Carbonate sedimentary rocks classification

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Author	Zainab Al Abdulwahab
Affiliation	Saudi Aramco
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In oil and gas industry, classifying carbonate rocks is a major step in core and thin section description. Two classification systems have been widely used in the industry:

1. Folk (1959, 1962) Classification
2. Dunham (1962) Classification

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

History

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

• Wentworth (1922)Template:Note [a]
• Bramkamp & Powers (1958)
• Folk (1959, 1962)
• Dunham (1962)
• Leignton & Pendexter (1962)
• Todd (1966)
• Embry & Klovan (1971)
• Wright (1992)
• Hallsworth & Knox (1999)

Among all of the attempts, the Folk and Dunham, extended by Embry & Klovan, classifications were widely accepted.

Folk Classification

Folk’s Criteria

Folk's 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.
• Microcrystalline lime mud: matrix component of the rock < 4 μm in size.
• Sparry calcite cement (sparite): pore-filling component precipitated during deposition.

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).

• Family I: sparry allochemical limestone that consists of allochems cemented by sparry calcite cement.[b]
• Family II: microcrystalline allochemical limestone that consists of > 10% allochems within lime mud.
• 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]

• 

  Figure 1 Folk classification system for carbonate rock (1959)

• 

  Figure 2 Folk subdivisions 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 subdivisions

• Intrasparite: consists of intraclasts with sparry calcite cement (Figure 3).
• Oosparite: consists of oolites with sparry calcite cement (Figure 4).
• Biosparite: consists of fossils with sparry calcite cement (Figure 5).
• Pelsparite: consists of pellets with sparry calcite cement (Figure 6).

• 

  Figure 3 Intrasparite

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  Figure 4 Oosparite

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  Figure 5 Biosparite

Family II subdivisions

• Intramicrite: consists of intraclasts with microcrystalline matrix (Figure 7).
• Oomicrite: consists of oolites with microcrystalline matrix (Figure 8).
• Biomicrite: consists of fossils with microcrystalline matrix (Figure 9).
• Pelmicrite: consists of pellets with microcrystalline matrix (Figure 10).

• 

  Figure 7 Intramicrite

• 

  Figure 8 Oomicrite

• 

  Figure 9 Biomicrite

• 

  Figure 10 Pelmicrite

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] 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

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).

Criteria

• For rocks dominated by lime mud, Folk considered the percentage of allochems presented.
• For rocks dominated by sparry calcite cement, Folk considered the sorting and rounding of allochems presented.

Subdivisions

Folk proposed a total of eight types representing a gradational transition of energy level from low to high. Considering, for instance, fossils to be the major allochems in the system[i], the sequential subdivisions are:

1. Micrites and dismicrites: rocks consist entirely of lime mud.
2. Fossiliferous micrites and dismicrites: rocks consist of 1-10% allochems.
3. Sparse biomicrites: rocks consist of 10-50% allochems floating in the matrix.
4. Packed biomicrites: rocks consist of > 50% packed allochems.
5. Poorly washed biosparites: rocks consist of almost equal amounts of lime mud and sparry calcite cement in which the current was not strong enough to winnow away all lime mud particles.
6. Unsorted biosparites: poorly sorted rocks in which the current was strong enough to winnow away lime mud particles, but the allochems are still not sorted.
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.

Dunham Classification

Dunham’s Criteria

Dunham's classification system is based on three depositional textural features:

• The presence or absence of lime mud.
• The abundance of grains.[j]
• The presence of in situ organic binding

Dunham (1962) Classification

Figure 12 Dunham classification system for carbonate rock (1962)

The original Dunham classification system composes of six classes summarized in Figure 12:

• Mudstone: a mud-supported rock with < 10% grains, and original components are not organically bounded together during deposition (Figure 13).
• Wackestone: a mud-supported rock with > 10% grains, and original components are not organically bounded together during deposition (Figure 14).
• Packstone: a grain-supported rock with lime mud presented, and original components are not organically bounded together during deposition (Figure 15).
• Grainstone: a grain-supported rock with lime mud absent, and original components are not organically bounded together during deposition (Figure 16).
• Boundstone: a rock with the presence of organic components bounded together during deposition.
• Crystalline: a rock with unrecognizable depositional texture (Figure 17).

