ABSTRACT

We conducted a laboratory hydrothermal experiment that simulated generation of low molecular-weight hydrocarbons during seafloor sediment alteration at 275–361°C and 30 MPa. The abundance and carbon and hydrogen stable isotope composition of low molecular weight thermogenic hydrocarbons in the fluids were determined. In general, the abundance of C₁−C₄ alkanes increased with time. The abundance of CH₄ relative to C₂−C₄ alkanes as reflected by C₁/C₂₊ ratios showed progressive increases from 1.2 to 4.3 with continued sediment heating. Alkenes were enriched in early phase and decreased with time. Carbon isotope ratios (δ¹³C) of thermogenic CH₄ ranged between –42.0~−24.2‰. Carbon isotope ratios of C₂H₆ and C₃H₈ were similar to each other throughout the experiment (δ¹³C = –28.0~−20.3‰). In general, the carbon isotope ratios of C₁−C₄ alkanes were more close to those of substrate organic matter in larger carbon numbers and at later periods of the experiment. Hydrogen isotope ratios (δD) of CH₄ varied from –325~−262‰, more negative than those expected at the isotope equilibrium between CH₄ and H₂O. Compared with results from the experiment, natural hydrothermal fluids show higher C₁/C₂₊ ratio, more diverse δ¹³C_CH4 values among the fields, higher δ¹³C_C2 values, and higher δD_CH4 values. The differences likely result from lower maturity of the experimental fluid and biogenic methane contribution to the natural fluids.