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

Abstract

Water (H2O) and minerals involved organic-inorganic reactions are critical for hydrocarbon evolution in sedimentary basins and deep in the Earth. The mechanism and isotope fractionation for H2O-mineral-hydrocarbon interactions remain unclear. In this study, isothermal pyrolysis of 1-methylnaphthalene (1-MNa) involving H2O and three Fe-bearing minerals (pyrite–FeS2, magnetite–Fe3O4, and siderite–FeCO3), was conducted using a gold-tube system at 330–400 °C and a pressure of 50 MPa. It was observed that the presence of FeS2 and Fe3O4 led to an increase in the yield of hydrocarbon gases (C1-5). The isomeric ratios (iC4/nC4 and iC5/nC5) were much higher in the presence of Fe-minerals. In addition, 13C of methane (CH4) in pyrolysis with H2O-FeS2 and H2O-Fe3O4 was evidently more depleted than that in pyrolysis with only water. Kinetic calculations and carbon isotope fractionation revealed that FeS2 and Fe3O4 enhanced the H2O-hydrocarbon reaction via either ionic or free radical mechanisms at elevated temperatures. Moreover, the presence of FeS2 and Fe3O4 promoted hydrogen (H) transfer from H2O, causing the enrichment in the 2H of CH4. Furthermore, we first established a correlation between the H transfer rate constant (kH) and temperature (T): ln kH = −27.545 × 1000/T + 27.077. On this basis, it can be deduced that the time needed for equilibrium H transfer from H2O to CH4 at 200 °C and 400 °C with Fe-bearing minerals is approximately 4.8 Ma and 54.4 d, respectively.