Changes

Both classifications provide an indication of depositional energy setting and reservoir quality.

Both classifications provide an indication of depositional energy setting and reservoir quality.

 

==History==

 

 

History

Various classification systems had been proposed for both academia and industry purposes:

Various classification systems had been proposed for both academia and industry purposes:

• Wentworth (1922) [a]

• Wentworth (1922) [a]

• Hallsworth & Knox (1999)

• Hallsworth & Knox (1999)

Among all of the attempts, Folk (1959, 1962) and Dunham (1962) extended by Embry & Klovan (1971) classifications were widely accepted.  

Among all of the attempts, Folk (1959, 1962) and Dunham (1962) extended by Embry & Klovan (1971) classifications were widely accepted.  

Folk Classification  

 

Folk’s Criteria  

==Folk Classification==

===Folk’s Criteria===

Folk classification system is based on three basic components of limestone:  

Folk classification system is based on three basic components of limestone:  

• Allochems (sediment grains): framework component of the rock. There are four major types of allochems in Folk system: intraclasts, oolites, fossils, and pellets.  

• Allochems (sediment grains): framework component of the rock. There are four major types of allochems in Folk system: intraclasts, oolites, fossils, and pellets.  

• Microcrystalline lime mud: matrix component of the rock < 4 μm in size.  

• Microcrystalline lime mud: matrix component of the rock < 4 μm in size.  

• Sparry calcite cement (sparite): pore-filling component precipitated during deposition.  

• Sparry calcite cement (sparite): pore-filling component precipitated during deposition.  

Major Families Classification

 

===Major Families Classification===

Based on the relative proportions of matrix and pore-filling components surrounding the framework component, Folk defined three major families plotted on a triangular diagram (Figure 1).   

Based on the relative proportions of matrix and pore-filling components surrounding the framework component, Folk defined three major families plotted on a triangular diagram (Figure 1).   

• Family I: sparry allochemical limestone that consists of allochems cemented by sparry calcite cement.[b]

• Family I: sparry allochemical limestone that consists of allochems cemented by sparry calcite cement.[b]

• Family III: microcrystalline limestone (micrite) that consists of 0-10% allochems within mud. In other words, composes almost entirely of mud.[c]  

• Family III: microcrystalline limestone (micrite) that consists of 0-10% allochems within mud. In other words, composes almost entirely of mud.[c]  

Folk defined a fourth family based on in situ organic growth deposits: biolithite.[d]

Folk defined a fourth family based on in situ organic growth deposits: biolithite.[d]

   

   

===Subdivision of Major Families===

Subdivision of Major Families

Family I and II of Folk’s system could be subdivided based on the type of allochems presented. The most important types of allochems are: intraclasts, oolites, fossils, and pellets respectively. A total of eight subdivisions are diagrammatically presented in Figure 2.[e]   

Family I and II of Folk’s system could be subdivided based on the type of allochems presented. The most important types of allochems are: intraclasts, oolites, fossils, and pellets respectively. A total of eight subdivisions are diagrammatically presented in Figure 2.[e]   

[[File:GeoWikiWriteOff2021-Abdulwahab-Figure2.png|thumbnail|Fig. 2 Folk subdivisions of major families [2] ]]

Fig. 2 Folk subdivisions of major families [2]  

 

Family I subdivisions

Family I subdivisions

• Intrasparite: consists of intraclasts with sparry calcite cement (Figure 3).

• Intrasparite: consists of intraclasts with sparry calcite cement (Figure 3).

• Biomicrite: consists of fossils with microcrystalline matrix (Figure 9).

• Biomicrite: consists of fossils with microcrystalline matrix (Figure 9).

• Pelmicrite: consists of pellets with microcrystalline matrix (Figure 10).

• Pelmicrite: consists of pellets with microcrystalline matrix (Figure 10).

Depositional Energy Setting and Reservoir Quality Indications

 

===Depositional Energy Setting and Reservoir Quality Indications===

Generally, Family I rocks are formed in high-energy settings and show good reservoir quality, especially if they are partially cemented. Oosparite is considered to be the best reservoir quality rock. It is usually characterized by good porosity & permeability, good sorting, and cross-bedding. It is mainly formed near strong offshore currents such as submarine shoals.[f]

Generally, Family I rocks are formed in high-energy settings and show good reservoir quality, especially if they are partially cemented. Oosparite is considered to be the best reservoir quality rock. It is usually characterized by good porosity & permeability, good sorting, and cross-bedding. It is mainly formed near strong offshore currents such as submarine shoals.[f]

Family II & III rocks are formed in low-energy settings and show poor reservoir quality. They usually formed in deep water and restricted settings where the current is not strong enough to winnow away the lime mud.[g] [h]     

Family II & III rocks are formed in low-energy settings and show poor reservoir quality. They usually formed in deep water and restricted settings where the current is not strong enough to winnow away the lime mud.[g] [h]     

Spectral Subdivision of Limestones  

 

===Spectral Subdivision of Limestones===

In 1962, Folk proposed the spectral subdivision. He subdivided the 3 major families into eight types forming a sequential transitional spectrum of energy level in which the first type represents the lowest energy, and the last type represents the highest energy (Figure 11).

In 1962, Folk proposed the spectral subdivision. He subdivided the 3 major families into eight types forming a sequential transitional spectrum of energy level in which the first type represents the lowest energy, and the last type represents the highest energy (Figure 11).

[[File:GeoWikiWriteOff2021-Abdulwahab-Figure11.png|thumbnail|Figure 11. [11]] ]]

Fig. 11 [11]

 

Criteria

Criteria

• For rocks dominated by lime mud, Folk considered the percentage of allochems presented.   

• For rocks dominated by lime mud, Folk considered the percentage of allochems presented.   

7. Sorted biosparites: well-sorted rocks in which the allochems are still not well-rounded and abraded.

7. Sorted biosparites: well-sorted rocks in which the allochems are still not well-rounded and abraded.

8. Rounded biosparites: well-sorted and well-rounded rocks in which the allochems underwent intense abrasion.   

8. Rounded biosparites: well-sorted and well-rounded rocks in which the allochems underwent intense abrasion.   

Dunham Classification  

 

==Dunham Classification==

Dunham’s Criteria

Dunham’s Criteria

Dunham classification system is based on three depositional textural features:

Dunham classification system is based on three depositional textural features: