Margie Kloska1,
W. S. Holbrook2
(1) ExxonMobil, Houston, TX
(2) University of Wyoming, Department of Geology and Geophysics, Laramie, WY
Abstract: Characterizing volumes and distribution of methane hydrate and underlying free gas with wide angle seismic data on the Blake Ridge, offshore South Carolina
To assess the potential of
methane hydrate as a future energy resource, accurate determination of hydrate
volumes and distribution is critical. The Blake Ridge, a lithologically
monotonous sediment drift deposit, is ideal for seismic determinations of
hydrate/gas distribution, in that variations in seismic velocity
and reflectivity
can be directly attributed to concentrations of hydrate and/or free gas. We
performed full waveform and travel-time inversions on high quality wide-angle
seismic data acquired in a criss-crossing star pattern. This unique acquisition
pattern produced seismic images of the hydrate zone in eight directions
radiating from a single point, giving the first-ever 360-degree view of the
variation of the physical properties over short (<1 km) lateral distances.
Angles of incidence out to 70 degrees were recorded due to the long-offset
ocean-bottom seismic geometry, allowing unusually good constraints on
fine-scale
velocity
structures. A total of 16 inversions were done at two
locations separated by 7 km. Results show clear lateral variations in BSR
velocity
structure over short distances, with some locations showing a distinct
high
velocity
"hydrate wedge" and others lacking a wedge. Average
hydrate concentrations within the wedge are 23% +/- 5% of the total rock
volume, significantly higher than previous estimates. Average underlying free
gas (2-3% of the total rock volume of a ~110 m thick layer) is the principal
cause of the high impedence contrast at the BSR (bottom simulating reflector).
Preliminary observations show a correlation between dipping beds of free gas
and high
velocity
hydrate wedges, suggesting that updip
migration
of gas
bubbles causes locally high concentrations of hydrate.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana