Linking provenance and diagenesis to reservoir quality evolution of sandstones: The Paleocene-Eocene Kerri-Kerri Formation, northeastern Nigeria

Abstract

A comprehensive understanding of the controls on sandstone reservoir quality requires the integration of provenance, detrital mineralogy, and diagenesis to effectively optimize their utilization for groundwater and hydrocarbon explorations or CO₂ storage. However, studies on the roles of sedimentary provenance and diagenesis in reservoir quality evolution of sandstones are often conducted separately, even though combining both is crucial for a comprehensive understanding of how these processes jointly influence reservoir quality evolution in sandstones. The Paleocene-Eocene Kerri-Kerri Formation in the Gongola Sub-basin, Upper Benue Trough (NE Nigeria), has yet to be assessed for how the interplay between sedimentary provenance and diagenesis influences its reservoir quality evolution. Therefore, this study aims to investigate the provenance of the Kerri-Kerri Formation and its influence on diagenetic alterations and reservoir quality through thin section petrography, scanning electron microscopy, X-ray diffraction, and X-ray fluorescence analyses. The findings of the study reveal that the sediments were primarily sourced from a quartzose sedimentary provenance and were deposited under warm, humid climatic conditions, influencing diagenesis and reservoir quality by resulting in a chemically stable, quartz-rich mineralogy characterized by near-complete dissolution and alteration of labile feldspar and mica grains. The provenance of the sandstones has significantly contributed to an unusual accumulation of kaolinite (2–46%; average 12.9%) due to intense rainfall, which released previously authigenic kaolinite from the source sandstones and incorporated it into the Kerri-Kerri sediments as detrital clay. The stable quartz-rich mineralogy limited the effects of mechanical compaction in the sandstones, aided by the absence of ductile grains. Minor authigenic kaolinite was formed as eogenetic clay due to the very low reactive feldspar content in the sandstones, while illite and authigenic quartz overgrowths developed as deep-burial cements. However, their effects are largely insignificant due to the scarcity of K-feldspar needed to supply K⁺ for illitization and the presence of well-developed clay coatings that significantly inhibited quartz cementation. Illitization of kaolinite preferentially occurred where discrete kaolinite crystals arranged themselves in a chain-like fashion near K-feldspar, providing the surface area for kaolinite dissolution and K⁺ adsorption in the formation of authigenic illite. Additionally, while grain size and sorting had minimal control over reservoir quality due to the similarities in grain size and sorting classes among the sandstones, detrital matrix content was the primary factor influencing reservoir quality. Channel fill sandstones exhibited the best reservoir quality owing to their low clay matrix, whereas occasional ripple-laminated sandstones from floodplain settings demonstrated the poorest reservoir quality due to their high matrix content. The findings of this study can enhance the understanding of reservoir quality controls in the subsurface equivalents of the Kerri-Kerri Formation in the basin or elsewhere, thereby facilitating their optimal utilization for hydrocarbon and groundwater explorations or CO₂. Our findings have significant implications for understanding the interplay between provenance and diagenesis in controlling the reservoir quality evolution of the Kerri-Kerri Formation. Additionally, this study offers valuable insights for assessing similar continental sandstones globally, helping to optimize their use in subsurface energy and water resource explorations. The quantitative data provided can also be applied in reservoir quality prediction modeling for the subsurface Kerri-Kerri Formation and analogous formations elsewhere.