamondoid hydrocarbons were all indicative of secondary oil cracking (Zhao et al., 2005). However, a mixed origin (55% kerogen cracking gas and 45% oil cracking gas) was interpreted for this area according to laboratory thermal simulation experiments of gas generation from source rock and oil precursors, which allowed kinetics of gas generation to be calculated and which also showed d13C1 values of 37.5‰ to 36.2‰ (Tian et al., 2007). Pyrolysis experiments on kerogen or oils can effectively simulate the evolution of hydrocarbon gases and allows the abundance and stable carbon isotopic composition of the gas products to be monitored (Behar et al., 1997; Seewald et al., 1998; Hill et al., 2003). Gloeocapsomorpha prisca, typically via Kukersite shales, has made a huge contribution to Ordovician source rocks all over the world (Stasiuk et al., 1993; Fowler et al., 2004), including in the Tarim Basin (Pu et al., 1998). Thus, immature and readily oil prone Kukersite kerogen has been used in several laboratory thermal treatment studies concerned with respective significance of kerogen versus oil. Closed pyrolysis systems (Horsfield et al., 1992; Berner et al., 1995; Behar et al., 1997) can generally support the identification of important intermediate products and reaction pathways (Burnham et al., 1987; Behar et al., 1992, 1997). In the present study, Kukersite kerogen was pyrolysed using the closed gold tube procedure (Pan et al., 2007) over a series of pyrolysis temperatures and the respective yields of inorganic (CO2) and organic hydrocarbon gases (C1, C2, C3, i-C4, n-C4, i-C5, n-C5) as well as higher molecular weight (MW) hydrocarbons (C6–C14) were measured. There is ongoing uncertainty about the sources of the economically significant Kukersite gases in the Tarim Basin, and the abundances and d13C values of these gases may help to clarify the origins of the gas and also the mechanism of hydrocarbon generation from overmature source rocks. Plots of ln C2/C3 versus ln C1/C2 plot and d13C2–d13C3 versus ln C2/C3 plot, used in previous studies of natural gas production (Prinzhofer and Huc, 1995), were applied to investigate the evolutional behaviours of gas hydrocarbons, distinguish the features of kerogen and oil cracking gases (Prinzhofer and Huc, 1995; Zhang et al., 2000) and try to provide

75

clues for the origin of the overmature natural gas in the Tarim Basin. 2. Experimental 2.1. Sample The Ordovician Kukersite shale sample was collected from an outcrop of northern Estonia. The shale was ground and Soxhlet extracted for 72 h with a dichloromethane:methanol solvent mix (95:5 v:v; 400 ml). The kerogen residue from the extraction process was acid digested in concentrated HCl for 24 h and then in 48% HF several times for 72 h to remove minerals. The acids were subsequently removed by exhaustive washing with distilled water. The elemental composition (wt%, C–77.0; H–9.68; O–10.6; S–1.7; Cl–0.72; N–0.36) and thermal maturity (VRo = 0.48%) of Kukersite kerogen (d13C = 31.5‰) has been previously reported (Lille et al., 2003; Mastalerz et al., 2003). 2.2. Gold tube reactor All the pyrolysis experiments were conducted in sealed gold tubes (24 mm  4.2 mm  2.2 mm i.d.) under anhydrous conditions. Prior to sample treatment, empty gold tubes were heated to 800 °C in a muffle furnace for 1 h to remove any residual organic material. One end of the tube was welded closed, the tube flushed with argon to remove air and the kerogen sample (20–80 mg) was then loaded. The open end was crimped and welded under a flow of argon while the other end was submerged in water at ambient temperature to help trap volatiles created during the welding process. The tubes were accurately weighed before and after pyrolysis to confirm their structural integrity (Pan et al., 2007; Lu et al., 2011). 2.3. Isothermal program The samples in the closed tubes were heated in a furnace equipped with 12 separate autoclaves maintained at a constant pressure of 50 MPa to prevent explosion during the pyrolysis process. A detailed description of this procedure can be found elsewhere (Pan et al., 2007; Lu et al., 2011). Two series of

Fig. 1. Map of the Tarim basin including major structural elements (after Liu et al., 2009). Well locations are shown as pentagrams. Major cities are shown as squares.

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Q. Wang et al. / Organic Geochemistry 65 (2013) 74–82

