Evaluation of the Tight Brown Limestone as Unconventional Reservoir, Northwest October Field, Gulf of Suez, Egypt

ABSTRACT The Gulf of Suez remains the most prolific and established petroleum province in Egypt. Within its central province, the Northwest October field has emerged as a key asset, sustaining commercial Eocene and Miocene oil production since 2019 following an extensive well‐workover campaign. A major exploration breakthrough occurred in 2021 when the Petrosalam Company achieved a structural discovery within the Upper Cretaceous Brown Limestone Formation, which initially flowed at approximately 700 barrels of oil per day (bopd). This discovery is highly significant because the organic‐rich Brown Limestone traditionally functions strictly as a primary source rock in neighboring structures, such as the October and Issran fields. This study evaluates the reservoir potential of the tight Brown Limestone as an unconventional, fracture‐dependent play and delineates the precise structural and stratigraphic factors controlling local hydrocarbon entrapment. The structural and reservoir models were constructed using a high‐fidelity 3D Pre‐Stack Depth Migration (PSDM) seismic survey—re‐processed in 2023—integrated with comprehensive sub‐surface data, including wireline logs, dipmeter profiles, and oil‐based mud microimager (OBMI) data. Structural contour mapping on top of the Brown Limestone Formation reveals a complex tectonic architecture dominated by NW–SE‐trending Oligo–Miocene Clysmic normal faults that are subsequently segmented by younger, NE–SW‐trending cross‐faults. This late‐stage fault intersection divides the target interval into distinct, isolated structural compartments, accounting for the abrupt lateral variations in productivity observed between adjacent well penetrations. Kinematic and petrophysical analyses demonstrate that stratal dip and fault displacement magnitude dictate lateral seal efficiency and volumetric column preservation. Within a critical vertical throw window of 600–950 ft, juxtaposition against impermeable hanging‐wall sequences (Esna Shale and Sudr Chalk) provides an effective lateral seal for the upthrown footwall blocks. Conversely, displacements exceeding 950 ft facilitate cross‐fault fluid communication with highly fractured Eocene and Nukhul sequences, acting as a critical migration pathway. Hydrocarbon charge migrates laterally from the deep western fetch basin, charging synthetic tilted fault blocks and three‐way dip closures. Risk‐weighted volumetric modeling indicates that validating this structural trapping model across unpenetrated up‐dip compartments unlocks a significant Proved plus Probable (2P) reserve upside of 2.13 MMSTB, expanding to a Proved plus Probable plus Possible (3P) resource potential of 7.54 MMSTB, which provides clear targets to optimize future field development.