Abstract: Deep-Water Sands, Pliocene, Niger Delta: Depositional Processes, Sand Body Geometry, and Sand Body Stacking Patterns

Kreisa, R. D.; R. B. Bloch; S. D. Joiner; J. B. Paul; and D. M. Jurick - Mobil Technology Company

The combination of hundreds of closely spaced wells, thousands of feet of core, and recent 3-D seismic data provide an unparalleled opportunity to document depositional patterns of Pliocene deep-water sands of the eastern Niger Delta and to understand the processes responsible for these patterns. In Mobil's Joint Venture acreage, sand was fed from distributaries, through numerous canyons incised into the shelf edge, to channel-levee complexes on the slope. Some of these are single channels deposited on the open slope whereas others comprise multi-layer stacked-channel complexes within incised slope-valley fairways. Individual channels are straight, becoming sinuous within a few km down slope and some channels show evidence of down slope migration via aggradation. Others broke through levees yielding anastomosing channel patterns.

Stacking patterns are distinctly cyclic. Allo-cyclic deposition relates to four low-stands of relative sea level, but these are punctuated by higher frequency cycles that are both allo- and auto-cyclic. Overall within the deep-water succession, grain size is a function of stratigraphic forcing mechanisms and climate cycles.

Turbidity flows were the dominant process for transport of sediment into the basin. Channel deposits (3 to 40 m thick) comprise upward-fining successions ranging from: 1) graded, pebbly coarse-grained sands at the base (beds up to 3 m), to 2) fine- and very fine-grained sands displaying complete Bouma sequences, to 3) interbedded very fine-grained sand and shale displaying partial Bouma sequences, to 4) silty shale at the top. The turbidity flows appear to emanate from near distributary channel mouths, perhaps via hyperpycnal flow. Channel deposits may also contain relatively thin intervals of intraslope-slumps and debris flows at their base. These formed due to bed shear from the coarse-grained turbidity flows during the initial stages of depositional cycles. Levee facies consist of shale with thin beds of very fine-grained sand or graded silt laminae. Clay-dominated condensed sections cap the channel/levee depositional-cycles.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil