|
Exploration Strategy and Reservoir Evaluation Methodology |
Osservatorio Geofisico Sperimentale, Dipartimento di Geofisica della Litosfera, P.O. Box 2011, 34016 Trieste, Italy
Reflection tomography techniques have been applied to two multi-channel seismic profiles, acquired across the accretionary prism of the South Sheltand margin, in order to reconstruct the velocity field associated with gas hydrate and free gas layers in the sedimentary sequence. Data show the presence of a strong bottom simulating reflector (BSR), running along the slope in water depths ranging from 1000 to 4600 m, locally underlain by a weak normal polarity reflector about 80 ms deeper in the section. The analysis indicates a velocity trend from the sea floor to the BSR generally consistent with that of normally compacted marine sediments, with an abrupt decrement between the BSR and the underlying reflector, indicating the presence of free gas in the sediment pore spaces. The calculated thickness of this gas-bearing layer is approximately 50 m. Local increments of tomographic velocity above the BSR can be related either to gas hydrate abundances in normally compacted slope basin sediments or to overcompaction in accreted sediments, as imaged by the pre-stack depth migrated sections. We conclude that clathrates and free gas distribution on the South Shetland continental slope are strongly controlled by the structural setting of the accretionary prism, where faults act as conduits for migration of natural gas towards the surface. A brief description of the adopted tomography method is also presented.