experiments were conducted with heating rates of 20 °C/h and 2 °C/h. Each series comprised 12 sample tubes separately heated from an initial 250 °C to a final temperature (336 °C, 360 °C, 384 °C, 408 °C, 432 °C, 456 °C, 480 °C, 504 °C, 528 °C, 552 °C, 576 °C or 600 °C) without a holding time. 2.4. GC analysis of gas products Following the off-line pyrolysis an auxiliary instrument was used to release the gas from the gold tube and introduce it to the gas chromatograph (Pan et al., 2007). GC analyses was conducted with a two-channel Hewlett–Packard 6890 GC custom configured (Wassen ECE) with eight (two capillary and six packed) columns and one FID and two TCD providing simultaneous detection of several organic (6 C5) and inorganic (CO2) gases (Lu et al., 2011). Helium (for inorganic gas analysis) or nitrogen (organic) were used as carrier gases and the GC oven was programmed from 70 °C (held for 5 min) to 130 °C at 15 °C/min, then to a final 180 °C at 25 °C/min (held for 4 min). 2.5. GC quantification of C6–14 hydrocarbon products The procedure used to analyze and quantify the higher MW (C6–C14) hydrocarbon products has been reported elsewhere (Pan et al., 2007). Briefly, the tubes were cooled in liquid nitrogen for 5 min, then cut swiftly into several pieces under pentane (3 ml in a vial which was subsequently capped) and subjected to 5  5 min ultrasonic treatments and left to stand for 72 h. Deuterated n-C24 was added as an internal standard and the pentane extract was analyzed using an Agilent 6890 GC fitted with a 50 m  0.32 mm i.d. column coated with a 0.40 lm film of CPSil5CB, employing nitrogen as carrier gas. The oven temperature was programmed from 40 °C (held for 5 min) to 290 °C (held for 45 min) at 4 °C/min. 2.6. GC-irMS analysis of stable carbon isotopes Stable carbon isotope analysis was conducted on an aliquot of the gas released from the pierced tubes which had been captured in a valve-sealed syringe and injected on a HP 5890 GC interfaced to a VG Isochrom II mass spectrometer. The GC was fitted with a Poraplot Q column (30 m  0.32 mm i.d.) and helium was used as the carrier gas. The oven was programmed from an initial 50 °C (4 min) to 190 °C (held 5 min) at 20 °C/min. The stable carbon isotope value of each sample was measured three times within a calculated precision of ± 0.3‰. 3. Results and discussion The concentration profiles with pyrolysis temperatures of the total C1–C5, C6–C14 as well as the individual gases are shown in Fig. 2, with all concentration values also listed in Table 1. The evolution of all gases were similar for both heating rates, but profiles of the 2 °C/h heating rate consistently spanned slightly lower temperatures than the 20 °C/h heating rate. The slightly higher temperature profile with the faster heating rate can be attributed to a slight lag in the thermal response of the sample. The total C1–5 products (Fig. 2a) and also yield of methane (Fig. 2c) continued to increase with pyrolysis temperature over the entire applied temperature range. The yields of C2, C3, C4 and C5 hydrocarbons also showed an initial increase with pyrolysis temperature, but after peaking at a critical pyrolysis temperature their yields started to decrease at higher temperatures. This decrease in concentration can be attributed to a higher rate of loss from direct cracking than rate of production from cracking of higher MW precursors.

3.1. Liquid (C6–14) and gas (C1–5) evolution The pyrolysis generation of gaseous and higher MW hydrocarbons needs to be collectively considered when investigating formation mechanisms. The yields of liquid hydrocarbon (Fig. 2b) peaked at 408 °C for 2 °C/h and 432 °C for 20 °C/h heating rates, respectively, whereas methane increased over the entire range of pyrolysis temperatures. The high yields of the liquid hydrocarbons (C6–14) reflect the oil proneness of the Kukersite ‘‘oil shale’’. These are thought to be lower MW products from the depolymerization of primary structures of the kerogen (Behar et al., 1992). The reduced yield of liquid (C6–14) hydrocarbons at high pyrolysis temperature also showed their subsequent susceptibility to crack into gaseous range hydrocarbons (Tissot and Welte, 1984). It has also been shown that such secondary gas products can participate in condensation reactions to produce polyaromatic hydrocarbons and coke (Hill et al., 2003). The gas hydrocarbons could potentially arise from all higher MW analogues (e.g., methane from P C2) including primary and secondary cracked products of the kerogen (Price and Schoell, 1995). The total yield of C1–5 hydrocarbon at 408 °C (2 °C/ h, peak temperature of liquid hydrocarbon production), had only reached 37% of the maximum level which was obtained at the highest pyrolysis temperature (600 °C). From 408–456 °C, the C1–5 hydrocarbons (dominated by methane generation) increased in yield to 86% of their maximum level, while the yields of C6–14 hydrocarbons decreased by 43% (for 2 °C/h). This behaviour is consistent with the production pathway of oil cracking into gas hydrocarbons (Behar et al., 1992). 3.2. Yields of gases The C2–5 hydrocarbons generated from source rocks have been traditionally considered as a whole (Campbell et al., 1980; Horsfield et al., 1992; Behar et al., 1997), but here we separately considered the evolution of C1, C2, C3, C4 and C5 hydrocarbons. Methane yields from a previous open pyrolysis experiment were reported to decline above a critical temperature (Huss and Burnham, 1982). However, the yield of methane in the present closed pyrolysis experiments increased continuously with pyrolysis temperatures. Pyrolysis experiments can be conducted in either open or closed systems. Closed pyrolysis and the continual increase in methane yield would be more indicative of natural systems than the open pyrolysis, where methane yield increases first and then decreases. The final methane yield of 304 mg/g OC (at 600 °C with 2 °C/h heating rate) was significantly larger than the yield of all other gas products (Fig. 1d–h), including ethane (91 mg/g OC at 504 °C and 2 °C/h) and CO2 (117 mg/g OC at 600 °C and 2 °C/h). The initial increase in CH4 at low temperatures (e.g. < 410–430 °C) was relatively modest. Methane yield increased more rapidly above 450 °C. Previous closed system pyrolysis studies (Huss and Burnham, 1982; Behar et al., 1992) have similarly shown the main accumulation of methane occurs after 480 °C. The increase in methane in the present experiments above 450 °C was coincident with a sharp decline in the yields monitored for all PC2 hydrocarbons. This indicates methane is collectively sourced from the secondary cracking of C6þ , C5, C4, C3 and (less so) C2 hydrocarbons. The thermal evolution of the C2 and C3 alkanes showed a similar relationship, but with the slightly different critical pyrolysis temperatures of 504–528 °C (C2) and 456–504 °C (C3). The critical pyrolysis temperature was again also notably lower for the slow heating rate. The critical temperatures (CTs) at which C4 and C5 hydrocarbons started to decline in yield were in the range of 432–504 °C and was again lower for the slower heating rate and higher MW (i.e. CTC5 < CTC4). The CT observed for all hydrocarbon gases

