Pyrolysis of 1-methylnaphthalene involving water: Effects of Fe-bearing minerals on the generation, C and H isotope fractionation of methane from H2O-hydrocarbon reaction
Introduction
As known, the occurrence of petroleum occurs in complex inorganic environments (Tissot and Welte, 1984). To date, numerous hydrous and hydrothermal experiments have been conducted to simulate organic-inorganic interactions. The mechanism and kinetics for high-temperature reactions in experiments likely differ from those for natural maturation in sedimentary basins, where lower temperatures exist over long periods of geologic time. However, pyrolysis experiments are beneficial for understanding the effects of inorganic substances on petroleum generation. Previous experimental studies have shown that inorganic fluids and minerals affect and even govern the generation and evolution of hydrocarbons at elevated temperatures (Hoering, 1984, Lewan, 1997, McCollom and Seewald, 2001, Seewald, 2001, Seewald, 2003, Pan et al., 2009, Lewan and Roy, 2011, Cai et al., 2017). It has been suggested that water (H2O) participates in the maturation of organic matters and the decomposition of hydrocarbons (Lewan, 1997, Seewald, 2001, Seewald, 2003). The hydrous pyrolysis of low-mature organic matters or kerogens led up to a 20–30 wt% increase in the yields of liquid products or oils compared with anhydrous pyrolysis (Lewan, 1997). The presence of H2O also resulted in higher yields of gas products in pyrolysis experiments involving hydrocarbons (He et al., 2018, Hoering, 1984), where inorganic hydrogen (H or 2H) and oxygen (O) were transferred from H2O to pyrolysis products and organic matters (Lewan, 1997, Schimmelmann et al., 2001, Lewan and Roy, 2011, He et al., 2018). As a result, the chemical and isotopic compositions of oil and gas can be altered by H2O-organics reactions via ionic or free radical mechanism (Lewan, 1997, Lewan and Roy, 2011, He et al., 2018). In several hydrothermal experiments, the hydrogen isotopic ratios (δ2H) of hydrocarbon gases were affected by H transfer from H2O (Reeves et al., 2012, Gao et al., 2014, He et al., 2019). In this regard, the hydrogen isotope rollover of methane (CH4) at high maturity is always attributed to the contribution of H from H2O (Burruss and Laughrey, 2010, Zumberge et al., 2012, Zhang et al., 2018). However, Wei et al. (2018) suggested that H2O had a limited effect on the δ2H of CH4 (δ2H1) during the pyrolysis of organic matters in the presence of H2O at low temperatures (60–100 °C). Such a discrepancy may be interpreted by two isotope fractionation effects dominating H exchange or transfer, including the equilibrium isotope effect (EIE) and kinetic isotope effect (KIE) (Horibe and Craig, 1995, Sessions et al., 2004, Wang et al., 2009, Reeves et al., 2012, Suda et al., 2014, He et al., 2019). For EIE, δ2H1 due to H transfer from H2O is governed by the equilibrium constant (αEIE) and the δ2H of water (Horibe and Craig, 1995, Wang et al., 2009, Suda et al., 2014). For KIE, δ2H1 is controlled by the H transfer rate, which is highly dependent on the experimental temperature (Sessions et al., 2004, Reeves et al., 2012). The equilibrium effect and αEIE have been previously well chronicled by theoretical calculations and hydrothermal experiments (He et al., 2018, Wang et al., 2009, Horibe and Craig, 1995). Unfortunately, the kinetic effect and H transfer rate from H2O to CH4 still remain unclear, making it challenging to determine whether H transfer from H2O to CH4 reaches equilibrium in particular geological settings.
