Abstracts: Deposition of Deep-Water Sands, Pliocene, Niger Delta: Sequence Stratigraphy, Depositional Facies, and Sand Body Geometry and Stacking Patterns

KREISA, RONALD D., with acknowledgments to R. B. BLOCH, J. B. PAUL, D. M. JURICK, AND S. D. JOINER

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 factors responsible for these patterns. The Niger is a mixed energy delta, with wave, tidal, and fluvial energy in near equilibrium, resulting in a radial pattern of distributaries. In Mobil's Joint Venture acreage, sand from these distributaries was fed through numerous canyons incised into the shelf edge and upper slope, rather than from a single point source. Most sand deposition occurred in fairways both within canyons and in channel-levee complexes on the open slope. Individual channels are straight to sinuous and confined by levee deposits or canyon walls. They show little evidence of lateral migration, but commonly broke through levees, yielding anastomosing channel patterns. Multiple incisions within canyons were common. Deposition was also influenced by bathymetry inherited from an earlier shelf-margin collapse and by movement on faults.

Stacking patterns are distinctly cyclic. Allocyclic deposition relates to four lowstands of relative sea level, but these are punctuated by higher frequency cycles that are both allo- and autocyclic. The overall deep-water succession (1--2 km thick) coarsens upward, reflecting progradation. Typically, facies in channel deposits (3--40 m thick) comprise upward-fining successions. They may contain relatively thin intervals of intraslope slumps and debris flows at the base, overlain by turbidite sands. Turbidite intervals range from graded, pebbly, coarse-grained sands at the base (beds up to 2 m), to fine and very fine grained sands displaying complete Bouma sequences. Many of the slumps and debris flows were apparently generated by bed shear from the coarse-grained turbidity flows. Mass movements of shelf facies or processes for transport of sand into the basin other than by turbidites were rare.

AAPG Search and Discovery Article #90938©1997-1998 AAPG Distinguished Lecturers