If we are interested in fluids that a carbonate rock may contain (water, oil, gases) then it's important to remember that pores constitute the part of the rock that contains those fluids. Assessment of the character, origin, and history of pores is essential for understanding the reservoir quality and productivity of a carbonate rock.
Porosity is the portion of the bulk volume made up of void space between mineral components. It may be expressed a percentage of the total volume, a volume fraction, or the void ratio (Vpore/Vmineral). A pore is an observable open space within the rock. It is important for students to learn to use these terms correctly: "porosity" for the bulk property of the rock to which a numerical value can (at least theoretically) be attached; "pore" for the feature that you observe through the microscope.
As in siliciclastic rocks, pores in carbonate rocks can be primary or secondary:
Primary Pore: A primary pore exists at the time of deposition. Application of this concept in carbonate rocks is more complex than in siliciclastic rocks, mainly because carbonates contain abundant biogenic components that have very complex shapes and abundant internal voids. In sandstones, primary porosity consists mostly of intergranular pores, the spaces between the grains, with only a minor (even trivial) component of primary intragranular (within the grain) pores.
Secondary Pore: These form after deposition and are the result of post-depositional chemical and mechanical processes (diagenesis).

The following cartoons and images give a primer on the some of the complex terminology applied to the description of pores in carbonate rocks. In these cartoons pore space is aqua blue (a common color for the impregnation media used to highlight the pores in thin sections), grains or replacement crystals are yellow, and mud is brown or green. See Scholle & Ulmer (2004) for a more complete summary of various pore classifications that have been devised. The cartoons are fairly generic with respect to scale---in general, they represent fields of view between about 0.2 and 2 cm across.

In the following list of examples click the thumbnails to open a larger view of the image.

Click the thumbnails below to open a larger view of the image. Open image windows can be re-sized and scrolled.

example of primary pores


Primary pores: intergranular and intragranular

Primary intergranular pores are simply those pores that exist between the particles of sediment at the time of deposition. Again, because of the role of complexly shaped biologic components, these pores in carbonate rocks display a range of form more complex than what is typically observed in siliciclastic rocks.

Primary intragranular pores occur in skeletal fragments and are more abundant, larger, and proportionally more significant as a fraction of the grain volume (and, hence, the rock volume), than in siliciclastic grains such as quartz or feldspar. In skeletal grains this type of pore arises from the internal space occupied by the soft tissues of the organism. This type of pore is intrinsic to the grain, and not a result of weathering, and exists from the moment of particle creation.

example of shelter
Sheltered pores:

Larger-than-average pores beneath large skeletal fragments that prop open a somewhat sediment-free space are called sheltered pores.
example of pores in mud
Pores in mud :

How do pores form in mud? There are several possibilities. There are tiny intergranular pores between the mud particles at the time of deposition, of course. These tiny pores are subject to compaction and cementation just like larger ones, but pores smaller than the thickness of the thin section cannot be studied by light microscopy. Larger pores in mud-rich rocks can form by dissolution, as described in the moldic pore example above. Another type of pore in mud arises from gas bubbles released from decomposing organic matter. Such pores may be small and ovoid in shape, called bird's-eye pores, or larger and more irregular in shape, fenestral pores. If filled with cement, these spaces are sometimes called "fabric", as in, "this wackestone displays bird's-eye fabric". Biological processes such as burrowing and rooting are implicated in the formation of Pores in mud.
example of Fenestral pores
Fenestral pores:

These fenestral pores are rather large, on the order of 1 to 2 mm.
example of moldic
Moldic pores:

An extreme case of a secondary intragranular pore is called a moldic pore. This type of pore forms when a grain completely dissolves away. This pore type is easiest to recognize in a muddy lithology such as a wackestone or packstone. Moldic pores within grainstones may collapse from compaction. If filled with cement, molds in grainstones are distinguished by either a contrasting cement crystal size than in the adjacent intraparticle pores, or, by rims of micrite formed before the dissolution event. In some cases the spar-filled mold is nearly indistinguishable from the adjacent spar-filled primary pores.
example of Moldic pores in ooid grainstone
Moldic pores in ooid grainstone:

The rounded shapes and also the surviving ooids allow us to identify these moldic pores as former ooids. Some host cement crystals, others do not.
example of Skeletal molds
Skeletal molds:

It's difficult to say for certain, but these crescent-shaped pores were, most likely, aragonitic pelecypods.
example of vugs & cracks
Vugs & cracks :

Large, irregular pores that cross-cut grains and cement and that typically arise from dissolution are called vugs. The material dissolved may be the limestone itself or something like an anhydrite nodule that was once enclosed in the limestone. In a way, vugs are just tiny caves. Mechanical breakage that opens space within a lime creates fracture pores.
example of intercrystalline
Intercrystalline pores:

In rocks that have undergone massive replacement, it is may no longer be possible to discriminate primary and secondary pores. Textural evidence that allows us to discern whether crystals are replacement versus fillings of primary pore space may not be preserved. In this case it is possible to simply describe pores as "intercrystalline".