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Influence of minerals and iron on natural gases generation during pyrolysis of type-III kerogen
Marine and Petroleum Geology xxx (2017) 1e9
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Research paper
Influence of minerals and iron on natural gases generation during pyrolysis of type-III kerogen Xiangxian Ma a, Guodong Zheng a, *, Wasim Sajjad a, b, Wang Xu a, b, Qiaohui Fan a, Jianjing Zheng a, Yanqing Xia a a Key Laboratory of Petroleum Resources, Gansu Province / Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2016 Received in revised form 22 December 2016 Accepted 13 January 2017 Available online xxx
Pyrolysis experiments in a closed system were conducted on kerogen (isolated from a low mature coal sample) either in the presence or absence of clay minerals, iron-bearing minerals, transition metal and their mixture, respectively. The generated gases were collected and analyzed for chemical compositions by using gas chromatography and mass spectrometer; the solid residues were also quantitatively € ssbauer spectroscopy. These experiments revealed that collected and analyzed for iron species using Mo the hydrocarbon gases generation from kerogen was significantly increased by catalysis of minerals and transition metal (iron). The generation of hydrocarbon gases could be potentially increased by 0.1e1.5 times in the presence of minerals and iron, and the catalytic efficiency was in the order of kerogen þ mixed catalysts > kerogen þ pyrite > kerogen þ smectite > kerogen þ iron > kerogen. The dryness ratios, including ethene/ethane, propene/propane, isobutane/n-butane and isopentane/npentane, were all decreased due to the catalysis of smectite, pyrite and mixed catalysts, and slightly increased with iron addition. The conversion of pyrite into pyrrhotite and elemental sulfur started at 300 C during the experiments, for which pyrite might be as an inducer indirectly via sulfur and thus enhance the free radical formation and improve the hydrocarbon yield. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Iron-bearing minerals Transition metal Clay minerals Organic-inorganic interaction €ssbauer spectrum Mo
1. Introduction Numerous studies have been established that the transition metals and mineral matrixes are important factors to control the processes of hydrocarbon gases generation, and even influence the resources evolution of crude oils and natural gases (Hunt, 1987; Huizinga et al., 1987). Conventionally, two types of geological catalysts have been mainly considered in terms of active effects on hydrocarbon gases generation, which are natural minerals and elements (Tissot et al., 1974; Johns, 1979; Mango, 1996; Vassileva and Vassilev, 2006). The minerals mentioned for such effectivity include clay minerals (i.e. kaolinite, smectite and illite), iron-bearing minerals (pyrite and siderite), carbonate minerals (like calcite and dolomite) and others (Espitalie et al., 1980; Li et al., 1998; Chen et al., 2000; Pan et al., 2008); and the elements mainly include transition metals (i.e. Fe, Co and Ni) and elemental sulfur and/or
* Corresponding author. E-mail address: [email protected] (G. Zheng).
other species of sulfur (Attar, 1978; Mango and Hightower, 1997; Lewan, 1998; Medina et al., 2000). Kerogen with smectite or kaolinite maybe typically able to stimulate the conversion of previously generated hydrocarbon gases into low-molecular-weight hydrocarbons (C7eC12), being probably due to Lewis acidic activity caused by smectite and kaolinite (Johns, 1979; Pan et al., 2010). The role of pyrite in kerogen pyrolysis has been discussed in few studies and reached the following agreements: (1) the decomposition of pyrite in coals could be decreased at 100 C than that without hydrocarbons, suggesting the indigenous hydrocarbons with hydrogen donor as a key factor to determine the transformation of pyritic sulfur into organic sulfur (Chen et al., 2000); (2) pyrite might directly influence the evolution of wet shale gases (C2eC5) and also the hydrocarbon gases generation associated with H2S through low valence sulfur species such as S0, and pyrite might also lead to the reversal of stable carbon isotope at temperatures exceeding 504 C (Wang et al., 2014); and (3) pyrite, whether endogenous or exogenous, could enhance coal conversion and improve hydrocarbon gases yield under hydro-liquefaction
http://dx.doi.org/10.1016/j.marpetgeo.2017.01.012 0264-8172/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ma, X., et al., Influence of minerals and iron on natural gases generation during pyrolysis of type-III kerogen, Marine and Petroleum Geology (2017), http://dx.doi.org/10.1016/j.marpetgeo.2017.01.012
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X. Ma et al. / Marine and Petroleum Geology xxx (2017) 1e9
conditions (Baldwin and Vinciguerra, 1983). In addition, calcite and dolomite are positively effective for the a-olefines generation, but negatively effective for methane generation (Pan et al., 2008). Transition metals such as Ni in source rocks have been advocated as catalysts in determining extent, composition, and timing of natural gases generation (Mango, 1996). However, some recent studies revealed that the transition metals could enhance gases generation and oil cracking, but had no effect on methane yield or enrichment (Lewan et al., 2008). Sulfur, a unique active element, is significant for hydrocarbons generation and also considered as a key factor for low matured crude oils formation (Lewan, 1998). However, the catalytic role of minerals/elements for hydrocarbon gases generation is still unclear. Different viewpoints, experimental designs and methods may lead to different opinions about the catalytic action of the same substances. The existence of clay minerals may play a significant role in the formation of isomeric hydrocarbons, but may not do so for the natural gases dominated by n-alkanes. Such a disagreement may suggest that the effect of free radicals on hydrocarbon gases generation should be important, but received less attention (L.M. Wu et al., 2012). In comparison with clay minerals, the catalytic effect of iron-bearing minerals has been limitedly studied and the action mechanism is still unclear (Bakr et al., 1991; Larsen and Hu, 2006). There is always more than one geological catalyst together with organic matters in natural source rocks. By considering various kinds of minerals and metal elements enriched and co-existed in coal, it should be emphasized that a mixed or complex catalytic effect of these minerals/metal elements may contribute to hydrocarbon gases generation. Based on an investigation of coal stratum in China, smectite, pyrite and iron are the three major minerals/ transition metal catalysts in coal with contents of 10e30%, 2e15%, and 0.2e0.4%, respectively (Li, 1992; Tomkins, 2010; Y.Y. Wu et al., 2012; Ma et al., 2015). According to the mineral and elemental concentrations in the source rocks mentioned above, a mixed catalyst set was prepared for the pyrolysis experiments, (smectite 30%) þ (FeS2 10%) þ (Fe 0.5%) with kerogen, to reveal the effect of minerals/transitional metal on natural gases generation and explain their mechanisms.
using a Rock-Eval instrument, which confirmed the kerogen used as typical type-III accordingly. The pyrite and smectite used in this study were purchased from National Research Center for Certified Reference Material (NRCCRMS) of China. The purity of the pyrite was more than 95%. Chemically pure iron (powder or brocks) was produced by Tianjin Chemical Reagent Co., Ltd. 2.2. Pyrolysis experiments Due to shortage of suitable commercial tools, the homemade glass tubes were used for all the experiments, whose size is approximately 2 cm of outside diameter, 1.8 cm of inside diameter and 23 cm in length. Five groups (sets) of pyrolysis experiments were performed for different test samples, including group A, the kerogen only (about 200e950 mg); group B, kerogen and smectite (about 200e950 mg kerogen and 30 wt.% smectite); group C, kerogen and pyrite (about 200e950 mg kerogen and 10 wt.% pyrite); group D, kerogen and iron (about 200e950 mg kerogen and 0.5 wt.% iron); group E, kerogen and compositional catalysts (about 200e950 mg kerogen þ 30 wt.% smectite þ 10 wt.% pyrite þ 0.5 wt.% iron). After the test samples loaded, the open end of each tube was purged with argon before being squeezed in a water stopper to create an initial seal, which was subsequently welded in the presence of argon. During welding, the previously welded end was submerged in cold water to prevent reactant heating and then the tubes with samples were heated to a desired temperature for 3 h (initial heating) followed by isothermally heating for 72 h. The temperature control was within ±1 C. After the pyrolysis experiment, gases were qualitatively collected by using a proper displacement method. 2.3. Gas composition measurement
2. Experimental
All gas samples were analyzed for chemical and isotopic composition at Key Laboratory of Petroleum Resources Research, Chinese Academy of Sciences (Lanzhou, China). The composition of major gases were determined by using both static mass spectrometer (model MAT271) and gas chromatography (model GC5890A), with errors below 5% based on repeated measurements of in-house standards, and with a main gas limit of 1 ppm.
2.1. Samples
€ssbauer spectroscopy 2.4. Mo
The kerogen (Ker) used in this study was obtained from a brown coal (Ro ¼ 0.47%) from the eastern Junggar Basin, NW China, whose geochemical properties were summarized in Table 1. The basic geochemical parameters suggested that it was a low-matured sample dominated by humic matters (type III) and suitable for the study on hydrocarbon gases generation by pyrolysis experiment. Soxhlet extraction of the coal was performed for 72 h by using trichloromethane to remove the original soluble bitumen. The bitumen-free powder sample (approximately 200 meshes) was treated with HCl and HF to remove the carbonate and silicate minerals present in the sample by using the methods described in detail in the literature (Pan et al., 2008). Clay minerals and ironbearing minerals were not detected in the prepared kerogen by X-Ray powder Diffraction (XRD, data not shown here). The organic carbon content of the prepared kerogen was 85.6%, as measured by
€ssbauer spectra were measured at 293 K using a MA-260 The Mo €ssbauer spectrometer with a g-ray source of (Bench MB-500) Mo 0.925 GBq, 57Co/Rh. The measurement and curve fitting procedures were described elsewhere (Matsuo et al., 1994). The measured spectra were fitted to Lorentzian line shapes using standard line shape fitting routines. The half-width and peak intensities of the quadruple doublet were constrained to be equal. Isomer shifts were expressed with respect to the centroid of the spectrum of metallic iron foil (56Fe > 99.85%). 3. Results 3.1. Gas composition The amounts of SC1e5 were consistently increased from a range
Table 1 Total organic carbon (TOC) and Rock-Eval pyrolysis data from the experimental samples. Sample No.
