The preservation and evolution of organic matter in carbonate rocks are controlled by the depositional environments, eogenesis, mesogenesis, and telogenesis. Tidal flat, restricted lagoon, and basinal environments

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are favorable sites for generation and accumulation of organic biomass.

The prolonged exposure to the freshwater diagenesis in regressive phases of depositional megacycles results in destruction of organic matter in tidal-flat and lagoonal environments. The transgressive phase of depositional megacycles provides for short-lived exposures to the freshwater diagenesis. Consequently, tidal-flat and astricted lagoonal deposits of transgressive phase can be considered as petroleum source rocks.

Bitumens and hydrocarbons, which evolve from the thermal degradation of kerogens, migrate to the nearest available microporosities such as recrystallized patches of limestones, calcite cements, and dolomitic mosaics. Thus, early and late diagenesis helps to bring about a continuous segregation of bitumens and hydrocarbons from kerogens in carbonate source rocks. Tectonic fracturing can provide avenues for secondary migration from source to reservoir facies.

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