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(a)

200

100

336

384

432

480

528

576

100 20 o C/h 2 o C/h

384

100

432

480

528

576

624

(d) 20 o C/h 2 o C/h

80

200

60 40 20 0

0 336

384

432

480

528

576

336

624

(e) i-C4, n-C4 mg/g TOC

20 o C/h 2 o C/h

60

C3 mg/g TOC

40

336

C2 mg/g TOC

C1 mg/g TOC

300

40

20

30

384

432

480

528

(f)

576

624

20 o C/h for nC 2 o C/h for nC 20 o C/h for iC 2 o C/h for iC

20

10

0

0 336

384

432

480

528

576

336

624 120

(g)

20 oC/h for nC 2 oC/h for nC 20 C/h for iC 2 o C/h for iC

8

4

384

432

480

528

576

624

432

480

528

576

624

(h) 20 oC/h 2 oC/h

o

CO2 mg/g TOC

i-C5, n-C5 mg/g TOC

80

624

(c)

12

120

0

0

80

20 o C/h 2 o C/h

160

C6~14 mg/g TOC

C1-5 mg/g TOC

(b) 20 o C/h 2 o C/h

300

80

40

0

0 336

384

432

480

temperature

528

576

624

(oC)

336

384

temperature (oC)

Fig. 2. Yields of total gaseous (C1–5), total liquid hydrocarbon (C6–14), and individual hydrocarbon (C1, C2, C3, C4, C5) and CO2 gases in the simulation experiments.

indicated activation energies in the order: E(i-C5, n-C5) < E(i-C4, n-C4) < E(C3) < E(C2) (Tang et al., 2000), reflecting the preferential production of higher MW gas products from the thermal cracking of kerogen and oil. The ratios of i-C4/n-C4 and i-C5/n-C5 were both < 0.4. Previous studies have suggested that n-alkanes are formed by free radical reactions of n-alkane moieties in kerogen, whereas the branched iso-alkanes can originate from either (1) free radical cracking of branched hydrocarbon structures within kerogen or bitumen; or (2) carbonium ion reaction of a-olefins with protons (Kissin, 1987). The structure of Kukersite kerogen has been investigated with a range of analytical techniques including solid state 13C NMR spectra (Lille et al., 2003), RuO4 chemical degradation, FTIR and flash pyrolysis–GC–MS (Blokker et al., 2001; Lille, 2004). These studies have consistently identified a higher proportion of long aliphatic chains than side chains in the structure of Kukersite

kerogen, which explain the very small ratios (< 0.4) of i-C4/n-C4 and i-C5/n-C5 in the present data. CO2 is a common product of the thermal treatment of organic matter and large amounts of CO2 typically accompany the generation of hydrocarbons on the cracking of kerogen, probably due to decarboxylation of organic acids and esters (Huss and Burnham, 1982). Carbonate minerals were removed from the Kukersite sample by HCl treatment, so the CO2 measured can all be attributed to an organic origin. The CO2 yield increased constantly with pyrolysis temperature, consistent with the results of previous studies (Burnham et al., 1987), but the final concentrations of CO2 (e.g., 117 mg/g OC at 600 °C and 2 °C/h) were much lower than methane (304 mg/g OC at 600 °C and 2 °C/h). According to Monthioux et al. (1985), Monthioux (1988) and Seewald et al. (1998), the oxygen in CO2 produced at temperatures below 375 °C is directly sourced from the kerogen (reflected by a reduction in its O/C ratio),

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Table 1 Hydrocarbons yields and their stable carbon isotopes from the gold tube pyrolysis of Kukersite with two heating rates under a constant pressure of 50 MPa. Heating T C1 rate (°C/h) (°C) (mg/g OC)

C2 (mg/g OC)

