Gas Hydrate Distribution and Amounts on the South Shetland Continental Margin, Antarctic Peninsula

About 800 km of multi-channel seismic data acquired on the South Shetland Margin, Antarctic Peninsula in 1993 has been reprocessed to examine the seismic character and velocity structure of gas-hydrate-bearing sediments in detail. Data show the presence of strong and widespread Bottom Simulating Reflectors (BSRs) in the area. Underlying free gas layers are locally associated with BSR. For estimating volume of gas hydrate in this area, interval velocities were determined by 1-D velocity inversion method and porosities were deduced from relationships with respect to the sub-bottom depth for terrigenous sediments. The velocity jump observed at the shallow sub-bottom depth may be partly due to the gas hydrates and partly to the unconformity. Because of no available well logs and significant geological complexity, we made two models for the baseline of the velocities of non-gas-hydrate-bearing sediments in this area. The first model assumes that part of the velocity increase is due to consolidation of sediments with porosity (45 %) predicted from the terrigenous sediments. The second model assumes that the velocity increase is due to porosity (35 %) reduction by 10 % owing to the unconformity and unconsolidated sediments. The difference between the results of two models is not significant. A total of 145 km of BSRs is identified on the seismic profiles. To estimate the total volume of gas hydrate in the study area, a 300 m of total thickness and 15 km of the width of gas-bearing sediments along the seismic lines, and the 0.063 of the average volume gas hydrate concentration (based on the second base line model) are used. Under the assumption that gas hydrates exist only where BSRs are observed, the total volume of gas hydrates along the seismic lines in the area is about 4.1x10¹⁰ m³ (6.7x10¹² m³ in a volume of methane).