Depth (m)
Stratum
TOC (%)
S1-peak (mg/g rock)
S2-peak (mg/g rock)
Ro (%)
Tmax ( C)
OS
1846
J1b
73.8
13.6
177.1
0.47
428
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Table 2 Types and concentrations (mg/g Ker) of gas generated during the pyrolysis experiments at 250 C/72 h, 300 C/72 h, 350 C/72 h and 400 C/72 h. Gases content (ml/g)
Ker
Ke þ FeS2
Ke þ Fe
Ker þ Sm
Ker þ mixed Catalysts
250 C Hydrogen sulfide Carbon dioxide Methane Ethane Ethene Propane Propene i-Butane n-Butane neo-Pentane i-Pentane n-Pentane Hydrocarbon total P Dryness (C1/[ C1eC4]) i/n-Butanes i/n-Pentane Ethene/Ethane Propene/Propane
n.d. 38.1 0.41 0.07 0.05 0.24 0.08 0.10 0.26 n.d. 0.26 0.10 1.57 0.34 0.38 2.60 0.71 0.33
n.d. 36.3 4.56 2.12 0.13 1.81 0.43 0.28 0.54 Trace 0.23 0.32 10.4 0.46 0.52 0.72 0.06 0.24
Ker þ FeS2
Ker þ Fe
Ker þ Sm
Ker þ mixed catalysts
n.d. 49.5 1.30 0.68 0.28 0.77 0.30 0.08 0.10 Trace 0.06 0.03 3.60 0.37 0.80 2.00 0.41 0.39
n.d. 24.8 0.77 0.35 0.10 0.31 0.09 0.05 0.03 n.d. 0.03 0.04 1.77 0.45 1.67 0.75 0.29 0.29
n.d. 16.4 1.13 0.51 0.33 0.45 0.26 0.21 0.08 n.d. 0.19 0.03 3.19 0.38 2.63 6.33 0.65 0.58
n.d. 59.5 1.91 0.74 0.53 0.75 0.53 0.18 0.08 0.03 0.12 0.03 4.90 0.40 2.25 4.00 0.72 0.71
0.05 62.1 32.4 13.5 0.08 10.7 0.39 1.92 4.07 0.32 0.80 1.32 65.5 0.51 0.47 0.61 0.01 0.04
n.d. 52.3 25.1 6.58 0.44 7.09 0.15 0.54 0.98 0.10 0.45 0.58 42.0 0.61 0.55 0.78 0.07 0.02
n.d. 77.2 27.0 14.5 0.79 9.31 0.72 1.92 2.69 0.10 0.77 0.74 58.5 0.47 0.71 1.04 0.05 0.08
Trace 84.9 37.4 16.7 0.76 11.9 0.61 3.26 4.63 0.23 1.74 2.06 79.3 0.50 0.70 0.84 0.05 0.05
300 C n.d. 44.2 0.39 0.07 0.05 0.16 0.06 0.05 0.10 n.d. 0.13 0.06 1.07 0.44 0.50 2.17 0.71 0.38
n.d. 35.8 0.32 0.05 0.01 0.05 0.01 0.03 0.20 n.d. 0.13 0.16 0.96 0.48 0.15 0.81 0.20 0.20
n.d. 40.3 0.46 0.07 0.10 0.24 0.12 0.06 0.19 n.d. 0.18 0.12 1.54 0.37 0.32 1.50 1.43 0.50
n.d 39.8 0.56 0.09 0.18 0.22 0.15 0.08 0.13 0.13 0.29 0.16 1.99 0.40 0.62 1.81 2.00 0.68
350 C Hydrogen sulfide Carbon dioxide Methane Ethane Ethene Propane Propene i-Butane n-Butane neo-Pentane i-Pentane n-Pentane Hydrocarbon total P Dryness (C1/[ C1eC4]) i/n-Butanes i/n-Pentane Ethene/Ethane Propene/Propane
Ker
n.d. 35.6 0.71 0.31 0.15 0.33 0.13 0.05 0.03 n.d. 0.03 0.01 1.75 0.42 1.67 3.00 0.48 0.39 400 C
Trace 49.1 7.43 3.94 0.14 3.14 0.54 0.49 0.91 Trace 0.32 0.45 17.4 0.45 0.54 0.71 0.04 0.17
n.d. 40.7 6.00 2.48 0.19 1.10 0.21 0.18 0.34 n.d. 0.06 0.39 11.0 0.57 0.53 0.15 0.08 0.19
n.d. 33.2 6.36 4.37 0.66 2.14 0.71 0.54 0.85 n.d. 0.71 0.58 16.9 0.41 0.64 1.22 0.15 0.33
n.d. 28.5 8.50 5.45 0.80 2.59 0.77 0.65 0.85 0.03 1.00 0.32 21.0 0.43 0.76 3.13 0.15 0.30
n.d. 59.5 24.9 10.7 0.08 8.21 0.39 1.40 2.80 Trace 0.61 0.90 50.0 0.51 0.50 0.68 0.01 0.05
Sm-smectite.
of 0.96e1.99 to 50.0e79.3 mg/g Ker during the pyrolysis experiments along with temperature increased from 250 to 400 C. The methane production was consistently increased from a range of 0.32e0.46 to 27.0e37.4 mg/g Ker. The lowest production of C1eC5 was in a range of 0.01e0.56 mg/g Ker for the five sets of experiments at 250 C while the lowest synthesis of C4 and C5 appeared at 300 C, being 0.08 and 0.04 mg/g Ker. The highest yields of C1-5 were observed at 400 C, and the values decreased gradually with increase in carbon numbers during all experiments (Table 2, Fig. 1 and Fig. 2). The produced carbon dioxide was increased from 38.1 to 44.2 to 52.3e84.9 mg/g Ker with temperature increased from 250 to 400 C. The hydrogen sulfide (H2S) was detected in the kerogen þ pyrite experiment at 400 C (Table 2 and Fig. 2). A smell of H2S gas was distinguished after heated the samples at 300 and 350 C, indicating the decomposition of pyrite probably started during the pyrolysis experiments since 300 C and onward. In comparison with gas composition, smectite, pyrite and their complex catalysts were all shown with positive effects on the production of SC1e5 hydrocarbons. The mixed catalysts showed the best effects with the highest value of 79.3, 21.0, 4.90, 1.99 mg/g from 400 to 250 C, respectively. Pyrite was the second positive catalyst with the highest value of 65.5, 17.4, 3.60, 1.07 mg/g from 400 to 250 C, respectively. And then smectite had the catalytic effect in the highest values of 58.5, 16.9, 3.19, 1.54 mg/g from 400 to 250 C, respectively. However, there was almost no obvious effect for iron itself as catalyst, the yields of SC1e5 hydrocarbons were almost the
same as Ker alone. The mixture of Ker þ Fe had the highest values of C1/SC1e4 ratios at any temperature stage, ranging from 0.34 to 0.61 with temperature increased from 250 to 400 C. The mixture of Ker þ smectite had the lowest value (0.41 and 0.47) at 350 and 400 C (Table 2 and Fig. 3). The dryness of produced gases was sequentially increased from 250 to 400 C in kerogen alone whereas the ratio was slightly increased at 250 C and decreased at 300 C and then increased again from 350 C to 400 C in the presence of pyrite, smectite and mixed catalysts. The ratio of methane/ethane was the highest at 250 C, in a range of 5.57e6.57. The highest ratio of ethane/propane was observed in the experiment at 350 C, in a range of 1.17e2.25. In the experiment with Ker þ FeS2, this ratio was increased with temperature raised. However, other four sets of experiments revealed this ratio firstly increased from 250 to 350 C and then decreased at 400 C. Based on the iC4/nC4 and iC5/nC5 ratios, the five kinds of experiments could be classified into three groups. Group one included the two experiments using smectite or mixed catalysts, having the highest ratios. Group two was the only experiment with kerogen alone, with medium ratios. And group three, including the two experiments using pyrite or iron, had the lowest ratios except at 250 C. The highest ethene/ethane ratio appeared in the experiment with smectite at 300 C, the other four sets of experiments, however, had the highest value at 250 C, in a range of 0.2e2.6, and then all decreased to a value below 0.1 with temperature increased to 400 C. The highest ratio of propene/propane
Please cite this article in press as: Ma, X., et al., Influence of minerals and iron on natural gases generation during pyrolysis of type-III kerogen, Marine and Petroleum Geology (2017), http://dx.doi.org/10.1016/j.marpetgeo.2017.01.012
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also appeared in the experiment with smectite at 300 C whereas the ratios in other three sets of experiments (Ker alone, Ker þ Fe and Ker þ FeS2) increased at 300 C, and then decreased at 350 C and 400 C. However, this ratio in the experiment of Ker þ mixed catalysts was decreased gradually with temperature raised. Finally, the lowest value appeared in all sets of experiments at 400 C, ranging from 0.02 to 0.08 (Table 2). 3.2. Mature scale The results of Rock - Eval experiments showed that Tmax was rose from 435 to 467 C during various pyrolysis experiments. The values of Tmax were about 436 ± 2 C, 443 ± 2 C, 456 ± 2 C and 466 ± 3 C after the pyrolysis experiments at 250 C, 300 C, 350 C and 400 C, respectively (Fig. 6). These results indicated that (1) the maturity of the sample rose with the increase of temperature; and (2) the catalysts used in this study might not change the maturity obviously.
Fig. 1. Total amounts of C1eC5 hydrocarbon gases produced at 250e400 C.
€ssbauer spectroscopy 3.3. Mo € ssbauer spectra for the residue materials of the pyrolysis Mo experiments are shown in Fig. 7 and their parameters summarized
Fig. 2. C1eC5 hydrocarbon gasesyeilds during various kerogen and minerals/metal pyrolysis: (aed) the experiments at 250, 300, 350, and 400 C, respectively.
