Most of the world’s giant fields produce hydrocarbons from carbonate reservoirs.
Distinctive and unique aspects of carbonate rocks are their predominantly
intrabasinal origin, their primary dependence on organic activities for their
constituents and their susceptibility to modification by post-depositional
mechanisms. These three features are significant such as to distinguish the
productivity of carbonate rocks from other sedimentary rocks including sandstone
and shale.

Most of the world’s giant fields produce hydrocarbons from carbonate reservoirs.
Distinctive and unique aspects of carbonate rocks are their predominantly
intrabasinal origin, their primary dependence on organic activities for their
constituents and their susceptibility to modification by post-depositional
mechanisms. These three features are significant such as to distinguish the
productivity of carbonate rocks from other sedimentary rocks including sandstone
and shale.

Carbonate rocks contain more than 50% of the world’s hydrocarbon reserves.
Carbonates are formed in special environments and they are biochemical in origin.

Organisms play an important role and have direct role in determining the reservoir
quality. Processes like compaction, lithification and other diagenetic events result
in large variations in the reservoir quality of carbonates.

Sedimentology
Carbonate sediments are particularly sensitive to environmental changes.
Carbonate sedimentation is rapid but easily inhibited. Temperature variations
influence biogenic activity and affect sediment production; thus most carbonate
production is strongly depth dependent. When conditions are favorable for
carbonate sedimentation, organic productivity is high; when unfavorable, organic
productivity ceases.

Carbonates form in special environments, and, in contrast with sandstones, are
biochemical in nature. Environments range from near-shore lagoons, platform
organic buildups, and shelf margin shoals to slope and basinal settings.
Carbonates typically are found in warm, shallow, clear marine water in low
latitudes.

Carbonates are essentially autochthonous, as they form very close to the final
depositional sites. Texture is more dependent on the nature of the skeletal grains
than on external influences. Intrabasinal factors control facies development. In
contrast, sandstone and shale were formed of sedimentary particles derived from
sources outside the depositional basin. Reefs, bioherms and biostromes are
examples of in-place local deposition where organisms have built wave-resistant
structures above the level of adjacent time-equivalent sediments. Many reefal
deposits are commonly composed of fragmented, locally-transported skeletal
debris and a minor volume of in situ framework organisms. Biofacies and
lithofacies often correlate, or in other words, organisms produce typical
lithofacies. Substrates control inhabiting organisms.

Basin configuration and water energy are the dominant controls on carbonate
deposition. Organic productivity varies with depth and light (photic zone);
upwellings and water agitation influence organic productivity.

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