Sequence Stratigraphy of a Shelf-Edge Depositional Complex Based on the Integration of High-Resolution Seismic with Core Data
Janet M. Combes, Susan E. Nissen, Robert W. Scott
Correlation of corehole and high resolution seismic data sets obtained
offshore Louisiana by a cooperative consortium of Louisiana State University and
ten petroleum industry partners has resulted in a detailed sequence
stratigraphic interpretation of a Late Pleistocene shelf margin delta system.
High resolution stratal geometries have been interpreted within this framework
of genetically related facies and key sequence surfaces have been identified
both in the core data and on the high resolution seismic lines. Regional
expressions of chronostratigraphically identified sequence-bounding
unconformities and transgressive ravinement surfaces emphasize the importance of
these surfaces in determining stratigraphic relationships. The geologic and
seismic characteristics of reg onal subaerial erosional surfaces, transgressive
surfaces, condensed sections, and maximum flooding intervals vary areally with
depositional setting and, in any one location, may not uniquely identify these
surfaces. This study documents examples of these characteristics and their
stratigraphic control within a clastic shelf to shelf margin setting. It also
provides details of stratal geometries of shelf edge delta systems never before
attained on a regional scale. Deltaic
foresets at a resolution of 3 feet (1
meter) and dipping seaward at 5° to 8° are delineated by seismic data of
frequencies up to 1200 Hz and can be correlated with log- and core-identified
lithologies. Within the progradational section, coarser grained (sand-rich)
sediments generally are located in the updip hird of a clinoform unit. The basal
two-thirds of the progradational package are predominantly finer grained
(mud-rich) deposits, although slumped sandy deposits may accumulate at the toe
of the clinoform slope. When progradation extends deposition past the shelf
break, the toe of the clinoforms merges with upper slope deposits and feeds
basinal deposits. The upper surface of a lowstand shelf edge delta is reworked
by subsequent transgressive processes. Shelf edge deltas do not spread laterally
to any degree because an essentially limitless accommodation space is available
down slope. The combined sediment influx to a basin from multiple small shelf
edge deltas is significant. Several key conclusions resulted from this study: 1)
The optimum location for interpretation of sequence surface is within or near
the locus of maximum deposition. 2) At a distance from a depocenter the
characteristic features of sequence surfaces lose seismic resolution and minor,
subtle variations in the reflection character are the only seismic indicators of
major boundaries. 3) Shelf edge
deltaic
deposits are known to contain important
hydrocarbon
reservoirs
and this latest Pleistocene system provides an excellent
model for older Cenozoic systems. 4) Potential deep sea fan
reservoirs
may
accumulate seaward of shelf margin deltas during both falling and rising sea
level stages depending upon local sedimentological conditions.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995