A.J. Polutranko, 1998. "Causes of Formation and Distribution of Abnormally High Formation Pressure in Petroleum Basins of Ukraine", Abnormal Pressures in Hydrocarbon Environments, B.E. Law, G.F. Ulmishek, V.I. Slavin
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The maintenance of abnormally high formation pressure (AHFP) over long periods of geologic time cannot be explained by compaction or structural deformation without the addition of fluids from underlying rocks. Abnormally high pressures develop in the deepest parts of basins that contain 8-10 km of sedimentary rocks. The deeper part of the sedimentary fill typically occurs in the zones of late catagenesis and incipient metamorphism with temperatures ranging from 200° to 300°C. The observed increase of formation pressure above normal hydrostatic pressure with increasing depth and temperature, in conjunction with other factors, indicates that extended zones of AHFP first appear at temperatures of 175°C and higher. At lower temperatures, abnormally high pressures occur only locally at the crests of anticlinal structures.
The main cause of abnormal pressure in the zone of late catagenesis is the generation of large volumes of methane, carbon dioxide, hydrogen, water, and other volatile components. The devolatilization process is commonly associated with elevated temperatures, loss of porosity and permeability, and fracturing of rocks. In rocks which have experienced a high level of catagenesis, such as those in the structurally inverted parts of the Donbas foldbelt and the western Lviv depression adjacent to the Carpathian foldbelt, abnormally high formation pressures are absent.
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Abnormal Pressures in Hydrocarbon Environments
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.