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4. Discussion 4.1. Effect of minerals and transition metal on hydrocarbon gases generation In the presence of minerals and iron, the methane and C1-5 yields were almost increased by 20e150% (except at 250 C), and the catalytic efficiency was shown in the following order, mixed materials > pyrite > smectite > iron. 4.1.1. Dryness coefficient (C1/SC1e4) As demonstrated in Table 2 and Fig. 1b, the ratio of C1/SC1e4 was continuously increased from the lowest (0.34) at 250 C to the highest (0.51) at 400 C during the experiments of kerogen alone, whereas the ratio of C1/SC1e4 decreased from higher value at 250 C to the lowest value at 300 C, and then again, this ratio was increased at 350 C and reached the highest value at 400 C in the presence of smectite, pyrite, metal iron and mixed catalysts. This might suggest that the minerals and iron could be able to induce cracking of methyls and related functional groups of kerogen at 250 C and this ratio increased with temperature raised and/or maturation increased. The ratio of C1/SC1e4 was very similar at temperature range of 350e400 C among the five cases except the experiment using iron, being relatively higher as discussed before (Fig. 3), indicating that mineral acidity could influence this ratio considerably in the pyrolysis experiments after the amount of produced bitumen reached to the highest level. The average ratio of C1/SC1e4 for natural gas was about 85 wt% (Mango, 1994). This ratio was substantially lower in our experiments in comparison with natural gases (Table 1 and Fig. 3) (Mango, 1994). However, it is a common phenomenon that the gaseous hydrocarbons produced by experimental pyrolysis are relatively poorer in methane than in the natural gases (Mango, 1991). Lewan, 1998 also revealed the lower ratio for Woodford Shale and its kerogen under various conditions ranged from 40.4 to 51.5% at about 330 C (72 h) and from 50.1 to 66.6% at 350 C (72 h).
Fig. 3. Dryness of gaseous hydrocarbon produced at 250e400 C against temperature.
in Table 3. Three sets of absorption peaks appeared in the spectra, including one doublet and two sextets. The doublet with isomer shift (IS) approximately 0.30 mm/s and quadrupole splitting (QS) approximately 0.60 mm/s was ascribed to pyritic iron (pyr-Fe2þ) (Dyar et al., 2006). The two sextets with magnetic hyperfine field (Bhf) approximately 28.0 T and 24.0 T were attributed to pyrrhotite (Dyar et al., 2006). In the Ker þ pyrite experiments, the appearance of doublet was measured as pyr-Fe2þ at 250 C, since the pyrite was initial material and there were almost no phase transformation at €ssbauer low temperature stage. With temperature raise, the Mo spectrum data revealed that the transformation of pyrite into pyrrhotite initiated at 300 C, and 40.0% and 79.9% of pyrite were transformed to pyrrhotite at 350 and 400 C, respectively, revealed significantly the pyrite initiation to phase transformation (Table 3 and Fig. 7). Furthermore, the IS of pyrite was gradually increased from 0.290 to 0.301 mm/s with the raise of pyrolysis temperatures from 250 to 400 C, indicating that the Coulomb force between the nucleus and density distribution of extra-nuclear electrons gradually decreased (Dyar et al., 2006). This variation trend suggested the higher chemical activity of pyrite. In addition, with increase of pyrolysis from 250 to 400 C, the Bhf of pyrrhotite was also increased, probably representing an increasing degree of pyrrhotite crystallization (Ma et al., 2016).
Table 3 Values of the hyperfine parameters from the best fits of
57
4.1.2. Ratios of iC4/nC4 and iC5/nC5 As demonstrated in Table 2 and Fig. 4, the ratios of iC4/nC4 and iC5/nC5 were substantially higher at 250e400 C in the presence of smectite and mixed catalysts than that in kerogen alone. This phenomenon might be interpreted as many acidic sites on the structural surface of smectite. Previous pyrolysis studies suggest that during petroleum formation process, n-alkanes are formed by free radical reactions, whereas branched iso-alkanes formed via two processes: (1) free radical cracking of kerogen and bitumen components and branched fragments; and (2) the carbonium ion reaction of a-olefins with protons promoted under acidic
€ssbauer spectra for the samples at room temperature (Ma et al., 2016). Fe Mo
Sample ID.
Iron species
Relative content, %
IS (mm/s)
Pyrite PS-250 PS-300
pyr-Fe2þ pyr-Fe2þ pyr-Fe2þ
PS-350
pyr-Fe
2þ
PS-400
pyrrhotite pyr-Fe2þ
100 100 93.2 ± 0.1 4.0 ± 0.1 2.8 ± 0.2 60.0 ± 1.0 24.5 ± 0.9 15.5 ± 0.8 20.1 ± 0.9 50.5 ± 0.8 29.4 ± 0.6
0.29 0.29 0.29 0.44 0.58 0.30 0.44 0.83 0.30 0.69 0.58
pyrrhotite
± ± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.01 0.04 0.01 0.02 0.04 0.023 0.01 0.01 0.02
QS (mm/s)
HW (mm/s)
Hi/T and compound
0.60 ± 0.00 0.60 ± 0.00 0.60 ± 0.00 0.19 ± 0.06 0.13 ± 0.03 0.60 ± 0.00 0.19 ± 0.06 0.38 ± 0.05 0.60 ± 0.00 0.29 ± 0.05 0.13 ± 0.03
0.31 ± 0.01 0.34 ± 0.02 0.31 ± 0.01 0.37 ± 0.02 0.19 ± 0.03 0.35 ± 0.02 0.37 ± 0.02 0.39 ± 0.03 0.31 ± 0.010 25 ± 0.03 0.19 ± 0.03
pyrite pyrite pyrite 27.50 ± 28.00 ± pyrite 27.64 ± 28.02 ± pyrite 27.76 ± 28.10 ±
0.05 0.02 0.15 0.11 0.09 0.02
Note: where IS the isomer shift (relative to a-Fe at RT), QS is the quadrupole splitting, HW is the half width at half maximum, Hi is the hyperfine magnetic field, relative content is the relative spectral absorption area for each specie.
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Fig. 4. Isoparaffin and n-alkane gas ratios from the pyrolysis experiments at 250e400 C.
Fig. 5. Olefin and alkane gas ratios from the pyrolysis experiments at 250e400 C.
conditions (Kissin, 1987). Some pyrolysis experiments and chemical degradation studies revealed that the coal type of kerogen rarely bear branched fragments (Blokker et al., 2001). The carbonium ion reaction should be primarily responsible for the branched isoalkane formation in the pyrolysates from kerogen. Tannenbaum et al. (1986) believed that this phenomenon is indicative of cracking via carbonium ion mechanism that is induced by acidic sites of clay during anhydrous pyrolysis. The ratios of iC4/nC4 and iC5/nC5 were generally low for the experiments in the presence of pyrite and iron than those in kerogen alone (Table 2 and Fig. 4a and b), which might reflect the nonacidic (basic) nature of these two kinds of minerals. Therefore, the traditional theory did not explain the iC4/nC4 and iC5/nC5 abnormalities witnessed at 300 C during the pyrite and iron experiments. We proposed that pyrite and iron could be able to induce the rearrangement of alkanes during kerogen degradation at 300 C, and new alkanes might be formed with high isomers and stable under such conditions.
Fig. 6. Mature scale (Tmax) of samples after pyrolysis experiments at 250e400 C.
4.1.3. Hydrogenation of gas olefins It was suggested that anhydrous pyrolysis could produce a large amount of olefins (Seewald, 2003). The abundance of produced olefins in the present experiments was quite consistent with
Please cite this article in press as: Ma, X., et al., Influence of minerals and iron on natural gases generation during pyrolysis of type-III kerogen, Marine and Petroleum Geology (2017), http://dx.doi.org/10.1016/j.marpetgeo.2017.01.012
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pyrrhotite
Relative Intensity
Pyr-Fe2+
7
(Schematic 1). During the transformation of organic matters in the source rocks into hydrocarbons, the clay minerals may exhibit different catalytic mechanisms in decarboxylation reaction and thermal cracking reaction of organic matters. In the decarboxylation reaction, Lewis acid of clay minerals may receive an electron from organic matter and the carboxylic acid will loss CO2, leading to the formation of free-radicals. The free-radicals may further cause rearrangement, leading to the cleavage of CeC bonds and formation of shorter-chain free hydrocarbons. The reaction was proposed as in eq. (1) (Johns, 1979).
CH3 ðCH2 Þn COOH/CH3 ðCH2 Þn1 $CH2 þCO2 þ $H
(1)
In the thermal cracking reaction of organic matters, the clay minerals act as a Brønsted acid and provide proton (Hþ) for organic matters. The Hþ may be derived from the dissociation of adsorbed water and interlayer water molecules. Mainly through the formation of transition state of the carbonium-ion reaction can be illustrated in eq. (2) (L.M. Wu et al., 2012).
k n MðH2 OÞx /n MðH2 OÞx1 $OHz1 þ nHþ
-10 -8 -6 -4 -2 0 2 4 6 Velocity (mm/s)
8 10
€ ssbauer spectra of pyrolysis experiment samples analyzed at room Fig. 7. The Mo temperature (modified from Ma et al., 2016).
previous studies (Table 1, Fig. 5a and b). Gaseous olefins were unstable and might be readily hydrogenated into saturated hydrocarbons in the presence of iron and pyrite. The presence of pyrite and iron should be essential for the generation of n-alkanes from the n-alkyl components and brown coal might play multiple roles for these conversions, a major one being the role of a hydrogen donor. Therefore, it might be suggested that a hydrogen radical from the intermediated compounds to the organic radical by a radical mechanism. The hydrogenation rate of olefins substantially influenced the ratio of iso-alkane/n-alkane, olefins released from kerogen and bitumen degradation were more likely hydrogenated into n-alkanes and less likely transformed into iso-alkanes when the hydrogenation rate of olefins was higher. As a result, the iC4/nC4 and iC5/nC5 ratios were low in the experiments for kerogen alone. In addition, the low ratios of ethene/ethane and propene/propane might indicate high hydrogenation rate of olefins in the experiment by using iron and pyrite in comparison with that by using smectite and mixed catalysts (Fig. 5a and b).
