The origin of thermogenic gas hydrates on the northern Cascadia Margin as inferred from isotopic ( 13C/ 12C and D/H) and molecular composition of hydrate and vent gas

Abstract

The isotopic ( δ 13C and δD) and hydrocarbon compositions of hydrate-bound and vent gas collected from the seafloor of Barkley Canyon (northern Cascadia Margin, offshore Vancouver Island, Canada) were evaluated to characterize the gas and infer the type and maturity of the source rock kerogen. The hydrate gas contained methane having δ 13C values from −43.4‰ to −42.6‰, δD values from −143‰ to −138‰ and a large percentage (14.9% to 31.9%) of C 2 to C 5+ hydrocarbons. These data are consistent with a thermogenic gas source. The data from Barkley Canyon are interpreted within the context of similar data from the Gulf of Mexico and Caspian Sea thermogenic hydrates, which occur in regions where the petroleum systems supporting gas and oil generation are better understood. A stable carbon isotope-based natural gas plot model and D/H data from the hydrate gas indicate that the source rock for the Barkley Canyon hydrate and vent gas had primarily Type III kerogen mixed with a small fraction of Type II kerogen. Oil from a seep having a pristane/phytane ratio of 3.2 was identified as a gas condensate, which supports the conclusions drawn from the gas data. A mechanism for explaining how fluids are conducted from the deep petroleum reservoir to the seafloor of Barkley Canyon is proposed.