ROBERTS, HARRY H., and WILLIAM J. WISEMAN, JR.
Coastal Studies Institute, Louisiana State University, Baton Rouge, LA

Abstract: Surficial Gas Hydrates of the Northern Gulf of Mexico: First Insight into Formation-Dissociation Cycles

Massive influx of sediment coupled with plastic deformation of salt has created a unique geologic framework for gas hydrate formation in the northern Gulf. Complex and widespread faulting as well as continuing salt adjustment to sedimentary. loads has resulted in a deepwater province characterized by near-continuous fault-related vertical flux of fluids and ases to the modern seafloor. Since their discovery in 1992 as surficial deposits, gas hydrates have been observed at many sites below ~500m in water depth on the upper-middle slope. Geochemistry suggests that are usually mixtures of methane with smaller amounts of other thermogenic gases. Qualitative observations of surficial gas hydrate deposits indicates as that they may form and disappear within the return observation period of a year. In situ measurements (gas emission, seafloor, and water column temperature, and current-speed/direction) designed for a 1-year monitoring period (GC185 and MC929) suggest important impacts on the formation-dissociation cycles for surficial hydrates. Experimental data suggest gas emissions occur across a spectrum of periods, as short as a few minutes to longer than 2 months. Heat fluxes accompanying rapid migration of fluids and gases from the deep subsurface can cause hydrate dissociation from below while water column temperature changes can cause similar changes from above. Diurnal fluctuations are typical less than 1°C, but fluctuations exceeding 2°C have been observed. Fluctuations of the order of 0.5-1.0°C occur on time scales on the order of two days. Fluctuations on the order of 1.0-3.0°C occur on time scales of 1-2 weeks. On longer time scales of 1-2 months, in association with Loop Current eddies and direct intrusions, bottom water temperatures could be expected to increase as much as 3-8°C resulting in dissociation of surficial deposits. Measurements relating to as hydrate stability at the seafloor suggest a wide range of periods an both subsurface and water column forcing processes. It is clear, however, that long data sets, greater than a single year, will be necessary to accurately characterize the process environment that affects gas hydrate formation and dissociation cycles.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas