Assessment of pore pressure in the Oligocene–Pleistocene stratigraphy of the West Delta Deep Marine, offshore Nile Delta, Egypt

Abstract

One of the main drilling challenges in the offshore deep-water Nile Delta is the overpressured Tertiary shales, which causes formation fluid influxes, kicks, and very narrow drilling window, thus contributes to non-productive times and enhanced drilling risk. Accurate understanding of pore pressure distribution is crucial for casing design, mud optimization and safe and successful drilling. This study presents first ever in-depth analysis of the pore pressure distribution within the 4500 m thick Oligocene-Pleistocene stratigraphy from the West Delta Deep Marine block in deep-water Nile Delta. Direct formation pressure measurements indicated around 0.06–0.1 PSI/ft (1.36–2.26 MPa/km) gas gradient in the Pliocene El Wastani and Kafr El Sheikh sandstone reservoirs, while the Miocene Qantara sandstones are water-bearing with a 0.42 PSI/ft (9.5 MPa/km) pressure gradient. Shale porosity distribution exhibited additional porosity retention within the montmorillonite and mixed clay-dominated Late Pliocene and deeper sediments and marks the onset of overpressure at the top Kafr El Sheikh Formation. Based on the loading trends and acoustic slowness-density relationship, we inferred compaction disequilibrium as the primary overpressure generating mechanism resulted from high sedimentation rate. Shale pore pressure was interpreted by utilizing wireline logs by utilizing compaction trendline-based approach and calibrated with drilling events and mudlog data. Qantara and Tineh formations are characterized by 0.75–0.77 PSI/ft (16.96–17.41 MPa/km) pore pressure gradient leaving a narrow drilling mud window of 1.7–2 PPG. Based on vertical effective stresses, two significant overpressure compartments were identified in the Late Pliocene and Early Miocene-Late Oligocene, which were separated by the Middle Miocene Sidi Salem Formation acting as a pressure seal.