What are reservoir rocks?

A reservoir rock is a rock that has the ability to store fluids. Reservoir rocks have two key characteristics that give them the ability to store fluids: porosity and permeability.
Porosity is a measure of how porous a rock is. In other words, porosity measures how much of a rock is open space.
Permeability is a measure of how easily a fluid can move through a porous rock.
Of course, it is possible for a rock to have both high permeability and porosity and still not be a reservoir rock. The rock has to contain enough hydrocarbon accumulation sealed inside of it to make it commercially worthwhile to obtain. Generally speaking, sedimentary rocks make good reservoir rocks. Sandstone, carbonate, and dolomite sedimentary rock are common reservoir rocks.
http://www.dnr.louisiana.gov/assets/TAD/education/BGBB/4/rocks.html

https://connect.spe.org/blogs/donatien-ishimwe/2014/09/29/reservoir-rock-properties