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Velocity
Problems; Ideas from the Tertiary Cool-Water Carbonate Seaspray
Group, Gippsland Basin, Southeast Australia
The 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