Litho-Petro-Facies studies of the Cenomanian Awe/Keana sandstone, Middle Benue Trough, Nigeria: Implications for palaeodepositional environment and preliminary reservoir assessment

Abstract

To assess the reservoir potential and distribution of the Cenomanian Awe/Keana sandstone in the Middle Benue Trough, Nigeria, this study focuses on the litho- petro-facies examination of the rock. To evaluate the facies characteristics, mineralogical composition, grain size distribution, diagenetic processes, and reservoir attributes at the microscale, field geological mapping, thin section petrography, and scanning electron microscopy (SEM) investigations were used. The study discovered two lithofacies associations in addition to nine lithofacies, including sandstone, shale, mudrock, siltstone, and heterolith facies. The glauconitic, fine-grained, and very fine-grained facies of the Awe/Keana Sandstone were further separated into arkose, subarkose, and feldspathic graywacke based on mineralogy. The subarkose falls within the craton interior margin, the arkose within the dissected arc, and the feldspathic graywacke within the recycled orogeny, with some samples exhibiting calcareous characteristics due to the presence of carbonate minerals, according to the QFL (Qt-Ft-Lt and Qm-Ft-Lt) ternary diagram. Detrital   minerals   such   quartz,   orthoclase,   microcline,   plagioclase,   biotite, muscovite, and clay minerals were found in the Awe/Keana Formation when the mineralogical makeup and diagenetic processes of the formation were also investigated. The primary diagenetic processes were determined to be cementation, mechanical compaction, dissolution, grain replacement, and recrystallization. Primary and secondary porosity, including primary intergranular and matrix micropores, were identified using petrographic and SEM analysis. The moderate porosity and permeability of the Awe/Keana sandstones point to their potential as advantageous hydrocarbon reservoirs, possibly requiring stimulation or fracking for the best extraction. The study also evaluated the sandstones' provenance and found that they could have originated from the crystalline basement complex rocks of the Oban and Cameroon massifs, which are found in the interior of cratons, dissected arks, and transitional continental settings. The study's findings offer important information about reservoir quality, exploration risks, and decision-making for both academic and industrial drilling projects in the Middle Benue Trough.