In addition, H2O-organics reactions can occur via indirect pathways initiated by H2O-minerals interactions. For example, Fe-bearing species (i.e., pyrite, pyrrhotite, magnetite, siderite) are widely distributed in organic-rich shales and even in hydrocarbon reservoirs (Habicht and Canfield, 1997, Posth et al., 2014, Abubakar et al., 2015). Fe-bearing species or minerals exhibit catalytic effects on hydrocarbon generation both in the Fischer-Tropsch synthesis and the maturation of organic matters (Fu et al., 2007, Taran et al., 2007, Zhang et al., 2013, Ma et al., 2016). Studies on the pyrolysis of organic matters revealed that Fe-bearing minerals alter the yields and, chemical and isotopic compositions of gas products (Su et al., 2006, Ma et al., 2016, Cai et al., 2017). Moreover, some reducible Fe-bearing minerals can be oxidised by H2O to release H2, which then reacts with organic matters or hydrocarbons under hydrothermal conditions (Kishima, 1989, Seewald, 2001, Milesi et al., 2016). The indirect hydrogenation by H2 derived from H2O is critical for the conversion of carbon dioxide (CO2) to abiotic CH4 in oceanic peridotites, volcanoes and deeply buried formations (Tassi et al., 2012, McCollom, 2016, Milesi et al., 2016, Etiope et al., 2017). Hydrothermal experiments with reduced-Fe minerals presented an increase in hydrocarbon gas yield from kerogens (Jin et al., 1999). Furthermore, the different mechanisms for H2O-organics reactions initiated by Fe-bearing minerals resulted in distinct C- and H-isotope fractionation (i.e., H transfer from H2O to CH4) (Cai et al., 2017). Therefore, it is necessary to understand the effects of different Fe-bearing minerals on gas generation from H2O-organics reactions to potentially evaluate and identify the origin of natural gases.
Herein, we conducted a series of isothermal pyrolysis experiments involving 1-methylnaphthalene (1-MNa), H2O, and three Fe-bearing minerals (i.e., FeS2, Fe3O4, and FeCO3) using a gold-tube pyrolysis system. Combined with the analysis of gas products and kinetic calculations, the effects of different Fe-bearing minerals on gas generation were studied. X-ray diffraction (XRD) analysis was performed to ascertain the evolution of these minerals. We also addressed the mechanisms and isotope fractionation for CH4 generation from the H2O-hydrocarbon reaction with Fe-minerals.
Section snippets
Sample and reagents
The model hydrocarbon 1-MNa used in this study had a purity of over 99.5% (J&K Chemical Ltd.). The carbon (δ13C) and hydrogen (δ2H) isotopic ratios of 1-MNa were −28.3‰ and −120.3‰, respectively. Deionized water with a δ2H of −64.0‰ was used. Inorganic minerals included commercially available pyrite (FeS2), magnetite (Fe3O4) and siderite (FeCO3). The XRD spectra indicated that these minerals had a purity of over 99%.
Gold-tube pyrolysis experiments
All pyrolysis experiments were conducted using a gold-tube pyrolysis system (He
Yields and chemical compositions of gas products
Table 1 shows the yields and chemical and isotopic compositions of individual gas products in isothermal pyrolysis of 1-MNa involving H2O with and without the Fe-bearing minerals. For comparison, the units for all gas yields were converted to millilitres (mL) per gram (g) of 1-MNa (mL/g). With Easy%Ro ranging from 0.79% to 2.31%, the yield of hydrocarbon gases (C1-5) in pyrolysis with only H2O gradually increased from 0.68 to 72.64 mL/g. The presence of FeS2 and Fe3O4 resulted in a substantial
Methane generation kinetics
The decomposition of 1-MNa (C11H10) occurs through two processes: 1) condensation that leads to the formation of polycyclic aromatics, and 2) C-C cleavage that causes the formation of hydrocarbon gases (Leininger et al., 2006, Bounaceur et al., 2017). Two polycyclic aromatics, benzopyrene (BP, C21H14) and dibenzanthracene (DBT, C22H14) were suggested to be the main products of condensation (Behar et al., 1993, Leininger et al., 2006).
Conclusions
Based on hydrothermal experiments conducted at 330–450 °C, we confirmed that the presence of FeS2 and Fe3O4 led to an apparent increase in hydrocarbon gas (C1-5) yield during the pyrolysis of 1-MNa with H2O. CH4 in the hydrothermal experiments with H2O-FeS2 and H2O-Fe3O4 was more 13C-depleted and 2H-enriched. The activation energies (Ea) for CH4 generation in the hydrothermal experiments with H2O-FeS2 and H2O-Fe3O4 were lower than those in the experiments with only H2O. Although FeCO3 had a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful for the supports by the National Key R&D Program of China (2017YFC0603102), the National Natural Science Foundation of China (41973068), the Scientific Research and Technological Development Project of CNPC (2019A-0208) and the Scientific Research Project from RIPED (2018ycq01). The careful reviews and constructive comments from two anonymous reviewers are greatly appreciated. We also thank Associate Editor Dr. Cliff Walters for his careful scutiny and editing.
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