Carbonate sediments and rocks Introduction to sedimentary petrology

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Diagenesis and its Influence on

Petrophysical Properties of Carbonates

Diagenesis describes all physical and chemical

processes that affect a sediment after deposition

Carbonate Diagenesis¾ Six major processes can be distinguished:1. Cementation 2. Microbial micritization3. Neomorphism4. Dissolution 5. Compaction6. Dolomitization¾ Carbonate diagenesis mostly involves the carbonate minerals:

aragonite, calcite, and dolomite; but other minerals such as quartz, feldspar, clays, phosphates, iron oxides and sulphides, and evaporitesalso may be involved.

¾ The diagenesis of carbonate begins at the sea floor; in fact depositional and diagenetic processes may be going on at the same time.

¾ As a reef is growing or carbonate sand is being moved by the waves, cements may be precipitated within interskeletal cavities and grains altered by micritization.

Carbonate Diagensis

Carbonate Diagenesis¾ Six major processes can be distinguished:1. Cementation 2. Microbial micritization3. Neomorphism4. Dissolution 5. Compaction6. Dolomitization¾ Carbonate diagenesis mostly involves the carbonate minerals:

aragonite, calcite, and dolomite; but other minerals such as quartz, feldspar, clays, phosphates, iron oxides and sulphides, and evaporitesalso may be involved.

¾ The diagenesis of carbonate begins at the sea floor; in fact depositional and diagenetic processes may be going on at the same time.

¾ As a reef is growing or carbonate sand is being moved by the waves, cements may be precipitated within interskeletal cavities and grains altered by micritization.

Carbonate Diagensis

Carbonate Diagenesis¾ Six major processes can be distinguished:1. Cementation 2. Microbial micritization3. Neomorphism4. Dissolution 5. Compaction6. Dolomitization¾ Carbonate diagenesis mostly involves the carbonate minerals:

aragonite, calcite, and dolomite; but other minerals such as quartz, feldspar, clays, phosphates, iron oxides and sulphides, and evaporitesalso may be involved.

¾ The diagenesis of carbonate begins at the sea floor; in fact depositional and diagenetic processes may be going on at the same time.

¾ As a reef is growing or carbonate sand is being moved by the waves, cements may be precipitated within interskeletal cavities and grains altered by micritization.

Carbonate Diagenesis¾ Six major processes can be distinguished:1. Cementation 2. Microbial micritization3. Neomorphism4. Dissolution 5. Compaction6. Dolomitization¾ Carbonate diagenesis mostly involves the carbonate minerals:

aragonite, calcite, and dolomite; but other minerals such as quartz, feldspar, clays, phosphates, iron oxides and sulphides, and evaporitesalso may be involved.

¾ The diagenesis of carbonate begins at the sea floor; in fact depositional and diagenetic processes may be going on at the same time.

¾ As a reef is growing or carbonate sand is being moved by the waves, cements may be precipitated within interskeletal cavities and grains altered by micritization.

Major Controls on Diagenesis of Carbonates

a. Original mineralogy of sedimentb. Pore fluid chemistry and flow rate c. Sea level history d. Burial history e. Time

Various diagenetic environments

Can CREATE or DESTROY porosity

Diagenesis early diagenesis on the seafloor or close to sediment surface; Meteoric Marine Deep marine bottom (rare) late (burial) diagenesis During burial

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diagenetic environments

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S. Cirilli: Petrografia del Sedimentario

S.Cirilli-SedPetr

Isopachus Marine Cement

S.Cirilli-SedPetr

Isopachus Marine Cement

Loose sediment cemented rock

S. Cirilli

Cements and cementation

Porosity decreases

DA FARE

•  Crystals precipitated from solutions within a pore or void space.

•  Major process involved in lithification of carbonate sediments.

•  Can be syndepositional to late diagenetic.

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cement mineralogy

Crystal size partially dependent on

Crystal form

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Many: Small size (from 10 to less than 50-80 microns)

Few: larger size (from few mm to cm)

Availability of CaCO3 in the waters

Number of nuclei or seed crystals

Shape classified based on crystal relative length to width

ratio.

•  Fibrous or acicular.

•  Bladed or prismatic.

•  Equant.