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Early carbonate diagenesis (view source)
Revision as of 14:01, 5 February 2014
, 14:01, 5 February 2014no edit summary
==Summary diagram==
==Summary diagram==
[[file:predicting-reservoir-system-quality-and-performance_fig9-70.png|thumb|{{figure number|9-70}}.]]
[[file:predicting-reservoir-system-quality-and-performance_fig9-70.png|thumb|{{figure number|1}.]]
The block diagram in [[:file:predicting-reservoir-system-quality-and-performance_fig9-70.png|Figure 1]] summarizes early diagenetic processes and products that occur in carbonate environments.
==Marine phreatic environment==
==Marine phreatic environment==
==Active marine phreatic zone==
==Active marine phreatic zone==
[[file:predicting-reservoir-system-quality-and-performance_fig9-71.png|thumb|{{figure number|2}}After .<ref name=ch09r35 />]]
The cementation rate is greatest in the active marine phreatic environment where three conditions occur:
The cementation rate is greatest in the active marine phreatic environment where three conditions occur:
Magnesium calcite or aragonite are the only cements that precipitate in the active marine phreatic zone. Both are unstable in Mg-deficient water regardless of whether it is marine, brackish, or fresh and tend to alter to low-Mg calcite because most water is magnesium deficient. Their common form is isopachous coatings on grains. Micritization of grains occurs in the active marine phreatic zone.
Magnesium calcite or aragonite are the only cements that precipitate in the active marine phreatic zone. Both are unstable in Mg-deficient water regardless of whether it is marine, brackish, or fresh and tend to alter to low-Mg calcite because most water is magnesium deficient. Their common form is isopachous coatings on grains. Micritization of grains occurs in the active marine phreatic zone.
The diagram below summarizes the diagenesis of this zone.
The diagram in [[:file:predicting-reservoir-system-quality-and-performance_fig9-71.png|Figure 2]] summarizes the diagenesis of this zone.
[[file:predicting-reservoir-system-quality-and-performance_fig9-71.png|thumb|{{figure number|9-71}}After .<ref name=ch09r35 />]]
==Stagnant marine phreatic==
==Stagnant marine phreatic==
==Vadose precipitation zone==
==Vadose precipitation zone==
[[file:predicting-reservoir-system-quality-and-performance_fig9-72.png|thumb|{{figure number|3}}. Copyright: Purser, 1978; courtesy Journal of Petroleum Geology.]]
The vadose precipitation zone begins where water in the vadose zone becomes saturated with CaCO<sub>3</sub>. Slight temperature increases or CO<sub>2</sub> degassing causes calcite to precipitate. Cementation is generally minor and reflects pore-water distribution. Meniscus cement precipitates where water clings between grains in a meniscus manner. Pendulous or microstalactitic cement precipitates where water droplets form underneath grains. Cement tends to be very fine equant calcite crystals. If magnesium is present in pore water, then calcite precipitation may be inhibited and aragonite or even dolomite may precipitate.<ref name=ch09r35 />
The vadose precipitation zone begins where water in the vadose zone becomes saturated with CaCO<sub>3</sub>. Slight temperature increases or CO<sub>2</sub> degassing causes calcite to precipitate. Cementation is generally minor and reflects pore-water distribution. Meniscus cement precipitates where water clings between grains in a meniscus manner. Pendulous or microstalactitic cement precipitates where water droplets form underneath grains. Cement tends to be very fine equant calcite crystals. If magnesium is present in pore water, then calcite precipitation may be inhibited and aragonite or even dolomite may precipitate.<ref name=ch09r35 />
The sketches below illustrate recent limestones from the intertidal-supratidal zone. They show petrographic aspects of vadose precipitation zone cements.
The sketches in [[:file:predicting-reservoir-system-quality-and-performance_fig9-72.png|Figure 3]] illustrate recent limestones from the intertidal-supratidal zone. They show petrographic aspects of vadose precipitation zone cements.
[[file:predicting-reservoir-system-quality-and-performance_fig9-72.png|thumb|{{figure number|9-72}}. Copyright: Purser, 1978; courtesy Journal of Petroleum Geology.]]
==Freshwater phreatic zone==
==Freshwater phreatic zone==
===Active and stagnant zones===
===Active and stagnant zones===
[[file:predicting-reservoir-system-quality-and-performance_fig9-73.png|thumb|{{figure number|4}}.]]
Based on diagenesis, the freshwater phreatic zone can be divided into two major zones:
Based on diagenesis, the freshwater phreatic zone can be divided into two major zones:
The '''stagnant freshwater phreatic zone''' occurs where there little to no movement. Pore water is near equilibrium with surrounding rock and lack of water movement means little cementation occurs. Consequently, primary porosity is generally preserved.
The '''stagnant freshwater phreatic zone''' occurs where there little to no movement. Pore water is near equilibrium with surrounding rock and lack of water movement means little cementation occurs. Consequently, primary porosity is generally preserved.
==See also==
==See also==