Large Illinoisan Shelf-Margin Channel in the South Timbalier and Ewing Bank Areas, Offshore Gulf of Mexico

During an Illinoisan lowstand, a major channel became entrenched on the shelf and deposited deltaic sediment of the shelf-slope break in the South Timbalier and Ewing Bank areas. This shelf-edge delta overloaded the margin, creating a major slope failure. This 'channel,' or slump canyon, which was originally deep and narrow, evolved through retrogradational wasting of the shelf into a broader feature during back-filling and abandonment. Mapping with integrated seismic and well data indicates a feature that is 6 mi wide and has 2300 ft of relief, from the base of the channel to the highest margin. Gravel lag fills the deepest portion of the scour and constitutes an excellent, though restricted, gas reservoir. The limits of the reservoir are defined by mapping of a seismic amplitude anomaly. The balance of the back-fill is comprised of thinly bedded silstone and sandstone sequences that generally thin and fine upward; each is capped by marine shales. The fill geometry is primarily nested or stacked channels. Positive topographic features on the outer shelf tower above the channel floor, either as uneroded remnants or as part of channel margin slump mounds. Sand-prone sediment onlapped these highs, thereby creating numerous stratigraphic traps. As sea level rose,more » the locus of deposition within the channel shifted progressively shoreward, and the fill was capped by a clean, transgressive shale. Further work is necessary to ascertain the location of the submarine fan far down-dip of this shelf-edge channel. Laterally extensive reservoirs can be expected in more distal depositional environments.« less