I've tried to tell people that percolating petro from deep under ground gets trapped in some formations, like salt domes and shale beds. That sedimentary formations get polluted with old compressed plant life. That oil, were it not dirtied up from these traps is greenish yellow, low sulfur, bunker grade crude. But they ain't having any.
I hope it starts out as better than bunker grade crude or we could never have light oil!