Abnormal Pressure and the Occurrence of Hydrocarbons in Offshore Eastern Trinidad, West Indies
P.D. Heppard, H.S. Cander, E.B. Eggertson, 1998. "Abnormal Pressure and the Occurrence of Hydrocarbons in Offshore Eastern Trinidad, West Indies", Abnormal Pressures in Hydrocarbon Environments, B.E. Law, G.F. Ulmishek, V.I. Slavin
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Abnormal pore pressure is widespread in the Tertiary through upper Mesozoic, clastic-dominated section of the Eastern Venezuelan Basin and the eastern extension of the basin into Trinidad. Some of the largest oil and gas columns are found within abnormally pressured sandstones which account for 43 million bbl (6.8 million m3) of oil in Poui field and 882 billion ft3 (24.98 billion m3) of gas in Cassia field. Abnormal pressure within the Tertiary to Upper Cretaceous rocks resulted from the transfer of overburden stress to the pore system during the rapid subsidence and infilling of the foredeep basin during the Miocene and Pliocene. Primary migration from thick, Upper Cretaceous source rocks and secondary migration through the thick Tertiary elastics occurred principally through hydraulically induced fractures within a highly overpressured section. Final migration out of the overpressured section and charging of present-day reservoirs off the east coast of Trinidad occurred during the late Pliocene to Pleistocene uplift and associated complex normal faulting. The multiple pressure compartments within the six fields studied are separated by relatively thin, abnormally pressured shale. The shale seals are most effective in trapping hydrocarbons when the pressure difference across the shale is less than 4 psi/ft (90 kPa/m) regardless of the shale thickness. Normal faults form effective pressure seals throughout the basin, separating porous sandstone pressure compartments with pressure differences as great as 1,856 psi (12.8 MPa). The oil and gas fields of offshore Trinidad reveal a widely varying depth to the top of abnormal pressure, large pressure differences across faults, pressure reversals, and a narrow zone of transition from mild abnormal pressure (<11 PPG [lb/gal] equivalent) to highly overpressured conditions (>14 PPG equivalent).
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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.