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| | isbn = 0-89181-602-X | | | isbn = 0-89181-602-X |
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− | 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.
| + | [[:[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. |
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− | [[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.]] |
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| ==Method== | | ==Method== |
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| ==Strain-time plots== | | ==Strain-time plots== |
− | 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.
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− | [[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.]] |
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| + | 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. |
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| ==Paleoductility analysis== | | ==Paleoductility analysis== |
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| ==Strain threshold for failure== | | ==Strain threshold for failure== |
− | 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
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− | [[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.]] |
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| + | 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 |
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| 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. |