Provenance, tectonic settings, depositional conditions, and diagenetic features of the late Cretaceous (Coniacian–Santonian) Fika Shale, Upper Benue Trough, Northeastern Nigeria: Implications for hydrocarbon potential

Fine-grained clastic sedimentary rocks (e.g., shale and mudstone) play critical roles in subsurface traditional and transitional energy systems. They serve as hydrocarbon source rocks, top seals in petroleum systems, cap rocks for CO₂ sequestration, and potential repositories for radioactive waste disposal. Their effectiveness in these roles is strongly influenced by factors such as provenance, paleoclimate and chemical weathering, mineralogical composition, textural characteristics, mechanical integrity, and diagenetic history. The Late Cretaceous Fika Shale, located within the Upper Benue Trough of northeastern Nigeria, is considered a potential hydrocarbon source rock and cap rock for CO₂ storage. However, its provenance, paleoweathering characteristics, mineralogy, and diagenetic evolution remain poorly constrained. To address this knowledge gap, this study employs an integrated multi-technique approach involving X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), thin-section petrography, and total organic carbon (TOC) analysis. Provenance analysis indicates that the Fika Shale was derived predominantly from intermediate igneous rocks and deposited under semi-arid to humid climatic conditions. The sediments accumulated in a shallow marine environment characterized by brackish to saline water, oxic to dysoxic redox conditions, low detrital influx, and reduced paleoproductivity, resulting in poor organic matter accumulation and preservation (average TOC = 0.2 wt%). The predominance of kaolinite, along with elevated chemical weathering indices, confirm that the sediments experienced intense chemical weathering in the source area. Tectonic setting analysis using multidimensional discriminant function diagrams reveals that deposition occurred in a rift-related passive margin setting. Diagenetic features include cementation, mineral dissolution, and minor mechanical compaction, despite the shale's exposure to a deep burial diagenetic regime. Localized deformation was observed around cone-in-cone calcite concretions, where shale laminae were bent. Kaolinite and smectite have been partially transformed into illite, which is the dominant deep-burial diagenetic clay. The occurrence of minor illite indicates limited thermal maturity, consistent with the absence of hydrocarbon generation in the studied shale. Overall, the results indicate that the Fika Shale was deposited under oxidizing conditions and exhibits limited hydrocarbon generation potential.