article · Scientific Reports
This study examines the Messinian Qawasim gas-bearing deltaic sequence in the Egyptian Nile Delta to determine how lithofacies and microfacies influence reservoir characteristics across different deltaic depositional stages. The reservoir was divided into upper and lower clastic units and analysed at both borehole and core-plug scales using geophysical logs and core measurements. Across the sequence, five lithofacies and four microfacies were identified and classified into five distinct reservoir rock types. The upper unit contains the highest-quality reservoir rocks, primarily pebbly massive and cross-bedded sandstones made of sublithic and subarkose arenites. In contrast, the lowest quality occurs in the lower unit, dominated by laminated mudstones and siltstones. By integrating core, petrographic, borehole, and seismic datasets, the study establishes an evaluation workflow previously unapplied to onshore Nile Delta deltaic systems.
Accurately understanding rock quality across varied deltaic environments helps geoscientists identify the most productive intervals within gas reservoirs. By pinpointing the specific rock types that yield superior storage and flow properties, exploration and development teams can better target sweet spots, refine geological models, and improve resource estimation in both the Nile Delta and analogous deltaic basins worldwide.
The findings and integrated multi-scale workflow can be applied by upstream energy companies and subsurface consultants evaluating deltaic gas reservoirs. Judged from the abstract, the methodology represents applied and tested research validated on core and well-log datasets in the Nile Delta. It can serve as an operational template for reservoir quality assessment, assisting teams in targeting productive intervals and reducing appraisal uncertainty in similar deltaic plays globally.
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The Messinian Qawasim sequence, as one of the most important reservoirs in the Egyptian Nile Delta, represents a typical gas-bearing deltaic reservoir sequence. It aims at delineating the implication of the litho- and microfacies associations on the reservoir characteristics of the different stages of the fluvial deltas including the prodelta, proximal/distal delta front, and the delta plain depositional sequences. The studied reservoir sequence was divided into two units; upper and lower clastic units. The petrophysical properties of these two units were studied on the borehole scale using the gamma-ray, caliper, sonic, density, neutron, and resistivity logs to estimate the reservoir parameters including the total and effective porosities, water saturation, shale volume, and the net-pay thickness. For more details, they are also examined on the plug-scale using the core dataset including helium porosity, density, permeability, and fluids saturations, where the flow zone indicator, the reservoir potentiality index, the reservoir quality index, and the average reservoir pore radius were then estimated. The entire sequence is divided into five reservoir rock types (RRT1-RRT5) where, the best quality is assigned to the first RRT (the upper reservoir unit), and the lowest quality is assigned to the RRT5 of the lower reservoir unit. Based on core description and the petrographical studies five lithofacies and four microfacies have been identified. The lithofacies are (1) laminated mudstones/siltstones, (2) ripple laminated sandstones, (3) cross-laminated sandstones, (4) cross-bedded sandstones, and (5) pebbly massive sandstones. These lithofacies are primarily composed of four microfacies; sublithic arenites, subarkose arenite, glauconitic quartz wacke, and sandy mudstone/siltstone. Based on this study, the pebbly massive and the cross-bedded sandstones of the upper unit which is composed of the sublithic and subarkose arenites (RRT1-RRT2) have the best reservoir quality. On the other hand, the lowest quality is assigned to the RRT5 (sandy mudstone/siltstone microfacies) which is represented on the macro scale by the laminated mudstones/siltstones lithofacies. The integration between multi-scale datasets (core-well, petrography, well logs, and seismic) gives a precise picture of the deltaic Qawasim reservoir rock units in the Nile Delta. This workflow has never been applied to the deltaic system in the onshore Nile delta and North Africa. Thereby, this study is considered a standard case study for the deltaic sequences and its proposed workflow is applicable to the Nile Delta elsewhere and worldwide for similar reservoir sequences.
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DOI: 10.1038/s41598-023-35660-0
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