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.

Analysis of intermediate-scale reservoir heterogeneity in a well exposed outcrop analogue of a point bar deposit within the Enagi Formation was carried out in order to reveal its reservoir dimensions. The study area is located in northern part of Bida basin, Nigeria. Three lithofacies were identified: (1) conglomerate facies interpreted as channel lag deposits; (2) epsilon and trough cross-bedded, fine to medium grained, sandstone facies interpreted as products of lateral accretion and 3D migration of subaqueous dunes in a mixed-load channel system; and (3) mudstone facies recording overbank deposits. Outcrop measurements reveal reservoir dimensions as follows: 1) bankfull channel depth of 3.9 m; 2) bankfull channel width of 31.5 m; 3) reservoir width (meander amplitude) of 132 m; and 4) reservoir length (meander wavelength) of 911 m. Analysis of internal lithologic variability within the point bar deposit allowed the detection of three main internal reservoir heterogeneities. They include: a) vertical reservoir heterogeneity resulting from overall fining-up trend within the point bar deposits; b) small scale vertical heterogeneity owing to fining-up grain size within each lateral accretion unit; c) lateral reservoir compartmentalization due to mud-drapes observed on some of the lateral accretion surfaces. Information provided in this work has implications for improved computation of Stock Tank Oil in Place (STOIP) at the initial stage of exploration and in planning of well spacing during development and production stages. This work, also suggests that field measurements on outcrop analogues combined with empirical computation of reservoir dimensions provides valuable information useful at all stages of exploration and development of an oil field.