• 

  Figure 13 Mudstone

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  Figure 14 Wackestone

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  Figure 15 Packstone

• 

  Figure 16 

• 

  Figure 17 Crystalline rock

Extended Dunham Classification (1971)

Embry and Klovan expanded the original Dunham classification due to the lack of:

• Classifying coarse-grained (>2mm) rocks.
• Subdividing organically-bound rocks (boundstones).

The extended Dunham Classification is shown in Figure 18.

Figure 18 Extended Dunham classification (1971)

Coarse-Grained Rocks Classification

• Floatstone: a matrix-supported rock with > 10% of grains >2mm and original components are not organically bounded together during deposition (Figure 19).
• Rudstone: a grain-supported rock with > 10% of grains >2mm and original components are not organically bounded together during deposition (Figure 20).

• 

  Figure 19 Floatstone

• 

  Figure 20 Rudstone

Organically-Bound Rocks Subdivision

• Bafflestone: in situ organically-bound rock in which organisms act as baffles (Figure 21).
• Bindstone: in situ organically-bound rock in which organisms encrust and bind (Figure 22).
• Framestone: in situ organically-bound rock in which organisms build a rigid framework (Figure 23).[k]

• 

  Figure 21 Bafflestone

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  Figure 22 Bindstone

• 

  Figure 23 Framestone

Depositional Energy Setting and Reservoir Quality Indications

Generally, mud-supported rocks: mudstones and wackestones are formed in low-energy settings and show poor reservoir quality. Grain-supported rocks: packstones and grainstones are formed in high-energy settings and show good reservoir quality. Grainstones is considered to be the best reservoir quality rock, especially if it is partially cemented. It is usually characterized by good porosity & permeability, and good sorting. Floatstones and bindstones are usually formed in medium-energy settings (below average wave base). Rudstones and framestones are usually formed in high-energy settings (above wave base). Organically-bound rocks show good reservoir quality if the skeletal growth is interconnected.

Notes

1. [a] Early carbonate classification is grain-size based system: Calcilutite (grains < 63 μm), calcarenite (63 μm < grains < 2mm), calcirudite (grains > 2mm).
2. [b] This family has restricted limits because cement by itself cannot form a rock and the supported framework component (allochems) has limited packing.
3. [c] If the microcrystalline limestone had been disturbed either by bioturbation or soft-sediment deformation, the term “disturbed microcrystalline limestone” or “dismicrite” could be assigned.
4. [d] Examples of biolithites are: algal biolithite and coral biolithite.
5. [e] Naming of a subdivision is a composite of two parts. The first part refers to the name of the allochem. The second part refers to the family type. For Example, intrasparite: intra- refers to intraclasts, and –sparite refers to family I.
6. [f] Oosparites are more common than Oomicrites because the need of strong current to form these rocks.
7. [g] Intrasparites are more common than intramicrites because the need of strong current to transfer course rock fragments.
8. [h] Both biosparites and biomicrites are common. However, biosparites are usually more rounded and abraded because of the strong current.
9. [i] The subdivisions could be applied considering other types of allochems: intraclasts, oolites, and pellets.
10. [j] The abundance of grains defines the nature of the framework. In other words, whether it is mud-supported or grain-supported.
11. [k] The term “boundstone” is retained if the organic bounding type is not recognized.

Other Sources

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• K. Bjørlykke (ed). Petroleum Geoscience: From Sedimentary Environments to Rock Physics, DOI 10.1007/978-3-642-34132-8_5, Ⓒ Springer-Verlag Berlin Heidelberg 2015.
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