FRACTIONATION OF METHANE ISOTOPOLOGUES DURING GAS HYDRATE DISSOCIATION: EXAMPLE FROM A HIGH-SATURATION RESERVOIR IN THE NORTHERN GULF OF MEXICO

We analyzed the multiply-substituted (aka “clumped”) isotopic signature (Δ¹³CH₃D and Δ¹²CH₂D₂) of methane in pore-filling gas hydrate from a sandy silt reservoir in the northern Gulf of Mexico (Green Canyon block 955, 418 meters below seafloor in 2032 m water depth). Clumped methane isotopes can reflect gas formation temperatures but can also be affected by kinetic and mixing effects. The gas was collected via controlled, stepwise depressurization of a pressure core in which 83% of the pore space was filled with methane hydrate (99.99% CH₄). To investigate fractionation of clumped isotopes during dissociation, we collected four gas samples over the course of depressurization. Over the course of dissociation, Δ¹³CH₃D increases from 4.84 to 5.85 ‰ corresponding to apparent isotopic temperatures decreasing from 58 to 23 °C. Δ¹²CH₂D₂ increases from 9.65 to 14.71 ‰ corresponding to apparent isotopic temperatures decreasing from 123 to 64 °C. If the methane was formed and transported in the absence of kinetic or mixing effects, the apparent isotopic temperatures would correspond to methane formation temperatures higher than the in situ reservoir temperature of ~19 °C. Our results suggest that clumped isotopes of methane in hydrates can yield reasonable temperature estimates, but that the role of fractionation during hydrate formation and dissociation needs to be better constrained. The range in isotopic temperatures observed over the course of dissociation would correspond to a ~1 km difference in depth of methane formation, based on the estimated geothermal gradient at Green Canyon 955. We are currently working to understand possible distillation effects and end-member compositions that may explain this fractionation. The fractionation in clumped isotopes during hydrate dissociation suggests the possible utility of a clumped isotope signal that could be used to fingerprint methane in the ocean or atmosphere that was formerly contained in hydrate.