Print this pageThe Seaspray group is a succession of cool-water marls and limestones that records the development of the continental shelf from the Oligocene to the Recent in the offshore Gippsland Basin, Southeast Australia. It disconformably overlies and forms the seal to the Cretaceous to Eocene sediments of the Latrobe group - the major hydrocarbon-bearing facies of the Gippsland basin. Typically, seismic velocity in sedimentary sequences increases with depth. However, in parts of the Seaspray Group, high (>2000 m/sec) inverse velocity-depth gradients are associated with mid-Miocene submarine canyons. These high velocity sediments cause depth conversion problems and create problems with the seismic interpretation of underlying hydrocarbon-prone structures.
There is a clear relationship between sonic velocity, carbonate content, age and depth of burial in Seaspray group sediments. High sonic velocities are recorded in intervals where older carbonate-rich sediments have undergone pressure solution and stylotisation.
The highest velocity sediments are associated with carbonate-rich sequences, where older (Miocene) submarine canyons are buried in the deepest parts of the basin above a major mid-Miocene erosional surface. These submarine canyons appear to have migrated laterally through time so that erosional surfaces seen in seismic section are, in-parts, diachronous. These meandering submarine canyons seem to be partially controlled by local basin structures, so-that submarine canyons and their seismic pull-up effects are associated with some large oil and gas fields.
AAPG Search and Discovery Article #90925©1999 AAPG Foundation Grants-in-Aid