Facies Architecture and Sand Body Connectivity in Meandering River Deposits: Example from Enagi Formation, Northern Bida Basin, Northwestern Nigeria

Abstract

Meandering river deposits of the Enagi Formation constitute potential reservoir rocks within the northern part of the Bida Basin, NW Nigeria. Well exposed parts of this deposit located on a road-cut around Kawo, Niger State was studied. Field assessment of their lithofacies, external and internal characteristics and bounding surfaces reveal the presence of conglomerate, sandstone and mudstone lithofacies as well as channel (CH), lateral accretion (LA), downstream accretion (DA), floodplain (FF) and abandoned channel (FF (CH)) architectural elements. The CH, LA, and DA elements are expected to constitute the potential reservoirs. They display multistory and multilateral stacking patterns with resultant vertical and lateral connectivity of the channel sandstone bodies which can be explained using the string-of-beads model as against the commonly used isolated sandstone geobodies. Extensive floodplain mudstones constitute vertical permeability barriers while abandoned channels, mud lenses, mud drapes on lateral accretion surfaces and fining-up grain size trend within channel sandstones are considered potential permeability baffles at interwell to individual well-scales. This study suggests that detailed field assessment in addition to architectural element analysis of well exposed meandering river deposits is vital to understanding sub-seismic to field– scale heterogeneities that may not be possible even in mature oil fields where subsurface data are abundant