Question

Petroleum occurs deep inside the earth floating over water. Which property makes it possible to form a separate layer over water?

Answer 1

Because of the difference of density of crude and water

The density of water is 1000kg/Cu m, where as the crude density ranges from 870–920kg/cu.m.
As you can see the lighter fluid will always float with respect to denser fluid.

Answer 2

Petroleum is less denser than water so it floats on the top of water and is insoluble in water