THERMOGENICGAS HYDRATES, GULFOF MEXICO CONTINENTALSLOPE
Roger Sassen and Ian R. MacDonald
Geochemical and Environmental Research Group, Texas A&M University, College Station, Texas 77845
Keywords: gas hydrates, thermogenic hydrocarbons, Gulf of Mexico
INTRODUCIION The Gulf of Mexico continental slope is a natural laboratory for gas contain hydrocarbons from deeply buried thermogenic sources.
hydrates that Thermogenic
hydrocarbons (oil and gas) from actively generating Mesozoic source rocks (>6 km burial depth) migrate vertically along conduits associated with actively-moving salt
structures and faults hydrocarbon trapping hydrocarbons reach the
to subsurface system is so sea floor', and
reservoirs (2-4 km) of Tertiary age'.
"leaky" that large volumes enter the water column*.
Although biogenic gas hydrates are abundant on the Gulf slopes, oil and gas from deep source rocks create a geochemically complex and physically dynamic environment for thermogenic gas hydrates at the sea floor. Structure II gas hydrate containing C1-C4 thermogenic hydrocarbon gases was first sampled in 1984 by piston cores in 530-560 m water depths on the Gulf slope offshore Louisiana4. Identification of the hydrate as structure I1 was based on the relative abundance of the C3 and i-C4 hydrocarbons4. The structural assignment was corroborated using solid-state nuclear magnetic resonance (NMR)5. Research on gas hydrates of the Gulf slope, however, has advanced rapidly in the last few years, and our objective here is to summarize new results.
THE BUSH HILL STUDYAREA The Bush Hill site in the Green Canyon area of the Gulf slope offshore Louisiana is a well-documented site for study of thermogenic gas hydrates (27O47.5' N and 91O15.0' W). Bush Hill is a fault-related sea-floor mound about 500 m wide, with relief of about 40 m6. Water depth of the mound crest is about 540 m, where mean water temperature is about 7" C (range = 6 to 11" C) Phase equilibria models indicate that Bush Hill is within the stability zone of thermogenic gas hydrates (Sloan, E.D.,
Sea-floor sediments contain crude oil and related free hydrocarbon gases.
oxidation of these hydrocarbons produces COz which precipitates as authigenic
carbonate rock with isotopically-light 8l3C values7. The crest of the mound is colonized by seep-dependent chemosynthetic organisms including bacterial mats, vestimentiferan tube worms, and methanotrophic mussels8. Persistent natural oil slicks appear on satellite remote sensing images of the sea surface over Bush Hi@.
Thermogenic gas hydrates and gases that vent to the water column at the mound crest are readily sampled by research submarines. Copious streams of gas vent continuously to the water column where subsurface migration conduits intersect the sea floor9Jo. Thermogenic gas hydrates form around the orifices of gas vents. The gas hydrates at vents are not dispersed in sediments as nodules or thin seams, but instead occur as continuous masses. Lens-shaped masses of yellow to orange gas hydrates breach the sea-floor at numerous locations on the crest of Bush HillgJo. The hydrates form sediment-draped mounds 30-50 cm high and up to several m in width, with exposed gas hydrate visible at the edges of mounds'o.
Vent Gases The C1-C5 hydrocarbons of the vent gases are dominated by methane (Cl = 91.1-
94.7%), and 813C values of C1 are within the narrow range of -42.4 to -45.6 %o PDB
(Table 1).The Ci-CS distributions and 813C of C1 of the vent gases (Table 1)correlate to gases from underlying subsurface reservoirs of Jolliet Fieldll.