The deeper you go, the more pressure is generated by the weight of the mud.
To the OP: The oil is spewing out not so much because it is a bit lighter than water, but mostly because it is MUCH lighter than the rock, sand, etc. on the sea floor. If it were the same density as the rock above, then it would not be at hardly any pressure when it got up to the sea floor. This is how the mud works to seal the well…not because it is thick and gooey but because it is nearly as dense as rock.