W. Steven Holbrook1, Ingo A. Pecher2, Daniel Lizarralde3, Matt Hornbach1, Andrew Gorman1, Kara Hackwith1
(1) University of Wyoming, Laramie, WY
(2) University of Texas, Austin, TX
(3) Georgia Tech, Atlanta, GA
ABSTRACT: New 2D and 3D Seismic Data from a Dynamic Gas Hydrate/Free Gas System, Blake Ridge
Recent active-source seismic investigations, including 2D and 3D multichannel seismic reflection, show that the Blake Ridge, offshore South Carolina, is a highly dynamic, three-dimensional, gas-charged system that may have expelled methane into the ocean/atmosphere system within the past 1-2 m.y. Seismic velocities indicate that hydrate occupies 3-4% of total rock volume (5-7% of porosity) in a 250-m-thick zone on the crest of the ridge and only about 1-2% of total rock volume 7 km off the crest of the ridge. A wedge of concentrated hydrate (~20-25% bulk hydrate) is locally present just above the BSR, reflecting active upward methane migration. A prominent paleo-BSR within the hydrate stability zone on the northeast flank of the ridge probably represents a reflection from a concentrated hydrate zone, formed when the phase boundary deepened during an erosional event, trapping free gas as hydrate. The Blake Ridge "collapse structure" lacks the thick free gas zone that underlies the rest of the ridge crest, strongly suggesting that large quantities of methane were expelled into the ocean/atmosphere system. However, new data show that the "collapse" is not a structural collapse, but rather a depositional/erosional construct comprising rapidly deposited sediment waves. We propose that the high deposition rate of these strata, combined with the availability of high-permeability pathways at the base of sediment wave packages, led to escape of free gas from the ridge crest. Similar wave packages are common on the northeast ridge flank, suggesting that gas release may be a fundamental, recurring part of the hydrate cycle on the Blake Ridge.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado