KLOSKA, MARGIE, and W. STEVEN HOLBROOK
University of Wyoming, Department of Geology and Geophysics, Laramie, WY

Abstract: Determining Amounts of Methane Hydrate and Free Gas from Seismic Studies, Blake Ridge, Offshore South Carolina

Methane hydrate and associated free gas zone has gained considerable attention because of their potential as an energy resource as well as a drilling hazard. However, to thoroughly understand its potential as a resource and drilling hazard, accurate estimations of hydrate and free gas amounts must be determined. In a well studied field area, the Blake Ridge, seismic velocities measured in three boreholes indicated the presence of a much thicker layer of free gas (~250 m) than previously expected.

Coincident single channel and wide-angle ocean bottom seismic data collected on Blake Ridge will be used to constrain the amount of hydrate and free gas. This study uses three ocean bottom hydrophones and 10 seismic lines criss-crossing in a star pattern. Waveform inversion was performed to determine the concentration of hydrate and saturation of trapped free gas below the hydrate. Preliminary results suggest that hydrate concentrations at the base of the hydrate stability zone are less than 20% while trapped free gas saturation is less than 10%. By using traveltime inversion methods on new observations from the wide-angle seismic data, we can predict the thickness of the free gas layer beneath the BSR without drilling. Preliminary seismic measurements of free gas thickness reasonably agree with observed thickness from the drilling results. Combining vertical incidence and wide-angle data with traveltime and waveform inversions can provide a basis for estimating hydrate and free gas amounts prior to drilling.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas