Thermogenic gas hydrates in the northern Cascadia margin

Abstract

Gas hydrates are ice‐like solids that form in rigid cage structures under specific conditions of pressure, temperature, and gas and water concentration. Marine gas hydrates are stable in pore spaces of sediments in water depths greater than ∼300 m beneath the slopes of active and passive continental margins [ Kvenvolden , 1988]. The lower limit of hydrate occurrence in marine sediments is determined by the geothermal gradient, so that the zone of hydrate stability is generally contained within the first few hundred meters of sediment. Continental hydrates occur in polar permafrost regions in the Arctic and Siberia. Most of the hydrates that have been discovered contain methane derived from microbial processes. Other hydrocarbons can also form hydrates, but in different structures of the surrounding water cages. Structure I, the most prevalent form, contains mostly (>99%) microbial methane, a small amount of ethane, and traces of C 2+ hydrocarbons [ Sassen et al , 2001]. Structure II and structure H hydrates contain significant quantities of thermogenic methane and larger, more complex hydrocarbons formed at high temperatures from fossil organic matter (i.e.,kerogen) or oil [ Sassen and MacDonald , 1994].The gas origin is inferred from measurements of the carbon‐13 isotopic ratio (δ 13 ); microbial methane is depleted in 13 C (δ 13 <−60‰) relative to thermogenic methane (δ 13 from −20‰ to −50‰).