HR: 14:00h
AN: NS13A-03    [Abstracts]
TI: Geophysical Evidence for Large Scale Fluid Vent Structure in the Abyssal Gulf of Mexico
AU: * Dohmen, T E
EM: tdohmen@br-inc.com
AF: Burlington Resources, 717 Texas Ave., suite 2100, Houston, TX 77002 United States
AB: This talk examines geophysical evidence for the appearance of a large-scale fissure or fluid vent structure in the abyssal Gulf of Mexico, located in Lund Area. We notice a near vertical fissure 20,000 ft tall and about 100km long, which directly underlies the thickest portion of the Mississippi River Fan. Disruption of the sedimentary layers below the fan resembles fluid vent structures normally seen as localized chimneys on seismic from deep water. In this case, the feature has developed into a long, linear structure, the upper portion of which appears to contain hydrates. We expect a relationship between the formation of the fissure and the rapid deposition of the Mississippi Fan during Pleistocene and Holocene glacial lowstands. The expanding fan would have led to uneven loading of the pre-existing, layered sediments of the abyssal plain. This may have caused a zone of low effective stress to develop at the leading edge of the levee, reaching deep into the subsurface due to the large spatial wavelength of the load, and bringing pore pressures close to the fracture gradient in much the same manner as envisioned by Dugan & Flemings, 2000 for formation of the box canyons off the New Jersey coast. Geophysically, we can document: 1) A long, linear time structure appearing at all levels underlying the Mississippi Fan and above the known source rock section (i.e., Top K through Pliocene). 2) Isopachs show no thinning implying a late, catastrophic event. 3) Banded, high amplitude reflections crowded into the top of the vent may be hydrates or alternating layers of hydrates and free gas trapped under the fan sediments. 4) Low impedance, low frequency, chaotic seismic "halo" 20-25miles wide surrounds the vent. 5) Near and far offset stacks of seismic data resemble one another enough to prove the feature is not just a shallow velocity anomaly. Far offset raypaths would undershoot a near surface feature causing deep reflectors from a far offset record to differ from a near offset record if it were simply a shallow velocity anomaly. 6) Congruence of the subsurface feature with the bathymetric maximum thickness of the Mississippi Fan combined with the apparent late-stage deformation suggests a causal relationship. Understanding the formation of this vent feature may provide insight into large-scale petroleum migration routes, the origins of overpressure in the shallow sedimentary section, effects on the distribution of hydrates, and possibly rates of petroleum sourcing.
DE: 3025 Marine seismics (0935)
DE: 4219 Continental shelf processes
DE: 8045 Role of fluids
DE: 8105 Continental margins and sedimentary basins
DE: 9350 North America
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly