Print this pageGas hydrates and associated free gas are scientifically important for three reasons: (1) A large amount of methane is held in the hydrate, so that gas from the hydrate and underlying free gas could become a major energy resource as well as a drilling hazard. (2) If released from the sediment, gas may contribute atmospheric methane and thus influence global climate. (3) Hydrate breakdown may reduce sediment cohesion and initiate sea floor landslides. 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 travel-time inversion methods on new observations termed "the free gas envelope" from the wide-angle seismic data, we can now predict the thickness of the free gas layer beneath the BSR without drilling. Seismic measurements of free gas thickness 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 #90925©1999 AAPG Foundation Grants-in-Aid