(2)
where M is the exchangeable cation, Z is the charge, x is the amount of water molecules coordinated with M, n is the amount of exchangeable cations, k is the ionization (dissociation) constant for the interlayer system. Actually, Brønsted acid and Lewis acid cannot be isolated, and they can be transformed into each other under certain conditions. When the surface of clay minerals adsorbed water molecules, Lewis acid will have strong affinity for electrons, and may share a pair of electrons with the hydroxyl (eOH) in the water and make the hydroxyl (eOH) firmly adsorbed on the Lewis acid surface. The remaining Hþ will be easily released for weak binding force, leading to the conversion of Lewis acid to Brønsted acid. When the clay minerals are dehydrated, the Brønsted acid will be gradually transformed into Lewis acid (Wang et al., 2006). From the thermodynamic point of view, the loss of Hþ in the Brønsted acid need to overcome the binding of intramolecular hydrogen bond, leading to increase of the system energy. Therefore, it could be occurred when the external energy is sufficient. However, the Lewis acidic sites get an electron and the energy of the system will be reduced, and the occurrence of this reaction could not require higher external energy. Therefore, Lewis acid sites on the clay minerals are more active at low temperature (250e300 C) conditions. Furthermore, the oil and gas formation processes are associated with the transformation process of montmorillonite to illite. During dehydration of montmorillonite, the activity of Brønsted acid is lower and the activity of Lewis acid is higher. In a summary, the main catalytic mechanism of clay minerals in nature is free-radical mechanisms at low temperature.
4.2. Action mechanism 4.2.1. Clay minerals (Smectite) The catalytic mechanism of clay minerals is mainly related to the Brønsted acid and Lewis acid which are on the clay surface. These two acidic sites could form two kinds of catalytic mechanism in hydrocarbon generation, such as carbonium-ion mechanism and free-radical mechanism (Brooks, 1948; Johns, 1979; Lei et al., 1997; Reddy et al., 2009; L.M. Wu et al., 2012). In the catalytic reaction for hydrocarbons generation, Brønsted acid and Lewis acid are normally active, but their role in the catalytic reaction process is different. The previous studies showed that the cracking reaction, polymerization reaction and hydrogenation reaction are mainly presented in the Brønsted acidic sites, and the decarboxylation reaction of organic matters is mainly related to the Lewis acid sites
4.2.2. Pyrite €ssbauer parameters, there was no change According to the Mo occurred in pyrite during pyrolysis at 250 C, and there was limited contribution of pyrite in hydrocarbon gases generation. The transformation of pyrite into pyrrhotite initiated at 300 C and the transformation was significantly enhanced with the raise of temperature to 350 and 400 C. Therefore, a clear increase of large radical concentration was observed in the kerogen þ pyrite system which was steady with the transformation of pyrite into pyrrhotite. This FeS2 based amplification in radical concentration might be ascribed to the production of nascent sulfur during the pyrite transformation to pyrrhotite (sulfur production in kerogen þ pyrite experiment have been shown by GC-MS results). This sulfur has strong affinity for hydrogen, and by a free radical procedure it
Please cite this article in press as: Ma, X., et al., Influence of minerals and iron on natural gases generation during pyrolysis of type-III kerogen, Marine and Petroleum Geology (2017), http://dx.doi.org/10.1016/j.marpetgeo.2017.01.012
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X. Ma et al. / Marine and Petroleum Geology xxx (2017) 1e9
excerpts hydrogen from kerogen (Ma et al., 2016). The process may occur as following:
FeS2 ðpyriteÞ/Fe1x S ðpyrrhotiteÞ þ S ð0 < x < 1Þ
(3)
S þ RH/HS$ þ R
(4)
HS$ þ RH/H2 S þ R$ðR kerogenÞ
(5)
It is clearly proposed that the enhanced formation of free radicals occurs through hydrogen transfer from kerogen to sulfur. Obviously, under such conditions the sulfur operated as a catalyst
M
CH3
CH3
conventional first-order kinetics (Lewan et al., 1979); and then the olefins were either hydrogenated to n-alkanes or catalytically converted into natural gas (eq. (6) and (7)). In our experiment, the ethene/ethane and propene/propane ratios were decreased with iron addition, but the increase degree of methane was very limited, indicating the catalytic effect of transition metal in low contents was inefficient. In addition, such phenomena could be related to the poisoning of catalytic sites by polar-rich bitumen (Lewan et al., 2008).
i h Ker/ n Cx¼ þH2 Fe C1 þC2 þC3 …nCx1 !
+ H2
M
H M
(6)
+ CH4
(7)
for radical production in kerogen. Similar enhanced radical formation by pyritic sulfur has been studied in coal (Ma et al., 2016).
4.3. Mixed catalyst
4.2.3. Transition metal (iron) The chemical state of iron was not changed before and after the €ssbauer parameters (Table 3). The pyrolysis experiment from Mo catalytic scheme of transition metal on natural gas produced from organic matter had been proposed of hydrogenation reaction (Mango, 1996). Kerogen was thermally converted into olefins by
In comparison with other geological catalysts, the mixed catalysts have the highest catalytic efficiency on the generation of methane and the amount of C1-5. Furthermore, although other geological catalysts could be able to improve the yield of gaseous hydrocarbons, the dryness of natural gas would be decreased whereas the composite catalysts were not. The pyrolysis results also suggested that the mixed catalysts could have better catalytic
Schematic 1. Catalytic mechanism of clay minerals based on carbonium ion reaction and free radical cracking (Modified from Johns, 1979; L.M. Wu et al., 2012).
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X. Ma et al. / Marine and Petroleum Geology xxx (2017) 1e9
effect on methane generation and the amount of C1-5 at low temperature stage (250 C) which are quite similar to natural gases. Therefore, the efficiency of mixed catalysts to natural gases generation should be very important based on the present study, and enough attention must be paid on this matter once to evaluate gas formation under really geological conditions because there are often more than one kind of catalysts in natural source rocks. 5. Conclusions The effects of smectite, pyrite, iron and their mixed catalysts on the gaseous hydrocarbons generation from coal type of kerogen were performed in this study and some new and improved findings about the processes can be summarized as the followings: (1) Smectite, pyrite, iron and their mixture catalysts could all be able to improve the gaseous hydrocarbons generation and also reduce the dryness co-efficiency of produced gases for almost all experiments except that at 250 C. (2) The ethene/ethane and propene/propane ratios were generally decreased with temperature increasing. However, these ratios were increased in the experiments by adding smectite and mixed catalysts and slightly decreased by adding pyrite and iron. The interactions among minerals, transition metals, and kerogen may lead to alkane's rearrangement, resulting in high isobutane/n-butane and isopentane/n-pentane ratios with smectite and mixed catalysts and low isobutane/n-butane and isopentane/n-pentane ratios with pyrite and iron addition. (3) Pyrite may act indirectly as a catalytic agent or inducer via sulfur, which may enhance free radical formation and hydrocarbons generation. Mixed catalysts have not only the effects of each catalyst but also the highest catalytic efficiency, which may reflect the realistic geological situation of minerals and transition metals on hydrocarbons generation. Acknowledgments This study was supported by China National Major S&T Projects (2016ZX05007-01) and the National Natural Science Foundation of China (41402129), the Key Laboratory of Petroleum Resources, Gansu Province (1309RTSA041, Y623JJGJ1MXX). Chinese Academy of Sciences issued the “100-Talent” Program for QH and Japan Society for the Promotion of Science (JSPS) awarded a fellowship to GZ (BR161304). References Attar, A., 1978. Chemistry, thermodynamics and kinetics of reactions of sulphur in coal-gas reactions: a review. Fuel 57 (4), 201e212. Bakr, M., Akiyama, M., Yokono, Y., Sanada, M., 1991. Role of pyrite during the thermal degradation of kerogen using in situ high-temperature ESR technique. Energy & Fuels 5, 441e444. Baldwin, R., Vinciguerra, S., 1983. Coal liquefaction catalysis: iron pyrite and hydrogen sulphide. Fuel 62, 498e501. Blokker, P., van Bergen, P., Pancost, R., Collinson, M.E., de Leeuw, J.W., Damste, J.S.S., 2001. The chemical structure of Gloeocapsomorpha prisca microfossils: implications for their origin. Geochim. Cosmochim. Acta 65, 885e900. Brooks, B.T., 1948. Active-surface catalysts in formation of petroleum. AAPG Bull. 32 (12), 2269e2286. Chen, H., Li, B., Zhang, B., 2000. Decomposition of pyrite and the interaction of pyrite with coal organic matrix in pyrolysis and hydropyrolysis. Fuel 79 (13), 1627e1631. €ssbauer specDyar, M., Agresti, D., Schaefer, M.W., Grant, C.A., Sklute, E., 2006. Mo troscopy of Earth and planetary materials. Annu. Rev. Earth Pl. S. C. 34, 83e125.
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Espitalie, J., Madec, M., Tissot, B., 1980. Role of mineral matrix in kerogen pyrolysis: influence on petroleum generation and migration. AAPG Bull. 64 (1), 59e66. Huizinga, B.J., Tannenbaum, E., Kaplan, I.R., 1987. The role of minerals in the thermal alteration of organic matterdIII. Generation of bitumen in laboratory experiments. Org. Geochem 11, 591e604. Hunt, J.M., 1987. Generation and migration of light hydrocarbon. Science 226, 1265e1270. Johns, W.D., 1979. Clay mineral catalysis and petroleum generation. Annu. Rev. Earth Pl. S. C. 7, 183e198. Kissin, Y., 1987. Catagenesis and composition of petroleum: origin of n-alkanes and isoalkanes in petroleum crudes. Geochim. Cosmochim. Acta 51, 2445e2457. Larsen, J., Hu, J., 2006. Kerogen Chemistry Hydrous pyrolysis of rundle kerogen: source of the oxygen in CO2 and mineral catalysis. Energy & Fuels 20, 278e280. Lei, H.Y., Shi, Y.S., Guang, P., Fang, X., 1997. Catalysis of aluminosilicate clay minerals to the formation of the transitional zone gas. Sci. China (Series D) Earth Sci. 40, 130e136. Lewan, M.D., 1998. Sulfureradical control on rates of natural petroleum formation. Nature 391, 164e166. Lewan, M.D., Winters, J.C., McDonald, J.H., 1979. Generation of oil-like pyrolysates from organic-rich shales. Science 203, 897e899. ski, A., 2008. Evaluating transitionLewan, M.D., Kotarba, M.J., Wie˛ cław, D., Piestrzyn metal catalysis in gas generation from the Permian Kupferschiefer by hydrous pyrolysis. Geochim. Cosmochim. Acta 72, 4069e4093. Li, Z., 1992. On the catalysis of clay minerals in the process of oil and gas generation. Petro. Geol. Expt 14, 59e63 (in Chinese with English abstract). Li, S.Y., Guo, S.H., Tan, X.F., 1998. Characteristics and kinetics of catalytic degradation of immature kerogen in the presence of mineral and salt. Org. Geochem 29, 1431e1439. Ma, X.X., Zheng, J.J., Wang, X.F., Wang, Z.D., Wu, Y.Z., Qian, Y., 2015. Research progress and new direction: catalysis of clay minerals on oil and gas generation. Lithol. Reserv. 27, 55e61 (in Chinese with English abstract). Ma, X.X., Zheng, J.J., Zheng, G.D., Xu, W., Qian, Y., Xia, Y.Q., Wang, Z.D., Wang, X.F., Ye, X.Y., 2016. Influence of pyrite on hydrocarbon generation during pyrolysis of type-III kerogen. Fuel 167, 329e336. Mango, F.D., 1991. The stability of hydrocarbons under the time-temperature conditions of petroleum genesis. Geochim. Cosmochim. Acta 352, 146e148. Mango, F.D., 1994. Role of transition-metal catalysis in the formation of natural gas. Nature 368, 536e538. Mango, F.D., 1996. Transition metal catalysis in the generation of natural gas. Org. Geochem 24, 977e984. Mango, F.D., Hightower, J., 1997. The catalytic decomposition of petroleum into natural gas. Geochim. Cosmochim. Acta 61, 5347e5350. € ssbauer spectroscopic characterMatsuo, M., Kobayashi, T., Tsurumi, M., 1994. Mo ization of iron compounds in paddy soil. Hyperfine Interact. 84, 533e537. Medina, J.C., Butala, S.J., Bartholomew, C.H., Lee, M.L., 2000. Low temperature ironand nickel-catalyzed reactions leading to coalbed gas formation. Geochim. Cosmochim. Acta 64, 643e649. Pan, C., Geng, A., Zhong, N., Liu, J., Yu, L., 2008. Kerogen pyrolysis in the presence and absence of water and minerals-I. Gas components. Energy & Fuels 22, 416e427. Pan, C., Jiang, L., Liu, J., Geng, A., 2010. The effects of calcite and montmorillonite on oil cracking in confined pyrolysis experiments. Org. Geochem 41, 611e626. Reddy, C.R., Bhat, Y.S., Nagendrappa, G., Prakash, B.S., 2009. Brønsted and Lewis acidity of modified montmorillonite clay catalysts determined by FT-IR spectroscopy. Catal. Today 141, 157e160. Seewald, J.S., 2003. Organic-inorganic interactions in petroleum-producing sedimentary basins. Nature 426, 327e333. Tannenbaum, E., Huizinga, B.J., Kaplan, I.R., 1986. Role of minerals in thermal alteration of organic matter-II: a material balance. 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Research paper
Influence of minerals and iron on natural gases generation during pyrolysis of type-III kerogen Xiangxian Ma a, Guodong Zheng a, *, Wasim Sajjad a, b, Wang Xu a, b, Qiaohui Fan a, Jianjing Zheng a, Yanqing Xia a a Key Laboratory of Petroleum Resources, Gansu Province / Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2016 Received in revised form 22 December 2016 Accepted 13 January 2017 Available online xxx
Pyrolysis experiments in a closed system were conducted on kerogen (isolated from a low mature coal sample) either in the presence or absence of clay minerals, iron-bearing minerals, transition metal and their mixture, respectively. The generated gases were collected and analyzed for chemical compositions by using gas chromatography and mass spectrometer; the solid residues were also quantitatively € ssbauer spectroscopy. These experiments revealed that collected and analyzed for iron species using Mo the hydrocarbon gases generation from kerogen was significantly increased by catalysis of minerals and transition metal (iron). The generation of hydrocarbon gases could be potentially increased by 0.1e1.5 times in the presence of minerals and iron, and the catalytic efficiency was in the order of kerogen þ mixed catalysts > kerogen þ pyrite > kerogen þ smectite > kerogen þ iron > kerogen. The dryness ratios, including ethene/ethane, propene/propane, isobutane/n-butane and isopentane/npentane, were all decreased due to the catalysis of smectite, pyrite and mixed catalysts, and slightly increased with iron addition. The conversion of pyrite into pyrrhotite and elemental sulfur started at 300 C during the experiments, for which pyrite might be as an inducer indirectly via sulfur and thus enhance the free radical formation and improve the hydrocarbon yield. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Iron-bearing minerals Transition metal Clay minerals Organic-inorganic interaction €ssbauer spectrum Mo
1. Introduction Numerous studies have been established that the transition metals and mineral matrixes are important factors to control the processes of hydrocarbon gases generation, and even influence the resources evolution of crude oils and natural gases (Hunt, 1987; Huizinga et al., 1987). Conventionally, two types of geological catalysts have been mainly considered in terms of active effects on hydrocarbon gases generation, which are natural minerals and elements (Tissot et al., 1974; Johns, 1979; Mango, 1996; Vassileva and Vassilev, 2006). The minerals mentioned for such effectivity include clay minerals (i.e. kaolinite, smectite and illite), iron-bearing minerals (pyrite and siderite), carbonate minerals (like calcite and dolomite) and others (Espitalie et al., 1980; Li et al., 1998; Chen et al., 2000; Pan et al., 2008); and the elements mainly include transition metals (i.e. Fe, Co and Ni) and elemental sulfur and/or
* Corresponding author. E-mail address: [email protected] (G. Zheng).
other species of sulfur (Attar, 1978; Mango and Hightower, 1997; Lewan, 1998; Medina et al., 2000). Kerogen with smectite or kaolinite maybe typically able to stimulate the conversion of previously generated hydrocarbon gases into low-molecular-weight hydrocarbons (C7eC12), being probably due to Lewis acidic activity caused by smectite and kaolinite (Johns, 1979; Pan et al., 2010). The role of pyrite in kerogen pyrolysis has been discussed in few studies and reached the following agreements: (1) the decomposition of pyrite in coals could be decreased at 100 C than that without hydrocarbons, suggesting the indigenous hydrocarbons with hydrogen donor as a key factor to determine the transformation of pyritic sulfur into organic sulfur (Chen et al., 2000); (2) pyrite might directly influence the evolution of wet shale gases (C2eC5) and also the hydrocarbon gases generation associated with H2S through low valence sulfur species such as S0, and pyrite might also lead to the reversal of stable carbon isotope at temperatures exceeding 504 C (Wang et al., 2014); and (3) pyrite, whether endogenous or exogenous, could enhance coal conversion and improve hydrocarbon gases yield under hydro-liquefaction
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X. Ma et al. / Marine and Petroleum Geology xxx (2017) 1e9
conditions (Baldwin and Vinciguerra, 1983). In addition, calcite and dolomite are positively effective for the a-olefines generation, but negatively effective for methane generation (Pan et al., 2008). Transition metals such as Ni in source rocks have been advocated as catalysts in determining extent, composition, and timing of natural gases generation (Mango, 1996). However, some recent studies revealed that the transition metals could enhance gases generation and oil cracking, but had no effect on methane yield or enrichment (Lewan et al., 2008). Sulfur, a unique active element, is significant for hydrocarbons generation and also considered as a key factor for low matured crude oils formation (Lewan, 1998). However, the catalytic role of minerals/elements for hydrocarbon gases generation is still unclear. Different viewpoints, experimental designs and methods may lead to different opinions about the catalytic action of the same substances. The existence of clay minerals may play a significant role in the formation of isomeric hydrocarbons, but may not do so for the natural gases dominated by n-alkanes. Such a disagreement may suggest that the effect of free radicals on hydrocarbon gases generation should be important, but received less attention (L.M. Wu et al., 2012). In comparison with clay minerals, the catalytic effect of iron-bearing minerals has been limitedly studied and the action mechanism is still unclear (Bakr et al., 1991; Larsen and Hu, 2006). There is always more than one geological catalyst together with organic matters in natural source rocks. By considering various kinds of minerals and metal elements enriched and co-existed in coal, it should be emphasized that a mixed or complex catalytic effect of these minerals/metal elements may contribute to hydrocarbon gases generation. Based on an investigation of coal stratum in China, smectite, pyrite and iron are the three major minerals/ transition metal catalysts in coal with contents of 10e30%, 2e15%, and 0.2e0.4%, respectively (Li, 1992; Tomkins, 2010; Y.Y. Wu et al., 2012; Ma et al., 2015). According to the mineral and elemental concentrations in the source rocks mentioned above, a mixed catalyst set was prepared for the pyrolysis experiments, (smectite 30%) þ (FeS2 10%) þ (Fe 0.5%) with kerogen, to reveal the effect of minerals/transitional metal on natural gases generation and explain their mechanisms.
using a Rock-Eval instrument, which confirmed the kerogen used as typical type-III accordingly. The pyrite and smectite used in this study were purchased from National Research Center for Certified Reference Material (NRCCRMS) of China. The purity of the pyrite was more than 95%. Chemically pure iron (powder or brocks) was produced by Tianjin Chemical Reagent Co., Ltd. 2.2. Pyrolysis experiments Due to shortage of suitable commercial tools, the homemade glass tubes were used for all the experiments, whose size is approximately 2 cm of outside diameter, 1.8 cm of inside diameter and 23 cm in length. Five groups (sets) of pyrolysis experiments were performed for different test samples, including group A, the kerogen only (about 200e950 mg); group B, kerogen and smectite (about 200e950 mg kerogen and 30 wt.% smectite); group C, kerogen and pyrite (about 200e950 mg kerogen and 10 wt.% pyrite); group D, kerogen and iron (about 200e950 mg kerogen and 0.5 wt.% iron); group E, kerogen and compositional catalysts (about 200e950 mg kerogen þ 30 wt.% smectite þ 10 wt.% pyrite þ 0.5 wt.% iron). After the test samples loaded, the open end of each tube was purged with argon before being squeezed in a water stopper to create an initial seal, which was subsequently welded in the presence of argon. During welding, the previously welded end was submerged in cold water to prevent reactant heating and then the tubes with samples were heated to a desired temperature for 3 h (initial heating) followed by isothermally heating for 72 h. The temperature control was within ±1 C. After the pyrolysis experiment, gases were qualitatively collected by using a proper displacement method. 2.3. Gas composition measurement
2. Experimental
All gas samples were analyzed for chemical and isotopic composition at Key Laboratory of Petroleum Resources Research, Chinese Academy of Sciences (Lanzhou, China). The composition of major gases were determined by using both static mass spectrometer (model MAT271) and gas chromatography (model GC5890A), with errors below 5% based on repeated measurements of in-house standards, and with a main gas limit of 1 ppm.