C3 (mg/g OC)

i-C4 (mg/g OC)

n-C4 (mg/g OC)

i-C5 (mg/g OC)

n-C5 (mg/g OC)

C6–14 (mg/g OC)

H2 (mg/g OC)

20 20 20 20 20 20 20 20 20 20 20 20

336 360 384 408 432 456 480 504 528 552 576 600

0.15 0.63 1.88 5.23 16.18 33.03 54.40 92.79 139.96 197.88 245.97 269.81

0.19 0.68 3.22 8.62 22.27 38.54 60.36 86.58 94.78 83.84 62.62 34.94

0.19 0.63 3.82 10.53 27.17 47.20 69.99 74.14 45.26 12.36 1.69 0.54

0.04 0.06 0.33 0.86 2.37 4.93 8.51 9.62 4.76 0.55 0.03 0.01

0.07 3.32 10.96 18.69 30.50 26.07 5.02 0.61 0.11 0.03 0.01 0.00

0.01 0.02 0.27 0.85 2.20 3.67 3.85 0.72 0.20 0.04 0.00 0.00

0.01 0.05 1.13 3.90 8.94 11.31 8.21 0.51 0.15 0.05 0.01 0.00

7.00 14.65 34.07 71.20 110.41 103.75 73.80 60.12 44.33 29.89 21.56 12.68

0.01 0.01 0.04 0.08 0.18 0.30 0.40 0.58 0.78 0.89 1.23 1.65

2 2 2 2 2 2 2 2 2 2 2 2

336 360 384 408 432 456 480 504 528 552 576 600

0.28 2.82 9.65 22.70 44.73 75.57 118.00 173.00 231.29 266.49 295.54 304.29

0.36 5.19 14.60 26.44 47.85 73.30 90.33 91.23 63.01 34.84 13.73 5.02

0.34 6.10 16.90 30.87 58.60 76.45 65.34 27.71 2.30 0.58 0.20 0.05

0.02 0.51 1.33 2.73 7.39 11.08 8.95 2.20 0.05 0.01 0.00 0.00

0.07 3.32 10.96 18.69 30.50 26.07 5.02 0.61 0.11 0.03 0.01 0.00

0.01 0.41 1.37 2.55 4.52 2.57 0.30 0.09 0.00 0.00 0.00 0.00

0.02 1.57 6.27 9.30 10.05 3.47 0.20 0.08 0.01 0.00 0.00 0.00

22.58 45.52 77.44 120.57 96.17 69.00 50.24 26.80 15.77 8.70 4.47 2.18

0.00 0.03 0.09 0.17 0.25 0.38 0.55 0.69 0.91 1.08 1.29 2.15

-32

(a)

0

20 o C/h 2 o C/h

-40 -44

-52

d13C3 (‰)

d13CO2 d13C2– d13C3 (‰) (‰)

7.29 11.95 20.84 31.61 45.03 52.89 55.32 62.27 68.12 76.69 88.40 93.62

0.22 0.78 3.43 9.55 25.80 29.79 26.14 23.35 21.15 19.90 15.80 20.15

0.40 0.55 0.09 0.13 0.31 0.47 0.53 0.70 1.02 1.47 2.00 2.67

0.37 0.46 0.21 0.18 0.18 0.18 0.24 0.54 1.12 2.30 4.00 4.55

n.d. 35.1 39.89 41.68 50.66 48.48 46.96 44.82 43.14 40.91 39.45 37.55

34.82 34.73 34.28 31.97 30.83 29.47 28.61 28.35 28.33 27.88 29.59 29.26

n.d. 38.25 37.94 36.96 36.48 36.52 36.22 33.09 29.35 23.27 16.79 11.55

n.d. n.d. 35.68 2.57 35.76 2.18 35.42 1.55 34.51 1.97 33.39 3.13 32.49 3.74 26.83 6.26 18.47 10.88 n.d. n.d. n.d. n.d. n.d. n.d.

4.30 25.82 37.62 45.74 49.56 55.79 64.91 76.30 88.06 98.44 108.40 117.24

0.05 4.46 10.87 20.11 26.51 23.39 23.14 21.43 23.25 23.20 22.66 24.64

0.36 0.02 0.22 0.48 0.56 0.66 0.90 1.27 1.91 2.66 3.70 4.73

0.46 0.22 0.24 0.23 0.18 0.34 0.71 1.58 3.70 4.48 4.63 5.09

38.01 49.53 51.15 49.38 46.67 45.03 43.62 41.2 39.05 37.57 36.36 34.68

31.51 31.74 31.5 30.82 29.88 28.59 28.31 27.56 27.61 27.64 28.14 29.36

35.45 38.76 37.29 36.48 36.21 35.2 31.63 26.39 17.96 11.72 8.25 n.d.

