Anatomy and Facies Analysis of Fluvial Body, Western Flank of Anambra Basin, Southern Nigeria: An Outcrop Study

Abstract

Fluvial sediments have complex body geometry. This paper describes sediment facies and architectural elements of a mud-prone fluvial body and showcases a conceptualised stratigraphic framework that reflects sandstone geometry and dimensions in the rock record, drawing valuable lessons for evaluating reservoir potential in subsurface analogues. Geological attributes collected for this purpose are lithology, sandstone-mudstone ratio, internal geometry and dimensions, grain size and sorting, small-scale sedimentary structures, and bed thickness. We recognised four channel surfaces of different magnitudes in the siliciclastic body. Channel complex comprises multilaterally stacked conglomerate-sandstone units encased in floodplain mud. Single-storey channel consists of vertically stacked lenticular granular-to-pebbly sandstone that is associated with wavy, non-parallel coarse-to-very-coarse sandstone. Channel-fills are composed of cross-bedded hemispheric-shaped medium-to-coarse sandstone, whereas lens consists of planar cross-bedded tabular medium sandstone. Three groups of architectural elements are defined, namely: channel, overbank, and floodplain elements. The geometry of the fluvial body reflects an intricate labyrinth of interconnected sandstone bodies typical of meandering river systems. In the Lower Zone, a compound ribbon of vertically-stacked coarser facies progressively thins from centre to margin and terminates into floodplain mud, creating a huge risk for horizontal sweep. The Middle Zone comprises sheet sandstone bodies with moderate-to-high lateral continuity and potential for better horizontal sweep. Sandstone-mudstone ratio and visual porosity in the Upper Zone are <25% and <10%, respectively indicating poor reservoir potential. Upward-decreasing permeability trends established for this geometry type may cause injected water to flood the high-permeability basal part while bypassing the upper mud-prone interval. Understanding the style of deposition is, therefore, critical to predicting internal barriers for successful management of reservoirs with similar architecture.