That's not what it is. A reservoir occurs in a rock that looks on the suface like any other. This rock, which to you and me looks like a solid, has tiny little pore spaces, and that's where the oil and gas pool. And it's out of rocks like this that we have to get the oil and gas that we use every day. A porous rock.
Pumice is a good choice and is available in the foot care section of many pharmacies.
Chalk is also excellent for this experiment. A scale or balance that can weigh in units of one gram or less. A container of water that is big enough to hold the stone submerged in water. Put the rock in water and leave it there for at least an hour. Take the rock out and shake off the excess water.