Facies analysis, diagenesis, and petrophysical controls on the reservoir quality of the low porosity fluvial sandstone of the Nubian formation, east Sirt Basin, Libya: Insights into the role of fractures in fluid migration, fluid flow, and enhancing the permeability of low porous reservoirs. (January 2023)

• Record Type: Journal Article
• Authors: Marghani, Mouna M.A.; Zairi, Moncef; Radwan, Ahmed E.
• Is Part Of: Marine and petroleum geology. Volume 147(2023)
• Publication Date: 2023-01
• Subjects: Reservoir quality -- Low porous reservoir -- Fractures -- Fluvial sandstone -- Facies analysis -- Diagenesis -- Nubian formation -- Sirt basin -- Libya; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2022.105986
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: A better understanding of the factors controlling reservoir quality in low-porous heterogeneous reservoirs is very important for petroleum exploration and production. This study investigates the depositional and post-depositional factors that control the sandstone reservoir quality of the Upper Nubian Member, North Gialo Field, East Sirt Basin, Libya, using an integrated facies analysis, diagenesis, and petrophysical analysis. To investigate the relationship between facies and petrophysical parameters and identify the role of fractures in the pore system enhancement of low-porous reservoirs, we incorporate core analysis, petrography analysis, and petrophysical measurements. 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. Comparison among different FAs of the fluvial setting shows that the fluvial meandering and braided channel sandstones have the best reservoir quality, while the crevasse splay FAs display moderate reservoir quality, and the flood plain FA, as expected, has the lowest reservoir quality due to the high detrital clay content. The sedimentological analysis reveals that the primary textural characteristics of the lithofacies (i.e., grain size, sorting, and detrital clay content) have a medium effect on the distribution of porosity and permeability in the fluvial reservoir, while diagenetic events may have a large effect, contributing to the delivery of a heterogeneous reservoir. The facies analysis, petrographic analysis, and routine core analysis (RCAL) results revealed that the main factors controlling the degradation of reservoir quality are mechanical compaction, quartz overgrowth, and clay cement precipitation, while the feldspar and anhydrite dissolution and fractures resulted in enhanced reservoir quality. The fracture characterization and image log analysis revealed that fractures play a significant role in the reservoir permeability of low porous fluvial sediments, with five types of fractures dominating the fracture network in the studied reservoir. The research defines the fractures' origin and their role in fluid migration, fluid flow and reservoir quality enhancement, as well as demonstrates how the fractures work as conduits across the low porous sand units. Furthermore, it can provide a reference and direction for reservoir development in the search for the high-quality sweet spot in low-porosity fluvial reservoirs. Highlights: The Upper Nubian unit is a very heterogeneous reservoir with major lateral and vertical quality variations. The fluvial meandering and braided channel sandstones have the best reservoir quality, while the flood plain FAs have the lowest reservoir quality. The reservoir quality degradation is controlled by mechanical compaction, quartz overgrowth, and clay cement precipitation. The fractures worked as conduits across the low porous sand units and enhanced the oil flow through the reservoir. Insights into the role of fractures in oil migration and flow and enhancing the reservoir quality.