Abnormally High Formation Pressures in Petroleum Regions of Russia and Other Countries of the C.I.S.
M.D. Belonin, V.I. Slavin, 1998. "Abnormally High Formation Pressures in Petroleum Regions of Russia and Other Countries of the C.I.S.", Abnormal Pressures in Hydrocarbon Environments, B.E. Law, G.F. Ulmishek, V.I. Slavin
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Statistical analyses of oil and gas pools in ancient platforms (Precambrian), young platforms (post- Hercynian),and mobile belts (foreland troughs, and intermontane depressions) of the Commonwealth of Independent States (C.I.S.), do not reveal any significant differences in the relationships between commercial production and the magnitude of overpressure. However, pressure data from all three structural provinces indicate 90% of all oil and gas pools are in reservoirs with pressure abnormality coefficients (Ac—the quotient of the measured pressure divided by the hydrostatic pressure) less than 1.8 (0.81 psi/ft, 18.7 kPa/m), suggestive of a commercially practical upper limit for productive reservoirs.
In ancient platforms, more than 90% of hydrocarbon pools are present in reservoirs in which the Ac is less than 1.7 (0.77 psi/ft, 17.8 kPa/m). Oil pools in ancient platform provinces are more common in reservoirs where the abnormality coefficient ranges from 1.1-1.3 (0.50-0.54 psi/ft, 11.5-12.4 kPa/m), whereas most gas pools occur in reservoirs with an Ac ranging from 1.06-1.1 (0.48-0.50 psi/ft, 11-11.5 kPa/ m). Very few oil and gas pools in ancient platforms occur in reservoirs with abnormality coefficients greater than 1.6 (0.72 psi/ft, 16.6 kPa/m). On young platforms, the majority of oil pools occur in reservoirs in which the Ac is less than 1.6 (0.72 psi/ft, 16.6 kPa/m). Gas pools in young platforms are found in reservoirs with abnormality coefficients as high as 2.0 (0.9 psi/ft, 20.8 kPa/m). In mobile belt provinces, 90% of oil pools occur in reservoirs with Ac values less than 1.8 (0.81 psi/ft, 18.7 kPa/m). A small number of pools occur in reservoirs where the abnormality coefficient is as high as 2.0 (0.9 psi/ft, 20.8 kPa/m). In mobile belt provinces, 90% of all oil and gas pools occur in reservoirs with Ac values less than 1.8 (0.81 psi/ft, 18.7 kPa/m) and are most common where the abnormality coefficient ranges from 1.2 to 1.3 (0.54-0.59 psi/ft, 12.4-13.6 kPa/m). The statistical data demonstrate that as the Ac increases, the frequency of occurrence of commercial oil and gas accumulations generally decreases. Very few oil and gas pools, regardless of structural setting, occur in reservoirs with abnormality coefficients greater than 1.8 (0.81 psi/ft, 18.7 kPa/m).
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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.