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Geology/importance of carbonate mud

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Question
Assalamualaikum.
Dear Sir Tariq.


I'm second year student, studying degree in Petroleum geoscience in Universiti Teknologi Petronas.

I would like to ask you about the importance of carbonate mud in deep marine environment?

Thank you.

Answer
Dear Siti,

Thanks for sending me your question.

Please be aware that deposition process of carbonate is very different than sandstones both in terms of sediments supply and processed that lead to deposit these rocks. carbonates are highly sensitive to their physiographic setting whereas sandstone are not.

Carbonate mud is deposited in warm, shallow marine environments is derived from the detritus of the  calcareous algae, precipitation from seawater and  breakdown of larger skeletal particles. The carbonates are mostly accumulate in quiet water environments (below wave base, lagoons) but can also form in higher energy environments where it is held in place or baffled by sea grasses or similar. carbonate mud is ideal to be  found in  such environments.

Carbonates have certain limitations while depositing which depend upon the depth of sea level.
Carbonate can only deposit in the shallower parts of the deep ocean unless organic productivity is so high that the remains of organisms are quickly buried and protected by dissolving.  This is because at depths between 3,000 and 5,000 m, in the deep oceans the rate of dissolution of carbonate is so high and the water has so little calcium carbonate, that carbonates cannot accumulate.

This depth is called the carbonate compensation depth (CCD). The main type of carbonate deposition in the deep oceans consists of the accumulation of the remains of planktonic foraminifera to form a carbonate ooze or Chalk.  Upon burial, this ooze undergoes diagenetic recrystallization to form micritic limestones or carbonate mud. These deeper burial carbonate ooze have excellent porosity but usually not remarkably well developed permeability,  forms the world class oil bearing reservoirs. There are two world class chalk reservoirs in the world, Cretaceous age chalk formation in North Sea's Ekofisk oilfield where recorded porosity is up to 30 percent whereas recorded  permeability upto 2mD and up to 150 mD in fractured zones.

Austin Chalk of Cretaceous age is yet another remarkable reservoir present in Texas, USA. According to the Chen. H. Y et al,. (1986)this chalk reservoir  found to be productive and  commercially viable only when it is fractured as average porosity is 5% and less than 0.1 mD permeability.

Hope this answer will be helpful.

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Regards

Tariq Hasany  

Geology

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