Sedimentary controls on the reservoir properties of the Paleogene fluvio-tidal sands of the Anambra Basin, southeastern Nigeria- implication for deepwater reservoir studies

Abstract

Reservoir sands from selected outcrops of the Eocene Nanka Formation were subjected to detailed field and laboratory studies in order to determine their reservoir properties.From the field studies, three lithostratigraphic units were delineated, namely-(I) basal sandstone unit of tidally influenced conglomeritic fluvial channel sandstones, (II) middle sandstone unit of tidally influenced cross-bedded sandstone unit with a thin mudrock unit, and (III) upper sandstone unit dominated by tidally influenced ferruginized sandstones with estuarine heterolith interval.The architecture of these units depicts south-easterly deposition in a tidally influenced fluvial environment.Granulometric analyses also reflect an upward decline in the sediment sorting, and an increase in clay content.The permeability values, which were generated using empirical equations, range between 49-121darcies, 37-45darcies and 13-37darcies, for the lower, middle and upper units respectively.The porosity also shows the same trends.The result of the petrographic studies shows that the reservoir heterogeneity at the microscale are mainly controlled by compaction, fracturing of the grains, chemical dissolution of unstable minerals, pressure solutions, and cementation, while at the megascale, major controls are bioturbations, clay drapes on foreset planes of cross-beds, clay dispersal within the sands and coarse grained lags at the base of channels.The reservoir sands are compartmentalized, especially by cm-thick elongate ferruginized and lateritic bands of sand, especially at the lower and middle sandstone units.The study of outcrop models of the inland basins is important because they increase the knowledge of reservoir heterogeneities and provides the foundation on which the rest of the subsurface reservoir simulation models can be built, especially for the petroliferous Tertiary Niger Delta Basin.