Hydrocarbon Gas Generation from Direct and Indirect Hydrogenation of Organic Matter: Implications from Hydrothermal Experiments

Abstract

It is generally accepted that organic–inorganic interactions involving H-rich fluids (i.e., H2O and H2) contribute significantly to hydrocarbon (HC) generation in sedimentary basins. However, the effects of two hydrogenation processes involving H2O and H2 on the generation and C/H isotope fractionation of HC gases from organic matter (OM) remain unclear. In this study, two groups of hydrothermal experiments involving low-mature kerogen without (Group 1) and with FeS (Group 2) at 330–420 °C and 50 MPa were conducted to simulate the hydrogenation of OM by H2O and H2, respectively. The experimental results show that the redox reactions between H2O and FeS lead to the generation of considerable amounts of H2 in the Group 2 experiments. HC gas yield in the Group 2 experiments reaches 1.8–3.6 times that in the Group 1 experiments at Easy%Ro of 1.05–2.50%. In addition, indirect hydrogenation via H2O-derived H2 generates HC gases with smaller 13C fractionation and more negative δ2H compared with direct hydrogenation via H2O. On this basis, the mechanisms for HC gas generation from two hydrogenations were addressed. Additionally, it is demonstrated that the equilibrium isotope effect (EIE) is responsible for the 13C and 2H isotope fractionation in the hydrogenation of OM by H2.