Abstract:
Gas hydrate traps large quantities of hydrocarbon gasses on the continental margins and have potentially important effects on sedimentary geology, climate, slope instability, the carbon cycle, and offshore oil production. The Ocean Drilling Program (ODP) has expressed interest in coring gas hydrate in a variety of environments, including a mature petroleum basin. This article describes a proposal submitted to ODP for a Gulf of Mexico gas hydrate drilling cruise to address this goal. The Gulf is a unique gas hydrate environment because hydrocarbon gas flow is focused along faults which control the location and concentration of hydrate occurrence. As a result, massive gas hydrate deposits occur all the way to the seafloor, in contrast to widely-disseminated layers buried by a few hundred meters as found in many other settings. The drilling proposal targets hydrocarbon seep sites where gas hydrates occur at the seafloor. It is planned that 5-7 locations will be drilled in 1-2 transects at two sites, one at the upper edge of the gas hydrate stability zone (550-600 m depth) and another deeper, where the hydrate layer is thicker and more stable (1000-2000 m depth). At each drilling location, three holes are planned: one to collect core for stratigraphy and conventional sampling, one to fill gaps in the record of the first hole and allow pressure samples and heat flow measurements, and one to acquire logging-while-drilling data. Four sites on the northern Gulf of Mexico slope have been identified for further investigation. Two of these sites are planned for drilling, the others as alternates. One site, at a mud volcano on the border of Garden Banks leaseblocks 424 and 425, typifies a major, active, high flux seep in the shallow gas hydrate zone. The other shallow zone site is at the boundary of Green Canyon blocks 184 and 185 and includes the well-known "Bush Hill" hydrate mound. A site at the border between Mississippi Canyon blocks 852 and 853 is a major seep site in deeper water depths, whereas another located in Atwater blocks 381, 382, 425 and 426 is a deep site inferred to have lesser flux. Currently the proposal is under review by ODP advisory panels, and if approved, it could be drilled in 2002 or 2003.