Oil is a very valuable resource that can be used for several purposes. It can be found underground, both in small surface pockets from y=40 to y=130, and larger pockets from y=-30 to y=70. Both types of pocket can only be found in Sedimentary rock.
Crude oil is either sweet or sour. Sweet oil is pure and easy to refine further, but sour oil needs to be processed a bit before it can be refined.
Multiblock
Multiblock