ABSTRACT

Entrapment of oil in carbonate reservoirs can be explained by analysis of the following geological factors: (a) depositional environment; (b) diagenetic changes; (c) structural history; and (d) fluid mechanics.

Environments favorable to deposition of reservoirs in carbonate formations include reefs, bioherms, oolite bars, and porous skeletal calcarenites. Production of organic material in such environments (with the exception of oolite bars) is prolific, but under normal conditions a major portion 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 calcareous 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 subject to recurrent sea level fluctuations with periodic influxes of terrigenous clastics. Under cyclic regime of sedimentation, a reservoir type carbonate facies can be covered by sapropelic shales, evaporites or basinal facies. This stratigraphic relationship, in addition to providing adequate seal, can also be suitable to preservation of organic soft parts within the reservoir facies. Cyclic sedimentation in carbonates could explain in situ accumulation of hydrocarbons in carbonate rocks under certain favorable conditions.

The oil generated in carbonate rocks will be subject to secondary migration as a result of structural deformation. Understanding of fluid mechanics is very useful for explaining some of the 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.