Integrated geochemical approach to determine the source of methane in gas hydrate from Green Canyon Block 955 in the Gulf of Mexico

Abstract

Massive volumes of gas are sequestered within gas hydrate in subsurface marine sediments in the Gulf of Mexico. Methane associated with gas hydrate is a potentially important economic resource and a significant reservoir of carbon within the global carbon cycle. Nevertheless, uncertainties remain about the genetic source (e.g., microbial, thermogenic) and possible migration history of natural gas incorporated into hydrate. Previous studies have primarily used the hydrocarbon molecular (CH4/C2H6+) and isotopic (δ13C-CH4, δ2H-CH4) compositions of natural gas to address these uncertainties. However, hydrocarbon tracers are altered by mixing, oxidation, secondary methanogenesis, or fluid migration, which presents challenges when deciphering the mechanisms responsible for methane formation and accumulation. To evaluate the genetic source of natural gases from Green Canyon Block 955 (GC 955), east of the Sigsbee escarpment, we collected and analyzed samples from the first pressurized hydrate-bearing sediment cores collected from a coarse-grained hydrate reservoir in the Gulf of Mexico. Gas samples were analyzed for hydrocarbon (C1- C5), major gas (e.g., N2, CO2), and noble gas (He-Xe) abundance and isotopic (e.g., δ13C-CH4, δ2H-CH4, δ13C-CO2, δ15N-N2, 3He/4He, 4He/20Ne) compositions. We determined that natural gas in hydrates from this location are predominantly of primary microbial origin (conservatively at least 76%) and are formed by the hydrogenotrophic (CO2 reduction) methanogenesis pathway. We also note increased thermogenic proportions (~6%) in a hydrate-bearing layer below the main hydrate-bearing interval (separated by a 5 m water-bearing layer). Our results suggest that microbial methane may be abundant below the base of gas hydrate stability at GC 955.