Also note lots of oil found thousands of miles from subduction zones.
And still doesn’t explain how in the sedimentary layer, different types of oil found under different layers of sealing rock.
Go look at continental drift maps over the three to four billion years the earth has been around. Best guess is when north America first formed, the west coast and current Rockies did not exist. The was another mountain range east of the Rockies that is completely gone, worn away.
Induction of the Pacific Plate under the West Coast has built up everything from the plains to the west coast since after the north American Continent formed.
Just because the subduction zone has moved doesn’t mean there was never one there.
If my hypothesis is true you would find older oil at shallower depths and newer oil deeper. Depending on the seal layers density/porousness, you could have ligher oils where the light ends could not be bleed off and heavier oil where light ends could migrate out. Oil could also vary from what got subducted with it. Had there been a recent meteorite or volcanic event close by?
The other hypothesis is that oil is also forming from carbon and hydrogen trapped in the earth’s core since it was formed. It could be a combination.
I’m simply not sure we have enough data at present to rule out any of it, including the possibility that all three are occurring.