Abstract
A field mapping and laboratory study was undertaken to characterize the lower portion of Bida Formation around Jima and Doko areas near Bida, central Nigeria. Field mapping revealed four lithofacies for the sandy braided river deposits. They are: (1) lithofacies Fa consists of gravely, very coarse sandstones; (2) lithofacies Fb is characterized by gravely, medium to very coarse grained sandstones; (3) lithofacies Fc is identified by coarse silt to fine grained sandstones; (4) lithofacies Fd is recognized by sheet-like beds of fine to medium grained sandstones. They record channel flow (Fa), subaqueous dune (Fb), overbank (Fc) and crevasse splay deposits (Fd) respectively. The four lithofacies allowed the definition of two architectural elements: (1) channel architectural element (CH) and (2) overbank architectural element (OF). A CH element overlain by an OF element defines a unit bar; while stacked, multiple CH overlain by an OF delineate a compound bar. Computation of grain size data for Fa and Fb facies yielded permeability values of 19.4 to 352.3 millidarcies using empirically generated equation. Because permeability values do not vary much, it is suggested that Fa and Fb be treated as one homogeneous entity if object based reservoir modelling of the sandstones is to be done. This work shows that detailed field mapping integrated with grain size data can offer valuable information useful for aquifer and reservoir studies of the sandstones.
Highlights
The distinction between single and stacked channel floor, channel bar and overbank units in fluvial successions is key to understanding the dynamics of fluid flow within them
Four main lithofacies have been identified and they can be related to lithofacies codes and architectural elements of Mial [12, 4]
The single fining up channel architectural element (CH) plus overlying OF elements of the present work constitute the lower and upper bar portions of unit bars while the composite fining up units formed from stacked CH elements with the overlying OF element are the compound bars
Summary
The distinction between single and stacked channel floor, channel bar and overbank units in fluvial successions is key to understanding the dynamics of fluid flow within them. Fluvial sandstones and conglomerates represent important hydrocarbon reservoirs and water aquifers the world over. The study area is situated near the central part of Bida basin, a major inland sedimentary basin in Nigeria (Fig. 1). The Bida basin is a NW-SE trending structure which extends from Kontagora in the northern sector to Lokoja in the southern one covering a distance of about 400 km. It has a sedimentary fill of up to 4.7 km with an average of 3 km [6]
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