Not to mention the presence of oil does not mean there's always enough to be economically viable.
Usually it's not a question of quantity, but of quality. A minority percentage of all oil is "usable" for commercial purposes. Not enough temperature or pressure and it's "undercooked", too much and it's "overcooked".
For decades oil companies struggled to understand whether a petroleum deposit was commercially viable absent the expense of drilling and testing. It was a petrologist and paleontologist from Brooklyn (of all places) named Anita Harris who is recognized with discovering a simple test. She spent years studying conodonts - the toothlike structures belonging to a type of eel that lived across the Paleozoic / Triassic boundary from ~541 million years ago to ~252 million years ago. Conodonts are contemporaneous with the marine algae deposits in warm, shallow seas that would eventually become oil. The eels lived in those seas. So where you find conodont fossils, there's usually oil.
More important is the fact that conodonts are found in a spectrum of colors from pale / almost white to nearly black. Paleontologists for many years simply wrote the difference off as local variation, species variation, etc. Turns out they change color depending on temperature and pressure and there is a direct correlation between conodont color and the commercial viability of adjacent petroleum - the favored color being "coffee with light cream".
Since 1977 every oil company in the world has employed a fleet of paleontologists specializing in conodonts.