Abnormally High Formation Pressure and Seal Impacts on Hydrocarbon Accumulations in the Nile Delta and North Sinai Basins, Egypt
M. Nashaat, 1998. "Abnormally High Formation Pressure and Seal Impacts on Hydrocarbon Accumulations in the Nile Delta and North Sinai Basins, Egypt", Abnormal Pressures in Hydrocarbon Environments, B.E. Law, G.F. Ulmishek, V.I. Slavin
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The Nile Delta and North Sinai Basins are active geodynamic (high subsidence rate) basins with a thick, clay-dominated Oligocene to Recent sedimentary section. Abnormally high formation pressures have developed in this section and in the underlying pre-Tertiary section primarily due to rapid sedimentation. Secondary mechanisms may be locally superimposed where the Messinian evaporite super seal is present. The abrupt development of pore pressure in the southern part of the Nile Delta is believed to be due to changes in the volume of pore fluids or rock matrix as a consequence of either aquathermal expansion, hydrocarbon generation, or thermal cracking of oil to gas in the lower Miocene-upper Oligocene compartment. Fluid flow in the Nile Delta and North Sinai Basins is mainly due to compaction-and thermal-driven forces.
The sedimentary sequence in the study area is divided into eight pressure compartments, separated by seals, some of which are associated with major unconformities. Four seals are clearly demonstrated in the North Sinai, Early Cretaceous basin and are referred to as: 1) Upper Jurassic-Lower Cretaceous; 2) Aptian; 3) Albian; and 4) Upper Cretaceous-Eocene carbonates. A total of four seals are also recognized in the Nile Delta and are referred to as: 5) Aquitanian-Burdgalian; 6) Langian; 7) Serravalian-Tortonian; and 8) Messinian.
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Abnormal pressures, pressures above or below hydrostatic pressures, occur on all continents in a wide range of geological conditions. According to a survey of published literature on abnormal pressures, compaction disequilibrium and hydrocarbon generation are the two most commonly cited causes of abnormally high pressure in petroleum provinces. In young (Tertiary) deltaic sequences, compaction disequilibrium is the dominant cause of abnormal pressure. In older (pre-Tertiary) lithified rocks, hydrocarbon generation, aquathermal expansion, and tectonics are most often cited as the causes of abnormal pressure.
The association of abnormal pressures with hydrocarbon accumulations is statistically significant. Within abnormally pressured reservoirs, empirical evidence indicates that the bulk of economically recoverable oil and gas occurs in reservoirs with pressure gradients less than 0.75 psi/ft (17.4 kPa/m) and there is very little production potential from reservoirs that exceed 0.85 psi/ft (19.6 kPa/m). Abnormally pressured rocks are also commonly associated with unconventional gas accumulations where the pressuring phase is gas of either a thermal or microbial origin. In underpressured, thermally mature rocks, the affected reservoirs have most often experienced a significant cooling history and probably evolved from an originally overpressured system.