Date of Award
Open Access Dissertation
James H Knapp
Cold seeps are areas where methane is transferred from the lithosphere into the hydrosphere, accounting for the major source of hydrocarbons in seawaters. Formation of gas hydrate in cold seeps modulates the global discharge of methane to the environment. However, cold seeps are dynamic settings where hydrates dissociate on short and long time-scales triggering substantial methane fluxes to the oceans. These methane vents sustain unique ecosystems at the ocean floors and contribute to ocean acidification. Also, the methane can potentially reach the sea surface and be exchanged with the atmosphere contributing to global warming. Understanding how cold seep-hydrate systems (CSHSs) operate through time and space is therefore crucial to evaluate their global impact on ocean biogeochemistry and climate.
The area investigated is Woolsey Mound, a CSHS located in the Northern Gulf of Mexico. For the first part of the research, the goal was to determine the spatial distribution of subsurface gas hydrate at this site. In terms of hydrate-reservoir category, Woolsey Mound is classified as "seafloor mound" and "fractured mud". To date, these two categories are poorly constrained worldwide. This study documents a successful integration of high-resolution seismic and core data to detect the spatial distribution of hydrates in such settings. The approach adopted and the model may be applied globally for these reservoir categories.
The aim of the second part was to untangle the contentious long-term (thousands to millions of years) dynamics driving methane hydrate dissociation and seepage in CSHSs. Analyses on high-resolution seismic data suggest that tectonics is the main forcing mechanism and that CSHSs may operate independently from eustatic fluctuations. This contradicts the broad consensus in the literature about methane seepage in CSHSs being systematically triggered during sea-level lowstand.
The third part of the research aimed to characterize the short-term (years) dynamics of Woolsey Mound via time-lapse seismic monitoring. Quantitative 4-D seismic analysis through amplitude differencing of two sets of 3-D data suggests that CSHSs may release considerable volumes of methane on a 3-year time-scale. Also, short-term methane hydrate destabilization and seepage appear to be triggered primarily by transient migration of overpressure thermogenic methane through the system.
Simonetti, A.(2013). Spatial and Temporal Characterization of A Cold Seep-Hydrate System (Woolsey Mound, Deep-Water Gulf of Mexico). (Doctoral dissertation). Retrieved from http://scholarcommons.sc.edu/etd/2502