Shale oil is an oil distilled from shales and used as fuel

The reason that oil shale is not used to produce oil is because this is what keeps the oil well from collapsing after the oil is removed. If the oil shale were mined, the oil well would collapse into a large trench in the ground.

one has oil shale and one dosent

It does not matter where oil comes from it is bad for the environment shale oil is just a Les efficient source of oil.

Oil is not mined it is drilled except for oil shale which is mined. Shell in the past has mined for oil shale. Click the link.