METHOD OF ESTIMATING THE AMOUNT OF IN-SITU GAS HYDRATES IN DEEP MARINE-SEDIMENTS

The bulk volume of gas hydrates in marine sediments can be estimated b y measuring interval velocities and amplitude blanking of hydrated zon es from true amplitude processed multichannel seismic reflection data. In general, neither velocity nor amplitude information is adequate to independently estimate hydrate concentration. A method is proposed th at uses amplitude blanking calibrated by interval velocity information to quantify hydrate concentrations in the Blake Ridge area of the US Atlantic continental margin. On the Blake Ridge, blanking occurs in co njunction with relatively low interval velocities. The model that best explains this relation linearly mixes two end-member sediments: hydra ted and unhydrated sediment. Hydrate concentration in the hydrate end- member can be calculated from a weighted equation that uses velocity e stimated from the seismic data, known properties of the pure hydrate, and porosity inferred from a velocity-porosity relationship. Amplitude blanking can be predicted as the proportions of hydrated and unhydrat ed sediment change across a reflection boundary. Our analysis of a sma ll area near DSDP 533 indicates that the amount of gas hydrates is abo ut 6% in total volume when the interval velocity is used as a criterio n and about 9.5% when amplitude information is used. This compares wit h a calculated value of about 8% derived from the only available measu rement in DSDP 533.