The interplay of tectonics, thermal regime, and depositional history generally determines the petroleum potential of individual basins. The application of these factors to a review of several extensional basin systems (Rhine graben, North Sea basin, Reconcavo basin, Gulf of Suez basin, Red Sea basin, Bass basin, and Gippsland basin) provides the basis for this analysis.

Extensional tectonic systems provide a mechanism for thinning the crust and upper mantle (the lithosphere). The result is a progressive rift-basin evolution starting with graben formation and ending in the development of continental margins. However, if tensional stress stops at any stage, a post-rift phase of subsidence, regulated by thermal decay, begins.

Sedimentary rocks deposited prior to extension (pre-rift sediments) are preserved in the graben areas but eroded from the horsts. Rift sediments (usually clastics) deposited during extension are eroded from the uplifted areas, but carbonate rocks and evaporites may be deposited if the climatic environment is favorable. Sediment deposition is continued after extension stops (post-rift sediments).

Source beds can be in the pre-rift, rift, or post-rift sediments but generation does not occur until they are buried to the depth of the generative window. Generation is aided by the high heat flow caused by the thinned crust. The kind of petroleum generated is dependent on the type of organic material present and the deepest zone of generation reached.

Extensive normal faults contribute to trap geometry but inhibit long distance migration. Consequently, except in the post-rift section, entrapment of petroleum requires that the source and reservoir rocks are in close proximity, which can be accomplished by faulting, interfingering, or the superposition of source rocks on reservoir rocks at unconformities. The most favorable conditions for generating and trapping large oil fields are commonly in or near the deepest part of the basin.

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