Formation Evaluation of the Lower Cretaceous Nubian Formation, East Sirt Basin, Libya

It is crucial to gain a deeper comprehension of the factors that influence reservoir quality in low-porous heterogeneous reservoirs for the purpose of petroleum exploration and production. This research examines the depositional and post-depositional influences on the sandstone reservoir quality of the Upper Nubian Member in the North Gialo Field, East Sirt Basin, Libya. To explore the connection between facies and petrophysical parameters and to determine the impact of fractures on enhancing the pore system in low-porous reservoirs, core analysis, petrography analysis, and petrophysical measurements are integrated. The study revealed that the Upper Nubian Member is a very heterogeneous reservoir with major lateral and vertical quality variations, and has low porosity and fair permeability. The reservoir can be subdivided into seven lithofacies and grouped into four dominant facies associations (FAs) (i.e., flood plain, crevasse splay, meandering channel, and braided channel) based on sedimentological analysis. Analysis of various lithofacies associations within the fluvial environment indicates that the fluvial meandering and braided channel sandstones exhibit superior reservoir quality. In contrast, the crevasse splay lithofacies associations demonstrate moderate reservoir quality, whereas the flood plain lithofacies association, as anticipated, showcases the poorest reservoir quality owing to its high detrital clay content. The examination of facies, petrography, and routine core analysis (RCAL) data indicated that mechanical compaction, quartz overgrowth, and clay cement precipitation are the primary factors influencing the deterioration of reservoir quality, whereas feldspar and anhydrite dissolution, along with fractures, contributed to the improvement of reservoir quality. The analysis of fracture characteristics and image logs demonstrated the substantial impact of fractures on the permeability of low-porosity fluvial sediments, with five predominant fracture types shaping the fracture network within the reservoir under investigation. The study delineates the source of fractures and their impact on fluid migration, fluid flow, and the improvement of reservoir quality. It also illustrates the function of fractures as pathways through the less porous sand units. Additionally, it offers guidance and insight for reservoir development in the quest for high-quality sweet spots in low-porosity fluvial reservoirs.