Evolution of Large Deep Water Depositional Systems: Implications for Reservoir Architecture and Connectivity in the Nile Delta Offshore

Abstract

Topographic gradients exert a primary control on deposition, erosion and bypass of sediment gravity flows. This fundamental control translates into predictable stratigraphic architectures and distribution of sediment properties, not only at the bed-scale, but at the scale of entire deep-water fans. High gradient systems (such as the modern Golo fan shown in this study) tend to develop short bypass channels and deposit large volumes of sand in aggrading and back-stepping lobes, while mud deposits are common in distal fringes. Low gradient systems (such as the modern Zaire fan shown in this study) concentrate sand in robustly developed leveed feeder channels, and tsmaller distributary channels. Mud is concentrated in levees and advancing lobes have high concentrations of slurry deposits with diminished Net-to-Gross and reservoir quality. This fundamental control of depositional gradient on depositional style governs the distribution of sand versus mud in different parts of the system, the sorting of the sediment and ultimately the static and dynamic connectivity of stacked reservoirs. Typically, during early stages of basin evolution, systems start off as high-gradient but tend to heal topography and evolve into lower-gradient systems. As such, it is crucial to understand the paleo-bathymetry during deposition and at all stages of the system evolution. In the case of the offshore deepwater Nile Delta fan, this motif of an evolving depositional system is complicated by active tectonics associated with inversion events during deposition, the Messinian salinity crisis and significant slope failure and salt movement. Here we demonstrate the strong coupling between depositional response and gradients through examples of different styles of reservoir architecture in the context of the paleo-bathmetric setting of discrete Oligocene through Pliocene reservoir intervals in the outboard area of the Nile Delta deepwater fan.

AAPG Datapages/Search and Discovery Article #90355 © 2019 AAPG Africa Region, The Eastern Mediterranean Mega-Basin: New Data, New Ideas and New Opportunities, Alexandra, Egypt, September 6-7, 2019