These two words represent what we know as a type of unconventional reservoirs which have revolutionized the traditional ways of finding, assessing and predicting the hydrocarbons production and their storage and flow mechanisms within the rocks.
Shales are rocks with high organic content, traditionally considered as a source rock, where the oil and gas were “cooked” in order to later migrate to a reservoir rock, which would be more permeable, but, what does it mean more permeable?
It means, in basic terms, that the grains conforming it allow to have a space between each other forming natural conduits sufficiently big to be approximated as a bunch of tortuous tubes sufficiently communicated with each other and continuous enough to permit the fluids to get naturally, without fracturing the rock to create this necessary communication, from their original reservoir location to the wellbore created for its extraction. In fact, shales do not have significant matrix permeability, but a set of natural microfractures and internal surfaces where the oil or gas is not only trapped, but also adsorbed onto the surface of the rock by intermolecular forces, and this is happening in the most of the rock. Therefore, in general cases, an increase in permeability process through matrix stimulation (as with fractures) is needed in order for this type of projects to suceed.
In the case of the gas molecules, because of the small size of the natural paths in the matrix created by the microfractures or the nanopores, micropores, etc., there are several phenomena, like molecular sieving, that make the gas flow in shales so hard to describe, that today there is no sufficiently accurate model or equation that fully describes the production of gas from a shale rock, as it exists in the conventional rocks (i.e. Darcy´s law), so today, the shale reservoir characterization topic is one of the most complicated and most studied due to the promising future of this type of hydrocarbons reservoir exploitation.