Petrography, Diagenetic Evolution and Reservoir Quality Implications of the Bima Formation Sandstones in the Yola Sub-basin, Northern Benue Trough, Northeastern Nigeria

Abstract

The Lower Cretaceous Bima Formation constitutes the basal siliciclastic succession of the 
Upper Benue Trough and represents one of the most significant continental sandstone 
reservoirs and reservoir analogues in northeastern Nigeria. This study integrates field 
sedimentology, grain-size analysis, petrographic examination, and scanning electron 
microscopy (SEM) to investigate the depositional characteristics, diagenetic evolution, and 
reservoir quality of selected Bima Formation sandstones exposed within the Yola Sub-basin. 
Field observations reveal thick-bedded to massive sandstone bodies characterized by planar 
stratification, erosional bounding surfaces, and vertically stacked channel architectures 
indicative of deposition within high-energy braided fluvial systems. Grain-size analyses 
demonstrate predominantly fine- to coarse-grained sediments exhibiting moderate to good 
sorting, reflecting variable hydraulic conditions and fluctuating sediment transport regimes 
during deposition. Petrographic observations indicate that the sandstones are composed 
predominantly of monocrystalline and polycrystalline quartz with subordinate feldspar and 
lithic fragments, suggesting moderate to high compositional maturity resulting from 
prolonged transport and sediment recycling. SEM investigations reveal a complex diagenetic 
history involving mechanical compaction, grain rearrangement, quartz overgrowth 
development, authigenic clay mineral precipitation, carbonate cementation, and localized 
dissolution. Mechanical compaction and cementation constitute the principal porosity
reducing processes through progressive pore-throat occlusion and destruction of primary 
intergranular pore networks. Conversely, dissolution of unstable framework grains and 
cements has generated localized secondary porosity, partially enhancing pore connectivity and 
storage capacity. The integrated sedimentological and petrographic evidence indicates that 
reservoir quality evolution in the Bima Formation is controlled by the combined influence of 
depositional texture and subsequent diagenetic modification. Reservoir quality varies from 
moderately good intervals characterized by preserved intergranular porosity to tight 
sandstone zones dominated by intense compaction and cementation. The study establishes a 
diagenetic framework for understanding pore system evolution and reservoir heterogeneity 
within the Bima Formation and provides valuable insights for the exploration and 
characterization of fluvial sandstone reservoirs in the Northern Benue Trough and analogous 
continental rift basins.