2.1. Samples
€ssbauer spectroscopy 2.4. Mo
The kerogen (Ker) used in this study was obtained from a brown coal (Ro ¼ 0.47%) from the eastern Junggar Basin, NW China, whose geochemical properties were summarized in Table 1. The basic geochemical parameters suggested that it was a low-matured sample dominated by humic matters (type III) and suitable for the study on hydrocarbon gases generation by pyrolysis experiment. Soxhlet extraction of the coal was performed for 72 h by using trichloromethane to remove the original soluble bitumen. The bitumen-free powder sample (approximately 200 meshes) was treated with HCl and HF to remove the carbonate and silicate minerals present in the sample by using the methods described in detail in the literature (Pan et al., 2008). Clay minerals and ironbearing minerals were not detected in the prepared kerogen by X-Ray powder Diffraction (XRD, data not shown here). The organic carbon content of the prepared kerogen was 85.6%, as measured by
€ssbauer spectra were measured at 293 K using a MA-260 The Mo €ssbauer spectrometer with a g-ray source of (Bench MB-500) Mo 0.925 GBq, 57Co/Rh. The measurement and curve fitting procedures were described elsewhere (Matsuo et al., 1994). The measured spectra were fitted to Lorentzian line shapes using standard line shape fitting routines. The half-width and peak intensities of the quadruple doublet were constrained to be equal. Isomer shifts were expressed with respect to the centroid of the spectrum of metallic iron foil (56Fe > 99.85%). 3. Results 3.1. Gas composition The amounts of SC1e5 were consistently increased from a range
Table 1 Total organic carbon (TOC) and Rock-Eval pyrolysis data from the experimental samples. Sample No.
Depth (m)
Stratum
TOC (%)
S1-peak (mg/g rock)
S2-peak (mg/g rock)
Ro (%)
Tmax ( C)
OS
1846
J1b
73.8
13.6
177.1
0.47
428
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3
Table 2 Types and concentrations (mg/g Ker) of gas generated during the pyrolysis experiments at 250 C/72 h, 300 C/72 h, 350 C/72 h and 400 C/72 h. Gases content (ml/g)
Ker
Ke þ FeS2
Ke þ Fe
Ker þ Sm
Ker þ mixed Catalysts
250 C Hydrogen sulfide Carbon dioxide Methane Ethane Ethene Propane Propene i-Butane n-Butane neo-Pentane i-Pentane n-Pentane Hydrocarbon total P Dryness (C1/[ C1eC4]) i/n-Butanes i/n-Pentane Ethene/Ethane Propene/Propane
n.d. 38.1 0.41 0.07 0.05 0.24 0.08 0.10 0.26 n.d. 0.26 0.10 1.57 0.34 0.38 2.60 0.71 0.33
n.d. 36.3 4.56 2.12 0.13 1.81 0.43 0.28 0.54 Trace 0.23 0.32 10.4 0.46 0.52 0.72 0.06 0.24
Ker þ FeS2
Ker þ Fe
Ker þ Sm
Ker þ mixed catalysts
n.d. 49.5 1.30 0.68 0.28 0.77 0.30 0.08 0.10 Trace 0.06 0.03 3.60 0.37 0.80 2.00 0.41 0.39
n.d. 24.8 0.77 0.35 0.10 0.31 0.09 0.05 0.03 n.d. 0.03 0.04 1.77 0.45 1.67 0.75 0.29 0.29
n.d. 16.4 1.13 0.51 0.33 0.45 0.26 0.21 0.08 n.d. 0.19 0.03 3.19 0.38 2.63 6.33 0.65 0.58
n.d. 59.5 1.91 0.74 0.53 0.75 0.53 0.18 0.08 0.03 0.12 0.03 4.90 0.40 2.25 4.00 0.72 0.71
0.05 62.1 32.4 13.5 0.08 10.7 0.39 1.92 4.07 0.32 0.80 1.32 65.5 0.51 0.47 0.61 0.01 0.04
n.d. 52.3 25.1 6.58 0.44 7.09 0.15 0.54 0.98 0.10 0.45 0.58 42.0 0.61 0.55 0.78 0.07 0.02
n.d. 77.2 27.0 14.5 0.79 9.31 0.72 1.92 2.69 0.10 0.77 0.74 58.5 0.47 0.71 1.04 0.05 0.08
Trace 84.9 37.4 16.7 0.76 11.9 0.61 3.26 4.63 0.23 1.74 2.06 79.3 0.50 0.70 0.84 0.05 0.05
300 C n.d. 44.2 0.39 0.07 0.05 0.16 0.06 0.05 0.10 n.d. 0.13 0.06 1.07 0.44 0.50 2.17 0.71 0.38
n.d. 35.8 0.32 0.05 0.01 0.05 0.01 0.03 0.20 n.d. 0.13 0.16 0.96 0.48 0.15 0.81 0.20 0.20
n.d. 40.3 0.46 0.07 0.10 0.24 0.12 0.06 0.19 n.d. 0.18 0.12 1.54 0.37 0.32 1.50 1.43 0.50
n.d 39.8 0.56 0.09 0.18 0.22 0.15 0.08 0.13 0.13 0.29 0.16 1.99 0.40 0.62 1.81 2.00 0.68
350 C Hydrogen sulfide Carbon dioxide Methane Ethane Ethene Propane Propene i-Butane n-Butane neo-Pentane i-Pentane n-Pentane Hydrocarbon total P Dryness (C1/[ C1eC4]) i/n-Butanes i/n-Pentane Ethene/Ethane Propene/Propane
Ker
n.d. 35.6 0.71 0.31 0.15 0.33 0.13 0.05 0.03 n.d. 0.03 0.01 1.75 0.42 1.67 3.00 0.48 0.39 400 C
Trace 49.1 7.43 3.94 0.14 3.14 0.54 0.49 0.91 Trace 0.32 0.45 17.4 0.45 0.54 0.71 0.04 0.17
n.d. 40.7 6.00 2.48 0.19 1.10 0.21 0.18 0.34 n.d. 0.06 0.39 11.0 0.57 0.53 0.15 0.08 0.19
n.d. 33.2 6.36 4.37 0.66 2.14 0.71 0.54 0.85 n.d. 0.71 0.58 16.9 0.41 0.64 1.22 0.15 0.33
n.d. 28.5 8.50 5.45 0.80 2.59 0.77 0.65 0.85 0.03 1.00 0.32 21.0 0.43 0.76 3.13 0.15 0.30
n.d. 59.5 24.9 10.7 0.08 8.21 0.39 1.40 2.80 Trace 0.61 0.90 50.0 0.51 0.50 0.68 0.01 0.05
Sm-smectite.
of 0.96e1.99 to 50.0e79.3 mg/g Ker during the pyrolysis experiments along with temperature increased from 250 to 400 C. The methane production was consistently increased from a range of 0.32e0.46 to 27.0e37.4 mg/g Ker. The lowest production of C1eC5 was in a range of 0.01e0.56 mg/g Ker for the five sets of experiments at 250 C while the lowest synthesis of C4 and C5 appeared at 300 C, being 0.08 and 0.04 mg/g Ker. The highest yields of C1-5 were observed at 400 C, and the values decreased gradually with increase in carbon numbers during all experiments (Table 2, Fig. 1 and Fig. 2). The produced carbon dioxide was increased from 38.1 to 44.2 to 52.3e84.9 mg/g Ker with temperature increased from 250 to 400 C. The hydrogen sulfide (H2S) was detected in the kerogen þ pyrite experiment at 400 C (Table 2 and Fig. 2). A smell of H2S gas was distinguished after heated the samples at 300 and 350 C, indicating the decomposition of pyrite probably started during the pyrolysis experiments since 300 C and onward. In comparison with gas composition, smectite, pyrite and their complex catalysts were all shown with positive effects on the production of SC1e5 hydrocarbons. The mixed catalysts showed the best effects with the highest value of 79.3, 21.0, 4.90, 1.99 mg/g from 400 to 250 C, respectively. Pyrite was the second positive catalyst with the highest value of 65.5, 17.4, 3.60, 1.07 mg/g from 400 to 250 C, respectively. And then smectite had the catalytic effect in the highest values of 58.5, 16.9, 3.19, 1.54 mg/g from 400 to 250 C, respectively. However, there was almost no obvious effect for iron itself as catalyst, the yields of SC1e5 hydrocarbons were almost the
same as Ker alone. The mixture of Ker þ Fe had the highest values of C1/SC1e4 ratios at any temperature stage, ranging from 0.34 to 0.61 with temperature increased from 250 to 400 C. The mixture of Ker þ smectite had the lowest value (0.41 and 0.47) at 350 and 400 C (Table 2 and Fig. 3). The dryness of produced gases was sequentially increased from 250 to 400 C in kerogen alone whereas the ratio was slightly increased at 250 C and decreased at 300 C and then increased again from 350 C to 400 C in the presence of pyrite, smectite and mixed catalysts. The ratio of methane/ethane was the highest at 250 C, in a range of 5.57e6.57. The highest ratio of ethane/propane was observed in the experiment at 350 C, in a range of 1.17e2.25. In the experiment with Ker þ FeS2, this ratio was increased with temperature raised. However, other four sets of experiments revealed this ratio firstly increased from 250 to 350 C and then decreased at 400 C. Based on the iC4/nC4 and iC5/nC5 ratios, the five kinds of experiments could be classified into three groups. Group one included the two experiments using smectite or mixed catalysts, having the highest ratios. Group two was the only experiment with kerogen alone, with medium ratios. And group three, including the two experiments using pyrite or iron, had the lowest ratios except at 250 C. The highest ethene/ethane ratio appeared in the experiment with smectite at 300 C, the other four sets of experiments, however, had the highest value at 250 C, in a range of 0.2e2.6, and then all decreased to a value below 0.1 with temperature increased to 400 C. The highest ratio of propene/propane
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also appeared in the experiment with smectite at 300 C whereas the ratios in other three sets of experiments (Ker alone, Ker þ Fe and Ker þ FeS2) increased at 300 C, and then decreased at 350 C and 400 C. However, this ratio in the experiment of Ker þ mixed catalysts was decreased gradually with temperature raised. Finally, the lowest value appeared in all sets of experiments at 400 C, ranging from 0.02 to 0.08 (Table 2). 3.2. Mature scale The results of Rock - Eval experiments showed that Tmax was rose from 435 to 467 C during various pyrolysis experiments. The values of Tmax were about 436 ± 2 C, 443 ± 2 C, 456 ± 2 C and 466 ± 3 C after the pyrolysis experiments at 250 C, 300 C, 350 C and 400 C, respectively (Fig. 6). These results indicated that (1) the maturity of the sample rose with the increase of temperature; and (2) the catalysts used in this study might not change the maturity obviously.
Fig. 1. Total amounts of C1eC5 hydrocarbon gases produced at 250e400 C.
€ssbauer spectroscopy 3.3. Mo € ssbauer spectra for the residue materials of the pyrolysis Mo experiments are shown in Fig. 7 and their parameters summarized
Fig. 2. C1eC5 hydrocarbon gasesyeilds during various kerogen and minerals/metal pyrolysis: (aed) the experiments at 250, 300, 350, and 400 C, respectively.
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5
4. Discussion 4.1. Effect of minerals and transition metal on hydrocarbon gases generation In the presence of minerals and iron, the methane and C1-5 yields were almost increased by 20e150% (except at 250 C), and the catalytic efficiency was shown in the following order, mixed materials > pyrite > smectite > iron. 4.1.1. Dryness coefficient (C1/SC1e4) As demonstrated in Table 2 and Fig. 1b, the ratio of C1/SC1e4 was continuously increased from the lowest (0.34) at 250 C to the highest (0.51) at 400 C during the experiments of kerogen alone, whereas the ratio of C1/SC1e4 decreased from higher value at 250 C to the lowest value at 300 C, and then again, this ratio was increased at 350 C and reached the highest value at 400 C in the presence of smectite, pyrite, metal iron and mixed catalysts. This might suggest that the minerals and iron could be able to induce cracking of methyls and related functional groups of kerogen at 250 C and this ratio increased with temperature raised and/or maturation increased. The ratio of C1/SC1e4 was very similar at temperature range of 350e400 C among the five cases except the experiment using iron, being relatively higher as discussed before (Fig. 3), indicating that mineral acidity could influence this ratio considerably in the pyrolysis experiments after the amount of produced bitumen reached to the highest level. The average ratio of C1/SC1e4 for natural gas was about 85 wt% (Mango, 1994). This ratio was substantially lower in our experiments in comparison with natural gases (Table 1 and Fig. 3) (Mango, 1994). However, it is a common phenomenon that the gaseous hydrocarbons produced by experimental pyrolysis are relatively poorer in methane than in the natural gases (Mango, 1991). Lewan, 1998 also revealed the lower ratio for Woodford Shale and its kerogen under various conditions ranged from 40.4 to 51.5% at about 330 C (72 h) and from 50.1 to 66.6% at 350 C (72 h).
Fig. 3. Dryness of gaseous hydrocarbon produced at 250e400 C against temperature.
in Table 3. Three sets of absorption peaks appeared in the spectra, including one doublet and two sextets. The doublet with isomer shift (IS) approximately 0.30 mm/s and quadrupole splitting (QS) approximately 0.60 mm/s was ascribed to pyritic iron (pyr-Fe2þ) (Dyar et al., 2006). The two sextets with magnetic hyperfine field (Bhf) approximately 28.0 T and 24.0 T were attributed to pyrrhotite (Dyar et al., 2006). In the Ker þ pyrite experiments, the appearance of doublet was measured as pyr-Fe2þ at 250 C, since the pyrite was initial material and there were almost no phase transformation at €ssbauer low temperature stage. With temperature raise, the Mo spectrum data revealed that the transformation of pyrite into pyrrhotite initiated at 300 C, and 40.0% and 79.9% of pyrite were transformed to pyrrhotite at 350 and 400 C, respectively, revealed significantly the pyrite initiation to phase transformation (Table 3 and Fig. 7). Furthermore, the IS of pyrite was gradually increased from 0.290 to 0.301 mm/s with the raise of pyrolysis temperatures from 250 to 400 C, indicating that the Coulomb force between the nucleus and density distribution of extra-nuclear electrons gradually decreased (Dyar et al., 2006). This variation trend suggested the higher chemical activity of pyrite. In addition, with increase of pyrolysis from 250 to 400 C, the Bhf of pyrrhotite was also increased, probably representing an increasing degree of pyrrhotite crystallization (Ma et al., 2016).
Table 3 Values of the hyperfine parameters from the best fits of
57
4.1.2. Ratios of iC4/nC4 and iC5/nC5 As demonstrated in Table 2 and Fig. 4, the ratios of iC4/nC4 and iC5/nC5 were substantially higher at 250e400 C in the presence of smectite and mixed catalysts than that in kerogen alone. This phenomenon might be interpreted as many acidic sites on the structural surface of smectite. Previous pyrolysis studies suggest that during petroleum formation process, n-alkanes are formed by free radical reactions, whereas branched iso-alkanes formed via two processes: (1) free radical cracking of kerogen and bitumen components and branched fragments; and (2) the carbonium ion reaction of a-olefins with protons promoted under acidic
€ssbauer spectra for the samples at room temperature (Ma et al., 2016). Fe Mo
Sample ID.
Iron species
Relative content, %
IS (mm/s)
Pyrite PS-250 PS-300
pyr-Fe2þ pyr-Fe2þ pyr-Fe2þ
PS-350
pyr-Fe
2þ
PS-400
pyrrhotite pyr-Fe2þ
100 100 93.2 ± 0.1 4.0 ± 0.1 2.8 ± 0.2 60.0 ± 1.0 24.5 ± 0.9 15.5 ± 0.8 20.1 ± 0.9 50.5 ± 0.8 29.4 ± 0.6
0.29 0.29 0.29 0.44 0.58 0.30 0.44 0.83 0.30 0.69 0.58
pyrrhotite
± ± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.01 0.04 0.01 0.02 0.04 0.023 0.01 0.01 0.02
QS (mm/s)
HW (mm/s)
Hi/T and compound
0.60 ± 0.00 0.60 ± 0.00 0.60 ± 0.00 0.19 ± 0.06 0.13 ± 0.03 0.60 ± 0.00 0.19 ± 0.06 0.38 ± 0.05 0.60 ± 0.00 0.29 ± 0.05 0.13 ± 0.03
0.31 ± 0.01 0.34 ± 0.02 0.31 ± 0.01 0.37 ± 0.02 0.19 ± 0.03 0.35 ± 0.02 0.37 ± 0.02 0.39 ± 0.03 0.31 ± 0.010 25 ± 0.03 0.19 ± 0.03
pyrite pyrite pyrite 27.50 ± 28.00 ± pyrite 27.64 ± 28.02 ± pyrite 27.76 ± 28.10 ±
0.05 0.02 0.15 0.11 0.09 0.02
Note: where IS the isomer shift (relative to a-Fe at RT), QS is the quadrupole splitting, HW is the half width at half maximum, Hi is the hyperfine magnetic field, relative content is the relative spectral absorption area for each specie.
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Fig. 4. Isoparaffin and n-alkane gas ratios from the pyrolysis experiments at 250e400 C.
Fig. 5. Olefin and alkane gas ratios from the pyrolysis experiments at 250e400 C.
conditions (Kissin, 1987). Some pyrolysis experiments and chemical degradation studies revealed that the coal type of kerogen rarely bear branched fragments (Blokker et al., 2001). The carbonium ion reaction should be primarily responsible for the branched isoalkane formation in the pyrolysates from kerogen. Tannenbaum et al. (1986) believed that this phenomenon is indicative of cracking via carbonium ion mechanism that is induced by acidic sites of clay during anhydrous pyrolysis. The ratios of iC4/nC4 and iC5/nC5 were generally low for the experiments in the presence of pyrite and iron than those in kerogen alone (Table 2 and Fig. 4a and b), which might reflect the nonacidic (basic) nature of these two kinds of minerals. Therefore, the traditional theory did not explain the iC4/nC4 and iC5/nC5 abnormalities witnessed at 300 C during the pyrite and iron experiments. We proposed that pyrite and iron could be able to induce the rearrangement of alkanes during kerogen degradation at 300 C, and new alkanes might be formed with high isomers and stable under such conditions.
Fig. 6. Mature scale (Tmax) of samples after pyrolysis experiments at 250e400 C.
4.1.3. Hydrogenation of gas olefins It was suggested that anhydrous pyrolysis could produce a large amount of olefins (Seewald, 2003). The abundance of produced olefins in the present experiments was quite consistent with
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pyrrhotite
Relative Intensity
Pyr-Fe2+
7
(Schematic 1). During the transformation of organic matters in the source rocks into hydrocarbons, the clay minerals may exhibit different catalytic mechanisms in decarboxylation reaction and thermal cracking reaction of organic matters. In the decarboxylation reaction, Lewis acid of clay minerals may receive an electron from organic matter and the carboxylic acid will loss CO2, leading to the formation of free-radicals. The free-radicals may further cause rearrangement, leading to the cleavage of CeC bonds and formation of shorter-chain free hydrocarbons. The reaction was proposed as in eq. (1) (Johns, 1979).
CH3 ðCH2 Þn COOH/CH3 ðCH2 Þn1 $CH2 þCO2 þ $H
(1)
In the thermal cracking reaction of organic matters, the clay minerals act as a Brønsted acid and provide proton (Hþ) for organic matters. The Hþ may be derived from the dissociation of adsorbed water and interlayer water molecules. Mainly through the formation of transition state of the carbonium-ion reaction can be illustrated in eq. (2) (L.M. Wu et al., 2012).
k n MðH2 OÞx /n MðH2 OÞx1 $OHz1 þ nHþ
-10 -8 -6 -4 -2 0 2 4 6 Velocity (mm/s)
8 10
€ ssbauer spectra of pyrolysis experiment samples analyzed at room Fig. 7. The Mo temperature (modified from Ma et al., 2016).
previous studies (Table 1, Fig. 5a and b). Gaseous olefins were unstable and might be readily hydrogenated into saturated hydrocarbons in the presence of iron and pyrite. The presence of pyrite and iron should be essential for the generation of n-alkanes from the n-alkyl components and brown coal might play multiple roles for these conversions, a major one being the role of a hydrogen donor. Therefore, it might be suggested that a hydrogen radical from the intermediated compounds to the organic radical by a radical mechanism. The hydrogenation rate of olefins substantially influenced the ratio of iso-alkane/n-alkane, olefins released from kerogen and bitumen degradation were more likely hydrogenated into n-alkanes and less likely transformed into iso-alkanes when the hydrogenation rate of olefins was higher. As a result, the iC4/nC4 and iC5/nC5 ratios were low in the experiments for kerogen alone. In addition, the low ratios of ethene/ethane and propene/propane might indicate high hydrogenation rate of olefins in the experiment by using iron and pyrite in comparison with that by using smectite and mixed catalysts (Fig. 5a and b).
(2)
where M is the exchangeable cation, Z is the charge, x is the amount of water molecules coordinated with M, n is the amount of exchangeable cations, k is the ionization (dissociation) constant for the interlayer system. Actually, Brønsted acid and Lewis acid cannot be isolated, and they can be transformed into each other under certain conditions. When the surface of clay minerals adsorbed water molecules, Lewis acid will have strong affinity for electrons, and may share a pair of electrons with the hydroxyl (eOH) in the water and make the hydroxyl (eOH) firmly adsorbed on the Lewis acid surface. The remaining Hþ will be easily released for weak binding force, leading to the conversion of Lewis acid to Brønsted acid. When the clay minerals are dehydrated, the Brønsted acid will be gradually transformed into Lewis acid (Wang et al., 2006). From the thermodynamic point of view, the loss of Hþ in the Brønsted acid need to overcome the binding of intramolecular hydrogen bond, leading to increase of the system energy. Therefore, it could be occurred when the external energy is sufficient. However, the Lewis acidic sites get an electron and the energy of the system will be reduced, and the occurrence of this reaction could not require higher external energy. Therefore, Lewis acid sites on the clay minerals are more active at low temperature (250e300 C) conditions. Furthermore, the oil and gas formation processes are associated with the transformation process of montmorillonite to illite. During dehydration of montmorillonite, the activity of Brønsted acid is lower and the activity of Lewis acid is higher. In a summary, the main catalytic mechanism of clay minerals in nature is free-radical mechanisms at low temperature.
4.2. Action mechanism 4.2.1. Clay minerals (Smectite) The catalytic mechanism of clay minerals is mainly related to the Brønsted acid and Lewis acid which are on the clay surface. These two acidic sites could form two kinds of catalytic mechanism in hydrocarbon generation, such as carbonium-ion mechanism and free-radical mechanism (Brooks, 1948; Johns, 1979; Lei et al., 1997; Reddy et al., 2009; L.M. Wu et al., 2012). In the catalytic reaction for hydrocarbons generation, Brønsted acid and Lewis acid are normally active, but their role in the catalytic reaction process is different. The previous studies showed that the cracking reaction, polymerization reaction and hydrogenation reaction are mainly presented in the Brønsted acidic sites, and the decarboxylation reaction of organic matters is mainly related to the Lewis acid sites
4.2.2. Pyrite €ssbauer parameters, there was no change According to the Mo occurred in pyrite during pyrolysis at 250 C, and there was limited contribution of pyrite in hydrocarbon gases generation. The transformation of pyrite into pyrrhotite initiated at 300 C and the transformation was significantly enhanced with the raise of temperature to 350 and 400 C. Therefore, a clear increase of large radical concentration was observed in the kerogen þ pyrite system which was steady with the transformation of pyrite into pyrrhotite. This FeS2 based amplification in radical concentration might be ascribed to the production of nascent sulfur during the pyrite transformation to pyrrhotite (sulfur production in kerogen þ pyrite experiment have been shown by GC-MS results). This sulfur has strong affinity for hydrogen, and by a free radical procedure it
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X. Ma et al. / Marine and Petroleum Geology xxx (2017) 1e9
excerpts hydrogen from kerogen (Ma et al., 2016). The process may occur as following:
FeS2 ðpyriteÞ/Fe1x S ðpyrrhotiteÞ þ S ð0 < x < 1Þ
(3)
S þ RH/HS$ þ R
(4)
HS$ þ RH/H2 S þ R$ðR kerogenÞ
(5)
It is clearly proposed that the enhanced formation of free radicals occurs through hydrogen transfer from kerogen to sulfur. Obviously, under such conditions the sulfur operated as a catalyst
M
CH3
CH3
conventional first-order kinetics (Lewan et al., 1979); and then the olefins were either hydrogenated to n-alkanes or catalytically converted into natural gas (eq. (6) and (7)). In our experiment, the ethene/ethane and propene/propane ratios were decreased with iron addition, but the increase degree of methane was very limited, indicating the catalytic effect of transition metal in low contents was inefficient. In addition, such phenomena could be related to the poisoning of catalytic sites by polar-rich bitumen (Lewan et al., 2008).
i h Ker/ n Cx¼ þH2 Fe C1 þC2 þC3 …nCx1 !
+ H2
M
H M
(6)
+ CH4
(7)
for radical production in kerogen. Similar enhanced radical formation by pyritic sulfur has been studied in coal (Ma et al., 2016).
4.3. Mixed catalyst
4.2.3. Transition metal (iron) The chemical state of iron was not changed before and after the €ssbauer parameters (Table 3). The pyrolysis experiment from Mo catalytic scheme of transition metal on natural gas produced from organic matter had been proposed of hydrogenation reaction (Mango, 1996). Kerogen was thermally converted into olefins by
In comparison with other geological catalysts, the mixed catalysts have the highest catalytic efficiency on the generation of methane and the amount of C1-5. Furthermore, although other geological catalysts could be able to improve the yield of gaseous hydrocarbons, the dryness of natural gas would be decreased whereas the composite catalysts were not. The pyrolysis results also suggested that the mixed catalysts could have better catalytic
Schematic 1. Catalytic mechanism of clay minerals based on carbonium ion reaction and free radical cracking (Modified from Johns, 1979; L.M. Wu et al., 2012).
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effect on methane generation and the amount of C1-5 at low temperature stage (250 C) which are quite similar to natural gases. Therefore, the efficiency of mixed catalysts to natural gases generation should be very important based on the present study, and enough attention must be paid on this matter once to evaluate gas formation under really geological conditions because there are often more than one kind of catalysts in natural source rocks. 5. Conclusions The effects of smectite, pyrite, iron and their mixed catalysts on the gaseous hydrocarbons generation from coal type of kerogen were performed in this study and some new and improved findings about the processes can be summarized as the followings: (1) Smectite, pyrite, iron and their mixture catalysts could all be able to improve the gaseous hydrocarbons generation and also reduce the dryness co-efficiency of produced gases for almost all experiments except that at 250 C. (2) The ethene/ethane and propene/propane ratios were generally decreased with temperature increasing. However, these ratios were increased in the experiments by adding smectite and mixed catalysts and slightly decreased by adding pyrite and iron. The interactions among minerals, transition metals, and kerogen may lead to alkane's rearrangement, resulting in high isobutane/n-butane and isopentane/n-pentane ratios with smectite and mixed catalysts and low isobutane/n-butane and isopentane/n-pentane ratios with pyrite and iron addition. (3) Pyrite may act indirectly as a catalytic agent or inducer via sulfur, which may enhance free radical formation and hydrocarbons generation. Mixed catalysts have not only the effects of each catalyst but also the highest catalytic efficiency, which may reflect the realistic geological situation of minerals and transition metals on hydrocarbons generation. Acknowledgments This study was supported by China National Major S&T Projects (2016ZX05007-01) and the National Natural Science Foundation of China (41402129), the Key Laboratory of Petroleum Resources, Gansu Province (1309RTSA041, Y623JJGJ1MXX). Chinese Academy of Sciences issued the “100-Talent” Program for QH and Japan Society for the Promotion of Science (JSPS) awarded a fellowship to GZ (BR161304). References Attar, A., 1978. Chemistry, thermodynamics and kinetics of reactions of sulphur in coal-gas reactions: a review. Fuel 57 (4), 201e212. Bakr, M., Akiyama, M., Yokono, Y., Sanada, M., 1991. Role of pyrite during the thermal degradation of kerogen using in situ high-temperature ESR technique. Energy & Fuels 5, 441e444. Baldwin, R., Vinciguerra, S., 1983. Coal liquefaction catalysis: iron pyrite and hydrogen sulphide. Fuel 62, 498e501. Blokker, P., van Bergen, P., Pancost, R., Collinson, M.E., de Leeuw, J.W., Damste, J.S.S., 2001. The chemical structure of Gloeocapsomorpha prisca microfossils: implications for their origin. Geochim. Cosmochim. Acta 65, 885e900. Brooks, B.T., 1948. Active-surface catalysts in formation of petroleum. AAPG Bull. 32 (12), 2269e2286. Chen, H., Li, B., Zhang, B., 2000. Decomposition of pyrite and the interaction of pyrite with coal organic matrix in pyrolysis and hydropyrolysis. Fuel 79 (13), 1627e1631. €ssbauer specDyar, M., Agresti, D., Schaefer, M.W., Grant, C.A., Sklute, E., 2006. Mo troscopy of Earth and planetary materials. Annu. Rev. Earth Pl. S. C. 34, 83e125.
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Please cite this article in press as: Ma, X., et al., Influence of minerals and iron on natural gases generation during pyrolysis of type-III kerogen, Marine and Petroleum Geology (2017), http://dx.doi.org/10.1016/j.marpetgeo.2017.01.012