Abstract: Variations and Controls on the Distribution of Reservoir Facies within Confined Channels, Leveed Channels, and Distributive Systems in the Middle Miocene to Pliocene of the Eastern Niger Delta Slope, Offshore Nigeria

Sprague, Anthony R. - Exxon Production Research Co.; C. Randall Bissell* and Derek N. Reiber - Exxon Exploration Co.

Miocene and Pliocene strata in the eastern offshore Niger Delta, Nigeria, were deposited in a middle to lower slope deep-water setting. Coarse clastic sediment was delivered southwards across the upper slope through several established transport corridors. Sandstone-rich facies were deposited down the slope along these feeder systems in three principal depositional environments: terminal lobes/distributive sheets, confined channels, and leveed channels.

The terminal lobe/distributive sheet environment consists of thick, shingled, multi-cycle lenses in the lower and middle Miocene. Distributive systems become smaller, single-cycle, elongate sheets in the upper Miocene, and small distributive lobes in the lower Pliocene. In contrast, the confined channel environment consists of strongly erosional and linear, single "feeder" channels in the upper Miocene. Single channels transition to multiple, radiating channel complexes, becoming smaller in the lower Pliocene. The leveed channel environment is characterized by low-sinuosity single-cycle channels with narrow levees in the upper Miocene, which become wider, thicker, and better developed in the Pliocene.

Across the eastern Niger Delta slope there is also an upward change in the proportion of the three depositional environments. Sand-prone depositional systems in the middle Miocene are dominated by terminal lobes. Into the upper Miocene, terminal lobe/distributive sheet environments become smaller and are commonly associated with a well established confined channel environment. Confined channels are strongly erosional and often exhibit depositional mounding. Confined channels have a multi-cycle seismic character which often appears to exhibit a backstepping fill character. This seismic expression is interpreted to be related to sandy debris flow deposits back-filling the established channel. In the lower Pliocene, the confined channels become one part of a feeder system that includes or may even be dominated by leveed channel systems.

Sand-prone facies were deposited in linked chains of environments, coursing north to south down the depositional slope. The localization of the sand-prone facies along down-slope paths is related to the fill of erosional conduits which were cut as composite sequence boundaries during major sea-level falls. The fill of these conduits consists of high-frequency sequences commonly deposited in a backfilling or retrograding fashion. The distribution of the confined channels and terminal lobe/distributive sheet systems along the composite sequence boundaries is related to deposition of sand-prone lowstand sediments at abrupt decreases in depositional slope. These changes in slope are most likely due to the earliest seafloor expressions of the present-day shale-supported and thrust-cored structural highs.

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