Sedimentary rocks and their classification

Quick refresher

By how they formed, sedimentary rocks fall into clastic (terrigenous), carbonate, evaporites, siliceous, organic (coals, oil shales), iron-rich and phosphatic, and volcaniclastic rocks. Reservoirs are mostly sandstones and carbonates. Seals are mudrocks (clay-rich rocks such as shale and mudstone) and evaporites. Source rocks are dark mudrocks and clayey carbonate rocks.
Clastic rocks are named by grain size. Grains coarser than 2 mm give conglomerates, gravelstones and breccias. Sand gives sandstones, silt gives siltstones, and clay gives clays and mudstones. In international practice the boundaries are 2, 1/16 and 1/256 mm. Russian-language descriptions also use decimal scales.
A sandstone is named by two things. One is the share of fine matrix (the fine material between the grains): arenite (less than 15%) or wacke (15–75%). The other is the composition of the framework of quartz (Q), feldspars (F) and rock fragments (L): quartz arenite, subarkose, sublitharenite, arkose, lithic arenite.
In the Dunham scheme, a carbonate rock is named by what holds up its framework: mud (mudstone, wackestone), grains (packstone, grainstone) or organisms that bound the sediment together (boundstone). In the Folk scheme, it is named by the main grains (intra-, oo-, bio-, pel-) and by the filling (sparite, a coarse clear calcite cement, or micrite, fine lime mud).
In Russian-language descriptions, a mixture is named after the component that makes up more than half of it, and admixtures are marked with adjectives: 5–25% — the weaker form (slightly dolomitic), 25–50% — the stronger form (dolomitic). A rock with 25–75% carbonate and 25–75% clayey material is a marl.
Only about 13 m of rock salt precipitates from a 1 km column of sea water. So salt sequences several kilometres thick build up only in basins that receive sea water for a long time and where it evaporates.

Key relationships

Q = \frac{N_Q}{N_Q + N_F + N_L}\cdot 100\,\%

share of quartz in the framework of a sandstone (likewise F and L)

M = \frac{N_M}{N_Q + N_F + N_L + N_M}\cdot 100\,\%

share of matrix: arenite — less than 15%, wacke — 15–75%

h = H\,\frac{\rho_{sw}\,c}{\rho_{min}}

layer of a mineral from a column of sea water of height $H$ with content $c$

SE = \sqrt{p\,(1 - p)/n}

error of a proportion counted at $n$ points of a thin section

Common pitfalls

Mixing up struktura and tekstura. In Russian-language petrography struktura is the size, shape and combination of grains (in English, texture), and tekstura is bedding and other features of how the rock is put together (in English, sedimentary structures).
Assuming that “greywacke” means the same thing everywhere. In one widely used English-language scheme it is a sandstone with a large share of matrix; in Russian-language literature it is a dark sandstone rich in rock fragments.
Reading mudstone from the carbonate classification as an argillite: in Dunham's scheme it is a limestone made of lime mud.
Confusing matrix — fine clastic material deposited together with the grains — with cement — minerals that grew in the pores after burial.

After this lesson you can

put a sedimentary rock in a class by how it formed, and know which classes make reservoirs, seals and source rocks
name clastic rocks by grain size, and sandstones by the composition of the framework (quartz, feldspars, rock fragments) and the share of matrix
name a carbonate rock by the Dunham and Folk schemes from a description of a thin section (a slice of rock 30 µm thick, seen under a microscope), and name mixtures of calcite, dolomite and clayey material by how much of each they contain
estimate how much salt evaporating sea water leaves, and explain what this means for thick salt sequences
explain what each word in a rock's name says about the reservoir, and read the lithological description of core and cuttings

Classes of sedimentary rocks

Sedimentary rocks cover about three-quarters of the land surface, although they make up only a few per cent of the volume of the crust. They contain almost all oil and gas accumulations (the three classes of rock). The most widespread are mudrocks: they make up more than half of the volume of the sedimentary shell. Sandstones and carbonate rocks make up roughly a fifth each, and all the others only a few per cent.

The first division is by the main process that created the rock: transport and deposition of fragments, the work of organisms, precipitation from brine (very salty water), accumulation of organic matter, or volcanic eruptions. The class at once suggests the role of the rock in a petroleum system.

Classes of sedimentary rocks

Class: Clastic (terrigenous)
How it forms: breakdown of rocks on land, transport by water, wind or ice, deposition
Main rocks: conglomerates, sandstones, siltstones, clays and mudstones
Role in the petroleum system: sandstones are reservoirs; mudrocks are seals and source rocks

Class: Carbonates
How it forms: skeletons of organisms, and carbonate that organisms help to precipitate; dolomite mainly forms when it replaces limestone
Main rocks: limestones, dolomites, chalk
Role in the petroleum system: reservoirs; dark clayey limestones are source rocks; tight ones are seals

Class: Evaporites
How it forms: precipitation of salts from evaporating sea or lake water
Main rocks: gypsum, anhydrite, rock salt, potash salts
Role in the petroleum system: the best seals

Class: Siliceous
How it forms: skeletons of siliceous organisms, precipitation of silica
Main rocks: diatomites, opokas (light opaline rocks), radiolarites, cherts
Role in the petroleum system: occasionally reservoirs; siliceous–clayey rocks are source rocks

Class: Organic (caustobioliths, combustible organic rocks)
How it forms: accumulation of the remains of plants and plankton
Main rocks: coals, oil shales
Role in the petroleum system: source rocks; coals are a source of gas

Class: Iron-rich and phosphatic
How it forms: chemical and biochemical precipitation
Main rocks: sedimentary iron ores, phosphorites
Role in the petroleum system: minor role

Class: Volcaniclastic
How it forms: volcanic ash and fragments redeposited by water
Main rocks: tuffs, tuffites
Role in the petroleum system: occasionally reservoirs; the ash alters to clay minerals and zeolites

