Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Predicting reservoir system quality and performance

  | chapter = Predicting reservoir system quality and performance

  | frompg  = 9-1

  | frompg  = 9-113

  | topg    = 9-156

  | topg    = 9-114

  | author  = Dan J. Hartmann, Edward A. Beaumont

  | author  = Dan J. Hartmann, Edward A. Beaumont

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

Most of the diagenesis seen in carbonate rocks can be attributed to processes that occurred during short-term fourth- or fifth-order cycles (10–500 k.y) and, to a lesser extent, long-term third-order cycles (1–10 m.y). Early diagenesis that takes place during fourth- or fifth-order cycles can have a great deal of influence on the amount of paleohydrology and diagenesis that occurs during the longer term third-order cycles. For example, if [[porosity]] is occluded early, then later diagenesis does not occur or is greatly reduced because there is little or no pore-water movement.

Most of the [[diagenesis]] seen in carbonate rocks can be attributed to processes that occurred during short-term fourth- or fifth-order cycles (10–500 k.y) and, to a lesser extent, long-term third-order cycles (1–10 m.y). Early diagenesis that takes place during fourth- or fifth-order cycles can have a great deal of influence on the amount of paleohydrology and diagenesis that occurs during the longer term third-order cycles. For example, if [[porosity]] is occluded early, then later diagenesis does not occur or is greatly reduced because there is little or no pore-water movement.

==High-frequency cycles==

==High-frequency cycles==

High-frequency sea level cycles (fourth or fifth order) cause the cyclical [[migration]] of the vadose, freshwater phreatic, and marine phreatic zones and the diagenetic processes occurring within those zones. Cycle amplitude and frequency determine the thickness and number of times a sequence is subjected to the processes that characterize each diagenetic zone. Each cycle leaves a diagenetic sequence mainly seen in the form of different cement types.<ref name=ch09r49>Read, J., F., 1995, Overview of carbonate platform sequences, cycle stratigraphy and reservoirs in greenhouse and ice-house worlds, in Read, J., F., Kerans, C., Webber, L., J., Sarg, J., F., Wright, F., M., eds., Milankovitch Sea-level Changes, Cycles, and Reservoirs on Carbonate Platforms in Greenhouse and Ice-house Worlds: SEPM Short Course 35, 183 p. Good summary of concepts of climatic effect on sea level cycles, carbonate deposition, and reservoir development.</ref>

High-frequency sea level cycles (fourth or fifth order) cause the cyclical migration of the vadose, freshwater phreatic, and marine phreatic zones and the diagenetic processes occurring within those zones. Cycle amplitude and frequency determine the thickness and number of times a sequence is subjected to the processes that characterize each diagenetic zone. Each cycle leaves a diagenetic sequence mainly seen in the form of different cement types.<ref name=ch09r49>Read, J., F., 1995, Overview of carbonate platform sequences, cycle stratigraphy and reservoirs in greenhouse and ice-house worlds, in Read, J., F., Kerans, C., Webber, L., J., Sarg, J., F., Wright, F., M., eds., Milankovitch Sea-level Changes, Cycles, and Reservoirs on Carbonate Platforms in Greenhouse and Ice-house Worlds: SEPM Short Course 35, 183 p. Good summary of concepts of climatic effect on sea level cycles, carbonate deposition, and reservoir development.</ref>

==Example==

==Example==

[[file:predicting-reservoir-system-quality-and-performance_fig9-79.png|thumb|{{figure number|1}}Modified. Copyright: Read, 1995; courtesy SEPM.]]

[[file:predicting-reservoir-system-quality-and-performance_fig9-79.png|300px|thumb|{{figure number|1}}Sequence of diagenetic events occurring in a prograding tidal flat during a high-frequency sea level cycle. Modified from Read.<ref name=ch09r49 /> Courtesy SEPM.]]

The diagram in [[:file:predicting-reservoir-system-quality-and-performance_fig9-79.png|Figure 1]] shows the sequence of diagenetic events occurring in a prograding tidal flat during a high-frequency sea level cycle. Also shown are two pre-existing topographic highs that were propagated upward through the section by differential compaction. The stippled pattern represents limestone; the brick pattern is dolomite.

The diagram in [[:file:predicting-reservoir-system-quality-and-performance_fig9-79.png|Figure 1]] shows the sequence of diagenetic events occurring in a [[Depocenter#Sediment_supply_rate_and_facies_patterns|prograding]] tidal flat during a high-frequency sea level cycle. Also shown are two pre-existing topographic highs that were propagated upward through the section by differential compaction. The stippled pattern represents [[limestone]]; the brick pattern is [[dolomite]].

The following diagenetic processes occurred during this period.

The following diagenetic processes occurred during this period.

==Diagenesis related to longterm cycles==

==Diagenesis related to longterm cycles==

Major unconformities that form as a result of uplift or third-order sea level lowstand can cause extensive diagenesis. In humid regions diagenesis can extend downdip for many miles and down section for hundreds to thousands of feet..<ref name=ch09r50>Read, J., F., Horbury, A., D., 1993, Eustatic and tectonic controls on porosity evolution beneath sequence-bounding unconformities and parasequence disconformities on carbonate platforms, in Horbury, A., D., Robinson, A., G., eds., Diagenesis and Basin Development: AAPG Studies in Geology 36, p. 155–197.</ref> Early down-section water flow is diffusive, mainly moving through intergranular pores. Later, down-section water movement becomes dominated by conduit flow. The change in flow character is a result of plugging intergranular pores and leaching that widens joints and turns vugs into caves.

Major [[Unconformity|unconformities]] that form as a result of uplift or third-order sea level lowstand can cause extensive diagenesis. In humid regions diagenesis can extend downdip for many miles and down section for hundreds to thousands of feet.<ref name=ch09r50>Read, J., F., Horbury, A., D., 1993, [http://archives.datapages.com/data/specpubs/resmi1/data/a067/a067/0001/0150/0155.htm Eustatic and tectonic controls on porosity evolution beneath sequence-bounding unconformities and parasequence disconformities on carbonate platforms], in Horbury, A., D., Robinson, A., G., eds., Diagenesis and Basin Development: AAPG Studies in Geology 36, p. 155–197.</ref> Early down-section water flow is diffusive, mainly moving through intergranular pores. Later, down-section water movement becomes dominated by conduit flow. The change in flow character is a result of plugging intergranular pores and leaching that widens joints and turns vugs into caves.

Reservoirs associated with long-term karsting tend to be highly compartmentalized<ref name=ch09r50 /> Vadose karst reservoirs tend to be of poor quality contrasted with phreatic reservoirs, which tend to be of high quality (Richard Fritz, personal communication). The amount of karstification decreases with decreasing rainfall. In very arid regions there may be relatively little diagenesis during emergence.

Reservoirs associated with long-term karsting tend to be highly compartmentalized<ref name=ch09r50 /> Vadose karst reservoirs tend to be of poor quality contrasted with phreatic reservoirs, which tend to be of high quality (Richard Fritz, personal communication). The amount of karstification decreases with decreasing rainfall. In very arid regions there may be relatively little diagenesis during emergence.

* [[Carbonate facies]]

* [[Carbonate facies]]

* [[Carbonate diagenetic stages]]

* [[Carbonate diagenetic stages]]

* [[Early carbonate diagenesis]]

* [[Early carbonate diagenesis]]

* [[Basics of carbonate porosity formation and preservation]]

* [[Basics of carbonate porosity formation and preservation]]

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting reservoir system quality and performance]]

[[Category:Predicting reservoir system quality and performance]]