Relation of Shale Porosities, Gas Generation, and Compaction to Deep Overpressures in the U.S. Gulf Coast
John M. Hunt, Jean K. Whelan, Lorraine Buxton Eglinton, Lawrence M. Cathles, III, 1998. "Relation of Shale Porosities, Gas Generation, and Compaction to Deep Overpressures in the U.S. Gulf Coast", Abnormal Pressures in Hydrocarbon Environments, B.E. Law, G.F. Ulmishek, V.I. Slavin
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Direct measurements of porosities from Tertiary and Cretaceous shales in the Texas-Louisiana Gulf Coast show that in many areas shale porosity is either constant or increasing at the depths where high overpressures occur andwhere hydrocarbons are being generated. In the absence of a decrease in porosity with sediment load (depth), gasgeneration becomes the principal cause of overpressures and hydrocarbon expulsion.
Gulf Coast shale porosities decrease exponentially in normally compacting shales only down to porosities of about 30%, after which the decrease is linear until a constant porosity is reached. These linear trends are believedto be related to the high quartz content (74%) of the clay-size fraction (=4 microns).
The depths at which shales reach relatively constant porosity values appear to depend on the internal surface areas of the shales. Shales containing minerals with small, internal surface areas, such as finegrained quartz andcarbonates, stop compacting at porosities around 3%, whereas shales containing minerals with large surface areas, such as smectite and illite, stop compacting around 10%. This interval of no compaction usually is reached at depths around 3 to 4 kilometers (temperatures of 85° to 110°C) prior to the development of deep high overpressures and the generation of large quantities of hydrocarbons in the Gulf Coast. Model studies indicate that gas generation is the dominant process creating these deep overpressures.
The porosity-depth profiles that show a linear decrease with depth followed by a constant porosity do not conform to the hypothesized exponential profiles used in many modeling programs today. This means that more direct shale porosity measurements are needed to confirm the type of profiles that actually exist and should be used in any basin modeling program.
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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.