Oil and gas develop in a sedimentary basin. They are generated then migrate within sedimentary rocks.
These rocks have one characteristic in common: they are all deposited in water (an ocean, a sea, a lagoon, a lake..) in the form of grains. These grains can be very large (gravel, for example), finer (sand) or of minuscule size, forming mud.
These grains are in contact
one with the other, but empty space remains between them, space that defines the porosity of a rock. This is measured as a percentage of the total volume of the rock.
Why are oilmen so interested in the porosity and the permeability of rocks? Quite simply because, for a rock to contain large quantities of oil or gas, it must have good porosity (sufficient empty space where the hydrocarbons go to replace the water), and good permeability (so that the petrol and gas can move rapidly when the time comes to pump them in order to exploit them).
A rock that posses both a good porosity and a good permeability, is a reservoir. The better these two petro-physical characteristics of the rock, the better will be the reservoir. If the rock is fractured, its qualities as a reservoir are improved. Rocks that make a good reservoir are, in the majority of cases, sandstones or carbonates (calcareous and dolomite).
Clays possess a lot of space between the particles of which they are composed but these particles take the form of flat sheets that are piled tightly against each other. Their permeability is virtually zero. |