Focused Accumulations of Methane Hydrate Above Deeply-Buried Basement Highs in the Deep-Water Bering Sea Basins

Ginger A. Barth¹, David W. Scholl², and Jonathan R. Childs^1
1Coastal and Marine Geology Team, U. S. Geological Survey, Menlo Park, CA
²U.S.G.S. and College of Natural Science and Mathematics, U. of Alaska, Fairbanks, AK

Velocity-amplitude anomalies (VAMPs), comprising coincident seismic travel-time anomalies and gas bright spots, are features widely identified in seismic-reflection images from the deep-water Aleutian and Bowers Basins in the Bering Sea. The chimney-like acoustic structures are interpreted as images of large deposits of natural methane hydrate directly overlying columns of ascending fluids that deliver methane gas to the hydrate-stability zone. Because these chimneys extend downward to depths where thermocatalytic processes are underway, methane involved in Bering Sea VAMP structures is presumed to be petroleum generated.

Interval travel-time anomalies have been used to rigorously detect these features. Relative travel-time variation in the sedimentary intervals above and below a gas hydrate bottom-simulating reflection (BSR) are a selected diagnostic for VAMP detection, measuring velocity pull-up in the hydrate stability zone and push-down in the underlying gas zone. Travel-time analysis has also been used to quantify implied resource content in several example cases. Individual large VAMP structures appear to involve a volume of methane (~0.6-0.9 TCF within an area ~4-8 km in diameter) equivalent to that of a large gas field.

The largest VAMP anomalies studied, including all of those associated with hydrate indicators, are located above prominent basement highs or above steep edges of high basement blocks. This association suggests that long-lived fluid migration patterns in these undeformed deep-water basins were originally established in response to sedimentation and compaction over the oceanic basement topography, perhaps augmented by hydrothermal circulation involving the crust itself. Basin-scale fluid migration patterns appear to be responsible for the natural-gas concentration process that creates these localized hydrate targets.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas