Slope fan depositional elements evaluation: Implication for reservoir depositional origin in the deep offshore Niger Delta Basin, Nigeria

Abstract

A study on the slope fan depositional element evaluation with the implication for reservoir depositional origin from the deep offshore Niger Delta Basin, Nigeria has been carried out. This involved an evaluation of the spatial distribution of reservoir depositional facies and their textural properties from deepwater cores obtained at different stratigraphic intervals. Six lithofacies were sedimentologically described based mainly on the lithological composition, textural characteristics, sedimentary structures, sediment concentration, particle cohesion and particle support mechanisms namely coarse-grained clast-supported sandstones, medium-grained normally graded sandstones, parallel laminated sandstones, cross to convolute laminated sandstones, mud-rich heterolithic and massive mudstones. They were genetically grouped into debris flow, turbidity current flow and overbank mud lithofacies associations. Depositional architectures varied from amalgamated channel sands and incised canyon fills to lobate sands. The lobate deposits were dominated by suspension settling and traction depositions due to fluid turbulence and bottom current reworking respectively, and were documented within the mid fan and outer fan complexes. The inner fans and the canyon fill deposits were dominated by debris flows associated with high density, clast-supported laminar flows. The study inferred that the erosion and channelization of the subaerially exposed Niger Delta continental shelf during an early forced regressive event triggered the accumulation of the Q-Sands by debris flow process in an inner fan environment while a short-lived early rise normal regressive event that followed an earlier forced regressive deposits brought about the mobilization of the R-Sands in an entrenched channel levee setting in a mid-fan environment. The continued aggradation of the continental shelf in the fluvial to shallow-marine environments during the early rise normal regression resulted in the delivery of mudstones of low density turbidity flows to the outer fan environment.