Class	How it forms	Main rocks	Role in the petroleum system
Clastic (terrigenous)	breakdown of rocks on land, transport by water, wind or ice, deposition	conglomerates, sandstones, siltstones, clays and mudstones	sandstones are reservoirs; mudrocks are seals and source rocks
Carbonates	skeletons of organisms, and carbonate that organisms help to precipitate; dolomite mainly forms when it replaces limestone	limestones, dolomites, chalk	reservoirs; dark clayey limestones are source rocks; tight ones are seals
Evaporites	precipitation of salts from evaporating sea or lake water	gypsum, anhydrite, rock salt, potash salts	the best seals
Siliceous	skeletons of siliceous organisms, precipitation of silica	diatomites, opokas (light opaline rocks), radiolarites, cherts	occasionally reservoirs; siliceous–clayey rocks are source rocks
Organic (caustobioliths, combustible organic rocks)	accumulation of the remains of plants and plankton	coals, oil shales	source rocks; coals are a source of gas
Iron-rich and phosphatic	chemical and biochemical precipitation	sedimentary iron ores, phosphorites	minor role
Volcaniclastic	volcanic ash and fragments redeposited by water	tuffs, tuffites	occasionally reservoirs; the ash alters to clay minerals and zeolites

A division by the main process; in nature there are many transitional rocks — for example, clayey–carbonate marls or sandy limestones.

Within a class, there are two ways to name a rock. A name by texture (the size and shape of the grains) tells in what environment the rock was deposited and what its porosity may be. “Sandstone” means that the grains are of sand size. A name by composition tells what the grains are made of, where the material came from, and how the rock will behave when it is buried and when the reservoir is treated. “Quartz sandstone” and “arkose” are examples. A full name combines both and adds colour, cement and other features. So in a core description it grows into a short account.

“Structure” and “texture”: false friends for the translator

In Russian-language petrography the struktura (“structure”) of a rock is the size, shape and mutual arrangement of grains and crystals, and the tekstura (“texture”) is bedding, cross-bedding and other features of how the rock as a whole is put together. In English it is the other way round: the size, sorting and shape of grains are called texture, and bedding is called sedimentary structures. So the Russian “fine-grained structure” is fine-grained texture in English, and the Russian “cross-bedded texture” is cross-bedding (layers laid down at an angle to the main bedding). Bedding and other structures, and what they say about the conditions of deposition, are a separate topic (primary sedimentary structures).

Clastic rocks: names by grain size

We name clastic rocks first by the size of their fragments, because size reflects the strength of the current. A fast stream carries away small particles and leaves gravel and sand, while mud settles in quiet water. To a first approximation, grain size also decides the porosity, and especially the permeability, of the future reservoir (Rock permeability).

Main classes of clastic rocks by grain size

Grain size: coarser than 2 mm
Loose sediment: gravel, pebbles, boulders; rubble (angular fragments)
Cemented rock: gravelstone and conglomerate (rounded fragments), breccia (angular)

Grain size: 2 to 1/16 mm (0.0625 mm)
Loose sediment: sand
Cemented rock: sandstone

Grain size: 1/16 to 1/256 mm (about 0.004 mm)
Loose sediment: silt
Cemented rock: siltstone

Grain size: finer than 1/256 mm
Loose sediment: clay
Cemented rock: clay, mudstone (argillite)

Grain size	Loose sediment	Cemented rock
coarser than 2 mm	gravel, pebbles, boulders; rubble (angular fragments)	gravelstone and conglomerate (rounded fragments), breccia (angular)
2 to 1/16 mm (0.0625 mm)	sand	sandstone
1/16 to 1/256 mm (about 0.004 mm)	silt	siltstone
finer than 1/256 mm	clay	clay, mudstone (argillite)

The boundaries are those of the Udden–Wentworth scale, in which each boundary is twice the next one. The detailed division and the sorting of grains (how uniform their sizes are) are in the topic on grain size, sorting and shape.

Russian-language literature also uses other scales. For example, the boundary between sand and silt may be at 0.1 or 0.05 mm, and between silt and clay at 0.01 or 0.005 mm. So a “coarse-grained siltstone” in one description may be a “very fine-grained sandstone” in another. When you compare core descriptions from different years or organisations, check which scale each one uses.

In conglomerates and breccias, it matters what holds up the framework. If the large fragments touch one another (clast-supported), large pores or gaps filled with clean sand may remain between them, and such a rock can be a good reservoir. If the fragments “float” in a filling of clay and sand (matrix-supported), as in the deposits of mudflows and glaciers, fine material chokes the voids. By the composition of the fragments, we distinguish oligomictic conglomerates (made of stable quartz, chert and quartzite) and polymictic ones (made of fragments of many rocks).

Mudrocks are mixtures of clay minerals, fine quartz, mica and organic matter. A rock that is plastic and softens in water is called a clay. A compacted rock that does not soften is a mudstone (argillite). A rock with a noticeable share of silt is a silty clay or a siltstone. The English word shale means an argillite that splits into thin plates. It does not mean a metamorphic slate (more). The colour of a mudrock often tells you what it is made of:

Black and dark grey — a lot of organic matter and pyrite. The rock was deposited in stagnant water without oxygen. Such rocks may be source rocks (Source rocks, maturation and migration).
Red and brown — ferric iron oxides (rust-like compounds). The sediment was deposited, or lay for a long time, in an oxygen-rich (oxidising) environment, often on land. Organic matter does not survive in it.
Green and grey-green — iron kept in a reduced form (with little oxygen attached) in clay minerals and chlorite, or grains of glauconite. The environment had no excess oxygen and was often marine.

Mudrocks are the main seals. Their pores are so fine that oil and gas cannot enter them without a large pressure difference (Reservoir and seal). Thin interbeds (thin layers) and clay scattered through sandstones spoil the reservoir and make log interpretation harder (Determining shale volume).

Sandstones: framework, matrix and cement

Sandstones are the main reservoirs of clastic sections. To name a sandstone by composition, you tell four constituents apart in thin section:

Framework: The sand grains: quartz ( $Q$ ), feldspars ( $F$ ) and rock (lithic) fragments ( $L$ ). The rock fragments come from chert, slate and shale, mica schist and volcanic rocks. The framework count usually leaves out micas, heavy minerals, glauconite and shell fragments, but the name mentions them if there are many.
Matrix: Fine clastic material: clay particles and fine silt finer than about 0.03 mm, deposited together with the grains. The matrix fills the spaces between the grains and sharply reduces permeability.
Cement: Minerals that grew in the pores after burial: quartz growing onto the grains, calcite, dolomite, siderite, anhydrite, authigenic clay minerals (clay minerals that grew in place). Cement binds the rock and reduces porosity.
Pores: The voids that remain: primary intergranular ones, and secondary ones formed when grains and cement dissolved.

