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  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

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[[:file:evaluating-top-and-fault-seal_fig10-39.png|Figure 1]] is an example of evaluating, or risking, top seal integrity using two traps in the Central Graben, North Sea. One trap was dry; one trap is now a producing field. Both are low-relief salt structures, a common trap style in the Central Graben, with Upper Jurassic reservoirs sealed by Upper Jurassic-Lower Cretaceous shales. A seismic line across both traps is shown in the following figure. Trap A is at the left; trap B is at the center.

[[:file:evaluating-top-and-fault-seal_fig10-39.png|Figure 1]] is an example of evaluating, or risking, top seal integrity using two traps in the Central Graben, North Sea. One trap was [[dry]]; one trap is now a producing field. Both are low-relief salt structures, a common trap style in the Central Graben, with Upper Jurassic reservoirs sealed by Upper Jurassic-Lower Cretaceous shales. A seismic line across both traps is shown in the following figure. Trap A is at the left; trap B is at the center.

[[file:evaluating-top-and-fault-seal_fig10-39.png|300px|thumb|{{figure number|1}}Evaluating, or risking, top seal integrity using two traps in the Central Graben, North Sea. Copyright: Esso Exploration and Production U.K.]]

[[file:evaluating-top-and-fault-seal_fig10-39.png|300px|thumb|{{figure number|1}}Evaluating, or risking, top seal integrity using two traps in the Central Graben, North Sea. Copyright: Esso Exploration and Production U.K.]]

[[file:evaluating-top-and-fault-seal_fig10-40.png|thumb|300px|{{figure number|2}}Strain-time plots showing the incremental strains affecting the top seal above each trap and the wide variation in strain magnitude as well as strain history in these traps.]]

[[file:evaluating-top-and-fault-seal_fig10-40.png|thumb|300px|{{figure number|2}}Strain-time plots showing the incremental strains affecting the top seal above each trap and the wide variation in strain magnitude as well as strain history in these traps.]]

The strain-time plots in [[:file:evaluating-top-and-fault-seal_fig10-40.png|Figure 2]] show the incremental strains affecting the top seal above each trap and the wide variation in strain magnitude as well as strain history in these traps. Trap A has a maximum strain of 4.5%, which contributes to the risk of top seal fracturing. The time of maximum strain occurred 97-60 Ma. Today, trap Ais a dry hole because of fracturing and loss of top seal integrity. Trap B has a maximum strain of only 1%, which occurred before 100 Ma, and only a 0.2% strain that occurred 97-60 Ma. Today, trap B is a producing structure with an intact top seal capable of trapping hydrocarbons in the underlying Upper Jurassic sands.

The strain-time plots in [[:file:evaluating-top-and-fault-seal_fig10-40.png|Figure 2]] show the incremental strains affecting the top seal above each trap and the wide variation in strain magnitude as well as strain history in these traps. Trap A has a maximum strain of 4.5%, which contributes to the risk of top seal fracturing. The time of maximum strain occurred 97-60 Ma. Today, trap Ais a [[dry hole]] because of fracturing and loss of top seal integrity. Trap B has a maximum strain of only 1%, which occurred before 100 Ma, and only a 0.2% strain that occurred 97-60 Ma. Today, trap B is a producing structure with an intact top seal capable of trapping hydrocarbons in the underlying Upper Jurassic sands.

==Paleoductility analysis==

==Paleoductility analysis==

==Strain threshold for failure==

==Strain threshold for failure==

[[file:evaluating-top-and-fault-seal_fig10-41.png|thumb|300px|{{figure number|3}}traps with a maximum post-100 Ma incremental strain > 1.6% are dry (traps H, I, J). Traps with a strain <1.6% are still intact and contain hydrocarbons (traps B-G). Trap A, a structure with very low strain, is dry because of natural hydraulic fracturing. Traps that were dry ue to other causes such as lack of reservoir or source are not included in this plot.]]

[[file:evaluating-top-and-fault-seal_fig10-41.png|thumb|300px|{{figure number|3}}traps with a maximum post-100 Ma incremental strain > 1.6% are [[dry]] (traps H, I, J). Traps with a strain <1.6% are still intact and contain hydrocarbons (traps B-G). Trap A, a structure with very low strain, is dry because of natural hydraulic fracturing. Traps that were dry ue to other causes such as lack of reservoir or source are not included in this plot.]]

Incremental strain analysis of a group of successes and failures in the Central Graben defines a strain threshold at approximately 1.6%. In [[:file:evaluating-top-and-fault-seal_fig10-41.png|Figure 3]], traps with a maximum post-100 Ma incremental strain > 1.6% are dry (traps H, I, J). Traps with a strain <1.6% are still intact and contain hydrocarbons (traps B-G). Trap A, a structure with very low strain, is dry because of [[Natural hydraulic fracturing of top seals|natural hydraulic fracturing]]. Traps that were dry ue to other causes such as lack of reservoir or source are not included in this plot.

Incremental strain analysis of a group of successes and failures in the Central Graben defines a strain threshold at approximately 1.6%. In [[:file:evaluating-top-and-fault-seal_fig10-41.png|Figure 3]], traps with a maximum post-100 Ma incremental strain > 1.6% are dry (traps H, I, J). Traps with a strain <1.6% are still intact and contain hydrocarbons (traps B-G). Trap A, a structure with very low strain, is dry because of [[Natural hydraulic fracturing of top seals|natural hydraulic fracturing]]. Traps that were dry ue to other causes such as lack of reservoir or source are not included in this plot.