31.82 3.64 35.92 2.85 35.1 2.19 34.61 1.87 34.03 2.18 29.91 5.29 23.55 8.08 10.32 16.07 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

(b) 20 o C/h 2 o C/h

-20

-40 336

384

432

480

528

576

336

624

-24

(c)

20 o C/h 2 o C/h

-20

-30

384

432

480

528

576

624

432

480

528

576

624

(d) 20 o C/h 2 o C/h

-28

δ13C-CO2 (‰)

δ13C3 (‰)

d13C2 (‰)

-30

-48

-10

ln C1/ ln C2/ d13C1 C2 C3 (‰)

-10

δ13C2 (‰)

δ13C1 (‰)

-36

H2S (mg/g OC)

CO2 (mg/g OC)

-32

-36

-40 336

384

432

480

528

576

624

temperature (oC)

336

384

temperature (oC)

Fig. 3. Stable carbon isotope values for methane, ethane, propane and CO2 with increasing thermal treatment at two heating rates of 20 °C/h and 2 °C/h.

whereas at higher temperatures (the O/C ratio of the kerogen remains relatively constant) more oxygen may be sourced from H2O. The oxygen released from Kukersite kerogen (13.9% oxygen content) probably explained the production of CO2 at low temperatures. At higher temperatures continued increase of CO2 was probably due to supply of bound water in clay minerals, since no water was added to the present pyrolysis experiments.

3.3. Stable carbon isotopes for the gases The stable carbon isotope value (31.1‰) of the Kukersite kerogen sample studied here concurs with previously documented values for Estonian Kukersite (e.g., d13C = 31.5‰; Mastalerz et al., 2003). The d13C profiles of all components (C1–C3 and CO2) over the 20 °C/h and 2 °C/h pyrolysis series are illustrated in

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Q. Wang et al. / Organic Geochemistry 65 (2013) 74–82 Table 2 Increase ratio values for C1–C5 hydrocarbons over the pyrolysis temperature sequence. Increase ratio

Heating rate = 20 °C/h

Y(360 °C)/Y(336 °C) Y(384 °C)/Y(360 °C) Y(408 °C)/Y(384 °C) Y(432 °C)/Y(408 °C) Y(456 °C)/Y(432 °C) Y(480 °C)/ (456 °C) Y(504 °C)/Y(480 °C) Y(528 °C)/Y(504 °C) Y(552 °C)/Y(528 °C) Y (576 °C)/Y(552 °C) Y (600 °C)/Y(576 °C)

Heating rate = 2 °C/h

C1

C2

C3

C4

C5

C1

C2

C3

C4

C5

4.2 3.0 2.8 3.1 2.0 1.6 1.7 1.5 1.4 1.3 1.1

3.6 4.7 2.7 2.6 1.7 1.6 1.4 1.1 0.9 0.7 0.6

3.3 6.1 2.8 2.6 1.7 1.5 1.1 0.6 0.3 0.1 0.3

2.9 10.8 3.2 2.5 1.6 1.3 0.5 0.3 0.2 0.1 0.2

3.6 20.7 3.4 2.3 1.3 0.8 0.1 0.3 0.2 0.1 0.0

10.2 3.4 2.4 2.0 1.7 1.6 1.5 1.3 1.2 1.1 1.0

14.3 2.8 1.8 1.8 1.5 1.2 1.0 0.7 0.6 0.4 0.4

18.2 2.8 1.8 1.9 1.3 0.9 0.4 0.1 0.3 0.3 0.2

42.8 3.2 1.7 1.8 1.0 0.4 0.2 0.1 0.2 0.1 0.0

93.3 3.9 1.6 1.2 0.4 0.1 0.3 0.1 0.0 0.0 0.0

Fig. 3. Generally, the stable carbon isotope values of the gases become enriched with increasing pyrolysis temperature, with a slightly lower temperature offset to this trend for the 2 °C/h data series (cf. 20 °C/h). The 2 °C/h data series is henceforth referred to in describing the stable carbon isotopic fractionations evident with pyrolysis. The d13C1 values generated from Kukersite become enriched in 12C from 336–360 °C at which point it showed the lightest d13C1 value of 51.2‰, and beyond 360 °C became enriched in 13C. A decreasing trend of d13C1 values as initially observed here at low temperatures has also been reported in many other pyrolysis experiments and generally attributed to the mixing of gas precursors (Berner et al., 1995; Lorant et al., 1998; Tang et al., 2000; Hill et al., 2003; Tian et al., 2007, 2010). Likewise, similar increases in d13C1 values to those evident with increasing pyrolysis temperatures 100

(a) 2 o C/h

3

80

increase ratio

above 360 °C have been reported (Berner et al., 1995) and attributed to a general reduction in the isotopic fractionation between methane and the kerogen (or oil) precursor at high temperatures. The two distinct trends reflected by d13C1 profiles suggest the initial structural precursors of the methane are isotopically more depleted than the more thermally stable aromatic structures of the kerogen (Galimov, 2006). At all pyrolysis temperatures the d13C1 values were lighter than the initial stable carbon isotope value of the Kukersite kerogen (31.1‰). The d13C values of ethane and propane remain relatively constant at low pyrolysis temperatures, but both start to increase above 456 °C. Notably, propane showed a more rapid 13C enrichment than ethane with increasing pyrolysis temperature (Fig. 3b and c). These d13C trends reflect preferential cracking of 12C–12C bonds, such that the non-cracked residue of the C2 and C3 gases

C1 C2 C3 C4 C5

2

60 1

40 0 384

20

432

480

528

576

624

0 336

384

432

480

528

576

624

temperature (oC) 25

(b) 20 oC/h

5

C1 C2 C3 C4 C5

4

20

Increase ratio

3

15

2 1

10

0 384

5

432

480

528

576

624

0 336

384

432

480

528

576

temperature (oC) Fig. 4. The increase ratios for C1–C5 hydrocarbons at two heating rates of 2 °C/h and 20 °C/h.

