Entrapment of oil in carbonate reservoirs can be explained by analysis of (a) depositional environment, (b) diagenetic changes, (c) structural history, and (d) fluid mechanics.

Favorable reservoir rocks in carbonate environments include reefs, bioherms, oolite bars, and porous skeletal calcarenite. Production of organic material in such environments (with the exception of oolite bars) is prolific, but under normal conditions a major part of the organic soft parts are destroyed by bacteria scavengers and early diagenesis, whereas skeletal parts are preserved. Early diagenesis modifies the texture and the original porosity of carbonate deposits by recrystallization, solution, cementation, and replacement. Under favorable conditions, dolomitization enhances the reservoir characteristics of the carbonate sediment.

Hydrocarbons are found in cyclic carbonates which were deposited on unstable shelves and subjected to recurrent sea-level fluctuations with periodic influxes of terrigenous clastic material. Under a cyclic regime of sedimentation, a reservoir-type carbonate facies can be covered by sapropelic shale, evaporite, or basinal facies. This stratigraphic relation, in addition to providing an adequate seal, also can be suitable to preservation of organic soft parts within the reservoir facies. In the writer's opinion, cyclic sedimentation in carbonate rocks could explain in situ accumulation of hydrocarbons in carbonate rocks under certain favorable conditions.

The oil generated in carbonate rocks is subject to secondary migration as a result of structural deformation. An understanding of fluid mechanics is very useful for explanation of some peculiarities of oil distribution within the carbonate traps.

Tectonic setting of the carbonate shelf, relative to the stable nuclei and the mobile margins of the continents, has a profound influence on the type of trapping mechanisms likely to be found in the carbonate rocks.

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