one has oil shale and one dosent
That would be "Oil shale" - shale is a kind of rock. Anyway, the oil shale is dug up and then hot, hot steam or other substance is run through it to make the oil runny enough to flow down and out, where it is then collected.
Oil shale.
The main one is oil shale.
coal
Oil shale rock
shale
Oil rich Mississippian!
Shale
Shale rock / Roof rock
They are locked in shale rock.
Shale is a sedimentary rock that is structured in thin layers, like pages in a book. The oil is trapped between these "pages". Since the oil can't flow between the layers it only can be extracted at the broken edges of the shale deposit or by breaking the shale ip into tiny bits and washing the oil off for collection.