624

80

Q. Wang et al. / Organic Geochemistry 65 (2013) 74–82

5

(a) 2 o C/h

1.6

600oC

(b) 2 oC/h

504oC

552oC

4

1.2

lnC2/C3

3 0.8

(3)

480oC

2

336oC

504oC

enlarged

1

456oC

0.4

384oC

408oC

336oC 360oC 360oC

0

0

432 oC

Stage (2)

0

432oC

1

2

3

4

0

5

0.4

0.8

5

(d) 20 o C/h

600oC

1.2

1.6

528oC

576oC

0.8

3 552oC

2

0.6

(3)

504oC 336oC 360oC

0.4

480oC

528oC

1

enlarged

0.2

432oC

384oC

456oC

408oC

Stage (2)

0

0

456 oC

0

1

2

3

4

5

0

0.4

0.8

lnC1/C2

0

-4

408oC

lnC1/C2

0

(e) 2 o C/h

384oC

b)

360oC

434oC

336oC

434oC

δ13C2-δ13C3 (‰)

δ13C2-δ13C3 (‰)

1.6

1

4

lnC2/C3

1.2

(c) 20 o C/h

1.2

lnC1/C2

lnC1/C2

456oC

-8 -12

456oC

-4

(f) 20 o C/h

408oC 384oC 360oC 480oC

504oC

-8

480oC

-16

528oC

-20

-12 0

0.4

0.8

1.2

1.6

lnC2/C3

0

0.4

0.8

1.2

lnC2/C3

Fig. 5. Plots of ln C2/C3 versus ln C1/C2 and d13C2–d13C3 versus ln C2/C3 for gases from Kukersite kerogen pyrolysis. The arrows and the numbers refer to the four stages defined according to the increasing/decreasing trend of C2/C3 and C1/C2 ratios.

is isotopically heavy. In fact, the d13C values of ethane and propane become heavier than that of the parent Kukersite kerogen (> 31.1‰) above 480 °C. Relatively heavy d13C values of ethane and propane have generally been related to their cracking at high temperatures (Lorant et al., 1998; Tian et al., 2007). The d13C values for CO2 showed a moderate but consistent increase with temperature apart from a slight decrease above 576 °C (Fig. 3d). The general increase was less significant than the d13C observed for the hydrocarbon gases, and the d13CCO2 profiles obtained with the different heating rates show a convergence at higher pyrolysis temperatures.

3.4. Increase ratios for C1–C5 hydrocarbons The hydrocarbon yields from the thermal cracking of kerogen are controlled by a number of factors including the relative thermal

stabilities of different alkanes and the mechanism of cracking. Here we define a new parameter called the increase ratio to describe the variance in yield (increase > 1 > decrease) of a product measured between two pyrolysis temperatures as described by Eq. (1).

Increase ratioðiÞ ¼ YðT iþ1 Þ=YðT i Þ

ð1Þ

where Ti stands for the sequence number of the sampling temperature; Y(Ti+1)/Y(Ti) stands for the ratio of yields of the component at the next sampling temperature relative to current temperature. The increase ratio values for the C1–C5 hydrocarbons were separately calculated for each pyrolysis temperature and are listed in Table 2. The increase ratios of the C1–C5 hydrocarbons reflected a general increase with MW (C5 > C4 > C3 > C2 > C1) at lower pyrolysis temperatures (< 384–408 °C; Fig. 4a and b), but then a general decline with MW at higher pyrolysis temperatures (C1 > C2 > C3 > C4 > C5). This is consistent with the main cracking products of the

81

Q. Wang et al. / Organic Geochemistry 65 (2013) 74–82 Table 3 Different performances of the four profiles in many indicators. Indicator

Profile 1

Profile 2

Profile 3

Profile 4

ln C1/C2 ln C2/C3 d13C2–d13C3 C1/C1–5 Increase ratio

Decrease Decrease Increase Decrease C3 > C2 > C1

Increase Constant Increase Increase C1 > C2C3

Increase Increase sharply Decrease Increase C1 > C2 > C3

Increase Increase slightly

Source from Temperature range (2 °C/h) Temperature range (20 °C/h)

