Carbon Isotope Reversal Gas Generated in the Hydrothermal Experiments Involving 1-MNa and MnCO₃

Abstract

To learn about the gas generation from water−mineral−hydrocarbon interactions, isothermal pyrolysis of 1-methylnaphthalene involving water (H 2 O) with and without rhodochrosite (MnCO 3 ) was conducted by a gold tube system at 330−450 °C and 50 MPa in this study. By determination of the yields of individual gas products, it was observed that the presence of MnCO 3 led to an apparent increase in the yields of hydrocarbon gases (C 1−5 ). The dryness (C 1 /C 1−5 ) of gas products in hydrothermal experiments with the mineral was much lower compared with that without the mineral. Kinetic calculations indicated that the presence of MnCO 3 reduced the activation energy for the generation of hydrocarbon gases at elevated temperatures. The higher isomeric ratios of hydrocarbons (i-C 4 /n-C 4 and i-C 5 /n-C 5 ) implied that water−hydrocarbon reactions in the presence of MnCO 3 should mainly occur via the ionic mechanism. It is noticed that hydrocarbon gases generated in the hydrothermal experiments with MnCO 3 show a partial reversal in the 13 C isotope (i.e., δ 13 C 1 < δ 13 C 2 > δ 13 C 3 ). Moreover, the presence of MnCO 3 led to the depletion of 2 H for methane. Therefore, it is reasonable that this mineral can control the mechanisms and C/H isotope fractionation for hydrocarbon gas generation from water−hydrocarbon reactions. Most importantly, our study may provide a possible interpretation for 13 C isotope reversal gas generated at high-over maturities in deep formations.