Porosity and permeability are two terms that are commonly mentioned together and sometimes, even mistakenly interchanged. It is certain, however, that these two terms are completely different and pertain to different rock, sediment, or soil characteristics. The volume of openings in relation to its total volume determines porosity. The rule of thumb is that, the more tightly packed a rock is, then the less porous it is.
As an example, crystals are less porous than volcanic rocks since the crystalline structure allows for compact organization, while volcanic rocks usually form in the presence of gases and therefore contain air spaces. Permeability, on the other hand, pertains to the degree of capacity to enable fluid to pass through. Thus, for example, if a rock has numerous holes in its structure and lets fluids to pass through effectively, then it can be considered as something that is highly permeable. On the other hand, if a rock has holes yet fluid still cannot pass, then it cannot be considered to have good permeability.
In this sense, not only are possible passageways required for having good permeability, but the interconnectivity between the passageways is also important in order to allow the fluid to flow through. A sedimentary rock such as shale, which is composed of lithified clay, can be considered to have high porosity and low permeability. Since it is characterized by the numerous irregular spaces and breaks in its structure, it considered to be is rather porous just like other sedimentary rocks.
However, due to the fact that the spacing pattern does not convey a continuous path wherein fluids may travel through, it cannot be considered to be permeable; hence, it has low permeability. Therefore, porosity and permeability are indeed distinct from each other, but a directly proportional relationship may commonly be observed between the two; however, there are cases where the relationship does not hold true due to the lack of proper interconnectivity between pores.
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