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.