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