Kerogen cracking (only) – –

360 384

Kerogen cracking (only) – –

Kukersite kerogen being higher MW gaseous hydrocarbons (e.g., n-C5, n-C4) at low pyrolysis temperatures (< 408 °C), and low MW gases (e.g., C1, C2, C3) at higher pyrolysis temperatures. 3.5. Gas evolution during pyrolysis 3.5.1. ln C2/C3 versus ln C1/C2 (Fig. 5a–d) A comparison of the C1–C3 composition of natural gases at different thermal maturities can help distinguish whether methane is directly sourced from kerogen or alternatively is a secondary product of oil cracking (Prinzhofer and Huc, 1995). The C1/C2 ratio is particularly sensitive to thermal maturity, since methane production generally increases more rapidly than ethane production with higher maturity. Furthermore, kerogen generally cracks into similar proportions of ethane and propane leading to a relatively constant C2/C3 ratio. By contrast, the C2/C3 ratio of gases from secondary oil cracking increase with maturity due to enhanced C2 production. Plots of ln C2/C3 versus ln C1/C2 for Kukersite kerogen obtained with the two heating regimes are shown in Fig. 5. Both plots show the same four distinct stages (Fig. 5), albeit with very slight differences in actual temperature ranges. The four stages are as follows. (1) An initial decrease in both parameters is evident at pyrolysis temperatures below 360 °C, which can be attributed to preferential production of C3 > C2 > C1. This is consistent with the initial cracking of kerogen into hydrocarbon gases of greater MW than methane (Section 3.4). (2) At 360–408 °C, ln C1/C2 increases while ln C2/C3 remains relatively constant (0.2). This indicates increased production of methane compared to ethane, typical of kerogen cracking, but little net change between ethane and propane production. (3) During the third stage (432–528 °C), both parameters show a sharp increase. The ln C2/C3 values increase from 0.2 to 4.5 and the ln C1/C2 values from 0.6 to 2.0. These trends indicate increasing production of both ethane and methane within this maturity range, which is typical of secondary oil cracking (Prinzhofer and Huc, 1995), although the possibility that kerogen cracking also contributes to some of the methane cannot be excluded. The critical point of maximum C3þ hydrocarbon production occurs near the middle point of this temperature range, whereas the yields of methane and ethane continue to increase (Fig. 2c–e). (4) From 552–600 °C both gas indices show a moderate increase and a reduction in C2 and C3 yields relative to C1 and C2, respectively, reflects the continued cracking of C3 and to a lesser extent for C2 at these very high pyrolysis temperatures (Fig. 2d and e). 3.5.2. d13C2–d13C3 versus ln C2/C3 (Fig. 5e and f) A d13C2–d13C3 versus ln C2/C3 plot can also help distinguish gases from kerogen or oil cracking (Prinzhofer and Huc, 1995; Lorant et al., 1998; Hill et al., 2003; Tian et al., 2010). Kerogen cracking usually leads to greater d13C differences between ethane and propane, although d13C values of these C2 and C3 hydrocarbons can also be influenced by system openness. A previous study showed C2–3 gases generated in a closed system had diverging

408 456

Oil cracking (dominate) – –

552 576

Increase C1 > C2 C3þ almost vanished C2 cracking; kerogen cracking – –

d13C values with increasing maturity, while in an open system they converged (Lorant et al., 1998; Hill et al., 2003). Oil cracking is typically characterized by significant differences in the concentrations of ethane and propane (Prinzhofer and Huc, 1995). Our data (Fig. 5e and f) were typical of the fractionation associated with thermogenic gases (Prinzhofer and Huc, 1995). Oil cracking does not occur at low temperatures, but above 408 °C our data show a decrease in d13C2–d13C3 values with increasing ln C2/C3 (Fig. 5e and f) was similar to previous reports from oil cracking experiments (Hill et al., 2003; Tian et al., 2010). The thermally initiated evolution of C1–5 gaseous hydrocarbons from Kukersite kerogen (Figs. 4 and 5) thus can be described by several different pathways separately expressed by the following reactions:

Kerogen ! C3þ > C2 > C1

ð2Þ

Kerogen ! C1 > C2 > C3þ

ð3Þ

Kerogen and bitumen ðdominantÞ ! C1 > C2 > C3þ

ð4Þ

Bitumen ðC6þ Þ ! C25 þ methane þ precoke ðthe intermediateÞ

ð5Þ

C3 ! C1 þ C2

ð6Þ

C2 ! C1

ð7Þ

A similar multi-reaction scheme for the production of gas hydrocarbons from pyrolysis of kerogen, oil and lignite in a closed system was proposed by Behar et al. (1992, 2008). Our gas yield and carbon isotopic data does distinguish stages 1 and 2 (Figs. 4 and 5) which are mainly due to kerogen cracking (Eqs. (4) and (5)), from stage 3 which involves a combination of both kerogen and oil cracking (Eqs. (6) and (7)) of significant magnitude and a stage 4 mostly involved in secondary gas cracking (e.g., C2–3; Eqs. (6) and (7)). These four stages of thermal cracking were reflected by established molecular (e.g., ln C1/C2, ln C2/C3, increase ratio) and stable isotopic (e.g., d13C2–d13C3) indicators as summarized in Table 3. 4. Conclusions Gold tube pyrolysis was conducted with Kukersite kerogen to study the reaction pathways of gas range hydrocarbons thermally generated from overmature organic sources. Monitoring the yields and d13C values of inorganic (CO2) and organic (C1, C2, C3, i-C4, n-C4, i-C5, n-C5) gas products as well as liquid hydrocarbon (C6–C14) products helped illuminate the competing mechanisms of kerogen and secondary oil cracking. An increase ratio was used to provide a measure of the degree to which each of the C1–C5 hydrocarbons increased with maturity. The increase ratios and isotopic values distinguished four separate stages of gas product generation and generally reflected an order of C5 > C4 > C3 > C2 > C1 at low maturities (pyrolysis temperature

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Q. Wang et al. / Organic Geochemistry 65 (2013) 74–82

6 408 °C) where the cracking of higher MW products was favored and the inverse order of C1 > C2 > C3 > C4 > C5 at higher maturities (P 420 °C) due to increased production of methane and ethane which is typical of oil cracking. Consistent with the different yields obtained, the d13C values of ethane and propane (typical plots of d13C2–d13C3 versus ln C2/C3) showed trends consistent with Kukersite kerogen cracking at lower thermal maturities and subsequent secondary oil cracking at higher maturities. The Kukersite production of high concentration of C6–14 hydrocarbons is typical of ‘‘oil shale proneness’’ for hydrocarbon generation. The very low proportions of iso-alkanes compared to nalkanes (e.g., i-C4/n-C4 and i-C5/n-C5 < 0.4) reflect the scarcity of chain structures within the Kukersite kerogen. The yields of CO2 increased continually with pyrolysis temperatures, but were of significantly lower concentration than methane possibly due to limited supply of oxygen (also reflecting a lack of free water). Acknowledgements The authors are indebted to two anonymous reviewers for their insightful comments and suggestions, which significantly improved on an earlier version of our manuscript. Especially we thank Associate Editor Andrew Murray for his patient editorial work and Editor-in-Chief L.R. Snowdon for constructive advice and prompt editorial processing of our manuscript. Financial support for this work was gratefully received from State 973 Project (2012CB214706), Major National Science and Technology Projects (2011ZX05008-002-33) and NSFC Project (40873048, 41173053). This is contribution No. IS-1762 from GIGCAS. Associate Editor—Andrew Murray References Behar, F., Kressmann, S., Rudkiewicz, J.L., Vandenbroucke, M., 1992. Experimental simulation in a confined system and kinetic modelling of kerogen and oil cracking. Organic Geochemistry 19, 173–189. Behar, F., Vandenbroucke, M., Tang, Y., 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. Organic Geochemistry 26, 321–339. Behar, F., Lorant, F., Lewan, M., 2008. Role of NSO compounds during primary cracking of a Type II kerogen and a Type III lignite. Organic Geochemistry 39, 1– 22. Berner, U., Faber, E., Scheeder, G., Panten, D., 1995. Primary cracking of algal and landplant kerogens: kinetic models of isotope variations in methane, ethane and propane. Chemical Geology 126, 233–245. Blokker, P., van Bergen, P., Pancost, R., Collinson, M.E., de Leeuw, J.W., Sinninghe Damsté, J.S., 2001. The chemical structure of Gloeocapsomorpha prisca microfossils: implications for their origin. Geochimica et Cosmochimica Acta 65, 885–900. Burnham, A.K., Braun, R.L., Gregg, H.R., Samoun, A.M., 1987. Comparison of methods for measuring kerogen pyrolysis rates and fitting kinetic parameters. Energy & Fuels 1, 452–458. Campbell, J.H., Gallegos, G., Gregg, M., 1980. Gas evolution during oil shale pyrolysis. 2. Kinetic and stoichiometric analysis.. Fuel 59, 727–732. Chen, J., Xu, Y., Huang, D., 2000. Geochemical characteristics and origin of natural gas in Tarim Basin, China. American Association of Petroleum Geologists Bulletin 84, 591–606. Espitalié, J., Ungerer, P., Irwin, I., Marquis, F., 1988. Primary cracking of kerogens. Experimenting and modelling C1, C2–C5, C6–C15 and Cþ 15 classes of hydrocarbons formed. Organic Geochemistry 13, 893–899. Fowler, M.G., Stasiuk, L.D., Hearn, M., Obermajer, M., 2004. Evidence for Gloeocapsomorpha prisca in Late Devonian source rocks from Southern Alberta, Canada. Organic Geochemistry 35, 425–441. Galimov, E., 2006. Isotope organic geochemistry. Organic Geochemistry 37, 1200– 1262. Hill, R.J., Tang, Y.C., Kaplan, I.R., 2003. Insights into oil cracking based on laboratory experiments. Organic Geochemistry 34, 1651–1672. Horsfield, B., Schenk, H.J., Mills, N., Welte, D.H., 1992. An investigation of the inreservoir conversion of oil to gas: compositional and kinetic findings from

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