Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Evaluating top and fault seal

  | chapter = Evaluating top and fault seal

  | frompg  = 10-1

  | frompg  = 10-51

  | topg    = 10-94

  | topg    = 10-52

  | author  = Grant M. Skerlec

  | author  = Grant M. Skerlec

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

Once the ductility or paleoductility of a seal is known, the amount of strain that has affected the seal must be determined. One of the most useful techniques is '''incremental strain analysis'''. Originally applied to fractured reservoirs<ref name=ch10r91>Watts, N., L., 1983, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0067/0002/0200/0201.htm Microfractures in chalks of the Albuskjell field, Norwegian sector, North Sea: possible origin and distribution]: AAPG Bulletin, vol. 67, no. 2, p. 201–234.</ref> the technique is equally applicable to top seal analysis (<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> Koch et al., 1992).

Once the [[ductility]] or paleoductility of a seal is known, the amount of strain that has affected the seal must be determined. One of the most useful techniques is '''incremental strain analysis'''. Originally applied to fractured reservoirs<ref name=ch10r91>Watts, N., L., 1983, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0067/0002/0200/0201.htm Microfractures in chalks of the Albuskjell field, Norwegian sector, North Sea: possible origin and distribution]: AAPG Bulletin, vol. 67, no. 2, p. 201–234.</ref> the technique is equally applicable to top seal analysis (<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> Koch et al., 1992).

==Incremental strain analysis==

==Incremental strain analysis==

[[:file:evaluating-top-and-fault-seal_fig10-38.png|Figure 1]] is a simple [[cross section]] of a low-relief salt structure showing three seismic sequences. The figure shows how strain is calculated: by comparing the original undeformed line length with the deformed line length between two arbitrary points, A and B. In this figure, 1₀ is the initial, undeformed line length; 1₁ is the deformed line length of the 60-m.y sequence boundary; and 1₂ is the deformed line length of the 131-m.y. sequence boundary. The strain (ε) on the 131-m.y sequence boundary is ε = (1₂ – 1₀)/1₀ = 2.5%. The strain on the 60-m.y. sequence boundary is ε = (l₁ – 1₀)/1₀ = 0.5%. All strains are extensional.

[[:file:evaluating-top-and-fault-seal_fig10-38.png|Figure 1]] is a simple [[cross section]] of a low-relief salt structure showing three seismic sequences. The figure shows how strain is calculated: by comparing the original undeformed line length with the deformed line length between two arbitrary points, A and B. In this figure, 1₀ is the initial, undeformed line length; 1₁ is the deformed line length of the 60-m.y sequence boundary; and 1₂ is the deformed line length of the 131-m.y. sequence boundary. The strain (ε) on the 131-m.y sequence boundary is ε = (1₂ – 1₀)/1₀ = 2.5%. The strain on the 60-m.y. sequence boundary is ε = (l₁ – 1₀)/1₀ = 0.5%. All strains are extensional.

This method provides both an estimate of quantitative strain and the timing of strain. The incremental strain is the strain occurring during the time interval between two sequence boundaries. In the preceding figure, 0.5% strain occurred between 60 Ma and the present, while 2.5% strain occurred between 131 Ma and the present. Consequently, 2.0% strain occurred between 60 and 131 Ma. Strain–time plots display the strain affecting the top seal from the onset of deformation to the present.

This method provides both an estimate of quantitative strain and the timing of strain. The incremental strain is the strain occurring during the time interval between two sequence boundaries. In the preceding figure, 0.5% strain occurred between 60 Ma and the present, while 2.5% strain occurred between 131 Ma and the present. Consequently, 2.0% strain occurred between 60 and 131 Ma. Strain–time plots display the strain affecting the top seal from the onset of [[deformation]] to the present.

==Correction for compaction==

==Correction for compaction==

Analysis requires decompaction of the sediments to correct for the apparent strain caused by compaction. In the Central Graben, the effect is generally minimal.

Analysis requires [[decompaction]] of the sediments to correct for the apparent strain caused by compaction. In the Central Graben, the effect is generally minimal.

==Empirical threshold for top seal failure==

==Empirical threshold for top seal failure==

How much strain can a top seal withstand? In a mature basin, risk assessment of the strain threshold at which failure occurs is determined empirically by analyzing successes and failures. The empirical threshold varies from basin to basin. In a frontier setting, estimates of the strain required for failure must be based on (1) laboratory data on seal properties and (2) estimates of the confining pressure and seal ductility at the time of trap formation.

How much strain can a top seal withstand? In a mature basin, risk assessment of the strain threshold at which failure occurs is determined empirically by analyzing successes and failures. The empirical threshold varies from basin to basin. In a frontier setting, estimates of the strain required for failure must be based on (1) laboratory data on seal properties and (2) estimates of the confining pressure and [[seal ductility]] at the time of trap formation.

==Limitations of incremental strain analysis==

==Limitations of incremental strain analysis==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Evaluating top and fault seal]]

[[Category:Evaluating top and fault seal]]