What is the primary natural resource which is extracted in the process known as “fracking”?

The term refers to gas extraction by "fracturing" the rock apart. With the help of special hard rock drills, people make a hole and then direct a water-jet into the created well. It's obvious that these procedures are performed in order to release the gas.

It's an interesting fact that this "water-jet" is not just a stream of water as it also contains sand and even some chemical elements. The pressure of the water mixture is so high that during the process of its injection the gas is flowing out on its own.