First you estimate the share of matrix: it shows how well the current sorted the sediment. Arenites have less than 15% matrix, and wackes have 15 to 75%. With more matrix than that, the rock is a mudrock. Then you recalculate the framework composition to 100% and place the rock on the quartz–feldspar–rock fragment triangle.

Composition of a sandstone framework

Q = \frac{N_Q}{N_Q + N_F + N_L}\cdot 100\,\%,\qquad F = \frac{N_F}{N_Q + N_F + N_L}\cdot 100\,\%,\qquad L = \frac{N_L}{N_Q + N_F + N_L}\cdot 100\,\%

where:

$N_Q,\ N_F,\ N_L$ number of points (or volume fractions) counted in thin section as quartz, feldspars and rock fragments, —
$Q,\ F,\ L$ shares of quartz, feldspars and rock fragments in the framework; $Q + F + L = 100$ %, %

Units: percentages of the sum of the three framework constituents

Holds when

Matrix, cement, pores, micas and other grains are not included in the sum.
Some schemes count chert as quartz and others count it as rock fragments, so a report should state how it was counted.

Recalculating to the framework lets you compare sandstones whatever their cement and pore content. Cement and pores record the history after deposition, whereas the framework records the source of the material and how it was transported.

Arenite classes by framework composition

Name: Quartz arenite
Quartz, % of framework: 95 or more
Which of the other grains is more abundant: —

Name: Subarkose
Quartz, % of framework: 75–95
Which of the other grains is more abundant: feldspars

Name: Sublitharenite
Quartz, % of framework: 75–95
Which of the other grains is more abundant: rock fragments

Name: Arkose
Quartz, % of framework: less than 75
Which of the other grains is more abundant: feldspars

Name: Lithic arenite
Quartz, % of framework: less than 75
Which of the other grains is more abundant: rock fragments

Name	Quartz, % of framework	Which of the other grains is more abundant
Quartz arenite	95 or more	—
Subarkose	75–95	feldspars
Sublitharenite	75–95	rock fragments
Arkose	less than 75	feldspars
Lithic arenite	less than 75	rock fragments

Wackes are divided in the same way: quartz wacke (95% quartz or more), feldspathic and lithic wackes. The boundaries are those of one widely used scheme; other schemes draw them slightly differently (advanced section).

CalculatedArenite classes on the quartz–feldspar–rock fragment triangle and three samples: A is from the example below, B and C are for comparison. The sample compositions are assumed for illustration.

Worked exampleNaming a sandstone from a point count in thin section

Given (assumed data)

Points counted: 400
Quartz, feldspars, rock fragments: 196, 62 and 34
Mica: 6
Matrix: 22
Cement (calcite and quartz overgrowths): 36
Pores: 44

Solution

Framework: $196 + 62 + 34 = 292$ points; mica is not included in the framework.
Framework composition: $Q = 196/292 = 67.1$ %, $F = 62/292 = 21.2$ %, $L = 34/292 = 11.6$ %.
Matrix: $22/(292 + 22) = 7.0$ % — less than 15%, so the rock is an arenite.
Quartz is less than 75% and feldspars outnumber rock fragments: arkose.
Visible porosity in thin section: $44/400 = 11$ %; cement — $36/400 = 9$ %. The ratio $Q/(F + L) = 196/96 = 2.04$ is a measure of mineralogical maturity (how far transport and weathering have left mainly stable quartz).

Answer: Arkose: quartz 67.1%, feldspars 21.2%, rock fragments 11.6% of the framework, matrix 7.0%; visible porosity about 11%.

What it means. The name suggests what to check in the reservoir. Potassium feldspars contain potassium, so the gamma ray log of this clean sandstone may read high, like that of a shale (Gamma ray log and natural gamma ray spectrometry). As the rock is buried, feldspars partly dissolve and leave secondary pores, and they turn into kaolinite. Kaolinite particles can break loose and plug the pore channels during production (Formation damage and damage skin). Calcite cement dissolves in hydrochloric acid. The porosity seen in thin section is usually lower than the porosity measured on core, because fine pores between clay particles are not visible under the microscope. A count of 400 points gives the proportions with an error of about 2–3% (advanced section). The data in this example are assumed for illustration.

The framework composition shows the mineralogical maturity of a sand. The longer weathering and transport worked on it, the fewer unstable feldspars and rock fragments it has, and the more quartz. The ratio $Q/(F + L)$ measures this. Textural maturity shows in the sorting and rounding of the grains and in the absence of matrix. These are different properties: a well-sorted beach sand may be rich in feldspars, and a quartz sand may be poorly sorted (the two “maturities”). Immature sandstones lose quality faster as they are buried. Feldspars and fragments of volcanic rocks turn into clay minerals, and soft fragments of shale and slate are crushed and close the pores.

Carbonate rocks: the Dunham and Folk classifications

Carbonate rocks are made of calcite and dolomite. Modern carbonate sediments contain a lot of aragonite and magnesian calcite, because many organisms build their skeletons from them. After burial they change to calcite. Dolomite forms mainly later, by replacing calcite (main minerals). Unlike clastic rocks, carbonate grains are made in the basin itself, most often with the help of organisms. So grain size says little here about the strength of the current: a large shell can lie in a quiet lagoon too.

Bioclasts: Whole skeletons and fragments of shells, corals, algae and echinoderm spines.
Ooids: Rounded grains, usually 0.25–2 mm across, made of concentric layers of carbonate around a nucleus (a shell fragment or a sand grain). They form on shoals (shallow banks), where water keeps rolling the grains about. A rock made of ooids is an oolite.
Peloids: Small rounded grains of microcrystalline carbonate without internal structure. Many of them are coprolites (pellets, fossil droppings) of bottom-dwelling organisms.
Intraclasts: Fragments of semi-lithified (half-hardened) carbonate sediment from the same basin, torn off by a storm or a current.
Micrite: Lime mud — calcite with crystals a few micrometres across (usually less than 4–5 µm); in Russian-language descriptions, pelitomorphic calcite. A sign of quiet water.
Sparite: Clear crystalline calcite with crystals tens of micrometres across and larger; usually a cement that grew in the pores between grains.

The main idea of the Dunham classification (Dunham, 1962) is that two things matter most: how much lime mud the rock contains, and what holds up its framework, mud or grains. Mud settles only where the water is calm; constant waves and currents wash it out. So a Dunham name tells you directly about the energy of the depositional environment, and about what the primary pores were like.

Carbonate classes after Dunham

Name: Mudstone
What supports the framework: mud
Grains: less than 10%
Mud: present
What it means: quiet water

Name: Wackestone
What supports the framework: mud
Grains: 10% or more, “floating” in the mud
Mud: present
What it means: quiet water

Name: Packstone
What supports the framework: grains
Grains: touching
Mud: present between the grains
What it means: grains accumulated, mud settled between them

Name: Grainstone
What supports the framework: grains
Grains: touching
Mud: absent (less than 1%)
What it means: waves and currents washed the mud out

Name: Boundstone
What supports the framework: organisms bound the sediment together as it was deposited
Grains: —
Mud: —
What it means: reefs, algal buildups

Name: Crystalline carbonate
What supports the framework: —
Grains: the original fabric cannot be recognised
Mud: —
What it means: recrystallisation, often dolomitization

Name	What supports the framework	Grains	Mud	What it means
Mudstone	mud	less than 10%	present	quiet water
Wackestone	mud	10% or more, “floating” in the mud	present	quiet water
Packstone	grains	touching	present between the grains	grains accumulated, mud settled between them
Grainstone	grains	touching	absent (less than 1%)	waves and currents washed the mud out
Boundstone	organisms bound the sediment together as it was deposited	—	—	reefs, algal buildups
Crystalline carbonate	—	the original fabric cannot be recognised	—	recrystallisation, often dolomitization

In Dunham's scheme, mud means particles finer than about 0.02 mm; in the extended scheme of Embry and Klovan, finer than 0.03 mm. In the extended scheme, a rock with more than 10% of grains coarser than 2 mm is a floatstone (large fragments floating in mud) or a rudstone (a framework of large fragments). Boundstones are divided into framestones, bindstones and bafflestones, depending on whether the organisms built a rigid framework, encrusted the sediment or trapped it.

SchematicDunham classes in thin section: from mud with rare grains to a framework of grains without mud, and a skeletal buildup. Schematic.

The Folk classification (Folk, 1959–1962) answers a different question: what are the grains made of, and what fills the space between them? A name has two parts: a prefix for the main grains and an ending for the filling. The ending is “-sparite” if there is more sparite cement than micrite, and “-micrite” if it is the other way round. You choose the prefix in order. If intraclasts make up at least a quarter of the grains, the prefix is “intra-”. If not, but at least a quarter are ooids, it is “oo-”. Otherwise the ratio of bioclasts to peloids decides: more than 3 : 1 gives “bio-”, less than 1 : 3 gives “pel-”, and anything in between gives “biopel-”. If grains make up less than 10% of the rock, it is called a micrite. A reef buildup of skeletons that grew in place is a biolithite.

Names after Folk

Main grains: Intraclasts
Filling — sparite cement: intrasparite
Filling — micrite: intramicrite

Main grains: Ooids
Filling — sparite cement: oosparite
Filling — micrite: oomicrite

Main grains: Bioclasts
Filling — sparite cement: biosparite
Filling — micrite: biomicrite

Main grains: Peloids
Filling — sparite cement: pelsparite
Filling — micrite: pelmicrite

Main grains	Filling — sparite cement	Filling — micrite
Intraclasts	intrasparite	intramicrite
Ooids	oosparite	oomicrite
Bioclasts	biosparite	biomicrite
Peloids	pelsparite	pelmicrite

The names in the two schemes complement each other, so descriptions often give them together: “oolitic grainstone (oosparite)”. Today Dunham's scheme is used more often.

Worked exampleTwo thin sections of limestone: names after Dunham and after Folk

Given (assumed data)

Thin section A, 300 points: ooids 156, bioclasts 30, peloids 9, intraclasts 3, sparite cement 72, pores 30, no micrite; the ooids touch
Thin section B, 300 points: bioclasts 66, peloids 12, micrite 198, sparite 9, pores 15; the bioclasts are scattered in the mud and do not touch

Solution

Thin section A: grains $156 + 30 + 9 + 3 = 198$ , which is 66% of the rock. Intraclasts are $3/198 = 1.5$ % of the grains, less than a quarter. Ooids are $156/198 = 79$ %, more than a quarter, so the prefix is “oo-”. There is no micrite, and there is sparite between the grains. By Folk's scheme this is an oosparite.
By Dunham's scheme: the ooids form the framework and there is no mud, so this is a grainstone (oolitic). Visible porosity $30/300 = 10$ %.
Thin section B: grains $66 + 12 = 78$ , which is 26% of the rock, more than 10%. There are no intraclasts or ooids. There are $66/12 = 5.5$ times as many bioclasts as peloids, which is more than three, so the prefix is “bio-”. There are 198 points of micrite against 9 of sparite. By Folk's scheme this is a biomicrite.
By Dunham's scheme: the grains are scattered in the mud and do not touch, and there are more than 10% of them, so this is a wackestone. Visible porosity $15/300 = 5$ %.

Answer: Thin section A is an oolitic grainstone (oosparite), thin section B is a wackestone (biomicrite).

What it means. The first rock was deposited where waves kept reworking the sea floor, on an oolite shoal. Pores remain between the grains, and unless early cement filled them, it is a good reservoir. The second rock was deposited in quiet water, where mud settled. Its pores are microscopic and its permeability is low, until dissolution, dolomitization or fractures create secondary porosity (pores that formed after deposition). Note that for a Dunham name you have to see whether the grains touch: percentages alone are not enough. The data in this example are assumed for illustration.

Russian-language descriptions of carbonates use their own words for the fabric. Organogenic (biomorphic) and organogenic–clastic limestones are made of skeletons and their fragments. Oolitic limestone is made of ooids. Clotted limestone is made of peloids and clumps. Pelitomorphic limestone is made of micrite. Crystalline–granular limestone is recrystallised, and the description states the crystal size. Chalk is a special case. It is a soft, fine-grained limestone made of the skeletons of planktonic algae a few micrometres across. Its porosity can be very high, but its permeability is only a few millidarcies, and the commercial flows from it often come through fractures.

Mixed rocks: dolomitic limestones and marls

Many rocks are mixtures: of calcite and dolomite, of carbonates and clayey material, of sand and carbonate. Russian-language descriptions name a mixture after the component that makes up more than half of the rock, and they mark admixtures with adjectives. At 5–25% they use the weaker form of the adjective (Russian -istyi: slightly dolomitic, clayey, sandy). At 25–50% they use the stronger form (Russian -ovyi or -nyi: dolomitic, sandy). They usually do not mention an admixture of less than 5%, or they write “with an admixture of”. The rule is not followed everywhere: clayey limestones and marls have their own scale (the second table).

Mixtures of calcite and dolomite

Name: Limestone
Dolomite, % of the carbonate part: less than 5

Name: Slightly dolomitic limestone
Dolomite, % of the carbonate part: 5–25

Name: Dolomitic limestone
Dolomite, % of the carbonate part: 25–50

Name: Calcareous dolomite
Dolomite, % of the carbonate part: 50–75

Name: Slightly calcareous dolomite
Dolomite, % of the carbonate part: 75–95

Name: Dolomite
Dolomite, % of the carbonate part: more than 95

Name	Dolomite, % of the carbonate part
Limestone	less than 5
Slightly dolomitic limestone	5–25
Dolomitic limestone	25–50
Calcareous dolomite	50–75
Slightly calcareous dolomite	75–95
Dolomite	more than 95

The shares are percentages of the carbonate part of the rock. Some reference books use fewer classes: limestone, dolomitic limestone, calcareous dolomite and dolomite, with boundaries at 5, 50 and 95%. English-language literature often uses boundaries of 10, 50 and 90% for such mixtures (dolomitic limestone, calcitic dolomite).

Mixtures of carbonates and clayey material

Rock: Limestone (dolomite)
Carbonates, %: 95–100
Clayey material, %: 0–5

Rock: Clayey limestone (dolomite)
Carbonates, %: 75–95
Clayey material, %: 5–25

Rock: Marl (dolomitic marl)
Carbonates, %: 25–75
Clayey material, %: 25–75

Rock: Calcareous (dolomitic) clay
Carbonates, %: 5–25
Clayey material, %: 75–95

Rock: Clay
Carbonates, %: 0–5
Clayey material, %: 95–100

Rock	Carbonates, %	Clayey material, %
Limestone (dolomite)	95–100	0–5
Clayey limestone (dolomite)	75–95	5–25
Marl (dolomitic marl)	25–75	25–75
Calcareous (dolomitic) clay	5–25	75–95
Clay	0–5	95–100

The boundaries are those of a widely used Russian-language scale. Marls and clayey limestones are common seals and source rocks; they usually become reservoirs only when fractured.

Worked exampleHow to name a rock from a calcimetry result

Given (assumed data)

Calcimetry of cuttings (from the example in the lesson on minerals): calcite 53.2%, dolomite 29.1%, non-carbonate part 17.7%
Non-carbonate part: according to the description of the cuttings, mainly clayey material (an assumption of the example)

Solution

Carbonate part: $53.2 + 29.1 = 82.3$ %.
Share of dolomite in the carbonate part: $29.1/82.3 = 35.4$ % — between 25 and 50%: a dolomitic limestone.
Carbonates 82.3%, clayey material 17.7% — carbonates between 75 and 95%: a clayey limestone.

Answer: The rock is a dolomitic, clayey limestone: dolomite is 35.4% of the carbonate part, clayey material 17.7%.

What it means. The name changes with the nature of the admixture: if the non-carbonate part were quartz silt, the rock would be a dolomitic, silty limestone. So we add a description of the cuttings and of thin sections to the calcimetry. This matters all the more because calcimetry cannot tell dolomite from slowly reacting siderite, and it depends on an arbitrary time limit between “fast” and “slow” gas (calcimetry). The data in this example are assumed for illustration.

Evaporites and other sedimentary rocks

Evaporites settle out of water that evaporates faster than it is replaced. This happens on the tidal salt flats of a hot coast (sabkhas), in lagoons and salt lakes, and in isolated marine basins. As the water evaporates, the brine becomes more concentrated, and minerals precipitate (settle out as solids) in a fixed order, the least soluble first.

What precipitates as sea water evaporates

Mineral: Calcite, aragonite
Brine concentration relative to sea water: about 1.8 times
Water evaporated: about 45%

Mineral: Gypsum; at depth it changes to anhydrite
Brine concentration relative to sea water: about 3.8 times
Water evaporated: about 74%

Mineral: Rock salt (halite)
Brine concentration relative to sea water: about 10.6 times
Water evaporated: about 91%

Mineral: Potash and magnesium salts (sylvite, carnallite and others)
Brine concentration relative to sea water: 70–90 times and more
Water evaporated: more than 98%

Mineral	Brine concentration relative to sea water	Water evaporated
Calcite, aragonite	about 1.8 times	about 45%
Gypsum; at depth it changes to anhydrite	about 3.8 times	about 74%
Rock salt (halite)	about 10.6 times	about 91%
Potash and magnesium salts (sylvite, carnallite and others)	70–90 times and more	more than 98%

The values come from observations of sea water evaporating in salt works, and are rounded. The fraction of water that has evaporated is taken as $1 - 1/k$ , where $k$ is the concentration of the brine.

About 35 g of salts are dissolved in 1 kg of sea water. About 27.4 g of this is sodium chloride; this is the amount that the sodium allows, because chlorine is in excess. This leads to a simple and unexpected conclusion: if you evaporate even a very deep body of water completely, you get only a thin layer of salt.

Thickness of a mineral layer from a column of sea water

h = H\,\frac{\rho_{sw}\,c}{\rho_{min}}

where:

$h$ thickness of the mineral layer without pores, m
$H$ height of the column of evaporated sea water, m
$\rho_{sw}$ density of sea water at the surface, about 1025. (Basin subsidence calculations use a rounded 1030, because in a column kilometres deep the water is colder and compressed. The effect on the thickness of the salt layer is less than 1%.), kg/m³
$c$ mass of mineral that 1 kg of sea water can yield: NaCl — about 0.0274, CaSO₄ — about 0.0014, kg/kg
$\rho_{min}$ density of the mineral: halite 2170, anhydrite 2970, gypsum 2320, kg/m³

Units: SI

Holds when

The water evaporates completely and the whole dissolved component precipitates as a single mineral.
The salt is not redissolved and is not carried away with the brine; nothing is lost into pores or brine.

A 1 km column of sea water gives about 13 m of rock salt and less than half a metre of anhydrite. Salt sequences several kilometres thick need hundreds of kilometres of water to evaporate. This means the basin received sea water for a long time.

Worked exampleHow much sea water has to be evaporated to make a salt sequence

Given (assumed data)

Sea water: 1025 kg/m³; NaCl 27.4 g/kg; CaSO₄ 1.40 g/kg
Density of halite and anhydrite: 2170 and 2970 kg/m³ (gypsum 2320)
Rock salt sequence: 4.5 km; basin depth 2 km

Solution

A column of sea water 1000 m high with a cross-section of 1 m² weighs $1000\cdot 1025 = 1.025\cdot10^6$ kg.
Rock salt: $1000\cdot 1025\cdot 0.0274/2170 = 12.94$ m.
Anhydrite: $1000\cdot 1025\cdot 0.00140/2970 = 0.48$ m; had gypsum precipitated, the layer would be 0.78 m.
For 4.5 km of salt we have to evaporate $4500/12.94 = 348$ columns of 1 km, that is, about 348 km of sea water.
A basin 2 km deep would have to be filled and evaporated to dryness again and again; the number of fillings is $4500/(2\cdot 12.94) = 174$ .

Answer: A 1 km column of sea water leaves about 12.9 m of rock salt and 0.48 m of anhydrite; for 4.5 km of salt about 348 km of sea water has to be evaporated.

What it means. Thick salt sequences are therefore the trace of basins into which sea water flowed for a long time, without a break, through a narrow strait. Evaporation concentrated the water until salt precipitated. This is how the Kungurian salt of the Pre-Caspian Basin built up. Its original thickness is estimated at 4–5 km. Collisions on the margins had almost cut the basin off from the ocean (Plate tectonics and sedimentary basins). The calculation is only an estimate. In nature, part of the salt dissolved and was deposited again, and concentrated brine could partly leave the basin and carry dissolved salts away. In that case even more sea water was needed. Potash and magnesium salts precipitate only from the last, most concentrated brines. The composition of sea water is a reference value, rounded.

As gypsum is buried, usually by a depth of about a kilometre, it loses its water and turns into anhydrite. The volume of the solid phase falls by almost 40% (calculation). Rock salt flows under load. So salt sequences form domes and walls that create traps (Traps and timing), and they are the best seals. They also complicate drilling: fresh drilling fluid dissolves the salt and makes washouts, and creeping salt narrows the wellbore (Drilling fluids: functions and properties).

Siliceous rocks are made of opal and quartz from the skeletons of diatoms, radiolarians and sponges. They include diatomites, opokas and radiolarites, and also dense cherts and jaspers. As the rock is buried, amorphous opal (opal with no crystal structure) recrystallises to quartz, and the rock loses porosity. Young diatomites can be reservoirs with very high porosity but low permeability.
Organic rocks are coals and oil shales. Coals go through the series peat, brown coal, hard coal and anthracite, and they release gas as they mature. Oil shales are immature clayey or marly rocks rich in kerogen (the solid organic matter that turns into oil). You can get oil from them only by heating. This is not the same as “shale oil” (tight oil), which is produced from mature source rocks and tight rocks by horizontal wells with hydraulic fracturing.
Volcaniclastic rocks are tuffs and tuffites made of volcanic ash. Ash quickly alters to smectite and zeolites. Thin bentonite interbeds swell and make good correlation markers, while in reservoirs ash material reduces permeability.
Iron-rich and phosphatic rocks are sedimentary iron ores and phosphorites; in a petroleum section they occur as thin interbeds and nodules.

What the name says about the reservoir

Rock name and what to expect of the reservoir

Rock: Quartz arenite, well sorted
What to expect: high porosity and permeability, low gamma ray; at depth, quartz cement
What to check: how far the pores have been filled with quartz during burial

Rock: Arkose, subarkose
What to expect: dissolution of feldspars gives secondary pores, but also kaolinite; raised gamma ray
What to check: clay minerals in the pores, fines migration, reaction with acid

Rock: Lithic arenite
What to expect: soft fragments are crushed: porosity falls quickly with depth
What to check: the share of soft, easily squashed fragments, compaction

Rock: Wacke (greywacke)
What to expect: matrix plugs the pores: permeability is low
What to check: fractures; otherwise unlikely to be a reservoir

Rock: Grainstone, oolite
What to expect: intergranular pores, high permeability
What to check: early cement, dissolution

Rock: Wackestone, mudstone
What to expect: micropores: porosity can be high, permeability low
What to check: secondary porosity, dolomitization, fractures

Rock: Boundstone, reef limestone
What to expect: large primary and secondary voids, strong heterogeneity
What to check: vugs, fractures, their connectivity

Rock: Dolomite
What to expect: often better than the original limestone: intercrystalline pores
What to check: crystal size, anhydrite cement

Rock: Clays, mudstones, marls, salt, anhydrite
What to expect: seals; dark clays and marls are source rocks
What to check: continuity of the seal, total organic carbon content

Rock	What to expect	What to check
Quartz arenite, well sorted	high porosity and permeability, low gamma ray; at depth, quartz cement	how far the pores have been filled with quartz during burial
Arkose, subarkose	dissolution of feldspars gives secondary pores, but also kaolinite; raised gamma ray	clay minerals in the pores, fines migration, reaction with acid
Lithic arenite	soft fragments are crushed: porosity falls quickly with depth	the share of soft, easily squashed fragments, compaction
Wacke (greywacke)	matrix plugs the pores: permeability is low	fractures; otherwise unlikely to be a reservoir
Grainstone, oolite	intergranular pores, high permeability	early cement, dissolution
Wackestone, mudstone	micropores: porosity can be high, permeability low	secondary porosity, dolomitization, fractures
Boundstone, reef limestone	large primary and secondary voids, strong heterogeneity	vugs, fractures, their connectivity
Dolomite	often better than the original limestone: intercrystalline pores	crystal size, anhydrite cement
Clays, mudstones, marls, salt, anhydrite	seals; dark clays and marls are source rocks	continuity of the seal, total organic carbon content

These are tendencies, not rules. Diagenesis (the changes in a rock after burial), fractures and measurements on core finally decide the quality of a reservoir (Core analysis).

The subsalt buildups of the Pre-Caspian Basin are an example of a carbonate reservoir. Tengiz, Kashagan and Karachaganak are linked to carbonate platforms and reef buildups (atolls and reefs) of the Late Devonian, Carboniferous and Early Permian (basins of Kazakhstan). Geologists describe such reservoirs by combining the Dunham classification, the types of voids (intergranular pores, vugs, fractures) and data on diagenesis (Carbonate sediments, environments and carbonate porosity). They describe the Jurassic and Cretaceous sandstones of Mangyshlak and the South Turgai Basin by framework composition and shale volume.

How to read a core description

1
“Sandstone light grey, fine-grained”
a class by grain size: fine sand (0.125–0.25 mm on the international scale; check which scale the description uses). A light colour means little organic matter and little clay.
2
“well sorted”
grains of similar size: other things being equal, porosity and permeability are higher (on sorting).
3
“oligomictic quartz”
a framework mainly of quartz with a small admixture of other grains: a mature, chemically stable sandstone with a low gamma ray.
4
“slightly clayey”
a little clayey material, either matrix or authigenic clays. Which one it is matters: kaolinite is prone to fines migration (loose particles moving with the flow), and smectite swells in fresh water (clay minerals).
5
“on quartz-overgrowth and carbonate cement”
the pores are partly filled with quartz that grew onto the grains, and with calcite. Porosity is lower than the original, and carbonate cement dissolves in hydrochloric acid.
6
“with an oil smell, patches of oil saturation”
direct evidence of hydrocarbons in the sample (oil shows); it is confirmed by fluorescence under ultraviolet light and by laboratory studies.

Terms and symbols

Clastic (terrigenous) rocks: Rocks made of fragments of minerals and rocks carried and deposited by water, wind or ice.
Texture and structure of a rock: In Russian-language petrography: struktura — the size, shape and combination of grains (English texture), tekstura — bedding and the fabric of the rock (English sedimentary structures).
Framework, matrix, cement: The sand grains; fine clastic material finer than about 0.03 mm, deposited with the grains; minerals that grew in the pores after burial.
Arenite and wacke: Sandstones with a matrix share of less than 15% and of 15 to 75%.
QFL triangle: A diagram of the framework composition of a sandstone: quartz — feldspars — rock fragments.
Arkose: A sandstone whose framework has less than 75% quartz and more feldspars than rock fragments.
Lithic arenite: A sandstone whose framework has less than 75% quartz and more rock fragments than feldspars.
Mineralogical and textural maturity: Mineralogical maturity is how rich a sand is in stable quartz; textural maturity is how well sorted and rounded it is, and how free of matrix.
Micrite and sparite: Lime mud made of crystals a few micrometres across, and clear crystalline calcite, usually a cement.
Dunham classification: A name for a carbonate by what supports the framework: mudstone, wackestone, packstone, grainstone, boundstone, crystalline carbonate.
Folk classification: A name for a carbonate by the main grains and the filling: intra-, oo-, bio-, pel- + sparite or micrite.
Marl: A rock with 25–75% carbonate and 25–75% clayey material.
Evaporites: Gypsum, anhydrite, rock salt and potash salts precipitated from evaporating water.

Common mistakes and misconceptions

Wrong

“It is a sandstone, so it is a good reservoir.”

In fact

A wacke with a clayey matrix, or a sandstone whose pores are filled with quartz cement, can be almost impermeable. A name by composition and texture suggests what to expect, but the quality is decided by matrix, cement and diagenesis.

Wrong

“Mudstone in a description of carbonates is an argillite.”

In fact

In Dunham's scheme a mudstone is a carbonate rock made of lime mud with rare grains. An argillite is a clastic clayey rock.

Wrong

“If there are many pores in thin section, the rock is a grainstone.”

In fact

Dunham's classes describe the primary fabric — what supported the framework and whether there was mud — and not the present porosity. A porous wackestone with dissolution vugs remains a wackestone.

Wrong

“A salt sequence 1 km thick is an evaporated sea about 1 km deep.”

In fact

The evaporation of 1 km of sea water gives only about 13 m of rock salt. Sequences a kilometre thick accumulate in basins into which sea water flows for a long time and where it evaporates continuously.

Wrong

“Oil shale and shale oil are the same thing.”

In fact

Oil shales are immature rocks rich in kerogen: oil is obtained from them by heating. “Shale oil” is produced from mature source rocks and tight rocks, in which it has already formed, by horizontal wells with hydraulic fracturing.

In depth: advanced detail

How many points to count in a thin section

You find the composition of a rock in thin section by point counting. You move the section over a grid, and at each point you record what lies under the cross-hair of the eyepiece. The share of points that fall on a component estimates its share of the volume. This is a sample, so it has a random error, like any proportion in a binomial sample (a sample of yes/no counts) (Distributions and sample size).

Error of a proportion found by point counting

SE = \sqrt{\frac{p\,(1 - p)}{n}}, \qquad n \approx \frac{4\,p\,(1 - p)}{\delta^2}

where:

$SE$ standard error of the proportion, fraction
$p$ proportion of the component from the count, fraction
$n$ number of points counted, —
$\delta$ desired half-width of the $\pm 2\,SE$ interval (about 95%), fraction

Units: fraction

Holds when

The points are independent: the grid spacing is greater than the grain size, otherwise one grain falls under several points and the error is greater than calculated.
The thin section is representative of the sample, and the sample of the bed; the number of points does not reduce this error.

The error falls as $1/\sqrt{n}$ : to halve it, four times as many points are needed.

Worked exampleArenite or wacke?

Given (assumed data)

Thin section from the example in the lesson: 22 matrix points out of 314 framework and matrix points
Another sample: matrix 13% with the same number of points

Solution

First sample: $p = 22/314 = 0.070$ ; $SE = \sqrt{0.070\cdot(1 - 0.070)/314}$ , that is, 1.44%; the $\pm 2SE$ interval is from 4.1 to 9.9%: confidently below 15%.
Second sample: $SE = \sqrt{0.13\cdot 0.87/314}$ , that is, 1.90%; the interval is from 9.2 to 16.8%. The 15% boundary lies inside it, so we cannot say whether the rock is an arenite or a wacke.
To narrow the interval to ±2%, the number of points must be $n = 4\cdot 0.13\cdot 0.87/0.02^2 = 1131$ .

Answer: With 314 points, a share of 13% is known to within about ±3.8%, so we cannot put the sample in either the arenites or the wackes. For an accuracy of ±2% we need about 1131 points.

What it means. For an ordinary description, 300–500 points are enough. But when a rock lies near the boundary between classes, its name is only a convention, so it is safer to report the proportions themselves. The data in this example are assumed for illustration.

One rock — different names

There are many schemes for naming sandstones. They all use the same quartz–feldspar–rock fragment triangle, but they draw the boundaries differently. In the scheme used in this lesson, the field below 75% quartz is divided in half: arkose or lithic arenite. In another widely used scheme (Folk's), this field is divided into four parts by the ratio of feldspars to rock fragments, $F : L$ , with boundaries at 3 : 1, 1 : 1 and 1 : 3. The four classes are arkose, lithic arkose, feldspathic litharenite and litharenite. This scheme also assigns some grains differently: for example, it counts fragments of granite as feldspars.

Worked exampleThe sandstone from the lesson's example, named by another scheme

Given (assumed data)

Framework: quartz 196, feldspars 62, rock fragments 34 points

Solution

Quartz is 67.1% — less than 75%.
The share of feldspars among feldspars and rock fragments: $62/(62 + 34) = 64.6$ %, that is, $F : L \approx 1.8 : 1$ — between 1 : 1 and 3 : 1.

Answer: By the scheme of the lesson, an arkose; by the scheme with four fields, a lithic arkose.

What it means. Neither name is wrong, but you can compare sandstones from different reports only if you know which scheme named them. So reports should state the scheme and give the proportions of $Q$ , $F$ and $L$ themselves. The data in this example are assumed for illustration.

Connections to other topics

Minerals and rocks — minerals of sedimentary rocks, the three classes of rock and calcimetry.
Reservoir and seal — what makes a rock a reservoir and a seal; the two “maturities” of a sandstone.
Plate tectonics and sedimentary basins — in which basins sedimentary sequences accumulate, and why.
Core analysis — laboratory studies of core, which check what the rock's name leads you to expect.
Gamma ray log and natural gamma ray spectrometry — why arkoses and shales give a high gamma ray.
Lithology and porosity from crossplots — how lithology is determined from logs.
Sediment texture: grain size, sorting and shape — grain size, sorting and shape.
Primary sedimentary structures — bedding and other structures of sedimentary rocks.
Carbonate sediments, environments and carbonate porosity — environments of carbonate deposition and the types of voids in carbonates.

Check yourself

1
The framework of a sandstone: quartz 80%, feldspars 14%, rock fragments 6%; the matrix is 20% of the sum of framework and matrix. What is it called?
1. A.Subarkose
2. B.Feldspathic wacke
3. C.Quartz arenite
4. D.Arkose
Show the answer
Correct answer: B. Feldspathic wacke
Look at the matrix first. 20% is more than 15%, so this is a wacke, not an arenite. Quartz is less than 95%, and feldspars outnumber rock fragments, so it is a feldspathic wacke. If the matrix were less than 15%, the same framework (75–95% quartz, with more feldspars) would give a subarkose.
2
In thin section the ooids touch one another, the spaces between them are filled with clear crystalline calcite, and there is no lime mud. How is the rock named after Dunham and after Folk?
1. A.Boundstone; biolithite
2. B.Wackestone; oosparite
3. C.Packstone; oomicrite
4. D.Grainstone; oosparite
Show the answer
Correct answer: D. Grainstone; oosparite
The grains hold up the framework and there is no mud, so it is a grainstone. The main grains are ooids and the filling is sparite cement, so it is an oosparite. A packstone and an oomicrite would have mud between the grains. A wackestone has grains scattered in mud. A boundstone has organisms that bound the sediment together.
3
A limestone: bioclasts 18%, peloids 2%, micrite 70%, sparite 4%, pores 6%; the bioclasts are scattered in the mud and do not touch. Name the rock after Dunham and after Folk.
Show the answer
Grains are $18 + 2 = 20$ %, which is more than 10%, and they “float” in the mud. By Dunham's scheme this is a wackestone. By Folk's scheme: there are no intraclasts or ooids, and there are 9 times as many bioclasts as peloids (more than 3 : 1), so the prefix is “bio-”. There is more micrite than sparite, so the ending is “-micrite”: a biomicrite.
4
How much rock salt and anhydrite precipitate when a layer of sea water 250 m thick evaporates completely?
Show the answer
A 1 km column of sea water gives about 12.9 m of halite and 0.48 m of anhydrite. A 250 m column gives a quarter of that: about 3.2 m of rock salt and 0.12 m of anhydrite. For a salt sequence hundreds of metres thick, sea water must flow into the basin for a long time, without a break.
5
By calcimetry the carbonate part of a rock is 60% dolomite and 40% calcite, with almost no non-carbonate admixture. What is the rock called in Russian-language practice?
1. A.Marl
2. B.Dolomitic limestone
3. C.Calcareous dolomite
4. D.Slightly calcareous dolomite
Show the answer
Correct answer: C. Calcareous dolomite
Dolomite is more than half, so the rock is a dolomite. Calcite is 40%, between 25 and 50%, so the adjective takes the stronger form (Russian -ovyi): calcareous dolomite. “Slightly calcareous” would mean 5–25% calcite, and “marl” would mean 25–75% clayey material.
6
A core description translated word for word from Russian reads: “structure fine-grained, texture cross-bedded”. What does each word describe and how should it be said in English?
Show the answer
The Russian struktura (“structure”) is the size, shape and combination of grains: “fine-grained” means the grains are of fine-sand size, which in English is fine-grained texture. The Russian tekstura (“texture”) is the fabric of the rock as a whole, here cross-bedding — in English, a sedimentary structure. Translating tekstura as “texture” is a common mistake.

Updated: 10 October 2026