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Line 14: − Introduction Following 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.+ − + Line 22:
Line 22: − The following strain-time plots 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. − + + + Line 30:
Line 31: − Incremental strain analysis of a group of successes and failures in the Central Graben defines a strain threshold at approximately 1.6%. In the following figure, traps with a maximum post-100 Ma incremental strain > 1.6% are dry (traps H, I, J). Traps with a strain − + + +
Top seal integrity: Central Graben example (view source)
Revision as of 14:54, 6 February 2014
, 14:54, 6 February 2014no edit summary
| 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|thumb|{{figure number|10-39}}. Copyright: Esso Exploration and Production U.K.]]
[[file:evaluating-top-and-fault-seal_fig10-39.png|thumb|{{figure number|1}}. Copyright: Esso Exploration and Production U.K.]]
==Method==
==Method==
==Strain-time plots==
==Strain-time plots==
[[file:evaluating-top-and-fault-seal_fig10-40.png|thumb|{{figure number|10-40}}See text for explanation.]]
[[file:evaluating-top-and-fault-seal_fig10-40.png|thumb|{{figure number|2}}See text for explanation.]]
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|{{figure number|10-41}}See text for explanation.]]
[[file:evaluating-top-and-fault-seal_fig10-41.png|thumb|{{figure number|3}}See text for explanation.]]
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
This empirical threshold provides an important tool for evaluating top seals. Prospects can be evaluated before drilling using incremental strain analysis. In this case, high-risk prospects are those with strains exceeding the threshold of 1.6%. Low-risk prospects are those with strains below the threshold.
This empirical threshold provides an important tool for evaluating top seals. Prospects can be evaluated before drilling using incremental strain analysis. In this case, high-risk prospects are those with strains exceeding the threshold of 1.6%. Low-risk prospects are those with strains below the threshold.