Abstract

Based on comparisons between structural histories and the distribution of current and paleo-oil accumulations, it is proposed that the partitioning of postrift strain between faults in relation to trap geometry was critical in determining oil preservation during Neogene fault reactivation in the Timor Sea. Most of the trap-bounding faults in the region have been reactivated; however, the distribution of postrift displacements is heterogeneous and depends heavily on rift-phase fault size, location, and interaction with nearby faults. Preferential localization of postrift strain onto larger faults in the population resulted in the partial protection of some fault-bound traps with favorable geometries, but promoted breaching of others. Oil columns tend to be preserved where the crest of the trap is bound by a fault segment that has accommodated relatively low postrift displacements (less than about 60 m [196 ft]) during reactivation, typically where smaller rift faults are overlapped by larger rift faults. Complete loss of oil column is generally observed where the crest of the trap is bound by a typically large fault with high postrift displacements (greater than about 60 m [196 ft]). Where faults with high postrift displacements are located downdip of the trap crest, hydrocarbon columns are preserved down to the depth of the intersection between this fault and the top reservoir horizon. A simple trap integrity model based on these observations was found to be largely consistent with a database of 69 drilled traps in the region. The mechanisms and models discussed in this study are likely to apply to other petroleum systems where fault reactivation represents a risk to hydrocarbon preservation.

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