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We often assume that a structure remains static when charged by petroleum. Traps may be charged during structural growth, and accumulations can be partially or completely spilled by later structural deformation.

We often assume that a structure remains static when charged by petroleum. Traps may be charged during structural growth, and [[accumulation]]s can be partially or completely spilled by later structural deformation.

Traps charged during structural growth are not destroyed by spillage as long as the trapping geometry is maintained during deformation because petroleum migrates with the structural closure much faster than the rate of structural growth.<ref name=ch11r15>Hubbert, M., K., 1953, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0037/0008/1950/1954.htm Entrapment of petroleum under hydrodynamic conditions]: AAPG Bulletin, vol. 37, p. 1954–2026.</ref> Conversely, if structural closure is destroyed during deformation, spillage occurs rapidly.

Traps charged during structural growth are not destroyed by spillage as long as the trapping geometry is maintained during deformation because petroleum migrates with the structural closure much faster than the rate of structural growth.<ref name=ch11r15>Hubbert, M., K., 1953, [http://archives.datapages.com/data/bulletns/1953-56/data/pg/0037/0008/1950/1954.htm Entrapment of petroleum under hydrodynamic conditions]: AAPG Bulletin, vol. 37, p. 1954–2026.</ref> Conversely, if structural closure is destroyed during deformation, spillage occurs rapidly.

==Change in a fold trap==

==Change in a fold trap==

[[file:predicting-preservation-and-destruction-of-accumulations_fig11-1.png|300px|thumb|{{figure number|1}}Continued growth of a foreland-sloping duplex preserves an accumulation in an early duplex but displaces the accumulation relative to the reservoir rock. Modified from Mitra;<ref name=Mitra1986>Mitra, S., 1986, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0070/0009/1050/1087.htm Duplex structures and imbricate thrust systems: geometry, structural position, and hydrocarbon potential]: AAPG Bulletin, vol. 70, p. 1087–1112.</ref> courtesy AAPG.]]

[[file:predicting-preservation-and-destruction-of-accumulations_fig11-1.png|300px|thumb|{{figure number|1}}Continued growth of a foreland-sloping duplex preserves an [[accumulation]] in an early duplex but displaces the accumulation relative to the reservoir rock. Modified from Mitra;<ref name=Mitra1986>Mitra, S., 1986, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0070/0009/1050/1087.htm Duplex structures and imbricate thrust systems: geometry, structural position, and hydrocarbon potential]: AAPG Bulletin, vol. 70, p. 1087–1112.</ref> courtesy AAPG.]]

[[:file:predicting-preservation-and-destruction-of-accumulations_fig11-1.png|Figure 1]] shows how continued growth of a foreland-sloping duplex preserves an accumulation in an early duplex but displaces the accumulation relative to the reservoir rock. The stippled area outlining the initial accumulation is fixed relative to the rock. The solid area on the lower figure marks the accumulation at the top of the structure after movement.

[[:file:predicting-preservation-and-destruction-of-accumulations_fig11-1.png|Figure 1]] shows how continued growth of a foreland-sloping duplex preserves an accumulation in an early duplex but displaces the accumulation relative to the reservoir rock. The stippled area outlining the initial accumulation is fixed relative to the rock. The solid area on the lower figure marks the accumulation at the top of the structure after movement.

==Evaluating spillage==

==Evaluating spillage==

Structural spillage is avoided if trapping geometry is maintained during deformation after charging. Structural closure must be maintained at all times during subsequent deformation. Throws on faults likely to cut the seal at the accumulation should be less than the seal thickness to avoid spillage by juxtaposition across the fault plane.

Structural spillage is avoided if trapping geometry is maintained during deformation after charging. Structural closure must be maintained at all times during subsequent deformation. Throws on faults likely to cut the seal at the [[accumulation]] should be less than the seal thickness to avoid spillage by juxtaposition across the fault plane.

Spillage potential can be evaluated by combining geohistory analysis and structural analysis. '''Geohistory analysis''' (combined analysis of burial, thermal, and generation history) of gathering areas for prospects gives the range of charging times for the prospect [essentially the time of [[Petroleum generation|generation]] in nearby gathering areas<ref name=ch11r8>England, W., A., Mann, A., L., Mann, D., M., 1991, [http://archives.datapages.com/data/specpubs/geochem1/data/a037/a037/0001/0000/0023.htm Migration from source to trap], in Merrill, R., K., ed., Source and Migration Processes and Evaluation Techniques: [http://store.aapg.org/detail.aspx?id=436 AAPG Treatise of Petroleum Geology Handbook of Petroleum Geology], p. 23–46.</ref>]). '''Structural analysis''', using [[Balanced cross sections|balanced structural cross sections]] as well as cross-cutting and superposition relationships, gives the range of times for trapping geometry formation and failure.

Spillage potential can be evaluated by combining geohistory analysis and structural analysis. '''Geohistory analysis''' (combined analysis of burial, thermal, and generation history) of gathering areas for prospects gives the range of charging times for the prospect [essentially the time of [[Petroleum generation|generation]] in nearby gathering areas<ref name=ch11r8>England, W., A., Mann, A., L., Mann, D., M., 1991, [http://archives.datapages.com/data/specpubs/geochem1/data/a037/a037/0001/0000/0023.htm Migration from source to trap], in Merrill, R., K., ed., Source and Migration Processes and Evaluation Techniques: [http://store.aapg.org/detail.aspx?id=436 AAPG Treatise of Petroleum Geology Handbook of Petroleum Geology], p. 23–46.</ref>]). '''Structural analysis''', using [[Balanced cross sections|balanced structural cross sections]] as well as cross-cutting and superposition relationships, gives the range of times for trapping geometry formation and failure.