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  | isbn    = 0891816607

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Normal reservoir pressure is the pressure in the reservoir fluids necessary to sustain a column of water to the surface.<ref name=pt03r19>Fertl, W. H., 1976, Abnormal formation pressures: New York, Elsevier Scientific Publishing Company, 382 p.</ref> Normal pressures range between 0.43 and 0.50 psi/ft. Normal drilling muds weigh about 9 ppg (pounds per gallon) and exert a bottom hole pressure of approximately 0.47 psi/ft of depth.

Normal reservoir pressure is the pressure in the [[reservoir fluids]] necessary to sustain a column of water to the surface.<ref name=pt03r19>Fertl, W. H., 1976, Abnormal formation pressures: New York, Elsevier Scientific Publishing Company, 382 p.</ref> Normal pressures range between 0.43 and 0.50 psi/ft. Normal drilling muds weigh about 9 ppg (pounds per gallon) and exert a bottom hole pressure of approximately 0.47 psi/ft of depth.

By convention in the [[petroleum]] industry, ''overpressure'' refers to pressures higher than normal that require heavy drilling mud to keep formation fluids from entering the borehole. Pressures lower than normal are called ''subnormal''.

By convention in the [[petroleum]] industry, ''overpressure'' refers to pressures higher than normal that require heavy drilling mud to keep formation fluids from entering the borehole. Pressures lower than normal are called ''subnormal''.

The distribution of reservoirs and overpressuring is strongly controlled by the depositional environment ([[:file:pressure-detection_fig1.png|Figure 1]]). Overpressured reservoirs are commonly found where there are thick deposits of shaly sediments.

The distribution of reservoirs and overpressuring is strongly controlled by the depositional environment ([[:file:pressure-detection_fig1.png|Figure 1]]). Overpressured reservoirs are commonly found where there are thick deposits of shaly sediments.

[[file:pressure-detection_fig2.png|thumb|300px|{{figure number|2}}Common patterns of increasing pressure with depth. In the case illustrated by line A, the pressure increases normally to a certain depth, then increases abruptly to almost the weight of the overburden, which it then parallels. In the case of line B, the increase of pressure above normal follows the aquathermal gradient (constant water density) and then follows the fracture gradient.<ref name=pt03r8>Barker, C., Horsfeld, B., 1982, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0001/0050/0099.htm Mechanical versus thermal cause of abnormally high pore pressures in shales— discussion]: AAPG Bulletin, v. 66, n. 1, p. 99–100.</ref>]]

[[file:pressure-detection_fig2.png|thumb|300px|{{figure number|2}}Common patterns of increasing pressure with depth. In the case illustrated by line A, the pressure increases normally to a certain depth, then increases abruptly to almost the weight of the overburden, which it then parallels. In the case of line B, the increase of pressure above normal follows the aquathermal gradient (constant water density) and then follows the fracture gradient.<ref name=pt03r8>Barker, C., and B. Horsfeld, 1982, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0001/0050/0099.htm Mechanical versus thermal cause of abnormally high pore pressures in shales— discussion]: AAPG Bulletin, v. 66, n. 1, p. 99–100.</ref>]]

====Aquathermal effects====

====Aquathermal effects====

====Tectonic phenomena====

====Tectonic phenomena====

Tectonic phenomena also produce overpressures. In the Gulf of Alaska, fluid pore pressures up to 0.85 psi per ft were found due to horizontal compressive stress in the rocks. In Western Alberta, large thicknesses of Paleozoic carbonates have been thrust over soft Cretaceous shales, resulting in overpressuring of the lenticular oil-bearing sandstones that extend under the overthrust (such as Leafland and Pembina).

Tectonic phenomena also produce overpressures. In the Gulf of Alaska, fluid pore pressures up to 0.85 psi per ft were found due to horizontal compressive stress in the rocks. In Western Alberta, large thicknesses of Paleozoic carbonates have been thrust over soft Cretaceous shales, resulting in overpressuring of the lenticular oil-bearing sandstones that extend under the [[overthrust]] (such as Leafland and Pembina).

====Thermal cracking of organic matter====

====Thermal cracking of organic matter====

[[Category:Wellsite methods]]

[[Category:Wellsite methods]]