The Diversity of Bottom-Current Influenced Submarine Slope Channel Complexes: Insights From Offshore Tanzania
The interaction of bottom currents with turbidity currents and their deposits can affect the distribution of submarine channel and lobe complexes as well as the facies distribution within them. This study combines high resolution 3D seismic, well log and core data of Block 2 area, offshore Tanzania, to investigate bottom current influence on Upper Cretaceous and Paleogene slope channel complexes. In the Upper Cretaceous, tectonic and drift related topography governed the distribution of channel and lobe complexes. Large drift deposits, 10s of km wide and up to 1.6 km thick, form the pathways for slope channel complexes that in turn erode and modify the drifts. Channel complexes striking in a NW-SE orientation are confined by long-lived drifts throughout the Turonian and Campanian, and step up-slope towards the SE. During the Paleogene the sedimentary system underwent significant modification, becoming dominated by extensive, unconfined lobe complexes on the lower slope that were incised into by two slope channel complexes. These slope channel complexes are characterized by an early stage of erosion and lateral migration, and late-stage aggradation of stacked channel complexes. Both channel complexes have asymmetrical levees being larger towards the NW leading to unilateral migration of the channel complex to the SE. The asymmetrical levees are interpreted as fine-grained deposits of low-energy bottom currents and overspilling turbidity currents. The depositional mechanism is a combination of: (i) the fallout of fines due to waning of the bottom current flow along relief; (ii) flow stripping of the turbidity current and subsequent reworking of unconsolidated sediments on the channel flank. The toes of the levee/drifts interfingered with the channel fill deposits and progressively stepped into the channel fills, pinning the channels to the slope over long periods of time. We infer that the bottom current drift deposits developed as lee waves, with accretion predominantly on the bottom-current upstream side, and consequently drift migration was in the upstream direction. The sedimentary characteristics of these mixed contourite and turbidite systems have several implications for reservoir facies and trapping mechanisms: (i) the potential of high net-to-gross sandstone bodies along steep slopes; (ii) the likelihood of stratigraphic trapping due to the fine grained drift facies surrounding reservoir facies; (iii) the risk of fluid flow baffles and barrier resulting from the “toes” of the drift stepping into the slope channel complexes. Understanding the influence of bottom currents on deep-marine depositional systems is crucial for the understanding of hydrocarbon plays on the East African margin and other plays globally.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019