Rock typing and reservoir characterization of the Messinian Abu Madi Formation in the onshore South Abu El Naga Gas Field, Nile Delta, Egypt

Abstract

Abstract Exploration of the heterogeneous sandstone reservoirs presents a significant opportunity within the Nile Delta Basin. This study uses the well log, core, and petrographical data to describe the different rock types and characterizes the heterogenous sandstone of the Late Miocene Messinian Abu Madi reservoir as one of the main prolific reservoirs in the South Abu El Naga Gas Field in the Nile Delta. However, accurate assessment of the potential of these complex and heterogeneous sandstone reservoirs requires a meticulous approach. The available data was imported from four wells: SAEN-2, SAEN-4, SAEN-6, and SAEN-9. A total of 35 core plugs, which were derived from two cored intervals in the SAEN-2 well, were used in a well-integrated workflow for reservoir characterization, facies analysis, and rock typing. Core analysis (grain density ‘ρg’, helium porosity ‘∅He’, horizontal and vertical permeabilities ‘kH & kV’, and water saturation ‘Sw’) and well log data (caliper, gamma-ray, spontaneous potential, PEF, density, neutron, and resistivity logs) provided crucial insights into the lithology, pore systems, and textures. This information allowed us to define the dominant microfacies types as quartz arenite, feldspathic arenite, quartz wacke/wacke, feldspathic wacke, and subfeldspathic wacke. With the core data, it was also possible to estimate the reservoir quality index (RQI), flow zone indicator (FZI), and the effective pore radius (R35) from core data, while the net pay thickness, the effective porosity, the shale volume (Vsh), and the water saturation (Sw) from the well log data. It also enabled the identification of the potential zones of the gas-bearing reservoirs. Hydraulic flow units (HFUs) were established using well logs and core data. These units represent zones with similar fluid flow properties, facilitating the prediction of gas deliverability. Additionally, the flow zone indicator (FZI) that derived from the well logs further characterized the flow regime within the reservoir. Sedimentological studies, including thin section petrography, XRD, and SEM, complemented with the well log interpretation. This integrated workflow provided a comprehensive perspective of the reservoir, including pore structures, mineral composition, and textures. The Abu Madi Formation in the SAEN-9 well, to the northeast of the field, has the lowest net pay (7.3 m), while the SAEN-2 well, in the center of the field, has the highest net pay thickness (16.6 m). The core studies indicate that SAEN samples could be divided into four reservoir rock types (RRTs). The RRT1 has the lowest reservoir quality (0.12 ≤ ∅He ≤ 0.26, 2.4 ≤ kH ≤ 429 mD, 54.9 ≤ Sw ≤ 70.5%, 0.14 ≤ RQI ≤ 1.22 μm, 0.82 ≤ FZI ≤ 3.863 μm, and 1.055 ≤ R35 ≤ 11.41 μm), while the RRT4 has the best reservoir quality (0.25 ≤ ∅He ≤ 0.28, 2680 ≤ kH ≤ 4893 mD, 45.4 ≤ Sw ≤ 55.3%, 3.24 ≤ RQI ≤ 4.13 μm, 9.72 ≤ FZI ≤ 10.59 μm, and 34.668 ≤ R35 ≤ 44.78 μm). This study demonstrates the effectiveness of an integrated approach in comprehensively assessing the potential gas-bearing reservoirs and defining their quality in the Abu Madi Formation in the Nile Delta, which is characterized by very good reservoir quality (net pay thickness = 7.3–16.6 m, av. porosity = 23.3–30.35%, and av. water saturation = 31.7–64.0% for the various wells). The findings contribute significantly to optimizing exploration and development strategies for gas-bearing hydrocarbon resources in the Nile Delta Basin, especially for the Abu Madi reservoir.