Changes

We don't really know how high the pore pressure must be to induce [[Fracture|fracturing]]. In field examples, failure seems to occur below the fracture pressure.<ref name=ch10r70>Skerlec, G. M., 1982, Risking top seals in the Central Graben: Exxon Production Research Company internal report.</ref><ref name=ch10r71>Skerlec, G. M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.</ref><ref name=ch10r21>Dutta, N. C., 1987, Fluid flow in low [[permeability]] porous media, in B. Doligez, ed., [[Migration]] of Hydrocarbons in Sedimentary Basins: Paris, Editions Technip, p. 567–596.</ref><ref name=ch10r49>Lerche, I., 1990, Basin Analysis I: San Diego, Academic Press, 562 p.</ref><ref name=ch10r84>Ungerer, P., J. Burrus, B. Doligez, P. Y. Chenet, and F. Bessis, 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0003/0000/0309.htm Basin evaluation by integrated two-dimensional modeling of heat transfer, fluid flow, hydrocarbon generation, and migration]: AAPG Bulletin, vol. 74, no. 3, p. 309–335.</ref> Risk increases as overpressure increases relative to P_f, but failure may occur below the theoretical fracture pressure. Other factors, not yet understood, control the point at which failure occurs.

We don't really know how high the pore pressure must be to induce [[Fracture|fracturing]]. In field examples, failure seems to occur below the fracture pressure.<ref name=ch10r70>Skerlec, G. M., 1982, Risking top seals in the Central Graben: Exxon Production Research Company internal report.</ref><ref name=ch10r71>Skerlec, G. M., 1990, SEALS: A short course for risking top seal and fault seal: Franklin, Pennsylvania, SEALS International, 600 p.</ref><ref name=ch10r21>Dutta, N. C., 1987, Fluid flow in low [[permeability]] porous media, in B. Doligez, ed., [[Migration]] of Hydrocarbons in Sedimentary Basins: Paris, Editions Technip, p. 567–596.</ref><ref name=ch10r49>Lerche, I., 1990, Basin Analysis I: San Diego, Academic Press, 562 p.</ref><ref name=ch10r84>Ungerer, P., J. Burrus, B. Doligez, P. Y. Chenet, and F. Bessis, 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0003/0000/0309.htm Basin evaluation by integrated two-dimensional modeling of heat transfer, fluid flow, hydrocarbon generation, and migration]: AAPG Bulletin, vol. 74, no. 3, p. 309–335.</ref> Risk increases as overpressure increases relative to P_f, but failure may occur below the theoretical fracture pressure. Other factors, not yet understood, control the point at which failure occurs.

Theoretically, [[Fracture|fracturing]] occurs when the pore pressure reaches P_f. However, P_f increases as pore pressure increases. Although the theory generally is described in terms of the pore pressure needed to overcome the horizontal stress keeping the fractures closed, in practice the pore pressure must approach the [[Geostatic and lithostatic pressure|lithostatic pressure]] for [[Brittleness|brittle failure]] to occur.<ref name=ch10r52>Lorenz, J., C., Teufel, L., W., Warpinski, N., R., 1991, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0075/0011/0000/1714.htm Regional fractures: a mechanism for the formation of regional fractures at depth in flat-lying reservoirs]: AAPG Bulletin, vol. 75, no. 11, p. 1714–1737.</ref>

Theoretically, [[Fracture|fracturing]] occurs when the pore pressure reaches P_f. However, P_f increases as pore pressure increases. Although the theory generally is described in terms of the pore pressure needed to overcome the horizontal stress keeping the fractures closed, in practice the pore pressure must approach the [[Geostatic and lithostatic pressure|lithostatic pressure]] for [[Brittleness|brittle failure]] to occur.<ref name=ch10r52>Lorenz, J. C., L. W. Teufel, and N. R. Warpinski, 1991, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0075/0011/0000/1714.htm Regional fractures: a mechanism for the formation of regional fractures at depth in flat-lying reservoirs]: AAPG Bulletin, vol. 75, no. 11, p. 1714–1737.</ref>

[[:file:evaluating-top-and-fault-seal_fig10-42.png|Figure 1]] charts P_f vs. pore pressure for a range of overburden stress gradients (0.5–1.0). The pore pressure equals or exceeds P_f only when the pore pressure is equal to or greater than the lithostatic stress. Fracture occurs when P_f equals the pore pressure.

[[:file:evaluating-top-and-fault-seal_fig10-42.png|Figure 1]] charts P_f vs. pore pressure for a range of overburden stress gradients (0.5–1.0). The pore pressure equals or exceeds P_f only when the pore pressure is equal to or greater than the lithostatic stress. Fracture occurs when P_f equals the pore pressure.

==Pore pressure variations==

==Pore pressure variations==

Pore pressure (p) is not always a smooth curve.<ref name=ch10r26>Engelder, T., Leftwich, J., 1993, An analysis of geopressure profiles from south Texas: the search for higher quality reservoirs in the Tertiary sections of the Gulf Coast: AAPG Hedberg Research conference, unpublished abstract.</ref> Decompaction is controlled in part by the permeability of sediment layers. Shales adjacent to high-permeability layers may undergo rapid decompaction while thick shale sequences with no immediate access to high-permeability layers will decompact more slowly and have high pore pressures. Local variations in pore pressure may be important in evolving seal integrity during basin subsidence.

Pore pressure (p) is not always a smooth curve.<ref name=ch10r26>Engelder, T., and J. Leftwich, 1993, An analysis of geopressure profiles from south Texas: the search for higher quality reservoirs in the Tertiary sections of the Gulf Coast: AAPG Hedberg Research conference, unpublished abstract.</ref> Decompaction is controlled in part by the permeability of sediment layers. Shales adjacent to high-permeability layers may undergo rapid decompaction while thick shale sequences with no immediate access to high-permeability layers will decompact more slowly and have high pore pressures. Local variations in pore pressure may be important in evolving seal integrity during basin subsidence.

==Effect of water depth, stratigraphy, and facies changes==

==Effect of water depth, stratigraphy, and facies changes==