NOBLE GAS AND HYDROCARBON GEOCHEMICAL COMPOSITION OF FLUIDS ASSOCIATED WITH GAS HYDRATE FORMATION IN CORES FROM GULF OF MEXICO GREEN CANYON, BLOCK GC955

Methane hydrates hosted in continental slope and permafrost sediments constitute a large and important hydrocarbon reservoir globally (~10¹⁵ m³). More work is needed to determine the genetic source(s) (i.e., biogenic, thermogenic, mixed) of hydrocarbon gases, the hydrocarbon source rocks, the rates of clathrate formation, and residence time of natural gases in these hydrate systems. Here, we examine gases released from hydrate in two pressurized cores extracted from coarse silt/sand reservoirs ~430 m below the seafloor within the GC955 block of the Green Canyon area in the Gulf of Mexico to resolve the genetic source of natural gas within hydrates and the residence times of natural gas in this area.

Previous industry work has inferred the genetic source of natural gases from this area using analyses of hydrocarbon molecular (C₁/C₂+) and stable isotopic composition of gases (e.g., δ¹³C-CH₄, δ²H-CH₄) and water (e.g., δ²H-H₂O, δ¹⁸O-H₂O), and trace elements. Because these parameters can be affected by mixing, migration, microbial activity, and oxidation, we integrate measurements of noble gases to better constrain the genetic source of hydrocarbons. Preliminary noble gas and hydrocarbon molecular data indicates at least two end-members including a) a nearly pure biogenic end-member with low ⁴He and high C₁/C₂+ (>10,000) and b) an apparently mixed end-member consisting of moderate ⁴He and intermediate C₁/C₂+ ranging from 800-3,000.

As part of this work, we developed and validated an improved method for sample collection during core depressurization that dramatically reduced atmospheric contamination by >90%. This reduction in atmospheric contamination improves residence time model estimates in this study. Our preliminary noble gas measurements and residence time models suggest unexpectedly young residence times ranging from ~50,000 to 300,000 kyr precluding significant contributions from the long-range migration of “older” thermogenic gas from deeper formations. In general, we do observe a general trend of higher apparent ages in the mixed genetic end-member.