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