Porosity of Rocks

the sum of interstices, or pores, enclosed in rocks. Quantitatively, the porosity of rocks is expressed as the ratio of the volume of all pores to the total volume of the rocks; this ratio is given as a fraction or percentage. It is customary to divide the pores in rocks by size into subcapillary (less than 0.2 μ), capillary (0.2–100 μ), and supercapillary (more than 100 μ).

Pores may be shaped like bubbles, canals, slits, branches, and so on. The shape and size of the individual pores and their mutual relationship determine the geometry of the pore space of the rocks.

Geologists distinguish between total, open, and closed porosity. Total (absolute, physical, full) porosity is the total of all pores in the rocks. Open (saturation) porosity is the volume of interconnected pores, whereas closed porosity is the total volume of pores that are not interconnected. Petroleum geology also distinguishes effective porosity, which is the total volume of pores filled with petroleum and gas, and dynamic porosity, which is the volume of pores through which saturating liquids or gases pass at a given pressure and temperature. Dynamic porosity is always less than the total porosity of the rocks.

The highest porosity—to 60–80 percent and more —is characteristic of soils and loose sediments, such as sand and clay. Sedimentary and igneous rocks, including sandstones, limestones, lavas, and tuffs, range in porosity from 50 to 10 percent and less. Magmatic and metamorphic rocks ordinarily have low porosity, approximately 0.1–3 percent. Porosity usually decreases in rocks found at greater depths, especially in the case of sedimentary rocks. At very great depths porosity may be very low.

The porosity of rocks is analyzed in the laboratory by free, vacuum, and forced saturation of rocks with a liquid. Other methods are based on the expansion of gas. Under field conditions, various types of well logging are used to estimate the porosity value of rocks. Estimates of rock porosity are used both to calculate reserves of petroleum, gas, and other useful minerals and to select the technology for extraction.